摘要:

Russian Chemical Reviews 69 (4) 279 ± 305 (2000) The chemical and adsorption effects of foreign molecules on the properties of high-temperature superconductors L L Makarshin, D V Andreev, V N Parmon Contents I. Introduction II. Some specific features of the physicochemical properties of high-temperature superconductors III. The effects of adsorption and intercalation of simple compounds on the properties of high-temperature superconductors IV. The effects of adsorption and intercalation of organic molecules on the properties of high-temperature superconductors V. The effect of superconductivity on the electronic state of the adsorbed molecules and on adsorption processes VI. Conclusion Abstract. of intercalation and adsorption of effect the on Data Data on the effect of adsorption and intercalation of foreign high-temperature of properties the on molecules foreign molecules on the properties of high-temperature super- super- conducting of effect reciprocal the on and materials conducting materials and on the reciprocal effect of superconduc- superconduc- tivity are molecules adsorbed of state electronic the on tivity on the electronic state of adsorbed molecules are considered.considered. Particular of analysis the to given is attention Particular attention is given to the analysis of experimental experimental conditions An obtained. results the of interpretation and conditions and interpretation of the results obtained. An effect effect of the and state electrophysical the on molecules adsorbed of adsorbed molecules on the electrophysical state and the critical critical parameters of features Specific noted.is superconductors of parameters of superconductors is noted. Specific features of the the interaction state superconducting the in phases the between interaction between the phases in the superconducting state and and the bibliography The discussed. are molecules external the external molecules are discussed. The bibliography includes includes 280 280 references. references. I. Introduction The discovery of oxide materials which become superconductors at the liquid nitrogen temperature 1, 2 has resulted in a rapid development of studies of their physicochemical properties. These studies showed that the high-temperature superconductiv- ity (HTSC) is extremely sensitive to the effect of the environment.An opinion exists that superconductors are thermodynami- cally unstable systems.3 On the other hand, it was shown in one of the first reviews on the thermodynamic properties and stability of yttrium-containing superconducting ceramics 4 that, although the YBa2Cu3O7 phase is somewhat unstable thermodynamically, it can exist almost infinitely in this state. As a result of interaction of oxide ceramics with foreign molecules, superconductivity can both disappear and appear. Both of these phenomena have been observed experimentally in a study of the effect of the external molecules on the properties of HTSC oxides. Systematic analysis of the effect of foreign molecules on HTSC materials can help to reveal new ways for reaching high superconducting transition temperatures. From the applied viewpoint, an important task is to obtain HTSC materials with high critical superconductivity parameters, L L Makarshin, D V Andreev, V N Parmon G K Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, prosp.Akad. Lavrent'eva 5, Novosibirsk 630090, Russian Federation. Fax (7-383) 234 30 56. Tel. (7-383) 234 28 31. E-mail: makarshin@catalysis.nsk.su (L L Makarshin), andreev@catalysis.nsk.su (D V Andreev), Tel. (7-383) 234 32 69. E-mail: parmon@catalysis.nsk.su (V N Parmon) Received 24 March 1999 Uspekhi Khimii 69 (4) 307 ± 336 (2000); translated by S S Veselyi #2000 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2000v069n04ABEH000521 279 279 282 292 297 301 viz., temperature (Tc), current (Ic) and current density (Jc).Research on both classical low-temperature superconductors and HTSC materials has shown that high critical currents can be obtained if the superconductor structure incorporates an opti- mum number of defects acting as pinning centres for the Abrikosov vortices. Systematic studies of processes for the con- trolled formation of defects in superconducting materials has led to the conclusion that the intercalation of various chemical admixtures produces additional pinning centres, and hence increases the critical current density. Recently, great interest arose in new electronic sensors which contain layers of organic molecules or polymeric films deposited on the surfaces of semiconductors or metals.5 ±7 Such elements have a significant advantage over the usual inorganic solid-state sensors, i.e., enhanced sensitivity and selectivity with respect to environment.8± 10 A novel class of oxide materials, viz., high-tem- perature superconductors that possess a combination of unique physical properties at relatively high temperatures, has also become the subject of intense studies aimed at their application in similar electronic elements.For instance, it was found that molec- ular dye layers deposited on a Josephson junction made of super- conductors increase considerably the sensitivities and selectivities of light sensors with respect to visible light.11 ± 13 This was the first documented example of the effect of the properties of an HTSC material on the electron state of adsorbed organic molecules.In this review, we have analysed the reported data on the effect of adsorption and intercalation of molecules on the physicochem- ical properties of ceramic and film HTSC materials and on the effect of the superconducting state on the low-temperature adsorption, catalysis and the electronic state of the molecules adsorbed on the HTSC material surfaces. Intensive studies are now being carried out along these lines, hence our review cannot claim to be comprehensive. In some cases, the interpretation of certain experimental results can be debatable. Nevertheless, the possibility of the reciprocal influence of the superconducting state and the molecular processes on the surface of HTSC materials is an established fact, which however requires detailed studies.II. Some specific features of the physicochemical properties of high-temperature superconductors All high-temperature superconductors are copper-containing oxides of complex chemical composition. As a rule, each unit cell of an HTSC phase incorporates from four to six chemical elements.14280 The data from small angle neutron scattering indicate that polycrystalline HTSC materials contain rather branched systems of pores connected to the surfaces of samples.15 Hence, molecules from the environment can readily penetrate into the specimen and react with the superconducting phase.The specific surface of such specimens measured by the Brunauer ± Emmett ± Teller (BET) method on the basis of the low-temperature nitrogen adsorption is from 0.2 to 1.5 m2 g71, which is rather high for compact ceramic materials. The thermodynamic instabilities of HTSC materials can manifest themselves in their smaller stabilities in comparison with those of simpler low-temperature superconduc- tors, viz., metals and alloys. Obviously, the effect of foreign molecules on HTSC materials can be rather complex. In addition, the study of these effects and particularly their interpretation can be complicated considerably because the surface chemical composition of an HTSC material differs significantly from its bulk composition.In order to classify possible mechanisms of the interaction between the HTSC materials and the external molecules, it is worthwhile to consider some characteristic features of the struc- ture of these materials and the mechanism of the initiation of the superconducting state. 1. Specific features of the structure of high-temperature superconducting oxides Such multi-component oxide systems as La7Ba(Sr)7Cu7O, Y7Ba7Cu7O, Bi7Sr7Ca7Cu7O with perovskite-like or defective perovskite-like structures (Fig. 1) represent typical HTSC materials.16, 17 Virtually all HTSC materials are characterised by the exis- tence of copper ± oxygen layers and chains. The first and most important factor enabling superconductivity to appear is the existence of at least two types of copper coordination in the structural cell of the material; this results from the ability of the lattice copper ions to exist in several oxidation states, namely Cu(I), Cu(II) and Cu(III).Overlapping of the 3d orbitals of copper with the 2p orbital of oxygen results in the formation of a valence band and strong delocalisation of the holes.18 In the structure of CuO2 (the copper ± oxygen layers), the holes migrate along the oxygen anions, which makes the 3d 9L-type electron configuration much more probable than 3d 10L (where L is the oxygen vacancy).19 This specific behaviour of copper ions (in particular, the possibility of existence in several different coordinations) is a c b Figure 1. Unit cells of typical HTSC oxides.(a), La27xSrx(Bax)CuO4; (b), YBa2Cu3O77x; (c), Bi2(Ca,Sr)3Cu2Oz (one half of the unit cell is shown).16, 17 b c BaY La(Sr, Ba) O(2) O(2) O(1) a O(5) b L L Makarshin, D V Andreev, V N Parmon closely related to a manifestation of the Jahn ± Teller effect by these ions. The second factor responsible for the superconductivity of oxide systems is the low spatial dimensionality of the electronic structure, which is nearly two-dimensional. The low effective dimensionality of the electronic structure of HTSC materials results from the presence of the copper ± oxygen layers (see Fig. 1). It is in these layers that the electrons providing the superconductivity are localised. The low structural dimensionality of an HTSC material is directly proved by strong anisotropy of superconductivity: the critical current along the crystallographic axes a and b is by an order of magnitude higher than that along the c axis.20, 21 The low dimensionality of the electronic structure is not the only factor which enables the possibility of existence of the superconducting state.For example, it has been shown 22 that the superconductivity of La2CuO4 is much more strongly affected by violation of the stoichiometry of this oxide with respect to oxygen than by structural changes. The non-stoichiometry effect becomes more pronounced as the La :Cu atomic ratio decreases. The excess of copper favours the formation of additional oxygen vacancies; this facilitates the incorporation of oxygen during synthesis, resulting in partial oxidation of Cu2+ into Cu3+.Unlike La27xCuIIO4, the La27xCuII/IIIO4 compound is a superconductor. In YBa2Cu3O77x, violation of stoichiometry occurs due to the filling of the oxygen rows at the O(4) position (see Fig. 1 b).23 ± 25 The superconducting orthorhombic structure of YBa2Cu3O77x exists at the oxygen index x40.6. For x>0.6, the unit cell of this oxide is characterised by a non-superconducting tetragonal struc- ture. Figure 2 shows the dependence of the superconducting transition temperature on the oxygen index x.24 One can see that the quantity of the excess (or mobile) oxygen affects substantially the ceramics superconductivity. The role of oxygen in HTSC materials has been discussed in several publications (see, e.g., Refs 23, 24, 26 ± 28).In the ortho- rhombic phase of YBa2Cu3O77x , oxygen can occupy different crystallographic positions:27 the O(2) and O(3) positions in the [CuO2] plane and two positions, O(1) and O(4), in the Cu7O chains (see Fig. 1 b). The oxygen states O27, O7 and O2¡ 2 , which exist in dynamic equilibrium and affect the superconducting and chemical properties of the majority of HTSC materials, have been predicted theoretically and found experimentally.28 The high oxygen mobility in the material bulk is provided by the oxygen c c O(2) O(3) Cu(2) O(3) O(1) O(1)a Cu(1) a O(4) b Bi Sr,Bi Ca,Sr CuThe chemical and adsorption effects of foreign molecules on the properties of high-temperature superconductors Tc /K 80 400 0.6 x 0.4 0.2 Figure 2.Dependence of Tc of the YBa2Cu3O77x ceramic on the oxygen index x.24 forms which have the smallest charge or even no charge at all, whereas electron migration is provided by the O27 ions. 2. Structures of polycrystalline specimens Ceramic polycrystalline HTSC materials are of the greatest interest. This is due to their accessibility and relative simplicity of their preparation, which enables the synthesis of single-phase specimens with high superconducting transition temperatures. The main obstacles to the wide practical application of polycrys- talline superconductors include the low critical currents and their rapid decrease as the field increases.In addition, ceramic HTSC materials have low corrosion resistance. A polycrystalline HTSC material (ceramic) is a system of anisotropic superconducting granules with sizes ranging from 1 to 100 mm; they are linked in a three-dimensional network of weak Josephson junctions.27, 28 In a general case, a sandwich-like structure of the SNISN type appears on the boundaries between the granules (where S is a superconductor;Nis a layer with normal metallic conductivity or a superconductor layer with a lower critical temperature; I is a dielectric or semiconductor layer). The thickness of the boundary layer allowing superconductivity cur- rent to flow between the granules should not exceed the coherence length along the crystallographic axis c [ec(0 K)=0.2 nm] and in the ab plane [eab(0 K)=1.64 nm].29 Weak bonds between the granules of ceramic HTSC materials differ in nature.In this review, we shall consider only those typical features of the structure and properties of inter-granule layers that can be affected by molecules foreign to the given material, namely: (1) non-stoichiometry of granules with respect to oxygen in the near-boundary layers; (2) high probability of admixture segrega- tion at the inter-granule boundaries; (3) possibility of defective packing; (4) presence of `foreign' or amorphous phases at the inter-granule boundaries. The oxygen non-stoichiometry is due to surface rearrange- ment because of a decrease in the surface energy at the granule boundaries.The boundary layers with a high fraction of the lattice sites belonging to both contacting microcrystals are also energeti- cally favourable. Studies of admixture-free boundary granule layers of single- phase ceramic with composition YBa2Cu3O77x (Refs 30 ± 33) showed that enrichment of granules with copper and their depletion of oxygen occurs in a boundary layer no thicker than 5 nm. The phase formed at the granule boundaries has a ortho- rhombic structure. Within the percolation theory based on the random distribu- tion of oxygen defects, the critical oxygen deficiency (the oxygen index x) was determined; above this value, the oxygen-enriched structures (superconducting granules) are separated with insulat- ing layers and the inter-granule superconductivity transport current is absent.34 The value of x for the YBa2Cu3O77x HTSC ceramic found by the Monte Carlo method for a square lattice (two-dimensional superconductivity) was 0.26.34 Segregation of admixture atoms and phases at granule boun- daries seems to be the simplest and most natural reason for the formation of inter-granular layers possessing metallic conductiv- 281 ity and/or dielectric properties. In most cases, carbon and/or carbon-containing compounds constitute one of the main admix- tures on the granule surfaces.Carbon can originate not only from the starting compounds used for the material synthesis, e.g., barium carbonate, but also from atmospheric carbon diox- ide.35 ± 37 If excess BaCO3 is present in the starting mixture for the ceramics synthesis, carbon is segregated on the granule boundaries and forms a coating 1 ± 4 monolayers thick.38 It is of note that carbon is not present on intergranule fractures. In addition to carbon and its compounds, non-superconduct- ing phases formed from the starting compounds due to incomplete synthesis were found on granule boundaries.39, 40 For example, local melting at granule boundaries in copper ± barium ± yttrium HTSC materials at synthesis temperatures above 930 8C results in the formation of regions enriched with barium (the BaCuO2 phase), yttrium (the BaY2CuO5 phase) or copper (the liquid phase).41 The formation of defective packing in the crystal structure at inter-granule boundaries and the formation of a twinning struc- ture in the ceramic microcrystals are accompanied by relaxation of elastic stresses that appear upon structural phase transitions in the temperature range from 873 to 973 K.For example, the absolute majority of the granule boundaries of the YBa2Cu3O77x ceramics contain inter-layers consisting of blocks of atomic planes CuO27 BaO7(CuO2)7BaOx ca. 1.5 nm thick; this is explained by the breaking of the layered structure in YBa2Cu3O77x. This process is similar to the formation of the YBa2Cu4O77x phase with a critical temperature of 80 K.42 ± 44 Since the coherence length along the axis c is 0.7 nm, the presence of such inter-layers on the bounda- ries of the majority of granules should have a strong adverse effect on the ability of the ceramics to conduct electric current.The presence of the twinning structure inside the microcrystal- line granules favours the formation of domains shaped as long (up to 1 nm) narrow `bands' parallel to the (110) or (110) axes. The width of these domains varies from 5 to 300 nm and depends on the technique for the ceramic's synthesis and on the granule size.45 These structures also deteriorate the current-carrying ability of HTSC materials. 3. Properties and structures of real high-temperature superconducting oxide surfaces The interaction of the external molecules with an HTSC material starts at its surface. It has been noted above that, generally, the chemical composition, state, and structure at the surface and in the bulk of a superconductor differ considerably.However, the properties of the surface layer of an HTSC material affect directly the electronic properties and superconductivity parameters of the bulk phase.46 The surfaces of the specimens are highly non-uniform. They contain macroscopic defects, such as growth steps, cracks, pores, dislocation exits, etc. Due to the interaction of coordination- and/or valence-unsaturated surface atoms with the external molecules, various chemical components are invariably present at the surface. The adsorption of molecules and atoms often changes the properties of the surface itself, particularly the electron state of the surface atoms. The study of the YBa2Cu3O77x ceramic by photo- emission spectroscopy revealed a difference in the state of the `pure' and `contaminated' surfaces.47 Typical contaminations appear due to the chemisorption of the H2O, CO, and CO2 molecules.The content of surface carbonates on `contaminated' specimens is 10 at.% ± 15 at.%, while that on a `pure' specimen is 1 at.%. The surface also contains barium hydroxide, copper oxides, various yttrium compounds and the so-called `green' phase, i.e., Y2BaCuO5. The surface copper exists in the Cu+ and Cu2+ states;48 the near-surface layer does not possess super- conductivity.49 It is therefore obvious that controlled surface cleaning is an important problem in studies on the effect of the surface states on the material superconductivity.282 It is well known that chemical etching of polycrystalline films of YBa2Cu3O77x with solutions of halogens and hydrogen halides in alcohols improves the stoichiometric composition of the surface and decreases considerably its contamination with carbonates and hydroxides.50 For example, it was shown 51 that the electric characteristics of the tunnel contact between the YBa2Cu3O77x film and metallic niobium are improved considerably after chem- ical etching of the film.In this case, an inter-layer of the SNS type is formed in which the surface resistance is almost 30 times smaller and the critical current is 40 times greater than those in contact via a non-cleaned surface. An inter-layer between the superconduc- tors can be fabricated by chemical etching. Probably, this type of treatment of superconducting ceramics not only decreases consid- erably the surface contamination but also brings about certain chemical modification of the surface layer of the YBa2Cu3O77x system due to reactions of halogens and hydrogen halides with metal oxides on the surface.In fact, treatment of an YBa2- Cu3O77x film surface with a 1%±2% solution of Br2 or HBr in ethanol results in the following surface compounds: YBr3, BaBr2 and CuBr.52 Scanning electron microscopy did not reveal any significant changes in the surface morphology after chemical etching. It is important that surface treatment of this type increases its resistance against atmospheric gases. Interesting results were obtained by bombardment of HTSC film surfaces with argon and oxygen ions.Bombardment with argon ions with an energy of 500 eV results in strong damage of the surface layer.23, 53 Oxygen atoms are preferentially removed from the surface and hence copper is reduced (while electro- neutrality is maintained). This damage affects directly the oxygen vacancies and causes disordering of the near-surface layer atoms. As a result, the superconductivity characteristics of the films become much worse. However, the original properties of the films are completely restored after annealing in pure oxygen. Quite a different picture is observed if the surface of an HTSC material is bombarded with oxygen ions.54 In this case the surface etching rate is higher than that in the case of argon bombardment.After a brief bombardment, the dependence of the superconduct- ing transition temperature for the YBa2Cu3O77x film on the etching time passes through a maximum with DT=2 K and then tends to the original value. Hence, cleansing of the surface of an HTSC material by bombardment with oxygen ions is preferable to cleansing with argon ions. The mechanism of the effect of high-energy oxygen ions on the superconductivity transition temperature is not quite clear yet. However, it can be assumed that during bombardment with O7 ions, the surface layer, which is originally depleted of oxygen, is forcedly saturated with it, which favours the formation of the superconducting phase. Although scribing (mechanical surface cleaning) of the YBa2Cu3O77x ceramic decreases the contamination with foreign molecules, it damages the surface considerably.55 Studies of the surface by characteristic electron energy loss reflection spectro- scopy showed that the near-surface layer contains an excess of barium and oxygen atoms, while the surface concentration of the copper ± oxygen planes is insignificant.This is explained by the fact that scribing mostly results in microcrystal breaking along the inter-granule boundaries or at the Ba7O planes. In addition, preferential diffusion of the Ba7O complexes over the surface occurs. One should pay attention to the nature and structure of the adsorption-active sites capable of adsorbing foreign molecules on the surfaces of HTSC materials. As shown in earlier studies,56 the structures of real surfaces of oxide semiconductors and dielectrics are rather complex.The nature of the adsorption-active sites is determined by the state of oxygen in the near-surface region: deficiency or excess of oxygen give rise to electron-donating or electron-withdrawing active sites, respectively. Studies of the YBa2Cu3O77x ceramic surface revealed active sites consisting of Cu+ cations and an oxygen vacancy (&, hole), which is an electron acceptor.57 At room temperature, an active L L Makarshin, D V Andreev, V N Parmon 2 site of the [Cu+, &] type is stabilised by the elastic strain field of dislocations or by boundaries of twinning structures.58 At 400 K, diffusion of the interstitial O7 ions begins to the sinks on the surface (holes or dislocation exits).Trapping of the O7 ion by the oxygen vacancy results in electrically neutral oxygen and a free electron. Afraction of the O7 ions reaching the Cu2+ surface ions can transfer a hole to them; this probably results in Cu3+. Subsequent evolution of the active site at 580 K involves migra- tion of neutral oxygen from the bulk; it `catalyses' the decay of the [2Cu3+, 2O27] site accompanied by the emission of the O27 and Cu2+ ions to the surface.59 The disappearance of the O2¡ bipolarons (local hole pairs) after the first heating in vacuo at 623 Kmakes the transition of this ceramic to the superconducting state at T>77.4 K impossible. The evolution of the state of oxygen on the YBa2Cu3O77x surface at low temperatures was studied 59 by recording the trans- formation of the O(1s) X-ray photoemission spectrum upon temperature decrease from 300 to 110 K.At 110 K, the O7 ions in the near-surface region are dimerised to give the O2¡ 2 dimer, but the oxidation state of the copper ions remains unchanged. Simultaneously, the surface electric conductivity increases. III. The effects of adsorption and intercalation of simple compounds on the properties of high- temperature superconductors Intercalation of simple molecules into HTSC ceramics is accom- panied by various physicochemical transformations: replacement of nodal atoms in the crystal lattice, intrusion of foreign molecules into the interstitial space, diffusion of molecules along the ceramic granule boundaries and formation of strong chemical bonds at the surface.Unfortunately, the available experimental data do not allow yet an unambiguous conclusion regarding the nature of the interaction between small molecules and the superconductor material to be made. For example, it is believed that the low-temperature adsorp- tion of molecules on oxides is generally of physical character, hence it is non-specific and its dependence on the availability of adsorption sites and the electron state of the surface is insignif- icant. Physical adsorption can result in only small changes in the surface charge and the vibration modes of the surface atoms. However, in reality the adsorption of certain molecules on HTSC materials at T<300 K affects their superconductivity; this can- not be explained by simple physical adsorption.As a rule, the changes in the superconductivity characteristics observed in these cases are related to intercalation or inclusion phenomena of molecules into the structure. The possibility of intercalation and chemical interaction of simple gases with HTSC materials at high temperatures (> 473 K) has been established reliably and does not raise doubts (see below). 1. The effect of oxygen Oxygen plays a particularly important role in the formation of the structure of perovskite-like complex oxide materials and in their transition to the superconducting state. In addition, the oxygen molecule is a paramagnetic species, its adsorption on a super- conductor surface can affect directly the superconducting proper- ties.The techniques for obtaining HTSC materials and the role of oxygen in the formation of superconducting phases have been well covered in the literature. The reaction pathways of oxygen with HTSC materials during the synthesis and high-temperature treat- ment are so diverse that, despite a great number of publications, this phenomenon still remains insufficiently studied because the problem of non-stoichiometry with respect to oxygen and the related phenomena for the RBa3Cu3Oz+O2 systems (where R is a rare-earth element) are excessively complex.60 We shall only discuss the main aspects of such interactions.The chemical and adsorption effects of foreign molecules on the properties of high-temperature superconductors One of the first Russian reviews devoted to the role of oxygen in HTSC materials was published in 1989.61.In the final stage of the synthesis, the oxide YBa2Cu3O77x is saturated with oxygen. When compact ceramics are being obtained, this process is rather complicated, particularly in the synthesis of highly oxidised speci- mens (x?0). It was shown in a study of the oxidation kinetics of ceramic specimens of various densities at 773 K that slow oxygen absorption by compact ceramics results from transition of the reaction from the diffusion mode to the kinetic mode due to accumulation of considerable elastic strain in the material.62 Thermodynamic estimates showed that the elastic strain energy upon oxygen absorption by compact ceramics can affect the oxidation kinetics only in the case where the base planes contain oxygen ions of two types, namely, those strongly and weakly bound to the lattice.The diffusion of oxygen into the YBa2Cu3O77x ceramic was described 63 within a one-dimensional diffusion model. The equa- tions obtained provide a good description of the experimental results on the oxygen exchange kinetics. The transformation of the specimens during oxygen diffusion in the temperature range of 423 ± 973 K and at oxygen pressures of 1.4 ± 10.7 atm involves three stages. A solid solution of oxygen in the tetragonal phase YBa2Cu3O6 is formed in the first stage.64 In this case, the diffusion factor is described by an Arrhenius dependence with the activation energy of 0.14 eV, while the pre-exponential factor depends on the oxygen pressure.In the second stage, oxygen starts to diffuse along unordered CuO chains in the tetragonal phase. The diffu- sion factor is also described by an Arrhenius dependence, but the activation energy is 0.6 eV and the pre-exponential factor no longer depends on the oxygen pressure. In the third (final) stage where the tetragonal phase is transformed to the orthorhombic one the diffusion factor becomes smaller than that in the tetrag- onal phase and gradually decreases to zero as the transition to the ideal orthorhombic phase of YBa2Cu3O77x occurs. Subsequent systematic studies of oxygen desorption from the YBa2Cu3O77x ceramic at high temperatures showed 65, 66 that this process probably occurs according to a different mechanism.The main features of this process will be described in more detail below; for now, let us note that the adsorption of molecular oxygen on the surface of an HTSC material occurs according to the dissociation mechanism, whereas desorption occurs according to the association mechanism. The oxygen atoms occupy the vacant positions, i.e., O(1) or O(5), in the near-surface layer of the crystal lattice. The exchange and transport of oxygen during the adsorption±desorption processes on the YBa2Cu3O77x ceramic probably involves oxygen vacancies according to the relay-race mechanism. The kinetics of oxygen evolution from the YBa2Cu3O6.927x ceramic with x<0.54 was studied in the temperature range of 600 ± 950 K using the thermo-programmed desorption method.65 At T<Tm (where Tm is the temperature corresponding to the desorption peak maximum), the desorption rate is described fairly well by a second-order kinetic equation which is formally con- sistent with associative desorption.Oxygen-deficient specimens with x<0.25 are characterised by a noticeable increase in the activation energy accompanied by an increase in the pre-exponen- tial factor; this implies the presence of a compensation effect. Many authors (see, e.g., Refs 26, 67 ± 69) consider the surface potential barrier as the main obstacle to the release of oxygen from HTSC materials. It was shown convincingly that the nature of this barrier is determined by the surface energy parameters.69 For example, the oxygen desorption rate increased, whereas the desorption activation energy decreased by 0.20.03 eV after vapour phase deposition of several monolayers of silver on the surface of a sample of YBa2Cu3O77x ceramic.Changes in the surface properties should not affect the oxygen desorption rate if oxygen diffusion in the granules is the limiting step of this process. UV photoelectron spectroscopy data suggest that the electron work function for the yttrium ceramic's surface decreases by 0.3 eV upon vapour deposition of silver. It is thought that this 283 decreases the surface energy barrier.69 The results of this experi- ment also indicate that the barrier is in fact energy-related rather than resulting from the formation of a particular surface structure. The temperature where oxygen starts to enter the ceramic was observed to decrease upon `mild' treatment of the ceramic with molecular hydrogen.70, 71 This is also evidence of the importance of surface barriers.The temperature of oxygen penetration into HTSC ceramics also decreased upon thermal vacuum treatment 72 and after plasma treatment.73 Several parameters characterising the surface potential barrier for the release of oxygen atoms were determined.74 It was found by isotope exchange between the gas phase and the surface of a YBa2Cu3O77x single crystal under the conditions of isochronous and isothermal annealing of the specimens that the activation energy for oxygen desorption is 1.08 eV.In the authors' opinion, this indicates that the sorption mechanism of the formation of the surface barrier upon isotope exchange is somewhat preferable. However, an unambiguous conclusion on the surface barrier nature could not be made within the scope of this study. The use of polycrystalline specimens instead of single crystals does not affect the main results. Degradation of HTSC materials at room temperature in the presence of water vapour is accompanied by the loss of oxygen from the HTSC phase. The water layer adsorbed on the super- conductor surface probably favours the acceleration of oxygen exchange between the HTSC material and the gaseous environ- ment.75, 76 It was confirmed 75 that oxygen is desorbed at room temperature from the YBa2Cu3O77x ceramic covered with an adsorbed water layer.In this case, the basic orthorhombic phase OI is coated with a modified orthorhombic OII phase with Tc=60 K. The proportion of the OII phase increases with an increase in the degree of hydration of the material and decreases upon annealing the specimen at 913 K. The low-temperature oxygen exchange between the YBa2Cu3O77x ceramic and the gas phase is limited by the surface potential barrier. Probably, the adsorbed layer of water molecules lowers this barrier significantly. On the other hand, it was shown theoretically 77 that the diffusion of On7 oxygen ions inside the crystal lattice `channels' [1/2, b, 0], which can be regarded as the oxygen sublattice, occurs without activation and does not limit the redox processes.The hypothesis on the high mobility of oxygen in the YBa2Cu3O77x lattice at low temperatures was confirmed.76 It should be noted that the high mobility can also involve a different mechanism. Based on thermogravimetric and X-ray diffraction analyses and electric conductivity measurements, a scheme for YBa2Cu3O77x reduction at room temperature was suggested. According to this scheme, the oxygen diffusion is considered as an activationless process. Depending on the water vapour pres- sure, the stage limiting the oxygen exchange rate also changes. The oxygen exchange between YBa2Cu3O77x and the gas phase at pH2O5760 Pa is limited due to the presence of a chemisorbed water surface barrier. In the pressure range 250<pH2O<760 Pa, the rate of YBa2Cu3O77x reduction is limited by the step of On7 anion diffusion through the layer of physically adsorbed H2O molecules, while at 80<pH2O<250 Pa, the movement of the On7 anions is hindered by the twinning boundaries.At the lowest humidities (pH2O<80 Pa), all the above factors no longer oper- ate, and the reduction rate exceeds 0.15 mass% min71. However, a layer of the tetragonal phase is formed after some time on the YBa2Cu3O77x surface; this layer does not conduct oxygen ions at low temperatures. A study 66 of the oxygen isotope exchange between 18O from the gas phase and 16O from the YBa2Cu3O6.92 ceramic in the temperature range of 293 ± 923 K aimed at the elucidation of the nature of labile oxygen, which plays an important role in hetero- geneous oxidation.The maximum oxygen isotope substitution at 923 K was 3.80.3 a.u. per formula unit of the cuprate. It was assumed that the chain [O(4)] and bridging [O(1)] oxygen atoms in the cuprate crystal lattice are replaced almost completely. Hence, it is in these positions that labile oxygen, which probably partic-284 ipates in catalytic oxidation on the surface of these ceramics at T>800 K, is localised. The inequality of the oxygen atom positions in the CuO2 and ab planes results from the orthorhombic structure of the YBa2Cu3O77x crystal. According to the experimental data 78 ¡¾ 81 and theoretical calculations,82 the activation energies EaOO3U and EOa O2U for oxygen exchange between the O(3) and O(2) positions (see Fig.1 b) are equal to 1.02 eV and 1.18 eV, respectively. The transition from the O(3) to the O(2) position occurs with an activation energy of EOa O3U=E07DE, and the transition from the O(2) to the O(3) position occurs with an activation energy of EOa O2U=E0+DE. The difference between these energies, 2DE=0.16 eV, is explained by the unequal O(3) and O(2) oxygen positions in the orthorhombic lattice, where the oxygen ¡¾ copper bond along the a axis is longer than that along the b axis. To the first approxima- tion, the ratio of the occupancies of these positions with oxygen is described by the Boltzmann equation , a exp ¢§2DE kT NOO3U 0 NOO2U 0 0 0 while the oxygen index is described by the expression x=NOO3U a NOO2U 71 .The features of oxygen ordering in YBa2Cu3O77x (x&0.1) were studied by transmittance electron spectroscopy and electron microdiffraction.83 Near the superconducting transition point, oxygen ordering occurs in the YBa2Cu3O77x crystal; this is accompanied by an increase in the orthorhombic parameter R a ja ¢§ bj a a b . These results suggest that oxygen atoms can diffuse in the crystal even at liquid nitrogen temperature. Based on elastic relaxation measurements in the YBa2Cu3O77x ceramic in the temperature range of 283 ¡¾ 1073 K, the activation energy of oxygen exchange between the O(4) and O(5) positions (see Fig.1 b) Ea=1.07 eV and the pre-exponential factor in the Arrhenius equation for the self-diffusion factor D0=1.861074 cm2 s71 were found for various oxygen partial pressures.78 The activation energy decreases with an increase in the oxygen content in the ceramics. More detailed data on Ea and D0 can be found else- where.60 The phase stability of copper cuprates is directly related to the diffusion of labile oxygen. A novel technique based on direct coulometric titration was developed for the study of the redox kinetics of YBa2Cu3O77x and for the measurement of the oxygen diffusion factor under various conditions.84 It was found exper- imentally that the oxygen diffusion factor in the specimens decreased monotonically with a decrease in x from 0.8 to 0.1.In the authors' opinion, this phenomenon can be explained by the vacancy-related oxygen diffusion mechanism. The temperature dependences of the measured diffusion factors obey the Arrhenius equation; these are in good agreement with previous results (see, e.g., the review 26). In continuation of these studies,85 the phase stability limits of the compounds RBa2Cu3O77x (R=Y, Nd) were determined over a broad range of oxygen partial pressures. ForR=Nd, the stability limit lies at higher temperatures than for R=Y. This is explained by a decrease in the energy of repulsion between the oxygen ions due to the larger distance between them L L Makarshin, D V Andreev, V N Parmon because of the larger ionic radius of Nd3+.The dependences of the diffusion parameters (D0 , Ea) on the oxygen non-stoichiometry index (x) and on the oxygen partial pressure were found. The diffusion of oxygen in the orthorhombic phase occurs faster than in the tetragonal phase. Practical recommendations were given for reducing the probability of microcrack formation, improving the contacts between the crystallites and shortening the low-temper- ature annealing when the HTSC ceramics are oxidised in the final stage of synthesis. Studies on oxidation of the YBa2Cu3O77x ceramic under polythermal conditions showed that the temperature dependence of oxidation obeys the Arrhenius function, while the activation energy of this process changes monotonically as x increases.86 In the ranges 0.2<x<0.3 and 0.6<x<0.7, the Ea(x) dependence displays characteristic breaks. The authors suggest that this is closely related to a change in the mechanism of oxygen ion ordering in the CuOz planes.In the first case, a change in the oxidation mechanism results from the fact that 1/9 of the oxygen vacancies residing in the planar planes are occupied and their subsequent chaotic occupation is no longer possible. The second break most probably corresponds to the transformation of the tetragonal phase to the orthorhombic phase. The diffusion of oxygen in Y17yPryBa2Cu3O77x was studied 87 by measuring the electric conductivity of a sample in the temperature range of 300 ¡¾ 1000 K. The desorption of oxygen from Y17yPryBa2Cu3O77x was much slower than that from the pure YBa2Cu3O77x phase and decreased with an increase in y.The number of charge carriers in the ceramics studied was controlled by the oxygen content and did not depend on the Pr concentration. The specific features of the thermal destruction of HTSC materials were analysed.88 Analytical expressions for the depend- ence of the extent of oxygen non-stoichiometry on temperature and pressure were suggested on the basis of pseudo-chemical reaction models with consideration of the formation of point defects on copper ions and oxygen vacancies. It was found that the thermal destruction kinetics of high-temperature superconductors are described by chain nucleation equations. A study of the processes of oxygen evolution from the YBa2Cu3O77x ceramic upon thermal desorption in vacuo (1075 Pa) revealed three characteristic temperature regions of oxygen evolution with maxima at 517, 673 and 932 K.89 The first region corresponds to the loss of the chemisorbed, weakly bound oxygen; the second region, to the transformation of the ceramic from the orthorhombic phase to the tetragonal phase involving cleavage of the O7Cu(III) bonds; the third region, to the decom- position of this phase to give yttrium and barium cuprates as well as copper oxides with cleavage of the O7Cu(II) bonds.Thermog- ravimetric analysis of the YBa2Cu3O77x ceramic in vacuo at temperatures up to 1273 K revealed two temperature regions (673 ¡¾ 873 and 1173 ¡¾ 1233 K) where mass loss occurred due to the loss of oxygen.90 The first region corresponds to the reversible oxygen loss related to a change in the copper oxidation state, Cu2+?Cu+, whereas the second region corresponds to the irreversible oxygen loss accompanied by the destruction of the perovskite structure of the ceramics and formation of barium and yttrium cuprates.Annealing of the YBa2Cu3O77x ceramic in vacuo at 1073 Pa and 1073 K results in the destruction of the orthorhombic structure.91 On average, up to two oxygen atoms are removed from each unit cell, and the ceramic acquires a composition described by the formula YBa2Cu3O6. Oxygen is completely removed from the O(4) positions (one atom per cell) and partially from other positions (see Fig.1 b). The oxidation state of one- third of all copper ions becomes equal to unity. Annealing of the sample in air at 773 K results in the intercalation of oxygen into the material crystal structure; the overall number of oxygen atoms per unit cell reaches six. The isothermal oxidation of the YBa2Cu3O77x ceramic com- prises two stages, viz., a diffusion-controlled stage and a `slow' oneThe chemical and adsorption effects of foreign molecules on the properties of high-temperature superconductors 2 where the oxide is in equilibrium with the gas phase. The stepwise increase in the parameter x over time during the `slow' stage was studied.92, 93 This stepwise change in x is due to the structural transformation of the tetragonal phase to the orthorhombic one; as a result, mechanical stress is generated in the crystal, which is followed by its relaxation.This effect is described fairly well within the framework of a model which takes not only the T and pO parameters but also the elastic intercrystallite interaction energy into account. The kinetics of oxidation of the YBa2Cu3O77x ceramic in the temperature range of 748 ± 873 K was explained 94 using a concept according to which the oxygen diffusion in the tetragonal cuprate phase occurs at the O(4) and O(5) vacancies and is thermally activated, while in the orthorhombic phase it occurs along specific structure `channels' and does not require thermal activation. The oxygen migration in the orthorhombic phase is controlled by the interaction of the oxidant with the domain boundaries.A study of the thermal stability of the orthorhombic phase of YBa2Cu3O77x in vacuo and in oxygen showed that transforma- tion of the orthorhombic phase to the tetragonal phase in vacuo (1073 Pa) occurs at 973 K.95 An increase in the oxygen pressure to 1 Pa increases the transformation temperature to 1073 K, while in pure oxygen, at a pressure of 105 Pa, this value is as high as 1173 K. A phase diagram that characterises the dependence of the temperature of transformation of the orthorhombic phase to the tetragonal phase on the oxygen pressure has been published.96 Diffusion processes in thin superconducting films differ from those in bulk ceramic materials and single crystals.The diffusion of oxygen in thin epitaxial films of GdBa2Cu3O77x was studied by measuring the electric resistance under isobaric and isothermal conditions in the temperature range of 650 ± 1050 K.97 Fast oxy- gen diffusion occurs preferentially at defects located along the c axis and with low activation energies. Chemical diffusion at vacancies occurs in the ab plane. The diffusion factor D in films thicker than 10 nm is*10713 cm2 s71. The thermal activation of diffusion is well described by an Arrhenius-type dependence and does not depend on the film thickness. At oxygen pressures around 102 Pa, the activation energy is 1.5 eV. However, if the film is thinner than 10 nm, the oxygen diffusion along the c axis is hindered due to the effect of the intermediate layer at the super- conducting film ± support interface.It is important to establish the mechanism of the oxygen effect on the superconductivity parameters because it is necessary to solve the problem of the nature of the superconducting state. 298, S298 , and Cp298 Methods for the determination of the DH values and for the evaluation of the upper temperature threshold of superconducting transition of Y7Ba7Cu7O HTSC systems were developed with the use of thermodynamic simulation.98 The oxygen content decreases abruptly in the temperature range of 200 ± 500 K in complete agreement with the temperature depend- ences of Cu3+ concentration in the solutions studied. The Cu3+ concentration (at.%) as a function of the x index is described by linear equations for all solutions in the range of 100 ± 900 K.For example, the equation for YBa2Cu3Oz is (1) [Cu3+]=7433.866+66.71538z , where z56.5. Direct comparison of the resulting Cu3+ concentrations with the z values for real superconductors is incorrect, since the structures of a superconductor and a model solution should be different, in accordance with the calculation conditions. However, the authors give certain recommendations regarding the applic- ability of the results obtained to the study of real superconductors. An equation was suggested for the superconducting Y7Ba7Cu7O system relating the Cu3+ concentration to the superconducting transition temperature (in K): (2) Tc=45.32265+1.4333[Cu3+] .The inaccuracy of Tc calculation using Eqn (2) is considerable, since the phases obtained are usually thermodynamically unstable 285 under the conditions of synthesis. The correlation Eqns (1) and (2) can apparently be useful for rough estimates of Tc if the z index is known. The application of the Langmuir ± Maclin model made it possible to determine more accurately the oxygen diffusion factors in YBa2Cu3O77x powdered superconducting ceramics at various temperatures.99 The overestimated values for powders with par- ticle sizes of 100 mm were explained by the existence of `easy pathways' for diffusion such as cracks, grain boundaries, etc., the role of which is particularly great for particles larger than separate superconductor crystallites.A study of the physicochemical nature of oxygen dissolution and ordering of oxygen vacancies determining the superconduct- ing properties of the YBa2Cu3O77x ceramic 100 showed that an oxygen solution in the ceramic is somewhat comparable with an ideal gas (i.e., obeys the Fermi ± Dirac distribution), which is an important disagreement with the widespread concept of the `regular' character of solid solution. The authors believe that the ordering of oxygen vacancies in the YBa2Cu3O77x ceramic, which provides high-temperature superconductivity, results from struc- ture transformations according to a mechanism related to the Jahn ± Teller effect rather than from the interaction (repulsion) between the dissolved oxygen atoms. The evolution of oxygen from the YBa2Cu3O77x, La27yBayCuOx, Bi4 Ca3Sr3Cu4Ox, Bi27yPbyCa2Ba2Cu3Ox and Tl2Ca2Ba2Cu3Ox ceramics in vacuo (1074 Pa) with heating of the specimens to 1173 K was studied.101 At 773 ± 923 K, oxygen evolution from the yttrium and thallium ceramics was observed.Oxygen was not evolved from the bismuth and lanthanum ceramics in the temperature range studied. According to the published data,89 the evolution of oxygen from the Bi2Sr2CaCu2Ox system in vacuo occurs only in the temperature range of 1223 ± 1323 K. Thermogravimetric showed 102 that the analysis Bi2Sr2Ca2Cu4Ox ceramic contains over-stoichiometric oxygen: a specimen prepared in an oxygen atmosphere has the composition Bi2Sr2Ca2Cu4O11.38. The existence of the excess oxygen is explained by the presence of copper ions in a high oxidation state (Cu3+).This observation was also confirmed in other studies.103 ± 105 A considerable increase in the superconductor transition temperature of the Bi2Sr2CaCu2O8+x ceramic was observed after it was annealed in oxygen.106 Raman spectroscopy studies showed that oxygen is included in the Bi7O layers and the increase in Tc is due to the oxygen exchange between the Bi7O and Cu7O layers (see Fig. 1 c). The authors of the present review studied the effect of low- temperature (77.4 ± 100 K) adsorption of O2, Ar and N2 on the superconductivity of a GdBa2Cu3O77x thin film.107 All experi- ments were conducted in situ in an adsorption reactor placed in the probe of a magnetic susceptibility measuring device.The temper- ature dependence of the real and imaginary parts of magnetic susceptibility, w 0(T) and w 00(T), were recorded after the next gas portion was injected into the reactor at 77 K. All of the gases mentioned above affected the shape of the w 0(T) curve in the vicinity of Tc and `shifted' the maximum of the w 00(T) curve towards the low-temperature region. The half-width of the w 00(T) peak decreased with an increase in the quantity of injected argon and nitrogen and increased upon injection of oxygen; this meant an increase and decrease in the critical currents in the super- conducting film with an increase in the degree of its coverage with the gas molecules. The effects of the gases were reversible: removal of the adsorbed molecules by evacuation or by purging with helium at room temperature resulted in complete restoration of the original shapes of the w 0(T) and w 00(T) curves.Comparison of experimental w 00(T) curves 107 with results of theoretical calculations for the hysteresis losses in thin super- conductors 108, 109 afforded critical current densities extrapolated to 0 K, Jc(0), and their dependence on the degree of surface coverage with adsorbed gas molecules (Fig. 3). The adsorption of286 10710Jc(0) /A m72 12 3.2 2.8 3 2.4 0 0.4 0.8 1.2 Y Figure 3. Dependence of Jc extrapolated to 0 K on the degree of cover- age of the GdBa2Cu3O77x film surface with N2 (1), Ar (2) and O2 (3) molecules.107 argon and nitrogen is accompanied by a small increase in Jc(0), whereas the adsorption of oxygen decreases this parameter.On the other hand, the superconducting transition temperature did not vary with an increase in the degree of surface coverage; only `blurring' of this transition in the vicinity of Tc increased. The decrease in the critical current density in the case of low- temperature oxygen adsorption is due to the effect of the para- magnetic O2 molecule on the transport characteristics of the intergranular layers. In fact, the magnetic field strength at a distance from the adsorption site of an oxygen molecule equal to the diameter of this molecule is*10 T. This field strength is quite sufficient for a total perturbation of superconductivity of the intergranular layers.The rather small decrease in the critical current density observed probably results from the fact that the distances of the adsorption sites of the oxygen molecules from the superconducting phase exceeds substantially the molecule size, e.g., due to the presence of a `buffer' non-superconducting oxide layer on the film surface. The `blurring' of the superconducting transition due to the low-temperature adsorption of oxygen, argon or nitrogen can be caused by many reasons. A most likely reason involves inclusion of the molecules of these gases directly into the crystal lattice of the material. This hypothesis was suggested long ago (see below) and was partially confirmed experimentally for argon adsorption.110 It is likely that the inclusion of simple molecules into polycrystalline HTSC oxides at low temperatures is facilitated by the presence of numerous defects and oxygen vacancies in the crystal lattices of these materials.For instance, the high surface defectiveness is indicated by experimental results of a study of electron photo- emission from the YBa2Cu3O77x surface 111 which showed that if the ceramic is crushed, the lattice oxygen leaves the surface instantly even at the liquid nitrogen temperature. Similar changes in the superconductivity characteristics were recorded for the low-temperature adsorption of oxygen on the YBa2Cu3O77x ceramic.112 At 77.4 K, this adsorption results in a strong decrease in the transport critical current.The amplitude of the changes was much greater than that in the case of a super- conducting film; this is probably due to the presence of a large number of weak bonds between the granules. 2. The effect of hydrogen The specifics of the interaction of hydrogen with metal oxide superconductors primarily result from the presence of the O27 and O7 ions and possibly O0 in the anion sublattice; hydrogen readily forms chemical bonds with these species to give OH7 ions or water molecules.113 Hydrogen that has penetrated into the crystal lattice of an HTSC material reacts with the metal ions to change their effective oxidation state. All of these changes affect, directly or indirectly, the hole concentration in the CuO2 plane, and hence, the superconducting transition temperature. L L Makarshin, D V Andreev, V N Parmon The neutron diffraction method was used to study hydrogen adsorption on the YBa2Cu3O77x ceramic in the temperature range from 333 to 593 K.114 At low temperatures, hydrogen is preferentially adsorbed on the granule boundaries, but above 493 K, it undergoes chemical reactions with the superconductor material.Hydrogen adsorption at elevated temperatures results in partial amorphisation of YBa2Cu3O77x accompanied by the evolution of a considerable amount of metallic copper. According to the reported data,115 treatment of the YBa2Cu3O77x ceramic with hydrogen at 393 ± 473 K results in a strong decrease in the low-field magnetic susceptibility and suppression of magnetic hysteresis at high fields.The superconducting transition temper- ature is not changed in this case. The suppression of the magnetic properties of the HTSC material is explained by the formation of metal hydrides in the inter-granular layers; the invariance of Tc is explained by the stable superconductivity of single crystals of the ceramic superconductor. The EuBa2Cu3O77x ceramic was treated with hydrogen at pressures of 1.3 ± 10.0 kPa and temperatures of 393 ± 483 K.116, 117 This did not change the superconducting transition temperature. However, the formation of hydride and hydroxide phases [e.g., CuH2, Ba(OH)2 and others] on the granule surfaces resulted in a decrease in the fraction of the superconducting phase, modification of the Josephson contacts, an increase in their electric conductivity and a decrease in the percolation conductiv- ity. It was found for the first time that the dependence of the critical current Ic on the quantity of hydrogen absorbed by the YBa2Cu3O77x ceramic is non-monotonic and passes through a maximum at low concentrations of the latter.118 The position of the maximum depends on the specimen density and correlates with the minimum position on the electric resistance curve.Based on these data, the conclusion was made that the enhanced conductivity of the inter-granular layers at low concentrations of absorbed hydrogen is due to the reduction of oxides with hydro- gen, which favours the formation of metallic copper and compac- tion of the layers between the granules, and due to additional ionic conductivity resulting from the presence of theH+andOH7ions.The superconductivity parameters of the YBa2Cu3O77x ceramic depend strongly on the oxygen concentration in the material lattice. It is well known that the oxygen-deficient YBa2Cu3O6 ceramic is a semiconductor and displays antiferro- magnetic properties, while the superoxidised YBa2Cu3O7 ceramic has metallic properties. The effect of hydrogen inclusion into the YBa2Cu3O77x ceramic with various oxygen contents at a temper- ature of 441 K in the pressure range from 1 to 3 atm was studied by the muon spin rotation method.119 It was shown that hydrogen intercalation results in magnetic ordering of the copper atoms; the amount of hydrogen required for this depends on the oxygen index: the more oxygen the ceramic contains the more hydrogen is required.Thus, hydrogen intercalation results in a formal decrease in the oxygen index. This requires one hydrogen atom per two oxygen atoms. It was concluded that hydrogen in YBa2Cu3O77x plays the role of an electron donor which decreases the number of holes. The intercalation of hydrogen into the YBa2Cu3O77x and Bi2Ca2SrCu2O8+x ceramics at 473 ± 503 K and at a pressure of 0.9 atm was studied.120 The concentration of holes in the super- conductors decreased in a ratio of one hole per one hydrogen atom upon hydrogen intercalation. In addition, the superconducting transition temperature depended strongly on the position occu- pied by hydrogen in the lattice.It was thus concluded that Tc is primarily determined by the mobility of holes in the CuO2 planes. This was confirmed by an X-ray diffraction study of the EuBa2Cu3O77x ceramic.121 It was shown that the penetration of hydrogen into the material crystal structure along the CuO2 planes begins at relatively low temperatures, 393 ± 423 K, and at pres- sures of 5 ± 10 kPa. A strong decrease in the thermal expansion factor was observed, which is believed to result from changes in the electronic structure of the material and the metal ± dielectricThe chemical and adsorption effects of foreign molecules on the properties of high-temperature superconductors transition due to a decrease in the concentration of the oxygen vacancies.A more detailed analysis of the hydrogen effect on the structure of yttrium and bismuth ceramics was made.122 The inclusion of hydrogen results in structural changes in the YBa2Cu3O77x lattice depending on the oxygen index.123 ± 125 Subsequent saturation of the ceramic with hydrogen results in the formation of amorphous hydride phases where hydrogen occupies interstitial positions. A solid solution of hydrogen in the ceramic, similar to metal ± hydrogen solid solutions, can be formed. According toNMRdata,126 the diffusion of hydrogen into the YBa2Cu3O77x ceramic occurs on the oxygen vacancies in the Cu7O chains and along the a or b axes on the interstices in the Y planes. The activation energies of these processes are 0.29 and 0.25 eV, respectively.The diffusion of hydrogen ceases following exposure of the ceramic to hydrogen at room temperature for three to four months, and the hydrogen atoms form strong O7H bonds. It was found by 1H NMR spectroscopy that hydrogen diffusion also occurs in hydrogenated YBa2Cu3O77x ceramic at T<200 K.127 At temperatures from 130 to 92 K, the hydrogen atoms are mostly localised at the O(4) and O(5) lattice points. The inelastic neutron scattering and diffraction methods afforded the vibration spectra of hydrogen atoms in YBa2Cu3O77x ceramic treated with hydrogen at 373 K.128 Only several percent of the absorbed hydrogen participated in the formation of a solid solution; the H atoms occupy the octahedral positions formed by four Y atoms and two Cu atoms at the Cu(2) positions (Fig.4). The rest of the hydrogen atoms are involved in an amorphous non-superconducting phase. The NMR and X-ray diffraction methods were used to study the absorption of hydrogen and deuterium with the YBa2Cu3O7 and YBa2Cu3O6 ceramics at 473 K.129 The incorporation of the hydrogen isotopes into YBa2Cu3O7 results in partial removal of the chain oxygen from the O(1) positions and formation of O7H Ba YOCu HIII Cu(2) O(2) O(3) O(1) HII Cu(1) O(4) c HI b a Figure 4. Possible locations of hydrogen atoms in the crystal lattice of YBa2Cu3O77x.138 287 bonds. No new bonds are formed in the YBa2Cu3O6 ceramic, and no changes in the lattice parameters with increase in the hydrogen content are observed.Pre-treatment of an yttrium HTSC ceramic in vacuo followed by treatment with hydrogen at 400 ± 600 K and 2.6 kPa caused a considerable decrease (by 150 K) in the temperature at which oxygen enters the tetragonal YBa2Cu3O77x phase.130 The speci- men became a superconductor (Tc=80 K) after such low-tem- perature intercalation of oxygen; the `width' of transition into the superconducting state was 15 K. If treatment of the specimen with oxygen is carried out at the same temperatures immediately after evacuation, it does not become a superconductor. It was shown 131 that the decrease in the temperature at which oxygen enters the ceramics is caused by the transformation of the tetragonal phase of YBa2Cu3O77x into the orthorhombic phase upon hydrogen treatment.The lattice parameter b decreases in this case since the hydrogen atoms occupy the oxygen vacancies along the direction b in the plane formed by CuO chains. Nevertheless, superconduc- tivity is absent down to 4.2 K on cooling of the ceramics treated only with hydrogen. This is explained 131 by the small orthorhom- bic parameter R which is 1073 for the hydrogenated ceramics, whereas it is above 361073 for the superconducting yttrium ceramics. Similar changes in the physicochemical properties were observed upon treatment of superconducting bismuth oxides. The fraction of the superconducting phase and the superconduct- ing transition temperature were found to decrease upon reduction of a Bi2Sr2CaCu2Oz single crystal with hydrogen (a 10 : 90 H2:N2 mixture) at 673 K.132 Analysis of the X-ray absorption spectra led to the conclusion that reduction with hydrogen decreases the formal oxidation state of copper and the number of holes in the CuO2 planes; this is the reason for the decrease in Tc.In a study of the effect of hydrogen treatment on the structure of the bismuth-containing HTSC ceramic, the specimens were kept for several hours in a stream of hydrogen at 573 K.133 Measurements of the electric conductivity of the specimens showed that the ceramic loses superconductivity completely and becomes a semiconductor. Hydrogenation of the ceramic results in a decrease in the unit cell volume, as shown by X-ray diffraction analysis.It should be noted that, on the contrary, hydrogenation of the yttrium HTSC ceramic increases the unit cell volume. Thus, the type of effect of hydrogen on the properties of an HTSC material depends strongly on the treatment mode and temperature. At relatively low temperatures (up to 473 K), hydro- gen mostly reacts with the granule surfaces and forms amorphous hydride phases involving metal atoms of the ceramic. In some cases, insignificant intercalation of hydrogen into the crystal structure of the material is observed. This phenomenon is accom- panied by degradation of the magnetic properties of the super- conductor and a decrease in the transport current density. As a rule, the superconducting transition temperature changes only slightly.As the temperature of hydrogen treatment is increased, the superconductivity of HTSC materials decreases abruptly, until it is lost completely. This results from the incorporation of hydrogen into the crystal structure of the material which changes the Tc and the formal oxidation state of copper and decreases the concen- tration of holes in the CuO2 planes. High reactivity of hydrogen in the 400 ± 470 K range found in a series of studies 134 ± 136 gives reasons to assume that the protonic conductivity of hydrogen-containing barium ± yttrium cuprates is rather high. However, the originally high metallic conductivity of cuprates of the LnBa2Cu3O7 family (where Ln indicates yttrium or another rare-earth element) `masks' strongly weaker ionic con- ductivity.Studies of hydrogenated cuprates of the general formula HyYBa2Cu3O77x showed 137 that their specific resistivities increase by almost six orders if the hydrogen index y increases from 0 to 2 and decrease by three orders upon oxidation of the ceramics with y=2 if the x index changes from 0.1 to 0.8.288 Almost all aspects of the physicochemical interaction of hydrogen with superconducting cuprates were covered in one of the latest reviews on the effect of hydrogen on HTSC materials.138 Figure 4 shows the crystal structure of the orthorhombic phase of YBa2Cu3O77x with indication of the positions where the hydro- gen atoms are localised according to numerous studies. One can see that the intercalation of hydrogen atoms occurs along three main directions, i.e., into the copper ± oxygen bridges, chains and oxygen vacancies.However, the conductive CuO2 layers remain unaffected. Hence, the observed changes in the electrophysical properties of HTSC materials are due to the electron density redistribution between the CuO2 layers and the copper ± oxygen chains O7Cu(1)7O7Cu(1)7O. As a rule, hydrogen intercala- tion into HTSC materials suppresses the superconductivity, while the superconducting transition temperature decreases (with rare exceptions). Figure 5 shows the dependence of the temperature of the onset of superconducting transition Tc,on of an HyYBa2Cu3O77x thin film on the hydrogen content. If the hydrogen concentration increases, Tc,on decreases monotonically, while the transition width increases from 0.8 to 6.2 K.It should be noted that at the hydrogen content of *0.3 atoms per cell, a sharp bend of the curve is observed; this correlates with the bend on the dependence of Tc on the oxygen content (see Fig. 2). This probably indicates that the hydrogen atoms (donors) react readily with weakly bound oxygen ions (electron acceptors), thus chang- ing the concentration of the current carriers in the HTSC material. Tc,on /K 12 80 60 40 y 0.4 0.2 Figure 5. Dependence of the temperature of the onset of superconduct- ing transition (Tc,on) of thin HyYBa2Cu3O77x film on the hydrogen content (y) in unit cell;138 (1) from electric resistance measurements; (2) from magnetic susceptibility measurements.3. The effect of halogens Numerous studies on the effect of the crystal structure and oxidation states of the chemical elements constituting an HTSC material on superconductivity showed that the holes serve as charge carriers in the superconducting state; these holes are genetically related to the p states of the oxygen ions, and their concentration is an important characteristic of a superconduc- tor.139, 140 It thus follows that the anionic sublattice plays an essential role in superconductivity. One of the ways for changing the anionic sublattice defectivity involves doping with other anions, e.g., halides. The disappearance of electrical resistance at 155 K on YBa2Cu3O77x specimens synthesised with the use of BaF2 as one of the starting reagents was detected for the first time in 1987 (Ref.141). Since that publication, numerous studies on the replacement of the anionic sublattice by halogens have appeared. The presence of fluorine can both improve superconductiv- ity 142 ± 149 and impair it.150, 151 For example, treatment of the YBa2Cu3O6.7 ceramic with Tc=65 K with NF3 at 613 K and with CCl4 at 523 ± 573 K increased Tc to 90 K in both cases.152 Presumably, the halogen atoms are incorporated in the O(4) position along the CuO chains, i.e., in a sense, halogenation is similar to saturation of oxygen-deficient ceramics with oxygen. If L L Makarshin, D V Andreev, V N Parmon treatment is continued, the halogen anions occupy the O(5) positions but Tc is not increased any more.The effect of `soft' fluorination on the degradation of powders of the YBa2Cu3O7-x composition in a humid atmosphere was studied.153 The fluorina- tion of powders was carried out in 49% hydrofluoric acid vapour at room temperature in a closed vessel. The physical properties and superconductivity of the powders were almost unchanged, but their stability in humid atmosphere increased considerably. This phenomenon is rationalised as being due to the higher stability of the resulting fluorides, BaF2 and CuF2, to water and prevention of further interaction of water with the superconductor granules. The presence of barium fluoride in synthesised thallium-contain- ing ceramic cuprates with composition TlBa2Ca2Cu3Ox doped with zirconium, hafnium, ruthenium and tin oxides improved their critical parameters noticeably.154 However, there is no satisfactory explanation of the effect of barium fluoride on the properties of thallium-containing cuprates. In a continuation of this work, the composition and properties of thallium-containing HTSC ceramics obtained in the presence of metal fluorides were studied.155 The specimens modified in this way are characterised by higher content of the superconducting phase, smaller granule sizes and more pronounced transition in the superconducting state in comparison with the specimens obtained under the same conditions but without addition of fluorides.Enrichment of the (Tl0.5Pb0.5)Sr1.6Ba0.4Ca2Cu3Oz ceramic with fluorine carried out by the addition of CuF2 during the synthesis improves considerably its superconductivity.156 The superconducting transition temperatures increase from 120 to 125 K, and the critical current density increases by 300%.According to X-ray diffraction data, the fluorine atoms replace the oxygen atoms in the CuO2 plane. Changes in the morphology of the specimens due to the increase in the ceramic granule sizes are observed. The increase in Tc is explained 156 by optimisation of the concentration of charge carriers in the CuO2 plane by the fluorine atoms, whereas in ceramics devoid of fluorine, the concentration of these carriers is apparently far from optimum because of the high oxygen content. NMRand EPR spectroscopic methods were used to study the intercalation of halogens into the crystal lattice of YBa2Cu3O77x.157 The halogenation was carried out by exposing the specimens in a stream of gaseous NF3 or CCl4 diluted with N2 at 573 and 523 K, respectively.The intercalated chlorine atoms are inserted in the chains between two copper atoms, while the fluorine atoms occupy the oxygen vacancies, as follows from the higher fluorine concentration in the materials with smaller oxygen indices. The mechanism of the formation of halogen-containing HTSC phases based on YBa2Cu3O77x was studied.158 The intercalation of halogens (Cl2, Br2 , I2) into the YBa2Cu3O77x lattice was carried out in the temperature range of 423 ± 523 K. An X-ray anomalous dispersion andMoÈ ssbauer spectroscopic studies of the specimens obtained showed that the interaction of halogens with the tetragonal phase of YBa2Cu3O77x occurs as intercalation of halogens into the anionic vacant positions of the Cu(1) layer (see Fig.1 b). Charge transfer from the CuO2 plane to halogens results in the formation of holes, which favours the manifestation of superconductivity (60<Tc<93 K) acquired by the resulting intercalation compounds. The chloride anion, unlike other halide anions, changes the ceramic's microstructure owing to its large size; this results in the formation of stresses and dislocations and manifests itself as a decrease in the critical current. A study of the surface layers of the YBa2Cu3O77x tetragonal phase after its bombardment with chloride ions showed that implantation of chloride anions results in the formation of a orthorhombic phase layer near the surface; its thickness approximately equals the anion penetration depth.159 This layer has the properties of a superconductor with a critical temperature of ca.70 K. In a study of chlorine-containing phases of the type YBa2Cu3O77xCly synthesised with the use of PCl5 as a chlorinat-The chemical and adsorption effects of foreign molecules on the properties of high-temperature superconductors ing agent, it was assumed 160 that at low temperatures the halogens are localised in chains around the copper atoms Cu(1) and the O(5) vacant positions are occupied first. An increase in the halogenation depth results in replacement of the O(4) chain oxygen atoms.Further halogenation under more drastic condi- tions results in replacement of the O(1) atoms, and the resulting phase has the limiting composition YBa2Cu3O4Cl4. The conclu- sion was made 160 that the decisive role in maintaining the super- conductivity of halogen-containing phases belongs to the nature of the bridging atoms located at the O(1) position, which link the CuO2 layers to the Cu(1) chain atoms: the electron-withdrawing properties of these bridging atoms induce superconductivity. The change in the superconducting transition temperature due to the intercalation of HgBr2 into polycrystalline Bi2Sr1.57yLayCa1.5Cu2Ox specimens with 0.04y40.4 was studied.161 The specimens were obtained by keeping a tube containing certain weighed samples of HgBr2 and the original superconducting ceramic at 508 K for 4 h in vacuo.An X-ray diffraction study showed that HgBr2 is intercalated between the Bi7O planes. Figure 6 shows the dependence of Tc on y for the original and treated specimens. One can see that after treatment Tc decreases for the specimens with y<0.2 and increases for the specimens with y>0.2. It is believed that the intercalation of HgBr2 changes the charge on the copper ions located in the conducting planes. The correlation relationships between the change in the copper charge (Dn) and Tc are reported;161 it follows from this relationship that Dn depends linearly on the concen- tration of the added compound and correlates with the concen- tration of the charge-carrying holes.Upon treatment of non-superconducting ceramic specimens of YBa2Cu3O77x (tetragonal phase) with bromine vapour at 503 K and iodine vapour at 673 K without contact with air, they acquired superconductivity; the Tc values were 80 ± 85 and 55 K, respectively.162 The chemical interaction of iodine with the lattice copper ions can be a possible reason for the appearance of superconductivity in iodinated specimens. A similar assumption was made in a Raman spectroscopic study of the tetragonal YBa2Cu3O6.1 ceramic treated in a iodine atmosphere. Iodine reacts with copper to `extract' it from the lattice.163 Oxygen is redistributed over the lattice, occupies the vacant sites in the chain and thus provides the possibility for superconductivity to occur.The same conclusion was also made in yet another study.164 X-Ray diffraction analysis performed after annealing of the non-superconducting phase YBa2Cu3O6 in iodine vapour at 553 ± 653 K showed that the superconducting orthorhombic phase had appeared and reflexes of foreign phases were detected, which possibly corresponded to copper, barium and yttrium iodides. Treatment of the YBa2Cu3O7 orthorhombic phase under similar conditions was not accompa- nied by either structural parameter changes or formation of Tc /K 80 2 75 1 70 65 y 0.2 0.1 0.3 0.4 Figure 6. Dependence of the superconducting transition temperature of the Bi2Sr1.57yLayCa1.5Cu2Ox ceramic on y;161 (1) starting specimen; (2) sample after treatment with HgBr2.289 foreign phases. Annealing a powder mixture of the YBa2Cu3O6 and YBa2Cu3O7 phases in vacuo at 493 K (which is much lower than the temperature of oxygen liberation, 623 K) was accompa- nied by the intergranular oxygen transfer from the YBa2Cu3O7 phase to the YBa2Cu3O6 phase and resulted in a phase with composition YBa2Cu3O6.65. This suggests the high diffusion mobility of oxygen at low temperature in the lattices of the ceramics studied. Thus, the basic mechanism of the effect of iodine on the superconducting properties of yttrium ceramics apparently involves oxygen transfer into the tetragonal phase; this oxygen can be liberated from the neighbouring phases under the effect of halogen atoms.Treatment of the tetragonal phase with iodine vapour at 543 K resulted in a continuous series of YBa2Cu37yOxIz solid solutions which possessed superconductiv- ity.165 The reaction of iodine with the crystal lattice of YBa2Cu3O77x single crystals depends essentially on the initial oxygen content.166 At the maximum oxygen content (the specimen with Tc=92 K), no reaction of the crystals with iodine was observed. However, the stoichiometric cationic compositions of the crystals of orthorhombic (Tc=55 ± 60 K) and tetragonal phases changed and an amorphous layer of varying thickness was formed on the surface; this was followed by complete destruction of the specimens. The penetration of iodine into the matrix was determined by the presence of oxygen vacancies in the principal planes.Ordering of the iodine atoms included in the crystal lattice was not observed. Two-phase specimens were formed upon bromination: the original tetragonal phase coexisted with a orthorhombic phase in the ratio*1 : 1. The iodinated specimens consisted completely of a orthorhombic phase. The parameters of the orthorhombic phases in the iodinated and brominated specimens are similar. Thus, the halogen atoms occupy presumably the vacancies of the O(4) type along the b axis (see Fig. 1 b).162 It was assumed on the basis of EPR spectra that halogenation results in the transfer of negative charge to the CuO chains and formation of holes at the CuO2 planes. A study of iodine intercalation at 23 K into the Bi2Sr2Ca17yYyCu2O8 ceramic by X-ray diffraction analysis showed that iodine is incorporated in the BiO7OBi double layers in such a way that the distance between them increases.167 The superconducting transition temperature decreases by 10 K because the concentration of the hole carriers exceeds the opti- mum level.The effect of iodine and oxygen intercalation on the physical properties and superconductivity of Bi2Sr2CaCu2O8+x single crystals was studied.168 The starting single crystals and elementary iodine were sealed in an evacuated glass tube, heated to 453 Kand kept for 330 h at the same temperature. The oxidation of single crystals with oxygen was carried out at 10 atm and 823 K for 140 h, or at 600 atm and 673 K for 320 h.The resistance and magnetic measurements of the thus treated samples showed that the dimensionality of the superconducting system changes from two to three, Tc decreases, and the activation energy of magnetic flux creep changes. According to the model suggested for the explanation of these phenomena, the HTSC material acquires a multilayered structure with alternation of the superconducting and normal layers due to the intercalation of iodine and oxygen. The `proximity' effect results in the redistribution of the charge carriers between the layers and hence in changes in the physical properties and superconductivity parameters. The structural ordering of atoms in superconducting Bi2Sr2CaCu2IyOx single crystals containing intercalated iodine was studied by low-energy electron diffraction, Auger and photoelectron spectroscopy on chipped surfaces, as well as by transmission electron spectro- scopy.169 It was shown that iodination results in a superstructure with ordered aggregates of iodine molecular complexes in the empty channels formed upon distortion of the original lattice by structural modulation.290 In yet another study,170 the intercalation of iodine in bismuth- based ceramics was carried out in a sealed tube, which was heated to 458 K over a period of 6 h and kept for 240 h at this temper- ature.According to the nuclear quadrupole resonance data, the iodinated Bi2Sr2CaCu2Ox specimens contain no chemical bonds of the Bi7I and Cu7I types, i.e., iodine in them is in atomic or molecular form and is truly intercalated.The superconducting transition temperature of the iodinated ceramic was 73 K. Thus, halogenation of the yttrium-containing oxygen-defi- cient ceramic consisting of a YBa2Cu3O6+x tetragonal phase results in orthorhombic phases with YBa2Cu3O6+x Haly compo- sition. The new phases have superconducting properties with approximately the same Tc as the usual superconducting ceramics, YBa2Cu3O77x. The iodinated ceramic is an exception, its Tc is no higher than the liquid nitrogen temperature. The formation of superconducting phases upon ceramics' halogenation is explained by inclusion of halogens at the oxygen O(1) positions; the halogens `withdraw' the electronic charge from the CuO2 planes onto themselves, which in turn increases the concentration of holes.4. Degradation of high-temperature superconducting materials exposed to CO2 and H2O Exposure of HTSC materials to atmospheric carbon dioxide and water results in their degradation accompanied by the loss of superconductivity and degradation of the superconducting phases. Therefore, the problem of storage and stabilisation of HTSC materials is of major importance. One of the first reviews on the chemical degradation of superconductors is found in Ref. 171. The reaction of CO2 with the YBa2Cu3O77x ceramic starts at room temperature.172 The decrease in the critical intergranular current correlates with the intensity of the COá2 peak in the mass spectra of the gases evolved from the ceramic; this suggests that carbonates are localised at the granule boundaries.When syn- thesis is carried out in the air at high temperatures, YBa2Cu3O77x absorbs CO2. The maximum absorption is observed at 1173 K, and the amount of absorbed CO2 with respect to carbon reaches 0.05% of the ceramic mass.173Astudy of the decomposition of the YBa2Cu3O77x ceramic in a 5% CO2+95% O2 mixture at 1088 Kshowed that the nuclei of the Y2Cu2O5 and BaCO3 phases are initially formed very fast, and then a `green' phase of Y2BaCuO5 and CuO appears. The formation of BaCO3 at the boundaries between the granules upon treatment of YBa2Cu3O77x with pure CO2 at 1123 K for 2 h was reported.174 The powdered YBa2Cu3O77x ceramic reacts with CO2 at 1273 K both in the absence and in the presence of moisture to give decomposition products such as BaCO3, Y2Cu2O5, CuO, Cu2O and Y2O3.175 In the 90% CO2+10% O2 mixture, the decomposition products differ somewhat: they do not contain Cu2O and Y2O3.A detailed study of the kinetics of the reaction between powdered YBa2Cu3O77x and pure CO2 in the temperature range of 823 ± 1163 K was carried out.176 The reaction of the ceramic with CO2 can be described by the equation: 2 YBa2Cu3O77x+3CO2=Y2BaCuO5+3 BaCO3+3 CuO+ +Cu2O + (17x)O2 , where Y2BaCuO5 is an intermediate. In the temperature range of 823 ± 1088 Kthis reaction limits the overall rate of the process and has an order of 1.5 with respect to the ceramic; its activation energy is 95 kJ mol71.Subsequent `carbonisation' of the ceramic occurs according to the equation: Y2BaCuO5+CuO +CO2=Y2Cu2O5 +BaCO3 . The presence of an alkaline-earth element in the HTSC ceramics provides a material with basic properties and hence makes it capable of absorbing CO2 when in contact with air. L L Makarshin, D V Andreev, V N Parmon Calcination of the YBa2Cu3O77x ceramic during synthesis at 1223 Kin air containing more than 1%carbon dioxide results in a non-superconducting tetragonal phase.177 It is formed due to hindered oxygen exchange because of the formation of carbonate films on the material surface. The carbonates and decomposition products are mainly formed at the granule boundaries; this decreases considerably the critical intergranular current of the superconductor.In the initial stage of the reaction of YBa2Cu3O77x with water vapour and carbon dioxide, the kinetics of water sorption is consistent with a process occurring at the internal grain bounda- ries and it is well described by a first order equation.178 The diffusion of water vapour to the internal layers of the ceramic is the rate-determining step for CO2 transfer. If the ceramic is pre- treated with water vapour, the CO2 transfer occurs extremely quickly and is characterised by an effective diffusion factor exceeding 1076 cm2 s71. This is assumed to be due to water vapour condensation in the material pores. Prolonged annealing of a Bi1.6Pb0.4Sr1.7Ca2.3Cu3.0O10+x pow- der at 1123 K decreased slightly the carbon content in the speci- men from 0.02 mass% after 24 h to 0.01 mass% after 192 h.179 The powdered HTSC ceramic quickly absorbs carbon dioxide if stored in air: the content of carbon in the ceramic increases by a factor of two in 15 h, reaches 0.1 mass% in 240 h and then remains almost unchanged.An increase in the carbon content in bulk specimens from 0.019 mass% to 0.13 mass% is accompa- nied by a decrease in the critical current density from 280 to 150 A cm72. The YBa2Cu3O77x ceramic readily reacts with water vapour even at room temperature to form the `green' phase Y2BaCuO5 according to the equation:180, 181 2 YBa2Cu3O77x+3H2=5 CuO+3 Ba(OH)2+Y2BaCuO5+ +(0.57x)O2 . If this occurs in the presence of CO2, the resulting barium hydroxide immediately reacts with carbon dioxide:182 CO2+Ba(OH)2=BaCO3+H2O .Thus, in this case water acts as a catalyst of carbonate formation. Owing to the high basicity of barium oxide, YBa2Cu3O77x can react with CO2 in humid atmospheres even at very small CO2 partial pressures.183 The products of hydration ageing of the YBa2Cu3O77x ceramic were studied.184 The morphology of the new formations was studied using a scanning electron microscope. The original grain surface was rather smooth, but small protru- sions and cracks appeared there after exposure to water vapour for one hour. If exposure was increased to 2.5 h, the ceramic's grains became covered with thin needles. An even longer exposure promoted the integration of the new formations which gradually covered the entire grain surfaces.X-Ray diffraction analysis showed that the new formations consist of barium hydroxide, a-Ba(OH)2, and the Y2BaCuO5 `green' phase. Similar studies carried out using electromagnetic radiation absorption in the range of 1 ± 3 MHz made it possible to follow the character of intergranular medium changes in specimens of the YBa2Cu3O77x ceramic exposed to moisture.185 The changes in the chemical composition (degradation) of certain granules can occur at a depth of 0.3 ± 1.0 mm from the surface depending on the specimen density. The formation of such a layer interferes with subsequent changes. Noticeable degradation of specimens with a density above 5.4 g cm73 is observed in water after 10 h whereas in ordinary atmosphere it takes 5 months.Subsequent exposure to water little affects the superconductivity characteristics of the specimens. The surface state and microstructure affect the reaction of the YBa2Cu3O77x ceramic with water.186 Generally, the average pore size in the YBa2Cu3O77x ceramic is *1 mm at an average grain size of 3 ± 20 mm.187 Analysis of data on small angle neutron scattering at the liquid nitrogen temperature led to the conclusionThe chemical and adsorption effects of foreign molecules on the properties of high-temperature superconductors that the majority of pores in the yttrium ceramic are open.188 Thus, water can react not only with the outer but also with the inner surface by penetrating through the open pores.Water diffusion is an activation process. The activation energy is *0.033 eV, this order of magnitude corresponding to the energy of a thermal quantum. Surface polishing of pellets of a sample accelerates the degradation three- to five-fold; apparently, this is caused by an increase in the number of open pores connected to the surface as a result of polishing. Hydration of textured speci- mens is strongly hindered, probably due to the small number of boundaries because of strong binding between the oriented grains, the concentration of oxygen vacancies in the intergranular spaces is rather small, and penetration of hydroxy groups inside the pellet is hindered. The penetration depth of water molecules into a textured specimen after ageing in water for 10 h at room temper- ature is 5 times smaller than the depth of its penetration into an isotropic ceramic.Isothermal oxidation of hydrated YBa2Cu3O77x ceramic is accompanied by a stepwise increase in the sample mass, which indicates the presence of two oxidation steps, `slow' and `fast'.189 The oxidation rate decreases proportionally to the water concen- tration in the material. The OH7 ions play a role of stoppers for defects of different dimensionality (oxygen ions, twinning struc- ture boundaries) in their movement through the YBa2Cu3O77x lattice. Presumably, during the `slow' oxidation stage of the yttrium ceramic, the boundaries of the twinning structure between the crystallites of this compound are in the state of accelerated movement due to thermal activation and `stripping' of these boundaries from localised proton-containing ions in the near- surface zone.Subsequent movement of these boundaries occurs in the matrix free of impurities. The YBa2Cu4O8 ceramic and YBa2Cu3O77x specimens with an admixture of silver are much more stable against moisture than non-doped YBa2Cu3O77x ceramic.190, 191 This is explained by a more compact arrangement of granules consolidated by metallic silver particles in doped ceramics; this evidently interferes with fast diffusion of the water molecules in the material bulk. The effect of silver on the stability of powders with composition YBa2Cu3O77x in a humid atmosphere was studied.192 The composition of the surface was studied using an electron probe X-ray microanalyser and X-ray photoelectron spectroscopy.If the water content in the powder is above 1.6 mass %, silver (5 mass%± 15 mass %) has a stabilising effect. In order to increase the critical current, it is recommended to add 15 mass% of silver to the moist powder (1.6 mass% H2O). On the surfaces of ceramics containing this amount of silver, the latter is present in the form of separate inclusions. At the Ag content of 5 mass%± 15 mass %, super- conductivities of the ceramics remain practically the same as those for the original YBa2Cu3O77x specimen. If the EuBa2Cu3O77x ceramic is treated with water, the evolution of oxygen is preceded by dissolution of an insignificant amount of Ba2+ ions (*7% of the total amount of barium in the ceramic).193 The europium ceramic is much more stable than the YBa2Cu3O77x ceramic, which undergoes 50% decomposition in a humid atmosphere at room temperature. Noticeable decomposi- tion of EuBa2Cu3O77x (i.e.evolution of gaseous O2 and passage of Ba2+ cations into solution) starts 2 h after the contact with water at 333 K and occurs according to the equation: 2EuBa2Cu3O77x+3H2=5 CuO+3 Ba(OH)2+Eu2BaCuO5+ +(0.57x)O2 . The decomposition of the ceramic at 353 K starts almost immediately. Complete reduction of Cu3+ to Cu2+ occurs only at 373 K. The decomposition of the YBa2Cu3O77x ceramic in distilled water and in solutions of sodium sulfate and barium chloride was studied.194 It was shown that the presence of barium ions interferes with the hydrolytic decomposition of YBa2Cu3O77x; after treatment with 1072 M aqueous solution of BaCl2, the ceramic retains superconductivity even after boiling.291 Hydrolysis of the ceramic is accompanied by leaching of barium cations, and addition of sulfate ions to water which binds barium ions causes complete decomposition of the superconductor. This is explained by equilibrium displacement upon changes in the reaction product concentration in accordance with the thermo- dynamic laws. An interesting feature of the behaviour of water present in the YBa2Cu3O77x ceramic was noted.195 It was found that the super- conducting phase cannot release water at temperatures of oxida- tive annealing (673 ± 873 K).It is believed that this results from geometrical factors related to lattice distortion as a result of oxidative annealing, which hinders the exit of proton-containing ions from the O(5) positions. It was found by thermal desorption methods that water on the surface of the YBa2Cu3O77x ceramic can be physically adsorbed and structurally bound (in the latter case, the bond energy equals 145 kJ mol71).196, 197 The water molecules can occupy two types of positions, viz., the position in the yttrium plane between the CuO2 planes and the position between two Cu(1) copper atoms (see Fig. 4). EPR studies showed 198 that inclusion of water molecules into the YBa2Cu3O77x lattice results in the appearance of the Cu2+ paramagnetic centres at the Cu(2) positions in the CuO2 layers.The EPR signal intensity depends on the water content in the ceramic. It is assumed that these paramagnetic centres are formed due to non-uniform deformation along the CuO2 plane rather than due to decomposition of the material. A radiothermoluminescence study of the moisture-induced degradation of the RBa2Cu3O77x type HTSC materials (R=Y, Sm, Eu, Ho) showed that barium hydroxide is the compound responsible for radiothermoluminescence.199 The radiation inten- sity is much lower if the specimen contains a minor amount of the original compounds which did not react under the conditions of solid-phase synthesis of the HTSC material. The possibility of protection of materials based on YBa2Cu3O77x from degradation in water vapour by applying coatings of paraffin, poly(vinylbutyral) and oxidised triglycerides of unsaturated fatty acids was studied.200 The last named was found to be the most effective.5. The effect of Ar, He and N2 adsorption Noble gases, viz., Ar and He, as well asN2, do not react chemically with the material, and hence, the major factors of their effect on superconductivity can involve inclusion into the crystal structure of the material or surface `charging' due to adsorption. The effect of argon on the electric conductivity of the YBa2Cu3O77x ceramic was studied.110 The specimens were kept for several days in vacuo (1073 Pa) at room temperature, then purged for three days with pure argon at the same temperature.After that, they were analysed by the resistance method at currents of 50, 500 and 5000 mA. An abnormal dependence of the electric conductivity on temperature was found when the latter was increased from 200 to 300 K: the resistance of a sample abruptly almost doubled at a current of 50 mA; the increase was only several percent at 500 mA, whereas no abnormal increase in resistance at all was observed at 5000 mA. Apparently, this dependence of the electric resistance on current in this temperature range cannot be due to any manifestation of superconductivity. An increase in the current flowing through a superconducting specimen should result in the violation of the superconducting state and in the increase in resistance, rather than in the opposite effect observed in the experiment.An Auger scanning microscopic study of specimens treated with argon revealed the presence of argon atoms in the near- surface layer (*100 A). The observed phenomenon is explained 110 by the oxygen disordering in the lattice on the superconductor surface and the formation of a tetragonal non- superconducting phase, which is responsible for the resistance increase, in the gaps between the granules. However, the resistance of the specimens treated with argon decreased with a decrease in292 temperature. Apparently, this is due to the fact that below room temperature, oxygen and oxygen vacancies can become ordered, and the semi-conducting tetragonal phase can be transformed to the orthorhombic phase, which displays metallic properties.201 An increase in Tc and Jc due to intercalation of simple gases into HTSC ceramics was observed repeatedly.110, 202 ± 204 How- ever, no reliable information on this phenomenon is available so far.Apparently, this can be associated with a very strong depend- ence of superconductivity of a material on its structure; because of this, small and hardly controlled changes in the crystal lattice parameters due to intercalation can change essentially the proper- ties affecting superconductivity. A substantial increase in Tc was observed upon low-temper- ature (*78 K) treatment of the YBa2Cu3O77x superconductor in N2, Ar, O2 and He streams.203 In particular, Tc increased from 90 to 141 Kupon treatment with nitrogen.Treatment with hydrogen did not affect Tc. The change in the superconducting transition temperature correlates with the duration of treatment. The change in the parameters of YBa2Cu3O77x superconductivity was assumed to be due to the inclusion of the gas molecules in the superconductor lattice. This changes the energetic state of the lattice and hence increases the energy of interaction between the carriers of superconductivity current, i.e., the Cooper pairs. If the temperature increases, the gas starts to desorb; the Cooper pairs become unstable, and the superconductor passes in the normal (non-superconducting) state. However, it is unclear from this explanation why gases as different in nature as nitrogen, argon, oxygen and helium behave similarly.It is known, for example, that helium is not adsorbed at all on solid surfaces at the liquid nitrogen temperature.205 In at attempt to reproduce the effects described above, Granados et al.206 failed to observe an increase in Tc during low- temperature treatment of the YBa2Cu3O77x ceramic with helium. Therefore, the results considered above are attributed to exper- imental errors. A reason for the Tc increase could involve the rather fast heating of the specimen, which at certain helium pressures usually results in overestimation of Tc in resistance measurements. Low-temperature treatment (77.4 K) of the YBa2Cu3O77x ceramic with nitrogen at normal pressure and with hydrogen at pressures up to 300 atm resulted in changes in Tc which increased or decreased depending on the duration of treatment.204 Similar results were obtained upon treatment of YBa2Cu3O77x with benzonitrile.After keeping the specimen in liquid benzonitrile for 11 h at room temperature, the Tc increased by 2 K. The Tc value was determined by measuring the electric resistance and magnetic susceptibility of a superconductor heated from 77.4 to 100 K. After several measurements, the original Tc value was almost restored. The similar effect of foreign molecules with completely different chemical properties on the superconductivity of the oxide YBa2Cu3O77x led to the conclusion 204 that foreign molecules penetrate into the ceramics due to diffusion along the granule boundaries and microcracks, thus creating mechanical stress.207 ± 209 The adsorbed phase propagating along the surface of a granule or a microcrack `bursts open' the granules at the contact site, and the resulting mechanical stress can be quite strong.In the zones adjacent to the crack mouths, the ceramic material appears to be under huge pressure, which is believed to result in a local change in the superconducting transition temper- ature. IV. The effects of adsorption and intercalation of organic molecules on the properties of high-temperature superconductors The effects of organic compounds on the properties of super- conductors are considerably more diverse than those of simple molecules. They can affect HTSC materials due to a number of their specific properties (ability to form complexes with metal L L Makarshin, D V Andreev, V N Parmon ions, to create regular layered structures that have electric conductivity, to be mechanically implanted in the superconductor material, to form charge transfer complexes), and also due to the possible presence of stable paramagnetic centres in their mole- cules.1. Adsorption of organic compounds The effects of adsorption of organic compounds on superconduc- tivity was first studied on films of low-temperature metal super- conductors.210 ± 214 The adsorption of 3,4,7,8-tetramethyl-1,10- phenanthroline (TMP) on a thin vanadium film increased the superconducting transition temperature by 0.09 K.210 The assumption was made that this effect is caused by the electronic properties of the adsorbed molecule, namely, the electronegativity parameter c à I á A , 2 where I is the ionisation potential and A is the electron affinity.Subsequent studies covered a much wider range of organic molecules of various nature with a very wide range interval of electronegativities (Table 1).211 ± 214 Thin vanadium and indium films (*10 nm thick) with Tc=2.4 and 4.1 K, respectively, were used as superconductors. The films were obtained by vapour deposition of the pure (99.999%) metals in a high vacuum (1076 Pa) onto a single-crystal quartz substrate. The molecules of organic compounds, mainly those with planar structures, were deposited on the films by evaporation in vacuo or by chemical methods. The superconducting transition temperatures and the critical currents before and after the adsorption were determined by the contact resistance method.For a vanadium film, good correlation was observed between the electronegativity and the sign of the Tc change upon adsorption of 14 different organic compounds. Adsorption of electron-donating molecules (anthra- cene, tetracene, perylene, etc.) with low electronegativity (c&3.9 eV) caused an increase in Tc. Conversely, electron- acceptor molecules, for example, tetracyanoquinodimethane with c55.5 eV , decreased the Tc. Similar regularities were also noted for an indium film. However, there are exceptions. For example, the supercon- ducting transition temperature increased upon deposition of Table 1.Changes in Tc upon adsorption of organic compounds on low- temperature metallic superconductor films and certain physicochemical parameters of these compounds.214 Adsorbed compound Tc /K DTc /K I /eV A /eV c /eV Thin vanadium film +0.10 0.42 1.0 0.85 +0.08 +0.09 +0.09 +0.06 3.83 3.94 4.0 7 7 7 7<4.9 >5.0 7.23 +0.075 6.88 7.15 7.28 8.65 <9.34 <0.5 +0.025 >9.34 0.66 2.33 2.76 2.92 2.20 2.45 2.62 2.23 0.85 2.25 7 7 70.11 1.7 7 >5.5 7 7 7 7 7 7 2.25 3.6 2.42 70.10 +0.05 70.05 Anthracene Tetracene Perylene Phenothiazine TMP 2-Aminoanthraquinone 1,4,5,8-Tetrachloro- anthraquinone Pyromellitic dianhydride TCQDM Tripyridyl-s-triazine Titanium dicyclopenta- dienyl dichloride Thin indium film 8.65 7 0.00 70.23 4.1 4.09 TMP TCQDM 7 7>5.5 1.7 Note.TMP is 3,4,7,8-tetramethyl-1,10-phenanthroline; TCQDM is tetra- cyanoquinodimethane.The chemical and adsorption effects of foreign molecules on the properties of high-temperature superconductors electron-accepting tripyridyl-s-triazine molecules on a vanadium film. The electron-donating TMP molecules affected vanadium and indium films differently: the adsorption of TMP on the vanadium film increased Tc, whereas that on the indium film did not change Tc.214 This was explained by the formation of a `metal atom ± organic molecule' complex. In the case of vanadium, which is a transition metal, a bond is formed between an unoccupied d orbital and a molecular p orbital of the organic molecule; this favours the transfer of an electron to the superconductor film and hence changes its superconductivity.Unlike vanadium, indium does not have a similar d orbital, therefore TMP adsorption does not affect its properties. In yet another work,214 the effects of the adsorption of organic compounds on the critical current of superconducting films was also studied. The adsorption of electron donors, viz., tetracene and perylene, on a vanadium film resulted in an increase in the critical current. Although a rather good correlation exists between the electronegativity of organic molecules, c, and the critical parameters of superconductivity of the vanadium film, Ic and Tc, the detailed mechanism of this phenomenon remains unclear.This can be due to charging of the superconducting film surface upon adsorption of organic compounds. For example, is was shown 215 that the superconducting transition temperature of an indium film depends on the surface charge sign. A strong acceptor, TCQDM, `withdraws' electrons from the superconductor bulk, the concen- tration of the charge carriers decreases, and Tc decreases by 0.23 K.Adecrease in Tc was also observed in an external electrical field where the film surface was charged positively. With the advent of oxide-based HTSC materials, research into the effect of adsorption of organic compounds on the critical parameters of superconductivity has gained new impetus.An organic dye, octaethylporphyrin, was applied onto a plate 3 mm long and 50 mm wide made of a 100 nm thick YBa2Cu3O77x film on a MgO(100) single crystal substrate.216 After cooling the specimen below Tc, the superconductor was irradiated, and the dependence of the current on the wavelength of incident light was measured. The system response to the light and the wavelength selectivity were much more significant at temperatures below Tc. The strongest response estimated as the current passing through the plate was observed at a wavelength of 600 nm corresponding to the maximum absorption of the dye. The effect of a deposited conducting polymer, polypyrrole, on superconductivity of an YBa2Cu3O77x film was studied.217 The electric conductivity of the film at temperatures higher than Tc, as well as the Tc itself, depend on the electric properties of the polymer.The dielectric polymer little affects the electric properties of the film. The polymer in the conducting or oxidised forms decreases Tc of the superconducting film by 15 K. Chemical or electrochemical reduction of the polymer results in restoration of the original Tc value. In order to explain these phenomena, the authors put forward two assumptions: (1) the polymer in the oxidised state does not affect the concentration of weakly bound oxygen in the YBa2Cu3O77x film, and hence the change in Tc is not related to oxygen; (2) the observed change in Tc is caused by the `proximity' effect, i.e. partial penetration of the Cooper pairs into the conducting polymer layer.In continuation of these experiments, temperature depend- ences of the contact resistance at the boundaries between the YBa2Cu3O77x, GdBa2Cu3O77x and Bi1.7Pb0.3Sr1.6Ca2.4Cu3O10 HTSC ceramics, on the one hand, and the oxidised form of a deposited conducting polymer, poly(3-hexylthiophene), on the other hand, were studied.218 The electric resistance of this contact decreases around Tc. It was assumed that ceramics in the super- conducting state can induce a kind of superconductivity in the polymer, in other words, this phenomenon can be directly related to the `proximity' effect. Exposure of an YBa2Cu3O77x powder to organic donor- acceptor electron-transport (DAET) systems (for example, DMF± CCl4, 2,20-bipyridyl ± monoethanolamine, DMSO± CCl4, etc.) was accompanied by partial etching of the ceramic without 293 considerable change in its phase composition.219, 220 Etching removes the labile near-surface oxygen and exposes the more perfect structured layers; the contact between the granules and their ability to sinter are improved.The concentration of oxygen in the material bulk does not change. Thus, etching of HTSC ceramics powder with DAET-systems under mild conditions results in modification of the surface layers of the species and is accompanied by stabilisation of the superconducting state and improvement of the superconductor quality. Room-temperature treatment of an YBa2Cu3O77x powder with an excess of 1-hydroxy-2,2,6,6-tetramethylpiperidine (HTMP), which is readily oxidised into a stable nitroxyl radical even on treatment with atmospheric oxygen, is accompanied by extraction of weakly bound oxygen from the specimen bulk.221 An EPR study of the radical accumulation kinetics made it possible to determine the amount of the oxygen extracted and to estimate the activation energy of this process.The determined activation energy, *16 kJ mol71, is comparable with Ea for the diffusion of highly reactive oxygen in YBa2Cu3O77x. It is much smaller than the values that characterise the evolution of weakly bound oxygen to the gas phase upon thermal desorption (at x<0.006, Ea=120 kJ mol71). Apparently, this difference can be explained by the fact that HTMP adsorbed on the ceramic lowers the activation barrier for oxygen exit from the surface layer of the specimen.As the weakly bound oxygen is extracted, the super- conductivity is impaired and disappears completely if more than 0.03 a.u. of oxygen is lost. Since a rather small change in the oxygen content cannot cause such a strong degradation of the superconductor, the actual major role is probably played by the magnetic moment of stable nitroxyl radicals formed during HTMP oxidation (see below). In another study,222 the YBa2Cu3O77x ceramic was converted into a metastable state by mild removal of oxygen from the O(4) position (see Fig. 1 b) at 523 K using a different organic reducing agent, namely, azoben- zene. The type of the kinetic dependence for oxygen removal from the Cu(1) layers allows one to assume that it involves the diffusion mechanism.The dynamics of the change in the crystal lattice parameters a, b and c depending on the oxygen concentration was followed. Subsequent treatment of the ceramic with iodine resulted in restoration of the lattice parameters to the original values (i.e. to those before the reaction with azobenzene). Treatment of YBa2Cu3O77x and Bi2Sr2CaCu2O8+x with methanol 223 containing *20 mass% of HTMP and 10% KOH as an HTMP oxidation catalyst 224 was also accompanied by extraction of weakly bound oxygen from the ceramics. The superconductivity parameters of the ceramics after this treatment strongly depended on their initial properties and on the methods of preparation.The superconductivity parameters of YBa2Cu3O77x specimens obtained by cryochemical technology (A) and by self-propagating high-temperature synthesis (B) changed essentially upon contact with HTMP and KOH in methanol at 313 K for 120 min. The superconducting transition temperature of the specimen A decreased strongly (to 60 K). This was explained 223 by removal of reactive oxygen from the O(4) and O(5) positions of the YBa2Cu3O77x crystal lattice. The specimen B displayed no trend to form a phase with Tc=60 K, and the change in its superconducting transition temperature was smaller. This phenomenon is due to the specifics of structural trans- formations on the ceramic's surface, which hinder oxygen diffu- sion from the specimen bulk.225, 226 These transformations were not observed in the oxygen extraction from the Bi2Sr2CaCu3O8+x ceramic, and the superconductivity was suppressed completely.However, brief treatment with HTMP (15 min) increased the superconducting transition temperature by 13 K. It should be noted that a similar phenomenon was observed upon removal of a small amount of oxygen (x&0.03) from the Bi2Sr2CaCu2O8+x ceramic.227 ± 230 This is probably due to the extraction of the highly reactive oxygen intercalated between the BiO7OBi layers and the essential increase in the unit cell parameter c. Prolonged treatment294 did not change Tc , but the fraction of the superconducting phase decreased to complete loss of superconductivity.The much weaker effect of treatment with methanol contain- ing 10% KOH on the specimen A than on the specimen B was explained by the higher oxidising and catalytic activity of the YBa2Cu3O77x ceramic obtained by self-propagating high-tem- perature synthesis.231 The extraction of oxygen from Bi2Sr2CaCu2O8+x was accompanied by an insignificant decrease in the fraction of the superconducting phase and, as in the previous case, an increase in the superconducting transition temperature by 11 K. The difference between the effect of the 10% KOH solution in methanol and the effect of the HTMP solution in methanol is due to the fact that methanol cannot react with the weakly bound oxygen responsible for the material superconductivity. In one of the first studies on the chemical modification of HTSC material surfaces,232 various ferrocene derivatives contain- ing aminoalkyl, aminoaryl, thiol, phosphino, amide and hydroxy- methyl functional groups were adsorbed at room temperature onto the surfaces of the YBa2Cu3O77x ceramic and thin film from a solution in acetonitrile.The solution concentration of the reagents was 1073 mol litre71. It was shown by cyclic voltamme- try that only molecules containing aminoalkyl, aminoaryl and thiol groups are adsorbed. This implies that preferential adsorp- tion occurs on the YBa2Cu3O77x surface. Ferrocenes containing such substituents as amide, phosphino and hydroxymethyl groups are virtually not adsorbed on HTSC materials. The pronounced ability of YBa2Cu3O77x to adsorb amino groups is explained by the presence of the Cu(II) and Cu(III) ions on its surface.The adsorption of ferrocene derivatives with thiol groups is explained by the high oxidising ability of the superconducting material, on the one hand, and by the reducing properties of thiol groups, on the other hand. Treatment of thin YBa2Cu3O77x film with solutions of various ferrocene derivatives in acetonitrile for 3 h at room temperature did not affect its superconductivity. An increase in the treatment time to 48 h decreased Tc by 2 K. It was noted that covering of the YBa2Cu3O77x surface with a mono- layer of molecules of substituted ferrocenes makes the HTSC material resistant against moisture. Cyclic voltammetry was used to study the effect of small amounts of water on the adsorption of 11-mercaptoundecanoyl- ferrocene (MUF) on the surface of the Tl1.4Ba2CaCu2Oz ceramic.233 The presence of 5% water in a 0.1 M solution of MUF in acetonitrile decreased considerably the degree of cover- age of the ceramic's surface with MUF molecules.If ethanol was used as the solvent, adsorption virtually did not occur. However, adsorption of MUF from dehydrated solvents gave a dense and stable monomolecular layer of chemisorbed MUF. It is believed that water affects adsorption due to a BaCO3 layer formed as a result of corrosion, which does not adsorb MUF. Direct proof of the chemical interaction between the amino groups and the YBa2Cu3O77x surface was obtained in studies using Raman spectroscopy.234 It was shown that 4-aminopyridine molecules are chemisorbed on the superconductor surface to form a dense monomolecular layer of vertically arranged molecules; the amino group is coordinated with the surface copper atoms of the superconductor (Fig.7). A small review of the early studies on the adsorption of organic molecules on HTSC materials 235 presented data on the chemisorption of ferrocenes containing various ligands, on the Raman spectra of monomolecular layers on HTSC material surface and on the kinetics of adsorbent ± adsorbate exchange. Special attention was given to the practical use of HTSC materials modified with adsorbed organic molecules. For example, mono- layer adsorption of hydrophobic molecules on HTSC materials can be used to prevent their corrosion in a humid atmosphere.In addition, this treatment increases considerably the adhesion between the polymer and the HTSC material. Superconductors coated with a monolayer of organic molecules can serve as the basis for obtaining synthetic tunnelling contacts used in super- L L Makarshin, D V Andreev, V N Parmon N N N NH2 NH2 NH2 YBa2Cu3O77x Figure 7. Coordination of adsorbed 4-aminopyridine molecules on the surface of the YBa2Cu3O77x HTSC ceramic.234 conducting quantum interferometer probes. They can also be useful for solving a number of fundamental problems of the physics of HTSC materials. Adsorption of naphthalene on the YBa2Cu3O77x ceramic was accompanied by an increase in Tc and the transport critical current.236 The treatment was carried out by immersion of samples in chloroform solutions of naphthalene of different concentrations.According to gravimetric data, this treatment results in covering of the surface of the samples with a shell of naphthalene molecules from one to several hundred monolayers thick. The transport critical current was measured with a vibrating magnetometer using a ring cut from the ceramic under study; the superconducting transition temperature was determined by recording the real part of magnetic susceptibility. Adsorption of naphthalene from 0.2 M solution in chloroform did not affect the critical superconductivity parameters of the specimen. An increase in the naphthalene concentration to 1 mol litre71 increased the Tc by 1.5 K.Further increase in the naphthalene concentration did not change Tc considerably. For example, the Tc increase due to treatment of the specimen with a saturated solution (3.5 mol litre71) was 1.62 K. If naphthalene is washed off from the specimen with pure chloroform, the original superconducting transition temperature was restored almost completely (Fig. 8). It has to be noted that the absolute value of the signal amplitude w 0 did not depend on the treatment of the specimen. Hence, naphthalene adsorption does not change the fraction of the superconducting phase in the specimen. The transport critical current after naphthalene adsorption from a saturated solution increased by 20%.Additional experiments on measurements of the total current decay in a superconducting ring were carried out in order to analyse this interesting phenomenon. It was found that treatment of a specimen with naphthalene does not affect the decay rate and hence does not affect the pinning energy of the Josephson vortices. The mechanism of the effect of adsorbed naphthalene on superconductivity has not been determined so far. Probably, a bond between an unoccupied d orbital of the Cu(III) ion in the superconductor and the molecular p orbital of naphthalene is w 0 1 70.4 2 3 70.8 88 80 92 T /K 84 Figure 8. Temperature dependence of the real part (w 0) of magnetic susceptibility of the YBa2Cu3O77x ceramic;236 (1) original specimen; (2) after treatment with saturated solution of naphthalene in chloroform; (3) after washing the treated specimen with pure chloroform.The chemical and adsorption effects of foreign molecules on the properties of high-temperature superconductors formed, which favours electron transfer to the superconductor and hence changes its properties.The change in the superconducting transition temperature caused by the naphthalene adsorption cannot be explained by donor-acceptor interactions, since interactions of electron donors with superconductors having hole-type conductivity most prob- ably decrease the concentration of the charge carriers and hence decrease rather than increase Tc. The increase in Tc observed in the experiment is probably determined by the effective charging of the superconductor granule surfaces upon adsorption of naphthalene molecules.237 As shown above, the effect of adsorption of organic molecules on the properties ofHTSC materials primarily results from donor- acceptor interactions or redox reactions involving copper and oxygen ions.On the other hand, the high sensitivity of the critical parameters of superconductivity to external magnetic field pro- vides a reason to assume that the superconductor properties are strongly affected by paramagnetic species, i.e., radicals possessing magnetic moments. The effect of adsorption of a stable nitroxyl radical, 2,2,6,6- tetramethylpiperidin-N-oxyl (TEMPO), on the transport proper- ties of the YBa2Cu3O77x superconducting ceramic was studied.238, 239 Adsorption was carried out at room temperature by immersing the ceramics in a 0.2 ¡¾ 2.0 M chloroform solution of TEMPO. The specimen was kept in the solution for 12 h and dried in air.The critical current was measured as a function of temper- ature using a vibrating magnetometer. In reference experiments, the specimens kept in pure chloroform for 30 days did not show any effect of chloroform on the ceramic's properties. The super- conducting transition temperature was measured by recording the magnetic susceptibility on an alternating current magnetometer. Figure 9 shows the dependences of the transport current Ic on temperature at various radical concentrations in the starting solution. It is evident that radical adsorption affects rather strongly the critical current in the temperature range from 77.4 to 86 K.At a content of adsorbed radicals in the ceramic equal to 1.761072 mass %, the critical current was higher than that in the original non-treated specimen over the entire temperature range studied. Further increase in the content of radicals decreased Ic (see Fig. 9). The superconducting transition temperature of the ceramic before and after adsorption of radicals remained the same. The mechanism of the effect of adsorbed molecules with their own magnetic moments on superconductivity is probably rather complicated. The results of additional experiments carried out specially to reveal the characteristic features of this effect were analysed within the framework of the intergranular conductivity Ic /A 0.8 12345 0.40 T /K 84 80 Figure 9.Temperature dependence of the critical current in the TEMPOy ¡¾ YBa2Cu3O77x composite material at the following concen- trations of the stable radical: 0 mass% (1), 1.761072 mass% (2), 4.261072 mass% (3), 8.561072 mass% (4), 1.761071 mass% (5).238, 239 295 n Ic(0) /A 100 1.9 80 1.7 60 15 102 [TEMPO] (mass %) 10 0 5 Figure 10. Dependence of the critical current extrapolated to 0 K and the exponent n in the Eqn (3) on the concentration of stable radicals TEMPO in the YBa2Cu3O77x ceramic.238, 239 (3) IcOTU a IcO0U 1 ¢§ T n , Tc model.240 The general temperature dependence of the critical current for various contacts between the granules has the form where Ic(0) is the critical current obtained by extrapolation to 0 K; n is an index reflecting the nature of contacts between the granules, which can vary from 1 to 2.Figure 10 presents the calculated dependences of the critical current Ic(0) and the index n on the concentration of radicals.238, 239 It is evident that n is equal to 2 in the absence of radicals and n<2 at their low concentrations. This means that upon adsorption of a radical, an SNS contact is transformed to an SNINS contact, or, in other words, the intergranular bond is improved and approaches a character close to the classical Josephson transition.240 On the other hand, the absolute value of the critical current also decreases, apparently due to the effect of the radical magnetic field on the intergranular bond. If the radical concentration is increased further, n even- tually becomes equal to two.The change in the intergranular conductivity correlates with the change in the critical current Ic(0). The increase in the latter at high radical concentrations is probably due to a specific behaviour of the highly concentrated magnetic media. For example, it is known that at a concentration of paramagnetic species above a certain value a significant change in the spin orientation is observed and the total magnetic moment of the medium decreases. Hence, in our case the increase in the concentration of stable radicals should weaken the effect of the magnetic moments on the conductivity of the intergranular layer, which is confirmed experimentally.A study of the transport properties of the YBa2Cu3O77x HTSC ceramic upon magnetic field trapping by granules of the specimen showed that an increase in the critical current between the granules is actually observed at certain parameters of the magnetic field `uptaken' by the granules.242 Moreover, treatment of a superconducting ceramic ring with stable radicals does not change the decay rate of the current induced in the ring. Hence, additional pinning centres are not formed, and probably the increase in the transport critical current in the YBa2Cu3O77x ceramic after adsorption of stable radicals mostly occurs due to `uptake' of magnetic field of a radical in the specimen granules. It was shown experimentally 241 that interaction of the Josephson and Abrikosov vortices can increase the critical current essen- tially.An effect on the intergranular bonding and the donor- acceptor properties of the stable radical, which probably mani- fests itself as a change in the index n in Eqn (3), cannot be ruled out either. However, the major role still belongs to the `uptake' of the radical magnetic field into the specimen granules. The critical parameters of superconductivity of the YBa2Cu3O77x ceramic containing adsorbed radicals depend on296 the temperature to which a specimen has been cooled prior to the measurements.243 The type of the temperature dependence of magnetic susceptibility indicates an increase in the critical param- eters of superconductivity upon cooling of the specimen.The explanation for this is that at lower temperatures, greater mag- netic flux from the radical molecules is `uptaken' by the granules, resulting in an increase in the critical parameters. The effect of the adsorption of a stable diphenylpicrylhydrazyl radical (DPPH) on the superconductivity of the Bi1.8Pb0.5Sr1.9Ca2.5Cu3.2Oz ceramic was studied.244 Before an experiment, the surface of the specimen to be studied was freed from adsorbed water, CO2 and organic contaminants by calcina- tion in an oven at 473 K in vacuo for one hour. Then the cleaned specimen was transferred into a glass tube containing DPPH powder and sealed. The adsorption of DPPH was carried out by heating the tube to 473 K for 20 min.After that, the tube was broken, and the specimen was transferred into a cell for measure- ment of magnetic properties. For reference purposes, the procedure described above was also carried out in the absence of DPPH. The superconductivity parameters of the specimen under study were measured with an alternating current magnetometer. Figure 11 presents the temperature dependence of the magnetic susceptibility of the Bi1.8Pb0.5Sr1.9Ca2.5Cu3.2Oz ceramic. It is evident that upon adsorption of the radical, the curve shifts towards the low-temperature region; this indicates that the current-carrying ability of the specimen decreases. Thermal treat- ment of the ceramic also deteriorates superconductivity, but in this case the change is much less.The experimental values of magnetic susceptibility were compared with the real part of the magnetic susceptibility calculated within the framework of the simple Kim model.245 Good agreement between the experimental results and the calculations were achieved with the assumption that the intergranular layer is affected by a static magnetic field of 0.13 mT (see Fig. 11, curve 2). Given the magnetic induction B, one can estimate the distance d between an adsorbed radical molecule and an intergranular contact: 2m d à , (4) B 1=3 where m=gbS is the magnetic moment of the radical, g=2 is the g-factor of a free electron, b is the Bohr magneton, S=1/2 is the spin quantum number.Substituting the above value of B=0.13 mT in the formula (4), we obtain d=5.6 nm. This distance is in good agreement with the thickness of the non- superconducting layer, which is generally located on the super- conductor surface and is estimated as 3 ± 5 nm.246 Such consis- tency supports the suggested model of the effect of the magnetic w 0 70.2 Tc 123 70.6 71.0 80 T /K 90 Figure 11. Temperature dependence of magnetic susceptibility of the Bi1.8Pb0.5Sr1.9Ca2.5Cu3.2Oz ceramic;244 (1) original specimen; (2) after adsorption of the DPPH radical; (3) after heating in a sealed tube for 1 h. Solid lines � experiment, dashed lines � calculated real part of magnetic susceptibility within the simple Kim model. L L Makarshin, D V Andreev, V N Parmon 65 3 21 332.6 B /mT 332.0 Figure 12. EPR spectra of the DPPH radical adsorbed on the surface of the Bi1.8Pb0.5Sr1.9Ca2.5Cu3.2Oz superconducting ceramic recorded at the temperatures: 77.4 (1), 89 (2) and 125 K (3).244 field of adsorbed radicals on the intergranular current in super- conducting ceramics.The EPR spectrum of the DPPH radical adsorbed on the surface of the Bi1.8Pb0.5Sr1.9Ca2.5Cu3.2Oz ceramic in supercon- ducting state at 77.4 K consists of two lines (Fig. 12). As the specimen is heated, the high-field line of the doublet starts to shift downfield and, since the position of the low field line remains unchanged, splitting decreases. At 125 K, i.e., at a temperature above Tc, the high-field line coalesces with the low-field one, and a singlet corresponding to the line in the EPR spectrum of the DPPH radical remains.It follows that the low-field line of the doublet, the position of which is not changed, corresponds to the adsorbed radical located at a great distance from the super- conducting phase. For this reason, this phase does not affect the low-field line. The shift of the high-fielne with the increase in temperature can be caused by the following reasons. First, the magnetic flux trapped by granules of a superconducting specimen (the Abrikosov vortices) located in an external magnetic field with B0&0.33 mT changes the magnetic field parameters in the vicinity of the adsorbed radical. Second, the magnetic moment of the adsorbed radical has a `mirror' image in the diamagnetic surface of the superconducting specimen, and probably the line position in the EPR spectrum of the radical is actually determined by the magnetic field of this `mirror' dipole. 2.Intercalation of organic molecules into the crystal structure of high-temperature superconducting materials A characteristic feature of HTSC materials is their layered structure which plays an important role in the transition to the superconducting state. As noted above and as has been shown by Bulaevskii,247 intercalation of atoms and molecules into the structure of layered solids changes essentially their chemical and physical properties. For example, intercalation of pyridine mole- cules into the TaS2 superconductor was accompanied by a significant increase in the superconducting transition temperature (from 0.5 to 4.5 K).248 Hence, one can expect that intercalation of organic molecules in an HTSC material would affect strongly the critical parameters of superconductivity.It was even suggested that intercalation of organic molecules in layered inorganic materials can result in superconductors with high critical temperatures.249 Bismuth oxide- and thallium oxide-containing superconduc- tors can be suitable objects for studying the effect of intercalation on superconductivity. In fact, the magnetic properties of the Bi1.7Pb0.3Sr2Ca2Cu3Oz ceramic with Tc=105 Kunderwent inter- esting changes on treatment with benzene.250 A powder with a particle size of *50 mm obtained by grinding bulk ceramic specimens was placed in benzene and kept at room temperature for a period from 10 min to 2.5 h.Then benzene was evaporated at 308 K, and the powder was placed in molten paraffin in order toThe chemical and adsorption effects of foreign molecules on the properties of high-temperature superconductors prevent electrical contact between the granules. As a rule, the specimens obtained by this method manifested weak paramagnet- ism with an absolute susceptibility of 1077 or ferromagnetism with a magnetisation of 1079 A m2 g71. Specimens of the ceramic that had been kept in benzene for 20 ± 30 min had unusual electrophysical properties. First, rather high diamagnetic response was recorded at room temperature and hysteresis phenomena were observed in low magnetic fields.For example, the magnetisation in a field of 10 mT at 300 K was 861076 A m2 g71. Second, a stepwise change in the external magnetic field was accompanied by magnetisation relaxation. Third, if the temperature of the specimen was cycled in the range from 77.4 to 300 K, the initial magnetisation decreased gradually. For example, the maximum magnetisation of a specimen was 261075 A m2 g71 after the first cooling, whereas it was as low as 861076 and 461076 A m2 g71, respectively, after the second and third cooling. Fourth, the EPR spectrum of the specimen at 300 K contained a low-field microwave absorption signal. Repeated cyclic changing of the specimen temperature decreased this signal.X-Ray diffraction analysis revealed additional reflec- tions, apparently due to a new phase that was formed in the specimen. Such unusual physical properties of the Bi1.7Pb0.3Sr2Ca2Cu3Oz ceramic containing intercalated benzene can be a manifestation of superconductivity. Therefore, the authors of this study 250 believed that they could detect a small fraction of a superconducting phase in a specimen at room temperature. Unfortunately, the phenomena described above were only observed on some specimens. They could not be reproduced in other specimens, and the fraction of the super- conducting phase could not be increased by varying the treatment conditions. Later, a detailed study of the intercalation of planar aromatic molecules into superconducting ceramics was undertaken.251 Intercalation of benzene and phthalocyanine complexes of copper and nickel into bismuth-containing ceramics, Bi2Sr2CaCu2Oz and Bi2Sr2Ca2Cu3Oz, and into a thallium-containing ceramic, Tl2Ca2BaCu2Oz, were carried out in sealed tubes at 273 ± 423 K.As in yet another study,250 the ceramics studied displayed unusual physical properties at room temperature: the specimens retained magnetisation for some time after the external magnetic field was turned off; a low-field microwave absorption signal was observed; the magnetisation curves had unusual shapes; negative magnet- isation was recorded in low magnetic fields (1 ± 2 mT). X-Ray diffraction and Raman spectroscopic data showed that these organic compounds were located between the Tl7O or Bi7O double layers (see Fig.1 c). Intercalation of zinc and nickel phthalocyanine complexes also affected the properties of bismuth- and thallium-containing HTSC ceramics. In Ref. 252, as in previous studies,250, 251 unusual physical properties of these ceramics at room temperature were reported. For example, at 300 K the magnetisation of a metal phthalocyanine intercalated in the ceramics was found to be anomalously high, two orders of magnitude higher than the magnetisation of a free metal phthalocyanine complex. In the superconducting state (at 90 K), the magnetisation of the ceramics with the intercalated complex was one order of magnitude smaller and was equal to 1072 A m2 g71.The temperature dependence plots of the magnetisation of (NiPc)0.1Bi2Sr2CaCu2Oz and (NiPc)0.09Tl2Ca2BaCu2Oz specimens (Pc=C32H16N8) obtained on cooling in magnetic fields and without a field showed notice- able differences starting from 250 K. Such unusual magnetic properties can indicate that the specimens display superconduc- tivity at rather high temperatures. Certain assumptions on the nature of the observed phenomena were made.250 ± 252 It was assumed that molecules of aromatic compounds characterised by strong electron delocalisation form new composite materials when introduced into the structure of a layered superconductor. These materials manifest unusual physical properties which resemble in certain respects the properties of superconductors; this is primar- ily due to the possible formation of quasi-species�`anyons' with 297 half-integer statistics.It was shown theoretically 253 that similar quasi-species present in two-dimensional electronic systems can generate a superconducting state, the transition to which occurs by a mechanism different from the generally accepted one and which is characterised by a higher Tc. In addition, the superconductivity of intercalated composites can be explained using the exciton mechanism.254 In fact, inter- calation of organic molecules into layered crystal systems results in `sandwiches' that consist of alternating thin metal layers and dielectric intermediate layers. An electron ± hole pair (exciton) can be formed in such systems.High magnetisation of intercalated bismuth- and thallium-containing HTSC oxides can also be due to the joint orbital contribution of p-electrons of aromatic mole- cules, which can form two-dimensional conducting layers due to the structural features of the material. According to X-ray diffraction data,252 intercalation changes the material structure substantially. Not only do the distances between the structural Tl7O and Bi7O layers change, but also those between the atoms along the a and b axes (see Fig. 1 c). These transformations affect the electronic condition of the copper ion and result in transition from the usual phase with antiferromagnetic ordering of the copper spin states to the ferromagnetic phase. The effect of the amount of the metal phthalocyanine intercalated in the ceramics on this process was observed for the (NiPc)yBi2Sr2CaCu2Oz system.252 V.The effect of superconductivity on the electronic state of the adsorbed molecules and on adsorption processes The effect of the substrate superconductivity on the fluorescence fetime of an adsorbed dye, 1,4-bis(5-phenyloxazol-2-yl)benzene (POPOP), was studied.255 Adsorption of the dye on a YBa2Cu3O77x ceramic substrate was carried out by treating the specimen with a 1073 M POPOP solution for 100 s in toluene followed by drying in air. As a result of this treatment, the width of the superconducting transition temperature of the substrate, DT=1 K, expanded to DT=8 K towards low temperatures.The fluorescence lifetime t at each particular temperature was calculated using an exponential kinetic curve of the decay of the dye fluorescence excited by light with a wavelength of l=330 nm. Figure 13 presents the temperature dependences of the relative specific resistance of the substrate (r/r298, where r298=261073 O cm is the substrate specific resistance at 298 K) and the lifetime of fluorescence of adsorbed POPOP (t). If temperature is decreased, an abrupt increase in t is observed in the region of transition into the superconducting state. In control experiments, where the fluorescence lifetime of POPOP molecules adsorbed on a non-superconducting substrate with similar com- r/r298 t /ns 3.25 0.3 0.2 3.00 0.1 2.75 0 T /K 100 90 80 Figure 13.Temperature dependences of the relative specific resistance of the YBa2Cu3O77x support and the fluorescence lifetime of POPOP adsorbed on this support.255298 position was measured, no such dependence of t on temperature was observed. Thus, the abrupt increase in the lifetime of the electronically excited singlet state of POPOP upon temperature decrease is due to the specific features of the superconducting state of the substrate. Probably, the following aspects are most important. First, the transition of the ceramic in the superconducting state is accompanied by small structural changes of its crystal lat- tice,256, 257 which can affect the geometry of the adsorbed molecule and thus change the lifetime of the excited electronic state.Second, the transition into the superconducting state changes the dielectric constant of the surface layer of the HTSC material;258 this results in a change in the electric field strength near the surface and hence can affect the probability of nonradiative transitions of the excited molecules of the adsorbate. Third, the transition of the substrate in the superconducting state can weaken the interaction of the electronically excited adsorbate molecule with the conductivity electrons of the substrate, which also affects the luminescence lifetimes. The effect of the substrate superconductivity on the electronic state of the adsorbed molecule was observed:258 the fluorescence quantum yield of an organic dye, erythrosine, adsorbed on a superconductor surface changed in a stepwise manner at the superconducting transition temperature of the substrate.A bulk ceramic specimen of YBa2Cu3O77x (Tc=87 K), an epitaxial polycrystalline film of YBa2Cu3O77x (Tc=50 K) and a low- temperature superconductor film of NdN (Tc=13.5 K) were used as the superconductors. The fluorescence quantum yield of erythrosine applied on the low-temperature superconductor NdN underwent a small (5%) stepwise change at Tc, while the quantum yield above and below this point virtually did not depend on temperature. A different behaviour was observed where the dye was adsorbed on HTSC substrates. The fluorescence quantum yield of erythrosine adsorbed on a YBa2Cu3O77x polycrystalline film increased by 100% as the latter has passed in the super- conducting state.The quantum yield of erythrosine adsorbed on the bulk YBa2Cu3O77x ceramic changed substantially around the temperatures of 50 and 90 K, which correspond to two super- conducting subsystems, viz., the intergranular layers and granules, respectively. The conclusion was made 258 that the fluorescence quantum yield of the dye changes upon transition of the adsorbate in the superconducting state owing to the change in the dielectric constant of the material. An original way for the separation of diamagnetic and para- magnetic molecules on a porous membrane made of a super- conducting material, YBa2Cu3O77x, was reported.259 In the superconducting state, the porous ceramic is `transparent' for nitrogen molecules but does not pass the paramagnetic molecules of oxygen.TheO2:N2 concentration ratio decreased by a factor of six after a gas mixture (air) has passed through such a membrane. There was no gas separation effect with a non-superconducting membrane at the same temperatures, which is explained by high (comparable with the energy of thermal movement of molecules) energy of interaction between the magnetic moment of a triplet oxygen molecule and its mirror image in the superconducting surface of the membrane material. As a result, the effective cross- section of interaction of the oxygen molecule with the super- conductor increases and a paramagnetic oxygen molecule pene- trates the membrane with greater difficulty than a diamagnetic nitrogen molecule. The features of interaction of HTSC materials with charged or magnetoactive species provide the possibility of using these materials for practical purposes. For example, the focusing of electron and ion beams passing through a superconducting tube made of HTSC ceramics was identified and studied.260 ± 262 A theoretical estimate 260 of the focusing effect of a superconducting surface on the beam of free electrons or ions showed that at certain velocities of the beam the focusing force can be comparable to, or even exceed, the force of Coulomb repulsion of the charged species in the beam.The vast experimental and theoretical material that L L Makarshin, D V Andreev, V N Parmon has been accumulated shows the possibility of using HTSC materials for these purposes.261 For example, the passage of an electron beam through a superconducting tube from bismuth- containing ceramics with Tc=105 K was studied experimen- tally:262 an electron beam with an energy of 340 keV, an intensity of 0.8 kA and a diameter of 8 mm was directed into a super- conducting tube.The electron beam leaving the tube was focused to a diameter of 1.1 mm. The authors of the present review have thoroughly studied the interaction of a superconductor with magnetoactive species.263, 264 Specific features of the low-temperature adsorption and desorp- tion of oxygen and argon on a 0.25 ± 0.50 mm fraction of powder of the YBa2Cu3O77x superconducting ceramic were studied.The adsorption isotherms of argon and oxygen in the temperature range from 77.4 to 98 K were obtained. The desorption curves coincided completely with the adsorp- tion curves. Hence, the adsorption of argon and oxygen on the oxide YBa2Cu3O77x is reversible and can be regarded as physical adsorption. The experimental isotherms of the argon and oxygen adsorption in the entire temperature range studied correspond to the `second type' 265 and are well described by the BET polymo- lecular adsorption theory.266 The temperature dependence of the superconducting adsorbent specific surface calculated using the BET equation for oxygen adsorption differs essentially from the similar dependence obtained for argon adsorption (Fig. 14). According to the BET theory, the geometrical dimensions of the surface do not depend on temperature and are determined by the nature of the material.This assumption is totally consistent with the low-temperature adsorption of argon. The specific surface of the specimen measured with respect to argon was found to be 1.05 m2 g71. S /m2 g71 1 1.0 3 2 0.8 0.6 T /K 90 80 Figure 14. Temperature dependences of the specific surface measured by the BET technique from adsorption of Ar (1) and O2 (2, 3) on the YBa2Cu3O77x adsorbent in superconducting (1, 3) and non-supercon- ducting states (2).263 A different situation is observed for the adsorption of para- magnetic oxygen molecules. At 77.4 K where the adsorbent is in the superconducting state, the amount of oxygen adsorbed in a monolayer is more than twice as small as that in the case of argon.As temperature increases, the amount of adsorbed oxygen increases, and when the adsorbent is no longer in the super- conducting state, oxygen is adsorbed exactly in the same way as diamagnetic argon (see Fig. 14). Adsorption of oxygen on a non- superconducting adsorbent prepared by heating the original superconducting adsorbent in air to 873 Kfollowed by quenching to room temperature was studied for reference purposes. In this case, the specific surface does not depend on temperature, and its small decrease in comparison with the specific surface of the original superconducting adsorbent is apparently due to partial sintering of small granules during thermal treatment of the ceramic at 873 K.The anomaly observed in the low-temperature adsorption of molecular oxygen on HTSC ceramics is undoubtedly due to theThe chemical and adsorption effects of foreign molecules on the properties of high-temperature superconductors paramagnetism of O2 molecules and the superconducting state of the adsorbent. It is believed 259 that perfect diamagnetism of the adsorbent in the superconducting state favours its additional interaction with the paramagnetic oxygen molecule. However, estimates show that the energy of repulsion between the magnetic moment of oxygen and its mirror image in the superconducting adsorbent can be comparable with the energy of thermal move- ment only at distances equal to the molecular diameter.This does not consider the London penetration depth of magnetic field into the superconductor and the thickness of the possible non-super- conducting phase layer on the adsorbent surface. A more reason- able assumption is that the repulsive force arises due to the interaction between the magnetic moment and the magnetic flux `uptaken' by the granules of the superconducting adsorbent.267 As the temperature is increased, the adsorbent diamagnetism decreases and the amount of adsorbed oxygen increases synchro- nously, tending to a limit which corresponds to complete surface coverage. Measurements of the `pure' heat of adsorption Qa=qst7qL , (where qst is the isosteric heat of adsorption and qL is the heat of condensation of the adsorbate on a planar surface) also indicate a significant difference between the adsorption of molecular oxygen on superconducting and non-superconducting adsorbents.As the surface of a non-superconducting adsorbent is covered with oxy- gen, Qa decreases monotonically (Fig. 15). This agrees with the common concept of adsorption on non-porous solids with the `classical' non-uniform surface. Conversely, Qa increases in the case of oxygen adsorption on superconducting adsorbents. This can be a consequence of the formation of new adsorption sites as the surface is covered with oxygen molecules due to violation of superconductivity in the near-surface zone by the strong magnetic field of the adsorbed molecules. In continuation of previous works,263, 264 the heat transfer from a heater to a superconductor by argon, nitrogen, helium and oxygen molecules was studied.268 It is well known that the heat transfer from gas to solid occurs efficiently only in the case if the collision of the gas molecules with the surface is inelastic.There- fore, based on the above data on the adsorption of paramagnetic molecules,112, 263 it can be assumed that oxygen transfers heat to a superconductor less efficiently than molecules which have no magnetic moment. The efficiency of heat transfer by gas molecules to the surface of the YBa2Cu3O77x superconducting ceramic was estimated as the time required for heating the specimen to Tc. Specimens of the YBa2Cu3O77x ceramic had different critical parameters: the specimen Y1 was characterised by a large strength of the first critical magnetic field of the granules Hc1g=1.026104 A m71 and Tc=89.1 K, while the specimen Y2 had the following parameters: Hc1g=1.46103 A m71 and Tc=91.3 K.The electron micrographs of the cross-sections of specimens Y1 and Y2 taken with a scanning electron microscope showed that their morphologies differ essentially: the specimen Y1 Qa /kJ mol71 1 5.0 4.0 2 3.0 0 0.4 0.8 Y Figure 15. Dependence of the `pure' heat of adsorption (Qa) of oxygen on the YBa2Cu3O77x ceramic in superconducting (1) and non-superconduct- ing (2) state on the degree of surface coverage (Y) with molecular oxygen in the temperature range of 77.4 ± 96 K.263 1 6.0 mm 1 11.0 mm Figure 16.Scheme of measuring cell for recording the heating rate of HTSC materials in nitrogen or oxygen media;268 (1) external glass flask of the measuring cell; (2) heater; (3) quartz tube; (4) specimen; (5) capron thread; (6) thermal insulation. consists of distinct oriented monolithic blocks with characteristic cross sizes of*406100 mm, whereas the specimen Y2 consists of randomly oriented granules with an average size of*10 mm. The period from the moment when the heater has been turned on to the moment when the ceramic passes into the normal state was measured in different gaseous media in a cell (Fig. 16) placed in a mutual inductance coil for recording the magnetic suscepti- bility of the specimens. Figure 17 shows the experimental depend- a tc /s 580 540 Dtc 500 1 0.5 0 b 540 500 0.3 0.15 0 c 540 520 0.5 0 0.25 Figure 17.Dependence of the time required for heating a specimen to the superconducting transition temperature on the pressure of oxygen (1) and nitrogen (2) in the measuring cell;268 (a) measurements on the Y1 speci- men; (b) measurements on the Y1 specimen coated with a paraffin film; (c) measurements on the Y2 specimen. 299 65 4 11.5 mm 321 Liquid nitrogen 12 2 1.5 0.6 0.45 0.75 p /kPa300 tc /s 540 520 Hclg 500 2.0 4.0 1074H /A m71 0 Figure 18. Dependence of the time of the Y1 specimen heating to the temperature of superconductivity loss on the external magnetic field strength.268 ences of time (tc) required for heating the specimen to the super- conducting transition temperature in a nitrogen or oxygen atmos- phere on the pressure of these gases in the measuring cell.When the specimen Y1 was heated in oxygen, the time tc was obviously greater than that in nitrogen. On the specimen Y2 and on the specimen Y1 coated with a paraffin film, the time tc did not depend on the atmosphere in which heating was carried out. In addition, the time of heating of the specimen Y1 in oxygen depended on the strength of external magnetic field imposed on the specimen after cooling (Fig. 18). The time tc remained unchanged if the strength of the external magnetic field did not exceedHc1g for the particular specimen. Further increase in field strength resulted in a decrease in tc.Based on these experiments, it can be concluded that heat transfer by oxygen molecules to a superconducting specimen with high critical parameters occurs with lower efficiency. The essential difference of the oxygen molecule from the nitrogen molecule is its magnetic moment, therefore it can be believed that its interaction with the superconductor is responsible for the observed phenomenon. As shown above,263 the magnetic moment of oxygen can favour the decrease in its adsorption on the superconductor surface. Apparently, in this case the oxygen molecules transfer heat less efficiently because they are elastically reflected from the superconductor surface. Taking into account that the character of collisions of oxygen molecules with a super- conductor surface depends on its critical parameters, it is possible to suggest a new model of the interaction of the magnetic moment of oxygen with superconductors. In contrast to the previously suggested model of interaction between the magnetic moment and the Meissner superconductor phase,259 it is assumed 267, 269 that the repulsive force of the magnetic moment from the super- conductor surface is described by the power function F / d¡¦n , where the index n depends on the distance d between the magnet creating the magnetic field and the superconductor.At long distances and in the cases where the magnetic field strength of the magnet does not exceed the first critical field strength of the superconductor, n=4; this corresponds to the ordinary dipole ¡À dipole interaction.At short distances, i.e., in sufficiently strong magnetic fields, the character of interaction of the magnet with the superconductor changes substantially. The magnetic field of the magnet penetrates into the superconductor bulk; as a result, the superconductor passes into a mixed state. In this case, interaction is described by the vortical model, according to which the magnetic flux from the magnet is `uptaken' by the bulk of the granules, creating a grid of vortical lines reaching the superconductor surface and capable of interacting with the magnet. The interaction energy is expressed by the formula 270 (5) E �� mHclgd , pd L L Makarshin, D V Andreev, V N Parmon where m is the magnetic moment and d is the superconductor thickness.It should be noted that in this case the energy of interaction depends on distance much more weakly than in the Meissner model (d71 instead of d73). According to the estimate made, the energy of interaction between the magnetic moment of oxygen and the surface of the specimen Y1 is rather high and is comparable with the thermal energy kT at a distance d&5 nm. This distance is much larger than the oxygen molecule size and the thickness of the possible non-superconducting phases usually present on HTSC surfaces. The energy of interaction with the surface of the specimen Y2 becomes equal to the thermal energy kT only at d&0.05 nm, which is less than the oxygen molecule size, hence repulsion does not occur in this case. This agrees with the experimental results mentioned above.Thus, superconductivity affects the adsorption and collision of paramagnetic molecules with the superconductor sur- face.263, 264, 268 It is important that these processes are also controlled by the external magnetic field.271 Heterogeneous chemical reactions are very `sensitive' to the state of the surface. The recoil force, which appears during desorption of hydrogen molecules which acquire nonequilibrium translational energy upon their recombination at the surface was measured after keeping a YBa2Cu3O77x superconductor in a stream of hydrogen atoms.272 The measurements were carried out by the weighing method in the temperature range of 78 ¡À 350 K.The recoil force did not depend on temperature as long as the superconductor was in the ordinary state. After its transition to the superconducting state, a decrease in the recoil force was recorded. This was explained by a change in the magnetic state of the surface. The temperature dependence of the recoil force correlates well with changes in the relative intensity of the ferromagnetic resonance absorption of the superconductor. This correlation suggests that the recombination of hydrogen atoms occurs at the antiferromagnetically ordered Cu+ surface ions. Cooling of the substrate changes the magnetic ordering and hence affects the concentration of the Cu+ ions at the surface.The decrease in the recoil force below the superconducting transition temperature of the substrate is apparently due to magnetic shielding of the Cu+ ions due to of the total diamagnetism of the superconductor. The low-temperature catalytic reaction of the ortho ¡À para conversion of molecular hydrogen on the superconducting cata- lyst YBa2Cu3O77x was studied.273, 274 The catalytic activity of the superconductor at various temperatures was estimated by chro- matographic measurement of the amplitude of para-hydrogen signal on exit from the reactor. The temperature dependence of the effective rate constant of catalytic ortho ¡À para conversion is 7ln k (mol m72 s71) w 0/w 00 140K 90K 13 70.8 1 15 70.4 Tc 2 0 17 7 9 11 103/T /K71 Figure 19.Temperature dependences of the effective rate constant of the ortho ¡À para hydrogen conversion on the superconducting catalyst YBa2Cu3O77x (1) and the real part of the relative magnetic susceptibility of this catalyst (2).273The chemical and adsorption effects of foreign molecules on the properties of high-temperature superconductors presented in Fig. 19. One can see that the conversion rate decreases abruptly starting from 90 K. This temperature is close to the superconducting transition temperature of the catalyst determined from magnetic susceptibility (see Fig. 19, curve 2). Analysis of the possible magnetic mechanisms of hydrogen ortho ± para conversion on oxide catalysts 275 ± 278 made it possible to conclude that the magnetic state of the catalyst's surface plays an important role in these processes.Before a catalyst is trans- ferred to the superconducting state, ortho ± para conversion of hydrogen occurs on the paramagnetic centres; the Cu2+ cations play this role in HTSC materials. Superconductivity changes essentially the magnetic state of the catalyst and it acquires a new property, i.e., becomes a pure diamagnetic. The currents flowing along the surface shield the magnetic field of the para- magnetic centres and thus `switch off' the basic mechanism of the low-temperature catalytic ortho ± para conversion. As a result, the effective reaction rate constant decreases by several orders of magnitude. VI. Conclusion Analysis of the experimental and theoretical data presented above shows that the specific features of interaction of external mole- cules with HTSC materials are determined by a wide range of various physicochemical processes.The thermodynamic meta- stability of almost all of the known high-temperature super- conductors 60, 279, 280 probably has certain impact on the processes involved in the synthesis of HTSC materials and on the effects of external molecules on these processes. The study of superconductivity in HTSC materials requires rather complex problems to be solved. Of these, the reproducible synthesis and chemical stability of the final material present important prob- lems. Apparently, the wide scatter of the experimental results for the thermodynamic characteristics, critical parameters of super- conductivity, etc. reported by different authors is due to this problem.Nevertheless, a number of important conclusions can be made. 1. Depending on the temperature of treatment of oxide HTSC materials by foreign molecules of simple compounds, the phe- nomena resulting from their effect on the superconductivity of these materials can be separated in two groups. The phenomena observed on treatment of HTSC materials at temperatures up to 473 K correspond to the first group. As a rule, such treatment little affects superconductivity. It is important that this problem is far from clarified even now, despite numerous publications on this matter. The experimental results obtained by different groups of authors and the conclusions made on this basis on the nature of the effect are usually contradictory and incon- sistent.Nevertheless, studies in this temperature range are very important for understanding the physicochemical processes that occur on superconductor surfaces. The second group includes phenomena caused by treatment of HTSC materials at temperatures above 473 K. Such treatment often affects strongly the parameters of superconductors. As a rule, the experimental results obtained in different studies are reproduced satisfactorily. The effect of simple molecules on the properties of superconductors in this temperature range is caused by the intercalation of molecules in its crystal lattice and/or by their chemical reactions with the ions of an HTSC material.As a rule, the action of simple molecules at temperatures above 473 K results in superconductivity degradation. Intercalation of oxygen, which plays a key role in the formation of the superconducting phase, deserves special atten- tion. Numerous studies showed that halogen molecules or atoms incorporated in the lattices of HTSC ceramics can play the same role as the oxygen atoms. For example, the non-superconducting tetragonal phase of YBa2Cu3O77x passes in the superconducting orthorhombic phase upon intercalation of halogens. 2. The range of effects of organic compounds on the character- istics of superconductivity is much more diverse. The first studies 301 along this line carried out on low-temperature superconductors showed that the donor ± acceptor interaction of organic molecules with the superconductor material results in a considerable change in the critical parameters of supeductivity.The changes in Tc and Ic correlate with the electronegativity of the adsorbed mole- cules. Similar phenomena were observed later in the studies of HTSC materials. The superconductivity parameters of oxide materials depend on the electronic properties of adsorbed molecules or a deposited polymeric film. Dielectric polymers little affect superconductivity, whereas conducting polymers decrease Tc. This effect is appa- rently due to the `proximity' effect. It has been proven that intercalation of planar organic molecules into the space between the crystal lattice layers in HTSC materials changes their characteristics considerably.In some cases the specimens display unusual physical properties. Adsorption of stable organic radicals affects the magnetic state of the superconductor surface and the electric resistance of the intergranular layers; as a result, Ic changes. 3. In turn, superconductivity can affect noticeably various physicochemical processes occurring on the superconductor sur- face. For example, transition of a material into the superconduct- ing state changes the luminescence characteristics of organic dyes adsorbed on the surface of an HTSC material (the quantum yield and the position of the maximum in the luminescence spectrum). Systems in which the energy of interaction between the super- conductor and paramagnetic molecules from the environment is comparable with the energy of the adsorption interaction were found among HTSC materials. Owing to this, superconducting membranes can separate gas mixtures of paramagnetic and diamagnetic molecules.At the liquid nitrogen temperature, the superconducting adsorbent adsorbs paramagnetic oxygen mole- cules several-fold more weakly than diamagnetic argon atoms. This is probably due to the repulsive interaction of the magnetic moment of the molecule with the magnetic flux `uptaken' by the specimen granules. 4. 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出版商:RSC    

年代:2000

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Russian Chemical Reviews 69 (4) 307 ± 323 (2000) The synthesis of porphyrins from dipyrrolylmethanes N Zh Mamardashvili, O A Golubchikov Contents I. Introduction II. Synthesis of pyrroles III. Synthesis of dipyrrolylmethanes IV. Synthesis of porphyrins V. Synthesis of bisporphyrins VI. Conclusion Abstract. dipyrrolyl- pyrroles, of synthesis the for methods The The methods for the synthesis of pyrroles, dipyrrolyl- methanes analysed, are them from derived porphyrins and methanes and porphyrins derived from them are analysed, sys- sys- tematised includes bibliography The generalised. and tematised and generalised. The bibliography includes 82 82 references. I. Introduction The methods for the synthesis of porphyrins are highly diverse and their choice is determined by the nature and mutual arrangement of substituents to be incorporated into the porphyrin molecule.Those suitable for the preparation of relatively simple porphyrins can be inapplicable for the synthesis of more complex compounds. Consideration of these methods is appropriate to begin with a brief survey of different types of porphyrins (compounds 1 ±4). The first type of porphyrins includes those which comprise identical substituents in the b-positions of the pyrrole rings and symmetrical meso-substituted porphyrins (compounds 1). These compounds are usually prepared by one-step condensation of four identical pyrrole molecules into porphyrin. The second type of porphyrins (porphyrins 2) have a more complex structure which retains certain features of symmetry.These compounds are synthesised via intermediate dipyrrolyl- methanes, -methenes and pyrroketones; since these molecules contain two different substituents, four isomeric products can be formed. The third group of porphyrins includes porphyrins 3 with three different substituents and symmetrical dipyrrolic fragments. In some cases, dipyrroles can be used as precursors of porphyr- ins 3. The fourth type of porphyrins (porphyrins 4) comprise more than three different substituents. Their synthesis is preceded by the formation of linear tetrapyrrole molecules which are closed to form the corresponding cyclic structures in the final stage. Thus, a- and b-oxobilanes, b-bilenes and a,c-biladienes are typically used as intermediate compounds.NZh Mamardashvili Institute of Solution Chemistry, Russian Academy of Sciences, ul. Akademicheskaya 1, 153045 Ivanovo, Russian Federation. Fax (7-093) 237 85 09. Tel. (7-093) 237 85 12 O A Golubchikov Ivanovo State University of Chemistry and Technology, prosp. Engelsa 7, 153460 Ivanovo, Russian Federation. Fax (7-093) 241 79 95. Tel. (7-093) 232 73 78 Received 11 August 1999 Uspekhi Khimii 69 (4) 337 ± 354 (2000); translated by R L Birnova #2000 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2000v069n04ABEH000550 R1 R1 R2 R1 R1 NH N R2 R2 HN N R1 R1 R1 R1 R2 1 R2 R2 R1 R1 NH N HN N R1 R1 R3 R3 3 To date, numerous novel procedures for the synthesis of porphyrins have been developed.However, all of them include the formation of methine bridges from methyl or bromomethyl groups in the a-position of pyrrole, dipyrrolylmethane or -meth- ene. The choice of a synthetic procedure is largely determined by the desired symmetry of the porphyrin molecule. The syntheses based on the use of symmetrical dipyrrolylmethanes the cyclisa- tion of which yields porphyrins with a high degree of symmetry (C2n) are of particular interest in this respect. The obvious advantages of this method are the simplicity of preparation of starting dipyrrolylmethanes and sufficiently high yields (50% ± 60%) of porphyrins. Bisdipyrrolylmethanes were used in the synthesis of sterically hindered, dimeric and polymeric por- phyrins.II. Synthesis of pyrroles Pyrroles are the key compounds in the synthetic chemistry of porphyrins. These compounds are readily available and yield porphyrins upon condensation. This circumstance determined the wide application of pyrroles in the synthesis of meso-alkyl- and meso-arylporphyrins including a whole array of sterically hindered porphyrins. 307 307 310 314 318 322 R2 R2 R1 R1 NH N HN N R1 R1 R2 R2 2 R2 R1 R3 R1 NH N HN N R1 R5 R4 R4 4308 Unsubstituted pyrrole is usually prepared by pyrolysis of ammonium mucate.1 Substituted porphyrins can be synthesised in two ways, viz., by chemical modification of the macrocycle and by condensation of pyrroles with a predetermined set of substitu- ents.The latter approach is more advantageous, since it provides greater scope for synthesis. Avast variety of pyrroles have been described in the literature. The derivatives 5 containing electron-withdrawing groups are synthesised, as a rule, by the Knorr method,2 viz., by condensation of aminoketones 6 with b-dicarbonyl compounds 7. R2 COR4 O COR4 R2 + R3 R1 O R3 R1 NH2 7 5 6 HN The aminoketones 6 are usually prepared by the reduction of the oximes which, in turn, are formed upon nitrosation of ketones or b-dicarbonyl compounds with sodium nitrite in glacial acetic acid.2 Zinc in acetic acid is typically used as the reducing agent. Although the yields in this synthesis do not exceed 40%± 60%, this procedure has found wide application owing to simplicity and availability of the starting compounds.Kleinspehn 3 performed the synthesis of a-ethoxycarbonyl- (10a) or a-cyanopyrroles 10b by condensation of oximes 8 with b-diketones 9. O Me R2 Me EtO2C Zn, AcOH R2 NOH+ Me R1 Me R1 8 O 9 NH 10a,b R1=CO2Et (a), CN (b). Later, it was shown 4 that a-hydroxyimino b-keto esters, e.g., compound 11, can also enter into the reaction of reductive condensation with alkyl-b-diketones. This method was used in the synthesis of pyrrole 10a. R2 Me NOH O 9, Zn, AcOH Me Me EtO2C CO2Et 11 NH 10a Pyrroles containing a halogenomethyl or an acetoxymethyl group in one of the a-positions are most commonly used in the synthesis of dipyrrolylmethanes. Different approaches to the synthesis of these compounds were compared by Semeikin et al.5 a-Chloromethylpyrroles 12a are prepared by treatment of pyr- roles 10a with sulfuryl chloride or tert-butyl hypochlorite on cooling.5 The yields of reaction products strongly depend on the temperature of the reaction mixture.Diethyl ether, dichloro- methane or glacial acetic acid are used as the solvents. Bromina- tion of a-methylpyrroles 10a with molecular bromine in diethyl ether or acetic acid results in strong resinification which signifi- cantly reduces the yields of pyrroles 12b. R1 R1 Me Me a or b or c Me EtO2C EtO2C CH2R2 HN 10a HN 12a ± c (a) SO2Cl2 [12a, R2=Cl (85%)]; (b) Br2 [12b, R2=Br (73%)]; (c) Pb(OAc)4 [12c, R2=OAc (95%)]. Acetoxymethylpyrroles 12c can be prepared by the reaction of the halogenomethyl derivatives 12a,b with sodium acetate in acetic acid; however, direct oxidation of a-methylpyrroles 10a with lead tetraacetate is more efficient.5 Acetoxymethyl deriva- tives are less readily hydrolysed and oxidised in air than com- pounds 12a,b, which makes them especially attractive for further syntheses.N Zh Mamardashvili, O A Golubchikov The methods for the synthesis of isomeric butylpyrrole deriv- atives in high yields have been proposed.6±10 The reaction of acetylacetone with butyl bromide in DMF or acetone in the presence of anhydrous potassium carbonate yields 3-butylpen- tane-2,4-dione; its condensation with ethyl hydroxyiminoacetoa- cetate 11 in the presence of acetic acid and zinc dust gives 4-butyl- 2-ethoxycarbonyl-3,5-dimethylpyrrole (13).Me Me O O 11, Zn, AcOH BuBr, K2CO3 Bu O O Me Me Me Bu Me Bu I2, NaHCO3 SO2Cl2 Me CO2Et HO2C CO2Et HN HN 13 Me Bu Me Bu SnCl2, H+ I CO2Et CO2Et HN HN 14 As a rule, a-monosubstituted pyrroles are the target products in such syntheses. The oxidation of the a-methyl group of pyrrole 13 with sulfuryl chloride in diethyl ether results in 4-butyl-5- carboxy-2-ethoxycarbonyl-3-methylpyrrole. Its decarboxylation is carried out in two steps, viz., by substitution of iodine for the carboxy group by treatment with iodine and sodium hydrogen carbonate in aqueous ethanol and subsequent reduction of the a-iodopyrrole formed with tin dichloride.These reactions result in 4-butyl-2-ethoxycarbonyl-3-methylpyrrole (14) in sufficiently high yield.6±10 Areaction sequence was proposed 6±10 which leads to 3-butyl- 2-ethoxycarbonyl-4-methylpyrrole (15), a key compound in the synthesis of many di- and tetrapyrroles,{ from the readily avail- able a-ethoxycarbonylpyrrole 13. To this end, compound 13 was transesterified with benzyl alcohol; the resulting ester was oxidised with sulfuryl chloride at the a-methyl group and esterified with ethanol. The diester formed was subjected to hydrogenolysis on a palladium catalyst. The resulting 3-butyl-5-carboxy-2-ethoxycar- bonyl-4-methylpyrrole was decarboxylated via the iodo derivative using a standard procedure: CO2Bn CO2Bn Me Me 1) SO2Cl2 2) EtOH Pd/C, H2 BnOH, Na NH 13 NH Bu Bu CO2Et Me I CO2H Me Me I2, NaHCO3 SnCl2, H+ NH NH Bu Bu CO2Et CO2Et Me NH Bu 15 CO2Et The pyrroles 14 and 15 can be used for further transformations both directly and in the form of formyl derivatives which can be { It is of note that only a-unsubstituted pyrroles containing a small methyl group in the vicinal b-position have previously been used in the synthesis of dipyrrolylmethanes.11 ± 20 However, later it was found that the steric factor has no effect on the condensation reaction.7, 21 ± 23The synthesis of porphyrins from dipyrrolylmethanes easily obtained by the reaction of these pyrroles with dimethyl- formamide in the presence of phosphorus oxychloride or benzoyl chloride.In many reactions, e.g., in chlorination with thionyl chloride, the aldehyde group is protected.7, 24 To this end, for- mylpyrroles are treated with dicyanomethane; deprotection is carried out by alkaline treatment. This procedure was used in the synthesis of 4-butyl-5-chloromethyl-3-methylpyrrole 2-carbalde- hyde from 3-butyl-2,4-dimethylpyrrole. Me Bu Me Bu DMF, POCl3 CH2(CN)2 Me CHO Me NH NH Me Me Bu Bu SOCl2 KOH ClCH2 Me NH NH CN CN NC NC Me Bu CHO ClCH2 NH Dipyrrole structures in which pyrrole rings are linked through b-positions can be prepared from more readily available b-unsub- stituted pyrroles.25 ± 27 These structures and the possibilities of their chemical modification present special interest, since they permit the design of novel types of porphyrin molecules.The acylation of 3,5-dimethyl-2-ethoxycarbonylpyrrole with the dicarboxylic acid chloride 16 and subsequent reduction of the carbonyl groups of the diketone 17 formed with diborane result in compound 18. The bispyrrole 18 is further converted into the benzyl ester, which is further subjected to hydrogenolysis followed by decarboxylation and formylation, eventually resulting in the aldehyde 19.25 The formyl groups of the dialdehyde are protected with dicyanomethane as described above, which is followed by chlorination with sulfuryl chloride. This method was used in the synthesis of the dipyrrole derivatives 20.25 Cl Me O + (CH2)n72 Me EtO2C O 16 NH Cl Cl Me Me O SnCl4, CH2Cl2, 0 8C + (CH2)n73 Me EtO2C O EtO2C OEt NH NH + Me Me (CH2)n71PPh3I7 (CH2)n71I ...Me EtO2C EtO2C 22 NH NH 23 OMe Me Me (CH2)n72 (CH2)n72 EtO2C NHMe HN 24 n=5, 6. O Me (CH2)n72 EtO2C Me HN Me (CH2)n EtO2C Me Me NH 18 Me (CH2)n OHC Me NH Me 19 Me (CH2)n NH NC Cl CN 20 n=9 ± 11. Later, Morgan and Dolphin 27 synthesised b-linked dipyrrolic derivatives containing an aryl fragment in their bridges. The acylation of 3,5-dimethyl-2-ethoxycarbonylpyrrole and subse- quent reduction of the carbonyl group result in the hydroxy derivative 21, which is converted into the iodo derivative 22 on substitution of iodine for the hydroxy group via the corresponding mesylate. The methyl group in compound 22 is eliminated from position 5 using the standard procedure, i.e., via 5-iodopyrrole.Subsequent reaction with triphenylphosphine in boiling toluene affords the phosphonium salt 23. Its condensation with 2,6- diformyl-4-methylanisole in dioxane in the presence of potassium carbonate results in the diene 24. The final step in this synthesis is the catalytic hydrogenation of compound 24 leading to the target dipyrrole derivative 25 (Scheme 1). Syntheses based on b-unsubstituted pyrroles are multistage processes, which is the common drawback, chemical conversions involve alternately a- and b-positions of the ring, which signifi- cantly complicates the synthesis. In our opinion, the use of relatively available a-unsubstituted pyrroles is more advanta- geous.O (CH2)n73CO2Et BF3 . Et2O, NaBH4, THF Me OMeCHO OHC Me Me (CH2)n H2, Pd/C CO2Et EtO2C NH 309 O Me CO2Et Me NH 17 Me ... CO2Et NH Me CHO NH Me NH CN Cl NC Scheme 1 Me (CH2)n71OH Me EtO2C NH 21 OMe Me (CH2)n CO2Et HN Me 25310 III. Synthesis of dipyrrolylmethanes Porphyrins can also be synthesised from dipyrrolylmethanes and -methenes. However, both of them possess exceedingly high reactivity. This disadvantage can be overcome through the intro- duction of electron-withdrawing substituents. Synthesis of dipyr- rolylmethenes containing such substituents is problematic. On the other hand, there exist simple and efficient procedures for the synthesis of stabilised dipyrrolylmethanes; their advantages in the synthesis of structurally complex porphyrins leave no doubt.28, 29 It has been established that the condensation of pyrroles 12a ± c occurs via pyrrolylmethyl carbocations stabilised due to the delocalisation of the positive charge.2 Such cations are relatively stable both in inert solvents and in alcohols, carboxylic acids and even in water.+ + CH2 CH2 NH NH + + CH2 CH2 NH NH On boiling in ethanol in the presence of hydrobromic acid, pyrroles 12a ± c are converted into dipyrrolylmethanes in high yields.2, 5, 7, 30, 31 If such pyrroles contain alkyl substituents in the b-positions, dipyrrole compounds are obtained in up to 90% yields.Me Me R1 R1 H+ +CH2 EtO2C EtO2C CH2R2 12a ± c NH NH R1 R1 HO R1 Me Me Me + HN NH CH2OH EtO2C EtO2C CO2Et NHR1 R1 Me Me HN NH EtO2C CO2Et An efficient procedure for the synthesis of dipyrrolylmethanes containing butyl groups in positions 3 and 30 has been pro- posed.7 ± 10, 24, 29, 32 Bromination of the a-methyl group of the pyrrole 13 with subsequent condensation of 5-bromomethyl-4- butyl-2-ethoxycarbonyl-3-methylpyrrole in ethanol in the pres- ence of an acid results in the 5,50-diethoxycarbonyl derivative 26; its alkaline hydrolysis affords bis(3-butyl-5-carboxy-4-methylpyr- rol-2-yl)methane. As in the case of synthesis of bis(5-carboxy-3,4- dimethylpyrrol-2-yl)methane, lead tetraacetate proved to be the best oxidant for the starting pyrrole.5 Me Me Bu Bu Br2 Me CO2Et BrCH2 CO2Et NH 13 NH Bu Bu Me Me HN NH CO2Et EtO2C 26 Symmetrical dipyrrolylmethanes can also be prepared by the condensation of a-monosubstituted pyrroles with aldehydes.It is N Zh Mamardashvili, O A Golubchikov self-evident that the b-positions of the pyrrole ring must be substituted with at least one electron-withdrawing substituent, which thus stabilises the product.33 This reaction occurs under the same conditions as the self-condensation of the pyrroles 12a ± c. R2 R2 R3 R3 HCHO, H+ H+ R1 R1 CH2OH NH NH R2 R3 R3 R3 R2 R3 R1 R2 R2 NH + HN R1 NH CH2 R1 R1 NH The reaction of 2-benzyloxycarbonyl-3,4-dimethylpyrrole with benzaldehyde in ethanol in the presence of hydrochloric acid results in meso-phenyldipyrrolylmethane 27a.Under condi- tions of acid catalysis, 3-butyl-2-ethoxycarbonyl-4-methylpyrrole (15) reacts with benzaldehyde to give dipyrrolylmethane 27b.8 It was shown 7, 23 that the reaction of 4-butyl-2-ethoxycarbonyl-3- methylpyrrole (14) with benzaldehyde and its derivatives (XC6H4CHO, X=OMe, NO2, NMe2) in the ethanol ± hydro- chloric acid system results in meso-aryl-substituted dipyrrolyl- methanes 27c ± i in good yields. Similar to compounds 27a,b, these products (except for compound 27i which undergoes resinifica- tion) are converted into the corresponding dicarboxy derivatives upon alkaline hydrolysis. C6H4X R2 R2 CHO R1 R2 R1 R1 H+ + R3 HN NH NH R3 R3 X 27 X R3 Compound 27 R2 R1 HHH4-OMe 3-OMe 2-OMe 4-NO2 3-NO2 3-NMe2 Me Me Bu Bu Bu Bu Bu Bu Bu Me Bu Me Me Me Me Me Me Me CO2Bz CO2Et CO2Et CO2Et CO2Et CO2Et CO2Et CO2Et CO2Et abcdefghi The structures of a series of meso-substituted dipyrrolylme- thanes were calculated by the molecular mechanics methods (the force field MM+).34 ± 36 The strain energies of isomeric com- pounds of the type 27 are listed in Table 1.Table 1. The strain energies of isomeric dipyrrolylmethanes 27 (EMM /kJ mol71) and their constituents, viz., bond strain energies (Ebnd), bond angle energies (Eang), torsion angle energies (Edih) and Van der Waals energies (Evdw).Ya R3 R2 R1 Evdw Edih Eang Ebnd EMM H 76.0 H Me Bu Me Bu Bu 3.66 2.28 1.49 3.20 2.83 2.56 0.73 1.49 1.78 97 139 111 106 104 105 86 83 87 HHHPh Ph Ph Ph Ph Ph 86.1 712.0 77.0 16.8 2.3 20.2 10.2 10.1 8.0 4.3 4.1 133.5 712.8 93.4 100.3 731.7 92.3 728.1 87.5 724.2 67.8 717.7 75.6 721.4 72.4 716.2 CO2Et CO2Et Me CO2H CO2H CO2H HCO2Et CO2H Me CO2Et Me Bu Me Et Bu Me Bu Me Et aY is meso-substituent.The synthesis of porphyrins from dipyrrolylmethanes The above-described procedure is widely employed in the synthesis of bisdipyrrolylmethanes.13 ± 16, 18, 22 The reaction of pyrroles 28a ± c with dialdehydes in acid media gives high yields of compounds 29 the dipyrrolic fragments of which are linked through the meso-positions.RO2CHN CHO Me H+ + X NH Me CHO HN CO2R 28a ± c RO2C R = H (a), Et (b), Bn (c); X=(CH2)n (n=4, 6, 12), , , , , , bispyrroles 32 containing chloromethyl groups with 2-benzyloxy- , , , , , . 3-Methyl-2-ethoxycarbonyl-4-ethylpyrroles are easily con- densed in acid media with dialdehydes 30 the phenyl fragments CH C(CN)CO2Me NH Me Cl + (CH2)n Cl Me NH CH C(CN)CO2Me 32 n=9 ± 11. MeO HN EtO2C (CH2)n Me 34 Me NC HN CN HN EtO2C Me n=5, 6. of which are linked by ether bridges [O(CH2)nO] of different lengths (n=2 ± 4).23 The resulting esters 31 are easily hydrolysed to the corresponding acids in alkaline media.Et O(CH2)nO NH + Me Me CO2R Me CHO OHC 30 CO2Et Me Me NH Me X EtO2C Et NH HN Me Me O(CH2)nO CO2R Me Me 29 HN Et EtO2C Me 31 , , , Y n=2±4. Compounds with fundamentally different structures can be prepared on the basis of b-linked pyrroles. The condensation of carbonyl-4-ethyl-3-methylpyrrole in acetic acid results in bisdi- pyrrolylmethanes 33 the dipyrrole fragments of which are linked by methylene bridges through the 3,30-positions (n=9 ± 11) (Scheme 2).25 An alternative approach involves the condensation of 5,50-unsubstituted bispyrrole 34 with the a-chloromethyl deriva- tive 35 (Scheme 3).27 The reaction of compounds of the type 12a ± c with a-unsub- stituted pyrroles gives nonsymmetrical dipyrrolylmethanes.It was proposed 37 to start from bromomethylpyrrole in a sodium acetate ± glacial acetic acid buffer. It is the intermediate acetoxy- methylpyrrole formed that enters into the reaction. Unfortu- Me Me Et Et BnO2C Me Et NH HN CO2Bn NH HN NH (CH2)n MeO2C(CN)C HC Me Me 33 CN Me CN NH AcOH, 80 8C + NH CO2Et (CH2)n Et Me CH2Cl 35 OMe Me Et Et CN NH NC MeO NH CO2Et (CH2)n (CH2)n Me OMe 311 H+Me CO2Et Et NH NH Et CO2Et Me Scheme 2 CO2Bn CH C(CN)CO2Me Scheme 3312 nately, this reaction yields a large amount of the symmetrical dipyrrolylmethane as the side product. R3 R4 R6 CH2Br R5 R3 R2 R5 NH NH + HN NH R4 R2 R6 R1 R1 The reaction of pyrroles 36 and 37 in the presence of p-toluenesulfonic acid in AcOH gives dipyrrolylmethane 38 in 85% yield.38 Cl(CH2)2 D3C CD3 (CH2)2Cl + BnO2C ButO2C CHDOAc 37 NH HN 36 CD3 (CH2)2Cl (CH2)2Cl D3C HN NH CO2But BnO2C 38 Other methods for the synthesis of dipyrrolylmethanes are less popular.If chemical modification of dipyrrolylmethanes is envis- aged, allowance must be made for the fact that their stabilities and reactivities with respect to electrophilic reagents are similar to those of pyrroles. Dipyrrolylmethanes are stable if they contain at least one electron-accepting group in each of their pyrrole nuclei. Also the methylene unit in dipyrrolylmethanes is readily oxidised to form dipyrrolylmethenes or ketone.Dipyrrolylmethane 5,50-dicarboxylic acids are prepared by heating the corresponding 5,50-diethyl esters with aqueous alkalis with subsequent precipitation of the reaction product with a mineral acid or by hydrogenation of 5,50-dibenzyl esters on a palladium catalyst in THF at atmospheric pressure and at room temperature.7, 32 Heating of dry bis(5-carboxypyrrol-2-yl)methanes is accom- panied by profound destruction; therefore, McDonald et al.39 proposed to conduct the decarboxylation in sealed tubes in aqueous solutions of alkalis at 170 ± 180 8C, which causes only partial destruction. In an alternative procedure,40 the decarbox- ylation is carried out by boiling the dicarboxylic acids prepared from diethyl esters in DMF in an inert atmosphere.N,N-Diethylformamide was used as a solvent for increasing the boiling temperature of the reaction mixture.41 Unfortunately, the decarboxylation of dipyrrolylmethane 39 is accompanied by intramolecular rearrangements. Subsequent formylation gives a mixture of dialdehydes 40 and 41 in a total yield of 60%. MeO2C(CH2)2 CH2CO2Me 1) DEF, D 2) PhCOCl, DMF 3) NaHCO3, H2O Me (CH2)2CO2Me HN NH CO2H HO2C 39 MeO2C(CH2)2 CH2CO2Me Me (CH2)2CO2Me + HN NH OHC CHO 40Me CH2CO2Me MeO2C(CH2)2 (CH2)2CO2Me + HN NH CHO OHC 41 DEF is N,N-diethylformamide. N Zh Mamardashvili, O A Golubchikov If dipyrrolylmethanes have different substituents in the b-positions of the pyrrole ring, a mixture of isomeric products can be formed.To avoid this, the decarboxylation is carried out in two steps. First, the carboxy groups are substituted by iodine and then the iodo derivatives are reduced, e.g., with tin(II) chlor- ide.2, 7, 29 R2 R3 I2, NaHCO3 R1 R4 HN NH CO2H HO2C R2 R3 SnCl2, HCl R1 R4 EtOH HN NH I I R2 R3 R1 R4 HN NH Dipyrrolylmethane dicarboxylic acids can be decarboxylated by trifluoroacetic acid (TFA). It was found 42 that 5,50-unsubsti- tuted dipyrrolylmethanes thus formed are stable in TFA at room temperature for a week. Trifluoroacetic acid was used, for example, for decarboxylation of 5-benzyloxycarbonylpyrrol-2- yl-50-tert-butoxycarbonylpyrrol-20-ylmethane.42 However, TFA can induce intramolecular rearrangements in dipyrrolylmethanes.CF3CO2H NH NH HN HN CO2But BnO2C BnO2C Heating of carboxy derivatives in aqueous alkalis in the presence of hydrazine as a stabiliser seems to be the method of choice for decarboxylation of b-alkylated dipyrrolylmethanes.8, 29 In this case, the yield of the target product reaches 85%. Bis(5-formylpyrrol-2-yl)methane was obtained from bis(5- benzyloxycarbonylpyrrol-2-yl)methane by the Wilsmeyer reac- tion. The use of phosphorus oxychloride or benzoyl chloride results in dialdehydes with practically equal yields (45% ± 50%).33 H2, THF D NH HN HN NH CO2H HO2C CO2Bn BnO2C R2NCHO PhCOCl NH HN NaHCO3 H2O, D NH HN NH HN + + NR2 R2N CHO OHC An alternative procedure for the synthesis of dipyrrolylme- thanedialdehydes was proposed by Mamardashvili et al.24 who used triethyl orthoformate in TFA as the formylating reagent.5,50-Dicarboxy derivatives of dipyrrolylmethanes obtained by hydrogenolysis of dibenzyl esters are first dissolved 5 ± 10 minutes after the completion of decarboxylation and triethyl orthoformate is added to the reaction mixture. This enters into the reaction with the 5,50-unsubstituted dipyrrolylmethane formed. Apparently, a part of dipyrrolylmethane undergoes rearrangement. The synthesis of dipyrrolylmethanes from pyrroles containing electron-withdrawing groups, e.g., cyano groups, is of particular interest. This method was developed by Dolphin et al.43 for theThe synthesis of porphyrins from dipyrrolylmethanes synthesis of bis(5-formyl-3,4-dimethylpyrrol-2-yl)methane.This reaction consists in the condensation of pyrroles 42 and 43 in dry dichloromethane in the presence of catalytic amounts of tin(IV) chloride resulting in dipyrrolylmethane 44; its hydrolysis by heating on a water bath with a solution of KOH in aqueous ethanol gives a poorly soluble target dipyrrolylmethane. The yield of the final product can be significantly increased by performing deprotection in an inert atmosphere. CH2Cl Me Me + NH NH Me Me CH C(CN)CO2Me 42 CH C(CN)CO2Me 43 Me Me Me Me NH HN HC MeO2C(CN)C CH C(CN)CO2Me 44 Me Me Me Me NH HN OHC (65%) CHO The synthesis of 5-formyl-50-unsubstituted dipyrrolylme- thanes deserves special attention. The first compound of this series, viz., 3,30-diethyl-5-formyl-4,40-dimethyldipyrrol-2-ylme- thane, was obtained by Dolphin et al.44 using decarboxylation of Et Et Me Me Me D HN NH EtO2C EtO2C CO2H Et Et Me Me HN NH CHO HO2C R3 R2 CF3CO2H R4 R1 NH HN CO2But BnO2C R3 R2 R4 R1 NH HN CHO BnO2C R3 R2 H2, Pd/C R4 R1 NH HN CO2Bn EtO2C 5-carboxy-50-ethoxycarbonyl-3,30-diethyl-4,40-dimethyldipyrrol- 2-ylmethane with subsequent Wilsmeyer formylation of the resulting product (Scheme 4).In the synthesis of the monoaldehyde 45, the formyl group of the original pyrrole was protected by reaction with malononi- trile.45 CHO CH C(CN)2 Me Me NH NH R R SnCl4, CH2Cl2 Me Me Ac Me CH C(CN)2 Me Ac NH NH KOH, H2O, D R CH2Cl R Me Ac Me NH HN CHO 45 R=(CH2)2CO2Me.The monoformyldipyrrolylmethanes of the type 46 commonly employed in the synthesis of linear tetrapyrrolic compounds 42 can be prepared from esters of the corresponding dicarboxylic acids using various approaches (Schemes 5 and 6). Et Et Et Et POCl3 Me Me DMF HN NH NH HN EtO2C Et Et NaOH, D Me Me HN NH CHO R3 R2 R2 1) DMF, PhCOCl 2) CH2(CN)2 R4 R1 R1 NH HN BnO2C BnO2C R2 R3 R2 H2, Pd/C D R1 R4 R1 NH HN CHO HO2C R3 R2 46 R4 R1 NH HN 1) D 2) DMF, POCl3 3) OH7 4) NaOH, D CO2H EtO2C 313 R Me Me NH HN CH C(CN)2 Scheme 4 OH7 Me +NMe2 Scheme 5 R3 R4 NH HN CH C(CN)2 R3 R4 NH HN CHO 46 Scheme 6314 R3 R2 One of these approaches consists in thermal decarboxylation of intermediate 5-formyl-50-carboxydipyrrolylmethanes; how- ever, if the carboxypyrrolyl fragment contains electron-withdraw- ing groups, the reaction is carried out via iodo derivatives.The presence of even one electron-withdrawing aldehyde group increases the resistance of the methylene group of dipyrrolyl- methane to oxidants and prevents intramolecular rearrangements, fragmentation and resinification. The monoformyldipyrrolylme- thane 46 formed can be stored for a long period of time in a dry state in the absence of oxygen.42 R3 Unsymmetrical dipyrrolylmethanes 47 are formed in high yields in the reaction of a-ethoxycarbonyl-substituted pyrroles with compounds 48 containing chloromethyl and cyanovinyl (or cyanoacryl) groups in the a-positions.The latter deactivate pyrrole 48 and prevent its self-condensation. The condensation is carried out in inert solvents, e.g., dichloromethane, dichloro- ethane, benzene, etc. The dipyrrolylmethanes of the type 49 are often decarboxylated upon heating with alkalis.46 It should be noted that the cyanoacryl group which is hydrolysed more readily than the cyanovinyl group provides more efficient protection. R2 R1 R3 R2 R4 CO2Me SnCl4, D + CN EtO2C ClCH2 NH NH 48 R3 R3 R2 OH7 R4 R1 NH HN CO2Me R2 EtO2C CN 47 R3 R2 OH7, D R4 R1 NH HN CO2Me HO2C CN 49 The condensation of bis(3-butyl-4-methylpyrrol-2-yl)me- thane with bis(3-butyl-5-formyl-4-methylpyrrol-2-yl)methane in ethanol in the presence of hydrobromic acid and subsequent oxidation of the reaction mixture with o-tetrachloroquinone results in 3,7,13,17-tetrabutyl-2,8,12,18-tetramethylporphyrin in 24% yield.9 R3 R2 R4 R1 NH HN A method for the synthesis of porphyrins from 5-formyl-50- unsubstituted dipyrrolylmethanes has been proposed.45 Self-con- densation of compound 45 in glacial acetic acid in the presence of small quantities of acetic anhydride and hydrobromic acid gives 2,6-diacetyldeuteroporphyrin-II 54.CHO 46 The common disadvantage of the syntheses based on mono- formyl derivatives of dipyrrolylmethanes is the necessity to protect the aldehyde groups prior to chemical transformations.46 Removal of protecting groups may provoke side reactions.Me IV. Synthesis of porphyrins R=(CH2)2CO2Me. Dipyrrolylmethanes have been widely used in the synthesis of porphyrins since 1960. McDonald et al.39 developed a procedure for the condensation of bis(5-formylpyrrol-2-yl)methanes with 5,50-unsubstituted dipyrrolylmethanes under mild conditions. This reaction is carried out in glacial acetic acid in the presence of hydroiodic acid in a highly dilute solution (Scheme 7). This process involves the formation of b-bilene 50 and porphodimethene 51 which is partially converted into phlorine 52. Both the intermediate products and the phlorine 52 can be oxidised with atmospheric oxygen to yield porphyrin 53. In some cases, self-condensation of the original bis(5-formylpyrrol-2- yl)methanes may occur to a certain extent.The yields of the porphyrins 53 prepared by condensation of symmetrical dipyrro- lylmethanes by the method of McDonald are given in Table 2. N Zh Mamardashvili, O A Golubchikov R4 R5 CHO R6 HN NH + HN NH R7 CHO R8 R1 R4 R5 CHO R3 R6 NH HN H+ + NH HN R2 R7 R1 50 R8 R4 R5 R3 R6 NH HN O2 + NH HN R2 R7 R1 R8 52 Me Me CHO R Ac NH H+ NH Me R Ac 45 Me R Ac NH N N HN Me R Me 54 (80%) Scheme 7 H+ R4 R5 R6 NH HN + 7H+ NH HN+ R7 R1 R8 51 R4 R5 R6 N HN NH N R7 R1 53 R8 R Me NH HN 1) H+ 2) O2 CHO +NH HN Ac Me Me AcThe synthesis of porphyrins from dipyrrolylmethanes Table 2. The porphyrins 53 synthesised by the McDonald method.39, 47, 48 R4 R3 R2 R1 CO2Et CO2Et CH=CH2 CH=CH2 (CH2)2CO2Me CH2CO2Me CH2CO2H Me Me Me CH(OH)CH2CO2Me Me Me CH2CO2Me (CH2)2CO2Me CH2CO2Me (CH2)2CO2Me Me CH2CO2Me (CH2)2CO2Me Me CH(OH)CH2CO2Me Me Et (CH2)2CO2Me CH2CO2Me (CH2)2CO2Me CH2CO2Me Me (CH2)2CO2Me CH2CO2Me Me Me Et Me Note.The reaction was carried out in glacial acetic acid containing HI. a HCl was used instead of HI. A series of sterically hindered (strapped) porphyrins 55 were synthesised by condensation of bisformyldipyrrolylmethanes 56 in a dichloromethane ± methanol mixture (30 : 1) in the presence of p-toluenesulfonic acid.27 Me Me Me Me CHO Me Me NH N HN NH H+ X X N HN HN NH Me Me OHC Me Me 56 Me Me55 (40% ± 50%) X is the residue of a saturated or an aromatic hydrocarbon.Monoformyldipyrrolylmethanes have not found wide application in the synthesis of porphyrins due to their unavailability and the necessity to perform multistep syntheses. The condensation of a symmetrical 5,50-unsubstituted dipyr- rolylmethane with triethyl orthoformate in formic acid was carried out.9 In this reaction, first the formylation of dipyrrolyl- methane occurs followed by formation of the porphyrin. As this reaction occurs under drastic conditions, it can be accompanied by rearrangements of dipyrrolylmethane resulting in a mixture of isomeric porphyrins. The condensation reactions of 3,304,40-tetraalkyl-5,50-unsub- stituted dipyrrolylmethanes with aldehydes resulting in 5,15- dialkyl- and 5,15-diaryl derivatives of porphyrins are well docu- mented in the literature.7±10 Initially, the oxidative condensation was carried out in one step, as in the case of synthesis of meso- tetraphenylporphyrins, viz., in a boiling solvent in the presence of atmospheric oxygen.33 In a pyridine ± acetic acid mixture, the yield of porphyrins was rather low (*3%), but it was increased when the reaction was performed in benzene in the presence of TFA with high dilution of the reagents.It was shown 49 ± 52 that the use of a two-step procedure gives better results. The reaction of dipyrrolylmethane with benzaldehyde results in porphyrinogen 57, which is further oxidised by benzoquinone derivatives to porphyrin 58.The yields of porphyrins 58 synthesised by this method are given in Table 3. The yield of porphyrin 58 (R1=Me, R2=Bun, X=2-OMe) does not depend on the solvating properties of the solvent (Table 4), which suggests high complexity of the reaction mechanism.52 The strength of the organic acid used as a catalyst only slightly affects the yield of the target product but the nature of the oxidant is a critical factor (Table 4). If the condensation is performed in a boiling solvent, the rate of the reaction is naturally increased with no decrease in the yield.50 315 R8 R7 R6 R5 Yield (%) CH2CO2Me CO2Et CO2Et CO2Et CO2Et (CH2)2CO2Me 65 (CH2)2CO2H (CH2)2CO2H CH2 CO2H 5566 40 45 29 a (CH2)2CO2Me 36 a Me Me (CH2)2CO2Me (CH2)2CO2Me Me (CH2)2CO2Me (CH2)2CO2Me Me CH2CO2Me (CH2)2CO2Me CH2CO2Me X R1 R1 R1 R2 R2 CHO R2 NH HN NH [O] H+ + NH HN NH X R2 R2 R2 R1 R1 R1 X X 57 R1 R1 R2 R2 HN N N NH R2 R2 R1 R1 X 58 R1, R2=Me, Et, Bu.It is of note that rearrangements in dipyrrolylmethanes and/or intermediate tetrapyrroles in acid media result in the formation of monoarylporphyrins in addition to diarylporphyrins.33 In some cases, the monosubstituted porphyrin is the main or even the only reaction product, especially at elevated temperatures. The nature of the substituent in the original dipyrrolylmethane and the presence of a para-substituent in the benzaldehyde molecule have little effect on the yield of the target porphyrins (Table 5).The reaction of 5,50-unsubstituted dipyrrolylmethanes with an equimolar mixture of two aromatic aldehydes gives unsymmet- rical 5,15-diphenyloctaalkylporphyrins and a mixture of all the three feasible products, which have to be further separated by chromatography or, in some cases, by chemical modification. The closer are the electronic effects of the substituents in the aldehydes, the higher are the yields of unsymmetrical porphyrins.33, 53 The condensation of the dialdehydes 59 with 5,50-unsubsti- tuted dipyrrolylmethanes affords `capped' porphyrins 60 in suffi- ciently high yields.14 This method can be used for the synthesis of other complex systems containing octamethylporphyrin and etio- porphyrin fragments.13 ± 16, 18, 22 Unfortunately, porphyrins con- taining methyl or ethyl groups in the b-positions of the macrocycle316 Table 3.The conditions of synthesis and the yields of 5,15-diaryl(hetaryl)octaalkylporphyrins 58. X R R2 Yield (%) HHHHH2-Me 2-Me 3-Me 3-Me 4-Me 4-Me 4-Me 4-Me 4-Me 3-Et 2-OMe 2-OMe 3-OMe 4-OMe 4-OMe 4-OMe 4-OMe 2,6-(OMe)2 O C N(CH2)3O (see a) CO 4-OH 2-Cl 2,4-(Cl)2 2-NO2 4-NO2 4-NMe2 (see a) N (see a) N Note. DDQ is 2,3-dichloro-5,6-dicyano-1,4-benzoquinone; a meso-substituent. Table 4. The dependence of the yield of 5,15-diaryloctaalkylporphyrin 58 (R1=Me, R2=Bun, X=2-OMe) on the nature of the solvent, the catalyst and the oxidant.33, 50 Catalyst Solvent C6H14 CCl4 PhH CHCl3 MeCN CF3CO2H CF3CO2H " CF3CO2H " CF3CO2H " CF3CO2H " Et2O CF3 CO2H " MeOH Me2CO CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 CF3CO2H " CF3CO2H " CCl3CO2H " CH2ClCO2H " CH2ClCO2H CH2ClCO2H CH2ClCO2H1 HMe Et Et Bu HEt HEt HMe Et Et Et Et HEt Et HMe Et Et Et HMe Me Et Me HEt HEt HMe Me Me Et Me HEt Et HMe Me Et Me H H Et HHHMe Me Me HHHMe Me Et Et Et Et Oxidant o-tetrachloroquinone 44242740243437294444 p-tetrachloroquinone 32 p-tetrabromoquinone 28 42 DDQ Solvent Py MeOH MeOH PhH MeOH CH2Cl2 PhH CH2Cl2 PhH CH2Cl2 MeOH MeOH MeOH PhH CH2Cl2 CH2Cl2 PhH PhH CH2Cl2 MeOH MeOH PhH MeCN 3 58 56 30 11 81 40 86 30 89 54 56 58 30 77 86 30 40 83 50 56 406 87 CH2Cl2 62 80 79 73 69 46 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 MeOH MeOH MeOH 55 MeOH 44 are poorly soluble in organic solvents, which significantly restricts their investigation and practical application.60 ± 66 R1 Me Yield (%) R1=Me, Bu; R2=CH2 Some attempts were undertaken to synthesise porphyrins with bulky alkyl substituents in the b- and meso-positions starting from N Zh Mamardashvili, O A Golubchikov Oxidant Catalyst air p-tetrachloroquinone AcOH CCl3CO2H CCl3CO2H "air o-tetrachloroquinone p-tetrachloroquinone air p-tetrachloroquinone air p-tetrachloroquinone o-tetrachloroquinone air DDQ p-tetrachloroquinone air p-tetrachloroquinone CF3CO2H TsOH CF3CO2H CF3CO2H CF3CO2H CF3CO2H CF3CO2H CCl3CO2H " CCl3CO2H " TsOH CF3CO2H CF3CO2H CF3CO2H CF3CO2H CF3CO2H " CF3CO2H CCl3CO2H " CCl3CO2H "air p-tetrachloroquinone CF3CO2H CCl3CO2H CCl3CO2H " CF3CO2H " CF3CO2H " CF3CO2H " CF3CO2H " CCl3CO2H " CCl3CO2H "o-tetrachloroquinone TsOH TsOH " R1 O O R2 Me+OHC NH HN 59 R2 R1 R1 O O Me Me NH N HN N Me Me R1 R1 60 CH2.Ref. 54 555556 32 55 56 55 56 55333333 56 33 55 565655333356 57555855555555959 59 H+ CHOThe synthesis of porphyrins from dipyrrolylmethanes Table 5.The effects of substituents on the yields of 5,15-diaryloctaalkyl- porphyrins 58.33, 50 1 Yield (%) X R R2 Solvent�MeOH Me H Me H 66 H Me Me 64 H Me Et 64 HO2C 4-OMe Me 59 Solvent�CHCl3 61 H H Me H Me Me 68 H Et Me 33 5,50-unsubstituted dipyrrolylmethanes aimed at enhancement of their solubilities.7 ± 10, 24, 32, 67, 68 The yields of porphyrins in the reaction of bis(4-butyl-3-methylpyrrol-2-yl)methane with benzal- dehyde in o-xylene ± TFA, methanol ± p-toluenesulfonic acid, chloroform ± acetic acid systems and in acetic acid do not exceed 2.2%, 3%, 10% and 4.1%, respectively. In condensation of bis(dimethyl- and butylmethylpyrrol-2-yl)methanes with aliphatic aldehydes RCHO [R=Alk(C2±C7), C11H23] in TFA-containing chloroform followed by oxidation of the resulting porphyrinogens with p-tetrachloroquinone, the yield of porphyrins did not exceed 5%.9 The synthesis of porphyrins from 5,50-dicarboxy derivatives of dipyrrolylmethanes, the synthetic precursors of 5,50-unsubstituted compounds and 5,50-diformyldipyrrolylmethanes, appeared to be more efficient.5,50-Dicarboxylic acids are far more stable than 5,50-unsubstituted derivatives, and porphyrins are formed from them in higher yields and the amounts of poorly separable admixtures are much less than in the synthesis of porphyrins from the corresponding unsubstituted and formyl derivatives of dipyrrolylmethanes. Jackson et al.69 were the first to carry out the condensation of dipyrrolylmethane 61 with triethyl orthoformate in dichlorome- thane in the presence of trichloroacetic acid.This reaction gives coproporphyrin-II 62 in 25% yield. A series of meso-disubstituted porphyrins were synthesised 7, 8 by a modified Jackson ± Kenner method.69 In the condensation of 3,30-dibutyl derivatives of meso- aryldipyrrolylmethanes 63a with triethyl orthoformate in chloro- form in the presence of TFA, the yield of porphyrins 64a was the highest (*35%). The substitution of monochloroacetic acid for TFA decreases the yield to 17% and in the chloroform ± acetic acid system the yield is 10%. The yield of porphyrins 64b in the reaction of 3,30-dimethyl derivatives of meso-alkyldipyrrolylme- thanes 63b with triethyl orthoformate in chloroform in the presence of TFA is 20% ± 25%.8, 10 R1 R2 R1 Me Me R1 R1 R2 N HN Me Me NH HN NH N HO2C CO2H Me Me 61, 63a,b R2 R1 R1 62, 64a,b R1=(CH2)2CO2Me, R2= H (61, 62); R1=Bu: R2=Ph, C6H4OMe-2, C6H4OMe-3, C6H4OMe-4, C6H4NO2-3, C6H4NO2-4 (63a, 64a); R1=Me: R2=Alk(C1±C7), C11H23 (63b, 64b).317 This method is widely used in the synthesis of `capped' porphyrins.11,27 For example, the cyclisation of bisdipyrrolyl- methane 65 results in porphyrin 66.11 The methoxy groups in the substituent X in compound 66 were converted into the carbonyl groups by demethylation with boron tribromide in dichloro- methane with subsequent oxidation with 2,3-dichloro-5,6- dicyano-1,4-benzoquinone (the yield of the target product is 54%).CO2H CO2H HO2C NH HN NH HN Me Me Me Me Me Me Me Me (CH2)4X(CH2)4 65 X (CH2)4 (CH2)4 Me Me Me Me NH N HN N Me Me Me 66 (11%) Me OMe . X=MeO As noted above, octamethyl derivatives of porphyrins mani- fest low solubility. However, the yield of porphyrins decreases drastically if the methyl groups are substituted by bulkier alkyl groups in order to increase the solubility. It was found 32 that condensation of 5,50-dicarboxy derivatives of dipyrrolylmethanes with aldehydes in pyridine in the presence of zinc acetate can increase the yield up to 20%. The highest yields are obtained with nitrobenzoaldehydes, which can be attributed to the oxidising ability of the nitro group. Evidence in favour of this hypothesis can be derived from the increase in the yield of 5,15-diphenylpor- phyrin 68a after addition of small quantities of nitrobenzene.The (dipyrrolylmethane the of ratio optimum reagents 67 :PhCHO:PhNO2) is 1 : 3.6 : 3.6 (Fig. 1).R1 R1 Me Me R1 R1 R2CHO N N Me Zn R2 Me R2 NH HN N N Me HO2C 67 CO2H Me R1 68a ±m R1 R1 R2 Compound 68 Ph C6H4OMe-4 C6H4OMe-3 C6H4OMe-2 C6H4NO2-4 C6H4NO2-3 Me Et Pr Bu C5H11 C6H13 C11H23 Bu Bu Bu Bu Bu Bu Me Me Me Me Me Me Me abcdefghijklm6 2 1 20 15 105 PhCHO: 67 2 3 4 5 1 Figure 1. The dependence of the yield of porphyrin 68a on the molar ratio of the reaction mixture components. This method was used in the synthesis of 2,8,12,18-tetrabutyl- 3,7,13,17-tetramethyl-5,15-diphenylporphyrin (68a) and its deriv- atives 68b &plu f containing different substituents in the benzene rings.9, 32, 65 The yields of porphyrins (*20%) practically do not depend on the nature of the substituent in the benzaldehyde. 2,8,12,18-Tetrabutyl-5,15-di(2-methoxyphenyl)-3,7,13,17-tetra- methylporphyrin (68d) is an exception; its lower yield (10%) can be attributed to steric hindrance in the condensation of dipyrrolyl- methane with 2-methoxybenzaldehyde. This porphyrin exists in the form of two atropoisomers (a,a and a,b), which differ in relative positions of substituents in the phenyl fragments relative to the macrocycle plane.The reaction of dipyrrolylmethane 67 with aliphatic aldehydes in pyridine in an inert atmosphere (180 8C, 0.5 ± 2 h) resulted in 5,15-dialkyl derivatives of octamethylporphyrin 68g ±m.65 The presence of a coordinating reagent (zinc acetate) in the reaction mixture markedly facilitates the cyclocondensation.The yields of porphyrins 68g ±m are maximum at the Zn(OAc)2 : dipyrrolyl- methane 67 :RCHO molar ratio of 1.5 : 1 : 1.25. It is noteworthy that depending on the chain length of alkyl substituents, the yields of porphyrins vary in the range of 15% ± 33%, i.e., are relatively high (cf. tetraalkyl-substituted porphyrins, which are obtained by condensation of monopyrroles in 5% to 15% yields 33). A salient advantage of this method is that reaction products do not contain even trace amounts of meso-monosubstituted porphyrins.The condensation of meso-aryl-substituted dipyrrolylme- thanes 69a,b with aliphatic aldehydes was used in the synthesis of 5,15-dialkyl-10,20-diaryl-b-octamethylporphyrins 70a ± d.24 These compounds are characterised by high solubility due to the presence of meso-alkyl substituents; their aryl fragments can be subject to chemical modification. X Me Me RCHO Me Yield (%) 318 5 PhNO2 : 67 4 3 Me Py, Zn(OAc)2 NH HN 69a,b HO2C CO2H Me R Me Me Me N N Zn X X N N Me Me Me Me R70a ± d 69: X = H (a), 4-OMe (b); 70: X =H, R=Me (a), Et (b), C6H13 (c); X=4-OMe, R=C6H13 (d). The condensation of compounds 69a,b with acetaldehyde and heptanal was carried out in pyridine at elevated temperatures and high pressures.The yields of meso-tetrasubstituted porphyrins 70a,c,d are low (3% to 5%), which can be ascribed to steric hindrance in the cyclocondensation, and do not depend on the chain length of the alkyl substituent. Similarly, the yields of porphyrins obtained by the condensation of 5,50-unsubstituted meso-aryldipyrrolylmethanes with benzaldehyde and formalde- hyde are 5%± 6% and do not depend on the nature of the aldehyde.24 It may thus be concluded that 5,50-dicarboxy derivatives of dipyrrolylmethanes are promising reagents in the synthetic chem- istry of porphyrins. They compare favourably with 5,50-unsub- stituted dipyrrolylmethanes and formyl derivatives by relative accessibility and, which is particularly important, by their resist- ance to oxidation.These compounds can be used in the synthesis of porphyrins of complex structure, including position isomers. V. Synthesis of bisporphyrins A vast variety of bisporphyrins with different spatial arrangement of macrocycles have been synthesised from dipyrrolylmethanes. Such syntheses are typically performed by a modified McDonald method.11, 13, 15, 16, 70 Sessler et al.12 were the first to synthesise the so-called `gable' bisporphyrins the porphyrin fragments in which are linked through the meta-positions of the benzene ring. This synthesis involves the formation of intermediate dipyrrolyl struc- tures. The reaction of compound 71 with 5,50-diformyl derivatives of dipyrrolylmethane in methanol in the presence of p-toluenesul- fonic acid and subsequent oxidation of the condensation product with tetrachloroquinone result in bisporphyrin 72.15 Et Me Me HN HN Me Me Et 71 Me Prn N HNN NH Prn Me In order to construct models which are geometrically identical to the active centres involved in photosynthesis, Sessler et al.16 carried out the condensation of the 5,50-diformyl derivative of dipyrrolylmethane 73 with bisdipyrrolylmethanes 74a,b to yield bisporphyrin 75a with spatially approximated porphyrin nuclei and its linear analog 75b.The same procedure was used in the synthesis of a bisporphyrin in which the porphyrin macrocycles were linked through a quinone fragment. The yield of porphyrins in the first stage of this reaction was*5% (Scheme 8).The synthesis of compounds in which porphyrin fragments are linked by aromatic bridges, which stabilise their spatial structure has been described.13 This synthesis consisted in the condensation of bisdipyrrolylmethanes 76 with bis(5-formyl-3,4-dimethylpyr- rol-2-yl)methane and resulted in bisporphyrins 77 (Scheme 9). The reaction of bis(5-formyl-3,4-dimethylpyrrol-2-yl)me- thane with stereoisomeric bisdipyrrolylmethanes 78 in methanol in the presence of p-toluenesulfonic acid and subsequent oxidation N Zh Mamardashvili, O A Golubchikov Me Et CHO Prn NH NH H+ + NH NH Prn CHO Me Et Et Me Et Me Me Prn NH N HN N Me Me Prn Et Me 72 EtThe synthesis of porphyrins from dipyrrolylmethanes Et COH Me OMe NH + NH MeO Me COH 73 Et Ar=m-C6H4 (a), p-C6H4 (b); (a) H+; (b) BBr3, CH2Cl2; (c) [O].Me CHO Me NH + NH Me CHO Me , R=Me Et Me MeEt HN HN Et Et Me Me Me 78 Me Me R NH N HN N R Me Me R=C6H13. of the reaction mixture with tetrachloroquinone afforded spiro derivatives 79a,b in a total yield of 5% (Scheme 10).22, 70 Owing to the presence of a rigid spirobisindan bridge between the macro- cycles, bisporphyrins acquired fixed spatial orientation. The distances between the centres of the macrocycles are equal to 1.80 and 1.00 nm, respectively; the porphyrin fragments of com- pound 79b partially overlap at an angle (see Scheme 10). Osuka and Maruyama 71 synthesised a series of bisporphyrins 80 the porphyrin fragments of which were linked through different positions of the naphthalene nucleus.The original 5,50-unsubsti- tuted bisdipyrrolylmethanes were prepared by the reaction of 5-unsubstituted pyrrole with the corresponding dialdehydes according to the known procedure.13 The naphthalene fragments ensure rigid binding of porphyrin macrocycles, which are perpen- dicular to their plane. Me Et Me Et HN NH a, b, c Ar HN NH Et Me Me Et 74a,b Me Me Me Me HN NH R HN NH Me Me Me Me 76 . , Me Me OHCHN NH + HN NH OHC Me Me MeMe Me Et Et NH N Et Et Me Me Me79a Et Me O O Me Et Me Me N H+ NH Me Me R H+ R Me Me R R N + HN R R Me Et Et R= Me Me Et HN N Ar N NH Et Me Me 75a,b Me Me Me Me HN R N Me Me Me 77 Me Me Et N NH NH N Et Me Me Me Me Me Me Et N HN R N NH Et Me Me 80 , ,, , .319 Scheme 8 Et Et Me O NH N HN N O Me Et Et Scheme 9 Me Me NH N HN N Me Me Scheme 10 Me R R NH N Me Me N NH Et Me Et Me Me 79b Me Me Et Et NH N HN N Et Et Me Me , ,320 A series of bisporphyrins with a `face-to-face' orientation of fragments (FTF-porphyrins) containing a conformationally rigid backbone between the macrocycles has been synthesised.18, 72, 73 The reaction of bisdipyrrolylmethane 81 with bis(3-ethyl-4- methyl-5-methoxymethylpyrrol-2-yl)methane in boiling benzene with its subsequent oxidation with tetrachloroquinone or the reaction of compound 82 with bis(3-ethyl-5-formyl-4-methylpyr- rol-2-yl)methane in methanol in the presence of perchloric acid yields bisporphyrins 83 and 84.Et Me Et Me Et H NH Me NH N HN N Me Me Et NHH X Et Me Me Et X Et Et Me Et H Me NH NH N HN N Me Me Et NHH Et Me 83, 84 Et 81, 82 [84 (20%)]. [83 (10%)], X=It has been noted 18, 72 that bisporphyrins 83 and 84 are devoid of stereoisomers. Spectral data provide evidence in favour of the FTF-orientation of the macrocycles. Collman et al.74 synthesised binuclear ruthenium and molybdenum complexes 85, which generate coordination compounds with intramolecular metal ± metal bonds upon photolysis in pyridine.Et Et Me Me N M NN N Me Me Et Et Et Et Me Me N M NN N Me Me Et Et 85 (M=Ru, Mo) The condensation of formylanthrylporphyrin 86 with bis(3-ethyl-4-methylpyrrol-2-yl)methane has led to a compound of the FTF-type (87).17 Osuka et al.75 carried out the synthesis of tetrakisporphyrin 89 from compound 88 and bis(5-formyl-3- hexyl-4-methylpyrrol-2-yl)methane under identical conditions. CHOEt Me Me Et NH N HN N Me Et Et Me 86 HHN HN HHHN HN HPPA two-step synthesis of cyclophanes 90 containing two bridg- ing O(CH2)nO groups connecting the p-positions of the benzene nuclei, has been described.76 ± 78 The condensation of dialdehydes R Me O (CH2)n O R=Et, Bu; n=2±4.N Zh Mamardashvili, O A Golubchikov Me Et Me NH N HN N Me Et Me Et Et Et Me NH N HN N Me Et Me Et Et Et NH N HN N Et Me Et MeMe Me Me MeMe Me Me 88PP 89 R + Me NH HN R Me NH N Me R R Me NH N Me R 90 (5% ± 10%) Et Me Me Me Me 87 H Me Me NH Me NHH H Me Me Me NH Me NHH Me Me Me Me C6H13 NH N . P= N HN C6H13 Me Me Me CHO CHO 1) H+ 2) [O] O (CH2)n O 91 R Me N O HN Me R (CH2)n R Me N O HN Me RThe synthesis of porphyrins from dipyrrolylmethanes 91 with dipyrrolylmethanes was carried out in TFA-containing acetonitrile in an atmosphere of nitrogen.The freely soluble porphyrinogens formed were oxidised in situ with o-tetrachloro- quinone. Cyclophane 90, the only reaction product, could be easily separated from admixtures by column chromatography. 5,50-Dicarboxy derivatives of dipyrrolylmethanes were also used in the synthesis of bisporphyrins. The procedure used by Collman 79 in the synthesis of cyclophanes 92 linked with the bridging (CH2)n (n=4, 6) groups through the meso-positions is a modification of the Jackson ± Kenner synthesis.69 Heating of bisdipyrrolylmethanes 93 (n=4, 6) with triethyl orthoformate in dichloromethane in the presence of trichloroacetic acid results in the formation of compounds 92 in very low yields. HO2CHN HN HO2C n=4, 6. The same procedure was used 80 ± 82 in the synthesis of cyclo- phanes 94 in which bridging groups connect the meta-positions of the benzene nuclei.They were synthesised by the reaction of bisdipyrrolylmethanes 95 with triethyl orthoformate in boiling chloroform containing 2% TFA. The yield of compound 94 was *5%; the monomeric porphyrins were not formed. Cyclophane 94 exists in the form of three atropoisomers (their structures are shown in Fig. 2). In bisdipyrrolylmethanes, the phenoxy groups easily rotate around the C7C bond of the bridges, thus giving rise to one or another spatial isomer depend- ing on their orientation at the moment of cyclisation. The calculation of the structure of octamethyl analogues carried out by the molecular mechanics method 82 confirmed the possibility of HO2CHN HN HO2C R1, R2=Me, Et; n=3, 4.Et Et CO2H Me Me NH (CH2)n NH Me Me CO2H Et Et 93 Et Et Me Me NH N HN N Me Me Et Et (CH2)n (CH2)n Et Et Me Me NH N HN N Me Me Et Et 92 R1 R1 CO2H R2 R2 O(CH2)4O NH CH(OEt)3, H+ NH R2 R2 CO2H R1 95 R1 321 OO OO a,a,a,a-Atropoisomer O OO O a,b,a,a-Atropoisomer O O O O a,b,a,b-Atropoisomer Figure 2. The atropoisomers of cyclophane 94 containing two confor- mationally active ester bridges. existence of a,a,a,a-, a,b,a,a- and a,b,a,b-atropoisomers. The insufficient length of bridging groups excludes the formation of the a,a,b,b-isomer. Atropoisomers manifest different mobility in thin-layer chromatography (pyridine ± hexane, 1 : 4) which allowed their separation.The assignment of atropoisomers was performed on the basis of 1H NMR data as well as on the basis of analysis of their coordination properties and chromatographic behaviour.80 A simpler procedure for the synthesis of analogous com- pounds has been proposed.76 The condensation of 5,50-dicar- boxy-substituted dipyrrolylmethanes with dialdehydes in pyridine in the presence of zinc acetate resulted in cyclophanes 96 (which can also exist in the form of three atropoisomers) with R1 R1 R2 R2 NH N HN N R2 R2 O O R1 R1 (CH2)n (CH2)n R1 R1 O O R2 R2 NH N HN N R2 R2 R1 R1 94322 bridging O(CH2)nO (n=2 ± 4) groups connecting the ortho-posi- tions of the benzene rings.Me CHO HO2C R O HN 1) Zn(AcO)2, Py 2) CF3CO2H + (CH2)n HN O R HO2C CHO Me R R Me NH N HN N Me O (CH2)n RR RR OMe NH N HN N Me R R 96 R=Et, Bu; n=2±4. The zinc complexes formed by template synthesis were destroyed in situ by treatment with TFA. The molar ratio of the reaction mixture components is critical for the yields of cyclophanes 96. Using the mathematical simulation method, it was demonstrated that the maximum yield of these compounds varied from 8% to 15% and was reached at the dipyrrolylmethane : dialdehyde : zinc acetate molar ratio of 1 : 2 : 1. These yields decrease regularly with a decrease in the number of methylene units, n. The same applies to cyclophanes which contain bridging groups connecting the meta- and para-positions of the benzene nuclei (compounds 90 and 94) and may occur as a result of stronger spatial restrictions in cyclocondensation.The optimum duration of the reaction is 2.5 to 3 h. With a drop in temperature from 120 to 80 8C, the reaction products contain only trace amounts of cyclophanes. It should be noted that the cyclophane 96 containing a shortened bridge O(CH2)2O cannot form isomers; all the oxygen atoms of the ether group are arranged inwards towards the interporphyrin cavity. Cyclophanes containing methyl and ethyl groups in the b-positions of the macrocycles are extremely poorly soluble in the majority of organic solvents; tetrabutyl derivatives are more soluble.VI. Conclusion The use of dipyrrolylmethanes opens up good perspectives for the synthesis of different types of mono- and bisporphyrins. At the same time, the potentialities of syntheses based on dipyrrolyl- methanes are far from being exhausted. These methods hold a prominent place in the synthetic chemistry of porphyrins. The choice of particular synthetic procedures is largely determined by the symmetry of the porphyrin molecule, the mode and mutual arrangement of substituents in the macrocycle as well as by the availability of the starting compounds. Me Me O (CH2)n O Me Me N Zh Mamardashvili, O A Golubchikov References 1. L F Fieser, M Fieser Advanced Organic Chemistry (London: Chapman and Hall, 1964) 2.H Fischer, H Orth Die Chemie des Pyrrols (Leipzig: Akademie Verlag, 1934) 3. G G Kleinspehn J. Am. Chem. Soc. 77 1546 (1955) 4. G G Kleinspehn, A H Corwin J. Org. Chem. 25 1048 (1960) 5. A S Semeikin,N G Kuz'min,O I Koifman Izv. Vysh. Ucheb. Zaved., Khim. Khim. Tekhnol. 31 (6) 39 (1988) 6. N Zh Mamardashvili,M E Kluyeva, O A Golubchikov Molecules 5 89 (2000) 7. N Zh Mamardashvili, A S Semeikin, L V Klopova, O A Golubchikov, in XIII Vsesoyuz. Seminar po Khimii Porfirinov i ikh Analogov (Tez. Dokl.), Samarkand, 1991 [The XIIIth All-Union Seminar on Chemistry of Porphyrins and Their Analogs (Abstracts of Reports), Samarkand, 1991] p. 98 8. N Zh Mamardashvili, A S Semeikin, O A Golubchikov, B D Berezin, in XVII Vsesoyuz. 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N Zh Mamardashvili, O A Golubchikov, in Proceeding of the 1st International Conference on Supramolecular Science and Technology, Zakopane, 1998 p. 153 68. N G Mamardashvili, A S Semeikin, B D Beresin, O A Golubchikov, in Proceedings of the 3rd European Symposium on Organic Reactivity, GoÈteborg, 1991 p. 211 69. A H Jackson, G W Kenner, J Wass J. Chem. Soc., Perkin Trans. 1 1475 (1972) 70. A Osuka,K Maruyama, I Yamazaki,N Tamai J. Chem. Soc., Chem. Commun. 1243 (1988) 71. A Osuka, K Maruyama J. Am. Chem. Soc. 110 4454 (1988) 72. C K Chang, I Abdalmuhdi J. Org. Chem. 48 5388 (1983) 73. J Weiser, H A Staab Angew. Chem. 96 602 (1984) 74. J P Collman, K Kim, J M Garner J. Chem. Soc., Chem. Commun. 1711 (1986) 75. A Osuka, K Ida, K Maruyama Chem. Lett. 741 (1989) 76. N Zh Mamardashvili, S A Zdanovich, O A Golubchikov Zh. Org. Khim. 32 934 (1996) a 77. N Zh Mamardashvili, S A Zdanovich, O A Golubchikov, in VII Mezhdunar. Konf. po Khimii Porfirinov i ikh Analogov (Tez. Dokl.), S.-Peterburg, 1995 [The VIIth International Conference on Chemistry of Porphyrins and Their Analogs (Abstracts of Reports), St. Petersburg, 1995] p. 22 78. N Zh Mamardashvili, S A Zdanovich, O A Golubchikov, in Yubileinaya Konf. IGU `PLZhK-20' (Tez. Dokl.), Ivanovo, 1997 [Anniversary Conference of Ivanovo State University `Problem Laboratory on Liquid Crystals-20' (Abstracts of Reports), Ivanovo, 1997] p. 19 79. BMTrost, C R Hutchinson (Eds) Organic Synthesis Today and Tomorrow (Madison, WI: University of Wisconsin, 1980) 80. O A Golubchikov, N Zh Mamardashvili, A S Semeikin Zh. Org. Khim. 29 2445 (1993) a 81. N G Mamardashvili, A S Semeikin, S A Zdanovich, O A Golubchikov, in Proceedings of the Vth European Symposium on Organic Reactivity, Santiago de Compostela, 1996 p. 170 82. N Zh Mamardashvili, S A Zdanovich, O A Golubchikov Struktura Konformerov Dimernykh b-Oktalkil-ms-Difenilporfirinov s Mostiko- vymi Gruppami v o- i p-Polozheniyakh Benzol'nykh Fragmentov. Raschet Metodom Molekulyarnoi Mekhaniki (The Structure of Conformers of Dimeric b-Octaalkyl-ms-Diphenylporphyrins with Bridging Groups in the o- and p-Positions of Benzene Fragments. Molecular Mechanics Calculations); article deposited at the VINITI No. 2941-V95 (Moscow, 1995) a�Russ. J. Org. Chem. (Engl. Transl.) b�Russ. J. Gen. Chem. (Engl. Transl.) c�Russ. J. Appl. Chem. (Engl. Transl.) d�Russ. J. Coord. Chem. (Engl. Transl.) e�Russ. J. Phys. Chem. (Engl

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年代:2000

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Russian Chemical Reviews 69 (4) 325 ± 346 (2000) Synthesis, structure and properties of coordination compounds of iron phthalocyanines and their analogues VNNemykin, INTret'yakova, S V Volkov, VDLi,NGMekhryakova, O L Kaliya, E A Luk'yanets Contents I. Introduction II. Synthesis and properties of iron phthalocyanines and their analogues III. Bisaxially coordinated complexes of iron phthalocyanine and its analogues IV. m-Oxo(-nitrido, -carbido) binuclear complexes of iron phthalocyanine and its analogues V. Some features of coordination chemistry of substituted iron phthalocyanines and their analogues VI. Major fields of application of iron phthalocyanines and their analogues Abstract. coordination various of synthesis the for Methods Methods for the synthesis of various coordination compounds closest their and phthalocyanines iron of compounds of iron phthalocyanines and their closest structural structural analogues, their naphthalocyanines, and porphyrazines iron viz., ., iron porphyrazines and naphthalocyanines, their structures, The surveyed.are applications and properties structures, properties and applications are surveyed. The biblio- biblio- graphy 230 includes graphy includes 230 references. references. I. Introduction Haem-containing biological systems play a unique role in living nature. These systems are involved in oxygen binding and trans- port (haemoglobin and myoglobin) and in catalysis of vital reactions (oxygenases, catalases, peroxidases and cyto- chromes).1, 2 The closest synthetic analogues of porphyrins, viz., phthalo- cyanines (tetraazatetrabenzoporphins) and related compounds, find wide applications primarily as dyes, pigments and catalysts of oxidative desulfurisation of hydrocarbons.3, 4 In recent years, these compounds have also been examined and used as compo- nents of gas sensors, electrochromic materials, passive laser valves and isolators for amplifying stages of solid-state and chemical lasers.These compounds have also found use as antimicrobial agents and photosensitisers in the photodynamic therapy of cancer.5 In the last decades, catalysts exhibiting high oxidase and peroxidase activities have been constructed based on phthalo- cyanines.3 Iron-containing complexes have particular promise.Increased scientific and practical interest in these complexes stems from the fact that they are characterised by a great diversity of coordination and redox forms.6±11 In spite of the fact that differ- ent aspects of chemistry and applications of iron phthalocyanines V N Nemykin, I N Tret'yakova, S V Volkov V I Vernadsky Institute of General and Inorganic Chemistry, National Academy of Sciences of Ukraine, prosp. Palladina 32/34, 252680 Kiev, Ukraine. Fax (38-044) 444 30 70. Tel. (38-044) 444 32 70 E-mail: victor_nemykin@yahoo.com (V N Nemykin), (38-044) 444 10 11 (S V Volkov) V D Li, N G Mekhryakova, O L Kaliya, E A Luk'yanets State Scientific Centre of the Russian Federation `NIOPIK', ul. Bol'shaya Sadovaya 1/4, 103787 Moscow, Russian Federation.Fax (7-095) 254 12 00. Tel. (7-095) 408 59 47 (N G Mekhryakova), (7-095) 254 98 66 E-mail: jerrik@cityline.ru (O L Kaliya), rmeluk@cityline.ru (E A Luk'yanets) Received 30 June 1999 Uspekhi Khimii 69 (5) 355 ± 377 (2000); translated by T N Safonova #2000 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2000v069n04ABEH000545 325 325 328 339 343 343 and their analogues have been considered in a great number of publications and patents, many questions about the structures and the valent and spin states of the major coordination forms of these compounds remain as the subjects of discussion. The properties and applications of selected coordination forms of complexes of iron phthalocyanines and their analogues have been surveyed in several fundamental reviews.3, 4, 9 ± 11 How- ever, the systematic analysis of a wide range of these compounds is lacking in the literature.In the present review, modern data on the synthesis and coordination chemistry of iron phthalocyanines and their analogues are considered and analysed. II. Synthesis and properties of iron phthalocyanines and their analogues In 1928, chemists from the `Scottish Dyes Ltd.' found a dark-blue compound deposited on the walls of a reactor in the synthesis of phthalimide from phthalic anhydride and ammonia. This com- pound was extremely stable with respect to acids and alkalis, and it was suggested that it has the structure of PcFe (PcRFe, where R1=R2=R3=R4=H) based on the results of a detailed study of its properties and structure.R2 R3 4 5 3 R1 R4 6 R4 R1 N N N R3 R2 N Fe N R2 R3 N N N R1 R4 R4 R1 R3 R2 PcRFe As a result, a new class of macrocyclic tetrapyrrole com- pounds, viz., phthalocyanines, has been discovered.15 ± 17 The synthesis of PcFe was described in detail for the first time in the classical papers of Linstead.16, 17 Presently, PcFe and its numerous R-substituted derivatives (PcRFe) are prepared by reactions of various derivatives of phthalic acid with iron or its compounds, which are often performed in the presence of catalysts (Scheme 1). In all cases, the reactions are carried out either in a melt of the reagents (the sintering method) or in high-boiling organic sol-V N Nemykin, I N Tret'yakova, S V Volkov, V D Li, N G Mekhryakova, O L Kaliya, E A Luk'yanets 326 Scheme 1 N N HN : B N N NH N N CN Pc CN MX2 a or b or c or d PcFe (a) FeX2; (b) Fe(CO)5; (c) FePy4Cl2; (d ) Fe.O FeX2, OC(NH2)2, cat. PcFe Y O Y=O, NH. : B NH2 Pc N MX2 FeX2 PcFe NH vents.3, 4, 10, 11 Thus heating of phthalonitrile with iron(II) hydrox- ide in trichlorobenzene containing 10% of quinoline afforded PcFe in 73% yield.18 Boiling of phthalonitrile with iron pentacar- bonyl in 1-chloronaphthalene gave PcFe in 50% yield. In the latter case, there was virtually no need for further purification of the product.19, 20 The syntheses of PcFe from phthalonitrile and FePy4Cl2 (in yields of up to 70%) 21 or from phthalic anhydride, urea and powdered iron (or its compounds) in the presence of different catalysts (boric acid, ammonium molybdate or titanium chloride) (in yields of up to 65%) were described; however, the purities of the final product were often not reported.22 ± 24 Iron phthalocyanine can also be synthesised by metallation of metal- free phthalocyanine (Pc) prepared from the diimino derivative of isoindolinedione or phthalonitrile in the presence of a base (:B).A convenient procedure for the preparation of PcFe and its deriva- tives by heating the reagents with the use of microwave radiation Table 1. Structures of iron phthalocyanine, its biaxially coordinated complexes and their analogues established by X-ray diffraction studies.Complex Coordination polyhedron Oxidation state of iron PcFe [PcFe]7 [PcFe]27 PcFe(DMSO)2 PcFe(4-MeC5H4N)2 PcFe(MeIm)2 PcFe(4-MeC5H10N)2 PcFe(CO)(DMF) 1,2-NcFe(cyclo-C6H11NC)2 1,2-NctFe(ButNC)2 N4 N4 N4 N4S2 N6 N6 N6 N4OC N4C2 N4C2 +2 +1/+2 +1/+2 +2 +2 +2 +2 +2 +2 +2 PNP[PcFe(CN)2] +3 N4 C2 1.939, 1.957 Note. MeIm isN-methylimidazole, 1,2-NcFe is iron 1,2-naphthalocyanine, 1,2-NctFe is tetrakis(7-tert-butyl)-substituted 1,2-NcFe, PNP is the triphenyl- phosphineimmonium cation. was proposed.25 The latter procedure afforded the highly pure (up to 97%) target product in high yields (up to 80%). Iron phthalo- cyanine was also formed as a result of macrocycle contraction in the reaction of the so-called uranyl superphthalocyanine, which is a pentadentate analogue of phthalocyanine, with iron salts.26 Substituted iron phthalocyanines are synthesised according to procedures virtually identical to those used for the preparation of PcFe.Complexes PcRFe containing different numbers of sub- stituents R, where R is the halogen atom or the alkyl, aryl, nitro, alkoxy, aryloxy, alkylthio, arylthio, arylsulfonyl, polyfluoroal- koxy, polyfluorosulfamoyloxy, carboxy, sulfo, etc. groups, were described in the literature.4, 6 ± 8, 27 ± 36 Final purification of complexes was carried out by high- temperature sublimation in vacuo or by removal of ligands (L) from bisaxially coordinated complexes of the general formula PcRFeL2 on heating in vacuo or under a stream of an inert gas.6±14 Polymeric compounds containing the PcFe fragments can be prepared in a way analogous to that for the preparation of substituted iron phthalocyanines.37 ± 40 The formation of poly- meric PcFe by the reaction of pyromellitic acid, an iron salt and urea in the presence of a catalyst was studied in detail.40 The MoÈ ssbauer spectra of the resulting polymer contain four doublets belonging to polymeric and monomeric complexes as well as to oxidation products containing the phthalocyanine fragments.After purification of the polymer by gel permeation chromatog- raphy and its heating to 300 8C, the MoÈ ssbauer spectra exhibit only one doublet with the parameters characteristic of the [PcFeII]n complex.Like the majority of phthalocyanine complexes with other metals, PcFe can occur in different crystal modifications.4, 12, 14 According to the data from X-ray diffraction analysis, the environment about the iron ion in one of these modifications, viz., in the b-modification of PcFe, is a pseudooctahedron formed by four isoindole nitrogen atoms of the virtually planar phthalo- cyanine ligand and two meso-nitrogen atoms of the adjacent PcFe molecules (Table 1).41 The magnetic moments (meff) for different specimens of PcFe at room temperature are in the range of 3.7 ± 4.5 mB.51 ± 53 The MoÈ ssbauer spectra of a series of PcRFe complexes in which the s-donor and p-acceptor properties of the peripheral substituents vary over a rather wide range were examined (Table 2).The parameters of the MoÈ ssbauer spectra d and DEQ for these complexes, which reflect the character of the Pc7Fe interaction, should correlate with the Hammett constants (s). However, with the aim of giving an adequate account of the effect of substituents in the phthalocyanine macrocycle on the MoÈ ssbauer parameters, the authors 27, 28 had to introduce two arbitrary additional parameters describing the s-donor and p-acceptor properties of the substituents (S and P, respectively). Ref. Bond lengths /A Fe7L Fe7N(Pc) 41 42 42 43 44 45 46 47 48 49 50 1.926, 1.927 1.910 1.910 1.931, 1.951 1.932, 1.937 1.926, 1.955 1.929, 1.930 1.889 1.936, 1.949 7 2.308 2.040 1.946 2.119 1.72 (CO), 2.07 (O) 1.911 1.913 1.976Synthesis, structure and properties of coordination compounds of iron phthalocyanines and their analogues Table 2.Parameters of MoÈ ssbauer spectra of the PcRFe complexes. R3 R1 R2 R4 T /K d a DEQ /mm s71 /mm s71 H H H H 111 163 203 H H H CO2 C10H21 CO2C10H21 H H CO2 C10H21 H H C(O)N(C10H21)C(O) H H H NO2 Cl Cl Cl Cl H CN H CN H Me H Me H H OCH2O H OMe H OMe 298 298 298 298 298 291 298 79 297 79 292 79 297 79 HH HH H But H OBui 79 299 H H H SPh H CO2H CO2 H H 79 296 77 293 77 H HH H CO2 H H (CH2)4 79 299 293 2.70 2.601 2.609 2.604 2.581 2.78 2.71 2.61 2.48 2.50 2.48 1.96 1.96 1.96 1.99 1.99 2.00 1.72 1.65 1.69 1.56 1.56 1.56 2.66 2.67 2.58 2.88 2.80 2.61 2.54 2.53 2.49 2.42 2.91 2.91 2.53 1.38 1.37 1.36 2.554 2.498 2.923 2.36 2.31 2.28 2.21 a Relative to metallic iron; b relative to sodium nitroprusside.4.2 0.48 0.736 b 0.691 b 0.667 b 0.634 b 4.3 0.29 78.8 0.29 0.22 4.2 0.23 79.3 0.20 0.17 4.3 0.30 79.3 0.27 0.16 4.3 0.22 78.5 0.22 0.15 4.5 0.32 78.7 0.32 0.25 4.3 0.27 79.0 0.27 0.20 5.1 0.49 0.48 0.38 4.6 0.47 0.47 0.37 4.2 0.48 0.46 0.33 0.43 4.2 0.28 0.28 0.20 4.4 0.24 0.23 0.14 0.238 0.137 0.297 4.3 0.47 0.46 0.37 0.36 The S value is larger for stronger s-donors, whereas thePvalue is larger for stronger p-donor and weaker p-acceptor.The MoÈ ssba- uer parameters are related to the S and P constants by the following equations: d=7S+P, DEQ=aS7bP+c, where a, b and c are constants and a>b>0. The MoÈ ssbauer spectra of all the PcRFe complexes studied are characterised by high and virtually temperature-independent quadrupole splitting (see Table 2) typical of iron(II) complexes with the intermediate (S=1) spin state.27, 28, 41, 51 327 D 2 Ref. 1.0 1 28 54 0.5 3 28 28 800 700 600 500 400 300 l /nm 28 Figure 1. Electronic absorption spectra of iron(II) tetrakis(4-tert- butyl)phthalocyanine (PctFe) in dry degassed benzene (1), PctFePy2 in benzene with an admixture of pyridine (2) and HPctFeCl in benzene (3) (c&1073 mol litre71, l=0.01 cm).28 28 28 27 27 27 27 27 2799 27 55 The electronic absorption spectra of PcFe in the gas phase and in noncoordinating solvents depend on the procedure for the preparation and the purity of the specimen. In our opinion, the most reliable data were obtained for sublimed specimens of PcFe dissolved in naphthalene at 110 8C in an atmosphere of helium.56 Bands similar in shape and position were observed in the elec- tronic absorption spectra of benzene solutions of PcRFe under an inert atmosphere (Table 3, Fig. 1) 6, 7 The spectra of PcRFe both in the gas phase 61 and in noncoordinating solvents 6, 7, 56 are untypical of the PcRMcomplexes (M is metal).61, 62 For example, the electronic absorption spectra of iron(II) tetrakis(4-tert- butyl)phthalocyanine (PctFe) always have several overlapping bands with approximately equal intensities in the long-wavelength region (580 ± 750 nm), whereas this region in the spectrum of its bisaxially coordinated adduct PctFePy2 is typical of the PcRM complexes (Fig. 1).(It should be noted that axial coordination in the PcRM complexes of other metals affects rather slightly the electronic absorption spectral patterns.) Originally, this anomaly was attributed to the fact that the PcRFe complexes are strongly aggregated in noncoordinating solvents.29 However, more recent studies of PcRFe complexes containing bulky substituents at the a position of the phthalocyanine macrocycle did not confirm this suggestion. Thus the spectral patterns of both sterically unhin- dered complexes and complexes containing bulky substituents, which prevent intermolecular aggregation (for example, the 2,4,6- trimethylphenyl substituents), are approximately identical.7 In this connection and taking into account that the planar-square environment is untypical of iron(II) complexes, it was suggested 7 that PcRFe complexes have a tetrahedrally distorted geometry, which leads to a change in the electronic absorption spectra.The configuration of the ground electronic state of PcRFe has been discussed in the literature over a long period. In the late 1960s, based on the results of MoÈ ssbauer spectroscopy (including the data obtained in magnetic fields), Dale postulated that the ground electronic state of the iron ion in PcFe has the 3Eg[(dxy)2(dxz,dyz)3(dz2)1] configuration.51, 63 Using the data on magnetic susceptibility of PcFe, Lever 53 postulated another configuration of the iron ion, viz., 3A2g[(dxy)2(dxz,dyz)2(dz2)2]. Subsequent quantum-chemical calculations 64, 65 and studies of the MoÈ ssbauer spectra of a wide range of substituted iron phthalocyanines 27, 28 made it possible to state reliably that the ground electronic state of the iron ion in the compounds under study has the 3Eg[(dxy)2(dxz,dyz)3(dz2)1] configuration.Apparently, the sole exception is iron terakis(4-phenylthio)phthalocyanine, which has anomalous MoÈ ssbauer parameters (see Table 2). As a consequence, the configuration of the ground electronic state ofV N Nemykin, I N Tret'yakova, S V Volkov, V D Li, N G Mekhryakova, O L Kaliya, E A Luk'yanets 328 Table 3.Electronic absorption spectra of the PcRFe and TAPRFe complexes in benzene. R4 R3 R2 R1 Positions of bands in electronic absorption spectra/nm (1075e) HH t PcRFe H ButH 2,4,6-Me3C6H2 H H 2,4,6-Me3C6H2 H Bu H i 690 sh, 647, 550, 350 690 sh, 647(0.60), 550(0.18), 460 sh, 410 sh, 352(0.46) 710 sh, 653, 550, 490 sh, 450 sh, 354 710 sh, 660(0.72), 562(0.16), 470 sh, 420 sh, 345(0.55) H H Bu Bui TAPRFe 659 sh, 590, 561 sh, 537 sh, 489, 474 sh, 427, 339 626, 443, 330 (seea) 58 Et Ph Et Ph 648(1), 550(0.34), 386(1.24), 322(1.93) 635(4.41), 595(4.30), 455(4.38), 345(4.85) 4-ButC6H4 4-ButC6H4 a The parameters of MoÈ ssbauer spectra at 300 K relative to metallic iron: d=0.05 mm s71, DEQ=2.53 mm s71.58 R2 R1 the iron ion in this compound was described as 3A2g[(dxy)2(dxz,dyz)2(dz2)2].27 The reaction of PcFe with potassium tert-butylthiolate in the presence of 2,2,2-cryptand (C(222) afforded the one-electron- reduced complex [C(222]+[PcFe]7.42 The reaction of PcFe with LiAlH4 and 18-crown-6 (18-C-6) in a THF solution yielded the two-electron-reduced complex [Li2(THF)(18-C-6)2]2+[PcFe]27.42 In both cases, the central iron atom is coordinated by four nitrogen atoms of the isoindole fragments of the planar phthalo- cyanine ligand (see Table 1).Based on the results of spectral studies, the following two schemes of the equilibrium of isoelec- tronic forms of the one- and two-electron-reduced complexes were proposed: [FeIPc27]>[FeIIPc37] and [FeIPc37]>[FeIIPc47].Complexes of iron tetraazaporphin (TAPRFe) have been poorly studied.6, 57 ± 60, 66 R2 R1 N N N R2 R1 N Fe N R1 R2 N N N TAPRFe R1 R5 R2 R4 Presently, only octaphenyl- 58, 60 octaethyl-,57 dibenzobarre- lene- 59, 66 and octakis(4-tert-butylphenyl)-containing 60 com- plexes (see Table 3 and notes therein) are reliably characterised by electronic absorption spectra,MoÈ ssbauer spectra and magnetic susceptibility data. On the basis of the data from magnetic susceptibility (3.8 58 and 3.82 mB 57 for octaphenyl- and octaethyl- porphyrazine iron complexes, respectively) and MoÈ ssbauer spec- troscopy, all TAPRFe complexes were assigned to iron(II) compounds characterised by the intermediate (S=1) spin state. Fused aza analogues of PcFe, viz., complexes of tetrakis(2,3- quinoxalino- and -quinolino)porphyrazines, were also synthesised and their thermal stabilities have been examined.67 III.Bisaxially coordinated complexes of iron phthalocyanine and its analogues Due to the low stability of iron 1,2-naphthalocyanine com- plexes (1,2-NcRFe) and, particularly, iron 2,3-naphthalocyanine complexes (2,3-NcRFe), which are prone to oxidation, reliable data on their properties in solutions are lacking. However, these complexes are stable in the solid state, which made it possible to measure their MoÈ ssbauer spectra.According to the available results, 2,3-NcFe belongs to iron(II) compounds with the inter- mediate (S=1) spin.27 1. Iron(II) complexes Coordinatively unsaturated compounds PcRFe readily form axially coordinated diamagnetic low-spin (S=0) complexes of R2 R15 6 4 R3 7 3 8 R3 R4 R4 N N NFe N N N N N R4 R4 R3 R2 1,2-NcRFe R1R4 R5 6 7 R3 R6 5 81 4 R2 R1 R6 R1 N N NFe N N N N N R3 R2 R2 R1 R3 R6 R5 R4 Ref. 6777 57 59 60 R1 R2 R3 R3 R2 R4 R5 R1 R6 2,3-NcRFeSynthesis, structure and properties of coordination compounds of iron phthalocyanines and their analogues the general formula PcRFeL2 with nitrogen-,15, 17, 46, 68 ± 73 phos- phorus-,74 sulfur- 47 and carbon-containing 75, 76 s-donor and (or) p-acceptor ligands (L).In addition, mixed complexes with differ- ent axial ligands of the general formula PcRFeL1L2 can be formed.76 ± 78 The formation of bisaxially coordinated PcFeL2 complexes, where L are nitrogen bases, was investigated in detail by kinetic methods.77 ± 84 According to concepts developed in these studies, the stepwise successive coordination of two ligand molecules proceeds by a dissociation mechanism L L (solv)PcFeL PcFeL (solv)PcFe (solv)2PcFe PcFeL2. Among complexes of the PcRFeL2 type, compounds contain- ing nitrogen bases of the heteroaromatic or aliphatic series as axial ligands were studied in most detail.46, 68, 69, 71, 72 Complexes with ligands containing the pyridine nitrogen atoms (which act simul- taneously as s-donors and p-acceptors) are more stable compared to complexes containing aliphatic amines (pure s-donors).46, 71, 73 Stynes 78 studied the kinetics of replacement of the axial ligand in PcFeL2, where L is methylimidazole, pyridine or piperidine, by benzyl isocyanide. The direct replacement of one axial ligand was observed only in the dark.When illuminated in the presence of L, the resulting complex PcFeL(BnNC) reverted to PcFeL2.78 Pyridine derivatives containing substituents at the b and g positions enter into the reaction of axial coordination, whereas substituents at the a position inhibit this reaction.69 Iron phtha- locyanine also readily reacts with primary aliphatic amines, including such sterically hindered compounds as tert-butyl- amine.71 The range of dialkylamines, which can be axially coordinated to PcRFe, is narrower.Branching of the carbon skeleton at the a-carbon atom [Pri2NH, (cyclo-C6H11)2NH] pre- vents the preparation of axially coordinated complexes of the PcFeL2 type.46, 71 In other words, as in the case of pyridine-type ligands, the choice of dialkylamines which can be axially coordi- nated is limited by steric factors. It is even more difficult to prepare axially coordinated complexes of this type with trialkylamines. Thus an unstable complex with quinuclidine is the only compound of this type, which was isolated and characterised by spectroscopic methods.72 Unlike tertiary amines, trisubstituted phosphines and phos- phites readily form complexes with PcFe.74, 85, 86 Compounds of the PcRFeL2 type, where L are aliphatic or aromatic isocyanides,75 carbon monoxide 47, 76 ± 78 or the cyanide ion,68, 87, 88 are also well studied.Of complexes containing the Fe7S bond, only those with tetrahydrothiophene and DMSO have been described.43, 47 Data on analogous complexes containing the Fe7O bonds are lacking in the literature. Among mixed-ligand complexes of the PcRFeL1L2 type, compounds PcFe(CO)L (where L is nitrogen-, sulfur- or oxygen-containing ligands),47, 76, 78, 87 PcRFe(CN)L (where L is a nitrogen-containing ligand)50 and PcFeL1L2 (where L1 and L2 are either two different nitrogen-containing ligands 77 or isocyanide and nitrogen-containing ligands 78) were studied. The structures of several complexes of the PcFeL2 type, viz., PcFe(4-MeC5H4N)2, PcFe(MeIm)2, PcFe(DMSO)2, PcFe(4- MeC5H10N)2 and PcFe(CO)(DMF), were established by X-ray diffraction analysis (see Table 1).43 ± 47 It should be noted that the axial ligands in PcFe(4-MeC5H4N)2 and PcFe(MeIm)2 are located in two mutually perpendicular planes, whereas these ligands in PcFe(4-MeC5H10N)2 are virtually coplanar.This suggests a sub- stantial contribution of back p-bonding in the PcFe(4- MeC5H4N)2 and PcFe(MeIm)2 complexes. For the same reason, the Fe7CO distance in the PcFe(CO)(DMF) complex is shorter than the corresponding distances in the complexes with other axial ligands and shorter than the Fe7N(Pc) distance (see Table 1).Analogous effects were also observed in complexes with strong p-acceptors, such as isocyanide ligands (see Table 1).48, 49 The electronic absorption spectra of PcFeL2 containing axial ligands of virtually all known types were meas- ured.46, 62, 68 ± 74, 77 ± 84, 87 These spectra consist of a long-wave- 329 length Q band at 660 nm which has vibrational structure, a charge-transfer band (CTB) at 425 nm and a B band at 330 nm. The s-and p-acceptor properties of the axially coordinated ligands in these complexes most substantially affect the positions of the charge-transfer bands (Table 4).68, 69, 71, 87 The above- mentioned effect has been examined many times (see, for example, Refs 46, 68, 69 and 71). Based on the results of calculations by the extended HuÈ ckel method, Goutermann and coworkers 64 were the first to assign this band to charge transfer from the central iron ion to the unoccupied orbital of the axial ligand (CTB Fe!L).More recently, Dale 68 studied a series of PcFeL2 complexes, where L is pyridine, imidazole, butylamine, piperidine, ammonia or the cyanide ion, and assigned this band to Fe!Pc charge transfer and related its position to the basicity of the axial ligand. Analogous correlations were also observed for a wide range of pyridine 69 and alkylamine 71 ligands. Ouedraogo and cow- orkers 69 followed Goutermann in attributing this band to Fe!L charge transfer. However, Stillman and coworkers 87 examined electronic absorption spectra as well as magnetic circular dichroism spectra of a series of PcFeL2 complexes and demonstrated that the above-mentioned band belongs to a trans- fer in which one of the states is degenerate.The transition from the 3dp orbitals of the central iron atom with symmetry eg to the unoccupied p* orbital of the phthalocyanine ligand with the symmetry b1u is the only allowed transition for the low-spin d 6 configuration of the Fe(II) ion in a complex with D4h symmetry. The axial coordination of the purely s-donor ligands, for exam- ple, of alkylamines, occurs primarily owing to the interaction between the dz2 orbital of the central atom and the n orbital of the ligand. The enhancement of the s-donor character of the axial ligand leads to an increase in the population of the dz2 orbital of the iron atom and, consequently, to its destabilisation as well as to destabilisation of the dp orbitals, which, in turn, leads to a bathochromic shift of the charge-transfer band.71, 87 The enhance- ment of the p-acceptor character of the axial ligands results in direct stabilisation of the dp orbitals of the central atom and, as a consequence, in an increase in the energy of the charge-transfer band. Thus this band is observed at 450, 430, 415 and 400 nm for the PcFeL2 complexes with the cyanide (s- and p-donors),87, 88 alkylamine (pure s-donors),46, 68, 71 pyridine (s-donors and p-acceptors) 68, 69, 87, 90 and isocyanide (weak s-donors and strong p-acceptors) 75, 76, 78 ligands, respectively, (see Table 4).To obtain an adequate correlation between the basicity of the axial ligands and the position of the charge-transfer band, the steric properties of these ligands should also be taken into account.71 A new approach to the estimation of the positions of charge-transfer bands in the spectra of the PcFeL1L2 complexes with ligands of all types on the basis of their electronic structures and steric proper- ties using second-order perturbation theory was proposed.46, 92 The electronic absorption spectra of the PcFeL2 complexes with axial pyridine-type ligands containing acceptor substituents have an additional band at 500 nm. This band was assigned to Fe!L charge transfer.69 It should be noted that the nature of the solvent has differing effects on the position of the charge-transfer band in PcRFeL2 complexes depending on the type of axial ligand.Thus if the axial ligands can form a dative p bond (pyridine-type ligands, isocyanides, etc.), the position of the charge-transfer band depends only slightly on the nature of the solvent. For example, the charge-transfer band for the PcFePy2 complex was observed at 415, 413.5 and 412 nm in DMSO, pyridine and dichloromethane, respectively.68, 69, 79, 87 In the case of alkylamines and, particularly, polyamines, which can form an intramolecular hydrogen bond, the position of this band varies over a wide range depending on the nature of the solvent.68, 69, 71, 87 For example, the charge-transfer band for the PcFe(NH3)2 complex is observed at 440 and 426 nm in DMSO68 and dichloromethane,87 respectively.Probably, this behaviour reflects a decrease in the bond energy on going to the PcRFeL2 complexes with pure s-donor ligands. However, there is no direct evidence for this suggestion.330 Table 4. Electronic absorption spectra of PcFeL2 complexes. LNH3 MeNH2 PrnNH2 PriNH2 BunNH2 BusNH2 BuiNH2 ButNH2 n-C5H11NH2 n-C6H13NH2 (EtO)3Si(CH2)3NH2 CF3CH2NH2 cyclo-C6H11NH2 BnCH(NH2)Me (CH2)2NH2 NH Ph(CH2)2NH2 BnNH2 NH2 HN N(CH2)2NH2 HN cyclo-(C6H10)(NH2)2 NH2(CH2)2NH2 NH2(CH2)3NH2 NH2(CH2)4NH2 NH2(CH2)5NH2 NH2(CH2)7NH2 HO(CH2)2NH(CH2)3NH2 Pip 4-MeC5H10N Mor Et2NH Pr2NH Py 3-MeC5H4N 4-MeC5H4N 3,4-Me2C5H3N 3,5-Me2C5H3N 3-OHC5H4N 4-OHC5H4N 3-ClC5H4N 4-ClC5H4N 3,5-Cl2C5H3N 3-CHOC5H4N 4-CHOC5H4N 3-CNC5H4N 4-CNC5H4N Im MeIm NNH V N Nemykin, I N Tret'yakova, S V Volkov, V D Li, N G Mekhryakova, O L Kaliya, E A Luk'yanets Solvent PhH ±L PhH ±L PhH ±L PhH ±L PhH ±L PhH ±L PhH ±L PhH ±L PhH ±L PhH ±L PhH ±L PhH ±L PhH ±L PhH ±L PhH ±L PhH ±L PhH ±L PhH ±L PhH ±L PhH ±L CH2Cl2 PhH ±L PhH ±L PhH ±L PhH ±L PhH ±L PhH ±L DMSO± L CH2Cl2 PhH ±L PhH ±L PhH ±L PhH ±L DMSO± L CH2Cl2 CHCl3 CHCl3 CHCl3 CH2Cl2 CHCl3 CHCl3 DMSO DMSO CHCl3 DMSO CHCl3 DMSO DMSO CHCl3 CHCl3 CHCl3 CH2Cl2 CH2Cl2 PhH ±L PhH ±L Positions of bands in electronic absorption spectra/nm (log e) 664, 637, 604, 425, 335 664, 637, 603, 427.5, 335 664, 637, 605, 427.5, 334 663, 636, 604, 424.5, 334 664, 637, 605, 427, 334 663, 636, 604, 425, 334 663, 635, 606, 427, 332 661, 634, 600, 421.5, 331 665, 636, 606, 428, 331 663, 636, 604, 427.5, 333 664, 637, 605, 428, 334 655, 628, 595, 416, 330 663, 636, 603, 425, 332 662, 635, 602, 425, 335 664, 637, 604, 427.5, 333 664, 635, 603, 426.5, 333 663, 636, 604, 424, 334 664, 637, 604, 426.5, 330 665, 640, 607, 430, 334 666, 639, 605, 432, 335 667, 640, 606, 435, 330 665, 639, 607, 433, 334 666, 640, 608, 433, 334 665, 636, 604, 429, 337 664, 636, 603, 428, 334 670, 640, 608, 437, 338 663, 636, 605, 426.5, 334 663, 642, 602, 434 659, 598, 425, 340 662, 635, 602, 427, 335 660, 633, 601, 423, 332 662, 637, 602, 426, 333 663, 637, 603, 426.5, 334 655(5.07), 593(4.47), 413(4.32), 332(4.82) 651, 591, 412, 328 655, 630, 595, 415 656, 630, 595, 412.5 656, 594, 410, 328 652, 592, 413, 331 656, 630, 595, 412.5 656, 630, 595, 415 656, 630, 595, 412 662, 635, 602, 432 653, 627, 592, 446, 407 680, 642, 613, 555 650, 625, 591, 465, 397 655, 630, 595, 410 654, 630, 595, 520 sh, 410 651, 625, 59,1 482, 402 652.5, 625, 591, 527 sh, 405 663, 640, 602, 430 657, 596, 423, 339 658, 597, 423, 338 659, 632, 597, 423.5, 340 655, 594, 408, 335 Relative intensity 2.08 : 0.72 : 0.61 : 0.3 : 1 2.05 : 0.56 : 0.47 : 0.31 : 1 1.47 : 0.81 : 0.57 : 0.28 : 1 1.64 : 0.7 : 0.64 : 0.28 : 1 1.58 : 0.71 : 0.6 : 0.35 : 1 2.06 : 0.8 : 0.69 : 0.32 : 1 1.37 : 0.61 : 0.52 : 0.25 : 1 2.23 : 0.68 : 0.57 : 0.32 : 1 1.45 : 0.55 : 0.47 : 0.24 : 1 1.75 : 0.72 : 0.51 : 0.21 : 1 1.65 : 0.64 : 0.55 : 0.29 : 1 1.82 : 0.6 : 0.52 : 0.29 : 1 1.61 : 0.56 : 0.49 : 0.33 : 1 1.48 : 0.57 : 0.48 : 0.27 : 1 1.70 : 0.52 : 0.46 : 0.29 : 1 71.45 : 0.65 : 0.55 : 0.29 : 1 1.50 : 0.57 : 0.49 : 0.24 : 1 1.44 : 0.66 : 0.58 : 0.29 : 1 2.39 : 0.74 : 0.63 : 0.36 : 1 71.72 : 0.75 : 0.69 : 0.32 : 1 1.84 : 0.81 : 0.71 : 0.35 : 1 2.51 : 0.78 : 0.66 : 0.37 : 1 3.41 : 1 : 0.87 : 0.43 : 1 1.88 : 0.67 : 0.6 : 0.45 : 1 2.01 : 0.71 : 0.65 : 0.29 : 1 772.02 : 0.61 : 0.51 : 0.35 : 1 2.05 : 0.82 : 0.66 : 0.28 : 1 1.61 : 0.53 : 0.45 : 0.32 : 1 1.61 : 0.52 : 0.44 : 0.28 : 1 777777777777777777772.89 : 0.8 : 0.69 : 0.34 : 1 1.48 : 0.38 : 0.27 : 1 Ref.71 71 71 71 71 71 71 71 71 71 71 71 71 71 71 71 71 71 71 71 89 71 71 71 71 71 71 69 87 71 71 71 71 79 68 69 69 90 87 69 69 69 69 69 69 69 69 69 69 69 69 87 87 76 76Synthesis, structure and properties of coordination compounds of iron phthalocyanines and their analogues Table 4 (continued). LN NH N CN7 CN7/N2H4 NH3/CO N2H4 ButNC cyclo-C6H11NC PhNC Me2C6H3NC o-MeC6H4NC m-MeC6H4NC p-MeC6H4NC PriC6H4NC Note. Pip is piperidine, Mor is morpholine and Im is imidazole.In some cases, the position of the charge-transfer band can by used for analytical purposes, for example, for the determination of CO concentration. Thus when CO was bubbled through a solution of PcFePy2, the intensity of the charge-transfer band decreased as the amount of the resulting PcFe(CO)Py complex increased.78 Analogously, in the reaction of the PcFePy2 complex with isocyanides, the intensity of the band of the initial complex (at 420 nm) first decreased [the formation of PcFePy(NCR), which gave weak bands at 420 and 394 nm] and then disappeared [the formation of PcFe(NCR)2, which gave a charge-transfer band at 394 nm]. The MoÈ ssbauer spectra of polycrystalline samples or frozen solutions of the PcFeL1L2 complexes were examined (Table 5).9, 46, 47, 69, 71, 76, 86, 93 ± 98 The spectra of freshly prepared samples of PcFeL1L2 are manifested as doublets and their parameters are characteristic of low-spin (S=0) iron(II) com- plexes.99, 100 The d values vary rather slightly, whereas the DEQ value varies over a wider range depending on the nature of the ligand (see Table 5).The isomer shift reflects the s-electron density at the iron nucleus.99 Consequently, an increase in the population of the 4s orbital of the iron atom leads to a decrease in the d value. For the complexes under consideration, an increase in the population of the 4s orbital of the central atom results directly from an increase in the s-donor character of the axial ligand and from the fact that the 3dz2 and 4s orbitals of the central iron atom have the same symmetry and can be mixed.99 The DEQ value, which characterises the electric field gradient about the iron nucleus, is more sensitive to the structure of the axial ligand (see Table 5).99 DEQ=1/2eQVzz(1+1/3Z2)1/2, where Vzz=eq, Z=(|Vxx7Vyy|)/|Vzz|.For the PcFeL2 complexes, Vxx&Vyy, and, consequently, Z&0;76, 95, 101 the electric field gradient has a positive sign.95, 101 For the low-spin iron(II) complexes, the q parameter can be represented as the sum q=qlat+qval+qmo.99, 102, 103 The first member qlat corresponds to the lattice contribution to the electric field gradient. This value decreases proportionally to the cube of the distance and is negligibly small for bulky complexes.The third member qmo is associated with the difference in the population of different MOs belonging to the same irreducible representation. For the low-spin iron(II) complexes, the ground electronic state has the (dxy)2(dxz,dyz) configuration,4 and, consequently, this term is equal to zero. The term qval reflects the symmetry of the population of the atomic orbitals of the iron atom and its s and p interactions with the ligands. This term can be written as follows 99, 103 Solvent PhH ±L CH2Cl2 CH2Cl2±N2H4 CH2Cl2 CH2Cl2±N2H4 CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 CHCl3 Positions of bands in electronic absorption spectra/nm (log e) 654, 595, 412, 327 664, 602, 426, 394, 310 663, 635, 600, 425, 357, 310 659, 596, 363, 317, 288 660, 633, 597, 417, 327 658, 598, 387 sh, 326 658, 598, 386 sh, 325 658, 598, 325 658, 598, 393 sh, 325 663, 601, 388 sh, 319 662, 598, 391, 324 663, 599, 392, 324 662, 598, 391, 325 (1) Relative intensity 1.71 : 0.45 : 0.23 : 1 777777777777 qval=4/7(17R)<r73>3d [nx27y2+nxy7nz27 71/2(nxz+nyz)], where R is the Sternheimer factor for antishielding of the iron nucleus, <r73>3d is the radial component of the 3d wave function and n is the population of the corresponding AO.If the coordination environment about the iron atom is nearly octahedral, the qval term for the low-spin t2g configuration is equal to zero.99, 103 In the case of axial distortion, the qval term makes the major contribution to the electric field gradient in complexes of the PcRFeL2 type.Actually, the quadrupole splitting in nearly octahedral complexes with isocyanide ligands is small,9, 90 whereas this splitting in complexes with nitrogen-centred axial ligands is significant (see Table 5).46, 68, 69, 71, 93, 95 ± 97 An increase in the DEQ value is observed in the following series of axial ligands: isocya- nides&cyanides<CO<phosphites<(CO)L&phosphines< pyridines, alkylamines and other nitrogen-containing ligands. Analysis of Eqn (1) demonstrates that the quadrupole splitting should decrease as both the s-donor and p-acceptor properties of the axial ligands are strengthened. However, the observed DEQ values for some complexes with ligands exhibiting pronounced p-acceptor properties (CO or isocyanides) are higher than the expected values (see Table 5).47, 76 To account for this anomalous behaviour, hypotheses for the cis and trans effects of the axial ligands on the phthalocyanine macrocycle were invoked.76 How- ever, the mechanism that causes a decrease in the quadrupole splitting is still an open question.Attempts were also made to interpret the available results with the use of empirical and semiempirical approaches. Thus a simple empirical procedure was proposed for the estimation of the DEQ value for the PcFeL2 complexes using absolute partial quadrupole splittings.104 A reasonable correlation between the DEQ values and the basicity was obtained.46, 92 A correlation between the DEQ values and the steric properties of alkylamines was also found.71 An approach based on perturbation theory was also proposed for the estimation of the DEQ values for all types of axial ligands.46, 92 The effect of circulating currents of the extended p system on the chemical shifts is clearly manifested in the 1H NMRspectra of the PcRFeL2 complexes. In all cases, the signals of the coordinated ligands are shifted upfield compared to the signals of the initial amines (Table 6) 72, 90, 105 ± 109 The high selectivity of axial coordi- nation, high induced shifts of the proton signals and the possibility of the prediction of their positions on the basis of the results of quantum-chemical calculations allow one to solve successfully a number of analytical problems.105 ± 108 331 Ref.76 87 50 87 50 91 91 91 91 91 91 91 91332 Table 5. Parameters of MoÈ ssbauer spectra of the PcFeL1L2 complexes. L2 L1 PrnNH2 PriNH2 BunNH2 BusNH2 ButNH2 CF3CH2NH2 PrnNH2 PriNH2 BunNH2 BusNH2 ButNH2 CF3CH2NH2 (CH2)NH2 (CH2)NH2 NH NH NH2 NH2 HN HN N(CH2)3NH2 N(CH2)3NH2 NH2(CH2)4NH2 Mor Et2NH NH2(CH2)4NH2 Mor Et2NH Pip Pip 4-MeC5H10N MeIm N 4-MeC5H10N MeIm N NH NH N N O O NNH NNH N N Im Im Py Py 3-MeC5H4N 3-MeC5H4N 4-MeC5H4N 4-MeC5H4N 3,4-Me2C5H3N 3,5-Me2C5H3N 3-OHC5H4N 4-OHC5H4N 3-ClC5H4N 4-ClC5H4N 3,5-Cl2C5H3N 3,4-Me2C5H3N 3,5-Me2C5H3N 3-OHC5H4N 4-OHC5H4N 3-ClC5H4N 4-ClC5H4N 3,5-Cl2C5H3N V N Nemykin, I N Tret'yakova, S V Volkov, V D Li, N G Mekhryakova, O L Kaliya, E A Luk'yanets T /K d a /mm s71 0.50 0.51 0.34 b 0.52 0.61 0.62 300 298 77 298 298 298 0.48 298 0.51 298 0.46 298 0.48 0.53 0.51 0.52 0.364 b 0.33 b 0.28 b 0.36 b 0.34 b 0.52 0.53 0.48 298 298 300 298 77 77 295 1154.2 300 298 298 0.51 0.57 298 77 0.53 298 298 77 77 77 295 195 77 77 293 77 300 295 195 77 77 295 195 77 77 295 195 0.51 0.55 0.313 b 0.29 b 0.54 0.59 0.63 0.322 b 0.26 b 0.33 b 0.53 0.53 0.58 0.62 0.321 b 0.55 0.60 0.61 0.347 b 0.55 0.58 0.342b 0.338b 0.355b 0.265b 0.343b 0.389b 0.360b 77 77 77 77 77 77 77 DEQ /mm s71 1.97 2.02 1.94 1.97 2.38 2.49 1.95 1.96 1.76 1.84 2.31 2.22 2.20 2.19 2.21 2.34 2.25 2.24 2.22 2.27 1.71 1.79 1.76 1.94 1.73 1.68 1.74 1.75 1.79 1.76 1.71 1.94 2.02 1.97 2.01 2.05 2.00 1.89 1.87 1.95 1.88 1.81 1.97 1.96 1.89 1.98 1.95 1.90 1.80 1.91 2.45 2.05 Ref.97 71 93 71 71 71 71 71 71 71 71 97 71 69 93 95 98 71 71 73 73 73 69 93 96 69 93 98 96 69 96 69 96 69 69 69 69 69 69 69Synthesis, structure and properties of coordination compounds of iron phthalocyanines and their analogues Table 5 (continued).T /K L2 L1 d a /mm s71 77 77 77 77 298 298 298 298 298 0.345b 0.325b 0.355b 0.340b 0.5 0.498 0.498 0.513 0.504 3-CHOC5H4N 4-CHOC5H4N 3-CNC5H4N 4-CNC5H4N Pyz MeC4H3N2 2,3-Me2C4H2N2 ClC4H3N2 EtC4H3N2 N 3-CHOC5H4N 4-CHOC5H4N 3-CNC5H4N 4-CNC5H4N Pyz MeC4H3N2 2,3-Me2C4H2N2 ClC4H3N2 EtC4H3N2 N N N NH NH 298 0.15b N N S S 0.53 298 DMSO PEt3 DMSO PEt3 PBu3 PBu3 P(OEt)3 P(OEt)3 P(OBu)3 P(OBu)3 0.50 0.16 b 0.25 b 0.15 b 0.24 b 0.23 b 0.13 b 0.17 b 0.18 b 0.42 0.48 0.36 0.37 0.36 0.36 0.36 0.35 0.37 298 291 78.6 291 78.8 4.3 291 78.8 4.3 298 77 298 298 298 298 298 298 298 THF H2O OPPh3 HMPT DMSO DMF MeOH CO CO CO CO CO CO CO S 0.38 298 CO CO CO CO CO CO CO CN ButNC cyclo-C6H11NC PhNC Me2C6H3NC Cl4C6HNC Me2C6H2(NC)2-1,4 C6Me4(NC)2-1,4 0.35 0.37 0.37 0.36 0.38 0.36 0.19b 0.16b 0.13b 0.11b 0.12b 0.09b 0.11b 0.12b 298 298 298 298 298 298 77 298 298 298 298 298 298 298 Et2NH Pip Py PrnNH2 NH3 CO CN ButNC cyclo-C6H11NC PhNC Me2C6H3NC Cl4C6HNC Me2C6H2(NC)2-1,4 C6Me4(NC)2-1,4 , HMPT is hexamethylphosphotriamide; a relative to sodium nitroprusside; b relative to metallic iron.N Note. Pyz is N Studies of the thermal stability of a number of the PcRFePy2 complexes demonstrated that both ligand molecules are elimi- nated in one stage.Electron-withdrawing substituents in the macrocycle increase thermal stability, whereas electron-donating substituents decrease this stability compared to that of the unsubstituted analogue.70 we restrict ourselves to consideration of the major representatives of this group. Polymeric complexes (PcRFeL)n were synthesised by the reactions of PcRFe with different bidentate ligands, such as pyrazine, tetrazine, 4,40-dipyridyl, 1,4-diisocyanobenzene, 4-phe- nylenediamine and 9,10-anthracenodiisocyanide, which serve as cross-linking agents. These complexes can be divided into two groups according to their semiconducting properties. The first group involves complexes exhibiting good semiconducting prop- erties only upon doping with halogens or other electron acceptors.The second group consists of complexes exhibiting semiconduct- Among axially coordinated PcFe(II) complexes, polymeric compounds of the general formula (PcRFeL)n belong to a special group.9, 13, 109 ± 113 These compounds attract great interest because of their semiconducting properties.114 ± 119 Since several compre- hensive reviews devoted to this subject are available,9, 13, 110, 112 DEQ /mm s71 1.97 1.84 2.17 1.90 2.006 1.895 1.968 2.149 2.016 1.79 2.20 2.08 1.54 1.47 1.57 1.47 1.45 1.07 0.99 0.95 1.05 0.96 1.82 1.75 1.69 1.60 1.56 1.56 1.56 1.55 1.45 1.27 1.19 1.11 1.02 0.82 0.56 0.79 0.69 0.67 0.70 0.67 0.65 0.66 333 Ref.69 69 69 69999999 47 47 86 86 86 86 76 76 76 76 76 76 76 76 76 76 76 76 76 76 939999999334 Table 6. Parameters of 1H NMR spectra of the axially coordinated ligands in the PcFeL2 complexes. LMeNH2 PriNH2 BusNH2 BunNH2 BuiNH2 n-C5H11NH2 n-C6H13NH2 BunCH(Et)CH2NH2 (EtO)3Si(CH2)3NH2 NH2 Me Me NH Pip Et2NH NH2(CH2)3NH2 NH2(CH2)4NH2 NH2(CH2)5NH2 NH2(CH2)7NH2 N(CH2)2NH2 HN N(CH2)3NH2 HO(CH2)2NH2 HO(CH2)2NH(CH2)2NH2 4-MeC5H4N Pyz ing properties without doping (Table 7). Polymeric compounds of the first type are stable up to 100 ± 130 8C and lose their semi- conducting properties in the absence of the doping agent.All complexes except for compounds containing tetrazine as the axial ligand belong to this type. Tetrazine-containing polymers belong to the second type. The latter are thermally more stable and have no need of doping.9, 13, 110, 112 Of the axially coordinated TAPRFeL2 complexes reported in the literature, only complexes of iron octaphenylporphyrazine, where L is a pyridine-type ligand, alkylamine or the cyanide ion, were systematically studied by electronic and MoÈ ssbauer spectro- scopy (Table 8).58, 120 ± 122 All complexes of this type belong to low-spin iron(II) compounds. The d and DEQ values (relative to sodium nitroprusside) were determined for some octaphenyl- substituted complexes: Complex TAPPhFePy2 TAPPhFe(4-MeC5H4N)2 TAPPhFeIm2 V N Nemykin, I N Tret'yakova, S V Volkov, V D Li, N G Mekhryakova, O L Kaliya, E A Luk'yanets Solvent CD2Cl2 CDCl3 C6D6 CD2Cl2 CDCl3 CD2Cl2 CDCl3 CD2Cl2 CDCl3 CD2Cl2 CDCl3 CD2Cl2 CD2Cl2 CDCl3 CD2Cl2 CD2Cl2 CD2Cl2 CD2Cl2 CD2Cl2 CD2Cl2 CD2Cl2 CDCl3 CD2Cl2 CD2Cl2 CD2Cl2 CDCl3 C6D6 CDCl3 CD2Cl2 CD2Cl2 CDCl3 CDCl3 DEQ/mm s71 d/mm s71 2.26 2.30 2.12 2.10 0.47 0.45 0.18 0.50 Positions of signals for protons of the coordinated ligand NHn a-CHn 72.73 72.73 73.70 73.09 73.15 73.49 73.49 72.98 73.01 73.21 73.26 73.01 73.00 73.00 73.01 72.98 77.28 77.28 78.75 77.46 77.67 77.36 77.78 77.28 77.44 77.29 77.45 77.17 77.17 77.42 77.38 77.18 73.23 77.48 73.12 73.12 72.86 73.01 73.05 73.00 73.09 77.89 77.97 76.43 76.46 76.58 77.06 77.15 72.86 72.80 73.22 76.65 76.81 77.10 72.9 76.18 76.85 76.06 72.95 72.74 1.95 1.92 The electronic absorption spectra of the octaphenyl-substi- tuted TAPPhFeL2 complexes with pyridine-type ligands are anal- ogous in general appearance to those of the PcFeL2 complexes (Fig.2).6, 57, 58 The band at 550 nm was attributed to the D 1 1 23 Ref. 0 300 58 58 58 120, 121 Figure 2. Electronic absorption spectra of the complexes TAPPhFePy2 (1), PcFePy2 (2) and 2,3-NcFePy2 (3) in benzene. b-CHn 71.94 71.94 72.06 72.06 71.41 71.41 71.31 71.16 71.42 71.41 71.42 71.49 71.26 72.23 71.46 71.80 71.24 71.31 71.4 71.39 71.43 70.70 70.3 71.18 70.98 72.14 71.18 4.78 6.01 500 400 Ref. 108 106 107 108 73 108 73 108 73 108 73 108 108 106 108 108 108 108 108 108 108 105 108 108 108 108 108 108 108 108 90 90 600 700 l /nmSynthesis, structure and properties of coordination compounds of iron phthalocyanines and their analogues Table 7.Selected characteristics of the polymeric complexes of iron phthalocyanines (PcRFeL)n and iron naphthalocyanines (NcRFeL)n.9, 13, 110 ± 114 L R4 R3 R2 R1 Phthalocyanines n-C7H15 n-C5H11 n-C7H15 HHHH n-C5H11 n-C7H15 n-C5H11 n-C6H13O H Pyz Tz HH Me Me HH CN CN HH tt Pyz Tz Pyz Tz Pyz HHHHH Et HH H H H C6 H4(NC)2-1,4 H H H C6 H4(NC)2-1,4 H H C6 H4(NC)2-1,4 H C6H4(NC)2-1,4 H C6Me4(NC)2-1,4 H C6 Me4(NC)2-1,4 H H H H C6 Me4(NC)2-1,4 H H H H Pyz H H H H Tz Me Me H H H H Me2 Tz CN CN H Bu H H Bu HH H Et H H Tz HH dabco bipy HH HH n-C5H11 H C6Me4(NC)2-1,4 H C6 Me4(NC)2-1,4 n-C5H11 n-C6H13O H HH Doped phthalocyanines H H H H Pyz H H H H Pyz H H H H Pyz H H H H Pyz H H H H Pyz H H H H Pyz H H H H C6 H4(NC)2-1,4 H H H H C6 H4(NC)2-1,4 H H H H C6 Me4(NC)2-1,4 H H H H C6 Me4(NC)2-1,4 HH1,2-Naphthalocyanine H H H H C6 H4(NC)2-1,4 2,3-Naphthalocyanines (R5=R6=H) H H H H C6 H4(NC)2-1,4 H n-C6H13O H n-C6H13O n-C6H13O H HH H C6 H4(NC)2-1,4 H C6 H4(NC)2-1,4 n-C6H13O H C6 H4(NC)2-1,4 H H H H Pyz H Tz H H HNote.Tz is tetrazine, Me2Tz is dimethyltetrazine, dabco is 1,4-diazabicyclo[1.2.2]octane, bipy is 2,20-bipyridyl; a relative to metallic iron; b nNC in cm71, lmax (electronic absorption spectrum) in nm; c relative to sodium nitroprusside. Fe!TAP [eg(dp)!eg(p*)] charge transfer.58 However, this assignment has recently come under strong criticism and this band was attributed 120, 121 to the Fe!TAP [eg(dp)!b1u(p*)] charge transfer. In the electronic absorption spectra of the TAPRFeL2 complexes containing alkylamines or the cyanide ion as the axial ligands, the intensity of the charge-transfer band at 500 nm is virtually equal to that of the Q band.However, taking into account that the above-mentioned complexes were not characterised by other spectroscopic methods, these data must be considered with caution.120 ± 122 Among complexes of iron naphthalocyanines, compounds containing isocyanide axial ligands have been studied in most detail (Table 9).48, 94, 123 ± 126 T /K d a DEQ /mm s71 /mm s71 s/S cm71 0.68 0.57 0.48 0.45 0.12 0.18 0.18 0.18 293 82 82 82 261075 2610710 261076 461079 <10712 <10712 161077 2 298 61075 2.009 2.23 0.5 c 0.13 2.00 0.24 298 261072 298 961076 161073 461073 561079 161076 5610711 961079 861079 261074 161079 261078 461072 361072 161075 161076 361073 261071 761073 361072 161073 261072 961077 2610710 6610710 0.58 0.51 0.47 293 0.13 82 0.17 82 0.18 261073 461076 161075 261076 561075 298 0.26 298 0.19 1.90 1.97 0.3 R2 R1N N R3 X Fe N R2 N N R1 L2 XR3 335 Other characteristics b nNC 2100 lmax 671, 607, 394, 321; nNC 2095 lmax 679, 610, 466, 395, 342, 303; nNC 2098 lmax 680, 612 sh, 459, 331, 302; nNC 2100 doped with (BF4)0.45n (PF6)0.5n (HSO4)0.4n (SCN)0.3n (ClO4)0.3n I2.5n I1.4n I3.0n I1.5n I3.0n I1.8n I2.5n lmax 712, 387; nNC 2090 nNC 2124 lmax 771, 690 sh, 346; nNC 2102 lmax 778, 692 sh, 365; nNC 2093 lmax 765, 686, 359; nNC 2095 R3 X L1 R1 R2 X R3 N NNR1 R2V N Nemykin, I N Tret'yakova, S V Volkov, V D Li, N G Mekhryakova, O L Kaliya, E A Luk'yanets 336 Table 8.Characteristics of the TAPRFeL2 complexes (R1=R2=Et or Ph). Relative intensity L R Positions of bands in electronic absorption spectra/nm (log e) CN Ph Py 4-MeC5H4N 4-EtC5H4N 3,4-Me2C5H3N 4-CH2=CHC5H4N 3-ClC5H4N 4-ClC5H4N Im MeIm PhNH2 NH3 Pip Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph cyclo-C6H11NH2 PriNH2 Prn2 NH 0.97 : 0.3 : 0.33 : 1 0.84 : 0.33 : 0.33 : 1 0.8 : 0.4 : 0.29 : 1 0.77 : 0.34 : 1 0.75 : 0.33 : 0.33 : 0.35 : 1 0.89 : 0.36 : 0.31 : 0.37 : 0.42 : 1 0.69 : 0.25 : 1 0.69 : 0.48 : 1 0.77 : 0.43 : 1 0.53 : 0.34 : 1 0.68 : 0.37 : 1 0.68 : 0.45 : 1 0.52 : 0.44 : 1 (see c) 0.53 : 0.35 : 1 0.82 : 0.70 : 1 (see c) 0.82 : 0.49 : 1 (see c) 0.81 : 0.56 : 1 (see c) 0.81 : 0.59 : 1 (see c) 0.83 : 0.71 : 1 643(4.39), 572(4.38), 451 sh, 383(4.46) (see a) 643(4.27), 574(4.59), 457 sh, 374(4.62) (see b) 610, 595 sh, 571, 513, 483 sh, 434 sh, 364 621, 599 sh, 571, 519, 485 sh, 422 sh, 363 621, 599 sh, 571, 517, 483 sh, 424 sh, 361 621, 595 sh, 575 sh, 520, 424 sh, 365 621, 595 sh, 570, 510, 481, 425 sh, 362 616, 592 sh, 567, 488, 454, 423, 362 615, 592 sh, 568, 424 sh, 356 627, 599 sh, 575 sh, 539, 445 sh, 366 629, 581 sh, 539, 450 sh, 365 627, 602 sh, 581 sh, 530, 450 sh, 370 631, 602 sh, 585 sh, 537, 424 sh, 365 629, 602 sh, 581 sh, 537, 420 sh, 366 623, 535, 367 631, 606 sh, 585 sh, 536, 420 sh, 363 627, 545, 371 622, 538, 364 622, 536, 364 623, 533, 365 628, 541, 366 586, 564 sh, 541, 458, 332 (see d) 57 590, 574 sh, 550 sh, 491, 333, 315 sh (see e) 57 Ph Ph Ph Ph Ph Ph Et Et Et2NH Mor n-C5H11NH2 Py MeIm a In CH2Cl2; b in acetone; c in acetonitrile; d in PhH ± Py; e in PhH.annelation of an additional benzene ring (contrary to angular annelation resulting in 1,2-NcFe) leads to a sharp change in the properties of these compounds, which is manifested, for example, in a strong bathochromic shift of the Q band in the electronic absorption spectra and in enhancement of the semiconducting properties (see Tables 7 and 9, Fig. 2).9, 13, 110, 112 An extension of the p-system of the macrocycle leads to a decrease in the stability of the complexes containing the 2,3-NcRFe fragment.123 ± 126 Four structural isomers are possible for derivatives of 1,2- NcRFeL2.An individual isomer, which contains the tert-butyliso- cyanide and cyclohexylisocyanide ligands and is characterised by C4h symmetry, was isolated and its structure was established.48, 49 Due to the strong p-acceptor properties of the axial ligands, the Fe7Cax bond length is smaller than the Fe7N(Pc) distance (see Table 1). On the whole, the spectral characteristics and properties of the 1,2-NcRFeL2 complexes are very similar to those of PcRFeL2 (see Tables 4 ± 7). In the 2,3-NcRFeL2 complexes, linear Table 9. Properties of axially coordinated complexes of iron naphthalocyanines and pyridinoporphyrazines.L X R3 R2 R1 Positions of bands in electronic absorption spectra /nm 1,2-Naphthalocyanines CH H CH=CHCH=CH CH H CH=CButCH=CH 653, 594, 353 654, 594, 354 654, 595, 354 666, 604, 353 662, 602, 354 668, 606, 354 670, 607, 364 ButNC cyclo-C6H11NC BunNC Py ButNC Py Py CH CH HH CH=CMeCH=CH CH=CPhCH=CH 2,3-Naphthalocyanine CH CH=CHCH=CH H C6H4(NC)2-1,4 Tetrakis(2,3-pyridino)porphyrazines 627, 573, 339 628, 574 sh, 398 sh, 339 633, 577 sh, 332 632, 577 sh, 393 sh, 331 634, 579 sh, 393 sh, 331 634, 580 sh, 330 633, 579 sh, 391 sh, 332 634, 579 sh, 394 sh, 332 H H H N ButNC cyclo-C6H11NC PhNC Me2C6H3NC o-MeC6H4NC m-MeC6H4NC p-MeC6H4NC p-PriC6H4NC Ref.122 58 58, 120, 121 58 58 58 58 58 58, 120, 121 58 58 58 58 58 120, 121 120, 121 120, 121 120, 121 120, 121 Ref. nNC /cm71 48 2140 2155 2153 49 49 49 94 2145 91 2157 2171 2141 2140 2136 2134 2144 2144Synthesis, structure and properties of coordination compounds of iron phthalocyanines and their analogues Complexes of iron tetrakis(2,3-phenanthro)porphyrazine were prepared by photochemical oxidation of trans-1,2-dicyano- 1,2-diphenylethylene in the presence of iodine followed by tem- plate condensation with iron compounds and axial coordination with different isocyanides.127 These complexes were characterised by electronic and MoÈ ssbauer spectroscopy. In addition, com- plexes of iron tetrakis(2,3-pyridino)porphyrazine with isocya- nides as axial ligands were prepared and characterised (see Table 9).91 2.Iron(III) complexes In spite of the fact that azaporphin iron(III) complexes have been considered in the literature over several decades, convincing evidence for the existence of Pc complexes of iron(III) has been reported, and furthermore their properties, have been described only recently. Oxidation of a solution of Na2[PcFeII(CN)2] in dichloromethane with bromine afforded the six-coordinate Na[PcFeIII(CN)2] complex.55 In the electronic absorption spectra, this process is characterised by distinct isosbestic points (Fig. 3). The Na[PcFeIII(CN)2] complex can also be prepared by oxidation of Na2[PcFeII(CN)2] with cumene hydroperoxide128 or by the reactions of Na[PcFeIII(OH)2] with cyanides.50 The [PcFeIII(CN)2]7 complex was characterised in detail by electronic absorption, MoÈ ssbauer, IR, Raman, ESR and circular dichroism spectra, magnetic susceptibility data and X-ray diffraction anal- ysis (Table 10).50, 55, 129 The magnetic susceptibilities, the param- eters of the MoÈ ssbauer spectra and the character of the electronic absorption spectra demonstrate that this compound can be described as a low-spin (S=1/2) iron(III) complex with the (dxy)2(dxz,dyz)3 electronic configuration.The presence of a hole in the dp orbital of the central atom results in the appearance of a series of L(Pc)7Fe charge-transfer bands in the electronic absorption spectra.The most detailed assignment of the bands in the electronic absorption spectrum of the Na[PcFeIII(CN)2] com- plex was made by Ough and Stillman.55 Based on the analysis of the magnetic circular dichroism and electronic absorption spectra, two Fe!L(Pc) and four Fe!Pc charge-transfer bands were identified.55 According to the data from X-ray diffraction analy- sis 50 (see Table 1), the Fe7Cax bond length in PNP[PcFeIII(CN)2] is larger than the Fe7N(Pc) bond length. This complex is more similar in structure to compounds of the PcFeIIL2 type with nitrogen-containing axial ligands 43 ± 46 and differs from analo- gous complexes with axial ligands exhibiting strong p-acceptor properties.48, 49 The reactions of [PcFeIII(CN)2]7 with pyridine and imidazole afforded compounds with composition PcFeIII(CN)L, whereas the reaction with hydrazine was accom- panied by reduction of the iron ion to form Table 10.Physicochemical properties of the [PcFeIIIXY]n complexes (S=1/2).89, 128, 130 n X Y T /K d a DEQ /mm s71 /mm s71 m /mB (T /K) Factor g in ESR spectra 2.22 0.18 4.2 OH OH 71 2.64 2.25 0.22 0.29 4.2 4.2 OPh NCO OPh NCO 71 71 71 2.65 2.49 0.26 0.21 4.2 4.2 NCS N3 NCS 71 N3 0.99 0.11 4.2 CN CN 71 2.31 (300) 2.06 (4.2) 2.3 (300) 2.44 (300) 1.96 (4.2) 2.4 (300) 2.05 (300) 1.76 (4.2) 2.49 (300) 1.89 (4.2) Py 00 ButPy 1.57 1.57 1.60 0.04 0.04 0.05 293 293 293 CN CN CN Pyz 0a Relative to sodium nitroprusside; b nCN 2131 cm71, sRT=1610710 S cm71; c nCN 2132 cm71, sRT=661075 S cm71.337 D 1.0 0 500 400 350 600 700 l /nm Figure 3. Dynamics of oxidation of the [PctFeII(CN)2]27 complex with cumene hydroperoxide to form [PctFeIII(CN)2]7. [PcFeII(CN)(N2H4)]7.50 Complexes of the PcFeIII(CN)L type can also be prepared by the reactions of the ligand L with the cyanide complex [PcFeIII(CN)]n.130 The reactions of Na[PcFeIII(CN)2] with acids yielded complexes of the Pc+.FeIII(CN)X type containing the phthalocyanine radical cat- ion. Finally, Na[PcFeIII(CN)2] can be reduced to the cyanide iron(II) complex under the action of different reducing agents.50 Complexes of the general formula PNP(PcFeIIIXY) (X=Y) were synthesised 89, 129 and characterised by electronic absorption, MoÈ ssbauer and ESR spectra as well as by magnetic susceptibility data (see Table 10).These compounds exhibit the following characteristic features: (1) the electronic absorption spectra of all complexes have additional Pc!Fe and Fe!Pc charge-transfer bands in the region of 400 ± 800 nm, and the Q band is bath- ochromically shifted compared to that for the PcFeL2 complexes; (2) in most cases, the electronic absorption spectra are typical of axially coordinated low-spin (S=1/2) iron(III) complexes;131, 132 (3) in all the studied complexes, except for the cyanide complex, the quadrupole doublets in the MoÈ ssbauer spectra are character- ised by substantial asymmetry; a low-intensity broad component of the doublet for the complexes with the Fe7Oax bond is observed at low rates, whereas this component for the complexes with the Fe7Nax bonds is observed at higher rates,129 as in the case of the well-studied iron(III) complexes with porphyrin ligands 100, 133 {for the azide complex anion [PcFeIII(N3)]7, the sign of the electric field gradient was found to be negative 134}; (4) magnetic moments for all the complexes studied are in the range of 2.0 ± 2.5 mB at room temperature.Based on the data from mag- Positions of bands in electronic absorption spectra /nm 800, 690, 630, 575, 450, 360 2.31, 2.11, 1.96 2.31, 2.12, 1.93 broad lines 775, 685, 673, 655, 645, 610, 555, 435, 375, 330, 280 790, 690, 673, 645, 610, 565, 525, 485, 385, 330 the same 2.41, 2.07, 1.79 2.41, 2.07, 1.79 2.00 (broad peak) 775, 685, 650, 620, 600, 540, 500, 425, 400, 322 781, 755, 688, 663, 646, 623, 603, 545, 421, 399, 325, 285 783, 690, 651, 595, 555, 510, 412, 317 (see b) see cV N Nemykin, I N Tret'yakova, S V Volkov, V D Li, N G Mekhryakova, O L Kaliya, E A Luk'yanets 338 netic circular dichroism and electronic absorption spectroscopy, it was concluded 55 that the phthalocyanine ligand in the [PcFeII(CN)2]27 complex behaves as a s-donor and a p-acceptor and the result of oxidation of the iron ion yielding [PcFeIII(CN)2]7 is that the phthalocyanine ligand behaves as a s- and p-donor.Of special note is the PcFeIII(NO)Py complex, which was prepared by a ligand exchange reaction. Based on the data from magnetic susceptibility and MoÈ ssbauer spectroscopy, this complex should be assigned to low-spin (S=1/2) iron(III) compounds. For this complex, two isomeric forms that differ in the site of localisation of the electron hole are possible: PcFeIII(NO)Py.97 PcFeII(NO*)Py Polymeric iron(III) complexes linked through bridging [PcFeIII(CN)]n, ligands, [2,3-NcFeIII(CN)]n viz., and [PcFeIII(SCN)]n, were synthesised 110, 112 analogously to poly- meric iron(II) complexes.The resulting polymers are characterised by high thermal stabilities (up to 250 8C) and rather high conductivities at room temperature (1075 ± 1073 S cm71). The octaphenyl complex (Bu4N)[TAPPhFeIII(CN)2] 122 is the only tetraazaporphin complex of this type characterised by IR and electronic absorption spectroscopy and elemental analysis. Oxidation of the Fe2+ ion in this complex to the Fe3+ ion, unlike oxidation of the corresponding phthalocyanine complexes, results in a hypsochromic rather than bathochromic shift of the Q band.3. `Acidic' complexes of iron phthalocyanine and its analogues Barrett and coworkers 17 were the first to prepare the so-called `chlorine-containing' PcFe complex by heating PcFe with con- centrated hydrochloric acid. This compound was also synthesised by the reaction of the PcFe m-oxo dimer,6, 56, 135 PcFe(en)2 com- plexes,50 where en is ethylenediamine, or the PNP[PcFeIII(OH)2] complex 89, 129 with hydrochloric acid as well as in a direct reaction, for example, upon boiling of a mixture of anhydrous FeCl3 with phthalonitrile in 1-chloronaphthalene.89 Originally, a structure with the chlorine atom at the FeIII atom, viz., PcFeIIICl (1), was assigned to the resulting com- pound.17 However, Lever 53 gave preference to the hydrochloride structure PcFeII .HCl (2), because an attempt to sublime the resulting complex in vacuo resulted in the formation of PcFe and elimination of hydrogen chloride. Later, Lever, on the basis of magnetic susceptibility data, returned to the initial hypothesis that the complex has the structure of `chlorine-containing' iron(III) phthalocyanine.52 Since then, the above-considered notions of the structures of these compound and analogous five-coordinate complexes with axial acido ligands X co-exist.Thus on the basis of spectral data alone (Table 11), `acidic' complexes of iron phthalocyanine were assigned to iron(III) derivatives with the mixed-spin (S=5/2 ± 3/2) electronic state (structure 1).89, 129 Based on the chemical properties and spectral data, these com- plexes were alternatively considered as iron(II) derivatives with intermediate (S=1) spin (structure 2),6, 7, 56, 136 ± 138 and it was Table 11. Spectral characteristics of `acidic' complexes of iron phthalocyanine of the HPcFeX type.56, 129 X Positions of bands in electronic absorption spectra /nm d (4.2 K) /mm s71 832, 758, 710, 655, 615, 600, 465, 368 832, 758, 655, 595, 485, 450 840, 710, 658, 590, 450 848, 710, 690, 595, 465 0.28 0.28 0.28 0.28 0.29 FCl Br ICF3CO2 CCl3CO2 HCO2 CF3SO3 Ts believed that the proton is most likely localised on the pyrrolenine nitrogen atom.6, 138 The electronic absorption spectra of the HPcFeX complexes are characterised by the presence of a charge-transfer band (at *820 ± 860 nm) in the near IR region along with an intense Q band in the long-wavelength region (at *660 nm). The position of this charge-transfer band depends on the nature of the ligand X (see Fig.1, Table 11).56, 89 The presence of the low-energy charge- transfer band in the near IR region is easily explained in the framework of both structures described above. The position of the Q band in the spectra of HPcFeX virtually coincides with those in the electronic absorption spectra of the PcFeIIL2 (see Refs 47 and 68 ± 77) and PcFeIIL(DMSO) 79 ± 84 complexes and differs sub- stantially from the position of the Q band in the electronic absorption spectra of the bisaxially coordinated iron(III) com- plexes [PcFeIIIX2]7 (see Table 10).89, 129 In theMoÈ ssbauer spectra (see Table 11) of the `acidic' complexes, the DEQ values are very large, which is untypical of both high- and low-spin iron(III) complexes with macrocyclic ligands.86, 99, 100, 103, 139 However, these values correlate well with the hypothesis that the iron ions in these complexes exist in an intermediate spin state (S=3/2).99, 100, 103 The only principal fact, which is inconsistent with this suggestion, is that the asymmetry of the doublet, which is characteristic of iron(III) complexes, is absent in the MoÈ ssbauer spectra.100, 133 At the same time, the parameters of the MoÈ ssbauer spectra of the `acidic' complexes are similar to those of the iron(II) complexes containing macrocyclic ligands and the iron ion in the intermediate spin state (S=1).99, 100, 103 The electronic absorption spectra and magnetic susceptibility data of the `acidic' complexes are avaialble only for unsubstituted phthalocyanines (see Table 11).89, 129 Unlike the low-spin [PcFeIIIX2]7 complexes whose electronic absorption spectra are typical of the d 5 electronic configuration (see Table 10),129 most of the `acidic' complexes either give broad lines or do not give signals at all.The spectra of solid samples differ from each other (see Table 11).Moreover, even the complexes with the same ligand X gave different spectra.129 At the same time, the electronic absorption spectra of the analogous high-spin porphyrin complexes 100 and the corre- sponding porphyrazine complexes containing iron in the inter- mediate spin state (which are considered below) both in frozen matrices and in the solid state are identical and typical of axial iron(III) complexes. However, if the structure 2 is correct, the electronic absorption spectra should not be observed due to the rapid spin-lattice relaxation typical of paramagnetic iron(II) complexes.132 Based on the data from magnetic susceptibility (see Table 11), the `acidic complexes' were assigned to compounds in which the central iron ions are in the mixed-spin (S=3/2 ± 5/2) electronic state 89, 129 because the magnetic susceptibilities differ from the spin-only values (S=3/2 mB=3.87; S= 5/2 mB=5.92).However, the experimental values are close to those for iron(II) complexes with intermediate spin in the case of strong coupling between ground state spin and orbital angular momenta.51 ± 53 For example, the magnetic susceptibilities of the meff /mB Factor g in ESR spectra DEQ (4.2 K) /mm s71 4.2K 300K 3.25 3.2 2.32 3.39 3.06 2.94 3.12 3.23 3.08 3.07 4.67, 2.0 broad lines 7signal was not observed 2.0 *6.3,*4.3 signal was not observed the same 4.53 4.09 3.62 4.08 3.90 4.09 4.14 4.17Synthesis, structure and properties of coordination compounds of iron phthalocyanines and their analogues Table 12.Spectral characteristics of the TAPRFeX complexes (R1=R2=Et, EtS or Ph).140, 142 ± 144 REt Et Et Et Et Ph Ph Ph Ph Ph EtS Et 4.3, 2.36;b 2.17, 1.96;b 3.92, 1.96 b 1.99 a Temperature at which the MoÈ ssbauer spectra were recorded; b data from different studies. PcFe complexes, iron octaethylporphyrazine and iron tetraphe- nylporphin at room temperature (3.85,51 ± 53 3.82 57 and 4.4 mB,139 respectively) are close to the meff values for the `acidic' complexes (see Table 11). The chemical behaviour of the `acidic' complexes, viz., the elimination of HCl and the formation of PcFe upon sublimation of the `chlorine-containing' PcFe complex,53 the absence of products of radical nature and hydrogen in the reaction of PcFe with HCl,6, 56 the formation of `acidic' complexes from the m-oxo iron(II) dimer (the form 690) 56, 135 and the absence of redox-conversion products, indicate that the iron ions in these complexes exist, most likely, in the divalent rather than trivalent state.Moreover, the reactions of the m-oxo PcFe(III) dimer (the form 630) with acids afforded unstable products whose electronic absorption spectra are quite different from those of the `acidic' complexes.7 Therefore, taking into account all the above-considered facts, the structure 2 seems to be preferable. However, the unambigous elucidation of the structures of `acidic' complexes of readily soluble PcFe derivatives calls for further investigations of these compounds both by physical (primarily, electronic absorption spectroscopy and magnetic susceptibility studies) and chemical methods. In the series of `acidic' porphyrazine iron complexes, octaethyl-,57, 140 octaphenyl- 141, 142 and octa(ethylthio)porphyr- azine 143 derivatives were studied.These complexes were prepared either by the reactions of the corresponding m-oxo dimers 57, 141 ± 143 or anionic dihydro complexes 143 with acids or by ligand exchange reactions.140 Based on the data from magnetic susceptibilities and on the parameters of the MoÈ ssbauer and electronic absorption spectra in frozen matrices (Table 12), the octaethyl- and octaphenylporphyrazine complexes were assigned to iron(III) derivatives with intermediate (S=3/2) spin.This assignment is consistent with the data from X-ray diffraction analysis (see Table 12).57, 140 However, the chemical properties and the electronic absorption spectra of `acidic' complexes of tert- butyl-substituted analogues 6 can be better explained on the basis of the structure 2. Therefore, we believe that final conclusions cannot be made. X Ta /K DEQ d/mm s71 Factor g /mm s71 in ESR spectra Positions of bands in electronic absorption spectra /nm 318, 364, 436, 555, 672 Cl 2.98 3.0 4.38 4.02 0.28 0.37 0.35 0.3 298 80 300 300 ClO4 PF6 SbF6 312, 375, 497, 582, 711 312, 372, 505, 558, 687 322, 372, 509, 588, 722 CF3CO2 310, 371, 445, 566, 684 665, 533, 435, 318 F 709, 544, 431, 335 Cl 716, 548, 432, 329 Br 723, 561, 444, 328 I 721, 563, 440, 329 HSO4 3.36 3.0 2.99 2.97 2.15 2.85 2.75 3.04 3.21 3.39 3.25 3.58 0.23 0.2 0.28 0.29 0.23 0.26 0.19 0.25 0.19 0.24 0.16 0.28 300 300 140 82 300 82 300 82 300 82 300 82 Cl 1.95 0.21 100 590, 350, 316 NO 339 meff /mB Data of X-ray diffraction analysis (bond lengths /A) Fe7X Fe7N4 Fe7N 1.929 0.352 2.278 3.89 3.98, 1.99 1.913 0.263 2.091 3.87 3.44 ± 3.94 3.55 ± 3.79 3.80 ± 4.06 3.98, 1.99 4.05, 2.04 4.07, 2.01 3.55, 2.01 3.44 2.98; 4.66 b 1.936 0.286 2.308 1.721 0.308 1.931 An unusual coordination about the central atom was observed in the `acidic' complex of octa(ethylthio)porphyrazine.143 Accord- ing to the data from X-ray diffraction analysis, the coordination sphere about the central atom is formed by four nitrogen atoms of the macrocyclic ligand, the chloride ion and the sulfur atom of the peripheral substituent of the adjacent molecule.The different behaviour of this complex in the polycrystalline state and in solution was attributed to the differences in the electronic struc- ture of these two states.143 Thus it was suggested that the solid complex exists in the mixed-spin (S=5/2 ± 1/2) state, whereas this complex in solution was described as in an intermediate-spin state (S=3/2).143 Recently, the spectroscopic parameters of the TAPEtFe(NO) complex, which has been synthesised by reductive nitrosation of the `acidic' TAPEtFe complex in the presence of 2,6-dimethylpyr- idine, were reported (see Table 12).144 These data made it possible to assign the complex under consideration to five-coordinate iron(II) derivatives with an angular structure of the nitrosyl ligand.IV. m-Oxo(-nitrido, -carbido) binuclear complexes of iron phthalocyanine and its analogues In 1965, the reversible addition of oxygen to iron(II) tetrasulfoph- thalocyanine (TSPcFeII) in aqueous solution yielding a m-peroxo dimeric complex was reported.145 TSPcFeIIOOTSPcFeII. 2TSPcFeII+O2 However, the structure TSPcFeIIIX was assigned to the product of the reaction of TSPcFeII (lmax=670 nm) with oxygen (lmax=630 nm), although attempts to isolate and characterise this product have failed.146, 147 In 1971, specimens of PcFe synthesised by different procedures were studied by MoÈ ssbauer spectroscopy and it was found that the spectra of the products obtained upon treatment of PcFe with 2-picoline in air and upon heating of PcFe in air (d=0.29 and 0.348 mm s71, DEQ=0.36 and 1.13 mm s71, respectively) differ substantially from the spectra of the initially freshly prepared PcFe (d=0.385 mm s71, DEQ=2.58 mm s71).98 In the following, we will demonstrateV N Nemykin, I N Tret'yakova, S V Volkov, V D Li, N G Mekhryakova, O L Kaliya, E A Luk'yanets 340 that the first two complexes described as `other forms of PcFe' 98 are actually m-oxo dimeric iron complexes.More recently, in attempting to isolate PctFe by chromatog- raphy, Bundina et al.56 obtained an oxygen-containing compound as the major product. Its molecular weight was approximately twice as large as that of PctFe and its spectral characteristics differed significantly from those reported for PcFe and its substituted analogues. Based on the spectral data and the chemical properties of chromatographically pure specimens, it was sug- gested that the compound under consideration had the structure of the m-oxo dimer with an Fe7O7Fe bridge analogous to the structures of the well-studied m-oxo dimeric porphyrin iron complexes.148 ± 151 In the first study,56 the authors considered two alternative structures for the m-oxo dimeric PcFe complexes, viz., PcFeIIIOFeIIIPc (3) and HPcFeIIOFeIIPcH (4), which differ in the oxidation state of the iron atom. Based on the data from elemental analysis and IR, X-ray photoelectron and electronic absorption spectroscopy as well as on the chemical behaviour of this complex, preference was given to the structure 4.It was suggested that the hydrogen atoms were located at the pyrrolenine nitrogen atoms of the phthalocyanine ligands.56, 152 other organic solvents;56, 154, 155 (2) by short-term boiling of a suspension of PcFe with sterically hindered aliphatic amines 157 or a-pyridines 98 in the medium of the amine or in organic solvents; (3) the solid-phase reaction of PcFe with oxygen with heating;98 or (4) dimerisation of monomeric bisaxially coordinated PcFe com- plexes.89, 156 The IR spectra of the m-oxo dimeric PcFe complex have bands at 851 and 822 cm71, which were assigned to vibrations of the Fe7O7Fe fragment.56, 135, 155 ± 157 m-Oxo dimeric PcRFe complexes were prepared by chromatography of the PcRFe complexes (using eluents which did not contain potential ligands, such as organic bases or acids) 56, 135, 152, 163 or according to one of the above-mentioned procedures for the synthesis of the m-oxo dimer of PcFe.In the electronic absorption spectra of m-oxo dimeric PcFe complexes in noncoordinating solvents, the Q and B bands are bathochromically shifted compared to their positions in the spectra of other forms of PcFe (Table 13). This behaviour is untypical of m-oxo dimeric phthalocyanine complexes of other metals.62 TheQ band is observed at 700 nm.In all cases, this band is broadened and has a short-wave shoulder.6, 7, 56, 135, 152 To explain the bathochromic shift on the basis of the theory of exciton splitting, it was assumed that the phthalocyanine ligands in these complexes are nonparallel or are shifted relative to each other (slipped-stack),128 which is untypical of the m-oxo dimeric iron(III) complexes.148, 149 In 1979, a stable dimeric complex was prepared by the reaction of PcFe with oxygen in DMSO or DMF, and the structure of the m-peroxo dimer PcFeIIIOOFeIIIPc was assigned to the resulting compound.153 However, thereafter this suggestion was rejected 154, 155 based on spectroscopic data, in particular, on the IR spectra of the complex containing the 18O isotope in the m-bridge, and the m-oxo dimeric structure was proposed. Taking into account the results of volumetric analysis of oxygen absorp- tion by the PcFe complex, the structure 3 with the central iron(III) ions was assigned to the m-oxo dimer.This point of view has received support from many researchers.135, 156 ± 162 At present, this view predominates in spite of the fact that it has recently come under strong criticism.7, 128, 163 Although m-oxo dimeric complexes belong to one of the most stable coordination forms of PcFe, their electronic structures are still questionable. In this connection, let us consider the properties of these complexes in more detail.The unusual geometry of the m-oxo dimeric PcFe complexes was confirmed by 1H and 13C NMR spectroscopy. As was noted in a number of studies,6, 7, 56, 128 the spectra of these compounds are more complicated than those of other PcFe forms containing the m-oxo fragment (see below) and are similar to those of m-oxo dimeric silicon phthalocyanines.164 The spectral patterns indicate that the protons of the peripheral substituents are substantially nonequivalent. This nonequivalence can be explained only on the assumption that the geometry of the m-oxo dimeric PcFe com- plexes differs substantially from that typical of other m-oxo dimeric iron complexes with macrocyclic ligands (the Fe7O7Fe angle is close to 180 8 and the macrocycles are parallel to each other 148, 149).The m-oxo dimeric PcFe complex can be prepared according to the following procedures: (1) by bubbling air or oxygen through a solution of PcFe in sulfuric acid, as well as in DMSO, DMF or m-Oxo dimeric PcFe complexes in the solid state, unlike those in solution, exist as two isomeric forms possessing different Table 13. MoÈ ssbauer and electronic absorption spectra of m-oxo dimeric PcFe complexes (form 690). R1 Ta /K R4 R3 R2 d /mm s71 DEQ /mm s71 Positions of bands in electronic absorption spectra b /nm 77 H H H HH But82 H H 82 H H Et H 100 H H H EtOCH2 82 H H H CO2 Et 82 H H H CO2 C6H13-n H 82 H H OCH2CH(Et)Bun 82 82 HH HH HH OCH2But OCH2C(Me)2Bn 82 H OC8H17-n H OC8 H17-n 0.44 1.26 1.39 0.39 1.35 0.45 1.34 0.43 1.35 0.41 1.35 0.53 1.33 0.45 0.41 1.34 0.41 1.34 0.52 0.52 82 H H OCH2CH(Et)Bun OCH2CH(Et)Bun 0.36 0.25 0.22 0.33 0.22 0.35 0.22 0.35 0.22 0.33 0.22 0.33 0.22 0.36 0.35 0.22 0.36 0.22 0.34 0.36 a Temperature at which the MoÈ ssbauer spectra were recorded; b toluene as the solvent.Ref. 156, 157 56, 128, 135 363, 590, 687, 704 135 363, 588, 687, 703 135 290, 358, 600, 692, 704 135 345, 586, 690, 710 135 285, 350, 586, 697, 710 135 300, 370, 410, 610, 715 135 135 295, 368, 400, 610, 711 289, 367, 593, 697, 709 300, 360, 425, 600, 670, 714 135 300, 360, 425, 600, 670, 714 135Synthesis, structure and properties of coordination compounds of iron phthalocyanines and their analogues properties.135, 156, 157 Dimerisation of the monomer PNP[PcFeIII(OH)2] afforded predominantly the so-called oxo(1) isomer of the m-oxo dimeric PcFe complex, while bubbling of air through an oxo(1) solution in sulfuric acid or short-term heating of PcFe with sterically hindered aliphatic amines or a-substituted pyridines yielded predominantly the oxo(2) isomer.89, 129, 156, 157 According to the data from MoÈ ssbauer spectroscopy (see Table 13), chromatographically pure specimens of readily soluble m-oxo dimeric PcRFe complexes occur as mixtures of the oxo(1) and oxo(2) isomers, which have not been isolated in the individual state.56, 128, 135, 152 The parameters of the MoÈ ssbauer spectra of the m-oxo dimeric PcRFe complexes depend only slightly on the nature of the peripheral substituents; however, the electric field gradients have negative and positive signs for the oxo(1) and oxo(2) isomers, respectively.This fact provides additional evidence for the assumption as to the difference in the geometry of the isomers.165 In addition, the MoÈ ssbauer spectra of the m-oxo dimeric PctFe complex in frozen matrices (for example, in toluene) at 77 K have only one signal belonging to the oxo(1) isomer.6 The proponents of the hypothesis for the trivalent state of the central iron atoms believe that the oxo(1) isomer has an angular geometry, whereas the Fe7O7Fe fragment in the oxo(2) isomer is quasi-linear and both isomers are high-spin iron(III) complexes [in the oxo(1) and oxo(2) isomers, S=5/2 for both Fe atoms].157 The supporters of the concept of the divalent state suggest that the phthalocyanine ligands in the oxo(1) isomer are, most probably, in the slipped- stack orientation, whereas an angular geometry of the ligands is typical of the oxo(2) isomer.Both iron(II) ions of the latter possess intermediate spin (S=1).128, 163 In our opinion, the results obtained in magnetic susceptibility studies and the ESR spectra of the m-oxo dimeric PcFe oxo(1) and oxo(2) complexes are less reliable.135, 155 ± 157 The magnetic susceptibilities of different speci- mens of the oxo(1) and oxo(2) isomers at room temperature vary from 2.19 to 2.23 mB and from 1.38 to 1.42 mB, respectively.The antiferromagnetic interaction constants J calculated from the magnetic susceptibilities are 7120 and 7195 cm71, respec- tively.156, 157 On the contrary, the results of measurements of the magnetic susceptibilities for ten chromatographically pure readily soluble specimens of the m-oxo dimeric PctFe complex, which have been synthesised independently,6, 56, 152 suggest the diamagnetism of this compound. The magnetic moments are no higher than 0.48 mB. Therefore, it should be admitted that specimens of the m- oxo dimeric PcFe complexes studied in Refs 135 and 155 ± 157 are very likely to contain substantial amounts of monomeric high- spin impurities, which introduce errors into the true values of magnetic susceptibility.This conclusion agrees well with the data from ESR and MoÈ ssbauer spectroscopy.156, 157, 163 The ESR spectra of powders of m-oxo dimeric PcFe complexes at the boiling temperatures of nitrogen or helium have signals with a g factor of *6, which is characteristic of monomeric high-spin iron(III) complexes. Some specimens give additional signals with a g factor of*4.3, characteristic of monomeric complexes of iron(III) in the intermediate (S=3/2) spin state.7, 132 The ESR spectrum of a powder of the m-oxo dimeric PctFe complex at 77 K has a set of lines with a g factor approximately equal to 2.135 Under analogous conditions, very weak signals with a g factor approximating 6 were additionally observed for this compound.163 It should be noted that all ESR data were obtained only for powdered samples.However, as has been mentioned many times,128, 166 the ESR spectra of the phthalocyanine complexes in the solid state should be considered with caution. Thus signals with g factors close to 2 were observed for many diamagnetic phthalocyanines in the solid phase.166 Moreover, the lack of reliable data of ESR spectroscopy, in particular, for compounds containing the Fe7O7Fe frag- ment, was emphasised in a recent review 149 devoted to the synthesis and properties of binuclear iron complexes. Actually, some researchers believed 163, 167 that solutions of the m-oxo dimeric PctFe complex are inactive for ESR spectroscopy both at room temperature and at 77 K. 341 The reaction of m-oxo dimers of PcFe with nitrogen-contain- ing compounds afforded m-oxo dimeric complexes in the first stage.According to the hypotheses for the structures of the initial m-oxo dimers, the structures LPcFeIIIOFeIIIPcL (5),168, 169 H2(LPcFeIIOFeIIPcL) or (LH)2[PcFeIIOFeIIPc] (6) 6, 7, 56, 128, 152 were assigned to the above-mentioned complexes. Complete X-ray diffraction analysis of one of the complexes of this type (L=MeIm) demonstrated that the phthalocyanine ligands in this compound are located parallel to each other.45 The Fe7N(MeIm) distance (2.039 A) virtually coincides with that in the PcFe(4- MeC5H4N)2 complex (2.04 A).44 The Fe7O7Fe angle is 178.9 8.45 The structure of the axially coordinated m-oxo dimeric PcFe complex exhibits two major distinctions from the structures of the well-characterised m-oxo dimeric complexes of iron por- phyrins without axial ligands:148 ± 151 (1) central iron atoms lie virtually in the plane of the phthalocyanine ligand, which is in marked contrast to the structures of the porphyrin analogues in which the high-spin iron(III) ions deviate from the plane of the ligand by 0.3 ± 0.5 A;100, 148, 149 (2) coordination polyhedra about the central atoms are tetragonal bipyramids, whereas the coordi- nation polyhedra in all m-oxo dimeric iron complexes with macro- cyclic ligands are tetragonal pyramids.148, 149 In studies 45, 168 in which the structure 5 was assigned to the axially coordinated m-dimeric PcFe complex, the first anomaly was attributed to the fact that this complex contains low-spin (S=1/2; S=1/2) rather than high-spin iron(III) ions.168 The unusual coordination poly- hedron was assigned to the `unique' properties of this class of compounds.45 However, it was noted 128 that the above-men- tioned anomalies can be readily explained assuming that the central iron atoms in the m-oxo dimeric PcFe complexes exist in oxidation state +2 (the structure 6).Their electronic absorption spectra are typical of m-oxo dimeric PcFe complexes with the parallel arrangement of the phthalocyanine ligands.62 Thus the Q band (lmax&627 nm) is shifted hypsochromically compared to the Q band of monomeric PcFe complexes and its position depends only slightly on the nature of the axial ligand. The data on magnetic susceptibility and the parameters of the MoÈ ssbauer spectra of these compounds are given in Table 14.Note once again that all specimens occur (according to the data from MoÈ ssbauer spectroscopy) as mixtures of different coordination forms of PcFe,45, 168, 169 and, consequently, magnetic susceptibil- ity data must be considered with caution. However, the parame- ters of the MoÈ ssbauer spectra most probably correspond to low- spin iron(II) complexes (see Tables 2 and 5) rather than to low- spin iron(III) complexes (see Table 10). The electric field gradient of the m-oxo dimeric PcFe complex, which is axially coordinated by 4-MeC5H4N, has a positive sign.134 Comparison of the MoÈ ssbauer spectra of the form 627 with the spectra of the oxo(1) and oxo(2) isomers suggests that the oxo(2) isomer and the axially coordinated m-oxo dimeric complex are identical in geometry.The chemical behaviour of the m-oxo dimeric PcFe complexes was studied in detail with the aim of determining the valence state of the central iron atoms.6, 7, 56, 128, 152 The reactions of the initial m-oxo dimeric form of both unsubstituted and substituted PcFe (the form 690) with nitrogen-containing heterocyclic com- Table 14. MoÈ ssbauer spectra of the form 627 of the iron phthalocyanine complexes.168 T /K L DEQ /mm s71 meff /mB d /mm s71 4-MeC5H4N 1.99 77 1.81 1.75 1.76 1.61 1.58 1.73 0.12 0.19 0.20 0.19 0.17 0.18 295 774.2 4.2 4.2 4.2 Pip MeIm Py 2.11 2.16 1.86V N Nemykin, I N Tret'yakova, S V Volkov, V D Li, N G Mekhryakova, O L Kaliya, E A Luk'yanets 342 pounds6, 7, 128, 152, 168, 169 or isocyanides 135 afforded complexes with composition PcFeIIL2 as the final products. On the assump- tion that the form 690 has the structure 3, this reaction should be a redox process; however, oxidation products have never been detected. Therefore, the structure 4 is more probable for the form 690.The reactions of the form 690 of the PctFe complex or its axially coordinated analogue (the form 627) with cyanide anions in organic solvents afforded the well-known Na2[PctFe(CN)2] complex.128, 167 The electronic absorption spec- tra for these reactions have distinct isosbestic points. Data on the oxidation of triphenylphosphine with the m-oxo dimer of PcFe (the form 690) under an inert atmosphere have been published:170 (PcFeIII)2O+PPh3+4 Py=2 PcFeIIPy2+Ph3PO. The formation of Ph3PO suggests the trivalent state of the central iron atoms in the initial compound.These results are in contradiction with those obtained in the study of the reaction of the form 690 with diphenylpicrylhydrazine under oxygen-free conditions.6 Thus it was found that the oxidation of diphenylpi- crylhydrazine under strictly oxygen-free conditions did not occur at all. However, when a controlled amount of oxygen was introduced into a reactor, oxidation catalysed by the metal complex occurred. Moreover, it was demonstrated that the reaction of the form 690 with quinoline (which apparently con- tained an impurity of isoquinoline) yielded water (in an equimolar amount with respect to the initial form 690).6, 152 H2(PcFe)2O+4L=2 PcFeL2+H2O This behaviour is consistent with the structure 4 for the form 690.However, it was mentioned 154, 162 that when m-oxo dimeric PcFe complexes were prepared by the reactions with O2, the oxygen was consumed in a ratio O2 : PcFe=1 : 4, and the only reaction product, viz., the m-oxo dimer, was formed in quantitative yield. This fact contradicts the structure [HPcRFe]2O (4), which can be formed from PcRFe and O2 if a source of hydrogen atoms exists. With the aim of refining the structure of the form 690, the reaction of PctFe with oxygen in noncoordinating solvents (ben- zene and cyclohexane) was studied.It was found that: (1) the amount of oxygen consumed per mole of PctFe introduced into the reaction depends on the concentration of the latter, and the O2 : PctFe ratio changes from 1 : 5.0 to 1 : 2.7 in the concentration range of 6.361073 ± 1.161071 mol litre71; (2) the m-oxo dimer is the major but not the only reaction product; the yield of this dimer varies from 78% to 95%. It was found that the reagents were consumed in side reactions, including oxidative destruction of the macrocycle by oxygen, which is strongly activated due to coordination in the complex with PctFe. Therefore, side oxidation reactions of the phthalocyanine macrocycle, substituents in this ring or other substrates can serve as a source of protons for the formation of the form 690.The formation of the form 690 from the iron(III) derivative, viz., [PcFe(OH)2]7,128, 156 also calls for explanation. In our opinion, this process is accompanied by reduction of the iron atom with the hydroxide anion, which is one of the strongest reducing agents for PcFe complexes.171 Oxidation of the form 690 was examined for the first time in the study.7 The m-oxo dimeric PctFe complex (hereinafter, the form 630), which was prepared by the reactions of the form 690 with iodosobenzene or peracetic acid, was characterised by electronic absorption spectroscopy (lmax=630 nm) and MoÈ ss- bauer spectroscopy (d=0.58 mm s71, DEQ=0.34 mm s71 at 80 K). Later on, a procedure for the preparation of this com- pound was improved.128 Thus this complex was prepared in virtually quantitative yield by the reaction of the form 690 with cumene hydroperoxide generated in situ. These authors were also the first to analyse theNMRspectra of the form 630. The reaction of the form 630 with cyanide anions afforded the known low-spin iron(III) complex, viz., Na[PctFe(CN)2],128 which can be also prepared by oxidation of the cyanide complex Na2[PctFe(CN)2] with bromine 55 or cumene hydroperoxide.128 The reaction of the form 630 with hydrogen chloride yielded derivatives of iron(II) and (formally) iron(IV), viz., hydrochloride PctFe .HCl and the radical cation Pc+.FeCl2, respectively.7 All the aforesaid provides evidence for the trivalent state of the central iron atoms in the form 630.Therefore, on the basis of the spectral data and the chemical properties of the m-oxo dimeric PcFe complexes, the forms 690 and 627 were characterised as PcFe(II) derivatives and the form 630 was characterised as the m-oxo dimer of iron(III) phthalocyanine.6, 7, 56, 128, 152, 163 The reactions of PcFe with azides afforded m-nitride PcFe complexes of the general formula PcFe+3.5NFe+3.5Pc (Table 15).172 ± 174 These compounds can be oxidised electro- chemically or chemically to form complexes with composition (PcFe+4NFe+4Pc)+X7 (see Table 15).172 The structure of one of the resulting compounds, viz., [(PcFe)2N]Br, was established by X-ray diffraction analysis.175 Table 15. Parameters of MoÈ ssbauer spectra of the m-nitrido and m-carbido dimeric PcRFe complexes.172, 173 Complex DEQ /mm s71 d /mm s71 (PcFe)2N (PcFe)2NPF6 (PcFe)2C [(4-MeC5H4N)PcFe]2C 0.06 0.10 70.16 0.03 0.12 0.01 0.01 0.01 1.76 2.06 2.69 1.19 0.25 1.16 1.11 0.94 1.76 1.76 1.73 1.52 (PyPcFe)2C (PipPcFe)2C [(MeIm)PcFe]2C [(4-MeC6H4N)PcFe]2NPF6 (PyPcFe)2NPF6 (PipPcFe)2NPF6 [(MeIm)PcFe)2NPF6 70.10 70.09 70.09 70.09 Note.The spectra were recorded at 77 K relative to metallic iron. The reaction of PcFe with tetraiodomethane in the presence of reducing agents, such as sodium dithionite or sodium borohy- dride, afforded m-carbide PcFe complexes of the general formula PcFeIVCFeIVPc (see Table 15).176 ± 179 This synthetic procedure has been improved quite recently, viz., the reaction of [PcFeIII(OH)2]7 with dichlorocarbene generated in situ was used.171, 180 Unfortunately, the poor quality of crystals of the [(MeIm)PcFe]2Ccomplex did not allow the researchers to perform a complete structure analysis of this complex.178 However, the values of selected bond lengths and bond angles were determined.In particular, it was demonstrated that two phthalocyanine ligands in the complex are virtually parallel to each other (the Fe7C7Fe angle is 178 8). Heterobimetallic oxo dimeric complexes of the PCrOFePc type, where P is the dianion of a-substituted porphyrin, were synthesised and their properties were studied.181 These com- pounds were prepared by the reactions of PCrIV=O with PcFe.The synthesis, structures and properties of the m-nitride hetero- dimer (TPP)Fe+3.5NFe+3.5Pc, where TPP is tetraphenylporphin, as well as of its oxidised forms were described.182 Among m-oxo dimeric complexes of iron porphyrazines, derivatives of tetra-tert-butyl-,6 octaphenyl-,141 octaethyl- 57 and octa(ethylthio)-substituted 143 porphyrazines are presently avail- able. All these compounds were characterised by electronic, IR and NMR spectra. The reaction of the pyridine adduct of iron octaphenylporphyrazine with perchloric acid followed by the addition of HN3 afforded the corresponding m-nitride dimeric complex, which was characterised by electronic absorption, IR, MoÈ ssbauer and ESR spectra.183 The pyridine monoadduct of thisSynthesis, structure and properties of coordination compounds of iron phthalocyanines and their analogues complex formed in frozen matrices adds reversibly one oxygen molecule per molecule of the initial complex.Recently, the synthesis and selected properties of the hetero- binuclear complex TAPPhMn=NFeTAPPh were reported.184 However, the data from electronic absorption spectroscopy and mass spectrometry (primarily, the absence of the molecular ion and the TAPPhFeN fragment) and the lack of MoÈ ssbauer spectra require that the suggested structure be considered with particular caution. V. Some features of coordination chemistry of substituted iron phthalocyanines and their analogues The interconversions of the coordination forms of iron phthalo- cyanines are shown in Scheme 2.As expected, the peripheral substituents in the macrocycle often break down these general regularities. Thus when bulky substituents (for example, the 2,4,6- trimethylphenyl substituents) were introduced at the a positions of the macrocycle, the m-oxo dimeric Fe(II) and Fe(III) complexes were not formed at all due to steric hindrances caused by the substituents.7, 8 In this case (under conditions of formation of m- oxo dimeric complexes), the corresponding monomeric hydroxo complexes HPcRFe(OH) can be isolated. In an alkaline medium, the latter are converted into the corresponding oxoferryl deriva- tives, which undergo polymerisation upon prolonged storage.7, 8 Scheme 2 [H] L H2[LPcFeII]2O [HPcFeII]2O [PcFeIII]2O [O] H2O H2O in vacuo H2O L L PcFeIIL2 PcFeII in vacuo in vacuo HX L HX L HLPcFeIIX HPcFeIIX HX The effect of the electronic properties of substituents on the behaviour of iron phthalocyanines is generally manifested only in the relative stabilities of particular forms and in the positions of the bands in the electronic absorption spectra.The qualitative characteristic features of this effect can be observed only in special cases. For example, due to the presence of eight electron-with- drawing substituents, iron (3,5)-octakisnitrophthalocyanine 7, 8 reacts with nitrogen bases to give initially the PcFeIIL2 complex, which is converted into the isoelectronic form Pc37.FeIIIL2 containing the stable phthalocyanine radical anion.Due to the presence of four SPh substituents in iron tetrakis- (phenylthio)phthalocyanine, iron ion has the the 3A2g[(dxy)2(dxz,dyz)2(dz2)2] electronic configuration 27 rather than the 3Eg[(dxy)2(dxz,dyz)3(dz2)1] configuration typical of PcRFe. When eight electron-donating alkoxy groups are introduced into PcFe at positions adjacent to the macrocycle (at positions 1 and 4), the central iron atom is initially subjected to electrochemical oxidation, whereas the phthalocyanine ligands are oxidised in the complexes containing other substituents in the macrocycle.109 VI. Major fields of application of iron phthalocyanines and their analogues High catalytic activity of PcFe in decomposition of peroxides and in oxidation of some classes of organic compounds had been discovered by Cook 185 ± 187 back in the late 1930s.The use of iron phthalocyanines in catalysis has been expanded substantially since these years.3, 188 Thus these compounds were used as homoge- 343 neous and heterogeneous catalysts of epoxidation of alkenes,188, 189 oxidation of hydrocarbons to alcohols, aldehydes, quinones and ketones 190 ± 194 and oxidation of arylamines,195 phosphines,162, 196 thiols 197 ± 199 and conjugated aromatic sys- tems.200 Electrodes modified with iron phthalocyanines and their analogues were used for electrooxidation of thiols,197 electro- reduction of molecular nitrogen to ammonia 201, 202 and electro- reduction of carbon dioxide,203 sulfur dioxide,204 nitrogen oxides 205 and molecular oxygen.206 ± 208 The chemo- and electrochromic properties of iron phthalo- cyanines and their analogues as well as variations in the conduct- ing properties of these complexes depending on the medium made it possible to use these compounds in sensors for halogens,209 nitrogen oxides,210 oxygen 211 and aldehydes.212 Photochemical reactions of PcFe are well studied, which allows the use of this compound for the preparation of photosensitive materials (including those operating in the near IR region 213), photo- conducting devices,214 photosensitive inks 215 and materials for optical data recording and storage 216, 217 as well as for the production of high-contrast photographic materials.218 Since axially coordinated complexes of iron phthalocyanines are readily available compounds, highly sensitive and selective membranes for gas separation 219 and food storage 220 were constructed on the basis of these complexes.The use of cathodes modified with PcFe and its analogues in lithium cells with aggressive electrolytes, such as thionyl chloride and sulfur dioxide, made it possible to improve substantially their discharge charac- teristics.221 ± 225 Intercalation of iron phthalocyanine into the structures of inorganic oxides leads to the appearance of magnetic properties in the resulting materials.226 In the reactions with reducing agents, such as ascorbic acid, catalysed by iron phthalocyanines, different so-called active oxy- gen forms (primarily, oxygen-centred radicals) are generated.This made it possible to initiate studies of complexes of iron phthalo- cyanines and their conjugates with proteins, DNA and RNA as potential drugs in catalytic (dark) therapy of cancer.21, 227, 228 In addition, high antihistamine and antiserotonin activities of PcFe derivatives were observed in a number of studies.5, 229 The use of these compounds as deodorising agents is based on their high oxidase activity.230 Semiconducting properties of iron phthalo- cyanines and their analogues have received little use.9, 13 Selective axial coordination of iron phthalocyanine to nitrogen-containing compounds allows one to solve a number of analytical problems by NMR spectroscopy.105 ± 108 This review was financially supported (in part) by JSPS (Grant No.P98418). 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ISSN:0036-021X    

出版商:RSC    

年代:2000

数据来源: RSC

摘要:

Russian Chemical Reviews 69 (4) 347 ± 366 (2000) Fractal analysis of macromolecules V U Novikov, G V Kozlov Contents I. Introduction II. Modelling of macromolecules III. Fractal characteristics of macromolecules IV. Conclusion Abstract. the macromolecules, of forms fractal the on Data Data on the fractal forms of macromolecules, the existence thermodynamic by predetermined is which of existence of which is predetermined by thermodynamic nonequli- nonequli- brium are order, deterministic of presence the by and brium and by the presence of deterministic order, are considered. considered. The fractal polymer of concept the of limitations The limitations of the concept of polymer fractal (macromolecular (macromolecular coil), of the Vilgis concept, and of the possibility of modelling in coil), of the Vilgis concept, and of the possibility of modelling in terms diffusion-limited and theory percolation the of terms of the percolation theory and diffusion-limited irreversible irreversible aggregation only not that noted is It discussed.are aggregation are discussed. It is noted that not only macromolec- macromolec- ular macromolecules of segments the also but coils ular coils but also the segments of macromolecules between between topological are entanglements) (cross-links, points fixing topological fixing points (cross-links, entanglements) are stochas- stochas- tic structure of model the by confirmed is this fractals; tic fractals; this is confirmed by the model of structure formation formation in 141 includes bibliography The polymer.network a in a network polymer. The bibliography includes 141 references. references. I. Introduction The theory of fractals and its application to physical and chemical processes has been vigorously developing in recent years.1 ±8 To facilitate the understanding of the results presented in this review, we shall introduce some notions and definitions and consider briefly the grounds for applying the principles of synergetics and fractal analysis to the description of structures and properties of polymers. Fractals are self-similar objects invariant with respect to local dilatations, i.e. objects that reproduce the same shape during observation at various magnifications. The concept of fractals as self-similar ensembles was introduced by Mandelbrot,1 who defined a fractal as a set for which the Hausdorff ± Besicovitch dimension always exceeds the topological dimension. The fractal dimension D of an object inserted in a d-dimensional Euclidean space varies from 1 to d.Fractal objects are the natural filling of the gap between known Euclidean objects with integer dimensions 0, 1, 2, 3... The majority of naturally existing objects proved to be fractal, which is the main reason for the vigorous development of the methods of fractal analysis. According to the Family classification,9 fractal objects can be divided into two main types, namely, deterministic and statistical objects. Deterministic fractals are self-similar objects which can be precisely constructed on the basis of several fundamental laws.V U Novikov Moscow State Open University, ul. P Korchagina 22/2, 129805 Moscow, Russian Federation. Fax (7-095) 283 80 71. E-mail: vknovik@cityline.ru G V Kozlov Kabardino-Balkarian State University, ul. Chernyshevskogo 173, 360004 Nal'chik, Russian Federation. Fax (7-866) 225 44 75. Tel. (7-866) 222 41 44 Received 12 November 1999 Uspekhi Khimii 69 (4) 378 ± 399 (2000); translated by Z P Bobkova #2000 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2000v069n04ABEH000523 347 350 356 364 Typical examples of these fractals are the Cantor set (`dust'), the Koch curve, the Sierpinski gasket, the Vicsek snowflake, etc. Two properties of deterministic fractals are most important, namely, the possibility of exact calculation of the fractal dimension and the infinite range of self-similarity (7?; +?).Since a line, a plane, or a volume can be divided into an infinite number of fragments in different ways, it is possible to construct an infinite number of deterministic fractals with different fractal dimensions. Therefore, deterministic fractals cannot be classified without introducing other parameters, apart from the fractal dimension. Statistical fractals are generated by disordered (random) processes. An element of disorder is typical of most real physical phenomena and objects. The fact that disorder, i.e. the absence of any spatial correlation, is a sufficient condition for the formation of fractals was first noted by Mandelbrot.1 A typical example of this type of fractal is the random-walk path.However, real physical systems are often inadequately described by purely statistical models. Among other reasons, this is due to the effect of excluded volume. The essence of this effect lies in the geometric restriction which forbids two different elements of a system to occupy the same volume in space. This restriction is to be taken into account in the corresponding modelling.10, 11 The best-known examples of this type of models are self-avoiding random walk, `lattice animals' and statistical percolation. In a definite range of scales, fractals have different topological structures depending on the maximum number of elements con- nected with the given element of the system. If each element can be connected to not more than two other elements, the resulting structure has no branching.By analogy with linear polymers, Family 9 has called these types of fractals linear fractals. If branching does occur, the resulting fractal has a framework structure; these types of fractals are referred to as branched fractals. The microstructure of polymers can possess a high degree of self-ordering of either natural or artificial origin;12 this represents one extreme case. The other extreme case is represented by chaos, which is the antithesis of order. Fractal analysis considers inter- mediate situations, i.e. those located between full order and full chaos. As a rule, these systems are obtained under conditions far from thermodynamic equilibrium; they fill the gap between periodic structures and fully disordered systems.13 In other words, fractal structures should possess a certain level of inter- mediate order.Therefore, when studying thermodynamically nonequilibrium solids (the class to which polymers normally belong 14) possessing a local order,15 it is important to consider the relationship between the level of local order and the degree of fractality.348 At present, it is beyond doubt that the approximation of a real polymer by a continuous medium is inadequate.16 Even during the synthesis of polymers, numerous micro-, meso- and macro-defects arise; during performance, they can be further developed. More- over, it has been found empirically that even those media that are initially homogeneous acquire upon deformation a hierarchical block structure, the characteristic dimensions of which Li obey, with rather high accuracy, the relation Li a Lia1=const, Li where Li is the automodelling coefficient, i = 0, 1, 2, ...3, 17, 18 An important characteristic of structural elements of a solid is Euclidean dimension d, which can take on the following values: d = 0 implies point defects, d = 1 corresponds to linear defects (dislocations), d=2 means planar defects (grain boundaries, twins, etc.), and d=3 describes three-dimensional structures. The Euclidean dimension can characterise highly ordered symmetrical microstructures which do not often arise even in materials produced under quasi-equilibrium conditions. Non- equilibrium systems produced under essentially nonequilibrium conditions, which represent a sort of casts of dynamic dissipative structures, cannot at all be adequately described in terms of classical methods of metallography or radiography.17 ¡¾ 21 This is even more so for solid (vitreous) polymers, which are, by defi- nition, thermodynamically nonequilibrium systems. Indeed, the atomic structure of nonequilibrium materials can possess a quasi- crystalline order, characterised by five-, seven-, thirteen- or higher-fold elements of symmetry 21 forbidden by the Brouwer theorem, which underlies the classical techniques of X-ray dif- fraction analysis.Moreover, numerous fractographical 4, 17, 21 and geophysical 18 studies have revealed fractality of the structures of many materials and essentially non-Euclidean geometry of the deformation and the fracture of solids.Therefore, the traditional methods of the mechanics of continua, which are based on the assumption of homeomorphism of deformations, regarded as imaging of the body being deformed to the Euclidean space, are unable to describe adequately the rheological behaviour and fracture of real materials, in particular, polymers. A significant breakthrough in tackling this problem is asso- ciated with the introduction into physicochemical analysis prac- tice of the ideas and methods of synergetics { and fractal analysis.3, 5, 6, 8, 20, 21 In particular, it was shown that chaotic systems (amorphous phases, fractured surfaces, etc.) formed under nonequilibrium conditions which are totally disordered at first glance, actually display unusual elements of order.2, 20 Whereas crystals possess a certain symmetry and translational invariance, nonequilibrium systems can possess chirality even if they have no quasicrystalline structure 3 and, what is the most important, these systems are scale invariant in a definite range of self-similarity.2, 3 Self-similarity is characterised quantitatively by the so-called Renyi dimension dq .Unlike the topological dimen- sion, it can be both an integer and a fraction 1 and is defined by the relations lim e? ? ia1 Pqi OeU, q 6a 1, lnPM O1 ¢§ qUlne dq= lim e? ? ia1 PiOeUlnPiOeU , q a 1, lne lnPM where M is the minimum number of d-meric boxes with side e needed to cover all the elements of a structure, Pi (e) is the { Synergetics is an interdisciplinary field of knowledge dealing with identification of the general features of the processes of formation, stability and destruction of ordered time and space patterns in complex nonequilibrium systems of diverse natures.V U Novikov, G V Kozlov probability that a point of the structure belongs to an ith element of the bulk coverage e d, and q is the parameter of transformation of the measure (`magnification parameter'). For Euclidean objects (smooth curves or regular lattices in a plane or in a bulk, etc.), the following identity holds:16 dq:d, where7?<q<?.In the case of regular mathematical fractals such as the Cantor set, the Koch curves and Sierpinski gaskets constructed by recurrent procedures, the Renyi dimension dq does not depend on q but16 dq = dH = const<d,7?<q<?, where dH is the Hausdorff ¡¾ Besicovitch dimension (Hausdorff dimension). According to the mathematical definition,4 the Haus- dorff dimension dH is a local characteristic of a set in the range of scales chosen, which can be covered by `spheres' not necessarily having identical diameters, provided that the diameter of each sphere is smaller than a chosen diameter. The fractal dimension of the structure of a macromolecule df can be determined physically in the range of scales in which the structure elements are self- similar, i.e.when they are fractals. Since the ranges of df and dH coincide, the df=dH is referred to as the fractal (Hausdorff) dimension of a macromolecule. Natural fractals such as clouds, polymers, aerogels, porous media, dendrites, colloidal aggregates, cracks, fractured surfaces of solids, etc., possess only statistical self-similarity, which, furthermore, takes place only in a restricted range of sizes in space.1 ¡¾ 4, 16 It has been shown experimentally for solid poly- mers 22 that this range is from several angstroms to several tens of angstroms. The relationship between the level of local order and the degree of fractality of disordered solids can be described using general mathematical terms.3 In particular, as regards the struc- ture of solid polymers, it is noteworthy that the majority of researchers take for granted that segment-size structural associ- ates are formed in polymers, although the particular type of packing in these associates is still debated.15, 23 It should be emphasised that the view on the amorphous state as being totally disordered is incorrect.According to the Ramsay theorem,3 any rather great number i>R(i, j ) of points or objects (in this particular case, structural elements) contain necessarily a highly ordered subsystem of Nj4R(i, j ) elements. Thus, no totally disordered systems (structures) exist. It can be shown in a similar way that any structure consisting of N elements, where Nj>BN( j ), is a set comprising a finite number k4j of self- similar structures embedded into one another and having, in the general case, different Hausdorff dimensions.This means that, irrespective of its physical nature, any system consisting of a relatively great number of elements is a multifractal (in a special case, monofractal) characterised by a spectrum of Renyi dimen- sions dq, q =7?to ?. 3 The tendency of condensed systems to self-organisation into scale invariant multifractal forms is a consequence of fundamental principles of thermodynamics of open systems and dq values are determined by the competition between short- and long-range interatomic interactions, which stipulate the volume compressibility and the shear rigidity of solids, respectively.24 Since the physicochemical properties of polymers depend appreciably on the topology of the structure, the real polymer structures can be studied using the results of physical and mechanical measurements.25, 26 Yet another important property of fractals which distin- guishes them from traditional Euclidean objects is that at least three dimensions have to be determined, namely, d, the dimension of the enveloping Euclidean space, df, the fractal (Hausdorff) dimension, and ds, the spectral (fracton) dimension, which char- acterises the object connectivity.[For Euclidean spaces, d=df=ds ; this allows Euclidean objects to be regarded as a specific (`degenerate') case of fractal objects. Below we shall repeatedly encounter this statement.]27 This means that twoFractal analysis of macromolecules fractal dimensions, df and ds, are needed to describe the structure of a fractal object (for example, a polymer) even when the d value is fixed.This situation corresponds to the statement of non- equilibrium thermodynamics according to which at least two parameters of order are required to describe thermodynamically nonequilibrium solids (polymers), for which the Prigogine ± Defay criterion is not met.28, 29 In conclusion, we consider the relationship between the characteristics of percolation and fractal clusters. First of all, note that a percolation cluster near the percolation threshold is a fractal with a dimension of*2.5 (for d=3).7, 30, 31 This value can be easily derived from the known relationship between d and the critical percolation indices b and n.9, 30 The relationships between the critical percolation indices b, n and t and fractal dimension of a percolation cluster df have been reported 7 b à 1 , n à 2 , t à 4 . d d df f f The critical indices estimated from these relations fall into the admissible ranges of variation � b=0.39 ± 0.40, n=0.8 ± 0.9, and t=1.6 ± 1.8, determined in terms of the percolation model for three-dimensional systems.The researchers 7 noted that not only numerical values but also the meanings of these values coincide. Thus the index b characterises the chain structure of a percolation cluster. The 1/df value, which serves as the index of the first subset of the fractal percolation cluster in the model considered,7 also determines the chain structure of the cluster.The index n is related to the cellular texture of the percolation cluster. The 2/df index of the second subset of the fractal percolation cluster is also associated with the cellular structure. By analogy, the index t defines the large-cellular skeleton of the fractal percolation cluster.{ Thus, these critical indices are universal and significant for analysis of complex systems, the behaviour of which can be interpreted in terms of e percolation theory. It has been noted above that the structures and properties of polymers are studied using the principles of synergetics and methods of fractal analysis. This is based on several prerequisites.Firstly, amorphous glassy polymers have thermodynamically nonequilibrium structure.32 SchaÈ fer and Keefer (see Ref. 10, p. 62) showed that fractal structures are formed in nonequilibrium processes. Therefore, there is good reason to believe that there are fractal structures in glassy polymers and that they can be described using the methods of synergetics. These assumptions have been repeatedly confirmed by experiments.32 ± 39 For example, the low-frequency region of the spectra of inelastic light scattering of amorphous polymers is a broad structurless plateau.22 This is due to the fractal structure of polymers on small linear scales.22, 35 Fractal objects are characterised by the following relation between the mass M (or density r) and the linear scale of measurement L:3 (1) M(L)!Ldm, where dm is the mass scaling index.Unlike mathematical fractals, real fractals (including poly- mers) have two natural scale lengths, Lmin and Lmax (Fig. 1); for lengths outside this range, the object is not a fractal.3 The lower limit Lmin is related to the finite size of structural elements, while the upper limit Lmax is associated with the tending to the limit of df. It has been found experimentally for polymers 22, 35, 40 that Lmin has an order of several angstroms and Lmax is about several tens of angstroms. It is noteworthy that the same range of existence of a local order is typical of the cluster model in which the size (length) { The relationship between the critical percolation indices and the fractal dimension of the percolation cluster for three-dimensional systems and examples of determination of these values for filled polymers are consid- ered in more detail in the book cited.7 349 ln r ln L Lmax Lmin Figure 1.Density r vs. the length scale L of the real fractal. The range Lmin ±Lmax is the region of existence of the fractal.3 of a statistic segment lst is the lower limit and the distance between the clusters Rcl is the upper limit. It often turns out in studies of polymers that strictly derived relations describe adequately the behaviour of rubbers (i.e., polymers at temperatures above the glass transition temperature Tg) but do not hold for the vitreous state. This is explained, first of all, by the sharp decrease in the mobility of chains below Tg or by `freezing' of the structure.41 Strictly speaking, there is no funda- mental difference between the molecular structure above and below Tg; in both cases, the structure is formed by long-chain macromolecules.They differ in the fact that the thermodynami- cally unstable state of the polymer existing below Tg is replaced by a quasi-equilibrium state above Tg . Within the framework of fractal analysis, this means that the polymer structure ceases to be a fractal at T5Tg and becomes Euclidean body (or, at least, a fairly close approximation). As noted by Flory,42 the ability to withstand severe deformations and restore entirely the initial characteristics when the stress has been relieved is a property exhibited, under appropriate conditions, by almost any polymer consisting of long-chain macromolecules.This feature is also manifested to some degree beyond the temperature range in which the `rubber elasticity' phenomenon is observed. In other words, the macromolecular `essence' of the polymer is more significant than the state in which the polymer occurs. Therefore, the use of the real (fractal) dimension of the object in the relations derived for rubbers 42 is expected to provide fairly accurate results when applied to vitreous polymers. Secondly, polymers are known to possess multilevel structures (molecular, topological, supermolecular, and floccular or block levels), the elements of which are interconnected.42, 43 In addition, an external action on a polymer can induce the formation of new (secondary) structural elements � cracks, fractured surfaces, plastic flow regions, etc.These primary and secondary structural elements as well as the processes forming them are characterised by miscellaneous parameters; therefore, only empirical correla- tions have been obtained, at best, between these parameters. If each of the above-mentioned elements (processes) is described by a standard parameter, for example, fractal dimension, one can derive analytical equations relating them to one another and containing no adjustable parameters. This is very significant for the computer synthesis of structure and for the prediction of properties and behaviour of polymeric materials during perform- ance.Note that fractal analysis has been employed successfully to describe the phenomena of rubber elasticity 16, 44, 45 and fluid- ity.25, 46 ± 48 Thirdly, legitimate application of these methods requires the use of a physically justified number of parameters describing the polymer structure. In this sense, the Euclidean and fractal objects are fundamentally different: the former require only one space dimension (Euclidean), whereas fractal objects (spaces) require not less than three dimensions. Yet another important aspect is the change in the fractal dimension of polymers when they are simulated on fractal rather than Euclidean lattices. This fact is also important from the350 practical standpoint for multicomponent polymer systems.The introduction of a dispersed filler into a polymer matrix results in structure `perturbation'; in terms of fractal analysis, this is expressed as an increase in the fractal dimension of this structure. As shown by Novikov et al.,25 the particles of a dispersed filler form in the polymer matrix a skeleton which possesses fractal (in the general case, multifractal) properties and has a fractal dimen- sion. Thus, the formation of the structure of the polymer matrix in a filled polymer takes place in a fractal rather than Euclidean space; this accounts for the structure modifications of the polymer matrix in composites. Proceeding from general statements, note that the term `multi- fractal' can be understood in two ways as applied to polymers.It is shown below that the fractal dimension of a chain section between points of chemical cross-linking (or between physical entangle- ments) D is determined by the molecular mass of this section Ms (Men) or by the cross-linking density ns (or density of the entanglement network nen). The Ms (Men) value has a definite distribution;27 hence, the dimensionDwould be characterised by a similar distribution.49 Since the parameter D is uniquely related to the fractal dimension df of the supermolecular structure, df would be also characterised by a certain distribution. In other words, we are dealing with the distribution of fractal dimensions or multi- fractals. In addition, all the primary and secondary structural elements mentioned above are also multifractals; therefore, a polymer can be represented as a combination of elements having different weights, i.e., again as a multifractal.Below we shall use averaged values of only the fractal dimensions of particular elements, at least as long as this approximation is useful for the understanding of the physical essence of the problem in question but does not decrease substantially the accuracy of estimates. Thus, the purpose of this review is to describe systematically the results obtained within the framework of fractal formalism and concerned with the studies of structure and properties of polymer macromolecules (linear, cross-linked, amorphous, etc.). II. Modelling of macromolecules 1.Fractal dimension The complexity of the polymer structure is reflected in the large number of dimensions needed to describe it. Alexander and Orbach 28 proposed the use of spectral or fracton dimension ds for the description of the density of states on a fractal. The necessity of introducing ds is due to the fact that the fractal dimension defined by Eqn (1) does not reflect this parameter. The investigators made use of the fact that anomalous diffusion of pticles is expected on a fractal and, hence, (2) hr2(t)i!t2/(2+d), where r(t) is the distance covered by a diffusing particle in time t and d is a scaling index for the diffusion constant. Having omitted the intermediate calculations, we present the final result obtained by Song and Roe 14 (3) ds= 2df 2 �¢ d .The spectral dimension ds is a true property of a fractal and is determined only by its connectivity. It differs from the mass scaling index [see Eqn (1)] or fractal dimension df and from the scaling index of the diffusion constant d by the fact that it does not depend on the way in which a fractal has been inserted into the Euclidean space with dimension d. The dependence of ds and d on df is described by Eqn (3). Alexander and Orbach 28 found that for a separate linear polymer chain, ds=1. In the case of a critical percolation cluster, the ds value does not depend on d and amounts to*4/3 (Table 1). The dimensions of a linear macromolecule in different states have been considered.50 The following states are known 50 ¡À 53 to be the most typical: 1Dcompact globule, 2Dcoil at theY-point, 3 D impermeable coil in a good solvent, 4 D permeable coil (the V U Novikov, G V Kozlov Table 1.Values of d, ds, df and d for percolation clusters.28 df d ds d0.80 1.55 2.71 3.30 4.00 1.9 2.5 3.3 3.8 4.0 1.36 1.42 1.39 1.44 4/3 2345?state typical of rigid-chain macromolecules), 5 D completely uncoiled rod-like macromolecule. The radius of gyration of a macromolecule Rg depends on the molecular mass M and on the geometry of distribution of the molecule in space and is characterised by the scaling index n (see Refs 32, 50). (4) Rg!Mn From Eqns (1) and (4), it follows that 25 (5) df=1n . Baranov et al.50 introduced the dimension dF, determined from the Flory formula 51 for good solvents (6) dF �� 3 ¡¦ 2nF .nF The dF value reflects the dimension of the sublattice in which the macromolecule is arranged, i.e., it is a fractal dimension of the medium in which the molecule is located rather than of the molecule itself; dF does not always coincide with the Euclidean dimension. The n, df and dF values of the three-dimensional space and the states of the macromolecule considered above are listed in Table 2.52 State 1 is encountered in globular proteins or on precipitation (below the Y-point) of linear macromolecules from highly dilute solutions.53, 54 Conformation 5 is typical of some proteins or linear chains exposed to particular types of external action; conformations 2 ¡À 4 are more trivial and fairly well studied.51 Analysis of the data of Table 2 leads to the conclusion that, for example, a macromolecular coil in the most frequently encountered states 2 ¡À 4 is always a fractal (at d=3).Table 2. Values of n, df, dF (for d=3) for various states of a macro- molecule.50 No. State of macromolecule dF df n Poor solvent 1/3 1/2 12 32 77 Globule Coil at the Y-point Good solvent 3/5 2/3 1 345 32.5 1 5/3 3/2 1 Coil Permeable coil Uncoiled chain 2. Polymer fractal Cates 55 introduced the notion `polymer fractal' by replacing the rigid bonds in a percolation cluster by flexible (phantom) links. This model can be used to describe the gelation process (Fig.2).56 Within the framework of this approach, Cates 55 described varia- tions of the structure and the dynamics of dilute solutions of polymers. This approach is based on the following main ideas. The replacement of rigid bonds in a percolation cluster by long phantom chains does not change the connectivity of a fractal (therefore, additional contacts between chains are regarded asFractal analysis of macromolecules a b x c x Figure 2. Scheme of the percolation of bonds during gelation:56 (a) pre-gel state, (b) gelation point, (c) post-gel state; x is the characteristic linear scale. imaginary) but does influence the dimension and the dynamic properties of the object. To describe the changes, at least three fractal dimensions are required.28 The dimension of the initial fractal (in the percolation lattice) is determined by the fractal dimension df0 using a mass scaling relation similar to (1).57 The subscript 0 corresponds to the initial fractal with rigid bonds.55 The dynamic properties can be studied using the spectral dimen- sion ds or (which is equivalent) using random walkers (the dimension dw) arranged on a fractal.The latter version implies a scaling law for the root-mean-square displacement r [see Eqn (2)] of a random walker on a fractal in a time t 56 (7) r dw0!t, where dw0 is the random-walk dimension at d=2. The case where dw0=2 corresponds to the classical Einstein diffusion. The r value is expressed in Euclidean distances; the process with dw0>2 is usually referred to as slow anomalous diffusion.The density of states is determined by the dimension ds (8) ds=2df0 . dw0 It has been noted above that ds is a true parameter of a fractal determined completely by the object connectivity. The replace- ment of rigid bonds by flexible links in a percolation cluster does not change ds, i.e. in the Cates procedure,55 ds is an invariant value. However, the fractal dimension df and the walk dimension dw change upon execution of this procedure. By using the `Einstein correlation' relating the scaling indices of a fractal (in this case, df is used as the index) to the resistivity (a component of viscosity) x¡¾ , it can be shown 55, 58 that for polymers with phantom bonds (i.e.for polymers with no excluded volume, for which x¡¾=2) the dw value can be represented by the sum (9) dw=df+2. Cates showed 55 that relation (9) is also valid for structures containing loops. In the case of an ideal polymer fractal with Gaussian chains and the connectivity parameter dl, the fractal dimension df of a percolation cluster with phantom bonds can be expressed as follows: l (9a) l df a 2 ¢§ d . 2d For dl=1 (linear chains), Eqn (9a) provides the correct value, df=2, corresponding to a macromolecular coil at theY-point (see Table 2). As noted above, ds=4/3 for a percolation cluster 351 irrespective of the dimension of the Euclidean space (see Table 1); therefore, from Eqn (9a), we obtain df=4, which is consistent with the Flory ¡¾ Stockmayer theory 59 for phantom chains.For three-dimensional space, df>3 has no physical mean- ing because the object cannot be packed more densely than an object having Euclidean dimension. It is evident that this discrep- ancy is due to the phantom nature of the polymer chains postulated by Cates;55 it is necessary to take into account self- interactions of chains due to which the dimension of a polymer fractal assumes a value which has a physical meaning. In Eqn (9a), df has the same meaning as the fractal dimension of phantom Gaussian chains; therefore, the standard mean-field theory, taking account of the factor of excluded volume, can be applied to a polymer fractal.56 The Flory approach can be used as the first approximation for this purpose.59 In this case, the free energy F of an object with self- interactions can be represented by the sum 52 2 (10) uM2 Rd , a n F a R R0 where R is the size of a real fractal, R0 is the size of an ideal phantom fractal without allowance for the interactions of excluded volume, nu is an excluded volume parameter, M is the total mass of the fractal (RD0 !M), and d is the dimension of the Euclidean space in which the fractal has been inserted.The first term in Eqn (10) is the elastic (or entropic) component of the free energy and the second term takes into account the volume interactions in terms of the mean-field theory.52, 59 By minimising the free energy with respect to R, one can find the fractal dimension D of a swollen fractal, which is determined by the equation (10a) D a dsOd a 2U .ds a 2 In the case of percolation clusters, Eqn (10a) gives D=2 for ds=4/3 and d=3 (see Ref. 56), while for linear polymers, D=5/ 3 for ds=1 and d=3 (see Ref. 60), which corresponds to an impermeable coil in a good solvent (see Table 2, state 3). The resulting dimensions are typical of macromolecular coils in monomeric solvents. Using the concept of polymer fraone can answer the question of what would happen if the monomeric solvent is replaced by a polymeric one, i.e., whether the polymer clusters and the clusters of a high-molecular-mass `solvent' would be separated from one another or `entangled'. This question can be answered by virtue of the equation 60 (10b) N=RD1aD2¢§d, g where N is the number of intersections of two arbitrary fractals with dimensions D1 and D2 .The final relations for the dimensions of systems involving mixtures of polymer fractals (fractals in low- and high-molecular- mass solvents, melts of identical and arbitrary fractals) are given in Table 3.60 The problems of penetration of polymers with arbitrary connectivity into confined spaces (for examples, pores or slits) have been discussed.61 It has been noted that the Flory theory, the blob model 61 and the scaling concept provide identical results for linear chains; however, in the case of branched polymers, the Flory theory and scaling can lead to contradictory results when applied without invoking additional information.For branched (and, hence, for cross-linked) polymers, the equation for the index n in relation (4) has the form 61 (11) n a ds a 2 d a 2 , and the fractal dimension D is determined by the ratio (12) D a ds n .352 Table 3. Final relations for the dimensions of systems comprising mixtures of polymer fractals.60 Parameter Melt of arbitrary fractals Mono- Poly- Melt of meric- meric identical solvent solvent fractals ds=df=0 ds=1, ds=ds , df=2 df=df d d d Fractal dimension in terms of ds sOd a 2U d sOd a 2U 2 d a 2 2 s a 2 sOd a 1UO2 ¢§ dsU 2O2 a ds ¢§ 2U in terms of df d a 2 2 f fU d a 2 2 a 1=d d a 2 2 ¢§ Odf ¢§ 2U=df d a 2 2O1 a 1=d d 7 d42 d s5 Saturation condition a f52 a dfOd ¢§ 1U d 2d d a 2 Note.The ds and df values are the spectral and fractal dimensions of the interglobular space in a solvent. a Transition from fractal to non-fractal behaviour. Equation (5) is a special case of Eqn (12) for linear chains (ds=1). m , In order to avoid the use of values such as the connectivity parameter (or chemical dimension) dl or spectral dimension ds , Lhuillier 62 proposed a d-independent universal index r in the dependence Rg max!Nr (13) where Rg max is the maximum radius of gyration of a macro- molecular coil and Nm is the number of base units (monomers) in the chain. The r value is supposed to vary in the range 1/d4r41. It is believed that the r-fractals thus introduced provide a simple description for real polymers: r=1 for linear chains and r=3/4 for branched chains.However, Vilgis 63 have found that, in essence, the following relation holds: (14) r71=ds ; this, in turn, implies that r-fractals can be reduced to polymer fractals, postulated by Cates.55 A study by Stepleton et al.64 was among the first which confirmed experimentally the fractal nature of a polymer chain taking a protein as an example. In a study of a frozen aqueous solution of a hemoprotein by electron spin relaxation, its fractal dimension was estimated to be 1.650.004. In the opinion of the researchers, this value is in good agreement with the fractal dimension of a self-avoiding random walk, which is equal to 5/3.57 This conclusion was drawn using the equation for the phonon density of states r(o) r(o)!ods71, where o is frequency.It has been suggested 64, 65 that the exponent in the equation for r(o) is determined by the fractal dimension. However, Alexander and Orbach 28 showed that in this case, the spectral dimension ds is involved, which is not equal to df [see Eqn (8) and Table 1]. Note that the publication by Stepleton et al.64 appeared earlier than the study by Alexander and Orbach.28 In the case of self-avoiding random walk, ds=1; substitution of this value into the equation for r(o) gives a result which, as shown by Young (see Ref. 10, p. 168), is at variance with experimental results. Attempts to resolve this contradiction have been under- taken.66, 67 Maritan and Stella (see Ref.10, p. 151) calculated the fractal dimensions for 50 proteins using two methods. In one method, fractal dimension is a scaling index for the contour length of a macromolecule with respect to the distance between its ends V U Novikov, G V Kozlov (it is equal to 1.19 ¡¾ 1.82), while in the other method, the fractal dimension is a scaling index for the total mass with respect to the length, which is equal to 1.62 ¡¾ 2.24. The calculations were performed using X-ray diffraction crystallographic parameters of proteins. In our opinion, the results obtained for proteins 64, 65 do not appear contradictory. Undoubtedly, the fractal dimension pre- sented in the study 64 is the spectral dimension ds .Its value (ds=1.65) does not seemtoo great either, if one takes into account that the values ds=1.65 ¡¾ 1.80 were obtained for block poly(- methyl methacrylate).35 These large values of spectral dimension can be due to several reasons, first of all, to the high connectivity of the polymer chain.28 In addition, the polymer chain cannot be modelled by self- avoiding random walk, as has been done by Young. Finally, Cappelli et al. (see Ref. 10, p. 156) demonstrated that the effective ds value can be influenced by the non-homogeneity of block specimens, for example, porosity or inter-grain boundaries. The difference between the dl and df values found by Colvin and Stepleton 65 can also be easily explained. Indeed, dl is a chemical dimension, as follows from the way of its determina- tion,27 df is the Hausdorff dimension.These values do not necessarily coincide. By using Eqn (10a) for a fractal swollen in a monomeric solvent, we obtain D&2.26 for ds=1.65 and d=3, which is in good agreement with both experimental data and the limiting D value for branched polymers.9 The following relation has been proposed:62 (15) dl=df z , where (16) z a 4 a 3D , for 4/3<D<4. 8 When D&2.26, the parameters z and dl are 1.348 and 1.68, respectively, which is consistent with the estimate made by Colvin and Stepleton.65 Calculation by an equation similar to (8) gives dw ^ 2.74, i.e. slow anomalous diffusion on the fractal takes place.28, 56 The following expression relating dw to dl has been pro- posed:31 (17) dw a df l 1 a 1 .d Assuming that dw=3.37 (`lattice animals' 31), we obtain dl=0.93 ¡¾ 1.98. This interval agrees with the range of dl found experimentally.65 Thus, there are no contradictions suggested by Young between the estimates made in the two studies cited above 64, 65 but the large number of fractal dimensions used to describe the polymer structure creates certain difficulties. This is not a matter of principle because the dimensions considered above are interre- lated.28, 55, 56, 60 ¡¾ 63, 68 ¡¾ 71 3. Statistical fractal Family 9 considered the conformations of statistical branched fractals (which simulate branched polymers) formed in equili- brium processes in terms of the Flory theory.Using this approach, he found only three different states of statistical fractals, which were called uncoiled, compensated, and collapsed states. In particular, it was found that in thermally induced phase transi- tions, clusters occur in the compensated state and have nearly equal fractal dimensions (*2.5). Recall that the value df=2.5 in polymers corresponds to the gelation point; this allows gelation to be classified as a critical phenomenon. The fractal dimension of purely statistical models, i.e. models without the effect of excluded volume, can be determined accu- rately [see Eqn (9a)]. For linear polymers, this model corresponds to phantom random walk. In the case of branched statistical fractals, the corresponding model is a statistical branched cluster,Fractal analysis of macromolecules whose branching obeys the random-walk statistics.Since the root- mean-square distance between the random-walk ends is propor- tional to the number of walk steps N, then D=2 irrespective of the space dimension. These types of structures have been studied.60, 72 ± 74 The value D=4 irrespective of d was obtained for a branched fractal. Unlike `ideal' statistical models, models with excluded volume, i.e. those involving correlations, cannot be accurately solved in the general case. The Df values for these systems are usually found either using numerical methods such as the Monte Carlo method or taking into account the spatial position of a re-normalisation group.What are the physical conditions under which a statistical fractal has the most branched structure? It is clear that, when a large number of clusters are present in the system, they all occupy the available volume. The presence of other clusters restricts the degree of branching of each cluster; a cluster is more branched in the isolated state than in a concentrated solution. For polymer coils, this is expressed as an increase in D from its value in a dilute solution in a good solvent to the value at the Y-point in concentrated solutions.70 Yet another factor which also influences the shape of the cluster is whether or not attractive interactions are present. As the temperature increases, the attractive interactions diminish. For example, no interactions of this sort are found in isolated macro- molecular coils in good solvents at high temperatures.74 Family 9 defined this state of a statistical fractal as an uncoiled state because in this case, it is characterised by the smallest fractal dimension.Then Family used the version of the Flory theory proposed by Isaacson and Lubensky 75 for determination of the fractal dimen- sion of an isolated statistical cluster. The method is based on the determination of the most probable cluster conformation using the free energy of repulsion. Under the influence of elastic free energy, the radius of the real cluster `tends' to the radius of the ideal cluster which involves no repulsive interaction. After mini- misation of the total free energy, the Df value for an uncoiled cluster can be found from the expression (18) D à 2Öd á 2Ü , 5 which was first obtained by Isaacson and Lubensky 75 for branched polymers in dilute solutions.Equation (18) has two peculiar features: firstly, D depends appreciably on d and, secondly, there exists a critical dimension of Euclidean space dc=8 for which D=4 in accordance with the ideal statistical model, i.e. the model without correlations. At dc>8, the correlations caused by the effect of excluded volume are no longer significant and Df does not change. The value dc=8 was found in studies of branched polymers and `lattice animals'.76 Calculations by formula (18) are in good agreement with the known results for `lattice animals.' To elucidate the effects which increase D, Family considered conformations of one big cluster comprising N particles in the presence of other clusters with a size distribution.The large cluster does not `feel' the presence of other clusters as long as their average diameter is small compared to its diameter. On the scale of lengths much greater than the radius of an average cluster, the big cluster interacts only with clusters similar to itself. It was found for polymers that, as the sizes of other clusters increase, they start to screen the effect of excluded volume, which is effective in the big cluster. The screening can change the fractal dimension of this cluster. The best examples of processes with a cluster distribution are percolation and thermal critical phenomena, which are reflected, for example, by the Ising, Potts, and other models.Under equilibrium conditions in a system consisting of clusters with a size distribution, the average cluster size deviates in the vicinity of the critical point. 353 The state of a statistical fractal in which the excluded volume effect is compensated by the screening effect is referred to as the compensated state. For this state, Family has found that (19) D à d á 2 . 2 This expression is expected to be valid for any statistical critical cluster in the presence of clusters having a size distribution. The upper critical dimension for critical clusters can be deter- mined assuming that in Eqn (19), D=4. The result thus obtained, dc=6, is in good agreement with the data for percolation.75 The above results confirm that the presence of numerous clusters and the deviation of the mean cluster size near the critical point results in the screening of the critical volume effect and in an increase in D.Another way of decreasing the repulsive interac- tions is to enhance the attractive interactions between the elements of an isolated cluster. In the general form, the free repulsive energy can be represented as a power series with respect to density with virial coefficients w2, w3, ..., which describe the effects of inter- action between pairs, triplets, etc.77 The first term (with w2) in this power series predominates. However, in the presence of attractive interactions, the second virial coefficient w2 tends to zero and the term with w3 should be taken into account.When w2 ? 0, the polymer occurs in the vicinity of the Y-point.74 In this case, with minimisation of the total free energy with respect to R (20) D à 4Öd á 1Ü . 7 This relation was first obtained by Bantle 78 for the conforma- tion of an isolated branched polymer at the Y-point. Calculation in terms of Eqn (20) forD=4 gives dc=6, as for the compensated state. Thus, the fractal dimension D increases near the compensa- tion point; this is due either to the geometric screening of the excluded volume effect or to the enhancement of the attractive interaction. Big clusters near the critical point in thermally induced phase transitions and polymers at the Y-point are examples of statistical fractals in the compensated state.9 Study of the conformation of a statistical fractal in a system occurring below the critical point and in the isolated state at w2<0 showed that the fractal is very compact and has a globular conformation in both cases.This state of a fractal was called collapsed, by analogy with the collapsed state of polymers. The conformation of clusters under these conditions was determined using the approach proposed 77 for studying the conformation of a branched polymer in a monodisperse melt, i.e. in a solution of coils with identical sizes. In the case of minimisa- tion of the free energy with respect to R, D=d (21) For a purely statistical fractal for which D=4, we obtain dst=4. This means that this type of statistical cluster is compact up to d=4. Therefore, Family 9 called this state of statistical fractals the collapsed state.In the case of an isolated fractal, attractive interactions predominate below the compensation point; the second virial coefficient w2 is negative and cannot be neglected in the power series mentioned above. The conformation of a fractal is deter- mined by the balance between the first two terms in this series. The results are identical to Eqn (21) for dst=4. Thus, the conforma- tion of an isolated statistical fractal below the compensation point is the same as the conformation of a big cluster below the critical point. 4. Experimental determination of fractal dimensions Let us consider several experimental estimates of various fractal dimensions for polymers with allowance for the above theoretical statements.The index n in Eqn (4) has been determined by small- angle neutron scattering for epoxy polymers;78 it was found that354 n=0.6. It has been suggested 79 that so great a n value is due to the enhanced chain rigidity of epoxy polymers. However, the use of Eqn (11) with ds=4/3 and d=3 gives n=0.666, which is close to the experimental n value (see Table 2, the permeable coil model). Calculation using Eqn (12) for ds=4/3 and n=0.6 gives D&2.22, which is close to the fractal dimension for the compen- sated state of branched polymers in accordance with Eqn (20). Silicate gels and epoxy polymers have been studied by Raman light scattering.34, 79 The spectral dimension ds of silicate gels falls within the range from 1.11 to 1.43, depending on the method of calculation, which is consistent with the conclusion drawn by Alexander and Orbach.28 The results of estimates of ds from the data of Raman scattering and small-angle neutron scattering for two epoxy polymers with cross-linking densities differing by a factor of two and for the monomer used to prepare these epoxy polymers are presented in Table 4.Attention is drawn to the fact that the ds values for all the three specimens are virtually equal [if the inaccuracy of determination of ds is taken to be 0.05 (see Ref. 35)], the slight difference being due rather to the determi- nation method used.Table 4. Value of ds for the DGEBA monomer a and for epoxy polymers based on it determined by Raman light scattering (I) and small-angle neutron scattering (II) methods.79 II I Example 1.44 1.44 1.52 1.37 1.33 1.32 DGEBA EDAb 1X DGEBA EDAb 2X DGEBA a Diglycidyl ester of bisphenol A. b Ethylenediamine. The results of investigation of the transfer of energy between molecules acting as electron density donors and acceptors have been reported.80, 81 The Raman scattering method gives actually the fractal dimension of the surface on which the energy transfer takes place.81 The fractal dimensions found for silicates are in the range of 2.23 ± 2.82;81 that for a polystyrene film is 2.2.80 Cappelli et al.(see Ref. 10, p. 156) have studied the annihila- tion of the triplet excitons in naphthalene aggregates located in the pores of various materials including polymers. A common feature of these systems is non-classical annihilation kinetics, which resemble the annihilation kinetics of fractals. The coefficient of the annihilation rate ka varies with time t (22) ka!t7h, where h is the non-homogeneity index (0<h<1), which is equal to zero only for homogeneous specimens. The effective spectral dimension of the medium d 0s is given by the relation 70 (23) d 0s=2(17h). The d 0s values for several polymers are listed in Table 5. Cappelli et al.10 also measured h as a function of temperature. At low temperatures, all the studied polymers exhibit properties similar to the properties of fractals.As the temperature increases, h decreases, i.e. the d 0s value increases. In some specimens, h ? 0 Table 5. Effective spectral dimensions d 0s for some polymers.10 h d T/ K Polymer 0s Pore size mm Acetate A8 Acetate A1 Acetate A3 Nylon B214 1.2 ± 1.7 0.8 ± 1.1 0.9 ± 1.1 1.2 ± 1.6 1.8 0.16 0.47 0.44 0.21 0.1 0.2 0.1 1.2 0.2 Poly(methyl methacrylate) 7 4444 77 V U Novikov, G V Kozlov as the temperature is raised. This implies that all the effects described by fractional dimensions are associated with disorder.82 A number of specimens behave as fractals also at room temper- ature. It is noteworthy that the d 0s values for the polymers studied vary over wide limits, from 0.8 to 1.8.In the case of poly(methyl methacrylate), d 0s exactly corresponds to the spectral dimension determined by Raman scattering measurements;22, 34 it is*1.8 in both cases. The theoretical concepts outlined above are confirmed rather accurately by experimental data. Nevertheless, it is worth noting that the concept of polymeric (macromolecular) fractal 55 has been developed for polymer solutions, while the Vilgis theory 60, 61 is applicable only to rubbers and polymer melts, although it has been developed for condensed media. Therefore, it can be expected that extension of these concepts to the vitreous state of polymers would require some corrections taking into account the specific features of this state.Some possible approximations have been considered by Cates,55 who concentrated attention on macromolecular entan- glements, which play an important role in the description of the behaviour of block polymers.83 ± 86 Cates believes that the fact that the concept of polymer fractal neglects the effects of macro- molecular entanglements is the main drawback of this theory. Nevertheless, Cates 55 introduced several simplifications which make it possible to ignore these effects for dilute solutions and relatively low molecular masses. However, in the opinion of Cates, even in the case of predominant influence of entanglements, theoretical interpretation of this phenomenon is impossible with- out preliminary investigation of the properties of the system in terms of Rouse ± Zimm dynamics, which can serve as the basis for a more complex theory.It was assumed 55 that the effects of entanglement can be due to the substantially enhanced local friction of macromolecules. 5. Levels of fractality Polymers exhibit the fractality inherent in them at different structural levels; both primary and secondary structural elements can be fractals. For example, macromolecular coils in Y or in good solvents with df&1.66 (with the Euclidean dimension d=3) are fractals.1, 29 The fractality of a polymer at the molecular level can be demonstrated as follows. Lebedev 87 reported data on the change in the radius of gyrationRg of the macromolecules of block polystyrene, poly(methyl methacrylate) and polyethylene with the molecular mass Mw.The Rg value is related to the length of a macromolecule Lw in the following way:88 (24) Lw!Rdf g . The length of a macromolecule can be determined from the knownMw, the molecular mass of one unit m0, and the projection of the bond length l0 on the macromolecular axis using the simple dependence (25) Lw=Mwl0 . m0 The m0 and l0 values for the polymers mentioned above were taken from Ref. 89 and Ref. 90, respectively. Figure 3 a shows the variation of Lw vs. Rg. Taking account of this dependence, it was found that df ^ 1.60, which is close to the corresponding value for a macromolecular coil in a Y-solvent (df&1.66, see Table 2). The elements of Koch figures (Fig. 3 b) resemble most closely the freely jointed chain model, which is normally used to simulate macromolecules.91 For this version, df=1.61.Thus, the fractality of a block polymer at a molecular level can be regarded as proven. The cluster model of the structure of amorphous polymers (Fig. 4) provides a quantitative description of supermolecular (supersegmental) structures in this type of polymer. The cluster with chains protruding from it (`interpenetrating' molecules) assumed in this model resembles the well-known cluster used inFractal analysis of macromolecules b a ln Lw 8 D1 D2 D3 63 5 lnRg Figure 3. Chain length Lw vs. the radius of gyration Rg for a macro- molecular coil of polystyrene (1), poly(methyl methacrylate) (2) and polyethylene (3) (a) and a variant of elements of the Koch curves having df=1.61 (b).88 Rw Figure 4.Cluster model of the structure of an amorphous polymer. the Witten ¡À Sander (WS) model.77, 78 Such a cluster distinguished in a polymer bulk restricted by the radius Rw would possess at least two features typical of fractal clusters. One feature is self- similarity, caused by the presence of unstable clusters consisting of fewer segments than thermodynamically stable clusters (Fig. 4 shows them consisting of two segments). The other feature is that the density of particles (statistical segments) comprising the cluster would decrease on moving away from the cluster centre and thus stipulate the r(L) dependence typical of fractals (see Fig. 1).In conformity with this dependence, it is assumed that the structure of an amorphous polymer contains sorts of `thickened' and `rarefied' sections. This `thickening' of the material can be explained by assuming the existence of a local order (as opposed to the model of interpenetrating macromolecular coils, i.e., the `felt' model, proposed by Flory).92, 93 Thus, the fractality of the structure of amorphous polymers in the *0.3 ¡À 5.0 nm range, shown experimentally in a number of studies,22, 35 is confirmed indirectly by the presence of local order. The size of a fractal cluster Rw and the number of particles in it Nr are related to each other by the following expression:88 (26) Nr!Rdf w . Let us consider the procedure for estimation of the Nr and Rw values in terms of the cluster model.15, 86 A cluster is regarded as a multifunctional unit in a fluctuation network of macromolecular entanglements; therefore, according to Flory,42 the reduced molecular mass M per cluster is estimated from Eqn (4).By dividing M into the molecular mass of a statistical segment, we obtain the number of particles (statistical segments) Nst per cluster. The distance between the clusters Rcl should be taken as Rw ¡¦1=3, (26a) F 2Vcl Rcl=18 355 lnNst 64 12 2 3.0 lnRw 2.0 Figure 5. Number of statistical segments Nst in a cluster vs. its radius Rw for amorphous (1) and amorphous-crystalline (2) polymers under the assumption that the sizes of fractal and percolation clusters are equal.where Vcl is the density of the cluster network and F is the cluster functionality. Figure 5 presents the dependence of Nst on Rw for five amorphous vitreous and five amorphous-crystalline polymers.8 It can be seen that this correlation is fully consistent with relation (26). The df value of the cluster structure determined from the slope of the straight line is*2.75. The fractal dimension of the cluster in the WS model lies in the range of 2.25 ¡À 2.75,88 which is in agreement with the above estimate. (27) The fractal dimension for the structure of the amorphous polymer can be found from the relation 94 df=(d71)(1+m), where m is the Poisson coefficient. For a cluster structure with df&2.75, the m value is equal to *0.375, which is consistent with the known m values for amor- phous vitreous polymers.95, 96 As an example of fractality displayed in polymers at a macro- scopic level (a secondary structural element), we shall consider the growth of a crack in a film of amorphous vitreous polyarylate- sulfone.97 The surfaces of cracks have irregularities of at least two scales: *20 mm (Fig.6 a) and *20 mm (Fig. 6 b). This provides grounds for describing them in terms of fractal models.98 ¡À 100 The triangular profile throughout the whole period of growth of the crack allows the use of the relationship proposed by Mosolov 98 (28) dsk!rDsk=2, sk where dsk is the crack opening, rsk is the distance from the crack top and Dsk is the fractal dimension of the crack.b a c 2 lndsk 1 2 6420 7 lnrsk 3 Figure 6. Crack coast-line (a), general view of a stable crack (b) and opening of the crack dsk vs. the distance to its top rsk (c) for polycarbonate (1) and polyarylate (2).8356 Figure 6 c shows the plots for the variation of dsk vs. rsk for two amorphous polymers; the plots correspond to relation (28). In other words, a stable crack in polymer film samples is a stochastic fractal with the dimensionDsk&1.48. The linearity of plots shown in Fig. 6 c reflects the self-similarity of the crack at different stages of its growth. Thus, polymers also exhibit fractal properties at the macroscopic level. The examples considered above do not reflect the whole diversity of manifestation of fractality in polymers.Nevertheless, they help to understand the meaning of the term `multifractality' as applied to polymers. III. Fractal characteristics of macromolecules Condensed systems start to behave as polymers after a certain molecular mass has been attained and a network of macro- molecular entanglements or chemical nodes has been formed.56 In consideration of condensed polymers, it is especially important to describe the sections of the macromolecule between the points of chemical cross-linking or entanglements.55 The deformability and mobility of these sections largely determine the macroscopic properties of polymers.56, 101 The general physical grounds which account for the fractality of these chain sections are considered below in relation to epoxy polymers.To determine the fractality of an object, three aspects are usually considered.12 First, as noted above, a real physical object possesses fractal properties only in a definite scale range (see Fig. 1). The boundary sizes for a macromolecular section can be determined using the data of several publications.22, 35, 102 The lower boundary is related to the finite size of a structural element; in terms of the cluster model, this is the length of the statistical segment lst (29) lst=l0C?, where l0 is the length of a chemical bond and C? is the character- istic ratio. For most polymers, l0=0.125 nm (see Ref. 90), the minimum C?=2 (see Ref. 91); therefore, the minimum possible lst is 0.25 nm. This value is in good agreement with the experimental estimate of the lower limit of fractality in polymers (*0.3 nm22) and should be accepted as the Lmin for the chain section between chemical cross-links.The length of this section Ls can be taken as the upper limit. For most of the epoxy polymers considered, this value is in the range from*1.6 to 7.4 nm, which is also consistent with the data of Refs 22 and 35. The second aspect is associated with the self-similarity of the object considered. A section of the macromolecule between chemical cross-linking points represented by a freely jointed Kuhn chain or by a chain with free or hindered rotation about the bond 91 is simulated by a broken line having the topological dimension dt=1.3 In the case of simulation by the freely jointed Kuhn chain, the broken line consists of Ns statistical segments of length A, each of them being simulated by a straight-line segment. These straight-line segments are standard self-similar objects but the chain sections consisting of them are self-similar only when their lengths Ls and the A values vary proportionally.For the epoxy polymers in question, this condition is fulfilled;103 there- fore, these sections are self-similar objects.104 Finally, the third aspect concerns determination of the fractal dimension D of an object.12 For fractals, the condition D>dt . should hold.16 In the case where a macromolecular section between chemical cross-linking points is represented by the freely jointed Kuhn chain, the following equality should hold:101 (30) R2s =LsA, where Rs is the smallest (straight-line) distance between chemical cross-links.V U Novikov, G V Kozlov A fractal broken line can be described by the relation 88 D, (31) La a Ra where L is the length of the line, R is the straight-line distance between its ends, a is the scale of measurements in the macro- molecule. 2 (32) , a LAs a Rs By dividing both parts of Eqn (30) by A2, we find that i.e., a freely jointed Kuhn chain is a broken line with dimension 2 provided that condition (30) is fulfilled. Note that in this case, the chain is a fractal because D>dt . Since the random-walk dimen- sion dw for this model of a macromolecule is also equal to 2,66 the spectral dimension ds of the chain can be estimated [see Eqn (8)].For a freely jointed Kuhn chain, it is also equal to 2. Let us consider conditions under which ds=2. For a real polymer, with allowance for the effect of excluded volume, one can write that 56 s (33) s D a a ¢§ d , 2d where (34) a=ds+2Ods a 2U . d a 2 When D=ds=2, Eqns (32) and (34) are correct only when d=2. This means that the ranges of variation of the fractal dimension of a macromolecule section between cross-linking points are dt<D4d or 1<D42. rot However, the freely jointed Kuhn chain is the most idealised model for a real macromolecule; therefore, there exist several corrections taking into account specific features of the polymer structure.Thus, for chains in which Ls/A<10, condition (30) no longer holds. The modified condition is written as follows 91 Rs2 a LsA ¢§ AO1 ¢§ e¢§2LsAU . (35) 2 Equation (35) points to a decrease in Ls at a constant Rs and, hence, to a decrease in D, in conformity with Eqn (31). Note that the condition Ls/A<10 holds for nine of the ten specimens of epoxy polymers; this automatically means that D<2.8 The hindrance of rotation of the macro-chain around bonds, which necessarily occurs in a vitreous polymer, accounts for the decrease in the real A value to Arot according to the relation 91 s2 a A , (36) A The hindrance parameter s can be determined from the empirical equation 105 Tg=C(s71), (37) where Tg is the experimental glass transition temperature and C is a constant (for carbon-chain polymers, C=270 K).Now we can calculate non-fractal values Rs and Ls as functions of the cross-linking density ns using Eqn (29) and the relation 104 L (38) s a Fch , 2Sns where Fch is the functionality of a chemical cross-linking point (hereinafter, we assume that Fch=4) and S is the cross-section area of the macromolecule. The results of calculation of D as a function of ns for the epoxy polymers considered here using Eqn (31) with a=Arot , L=Ls and R=Rs are shown in Fig. 7, curve 1. It can be seen that the allowance for the hindered character of rotation has resulted in a monotonic decrease in D following an increase in ns. As has been shown,101 this implies a decrease in the mobility (deformability) ofFractal analysis of macromolecules D 2 1.6 13 1.2 0 10 10726 ns /m73 Figure 7.Fractal dimension D of a macromolecule section between chemical cross-linking points vs. cross-linking density ns for epoxy polymers based on epoxy oligomers and amino-containing curing agents. The D values were found from Eqn (31) for a=Arot (1), A (2) and 0.25 nm (3). the macromolecule section between the points of chemical cross- linking. The calculation of D from Eqn (31) but for a=A gives D&2 irrespective of ns (Fig. 7, curve 2). Finally, when persistence of the chain section is taken into account, i.e. the chain parameters vary according to Eqn (35), a somewhat higher D value is obtained, although the difference does not exceed *3%.The change in the scale of measurements also changes the fractal dimension of the chain. Curve 3 in Fig. 7 reflects the dependence of D on ns for a=0.25 nm (i.e., for the minimum size). This curve runs parallel to curve 1 but is characterised by somewhat lower D (by *3%¡À 13%). Our choice of the scale of measurement of Arot (or lst) is due to the fact that it reflects the real state of the chain under particular conditions (`natural' scale of measurements). Let us discuss in more detail the physical meaning of the fractal dimension D of a chain section between chemical cross- linking points (or between entanglement points). Substitution L=Ls , R=Rs and a=lst in Eqn (31) gives the relation 101, 106 D (39) , Ls l �� lst st Rs which implies that whenD=1, Ls=Rs , i.e.the chain between the cross-links is completely stretched and loses the capacity for deformation (mobility). This is consistent with the data of a publication,51 according to which D=1 corresponds to a com- pletely uncoiled chain (see Table 2). Recall that the condition D=1 means that the given section of the macromolecule has completely lost fractal properties. When D=2, we obtain the maximum possible length of the chain section Ls for a fixed Rs and the greatest lst, i.e. the chain deformability (mobility) is the most pronounced in this case. Note that the highest stretching ratio lmax for a given chain section is equal to Ls/Rs . 107, 108 It is clear that D=2 is attained for rubbers, and intermediate values, 1<D<2, describe different degrees of mobility of a macromolecule section between chemical cross-links (entanglements).This quantitative measure of mobility is convenient because the fractional part of D provides actually a relative measure of mobility, irrespective of the structure of the macromolecule or other features of the poly- mers.82 Let us consider the physical reasons for the fractality of the macromolecule, i.e. for the dependence of the chain length Ls on the scale of measurement Arot or lst . The first reason is that Ls can change upon conformational changes in the macromolecule due to variation of C?, i.e. the bond angle yv can change according to the equation 91 v .(40) s2 �� C? 1 ¡¦ cos y cos yv 357 a l0 y l0 b lpr Figure 8. Schemes of two adjacent bonds of a macromolecule (a) and the enveloping tube (b). Figure 8a shows schematically two consecutive bonds with the length l0 . The total length of such a section l projected onto the axis of one of the bonds is equal to (41) l=l0+l0 cos yv . It is clear that the smaller yv , the greater the second term in Eqn (41) and the greater Ls. The maximum Ls value is attained for tetrahedral angles (yv=70.5 8).91 The distance between the cross- links determined in this way (we shall call it conformational length Lconf) is obviously equal to L (42) conf �� Lsl . 2l0 Another physical representation of the effect in question is the use of the reptation model,41 in which a macromolecule is assumed to be encapsulated in a `tube' of a finite thickness (Fig.8 b). In this case, the topological skeleton of the chain consists of Npr units of length lpr , which form a primitive path with the length Lpr=Nprlpr . It is also assumed that the ends of fragments of the primitive path with the length lpr are fixed in a definite manner; therefore, the Npr and Lpr values can be found fairly easily using lpr . The Lpr value can be expressed as follows:41 (43) Al rot , Lpr �� Ls pr where lpr is assumed to be equal to the distance between clusters Rcl , which can be calculated from Eqn (26a). The length of the fractal broken line Lf is expressed in the following way:3 (44) Lf=L 0a17D, where L0>1 is the length of the non-fractal line.Since the Ls value determined using Eqn (38) is calculated, in the first approximation, as the length of a cylinder with the cross- section area S, i.e. of a non-fractal object, it can be assumed that L0&Ls . If the two above examples point to the presence of fractal properties of a section between chemical cross-links (entangle- ments), the following relation should be obeyed: Lf&Lconf&Lpr . (45) Relation (45) actually holds (with a deviation of 20%) for epoxy polymers based on bisphenol A and cured by diamine (EP-1) or anhydride (EP-2) (Fig. 9).47 This indicates that the fractality of the macromolecule section between chemical cross- linking points (entanglements) in a vitreous polymer is due to known mechanisms of the change in the statistical rigidity. It follows from Eqn (39) that the loss of fractality by a chain section, irrespective of the scale of measurements, takes place in the case of full stretching of this section between cross-linking358 Lconf, Lpr/ nm 30 1234 30 20 15 Lf/ nm Figure 9.Relationships between the conformational length Lconf (1, 3) and primitive path Lpr (2, 4) of the chain section between chemical cross- linking points and its fractal length Lf for EP-1 (1, 2) and EP-2 (3, 4). D 3 1 2 2.0 1.6 1.2 a/ nm 0 10 Figure 10. Fractal dimension of the macromolecule section between chemical cross-linking point (D) vs. scale of measurement a for EP-1.ns610727/ m73: (1) 0.2, (2) 0.6, (3) 1.7. points (entanglements), i.e. when Ls=Rs . In addition, for a? 0, the L(a) value would tend to a final limit equal to the length of the straight line. Figure 10 shows the D(a) plots for EP-1. It can be seen that, when a ? 0, the fractality of the chain section between cross-linking points is retained. Therefore, the behaviour of this section of a macromolecule is more intricate than that of a mere fractal broken line; this can be due to a more complex structure of the macromolecule. Indeed, transition to a non-fractal behaviour requires not only that condition Ls=Rs be fulfilled, as for an ordinary broken line, for example, for a coast-line, but also that a conformation condition�Eqn (30)�be fulfilled.In this case, it is necessary that the bond angle yv reach a value which complies with the equality (for lst=A) (46) C?=Acos yv 2 . l0 A chain for which D=1 can be referred to as `true fractal'. Examination of Eqns (26a) and (38) leads to yet another, at least formal, reason for the fractal properties of the section of a macromolecule between the cross-links. For a given cross-linking density, the Ls value (unlike Rs) depends on the cross-sectional area S of the macromolecule, which should also result in the variation ofD. Figure 11 shows the dependences ofDon S for two arbitrary ns values, namely, 161027 and 261027 m73. It can be seen that D rapidly decreases as S increases; when S&0.80 and 0.55 nm2, respectively, D reaches the lower limit (D=1).This implies full `freezing' of the molecular mobility of the chain,101 which should result in a high fragility of the polymer. This assumption is confirmed in practice. It is known that polystyrene and poly(methyl methacrylate) are fragile polymers (S=0.698 andvely) and poly(n-dodecyl methacrylate) and poly(n-octyl methacrylate) are highly fragile polymers (S=1.766 and 1.351 nm2, respectively).109 Thus, the section of the macromolecule between chemical cross-links and physical entanglements possesses fractal proper- ties. This is due to the fact that the length of this section is shorter than the size of the macromolecule. This factor is due to various V U Novikov, G V Kozlov D 2.0 1.0 12 S/ nm2 0 1.0 Figure 11.Fractal dimension of a section of macromolecule between chemical cross-linking points (D) vs. cross-sectional area of the macro- molecule S. ns610727/ m73: (1) 1, (2) 2. steric restrictions, which induce deviations from the behavior predicted by Eqn (30). In the general case, Eqn (30) is valid for a non-perturbed (quasi-equilibrium) coil;32 therefore, the fractality of the chain section considered can be interpreted as being due to the deviation from the quasi-equilibrium state.12 The deviation changes the degree of the `sinuosity' of the macromolecular section (i.e. the length of the statistical segment); therefore, the lst (or A, or Arot) value can be considered to be a `natural' scale of measurement.Since the structure of a macromolecule is more complex than the geometric image represented by a broken line, its behaviour would also be more complicated; this is confirmed by the change in D when a ? 0. In other words, the section of a macromolecule between cross-links has configurational fractal properties, stipulated by the mutual arrangement of the rigidity sections of the macromolecule, and conformational fractal prop- erties, caused by the state of the bonds within this rigidity section (statistical segment). 1. The dimension of the sections of a macromolecule between topological fixing points A cross-linked polymer is formally one giant macromolecule (fractal cluster); therefore, the models considered above provide the dimension df of exactly this cluster.The Vilgis concept 60, 61 assumes the existence of several clusters of this type and linear macromolecules between them. In practice, parameters used more widely are the density of the network of nodes (chemical cross- links) ns or the molecular mass of the macromolecular section between the cross-links Ms, which are related to each other in the following way: (46a) ns=rNA , Ms where NA is the Avogadro number and r is the polymer density. In a study by Sandakov et al.,49 the term `molecule' for cross- linked polymers was used to denote only a section of the macro- molecule with the molecular massMs, because it is known 43 that it is the parameters ns and Ms that determine the properties of network polymers.In addition, the difference between the scales of fractality of a big macromolecular cluster and the polymer structure in the vitreous state should be taken into account, i.e. one should have in view lengths of the order of *0.3 ± 5.0 nm where a macromolecule is a fractal.22 Below we consider practical aspects of the estimation of the fractal dimension D of a macromolecule section between chemical cross-linking points with allowance for these statements as well as for those considered in the preceding Sections. This approach differs fundamentally from that of the Cates 55 and Vilgis mod- els.60, 61 All the foregoing is valid not only for a network of chemical bonds but also for a network of macromolecular entanglements in linear polymers.Fractal analysis of macromolecules Several methods for the estimation of the D value exist.One method makes use of the relation 110 (47) D=ln nst , ln lst where nst is the number of statistical segments of length lst in a chain fragment between entanglements (chemical cross-links). Yet another variant of calculation of D is given by Eqn (39). The following relation has been obtained:111 (48) ?, 2 jcl a CD where jcl is the relative part of local order (clusters) in the polymer. The relationship betweenDand df [see Eqn (27)] is reflected by the expression:112, 113st (49) D=lnO4 ¢§ dfU ¢§ lnO3 ¢§ dfU . ln n The D values estimated using Eqns (39) and (47) ¡¾ (49) vary from 1 to 2. The use of fractal analysis makes it possible to relate molecular parameters to characteristics of supermolecular structure of polymers.Figure 12 illustrates the linear correlation between D and df [df was estimated from Eqn (27)] for epoxy polymers. When the molecular mobility is suppressed (D=1), the structure of the polymer has the fractal dimension df=2.5, which corresponds to m=0.25. The given value of the Poisson coefficient corresponds to the boundary of ideally brittle structure; at m<0.25, the polymer is collapsed without viscoelastic or plastic dissipation of energy.3 This is fully consistent with the Kausch conclusion114 stating that any increase in the molecular mobility enhances dissipation of the mechanical energy supplied from the outside and, as a consequence, increases plasticity of the polymer.When D=2 the df value is equal to 3, which corresponds to m=0.5, typical of the rubbery state. Figure 12 b shows the variation of D vs. ns for the epoxy polymers considered here. The dependence is non-linear, the D value decreases with an increase in ns; even for small ns values, a substantial decay in D is observed. It is of interest that the end of this decay and transition to a much slower decrease in D takes place at ns&861026 m73, which corresponds to *10 monomer units between the cross-links. Actually, this can be interpreted as transition to densely cross-linked epoxy polymers in which an increase in ns influences the suppression of the molecular mobility to a substantially lower degree than in the region where ns=0.861027 m73 (see Ref.8). If the conformational and/or physical state of the section between cross-links are assumed to coincide with that of the whole df a b D 2.7 �¢1 �¢2 1.6 2.6 2.5 0 1.2 D 1.0 1.4 1 210727ns /m73 Figure 12. Correlation between fractal dimensions of the structure df and the chain section between chemical cross-linking points D(a) and theD(ns) dependence (b) for EP-1 (1) and EP-2 (2).106 The straight line was drawn through the points: D=1.0, df=2.5 and D=2.0, df=3.0. 359 macromolecule (see Table 2), the plot shown in Fig. 12 b allows one to judge the nature and the ranges of variation of these states. The first characteristic dimension D=2 corresponds to the state of a statistical coil in an ideal solvent, the second one D&1.67 corresponds to an impermeable coil in a good solvent, the third one D=1.50 is matched by a permeable coil (the state typical of rigid-chain polymers), and the fourth value, D=1, describes a completely uncoiled macromolecule.50 It can be assumed that the increase in the cross-linking density between the above D values is accompanied by gradual transition from a state with greaterDto a state with smaller D. Thus, depending on ns, a network polymer can be both a rubber and a rigid-chain polymer.115 Therefore, it is evident that the knowledge of these regularities is important for controlling the properties of network polymers.Now we attempt to estimate D within the framework of the theoretical views on polymer fractals.22, 35, 55, 60 ¡¾ 63 It is assumed 66 that the fractal dimensions ds , dw and D are related to one another via an expression similar to Eqn (8).The relationship between the parameters ds and D has been considered.65 It is believed that ds is a dynamic value which responds to a change in the conditions of the interaction of a macromolecule with its surrounding andDis a static parameter. However, in our opinion, the opposite situation occurs in reality; this is indicated by the following facts, some of which have been noted above. It has been shown experimentally 69 that a twofold increase in the cross-linking density does not change ds , while, according to the plot shown in Fig. 12 b, ns has an appreciable influence on D.Moreover, the monomer and the cross-linked epoxy polymer have nearly identical ds values (see Table 4). Helman et al.66 proposed a fractal model in which they employ the notion of weightless bridges. In accordance with this m, ds is equal to D when the concentration of these bridges is relatively high. However, theoretical analysis and computer simulation 10, 67 show that the inclusion into consideration of bridges of a finite length does not change the spectral dimension of the system. With allowance for this fact, it was assumed that the ds value would be equal to that determined experimentally for an epoxy polymer in a three-dimensional space, ds=1.44 (see Ref. 69). Two hypotheses can be used to estimate dw�¢the Alexander ¡¾ Orbach (AO) and Aharoni ¡¾ Stauffer (AS) hypotheses.10 The AO hypothesis is based on the following relation: dw=32 df , while the AS hypothesis assumes the relation dw=1+df .It is considered that dw is determined by the properties of the supermolecular structure of epoxy polymers, namely, by the degree of local order in them. This assumption arises due to the well-known succession of relationships between the diffusion process and dw , 28 between the diffusion process and the fluctua- tion free volume ffv , 116 and between ffv and the cluster struc- ture.117 Therefore, the fractal dimension of the cluster structure of cross-linked polymers,102, 107 estimated from Eqn (27), is taken to be df. Since the D value is determined for an Euclidean space with d=2, it was also assumed in Eqn (27) that d=2.The ds value was found for a three-dimensional space; therefore, a graph 118 for converting ds from three- to two-dimensional space was used in subsequent calculations. The D values were further calculated from Eqn (8) in terms of the AO and AS hypotheses. Figure 13 shows the dependences of D on ns calculated from Eqns (8) and (39). The dependences ofDon ns and the absolute magnitudes ofD are in good agreement, especially when the AO hypothesis is used. When ns is low, the D values calculated from Eqn (39) deviate towards the curve estimated using the AO hypothesis. Never- theless, the discrepancy between the calculations by two methods does not exceed 10%.Thus, the fractal dimension of a section of the macromolecule between chemical cross-linking points in network polymers varies360 D 1.3 1234 1.1 1.0 0 10727 ns /m73 Figure 13. Fractal dimension of the chain section between chemical cross- linking points D vs. cross-linking density ns . Calculation using Eqn (39) for EP-1 (1) and EP-2 (2); calculation using Eqn (8) for EP-1 and EP-2 in terms of the AS (3) and AO (4) hypotheses. over limits close to those predicted for a fractal broken line, i.e. 14D42. The dimension D can serve as a measure of the molecular mobility and is related to the fractal dimension of the cluster structure of epoxy polymers df . The dependence of D on df can also be derived theoretically. The calculation procedures proposed take into account the influence on D of characteristics of the molecular and topological structural levels of cross-linked polymers.2. The concept of macromolecular skeletons a. The model of a network polymer The model of network polymers considered below describes quantitatively the formation of structure (and, hence, properties) and is based on fundamental principles of physics. The initial parameters used for cross-linked systems are the ns value (topo- logical characteristics) and some generally accepted molecular parameters. Since the chemical cross-linking processes are com- plex, the model makes use of the simplest scheme of structure formation, which does not take into account, for example, the decrease in the rate of structure formation with time when one structure element starts to influence the conditions of formation of an adjacent element. An example of taking account of effects of this type has been reported.118 Although this scheme is simple, it provides a quantitative description of the processes considered and the structures formed.One point of the model under discussion is estimation of the relationship between ns and the level of local order in the structure of epoxy polymers, which is characterised by the density Vcl of the cluster network of macromolecular entanglements.101 ± 103 A parameter characterising the level of local order more rigorously is the fraction of clusters jcl in the polymer. Curing is supposed to be a turbulent process because it requires mobility of structural elements and takes place in a viscous medium.In addition, transition to a turbulent mode implies an increase in the degree of ordering, i.e. self-organisation of dissipative structures such as clusters.119 A fundamental property of turbulent flows is fractality of turbulent structures.3, 10 It is generally believed that dissipation of energy in three-dimen- sional turbulent flows is concentrated on a set with a non-integral fractal dimension. However, experimental data on the beginning of fluctuations indicate that the small-scale properties of a turbulent flow cannot be described by virtue of fractals.3 There- fore, turbulent dissipative structures are described using `non- homogeneous fractals,' the rules of formation of which are chosen at random at each step of scale hierarchy in accordance with some probability distribution.120, 121 In this case, the transfer of energy is described by virtue of random fragmentation model with the assumption that no correlations exist between stages of the process.Nevertheless, it is possible to calculate the fractal dimen- sion of turbulent structures (Df), which is defined by the relation (50) n hNni*L¡Df , V U Novikov, G V Kozlov where Nn is the number of active vortices at an n-th fragmentation step, Ln is the scale of the n-th vortex, and D is estimated using the following equation:10 (51) Df=3+log2b, in which b is the fraction of the volume occupied by vortices of the scale Ln .As a first approximation, it can be assumed that Df=d tf , where d tf is the fractal dimension of the topological structure of the polymer, which can be found from Eqn (27). It is clear that the chain fragments with the molecular massMs are considered in a plane (d=2) and the formation of clusters is discussed in a volume (d=3). The D value calculated from Eqn (39) for a two-dimensional fractal can be used to find the value for a three-dimensional fractal by applying Eqn (27) to the dimension d tf . This method is not highly accurate but simple. Equation (51) makes it possible to determine the fraction of volume b occupied by vortices of the Ln scale. Assuming that the b and jcl values are equal, the Vcl value can be calculated from the relation jcl Vcl=Sl0C? .Comparison of the Vcl values found in this way with those obtained experimentally 47 for systems EP-1 and EP-2 showed that the two methods provide the same order of magnitude. Never- theless, to attain better agreement between the results, it can be assumed that not all the active turbulent vortices are converted into local order regions; in general, this is obvious due to the fluctuational nature of these regions.89 This means that jcl=Cb, where C is a proportionality coefficient. Assuming that for EP-1 and EP-2, C=0.6, we obtain a much better quantitative agree- ment between experimental and calculated data. It is clear that the coefficient C reflects the influence of variable characteristics of curing such as temperature, time, viscosity of the medium, etc.The fact that the dimension of non-homogeneous fractal Df is always greater than the dimension of the corresponding homogeneous fractal d tf might serve as yet another reason for the inconsistency between the jcl and b values, mentioned above.122 The condition Df>d tf implies a decrease in b in the calculations using Eqn (51). Fractality of the structures of polymers influences their properties. According to modern concepts,123 ± 125 the glass tran- sition of amorphous polymers is regarded as the formation of `frozen' local order (dissipative structures). Generally, for amor- phous systems, this is considered to be a complex and hierarchi- cally multistage process.The first stage involves the formation of seeds of microcrystals, which are unable to grow and serve as a sort of substrate for deposition of amorphous clusters. The second stage is characterised by lf-organisation of mesoclusters.126 As applied to the glass transition of polymers, these processes can be specified in the following way. The first stage starts at the temperature of so-called liquid ± liquid transition Tll , in which regions of dynamic short-lived local order are formed.23 At the glass transition temperature, avalanche-like accumulation of clusters takes place, which characterises the formation of `frozen' local order (the second stage). For this process, (51a) t*!(R*)df , where t* is the time before the onset of this process and R* is the critical cluster size.98 Since the temperature ± time superposition is correct for polymers, we assume that R*!lst and write relation (51a) in the following form Tg!l¡a st .The dependence of the experimental Tg values 112 on lst for epoxy polymers can be represented by a correlation function. In addition, a ^ 0.28, which complies with the necessary conditionFractal analysis of macromolecules a=d7df . 88 Hence, the change in Tg upon the variation of ns is also an indication of the fractal structure of network polymers. Note that the behaviour of network polymers is in agreement with several other fundamental physical principles. Let us illus- trate this by two examples. An integral stoichiometry implies stability and ordering of the structure of a compound.3 This is confirmed by the data of Beloshenko et al., 102 according to which the maximum degree of local ordering in epoxy polymers is attained when an oligomer and a curing agent are taken in stoichiometric amounts, i.e.when K=1.0 (or is close to 1). Structure parameters and characteristics of properties of epoxy polymers are not random values either. Thus the length of statistical segment lst obeys the automodelling relation l st i (52) a D1=m , l stOia1U where D=10 and m varies according to a geometric progression law (53) m=2, 4, 8, 16, 32, ..., 2n, ...,?. Hence, formation of the structure and properties of epoxy polymers during curing is determined by fundamental physical principles.This is accompanied by the change in the characteristic ratio C? (molecular characteristics), although the structure of the macromolecule remains invariant. The use of the above physical principles even in the simplest version provides a correct descrip- tion of the structure and properties of network polymers. b. Scaling representation of a macromolecule A number of methods exist for the modelling of polymers at a molecular level. One of the most widely used methods is scaling presentation,52 according to which the diameter of the `tube' in which the macromolecule is enclosed (it is equal to the distance between the entanglement points) can be estimated from the ratio 72 sl 20. (54) 2=C? Mm0 The main difference between physical entanglements (loops) and chemical cross-links is that the former allow creeping of chains.The density of both types of link are determined using the same rubber elasticity equation, a factor of 0.8 being frequently added to take creeping into account.72 Yet another representation of a macromolecular chain as a fractal is considered below. Olemskoy and Flat 127 have noted that the concept of fractals rather than description of an observed geometric image is usually applied to condensed media. As regards a macromolecule, modelled by a freely jointed chain consisting of statistical segments, one can hardly find a geo- metrical image more vivid than a fractal broken line. In addition, it should be noted that representation of a macromolecule by either one-dimensional (as a curve) or three-dimensional (as a cylinder) object appears to be a fairly rough approximation in view of the real structure of a macromolecule, i.e.the presence of side chains, flexible bonds, rigid segments, etc. As noted above, a change in the density of chemical cross- linking ns in epoxy polymers results in an extremal variation of the characteristic ratio C? with the minimum K ^ 1.0.115 If the section of a macromolecule between chemical cross-links is modelled by a freely jointed chain, the use of known Ls and lst values results in a typical fractal dependence 119 (55) Nst(xlst)=x7DNst(lst), x<1, where Nst is the number of statistical segments per fragment of a macromolecule with the molecular mass Ms, and x is a similarity parameter.With the assumption that Ls=const and K=1.0, x=0.85 was obtained; with Ls=const and K>1.0, the calculations gave x=(0.85)1/2=0.926. The calculation of the dimension D from 361 Table 6. Calculated molecular characteristics of epoxy polymers.8 K D C Polymer ? III II I EP-1 EP-2 5.28 3.50 3.68 3.19 3.34 5.35 4.69 3.37 3.90 4.29 4.83 6.97 9.94 8.89 7.68 6.54 8.89 9.24 9.24 7.96 5.23 4.32 3.38 3.75 4.05 5.27 3.98 3.40 3.70 4.10 2.06 1.60 1.25 1.35 1.50 1.85 1.43 1.32 1.35 1.55 0.50 0.75 1.00 1.25 1.50 0.50 0.75 1.00 1.25 1.50 Note. The following designations are used: I �¢ data of Ref. 121, II �¢ calculation by Eqn (54), III�¢calculation by Eqn (57).Eqn (55) gave 1.17. Thus, when the condition mentioned above is fulfilled, the section of a macromolecule between chemical cross- linking points can be represented as a fractal. Variation of K changes the cross-linking density ns and, hence, Ls . As a conse- quence, the chain ceases to be a self-similar fractal. Nevertheless, it still can be modelled by a non-homogeneous fractal with the same fragmentation step but with a variable D value, determined from relation (39). The D values calculated in this way are listed in Table 6. The results of calculation of the characteristic ratio C? from Eqn (54) are inconsistent in kind with the data obtained by Jean et al.128 Indeed, the maximum rather than the minimum C? is attained at K=1.0.Since 2!ns¢§2=3,Ms!n¢§1 s , then, according to Eqn (54), the C? value should be proportional to n1s =2. In other words, an increase in the density of cross-links should increase the chain flexibility, which is inconsistent with the extremal increase in the glass transition temperature of epoxy polymers.115, 129 Let us consider the reasons for this inconsistency. Since C?l0=lst , Ml0 m =L0 , multiplication and division of Eqn (54) by l 2st gives the relation 8 2 s , (56) L0 l a lst st R which is similar to Eqn (39). Comparison of Eqns (39) and (56) shows that the scaling relation (54) is a specific case of the fractal formula (39) for D=2. For the epoxy polymers considered, condition D=2 does not hold (see Table 6); this may account for the discrepancy found. Using relations (39) and (56), the scaling equation (54) can be written in a more general form sl D .(57) ? M 0 m aD a CD¢§1 The calculation of C? using Eqn (57) give values that are in good agreement with the results obtained by Jean et al.128 It has been shown 90 that in the case of linear polymers, the following empirical correlation is obeyed for physical entangle- ments (58) Rs ^ 10C?. Taking into account the data presented above, the same (or similar) correlation should be expected for chemical cross-links in network polymers.362 Rs/ nm 30 20 �1 �2 10 2 4 6 C? Figure 14. Distance between chemical cross-linking points Rs vs.charac- teristic ratio C? for EP-1 (1) and EP-2 (2).8 The relationship between the C? values found using Eqn (57) and Rs calculated using Eqn (26a) is linear Rs ^ 5C?, (59) which is illustrated by Fig. 14. Equations (58) and (59) are qualitatively similar and the discrepancy of numerical coefficients can be due to the approx- imate character of these relations and to the difference between the methods used to calculate Rs.130 Recall that the parameter D characterises the degree of deformability (mobility) of a macromolecule fragment between chemical cross-linking points. Indeed, an increase in the chain flexibility brings about an increase in C?, i.e. an increase in D. It should be noted that extrapolation of straight lineD(C?) toD=1 gives C?^ 2.55; evidently, this is the limiting value for epoxy polymers considered.It is also noteworthy that it is close toC?for a linear polymer with a similar structure, for example, for polycarbonate, which has C?=2.4.90 Apparently, the deformabilitromolecule in vitreous epoxy polymers is determined not only by their molecular characteristics but also by supermolecular (supersegmental) `fro- zen' structure, which can be characterised quantitatively in terms of the cluster model. It should be borne in mind that an amorphous polymer contains regions of local order (clusters) and that fixing of one or several statistical chain segments in a cluster should change the D value. The most probable reason for the change in D is that the fixed segment becomes expelled from the freely jointed chain, as it has lost its mobility in the junction points.Evidently, this decreases the Ls value in Eqn (39) by lst and, finally, induces a decrease in D. Thus in the case of epoxy polymer EP-1, elimination of one statistical segment induces a decrease in D from 2.06 to 1.73; when two segments are eliminated, D decreases to 1.65; in the case of three segments, it decreases to 1.56, etc. In the absence of `frozen' local order, i.e., for rubbers (jcl=0), D=2 and the loss of chain mobility (deformability) is attained at jcl ^ 0.65. The opposite effect is also possible; an increase in ns induces a decrease in D, i.e. the chain becomes more taut, which should increase the level of local order.131 In the case of linear polymers, the network of macromolecular entanglements is the analogue of the network of chemical bonds; their density Ven is determined at T>Tg .Thus, relation (54) is a specific case of a more general fractal relation (57) and is applicable only to rubbers, for which it has actually been derived. c. Estimation of structural parameters The cited examples demonstrate the capacity and the prospects of using fractal concepts for the description of macromolecular characteristics of network and linear polymers. The change in the C? value induced by variation of ns or Ven implies that an absolutely different polymer can be formed although the chemical composition of the macromolecule has remained almost the same.This `new' polymer would have different molecular characteristics (C?, D), a different supersegmental structure (Vcl , jcl), and other properties (glass transition temperature Tg, elasticity modulus E, V U Novikov, G V Kozlov etc.). Equation (54) gives the maximum C? value possible for the given ns (see Table 6); the minimum values can be obtained by extrapolating the straight line D(C?) to D=1. At present, vitreous network polymers are mainly character- ised by the ns value.43 However, one network polymer with constant ns can possess different properties, for example, different glass transition temperatures.132 This is explained by the change in the supermolecular structure of a network polymer during phys- ical ageing, namely, by an increase in the level of local order.This effect has been described quantitatively in terms of the cluster model. In turn, the level of local order determines the fractal dimension df of the supermolecular (supersegmental) structure of the polymer.8 Thus, it becomes possible to predict structural characteristics (and, hence, properties) of network polymers in the vitreous state based on the degree of chemical cross-linking up to the gelation point. Analysis of the relationship between ns and df for the epoxy polymers considered showed that an increase in the fractal dimension of the framework to the gelation point induces a decrease in the fractal dimension of the supermolecular (super- segmental) structure of the network polymer in the glassy state.It has been noted in the Introduction that description of fractal structures requires, at least, two dimensions, ds and df, at a constant d value.27 However, the fractal concept is a mathemat- ical model, which does not provide identification of the specific features of the polymer structure. Therefore, in order to use the parameters ds and df in practice (for instance, in computer simulation), one has to find out which elements of the polymer structure (in particular, of network polymers) they embody. It is known 55 that ds characterises the degree of connectivity of the framework and df (which might be time-dependent) refers to the structure of this framework. The df value can be found from Eqn (27), which indicates that df is a function only of the Poisson coefficient m (for fixed d ).For amorphous polymers, m is deter- mined by the degree of local order;46 hence, this structural factor also determines df.As regards the spectral dimension ds , it appears to be related in one way or another to the framework formed by chemical cross-links. This suggestion was based on the results of two studies by Kozlov et al.115, 133 In the former study, the relationships between ns and the degree of local order and between ds and df were elucidated. The latter study demonstrated the variation of properties of epoxy polymers with time (during physical ageing) following the change in the level of local order (or df) with ns remaining invariant. The ds value can be calculated from Eqn (8).For estimating the dimension dw , needed for this purpose, several relations exist;23, 36, 55, 60, 122, 134, 135 they give somewhat different (to within *20%) absolute magnitudes of this parameter. If the variant of constant dw is chosen and the model of `lattice animals' (d=3) is used for describing branched polymers, the dependence of ds on ns would follow a somewhat unexpected pattern. According to this dependence, an increase in the density of cross-linking points results in lower ds and, hence, in lower connectivity of the macro- molecular framework of chemical cross-links. It is noteworthy that for great ns, the ds value approaches asymptotically the experimental spectral dimensions of epoxy polymers.22, 35, 69 This finding can be interpreted in terms of the generally accepted models of the structure (or, more precisely, morphology) of network polymers.136 The main morphological unit of densely cross-linked epoxy polymers is a globule. It is a local thickening of a molecular space network the packing density in which is higher than the average density.During etching of fractured surfaces of epoxy polymers, this thickening is revealed as a discrete particle � a globule. The non-homogeneity of network polymers is due to both the kinetics of formation of a three-dimensional network and the presence of associates in the initial oligomers.136 Two variants of construction of a supermolecular (globular) structure are possible: (a) accrete globules with a hexagonal or cubic packing form a strong continuous skeleton in which the roleFractal analysis of macromolecules of inter-globule defective areas is insignificant; (b) densely cross- linked globules (grains) are located in a sparse-network matrix and do not contact directly one another.The former variant of morphology seems to be more fre- quently encountered. An increase in the concentration of a cross- linking agent induces simultaneous generation of a large number of chemical reaction centres; this is confirmed indirectly by a decrease in the size and the increase in the number of globules.116 This results in a smaller size and a greater number of inter-globule defective areas; thus, the connectivity of the skeleton diminishes (defective areas act as sorts of skeletal discontinuities) and, hence, ds decreases.In the case where the morphology of a network polymer corresponds to the second variant, its structure can be represented as a mixture of arbitrary polymer fractals in the vicinity of the gelation point; the spectral dimension of the globule is ds and that of the inter-globule area is ds. The ds value can be estimated using the relation 60 df=ds Od a 1UO2 ¢§ dsU O2 a ds ¢§ 2dsU . As a first approximation, it was assumed in this relation that the spectral dimensions of the structures of the epoxy polymer and the globules are equal. It was found that the dependence of ds on ns for epoxy polymers shows parallel variation of ds and ds upon the variation of ns provided that ds>ds .60 This condition indicates that the skeleton in the inter-globular gaps is less connected than the globules themselves because it is more defective. Thus, the density of emical cross-linking points cannot serve as an index for the connectivity of the macromolecular skeleton of network polymers. This makes impossible the use of ns to characterise the structure of network polymers in a computer simulation, which follows from the results presented above. The ds value, which provides determination of elastic properties, may serve as a suitable parameter. However, to estimate other proper- ties, one more parameter is required, which would characterise the degree of thermodynamic nonequilibrium of the structures of vitreous polymers.This role can be played by df or the density of the cluster network of physical entanglements Vcl , 47 or by the proportion of clusters jcl . 137 For instance, the necessity to take into account ds , Vcl or jcl for calculating the yield point from known elasticity modulus has been demonstrated by Beloshenko and Kozlov. 47 Therefore, the frequently encountered correlations between properties of network polymers and ns can serve only as an illustration. The use of two parameters (for example, ds and df with a constant d ) is a mandatory (fundamental) condition which follows from the fractality of intermolecular frameworks of vitreous polymers; in turn, the fractality is a consequence of thermodynamic nonequilibrium of this structure.3. Description of molecular mobility using fractal characteristics The problems of molecular mobility in polymers have always received considerable attention.32, 95, 138 The reasons are obvious �¢ polymers are thermodynamically nonequlibrium sol- ids, their physical properties being determined by molecular relaxation processes occurring in them; these processes, in turn, depend on specific structural features of molecular chains and the structural organisation of polymers.138 However, there is no consensus regarding the parameters describing these processes. In particular, it is assumed that fast relaxations are determined by the mobility of free chains located between densely packed regions, which are simultaneously the nodes of the network of physical entanglements in a macromolecule.This interpretation fully complies with the main statements of the cluster model for the structure of amorphous polymers,15 which provides quantita- tive description of structural elements. The relationship between the cluster model and fractal analysis can be formulated as follows: fractal analysis gives general physical parameters, while 363 the cluster model makes them more specific using particular characteristics of polymers. Yet another important aspect is worth noting. The above interpretation of the structure of amorphous polymers refers to elastomers. Extension of these notions to vitreous amorphous polymers implies `freezing' of densely packed regions, i.e. a sharp increase in their lifetimes.The fractal forms of macromolecules, macromolecular coils formed during nonequilibrium physico- chemical processes are retained (remain `frozen') in polymers. This suggests that the main factor determining the molecular mobility in the vitreous state is the mobility of chain fragments between the points of fixing.111 The fractal dimension D has been chosen as the structural factor characterising the level of molecular mobility;111 this was done for the following reasons. First, a chain fragment between the points of fixing is a self-similar object having a dimension differing from the topological dimension, i.e. it is a fractal by definition.107 Second, it was shown101 that D with the range of variation 1<D42 characterises exactly the molecular mobility of a chain fragment in loosely packed regions.138 The condition D=1 implies stretching of this chain fragment and loss of fractal properties and mobility.In terms of relaxation spectroscopy, this means that tan d =0, where tan d is the mechanical (or dielectric) loss tangent. The condition D=2 means the maximum possible mobility of the chain section, which corresponds to a rubbery state of the polymer, i.e. it is consistent with the maximum value of tan d at the glass transition temperature Tg . Figure 15 shows the dependence of tan d on D for several copolymers 111 [the D values were calculated using Eqn (48)] for a measurement frequency of 1 kHz. The dependence has clearly defined limits.For D=1, it is extrapolated to tan d =0. When D=2, tan d is roughly equal to the corresponding value at Tg. Thus, linear dependences with the limiting tan d values indicated above can be used to predict the fractal dimension. (tan d)6102 106 �¢1 �¢2 �¢3 �¢4 �¢5 �¢6 2 D 1.4 1.2 1.0 Figure 15. Dielectric loss tangent tan d vs. fractal dimension D of the macromolecule section for copolyethersulfoneformals with the content of formal blocks of 0 (1), 5 (2), 10 (3), 30 (4), 50 (5) and 70 (6) mol.%. Measurement frequency 1 kHz.111 Now we shall consider examples of using the dimension D for the solution of applied problems. When polymer samples are obtained by solid-phase extrusion, the macroscopic (extrusion) draw ratio le is determined by the ratio 119, 139, 140 (60) le= 2b , 2spin where b , spin are the diameters of the billet and the spinneret, respectively. However, the structure and properties of oriented polymers are characterised much more adequately by the molecular draw364 ratio lmol .122 The following expression relating le, lmol and D to one another has been obtained:120 (61) lmol=lstl2e =D .Rs It follows from this relation that the lower D, i.e. the more `frozen' the molecular mobility of the chain during drawing, the greater lmol (the more efficient the orientation process), all other factors being the same. The limiting draw ratio l? attained by stretching a polymer is determined, in terms of the rubbery high-elasticity theory, by the equality 121 st , l? = n 1=2 (62) where nst is the number of statistical segments.This equation has been applied repeatedly to vitreous poly- mers. The following fractal relation has been obtained:38 (63) l? = n D=4 st . It can be easily seen that an increase in D for nst=const enhances the deformability of polymers. Equations (62) and (63) are identical when D=2, i.e. for rubbers. An important difference between Eqns (62) and (63) is that in the former, l? depends on one parameter, which is typical of equilibrium Euclidean objects, while in the latter, it is a function of two parameters, which is typical of thermodynamically nonequili- brium fractal objects. Therefore, the use of Eqn (62) in the latter case is improper. The relationship between the statistical chain flexibility, characterised by C?, and the dimension D has been found 141(64) D=27 1 .C? Using published data for C? (see, for example, Ref. 90) and Eqns (27) and (49), the lengths of chain sections between entan- glements Len were calculated. The results were in agreement with experimental data (Table 7). Thus, the increase in the density of the network of macromolecular entanglements following an increase in the statistical chain rigidity was confirmed within the framework of fractal analysis. By combined use of Eqns (27) and (49), one can predict the increase in the number of points of topological fixing of a macromolecule in the vitreous state with respect to the rubbery state.141 The fractal dimension D of a chain fragment between the points of topological fixing (entanglements, clusters, cross-links) is an important structural parameter, which controls the molec- Table 7.Characteristic ratios C? and Len . 131 Polymer Len/ nm C?90, 132 experiment calculation 226 177 104 Poly(n-dodecyl methacrylate) 13.4 10 109.4 9.1 8.6 6.8 6.7 6.3 5.3 5.1 4.2 4.2 3.3 2.4 Poly(n-octyl methacrylate) Polystyrene Poly(vinyl acetate) Poly(n-butyl methacrylate) Poly(methyl methacrylate) Polyethylene Poly(vinyl chloride) Polytetrafluoroethylene Polyamide-6 Poly(propylene oxide) Poly(ethylene oxide) Poly(ethylene terephthalate) Polyetherterephthalate Polycarbonate 88 130 97 56 31 41 46 60 45 21 28 29 208 149 149 136 131 92 88 87 80 61 58 43 36 30 17 V U Novikov, G V Kozlov ular mobility and deformability of polymers.Crucial factors accounting for the use of the dimension D are clearly definariation (1<D42) and dependence on the super- molecular structure of the polymer. It should be emphasised that all fractal relations contain at least two variables. Therefore, correlations between some property of polymers and the density of the network of entanglements (cross-links), which are fre- quently encountered in the literature, can serve only as illustra- tions. The necessary use of at least two parameters (for instance, ds and df with a constant d) is a fundamental condition, which follows from the fractality of the intermolecular skeletons in vitreous polymers.IV. Conclusion The fractal properties of a macromolecule in a polymer are predetermined by thermodynamic nonequilibrium and by the presence of a certain deterministic order. Proceeding from this fact, we considered the limitations of the scope of the concept of polymer fractal, developed for polymer solutions, and the Vilgis theory for condensed media (melts and rubbers). The major drawback of these approaches is that they neglect the effects of macromolecular entanglements. It is demonstrated that a descrip- tion of the fractal properties of macromolecules cannot be physi- cally correct without allowance for entanglements.Examples are given to illustrate the diversity of fractality in polymers. In particular, it is shown that not only macromolecular coils but also sections of macromolecules between the points of topological fixing (cross-links, entanglements) are stochastic fractals. Conditions required for the fractality to be manifested are discussed. They include the range of scales, self-similarity and fractional dimension. It is shown that a section of a macro- molecule between the points of topological fixing possesses con- figurational fractal properties, caused by the mutual arrangement of fragments of the macromolecule, and conformational fractal properties, conditioned by the state of bonds within a section of rigidity (i.e. statistical segment).In this connection, a model of structure formation in network polymers is considered; this model includes a quantitative determination of the relationship between the cross-linking density and the level of local order of the structure. It is demonstrated that, in addition to the parameters of local order, fractal dimensions of the polymer structure should also be taken into account. Examples of practical use of the fractal dimension of the section of a macromolecule between chemical cross-linking points are given; it is shown that this value can serve as a measure of molecular mobility. 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ISSN:0036-021X    

出版商:RSC    

年代:2000

数据来源: RSC