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FEATURE ARTICLE Concave reagents Ulrich Lu�ning† Institut fu� r Organische Chemie, Olshausenstr. 40, D-24098 Kiel, Germany The concave geometry of enzymes is important for the high selectivity of their reactions. By analogy, the incorporation of standard reagents of organic chemistry into a concave environment gives concave reagents with a geometry resembling that of a light bulb in a lampshade. Selectivity enhancements have been realized in protonations, base catalysed reactions and metal ion catalysed reactions.The geometry of enzymes is one factor which determines their of the reaction has been achieved with a cyclophane which is able to bind positively charged guests.3 high selectivity. The reactive centre is embedded in a concave pocket of the protein, and these geometrical factors are respon- Other approaches to mimicking enzymatic activity have taken coenzyme moieties, especially the coenzymes of the sible for a large part of their extraordinary selectivity (lock and key principle1).vitamin B series, and incorporated them into macrocycles or attached them to binding sites.2 Hydrolases have been mim- Therefore enzymes have been more and more often used as reagents in organic chemistry.However, they cannot be used icked by placing acidic and basic centres into geometrically defined host molecules.2 A well defined orientation of an acidic in all cases for the following reasons: (i) many reactions exist for which no enzyme is known; (ii) enzymes are very specific, and a basic centre is also responsible for an enolization and a hemiacetal cleavage catalysed by an artificial host system.4 and are often unreactive when the substrate has been modified slightly, or the selectivity diminishes drastically when the The accelerated oxidation of a diphenolic benzyl alcohol5 is also caused by geometrically well defined binding regions and substrate is varied; (iii) enzymes are labile, and often cannot be handled in organic solvents, at low and high pH values, in reactive sites.Further mimicking activities have concentrated on the acti- solution with high ionic strength or at high temperatures. Therefore, many approaches have been used to try to under- vation of a substrate by binding it to a host system. Also effective are systems that bind two components in close proxim- stand the action of enzymes and to mimic them with arti- ficial systems.2 ity to allow and accelerate a bimolecular reaction.Successes have been observed in hydrolyses,6 regioselective 1,3-dipolar Enzymes accelerate reactions by binding the transition state and thus lowering its energy. For the methylation of quinoline, cycloadditions,7 Diels–Alder reactions and acyl transfer the stabilization of the transition state and thus an acceleration Reproduced from ref. 3 with permission of VCH Verlagsgesellschaft mbH, Weinheim † E-mail: noc03@rz.uni-kiel.d400.de Reprinted with permission from ref. 5, Copyright 1994 American Chemical Society J. Mater. Chem., 1997, 7(2), 175–182 175Reproduced from ref. 4 with permission of VCH Verlagsgesellschaft mbH, Weinheim reactions.8 Non-macrocyclic templates have also been synthe- host–guest complexes, can prevent this (inhibition).For effecsized for nucleophilic substitution reactions.9 tive reactivity or catalytic activity this reaction must be If the product of such a reaction is the template itself, minimized. replication may be observed, an aspect of the well studied field For the results presented here, the geometry of enzymes has of molecular self-organization.10 The systems first used to been our inspiration.The work presented does not at all try investigate self-replication were nucleotides (as in nature),11 to mimic enzymes, but instead tries to use the concept of but the formation of amides12 and Schiff bases13 can also be concavity to increase the selectivity of organic reactions.This catalysed autocatalytically. article will demonstrate that improved selectivities can be The interpretation of the data obtained from reactions achieved when the standard reagents of organic chemistry are involving catalysts, especially autocatalysts, is difficult. combined with aconcave geometry such as is found in enzymes. Catalysis by other molecules in the mixture and/or product The approach is therefore to take a reagent and to incorporate inhibition8 leads to complex kinetic behaviour.14 it into a concave environment in the same way as a light bulb In summary, current efforts to mimic enzymes have met sits in a lamp shade (Fig. 1).We call these compounds concave with limited success, and the catalytic efficiency of enzymes reagents.15,16 has yet to be reached for artificial systems.2a However, the In comparison to enzymes this approach has the following insight gained into the mechanisms of enzymes gives guidelines advantages: (i) the nature of the lampshade is not restricted to for the development of non-enzyme catalysts and reagents amino acids—the building blocks are very variable; (ii) there- which will be especially useful if the reaction cannot be carried fore substitution and tailoring of the geometry of the active out by enzymes.site is easier; (iii) lower molecular weights will be possible It should be noted that three different types of interaction because the task of most amino acids in an enzyme is to between a substrate and a concave host are possible: reaction, stabilize the reactive conformation by building up a large catalysis and host–guest complex formation.In a reaction as backbone; (iv) due to the large variability of the building well as during catalysis, the concave molecule induces a change blocks, concave reagents can be made solvent, pH and tempera- in the substrate. But during a reaction the host molecule itself ture resistent. is also altered, e.g.oxidized or reduced, protonated or depro- Let us now examine the model of the concave reagent: a tonated. During catalysis, however, the host molecule leaves light bulb in a lampshade.17 The light bulb stands for the the reaction unchanged and will be able to participate in the reactive centre, the lampshade is the concave shielding. If this next catalytic cycle.But the third interaction, the formation of geometry is translated into molecular dimensions, some requirements have to be met: (i) the rim of the lampshade must be a ring; (ii) in order to let molecules or parts of molecules pass through this ring to reach the active site, it must be macrocyclic; (iii) this macrocycle must be spanned by a bridge which carries the functional group.The minimal requirement for a molecular lamp is therefore that it must be at least bimacrocyclic.18 A concave reagent can in principle possess any functional group. So far we have incorporated acids, bases and catalytically active metal ions. These functional groups have to be fixed into the spanning bridge in such a defined way that they are inside. This can be achieved by using stiff aromatics substituted in the ortho- or a-positions, e.g. 2,6-disubstituted benzene derivatives, 2,6-disubstituted pyridines or 2,9-disubstituted 1,10-phenanthrolines. As stated above, concave reagents are at least bimacrocyclic. Three bridges have therefore to be connected by bridgeheads. This task can be mastered by using nitrogen atoms or trisubsti- Reproduced from ref. 2(a) with permission of VCH Verlagsgesellschaft mbH, Weinheim tuted aryl rings. 176 J. Mater. Chem., 1997, 7(2), 175–182Reproduced from ref. 4 with permission of VCH Verlagsgesellschaft mbH, Weinheim Reproduced from ref. 13 with permission of VCH Verlagsgesellschaft mbH, Weinheim Fig. 2 Strategies forthe synthesis of bimacrocyclic compounds containing a functional group in one chain (here pyridine) case, two strategies can be used: the construction of a macrocycle containing the functional group followed by a second macrocyclization, and the use of presynthesized macrocycles which are bridged in a second reaction step with a building block containing the functional group.All three methods have been realized (see Fig. 2 and 3): (i) bislactams and sulfonamides Fig. 1 Light bulb and lampshade model of concave reagents have been synthesized by a wise reaction starting from a precursor containing a pyridine or a 1,10-phenanthroline unit;19 (ii) in the pyridine-bridged calixarenes a pyridine- The task was therefore to synthesize bimacrocyclic 2,6- substituted acids, bases or ligands for metal ions. In general, containing unit has been used to bridge the calixarene macrocycle; 20 (iii) bimacrocyclic diaryl pyridines,21 diaryl 1,10-phen- three possibilities exist for the synthesis of such a bimacrocyclic reagent: stepwise macrocyclizations or simultaneous construc- anthrolines22 and diaryl benzoic23 and benzenesulfinic acids,24 as well as a diaryl-m-terphenylthiol25 and its acetate,26 have tion of both macrocycles in one reaction step.In the former J. Mater. Chem., 1997, 7(2), 175–182 177Fig. 4 Space-filling drawings of a concave 1,10-phenanthroline cyclophane with octamethylene side chains based on X-ray data [ref. 22(b)]. In the side view, the nitrogen atoms of the 1,10-phenanthroline are hidden behind the octamethylene chains. They are accessible from below through the ‘rim of the lampshade’.The accessibility of the functional groups of those concave acids and bases for which X-ray data are available has been studied by computer analysis (Connolly routine).27 Spheres of varying sizes have been rolled over the van der Waals surfaces generated from the X-ray data, and the resulting contact surface has been monitored. The simulations show clearly that small spheres are able to enter the cavity and can contact the functional group(s), whereas larger spheres are too bulky.The cut-off radius for a sphere to contact the nitrogen atoms of the concave 1,10-phenanthroline of Fig. 4 is 2.8 A° . Model reactions Many concave reagents have been synthesized and are now accessible in useful quantities. They have been tested in a number of model reactions to learn which geometries lead to an increase in selectivity. In all model reactions an organic transformation has been chosen which can be carried out with anon-concave standard reagent (acid, base, metal ion complex). The changes in reactivity and selectivity caused by the incorporation of the functional group into the concave environment have been investigated.These results have then been compared to those obtained with non-concave molecules and other sterically shielded, related molecules.In the reactions chosen, the concave molecules have been applied as reagents and as catalysts. In contrast to a catalyst, a reagent is used up in a reaction. It is therefore necessary to find methods for ‘recharging’ the reagents because their synthesis is too tedious for them to be used only once.Such recharging is very easy for acids and bases. Therefore, the only reaction so far in which concave reagents have been employed Fig. 3 Examples of concave pyridines (refs. 19,20,21), 1,10-phenanthro- in amounts equal to or exceeding the stoichiometry were lines (ref. 19, 22), benzoic acids (ref. 23), sulfinic acids (ref. 24), thiols proton transfer reactions.In all other reactions, the concave (ref. 25) and thiol acetates (ref. 26) reagents were used as catalysts. Protonations been synthesized in one reaction step starting from appropriate tetrafunctionalized precursors. Most asymmetrically substituted carbon atoms have a hydrogen atom as one of the four different substituents. If this To achieve high yields in these cyclizations either high dilution principle conditions or template effects have been hydrogen atom could be attached selectively to a prochiral carbon atom by a reagent controlled protonation, many exploited.The yields vary but in many cases yields of 30–50% for the bismacrocyclizations have been achieved in both the enantio- and diastereo-selectivity problems would be solved. However, there are always two mechanistical pathways one step and the two step reactions.Therefore concave reagents are relatively easy to obtain, and many of them have been possible for a kinetically controlled protonation: general and specific protonation (Scheme 1).28,29 The proton can either be synthesized in multigram quantities. Some of the concave reagents synthesized in past years are compiled in Fig. 3. transferred directly from the acid to the substrate (reagent control, general protonation) or via the solvent or co-solvent After the synthesis of the concave reagents, the question of whether these new molecules are indeed concave had to be (shuttle mechanism, specific protonation). Therefore for a reagent controlled protonation, reagents and reaction con- answered.A number of X-ray analyses have been carried out, proving the lamp-like concave structure of the bimacrocyclic ditions have to be worked out to allow general protonation exclusively. compounds. Fig. 4 shows the X-ray structure of a concave diaryl-1,10-phenanthroline with two octamethylene side chains The protonation of three different groups of anions has been carried out: nitronate ions,30 ester enolates25a,31 and allyl X22b as an example. 178 J. Mater. Chem., 1997, 7(2), 175–182a hydrogen atom, which explains the contra-thermodynamic course of this protonation. Surprisingly, acids successful in one of the reactions in Scheme 2 are often not suitable for the general protonation of another anion. The c/a-selectivity of the protonation of the allyl anions could be increased by a variety of acids.Tetra-ortho-methylsubstituted m-terphenyls carrying an acidic group in the 2¾-position Scheme 1 General and specific protonation of an anion A-. Direct were successfully applied, leading to selectivities between 90510 proton transfer from the acid of the buffer (X–H) to the anion A- (X=CH2OH) and 9654 (X=SH). The results were almost leads to a reagent controlled general protonation, while dissociation independent of the nature of the group X.of the acid and proton transfer via the protonated solvent So H+ gives an uncontrollable specific protonation. To avoid thermodynamic control the deprotonation of A–H must not occur. anions.25a,32 Scheme 2 summarizes the investigated reactions and shows which stereo- and regio-isomers might be formed.A variety of concave and other acids have been used for the reactions in Scheme 2. Not all of them led to a change in However, when the same acids were tested for the stereoselec- selectivity, but for each anion acids could be found with which tive protonation of the cyclohexane ester enolate, only the acid a reagent controlled, general protonation was achieved and with the smallest acidity, the alcohol (X=CH2OH), gave a with which the regio- or stereo-selectivities could be altered.good cis-selectivity (9456). m-Terphenyls carrying groups with Steric shielding of the proton in the concave acid caused an a higher acidity showed no selectivity. Presumably the unselec- increase in c-protonation of the allyl anions, while the protive specific protonation is faster than the general protonation tonation of the cyclic anions by sterically shielded acids led to if the acidity of the m-terphenyl is higher.an increase of the cis-products. This competition between general and specific protonation In the latter case the thermodynamically less stable products can be exploited. If the proton in a concave acid is extremely have been formed.This can be rationalized by inspecting the shielded, a general protonation will be retarded, which should transition state of such a protonation (Fig. 5). The proton is allow a specific protonation even if the concentration of still partly bound to the concave acid, a m-terphenyl deriva- protonated solvent is very low (high pH) (Scheme 3). This has tive25a,31 or a protonated 2-aryl-1,10-phenanthroline.30 The been used to carry out the Nef reaction33 which usually proton is therefore a large pseudo-substituent for which an requires strongly acidic conditions in a buffered medium.Due equatorial orientation is favoured. In the product, however, to the mild reaction conditions this reaction has been called the large pseudo-substituent becomes the smallest substituent, the soft Nef reaction.30 The Nef reaction is an important method for the conversion of nitro compounds into carbonyl compounds.The strong acidic conditions needed for this reaction have often prohibited its application to molecules with acid labile groups. However, the reaction conditions of the soft Nef reaction are mild enough that protective groups such as tert-butyldimethylsilyl (TBDMS), methoxymethyl (MOM) or acetals survive.30d Base catalysis In contrast to protonations, which have to be carried out with an at least equimolar amount of acid, in a base catalysed reaction less concave reagent is needed if the reaction occurs fast enough.In addition, the catalyst will remain unaltered during the reaction. For example, the base catalysed addition of alcohols to Scheme 2 A general protonation of allyl anions by an acid X–H leads ketenes has been investigated for concave and non-concave to c- and a-regioisomers, while a general protonation of the cyclic anions gives cis- and trans-products pyridines.34 By formation of a hydrogen bond between the alcohol and the nitrogen atom of the pyridine, the nucleophilicity of the alcohol is increased and the rate of its addition to the ketene is increased.The net reaction is an acylation of the hydroxy group (the formation of an ester) which may serve as a protective group for the alcohol. Fig. 5 Alternative transition states for protonation of cyclic anions by Scheme 3 a concave acid J. Mater. Chem., 1997, 7(2), 175–182 179Table 2 Acylation of a chinovose derivative in the presence of a The rates of the catalysed addition depend on the geometry concave catalyst and the basicity of the catalyst.34 By plotting the logarithms of the rates of addition versus the basicities (Brønsted plot), geometrical and basicity influences on the reactivity of the pyridines could be separated.34,35 For pyridines of the same geometry, linear correlations exist between the basicity and product yield (%) the rate.The more shielded a pyridine is, the smaller is the catalyst (R2,R4=H,R3=Ph2CHCO) rate of addition.34 However, when a size variation in the concave pyridines pyridine 29 gives rise to a change in rate, size variation of the alcohol should also have an influence on the reaction rate.In other words, the addition of primary, secondary or tertiary alcohols should be catalysed differently (Fig. 6). Therefore the base catalysed addition of different hydroxy species (EtOH, PriOH, ButOH or propane-1,2-diol) has been compared and indeed a selectivity increase in favour of the acylation of primary hydroxy groups has been found when concave pyridines have R=OMe 60 been used.36 R=NEt2 48 Thus concave pyridines are able to discriminate between different alcohols, which would be very useful if it could be exploited in intramolecular competition reactions.For example, two carbohydrate derivatives were reacted with diphenylketene in the presence of concave catalysts. The results of the acylation of a glucose derivative, and those of the acylation of a chinovose derivative are listed in Tables 1 and 2, respectively.In both carbohydrates, the hydroxy groups are secondary and equatorial. The use of a concave pyridine leads to the predominant formation of only one product (>951 in the glucose case; one of seven possible acylation products in 60% yield in the chinovose case).36 Metal ion catalysis Fig. 7 A metal ion bound to a concave 1,10-phenanthroline The free electron pair(s) in concave pyridines, and especially in concave 1,10-phenanthrolines, are not only able to bind a proton, they may also complex a metal ion (Fig. 7). For already been generated.22b,37 They form readily in acetonitrile concave 1,10-phenanthrolines, transition metal complexes have solution with binding constants of 104–107 and larger.These complexes have been applied in two transition metal ion catalysed reactions: the Lewis acid catalysed Diels–Alder reaction37 and the Cu+-catalysed cyclopropanation of alkenes by diazo compounds.38 Scheme 4 and Fig. 8 show the Ni2+ and Co2+-catalysed cycloaddition of pyrazole-substituted acrylamides with cyclopentadiene and the increase in exoselectivity when concave ligands are used.Fig. 6 Model of a concave pyridine–alcohol complex Table 1 Acylation of a glucose derivative in the presence of a concave catalyst product yield (%) R2=H R2=Ph2CHCO R2=Ph2CHCO catalyst R3=Ph2CHCO R3=H R3=Ph2CHCO none 36 42 3 pyridine 12.5 50 12.5 7 67 — Scheme 4 The reaction of pyrazole-substituted acrylamides with cyclopentadiene can be catalysed by transition metal salts.With increasing shielding of the metal ion by the use of concave 1,10-phenanthrolines, the exo/endo-selectivity can be shifted towards exo. 180 J. Mater. Chem., 1997, 7(2), 175–182Table 3 The Cu+-catalysed cyclopropanation of alkenes by diazo compounds trans/cis ratio alkene without ligand with ligand Fig. 8 Plot of ln(exo/endo) for the reaction of pyrazole-substituted acrylamides with cyclopentadiene catalysed by transition metal salts and various concave ligands (Scheme 4): (() M=Ni, R=Cl; (,) M= Co, R=Cl; (%) M=Ni, R=Me; (&) M=Co, R=Me; (') M=Ni, R=H; (+) M=Co, R=H Without the presence of a ligand, endo-norbornenes are usually the main products.But when the metal ion is sterically shielded an exo-preference is found, which can be rationalised as shown in Fig. 9. In the transition state leading to the endonorbornene, the cyclopentadiene has to approach with the atoms C-2 and C-3 foremost, which is sterically disfavoured compared to the orientation leading to the exo-compound (C- 5 foremost). The second transition metal ion catalysed model reaction is shown in Table 3, the Cu+-catalysed cyclopropanation of alkenes by ethyl diazoacetate. With CuI salts alone a mixture 1.1 3.0 1.4 4.7 3.3 56 5.2 66 2.1 73 1.3 12 of cis- and trans-cyclopropanes or endo- and exo-cyclopropanes is formed. The use of concave ligands leads to the preferred formation of the trans-product, e.g. 7351 for indene when a diaryl-1,10-phenanthroline was used as ligand for the catalyst instead of 2.151 without concave ligand.38 The increase in the formation of the trans-products of the cyclopropanation of a cis-alkene is explained in Fig. 10. First Cu+ reacts with a diazoacetate molecule and forms a carbenoid, which is then transferred to the alkene. Due to the concave shielding, the sterically most favoured orientation is unproductive because the CNC and CuNC bonds are orthogonal. Therefore the alkene must rotate with respect to the carbenoid, leading to two twisted transition states in which Fig. 10 Due to the concave shielding of the copper ion the least two orientations of the substituents of the alkene are possible: hindered approach of the alkene to the carbenoid formed by the anti to the ester leading to a trans-product or syn leading to reaction of Cu+ with ethyl diazoacetate is the unproductive orthogonal approach.A clockwise rotation of the alkene diminishes the steric the cis-product. The greater steric repulsion in the syn-orien- interactions between the substituents R of the alkene and the ester tation explains the large trans-selectivity. group, favouring the formation of the trans-products. Outlook limited by the macrocyclization steps. Therefore for practical As outlined in this article, various classes of concave reagents use these reagents are quite ‘expensive’ and recovery and may be synthesized in multigram quantities.However, the recycling is necessary. syntheses are multistep sequences and the yields are often In order to be able to reisolate the concave reagents by filtration we have attached 4-substituted concave pyridines to a polymer (Merryfield resin) (Scheme 5).30d,39 The amount of concave pyridine bound to the polymer has been determined to be ca. 20% (w/w). The polymer-bound concave pyridines are catalytically active and the selectivity in the base-catalysed addition of hydroxy groups to ketenes is comparable to that of non-bound concave pyridines. The concave pyridine-loaded polymer resin has been placed in an HPLC column and the selective acylation of the hydroxy groups in the glucose derivative used in Table 1 with diphenylketene has been successfully carried out.For further application of the concave acids, bases or complexes presented in this article, these compounds need to Fig. 9 The approach of cyclopentadiene towards the complexed acrylamide determines the exo/endo-selectivity be investigated more thoroughly.As seen in the protonation J. Mater. Chem., 1997, 7(2), 175–182 18110 D. Philp and J. F. Stoddart, Angew. Chem., 1996, 108, 1242; Angew. Chem., Int. Ed. Engl., 1996, 35, 1154. 11 (a)W. S. Zielinski and L. E. Orgel,Nature (L ondon), 1987, 327, 346; (b) T. Achilles and G. von Kiedrowski, Angew. Chem., 1993, 105, 1225; Angew. Chem., Int.Ed. Engl., 1993, 32, 1198. 12 J. S. Nowick, Q. Feng, T. Tjivikua, P. Ballester and J. Rebek, Jr., J. Am. Chem. Soc., 1991, 113, 8831. 13 A. Terfort and G. von Kiedrowski, Angew. Chem., 1992, 104, 626; Angew. Chem., Int. Ed. Engl., 1992, 31, 654. 14 (a)M.M. Conn, E. A. Wintner and J. Rebek, Jr., J. Am. Chem. Soc., 1994, 116, 8823; (b) F.M. Menger, A. V. Eliseev, N. A. Khanjin and M.J. Sherrod, J. Org. Chem., 1995, 60, 2870; (c) D. N. Reinhoudt, D. M. Rudkevich and F. de Jong, J. Am. Chem. Soc., 1996, 118, 6880. 15 U. Lu�ning, L iebigs Ann. Chem., 1987, 949. 16 Review: U. Lu�ning, T op. Curr. Chem., 1995, 175, 57. 17 Our concept of concave reagents (ref. 15) has been adopted and ‘reaction bowls’ have been synthesized: K. Goto, N. Tokitoh and Scheme 5 Attaching a concave pyridine to a polymer R.Okazaki, Angew. Chem., 1995, 107, 1202; Angew. Chem., Int. Ed. Engl., 1995, 34, 1124. 18 The term bimacrocycle will be used instead of macrobicycle reactions, equilibria between bound and free protons exist and because bimacrocycles often also contain smaller rings (e.g. aryl can alter the course of the reaction. Therefore the association rings).They are thus macropolycyclic although only two macro- of the reagents inside the lamp shade must be guaranteed. In cycles exist. (In addition, no bicycle company has offered sponsor- other words, large pKa or Kass binding constants are required. ship yet.) 19 (a) U. Lu� ning, R. Baumstark, K. Peters and H. G. v. Schnering, L iebigs Ann. Chem., 1990, 129; (b) U. Lu� ning, R.Baumstark and M. Mu�ller, L iebigs Ann. Chem., 1991, 987; (c) U. Lu� ning and M. Mu�ller, L iebigs Ann. Chem., 1989, 367. 20 H. Ross and U. Lu�ning, Angew. Chem., 1995, 107, 2723; Angew. Chem., Int. Ed. Engl., 1995, 34, 2555; (b) H. Ross and U. Lu�ning, L iebigs Ann. Chem., 1996, 1367; (c) H. Ross and U. Lu�ning, T etrahedron, 1996, 52, 10879. 21 U. Lu�ning, R. Baumstark and W.Schyja, T etrahedron L ett., 1993, 34, 5063. 22 (a) U. Lu� ning and M. Mu�ller, Chem. Ber., 1990, 123, 643; (b) U. Lu� ning, M. Mu�ller, M. Gelbert, K. Peters, H. G. von Schnering and M. Keller, Chem. Ber., 1994, 127, 2297. 23 (a) U. Lu� ning, C. Wangnick, K. Peters and H. G. v. Schnering, Chem. Ber., 1991, 124, 397; (b) U. Lu� ning and C.Wangnick, L iebigs Ann. Chem., 1992, 481. 24 U.Lu�ning and H. Baumgartner, T etrahedron, 1996, 52, 599. 25 (a) U. Lu� ning, H. Baumgartner, C. Manthey and B. Meynhardt, J. Org. Chem., 1996, 61, 7922; (b) H. Baumgartner, Dissertation, Universita�t Freiburg, 1996. 26 U. Lu�ning and H. Baumgartner, Synlett, 1993, 571. This work was carried out by my talented co-workers whose 27 QCPE program No. 429 by M. L. Connolly, used with Chem-X, names appear in the references.Financial support came from developed and distributed by Chemical Design Ltd, Oxford, the Deutsche Forschungsgemeinschaft, the Fonds der England. Chemischen Industrie, the Schwerpunkt des Landes Baden- 28 The terms general and specific protonation are used in the same sense as the terms general and specific acid catalysis are used Wu�rttemberg: Sensoren and the Wissenschaftliche (ref. 29): specific means a specific protonated solvent molecule is Gesellschaft Freiburg. the reacting species while general means that in general all acids in solution contribute to the reaction. 29 For general and specific acid catalysis, see H. Maskill, T he Physical Basis of Organic Chemistry, Oxford University Press, New York, References 1993.For general and specific protonation see ref. 32(a). 30 (a) U. Lu� ning, R. Baumstark, M. Mu�ller, C. Wangnick and 1 100 years of lock and key principle: (a) A. Eschenmoser, Angew. F. Schillinger, Chem. Ber., 1990, 123, 221; (b) U. Lu� ning and Chem., 1994, 106, 2455; Angew. Chem., Int. Ed. Engl., 1994, 33, F. Schillinger, Chem. Ber., 1990, 123, 2073; (c) U. Lu� ning and 2363; (b) F.W. Lichtenthaler, Angew. Chem., 1994, 106, 2456; M. Mu�ller, Angew. Chem., 1992, 104, 99; Angew. Chem., Int. Ed. Angew. Chem., Int. Ed. Engl., 1994, 33, 2364; (c) D. E. Koshland, Jr., Engl., 1992, 31, 80; (d) W. Hacker, Dissertation, Universita�t Angew. Chem., 1994, 106, 2468; Angew. Chem., Int. Ed. Engl., 1994, Freiburg, 1996. 33, 2408. 31 C. Manthey, Diploma Thesis, Universita�t Kiel, 1995. 2 Reviews: (a) A. J. Kirby, Angew. Chem., 1996, 106, 770; Angew. 32 (a) U. Lu�ning, C. Wangnick and M. Ku�mmerlin, Chem. Ber., 1994, Chem., Int. Ed. Engl., 1996, 35, 705; (b) Y. Murakami, J. Kikuchi, 127, 2431; (b) B. Meynhardt, Diploma Thesis, Universita�t Kiel, Y. Hisaeda and O. Hayashida, Chem. Rev., 1996, 96, 721. 1995. 3 D. A. Stauffer, R. E. Barrans, Jr. and D. A. Dougherty, Angew. 33 Review: H. W. Pinnick, Org. React., 1990, 38, 655. Chem., 1990, 102, 953; Angew. Chem., Int. Ed. Engl., 1990, 29, 915. 34 U. Lu�ning, R. Baumstark and W. Schyja, L iebigs Ann. Chem., 4 J. Rebek, Jr., Angew. Chem., 1990, 102, 261; Angew. Chem., Int. Ed. 1991, 999 Engl., 1990, 29, 245. 35 (a) W. Schyja, Dissertation, Universita�t Freiburg, 1995; 5 C. F. Martens, R. J. M. Klein Gebbink, M. C. Feiters and (b) W. Schyja, S. Petersen and U. Lu�nig, L iebigs Ann., 1996, 2099. R. J. M. Nolte, J. Am. Chem. Soc., 1994, 116, 5667. 36 U. Lu�ning and W. Schyja, NATO ASI Ser., Ser. C, 1995, 473, 223. 6 Review: M. W. Go�bel, Angew. Chem., 1994, 106, 1201; Angew. 37 M. Gelbert, Dissertation, Universita�t Freiburg, 1995. Chem., Int. Ed. Engl., 1994, 33, 1141. 38 (a) M. Hagen, Diploma Thesis, Universita�t Kiel, 1996; 7 W. L. Mock, T. A. Irra, J. P.Wepsiec and M. Adhya, J. Org. Chem., (b) M. Hagen and U. Lu�ning, Chem. Ber., in the press. 1989, 54, 5302. 39 U. Lu�ning and M. Gerst, J. Prakt. Chem./Chem. Ztg., 1992, 334, 8 H. L. Anderson and J. K. M. Sanders, J. Chem. Soc., Perkin T rans. 656. 1, 1995, 2223. 9 T. R. Kelly, G. J. Bridger and C. Zhao, J. Am. Chem. Soc., 1990, 112, 8024. Pap

ISSN:0959-9428    

DOI:10.1039/a603773i

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

MATERIALS CHEMISTRY COMMUNICATION A supramolecular cation in an electrically conducting nickel dithiolate salt Tomoyuki Akutagawa,*a Takayoshi Nakamura,*a Tamotsu Inabeb and Allan E. Underhillc aResearch Institute for Electronic Science Hokkaido University Sapporo 060 Japan bDepartment of Chemistry Faculty of Science Hokkaido University Sapporo 060 Japan cDepartment of Chemistry University of Wales Bangor Gwynedd UK L L 57 2UW [Ni(dmit)2]2 2H2O5 and [NH4+][18C6][Ni(dmit)2]3,6 which contain regular stacks of 15C5 macrocyclic molecules arranged A novel dimeric penta-coordinated ion cavity of [Li+]2[12- crown-4-ether]3 has been formed in the highly conducting parallel to the electron conduction path formed by the [Ni(dmit)2] ions and NH4+ ions completely included at the Ni(dmit)2 salt. This salt showed a room-temperature conductivity of 30 S cm-1 and exhibited a semiconductor– centre of 18C6 to form supramolecular cations respectively.We have now studied the structure and properties of the semiconductor phase transition on the application of pressure or on lowering the temperature. material with [Li+][12-crown-4(12C4)] as the counter cation. The resulting material contains a complex supramolecular entity involving dimeric penta-coordinated ion cavities situated between the electrically conducting [Ni(dmit)2] sheets. The electrocrystallisation of [NBu4+] [Ni(dmit)2-] with a The electronic magnetic and optical properties of organic– mixture of lithium tetrafluoroborate and 12C4 in acetonitrile– inorganic hybrid molecular conductors have been studied acetone (151) solution for two weeks at a constant current widely.1 Within this class of materials the [M(dmit)2] com- (1 mA) gave black single crystals with typical dimensions plexes (M=Ni Pd Pt Cu Au; dmit=2-thioxo-1,3-dithiol-4,5- of 1.0×0.4×0.1 mm3.Elemental analysis indicated a dithiolate) have provided a number of organic metals or composition of [Li+]2[12C4]3 [Ni(dmit)2]7·2Me2CO C72H60 superconductors through the formation of electron-conducting O14S70Ni7Li2 (Calc. C 22.65; H 1.57; N 0.00%. Found C molecular columns or layers in the solid state.1a In the molecu- 22.87; H 1.65; N 0.00%). X-Ray crystal structural analysis lar conductors of the type [C+][M(dmit)2]x (x>1 C+ denotes also supports this composition.† The broad absorption band a closed shell monovalent cation) salts the non-integral stoichi- ascribable to the intermolecular transition from monovalent ometry involving the counter cation compensates for the [Ni(dmit)2-] to neutral [Ni(dmit)20] was observed with the partially filled conduction band.absorption maximum at 0.21 eV.7 Electrical resistivities were Macrocyclic compounds exhibit many characteristic proper- measured by a standard dc four-probe method using gold ties e.g. selective capture of ions and transport of ions through paste (Tokuriki 8560) to connect the gold wire (10 mm) to the artificial or natural membranes.2 In the solid state the cavity crystal. Conductivity measurements under high pressure were within the macrocycle will offer an environment in which ions carried out using a cramp cell with the pressure medium (dufni can be isolated from the chemical environment outside the oil Idemitu 7573).8 macrocycle and in some cases the ions may have freedom to The salt showed a high electrical conductivity of 30 S cm-1 move between cavities in the solid state.at room temperature (sRT). The sRT value is enhanced by We have been interested in synthesising materials which can about two orders of magnitude compared with [Li+] provide a cavity for counter ions in highly conducting molecu- [Ni(dmit)2]2·2MeCN (sRT=0.5 S cm-1).1a The large lar solids. Counter ions exert a strong influence on the conduc- molecular polarizability of the Li+–macrocyclic supra- tion electrons through the formation of a potential field. molecular cation in contrast to the bare Li+ present in Control of the arrangement and motion of ions in and between [Li+][Ni(dmit)2]2·2MeCN could be one of the reasons for the cavities could provide new possibilities for regulating the the high sRT value.The temperature dependence of the conduc- conduction electrons in the crystal. To facilitate this possibility tivity at 1 bar showed semiconducting behaviour (Fig. 1). A it is important to investigate the solid-state structure of con- transition is observed at around 230 K which separates two ducting molecular complexes containing macrocyclic ligands. In the 1980s radical anion salts of 7,7,8,8-tetracyanoquinodimethane (TCNQ) containing various kinds of macrocyclic † Crystal data C72H60O14S70Ni7Li2 M=3818.12 crystal dimensions 0.5×0.3×0.1 mm3. Mac Science MXC18 diffractometer Mo-Ka radi- molecules were prepared.3 The crystal structures were reported ation (l=0.710 73 A° ) triclinic space group P1� (no.2) a=9.075(4) for the insulating salts [K+/Rb+][18-crown-6 (18C6)] b=18.370(5) c=20.677(5) A° . a=91.376(21) b=93.829(28) c= [TCNQV-] and [K+][15-crown-5 (15C5)]2 [TCNQV-].4 103.857(29)°. U=3336(2) A° 3. T=298 K Z=1 Dc=1.904 g cm-3 Coordination of K+ (Rb+) with the crown ether gave rise F(000)=1926 m(Mo-Ka) 12.044 cm-1. Lorentz polarisation and to typical planar or sandwich-type structures for absorption corrections applied 16 887 reflections measured 11 860 [K+/Rb+][18C6] and [K+][15C5]2 respectively. We have independent reflections 7698 reflections with I>3.00s(l ) used in refinement. Calculations were performed using the Crystan-GM crys- recently reported the structure of [Li+]0.6[15C5] tallographic software packages with refinements based on F.Weighting scheme employed w=1/s2(Fo).Solution by direct methods; nonhydrogen atoms refined anisotropically and no refinement of hydrogen atoms. (Dr)max=1.09 e A° -3 (Dr)min=-1.33 e A° -3 R=0.0757 R¾= 0.0753. Atomic coordinates thermal parameters and bond lengths and angles have been deposited at the Cambridge Crystallographic Data Centre (CCDC). See Information for Authors J. Mater. Chem. 1997 Issue 1. Any request to the CCDC for this material should quote the full literature citation and the reference number 1145/26. J. Mater. Chem. 1997 7(2) 183–185 183 Fig. 1 Electrical conductivity of [Li+]2 [12C4]3 [Ni(dmit)2 ]7 ·2Me2CO. Logarithmic resistivities normalised by the room-temperature value (r/rRT) vs. inverse temperature (T -1) at 1 bar (') 5.2 ($) and 10.2 (#) kbar.Inset shows the pressure (p) dependence of conductivity at room temperature (sRT) and the jump of sRT is indicated by an arrow. semiconducting phases denoted as the a (>230 K) and b phase (<230 K). The activation energy (Ea) of the a phase (0.10 eV) was lower than that of the b phase (0.17 eV). The conduction behaviour at 5.2 kbar was almost the same as that at 1 bar with Ea values of 0.09 and 0.15 eV for the a and b phase respectively. However the temperature-dependent conductivity at 10.2 kbar showed no evidence of a transition with a linear Arrhenius plot over the whole temperature range with Ea= 0.07 eV. An anomaly was also observed in the pressure dependence of sRT. Application of pressure increased sRT up to 80 S cm-1 at 10.2 kbar with a jump at 5.8 kbar (Fig.1 inset). The a phase changes to a c phase above 5.8 kbar. It is not clear at present whether this high-pressure c phase is identical to the b phase. Fig. 2(a) shows a projection of the unit cell viewed along the b axis. Four [Ni(dmit)2] (A–D) two 12C4 (E and F) one acetone molecule and the Li+ ion are crystallographically independent and one [Ni(dmit)2] (A) and one 12C4 (F) molecule are located on the centre of inversion. The segregated stacks of [Ni(dmit)2 ] and 12C4 molecules account for the observed optical properties and the high conductivity. The stacking mode in the [Ni(dmit)2 ] column is not uniform which is one of the main reasons for the observed semiconducting nature of this salt regardless of the high sRT value. The 12C4 molecules which are located in the ab plane in the interlayer space between [Ni(dmit)2] columns form an ion cavity which includes the Li+.Fig. 2(b) illustrates the [Ni(dmi2] arrangements in the conducting layer viewed along the long axis of [Ni(dmit)2]. The overlap integrals (10-3 t) were obtained from the extended Hu�ckel molecular orbital calculations of the LUMO of each [Ni(dmit)2 ].9 The [Ni(dmit)2] column elongates in the 2a-b direction in the stacking order A–B–C–D–D¾–C¾–B¾–A¾ (the primes indicate molecules generated by inversion centres). The mean interplanar distances (absolute t) between A–B B–C Fig. 2 Crystal structure of [Li+]2 [12C4]3 [Ni(dmit)2 ]7·2Me2CO. C–D and D–D¾ in the stack are 3.21 (t1=15.5) 3.78 (t2=1.8) (a) Packing motif in the unit cell viewed along the b axis. (b) Stacking 3.60 (t3=1.9) and 3.57 A° (t4=0.7) respectively.mode of Ni(dmit)2 layer viewed along the long axis of Ni(dmit)2. The The overlap modes of A–B and C–D are metal-over-metal molecules A–D are crystallographically independent of each other. type while those of B–C and D–D¾ are of the metal-over-ring The mean interplanar distances of the Ni(dmit)2 planes and the transfer type. Owing to the b2u symmetry of the LUMO of the integral (10-3 t) are indicated. (c) Fundamental structure of the ion [Ni(dmit)2 ] molecule the effective overlap of the molecular cavity with dimeric pentacoordinated Li+. The oxygen atoms are shaded and the dashed lines indicate the Li+,O contact. orbitals in the later case is much smaller than in the former. 184 J. Mater. Chem. 1997 7(2) 183–185 Within the column the close overlap of A–B enhances the t1 Studies using other crown ethers and cations are now in progress to create various types of ion cavities in the con- interaction while the elongated mean interplanar distance of B–C decreases t2.Also the overlaps at C–D and D–D¾ are ducting crystals. decreased due to the symmetry of the LUMO. Thus the This work was partly supported by a Grant-in-Aid for Science column is composed of tightly bonded triads (B–A–B¾) and Research from Ministry of Education Science Sports and loosely interacting molecules C and D. Culture of Japan. The network of sulfur–sulfur interatomic contacts extends in the ab plane through side-by-side interactions. Two types of side-by-side networks I and II indicated by transfer integrals References tI and tII respectively extend in the ab plane.The absolute t 1 (a)P. Cassoux L. Valade,H. Kobayashi A. Kobayashi R. A. Clark values in the I and II networks are 0.22 (tIAB) 0.26 (tIBC) 2.72 and A. E. Underhill Coord. Chem. Rev. 1991 110 115; (tICD) and 0.53 (tIDD) for I and 0.64 (tIIAC) 3.09 (tIICD) and 0.83 (b) A. T. Coomber D. Beljonne R. H. Friend J. L. Bre�das (tIIDB) for II. The strong overlaps (tICD and tIICD ) form the cyclic A. Charlton N. Robertson A. E. Underhill M. Kurmoo and tetrad of C–D¾–C¾–D. The electron-conducting layer in the ab P. Day Nature (L ondon) 1996 380 144; (c) C. S. Winter C. A. S. plane is composed of two units of the tightly connected triad Hill and A. E. Underhill Appl. Phys. L ett. 1991 58 107. B–A–B¾ and cyclic tetrad C–D¾–C¾–D and the transfer of a 2 R. M. Izatt J. S. Brandshaw S. A.Nielsen J. D. Lamb J. J. Christensen and D. Sen Chem. Rev. 1985 85 271. conduction electron between a triad and a tetrad is possible 3 (a) M. Morinaga T. Nogami Y. Kanda T. Matumoto through the weak interactions t2 and t3. K. Matsuoka and H. Mikawa Bull. Chem. Soc. Jpn. 1980 53 1221; The 12C4 acetone molecules and Li+ ions form the 12C4 (b) K. Matsuoka T. Nogami T. Matsumoto H. Tanaka and (E)–Li+–12C4 (F)–Li+–12C4 (E) structure [Fig. 2(c)]. A care- H. Mikawa Bull. Chem. Soc. Jpn. 1982 55 2015; (c)M. C. Grossel ful examination of the differential Fourier map showed no F. A. Evans I. A. Hriljac J. R. Morton Y. LePage K. F. Preston evidence of another Li+ ion in the cavity of the 12C4 (F) L. H. Sutcliffe and A. J. Williams J. Chem. Soc. Chem. Commun. 1990 439. molecule. The coordination sphere around the Li+ ion consists 4 (a) T.Nogami M. Morinaga H. Mikawa H. Nakano of four basal oxygen atoms of 12C4 (E) and an apical one of M. Morioka H. Horiuchi and M. Tokonami Bull. Chem. Soc. Jpn. 12C4 (F) resulting in the pentagonal oxygen coordinated 1990 63 2414; (b) M. C. Grossel and S. C. Weston J. Phys. Org. nature of Li+ ion. Similar penta-coordination of Li+ by four Chem. 1992 5 533. basal oxygen of 12C4 and one apical nitrogen of bis(trimethyl- 5 T. Nakamura T. Akutagawa K. Honda A. E. Underhill silyl)amide and thiocyanate have been reported.10 Two sym- A. T. Coomber and R. H. Friend Nature (L ondon) submitted. 6 T. Akutagawa T. Nakamura T. Inabe and A. E. Underhill Synth. metrical oxygens of 12C4 (F) act as a spacer unit connecting Met. in press. two penta-coordination spheres.The distances between the 7 (a) J. B. Torrance B. A. Scott B. Welber F. B. Kaufman and Li+ and the oxygen atoms of 12C4 (E) are 1.986 2.051 2.072 P. E. Seiden Phys. Rev. B 1979 19 730; (b) E. M. Conwell and 2.085 A° which are longer than that of the apical oxygen I. A. Howard J. P. Pouget C. S. Jacobsen J. C. Scott and (1.939 A° ). The average Li+,O distance (2.027 A° ) is shorter L. Zuppiroli in Semiconductors and Semimetals ed. E. M. Conwell than the sum of the van der Waals radius of oxygen and the Academic Press New York 1988 vol. 27 p. 293. 8 K. Murata M. Tokumoto H. Anzai H. Bando K. Kajimura ion radius of Li+ (2.12 A° ) or of typical Li+,O distances for T. Ishiguro and G. Saito J. Phys. Soc. Jpn. 1985 54 1985. penta-coordination geometry (2.06 A° ).11 These results indicate 9 (a) T.Mori A. Kobayashi Y. Sasaki H. Kobayashi G. Saito and that the Li+ ion is tightly coordinated by five oxygens of the H. Inokuchi Bull. Chem. Soc. Jpn. 1984 57 627; 12C4 cavity. The acetone molecule on the 12C4 (E) molecule (b) R. H. Summerville and R. Hoffmann J. Am. Chem. Soc. 1976 isolates the ion cavities of 12C4 molecules from each other. 98 7240. The dimeric penta-coordinated 12C4 units are arranged along 10 (a)H. Hope M. M. Olmstead P. P. Power and X. Xu J. Am. Chem. Soc. 1984 106 819; (b) H. Hope M. M. Olmstead P. P. Power a nearly perpendicular direction to the stacking axis of J. Sandell and X. Xu J. Am. Chem. Soc. 1985 107 4337; [Ni(dmit)2 ] or the same direction to the side-by-side inter- (c) P. Groth Acta Chem. Scand. Ser. A 1981 35 463. action of II. There are no interatomic contacts shorter than 11 E. Weber J. L. Toner I. Goldberg F. Fo� gtle D. A. La�idler the sum of van der Waals radii between [Ni(dmit)2] and 12C4 J. F. Stoddart R. A. Bartsch and C. L. Liotta in Crown ethers and layers. The dimeric penta-coordinated unit and acetone exist analogs ed. S. Patai and Z. Rappoport Wiley New York 1989. at the neighbouring spaces of the cyclic C–D¾–C¾–D tetrad and B–A–B¾ triad respectively. Communication 6/07600K; Received 8th November 1996 J. Mater. Chem. 1997 7(2) 183&ndash

ISSN:0959-9428    

DOI:10.1039/a607600k

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Immobilisation of phosphomolybdic (PM) acid by Nafion and the electrochromism of the resulting PM–Nafion films B. H. Pan and Jim Y. Lee Department of Chemical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 The electrochromism of phosphomolybdic (PM) acid in solution and immobilised in Nafion has been investigated. Immobilisation made use of two simple dip-coating techniques which allowed easy modification of the electrochemical activity of the composite films.PM in solution changed from light yellow to blue upon reduction, whereas immobilisedPM showed a change from pale yellow to light blue under identical conditions. Electrostatic repulsions between PM anions and negatively charged sulfonic groups in Nafion limited the amount of PM that could be molecularly dispersed in Nafion.This, coupled with the poor conductivity inherent in the Nafion matrix, resulted in suppressed electrochemical reactivities for the composite films. Despite these results, UV–VIS absorption spectra showed marked differences in light absorption between the pristine and coloured forms of immobilised PM, indicative of application potentiality. Electrochromism is the phenomenon related to changes in Experimental colour induced in selected materials by reversible electrochemi- All chemicals used were of AR grade.Crystalline phosphomo- cal processes, where an electron-transfer or a redox reaction lybdic acid (PM, Merck) was dissolved in deionized water to takes place.1–4 Numerous materials, including both inorganic the desired concentrations.A conventional three-electrode cell and organic liquids and solids, exhibit electrochromism. was used for all electrochemical experiments. Indium tin oxide Phosphomolybdic (PM) and phosphotungstic (PW) acids of (ITO) glass slides (Hampton Scientific) of dimensions formula P2O5 24MO3 nH2O (M=Mo, W) are crystalline 1.2×0.8 cm2 were used as working electrodes.The counter compounds belonging to the family of heteropolyacids and reference electrodes were a platinum basket and a saturated (HPAs).5 Several HPAs have been shown to be electrochromic calomel electrode (SCE) respectively. Prior to their use, the and follow similar colouration and bleaching mechanisms as ITO glasses were cleaned ultrasonically in deionised water that of MO3 (M=Mo, W).6 followed by rinsing with ethanol.In recent years, the observed capability of HPAs to modify An Aldrich Nafion solution containing 5 mass% of pro- persistently metallic, as well as semiconductor, surfaces has tonated Nafion 117 (molar mass 1100) in a mixture of low prompted electrochemists to use these compounds as surface aliphatic alcohols and water (951), was used without dilution.promoters in electrochemistry.7–9 Moreover, HPAs are of con- The concentration of sulfonic acid groups was 39.7 mmol siderable interest as redox catalysts in a variety of oxidation dm-3. With the pH of the solution at 1.7, approximately 50% reactions.10 The electrocatalytic properties of these compounds of the sulfonic acid was dissociated. Nafion was used to as heterogeneous vapour-phase oxidation catalysts have been immobilisePM on ITO electrodes by two very simple methods. investigated extensively and are attributed to their reducibility and reoxidisability, strong dissociation constants and ionic Method A.Bare ITO slides were precoated with Nafion and conductivities.11–14 Most previous studies on HPA modified the Nafion-coated ITO slides were exposed to the PM solution electrodes were conducted with the aim of substituting noble- for different periods of time.The resulting composite films metal-based catalysts with less expensive materials activated were identified as type A films. by such compounds. Promisingly, several HPAs exhibit colouration upon Method B. Bare ITO slides were coated with Nafion solu- reduction.Thus, there is interest in the study of their electro- tions containing different amounts of dissolved PM. The chemical and electrochromic performance as well as their resulting composite films were identified as type B films. possible applications in display devices. The electrochromisms of PW powders and PW solutions15 have been documented. All the PM–Nafion composite films were dried in air.The experimental results showed proton conduction and blue Cyclic voltammetry (CV) and chronoamperometry (CA) colouration of the materials upon electroreduction. It has been were carried out at room temperature using the EG&G model commented that HPAs will be useful as electrochromic mate- 270 electrochemical measurement system. Aqueous 0.1 mol rials only if they can be rendered translucent in the bleached dm-3 LiClO4 was used as the supporting electrolyte.CV began state and be obtainable as thin films instead of compressed with a cathodic sweep from 1.0 V after the system had been powders or in solutions.1,4,16 These attributes are difficult to conditioned at this potential for 15 s. A Shimadzu UV-3101PC attain in the pristine form of such materials.Thus far only spectrophotometer was used to obtain the UV–VIS spectra of Shimidzu et al.17 have reported the formation of a composite pristine and blue PM–Nafion composite films in aqueous of PW with polypyrrole for electrochromic applications. In solutions. comparison, the electrochromism of PM has rarely been addressed and this work aspires to fill some of the voids.To Results and Discussion this end, this paper reports very simple but effective means to immobilize PM as thin films on transparent substrates by Electrochromism of PM in solutions Nafion, and the electrochromism of the resulting composite films. In addition, the electrochromism of PM in solution, PM was dissolved in deionised water to give a 1 mmol dm-3 which has rarely been mentioned in the literature, is also solution.The solution at this concentration was light yellow. The solution next to the electrode was immediately darkened described. J. Mater. Chem., 1997, 7(2), 187–191 187to blue when a small and negative potential (ca. -0.05 V) was ITO electrode was preconditioned at 0.8 V for 15 s to keep the PM solution in its pristine light yellow colour.The potential applied. The blue colouration could be bleached by applying a positive potential to the electrode, but at a much slower rate was then stepped to -0.1 V for 4 s and the solution next to the electrode immediately turned blue. This was followed by relative to colouration. No solid deposit was formed on the electrode. The colouration was due to the localization of a an anodic step to 0.8 V for 4 s during which the blue species was progressively bleached and the solution returned to the soluble coloured species near the electrode, which would eventually diffuse away and disappear if left alone.pristine colour of the PM solution. The ease of PM colouration enabled the use of a small negative potential (-0.1 V) to The cyclic voltammogram of a 1 mmol dm-3 PM aqueous solution at a scan rate of 50 mV s-1 is shown in Fig. 1. LiClO4 reduce the effects of hydrogen evolution. It can be seen from the current transient that colouration and bleaching proceeded (0.1 mol dm-3) was added as the supporting electrolyte. In the cathodic scan, the current began to increase significantly at at noticeably different rates, with more time being needed for bleaching than for colouration.The higher residual current in 0.4 V until the end of the scan was reached. Some small reduction peaks also emerged as shoulders during the scan. the electroreduction step could be due to the evolution of small amounts of hydrogen. The chronoamperometric results The solution next to the electrode turned increasingly blue as the current increased.On the reverse scan, several correspond- are therefore in general agreement with the cyclic voltammetric results. ing oxidation peaks appeared, and the blue PM species was bleached slowly and incompletely relative to colouration. If the reverse scan was not started immediately after the end of Electrochromism of type A PM–Nafion films the forward scan, and the system was left to stand for some Crystalline PM powder was dissolved in deionized water to time, the blue colouration began to spread and decolourisation concentrations of 1, 5 and 10 mmol dm-3, resulting in light could not be completed in the reverse scan.The species formed yellow solutions. Nafion-coated ITO electrodes were prepared upon PM reduction was therefore water-soluble, and depos- by dip-coating.Bare ITO electrodes were lowered into the ition of a solid was not possible in this case. Chang et al.18 Nafion solution and withdrawn immediately. The Nafion- reported an analogous system of polytungstic anions, and its coated ITO electrodes were kept vertically straight and the electrochemical colouration and bleaching by acidified H2O2.solvent was allowed to evaporate in air overnight. PM was Cho et al.19 observed a complex anodic voltammetric pattern subsequently introduced into the Nafion layer by exposing the for PM electrocatalysts at slow scan rates and attributed it to Nafion-coated ITO electrodes to a given PM solution for a slow reaction kinetics in the formation of unstable higher predetermined time, followed by washing in deionized water oxidation state species of PM.Cyclic voltammograms of PM and drying in air. All resulting PM–Nafion films were pale electrocatalysts in a 50% (v/v) water–dioxane mixture with yellow. 0.5 mol dm-3 H2SO4 also showed similar anodic behaviour.7 The incorporation of PM into the Nafion layer was hindered Double-step chronoamperometry was used to assess the by electrostatic repulsions between anionic PM species and speed of electrochromic response in solution.Fig. 2 shows the the negatively charged sulfonate groups of Nafion. However, I–t transient of a 0.5 mmol dm-3 PM aqueous solution. The a certain amount of PM anions could still be incorporated into Nafion by virtue of a concentration difference across the ion-permeable Nafion membrane.1 Honda et al.20 fabricated Prussian Blue (PB)-based electrochromic devices through sequential immersions of a Nafion membrane in Fe2+ and Fe(CN)63- solutions.The resulting PB-containing Nafion composite film showed very satisfactory electrochromism. Nafion has also been used to reduce the slow dissolution of electrochromes under application conditions. A good example is given by Shen et al.21 who coated electrochromic WO3 by Nafion to prolong the life of the former in acidic media.Fixed PM concentration, varying immersion time (type A1 films). Fig. 3 shows the cyclic voltammograms of Nafionimmobilised PM on ITO slides in aqueous 0.1 mmol dm-3 LiClO4 at 50 mV s-1. The electrodes were prepared by Fig. 1 Cyclic voltammogram of a 1 mmol dm-3 PM aqueous solution exposing Nafion-coated ITO to a 5 mmol dm-3 aqueous PM at 50 mV s-1. 0.1 mmol dm-3 LiClO4 was added as the supporting electrolyte. Fig. 3 Cyclic voltammograms of type A1 immobilised PM films in Fig. 2 I–t Transient of a 0.5 mmol dm-3 PM aqueous solution. The 0.1 mmol dm-3 LiClO4–H2O at 50 mV s-1. The electrodes were prepared by immersion of Nafion-coated ITOs in 5 mmol dm-3 PM electrode was conditioned at 0.8 V for 15 s before the potential was stepped to -0.1 V for 4 s and then to 0.8 V for another 4 s.aqueous solution for different times [(a) 30 s, (b) 5 min and (c) 10 min]. 188 J. Mater. Chem., 1997, 7(2), 187–191solution for 30 s, 5 min and 30 min. All voltammograms dis- evolution reaction (HER) and the matrix effect of Nafion intervened.The poor conductivity of Nafion hampered the played characteristic PM behaviour (see Fig. 1). The electrochemical activity of the composite films was lower than that effectiveness of charge-transfer reactions except for those occurring immediately next to the electrode.1,21 As a result, colour- of PM in solution but their reversibility, in terms of the closeness of the anodic peak to the cathodic end, was better.ation and bleaching became sluggish and slower than the corresponding processes in aqueous solution. At the shortest immersion time (30 s), both the cathodic colouration and anodic bleaching currents were relatively small. This was primarily due to limited incorporation of PM Fixed immersion time, varying PM concentration (type A2 films).Experiments were also carried out using a fixed immer- in the Nafion, resulting in a low concentration of PM in the composite film. When the immersion time was lengthened to sion time of 5 min and different PM concentrations (1, 5 and 10 mmol dm-3). The choice of 5 min was arbitrary (although 5 min, more PM permeated and incorporated into the Nafion layer and the resulting PM–Nafion film showed higher colour- it did correspond to the near-optimal immersion time for the 5 mmol dm-3 solution), and no attempt was made to use ation and bleaching currents.However, corresponding to the increase in currents, the anodic peak was shifted to a more factorial experimental design procedures to obtain the optimal preparation conditions from the experimental variables.positive potential, indicating a decrease in electrochemical reversibility. Increasing the immersion time to beyond 5 min The effect of concentration on the voltammograms of PM–Nafion composite films is shown in Fig. 5. For the 1 mmol did not bring about any significant changes in the voltammograms. This implies that PM incorporation into Nafion has dm-3 case, both the cathodic colouration and the anodic bleachingcurrents were smaller.The anodic peak was, however, reached saturation after 5 min for the 5 mmol dm-3 solution. The saturation time is, of course, dependent on the Nafion closer to the cathodic end of the scan, indicating better electrochemical reversibility for electrochromism. This obser- layer thickness and the concentration of the PM solution.There are no economic incentives to use an immersion time vation can be understood in terms of a low concentration of PM in the Nafion layer. Immersion for 5 min was probably greater than that needed to saturate Nafion with PM. Double-step chronoamperometry was carriedout to evaluate inadequate for a dilute PM solution to fully saturate the Nafion layer. A subsequent increase in electrochromic activity the electrochromic response of PM–Nafion films to potential steps.The films were initially conditioned at 1.0 V for 15 s. was possible with increased immersion time. On the other hand, there were no substantial changes in the voltammetric The potential was then stepped to -1.0 V for 4 s and returned to 1.0 V for another 4 s. Larger potential steps were needed to features when the PM concentration was increased beyond 5 mmol dm-3.In relation to the 1 mmol dm-3 solution, the initiate the electrochromic response because of the increased IR drop introduced by the Nafion layer. Fig. 4 shows the cathodic and anodic currents were higher and the anodic peak was located at a more positive potential, indicating that more response of PM–Nafion films prepared using different immersion times, as indicated.The supporting electrolyte was PM had permeated into the Nafion layer resulting in a higher concentration of immobilised PM. Since 5 min immersion was 0.1 mmol dm-3 LiClO4 in water. The films after conditioning were pale yellow. The first step (1.0 V�-1.0 V) coloured the adequate for the 5 mmol dm-3 PM solution to saturate the Nafion layer, further increasing the PM concentration to films to the characteristic pale blue of reduced PM and the second step (-1.0 V�1.0 V) restored the pristine colour of 10 mmol dm-3 would only lead to a faster approach to saturation, but not to any increase in the electrochromic the films.With increased immersion time, the response time increased activity of the composite films.If statistical design of experiment procedure is followed, an optimal electrochromic response (in and the response speed decreased. Increasing the immersion time beyond 5 min did not bring about significant changes in terms of activity, reversibility, response time and cost-effectiveness of preparation) from a defined domain of immersion the response speed, which agreed well with the voltammetric findings.For each case, the response during the initial stage of time and PM concentration could be identified. Nevertheless, the present unoptimised resudo verify that Nafion is PM- colouration (ca. 0.5 s) was quite fast and distinct but became slower thereafter. In contrast, all PM coloured in the first step permeable and that it is easily saturated by PM because of the electrostatic repulsions between PM anions and the negatively could be completely bleached in ca. 1.5 s in the decoloration step. charged sulfonate groups of Nafion. The electrochromic performance of these films was again As has been mentioned previously, an aqueous PM solution showed faster colouration than bleaching, inferring that PM tested by chronoamperometry and the resulting I–t transients are shown in Fig. 6. The experimental conditions were the was easier to reduce than to reoxidise. This behaviour was reflected only within the first 0.5 s of the colouration of same as those used for Fig. 4. The films after conditioning showed a pale yellow colouration, which changed to pale blue PM–Nafion films, thereafter the occurrence of the hydrogen Fig. 5 Cyclic voltammograms of type A2 immobilised PM films in Fig. 4 Double-step chronoamperometric curves of type A1 immobilised PM films in 0.1 mol dm-3 LiClO4–H2O. The potential was 0.1 mol dm-3 LiClO4–H2O at 50 mV s-1. The electrodes were prepared by immersing Nafion-coated ITOs in aqueous PM solutions stepped between -1.0 and 1.0 V for 4 s in each interval; (a) 30 s, (b) 5 min, (c) 10 min.of different concentrations [(a) 1, (b) 5 and (c)10 mmol dm-3] for 5 min. J. Mater. Chem., 1997, 7(2), 187–191 189the PM concentration again brought about a positive shift in the first anodic peak. Similar to Fig. 1, more than one anodic peak was detected, which may be indicative of the presence of a separate PM phase in the composite. Again the increase in activity came at the expense of a decrease in electrochemical reversibility.Although the electrochromic reversibility of these films was reduced by virtue of a higher concentration of immobilised PM, most major voltammetric features are qualitatively similar to those of type A films. The chronoamperometric response of the type B PM–Nafion films is shown in Fig. 8. The films after conditioning were light yellow, and changed to light blue during colouration and returned to light yellow after bleaching.The chronoamperometric response was similar to that of the type A films, except Fig. 6 Chronoamperometric curves of type A2 immobilised PM films that the response times for colouration and bleaching were in 0.1 mol dm-3 LiClO4–H2O. The potential was pulsed between-1.0 generally longer, and the peak currents were more similar.and 1.0 V for 4 s in each step; (a) 1, (b) 5, (c) 10 mmol dm-3. These changes arose because of the higher concentration of PM in Nafion which imposed a negative effect on the respond during colouration but returned to the pristine colour after speed. The current response in the first 0.5 s of colouration bleaching.was still fast, but a subsequent slowing of response was Similar to Fig. 4, the current transient slowed after the first nonetheless observed. In the following bleaching step, all the 0.5 s of colouration. Bleaching, on the other hand, was more PM coloured in first step could be completely bleached in efficient and all of the PM coloured in the first potential step ca. 3 s.could be completely bleached in ca. 1.5 s. The peak current in With increasing PM concentration, the response time bleaching was also higher than that in colouration. With the increased slightly, which was consistent with the slight increase increase in PM concentration, the response time increased and of both colouration and bleaching currents in the voltammo- the response speed decreased, but the increase was less con- grams.The changes in the response currents and response spicuous when the solution concentration was increased from times with concentration were smaller because of a larger 5 to 10 mmol dm-3. The parallel with the voltammetric immobilised PM fraction in these films compared to type A behaviour of these films is quite evident. films. Lower electrochemical reversibility aside, method B does offer a more expedient and convenient method for the immobil- Electrochromism of type B PM–Nafion Films isation of PM in Nafion.The PM loading in the film can be adjusted through the PM concentration in the solution. The In this alternative method of PM immobilisation, PM was homogeneity of the composite layer may, however, be compro- dissolved in a Nafion solution and applied to the surface of mised at too high a loading level.ITO electrodes by dip-coating. The resultant PM–Nafion composite films after solvent evaporation were light yellow. UV–VIS spectroscopy Their electrochemical characterizations in 0.1 mol dm-3 aqueous LiClO4 followed the same procedures as described earlier. The optical absorption spectra of a type B film prepared from As PM was dissolved directly into the Nafion solution, the a 5 mmol dm-3 PM solution in Nafion are shown in Fig. 9. limit of PM incorporation was determined by the solubility of The pristine form of the film was light yellow and was held at PM in Nafion, although electrostatic repulsions could still -0.5 V for 2 min in 0.1 mol dm-3 LiClO4 until it turned light operate on the molecular level to result in separate microscopi- blue.The absorbance of the pristine film decreased with cally dispersed phases of Nafion and PM in the composite. wavelength in the region 350 to ca. 500 nm and remained Fig. 7 shows the voltammograms of several PM–Nafion films nearly constant thereafter ata relatively low level of absorption. which were prepared by coating ITO directly with PM–Nafion A broad absorption occurred, however, for the light blue film, solutions of different PM concentrations (1, 5 and 10 mmol the absorbance decreased from 350 to ca. 500 nm and then dm-3 PM). With increasing PM concentration in Nafion, increased with further increase in the wavelength. In addition, both cathodic and anodic currents continued to increase.This the intensity of absorption increased with increasing negative is unlike type A films, where the increase in electrochromic potentials in steps with the response of the cathodic current activity stopped at 5 mmol dm-3. Only slight increases were to the same electrical stimulations. All type A films also possible due to the poor conductivity of Nafion. Increasing Fig. 8 Double-step chronoamperometric curves of type B immobilised Fig. 7 Cyclic voltammograms of type B immobilised PM films in 0.1 mol dm-3 LiClO4–H2O at 50 mV s-1. The electrodes were PM films in 0.1 mol dm-3 LiClO4–H2O. The potential was stepped between -1.0 and 1.0 V for 4 s in each interval; (a) 1, (b) 5, prepared by dip-coating bare ITO in mixed PM and Nafion solutions of different PM concentrations [(a) 1, (b) 5 and (c) 10 mmol dm-3].(c) 10 mmol dm-3. 190 J. Mater. Chem., 1997, 7(2), 187–191as desired by changing the PM concentration in aqueous solution and/or the immersion time of Nafion-coated ITO in this solution as well as the concentration of PM dissolved directly in the Nafion solution. However, owing to electrostatic repulsions between PM anions and the negatively charged sulfonate groups of Nafion, and the poor conductivity of the Nafion matrix, the extent of incorporation was limited and Nafion was easily saturated by PM, leading to relatively low electrochromic activity for the composite films.On the other hand, the electrochromic reversibility of PM–Nafion films was better than that of PM solution because immobilisation led to increased availability of electroactive species in the electrode proximity.UV–VIS spectroscopy showed marked differences in light absorption between as-prepared and coloured immobilised PM species that are suitable for practical applications. Fig. 9 UV–VIS spectra of coloured (a) and pristine (b) type B films. The films were prepared from a 5 mmol dm-3 PM solution in Nafion, and held at -0.5 V for 2 min to change from light yellow to light blue.References exhibited similar optical characteristics under identical 1 P. M. S. Monk, R. J. Mortimer and D. R. Rosseinsky, conditions. Electrochromism: fundamentals and applications, VCH, Weinheim, 1995. The marked difference in light absorption between the two 2 C. G. Granqvist, Appl. Phys. A, 1993, 56, 1.forms of PM in the wavelength range 350–800 nm is desirable 3 B. W. Faughnan and R. S. Crandall, in T op. Adv. Phys., 1980, for practical electrochromic applications such as smart win- 40, 181. dows. The optical absorbance in the coloured state can be 4 C. M. Lampert, Sol. Energy Mater., 1984, 11, 1. varied through manipulation of the PM concentration in 5 F. A. Cotton and G.Wilkinson, Advanced Inorganic Chemistry, Nafion and the magnitude of the applied cathodic potential. Interscience, New York, 1972, p. 950. 6 B. Tell and S.Wagner, Appl. Phys. L ett., 1978, 33, 837. A good colouration can therefore be attained with a smaller 7 B. Keita and L. Nadjo, J. Electroanal. Chem., 1988, 243, 87. amount of electrochrome and/or lower energy input, resulting 8 B.Keita, L. Nadjo and J. M. Saveant, J. Electroanal. Chem., 1988, in substantial savings in the operating cost of the electro- 243, 105. chromic devices. The simple methodologies outlined here have 9 O. Savadogo, J. Electrochem. Soc., 1992, 139, 1082. removed a major hindrance to the use of HPAs for electro- 10 R. Standberg, Acta Chem. Scand., Ser. A, 1975, 29, 359. chromic applications, namely their immobilisation on trans- 11 M.Akimoto, H. Ikeda, A. Okabe and E. Echigoya, J. Catal., 1984, 89, 196. parent optical substrates. 12 M. Ai, J. Catal., 1981, 71, 88. 13 M. Akimoto, K. Shima, H. Ikeda and E. Echigoya, J. Catal., 1984, Conclusions 86, 173. 14 M. Mizuno, T. Watanabe and M. Misono, J. Phys. Chem., 1985, The electrochromism of phosphomolybdic (PM) acid in solu- 89, 80. tion and in a series of PM–Nafion composite films was 15 P. Stonehart, J. C. Koren and J. S. Brinen, Anal. Chim. Acta, 1968, 40, 65. investigated by means of cyclic voltammetry, chronoampero- 16 A. Donnadieu, Mater. Sci. Eng. B, 1989, 3, 185. metry and UV–VIS spectroscopy. PM in solution changed 17 T. Shimidzu, A. Ohtani, M. Aiba and K. Honda, J. Chem. Soc., from light yellow to blue upon reduction, whereas the com- Faraday T rans. 1, 1988, 84, 3941. posite films showed a change from pale yellow to light blue 18 I. F. Chang, B. L. Gilbert and T. J. Sun, J. Electrochem. Soc., 1975, under identical conditions. By using Nafion, PM could be 122, 955. immobilized on ITO substrates as thin films by two very 19 K. Cho, S. O. Chung, K. Ryu, K. Kim, J. H. Choy and H. Kim, Synth. Met., 1995, 69, 481. simple but effective methods, thereby removing a major hin- 20 K. Honda and H. Hayashi, J. Electrochem. Soc., 1987, 134, 1330. drance to the use of PM in electrochromic applications. 21 P. K. Shen, H. T. Huang and A. C. C. Tseung, J. Mater. Chem., The experimental results showed that the composition of 1992, 2, 497. the PM–Nafion composite films could be tailored and hence their electrochromic properties (such as activity, reversibility, response speed and optical contrast) could also be manipulated Paper 6/03981B; Received 6th June, 1996 J. Mater. Chem., 1997, 7(2), 187–191 191

ISSN:0959-9428    

DOI:10.1039/a603981b

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

13C NMR spin–lattice relaxation times as a probe of local polymer dynamics in plasticized polyethers M. Forsyth,*b P. Meakina and D. R. MacFarlanea aDepartment of Chemistry,Monash University, Clayton, V ictoria 3168, Australia bDepartment ofMaterials Engineering,Monash University, Clayton, V ictoria 3168, Australia 13C NMR spin–lattice relaxation times T1 are used to investigate the effect of low molecular weight diluents, including N,Ndimethylformamide, N-methylformamide, propylene carbonate, c-butyrolactone, triglyme and tetraglyme, on the local polymer segmental motion in polyether–urethane networks.In all cases, an increase in the local mobility is deduced from the increasing T1 measurements consistent with a decreasing glass transition temperature. The extent of plasticization, however, is dependent on the nature of the small molecules.Those molecules which can either form strong polymer-diluent interactions (for example through dipolar interactions) or are themselves rigid, give the least enhancement of polymer mobility and the greatest deviation from the Fox equation for Tg. In the presence of alkali metal salts, N,N-dimethylformamide and propylene carbonate are shown to have opposite effects on the local polymer motion, as seen from the T1 measurements.In both cases, addition of the plasticizers increases the 13C T1 relaxation times for the plasticizer. However, propylene carbonate decreases the polymer 13C T1 whilst N,Ndimethylformamide results in the expected increase in polymer 13C T1. It has been well established that the dissolution of alkali metal 13C studies have been reported, although these have also given salts in polyethers results in ionically conductive materials great insight into the polymer–salt interactions15–18 and the which have great potential as solid polymer electrolytes in all- effect of crosslink density in amorphous network polyethers.19 solid battery and other electrochemical applications. The high- For example, Spindler and Shriver15 have shown that midest conductivity achieved in purely polyether-based solid elec- chain polyether oxygens in a copolymer of polysiloxane and trolytes is less than 10-4 S cm-1,1 which is at the lower end polyether are capable of strong interactions with cations which of the useful range of conductivities.However, the addition of result in upfield 13C chemical shift changes.Similar results low molecular weight liquids, or plasticizers, have been shown have been found by Forsyth et al.20 Spin–lattice relaxation to significantly enhance the ionic conductivity while still main- times T1 have also shown a strong interaction between the salt taining useful material properties.2–4 and the polymer as indicated by a decreasing T1 which reflects In polymer language the term plasticizer refers to a species a slowing down of the local polymer dynamics.15,16,20 which will decrease the glass transition temperature of a Schantz et al.16 investigated the effects of divalent cation polymer.It was shown at an early stage in the development addition to poly(ethylene oxide) (PEO) and found that the of polymer electrolytes that lower glass transition temperatures interactions between Ba2+ and PEO were stronger than the resulted in higher conductivities since, it was believed, the alkali metal–PEO interactions as indicated by the significant mechanism of ionic conduction was strongly coupled to the reduction of the segmental motion in the amorphous phase.segmental motion of the polymer backbone.1,5 In recent years The effect of siloxane crosslinking on polyether mobility has it has been shown that, at the salt concentration levels used, also been reported by Lestel et al.19 These experiments have ionic aggregation also plays a major role in limiting conduc- illustrated the fact that polyether segmental motion remains tivity by decreasing the available number of charge carriers.6–8 relatively unrestricted in the case of cyclosiloxane crosslinked This is a result of the low relative permittivity of the polyethers.polymers as determined by linewidth and 13C T1 measurements. Therefore, the addition of plasticizers with higher relative 13C NMR relaxation times have also been used to probe the permittivities may have an additional effect on the ionic molecular motion of glassy polymers after blending and on conductivity of polymer electrolytes. Previous work in our the addition of low molecular weight diluents.21,22 In all of the laboratories2,9 has shown that the nature of the plasticizer is above studies, the T1 measurements can be related to molecular important in determining the level of conductivity enhancement motion through the correlation time t by assuming that the in polyether-based polymer electrolytes.In addition, it has relaxation time is governed by the 13C–1H dipolar relaxation been shown that the level of ion association is dependent on mechanism16,23 [eqn. (1) and (2)], the type of plasticizer, where solvents such as N,N-dimethylfor- 1/T1(C)=N/10 (mocCcHh/4pr3)2 (1) mamide, with a high relative permittivity as well as strong ×[J(vH-vC)+3J(vC)+6J(vH+vC )] solvating ability, result in a greater fraction of free ions compared to tetraglyme.10 J(v)~f (t).For simple cases J(v)= t 1+v2t2 (2) Nuclear magnetic resonance (NMR) is a useful tool for characterizing polymer electrolytes. Its nucleus specificity where N is the number of directly bonded protons, mo is the allows the separate investigation of the structure and dynamics vacuum magnetic permeability, cC and cH are the magnetogyric of cations, anions and polymer by using chemical shifts and ratios of the 13C and 1H nuclei respectively, r is the C–H relaxation measurements for nuclei such as 23Na, 19F, 13C and internuclear distance, J(v) is the spectral density of motions 1H.Ionic structure and mobility have been investigated in a and vC and vH are the 13C and 1H Larmor frequencies range of systems as a function of temperature and ion concenrespectively. Eqn. (2) has the additional assumption that the tration using 23Na and 7Li.11–13 1H NMR relaxation measurespectral density is governed by isotropic motion and hence a ments have also been invaluable in determining the correlation single correlation time t.In polymer systems motion of the between the amorphous nature of the polymer, as well as polymer mobility, and ionic conductivity.14 Relatively fewer polymer chain has been shown to be distinctly anisotropic and J. Mater. Chem., 1997, 7(2), 193–201 193therefore eqn. (2) is an oversimplification for the spectral pared as above, except that, due to the viscosity of the salt stock solution, these samples required heating to attain a density function.More complex functions based on the Havriliak–Nagami or Cole–Cole distribution of correlation homogeneous mix. To the systems containing various concentrations of salt and the salt–plasticizer–polymer systems a times are perhaps more valid.16 Nevertheless, in all cases, since the relaxation time is directly related to the spectral density stoichiometric amount of HDI was then added and samples stirred further.Finally, Thorcat 535 catalyst (nonadecanoate function and since this is in turn related to the correlation time for segmental motion (or distribution of correlation times), carboxylicester, approximately 0.5%) was stirred in to promote the reaction of the diisocyanate with the 3PEG over any it is clear that T1 measurements provide an important means of probing molecular motion in polymer systems.Moreover, residual water in addition to catalysing the reaction between the diisocyanate and the 3PEG. The samples were pipetted high resolution 13C NMR techniques make it possible to study individual carbons and hence the relative effect of additives into the NMR tubes and these, including the samples used for glass transitiontemperature measurements, were cured between such as salt and plasticizers on the motions of different parts of a polymer chain. 35–45 °C in an oven in the dry box. The spin–lattice relaxation times T1 of the 13C in the polymer In a previous communication, we presented some preliminary results of the effects of different plasticizers on the motion and plasticizer were measured at 22°C on the 200 MHz Bruker AC-200 spectrometer operating at 50.32 MHz for carbon, by of urethane crosslinked polyethers which are commonly used as host polymers for polymer electrolytes, using 13C T1 the inversion recovery method and analysed with a singleexponential curve fitting routine.The T1 measurements for measurements.9 This paper provides a more extensive discussion of this work with the intention of understanding the systems having both very long and very short T1 times were carried out in two steps. The polymer T1 times varied from role of plasticizers in the conduction mechanisms of polyetherbased solid polymer electrolytes.>100 to 600 ms and the plasticizer T1 times from 100 ms to 72 s (PC carbonyl group). Thus for systems containing zero or low mass percent of plasticizer, which exhibited broad, low Experimental intensity peaks for the polymer carbons and low T1 times the number of scans used was increased from 32 to 128. The The polyether was a poly(oxyethylene-co-oxypropylene)triol number of experiments, with the appropriate variable delay of molecular weight ca. 5000 g mol-1 (3PEG), obtained from times, was also increased to minimize the error in the fitted ICI Australia. curve. The chemical shifts of the spectra were obtained by external reference to chloroform (d 77). The glass transition temperatures Tg were measured by differential scanning calorimetry (DSC) on a Perkin Elmer DSC-7 at a heating rate of 10°C min-1, between -140 and 20°C, on sample sizes varying from 5 to 15 mg.Reproducible traces to ±0.5 °C or better were obtained by quenching the samples, sealed in aluminium pans, in liquid nitrogen before placing them into the DSC head which had been pre-cooled to -140 °C. Results 13C spin–lattice relaxation times of plasticized 3PEG A typical 13C NMR spectrum for a plasticized crosslinked 3PEG sample is shown in Fig. 1. The 13C resonances are slightly broadened as a result of restricted mobility due to crosslinking, however, the resolution and intensities are still adequate for measurement of relaxation times and chemical shifts. The addition of plasticizer usually enhanced the resolution, as a result of the decreased linewidth (increased local O O O O O N (CH2)6 NH O O Me O O O O O O ...O ... Me x MeHN O H y 1 2 3 4 N O H 1 2 5 3 4 Me Me 1 2 3 Me O O O O Me 1 g-butyrolactone propylene carbonate x 4 N-methylformamide 2 3 4 1 2 1 2 3 2 3PEG N,N-dimethylformamide triglyme x = 1 tetraglyme x = 2 PO EO The ethylene oxide:propylene oxide ratio in this material is approximately 351 on a random basis.The liquid polyether was dried under vacuum for three days, resulting in a moisture content undetectable with Karl Fisher reagent (<0.1% m/m). The plasticizers [propylene carbonate (PC) and N,N-dimethylformamide (DMF)], the salts [lithium perchlorate and sodium trifluoromethanesulfonate (triflate)] and the cross linking agent [hexamethylene diisocyanate (HDI)] were obtained from Aldrich, and were of 99% purity.The salts were dried to constant weight in a vacuum oven at 80°C. All samples were prepared in a dry box under a nitrogen atmosphere. The plasticized polymer blends were prepared on a mass percent basis from stock polymer and salt solution containing a stoichiometric amount of HDI and magnetically stirred for 30 min.The salt–3PEG 5000 solution was heated to 60°C to dissolve the salt, and the lithium perchlorate–3PEG 5000 solution was further dried at 80°C in the vacuum oven for Fig. 1 Typical 13C NMR spectra for 3PEG5000 (crosslinked) at 50.32 MHz three days. The salt-containing plasticized samples were pre- 194 J. Mater. Chem., 1997, 7(2), 193–201mobility). Table 1 presents the chemical shifts and assignments for the pure polymer and plasticizers.The behaviour of the polymer 13C T1 times as a function of DMF, NMF, PC, c-butyrolactone, tetraglyme and triglyme content are illustrated in Figs. 2(a)–7(a)respectively. Figs. 2(b)–7(b) show the same data for the plasticizer carbons. In Table 1 Structure and NMR assignments of 13C resonances in crosslinked 3PEG 5000 polymer, triglyme, tetraglyme, c-butyrolactone, propylene carbonate, N-methylformamide and N,Ndimethylformamide carbon d/ppm 3PEG CH3=3PEG C1 17.7 CH2=3PEG C2 68.8 CH2=3PEG C3 70.8 CH=3PEG C4 75 CH2=3PEG C5 75.3 c-butyrolactone CH2=BUT 3 22.2 CH2=BUT 2 27.7 CH2=BUT 4 68.6 C=O=BUT 1 178 DMF CH3=DMF 1 31.1 CH3=DMF 2 36.2 Fig. 3 T1 versus DMF content for (a) each polymer carbon type [3PEG C=O=DMF 3 162 C1 ($), C2 (%), C3 ('), C4 (&), C5 (1)] and (b) each plasticizer propylene carbonate carbon type [DMF C1 (#), C2 (&), C3 (+)] (T=295 K) CH3=PC 1 19.1 CH2=PC 2 70.8 CH=PC 3 73.9 C=O=PC 4 155 tetraglyme CH3=TET 1 58.5 CH2=TET 2 70.6 CH2=TET 3 70.8 CH2=TET 4 72.2 triglyme CH3=TRI 1 58.4 CH2= TRI 2 70.6 CH2=TRI 3 70.8 CH2=TRI 4 72.2 Fig. 4 T1 versus NMF content for (a) each polymer carbon type [3PEG C1 ($), C2 (%), C3 ('), C4 (&), C5 (1)] and (b) each plasticizer carbon type [NMF C1 ($), C2 (%)] (T=295 K) all cases, the error in the measured T1, as obtained from the curve fitting, is less than the size of the data symbol used.The results show that as the low molecular weight plasticizer is incorporated into the crosslinked 3PEG, the relaxation times of all the polymer carbons increase. This is consistent with an increasing mobility of the polymer and the narrowing of the 13C linewidths.The magnitude of the influence of plasticizer on the individual carbons appears to be independent of the Fig. 2 T1 versus propylene carbonate content for (a) each polymer nature of the carbons. When comparing the rate of change of carbon type [3PEG C1 ($), C2 (%), C3 ('), C4 (&), C5 (1)] and T1 with increasing plasticizer content for both the main chain (b) each plasticizer carbon type [PC C1 (#), C2 (&), C3 (+)] (T=295 K) polymer carbons (C2, C3) with that of the plasticizer carbons, J.Mater. Chem., 1997, 7(2), 193–201 195Fig. 5 T1 versus tetraglyme content for (a) each polymer carbon type Fig. 7 T1 versus c-butyrolactone content for (a) each polymer carbon [3PEG C1 ($), C2 (%), C3 ('), C4 (&), C5 (1)] and (b) each type [3PEG C1 ($), C2 (%), C3 ('), C4 (&), C5 (1)] and (b) each plasticizer carbon type [TETRA C1 (#), C2 (&), C3 (+)] (T=295 K) plasticizer carbon type [BUT C1 (#), C2 (&), C3 (+)] (T=295 K) Fig. 8 Comparison of reduced T1 times for the main chain methylene resonance (C3) with different plasticizers.[N,N-dimethylformamide (#), c-butyrolactone (%), N-methylformamide ('), propylene carbonate (&), tetraglyme (2)]. The C2 T1 times are given in the case of tetraglyme. carbon. This is not unexpected since these plasticizers are low molecular weight analogues of the polymer itself. Only a single resonance can be resolved, at approximately d 72, in these Fig. 6 T1 versus triglyme content for (a) each polymer carbon type cases and therefore the T1 measured will be a weighted average [3PEG C1 ($), C2 (%), C3 ('), C4 (&), C5 (1)] and (b) each plasticizer carbon type [TRI C1 (#), C2 ('), C3 (+), C4 (%)] of the two different species. Similarly, an overlap was found (T=295 K) between the 3PEG C2 carbon and the c-butyrolactone resonance at d 68.In order to directly compare the effects of each of the it appears that a twofold increase in the polymer relaxation rate is accompanied by a comparable increase in the plasticizer plasticizers on the polymer 13C T1 times and hence on polymer mobility, the reduced relaxation times [T1/T1(0)] have been carbon T1 times. In the case of tetraglyme and triglyme additions to 3PEG, calculated, where T1(0)=T1 at 0% plasticizer content.These are plotted for the main methylene resonance (C3) of the the rate of increase of the spin–lattice relaxation time for the C3 carbon is considerably greater than with any of the other polymer as a function of plasticizer concentration in Fig. 8. In the case of tetraglyme, the C2 resonance is shown.This diagram plasticizers. In addition, the relaxation time of the 3PEG C3 carbon appears to be influenced to a greater extent than the shows that DMF addition results in the greatest enhancement of polymer mobility whereas PC gives the least. All other remaining polymer carbon relaxation times. This anomaly is most likely due to the fact that the methylene groups of plasticizers show similar relative increases in the C3 or C2 T1 times.In particular, PC appears least capable of ‘plasticizing’ triglyme and tetraglyme are coincident with the 3PEG C3 196 J. Mater. Chem., 1997, 7(2), 193–201Table 2 Comparison of fluidity of various plasticizers and their effect phase relative to the crystalline phase. It is interesting to note on polymer 13C T1 for C3 carbon in 50% plasticizer–50% 3PEG that for samples containing tetraglyme and triglyme plasticiz- systems ers, crystallization occurred almost immediately after passing through Tg.This indicates that these plasticizers readily crys- fluidity/ tallize and is consistent with the difficulty in quenching the plasticizer 103 Pa-1 s-1 T1(50)/T1 (0) pure plasticizers into a glassy state.In all mixtures investigated, N,N-dimethylformamide 1.266 3.1 with the exception of high concentrations of DMF, only a N-methylformamide 0.606 2.1 single Tg was observed, suggesting a homogeneous single phase c-butyrolactone 0.571 2.1 system. This is consistent with a single T1 observed for all propylene carbonate 0.394 1.4 carbons in the 3PEG–plasticizer systems. The two Tg values tetraglyme 0.244 2.3 observed for higher DMF contents are also consistent with two 13C T1 times measured for the polymer and suggest a phase separated system.the motions of the polymer. This is evident both from the Fig. 10 shows the dependence of inverse Tg on plasticizer small slope of T1 versus plasticizer concentration curve, and concentration for 3PEG–plasticizer blends.The straight lines also from the fact that the polymer C3 T1 remains unchanged in Fig. 10 are the theoretical lines expected for an intimately up to almost 30 mass% PC, whereas all other curves are mixed system following Fox’s equation, i.e. 1/Tg blend= increasing with concentration. These differences cannot be w1/Tg1+w2/Tg2. This behaviour would be expected if the inter- totally explained by comparing the fluidity of the plasticizers action energies between plasticizer and polymer segments were (Table 2).comparable to polymer–polymer and plasticizer–plasticizer At DMF concentrations higher than 85 mass% the polymer interactions; in other words, DE for mixing was close to zero. 13C T1 is observed to decrease for 3PEG–DMF systems and In some cases, the Fox equation is obeyed and linearity is two separate resonance are observed.This suggests two differ- observed, however, in the cases of PC and c-butyrolactone ent polymer environments and the possibility of phase separa- significant deviations are observed. These deviations suggest tion in the high DMF content samples. In all other cases, a an interaction which results in ‘antiplasticization’ since the Tg single resonance with a perfectly exponential decay curve (and of these systems is above that predicted by the Fox equation.hence a single relaxation time) is obtained consistent with an Indeed in the case of PC, the lower than expected decrease in intimately mixed, homogeneous system. Phase behaviour will Tg, particularly at low concentrations, is consistent with the be addressed further in the next section. lack of dependence of the 13C T1 measurements for the polymer in the 3PEG–PC system when up to 30% PC is added.It is Thermal analysis measurements of plasticized 3PEG difficult to determine whether the ‘antiplasticization’ effect is a The most common method for testing whether a multicompon- result of strong interactions between the plasticizer and poly- ent polymer system is miscible or immiscible is via measure- mer segments, which can restrict segmental motion of the ment of its glass transition temperature Tg.The glass transition polymer (e.g. via a bulky side group), or whether molecules temperature is also a measure of flexibility in a polymer system such as PC and c-butyrolactone, which are quite rigid, simply and generally decreases as a low molecular weight diluent is occupy free volume that was once available to the polymer added.Polymer electrolyte systems with lower glass transition and thereby hinder polymer segmental motion.24 Recent posi- temperatures have been shown to result in higher conductivities tron annihilation lifetime spectroscopy (PALS) experi- since the ionic motion is intimately linked to the motion of ments25,26 in these systems have in fact indicated that the the polymer.1,5 Fig. 9 shows a typical DSC trace for the pure overall free volume, as measured by PALS, is reduced when 3PEG polymer crosslinked with HDI. A well defined Tg is certain plasticizers are added. observed at 209 K followed by a crystallization exotherm with The effect of the addition of salt to the 3PEG–plasticizer onset temperature Tc at 234 K and a melting endotherm at 251 K.The relative sizes of the heat flows at Tg and Tm suggest that the majority of the sample remains amorphous below 251 K and is clearly 100% amorphous at room temperature. This pattern of Tg followed by crystallization and melting is observed in all of the plasticized systems, with the exception of those containing PC as plasticizer.The greater the difference between Tc and Tg, the greater is the stability of the amorphous Fig. 10 Inverse Tg versus plasticizer content for (a) triglyme ('), tetraglyme ($) and NMF (%) and (b) c-but ('), PC (%) and DMF ($). The straight dotted line is the behaviour predicted by the Fig. 9 Typical DSC trace for crosslinked 3PEG showing Tg, Tc and Tm Fox equation.J. Mater. Chem., 1997, 7(2), 193–201 197Fig. 12 T1 of the polymer main chain CH2 (C3 carbon) as a function of plasticizer content with 1 mol kg-1 salt [PC–LiClO4 ($), PC–NaOSO2CF3 (&), DMF–LiClO4 (#), DMF–NaOSO2CF3 (%)] iour observed in the salt-free systems and the DMF plasticized polymer (with and without salt).Whereas the addition of DMF increases the spin–lattice relaxation time of the polymer C3 carbon, consistent with decreasing Tg and increased segmental motion, the addition of PC to both LiClO4 and NaCF3SO3 containing 3PEG results in a decreased polymer T1. These measurements were reproduced several times and the uncer- Fig. 11 Glass transition temperatures as a function of plasticizer tainty in the values is indicated in Fig. 12. content for systems with (filled symbols) and without (open symbols) The nature of the salt also appears to affect the rate of 1 mol kg-1 LiClO4; (a) tetraglyme, (b) DMF and (c) PC increase of the polymer C3 T1 upon addition of DMF with a 300% increase in the case of LiClO4 when 60% DMF is added as compared with only 150% increase in the sodium salt.systems has previously been reported20,26 in the case of the Similar trends in behaviour were observed in all the polymer 3PEG–LiClO4 as a function of PC, DMF and tetraglyme 13C resonances. concentration. A single glass transition temperature was observed in all cases. It is notable that PC and DMF produce similar reductions in glass transition temperatures of the salt- Discussion containing sample. It is of interest here to compare the effect The glass transition data indicate that the plasticized samples of adding plasticizer to pure 3PEG and 3PEG–LiClO4 investigated in this work are single phase over most of the (Fig. 11). In the case of DMF and tetraglyme additions, the composition region ranging from pure polymer through to rate of decrease of Tg with increasing DMF content is only pure plasticizer both with and without salt present.There is slightly greater in the presence of the salt. PC displays more thus no evidence of domains which are sufficiently extensive complex behaviour, almost sigmoidal in shape, and the Tg of spatially to produce a separate Tg. This domain size needed both 3PEG–PC and salt-containing complexes approach the to produce a distinct Tg is often taken to be of the order of same value at high PC concentrations. These differences are 100 A° 3.27 The conduction in these plasticized polyether samples likely to reflect differences in interactions between plasticizer where the plasticizer is not the major component (e.g.50 and salt and are discussed further in the following sections.mass%) is therefore distinctly different from gel electrolytes such as poly(methyl methacrylate)–PC28 and poly(acrylonitr- 13C T 1 relaxation measurements in 3PEG–plasticizer–salt ile)–PC29 discussed by others. In the latter cases the low complexes molecular weight solvent, which is by far the major component (ca. 80%), in all likelihood exists in channels which can conduct We have previously reported9,20 that the addition of salt to both pure 3PEG and 3PEG–50 mass% plasticizer results in a ions.As shown previously4 in work from this laboratory involving mixtures of this type, the conduction process passes decrease in the spin–lattice relaxation time which is interpreted as a decreased mobility (and increased Tg) of both the polymer smoothly from a realm dominated by the polymer to one dominated by the low molecular weight solvent.No obvious backbone and the plasticizer. In Figs. 12 and 13 the effect of plasticizer concentration on the spin–lattice relaxation times percolation threshold is observed, as might be expected if enhanced conduction was an event controlled by connected of the C3 3PEG carbon and the plasticizer carbons respectively are given for 1 mol kg-1LiClO4 and 1 mol kg-1 NaCF3SO3 solvent channels.Instead, we were able to show that the behaviour was as expected on the basis of the increased systems. With increasing PC or DMF concentration, the spin– lattice relaxation times of the 13C resonances associated with configurational entropy contributed to the system by the presence of the plasticizer.4 the plasticizer increase; the rate of increase of T1 is greatest at the higher plasticizer concentrations.The T1 times for the Furthermore, our previous NMRwork9,20 and the relaxation measurements presented in this work, have shown that there carbonyl carbons of both plasticizers were also determined and displayed similar behaviour, however, due to extremely is significant interaction between the polymer and salt in the plasticized polymer electrolytes across the whole phase dia- long relaxation times (more than 10 s in samples containing greater than 50% plasticizer) the absolute values are not gram.Hence the plasticizer does not become the predominant solvent species even at quite low polymer contents. reliable since often the delay time between experiments was less than 5×T1.The relaxation times of the polymer carbons Having established that polymer–salt and polymer–plasticizer interactions exist in these systems, it is of interest to show considerable deviation on PC addition from the behav- 198 J. Mater. Chem., 1997, 7(2), 193–201Fig. 13 T1 of the plasticizer carbons as a function of plasticizer content with 1 mol kg-1 salt; (a) PC [$=PC 1 (d 19.1), %=PC 2 (d 70.8), 1=PC 3 (d 73.9)] and (b) DMF [#=DMF 1 (d 31), %=DMF 2 (d 35), '=DMF (d 162)] understand what the actual role of the plasticizer is in the oxygen on the same neighbouring backbone.The displacement of ether oxygen coordination by plasticizer co-ordination of ionic conduction mechanism in these polymer electrolytes.If we consider that the low molecular weight component serves the cations can thus be expected to have a strong effect on the local segmental mobility of the backbone. DMF is a clear case merely as a traditional plasticizer, i.e. it increases the free volume and/or the configurational entropy of the binary in point. PC, on the other hand, appears to be a less effective coordinating solvent than the ether oxygens and its presence solvent such that the overall mobility of the system is enhanced (as indicated by lower Tg values), then the polymer segmental at fixed salt concentration may cause more ether oxygens per unit volume (because there are less of them) to become involved motion will increase, and the conductivity will therefore increase.Table 2 gives the relative increases in 13C polymer T1, in cation coordination.The polymer mobility therefore, which is reflected in T1, is slightly decreased when PC is added which reflect the increase in polymer segmental motion at 50% plasticizer concentration in the absence of salt. Propylene whereas DMF increases the polymer segmental motion. The solvent–cation interactions also depend on the nature of the carbonate has the least effect on the polymer segmental motion, and tetraglyme, c-butyrolactone and NMF all have a similar cation.This is seen in the relative increases of polymer 13C T1 when DMF is added to lithium-containing polyether as com- effect, whereas DMF has a distinctly greater influence than any of the others. To a large extent, this is a result of the pared with sodium.The lithium ion appears to be coordinated more strongly by the DMF than the sodium ion and hence higher mobility of the pure DMF relative to the other pure plasticizers. However, this cannot be the only factor in enhanc- the polymer mobility increases more rapidly when LiClO4 is present, upon addition of DMF. ing polymer mobility since the T1 ranking does not follow the plasticizer fluidity ranking.Hence a number of specific factors Although both plasticizers decrease Tg and both increase the conductivity of the polymer electrolyte, their effect on the must be influencing the overall effect. Some of these are further discussed below. polymer in the presence of salt is quite distinct. In one case, the plasticizer competes with the polymer for the coordination In the presence of salt the addition of plasticizer in some cases, for example PC, restricts the mobility of the polymer of the alkali metal ion thereby increasing the ion and polymer mobility, whilst in the case of PC, which itself is a poor donor relative to its unplasticized state.This observation was somewhat unexpected given that the glass transition temperature solvent and therefore is itself inefficient in alkali metal ion solvation, the addition of plasticizer enhances the coordination of these samples continues to decrease with addition of PC.Furthermore, the 13C spin–lattice relaxation times of the PC of the cation by the polymer and hence restricts polymer motion. It should be noted that both DMF and PC have itself still increases with increasing PC content.An explanation for this behaviour can be found when the chemical shift results considerably higher relative permittivities than the polymer and hence are expected to increase the number of charge are also considered.20 Previous work discussing the behaviour of 13C chemical shifts of polyethers suggested that when PC is carriers. Recent FTIR work10 which has investigated the effect of adding tetraglyme and DMF to lithium triflate-containing added, in the presence of alkali metal ions, the ion–polymer interactions are enhanced for both Li and Na salts.In contrast, polyether has shown that in the case of DMF the number of ‘free’ anions, that is, anions which are not coordinated by the addition of DMF diminishes the coordination of the cations by the ether oxygens.DMF, which is known to be cations, increases as the concentration of DMF increases. This of course will result in an increased conductivity as given by good donor solvent, would appear to be able to displace the ether oxygen from the Li ion solvation sphere; PC on the the Nernst–Einstein expression. The combined increase in the number of charge carriers and the relative increase in polymer other hand appears to be less effective in this sense but nonetheless can provide more effective anion–cation charge mobility upon addition of DMF is not, however, sufficient to explain the large increase in ionic conductivity.In addition, shielding than can the polyether alone. Ether oxygen coordination to a metal cation is likely to have a restricting effect FTIR has shown that the addition of tetraglyme actually decreases the number of ‘free’ ions and hence the total number on the backbone’s local motional freedom, especially if the cation is simultaneously coordinated to a second or third ether of available charge carriers.Since the total enhancement in J. Mater. Chem., 1997, 7(2), 193–201 199conductivity is at least four times as large as the polymer T1 the polymer segmental motion whilst DMF has the greatest effect.This is in part due to the high mobility (higher fluidity) enhancement, a further explanation is required to account for the conduction in the tetraglyme systems. Vincent et al.30,31 of the DMF molecule, however, a comparison of the fluidity of, for example tetraglyme and PC, indicates that these two have recently measured the diffusion coefficient of lithium and PF6- ions in a high molecular weight polyether by 7Li and should have comparable effects on polymer plasticization.The high relative permittivity may lead to strong plasticizer–poly- 19F NMR. The addition of tetraglyme was also investigated and it appears that a 50% addition of tetraglyme results in mer interactions (particularly at low PC contents) which restrict polymer segmental motion. This is even more notice- approximately an 8-fold increase in diffusion coefficient at 80°C.This accords with the approximately 8-fold increase in able in the case of alkali metal salt-containing polyether networks where PC additions lead to an increased polymer conductivity we have found in previous work.2 It appears, therefore, that at least in the case of tetraglyme systems, the 13C relaxation rate (decreasing T1).This has been shown previously to be at least in part a result of stronger metal ion– increased conductivity is predominantly a result of increasing mobility of the charge carriers. It remains then to explain what polymer interactions as a result of an increased cation–anion screening provided by the high relative permittivity of PC.governs the increasing diffusion coefficients of the ions. In traditional polymer electrolytes, ionic conduction relies on Thermal analysis data also shows evidence of strong specific interactions in some plasticizer–polyether systems, with only high segmental motion of the polymer chains, however, in the presence of plasticizers the motion of the ions is likely to be DMF, NMF and the ether based systems showing linear Fox behaviour in the inverse Tg versus composition plots.The complicated by the presence of the cosolvent. If the absolute spin–lattice relaxation times for the plasticizer remaining plasticizers result in a higher Tg than would be predicted by the Fox equation, indicating strong polymer– carbons are compared with those of the polymer it is clear that these can differ by more than an order of magnitude.The plasticizer interactions. The differences observed between the plasticizers in their effects on T1 are consistent with Tg. This approximate motional quantity to compare in these circumstances is the correlation time t for both plasticizer and therefore supports the notion that T1 is a good local probe of polymer mobility.polymer backbone. This would give a direct measure of the timeframe for plasticizer versus polymermotion. Unfortunately, This work was initiated from the desire to understand the effects of plasticization on the polymer mobility and hence on this requires extensive temperature-dependent T1 data.This was not possible to achieve in the present work since at the the conductivity behaviour of polymer electrolytes. The data presented here suggest that, on the basis of enhancement of lower temperatures required to reach the T1 minimum, the 13C linewidth became too broad and the signal eventually was lost. polymer motion (T1) in the absence of salt, DMF should have the greatest effect on conductivity enhancement followed by Nevertheless, the absolute T1 values for the polymer and plasticizers indicate that the plasticizer, although restricted in NMF, triglyme, tetraglyme and c-butyrolactone, whilst PC should have the least effect on the conductivity.Indeed, a its mobility relative to its pure state, nonetheless maintains a higher degree of mobility as is evidenced by higher T1 compared comparison of the effects of PC and DMF on the polymer 13C T1 times in salt-containing systems would suggest that PC with the polymer.Thus the model suggested by this work for conduction in plasticized polyether electrolytes requires that restricts polymer segmental motion and hence would not have a positive effect on ionic conductivity in these systems.This is ionic mobility is governed by both polymer and plasticizer mobility, however for that interval of time that the ions are not the case, however. Although DMF does give a greater enhancement than PC when added to polyether–urethane completely within the plasticizer environment they move at a faster rate than those times when they are coordinated (in the polymer electrolytes, NMF gives the greatest enhancement and PC still improves the conductivity to a larger degree than case of cations) or in the vicinity of (in the case of anions) the polymer segments.do triglyme and tetraglyme.2 In addition, the relative increase of polymer mobility as indicated by the increased T1 is not as great as the relative conductivity enhancements.These results Conclusions therefore indicate that the role of the plasticizer in improving ionic conductivity in polymer electrolytes is not simply in NMR 13C spin–lattice relaxation time measurements have illustrated the effects of the incorporation of small molecules improving the polymer segmental motion and decreasing Tg. The specific interactions between plasticizer molecules, polymer on the local polymer segmental motion in polyether–urethane networks both in the presence and absence of alkali metal segments and ionic species will affect the final conduction mechanism and the magnitude of the conductivity in polymer salts.In the case of unsalted networks, large additions of all the plasticizers investigated result in an increasing T1 consistent electrolytes.For example the DMF–cation interactions and the PC shielding will have an influence on, and determine the with an increased polymer mobility (assuming that the interactions which govern the relaxation are not significantly chang- number and nature of, the major charge-carrying species. The mobility of the plasticizer molecules themselves must also play ing upon addition of plasticizer).In most cases, with the exception of higher DMF concentrations, the data gave an a large role in the conduction mechanism. Although T1 of the polymer is increased, indicating greater polymer segment excellent fit to a single relaxation, the T1 of which varies smoothly with composition, indicating a single phase system. mobility, the plasticizer molecules will still be more mobile and hence ions which are in the vicinity of the plasticizer In the case of DMF, at concentrations higher than 70% a decrease in the polymer T1 is observed whilst the DMF T1 molecules will have a higher mobility and result in higher conductivities.times continue to increase. This is suggestive of phase separation, and is consistent with the observation of two Tg values in the thermal analysis experiments.References The degree to which the polymer relaxation time is affected is dependent upon the nature of the plasticizing molecule. In 1 F. M. Gray, Solid Polymer Electrolytes, Fundamentals and T echnological Applications, VCH, New York, 1991. particular the molecules that are capable of strongly interacting 2 M.Forsyth, D.R. MacFarlane, J. M. Hey and P. Meakin, Advances with the polymer backbone, or which are themselves sterically in Science and T echnology, New Horizons for Materials, vol 4, ed. hindered (or bulky) have less influence on the polymer relax- P. Vincenzini, Techna Srl, 1995, pp. 265–272. ation rate and hence on the local mobility. Furthermore, 3 D. R. MacFarlane, J. Hey and M.Forsyth, Mater. Res. Soc. Symp. certain plasticizers show distinct antiplasticization effects at Proc., 1991, 210, 197. low concentrations (less than 20%) as indicated by unchanging 4 D. R. MacFarlane, J. Sun, P. Meakin, P. Fasolopoulos, J. Hey and M. Forsyth, Electrochim. Acta, 1995, 40, 2131. polymer 13C T1. PC additions seem to have the least effect on 200 J. Mater. Chem., 1997, 7(2), 193–2015 M.B. Armand, Annu. Rev. Mater. Sci., 1986, 4, 245. 19 L. Lestel, P. Guegan, S.Boileau, H. Cheradame and F. Laupretre, 6 S. H. Chung, K. Such, W. Wiezoreck and J. R. Stevens, J. Polym. Macromolecules, 1992, 25, 6034. Sci., Part B: Polym. Phys., 1994, 32, 2733. 20 M. Forsyth, D.R. MacFarlane and P.M. Meakin, Electrochim. 7 J. P. Southhall, H. V. St. A.Hubbard, S. F. Johnston, V. Rogers, Acta, 1995, 40, 2339. G. R. Davies, J. E. McIntyre and I. M. Ward, presented at T he 21 H. Feng, Z. Feng, H. Ruan and L. Shen, Macromolecules, 1992, First Electronic Conference on Solid Electrolytes, 1995, to be pub- 25, 5981. lished in Solid State Ionics. 22 L. A. Belfiore, P. M. Henrichs, D. J. Massa, N. Zumbulyadis, W. 8 L. M. Torell, in Handbook of Solid State Batteries and Capacitors, P. Rothwell and S.L.Cooper, Macromolecules, 1983, 16, 1744. ed. M. Z. A. Munshi and P. S. S. Prasad, World Scientific Publ. 23 R. A. Komoroski, High Resolution NMR Spectroscopy of Synthetic Comp., in the press. Polymers in Bulk, VCH, Florida, 1986. 9 M. Forsyth, P. Meakin and D. R. MacFarlane, J. Mater. Chem, 24 W. J. Jackson, Jr. and J.R. Caldwell, J. Appl. Polym. Sci., 1967, 1994, 4, 1149. 11, 211. 10 A. Bishop, D. R. MacFarlane, D. MacNaughton and M. Forsyth, 25 M. Forsyth, A. J. Hill, D. R. MacFarlane and P. M. Meakin, J. Phys. Chem., 1996, 100, 2237. Electrochim. Acta, 1995, 40, 2349. 11 M. Forsyth, M. E. Smith, P. Meakin and D. R. MacFarlane, 26 M. Forsyth, P. Meakin, D. R. MacFarlane and A. J. Hill, J. Phys: J. Polym. Sci. Part B: Polym. Phys., 1994, 32, 2077. Condens. Matter, 1995, 7, 7601. 12 F. Ali, M. Forsyth, M. C. Garcia, M. E. Smith and J. H. Strange, 27 L. A. Utracki, Polymer Alloys and Blends, Oxford Science Solid State Nucl. Magn. Reson., 1995, 5, 217. Publications, Oxford, 1988. 13 M. Forsyth, D. R. MacFarlane, P. Meakin, M. E. Smith and T. 28 O. Bohnke, G. Frand, M. Rezrazi, C. Rousselot and C. Truche, J. Bastow, Electrochim. Acta, 1995, 20, 2343. Solid State Ionics, 1993, 66, 105. 14 C. Berthier, W. Gorecki, M. Minier, M. B. Armand, J. M. 29 P. E. Stallworth, J. Li, S. G. Greenbaum, F. Croce, S. Slane and Chabagno and P. Rigaud, Solid State Ionics, 1983, 11, 91. M. Salomon, Solid State Ionics, 1994, 73, 119. 15 R. Spindler and D. F. Shriver, J. Am. Chem. Soc., 1988, 110, 3036. 30 P. G. Bruce and C. A. Vincent, J. Chem. Soc., Faraday T rans., 1993, 16 S. Schantz and S. L. Maunu,Macromolecules, 1994, 27, 6915. 89, 3187. 17 S. L. Maunu, K. Soljamo, M. Laantera and F. Sundholm, 31 C.A. Vincent, Electrochim. Acta, 1995, 13–4, 2035. Macromol. Chem. Phys., 1994, 195, 723. 18 J. P. Manning, C. B. Frech, B. M. Fung and R. E. Frech, Polymer, 1991, 32, 2939. Paper 6/04781E; Received 8th July, 1996 J. Mater. Chem., 1997, 7(2), 193–201 201

ISSN:0959-9428    

DOI:10.1039/a604781e

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Bulk thermal polymerisation of diethylene glycol bis(allyl carbonate) as studied by dielectric relaxation spectroscopy Ian K. Smith,a Stuart R. Andrews,a Graham Williams*a and Paul A. Holmesb aDepartment of Chemistry, University College of Swansea, Singleton Park, Swansea, UK SA2 8PP bPilkington T echnologyManagement L imited, Hall L ane, L athom, Ormskirk, L ancashire, UK L 40 5UF Broad-band dielectric relaxation spectroscopy (DRS) has been used to study the changes in molecular relaxation behaviour when diethylene glycol bis(allyl carbonate) (CR39 monomer) is polymerised thermally in its bulk state.As polymerisation proceeds the dielectric a-relaxation process broadens markedly and moves to higher temperatures while, at the same time, a small lowtemperature b-relaxation process increases in its intensity and moves to higher temperatures.The dielectric loss spectra are shown to provide a convenient fingerprint that indicates the extent of cure in partially cured materials. Polymer glasses formed from the CR39 monomer are used the free-radical initiator. The materials were supplied by Akzo Chemicals and used in their as-received state without further widely as materials for prescription lenses, safety glasses and as synthetic glasses in a variety of applications because of their purification.A 3% solution of IPP in CR39 monomer was prepared and portions were used for all subsequent studies excellent optical clarity and mechanical strength. Few studies have been reported on the nature of the free-radical polymeri- reported here.The dielectric measurements were made using a Novocontrol dielectric spectrometer that incorporated a sation of the CR39 monomer. Dial et al.1 used idiometry as a method to analyse the polymerisation kinetics, while Schnarr Solartron SI 1260 impedance/gain-phase analyser with a Chelsea dielectric interface, allowing measurements of the and Russell2 used dilatometry to study the early stages of polymerisation.Starkweather and Eirich3 also used dila- complex permittivity, e*=e¾(v)-ie(v) in the frequency range 10-3 to 106 Hz. Fig. 2 shows a schematic diagram of the tometry while Hill et al.4 used density and refractive index methods together with NIR and EPR spectroscopies to moni- measuring system. The Novocontrol Quatro unit controlled sample temperature using N2 gas and allowed measurements tor the polymerisation process.While such methods provide essential information on changes in physical and chemical to be made in the range -180 to 400 °C with a precision of 0.1°C. The measuring system was computer controlled using structures during polymerisation, they are fairly insensitive to the important changes in the mechanical properties that occur the Novocontrol WinDETA software, allowing a wide range of heating/cooling cycles and frequency sweeps to be made in the late-stages of cure that lead to glass formation. The technique of dielectric relaxation spectroscopy (DRS) provides automatically. The liquid monomer was placed in a brass cup with an inside diameter of 36 mm, which formed the lower a convenient and sensitive means of monitoring changes in that range, since it provides information on molecular motions of chain dipoles in the developing cross-linked network.De Meuse5,6 reported dielectric studies of the polymerisation of the CR39 monomer. Two relaxation processes were observed in the cured polymer glass: (i) an a-relaxation process associated with the glass transition, and (ii) a b-relaxation process due to localised motions of ether and carbonyl dipoles.He showed6 that the location of the dielectric b-process in plots of loss factor (e) vs. temperature (T ) was sensitive to the cure cycle used and the initiator employed. Frounchi et al.,7 using dynamic-mechanical relaxation methods, studied the cured polymer and observed a- and b-processes in the region of Fig. 1 100 °C and -50 °C, respectively. In the present work, we present extensive dielectric data for materials polymerised to different extents, starting with the uncured CR39 monomer, which is a glass forming liquid, extending to the fully cured polymer glass. The variations of the different dielectric relaxation processes with sample preparation are documented and are shown to provide a convenient means of monitoring the state of cure of samples.In an earlier paper8 we reported the dielectric relaxation behaviour of CR39 monomer as studied over the frequency range 10-2 to 104 Hz and the temperature range -110 °C to -50°C, giving information on the a-relaxation process, its form and dependence on temperature. Experimental The CR39 monomer (see Fig. 1) was polymerised in its bulk Fig. 2 state using diisopropyl peroxydicarbonate (IPP, see Fig. 1) as J. Mater. Chem., 1997, 7(2), 203–209 203Table 1 The CR39 cure cycle showing the stages up to which different samples of the master solution were cured cure temperature/°C cure time/h, min product 40 16,00 P1 45 16,20 50 16,40 55 17,00 P2 60 17,20 65 17,40 70 18,00 P3 75 18,20 80 18,40 P4 85 21,00 90 21,20 95 21,40 100 22,00 105 22,20 110 22,40 115 23,00 120 44,00 P6 electrode, and a brass disc of 30 mm diameter was placed on Scheme 1 Intramolecular cyclisation top and acted as the upper electrode.The electrode separation was maintained by two thin strips of PTFE, thickness 760 mm. Since for our system we have a concentrated monomer solution This electrode assembly was mounted in the Novocontrol (97%) and the distance between the two allyl groups in the BDS1200 sample holder which was placed in the cryostat head monomer is relatively large,9 then the cyclisation reaction only of the instrument.Calculations of the permittivity, e¾, and loss occurs to a small extent. factor, e, for the sample in the cell were made using standard A general feature in the polymerisation of allyl monomers sample-out and sample-in measurements of the cell equivalent is a chain transfer reaction (Scheme 2) in which the growing parallel capacitance and conductance at each frequency.In a chain (Pn ) is terminated and an allyl radical is produced. frequency sweep, measurements were made at 51 frequencies Since such species are resonance stabilised, they are less likely in the range 10 to 105 Hz, while in a temperature scan to cause further chain growth than chain termination by measurements were made at 3°C intervals in the range -130 combination with other propagating radicals or similar allyl to 100 °C.Each measurement of (e¾, e) at each frequency and radicals. When this occurs the chain transfer reaction it is temperature was recorded during each experiment thus termed degrative since it leads to termination of the growing allowing post processing of data to give [e( f,T ), e( f,T )] for chains.For this reason degrative chain transfer is perhaps the each data point for use in two- and three-dimensional plots. most important kinetic step in the polymerisation of the CR39 Samples were polymerised as follows.A portion of the monomer. Hill et al.4 have shown that the major radical species master solution (3% IPP in CR39 monomer) was placed in present at conversions below 45% is the allyl radical. However, the dielectric cell and then incorporated into the measuring Starkweather and Eirich3 have shown that during the later system as described above.It was then heated according to stages of the reaction, the importance of the degrative chain the cure cycle given in Table 1, up to a chosen point and then transfer is less prevalent in the polymerisation of diallyl cooled rapidly to -130°C which stopped the reaction (e.g. compounds than monoallyl compounds, and have shown that sample P2 was obtained by heating a portion of the sample the ultimate conversion of double bonds for the CR39 mon- mixture for 16 h at 40°C, followed by 20 min at 45°C, 20 min omer is greater than that expected.This is explained as being at 50°C and finally 20 min at 55°C, then rapid cooling that due to reinitiation caused by the allyl radicals. At the later sample to -130 °C). It was then studied as a function of stages of cure, a three-dimensional network of cross-linked frequency during a heating run at every 3°C in the range polymer chains is formed in which reactions involving a 130 °C to Tend, where Tend is chosen to be sufficiently high for propagating radical and a vinyl group will become progress- a sample to allow full characterisation of the dipole relaxation ively more difficult.Active species will become isolated from processes. each other since they are chemically bonded to the network.Such a network will contain unreacted vinyl groups together with allyl radicals. Thus, the allyl radicals are less likely to Reaction mechanism and dielectric behaviour cause chain termination by combination with other radical It is appropriate to describe the essential chemical processes species than a reaction with the unreacted CNC bonds causing involved in the thermal free-radical polymerisation of the CR39 reinitiation (Scheme 3).The importance of reinitiation has also monomer. The initiator material IPP is effective in the range been recognised by other workers in other studies of diallyl 35–50°C. On heating the reaction mixture to temperatures in polymerisation.10–12 Thus in the later stages of the reaction this range, IPP decomposes into two free radicals which then initiate the polymerisation in the usual way, by adding to the CNC bond of the allyl group of the CR39 monomer.Since the CR39 monomer is bifunctional, the resulting propagating chain will have side-groups that contain active CNC groups. During polymerisation, an intramolecular cyclisation reaction may occur2,9 (Scheme 1). During the initial stages of the reaction this has been shown1 to occur only to a slight extent in comparison with the main addition process.In addition, Holt and Simpson9 have shown that the extent of intramolecular cyclisation is inversely dependent on the initial monomer Scheme 2 Chain transfer concentration and the distance between the two allyl groups. 204 J. Mater. Chem., 1997, 7(2), 203–209Scheme 3 Reinitiation of the reaction by the allyl A radical the allyl radicals are an important source of cross-linking reactions and the decrease in the concentration of CNC bonds in the system. In a previous publication8 by our group, it was shown that the glass-forming CR39 monomer liquid exhibited a well defined a-relaxation due to the large-scale reorientational motions of the dipoles, and a weakly defined b-relaxation due to the localised motions of the dipoles.Therefore, in relation to the dielectric properties of the reacting system, we expect to observe multiple relaxation processes associated with the large-scale and localised motions of the ester and ether dipoles, which of course throughout the reaction remain chemically unchanged.The complex permittivity of a material can be expressed using a Fourier transform relation13 according to eqn. (1): e*(v)-e2 e0-e2 =1-iv P2 0 w(t)exp(-ivt)dt (1) where v=2pf/Hz, e0 and e2 are the limiting low and high frequency permittivities with respect to the relaxation range, and w(t) is a decay function for the polarisation that describes the relaxation of a material following the step-removal of an electric field.To a good approximation w(t) corresponds to the dipole moment correlation function Cm(t), for the reorientational motions of dipoles in a material14,15 which may be written as: Fig. 3 (a) Permittivity and (b) loss as a function of frequency and temperature for P1 Cm(t)= .N i,j <mi(0)mj(t)> .N i,j <mi (0)mj(0)> (2) Comparison of these data with those reported for the monomer8 shows that the a-loss peak has moved to higher tempera- Here the sum is taken over all dipoles N in a macroscopic tures by ca. 20°C, reflecting an increased Tg for this partially volume V in the material. In regards to the system studied cured material, and has broadened significantly in both the here, eqn.(2) contains autocorrelation terms for the motions frequency and temperature domains, reflecting that dipole of ether and ester dipoles (mether, mester) and cross-correlation motions are now occurring in a range of local environments terms between the two dipoles. As explained in detail for glass- in this material (see also the discussion of Fig. 6 below). forming liquids and polymers,16 the motions of such dipoles Fig. 4(a) and (b) shows data for sample P4. Comparison of the may occur by local processes that only partially randomise permittivity data shows that although the high-temperature the dipole vector, giving rise to secondary (b, c, etc.) dielectric permittivities are approximately the same for both samples relaxations, or by micro-Brownian motions (a-process) that (and for the CR39 monomer8) the breadth of the dielectric totally relax the remainder of the mean square dipole moment dispersion is increased markedly by further curing.The comp- not relaxed by the secondary processes. Thus, a dielectric study lementary loss data in Fig. 4(b) shows how broad the a-process of the uncured, partially cured and fully cured CR39 monomer has become and how the shift of the process to higher materials will provide valuable information on how the temperatures is continued.Importantly, a small loss feature is motions of the dipoles are affected by the extent of polymeris- observed at low temperatures and frequencies, which at 10 Hz ation and cross-linking. is centred at ca.-100°C. Fig. 5(a) and (b) shows data for sample P6. The permittivity curves show a gradual decrease from the high-temperature values for T#100°C and a small Results and Discussion but definite low-temperature dispersion region is now apparent We have already shown that the CR39 monomer exhibits a below -80°C. The loss data in Fig. 5(b) show a complex well defined a-relaxation process.The frequency of maximum pattern of behaviour and it is apparent that the low-tempera- dielectric loss factor in plots of e vs. log( f/Hz) obeys the ture process is enhanced further and the high-temperature Vogel–Fulcher equation, process is extremely broad and has continued to move to higher temperatures than in sample P4. The qualitative features log( f )=f0- C B T-T0D (3) of Fig. 3–5, taken together with our earlier results8 for the CR39 monomer (Fig. 3 of ref. 8) may be summarised as follows. (1) The magnitude of the dielectric dispersion for the mix- with parameters f0=13.09, B=1236 K and T0=147.4 K. On polymerisation the reaction mixture contains a range of poly- tures of dipoles, De=e(high T )-e(low T ), does not change markedly as the monomer is polymerised.Thus, although the meric species in addition to the unreacted monomer. Fig. 3(a) and (b) shows three-dimensional plots of e¾ and e as a function nature and timescale for the motions of ether and ester dipoles changes markedly, the spatial extent of their overall motions of frequency and temperature for the least cured product P1 (Table 1). One dipole relaxation process is clearly observed is not changed appreciably when a comparison is made at a fixed high temperature (e.g. 120 °C) say, which is above the Tg together with a rising loss process at low frequencies and high temperatures due to ionic conduction in the material. of all samples. J. Mater. Chem., 1997, 7(2), 203–209 205(2) The three-dimensional dielectric landscapes provided by the figures reveal changes taking place in relaxation processes that were not readily obtainable previously (although Reddish in early studies of the dielectric behaviour of polyethylene terephthalate had previously given a three-dimensional representation of the loss data as a function of frequency and temperature, see ref. 3). From the loss data in Fig. 3–5 we see (i) that the a-process broadens remarkably and its location moves to higher temperatures and (ii) a new low-temperature b-process appears and increases in relaxation strength with increased curing.While the loss peaks for the a- and bprocesses are clearly defined in the temperature domain in all cases, the loss peaks become so broad in the frequency domain for samples P3–P6 that it is difficult to determine the frequency of maximum loss in such cases.We consider first the changes in plots of e vs. T at fixed frequencies for the different samples. Fig. 6(a) shows the data for CR39 monomer together with those for P1, P2 and P3 all measured at 1 kHz. The height of the a-loss peak decreases from 0.61 to 0.165 to 0.125 on going from the monomer through to P3. Fig. 6(b) shows the results for P1–P3 on an expanded scale that reveals the emergence of the b-process Fig. 4 (a) Permittivity and (b) loss as a function of frequency and temperature for P4 Fig. 6 Comparison of the loss factor temperature profiles: (a) for the monomer ($) and P1 (#), P2 (&) and P3 (%); (b) for P1 (#), P2 (&) Fig. 5 (a) Permittivity and (b) loss as a function of frequency and and P3 (%); and (c) for P4 ($), P5 (,) and P6 (+), all measured at a frequency of 1 kHz temperature for P6 206 J.Mater. Chem., 1997, 7(2), 203–209centred at ca. 180 K. Fig. 6(c) shows the loss plots for P4, P5 and P6; the b-process continues to grow and moves slightly to higher temperatures while the a-process is now of small magnitude and its peak position moves markedly to higher temperatures, being centred at 350 K for the most cured sample P6.The a-process moves from 195 to 350 K on going from the monomer to the fully cured glassy product P6. Its appearance for the uncured monomer is consistent with the normal a-relaxation observed in glass-forming liquids, which is characterised by the Vogel–Fulcher law for the dependence of the average relaxation frequency with temperature8 and the Kohlrausch–Williams–Watt (KWW) relaxation function17–19 with spread parameter b�=0.55.However, as we cure the samples to greater extents the loss curves for the a-process broaden remarkably and shift by 150 K to higher temperatures (Fig. 6). This clearly implies a change in mechanism on curing through the series of samples. At the same time the b-process, which is absent for the monomer, appears on curing and is a larger feature than that for the a-process in the highly cured samples [Fig. 6(c)]. It is well known that permittivity and loss data are frequencyand temperature-dependent for amorphous polymers.13 In the present case, the plots of e vs. T for fixed frequencies act as a fingerprint for the condition of a particular sample and may act as a guide for processing of the material from monomer to cross-linked polymer product. Fig. 7 shows the fingerprint loss spectra of two samples cured under identical conditions. P5 was made according to Table 1 using a fresh monomer– initiator mixture, while the ‘aged’ product was made using a monomer–initiator mixture that had been stored for six months Fig. 8 (a) Permittivity (', -40; ,, -46; +, -52; %, -58; &, -64; at -10 °C in a refrigerator prior to curing.The differences are #, -70; $, -76 °C) and (b) loss factor ($, -40; #, -46; &, -52; evident. First, the b-peak is smaller than the a-peak for the %, -58; +, -64; ,, -70; ', -76 °C) measured as a function of frequency for the least cured product, P1 aged specimen while the reverse is true for P5.Secondly, the b-peak for the aged sample is smaller than that for P5, while the reverse is true for the a-process. Furthermore, the a- and b-processes for the aged sample peak at lower temperatures than those for P5. These results all show that the aged sample decreased. The half-widths of the loss curves shown in Fig. 8(b) are in excess of four decades of frequency, which compares is not cured to the same extent as P5.The storage of the reaction mixture for this length of time clearly has reduced the with two decades of frequency for the CR39 monomer,8 and would correspond to KWW b� values less than 0.25. These effectiveness of the free-radical initiator and led to this result. Thus an important conclusion of this work is that the physical data [and those of Fig. 6(a) for P1] confirm that the arelaxation mechanism is changed markedly when the cross- condition of cured samples may be compared with those of fully cured samples via plots of e vs. T , as shown in Fig. 7. linked material is formed initially. Clearly, the cooperative motions of the ester and ether dipoles occur in a range of Thus far we have described plots of e vs.T for constant frequencies. As indicated above, we have reported previously8 environments; hence the broadening seen in Fig. 8(b). The increased broadening of the loss curves with decreasing tem- plots of e¾ and e vs. log( f/Hz) for the CR39 monomer that show KWW behaviour with b�=0.55. Fig. 8(a) and (b) shows perature is consistent with a spectrum of relaxation processes, each having their own apparent activation energy. The conse- plots of e¾ and e vs.log( f/Hz) respectively for P1. The loss curves are broad and become broader as the temperature is quence is that the plots of log( fm) vs. T -1 and log( f ) vs. Tm-1, where fm is the frequency of maximum loss at a fixed temperature and Tmax is the temperature of maximum loss at a fixed frequency, do not lie on a common curve.In practice, this means that for samples cured beyond P3 we cannot obtain reliable plots of log( fm) vs. T -1 but are able to do so for plots of log( f ) vs. Tmax-1 for all samples. Fig. 9 shows such plots for the a-process. The plots for P1–P3 are seen to be curved in the Vogel–Fulcher (VF) sense and all are shown to move strongly to higher temperatures, reflecting the increase in the Tg of the system that occurs as we proceed from uncured monomer through P1 to fully cured polymer P6. The VF equation [eqn.(1)] was fitted, using a computational reiterative procedure to produce a line of best fit, to the plots for P1, P2 and P3. The results are shown as the solid lines in the plots in Fig. 9 for these materials and the derived VF parameters from these plots are shown in Table 2.The quality of the fits shown is excellent for each data set, which is remarkable when Fig. 7 Comparison of the loss factor temperature profiles (measured it is remembered that the broadening of the curves is consider- at a frequency of 1 kHz) for two products obtained at the same time able on going from P1 to P3 [see Fig. 6(b)].Using the in the cure cycle. The ‘aged’ product was obtained using a stock parameters of eqn. (3) we calculated the apparent activation solution of reaction mixture that had been stored for six months prior to curing. energies, Qapp, for different measuring frequencies for P1–P3 J. Mater. Chem., 1997, 7(2), 203–209 207where f0¾ is an empirically determined constant and Qapp¾ is the apparent activation energy.The resulting fits for P4–P6 are shown as the solid lines in Fig. 9. Table 4 lists the Qapp¾ values calculated from the gradient of each of the lines. These values should be compared to the Qapp values shown in Table 3 for P1–P3 at a frequency of 103 Hz. During the later stages of reaction Table 4 shows that the activation energy of the arelaxation process undergoes a further increase as we progress from the partially cured material P4 to the fully cured, highly cross-linked material P6.It should be remembered that all plots in Fig. 9 should show VF behaviour and converge to a common line at high temperatures when the bulk relaxation process is at frequencies in excess of ca. 1010 Hz. At such high temperatures, however, the materials will be chemically unstable.Fig. 10 shows the corresponding plots of log( f ) vs. Tmax-1 Fig. 9 Activation energy plots of log (frequency/Hz) vs. T max-1 for the a-process for P1 ($), P2 (#), P3 ((), P4 (&), P5 (%) and P6 (+). for the b-process observed in each material. The linearity of The VF fits for P1–P3 and the Arrhenius fits for P4–P6 are shown as each plot is excellent showing that the b-process obeys an solid lines.Arrhenius equation and suggesting that the distribution of relaxation times is equivalent to a distribution of pre-ex- Table 2 Parameters of the Vogel–Fulcher relation fitted to the loss ponential terms in the Arrhenius equation. As such, eqn. (5) data in the temperature domain for the a-relaxation process of was fitted to the plots in Fig. 10 and the results are shown as products P1–Psolid lines in each case. The values of the apparent activation energies calculated from the slopes of the fits are product B/K To/K given in Table 5. As the b-process moves to higher temperatures P1 1077±1 161.0±0.1 on increased curing so Qapp¾ increases. Note that a similar P2 1164±4 162.7±0.1 trend has been observed for the mechanical b-relaxation pro- P3 975±3 195.9±0.1 cess observed in the dynamic mechanical studies on the effect Table 4 Activation energies for the a-process for products P4–P6, through the relation calculated using the Arrhenius relation fitted to the plots shown in Fig. 9 Qapp= RB A1- T0 TmaxB2 ; TT0 (4) product Q¾app/kJ mol-1 P4 213.8 and the results are given in Table 3.For each product it can P5 344.1 be seen that as the frequency of measurement is decreased, the P6 614.0 peak position of the loss curves decreases and the activation energy increases. Such an increase in Qapp is typical of that for a-relaxations in glass-forming liquids and amorphous polymers. Table 3 also shows that the apparent activation energies for the a-process of P1 and P2 atall frequencies of measurement are essentially the same.In comparison, those for P3 are increased appreciably. We now consider the plots for products P4–P6 in Fig. 9. At low frequencies the a-process becomes overlapped by the ionic conductivity process for these materials which masks the aloss peak position. At higher frequencies the a-process becomes overlapped by the lower temperature b-process, making defi- nition of the loss peak positions for both processes difficult.The plots for P4–P6, therefore, appear as limited data lying about a straight line. In order to gain an indication of the effect of cure on the apparent activation energies for the aprocess of these systems, the plots for P4–P6 shown in Fig. 9 were fitted to the Arrhenius relation given by eqn.(5), Fig. 10 Activation energy plots of log (frequency/Hz) vs. T max-1 for the b-process for P1 ($), P2 (#), P3 ((), P4 (&), P5 (%) and P6 (+). The Arrhenius fits are shown as solid lines. log( f )=f0¾- Qapp¾ RTmax (5) Table 3 Apparent activation energies (calculated using the VF parameters in Table 2) at each decade of frequency for the a-process of products P1–P3, together with the loss peak maxima P1 P2 P3 log (f/Hz) Tmax/K Qapp/kJ mol-1 Tmax/K Qapp/kJ mol-1 Tmax/K Qapp/kJ mol-1 1 205.9 188.0 211.4 180.3 240.0 239.7 2 210.8 160.5 216.8 155.0 245.4 199.2 3 216.9 134.9 222.8 133.4 251.9 164.1 4 224.2 122.7 231.0 111.7 260.8 130.9 5 234.1 91.8 241.9 91.7 272.6 102.3 208 J.Mater. Chem., 1997, 7(2), 203–209Table 5 Activation energies for the b-process for products P1–P6, (ii) The b-process that emerges in the range 180–230 K (see calculated using the Arrhenius relation fitted to the plots shown Fig. 6) is a surprising observation. As the network is formed in Fig. 10 the localised motions of the dipolar groups at these low temperatures increase in activity. The origin of this behaviour product Q¾app/kJ mol-1 is unclear.One may speculate that at long cure times, the P1 30.9 continual reaction, leading to new polymer chains and P2 30.5 increased cross-linking, is accompanied by physical annealing P3 35.2 of the material which changes the environment of the ether P4 40.7 and ester groups to give them additional freedom compared P5 41.1 with the situation at earlier times. Certainly one result of this P6 47.7 work is that the dielectric absorption spectra act as a clear diagnostic fingerprint of the extent to which cure has been achieved and is of practical use to compare different cured of increased cross-linking on molecular relaxations in diepoxide –diamine systems studied by Charlesworth.23,24 For products (see Fig. 6 and 7). amorphous polymers, local reorientational processes partially relax the auto- and cross-dipole moment correlation functions. The authors gratefully acknowledge the provision of a grant from the EPSRC for the purchase of the dielectric spectrometer The extent to which this occurs increases with increasing temperature.13,16 This is also the case for the samples studied and for the award of a PDRA to S.A.and a case studentship to I.K.S.here. The remarkable feature of the present work is the increase in relaxation strength of the b-process as the extent of cure is increased. De Meuse5 observed a strong b-process for two References cured CR39 monomer samples made using benzyl peroxide as initiator. Note also that Mangion and Johari20–22 studied the 1 W. R. Dial, W. E. Bissinger, B. J. DeWitt and F.Strain., Ind. Eng. Chem., 1955, 47, 2447. changes in dielectric behaviour of a diepoxide–diamine system. 2 E. Schnarr and K. Russell, J. Polym. Sci., 1980, 18, 913. In addition to the changes in the frequency–temperature 3 H. W. Starkweather and F. R. Eirich, Ind. Eng. Chem., 1955, 47, location of the a-process, they observed a b-process which for 2452. their DGEBA–DDM system increased in height and shifted to 4 D.J. T. Hill, D. I. Londero, J. H. O’Donnell and P. J. Pomery, Eur. higher temperatures, while for their DGEBA–DDS system it Polym. J., 1990, 26, 1157. 5 M. De Meuse, Polym. Eng. Sci., 1993, 33, 1049. increased in height but shifted to lower temperatures on curing. 6 M. De Meuse, J. Polym. Sci., B Polym. Phys., 1994, 32, 1749. They interpreted their data for the b-process in terms of 7 M.Frounchi, R. P. Chaplin and R. P. Burford, Polymer, 1994, changes in the local environment of the regions surrounding 35, 752. the dipole segments. The difference in behaviour of the DDS 8 I. K. Smith, S. R. Andrews, G. Williams and P. A. Holmes, and DDM systems is difficult to ascertain in terms of molecular J. Mater. Chem., 1996, 6, 539.structure. 9 T. Holt and W. Simpson, Proc. R. Soc. L ondon A, 1956, 238, 156. 10 A. Matsuoto and M. Iowa, J. Polym. Sci., Part A-1, 1970, 8, 751. 11 A. Matsuoto and M. Iowa, J. Polym. Sci., Polym. Chem., 1976, 14, 2383. Conclusions 12 L. K. Kostanski and W. Krolinkowski, J. Polym. Sci., Polym. By studying a series of samples ranging from the uncured Chem., 1985, 23, 605. 13 N. G.McCrum, B. E. Read and G. Williams, Anelastic and monomer to the cured polymer over wide ranges of frequency Dielectric Effects in Polymeric Solids, Dover Publications, New and temperature, a complex pattern of behaviour has been York, 1991. revealed which may be summarised as follows. 14 G. Williams, Chem. Rev., 1972, 72, 55. (i) The simple a-process observed for this monomer trans- 15 G. Williams, Chem. Soc. Rev., 1978, 7, 89. forms on curing to a broad feature that moves to higher 16 G. Williams, Adv. Polym. Sci., 1979, 33, 59. temperatures and broadens considerably, to the extent that 17 R. Kohlrausch, Pogg. Ann. Phys., 1854, 4, 77. 18 G. Williams and D. C. Watts, T rans. Faraday. Soc., 1970, 66, 80. the a-process of monomer and polymer should be regarded as 19 G. Williams, D. C. Watts, S. B. Dev and A. M. North, T rans. having very different mechanisms. That for the cured polymer Faraday. Soc., 1971, 67, 1323. corresponds to micro-Brownian motions of units in a range of 20 M. B. M. Mangion and G. P. Johari, J. Polym. Sci., Part B, Polym. environments and suggests that the cured network is hetero- Phys., 1990, 28, 71. geneous at the mesoscopic level with local heterogeneities 21 M. B. M. Mangion and G. P. Johari, J. Polym. Sci., Part B, Polym. smaller that the wavelength of light. While the a-process is Phys., 1991, 29, 437. 22 M. B. M. Mangion and G. P. Johari, Macromolecules, 1990, 23, extremely broad in plots of e vs. log( f/Hz) for partially cured 3687. materials, the corresponding plots of e vs. T clearly show the 23 J. M. Charlesworth, Polym. Eng. Sci., 1988, 28, 221. location of the a-process and hence allow Fig. 9 to be con- 24 J. M. Charlesworth, Polym. Eng. Sci., 1988, 28, 230. structed, which shows the shift of the a-process to higher temperatures and its sustained large apparent activation energy Paper 6/06296B; Received 12th September, 1996 in cured samples. J. Mater. Chem., 1997, 7(2), 203–209 209

ISSN:0959-9428    

DOI:10.1039/a606296b

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Supramolecular chiral liquid crystals. The liquid crystalline behaviour of mixtures of 4,4¾-bipyridyl and 4-[(S)-(-)-2-methylbutoxy]benzoic acid Maren Grunert,a R. Alan Howie,a Annett Kaedingb and Corrie T. Imrie*,a aDepartment of Chemistry, University of Aberdeen,Meston Walk, Old Aberdeen, Scotland, UK AB9 2UE bInstitut fu� r Physikalische Chemie, T echnischen Universita�t Clausthal, Arnold-Sommerfeld-Strasse 4, D-38678 Clausthal-Zellerfeld, Germany The thermal behaviourof binary mixtures of 4,4¾-bipyridyl and 4-[(S)-(-)-2-methylbutoxy]benzoicacid has been investigated.The two components are not liquid crystalline. Bycomparison, mixtures containingbetween 0.1 and 0.5 mole fraction of 4,4¾-bipyridyl exhibit mesogenic behaviour;specifically, chiral nematic, blue and smectic A phases are observed.The induced mesogenic behaviour is attributed to the formationof elongated hydrogen bonded complexes and this view is supported by infrared spectroscopy. The crystal structure of the complex containing 0.3 mole fraction of 4,4¾-bipyridyl was determined using X-ray diffraction. This confirmed the formation of linear hydrogen bonded complexes in which two acid fragmentsare attached to a central 4,4¾-bipyridyl core and supports the view that such complexes are responsible for the liquid crystalline behaviour of these mixtures.A conventional low molar mass liquid crystal comprises Results and Discussion molecules consisting of a semi-rigid anisometric core attached The transition temperatures of the mixtures of 4,4¾-bipyridyl to which are normally one or two alkyl chains.1 In essence it and 4-[(S)-(-)-2-methylbutoxy]benzoic acid are listed in is the anisotropic interactions between the cores which give Table 1.The two pure components are not mesogenic. By rise to liquid crystalline behaviour, while the role of the alkyl contrast, the mixtures containing between 0.10 and 0.50 mole chains is largely to reduce the melting point of the material.fraction of 4,4¾-bipyridyl do exhibit liquid crystalline behaviour. The mesogenic core often contains phenyl rings connected via Phase identification was performed using polarised light short unsaturated linkages.1 In recent years, however, increasmicroscopy; clear characteristic textures were obtained for ing research activity has focused on materials in which the smectic A, chiral nematic and blue phases.15,16 The entropy core is assembled via noncovalent bonding,2–4 although this is by no means a new concept; for example, some forty years ago changes associated with the transitions support the phase Gray and Jones attributed the mesogenic behaviour of the 4- assignments shown in Table 1.The mixtures containing 0.10 alkyloxybenzoic acids to the formation of hydrogen bonded and 0.20 mole fraction of 4,4¾-bipyridyl exhibit a blue phase; dimers.5 The novelty of the recent work lies with the use of identical platelet textures were observed for both mixtures (see hetero-intermolecular bonds in order to assemble the meso- Plate 1).To the best of our knowledge, this is only the second genic core, i.e.the two interacting molecules are not identical, reported example of the observation of blue phases in hydrogen and such studies provide excellent examples of specific molecu- bonded complexes.11 Mixtures containing greater than 0.50 lar recognition.6,7 In addition, the two interacting molecules mole fraction of 4,4¾-bipyridyl were not mesogenic, although need not be, and indeed are often not, mesogenic although the miscibility was observed across the complete composition resulting complex is liquid crystalline.This amplification of a range. specific molecular interaction into a macroscopic observ- The dependence of the transition temperatures on the mole able phenomenon is a central theme of supramolecular chem- fraction of 4,4¾-bipyridyl in the mixture is shown in Fig. 1. The istry.6,7 The most commonly used interaction for the construc- most striking feature of this phase diagram is the strong tion of supramolecular liquid crystals is the hydrogen bond induction of liquid crystallinity for mixtures containing and of particular significance have been the studies reported between 0.10 and 0.50 mole fraction of 4,4¾-bipyridyl.Indeed, by Kato and Fre� chet describing the use of hydrogen bond the mixture containing 0.05 mole fraction of 4,4¾-bipyridyl formation between pyridyl and carboxylic acid fragments.4 could be supercooled to ca. 100 °C prior to crystallisation There is now a wealth of literature describing this class of without the observation of liquid crystallinity while the iso- hydrogen bonded mesogens,2,4 but surprisingly few of these tropic phase of the mixture containing 0.60 mole fraction of reports considerthe induction of technologically relevant chiral 4,4¾-bipyridyl could be supercooled to ca. 90°C before crystal- phases.8–12 Thus, we have characterised the thermal behaviour lisation occurred. The strong induction of liquid crystallinity of mixtures of 4,4¾-bipyridyl 1 and 4-[(S)-(-)-2-methylbutoxy] for this binary system presumably arises from the formation benzoic acid 2.This particular system was chosen for study of a linear, hydrogen bonded complex between the two compo- because Kato, Fre� chet and their co-workers have shown that nents. This view is supported by infrared spectroscopy; specifi- mixtures of 4,4¾-bipyridyl and substituted benzoic acids exhibit cally, the spectra of the complexes contain bands centred at induced or enhanced liquid crystalline behaviour.13,14 ca. 2490 and 1900 cm-1, indicative of strong hydrogen bonding. 17–20 In addition, the carbonyl band is shifted to 1693 cm-1, a characteristic value for free carbonyl groups. 4,4¾-Bipyridyl is a difunctional hydrogen bond acceptor and N N 1 thus it would be expected that the formation of the hydrogen bonded complex would be maximised for the mixture containing 0.33 mole fraction of 4,4¾-bipyridyl and 0.67 mole fraction of the monofunctional hydrogen bond donor, 4-[(S)-(-)-2- methylbutoxy]benzoic acid.If the clearing temperature of the mixture simply reflected the interaction strength parameter O O OH 2 J. Mater. Chem., 1997, 7(2), 211–214 211Table 1 Transition temperatures and the entropies associated with the clearing transition for mixtures of 4,4¾-bipyridyl and 4-[(S)-(-)-2- methylbutyloxy]benzoic acid transition temperature/°C mole fraction of 4,4¾-bipyridyl C–Ia SA–N*b N*–BPc BP–Ic N*–Id SA–Id DS/R 0.00 110.5 0.10 105.2 104.7 105.6 0.16 0.20 142.4 126d 134.8e 1.21 0.30 142.5 132 134.8 1.12 0.40 136.0 134.1 1.10 0.50 141.4 133.3 0.90 0.60 142.0 0.70 118.8 0.80 118.8 0.90 107.5 1.00 112.2 aExtracted from reheating DSC trace, heating rate=5°C min-1.bMeasured using optical microscopy. cExtracted from cooling DSC trace, cooling rate=0.2 °C min-1. dExtracted from cooling DSC trace, cooling rate=5°C min-1. eCombined N*–BP–I transition. between these complexes, a maximum in the clearing temperature curve would be expected at 0.33 mole fraction of 4,4- bipyridyl.Indeed, such behaviour was found for the binary phase diagrams of trans-1,2-bis(4-pyridyl)ethylene, a difunctional hydrogen bond acceptor, and the monofunctional hydrogen bond donors 4-methoxy-, 4-hexyloxy- and 4-butoxybenzoic acid.14 In contrast to such expected behaviour, the clearing temperatures exhibited by the 4,4¾-bipyridyl–4-[(S)- (-)-2-methylbutoxy]benzoic acid mixtures do not pass through a maximum on varying composition but instead the clearing temperature curve is rather flat (Fig. 1). We will return to this observation later. The temperature range exhibited by the blue phase is reduced on increasing the concentration of 4,4¾-bipyridyl and, indeed, a blue phase is not observed for the 0.30 mole fraction mixture; the molecular significance of this behaviour is unclear. The temperature range of the chiral nematic phase is also reduced on increasing the concentration of 4,4¾-bipyridyl and the 0.40 and 0.50 mole frixtures exhibit only a smectic A phase.This behaviour may be understood by considering the role played by the component in excess in determining phase structure and stability.For the mixture containing 0.30 mole fraction of 4,4¾-bipyridyl the number of hydrogen bonds is maximised and a linear species, containing a central 4,4¾- Plate 1 The platelet texture of the blue phase exhibited by the mixture bipyridyl core attached to which are two terminal acid mol- containing 0.20 mole fraction of 4,4¾-bipyridyl ecules, is formed; as we will see this view is supported by the X-ray diffraction study of the crystal phase exhibited by this mixture.The mixture exhibits a smectic A and a chiral nematic phase. Smectic phase formation may be thought of as a microphase separation in which the mesogenic cores form one region while the alkyl chains constitute another. Thus, the smectic A phase exhibited by the complex would be expected to exhibit a monolayer structure.If we now increase the mole fraction of 4,4¾-bipyridyl in the mixture then the excess 4,4¾- bipyridyl can readily be accommodated in the smectic layer in the largely aromatic domains. A dynamic equilibrium is presumably established in which fully complexed, mono-complexed and uncomplexed 4,4¾-bipyridyl is present.The 4,4¾- bipyridyl preferentially dissolved in the aromatic regions stabilises the microphase separation and the chiral nematic phase is extinguished. Conversely, if the acid component is in excess then hydrogen bonded acid dimers will be present. These dimeric species cannot be readily accommodated into the layered smectic structure, and hence destabilise the smectic phase relative to the chiral nematic phase.The similarity in the clearing temperatures for mixtures containing between 0.20 Fig. 1 Dependence of the transition temperatures on the mole fraction and 0.50 mole fraction of 4,4¾-bipyridyl suggests that the of 4,4¾-bipyridyl for mixtures of 4-[(S)-(-)-2-methylbutyloxy]benzoic stabilisation of the smectic phase by the presence of excess acid and 4,4¾-bipyridyl.Melting temperatures have been omitted for 4,4¾-bipyridyl is similar to that of the nematic phase by the sake of clarity. Transitions: (×) blue phase–N*; (&) blue phase–I; (+) SA–N*; (E) blue phase–N*–I; (#) N*–I; ($) SA–I. excess acid. 212 J. Mater. Chem., 1997, 7(2), 211–214Fig. 2 The structure of the complex formed between 4-[(S)-(-)-2-methylbutoxy]benzoic acid and 4,4¾-bipyridyl, showing the atom numbering scheme and ellipsoids at the 40% probability level The 251 molar ratio of 4-butoxybenzoic acid and 4,4¾- bipyridyl exhibits a smectic A–nematic transition at 150 °C and a nematic–isotropic transition at 159 °C.13,14 The corresponding temperatures for the 4-[(S)-(-)-2-methylbutyloxy] benzoic acid–4,4¾-bipyridyl complex are 132 and 135 °C, respectively.Thus chain branching has reduced the clearing temperature by 24°C and the smectic A–nematic transition temperature by 18°C. These reductions are smaller, however, than normally observed on the addition of a 2-methyl branch in conventional low molar mass liquid crystals,21,22 but the Fig. 3 A layer of molecules at z/c=0 viewed down c. Atoms are generality of this observation must be tested for a wider range drawn as circles of arbitrary radius increasing in the order of materials.H<C<N<O. The cell edge b and [100] are indicated. The crystal structure of the 0.3 mole fraction bipyridyl complex was studied using X-ray diffraction; the space group used in the refinement was P21 /c. The R-value for this analysis (0.112) is rather high but attributable to disorder within the crystal.The ‘molecular’ unit is essentially a linear hydrogen bonded complex consisting of a central 4,4¾-bipyridyl core and two terminal 4-(2-methylbutoxy)benzoic acid molecules (see Fig. 2). The most interesting feature of the complex is the nature of the hydrogen bonding. The O(1)–H distance is 0.83 (±0.13) A° , the H–N distance is 1.88 (±0.13) A° and the O(1)–H–N bond angle is 161 (±12)°.Thus, these data support the view that the complex is assembled via hydrogen bonding Fig. 4 As Fig. 3 except that the complexes are at z/c=1/2 rather than via salt formation. The packing of the complexes is shown in Fig. 3 and 4. Both figures show layers of complexes viewed down c, one layer at Preparation of complexes z/c=0 (Fig. 3) and the other at z/c=1/2 (Fig. 4). In both The mixtures were prepared by co-dissolving the components figures the complexes are arranged head-to-tail to form infinite in hot EtOH. The cooled solution was allowed to evaporate chains, but in Fig. 4 the chains are rotated by approximately to dryness over several days in a desiccator over activated 40° and translated by b/2 compared with Fig. 3.Thus the silica gel. The desiccator was evacuated approximately every crystal consists of alternating layers of complexes propagated 12 h using a water pump. The crystals obtained were dried in the direction of c. A second feature common to Fig. 3 and under vacuum. 4 is the occurrence of ribbons of molecules lying side by side (R and R¾). The stacking of the ribbons in the c direction Thermal characterisation creates blocks one molecule thick which lie perpendicular to a.The only interactions between adjacent ribbons within a The thermal behaviour of the materials was characterised by layer, and therefore between the blocks, are the comparatively differential scanning calorimetry using a Mettler-Toledo DSC weak van der Waals forces between the alkyl chains of the 820 system equipped with an intracooler accessory and cali- acid fragments.This is very similar to the situation described brated using an indium standard. Phase identification was for monoclininc zinc butanoate,23 for which disorder in the performed by polarised light microscopy using an Olympus stacking of double-sided parafinnic layers was clearly detect- BH-2 optical microscope equipped with a Linkham THMS able.Thus there are grounds for anticipating disorder in the 600 heating stage and TMS 91 control unit. stacking of the parafinnic blocks in the crystal structure of the complex giving rise to the rather high R-factor. Crystal structure determination The crystal structure exhibited by the mixture containing 0.3 Experimental mole fraction of 4,4¾-bipyridyl was determined by X-ray diffraction using a Nicolet P3 four-circle diffractometer with Materials Mo-Ka radiation.Data collection and cell refinement were performed using Nicolet P3 software.25 Data collection used 4,4¾-Bipyridyl (98%) was obtained from Aldrich. 4-[(S)-(-)-2- Methylbutoxy]benzoic acid was synthesised using well docu- 2h scan rates of 5.33 (Ip<150) to 58.6° min-1 (Ip>2500), where Ip was the prescan intensity.Scan widths were 2.4 to mented procedures: specifically, (S)-(-)-2-methylbutan-1-ol was brominated using phosphorus tribromide to yield (S)-(-)- 2.7° 2h. Data reduction was performed using RDNIC.26 The structure was solved by direct methods and refined by full- 2-methyl-1-bromobutane, which was subsequently reacted with 4-hydroxybenzoic acid to give 4-[(S)-(-)-2-methylbutoxy]ben- matrix least squares.All non-H, with the exception of C(10A), C(10B) and C(11), of the highly disordered 2-methylbutoxy zoic acid.24 J. Mater. Chem., 1997, 7(2), 211–214 2138 H. Kihara, T. Kato, T. Uryu, S. Ujiie, U. Kumar, J. M. J. Fre� chet, terminal chains were refined anisotropically (atom numbers D.W. Bruce and D. J. Price, L iq. Cryst., 1996, 21, 25. are defined in Fig. 2). Aromatic protons were placed in calcu- 9 Y. Tian, F. Su, Y. Zhao, X. Luo, X. Tang, X. Zhao and E. Zhou, lated positions with the CMH bond length at 0.95 A° and L iq. Cryst., 1995, 19, 743. refined riding upon the C to which they were attached with 10 U. Kumar, J. M. J. Fre� chet, T.Kato, S. Ujiie and K. Timura, separate group Uiso values for pyridyl and phenyl H [refined Angew. Ch Int. Ed. Engl., 1992, 31, 1531. 11 L. J. Yu, L iq. Cryst., 1993, 14, 1303. values 0.075(±0.016) and 0.058(±0.014) A° 2, respectively]. The 12 T. Kato, H. Kihara, T. Uryu, S. Ujiie, K. Iimura, J. M. J. Fre� chet hydroxy H (OH) was found in a difference map and refined and U.Kumar, Ferroelectrics, 1993, 148, 161. isotropically in the usual manner [Uiso=0.13(±0.06) A° 2]. No 13 T. Kato, P. G. Wilson, A. Fujishima and J. M. J. Fre� chet, Chem. attempt was made to position the alkyl protons because of the L ett., 1990, 2003. disorder apparent, particularly in the terminal ethyl group of 14 T. Kato, J. M. J. Fre� chet, P. G. Wilson, T. Saito, T. Uryu, A.Fujishima, C. Jin and F. Kaneuchi, Chem. Mater., 1993, 5, 1094. the 4-(2-butoxy) residue, for which C(10A) and C(10B) rep- 15 G. W. Gray and J. W. Goodby, Smectic L iquid Crystals—T extures resent two approximately equally occupied positions for the and Structures, Leonard-Hill, Glasgow, 1984. methylene carbon. The structure solution and refinement 16 D. Demus and L. Richter, T extures of L iquid Crystals, Verlag software used was SHELXS8627 and SHELX7628 and for Chemie,Weinheim, 1978.molecular graphics PLOTAID29 and ORTEX30 were used. 17 S. L. Johnson and K. A. Rumon, J. Phys. Chem., 1965, 69, 74. 18 S. E. Odinokov, A. A. Mashkovsky, V. P. Glazunov, A. V. Atomic coordinates, thermal parameters, and bond lengths Iogansen and B. V. Rassadin, Spectrochim.Acta, Part A, 1976, and angles have been deposited at the Cambridge 32, 1355. Crystallographic Data Centre (CCDC). See Information for 19 J. Y. Lee, P. C. Painter and M. M. Coleman,Macromolecules, 1988, Authors, J. Mater. Chem., 1997, Issue 1. Any request to the 21, 954. CCDC for this material should quote the full literature citation 20 T. Kato, T. Uryu, F. Kaneuchi, C. Jin and J.M. J. Fre� chet, L iq. Cryst., 1993, 14, 1311. and the reference number 1145/21. 21 G. W. Gray, in T he Molecular Physics of L iquid Crystals, ed. G. R. Luckhurst and G. W. Gray, Academic Press, London, 1979, ch. 12. We are pleased to acknowledge support from the EPSRC, 22 G. W. Gray and K. J. Harrison, Symp. Faraday Soc., 1971, 5, 54. 23 J. Blair, R. A. Howie and J. L. Wardell, Acta Crystallogr., Sect.C, grant number GR/J32701, and from the University of Aberdeen 1993, 49, 219. for the award of a grant to purchase the Mettler-Toledo DSC 24 S. Takenaka, T. Ikemoto and S. Kusabayashi, Bull. Chem. Soc. 820. M.G. thanks the ERASMUS programme for travel funds. Jpn., 1986, 59, 3965. 25 Nicolet P3/R3 Data Collection Operator’s Manual, 1980, Nicolet XRD Corporation, 10061 Bub Road, Cupertino, CA 95014, USA. 26 R. A. Howie, RDNIC, Data Reduction Program for Nicolet P3 References Diffractometer, 1980, University of Aberdeen, Scotland. 1 G. W. Gray, in T he Molecular Physics of L iquid Crystals, ed. G. R. 27 G. M. Sheldrick, Acta Crystallogr., Sect. A, 1990, 46, 467. Luckhurst and G. W. Gray, Academic Press, London, 1979, ch. 1. 28 G. M. Sheldrick, SHELX76, Program for Crystal Structure 2 C. M. Paleos and D. Tsiourvas, Angew. Chem., Int. Ed. Engl., 1995, Determination, 1976, University of Cambridge, England. 34, 1696. 29 P. D. G. Cradwick, PLOTAID, A Fortran Program for the 3 C. T. Imrie, T rends Polym. Sci., 1995, 3, 22. Preparation of Molecular Drawings, 1970, Macaulay Land Use Research Institute, Aberdeen, Scotland. 4 T. Kato and J. M. J. Fre� chet,Macromol. Symp., 1995, 98, 311. 30 P. McArdle, J. Appl. Crystallogr., 1994, 27, 438. 5 G. W. Gray and B. J. Jones, J. Chem. Soc., 1953, 4179. 6 J.-M. Lehn, Angew. Chem., Int. Ed. Engl., 1988, 27, 89. 7 J.-M. Lehn, Angew. Chem., Int. Ed. Engl., 1990, 29, 1304. Paper 6/05692J; Received 14th August, 1996 214 J. Mater. Chem., 1997, 7(2), 211

ISSN:0959-9428    

DOI:10.1039/a605692j

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Liquid-crystalline behaviour of di- and mono-palladium organyls: two ways of lyomesophase formation Nadejda Usol’tseva,*a,b Pablo Espinet,*b,† Julio Bueyb and Jose� Luis Serranoc aL iquid Crystal L aboratory, Ivanovo State University, C.I.S.-153025 Ivanovo, Russia bDepartamento de Quý�mica Inorga� nica, Facultad de Ciencias, Universidad de Valladolid, E-47005 Valladolid, Spain cDepartamento de Quý�mica Orga�nica, Facultad de Ciencias–ICMA, Universidad de Zaragoza–CSIC, 50009–Zaragoza, Spain Three members of a series of lath-like dinuclear complexes [Pd2(m-Cl)2(C,N)2 ] 1, (C,N=orthopalladated imine R1MC6H3MCHNNMC6H4MR2; R1=alkoxy chain; R2=alkyl or alkoxy chain), a dinuclear cyclopalladated complex with mixed bridges (acetate/thiolate) 2 and a mononuclear orthopalladated complex incorporating a b-diketonate ligand 3, have been synthesized and their lyotropic phase behaviour in mixtures with apolar organic solvents (linear alkanes, cycloocta-1,5-diene or limonene) has been studied.The lyotropic mesomorphism is determined by the combination of the length of the metalloorganyl side chains and the length of alkane used as solvent. Two ways of formation of lyotropic phases are found.The first case of lyotropic chiral nematic (cholesteric, N*) phase formation in a binary system composed of a non-chiral di- or mono-palladium organyl and a chiral apolar organic solvent is reported. The geometrical proportions (shape, length, width, thickness) behaviourof the dipalladium organyl 2 and the monopalladium organyl 3 in apolar organic solvents of different types.of mesogenic molecules are a main factor in determining their supramolecular packing in the mesophase. Thus, disc-like octasubstituted metal complexes of phthalocyanine and por- Experimental phyrin derivatives,1 or sheet-like tetra-metal organyls,2 give the columnar type of supramolecular structure, both in ther- The dinuclear PdII complexes 1a–c and monopalladium complex 3 have been reported previously.7a,b Complex 2 was motropic and in the lyotropic liquid crystalline phases.For rod-like metal-containing mesogens the layered kinds of thermotropic phases,1a,3 and the lamellar or the hexagonal arrangements of lyotropic phases are typically formed.1a,4 Notwithstanding that, recently it has been shown that by elongating a porphyrin core along one axis through substitution at the 5 or 15 positions, the basically disc-shaped molecule behaves as a rod-like one, displaying smectic A or calamitic nematic thermotropic phases.5 Moreover, the discovery of biforked mesogens, with a molecular form also intermediate between the two typical extreme shapes, showed their rich thermotropic polymorphism: nematic, lamellar, columnar and cubic thermotropic phases have been observed within the same series.6 The lyotropic phase behaviour of these ‘intermediate-form’ mesogens was not studied.In this respect it is difficult to predict the lyotropic mesomorphism of orthopalladated dinuclear complexes (‘dipalladium organyls’), or palladium b-diketonato complexes (‘monopalladium organyls’). Molecules of this type are known to show thermotropic behaviour similar to that of typical rodlike mesogens,7 but they possess molecular structures somehow intermediate between disc-like and rod-like.By analogy with the nomenclature of some lyomesogens, such types of molecular shape could be classified as lath-like in order to stress that they are definitely bidimensional molecules, not just elongated molecules.The investigation presented here is the extension of previous research on the connection between the molecular structure of metal-containing compounds and the lyomesomorphism they display in binary and in multicomponent systems. We report and discuss the first data on the influence of the length of the lateral chains, or the composition of the mesogen–solvent system on the lyotropic properties of chlorobridged dipalladium organyls 1.We also compare the lyotropic mesomorphism of these compounds with the lyotropic phase N N R1 R2 Pd Cl Cl Pd R1 R2 N OC6H13 N OC6H13 Pd S Pd OAc C6 OC6H13 OC6H13 a R1 = OC10; R2 = C6 b R1 = OC6; R2 = C6 c R1 = OC10; R2 = C10 1 2 N O OC10H21 OC10H21 Pd O OC10H21 OC10H21 3 Scheme 1 † Email: espinet@cpd.uva.es J.Mater. Chem., 1997, 7(2), 215–219 215Table 1 Optical, thermal and themodynamic data for compounds 1–3a synthesized as described previously for similar compounds,7c and characterized by IR, 1H NMR, and C,H,N analyses.† The compound transition T /°C DH/kJ mol-1 phase behaviour of the compounds was investigated by polarizing microscopy employing a Leica DMRB microscope with 1a K–SA 106.6 31.8 crossed polarizers equipped with a Mettler FP 90 hot stage, SA–I 263.4c — 1b K–K¾ 101.2 7.1 at heating rates of 1–5 K min-1, and by differential scanning K¾–SAb 127.5 26.1 calorimetry (Perkin Elmer DSC 7, heating rate 5 K min-1).SA–I 210.0c — The photographs of textures shown were taken using a CCD 1c K–K¾ 95.0 26.7 Hitachi KP–C 501 video camera equipped with a Sony UP K¾–SC 112.4 3.3 5000 P videoprinter.SC–SAb 135.0 — We applied the contact method to explore the lyomesophase SA–I 237.6c — 2 K–SA 154.4 13.7 behaviour of these palladium organyls with the following SA–N 168.9 apolar organic solvents: linear alkanes (octane, decane, N–I 173.2 2.5d dodecane, pentadecane), cycloocta-1,5-diene and (R)-(+)-lim- 3 K–SC 80.0 61.9 onene.This method has been used successfully to identify SC–SAb 150.0 0.4 mesophases in liquid-crystalline mixtures,8a in amphiphile– SA–N 154.0 water systems,8b and in metallomesogen–organic solvent com- N–I 155.0 2.5d positions.2c,d The phase diagram of the 1c–pentadecane system aK,K¾=crystal; SA=smectic A; SC=smectic C; N=nematic; I= was obtained on the basis of polarizing microscopy and DSC isotropic. bOptical microscopy data.cDecomposition. dCombined studies of eleven mixtures containing the dipalladium organyl enthalpies. between 15 and 89 mass% in the compositions. The preparation of homogeneous mixtures was achieved as described previously.2c Results and Discussion Thermotropic behaviour Compound 1a (R1=OC10, R2=OC6) exhibits a smectic A (SA) phase with myelinic figures and spherulites, between 106.6 and 263.4 °C.Compound 1b, with shorter chains but more similar in length (R1=OC6, R2=C6 ) , also displays the SA phase although in a shorter temperature range. 1c (R1=OC10, R2= C10) possesses a smectic C (SC) phase with a broken fanshaped texture, as well as an SA phase with myelinic figures and spherulites. The non-planar dinuclear Pd complex 2 shows, besides the SA phase, the schlieren texture of a nematic (N) phase between 168.9 and 173.2°C.The monopalladium organyl 3 displays SC , SA and N phases, although the nematic phase is observed in a very short temperature range (ca. 1°C only). The data of transition temperatures, together with their corresponding Fig. 1 Relationship of the lamellar phase existence with the chain enthalpies and types of mesophases, are summarized in Table 1.length n of the solvent in the binary systems of dipalladium organyls 1b or 1c with linear alkanes, based on the observation of contact preparations on heating Lyotropic behaviour The dipalladium organyl 1a does not form any lyomesophase in contact preparations with the linear alkanes octane, decane, Looking at the phase diagram of the 1c–pentadecane system dodecane, or pentadecane.Only dissolution of the substance (Fig. 2) it is possible to conclude that the supramolecular (on heating) or crystallization (on cooling) was observed. When packing in the lyotropic phase with myelinic figures remains 1b or 1c were mixed with the above-mentioned linear alkanes basically the same as in the thermotropic smectic phase.This one type of lyomesophase was observed, with myelinic figures process is accompanied by a reductionof the SC to SA transition and spherulites in the contact area. tem On heating, for the contact preparations of 1b with octane In the contact preparations of compound 2 with the above or decane the phase transitions temperatures to the lyomeso- mentioned linear alkanes a nematic lyotropic phase with a phase are lower (42 and 64.3°C respectively) than for the schlieren texture appears.The widest temperature area of analogous contact preparations of 1c (76 and 80°C respect- N phase existence was observed in the 2–pentadecane system ively, see Fig. 1). However, for contact preparations with (48°C on heating or 143°C on cooling).Moreover, on cooling dodecane or pentadecane the reverse was observed: the phase the nematic phase remains in this system until room tempera- transition temperatures with 1b are higher (120 and 130.5°C) ture (Fig. 3). In the 3–linear alkane systems the formation of than for 1c (90 and 99°C). On cooling, the lyomesophases a homeotropically oriented nematic phase and a lamellar phase stay in the contact preparations until 25–27 °C. with spherulites and myelinic figures was observed, the last one monotropically. In contrast to the thermotropic phase behaviour of compound 3, displaying the nematic phase in a † Characterization data for complex 2: yield: 73%.Elemental analysis: narrow range of temperatures, the lyotropic nematic area of found: C, 60.51; H, 7.22; N, 2.48%.Calc. for C58H84N2O6Pd2S: C, 3–linear alkane systems exists in a wide temperature range, 60.57; H, 7.36; N, 2.43%. IR (KBr disc): 1605, n(CNN); 1522, e.g. 92°C in the 3–decane system, on heating. The temperature 1458 cm-1, n(CNO). 1H NMR (300 MHz, CDCl3 , d): 7.95 (s, 2H, range of the nematic phase was smaller when the length of the HMCNN), 7.10 (d, 2H), 6.59 (dd, 2H), 7.19 (d, 2H, cyclometallated solvent molecule and the length of the substituents in the ring), 7.25, 6.84 (8H, AA¾XX¾ system), 4.14–3.90 (m, 8H, OCH2), 2.47 (m, 2H, SCH2), 1.60 (s, 3H, CH3MCO2 ).metalloorganyl 3 were more different (Table 2). 216 J. Mater. Chem., 1997, 7(2), 215–219Fig. 4 Myelinic figures and spherulites in the binary system 1c–(R)- (+)-limonene; contact preparation, T=32°C, on cooling, magnifi- cation ×200 1a/1b–(R)-(+)-limonene systems do not show lyomesomorphism, but a lyotropic phase with myelinic figures appears in the contact area of the 1c–(R)-(+)-limonene system (Fig. 4). In 2/3–(R)-(+)-limonene preparations the existence of a chiral nematic lyomesophase (cholesteric, N*) is observed. This Fig. 2 Simplified phase diagram of the binary system 1c–pentadecane phase displays fingerprint, Grandjean and focal-conic choles- in the region of lyomesophase formation, based on optical microscopy teric textures (Fig. 5). Qualitatively, the cholesteric pitch in the and DSC measurements of the mixtures with defined contents of the 2–(R)-(+)-limonene system is larger than in the 3–(R)-(+)- components (on heating): K1–4=crystalline phases, SC=smectic C limonene system (at the same temperatures).phase, SA=smectic A phase, L=lamellar phase, I=isotropic region In both these compositions the N* phase exists in a very broad range of temperatures, including room temperature on cooling (Table 2). When a two-component mixture (limonene– pentadecane) was used as the solvent, the increase of the content of the pentadecane in this mixture (151, 152 or 153 mol%) caused an increase of the cholesteric pitch in the contact area.The occurrence of myelinic figures in the contact preparations of 1b, 1c and 3 with apolar organic solvents is connected most probably with the formation of a lamellar lyotropic phase. In this case the general symmetry of the layered structure of the thermotropic smectic phase is kept in the lamellar lyotropic one, and lyotropic phase formation could be associated with an expansion of the arrangement existing in the absence of solvent (swollen process).The same manner of lyotropic nematic phase appearance is possible for the metalloorganyls 2 and 3, which display nematic mesomorphism both in the thermotropic and in the lyotropic state.Fig. 3 Schlieren texture of the nematic phase in the binary system This is the first way of lyotropic phase formation in the 2–pentadecane; contact preparation, T=32 °C, on cooling, magnifi- metalloorganyls 1–, 2– and 3–apolar organic solvent systems. cation ×200 Note that all contact preparations 3–apolar organic solvent developed a very strong homeotropic orientation of the director in the nematic phase.The dark nematic pseudo-isotropic area In contact preparations with cycloocta-1,5-diene as solvent neither the dipalladium organyls 1 and 2, nor the monopalla- on the border with the isotropic solution can be detected because of the strong thermal fluctuations, or by the schlieren dium organyl 3 display any lyomesophase. However, in binary systems with another cyclic solvent, (R)-(+)-limonene, the texture existing 2–3 s after shearing of the sample.Usually, a special surface treatment is necessary for such an orientation lyomesophase appearance and the type of lyomesophase depend on the structure of the palladium organyls. The of the nematogens.9 Table 2 Lyotropic phase behaviour of metalloorganyls 2/3–apolar organic solvent systemsa solvent compound cycle octane decane dodecane pentadecane (R)-(+)-limonene 2 heating K 86 I K 100 N 145 I K 114 N 150 I K 125 N 173 I K 93 N* 147 I cooling I 79 N(r.t.) I 115 N 82 N+K(r.t.) I 118 N 89 N+K(r.t.) I 165 N(r.t.) I 143 N*(r.t.) 3 heating K 60 N 110 I K 58 N 150 I K 57 N 96 I K 70 S 85 N 100 I K 65 N* 136 I cooling I 106 N 38 S(r.t.) I 131 N 50 L 26 K I 90 N 40 L 25 K I 81 N 51 L 25 K I 120 N*(r.t.) aN=nematic phase; N*=chiral nematic phase; L=lamellar phase; S=smectic phase; K=crystalline phase; I=isotropic phase; (r.t.)=mesophase maintained at room temperature.J. Mater. Chem., 1997, 7(2), 215–219 217Table 3 Types of lyomesophases displayed in the binary systems of palladium organyls 1–, 2– and 3–apolar organic solventa compound (R1, R2) 1a 1b 1c 2 3 solvent (OC10, C6) (OC6, C6) (OC10, C10 ) (OC6, OC6, R3=C6) (OC10, OC10) octane — L L N (m) N decane — L L N N, L (m) dodecane — L L N N, L (m) pentadecane — L L N N, L (m) cycloocta-1,5-diene — — — — — (R)-(+)-limonene — — L N* N* aN=nematic phase; N*=chiral nematic phase; L=lamellar phase; (m)=monotropic.nematic phase formation in the binary system non-chiral lathlike di- or mono-palladium organyl–chiral apolar organic solvent.Conclusions Dipalladium organyls of types 1 and 2 and monopalladium organyl 3 can display both thermotropic (Table 1) and lyotropic (with apolar organic solvents; Table 3) mesomorphic properties. Both the ratio of the metallomesogen chain lengths to the length of alkanes used, and the structure of the apolar organic solvent, seem to have significant influences on the type of lyomesophase formed and on the temperatures of the phase transitions (see Table 2, Fig. 2). Two ways of lyotropic phase formation are possible in these above-mentioned lyotropic binary systems: (1) the preservation in the lyotropic mesomorphic state of the general symmetry existing in the thermotropic mesophase; and (2) the occurrence in binary systems of a new type of supramolecular packing, not typical for the thermotropic state of the pure substance, in this case a twisted nematic one.The size of the cholesteric pitch in these systems can be changed via addition to the chiral solvent of controlled amounts of the above mentioned linear alkanes. Financial support of this work by the Spanish Direccio�n General de Investigacio�n Cientý�fica y Te� cnica (SAB 95–0131) and the Comisio�n Interministerial de Ciencia y Tecnologý�a (project MAT 93–0329) is gratefully acknowledged.References 1 (a) C. Piechocki and J. Simon, New. J. Chem., 1985, 3, 159; (b) H. Groothues, F. Kremer, P. G. Schouten and J. W. Warman, Fig. 5 Fingerprint textures formed in contact preparations of metal- Adv. Mater., ; (c) F.Lelj, G. Morelli, G. Riccardi, loorganyls with (R)-(+)-limonene: (a) chiral nematic phase of the 2–(R)- A. Roviello and A. Sirigu, L iq. Cryst., 1992, 12, 941; (+)-limonene system, T=22°C, on cooling; (b) chiral nematic phase (d) D. W. Bruce, in Inorganic Materials, ed. D. W. Bruce and D. of the 3–(R)-(+)-limonene system, T=59°C, on cooling; magnifi- O’Hare, Wiley, Chichester, 1992; (e) N.V. Usol’tseva and cation ×200 V. V. Bykova, Mol. Cryst. L iq. Cryst., 1992, 215, 89; (g) N. B. McKeown and J. Painter, J.Mater. Chem., 1994, 4, 1153; ( f ) N. Usol’tseva and K. Praefcke, oral presentation at the More interesting is the occurrence of a new type of supramol- International Symposium on L iquid Crystals and Supramolecular ecular organization in the lyotropic state, not found in the Order, January 3–5, 1996, Bangalore, India, to be published inMol.pure substance, which is the second way of lyotropic phase Cryst. L iq. Cryst., 1996. formation. This is the case of the formation of a lyotropic 2 (a) K. Praefcke, D. Singer and B. Gu�ndogan, Mol. Cryst. L iq. Cryst., 1992, 223, 181; (b) K. Praefcke, B.Bilgin, J. Pickardt and cholesteric phase in the systems composed of di- or mono- M. Borowski, Chem. Ber., 1994, 127, 1543; (c) N. Usol’tseva, palladium organyls 2 and 3 with the chiral apolar organic K. Praefcke, D. Singer and B. Gu�ndogan, L iq. Cryst., 1994, 16, 601; solvent (R)-(+)-limonene. It is usual for the lyotropic aqueous (d) K. Praefcke, B. Bilgin, N. Usol’tseva, B.Heinrich and compositions of amphiphiles that chiral nematic phases appear D. Guillon, J.Mater. Chem., 1995, 5, 2257. by adding chiral dopants (e.g. brucine sulfate) to the solvent 3 P. Espinet, M. A. Estruelas, L. A. Oro, J. L. Serrano and E. Sola, forming a lyotropic nematic phase,10a–c or by dissolving chiral Coord. Chem. Rev., 1992, 117, 215. 4 (a) D. W. Bruce, D. A.Dunmur, P. M. Maitlis, J. M. Watkins and mesogens in aqueous media.10d–f Recently, one of us reported G. J. Tiddy, L iq. Cryst., 1992, 11, 127; (b) D. W. Bruce, the first case of the lyotropic twisted nematic (N*) phase J. D. Holbray and G. J. Tiddy, 9th Ann. Conf. British L iq. Cryst. induced by a chiral charge-transfer complex composed of Soc., University of Hull, Hull, 1994, poster 49.sheet-like tetrapalladium or tetraplatinum organyls with (+)- 5 D. W. Bruce, M. A. Wali and Q. M. Wang, J. Chem. Soc., Chem. or (-)-TAPA [TAPA=2-(2,4,5,7–tetranitro–9–fluorenyliden- Commun., 1994, 2089. eaminooxy) propionic acid] in apolar organic solvents.2d,10g 6 H. T. Nguen, C. Destrade and J. Malthe`te, L iq. Cryst., 1990, 8, 797. 7 (a)M. J. Baena, P. Espinet, M. B.Ros and J. L. Serrano, J.Mater. The case reported here is the first case of lyotropic twisted 218 J. Mater. Chem., 1997, 7(2), 215–219Chem., 1996, 6, 1291; (b) N.J. Thompson, J. L. Serrano, M. J.Baena 10 (a) K. Radley and A. Saupe, Mol. Phys., 1978, 35, 1405; (b) L. J. Yu and A. Saupe, J. Am. Chem. Soc., 1980, 102, 4879; (c) T. M. H. and P. Espinet, Chem. Eur. J., 1996, 2, 186; (c)M. J. Baena, J. Buey, P. Espinet, H-S. Kitzerow and G. Heppke, Angew. Chem., Int. Ed. DoAido, M. R. Alcantara, O. Felippe, Jr., A. M. J. Pereira and J. A. Vanin, Mol. Cryst. L iq. Cryst., 1993, 226, 195; (d) K. Radley Engl., 1993, 32, 1201. 8 (a) C. Casagrande, M. Veyssie and H. Finkelman, J. Phys. L ett., and H. Cattey, L iq. Cryst., 1992, 12, 875; (e)Mol. Cryst. L iq. Cryst., 1993, 226, 195; ( f ) M. R. Alca�ntara and J. A. Vanin, L iq. Cryst., 1980, 43, 671; (b) P. Ekwall, in Advances in L iquid Crystals, ed. G. Brown, Academic Press, New York, San Francisco, London, 1995, 18, 207; (g) N. Usol’tseva, K. Praefcke, D. Singer and B. Gu�ndogan, L iq. Cryst., 1994, 16, 617. 1975, vol. 1, p. 1. 9 H. Kelker, Handbook of L iquid Crystals, ed H. Kelker and R. Hatz, Weinheim, Deerfield, Verlag Chemie, 1980, p.14. Paper 6/05361K; Received 31st July, 1996 J. Mater. Chem., 1997, 7(2), 215&ndash

ISSN:0959-9428    

DOI:10.1039/a605361k

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Semiconducting charge-transfer salts of BEDT-TTF [bis(ethylenedithio)tetrathiafulvalene] with hexachlorometallate(IV ) anions Cameron J. Kepert,† Mohamedally Kurmoo‡ and Peter Day* Davy Faraday Research L aboratory, T he Royal Institution, 21 Albemarle Street, L ondon, UK W1X 4BS Two new crystalline charge-transfer salts of BEDT-TTF [bis(ethylenedithio)tetrathiafulvalene] containing hexachlorometallate anions of 5d elements have been synthesised, characterised structurally and their magnetic and conducting behaviour investigated.a-(BEDT-TTF)4[ReCl6] C6H5CN is triclinic [P1�; a=9.455(2), b=11.306(3), c=18.193(5) A° ; a=101.85(2), b=92.74(2), c=110.52(2)°; Z=1]. Its structure consists of alternate layers of BEDT-TTF and of [ReCl6]2- C6H5CN, the former consisting of two crystallographically independent stacks, in one of which the molecules carry a charge close to+1 while in the other they are approximately neutral.The cations lie between the [ReCl6]2- moieties while the neutral molecules are closest to the C6H5CN species showing that the structure is determined by electrostatic interactions. (BEDT-TTF)2[IrCl6 ] is likewise triclinic [P1�; a=8.721(1), b=10.257(2), c=11.086(2) A° ; a=111.00(1), b=98.31(1), c=103.32(1)°; Z=1].Its packing motif is new among BEDT-TTF salts, containing no discrete layers of anions and cations. Face-to-face BEDT-TTF dimers form an infinite threedimensional network. Both compounds are semiconducting and their magnetic properties are dominated by the Curie–Weiss behaviour of the anions. Although they give rise to a large number of superconducting and recrystallised from dilute hydrochloric acid to form small phases with the TMTSF (tetramethyltetraselenafulvalene) block-like crystals of (NEt4)2[IrCl6].donor molecule, octahedral anions have received relatively little attention among the charge-transfer salts of BEDTTTF. The only BEDT-TTF salts with dinegative octahedral Electrochemical synthesis anions are the k-(BEDT-TTF)4[MX6] C6H5CN series Crystals were grown electrochemically on Pt electrodes in ([MX6]2-=[PtCl6]2-, [PtBr6]2-, [TeCl6]2-, [SnCl6 ]2-).1 conventional H-shaped cells (45 cm3 capacity).Approximately The [PtCl6 ]2- salt is unique among these in having semi- 20 mg of BEDT-TTF, twice recrystallised from CHCl3, was metallic properties, although it undergoes a first-order metal– added to the anodic section of the cell, and a solution of the insulator transition near 250 K.It differs structurally from the anion salt (ca. 20 mg) was added to all three compartments. other members by a doubling of the unit cell at high tempera- The cells were set up under an atmosphere of dry Ar and ture,2 and appears to be the only salt of BEDT-TTF with two placed in a dark enclosure on a vibration free table.With independent BEDT-TTF layers showing semi-metallic and (NBun4)2[ReCl6] as electrolyte, benzonitrile as solvent and a semiconducting properties. It is of interest to attempt to current of 0.5 mA, high-quality, thick, black, hexagonal plates stabilise the metallic k-phase layer to low temperature by (ca. 2×2×0.2 mm3) of a-(BEDT-TTF)4[ReCl6] C6H5CN substituting different octahedral anions. To combine the pos- formed on the anode. sibility of including magnetic moments and anions with redox With (NBun4)2[IrCl6] as electrolyte and dichloromethane possibilities we have synthesised new BEDT-TTF salts of as solvent, low-quality, black, polycrystalline material coated [ReCl6]2- and [IrCl6 ]2-.Low-spin 5d5 [IrCl6 ]2- is a strong the anodes on passing a current of 0.3–0.5 mA. On the other oxidising agent, converting to the diamagnetic d6 [IrCl6]3- hand, in benzonitrile with a current of 0.5 mA (voltage ca. anion, and 5d3 [ReCl6]2- has previously been reported to 2.6 V), high-quality, black, block-like crystals (1×1×1 mm3) form a semiconducting salt with dibenzotetrathiafulvalene.3 of (BEDT-TTF)2[IrCl6] formed on the anode.Most of these This paper presents the synthesis, crystal structure and physical were kite-shaped, owing to the presence of a clearly visible properties characterisation of a-(BEDT-TTF)4[ReCl6] diagonal twin plane. C6H5CN and (BEDT-TTF)2[IrCl6]. Experimental Crystal structure determination Syntheses a-(BEDT-TTF)4[ReCl6] C6H5CN.The unit cell was refined (NBun4)2[ReCl6] was prepared by metathesis. Solutions of from 25 reflections (22.9<2h<30.6°). The hydrogen atoms of K2[ReCl6 ] (0.96 g, Aldrich) and NBun4Cl xH2O (1.52 g, the BEDT-TTF ethylene groups were fixed with regular excess) in dilute hydrochloric acid were added and evaporated geometry at a distance of 0.96 A° from the carbon atoms.PSI- to small volume. Recrystallisation from dichloromethane and scan data were collected on ten reflections. In the least-squares diethyl ether produced long green–yellow needles. structural refinement the analytical absorption correction was Concentrated aqueous solutions of H2[IrCl6] xH2O(0.21 g, superior to the laminar empirical correction, giving a lower R- Aldrich) and NEt4Cl (0.23 g) were mixed and evaporated to a factor of refinement [for reflections from the full data set with small volume and the dark brown precipitate was collected I3s(I), R=5.39% cf. 5.56%] and preventing the omission of 745 reflections. The reflections (2,0,1), (2,2�,1), (2,3�,1), (2,3�,2) and (2,1,1) were found to suffer significantly from extinction † Present address: Inorganic Chemistry Laboratory, South Parks (each with -dev/s>8), and were omitted from the least- Road, Oxford, UK OX1 3QY.squares refinement. Despite the general bad agreement of the ‡ Present address: IPCMS-GMI, 23 rue Loess, BP 20/CR, 67037 Strasbourg Cedex, France. intense reflections, the application and refinement of the J. Mater. Chem., 1997, 7(2), 221–228 221weighting scheme Magnetism The magnetic susceptibilities of polycrystalline samples of both w(S)= 1 s2(S)+g|Fobs(S)|2 salts were determined from 300 to 2 K in fields of 250 and 1000 G (ReCl6) and 5000 G (IrCl6) using a Quantum Design where s(S) is the estimated standard deviation of F(S), and g MPMS 7 SQUID magnetometer.Samples were mounted must be positive, led to a significant increase in the R-factors inside a Perspex rod centred on the axis of the magnet solenoid.and only a small decrease in the e.s.d.s. The weighting param- Magnetisation measurements were also made in fields from eter g was subsequently set to zero. Further details of the data 0.1 to 7.0 T at 2 and 5 K. Corrections were made for the collection, processing and structural refinement may be found diamagnetic response of the sample rod and of the sample in Table 1.using Pascal’s constants. (BEDT-TTF)2[IrCl6]. A region was cut from a large diag- Electronic band structure onally twinned crystal, and confirmed to be single-phase by Band-structure calculations were performed in the tight binding X-ray oscillation photography. The unit cell was refined from formalism within the dimer-splitting approximation using the 25 reflections (17.6<2h<26.8°).PSI-scan data were collected extended Hu�ckel method, with a double-zeta basis set.4 on ten reflections. The data were corrected for absorption by the empirical ellipsoid method. No significant change was seen in the refinement indices, apart from a slight lowering of the Results and Discussion goodness of fit.All BEDT-TTF ethylene group hydrogen atoms were located in the Fourier difference map. The pos- Crystal and electronic band structures of a-(BEDTTTF) 4 [ReCl6 ] C6H5CN itional and isotropic thermal parameters for H(10B) were unstable to refinement and hence were fixed, whilst all others The salt crystallises in the P1� space group, and has a compar- refined to give reasonable values.Further details of the data able unit cell to the low-temperature (single-layer) phase of k- collection, processing and structural refinement may be found (BEDT-TTF)4[PtCl6] C6H5CN {a-(BEDT-TTF)4[ReCl6] in Table 1. C6H5CN: a=9.455, b=11.306, c=18.193 A° , a=101.85, b=92.74, c=110.52°, V=1767.3 A° 3; k-(BEDT-TTF)4 [PtCl6] Electrical conductivity C6H5CN at 218 K:1.891, c=17.512 A° , a= 81.55, b=86.65, c=94.44°, V=1758 A° 3}.The asymmetric unit The electrical conductivities of single crystals of both salts were determined, for the ReCl6 salt by both two- and four- contains two BEDT-TTF molecules, one half of the [ReCl6] unit, and a 50% occupied benzonitrile molecule [see Fig. 1(a) probe dc methods parallel and perpendicular to the crystal plates from 300 to 85 K, and for the IrCl6 salt by two-probe for the atomic numbering scheme].Ionic layers of a-packed 2(BEDT-TTF2 )+ and [ReCl6]2- C6H5CN alternate along the measurements from 300 to 180 K. Table 1 Crystal and refinement data for a-(BEDT-TTF)4 [ReCl6 ] C6H5CN and (BEDT-TTF)2 [IrCl6] a-(BEDT-TTF)4 [ReCl6] C6H5CN (BEDT-TTF)2[IrCl6 ] crystal data chemical formula C47H37Cl6N1Re1S32 C20H16Cl6Ir1S16 M/g mol-1 2040.7 1174.2 crystal system triclinic triclinic space group P1� P1� a/A° 9.455(2) 8.721(1) b/A° 11.306(3) 10.257(2) c/A° 18.193(5) 11.086(2) a/degrees 101.85(2) 111.00(1) b/degrees 92.74(2) 98.31(1) c/degrees 110.52(2) 103.32(1) V /A°3 1767.3(8) 872.2(3) Z 1 1 F(000) 1015 571 Dc/g cm-3 1.92 2.25 crystal size/mm3 0.10×0.40×0.50 0.15×0.15×0.30 m(Mo-Ka)/cm-1 29.2 52.2 data collection and processing data measured 8213 3290 unique data 6233 3078 sigma cut (c) 5 3 no.with Ics(I) 5298 2910 structural analysis and refinement no. parameters 390 225 absorption corr. analytical empirical crystal faces (distance from crystal centre) 001 (0.05) — 010 (0.25) — 11�0 (0.20) — mR — 0.65 min., max. transmission 0.35, 0.75 0.42, 0.46 g (weighting) 0.00 0.00 R(%) [R(all) (%)] 4.97 (6.22) 3.27 (3.68) Rw (%) 4.41 2.87 goodness of fit 3.70 1.81 largest peak/e A° -3 1.2, -1.7 0.9, -0.5 222 J.Mater. Chem., 1997, 7(2), 221–228Fig. 1 Atomic numbering scheme of (a) a-(BEDT-TTF)4[ReCl6 ] C6H5CN and (b) (BEDT-TTF)2[IrCl6] (both showing 50% thermal ellipsoids) c-direction (see Fig. 2). Atomic coordinates, thermal parameters, and bond lengths and angles have been deposited at the Cambridge Crystallographic Data Centre (CCDC).See Information for Authors, J. Mater. Chem., 1997, Issue 1. Any request to the CCDC for this material should quote the full literature citation and the reference number 1145/22. Selected intramolecular bond lengths and angles are listed in Table 2, and the shortest intermolecular SMS distances may be found adjacent to the calculated intermolecular transfer integrals in Table 3.BEDT-TTF layer. The BEDT-TTF molecules form a layer in which the packing is of the a-type.5 The layer is constructed of two crystallographically independent stacks of BEDT-TTF, with a dihedral angle of 120.0°. Inversion centres are positioned midway between each parallel BEDT-TTF pair, leading to AA* and BB* stacks that propagate along the a-direction. From Fig. 2 it can be seen that the two BEDT-TTF molecules lie at approximately the same height within the layer. The AA*A and BB*B pairs of stacking interactions are very similar, with alternating translations of 0 and 1/4 along the long axis of the molecule. The intermolecular spacings within each stack are unusually wide, with at most one intermolecular SMS distance less than 4 A° for each interaction (see Table 3).In contrast, there are numerous short interstack SMS distances. It seems likely that anion–anion repulsion causes the enhanced separation between the BEDT-TTF in this direction. The long axes of molecules A and B make angles of 66.9° and 65.7° respectively with the plane of the BEDT-TTF layer, giving an effective perpendicular cross-sectional area per molecule of 22.9 A° 2, similar to many other close-packed BEDTTTF salts.All four independent ethylene groups are ordered in the chair form and both molecules adopt the staggered conformation. Analysis of the intramolecular bond lengths of the two independent BEDT-TTF molecules suggests that the two Fig. 2 Projection of a-(BEDT-TTF)4[ReCl6] C6H5CN (a) down the stacks have quite different degrees of ionicity, with calculated a axis; (b) down the b axis (for clarity only one orientation of the disordered C6H5CN molecule is shown) charges Q(A)=0.8(2)+ and Q(B)=0.4(2)+ as estimated by J. Mater. Chem., 1997, 7(2), 221–228 223Table 2 Selected intramolecular bond lengths (A° ) and angles (degrees) of a-(BEDT-TTF)4 [ReCl6 ] C6H5CN Re(1)MCl(1) 2.365(3) N(1)MC(21) 1.01(5) Re(1)MCl(2) 2.371(3) C(21)MC(22) 1.54(6) Re(1)MCl(3) 2.361(2) C(22)MC(27) 1.46(4) C(23)MC(24) 1.37(5) Cl(1)MRe(1)MCl(2) 91.5(1) C(24)MC(25) 1.36(4) Cl(1)MRe(1)MCl(3) 88.5(1) C(25)MC(26) 1.46(5) Cl(2)MRe(1)MCl(3) 88.9(1) C(26)MC(27) 1.23(5) S(1)MC(1) 1.727(8) S(11)MC(11) 1.731(13) S(1)MC(3) 1.760(11) S(11)MC(13) 1.776(12) S(2)MC(1) 1.723(10) S(12)MC(11) 1.746(13) S(2)MC(4) 1.735(10) S(12)MC(14) 1.759(11) S(3)MC(2) 1.725(11) S(13)MC(12) 1.748(13) S(3)MC(5) 1.746(10) S(13)MC(15) 1.760(11) S(4)MC(2) 1.737(9) S(14)MC(12) 1.737(13) S(4)MC(6) 1.743(10) S(14)MC(16) 1.765(12) C(1)MC(2) 1.390(15) C(11)MC(12) 1.376(15) C(3)MC(4) 1.342(12) C(13)MC(14) 1.322(18) C(5)MC(6) 1.346(12) C(15)MC(16) 1.327(18) the d-model.6 The A stacks are sandwiched between chains of [ReCl6]2- anions, whereas the B stacks lie between the neutral benzonitrile solvent molecules [Fig. 3(a)].Thus electrostatic interaction with the anion layer may influence electronic localisation strongly within the BEDT-TTF layer, as in k- (BEDT-TTF)4[PtCl6 ] C6H5CN. Such an effect (not previously noted) is a pervasive contributor to localisation in BEDT-TTF salts in general.Fig. 3 Projection of the cation (a) and anion (b) layers of a-(BEDT- Anion layer. The structure of the anion–solvent layer TTF)4[ReCl6] C6H5CN down the BEDT-TTF axis (for clarity only [Fig. 3(b)] is the same as that found in k-(BEDT- one orientation of the disordered C6H5CN molecule is shown) TTF)4[PtCl6] C6H5CN,1 although modified slightly owing to changes to the a, b and c unit-cell parameters (brought about by the different donor packing arrangement).The average Band structure. There are eight independent nearest-neighbour interactions between the two independent BEDT-TTF ReMCl bond length of 2.366 A° is slightly longer than the PtMCl length of 2.318 A° .The area of the ab plane is 100.1 A° 2, molecules, as defined in Fig. 4. The calculated transfer integrals closely reflect the variations in intermolecular SMS distances, in comparison to the similar value of 103.2 A° 2 for k-(BEDTTTF) 4[PtCl6] C6H5CN. The [ReCl6]2- species lie on the being moderate between the stacks, and only very small in the stacking direction (Table 3). The bond-over-bond stacking (0,1/2,0) inversion centre, oriented with a triangular face of the Cl octahedron parallel to the BEDT-TTF layer, and are interactions F and H are significantly stronger than the translated interactions E and G.The estimated energy differ- compressed slightly in the direction parallel to the BEDT-TTF long axes. The benzonitrile is disordered about the (1/2,0,0) ences DEHOMO between the HOMOs of molecules of type A and B are large with respect to the calculated transfer integrals, inversion centre, with 50% occupation of each of the inversionrelated positions.Unlike k-(BEDT-TTF)4 [PtCl6] C6H5CN indicating the extent of localisation between the independent stacks, as also evidenced by the charge estimated through salt, where the six-fold carbon ring is centred about the inversion centre, the inversion-related atoms of the benzonitrile bond length analysis.A value for DEHOMO of 200 meV was chosen for the purpose of the band-structure calculation. molecule are not superposed. Attempts at anisotropic refinement of the benzonitrile molecule gave very high correlation The calculated band structure (shown with the Fermi surface in Fig. 5) predicts a semi-metallic state, with pairs of electron owing to the close atomic proximity of the two inversionrelated orientations. Isotropic modelling led to a successful pockets located at ±b*/2 and hole pockets at ±a*/2. Measurement of electrical conductivity, magnetic susceptibility distinction between the two, giving the expected geometry and sensible tempe factors. and infrared reflectivity, however, show the material to be Table 3 Calculated transfer integrals, t¾, HOMO energy differences and intermolecular SMS distances of a-(BEDT-TTF)4 [ReCl6 ] C6H5CN intermolecular SMS distances 4.0 A° /A° interactiona t¾ DEHOMO/eV inner–innerb inner–outerb outer–outerb A 140 270 3.73, 3.80, 3.85 3.40, 3.67 3.46, 3.58 B 97 120 3.83, 3.86, 3.87, 3.94 3.57, 3.91 3.56, 3.70 C 68 300 3.82, 3.86, 3.93 3.71 3.53 D 64 140 3.73, 3.82, 3.95 3.51, 3.53 3.45, 3.52 E 6 0 3.78 — — F 32 0 3.86 — — G 9 0 3.69 — — H 28 0 — — — aThe interactions A–H are defined in Fig. 4. bInner and outer denote S atoms of the TTF and bis(ethylenedithio)moieties respectively. 224 J. Mater. Chem., 1997, 7(2), 221–228Table 4 Selected intramolecular bond lengths (A° ) and angles (degrees) of (BEDT-TTF)2[IrCl6] Ir(1)MCl(1) 2.320(2) S(1)MC(1) 1.717(5) Ir(1)MCl(2) 2.328(2) S(1)MC(3) 1.746(7) Ir(1)MCl(3) 2.322(2) S(2)MC(1) 1.719(6) S(2)MC(4) 1.743(6) Cl(1)MIr(1)MCl(2) 90.6(1) S(3)MC(2) 1.720(6) Cl(1)MIr(1)MCl(3) 88.3(1) S(3)MC(5) 1.740(7) Cl(2)MIr(1)MCl(3) 89.0(1) S(4)MC(2) 1.713(5) S(4)MC(6) 1.748(7) C(1)MC(2) 1.400(9) C(3)MC(4) 1.344(6) C(5)MC(6) 1.358(7) the end-to-end interactions with neighbouring cells.The [IrCl6]2- sits on the inversion centres at the corners of the Fig. 4 Definition of the eight independent intermolecular BEDT-TTF cell, and is compressed slightly in the direction parallel with interactions (A–H) in a-(BEDT-TTF)4 [ReCl6] C6H5CN the BEDT-TTF axis. The IrMCl bond lengths are approximately equal.The shortest intermolecular SMS contacts occur within the dimer, which is distorted to minimise the distances between the inner S atoms (Table 5). The two independent inner SMS distances of 3.37 and 3.49 A° are extremely short, being much less than the sum of the van derWaals radii (3.7 A° ). In contrast, there are few short SMS contacts between neighbouring dimers. There is a potential ambiguity in the charge assignment in this salt, since the hexachloroiridate anion may exist in either the dinegative or the trinegative state.7 Analysis of the intramolecular BEDT-TTF bond lengths by the d-method6 gives a charge of 1.05(1)+, suggesting that the correct formulation is (BEDT-TTF+)2[IrIVCl6] rather than (BEDT- Fig. 5 Calculated band structure (a) and Fermi surface (b) of a- TTF1.5+)2[IrIIICl6]. The rather long BEDT-TTF central CMC (BEDT-TTF)4[ReCl6] C6H5CN bond (1.400 A° ) suggests a further concentration of positive charge owing to the strong face-to-face dimer interaction.Accordingly, the neighbouring bond lengths are compatible semiconducting. This discrepancy arises from the inadequacy of the extended Hu�ckel model in taking account of electron with a lower charge.The average IrMCl bond length of 2.323 A° is very similar to the PtMCl length of 2.318 A° . A further correlation leading to localisation. However, important features that can be extracted include the high value of DEHOMO, uncommon aspect is that both BEDT-TTF ethylene groups are well ordered in the boat conformation [Fig. 6(b)] because and the small intrastack transfer integrals E, F, G and H, which alternate in magnitude along the (BEDT-TTF) stacks.of the steric interaction with the anion. The large difference in ionicity observed and the high DEHOMO value suggest that there is only a very small degree of Intermolecular overlap. (BEDT-TTF)2 [IrCl6] is one of the few examples of BEDT-TTF charge-transfer salts where sig- orbital mixing between the neighbouring donor stacks. Hence, the band structure may be crudely approximated by isolated nificant intermolecular overlap occurs in all three directions (Table 5).However, despite the large number of neighbours, one-dimensional stacks of A+ and B0. Semiconducting properties may be explained either in terms of an alternating delocal- only four significant independent interactions occur.The very strong bond-over-bond intra-dimer interaction vastly exceeds ised A+ stack, or by an A+ Hubbard band (if the intrastack transfer integrals are sufficiently small to favour localisation). all those to neighbouring cells, which are smaller owing to the end-to-end orientation and the longer SMS contacts. Hence localisation of charge in the form of (BEDT-TTF)22+ is Crystal and band structures of (BEDT-TTF)2[IrCl6] expected to dominate the electrical properties.Despite employing the same temperature and similar electrolyte concentrations as in the synthesis of a-(BEDT- Electrical conductivity TTF)4[ReCl6] C6H5CN, a different phase is formed by [IrCl6]2-. (BEDT-TTF)2[IrCl6 ] crystallises in the P1� space Both the ReCl6 and IrCl6 salts are semiconducting, the former with a temperature-dependent activated behaviour [Fig. 7(a)]. group, with an asymmetric unit containing (BEDT-TTF)+ and half the [IrCl6]2- units. There is no solvent of crystallisation. Room-temperature conductivities of the ReCl6 salt are relatively high for a semiconductor [sRT(along plate)#3 S cm-1 Atomic coordinates, bond lengths and angles, and thermal parameters have been deposited at the Cambridge and sRT (through plate)#0.03 S cm-1].There is evidence for a subtle transition at 250 K [see inset to Fig. 7(a)], above which Crystallographic Data Centre (see earlier; ref. no. 1145/22). Selected intramolecular bond lengths and angles are listed in there is a constant activation energy, Ea, of 0.07 eV. Below the transition the activation energy is larger, varying between 0.1 Table 4, and the shortest intermolecular SMS distances may be found adjacent to the calculated intermolecular transfer and 0.15 eV.The sharp increase at 250 K may be due to localisation towards BEDT-TTFA1+ and BEDT-TTFB0, rep- integrals in Table 5. The structure of (BEDT-TTF)2[IrCl6] represents a new resenting a transition from two-dimensional to one-dimensional behaviour.The gradual decrease in Ea below the 250 K packing motif among BEDT-TTF salts, and is unusual in containing no discrete layers of cations and anions. Face-to- transition is reminiscent of many one-dimensional conducting materials,8 and is consistent with the one-dimensional electron face BEDT-TTF dimers oriented in the ring-over-bond conformation lie diagonally across the unit cell (see Fig. 6). A three- hopping model. The upturn below ca. 110 K, however, represents a deviation from this behaviour. The strong crystallo- dimensional interacting network of such dimers is formed by J. Mater. Chem., 1997, 7(2), 221–228 225Table 5 Transfer integrals, t, and intermolecular SMS distances of (BEDT-TTF)2 [IrCl6 ] interaction intermolecular SMS distances 4.0 A° /A° cell molecule t/meV inner–innera inner–outera outer–outera 0,0,0 A* 672 3.37, 3.49 — 3.92 1,0,0 A* 140 — 3.74, 3.84, 3.94, 4.03 — 0,1,0 A 19 — 4.05 3.88 A* 0 — 3.66 3.37 0,-1,0 A 19 — 4.05 3.88 0,0,-1 A* 39 — 3.66 3.60 1,0,1 A 0 — — 3.52 aInner and outer denote S atoms of the TTF and bis(ethylenedithio) moieties respectively.Fig. 6 Projection of the crystal structure of (BEDT-TTF)2 [IrCl6 ] (a) down the a axis; (b) down the BEDT-TTF axis (the anion is Fig. 7 Electrical transport of (a) a-(BEDT-TTF)4 [ReCl6] C6H5CN omitted for clarity) (along plate measurement), (b) (BEDT-TTF)2[IrCl6 ] graphic evidence of ionicity, and the temperature-dependence For the more general case of an antiferromagnetic chain with of the activation energy, favours the Mott–Hubbard over the exchange constants J1 and J2 alternating along the chain, the delocalised band picture.The IrCl6 salt has a temperature- Hamiltonian becomes independent activation energy, Ea=0.23 eV, from 180 to 300 K and a room-temperature conductivity, sRT#0.01 S cm-1 H=-2J1 .n /2 i=1 (S2i-1S2i+aS2iS2i+1) [Fig. 7(b)]. where a=J2/J1.Magnetic properties The temperature dependence of the susceptibility of a uni- a-(BEDT-TTF)4[ReCl6] C6H5CN. The low-temperature form Heisenberg antiferromagnetic linear chain was given by magnetic properties of a-(BEDT-TTF)4[ReCl6] C6H5CN are Bonner and Fisher,10 and, for the high-tempere region, dominated by the Curie–Weiss behaviour of the anion was fitted to the expression [Fig. 8(a)], while at higher temperature a further Curie-like component due to the BEDT-TTF is superimposed. Based on xm=Ng2mB2 kT A+Bx+Cx2 1+D+Ex2+Fx3 the crystal structure and the transfer integral calculation, the susceptibility was modelled as the sum of Curie–Weiss and where x=|J|/kT . alternating antiferromagnetic chain contributions at high tem- The values A=0.25, B=0.14995, C=0.30094, D=1.9862, perature.9 For the case of a linear chain, the Hamiltonian for E=0.68854, F=6.0626 fit with a maximum disagreement of Heisenberg exchange interaction is only 0.5% for T>J/2k. The susceptibility of the alternating Heisenberg antiferromagnetic chain obeys this equation,11 with H=-2J .n i=1 Si-1Si coefficients defined as polynomial expansions in a.12 The 226 J.Mater. Chem., 1997, 7(2), 221–228ture. A large broadened feature centred at 1370 cm-1 arises from coupling of the free electron with the donor CNC stretching mode, as has been seen in many other salts.13 Further weak sharp vibrational peaks occur at 780, 885, 1018, 1267 and 1410 cm-1. The unpolarised spectrum of (BEDTTTF) 2[IrCl6] has a reflectivity lower than 20% in the range 700–4300 cm-1, characteristic of a semiconductor. The most prominent vibrational peak at 1410 cm-1 arises from the n(CNC) mode of BEDT-TTF.Further weak sharp peaks occur at 705, 805, 879, 993, 1024, 1175, 1236, 1279 and 1343 cm-1. Conclusions Two new crystalline BEDT-TTF charge-transfer salts, with octahedral transition-metal anions, a-(BEDT-TTF)4 [ReCl6] C6H5CN and (BEDT-TTF)2[IrCl6 ], have been synthesised and structurally characterised. With these anions no salts crystallised in the form k-(BEDTTTF) 4[MCl6] C6H5CN, the only previously reported BEDTTTF phase with hexachlorometallate anions.1 That three different phases form from benzonitrile solution with anions of similar size and charge is evidence for the subtle energetics of crystal growth in this series of compounds.A general structure–property relationship has emerged for a-phase BEDT-TTF salts such that those with angles between neighbouring stacks of ca. 50° undergo a metal–insulator transition at intermediate temperature, whilst those with Fig. 8 (a) Temperature dependence of the effective Curie constant of an angle of ca. 80° remain metallic to low temperature a-(BEDT-TTF)4 [ReCl6] C6H5CN (x, 250 G; $, 1000 G).(b) Fitting or may become superconducting.14 Therefore a-(BEDT- of the residual susceptibility of a-(BEDT-TTF)4 [ReCl6] C6H5CN to TTF)4[ReCl6] C6H5CN represents an unusual deviation from the alternating AF chain model (T>8 K, J1=16 K, J2=13–5 K, a= expected behaviour, being a semiconductor no doubt because 0.84) and the Bleaney–Bowers model (T<8 K, J<13 K).(#, 250 G; of its low symmetry and the presence of dinegative anions. $, 1000 G.) The intramolecular bond lengths indicate that a high degree of ionicity exists within the BEDT-TTF layers of both the a- ReCl6 and k-PtCl6 phases, which clearly match the varying coefficients are valid for T>J1/2k. At 6.5 K there is a definite charge distribution within the anion layers.We propose that break in the susceptibility, so below 6 K it was fitted to the the electrostatic interaction between layers influences the sum of Curie–Weiss and dimer components. For the latter, HOMO energies of the molecules, thereby moderating the the susceptibility is given by degree of orbital mixing. In the extreme case of the energy difference between the HOMO dominating over the intermol- xm= 2Ng2mB2/kT 3+exp(-2J/kT ) ecular transfer integrals the band structure approximates to separate lattices of independent BEDT-TTF. In a-(BEDT- Iterative refinement gave, for the anion, C=1.10(1) emu TTF)4[ReCl6] C6H5CN the band structure corresponds to K mol-1 and h=-0.135(5) K, and, for the BEDT-TTF chains, the simple case of alternate one-dimensional stacks of (BEDT- a=0.84, J1=16 K, J2=13 K and n=62%.The residual suscep- TTF)+. The semiconducting behaviour is therefore attributed tibility after subtracting the Curie–Weiss component due to to the strongly varying charge distribution in the anion layer the anion is shown in Fig. 8(b) with fittings to the alternating owing to the presence of the neutral solvent molecules, which chain model (T>8 K) and dimer model (T<6 K).significantly influences the electronic energies of the BEDT- Finally, there is evidence for a subtle transition at 250 K, TTF cation layers. above which the cation susceptibility falls below the expected In (BEDT-TTF)2[IrCl6] we have found a new packing behaviour [Fig. 8(a)]. This feature matches the small sharp arrangement of BEDT-TTF, with face-to-face dimers forming increase in the activation energy of electrical conduction. a three-dimensional interacting network.The BEDT-TTF bond lengths strongly favour a monopositive charge, a con- (BEDT-TTF)2[IrCl6]. The magnetic properties of (BEDT- clusion substantiated by the absence of any observable reaction TTF)2[IrCl6] are dominated by an S=1/2 Curie–Weiss com- between neutral BEDT-TTF and [IrCl6]2- in solution.The ponent, confirming the (BEDT-TTF+)2[IrCl6]2- charge magnetic susceptibility also favours the charge assignment assignment. The total susceptibility was modelled at high (BEDT-TTF)22+[IrCl6]2- over (BEDT-TTF)23+[IrCl6]3-. temperature by the sum of the Curie–Weiss and Bleaney– The very strong face-to-face intra-dimer interaction governs Bowers equations, giving C=0.234 emu K mol-1, h=-0.37 K the electronic and magnetic properties of this salt, producing for the anion and J=670 K.For (BEDT-TTF+)2 the exchange spin-paired (BEDT-TTF+)2. constant is exceptionally high for a BEDT-TTF salt, and The relatively high charge-to-size ratio of the [MCl6]2- is results from the very short face-to-face interaction within the an important factor in their crystallisation with BEDT-TTF.dimer pair. The 50-fold increase of the inter-dimer exchange Whilst the inclusion of the large benzonitrile solvent molecule constant of the IrCl6 salt over the ReCl6 one arises from the facilitates the formation of charge-ordered stacks of BEDT- smaller interplanar separation in the former (ca. 3.35 compared TTF in the ReCl6 salt, a phase containing BEDT-TTF+ is to ca. 3.8 A° ). obtained when the dinegative anion crystallises alone, as in the IrCl6 salt. Infrared reflectance The contribution of the BEDT-TTF to the magnetic susceptibility is difficult to separate from the Curie–Weiss contri- Polarised spectra of a-(BEDT-TTF)4[ReCl6] C6H5CN suggest moderate anisotropy in the room-temperature band struc- bution due to the paramagnetic anions, although it is clear J.Mater. Chem., 1997, 7(2), 221–228 2272 V. E. Korotkov, V. N. Molchanov and R. P. Shibaeva, Sov. Phys. that the cations form antiferromagnetic chains. In a-(BEDTCrystallogr., 1993, 37, 776. TTF)4[ReCl6] C6H5CN bond length considerations suggest 3 M. Z. Aldoshina, L. S. Veretennikova, R.N. Lubovskaya and that the orbital components of the conduction and valence M. L. Khidekel, T ransition Met. Chem., 1980, 5, 63. bands derive primarily from the A+ molecule. Hence, magneti- 4 L. Ducasse, A. Abderrabba, J. Hoaran, M. Pesquier, B. Gallois and cally the salt may be considered as containing a one-dimen- J. Gaultier, J. Phys. C: Solid State Phys., 1986, 19, 3805; L. Ducasse sional alternating chain of A+ with a=0.84 and J1=16 K.In and A. Fritsch, Solid State Commun., 1994, 91, 201. (BEDT-TTF)2[IrCl6 ] the one-dimensional stacks along the a 5 K. Bender, I. Hennig, D. Schweitzer, K. Dietz, H. Endres and H. J. Keller, Mol. Cryst. L iq. Cryst., 1984, 108, 359. direction are dimerised strongly, and the small increase in 6 P. Guionneau, C.J. Kepert, D. Chasseau, M. R. Truter and P. Day, susceptibility at high temperature fits a dimer model with Synth. Met., in press. J#670 K. The results therefore reveal a large increase in 7 F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, magnetic exchange interaction on shortening the face-to-face Wiley, New York, 1988, 5th edn. separation between the BEDT-TTF units. To clarify the extent 8 E. M. Conwell, in Semiconductors and Semimetals: Highly of any interaction between the organic and inorganic spin Conducting Quasi-One-Dimensional Organic Crystals, ed. E. M. systems in these novel phases it will be necessary to synthesise Conwell, Academic Press, New York, 1988, vol. 27, pp. 1–27. 9 W. E. Hatfield, W. E. Estes, W. E. Marsh, M. W. Pickens, L. W. ter isostructural salts with diamagnetic [MCl6]2-. Haar and R. R.Weller, in L inear Chain Compounds, ed. J. S. Miller, Plenum Press, New York, 1983, vol. 3, p. 43. We acknowledge support from EPSRC and the EU Human 10 J. C. Bonner and M. E. Fisher, Phys. Rev. A, 1964, 135, 640. Capital and Mobility Programme. C.J.K. is the recipient of a 11 J. C. Bonner and H. W. J. Blo�te, Phys. Rev. B, 1982, 25, 6959. Hackett Scholarship of the University of Western Australia. 12 W. E. Hatfield, J. Appl. Phys., 1981, 52, 1985. We are grateful to Dr. W. Hayes for access to the infrared 13 F. L. Pratt, W. Hayes, M. Kurmoo and P. Day, Synth.Met., 1988, spectrometer, and to Drs. P. Guionneau and L. Ducasse for 27, 439. 14 M. Kurmoo, P. Day, A. M. Stringer, J. A. K. Howard, L. Ducasse, assistance with the transfer integral and band-structure F. L. Pratt, J. Singleton and W. Hayes, J. Mater. Chem., 1993, calculations. 3, 1161. References Paper 6/05878G; Received 27th August, 1996 1 A. A. Galimzyanov, A. A. Ignatev, N. D. Kushch, V. N. Laukhin, M. K. Makova, V. A. Merzhanov, L. P. Rozenberg, R. P. Shibaeva and E. B. Yagubskii, Synth.Met., 1989, 33, 81. 228 J. Mater. Chem., 1997, 7(2), 221–2

ISSN:0959-9428    

DOI:10.1039/a605878g

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

A theoretical examination of the molecular packing, intermolecular bonding and crystal morphology of 2,4,6-trinitrotoluene in relation to polymorphic structural stability Hugh G. Gallagher,a Kevin J. Roberts,b John N. Sherwoodc and Lorna A. Smithd aDepartment of Physics and Applied Physics, University of Strathclyde, Glasgow, UK G1 1XL bCentre for Molecular and Interface Engineering, Department ofMechanical and Chemical Engineering, Heriot-Watt University, Edinburgh, UK EH14 4AS cDepartment of Pure and Applied Chemistry, University of Strathclyde, Glasgow, UK G1 1XL dDepartment of Pure and Applied Chemistry, University of Strathclyde, Glasgow, G1 1XL and Centre forMolecular and Interface Engineering, Department ofMechanical and Chemical Engineering, Heriot-Watt University, Edinburgh, UK EH14 4AS 2,4,6-Trinitrotoluene (TNT) can crystallise in both monoclinic and orthorhombic polymorphic forms characterised by two distinct molecular packing configurations in the solid-state associated with two conformationally unique molecules (A and B) in the asymmetric unit with four asymmetric units in the unit cell.In this the centrosymmetric monoclinic (pseudo b-glide) structure (P21/c) adopts an AABBAABB packing motif and the orthorhombic (pseudo-centrosymmetric structure Pb21a) adopts an ABABABAB packing motif.The close molecular similarity between the conformations of the A and B molecules also gives rise to twinning in the monoclinic phase due to defects in the stacking sequence. The two molecular structures differ in the degree to which the 2,4,6 nitro groups are twisted out of the plane of the benzene ring and there are also subtle differences in the molecular conformation of the two molecular types between the two polymorphs.Lattice energy calculations based on the solved structures reflect the similarity of the packing sequences, suggesting a relative stability of monoclinic>twinned monoclinic>orthorhombic in agreement with experimental studies.The small energy difference between the twinned and non-twinned monoclinic polymorphs explains the abundance of twinning in TNT. Attachment energy calculations predict needle-like crystal morphologies with a platelike cross-section. Polymorphism plays a major role in the performance, pro- structuresof the polymorphic forms with a view to rationalising duction and long-term stability of many chemical and bio- the experimental data of Gallagher and Sherwood in terms of chemical products.The polymorphic form of a material can the packing energetics and hence relative stability of the two affect a wide range of physical, chemical and biochemical structures. Lattice energy calculations have also been perproperties, 1–3 including density, hardness, ductility and colour, formed on the previously proposed model20–22 of the twinned solid-state reactivity and bioavailability.Studies of the struc- structure and have been used to explain the energetic basis for tural aspects of polymorphism have been carried out by the observed twinning. In addition, predictions of the morphoa number of workers,4–6 e.g.on 1,4-dichlorobenzene,7 o- logically important crystal faces have been made in order to acetamidobenzamide8 and benzylideneaniline.9–12 Crystallo- understand the observed variation in morphology with differgraphic analyses, via studies of molecular conformation and ent growth conditions. packing motif, have been found to be invaluable in elucidating the nature and transformation mechanisms associated with Computational methods polymorphic behaviour in the organic solid state.The crystallisation behaviour of TNT has been the subject Structural parameters of considerable attention since the late eighteenth century The structural coordinates for the monoclinic and orthorhom- when various authors13–19 observed orthorhombic and monobic forms were determined by Duke14 and are given in supple- clinic polymorphs as well as numerous variants of the two mentary material.† The monoclinic form has the space group modifications depending on the method of preparation. More P21 /c and unit-cell dimensions a=21.275 A° , b=6.093 A° , c= recently Gallagher and Sherwood20–22 systematically investi- 15.025 A° , b=110.23°. The orthorhombic form has the space gated the structural nature, relative stability and morphology group Pb21a and unit-cell dimensions a=15.075 A° , b= of TNT crystals grown from solution.Three basic forms were 20.024 A° and c=6.107 A° . As no structural data existed for the found to exist with monoclinic, orthorhombic and twinned twinned crystal the molecular arrangement about the twin monoclinic structures respectively.Subsequent examination fault was generated by imbedding a unit cell of the orthorhom- revealed the monoclinic polymorph to be the thermobic polymorph within the monoclinic structure using the dynamically more stable form under ambient conditions. A modelling system INTERCHEM23 and the program model of the twinned monoclinic structure was presented CRYSTLINK.24 Within HABIT9529–31 these coordinates were which attributed the predominance of twinning to the presence generated repeatedly along the twin fault direction whilst of pseudo-symmetry elements between successive molecular monoclinic unit cells were stacked repeatedly above and below layers in the packing arrangement of the monoclinic form.Variations in both structure and morphology with growth conditions were observed.In this paper molecular and crystallographic modelling † Deposited at the British Library: SUP 57198 (5 pp.). Details available from the Editorial Office. techniques have been used to examine in detail the crystal J. Mater. Chem., 1997, 7(2), 229–235 229the twin fault. The atomic coordinators for all these structures mational forms, A and B.Both structures have four asymmetric units in the unit cell, each containing one type A and one type are also given in the supplementary material.† B molecule. The monoclinic polymorph has a pseudo b-glide plane with a real centre of symmetry whilst the orthorhombic Calculation of the intermolecular interactions, lattice energy polymorph has a real b-glide plane and a pseudo-centrosym- and the crystal morphology metric structure.The A and B type molecules of the monoclinic Using the atom–atom method,25 the strength of an intermol- form are stacked in the sequence AABBAABB perpendicular ecular interaction can be approximated to a summation of all to the bc plane, while the orthorhombic form has a stacking the constituent atom–atom interactions between the two mol- sequence of ABABABAB parallel to the b axis.These pseudo- ecules. The lattice energy is simply the summation of all the elements cause the packing differences between the two poly- intermolecular forces in the crystal lattice. For a central morphs to be small. molecule (with n atoms) surrounded by N molecules (each Fig. 1 shows the intramolecular interaction distances and containing n¾ atoms) the lattice energy can be given by: angles for the A and B type molecules of both the monoclinic and orthorhombic polymorphs.The interior angles of the E=1 2 .N k=1 .n i=1 .n ¾ j=1 Vkij carbon atoms attached to the nitro groups are all greater than 120° in both polymorphs, reflecting distortion of the benzene where Vkij is the interaction energy between atom i of the rings.The nitro groups in both polymorphs form non-zero central molecule and j of the kth surrounding molecule, with dihedral angles with the plane of the benzene ring. In the the factor 1/2 reflecting the summed total coming from a pair monoclinic polymorph the 2,4,6 nitro groups are twisted out of interactions. of the plane by 59.4, 32.2 and 40.1° for the A form and by The atom–atom interactions were calculated using the para- 49.9, 22.2 and 46.1° for the B form.For the orthorhombic meter set of Scheraga et al.26 which was derived for hydro- polymorph they are twisted out of the plane by 58.8, 32.2 and carbons, carboxylic acids and amides and has been shown to 41.3° for the A form and by 50.1, 21.7 and 45.7° for the B give accurate results for these systems.The partial atomic form. A similar twisting has been found in both 2,4,6-trinitro- charges of each molecule were calculated using the self-consist- aniline38 and 2,3,4,6-tetranitroaniline39 where it was argued ent force-field MNDO27 (modified neglect of differential over- that it was a result of crowding between groups. This crowding lap) method within MOPAC28 with the actual values given in is reflected in the high densities of these species (1.762 g cm-3 the supplementary material.† for trinitroaniline and only 1.87 g cm-3 for tetranitroaniline) The intermolecular bond strengths and resulting lattice and hence this argument can be extended to both the mono- energies were calculated using the program HABIT95.29–31 clinic and orthorhombic forms of TNT which also exhibit high The program identifies the atom–atom pairs between a central densities (1.650 g cm-3 and 1.645 g cm-3 respectively).These molecule and all the surrounding molecules within a sphere observations are in agreement with those of Carper et al.40 for and evaluates their interaction energies. orthorhombic TNT. The extent to which the nitro groups twist The validity of the intermolecular force-field was assessed out of the plane of the benzene ring is similar for both the A with respect to thermodynamic parameters via the calculation type molecules of the different polymorphs and for both the B of the experimental sublimation enthalpy32,33 (Vexp) which is type molecules.The twisting is, however, considerably different related to the sublimation enthalpy (DsubH) by the equation: between molecules of type A and B within the same polymorph.Root-mean-square fit values have been calculated using Vexp=-DsubH-2RT INTERCHEM and are reported in Table 1. Values are calcu- where Vexp is the lattice energy, DsubH is the enthalpy of lated between similar types of molecules in different poly- sublimation and 2RT represents a correction factor for the morphs and between different types of molecules within the difference between the gas-phase enthalpy and the vibrational same polymorph.These values emphasise the greater similarity contribution to the crystal enthalpy.34 The accuracy of the of the A type molecules of both polymorphs and of the B type calculations depends upon the force-field utilised, a reasonable molecules of both polymorphs in comparison to A and B type error being ±0.25 kcal mol-1 (1 cal=4.184 J).Despite this, molecules of the same polymorph. The similarity between lattice energy calculations usually provide a reliable method the molecules of different polymorphs emphasises the of predicting the order of thermodynamic stability of a series conformational similarity of the two polymorphs.of systems.4,7 The crystal morphology was predicted via an attachment Molecular packing in the crystal structure energy (Eatt) calculation where Eatt is the fraction of the total lattice energy released on the attachment of a slice to a growing The monoclinic structure consists of pairs of identicalmolecules crystal surface.HABIT9529–31 calculates the attachment energy related by symmetry centres arranged in layers perpendicular of suitable slices by summing the individual interactions to the bc plane. The centres of symmetry occur at positions between the central molecule and all the molecules outwith a (0,0,1/2) and (0,1/2,0) for molecules of type B and at (1/2,0,1/2) slice of thickness dhkl , but within the limiting sphere radius. and (1/2,1/2,0) for molecules of type A.The layers are linked Suitable slices (hkl) for these calculations were obtained using by a pseudo-glide plane. Fig. 2 shows a projection of the (001) a Bravais–Freidel–Donnay–Harker35–38 analysis via the com- plane of the orthorhombic polymorph. In the orthorhombic puter program MORANG.24 polymorph the layers contain pairs of the two A and B forms.These are related by pseudo-centres of symmetry at the approximate coordinates (23/8,0,1/4). A true glide plane, paral- Results and Discussion lel to the b axis, exists between successive layers. Fig. 3 shows Molecular conformation a projection of the (010) plane of the monoclinic polymorph. In both polymorphs hydrogen bonds of the type CMH,O Examination of the crystal structures determined by Duke14 exist.In the case of the orthorhombic polymorph seven hydro- reveals a similarity between the orthorhombic and monoclinic gen bonds occur, three of these being of the type B,B, two polymorphs in both their packing and in their molecular of the type A,A and two of the type A,B. Table 2 gives the configurations.The TNT molecules in both the orthorhombic calculated strengths of these intermolecular bonding inter- and monoclinic polymorphs exist in two unique confor- actions. The B,B type bonds are weaker than the A,A bonds and the A,B bonds are weaker than both. The monoclinic polymorph has two A,A type bonds, one B,B type † See footnote on previous page. 230 J. Mater. Chem., 1997, 7(2), 229–235Fig. 1 (a) Molecular form A of monoclinic TNT, (b) molecular form B of monoclinic TNT, (c) molecular form A of orthorhombic TNT and (d) molecular form B of orthorhombic TNT Table 1 Root-mean-square fit values for TNT polymorph molecules subgrouprelationship. The phase transformation between these overlaid involves the loss of one symmetry element and the addition of another.Hence this must involve a first-order process and is molecule 1 molecule 2 rms fit/A° likely to be a rather disruptive event. A pure solid–solid phase transformation can thus not take place, with the phase trans- monoclinic, molecule A orthorhombic, molecule A 0.0969 monoclinic, molecule A monoclinic, molecule B 0.1427 formation likely to involve the complete dissolution of the first monoclinic, molecule B orthorhombic, molecule B 0.0828 form followed by the regrowth of the second.Given the orthorhombic, molecule A orthorhombic, molecule B 0.1790 energetic nature of the material the implication of such a disruption is important. The similarity of the two molecular forms (A and B) gives rise to good correspondence between bond and two A,B type bonds.The strengths of all these the two structural modifications. If the two structures were bonds are fairly similar, with the exception of one weak A,A superimposed in space the first two layers would coincide type bond. almost exactly, while the second two would be displaced relative to one another by c/2 with respect to the monoclinic Group–subgroup relationship between the two polymorphic axis system, repeated throughout the structure.forms Both polymorphs can be regarded as being related to the same mother-phase Pbma, i.e. to a phase containing both the Examination of the two space groups (P21/c and Pb21a) reveals the two polymorphic structures not to be related by a group– suppressed symmetry elements. Whilst there is no evidence for J. Mater. Chem., 1997, 7(2), 229–235 231Fig. 4 Crystal packing diagram showing generation of 180° twin plane in the monoclinic (stacking sequence AABBAA) polymorphic form of TNT associated with the embedding of an orthorhombic unit cell (stacking sequence ABAB) on the twin plane boundary Fig. 2 Projection of the 001 plane of the orthorhombic polymorphic phase of TNT Twinning in the monoclinic phase Twinning in monoclinic TNT has been identified by Gallagher and Sherwood,22 who also proposed a model for the twinned structure.In the monoclinic phase, the pseudo-glide plane relates alternate molecular layers perpendicular to the bc plane. During crystallisation, this element of pseudo-symmetry can promote deviation from the normal stacking sequence, corresponding to a relatively minor conformational change in the molecules of the faulted layer.Subsequent layers continue in the normal sequence, although they are displaced in a symmetrically related direction. This produces a twin fault within the crystal as shown in Fig. 4. The regions above and below the twin plane follow the usual monoclinic stacking sequence, although they are now related to each other by an angle of 180° around an axis perpendicular to the {100} plane.The new stacking sequence across the boundary corresponds to the orthorhombic structure. The nature of the twinning creates an orthorhombic cell (ABAB stacking sequence) within a monoclinic structure (i.e. A+AABBAABB�A+ABAB+AABB, etc.). This causes a switch in the stacking sequence with A molecules either side of the orthorhombic cell which can be regarded as a 180° Fig. 3 Projection of the 010 plane of the monoclinic polymorphic rotation about an axis perpendicular to the (100) plane. phase of TNT Calculation of the strength of the intermolecular interactions and lattice energy Table 2 Intermolecular interactions for both the monoclinic and orthorhombic polymorphs Lattice energy convergence was reached via summing the intermolecular interaction to a limit of 30 A° for both poly- orthorhombic monoclinic morphs. The lattice energies were calculated using both the A and B molecules as the origin molecule and then averaged.strength/ strength/ type length/A° kcal mol-1 length/A° kcal mol-1 The lattice energies of the A and B molecules were -29.01 and -28.56 kcal mol-1 respectively for the monoclinic poly- A,A 2.60 -1.428 2.59 -0.244 morph and -28.29 and -28.18 kcal mol-1 respectively for A,A 2.99 -0.949 2.72 -0.911 the orthorhombic polymorph.These variations reflect the B,B 2.75 -0.799 2.66 -0.920 different atomic positions for the A and B molecules in the B,B 2.60 -0.315 — — two polymorphs. The calculated lattice energy of the mono- B,B 2.98 -0.828 — — A,B 2.73 -0.429 2.54 -0.887 clinic polymorph is -28.83 kcal mol-1, and that of the A,B 2.73 -0.166 2.77 -0.953 orthorhombic polymorph is -28.24 kcal mol-1.These values are in good agreement with the sublimation enthalpy (-2RT ) of 28.3±1.0 kcal mol-1 reported by Edwards41 and compare well with that reported by Lenchitz and Velidly42 (24.7 kcal the formation of this phase under ambient conditions prior to melting it can be speculated that such a phase might be mol-1).The similarity between the lattice energies reflects the similarity between the crystal structures. The fact that the observed at higher pressures and that this might be reflected in the energetic nature of such a phase. monoclinic polymorph has a slightly more negative lattice 232 J.Mater. Chem., 1997, 7(2), 229–235energy suggests it is the most stable, confirming the previous the strengths of these bonds reflects the similarity in conformation and packing motif between the two polymorphs. The experimental observations.20–22 The difference in the lattice energies of the two polymorphic forms, 0.59 kcal mol-1, twinned crystal exhibits eight intermolecular bonds, similar in nature and strength to those in the monoclinic polymorph. represents the energy of transformation from the monoclinic to the orthorhombic polymorph.This can be compared to the two reported experimental values of DtrH=0.22 kcal mol-1 22 Morphological prediction and DtrH=0.27±0.07 kcal mol-1.43 The discrepancy between the experimental and calculated values may be accounted for Table 4 shows the attachment energies and associated dspacings for the most morphologically important faces together by the presence of twinning or crystallographic impurities in the samples.with the percentage of the surface area for the three polymorphs. Fig. 5 shows the resulting predicted morphologies. The average lattice energy for the twinned structure was calculated as -28.43 kcal mol-1, with a lattice energy of For the monoclinic polymorph, a plate-like habit dominated by the {100} face is predicted.The plate-like habit suggests -28.57 kcal mol-1 calculated using molecule A as the origin molecule and a lattice energy of -28.28 kcal mol-1 using that the monoclinic polymorph crystallises in a layer-like structure in which the bonding between adjacent {100} layers molecule B as the origin molecule.The small difference in the lattice energy of the twinned is considerably weaker in comparison to the bonding interactions contained within the layers. Examination of the inter- crystal and the monoclinic polymorph explains the abundance of twin faults in TNT crystals. As a consequence of the small molecular bonds confirms this observation with all the principal intermolecular interactions (Ma,Mh) being involved energy difference, the crystallisation conditions become increasingly important so that minor variations in growth rate or the within the slice and none perpendicular to the slice.The resulting small attachment energy of the {100} layers causes presence of impurities may result in the occurrence of twinning. For example, under unstable growth conditions it is highly slow growth perpendicular to the {100} face.Rapid growth occurs perpendicular to the {110}, {1–1–1} and {011} faces, likely that the molecular selectivity needed to differentiate between type A and B molecules in order to achieve a strict owing to the presence of strong intermolecular interactions parallel to the b axis associated with the strong type Ma, Mb monoclinic AABBAA packing might not always be achieved.Table 3 give the major types and strengths of the intermol- and Md interactions which link these chains (4Ma+2 Mb+Md). The resulting bonding network creates a series of ecular bonds involved in the monoclinic, orthorhombic and twinned polymorphs. The bond strengths are average values, strong bonding chains, leading to rapid growth, perpendicular to faces {110}, {1-1-1} and {011}. These observations are taken by summing all the constituent atom–atom interactions. Slight variations exist in the strength of the intermolecular consistent with the experimental observation that perfect cleavage occurs readily parallel to these planes.22 interactions for the same type of bonds, i.e.A,A, A,B and B,B, along different directions. Both the monoclinic and A similar situation occurs for the orthorhombic polymorph, where a plate-like habit dominated by the {010} is predicted. orthorhombic polymorphs are dominated by eight types of intermolecular interactions, accounting for 87.95 and 88.22% The dominance of the {010} face is due to all the principal intermolecular interactions being involved in bonding within respectively of the total lattice energy.The similarity between Table 3 Type and strengths of the intermolecular interactions of the monoclinic, orthorhombic and twinned polymorphs interaction energy/ (kcal mol-1) interactionsa I J U V W Z Jb typec no. of bonds distanced/A° Coulombic total % total energy Ma 1 1 0-1 0 1 1 like 2 6.1 -0.49 -2.83 19.6 Mb 1 2 0 0 0 2 2 like 2 6.2 -0.29 -2.76 19.2 Mc 2 1 0 1 0 2 2 unlike 1 6.0 -0.40 -2.71 9.4 Md 2 1 0 2 1 3 2 unlike 1 6.9 -0.63 -2.60 9.0 Me 2 1 0 3 0 3 1 like 2 8.1 -0.29 -1.67 11.6 Mf 4 1 0-1 1 1 1 like 2 8.1 -0.26 -1.49 10.3 Mg 2 1-1 2-1 4 1 like 1 8.0 -0.16 -1.37 4.7 Mh 4 1 0-2 0 4 2 unlike 1 8.5 -0.20 -1.17 4.1 Oa 2 1 0 0 1 2 1 like 2 6.1 -0.42 -2.61 18.5 Ob 1 1 0 0 0 2 2 unlike 2 6.2 -0.31 -2.75 19.5 Oc 2 1 0 0 0 2 2 unlike 1 6.1 -0.44 -2.74 9.7 Od 4 1 1 0 1 2 2 unlike 1 6.9 -0.65 -2.64 9.3 Oe 2 1-1 0-1 4 1 like 2 8.1 -0.31 -1.68 11.9 Of 4 1 0 0 0 2 1 like 2 8.1 -0.25 -1.48 10.5 Og 2 1-1 1 0 3 2 unlike 1 8.0 -0.17 -1.33 4.7 Oh 2 1 0 0-1 2 2 unlike 1 8.6 -0.17 -1.18 4.2 Ta 1 5 0-1 0 1 5 like 2 9.09 -0.48 -2.80 19.71 Tb 1 4 0 1 0 1 6 unlike 2 6.17 -0.30 -2.77 19.47 Tc 1 1 0-1 0 1 3 unlike 1 6.04 -0.41 -2.58 9.07 Td 1 4 0 0 0 1 1 unlike 1 6.89 -0.64 -2.41 8.46 Te 1 7 0-1-1 1 8 like 2 8.09 -0.29 -1.66 11.64 Tf 1 6 0 0 0 1 5 like 2 8.13 -0.32 -1.55 10.90 Tg 1 2-1 0 1 1 7 unlike 1 7.99 -0.15 -1.37 4.82 Th 1 1 0-2 0 1 3 unlike 1 8.54 -0.22 -1.18 4.17 aMx, Ox and Tx represents monoclinic, orthorhombic and twinned bonds respectively.bI and J refer to the asymmetric unit and molecule representing the origin, while Z and J represent the asymmetric unit and molecule of the other molecule. UVW represent the unit-cell vector. The origin vector is 000. cThis refers to the nature of the bond. A bond labelled ‘like’ is a bond between either all A type molecules or all B type molecules.A bond labelled ‘unlike’ refers to an interaction between both A and B bonds. dThis refers to e distance between the centres of gravity of the two independent molecules. J. Mater. Chem., 1997, 7(2), 229–235 233Table 4 Slice shift, attachment energies, d-spacing and % area of the crystal for the growth faces of the monoclinic, orthorhombic and twinned polymorphs polymorph growth face slice shift/A° Eatt/kcal mol-1 d-spacing % area monoclinic {1 0 0} 0.0 0.40 20.0 44.7 monoclinic {0 0 1} 1.4 4.39 14.1 2.4 monoclinic {1 0 -1} 1.4 4.41 14.0 1.7 monoclinic {1 1 0} 0.3 16.53 5.8 0.4 monoclinic {1 -1 -1} 0.6 16.94 5.6 0.1 monoclinic {0 1 1} 0.8 16.95 5.6 0.1 monoclinic {1 0 -2} 0.0 13.08 7.5 — monoclinic {1 0 2} 0.0 15.09 6.0 — monoclinic {1 1 1} 1.6 16.96 5.2 — orthorhombic {0 1 0} 1.0 0.40 20.0 44.3 orthorhombic {1 0 0} 0.8 3.91 15.0 4.6 orthorhombic {0 1 -1} 0.3 16.01 5.8 0.6 orthorhombic {1 1 0} 0.6 6.77 12.0 — orthorhombic {2 1 0} 0.0 13.03 7.0 — orthorhombic {2 1 -2} 0.3 21.63 2.8 — twinned {1 0 0} 0.0 -0.40 20.0 44.7 twinned {0 0 1} -1.4 -4.40 14.1 2.4 twinned {1 0 -1} 1.4 -4.41 14.0 1.8 twinned {1 1 0} 0.6 -16.4 5.8 0.9 twinned {1 -1 -1} -0.6 -16.8 5.6 0.1 twinned {0 1 1} -1.1 -16.8 5.6 0.1 twinned {1 0 -2} 0.0 -12.9 7.5 — twinned {1 0 2} 0.0 -14.8 6.0 — twinned {1 1 1} -1.6 -16.8 5.2 — compared favourably with the experimentally determined morphologies of Gallagher and Sherwood.20–22 They found that in general the faces {100}, {102}, {001}, {101}, {011}, {111} and {1-1-1} were observed for crystals grown from a variety of solvents, with {100} dominating.Variations in the crystal morphology were found with changes in supersaturation and solvent. Changes in the relative sizes of faces were observed and in some cases the {111} and {011} facets were absent. This phenomenon can be explained by the fact that there were only small differences in the attachment energies of the {102}, {011}, {110}, {1–1–1} and {111} faces.Crystals grown from methanol also exhibited the {10–2} and {20–3} faces, which can also be explained by the similarity of their attachment energies to those of the {102}, {011}, {110}, {1–1–1} and {111} faces. The predicted morphology for the twinned monoclinic form is also very similar to those of the monoclinic polymorph.The intermolecular bonds Ta, Tb and Td form a strong intermolecular bonding network perpendicular to the {110}, {1–1–1} and {011} faces causing rapid growth perpendicular to these faces and hence faces of small morphological importance, whilst all the principal intermolecular interactions contribute to bonding within the {100} face and none perpendicular to it, creating a very slow growing dominant {100} face.Despite the agreement between predicted and observed crystallographic form being acceptable, the experimental crystal habits in general tend to be more prismatic when compared to theoretical simulations. The reasons for this are not clear at this time, but are likely to reflect the effect of the growth environmental factors such as solvent and supersaturation.Fig. 5 (a) Predicted morphology of orthorhombic TNT and (b) Conclusions predicted morphology of monoclinic phase of TNT; (c) predicted morphology of the twinned monoclinic phase Previous experimental studies have indicated difficulties in obtaining consistent crystal forms of TNT. The calculations show that the difference in lattice energies between the two the slice and none contributing to bonding in the growth direction perpendicular to the {010} plane.In contrast to this, forms is quite small and that subtle differences can affect the form of the crystal and can cause twinning. The results indicate the small area of the {01-1} face is due to the presence of a strong network of intermolecular bonds of type Oa, Oc and that such calculations can be of value in predicting crystal morphologies, with a plate-like morphology dominated by the Od (4Oa+Oc+Od) perpendicular to the {01-1} face.The predicted faces of the monoclinic polymorph can be {100} face predicted for the monoclinic form and a plate-like 234 J. Mater. Chem., 1997, 7(2), 229–23516 E. Hertel and G. Ro�mer, Z. Phys. Chem. B, 1930, 2, 77. morphology dominated by the {010} face predicted for the 17 R.Hullgren, J. Chem. Phys., 1936, 4, 84. orthorhombic form. Thus, for an energetic material such as 18 E. Artini, R. C. Acad. L incei, 1915, 24, 274. TNT which can be experimentally difficult to study, compu- 19 W. C. McCrone, Anal. Chem., 1949, 21, 1583. tational modelling can provide a useful tool to aid understand- 20 H. G.Gallagher and J. N. Sherwood, T he Influence of L attice ing, and in potentially controlling the properties such as crystal Imperfections on the Chemical Reactivity of Solids: T he Growth and Purification of TNT Single Crystals., Avail. NTVS. Gov. Rep. form and morphology for TNT. Announce. Index (NS), 1984, 84, 83. Further work is currently in hand to assess the role of 21 H. G.Gallagher and J. N. Sherwood, T he Growth and Purification growth environmental factors with a view to rationalising of Single Crystals of TNT, Mater. Res. Soc. Symp. Proc., 1993, discrepancies between the morphological simulations and the 296, 215. experimentally observed crystal morphology. 22 H. G. Gallagher and J. N. Sherwood, J. Chem. Soc., Faraday T rans., 1996, 92, 2107. 23 P.Bladon and R. Breckinridge, INTERCHEM, University of The authors would like to acknowledge J. R. C. Duke of Strathclyde, UK, 1986. PERME, Waltham Abbey, Essex, for allowing access to his 24 R. Docherty, PhD Thesis, Strathclyde University, 1989. fractional coordinates of the monoclinic and orthorhombic 25 R. Buckingham and J.Corner, Proc. R. Soc. L ondon, 1947, 189, 118. polymorphs of 2,4,6-trinitrotoluene prior to publication.The 26 G. Nemethy, M. S. Pottle and H. A. Scheraga, J. Phys. Chem., 1983, 87, 1883. involvement of R. Docherty and G. Clydesdale in the early 27 M. J. S. Dewar, E. G. Zoebisch, E. F. Healy and J. J. P Stewart, stages of this work is also gratefully acknowledged. K. J. R. J. Am. Chem. Soc., 1985, 107, 3902. also acknowledges the Engineering and Physical Sciences 28 J.J. P. Stewart, J Comput.-AidedMol. Design, 1990, 4, 1. Research Council (UK) for the current support of a senior 29 G. Clydesdale, R. Docherty and K. J. Roberts, Comput. Phys. fellowship. Commun., 1991, 64, 311. 30 G. Clydesdale, R. Docherty and K. J. Roberts, QCPE Program No. 670, Quantum Chemistry Program Exchange, Indiana University, Bloomington, Indiana, USA.References 31 G. Clydesdale, R. Docherty and K. J. Roberts, Proc. Conf. Crystal 1 J. A. R. P. Sarma and G. R. Desiraju, J. Chem. Soc., Perkin T rans. Growth Organic Materials, ed. A. S. Myerson, D. A. Green and 2, 1987, 1187. P. MeenenWashington, DC, 1995; J. Crystal Growth, 1996, 166, 78. 2 W. Jones, C. R. Theocharis, J. M. Thomas and G. R. Desiraju, 32 F.A. Momany, R. F. McGuire, A. W. Burgess and H. A. Scheraga, J. Chem. Soc., Chem Commun., 1983, 1443. J. Phys. Chem., 1975, 79, 2361. 3 M. C. Etter, R. B. Kress, J. Bernstein and D. J. Cash, J. Am. Chem 33 S. Lifson, A. T. Hagler and P. Dauber, J. Am. Chem. Soc., 1979, Soc., 1984, 106, 6921. 101, 5111. 4 A. Gavezzotti and G. Filippini, J. Am. Chem. Soc., 1995, 117,12299. 34 D. E. Williams, J. Chem. Phys., 1966, 45, 3370. 5 V. Benghiat and L. Leiserowitz, J. Chem. Soc., 1972, 1763. 35 A. Bravais, Etudes Crystallographiques, Paris, 1913. 6 J. D. Dunitz and J. Bernstein, Acc. Chem. Res., 1995, 28, 193. 36 G. Freidel, Bull. Soc. Fr. Mineral., 1907, 30, 326. 7 G. Desiraju, Crystal Engineering, the Design of Organic Solids, 37 J. D. H. Donnay and D. Harker, Am.Mineral., 1937, 22, 463. Mater. Sci. Monogr., 54, Elsevier, Amsterdam, 1989. 38 J. R. Holden, C. Dickinson and C. M. Bock, J. Phys. Chem., 1972, 8 L. A. Errede, M. C. Etter, R. C. Williams and S. M. Darnauer, 76, 3597. J. Chem. Soc., Perkin T rans. 2, 1981, 233. 39 C. Dickinson, J. M. Stewart and J. R. Holden, Acta Crystallogr., 9 J. Bernstein, I. Bar and A. Christensen, Acta Crystallogr., Sect. B, 1966, 21, 663. 40 W. R. Carper, L. P. Davis and M. W. Extine, J. Phys. Chem., 1982, 1976, 32, 1609. 86, 459. 10 I. Bar and J. Bernstein, Acta Crystallo., Sect. B, 1977, 33, 1738. 41 G. Edwards, T rans. Faraday Soc., 1950, 46, 423. 11 I. Bar and J. Bernstein, Acta Crystallogr., Sect. B, 1982, 38, 121. 42 C. Lenchitz and R. W. Velidly, J. Chem. Eng. Data, 1920, 15, 3, 401. 12 J. Bernstein, M. Engel and A. T. Hagler, J. Chem. Phys., 1981, 43 D. G. Graber, F. C. Rauch and A. J. Fanelli, J. Phys. Chem., 1969, 75, 2346. 73, 3514. 13 L. A. Burkardt and J. H. Bryden, Acta Crystallogr., 1954, 1, 135. 14 J. R. C. Duke, 1981, unpublished work. 15 P. Freidlander, Z. Kristallogr., 1879, 3, 169. Paper 6/03983I; Received 6th June, 1996 J. Mater. Chem., 1997, 7(2), 229–235 235

ISSN:0959-9428    

DOI:10.1039/a603983i

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Crystallisation in polymer films: control of morphology and kinetics of an organic dye in a polysilicone matrix Roger J. Davey,*a Simon N. Black,b Alan D. Goodwin,b Duncan Mackerron,b Steven J. Maginnb and Emma J. Millerb aColloids, Crystals, and Interfaces Group, Department of Chemical Engineering, UMIST , PO Box 88,Manchester, UK M60 1QD bResearch and T echnology Department, ICI Chemicals and Polymers, T he Heath, Runcorn, Cheshire, UK WA7 4QD The successful selection, design and synthesis of additives for the control of nucleation and crystallisation of a red anthraquinone based dye is described.This procedure is based on the crystal structure and morphology of solution grown crystals and in this study the possibility of using the same approach to control crystallisation within an amorphous polymeric matrix is explored.The work has been motivated by the desire to prepare stable dye sheets for thermal transfer printing applications, yet the parallel with natural biomineralisation processes is clear. Recent years have seen significant advances in the understand- Experimental ing of structural mechanisms available for the control of The dye chosen for this work was the red, anthraquinone- crystallisation processes.1–3 Thus, the factors leading to the based 1-amino-4-hydroxy-2-phenoxyanthraquinone 1, sup- design of auxiliary molecules for the control of nucleation and plied by ICI. Crystallisation was carried out both from growth of both organic and inorganic materials are now well solutions in butanone and from films comprising crosslinked understood.4,5 One area of particular interest, largely in the polydimethylsiloxane, also supplied by ICI.These films were context of materials design, has been biomineralisation. Here, prepared by coating a solution of dye, polymer, crosslinker biological systems are seen as models for the expression of and catalyst (dibutyltin) in CH2Cl2 onto a 6 mm melinex highly organised structures comprising simple inorganic crys- support [biaxially oriented poly(ethylene terephthalate)] using tals located in macromolecular matrices(e.g.calcium phosphate a K-bar (R K Print-Coat Instruments Ltd.) to give coatings in collagen). A number of studies have already been reported 24–54 mm in thickness. After spreading, the solvent was which attempt to mimic these processes using synthetic poly- evaporated using a hot air drier and the film cured at 100 °C mers as matrices in which simple inorganic species are for 30 s to complete the crosslinking.This left a final film of nucleated and grown.6,7 In this study we present for the first ca. 1 mm thickness. By carrying out the final cure for variable time an example of an organic molecule crystallising within a times and on a microscope hot stage, the nucleation and polymer film and show how the existing strategies for the growth of dye crystals at 100°C in these initially amorphous control of crystallisation from solutions and melts can be films was monitored.Dye concentrations in the original successfully transferred to crystallisation from an amorphous solutions were chosen to be 0.28, 0.56 and 0.85 wt.% and polymeric matrix.The work was motivated by the commercial each measurement reported is the mean of seven experiments. need to develop dye sheets for use in thermal transfer colour Crystals of the dye were grown from solution by slow printing,8–10 in which the dye was stabilised against crystallisation during storage.evaporation of 1% butanone solutions at 30°C. Owing to the J. Mater. Chem., 1997, 7(2), 237–241 237colour intensity of such solutions it was not possible to monitor added at levels of 1 wt.% on dye, while in the polymer films levels up to 15 wt.% on dye were used. the progress of these experiments. When only a little solvent remained the solutions were filtered and the crystals examined by SEM.Crystals grown from pure solutions were examined Crystal structure and additive design by optical microscopy and optical goniometry in order to define their morphology. The theoretical morphology was The crystal structure of the dye used here has been reported calculated using the program HABIT3 and the potentials of previously.10 It belongs to the monoclinic space group P21/c Momany et al.11 with four molecules in the unit cell and a=1.4972, b=0.5141 Small molecule additives [1-amino-2-(4-tert-butylphenoxy)- and c=1.9853 nm and b=94.15°.The anthraquinone and 4-hydroxyanthraquinone 2 and 1-amino-2-phenoxy-4-toluidi- phenoxy rings adopt a dihedral angle of 97° and the molecules noanthraquinone 3] were supplied by ICI. These materials are held in the structure by van derWaals and p–p interactions.were characterised by NMR spectroscopy, thin layer chroma- There are no intermolecular hydrogen bonds. Fig. 1(a) viewed tography (TLC) and microanalysis and found to have purities down the a-axis shows the herringbone motif and indicates in excess of 98%. The polymeric additives were the products that the p–p interactions lie along [010].Fig. 1(b) is a view of grafting 1-amino-2-bromo-4-hydroxyanthraquinone onto down the b-axis. Fig. 2(a) is a scanning electron micrograph of copolymers of styrene and vinylphenol (4). Poly(vinylphenol)s solution grown crystals which adopt a needle morphology in of varying vinylphenol contents were synthesised by reacting which the needle axis corresponds to [010] with large {101} vinylphenol (prepared by the method of Hatakeyama et al.12) and smaller {101} and {002} side faces, as determined by (0.044, 0.11, and 0.15 mole fraction) with styrene at 60°C for optical goniometry.The predicted morphology is shown in 50 h using azoisobutyronitrile (AIBN) as an initiator. The Fig. 2(b) and is in reasonable agreement with experiment with copolymers were recovered by dissolving the reaction mixture elongation along [010] as expected from the p–p interactions, in butanone and precipitating the polymer with MeOH. 1- with {002}, {200} and {101} as the most important side faces. Amino-2-bromo-4-hydroxyanthraquinone was then grafted The overall objective of this study was to prevent crystallis- onto these polymers by reaction in N-methylpyrrolidone at ation of dye molecules within the polymeric films by including 135 °C for 20 h.The resultant grafted polymers were recovered in the formulations additives which would inhibit nucleation by precipitation with MeOH. Characterisation was performed and growth of crystals. It was on the basis of the above by gel permeation chromatography (GPC), to measure the structural and morphological data that such additives were molecular weight relative to a polystyrene standard, and proton selected and designed as molecules likely to inhibit crystallis- NMR, FTIR and UV–VIS spectroscopy to assess the extent ation of specific faces of the dye crystals.For example, in the of grafting. {101} faces molecules present either their phenoxy or their For subsequent solution growth experiments additives were anthraquinone rings to the growth environment [Fig. 1(b)]. Hence substitution at the para position on the phenoxy ring by a bulky group should yield a molecule that is recognised by the crystal surface but which, once in position, will sterically Fig. 2 The morphology of 1-amino-4-hydroxy-2-phenoxyanthraqui- Fig. 1 The crystal structure of 1 (a) viewed down [100], and (b) viewed none crystals: (a) solution grown and (b) predicted from the crystal structure down [010] 238 J.Mater. Chem., 1997, 7(2), 237–241prevent addition of further molecules to the growing crystal. was designed to inhibit growth on the (101) face, and a second in which the hydroxy group was replaced by a toluidino group For {020} there are four different molecular orientations [Fig. 1(a)], however in two of these the hydroxy group is (3) designed to inhibit (020). Their likely positions in the two surfaces are shown in Fig. 3(a) and (b). In addition, the directed away from the surface such that its replacement by a bulkier substituent might be expected to inhibit growth in this polymeric additives (4) described above were synthesised with the aim of providing the possibility of multidentate binding to direction.Bearing these conclusions in mind two potential additives were selected, one in which the phenoxy ring was the surfaces to compare with the small unidentate molecules. This is shown schematically in Fig. 3(c) for the {101} faces. substituted at the para position by a tert-butyl group (2) and Fig. 3 Possible binding sites of additives: (a) 2 on (101), (b) 3 on (020), and (c) 4 on (101) Fig. 4 Crystals grown from solution in the presence of (a) 2, (b) 3, (c) a mixture of 2 and 3 and (d) polymer 4c J. Mater. Chem., 1997, 7(2), 237–241 239Crystallisation in polymer films Results In polymer films dye nucleation was followed by radial growth, Characterisation of polymers giving polycrystalline spherulitic or rosette type morphologies Three copolymers were prepared, having mole fractions (as which eventually spread through the entire film.This is seen determined by FTIR) of 0.048, 0.11, and 0.16 vinylphenol. This in Fig. 5. Similar behaviour has been reported previously for suggests a reactivity ratio of vinylphenol with styrene of unity, anthraquinone crystallising from polystyrene films.14 Powder in agreement with related studies.13 After grafting, 2.25, 4.7 X-ray diffraction of crystallised films had major reflections and 12.8% (w/w) of 1-amino-2-bromo-4-hydroxyanthraqui- corresponding to d-spacings of 1.23, 1.15, 0.62 and 0.49 nm as none had been incorporated, and the copolymers had weight expected for b-axis needles of this dye having predominant mean molar masses (Mw) of 168100, 103300 and 85700, respect- {101} and {002} faces.13 This implies radial growth of the ively.They are referred to subsequently as polymers 4a, b and c. spherulites along the fast growing [010] direction. Increasing the dye concentration from 0.28 to 0.85 wt.% led, as expected, to an increase in the nuclei density from 0.7 to 3.5 rosettes per mm2 due to the associated rise in supersaturation. Solution grown crystals In agreement with the solution growth experiments reported above, all the additives influenced the crystallisation of the dye Fig. 4(a)–(d) shows crystals grown in the presence of the tertin the polymer film. The morphological changes are shown in butyl derivative 2, the 4-methylaniline derivative 3, a mixture Fig. 6. Fig. 6(a) and (b) show the effect of increasing levels of 2 of 2 and 3, and polymer 4c, respectively. When compared with while Fig. 6(c) and (d ) show the effects of 3 and polymer 4c, crystals grown from pure solutions [Fig. 2(a)] it is quite clear respectively. Clearly, all these additives inhibit the formation that all the additives tested had a significant effect on the of the space-filling rosettes and appear to inhibit the crystallis- crystallisation process.The tert-butyl derivative appeared to ation process such that large areas of the film remain prevent the formation of well defined crystals, yielding a uncrystallised. powder of irregular plate-like crystals. The toluidino derivative significantly reduced growth along the b-axis as expected, while In order to quantify the magnitude of crystallisation inhipolymer 4c appeared to give enhanced nucleation (smaller bition, simple kinetic studies were undertaken in which the crystals) but little morphological change.Overall these results rosette numbers and radii were measured. The resulting crystalindicated that the additives chosen were of sufficient potency lisation times and nucleation densities are shown in Fig. 7(a) to test in polymer films. and (b), where it is clear that stabilisation of the dye films is enhanced by increasing concentrations of additive. Compound 3 is most active in this respect, presumably because it was designed to inhibit the fastest growth direction, [010]. The polymers appear to be particularly successful at inhibiting nucleation, an observation that appears to be in complete contrast to their solution behaviour, where they appear to enhance the nucleation rate. This may well be related to reduced mobility and conformational flexibility in the polymer film.Finally, the behaviour of the three grafted copolymers was compared and the kinetic data are shown in Fig. 7(c). For comparison, polystyrene was also tested and found to be ineffective in modifying the crystallisation and also to be immiscible with the silicone binder system used here.Given that these copolymers contain significant polystyrene blocks it would be expected that their solubility in the polymer film would be a key factor in their efficacy. For example, polymer 4a is 95.5% polystyrene, which implies a low solubility in the film and hence explains its poor efficacy in Fig. 7(c). Polymers 4b and 4c showed improved performance, although at high (10%) levels they also phase separate in the film. It is perhaps surprising that the efficacy does not increase monotonically Fig. 5 Spherulitic crystallisation of 1 in a polymer film with level of grafting, yet it is encouraging that all the polymers Fig. 6 The influence of additives on the morphology of crystallised polymer films: (a) 5%of 2, (b) 14% of 2, (c) 2% of 3 and (d) 5% of 4c 240 J. Mater. Chem., 1997, 7(2), 237–241Conclusions This study has applied the structurally based strategy for selection and design of tailor-made growth auxiliaries to the case of an anthraquinone dye crystallising from an amorphous polymer film. Crystallisation inhibitors, selected on the basis of the known crystal structure of the dye, were tested for efficacy in solution growth experiments and shown to be powerful crystallisation modifiers.It is interesting to note that unlike previous studies of tailor-made inhibitors the molecular recognition features involved utilise only van der Waals and p–p interactions rather than more specific hydrogen bonding.It was found that this behaviour could be transferred directly to crystallisation in a polymer matrix, with the result that crystallisation could be significantly inhibited and the dye films stabilised even at temperatures as high as 100°C. This correspondence between solution and polymeric media has not been addressed before, although it is clearly of significance both technologically and from the biomineralisation viewpoint.The authors acknowledge the assistance of Drs. B. D. Chen and H. Lieberman in the preparation of this manuscript. References 1 I. Weissbuch, R. Popoviz-Biro, L. Leiserowitz and M. Lahav, in T he L ock and Key Principle, ed. J. P. Behr, John Wiley and Sons, Chichester, 1994, ch. 6. 2 R. J. Davey, in Separation T echnology T he Next T en Years, ed. J. Garside, I. Chem E., Rugby, 1994, ch. 4. 3 G. Clydesdale, K. J. Roberts and R. Docherty, in Colloid and Surface Engineering: Controlled Particle, Droplet, and Bubble Formation, ed. D. Wedlock, Butterworth-Heineman, London, 1993, pp. 95–135. 4 A. L. Rohl, D. H. Gay, R. J. Davey and C. R. A. Catlow, J. Am. Chem. Soc., 1996, 118, 642. 5 S. Mann, Nature (L ondon), 1993, 365, 499. 6 J. Lin, E. Cates and P. A. Bianconi, J. Am. Chem. Soc., 1994, 116, 4738. 7 B. J. Brisdon, B. R. Heywood, A. G. W. Hodson, S. Mann and K. W.Wong, Adv. Mater., 1993, 5, 49. 8 P. Gregory, Chem. Br., 1989, January, 47. 9 R. J. Davey and D. H. Mackerron, Eur. Pat. Appl., EP 294109, 1988. Fig. 7 The influence of additives on the kinetics of crystallisation of 1 10 S. N. Black, R. J. Davey, C. A. O’Mahoney and D. J. Williams, in polymer films: (a) the effect of (%) 2, (') 3, and (+) polymer 4c on Acta Crystallogr., Sect. C, 1992, 48, 321. times taken for films to fully crystallise, (b) the effect of (%) 2 and 11 F. A. Momany, L. M. Carruthers, R. F. McGuire and H. A. polymers (×) 4a and (+) 4c on the nucleation in films, and (c) the Scheraga, J. Phys. Chem., 1974, 78, 1595. effect of (*) polystyrene, (%) 2 and polymers (×) 4a, (2) 4b and (+) 12 T. Hatakeyama, K. Nakamura and H. Hatakeyama, Polymer, 4c on crystallisation rates 1978, 19, 593. 13 A. D. Jenkins, K. Petrak, G. A. F. Roberts and D. R. M. Walton, Eur. Polym. J., 1975, 11, 653. show enhanced activity over the single small molecule tert- 14 Y. Murata and T. Kiyotsukuri, Kobunshi Ronbunshu, 1984, 41, 111. butyl derivative. This supports the idea that efficacy may be Paper 6/05614H; Received 12th August, 1996 enhanced through multidentate binding. J. Mater. Chem., 1997, 7(2), 237–241 241

ISSN:0959-9428    

DOI:10.1039/a605614h

出版商:RSC    

年代:1997

数据来源: RSC