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J O U R N A L O F C H E M I S T R Y Materials Feature Article Layered molecular optoelectronic assemblies Itamar Willner* and Bilha Willner Institute of Chemistry and The Farkas Center for Light-Induced Processes, The Hebrew University of Jerusalem, Jerusalem 91904, Israel. E-mail: willnea@vms.huji.ac.il Received 2nd June 1998; Accepted 20th August 1998 Layered functionalized electrodes are used as optoelec- of binding processes,9,10 translocation,11 redox properties,12 tronic assemblies for the electronic transduction of catalytic reactions13 etc.represent chemical functions that are recorded photonic signals. Functionalization of a Au elec- triggered by external signals. (iii) The functions of the molecutrode with a photoisomerizable redox-activated monolayer lar device should be externally controlled.That is, external enables the amperometric transduction of the photonic signals block the chemical functions of the system and regenerinformation recorded by the interface. This is exemplified ate the device for a secondary operation cycle. (iv) The with the organization of a phenoxynaphthacenequinone molecular device should include a transduction element that monolayer (1a).Organization of a photoactivated comprovides information-transfer from the molecular system to mand layer on an electrode can be used to control interfathe macroscopic environment regarding its physicochemical cial electron transfer and might be applied for the electrical state, e.g. sensory level, chemical state or structural position. transduction of recorded optical signals.This is addressed The transduction signals may include, for example, optical, with the assembly of a nitrospiropyran photoisomerizable electrical, or piezoelectrical signals. monolayer (2a) on a Au electrode which acts as a command surface for controlling by light interfacial electron transfer. Molecular switches,14,15 molecular brakes,16 molecular The monolayer undergoes photoisomerization between the ratchets17 and molecular shuttles18 represent a few chemical neutral state (2a) and the positively charged protonated assemblies duplicating the functions of macroscopic devices.merocyanine state (2b). The charged interface controls the Electrochemical,12,19 photonic,14,20 thermal21 and pH22 signals oxidation of dihydroxyphenylacetic acid, DHPAA (3), and were used to trigger molecular devices.For example, a bisof 3-hydroxytyramine, DOPA (4), and the system is used bipyridinium cyclophane threaded and stoppered on a p- for the electrochemical transduction of optical signals phenylenediamine-p-dialkoxybiphenyl wire is reversibly trans- recorded by the monolayer. Functionalization of electrodes located on the molecular wire by cyclic oxidation and reduction with a b-cyclodextrin monolayer or with an eosin p-donor of the p-donor diamine unit, Fig. 1(A).18a Light-stimulated layer enables the light-stimulated association or disassociation and dissociation of ions,9,23 e.g. Fig. 1(B),23b to sociation of the photoisomerizable N,N¾-bipyridinium azophotoisomerizable receptors represents a light-activated ion benzene (5t) and of bis-pyridinium azobenzene (8t) to or from the modified surfaces.Association and dissociation switch. Oxidation or reduction of a metal ion may induce its of the surface-associated supramolecular complexes are internal translocation between two ligation sites,15b respectively transduced by electrochemical or piezoelectrical signal [Fig. 1(C)].However, most of the reported systems are not outputs. The organization of a supramolecular system integrated in a configuration meeting all the characteristic where a molecular component is translocated by light- elements of molecular devices. The systems were assembled as signals between two distinct positions enables one to design supramolecular complexes in solution, and their signal-trig- ‘molecular machines’.This is exemplified by the organizgered properties were monitored by diVerent spectroscopic ation of a molecular assembly consisting of a ferrocenemeans. functionalized b-cyclodextrin (11) threaded onto an azo- Among the possible molecular devices, photoactivated benzene-alkyl chain wire and stoppered with an anthramolecular switches or phototriggered molecular machines play cene barrier which acts as a nanoscale molecular machine, a central role in future nanoscale optoelectronics. Tailoring of a light-stimulated ‘molecular train’.The ferrocene-functionalized b-cyclodextrin is reversibly translocated between photoactivated molecular switches (PMSs) requires the intethe trans-azobenzene and the alkyl chain by cyclic light- gration of a light-sensitive chemical assembly with a solid induced isomerization of the photoactive monolayer.The support acting as a transduction element. Photochemicallyposition of the b-cyclodextrin receptor is transduced by its induced excitation, electron transfer,24 or isomerization of the chronoamperometric response. chemical component,25 stimulate electronic redox or structural changes in the chemical component.These chemical perturbations are electronically transduced by the solid support, e.g. Miniaturization of devices to the molecular level or nanoscale by amperometric, potentiometric, conductometric, impedance dimensions represents one of the most challenging research or piezoelectric signals. Photoactive or electroactive chemical subjects in modern science.1,2 Progress in tailoring molecular functionalities were assembled on solid supports by polymeriz- machines may have important implications in molecular-based ation or layer deposition of thin films.26 Covalent association logic gates,3 computer technologies,4,5 sensory methods,6 of monolayers onto surfaces,27 and specifically, the linkage of bioengineering7 and biomimetic architecture.8 The organizthiolated monolayers to metal supports, e.g. Au surfaces,28,29 ation of molecular devices requires the assembly of integrated opens the possibility of organizing ordered two-dimensional molecular systems exhibiting several elements: (i) The molecumonolayer arrays on solid surfaces.We have recently discussed lar assembly should communicate with its macroscopic our activities in tailoring optobioelectronic devices by the environment. That is, external signals should activate the assembly of bioactive monolayers30,31 on electrode supports.physicochemical functions of the molecular system. External The functions of the integrated biomaterial/transducer systems signals that activate the molecular systems may include phoin the electronic transduction and amplification of recorded tonic signals, electrical signals, magnetic inputs etc.(ii) The optical signals32,33 as well as biosensor34 and bioelectronic35 molecular assembly should respond to the external triggering signals by altering its physicochemical properties. Activation applications were addressed. J. Mater. Chem., 1998, 8, 2543–2556 2543Fig. 1 Signal-controlled chemical switches: (A) Reversible electrochemical translocation of an electron acceptor between two distinct electron donor sites. (B) Reversible photochemical binding and dissociation of ions by a photoisomerizable ion-chelator. (C) Electrochemical translocation of an ion between distinct ligation sites. The present article summarizes recent progress in the assembly of thiolated monolayers on Au surfaces for the electrochemical or piezoelectric transduction of the photonic organization of molecular optoelectronic systems and, specifi- cally, PMS devices and phototriggered molecular machines, signals.Tailoring of integrated molecular optoelectronic assemblies using photoisomerizable chemical components as the active ingredient for recording the photonic information.We emphas- that act as PMSs was achieved by three general approaches schematically outlined in Fig. 2. One approach involves the ize the integration of molecular optoelectronic systems by the 2544 J. Mater. Chem., 1998, 8, 2543–2556none was covalently linked to the base layer in the presence of 1-ethyl-3-(3-dimethylaminopropyl )carbodiimide (EDC).Fig. 3 (curve a) shows the cyclic voltammogram of the resulting monolayer. An ill-defined redox wave of the trans-quinone state (1a) is observed. This ill-defined wave is attributed to the fact that a non-densely-packed monolayer of the quinone is formed. DiVerent orientations of the quinone relative to the electrode, or eventually non-specific adsorption of the quinone to the surface, yield a mixture of non-electroactive quinone units and species of diVerent electrochemical features that lead to the broad voltammogram.38 Treatment of the trans-quinonefunctionalized electrode with tetradecanethiol (C14SH) results in the association of the long-chain, hydrophobic thiolate to surface pinhole defects and the formation of a densely-packed, two-dimensional mixed monolayer consisting of C14SH and the trans-quinone.39 Fig. 3 (curves b–e) show the stepwise rigidification of the quinone component upon treatment of the quinone monolayer with C14SH. The quasi-reversible redox wave, E°=-0.62 V vs. SCE (at pH 7.0), is attributed to the two-electron redox process of the trans-quinone in a rigidified, aligned configuration in the monolayer assembly. Coulometric assay of the charge associated with the reduction (or oxidation) Fig. 2 Schematic configuration of photoactivated molecular switches: of the trans-quinone component reveals a surface coverage of (a) Redox activation of a photoisomerizable monolayer electrode. (b) Amperometric transduction of light-induced association of a the electrode by the quinone corresponding to 2×10-10 redox-active substrate to a photoisomerizable command interface.mol cm-2. The electron transfer rate from the electrode to the (c) Light-induced association of a redox-activated photoisomerizable quinone was estimated to be ket#2.5 s-1 by following the substrate to a functionalized monolayer electrode. peak-to-peak separation of the redox wave at diVerent scan rates. Fig. 4 shows the cyclic voltammogram of the trans-quinone immobilization of a photoisomerizable component on an monolayer, curve a, and that of the ‘ana’-quinone (1b) state, electrode as the solid support [Fig. 2(a)]. In the photoisomer curve b, formed upon photoisomerization of the initial mono- state ‘A’, the molecular unit is redox-inactive and no electronic layer, 305 <l<320 nm. In the presence of the ‘ana’-quinone signal is transduced.Photoisomerization of the chemical commonolayer, only the background current of the electrolyte is ponent to state ‘B’ generates a redox-active assembly, and the observed, implying that this photoisomer monolayer is redox- electron transfer between the electrode and the chemical inactive. Irradiation of the ‘ana’-quinone monolayer, interface yields the amperometric (electrochemical ) transducl< 430 nm, restores the trans-quinone monolayer and reactiv- tion of the photonic signal that activates the system.The ates the amperometric response of the monolayer. By cyclic second approach to organize an integrated photoactivated photoisomerization of the monolayer between the trans-quin- optoelectronic switch is schematically detailed in Fig. 2(b), one and ‘ana’-quinone states, the transduced current is and includes the organization of a photoisomerizable monoswitched reversibly between ‘ON’ and ‘OFF’ states [Fig. 4 layer on the solid support that acts as a ‘command interface’ (inset)]. Activation of the electrical redox response of the for controlling electron transfer at the solid interface. In one functionalized monolayer by photonic isomerization of the photoisomer state, electron transfer to a redox probe solubil- ‘ana’-quinone state to the trans-quinone configuration rep- ized in the electrolyte solution is prohibited, whereas in the resents the electronic transduction of the recorded optical complementary state of the monolayer the interfacial electron signal.The back photoisomerization of the trans-quinone to transfer is allowed.The latter process transduces the recorded the ‘ana’-quinone state represents the erasure of the optical optical signal as a current output. The third approach to signal recorded by the monolayer and the regeneration of the assemble a molecular optoelectronic switch is shown in photosensitive recording interface. Thus, the phenoxynaph- Fig. 2(c) and involves the assembly of a chemically functhacenequinone monolayer assembled onto the Au support tionalized surface and its integration with a photoisomerizable exhibits Write-Read-Erase features. molecular component. In configuration ‘A’ of the molecular An important aspect in the development of molecular component, no aYnity interactions with the modified surface optoelectronic systems involves the amplified electronic trans- exist, and the system is in a mute state.Photoisomerization of duction of the recorded photonic signals. Such systems could the substrate to state ‘B’ activates the aYnity binding of the find interesting future applications in designing electronic molecular component to the surface, a process that is electronically transduced (e.g.an amperometric, impedance or amplifiers for weak light signals or sensitive actinometers. The piezoelectric signal ). Our research group has recently exem- photoswitchable redox functions of a photoisomerizable plified36 the organization of photoactivated molecular switches monolayer can be amplified by coupling of the electroactive (PMSs) based on these concepts, and these ‘simple’ systems component to an electron transfer cascade (Scheme 2). The served as a guideline for tailoring more complex nano-struc- mixed monolayer, consisting of C14SH and the ‘ana’-quinone tured molecular optoelectronic systems acting as light-driven state, provides an insulating layer on the conductive support.molecular machines. This yields a barrier for the direct electron transfer from the electrode to a secondary electron relay solubilized in the electrolyte.Photoisomerization of the monolayer to the Photoisomerizable redox-activated monolayer trans-quinone state activates the vectorial electron transfer electrodes from the redox-active unit to the diVusional relay. This stimulates the electrocatalyzed reduction of the relay solubilized in A phenoxynaphthacenequinone monolayer was assembled on the electrolyte solution.The regeneration of the electroactive a Au electrode as outlined in Scheme 1.37 A cystamine monoquinone unit in the monolayer yields an electrocatalytic layer was assembled on the Au surface as base monolayer, and 6-[(4-carboxymethylphenyl )oxy]-5,12-naphthacenequi- cathodic current that represents the amplified amperometric J.Mater. Chem., 1998, 8, 2543–2556 2545Scheme 1 Assembly of the phenoxynaphthacenequinone/C14SH mixed monolayer on a Au electrode and its photoisomerization. Fig. 3 Cyclic voltammograms of the trans-quinone monolayer electrode at diVerent time intervals of treatment with long-alkyl mercaptan C14SH (1 mM in ethanol ): (a) before treatment, (b) 1, (c) 4, (d) 10, and (e) 30 min of treatment, respectively and (f ) cyclic voltammogram Fig. 4 Cyclic voltammograms of the photoisomerizable of a Au electrode modified with C14SH only (1 mM, 30 min). quinone/C14SH mixed-monolayer electrode in the diVerent isomeric Electrolyte composition 0.01 M phosphate buVer, pH 7.0, and 0.1 states: (a) trans-quinone (1a) produced by irradiation, l>430 nm; Na2SO4; potential scan rate, 50 mV s-1.(b) ana-quinone state (1b) formed by illumination, 305<l>320 nm. Electrolyte composition 0.01 M phosphate buVer, pH 7.0, 0.1 M Na2SO4, scan rate 50 mV s-1. Inset: Cyclic amperometric transduction transduction of the recorded photonic signal. The amplification upon photoisomerization of the monolayer electrode ($) in the 1a of photonic signals recorded by the phenoxynaphthacenequistate, and (&) in the 1b-state.none monolayer will be addressed here by the application of N,N¾-dibenzyl-4,4¾-bipyridinium, BV2+, as a secondary electron relay. The reduction potential of BV2+ is pH-independent, electron relay. Photoisomerization of the monolayer to the ‘ana’-quinone state, 305 <l<320 nm, in the presence of and corresponds to -0.58 V (vs.SCE), where the formal reduction potential of the trans-quinone component at pH 7.5 BV2+ results in only the background current (curve c), implying that the direct electron transfer to BV2+ is prohibited. is E°=-0.65 V. This allows the vectorial reduction of BV2+ by the electroactive trans-quinone monolayer, and the acti- By cyclic photoisomerization of the monolayer between the trans-quinone and ‘ana’-quinone states, the reversible amplified vation of the respective electron transfer cascade.Fig. 5 shows the cyclic voltammograms of the photoisomerizable trans- amperometric transduction of the recorded optical signals is observed. quinone monolayer electrode in the absence (curve a) and presence (curve b) of BV2+.With the relay unit, BV2+, an The reduction potential of the trans-quinone monolayer is controlled by the pH of the electrolyte and is positively shifted electrocatalytic cathodic current is observed, indicating the vectorial electrocatalyzed reduction of the relay by the trans- as the pH decreases. For example, the formal potential of the trans-quinone monolayer corresponds to E°=-0.65 V and hydroquinone component.Also, it is evident that the transduced current is ca. 10-fold enhanced in the presence of the E°=-0.51 V (vs. SCE) at pH 7.5 and 5.0, respectively. This 2546 J. Mater. Chem., 1998, 8, 2543–2556Fig. 6 Photochemical and pH-controlled reduction of benzyl viologen (BV2+) by the 1a monolayer electrode. An ‘AND’ molecular electronic gate. trode acts in the presence of BV2+ as a gated molecular optoelectronic system, duplicating the function of an ‘AND’ gate in an electronic circuit (Fig. 6). In the ‘ana’-quinone state at pH 5.0, the system is in a mute configuration. Photoisomerization of the monolayer to the trans-quinone state does not yield the amplified amperometric response, since the electron transfer cascade that reduces BV2+ is not activated.Photoisomerization of the monolayer to the transquinone state, and alteration of the pH of the system to pH7.5 facilitates the mediated electrocatalyzed reduction of BV2+ and the transduction of the amplified electronic current signal. Photoisomerizable monolayer electrodes controlling interfacial electron transfer Photoisomerizable monolayers associated with conductive surfaces can act as active interfaces for controlling electron transfer at the electrode–solution interface [Fig. 2(b)]. The Scheme 2 Photoswitchable electrocatalytic reduction of an electron associative interactions between one photoisomer state of the acceptor at the phenoxynaphthacenequinone/C14SH mixed monolayer monolayer and the redox probe present in the electrolyte lead electrode interface.to the formation of a complex between the electroactive substrate and the monolayer. Concentration of the redox probe at the functionalized support electrically contacts the substrate and the electrode and stimulates interfacial electron transfer. Various interactions, such as electrostatic, host–guest or donor–acceptor interactions, could yield the aYnity binding of the redox probe to the photoisomerizable monolayer.Charged monolayers were employed as active interfaces for controlling specific electron transfer at electrode supports.40 Negatively charged monolayers associated with electrodes were used to discriminate the electrochemical reactions of a mixture of positively and negatively charged substrates.41 Accordingly, we have designed a photoisomerizable monolayer on a Au electrode that alters the electrical charge on the conductive support.42 1-(4-Mercaptobutyl )-3,3-dimethyl-6¾- nitrospiro[2¾H-1-benzopyran-2¾,2-indoline], mercaptobutylnitrospiropyran 2a, was assembled on a Au electrode (Scheme 3).The nitrospiropyran monolayer SP state is neutral. Photoisomerization of the monolayer, 320<l<350 nm, yields at pH 7.0 the protonated nitromerocyanine (2b) monolayer Fig. 5 Cyclic voltammograms of the quinone photo- MRH+ state. The latter monolayer is positively charged and isomerizable/C14SH monolayer electrode: (a) electrode in trans-quin- thus the electrochemistry of charged redox substrates could be one 1a state in pure electrolyte solution; (b) electrode in trans-quinone discriminated.Positively charged redox probes will be repelled 1a state with benzyl viologen (BV2+), 1×10-3 M, in electrolyte by the functionalized electrode, where negatively charged solution; (c) electrode in ‘ana’-quinone 1b state with BV2+, 1×10-3 M species are attracted by the monolayer and enhance the in the electrolyte solution. Electrolyte composition as in Fig. 4, scan electron transfer at the electrode surface (Scheme 4).The rate 5 mV s-1. Inset: Cyclic amplified amperometric transduction of reversible photoactivated isomerization of the monolayer: ($) 1a photostimulated oxidation of the negatively charged substrate, monolayer state; (&) 1b monolayer state. 3,4-dihydroxyphenylacetic acid at pH 7.0, DHPAA (3), and the positively charged substrate 3-hydroxytyramine (dopamine), DOPA (4) (at pH 7.0), was examined in the presence allows the control of the interfacial electron transfer features of the functionalized monolayer electrodes by external pho- of the photoisomerizable monolayer electrode.Fig. 7(A) (curve a) shows the cyclic voltammogram corresponding to tonic and pH signals. At pH 5.0 the trans-quinone monolayer is thermodynamically prohibited from stimulating the electron the electrochemical oxidation of DHPAA by the SP monolayer- functionalized electrode.Photoisomerization of the transfer to BV2+ (E°=-0.58 V). Only the weak electrical response of the trans-quinone monolayer is observed, without monolayer to the MRH+ monolayer state, 320 <l<350 nm, results in the cyclic voltammogram shown in Fig. 7(A) (curve the activation of the electron transfer cascade. Thus, the phenoxynaphthacenequinone-functionalized monolayer elec- b). Regeneration of the SP monolayer by photoisomerization J. Mater. Chem., 1998, 8, 2543–2556 2547Scheme 3 Assembly of the nitrospiropyran monolayer on a Au electrode. electrode (curve c). By reversible photoisomerization of the monolayer between the SP and MRH+ states, the amperometric responses of the electrode are cycled between high and low values, respectively [Fig. 7(B), inset]. Note that for DOPA, the high amperometric responses of the functionalized electrode are observed with the SP monolayer electrode. The retardation of the electrochemical oxidation of DOPA in the presence of the MRH+ monolayer electrode is attributed to the electrostatic repulsion of DOPA by the positively charged interface.Thus, the photoisomerization of the monolayer between the SP state and the protonated nitromerocyanine MRH+ state provides a means to control the electrical features of the electrode surface, thereby regulating the electron transfer at the electrode interface. The SP monolayer results in a neutral electrode surface where the MRH+ monolayer charges positively the surface and results in the formation of an electrical double-layer at the electrode interface.Functionalized monolayer electrodes coupled to Scheme 4 Photochemically-controlled oxidation of DHPAA (3) and photoisomerizable substrates for the physical DOPA (4) at a nitrospiropyran-functionalized electrode. transduction of photonic signals The assembly of monolayers on solid supports could lead to a functionalized interface that regulates the aYnity interactions of a photoisomerizable substrate with the monolayer [Fig. 2(c)]. In one photoisomer state, the substrate associates to the functionalized monolayer, and concentration of the substrate at the solid support is physically transduced (e.g.an electrochemical, piezoelectric, impedance, surface plasmon resonance or ellipsometric, eVect). In the complementary photoisomer state of the substrate, it lacks aYnity for the monolayer, and the respective transduced signal is blocked. This approach of the MRH+ interface restores the cyclic voltammogram shown in curve c. By reversible photoisomerization of the for tailoring molecular optoelectronic assemblies was demonstrated by us, using diVerent photoisomerizable substrates.43,44 monolayer between the SP and MRH+ states, the amperometric responses of the electrode are cycled between low and Two systems will be addressed here to exemplify the method.trans-N-Methyl-N¾-(1-phenylazobenzyl )-4,4¾-bipyridinium (5t) high values, respectively [inset, Fig. 7(A)]. The high amperometric responses of the MRH+ monolayer in the presence of exhibits reversible photoisomerizable properties. Irradiation of 5t, l=355 nm, yields the cis-isomer (5c), and further illumi- DHPAA are attributed to the electrostatic attraction of the redox-active substrate to the positively charged monolayer nation of 5c, l>375 nm, restores the trans-bipyridinium azobenzene substrate (5t).The two photoisomers diVer sub- interface. Concentration of DHPAA at the electrode surface facilitates electron transfer and results in high amperometric stantially in their binding features to the b-cyclodextrin (b- CD) receptor. The association constants of 5t and 5c to b-CD transduction of the attractive aYnity interactions. With the positively-charged electroactive substrate DOPA, the direction are Ka=1700 and 180 M-1, respectively.Accordingly, aminob- cyclodextrin (6) was synthesized and assembled on a Au of the transduced amperometric signals is reversed. Fig. 7(B) shows the cyclic voltammograms of DOPA in the presence of electrode43 (Scheme 5). The association of 5t to the b-CD receptor monolayer is reflected by the high amperometric the photoisomerizable monolayer electrode.With the SP monolayer-functionalized electrode, a high amperometric response of the electrode [Fig. 8 (curve a)]. Photoisomerization of the substrate to the cis-isomer state (5c) results in a response is observed (curve a). Photoisomerization of the monolayer to the protonated nitromerocyanine MRH+ state substantially lower amperometric response [Fig. 8 (curve b)]. By cyclic photoisomerization of the substrate between the retards the electrochemical oxidation of DOPA (curve b). Back photoisomerization of the MRH+ monolayer to the SP states 5t and 5c, reversible high and low current signals are transduced by the b-CD-functionalized electrode, respectively state regenerates the high amperometric response of the 2548 J.Mater. Chem., 1998, 8, 2543–2556Fig. 8 Cyclic voltammograms at the cyclodextrin-functionalized Au electrode of (a) [5t]=1×10-6 M and (b) [5c]=1×10-6 M in 0.01 M phosphate buVer, pH 10.8, scan rate 100 mV s-1, T=25 °C. Inset: cyclic cathodic responses of the 6 monolayer electrode measured at 0.6 V upon reversible photoisomerization of the diad between 5t and 5c, respectively.Light-controlled formation and dissociation of donor– acceptor complexes at solid supports provides an alternative route for the physical transduction of optical signals. The formation of donor–acceptor complexes between bipyridinium salts (electron acceptors) and xanthene dyes (electron donors) (e.g. eosin, Rose Bengal ) was extensively characterized in our laboratory.45 We have identified the crystal structure of these complexes and characterized the structural features of the donor–acceptor complexes in solutions using NMR spectroscopy.We reported that the xanthene dye–bipyridinium donor–acceptor complexes are stabilized by charge-transfer interactions, p–p overlap, and attractive electrostatic interactions between the electron donor and electron acceptor units.46 It was also demonstrated that the complexation features of photoisomerizable bipyridinium and bis-pyridinium Fig. 7 Cyclic voltammograms of the nitrospiropyran 2 photoisomerizable electrode in the presence of charged electroactive substrates: (A) electron acceptors to the xanthene dye are controlled by the in the presence of DHPAA (5×10-4 M). Curves (a) and (c)—mono- photoisomer state of the electron acceptor.14,44b The formation layer electrode in the nitrospiropyran state (2a).Curves (b) and (d)— and the dissociation of the supramolecular donor–acceptor monolayer electrode in the protonated merocyanine state (2b). Inset: complex between the xanthene dye and the bipyridinium unit Cyclic amperometric transduction upon reversible photoisomerization could be triggered by the light-induced transformation of the of the monolayer between the nitrospiropyran (2a) ($) and protonated latter component to photoisomers exhibiting high or low merocyanine (2b) (&) states.(B) In the presence of DOPA (1×10-4 M). Curves (a) and (c)—monolayer electrode in the nitrospir- aYnities for the electron donor, respectively. In these systems, opyran state (2a).Curves (b) and (d)—monolayer electrode in the reversible photoisomerization of the electron acceptor led to protonated merocyanine state (2b). Inset: Cyclic amperometric trans- cyclic photoswitchable formation or dissociation of the duction upon photoisomerization of the monolayer electrode between respective donor–acceptor complexes in solution. the nitrospiropyran state (2a) ($) and protonated merocyanine state From the diVerent systems that were developed in our (2b) (&).For all experiments, electrolyte compsoition 0.02 M phoslaboratory, 44 the organization of a molecular optoelectronic phate buVer, pH 7.0. Potential scan rate 200 mV s-1. Nitrospiropyran monolayer state (2a) was generated by irradiation, l>495 nm. system based on the eosin, Eo2- (7), electron donor and the Protonated merocyanine monolayer state (2b) was formed by trans- or cis-3,3¾-bis(N-methylpyridinium) azobenzenes 8t or illumination 320<l< 350 nm. 8c will be addressed.44c Fig. 9(A) shows the spectral changes of the eosin chromophore upon addition of trans-3,3¾-bis(Nmethylpyridinium) azobenzene 8t. The absorbance maximum (Fig. 8, inset).The high amperometric response of the electrode in the presence of 5t is attributed to the concentration of the Eo2- chromophore decreases, and from the spectral changes of Eo2- at diVerent concentrations of 8t the associ- of the substrate at the electrode surface by its binding to the cyclodextrin receptor. The association of 5t to the receptor ation constant of the donor–acceptor complex was derived (Ka=7.8×104 M-1).Fig. 9(B) shows the spectral changes of monolayer, and its surface confinement, is supported by the fact that the cathodic peak currents (or anodic peak currents) the eosin chromophore upon addition of cis-3,3¾-bis(N-methylpyridinium) azobenzene 8c, electron acceptor. The latter is observed in the cyclic voltammogram relate linearly to the scan rate (ip3n).It should be noted that trans-bipyridinium obtained upon photoisomerization of 8t, l=355 nm. Photoisomerization of 8t to 8c yields a photostationary state, azobenzene substrate 5t yields, upon photoisomerization to 5c, a photostationary equilibrium that includes ca. 10% of 5t. where ca. 15% of 8t is in equilibrium with 8c. From the spectral changes of the Eo2- upon addition of 8c, and knowing Thus, it was estimated that ca. 40% of the current observed in the presence of 5c originates from the residual 5t that is the composition of the photostationary equilibrium, the association constant of the donor–acceptor complex between Eo2- present in the system as a result of the photostationary equilibrium. and 8c was derived (Ka=1.4×104 M-1). J. Mater.Chem., 1998, 8, 2543–2556 2549Scheme 5 Assembly of the amino-functionalized b-cyclodextrin (6) monolayer onto a Au surface. The high aYnity of 8t to the eosin chromophore, and the formation of the resulting donor–acceptor complex, is reflected by an absorbance decrease at l=520 nm. With 8c the decrease is substantially lower. Thus, the system includes an internal optical transduction signal (absorbance) for the photoswitchable formation or dissociation of the donor–acceptor complex.Fig. 10 shows the cyclic absorbance changes of the eosin Fig. 9 (A) Spectral changes of an aqueous solution of eosin chromophore upon reversible photoisomerization of the elec- (1×10-5 M), at diVerent concentrations of added 8t corresponding to: tron acceptor between the states 8c and 8t.In the presence of (a) 0, (b) 6.6×10-6, (c) 1.32×10-5, (d) 1.96×10-5, (e) 2.6×10-5, 8c the system reveals a high absorbance (OD=0.65) while in (f ) 3.2×10-5 and (g) 3.85×10-5 M. (B) Spectral changes of an the presence of 8t the eosin absorbance is quenched (OD= aqueous solution of eosin (1×10-5 M) at diVerent concentrations of 8c corresponding to (a) 0, (b) 6.6×10-6, (c) 1.32×10-5, 0.35), indicating the formation of the supramolecular donor–- (d) 1.96×10-5, (e) 2.6×10-5, (f ) 3.2×10-5 and (g) 3.85×10-5 M.acceptor complex between eosin and 8t. By photoisomerization of the electron acceptor between the states 8t and 8c, the reversible photostimulated formation of the donor–acceptor complex and its dissociation are observed, respectively.Tailoring of molecular optoelectronic systems based on the photoswitchable complexation features of Eo2- and 8t or 8c requires the integration of the molecular components with a transducing support to the extent that the light-stimulated formation and dissociation of the respective donor–acceptor complexes are electronically transduced. Since the electron acceptors 8t or 8c are electrochemically inactive, the amperometric detection of the formation or dissociation of the complexes at electrode surfaces is impossible.To overcome this diYculty, we have applied the piezoelectric phenomenon and the microgravimetric detection of the formation and dissociation of the donor–acceptor complexes at a piezoelectric crystal as an electronic transduction means.44c For a piezo- Fig. 10 Reversible spectroscopic transduction of light-stimulated electric quartz crystal, the change in resonance frequency (Df ) dissociation and association of the photoisomerizable electron acceptors 8c and 8t to eosin. [Eo2-]=1×10-5 M, [8t]=3.8×10-5 M. upon a mass change (Dm) occurring on the crystal is given by 2550 J. Mater. Chem., 1998, 8, 2543–2556Scheme 6 Assembly of an eosin monolayer on Au electrodes associated with a quartz crystal.the Sauerbrey equation [eqn. (1)]. In this relation, fo is the basic frequency of the crystal, rq is the quartz density, mq is the Df=-C 2nfo2 (rqmq)1/2DDm=-Afo2 NrqBDm=-CfDm (1) shear-modulus of the crystal, and n is the wave number. Thus, an increase in the crystal mass by Dm is associated with a decrease of the crystal resonance frequency by -Df/Cf Hz.Similarly, a decrease in the crystal mass will be reflected by an increase in the crystal frequency. For piezoelectric quartz crystals, AT-cut, exhibiting the characteristic resonance frequency of fo=9×106 Hz, the value Cf corresponds to 2.66×108 Hz cm. Fig. 12 Cyclic frequency changes of the eosin-modified quartz crystal upon photoisomerization of the electron acceptor between states 8t The eosin electron donor was assembled on Au electrodes and 8c.All experiments were performed in an aqueous solution, [8t]= associated with a quartz crystal by the coupling of eosin [8c]=7.35×10-5 M. Frequencies were determined 10 min after inter- isothiocyanate (9) to a cystamine monolayer linked to the Au action between the functionalized crystal and the respective isomer surfaces, Scheme 6.The linkage of the eosin component to the state of the electron acceptor. base cystamine monolayer is associated with a decrease of the crystal frequency that corresponds to Df=-300 Hz. This value translates to an eosin coverage that corresponds to to a new decreased constant frequency that relates to the 1.6×10-9 mol cm-2 on the Au surface.Fig. 11(A) shows the equilibrium coverage of the monolayer by 8t. Fig. 11(B) shows crystal frequency changes upon interaction of the eosin-func- the crystal frequency changes upon challenging the eosin tionalized Au-quartz crystal with diVerent concentrations of monolayer with diVerent concentrations of 8c. Note that for 8t. As the bulk concentration of 8t is elevated, the decrease in the 8c electron acceptor, the decrease of the crystal frequency the crystal frequency is enhanced, indicating that the associ- is substantially lower as compared to the identical system with ation of the electron acceptor to the monolayer increased.For the 8t isomer. This implies that 8c exhibits a lower binding each bulk concentration of 8t, the crystal frequency levels oV aYnity to the eosin monolayer as compared to the 8t photoisomer state.Fig. 12 shows the cyclic microgravimetric, QCM transduction of the photostimulated binding of 8t to the eosin monolayer and the dissociation of the complex at the crystal interface upon photoisomerization of the electron acceptor to 8c. With 8c, a frequency decrease of ca.Df=-(12±3) Hz is observed. Photoisomerization of the electron acceptor to the 8t state results in the decrease of the crystal frequency Df= -(25±3) Hz, implying the binding of the electron acceptor to the monolayer assembly. Further isomerization of the electron acceptor to the 8c state yields the original frequency decrease characteristic for this photoisomer, indicating the release and dissociation of 8c from the monolayer interface.Using eqn. (1) and the observed frequency changes in the presence of 8t and 8c we estimate the surface coverage of the monolayer to be 2.9×10-10 and 1.4×10-10 mol cm-2 by the two electron acceptors, respectively. Molecular optoelectronic machines The previous sections addressed diVerent methods to organize molecular optoelectronic systems based on the use of a lightactive compound that is present in one of two photoisomer states.The interactions of these photoisomers with the electrode or functionalized electrode interfaces were electronically transduced and yielded the integrated molecular optoelectronic Fig. 11 Frequency changes of the eosin-modified quartz crystal upon systems. One could, however, envisage more complex assembl- interaction with diVerent concentrations of (A) [8t] corresponding to ies duplicating the functions of macroscopic machines and (a) 1.47×10-4, (b) 2.20×10-4 and (c) 2.94×10-4 M; (B) [8c] corresponding to (a) 7.35×10-5, (b) 1.47×10-4 and (c) 2.2×10-4 M. acting as molecular optoelectronic system, (MOSs).J. Mater. Chem., 1998, 8, 2543–2556 2551Macroscopic machines include several basic features that can electrode.The two bipyridinium salt configurations exhibit similar redox potentials, and thus cannot be resolved by the be adopted in tailoring of MOSs: (i) The machine is triggered by an external signal to the ‘ON’ and ‘OFF’-states, e.g. an pulsed voltammetric method. The electron transfer rates to the two electron-acceptor configurations diVer suYciently and electrical or photonic signal.(ii) The activated machine performs a dynamic process, e.g. rotation or translocation. (iii) enable the kinetic resolution of the electron transfer by chronoamperometry. Fig. 13 shows the chronoamperometric Usually, the machine exhibits the ability to transduce its position and its phase of activity. response of the generated monolayer.The current decay follows biexponential kinetics, consistent with two electron Chemistry has reached high levels of molecular architecture, ingenious synthesis and molecular assemblage. It is, however, transfer rates to the diVerent populations of the electron acceptor configurations [eqn. (2)]. The faster rate constant, a future scientific challenge to organize ‘molecular machines’.This goal involves the immobilization of molecular systems k1=900±30 s-1, is attributed to electron transfer to twopoint- attached bipyridinium salt, while the slower rate con- on a transducer interface. External signals trigger the movement, translocation or structural perturbation between distinct stant, k2=500±30 s-1, corresponds to the electron transfer to one-point attached bipyridinium salt.This explanation was molecular states that are recognized, identified and transduced by the transducer element. supported by the selective production of single-point and twopoint monolayer-linked bipyridinium relay units using a high We have shown that chronoamperometric kinetic resolution of the electron transfer rates to or from a redox-active mono- concentration or a low concentration of the bipyridinium salt in the modification procedure, respectively.These experimental layer, associated with an electrode, provides a sensitive means to characterize the composition and configuration of the electroactive monolayer.47 For a single kind of redox-active monolayer associated with an electrode, the current transient upon the application of a reductive (or oxidative) potential step follows an exponential decay with a time constant that corresponds to the interfacial electron-transfer rate constant.For two very similar redox species in the monolayer array that diVer slightly in their chemical structure or position, the current transient upon the application of the potential step is expected to follow a biexponential decay [eqn.(2)], where k1 and k2 and Q1 and Q2 are the interfacial electron transfer rates, and the monolayer coverage of the two redox-active species, respectively. For example, I (t)=(Q1k1)e-kit+(Q2k2)e-k2t (2) N,N¾-bis(carboxyalkyl )-4,4¾-bipyridinium (10) was coupled to a base cystamine monolayer linked to a Au electrode, Scheme 7. The synthesis of the monolayer yields two modes Fig. 13 Current response of the Au electrode modified with N,N¾- of attachment of the bipyridinium salt to the monolayer. One bis(7-carboxyheptyl )-4,4¾-bipyridinium monolayer rigidified with mode includes the one-point attachment of the redox-active C14SH following a potential step from -0.3 to -0.69 V. Electrolyte unit to the monolayer, while the second mode involves the composition 0.1 M phosphate buVer, pH 7.0.Inset: semilogarithmic plot for the current response. two-point, bridged, linkage of the electron acceptor to the Scheme 7 Assembly of N,N¾-bis(7-carboxyheptyl )-4,4¾-bipyridinium as a monolayer via single- and double-point coupling to a cystaminemodified Au electrode. 2552 J. Mater. Chem., 1998, 8, 2543–2556increasing the distance between the redox-active unit and the electrode surface retards the electron-transfer rate.Thus, the chronoamperometric response of a signal-translocated electroactive component on a molecular network is anticipated to transduce its steric position in the assembly. The molecular optoelectronic system mimicking functions of a molecular machine is schematically described in Scheme 8, where a light-activated ‘molecular train’ is assembled on an electrode.48 The assembly consists of a ‘station’, A, and a ‘railway’, B, linked to the conductive support.The molecular component, ‘C’, acting as the ‘locomotive’ is threaded onto the ‘railway-station’ assembly and protected by a barrier (‘stopper’) from dissociation. The preferred ‘locomotive’ position is at the station due to aYnity interactions. Photoisomerization of the station to a state that lacks aYnity for the ‘locomotive’ component results in its translocation to the ‘railway’ position.Back photoisomerization of the station unit to its original state restores the high-aYnity site for the ‘locomotive’ and regenerates the primary state of the moveable molecular assembly.Provided the threaded molecular shuttle is redox-active, then its chronoamperometric response provides a means for the transduction of the ‘locomotive’ position on the molecular ‘railway-station’ wire. The molecular shuttle or ‘locomotive’ is ferrocenylcarboxamide –b-cyclodextrin, Fc–b-CD (11). This substrate includes Scheme 8 Assembly of a nanoscale molecular assembly acting as a the b-cyclodextrin receptor site and the redox-active ferrocene light-stimulated ‘molecular-train’.label. Scheme 9 shows the assembly of the ‘molecular train’. 4,4¾-Dicarboxy-trans-azobenzene was covalently coupled to a cystamine monolayer associated with a Au electrode. 1,12- conditions favor the single-point and two-point linkage modes, Diaminododecane was then linked to the azobenzene unit.respectively, and accordingly led to the faster and slower single The trans-azobenzene alkyl chain molecular wire monolayer exponential electron transfer rates of the resulting monolayers. includes the trans-azobenzene site, exhibiting a high-aYnity This approach of kinetic resolution of electron transfer rates binding site for b-cyclodextrin, and the alkyl chain component at redox-active monolayers was applied to characterize various of lower binding properties to b-cyclodextrin.In order to electroactive composite monolayers. As the electron transfer generate a single molecular shuttle on the ‘railway-station’ rate is sensitive to the position of the redox-active component array, the trans-azobenzene site was photoisomerized to the with respect to the electrode surface, any structural change or cis-azobenzene state.The receptor Fc–b-CD was then threaded translocation of the electroactive unit is anticipated to be reflected by the interfacial electron transfer rate. In general, onto the alkyl-chain, and the supramolecular complex was Scheme 9 Assembly of a monolayer consisting of ferrocene-functionalized b-cyclodextrin threaded onto an azobenzene-alkyl chain monolayer and blocked with an anthracene barrier on a Au electrode.J. Mater. Chem., 1998, 8, 2543–2556 2553Fig. 14 Cyclic voltammograms of the assembly consisting of ferrocenefunctionalized b-cyclodextrin threaded onto the azobenzene-alkyl chain and blocked by the anthracene barrier. (a) Monolayer in the cis-azobenzene configuration.(b) Monolayer in the trans-azobenzene configuration. Data recorded in 0.1 M phosphate buVer, pH 7.3. Scan rate 500 mV s-1. stoppered by coupling of anthracene-9-carboxylic acid to the amino function of the ‘railway’ element. The organization of the supramolecular monolayer assembly was confirmed by electrochemical means (Fig. 14). The cyclic voltammogram of the monolayer consists of the two-electron and two-proton reduction of the trans-azobenzene to hydrazobenzene (and the respective oxidation process), and the one-electron redox process of the ferrocene unit at E°= 0.31 V vs.SCE of the threaded Fc–b-CD. Fig. 15 (A) Chronoamperometric responses of the assembly consisting Scheme 10 shows schematically the light-induced of the ferrocene-functionalized b-cyclodextrin threaded onto the azotranslocation of the Fc–b-CD unit on the trans-azobenzene benzene-alkyl chain and blocked by the anthracene barrier.(a) Monolayer in trans-azobenzene state. (b) Monolayer in cis- alkyl chain path, and the electrochemical transduction of the azobenzene state. (B) Cyclic variation of the electron transfer rates of position of the ‘molecular shuttle’ in the system.In the transthe ferrocene-functionalized b-cyclodextrin in the molecular ‘train- azobenzene state of the molecular wire, the ‘locomotive’, Fc–bshuttle’ upon reversible photoisomerization of the monolayer: (+) CD, is associated to the azobenzene ‘station’. The close monolayer in trans-azobenzene state, ($) monolayer in cis-azobenposition of the redox label to the electrode surface is expected zene state. to yield a fast interfacial electron transfer.Photoisomerization of the trans-azobenzene unit to cis-azobenzene is expected to in the presence of the trans-azobenzene monolayer assembly. translocate Fc–b-CD to the alkyl chain, or ‘railway position’. A fast current decay, ket(1)=65 s-1, is observed, implying that Spatial separation of the ferrocene redox label with respect to the molecular shuttle is close to the electrode surface.the electrode surface is anticipated to retard the interfacial Photoisomerization of the monolayer to the cis-azobenzene electron transfer rate. Fig. 15(A) (curve a) shows the configuration, 320<l<380 nm, results, for Fc–b-CD, in the chronoamperometric response of the ferrocene–b-cyclodextrin chronoamperometric transient shown in Fig. 15(A), (curve b). A substantially lower electron transfer rate constant is observed (ket(2)=15 s-1) indicating that the threaded receptor is positioned on the monolayer array in a spatially separated con- figuration with respect to the electrode surface. This is consistent with the translocation of the ferrocene–b-cyclodextrin from the trans-azobenzene ‘station’-position to the alkyl chain ‘railway’ site.Further photoisomerization of the cisazobenzene unit to the trans-azobenzene configuration, l>420 nm, restores the chronoamperometric response, characteristic for the receptor positioned on the trans-azobenzene unit. By cyclic photoisomerization of the monolayer between the trans-azobenzene and cis-azobenzene configurations, the threaded receptor is reversibly moved between the transazobenzene ‘station’ site and the alkyl chain ‘railway’ component [Fig. 15(B)]. The controlled light-stimulated dynamic translocation of the receptor between the two monolayer states is electronically transduced by the chronoamperometric response of the redox-labeled receptor. Conclusions and perspectives Scheme 10 Photoinduced translocation of ferrocene-functionalized The present article summarizes our recent research activities b-cyclodextrin between the azobenzene-alkyl chain sites by cyclic lightinduced isomerization of the monolayer.in tailoring molecular optoelectronic assemblies where light 2554 J. Mater. Chem., 1998, 8, 2543–2556signals are recorded in the form of a chemical event on a References monolayer assembly associated with a conductive support. 1 (a) J.-M. Lehn, Angew. Chem., Int. Ed. Engl., 1990, 102, 1347; The electronic transduction of the chemical feature stored in (b) F. L. Carter, A. Schultz and D. Duckworth, in Molecular the monolayer provides a means to read the stored photonic Electronic Devices, ed. F. L. Carter, Marcel Dekker, New York, information.The reversible photoisomerizable properties of 1987, p. 183. the monolayer allow us to erase the stored information, and 2 (a) V. Balzani, L. Moggi and F. Scandola, in Supramolecular to reactivate the monolayer interface for the renewal of optical Photochemistry, ed. V. Balzani, Reidel, Dordrecht, 1987, pp. 1–28; storage. Thus, the systems reveal ‘Write-Read-Store’ features.(b) Molecular Electronic Devices, ed. F. L. Carter, R. E. Siatkowsky and H. Woltjien, Elsevier, Amsterdam, 1988; To date, photoswitchable molecular assemblies have been (c) P. Ball, Nature, 1993, 362, 123. tailored in the solution phase, and the state of the molecular 3 A. P. DeSilva, H. Q. N. Gunaratne and C. P. McCoy, Nature, switch was elucidated by spectroscopic assay, e.g. NMR 1993, 364, 42.analysis. Our scientific eVorts are aimed to integrate the 4 P. Ball and L. Garwin, Nature, 1992, 355, 761. photoswitchable molecular features with a solid support that 5 D. Bradley, Science, 1993, 259, 890. permits the electronic transduction of the photonic infor- 6 (a) R. A. Bissell, A. P. DeSilva, H. Q. N. Gunaratne, P. L. M. Lynch, G. E. M.Maguire, C. P. McCoy and mation. This is a desirable evolutionary step from solution K. R. A. S. Sandanayake, Top. Curr. Chem., 1993, 168, 223–264; photochemistry to an optoelectronic device.We have addressed (b) W. Go� pel and P. Heiduschka, Biosens. Bioelectron., 1995, 10, two electronic transduction means of the stored optical infor- 853. mation, e.g. amperometric and piezoelectric signaling.Other 7 (a) I. Willner, E. Katz and B. Willner, Electroanalysis, 1997, 9, transduction signals, such as impedance, surface plasmon 965; (b) E. Katz, V. Heleg-Shabtai, B. Willner and I. Willner, resonance (SPR) or ellipsometry, can be envisaged to follow Bioelectrochem. Bioenerg., 1997, 42, 95; (c) I. Willner, R. Blonder, the photoswitchable properties at the functionalized solid E.Katz, A. Stocker and A. F. Bu�ckmann, J. Am. Chem. Soc., 1996, 118, 5310. surface. We have followed several methods to design and tailor 8 K. E. Drexler, Nanosystems, Molecular Machinery, Manufacturing molecular optoelectronic systems. These included the organizand Computation, Wiley, New York, 1992. ation of photochemically-triggered ‘command surfaces’ that 9 (a) G.De Santis, L. Fabbrizzi, M. Licchelli, M. Pallavicini, control interfacial electron transfer, the functionalization of A. Perotti and A. Poggi, Supramol. Chem., 1994, 3, 115; electrodes with receptor sites that control the association (b) L. Fabbrizzi and A. Poggi, Chem. Soc. Rev., 1995, 24, 197. of photoisomerizable molecular species with the modified sur- 10 (a) J. D. Winkler, K.Deshayes and B. Shao, in Bioorganic Photochemistry–Biological Application of Photochemical Switches, face, and the assembly of photochemically redox-activated ed. H. Morrison, Wiley, New York, 1993, vol. 2, p. 169; monolayer systems. (b) J. Rosengaus and I. Willner, J. Phys. Org. Chem. 1995, 8, 54; Finally, we have demonstrated that the concept of molecular (c) F. Wu� rthner and J.Rebek, Jr., Angew. Chem., Int. Ed. Engl., optoelectronics can be expanded from static photoswitchable 1995, 34, 446. assemblies to dynamic photochemically-triggered molecular 11 (a) S. Shinkai, T. Minami, T. Kusano and O. Manabe, J. Am. systems. This has been exemplified by the tailoring of a Chem. Soc. , 1982, 104, 1967; (b) P. R. Ashton, R. Ballardini, ‘molecular train’ that includes a ‘station’ and a ‘railway’, and V.Balzani, A. Credi, M. T. Gandolfi, S. Menzer, L. Pe�rez-Garcý�a, L. Prodi, J. F. Stoddart, M. Venturi, A. J. P. White and a molecular ‘locomotive’ is translocated by light signals D. J. Williams, J. Am. Chem. Soc., 1995, 117, 11171. between the ‘station-railway’ sites. The molecular ‘locomotive’ 12 (a) S. L. Gilat, S. H. Kawai and J.-M.Lehn, Chem. Eur. J., 1995, electrically transduces its position on the molecular array. 1, 275; (b) S. H. Kawai, S. L. Gilat, R. Ponsinet and J.-M. Lehn, Such dynamic molecular assemblies, where light signals acti- Chem. Eur. J., 1995, 1, 285; (c) S. Liu, K. Hashimoto and vate molecular motion that is transduced to the macroscopic A. Fujishima, Nature, 1990, 347, 658; (d) K.Morigaki, Z. Liu, environment, represent the first steps towards ‘molecular K. Hashimoto and A. Fujishima, J. Phys. Chem., 1995, 99, 14771. 13 R. Blonder, E. Katz, I. Willner, V. Wray and A. F. Bu�ckmann, machinery’. Based on our results, one could envisage more J. Am. Chem. Soc., 1997, 119, 11747. complex assemblies that include several ‘stations’ and a ‘rail- 14 I. Willner, S.Marx and Y. Eichen, Angew. Chem., Int. Ed. Engl., way’, complementary electro- and opto-activation of the mol- 1992, 31, 1243. ecular translocation, and application of intermediary 15 (a) S. H. Kawai, S. L. Gilat and J.-M. Lehn, J. Chem. Soc., Chem. photosensitizer sites as signaling sites for the dynamic Commun., 1994, 1011; (b) L. Zelikovich, J. Libman and processes. A. Shanzer, Nature, 1995, 374, 790.The architecture of the molecular systems on the solid 16 T. R. Kelly, M. C. Bowyer, K. V. Bashkar, D. Debbington, A. Garcia, F. Lang, M. H. Kim and M. P. Jette, J. Am. Chem. supports involved the assemblies of photoactive monolayers. Soc., 1994, 116. 3657. The monolayer represents a two-dimensional array on the 17 T. R. Kelly, I. Tellitu and J. P. Sestelo, Angew.Chem., Int. Ed. solid support. The rapid progress in the development of Engl., 1997, 36, 1866. scanning nano-scale irradiation sources, e.g. scanning near- 18 (a) R.A. Bissell, E. Cordova, A. E. Kaifer and J. F. Stoddart, field optical microscope (SNOM), and the well-established Nature, 1994, 369, 133; (b) D. Philp and J. F. Stoddart, Synlett. scanning techniques to image a redox activated or conductive 1991, 445; (c) A.C. Benniston, A. Harriman and V. M. Lynch, interface, e.g. by STM or scanning electrochemical microscopy, J. Am. Chem. Soc., 1991, 117, 5275. 19 E. Zahavy and M. A. Fox, Chem. Eur. J., 1998, 4, 1647. open the way to generate addressable optoelectronic 2M. Huck and B. L. Feringa, J. Chem. Soc., Chem. nanostructures. Commun., 1995, 1095; (b) B.L. Feringa, W. F. Jager and B. de The future perspectives of this challenging project of Lange, J. Am. Chem. Soc., 1991, 113, 5468. molecular optoelectronics rests on the mandatory need for an 21 (a) E. Katz and I. Willner, Electroanalysis, 1995, 7, 417; interdisciplinary eVort that combines chemistry, physics, elec- (b) I. Moriguchi, K. Hanai, A. Hoshikuma, Y.Teraoka and S. tronics and material science. Kagawa, Chem. Lett., 1994, 691. 22 (a) E. Katz, M. Lion-Dagan and I. Willner, I. J. Electroanal. Chem., 1996, 408, 107; (b) E. Katz, A. L. De Lacey and The support of this study by The Israel Science Foundation V. M. Fernandez, J. Electroanal. Chem., 1993, 358, 261; (c) N. Nakashima and T. Tagushi, Colloids Surf. A., 1995, 103, founded by The Israel Academy for Science and Humanities, 159.and by The Israel Ministry of Science (MOS) under the 23 (a) S. Shinkai, T. Minami, T. Kusano and O. Manabe, J. Am. Infrastructure Project on Novel Materials and Functions, is Chem. Soc., 1982, 104, 1967; (b) M. Blank, L. M. Soo, gratefully acknowledged. Special thanks are directed to my N. H. Wassermann and B. F. Erlangler, Science, 1981, 214, 70.coworkers who contributed their skills, enthusiasm and eVorts 24 (a)M. P. Debreczeny, W. A. Svec and M. R.Wasielewski, Science, to the success of the project. Their names appear in the 1996, 274, 584; (b) R. W. Wagner, J. S. Lindsey, J. Seth. V. Palaniappan and D. F. Bocian, J. Am. Chem. Soc., 1996, 118, reference list. J. Mater. Chem., 1998, 8, 2543–2556 25553996; (c) L.De Cola, V. Balzani, F. Barigelletti, L. Flamigni, 35 I. Willner, G. Arad and E. Katz, Bioelectrochem. Bioenerg., 1998, 44, 209. P. Belser, A. Von Zelewsky, M. Frank and F. Vo� gtle, Inorg. Chem., 1993, 32, 5228. 36 I. Willner and B. Willner, Adv. Mater., 1997, 9, 351. 37 (a) A. Doron, M. Portnoy, M. Lion-Dagan, E. Katz and 25 A. Credi, V. Balzani, S. J.Langford and J. F. Stoddart, J. Am. I. Willner, J. Am. Chem. Soc., 1996, 118, 8937; (b) A. Doron, Chem. Soc., 1997, 119, 2679. E. Katz, M. Portnoy and I. Willner, Angew. Chem., Int. Ed. Engl., 26 (a) J. Daub, J. Salbeck, T. Kno� chel, C. Fisher, H. Kunkely and 1996, 35, 1535. K. M. Rapp, Angew. Chem., Int. Ed. Engl., 1989, 28, 1494; 38 A. T. Hubbard, Heterog. Chem. Rev., 1994, 1, 3. (b) J. Daub, C. Fischer, J. Salbeck and K. Ulrich, Adv. Mater., 39 (a) E. Katz, N. Itzhak and I. Willner, J. Electroanal. Chem., 1992, 1990, 2, 366; (c) J. Achatz, C. Fischer, J. Salbeck and J. Daub, 336, 357; (b) E. Katz, N. Itzhak and I. Willner, Langmuir, 1993, J. Chem. Soc., Chem. Commun., 1991, 504. 9, 1392. 27 A. Ulman, An Introduction to Ultrathin Organic Films from 40 (a) R. T. Lane and A. T. Hubbard, J. Phys. Chem., 1973, 77, 1411; Langmuir–Blodgett to Self-Assembly, Academic Press, San Diego, (b) K. Takehara and Y. Ide, Y. Bioelectrochem. Bioenerg. 1992, 1991. 27, 207. 28 (a) H. O. Finklea, in Electroanalytical Chemistry, vol. 19, ed. 41 F. Malem and D. Mandler, Anal. Chem., 1993, 65, 37. A. J. Bard and I. Rubinstein, Marcel Dekker, New York, 1996, 42 A. Doron, E. Katz, G. Tao and I. Willner, Langmuir, 1997, 13, pp. 109–335; (b) G. M. Whitesides and P. E. Laibinis, Langmuir, 1783. 1990, 6, 87. 43 (a) M. Lahav, K. T. Ranjit, E. Katz and I. Willner, Chem. 29 L. H. Dubois and R. G. Nuzzo, Annu. Rev. Phys. Chem., 1992, Commun., 1997, 259; (b) M. Lahav, K. T. Ranjit, E. Katz and I. 43, 437. Willner, Isr. J. Chem., 1997, 37, 185. 30 (a) I. Willner, Acc. Chem. Res., 1997, 30, 347; (b) I. Willner and 44 (a) S. Marx-Tibbon, I. Ben-Dov and I. Willner, J. Am. Chem. S. Rubin, Angew. Chem., Int. Ed. Engl., 1996, 35, 367. Soc., 1996, 118, 4717; (b) K. T. Ranjit, S. Marx-Tibbon, I. Ben- 31 I. Willner and B. Willner, Adv. Mater. 1995, 7, 587. Dov, B. Willner and I. Willner, Isr. J. Chem., 1996, 36, 407; 32 (a) I. Willner, M. Lion-Dagan, S. Marx-Tibbon and E. Katz, (c) K. T. Ranjit, S. Marx-Tibbon, I. Ben-Dov and I. Willner, J. Am. Chem. Soc., 1995, 117, 6581; (b) M. Lion-Dagan, S. Marx- Angew. Chem., Int. Ed. Engl., 1997, 36, 147. Tibbon, E. Katz and I. Willner, Angew. Chem., Int. Ed. Engl., 45 I. Willner, Y. Eichen, M. Rabinovitz, R. HoVman and S. Cohen, 1995, 34, 1604. J. Am. Chem. Soc., 1992, 114, 637. 33 M. Lion-Dagan, E. Katz and I. Willner, J. Chem. Soc., Chem. 46 I. Willner, Y. Eichen, A. Doron and S. Marx, Isr. J. Chem., 1992, Commun., 1994, 2741. 32, 53. 34 (a) I.Willner, R. Blonder and A. Dagan, J. Am. Chem. Soc., 1994, 47 E. Katz and I. Willner, Langmuir, 1997, 13, 3364. 116, 9365; (b) R. Blonder, S. Levi, G. Tao, I. Ben-Dov and 48 I. Willner, V. Pardo-Yissar, E. Katz, K. T. Ranjit, submitted for I. Willner, J. Am. Chem. Soc., 1997, 119, 10467; (c) A. Bardea, publication. A. Dagan, I. Ben-Dov, B. Amit and I. Willner, Chem. Commun., 1998, 839; (d) A. Bardea, E. Katz, A. F. Bu�ckmann and I. Willner, J. Am. Chem. Soc., 1997, 119, 9114. Feature Article 8/04135K 2556 J. Mater. Chem., 1998, 8, 2543&ndas

ISSN:0959-9428    

DOI:10.1039/a804135k

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Feature Article Inorganic lyotropic liquid crystals A. S. Sonin A. I. Nesmeyanov Institute of Elemento-Organic Compounds, Russian Academy of Sciences, Vavilova Str. 28, 117813 Moscow, Russia Received 7th April 1998, Accepted 5th May 1998 A detailed review of inorganic lyotropic liquid crystals gence when a flow was created in this dispersion by shaking (water or water–organic dispersions of certain non-organic of a retort or by the sol preparation.substances) is presented. The history of the discovery of Soon afterwards, Freundlich showed11 that V2O5 sols also these mesophases is described and their classification is become optically anisotropic in electric fields. Moreover, he considered. The structure and properties of all liquid proved11,12 that the Majorana eVect and the appearance of crystals of this class known to date are analysed in detail.optical anisotropy in an electric field and under a flow was caused by the same reason—the orientation of the anisometrical particles with their long axes in the direction of the flow Introduction or of the electric and magnetic field strength. The sol samples Liquid crystals were discovered among organic compounds were found to behave like optically uni-axial positively charged more than a century ago.In the beginning, mesophases were crystals with considerable dichroism. After switching oV the found by melting of cholesterol ethers and nitro compounds,1 orientation-inducing action, the sols returned to their initial and later by dissolving the long-chain acid salts.2 The former optically isotropic state.substances were named ‘thermotropic’, and the latter It was possible to observe the V2O5 sol particles with the ‘lyotropic’ liquid crystals. These mesophases were found to help of an ultramicroscope. They had the appearance of long exhibit orientational order in a certain range of temperatures sticks, with the length-to-width ratio equal to approximately (for thermotropic substances) and/or concentrations (for 1051.Freundlich called these particles ‘swarms’, in analogy lyotropic ones).3 with ‘swarms’ in liquid crystals, the existence of which was Over the following century more than 10 000 diVerent liquid considered to be proved at this time. crystals were discovered. The great majority of these substances The analogy between sols with induced optical anisotropy are organic compounds with highly variable chemical composi- and thermotropic liquid crystals was specially discussed by tions.This circumstance usually does not provoke any misun- Freundlich.11,12 He also pointed out the ‘similarity’ of both derstanding, since one of the necessary conditions of the phenomena, since liquid crystals were found to behave in a mesomorphism—the anisosymmetry of molecules—may, in magnetic field analogously to sols.However, he stressed that fact, be most easily realised in organic compounds. there were considerable diVerences between these two classes The anisosymmetry of forms of the structural units is equally of substances. Firstly, thermotropic liquid crystals are singlenecessary for formation of both thermotropic and lyotropic phase systems, while sols are two-component compounds.mesophases. Separate molecules usually serve as structural Secondly, liquid crystals form homogeneously oriented, units for thermotropic liquid crystals, while molecular aggre- optically anisotropic samples between two glass walls in a gates (micelles and columns) usually play this role for lyotropic plane-parallel capillary, while sols do not form such samples. liquid crystals.In some cases, the structural units may be Thirdly, in the initial state, sols are transparent, while liquid represented by single molecules of quite large size, oligomers crystals are opaque. Fourthly, sols acquire an optical aniand living organisms (viruses).Such systems are typically sotropy when flowing, while thermotropic liquid crystals of pcolloidal. azoxyanisol and p-azoxyphenol, as the Freundlich experiments In addition, there are quite a lot of colloidal systems with showed, do not exhibit any anisotropy.† However, he observed non-organic dispersion phases. Therefore, it should be logical optical birefringence in homeotropic samples of these subto expect that, if a non-organic dispersive phase of this kind stances, upon displacing the upper glass of a cell.consists of anisometric particles, then such a colloidal system From all the results mentioned above, Freundlich made the will exhibit liquid-crystalline properties. following prudent statement: ‘Everybody now comes to the conclusion that the V2O5 sol, in general, from diVerent points of view, is the magnified model of an anisotropic melt (i.e.of a thermotropic liquid crystal—A.S.)’.11 History The question of the nature of the ordered particles—the The historical idea of the possibility of existence of inorganic structural units of sols—had already arisen in these pioneering liquid crystals was formed gradually, mainly due to studies of works.This issue was discussed during the debates on the induced optical anisotropy in sols. This is the so-called Freundlich report at the meeting of the German Majorana eVect, which manifested itself by the appearance of Electrochemical Society.11 Freundlich himself considered this a birefringence in some sols under the action of a magnetic question to be unclear, since in spite of the proven (by the field4 (Majorana himself worked with FeOOH).The studies microscopic observations) crystalline nature of these particles, of this phenomenon5–9 showed that sols consisting of aniso- some amorphous residue appeared during coagulation. metric particles can be orientationally ordered under the action However, this fact, as it was pointed out by Zigmondi,11 did of a magnetic field.However, after switching oV the magnetic not contradict the statement about the crystalline character of field, the orientational order was destroyed by thermal motion. the sol particles. He insisted on this latter statement. A second way to obtain induced optical anisotropy of sols— orientation by flow—was found in 1915.10 It was shown that †In reality, as shown by Zo� cher,15 thermotropic liquid crystals can be oriented by a flow, and, then, their optical anisotropy increases.sols of vanadium pentoxide (V2O5) acquired strong birefrin- J. Mater. Chem., 1998, 8, 2557–2574 2557The ultramicroscopy studies13,14 proved, in general, the Zigmondi point of view. It was shown that, in the V2O5 sols, the structural units were elongated particles of a clearly crystalline character, with length 7 mm and width 0.5–2 mm.The particle length reached up to 19 mm in old sols (3.5 years). In an isotropic sol these particles were not situated chaotically, but in small areas were oriented parallel to each other, without intersections. The particle forms and sizes were also determined by means of an ultramicroscope in some other sols (mercury chloride, HgCl; lead iodide, PbI2) exhibiting birefringence in a flow.It was shown that in the first sol they were puck-shaped with a diameter of about 1 mm, while in the second sol they were needle-shaped with lengths between 0.25 and 2 mm. Inorganic sols with induced optical anisotropy were further investigated in detail by Zo�cher et al.15,16 They found the appearance of birefringence in flowing sols of AgCNO, in a clay suspension and in a sequence of mesogenic organic compounds.15 V2O5 was studied quantitatively in detail.16–18 The dependences of birefringence and dichroism upon the flow rate, temperature, sol concentration and age were measured in a pipe with a rectangular cross-section and between turning cylinders.The essential result was the proof of the fact that the freshly prepared V2O5 sol did not exhibit induced birefrin- Fig. 1 Tactoids in a vanadium pentoxide dispersion.21 gence; however, the birefringence developed with time. The ageing velocity was found to be described by a second order equation. This velocity increased sharply with increasing erature.The induced birefringence increased linearly with increasing flow rate and decreased linearly with increasing temperature. It was found, while studying induced birefringence,19 that old V2O5 sols began to rearrange into layered structures as time passed. The upper sol part (about two thirds of a sample) remained isotropic, while the lower part became strongly birefringent.At the same time, the concentration of the substance in the lower part increased, while in the upper part it decreased. Thus, a sol having an initial concentration of 1.43 mass%, after 9 days became arranged in layers so that the concentration in its upper part became 2.14 mass%, and that in its lower part became 1.39 mass%.20 If one mixes drops of the upper and lower phases, and observes the obtained isotropic sample under a polarising microscope, after 0.5–2 hours the optically anisotropic areas— linear areas of a spindle-like form, named tactoids20—will appear in a sample.20–24 They are clearly distinguished in the polarised-light micrographs (Fig. 1). Anisotropic areas, analogous in many features to tactoids, are also found in other inorganic sols: H2WO420 and Fig. 2 A drop with steps in an iron oxyhydroxide dispersion.21 FeOOH.21‡ The latter, in contrast to V2O5, possesses rainbow colours. This is due to the layered structure of such sols—the distance between these layers is of the order of magnitude of classical work:21 ‘In conclusion, one should note another surprising analogy. The aYnity between the birefringent sols the wavelength of visible light.21 This layered character is clearly visible in Fig. 2, where a micrograph of a 5 mass%, 15 and liquid crystals (mesomorphic substances, according to Friedel ) is so big, that, under a superficial consideration, one year old FeOOH sol is presented. This micrograph is surprisingly similar to the textures of smectic drops with steps. The can be taken for another. This is even more amazing, since two kinds of these sols correspond to the two main types of latter are reproduced in numerous textbooks on liquid crystals as proof of the layered structure of smectics.mesomorphic substances, studied by Friedel. The first, nematic type, has the structure with the long axes of its molecules The V2O5 tactoids, in contrast, look like nematic drops nucleating in an isotropic melt.However, in thermotropic situated parallel to each other. The second, smectic type, has the structure, where molecules are not only parallel, but are nematics, such drops have a spherical form; nevertheless, the optical anisotropy of tactoids leaves no doubt concerning the separated by equal distances in the direction of their long axes. The analogue to the first type is the structure of V2O5, similarity of these structures to those of nematic liquid crystals.In spite of that fact, Zo�cher and his co-authors did not the analogue to the second is the structure of FeOOH’. The same point of view was expressed by Zo�cher later, in venture to identify, in their early works on tactoids, anisotropic inorganic sols (they were called tactosols) with liquid crystals. 1954, in his special work ‘Tactosols and Mesophases’.26 He supposed that tactosols were similar to mesophases only This is what Zo� cher, himself, wrote in this connection in his phenomenologically, while microscopically these two classes of compounds were absolutely diVerent.Thus, ‘these both ‡Tactoids were found for a series of inorganic sols, such as benzopurkinds of anisotropic media were, indeed, totally diVerent’ puryne 4B and chryzophenine,21 and also for water solutions of the tobacco mosaic virus.25 (ref. 26, p. 85). 2558 J. Mater. Chem., 1998, 8, 2557–2574Only towards the end of the 1960s did Zo�cher realise27,28 gel. The concentration is usually determined by drying the the necessity to recognise the actual identity of anisotropic samples at a temperature of 500 °C.For its expression, besides inorganic sols and liquid crystals. However, he had classified the usual chemical units, one commonly uses the number of these sols separately, and called them ‘phases of a higher water molecules n which correspond to one vanadium pentoxorder’ or ‘superphases’. The reasons for this, according to ide molecule—V2O5·nH2O.Zo� cher, were not only the much larger size of structural units The critical concentration, which corresponds to the transin sols, but also the diVerent nature of forces acting between ition from sol to gel, equals approximately 0.2 mol l-1=18 the sol particles. In classical thermotropic liquid crystals, mass%=V2O5·250H2O. Below this concentration, a colloidal dispersion and repulsion forces prevail, while in superphases solution has the properties of a sol, above it has those of a gel.30 the forces of electrical ions (present in water) play a consider- Methods used to obtain vanadium pentoxide sols and gels able role together with dispersive interactions.are extremely variable. Some of them can be utilised to obtain Nevertheless, superphases, according to Zo� cher, should be either one phase or the other.attributed, with certain reservations, to liquid crystals. These Four main techniques are most commonly used. The first are his exact words: ‘Due to the close analogy, it is, probably, one is that originally elaborated by Biltz.31 This involves inevitable to include nematic and smectic superphases into the preparation of a V2O5 dispersion in water.V2O5 is obtained domain of liquid crystals. Nevertheless, the physico-chemical by means of treatment of ammonium vanadate with hydronature of superphases strongly diVers from the nature of low- chloric acid. The obtained colloidal solution is called a Biltz molecular organic substances, having the mesophases. The sol. The second method is preparation of a dispersion of latter can be obtained by changing the temperature and melted ammonium vanadate or vanadium pentoxide in water.concentration, the former—only by changing the The obtained solution is called a Mu� ller sol.32 The third concentration’.27 technique is to let vanadium salt solutions pass through an In fact, as we will see later, the influence of temperature on ion-exchange column.As a result, decavanadium oxide is the phase behaviour of inorganic liquid crystals is not consider- obtained. Then, this acid is decomposed in boiling water.33 able. However, the thermal behaviour is not the main feature Sometimes, the fourth method of Prandtl and Hess34 is utilised. of mesophases. The principle feature is that both inorganic This is the hydrolysis of the isoamyl ether of orthovanadium lyotropic superphases and classical thermotropic and lyotropic acid.This procedure allows one to obtain monodisperse sols. liquid crystals form, under certain conditions, thermo- X-Ray,35–41 electron diVraction,42,43 neutron diVraction,40,44 dynamically stable phases, which exhibit long-range orien- and spectroscopic45–48 studies together with electron microtational order.scopic observations49–52 have shown that monocrystals in the A boom in investigation and application of liquid crystals, form of small elongated solid particles, and/or of long flexible which began in the 1960s, absolutely did not include the class fibres or ribbons, depending on their formation conditions, of inorganic lyotropic mesophases. Until very recently they may serve as the structural units in sols and gels of vanadium seemingly did not exist, they were absolutely forgotten.It is pentoxide. suYcient to say that there is not any, even the slightest, The crystalline nature of these particles is clearly visible mention of inorganic lyotropic mesophases in numerous text- from the electron diVraction patterns (Fig. 3), obtained for books and monographs on liquid crystals.Only in the last few the Biltz, Mu� ller and ion-exchange sols.42 These electron years has there appeared a certain interest in their properties. diVraction patterns coincide, in their main features, with the It was shown that not only the substances studied by Zo� cher, ones obd V2O5 monocrystal.35–37 but also some other dispersions of inorganic materials pos- In most cases, freshly prepared V2O5 sols are optically sessed a liquid-crystalline structure.isotropic. The particles of such sols are amorphous45,53 and Consider now the properties of these mesophases in more have a maximum size of 100 A° . However, the crystallisation detail. The currently known inorganic dispersions, in which process begins after several hours—monocrystals are being the existence of mesophases has been proved, the mesophase formed.These monocrystals continue to grow later. This types and areas of concentration, are shown in Table 1. phenomenon is called an ‘ageing process’. Upon ageing, sol In addition to the substances summarised in Table 1, there particles reach a critical size, which makes their orientational is information about the existence of mesophases in water ordering possible. Then, birefringence appears, and diVerent dispersions of some mineral clays, such as montmorillonite or characteristic textures in the sample form (see below). bentonite and also in graphite acid and potassium phosphate.Sols age more rapidly under heating, or when an electrolyte (0.1 M NaCl, NH4Cl, CaCl2 or K2SO4) is added.23 However, the main factor which influences the ageing process is increas- Nematics ing in the V2O5 concentration.20 Vanadium pentoxide The V2O5 sol ageing process was mainly studied either by optical13,14 and electron49–52 microscopy, or by X-ray We will start the description of nematics with the classic methods.30,35,36 inorganic lyotropic liquid crystal—vanadium pentoxide. Its A freshly prepared Biltz sol (with a concentration of about colloidal solutions have been studied in detail for many years, 1 mass%) is practically monodisperse, with particles of length but not in connection with their liquid-crystalline properties.29 20–40 nm.After one month, this sol becomes polydisperse, It was shown that, depending on the V2O5 concentration, a colloidal solution exhibited the properties of either a sol or a and the maximum length of its particles reaches 0.2–0.3 mm; Table 1 Inorganic liquid crystals Dispersion phase Dispersion medium Concentration/mass% Type of mesophase V2O5 H2O 2–15 Nematic AlOOH H2O 1.5–3 Nematic Li2Mo6Se6 CH3CONH2 4–10 Nematic UO2F2 CH3COCH3+H2O 8–30 Nematic SiO2 Al2O3 H2O (imogolite) CH3COOH+H2O 2–5 Cholesteric FeOOH H2O 0.5–6 Smectic H2WO4 H2O 1 Smectic J.Mater. Chem., 1998, 8, 2557–2574 2559Table 2 V2O5 particle sizes in the Biltz sol35,36 Size/nm Ageing time A B C Freshly prepared 5 1 1 2 weeks 15 2 1 20 years 100 10 3 These sols possess a narrow particle-size distribution curve. Moreover, the particles in such sols probably remain unchanged during the course of the ageing process.The data given above show that the Biltz sols are mostly subject to ageing. The longest particles are also found in these sols. This is probably due to the presence of ammonium ions (NH4+) in these sols. The NH4+ ions remain after V2O5 synthesis. Ammonium ions are not found in other sols. Indeed, as is well known,23 the ageing process is accelerated considerably in the presence of salts of alkali metals and ammonium ions.In the course of the ageing process, V2O5 particles increase Fig. 3 Electron diVraction patterns for vanadium pentoxide obtained their size mainly in one direction. This direction is determined by the Biltz method (a), Mu� ller’s technique (b) and the ion-exchange by the specific character of the vanadium pentoxide crystal method (c) (reproduced with permission from ref. 42). structure. V2O5 monocrystals belong to the orthorhombic symmetry class. Their space symmetry group is Pmmm; the elementary after a further 4–6 months, it doubles; and after three years, cell parameters are: a=11.510, b=4.369 and c=3.563 A° .35–37 the particle length is equal to 2.5–3.0 mm. However, the particle The V2O5 structure is constructed from trigonal bipyramids width remains practically constant and is equal to 14 nm.(Fig. 6). As is shown in the inset to Fig. 6, five points of each The evolution of particles during the Biltz sol ageing process is clearly illustrated in Fig. 4, where micrographs obtained by means of an electron microscope52 are shown. It is evident from this Figure that when the sol ages, the long flexible particles start to interweave, forming a net.The old X-ray data,35,36 however, give much smaller particle sizes (Table 2). The particles in the freshly prepared Mu� ller sol have the form of fibres with length 50 nm and width 15 nm. During the ageing process, they increase in size much more slowly than the particles in the Biltz sol.Thus, after one year, their average length is found to be approximately 0.4 mm, and after two years it is ca. 1 mm.52 Needle-shaped particles with a length of 1.2 mm, a width of 10–15 nm and a thickness of 1 nm are found in an ionexchange sol.30,52 They form conglomerates, the centres of which serve as the intersection points for sol particles (Fig. 5). The characteristic feature of this sol is that its particles do not increase in size when sol ages.Finally, sols obtained by the Prandtl and Hess method (with Fig. 5 Electron microscopy images of textures of ion-exchanged a concentration of 0.1 mass%) contain rigid elongated particles vanadium pentoxide. The sol is two months old. (Reproduced with permission from ref. 52.) with the following mean sizes: A=150, B=10 and C=5 nm.52 Fig. 4 Electron microscopy images of Biltz sol textures: (a) two days old; (b) two weeks old; (c) one year old (reproduced with permission from ref. 52). 2560 J. Mater. Chem., 1998, 8, 2557–2574such pyramid are occupied by oxygen atoms, and the pyramid The step is a multiple of 2.8 A° (Fig. 8). Thus, one can understand the process of sol formation, by considering the centre is occupied by the vanadium atom.The O(2) atom binds the two neighbouring pyramids so mechanism of V2O5 powder hydration.44 The particles in the hydrated powder-like sample are compactly packed, and the that they form a zigzag chain in the [001] direction. The chains are joined into layers, which are parallel to the (010) plane. distance between them is approximately 10 A° .This corresponds to n#2H2O. The particles have the same thickness but In the V2O5 sols and gels, the structure is divided into layers, which form the 2D structure of colloidal particles diVerent widths. When a sample swells, the distance between the particles increases to 10–26 A° . Water penetrates only into (Fig. 7). To compare this structure with that of a monocrystal, it is necessary to change the polar axis c to b.The 2D structure cavities between the particles, which leads to their separation in the direction perpendicular to the layers. As the amount of consists of mutually associated rhombic V2O5 blocks. Each of these blocks contains five V2O5 groups, i.e. two acute-angle water increases (10<n<80), the inter-particles distance increases to 50–250 A° .The latter circumstance allows the VO5 pyramids connected to similar VO5 groups at the points. The block thickness, evaluated by means of X-ray38–41 and particles to rotate about their long axes. The ribbons may intertwine due to these multiple simultaneous rotations. neutron diVraction40,44 experiments, is about 8.8 A° , which is in good agreement with the particle sizes (see above).Finally, in the region where n=250–800 (a sol ), the interparticle distance equals 400–800 A° . This makes possible a free Now it is clear that when a sol is ageing, the V2O5 particles, in a 2D cell, grow preferably in the direction of the b axis rotation of the particles around their long axes, without any interference from the neighbours.and, to a smaller extent, in the direction of the a axis, while their thickness (the direction of the c axis) remains practically As a whole, this model was confirmed by small-angle X-ray experiments.38–41 The free, non-correlated motion of particles unchanged. In a dispersion, the particles are bound by meaf water was observed for sols with n=5000. However, beginning from n600, correlation in the positions of long axes was observed.molecules.40,46 In the limiting case, when two molecular layers are bound by one water layer, the distance between the The existence of long-range orientational order in sols and gels of V2O5 was proved by optical microscopic and diVraction particles is 2.8 A° . With increasing concentration of water, the increasing number of water molecules in the layers leads to a experiments.However, the character of the mesophase development was found to depend on the method of preparation stepwise increase of the distance between the V2O5 particles. of the colloidal solution. In sols, obtained using the Biltz technique, as we have already mentioned, the process of mesophase formation is accompanied by tactoid growth (Fig. 1). This process has been studied in detail by means of polarised optical microscopy54 and electron microscopy.49 Tactoids are formed for V2O5 concentrations above some critical value, which depends upon the ionic force of the electrolyte present in the solution. The rate of tactoid formation increases with increasing V2O5 concentration. When the V2O5 concentration is constant, this rate increases with increasing electrolyte concentration.54 The structure of a tactoid is clearly visible in the electron micrograph shown in Fig. 9.It is obvious that the V2O5 particles are assembled in bunches. This helps the mutual orientation of particles. The structure of tactoids (Fig. 10) may be imagined on the basis of the observations presented above and of the optical data.They typically have nematic organisation and may be considered as a nucleus of a nematic mesophase in a lyotropic solution. The spindle-like form of a tactoid means that the ratio between its surface tension and its viscosity is not very great.54 In the course of the ageing process, the tactoid form Fig. 6 Structure of crystalline vanadium pentoxide (reproduced with permission from ref. 37). Fig. 7 Structure of a 2D vanadium pentoxide particle, projected onto Fig. 8 Change of distance between the 2D layers in the process of the (001) (a) and (010) (b) planes (reprinted with permission from ref. 29. Copyright 1991 American Chemical Society). water absorption by V2O5 (reproduced with permission from ref. 40). J. Mater. Chem., 1998, 8, 2557–2574 2561Fig. 9 Vanadium pentoxide tactoid (electron microscopy image) (reprinted with permission from J. H. Watson, W. Heller and W. Wojtowicz, Science, 1949, 109, 274. Copyright 1949 American Association for the Advancement of Science. Readers may view, browse, and/or download this material for temporary copying purposes only, provided these uses are for noncommercial personal purposes.Except as provided by law, this material may not be further Fig. 11 Vanadium pentoxide dispersion, containing atactoids.21 reproduced, distributed, transmitted, modified, adapted, performed, displayed, published or sold in whole or in part, without prior written permission from AAAS). Fig. 10 Schematic arrangement of V2O5 particles inside a tactoid.21 sharpens, i.e. the elastic counteraction to the surface tension increases.Tactoids exhibit strong positive dichroism. In the case when their long axes are parallel to the polarisation plane of the Fig. 12 Atactoid phase of a vanadium pentoxide dispersion (electron incident light they seem to be yellow, if long axes are perpen- microscopy image) (reprinted with permission from J. H. Watson, W. dicular to this plane they are colourless. Heller and W.Wojtowicz, Science, 1949, 109, 274. Copyright 1949 V2O5 mesophases which have been aged for longer periods American Association for the Advancement of Science. Readers may view, browse, and/or download this material for temporary copying or have higher concentrations occupy practically the whole purposes only, provided these uses are for noncommercial personal sample area. However, ‘holes’ of linear form, which reproduce purposes.Except as provided by law, this material may not be further precisely the tactoid shape, are observed in this sample reproduced, distributed, transmitted, modified, adapted, performed, (Fig. 11). These tactoids are called ‘negative’ or ‘atactoids’. displayed, published or sold in whole or in part, without prior written The space between such tactoids is filled with the dilute sol, permission from AAAS).which acquires induced birefringence when flowing. The conclusion has been drawn, on the basis of this fact, that the sol Table 3 Ratios between axes (a/b) and the long axis sizes (a) for in the atactoid phase does not possess orientational order.20 tactoids and atactoids49 This has been proved by electron microscopy studies.49 Fig. 12 Type of Number of shows an electron micrograph of a 72 h old sol, showing the phase measurements a/b a/mm atactoid phase. The values of the long axes and the relationships between them for tactoids and atactoids (Table 3) are Tactoids 27 2.2–14.0 5.91–2.75 found using micrographs analogous to that shown in Fig. 12. Atactoids 6 0.2–0.6 4.0–1.6 It is clear from these data that the long axes for tactoids and 2562 J. Mater. Chem., 1998, 8, 2557–2574atactoids have quite similar values, while the ratios between the lengths of their long and short axes diVer considerably. Upon further ageing, the tactoid structure transforms into a discontinuous anisotropic mesophase, which shows fibrous structure under high magnification.These fibres have a transverse structure, clearly visible in Fig. 13. It consists of a number of alternating convexities and cavities with sizes of 65 and 35 A° , respectively. This structure corresponds to the actual arrangement of the V2O5 microcrystals. That is why the obtained picture is in accordance with the presence of nematic ordering in this mesophase.Magnetic and electric fields influence the tactoid struc- Fig. 14 Schematic arrangement of V2O5 particles inside a tactoid in an electric field.20 ture.20,55 Since V2O5 is paramagnetic, its particles are oriented in a magnetic field along the direction of the field. Tactoids are oriented in the same manner. Biltz sols is connected, to a great extent, with the presence of The influence of an electric field is more complex, since a considerable number of ammonium ions.together with purely orientational eVects, the particles are However, in some cases, low-concentration Biltz sols give, oriented by the flow. In ac fields (with a potential of 220 V), in the course of the ageing process, immediately after formation in the absence of a flow, the V2O5 particles are oriented of the isotropic solution, a discontinuous nematic phase perpendicular to the field.In dc fields (with a potential of (Fig. 15). Nevertheless, when this sol ages, tactoids form. This 120 V) areas with particle orientations both perpendicular and is possibly due to dividing of the sol into an isotropic phase parallel to the field are observed. The latter orientation is due and mesophase layers.This assumption seems to be credible, to the flow. if one takes into account that the initial sol concentration is In a dc field, tactoids are oriented by their long axes in the only slightly higher than the critical value, which is necessary direction of the flow (along the field). They move parallel for the formation of a mesophase.56 towards the anode, and are strongly deformed.Their head Sols and gels obtained by other methods do not form tactoid section is deformed into a sharp point (sometimes, into several mesophases, but exhibit typical nematic textures. Sols and gels sharp points), while their tail section becomes rounded. The obtained by the ion-exchange technique are studied in detail polarised optical microscopy observations allow one to imagine in ref. 30. Sols with concentrations lower than 0.12 mol l-1= the distribution of the V2O5 particles in such tactoids which V2O5·500H2O are optically isotropic; however, for higher is schematically shown in Fig. 14. Note that in the head part concentrations both sols (0.17 mol l-1=V2O5·350H2O) and of the tactoid, where the flow is practically absent due to the gels (0.85 mol l-1=V2O5·65H2O) give typical nematic textures narrowness of this area, the V2O5 particles are oriented (Fig. 16). A nematic mesophase also appears when a gel is perpendicular to the flow. All these data prove that dielectric diluted with toluene. The addition of NaCl electrolyte rapidly anisotropy is negative in the nematic mesophase of V2O5. leads to suspension flocculation. The appearance of textures Concluding this section, we will consider the influence of remains unchanged when the sample is heated to 80 °C and electrolytes upon the tactoids.The addition of small amounts also under the action of a magnetic field (1.7 T) and an ac (2–5 mmol) of NaCl and LiCl solutions leads to tactoid electric field (106V m-1, f=100 kHz). However, both sol and compression in the direction perpendicular to their long axes.gel can be easily oriented by shifting the upper cell glass. Moreover, the addition of arsenic acid totally prevents the Birefringence in a gel sample was measured at a concenformation of tactoids. Since electrolytes strongly influence the tration of 0.3 mol l-1. It was found to be equal to 10-2, which tactoid mesophase, one can suppose that its formation in the was in good agreement with values for other lyotropic nematics.57 A small-angle X-ray experiment gives a diVuse reflection typical for mesophases in capillaries with radial director orientation.It has been found from the X-ray data that the mean distance between the ribbons is about 160 A° . Thus, we do not possess much information on the original lyotropic liquid crystal, vanadium pentoxide, which seems to be quite well studied.We know only its type of superstructure (nematic), the organisation of its structural units, the absence of orientation in magnetic fields and athermal behaviour. Aluminium oxyhydroxide Aluminium oxyhydroxide sol was obtained by Zo�cher and Torok24 by dissolving aluminium foil in acetic acid in the presence of a small amount of mercury acetate.After boiling, the cooled studied solution became opaque and a strongly birefringent gel was formed. The addition of water during the boiling process transformed this gel into a sol. The latter exhibited birefringence only when flowing. However, the Fig. 13 Vanadium pentoxide dispersion: electron microscopy image, birefringence dispersion obtained from this sol was retained magnification×120 000 (reprinted with permission from J.H.Watson, W. Heller and W. Wojtowicz, Science, 1949, 109, 274. Copyright 1949 for quite a long time. American Association for the Advancement of Science. Readers may The Zo�cher sols and gels contain some acetic acid, which view, browse, and/or download this material for temporary copying can not be removed by boiling. Its presence has a considerable purposes only, provided these uses are for noncommercial personal eVect on the optical properties of the obtained sol (see below).purposes. Except as provided by law, this material may not be further A more recent method of preparation of the AlOOH sols reproduced, distributed, transmitted, modified, adapted, performed, and gels is the hydrolysis of isopropoxyaluminium in the displayed, published or sold in whole or in part, without prior written permission from AAAS).presence of HCl and NH3 (aq).58 The properties of the J. Mater. Chem., 1998, 8, 2557–2574 2563Fig. 16 Textures of a vanadium pentoxide dispersion: (a) gel, concentration 0.85 mol l-1; (b) sol, concentration 0.17 mol l-1 (reproduced with permission from ref. 30, Taylor and Francis). Fig. 15 Vanadium pentoxide dispersion with a concentration of about 1 mass%. The texture change in the course of the ageing process.56 Fig. 17 Boehmite lattice: octahedral layers connected by hydrogen obtained dispersions also depend, in this case, upon the bonds.62 quantity of HCl and NH3 (aq) participating in the reaction.X-Ray,58,59 electron diVraction60,61 and electron microscopy58 investigations have showed that the structural time.24 Fig. 18 shows micrographs of tactoids obtained in polarised light with diVerent angles of rotation of the polaris- units of aluminium oxyhydroxide are c-AlOOH (boehmite) crystals. ation plane with respect to the main axis of a tactoid. These conoscopic pictures agree well with the model of the nematic Boehmite belongs to the lepidocrocite structural type (Fig. 17). This is a rhombic structure of the Cmcm space arrangement of the AlOOH particles in a tactoid (see Fig. 10). Aluminium oxyhydroxide tactoids exhibit slight positive group and with the following unit cell parameters: a=2.86, b=12.2, c=3.69 A° .62–65 The aluminium ions lie in the centres birefringence.66 After some time, tactoids join together forming a large mesophase region, containing sometimes atactoid of octahedra. Each such ion is surrounded by five oxygen ions and one OH ion.The layers of octahedra are connected to inclusions. These mesophase areas have typical schlieren textures with clearly visible disclinations (Fig. 19). one another by hydrogen bonds.It is most probable that the hydrogen bonds are broken in dispersions, so the dispersion The tactoids, which contain a minimal amount of acetic acid remnant, show an interesting stripe structure.67 This phase consists of ribbon-like particles. The thickness of any of these ribbons is equal to the thickness of the bemite layer. structure is oriented perpendicular to the long axis of the tactoids (Fig. 20). The tactoid edges look jagged. As a rule, Similarly to vanadium pentoxide, the aluminium oxyhydroxide mesophase forms tactoids when it is stored for a long the distance between the neighbouring ‘teeth’ is about 3–6 mm. 2564 J. Mater. Chem., 1998, 8, 2557–2574Fig. 20 Aluminium oxyhydroxide tactoid with ‘teeth’ (reproduced with permission from ref. 67). Lithium molybdenoselenite Fig. 18 Aluminium oxyhydroxide tactoids. The angle between the microscope nicols: 90° (top); 10° (middle) and 45° (bottom). Lithium molybdenoselenite, Li2Mo6Se6, is a member of a (Reproduced with permission from ref. 24.) whole class of compounds having the general formula M2Mo6X6, where X=Se, Te and M=Li, Na, K, Rb, Cs, In, Ag, Cu.68,69 These substances are now intensively studied, since some of them appear to be superconductors. Lithium molybdenoselenite is synthesised from In2Mo6Se6 by a displacement reaction in the presence of lithium iodide.70 Indium molybdenoselenite is obtained from its elements by means of direct synthesis at 1050 °C.The isomorphic compounds M2Mo6Se6 exhibit a hexagonal symmetry with P63/m space group.They have a quite peculiar structure,68,69 constructed from (Mo3Se3)2 chains, formed on the basis of (Mo6Se6) icosahedra, which, in their turn, appear to be two (Mo6 and Se6) interpenetrating octahedra (Fig. 21). The (Mo6Se6) chains lie along the [001] axis as is shown in Fig. 21(b). Thus, each such chain may be considered as a quasi-one-dimensional structure. Fig. 19 Schlieren texture of aluminium oxyhydroxide (reproduced with It has been shown71 that some compounds of the M2Mo6Se6 permission from ref. 66). class can be dissolved in highly polar solvents, such as dimethyl sulfoxide and N-methylformamide. This produces gels, which can be further diluted by the same or a less polar solvent, such as acetonitrile, glycerol and methanol. However, the Separate layers of the transverse-stripe structure exhibit negative birefringence.This is clearly visible if the stripe obtained sols and gels are not stable. After 3–4 weeks from their formation, flocculation takes place and a flake-like insol- structure is placed parallel to the sample plane. When two such tactoids merge together, a network of layers may be uble residue is deposited.Electron microscopy studies of lithium molybdenoselenite71 formed. Separate elements of the transverse-stripe structure have the show that the stick-like crystalline particles with lengths of the order of 2 mm serve as the structural unit for the dispersion. form of tactoids with dimensions 20×30 mm. In this connection, the assumption has been made67 that These particlepresent, probably, separate chains surrounded by hydrated lithium ions.Electron microscopy also these tactoids are formed by two kinds of particles, which are needle-like and thread-like. These particles are placed perpen- reveals a tendency of the Li2Mo6Se6 crystals to orient themselves in one direction (Fig. 22). dicular one to another and exhibit birefringence of diVerent signs.In a narrow concentration range, between 4 and 10 mass%, the Li2Mo6Se6 dispersion in N-methylformamide exhibits a If a 1 M solution of acetic acid is added to this tactosol, the stripe texture disappears, and the tactoids return to their structure72 typical of nematic liquid crystals (Fig. 23). An analogous texture is observed when adding, in the ratio 153, initial form.67 If a low-strength dc electric field is applied, the tactosol is acetonitrile into the N-methylformamide solution.The obtained nematic structure is destroyed with time (from several displaced towards the cathode (the sol is charged positively). Then, in the cathode area, the pH increases and the aluminium hours to several months). It breaks into separate anisotropic and isotropic areas.oxyhydroxide concentration also increases. The transversestripe tactoids exhibit similar behaviour, but the stripe texture X-Ray investigations72 show the presence of a large variation in the particles size—from 10 to 1000 A° . The evaluation of disappears and tactoids acquire positive birefringence. There are still no data on the mesomorphic structure of the particle diameter allows one to assume that the chains are isolated and spontaneously oriented in one direction.Thus, a AlOOH gels; however, it is known to form easily ordered structures.58,60 nematic type of ordering occurs in this case. J. Mater. Chem., 1998, 8, 2557–2574 2565Fig. 23 Schlieren texture of lithium molybdenoselenite (reproduced with permission from ref. 72). Orientational ordering typical of a nematic mesophase was discovered in such a solution by studying the NMR spectrum of the acetone protons74 (Fig. 24). Our microscopy investigations have confirmed the formation of a nematic mesophase in this case. A typical texture of the water–acetone UO2F2 solution, obtained by the edge-drying method, is shown in Fig. 25. A detailed study of the NMR spectra73,75 allows one to determine the temperature and concentration ranges where the mesophase is present.Fig. 26 shows the dependence of the temperature range of the existence of the nematic mesophase upon uranyl fluoride concentration, for diVerent concentration Fig. 21 M2Mo6Se6 structure: (a) projection onto the ab plane; (b) proratios between acetone and D2O. The nematic mesophase jection onto the ac plane (reproduced with permission from ref. 68). region occupies an intermediate area between the isotropic phase and the frozen solution. When the UO2F2 concentration diminishes, this area becomes narrower due to lowering of the Fig. 24 NMR spectrum of the acetone protons in the uranyl fluoride– acetone–water system at T=4 °C (reproduced with permission from ref. 73). Fig. 22 Texture of a lithium molybdenoselenite gel (electron microscopy image) (reproduced with permission from ref. 71). Although the physical properties of the nematic phase of lithium molybdenoselenite have not yet been studied, one may suppose that this case is the first example of the manifestation of orientational ordering in a system of current-conductive particles. This promises interesting peculiarities in the behaviour of this nematic.Uranyl fluoride Uranyl fluoride was prepared by means of the dissolution of uranium(III) oxide in a stoichiometric volume of plavic acid.73 After multiple re-crystallisations, a water–acetone solution was prepared using heavy water. This solution was homogenised by heating to 40–50 °C. When acetone was added, the solution density was increased and light scattering occurred.When a certain acetone concentration was achieved, acetone was added Fig. 25 Texture of the uranyl fluoride–acetone–water system (edgedrying method). into the heated UO2F2 water solution in small portions. 2566 J. Mater. Chem., 1998, 8, 2557–2574The temperature and concentration dependences of the order parameter, S, for acetone in the nematic are obtained using the NMR technique.73 Fig. 28 shows such dependences, measured for diVerent concentrations of uranyl fluoride and an acetone concentration of 12.0 mol (kg D2O)-1. It is clear that these dependences are characteristic of the nematic liquid crystals. The value of S in this case, however, is considerably smaller than that obtained for typical lyotropic nematics.76 This is probably due to the fact that acetone is not included (or only partially included) within the structural units.According to ref. 73, the following dimer complexes serve as structural units for the mesophase considered: Fig. 26 Change of the temperature area of the existence of the nematic mesophase with changing uranyl fluoride concentration. Concentration of acetone: (1) 18.5; (2) 15.5; (3) 12.0; (4) 5.0 mol (kg UO2 H2O F F F H2O H2O UO2 F H2O H2O H2O UO2 H2O H2O F F F H2O UO2 H2O H2O H2O F D2O)-1.(Reproduced with permission from ref. 75.) Their rigidity is ensured by the bridges consisting of fluorine atoms of two kinds, which connect the two uranyl fluoride molecules. If this assumption is correct, then it will be easy to explain the low values of S.Indeed, the acetone molecules follow only sterically the nematic ordering of dimers. Further studies of 2H and 17O NMR spectra of water molecules introduced into such a system as a small-weight additive, allowed the detection77 of quadruple splitting (typical for nematics) for a whole range of uranyl fluoride-containing lyotropic compounds. Heavy and normal water, tributylphosphate and benzene were investigated as dispersion systems.In addition, analogous splittings were found in systems containing tributylphosphate and uranyl nitrate, perchlorate, sulfate and chloride. However, no other proofs of the existence of liquid crystalline phases in these systems have been found. Nevertheless, the possibility of the existence of mesophases in solutions containing some actinoid coordination compounds is of fundamental importance.Clays Natural aluminosilicates (clays) are widely spread rock-forming minerals. They have a layered structure,78,79 and, as a Fig. 27 Phase diagram of the uranyl fluoride–acetone–water system: (I ) isotropic phase; (II ) frozen solution area; (III ) nematic mesophase; (IV) phase separation area. (Reproduced with permission from ref. 75.) temperature of the nematic–isotropic liquid phase transition. For UO2F2 concentrations lower than 0.30 mol (kg D2O)-1 [for an acetone concentration in the system lower than 12–18 mol (kg D2O)-1] the nematic phase is no longer observed. Decreasing the acetone concentration in the system leads to narrowing of the temperature range of existence of the nematic mesophase.Studies of this system in the region of high acetone concentrations [ca. 28–40 mol (kg D2O)-1] allow one to assume75 the existence of another liquid crystalline mesophase in this region. This phase was characterised by an NMR signal typical for lamellar or hexagonal mesophases. However, detailed studies of this mesophase have not been carried out. Fig. 27 represents the phase diagram of the system studied.It is clear from this figure, that at T=-10 °C the nematic phase occupies a central part of the diagram, which lies between the isotropic phase and the frozen solution (II ). As Fig. 28 Temperature dependence of the order parameter for the ace- the temperature increases, the mesophase area becomes nartone molecules in the nematic mesophase of the uranyl fluoride– rower, while the isotropic phase area widens.Unfortunately, acetone–water system. The acetone concentration is 12.0 mol (kg it was not possible to investigate, by means of the NMR D2O)-1. Uranyl fluoride concentrations: (1) 0.5; (2) 0.6; (3) 0.7; method, a mesophase lying in the area of high concentrations (4) 0.8; (5) 0.94; (6) 1.21; (7) 1.35; (8) 1.88 mol (kg D2O)-1.(Reproduced with permission from ref. 73.) of uranyl fluoride. J. Mater. Chem., 1998, 8, 2557–2574 2567consequence, occur mainly in the form of finely dispersed powders, which are easily swollen in water. As a result, for some values of electrolyte concentration and pH, certain dispersive systems with liquid crystalline properties can be formed. 1. Imogolite. The dispersive particles of the clay mineral imogolite, Al2O3·SiO2·2H2O, are thin pipes of diameter 25.2 A° , with the structure shown in Fig. 29.80 The thickness of these pipes is equal to several thousand angstroms. The structure consists of gypsolite layers, in which the orthosilicate anion plays a connecting role. This anion occupies the vacant octahedral positions inside the gypsolite layer, separating the SiKOH hydrogen atom from three hydroxyl groups which surround each of these positions.Four SiMO bonds of this anion are from the layer, and they bind the protons into the SiOH structure. It is thus more correct to write the structural Fig. 30 A spherolite with a ‘fingerprint’ texture in the imogolite formula of imogolite as (OH)3Al2O3·SiOH. dispersion.The mass concentration is 0.0385. (Reproduced with Imogolite gel was prepared81 by means of dispersing the permission from ref. 82, Hu� thig and Wepf Publishers, Zug, powder in an aqueous solution of acetic acid (pH=3.5). This Switzerland.) procedure was carried out in an ultrasound disperser and lasted 10 min. Then the obtained solution was centrifuged for one hour, until the total deposition of the insoluble matter.The upper part was evaporated oV till its concentration became equal to 0.4 mass%. Then, acetic acid was added to the solution, till pH#3, and 0.02 mass% of NaN3 was added as a stabiliser. Polarisation optical microscopy observations have permitted the discovery82 of a typical cholesteric ‘fingerprint’ texture (Fig. 30). This striped fingerprints structure disappears, as the weight ratio of imogolite in the dispersion increases (Fig. 31). This means that the helix pitch increases. A transition to the gel phase, which still has a cholesteric structure consisting of bent ribbons, takes place. The helix pitch p dependence is well described by the following empirical relation: p=c-1.9, where c is the imogolite concentration. In the high-concentration area (imogolite D), the dispersion is divided into three phases.The lower two of these three phases are cholesteric, while the upper one is isotropic. The Fig. 31 Dependence of the helix pitch for the cholesteric mesophase phase behaviour and the helix pitch are not influenced by of the imogolite–water–acetic acid system with changing concentration: temperature.the imogolites D, E and F are dispersion samples which diVer in A clearly distinguished layered structure with periods of concentration (reproduced with permission from ref. 82, Hu� thig and 100, 15.6, 10, and 6.5 A° have been detected by means of Wepf Publishers, Zug, Switzerland). electron microscopy studies. The last period is clearly visible in the micrograph shown in Fig. 32. To explain the layered character of this structure, a theoretical model is proposed.82 According to this model, the cholesteric mesophase consists of structural units in the form of layers (‘rafts’). Each of these layers consists of four imogolite pipes. A schematic representation of this structure is shown in Fig. 32 Imogolite dispersion having a layered structure with a period Fig. 29 Schematic view of a part of an imogolite pipe (reproduced with permission from ref. 80, Hu� thig and Wepf Publishers, Zug, of 6.5 A° (reproduced with permission from ref. 82, Hu� thig and Wepf Publishers, Zug, Switzerland). Switzerland). 2568 J. Mater. Chem., 1998, 8, 2557–2574Fig. 33. It is easy to see that such packing of ‘rafts’ at an angle of 75° to one another, explains well the periodicity of the structure in the direction shown by the arrow.If the ‘raft’ packing is organised under an angle of 66.5°, then the layered structure will have a period equal (depending on the direction of observation) to 10 or 15.6 A° . The angle of ‘raft’ packing is determined by the imogolite concentration in the dispersion. The described model assumes each ‘raft’ to be rotated by some small angle with respect to the neighbouring ‘raft’.This leads to a cholesteric twist. According to ref. 82, the reason for this twist is connected to the mutual positioning of the end OH groups of the imogolite pipes. These groups are situated at the pipe surface in a spiral manner. The axes of these spirals coincide with the pipe molecular axes (Fig. 34).Thus, the imogolite structure is, indeed, chiral. The imogolite mesophase was found to be a good model system to verify the application of the Onsager and Flory theories to the description of ordering in a system consisting of rigid rods.81 It was shown that the phase behaviour of this dispersion agreed qualitatively with Onsager’s predictions. 2. Montmorillonite. An aluminosilicate of the smectite group, montmorillonite, has a complex chemical organisation.The pyrophyllite unit Al2Si4O10(OH)2 with a partial replacement of Al3+ by Mg2+, Fe2+ and Fe3+, serves as the basis of the montmorillonite structure. Montmorillonite exhibits a layered organisation (Fig. 35), consisting of tetrahedra [SiO4] and octahedra [AlO4(OH)2].78,79 Alkali and alkaline-earth cations and water molecules can be found in the inter-layer space. Montmorillonite easily absorbs water and electrolyte solu- Fig. 34 Molecular model of the gibbsite unit of the imogolite pipe. tions, which are localised between the aluminosilicate layers. The two continuous lines represent the OH group arrangement at 55° As a result, a gel is formed. It probably contains 2D montmoril- to the molecular axis, shown by the dashed line.The distance between lonite layers as a dispersive phase. the two lines is equal to 2.4 A° . (Reproduced with permission from ref. 82, Hu� thig and Wepf Publishers, Zug, Switzerland.) A liquid crystalline mesophase was detected83 in the montmorillonite gel. This gel was obtained as follows. The montmorillonite fraction containing particles with sizes of about 2 mm, was washed with 1 M sodium chloride solution, and freed later from chlorine ions by means of dialysis through a cellulose filter (pH=7).Then, the solid sample was placed into a 0.03% sodium chloride solution. A swollen gel exhibited an anisotropic stripe texture, which darkened when the crossed nicols were rotating at 90°. The stripe direction lay at an angle Fig. 35 Layered structure of montmorillonite. The positions of Al3+ ion substitution by Mg2+ and other ions are indicated. (Reproduced with permission from ref. 78.) of about 20° with respect to the maximal darkness direction, the width of the stripes was 2 mm and their length was 150 mm. When the sodium chloride concentration is reduced, the stripes width also decreases. If, in these experiments, one uses montmorillonite particles with sizes less than 0.5 mm, then more regular and wider stripes will be observed.Fig. 33 Scheme of ordering of the structural units in the cholesteric If a surface active substance, e.g. cetyltrimethylammonium mesophase of an imogolite dispersion (reproduced with permission from ref. 82, Hu� thig and Wepf Publishers, Zug, Switzerland). bromide, is added to the dispersion, then separate montmoril- J.Mater. Chem., 1998, 8, 2557–2574 2569lonite small crystals, perpendicular to the stripe structure, will this structure; their removal leads to the transformation of FeOOH into a-Fe2O3 (Fig. 37). appear in the optical field. In this connection, it is natural to make the assumption that in the montmorillonite gel the If, to obtain a sol, one uses a concentrated solution of iron- (III ) chloride, then, after 4–6 weeks, tactoids will be formed in particles are situated in the layers, which are parallel to the observed stripes.If this is true, then the montmorillonite gel the isotropic solution.87,92 FeOOH tactoids are similar V2O5 (Fig. 2). However, the former exhibit more pro- should be classified as a smectic liquid crystal.A bentonite dispersion, consisting of montmorillonite (79 nounced circular form. Typical sizes of FeOOH tactoids are 0.2×0.1 mm. mass%) with added quartz, calcite and gypsum, readily demonstrates birefringence when flowing. Areas of spontaneous However, low concentration Zo�cher sols (1–6 mass%) with low pH (1), which are isotropic just after their preparation, optical anisotropy have been observed in concentrated dispersions. 83,84 This allows one to assume the existence of a can be, after some time, divided into layers.These sols form dense iridescent residues, which are called ‘schiller’ (which mesophase in such a dispersion. means iridescent) layers.20,21,27 Optical microscopy allows one to observe typical smectic Smectics textures.Fig. 38(a) illustrates a step texture. The plane of this texture is perpendicular to the optical axis. Fig. 38(b) shows Iron oxyhydroxide a texture in which the layers are oriented perpendicular to the We will start the consideration of smectics with the FeOOH figure plane. These textures demonstrate convincingly the sol. This is a true and well studied lyotropic liquid crystal. layered smectic structure of the FeOOH sol.The method of FeOOH synthesis has been elaborated by The orientation of the particles with respect to the layers Zo�cher and Heller.85 It consisted of the slow hydrolysis of was studied by means of electron microscopy.89 It was assumed iron-(III ) chloride at low temperatures, in the presence of that the particles formed layers, orienting their long axes in ammonia solution.The hydrolysis lasted from 4 to 12 months. the layer plane. However, this structure was not in accordance As a result, a dense, coloured residue was precipitated at the with the observed optical properties—the direction of the bottom of the reaction vessel. It was possible to use this optical axis and the positive birefringence.Recently, it was residue directly for the investigations, or to centrifuge it until shown convincingly90 that FeOOH particles were oriented with it became a paste which was then redissolved by heating for their long axes perpendicular to the layer plane, as is the case 3 weeks. in typical smectics. The FeOOH crystals serve as structural units for an isotropic Calculations, realised with the help of the DLVO method, gel (Fig. 36). Their sizes lie in the range from 50×400 to showed that the particles are packed inside the layers according 140×700 nm. However, the issue of the FeOOH crystalline to the quadric law, due to the existence of the second minimum form is found to be quite diYcult. in the potential curve. The distance between the particles was Indeed, iron oxyhydroxide exists in three modifications: found to be approximately 300–400 A° .rhombic a-FeOOH (getite), tetragonal b-FeOOH (acagapite) There are two periods in the FeOOH smectic structure: the and rhombic c-FeOOH ( lepidocrite). X-Ray86,87 and electron distance between the layers and the distance between the microscopy88–90 investigations prove with no doubt that b- particles inside the layers.The inter-layer distance is of the FeOOH crystals are present in sols obtained by means of the order of magnitude of the wavelength of visible light. This Zo�cher method. However, earlier studies by Heller et al.91 leads to the clearly visible selective reflection in the homeohave shown that some complex processes in iron oxyhydroxide tropic texture.The freshly prepared Mu� ller layers look bright sols take place with time. In the beginning of the hydrolysis process, FeOCl is formed. It transforms gradually into b- FeOOH, and, later on, into a-FeOOH. Moreover, the time to the first transition is about 5–8 years, while that to the second ranges from several months up to 25 years. Since the majority of researchers work with sols which have been aged for several years, then it becomes clear that b- FeOOH is present in these sols only as an intermediate form.b-FeOOH crystals are tetragonal with unit cell parameters in: a=b=10.48 A° , c=3.023 A° space group I4/m.86,87 The structure of b-FeOOH consists of a network of tetrahedra, composed of O atoms and OH groups, with Fe atoms lying in the centre of the tetrahedra.Water molecules are present in Fig. 37 b-FeOOH structure projected onto the (001) plane (repro- Fig. 36 Electron microscopy image of b-FeOOH crystals (reprinted from ref. 86: Y. Maeda and S. Hachisu, Colloids Surf., 1983, 6, 1, duced with permission of the Mineralogical Society of Great Britain and Ireland). Copyright 1983, with permission from Elsevier Science). 2570 J. Mater. Chem., 1998, 8, 2557–2574Table 4 Mean sizes of the particles in the dispersion phase of the tungstic acid–water96 Type a/mm b/mm c/mm a5b5c A 11.7 4.05 0.15 10053551.3 B 14.6 5.0 0.35 10053452.4 D 2.27 0.75 0.044 10053351.95 oriented by a magnetic field. Temperature has no eVect on FeOOH sol behaviour.20,21,27 Tungstic acid Tungstic acid H2WO4 (WO3·H2O) is the second credible example of smectic mesophase formation.Its water dispersion is usually obtained as follows.20 Sodium tungstate solution is treated with hydrochloric acid. The obtained fraction of solid particles is centrifuged and diluted with water to a low volume. The residue is centrifuged again and is diluted with water, thoroughly mixing the solution. The obtained mixture is then centrifuged several times.As a result, a residue is obtained which is dissolved in a large amount (ca. 200 cm3) of water. Finally, an opaque dispersion, showing iridescent radiance when flowing, is obtained. Microscopy20,27 and electron microscopy94–97 studies have shown that quite complicated particles form the dispersive phase. Just after the formation of the dispersion, one can observe elliptical particles with maximum sizes of 0.2–0.3 mm and their length and width increase rapidly with time.These are termed A-type particles [Fig. 39(A)] which possess low positive birefringence and are optically homogeneous. It is possible that these particles may be small tactoids, since the particle edges are not smooth, but jagged. These notches can Fig. 38 Textures of the iron oxyhydroxide–water system: (a) step serve as outlets for the smectic layers of real structural units. texture (reproduced with permission from ref. 27 reprinted from As the considered dispersion ages, D-type particles are ref. 86: Y. Maeda and S. Hachisu, Colloids Surf., 1983, 6, 1, Copyright 1983, with permission from Elsevier Science); (b) layered texture. formed [Fig. 39(D)]. These are narrow crystals of absolutely perfect form. Comparison of the difractograms for D- and Atype particles shows that these objects are completely diVerent in structure. green under normal illumination. When the light incidence If the ageing time is short, then particles with intermediate angle is increased, they become sky-blue, green, blue and forms will appear.These are referred to as B-type [Fig. 39(B)] violet.21 The schiller layers, lying perpendicular to the glass and C-type [Fig. 39(C)] particles. The first of these types substrate surfaces, also exhibit selective reflection, if the includes particles with plane-parallel ribs, but with elliptical FeOOH particle diameter has the order of magnitude of the ends. The second type includes eight-sided particles with wavelength of visible light.90 straight ribs.The inter-layer distance increases with time, which is proved Particle size measurements give quite interesting results (see by the change of colour from orange to red.20 If the pH is Table 4). In spite of large distribution of particle sizes, the increased from 1.5 to 3, the selective reflection wavelength is ratio of particle dimensions is almost the same for each type, decreased twofold.27 This phenomenon is accompanied by the which proves the general nature of these particles.appearance of strong dichroism: the layers look bright green under the initial light polarisation, and bright yellow, under light polarisation perpendicular to the initial light.27 An atteundertaken93 to calculate the inter-layer distance for the homeotropic texture, using a simple interference formula.Data for sols of diVerent concentrations (from 0.5 to 0.3 mass%) and of diVerent ageing time (from 59 to 1695 days) were used. It was found that when a sol aged its inter-layer distance increased. With increasing concentration, the distance between layers could either increase or decrease.The mean inter-layer distance was found to be ca. 2000 A° . However, these estimations did not take into account changes in the particle sizes. When a low electric voltage is applied, the sol is displaced towards the cathode. The selective reflection wavelength is simultaneously red-shifted.21 Magnetic fields do not influence the selective reflection; however, areas with negative birefringence parallel to the field lines appear.21 This fact, together with the phenomenon of the rotation of the light polarisation plane (for incident light Fig. 39 Electron microscopy images of tungstic acid dispersion particles (reproduced with permission from ref. 94). normal to the field lines), prove92 that the sol particles are J. Mater. Chem., 1998, 8, 2557–2574 2571It is interesting that an isothermal transition between particles of diVerent types is observed.The principal direction of this transition is A�B�C�D. However, a direct A�D transition is also observed. Since a considerable eVect of the medium pH on these transitions is found, the assumption is made95 that a change in the habitus of the tungstic acid particles is connected with a modification of their structure.This latter structural change is analogous to the transition: FeOCl�b-FeOOH�a-FeOOH (see above). If the tungstic acid dispersion is stored for quite a long time, it will acquire the iridescent radiance characteristic of layered structures.20 The smectic layers and the orientation of particles inside them are clearly visible in Fig. 40.The inter-layer distance depends upon the concentration, pH and the presence of electrolytes in the solution.20,27,98,99 Qualitative studies (by means of interference order observation) show that the inter-layer thickness increases with the degree of dilution. If the solution is diluted approximately 40- fold the distance between the layers increases approximately threefold. This is accompanied by an increase in pH of the solution.Quantitative measurements were carried out by optical Fig. 41 Dependence of the inter-layer distance in the smectic phase of methods.20,98,99 It was shown that the inter-layer distance tungstic acid upon the electrolyte concentration: the solid points decreased with increasing number of layers. Thus, in a sample represent the experimental data of ref. 99; the open points show the containing 80 layers, the mean inter-layer distance was found experimental results of ref. 99; the continuous lines indicate the to be 0.83 mm, while in a sample with 800 layers it was 0.65 mm. theoretical estimations of refs. 98 and 99 (reproduced with permission from ref. 98). The dielectric conductivity of the dispersive medium also influences the inter-layer distance.Using alcohol instead of water as the solute decreases this distance by a factor of 0.58. However, the most considerable influence is caused by ganic liquid crystals. In fact, there are many more such electrolytes. It is found98,99 that diVerent electrolytes with the mesomorphic substances. However, the information on these same equivalent concentration, decrease the inter-layer dis- mesophases is often contradictory and, hence, needs careful tance to the same degree.If the electrolyte concentration is verification. As an example, one may consider the reports on changed from 1×10-8 to 64×10-8 equiv. cm-3, the inter- the observation of mesophases in the so-called graphite layer distance decreases from 0.9 to 0.2 mm. oxide100–102 and in calcium phosphate.103 All these facts prove the evident eVect of electrostatic forces In this connection, one needs to point attention to the old on the formation of such layers.An attempt has been made99 work of our compatriot P. P. von Veimarn.104,105 He obtained to calculate the equilibrium distance between the layers, taking anisotropic dispersions of such simple ion salts as barium electrostatic and gravitation forces into account.The calcu- sulfate, calcium chloride, etc. lation scheme is similar to that of the DLVO method. Fig. 41 Lyotropic mesophases, as we see, are formed by inorganic shows the obtained theoretical results together with the exper- compounds which are very diVerent in their chemical nature. imental data taken from refs. 98 and 99. It is clear that the However, they all have similar structures: layered or islandcalculated and experimental values of the inter-layer distances like, i.e. structures composed of separate elements of considerare in good agreement. able sizes. Examples of layered structures are vanadium pent- The above-described studies are, at the moment, the only oxide, aluminium oxyhydroxide, imogolite, montmorillonite ones on the smectic mesophase of tungstic acid.and other clays. An example of and island structure is lithium molybdenoselenite, which consists of long columns. When interacting with a solvent, such structures are easily Conclusion decomposed into separate fragments ( layers or columns), The above-considered mesophases of inorganic substances which become anisometric structural units of the forming represent most of the documented studies of lyotropic inor- mesophases.Such a scenario of the formation of the lyotropic mesophases does not exclude an alternative process, whereby mesophases may appear due to the ordered arrangement of the bulk anisometric crystals. This latter scenario is, probably, realised in the cases of iron oxyhydroxide and tungstic acid.At the moment, the situation with the uranyl fluoride dispersion is unclear; the structural unit in the form of the molecular dimer seems to be improbable for formation of the colloidal solution. The second peculiarity of inorganic liquid crystals is that they exhibit only one mesophase. This is probably a consequence of the large sizes of their structural units.Indeed, the reorganisation of such large structural elements requires considerable energy. However, as we have observed in the cases of aluminium oxyhydroxide and uranyl fluoride, there are some facts that can be interpreted in favour of the existence of highly ordered mesophases. It remains unclear, in this connection, why the studied gels and sols possess only one and the same mesophase.All these facts should be thor- Fig. 40 Smectic texture of the tungstic acid (reproduced with permission from ref. 27). oughly studied. 2572 J. Mater. Chem., 1998, 8, 2557–257416 H. Freundlich, F. Stapelfeld and H. Zocher, Z. Phys. Chem., The third characteristic feature, the narrow concentration 1925, 114, 161. interval of the existence of mesophases and their athermal 17 H.Freundlich, F. Stapelfeld and H. Zocher, Z. Phys. Chem., behaviour, are the consequences of the large sizes of the 1925, 114, 190. structural units. 18 H. Freundlich, H. Neukircher and H. Zocher, Kolloid Z., 1926, And, finally, the fourth peculiarity: the influence of electro- 38, 48. lytes. This last characteristic feature makes inorganic meso- 19 H.Zocher, Z. Phys. Chem., 1912, 98, 293. 20 H. Zocher and K. Jacobsohn, Kolloid Beih., 1929, 28, 167. phases similar to organic micellar and chromic liquid crystals. 21 H. Zocher, Z. Anorg. Allg. Chem., 1925, 147, 91. If we now consider the fact of the existence of inorganic 22 H. Zocher and K. Jacobsohn, Kolloid Z., 1927, 41, 220. lyotropic mesophases, taking into account all the above- 23 J. Jochims, Kolloid Z., 1927, 41, 215.described peculiarities, it is necessary to point out that the 24 H. Zocher and C. Torok, Kolloid Z., 1960, 170, 140. appearance of orientational ordering is a quite widespread 25 J. Bernal and J. Fankuchen, Nature, 1937, 139, 923. form of organisation of both organic and inorganic solutions. 26 H. Zocher, Kolloid Z., 1954, 139, 81. 27 H. Zocher and C. Torok, Acta Crystallogr., 1967, 22, 751. While organic mesophases have been stail, inor- 28 H. Zocher, Mol. Cryst. Liq. Cryst., 1969, 7, 177. ganic liquid crystals still wait for their turn. Unfortunately, 29 J. Livage, Chem. Mater., 1991, 3, 578. one can say that, since they have been neglected for so long, 30 P. Davidson, A. Garreau and J. Livage, Liq.Cryst., 1994, 16, we deal now with a quite new and unexplored area. 905. By this statement, I mean the following. Considerable 31 W. Biltz, Ber. Dtsch. Chem. Ges., 1904, 37, 1098. experimental material, concerning mainly the fact of the 32 E. Muller, Z. Chem. Ind. Kolloide, 1911, 8, 302. 33 J. Lemerle, L. Nejem and J. Lefebre, J. Inorg. Nucl. Chem., 1990, existence of mesophases in inorganic lyotropic systems, has 42, 17.been collected thus far. In some cases, there are even structural 34 W. Prandtl and L. Hess, Z. Anorg. Allg. Chem., 1913, 82, 103. proofs of mesomorphism. However, most attention has been 35 J. Ketelaar, Nature, 1936, 316. paid to the similarity between these mesophases and the well 36 J. Ketelaar, Chem. Weekbl., 1936, 33, 51. studied organic liquid crystals.There are diVerences, which 37 H. G. Bachmann, F. R. Ahmed and W. H. Barnes, are most interesting, since they are determined by the specific Z. Kristallogr., 1961, 115, 110. 38 P. Adelbert, N. BaYer, N. Gharbi and J. Livage, Mater. Res. character of the inorganic mesophases. After all, these diVer- Bull., 1981, 16, 669. ences are due to the unequal sizes of the structural units in 39 J.Legendre, P. Adelbert, N. BaYer and J. Livage, J. Colloid these two systems. One can say that the structural unit sizes Interface Sci., 1983, 94, 84. in inorganic lyotropic systems are one order of magnitude 40 P. Adelbert, H. Haesslin, N. BaYer and J. Livage, J. Colloid greater than in organic lyomesophases. The elastic behaviour Interface Sci., 1984, 98, 478.in these former systems is completely unknown. Here we will 41 T. Kamiyama, T. Iton and K. Suzuki, J. Non-Cryst. Solids, 1988, 100, 466. probably deal with, already discussed theoretically,106 but not 42 K. Takiyama, Bull. Chem. Soc. Jpn., 1958, 31, 555. observed experimentally, second-order elasticity: when not 43 J. Legendre and J. Livage, J. Colloid Interface Sci., 1983, 94, 75.only the structural units, as a whole, but also their constituent 44 N. BaYer, P. Adelbert, J. Livage and H. Haesslin, J. Colloid parts will participate in bending deformations. Interface Sci., 1991, 141, 467. The issue of elasticity is only one of many physicochemical 45 C. Scanchez, J. Livage and G. Lucazeau, J. Raman Spectrosc., problems concerning inorganic lyotropic liquid crystals.The 1982, 12, 68. 46 M. Vandenborre, R. Prost and J. Livage, Mater. Res. Bull., 1983, other, which is closely connected with this former: is the 18, 1133. Fredericks transitions. Here one should also expect some 47 L. Abello and G. Lucaseau, J. Chim. Phys., 1984, 81, 539. serious diVerences connected with the greater (than in the well 48 L. Abello, E. Husson, G.Lucaseau and Y. Repelin, J. Solid State studied mesophases) structural unit sizes. Chem., 1985, 56, 379. Finally, the phase transitions in the above-described systems 49 J. H. Watson, W. Heller and W. Wojtowicz, Science, 1949, 109, are athermal. However, the concentration areas of phase 274. 50 J. Donnet, Compt. Rend., 1948, 227, 508. stability are known, but they have not yet been studied 51 J.Donnet, H. Zbinden, H. Benoit, M. Daune, N. Dubois, in detail. J. Pouget, G. Scheibling and G. Vallet, Compt. Rend., 1950, 47, All these unsolved problems, together with the issues of 51. electric and magnetic field eVects, allow one to confirm the 52 K. Takiyama, Bull. Chem. Soc. Jpn., 1958, 31, 329. above-made statement, that inorganic lyotropic liquid crystals 53 J.Livage, N. Gharbi, M. C. Leroy and M. Michaud, Mater. Res. represent a new and unexplored area, in spite of the fact that Bull., 1978, 13, 1117. 54 W. Heller and W.Wojtowicz, J. Appl. Phys., 1949, 20, 343. they have been known for a long time. In this connection, the 55 S. Berkman and H. Zocher, Kolloid Z., 1927, 42, 315. aim of the present review is to stimulate studies in this new 56 E.V. Generalova, E. L. Kitaeva and A. S. Sonin, I All-Union and interesting field. conference of lyotropic liquid crystals, Abstracts, Ivanovo, 1990, p. 38 (in Russian). 57 A. S. Sonin, Usp. Fiz. Nauk., 1987, 152, 273 (in Russian). 58 Song Ki Chang and Chung In Gae, J. Non-Cryst. Solids, 1989, References 108, 37. 59 H. B. Weiser and W. O. Milligan, Adv. Colloid Sci., 1942, 1, 227. 1 F. Reinitzer, Sitzungsber. Bayer. Akad. Wiss. Math. Naturwiss. 60 W. O. Milligan and H. B. Weiser, J. Phys. Colloid Chem., 1951, Kl., 1886, 94, 719. 55, 490. 2 O. Lehmann, Ann. Phys., 1985, 56, 771. 61 L. H. Genner and K. H. Storks, Ind. Eng. Chem. Anal. Ed., 1939, 3 A. S. Sonin, Zh. Strukt. Khim., 1991, 32, 137 (in Russian). 11, 538. 4 Q. Majorana, Phys. Z., 1902, 4, 145. 62 F. J. Ewing, J. Chem. Phys., 1935, 3, 420. 5 A. Schmauss, Ann. Phys., 1903, 10, 658. 63 M. S. Goldsztaub, Bull. Soc. Fr. Mineral., 1935, 59, 348. 6 A. Schmauss, Ann. Phys., 1903, 12, 186. 64 R. Hocart and J. Lapparent, Compt. Rend., 1929, 189, 995. 7 Y. Bjornstahl, Philos. Mag., 1921, 42, 352. 65 P. P. Reichertz and W. J. Yost, J. Chem. Phys., 1946, 14, 495. 8 A. Cotton and H. Mouton, Ann. Chim. Phys., 1907, 40, 145. 9 W. Heller and H. Zocher, Z. Phys. Chem., 1933, 164, 365. 66 H. Zocher and C. Torok, Kolloid Z., 1960, 173, 1. 67 H. Zocher and C. Torok, Kolloid Z., 1962, 180, 41. 10 H. Diesselhorst, H. Freundlich and B. Leonard, Elster-Geitel- Fortschrift, 1915, 453. 68 M. Potel, R. Chevrel, M. Sergent, M. Decroux and O. Fisher, Compt. Rend., Ser. A, 1979, 288, 429. 11 H. Freundlich, Z. Elektrochem., 1916, 22, 27. 12 H. Diesselhorst and H. Freundlich, Phys. Z., 1915, 16, 419. 69 M. Potel, R. Chevrel, M. Sergent, J. C. Amici and O. Fiser, J. Solid State Chem., 1980, 35, 286. 13 W. Reinders, Kolloid Z., 1917, 21, 161. 14 A. Szegvari, Z. Phys. Chem., 1924, 112, 295. 70 J. M. Tarascon, G. W. Hull and F. J. Di Salvo, Mater. Res. Bull., 1984, 19, 915. 15 H. Zocher, Z. Phys. Chem., 1921, 98, 293. J. Mater. Chem., 1998, 8, 2557–2574 257371 J. M. Tarascon, F. J. Di Salvo, C. H. Chen, P. J. Carrol, 89 J. H. Watson and R. R. Cardell, J. Phys. Chem., 1962, 66, 1757. M. Walsh and L. Rupp, J. Solid State Chem., 1985, 58, 290. 90 Y. Maeda and S. Hachisu, Colloids Surf., 1983, 6, 1. 72 P. Davidson, J. C. Gabriel, A. M. Levelut and P. Batail, 91 W. Heller, O. Kratky and H. Nowotny, Compt. Rend., 1936, Europhys. Lett., 1993, 21, 317. 202, 1171. 73 V. A. Michalev and V. A. Tcherbakov, Zh. Obshch. Khim., 1985, 92 K. Coper and H. Freundlich, Trans. Faraday Soc., 1937, 33, 348. 55, 1223 (in Russian). 93 W. Heller, Compt. Rend., 1935, 201, 831. 74 V. A. Tcherbakov, L. L. Tcherbakova and B. V. Semakov, Zh. 94 J. H. Watson, W. Heller and W. Wojitowicz, J. Chem. Phys., Strukt. Khim., 1974, 15, 925 (in Russian). 1948, 16, 997. 75 V. A. Michalev and V. A. Tcherbakov, Zh. Strukt. Khim., 1985, 95 W. Heller, W. Wojtowicz and J. H. Watson, J. Chem. Phys., 55, 1229 (in Russian). 1948, 16, 998. 76 N. V. Kazakov, A. V. Kaznacheev and A. S. Sonin, Izv. Akad. 96 J. H. Watson, W. Heller and W. Wojtowicz, J. Chem. Phys., Nauk SSSR, Ser. Fiz., 1991, 55, 1731 (in Russian). 1948, 16, 999. 77 V. A. Tcherbakov and L. L. Tcherbakova, Radiokhimiya, 1984, 97 J. Turkievich and W. Heller, J. Anal. Chem., 1949, 21, 475. 26, 708 (in Russian). 98 K. Furusawa and S. Hachisu, J. Colloid Interface Sci., 1968, 78 U. Hofmann, Angew. Chem., Int. Ed. Engl., 1968, 7, 681. 28, 167. 79 V. A. Druch and A. G. Kossovskaja, Glinistie minerals. M., 99 P. Bergmann, P. Low-Beer and H. Zocher, Z. Phys. Chem. A, Nauka, 1990 (in Russian). 1938, 181, 301. 80 K. Kajiwara, N. Donkai, Y. Hiragi and H. Inagaki, Macromol. 100 H. Thiele, Z. Anorg. Allg. Chem., 1930, 190, 145. Chem., 1986, 187, 2883. 101 G. Ruess, Kolloid Z., 1945, 110, 17. 81 N. Donkai, H. Inagaki, K. Kajiwara, H. Urakawa and 102 H. Thiele, Kolloid Z., 1948, 111, 15. M. Schmidt, Macromol. Chem., 1985, 186, 2623. 103 P. Gaubert, Compt. Rend., 1922, 174, 1115. 82 K. Kajiwara, N. Donkai, Y. Fujiyoshi and H. Inagaki, 104 P. P. Von Weimarn, Zh. Russ. Fiz-Khim. Ova., 1908, 40, 1323 Macromol. Chem., 1986, 187, 2895. (in Russian). 83 R. Bradfield and H. Zocher, Kolloid Z., 1929, 47, 223. 105 P. P. Von Weimarn, Kolloid Z., 1927, 44, 279. 84 A. Buzagh, Kolloid Z., 1929, 47, 223. 106 A. A. Vedenov and E. B. Levchenko, Usp. Fiz. Nauk, 1983, 141, 85 H. Zocher and W. Heller, Z. Anorg. Allg. Chem., 1930, 186, 75. 3 (in Russian). 86 A. L. Mackay, Mineral. Mag., 1960, 32, 545. 87 A. L. Mackay, Mineral. Mag., 1962, 33, 270. 88 J. H. Watson and R. R. Cardell, J. Appl. Phys., 1961, 32, 1641. Feature Article 8/02666A 2574 J. Mater. Chem., 1998, 8, 2557–2574

ISSN:0959-9428    

DOI:10.1039/a802666a

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年代:1998

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J O U R N A L O F C H E M I S T R Y Materials Communication Rational design of new acid-sensitive organogelators Jean-Luc Pozzo,* Gilles Michel Clavier and Jean-Pierre Desvergne Photochimie Organique et Supramole�culaire, UMR 5802, Universite� Bordeaux I, 351 cours de la Libe�ration, 33405 Talence Cedex, France. E-mail: jl.pozzo@lcoo.u-bordeaux.fr Received 16th September 1998, Accepted 7th October 1998 2,3-Di-n-alkoxyphenazines were shown to act as acid- are reported in Table 1.The data revealed the trend of phenazine derivatives to fairly easily gelify polar solvents; it sensitive organogelators at ambient temperature in acetonitrile; the protonated yellow species formed in the also indicates that 2b, possessing two longer alkyl side-chains, acts as a more eYcient gelator than 2a.These compounds are presence of acid displays stronger aggregative properties and higher temperature resistance than the colourless neu- soluble in low polarity solvents (halogenated or aromatic)11 and hence cannot rigidify such solutions, with the exception tral phenazine; the protonation/deprotonation process is reversible. of compound 2b, which can gelify n-heptane at low temperature (Tgel=3 °C).Under the same conditions, the parent molecules (DAOA, 1) displayed, in general, similar behaviour and a Thermoreversible physical gels generated from low molecular mass organic compounds are an active field of research from slightly higher capacity to encage the alcoholic solvents.4 Thus the replacement of CH groups by nitrogen atoms on the both an academic viewpoint and because of their practical applications.1–3 Organogelators not only rigidify the solvents anthracene moiety does not deprive the dialkoxyaromatic from its gelling capacities.but also create supramolecular networks which could induce specific and unique properties to the resulting materials.4 Owing to the presence of basic sites on the phenazine moiety, addition of trifluoroacetic acid (TFA) to solutions of Although hydrogen-bonding donor and acceptor groups are necessary for most organogelators which self-aggregate in 2a and 2b strongly modifies both their 1H and 13C NMR signals, which are shifted upfield, and also the UV-visible water and organic solvents, it has been recently shown by us5 and others6,7 that small non-hydrogen bonding molecules absorption spectra, the maxima being bathochromically shifted from 249 and 390 nm to 260 and 413 nm, respectively.The could also exhibit gelling properties. Amongst the compounds investigated, 2,3-di-n-alkoxyanthracenes (DAOA, 1) were absorption spectra of 2a and 2b are superimposable both in the presence or absence of TFA. Taking into account the range reported to display, at very low concentrations, exceptional aggregative properties in various organic fluids.5 For these of acid concentration used (0 to 2×10-2 M) and according to the reported data for neutral, mono- and di-protonated phena- self-assembling systems, van der Waals interactions, dipole– dipole contributions and p-stacking are the main driving forces zine, 12 the yellow coloured species obtained upon acid addition is consistent with formation of the monoprotonated moiety.involved for building the fibrous aggregates. The gel-to-sol phase transition temperature of an More importantly, the gel-forming ability of 2a and 2b is significantly enhanced in the presence of acid, resulting in an organogelator is usually found to depend on the fluid and the gelator concentration, but it can also be influenced by external increase of Tgel by much as 60 °C in acetonitrile, as illustrated in Fig. 1. Reinforcement of the gelling properties could be stimuli ( light, pH etc.) as recently observed for some polymer containing hydrogels.8 Apart from light and cation com- due, in addition to other aggregative factors, to hydrogenbonding between nitrogen atoms and ammonium centres.As plexation, which were found to slightly modulate gelling abilities of cholesteric derivatives,7 such external controls observed by Weiss et al., formation of ammonium groups was shown to influence the gelation in alkyl substrates.6 The remain quite rare for low molecular mass gelling agents. It occurred to us that the introduction of basic sites on DAOA strongest eVect for a 10-2 M acetonitrile solution of 2b was recorded using one equivalent of TFA, as displayed in Fig. 2. might provide a new family of pH-sensitive organogelators. From our previous studies, it emerged that long alkyl chains It was observed that supplementary addition of acid decreases the gelling ability of the material, and that gel formation was and the oblong shape of the molecule characterized by the anthracene ring system were necessary for gel formation.We totally suppressed in strongly acidic medium; the strengthening of the aggregation through hydrogen-bonding is limited therefore decided to investigate the gel-forming abilities of the analogous phenazine derivatives 2a and 2b. by solubility of the gelling species. The role of the anion, which could be of importance in the construction of the In this communication, we report the straightforward synthesis9 of novel organogelators 2a and 2b whose gelling properties and colouration are significantly and reversibly modified by altering the acidity of the solution.Table 1 Gel-forming abilities and gel-to-sol phase transition temperatures (Tgel/°C) of phenazines 2a and 2b at 2×10-2 Ma Solvent 2a 2b MeOH 15 31 EtOH 14 (9) 33 (28) EtOH–H2O (451) pg 12 N N OR OR OR OR 1 2a R = n-C9H19 b R = n-C11H23 Acetonitrile -13 (-18) 44 (30) DMF p 9 The gelating capacity of 2a and 2b was screened at 2×10-2 M Acetone pg 9 by means of the inverted test-tube method.Phenazine (2a: n-Heptane s 3 9.2 mg; 2b: 10.2 mg) and solvent (1 ml ) were warmed in a as=soluble at ambient temperature, pg=partial gel, p=precipitate; septum-capped tube until complete dissolution of the solid.values in parentheses refer to Tgel determined with 1% wt of The solution was immersed in a thermocontrolled bath and organogelator (ca. 1.75×10-2 M and 1.50×10-2 M for 2a and 2b, slowly cooled at 2 °Cmin-1 until the gel formation occurred. respectively).The observed gel-to-sol phase transition temperatures (Tgel) J. Mater. Chem., 1998, 8, 2575–2577 2575Fig. 1 Tgel for 2b in acetonitrile as a function of organogelator concentration with or without trifluoroacetic acid: (%) [TFA]=0, (,) [TFA]=[2b], (#) [TFA]=10-2 M. Fig. 2 Tgel for 2b in acetonitrile as a function of TFA concentration ([2b]=10-2 M). tridimensional supramolecular network, is under current investigation and will be discussed in the forthcoming full paper. Moreover, it was also shown that the gel-forming ability of the system can be fine controlled by the reversible protonation of the phenazine ring.For example, bubbling ammonia through the gel provoked fading of the coloured protonated species and led to an optically translucent gel, the process being reversible upon addition of acid.The magnitude of the increase in Tgel obtained by addition of acid was found to depend on both fluid and concentration of organogelator. As shown in Fig. 1 for acetonitrile solution, the eVect is less pronounced at higher concentrations and most dramatic at Fig. 3 Transmission electron micrographs of a dried acetonitrile gel concentrations below 10-2 M.of 2b (a) in the absence and (b) in the presence of TFA. Direct evidence of the microscopic organization of the gel formed in acetonitrile was obtained from transmission electron fibres. Presumably, this latter is necessary to accommodate micrographs (TEM, Fig. 3). Numerous juxtaposed, fused, and hydrogen-bonding interactions formed upon protonation of intertwined thin straight fibres (several microns length) are the phenazine ring.formed by entanglement of long, slender aggregates with a In conclusion, the present study has demonstrated that the width of ca. 150 nm woven in a three-dimensional architecgelling abilities and colouration of low molecular mass organic ture. The diameter of the smallest fibres represents several compounds can be fine-tuned by variation in pH. Further gelator molecular l.Of note is the formation of nodes studies are in progress, devoted to the elucidation of the corresponding to areas of large gelator concentration where microscopic arrangement of these supramolecular assemblies. no diVraction pattern could be recorded, suggesting a non- We believe that these and other systems would open up new microcrystalline environment. These intertwined fibres are able prospects for signal-responsive chemistry in molecular to encage the solvent molecules.At 10-2 M, with one equivalent assembly systems. of TFA, TEM [Fig. 3(b)] shows the occurrence of longer We are indebted to Michel Chambon and Christine Corra elongated fibre-like structures without nodes, indicating that (Universite� Bordeaux I ) for TEM experiments. the molecules are apparently more eYciently packed.Compounds 2a and 2b diVer from other hydrogen-bond based organogelators in that the neutral species already form gels Notes and references through self-aggregation via dipole–dipole and van der Waals interactions, and that addition of acid induces a structural 1 R. Dagani, Chem.Eng. News, 1997, 75(23), 26. 2 P. Terech and R. G. Weiss, Chem. Rev., 1997, 97, 3133. reorganization of the molecular rearrangement within the 2576 J. Mater. Chem., 1998, 8, 2575–25773 J. van Esch, R. Kellogg and B. Feringa, Tetrahedron Lett., 1997, 7 K. Murata, M. Aoki, T. Nishi, A. Ikeda and S. Shinkai, J. Chem. Soc., Chem. Commun., 1991, 1715; K. Murata, M. Aoki, 38, 281; F.Placin, M. Colomes and J. P. Desvergne, Tetrahedron Lett., 1997, 38, 2665; K. Hanabusa, A. Kawakami, M. Kimura T. Suzuki, T. Harada, H. Kawabata, T. Komori, F. Ohseto, K. Ueda and S. Shinkai, J. Am. Chem. Soc., 1994, 116, 6664; S. and H. Shirai, Chem. Lett., 1997, 3, 191; J. E. S. Sohna and F. Fages, Chem. Commun., 1997, 327; H. Hachisako, H. Ihara, Shinkai and K. Murata, J.Mater. Chem., 1998, 8, 485. 8 Y. Osada and A. Matsuda, Nature, 1995, 376, 219. T. Kamiya, C. Hirayama and K. Yamada, Chem. Commun., 1997, 19; M. de Loos, J. van Esch, I. Stokroos, R. Kellogg and 9 As adapted from a literature procedure (ref. 10), o-phenylenediamine and 2,5-dihydroxybenzoquinone in refluxing ethanol B. Feringa, J. Am. Chem. Soc., 1997, 119, 12675. 4 W. Gu, L.Lu, G. Chapman and R. G. Weiss, Chem. Commun., yielded 2,3-dihydroxyphenazine, which was subsequently etherified into 2a and 2b using K2CO3 and alkyl bromides (1/4.5/3.5). 1997, 543; R. Hafkamp, B. Kokke, I. Danke, H. Geurts, A. Rowan, M. Feiters and R. J. M. Nolte, Chem. Commun., 1997, The new compounds purified by flash chromatography ( light petroleum–dichloromethane) were fully characterized by 1H and 545. 5 T. Brotin, R. Utermo� hlen, F. Fages, H. Bouas-Laurent and 13C NMR, IRFT, mass spectra and elemental analyses. 10 R. Nietzki and G. Hasteslik, Chem. Ber., 1890, 23, 1337. J. P. Desvergne, J. Chem. Soc., Chem. Commun., 1991, 416; H. Bouas-Laurent, J. P. Desvergne and P. Terech, J. Colloid 11 Compounds 2a and 2b were found to be soluble in benzene, toluene, chloroform and dichloromethane. Interface Sci., 1995, 174, 258; J. L. Pozzo, G. Clavier, M. Colomes and H. Bouas-Laurent, Tetrahedron, 1997, 53, 6377. 12 UV Atlas of organic compounds, Plenum Press, New York, 1968, vol. IV. 6 L. D. Lu and R. G. Weiss, Langmuir, 1995, 11, 3630; R. Mukkamala and R. G. Weiss, Langmuir, 1996, 12, 1474; L. D. Lu and R. G. Weiss, Chem. Commun., 1996, 2029. Communication 8/07237J J. Mater. Chem., 1998, 8, 2575–2577

ISSN:0959-9428    

DOI:10.1039/a807237j

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年代:1998

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摘要:

J O U R N A L O F C H E M I S T R Y Materials Communication 4-[Di(biphenyl-4-yl )amino]azobenzene and 4,4¾-bis[bis(4¾-tertbutylbiphenyl- 4-yl )amino]azobenzene as a novel family of photochromic amorphous molecular materials Yasuhiko Shirota,* Kazuyuki Moriwaki, Satoru Yoshikawa, Toshiki Ujike and Hideyuki Nakano Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadaoka, Suita, Osaka 565–0871, Japan.E-mail: shirota@ap.chem.eng.osaka-u.ac.jp Received 1st September 1998, Accepted 29th September 1998 We have created photochromic amorphous molecular temperature by the incorporation of a bulky substituent and materials containing an azobenzene chromophore, 4- hence, the backward cisAtrans thermal isomerization can be [di(biphenyl-4-yl )amino]azobenzene and 4,4¾-bis[bis(4¾- controlled by temperature for t-BuBBAB.tert-butylbiphenyl-4-yl )amino]azobenzene (t-BuBBAB), which readily form amorphous glasses above room temperature and exhibit photochromism in their amorphous films. We have shown that the photogenerated cis-form in the amorphous film can be stabilized at ambient temperature by the incorporation of a bulky group; the backward cisAtrans thermal isomerization can be controlled by temperature for t-BuBBAB. Photochromic materials have recently been attracting a great deal of attention because of their potential technological applications to optical recording for storage of information, optical switching, etc.In applying photochromic materials to practical applications, they may be used in the form of a solid film.Thus, photochromism in the solid state has been a subject of current interest. Extensive studies have been made of photochromism of polymers containing photochromic chromophores and composite polymer systems, where low molecular-weight photochromic compounds are dispersed in polymer binders.1–8 It is of interest and significance to create low molecularweight organic photochromic compounds that readily form stable amorphous glasses above room temperature, which we refer to as ‘photochromic amorphous molecular materials’ or ‘photochromic molecular glasses’.Photochromic amorphous molecular materials may constitute a novel class of photochromic materials that exhibit glass-transition phenomena usually associated with polymers. They form uniform amorphous films by themselves and have the advantage that there is no dilution of photochromic chromophores relative to photochromic polymers and composite polymer systems, where low molecular-weight photochromic compounds may crystallize at high concentrations. However, very few studies have been made of photochromic amorphous molecular materials because low molecular-weight organic compounds generally tend to crystallize readily.Spirooxazine photochromic compounds have recently been reported to form amorphous glasses DBAB was synthesized by the reaction of 4-aminoazobenzene (3.0 g) with 4-iodobiphenyl (20 g) in the presence of with glass-transition temperatures (Tgs) between -45 and 29 °C.9 copper powder (0.8 g) and potassium hydroxide (7.0 g) in decalin (10 ml ) at 150 °C for 8 h.t-BuBBAB was synthesized We have been studying the creation of amorphous molecular materials.10 In the present study, we have designed and synthe- by refluxing a THF solution (40 ml ) of N,N-bis(4¾-tert-butylbiphenyl- 4-yl )-4-nitroaniline (100 mg) in the presence of LiAlH4 sized novel photochromic compounds containing an azobenzene chromophore, 4-[di(biphenyl-4-yl )amino]azobenzene (68 mg) for 2 h.N,N-Bis(4¾-tert-butylbiphenyl-4-yl )-4-nitroaniline was prepared by the reaction of p-nitroaniline (0.72 g) (DBAB) and 4,4¾-bis[bis(4¾-tert-butylbiphenyl-4-yl )amino]- azobenzene (t-BuBBAB), for making photochromic amor- with 4-tert-butyl-4¾-iodobiphenyl (7.0 g) in the presence of copper powder (2.0 g), potassium carbonate (8.6 g), and 18- phous molecular materials.These compounds were found to form readily stable amorphous glasses with Tgs of 68 and crown-6 (1.5 g) in 1,2,4-trichlorobenzene (10 ml ) at 165 °C for 6 h. These new compounds were identified by various spectro- 177 °C, respectively, and to exhibit photochromism in their amorphous films. It is also shown that the photogenerated cis- scopic techniques, mass spectrometry, and elemental analysis.11 Both DBAB and t-BuBBAB were found to readily form isomer in the amorphous film can be stabilized at ambient J.Mater. Chem., 1998, 8, 2579–2581 2579Fig. 2 The electronic absorption spectral change of a DBAB amorphous film: a) before irradiation, b) photostationary state upon irradiation with 450 nm light from a 500 W Xenon lamp (UXL-500D, USHIO) through an interference filter (IF-S 450, Vacuum Optics Co.).Fig. 1 DSC curves of a) DBAB and b) t-BuBBAB. i) Polycrystalline samples obtained by recrystallization from acetonitrile (DBAB) and from the mixed solvent of benzene and hexane (t-BuBBAB). ii) The glass samples obtained by cooling the melt. amorphous glasses when the melt samples were cooled on standing in air, as evidenced by diVerential scanning calor- Fig. 3 Thermal decay of a) cis-DBAB and b) cis-t-BuBBAB at 30 °C imetry (DSC), X-ray diVraction, and polarizing microscopy. in the amorphous film. Fig. 1 shows DSC curves of DBAB and t-BuBBAB. When the crystalline samples of DBAB and t-BuBBAB were heated, an endothermic peak due to melting was observed at 223 and generated molecule should be stable at ambient temperature.Generally, the cis-form of azobenzene derivatives goes back 356 °C, respectively. When the melt samples were cooled on standing in air, amorphous glasses were obtained via the to the trans-form at ambient temperature in the dark. The rate constants for the backward cisAtrans thermal isomerization supercooled liquid state. When the glass samples of DBAB and t-BuBBAB were again heated, the glass-transition of polymers containing the pendant azobenzene moiety and low molecular-weight azobenzene derivatives dispersed in poly- phenomenon was observed at 68 and 177 °C, respectively.On further heating, crystallization took place around 162 °C, mer binders have been reported to be in the range from 10-3 to 10-2 min-1 at room temperature.3,5 A polyurethane con- followed by melting at 223 °C for DBAB, but no crystallization phenomenon was observed for the t-BuBBAB glass.taining the azobenzene chromophore in the main chain has been reported to undergo backward cisAtrans thermal iso- The amorphous film of DBAB (thickness: 0.10 mm, 1.6×10-5 mol cm-2) was prepared by vacuum deposition at merization with a slower rate constant of 1.7×10-4 min-1 at 3 °C.4 a rate of 2–3 A° s-1 at ca. 10-5 Torr. As Fig. 2 shows, irradiation of the film with 450 nm light (0.44 mW cm-2) at It was expected that the introduction of a bulky group may stabilize the photogenerated cis-form. Based on this idea, we room temperature caused a decrease in the absorbance around 450 nm due to the photoisomerization of trans-DBAB to the have designed and synthesized t-BuBBAB.The t-BuBBAB amorphous film (thickness: 0.05 mm, 5×10-6 mol cm-2), cis-form. The cis-fraction of DBAB in the amorphous film at the photostationary state attained by irradiation for ca. 15 min which was prepared by spin-coating from a benzene solution (ca. 3×10-2 mol dm-3), underwent transAcis photoisomeriz- at 30 °C was 0.53; this value is smaller than that (0.81) for the toluene solution.It is suggested that the local free volume ation on irradiation with 500 nm light (4.6 mW cm-2). The cis-fraction of t-BuBBAB at the photostationary state attained around the remaining trans-DBABmolecules in the amorphous film is not large enough to allow the isomerization from the by irradiation for ca. 1 h was 0.15, being smaller than that for the toluene solution (0.71). The cis-form of t-BuBBAB photo- trans- to the cis-form to the extent observed for the solution. When irradiation was stopped after the reaction system had generated in the amorphous film was found to be fairly stable; 80% of the photogenerated cis-t-BuBBAB still remained after reached the photostationary state, the electronic absorption spectrum of the film gradually recovered to the original one 5 days at room temperature (rate constant: less than 10-5 min-1 after slight decay at the initial time) (Fig. 3b).As in ca. 24 h at room temperature due to the backward cisAtrans thermal isomerization of DBAB (Fig. 3a).12 far as we know, this is the most stable example of a cisazobenzene derivative photogenerated in the amorphous In applying photochromic amorphous molecular materials to the rewritable information storage system, the photo- film.It is thought that most photogenerated cis-t-BuBBAB 2580 J. Mater. Chem., 1998, 8, 2579–25817 A. Yassar, C. Moustrou, H. K. Youssoufi, A. Samat, molecules have lost the space needed to isomerize to the R. Guglielmetti and F.Garnier, Macromolecules, 1995, 28, 4548. trans-form in the film because of the changes in the surrounding 8 A. Shishido, O. Tsutsumi, A. Kanazawa, T. Shiono, T. Ikeda and environment due to the motion of the bulky group in the N. Tamai, J. Am. Chem. Soc., 1997, 119, 7791. process of transAcis photoisomerization. When the film was 9 A. Zelichenok, F. Buchholtz, J. Ratner, E. Fischer and heated up to 140 °C, however, the photogenerated cis-t- V.Krongauz, J. Photochem. Photobiol. A, 1994, 77, 201. 10 Y. Shirota, T. Kobata and N. Noma, Chem. Lett., 1989, 1145; BuBBAB was transformed into the trans-t-BuBBAB within A. Higuchi, H. Inada and Y. Shirota, Adv. Mater., 1991, 3, 549; ca. 10 min. Thus, the backward cisAtrans thermal W. Ishikawa, H. Inada, H. Nakano and Y.Shirota, Chem. Lett., isomerization can be controlled by temperature. 1990, 1731; H. Inada and Y. Shirota, J. Mater. Chem., 1993, 3, In summary, novel photochromic amorphous molecular 319; Y. Shirota, Proc. SPIE-Int. Soc. Opt. Eng., 1997, 3148, 186, materials containing an azobenzene chromophore, DBAB and and references cited therein. 11 DBAB: Yield 45%.Mp 223 °C; dH (600 MHz, THF-d8, TMS) 7.20 t-BuBBAB, have been created.They undergo reversible (d, 2H), 7.27 (d, 4H), 7.28 (t, 2H), 7.40 (t, 4H), 7.41 (t, 1H), 7.47 transAcis and cisAtrans isomerizations in their amorphous (t, 2H), 7.63 (d, 8H), 7.84 (d, 2H), 7.86 (d, 2H); m/z (EI ) 501 films. The stabilization of the photogenerated cis-form and (M+); Calc. for C36H27N3: C, 86.20; H, 5.42; N, 8.38. Found: C, the control of the backward cisAtrans thermal isomeriza- 86.04; H, 5.41; N, 8.23%; lmax(toluene) (log e) for trans-DBAB: tion by temperature are achieved by introducing a bulky 335 nm (4.5), 435 nm (4.5); lmax(toluene) ( log e) for cis-DBAB: 343 nm (4.5), 451 nm (3.9).substituent. t-BuBBAB: Yield 21%. Mp 356 °C; dH (600 MHz, 1,4-dioxaned8, TMS) 1.34 (s, 36H), 7.23 (d, 4H), 7.28 (d, 8H), 7.46 (d, 8H), 7.54 (d, 8H), 7.57 (d, 8H), 7.80 (d, 4H); m/z (EI ) 1044 (M+); Calc.for C76H76N4: C, 87.31; H, 7.33; N, 5.36. Found: C, 87.11; Notes and references H, 7.24; N, 5.35%; lmax(toluene) ( log e) for trans-t-BuBBAB: 345 nm (4.7), 480 nm (4.7); lmax(toluene) ( log e) for cis-t- 1 Photochromism, Molecules and Systems, ed. H. Du� rr and BuBBAB: 342 nm (4.8), 488 nm (3.9). H. Bouas-Laurant, Elsevier, 1990. 12 The cisAtrans thermal isomerization in the amorphous film could 2 Applied Photochromic Polymer Systems, ed. C. B. McArdle, be analyzed in terms of the first-order kinetics consisting of two Blackie & Son Ltd., 1992. components. The rate constants for the faster component with a 3 C. S. Paik and H. Morawetz, Macromolecules, 1972, 5, 171. fraction of 0.05 and slower one with a fraction of 0.95 were 0.12 4 L. Lamarre and C. S. P. Sung, Macromolecules, 1983, 16, 1729. min-1 and 3.3×10-3 min-1, respectively. 5 I. Mita, K. Horie and K. Hirao, Macromolecules, 1989, 22, 558. 6 T. Seki, M. Sakuragi, Y. Kawanishi, Y. Suzuki, T. Takahashi, R. Fukuda and K. Ichimura, Langmuir, 1993, 9, 211. Communication 8/06802J J. Mater. Chem., 1998, 8, 2579–2581 25

ISSN:0959-9428    

DOI:10.1039/a806802j

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Communication A comparative study of cell attachment to self assembled monolayers and plasma polymers Ros Daw,a Ian M. Brook,b A. Jane Devlin,b Robert D. Short,*a Elaine Cooperc and Graham J. Leggettd aDepartment of Engineering Materials, University of SheYeld, Sir Robert Hadfield Building, PO Box 600, SheYeld, UK S1 4DU. E-mail: r.short@shef.ac.uk bDepartment of Oral Maxillofacial Surgery, University of SheYeld, UK S1 4DU cDepartment of Materials Engineering and Materials Design, The University of Nottingham, University Park, Nottingham, UK NG7 2RD dDepartment of Chemistry, University of Manchester Institute of Science and Technology, PO Box 88, Manchester, UK M60 1QD Received 24th August 1998, Accepted 29th September 1998 The first comparative study of cell attachment to 4×10-2 mbar.Films were deposited on Thermanox tissue self-assembled monolayers (SAMs) and plasma-deposited culture plastic coverslips. films is reported. Osteoblast-like cells attached extensively SAMs were prepared according to well-established proto acid-terminated alkyl thiol SAMs and to a plasma co- cedures, by the immersion of gold-coated (50 nm) chromiumpolymer of acrylic acid and octa-1,7-diene (acid-PCP). primed (2 nm) glass microscope slides in 1 mM ethanolic However, they attached poorly to methyl-terminated solutions of mercaptoundecanoic acid (MUA), 3-mercapto- SAMs and a plasma polymer of octa-1,7-diene (OD-PP).propanoic acid (MPA), butanethiol (BT), octanethiol (OT) and dodecanethiol (DDT) for ca. 16 h. BT, OT and DDT were obtained from Fluka, UK, and MPA was obtained from Cell attachment to synthetic substrata is strongly influenced Sigma, UK, and were used as received. MUA was synthesised by the surface chemical structure, both directly (in serum-free following a procedure adapted from the method of Bain et al.7 conditions) and through the adsorption of proteins from Following removal of the SAMs from the thiol solution, they culture medium.SAMs have been used as ‘model’ surfaces to were rinsed with ethanol and dried in a stream of nitrogen. probe cellular responses to specific functional groups. ROS 17/2.8 cells were donated by G.A. Rodan of Merck, Carboxylic acid-terminated SAMs have been shown to pro- Sharp and Dohme. Cells were removed from liquid nitrogen, mote the attachment and spreading of osteoblasts,1 fibroblasts2 and fast thawed.They were suspended in serum supplemented and endothelial cells,3 whereas methyl-terminated SAMs have medium, seeded in tissue culture flasks and incubated for seven not promoted attachment. days. The cells were then trypsinised using 0.05% trypsin A number of attachment studies have been undertaken with containing 0.5 mM disodium ethylenediaminetetraacetic acid plasma-deposited films.4 Plasma polymers cannot be con- (Na2EDTA), before centrifuging at 405g.The cell pellet was sidered ‘model’ surfaces for studies of this type as the technique re-suspended in fresh medium. Three samples of each film leads to the incorporation of functional groups not present in were placed into 24 well trays.Cells were seeded at a density the original monomer unit. However, plasma deposits are of 1.1×105 cells ml-1 (with 1 ml per well ), and allowed to important in biomaterials science: a wide range of compounds attach in an incubator at 37 °C and 5% CO2 for 90 min. Each can be plasma polymerised and deposition is possible onto sample was then washed three times with warmed PBS to virtually any substrate.Deposits are produced in a sterile remove unattached cells. Cells were counted using a micro- environment free of initiator and solvents, and they can be scope graticule and at least four fields of view for each sample. readily used to coat medical devices and implants. The mean cells per mm2 and standard deviation were calcu- It has been shown that plasma co-polymerisation using low lated.Total DNA content of the samples was estimated using plasma powers may be used to control surface functional a Hoechst stain. The technique is described elsewhere.6 group concentration. The attachment of keratinocytes5 and Statistical significance was calculated using the Students’ t-test. osteoblast-like cells6 to plasma co-polymerised surfaces has Data were taken to be significant when a p value of 0.05 or been explored in previous studies.Films with oxygen-containless was obtained (showing a 95% confidence limit). ing functional groups encouraged cell attachment,5,6 with Fig. 1 shows the mean number of osteoblast cells per mm2 keratinocyte attachment correlating best with the carboxylic attached to each sample.Two distinct levels of attachment acid functional group.5 Optimum attachment was found to were seen. Almost equal numbers of cells attached to the acid- surfaces containing 3% carboxylic acid. Pure hydrocarbon terminated SAMs and the acid plasma co-polymer, with the films (OD-PP) produced poor attachment.5,6 possible suggestion of higher levels of attachment to the The purpose of this study was to compare osteoblast-like plasma co-polymer (acid-PCP).On the methyl-terminated cell attachment on these plasma-polymerised materials with SAMs and OD-PP, statistically similar numbers of cells were attachment to self-assembled monolayers of carboxylic acid found to have attached, although slightly higher mean levels and methyl-terminated adsorbates.of attachment were observed on the OD-PP and BT surfaces. The reactor vessel used for the plasma polymerisations is The numbers of cells on the acid-terminated SAMs and the described elsewhere.6 The plasma was sustained by a radioacid- PCP surfaces were significantly greater than the numbers frequency (13.56 MHz) signal generator and amplifier ‘inducof cells attaching to the methyl-terminated SAMs and the tively’ coupled to the reactor vessel.The base pressure of the OD-PP surfaces. reactor vessel was 8×10-3 mbar. Acrylic acid (40%) and octa- On the hydrocarbon surfaces, cells were only weakly 1,7-diene (60%), from Aldrich UK, were co-polymerised at a attached, clumped together and had a rounded morphology. plasma flow rate of 2 cm3 min-1 (STP), and power of 2 W.The cells more strongly adhered to the acid surfaces; they were Octa-1,7-diene was polymerised using the same plasma parameters. The pressure during polymerisation was typically better spread and less clumped. J. Mater. Chem., 1998, 8, 2583–2584 2583The result from the OD-PP and methyl-terminated SAMs is also interesting. Structural irregularity in the surface of the OD-PP was anticipated.The surface will present a number of diVerent carbon–hydrogen bonding arrangements (methyl, methylene, olefinic etc.). Order in SAMs is thought to be influenced by adsorbate chain length. Long chain SAMs, such as those formed from DDT, exhibit ordered, crystalline structures in which the alkyl chains pack relatively rigidly. Short chain SAMs, such as those formed from BT, are thought to exist in a two dimensional liquid state, in which the alkyl chains are relatively mobile.Consequently, while the surface of a long chain SAM is composed almost exclusively of methyl groups, the short chain SAMs are much more disordered and the alkyl chain methylene groups are exposed as well as terminal methyl groups. In view of this it is not surprising that the data for OD-PP resemble those for the short-chain Fig. 1 Cell attachment to SAMs, plasma-deposited surfaces, TCPS SAM, BT, rather than those formed from longer adsorbates and gold. Values are the average of 12 measurements (three samples (OT and DDT). ×four fields of view) except for BT.† The statistical significance ( p) In serum-containing media, cell attachment is influenced by between the acid surfaces and methyl surfaces was 0.002; between the OD-PP and methyl-terminated SAMs, p=0.1.the adsorbed protein layer. Identification of the factors that control protein adsorption and retention of activity (conformation) once adsorbed, has been the subject of considerable Fig. 2 shows the measured DNA concentrations per mm2 research eVort, with much emphasis put upon surface hydro- on these samples.The amounts of DNA measured on the philicity/hydrophobicity. However, adsorption may also be acid-PCP and the acid-terminated SAMs were almost equal. controlled by factors at the molecular level, such as the There appeared significantly more DNA on the OD-PP than availability of a specific functional group that acts at a specific both the DDT- and OT-SAMs, but a similar value was binding site for the protein.In this study, no attempt was recorded for the BT-SAM. The amounts of DNA on the acid- made to control the nature of the layer of adsorbed protein containing surfaces were significantly higher than on the at the sample surfaces. Undoubtedly, the nature of the hydrocarbon surfaces. adsorbed protein layer, which forms rapidly following exposure Also included in Fig. 1 are data for cells cultured on gold of the samples to the culture medium, plays a critical role in and tissue culture polystyrene (TCPS). The former is the determining the ultimate outcome of the cell–material intersubstrate on which the SAMs were prepared. It has been action. Future studies must address this problem.However, previously observed that cells attach well to this surface. TCPS the contrasting eVects of the diVerent chemistries, and the is the usual substratum material used in tissue culture, although similarity in the response of the plasma co-polymer and the it may be subsequently coated with various extracellular matrix acid terminated SAM, are clearly demonstrated. (ECM) components, prior to cell culture.Good attachment This study draws attention to the importance of having to TCPS was expected, but as described elsewhere, the surface well-defined surfaces on which to work. It illustrates the chemistry of TCPS can be variable and it has only limited importance of the acid functionality, but also questions value as a control.5 whether cell attachment requires high concentrations of this The degree of correlation observed in the cell counts and in functionality at the surface.The comparison of the OD-PP the DNA between the acid-terminated SAMs and the acid- with the methyl-terminated SAMs illustrates how SAMs may PCP is perhaps surprising. Although both contain carboxylic possibly be used to probe structural disorder in PPs.acid, the numbers of carboxylic acid groups per 100 carbons are very diVerent for these two types of surface. Furthermore, R.D.S. and R.D. thank the EPSRC for a studentship (for R.D.); G.J.L. and E.C. thank the Leverhulme Trust for the in the SAMs, the acid groups are known to be at the liquid provision of a research grant and G.J.L. thanks the NuYeld interface; this has not been established for the acid-PCP.In Foundation for provision of a Science Research Fellowship. aqueous media, the acid-PCP can hydrate, as we will demonstrate elsewhere.8 This behaviour is not expected of the acidterminated SAMs. Significant re-orientation of acid functional Notes and references groups within the surface of the acid-PCP, to present a higher † Cell counts comparable to those made on gold were made on two concentration of acid at the liquid interface, can be discounted of the BT SAMs, but these values were discounted on the grounds as the advancing contact angle measured on the acid-PCP (80° that the BT SAM is the least stable of the SAM surfaces studied.for distilled water) reflects the essentially hydrocarbon nature We strongly suspected that regions of the gold substratum had of this surface.become exposed. The anomaly was verified by the respective DNA values, which were also subsequently discarded. 1 C. A. Scotchford, E. Cooper, S. Downes and G. J. Leggett, J. Biomed. Mater. Res., 1998, 41, 431. 2 E. Cooper, W. Robin, D. A. Hunt, L. Parker, G. J. Leggett and T. L. Parker, J. Mater. Chem., 1997, 7, 435. 3 C. D. Tidwell, A. M. Belu, B. D. Ratner, B. Tarasevich B. S. Atre and D. L. Allara, Transactions Soc. Biomater, 23rd Annual Meeting, 1997, vol. XX, 203. 4 S. I. Ertel, A. Chilkoti, T. A. Horbett and B. D. Ratner, J. Biomater. Sci., Polym. Ed., 1991, 3, 163. 5 R.M. France, R. D Short, R. A. Dawson and S. MacNeil, J. Mater. Chem., 1998, 8, 1, 37. 6 R. Daw, A. J. Beck, R. D. Short, A. J. Devlin and I. M. Brook, Biomaterials, 1998, in the press. 7 C. D. Bain, E. B. Troughton, Y.-T. Tao, J. Evall, G. M. Whitesides and R. G. Nuzzo, J. Am. Chem. Soc., 1989, 111, 321. 8 R. M. France, R. J. Heaton, M. C. Davies, S. J. B. Tendler, C. J. Roberts, P. Williams, R. Daw and R. D. Short, unpublished Fig. 2 Total (mg mm-2) measured on the SAMs, PP, PCP, TCPS and work. gold. The statistical significance ( p) between the acid surfaces and methyl surfaces was 0.04; between the OD-PP and methylterminated SAMs, p 0.05. Communication 8/06612D 2584 J. Mater. Chem., 1998, 8, 2583–2584

ISSN:0959-9428    

DOI:10.1039/a806612d

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Communication The synthesis and structure of Tl(Sr1.4La2.6)Ni2O9; a direct structural analogue of a superconducting cuprate Christopher S. Knee and Mark T.Weller* Department of Chemistry, The University of Southampton, Southampton, UK SO17 1BJ. Email: mtw@soton.ac.uk Received 21st October 1998 The nickelate Tl(Sr1.4La2.6)Ni2O9, synthesised by the reac- larger than expected value and this atom was therefore placed on a new 4-fold site (x,y,0) at quarter occupancy.This tion of Tl2O3, La2 O3 and Sr2Ni2O5, is isostructural with the equivalent superconducting cuprate phase, consisting displacement refined steadily with x=y#0.052, and a concomitant reduction in the thermal parameter to a reasonable of layers of apex-sharing, stoichiometric NiO6 octahedra separated by a TlO layer.value. Attention was then focused on the mixed Sr/La sites, in particular the (0,0,0.2) site, which had an unreasonably low temperature factor. The fractional occupancies of the La and The structural chemistry of complex nickelates has recently Sr sites were reciprocally linked and allowed to vary from the regained impetus from its relationship to that of cuprates initial 50550 ratio.The site was found to favour a marked exhibiting high temperature superconductivity. One area of increase in the La content to around 0.8 and the thermal particular interest has been complex nickel oxides adopting parameter now refined to a larger, more sensible, value. The the perovskite and K2NiF4 structures e.g. La2-xSrxMO4±d occupancy of the other Sr/La site (0,0,0.08) was probed in (M=Ni, Cu);1,2 superconductivity has been claimed in the this way however the variation was negligible and the site nickelate but this has never been fully substantiated.2 A few occupancy factors maintained at 0.5/0.5.The final refined other phases exist with structural analogies in copper and structural parameters are summarised in Table 1 and derived nickel chemistry for example Li2MO23,4 and ‘BaMO2’.5,6 These bond lengths given in Table 2.The final fit to the profile is structural analogues derive from the ability of both copper(II ) shown in Fig. 1, and Fig. 2 shows the structure of and nickel(II) to adopt four- (square planar), five- (square Tl(Sr2La2)Ni2O9. pyramidal ) and six-fold (octahedron based) coordinations to The refined stoichiometry of Tl(Sr1.4La2.6)Ni2O9 gives a oxygen though the five- and six-fold geometries are normally nickel oxidation state of +2.2 with full occupancy of the more distorted for copper due to the Jahn–Teller eVect. More oxygen sites; no evidence was found in this work of site recently the synthesis of TlSr2NiO4+d7,8 has further demondeficiencies and all refined oxygen atom temperature factors strated the ability of nickelates to adopt similar structures were reasonable.The nickel oxidation state is also dependent to cuprates, though in this case the oxygen stoichiometries on the La5Sr ratio and the slight strontium deficiency with and distributions diVer and no complete NiO2 layers exist. respect to the starting stoichiometry is consistent with the In this communication we report the synthesis and characterobservation of the small level of Tl2Sr4O7 impurity.A higher isation of a new layered complex nickel oxide, Tl(Sr2La2)- level of lanthanum on the A-sites is also observed in the Ni2O9, containing complete NiO2 layers, which is a direct reported isostructural cuprate, Tl(Ba1.6La2.4)Cu2O9.9 The analogue of a high temperature superconducting phase, eVects of diVerent starting ratios of Tl, La and Sr are currently Tl(Ba2-xLa2+x)Cu2O9, Tc=35 K.9 Tl2O3, La2 O3, and Sr2Ni2O5, synthesised following the literature method,10 were mixed in the ratio 15151 and ground Table 1 Final refined atomic coordinates for Tl(Sr1.4La2.6)Ni2O9 thoroughly. The reactants were pressed into 13 mm diameter (e.s.d.s are given in parentheses) pellets under ca. 10 tonne cm-2 and the pellets so formed sealed inside a gold tube. The capsule was then slowly ramped Atom Sitex y z B/A° 2 n to 900 °C and fired for 5 h and then allowed to furnace cool. Tl 8h 0.0517(2) 0.0517(2) 0.0 1.50(18) 0.25 The resulting black powder was examined using a Siemens La(1) 4e 0.5 0.5 0.0845(1) 1.82(9) 0.5 D5000 diVractometer (CuKa1 radiation) and the pattern Sr(1) 4e 0.5 0.5 0.0845(1) 1.82(9) 0.5 obtained was indexed on a tetragonal unit cell with a#3.8, La(2) 4e 0.5 0.5 0.2032(1) 1.19(7) 0.8 c#30.0 A° .Weak lines (I/I0<5%), that did not index using Sr(2) 4e 0.5 0.5 0.2032(1) 1.19(7) 0.2 this method, were identified as resulting from the impurity Ni 4e 0.0 0.0 0.1466(3) 1.20(13) 1.0 phase Tl2Sr4O7.O(1) 2b 0.5 0.5 0.0 4.1(1) 1.0 The structure of Tl(Sr2La2)Ni2O9 was refined from X-ray O(2) 4e 0.0 0.0 0.0721(8) 3.0(8) 1.0 powder diVraction data using the GSAS package.11 Data were O(3) 8g 0.5 0.0 0.1439(6) 2.3(4) 1.0 O(4) 4e 0.0 0.0 0.2178(7) 0.5(6) 1.0 collected in the 2h range 17–117° over a 17 h period with a step size of 0.02°. The starting structural model was taken Space group I4/mmm; a=3.8062(1), c=30.0854(6) A° ; RF=8.50%, from that of Tl(Ba2-xLa2+x)Cu2O9,9 in the space group x2=1.76. I4/mmm.Initial stages of the refinement placed all atoms on these special sites at full occupancy and proceeded with the variation of global parameters such as background and peak Table 2 Selected derived bond lengths profile coeYcients and the lattice constants.The contribution Tl–O(1) 2.970(9) La(2)–O(3)×4 2.610(1) from the Tl2Sr4O7 impurity was introduced as an additional Tl–O(1)×2 2.706(1) La(2)–O(4)×5 2.727(4) phase using literature data,12 and fitted with the refinement of Tl–O(1) 2.413(9) La(2)–O(4)×1 2.377(8) cell constants and phase fraction only. The isotropic tempera- Tl–O(2)×2 2.188(3) Ni–O(3)×4 1.905(1) ture factors of the atoms in the main phase were then La(1)–O(1) 2.543(4) Ni–O(2) 2.24(3) introduced, and, along with the refinement of atomic positions, La(1)–O(2)×4 2.717(4) Ni–O(4) 2.14(2) resulted in the expected improvement in the least-squares fit.La(1)–O(3)×4 2.610(2) The temperature factor for the thallium atom refined to a J. Mater. Chem., 1998, 8, 2585–2586 2585with an in-plane distance of 1.905(1) A° and two apical bonds of 2.24(3) and 2.14(2) A° .These units are linked by sharing equatorial apices to form infinite layers in the ab-plane of stoichiometry NiO2. There is no evidence of oxygen vacancies in these planes, in contrast to the structurally related 1201 phase TlSr2NiO4+d7,8 in which the asymmetric distribution of oxide vacancies leads to an orthorhombic structure.A TlO layer separates the NiO6 octahedra. The thallium coordination is complicated by the disorder of the thallium atom, which is a common feature of such single layer thallium materials.13 The average thallium environment may be most simply viewed as distorted octahedral. The Sr/La sites exhibit 9-fold coordination and the bond lengths are similar to those found in the (La,Sr)2NiO4 phases with the K2NiF4 type structure.Comparison of the nickel and copper coordination in the materials Tl(Sr1.4La2.6)Ni2O9 and Tl(Ba1.6La2.4)Cu2O9 reveal Fig. 1 Final profile fit for Tl(Sr1.4La2.6)Ni2O9. Crosses mark observed near identical M–O in-plane distances of 1.905(1) and intensities, upper continuous line the calculated profile, lower continu- 1.906(1) A° respectively.The apical metal–oxygen environment ous line the diVerence. Reflections are shown with tick marks for if more distorted for the cuprate, with two quite diVerent Tl(Sr1.4La2.6)Ni2O9 ( lower) and Tl2Sr4O7 (upper). apical distances of 2.63(3) and 2.25(4) A° compared to the more regular 2.24(3) and 2.14(2) A° observed for the nickelate, which may be attributed to Jahn–Teller eVects.The extension of the series of compounds Tl(Sr2-xLn2+x)Ni2O9, to Ln=Nd–Gd, is being undertaken. The materials show the expected reduction in lattice parameters, i.e. for Tl(Sr2Gd2)Ni2O9, a=3.7681(1), c= 29.4012(9) A° . In conclusion the end member of a new family of layered nickel oxides Tl(Sr1.4Ln2.6)Ni2O9, Ln=La has been synthesised and characterised using powder X-ray diVraction. The electronic and magnetic properties of this material and the structural characterisation of the full range of lanthanide derivatives will be reported in a full paper.We thank the EPSRC for a studentship for C.S.K. Notes and references 1 M. James and J. P. Attfield, J. Mater. Chem., 1996, 6, 57. 2 Z. Kakol, J. Spalek and J. M.Honig, J. Solid State Chem., 1989, 79, 288. 3 H. Rieck and R. Hoppe, Z. Anorg. Allg. Chem., 1972, 392, 193. 4 W. Losert and R. Hoppe, Z. Anorg. Allg. Chem., 1970, 379, 234. 5 M. A. G. Aranda and J. P. Attfield, Angew. Chem., Int. Ed. Engl., 1993, 32, 1454. 6 R. Gottscall and R. Scollhorn, Solid State Ionics, 1993, 59, 93. 7 C. S. Knee and M. T.Weller, J. Mater. Chem., 1996, 6, 1449. 8 C. S. Knee and M. T. Weller, The structure of TlSr2NiO4+d by high-resolution powder neutron diVraction, J. Solid State Chem., submitted. 9 C. Martin, A. Maignan, M. Huve, M. Hervieu, C. Michel and B. Raveau, Physica C, 1991, 179, 1. 10 Y. Takeda, T. Hashino, H. Miyamoto, F. Kanamaru, S. Kume and M. Koizumi, J. Inorg. Nucl. Chem. Lett., 1972, 34, 1599. 11 A. C. Larson and R. B. Von Dreele, MS-H805, Los Alamos Fig. 2 Structure of Tl(Sr2La2)Ni2O9. The nickel coordination is shown National Laboratory, Los Alamos, NM, 87545. as octahedra, thallium ions are shown as small dark spheres, Sr/La 12 R. von Schenck and H. Mueller-Buschbaum, Z. Anorg. Allg. ions as large dark spheres and oxygen as medium light spheres. Chem., 1973, 396, 113. 13 Morosin, E. L. Venturini and D. S. Ginley, Physica C, 1991, being probed but absolute phase purity is diYcult to achieve 183, 90. due to loss of thallium from the reactant mixture. The nickel coordination is an elongated NiO6 octahedron Communication 8/08182D 2586 J. Mater. Chem., 1998, 8, 2585–2586

ISSN:0959-9428    

DOI:10.1039/a808182d

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Communication Rb2Cu3CeTe5: a quaternary semiconducting compound with a two-dimensional polytelluride framework Rhonda Patschke,a Paul Brazis,b Carl R. Kannewurfb and Mercouri Kanatzidis*a aDepartment of Chemistry, Michigan State University, East Lansing, Michigan 48824, USA. Fax: Int. Code+(517) 3531793. E-mail: kanatzidis@argus.cem.msu.edu bDepartment of Electrical Engineering and Computer Science, Northwestern University, Evanston, Illinois 60208, USA Received 28th August 1998, Accepted 30th September 1998 Rb2Cu3CeTe5 has been synthesized from the reaction of Cu and Ce in a molten alkali metal/polytelluride flux.The compound crystallizes in the monoclinic space group, C2/m (no. 12) with a=18.6884(1) A° , b=6.2384(2) A° , c= 12.5264(3) A° , b=112.795(1)°, V=1346.34(5) A° 3, and Z= 4.Rb2Cu3CeTe5 is two-dimensional with 1 2 [Cu3CeTe5]2- layers built from one-dimensional 1 2 [Cu2CeTe5]3- chains that are ‘stitched’ together by distorted tetrahedral Cu atoms; the compound is paramagnetic and a narrow-gap p-type semiconductor. Over the past decade, the polychalcogenide flux method has become an established technique for discovering new solid state chalcogenides.1 Although many of the compounds form completely new structure types, others are reminiscent of, or can be considered derivatives of known chalcogenides.This is particularly true when lanthanide and actinide metals are Fig. 1 ORTEP representation of the structure of Rb2Cu3CeTe5 as involved. The binary LnQ3 phases (NdTe32 and ZrSe33 type), seen down the b-axis (90% ellipsoids).The ellipses with octant shading for example, are quite stable. Several new ternary phases have represent Ce and Rb, the crossed ellipses represent Cu and the open recently been reported in which the structural motifs are ellipses represent Te. related to these LnQ3 binaries. While NaLnS3 (Ln=La,Ce)4 and ATh2Q6 (A=Cs,Rb,K; Q=Se,Te)5 represent two diVerent variations of the ZrSe3 structure type, ALn3Te8 (A=Cs,Rb,K; Conceptually, these one-dimensional chains derive from the ZrSe3 structure type.By replacing one (Q22-) unit in the ZrSe3 Ln=Ce,Nd)6 is closely related to the structure of NdTe3. In an eVort to access quaternary phases which are less structurally framework with a Q2- unit, the coordination environment of the metal changes from bicapped trigonal prismatic to pentag- related to the LnQ3 binaries, another element was introduced into the synthesis.Copper proved to be well behaved in this onal bipyramidal. This change in coordination is accompanied by a conversion from two-dimensional layers to one-dimen- respect and we were able to isolate several compounds, whereas other elements gave phase-separated ternary compounds.sional chains. Within the 1 2 [CeTe5]5- chains exist empty distorted tetrahedral pockets of Te atoms which are large Reactions in the A/Cu/Ln/Q (Q=S,Se) system have produced several quaternary compounds, including K2Cu2CeS4,7 enough to accommodate Cu atoms. Each Cu atom is bonded at two points to the axial positions of two neighboring KCuCe2S6,7,8 KCuLa2S6,8 CsCuCe2S6,8 CsCuCeS38 and KCuUSe3.8 Other investigators have identified such com- pentagonal bipyramids, and at the remaining sites to the closest edge between these axial positions.The chains, once pounds as BaErAgS3,9 CsCuUTe3,10 BaLnMQ3 (Ln= La,Ce,Nd;M=Cu,Ag; Q=S,Se)11 and KCuEu2S6.12 Although extended to include the Cu atom, can be written as 1 2 [Cu2CeTe5]3-.Finally, the layers are formed when the second many of these phases are structurally unique, some still retain the components of the LnQ3 motif. It is apparent that the type of Cu atom ‘stitches’ these chains together in the adirection by coordinating to neighboring chains in a distorted greater the amount of copper in the framework, the more profound the eVect of breaking up the LnQ3 structure.Along tetrahedral arrangement. A view perpendicular to the layers is shown in Fig. 2(B). It is interesting that if one removes the these lines, we examined the A/M/Ln/Te (M=Cu,Ag) system using polytelluride fluxes and discovered several novel Ce atoms from the structure, the remaining [Cu3Te3] substructure remains contiguous. In this sense, the Ce atoms are compounds including KCuCeTe4,13 K2Ag3CeTe414 and K2.5Ag4.5Ce2Te9.15 We report here on Rb2Cu3CeTe5,16 a low- situated on both sides of a two-dimensional [CuTe]- substrate.In fact, this copper telluride framework, albeit distorted, bears dimensional compound in which the basic LnQ3 structure is substantially disrupted. a close resemblance to the layers of NaCuTe18 [Fig. 2(C)]. The magnetic susceptibility of Rb2Cu3CeTe5 was measured Rb2Cu3CeTe5 consists of 1 2[Cu3CeTe5]2- layers separated by Rb+ cations (Fig. 1.) The Ce atom is seven coordinate, over the range 5–300K at 6000 G, and a plot of 1/xm vs. T shows that the material exhibits nearly Curie–Weiss behavior exhibiting a distorted pentagonal bipyramidal geometry in which one g2-(Te22-) unit17 and three Te2- anions comprise with only slight deviation from linearity beginning below 50 K.Such deviation has been reported for several Ce3+ compounds the pentagon and two Te2- anions occupy the axial positions [Fig. 2(A)]. The pentagonal bipyramids share monotelluride and has been attributed to crystal field splitting of the 3F5/2 ground state of the cation.19 At temperatures above 150 K, ions, forming 1 2 [CeTe5]5- chains parallel to the b-axis.J. Mater. Chem., 1998, 8, 2587–2589 2587Fig. 2 (A) Schematic comparison of the two-dimensional layers of ZrSe3, the one-dimensional 1 2 [CeTe5]5- chains and the 1 2 [Cu2CeTe5]3- chains in Rb2Cu3CeTe5. The dotted line highlights the pentagonal bipyramidal coordination around Ce. Selected distances (A° ) are as follows: Ce–Te1 3.161(1), Ce–Te2 3.2538(5), Ce–Te3 3.246(2), Ce–Te4 3.253(2) and Te1–Te1 2.771(2).(B) View perpendicular to the layers of Rb2Cu3CeTe5, illustrating how the second Cu atom stitches together the 1 2 [Cu2CeTe5]3- chains to form two-dimensional layers. The ditelluride groups above and below the anionic layers are omitted for clarity. Selected distances (A° ): Cu1–Te2 2.820(2), Cu1–Te3 2.591(2), Cu1–Te4 2.593(2), Cu1–Ce1 3.332(2), Cu1–Cu2 2.650(2), Cu2–Te3 2.721(2), Cu2–Te4 2.721(2) and 2.593(2).(C) The distorted [CuTe]-, PbO-like layer in Rb2Cu3CeTe5. a meff of 2.64 mB has been calculated, which is in accord with the usual range for Ce3+ compounds (2.3–2.5 mB). The presence of Ce3+ is confirmed by IR spectroscopy with shows one well defined, broad peak at ca. 3420 cm-1 (0.42 eV) This absorption is electronic in origin and is attributed to an f–f or f–d transition within the f1 configuration of Ce3+. From this we can conclude that Rb2Cu3CeTe5 is a valence precise compound, and thus we expect semiconducting properties. The formal oxidation states are (Rb1+)2(Cu1+)3(Ce3+)- (Te2-)3(Te22-). The electrical conductivity of Rb2Cu3CeTe5 as a function of temperature measured on single crystals suggests that the material is indeed a narrow gap semiconductor with a room temperature conductivity value of 0.05 S cm-1 [Fig. 3(A)]. The log s vs. 1/T plot is non-linear over the entire temperature range of 8–300 K, suggesting the conduction mechanism varies in diVerent temperature regions, possibly due to diVerent types of mid-gap states.Thermoelectric power data as a function of temperature show a large Seebeck coeYcient at room temperature of +275 mVK-1 [Fig. 3(B)]. The increasing Seebeck coeYcient with decreasing temperature and its positive sign are consistent with a p-type semiconductor. Note added in proof. By the time we received proofs of this manuscript we became aware of the syntheses of BaDyCuTe3, K1.5Dy2Cu2.5Te5 and K0.5Ba0.5DyCu1.5Te3 (F.Q. Huang, W. Choe, S. Lee and J. S. Chu, Chem. Mater., 1998, 10, 1320). These compounds are not structurally related to the one reported here; however, they do belong in the broad quaternary family of A/Cu/Ln/Q compounds. Acknowledgments Financial support from the National Science Foundation (DMR-9527347 MGK) and (DMR-9622025 CRK) is gratefully acknowledged.The authors are grateful to the X-ray Fig. 3 (A) Variable temperature, four-probe electrical conductivity Crystallographic Laboratory of the University of Minnesota data for a single crystal of Rb2Cu3CeTe5. (B) Variable temperature thermopower data for a single crystal of Rb2Cu3CeTe5. and to Dr. Victor G. Young, Jr., for collecting the single 2588 J.Mater. Chem., 1998, 8, 2587–25890.447 g Te (7.0 mmol) which was sealed under vacuum in a carbon crystal X-ray data set. M.G.K. is a Henry Dreyfus Teacher coated quartz tube and heated to 850 °C for 10 days. The tube was Scholar 1993–1998. This work made use of the SEM facilities then cooled to 400 °C at -3 °C h-1, and then quenched to room of the center for Electron Optics at Michigan State University.temperature. The excess RbxTey flux was removed, under nitrogen At Northwestern University, this work made use of the Central atmosphere, with dimethylformamide to reveal black needle- Facilities supported by NSF through the Materials Research shaped crystals in 45% yield (based on Cu). The crystals are air and water stable. Phase homogeneity was confirmed by comparing Center (DMR-9632472).the power X-ray diVraction pattern of the product against that calculated using the crystallographically determined atomic coordinates. Microprobe analysis carried out on randomly selected Notes and references crystals gave an average composition of Rb2.46Cu3.29Ce1.0Te5.55. A Siemens SMART Platform CCD diVractometer was used to 1 M. G.Kanatzidis and A. C. Sutorik, Prog. Inorg. Chem., 1995, 43, collect data from a crystal of 0.160×0.035×0.010 mm dimensions 151 and references therein; M. G. Kanatzidis, Curr. Opin. Solid using Mo-Ka (l=0.71073 A° ) radiation. SMART16b software was State Mater. Sci., 1997, 2, 139; M. A. Pell and J. A. Ibers, Chem. used for data acquisition and SAINT16c for data extraction and Ber./Recueil, 1997, 130, 1.reduction. An absorption correction was performed using 2 B. K. Norling and H. Steinfink, Inorg. Chem., 1966, 5, 1488. SADABS.16d 3 V.W.Kro� nert and K. Plieth, Z. Anorg. Allg. Chem., 1965, 336, Crystal data at 173 K: a=18.6884(1), b=6.2384(2), c= 207. 12.5264(3) A° , b=112.795(1)°, V=1346.34(5) A° 3, Z=4, Dc= 4 A. C. Sutorik and M. G. Kanatzidis, Chem. Mater., 1997, 9, 387. 5.623 g cm-3, monoclinic, space group C2/m (no. 12), m= 5 J. A. Cody and J. A. Ibers, Inorg. Chem., 1996, 16, 3273; E. J.Wu, 25.741 mm-1, index ranges -22h20, 0k7, 0l14, M. A. Pell and J. A. Ibers, J. Alloys Compd., 1997, 255, 106; 2hmax=50°, total data 3427, unique data 1307 (Rint=0.044), data K.-S. Choi, R. Patschke, S. J. L. Billinge, M. J.Waner, M. Dantus with Fo2>2s(Fo2) 1087, no.of variables 60, final R/wR2= and M. G. Kanatzidis, J. Am. Chem. Soc., in press. 0.0461/0.1182, GOF 1.041. The structure was solved and refined 6 R. Patschke, J. Heising, J. Schindler, C. R. Kannewurf and using the SHELXTL-5 package of crystallographic programs;16e M. G. Kanatzidis, J. Solid State Chem., 1998, 135, 111. SHELXTL refines on F2. (b) SMART: 1994, Siemens Analytical 7 A.C. Sutorik, J. Albritton-Thomas, C. R. Kannewurf and Xray Systems, Inc., Madison, WI 53719 USA; (c) SAINT: Version M. G. Kanatzidis, J. Am. Chem. Soc., 1994, 116, 7706. 4, 1994–1996, Siemens Analytical Xray Systems, Inc., Madison, 8 A. C. Sutorik, J. Albritton-Thomas, T. Hogan, C. R. Kannewurf WI 53719 USA; (d) SADABS: G.M. Sheldrick, University of and M.G. Kanatzidis, Chem.Mater., 1996, 8, 751. Go� ttingen, Germany, to be published. (e) SHELXTL: Version 5, 9 P. Wu and J. A. Ibers, J. Solid State Chem., 1994, 110, 156. 1994, G.M. Sheldrick, Siemens Analytical X-ray Instruments, Inc. 10 J. A. Cody and J. A. Ibers, Inorg. Chem. 1995, 34, 3165. Madison, WI 53719. Full crystallographic details, excluding 11 A. E. Christuk, P. Wu and J.A. Ibers J. Solid State Chem., 1994, structure factors, have been deposited at the Cambridge 110, 330; P. Wu and J. A. Ibers, J. Solid State Chem., 1994, Crystallographic Data Centre (CCDC). Any request to the CCDC 110, 337. for this material should quote the full literature citation and the 12 W. Bensch and P. Du� richen, Chem. Ber., 1996, 129, 1489. reference number 1145/124. 13 R. Patschke, J. Heising, P. Brazis, C. R. Kannewurf and 17 The Te–Te stretch exhibits a Raman shift at ca. 160 cm-1. M. G. Kanatzidis, Chem.Mater., 1998, 10, 695. 18 G. Savelsberg and H. Scha�fer, Z. Naturforsch., Teil B. 1978, 33, 14 R. Patschke, P. Brazis, C. R. Kannewurf and M. G. Kanatzidis, 370. Inorg. Chem., in press. 19 N. N. Greenwood and A. Earnshaw, Chemistry of the Elements, 15 R. Patschke, P. Brazis, C. R. Kannewurf and M. G. Kanatzidis, Pergamon Press, New York, 1984, p. 1443. submitted for publication. 16 (a) Rb2Cu3CeTe5 was synthesized from a mixture of 0.448 g Rb2Te (3.0 mmol), 0.095 g Cu (3.0 mmol), 0.070 g Ce (1.0 mmol) and Communication 8/06729E J. Mater. Chem., 1998, 8, 2587–2

ISSN:0959-9428    

DOI:10.1039/a806729e

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Influence of composition and glass transition temperature on the diVusion and solubility behaviour of methyl ethyl ketone–isopropyl alcohol mixtures in poly(methyl methacrylate) Richard A. Pethrick* and Kathleen E. Rankin Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow, UK G1 1XL Received 31st March 1998, Accepted 14th October 1998 The process of removal by a solvent mixture of low molar mass polymer generated as a consequence of chain scission is a critical step in electron beam lithography.The development of the image depends on a number of factors, including the composition of the solvent and the nature of the polymer. In this paper the eVects of change in the composition of mixtures of methyl ethyl ketone and isopropyl alcohol, a common development solvent used in lithography, and the glass transition temperature of poly(methyl methacrylate) films on the mutual diVusion coeYcient are reported.The mutual diVusion coeYcient decreases and becomes asymmetric towards low volume fraction of solvent as the proportion of isopropyl alcohol in the mixture is increased. The higher glass transition temperature films are prone to exhibiting crazing on exposure to solvent.The diVusion of the mixture into the polymer film is selective and preferential for methyl ethyl ketone. DiVusion becomes complex as the content of the mixture moves towards a higher isopropyl alcohol composition. Also, there is evidence for both lowering of the glass transition temperature and re-precipitation of the polymer by the non-solvent (isopropyl alcohol ).Change in the initial Tp of the films leads to small changes in the swelling rate. In the development process of electron beam resist films used in semiconductor lithography, crazing probably plays as important a role in the overall development process as simple solvent driven dissolution.polymer undergoes progressive scission producing a lower Introduction molar mass product. Changes in the solvent mixture and The ability to achieve very large-scale integrated (VLSI) circuit temperature result in the mixture changing from being a good fabrication depends critically on the precision of the litho- to a poor solvent for the polymer. Since the solubility is a graphic processes used. While photolithography remains the function of molar mass, it is possible to select a mixture which primary tool for large-scale semiconductor fabrication, mask will selectively dissolve the low molar mass material without generation and specialist circuit fabrication is dominated by significantly swelling the unexposed, higher molar mass compoelectron beam technology.1 The lithography process is based nents.Despite the importance of electron beam lithography in on radiation induced changes in the dissolution rate of thin VLSI fabrication, little research appears to have been carried resist films. The type of development process is controlled by out on the mechanism of polymer dissolution in these mixed the nature of the interaction of the radiation with the resist, solvent systems.PMMA can exist in three diVerent stereochemwhich may be to either degrade or crosslink the polymer. In ical forms with diVerent values of the Tg. Isotactic PMMA general, there are several diVerent regimes for polymer dissolu- has a Tg of approximately 40 °C, the atactic form has a Tg tion, each of which requires a separate model to describe the of 117 °C and syndiotactic PMMA has a Tg of 125 °C.The process. The mechanism for poly(styrene) dissolution in a diVerences in the Tg values will also influence the solubility good solvent is clearly very diVerent from that for poly(methyl rates of polymers with the same molar mass.9 DiVerences in methacrylate) and both are diVerent from the dissolution of stereochemistry also influence the sensitivity of the polymer the inhibited phenolic polymers used in photoresists.1 Of these to electron beam irradiation.9–11 A combination of a more processes, only the first, dissolution of glassy amorphous marked dependence of the solubility on molar mass for the polymers like poly(styrene), is reasonably well understood.2,3 isotactic polymer and changes in the distribution of the molar Peppas and co-workers have applied scaling concepts4,5 to mass for the degraded polymer have a significant eVect on the the description of the dissolution of poly(styrene).Their theory electron beam sensitivity. Ignoring slight diVerences in the assumes the initial formation of a gel layer at the polymer– degradation mechanism, the solubility rate of the exposed solvent interface as the solvent diVuses into the film.Once this PMMA correlates well with the segmental mobility of the gel layer is formed, it propagates at constant thickness through polymer. Changes in the size of the solvent used in the the polymer film as dissolution occurs. The dissolution rate development process indicate that the solubility rates are and thickness of the gel layer are dependent on the molecular strongly aVected by the relative size of the interstitial distance weight of the polymer and are described accurately by reptation (free volume) between the highly interpenetrating chain of the theory.5 Experiments on high molar mass poly(styrene) indi- polymer and size of the solvent molecule.Plots of the solubility cate that methyl ethyl ketone (MEK) dissolves at a rate against molar mass of the solvent exhibit a sharp break in predicted by theory, but these predictions failed when applied slope between propyl acetate and butyl acetate.The power to poly(methyl methacrylate)6–8 (PMMA), where dissolution dependence of the solubility on the molar mass between methyl occurred without the formation of a significant gel layer.acetate and propyl acetate is relatively weak, but between Many electron beam resists are based on PMMA and butyl acetate and the higher acetate homologues is very strong. development of the lithographic pattern is achieved by the use For lower molar mass solvents there is relatively little correof mixtures of isopropyl alcohol (IPA) and MEK.Isopropyl lation between the motion of the polymer chain and that of alcohol is a non-solvent for PMMA, whereas MEK is a good the solvent whereas, above propyl acetate, the motion is hindered by the polymer and a high degree of correlation solvent. During the exposure process, the high molar mass J. Mater. Chem., 1998, 8, 2591–2598 2591between the chain and solvent is required for diVusion.A Fabry Perot interferometer experiment for assessment of solvent diVusion typical development mixture, used in practice, consists of 153 MEK to IPA, at room temperature. Selection of the composi- The basic interferometer used for assessment of the solvent tion of the solvent mixture is usually based in measurement diVusion process was constructed using two 6 mm thick glass of dissolution rates.It has been shown that the dissolution plates which measured 25×25 mm. These were coated with rate (S) is related to the molar mass through eqn. (1), chromium to achieve a transmission level of approximately 20%. The coating was carried out using an Edwards Coating S=KMa (1) System E306A and the transmission measured with a Perkin- Elmer 257 IR spectrometer.The polymer films were either where K and a are solvent dependent parameters that are spun coated onto the glass or, alternatively for thicker films, specific to the particular polymer system under consideration.14 cast by slow evaporation from an 8 wt% solution in MEK. A number of instruments have been developed for the measure- The film sandwiched between the plates was clamped using a ment of dissolution rates.The technique usually involves the normal IR plate holder. This construction was placed in a measurement of changes in thickness of the resist layer, loss water jacket connected to a thermostatted bath and held at of weight or time to complete dissolution measured by end 30±0.1 °C. A more detailed description of the experimental point analysis.15 However, these studies do not give a molecular apparatus used has been published elsewhere.16 The fringe interpretation of the dissolution process.A novel technique pattern was recorded using an Olympus CHC binocular micro- has been proposed16 which allows examination of the dissoluscope with an attached Olympus OM2 camera. The interfer- tion process by optical examination of the change that occurs ometer was illuminated with a sodium vapour lamp which has in the optical interference, as a function of time, for micron a strong band at 589 nm and the images recorded using a thick films sandwiched into a Fabry Perot interferometer Kodak black and white Tri X Pan film which had a speed of configuration.16 In this paper, the results of a study of the 400 ASA and was sensitive to yellow light.eVects of solvent variation on the dissolution process for atactic PMMA are reported. The polymer selected is typical of the type of material commonly used in many electron Operation of the Fabry Perot interferometer and data analysis beam resists. Mixtures of MEK–IPA, in the range of 352 w/w MEK–IPA, Solvent diVuses into the polymer, and the sharply defined edge are usually used for the development of electron beam resists.of the film and the associated interference pattern becomes Compositions of 151, 352 and 3159 w/w MEK–IPA were distorted, taking up a sigmoidal form which reflects the way investigated in an earlier paper.16 It was observed that, as the in which the fringe pattern changes as the solvent diVuses into amount of IPA is increased, the mutual diVusion coeYcient the polymer.Change in the number of fringes per unit dimenwas reduced. Also, it was noted this as the point at which the sion is a direct measure of the concentration–distance profile. volume fraction of solvent coincides with the maximum mutual This approach was initially proposed by Crank18 and by diVusion coeYcient moves towards lower volume fractions of Crank and Park.19 Each interference fringe represents a plot solvent in the films.In certain instances, the films exhibited of refractive index versus distance, over the concentration environmental stress cracking, consistent with the good solvent range from pure solvent to pure polymer. Assuming that the (MEK) shocking the films’ surface.This eVect could be refractive index is linearly proportional to concentration and overcome simply by not baking the film, thus allowing the that there is negligible volume change on mixing of polymer and solvent, then the refractive index plot represents the residual casting solvent to remain and to plasticise the film. A concentration profile. The profiles were recorded at 3 min two stage diVusion process was also observed for certain intervals and the fringes traced from the photograph.Where compositions. In this paper, an attempt will be made to explore the fringe is horizontal at the film edge, the concentration of further the nature of the two stage diVusion process and to diVusing solvent is zero (i.e. ws=0) and where it is horizontal quantify the eVect of baking on the properties of the films.on the solvent side, the polymer concentration is zero (i.e. ws=1.0). The two measurable features are the change in the concentration profile and movement of the polymer film edge, Experimental which is a direct measure of the rate of swelling of the polymer film. When there is a discontinuity in the fringe pattern, which Materials and thin film formation is a direct indication that the solvent is a poor solvent for the Poly(methyl methacrylate) was obtained from Merck (Poole, polymer (Fig. 1), then the maximum number of fringes, nT, UK), and had a nominal molar mass of 100 000 g mol-1. The that would be observed between pure solvent and pure polymer molar mass and its distribution were determined by gel per- is calculated using eqn.(2),18,19 meation chromatography and a value of M9 n of 96 000 g mol-1 and a heterogeneity index of 1.66 was obtained. Methyl ethyl ketone and isopropyl alcohol were used as solvents and were nT= 2l l (np-ns) (2) obtained from Merck (Poole, UK) as AnalaR grade reagents. The refractive indices of the solvent mixtures were measured where l is the film thickness, l is the wavelength of the using an Abbe refractometer. Films used in this study were monochromatic light source and np and ns are respectively the spun onto chromium-coated glass substrates from a 3 wt% refractive indices for polymer and solvent.The mutual solution of polymer in MEK, using a Headway Research diVusion coeYcient can be calculated using eqn. (3),18,19 Incorporated spinner operating with speeds between 1000 and 500 revolutions per minute (rpm). 50 ml of polymer solution were applied directly to the substrate (2×2 cm). All solutions Dm= -A1 2tB Adx dwsBws=ws¾ Pws¾ 0 xdws (3) used were filtered through a 0.22 mm filter prior to use. The best films were produced when 15 s was left between application of the droplet and spinning at a pre-set speed for 60 s.7 where ws is the volume fraction of solvent at some time t, at The films were baked at 120 °C for 1 h to remove excess a plane distance x away from the original boundary between solvent and to aid development of a smooth profile.These solvent and polymer. The original position of this boundary films are of similar dimensions to those used in electron beam is always set in the same place.The Boltzmann transformation for any fixed value, ws, versus x/Ót, for the data collected at lithography. 2592 J. Mater. Chem., 1998, 8, 2591–2598various times should all fall on a single average curve if the process is Fickian.3,18,19 In this case, eqn. (3) gives eqn. (4). Dm=- A1 2B Adxt 1 2 dws Bws=ws¾ Pws¾ 0 xt 1 2dws (4) Gradient of Area under tangent to the curve the curve at ws¾ from 0 to ws¾ This equation was used to calculate the concentration–distance profile for the system investigated. Results and discussion Secondary boundary phenomenon Investigation of the diVusion behaviour with a 352 w/w IPA–MEK mixture showed that the curves up to about 16 min conformed to a simple one stage process.However, after 25 min a second boundary appeared at the edge close to the solvent and rapidly diVused into the film which already contained solvent.This was very marked at about 36 min, where it was observed that the second boundary had now reached a point about half way between the film surface and edge of the solvent diVusion front into the polymer. It is not possible to calculate a diVusion coeYcient for the separate processes because it is not possible to determine the composition of the solvent mixture at the line between the two diVusion regions.In order to obtain an apparent mutual diVusion coeYcient, an average value over the whole curve was calculated in the manner presented in the previous paper.16 Two hypotheses can be proposed to explain the observation of two diVusion regions.Gel–glass boundary. It is possible that the feature is similar to that observed in poly(styrene),4 where a two stage diVusion process is associated with the solvent lowering the glass transition temperature. In the initial stages, the mixture diVuses into a glassy polymer and is controlled by the osmotic pressure –solubility of the solvent in the matrix. After suYcient solvent has diVused into the polymer matrix, the polymer is able to swell and changes from a glass into a gel state.In this initial stage, the solvent is diVusing into a mobile polymer state and diVusion is influenced by segmental motion and reptation of the polymer chains. Crank and Robinson20 have termed this process middle boundary behaviour. Preferential solvent absorption. When the solvent contains both good solvents and non-solvents, preferential absorption of the good solvent lowers the Tg.The expanded matrix allows access of the non-solvent which will reprecipitate the polymer and lead to the mixed solvent diVusion having a diVerent rate to that of the initially absorbed good solvent. The second boundary is then associated with precipitation of polymer rather than the occurrence of the glass to gel transition.In assessing the possible eVects of the solvent on depression of the Tg, measurements using a penetration technique were made, as described previously.21 The apparatus consisted of a 6 mm diameter glass rod with a round tip at one end, which was allowed to rest on the sample which had been previously plasticised with a known composition of solvent.A chromel– alumel thermocouple was attached to the tip of the rod and Fig. 1 Interference photographs for PMMA/MEK: (a) before solvent the temperature measured using a Digitron 3750-K digital added, and after (b) 3, (c) 5 and (d) 10 min; (e) concentration– thermometer. A Shlumberger linear variable diVerential transdistance curve for PMMA/MEK. ducer (LVDT) was placed on the top of the probe and its movement recorded against temperature, using a two pen recorder.The sample was cooled with a methanol–cardice mixture or heated with a paraYn oil bath. The temperature J. Mater. Chem., 1998, 8, 2591–2598 2593was changed at a rate of 3 K min-1. The samples, in sealed in samples 90 to 30%, with the excess solvent becoming cloudy in the 90 to 70% samples.phials, had been previously equilibrated for two days, in a temperature controlled oven at 40 °C, to allow the mixture The penetration technique was used to measure the solids which contained various levels of absorbed solvent. The results to penetrate into the solid completely, and were cooled to room temperature, before the phials were opened, to avoid are presented in Fig. 2. It is evident that the Tp, which is closely associated with the Tg, has essentially the value expected evaporation. The variations of the Tg, determined by the penetration for depressions produced by MEK for low values of polymer to solvent, and it is only at higher values of solvent absorp- method, are shown in Fig. 2. When MEK is used, the observed Tg deviates significantly from ideal mixing behaviour.Similar tion that deviations from the ‘ideal MEK’ curve are observed. The values of Tp are very close to those of the Tg measured experiments were performed, starting with solvent having compositions of 151, 352 and 753 w/w IPA–MEK. The previously on bulk samples.21 The liquid layer was analysed using 1H NMR signal intensities to determine its compositions polymer was equilibrated with the solvent for two days at 40 °C and then the samples were measured.The following (Table 1). Analysis of the composition of the excess liquid layer shows that, for solutions with low solids content, the solution observations were made. composition is identical to that of the solution. However, as the volume of solvent in the polymer mixture is reduced, the 151 w/w IPA–MEK/PMMAmixtures.At room temperature, for 60 to 90% solvent to polymer mixtures, not all of the good solvent is preferentially absorbed leading to changes in the measured composition compared with that originally used added solvent was absorbed. Below 60%, all the solvent was absorbed and the solid was clear. Below 0 °C, the excess solvent to produce the mixture.This implies that, at any point during the diVusion process, the composition can deviate from that is cloudy in the 90 to 80% samples, but is clear in the 70 and 60% samples. For 90 to 40% samples, there is a double layer of the contacting mixture as a consequence of preferential absorption of MEK. The average mutual diVusion coeYcient visible in the polymer layer. The lower part of the solid is clear.However, the upper layer is opaque. On cooling, the 30 must therefore be considered as reflecting the overall behaviour of the system and incorporates the eVects of the precipitation to 20% samples remain unchanged. as well as solubility–diVusion. 352 w/w IPA–MEK/PMMA mixtures. Two phase behaviour is visible at room temperature for the 90 to 40% solvent to EVect of glass transition temperature on the diVusion behaviour polymer mixtures.The excess solvent is clear at room temperature, but cloudy below 0 °C. The spun polymer resist is usually baked at a temperature above the Tp before being exposed to electron beam irradiation. The process is carried out to increase the mechanical rigidity, 753 w/w IPA–MEK/PMMA mixtures.At 40 °C, the polymer mixtures are clear in most cases. The exception is that in the flatness of the film and to improve the development characteristics, after electron beam exposure. For PMMA, baking is 90 to 60% solvent to polymer mixtures, there is a surface layer of opaque polymer. Excess solvent is evident in the 90 to 40% usually performed at between 130 and 160 °C for 1 h.Not all of the residual casting solvent is removed during the samples. At room temperature, two phase behaviour is evident Table 1 Variation in the composition with the volume of solvent added for various starting compositions of MEK and IPA Solvent Solvent IPA in Solvent Solvent IPA in added (%) absorbed (%) solvent residue (%) added (%) absorbed (%) solvent residue (%) 1:1 w/w IPA–MEK 90 56 55 80 53 55 70 55 57 60 55 60 50 50 61 40 40 no residual 30 30 no residual 20 20 no residual 3:2 w/w IPA–MEK 90 55 62 80 53 64 70 51 64 60 49 71 50 49 no residual 40 39 no residual 30 30 no residual 20 20 no residual 7:3 w/w IPA–MEK 90 54 71 80 45 76 70 46 77 60 46 80 50 42 78 40 40 77 30 30 no residual 20 20 no residual Table 2 Description of the PMMA films used in the study of the eVects of Tg on the mutual diVusion coeYcients Initial Final Film Tg/°C Thermal–time treatment Tg/°C A 66 7 days @ ambient T and P 73 B 59 12 days @ ambient T and P and 24 h in a 78 vacuum oven at ambient T C 61 vacuum oven; 40 °C/48 h, 60 °C/24 h, 98 80 °C/36 h D 60 ambient P, 130 °C/1 h 106 E 64 ambient P, 160 °C/1 h, 112 cooled straight from oven F 61 ambient P, 160 °C/1 h, 110 cooled very slowly over 24 h 2594 J.Mater. Chem., 1998, 8, 2591–2598Fig. 2 Variation of the penetration temperature (Tp) for various IPA–MEK mixtures with PMMA: (%) pure and (2) 151, (&) 352 and (1) 753 MEK, w/w IPA–MEK. The errors in the experimental points are ±3 K; the lines are guides to the data. spinning process and it is appropriate to examine the eVects of Tp and, hence, the residual solvent content on the development/ diVusion behaviour.Fig. 3 Boltzmann transformation curves. (a) Volume fraction of sol- A series of films was produced by casting. Their initial vent versus distance/Ótime for film A. (b) Mutual diVusion coeYcients values of Tp are presented in Table 2. These low Tp films which for diVerent solvent mixtures for film A.(%) 151, (2) 352 and (&) 753 w/w IPA–MEK. The errors are estimated to be ±0.05 in the contain residual casting solvent will slowly lose solvent and volume fraction in (a) and ±0.05×10-11 m2 s-1 for each data point increase their Tp at a rate of approximately 1 °C per day. in Dm in (b). Measurements were performed within 2 h of the Tp measurements. The values quoted in Table 2 are, therefore, the values of the films used in the diVusion study.Since the diVusion cell is a sandwich of heavy glass plates, further significant loss of 753 w/w IPA–MEK mixtures, the curves are crescent shaped and truncated at ws=0.7 and ws=0.5 respectively. The curves solvent is unlikely once the cell has been constructed. are almost indistinguishable up to ws=0.5 and, as a consequence, the mutual diVusion curves [Fig. 4(b)] are essentially Film A (Tp=73 °C). The concentration–distance profiles for the exposure of the films to three diVerent solvent compositions identical. are shown in Fig. 3(a). The plot for the 151 w/w IPA–MEK mixture is sigmoidal in shape and ws=0.5 when x/Ót=0, Film C (Tp=98 °C). The composition–distance curves obtained with the three solvent mixtures are shown in Fig. 5(a). indicating that the rate of solvent penetration into the film is equal to the rate of polymer dissolution into the solvent and In the case of the 151 w/w IPA–MEK mixtures, the film edge disappears after 25 min and recedes as dissolution takes place. that simple Fickian diVusion is observed. Consistent with this assumption is the disappearance of the film edge after about The 352 w/w IPA–MEK mixture relationship is again sigmoidal in behaviour and extends up to ws=1.0.The edge is 8 min of exposure to solvent. For the 352 w/w IPA–MEK mixture, there is a discontinuity at ws=0.8, indicating that the not well defined and abrupt kinks in the fringe pattern are observed rather than a distinct discontinuity. In the case of solvent mixture is no longer able to dissolve the polymer completely. The behaviour of the 753 w/w IPA–MEK solutions the 753 w/w IPA–MEK mixture, a secondary boundary was evident even after 120 min.The diVusion coeYcients for the is similar to that of the 352 w/w IPA–MEK mixture, except that it shows less penetration of solvent into the polymer and lower concentrations are shown in Fig. 5(b). less swelling. The calculated mutual diVusion coeYcient–concentration curves [Fig. 3(b)] show almost symmetrical behav- Film D (Tp=106 °C). The concentration–distance curves for two of the mixtures are shown in Fig. 6(a). As with film C, iour, the curves for the 352 and 753 w/w IPA–MEK mixtures being truncated, reflecting swelling without dissolution of the the shapes of the curves are similar to those observed for other films.A distinct edge receding with time was observed after polymer in the solvent. 16 min. This edge quickly disappears as dissolution of the polymer takes place. In the case of the 753 w/w IPA–MEK Film B (Tp=78 °C). From studies of 151 w/w IPA–MEK mixtures, the concentration–distance curves indicate, once mixture, it was not possible to calculate curves as the films exhibited both environmental stress cracking and also the more, approximately sigmoidal behaviour, with the disappearance of the film edge after 9 min [Fig. 4(a)]. For the 352 and presence of a secondary boundary. J. Mater. Chem., 1998, 8, 2591–2598 2595Fig. 4 Boltzmann transformation curves. (a) Volume fraction of sol- Fig. 5 Boltzmann transformation curves.(a) Volume fraction of solvent versus distance/Ótime for film B. (b) Mutual diVusion coeYcients vent versus distance/Ótime for film C. (b) Mutual diVusion coeYcients for diVerent solvent mixtures for film B. (%) 151, (2) 352 and (&) for diVerent solvent mixtures for film C. (%) 151, (2) 352 and (&) 753 w/w IPA–MEK. The errors are estimated to be ±0.08 in the 753 w/w IPA–MEK. The errors are estimated to be ±0.05 in the volume fraction in (a) and ±0.1×10-11 m2 s-1 for each data point volume fraction in (a) and ±0.15×10-11 m2 s-1 for each data point in Dm in (b).in Dm in (b). Film E (Tp=112 °C) and film F (Tp=110 °C). Film F would solid state. The non-equilibrium chain structure will attempt be expected to have a lower residual solvent content as it was to gain its equilibrium state as the film is swollen and, hence, kept at a higher temperature for a longer period of time.In will generate stresses that may lead to stress crazing. The both cases, environmental stress cracking made analysis of the removal of solvent will be accompanied by generation of diVusion behaviour very diYcult. In both cases, the film edge denser, more compact structures and a concomitant reduction receded with time as dissolution of the polymer molecules into in the diVusion coeYcient would be observed.The apparently the solvent occurred. The film edge disappeared after a period anomalous behaviour of the 106 °C film can be explained by of about 27 min for film E and after 100 min for film F. Stress the fact that, in this case, contact with poor solvent allows recracking is a consequence of the osmotic pressure increasing dissolution of the polymer molecules in the surface without quickly in the film edge and the polymer behind being unable significant re-swelling of the molecules that form the bulk of to release the stress which is generated.the material. Hence, the rate of dissolution of the polymer Comparison of the data from the various films exposed to becomes comparable to the rate of solvent penetration into the 151 w/w IPA–MEK indicates that all the curves are the polymer and crazing is not observed.virtually superimposable, with only slight diVerences, in the region ws=0.7–1.0 and in the range ws=0–0.3, being observed Swelling behaviour (Table 3).There is a marked decrease in Dm as the solvent mixture is changed from 151 to 753 w/w IPA–MEK, this eVect The development process is a combination of dissolution and swelling of the polymer matrix. The swelling rate can be being particularly marked for the low Tp film. There is slight reduction in the diVusion coeYcient with increase in the Tp. measured directly from the movement of the solvent polymer interface, obtained from the interferograms. Change in the Tp However, this eVect is not as marked as the solvent eVect.There are no data presented for films C and D for 753 w/w of the films leads to small changes in the swelling rate with the 352 w/w IPA–MEK (Fig. 7). There are two eVects which IPA–MEK as these exhibited marked crazing. As the composition of the solvent mixture is changed, the plots of Dm against need to be considered in interpretation of these data.Firstly, the polymer films are obtained from a good solvent. Also, it ws become asymmetric towards low values of ws. The polymer chains will be extended in the good solvent is probable that the polymer molecules may have retained their expanded conformations with the possible eVect of micro- used for spin casting the films and this extended structure, as a consequence of chain entanglement, will be retained in the crazing at the surface in the higher Tp films influencing the 2596 J.Mater. Chem., 1998, 8, 2591–2598Fig. 7 Swelling rate curves for (a) 352 and (b) 753 w/w IPA–MEK. Fig. 6 Boltzmann transformation curves. (a) Volume fraction of sol- (%) Film A, (2) film B, (&) film C and (1) film D.The errors in vent versus distance/Ótime for film D. (b)Mutual diVusion coeYcients the distances are of the order of 0.1×10-4. for diVerent solvent mixtures for film D. (%) 151 and (2) 352 w/w IPA–MEK. The errors are estimated to be ±0.05 in the volume fraction in (a) and ±0.1×10-11 m2 s-1 for each data point in Dm in (b).The errors at the extremes of the curves are greater than the and precipitation of polymer in the swollen layer by the average values. poorer solvent. Table 3 Mutual diVusion coeYcients for films with diVerent values of Acknowledgement Tg One of the authors (K.E.R.) wishes to acknowledge the Dm (max)/10-10 m2 s-1 support of the EPSRC in provision of support in the form of a studentship for the period of this study.IPA % Film A Film B Film C Film D 50 0.26 0.23 0.20 0.22 References 60 0.14 0.13 0.05 0.12 70 0.08 0.11 — — 1 P. C. Tsiartas, L. W. C. L. Henderson, W. D. Hinsberg, I. C. Sanchez, R. T. Bonnecaze and C. GrantWillson, Macromolecules, 1997, 30, 4656. 2 J. M. D. McElroy, DiVusion in Polymers, ed. P. Neogi, Marcel initial diVusion behaviour.Secondly, there are marked diVer- Dekker, New York, Basel, Hong Kong, 1996, p. 1. 3 J. Crank, The Mathematics of DiVusion, Oxford, 1985, p. 256. ences in the swelling rate with change in solvent. 4 N. A. Peppas, J. C.Wu and E. D. von Meerwell, Macromolecules, 1994, 27, 5626. Conclusions 5 P. G. de Gennes, Scaling Concepts in Polymer Physics, Cornell University Press, Ithaca, NY, 1979.Change in the composition of the solvent mixture used in the 6 A. C. Ouano and J. A. Carothers, Science and Technology of measurement of the mutual diVusion coeYcient for PMMA Polymer Processing, ed. N. P. Sung and N. H. Sung, MIT Press, Cambridge, MA, 1979, p. 755. films has significant eVects on the Tp of the film. The higher 7 A. C. Ouano and J. A. Carothers, Structure–Solubility the Tp of the films formed after baking, the more susceptible Relationships in Polymers, ed.F. W. Harris and R. B. Seymour, are the films to crazing. In the resist development process, Academic Press, New York, 1977, p. 11. crazing probably plays as important a role as dissolution in 8 A. C. Ouano, Y. O. Tu and J. A. Carothers, Polym. Prepr., 1976, the overall process. The diVusion of the solvent into the 17, 329. polymer film is a complex process which involves selective 9 A. C. Ouano, Polymers in Electronics, ed. T. Davidson, ACS Symp. Ser., 1984, 242, 84. diVusion of the better solvent, lowering the Tp of the films J. Mater. Chem., 1998, 8, 2591–2598 259710 R. A. Pethrick, Irradiation EVects on Polymers, ed. D. W. Clegg 16 K. E. Rankin and R. A. Pethrick, Microelectron. Eng., 1995, 26, and A. A. Collyer, Elsevier Applied Science, London, 1991, p. 403. 141. 11 V. K. Sharma, R. A. Pethrick and S. AVrossman, Polymer, 1982, 17 K. R. Dunham, Solid State Technol., 1971, 14(6), 41. 23, 1732. 18 J. Crank, The Mathematics of DiVusion, Oxford Science 12 T. Kato, Microelectronic Polymers, ed. M. S. Htoo, Marcel Publications, Oxford, 1985, p. 230. Dekker, New York, 1989, p. 1199. 19 J. Crank and G. S. Park, DiVusion in Polymers, ed. J. Crank and 13 M. M. O’Toole, Microelectronic Polymers, ed. M. S. Htoo, Marcel G. S. Park, Academic Press, New York, 1968, ch. 1, p. 4. Dekker, New York, 1989, p. 315. 20 J. Crank and C. Robinson, Proc. R. Soc. London, Ser. A, 1951, 14 M. J. Bowden, Materials for Microlithography, ACS Symp. Ser., 204, 549. 1984, 266, 71. 21 R. J. Elwell, D. Hayward and R. A. Pethrick, Polym. Int., 1993, 15 L. F. Thompson, Introduction to Microlithography, 2nd edn., ed. 30, 55. L. F. Thompson, C. Grant Willson and M. J. Bowden, ACS Professional Reference Books, ACS, Washington DC, 1994, p. 280. Paper 8/02466I 2598 J. Mater. Chem., 1998, 8, 2591–2598

ISSN:0959-9428    

DOI:10.1039/a802466i

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Influence of molar mass on the diVusion and solubility behaviour of methyl ethyl ketone–isopropyl alcohol mixtures in poly(methyl methacrylate) Richard A. Pethrick* and Kathleen E. Rankin Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow, UK G1 1XL Received 31st March 1998, Accepted 14th October 1998 Poly(methyl methacrylate) PMMA is used extensively as an electron beam resist.The development of the lithographic pattern after exposure of the polymer to electron beam irradiation is achieved by contacting the film with a solvent mixture of isopropyl alcohol and methyl ethyl ketone. In this paper the mutual diVusion coeYcients for isopropyl alcohol–methyl ethyl ketone mixed solvents into PMMA of narrow molar mass distribution with a range from 49000 to 400000 g mol-1 are reported.The mutual diVusion coeYcient indicates that the behaviour was both a function of molar mass and solvent composition. The ability for the PMMA to swell in the solvent was found to be molar mass dependent. Cloud point measurements on solutions indicated that change in the molar mass influenced the temperature at which precipitation occurred in the mixed solvents. The cloud points also depended on the quality of the solvent; occurring at lower temperature for the better solvent [151 w/w isopropyl alcohol (IPA)–methyl ethyl ketone (MEK)] and being higher for the poorer solvent (451 w/w IPA–MEK). The swelling characteristics indicate that an increase in the degree of chain entanglement leads to suppression of the degree of swelling.The electron beam sensitivity suggests a complex interplay of the eVects of the molar mass on the chain scission process, on the ability of solvent to enter the degraded matrix and on the solubility of the polymer on the solvent mixture. The casting solvent has an eVect on the structure of the solid and influences the behaviour of the films when exposed to the developing solvent.between 49000 and 400000 g mol-1 are examined. The range Introduction of molar masses was selected to simulate the diVerences in Many electron beam resists are based on PMMA and the molar mass which will be generated during the electron beam development of the lithographic pattern is achieved by the use degradation process.of mixtures of isopropyl alcohol (IPA) and methyl ethyl ketone (MEK). In a previous paper1 the eVects of change of composition of mixtures of IPA and MEK on the mutual diVusion Experimental coeYcients for poly(methyl methacrylate) (PMMA) were investigated. The diVusion of the solvent mixture into the Materials polymer film is selective and preferential for methyl ethyl Poly(methyl methacrylate) was obtained from Polymer ketone.The mutual diVusion coeYcient decreases and becomes Laboratories plc, Shrewsbury, UK. The samples of PMMA asymmetric towards low values of the solvent mixture as the had molar mass distributions between 1.10 to 1.06 and molar proportion of isopropyl alcohol is increased. The higher glass masses of 49000, 79000, 127000, 265000 and 400000 g mol-1.transition temperature films are prone to crazing and this Methyl ethyl ketone and isopropyl alcohol were used as influences the overall development process. The diVusion of solvents, obtained from Merck (Poole, UK) as AnalaR grade the solvent mixture into PMMA is complex and involves a reagents. The films used were produced by spinning a 3 wt% lowering of the glass transition temperature, precipitation of solution of PMMA in MEK onto a chromium-coated glass the polymer by the non-solvent—isopropyl alcohol—and polysubstrate, using a Headway Research Incorporated spinner, mer dissolution.Electron beam lithography is based on radioperating with speeds between 1000 and 500 revolutions per ation-induced degradation of the polymer producing regions minute.6 The spun films were baked at 130 °C for 1 h, before of lower molar mass materials.The development of the pattern being used in these studies. involves the selective removal of these areas of lower mass material without significantly modifying the unexposed areas. The mechanism of dissolution of PMMA is very diVerent from Fabry Perot interferometer experiment that appropriate for photoresis material.2 The interferometer construction and its operation have been Isopropyl alcohol is a non-solvent for PMMA, whereas described in a previous paper.6 The traces were used to MEK is a good solvent for PMMA.calculate the concentration–distance curves from which the Changes in the solvent mixture and in the temperature used mutual diVusion coeYcients were calculated.for the development process result in the mixture changing from being a good solvent to becoming a poor solvent for the polymer.3 Since solubility is a function of molar mass, it is Data analysis possible to select a mixture which will selectively dissolve the In the analysis, it was assumed that the refractive index is low molar mass material without significantly swelling the linearly proportional to concentration and that there is negli- unexposed higher molar mass components.4,5 gible volume change on mixing of polymer and solvent.The In this paper, the results of a study of the eVects of solvent profiles were recorded at 3 min intervals and the fringe traced variation on the dissolution process for narrow molar mass distribution PMMA, having a range of molar masses lying from the photographic representation. J.Mater. Chem., 1998, 8, 2599–2603 2599Measurement of the cloud points for the polymer solutions The cloud points were determined by placing 0.2 cm3 of polymer solution into a capped glass tube having 3 mm thick glass walls, a 3 mm internal diameter and 15 cm length. The sample was then immersed in a water bath and heated until the solution became clear.After equilibration for 1 h, the tube was then slowly cooled at a rate of 0.1 °Cmin-1 until the solution became cloudy. The temperature of the solution was observed using a resistance thermometer, attached intimately to the outside of the tube. The values quoted are the mean of four measurements and the average spread in the temperatures was less than ±1 °C.Electron beam characterisation of the PMMA films PMMA films, with a thickness of approximately 1 mm and an area 1.25×1.25 cm, were deposited on chromium-coated glass cover slips and baked for 1 h at 130 °C. Electron beam exposure was carried out in a modified Philips electron microscope which allowed scanning of the film using a computercontrolled raster drive.A pattern consisting of a series of boxes, with the doses ranging between 1×10-4 and 1×10-3 C cm-2, in 36 steps and having dimensions of 57×32 mm, was produced. A second pattern using dose rates between 1×10-5 to 1×10-4 C cm-2, in 35 steps and using boxes of size 192×108 mm was also used. Each experiment was repeated at least three times. The films were developed using a 753 w/w IPA–MEK mixture for periods of time ranging from 45 to 180 s.The developed films were baked at 100 °C for 30 min before measurement of the dissolution rates assessed from step sizes, obtained using a Dektac surface profile analyser. Results and discussion Mutual diVusion coeYcient measurements The diVusion behaviour is discussed in terms of three solvent compositions: 151 w/w IPA–MEK, 352 w/w IPA–MEK and 753 w/w IPA–MEK.These compositions span those typically Fig. 1 Boltzmann transformation curves. (a) Volume fraction of sol- used for the development of electron beam resists. vent versus distance/Ótime for 151 w/w IPA–MEK. (b) Mutual diVusion coeYcients for diVerent solvent mixtures for film A. (^) 49000, (2) 79000, (%) 127000, (1) 265000 and 151 w/w IPA–MEK (&) 400000 g mol-1, respectively.The errors are estimated to be It was observed in a previous paper1 that this mixture is a ±0.05 in the volume fraction in (a) and ±0.01×10-10 m2 s-1 for good solvent system for PMMA. This is also true of all the each data point in Dm in (b). The errors are greater at the extremes of the compositional variations.molar masses studied in this paper (Fig. 1). However, there are significant diVerences between the lower molar mass PMMA polymers (49000 and 79000 g mol-1) and the higher molar mass materials. The observation of two peaks, in the mutual diVusion coeYcient versus composition profile, indicates very rapid diVusion of the polymer molecules into the Table 1 Summary of Dm and ws as a function of molar mass and solvent composition solvent and solvent molecules into polymer.These processes are slowed down considerably when the molar mass value of Molar mass/ Dm/10-10 PMMA is above 127000 g mol-1. However, in the case of the Solvent composition g mol-1 m2 s-1 ws 400000 g mol-1 PMMA, an apparently anomalous higher value of Dm is observed. This may be attributed to the eVects 151 w/w IPA–MEK 49000 2.08 0.1 of chain entanglement, suppressing densification during the 79000 0.97 0.1 127000 0.32 0.25 spin casting process.The maximum value of the mutual 265000 0.23 0.3 diVusion coeYcients, Dm and their location on the composition 400000 0.50 0.5 axis are summarised in Table 1. Entanglement influences the film forming process and also the swelling and diVusion of 352 w/w IPA–MEK 49000 0.21 0.2 solvent during the development process. 79000 0.18 0.2 127000 0.12 0.5 265000 0.16 0.5 352 w/w IPA–MEK 400000 0.16 0.2 With this poorer solvent, a characteristic peaking of Dm at low 753 w/w IPA–MEK 49000 0.16 0.2 ws is observed, reflecting the relative importance of dissolution 79000 0.10 0.15 over solvent penetration in the diVusion process (Fig. 2). The 127000 0.03 0.1 magnitude of the peak decreases with molar mass (Table 1). 265000 0.09 0.15 Above the 127000 g mol-1 molar mass value for PMMA, there 400000 0.06 0.2 is a marked decrease in the magnitude of Dm. The absolute 2600 J. Mater. Chem., 1998, 8, 2599–2603Fig. 3 Boltzmann transformation curves. (a) Volume fraction of solvent versus distance/Ótime for 753 w/w IPA–MEK.(b) Mutual Fig. 2 Boltzmann transformation curves. (a) Volume fraction of soldi Vusion coeYcients for diVerent solvent mixtures for film A. vent versus distance/Ótime for 352 w/w IPA–MEK. (b) Mutual (^) 49000, (2) 79000, (%) 127000, (1) 265000 and diVusion coeYcients for diVerent solvent mixtures for film A. (&) 400000 g mol-1 respectively. The errors are estimated to be±0.05 (^) 49000, (2) 79000, (%) 127000, (1) 265000 and in the volume fraction in (a) and ±0.02×10-10 m2 s-1 for each data (&) 400000 g mol-1 respectively.The errors are estimated to be±0.05 point in Dm in (b). in the volume fraction in (a) and ±0.01×10-10 m2 s-1 for each data point in Dm in (b). magnitude of Dm is significantly lower than it is for the better mass PMMA. The swelling data for the remaining systems are solvent system, indicating a slowing down of the permeation all within experimental error.process. Thermodynamic eVects on the dissolution behaviour 753 w/w IPA–MEK Solubility is one of the factors in the dissolution process and In this much poorer solvent (Fig. 3), peaking at low ws is once can be quantified in terms of the thermodynamic interaction more observed.There is also a marked decrease in the diVusion parameter which influences the temperature at which precipicoe Ycients on increasing the molar mass. The non-monotonic tation of the polymer occurs.7 For polymer solutions, there decrease in Dm is an indication of the eVects of entanglement exist an upper critical solution temperature and a lower critical on the diVusion process.solution temperature which are a function of the nature of the interaction between polymer and solvent. Changing the solvent Swelling behaviour composition will change the nature of the solvent–polymer interaction parameter and, hence, the cloud point temperature The development process is a combination of dissolution and swelling of the polymer matrix.The swelling rate can be (Fig. 5). The concentration of the polymer solution (0.6 wt% of polymer in MEK) used in this study corresponds to the directly measured from the movement of the solvent polymer interface during the exposure process. The swelling rate with dilute solution region and the eVects of variation of the molar mass of the polymer used were investigated.There is a very the lower molar mass polymers in the better solvents (151 and 352 w/w IPA–MEK) are diYcult to measure as the boundary marked molar mass eVect and a solvent eVect on the cloud point. For the better solvent (151 w/w IPA–MEK) all the disappears almost immediately the experiment is started. It is only in the poorer solvent system and for the higher molar diVerent molar mass samples are soluble in the mixture at ambient temperature.At 310 K, as the quality of the solvent mass materials that a swelling rate becomes measurable (Fig. 4). The rate of swelling for the solvent mixture 753 w/w is decreased, the polymer ceases to be soluble in the solvent. A room temperature development system would use the IPA–MEK decreases from 49000 to 79000 g mol-1 molecular J.Mater. Chem., 1998, 8, 2599–2603 2601Fig. 5 Cloud point temperature versus molar mass for PMMA and various compositions of IPA–MEK. (^) 151, (2) 352, (&) 753 and (1) 451 w/w IPA–MEK. Fig. 4 Swelling rate curves for (a) 352 and (b) 753 w/w IPA–MEK. (^) 49000, (2) 79000, (%) 127000, (1) 265000 and (&) 400000 g mol-1 respectively. The definition of the distance is determined by the width of the boundary and is on average ±0.1×10-4 m.selective solubility to dissolve the lower molar mass polymer whilst not dissolving the higher molar mass material. Electron beam sensitivity curves The eVect of molar mass on the sensitivity of thin films of PMMA to electron beam irradiation is shown in Fig. 6. The more sensitive the film to electron beam irradiation the lower the dose at which the film is observed to be reduced to half its usual thickness, usually designated D50.Values for D50 are presented in Table 2. The films obtained from 265000 and Fig. 6 Electron beam sensitivity curves for narrow molar mass 12700 g mol-1 samples of PMMA exhibit the greatest sensi- PMMA. (^) 27000, (2) 49000, (%) 79000, (1) 127000, (&) 265000 tivity to electron beam irradiation.The films obtained from and (%) 400000 g mol-1 respectively. the lower molar mass samples (27000 and 49000 g mol-1 PMMA) are less easily degraded. The films obtained from the highest molar mass material (400000 g mol-1 PMMA) require Table 2 Values of D50 for PMMA of diVerent molar mass the highest electron beam dose for the generation of the pattern.The electron beam exposure produces chain scission Polymer Molar mass D50/mC cm-2 and lowers the average molar mass of the polymer. It would, therefore, be expected that, since solubility is a function of the PMMA 27000 2.315 49000 2.240 molar mass, the sensitivity should correlate with the molar 79000 2.390 mass of the original films. However, in practice, degradation 127000 2.230 of a high molar mass material could generate mass fractions 265000 2.220 which are still suYciently high to allow entanglement even in 400000 2.410 the irradiated film.Hence, the response of the films to the 2602 J. Mater. Chem., 1998, 8, 2599–2603developer would be influenced by entanglement eVects. The accepted theory of random choice scission and the eVects of molar mass on the solubility of the degraded polymer elements sensitivity will therefore a function of: (i) The structure of the polymer in the spin cast films. This and their interaction with the polymer matrix.However, there are anomalies which reflect the frozen-in conformational con- is a well-known eVect and indicates the influence of both the casting solvent and the Tg of the final film, dictated by the straints on formation of the solid films.pre-exposure, baking procedures. (ii) The solvent, time and temperature used in the develop- Acknowledgements ment process. With the solvent system and temperature held One of the authors (K.E.R.) wishes to acknowledge the constant, the time for development was varied so as to achieve support of the EPSRC in provision of support in the form of the optimum conditions for polymer removal.The exposure a studentship for the period of this study. time was varied between 45 and 90 s. The most sensitive films required a slightly longer exposure to developing solvent than was needed by the lower molar mass materials. Once more, References the eVects of the frozen-in, non-equilibrium conformation may 1 K.E. Rankin and R. A. Pethrick, submitted for publication. be playing a role in terms of defining the final ranking of the 2 P. C. Tsiartas, L. W. C. L. Henderson, W. D. Hinsberg, sensitivity of the polymers. From our previous studies,8 we I. C. Sanchez, R. T. Bonnecaze and C. Grant Willson, know that in the case of the 265000 and 129000 g mol-1 molar Macromolecules, 1997, 30, 4656.mass PMMA materials, irradiation with the electron beam 3 M. Kurata, Thermodynamics of Polymer Solutions, Gordon and would have generated material with molar masses in the range Breach, New York and London, 1987. 4 M. J. Bowden, Materials for Microlithography, ACS Symp. Ser., 40000–60000 g mol-1 which will be very soluble and hence 1984, 266, 71. easily removed.Irradiation of the lower molar mass PMMA 5 L. F. Thompson, Introduction to Microlithography, 2nd edn., ed. material could generate material with molar masses in the L. F. Thompson, C. Grant Willson and M. J. Bowden, ACS range 20000–30000 g mol-1 which, in principle, should be Professional Reference Books, ACS, Washington DC, 1994, more easily removed. However, having a lower degree of p. 280. entanglement could generate on spinning a more dense solid 6 K. E. Rankin and R. A. Pethrick, Microelectron. Eng., 1995, 26, 141. which will be less easy to remove than the more expanded, 7 A. H. Liddell and F. L. Swinton, Discuss. Faraday Soc., 1970, higher molecular weight material. The sensitivity is therefore 49, 115. not only a function of the dissolution characteristics of the 8 R. A. Pethrick, Irradiation EVects on Polymers, ed. D. W. Clegg regions formed on exposure, but is also controlled by the and A. A. Collyer, Elsevier Applied Science, London, 1991, p. 403. spinning and baking processes, prior to exposure. These find- 9 A. C. Ouano, Polymers in Electronics, ed. T. Davidson, ACS ings are in agreement with experimental observations.8–12 Symp. Ser., 1984, 242, 84. 10 M. M. O’Toole, Microelectronic Polymers, ed. M. S. Htoo,Marcel Dekker Inc, New York, 1989, p. 315. Conclusions 11 M. J. Bowden, Materials for Microlithography, ACS Symp. Ser., 1984, 266, 71. The eVects of the change of molar mass on the mutual diVusion 12 N. A. Peppas, J. C.Wu and E. D. von Meerwell, Macromolecules, coeYcient, solubility and electron beam sensitivity are reported 1994, 27, 5626. for a series of diVerent molar mass and diVerent solvent systems. The observed changes can be rationalised in terms of Paper 8/02465K J. Mater. Chem., 1998, 8, 2599–2603 2603

ISSN:0959-9428    

DOI:10.1039/a802465k

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Preparation, characterization and mesomorphic properties of nickel and copper complexes derived from N,N¾-bis[3-(3¾,4¾- dialkoxyphenyl )-3-oxopropenyl]ethylenediamine Chung K. Lai,* Yung-Shyen Pang and Chun-Hsien Tsai Department of Chemistry, National Central University, Chung-Li, Taiwan, ROC Received 14th May 1998, Accepted 24th September 1998 The preparation, characterization and mesomorphic properties of copper and nickel complexes derived from N,N¾- bis[3-(3¾,4¾-dialkoxyphenyl )-3-oxopropenyl ]ethylenediamine are reported.Liquid crystalline behavior for these structurally similar complexes was found to be strongly dependent both on the number of sidechains and metal centers incorporated. Nickel complexes with four or six alkoxy sidechains exhibited columnar phases.However, nickel complexes with two alkoxy sidechains and all copper complexes regardless the numbers of the sidechains were not liquid crystalline. The structure of the mesophases was confirmed as columnar hexagonal (Colh) by powder XRD diVraction. The data that the copper complexes have slightly lower isotropic temperatures than the analogous nickel complexes suggested that the lack of liquid crystallinity for the copper complexes may be attributed to weaker molecular interactions.The results also indicated that nickel complexes with four sidechains showed a wider range of mesophase temperature than complexes with six sidechains. molecular layers is extremely critical in the formation of Introduction columnar phases since the induction of the mesophase is Numerous metallomesogenic compounds with unique mainly controlled by a delicate balance of intermolecular geometries and molecular shapes have been generated by interactions.incorporation1 of a metal center or metal centers into organic Herein we report the preparation, characterization and moieties. In general the geometry of the complex is often mesomorphic properties of three series of copper and nickel determined by the metal center incorporated and the organic complex analogues derived from N,N¾-bis[3-(3¾,4¾-dialkoxychelating ligand, and it can vary from square-planar to tetra- phenyl )-3-oxopropenyl ]ethylenediamine.Of these, nickel comhedral structures for complexes2 with coordination numbers plexes 2 and 3 exhibited columnar hexagonal phases, and of four.Square-planar (i.e. Cu2+, Ni2 +, Pd2 +, Pt2 +, Zn2 +) copper complexes were not liquid crystalline. and square-pyramidal geometries (FeCl2+, VO2 +) generally give rise to liquid crystals, whereas tetrahedral geometries are often not mesomorphic. Some of these materials have been Results and discussion extensively studied as potential candidates in terms of appli- Synthesis cations and all the related physical properties may originate from the rich electronic configuration of the metal centers.The synthetic pathways to copper and nickel complexes 1, 2 Incorporation of a metal center can often induce the and 3 are summarized in Scheme 1. The preparation of 4- formation of mesophases by a non-mesogenic organic ligand, alkoxylacetophenones, 3,4-dialkoxyacetophenones and 3,4,5- and this diVerentiation in mesomorphic properties is generally trialkoxyacetophenones were via literature procedures.4 The attributed to the change of molecular shape and/or intermol- sodium salts of alkoxylphenyl-3-oxo-3-phenylpropionaldehyde ecular interaction.On the other hand the mesomorphic derivatives were obtained by the Claisen formylation5 reaction properties may be totally lost upon incorporation of a metal of the appropriate acetophenone, ethyl formate and sodium ion.metal dispersed in diethyl ether. The isolation of the neutral In previous studies3 we demonstrated the formation of forms of alkoxylphenyl-3-oxo-3-phenylpropionaldehydes was mesophases by use of a b-enaminoketonate framework as the not attempted owing to their relatively low thermal stability.core group in which the better planar core based on b- The ethylenediamine SchiV bases; N,N¾-bis[3-(3¾,4¾-dialkoxyenaminoketonato instead of b-diketonato structures was phenyl )-3-oxopropenyl ]ethylenediamines were obtained as applied to induce the mesophases. The separation between light yellow solids by reaction of the freshly prepared sodium salts with ethylenediamine in refluxing dried dichloromethane in high yields.The reaction9 of SchiV bases with copper(II ) acetate monohydrate or nickel(II) acetate tetrahydrate in refluxing THF–methanol produced the complexes. Recrystallization twice from ethyl acetate or THF–methanol gave yellow solids for the nickel complexes and green–gray solids for copper complexes.These SchiV base derivatives were characterized by 1H and 13C NMR spectroscopy. SchiV bases can potentially exist possibly in three diVerent keto–enol tautomeric6 forms; A, B and C (Fig. 1). The 1H NMR data in CDCl3, e.g., for 5 (n= 16) showed three characteristic peaks at d 5.64, 6.75 and 10.19, assigned to olefinic methine H (KCHLCK), aldehyde H (KCHLNK) and imine H (KCLNHK).In addition, the preference for the tautomeric A structure was also indicated by two O N RO X Y O N OR X Y H H M = Cu, Ni; R = (CH2) nH 1 X = Y = H 2 X = OR; Y = H 3 X = Y = OR M J. Mater. Chem., 1998, 8, 2605–2610 2605O O ONa RO OR X RO Y RO OH N RO X Y O HN RO X Y OH O RO OR RO OMe O RO OR RO O RO O HO O OR RO OR RO O N RO X Y O N RO X Y 4 X = Y = H 5 X = OR; Y = H 6 X = Y = OR b d g c e e e M = Cu, Ni 1 X = Y = H 2 X = OR; Y = H 3 X = Y = OR M f a Scheme 1 Reagents and conditions: a, KOH (2.0 equiv.), refluxing in THF–H2O (9/1), 12 h; b, CH3Li (1.1 equiv.), stirred in dried THF at 0 °C; then at RT, 12 h.c, CH3COCl (1.1 equiv.), AlCl3 (3.0 equiv.), stirred in CH2Cl2 at 0 °C, then at RT, 8 h; d, RBr (1.1 equiv.), K2CO3 (3.0 equiv.), refluxing in Me2CO, 18 h; e, ethyl formate (4.5 equiv.), Na (3.0 equiv.), stirred in Et2O at RT, 8 h; f, NH2(CH2)2NH2 (0.55 equiv.), CH3CO2H (3 drops), refluxing in CH2Cl2, 12 h; g, M(OAc)2 (1.1 equiv.); MLCu, Ni, refluxing in THF–CH3OH, 12 h.characterteristic peaks at d 189.63 and 153.73 in the 13C NMR spectrum. Copper (d9) compounds, which are paramagnetic, displayed only broad alkoxy signals in the 1H and 13C NMR spectra.However, the nickel complexes showed sharp signals, also indicating the diamagnetic configuration. Elemental analysis confirmed the purity of the complexes. Mesomorphic properties The liquid crystalline behavior for the metal complexes was studied by thermal analysis (DSC) and polarized optical microscopy.Nickel complexes exhibited columnar phases while copper complexes formed crystalline phases. Phase transitions and thermodynamic data for these metal complexes were summarized in Table 1. The nickel complexes 2 and 3 with four and six alkoxy sidechains exhibited enantiotropic liquid crystalline behavior, whereas complexes 1 with two sidechains were not liquid crystalline.This dependence of sidechain density has been commonly observed in other columnar phases.4a–c,7c Two transitions of crystal-to-columnar (KACol ) and columnar-to-isotropic (ColAI ) were typically observed in all nickel derivatives. The molecular shapes of these metal complex are roughly halfdisc, and the correlated columnar mesophases are formed by shape eVects and dative interaction between neighboring complexes.Two molecules rotated by 180° are stacked in an antiparallel organization7 within columns. O HN O HN H H OH N OH N H H OH N O HN H H H H H H H H keto (A) enol (B) keto-enol (C) DSC data showed that the crystal-to-mesophase transitions, Fig. 1 Three structures of keto-enol tautomeric forms for enaminoketone derivatives. for example, for nickel complexes 2, were observed in the 2606 J.Mater. Chem., 1998, 8, 2605–2610Table 1 Phase behaviora of metal complexes, 1, 2 and 3 n 1 Ni 8 K I ,bbb) 108.6 (40.0) 65.3 (41.3) 16 K I ,bbb) 113.5 (85.8) 98.0 (85.3) Cu 8 K I ,bbb) 105.3 (30.4) 54.1 (31.4) 16 K I ,bbb) 100.1 (73.9) 85.6 (88.1) 2 Ni 10 K I ,bbb) 110.0 (20.4) 105.7 (20.9) 12 K Colh I ,bbb) 57.7 (2.84) 54.4 (2.64) ,bbb) 108.5 (13.4) 105.9 (13.4) 14 K Colh I ,bbb) 62.9 (6.00) 58.8 (6.03) ,bbb) 105.6 (12.0) 100.1 (11.5) 16 K Colh I ,bbb) 66.8 (5.90) 59.3 (6.50) ,bbb) 100.5 (5.83) 88.9 (6.40) 18 K Colh I ,bbb) 73.4 (14.9) 63.5 (7.25) ,bbb) 102.1 (7.49) 93.1 (7.60) Cu 10 K I ,bbb) 93.9 (40.0) 60.1 (45.6) Fig. 2 Bar graph showing the phase behavior of the nickel complexes (2 and 3); n is the carbon number of the sidechains. 12 K I ,bbb) 89.6 (36.9) 75.7 (57.2) sidechains, and only a crystal-to-isotropic (KAI ) transition 16 K I ,bbb) 88.9 (34.1) 74.0 (34.0) was observed. Such drastic change in mesomorphic properties due to incorporation of diVerent metals has also been observed 3 Ni 14 K Colh I in other systems,1a and can be due to many factors. The lack ,bbb) 70.1 (3.55) 56.7 (3.71) ,bbb) 75.7 (5.45) 66.7 (5.16) of liquid crystallinity is generally believed to be attributed to 16 K Colh I weaker interaction between the copper complexes, and this ,bbb) 64.5 (1.60) 50.3 (1.54) ,bbb) 70.6 (2.13) 62.5 (2.90) suggestion was indicated by the fact that the clearing temperatures of the copper complexes were slightly lower than their Cu 14 K I ,bbb) 71.0 (16.5) 61.3 (16.5) nickel analogues.However, ca. 2–3 times larger enthalpies were observed for the crystal-to-isotropic transitions in the 16 K I copper relative to the nickel complexes. ,bbb) 44.8 (14.5) <20.0 Copper and nickel complexes of similar structures derived from imineketone derivatives with six alkoxy sidechains an Represents the number of carbons in the alkoxy chain. K=crystal phase; Colh=columnar hexagonal phase; I=isotropic.The transition reported by Swager’s group7b were found to form hexagonal temperatures (°C) and enthalpies (in parenthesis, kJ mol) are disordered columnar phases. Detailed comparison of DSC determined by DSC at a scan rate of 10.0 °Cmin-1. data for these two types of metal complexes showed that all imineketone-derived complexes had higher isotropic points and much larger enthalpies (2–4 times) of the crystal-toisotropic transitions than iminealdehyde-derived homologues.temperature range 58.0–73.0 °C on heating with the magnitude of transition enthalpies ranging from 2.84 to 14.9 kJ mol-1, These observations were rationalized in that the molecular interactions in columnar arrangements for imineketone- and isotropic points were all in the range 109.0–101.0 °C with a relatively large enthalpy (5.83–13.4 kJ mol-1).The relatively derived complexes should be much better or/and stronger owing to the presence of terminal methyl groups. On the large enthalpies indicated that the mesophases were in a highly ordered state.8 Similar thermal data were observed for nickel other hand molecular interactions in iminealdehyde-derived complexes are much weaker than in imineketone-derived com- complexes 3.Increasing the carbon length in the alkoxy sidechains decreased the clearing temperature, mainly due to plexes,9 and only little energy was needed to overcome such a small energy barrier as to pass into the liquid phase upon the greater dispersive forces associated with the longer alkoxy chains.The dependence of mesophase formation on the side- heating. These results also indicated that complexes without terminal tetrahedral methyl groups in iminealdehyde-derived chain density was also studied. Increasing the sidechain numbers of the nickel complexes from four (2) to six (3) increased materials preferred to form ordered hexagonal phases (Colho) over hexagonal disordered phases (Colhd) in imineketone- the melting temperatures and decreased the clearing temperatures. On the other hand the temperature range of the meso- derived complexes. The core–core distance was 3.68 A° (2, M= Cu, n=12), which is close to stacking distance in imineketone- phase was decreased from 28.7–51.0 °C for 2 to 5.6–6.1 °C for 3, as shown in Fig. 2. These complexes have clearing tempera- derived complexes (3.60 A° ).7b The assignment of a columnar hexagonal phase was tures relatively lower than most metallomesogenic complexes.Under a polarized optical microscope on very slow cooling confirmed by X-ray powder diVraction data. A summary of the diVraction peaks and lattice constants for the nickel from the isotropic point a mosaic texture was observed for a thin layer of samples between two glass plates, whereas, complexes 2 and 3 is given in Table 2. For example, as shown in Fig. 3, nickel complex 2 (n=12) displays a diVraction textures more like focal-conic with a large area of uniform homeotropic alignment was observed with thicker samples. pattern of a two-dimensional hexagonal lattice with one intense peak and two weak peaks at 35.09, 20.34 and 17.49 A° at 80 °C.Surprisingly, none of the copper complexes displayed any mesomorphic properties regardless of the numbers of the This type of diVraction pattern is characteristic of a hexagonal J. Mater. Chem., 1998, 8, 2605–2610 2607Table 2 Variable-temperature XRD diVraction data for nickel(II) for many disordered Colh systems. The temperature depencomplexes 2 and 3 dence of the lattice parameters in liquid crystals was also studied.We found that the low-angle reflection generally Lattice d-Spacing/ shifted to lower d-spacing at lower temperatures (i.e. d= spacing/ A° Miller 40.51 A° at 80 °C and d=41.01 A° at 104 °C for complex 2; n= Complex Mesophase A° obs. (calc.) indices 12). The hexagonal lattices are also well correlated with n increasing side chain lengths. 2 12 Colh 80 °C 40.51 35.09 (35.09) (100) 20.34 (20.26) (110) 17.49 (17.54) (200) Conclusion 4.47 (br) We have prepared three series of copper and nickel complexes 3.68 derived from N,N¾-bis[3-(3¾,4¾-dialkoxyphenyl )-3-oxoprop- Colh 104 °C 41.01 35.52 (35.52) (100) 20.55 (20.51) (110) enyl ]ethylenediamine.Liquid crystalline behavior was found 17.97 (17.76) (200) to be controlled by sidechain density and/or metal centres 4.53 (br) incorporated.Nickel complexes with four or six sidechains 3.54 exhibited columnar hexagonal phases, and complexes with 14 Colh 90 °C 42.78 37.05 (37.05) (100) four sidechains showed a much wider range of mesophase 21.83 (21.39) (110) than complexes with six sidechains. However, nickel complexes 18.64 (18.52) (200) 4.78 (br) with two sidechains and all copper complexes, regardless of 16 Colh 80 °C 49.32 42.71 (42.71) (100) the number of sidechains, were non-mesomorphic.This diVer- 24.57 (24.66) (110) ence might be attributed to the greater degree of molecular 21.23 (21.35) (200) interaction in nickel than in copper complexes. 4.63 (br) 3.89 18 Colh 57 °C 49.16 42.57 (42.57) (100) Experimental 24.80 (24.58) (110) 21.39 (21.29) (200) All chemicals and solvents were reagent grade from Aldrich 17.53 Chemical Co.and used without further purification. The 4.27 (br) solvents were dried by standard techniques. 1H and 13C NMR 3.96 spectra were measured on a Bruker DRS-200. DSC thermo- 3 14 Colh 60 °C 44.14 38.25 (38.25) (100) graphs were carried out on a Perkin-Elmer DSC-7 and cali- 19.16 (19.13) (200) 16.40 brated with a pure indium sample.All phase behaviors are 4.59 (br) determined at a scan rate of 10.0 °Cmin-1. Optical polarized 3.32 microscopy was carried out on Nikkon MICROPHOT-FXA 16 Colh 57 °C 46.57 40.33 (40.33) (100) with aMettler FP90/FP82HT hot stage system. X-Ray powder 23.30 (23.28) (110) diVraction (XRD) studies were performed on an INEL MPD- 20.25 (20.16) (200) diVractometer with a 2.0 kW Cu-Ka X-ray source equipped 4.45 (br) 3.70 with an INEL CPS-120 position sensitive detector and a variable temperature capillary furnace with an accuracy of ±0.10 °C in the vicinity of the capillary tube.The detector was calibrated using mica and silicon standards. The powder samples were charged in Lindemann capillary tubes (80 mm long and 0.01 mm thickness) from Charles Supper Co.with a inner diameter of 0.10 or 0.15 mm. The sample was heated above the isotropic temperature and allowed to stay at that temperature for 10 min. The sample was then cooled at a rate of 5.0 °Cmin-1 to the appropriate temperature and the diVraction data collected. Elemental analyses for carbon, hydrogen, and nitrogen were conducted on a Heraeus CHNO- Rapid elemental analyzer, and the results are listed in Table 3.The compounds of 1,2-dialkoxybenzenes, 4-alkoxyacetophenones, 3,4-dialkoxyacetophenones, methyl 3,4,5-trialkoxybenzoate esters, 3,4,5-trialkoxybenzoic acids and 3,4,5- trialkoxyacetophenones were prepared according to literature procedures.5,7 4-Hexadecyloxyacetophenone White crystals, yield 85%. 1H NMR (CDCl3): d 0.83(t, CH3, Fig. 3 Powder X-ray diVraction pattern of the columnar hexagonal 3H), 1.24–1.81(m, CH2, 28H), 2.45(s, COCH3, 3H), 3.93(t, phase (Colh) at 80°C for nickel complexes 2 (n=12). OCH2, 2H), 6.84(d, C6H4, 2H), 7.84(d, C6H4, 2H). 13C NMR (CDCl3): d 13.97, 22.56, 25.87, 26.07, 29.00, 29.25, columnar (Colh) phase with a d-spacing ratio of 1, (1/3)1/2 29.45, 31.76, 68.10(OCH2), 113.98(C3, C5), 129.99(C2, C6), and (1/4)1/2, corresponding to Miller indices (100), (110) and 130.39(C1), 163.01(C4), 196.37(CLO). (200), respectively.This corresponds to an intercolumnar distance (a parameter of the hexagonal lattice) of 40.51 A° . 1,2-Dihexadecyloxybenzene An additional weak halo peak at medium angle (d-spacing#4.47 A° ) was observed for most complexes.The White crystals, yield 92%. 1H NMR (CDCl3): d 0.83(t, CH3, 6H), 1.24–1.86(m, CH2, 56H), 3.94(t, OCH2, 4H), 6.86(s, presence of a distinct peak at ca. 3.68 A°indicated a relatively ordered mesophase which is consistent with DSC analysis of C6H4, 4H). 13C NMR (CDCl3): d 14.10, 22.68, 26.05, 29.36, 29.44, 29.70, 30.89, 31.92, 69.28, 114.13(C3, C6), 120.98(C4, large enthalpies for the columnar-to-isotropic transition.This peak reflects a more regular period within the columns than C5), 149.24(C1, C2). 2608 J. Mater. Chem., 1998, 8, 2605–2610Table 3 Elemental analysisa of metal complexes mixture turned clear and slightly acidic (pH paper). Ethylenediamine (0.15 g, 0.0025 mol) was added and the mix- Complex n C (%) H (%) ture was gently refluxed for 24 h.The solution was concentrated to give the crude product as a brown solid. Yellow 1 Cu 8 67.66 (67.74) 7.93 (7.89) needles were obtained after recrystallization from dichloro- 16 72.10 (72.39) 9.42 (9.58) Ni 8 68.28 (68.26) 8.00 (7.76) methane–methanol (253). Yield 78.0%, mp 98.0 °C. 1H NMR 16 72.58 (72.80) 9.65 (9.63) (CDCl3): d 0.85(t, CH3, 12H), 1.13–1.42(m, CH2, 104H), 2 Cu 10 71.72 (71.57) 9.87 (9.81) 1.80(t, CH2, 8H), 3.37(m, CNCH2, 4H), 3.98(t, OCH2, 4H), 12 72.92 (72.98) 9.93 (10.27) 4.00(t, OCH2, 4H), 5.64(d, COCH, 2H), 6.75(m, CHN, 2H), 14 74.08 (74.13) 10.76 (10.64) 6.83(d, C6H3, 2H), 7.36(d, C6H3, 2H), 7.45(s, C6H3, 16 75.18 (75.09) 10.92 (10.95) 2H), 10.19(m, CNH, 2H). 13C NMR (CDCl3): d 14.09, 22.67, 18 76.17 (75.90) 11.24 (11.22) Ni 10 71.72 (71.91) 9.87 (9.86) 26.00, 29.14, 29.35, 29.69, 31.91, 50.21(NCH2), 69.03(OCH2), 12 73.38 (73.29) 10.17 (10.31) 69.16(OCH2), 90.84(CHL), 112.03(C5¾), 112.17(C2¾), 14 74.57 (74.42) 10.78 (10.68) 120.78(C6¾), 132.25(C1¾), 148.72(C3¾), 151.91(C4¾), 16 75.15 (75.36) 10.96 (10.99) 153.73(CLN), 189.63(CLO).IR (thin film): 1636, 1590, 1545, 18 76.24 (76.16) 11.30 (11.25) 1516, 1464, 1374, 1335, 1269, 1219 cm-1. 3 Cu 14 75.57 (75.42) 11.54 (11.32) 16 76.27 (76.36) 11.58 (11.60) N,N¾-Bis[3-(4¾-hexadecanoxyphenyl )-3-oxopropenyl]ethyl- Ni 14 75.86 (75.65) 11.69 (11.35) 16 76.66 (76.57) 11.69 (11.63) enediamine. Light yellow solid, yield 84%, mp 172.0 °C. 1H NMR (CDCl3): d 0.85(t, CH3, 6H), 1.13–1.40(m, CH2, 52H), aCalculated values in parenthesis. 1.75(t, CH2, 8H), 3.34(m, CNCH2, 4H), 3.94(t, OCH2, 4H), 5.61(d, COCH, 2H), 6.72(m, CHN, 2H), 6.83(d, C6H4, 4H), 7.77(d, C6H4, 4H), 10.23(m, CNH, 2H). 13C NMR (CDCl3): 3,4-Dihexadecyloxyacetophenone d 14.05, 22.59, 25.94, 29.16, 29.27, 31.74, 50.12(NCH2), White crystals, yield 85%. 1H NMR (CDCl3): d 0.83(t, CH3, 68.03(OCH2), 90.71(CHL), 113.89(C3¾), 129.00(C2¾), 6H), 1.24–1.84(m, CH2, 56H), 2.52(s, CH3, 3H), 3.99(t, 131.89(C1¾), 153.76(C4¾), 161.63(CLN), 189.53(CLO).OCH2, 4H), 6.82(d, C6H3, 1H), 7.68(d, C6H3, 1H), 7.54(s, C6H3, 1H). 13C NMR (CDCl3): d 13.97, 22.56, 25.87, 26.07, N,N¾-Bis[3-(3¾,4¾,5¾-trihexadecyloxyphenyl )-3-oxopropenyl]- 29.00, 29.25, 29.45, 31.76, 68.10(OCH2), 111.49(C5), ethylenediamine. Light yellow solid, yield 80%, mp 49.0 °C. 1H 112.32(C2), 123.14(C6), 130.40(C1), 148.78(C3), 153.48(C4), NMR (CDCl3): d 0.85(t, CH3, 18H), 1.12–1.42(m, CH2, 197.73(CLO). 156H), 1.74(t, CH2, 12H), 3.39(m, CNCH2, 4H), 3.96(t, OCH2, 12H), 5.61(d, COCH, 2H), 6.77(m, CHN, 2H), 7.06(s, Methyl 3,4,5-trihexadecyloxybenzoate ester C6H4, 4H), 10.25(m, CNH, 2H). 13C NMR (CDCl3): d 14.10, 22.68, 26.09, 29.37, 29.71, 30.32, 31.91, 50.18(NCH2), White solid, yield 92%. 1H NMR (CDCl3): d 0.84(t, CH3, 69.15(OCH2), 73.46(OCH2), 91.02(LCH), 105.80(C2¾, C6¾), 9H), 1.26–1.80(m, CH2, 84H), 3.86(s, OCH3, 3H), 3.97(t, 134.37(C1¾), 141.15(C4¾), 152.82(C3¾, C5¾), 154.04(CLN), OCH2, 6H), 7.25(s, C6H2, 2H). 13C NMR (CDCl3): d 14.16, 189.80(CLO). 22.77, 26.14, 29.35, 29.45, 29.62, 29.72, 29.75, 29.81, 30.41, 32.01, 52.11, 69.22(OCH2), 75.50(OCH2), 108.03(C2, C6), Copper complexes of N,N¾-bis[3-(3¾,4¾-dihexadecanoxyphenyl )- 124.68(C1), 142.43(C4), 152.80(C3, C5), 166.91(COO). 3-oxopropenyl]ethylenediamine (general procedures for the copper complexes) 3,4,5-Trihexadecyloxybenzoic acid N,N¾-Bis[3-(3¾,4¾-dihexadecanoxyphenyl )-3-oxopropenyl ]- White solid, yield 83%. 1H NMR (CDCl3): d 0.83(t, CH3, ethylenediamine (0.50 g, 0.40 mmol) dissolved in dichloro- 9H), 1.24–1.84(m, CH2, 84H), 3.97(t, OCH2, 6H), 7.29(s, methane (5.0 ml ) was added to a hot methanol solution of C6H2, 2H). 13C NMR (CDCl3): d 14.10, 22.69, 26.09, 29.38, copper(II) acetate monohydrate (0.038 g, 0.20 mmol). Upon 29.72, 30.34, 31.93, 69.17(OCH2), 73.54(OCH2), 108.54(C2, addition a light brown solid began to appear, and the solution C6), 123.66(C1), 143.08(C4), 152.83(C3,C5), 171.87(COO).was gently refluxed for 6 h. The light brown solid was filtered oV, and recrystallized from dichloromethane–methanol to give 3,4,5-Trihexadecyloxyacetophenone a grey–green solid. Yield 72%. IR (thin film):1621, 1599, 1578, White solid, yield 82%. 1H NMR (CDCl3): d 0.82(t, CH3, 1495, 1466, 1437, 1401, 1383, 1327, 1271, 1231, 1202 cm-1. 9H), 1.24–1.86(m, CH2, 84H), 2.52(s, COCH3, 3H), 3.96(t, Anal. Calc. for C84H146O6N2Cu: C, 75.09; H, 10.95. Found: OCH2, 6H), 7.15(t, C6H2, 2H). 13C NMR (CDCl3): d 14.05, C, 75.18; H, 10.92%. 22.65, 26.1, 26.05, 26.30, 29.35, 29.69, 30.31, 31.68, 31.90, 69.29(OCH2), 73.47(OCH2), 107.11(C2, C6), 132.06(C1), Nickel complexes of N,N¾-bis[3-(3¾,4¾-dihexadecanoxyphenyl )- 3-oxopropenyl]ethylenediamine (general procedures for the 142.94(C4), 152.87(C3, C5), 196.92(CLO).nickel complexes) General procedures for the synthesis of SchiV bases N,N¾-Bis[3-(3¾,4¾-dihexadecanoxyphenyl )-3-oxopropenyl ]- ethylenediamine (0.50 g, 0.40 mmol) dissolved in dichloro- N,N¾-Bis[3-(3¾,4¾-dihexadecanoxyphenyl )-3-oxopropenyl]- ethylenediamine. A mixture of freshly cut sodium (0.34 g, methane (5.0 ml ) was added to a hot methanol solution of nickel(II ) acetate tetrahydrate (0.049 g, 0.20 mmol), and the 0.015 mol) suspended in dry diethyl ether (25.0 ml ) and 3,4- dihexadecyloxyacetophenone (3.0 g, 0.005 mol) was stirred for solution gently refluxed for 4 h.The solution was concentrated to dryness to give a brown solids. A yellow solid was obtained 0.5 h.Ethyl formate (1.85 g, 0.025 mol) was slowly added to the solution at room temperature and the mixture allowed to after recrystallization from ethyl acetate. Yield 73%. 1H NMR (CDCl3): d 0.88(t, CH3, 12H), 1.27–1.49(m, CH2, 104H), stir at room temperature under an N2 atmosphere for 18 h. The pale orange cloudy solution was carefully quenched with 1.84(m, CH2, 8H), 3.18(m, CNCH2, 4H), 4.00(t, OCH2, 4H), 5.63(d, COCH, 2H), 6.83(m, CHN, C6H3, 4H), 7.39(m, methanol to remove any excess sodium.The solution was concentrated to give a yellow solid, which was redissolved in C6H3, 4H). 13C NMR (CDCl3): d 14.07, 22.66, 26.03, 26.10, 29.25, 29.34, 29.35, 29.43, 29.51, 29.62, 29.65, 29.68, 29.71, dichloromethane (ca. 20.0 ml ). Then acetic acid was added slowly to neutralize the solution, and at this point the cloudy 31.90, 57.92, 69.03, 69.31, 92.02, 112.48, 112.62, 119.88, 130.33, J.Mater. Chem., 1998, 8, 2605–2610 2609C. K. Lai and T. M. Swager, Chem. Mater., 1994, 6, 2252; 148.42, 150.82, 156.14, 172.29. IR (thin film):1603, 1580, 1536, (e) C. K. Lai, C. H. Tsai and Y. S. Pang. J. Mater. Chem., 1998, 1511, 1493, 1443, 1381, 1362, 1329, 1273, 1241, 1206, 1164, 8, 1355. 1136 cm-1. Anal. Calc. for C84H146O6N2Ni: C, 75.36; H, 5 (a) W. Pyzuk, E. Go�recka and A. Krwczynski, Liq. Cryst., 1992, 10.99. Found: C, 75.15; H, 10.96%. 11, 797; (b) W. Pyzuk, E. Go�recka, A. Krwczynski and J. Przedmojski, Liq. Cryst., 1993, 14, 773; (c) E. Go� recka, W. Pyzuk, A. Krwczynski and J. Przedmojski, Liq. Cryst., 1993, Acknowledgements 14, 1837. 6 W. Pyzuk, A. Krowczynski and E. Go�recka, Liq. Cryst., 1991, We thank the National Science Council of Taiwan, ROC for 10, 593. funding (NSC-87–2113-M008–007) in generous support of 7 (a) S. T. Trzaska and T. M. Swager, Chem. Mater., 1998, 10, 438; this work. (b) H. Zheng, C. K. Lai and T. M. Swager, Chem. Mater., 1994, 6, 101; (c) C. K. Lai, A. G. Serrette and T.M. Swager, J. Am. Chem. Soc., 1992, 114, 7948. References 8 (a) K. E. Treacher, G. J. Clarkson and N. B. Mckeown, Liq. Cryst., 1995, 19, 887; (b) Chung K. Lai, Min-Yi Lu and Fun-Jane 1 (a) J. L. Serrano, in Metallomesogens; Synthesis, Properties, and Lin, Liq. Cryst., 1997, 23, 313; (c) A. Takada, N. I. T. Fukuda and Applications, VCH, New York, 1996; (b) D.W. Bruce and T. Miyamoto, Liq. Cryst., 1995, 19, 441; (d) N. B. McKeown and D. O’Hare, in Inorganic Materials, John Wiley & Sons, New York, J. Painter, J. Mater. Chem., 1994, 4, 1153; (e) C. Pugh and 1992, pp. 407–490. V. Percec, J. Mater. Chem., 1991, 1, 765. 2 (a) S. A. Hudson and P. M. Maitlis, Chem. Rev., 1993, 93, 861; 9 (a) B. Mu� hlberger and W. Haase, Liq. Cryst., 1989, 5, 251; (b) P. Espinet, M. A. Esteruelas, L. A. Oro, J. L. Serrano and (b) C. Cativiela, J. L. Serrano and M. M. Zurbano, J. Org. Chem., E. Sola, Coord. Chem. Rev., 1992, 117, 215; (c) P. Maitlis and 1995, 60, 3074; (c) X. Yang, Q. Lu, S. Dong, D. Liu, S. Zhu, A. M.Giroud-Godquin, Angew. Chem., Int. Ed. Engl., 1991, 30, F. Wu and R. Zhang, J. Phys. Chem., 1993, 97, 6726; 375. (d) S. N. Poelsma, A. H. Servante, F. P. Fanizzi and P. M. Maitlis, 3 C. K. Lai, M.-Y. Lu, R. Lin and K.-C. Kao, J. Chem. Soc., Dalton Liq. Cryst., 1994, 16, 675; (e) H. Zheng, P. J. Carroll and Trans., 1998, 1857. T. M. Swager, Liq. Cryst., 1993, 14, 1421; ( f ) K. Ohta, M. 4 (a) C. K. Lai, C. H. Chang and C. H. Tsai, J. Mater. Chem., 1998, Yokoyama and H. Mikawa, Mol. Cryst. Liq. Cryst., 1981, 73, 205. 8, 599; (b) C. K. Lai, F. G. Chen, Y. J. Ku, C. H. Tsai and R. Lin, J. Chem. Soc., Dalton Trans, 1997, 4683; (c) C. K. Lai and F. J. Lin, J. Chem. Soc., Dalton Trans, 1997, 17; (d) A. G. Serrette, Paper 8/03603I 2610 J. Mater. Chem., 1998, 8, 2605&

ISSN:0959-9428    

DOI:10.1039/a803603i

出版商:RSC    

年代:1998

数据来源: RSC