Supramolecular dimeric liquid crystals. The liquid crystalline behaviour of mixtures of a-(4-pyridyloxy)-v-[4-(4-butylphenylazo)phenoxy]alkanes and 4-octyloxybenzoic acid Marc J. Wallage and Corrie T. Imrie* Department of Chemistry, University of Aberdeen,Meston Walk, Old Aberdeen, UK AB24 3UE The thermal behaviour of binary mixtures of 1-(4-pyridyloxy)-5-[4-(4-butylphenylazo)phenoxy]pentane (Bu-azo-5-Pyr) and 1-(4- pyridyloxy)-6-[4-(butylphenylazo)phenoxy]hexane (Bu-azo-6-Pyr) with 4-octyloxybenzoic acid is reported.Both systems are miscible over the complete composition range; this miscibility is attributed to the formation of a hydrogen bond between the unlike species, a view confirmed by IR spectroscopy. Bu-azo-5-Pyr and Bu-azo-6-Pyr do not exhibit liquid crystallinity but enhanced liquid crystal behaviour is observed for the mixtures with the acid.In particular, smectic A behaviour is injected into the phase diagrams of both systems. The clearing temperatures of the mixtures containing Bu-azo-6-Pyr are considerably higher than those for the analogous mixture containing Bu-azo-5-Pyr. In addition, the entropy change associated with the clearing transition for the equimolar mixture containing Bu-azo-6-Pyr is significantly higher than that for the analogous mixture containing Bu-azo- 5-Pyr.These observations strongly suggest the formation of a supramolecular dimeric liquid crystal. For the Bu-azo-6-Pyr-based system the specific molecular interaction between the pyridyl and acid fragments has not only strongly enhanced the liquid crystalline behaviour but also increased the degree of molecular ordering.In recent years increasing research activity has focused on Experimental liquid crystalline systems consisting of supramolecular com- Bu-azo-5-Pyr, 1, and Bu-azo-6-Pyr, 2, were prepared using the plexes assembled via noncovalent interactions1–3 but this is by synthetic route shown in Scheme 1.no means a new idea. Indeed, some forty years ago Gray and Jones4 attributed the liquid crystalline behaviour of the 4- 4-Butyl-4¾-hydroxyazobenzene, 4 alkyl- and 4-alkoxy-benzoic acids to hydrogen bonded dimers rather than to discrete molecular units. The novelty of the Compound 4 was prepared as described in detail elsewhere.7 recent work, however, is that the two interacting species are not identical and a hetero-intermolecular bond is used to 1-Bromo-5-[4-(4-butylphenylazo)phenoxy]pentane, 5 assemble the liquid crystal unit.Furthermore, the two interacting molecules need not be, and are often not, liquid crystal- Compound 5 was prepared using the method described by line individually but the resulting supramolecular complex is Attard et al. elsewhere.8 Thus, a mixture containing 4 (4.5 g, mesogenic.This amplification of a specific molecular inter- 17.6 mmol), 1,5-dibromopentane (40.6 g, 176 mmol) and potaction into a macroscopically observable phenomenon, in this assium carbonate (20.68 g, 150 mmol) in acetone (100 ml) was case liquid crystallinity, is very much a central theme in refluxed with stirring overnight. The reaction mixture was supramolecular chemistry.5,6 allowed to cool, filtered and the acetone removed using a The most commonly used interaction in assembling liquid rotary evaporator.Light petroleum (bp 40–60°C) (300 ml) crystalline complexes is the hydrogen bond1–3 and particular interest has centred on mixtures of molecules containing pyridyl and carboxylic acid fragments.2 In these mixtures the pyridyl unit serves as the hydrogen bond acceptor and the carboxylic acid as the hydrogen bond donor.The focus of much of this research has been the role of the hetero-hydrogen bond in determining the transition temperatures and phase behaviour of the system. In comparison, complex formation has not been used in an attempt to manipulate the degree of molecular ordering within the mesophase.In order to investigate this possibility we have now characterised the thermal behaviour of mixtures of two a-(4-pyridyloxy)-v-[4-(4-butylphenylazo) phenoxy]alkanes, 1 and 2, with 4-octyloxybenzoic acid, 3. The acronyms used to refer to 1 and 2 are Bu-azo-5- Pyr and Bu-azo-6-Pyr, respectively. These particular structures were chosen because the analogous covalently bonded structures containing two mesogenic groups linked via a flexible spacer, the so-called dimeric liquid crystals, are known to exhibit mesophases in which the degree of molecular order is critically dependent on the length and parity of the alkyl spacer.Scheme 1 J. Mater. Chem., 1997, 7(7), 1163–1167 1163was added to the organic extracts and the solution cooled to Results and Discussion -20 °C for ca. 2–3 h. The resulting precipitate was collected, Bu-azo-5-Pyr–4-octyloxybenzoic acid mixtures washed with light petroleum and dried under vacuum. The product was recrystallised from ethanol. Yield: 3.93 g, 55%. dH Bu-azo-5-Pyr and 4-octyloxybenzoic acid were miscible over (CDCl3; J values in Hz throughout) 7.9, 7.8, 7.3, 7.0 (m, the complete composition range and this was presumably aromatic, 8H), 4.1 (t, ArOCH2, 2H, J 6.3), 3.4 (t, CH2Br, 2H, attributable to the formation of a hydrogen bond between the J 6.7), 2.7 (t, ArCH2 , 2H, J 7.8), 1.3–2.0 [m, CH3CH2CH2, unlike components in the mixture.This view was supported OCH2CH2)3CH2Br, 10H], 1.0 (t, CH3, 3H, J 7.3). by IR spectroscopy; specifically, the spectra of the complexes contain bands centred at ca. 2490 and 1900 cm-1, indicative 1-Bromo-6-[4-(4-butylphenylazo)phenoxy]hexane, 6 of strong hydrogen bonding.10–13 In addition, the carbonyl band has a shoulder at ca. 1690 cm-1, a characteristic value Compound 6 was prepared using the procedure described for for free carbonyl groups. Representative IR spectra of the 5. Yield: 5.54 g, 71%. dH (CDCl3) 7.9, 7.8, 7.3, 7.0 (m, aromatic, individual components and the equimolar complex are shown 8H), 4.0 (t, ArOCH2, 2H, J 6.4), 3.4 (m, CH2Br, 2H), 2.7 (t, as Fig. 1.Unfortunately, the complexity of the spectrum of the ArCH2, 2H, J 7.6), 1.3–2.0 [m, CH3CH2CH2, complex is such that it prevents even a semi-quantitative OCH2(CH2)4CH2Br, 12H], 0.9 (t, CH3, 3H, J 7.3). assessment of the equilibrium constant for the formation of the complex.Bu-azo-5-Pyr, 1 The dependence of the transition temperatures determined A mixture containing 5 (3.8 g, 9.5 mmol), 4-hydroxypyridine using dierential scanning calorimetry on the mole fraction of (0.92 g, 9.7 mmol) and caesium carbonate (15.6 g, 47.7 mmol) Bu-azo-5-Pyr in the mixture is shown in Fig. 2. Bu-azo-5-Pyr in N,N-dimethylformamide (DMF) (50 ml) was refluxed with melts directly into the isotropic phase at 89°C and can be stirring overnight.The reaction mixture was allowed to cool, supercooled to ca. 70°C without the observation of liquid poured into ice cold water (1 l) and stirred for ca. 30 min. The crystallinity. Similarly, the mixtures containing greater than mixture was extracted with chloroform; the organic layer was 0.5 mol fraction of Bu-azo-5-Pyr do not exhibit liquid crystal- washed with water, dried and the chloroform removed using line behaviour.In comparison, the remaining mixtures are a rotary evaporator. The crude product was passed through liquid crystalline; specifically nematic, smectic C and smectic A silica gel using acetone as the eluent. Yield: 0.6 g, 14.9%.Mp phases are observed. These phases were identified on the basis 85–86°C. dH (CDCl3 ) 8.4, 7.9, 7.3, 6.9 (m, aromatic, 12H), 4.0 of the observation of clear characteristic optical textures when (m, 2 ArOCH2, 4H), 2.7 (t, ArCH2, 2H, J 7.6), 1.2–2.0 [m, viewed through the polarised light microscope.14,15 Specifically, (CH2 )3, CH2CH2 , 10H], 1.0 (t, CH3, 3H, J 7.3). Bu-azo-6-Pyr, 2 Bu-azo-6-Pyr was prepared using the procedure described for Bu-azo-5-Pyr.Yield 0.53 g, 9.7%. Mp 118–120°C. dH (CDCl3) 8.4, 7.8, 7.3, 7.0, 6.8 (m, aromatic, 12H), 4.0 (m, ArOCH2, 4H), 2.7 (t, ArCH2, 2H, J 7.6), 1.3–1.9 [t, (CH2)4, CH2CH2, 12H], 0.9 (t, CH3 , 3H, J 7.3). 4-Octyloxybenzoic acid, 3 4-Octyloxybenzoic acid (Aldrich) was recrystallised from ethanol prior to use.The transition temperatures were in good agreement with those reported elsewhere:9 crystal–smectic C, 101 °C, smectic C–nematic, 108 °C, and nematic–isotropic, Fig. 1 IR spectra of (a) Bu-azo-5-Pyr, (b) 4-octyloxybenzoic acid and 147 °C. (c) the equimolar complex Preparation of complexes The mixtures were prepared by codissolving the components in pyridine and the solvent allowed to evaporate slowly.The complexes were dried under vacuum for at least 24 h prior to characterisation. Characterisation The proposed structures of all the compounds were verified using 1H NMR and IR spectroscopy. 1H NMR spectra were measured in CDCl3 on a Bruker AC-F 250 MHz NMR spectrometer. IR spectra were recorded using a Nicolet 205 FTIR spectrometer. Thermal characterisation The thermal behaviour of the materials was characterised by dierential scanning calorimetry using a Mettler-Toledo DSC Fig. 2 Dependence of the transition temperatures on the mole fraction 820 system equipped with an intracooler accessory and cali- of Bu-azo-5-Pyr 1 for mixtures of Bu-azo-5-Pyr and 4-octyloxybenzoic brated using an indium standard. The heating and cooling acid. The broken line represents the melting point; (#) rates in all cases were 10°C min-1.Phase identification was smectic A–isotropic transition; (1) smectic C–nematic transition, (%) performed by polarised light microscopy using an Olympus nematic–isotropic transition; (() smectic C–smectic A transition. C= BH-2 optical microscope equipped with a Linkam THMS 600 crystal; N=nematic; SC=smectic C; SA=smectic A; I=isotropic.Crystal–crystal transitions have been omitted for the sake of clarity. heating stage and TMS 91 control unit. 1164 J. Mater. Chem., 1997, 7(7), 1163–1167for the nematic phase a Schlieren texture was observed contain- and a nematic phase whereas the 0.7, 0.8 and 0.9 mixture exhibit solely nematic behaviour. The 0.95 mixture and ing both types of point singularity and which flashed when subjected to mechanical stress, while for the smectic C phase pure Bu-azo-6-Pyr do not exhibit liquid crystallinity.The virtual clearing temperature of Bu-azo-6-Pyr, estimated by a Schlieren texture was also observed but in which only one type of point singularity was evident. For the smectic A phase extrapolating the clearing temperature curve, is ca. 74°C. As with the Bu-azo-5-Pyr-based mixtures, the melting points a focal conical fan texture was observed in coexistence with regions of homeotropic alignment. of the Bu-azo-6-Pyr-based mixtures tend to be significantly higher than expected on the basis of a linear variation of the The mixtures containing 0.05 and 0.1 mol fraction of Buazo- 5-Pyr exhibit the same phase sequence as the pure acid, melting point on composition, see Fig. 3, and again this is indicative of a specific interaction between the unlike compo- namely smectic C–nematic–isotropic. The nematic–isotropic transition temperature falls on increasing the concentration of nents. The clearing temperatures for the Bu-azo-6-Pyr-based mixtures are also considerably higher than expected and Bu-azo-5-Pyr whereas the smectic C–nematic transition temperature appears to be insensitive to changes in composition.smectic A behaviour is injected into the phase diagram. For the 0.2 mol fraction Bu-azo-5-Pyr mixture, a smectic A phase is injected between the smectic C and nematic phase. Comparison of the systems The mixtures containing 0.3, 0.4 and 0.5 mol fraction of Bu- The estimated virtual transition temperature of Bu-azo-6-Pyr azo-5-Pyr exhibited exclusively smectic A behaviour.Thus, is ca. 22°C higher than that for Bu-azo-5-Pyr. This dierence increasing the concentration of Bu-azo-5-Pyr strongly pro- is in accord with the pronounced alternation observed for motes smectic A rather than nematic behaviour; we will return conventional low molar mass mesogens containing a bulky to this observation below.group attached via a flexible alkyl spacer to the mesogenic The melting points of the Bu-azo-5-Pyr–4-octyloxybenzoic core.16–19 Thus, these estimated values are both self-consistent acid mixtures are typically significantly higher than those of and reasonable. The miscibility observed for both systems the individual components, see Fig. 2. This behaviour strongly impliesthat the hydrogen bond between the unlike components suggests that the unlike components exhibit a specific inter- is more favourable than that present in the self-associated acid action. By comparison, the clearing temperature initially dimers. Furthermore, the strength of this hydrogen bond is decreases in essentially a linear fashion on increasing the sucient to counteract the unfavourable entropic term which concentration of Bu-azo-5-Pyr, see Fig. 2. The extrapolation tends to promote phase separation.9 of the linear segment of the clearing temperature curve allows The smectic A–isotropic transition temperature for the equi- the virtual clearing temperature of Bu-azo-5-Pyr to be esti- molar mixture containing Bu-azo-6-Pyr,135 °C, is considerably mated at ca. 52°C. The clearing temperatures for the 0.3, 0.4 higher than that observed for the Bu-azo-5-Pyr-based mixture, and 0.5 mol fraction of Bu-azo-5-Pyr mixtures deviate in a 109°C. This large dierence, similar to that observed for positive sense away from a linear dependence on composition. conventional dimeric liquid crystals,20,21 may be accounted for by considering the relative shapes of the complexes.Thus, for Bu-azo-6-Pyr–4-octyloxybenzoic acid mixtures the Bu-azo-6-Pyr–acid complex the mesogenic groups are coparallel if the spacer is in the all-trans conformation, see The dependence of the transition temperatures on the mole fraction of Bu-azo-6-Pyr in the mixtures is shown in Fig. 3. Fig. 4(a); this arrangement reinforces the intramolecular orientational correlations between the mesogenic cores and, hence, The phase assignments were performed using the arguments oered for the Bu-azo-5-Pyr-based mixtures.In addition, the enhances the clearing temperature. In contrast, for the Bu-azo- 5-Pyr–acid complex when the spacer is in the all-trans confor- IR spectra obtained for the mixtures were essentially identical to that shown in Fig. 1. The mixtures containing 0.05 and mation the mesogenic groups are constrained to lie at an angle with respect to each other, see Fig. 4(b), so reducing the 0.1 mol fraction of Bu-azo-6-Pyr exhibit smectic C and nematic phases and the associated transition temperatures are similar clearing temperature. All the mixtures containing Bu-azo-6- Pyr exhibit higher clearing temperatures than the correspond- to those of the pure acid.The 0.2 mol fraction of Bu-azo-6- Pyr mixture exhibits smectic C and A phases while the 0.3, 0.4 and 0.5 mol fraction mixtures exhibit exclusively smectic A behaviour. The 0.6 mol fraction mixture shows a smectic A Fig. 3 Dependence of the transition temperatures on the mole fraction of Bu-azo-6-Pyr 1 for mixtures of Bu-azo-6-Pyr and 4-octyloxybenzoic Fig. 4 Schematic representation of the eect on the shape of the acid. Crystal–crystal transitions have been omitted for the sake of clarity. (') Smectic A–nematic transition; all other symbols are as complex on introducing a single gauche defect into the spacer for (a) an even-membered and (b) an odd-membered spacer defined in Fig. 2. J. Mater. Chem., 1997, 7(7), 1163–1167 1165ing mixture containing Bu-azo-5-Pyr and this simply reflects the higher clearing temperature of the Bu-azo-6-Pyr–acid complex. The entropy change associated with the smectic A–isotropic transition, expressed as the dimensionless quantity DS/R, for the equimolar mixture containing Bu-azo-6-Pyr, 3.61, is considerably larger than that for the Bu-azo-5-Pyr-based equimolar mixture, 1.65; these values have been calculated assuming the formation of a 151 complex. This dramatic dierence in the entropy change associated with the clearing transition on adding just a single methylene unit is characteristic behaviour for conventional dimeric liquid crystals20,21 and may be accounted for by considering the inherent flexibility of the spacer.Indeed, the rationalisation of the dependence of the clearing temperatures on the parity of the spacer considered just the all-trans conformation of the spacer which is clearly unrealistic. If instead we consider the eects of introducing a single gauche defect into the spacer and allow it to move sequentially along the chain, then there are still conformations of an even-membered spacer in which the mesogenic units lie coparallel, see Fig. 4(a). In comparison, all the conformations of the odd-membered spacer constrain the mesogenic units to lie at an angle with respect to each other, see Fig. 4(b). The smectic A phase is an anisotropic environment which selects the more elongated conformations in which the mesogens are coparallel. Thus, at the smectic A–isotropic transition, there is a greater change in the conformational distribution of an evenmembered spacer than for an odd-membered spacer.Consequently, the conformational component of the overall smectic A–isotropic entropy change is higher for even-membered dimers, i.e. the equimolar Bu-azo-6-Pyr–acid mixture. We must remember, however, that are three main contributions Fig. 5 Probable local packing arrangements in the smectic A phase for mixtures containing (a) an equimolar ratio of the two components, to the overall entropy change, conformational, orientational (b) an excess of the acid and (c) an excess of 1. Shaded ellipses represent and translational, and it is the subtle interplay of these that azobenzene, open ellipses represent hydrogen bonded mesogenic units, determines the overall value.Indeed, calculations suggest that filled ellipses represent hydrogen bonded acid dimers and the pear- the conformational component may only be a relatively small shape represents free pyridyl units. component of the overall entropy change.21 Thus, further speculation on the molecular significance of the pronounced dierence in the smectic A–isotropic entropy change between these supramolecular complexes must now await further inves- chain interaction while entropically, the unfavourable inter- tigation.It is apparent, however, that the specific molecular action between a core and a chain which acts to order the interaction between the pyridyl and acid fragments in the Bu- chain drives phase separation. Thus, the equimolar complex azo-6-Pyr–acid equimolar mixture is manifested at the macro- would be expected to, and indeed does, exhibit smectic rather scopic level not only by the induction of smectic A behaviour than nematic behaviour. If we now increase the mole fraction into the phase diagram but also by the high degree of molecular of the acid component then hydrogen bonded acid dimers will ordering within the mesophase.It should be noted that if the be present. Initially, these may be accommodated within the two components are simply miscible but exhibit non-ideal smectic phase structure, see Fig. 5(b), but as their concentration behaviour then the clearing entropy would not be so critically increases the terminal octyl chains, which are too long to be dependent on spacer length.The non-ideality in the transition accommodated within the spacer domains, destabilise the temperatures of some liquid crystal mixtures is driven by an smectic phase and nematic behaviour is observed. Conversely, enthalpic term and the clearing entropy is essentially unaec- if the Bu-azo-Pyr component is in excess than the packing ted. Thus, the magnitude of the clearing entropies for the density in the smectic phase is reduced, see Fig. 5(c), and again systems discussed here are indicative of complex formation. nematic behaviour is favoured. The strong induction of smectic A behaviour in both phase diagrams, see Fig. 2 and 3, may be accounted for in terms of the relative tendencies of dimeric liquid crystals to form smectic Conclusion phases and the probable local structure within the smectic phase.It has been shown that if the lengths of the terminal In this study, we have seen that the phase behaviour and alkyl chains attached to a dimer exceed half the length of the transition temperatures of mixtures 1 and 2 with 3 are deter- flexible spacer, as, for example, in both equimolar complexes, mined by the formation of a hydrogen bond between the then a monolayer smectic phase results.21 The formation of unlike components; this is quite general behaviour.1–3 For the this phase may be understood in terms of a microphase first time, however, we have shown how the degree of molecular separation in which there exists three distinct regions: meso- order within the mesophase can also be controlled by hydrogen genic groups, terminal alkyl chains and spacers, see Fig. 5(a). bonding at the molecular level. In these dimeric structures the terminal chains are simply too long to be accommodated within the region comprising the spacers. The driving force resulting in this phase separation We are pleased to acknowledge support from the EPSRC, grant number GR/J32701 and from the University of Aberdeen may be considered as either energetic or entropic.Energetically, phase separation will occur if the mean of the core–core and for the award of a grant to purchase the Mettler-Toledo DSC 820. chain–chain interactions is more favourable than the core– 1166 J. Mater. Chem., 1997, 7(7), 1163–116713 S. E. Odinokov, A. A. Mashkovsky, V. P. Glazunov, References A. V.Iogansen and B. V. Rassadin, Spectrochim. Acta, 1976, 32A, 1 C. T. Imrie, T rends Polym. Sci., 1995, 3, 22. 1355. 2 T. Kato and J. M. J. Fre� chet,Macromol. Symp., 1995, 98, 311. 14 D. Demus and L. Richter, T extures of L iquid Crystals, Verlag 3 C. M. Paleos and D. Tsiourvas, Angew. Chem., Int. Ed. 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