Thermal and optical properties of chiral twin liquid crystalline bis (cholesteryl ) alkanedioates Antonius T. M. Marcelis, Arie Koudijs and Ernst J. R. Sudholter" Department of Organic Chemistry, Wageningen Agricultural University, Dreijenplein 8, 6703 HB Wageningen, The Netherlands The thermal and optical properties of a series of bis(cholestery1) alkanedioates were investigated. The melting points, the cholesteric to isotropic transition temperatures, the entropy changes at this temperature and the selective reflection wavelengths of the cholesteric phase all exhibit an odd-even effect as a function of spacer length. This is attributed to a difference in the ordering of the cholesteric phase as a function of the parity of the spacer. Because the properties of the members of the series with a short spacer could not be measured for several reasons, their properties were also determined as a 5 wt% solution in a cholesteric host.A similar odd-even influence on the cholesteric to isotropic transition temperatures and the optical properties of the host was observed as for the pure compounds. The influence on the properties of the host is stronger for compounds with a short spacer than for compounds with a longer spacer. An important property of chiral liquid crystals is that the planar oriented cholesteric phase selectively reflects The selective reflection wavelength strongly depends on the chiral centres present in the molecule, but also on the ordering of the molecules in the phase. Twin liquid crystals, for example, consisting of two mesogenic units connected by a flexible spacer, exhibit a strong alternating ordering of the nematic phase as a function of the parity of the Better ordered nematic phases are obtained for twin liquid crystals in which the mesogens are preferentially oriented parallel. This is usually the case when the number of flexible units separating the mesogens is even.Chiral twin liquid crystals can be obtained by introducing chirality in the spaceP9 or in the mesogens.lo-l2 Our recent studies on chiral twin liquid crystals containing steroid properties of a series of these esters will be presented. The study of the properties of these compounds with shorter spacers is complicated, because they are either monotropic and crys- tallize or they decompose due to the high temperatures needed to obtain the cholesteric phase.Therefore, the properties of these compounds were also investigated as a solution in a cholesteric host. Experimental Bis(cholestery1) alkanedioates (I-n) A mixture of 10mmol of the appropriate dicarboxylic acid chloride and 20mmol of cholesterol in 50ml of toluene was refluxed for 3 h. After cooling the solvent was evaporated and the residue was recrystallized from chloroform-ethanol. Yields cu . 80%. 1-3: 'H NMR (CDC13) 6 5.4 (m, 2 H, CH=), 4.65 (m, 2H, CHO), 2.35 (m, 4H, CH2C=0), 2.0-0.7 (m, 51H, aliphatic) (Calc. for C59H9604:C, 81.51; H, 11.13. Found: C, 81.82; H, 11.41%). n=odd Measurements.h mesogens have shown that the selective reflection wavelength of the cholesteric phase also exhibits an odd-even effect as a function of the parity of the connecting spacer.Compounds that give better ordered cholesteric phases give higher selective reflection wavelengths. Furthermore, a different temperature dependence was found for the selective reflection wavelength of compounds with odd and even spacers.13-15 The prototype of chiral twin liquid crystals with steroids as mesogens are the bis(cholestery1) esters of dicarboxylic acids. Although these compounds are known,16-18 as are the related bis(cholestery1) carbamates and carbonate~,l~*~~ the optical properties of the cholesteric phases of these compounds have not been investigated before. In this paper a study of the Melting points, thermal phase transition temperatures and optical inspection of the liquid crystalline phases were deter- mined on samples between ordinary glass slides using an Olympus BH-2 polarization microscope equipped with a Mettler FP82HT hot stage, controlled by a Mettler FP80HT central processor. The selective reflection wavelengths were determined by measuring the transmission spectra of the chiral nematic phases of the compounds.For the pure compounds this was done as a function of temperature by inserting the hot stage with a planar oriented cholesteric sample between parallel glass slides in the measuring beam of a Hewlett Packard 8452A photo diode array spectrophotometer. The selective reflection wavelengths of 5 wt% solutions of the bis(cholestery1) esters in an approximately 1 :2 (w/w) mixture of cholesteryl nonanoate and cholesteryl chloride2' were deter- mined in the same manner at room temperature.Differential scanning calorimetry (DSC) thermograms were obtained on a Perkin Elmer DSC-7 system. The entropy changes at the phase transition temperatures are expressed as AS/R, in which AS is calculated from AS=AH/T. AH is calculated in J mol-' and T is the corresponding phase transition temperature in Kelvin. J. Muter. Chem., 1996, 6(9), 1469-1472 1469 Results and Discussion All compounds gave correct 'H NMR spectra, elemental analyses and single spots on thin layer chromatography The thermal transitions temperatures of the compounds are pre- sented in Fig 1 As can be seen the melting points show an alternating behaviour with spacer length This odd-even effect is more clearly observed for the N*-I transition temperatures of the compounds with n34 The compounds with odd n are monotropic liquid crystals, whereas the compounds with even n usually show enantiotropic thermal behavior Upon cooling below 160°C the compounds 1-3 and 1-1 crystallize before a liquid crystalline phase is found The compounds 1-2 and 1-0 have a broad enantiotropic liquid crystalline range and become isotropic at high temperatures These results indicate that the odd-even effects observed for the N*-I transition temperatures of the compounds attenuate with spacer length This behavior is often observed for twin liquid crystals 22 A problem encoun- tered with studying the liquid crystalline phases of compounds 1-0 and 1-2 is that the compounds slowly decompose above 200 "C Generally, odd-even effects for the liquid crystalline proper- ties of twin liquid crystals are more clearly expressed in the entropy change at the (chiral) nematic to isotropic transition temperature than the clearing temperatures themselves The AS/R values for the pure compounds that could be obtained by DSC are presented in Fig 2 As stated before, the values 0 0 D 0 012345678 n Fig.1 Melting points (---0 ---) and cholesteric-lsotroplc transitlon temperatures (-0-) of compounds I-n as a functlon of the number of methylene units (n)in the spacer 10-s u,.4 05-012345678 n Fig. 2 Entropy change (expressed as AS/R)at the cholesteric-isotropic transition temperature of compounds I-n as a funtion of n 1470 J Muter Chem, 1996, 6(9), 1469-1472 for 1-1 and 1-3 could not be obtained because the compounds crystallized before a liquid crystalline phase was found and the value for 1-0 could not be accurately measured due to severe decomposition For the compounds with an odd spacer the AS/R values lie around 0 6 and for compounds with even n they lie around 09 These values agree with those found previously for two members of this series" and are comparable with those of a series of cholesteryl o-cyanobiphenylyloxy- alkanoates,13 l4 and confirm the difference in ordering of the cholesteric phase as function of the spacer length When the number of flexible units in the spacer is even, the rigid mesogens preferentially adopt a parallel orientation in the nematic phase This gives better ordered nematic phases with higher isotropization temperatures and associated entropy changes than homologues with an odd number of flexible units in the spacer The orientation of the mesogens is dependent on the number of flexible spacer groups For n=even the cholesteryl groups are preferentially oriented parallel, whereas they are not parallel for n = odd These preferential orientations strongly affect the ordering and therefore the macroscopic properties of the cholesteric phase The molecules indhe cholesteric phase are present in a helical arrangement, and the pitch of the helix IS influenced by the ordering of the molecules in the phase This can be studied by measuring the selective reflection of light by the planar oriented cholesteric phase, because the wavelength of the reflected light in the liquid crystalline phase equals the pitch of the cholesteric helix Therefore measurement of the selective reflection wavelength can in principle give information about the ordering of the phase as a function of temperature, whereas measurement of AS/R only provides information about the ordering change at the isotropization temperature The selec- tive reflection wavelengths of the bis(cholestery1) esters are plotted in Fig 3 as a function of the reduced temperature T/T(N*-I) (both in Kelvin) The selective reflection wavelength strongly depends on the spacer length For odd spacers, the selective reflection wavelength increases with spacer length and is almost temperature independent For even spacers, the selective reflection wavelengths are larger than for compounds with an odd spacer Furthermore, contrary to what is found for the compounds with odd spacers, it decreases with tempera- ture It is clearly seen that the selective reflection wavelength exhibits a strong odd-even effect which attenuates with spacer length The results obtained correspond well with those obtained for other series of cholesterol containing twin and triplet liquid crystals l3 l5 For small odd n the selective reflec- tion wavelength decreases, but for n = 8 it increases again The reasons for this could be that the reflection wavelengths for the compounds with a longer spacer are determined at lower temperatures where, due to the temperature dependence, the selective reflection wavelength is higher and that upon increas- '-* \8ool700 600 400-1-7 ................300 '. '. . 1-5 ... 7 3 I * 1.. " Fig. 3 Selective reflectlon wavelengths of the cholesteric phase of compounds I-n as a function of the reduced temperature TIT (N*-I) ing the spacer the mesogens become more diluted, resulting also in a higher selective reflection wavelength. A small amount of a compound (guest) dissolved in a nematic host often induces a change in its properties that depends on the nature of the guest and is proportional to its concentration. Chiral compounds, for example, when dissolved in nematics induce cholesteric phases whose inverse pitch is proportional to its concentration.This interesting property has been the subject of much research.lP3 Also, the nematic- isotropic transition temperature of a mixture of nematics is often intermediate between those of the components. Because these relations hold best for mixtures of similar compounds and because the properties of the compounds with the shorter spacers could not be determined for the pure compounds, we also studied the thermal and optical properties of the bis(esters) as 5 wt% solutions in a liquid crystalline host consisting of a 2 : 1 (w/w) mixture of cholesteryl nonanoate and cholesteryl chloride.This host was also chosen because compounds I-n can be dissolved in them and because it forms a cholesteric phase at room temperature that reflects light in the visible range.21 The isotropization temperatures of these cholesteric mix- tures are depicted in Fig. 4. The host itself has an isotropization temperature of 78°C. Clearly an odd-even effect is seen that becomes more pronounced with a shorter spacer. In most cases the guest increases the isotropization temperature of the mixture. This is to be expected because the (chiral) nematic isotropic transition temperatures of mixtures usually lie between those of its pure components. The same trend in the odd-even effect is found as for the pure compounds.The stabilizing effect on the cholesteric phase of the host is stronger for the compounds with an even spacer than for compounds with an odd spacer and compound 1-1 even destabilizes the cholesteric phase of the host. Upon increasing the concen-tration of the guest the odd-even effects on the isotropization temperature become more pronounced. If a linear dependence is assumed between the clearing temperatures of the host and the guest, the clearing temperatures of the guest can be calculated. Upon comparing these with the observed values virtual clearing temperatures for compounds 1-1 and 1-3can be estimated of approximately 60 and 160"C, respectively. This corresponds nicely with the fact that no cholesteric phase could be observed for the pure compounds 1-1 and 1-3.It also confirms that the odd-even effect is more pronounced for compounds with shorter spacers.Previously, odd-even effects have been observed for mixtures 90 85 P 80 75 70 I I I I I 012345678 n Fig. 4 Cholesteric-isotropic transition temperatures ( +) of a 5 wt% solution of compounds I-n in a cholesteric host consisting of a 2 : 1 (w/w) mixture of cholesteryl nonanoate and cholesteryl chloride as a function of n 40 -20 -\E-4 d O---20 --40 -SO! 1I I I 1 I I I I I -10123456789 n Fig. 5 Induced change in selective reflection wavelength (AA/nm) at room temperature by a 5 wt% solution of compounds I-n in a cholesteric host consisting of a 2: 1 (w/w) mixture of cholesteryl nonanoate and cholesteryl chloride as a function of n in which a chiral compound was dissolved in a nematic host of which the chain length was changed.23 The better ordered phases of the host give mixtures with a longer pitch. For cholesteryl alkanoates in a compensated cholesteric mixture an odd-even effect was found for the temperature at which the phases are exactly compensated depending on the length of the alkyl chain of the guest.24 The effects are, however, weak and for other mixtures of a series of chiral compounds in nematics no clear odd-even effects on the pitch have been ob~erved.~' The influence of compounds I-n on the optical properties of the cholesteric host have also been studied.The host itself reflects at 650nm at room temperature and the selective reflection wavelengths of the solutions deviate from this value.These deviations are plotted in Fig. 5. For the series of com-pounds a clear odd-even effect is observed as a function of the parity of the spacer. For the compounds with an odd spacer there seems to be a minimum for n =5 and for the compounds with an even spacer this minimum lies at n =6. This may be the result of specific interactions between the guest and the host. Because the guest and host have the same left-handed screw sense of the helix the compounds that have the lowest selective reflection wavelength as a pure compound also give the lowest reflection wavelength as a solution. Most com-pounds decrease the selective reflection wavelength of the host. From Fig.3 it is seen that most pure compounds also have a selective reflection wavelength that is smaller than that of the host although it has to be realized that this was measured well above room temperature. Only compound 1-2 has a measured selective reflection wavelength that is higher than that of the host and in solution it also has a higher selective reflection wavelength than the host. Compound 1-0 increases the selective reflection even more, indicating that the pure compound would have a selective reflection wavelength that is in the near infrared. Very recently, a substantial difference in the pitch of the cholesteric and also the Sc* phase of two members of a chiral twin liquid crystal dissolved in an achiral host was observed.' In these twins the chirality stems from a chiral centre in the spacer and the different pitch was attributed to the odd-even effect of the guest on the ordering of the host.Conclusions A series of chiral twin liquid crystalline bis(cholestery1) alkane- dioates show odd-even effects as a function of spacer length for the properties of the cholesteric phase, i.e. the cholesteric J. Muter. 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