Cholesteric sense

Further challenges need to be addressed for the functional development of liquid-crystalline cellulosics, coupled with the elucidation of fundamental aspects of their chiral nature. In addition, the potential thermotropicity of cellulose microcrystallites with the surfaces modified adequately and manipulation of the cholesteric sense (handedness) in the helically arranged molecular assembly are embryonic but fascinating subjects in this research field. 

The recent studies on the structure and properties of polypeptide liquid crystals, which are formed in solution as well as in the solid state, are reviewed in this article. Especially the cholesteric pitch and the cholesteric sense (right-handed or left-handed), which are characteristic factors of cholesteric liquid crystals, are discussed in detail in relation to the effects of temperature, concentration, and solvent. Further cholesteric liquid crystalline structure retained in cast fdms and thermotropic mesomorphic state in some copolypeptides are also discussed. 

A similar thermally-induced inversion of the cholesteric sense was observed for the PBLG liquid crystal in benzyl alcohol. In this solution, a gel-like opaque phase coexists with the cholesteric phase at lower temperatures. The opaque phase disappears around 70 °C, where endothermic peaks are observed in the differential scanning calorimetry curve. The value of S below 70 °C remains constant, and then changes with temperature above 70 °C. The compensation occurs at about 103 °C, and the transition from biphasic phase to the isotropic phase is observed above 150 °C in this case. The results are summarized in Fig. 12, where the reciprocal of the half-pitch is plotted against temperature. The sign of 1/S is taken as positive when the cholesteric sense is the right-handed. 

The thermally induced inversion of the cholesteric sense was considered to be related to the intermolecular hydrogen-bond formation of the side chain ester groups of PBLG with m-cresol, since it was first observed in m-cresol solutions. However, our subsequent investigations revealed that the sense inversion also occurs in 1,2,3-trichloropropane(TCP) which does not have any particular functional groups. A similar finding was recently reported in 1,1,2,2-tetrachloroethane. ... 

According to Keating s theory, the pitch decreases with increasing temperature. However, the pitch in the cholesteric liquid crystals of polypeptides increases below T. Furthermore the cholesteric sense inverts at T. These results can not been ex-... 

In thermotropic liquid crystals, the cholesteric sense is determined by the chirality of the constituent molecule the cholesteric sense of the liquid crystals of an optical isomer must be opposite to that of its mirror image isomer. When equal moles of both isomers are mixed (racemic mixture), the twisting power falls to zero and the... 

This equation provides a twisting power which varies linearly with composition. When Sj and Sjj have opposite signs, the inversion of the cholesteric sense should occur at a critical composition, x = S j /(S / — Sf/). 

In Fig. 23 the relationship between B/kg and XA/D are shown for the polypeptide liquid crystals in various solvents. Following Eq. (25), the compensation temperature is determined by the ratio (fi/kg)/(lA/D) at a constant polymer concentration. The solid and broken lines shown in Fig. 23 correspond to the theoretical values calculated for Tjj = 25 °C and 80 °C, respectively. The solvents located above and below the solid line support the right-handed and left-handed cholesteric liquid crystals, at 25 °C, respectively. The situation is the same for the broken line at 80 °C. The solvents located between the two straight lines invert the cholesteric sense from right-handed to left-handed in the range of measurements. 

We have discussed in detail the cholesteric pitch and the cholesteric sense in polypeptide liquid crystals, centering around our own work, which had not been investigated much since Robinson had presented excellent works about a quarter of a century ago. We have show that compensation of the cholesteric sense caused by temperature and solvent can be consistently explained by the theory of Kimura et al., in which the intermolecular force is assumed as the sum of a repulsion of a hardcore with shape of a twisted rod and of dispersion forces of Maier-Saupe-Goossens type. 

Watanabe J, Nagase T (1988) Thermotropic polypeptides. 5. Temperature dependence of cholesteric pitches exhibiting a cholesteric sense inversion. Macromolecules 21 171-175... 

Toriumi H, Kusumi Y, Uematsu I, Uematsu Y (1979) Thermally induced inversion of the cholesteric sense in lyotropic polypeptide liquid crystals. Polym J 11 863-869... 

Watanabe J, Nagase T (1988) ThermoLopic polypeptides. 5. Temperature dependence of cholesteric pitches exhibiting a cholesteric sense inversion. Macromolecules 21(1) 171-175 Wenzel RN (1936) Resistance of solid surfaces to wetting by water. Ltd Eng Chem 28(8) 988-994 Werbowyj RS, Gray DG (1976) Liquid crystalline structure in aqueous hydroxypropyl cellulose solutions. Mol Cryst Liq Cryst 34(4) 97-103... 

The cholesteric helices can be right-handed or left-handed and usually have a temperature-dependent pitch also, in a few cases the sense of the helix... 

Recently, a promising theoretical treatment was introduced by Ferrarini et al.22 which, in selected cases, leads to the effective calculation of the helical sense and pitch of the induced cholesteric phases.23 Attempts to relate the cholesteric handedness of lyotropic cholesterics to the helical sense of the polymers were first reported by Sato and co-workers.11... 

As mentioned in the introduction, the first empirical correlation between the absolute configuration of dopants and the handedness of induced cholesterics was proposed in 1975.20 The first attempt to find a general correlation was a few years later Krabbe et al.58 related the sense of the cholesteric to a stereochemical descriptor of the dopant based on the effective volume of the substituents and listed many compounds following this rule. However, exceptions were described at that time,59 and, furthermore, this approach neglects the role of the structure of the nematic solvent in determining the sense of the cholesteric. It is well known that chiral compounds may induce cholesterics of opposite handedness in different nematics.60,61... 

The CD reflection spectra are quite sharp at all temperatures, and the reflection wavelength, corresponding to the optical pitch of the TChLC phase, increased progressively with temperature from 500 nm at 70 °C to 1,000 nm at 140 °C. It was considered that the positive sign of the CD reflection band indicated M screw sense helicity of the cholesteric phase. Very recently, a smectic A-cholesteric phase transition was also observed for PDMBS.348... 

Vo and Zugenmaier (105) determined the pitch of cellulose tricarbanilate (CTC, D.P. = 100) in 2-pentanone and methyl ethyl ketone (MEK) and ethyl cellulose (EC) in glacial acetic acid as a function of temperature, concentration, solvent, and degree of polymerization. The pitch of the helicoidal structure of CTC/MEK and CTC/2-pentanone is right-handed but EC in glacial acetic acid is left-handed. This is the first report that the substituent will influence the sense of the cholesteric superhelicoidal structure. 

Guo and Gray (114) foimd that acetylation of the imsubstituted groups in ethyl cellulose changes the sense of the helicoidal cholesteric twist from leff-handed to right-handed in either CHCI3 or m-cresol. 

The Relationship Between the CD Sign and the Helical Sense in Cholesteric Phases... 

When linearly polarized light passes through a cholesteric phase having helical directors wound in a right-handed helix, RCP is scattered selectively, and the sactter-ing results in eL < eR, i.e., Ae < 0. Thus, the pitch-band CD with minus sign corresponds to a right-handed helical sense in the mesophase. 

Crown ethers of the type discussed in this section have been used as sensors, membranes, or materials for chromatography. Shinkai used cholesterol-substituted crown ether 10 as a sensor for chirality in chiral ammonium compounds (Scheme 16). It was found that the pitch of the cholesteric phase exhibited by 10 was changed upon addition of the chiral salt. As the wavelength of reflection for incident light depends on the pitch, a color change was observed that was visible to the naked eye [45, 46]. Such chirality sensing systems were known before but chromophores had to be bound to the crown ether in order to observe color changes [47]. This problem could be overcome by 10, which uses intrinsic properties of the chiral nematic phase. 

Photochemical modulation of the helical screw sense and pitch of a cholesteric phase was achieved with the combination of a nematic liquid crystalline host and an optically active photoresponsive guest as illustrated in Scheme 25.[92] Doping of 4 -(pentyloxy)-4-biphenylcarbonitrile 41 with P-trans-17b (1 wt%) converts the nematic phase into a cholesteric phase. 

The cholesteric screw sense was measured by the Grandjean-Cano method, which showed that M-cis-17a and P-trans-17b give cholesteric phases with opposite handedness. Observations of the switching behavior of 17 in a large number of LC materials,... 

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