Cholesteric screw sense

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,... 

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... 

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. 

Fig. 2 POM observations of a concentrated PCIBLA solution (25 wt%) in TCE. The birefringent cholesteric texture changes its sign from (a) left (90°C) to (c) right (102°C) with increasing temperature, (b) The cholesteric pitch diverges in the transition region (97°C). The screw sense of the polypeptide backbone transforms from right to left, accordingly. Reprinted with permission from [76]. Copyright 2005 Wiley-VCH...

As noted in Section 2 the d- and fi-isomers of optically active molecules exhibit somewhat different order parameters in liquid crystals possessing a local screw sense (such as cholesteryl derivatives). Accordingly d- and C-isomers should separate on cholesteric substrates. For 3,3,3-trichloropropylene oxide the difference in the order parameter is of the two isomers is AS z = 0.00015 (cf Section 2) for a compensated nematic mixture of cholesteryl derivatives. On the basis of Equ. (42) one would expect a separation factor of a 1.002. Up to the present the separation of optically active isomers on cholesteric substrates has not been achieved. 

The characteristic feature by which cholesterics differ from the nematics is the spontaneous formation of twisted structures, reflecting the existence of a preferred screw sense. For this reason, the defect lines no longer merge and cancel each other as in nematics instead, complicated stable networks of disclination lines may form. The "streaks" in planar cholesteric films that often form a crackle consist of bimdles of thin individual lines (Figure 6.18). A single line itself may show a number of complicated features. ... 

Among the several liquid crystal polymers that have been studied in recent years those containing intrinsically chiral elements with a prevalent chirality hold a particular position. Some of these, in fact, by virtue of their structural characteristics, assume a spatial array with nematic planes stacked in a superhelical structure characterized by a prevalent screw sense and are known as cholesteric phase. This kind of order can be controlled by either concentration in solution (lyotropic systems) or temperature in bulk (thermotropic systems). 

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