Optical rotatory power, cholesterics

The theory of quadratic variations in optical activity with respect to the electric field strength was first formulated for macromolecules by Tinoco and Hammerle," and then developed by others." The earliest e qperi ments are due to Tinoco in solutions of poly-y-L-sJutamate in ethjdene dichloride of late, this experiment has been extended to transient optical rotation changes by Jennings and Baily." Also, electric field effects on the optical rotatory power of a compensated cholesteric liquid crystal have been stuped." ... 

An analogous enhancement in the optical rotatory power of the isotropic phase of a thermotropic liquid crystal has been observed near the isotropic-cholesteric phase transition. Patel and DuPre concluded that it is attributable to short-range chiral ordering of the long axes of the macromolecules. 

Only a few solvents are known to dissolve cellulose completely, and solid cellulose decomposes before melting. Therefore, it is difficult to study the mesophase behavior of cellulose. Chanzy et al. [32] reported lyotropic mesophases of cellulose in a mixture of jV-methyl-morpholine-Af-oxide and water (20-50%), but were unable to determine the nature of the mesophase. Lyotropic cholesteric mesophase formation in highly concentrated mixtures of cellulose in trifluoroa-cetic acid + chlorinated-alkane solvent [33] and in ammonia/ammonium thiocyanate solutions [34] has been studied, and although poor textures were obtained in the polarizing microscope, high optical rotatory power has been measured in an optical rotation (ORD) experiment, which could be fitted to the de Vries equation [Eq. (3)] for selective reflection beyond the visible wavelength region and was taken as proof of a lyotropic chiral nematic phase. 

Fig. 12.1 Spectra of optical rotatory power (OR) and selective reflection (R) of a planar cholesteric texture for light propagation parallel to the helical axis.

We present here an approximate calculation of the optical rotatory power of the blue phase. As shown in the discussion of the cholesteric phase in Chapter 3, for a light beam propagating... 

This is the typical optical rotatory power of cholesteric liquid crystals with pitch shorter than the hght wavelength. In the blue phases, the double-twist cylinders orient along many different directions, and therefore the optical rotatory power is smaller than that of cholesteric phase. The thickness of blue phases displays are typically a few microns, and over this distance the hehcal structure in the blue phases does not change much the polarization state of light. 

Another example (which up to now seems very difficult to achieve) is based on chains dissolved in a cholesteric phase. This is a liquid where the molecules locally have one direction of alignment but where this direction has a helical twist in space. If we start with chains which are not optically active, crosslink them by an optically inactive agent, and then wash out the cholesteric solvent (replacing it by an achiral solvent), we should obtain a gel which has an optical rotatory power (a memory of its preparative state) although all its components do not distinguish right from left. ... 

FIGURE 6.4. Schematic representation of wavelength dependence of (a) optical rotatory power and (b) circular dichroism (reflection) in a planar cholesteric mixture. 

Outside the reflection band, cholesteric liquid crystals exhibit an extremely strong optical rotatory power which can amount to values as large as 20.000° or 50 complete rotations per millimeter. The sign of the rotatory power is different below and above the reflection band. 

From the stereochemical point of view, (3 characterizes a chiral compound similar to the classical rotatory power [a]0. However, the two quantities have a different origin The optical rotation is a consequence of the chiral interaction between light and matter, while the cholesteric induction originates from a... 

Werbowyj and Gray (79) examined the relationships between the cholesteric pitch and optical properties of HPC in water, CH3COOH and CH3OH. The reciproc pitch varied as the third power of the HPC concentration. Optical rotatory dispersion results show HPC has a right-handed superhelicoidal structure regardless of structure. As will be discussea below, a change in solvent can reverse the handedness of other cellulose derivatives. 

The cholesteric liquid crystal state consists of either all left-handed helices or all right-handed helices. The physical difference between these two states is manifested in their optical activities. See, for example, H. de Vries, Rotatory Power and Other Optical Properties of Certain Liquid Crystals, Acta. Cryst, 4, p. 219(1951). 

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