Cholesteric state

Many cellulose derivatives form Hquid crystalline phases, both in solution (lyotropic mesophases) and in the melt (thermotropic mesophases). The first report (96) showed that aqueous solutions of 30% hydroxypropylceUulose [9004-64-2] (HPC) form lyotropic mesophases that display iridescent colors characteristic of the chiral nematic (cholesteric) state. The field has grown rapidly and has been reviewed from different perspectives (97—101). 

The phase transitions of cholesteryl nonanoate have been studied with a new apparatus for thermal analytical microscopy. The enantiomer ratio of some chiral sulphoxides can be changed from racemic to a modest preference for one enantiomeric form by dissolution in a cholesteryl ester in its liquid-crystalline ( cholesteric ) state. 5,6-Epoxycholestan-3-yl p-nitrobenzoates exhibit liquid-crystal properties, but 5,6-diols and dibromides are inactive. ... 

As a first step towards the measurement of single molecule effects, Schrader and Korte (1972) reported the measurement of the infrared rotatory dispersion of carvone in liquid crystalline solution. They used a modified commercial spectrometer. They observed a huge effect which is not the result of the carvone itself but of the liquid crystal in which a helical arrangement (cholesteric state) is induced by the chiral solute (Sec. 4.6.4). In this case the liquid crystal acts as a kind of molecular amplifier which allows the absolute configuration of tiny amounts of solutes to be determined reliably. At about the same time Dudley et al., (1972) measured the infrared circular dichroism of (-)-menthol in a liquid crystal. Their equipment consisted of a normal infrared spectrometer supplemented by a Fresnel rhomb made from sodium chloride. 

Recently Krigbaum and coworkers prepared cholesteric polyesters from the reaction of 4,4 -dihydroxy-a-methylstilbene with mixtures of (-f) 3-methyladipic acid and adipic acid. They could change the morphology of the cholesteric state formed by the (4-) 3-methylapidic acid homopolymer by mixing it either with a low molecular... 

Table 3. Anisotropy of the magnetic susceptibility and twist elastic constants of some polypeptides in the cholesteric state...

Continued investigations led to the identification of three main types of mesophase a smectic state smegma in Greek, meaning soap), a nematic state nema in Greek, meaning thread), and a cholesteric state observed in systrans containing molecules with a chiral craitCT. 

The molecules arrange themselves in parallel layers in smectic systems. In these cases, the molecular axis is perpendicular to the layer plane. Within the layers, the molecule may be arranged randomly or ordered with respect to other molecules. The molecules are also arranged parallel to each other but not in layers in nematic systems. The cholesteric state is midway between nematic and smectic the molecules are arranged in layers, but with the molecular axis being parallel to the plane of the layer. 

Strong optical activity is exhibited, is known as the cholesteric state some workers regard this state as a special case of the nematic. The name arises from the fact that cholesteryl compounds form the majority of known examples. 

It is known that the trans isomer of azobenzene has elongated rod shape that is favorable for the stabilization of the LC phases, while the cis isomer in bent form is unfavorable and tends to destabilize the LC phases. The trans-cis photoisomerization will decrease the order parameters, if significantly enough, which could lead to the destruction of the ordered LC phase and formation of isotropic state. The reverse transition can be achieved with either visible light irradiation or thermal relaxation as a result of cis-trans isomerization. This principle was initially used to induce the phase transition from nematic to isotropic in azobenzene LCs [39]. Cholesteric mesophase is intrinsically similar to nematic with additional helical arrangement of nematic layers, thus the photoisomerization of azobenzenes can also bring out the phase transition from the cholesteric state to the photoin-duced isotropic (PHI) state. 

The decrease in the melting and/or clearing points of the monomers caused by the presence of the polymer was detected in both the smectic state polymerization of vinyl oleate (l8,19) and in the polymerization of cholesterylacrylate in the cholesteric state (ij). The polyvinyl oleate formed in this reaction was crystalline while the polycholesterylacrylate was amorphous. The temperat ore of the phase transition was reduced in both cases. 

The polymerization in the cholesteric state of this monomer is a heterogeneous phase reaction. The polymer formed is precipitated as a smectic phase, and as the conversion increases, the cholesteric monomer is dissolved in the isotropic system, and a homogeneous phase is formed. As the reaction proceeds further, the system becomes smectic ( 25) ... 

For a long time we searched without success for a homogeneous reaction in the cholesteric state until we began to study the cholesterylvinylfumarate monomer. In the following we present the polymer/monomer state diagram of this substance. 

Q he greatest changes in enthalpy are observed at Tp, but they are lower than would have been expected (on the basis of molecular dimensions) from crystalline-to-mesomorphic transitions. The cholesteric state was observed at the flow temperature under a polarizing microscope. The cholesteric reflections are observed at lower temperat ares than Tp in systems containing polymers or at Tp of the monomer itself. 

This means that there is a phase transformation in the cholesteric state too, which corresponds to the last peak ofAH2 ... 

The racemic axially chiral bicyclic ketone 4 reported by Schuster and Suarez underwent partial photoresolution with /-CPL irradiation and a PSS was achieved in 6.7 h with 0.4% ee (Fig. 5.8), which is in a good agreement with the calculated ee value from the anisotropy factor (g = 0.0105 at 305 nm) [38]. However, when this compound was employed as a dopant, the enantiomeric enrichment was not sufficient to induce the phase transition from nematic to cholesteric state probably due to the low HTP. The Schuster s group further designed and synthesized a series of these... 

Figure 10.24 Schematic diagram showing the transitions among the cholesteric states.

Most frequently, a planar orientation of CLCs is obtained by the rubbing technique. However, in cells coated with polyamide (without rubbing), a new cholesteric state, called an amorphous cholesteric structure, is formed [82]-[85]. The surface, covered by polyamide, orients the molecules parallel to the surface, but without any preferable direction in the surface plane. Thus, a structure with random orientation of liquid crystal molecules, and a helical axis oriented normal to the surfaces, is obtained (Figure 6.2(e)). All parts of the amorphous cholesteric structure with medium chirality rotate the polarization plane by the same angle. The whole structure can be considered as optically active. In this case, the angle of rotation is independent of the direction of the polarization plane of the incident light. This is the basic property used for electro-optic application of the amorphous cholesteric structure vrith medium chirality. Characteristic features of this effect are ... 

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