Achiral materials, nematics

When the nematic phase is composed of optically active materials (either a single component or a multicomponent mixture made up of chiral compounds or chiral compounds mixed with achiral materials), the phase itself becomes chiral and has reduced environmental space symmetry. The structure of the chiral nematic (or cholesteric) modification is one where the local molecular ordering is identical to that of the nematic phase, but in the direction normal to the director the molecules pack to form a helical macrostructure, see Fig. 5. As in the nematic phase the molecules have no long-range positional order, and no layering exists. The pitch of the helix can vary from about 0.1 x 10 m to almost infinity, and is dependent on optical purity and the degree of molecular... 

Photoresponsive molecules are important for the fabrication and photomodulation of photoresponsive CLC materials, no matter if they are employed as chiral mesogens, achiral nematic LC hosts, or chiral/achiral guest molecules. The most important property of these molecules in photoresponsive CLC materials is the changes in molecular shape (geometry/conformation) as a result of the light-induced isomerization, which constitutes the basis for photomning of the properties of CLCs [18], The first example of using photoresponsive molecules to modulate the properties of CLCs was reported by Sackmann in 1971 [19], where azobenzene was employed as the dopant molecule. To date, various kinds of photoisomerizable molecules have been developed for this purpose (Fig. 5.3). 

In smectic C materials, the relative twist of the planes is uncoupled to the layer thickness, giving rise to strong, nematic-like scattering [106]. There are relatively few light scattering studies of either achiral or chiral SmC phases, despite the technological importance of the ferroelectric SmC phase. This is in part due to the few discussions of the elastic theories of these phases, and in particular descriptions that include dynamic behaviour. Indeed it is only relatively recently that Leslie et al. [107] de-... 

Ferroelectric host systems are mixtures composed of materials that are achiral, and when blended together they exhibit a nematic-smectic A-smectic C phase sequence for the purposes of ease of alignment with the smectic C phase being available over a temperature range that includes room tem-... 

Thermotropic chiral nematics, also called thermotropic cholesterics, are liquid crystals formed by chiral compounds or by mixtures of chiral and achiral compounds with long range chiral orientational order but without long range positional order. Their properties have been extensively discussed and reviewed [1-3] and here the materials will be presented from the points of view of chirality and structure-property relations. 

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