Chiral nematics long range orientational order

The nematic phase is the simplest mesophase. It is described by the director n and the orientational order parameter S. In the nematic phase, the mesogens are arranged in such a way that their long axes lie preferentially in one direction (long-range orientational order). The long-range orientational order is responsible for the characteristic differences with respect to their isotropic melt. If the constituent compound is racemic, it is possible to form another phase from the enantiomericaUy pure compound. This is chiral nematic. 

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. 

Similar to cholesteric liquid crystals, the nematics have an orientational long-range order with the deviation that the direetion of the preferred orientation does not rotate (Fig. 2a). If, however, a chiral mesogen is dissolved in a nematic liquid crystal, the latter will be transformed into a eholesterie liquid crystal. 

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