The Synthesis of Chiral Nematic Liquid Crystals

Chiral nematic liquid crystals, as the name suggests, are optically active variants of nematic liquid-crystalline compounds the incorporation of a chiral centre imparts properties which are unique to the chiral nematic phase and are responsible for their utilisation in a variety of differing display technologies and other related applications. The term cholesteric liquid crystal was originally used to describe this phase, and originates from the structural nature of the earliest chiral nematic liquid crystals which were derivatives of cholesterol [1,2], Nowadays, the term chiral nematic is used primarily because the materials are clearly derived from nematic type liquid crystals [3, 4], Despite these differences in definition, the terms cholesteric and chiral nematic phase are interchangeable and it is common to find references to either term in the literature. 

The incorporation of a chiral centre into a formerly nematic structure (or alternatively of a chiral non-mesogenic dopant into a nematic host) results in the induction of a 

All interest in the unique optical properties of the chiral nematic phase stems from the two-fold optical activity of the phase (5, 6-9]. That is, the mesophase displays (1) molecular optical activity - the phase being composed of optically active molecules and (2) macromolecular optical activity - arising from the macroscopic helical twist induced by the chiral molecules in the phase (sometimes termed form chirality). These features are of immense importance technologically, as many of the applications of such materials depend on one if not both of these phenomena. 

Certain chiral nematic liquid crystals (or mixtures) are found to exhibit thermo-chromism, that is, temperature dependent selective reflection here the pitch length of such materials or mixtures is often found to have an inverse relationship with temperature and can depend on the nature of the material s mesomorphism. Thermochrom-ism is found to be at its most spectacular in materials which display chiral nematic phases and underlying smectic phases this is exemplified for a material with an I-N -SmA sequence (on cooling) and is shown schematically in Fig. 2. 

As an aside, it may be of interest to the reader that although selective reflection is usually a phenonemon associated with cal-amitic liquid crystalline systems, various multiyne materials have recently been demonstrated to show selective reflection as well as a helix inversion in chiral discotic nematic phases [15, 16]. 

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