Smectic phase lateral attractions

Lateral substituents in core units have to be kept small in order to preserve liquid crystallinity and the most generally useful lateral substituent is fluoro. The special features of fluoro substitution, which separately or in combination may be useful, include the following (a) a small size which usually has only a small effect on nematic thermal stability (b) electron-attraction which may give compounds of negative dielectric anisotropy (c) the ability to diminish or destroy anti-parallel molecular association and increase positive dielectric anisotropies (d) a depression of melting point (e) a reduction in the prominence of ordered smectic phases, frequently, with the consequent relevation of a nematic phase and... 

Terminal aUsyl chains increase the length-to-breadth ratio of the molecule and result in increased lateral attractive forces which stabilize liquid crystal phases and, in particular, the smectic phase. As the alkyl chain lengthens, the N-I transition exhibits an alternation in temperature (Figure 2.6) which is caused by the terminal methyl group of the alkyl chain being either in (higher N-I) or out of line with the long axis of the molecule. 

However, if the branch unit is polar (e.g. fluoro), then smectic phases are often supported to the exclusion of the nematic phase due to enhanced lateral attractions (e.g. compound 31 [33] compared with compound 15). In fact, superfluOTO and perfluoro chains are voy stiff which fiicilitates excellent lamellar packing and so excq)tionally high smectic phase stability is goi ted (e.g. compound 32) [34]. 

The use of latoal substituents in liquid crystals has proved to be very important, initially in nematic material and lata- in smectic C matoials. Clearly, anything that sticks off the side of a rod-like molecule will tend to reduce the liquid crystal phase stability, and genmlly the larg the lateral substituent the greater the reduction in liquid crystal phase stability. Usually, the smectic phase stability is much mmre affected than that of the nematic phas especially by larger substitumts because of the obvious reduction in lateral attractions, but increased lateral attractions associated with polar substituents cause a smaller reduction in smectic phase stability (see compounds 52-57) [46]. 

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