Liquid-crystalline properties of some chiral benzoate and phenylpropiolate esters with optically active centres derived from (S,S)-isoleucine and (S,R)-allo-isoleucine

Abstract

Four chiral mesogenic materials based on benzoate and phenylpropiolate core systems have been synthesized. The optically active moieties of the four materials were derived from the α-amino acids Isoleucine and allo-Isoleucine in order to facilitate the preparation of two compounds with an S,S configuration of sequential chiral centres and two compounds with an S,R configuration of sequential chiral centres in the terminal chain. In all four materials, the chiral centres located closest to the core of the mesogen were identical in terms of structure and absolute configuration. The absolute configuration of the second chiral centre was dependent on whether the material was derived from Isoleucine or allo-Isoleucine leading to both S and R forms being available. The study of these materials enabled a close study of the effects that the second chiral centre can have on mesogenic properties. The materials were analysed in terms of their phase behaviour, and in particular the incidence of frustrated mesophases. In addition, some of the properties of the smectic C* phase (helical twist sense and polarization direction) were determined for each material and the results analysed in terms of the predictions expected for the chiral centres when treated independently. Computer modelling of these systems enabled an explanation of the physical results to be formulated.