The dramatic influence of the location of bend and of lateral fluoro substitution on the mesomorphic properties of angular chiral esters based on a 1,3-disubstituted benzene ring

Abstract

The synthesis and mesomorphic properties of a range of 3-ring and 4-ring chiral esters based on a 1,3-disubstituted benzene unit are detailed. These materials all deviate from the usual linear molecular architecture of liquid crystals, and hence are all angular in nature. Some of these materials have the bend right at the end of the molecule where the chain deviates from the normal linear arrangement, and hence a ‘hockey stick’ molecular architecture is perhaps an accurate description. Other materials have a genuine bent-core construction where the bend is towards the centre of the molecule, and hence are best termed as ‘boomerang’ shape. In all cases, interesting comparisons of mesophase morphology and transition temperatures were found, both between the various angular materials and with their linear analogues. In particular, the influence on transition temperatures of lateral fluoro substitution in the novel angular materials was found to be wholly different to that found in the known linear analogues. The work forms part of a larger on-going research programme to investigate the mesomorphic and chirality-dependent properties of angular liquid crystals. The research revealed that no mesomorphism in parent compounds is possible when the bend is close to the centre of the molecule, however, lateral fluoro substitution of such compounds facilitates the generation of liquid crystal phases. Where the molecular bend is as a consequence of the terminal unit at the end of the core, then surprisingly high clearing points resulted, and such materials were found to show the potential for a high tilt angle, and a strong tendency towards helical mesophases. Lateral fluoro substitution of these latter examples resulted consistently in significantly higher clearing points, which is in marked contrast to the behaviour reported previously in known liquid crystals of linear molecular architectures.