Synthesis, chiral separation and physical properties of the cinnamic acid derivatives exhibiting short-pitch cholesteric and TGB phases

Abstract

ABSTRACT Two new photosensitive chiral liquid crystalline materials derived from cinnamic acid were synthesised in (R) and (S) enantiomer forms. The materials were characterised by NMR spectroscopy, purified by preparative liquid chromatography, and the chemical purity of the final products was determined by high performance liquid chromatography (HPLC). High optical purity was reached for all enantiomers. The enantiomer elution order was confirmed by the analysis of non-racemic (R) and (S) mixed samples. The cinnamic acid derivatives are capable of undergoing photoisomerisation. Sample irradiation was performed to realise E-Z isomerisation and investigate its effect on the enantioseparation. The conditions of the enantioselective HPLC method were optimised to achieve a baseline separation for all studied materials both before and after irradiation by the UV light. The mesogenic properties of the studied enantiomers and their racemic mixtures were obtained from differential scanning calorimetry measurements, optical textures observed under the polarising light microscope and X-ray diffraction measurements. Cholesteric and TGB phases have been found for the enantiomers and, analogously, nematic and smectic phases have been confirmed for their racemates. The temperature dependence of the optical birefringence has been studied for the racemates in the temperature range of the mesophases.