Influence of different linkages on the mesomorphic properties of aromatic ring system liquid crystals

Abstract

In order to study the effect of the central linkage on the mesomorphic behaviour of aromatic ring liquid crystals we have synthesized two homologues groups, group A includes compounds 1a-d and group B includes compounds 2a-e. Group A has azo, azomethine, azo-ester, and azomethine-ester central linkages whereas the group B has azomethine, ether-azomethine, heterocyclic, and methylene-azomethine central linkages. The synthesized compounds were identified and characterized by elemental analyses and spectroscopic methods (proton nuclear magnetic resonance and Fourier transform infrared). The mesomorphic properties were investigated by polarizing optical microscopy (POM) with a heating stage and differential scanning calorimetry (DSC) thermograms with an aim to investigate the influence of different types of linkages on the mesomorphic properties in such aromatic ring systems. The results revealed that the change of the central linkages has a crucial effect on the mesomorphic properties when compared to the influence of different types of linkages. We found that the synthesised compounds (1a-d) showed smectic phase with different thermal ranges, while the synthesised compounds (2a-e) showed nematic phase except the compounds 2c-e which did not exhibit any liquid crystalline phase.