Symmetrical trimeric star-shaped mesogens based on 1,3,5-trisubstituted benzene incorporating Schiff base and azobenzene fragments as the peripheral units: Synthesis and mesomorphic properties

Abstract

Two series of symmetrical three-armed star-shaped mesogens based on 1,3,5-trihydroxybenzene as a core unit, interconnecting three Schiff base or azobenzene moieties via oxymethylene spacers have been synthesized and characterized by spectroscopic techniques. Every member in these series possesses either chlorine (Cl) or bromine (Br) terminal atom, with different alkyl spacer length (C n H 2n whereby n ranging from 3 to 6). Their thermal stability and mesomorphic properties are investigated by employing DSC and POM. The dependence of phase transition in relation to the alkyl spacer length is shown by both series. These star-shaped mesogens exhibit only nematic and smectic phases. The difference between the two series lies on the structure of linking group in the peripheral units (-CH=N- for series PSB-X-n and -N=N- for series PAZ-X- n). Therefore, a comparison study of the mesomorphic properties between these two series of star-shaped mesogens is discussed whereby the azobenzene-based mesogens are thermally more stable than the Schiff base counterpart. In addition, soft crystalline phase is observed for the azobenzene-based star-shaped mesogens possessing hexyl alkyl spacer. © 2014 Indian Academy of Sciences.