Effect of non-chiral alkyl chain length on mesomorphic and photoresponsive behaviour of chiral azo-ester-based liquid crystals

Abstract

In the present study, a new homologous series of chiral azo-ester liquid crystal (SAzo8-14) has been designed to elucidate the role of terminal alkyl chain length on mesomorphic behaviour, photoisomerization, and nonlinear optical properties. The shortest member, SAzo8, exhibits a broad chiral nematic (N*) phase, whereas increasing chain length induces the emergence of chiral smectic C (SmC*) phases, which become enantiotropic for the longer homologues. Photoisomerization studies showed efficient and reversible E-to-Z photoisomerization under UV irradiation for all members of the series. Despite comparable HOMO-LUMO gaps across the series, SAzo8 exhibits the highest nonlinear optical response, suggesting that the terminal alkyl chain length plays a key role in modulating the nonlinear optical properties while the azo-ester core provides the fundamental electronic framework. These findings collectively demonstrate that terminal-chain engineering provides an effective strategy for tuning chiral mesomorphism, photoresponsiveness, and nonlinear optical performance in azo-ester based liquid crystalline systems.