Synthesis and Liquid Crystalline Study of a Novel Azo-Ester Homologous Series with Varying Terminal and Lateral Substituents

Abstract

This study investigates the structure–property relationships of photo-responsive soft materials to optimize the smectogenic mesophase for advanced electro-optical and photonic applications A novel homologous series of twelve azo-ester derivatives with the general structure. RO-C₆H₄-C₆H₃(CH₃)-N=N-C₆H₃(NO₂) (Cl) was synthesised to investigate its mesomorphic properties. This study aims to elucidate the relationship between molecular structure and liquid crystalline behaviour by systematically varying the terminal n-alkoxy chain length alongside lateral (–CH₃) and polar terminal (–NO₂, –Cl) substitutions. The molecular structures of the homologues were confirmed using ¹H NMR, FT-IR, and mass spectrometry. Phase transition temperatures were determined using a hot-stage polarising optical microscope (POM). Notably, all synthesised homologues exclusively exhibit an enantiotropic smectic phase; the nematic phase is entirely absent. The phase transition diagram behaves in a standard manner, exhibiting the characteristic odd-even effect. The series demonstrates a maximum thermal stability of 151.8 °C, with mesophase lengths varying between 7.5 °C and 40.3 °C. Finally, the mesomorphic properties of this novel azo-ester series are compared with structurally similar known series to further establish structure-property relationships.