Photoresponsive fluoro-substituted azo-ester liquid crystals with 2,6-dimethyl lateral groups: mesomorphic and DFT studies

A homologous series of fluoro-substituted azo-ester liquid crystals incorporating 2,6-dimethyl lateral groups were synthesised to investigate the effect of terminal alkyl chain length on mesomorphic and photophysical properties. The molecular structures were confirmed using 1H and 13C NMR spectroscopy along with elemental analysis. Thermal and mesomorphic behaviour studied using differential scanning calorimetry (DSC) and polarised optical microscopy (POM) revealed enantiotropic smectic C (SmC) phases across the series, with systematic variations in transition temperatures. Thermogravimetric analysis demonstrated good thermal stability, with decomposition temperatures above 300°C. UV – visible absorption studies exhibited characteristic π–π* and n–π* transitions, while reversible trans–cis photoisomerisation indicated efficient photoresponsive behaviour with high reversibility. Density functional theory (DFT) calculations supported the experimental findings, revealing nearly planar molecular geometries and consistent HOMO–LUMO energy gaps. The combined influence of ortho-fluoro substitution and lateral methyl groups enhances molecular polarity and stabilises smectic ordering, yielding thermally stable and photoresponsive materials suitable for optical switching and photonic applications.