Design, structural elucidation, and thermal behaviour of fluoro-substituted azo-based low-melting liquid crystals: a multi-technique and DFT study

A homologous series of 11 fluoro-substituted azo-ester liquid crystals, 4-(4′-n-alkoxy phenyl azo)-2-fluoro ethyl benzoates, was synthesised and investigated for their structural, mesomorphic, and photophysical properties. The molecular structures were confirmed by FT-IR, 1H NMR, 13C NMR, mass spectrometry, and elemental (CHN) analysis, where characteristic molecular ion and fragmentation peaks supported the proposed structures. The compounds exhibited low melting behaviour, and enantiotropic smectic A (SmA) phases were observed from the C4 homologue onward, while C2 and C3 were non-mesomorphic. The smectic mesophase range varied between 18.8°C and 40.8°C, as determined by differential scanning calorimetry (DSC) and polarised optical microscopy (POM). Variable-temperature powder X-ray diffraction (PXRD) showed sharp small-angle reflections at elevated temperatures, confirming layered smectic ordering. Photophysical studies using UV–Vis and fluorescence spectroscopy revealed characteristic absorption and emission behaviour associated with the azo chromophore. Density functional theory (DFT) calculations provided insight into molecular geometry, HOMO–LUMO energy gaps, and electrostatic potential distribution. Comparative analysis with non-fluorinated analogues demonstrated that lateral fluoro substitution reduces melting temperatures and enhances smectic stability, highlighting their potential for advanced liquid crystal applications.