Photo-Responsive Brominated Hydrogen-Bonded Liquid Crystals

This study reports on the preparation and comprehensive characterisation of new brominated hydrogen-bonded liquid crystalline (HBLC) materials. Two distinct series of supramolecular complexes were prepared by hydrogen-bond formation between 3-bromo-4-pentyloxybenzoic acid as the proton donor and non-fluorinated and fluorinated azopyridines with variable terminal chains as proton acceptors. The successful formation of a hydrogen bond was confirmed by FTIR spectroscopy. The impact of alkyl chain length and fluorination on the mesomorphic properties of the HBLCs was systematically investigated. The molecular self-assembly was thoroughly examined using polarised optical microscopy (POM) and differential scanning calorimetry (DSC), revealing the presence of smectic C (SmC), smectic A (SmA), and nematic (N) phases, with thermal stability being highly dependent on the molecular architecture. Notably, the introduction of fluorine atoms significantly influenced the phase transition temperatures and the overall mesophase range. Using bromine as a lateral substituent induces the formation of SmC phases in these HBLCs, a feature absent in their non-brominated analogues. Further structural insights were obtained through X-ray diffraction (XRD) investigations, confirming the nature of the observed LC phases. Additionally, the photo-responsive characteristics of these HBLCs were explored via UV-Vis spectroscopy, demonstrating their ability to undergo reversible photoisomerisation upon light irradiation. These findings underscore the critical role of precise molecular design in tailoring the properties of HBLCs for potential applications such as optical storage devices.