Photoresponsive palm-based azo-benzothiazole liquid crystals: Mesomorphic behavior and photoisomerization studies

Abstract

The technical feasibility of palm fatty acids as biobased flexible alkyl chain was investigated through the synthesis of azo-benzothiazole palm-based liquid crystals (PBLCs). Two series of PBLCs with ethoxy (−OCH₂CH₃, E1 – E4) and methoxy (−OCH₃, M2 − M4) terminal substituents at the sixth position on the benzothiazole moiety were successfully synthesized using palm fatty acids, namely, caprylic acid, palmitic acid, stearic acid and oleic acid. Optical microscopic study revealed that all synthesized PBLCs are purely nematogenic, where the azo linkages, terminal alkoxy substituent and biobased flexible alkyl chain facilitated the single mesophase formation. Notably, a longer flexible chain can decrease the nematic stability and the presence of unsaturation in the flexible chain lowers the nematic stability. UV–Visible spectroscopic analysis confirmed the reversible photoisomerization behavior in solution state of all PBLCs synthesized at 25 °C, 35 °C and 45 °C, with UV-A initiated E-Z isomerization and thermal Z-E relaxation in the dark. Kinetic studies revealed that the E-Z-E isomerization of all synthesized PBLCs follows first-order kinetics. While increasing temperature enhanced the isomerization rates, it concurrently reduced the E-Z photoisomerization conversion efficiency (CE). Substituting methoxy moiety with ethoxy terminal group of PBLCs increases their E-Z photoisomerization rate but decreases their Z-E isomerization rate. The results establish palm fatty acids as effective, renewable spacers for designing photoresponsive nematogens and provide structure–property relationship for optical information, display, and light-gated material applications. These findings also demonstrated the synergistic role of molecular structure and alkyl spacer design in modulating mesophase behavior and photoswitching kinetics, offering insights into the development of biobased-derived photoresponsive LC materials while showcasing the potential of oleochemicals in advanced functional materials.