Mesomorphic and optical properties of azo-ester liquid crystalline reactive mesogens with enhanced thermal stability

Abstract

A series of azo-ester-linked polymerizable liquid crystalline compounds M1-M3, containing lateral methyl substitution and different terminal alkoxy substituents, e.g., −OCH3, −OCH2CH3, and −OCH2CH2CH3 have successfully been synthesized and characterized. The chemical structures of the prepared compounds were confirmed by different spectroscopic techniques. Thermogravimetric analysis revealed that all the compounds exhibited excellent thermal stability and showed two-staged decomposition patterns. The optical microscopic study revealed that all compounds displayed only the nematic phase and lateral substituent played a crucial role in the formation of single mesophase. The broad X-ray scattering peaks for M1, M2, and M3 in the 2.5–5.5 nm−1 region was observed at temperatures of 154 °C, 85 °C, and 129 °C, respectively, upon cooling from the isotropic liquid, confirming the existence of nematic mesophase. The electronic spectra of M1-M3 showed two main absorption bands with λmax ranges of 263–266 and 369–377 nm. The E-Z photoisomerization study of M1-M3 showcased that the processes followed the first-order kinetic, and the isomerization rate decreased as the alkoxy chain length increased. On the contrary, the Z-E thermal isomerization rates for M1-M3 showed insignificant changes as the alkoxy chain length increased.