Theoretical, experimental and optical study of new thiophene-based liquid crystals and their positional isomers

Abstract

ABSTRACT A new group of thiophene-based derivative liquid crystals named, 4-[(thiophen-3-yl)methyleneamino]phenyl 4-alkoxyoxybenzoates were synthesised and analysed mesomorphic and optical behaviour as well as their thermal parameters. Schiff base/ester linkage as mesogens and the number of carbons in the terminal alkoxy chains varies between 6 and 16 carbons have been constructed. The phase transitions were investigated by differential scanning calorimetry (DSC) and mesophase types were confirmed by polarised optical microscopy (POM). The optimised structures of present thiophene derivatives and their previously prepared positional isomer have been deduced theoretically by the density functional theory (DFT). Each isomer differs from each other by the position of azomethine linkage on the terminal thiophene ring. The mesomorphic study revealed that all prepared compounds are exhibiting enantiotropic nematic phase except the longer alkoxy chain (n = 16) showed dimorphic mesophases, enantiotropic N phase and monotropic smectic A phase. DFT results illustrated that the mesophase stability and its type could be explained in the term of calculated values of dipole moment, polarisability, thermal energy and molecular electrostatic potential as well as the change in the entropy of the investigated compounds.