Mesomorphic and DFT diversity of Schiff base derivatives bearing protruded methoxy groups

Five three-rings azomethine/ester derivatives, namely, 4-(hexyloxy)phenylazomethine phenyl benzoate (1), 4MODIFIER LETTER PRIME-(4-hexyloxy)phenylazomethine phenyl-3-methoxy benzoate (2), 4MODIFIER LETTER PRIME-(4-hexyloxy)phenylazomethine phenyl-4-methoxy benzoate (3), 4MODIFIER LETTER PRIME-(4-hexyloxy) phenylazomethine phenyl-3,4-dimethoxy benzoate (4), and 4MODIFIER LETTER PRIME-(4-hexyloxy) phenylazomethine phenyl-3,5-dimethoxy benzoate (5), were synthesised and their mesophase behaviour investigated by differential scanning calorimetry (DSC) and polarised optical microscopy (POM). Their molecular structures were elucidated via FT-IR, (1) H-NMR and elemental analysis. All the compounds (1-5) were found to be enantiotropically monomorphic possessing SmA phase with high-temperature range and high thermal stability. Computational studies revealed that all compounds investigated are non-co-planar. The dipole moment of each derivative is a function of the number of the methoxy groups; it decreases in the order 2 > 5 > 1 > 3 > 4. On the other hand, the molecular polarisability is dependent on the number of the methoxy groups, which is accordingly dependent on the molecular aspect ratio. The results of the FMO calculations revealed that the HOMO orbitals energies of the methoxy-substituted compounds 2-5 are lower compared to the laterally neat compound, 1. Finally, the effect of introducing the methoxy group in different position and number affect the charge distribution that could explain the behaviour of the enhanced mesophases.