Effect of dipole moment and conformation on the mesophase behavior of di-laterally substituted phenylazophenyl benzoate liquid crystals

The dipole moments of the previously prepared 4-(3′-fluoro phenylazo)-2-(or 3-) substituted phenyl-4′′-alkoxybenzoates (Ina–d), have been determined in benzene at 30 °C. The data obtained were compared with those theoretically calculated by molecular modeling program to deduce the most probable conformations for each individual homologous series. Probable conformations deduced were found to vary according to type and position of the lateral substituent attached to the central benzene ring. The results were used to correlate the mesophase behavior, in pure and mixed derivatives, with the conformation deduced for each series. Each homologous series, that have in common a lateral fluorine atom on the first terminal ring, differs from the other by a second lateral group substituted on the central ring. The latter group varies between 2-CH3, 3-CH3, 2-Cl and 3-CN groups. Within each homologous series, the number of carbons in the other terminal alkoxy group varies between 8 and 16 carbons. The study aims to investigate the steric effect of the spatial orientation of the central lateral substituent, based on deduced conformations, on the mesomorphic properties in their pure or mixed states. The mesophase behavior was investigated via differential scanning calorimetry, DSC, and mesophases identified by polarized light microscopy, PLM.

► Four homologous series of di-laterally substituted derivatives were prepared.
► Measurement of the dipole moment and comparing it with those calculated for the various planner conformations.
► Binary phase diagrams were constructed for mixtures made from any two analogues of different central lateral substituents.
► Mesophase behavior is related to conformation.
► The two molecules in all mixtures investigated are arranged in a back-to-face pattern.