Polarity and steric effect of di-lateral substitution on the mesophase behaviour of some azo/ester compounds
Eight homologous series of 2-(or 3-)substituted phenyl 4ʹ-(4″-alkoxy (2ʹ-, or 3″-substituted phenylazo) benzoates (InXY) were prepared in which the suffix ‘X’ refers to the lateral substituent X attached to the terminal benzene ring that carries the alkoxy group, and the suffix ‘Y’ refers to the substituent attached to the other terminal phenyl group. Within each homologous series, the length of the terminal alkoxy group varies from 8 to 16 carbons, while the lateral polar substituents, X and Y, alternatively varies between CH3 and F. The mesophase behaviour was investigated by differential scanning calorimetry and identified by polarised optical microscopy. The results were discussed in terms of the polarity and steric effects of the two lateral substituents. Comparative correlations were made to investigate the effect of the second lateral substituent on the mesophase behaviour of the previously investigated mono-laterally substituted analogues. UV–vis spectroscopic study revealed that the compounds I8XY exhibited two absorption bands: low intense bands at 254–263 and a broad band at 364–376 nm. These bands are attributed to the π–π∗ transition of the phenyl rings and the whole mesogenic portion.