Phase Transitions and Structural Studies of New Compounds Belonging to Homologous Series

Since few years, we have been studying the synthesis and the physical properties of homologous series with the following general structure: CH3-(CH2)m -CO-C6H4-N=N-C6H4-O-CO-(CH2)n -CH3, with n ranging from zero to sixteen. In this work we report the results of the diffractometric study of the liquid crystalline series 4-butanoyl-4′-n-alkanoyloxy azobenzenes (m=2) and 4-pentanoyl-4′-n-alkanoyloxy azobenzenes (m=3): in particular all the ordered smectic phases of the m=3 series have been identified and some structural parameters are discussed. Considering the smectic A phase, we have determined the layer thickness as a function of n for both the m=2 and m=3 series: the results have been compared with those of the homologous series with m=0 and m=1. In particular an appreciable decrease of the thickness increment due to a CH2 group and a corresponding increase of the thickness of aromatic sublayer were observed: these results are interpreted by means of a proposed model.