Mesomorphism in Relation to Molecular Structure with Reference to a Laterally Substituted Homologous Series

Abstract

A novel homologous series 4-(4′-n-alkoxy benzoyloxy) 3-methyl phenyl azo-2″, 6″-dimethyl benzenes consists of 12 homologues. The methyl to hexyl derivatives are non-mesomorphic and the rest of the homologues are mesomorphic. Mesomorphism commences from heptyl homologue in a monotropic manner and continues as an enantiotropic nematic for octyl to dodecyl derivatives, and then monotropic mesomorphism as a nematic mesophase reappears for tetradecyl and hexadecyl derivatives without the exhibition of smectogenic character. The textures of the nematic mesophase are threaded, or are schlieren in type. The nematic–isotropic (or vice versa) transition curve of the phase diagram behaves in a normal manner without the exhibition of an odd-even effect. The average thermal stability for nematic to isotropic transition is 126.5°C. Analytical and spectral data confirm the structure of molecules. The mesomorphic to isotropic transition temperatures vary between 56°C and 130°C. The nematogenic mesophase appeared at the lowest temperature of 41.3°C. Transition and melting temperatures are determined using an optical polarizing microscopy equipped with a heating stage. The present novel series is of a low ordered melting type and predominantly nematogenic, without the exhibition of smectogenic character.