Comparison of dielectric properties of three alkyl and alkoxy azoxybenzenes (nAOBs and nOAOBs, n = 5,6,7) in the isotropic and liquid crystalline phases

Abstract

Results of dielectric studies of three 4,4′-di-n-alkyl-azoxybenzenes (nAOB) and 4,4′-di-n-alkoxy-azoxybenzenes (nOAOB), n = 5, 6 and 7, are presented. The nAOB compounds exhibit positive, and the nOAOBs negative dielectric anisotropy in the nematic phase. In the smectic A phase of 7AOB the anisotropy changes its sign, whereas in the smectic C phase of 7OAOB it persists negative; moreover, in the latter case the perpendicular component of the permittivity increases with respect to the value of the nematic phase. This may be due to parallel dipole-dipole correlation of the perpendicular components involved by the layer arrangements in both smectic phases. For all substances a systematic decrease of the permittivity components with chain length was observed. Different types of smectic order in the 7th members of both series are discussed in relation to the mean-field theory developed recently by Govind and Madhusudana. The dielectric relaxation times and activation enthalpies characterizing the molecular rotation around the principal inertia axes are given.