Dynamics of 4,4′-di-n-heptylazoxybenzene (HAB) studied using dielectric and 2H NMR relaxation measurements

Results of studies of 4,4'-di-n-heptylazoxybenzene (HAB) in the isotropic, nematic and smectic A phases are presented. Two experimental methods were employed: broad band dielectric spectroscopy and nuclear magnetic resonance spectroscopy. The complex dielectric permittivity, epsilon*(omega)= epsilon'(omega) iepsilon"(omega), was measured in the frequency range 1 kHz - 4 GHz. This allowed two main relaxation processes to be separated in all the phases studied: the low frequency (l.f.) process connected with molecular reorientations around the short axes, and the high frequency ( h.f.) process connected with the rotations around the long axes. The corresponding relaxation times and activation enthalpies were obtained. The l.f. relaxation time changes step-wise at the phase transitions, whereas the h.f. relaxation time passes smoothly through all the phases. The measurement of H-2 spin-lattice relaxation times was carried out throughout the mesophase range at 61.38 MHz. These data were analysed together with the relaxation times measured at 10.00 and 46.04 MHz, available from previous studies. Using suitable theoretical models the principal components of the diffusional tensor, D-parallel to and D-perpendicular to, as well as the diffusion coefficients D-R relative to the internal rotation of the phenyl rings, were determined. The results of both studies are compared and discussed.