Nuclear Zeeman and Dipolar Spin-Lattice Relaxation in the Reentrant Nematic Liquid Crystal OBBC

Proton spin-lattice relaxation rates were measured in Zeeman and dipolar fields in the liquid crystalline phases (N–S Ad –RN) of OBBC, 4-(4''-octyloxybenzoyloxy)-benzylidene-4'-cyanoaniline. The relaxation mechanisms were analyzed from the frequency- and temperature-dependences of the relaxation rates. The Zeeman relaxation mechanism in the high temperature nemanic (N) phase was characterized by the long-range fluctuation of the nematic director, whereas in the reentrant nematic (RN) phase the relaxation was predominated by translational self-diffusion of molecules. On the other hand, the relaxation in the dipolar frame caused by director fluctuation was observed in all of the three liquid crystalline phases. It is suggested that persistence of nematic director fluctuation in the smectic Ad mesophase is related to the reentrance phenomenon in polar mesogens.