Chain conformation and spin relaxation in nematogens

Abstract

Liquid crystals are composed of flexible organic molecules with many internal degrees of freedom; these molecules, with various alkyl chain lengths, may show different mesophases. When modelling molecular motions, the internal conformation motions of the chain may be superimposed onto the reorientation of the entire molecule. Transitions among different chain conformers have been described using a master equation. The nematic mean field, which determines the equilibrium probability of each conformer, can be modelled by fitting quadrupolar splittings observed in deuteriated liquid crystals. The above approach allows evaluation of spectral densities of motion necessary for interpreting nuclear spin–lattice relaxation times in nematogens.

We report on the measurements of deuterium T_1z and T_1Q as a function of temperature at 15.3 and 46 MHz in 4-[²H₁₁]pentyl-4′-cyano-[2,3,5,6-²H₄]biphenyl (5CB-d₁₅). The data are discussed based on the above motional model. In particular we focus on the site dependence of the relaxation rates along the alkyl chain, the frequency dependence of the relaxation rate, and the discontinuity of the relaxation rates at the nematic–isotropic transition.