ELECTRIC-DIPOLE INTERACTIONS OF CHAIN MOLECULES IN NEMATICS - THE ANALYSIS OF SEGMENTAL ORDERING IN ALPHA, OMEGA-DIBROMOALKANES

The residual electric dipole interactions recently detected in the deuterium NMR spectra of alpha,omega-dibromoalkanes dissolved in nematic liquid crystalline solvents [J. Phys. Chem. 96, 8176 (1992)] are analyzed in the context of the modular formulation of flexible molecule ordering. The chord model realization of this formulation, previously applied to the study of n-alkanes, is minimally extended to include the additional coupling of the dipole carrying segments of the solute molecules to the dipolar bias produced by the solvent. The ensuing simple model successfully reproduces the observed segmental order parameter profiles of the solute over the entire nematic temperature range of the solvents. Three types of solvents are considered (Phase V, 5CB, 5OCB) differing in their molecular dipole moment constitutions. It is found that in all solvent cases the orientational ordering is suppressed at the dipole moment carrying ends of the solute molecules whereas the ordering in the main body could either be reduced or elevated depending on the configuration of the dipole moments on the solvent molecules. Quantitative results for the effects of the dipolar bias on the potential of mean torque are presented and the mechanism of electric dipole driven ordering in these solvent-solute systems is discussed.