The Orientational Behaviour of Some Fluorinated Molecules in Nematic Liquid Crystal Solvents

Abstract

The analysis of the nmr spectrum of a molecule dissolved in a liquid crystal solvent leads to information about both the geometry of the molecule and its degree of orientation in the solvent'. The orientation is often considered as a by-product of the analysis, since it is usually the molecular structure whch is of primary interest, but useful information is contained within the orientation matrix and several studies have been reported which attempt to link the orientation to other molecular properties. The early work of Saupe^2,3 showed that, in the absence of specific solvent-solute interactions, a simple bond additive model gave approximate agreement for various fluoro- and chloro-substituted benzene derivatives. Later studies attempted to correlate the orientation with molecular shape,⁴ polarizability,⁵ moments of inertia,⁶ and by empirical relationships involving a form of substituent additivity constant.⁷ Very few studies have been reported which specifically examine the molecular orientation of the solute molecule as a function of temperature, concentration, solvent type, etc. Hillenbrand and Yu⁸ found that the two orientation parameters required to describe the orientation of fluorobenzene in hexyloxy-azoxybenzene were functions of reduced temperature in much the same way that the order of the liquid crystal solvent itself followed a universal curve for either different concentrations of the same solute^9,10 or for a wide variety of solute molecules at the same concentration. “A study of the temperature and concentration dependence of the orientation of trichlorobenzene in MBBA showed that the reduced temperature behaviour did depend upon the concentration.” More recently it has been claimed that the ability to fit the experimental orientation parameters in phenylcyclohexylcyanide type solvents with a potential based upon the anisotropy of the molecular polarizability determines whether dispersive or repulsive^12,13 occur.