Orientational Order of Spin Probe Molecules in Liquid Crystals

Abstract

We report here measurements of hyperfine splittings and g values of two nitroxide spin probes 4',4'-dimethyl-spiro-(5α-cholestane-3,2'-oxazolidin)-3'-yloxyl and 2-(10-carboxydecyl)-2-hexyl-4,4-dimethyl-3-oxazolidinyloxyl methyl ester dissolved in p-azoxyanisole. Using the principal axes values of g and A tensors available, we have determined all three diagonal components of the orientational order parameter tensor S for these spin probe molecules. We find that for neither of the spin probes used here has this tensor S the cylindrical symmetry that is normally assumed to determine the orientational order parameter tensor S^(p) defined by the long molecular axis from such measurements. We show further that this conclusion about S is independent of any uncertainty in the principal axes values of the g and A tensors, and hence the relations between S and S^(p) normally used do not seem to be well justified. Probable relations are also suggested.