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.