Anisotropy of rotation of nitroxide probes in nematogenic liquid crystals

An electron paramagnetic resonance (EPR) line shape simulation for nitroxide spin probes in the motional narrowing region was carried out assuming axially symmetric g and A tensors and with different anisotropies of rotation N (= R_∥/R_⊥) where R_∥ and R _⊥ are, respectively, elements of the diffusion tensor along and perpendicular to its principal axis z′. In addition, it was assumed that the principal axes of the diffusion tensor coincide with the molecular axes. Each of three cases z′ = x, z′ = y and z′ = z, which result from cyclic permutations of the molecular axes x, y and z with the z′, y′ and x′ axes of the diffusion tensor, yields its typical EPR spectrum characterized by the relative intensities of the low-, center- and high-field lines. The parameter δ defined by and calculable from the intensities of the three lines was found to vary linearly with N for the z′ = x and z′ = y cases and, as anticipated, to be practically constant at a value of 1 for the z′ = z case. This suggested a method for estimating N for a probe from its EPR spectrum. Experimental spectra over a narrow temperature range (1°C) in the vicinity of the nematic-to-isotropic transition (about 34.6°C) of N-(4-n-butylbenzilidene)-4-amino-2,2,6,6- tetramethylpiperidine-1-oxide at a mole fraction of 1·10^-3 in 4-n-pentyl-4′-cyanobiphenyl showed a pattern of peak heights characteristic of the z′ = x case with δ values that gave, neglecting effects of the mean field, higher and lower N values in the nematic and isotropic regions, respectively. Analysis of other similar systems in the literature gave similar results.