Angular dependent linewidths of ESR spin probes in oriented smectic systems. An application to oriented lecithin—cholesterol multibilayers

A theory, based on the diffusion model of molecular reorientation extended with an orienting potential, is developed for the angular dependence of the ESR linewidths of symmetrical top radicals in oriented smectic liquid crystals. The theoretical results have been compared with linewidth data obtained with the cholestane spin label in oriented multibilayers of equimolar quantities of dipalmitoyl lecithin and cholesterol. For the determiination of the linewidths, the ESR spectra were fitted to a line shape function, which includes unresolved proton hyperfine interactions.From the comparison of theory and experiment the rotational diffusion tensor and order parameter S can be found, giving D_⊥ = 3.4 × 10⁶ s^?1, D|/D_⊥ = 40 and S = 0.86 at 49°C. The high value of D|/D_⊥ indicates a strong anisotropic motion of the cholestane spin label. The order parameter found is in good agreement with the order parameter calculated from the angular dependence of the ESR line positions.