Spin relaxation for biradical spin probes in anisotropic environments

Abstract

The large zero-field splitting of certain biradicals makes them important candidates for spin probes of anisotropic systems such as liquid crystals and membranes. The electron resonance spectrum of a biradical dissolved in a liquid crystal may be influenced by the zero-field splitting in two quite distinct ways. Firstly the line positions are affected because the spin probe is partially oriented by the liquid crystal solvent. In addition the zero-field splitting coupled with the molecular reorientation constitutes a powerful spin relaxation process and so may determine the widths of the spectral lines. Here we develop a theory of such line broadening, within the limit of fast motion, and show how it results in an angular dependence of the linewidths. The application of the theory is illustrated by studying the line broadening for a nitroxide biradical dissolved in a nematic mesophase. An analysis of the angular dependence of the linewidths suggests that the diffusion model provides a better account of molecular reorientation in a liquid crystal than the strong-collision approach. Finally we draw attention to the potential value of the theory for understanding the linewidth variations in the deuteron magnetic resonance spectra of liquid crystals.