Heisenberg spin exchange and molecular diffusion in liquid crystals

Abstract

Heisenberg spin exchange (HE) studies of translational diffusion of the nitroxide radicals PD-tempone and P probe in two liquid crystalline solvents 6OCB–8OCB and 4O,6 are described. It is shown that while PD-tempone undergoes strong exchange in the two solvents, the more anisotropic P-probe exhibits a tendency toward weak exchange which becomes more prominent in the low temperature mesophases. The molecular diffusion rates measured from our HE studies are compared with rates measured over larger distances using electron-spin resonance (ESR) imaging methods; we find that, in similar thermotropic liquid crystals, the former are somewhat faster. Also, while diffusion rates for PD-tempone (using HE) in the ordered phases of 6OCB–8OCB are consistent with a single activation energy, those in 4O,6 show variations; suggesting that probe expulsion from core to chain regions in the former most likely occurs prior to SA formation, whereas in the latter it occurs in the SA phase. The absence of discontinuities in our diffusion data at the N–SA–RN transitions supports the belief that these transitions are subtle, with nothing dramatic occurring as the reentrant nematic (RN) phase is formed. The effect of including a potential of mean force U(r) between colliding radicals due to the liquid-crystal structure, is also considered. Our analyses indicate that the potential is of a repulsive nature [i.e., U(d)>0] suggesting the possibility of solvent molecules inhibiting collisions of radicals at distances shorter than the sum of their solvated radii. The influence of orientational ordering on HE involving nonspherical radicals is considered, but changes from strong to weak exchange in the ordered phases appear to depend on how τ1 the lifetime of the interacting radical pair is influenced by U(r). A careful effort is made to separate the HE effects from the intermolecular electron–electron dipolar (EED) interactions. It is suggested that anomalies in D obtained from HE vs EED in this and earlier studies may also be rationalized in terms of the effects of U(r).