A diffusive model for interpreting solvation dynamics in isotropic and ordered liquid phases

Solutions are discussed for a two-body diffusive model in which a rotating probe is coupled to a solvent polarisation field, in the case of anisotropic diffusion. The model describes the diffusional rotational behaviour of a rigid molecule, carrying a permanent electric dipole and coupled, via a dipole-field term, to a polarisation vector or reaction field coordinate, which is also relaxing diffusively. When the solvent relaxation coordinate relaxes faster or slower than the probe rotation, a semi-analytical treatment of the system is possible, based on the separation of timescales. This treatment can be applied in the presence of a mean field potential acting on the probe orientation, thus allowing to consider liquid crystalline phases. We specialize our treatment to the case of solvation dynamics: a theoretical transient Stokes shift (TSS) correlation function is defined and its behaviour is discussed.