Time-resolved luminescence spectra of dipolar excited molecules in liquid and solid mixtures. Dynamics of dielectric enrichment and microscopic motions

Abstract

In a liquid mixture the solvation shell of a dipolar molecule is enriched in the solvent of greater polarity (dielectric enrichment); this process requires the diffusion of solvent molecules near the solute dipole and its dynamics is related to the viscosity of the medium. Time-resolved luminescence spectra of dipolar excited states created by short laser pulses have been measured in the nanosecond to microsecond timescale in liquid mixtures and the dynamics of dielectric enrichment is described. The role of the solvent's viscosity is emphasised and the relationship between microscopic and macroscopic viscosities is clarified in a few instances. In mixed rigid matrices the time-resolved phosphorescence spectra of long-lived dipolar triplet states have been measured over times of several seconds in an effort to find evidence for slow microscopic motions of matrix molecules in the neighbourhood of dipolar solutes. Calculations based on the macroscopic viscosities of glassy solvents suggest that diffusion over approximately one solvent shell cannot be neglected over a time of ca. 1 s; the experimental results of time-resolved phosphorescence are considered from the point of view of the microscopic viscosity, and their relevance to the distance dependence of electron-transfer reactions is discussed.