Local polarity of solvent mixtures in the field of electronically excited molecules and exciplexes

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Abstract
In mixtures of solvents of different dielectric polarities a process of preferential solvation described as ‘dielectric enrichment’ occurs in the solvation shell of dipolar solute molecules. Dielectric enrichment requires the diffusion of solvent molecules, and its extent depends on solvent viscosity and on time; in the case of electronically excited molecules it may be limited by their lifetime. Measurements on short-lived ( < 1 ns) and long-lived ( > 10 ns) fluorescers show that in non-viscous liquid media dielectric enrichment reaches equilibrium in times of 1 ns to over 20 ns depending on the polar solvent mole fraction in the mixture. In the case of exciplexes the process of dielectric enrichment is complicated by the quenching action of polar solvents, which reduces the exciplex lifetime.

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