Transient dynamics of solvatochromic shift in binary solvents

Abstract

Using the picosecond spectro-streak method we monitored the dynamics of preferential solvation of charge-transfer dipoles in binary mixtures of solvents strongly different in dielectric constants. The observed slow (⩽1 ns) spectral red-shift of the charge-transfer (exciplex) fluorescence is considered to be mainly due to the formation and the dielectric enrichment of the solvation shell around the solute molecules in their dipolar excited charge transfer state by diffusion of polar molecules from the bulk of the solvent mixture. As a fluorescent probe molecule we used 8-N,N-(dimethylamino)-11H-indeno[2,1-a]pyrene. The characteristic time of the solvatochromic shift varies from 400 ps to 140 ps for toluene/DMSO (dimethylsulfoxide) mixtures as the volume fraction of the polar component increases. Typical Stokes shifts are of the order of 1000–3000 cm−1. A basic concept of the classical Onsager model has been modified and adapted for binary solvents considering a spherical cavity surrounded by a solvent layer with the permittivity of the polar component. To describe the transport of polar molecules to the first coordination shell of the excited dipole we applied the Smoluchowski diffusion model which leads to hyperbolic kinetics that compare reasonably well with the experimental Stokes shift data.