Solute–solute spatial distribution in hydrogen bonding liquids probed with time-dependent intermolecular electron transfer

Abstract

Solute–solute spatial distribution in strongly hydrogen bonding solvents is investigated using photoinduced electron transfer dynamics between rhodamine 3B (R3B) and N,N-dimethylaniline (DMA) in a series of monoalcohols, polyalcohols, and alcohol mixtures. Fluorescence up-conversion data are presented on electron transfer in ethylene glycol and are compared to data characterizing electron transfer in seven other solvents. The data are analyzed with a detailed statistical mechanical theory that includes a distance-dependent Marcus rate constant, diffusion with the hydrodynamic effect, and solute–solute radial distribution functions. When the standard assumption is made that for low concentration solutes the solute–solute spatial distribution follows that of the solvent’s radial distribution function, a single parameter fit to the electronic coupling matrix element results in the same value, independent of solvent, for data from five solvents. However, it is impossible to fit the data from the solvent ethylene glycol using the model based on the solvent radial distribution function. When the assumption that the solute–solute spatial distribution tracks the single molecule solvent radial distribution function is relaxed by using a large “effective” solvent diameter to establish the donor–acceptor distance distribution and hydrodynamic effect, excellent fits to the electron transfer data are obtained. The fits give the same parameters for ethylene glycol and two other solvents with high OH/C ratios as the five “normal” solvents. The results suggest that the solute–solute (donor–acceptor) spatial distributions in the high OH/C ratio solvents are determined by multiple hydrogen bond solvent “aggregates” that inhibit solute molecules from distributing freely among solvent molecules.