Experimental and Theoretical Analysis of Photoinduced Electron Transfer: Including the Role of Liquid Structure

Experimental determinations of the dynamics of photoinduced electron transfer from rubrene to duroquinone in three solvents, dibutyl phthalate, diethyl sebacate, and cyclohexanone are presented. Measurements of the donor (rubrene) fluorescence decays were made with time-correlated single-photon counting. The data are analyzed using recent theoretical developments that include important features of the solvent, i.e., the effects of finite molecular volume on local solvent structure and on the mutual donor-acceptor diffusion rates. Inclusion of the liquid radial distribution function (rdf) in the theory accounts for the significant variation of the acceptor concentration near a donor. Because the concentration of acceptors near a donor is substantially greater than the average concentration used in a featureless continuum liquid model, incorporating the rdf is necessary to properly analyze experimental data. Hydrodynamic effects, which slow the rate of donoracceptor approach at short distance, are important and are also included in the theoretical analysis of the data. The data analysis depends on a reasonable model of the rdf. A hard-sphere liquid rdf is shown to be sufficiently accurate by comparing model electron-transfer calculations using a hard-sphere rdf and an rdf from neutronscattering experiments reported in the literature. A method is presented to obtain the hard-sphere parameters needed to calculate the rdf. The method uses a self-consistent determination of the hard-sphere radius and diffusion constant and the solvent self-diffusion constant calculated from the Spernol and Wirtz equation. The Marcus form of the distance-dependent transfer rate is used. For the highest viscosity solvent (dibutyl phthalate), a unique set of the Marcus transfer parameters is obtained. For lower viscosity solvents, the transfer parameters are less well defined, but information on the distance and time dependence of charge separation is still acquired. These experiments, combined with the theoretical analysis, yield the first realistic description of through-solvent photoinduced electron transfer.