PHOTOINDUCED ELECTRON-TRANSFER AND GEMINATE RECOMBINATION IN LIQUIDS

The coupled processes of intermolecular photoinduced forward electron transfer and geminate recombination between donors (rubrene) and accep ters (duroquinone) are studied in two molecular liquids: dibutyl phtha late and diethyl sebacate. Time-correlated single-photon counting and fluorescence yield measurements give information about the depletion o f the donor excited state due to forward transfer, while pump-probe ex periments give direct information about the radical survival kinetics. A straightforward procedure is presented for removing contributions f rom excited-state-excited-state absorption to the pump-probe data. The data are analyzed with a previously presented model that includes sol vent structure and hydrodynamic effects in a detailed theory of throug h-solvent electron transfer. Models that neglect these effects are inc apable of describing the data. When a detailed description of solvent effects is included in the theory, agreement with the experimental res ults is obtained. Forward electron transfer is well-described with a c lassical Marcus form of the rate equation, though the precise values o f the rate parameters depend on the details of the solvents' radial di stribution function. The additional experimental results presented her e permit a more accurate determination of the forward transfer paramet ers than those presented previously.(1) The geminate recombination (ba ck transfer) data are highly inverted and cannot be analyzed with a cl assical Marcus expression. Good fits are instead obtained with an expo nential distance dependence model of the rate constant and also with a more detailed semiclassical treatment suggested by Jortner.(2) Analys is of the pump-probe data, however, suggests that the geminate recombi nation cannot be described with a single solvent dielectric constant. Rather, a time-dependent dielectric constant is required to properly a ccount for diffusion occurring in a time-varying Coulomb potential. A model using a longitudinal dielectric relaxation time is presented. Ad ditionally, previously reported theoretical results(3) are rederived i n a general form that permits important physical effects to be include d more rigorously.