Does molecular size matter in photoinduced electron transfer reactions?

Photoinduced electron transfers (ET) from trimethoxybenzene to excited quinones were studied in polar and apolar solvents by flash photolysis and photoacoustic calorimetry. In apolar solvents, back-ET was also observed. The enthalpy, entropy and volume changes of these ET were measured and compared with those from dimethylaniline to excited pyrene. ET to a triplet quinone in an apolar solvent involves a structural volume increase, indicating the formation of a loose ion pair, whereas ET to the singlet state of pyrene leads to a volume contraction. ET from tetramethylbenzidine to excited C-60 was also measured in benzonitrile and benzene and the formation of a ground-state complex in benzene was detected. The rates of the photoinduced and back-ET were measured and interpreted with nonadiabatic theories and with the intersecting-state model. It is shown that the size of the reactants has a negligible effect on the kinetics of these reactions and that nonspecific solvent effects are only significant for very exothermic reactions.