Modeling Kinetics of Ultrafast Photoinduced Intramolecular Proton-Coupled Electron Transfer

A model of photoinduced intramolecular proton-coupled electron transfer is derived. The model includes three states as follows: the ground, excited, and product states. The charge transfer is associated with both stages, photoexcitation and product formation. A larger part of the model parameters can be extracted from the stationary absorption and fluorescence spectra of a particular fluorophore. Two different reaction coordinates are associated with the two stages, which are not independent. The angle between the reaction coordinates strongly influences on the kinetics of ultrafast product formation. The stochastic multichannel approach is exploited for simulations of the kinetics. The simulations well reproduce the kinetics of ultrafast intramolecular proton- coupled electron transfer in 24(2-(2-hydroxyphenyObenzo[d]oxazol-6-yOmethylene)malononitrile in a few solvents. The transfer is shown to occur totally or partly, depending on the solvent, in the nonequilibrium regime. Analysis of the kinetics of the excited-state decay has uncovered a significant decrease in the magnitude of the reorganization energies of slow nuclear modes with increasing the solvent polarity. Such an unusual behavior of the total reorganization energy can be rationalized under the assumptions: (i) a slow intramolecular reorganization of a significant magnitude associates with the transition between excited and product states and (ii) intramolecular slow reorganization is accompanied by a change in the dipole moment of the fluorophore.