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Regularities of ultrafast charge recombination kinetics in photoinduced intramolecular electron transfer in polar solvent are investigated. The multichannel stochastic model is used to simulate kinetics of charge separation and ensuing ultrafast charge recombination. The rate constant of ultrafast charge recombination occurring in non-equilibrium mode is shown to depend on the free energy gap of the preceding charge separation stage. This is a clear demonstration of violation of one of the fundamental principles of formal chemical kinetics, namely, the postulate of independence of elementary chemical reactions. The simulations revealed two main trends: (i) the charge recombination rate constant typically increases with decreasing the charge separation free energy gap, (ii) the maximum of the charge recombination rate constant, as a function of the charge recombination free energy gap, |ΔGCR|, shifts toward smaller values of |ΔGCR| with decreasing the free energy gap at the previous, charge separation, stage of the reaction. The prediction of ultrafast charge recombination in donor–acceptor dyads with small charge recombination free energy gap is shown to be supported by the experimental data on the kinetics of charge recombination into the first excited state followed the charge separation from the second excited state.

We study the fluorescence spectral dynamics of a molecule with two excited fluorescent states (called the precursor and product) within the stochastic point transition model, which includes a consistent description of the dynamics of the particles distribution in excited states and transitions between them. Crucial changes in spectral dynamics with increasing the transition rate between the precursor and the product are discovered. Analysis of the results of spectral dynamics modeling allows us to determine how the transition rate between the precursor and the product affects the isoemissive point in the spectra collected with an increasing time delay. The following main regularities were revealed: fluorescence spectra with a short delay not exceeding several solvent relaxation times ( τ L ) may not pass through the isoemissive point; the isoemissive point shifts to the red with a decrease in reaction time constant ( τ eff ). The shifted isoemissive point exists in the range 10 τ L τ eff 40 τ L . For faster reactions with 5 τ L τ eff 10 τ L , the isoemissive point blurs. When the reaction proceeds in the ultrafast regime τ eff 5 τ L , the fluorescence spectrum shifts from a vicinity of the precursor spectrum to the product spectrum as a whole, changing its shape.