Influence of environment reorganization energy and its dynamic properties on vibrational spectral effect in the kinetics of photo-induced electron transfer

In this work the kinetics of photo-induced charge transfer in molecular donor- acceptor dyads is studied. The calculations are performed within the multichannel stochastic model. The model includes the reorganization of the environment, characterized by two relaxation times, the reorganization of high-frequency intramolecular vibrational mode and its relaxation as well. It is assumed that a single vibrational mode is active at the stage of molecule excitation and charge transfer stage and is characterized by the same value of the Huang- Rhys factor at both stages. The influence of the carrier frequency of the excitation pulse, leading to the population of different vibrational sublevels of locally-excited state on the charge transfer rate has been analyzed. For a quantitative description of this effect the concept of vibrational spectral effect is used. The effect is called positive if the charge transfer rate increases together with the carrier frequency of the excitation pulse, and negative in the opposite case. It is shown that the variation of dynamic properties of the solvent can change the value of the spectral effect significantly, leaving his sign unchanged. The growth of solvent reorganization energy leads to displacement of positive and negative effects’ areas towards the region of larger charge transfer exergonicity by an amount of change of the reorganization energy. When the reaction exergonicity is fixed, the variation of the solvent reorganization energy leads to a strong change in the effect magnitude. When the solvent reorganization energy is changed by 0.4 eV, vibrational spectral effect can vary from its minimum to maximum point. This finding allows controlling the charge transfer rate by varying the solvent polarity, that determine the value of the reorganization energy.