Control of thermal photoinduced electron transfer reactions in the activated and activationless regimes

Photoinduced electron transfer rates depend on the internal energy distribu tion of the locally excited donor state. This energy distribution may be ho t or cold relative to the temperature of the donor in the ground electronic state and is dependent on the photoexcitation frequency. In the activated regime, the electron transfer rate depends exponentially on the temperature of the locally excited donor state. Therefore, the electron transfer rate is sensitive to the photoexcitation frequency. In the activationless regime , even if the vibrational frequencies of the locally excited donor state an d the acceptor state differ, the electron transfer rate is rather insensiti ve to the internal energy distribution of the locally excited donor state. Therefore, changing the photoexcitation frequency does not lead to a signif icant change in the transfer rate. Model computations are presented to demo nstrate this qualitative difference between the two regimes, as well as to confirm that the photoinduced electron transfer rate is well-approximated a s a thermal electron transfer rate, but at an effective temperature of the locally excited donor state that depends on the photoexcitation frequency. (C) 2001 American Institute of Physics.