U‑Shaped Temperature Dependence of Rate Constant of Intramolecular Photoinduced Charge Separation in Zinc–Porphyrin–Bridge–Quinone Compounds

The multichannel stochastic point transition model of photoinduced electron transfer from both a vibrationally unrelaxed and a relaxed states involving the vibrational relaxation in donor–acceptor pairs has been elaborated. The U-shaped temperature dependencies of the rate constants of the intramolecular photoinduced charge separation from both the vibrationally unrelaxed and the relaxed states observed in Zn–porphyrin–bridge–quinone compounds in 2-methyltetrahydrofuran solvent have been reproduced in the framework of the proposed model that accounts for the temperature dependencies of the charge separation free energy gap and the medium reorganization energy. This modeling has allowed uncovering the mechanism of such a variation of the rate constant with the temperature. In the high temperature region, 310–125 K, the charge separation proceeds in the solvent controlled regime and its rate constant decreases with decreasing the temperature mirroring the temperature dependence of the medium relaxation rate. Further lowering the temperature leads to a rise of the reaction free energy gap so that it becomes larger than the medium reorganization energy. In this region the dynamic solvent effect is strongly suppressed and the charge separation rate constant becomes independent from the solvent relaxation rate. Although the medium relaxation rate continues to decrease with decreasing the temperature, the charge separation rate constant starts to rise because the reaction proceeds in the barrierless region.