Kinetics of charge separated state population produced by intramolecular electron transfer quenching of second excited state

Ultrafast photoinduced intramolecular charge separation from the second excited singlet state of Zn–porphyrin–amino naphthalene diimide dyad in toluene solution is theoretically studied in the framework of a multichannel stochastic model. The model incorporates four electronic states (the first and the second singlet excited, the charge separated, and the ground states) as well as their vibrational sublevels corresponding to the excitation of intramolecular high frequency vibrational modes. Electron transfer from the second excited state results in creation of the charge separated state with strongly non-equilibrium surrounding solvent and the intramolecular vibrations. The solvent motion to its equilibrium is described in the terms of three relaxation modes. The model explicitly describes the hot transitions from the charge separated state into the first excited state occurring in the course of the nuclear relaxation. Upon termination of the ultrafast decay of the second excited state as well as the relaxation of the solvent and intramolecular vibrations, the populations of the first excited, the ground, and the charge separated states can be far from the thermal equilibrium. Further, the populations approach their equilibrium values in the thermal regime. Simulations of the kinetics of the charge separated state population allow quantitative reproducing the two-humped kinetic curve observed in the experiment. The results of the fitting indicate that the reorganization energy of intramolecular low frequency modes for the intramolecular charge separation from the second excited state is extremely large in the porphyrin–diimide dyad.

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