Numerical simulations of ultrafast charge separation dynamics from second excited state of directly linked zinc-porphyrin-imide dyads and ensuing hot charge recombination into the first excited state.

A model of the intramolecular charge separation from the second singlet excited-state of directly linked Zn-porphyrin-imide dyads and following charge recombination into the first singlet excited-state has been constructed and investigated. The model incorporates three electronic states (the first and the second singlet excited and charge separated states) as well as their vibrational sublevels. Dynamics of the transitions between these states are described in the framework of the stochastic point-transition approach. The relaxation of the intramolecular high frequency vibrational mode is supposed to occur as a single-quantum transition between nearest states with a time constant depending on the number of the vibrational state. The medium relaxation is characterized by two timescales. A good fitting to experimentally observed population dynamics of both the first and the second singlet excited states has been obtained. The calculations show the charge recombination into the first excited-state to proceed in a hot stage in parallel with the relaxation of both the medium and the intramolecular high-frequency vibrational mode.