What Factors Control Product Yield in Charge Separation Reaction from Second Excited State in Zinc–Porphyrin Derivatives?

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 investigated in the framework of a model involving three electronic states (the first and the second singlet excited and charge separated states) as well as their vibrational sublevels. Kinetics of the transitions between these states are described in terms of the stochastic point-transition approach. The influence of the model parameters (free energy change of charge separation, magnitude of the reorganization energies of the medium and the high frequency intramolecular vibrations, the rate of relaxation of the medium and the intramolecular high frequency vibrational mode) on the kinetics of population of both the charge separated and the first singlet excited states has been explored. Simulations of the kinetics of the charge separated state population have allowed reproducing the distinctive features of the kinetics observed in the experiment [ Wallin S. ; Monnereau C. ; Blart E. ; Gankou J.-R. ; Odobel F. ; Hammarström L. J. Phys. Chem. A 2010, 114, 1709 ]: (i) two maxima on short time scale (hundreds of femtoseconds) and long time scale (tens of picoseconds), (ii) the magnitudes of both maxima, and (iii) the depth of the notch between the maxima.