Chapter 65 - Ultrafast charge separation and radiationless relaxation processes from S2 excited electronic states of directly linked Zinc-porphyrin-acceptor dyads

Abstract

Among various factors regulating the photo-induced ET reactions, the energy gap law dependence is a diagnostic tool in testing ET theories. Recently attempts were made to utilize a large energy gap for charge separation (CS) from S2 state of Zinc-porphyrin to directly linked electron acceptor (a series of imide compounds abbreviated as I) to confirm unambiguously the existence of the inverted region. By means of the detailed femtosecond fluorescence studies on the series of ZP-I compounds in polar solvents, such as, tetrahydrofurane (THE), triacetin (GTA) and acetonitrile (ACN), experts succeeded in demonstrating unequivocally as the first example for the photo-induced CS reaction the bell-shaped energy gap law (EGL) with a distinct inverted region. In addition, these results for EGL observed in polar solvents THE, GTA, ACN were reproduced satisfactorily by means of the Ulstrup-Jortner equation assuming the nonadiabatic mechanism, and the averaged single frequency hfm (intramolecular high frequency mode) coupled with CS reaction. This chapter confirms the modification of the EGL for the CS from S2 induced by the change of solvent polarity by comparing the EGL in toluene (Tol) and methylcyclohexane (MCH) solution with that in THE. Further, this chapter compares the rate constants (λp) of the S1 state formation by S2 excitation with the decay rate constants (λ1) of S2 state in ZP-I series and examines solvent polarity effects on these rate constants comparing THE with Tol or MCH solutions.