Ultrafast charge separation and radiationless relaxation processes from higher excited electronic states of directly linked porphyrin–acceptor dyads

Abstract

We have investigated photoinduced electron transfer (ET) and related processes from the higher excited electronic state (S₂) of Zn-porphyrin–imide acceptor directly linked supramolecular systems (ZP–I) designed especially for the critical studies of the energy gap law (EGL) of the charge separation (CS) from the S₂ state, effects of solvent dynamics and intramolecular vibrations on this CS, and competition or cooperation between this CS and S₂ → S₁ conversion, etc. In this study, we have confirmed the modification of the EGL for the CS from S₂ induced by the change of solvent polarity by comparing the EGL in toluene solution with that in THF, i.e. the EGL in toluene extends over a wider range of the energy gap for CS in the inverted region and becomes somewhat similar to the case of the weak coupling limit of an intramolecular radiationless transition. Moreover, we have compared the rate constants (λ_p) of the S₁ state formation by the S₂ excitation with the decay rate constants (λ₁) of the S₂ state in the ZP–I series and have also examined solvent polarity effects on these rate constants comparing THF and toluene solutions. Our studies have revealed that S₁ formation by S₂ excitation occurs mainly due to the CS in S₂ followed by charge recombination (CR) producing S₁, and these processes are affected by the modification of EGL owing to the solvent polarity, resulting in the smaller λ_p in toluene.