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

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 (lambda p) of the S1 state formation by the S2 excitation with the decay rate constants (lambda 1) 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 lambda p in toluene.