Porphyrin-Quinone Electron Transfer Revisited. The Role of Excited-State Degeneracy in Ultrafast Charge Transfer Reactions

Ultrafast time-resolved anisotropy experiments in which electronically excited [5-(1',4'-benzoquinonyl)-10,1 5,20-triphenylporphinato]magnesium (MgPQ) was probed with polarization both parallel and perpendicular to the pump pulse have been carried out in a variety of solvents. These results show that the photoinduced electron transfer process involving the locally-excited MgP*Q state is solvent independent, while the thermal charge recombination reaction is solvent dependent. Moreover, these studies indicate that the photoinduced charge separation reaction may be rate-limited by electronic population equilibration between near degenerate porphyrin Q(x) and Q(y) levels.