Ultrafast Electron Transfer Dynamics of a Zn(II)porphyrin-Viologen Complex Revisited: S2 vs S1 Reactions and Survival of Excess Excitation Energy

The photoinduced electron transfer reactions in a self-assembled 1:1 complex of zinc(II)tetrasulphonatophenylporphyrin (ZnTPPS4−) and methylviologen (MV2+) in aqueous solution were investigated with transient absorption spectroscopy. ZnTPPS4− was excited either in the Soret or one of the two Q-bands, corresponding to excitation into the S2 and S1 states, respectively. The resulting electron transfer to MV2+ occurred, surprisingly, with the same time constant of τFET = 180 fs from both electronic states. The subsequent back electron transfer was rapid, and the kinetics was independent of the initially excited state (τBET = 700 fs). However, ground state reactants in a set of vibrationally excited states were observed. The amount of vibrationally excited ground states detected increased with increasing energy of the initial excited state, showing that excess excitation energy survived a two-step electron transfer reaction in solution. Differences in the ZnTPSS•3−/MV•+ spectra suggest that the forward electr...