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 S 2 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 electron transfer from the S2 state at least partially produces an electronically excited charge transfer state, which effectively suppresses the influence of the inverted regime. Other possible reasons for the similar electron transfer rates for the different excited states are also discussed.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry