Variation of excitation energy influences the product distribution of a two-step electron transfer: S(2) vs S(1) electron transfer in a Zn(II)porphyrin-viologen complex.

We have, for the first time for molecular systems in solution, shown a case where the variation of excitation energy influences the product distribution of a two-step electron transfer. The photoexcitation was to the porphyrin-localized S(2) state or either of the S(1)(v = 1) or S(1)(v = 0) states of an aqueous Zn(II)-meso-tetrasulphonatophenyl-porphyrin-methylviologen (ZnTPPS(4-)/MV(2+)) complex. Both forward and back electron transfer occur on a subpicosecond time scale (tau(FET) approximately 0.2, tau(BET) = 0.7 ps). The excess energy of the higher excitations partially survives both electron transfer steps. This is seen by different distributions of unrelaxed ground states, which are generated by the back electron transfer and has unique UV-vis spectroscopic signatures. State selective electron transfer opens interesting possibilities for reaction control, and the results represent initial steps in that direction.