Time-resolved energy-dispersive XAS studies of photoinduced electron transfer intermediates in electron donor-acceptor complexes

The structure around the Fe atom of a photoinduced electron transfer intermediate in a di-porphyrin electron donor/acceptor complex is studied with time-resolved energy dispersive X-ray absorption spectroscopy. Photoinduced electron transfer from the lowest excited triplet state of a Zn porphyrin attached at a fixed 25 D distance by means of a long spacer molecule to an Fe(III) porphyrin occurs with a 6 ms time constant at 77 K. Time domain X-ray absorption measurements show that Fe(III) is transiently photoreduced to Fe(II) and that the bond between the Fe atom and a pyridine molecule ligated to it weakens substantially upon reduction of the Fe. Coupling of ligand loss to reduction of Fe(III) to Fe(II) provides a mean of stabilizing the reduced intermediate.