Mimicking the electron donor side of Photosystem II in artificial photosynthesis

This review focuses on our recent efforts in synthetic ruthenium - tyrosine - manganese chemistry mimicking the donor side reactions of Photosystem II. Tyrosine and tryptophan residues were linked to ruthenium photosensitizers, which resulted in model complexes for proton- coupled electron transfer from amino acids. A new mechanistic model was proposed and used to design complexes in which the mechanism could be switched between concerted and step- wise proton- coupled electron transfer. Moreover, a manganese dimer linked to a ruthenium complex could be oxidized in three successive steps, from Mn-2(II, II) to Mn-2(III, IV) by the photo- oxidized ruthenium sensitizer. This was possible thanks to a charge compensating ligand exchange in the manganese complex. Detailed studies of the ligand exchange suggested that at high water concentrations, each oxidation step is coupled to a proton- release of water- derived ligands, analogous to the oxidation steps of the manganese cluster of Photosystem II.