In Photosystem II (PSII), solar energy is used to extract the electrons needed to reduce CO2 (the greenhouse gas) from water. Water oxidation is catalyzed by a domain within PSII called the water oxidizing complex (WOC), which involves a tyrosine residue. To mimic the WOC and the redox reactions involved in photosynthetic water oxidation, principles from PSII into supramolecular complexes were incorporated. To increase the similarities between supramolecular complexes and the donor side of PSII, a triad system complex 2 containing a RuII(bpy)3 center coupled via a modified L-tyrosine to a Mn2II,II-bpmp dimer (1) (bpmp = 2,6-bis[[ N,N-di .(2-pyridylmethyl)amino]methyl]-4-methylphenol) was synthesized. The photochemistry and electrochemistry in two model (complexes complex 1 and 2) were investigated. Both complexes underwent photo-induced, stepwise manganese oxidation by electron transfer to a photosensitizer. In the presence of water, the Mn2II,II moiety was successively oxidized to the mixed-valence Mn2III,IV state. Electrochemical studies indicated that ligand exchange in the Mn2III,III state, e.g., substitution of the acetate bridges by water molecules, was most likely a prerequisite for oxidation to the Mn2III,IV state. The light induced Ru oxidation to the highly oxidizing RuII species involves in oxidizing potential quite similar to that of P680+ in the PSII reaction center. The redox potentials of the different manganese oxidation states of 1 and 2 were also quite comparable to those involved in the donor side chemistry in PSII with the potential of the highest oxidation state remarkably close to that which is necessary for oxidizing water. The successive recovery of RuII accompanied by stepwise Mn oxidation, possibly mediated by the tyrosine derivative in 2, thereby, highly resembled important steps in the function of the WOC in PSII.
ASJC Scopus subject areas