Mimicking photosystem II reactions in artificial photosynthesis: Ru(II)-polypyridine photosensitizers linked to tyrosine and manganese electron donors

The paper describes a project aiming at constructing functional mimics of the oxygen evolving complex in photosystem II, coupled to photoinduced charge separation. Biomimetic electron donors manganese complexes and tyrosine, have been linked to a Ru(II)-polypyridine photosensitizer. Oxidation of the donors by intramolecular electron transfer from the photooxidized Ru(III) complex was demonstrated using optical flash photolysis and EPR experiments. A step-wise electron transfer Mn(III,III)→tyrosine→Ru(III) was demonstrated, in analogy to the reaction on the donor side of photosystem II. Electron transfer from the tyrosine to Ru(III) was coupled to tyrosine deprotonation. This resulted in a large reorganization energy and thus a slow reaction rate, unless the tyrosine was hydrogen bonded or already deprotonated. A comparison with analogous reaction in photosystem II is made. Finally, light-induced oxidation of a manganese dimer linked to a Ru(II)-photosensitizer was observed. Preliminary results suggest the possibility of photooxidizing manganese dimers in several steps, which is an important step towards water oxidation.