BINUCLEAR RUTHENIUM-MANGANESE COMPLEXES AS SIMPLE ARTIFICIAL MODELS FOR PHOTOSYSTEM-II IN GREEN PLANTS

As part of a project aimed at developing models for photosystem II (PS II) in green plants, we have prepared a series of model compounds (7, 8, and 13). In these compounds, a photosensitizer, ruthenium(II) tris( bipyridyl) complex (to mimic the function of P-680 in PSII), was coval ently linked to a manganese(II) ion through different bridging ligands . The structures of the compounds were characterized by electron param agnetic resonance measurements and electrospray ionization mass spectr ometry. The interaction between the ruthenium and manganese moieties w ithin the complex was probed by steady-state and time-resolved emissio n measurements. When the binuclear complexes are exposed to flash phot olysis in the presence of an electron acceptor such as methylviologen (MV2+), it could be shown that after the initial electron transfer fro m the excited state of Ru(II) in compound 7, forming Ru(III) and MV+., an intramolecular electron transfer from coordinated Mn(II) to the ph otogenerated Ru(III) occurred with a first-order rate constant of 1.8 x 10(5) s(-1), regenerating Ru(II). This is believed to be the first s upramolecular system where a manganese complex has been used as an ele ctron donor to a photo-oxidized photosensitizer, Possible extensions t o develop the manganese donor, and thus to approach the function of re action center in PSII, are indicated.