Towards artificial photosynthesis — Light-induced intramolecular electron transfer from manganese (II) to ruthenium (III) in a binuclear complex

Abstract

One attractive way to harvest solar energy is to use the concepts of natural photosynthesis in an artificial system. In green plant photosynthesis, the solar energy is transformed into usable energy in the form of reduced compounds. The electrons come from water which is oxidized to molecular oxygen, thereby providing the plants with a never ending supply of reducing equivalents. In photosystem II, the photosensitizer is a chlorophyll species, P680, which is coupled to a cluster composed of four manganese ions that catalyses the water oxidation. We have tried to mimic this by the synthesis of a binuclear compound [Ru(bpy)2(Mebpy-Mebpy)MnCl2(H2O)2]Cl2 built on bipyridine ligands containing a Ru(II) moiety (the photosensitizer) and a Mn(II) ion (the donor) linked via a bridging ligand. In the complex, which is structurally defined by NMR, elemental analysis and electrospray mass spectroscopy, we have observed, (1) the Mn is sufficiently close (about 13Å) to interact with the Ru(II) ion, and (2) intramolecular, photochemically induced electron transfer from Mn(II) to the photogenerated Ru(III) moiety after a light flash in the presence of an electron acceptor. We suggest that the synthesis, characterization and observation of intramolecular electron transfer in this novel Ru-Mn compound is an important step towards artificial photosynthesis.