Electron and Energy Transfer in Supramolecular Complexes Designed for Artificial Photosynthesis

Abstract: In the society of today the need for alternative energy sources is increasing. The construction of artificial devices for the conversion of sunlight into electricity or fuel seems very attractive from an environmental point of view, since these devices are based on processes that does not necessarily generate any harmful biproducts. In the oxygen evolving photosynthetic process highly efficient energy and electron transfer reactions are responsible for the conversion of the sunlight into chemically stored energy and if the same principles can be used in an artificial device, the only electron supply required, is water. This thesis describes energy and electron transfer reactions in supramolecular complexes where the reactions are intended to mimic the basic steps in the photosynthetic process. All complexes are based on ruthenium(II)-trisbipyridine as photosensitizer, that is covalently linked to electron donors or electron or energy acceptors. The photochemical reactions were studied with time resolved transient absorption and emission measurements. In the complexes that mimic the donor side of Photosystem II, where a manganese cluster together with tyrosine catalyses the oxidation of water, intramolecular electron transfer was found to occur from Mn(II) or tyrosine to photo-oxidized Ru(III). Studies of a series of Ru(II)-Mn(II) complexes gave information of the quenching of the Ru(II) excited state by the coordinated Mn(II), which is important for the development of multi-nuclear Ru(II)-Mn complexes. In the supramolecular triad, PTZ-Ru2+-Q, the charge separated state, PTZ+?-Ru2+-Q-?, was rapidly formed, and further development where a second electron acceptor is linked to quinone is planned. Ultra fast energy transfer ?<200 fs), was obtained between ruthenium(II) and osmium(II) in a small artificial antenna fragment. Fast and efficient energy transfer is important in larger antennas or photonic wires where a rapid energy transfer is desired over a large distance.