Synthesis and characterization of dinuclear ruthenium complexes covalently linked to Ru^II Tris-bipyridine: An approach to mimics of the donor side of photosystem II

To mimic the electron-donor side of photosystem II (PSII), three trinuclear ruthenium complexes (2, 2a, 2b) were synthesized. In these complexes, a mixed-valent dinuclear Ru₂^II,III moiety with one phenoxy and two acetato bridges is covalently linked to a Ru^II tris-bipyridine photosensitizer. The properties and photoinduced electron/energy transfer of these complexes were studied. The results show that the Ru ₂^II,III moieties in the complexes readily undergo reversible one-electron reduction and one-electron oxidation to give the Ru ₂^II,III and Ru₂^III,III states, respectively. This could allow for photo-oxidation of the sensitizer part with an external acceptor and subsequent electron transfer from the dinuclear ruthenium moiety to regenerate the sensitizer. However, all trinuclear ruthenium complexes have a very short excited-state lifetime, in the range of a few nanoseconds to less than 100 ps. Studies by femtosecond time-resolved techniques suggest that a mixture of intra-molecular energy and electron transfer between the dinuclear ruthenium moiety and the excited [Ru(bpy)₃] ²⁺ photosensitizer is responsible for the short lifetimes. This problem is overcome by anchoring the complexes with ester- or carboxyl-substituted bipyridine ligands (2a, 2b) to nanocrystalline TiO ₂, and the desired electron transfer from the excited state of the [Ru-(bpy)₃]²⁺ moiety to the conduction band of TiO ₂ followed by intramolecular electron transfer from the dinuclear Ru₂^II,III moiety to photogenerated Ru^III was observed. The resulting long-lived Ru₂^III,III state decays on the millisecond timescale.