Synthesis and Characterization of Dinuclear Ruthenium Complexes Covalently Linked to RuII 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 , 2 a , 2 b ) were synthesized. In these complexes, a mixed‐valent dinuclear Ru II 44 III 2 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 II 44 III 2 moieties in the complexes readily undergo reversible one‐electron reduction and one‐electron oxidation to give the Ru II 44 II 2 and Ru III 44 III 2 states, respectively. This could allow for photooxidation 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 intramolecular 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 ( 2 a , 2 b ) 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 Ru II 44 III 2 moiety to photogenerated Ru III was observed. The resulting long‐lived Ru III 44 III 2 state decays on the millisecond timescale.