A Redox Asymmetric, Cyclometalated Ruthenium Dimer: Toward Upconversion Dyes in Dye-Sensitized TiO₂ Solar Cells

We have prepared the dinuclear ruthenium complexes [(R 3 -tpy)Ru(N ∧ C ∧ N-tpy)Ru(tpy)] 3+ (R = H ([ 5a ] 3+ ), CO 2 Me ([ 6a ] 3+ ), N ∧ C(H) ∧ N-tpy = 4′-(3,5-dipyridylphenyl)-2,2′:6′,2′′-terpyridine, tpy = 2,2′:6′,2′′-terpyridine) and [(R 3 -tpy)Ru(N′ ∧ C ∧ N′-tpy)Ru(tpy)] 3+ (R = H ([ 5b ] 3+ ), CO 2 Me ([ 6b ] 3+ ), N′ ∧ C(H) ∧ N′-tpy = 4′-(3,5-di(4- tert- butylpyridyl)phenyl)-2,2′:6′,2′′-terpyridine) in a stepwise manner. The directional nature of the bridging ligand, which is potentially cyclometalating on one side, induces large redox asymmetry in the resulting dinuclear complexes. One-electron oxidation gives rise to a strong metal-to-metal charge transfer transition from the [Ru(tpy 2 )] 2+ moiety to the cycloruthenated group, centered at 1034 nm for [ 6b ] 4+ . The localized nature of the oxidation processes, the shape of the NIR band, and TD-DFT calculations allow assignment of these systems to localized Robin−Day class II. Exclusive substitution of the terminal tpy ligand on the cyclometalated ruthenium with acid moieties allows selective attachment of the dye to a semiconductor surface, whereby a possible two-step upconversion path is created in dye-sensitized solar cells for the utilization of low-energy photons.