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

We have prepared the dinuclear ruthenium complexes [(R3-tpy)Ru(N∧C∧N-tpy)Ru(tpy)]3+ (R = H ([5a]3+), CO2Me ([6a]3+), N∧C(H)∧N-tpy = 4′-(3,5-dipyridylphenyl)-2,2′:6′,2′′-terpyridine, tpy = 2,2′:6′,2′′-terpyridine) and [(R3-tpy)Ru(N′∧C∧N′-tpy)Ru(tpy)]3+ (R = H ([5b]3+), CO2Me ([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(tpy2)]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.