A redox asymmetric, cyclometalated ruthenium dimer: toward upconversion dyes in dye-sensitized TiO2 solar cells

Abstract

Wehave prepared the dinuclear rutheniumcomplexes [(R3-tpy)Ru(N ∧ C ∧ N-tpy)Ru(tpy)]3þ (R=H ([5a]3þ ), CO2Me ([6a]3þ ), N ∧ C(H) ∧ N-tpy = 40-(3,5-dipyridylphenyl)-2,20:60,200-terpyridine, tpy = 2,20:60,200-terpyridine) and [(R3-tpy)Ru(N0∧ C ∧ N0-tpy)Ru(tpy)]3þ (R = H ([5b]3þ ), CO2Me ([6b]3þ ), N0∧ C(H) ∧ N0-tpy = 40-(3,5-di(4-tert-butylpyridyl)phenyl)-2,20:60,200-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)]2þ moiety to the cycloruthenated group, centered at 1034 nm for [6b]4þ . 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.