Cyclometalated Organoruthenium Complexes for Application in Dye-Sensitized Solar Cells

To investigate the viability of cyclometalation as a general tool in the design of new sensitizers for dye-sensitized solar cells, a series of (cyclometalated) ruthenium complexes was prepared. To this purpose we have prepared the carboxylate-functionalized 2,2':6',2"-terpyridine (tpy)-based tridentate ligands 4'-ethoxycarbonyl-2,2':6',2"-terpyridine (EtO2C-N boolean AND N boolean AND N, 5), methyl-3,5-di(2-pyridyl)benzoate (MeO2-C-N boolean AND C(H)boolean AND N, 6), 4-ethoxycarbonyl-6-phenyl-2,2'-bipyridine (EtO2C-C(H)boolean AND N boolean AND N, 7), and 4,4'-bis(methoxycarbony1)-6-phenyl-2,2'-bipyridine ((EtO2C)(2)-C(H)boolean AND N boolean AND N, 8), and the ruthenium complexes thereof, [Ru(EtO2C-tpy)(tpy)](PF6)(2), 1a, [Ru(MeO2C-N boolean AND C boolean AND N)(tpy)](PF6), 2a, [Ru(EtO2C-C boolean AND N boolean AND N)(tpy)](PF6), 3a, and [Ru((MeO2C)(2)-C boolean AND N boolean AND N)(tpy)](PF6)(2) 4a. In this series, cyclometalation results in a red shift as well as in a broadening of the electronic absorption features and is accompanied by a cathodic shift in the Rull/Ruill redox process. The complexes are photostable in both the Ru(I I) and the Ru(III) state. Deprotection of the esters and grafting onto TiO2 resulted in a small additional red shift of the absorption features. Incorporation of the free acids of the complexes into a standardized solar cell shows efficient sensitization for the complexes 3b and 4b, with the C,N,N'-bonding motif. The dicarboxylated complex 4b showed short circuit currents similar to those obtained for the benchmark compound N719. In contrast, for the free acid of la, with the N,N',N"-bonding motif, and for 2a, with the N,C,N'-bonding motif, low efficiencies were observed. To put these results into perspective, we have applied TD-DFT calculations. The optical assignments based on these calculations correlated well with the spectral changes observed during pK(a) determinations. The complexes with the C,N,N'-bonding motif possess an excited state associated with the cyclometalated ligand, allowing efficient charge injection, while the complex with the N,C,N'-bonding motif possesses a more isolated excited state located on the remote tpy ligand and, as a result, is not capable of efficient charge injection into the TiO2 conduction band. This shows that the covalent carbon-to-ruthenium bond can be utilized as a tool to shift the operational threshold of the individual sensitizer for dye-sensitized solar cells toward lower energy, as long as care is taken that the nature of the excited state is appropriate for electron injection.