Cyclometalated organoruthenium complexes for application in dye-sensitized solar cells

Article abstract of DOI:10.1021/om900481g

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^∧N ^∧N, 5), methyl-3,5-di(2-pyridyl)benzoate (MeO₂C-N ^∧C(H)^∧N, 6), 4-ethoxycarbonyl-6-phenyl-2,2′- bipyridine (EtO₂C-C(H)^∧N^∧N, 7), and 4,4′-bis(methoxycarbonyl)-6-phenyl-2,2′-bipyridine ((EtO ₂C)₂-C(H)^∧N^∧N, 8), and the ruthenium complexes thereof, [Ru(EtO₂C-tpy)(tpy)](PF ₆)₂, 1a, [Ru(MeO₂C-N^∧C ^∧N)(tpy)](PF₆), 2a, [Ru(EtO₂C-C ^∧N^∧N)(tpy)](PF₆), 3a, and [Ru((MeO ₂C)₂-C^∧N^∧N)(tpy)](PF ₆)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 Ru^II/Ru^III redox process. The complexes are photostable in both the Ru(II) 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 1a, 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.