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 (EtO 2 C-N ∧ N ∧ N, 5 ), methyl-3,5-di(2-pyridyl)benzoate (MeO 2 C-N ∧ C(H) ∧ N, 6 ), 4-ethoxycarbonyl-6-phenyl-2,2′-bipyridine (EtO 2 C-C(H) ∧ N ∧ N, 7 ), and 4,4′-bis(methoxycarbonyl)-6-phenyl-2,2′-bipyridine ((EtO 2 C) 2 -C(H) ∧ N ∧ N, 8 ), and the ruthenium complexes thereof, [Ru(EtO 2 C-tpy)(tpy)](PF 6 ) 2, 1a, [Ru(MeO 2 C-N ∧ C ∧ N)(tpy)](PF 6 ), 2a, [Ru(EtO 2 C-C ∧ N ∧ N)(tpy)](PF 6 ), 3a, and [Ru((MeO 2 C) 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 TiO 2 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 p K 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 TiO 2 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.