Cyclometalated organoruthenium complexes for application in dye-sensitized solar cells

Abstract

To investigate the viability of cyclometalation as a general tool in the design of new sensitizers for dyesensitized solar cells, a series of (cyclometalated) ruthenium complexes was prepared. To this purpose we have prepared the carboxylate-functionalized 2,20:60,200-terpyridine (tpy)-based tridentate ligands 40-ethoxycarbonyl-2,20:60,200-terpyridine (EtO2C-N ∧ N ∧ N, 5), methyl-3,5-di(2-pyridyl)benzoate (MeO2- C-N ∧ C(H) ∧ N, 6), 4-ethoxycarbonyl-6-phenyl-2,20-bipyridine (EtO2C-C(H) ∧ N ∧ N, 7), and 4,40-bis- (methoxycarbonyl)-6-phenyl-2,20-bipyridine ((EtO2C)2-C(H) ∧ N ∧ N, 8), and the ruthenium complexes thereof, [Ru(EtO2C-tpy)(tpy)](PF6)2, 1a, [Ru(MeO2C-N ∧ C ∧ N)(tpy)](PF6), 2a, [Ru(EtO2C-C ∧ N ∧ N)- (tpy)](PF6), 3a, and [Ru((MeO2C)2-C ∧ N ∧ 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 RuII/RuIII 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,N0-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,N0,N00-bondingmotif, and for 2a,with the N,C,N0-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 correlatedwellwith the spectral changes observed during pKa determinations. The complexes with the C,N,N0-bonding motif possess an excited state associatedwith the cyclometalated ligand, allowing efficient charge injection, while thecomplexwith the N,C,N0-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.