Effect of the CF₃ Substituents on the Charge-Transfer Kinetics of High-Efficiency Cyclometalated Ruthenium Sensitizers

Six thiocyanate-free complexes, DUY1-DUY6, were synthesized, and their application in a dye-sensitized solar cell was studied to explore the effect of the CF₃ substituent positioned in the ancillary ligand and the structure of the anchoring ligand on the physicochemical properties, charge-transfer kinetics, and photovoltaic properties of ruthenium sensitizers. When the electron-withdrawing groups were installed on the cyclometalating ligands and their π conjugation of the ancillary ligand was extended, the frontier orbital energy levels of the ruthenium complex appeared to be sufficient for effective electron injection and dye regeneration, at the same time having high light-harvesting ability. Two electron-withdrawing CF₃ groups meta to the cyclometalated position reduce the electron density at the metal center less seriously than o-CF₃ and p-CF₃ groups. The sensitizers containing a m-CF₃ group also reveal a more favorable distribution of β lowest unoccupied spin orbital for interaction between the oxidized dyes and the iodide ion, which promotes dye regeneration. The absorption profiles of DUY1-DUY4 adsorbed a TiO₂ film extended to longer wavelength compared to those in an N,N-dimethylformamide solution, especially DUY1 and DUY2 dyes, which have λ_max red shifts of up to 30 nm. The DUY2-dyed cell exhibited the highest efficiency of 9.03%, while the power conversion efficiencies of DUY1-, DUY3-, DUY4-, and N719-based devices were 7.40%, 7.01%, 8.92%, and 8.63%, respectively. DUY5 and DUY6 (the side products of DUY3 and DUY4) without anchoring groups have very weak physical adsorption on a TiO₂ anode. The corresponding cells exhibit very low efficiency (<0.1%), although both dyes have high light-harvesting ability and proper frontier orbital energy levels.