Interrelationship between the Ancillary Ligand Structure, Acid-Base Properties, and TiO2 Surface Coverage of RuII Dyes

The effect of donor/acceptor substituents of phenanthroline on the ground and excited state pK(a) of Ru-II tris-heteroleptic compounds, such as cis-[Ru(R-phen)(dcbH(2))(NCS)(2)], where R = H, Me-2, Me-4, Ph-2, and Cbz(2), was investigated, as well as their effect on dye adsorption on a TiO2 surface. It was determined that, aside from the long distance between the substituent and acid site on the molecule, the pK(a) followed the same trend found for uncoordinated ligands; slightly higher pK(a) values were determined for compounds with donor substituents. A linear correlation suggested that the acid-base properties of the dyes have a direct relationship to the amount of dye adsorbed onto the TiO2 surface as well as their molecular volumes. Solar cells prepared using such dyes exhibited IPCE values up to 77% at 520 nm. These results are important for understanding the effect of ancillary ligands on the acid-base properties of ruthenium compounds and the TiO2 surface coverage, which can lead to the development of new and efficient dyes for energy conversion devices.