New Dicarboxylic Acid Bipyridine Ligand for Ruthenium Polypyridyl Sensitization of TiO2

Journal article, 2012

An ambidentate dicarboxylic acid bipyridine ligand, (4,5-diazafluoren-9-ylidene) malonic acid (dfm), was synthesized for coordination to Ru(II) and mesoporous nanocrystalline (anatase) TiO2 thin films. The dim ligand provides a conjugated pathway from the pyridyl rings to the carbonyl carbons of the carboxylic acid groups. X-ray crystal structures of [Ru(bpy)(2)(dfm)]Cl-2 and the corresponding diethyl ester compound, [Ru(bpy)(2)(defm)](PF6)(2), were obtained. The compounds displayed intense metal-to-ligand charge transfer (MLCT) absorption bands in the visible region (epsilon > 11,000 M-1 cm(-1) for [Ru(bpy)(2)(dfm)](PF6)(2) in acetonitrile). Significant room temperature photo-luminescence, PL, was absent in CH3CN but was observed at 77 K in a 4:1 EtOH:MeOH (v:v) glass. Cyclic voltammetry measurements revealed quasi-reversible Ru-III/(II) electrochemistry. Ligand reductions were quasi-reversible for the diethyl ester compound [Ru(bpy)(2)(defm)](2+), but were irreversible for [Ru(bpy)(2)(dfm)](2+). Both compounds were anchored to TiO2 thin films by overnight reactions in CH3CN to yield saturation surface coverages of 3 X 10(-8) mol/cm(2). Attenuated total reflection infrared measurements revealed that the [Ru(bpy)(2)(dfm)](2+) compound was present in the deprotonated carboxylate form when anchored to the TiO2 surface. The MLCT excited states of both compounds injected electrons into TiO2 with quantum yields of 0.70 in 0.1 M LiClO4 CH3CN. Micro- to milli- second charge recombination yielded ground state products. In regenerative solar cells with 0.5 M LiI/0.05 M I-2 in CH3CN, the Ru(bpy)(2)(dfm)/TiO2 displayed incident photon-to-current efficiencies of 0.7 at the absorption maximum. Under the same conditions, the diethylester compound was found to rapidly desorb from the TiO2 surface.