Synthesis and Characterization of New Efficient Tricarboxyterpyridyl (β-diketonato) Ruthenium(II) Sensitizers and Their Applications in Dye-Sensitized Solar Cells

A new series of aryl-substituted β-diketonato-ruthenium(II)-polypyridyl sensitizers, [Ru(Htctpy)(tfpbd)(NCS){(C 4 H 9 ) 4 N} 2 ] ( 1 ), [Ru(Htctpy)(tffpbd)(NCS){(C 4 H 9 ) 4 N} 2 ] ( 2 ), and [Ru(Htctpy)(tfcpbd)(NCS){(C 4 H 9 ) 4 N} 2 ] ( 3 ), were synthesized and fully characterized by elemental analysis, UV−visible and emission spectroscopy, NMR, ATR-FTIR, cyclic voltammetric studies, and density functional theory (DFT) calculations (where tctpy = 4,4‘4‘ ‘-tricarboxy-2,2‘:6‘,2‘ ‘-terpyridine, tfpbd = 4,4,4-trifluoro-1-(phenyl)butane-1,3-dione, tffpbd = 4,4,4-trifluoro-1- (4-fluorophenyl)butane-1,3-dione, and tfcpbd = 4,4,4-trifluoro-1-(4-chlorophenyl)butane-1,3-dione). In ethanol−methanol solution, these complexes exhibit intense visible light absorption, with low-energy MLCT band maxima above 600 nm. A distinct shoulder at around 680 nm improved red-light absorptivity beyond 680 nm. Controlled shifting of the low-energy MLCT band and Ru oxidation potential can be achieved by changing the electron donor ability of the β-diketonate ligand using various aryl substituents of the β-diketonate ligand with differing electron-donating strengths. Density functional theory (DFT) calculation of complex 2 shows that the HOMO is localized on the NCS ligand and the LUMO on the tctpy ligand, which is anchored to the TiO 2 nanoparticles. The aryl-substituted β-diketonato-ruthenium(II)-polypyridyl sensitizers, when anchored to nanocrystalline TiO 2 films, achieve very efficient sensitization, with greater than 80% incident photon-to-current conversion efficiency (IPCE) in the whole visible range extending up to 950 nm. Under standard AM 1.5 irradiation (100 mW cm - 2 ) and using an electrolyte consisting of 0.6 M dimethylpropyl-imidazolium iodide, 0.05 M I 2, 0.1 M LiI, and 0.07 M tert -butylpyridine in acetonitrile, a solar cell containing complex 2 yielded a short-circuit photocurrent density of 19.1 mA cm - 2, an open-circuit photovoltage of 0.66 V, and a fill factor of 0.72, corresponding to an overall conversion efficiency of 9.1%.