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 beta-diketonato-ruthenium(II)-polypyridyl sensitizers, [Ru(Htctpy)(tfpbd)-(NCS) {(C4H9)(4)N}(2)] (1), [Ru(Htctpy)(tffpbd)(NCS){(C4H9)(4)N}(2)] (2), and [Ru(Htctpy)(tfcpbd)(NCS){(C4H9)(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 beta-diketonate ligand using various aryl substituents of the beta-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 TiO2 nanoparticles. The aryl-substituted beta-diketonato-ruthenium(II)-polypyridyl sensitizers, when anchored to nanocrystalline TiO2 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%.