Ruthenium Sensitizers with 2,2′-Bipyrimidine or a 5,5′-Disubstituted 2,2′-Bipyrimidine Ligand: Synthesis, Photo- and Electrochemical Properties, and Application to Dye-Sensitized Solar Cells



Novel ruthenium sensitizers with 2,2′-bipyrimidine (TUS-26) or a 2,2′-bipyrimidine derivative bearing two 5-hexyl-2,2′-bithiophene units at the 5,5′-positions (TUS-27) have been synthesized for application in dye-sensitized solar cells. TUS-27 shows a strong absorption at around 460 nm, mainly attributed to a π–π* transition in the 5,5′-disubstituted 2,2′-bipyrimidine ligand, and an onset wavelength of the absorption spectrum that is redshifted by about 50 nm relative to those of structurally analogous sensitizers (cis-[Ru(dcbpy)(bpy)(NCS)2] and N719). The energy level of the LUMO of TUS-27 is relatively close to that of the conduction band of TiO2. Dye-sensitized solar cells containing TUS-26 and TUS-27 showed much poorer performances than those containing cis-[Ru(dcbpy)(bpy)(NCS)2] and N719, even though an electrolyte containing no 4-tert-butylpyridine was used to adjust the energy of the conduction band of TiO2. The results of DFT calculations indicated that both unfavorable populations of LUMO and LUMO+1, and the lower energy level of LUMO+1, contribute to the much poorer solar cell performances of the TUS sensitizers. In addition, photovoltaic measurements also suggested that enhancement of backward electron transfer from the conduction band of TiO2 to the one-electron-oxidized form of the dye or to I3 in the electrolyte is another reason for the inferior performances of the TUS sensitizers.