Synthesis and Electrochemical Properties of 2,6-Bis(quinoline-2-yl)pyridyl Ruthenium Complexes as Near-Infrared Sensitizers for Dye-Sensitized Solar Cells

New ruthenium(II)–polypyridyl complexes 1a–1d (1a: X= Y= H, 1b: X= H, Y= Cl, 1c: X= H, Y= Me, 1d: X= COOH, Y= H) having 2,6-bis(4-carboxyquinolin-2-yl)pyridine derivatives as ligands were synthesized as sensitizers for dye-sensitized solar cells (DSCs) and their photophysical and photochemical properties were characterized. The photovoltaic performance of DSCs sensitized with 1a–1d was found to be different. The DSC sensitized with 1a exhibits higher efficiency and IPCE value than those sensitized with 1b–1d. The APCE value of the DSC sensitized with 1a was almost 80% in the 500–800 nm range. Therefore, the performance of the DSC sensitized with 1a might improve if the adsorption of 1a on the TiO₂ surface could be appropriately controlled by investigation of dye immersion condition or introduction of bulky substituents on the ligand in order to suppress dye aggregation.