Aromatic β‐Diketone as a Novel Anchoring Ligand in Iridium(III) Complexes for Dye‐Sensitized Solar Cells

A novel β‐diketone ligand bearing a thiophene moiety and an anchoring Ph‐COOCH₃ unit has been designed, prepared and used in synthesis of cyclometalated benzimidazole‐based iridium(III) complexes for application in dye‐sensitized solar cells (DSSC). The replacement of traditional 4,4'‐dicarboxy‐2,2'‐bipyridine by this aromatic β‐diketone results in excellent tuning the redox potentials and excited state properties of these Ir^III complexes, one of which exhibits good efficiency when used as a dye in DSSC. All the complexes demonstrate reversible redox behavior, with oxidation potentials (Ir⁴⁺/Ir³⁺) strongly depending on the electron‐donor/withdrawing nature of the substituents in the cyclometalated ligands. Surprisingly, the latter has just a little effect on their luminescence spectra, in which structured emission bands are observed. Time‐resolved spectroscopic studies in combination with DFT calculations show that the complexes emit light by mixed ³MLCT–³LC excited states predominantly composed of the diketone ligand‐centered triplet state.