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

A novel beta-diketone ligand bearing a thiophene moiety and an anchoring Ph-COOCH(3)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 beta-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(3)MLCT-(LC)-L-3 excited states predominantly composed of the diketone ligand-centered triplet state.