Cyclometalated dinuclear iridium(III) complex with 2,2'-bipyridyl-based bridging ligand as a photosensitizer for photochemical hydrogen evolution reaction

A new cyclometalated dinuclear iridium complex with a 1,2-bis(4′-methyl-2,2'-bipyridin-4-yl)ethane (bmbe) bridging ligand, [Ir₂(ppy)₄(bmbe)](PF₆)₂ (IrPS-D; ppy = 2-phenylpyridinate), which is anticipated to function as a highly efficient photosensitizer (PS) for the photochemical hydrogen evolution reaction, is synthesized and structurally characterized via ¹H NMR, ESI-TOF-MS, ATR-FT-IR, elemental analysis, and single crystal X-ray diffraction analyses. The combination of spectroscopic analysis and time-dependent density functional theory calculations clarifies that (i) the visible-light absorption spectrum of IrPS-D exhibits a shape and excitation features almost identical to those of [Ir(ppy)₂(dmbpy)]PF₆ (IrPS-M; dmbpy = 4,4'-dimethyl-2,2′-bipyridine), which is a well-known promising PS for the photochemical hydrogen evolution reaction and mononuclear counterpart of IrPS-D, whereas the spectral intensity of IrPS-D is approximately twice as high as that of IrPS-M; (ii) the emission energies of IrPS-D and IrPS-M are nearly identical; however, IrPS-D exhibits a slightly shorter emission lifetime and higher quantum yield compared with those of IrPS-M, resulting in relatively faster radiative and non-radiative rate constants for IrPS-D compared with those of IrPS-M; and (iii) the excited state of IrPS-D is quenched by triethylamine (TEA) faster than that of IrPS-M. Upon visible-light irradiation, IrPS-D serves as a highly efficient PS for the photochemical hydrogen evolution reaction in the presence of K₂PtCl₄ and TEA, which function as the hydrogen evolution catalyst and sacrificial electron donor, respectively. The maximum turnover number (TON_PS) of IrPS-D for hydrogen evolution is 2672, which is approximately 4.75 times greater than that of IrPS-M (TON_PS = 563) under the same reaction conditions.