Photophysical Properties and Photosensitizing Abilities for Hydrogen Evolution Reactions of Robust Cyclometalated Iridium(III) Complexes with 5,5'-bis(trifluoromethyl)-2,2'-bipyridine

Abstract Two luminescent cyclometalated iridium(III) complexes coordinated with electron-withdrawing 5,5'-bis(trifluoromethyl)-2,2'-bipyridine (dCF3bpy) ligand [Ir(C^N)2(dCF3bpy)]PF6 {H-C^N = 2-phenylpyridine (Hppy) for [1]PF6 and 2-(2,4-difluorophenyl)pyridine (HF2ppy) for [2]PF6}, which work as the robust photosensitizers, were synthesized and characterized by experimental and theoretical analyses. Obtained [1]PF6 and [2]PF6 showed yellow light emissions (559 and 578 nm, respectively) with relatively long lifetimes (728 and 739 nsec, respectively) at room temperature in CH2Cl2. Density functional theory (DFT) calculations clarified that these emissions were assigned as the phosphorescence derived from the mixed metal-ligand-to-ligand charge transfer (MLLCT) excited states. Potential photoredox abilities of [1]PF6 and [2]PF6 as the photosensitizers were also closely investigated by cyclic voltammetries (CVs), Stern-Volmer plots, and photochemical hydrogen evolution reactions. The results of these experiments indicated that [1]PF6 and [2]PF6 have substantial photosensitizing abilities; (i) reductive quenching of excited states of [1]PF6 and [2]PF6 by triethylamine (TEA) were observed in 10% aqueous THF, (ii) the systems involving of [1]PF6 (or [2]PF6), K2PtCl4, and TEA, which were served as the photosensitizer, H2 evolution catalyst, and sacrificial donor, respectively, showed highly efficient H2 evolution activities, and (iii) [1]PF6 and [2]PF6 have good durabilities in the photochemical H2 evolution reactions for a long time. The turnover numbers (TONs) of H2 evolution based on [1]PF6 and [2]PF6 at 96 h of irradiation were reached at 758 and 832 TONs, respectively, which were higher than that based on a well-known photosensitizer [Ir(ppy)2(bpy)]PF6 (659 TON).