Water Reduction Systems Associated with Homoleptic Cyclometalated Iridium Complexes of Various 2-Phenylpyridines

The photoreduction of water to hydrogen represents a promising method for generating sustainable clean fuel. The molecular processes of this photoreduction require an effective light absorber, such as the ruthenium polybipyridine complex, to collect and convert the solar energy into a usable chemical form. In the search for a highly active and stable photosensitizer (PS), iridium complexes are attractive because of their desirable photophysical characteristics. Herein, a series of homoleptic tris-cyclometalated iridium complexes, based on different 2-phenylpyridine ligands, were utilized as PSs in photocatalytic systems for hydrogen production with [Rh(dtb-bpy)(3)](PF6)(3) (dtb-bpy=4,4-di-tert-butyl-2,2-dipyridyl) serving as the water reduction catalyst (WRC) and triethanolamine (TEOA) as the electron donor. The photophysical and electrochemical properties of these complexes were systematically investigated. The excited state of neutral iridium complexes (PS*) could not be quenched by using TEOA as an electron donor, but they could be quenched by using [Rh(dtb-bpy)(3)](PF6)(3) as an electron acceptor, indicating that the PS* quenching pathway in catalytic reactions was most likely an oxidative quenching process. A set of long-lived and highly active systems for hydrogen evolution were obtained in Ir-III-Rh-III-TEOA systems. These systems maintained their activity for more than 72h with visible-light irradiation, and the total turnover number was up to 3040. Comparative studies indicated that the photocatalytic performance of these homoleptic tris-cyclometalated iridium compounds was superior to that of the cationic iridium complex [Ir(ppy)(2)(bpy)](PF6) (ppy=2-phenylpyridine, bpy=2,2-dipyridyl) (4), which was used as a reference. The significant increase in the photocatalytic efficiencies was in part attributed to the higher photostability of the neutral Ir-III complexes. This assumption was supported by their different coordination modes, photophysical, and electrochemical properties.