Dinuclear Iridium(III) Complexes Containing Bibenzimidazole and Their Application to Water Photoreduction

An efficient three-component catalytic system for visible-light-induced production of hydrogen from water was developed based on new dinuclear iridium photosensitizers (PSs), [Ir(tfdpyb)Cl](2)(BiBzIm) (P1) and [Ir(tfmppy)(2)](2)BiBzIm (P2) [tfdpyb = 1,3-di(2-pyridyl)-4,6-bis(trifluoromethyl)benzene, tfmppy = 2-(4-(trifluoromethyl)phenyl)-pyridine, BiBzIm = 2,2'-bibenzimidazole]. These iridium complexes were fully characterized by ill NMR and ESI-MS, and their photophysical properties and electrochemical behaviors were also investigated. To compare with this new type of iridium compounds, the mononuclear analogues Ir(tfdpyb)(BiBzImH)Cl (P3), Ir(tfmppy)(2)(BiBzImH) (P4), and Ir(dpyx)(BiBzImH)Cl (P5) (dpyx = 1,3-di(2-pyridyl)-4,6-dimethylbenzene) were also synthesized. The absorption spectra of dinuclear and mononuclear complexes are similar mainly in terms of their shape in the visible-light region, but as expected, the dinuclear species are more intense (approximately twice) corresponding mononuclear compounds. These complexes were tested as PSs with regard to their capacity to reduce water in the photocatalytic system for hydrogen production together with a series of water reduction catalysts and triethanolamine (TEOA) as a sacrificial electron donor. Turnover numbers up to 3780 for the dinuclear iridium complex P1 and 1020 for the mononuclear iridium compound P3 were obtained under the identical conditions. The results indicate that dinuclear iridium compounds can behave as PSs to be used for reducing water to hydrogen, and their activity was superior to that of the mononuclear compounds in this system. This work provides us a more general architectural guideline for constructing metal complexes as light-harvesting materials for visible-light-induced hydrogen production.