Tricyclometalated Iridium Complexes as Highly Stable Photosensitizers for Light-Induced Hydrogen Evolution

The development of an efficient and stable artificial photosensitizer for visible-light-driven hydrogen production is highly desirable. Herein, a new series of charge-neutral, heteroleptic tricyclometalated iridium(III) complexes, [Ir(thpy)2(bt)] (14; thpy=2,2-thienylpyridine, bt=2-phenylbenzothiazole and its derivatives), were systematically synthesized and their structural, photophysical, and electrochemical properties were established. Three solid-state structures were studied by X-ray crystallographic analysis. This design offers the unique opportunity to drive the metal-to-ligand charge-transfer (MLCT) band to longer wavelengths for these iridium complexes. We describe new molecular platforms that are based on these neutral iridium complexes for the production of hydrogen through visible-light-induced photocatalysis over an extended period of time in the presence of [Co(bpy)3]2+ and triethanolamine (TEOA). The maximum amount of hydrogen was obtained under constant irradiation over 72h and the system could regenerate its activity upon the addition of cobalt-based catalysts when hydrogen evolution ceased. Our results demonstrated that the dissociation of the [Co(bpy)3]2+ catalyst contributed to the loss of catalytic activity and limited the long-term catalytic performance of the systems. The properties of the neutral complexes are compared in detail to those of two known non-neutral bpy-type complexes, [Ir(thpy)2(dtb-bpy)]+ (5) and [Ir(ppy)2(dtb-bpy)]+ (6; ppy=2-phenylpyridine, dtb-bpy=4,4-di-tert-butyl-2,2-dipyridyl). This work is expected to contribute toward the development of long-lasting solar hydrogen-production systems.