Molecular Copper-Anthraquinone Photocatalysts for Robust Hydrogen Production.

The development of robust and inexpensive photocatalysts for H production under visible light irradiation remains a significant challenge. This study presents a series of square planar copper anthraquinone complexes (RN)CuL (R = ethyl, L = alizarin dianion (CuAA); R = -butyl, L = purpurin dianion (CuPP), (2-hydroxyanthraquinone)formamide dianion (CuAHA)) as molecular photocatalysts to achieve high long-term stability in visible-light-driven H production. These complexes are self-sensitized by the anthraquinone ligands and serve as proton reduction photocatalysts without additional photosensitizers or catalysts. Under irradiation of blue light, complex CuAA produces H in a mixture of HO/DMF with undiminished activity over 42 days, giving a turnover number exceeding 6800. Electrochemical and UV-vis studies are consistent with an EECC mechanism (E: electron transfer and C: protonation) in the catalytic cycle. The initial photochemical steps involve conversion of both anthraquinone ligands to hydroquinones. Further light-driven reductions of the hydroquinones followed by two protonation steps results in formation of H. Dependence of the catalytic rate on the concentration of HO suggests that either the generation of a Cu-H intermediate by protonation or heterocoupling between Cu-H and H to produce H is the turnover-limiting step in catalysis.