Molecular Copper-Anthraquinone Photocatalysts for Robust Hydrogen Production

The development of robust and inexpensive photocatalysts for H₂ production under visible light irradiation remains a significant challenge. This study presents a series of square planar copper anthraquinone complexes (R₄N)CuL₂ (R = ethyl, L = alizarin dianion (CuAA); R = n-butyl, L = purpurin dianion (CuPP), (2-hydroxyanthraquinone)formamide dianion (CuAHA)) as molecular photocatalysts to achieve high long-term stability in visible-light-driven H₂ 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 H₂ in a mixture of H₂O/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 H₂. Dependence of the catalytic rate on the concentration of H₂O suggests that either the generation of a Cu^II-H intermediate by protonation or heterocoupling between Cu^II-H and H⁺ to produce H₂ is the turnover-limiting step in catalysis.