Constructing Molecular Junctions with High-Spin Metal Sites in POMOFs to Trigger Heteropoly Blue States for Photocatalytic H₂ Production

The spin state of metal atoms may play a key role in the photocatalytic hydrogen evolution reaction (HER) of polyoxometalate (POM)-based catalysts. Herein, we construct two POM-based metal-organic frameworks (POMOFs) with molecular junction structures by integrating a metal-organic framework structure including high-spin state active Co or Ni sites with [GeW₁₂O₄₀]^4- (GeW₁₂) POM. These structures have a fast electron transfer channel, in which the electronic structures of the high-spin metal centers may determine whether the POM can produce reduced POM (heteropoly blue, HPB), leading to enhanced HER activity. Upon photoexcitation, the Co^II high-spin state has a vacant in the t_2g orbital, which transfers electrons through π-orbitals, enhancing the electron transfer rate and promoting the generation of HPB. Thus, Co₆-GeW₁₂ POMOF forms HPB species during the photocatalytic process and has a photocatalytic HER activity as high as 13.7 mmol g^-1 h^-1. This value is the highest one obtained for any photocatalyst with POM as a photosensitizer.