Confining a Single Water Molecule Through Molecular Crystal Engineering for Water Oxidation

Hydrogen-bonded water clusters (H₂O)_n obscure the intrinsic reactivity of monomeric H₂O (n = 1) by restricting molecular reorientation. Elucidating the catalytic behavior of isolated water remains a key challenge in aqueous-phase chemistry. Here, we address this by designing a molecular crystal that uniformly confines single water molecules in identical tetrahedral cavities. This platform, CB-H₂O, exhibits exceptional activity for photocatalytic H₂O-to-H₂O₂ conversion, achieving 7.03 mmol g^{- 1} h^{- 1} with pure water, representing an 11.6-fold enhancement over cavity-deficient controls and being markedly superior to existing photocatalytic systems. This performance advantage is directly attributed to the crystallographically defined monomeric water, as verified by isotopic labelling and in-situ spectroscopy. Theoretical calculations further demonstrate that cavity confinement eliminates hydrogen-bond reorganization penalties, substantially lowering the activation barrier for water oxidation. Our work establishes monomeric-water catalysis as a distinct and efficient paradigm, showcasing molecular crystal engineering as a versatile approach to tailoring water-involved reactions for sustainable catalysis.