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

ABSTRACT Hydrogen‐bonded water clusters (H 2 O) n obscure the intrinsic reactivity of monomeric H 2 O (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‐H 2 O , exhibits exceptional activity for photocatalytic H 2 O‐to‐H 2 O 2 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.