Metal cluster confinement in MOFs and COFs: Advanced synthesis strategies and applications in photocatalysis

Atomically precise metal nanoclusters, consisting of tens to hundreds of metal atoms, represent a unique class of catalytic materials with well-defined electronic structures and tunable surface chemistry. These features enable nanoclusters to act as versatile components in photocatalytic systems, where they regulate light absorption, charge separation, and interfacial reaction dynamics. Photocatalysis provides a sustainable pathway for converting solar energy into fuels and value-added chemicals; however, its practical application is limited by intrinsic thermodynamic and kinetic barriers, as well as catalyst stability and selectivity challenges. Recent advances have spotlighted the integration of nanoclusters with extended porous frameworks, including metal–organic frameworks (MOFs) and covalent organic frameworks (COFs), as a powerful strategy to overcome these limitations. These hybrid architectures allow precise control over active-site geometry, electronic environments, and substrate accessibility, while promoting synergistic effects such as enhanced charge transport and stabilization of reactive intermediates. This review highlights emerging synthetic methodologies, modification strategies, and recent photocatalytic applications (CO₂ reduction, H₂ evolution, H₂O₂ production, organic synthesis, pollutant removal, CH₄ conversion, etc) reported over the past three years. We discuss mechanistic insights, structure–function relationships, and critical challenges, including conductivity, robustness, and scalability. Finally, we propose integrated design principles for constructing hybrid nanocluster–framework photocatalysts with optimized efficiency, selectivity, and durability, offering a roadmap for the rational development of next-generation energy-conversion and chemical-synthesis platforms.