Transition-metal coordination engineering in photocatalysis: mechanistic principles, material platforms, and future perspectives

Heterogeneous photocatalysts have become important materials in solar-driven green catalysis owing to their excellent structural stability, recyclability, and compatibility with diverse reaction systems. In recent years, transition-metal coordination chemistry has provided important theoretical insights and feasible design strategies for overcoming the practical efficiency limitations of heterogeneous photocatalysts. Owing to their tunable d-derived electronic states, variable oxidation states, flexible coordination geometries, and rich metal-ligand charge-transfer behavior, transition-metal centers offer atomically tunable platforms for regulating the intrinsic electronic structures and interfacial catalytic processes of photocatalysts. In this review, we systematically summarize recent advances in transition-metal coordination engineering for photocatalytic applications. We first discuss the fundamental mechanistic roles of coordination interactions in photocatalysis, including energy-level modulation, ligand-to-metal and metal-to-ligand charge transfer, construction of charge-transport pathways, reactant adsorption and activation, and stabilization of key reaction intermediates. We then highlight representative transition-metal coordination photocatalysts, with emphasis on single-atom photocatalysts, metal–organic frameworks, covalent organic frameworks, and related coordination-based hybrid systems. Attention is paid to how coordination number, coordination geometry, ligand electronic structure, heteroatom coordination, dual-metal sites, and dynamic coordination evolution influence photocatalytic hydrogen evolution, CO 2 reduction, nitrogen fixation, methane conversion, pollutant degradation, and selective organic transformation. Finally, the current challenges and future opportunities in this field are discussed, including precise structure identification, dynamic mechanism elucidation, stability improvement, reaction-environment adaptability, and scalable material design. This review aims to provide a coordination-chemistry-centered perspective for the rational design of efficient, selective, and durable photocatalytic systems.