Mechanistic insights into tunable copper valence states as photocatalytic active sites

Copper-based photocatalysts, featuring low cost, versatile valence states (Cu⁰, Cu⁺, Cu²⁺), and outstanding photophysical and photochemical properties, hold great promise for organic synthesis, environmental remediation, and energy conversion. This review systematically examines the pivotal role of variable-valence copper species in photocatalysis, establishing a dual-dimensional framework that spans valence evolution and catalytic systems. It highlights advances in Cu⁰ active sites, photoexcited Cu⁺ and Cu²⁺ species, and heterogeneous platforms such as polymer ligands, metal-organic frameworks, and single-atom catalysts. In homogeneous systems, Cu⁺ complexes achieve long-lived metal-to-ligand charge transfer states that drive single-electron transfer for C–C/C–X bond formation and alkene/alkyne bifunctionalization. Cu²⁺ participates in oxidation and radical cycles via ligand-to-metal charge transfer. In heterogeneous systems, Cu⁰ nanostructures enhance light absorption via localized surface plasmon resonance, while Cu⁺/Cu²⁺ semiconductors form Z/S-scheme heterojunctions to improve charge separation. Key strategies, including ligand engineering, defect control, interface design, and single-atom anchoring, are discussed to enhance carrier dynamics and stability. Despite challenges in in-situ characterization, mechanistic elucidation, and scalable synthesis, copper-based photocatalysts offer strong potential for light-driven synthesis, CO₂ valorization, and pollutant mineralization. Future efforts should focus on operando spectroscopy and theory-guided design of robust, selective catalysts to bridge fundamental research and industrial application.