Single-atom photocatalyst for click reaction

Copper(I)-catalyzed azide-alkyne cycloaddition is the most prevalent click chemistry reaction, which is widely exploited for the synthesis of diverse organic molecules. Typically, the Cu(I) catalysts are generated in situ through the chemical reduction of Cu(II) ions. Unfortunately, such homogeneous systems are considered impractical for large-scale production, and the involvement of multiple equilibria among catalysts, reductants, and substrates complicates elucidation of the catalytic process. Inspired by photosynthesis, which involves transient redox cycle of magnesium porphyrin moiety in chlorophylls, a metal complex-based strategy is proposed for light-triggered click chemistry reactions, utilizing a single-atom Cu(II)-loaded covalent organic framework as a heterogeneous catalyst, wherein the organic ligand functions as a robust scaffold and photoinitiator. Upon light irradiation, photogenerated electrons within the ligand moiety undergo ligand-to-metal charge transfer, injecting into the 3 d orbital of Cu(II) and reducing it to Cu(I) as a short-lived transient species with submicrosecond lifetime, which acts as efficient active sites for the click chemistry reaction. This controllable and robust system shows promise in photochemical cycloaddition of benzylazide and phenylacetylene, a model click chemistry reaction, achieving 95% substrate conversion and >90% triazole yield. The findings are anticipated to be broadly applicable in areas such as sustainable photolithography and polymer chemistry.