π-Bridge Modulation in Three-Motif Covalent Organic Framework for Efficient H₂O₂ Photosynthesis From Water and Air

Rational integration of strong photosensitive moieties and multiple active sites into high performance photocatalysts is challenging due to uncontrollable intercrossing charge transfer. Herein, a series of imine-bonded two- and three-motif covalent organic frameworks, USTB-65∼USTB-68, have been studied, which feature triphenylamine unit as electron-donor (D) together with benzothiadiazole and/or 2,4,6-triphenyl-1,3,5-triazine moieties as electron-acceptor (A). Introduction of benzene rings as π-bridge into the D-A-A lattice of three-motif USTB-65 affords D-A-π-A USTB-66, leading to more facile exciton dissociation and efficient step-wise charge transfer among three motifs as revealed by various photophysical investigations and theoretical calculations. This, in combination with strong light absorption and multiple photocatalytic sites, results in the outstanding activity of USTB-66 in H₂O₂ photoproduction from water and air, achieving a production rate of 11.2 mmol g^‒1 h^‒1, an outstanding apparent quantum yield of 27.3% at 550 nm, and a solar-to-chemical conversion efficiency of 2.71%. Under solar concentrator, the H₂O₂ production rate based on USTB-66 in a flow reactor is further increased to 33.8 mmol g^‒1 h^‒1 during 24 h irradiation.