p-π Conjugated Covalent Organic Frameworks Expedite Molecular Triplet Excitons for H2O2 Production Coupled with Biomass Upgrading

High-efficiency production of triplet states in covalent organic framework photocatalysts is crucial for high-selectivity oxygen (O2) reduction to hydrogen peroxide (H2O2). Herein, fluorine and partial fluorine atoms are incorporated into an olefin-linked triazine covalent organic framework (F-ol-COF and HF-ol-COF), in which the adjacent fluorine (F) atoms-olefinic bond forms p-pi conjugation that induces spin-polarization under irradiation, thus expediting triplet excitons for activating O2 to singlet oxygen (1O2) and contributing to a high H2O2 selectivity (91%). Additionally, the feasibility of coupling H2O2 production with the valorization of 5-hydroxymethylfurfural (HMF) is exhibited. The F-ol-COF demonstrates a highly stable H2O2 yield rate of 12558 mu mol g-1 h-1 with the HMF-to-functionalized furan conversion yield of 95%, much higher than the partially fluorinated COF (HF-ol-COF) and the non-fluorinated COF (H-ol-COF). Mechanistic studies reveal that F-incorporation promotes charge separation, intensifies the Lewis acidity of the carbon atoms on the olefinic bond as active sites for O2 adsorption, and provides highly concentrated holes at the triazine unit for HMF oxidation upgrading. This study suggests the attractive potential of rational design of porous-crystalline photocatalysts for high-efficiency photocatalytic O2 reduction to H2O2 and biomass upgrading.