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

Abstract

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-π 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 µmol 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.