In covalent organic frameworks (COFs), the highly symmetric skeleton limits O2 adsorption and weakens the thermodynamic driving force of the two-electron oxygen reduction reaction (2e- ORR), thus restricting photocatalytic efficiency. In this study, we modulated the local arrangement of fluorine atoms in COFs (para- and ortho-fluorinated, named Fp-COFs and Fo-COFs) to create an asymmetric electronic distribution, which supplies effective O2-adsorption sites, strengthens the driving force for 2e- ORR and ultimately elevates the photocatalytic activity. Theoretical analysis shows that asymmetric fluorination delocalizes the lone-pair electrons of F atoms to adjacent carbons, producing a discretized electron distribution that improves O2 adsorption at imine bonds. The increased electron density on these carbons facilitates electron transfer into the π* orbital of adsorbed O2, accelerating ·OOH* intermediate formation and lowering the Gibbs free energy barrier of the 2e- pathway. Consequently, a quantum yield of 8.8 % for H2O2 photosynthesis in pure water is achieved. This work provides a new approach for tuning local electron distribution in COFs, offering guidance for the rational design of efficient photocatalytic materials and broadening the application prospects of asymmetric electronic structures.
Wu et al. (Sun,) studied this question.