Hydroxyl deficiency-induced mixed linkages in covalent pyrimidine frameworks towards enhancing stability during H2O2 photosynthesis

Abstract

Covalent organic frameworks (COFs) showed great potential as visible-light-responsive photocatalysts for the artificial photosynthesis of hydrogen peroxide (H₂O₂), but susceptible to self-degradation caused by photoinduced holes. Herein, a novel mixed-linkage strategy was developed to improve the stability of COFs during photocatalysis. The incorporation of a single hydroxyl deficiency in the building unit resulted in mixed imine and β-ketoamine linkages (50% each) within pyrimidine-based COFs (COF-BD). This modification endowed it with enhanced robustness and improved efficiency in H₂O₂ photosynthesis. Specifically, the photocatalytic production rate of H₂O₂ reached 5312 μmol g_cat⁻¹ h⁻¹ in pure water within one hour, accompanied by enhanced stability. In comparison, pure β-ketoamine-linked COFs (COF-TP) initially showed a high H₂O₂ production rate of 14 480 μmol g_cat⁻¹ h⁻¹ within the first 10 minutes, but this rapidly decreased to 6104 μmol g_cat⁻¹ h⁻¹ after one hour. The enhanced durability of COF-BD is attributed to its optimized electron distribution, which lowers the valence band maximum (VBM) from 1.88 V vs. the reversible hydrogen electrode (RHE) in COF-TP to 1.62 V vs. RHE in COF-BD. This shift changes the dominant H₂O₂ photosynthetic pathway from the 2e⁻ oxygen reduction reaction (ORR)/2e⁻ water oxidation reaction (WOR) in COF-TP to the 2e⁻ ORR/4e⁻ WOR in COF-BD. Theoretical calculations suggest that the mixed linkages promote the rapid consumption of photoinduced holes, thereby enhancing the performance of COF-BD in H₂O₂ photosynthesis. These results not only highlight an effective mixed-linkage strategy for boosting the robustness of organic photocatalysts but also underscore the importance of fine-tuning hydroxyl deficiencies to modulate the electronic properties of materials.