Keto-enol tautomerism as dynamic electron/hole traps promote charge carrier separation for hydrogen peroxide photosynthesis

Abstract

Covalent organic frameworks (COFs) are promising photocatalysts for H₂O₂ photosynthesis, but charge carrier separation remains a critical challenge. Donor-acceptor COFs enhance charge separation, but the slow kinetics of water oxidation and oxygen reduction reactions lead to carrier accumulation, thereby decreasing efficiency. Here, we report T-C type COFs (T = trap units, C = catalytic units), demonstrating that units with keto-enol tautomerism can serve as dynamic electron/hole traps (T) to mitigate Coulomb forces. This design effectively facilitates swift charge transfer and extends carrier lifetimes, thereby enhancing reactions at the C units. Imine COFs derived from 2,4,6-trihydroxybenzaldehyde (Tp) outperform those based on 1,3,5-benzenetricarboxaldehyde due to tautomerization. The optimal Tp COF (TpBpy) achieves an H₂O₂ generation rate of 37.9 μmol h⁻¹ (or 8350 μmol h⁻¹ g⁻¹) under simulated light, and a solar-to-chemical conversion efficiency of 0.038% in a flow reactor under natural sunlight. This work provides molecular design strategies and standard criteria for efficient H₂O₂ photocatalysts.