Keto-enol tautomerism as transformative electron/hole traps to promote charge carrier separation for record-high H2O2 photosynthesis in real world

Covalent organic frameworks (COFs) are excellent photocatalysts for hydrogen peroxide (H₂O₂) photosynthesis, but are often limited by retarded charge carrier separation. Presently, donor-acceptor (D-A) type COFs are commonly used to enhance the separation of photogenerated electrons and holes at the molecular level by promoting the migration of electrons towards the acceptor and the movement of holes towards the donor. However, the significantly slower kinetics of the synchronous water oxidation and oxygen reduction reactions (WOR and ORR) often result in the accumulation of photogenerated carriers, which induces strong Coulomb forces, in turn adversely affecting the carrier separation efficiency of D-A type COFs. Herein, it is observed that keto-enol tautomerism can function as dynamic traps for both electrons and holes, alternately capturing them, while the counterpart holes and electrons participate in and are consumed during asynchronous oxidation and reduction reactions. This represents the first example of T-C type COFs (T denotes traps units; C denotes catalytic active units), which can effectively weaken the Coulomb force by reducing charge carrier accumulation, resulting in rapid charge transfer and prolonged lifetimes of free charge carriers for efficient alternating photocatalytic WOR and ORR. Our in-depth research indicates that imine COFs based on 2,4,6-trihydroxybenzaldehyde-1,3,5-tricarbaldehyde (Tp series) exhibit enhanced photocatalytic activity compared to those based on 1,3,5-benzenetricarboxaldehyde (BT series), which can be attributed to the occurrence of keto-enol tautomerization. Notably, the optimal Tp imine COF (TpBpy) displays an ultrafast rate for H₂O₂ photosynthesis, reaching 3.79 mM h^-1, surpassing all previously reported photocatalysts. More importantly, when employed in a flow-reactor system, TpBpy also showcases exceptional effectiveness for continuous photocatalytic H₂O₂ synthesis, achieving a solar-to-chemical conversion (SCC) efficiency of 0.038%, representing the highest performance recorded to date under natural sunlight conditions. This work offers molecular-level guidance for designing efficient photocatalysts for H₂O₂ photosynthesis and proposes standardized criteria for obtaining reliable SCC values.