Acceptor Strength Modulates Keto-Enol Tautomerism in Donor-Acceptor COFs for Enhanced Photocatalytic Aerobic...

Donor-acceptor (D-A) covalent organic frameworks (COFs) have shown significant potential for photocatalytic applications; however, their performance is often hindered by limited charge separation efficiency. In this work, this challenge was addressed through dynamic modulation of keto-enol tautomerism in COFs by employing an acceptor-strength tuning approach. Three D-A COFs (B-COF, S-COF, and dimethylsulfone DS-COF) were synthesized using triformylphloroglucinol (TP) combined with systematically tuned diamine acceptors containing benzidine (B), sulfone (S), and dimethyl S groups. Both experimental and theoretical investigations reveal that the strongly electron-withdrawing S group in S-COF shifts the tautomeric equilibrium toward the keto form (O═C─C═C─N), thereby enhancing π-conjugation and promoting more efficient charge separation. As a result, S-COF exhibits a prolonged exciton lifetime of 2.4 ns and a reduced charge-transfer resistance, delivering superior catalytic performance compared to B-COF and DS-COF in four distinct aerobic transformation reactions. Notably, S-COF achieves up to 95% isolated yield in aldoxime dehydration. Mechanistic studies further confirm that keto-form predominance, induced by the acceptor group, plays a pivotal role in generating reactive oxygen species (ROS). This work demonstrates a dynamic structural modulation strategy for optimizing D-A COFs and provides valuable insight for the rational design of next-generation photocatalytic materials.