Carbonyl-Site-Mediated Oxygen Reduction in Fluorenone-Based Covalent Organic Frameworks for Efficient Hydrogen...

Hydrogen peroxide (H₂O₂) photosynthesis from molecular oxygen and water offers a sustainable route for decentralized oxidant generation, yet elucidating the molecular-level active sites for oxygen activation in covalent organic frameworks (COFs) remains challenging. Herein, we report a fluorenone-containing COF, FO-Tp, as an efficient photocatalyst for H₂O₂ production, delivering a rate nearly ten times higher than that of fluorene-based COF, highlighting the crucial contribution of fluorenone carbonyls to photocatalytic oxygen reduction. The fluorenone carbonyls facilitate photogenerated-electron trapping and the formation of persistent electron-rich radical states with ketyl radical-anion-like character, enabling FO-Tp to maintain excellent H₂O₂ production under alkaline conditions (AQY = 30.40%, pH = 13, at 400 nm) and in concentrated salt solutions. Extended fluorenone-based COFs validate the generality of this active-site design, reaching 2478.1 µmol·g^-1 h^-1 in pure water and up to 38.6 mmol g^-1·h^-1 in a water/benzyl alcohol biphasic system. Spectroscopic experiments and density functional theory calculations jointly confirm the reduced fluorenone carbonyl as a thermodynamically favored and generalizable electron-accumulating active site for oxygen reduction to H₂O₂. This work elucidates how the fluorenone carbonyl functions as a dynamic molecular active site and proposes a strategy for designing efficiently H₂O₂-producing COF photocatalysts by incorporating carbonyl activation.