Carbonyl‐Site‐Mediated Oxygen Reduction in Fluorenone‐Based Covalent Organic Frameworks for Efficient Hydrogen Peroxide Photosynthesis

ABSTRACT Hydrogen peroxide (H 2 O 2 ) 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 H 2 O 2 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 H 2 O 2 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 H 2 O 2 . This work elucidates how the fluorenone carbonyl functions as a dynamic molecular active site and proposes a strategy for designing efficiently H 2 O 2 ‐producing COF photocatalysts by incorporating carbonyl activation.