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

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.