Rational modification of hydroxy-functionalized covalent organic frameworks for enhanced photocatalytic hydrogen...

Covalent organic frameworks (COFs), a class of flexibly tunable crystalline materials, have fascinating potential in photocatalytic hydrogen peroxide (H₂O₂) evolution under visible light irradiation. However, achieving efficient catalytic activity by tuning the composition of COFs and the linkages of building blocks is still a challenge. Herein, four imine-linked COFs with different numbers of hydroxy-functionalized are constructed to unveil the latent structure-activity relationship between the reversibility of bonding in supramolecular chemistry and the photocatalytic H₂O₂ performance. As the optimized material, TAPT-HTA-COF (1H-COF) containing single hydroxy group in aldehyde node exhibits a highest ordered structure and conjugation degree along and across the plane in the extended frameworks originating from the flexibly reversible iminol-to-ketoenamine tautomerism than others, which broadens the visible light absorption and accelerates the dissociation of photogenerated carriers in 1H-COF. These merits ensure that 1H-COF has the highest H₂O₂ yield (44.5 μmol L^-1) and O₂ two-electron reduction pathway among the four COFs under visible light irradiation (λ > 420 nm, 10 vol% isopropanol aqueous solution). At the same time, the long-range ordered framework of 1H-COF is well preserved during the photocatalytic H₂O₂ evolution process assisted by the proton-induced tautomerization. This work facilitates the design and development of COF-based photocatalysts in the evolution of H₂O₂.