A Covalent Organic Framework as Photocatalyst for Smart Conversion Between Photooxidation and Photoreduction and H₂O₂ Production in Full pH Environment

Developing multifunctional photocatalysts with intelligent self-adjusting is of great significance in the photocatalytic process. Herein, a smart covalent organic framework (Por-HQ-COF) with a phenol-quinone conversion structure with pH changes is constructed for photooxidation, photoreduction, and H₂O₂ production. As a smart photocatalyst, Por-HQ-COF can convert into Por-BQ-COF intelligently with a trigger including solution pH, and vice versa. The reconstruction of phenol-quinone conversion not only significantly alters the morphologies and the specific surface areas of the COF, but also leads to an entirely change in the band energy and charge distribution to influence photoelectric properties. As a result, under acidic conditions, Por-BQ-COF converts into Por-HQ-COF automatically and can photoreduce high concentration Cr(VI) to Cr(III) efficiently. Under neutral conditions, the superoxide anions (·O₂ ^-) initiate the Por-HQ-COF reconstruction into Por-BQ-COF to accelerate photooxidation to degrade high-concentration TC. Under alkaline conditions, Por-HQ-COF converts into Por-BQ-COF, can effectively photosynthesize H₂O₂ (1525 µmol h^-1 g^-1 at λ > 420 nm) in the absence of any sacrificial reagents, and reveal the strong alkalinity lower the energy barrier of hydrogen extraction from H₂O and clarify active sites for H₂O₂ production. This work provides a new strategy for developing smart photocatalysts and fulfill the application across the full pH environment.