Nitrogen Modulation in Multipyrimidine- and Multipyridine-Based Covalent Organic Frameworks with Exceptional...

Solar energy-driven green synthesis of hydrogen peroxide (H₂O₂) from water and air is one of the most intensely studied subjects in the photocatalysis field. Herein, three new triamino monomers with a central core of pyridine or benzene surrounded by three pyrimidine or pyridine rings are successfully synthesized, and utilized to polycondense with 1,3,5-triformylphloroglucinol, respectively, to obtain three covalent organic frameworks (COFs) with large specific surface areas. Of interest is the observation that the photocatalytic efficiencies are dramatically promoted by elaborately modulating the position and number of electronegative nitrogen atoms in the N-heterocyclic building blocks of COF frameworks. Under the visible light catalysis, the benzene-cored multipyrimidine COF exhibits the H₂O₂ production rates of 10762 µmol g^-1 h^-1 (in water/O₂) and 9613 µmol g^-1 h^-1 (in water/air) at the reaction time of 1 h without the assistance of any sacrificial agent, representing the highest values among the photocatalysts already reported in the literature under the similar condition. The structure-property relationships and photocatalytic mechanism are studied by virtue of a series of controlled experiments and theoretical calculations, which provide new understanding on the molecular engineering of nitrogen-rich COFs for highly efficient H₂O₂ photosynthesis.