Nitrogen Modulation in Multipyrimidine- and Multipyridine-Based Covalent Organic Frameworks with Exceptional Promotion for Photocatalytic H2O2 Synthesis.

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.