Terpyridine- and Quarterpyridine-Based Cationic Covalent Organic Frameworks for Visible-Light-Catalytic H₂O₂...

This paper presents multipyridine-containing covalent organic frameworks (COFs) with precisely defined position and number of pyridinium cationic groups. Specifically, three terpyridine- and quarterpyridine-based trialdehydes were synthesized, and utilized as the starting monomers to polymerize with trimethylpyridinium bromide to yield vinylene-linked iTPy-COF, iTPPy-COF and iQPPy-COF, respectively. Thus constructed donor-acceptor cationic COFs exhibit considerably high visible-light catalytic efficiency for hydrogen peroxide (H₂O₂) synthesis by the dual-channel mechanisms of oxygen reduction reaction (ORR) and water oxidation reaction (WOR). In pure water and O₂ atmosphere, the H₂O₂ production rate (HPR) of iTPPy-COF after 1 h reaction is as high as 7955 μmol g^-1 h^-1. Even though in air, its HPR value still reaches 6249 μmol g^-1 h^-1. Moreover, it is found that changing the arm lengths and ratios of pyridine to benzene ring in the frameworks significantly affects the photocatalytic capability. The structure-property relationship is investigated in terms of the variations of electronic structures through the theoretical simulations and measurements of photophysical parameters such as fluorescence lifetimes, photocurrent intensities, and impedances of charge transfer, which offers new insights into the engineering of multipyridine-based cationic COFs for highly efficient H₂O₂ photosynthesis.