Linker length-dependent photocatalytic activity of β-ketoenamine covalent organic frameworks

Myriad endeavors have shifted towards covalent organic frameworks (COFs) for their aptitude in visible light photocatalysis. Herein, three β-ketoenamine COFs with different linker lengths, i.e., TpPa-COF, TpBD-COF, and TpDT-COF, were synthesized from 1,3,5-triformylphloroglucinol and 1,4-benzenediamine, 4,4'-biphenyldiamine, and 4,4''-p-terphenyldiamine, respectively. With the shortest linker, a noticeable conversion of benzylamine was observed by TpPa-COF photocatalysis. By prolonging the linker length, TpBD-COF performed a better conversion of benzylamine, which might be due to more suitable redox potentials than TpPa-COF. However, with the longest linker, TpDT-COF resulted in a lower conversion than TpBD-COF, which might be ascribed to the slightly inferior optoelectronic properties. With a moderate linker length, TpBD-COF convincingly delivered the best photocatalytic activity for the violet light-driven aerobic oxidation of amines. This work sheds light on the dimension of linker length in designing more active COF photocatalysts.