Structure fine-tuning of benzoxazole-linked covalent organic frameworks (COFs) for photocatalytic hydroxylation of phenyl boronic acids

Abstract

Recently, benzoxazole-linked COFs have received much attention due to their high stability, good crystallinity, and surprising photoelectrical properties. However, benzoxazole-linked COFs are conventionally constructed through the reaction of 2,5-diaminohydroquinone dihydrochloride and various polyaldehydes, so the fine regulation of the COF skeleton is limited. Herein, a novel monomer of 5,5′,5′′-(1,3,5-triazine-2,4,6-triyl)tris(2-aminophenol) (TTAP) was successfully synthesized, enabling it to condense with terephthalaldehyde (DT) and 2,5-dimethoxyterephthalaldehyde (DTM) to construct new benzoxazole-linked COFs with different substituents, i.e., H–BO-COF and MeO–BO-COF. Both COFs have good crystallinity, uniform pore size, excellent stability, and high specific surface area. It is found that the two COFs have different bandgaps and photocurrent responses derived from the precise adjustment of the skeleton structures of the COFs. As a photocatalyst of the oxidative hydroxylation of benzyl boronic acid, the performance of the MeO–BO-COF with a yield of 95% is obviously better than that of the H–BO-COF with 83%. This may result from the introduction of the MeO group in the MeO–BO-COF, which broadens its conjugation range, giving it a wider visible light absorption range and higher charge transfer efficiency. This monomer exchange strategy not only enriches the variety of benzoxazole heterocyclic-linked COFs but also fine-tunes the structure of benzoxazole-linked COFs for the first time.