Highly Conjugated Imine-Linked Donor–Acceptor Covalent Organic Framework for Efficient HCl Sensing and Photocatalytic Oxidation of Benzylamine

Covalent organic frameworks (COFs) show potential as photocatalysts for harnessing solar energy, primarily because of their adjustable band gap, which enhances their ability to harvest light energy. However, their efficiency as photocatalysts is frequently hampered by inadequate charge transfer and fast charge recombination. In this work, we integrated a donor-acceptor pair into the 2D COF material to enhance charge transfer and reduce charge recombination, thereby improving the overall photoactivity. The 2D COF (<b>OML-4</b>) was synthesized through the polycondensation of a 4',4‴,4‴″,4‴‴'-(ethene-1,1,2,2-tetrayl)tetrakis(([1,1'-biphenyl]-4-carbaldehyde)) (ETBC) linker serving as the donor and a 4,4',4″-(1,3,5-triazine-2,4,6-triyl)trianiline (TAPT) linker acting as the acceptor. This imine-linked 2D COF (<b>OML-4</b>) material shows photocatalytic activity toward benzylic amine oxidation in the presence of visible light for the formation of aldehyde molecules in an aqueous medium. This COF was also investigated for HCl gas sensing behavior based on the protonation of the imine linkage and triazine unit present in the COF material. This exhibits exceptionally good sensitivity up to the ppm level and naked eye detection of HCl by changing in color from yellow to red and turning back to yellow in ammonia vapor, showing the reversible and recyclable nature of the COF.