Photocatalytic benzylamine coupling dominated by modulation of linkers in donor-acceptor covalent organic frameworks

The strong excitonic effect is widely prevalent in covalent organic frameworks (COFs)-based catalysts, while the high exciton binding energies (Eb) of COFs seriously limit their photocatalytic performance. Herein, theoretical calculations are conducted to evaluate the Eb values of potential COFs derived from benzo[1,2-b:3,4-b'-b'']trithiophene-2,5,8-tricarbaldehyde (BTT) and 2,4,6-trihydroxybenzene-1,3,5-tricarbaldehyde (Bp) as aldehyde monomers. Diverse aromatic amine linkers including dipyridine diamine (BPy), biphenyl diamine (BPh), and p-phenylenediamine (MPh) with different molecular lengths and N-sites are used as linker models to regulate the excitonic effect. The obtained BTT-BPy-COF with a long BPy linker and the lowest calculated Eb value possesses the best photocatalytic performance for benzylamine coupling, with conversion and selectivity up to 99% accompanied by a high oxidation rate, outperforming most of the reported works. Further mechanistic investigation reveals that the bipyridine-N sites in BTT-BPy-COF exhibit strong electron transfer as well as enhanced O2 adsorption and activation that is beneficial to photocatalytic benzylamine coupling.