Molecular engineering of donor-acceptor sp2-carbon-linked covalent organic frameworks for enhancing photocatalytic hydrogen production

Donor-acceptor covalent organic frameworks (COFs) are promising photocatalysts due to their adjustable structures and band gaps. Meanwhile, constructing fully conjugated sp2-carbon-linked COFs can further promote charge carrier mobility. Herein, three sp(2)-carbon-linked D-A COFs (Ph-COF, Th-COF and TTh-COF) are successfully synthesized via a Knoevenagel condensation reaction using p-phenylene diacetonitrile as the acceptor and triphenylbenzene, trithiophenephenyl and benzotrithiophene as donors. By varying donor units, the band gaps, visible light absorption range and charge separation and transportation properties of the COFs are precisely regulated. Among these COFs, TTh-COF with the optimum-electron donor benzotrithiophene exhibits the narrowest band gap and the strongest built-in electric field, which induce excellent charge transfer, resulting in enhanced photocatalytic performance with an impressive hydrogen generation rate of 22.22 mmol g(-1) h(-1). This work provides an effective method to adjust the photocatalytic activity of COFs by changing the donor at the molecular level.