Structural Engineering of Covalent Organic Frameworks Comprising Two Electron Acceptors Improves Photocatalytic Performance.

Covalent organic frameworks (COFs) have recently attracted much attention as potential photocatalysts for hydrogen production. The effective separation of photogenerated charges is a key objective to improve the photocatalytic activity of COFs. Here, four COFs were synthesized through the Schiff-base reaction to investigate whether the presence (simultaneous or not) of triazine and ketone as acceptors in COFs improved electron-hole separation efficiency. Evidence indicated that charge separation was more efficient when triazine and ketone were simultaneously present in the COF. The COF comprising two acceptors displayed the highest photocatalytic hydrogen production rate (31.43 μmol h-1 ; 41.2 and 3.4 times as large as those of the COFs containing only triazine or ketone, respectively). Moreover, the effect of the distance between the two acceptors on the electron-hole separation was investigated by changing the length of a bridging biphenyl ring. It turned out that the transport distance of a single phenyl group was more favorable for the catalytic reaction. This work affords insight and support for the design of efficient COF photocatalysts.