Modulating the Polarity of Imine Bonds in Donor-Acceptor Covalent Organic Frameworks for Enhanced Photocatalytic H2 Production.

The development of imine-linked covalent organic frameworks (COFs) has also drawn attention to the influence of the nature of imine binding, including its orientation and possible protonation, on the chemical and physical properties of imine COFs. However, the influence of the polarity of imine bond (C=N) on the photocatalytic performance of COFs has not yet been described. In this work, we synthesized a series of imine COFs with differently polarized C=N bonds by altering the C=N bond orientation and regulating the donor-acceptor (D-A) strength, respectively. The less polarized C=N bond for one of the COFs possesses a faster charge separation with a decrease of exciton binding energy (Eb). In contrast, the imine COF with more polarized C=N bond exhibits a stronger internal electric field contributing to a more efficient charge transfer. Taking into account this trade-off between binding energy and charge transfer concerning the effect of C=N polarity on the photocatalytic performances, the imine COF with moderately polarized C=N bond shows the highest hydrogen evolution performance of 28.1 mmol h-1 g-1. This work provides evidence that the polarity of C=N should be taken into account when designing and tuning the properties of imine COFs.