Boosting photocatalytic hydrogen evolution of β-keto-enamine-based covalent organic frameworks by introducing electron-donating functional substituents

An understanding of the structure-property relationships of the organic photocatalysts at the molecular level is highly desirable to inspire the further development of novel high-performance organic photocatalysts. Herein, a series of β-keto-enamine-based covalent organic frameworks (COFs) with different substituents (named as Tp-DB-X-COFs, where X is -H2, -(CH3)2, -(OCH3)2 or -(NO2)2) has been rationally designed and successfully synthesized. Ample evidences reveal that the photocatalytic hydrogen evolution performance of the Tp-DB-X-COFs photocatalysts is closely related to the electronegativity of the attached substituents and introduction of electron-donating substituents into such COF photocatalysts is able to effectively inhibit the recombination of photogenerated electron/hole pairs, resulting in a great improvement on the photocatalytic hydrogen evolution performance. For an example, the Tp-DB-(OCH3)2-COF photocatalyst with the electron-donating substituent exhibits a hydrogen evolution rate of 1.23 mmol g−1 h−1, which is about 82 times higher than that (0.015 mmol g−1 h−1) of the Tp-DB-(NO2)2-COF photocatalyst with the electron-withdrawing substituent. This study provides some inspiration for the design and synthesis of the advanced organic photocatalysts through the molecular engineering strategy.