Regulating the Photoisomerization of Covalent Organic Framework for Enhanced Photocatalytic Hydrogen Evolution

Covalent organic framework (COF) is a desirable platform to tailor electronic properties for improving photocatalytic performances. However, the study on excited-state configurations that determine photogenerated carrier dynamics has long been neglected. Herein, we concentrate on the molecular design of beta-ketoenamine-linked COFs to drive their photoisomerization via the excited-state intra-molecular proton transfer (ESIPT), which can induce the partial keto-to-enol tautomerization and accordingly rearrange the photoinduced charge distribution. We demonstrate that the push-pull electronic effect of functional side groups attached on the framework linkers is directly correlated with the ESIPT process. The phenylene linkers modified with electron-withdrawing cyano-groups reinforce the ESIPT-induced tautomerization, leading to the in situ partial enolization for extended pi-conjugation and rearranged electron-hole distribution. In contrast, the electron-rich linkers limit the photoisomerization of COF and suppress the photoinduced electron accumulation. Thus, the maximum hydrogen evolution rate is achieved by the cyano-modified COF, reaching as high as 162.72 mmol<middle dot>g(-1)<middle dot>h(-1) with an apparent quantum efficiency of 13.44% at 475 nm, which is almost 11.5-fold higher than those of analogous COFs with electron-rich linkers. Our work opens up an avenue to control over the excited-state structure transformation for enhanced photochemical applications.