Photostimulated Covalent Linkage Transformation Isomerizing Covalent Organic Frameworks for Improved Photocatalytic Performances

Covalent organic frameworks (COFs) offer a desirable platform to explore multichoromophoric arrays for photocatalytic conversion. Symmetric arrangement of choromophoric modules over pi-extended frameworks enhances exciton delocalization while impairing excitation density and accordingly photochemical reactivity. Herein, a photoisomerization-driven strategy is proposed to break the excited-state symmetry of ketoenamine-linked COFs with multichoromophoric arrays. Incorporating electron-withdrawing benzothiadiazole facilitates the ultrafast excited-state intramolecular proton transfer (ESIPT) from enamine to keto within 140 fs, resulting in partially enolized COF isomers. The hybrid linkages containing imine and enamine bonds at the node of framework alter the symmetry of electronic structure and enforce the photoinduced charge separation. Increasing the imine-to-enamine ratio further promotes the electron transferred number in a long range, thereby affording the optimum photocatalytic hydrogen evolution rate. This work put forward an ESIPT-induced photoisomerization to build a symmetry-breaking COF with weakened exciton effect and enhanced photochemical reactivity. An ultrafast excited-state intramolecular proton transfer (ESIPT) can be triggered by incorporating electron-withdrawing benzothiadiazole into ketoenamine-linked covalent organic frameworks (COFs). The ESIPT-induced photoisomerization generates the partially enolized structure with hybrid linkages, leading to the excited-state symmetry-breaking for enhanced charge separation and long-range electron transfer. Furthermore, asymmetric COF is more favorable to boost the photocatalytic performance than symmetric analog by the EISPT-induced photoisomerization.image

Automated summary

This study presents a photoisomerization-driven strategy for the design of covalent organic frameworks (COFs) by introducing electron-withdrawing benzothiadiazole, aiming to break the excited-state symmetry and enhance the photochemical reactivity of COFs. The ultrafast excited-state intramolecular proton transfer (ESIPT) is utilized to generate partially enolized COF isomers, leading to symmetry-breaking and enhanced charge separation and long-range electron transfer through the hybrid linkages. Asymmetric COFs show better photocatalytic performance than symmetric ones in ESIPT-induced photoisomerization.