Regulating electron transfer through imine linkage orientations in two-dimensional covalent organic frameworks for efficient electrochromism

Two-dimensional covalent organic frameworks (2D-COFs) based on imine linkages have been shown to offer intriguing optical and electrochromic (EC) performance, attracting extensive attention in the field of advanced photo-electrochemistry. However, few studies have explored how the orientation of imine linkages in 2D-COFs regulates electron transfer behavior and subsequently affects EC performance. In this work, we report an isomeric pair (Py-C-TP-COF and Py-N-TP-COF), differing only in the orientations of the imine linkages. Experimental and theoretical results demonstrate that significant differences in the electronic orbital contributions and bandgaps of 2D-COFs with distinct imine linkage orientations. Furthermore, the different orientations of imine linkages play a crucial role in the distribution of the highest occupied and the lowest unoccupied molecular orbital (HOMO and LUMO), thus significantly influencing electron separation and charge transfer. This approach of altering imine linkage orientations allows for the fine control of the charge transport and band structure of the 2D-COFs. As a result, the Py-C-TP-COF film exhibits excellent EC performance with high coloration efficiency (η CE ,1316 cm 2 ·C -1 ), fast switching time (2.0/0.4 s) and outstanding cycling stability (99.5 % retention of EC cyclic stability after 150 cycles). This study provides new insights for further rational design and optimization of functional COFs as EC materials with atomically regulated charge transfer.