Abstract
Flexible covalent organic frameworks (COFs) have been studied for applications containing sorption, selective separation, and catalysis. How to correlate the microscopic structure with flexibility in COFs is a great challenge. Herein, we visually track the flexible deformation behaviors of single COF-300 and COF-300-AR particles in response to solvent vapour guests with dark-field microscopy (DFM) in an in operando manner. COF-300-AR with freely-rotating C-N single bonds are synthesized by the reduction of imine-based COF-300 consisting of rigid C=N double bonds without changing topological structure and crystallinity. Unexpectedly, we observe that the flexible deformation of COF-300 is extremely higher than that of COF-300-AR despite it bears many C-N single bonds, clearly illustrating the apparent flexibility decrease of COF-300 after reduction. The high spatiotemporal resolution of DFM enables the finding of inter-particle variations of the flexibility among COF-300 crystals. Experimental characterizations by variable-temperature X-ray diffraction and infrared spectroscopy as well as theoretical calculations demonstrate that the flexible deformation of COF-300 is ascribed to the pedal motion around rigid C=N double bonds. These observations provide new insights into COF flexibility.