Chi Hongbin, Liu Yang, Li Ziyi, Chen Wanxin, He Yi
School of Nuclear Science & Technology, Southwest University of Science and Technology, 621010, Mianyang, P. R. China.
Sichuan College of Architectural Technology, 618000, Deyang, Sichuan, P. R. China.
Nat Commun. 2023 Aug 21;14(1):5061. doi: 10.1038/s41467-023-40831-8.
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.
柔性共价有机框架（COFs）已被研究用于吸附、选择性分离和催化等应用。如何将COFs的微观结构与柔韧性相关联是一个巨大的挑战。在此，我们以操作数方式，通过暗场显微镜（DFM）直观地追踪单个COF-300和COF-300-AR颗粒响应溶剂蒸汽客体时的柔性变形行为。通过还原由刚性C=N双键组成的亚胺基COF-300合成了具有自由旋转C-N单键的COF-300-AR，且不改变拓扑结构和结晶度。出乎意料的是，我们观察到尽管COF-300含有许多C-N单键，但其柔性变形却比COF-300-AR高得多，这清楚地说明了还原后COF-300的表观柔韧性降低。DFM的高时空分辨率使得能够发现COF-300晶体之间柔韧性的颗粒间差异。通过变温X射线衍射和红外光谱进行的实验表征以及理论计算表明，COF-300的柔性变形归因于围绕刚性C=N双键的踏板运动。这些观察结果为COF的柔韧性提供了新的见解。
