Covalent organic frameworks are 2D or 3D porous crystals built by organic molecules through covalent bonds. Due to the framework stability and accurate control at molecular-level, this type of material shows potential applications in gas adsorption/separation, energy storage and conversion, drug delivery and so on. Recently, a collaborative work by SPST Assistant Professors Zhang Yuebiao and Ma Yanhang’s group reported atomic-level structural analysis of dynamics of a 3D COF, which was published as a supplementary cover story in Journal of the American Chemical Society.
COFs are constructed by strong covalent bonds, which leads to the poor reversibility of crystal growth. The crystal growth and structural analysis are big challenges in this area. Recently, large single COF crystals have been synthesized using amine-exchange strategy, which allows for single crystal X-ray diffraction analysis. However, this strategy is still not widely applied to most of COFs with different linkages and topologies. Therefore, developing new methods for structural analysis of COF nanocrystals are important to understand the reaction mechanism and dynamics of frameworks. In the previous work, Zhang Yuebiao and Wang Wei in Lan Zhou University reported the discovery and characterizations of guest-induced dynamics in 3D COFs, which opens the window for the study of dynamics in 3D COFs.
Recently, Zhang Yuebiao’s group developed a new and simple strategy for synthesizing COFs. COF-300 crystals with highly crystallinity, uniform morphology and well dispersity were synthesized using this approach. The guest-induced dynamics in COF-300 have been discovered using high-resolution powder XRD: retraction after absorbing water and expansion after absorbing organic solvents. The flexibility of COF can be understood at the molecular level through Rietveld refinement against PXRD patterns. The understanding of guest-host interaction and mechanism for framework flexibility will provide meaningful instructions for further design of new stimuli-responsive COFs.
Electron diffraction can be applied to structural analysis of nanocrystals due to its strong interaction with matters, which shows potential applications in structural biology, pharmacy and materials science. However, due to various problems including electron-beam sensitivity, sample dispersity and amorphous after losing guest molecules, the structural analysis of COFs at atomic level has been challenging. Ma Yanhang has rich experiences on the study of nanoporous materials using electron microscopes. By combining electron diffraction tomography (EDT) and high-resolution imaging, structures of some new COFs have been solved (Y. Liu,# and Y. Ma,# et al. Science 2016, 351, 365; JACS 2017, 139, 13166; JACS 2018, 140, 16015;JACS 2019, 141, 677). However, the resolution of EDT data has been limited by 2 Å before. In this study, electron diffraction with resolution better than 0.9 A has been collected by optimizing experimental parameters at cryo-condition, as a result of which all of the non-hydrogen atoms can be located in the ab initio structure solution. Moreover, water molecules have been fixed inside the COF-300 channel by fast freezing process. 3D ED data with resolution better than 0.8 resolution has been collected using cryo-transfer holder and optimized data collection strategy. As a result, the positions of guest molecules were located inside the framework. Due to the high vacuum in TEM, organic solvents can be easily moved from the pores of COFs, which leads to disorder of COF framework and usually poor quality data was obtained. After many tries, our team filled the COF-300 channels using polymers through in-situ polymerization, which give 1.5 Å resolution ED data. This work makes the study of guest-host interaction in COFs possible using electron diffraction technique and also provide a new method for studying materials absorbing water and explain the absorbing mechanism.
This work has been published in JACS and selected as one of three supplementary covers. Mr. Sun Tu and Mrs. Wei Lei are co-first authors, Mr. Chen Yichong is one of co-authors. Zhang Yuebiao and Ma Yanhang are co-corresponding authors. ShanghaiTech University is the only corresponding institution. We thank Professor Osamu Terasaki, Professor Peter Oleynikov and other SPST faculty for their great support. Professor Liu Chong and Dr. Luo Feng provided great support on the preparation of cryo-samples. Professor Chia-Kuang Tsung gave many helpful suggestions. This work was supported by NSFC, NSFC-Brics project, Shanghai Municipality and the Young Elite Scientist Sponsorship Program by CAST.
Figure 1. (a) The three structural segments correspond to the potential diagrams of the three phases respectively shown the geometric deformations of the tetrahedron center and the building unit (-Ph- N =C-Ph-C=N-Ph-). This suggests that the phase transition is due to the rotation of imine bonds rather than mechanical stretching. (b) The structure of the hydrated phase clearly reveals the interaction between the guest and the frame.
Figure 2. Atomic-level characterization of dynamics through electron diffraction tomography illustrated by COF-300 with single crystals at only micrometer size. The TEM images of the single crystals and the EDT data projected along the  or  direction for the activated phase COF-300-V with routine holder (a, b). With cryo-holder the resolution of electron diffraction data has been significantly improved (c, d). The TEM images of the single crystal for hydrated phase shows a small amount of vitrified ice around the crystal and electron diffraction images show ice diffraction rings (e, f). The aspect ratio of the crystal for COF-300-PMMA phase is significantly smaller than that of the other two phases (g and h).