Recently, SPST Assistant Professor Zhang Yuebiao’s group reported a guest-dependent dynamic Covalent Organic Frameworks (COFs). In this Issue, they developed a facial and scalable method to produce homogeneous and highly-crystallinity COFs. For the first time, scientists discovered the crystal contraction and expansion of COFs based on gases and guest molecules. Their study, “Guest-Dependent Dynamics in a 3D Covalent Organic Framework” was published on January 18th in Journal of the American Chemical Society.
The polymers’ swelling and shrinkage lead their important applications. However, it is difficult to uncover the structure-effect relationship due to the disordered structures and irreversible behavior. COFs are emerging crystalline porous materials which linked the building blocks by strong covalent bonds absolutely and form two or three dimensional networks. The devisable structure and chemical stability are becoming the frontier challenges of reticular chemistry, which is becoming a hot issue in material science. However, synthesis of COFs is extremely challenging because researchers need to adjust the crystal nucleation and growth while controlling the bonding rate and reversibility of bond formation, which is regarded as a crystallization problem. The chemists have made breakthroughs in synthesis of imine large-sized COFs, but the yield, efficiency and scale still need to be improved. Therefore, Zhang’s group aimed to develop a facial and scalable method for the research on structure effect relationship and large scale functional applications.
The synthetic method was developed by 4th year undergraduate student Chen Yichong independently. Having joined Professor Li Chaozhong’s lab in Shanghai Institution of Organic Chemistry to begin his research training at the beginning of his sophomore year, this experience trained his independent thinking and research abilities. During the holidays and spare time, he studied and practiced to synthesize and purify the organic building blocks which lay a solid foundation for the further research. Taking facial preparation method of MOFs as an example, they decided to choose the ambient pressure glass vial as reaction vessel to prepare COF-300. The experience did not go well at first. They used a good solvent 1,4-dioxane so they did not find obvious precipitation and crystallization. After scientific thinking and long discussion, they tried to mingle the bad solvent cyclohexane to the reaction. As a result, they discovered the solution began to turbid and a large amount of solid appeared slowly. The highly crystalline, high conversion, high yield and morphology COF-300 was characterized by FT-IR, 13C SS-NMR, XRD, SEM and ED. The synthesis method is scalable. Researchers used larger glass bottle to obtain grams highly crystalline samples. Compared with the traditional method, this new method avoids heterogeneity and high energy barrier of the two-step process. This enlightens a new method for controlling the crystalline of COFs by designing the crystallization conditions.
Figure 1. (a) the mechanism of COF crystallization for two methods. (b) Facial and scalable synthesis of COF-300.
However, the researchers did not observe the nitrogen adsorption behavior at 77 K when they began the further property study, which took research team wondering. Thanks to the convenience of Analytical Instrumentation Center in School of Physical Science and Technology, the team characterized the water and various organic vapor adsorption and gas adsorption behavior in different temperatures. They found the dynamic structure and multi-step behavior in such crystals. With the help of synchrotron X-ray diffraction in Shanghai Synchrotron Radiation Facility and Rietveld refinement, researchers determined three different frameworks structures in different chemical environments. They found that the organic solvents and water can induce the crystal expansion and contraction of COF-300 respectively. This work illustrated the structural adaptability and guest dependence of COFs. This solvent-induced dynamic behavior is similar to the response mechanism of substrates and solvents in biological enzymes catalytic process. It provides new ideas in the design of biomimetic materials. In addition, Zhang’s group used gas adsorption in suit powder X-ray diffraction for the first time to prove that framework structure undergone dynamic crystal structure transformation during gas adsorption. It provides a new thinking in application in gas storage and separation.
Figure 2. Guest-dependent crystal contraction and expansion behavior of COF-300 (a) and its difference of frameworks.
This work was supported by National Natural Science Foundation of China and National Postdoctoral Program for Innovative Talents. The 4th year undergraduate Chen Yichong, 5th year PhD Shi Zhaolin and 4th year PhD Wei Lei are the first authors. Professor Zhang Yuebiao and Dr. Zhou Haolong are corresponding authors. ShanghaiTech University is the first responsible institution. This work is also supported by ShanghaiTech University-Shanghai Advanced Research Institute, Chinese Academy of Sciences Low Carbon Energy Science Joint Laboratory, Shanghai Synchrotron Radiation Facility BL-14B1 beamline, Professor Osamu Terasaki in The Electron Microscopy Center, Dr. Yu Na and Ms. Long Liuliu in Analytical Instrumentation Center, Professor Wang Wei and Dr. Ding Sanyuan in Lanzhong University and other experts and scholars.