3D electron diffraction as an important technique for structure elucidation of metal-organic frameworks and covalent organic frameworks

Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have emerged as the most widely investigated classes of porous materials during the past two decades. The almost unlimited combination of building units (metal clusters and organic molecules) endows highly tuneable porosities and functionalities that are appealing for a wide scope of applications. The applications of MOFs and COFs depend on their physical and chemical properties, which in turn are determined by the arrangement of atoms – the crystal structures. Therefore, structure determination is arguably the most important characterization step for MOFs and COFs. While single crystal X-ray diffraction (SCXRD) is the most widely used method for structure determination, many MOFs and COFs are synthesized in too small sizes or their crystal qualities are too low for SCXRD. During recent years, three-dimensional electron diffraction (3DED) methods has undergone rapid developments and can be used for structure determination of nano- and submicro-sized crystals to overcome this fundamental drawback. In this review, we summarize the development of 3DED methods and their applications for structure elucidation of MOFs and COFs. Advances of 3DED data collection techniques are described, from step-wise rotation to continuous rotation of the crystal. The latter allows fast data collection which is crucial for beam sensitive materials including MOFs and COFs. Examples of ab initio structure determination of various MOFs and COFs by using 3DED are presented, with highlighted examples for solving the structures of mesoporous MOFs, mixed-metal MOFs, flexible MOFs, and for studying host–guest interactions. Finally, the accuracy and reproducibility of structure determination by 3DED are presented. We show the structure information obtained from 3DED provides crucial insights into structure–property relationships, which could further accelerate the development of new functional materials.