First single-crystal x-ray diffraction structures of covalent organic frameworks

The research team led by Wei Wang from Lanzhou University, Junliang Sun from Peking University, and Omar M. Yaghi from the University of California-Berkeley, report a general approach involving the use of aniline as a modulator to grow high-quality large single crystals of porous COFs held together by strong imine bonds. For the first time, they succeeded in synthesizing large single 3D COFs crystals, and determined the crystal structures with atomic precision through BL17B Synchrotron-based Single-crystal X-ray diffraction technique. Their findings, titled “Single-crystal x-ray diffraction structures of covalent organic frameworks," were published in Science in July 2018.

The research pioneers the growth of large single COFs crystals and the precise anisotropic structure determination through single-crystal X-ray diffraction techniques. This work has addressed the long-standing challenge in the development of COFs, and furnishes a novel case for theoretical and applied research in dynamic covalent chemistry.

The crystallization problem is an outstanding challenge in the chemistry of porous COFs. Progress in developing their chemistry depends on under-standing their structure, but their synthesis often yields polycrystalline or amorphous products rather than single crystals. Although structures modeled from powder x-ray diffraction have advanced the field, definitive structural solutions by single-crystal XRD are needed for unambiguous solution and precise anisotropic refinement.

Researchers developed a general approach involving the use of aniline as a modulator to grow high-quality large single crystals of porous COFs. The addition of aniline increases the overall concentration of amine in the reaction, enhancing the reversibility of imine bond formation and the process of error corrections, and ultimately, crystallization. As a result of adding aniline modulator to the reaction mixture, single crystals of various imine-based 3D COFs were obtained: COF-300, COF-303, LZU-79, and LZU-111. The high quality of the crystals allowed collection of SCXRD data of up to 0.83-angstrom resolution with synchrotron light source (BL17B), leading to unambiguous solution and precise anisotropic refinement. Characteristics such as degree of interpenetration, arrangement of water guests, the reversed imine connectivity, linker disorder, and uncommon topology were deciphered with atomic precision—aspects impossible to determine without single crystals.