The crystallization problem is an outstanding challenge in the chemistry of porous covalent organic frameworks (COFs). Their structural characterization has been limited to modeling and solutions based on powder x-ray or electron diffraction data. Single crystals of COFs amenable to x-ray diffraction characterization have not been reported. Here, we developed a general procedure to grow large single crystals of three-dimensional imine-based COFs (COF-300, hydrated form of COF-300, COF-303, LZU-79, and LZU-111). The high quality of the crystals allowed collection of single-crystal x-ray diffraction data of up to 0.83-angstrom resolution, 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.

We report herein the first example of interpenetration isomerism in covalent organic frameworks (COFs). As a well-known three-dimensional (3D) COF, COF-300 was synthesized and characterized by the Yaghi group in 2009 as a 5-fold interpenetrated diamond structure ( dia-c5 topology). We found that adding an aging process prior to the reported synthetic procedure afforded the formation of an interpenetration isomer, dia-c7 COF-300. The 7-fold interpenetrated diamond structure of this new isomer was identified by powder X-ray diffraction and rotation electron diffraction analyses. Furthermore, we proposed a universal formula to accurately determine the number of interpenetration degrees of dia-based COFs from only the unit cell parameters and the length of the organic linker. This work not only provides a novel example to the category of interpenetration isomerism but also provides new insights for the further development of 3D COFs.

Crystal size effect is of vital importance in materials science by exerting significant influence on various properties of materials and furthermore their functions. Crystal size effect of covalent organic frameworks (COFs) has never been reported because their controllable synthesis is difficult, despite their promising properties have been exhibited in many aspects. Here, we report the diverse crystal size effects of two representative COFs based on the successful realization of crystal-size-controlled synthesis. For LZU-111 with rigid spiral channels, size effect reflects in pore surface area by influencing the pore integrity, while for flexible COF-300 with straight channels, crystal size controls structural flexibility by altering the number of repeating units, which eventually changes sorption selectivity. With the understanding and insight of the structure-property correlation not only at microscale but also at mesoscale for COFs, this research will push the COF field step forward to a significant advancement in practical applications.

Growth of covalent organic frameworks (COFs) as single crystals is extremely challenging. Inaccessibility of open‐structured single‐crystal COFs prevents the exploration of structure‐oriented applications. Herein we report for the first time a non‐interpenetrated single‐crystal COF, LZU‐306, which possesses the open structure constructed exclusively via covalent assembly. With a high void volume of 80 %, LZU‐306 was applied to investigate the intrinsic dynamics of reticulated tetraphenylethylene (TPE) as the individual aggregation‐induced‐emission moiety. Solid‐state 2H NMR investigation has determined that the rotation of benzene rings in TPE, being the freest among the reported cases, is as fast as 1.0×104 Hz at 203 K to 1.5×107 Hz at 293 K. This research not only explores a new paradigm for single‐crystal growth of open frameworks, but also provides a unique matrix‐isolation platform to reticulate functional moieties into a well‐defined and isolated state.

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