We designed and synthesized A 2 B 2 type tetraphenyl benzenemonomers (p-, m-, and o-TetPB) whichhave the para-,meta, and ortho-substituted isomeric structures,for the direct construction of isomeric frameworks.I nterestingly,b oth kagome (kgm)a nd monoclinic square (sql)f ramework isomers are produced from either p-TetPB (C 2h symmetry) or m-TetPB (C 2v symmetry) by changing reaction solvents,w hile their isomeric structures are characterized by X-rayd iffraction, computational simulation, microscopy, and sorption isotherm measurements.O nly sql frameworks was formed for o-TetPB (C 2v symmetry), irrespective of reaction solvents.These results disclose au nique feature in the framework structural formation, that is,the geometry of monomers directs and dominates the lattice growth process while the solvent playsar ole in the perturbation of chain growth pattern. The isomeric frameworks exhibit highly selective adsorption of vitamin B 12 owing to pore shape and sized ifferences.
Structure determination of covalent organic frameworks (COFs) with atomic precision is a bottleneck that hinders the development of COF chemistry. Although three‐dimensional electron diffraction (3D‐ED) data has been used to solve structures of sub‐micrometer‐sized COFs, successful structure solution is not guaranteed as the data resolution is usually low. We demonstrate that the direct‐space strategy for structure solution, implemented using a genetic algorithm (GA), is a successful approach for structure determination of COF‐300 from 3D‐ED data. Structural models with different geometric constraints were considered in the GA calculations, with successful structure solution achieved from room‐temperature 3D‐ED data with a resolution as low as ca. 3.78 Å. The generality of this strategy was further verified for different phases of COF‐300. This study demonstrates a viable strategy for structure solution of COF materials from 3D‐ED data of limited resolution, which may facilitate the discovery of new COF materials in the future.
Electron microscopy has been widely used in the structural analysis of proteins, pharmaceutical products, and various functional materials in the past decades. However, one fact is often overlooked that the crystal structure might be sensitive to external environments and response manners, which will bring uncertainty to the structure determination and structure-property correlation. Here, we report the atomic-level ab initio structure determinations of microcrystals by combining 3D electron diffraction (3D ED) and environmental transmission electron microscope (TEM). Environmental conditions, including cryo, heating, gas and liquid, have been successfully achieved using in situ holders to reveal the simuli-responsive structures of crystals. Remarkable structural changes have been directly resolved by 3D ED in one flexible metal-organic framework, MIL-53, owing to the response of framework to pressures, temperatures, guest molecules, etc.
Targeted synthesis of kagome (kgm) topologic 2D covalent organic frameworks remains challenging, presumably due to the severe dependence on building units and synthetic conditions. Herein, two isomeric “two‐in‐one” monomers with different lengths of substituted arms based on naphthalene core (p‐Naph and m‐Naph) are elaborately designed and utilized for the defined synthesis of isomeric kgm Naph‐COFs. The two isomeric frameworks exhibit splendid crystallinity and showcase the same chemical composition and topologic structure with, however, different pore channels. Interestingly, C60 is able to uniformly be encapsulated into the triangle channels of m‐Naph‐COF via in situ incorporation method, while not the isomeric p‐Naph‐COF, likely due to the different pore structures of the two isomeric COFs. The resulting stable C60@m‐Naph‐COF composite exhibits much higher photoconductivity than the m‐Naph‐COF owing to charge transfer between the conjugated skeletons and C60 guests.
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