Observation of Interpenetrated Topology Isomerism for Covalent Organic Frameworks with Atom-Resolution Single Crystal Structures

Abstract

Rational control and understanding of isomerism are of significance but still remain a great challenge in reticular frameworks, in particular, for covalent organic frameworks (COFs) due to the complicated synthesis and energy factors. Herein, reaction of 3,3′,5,5′-tetra(4-formylphenyl)-2,2′,6,6′-tetramethoxy-1,1′-biphenyl (TFTB) with 3,3′,5,5′-tetrakis(4-aminophenyl)bimesityl (TAPB) under different reaction conditions affords single crystals of two 3D COF isomers, namely, USTB-20-dia and USTB-20-qtz. Their structures with resolutions up to 0.9−1.1 Å have been directly solved by three-dimensional electron diffraction (3D ED) and synchrotron single crystal X-ray diffraction, respectively. USTB-20-dia and USTB-20-qtz show rare 2 × 2-fold interpenetrated dia-b nets and 3-fold interpenetrated qtz-b frameworks. Comparative studies of the crystal structures of these COFs and theoretical simulation results indicate the crucial role of the flexible molecular configurations of building blocks in the present interpenetrated topology isomerism. This work not only presents the rare COF isomers but also gains an understanding of the formation of framework isomerism from both single crystal structures and theoretical simulation perspectives.

Dynamic Entwined Topology in Helical Covalent Polymers Dictated by Competing Supramolecular Interactions

“…Additionally, it has been demonstrated that minor changes to the structure, such as larger counter‐ions or the introduction of steric interactions, can change the interpenetration, stacking between layers, and topology [23–27] . Elucidating the crystal structures of the polymers through single‐crystal XRD (SCXRD) or electron diffraction techniques enabled the detailed analysis of polymeric structures at the atomic level [8,18,22,28–36] . This provides a deeper understanding of the synergistic interplay between DCvC and supramolecular chemistry.…”

Encoding ordered structural complexity to covalent organic frameworks

“…In this context, the structural complexity of COFs refers to the number of non-equivalent sites that can be used to encode chemical heterogeneity, which is essentially related to the size of the crystallographic asymmetric unit in the COF structure. In reticular chemistry, complexity can be achieved by topological design, where specific connectivity between molecular building blocks can direct them to form complex and ordered structures in mathematically predictable ways 7 – 9 . For COFs, pursuing new topologies often requires lengthy synthesis of multi-dentate molecular building blocks and faces formidable crystallization challenges associated with the strong covalent linkage.…”

“…In COF chemistry, covalent linkages play the dominant structural directing role, which is the chemical basis for topological synthetic design 14 . However, structural dynamics, transformation, and isomorphism of COFs all indicate that there is still plenty of room for structure design through non-covalent interactions, which is associated with different bonding conformations and packing modes of molecular building blocks 9 , 15 – 17 . Thus, even if the types of molecular building blocks and their covalent connectivity are pre-determined, there would still be a considerable number of plausible COF structures with different bonding conformations and molecular packing modes that span a chemical space available for structural design.…”

Toward the Next Generation of Permanently Porous Materials: Halogen-Bonded Organic Frameworks

“…Due to their highly exothermic connectivity, dynamic covalent and metal–ligand bonding often exhibit rapid nucleation with minimal defect correction during assembly, resulting in small crystallites . COFs generally form crystalline domains <1 μm, though recent advances have achieved crystallite sizes of ∼200 μm. , MOFs can achieve domains >100 μm when synthesized in the presence of coordinative modulators or when the coordination bonding is relatively weak . On the other hand, HOFs form through relatively weaker, more reversible noncovalent hydrogen bonds (2–10 kcal mol –1 ), allowing for greater defect correction and control over assembly to form larger crystallites (>1 cm) …”