Single-crystal X-ray diffraction is a powerful characterization technique that enables the determination of atomic arrangements in crystalline materials. Growing or retaining large single crystals amenable to it has, however, remained challenging with covalent organic frameworks (COFs), especially suffering from post-synthetic modifications. Here we show the synthesis of a flexible COF with interpenetrated qtz topology by polymerization of tetra(phenyl)bimesityl-based tetraaldehyde and tetraamine building blocks. The material is shown to be flexible through its large, anisotropic positive thermal expansion along the c axis (αc = +491 × 10–6 K–1), as well as through a structural transformation on the removal of solvent molecules from its pores. The as-synthesized and desolvated materials undergo single-crystal-to-single-crystal transformation by reduction and oxidation of its imine linkages to amine and amide ones, respectively. These redox-induced linkage conversions endow the resulting COFs with improved stability towards strong acid; loading of phosphoric acid leads to anhydrous proton conductivity up to ca. 6.0 × 10−2 S cm−1.
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All data supporting the finding of this study are available within this article and its supplementary information. Crystallographic data for the structures in this article have been deposited at the Cambridge Crystallographic Data Centre under deposition nos. CCDC 2162845 (USTB-5), 2162846 (USTB-5r), 2162847 (USTB-5o), 2162848 (recovered USTB-5), 2162849 (USTB-5r obtained in bulky production) and 2234439 (H3PO4@USTB-5o). Copies of the data can be obtained free of charge from https://www.ccdc.cam.ac.uk/structures/. Source data are provided with this paper. In addition, the data that support the findings of this study and the raw data for all the figures have been uploaded to Figshare59 at https://doi.org/10.6084/m9.figshare.20446014.
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J.J. was supported by the Natural Science Foundation of China (nos. 22235001, 22175020 and 21631003), the Fundamental Research Funds for the Central Universities (no. FRF-BD-20-14A) and the University of Science and Technology Beijing. H. Wang was supported by the Natural Science Foundation of China (no. 22131005), Xiaomi Young Scholar Program, and University of the Science and Technology Beijing. R.-B.L. was supported by the Natural Science Foundation of China (nos. 22101307 and 22090061) and Hundred Talents Program of Sun Yat-Sen University. G.X. was supported by the Natural Science Foundation of China (nos. 22171263 and 91961115). S.L. was supported by the Natural Science Foundation of China (no. 22178012). We thank M. O’Keeffe at the Arizona State University for helpful discussion on structure expression. In addition, we also thank the staff from BL17B1 beamline of National Facility for Protein Science in Shanghai (NFPS) at Shanghai Synchrotron Radiation Facility for assistance during data collection.
The authors declare no competing interests.
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Optical microscopy photos of USTB-5 to show the crystallization kinetics together with the SEM photos of USTB-5a, USTB-5ra, and USTB-5oa. [Note]: It should be noted that, under the optimized reaction conditions, the single crystal size of USTB-5 amounts to ca. 50 μm after only 9 hours reaction, which quickly increases to ca. 200 μm after 24 hours. Five days later, single crystals of USTB-5 with the size up to ca. 450 μm and scalability over 0.4 g could be obtained.
(a) Asymmetric unit in the single-crystal structure of USTB-5 (2162845). Thermal ellipsoids are drawn with 50% probability. Carbon atom C1 and nitrogen atom N1 in an imine bond are co-occupancy disordered, and only a carbon hydrogen atom therefore appears around the co-occupancy position. (b) The complicated helical tube structure in USTB-5 is composed of two-strand helices and two single helices with atoms in individual helix shown in pink, green, and blue color, respectively. (c) Crystal structure of USTB-5 with three-fold interpenetration nets with all atoms in individual framework shown in pink, green, and blue color, respectively. (d) A qtz topology for USTB-5 with TFPB and TAPB simplified as the same 4-connected node. [Note]: The intercrossing voids between different interpenetrated nets shown in Extended Data Fig. 2b allow both the guest-induced dynamic transformation and post-synthetic chemical conversion depending on the reagent diffused to the imine sites.
Temperature-dependent unit cell parameters a, b, c, and V for USTB-5.
(a) Asymmetric unit in the single-crystal structure of USTB-5r (2162846). Thermal ellipsoids are drawn with 50% probability. Carbon atom C1 and nitrogen atom N1 in amine bond are co-occupancy disordered, and two carbon hydrogen atoms and a nitrogen hydrogen atom therefore appear around the co-occupancy position. (b) Crystal structure of USTB-5r containing three-fold interpenetration nets with all atoms in individual framework shown in pink, green, and blue color, respectively. [Note]: The low diffraction resolution and structure disorder of USTB-5r prevent us from distinguishing the amine bond length in this compound and the imine bond length in USTB-5.
(a, b) Asymmetric unit in the single-crystal structure of USTB-5o (2162847), which is divided into two parts for clarity. Thermal ellipsoids are drawn with 50% probability. For Extended Data Fig. 5a, carbon atoms C5 and C5A and nitrogen atoms N2 and N2A in amide bond are co-occupancy disordered, respectively, and hydrogen atom on amide nitrogen atom is obscured by oxygen atom in the present case. (c) Crystal structure of USTB-5o containing three-fold interpenetration nets with all atoms in individual framework shown in pink, green, and blue color, respectively.
(a) Structural model of USTB-5a containing three-fold interpenetration nets with all atoms in individual framework shown in pink, green, and blue color, respectively. Unit cell parameters are a = b = 32.6300 Å, c = 24.6860 Å, α = β = 90°, γ = 120°, and V = 22762.3 Å3 (Z = 18). (b) Hysteretic N2 sorption isotherms at 77 K with the first and second step corresponding to the flexible structure of USTB-5a and USTB-5, respectively. Inset: the structural difference between USTB-5a and USTB-5 along c axis, as mainly indicated by the different pitches for their double-strand helical secondary building blocks.
(a) Structural model of USTB-5oa containing three-fold interpenetration nets with all atoms in individual framework shown in pink, green, and blue color, respectively. Unit cell parameters are a = b = 32.6000 Å, c = 25.3600 Å, α = β = 90°, γ = 120°, and V = 23340.8 (Z = 18). (b) Hysteretic N2 sorption isotherms at 77 K with the first and second step corresponding to the flexible structure of USTB-5oa and USTB-5o, respectively. Inset: the structural difference between USTB-5oa and USTB-5o along c axis, as indicated by the different pitches for their double-strand helical secondary building blocks.
Supplementary Figs. 1–59, Tables 1–14, references and disclaimer.
Crystallographic data for USTB-5.
Crystallographic data for USTB-5r.
Crystallographic data for USTB-5o.
Crystallographic data for recovered USTB-5.
Crystallographic data for USTB-5r obtained in bulky production.
Crystallographic data for H3PO4@USTB-5o.
Data for Supplementary Figs. 19, 21–23, 24a, 42a,b, 44, 45 and 47–50.
Data for Supplementary Fig. 43.
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Yu, B., Lin, RB., Xu, G. et al. Linkage conversions in single-crystalline covalent organic frameworks. Nat. Chem. (2023). https://doi.org/10.1038/s41557-023-01334-7