Heterogeneity represents one of the most prevalent polycrystalline phenomena in both the natural world and scientific realm, playing a significant role in functional materials. Due to variations in the spatial arrangement of identical structural units, different topological heterogeneities in MOFs or SMFs often exhibit distinct structural characteristics, consequently showcasing diverse functionalities. In contrast, heterogeneity is seldom observed in COFs, which may be attributed to the inherent strong interactions between constituent segments and the challenges in determining COF structures.

On November 9, 2023, Professor Jianzhuang Jiang's research team and Professor Hailong Wang's team from University of Science and Technology Beijing, in collaboration with Professor Wei Zhou's team from the Neutron Research Center at the National Institute of Standards and Technology in the United States, published an article titled "Observation of Interpenetrated Topology Isomerism for Covalent Organic Frameworks with Atom-Resolution Single Crystal Structures" in the "Journal of the American Chemical Society," with an impact factor of 15.0. The paper showcases rare isomers of Covalent Organic Frameworks (COFs) and employs 3D Electron Diffraction (3D ED) and Single Crystal X-ray Diffraction (SSXRD) techniques to characterize the crystal structure of these isomers. The research distinctly reveals the crucial role of flexible molecular configurations in the energetics of frameworks and the formation of COF isomers.

ReadCrystal Technology participated in this research, contributing to the microcrystal structure analysis of COF materials with the assistance of MicroED technology (also known as 3D ED technology or cRED technology).

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.

Construct USTB-20-dia and USTB-20-qtz

TFTB and TAPB underwent the following condensations under different conditions. USTB-20-dia single crystals in pale yellow blocks were obtained after a 10-day reaction at room temperature in tetrahydrofuran (THF); while white USTB-20-qtz was obtained after a 2-day reaction at 50°C in dichloromethane (DCM).

The single crystal structure of USTB-20-dia was determined using 3D electron diffraction (3D ED) technique at a low temperature of 100.0 K, yielding high-resolution data (∼0.9 Å).

MicroED (also known as 3D ED or cRED) analysis of the crystal structure was assisted by ReadCrystal. The COF material sent for testing had a minimum crystal size of approximately 500 nm. ReadCrystal  collected more than 12 sets of data, and the best 4 sets were selected for combined processing, completing the structure analysis in 7 days.

Successfully, SSXRD (Single-crystal X-ray Diffraction) was employed to analyze USTB-20-qtz, achieving a resolution as high as 1.1 Å.

From the obtained crystal structures of the two COF isomers, TFTB and TAPB exhibit flexible configurations in terms of variable dihedral angles between adjacent benzene rings. This results in the present structural isomerism.

Characterization of COF isomers and gas/vapor adsorption

In the Fourier-transform infrared (FT-IR) spectra of USTB-20-dia and USTB-20-qtz, characteristic C=N bands were observed at 1627 cm⁻¹. Additionally, a faint C=O stretching band was retained at 1700 cm⁻¹, while the stretching band of N−H bonds disappeared at 3344 ~ 3461 cm⁻¹. These changes between the monomers and COFs confirm the successful imine condensation between the aldehyde groups of TFTB and the amine groups of TAPB.

After activation to produce USTB-20a-dia, PXRD peaks at 3.95°, 4.13°, and 5.86° were observed, consistent with the prepared covalent organic framework, indicating a rigid framework for USTB-20-dia. In contrast, activation of USTB-20-qtz resulted in changes in the PXRD pattern of the activated USTB-20a-qtz compared to the fresh sample, with the activated phase having a more compressed structure compared to the prepared phase.

The active materials USTB-20a-dia and USTB-20a-qtz exhibited stable mass at 400°C, as determined by thermogravimetric analysis under N2 atmosphere, indicating their porous nature and good thermal stability. The N2 adsorption isotherm of USTB-20a-dia at 77 K exhibited a Type I curve, suggesting its microporous nature.

The BET specific surface area of USTB-20a-dia was 2436 m² g⁻¹. Compared to USTB-20a-dia, its isomer USTB-20a-qtz also exhibited a Type I N₂ adsorption curve, but with a lower N₂ volume at 1.0 bar and 77 K, measuring 443 cm³ g⁻¹ (Figure 3f). The BET surface area of activated USTB-20-qtz was only 1672 m² g⁻¹. It is worth noting that the experimental pore volume of USTB-20a-qtz was 0.69 cm³ g⁻¹, significantly lower than the theoretical value of 1.15 cm³ g^-1 obtained from synchrotron crystal structure simulations, indicating a compressed structure of the activated material compared to the prepared material.

Formation Mechanism of COF Isomers

For the dia-b topological framework, ranging from dia-c(1), dia-c(1 × 1)a, dia-c(1 × 1)b, dia-c(2 × 1) to dia-c(2 × 2) (where 'a' and 'b' denote different symmetries within interpenetrating double frameworks), an increase in the number of interpenetrating networks leads to increased density and decreased relative energy due to enhanced van der Waals interactions between adjacent networks. This trend is also observed for the qtz-b topological framework, from qtz-c(1) to qtz-c(3). The relative energy of the non-interpenetrating dia-c(1) framework is slightly lower than that of qtz-c(1), indicating better stability for materials with the dia-b topological structure. The comparison of crystal structures of COF isomers suggests the presence of tension in USTB-20-qtz, supporting this conclusion. The qtz-c(3) structure, with a triple-interpenetrating network, exhibits slightly higher relative energy than the dia-c(2 × 2) structure with a 2 × 2-interpenetrating network, indicating better stability for dia-c(2 × 2) (i.e., USTB-20-dia).