Precise Exfoliation of Covalent Organic Framework Monolayers and Bilayers

The happy marriage of pseudorotaxane and layered covalent organic framework (COF) gives birth to crystalline COF monolayers and bilayers.

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Covalent organic frameworks (COFs) are porous organic crystals connected via covalent bonds. Depending on how the covalent linkages propagate in space, COFs can be categorized as one, two and three-dimensional (1, 2 and 3D), respectively.1

2D COFs are “2D” because of their layered structures and non-covalent bonding in the c-axis compared to the in-plane direction. To make a truly 2D layer called “COFene” from COF would require the uncoupling of the binding forces between the layers.2, 3 Brute force method such as mechanical exfoliation or sonication usually gives ill-defined cleavage planes as well as non-uniformly thick flakes.4, 5

Can the space in 2D COF be partitioned using pre-fixed connections between a defined number of layers? We hypothesize that one way of achieving this is to introduce pseudorotaxane moieties into the backbone of COFs to weaken layer interaction, this would induce turbostratic stacking and facilitate their exfoliation subsequently. In our work entitled “Partitioning the interlayer space of covalent organic frameworks by embedding pseudorotaxanes in their backbones” in Nature Chemistry6, we designed crown-ether-based macrocyclic building units that can be connected with either one or two COF layers. Crown ethers complex with viologens to form pseudorotaxanes, therefore in the presence of viologen, the formation of host–guest complexes facilitate the self-exfoliation of the COFs into crystalline monolayers or bilayers (Fig. 1). The monolayer COF exhibits hydrophilic hydrazone linkages on one face and hydrophobic phenyl groups on the other face, forming a basis of research on Janus COF and a new type of organic Janus 2D materials.

Our studies show that mechanically interlocked molecular architectures are useful tools in partitioning the interlayer space in COFs to control the exfoliation of layers with well-defined thickness. In addition, pseudorotaxane COF bridges the field between molecular machines and 2D materials, paving the way for the integration of molecular shuttles that can be stimuli-responsive to pH, ions, light and electrons.

Fig. 1. Schematic synthesis of pseudorotaxane-based COF monolayers and bilayers.

To read more about our work at Nature Chemistry: https://www.nature.com/articles/s41557-020-00562-5

References

  1. Xu, H.-S. et al. Single crystal of a one-dimensional metallo-covalent organic framework. Nat. Commun. 11, 1434 (2020).
  2. Kandambeth, S., Dey, K. & Banerjee, R. Covalent Organic Frameworks: Chemistry beyond the Structure. J. Am. Chem. Soc. 141, 1807-1822 (2019).
  3. Li, X., Yadav, P. & Loh, K. P. Function-oriented synthesis of two-dimensional (2D) covalent organic frameworks – from 3D solids to 2D sheets. Chem. Soc. Rev. 49, 4835-4866 (2020).
  4. Li, X. et al. Molecular Engineering of Bandgaps in Covalent Organic Frameworks. Chem. Mater. 30, 5743-5749 (2018).
  5. Li, X. et al. Rapid, Scalable Construction of Highly Crystalline Acylhydrazone Two-Dimensional Covalent Organic Frameworks via Dipole-Induced Antiparallel Stacking. J. Am. Chem. Soc. 142, 4932-4943 (2020).
  6. Li, X. et al. Partitioning the interlayer space of covalent organic frameworks by embedding pseudorotaxanes in their backbones. Nat. Chem., DOI: 10.1038/s41557-020-00562-5.

 

Li Xing

Research Fellow, National University of Singapore

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