Synthesis of three-dimensional covalent organic framework films and their electroactivity

The high porosity and crystallinity of covalent organic frameworks (COFs) hints to a great potential for this class of material in both mass and charge transport. However, the powdered state of COFs from the conventional solvothermal synthesis hampers applications that require internal continuity, e.g. electroactive COFs where grain boundaries interrupt the successive charge transportation.[1] We have explored two methods to solve this problem, which we have successfully applied to make two and three-dimensional COFs, both of which builds upon a spatial kinetic control of bond formation. The first method is based on a solid-liquid interface. Anchoring groups are chemically attached to a surface, and as reagents are flown over the surface, the polymerization reaction occurs predominantly on the surface. We have shown that the key for making smooth films is to work at low concentrations.[2] This because the reaction between monomers and the surface follows first order kinetics,[3] while the coupling between monomers in the bulk follow second order kinetics. We have further shown that by flowing different monomers sequentially over the surface, it is possible to achieve layered COFs,[4] potentially enabling a method to incorporate different functions in different layers in a film. The electroactivity of the made COFs has also been explored.[3, 5] The second method is based on a liquid-liquid interfacial approach.[6] By distributing reactants into separate phases using hydrophobicity, the chemical reaction is confined to an interface that orients the crystal growth. A molecular-smooth interface combined with in-plane isotropic conditions enables the synthesis of films on a centimetre size scale with a uniform thickness of 10-100 nm, and with the lattice alignment clearly seen under transmission electron microscope.[6a] The fabricated films exhibit considerable mechanical robustness, for instance a 13 nm homogenously thick film can have a free-standing length of 37 µm. The explored method is supposed to be universal to all imine type three-dimensional covalent crystal films since it is based on general chemical considerations.

References

[1] Y. Yang, K. Börjesson, Trends Chem. 2022, 4, 60-75.

[2] M. Ratsch, C. Ye, Y. Yang, A. Zhang, A. M. Evans, K. Börjesson, J. Am. Chem. Soc. 2020, 142, 6548-6553.

[3] Y. Yang, S. Mallick, F. Izquierdo-Ruiz, C. Schäfer, X. Xing, M. Rahm, K. Börjesson, Small 2021, 17, 2103152.

[4] Y. Yang, M. Ratsch, A. M. Evans, K. Börjesson, J. Am. Chem. Soc. 2023, 145, 18668-18675.

[5] Y. Yang, C. Schäfer, K. Börjesson, Chem 2022, 8, 2217-2227.

[6] a) Y. Yang, Y. Chen, F. Izquierdo-Ruiz, C. Schäfer, M. Rahm, K. Börjesson, Nat. Commun. 2023, 14, 220; b) Y. Yang, A. P. Sandra, A. Idström, C. Schäfer, M. Andersson, L. Evenäs, K. Börjesson, J. Am. Chem. Soc. 2022, 144, 16093-16100.