Two-dimensional materials with monolayer thickness and extreme aspect ratios are sought for their high surface areas and unusual physicochemical properties1. Liquid exfoliation is a straightforward and scalable means of accessing such materials2, but has been restricted to sheets maintained by strong covalent, coordination or ionic interactions3,4,5,6,7,8,9,10. The exfoliation of molecular crystals, in which repeat units are held together by weak non-covalent bonding, could generate a greatly expanded range of two-dimensional crystalline materials with diverse surfaces and structural features. However, at first sight, these weak forces would seem incapable of supporting such intrinsically fragile morphologies. Against this expectation, we show here that crystals composed of discrete supramolecular coordination complexes can be exfoliated by sonication to give free-standing monolayers approximately 2.3 nanometres thick with aspect ratios up to approximately 2,500:1, sustained purely by apolar intermolecular interactions. These nanosheets are characterized by atomic force microscopy and high-resolution transmission electron microscopy, confirming their crystallinity. The monolayers possess complex chiral surfaces derived partly from individual supramolecular coordination complex components but also from interactions with neighbours. In this respect, they represent a distinct type of material in which molecular components are all equally exposed to their environment, as if in solution, yet with properties arising from cooperation between molecules, because of crystallinity. This unusual nature is reflected in the molecular recognition properties of the materials, which bind carbohydrates with strongly enhanced enantiodiscrimination relative to individual molecules or bulk three-dimensional crystals.
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The data that support the findings of this study are available from the corresponding authors.
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This work was financially supported by the National Science Foundation of China (grant nos 21875136, 91856204 and 91956124), the National Key Basic Research Program of China (grant nos 2021YFA1200402, 2021YFA1501501 and 2021YFA1200300), the Shanghai Rising-Star Program (grant no. 19QA1404300), the National Research Foundation Singapore (grant no. NRF-CRP14-2014-01), the National University of Singapore and the Ministry of Education of Singapore (grant no. C-261-000-207-532/C-261-000-777-532), and the University of Bristol. Y.H. acknowledges the support of the CCF grant (no. FCC/1/1972‐19) from King Abdullah University of Science and Technology.
The authors declare no competing interests.
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Supplementary Information, including Figs. 1–77, Tables 1–4 and References.
Molecular dynamics simulation of l-glucose (orange colour) and d-glucose (green colour) binding to zero-dimensional discrete metallacycles of (R)-1 in acetonitrile solution.
Molecular dynamics simulation of l-glucose (orange colour) and d-glucose (green colour) binding to 3D bulk crystals of (R)-1 in acetonitrile solution.
Molecular dynamics simulation of l-glucose (orange colour) and d-glucose (green colour) binding to 2D monolayer nanosheets of (R)-1 in acetonitrile solution.
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Dong, J., Liu, L., Tan, C. et al. Free-standing homochiral 2D monolayers by exfoliation of molecular crystals. Nature 602, 606–611 (2022). https://doi.org/10.1038/s41586-022-04407-8
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