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On-surface synthesis of aromatic cyclo[10]carbon and cyclo[14]carbon

Nature volume 623, pages 972–976 (2023)Cite this article

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Abstract

All-carbon materials based on sp2-hybridized atoms, such as fullerenes1, carbon nanotubes2 and graphene3, have been much explored due to their remarkable physicochemical properties and potential for applications. Another unusual all-carbon allotrope family are the cyclo[n]carbons (Cn) consisting of two-coordinated sp-hybridized atoms. They have been studied in the gas phase since the twentieth century4,5,6, but their high reactivity has meant that condensed-phase synthesis and real-space characterization have been challenging, leaving their exact molecular structure open to debate7,8,9,10,11. Only in 2019 was an isolated C18 generated on a surface and its polyynic structure revealed by bond-resolved atomic force microscopy12,13, followed by a recent report14 on C16. The C18 work trigged theoretical studies clarifying the structure of cyclo[n]carbons up to C100 (refs. 15,16,17,18,19,20), although the synthesis and characterization of smaller Cn allotropes remains difficult. Here we modify the earlier on-surface synthesis approach to produce cyclo[10]carbon (C10) and cyclo[14]carbon (C14) via tip-induced dehalogenation and retro-Bergman ring opening of fully chlorinated naphthalene (C10Cl8) and anthracene (C14Cl10) molecules, respectively. We use atomic force microscopy imaging and theoretical calculations to show that, in contrast to C18 and C16, C10 and C14 have a cumulenic and cumulene-like structure, respectively. Our results demonstrate an alternative strategy to generate cyclocarbons on the surface, providing an avenue for characterizing annular carbon allotropes for structure and stability.

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Fig. 1: Reaction schemes for the formation of cyclo[10]carbon and cyclo[14]carbon.
Fig. 2: Four possible structures of cyclo[10]carbon.
Fig. 3: On-surface generated precursor, intermediates and product (C10).
Fig. 4: On-surface generated intermediates, product (C14) and AFM simulations of C14 with varying BLAs.

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All data supporting the findings of this study are available within the paper and its online Extended Data files.

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Acknowledgements

We acknowledge financial support from the National Natural Science Foundation of China (22125203).

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  1. These authors contributed equally: Luye Sun, Wei Zheng

Authors and Affiliations

  1. Interdisciplinary Materials Research Center, School of Materials Science and Engineering, Tongji University, Shanghai, People’s Republic of China

    Luye Sun, Wei Zheng, Wenze Gao, Faming Kang, Mali Zhao & Wei Xu

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Contributions

W.X. conceived the research. L.S., W.G. and F.K. performed the STM/AFM experiments and carried out the DFT calculations. W.Z. synthesized the C14Cl10 molecules. All authors contributed to writing the manuscript.

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Correspondence to Wei Xu.

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Extended data figures and tables

Extended Data Fig. 1 The bond lengths, Mayer bond orders and bond angles in a cyclo[10]carbon.

Calculations were conducted at the ωB97XD/6-311 + + G(d,p) level. The bond lengths and bond order in C10 are all nearly the same, indicating the structure of consecutive carbon-carbon double bonds, i.e., cumulenic structure (BLA = 0).

Extended Data Fig. 2 Real-space function analysis of cyclo[10]carbon.

(a to d) Localized orbital locator calculated based on in-plane π MOs (LOL-πin) and out-plane π MOs (LOL-πout). (a) and (c) correspond to isosurface maps of LOL-π = 0.4. (b) shows LOL-πin in the ring plane, and (d) shows LOL-πout above 1 Bohr of the ring plane. (e) Interaction region indicator (IRI) isosurface and (f) color-filled map of C10 showing the homogeneous covalent interactions in carbon-carbon bonds. (g) Standard coloring method and chemical explanation of sign(λ2)ρ on interaction region indicator (IRI) isosurfaces.

Extended Data Fig. 3 STM images of the C10Cl8 precursor and the product cyclo[10]carbon.

(a) C10Cl8 molecules separately adsorbed on bilayer NaCl/Au(111) surface. A single CO molecule appeared as a small depression. (b) Close-up STM image of single C10Cl8 molecules on NaCl. (c) Close-up STM image of C10. Scanning condition: (a) I = 1 pA, V = 0.3 V. (b) I = 2 pA, V = 0.3 V. (c) I = 0.5 pA, V = 0.3 V.

Extended Data Fig. 4 Other intermediates observed during manipulation.

(a-c) C10Cl6, (d-f) C10Cl1. The Laplace-filtered AFM images are also shown. Reference set point of Δz: I = 0.5 pA, V = 0.3 V for (b), I = 1 pA, V = 0.3 V for (e). The double bonds indicated by blue and black in (d)represent two different bond lengths within the structure, respectively.

Extended Data Fig. 5 DFT relaxed C10 and C14 structures on NaCl surface.

(a, b) C10 on Cl-top site. (c, d) C10 on Na-top site. (e, f) C14 on Cl-top site. (g, h) C14 on Na-top site.

Extended Data Fig. 6 AFM images of C10 acquired at the oscillation amplitude A = 50 pm.

We tested the effect of different amplitude on the AFM imaging of C10. The AFM image (a, b) and AFM simulations (c, d) were carried out at the oscillation amplitude A = 50 pm. The Laplace-filtered AFM images of (a) and (b) are also shown in (e) and (f). Reference set point of Δz: I = 0.5 pA, V = 0.3 V. The scale bar in (a) applies to all experimental, simulated and Laplace-filtered AFM images.

Extended Data Fig. 7 The bond lengths, Mayer bond orders and bond angles in a cyclo[14]carbon.

Calculations were conducted at the ωB97XD/6-311 + + G(d,p) level, revealing a small bond length alternation (BLA = 0.05 Å) and bond angle alternation (BAA = 25.3°) within C14. The double bonds indicated by blue and black in C14 represent two different bond lengths within the structure, respectively.

Extended Data Fig. 8 The bond lengths, Mayer bond orders and bond angles in a cyclo[18]carbon.

Calculations were conducted at the ωB97XD/6-311 + + G(d,p) level, revealing a bond length alternation (BLA = 0.12 Å) within C18.

Extended Data Fig. 9 STM images of the C14Cl10 precursor and the product cyclo[14]carbon.

(a) C14Cl10 and CO molecules separately adsorbed on a bilayer NaCl/Au(111) surface. (b, c) STM and AFM images of an individual C14Cl10 molecule. (d) Spectra of frequency shift (Δf) as a function of tip height (Δz). The red and blue spectra were taken at the topmost Cl atom of the C14Cl10 molecule, and the Cl atom at the first layer of NaCl surface, respectively. Inset: schematics of the CO-tip approaching processes. The distance between two topmost Cl atoms of C14Cl10 molecule extracted from the AFM image and experimental absolute height extracted from the spectra reasonably agree with the theoretical values. (e) Close-up STM image of C14. Scanning condition: I = 1 pA, V = 0.3 V for (a); I = 0.5 pA, V = 0.3 V for (b); I = 1 pA, V = 0.3 V for (e). Reference set point of Δz for (c): I = 0.5 pA, V = 0.3 V.

Extended Data Fig. 10 AFM simulations of C14 with varying BLAs.

(a-h) A series of AFM simulations of C14 with varying BLAs from cumulenic to intermediate to polyynic structures at decreasing tip-sample distances from left to right (i.e., BLA = 0 Å, 0.03 Å, 0.05 Å, 0.07 Å, 0.09 Å, 0.11 Å, 0.13 Å, 0.15 Å) followed by the method developed in ref. 13. The simulated AFM images within blue and red boxes are assigned to cumulene-like and polyynic structures, respectively. All atomic coordinates keep the BAA as 25.3°. The double bonds indicated by blue and black in (b), (c) and (d) represent two different bond lengths within the structures, respectively. The scale bar in (a) applies to all simulated AFM images.

Extended Data Fig. 11 The bond lengths in the C14Cl4 and C14Cl1 intermediates.

The bond lengths of a ten-membered carbon ring in the C14Cl4 intermediate (a) and a fourteen-membered carbon ring in the C14Cl1 intermediate (b) are listed. Calculations were conducted at the ωB97XD/6-311 + + G(d,p) level. The double bonds indicated by blue and black in C14Cl1 represent two different bond lengths within the structure, respectively.

Extended Data Fig. 12 Other intermediates and side reaction products observed during manipulations.

(a-c) C14Cl8, (d-f) C14Cl3, (g-i) C14Cl5, (j-l) C14Cl6. The Laplace-filtered AFM images are also shown. Reference set point of Δz: I = 0.5 pA, V = 0.3 V for (b) and (e), I = 1 pA, V = 0.3 V for (h), I = 0.2 pA, V = 0.3 V for (k). The scale bar in (b) applies to all experimental and Laplace-filtered AFM images.

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Sun, L., Zheng, W., Gao, W. et al. On-surface synthesis of aromatic cyclo[10]carbon and cyclo[14]carbon. Nature 623, 972–976 (2023). https://doi.org/10.1038/s41586-023-06741-x

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