On-surface synthesis and characterization of anti-aromatic cyclo[12]carbon and cyclo[20]carbon

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Nature Communications volume 15, Article number: 7649 (2024 ) Cite this article Carbon Blocks Blast Furnace

On-surface synthesis and characterization of anti-aromatic cyclo[12]carbon and cyclo[20]carbon | Nature Communications

Cyclo[n]carbons have recently attracted significant attention owing to their geometric and electronic structures remaining largely unexplored in the condensed phase. In this work, we focus on two anti-aromatic cyclocarbons, namely C12 and C20. By designing two fully halogenated molecular precursors both including 4-numbered rings, we further extend the on-surface retro-Bergman ring-opening reaction, and successfully produce C12 and C20. The polyynic structures of C12 and C20 are unambiguously revealed by bond-resolved atomic force microscopy. More importantly, subtly positioning the C20 molecule into an atomic fence formed by Cl clusters allows us to experimentally probe its frontier molecular orbitals, yielding a transport gap of 3.8 eV measured from scanning tunneling spectroscopy. Our work may advance the field by easier synthesis of a series of cyclocarbons via on-surface retro-Bergman ring-opening strategy.

Molecular carbon allotropes, cyclo[n]carbons (Cn) have fascinated a number of experimentalists and theoreticians because of their unique structures and potential applications. Nevertheless, their synthesis in condensed phase is hardly achieved due to the high reactivity, resulting in the geometric and electronic structures of these species remaining largely unexplored1,2,3,4,5,6,7,8,9,10,11,12,13,14,15. On-surface synthesis has demonstrated advantage for the generation of a series of cyclocarbons, for example, C1816,17, C1618, C1419, C1019 on the NaCl surface, and their geometric structures have been revealed in real space. However, endeavors to synthesize larger cyclocarbons (e.g., C20 or C24) have encountered difficulties owing to the unstability of precursor molecules18. Meanwhile, it has been extremely challenging to probe the electronic structures of cyclocarbons due to their high mobilities on the NaCl surface.

For smaller anti-aromatic cyclocarbons, e.g., C12, the combination of increased strain and reduced stability further complicates their synthesis. Specifically, theory predicted that the bond angle alternation drastically changes from C16 (BAA = 0.03°) to C12 (BAA = 36.9°)11. On the other hand, for larger ones (e.g., n > 18), monocyclic, bicyclic or polycyclic isomers were predicted by theory or identified experimentally in the gas phase20,21,22. Especially, the carbon cluster (n = 20) is considered to be the transition point from monocyclic to bicyclic/polycyclic structures23,24,25,26. Thus, synthesizing and characterizing the geometric and electronic structures of C12 and C20 in the condensed phase are of particular interest.

Considering the anti-aromatic nature of these two cyclocarbons, herein, we designed and synthesized two molecular precursors both including 4-numbered rings, i.e., a fully halogenated biphenylene (1,4,5,8-tetraiodo-2,3,6,7-tetrabromobiphenylene, C12Br4I4) and dibenzo[b,h]biphenylene (perchlorodibenzo[b,h]biphenylene, C20Cl12). Through scanning tunneling microscopy (STM) tip-induced dehalogenation and accompanied retro-Bergman ring-opening reactions, C12 was successfully generated on the surface (Fig. 1a). Such a strategy is also very recently followed by Gross and Anderson’s groups to produce C1327. Moreover, through on-surface transformation from hexagon-tetragon (6–4) carbon rings to pentalene moiety (5–5) and followed by dehalogenation and more complicated ring-opening reactions, C20 was also achieved (Fig. 1b).

a Reaction scheme for the formation of C12 (tetragon carbon ring of the precursor colored in blue). b Reaction scheme for the formation of C20 through transformation from hexagon-tetragon (6–4) carbon rings (colored in blue) to pentalene moiety (5–5) (colored in green) and followed by dehalogenation and ring-opening reactions.

For anti-aromatic cyclo[12]carbon, two polyynic structures with D6h and C6h symmetries (Supplementary Fig. 1a, b) are considered. Based on modern theories11,12, it is predicted that the lowest energy geometry of C12 is C6h polyynic structure (Supplementary Fig. 1b), featuring a bond length alternation (BLA = 0.13 Å), and a bond angle alternation (BAA = 36.9°) (Supplementary Fig. 2).

To generate C12, C12Br4I4 molecules were introduced on the cold sample held at ~6 K. All molecules were studied on a 1-3 ML NaCl/Au(111) surface at 4.7 K. High-resolution atomic force microscopy (AFM) image acquired with CO-terminated tip revealed both the carbon skeleton (6-4-6-membered ring) and eight halogen atoms of the C12Br4I4 molecule (Fig. 2a II), which were also clearly shown in the AFM simulation and Laplace-filtered AFM images (Fig. 2a III and a IV). To trigger dehalogenation reactions, the tip was initially positioned on a single C12Br4I4 molecule, and retracted by ~3 Å from a setpoint (typically I = 0.4 pA, V = 0.3 V), after that, ~3 V pulse was applied on the molecule with currents on the order of a few pA. Normally, one or two iodine atoms were first removed, leading to the formation of C12Br4I3 (Supplementary Fig. 3a) or C12Br4I2 (Fig. 2b) intermediates. The AFM image (Fig. 2b II) revealed that the first-step retro-Bergman ring-opening reaction has already occurred in a C12Br4I2 molecule, leading to the formation of an 8-membered ring (also refer to Fig. 1a). In addition, two characteristic bright features corresponding to the carbon-carbon triple bonds were observed (also see our calculations in Supplementary Fig. 4a and the simulation in Fig. 2b III), which are more clearly shown in the Laplace-filtered AFM image (Fig. 2b IV). Further dehalogenation and accompanied second-step retro-Bergman reaction led to the formation of C12Br3 intermediate (Fig. 2c II), which was imaged as a larger carbon ring and three Br atoms attached, where four characteristic bright features corresponding to the triple bonds could be distinguished, as also shown in the AFM simulation and Laplace-filtered AFM image (Fig. 2c III and c IV). The bond lengths calculation of such a 12-membered ring was shown in Supplementary Fig. 4b. Other observed intermediates during pulses were shown in Supplementary Fig. 3b–d.

a I–a IV, b I–b IV, c I–c IV, d I–d IV Molecular structures, AFM images, AFM simulations, and Laplace-filtered AFM images of precursor, typical intermediates, and product. AFM simulations are based on gas-phase DFT-calculated geometries. The scale bar in (a II) applies to all images of precursor and intermediates, and the scale bar in (d II) applies to all images of product. All molecules in Fig. 2 were studied on a 3 monolayer (ML) NaCl/Au(111) surface. Reference set point of Δz for a II to d II: I = 0.4 pA, V = 0.3 V.

Subsequent voltage pulses ( ~ 3-3.5 V) could induce complete dehalogenation of intermediates (e.g., Fig. 2c and Supplementary Fig. 3b–d), resulting in the formation of the final products as shown in the AFM images (Fig. 2d and Supplementary Fig. 5). AFM images show a single carbon ring with no halogen atoms attached, which can be unambiguously recognized as a C12. More importantly, six pronounced characteristic bright features are clearly distinguished in the AFM images, which were assigned to six triple bonds of C12, that means, the anti-aromatic C12 also adopts an energetically favorable polyynic structure as the ground state, similar to the case of C1618. The assignment of six triple bonds is also supported by AFM simulation (Fig. 2d III). It is noticeable that C12 possesses a moderate bond length alternation (BLA C12 = 0.13 Å) (Supplementary Fig. 2) among already generated cyclocarbons on the surface (i.e., BLA C18 = 0.12 Å, BLA C16 = 0.15 Å, BLA C14 = 0.04 Å, BLA C10 = 0 Å, calculated at the ωB97XD/def2-TZVP level11). In the close tip-sample height (Supplementary Fig. 6), bright lines appear between triple bonds, which should originate from the tip-tilting effect28. The DFT calculations of C12 on the NaCl surface at different sites show near-planar adsorption configurations (Supplementary Fig. 7). Our experimental results verify the polyynic structure of C12 predicted by modern theories, while, the D6h or C6h symmetries cannot be distinguished in AFM images17.

During our experiments, we realized that the thickness of NaCl is a rather important point for the successful generation of C12. We have respectively tried different thickness of NaCl, e.g., 1 ML, 2 ML, 3 ML, and found out that the yield is highest on the 1 ML NaCl surface. We have attempted 15 precursors and successfully generated 7 individual C12 on the 1 ML NaCl surface with a yield of about 46.7%. We have attempted 8 precursors and successfully generated 1 individual C12 on the 2 ML NaCl surface with a yield of 12.5%. We have attempted 12 precursors and successfully generated 1 individual C12 on the 3 ML NaCl surface with a yield of about 8.3%. The main reason for the unsuccessful attempts is the disturbance of intermediates during imaging on the NaCl surfaces. At given STM setpoints the tip is closer to the molecule with increasing NaCl thickness, and thus more likely displaces the molecule during STM imaging.

To generate larger anti-aromatic cyclocarbon, e.g., C20, C20Cl12 molecules were introduced on the cold sample held at ~6 K. All molecules were studied on a 1 ML NaCl/Au(111) surface at 4.7 K. AFM and STM images (Fig. 3a II and a III, also Supplementary Fig. 8) revealed its nonplanar adsorption configuration on the NaCl surface due to steric hindrance (refer to the model in Fig. 3a IV)29. To induced dehalogenation reactions, the tip was initially positioned on a single C20Cl12 molecule, and retracted by ~4 Å from a setpoint (typically I = 5 pA, V = 0.3 V), after that, ~4 V pulse was applied on the molecule with currents on the order of a few pA. Subsequent high-resolution AFM images (Fig. 3b and also Supplementary Fig. 9) showed that the typically observed intermediates became more planar as a result of dehalogenation at different sites. In addition, it is noted that the carbon skeleton of each observed intermediate molecule has undergone the transformation from a 6–4 configuration to pentalene moiety (5-5) accompanied with dehalogenation (as shown in Fig. 1b), which was also observed in other on-surface reaction systems30 and gas-phase experiments31.

a I–a IV C20Cl12. b I–b IV C20Cl9. c I–c IV C20Cl2 with a 10-membered ring. d I–d IV C20Cl2 with 8- and 10-membered rings. e I–e IV C20Cl4 with a 9-membered ring. f I–f IV C20Cl2 with a 13-membered ring. g I–g IV C20Cl2 with a 16-membered ring. Structures (a I–g I), AFM images (a II–g II), AFM simulations (b III–g III), Laplace-filtered AFM images are shown (b IV–g IV). AFM simulations are based on gas-phase DFT-calculated geometries. The scale bar in (a II) applies to all images. Reference set point of Δz for a II to g II: I = 5 pA, V = 0.3 V.

Additional pulses ( ~ 4 V) can induce dehalogenation and occurrence of different kinds of ring-opening reactions, e.g., retro-Bergman19,32,33,34, formation of Sondheimer-Wong diyne35,36. Figure 3c shows the generation of a C20Cl2 intermediate consisting of a 10-membered ring (colored in blue) via a retro-Bergman reaction (the calculated BLAs within this 10-membered ring are shown in Supplementary Fig. 10). More interestingly, Fig. 3d shows the generation of another C20Cl2 intermediate consisting of a newly formed 8-membered ring, indicating the pentalene moiety has changed (additional AFM images in Supplementary Fig. 11). Such a reaction is understood to be the formation of Sondheimer-Wong diyne36 (the reaction scheme is shown in Supplementary Fig. 12 together with the calculated BLAs within this 8-membered and 10-membered rings).

Furthermore, intermediates with larger odd-membered carbon rings were also observed as shown in Fig. 3e, f. For example, a 9-membered ring was formed within the C20Cl4 intermediate (Fig. 3e), in which two characteristic bright features assigned to triple bonds were revealed (see calculations on BLAs in Supplementary Fig. 13a). An even larger 13-membered ring was also observed within another C20Cl2 intermediate (Fig. 3f), in which both characteristic bright features (for triple bonds) and uniform line features (for cumulene) were observed in this peculiar ring (see calculations on BLAs in Supplementary Fig. 13b, additional AFM images in Supplementary Fig. 14). Figure 3g shows another C20Cl2 intermediate with a 16-membered ring, which could arise from direct ring-opening of an 8- and a 10-membered ring, or a 5- and a 13-memebered ring. Seven characteristic bright features in AFM images point out the location of triple bonds (see calculations on BLAs in Supplementary Fig. 15), in agreement with the results of C16 intermediate (C18O2)18. In our experiments, the tip-induced dehalogenation could be related to anionic charge states of molecules or an applied electric field34,37,38. In addition, inelastic electron tunnelling may also help to trigger dehalogenative reactions16. With such voltage pulses, skeletal rearrangements can either be triggered, too, or happen spontaneously as a consequence of the dehalogenation34.

Subsequent voltage pulses could induce complete dehalogenation of intermediates, resulting in the formation of the final product as shown in the STM and AFM images (Fig. 4a). STM image (Fig. 4a I) shows a donut shape without much orbital information at a lower bias voltage (V = 0.3 V). AFM images (Fig. 4a II and a IV) clearly show a single carbon ring with no halogen atoms attached, which can be unambiguously recognized as a C20. We attempted 27 precursors and generated 5 individual C20 molecules on the 1 ML NaCl surface with a yield of about 18.5 %. In the unsuccessful attempts, we frequently found the edge benzoid rings opened as shown in Supplementary Fig. 16. More importantly, ten pronounced characteristic bright features are clearly distinguished in the AFM images, which are assigned to ten triple bonds of C20. Based on the calculations (Supplementary Fig. 17), we verify that the anti-aromatic C20 adopts an energetically favorable D10h polyynic structure (Supplementary Fig. 1c) as the ground state, and features a BLA of 0.14 Å, while, D10h or C10h symmetry cannot be distinguished in AFM images. The assignment of ten triple bonds is also supported by AFM simulations (Fig. 4a III). The DFT calculations of a C20 on the NaCl surface at different sites show near-planar adsorption configurations (Supplementary Fig. 18).

a I–a IV STM image, AFM image, AFM simulation, and Laplace-filtered AFM image of a C20. STM set points: I = 5 pA, V = 0.3 V. Reference set point of Δz: I = 5 pA, V = 0.3 V. b Calculated frontier orbitals of C20 and superposition of orbital densities of the nearly energetically degenerated orbitals. Calculations were conducted at the ωB97XD/def2-TZVP level. (c) Scanning tunneling spectroscopy (STS) of C20 conducted on NaCl. The differential conductance (dI/dV) signal shows two peaks that can be attributed to the PIR and NIR states. Inset: STM (I = 5 pA, V = 0.3 V, 0 to 1.6 Å) and AFM (Δz = −0.6 Å, reference set point: I = 5 pA, V = 0.3 V, −3.4 Hz to −0.2 Hz) images of a C20. Scale bar: 0.5 nm. STM images (d I–f I) and Laplace-filtered STM images (d II–f II) of C20 obtained with a CO-tip. (d I–d II) NIR, constant current mode; (e I–e II) PIR, constant height mode (Δz = 0 Å, reference set point: I = 5 pA, V = −2.25 V); (f I and f II) NIR, constant height mode (Δz = 0 Å, reference set point: I = 5 pA, V = 1.55 V). The dI/dV spectrum sweeping from zero to negative bias (blue line) was conducted on the triple bond of C20 (blue cross in Fig. 4c), and the dI/dV spectrum sweeping from zero to positive bias (red line) was conducted on the single bond of C20 (red cross in Fig. 4c).

From calculated molecular orbitals of C20 (Fig. 4b and Supplementary Fig. 19), it is seen that C20 exhibits a doubly anti-aromatic configuration. It has been difficult to measure the electronic states of cyclocarbons due to the high mobility on the NaCl surface. We succeeded in manipulating a C20 molecule trapped into an atomic fence formed by eliminated Cl atoms (Supplementary Fig. 20), thus allowing to probe its electronic states by applying relatively larger bias voltages. As shown in Fig. 4c, we have successfully measured the differential conductance as a function of voltage, dI/dV, of a C20. Two peaks show up at V = −2.25 V and V = 1.55 V, which corresponds to the positive ion resonance (PIR) and the negative ion resonance (NIR)39, respectively. The dI/dV spectrum gives a transport gap about 3.8 eV for C20. For PIR, ten characteristic lobes can be seen from STM images (Fig. 4e I and e II). From the calculations, it is seen that HOMO (in-plane) and HOMO-1 (out-of-plane) orbitals are nearly energetically degenerated, we deduce that the PIR state results from the superposition of HOMO and HOMO-1 orbitals (Fig. 4b), in which ten lobes are located over every triple bond of C20. Meanwhile, ten characteristic lobes are also visualized at NIR state (Fig. 4d I, d II, f I, and f II), which should result from the superposition of nearly degenerated LUMO (out-of-plane) and LUMO + 1 (in-plane) orbitals (Fig. 4b), in which ten lobes are located over every single bond. The superimposed structures of C20 (Fig. 4e II and f II) determined from theoretical calculations exhibited a slight rotation ( ~ 8°) compared to the experimental AFM image (the inset of Fig. 4c), and such a rotation might be induced during STM imaging. It is considered that the peaks at the PIR and NIR dominantly relate to the out-of-plane orbitals, that is, HOMO-1 and LUMO, respectively27. In addition, due to the energy broadening of the ionic resonances on NaCl ( ~ 0.3 V)39, nearly degenerated HOMO/HOMO-1 and LUMO/LUMO + 1 could not be resolved as separate peaks in dI/dV spectrum.

In conclusion, we have successfully generated two anti-aromatic cyclocarbons, i.e., C12 and C20, by atom manipulation and induced ring-opening reactions on the NaCl/Au(111) surface at 4.7 K. The polyynic structure of C12 and C20 was revealed by bond-resolved AFM imaging, in agreement with theoretical predictions. More importantly, manipulating C20 next to Cl clusters to hinder its diffusion makes it possible to probe the electronic structures of C20. We believe such an on-surface synthesis strategy could be further extended to the generation and characterization of other larger cyclocarbons.

STM and AFM measurements were carried out in a commercial (Createc) low-temperature system operated at 4.7 K with base pressure better than 1×10−10 mbar. Single crystalline Au(111) surface was cleaned by several sputtering and annealing cycles. The NaCl films were obtained by thermally evaporating NaCl crystals onto a clean Au(111) surface at room temperature, resulting in islands of one and two monolayer (ML) thickness. After annealing the sample to a temperature of about 373 K, large-area three ML NaCl surface was obtained. The precursor molecule 1,4,5,8-tetraiodo-2,3,6,7-tetrabromobiphenylene (C12Br4I4) was synthesized as detailed below using procedures in ref. 40. The precursor molecule perchlorodibenzo[b,h]biphenylene (C20Cl12) was synthesized as detailed below. C12Br4I4 and C20Cl12 molecules were separately deposited on a cold NaCl/Au(111) surface by thermal sublimation from a molecular evaporator. CO molecules for tip modification41 were dosed onto the cold sample via a leak valve. We used a qPlus sensor42 with a resonance frequency f0 = 29.49 kHz, quality factor Q ≈ 45,000 and a spring constant k ≈ 1800 N/m operated in frequency-modulation mode43. The bias voltage V was applied to the sample with respect to the tip. AFM images were acquired in constant-height mode at V = 0 V and an oscillation amplitude of A = 1 Å. The tip-height offsets Δz for constant-height AFM images are defined as the offset in tip-sample distance relative to the STM set point at the NaCl surface. The positive (negative) values of Δz correspond to the tip-sample distance increased (decreased) with respect to a STM set point. With increasing thickness of NaCl, the tip-sample distance decreased at given STM set-point conditions, which normally leads to larger tip-height offsets for constant-height AFM images (see AFM images of C12, C20 and intermediates on 1-3 ML NaCl surfaces in Supplementary Fig. 21).

1,4-Bis(trimethylsilyl)-2,3,5,6-tetrabromobenzene: 1,2,4,5-Tetrabromobenzene (10.0 g, 25.4 mmol) and trimethylsilyl chloride (7 mL, 106 mmol) were suspended in anhydrous THF (150 mL) in a flame-dried 500 mL Schlenk flask and cooled to −78 °C. Subsequently, a solution of LDA (2 M, 28 mL) was added to this suspension in the course of 30 min. The resulting solution was warmed up to room temperature overnight, quenched with 1 N HCl solution and extracted with CH2Cl2. The combined organic fractions were dried over MgSO4. After filtration and evaporation of the solvent, the residue was subjected to repeated column chromatography (silica, hexane). 1,4-Bis(trimethylsilyl)-2,3,5,6-tetrabromobenzene was obtained as a colorless liquid. (7.0 g, 70 %). 1H NMR (CDCl3, 400 MHz, δ): 0.60 (s).

2,3,6,7-Tetrabromo-1,4,5,8-tetra(trimethylsilyl)biphenylene: To a solution of 1,4-Bis(trimethylsilyl)-2,3,5,6-tetrabromobenzene (5.0 g, 9.29 mmol) in anhydrous THF (40 mL) at −78 °C under argon, nBuLi (5.8 mL, 9.29 mmol, 1.6 M in hexane) was added over one hour. After warming to room temperature overnight, the reaction mixture was quenched with 1 N aqueous HCl solution, extracted with CH2Cl2, the combined organic phases were dried over MgSO4 and filtered. The solvents were evaporated in vacuo and the residue was subjected to column chromatography (silica, hexane/CH2Cl2 8:1), followed by precipitation from MeOH to give 2,3,6,7-Tetrabromo-1,4,5,8-tetra(trimethylsilyl)biphenylene (0.8 g, 10%) as a yellow solid. 1H NMR (CDCl3, 400 MHz, δ): 0.40 (s).

1,4,5,8-tetraiodo-2,3,6,7-tetrabromobiphenylene (C12Br4I4): In the absence of light a 1 M iodine monochloride solution in CH2Cl2 (7.93 mL, 7.93 mmol) was added dropwise over 15 min to 2,3,6,7-Tetrabromo-1,4,5,8-tetra(trimethylsilyl)biphenylene (300 mg, 0.40 mmol) in anhydrous CH2Cl2 (20 mL) at 0 °C under argon. After warming to room temperature overnight, the reaction was quenched by the addition of an aqueous Na2SO3 solution. The precipitate was filtered and extensively washed with water, MeOH, CH2Cl2 and THF, yielding 1,4,5,8-tetraiodo-2,3,6,7-tetrabromobiphenylene as a hardly soluble light yellow solid (190 mg, 50%). The compound was used without further purification.

The perchlorodibenzo[b,h]biphenylene was obtained in two steps: the synthesis of dibenzo[b,h]biphenylene (C20H12) followed by ref. 44. Under an N2 atmosphere, a 10 mL dry round-bottom flask was charged with palladium acetylacetonate (6.1 mg, 0.02 mmol), ligand tri-tert-butylphosphonium tetrafluoroborate (11.6 mg, 0.04 mmol), trimethylsilyl oxanorbornadiene (43.2 mg, 0.2 mmol), 2-Bromonaphthalene (41.4 mg, 0.2 mmol), sodium phenoxide (34.8 mg, 0.3 mmol) and 2 mL DMF. The mixture was stirred at room temperature for 5 min, and then heated to 120 °C in oil bath. After 24 h, the reaction was cooled to room temperature, and diluted with ethyl acetate. The solution was filtered and concentrated under vacuum. The product was purified by flash chromatography (1:20 ethyl acetate/hexane) to obtain compound and then dissolved in 1 mL of CHCl3 and 2 mL of iso-propanol, and the solution was transferred into microwave tube with a rubber septum and a stir bar. The solution was flushed with argon for 3 times, then 0.1 mL concentrated HCl was added. The tube was covered by alumina foil and taken into oil bath at 80 °C. After 12 h, the reaction was cooled down and 3 mL methanol was added. The solid was filter to get a light yellow solid. 1H NMR (CDCl3, 500 MHz,): δ 7.60 (dd, J = 6.0, 3.4 Hz, 4H), 7.33 (dd, J = 6.1, 3.3 Hz, 4H), 7.27 (s, 4H). The perchlorinated compounds (C20Cl12) as follows:

C20Cl12 was obtained by dissolving the aromatic hydrocarbon (C20H12) in the BMC reagent consisting of a mixture of S2Cl2 and AlCl3 in a Cl equivalent ratio of 1:0.5 in 150 mL of SO2Cl2 and heating to 64 oC for 4 h. At the end of the reaction, the mixture was treated with icy water. After neutralization with NaHCO3 the product was extracted with CHCl345. MS (MALDI-TOF): m/z calculated for C20Cl12:659.62; found: 659.50. 13C NMR is not available because of extremely low solubility.

Density functional theory (DFT) calculations were carried out in the gas phase using Gaussian 16 program package46. ωB97XD exchange-correlation functional47 in conjunction with def2-TZVP48 basis sets was used for C12 and C20 related calculations in gas phase.

The AFM simulations were conducted by the PP-AFM code provided by Hapala et al.28. The detailed parameters were listed below. The lateral spring constant for CO-tip was 0.2 N/m, and a quadrupole-like charge distribution at the tip apex was used to simulate the CO tip with q = −0.1 e. In addition, e is the elementary charge and refers to |e | , and q is the magnitude of quadrupole charge at the tip apex. The amplitude was set as 1 Å.

The Vienna ab initio simulation package (VASP)49,50 was used to perform the DFT calculations on the NaCl surface. For describing the interaction between electrons and ions, the projector-augmented wave method51,52 was used, and the Perdew−Burke−Ernzerhof generalized gradient approximation exchange−correlation functional was employed53. Van der Waals corrections were also included using the DFT-D3 method of Grimme54. The kinetic energy cutoff was set to 400 eV. We used a bilayer NaCl(001) slab separated by a vacuum thicker than 20 Å and the bottom layer of the NaCl was fixed. The atomic structures were relaxed until the atomic forces were less than 0.03 eV/ Å.

All data that support the findings of this study are available from the corresponding authors upon request. Source data are provided with this paper.

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The authors acknowledge the financial support from the National Natural Science Foundation of China (22125203), the National Key R&D Program of China (2023YFE0101900), the Ministry of Science and Technology of the People’s Republic of China.

These authors contributed equally: Luye Sun, Wei Zheng.

Interdisciplinary Materials Research Center, School of Materials Science and Engineering, Tongji University, Shanghai, 201804, People’s Republic of China

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

Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, School of Physics Science and Engineering, Tongji University, Shanghai, 200092, People’s Republic of China

Tongde Wang & Guohua Gao

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W.X. conceived the research; L.S., F.K., and W.G. performed the STM/AFM experiments; L.S., T.W. and G.G. carried out the DFT calculations; W.Z. synthesized the C12Br4I4 and C20Cl12 precursors; all authors contributed to writing the manuscript. L.S. and W.Z. contributed equally to this work.

The authors declare no competing interests.

Nature Communications thanks the anonymous reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

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Sun, L., Zheng, W., Kang, F. et al. On-surface synthesis and characterization of anti-aromatic cyclo[12]carbon and cyclo[20]carbon. Nat Commun 15, 7649 (2024). https://doi.org/10.1038/s41467-024-52115-w

DOI: https://doi.org/10.1038/s41467-024-52115-w

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On-surface synthesis and characterization of anti-aromatic cyclo[12]carbon and cyclo[20]carbon | Nature Communications