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

doi:10.21203/rs.3.rs-3411934/v1

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

https://doi.org/10.21203/rs.3.rs-3411934/v1

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Molecular carbon allotropes, cyclo[n]carbons (C_n) have attracted significant attention from both experimentalists and theoreticians due to their extraordinary properties^1-9. 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 unexplored. Recently, on-surface synthesis has emerged as an efficient method for the fabrication of various cyclocarbons, including C₁₈^10,11, C₁₆¹², C₁₄¹³, C₁₀¹³. However, endeavors to synthesize larger cyclocarbons (e.g., C₂₀ or C₂₄) have encountered difficulties owing to the unstability of precursor molecules¹². Herein, we design a molecular precursor of a fully halogenated dibenzo[b,h]biphenylene (C₂₀Cl₁₂), and successfully produce the anti-aromatic cyclo[20]carbon (C20), via tip-induced dehalogenation and accompanied ring-opening reactions on a bilayer NaCl/Au(111) surface at 4.7 kelvin. The polyynic structure of C₂₀ was revealed by bond-resolved atomic force microscopy (AFM) and theoretical calculations. Furthermore, by subtly positioning the C₂₀ molecule into an “atomic fence” (i.e., formed by Cl atoms) to block its diffusion, we have achieved to experimentally probe the frontier molecular orbitals of C₂₀, with a measured bandgap of 3.8 eV. Our work demonstrates the versatility of such an on-surface synthesis strategy for the generation of cyclocarbons in the condensed phase, and for further characterizing their precise geometric and electronic structures.

Physical sciences/Chemistry/Physical chemistry/Chemical physics

Physical sciences/Chemistry/Surface chemistry/Scanning probe microscopy

Recently developed modern theories have predicated the structures of cyclocarbons^14-19. While, for larger carbon clusters (e.g., n > 18), monocyclic, bicyclic or polycyclic isomers were predicted by theory or identified experimentally in the gas phase^20-22. Especially, the carbon cluster (n = 20) is considered to be the transition point from monocyclic to bicyclic/polycyclic structures^23-26. Thus, investigating the precise structure of this specific carbon cluster in the condensed phase is of particular interest.

Scanning tunneling microscopy (STM) tip-induced decarbonylation/dehalogenation^10-12 and more recently developed dehalogenation with accompanied retro-Bergman ring-opening reaction¹³ have been demonstrated to be efficient strategies for generating the family of cyclocarbons in the condensed phase, offering the advantage of characterizing the geometric and electronic structures of these species on the thin insulating surface in real space. In this work, we synthesized perchlorodibenzo[b,h]biphenylene (C₂₀Cl₁₂) as the precursor. Through on-surface transformation from hexagon-tetragon (6–4) carbon rings to a pair of pentagon (5–5) rings and followed by dehalogenation and ring-opening reactions, cyclo[20]carbon (C20) was successfully generated on the surface (Fig. 1a).

For cyclo[20]carbon, four possible structures are considered (Fig. 1), i.e., two polyynic structures (BLA ≠ 0) with D_10h and C_10h symmetries (Fig. 1b, c), and two cumulenic structures (BLA = 0) with D_20h and D_10h symmetries (Fig. 1d, e), respectively. Based on modern theories, it is predicted that the lowest energy geometry of C₂₀ is D_10h polyynic structure (Fig. 1b), and featuring a bond length alternation (BLA = 0.14 Å) (Extended Data Fig. 1).

To generate C₂₀, C₂₀Cl₁₂ molecules were introduced on the cold sample held at ~6 K. All molecules were studied on a bilayer NaCl/Au(111) surface at 4.7 K. STM and AFM images (Extended Data Fig. 2) revealed its nonplanar adsorption configuration on the NaCl surface due to steric hindrance. 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. Subsequent high-resolution AFM images (Fig. 2a and Extended Data Fig. 3) acquired with CO-terminated tip showed that the typically observed intermediates became more planar, with accompanied dehalogenation at different sites. In addition, it is noted that the carbon skeleton of each observed molecule has undergone the transformation from a 6–4 configuration to a 5–5 one (as shown in Fig. 1a), which was also observed in other on-surface reaction systems²⁷. We did not observe the intact C₂₀Cl₁₂ molecule with a 6–5–5–6–6 configuration.

As the carbon skeleton becomes more complicated, additional pulses (~4 V) can induce occurrence of different kinds of ring-opening reactions (with accompanied further dehalogenation), e.g., retro-Bergman^13,28,29, formation of Sondheimer-Wong diyne^30,31. Fig. 2b 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 Extended Data Fig. 4). More interestingly, Fig. 2c shows the generation of another C₂₀Cl₂ intermediate consisting of a newly formed 8-membered ring, indicating the 5-5 configuration has changed. Such a reaction is understood to be the formation of Sondheimer-Wong diyne³¹ (the reaction scheme is shown in Extended Data Fig. 5 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. 2d and e. For example, a 9-membered ring was formed within the C₂₀Cl₄ intermediate (Fig. 2d), in which two characteristic bright features assigned to triple bonds were revealed (see calculations on BLAs in Extended Data Fig. 6a). An even larger 13-membered ring was also observed within another C₂₀Cl₂ intermediate (Fig. 2e), 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 Extended Data Fig. 6b). The results above provide insights into the extraordinary BLAs of odd-membered carbon rings¹⁷. Fig. 2f 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-related images point out the location of triple bonds (see calculations on BLAs in Extended Data Fig. 7), in agreement with the results of C₁₆ intermediate¹². Other observed intermediates are listed in Extended Data Fig. 8.

Subsequent voltage pulses could normally induce complete dehalogenation of intermediates, resulting in the formation of the final product as shown in the STM and AFM images (Fig. 3a). STM image shows a donut shape without much orbital information at a lower bias voltage (V = 0.3 V). AFM images clearly show a single carbon ring with no halogen atoms attached, which can be unambiguously recognized as a C₂₀. More importantly, ten pronounced characteristic bright features are clearly distinguished in the AFM images (Fig. 3a_II, b_II), which are assigned to ten triple bonds of C₂₀ (Fig. 3b_I). That means, the anti-aromatic C₂₀ also adopts an energetically favorable polyynic structure as the ground state, similar to the case of C₁₆. The assignment of ten triple bonds is also supported by AFM simulations (Fig. 3a_III, b_III). The DFT calculations of a C₂₀ on the NaCl surface at different sites show near-planar adsorption configurations (Extended Data Fig. 9). Our experimental results verify the theoretically predicted polyynic structure of C₂₀, while, the D_10h or C_10h symmetries cannot be distinguished in AFM images.

Apart from the nearly perfect circle of a C₂₀ shown above, we also observed some deformed C₂₀ induced by the eliminated Cl atoms as shown in the STM images (Fig. 3c_I, d_I). The AFM related images (Fig. 3c_II to c_IV, d_II to d_IV) present more clearly on the deformed shapes of C₂₀, where ten characteristic bright features corresponding to ten triple bondsare still visible, indicating the BLAs are not much influenced with the changes of BAAs. In addition, the deformation of C₂₀ also indicates its flexibility due to the larger size. It is noted that on the surface we did not generate other isomeric forms as observed in the gas phase.

It has been difficult to measure the electronic states of cyclocarbons due to the high mobility on the NaCl surface. Inspired from the scenario shown above (Fig. 3c_I, d_I), we succeeded in manipulating a C₂₀ molecule to be trapped into an “atomic fence” formed by eliminated Cl atoms (Extended Data Fig. 10), thus allowing to probe its electronic states by applying relatively larger bias voltages. As shown in Fig. 4a, 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 negative ion resonance (the highest occupied molecular orbital, HOMO) and the positive ion resonance (the lowest unoccupied molecular orbital, LUMO)³², respectively. The dI/dV spectrum gives a HOMO-LUMO bandgap about 3.8 eV for C₂₀. Moreover, STM images recorded at the corresponding voltages present the density of state of HOMO and LUMO, respectively (Fig. 4b-g). Notably, ten characteristic nodes over every triple bond of C₂₀ (cf. Fig. 4h) are seen at HOMO (Fig. 4c, f), in consistent with the DFT-calculated HOMO (Fig. 4k), which is also comparable with the ground-state structure of C₂₀ (Fig. 3a_IV). Meanwhile, ten characteristic nodes in between every triple bond are also visualized at LUMO (Fig. 4b, e, d, g), in good agreement with the DFT-calculated LUMO (Fig. 4l).

To gain more insights into the anti-aromatic C₂₀, we conduct the anisotropy of current-induced density (ACID) calculations to visualize the induced current after applied magnetic field. The ACID plot of C₂₀ shows a counterclockwise direction represented by the green arrows (Fig. 4i) (also indicated by the yellow curved arrow), which reflects its anti-aromaticity nature. The ZZ components of the iso-chemical shielding surface (ICSS) calculated at 1 Å above the plane of cyclocarbon, named ICSS(1)_ZZ, is also presented in Fig. 4j. It reflects the shielding of a system to the external magnetic field, showing positive/negative ICSS(1)_ZZ values at the shielding/deshielding regions, respectively. For the anti-aromatic C₂₀, a deshielding region is shown at the center of the ring, with a shielding region surrounding it.

In conclusion, we have successfully generated an anti-aromatic cyclo[20]carbon by atom manipulation on the bilayer NaCl/Au(111) surface at 4.7 K. The polyynic structure of 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 expect such an on-surface synthesis strategy could be further extended to the generation and characterization of other cyclocarbons, for example, odd-numbered cyclocarbons, predicated owning unique structures and properties.

Acknowledgments: The authors acknowledge the financial support from the National Natural Science Foundation of China (22125203).

Author contributions: W.X. conceived the research; L.S. and F.K. performed the STM/AFM experiments and carried out the DFT calculations; W.Z. synthesized the C₂₀Cl₁₂precursor; all authors contributed to writing the manuscript. L.S. and W.Z. contributed equally to this work.

Competing interests: Authors declare that they have no competing interests.

Data and materials availability: All data are available in the main text or the Extended Data.

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Experimental details for STM and AFM measurements

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^-10mbar. 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 two and three monolayer thickness. C₂₀Cl₁₂ molecules were deposited on a cold NaCl/Au(111) surface by thermal sublimation from a molecular evaporator. CO molecules for tip modification³³ were dosed onto the cold sample via a leak valve. We used a qPlus sensor³⁴ 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 mode³⁵. 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.

Solution synthesis of perchlorodibenzo[b,h]biphenylene

The perchlorodibenzo[b,h]biphenylene was obtained in two steps: the synthesis of dibenzo[b,h]biphenylene (C₂₀H₁₂) followed by ref. 36. Light yellow solid. ¹H NMR (500 MHz, Chloroform-d): 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 filtered or extracted with CHCl₃. 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 calculations and AFM simulations

Density functional theory (DFT) calculations were carried out in the gas phase using Gaussian 16 program package³⁷. ωB97XD exchange-correlation functional³⁸ in conjunction with def2-TZVP³⁹ basis sets was used for C₂₀ related calculations in gas phase. The ACID calculation⁴⁰ was performed at the B3LYP-CSGT/def2-TZVP level. ICSS(1)_ZZ^41,42 calculation was performed at the B3LYP-GIAO/def2-TZVP level combined with Multiwfn 3.8 code⁴³. The isosurfaces of AICD calculation in Fig. 4 is 0.05 a.u..

The AFM simulations were conducted by the PP-AFM code provided by Hapala et al⁴⁴. 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)^45,46 was used to perform the DFT calculations on the NaCl surface. For describing the interaction between electrons and ions, the projector-augmented wave method^47,48 was used, and the Perdew−Burke−Ernzerhof generalized gradient approximation exchange−correlation functional was employed⁴⁹. Van der Waals corrections to the PBE density functional were also included using the DFT-D3 method of Grimme⁵⁰. 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.

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

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