On-surface synthesis of anti-aromatic cyclo[12]carbon and aromatic cyclo[6]carbon

doi:10.21203/rs.3.rs-3411973/v1

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On-surface synthesis of anti-aromatic cyclo[12]carbon and aromatic cyclo[6]carbon

https://doi.org/10.21203/rs.3.rs-3411973/v1

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A novel family of carbon allotropes, known as cyclo[n]carbons (Cn), have attracted significant attention from both experimentalists and theoreticians because of their remarkable properties. Recently, certain aromatic cyclocarbons, such as cyclo[18]carbon (C18), cyclo[14]carbon (C14), and cyclo[10]carbon (C10), as well as an anti-aromatic cyclo[16]carbon (C16), have been successfully synthesized and characterized on the thin insulating NaCl surface. However, the characterization of even smaller cyclocarbons in real space, particularly those with anti-aromatic characteristics, still remains challenging. The substantial difficulties in synthesizing smaller anti-aromatic cyclocarbons are due to their inherent high reactivity, and on the other hand, stemming from the increased strain and reduced stability owing to their anti-aromatic nature. Here, we successfully produce a smaller anti-aromatic cyclocarbon, cyclo[12]carbon (C12), via tip-induced dehalogenation and accompanied retro-Bergman ring-opening reaction of a fully halogenated biphenylene (C12Br4I4), on a bilayer NaCl/Au(111) surface at 4.7 kelvin. Moreover, the smallest aromatic cyclocarbon, cyclo[6]carbon (C6), is also generated by atom manipulation of hexachlorobenzene (C6Cl6) molecule. The polyynic structure of C12 and cumulenic structure of C6 are revealed by bond-resolved atomic force microscopy (AFM) and theoretical calculations. Our work further demonstrates the feasibility of such an on-surface synthesis strategy for the generation of cyclo[n]carbons in the condensed phase, and to further characterize their precise structures with bond-resolved resolution.

Physical sciences/Chemistry/Surface chemistry/Scanning probe microscopy

Physical sciences/Chemistry/Physical chemistry/Chemical physics

Cyclo[n]carbons, consisting of rings of two-coordinate sp-hybridized carbon atoms, have fascinated a number of experimentalists and theoreticians because of their unique structures and potential applications^5-8. Due to their high reactivity, cyclocarbons are hardly synthesized and characterized in condensed phases, leaving open questions about their detailed structures (polyynic or cumulenic) and stabilities (linear or cyclic). Theoretical calculations have significantly contributed to the understanding of cyclocarbons, however, there exist debates regarding their structures, which are often contingent upon the employed levels of theory^9-13. Recently, a partial elucidation of this puzzle has been achieved through the successful synthesis of specific cyclocarbons, namely C₁₈^1,2, C₁₄³, and C₁₀³, on thin insulating surfaces via atomic manipulation of molecular precursors. In combination of recently developed modern theories on the prediction of structures of cyclocarbons up to C₁₀₀^14-19, the polyynic structure of C₁₈, Peierls-transition-intermediate cumulene-like C₁₄ and cumulenic C₁₀ were revealed by AFM imaging and simulations, respectively. According to Hückel's rule, the above mentioned cyclocarbons are all aromatic (i.e., n = 4 k + 2, with k being an integer), conferring them with enhanced stability^15,16,20-25. Nevertheless, the realm of anti-aromatic cyclocarbons (i.e., n = 4 k), presents a more challenging scenario^15,24,26. To date, only C₁₆ has been successfully synthesized and characterized on the surface, where a polyynic structure was revealed in real space⁴. For smaller anti-aromatic cyclocarbons, the combination of increased strain and reduced stability further complicates their synthesis and characterization. C₁₂, the last homologue of C₁₆, has thus been a potentially interesting candidate for studying the smaller anti-aromatic cyclocarbon.

For anti-aromatic cyclo[12]carbon, four possible structures are considered (Fig. 1), i.e., two polyynic structures (BLA ≠ 0) with D_6h and C_6h symmetries (Fig. 1a, b), and two cumulenic structures (BLA = 0) with D_12h and D_6h symmetries (Fig. 1c, d), respectively. Based on modern theories, it is predicted that the lowest energy geometry of C₁₂ is D_6h polyynic structure (Fig. 1a), and featuring a relatively large bond length alternation (BLA = 0.17 Å) (Extended Data Fig. 1).

The on-surface synthesis method^27,28 has become a promising approach for the generation of such kinds of annular carbon allotropes, offering the advantage of characterizing these species on the surface in real space^1-4. STM tip-induced decarbonylation/dehalogenation and more recently developed dehalogenation with accompanied retro-Bergman ring-opening reaction have been demonstrated to be efficient strategies for generating cyclocarbons on thin insulating surfaces. Moreover, the resulting structures, whether polyynic or cumulenic, can be distinctly identified using bond-resolved AFM. In this work, we still employed the universal reaction scheme (involving dehalogenation and retro-Bergman reaction) demonstrated previously³ for generating C₁₂. We then synthesized a fully halogenated biphenylene (1,4,5,8-tetraiodo-2,3,6,7-tetrabromobiphenylene, C₁₂Br₄I₄) as the precursor. Through tip-induced dehalogenation with accompanied twice retro-Bergman reactions, C₁₂ was successfully generated on the surface (Fig. 2a).

To generate C₁₂, C₁₂Br₄I₄ 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. High-resolution AFM image acquired with CO-terminated tip revealed both the carbon skeleton (six-four-six-membered ring) and eight halogen atoms of the C₁₂Br₄I₄ molecule (Fig. 2b _I), which were also clearly shown in the AFM simulation and Laplace-filtered AFM images (Fig. 2c _I to e _I). 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. Normally, one or two iodine atoms were first removed, leading to the formation of C₁₂Br₄I₃ (Extended Data Fig. 2a-c) or C₁₂Br₄I₂ (Fig. 2b _II) intermediates. The AFM image (Fig. 2b _II) revealed that the first-step retro-Bergman ring-opening reaction has already occurred in a C₁₂Br₄I₂molecule, leading to the formation of an eight-membered ring (also refer Fig. 2a). In addition, two characteristic bright features corresponding to the carbon-carbon triple bonds were observed (also see our calculations in Extended Data Fig. 3a and the simulation in Fig. 2c_II), which are more clearly shown in the Laplace-filtered AFM image (Fig. 2d _II, e _II). Further dehalogenation and accompanied second-step retro-Bergman reaction led to the formation of C₁₂Br₃ intermediate (Fig. 2b_III), which was imaged as a larger carbon ring and three Br atoms attached, where four characteristic bright features corresponding to the carbon-carbon triple bonds could be distinguished, as also shown in the AFM simulation and Laplace-filtered AFM image (Fig. 2c _III to e _III). The bond lengths calculation of such a twelve-membered ring was shown in Extended Data Fig. 3b. Other observed intermediates during pulses were shown in Extended Data Fig. 2d-i.

Subsequent voltage pulses (~3-3.5 V) could normally induce complete dehalogenation of intermediates, resulting in the formation of the final product as shown in the AFM images (Fig. 2b _IV, d _IV). 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 (Fig. 2e _IV), that means, the anti-aromatic C₁₂ also adopts an energetically favorable polyynic structure as the ground state, similar to the case of C₁₆. The assignment of six triple bonds is also supported by AFM simulation (Fig. 2c_IV). From detailed AFM simulations of C₁₄ as an example³, we learnt that the larger BLAs the more pronounced characteristic bright features over triple bonds. It is noticeable that C₁₂ possesses a comparatively large bond length alternation (BLA_C12 = 0.17 Å) (Extended Data Fig. 1) among already generated cyclocarbons on the surface (i.e., BLA_C18 = 0.14 Å, BLA_C16 = 0.15 Å, BLA_C14 = 0.05 Å, BLA_C10 = 0 Å), which is speculated to be in relation to its intrinsic anti-aromaticity, that is, to increase the stability of its cyclic form. In the close tip-sample height (Extended Data Fig. 4c), bright lines appear between the carbon atoms, which should originate from the tip-tilting effect²⁹. The DFT calculations of C₁₂ on the NaCl surface at different sites show near-planar adsorption configurations (Extended Data Fig. 5). Our experimental results verify the polyynic structure of C₁₂ predicted by modern theories, while, the D_6h or C_6h symmetries cannot be distinguished in AFM images².

To further explore the structure and stability of even smaller cyclocarbons, we try to generate the smallest aromatic cyclocarbon, C₆, via tip-induced dehalogenation (Fig. 3a). Previous experiments and theories have shown that its lowest energy geometry is D_3h cumulenic structure^12,16,30-32 (also see our calculation in Extended Data Fig. 6). The hexachlorobenzene (C₆Cl₆) molecule³³ was chosen as the precursor to be introduced on the cold NaCl/Au(111) held at ~6 K. Subsequent high-resolution AFM image acquired with CO-terminated tip revealed its chemical structure (Fig. 3b_I), which was also clearly seen in the Laplace-filtered AFM image with superimposed model (Fig. 3c_I, d_I). Conducting several voltage pulses (~ 4 V) on C₆Cl₆ molecules lead to the formation of typical intermediates (e.g., C₆Cl₅, C₆Cl₄, C₆Cl₃, C₆Cl₂) as shown in AFM and related-AFM images (Fig. 3b_II to b_V, c_II to c_V, andd_II to d_V). Further applied voltage pulses could induce complete dehalogenation of intermediates to the final product. High-resolution AFM images at different tip heights (Fig. 3b_VI, Extended Data Fig. 7) clearly show a single carbon ring with no halogen atoms attached, which can be unambiguously recognized as a C₆. In addition, the experimentally observed cumulenic structure of C₆ agreed well with theoretical calculations (Extended Data Fig. 6). We notice that in the AFM images of C₆ (Fig. 3 b_VI, c_VI) the contrast of the left part is apparently brighter, which is due to the energetically favorable tilted adsorption configuration of C₆ with respect to the NaCl surface (see our calculations in Extended Data Fig. 8). Interestingly, a ring-opening intermediate, C₆Cl₂ chain, was frequently observed (see reaction scheme in Fig. 3e). STM image was shown in Fig. 3f_I. AFM-related images (Fig. 3f_II to f_V) revealed a polyynic structure of the chain where three characteristic bright features corresponding to the triple bonds and two Cl atoms at the ends were shown (line profile of a C₆Cl₂ chain is shown in Extended Data Fig. 9). The frequent observation of this ring-opening intermediate indicates instability of C₆^23,34 due to the increased strain from a comparatively large BAA (≈ 59°) (Extended Data Fig. 6).

As sp-hybridized carbon allotropes, cyclocarbons possess two perpendicular π electrons that globally delocalize on the carbon ring. When the external magnetic field is applied, it is expected that the π electrons can generate a directional induced current, which can reflect the aromaticity of cyclocarbons. To visualize this induced current, we conduct the anisotropy of current-induced density (ACID) calculations on the cyclo[6]carbon, cyclo[10]carbon, cyclo[12]carbon and cyclo[14]carbon, respectively. The ACID plot of C₁₂ shows a counterclockwise direction represented by the green arrows (Fig. 4g) (also indicated by the yellow curved arrow), which reflects its anti-aromaticity nature. In contrast, the ACID plots of aromatic C₆, C₁₀ and C₁₄ all show clockwise directions (Fig. 4e, f and h). The gauge-including magnetically induced current (GIMIC) maps at 1 Å above the planes of cyclocarbons are shown in Fig. 4i to l, providing a complementary perspective to AICD results, in which the counterclockwise (C₁₂) and clockwise (C₆, C₁₀, C₁₄) currents are observed along the carbon skeletons as indicated by the blue curved arrows. The ZZ components of the iso-chemical shielding surface (ICSS) calculated at 1 Å above the planes of cyclocarbons, named ICSS(1)_ZZ, are also presented in Fig. 4m to p. 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 aromatic C₆, C₁₀, and C₁₄, significant shielding regions are shown at the center of rings, and in contrast, for the anti-aromatic C₁₂, a deshielding region is shown at the center of the ring, with a shielding region surrounding it. The inversion of shielding and deshielding regions arises from the aromaticity/anti-aromaticity nature of these cyclocarbons.

In conclusion, we have successfully generated a smaller anti-aromatic cyclo[12]carbon and the smallest aromatic cyclo[6]carbon by atom manipulation on the bilayer NaCl/Au(111) surface at 4.7 K. The polyynic structure of C₁₂ and cumulenic C₆ were revealed by bond-resolved AFM imaging, in agreement with theoretical predictions. We expect such an on-surface synthesis strategy could be further extended to the generation and characterization of larger cyclo[n]carbons, for example, cyclo[20]carbon, predicted to be the watershed of single ring or cage/bowl structures³⁵.

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

Author contributions: W.X. conceived the research; W.Z. synthesized the C₁₂Br₄I₄ precursor; L.S., F.K., and W.G. performed the SPM experiments and carried out the DFT calculations; 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 supplementary materials.

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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 a base pressure better than 1×10^-10mbar. The single crystalline Au(111) surface was cleaned by several sputtering and annealing cycles. NaCl films were grown on Au(111) held at room temperature, resulting in islands of two and three monolayer thickness. The precursor molecules (1,4,5,8-tetraiodo-2,3,6,7-tetrabromobiphenylene, C₁₂Br₄I₄) were synthesized using procedures in ref.³⁶. C₁₂Br₄I₄ and hexachlorobenzene (C₆Cl₆, purchased from Dr. Ehrenstorfer GmbH (Germany), > 99 %) molecules were separately deposited on cold NaCl/Au(111) surface by thermal sublimation. CO molecules for tip modification³⁷ were dosed onto the cold sample via a leak valve. We used qPlus sensors³⁸ with a resonance frequency f₀ = 29.49 kHz, quality factor Q ≈ 45,000 and a spring constant k ≈ 1800 N/m operating in frequency-modulation mode³⁹. 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. The STM and AFM images are slightly smoothed for better visualization.

Density functional theory calculations and AFM simulations

Density functional theory (DFT) calculations were carried out in the gas phase using Gaussian 16 program package⁴⁰. ωB97XD exchange-correlation functional⁴¹ in conjunction with def2-TZVP⁴² basis sets was used for all C₆, C₁₀, C₁₂ and C₁₄ related calculations in gas phase. The ACID calculations⁴³ were performed at the B3LYP-CSGT/def2-TZVP level. GIMIC⁴⁴ calculations were performed at B3LYP/def2-TZVP level with ParaView 5.9.1⁴⁵. ICSS(1)_ZZ^46,47 calculations were performed at the B3LYP-GIAO/def2-TZVP level combined with Multiwfn 3.8 code⁴⁸. The isosurfaces of AICD calculations in Fig. 4 are all 0.03 a.u..

The AFM simulations were conducted by the PP-AFM code provided by Hapala et al²⁹. 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 NaCl surface. For describing the interaction between electrons and ions, the projector-augmented wave method^51,52 was used, and the Perdew−Burke−Ernzerhof generalized gradient approximation exchange−correlation functional was employed⁵³. Van der Waals corrections to the PBE density functional 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.

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On-surface synthesis of anti-aromatic cyclo[12]carbon and aromatic cyclo[6]carbon

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