ReportSURFACE CHEMISTRY

An sp-hybridized molecular carbon allotrope, cyclo[18]carbon

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Science  20 Sep 2019:
Vol. 365, Issue 6459, pp. 1299-1301
DOI: 10.1126/science.aay1914

Abstract

Carbon allotropes built from rings of two-coordinate atoms, known as cyclo[n]carbons, have fascinated chemists for many years, but until now they could not be isolated or structurally characterized because of their high reactivity. We generated cyclo[18]carbon (C18) using atom manipulation on bilayer NaCl on Cu(111) at 5 kelvin by eliminating carbon monoxide from a cyclocarbon oxide molecule, C24O6. Characterization of cyclo[18]carbon by high-resolution atomic force microscopy revealed a polyynic structure with defined positions of alternating triple and single bonds. The high reactivity of cyclocarbon and cyclocarbon oxides allows covalent coupling between molecules to be induced by atom manipulation, opening an avenue for the synthesis of other carbon allotropes and carbon-rich materials from the coalescence of cyclocarbon molecules.

The discovery of fullerenes (1), carbon nanotubes (2), and graphene (3), all of which consist exclusively of 3-coordinate carbon atoms, has sparked a new field of synthetic carbon allotropes (4, 5). The only molecular allotropes of carbon that have been isolated are the fullerenes (1). Rings of 2-coordinate carbon atoms (cyclo[n]carbons, Cn) have been suggested as an alternative family of molecular carbon allotropes (46). There is evidence for the existence of cyclocarbons in the gas phase (4, 5, 7, 8), but these highly reactive species have not been structurally characterized or studied in condensed phases. A fundamental and controversial question concerns whether cyclocarbons are polyynic, with alternating single and triple bonds of different lengths, or cumulenic with consecutive double bonds (Fig. 1) (912).

Fig. 1 Two possible structures of cyclo[18]carbon.

The polyynic form with D9h symmetry and the cumulenic form with D18h symmetry are shown, along with visualizations of their perpendicular π-systems. BLA, bond length alternation.

A distinctive feature of sp-hybridized carbon allotropes is that they possess two perpendicular π-conjugated electron systems (Fig. 1). Hückel’s rule predicts an aromatic structure with no bond length alternation (BLA) for planar, cyclic conjugated systems with (4n + 2) π electrons (13). Hoffmann discussed the stability of C18 in 1966 (6), and later it was realized that this molecule could exhibit two orthogonal ring currents, causing double aromatic stabilization (4, 13). Since then, the structures of cyclo[n]carbons have been a topic of theoretical debate, and the results depend on the level of theory (9, 10, 13). Most density functional theory (DFT) and Møller-Plesset perturbation theory calculations predict that the lowest-energy geometry of C18 is cumulenic D18h (9, 10), whereas Hartree-Fock (4) and high-level Monte Carlo and coupled cluster methods predict that the polyyne form is the ground state (11, 12).

Most attempts at synthesizing cyclocarbons have used a masked alkyne equivalent incorporated into a cyclic precursor designed to generate cyclo[n]carbon when activated by heat or light. Methods of unmasking included a retro–Diels-Alder reaction (4), decomplexation (14), decarbonylation (15), and [2+2] cycloreversion (16). These attempts have given tantalizing glimpses of cyclo[n]carbon in the gas phase. Cyclo[n]carbons may coalescence to form fullerenes (8, 15), and gas-phase electronic spectra of C18, formed by laser ablation of graphite, have been measured, but these studies did not reveal whether the structures are cumulenic or polyynic (17).

An alternative approach for studying highly reactive molecules is to isolate them on an inert surface at low temperature. Developments in the field of scanning tunneling microscopy (STM) and atomic force microscopy (AFM), in particular tip functionalization, have enabled imaging of molecules with unprecedented resolution (18, 19), and atom manipulation can trigger chemical reactions on surfaces (1922). We report the synthesis and structural characterization of cyclo[18]carbon. A low-temperature STM-AFM was used to sequentially remove masking CO groups from the precursor C24O6 by atom manipulation (Fig. 2). We resolved the structure of cyclo[18]carbon and observed BLA in its ground state. We also demonstrated covalent sion by atom manipulation of cyclocarbon oxides, exploiting their high reactivity.

Fig. 2 Reaction scheme for the on-surface formation of C18.

Decarbonylation was achieved via voltage pulses that resulted in the loss of two, four, or six CO moieties.

Cyclocarbon oxides, developed by Diederich and co-workers, were selected as suitable candidates for on-surface cyclocarbon generation (14). We synthesized the cyclocarbon oxide C24O6 using procedures similar to those previously reported (see supplementary materials and figs. S1 to S7 for synthetic details and NMR spectra) and sublimed it from a Si wafer onto a cold (T ≈ 10 K) Cu(111) surface partially covered with (100)-oriented bilayer NaCl islands. All molecules were studied on bilayer NaCl, which provided an inert surface on which radicals and polyynes (22) were stable and could be imaged (19). The experiments were carried out in a combined STM/AFM system equipped with a qPlus force sensor (23) operating at T = 5 K in frequency modulation mode (24). We used CO tip functionalization to improve the resolution (18). AFM images were acquired at constant height, with the offset Δz applied to the tip-sample distance with respect to the STM setpoint above the bare NaCl surface. We simulated AFM images using the probe particle model (25) based on the structures relaxed in the gas phase, calculated by DFT (see also supplementary text).

After deposition, we found molecules of the precursor C24O6 on the NaCl surface, as well as some fragmented or (partially) decarbonylated molecules and single CO molecules (see fig. S8 for an overview image). This result indicated that partial decarbonylation and dissociation took place during sublimation. Figure 3 shows AFM data and corresponding simulations for C24O6 and products created by atom manipulation (STM images are shown in fig. S9). C24O6 molecules appeared as triangular objects with dark features at the corners and two bright protrusions at each side (Fig. 3B). The dark contrast, characteristic of ketone groups (26), was explained by a reduced adsorption height and a relatively small electron density in the region imaged above the O atoms, which have high electron affinity. Both effects led to comparably small Pauli repulsive forces above the O atoms (26). Figure 3B was recorded at moderate tip height (“AFM far”), at which differences in bond order were visible in the Δf signal, with high brightness (i.e., high Δf) indicating high bond order (27). The two bright features at each side of the molecule were assigned to triple bonds (22, 28). At decreased tip-sample distance (“AFM close”; Fig. 3C), repulsive forces made greater contributions, and tip relaxations (i.e., tilting of the CO at the tip apex) affected the AFM images substantially. These effects led to apparent sharpening of the bonds and pronounced differences in apparent bond lengths (25, 27, 29). The different contrasts of the three different sides of C24O6 (Fig. 3B) suggest an adsorption geometry not parallel to the surface (26).

Fig. 3 Precursor and products generated by tip-induced decarbonylation.

Structures are shown in column 1. AFM images (columns 2 and 3) were recorded with a CO-functionalized tip at different tip offsets Δz, with respect to an STM set point of I = 0.5 pA, V = 0.2 V above the NaCl surface. (A to E) Precursor. (F to J and K to O) The most frequently observed intermediates. (P to T) Cyclo[18]carbon. The bright features in the lower part of (L), (M), (Q), and (R) correspond to individual CO molecules. Columns 4 and 5 show simulated AFM images based on gas-phase DFT-calculated geometries. The difference in probe height between “sim. far” and “sim. close” corresponds to the respective difference between “AFM far” and “AFM close.” The scale bar in (B) applies to all experimental and simulated AFM images.

To decarbonylate C24O6, we positioned the tip in the vicinity (a few nanometers) of the molecule, retracted by about 3 Å from the STM setpoint (typically V = 0.2 V and I = 0.5 pA), and increased the sample bias voltage V to about +3 V for a few seconds. This procedure often led to the removal of two, four, or six CO moieties. Because of the nonlocality and the observed bias thresholds (see supplementary text), we tentatively propose that the reaction was mediated by inelastic electron tunneling through hot interface-state electrons (19). The most abundant products were C22O4 (Fig. 3F) and C20O2 (Fig. 3K). The removal of a masking group (2 CO) resulted in the formation of a longer bent polyynic segment.

Assigning the bright features in the “AFM far” images to the location of triple bonds, we observed curved polyyne segments with the expected number of triple bonds: five in C22O4 (Fig. 3G) and eight in C20O2 (Fig. 3L). At small tip height (Fig. 3, H and M), we observed sharp bond-like features with corners at the assigned positions of triple bonds and straight lines in between. This contrast was explained by CO tip relaxation, in that maxima in the potential energy landscape, from which the tip apex was repelled, were located above the triple bonds because of their high electron density. Between these maxima, ridges in the potential landscape led to straight bond-like features (25, 30, 31). The assignment of the intermediates was further supported by AFM simulations (Fig. 3, columns 4 and 5) and STM images within the fundamental gap and at the ion resonances (see figs. S10 to S13).

We removed all six CO moieties from C24O6, with 13% yield (calculated by evaluation of 90 atomic manipulation events; see table S1 for details on the statistics and procedure for on-surface decarbonylation), typically resulting in circular molecules (Fig. 3, Q and R). At moderate tip heights (Fig. 3Q), the resulting molecule exhibited a cyclic arrangement of nine bright lobes. One side of the molecule appeared brighter, indicating that its adsorption geometry was not parallel to the surface (see fig. S14). For smaller tip heights (Fig. 3R), the molecule appeared as a nonagon with corners at the positions of the bright lobes that were observed at larger tip-sample distance (Fig. 3Q). The contrast can be explained by analogy to the precursors: The bright lobes in Fig. 3Q and the corresponding corners in Fig. 3R were observed above triple bonds. The molecule was thus identified as cyclo[18]carbon. The AFM contrast provided evidence for a polyynic structure of cyclo[18]carbon on NaCl with the defined positions of triple bonds supported by AFM simulations (Fig. 3, S and T, and fig. S15). In the case of a D18h cumulenic structure, no BLA and an 18-fold symmetry of the molecule would be expected, in contrast to the experimental result. Our results support a polyynic structure with nine-fold symmetry, but they do not allow us to distinguish between polyyne C9h and D9h geometries.

In the nine-fold symmetric form described above, the cyclo[18]carbon molecule was uncharged (neutral). We found that cyclo[18]carbon exhibited charge bistability (32) on this surface and changed to a less symmetric and less planar geometry in the negatively charged state. The molecule could be reversibly switched between its two charge states and associated geometries (figs. S16 to S18). During charge-state switching, tip-induced decarbonylation, and STM imaging, the molecules often jumped to different locations on the surface, indicating a very small diffusion barrier on NaCl. Cyclo[18]carbon moved more frequently than the other molecules observed, and it was often found adjacent to step edges or individual CO molecules adsorbed on the surface, pinning the molecule (Fig. 3, Q and R).

The high reactivity of cyclo[18]carbon and its oxides makes them promising candidates for on-surface covalent molecular fusion by atom manipulation, of which very few previous examples have been reported (20). Applying an elevated bias voltage near two proximate molecules led to their fusion. For example, the two neighboring cyclocarbon oxide intermediates, C20O2 and C22O4 (Fig. 4A), were fused in this way. After constant-current imaging at a set point of V = 3 V and I = 0.5 pA, the molecules reacted through a [4+2] cycloaddition, as revealed by the AFM-resolved structure of the product shown in Fig. 4C (see fig. S19 for a possible mechanism and figs. S20 and S21 for more examples of on-surface fused molecules). These results demonstrated that the strained polyynic moieties of cyclo[18]carbon and its oxide intermediates allowed covalent coupling by atom manipulation. Our results provide direct experimental insights into the structure of a cyclocarbon and open the way to create other elusive carbon-rich molecules by atom manipulation.

Fig. 4 Molecular fusion by atom manipulation.

(A) STM image of two neighboring intermediates, identified as C22O4 and C20O2. Imaging the area at constant current (c.c.) with a set point of V = 3 V, I = 0.5 pA resulted in the reaction between the molecules. (B and C) STM and AFM images, respectively, of the resulting fused molecule. (D) Proposed reaction scheme. See supplementary materials for further examples of molecular fusion reactions.

Supplementary Materials

science.sciencemag.org/content/365/6459/1299/suppl/DC1

Materials and Methods

Supplementary Text

Table S1

Figs. S1 to S21

References (3351)

References and Notes

Acknowledgments: We thank N. Moll and R. Allenspach for discussions. Funding: Supported by ERC grants 320969 and 682144 and by Leverhulme Trust project grant RPG-2017-032. P.G. was supported by Swiss National Science Foundation Postdoc.Mobility fellowship P300P2-177829. Author contributions: L.M.S., P.G., and H.L.A. synthesized the precursors; K.K., F.S., L.M.S., and L.G. performed the AFM experiments; F.S., P.G., and K.K. carried out the DFT calculations; F.S. performed the AFM simulations; all authors contributed to conceiving the research and writing the manuscript. Competing interests: The authors declare no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are available in the main text or the supplementary materials.
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