On-surface synthesis and characterization
The C16Cl10 precursor was synthesized in solution through a one-step sequence as shown in Fig. 2 (see Methods for synthetic details) and then deposited onto a Au(111) single-crystal surface partially covered with bilayer NaCl held at approximately 6 K. On-surface synthesis and characterization by STM and AFM with CO-functionalized tip20 were performed at 4.7 K. Fig. 3b shows an AFM image of a precursor, and only two Cl atoms are imaged as two bright features implying nonplanar adsorption configuration, which is caused by steric hindrance of Cl atoms in such a highly strained molecule (also see Supplementary Fig. 1). This is further confirmed by AFM simulation (Fig. 3c).
To remove Cl atoms from the molecule, the tip was initially positioned on a single molecule, and retracted by about 4 Å from the STM set point (I = 3 pA, V = 0.3 V), and the sample bias then gradually increased from 0.3 V to 4 V. This process typically resulted in yielding C16Cl5, C16Cl4 or C16Cl3 intermediates (Fig. 3e-l, and Supplementary Fig. 2a-f). For further dehalogenation, larger bias voltages were required, typically about 4.2-4.4 V. Voltage pulses were applied for a short time (500 ms) at constant tip height on one specific Cl atom of the C16Cl3 intermediate. This process dissociates the remaining Cl atoms one by one, resulting in C16Cl2 and C16Cl intermediates (Fig. 3m-t, and Supplementary Fig. 2g-i). The structures of these intermediates were structurally characterized by AFM imaging and further supported by AFM simulations (Fig. 3e-t). We speculate that the tip-induced dehalogenations are related to the anionic charge states of molecules and the applied electric field26,27. Additionally, inelastic electron tunneling may also contribute to initiating the dissociation process12,19.
Further voltage sweeping at 4.5 V induced complete dehalogenation of the intermediates and generated the final product, C16 (Fig. 3u-x). The carbon backbone of the product was clearly resolved by AFM (Fig. 3v), exhibiting a graphene-shaped flake. We assigned this molecule as a new isomer of C16, called C16 flake, and to our knowledge, this is the first time C16 flake has been generated and structurally characterized in a condensed phase. Notably, two distinct bright features are observed on both the upper and lower sides of the flake in the AFM images (Fig. 3v and x), corresponding to two triple bonds21,28. The AFM contrast provided evidence for the molecular structure of the C16 flake with the defined positions of triple bonds supported by AFM simulation (Fig. 3w). The yield for the on-surface synthesis of the C16 flake was approximately 23%, and in unsuccessful attempts, the molecules underwent migration or were picked up by the tip.
Structural and aromaticity analysis
The molecular structure of the C16 flake was further analyzed by density functional theory (DFT) calculations. As illustrated in Fig. 4a, the C16 flake contains both sp- and sp2-hybridized carbon atoms (denoted by cyan and pink dots). We calculated the bond length and Mayer bond order of the C16 flake at the ωB97XD/def2-TZVP level (Fig. 4b, and Supplementary Fig. 3), suggesting quasi-cumulenic structures on both the left and right sides of the flake. The shortest bonds are calculated to be 1.22 Å between two sp-hybridized atoms 1 and 2 (8 and 9) as shown in Supplementary Fig. 3, exhibiting a bond order close to a triple bond, which is also visualized in our AFM results showing two characteristic bright features (Fig. 3v and x). It is interesting to note that the alkyne resonance dominates in the C16 flake, which is different from the arynes studied previously29, in that case, the cumulene resonance plays the dominant role.
The delocalization degree of π electrons in the C16 flake is directly related to the characteristics of the induced ring current under an external magnetic field, reflecting its aromaticity30. This ring current can be reflected by the pattern of shielding and deshielding, which can be visualized using the nucleus-independent chemical shift (NICS)31. The ZZ component of the NICS calculated at 1 Å above the plane of the C16 flake, named NICS(1)ZZ, is presented in Fig. 4c. It can be seen that there are significant deshielding regions protruding in the flake, and a shielding region surrounded it, reflecting its aromatic nature. Additionally, the color of the six-membered rings on the left and right sides is notably darker than the ones on the top and bottom, suggesting a greater aromatic character. Another aromatic indicator called AV1245 provided similar results (see Supplementary Fig. 4). The localized orbital locator (LOL) function32 was further performed to visualize the delocalization of π electrons in the C16 flake. As illustrated in the LOL-π color-filled map (Fig. 4d), the favorable delocalization pathways of the π electrons are clearly depicted by white dashed lines, which aligns well with the NICS results.
Chlorine migration and skeleton isomerization
During atom manipulations, it is interesting to observe that the chlorine atom could migrate. Such a migration was triggered by a short (500 ms) voltage pulse with a sample bias voltage of 4.2 V on the target C16Cl molecule as illustrated in Fig. 5a. After applying a voltage pulse, the chlorine atom was found to move from site-1 to site-2 of the carbon skeleton as revealed by AFM results (Fig. 5b-e). More interestingly, in another case, the carbon skeleton of the C16Cl molecule was observed to transform from 6-6 membered rings to 5-7 membered rings also triggered by a voltage pulse of about 4.2 V (Fig. 5g-k). To better understand the molecular transformation processes, we calculated the reaction barriers for chlorine migration and skeleton isomerization (Fig. 5f and l), obtaining barriers of 2.24 eV and 1.39 eV, respectively. It is worth noting that both of these processes are endothermic, demonstrating that the chlorine migration and skeleton isomerization would not occur without applying a voltage pulse. The reaction barriers could generally be overcome by electronic excitation and the weakening of bonds by the antibonding orbital upon electron tunneling24,33,34.