Carbon allotropes comprising sp-hybridized carbon atoms have been investigated for decades for their molecular structure. One of the unsolved mysteries is whether they should take a linear or cyclic configuration in condensed phases due to the lack of atomistic characterizations. Herein, we designed a molecule with a C6 skeleton as a model system to address this issue, which was achieved by eliminating Br atoms from hexabromobenzene (C6Br6) molecule on the Ag(111) substrate via thermal treatment. It is found that the C6 ring intermediate resulting from complete debromination is energetically unstable at room temperature based on theoretical calculations. It subsequently transforms into the C6 polyynic chain via a ring-opening process and ultimately polymerizes into the organometallic polyyne, whose triyne structural unit is revealed by bond-resolved noncontact atomic force microscopy. Theoretical calculations demonstrated an energetically favorable pathway in which the ring-opening process occurs after complete debromination of C6Br6. Our study provides a platform for the synthesis of elusive carbon-rich materials.

As a bottom-up strategy, on-surface synthesis shows great potential in constructing novel carbon-based nanostructures. Herein, on-surface reactions of 1,1,2,2-tetraiodoethylene and 1,1,4,4-tetrabromo-1,2,3-butatriene were systematically explored. A combination of scanning tunneling microscopy and noncontact atomic force microscopy demonstrates that it is feasible to achieve acetylenic and diacetylenic organometallic polyyne (i.e., acetylenic Ag−carbyne and diacetylenic Cu−carbyne) on Ag(110) and Cu(110) surfaces, respectively. Interestingly, after annealing the sample, the acetylenic Ag−carbyne was transformed to diacetylenic Ag−carbyne on Ag(110), while an opposite transformation process occurred between diacetylenic and acetylenic Cu−carbyne on Cu(110). The DFT calculations indicate that such a lattice dependence of skeleton reconstruction originates from the thermodynamic stability of diacetylenic Ag−carbyne/acetylenic Cu−carbyne on the corresponding surfaces.

Stereoselective synthesis of geometrically pure isomers is challenging and important in a variety of chemistry related disciplines and applications. The recently developed on‐surface synthesis strategy provides us opportunities in the fabrication of specific nanostructures which remain challenging by conventional solution chemistry. In this work, we choose alkenyl gem‐dibromides as molecular precursors, and demonstrate the formation of specific trans‐cumulene products via C−C couplings on Cu(110). This study supplements the database of on‐surface synthesis strategies by introducing the alkenyl gem‐dibromide group on the surface. More importantly, it provides a way to stereoselective synthesis of specific isomers involving C−C double bonds, which may be of substantial utility in stereochemistry.

From the structure point of view, there are a number of ways of tiling a carbon sheet with different polygons, resulting in prospects of tailoring electronic structures of lowdimensional carbon nanomaterials. However, up to now, the experimental fabrication of such structures embedded with periodic nonhexagon carbon polygons, especially ones with more than three kinds, is still very challenging, leaving their potential properties unexplored. Here we report the bottom-up synthesis of a nanoribbon composed of 4−5−6−8-membered rings via lateral fusion of polyfluorene chains on Au(111). Scanning probe microscopy unequivocally determines both the geometric structure and the electronic properties of such a nanoribbon, revealing its semiconducting property with a bandgap of ∼1.4 eV on Au(111). We expect that this work could be helpful for designing and synthesizing complicated carbon nanoribbons.

Cyclo[n]carbon allotropes built from sp-hybridized carbon atoms have aroused substantial interests, but until now even their structures (i.e., polyynic or cumulenic) and stabilities (i.e., cyclic or linear) are still under debate. Due to their high reactivity, it is rather difficult to synthesize in condensed phase, which hampers further real-space characterizations. Recently, isolated cyclo[18]carbon (C18) and cyclo[16]carbon (C16) molecules were generated on the surface by atom manipulation, which allowed to identify the polyynic structure of C18 and C16. More interestingly, for the smaller cyclo[n]carbon, questions arise: is it still stable in cyclic form? If so, what is the structure? Here, we successfully produce two aromatic cyclocarbons, that is, cyclo[10]carbon (C10) and cyclo[14]carbon (C14), via tip-induced dehalogenation and followed by retro-Bergman reaction of fully chlorinated naphthalene (C10Cl8) and anthracene (C14Cl10) molecules, on a bilayer NaCl/Au(111) surface at 4.7 kelvin. The cumulenic structure of C10 and Peierls-transition intermediate of C14, different from the polyynic structure of C18 and C16, are revealed by bond-resolved atomic force microscopy. Our results demonstrate a universal synthetic strategy to generate aromatic cyclo[n]carbons on the surface, opening an avenue for characterizing a series of annular carbon allotropes for the structure and stability.