On-Surface Debromination of C 6 Br 6 : C 6 Ring versus C 6 Chain.

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 C 6 skeleton as a model system to address this issue, which was achieved by eliminating Br atoms from hexabromobenzene (C 6 Br 6 ) molecule on the Ag(111) substrate via thermal treatment. It is found that the C 6 ring intermediate resulting from complete debromination is energetically unstable at room temperature based on theoretical calculations. It subsequently transforms into the C 6 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 C 6 Br 6 . Our study provides a platform for the synthesis of elusive carbon-rich materials.