On-Surface Synthesis and Characterization of Cumulene-Linked Stone-Wales Polymers

Structural, chemical, and extrinsic modifications of graphene-based nanostructures enable bandgap tuning, optoelectronics, spintronics, and quantum materials design. A well-known approach to modify their electronic properties involves introducing nonbenzenoid ring topologies in their ideal sp²-hybridized hexagonal lattice, such as azulene or Stone-Wales (SW) defects. However, despite the unique structural and electronic characteristics that these nonalternant defects induce, their systematic incorporation in graphene-based nanostructures remains challenging. Here, we demonstrate the on-surface synthesis of one-dimensional SW-based polymers linked through cumulene bonds on the Au(111) surface via thermal and visible-light-induced reactions of a tailored molecular precursor. Scanning tunneling and noncontact atomic force microscopies reveal the nonplanar structure of SW-based units within the polymer chain, while the chemical structure of the polymer has been verified by Raman spectroscopy in combination with theoretical modeling. Additionally, scanning tunneling spectroscopy measurements show an experimental bandgap of 1.8 eV, which significantly differs from its isostructural cumulene-bridged bisanthene analogs. Our results highlight the critical role of SW defects in the structural and electronic properties of carbon-based conjugated polymers, advancing their design with prospects in next-generation optoelectronic devices.