On-Surface Synthesis of a Low Bandgap Cyclopenta[c,d]azulene-Based Polymer

Nonbenzenoid topologies offer a compelling strategy to tailor the electronic properties of low-dimensional carbon materials. Here, we report the on-surface synthesis of a nonalternant polymer incorporating a [7-5-5] cyclopenta[c,d]azulene (CPA) motif on Au(111) via thermally induced Ullmann coupling. Bond-resolved scanning tunneling microscopy confirms the robustness of the [7-5-5] framework. Scanning tunneling spectroscopy, together with dI/dV mapping supported by density functional theory calculations, reveals a remarkable narrow polymeric bandgap of 1.5 eV. The electronic structure is governed by the intramolecular charge transfer from the heptagonal ring to the adjacent fused pentagonal rings, resulting in a pronounced dipole moment despite being composed exclusively of carbon and hydrogen. Aromaticity analyses indicate the coexistence of locally aromatic and slightly antiaromatic regions, highlighting the intrinsic electronic frustration associated with the nonalternant topology. These findings demonstrate how the controlled incorporation of [7-5-5] motifs enables the rational bandgap engineering in carbon-based polymers and expands the scope of nonbenzenoid design in on-surface synthesis.