Heterocyclic [8]circulenes are an important class of polycyclic aromatic hydrocarbon (PAH) molecules because of their unique structural properties and promising applications in organic semiconductors, organic light-emitting diodes, and liquid-crystalline materials. However, their synthesis is limited and thus a challenge.

Adam S. Foster, Aalto University, Finland, Shigeki Kawai, National Institute for Materials Science, Ibaraki, Japan, Shingo Ito, Nanyang Technological University, Singapore, and colleagues have synthesized for the first time a π-extended diaza[8]circulene (pictured right). The molecule is surrounded by and fused with six hexagons and two pentagons. The researchers reacted dibenzo[a,e]cyclooctatetraene with azomethine ylides formed from iminium salts (pictured left). They used a combined in-solution and on-surface synthetic strategy. The final cyclodehydrogenation of a precursor molecule could be achieved only by on-surface synthesis on an Au(111) surface.

High-resolution CO-STM (scanning tunneling microscopy with a CO-functionalized tip) showed that the molecule adopts a planar conformation on Au(111) and has a unique electronic structure due to the introduction of multiple nitrogen atoms into the PAH framework. Density functional theory (DFT) calculations fully supported these experimental results.

According to the researchers, this work demonstrates the power of a combined in-solution and on-surface approach for the synthesis of unprecedented nitrogen containing PAH molecules that could not otherwise be synthesized.

---

• On-Surface Synthesis of a π-Extended Diaza[8]circulene,
  Kimihiro Nakamura, Qiang-Qiang Li, Ondřej Krejčí, Adam S. Foster, Kewei Sun, Shigeki Kawai, Shingo Ito,
  J. Am. Chem. Soc. 2020.
  https://doi.org/10.1021/jacs.0c02534