Helicenes are polycyclic aromatic hydrocarbons (PAHs) with a nonplanar, “screw-shaped” structure. They are interesting research targets due to their structures, chirality, and optoelectronic properties. They can show, e.g., circularly polarized luminescence (CPL), which can be useful for different applications. π-Extended helicenes with larger conjugated structures could be useful as tailor-made chiral graphene-like materials. However, examples with full extension perpendicular to the helical axis are still limited.

Han-Yuan Gong, Beijing Normal University, China, and colleagues have developed a new chiral nanographene structure, i.e., a π-extended pentadecabenzo[9]helicene (or EP9H, pictured). The team started from 2,5,8,11,14-penta-tert-butyl-17-iodohexabenzocoronene, which underwent a Sonogashira coupling with ((4-ethynylphenyl)ethynyl)triisopropylsilane. A Diels–Alder addition with 3,4-bis(4-(tert-butylphenyl)-2,5-diphenylcyclopenta-2,4-dien-1-one then gave a polyphenylene intermediate, which was further converted to a terminal alkyne. Another round of Sonogoshira coupling and Diels–Alder reactions were used to extend the compound, followed by a Scholl oxidation to generate the desired product. It was obtained as a deep-red solid in a yield of 24 %.

The product has a fully extended π-system perpendicular to the helical axis. It features a pentadecabenzo[9]helicene core fragment and four fused hexabenzocoronene units. Single-crystal X-ray diffraction confirmed the helical structure. A chiral resolution of the racemic product mixture was achieved, e.g., using chiral high-performance liquid chromatography (HPLC). The structure extension significantly improved the fluorescence quantum yield and the circularly polarized luminescence properties of the compound. According to the researchers, π-system extension of helicenes could be a strategy to obtain improved CPL molecular emitters.

• A π‐Extended Pentadecabenzo[9]Helicene,
  Yun‐Jia Shen, Nai‐Te Yao, Li‐Na Diao, Yang Yang, Xu‐Lang Chen, Han‐Yuan Gong,
  Angew. Chem. Int. Ed. 2023.
  https://doi.org/10.1002/anie.202300840