Polyaromatic structures with multiple six-membered rings can be turned into molecular bowls when fused with five-membered rings. Such molecular bowls can have useful electronic and dynamic properties for applications, e.g., in responsive materials, and they can serve as building blocks for the synthesis of carbon-based nanostructures. Corannulene (C₂₀H₁₀) and Sumanene (C₂₁H₁₂) are well-known examples of molecular bowls with a carbon skeleton. Similarly small, but electron-rich molecular bowls are somewhat rare. The synthesis of e.g., nitrogen-doped analogues of small molecular bowls can be hampered, e.g., by the high strain in the products.

Qing Wang, Inner Mongolia University, Hohhot, China, and colleagues have synthesized a highly strained, electron-rich molecular bowl, as well as its dimer, which reresents a “wavy” nanographene (simplified structures of both pictured). The molecular bowl can be thought of as a perylene with two pyrrole rings at the armchair edges. The team synthesized the target compound starting from a bowl-shaped bispyrrole-fused perylene-diimide, which was first converted to an anhydride and then underwent decarboxylation to obtain the desired bowl. They prepared derivatives with different substituents at the nitrogen atoms, as well as functionalized bowls with substituents at peri positions. The dimer was obtained from a derivative with multiple mesityl substituents via a Scholl reaction.

According to the researchers, the synthesized molecular bowls are the smallest stable electron-rich bowls reported so far. The team found that the substituents on nitrogen atoms significantly affect the bowl depth, intermolecular interactions, and supramolecular behaviors of the bowls. For example, an n-butyl-substituted derivative forms unusual polar crystals with all bowls oriented similarly. It also has a high affinity towards fullerenes and forms a sandwich-like complex with C₇₀. The nanographene-type dimer can be oxidized to give a surprisingly stable dication with potentially useful optical properties.

• Highly Strained Electron-Rich Molecular Bowl and Nanographene,
  Wenhao Zhang, Yeda Ding, Shanzi Yu, Jiaqi Lyu, Jiaxuan Yu, Jiawen Li, Xin Zhao, Lina Feng, Jianguo Wang, Zheng Zhou, Qing Wang,
  Angew. Chem. Int. Ed. 2025.
  https://doi.org/10.1002/anie.202425355