Helicenes are polycyclic aromatic compounds in which benzene rings or other aromatic rings are bound together to give helically-shaped molecules. Non-benzenoid helicenes possess unique topological structures and properties that could be useful in organic optoelectronics. However, the synthesis of these molecules is challenging, and their structure–property relationships are not well understood.
Xike Gao, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Xiaodi Yang, Shanghai University of Traditional Chinese Medicine, both China, and colleagues have synthesized a series of non-benzenoid helicenes. The team created azulene-embedded [n]helicenes ([n]AzH, n = 5, 6 and 7; example with six rings pictured), in which one azulene subunit is fused with n–2 benzene rings. Azulene is a bicyclic system that can be thought to consist of one cyclopentadiene and one cycloheptatriene ring.
The azulene-embedded [n]helicenes were prepared using a bottom-up synthetic strategy from 2-ethynylazulene derivatives, which were coupled to building blocks that contain benzene rings using Suzuki–Miyaura coupling reactions. Platinum-catalyzed cycloisomerization reactions were then used to close the remaining rings and obtain the desired products.
The structures of the three resulting [n]AzHs were revealed by single-crystal X-ray diffraction. The team found that [n]AzHs possess different helical structures and properties than the corresponding benzenoid helicenes. - and AzH exhibit strong chiroptical responses with high absorption dissymmetry factors (gabs, a measure for the selective response of chiral molecules to left and right circularly polarized light) of ca. 0.02. This is among the highest gabs values for helicenes in the visible range.
- Azulene‐Embedded [n]Helicenes (n = 5, 6 and 7),
Chao Duan, Jianwei Zhang, Junjun Xiang, Xiaodi Yang, Xike Gao,
Angew. Chem. Int. Ed. 2022.