Radical Cation and Neutral Radical of Aza-thia[7]helicene with SOMO–HOMO Energy Level Inversion

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• 作者：Ying Wang ; Hui Zhang ; Maren Pink ; Arnon Olankitwanit ; Suchada Rajca ; Andrzej Rajca
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：June 15, 2016
• 年：2016
• 卷：138
• 期：23
• 页码：7298-7304
• 全文大小：475K
• 年卷期：0
• ISSN：1520-5126

文摘

We report a relatively persistent, open-shell aza-thia[7]helicene with cross-conjugated electron-rich π-system. The singly occupied molecular orbital (SOMO) energy levels of both radical cation and neutral radical of the [7]helicene are below the highest occupied molecular orbital (HOMO) energy levels, thereby violating the Aufbau principle. The aza-thia[7]helicene is prepared from β-hexathiophene by a three-step one-pot reaction, in which the pyrrole ring is constructed by two consecutive C–N bond formations. Chemical oxidation converts the helicene to its radical cation, while in the presence of base (Cs₂CO₃), the oxidation gives neutral aminyl radical, likely via proton dissociation from the aminium radical cation with a low pK_a. Reaction of the aza-thia[7]helicene with NaH provides the corresponding anion, which shows characteristic cyclic voltammetry wave at anodic peak potential E_p^a ≈ +0.2 V. Chemical oxidation of the anion with ferrocenium hexafluorophosphate at room temperature gives persistent neutral aminyl radical. Structure of the aza-thia[7]helicene is supported by NMR, IR, X-ray crystallography, and cyclic voltammetry. The radical cation and neutral radical are characterized by EPR and UV–vis-NIR spectroscopies. DFT computations of the radical cation and neutral radical predict the SOMO–HOMO energy level inversion, which is supported experimentally by electrochemical data for the radical cation.