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

We report a relatively persistent, open-shell aza-Thia[7]helicene with cross-conjugated electron-rich ?-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 Ep_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.