TY - JOUR
T1 - Radical Cation and Neutral Radical of Aza-Thia[7]helicene with SOMO-HOMO Energy Level Inversion
AU - Wang, Ying
AU - Zhang, Hui
AU - Pink, Maren
AU - Olankitwanit, Arnon
AU - Rajca, Suchada
AU - Rajca, Andrzej
PY - 2016/6/15
Y1 - 2016/6/15
N2 - 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 pKa. Reaction of the aza-Thia[7]helicene with NaH provides the corresponding anion, which shows characteristic cyclic voltammetry wave at anodic peak potential Epa ? +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.
AB - 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 pKa. Reaction of the aza-Thia[7]helicene with NaH provides the corresponding anion, which shows characteristic cyclic voltammetry wave at anodic peak potential Epa ? +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.
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U2 - 10.1021/jacs.6b01498
DO - 10.1021/jacs.6b01498
M3 - Article
C2 - 27219299
AN - SCOPUS:84975028954
VL - 138
SP - 7298
EP - 7304
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
SN - 0002-7863
IS - 23
ER -