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

Ying Wang, Hui Zhang, Maren Pink, Arnon Olankitwanit, Suchada Rajca, Andrzej Rajca

Research output: Contribution to journalArticle

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Abstract

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 (Cs 2 CO 3 ), 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.

Original languageEnglish (US)
Pages (from-to)7298-7304
Number of pages7
JournalJournal of the American Chemical Society
Volume138
Issue number23
DOIs
StatePublished - Jun 15 2016

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Molecular orbitals
Electron energy levels
Cations
Positive ions
Oxidation
Cyclic voltammetry
Anions
Negative ions
Pyrroles
Near-Infrared Spectroscopy
X ray crystallography
X Ray Crystallography
Carbon Monoxide
Discrete Fourier transforms
Paramagnetic resonance
Protons
Nuclear magnetic resonance
Spectroscopy
Electrons
Temperature

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

Cite this

Radical Cation and Neutral Radical of Aza-Thia[7]helicene with SOMO-HOMO Energy Level Inversion. / Wang, Ying; Zhang, Hui; Pink, Maren; Olankitwanit, Arnon; Rajca, Suchada; Rajca, Andrzej.

In: Journal of the American Chemical Society, Vol. 138, No. 23, 15.06.2016, p. 7298-7304.

Research output: Contribution to journalArticle

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