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Compressing Double [7]Helicene by Successive Charging with Electrons.

Zheng Zhou1, Lin Fu2, Yunbin Hu3,4

  • 1Department of Chemistry, University at Albany, State University of New York, 1400 Washington Ave, Albany, NY, 12222, USA.

Angewandte Chemie (International Ed. in English)
|June 4, 2020
PubMed
Summary

Chemical reduction of benzo-fused double [7]helicene using potassium and rubidium metals yielded three distinct reduced states. Structural analysis revealed compression and twisting of the helicene core upon electron addition, confirmed by theoretical calculations.

Keywords:
X-ray diffractionalkali metalschemical reductionhelicenestructural study

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Area of Science:

  • Organic Chemistry
  • Supramolecular Chemistry
  • Materials Science

Background:

  • Helicenes are chiral polycyclic aromatic hydrocarbons with unique three-dimensional structures.
  • Understanding the electronic and structural properties of reduced helicenes is crucial for developing novel functional materials.
  • Alkali metal reduction offers a pathway to access exotic electronic states in organic molecules.

Purpose of the Study:

  • To investigate the chemical reduction of a benzo-fused double [7]helicene using alkali metals.
  • To characterize the structural and electronic changes occurring upon stepwise electron addition.
  • To correlate experimental findings with theoretical predictions of charge distribution.

Main Methods:

  • Chemical reduction of helicene (1) using potassium (K) and rubidium (Rb) metals.
  • Single-crystal X-ray diffraction for structural determination of reduced helicene species.
  • Nuclear Magnetic Resonance (NMR) spectroscopy and theoretical calculations (DFT) for electronic structure analysis.

Main Results:

  • Three reduced states of helicene (1) were successfully synthesized and characterized: 12-, 13-, and 14-.
  • X-ray diffraction revealed distinct ion pairing (solvent-separated vs. contact-ion complexes) with alkali metal counterions.
  • Stepwise reduction led to significant structural changes: core compression (2.09 Å to 1.42 Å) and increased core twist (16.5° to 20.7°).

Conclusions:

  • The study demonstrates controlled access to multiple reduced states of a benzo-fused double [7]helicene.
  • Electron addition induces substantial structural reorganization, impacting the helicene's geometry and electronic properties.
  • Experimental and theoretical data provide a comprehensive understanding of charge distribution in reduced helicene frameworks.