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Simple aryl halides do not react with nucleophiles. However, nucleophilic aromatic substitutions can be forced under certain conditions, such as high temperatures or strong bases. The mechanism of substitution under such conditions involves the highly unstable and reactive benzyne intermediate. Benzyne contains equivalent carbon centers at both ends of the triple bond, each of which is equally susceptible to nucleophilic attack. This 50–50 distribution of products is...
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All ortho–para directors, excluding halogens, are activating groups. These groups donate electrons to the ring, making the ring carbons electron-rich. Consequently, the reactivity of the aromatic ring towards electrophilic substitution increases. For instance, the nitration of anisole is about 10,000 times faster than the nitration of benzene. The electron-donating effect of the methoxy group in anisole activates the ortho and para positions on the ring and stabilizes the corresponding...
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Treating arylamines with nitrous acid gives aryldiazonium salts that are effective substrates in nucleophilic aromatic substitution reactions. The diazonio group in these salts can be easily displaced by different nucleophiles, yielding a wide variety of substituted benzenes. The leaving group departs as nitrogen gas, and this easy elimination is the driving force for the substitution reaction.
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All meta-directing substituents are deactivating groups. These substituents withdraw electrons from the aromatic ring, making the ring less reactive toward electrophilic substitution. For example, the nitration of nitrobenzene is 100,000 times slower than that of benzene because of the deactivating effect of the nitro group. The first step in an electrophilic aromatic substitution is the addition of an electrophile to form a resonance-stabilized carbocation. The energy diagrams for...
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A reversible single-molecule switch based on activated antiaromaticity.

Xiaodong Yin1, Yaping Zang2, Liangliang Zhu3

  • 1Department of Chemistry, Columbia University, New York, NY 10027, USA.

Science Advances
|November 4, 2017
PubMed
Summary
This summary is machine-generated.

Researchers developed a novel single-molecule switch using antiaromaticity. This device exhibits low conductance in its neutral state and high conductance when electrochemically oxidized, creating a highly conducting antiaromatic structure.

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Area of Science:

  • Molecular electronics
  • Physical organic chemistry
  • Nanotechnology

Background:

  • Single-molecule electronic devices enable correlating physical phenomena with molecular structures.
  • Conventional devices utilize aromaticity for enhanced conductivity.
  • Hückel antiaromaticity is a classical concept in physical organic chemistry.

Purpose of the Study:

  • To demonstrate a single-molecule switch leveraging antiaromaticity.
  • To create a switchable molecular electronic device with tunable conductance.
  • To explore the relationship between antiaromaticity and high conductivity in single molecules.

Main Methods:

  • Fabrication of single-molecule devices using scanning tunneling microscope-based break-junction technique.
  • Electrochemical oxidation to induce changes in molecular electronic states.
  • Nuclear magnetic resonance (NMR) spectroscopy to confirm molecular structure and electronic properties.
  • Density functional theory (DFT) calculations to model and understand conductance mechanisms.

Main Results:

  • A single-molecule switch was successfully created using a thiophenylidene derivative.
  • The device demonstrated a reversible switching behavior with an on/off ratio of approximately 70.
  • Oxidation led to an antiaromatic state (6-4-6-π electrons) with significantly increased conductance.
  • NMR and DFT studies confirmed the antiaromatic character and high-conductance origin in the oxidized state.

Conclusions:

  • Antiaromaticity can be effectively utilized to design highly conducting single-molecule electronic devices.
  • The developed switch offers a novel approach for molecular electronic applications.
  • This work bridges fundamental physical organic chemistry concepts with practical molecular device engineering.