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Related Concept Videos

Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical Electrocyclic Reactions: Stereochemistry

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The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation
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Thermal and Photochemical Electrocyclic Reactions: Overview01:26

Thermal and Photochemical Electrocyclic Reactions: Overview

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Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.
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Cycloaddition Reactions: MO Requirements for Photochemical Activation01:12

Cycloaddition Reactions: MO Requirements for Photochemical Activation

2.5K
Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene molecules occurs only in the presence of light. It is photochemically allowed but thermally forbidden.
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Diels–Alder Reaction Forming Bridged Bicyclic Products: Stereochemistry01:29

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Diels–Alder reactions between cyclic dienes locked in an s-cis configuration and dienophiles yield bridged bicyclic products.
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[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction01:16

[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction

12.0K
The Diels–Alder reaction is an example of a thermal pericyclic reaction between a conjugated diene and an alkene or alkyne, commonly referred to as a dienophile. The reaction involves a concerted movement of six π electrons, four from the diene and two from the dienophile, forming an unsaturated six-membered ring. As a result, these reactions are classified as [4+2] cycloadditions.
12.0K
Thermal Electrocyclic Reactions: Stereochemistry01:17

Thermal Electrocyclic Reactions: Stereochemistry

2.4K
The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
2.4K

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Highly Efficient Photoswitch in Diarylethene-Based Molecular Junctions.

Imen Hnid1, Denis Frath1, Frederic Lafolet1

  • 1Université de Paris, ITODYS, CNRS, UMR 7086, 15 rue J-A de Baïf, 75205 Paris Cedex 13, France.

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|April 14, 2020
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Thin layers of diarylethene oligomers were deposited and formed molecular junctions. These junctions showed significant conductivity changes upon light-induced switching, demonstrating potential for molecular electronics.

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Area of Science:

  • Materials Science
  • Molecular Electronics
  • Organic Chemistry

Background:

  • Diarylethene oligomers (oligo(DAE)) are photochromic molecules with potential applications in molecular electronics.
  • Controlling conductivity in molecular junctions is crucial for device development.

Purpose of the Study:

  • To fabricate and characterize solid-state molecular junctions using thin layers of diarylethene oligomers.
  • To investigate the photoswitchable conductivity of these molecular junctions.

Main Methods:

  • Electrochemical deposition of oligo(DAE) on glassy carbon and gold electrodes.
  • Characterization using electrochemistry, XPS, and AFM.
  • Fabrication of molecular junctions with C-AFM tips and varying layer thicknesses (2-3 nm and 8-9 nm).

Main Results:

  • Successful deposition and characterization of oligo(DAE) thin layers.
  • Demonstrated light-induced switching between open (highly resistive) and closed (conductive) forms.
  • Achieved ON/OFF ratios of 2-3 for 3-nm junctions and 200-400 for 9-nm junctions.

Conclusions:

  • Diarylethene oligomer molecular junctions exhibit photoswitchable conductivity.
  • Junction thickness influences the ON/OFF ratio, with thicker junctions showing higher ratios.
  • Oligo(DAE) molecular junctions show promise for applications in molecular switches and memory devices.