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The reaction of weakly electrophilic aryldiazonium (also called arenediazonium) salts with highly activated aromatic compounds leads to the formation of products with an —N=N— link, called an azo linkage. This reaction, presented in Figure 1, is known as diazo coupling and occurs without the loss of the nitrogen atoms of the aryldiazonium salt. Highly activated aromatic compounds such as phenols or arylamines favor the diazo coupling reaction. The coupling generally occurs at the para...
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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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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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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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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.
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UV–Visible absorption spectra of conjugated dienes arise from the lowest energy π → π* transitions. The light-absorbing part of the molecule is called the chromophore, and the substituents directly attached to the chromophore are called auxochromes. A strong correlation exists between the absorption maxima, λmax, and the structure of a conjugated π system. The Woodward–Fieser rules predict the value of λmax for a given structure by adding the...
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Multi-Photochromic Molecules Based on Dihydroazulene Units.

Anne Ugleholdt Petersen1, Jonathan Kirschner Solberg Hansen1, Emilie Sperling Andreasen1

  • 1Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100, Copenhagen, Denmark.

Chemistry (Weinheim an Der Bergstrasse, Germany)
|February 25, 2020
PubMed
Summary

Researchers explored multi-photochromic systems using dihydroazulene/vinylheptafulvene (DHA/VHF) switches for data storage. Linker choice significantly impacts switching speed and reversibility, with azulene-1,3-diyl spacers showing slower thermal back-reactions.

Keywords:
azulenesmultistatesphotoswitchesthermoswitchestrimers

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Area of Science:

  • Molecular chemistry
  • Materials science
  • Organic electronics

Background:

  • Multi-photochromic molecular systems are crucial for advanced data storage applications.
  • The dihydroazulene/vinylheptafulvene (DHA/VHF) photo-/thermoswitch offers tunable switching properties.

Purpose of the Study:

  • To synthesize and characterize novel multi-photochromic systems based on DHA/VHF.
  • To investigate the influence of different linkers on the photochromic switching behavior.
  • To explore structural modifications for enhanced control over switching events.

Main Methods:

  • Synthesis of DHA-based molecular systems with varying linkers (meta-phenylene, azulene-1,3-diyl, thiophene-2,5-diyl).
  • Photoisomerization and thermal back-reaction studies.
  • X-ray crystallographic analysis of a DHA dimer-Cu(I) complex.

Main Results:

  • Azulene-1,3-diyl spacers significantly reduced photoisomerization and slowed thermal back-reactions compared to meta-phenylene.
  • A DHA trimer exhibited stepwise photoisomerization with concurrent thermal back-reactions.
  • Structural modifications at C-1 and C-7 positions of DHA were explored for further functionalization.

Conclusions:

  • Linker selection critically controls the photo- and thermal-switching dynamics in DHA/VHF systems.
  • Stepwise photoisomerization and controlled thermal reversibility are achievable.
  • These findings pave the way for designing sophisticated molecular switches for data storage.