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Related Concept Videos

Aryldiazonium Salts to Azo Dyes: Diazo Coupling01:11

Aryldiazonium Salts to Azo Dyes: Diazo Coupling

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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...
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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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Thermal and Photochemical Electrocyclic Reactions: Overview01:26

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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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Updated: May 29, 2025

High-Contrast and Fast Photorheological Switching of a Twist-Bend Nematic Liquid Crystal
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Arylazopyrazole Photoconversion Enables Tunable Morphology and Mechanical Properties.

Soumen Adak1, Ishita Ghosh1, Manik Lal Maity1

  • 1Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur, Nadia, West Bengal, 741246, India.

Small (Weinheim an Der Bergstrasse, Germany)
|February 4, 2025
PubMed
Summary

A novel photochromic molecule, arylazo-N-phenyl-3,5-diphenyl pyrazole (AAP1), exhibits rapid, reversible switching under red light. Its unique structure provides high thermal stability and distinct mechanical properties in solid-state applications.

Keywords:
arylazopyrazoleselastic flexibilitymechanical propertyphotoswitchingsingle‐crystal

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Area of Science:

  • Materials Science
  • Organic Chemistry
  • Photochemistry

Background:

  • Azoheteroarenes are promising photochromic materials.
  • Developing molecules with efficient and stable photoswitching is crucial for advanced applications.
  • Understanding structure-property relationships in photochromic systems is key to molecular design.

Purpose of the Study:

  • To report a novel azoheteroarene, arylazo-N-phenyl-3,5-diphenyl pyrazole (AAP1), with efficient two-way photoswitching.
  • To investigate the solid-state photoswitching behavior of AAP1 in thin films and patterned inks.
  • To elucidate the structural basis for the enhanced thermal stability and differing mechanical properties of AAP1 isomers.

Main Methods:

  • Synthesis and characterization of arylazo-N-phenyl-3,5-diphenyl pyrazole (AAP1).
  • UV-Vis spectroscopy and photochromic switching studies in solution and thin films.
  • Single-crystal X-ray diffraction (SCXRD) for structural analysis of both isomers.
  • Nanoindentation experiments to quantify mechanical properties.

Main Results:

  • AAP1 demonstrates fast, near-quantitative two-way photoswitching, switchable by red light, with a high thermal half-life for the cis-isomer.
  • Solid-state photoswitching was achieved in pristine thin films and photo-patterned paper using AAP1 as ink.
  • SCXRD revealed intramolecular π-stacking in the cis-isomer, enhancing thermal stability.
  • Nanoindentation showed significant differences in mechanical properties: trans-AAP1 crystals are soft and flexible, while cis-AAP1 crystals are brittle.

Conclusions:

  • AAP1 is a highly efficient photochromic molecule with excellent thermal stability and tunable mechanical properties.
  • The intramolecular π-stacking interaction in the cis-isomer is critical for its enhanced stability.
  • The distinct mechanical properties of the trans and cis isomers offer potential for novel applications in responsive materials and actuators.