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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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Cycloaddition Reactions: MO Requirements for Photochemical Activation01:12

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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

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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[3,3] Sigmatropic Rearrangement of 1,5-Dienes: Cope Rearrangement01:21

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The Cope rearrangement is classified as a [3,3] sigmatropic shift in 1,5-dienes, leading to a more stable, isomeric 1,5-diene. The reaction involves a concerted movement of six electrons, four from two π bonds and two from a σ bond, via an energetically favorable chair-like transition state.
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Thermal Electrocyclic Reactions: Stereochemistry01:17

Thermal Electrocyclic Reactions: Stereochemistry

2.5K
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.
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Radical Formation: Homolysis00:54

Radical Formation: Homolysis

4.2K
A bond is formed between two atoms by sharing two electrons. When this bond is broken by supplying sufficient energy, either two electrons can be taken up by one atom forming ions by the cleavage called heterolysis, or the two electrons are shared by two atoms, with one each creating radicals by the cleavage called homolysis.
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Determination of the Photoisomerization Quantum Yield of a Hydrazone Photoswitch
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Manipulating dynamic covalent bonds through direct photoisomerization.

Neil D Dolinski1, Alex E Crolais2, Nicholas R Boynton1

  • 1Pritzker School of Molecular Engineering, University of Chicago Chicago Illinois 60637 USA stuartrowan@uchicago.edu n.dolinski@columbia.edu.

Chemical Science
|November 10, 2025
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Summary
This summary is machine-generated.

Researchers developed light-sensitive benzalisoxazolone (BIOx) molecules that enhance thiol-bonding under visible light. This innovation enables light-triggered stiffening in dynamic polymer networks and organogels.

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Area of Science:

  • Materials Science
  • Polymer Chemistry
  • Organic Chemistry

Background:

  • Light is an attractive stimulus for controlling dynamic covalent chemistries due to its availability and spatiotemporal control.
  • Dynamic covalent chemistries offer unique properties for advanced materials but require precise control mechanisms.

Purpose of the Study:

  • To develop novel photo-isomerizable benzalisoxazolone (BIOx) thia-Michael (tM) acceptors.
  • To investigate the light-induced increase in thiol-bonding and system equilibrium (Keq).
  • To incorporate these photo-responsive units into dynamic polymer networks for light-triggered material properties.

Main Methods:

  • Synthesis of photo-isomerizable benzalisoxazolone (BIOx) thia-Michael (tM) acceptors.
  • Irradiation with visible light (455-470 nm) to induce photo-isomerization and thiol-bonding.
  • In situ photo-NMR experiments to quantify changes in Keq.
  • Computational studies to elucidate the mechanism of increased Keq.
  • Incorporation into dynamic polymer networks to form organogels.

Main Results:

  • Developed BIOx tM acceptors showing increased thiol-bonding upon visible light irradiation.
  • Photo-NMR confirmed significant increases in Keq for various electronically-substituted BIOx tM acceptors.
  • Steric interactions in the E-isomer were identified as a driving force for enhanced Keq.
  • Light intensity was shown to tune the system's response.
  • Developed organogels exhibiting reversible, on-demand, light-triggered stiffening.

Conclusions:

  • Photo-isomerizable BIOx tM acceptors provide a mechanism for light-controlled dynamic covalent chemistry.
  • Steric effects play a crucial role in driving the photo-induced increase in bonding.
  • The developed materials offer potential for creating smart, light-responsive polymer networks and organogels.