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Thermal and Photochemical Electrocyclic Reactions: Overview

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 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.
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Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition state. These reactions are stereospecific and regioselective. The stereochemistry of the products depends on the symmetry characteristics of the interacting orbitals and the reaction conditions. Accordingly, pericyclic reactions are classified as either symmetry-allowed or symmetry-forbidden. Woodward and Hoffmann presented the selection criteria for...
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Related Experiment Video

Updated: Jul 13, 2026

High-Contrast and Fast Photorheological Switching of a Twist-Bend Nematic Liquid Crystal
06:24

High-Contrast and Fast Photorheological Switching of a Twist-Bend Nematic Liquid Crystal

Published on: October 31, 2019

Novel photoswitchable rotaxanes.

Werner Abraham1, Karin Buck, Marziena Orda-Zgadzaj

  • 1Institute for Chemistry, Humboldt-University, Berlin, Germany. abraham@chemie.hu-berlin.de

Chemical Communications (Cambridge, England)
|July 20, 2007
PubMed
Summary

Researchers developed a novel photoresponsive rotaxane. This molecular machine utilizes the photoheterolysis of an acridane unit as a bulky end group for controlled movement.

Area of Science:

  • Supramolecular Chemistry
  • Photochemistry

Background:

  • Rotaxanes are mechanically interlocked molecules with potential applications in molecular machines.
  • Photoresponsive materials offer dynamic control over molecular structure and function.

Purpose of the Study:

  • To develop a novel photoresponsive rotaxane system.
  • To utilize photoheterolysis for controlled molecular motion within a rotaxane.

Main Methods:

  • Synthesis of a rotaxane incorporating an acridane unit.
  • Investigation of the photoheterolysis mechanism of the acridane end group.

Main Results:

  • Successfully developed a photoresponsive rotaxane.
  • The acridane unit functions as a bulky end group and undergoes photoheterolysis.

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Determination of the Photoisomerization Quantum Yield of a Hydrazone Photoswitch

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  • Demonstrated photo-induced changes in the rotaxane structure.

    • Conclusions:

    • The developed rotaxane is a promising platform for photo-switchable molecular devices.
    • Photoheterolysis provides a viable mechanism for actuating rotaxane systems.