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Thermal and Photochemical Electrocyclic Reactions: Overview

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.

Photochemical Electrocyclic Reactions: Stereochemistry

Photochemical Electrocyclic Reactions: Stereochemistry

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

Photosystem I

Photosystem I

Although structurally similar to photosystem II (PSII), photosystem I (PSI) is has a different electron supplier and electron acceptor.
Both these photosystems work in concert. An excited electron from PSII is relayed to PSI via an electron transport chain in the thylakoid membrane of the chloroplast, which is comprised of the carrier molecule plastoquinone, the dual-protein cytochrome complex, and plastocyanin. As electrons move between PSII and PSI, they lose energy and must be re-energized...

Photosystem II

Photosystem II

The multi-protein complex photosystem II (PS II) harvests photons and transfers their energy through its bound pigments to its reaction center, and ultimately to photosystem I (PSI) through the electron transport chain. The pigments responsible for caputirng the light energy in photosystems include chlorophyll a, chlorophyll b, and carotenoids.
The pigment molecules are arranged across two photosystem domains — the antenna complex and the reaction center. The main aim of the pigment molecules...

Thermal Electrocyclic Reactions: Stereochemistry

Thermal Electrocyclic Reactions: Stereochemistry

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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Related Experiment Video

Updated: May 12, 2026

A 'Plug and Play' Method to Create Water-dispersible Nanoassemblies Containing an Amphiphilic Polymer, Organic Dyes and Upconverting Nanoparticles

A 'Plug and Play' Method to Create Water-dispersible Nanoassemblies Containing an Amphiphilic Polymer, Organic Dyes and Upconverting Nanoparticles

Published on: November 14, 2015

A photoswitchable poly(3-hexylthiophene).

Peter Bauer¹, Michael Sommer, Johann Thurn

¹Applied Functional Polymers, Department of Macromolecular Chemistry I, University of Bayreuth, Universitätsstrasse 30, 95440 Bayreuth, Germany.

Chemical Communications (Cambridge, England)

|April 12, 2013

Summary

Researchers synthesized a photoswitchable polymer, poly(3-hexylthiophene) (P3HT). Its light emission can be reversibly controlled, decreasing by 70% when the photoswitch is closed.

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Monitoring the Effects of Illumination on the Structure of Conjugated Polymer Gels Using Neutron Scattering

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Published on: December 21, 2017

Area of Science:

Polymer Chemistry
Materials Science
Organic Electronics

Background:

Conjugated polymers like poly(3-hexylthiophene) (P3HT) are crucial for organic electronics.
Developing stimuli-responsive polymers is key for advanced material applications.
Controlling polymer properties with external triggers like light is an active research area.

Purpose of the Study:

To synthesize a well-defined, main-chain photoswitchable poly(3-hexylthiophene) (P3HT).
To investigate the reversible modulation of P3HT's photoluminescence properties using a photoswitch.
To quantify the optical property changes during photoswitching cycles.

Main Methods:

Synthesis of a main-chain conjugated photoswitchable P3HT with homogeneous hydrogen end groups.
Utilizing irradiation/spectroscopy sequences to monitor optical properties.
Characterizing the reversible changes in P3HT emission upon photoswitch actuation.

Main Results:

Successful synthesis of a photoswitchable P3HT derivative.
Demonstration of reversible modulation of P3HT emission by opening and closing the photoswitch.
Observation of a significant quenching (70%) of P3HT emission when the photoswitch is in the closed state.

Conclusions:

The synthesized photoswitchable P3HT offers a new platform for light-controlled polymer systems.
Reversible control over P3HT emission is achievable, with significant quenching upon photoswitch closure.
This work demonstrates the potential of incorporating photoswitches into conjugated polymers for tunable optical properties.