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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.

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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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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.
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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: Jul 12, 2025

Scale-up Chemical Synthesis of Thermally-activated Delayed Fluorescence Emitters Based on the Dibenzothiophene-S,S-Dioxide Core

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Published on: October 24, 2017

Entropy-driven charge-transfer complexation yields thermally activated delayed fluorescence and highly efficient

Chun-Yen Lin¹, Chao-Hsien Hsu¹, Chieh-Ming Hung¹

¹Department of Chemistry, National Taiwan University, Taipei, Taiwan.

Nature Chemistry

|October 26, 2023

Summary

Researchers created inclusion complexes with exciplex-like properties for organic light-emitting diodes. This study provides structural and thermodynamic insights into these systems, enabling property tuning and improved device performance.

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Published on: October 18, 2018

Area of Science:

Materials Science
Organic Chemistry
Physical Chemistry

Background:

Thermally activated delayed fluorescence (TADF) systems are crucial for organic light-emitting diodes (OLEDs).
Further development of TADF materials is often limited by insufficient structural and thermodynamic characterization.
Exciplexes are key to TADF but require detailed understanding of their formation and properties.

Purpose of the Study:

To synthesize and characterize inclusion complexes exhibiting exciplex-like TADF.
To investigate the structural and thermodynamic aspects of host-guest complex formation.
To explore the potential of these complexes in organic light-emitting diode applications.

Main Methods:

Generation of inclusion complexes between a macrocyclic electron-accepting host (A) and N-methyl-indolocarbazole electron-donating guests (D).
X-ray crystallography for cocrystal structure determination.
UV-visible titration for thermodynamic analysis (enthalpy and entropy).
Fabrication and testing of OLED devices using the developed D/A systems.

Main Results:

Successfully formed and structurally resolved D/A inclusion complexes.
Demonstrated exciplex-like TADF via through-space electron transfer.
Characterized complex formation as an endothermic and entropy-driven process.
Achieved a maximum external quantum efficiency of 15.2% in fabricated OLEDs, with 10.3% maintained at 1,000 cd/m².

Conclusions:

Inclusion complexation is a viable strategy for understanding exciplex structure-property relationships.
The study provides a foundation for designing novel TADF materials through controlled complexation.
The findings highlight the potential for fine-tuning emission properties by modifying donor molecular orbitals.