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Designing stable π-radicals
Arthur Houplin1, Cheng-Hao Liu1,2, Dmytro F Perepichka1
1Department of Chemistry, McGill University, Montreal, QC, Canada. dmytro.perepichka@mcgill.ca.
Chemical Society Reviews
|June 26, 2026
Summary
Stable π-conjugated radicals, molecules with unpaired electrons, are crucial functional materials. This review covers their history, design principles for stability, and applications in electronics and magnetism.
Area of Science:
- Organic Chemistry
- Materials Science
- Quantum Chemistry
Background:
- π-Conjugated radicals, possessing unpaired electrons, have evolved from curiosities to vital functional materials over 125 years.
- Their study has significantly advanced electronic structure theory and understanding of organic compound reactivity.
- Applications span organic conductors, semiconductors, magnets, and electroluminescent/quantum materials.
Purpose of the Study:
- To provide essential quantum-chemical background and practical insights into stable π-radical development.
- To inspire and guide new researchers entering the field of stable π-radical design.
- To systematize key examples and applications of stable neutral π-radicals and radical ions.
Main Methods:
- Discussion of the fundamental causes of π-radical reactivity.
- Exploration of strategies to suppress reactivity for enhanced stability.
- Systematic review of stable neutral π-radical classes (arylmethyl, polycyclic hydrocarbons, heteroatomic, di-/polyradicals) and radical ions.
Main Results:
- Unpaired electrons in π-radicals lead to unique electronic, optical, and magnetic properties.
- Key examples of stable neutral π-radicals and radical ions are highlighted.
- Established methods for designing stable π-radicals are presented.
Conclusions:
- Stable π-radicals are pivotal in advanced functional materials.
- Understanding radical reactivity and stability is key for future applications.
- This review consolidates historical knowledge to foster innovation in π-radical research.
Related Concept Videos
01:10Radical Reactivity: Steric Effects
The presence of electron-donating, electron-withdrawing, or conjugating groups adjacent to a radical center, imparts electronic stabilization to the radicals. Examples of such electronically-stabilized radicals are triphenylmethyl, tetramethylpiperidine‐N‐oxide, and 2,2‐diphenyl‐1‐picrylhydrazyl. These radicals are remarkably stable and are known as persistent radicals. Some of the persistent radicals can even be isolated and purified.
Along with electronic factors, steric factors also account...
Along with electronic factors, steric factors also account...
01:11Radical Reactivity: Overview
Radicals, the highly reactive species, gain stability by undergoing three different reactions. The first reaction involves a radical-radical coupling, in which a radical combines with another radical, forming a spin‐paired molecule. The second reaction is between a radical and a spin‐paired molecule, generating a new radical and a new spin‐paired molecule. The third reaction is radical decomposition in a unimolecular reaction, forming a new radical and a spin‐paired molecule. These three...
01:27π Molecular Orbitals of the Allyl Radical
Allyl radicals are three-carbon conjugated systems. They are readily formed as intermediates in halogenation reactions of alkenes involving the addition of halogen to the allylic carbon instead of the double bond. As seen in allyl cations and anions, each of the three sp2-hybridized carbon atoms in allyl radicals has an unhybridized p orbital. These orbitals combine to give three π molecular orbitals.
The allyl systems have identical molecular orbitals but differ in the number of π electrons.
The allyl systems have identical molecular orbitals but differ in the number of π electrons.
00:47Radical Formation: Addition
Radicals can be formed by adding a radical to a spin-paired molecule. This is typically observed with unsaturated species, where the addition of a radical across the π bond leads to the production of a new radical by dissolving the π bond. For example, the addition of a Br radical to an alkene yields a carbon-centered radical.
Similar to charge conservation in chemical reactions, spin conservation is implicit for radical reactions. Accordingly, the product formed must possess an unpaired...
Similar to charge conservation in chemical reactions, spin conservation is implicit for radical reactions. Accordingly, the product formed must possess an unpaired...
01:07Radicals: Electronic Structure and Geometry
This lesson delves into the geometry of a radical, which is influenced by the electronic structure of the molecule. The principle is similar to that of a lone pair, where the unpaired electron influences the geometry at the radical center.
Accordingly, the structure of a trivalent radical lies between the geometries of carbocations and carbanions. An sp2-hybridized carbocation is trigonal planar, while an sp3-hybridized carbanion is trigonal pyramidal. Here, the difference in geometry is...
Accordingly, the structure of a trivalent radical lies between the geometries of carbocations and carbanions. An sp2-hybridized carbocation is trigonal planar, while an sp3-hybridized carbanion is trigonal pyramidal. Here, the difference in geometry is...
01:03Radical Formation: Overview
A bond can be broken either by heterolytic bond cleavage to form ions or homolytic bond cleavage to yield radicals. A fishhook arrow is used to represent the motion of a single electron in homolytic bond cleavage. There are two main sources from which radicals can be formed:
Radicals from spin-paired molecules:
Radicals can be obtained from spin-paired molecules either by homolysis or electron transfer. While two radicals are formed in the former, an electron is added in the latter, also known...
Radicals from spin-paired molecules:
Radicals can be obtained from spin-paired molecules either by homolysis or electron transfer. While two radicals are formed in the former, an electron is added in the latter, also known...