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Related Concept Videos

Radical Reactivity: Overview01:11

Radical 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...
Radical Formation: Overview01:03

Radical 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...
Radical Formation: Addition00:47

Radical 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...
Radical Chain-Growth Polymerization: Mechanism01:09

Radical Chain-Growth Polymerization: Mechanism

The radical chain-growth polymerization mechanism consists of three steps: initiation, propagation, and termination of polymerization. The polymerization initiates when a free radical generated from the radical initiator adds to the unsaturated bond in the monomer. The unpaired electron of the free radical and one π electron in the unsaturated bond creates a σ bond between the free radical and the monomer. As a result, the other π electron in the unsaturated bond converts this species into the...
Electron Paramagnetic Resonance (EPR) Spectroscopy: Organic Radicals01:17

Electron Paramagnetic Resonance (EPR) Spectroscopy: Organic Radicals

Ideally, an unpaired electron shows a single peak in the EPR spectrum due to the transition between the two spin energy states. However, coupling interactions can occur between the spins of the unpaired electron and any neighboring spin-active nuclei. This hyperfine coupling results in hyperfine splitting, where the EPR signal is split into multiplets. The signals split into 2nI + 1 peaks, where n is the number of equivalent nuclei and I is the nuclear spin. These splitting patterns provide...
Radical Reactivity: Steric Effects01:10

Radical 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...

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Reverse Spin-Transition-Like Behavior in a Single-Molecule Organic Diradical.

Rina Takano1, Takayuki Ishida1

  • 1The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan.

The Journal of Physical Chemistry Letters
|May 14, 2026
PubMed
Summary
This summary is machine-generated.

Organic diradical PyPBN exhibits a spin transition linked to a structural phase transition near 330 K. This transition causes thermochromic and thermomagnetic effects, with ferromagnetic coupling significantly reduced in the high-temperature phase.

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Area of Science:

  • Organic electronics
  • Materials science
  • Physical chemistry

Background:

  • Organic diradicals are promising for molecular magnetism.
  • Spin transitions in organic materials can lead to unique electronic and magnetic properties.

Purpose of the Study:

  • To investigate the spin transition and associated structural phase transition in PyPBN.
  • To elucidate the relationship between molecular structure, spin coupling, and magnetic behavior.

Main Methods:

  • Experimental determination of exchange coupling parameters (2J/kB).
  • Density Functional Theory (DFT) calculations for exchange coupling.
  • Crystallographic analysis to study structural differences between phases.

Main Results:

  • A structural phase transition around 330 K in PyPBN is associated with a spin transition.
  • Significant quenching of ferromagnetic coupling occurs in the high-temperature phase.
  • Increased nitroxide-benzene torsion in the high-temperature phase suppresses pi-delocalization and destabilizes the triplet state.

Conclusions:

  • The spin transition in PyPBN is driven by structural changes affecting electronic delocalization.
  • The observed thermomagnetic behavior resembles a reverse spin-transition phenomenon.
  • The interplay between enthalpy and spin entropy drives the phase transition.