Conformationally constrained, stable, triplet ground state (S = 1) nitroxide diradicals. Antiferromagnetic chains of S = 1 diradicals

Andrzej Rajca, Masahiro Takahashi, Maren Pink, Gaëlle Spagnol, Suchada Rajca

Research output: Contribution to journalArticle

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Abstract

Nitroxide diradicals, in which nitroxides are annelated to m-phenylene forming tricyclic benzo-bisoxazine-like structures, have been synthesized and characterized by X-ray crystallography, magnetic resonance (EPR and 1H NMR) spectroscopy, as well as magnetic studies in solution and in solid state. For the octamethyl derivative of benzobisoxazine nitroxide diradical, the conformationally constrained nitroxide moieties are coplanar with the m-phenylene, leading to large values of 2J (2J/k > 200 K in solution and 2J/k ≫ 300 K in the solid state). For the diradical, in which all ortho and para positions of the m-phenylene are sterically shielded, distortion of the nitroxide moieties from coplanarity is moderate, such that the singlet-triplet gaps remain large in both solution (2J/k > 200 K) and the solid state (2J/k ≈ 400-800 K), though an onset of thermal depopulation of the triplet ground state is detectable near room temperature. These diradicals have robust triplet ground states with strong ferromagnetic coupling and good stability at ambient conditions. Magnetic behavior of the nitroxide diradicals at low temperature is best fit to the model of one-dimensional S = 1 Heisenberg chains with intrachain antiferromagnetic coupling. The antiferromagnetic coupling between the S = 1 diradicals may be associated with the methyl nitroxide C-H⋯O contacts, including nonclassical hydrogen bonds. These unprecedented organic S = 1 antiferromagnetic chains are highly isotropic, compared to those of the extensively studied Ni(II)-based chains.

Original languageEnglish (US)
Pages (from-to)10159-10170
Number of pages12
JournalJournal of the American Chemical Society
Volume129
Issue number33
DOIs
StatePublished - Aug 22 2007

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Ground state
Magnetic Resonance Spectroscopy
Temperature
X ray crystallography
X Ray Crystallography
Magnetic resonance
Nuclear magnetic resonance spectroscopy
Paramagnetic resonance
Hydrogen
Hydrogen bonds
Hot Temperature
Derivatives
Proton Magnetic Resonance Spectroscopy

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

Cite this

Conformationally constrained, stable, triplet ground state (S = 1) nitroxide diradicals. Antiferromagnetic chains of S = 1 diradicals. / Rajca, Andrzej; Takahashi, Masahiro; Pink, Maren; Spagnol, Gaëlle; Rajca, Suchada.

In: Journal of the American Chemical Society, Vol. 129, No. 33, 22.08.2007, p. 10159-10170.

Research output: Contribution to journalArticle

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