Conformationally Constrained, Stable, Triplet Ground State (S = 1) Nitroxide Diradicals. Antiferromagnetic Chains of S = 1 Diradicals

Journal Article · Mon Jun 30 04:00:00 EDT 2008 · J. Am. Chem. Soc.

DOI:https://doi.org/10.1021/ja0712017· OSTI ID:1007609

Rajca, Andrzej; Takahashi, Masahiro; Pink, Maren; Spagnol, Gaelle; Rajca, Suchada ^[1]

---

1. UNL

Nitroxide diradicals, in which nitroxides are annelated to m-phenylene forming tricyclic benzobisoxazine-like structures, have been synthesized and characterized by X-ray crystallography, magnetic resonance (EPR and {sup 1}H 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 {approx} 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 {hor_ellipsis} 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.

• Research Organization: Advanced Photon Source (APS), Argonne National Laboratory (ANL), Argonne, IL (US)
• Sponsoring Organization: USDOE
• OSTI ID: 1007609
• Journal Information: J. Am. Chem. Soc., Journal Name: J. Am. Chem. Soc. Journal Issue: (33) ; 08, 2007 Vol. 129; ISSN JACSAT; ISSN 0002-7863
• Country of Publication: United States
• Language: ENGLISH

Related Subjects

08 HYDROGEN
36 MATERIALS SCIENCE
ANTIFERROMAGNETIC MATERIALS
CHAINS
CRYSTALLOGRAPHY
GROUND STATES
HYDROGEN
MAGNETIC RESONANCE
SPECTROSCOPY
STABILITY
TRIPLETS