The Magnetic Properties of Verdazyl Free Radicals. III. The Anomalous Magnetic Behavior of Symmetrical Triphenylverdazyl

Azuma, Nagao; Yamauchi, Jun; Mukai, Kazuo; Ohya-Nishiguchi, Hiroaki; Deguchi, Yasuo

doi:10.1246/bcsj.46.2728

Cited by 71 publications

(21 citation statements)

References 31 publications

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“…Experimental studies on the magnetic properties of 2,4,6-TPV 1,4 have shown that the crystal exhibits a complex temperature dependence of susceptibility at the lower temperatures. While following the Curie-Weiss law above 20 K with a of 8 K, a broad maximum was reported at around 6.9 K (T max ), followed by a minimum a little below 2 K (T min ).…”

Section: The 246-tpv Crystalmentioning

confidence: 99%

“…Therefore, the exchange coupling breakdown temperatures (T C and T N ) of ferromagnetic ͑FM͒ and antiferromagnetic ͑AFM͒ molecular crystals are very low. These crystals often exhibit interesting magnetic behaviors, [1][2][3][4] which sometimes look like ͑and sometimes are inadvertently interpreted as͒ phase transitions. Experimentalists [1][2][3][4][5] have often tried to explain such properties on the basis of the linear Heisenberg chain model and related treatments.…”

Section: Introductionmentioning

confidence: 99%

“…These crystals often exhibit interesting magnetic behaviors, [1][2][3][4] which sometimes look like ͑and sometimes are inadvertently interpreted as͒ phase transitions. Experimentalists [1][2][3][4][5] have often tried to explain such properties on the basis of the linear Heisenberg chain model and related treatments. 6,7 It is noteworthy that the dependence of magnetic susceptibility on T has been studied for a lot of inorganic systems with small J values, and lower-dimensional ͑often, one-dimensional͒ Ising and Heisenberg models have been widely used in these cases to fit the experimental susceptibility versus temperature plots that show complex trends.…”

Section: Introductionmentioning

confidence: 99%

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On the variation of magnetic susceptibility of a molecular crystal with temperature: The 2,4,6-triphenylverdazyl system

Datta

Navada

2004

The Journal of Chemical Physics

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Magnetic susceptibilities of spin-1 2 systems of orthorhombic and higher crystal symmetries have been numerically investigated while taking possible anisotropy in the coupling constants along different crystal axes into account. The work relies on the magnon-based theory of ferromagnetic ͑FM͒ and antiferromagnetic ͑AFM͒ crystal systems of types FFF, AFF, AAF, and AAA ͓J. Chem. Phys. 111, 9009 ͑1999͔͒. The AAF crystal, in particular, shows interesting changes in the temperature dependence of magnetic susceptibility when the ferromagnetic exchange coupling constant is varied. We especially show that the susceptibility anomalies of molecular crystals fit naturally within the framework of the extended magnon-theoretical formalism, and do not necessarily imply a FM→AFM or a reverse phase transition. A real system, molecular crystal of 2,4,6-triphenylverdazyl ͑2,4,6-TPV͒, has been investigated here. It was previously interpreted as an AAF system from observed susceptibility data ͓Tomiyoshi et al., Phys. Rev. B 49, 16031 ͑1994͔͒. The trend of the temperature dependence of magnetic susceptibility studied in the present work also indicates that the crystal belongs to the AAF category with a less prominent FM exchange coupling constant. To reinforce our conclusions, we have adopted a two-pronged strategy. First, the geometry of the 2,4,6-TPV monomer has been optimized here by ab initio unrestricted Hartree-Fock ͑UHF͒ calculations using the STO-3G basis set. The optimized geometry is almost planar. A subsequent calculation has been carried out with the phenyl rings twisted out of the plane of the nitrogen atoms. The STO-3G optimized geometry, and the same geometry except for the twisted phenyl rings, have been used to perform ab initio coupled-cluster ͑UCCSD-T͒ calculations with the same basis, and UHF as well as density-functional ͑UB3LYP͒ calculations using the 6-31G basis set. The calculated data can easily rationalize the twists while the species remains in crystal. The magnetic category of the crystal has been unambiguously confirmed as AFA from ab initio UHF and UB3LYP calculations of the total energy in different spin states of dimers and trimers along the crystal axes. The computed energy values, however, fail to yield accurate estimates of the exchange coupling constants J a , J b , and J c , because the latter are on the order of 1k B K corresponding to energy differences on the order of 10 Ϫ6 hartree between different spin states. In the second approach, the observed features of the susceptibility minimum and maximum have been used to determine the best values of the exchange coupling constants from the theoretical formulas for an anisotropic AFA crystal. The AFM (J a and J c ) and FM (J b ) exchange coupling constants and the Néel temperature (T N ) found from this analysis correspond to J a ϩJ c ϭϪ1.05 k B K, J b ϭ1.35 k B K, and T N ϭ1.75 K. The calculated J values significantly differ from those estimated from a linear Heisenberg chain model, but generate a susceptibility versus temperature graph that mimics...

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Section: The 246-tpv Crystalmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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On the variation of magnetic susceptibility of a molecular crystal with temperature: The 2,4,6-triphenylverdazyl system

Datta

Navada

2004

The Journal of Chemical Physics

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“…Azuma et al 3 pointed out that the spin density is distributed over four nitrogen atoms in the verdazyl ring, which implies the delocalization of the unpaired electron over the TPV molecule.…”

Section: A Nonresonant X-ray Emission Spectramentioning

confidence: 99%

“…2,4,6-triphenylverdazyl ͑TPV͒ shows an antiferromagnetic ordering with a Néel temperature of 1.7 K accompanied by a weak ferromagnetic component, [2][3][4] whereas, 2,4,6-tri-ptolylverdazyl ͑TTV͒ is paramagnetic. 5 According to Ref.…”

Section: Introductionmentioning

confidence: 99%

Soft-x-ray fluorescence study of the quasi-one-dimensional Heisenberg antiferromagnet tetraphenylverdazyl

Shimada¹,

Endo²,

Stadler³

2000

Phys. Rev. B

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Soft-x-ray fluorescence measurements have been performed on a single crystal of the organic antiferromagnet 2,4,6-triphenylverdazyl. Resonant and nonresonant C K␣ and N K␣ (2p→1s transition͒ x-ray emission spectra ͑XES͒ were measured and compared with x-ray photoelectron valence band spectra and deMon density-functional theory calculations. It is shown that intramolecular interactions are much stronger than intermolecular ones and give the main contribution to the formation of C 2p density of states. We present evidence of a delocalization of unpaired N 2p electrons over the verdazyl ring. The excitation energy dependence of C K␣ and N K␣ XES observed below the C 1s and N 1s thresholds, respectively, is discussed in terms of symmetry selective resonant inelastic x-ray scattering.

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New concepts for molecular magnets

Pilawa

1999

Ann. Phys.

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Miller and Epstein (1994) define molecular magnets as magnetic materials which are prepared by the low-temperature methods of the preparative chemistry. This definition includes molecular crystals of neutral radicals, radical salts and charge transfer complexes as well as metal complexes and polymers with unpaired spins (Dormann 1995). The challenge of molecular magnets consists in tailoring magnetic properties by specific modifications of the molecular units. The combination of magnetism with mechanical or electrical properties of molecular compounds promise materials of high technical interest (Gatteschi 1994a, Mo È hwald 1996 and both the chemical synthesis of new molecular materials with magnetic properties as well as the physical investigation and explanation of these properties is important, in order to achieve any progress. This work deals with the physical characterization of the magnetic properties of molecular materials. It is organized as follows. In the first part molecular crystals of neutral radicals are studied. After briefly discussing the general magnetic properties of these materials and after an overview over the physical principles of exchange interaction between organic radicals I focus on the interplay between the crystallographic structure and the magnetic properties of various derivatives of the verdazyl and nitronyl nitroxide radicals. The magnetic properties of metal complexes are the subject of the second part. After an overview over the experimental and theoretical tools which are used for the investigation of the magnetic properties I shortly discuss the exchange coupling of transition metal ions and the magnetic properties of complexes of two and three metal ions. Special emphasis is given to spin cluster compounds. Spin cluster denote complexes of many magnetic ions. They are attractive as building blocks of molecular magnets as well as magnetic model compounds for the study of spin frustration, molecular super-paramagnetism and quasi one-dimensional magnets.

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The Magnetic Properties of Verdazyl Free Radicals. III. The Anomalous Magnetic Behavior of Symmetrical Triphenylverdazyl

Cited by 71 publications

References 31 publications

On the variation of magnetic susceptibility of a molecular crystal with temperature: The 2,4,6-triphenylverdazyl system

On the variation of magnetic susceptibility of a molecular crystal with temperature: The 2,4,6-triphenylverdazyl system

Soft-x-ray fluorescence study of the quasi-one-dimensional Heisenberg antiferromagnet tetraphenylverdazyl

New concepts for molecular magnets

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