Mechanism for spin alignments in organic molecule-based ferrimagnetics

Shiomi, Daisuke; Kaneda, C.; Kanzaki, Yuki; Satô, Kenji; Takui, Takeji

doi:10.1016/s0379-6779(02)01031-7

Cited by 5 publications

(3 citation statements)

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“…A large amount of computational and theoretical work on organic polyradicals has been done to characterize the intramolecular ferromagnetic (FM) and antiferromagnetic (AFM) interactions in π-conjugated species. − Magnetic anisotropy from dipolar interaction can be important for a hydrocarbon-based polyradical . Jiang and his co-workers carried out an extensive valence-bond study on a series of π-conjugated radical systems, which included the polyarylmethyl moiety.…”

Section: Introductionmentioning

confidence: 99%

Quantum Chemical Investigation of Calix[4]arene-Based Radicals with Bis(biphenylene)methyl Linkers as Precursors of Spin Glass and Superparamagnets

Pal

Datta

2014

J. Phys. Chem. C

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A computational investigation is carried out on a calix[4]arene based tetraradical and a similar hexaradical with and without bis(biphenylene)-methyl substituent(s). The dimeric forms of these radicals are studied with aromatic groups replaced by hydrogen atoms. We use the B3LYP/6-31G(d,p) scheme for optimization and the 6-311G(d,p) basis set for single point calculations on high-spin and broken-symmetry solutions. Magnetic exchange coupling constants (J) relevant to both Heisenberg exchange Hamiltonian and Heisenberg−Dirac−van Vleck (HDVV) Hamiltonian are calculated for the tetraradical, hexaradical, and their substituted forms. There is strong coupling between the radical sites within the macrocycles, and relatively weak coupling between the macrocycles and linker(s). Thus, the bis(biphenylene)methyl link can be efficiently used for ferromagnetic and ferrimagnetic coupling between different units of macrocycles in the dimers and further extended systems. Indeed we find that the Heisenberg coupling constants in the dimers are fairly large to ferromagnetically couple the macrocycles, yet sufficiently small to produce a ferrimagnetically coupled linkage. These couplings were originally contended by Rajca. The zero-field splitting parameters for the precursors are more or less in agreement with observed values, which suggest that the nonassociated tetraradical can behave like a weak single-molecular magnet. Our calculations strongly support the polymer of tetraradical to be a spin glass and that of hexaradical to be a superparamagnetic nanoparticle. ■ INTRODUCTIONOrganic molecules with very large spin S can be used in the formation of magnetically ordered polymers. 1−3 The process requires the coupling of different monomer units (radical centers) via bonds with strong magnetic exchange interaction, 4−6 which produces polyradicals with enhanced spin. 7−11 The magnetic exchange coupling between unpaired π-electrons of organic polyradicals via a bridging fragment is expected to be stronger than the through-space spin-dipolar interaction of radicals in a molecular solid. 6 Polyradicals are known to retain magnetic ordering at or above room temperature. 6 Thus, it is only natural that there has been a lot of interest generated in the recent past on the molecular design of organic polyradicals with magnetic ordering. 12−21 Chemical instability and structural defects may occur during the process of synthesis. These factors often lead to weakly coupled and smaller polyradicals. 22 This is why an actual synthesis of a high-spin organic polymeric radical often turns out to be quite difficult. These two difficulties were carefully circumvented by Rajca and his co-workers by designing polymers with "multiple coupling channels between the radical centers." 1−3 These authors synthesized a series of polyradicals with large spin quantum numbers, using bis-(biphenylene)methyl as a linker to connect calix[4]arene macrocycles. 1 A strong molecular magnet needs a repeating unit of high-spin, that is, with more than one unpaired electr...

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Section: Introductionmentioning

confidence: 99%

Quantum Chemical Investigation of Calix[4]arene-Based Radicals with Bis(biphenylene)methyl Linkers as Precursors of Spin Glass and Superparamagnets

Pal

Datta

2014

J. Phys. Chem. C

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“…Since the 1980s, much theoretical work has been published characterizing the ferromagnetic or antiferromagnetic coupling in π-conjugated polyradical molecules. − Nevertheless, it is still computationally challenging to quantitatively predict Δ E L - H for medium- to large-size polyradicals. At one end, high-level ab initio calculations can, in principle, provide the most reliable information, but due to their prohibitive computational cost, they are often not applicable to medium- to large-size molecules.…”

Section: Introductionmentioning

confidence: 99%

Theoretical Study of Very High Spin Organic π-Conjugated Polyradicals

Liu

Zhang

et al. 2007

J. Phys. Chem. A

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Different forms of pi-conjugated polyarylmethyl systems, such as diradicals, polyradicals, spin clusters, and polymers, were studied with valence bond (VB) calculations within the density matrix renormalization group (DMRG) framework. For these systems, the energy gap between the high-spin ground state and the lowest low-spin excited state (DeltaE(L-H)) was computed and found to correlate well with their stability. On the basis of our analysis, medium-sized polyarylmethyl cycles are suggested to be potential key building blocks of very high spin spin clusters and polymers.

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“…At the same time, it is well established that the theoretical studies can be very helpful in understanding of the magnetic coupling mechanisms of these -conjugated polyradical molecules and many theoretical works have been devoted to this target. [35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50] Yamanaka et al 50 and Arita et al 51 adopted high-level ab initio method to calculate the energy a͒ Author to whom correspondence should be addressed. Tel.…”

Section: Introductionmentioning

confidence: 99%

Magnetic properties of very-high-spin organic π-conjugated polymers based on Green’s function theory

Yao

Liu

2008

The Journal of Chemical Physics

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Magnetic properties of two very-high-spin organic pi-conjugated polymers have been investigated theoretically by means of the many-body Green's function method with random phase approximation. The polymers are designed with a large density of cross-links and alternating connectivity of radical modules with unequal spin quantum numbers (S), macrocyclic S=2 or 3, and cross-linking S=1/2 modules, which permit large net S values for either antiferromagnetic or ferromagnetic exchange coupling between the modules. The numerical results reveal that, ascribing to the zero-temperature spin fluctuations, the sublattice magnetizations of the two polymers are both smaller than their classical spin values and the ground-state magnetizations of them are also smaller than their predicted values in the antiferromagnetic exchange coupling case. However, these magnetic behaviors do not occur in the ferromagnetic exchange coupling case. On the basis of our synthesis of the temperature dependence of the magnetic susceptibility multiplied by temperature, and through comparing the theoretical results with the experimental measurements, it is concluded that the magnetic exchange couplings between the modules within the two high-spin polymers should be ferromagnetic exchange couplings, which are consistent with other theoretical results drawn from the investigations into the ground-state properties of the two organic polymers.

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Mechanism for spin alignments in organic molecule-based ferrimagnetics

Cited by 5 publications

References 0 publications

Quantum Chemical Investigation of Calix[4]arene-Based Radicals with Bis(biphenylene)methyl Linkers as Precursors of Spin Glass and Superparamagnets

Quantum Chemical Investigation of Calix[4]arene-Based Radicals with Bis(biphenylene)methyl Linkers as Precursors of Spin Glass and Superparamagnets

Theoretical Study of Very High Spin Organic π-Conjugated Polyradicals

Magnetic properties of very-high-spin organic π-conjugated polymers based on Green’s function theory

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