Laboratory for Superfunctionality Materials:  Magnetism of Molecule-Based Solids, Electron Magnetic Resonance Spectroscopy

Daisuke Shiomi (Associate Professor)


1. Current Research and Principal Research Interests

Research Interests

My principal research interests center on studies of genuinely organic molecule-based magnetism, and I strive to develop novel magnetic materials such as single-component organic "ferrimagnets" by quantum-chemistry-based crystal engineering and to unravel the underlying mechanism for inter- and intramolecular magnetic interactions in crystalline solid states on a quantum mechanics basis.

The last decades have witnessed a rapid development of molecule-based magnetism.  In inorganic transition metal-based magnets, materials with fascinating electrical and optical properties have been developed.  On the other hand, a purely organic molecule-based ferromagnet with a three-dimensional long-range ordering of unpaired electron spins has been a long-standing issue in terms of materials challenge in molecular science and chemistry.  The first example of organic ferromagnet was discovered in 1991 (M. Tamura et al., Chem. Phys. Lett., 186, 401(1991)); I was engaged in this discovery in my graduate school days.  Since that discovery, more than 30 ferromagnets have been well-documented in genuinely organic molecule-based materials.  Unpaired electrons occupy molecular orbitals made up of s or p atomic orbitals in these materials.  This is in a remarkable contrast to traditional, naturally found magnetic materials, which are atom-based, having d- or f-orbital spin sites.  My research interests lie in such molecule-based magnets.  The research is oriented towards basic molecular sciences based on quantum chemistry, which include syntheses of novel open-shell molecules, characterization of the molecules with magnetic resonance spectroscopy (ESR) and solid-state magnetic properties measurements, and quantum statistical or quantum chemical calculations.  These studies elucidate the mechanism of intermolecular magnetic interactions and the relation between chemical structures and magnetic functionalities of molecules or molecular assemblages in solid states. 

In organic magnets including the above-mentioned ferromagnet, electron spins are spread over many atomic sites in constituent open-shell molecules.  Thus, intermolecular magnetic interactions for organic magnets inherently have a multicentered nature.  This multicentered nature can be negligible in traditional atom-based magnets and thus has been overlooked historically so far.  In 1997, I pointed out in quantum terms, for the first time, that in genuinely organic molecule-based magnetics an essential feature is multicentered magnetic interaction within and between the molecules with two or more unpaired electrons, i.e., S > 1/2.  An antiparallel coupling of different spin quantum numbers, e.g., S = 1 and S = 1/2 has been believed, without any rationale, to give a ferrimagnetic spin alignment as schematically shown in Fig.1a.  A purely organic molecule-based ferrimagnet, however, has not been discovered yet, which I have attributed to its multicentered nature from theoretical calculations based on the spin Hamiltonian as schematically shown in Fig.1b.  I have been examining the ferrimagnetic spin alignment in organic heteromolecular systems from both theoretical and experimental sides.  I strive towards the development of new organic magnets while at the same time unraveling the underlying mechanism for magnetic interactions on the basis of quantum mechanics.

<Spin Hamiltonian>

Fig. 1. Schematic drawings of the heteromolecular assemblage as a model for organic molecule-based ferrimagnetics (a) and the Heisenberg spin Hamiltonian (b).

Current Research

My current research interests fall into the following four issues.

(a)Theoretical Elucidation of the Nature of Molecule-Based Ferrimagnetism.   Preceding theoretical and experimental studies on ferrimagnets are based on the simple picture in Fig.1a.  I have drawn attention to a magnetic degree of freedom remaining in S > 1/2 molecules; this degree of freedom has been overlooked in the history of molecule-based magnetism.  From numerical calculations of expectation values for the biradical spin <Sb2> with the model spin Hamiltonian of biradical-monoradical alternating chain (Fig.1b), a spin contraction is found to occur at the biradical:  The <Sb2> value deviates from the value of Sb = 1 as expected for isolated molecules.  An S = 1 is not a good quantum number for describing the biradical embedded in the molecular assemblages.  The contraction is the consequence of the internal magnetic degree of freedom in the S > 1/2 molecule with multicentered intermolecular interactions.  Experimental evidence for the internal magnetic degree of freedom has been found from ESR spectra of an organic biradical embedded in a hetero-spin molecular assemblage.

 I have shown generation of an effective S = 1/2 spin in the alternating chain of biradicals and monoradicals with a Heitler-London formulation.  Molecular dimer of a biradical and a monoradical, or the unit cell of the alternating chain, behaves as a magnetic supramolecule with S = 1/2.  Spin polarization of the magnetic supramolecules has been shown to bring about effectively ferromagnetic interactions between the supramolecules as schematically shown in Fig.2.  Thus, the ferrimagnetic spin alignment in the heteromolecular chains is shown to be equivalent to the ferromagnetic alignment of the effective S = 1/2 spins.  This equivalence has been explicitly derived for the first time in quantum terms.  Very recently, the equivalence was examined in a real open-shell molecular system of an organic triradical in terms of thermodynamic properties reflecting the magnetic degree of freedom.

<Spin Alignment>

Fig.2. Schematic diagrams for the ferromagnetic chain of spin polarized S = 1/2 radicals (a) and the ferrimagnetic chain of spin-polarized supramolecules with Seff = 1/2 composed of a biradical and a monoradical (b). The arrows indicate the positive or negative spin polarization.

(b)Single-Component Organic Molecule-Based Ferrimagnetics.   A practical difficulty in constructing organic molecule-based ferrimagnetics is co-crystallization of two kinds of molecules with different spin quantum numbers S’s, e.g., S = 1 and S = 1/2.  As a purposive molecular design for co-crystallizing distinct open-shell entities, I have proposed a strategy of "single-component ferrimagnetics".  An organic triradical 1 (Fig.3a) has been designed and synthesized, which is composed of a π-biradical with a triplet (S = 1) ground state and a π-radical with S = 1/2.  In the triradical, the π-conjugation between the S = 1 and the S=1/2 moieties is substantially truncated, giving a weakly coupled composite π-system with two kinds of magnetic degree of freedom for S = 1 and S = 1/2 spins.  An alternating chain of the S = 1 and the S = 1/2 moieties has been found in 1 from an X-ray crystal structure analysis (Fig.3b and Fig.3c).  Magnetic susceptibility of 1 has indicated the occurrence of ferrimagnetic spin alignment in the alternating chain.  Thus, the triradical 1 is the first example of an organic molecule with the σ-bonded composite π-system exhibiting a ferrimagnetic behavior.  Syntheses and magnetic characterization for triradicals 2, 3, and 4 (Scheme 1) are in progress

<Single-Component Ferrimagnet>

Fig.3. (a) Triradical 1 with a schematic drawing of the three-spin system. (b) Molecular packing of 1. (c) Schematic diagram of the chain. The dashed lines represent the intermolecular interactions between the biradical and the monoradical moieties along the chain.


Scheme 1. Triradicals as building blocks of single-component ferrimagnetics.

(c) Supramolecular Ferrimagnetics and Bio-Inspired Magnetics.   Noncovalent bonding such as hydrogen bonding and coulombic interaction between ionic charges can be a promising driving force for crystallization of open-shell molecules with differing S's, as shown in Scheme 2(a,b). We have designed and synthesized nitronyl nitroxide biradical with a pyridine substituent 5 as an S = 1 component for the supramolecular ferrimagnets. The molecular ground states of the neutral biradical 5 and the cationic biradical 6+ were found to be triplet (S = 1) from magnetic susceptibility measurements, indicating that both 5 and 6+ serve as building blocks for organic supramolecular ferrimagnets. The neutral biradical 5 was found to co-crystallize with p-benzoic acid substituted monoradical 7. The hydrogen-bonded complex 5-7 undergoes an antiferromagnetic phase transition at 5 K.

Another type of hydrogen bonding-based molecular complexation is pairing of nucleobases such as cytosine and guanine as found in DNA. Nitronyl nitroxide radicals substituted with cytosine and guanine bases, as shown in Scheme 2c, have been synthesized. The hydrogen-bonded assemblage of the nucleobase-substituted radicals uncovers a new category of bio-inspired molecule-based magnets and bionano-architecture. This is termed “bio-inspired magnetics”. Studies on open-shell molecular assembly which is templated by single stranded oligonucleotides are in progress. The DNA-templated architecture is a prototype of spin-mediated molecular/quantum information processing devices.

<Hydrogen Bonding>

Scheme 2. Model compounds for (a) hydrogen-bonded complex (X = phenolate, carboxylate), (b) organic-salt ferrimagnets, and (c) bio-inspired ferrimagnets.

(d) Generalization of Ferrimagnetism: Approaches towards Molecular Devices.   The deviation of spin value <Sb2> for organic biradicals from that of an isolated molecule has been found for ground-state singlet (Sb = 0) biradicals as well. In non-quantum terms the ground-state singlet biradicals would seemingly have no contribution to the magnetization. An Sb = 0 is, however, not a good quantum number when the biradical is interacting with neighboring molecules. Theoretical investigation for molecular assemblages of the ground-state singlet biradicals and monoradicals indicate that the biradicals recover their contribution to the bulk magnetization; <Sb 2> > 0, leading to a generalized ferrimagnetic high-spin state (Fig.4). Subtle modification of the intermolecular interactions results in possible switching between the ferrimagnetic high-spin and quantum mechanically disordered low-spin states. This switching gives a new category of magnetic materials, leading to spin-mediated molecular devices. Model compounds for generalized ferrimagnets 11-13 have been synthesized, which are based on single-component ferrimagnet approach, and their magnetic properties are elucidated on the basis of the crystal structures

<Molecular Devices>

Fig.4. Schematic drawings of the recovery of magnetic moment at the ground-state singlet biradical (a) and the resulting switching of bulk magnetism (b).

<Generalized Ferrimagnet>

Scheme 3. Model compounds for generalized ferrimagnets.

2. Selected Publications

1. "Magnetic Phase Transition in a Heteromolecular Hydrogen-Bonded Complex of Nitronylnitroxide Radicals", K. Hayakawa, D. Shiomi, T. Ise, K. Sato, and T. Takui, J. Phys. Chem. B, 109, 9195-9197 (2005).

2. "Theoretical Study on Spin Alignments in Ferromagnetic Heterospin Chains with Competing Exchange Interactions: A Generalized Ferrimagnetic System Containing Ground-State Singlet Biradicals", K. Maekawa, D. Shiomi, T. Ise, K. Sato, and T. Takui, J. Phys. Chem. B, 109, 9299-9304 (2005).

3. "Magnetic Ordering in a Genuine Organic Crystal of Triangular Antiferromagnetic Spin Units", K. Takeda, Y. Yoshida, Y. Inanaga, T. Kawae, D. Shiomi, T. Ise, M. Kozaki, K. Okada, K. Sato, and T. Takui, Phys. Rev. B, 72, 24435/1-6 (2005).

4. "Syntheses, Crystal Structures and Magnetic Properties of Nitronyl Nitroxide Triradicals Composed of Ground-State Singlet Biradicals and Monoradicals: Spin Clusters in the Crystal", T. Ise, D. Shiomi, K. Sato, and T. Takui, Chem. Mater., 17, 4486-4492 (2005).

5. "Exchange Interaction in Covalently-Bonded Biradical-Monoradical Composite Molecules", K. Maekawa, D. Shiomi, T. Ise, K. Sato, and T. Takui, J. Phys. Chem. B, 109, 3303-3309 (2005).

6. "Cytosine-Substituted Nitronylnitroxide Radical: A Key Component for Bio-Inspired Molecule-Based Magnetics", D. Shiomi, M. Nozaki, T. Ise, K. Sato, and T. Takui, J. Phys. Chem. B, 108, 16606-16608 (2004).

7. "Stable Iminonitroxide Biradical in the Triplet Ground State", K. Hayakawa, D. Shiomi, T. Ise, K. Sato, and T. Takui, Chem. Lett., 33, 1494-1495 (2004).

8. "A Molecular Quantum Description of Spin Alignments in Molecule-Based Ferrimagnets: Numerical Calculations of Thermodynamic Properties", D. Shiomi, K. Sato and T. Takui, J. Phys. Chem. A, 106, 2096-2103 (2002).

9. "Quantum Ferrimagnetism Based on Organic Biradicals with a Spin-0 Ground State: Numerical Calculations of Molecule-Based Ferrimagnetic Spin Chains", D. Shiomi, K. Sato, and T. Takui, J. Phys. Chem. B, 105, 2932-2938 (2001).

10. "Single-Component Molecule-Based Ferrimagnetics", D. Shiomi, T. Kanaya, K. Sato, M. Mito, K. Takeda, and T. Takui, J. Am. Chem. Soc., 123, 11823-11824 (2001).