Laboratory for Physical Chemistry 2: Molecular Spin Science

Kazunobu Sato (Professor)

1. Current Research and Principal Research Interests

My current research focuses on an understanding of electronic structures and magnetic properties of molecule based magnetic materials. On the basis of the electronic structures we are developing the theory of molecular design of molecule-based high-spin systems. Molecular systems applied are a wide range from organic high-spin systems to inorganic polynuclear complexes. The organic systems include polyradicals, poly-carbenes and nitrenes, and the polyionic states of the organic molecules. All compounds possess a high-spin ground or thermally accessible excited states. The inorganic systems include not only magnetically coupled transition metal systems but also rare earth metals. One of the rare earth metal systems is a europium(II)-doped aluminate which is a long-persistent phosphor. The aluminate is free of hazardous and radioactive substances, and has been attracting interest because of its potential for broad application.

Continuous wave/pulsed electron magnetic resonance (EMR) spectrometers are major research resources used to microscopically elucidate the electronic structures of magnetic materials. Various advanced EMR techniques such as single crystal EMR, electron multiple resonance (ENDOR and TRIPLE), and high-field/frequency EMR, are applied to the magnetic materials. Pulsed EMR experiments are also performed to obtain information about the magnetic materials in time domain. A multidimensional EMR method by various pulse sequences on the pulsed EMR experiments sometimes makes spectral assignments easier. Methodological developments in EMR spectroscopy are also important research themes for applying the sophisticated molecule-based magnetic materials. The two-dimensional electron spin transient nutation (ESTN) method based on pulsed EMR was developed in order to identify molecular spin multiplicity of the high-spin systems and to discriminate high-spin species in mixed spin systems. The 2D-ESTN method is based on pulsed electron magnetic resonance to measure the spin Hamiltonian in terms of the rotating frame. This method is capable of discriminating high-spin species by differences in the transition moment. 2D-ESTN spectroscopy is classified as a new type termed transition moment spectroscopy. The EMR techniques are valuable tools for studying the electronic structures of molecule-based magnetic materials.

Computational chemistry is another resource available to study the electronic structures of magnetic materials. Ab initio and density functional calculations are used to understand the magnetic materials. We consider the electronic structures of these materials theoretically in terms of modern computational chemistry, and are also interested in the evaluation of magnetic parameters of the high-spin systems. The evaluation of fine structure parameters of delocalized high-spin systems is one of the most difficult issues, and we are trying to elucidate new findings from both experimental and theoretical views.

Polyionic states of highly symmetric molecules have been attracting interest with respect to symmetry-lowering distortions by the Jahn-Teller effect in degenerate electronic states as well as to high-spin states resulting from degeneracy of frontier molecular orbitals. We are studying the electronic structure of the polyanionic high-spin states of the highly symmetric molecules which are π-spin delocalization systems. We have reported high-spin states of polyanionic C60 fullerene and decacyclene prepared by chemical reduction with alkali metals. We found the anomalous spin relaxation due to phonon bottleneck and quantum-mechanical tunneling phenomenon in the polyanionic C60 fullerene. In the case of the highly symmetric molecule, the pseudo Jahn-Teller interaction in the quartet state is closely related to the quantum tunneling phenomenon. The Jahn-Teller assisted quantum spin tunneling observed in polyanionic C60 fullerene supports a theoretical model proposed for high-temperature superconductivity in fullerides with alkali metals. On the other hand, decacyclene with three-fold symmetry has two degenerate LUMOs and a pseudo-degenerate MO above the LUMOs in the neutral molecule. It is known that the trianionic state of decacyclene is a doublet ground state. Theoretical calculations considering configuration interaction also supported the doublet ground state for the trianionic state of decacyclene. Nevertheless, detection of the quartet state has been a long-standing issue because knowledge of this state is important for understanding of the Jahn-Teller effect in the high-spin state. We have recently detected the quartet state using powder-pattern EMR spectroscopy. It is concluded that the trianionic state of decacyclene is in the quartet ground state, contrary to the previously documented results. Both polyanionic C60 fullerene and decacyclene possess two or three delocalized π-spins in the high-spin states. Spin-spin interaction in the three-spin system includes a fundamental issue for discussing the electronic structure of the delocalized high-spin system. The spin-spin interaction has been understood based on two-body interaction. The viewpoint that three-body interaction is essential for the spin-spin interaction in the three spin systems, which has not been recognized so far, is something of a missing link in high-spin chemistry. Our findings contribute to the understanding and theoretical considerations of the spin-spin interaction related to the three-body problem.

2. Selected Publications

1. "Electronic and molecular structures of dendrimeric high-spin polycarbenes as studied by CW and pulsed ESR-based electron spin transient nutation spectroscopy", K. Sato, D. Shiomi, T. Takui, M. Hattori, K. Hirai, and H. Tomioka, Mol. Cryst. Liq. Cryst., 376, pp. 549-556 (2002).

2. "Transition moment spectroscopy-based simulation of 2D electron spin transient nutation spectra for high spin chemistry", H. Matsuoka, K. Sato, D. Shiomi, T. Takui, Synth. Met., 121, pp. 1822-1823 (2001).

3. "Electronic quartet and triplet states of polyanionic C60 fullerene and their anomalous spin relaxation as studied by cw-ESR/2D-Electron spin transient nutation spectroscopy", M. C. B. L. Shohoji, M. L. T. M. B. Franco, M. C. R. L. R. Lazana, S. Nakazawa, K. Sato, D. Shiomi, T. Takui, J. Am. Chem. Soc., 122, pp. 2962-2963 (2000).

4. "Spin Characterization of Lanthanoid Ion Eu2+(8S7/2) in a CaF2 Single Crystal by Two-Dimensional Electron Spin Transient Nutation Spectroscopy", K. Sato, H. Matsuoka, D. Shiomi, T. Takui, and K. Itoh, Two-Dimensional Correlation Spectroscopy, pp. 337-340, Ed. by Y. Ozaki and I. Noda, American Institute of Physics, New York, 2000.

5. "Spin alignment in singly oxidized spin-polarized diradical donor: Thianthrene bis(nitronyl nitroxide)", A. Izuoka, M. Hiraishi, T. Abe, T. Sugawara, K. Sato, T. Takui, J. Am. Chem. Soc., 122, pp. 3234-3235 (2000).

6. "Persistent high-spin polycarbene (S=3) as studied by 2D-ESTN spectroscopy", K. Sato, D. Shiomi, T. Takui, K. Itoh, M. Hattori, K. Hirai, H. Tomioka, Synth. Met., 103, pp. 2269-2270 (1999).

7. "Persistent high-spin polycarbene. Generation of polybrominated 1,3,5-tris-[2-[4-(phenylcarbeno)phenyl]ethynyl]benzene (S = 3) and spin identification by two-dimensional electron spin transient nutation spectroscopy", H. Tomioka, M. Hattori, K. Hirai, K. Sato, D. Shiomi, T. Takui, K. Itoh, J. Am. Chem. Soc., 120, pp. 1106-1107 (1998).

8. "Polycationic high-spin states of one- and two-dimensional (diarylamino)benzenes, prototypical model units for purely organic ferromagnetic metals as studied by pulsed ESR/electron spin transient nutation spectroscopy", K. Sato, M. Yano, M. Furuichi, D. Shiomi, T. Takui, K. Abe, K. Itoh, A. Higuchi, K. Katsuma, Y. Shirota, J. Am. Chem. Soc.. 119, pp. 6607-6613 (1997).

9. "Electronic and molecular structures of quintet bisnitrenes as studied by fine-structure ESR spectra from random orientation: Are the documented zfs constants correct?", T. A. Fukuzawa, K. Sato, A. S. Ichimura, T. Kinoshita, T. Takui, K. Itoh, and P. M. Lahti, Mol. Cryst. Liq. Cryst., 278, pp. 253-260 (1996).

10. "FT Pulsed EPR/Transient Quantum Spin Nutation Spectroscopy Applied to Inorganic High-Spin Systems and a High-Spin Polymer as Models for Organic Ferromagnets", K. Sato, D. Shiomi, T. Takui, K. Itoh, T. Kaneko, E. Tsuchida, and H. Nishide, J. Spectrosc. Soc. Japan (Bunko Kenkyu), 43, pp. 280-291 (1994).