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).