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Supramolecular-Cation Assemblies in [Ni(dmit)2] Molecular Crystals: Novel Ion-Electron Correlated System

T. Nakamura* and T. Akutagawa

Research Institute for Electronic Science, Hokkaido University
Sapporo 060-0812, JAPAN

This is an abstract for a presentation given at the
Seventh Foresight Conference on Molecular Nanotechnology.
There will be a link from here to the full article when it is available on the web.

 

Salts of [Ni(dmit)2] complex have attracted much attention from the view points of molecular conductors and molecular magnets. In the crystal, the counter ions, usually they are closed shell cations or organic cation radicals, should exist to compensate the charge of electrons on [Ni(dmit)2] moleucules. By using crown ether macrocycles, we can introduce supramolecular cation structures (SC) in the molecular crystals. The structure and magnetic properties of novel monovalent salts will be presented.

We have recently reported that alkali ions included in crown ethers can be incorporated in electrically conducting [Ni(dmit)2] salts. When we used Li+ as a counter cation, the ions were included in an ion channel structure of 15-crown-5 ethers and in a dimeric penta-coordination ion cavity of 12-crown-4. The structures of M+-(18-Crown-6) in [Ni(dmit)2] salts could be controlled through adjusting preparation conditions. The salts with the composition of M+(18-crown-6)[Ni(dmit)2]3 grew with the cations K+, Rb+ and NH4+. The cations were fully included in 18-crown-6 cavity. The resultant inclusion moieties were isolated to each other acting as a kind of large disk-shaped counter cation. On the other hand, M+x(18-crown-6)[Ni(dmit)2]2 - type salts appeared when Li+, Na+, NH4+ or Cs+ was used as a cation. Molecules of 18-crown-6 were assembled in one-dimensional regular stacks forming an ion-channel structure in the crystal. In such channel structures, we can provide the cations a motional freedom in the crystal. The cations are strongly correlated with conduction electrons, and thus, we can control the conduction electrons through the ionic motion of the counter cations. The number of cation in the channel is also controrable. The possibility of the band-filling control of the [Ni(dmit)2] p- band will be addressed .

Monovalent Ni[dmit]2 salts with supramolecular cations

As described above, the [Ni(dmit)2] units form segregated stacks in the conducting crystals. This is because large energy gain is expected for the mixed valence state by the band formation through the p-p overlap in the stacks. On the other hand, the [Ni(dmit)2] monovalent salts, which have one unpaired electron on every [Ni(dmit)2] molecule, can no longer gain the energy from the band formation as in the case of the partial oxidized salts. In other words, [Ni(dmit)2] molecules do not necessary to form stacking structures in the monovalent salts. A large structural diversity in [Ni(dmit)2] arrangements in these crystals are expected, accordingly.

The assemblies of 1/2 spins (one unpaired electron on every [Ni(dmit)2]) in the crystal are also interesting from the view points of molecular magnets. In fact, a spin-ladder system is already reported in the [Ni(dmit)2] monovalent salt [21]. According to the diversity in SC structure, a variety of the arrangements of [Ni(dmit)2] is expected in the SC+[Ni(dmit)2] salts. We will describe here the example the SC+[Ni(dmit)2] salt in which 15-crown-5 forms a burrel-like SC structure [15].

References
  1. T. Akutagawa, T. Nakamura, T. Inabe and A. E. Underhill, J. Mater. Chem., 7, 183 (1997).
  2. T. Akutagawa, T. Nakamura, T. Inabe and A. E. Underhill, Synthetic Metals, 86, 1861(1997)
  3. T. Nakamura, T. Akutagawa, K. Honda, A. E. Underhill, A. T. Coomber and R. H. Friend, Nature, 394, 159 (1998).
  4. T. Akutagawa, T. Nakamura, T. Inabe and A. E. Underhill, Thin Sold Films, 331, 264-271 (1998)
  5. T. Akutagawa, Y. Nezu, T. Hasegawa, K. Sugiura, T. Nakamura, T. Inabe, Y. Sakata and A. E. Underhill; J. Chem. Soc. Chem. Commun., 2599 (1998)

*Corresponding Address:
Prof. Takayoshi Nakamura
Research Institute for Electronic Science, Hokkaido University
N12W6, Kita-ku, Sapporo 060-0812, Japan
Phone: +81-11-706-2849; Fax: +81-11-706-4972
e-mail: tnaka@imd.es.hokudai.ac.jp



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