Letters to Nature

Nature 394, 159-162 (9 July 1998) | doi:10.1038/28128; Received 17 April 1998; Accepted 13 May 1998

A molecular metal with ion-conducting channels

Takayoshi Nakamura1, Tomoyuki Akutagawa1, Kazumasa Honda2, Allan E. Underhill3, A. Treeve Coomber4 & Richard H. Friend4

  1. Research Institute for Electronic Science, Hokkaido University, Sapporo 060, Japan
  2. National Institute for Materials and Chemical Research, Tsukuba, Ibaraki 305, Japan
  3. Department of Chemistry, University College of North Wales, Bangor, Gwynedd LL57 2UW, UK
  4. Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE, UK

Correspondence to: Richard H. Friend4 Correspondence and requests for materials should be addressed to R.H.F. (e-mail: Email: rhf10@cam.ac.uk).

Metallic behaviour is well known in charge-transfer complexes that contain stacks of planar, partially oxidized (or reduced) pi-conjugated molecules. Electronic conduction occurs in the partially occupied, delocalized pi bands formed by intermolecular orbital overlap, and some of these materials exhibit superconductivity1,2. Counter-ions, present to achieve charge neutrality, usually play a passive role, although in some cases they couple to the electronic structure, for example by imposing a new structural periodicity (a superlattice) by orientational ordering1. The development of molecular solids that can simultaneously support the transport of both electrons and ions is important for several fields, including the development of solid-state batteries3,4, electroluminescent devices5 and biomimetic systems6,7. Crown ethers are promising components for such systems, as they provide cavities through which ion motion might occur. Here we report that the charge-transfer salt Li0.6(15-crown-5-ether)[Ni(dmit)2]2dotH2O exhibits both electron and ion conductivity: the stacks of the nickel complex (dmit is an organic molecule) provide a pathway for electron conduction, and stacks of the crown ethers provide channels for lithium-ion motion. Evidence for the latter above 250 K is provided by NMR and conductivity studies. We also see evidence for coupling of the electron and ion motions. This compound might serve as a model for the development of other hybrid electronic/ionic conducting materials.