A molecular metal with ion-conducting channels

Metallic behaviour is well known in charge-transfer complexes that contain stacks of planar, partially oxidized (or reduced) π-conjugated molecules. Electronic conduction occurs in the partially occupied, delocalized π bands formed by intermolecular orbital overlap, and some of these materials exhibit superconductivity. 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 ordering. 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 batteries, electroluminescent devices and biomimetic systems. 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 Li_0.6(15-crown-5-ether)[Ni(dmit)₂]₂·H₂O 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.