Molecular Design and Physical Properties of Single-Component Molecular Metals

Abstract

The design of metallic crystals consisting of single-component molecules ( single-component molecular metal ) is explained on the basis of the examinations of frontier molecular orbitals and the simple extended-Hückel tight-binding band pictures. To meet the conditions required to realize automatic carrier generation by self-assembly of the same kind of neutral molecules, a crystal of a transition metal complex molecule [Ni(tmdt) 2 ] with extended-TTF-type (TTF = tetrathiafulvalene) dithiolate ligands was synthesized, which was found to be the first single-component molecular metal. The X-ray structure analyses of the crystals of [(C 4 H 9 ) 4 N] 2 [Ni(tmdt) 2 ] and neutral [Ni(tmdt) 2 ] provided information on the symmetry of the frontier molecular orbitals [the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)] from which the metallic bands are formed. The infrared and visible spectra of a crystalline powder sample of [Ni(tmdt) 2 ] showed a broad electronic absorption maximum at around 2200 cm -1 , suggesting an extremely small HOMO–LUMO gap. The physical properties of some of the hitherto-developed single-component molecular conductors are also briefly described.