Molecular design and development of single-component molecular metals

Abstract

This article discusses the requirements for designing single component molecular metals, derived from the results of crystal structure analyses, electrical resistivity measurements and extended Huckel tight-binding band calculations, performed on molecular conductors composed of single-component molecules of [Ni(ptdt)2] (ptdt = propylenedithiotetrathiafulvalenedithiolate) with extended TTF-ligands. The design of π molecules with a small HOMO–LUMO gap and a TTF-like skeleton is a key step to developing single-component molecular metals. A new approach is proposed to reduce HOMO–LUMO gaps. The preparation and characterization of a single-component three-dimensional molecular metal based on an analogous neutral transition metal complex molecule, [Ni(tmdt)2] (tmdt = trimethylenetetrathiafulvalenedithiolate) are reported. The details of the procedures for its synthesis are presented. Black crystals of this compound were obtained by the electrochemical method. In the crystal, which has a triclinic unit cell containing only one molecule, the planar [Ni(tmdt)2] molecules are closely packed to form the lattice plane (02). There are intermolecular short S⋯S contacts which indicate that the system is a three-dimensional conductor. The resistivity measurements show that the system is metallic down to 0.6 K. The extended Huckel tight-binding band calculation gave three-dimensional semi-metallic Fermi surfaces. A metallic crystal was also prepared with an analogous molecule [Ni(dmdt)2] (dmdt = dimethyltetrathiafulvalenedithiolate). The formation of a single component molecular metal opens the possibilities of developing various types of unprecedented functional molecular systems such as single component molecular superconductors, ferromagnetic metals composed of single component magnetic molecules, molecular metals (or superconductors) soluble in organic solvent, etc.