Origin of the High Electrical Conductivity of Neutral [Ni(ptdt)₂] (ptdt^2- = propylenedithiotetrathiafulvalenedithiolate): A Route to Neutral Molecular Metal

A novel neutral nickel complex molecule with the extended TTF dithiolato ligand, propylenedithiotetrathiafulvalenedithiolate [ptdt2- = (S8C9H6)2-], was synthesized. In the [Ni(ptdt)2] crystal. [Ni(ptdt)2] molecules form one-dimensional columns along the a axis, having short intermolecular transverse S···S contacts. The crystal exhibited an extremely high electrical conductivity (7 S cm-1) at room temperature as a neutral molecular crystal. High-pressure resistivity measurements were made up to 72 kbar, which revealed that the resistivity could not be suppressed by applying high pressure unlike the usual highly conducting low-dimensional organic conductors. The tight-binding band structure calculation indicated that the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital) formed “crossing bands”, whose Fermi surfaces tended to vanish due to the HOMO-LUMO interactions. Only very small electron and hole pockets appeared in the Fermi surface due to the transverse interactions between neighboring columns. On the basis of these analyses, the requirements for the development of molecular metals composed of single molecules are discussed.本研究合成了一种带有扩展型四硫富瓦烯（TTF）二硫醇配体的新型中性镍配合物分子，该配体为丙二硫基四硫富瓦烯二硫醇根（ptdt²⁻ = (S₈C₉H₆)²⁻）。在[Ni(ptdt)₂]晶体中，该配合物分子沿a轴形成一维柱阵列，分子间存在短程横向S···S接触作用。作为中性分子晶体，该材料在室温下展现出极高的电导率（7 S·cm⁻¹）。我们开展了最高压力达72千巴的高压电阻率测试，结果表明，与常规高导电低维有机导体不同，该晶体的电阻率无法通过施加高压得到抑制。紧束缚能带结构计算结果显示，最高占据分子轨道（highest occupied molecular orbital, HOMO）与最低未占据分子轨道（lowest unoccupied molecular orbital, LUMO）形成了“交叉能带”，由于HOMO-LUMO之间的相互作用，其费米面趋于消失。由于相邻柱阵列间的横向相互作用，费米面中仅存在极小的电子与空穴口袋。基于上述分析，本文讨论了单分子基分子金属的开发所需满足的条件。