Development of π-Electron Systems Based on [M(dmit)2] (M = Ni and Pd; dmit: 1,3-dithiole-2-thione-4,5-dithiolate) Anion Radicals

Molecular conductors based on [M(dmit)2] (M = Ni and Pd) present a variety of π electron systems that pave the way for a higher stage of solid-state science. Supramolecular interactions between [Ni(dmit)2] anion and halogen-containing cations provide bilayer systems that are characterized by coexistence of two crystallographically independent anion layers with different molecular arrangements and contrasting (for example, metal/insulator and ferromagnetic/antiferromagnetic) properties. In [Pd(dmit)2] salts in the Mott insulating state, a small energy difference between HOMO and LUMO coupled with strong dimerization affords HOMO–LUMO band inversion. The dimer units [Pd(dmit)2]2− form a triangular lattice, and interplay of strong electron correlation and spin frustration generates a wide variety of magnetic/charge states including antiferromagnetic long-range order, quantum spin liquid, charge order, and valence bond order, depending on counter cations. The cation dependence is attributed to a systematic arch-shaped molecular distortion that tunes the anisotropy of interdimer transfer integrals. This means that the [Pd(dmit)2] molecular skeleton is sufficiently flexible within the crystal field, and molecular degrees of freedom play an important role in fine tuning of the electronic state.