Single-component molecular conductors can provide a variety of electronic states. We demonstrate here that the Dirac electron system emerges in a single-component molecular conductor under high pressure. First-principles density functional theory calculations revealed that Dirac cones are formed in the single-component molecular conductor [Pd(dddt)2] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate), which shows temperature-independent resistivity (zero-gap behavior) at 12.6 GPa. The Dirac cone formation in [Pd(dddt)2] can be understood by a tight-binding model. The Dirac points originate from the HOMO and LUMO bands, each of which is associated with different molecular layers. Overlap of these two bands provides a closed intersection at the Fermi level (Fermi line) if there is no HOMO–LUMO coupling. Two-step HOMO–LUMO couplings remove the degeneracy on the Fermi line, resulting in gap formation. The Dirac cones emerge at the points where the Fermi line intersects with a line on which the HOMO–LUMO coupling is zero.

Halopyridinium cations with multiple halogen and hydrogen bonding donor sites have been used as the counter ions for the Ni(dmit)2 anion (dmit = 1,3-dithiole-2-thione-4,5-dithiolate). Because of the competing supramolecular interactions, such as halogen and hydrogen bondings, some of the Ni(dmit)2 salts showed complicated structures with multiple functionality that originates from the intriguing molecular arrangements. In these multifunctional salts, Ni(dmit)2 molecules play two roles, conducting and magnetic, depending on the molecular arrangement in the layers that they belong to. The physical properties of these salts have been examined by conductivity measurements, with or without hydrostatic/uniaxial pressure, as well as magnetic susceptibility and band calculations. By analysing the low temperature conductivity, it can be concluded that the itinerant electrons in the conducting layer have magnetic coupling with the localized spins in the magnetic layers to result in a Kondo singlet formation.

The electronic properties of cation radical salts derived from organometallic mixed-ligand complexes [(ppy)Au(S−S)] (ppy− = C-dehydro-2-phenylpyridine(−); S−S2− = dithiolene ligand) with Au(III)−C σ-bond were investigated. A 2:1 salt complex [(ppy)Au(C8H4S8)]2[PF6] (C8H4S82− = 2-{(4,5-ethylenedithio)-1,3-dithiole-2-ylidene}−1,3-dithiole-4,5-dithiolate(2−)) exhibited semiconductive behavior under ambient pressure (ρrt = 2.6 Ω cm, Ea = 0.03 eV). Magnetic measurements show that it is a Mott insulator close to the metal−insulator boundary. Raman and infrared spectra have revealed that the complex has a quasi-one-dimensional dimeric structure consisting of uniformly charged donor molecules. The complex exhibits metallic behavior at pressures above 0.8 GPa. In contrast, a similar compound [(ppy)Au(C8H4S6O2)]2[BF4] (C8H4S6O22− = 2-{(4,5-ethylenedioxy)-1,3-dithiole-2-ylidene}-1,3-dithiole-4,5-dithiolate(2−)) is a band insulator.

Six materials, (EDT-TTF)4BrI2(TIE)5 (1, where EDT-TTF = ethylenedithiotetrathiafulvalene and TIE = tetraiodoethylene), (EDST)4I3(TIE)5 (2, where EDST = ethylenedithiodiselenadithiafulvalene), (MDT-TTF)4BrI2(TIE)5 (3, where MDT-TTF = methylenedithiotetrathiafulvalene), (HMTSF)2Cl2(TIE)3 (4, where HMTSF = hexamethylenetetraselenafulvalene), (PT)2Cl(DFBIB)2 (5, where PT = bis(propylenedithio)tetrathiafulvalene and DFBIB = 1,4-difluoro-2,5-bis(iodoethynyl)benzene), and (TSF)Cl(HFTIEB) (6, where TSF = tetraselenafulvalene and HFTIEB = 1,1′,3,3′,5,5′-hexafluoro-2,2′,4,4′-tris(iodoethynyl)-biphenyl), consisting of conducting nanowires were obtained by galvanostatic oxidation of the donor molecules in the presence of the corresponding halide anions and iodine-containing neutral molecules. We report their characterizations using single-crystal crystallography, electrical resistance measurements, and electron spin resonance. The structures are built on stacks of planar cations of the donors that are isolated electrically by an insulating network consisting of supramolecular assemblies of the halide anions and neutral molecules held together by a halogen bond. The size and shape as well as the orientation (tilt) of the donors are matched by the self-organization of the insulating sheaths in all cases, providing a pea-in-a-pod example in the field of supramolecular chemistry. The observed resistivities, resistivity anisotropies, and electron spin resonance behaviors of these salts are analyzed by tight-binding band calculations and resistance-array modeling. Crystal 6 with insulating layer of 1 nm thickness exhibits 8 orders of magnitude anisotropy in its resistivity, indicating high potential of the supramolecular network as sheathing material. The observation of such networks leads us to propose a roadmap for future development toward multidimensional memory devices.Keywords: anisotropic resistivity; halogen bond; molecular conductor; nanowire sheathing; supramolecular network