Crystal structure and metallization mechanism of the π-radical metal

An organic superconductor, (TEA)(HEDO-TTF-dc)2·2(H2C2O4), coupled with strong hydrogen-bonding interactions

A new organic superconductor (TEA)(HEDO-TTF-dc)₂·2(H₂C₂O₄) (H₂EDO-TTF-dc = ethylenedioxy-tetrathiafulvalene dicarboxylic acids) with an onset T_C of 4.0 K, was successfully obtained using oxalic acid and HEDO-TTF-dc anion donor. The crystal structure analysis indicated that strong π-π overlaps and very strong intra- and inter-molecular hydrogen-bonding interactions exist between the HEDO-TTF-dc anion donors and oxalic acid molecules.

Electronic structure and magnetic coupling in selenium substituted pyridine-bridged bisdithiazolyl multifunctional molecular materials

Bisdithiazolyl radicals have furnished in recent years multiple examples of molecular materials with promising conductive and magnetic properties. The electronic band structure and magnetic ordering in four different isostructural pyridine-bridged bisdithiazolyl and Selenium substituted compounds have been studied by means of hybrid DFT based methods as implemented in the CRYSTAL code. The full rationalization of the properties of these multifunctional magnetic molecular materials requires a careful description of their complex open-shell electronic structure. The results describe the systems as narrow band (0.2-0.3 eV dispersion) open-shell semiconductors with a gap of 1.15-1.40 eV between the valence and conducting bands. The bands defining the insulating gap are dominated by orbital contributions arising from the heteroatoms sitting in the outer rings. A low energy closed-shell metallic solution is found at 0.25-0.35 eV above the magnetic solutions thus suggesting a complex mechanism for electric conduction with band and hopping contributions. The observed trend of the conductivity is in line with the variation of the insulating gap but more rigorous modelling is required to take into account the details of the band structure of the systems. For all the systems the spin density is well localised on the molecular units and is independent of the magnetic solution. Thus the system can be described as an ensemble of well-defined S = 1/2 magnetic centres using a two-body Heisenberg-Dirac-van Vleck spin Hamiltonian. The lowest energy electronic solutions are in line with the observed magnetic behaviour at low temperature. The set of competing magnetic exchange interactions that emerges from using a suitable mapping to consistently describe the low energy magnetic solutions explains the variety of magnetic responses (absence of long-range magnetic order, antiferromagnetism or ferromagnetism) of the four studied compounds at low temperatures.

Organic Molecular Conductors Based on Tetramethyl-TTP: Structural and Electrical Properties Modulated by the Anion Size and Shape

The tetramethyl derivative of bis-fused tetrathiafulvalene, 2,5-bis(4,5-dimethyl-1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene (TMTTP), has been successfully synthesized. Most of the radical cation salts consisting of TMTTP feature a high electrical conductivity of σ_rt = 10¹ to 10² S cm^-1 on a single crystal. The anion shape and size of TMTTP conductors modulate the electrical properties. The PF₆^- and AsF₆^- salts exhibit metallic conductivity down to 10 K, while the ReO₄^- and AuI₂^- salts show metal-to-insulator (MI) transition at 126 and 11 K, respectively. Single-crystal X-ray structure analysis of (TMTTP)₂X (X = PF₆, AsF₆, BF₄, and ReO₄) and (TMTTP)₃AuI₂ reveals that the donor packing motif is categorized as the so-called β-type. The tight-binding band calculations of (TMTTP)₂X (X = ReO₄, BF₄, PF₆, and AsF₆) suggest these salts are characterized by quasi-one-dimensional Fermi surfaces. The band calculation based on X-ray structure analysis of (TMTTP)₂ReO₄ at 100 K demonstrates that the MI transition of this salt is associated with the charge ordering with the zigzag stripe pattern.

Proton-electron-coupled functionalities of conductivity, magnetism, and optical properties in molecular crystals

Proton-electron-coupled reactions, specifically proton-coupled electron transfer (PCET), in biological and chemical processes have been extensively investigated for use in a wide variety of applications, including energy conversion and storage. However, the exploration of the functionalities of the conductivity, magnetism, and dielectrics by proton-electron coupling in molecular materials is challenging. Dynamic and static proton-electron-coupled functionalities are to be expected. This feature article highlights the recent progress in the development of functionalities of dynamic proton-electron coupling in molecular materials. Herein, single-unit conductivity by self-doping, quantum spin liquid state coupled with quantum fluctuation of protons, switching of conductivity and magnetism triggered by the disorder-order transition of deuterons, and their external responses under pressure and in the presence of an electric field are introduced. In addition, as for the functionalities of proton-d/π-electron coupling in metal dithiolene complexes, magnetic switching with multiple PCET and vapochromism induced by electron transfer through hydrogen-bond (H-bond) formation is introduced experimentally and theoretically. We also outlined the basic and applied issues and potential challenges for development of proton-electron-coupled molecular materials, functionalities, and devices.

Introducing Selenium in Single-Component Molecular Conductors Based on Nickel Bis(dithiolene) Complexes

Two selenated analogues of the all-sulfur single-component molecular conductor [Ni(Et-thiazdt)₂] (Et-thiazdt = N-ethylthiazoline-2-thione-4,5-dithiolate) have been prepared from their precursor radical-anion complexes. Replacement of the thione by a selenone moiety gives the neutral [Ni(Et-thiazSedt)₂] complex. It adopts an unprecedented solid-state organization (for neutral nickel complexes), with the formation of perfectly eclipsed dimers and very short intermolecular Se···Se contacts (81% of the van der Waals contact distance). Limited interactions between dimers leads to a large semiconducting gap and low conductivity (σ_RT = 1.7 × 10^-5 S cm^-1). On the other hand, going from the neutral [Ni(Et-thiazdt)₂] dithiolene complex to the corresponding [Ni(Et-thiazds)₂] diselenolene complex gives rise to a more conventional layered structure built out of uniform stacks of the diselenolene complexes, different, however, from the all-sulfur analogue [Ni(Et-thiazdt)₂]. Band structure calculations show an essentially 1D electronic structure with large band dispersion and a small HOMO-LUMO gap. Under high pressures (up to 19 GPa), the conductivity increases by 4 orders of magnitude and the activation energy is decreased from 120 meV to only 13 meV, with an abrupt change observed around 10 GPa, suggesting a structural phase transition under pressure.