Takashi Shirahata

Supramolecular organic conductors based on diiodo-TTFs and spherical halide ion X−(X = Cl, Br)

Novel organic conductors based on four diiodotetrathiafulvalene derivatives, DIET (2-(4,5-diiodo-[1,3]dithiol-2-ylidene)-5,6-dihydro-[1,3]dithiolo[4,5-b][1,4]dithiine), DIETSe (2-(4,5-diiodo-[1,3]diselenol-2-ylidene)-5,6-dihydro-[1,3]diselenolo[4,5-b][1,4]dithiine), DIEDO (2-(4,5-diiodo-[1,3]dithiol-2-ylidene)-5,6-dihydro-[1,3]dithiolo[4,5-b][1,4]dioxine) and DIEDO-STF (2-(4,5-diiodo-[1,3]diselenol-2-ylidene)-5,6-dihydro-[1,3]dithiolo[4,5-b][1,4]dioxine) have been prepared using spherical halide ion X− (X = Cl, Br) as the counter anion. Crystal structure analyses have revealed that all halide salts contain supramolecular structures tailored by the strong I⋯X iodine bond and their molecular arrangement depends on the combination of the group 16 elements included in the donor molecule. The temperature dependence of the electrical resistivity of (DIET)2X(H2O)2 is metallic down to 4.2 K, and they have ideally two-dimensional Fermi surfaces within the donor layer. (DIETSe)2X(CH2Cl2) salts show small temperature dependence of the resistivity down to low temperatures and their “double column” structure is dominated by the Y-shaped architecture composed of the halide ion and the crystalline solvent. On the other hand, (DIEDO)2X and (DIEDO-STF)2X are semiconducting from room temperature. They have another type of “double column” structure, i.e. the adjacent donor molecules along the side-by-side direction are solid-crossing and two types of column are included in the donor layer. The packing motifs of the halide salts based on the oxygen-substituted donor molecules are the same but their electronic states are sensitive to changes in the chalcogen atoms on the inner TTF skeleton.

New organic conductors based on dibromo- and diiodo-TSeFs with magnetic and non-magnetic MX4 counter anions (M = Fe, Ga; X = Cl, Br)

Eight cation radical salts based on halogenated tetraselenafulvalene (TSeF) derivatives, dibromo(ethylenedithio)tetraselenafulvalene (DBrETSe) and diiodo(ethylenedithio)tetraselenafulvalene (DIETSe), were prepared using magnetic and non-magnetic MX4 counter anions (M = Fe, Ga; X = Cl, Br). Crystal structures of the DBrETSe salts depend on the halogen species on the MX4 counter anion. The MCl4 salts of DBrETSe are isostructural and crystallize in the orthorhombic space group Ibam and those of the MBr4 salts crystallize in the monoclinic space group C2/c. On the other hand, all four MX4 salts of DIETSe are isostructural and crystallize in the orthorhombic space group Ibam. In all eight crystals, donor molecules form a so-called β-type molecular arrangement and characteristic halogen bonds between the halogen atoms on the edge of the donor molecules and those of the counter anions are observed. (DBrETSe)2MX4 (M = Fe, Ga; X = Cl, Br) show stable metallic behaviour down to 4.2 K. This is in contrast to their iodinated analogues (DIETSe)2MCl4 (M = Fe, Ga), which show a metal–semiconductor transition at 11 K for the FeCl4 salt and at 12 K for the GaCl4 salt. No metal–semiconductor transition is observed for (DIETSe)2GaBr4, which contains the non-magnetic anion, but the transition is observed at 7.2 K for the corresponding FeBr4 salt, which contains a magnetic anion, indicating that the metal–semiconductor transition of (DIETSe)2FeBr4 correlates to the π–d electronic interaction through the I⋯Br halogen bonds. Antiferromagnetic orderings of d spins of the FeX4 anions are observed in (DBrETSe)2FeX4 and (DIETSe)2FeX4 (X = Cl, Br). In contrast to the low Neel temperature (TN ≈ 2.5 K) of the FeCl4 salts, the antiferromagnetic orderings occur at relatively high temperatures, i.e.TN = 7.5 K for (DBrETSe)2FeBr4 and TN = 7.0 K for (DIETSe)2FeBr4. Since the metallic state of (DBrETSe)2FeBr4 remains below TN, this salt is classified as a novel antiferromagnetic organic metal. On the other hand, the antiferromagnetic ordering of the d spins in (DIETSe)2FeBr4 takes place cooperatively with the metal–semiconductor transition around 7 K. These antiferromagnetic orderings of the d spins between the FeX4 anions cannot be explained by direct anion–anion interactions because of their long halogen⋯halogen distances between the FeX4 anions, and the importance of the π–d interaction between the donors and the counter anions through the halogen bonds is strongly suggested.

Dimeric and trimeric tetrathiafulvalenes with strong intramolecular interactions in the oxidized states.

New dimeric and trimeric TTF derivatives with methylenedithio spacers (1a,b, 2a, and 2b) have been synthesized. X-ray structure analysis revealed that TTF units of the dimer 1b adopted distorted face-to-face overlapping arrangement both in intra- and intermolecular stacking. Cyclic voltammetric study indicated that trimeric 2a was in favor of taking di- and tetracationic states, while the dimeric 1a was in favor of taking a monocation. The absorption spectroscopic study suggested an existence of the strong face-to-face interaction particularly in di-, tri-, and tetracationic state of the trimeric TTF derivatives.

Hexagonal supramolecular organic conductors based on diiodo(pyrazino)tetraselenafulvalene: high yield recovery of the neutral π-donor by a simple chemical reaction

Conducting cation radical salts based on a pyrazine-fused iodine-bonded π-donor diiodo(pyrazino)tetraselenafulvalene (DIPSe) and octahedral anions AF6 (A = P, As, Sb) have been prepared by electrochemical oxidation. X-Ray structure analysis revealed that these three salts are isostructural and crystallize in the hexagonal P63/mcm space group. There is a strong and directional I⋯N iodine bond among the donor molecules, which is 18–19% shorter than the sum of the van der Waals radii. The hexagonal lattice is filled with the equilateral triangles of the donor molecules tailored by the I⋯N iodine bond and there are also two types of supramolecular channels including the counter anion and the crystalline solvent. The room temperature resistivities of these three salts are in the order of 10−2 Ω cm with metallic behaviour and they show a gentle upturn at around 250 K. In addition to the high-symmetrical crystal structure and the metallic conductivity, high yield recovery of the neutral π-donor from the cation radical salts has been accomplished by a simple chemical reaction. In situ1H-NMR monitoring of the reaction process indicates that the DIPSe cation radical was reduced by water and the characteristic supramolecular channels, which allow water molecules to access the cation radicals deep in the crystal, are essential for smooth progress of the recovery process.

Structural Transitions from Triangular to Square Molecular Arrangements in the Quasi-One-Dimensional Molecular Conductors (DMEDO-TTF)2XF6 (X = P, As, and Sb)

A series of quasi-one-dimensional molecular conductors (DMEDO-TTF)(2)XF(6) (X = P, As, and Sb), where DMEDO-TTF is dimethyl(ethylenedioxy)tetrathiafulvalene, undergo characteristic structural transitions in the range of 130-195 K for the PF(6) salt and 222-242 K for the AsF(6) salt. The dramatic structural transition is induced by the order of the ethylenedioxy moiety, and the resulting anion rotation leads to the reconstruction of the H···F interaction between the methyl groups and the anions. The unique hydrogen bonds play a crucial role in the transition. As a result, the molecular packing is rearranged entirely; the high-temperature molecular stacks with an ordinary quasi-triangular molecular network transforms to a quasi-square-like network, which has never been observed among organic conductors. Nonetheless, the low-temperature phase exhibits a good metallic conductivity as well, so the transition is a metal-metal (MM) transition. The resistivity measured along the perpendicular direction to the conducting ac-plane (ρ(⊥)) and the calculation of the Fermi surface demonstrate that the high-temperature metal phase is a one-dimensional metal, whereas the low-temperature metal phase has considerable interchain interaction. In the SbF(6) salt, a similar structural transition takes place around 370 K, so that the quasi-square-like lattice is realized even at room temperature. Despite the largely different MM transition temperatures, all these salts undergo metal-insulator (MI) transitions approximately at the same temperature of 50 K. The low-temperature insulator phase is nonmagnetic, and the reflectance spectra suggest the presence of charge disproportionation with small charge difference (0.14).

Spin-flop switching and memory in a molecular conductor.

We report the first observation of spin-flop-induced sharp positive magnetoresistance as large as 100% and nonvolatile magnetoresistive memory in a π-d hybrid molecular conductor, (DIETSe)(2)FeCl(4) [DIETSe = diiode(ethylenedithio)tetraselenafulvalene]. The unprecedented magnetotransport phenomena originate from the coexistence of the spin density wave (SDW) of the quasi-one-dimensional (Q1D) π electrons and the antiferromagnetic order of d-electron spins, indicating the interplay between the electronic instability of Q1D π electrons and local moments of antiferromagnetic d-electron spins. These findings offer new possibilities in molecular electronics/spintronics.

Synthesis of novel selenium-containing donors as selenium analogues of diiodo(ethylenedithio)diselenadithiafulvalene (DIETS)

Novel selenium analogues of diiodo(ethylenedithio)diselenadithiafulvalene (DIETS) have been successfully derived from 1,3-diselenole-2-thione, which could be synthesized without the use of the highly toxic reagent CSe(2).

Synthesis, Structure, Optical, and Electrochemical Properties of Triple‐ and Quadruple‐Decker Co‐facial Tetrathiafulvalene Arrays

Understanding the details of the electronic structure in face-to-face arranged tetrathiafulvalenes (TTFs) is very important for the design of supramolecular functional materials and superior conductive organic materials. This article is a comprehensive study of the interactions among columnar stacked TTFs using trimeric (trimer) and tetrameric (tetramer) TTFs linked by alkylenedithio groups (-S(CH2 )n S-, n=1-4) as models of triple- and quadruple-decker TTF arrays. Single-crystal X-ray analyses of neutral trimeric TTFs revealed that the three TTF moieties are oriented in a zigzag arrangement. Cyclic voltammetry measurements (CV) reveal that the trimer and tetramer exhibited diverse reversible redox processes with multi-electron transfers, depending on the length of the -S(CH2 )n S- units and substituents. The electronic spectra of the radical cations, prepared by electrochemical oxidation, showed charge resonance (CR) bands in the NIR/IR region (1630-1850 nm), attributed to a mixed valence (MV) state of the triple- and quadruple-decker TTF arrays. In the trimeric systems, the dicationic state (+2; 0.66 cation per TTF unit) was found to be a stable state, whereas the monocationic state (+1) was not observed in the electronic spectra. In the tetrameric system, substituent-dependent redox processes were observed. Moreover, π-trimers and π-tetramers, which show a significant Davydov blueshift in the spectra, are formed in the tricationic (trimer) and tetracationic (tetramer) state. In addition, these attractive interactions are strongly dependent on the length of the linkage unit.

Synthesis and X-ray crystal structure of a selenophenoquinonoid-extended donor, BEDT–BDTS, affording highly conducting tetracyanoquinodimethaneand I3 complexes

2,5-Bis(4,5-ethylenedithio-1,3-dithiol-2-ylidene)-2,5-dihydroselenophe++ne (BEDT–BDTS), a selenophenoquinonoid-extended analogue of bis(ethylenedithio)tetrathiafulvalenes (BEDT–TTF), has been synthesized as a promising candidate for an electron-donating component of highTc organic superconductors. BEDT–BDTS is air-stable and has a significantly enhanced electron-donating ability compared with that of BEDT–TTF. The molecular and crystal structures of BEDT–BDTS have been determined by single crystal X-ray analysis in which short intermolecular SS contacts have been found in the side-by-side directions of the donor arrangements. A 1:1 TCNQ complex and a 3:1 I3– radical cation salt of BEDT–BDTS have been obtained and proved to exhibit fairly high room temperature conductivities.

New unsymmetrical donor dimethyl(ethylenedioxy)tetraselenafulvalene (DMEDO-TSeF) : Structures and properties of its cation radical salts

A novel unsymmetrical tetraselenafulvalene (TSeF)-type donor dimethyl(ethylenedioxy)tetraselenafulvalene (DMEDO-TSeF) has been synthesized without the use of the highly toxic reagent CSe2 and the structures and properties of its cation radical salts with octahedral anions are described.