Synthesis and Characterization of Novel Molecular Conductors Based on Sulfur, Selenium andTellurium-containing Donors

A large number of donor and/or acceptor molecules have been produced for the development of novel molecular conductors so far. Many organic conductors have columnar structures, where the conduction pathways are formed by intracolumnar and intercolumnar contacts. The dimensionality of the system is determined by the anisotropy of these interactions. The one-dimensional conductor has a tendency to show metal-insulator transition at low temperature. Therefore, it is very important in the molecular design to develop multi-dimensional band structure. The main purpose of this thesis is to produce novel types of molecular conductors on the basis of newly synthesized donors. The present thesis is composed of eight chapters. In Chapter 1, as an introduction, the author described the history of organic molecular conductors. In Chapter 2, a novel pentathio-substituted donor 1 was treated. The neutral molecules have the two-dimensional network through sulfur atoms between the pentathio groups and TTF moieties. The CIO4- salt has no intermolecular S…S contact between the donor columns owing to the anions and crystal solvents located between them, however, there are several short intermolecular contacts mediated by the sulfur atoms of TTF moieties in the dimer. This salt is semiconductor with an activation energy of 0.11-0.13 eV and room temperature electrical conductivity is comparatively high (3.7 x 10-2 - 3.8 ｘ 10-1 S･cm-1) for 1:1 salt because of its tight one-dimensional stacking. In Chapter 3, the first synthesis and structure of a novel cyclophane-like bridged TTF dimer 2 was described. The TTF dimer shows two pairs of reversible one-electron redox waves and one pair of reversible two-electron redox wave, suggesting the existence of intradimer interaction between two TTF moieties. The electrical property of its CIO4- salt was insulating because of the dicationic state of the donors and its undesirable crystal structure. In Chapter 4, the crystal structure and electrochemical properties of a novel unsymmetrical donor containing pyrazino-ring, PEDTTSeF, were investigated. The BF4-, CIO4-, GaCl4-, PF6- and AsF6- salts showed metallic behavior down to low temperature and several of them show metal-insulator transitions. The BF4- and CIO4- salts have the unique donor arrangement resembling to β"-like structure and 2:1 composition of donor-to-anion. They stack in the zig-zag manner with periods of three lavers of donors, giving a six-periodic sigmoidal stacking and there is, thus, no distinct column in the crystal. Furthermore there are many short intermolecular contacts and two-dimensional networks. The paramagnetic susceptibility of the BF4- salt is almost constant throughout the temperature range of 300-2 K, indicating the Pauli paramagnetism of the metal electrons. Chapter 5 treated the electrical and magnetic properties of to κ-(BETS)2FeBr4. The physical properties of κ-(BETS)2FwBr4 showed this system to be the first antiferromagnetic organic metal at ambient pressure (T N = 2.5 K). This salt showed metamagnetic behavior when the magnetic field is applied to the direction parallel to the a-axis, the magnetization increases very rapidly and tends to be saturated around 30 kOe. The small resistivity drop observed at T N clearly showed the existence of the interaction between n metal electrons and localized magnetic moments. In addition, this system undergoes a superconducting transition at about 1.1 K. That is, κ-(BETS)2FeBr4 is the first antiferromagnetic organic metal exhibiting a superconducting transition below Neel temperature. Furthermore, estimated GL coherent lengths suggest that the superconductivity in this system is strongly anisotropic similar to the cases of BEDT-TTF based organic superconductors. The λ-type large peak of specific heat was also observed, suggesting the bulk antiferromagnetic transition of high-spin Fe3+ ions. It is strongly indicated that the surerconductivity and the magnetic order coexist below T N. In κ-(BETS)2FeCl X Br4-X systems, the Neel temperature shifts to lower temperature with the increase of chlorine contents. Moreover, the direction of an easy spin axis is changed from the a-axis to the b-axis in the sample with Cl≧1.2. On the other hand, the broad peak of the electrical resistivity at 60 K, which is observed in to κ-(BETS)2FeBr4, was suppressed with increasing chlorine contents. Furthermore, the temperature corresponding to the small drop of resistivities was in good agreement with the Neel temperatures and shifted to lower temperature with the increase of the chlorine contents. On the other hand, the critical temperature of superconductivity shifts to lower temperature with the increase of the chlorine contents and appeared at 0.7 K in the samrle of Cl = 0.1. In the case of κ-(BETS)2FeCl0.5Br3.5, a slight decrease of resistivity was observed below 0.7 K. Chapter 6 treated the structures and physical properties of a newly synthesized telluro-cycle fused donor 3. The TCNQ complex and the cation radical salts were studied. The CV result of neutral molecule showed three pairs ofreversible redox waves. In the repeated cycling between 0.0 V and 1.5 V (higher than a shoulder peak around 1.4 V), only the third redox waves were gradually extinguished. The TCNQ complex showed semiconducting behavior because of its mixed-stacking structure. All the salts prepared using the telluro-cycle fused donor 3 by an electrochemical oxidation were composed of the symmetrically dimerized donor 4. Such a dimerization at the tellurium atoms could be conducted by the high oxidation state produced during the electrochemical oxidation. The obtained cation radical salts were also semiconductors with very small activation energies of 0.03-0.06 eV. Their room temperature electrical conductivities were relatively high (1-10 S･cm-1), because of the tight one-dimensional stacking in regard of the Au(CN)2- salt. In Chapter 7, the crystal structures and physical properties of various cation radical salts of TMTTeN were examined. In (TMTTeN)2M(CN)2(M = Ag and Au), the crystal structure analyses and tight-binding band calculations indicate these systems to be quasi three-dimensional conductors. The salts are highly conductive (720-760 S･cm-1) and keep the metallic states down to ca. 50 K. Furthermore, Ag(CN)2- salt exhibited constant Pauli paramagnetic behavior down to 2 K. In TMTTeN(SCN)0.88, the donor molecules are stacked to form columns and three-dimensional network is developed through the intermolecular Te…Te contacts between columns. The salt is highly conductive (590 S･cm-1) and keep the metallic behavior down to liquid helium temperature. Furthermore, the paramagnetic susceptibility of TMTTeN(SCN)0.88 is almost constant throughout the temperature range, indicating Pauli paramagnetism of the system. The Fermi surface of the SCN- salt is open along the b* and c* directions, however, the intermolecular interactions between the donor columns are not so weak due to the three-dimensional network mediated by the protruded tellurium. In Chapter 8, the author described the conclusion of this thesis