Electrical Conductivity, Thermoelectric Power and Crystal and Band Structures of EDOB-EDT-TTF Salts Composed of PF6 −, AsF6 − and SbF6 −

Novel 2:1 EDOB-EDT-TTF radical salts with different octahedral PF6, AsF6, and SbF6 anions were prepared by electrochemical oxidation. AsF6 salt was found to be isostructural to PF6 salt and had a triclinic crystal structure, while SbF6 salt was not isostructural with PF6 salt and had monoclinic crystal structure. PF6 salt had higher metal-to-semiconductor (MS) transition temperature, than that of AsF6 salt, while SbF6 salt exhibited semiconductive behavior throughout the temperature range of electrical conductivity measurements. To clarify MS transition of these salts, thermoelectric power measurements were also carried out. Thus, thermoelectric power apparatus was constructed and measurements were performed simultaneously with thermoelectric power and electrical resistivity measurements. Crystal structural features for EDOBEDT-TTF salts at 90, 293, 330 and 350 K, as well as conductivity, thermoelectric power measurements and band structures before and after MS transition are described.


Introduction
Highly conducting organic TTF·TCNQ complex composed of tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) was reported by Heeger et al. in 1973 [1] and has been studied by many physicists and chemists as a one-dimensional conducting system [2,3]. TTF·TCNQ provides highly anisotropic charge-transfer (CT) complex with metallic properties down to 58 K along b-axis, in which crystal structure [4] has two columns of TTF and TCNQ. Thermoelectric power of TTF·TCNQ along b-axis is negative and proportional to absolute temperature down to 140 K, but not along a-axis [5]. This is consistent with electrical conductivity measured along different axes. Apparatus for thermoelectric power measurement on organic single crystals has been reported by Chaikin and Kwak [6]. It is designed specifically for small fragile anisotropic samples, such as TCNQ salts. Measurements can be taken with a small (0.5 K) temperature gradient for good temperature resolution.
Bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) is a good electron donor, and CT complexes and radical salts composed of BEDT-TTF also grow excellent crystals. BEDT-TTF radical salts afford many superconductors as two-dimensional conducting system. In the case of such twodimensional system, thermoelectric power often exhibits complicated temperature dependence and anisotropy. Mori and Inokuchi have found an agreement between thermoelectric power and calculations for β-(BEDT-TTF) 2 I 3 and κ-(BEDT-TTF) 2 Cu(NCS) 2 [7].
Bis(ethylenedioxy)dibenzotetrathiafulvalene (BEDO-DBTTF) modified with strong electrondonating groups containing ethylenedioxy groups has been synthesized [8]. BEDO-DBTTF CT complexes and salts afforded no metallic compounds. Therefore, we have synthesized a new unsymmetrical EDOB-EDT-TTF donor, which consists of parts of BEDO-DBTTF and BEDT-TTF, and EDOB-EDT-TTF radical salts with octahedral PF 6 − , AsF 6 − , and SbF 6 − anions [9,10]. Based on electrical resistivity measurements, PF 6 and AsF 6 salts underwent a metal-tosemiconductor (MS) transition. X-ray analyses of these salts elucidated their crystal characteristics. Simultaneous measurements of thermoelectric power and electrical resistivity on a single sample were performed for these salts. MS transition temperatures of these PF 6 and AsF 6 salts were also determined from their thermoelectric power.

Synthesis of unsymmetrical EDOB-EDT-TTF donor
As shown in Figure 1, synthesis of EDOB-EDT-TTF was carried out using two synthetic methods: cross-coupling (I) and thermal decomposition (II), resulting in 30 and 27% yields, respectively.

Cross-coupling method
To the suspension of 1 (3.0 g, 12 mmol) and 2 (2.5 g, 12 mmol) in dry benzene (40 mL) was added triethyl phosphate (30 mL, 130 mmol). The mixture was refluxed for 5 h at 80°C. The resulting orange precipitate was removed by filtration, and the filtrate was purified by silica gel column chromatography using CHCl 3 /hexane (5:1) as the eluent. The second fraction was collected, and the resulting product was recrystallized using ethyl acetate to afford EDOB-EDT-TTF as orange crystals (30% yield): mp 260-262. 1

Thermal decomposition method
. To a solution of 1 (1.03 g, 4.25 mmol) in dry THF (150 mL) at −78°C under nitrogen was added solution of MeLi (5.1 mL, 6 mmol) dropwise via a syringe. After stirring for 5 h, the mixture was treated with acetic acid (2 mL) and allowed to warm to room temperature. The mixture was combined with water, and the desired compound was extracted using CH 3 Cl. The organic layer was dried using MgSO 4 and evaporated in vacuo to afford the crude product, which was purified using silica gel chromatography with tetrahydrofuran THF/n-hexane (1:3) as the eluent to afford 3 as a colorless oil (0.74 g, 66% Hexathioorthooxalate (4). To a stirred solution of 3 (2.60 g, 10 mmol) in dry THF (120 mL) at −78°C under nitrogen was added n-BuLi (6.35 mL, 9.5 mmol) dropwise using a syringe. After stirring for 1 h, 4,5-ethylenedithio-1,3-dithiol-2-thione (2.24 g, 10 mmol) in THF was added dropwise to the mixture, followed by addition of an excess of MeI (1.91 mL, 30 mmol) via a syringe to the mixture after 1 h. After further stirring for 1 h, the solution was treated with portions of 1 mol/dm 3 aq. NH 4 Cl. The mixture was allowed to warm to room temperature, and the aqueous layer was extracted with CH 2 Cl 2 . The organic phase was concentrated and purified using silica gel column chromatography with CH 3 Cl/n-hexane (1:2) as the eluent to afford 4 as a yellow solid (2.40  EDOB-EDT-TTF. After refluxing 4 in 1,1,2-trichloroethane (TCE) for 12 h, the crude product was purified by column chromatography with CH 2 Cl 2 /n-hexane (1:1) as the eluent followed by recrystallization from ethyl acetate to yield EDOB-EDT-TTF (27%). 1 H NMR and MS data were comparable to those obtained by the cross-coupling method.
The redox potentials of unsymmetrical EDOB-EDT-TTF appeared middle between that of BEDT-TTF and BEDO-DBTTF, as similar to other unsymmetrical TTF derivatives reported in [11]. The difference potential (ΔE) between the first redox potential (E 1/2(1) ) and the second redox potential (E 1/2 (2) ) is related to intramolecular on-site Coulomb repulsion energy, U. The ΔE of EDOB-EDT-TTF also showed middle between that of BEDT-TTF and BEDO-BDTTF. The U of EDOB-EDT-TTF decreased compared to that of BEDO-BDTTF.

Preparation of CT complexes and radical salts
Hot solutions of each donor and acceptor in acetonitrile were mixed. After the reaction mixture was cooled to room temperature (RT), the resulting precipitate was collected by filtration. Complexes were washed with the same organic solvent and dried in vacuo. Black octahedral PF 6 , AsF 6 and SbF 6 salts were obtained by electrochemical oxidation in distilled 1,2-dichloroethane or TCE under constant current of 1 μA in a mixture of the donor and the tetra-nbutylammonium salts of the corresponding anions at RT using H-shaped cell with Pt electrodes for 2 weeks. Stoichiometry of CT complexes and radical salts was determined using elemental analysis or X-ray crystallographic analyses.

Electrical conductivity measurement
DC conductivities were measured with standard four-or two-probe techniques, using Keithley 220 current source, Keithley 199 voltage/scanner, Keithley 195A voltmeter, and Scientific Instruments 9650 temperature controller. For powder samples, measurements were performed on compressed pellets, which were cut to form orthorhombic shape. Gold wires were glued to the samples with gold paint (Tokuriki Chemical, no. 8560).

Apparatus for simultaneous measurements of thermoelectric power and resistivity on organic conductors
Simultaneous measurements of thermoelectric power and resistivity by a two-probe method of PF 6 , AsF 6 , and SbF 6 salts were performed using computer-interfaced system, which schematic diagram is shown in Figure 2.
Software program for controlling the system was created in LabVIEW by Computer Automation Co. and System Approach Co. By applying different digital signals using relay and counter timer, we could perform simultaneous measurements of thermoelectric power and two-probe electrical resistivity on a single sample over the entire temperature range [12]. First, by opening the circuit through Keithley 2002 multimeter attached 2001-Scan scanner card as the relay, sample voltage (∆V) between V + and V − electrodes was measured by Keithley 2182 nanovoltmeter. Temperature gradient, ∆T, between two copper plates was measured by another Keithley 2182 nanovoltmeter. Thermoelectric power was determined. Second, after thermal equilibrium was sufficiently reached between two copper plates, by connecting the circuit through the relay, sample voltage was measured by Keithley 2182 nanovoltmeter, while constant current was supplied to the sample reversing leads to cancel thermal EMFs by Keithley 220 current source. Two-probe resistance measurement was determined by subtract-ing these two voltages and averaging. Using thermoelectric power stage made by MMR Technologies Inc. radical salts and the reference Cu-constantan or Au/Fe-Chromel thermocouple wire were glued onto copper plates using gold paint. Thermal gradient across the sample was applied by heating chip resistor (110 Ohm), which was applied from loop 2 of LakeShore 331S temperature controller. Thermoelectric power was determined from the slope of the line and was calculated as follows: S = ΔV/ΔT [13], where ΔT of 15 total points was typically < 0.5 K. Changing voltage value was measured at specific fixed time intervals by 15 points simultaneously using different nanovoltmeters. Therefore, it is necessary to apply synchronously digital signals to two Keithley 2182 nanovoltmeters. By using National Instruments PCI-6602 counter timer, we could measure 15 points synchronously to determine ΔV and ΔT. It was necessary to subtract the offset drift in order to obtain absolute thermoelectric power of the samples. At each measurement, temperature of the sample holder in the cryostat was controlled using loop 1 of LakeShore 331S temperature controller with sensor (DT-470). System was checked by measurements with Pt standard [14] and TTF·TCNQ complex [5,15] in temperature range 4-350 K. Simultaneous measurements of thermoelectric power and resistivity of (EDOB-EDT-TTF) 2 PF 6 were also performed on Quantum Design PPMS Model P670 Thermal Transport System (TTO) in temperature range 232-327 K. The crystal, which was glued to two-probe using bar-shaped copper leads, was mounted on a TTO sample puck.

Crystal structure
Single crystal structure analyses have been carried out for PF 6

Crystal structure of (EDOB-EDT-TTF) 2 PF 6
Crystal structure of 2:1 (EDOB-EDT-TTF) 2 PF 6 at 298 K is depicted in Figure 3 and belongs to triclinic P−1 space group. Cation layers of EDOB-EDT-TTF molecules and anion layers of PF 6 − anions are arranged alternately along the direction of a-axis. Donor molecules in the crystal are stacked in alternating orientations along the stacking axis. Average interplanar donordonor distances in the columns equal to 3.635 and 3.641 Å, respectively. This donor packing arrangement is so-called β-type structure [16] as in β-(BEDT-TTF) 2 I 3 [17]. Intermolecular sideby-side short contacts, less than van der Waals (vdW) [18] sum, are observed at S(6)…S (6)

Crystal structure of (EDOB-EDT-TTF) 2 SbF 6
(EDOB-EDT-TTF) 2 SbF 6 crystallizes in two forms, plate and needle. Needle form is too much small for X-ray crystal structure analysis. Crystal structure of plate (EDOB-EDT-TTF) 2 SbF 6 is not isostructural to AsF 6 and PF 6 salts and belongs to monoclinic C2/c space group, which was elucidated at 90 and 293 K. Crystal structure of plate form at 293 K is shown in Figure 5, and octahedral SbF 6 − anion does not show a disorder. Molecular arrangement of EDOB-EDT-TTF molecules is quite different from that in (EDOB-EDT-TTF) 2 AsF 6 . Two EDOB-EDT-TTF molecules are paired with molecular planes almost parallel, and adjacent pairs are nearly perpendicular to each other. This type of molecular arrangement tends to create two-dimensional networks [19]. However, SbF 6 salt is semiconductor. Intermolecular short S…S and O…H contacts are observed, but no S…F contacts are found at 293 and 90 K. S…S short contacts at 293 K are between S(4)…S (5) Table 2 summarizes appearances, component ratios, metal-to-semiconductor (MS) transition, room-temperature electrical conductivity, and activation energies of EDOB-EDT-TTF complexes and salts. A newly [10] and the previously [9] reported (EDOB-EDT-TTF) 2 PF 6 salts exhibited electrical resistivity decrease with heating (Figure 7) and showed resistivity minimum at 340 K, then gradual increase up to 350 K. As shown in Figure 8, new black plates (EDOB-EDT-TTF) 2 AsF 6 (σ RT = 2.6 S cm −1 ) exhibited distinct minimum in resistivity at 315 K confirming MS transition at this temperature. The semiconductive region (< 315 K) showed the activation energy of E a = 0.13 eV. It was found that T MS decreased with increase in anion size, as well as β-(BEDT-TTF) 2 X (X = PF 6 and AsF 6 ) [20,21]. Electrical conductivity of new black plate and fine needle SbF 6 salts at room temperature was 4.4 × 10 −2 S cm −1 (E a = 0.13 eV) and 2.9 × 10 −3 S cm −1 (E a = 0.13 eV), respectively. SbF 6 salts behaved as semiconductors from room temperature to 100 K. a Measured on a compressed pellet by a four-probe method. b by a four-probe method. c by a two-probe method. Table 2. Electric conductivities of EDOB-EDT-TTF complexes and salts.

Electrical conductivity
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Thermoelectric power
Chaikin et al. have described in references [15,22,23], that thermoelectric power coefficient in metallic state for a single one-dimensional band is given by: where τ(ε) is energy-dependent electron scattering time, ρ is amount of charge transfer, t is transfer integral (4t is the bandwidth), k B is Boltzmann constant, E F is Fermi energy, and T denotes temperature. When band structure contribution to thermoelectric power dominates in Eq. (1), the sign of thermoelectric power is negative for ρ < 1 and positive for ρ > 1.  shows linear temperature dependence in metallic region.
Thermoelectric power coefficient for semiconductor is given by: where b is the ratio of electron-to-hole mobility, and m h and m e are, respectively, effective mass of hole and electron.  shows T −1 temperature dependence for semiconductor.
Conwell has shown that near-constant  value close to −60 μV/K over wide temperature range is obtained with model, in which there are strong on-site correlations,  [20,24].
In this study, thermoelectric power measurements were carried out to clarify MS transition of these salts. Thermoelectric powers of PF 6 , AsF 6 , and SbF 6 salts were measured in temperature range 220-360 K.

Thermoelectric power of (EDOB-EDT-TTF) 2 PF 6
Positive value of thermoelectric power implies hole-like character of conduction charge carriers. The sign (•) of thermoelectric power of PF 6 salt along the crystal growth axis was positive above 235 K as shown in Figure 9. Thermoelectric power value of PF 6 salt jumps around 305 and 340 K. Inflection of thermoelectric power curve occurs around 270 K. These jumps and inflection were reproduced for different three samples. Thermoelectric power data (▲) measured by Quantum Design PPMS also shows jumps around 272 and 305 K. Thermoelectric power jumps of PF 6 salt shown in Figure 9 is not clear, however, that of PF 6 salt was proportional to absolute temperature down to 320 K, which is characteristic of metallic conduction. Below 320 K, value of thermoelectric power dropped gradually, indicating MS transition around 320 K. These transition temperatures of PF 6 salt corresponded with the results of electrical resistivity (▪). Thermoelectric power of PF 6 salt dropped below 305 K, decreasing rapidly below 270 K. Metallic properties denoted both by thermoelectric power and by electrical resistivity measurements. Figure 9. Simultaneous measurements of temperature dependence of thermoelectric power (•) and electrical resistivity (▪) of (EDOB-EDT-TTF) 2 PF 6 salt. Data of thermoelectric power (▴) and resistivity (▾) were measured by Quantum Design PPMS in temperature range 232-327 K. Solid line extrapolates to zero at T = 0 K. Figure 10 shows simultaneous measurements of temperature dependence of thermoelectric power and electrical resistivity of (EDOB-EDT-TTF) 2 AsF 6 salt. Thermoelectric power of AsF 6 salt along the crystal growth axis was linear with temperature down to 310 K, which is characteristic of a metal. Thermoelectric power value of AsF 6 salt seems to jump around 305 K and then drops below 300 K, indicating MS transition around 310 K. MS transition temperature observed in thermoelectric power seems to be slightly lower than that of electrical conductivity [25]. Thermoelectric power of AsF 6 salt again showed rapid decrease below 260 K. Inflection of thermoelectric power curve was detected around 260 K. These jumps and inflection were reproduced for different two samples. The sign of thermoelectric power of AsF 6 salt was positive above 220 K. Thermoelectric power of AsF 6 salt became negative below 220 K and decreased with decreasing temperature. MS transition temperatures of PF 6 and AsF 6 salts observed in thermoelectric power decreased in order of 320 and 310 K.

Thermoelectric power of (EDOB-EDT-TTF) 2 SbF 6
Simultaneous measurements of temperature dependence of thermoelectric power and electrical resistivity of (EDOB-EDT-TTF) 2 SbF 6 salt are shown in Figure 11. Thermoelectric power value of plate SbF 6 salt was negative below 335 K and decreased with decreasing temperature. Negative value of thermoelectric power implies electron-like character of conduction charge carriers. Thermoelectric power exhibits T −1 -temperature dependence, which is a characteristic of semiconductor. Figure 11. Simultaneous measurements of temperature dependence of thermoelectric power (•) and resistivity (▪) of (EDOB-EDT-TTF) 2 SbF 6 salt.

Band structure of AsF 6 salt
Band structures of PF 6 and AsF 6 salts were calculated on the basis of tight-binding approximation using intermolecular overlap integrals of HOMO, which were calculated by extended Hückel method [26]. Calculated intermolecular overlap integrals are listed in Table 3.
Arrangements of donor centers and intermolecular overlaps in EDOB-EDT-TTF salts viewed onto ab plane are depicted in Figure 12. Figure 13 shows energy band structures and Fermi surfaces of AsF 6 salt at 293 and 350 K. Energy bands of these salts are three-quarters filled and metallic. Short S…S contacts are observed between stacks, not within the stacks. Such structural feature provides isotropic two-dimensional electronic structures in Fermi surfaces [27,28].

Conclusion
Synthesis of unsymmetrical EDOB-EDT-TTF donors was accomplished by two methods. New radical salts with octahedral PF 6 − , AsF 6 − and SbF 6 − anions were prepared by electrochemical oxidation. Crystal structure of AsF 6 salt was isostructural to PF 6 salt, but crystal structure of plate SbF 6 salt was not. According to conventional electrical conductivity measurements, both PF 6 and AsF 6 salts exhibited MS transitions at 340 and 315 K, respectively. Electrical resistivity and thermoelectric power of PF 6 , AsF 6 , and SbF 6 salts were measured simultaneously on a single sample. Judging from the results of thermoelectric power measurements, MS transition temperatures of PF 6 and AsF 6 salts were around 320 and 310 K, respectively. From crystal structure analysis in AsF 6 salt follows that intermolecular O…H distance along the stacking b-axis at 350, 330, 293, and 90 K decrease in that order by the dimerization. Formation of dimers results in semiconductor transition. Short S…S contacts were found only between the stacks and not within the stacks. They provided isotropic two-dimensional electronic structure in Fermi surfaces. Two-dimensional electronic structure derived from β-type arrangement is not so stable against packing modification [28]. MS transition is associated with some structural transition.