Crystallographic data for EDOB-EDT-TTF salts.
Abstract
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 EDOB-EDT-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.
Keywords
- EDOB-EDT-TTF radical salts
- conductivity
- thermoelectric power
1. 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
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 two-dimensional system, thermoelectric power often exhibits complicated temperature dependence and anisotropy. Mori and Inokuchi have found an agreement between thermoelectric power and calculations for
Bis(ethylenedioxy)dibenzotetrathiafulvalene (BEDO-DBTTF) modified with strong electron-donating 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 PF6−, AsF6−, and SbF6− anions [9, 10]. Based on electrical resistivity measurements, PF6 and AsF6 salts underwent a metal-to-semiconductor (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 PF6 and AsF6 salts were also determined from their thermoelectric power.
2. Preparation of organic conductors
2.1. 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.
2.1.1. Cross-coupling method
To the suspension of
2.1.2. Thermal decomposition 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 (Δ
2.2. 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
3. Measurements
3.1. 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).
3.2. 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 PF6, AsF6, and SbF6 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 (∆
Simultaneous measurements of thermoelectric power and resistivity of (EDOB-EDT-TTF)2PF6 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.
4. Crystal structure
Single crystal structure analyses have been carried out for PF6 salt at 298 K, AsF6 salt at 90, 293, 330, and 350 K, SbF6 salt at 90 and 293 K. Crystallographic data are listed in Table 1.
PF6 salt at 298 K |
AsF6 salt at 90 K |
AsF6 salt at 293 K |
AsF6 salt at 330 K |
AsF6 salt at 350 K |
SbF6 salt at 90 K |
SbF6 salt at 293 K |
|
---|---|---|---|---|---|---|---|
Chemical formula | C28H20F6O4PS12 | C28H20AsF6O4S12 | C28H20F6O4S12Sb | ||||
Formula weight | 950.23 | 994.15 | 1040.99 | ||||
Crystal system | Triclinic | Triclinic | Monoclinic | ||||
Space group | |||||||
7.003 | 6.875 | 7.000 | 7.036 | 7.057 | 37.805 | 38.105 | |
8.074 | 7.914 | 8.061 | 8.092 | 8.112 | 8.204 | 8.340 | |
16.326 | 16.411 | 16.424 | 16.411 | 16.404 | 11.371 | 11.429 | |
76.02 | 103.40 | 76.07 | 76.23 | 76.34 | |||
78.07 | 98.34 | 78.41 | 78.46 | 78.49 | 103.23 | 102.77 | |
81.50 | 97.11 | 81.45 | 81.07 | 80.85 | |||
871.7 | 847.8 | 876.2 | 883.4 | 888.0 | 3433.0 | 3542.4 | |
1 | 1 | 1 | 1 | 1 | 4 | 4 | |
298 | 90 | 293 | 330 | 350 | 90 | 293 | |
CCDC no. | 819768 | 809939 | 809703 | 802121 | 799928 | 799214 |
4.1. Crystal structure of (EDOB-EDT-TTF)2PF6
Crystal structure of 2:1 (EDOB-EDT-TTF)2PF6 at 298 K is depicted in Figure 3 and belongs to triclinic
4.2. Crystal structure of (EDOB-EDT-TTF)2AsF6
Crystal structure of (EDOB-EDT-TTF)2AsF6 is isostructural to PF6 salt and was elucidated at various temperatures (90, 293, 330 and 350 K). Cation layers of donor molecules and anion layers of AsF6− anions are arranged alternately along the direction of
4.3. Crystal structure of (EDOB-EDT-TTF)2SbF6
(EDOB-EDT-TTF)2SbF6 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)2SbF6 is not isostructural to AsF6 and PF6 salts and belongs to monoclinic
5. Electrical conductivity
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)2PF6 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)2AsF6 (
Acceptor or anion | Appearance | Stoichiometry D:A | References | |||
---|---|---|---|---|---|---|
M2TCNQ | Black powder | 1:1 | Insulatora | [9] | ||
TCNQ | Black power | 1:1 | 1.9a | 0.085 | [9] | |
FTCNQ | Dark green powder | 9.6 × 10−2a | 0.13 | [9] | ||
F2TCNQ | Dark green powder | 4:1 | 7.1 × 10−2a | 0.13 | [9] | |
PF6− | Black plate | 2:1 | 340 | 1.7 × 10b | 0.23 | [9] |
PF6− | Black plate | 2:1 | 337 | 1.0c | 0.17 | [10] |
AsF6− | Black plate | 2:1 | 315 | 2.6c | 0.13 | [10] |
SbF6− | Black plate | 2:1 | 4.4 × 10−2c | 0.13 | [10] | |
SbF6− | Black fine needle | 2:1 | 2.9 × 10−3c | 0.13 | [10] |
6. 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
Thermoelectric power coefficient for semiconductor is given by:
where
Conwell has shown that near-constant
In this study, thermoelectric power measurements were carried out to clarify MS transition of these salts. Thermoelectric powers of PF6, AsF6, and SbF6 salts were measured in temperature range 220–360 K.
6.1. Thermoelectric power of (EDOB-EDT-TTF)2PF6
Positive value of thermoelectric power implies hole-like character of conduction charge carriers. The sign (•) of thermoelectric power of PF6 salt along the crystal growth axis was positive above 235 K as shown in Figure 9. Thermoelectric power value of PF6 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 PF6 salt shown in Figure 9 is not clear, however, that of PF6 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 PF6 salt corresponded with the results of electrical resistivity (▪). Thermoelectric power of PF6 salt dropped below 305 K, decreasing rapidly below 270 K. Metallic properties denoted both by thermoelectric power and by electrical resistivity measurements.
6.2. Thermoelectric power of (EDOB-EDT-TTF)2AsF6
Figure 10 shows simultaneous measurements of temperature dependence of thermoelectric power and electrical resistivity of (EDOB-EDT-TTF)2AsF6 salt. Thermoelectric power of AsF6 salt along the crystal growth axis was linear with temperature down to 310 K, which is characteristic of a metal. Thermoelectric power value of AsF6 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 AsF6 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 AsF6 salt was positive above 220 K. Thermoelectric power of AsF6 salt became negative below 220 K and decreased with decreasing temperature. MS transition temperatures of PF6 and AsF6 salts observed in thermoelectric power decreased in order of 320 and 310 K.
6.3. Thermoelectric power of (EDOB-EDT-TTF)2SbF6
Simultaneous measurements of temperature dependence of thermoelectric power and electrical resistivity of (EDOB-EDT-TTF)2SbF6 salt are shown in Figure 11. Thermoelectric power value of plate SbF6 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
7. Band structure of AsF6 salt
Band structures of PF6 and AsF6 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
Symbol | PF6 salt at 298 K | AsF6 salt at 293 K | AsF6 salt at 350 K |
---|---|---|---|
−3.886 | −3.864 | −3.896 | |
16.089 | 12.819 | 14.467 | |
12.464 | 16.414 | 13.013 | |
5.545 | 0.607 | 5.922 | |
0.596 | 5.489 | 0.605 | |
0.058 | 0.058 | 0.044 | |
0.056 | 0.006 | 0.052 |
8. Conclusion
Synthesis of unsymmetrical EDOB-EDT-TTF donors was accomplished by two methods. New radical salts with octahedral PF6−, AsF6− and SbF6− anions were prepared by electrochemical oxidation. Crystal structure of AsF6 salt was isostructural to PF6 salt, but crystal structure of plate SbF6 salt was not. According to conventional electrical conductivity measurements, both PF6 and AsF6 salts exhibited MS transitions at 340 and 315 K, respectively. Electrical resistivity and thermoelectric power of PF6, AsF6, and SbF6 salts were measured simultaneously on a single sample. Judging from the results of thermoelectric power measurements, MS transition temperatures of PF6 and AsF6 salts were around 320 and 310 K, respectively. From crystal structure analysis in AsF6 salt follows that intermolecular O…H distance along the stacking
Acknowledgments
The author wish to thank Drs. Maeda T and Yamashita K of Computer Automation Co. and Dr. Niwa N of System Approach Co. for programming the LabVIEW thermoelectric power measurement system and Emeritus Professor Matsumoto S of Aoyama Gakuin University for giving valuable comments.
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