2D05
Physical properties of the radical cation salts of mono-substituted EDO-TTFs
X.F. Shao a, c, Y. Nakano a, H.Yamochi a, c, G. Saito a, b, S. Koshihara c,da) Research Center for Low Temperature and Materials Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8502,
Japan. b) Division of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502,
Japan. c) Non-equilibrium Dynamics Project, ERATO, Japan Science and Technology Agency, c/o KEK, 1-1
Oho, Tsukuba, Ibaraki, 305-0801, Japan. d) Frontier Collaborative Research Center & Department of Materials
Science, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8551, Japan.

BO is known to provide metallic radical cation salts
OOSS
almost exclusively due to the self-assembling nature.[1] TheBO
molecular shape and the existence of ethylenedioxy groups
SSO
O
are assigned to the origin of this novel property. While thepartial suppression of the self-assembling nature has been
R R= H, EDOOS
S
attempted by removing one of the terminal groups, theMe, MeEDO
complete vanishing of the nature has been observed for EDO.
SMe, MeSEDOSSOIn fact, most of the radical cation salts so far obtained show
highly conducting but weakly metallic or semiconducting transport properties. Among them,(EDO)2X (X = PF6, AsF6) shows peculiar metal-insulator (MI) transitions associated with the distinct
molecular deformations, which should cause the observed mixing of the MI transition mechanisms.[2]Furthermore, ultra-fast and highly efficient photo-induced insulator-to-metal transition is observed forthe radical cation salt with PF6.[3] Aiming to investigate the self-assembling nature and find out newsystems which exhibit the similar peculiarities to that of (EDO)2PF6, the EDO derivatives having asmall substituent are studied.
MeEDO and MeSEDO were synthesized to show the similar electronic states of the neutral species
to that of EDO.[4] This report presents the overview of the radical cation salts obtained andcharacterized up to now.
The electrocrystallization in EtOH provided the radical cation salts only in the powder forms. The
stoichiometries listed in Table 1 were estimated by the elemental analyses. Despite the difference inthe compositions, whole the radical cation salts showed similar optical spectra (See Figure 1 for
MeEDO salts). The low energy charge-transfer band was observed for each salt at around 3300 cm-1.
The temperature dependences of conductivities,
however, were classified into three classes. The
conductivity (σ) was measured on the compressedpellet by usual four-probe and two-probe methods athigh and low temperature regions, respectively.
(MeEDO)2BF4 showed metallic behavior down to
the lowest temperature measured (10 K) to give the10 times enhancement of the σ as shown in the upper
panel of Figure 2. The transport property wasconsistent with the temperature dependence ofmagnetic susceptibility (χ).
Although a small
reduction of the χ was observed on cooling, which
Figure 1 Optical spectra of MeEDO radical cation salts
was similar to that of the metallic BO radical cation dispersed in KBr. For reference, that of neutral MeEDO
salts (e.g. (BO)5(HCTMM)(PhCN)2), the magnetic is indicated with the green solid line.
behavior is understood as Pauli paramagnetism.
(MeEDO)2AsF6, (MeEDO)2SbF6, and (MeSEDO)2SbF6 formed the second class of the radical
cation salts, which exhibited weak metallic or almost temperature independent conducting behaviors
from room temperature (rt) down to around 250 K associated with the weakly temperature dependent
χ like that of (MeEDO)2BF4 (See the bottom panel of Figure 2). Although the activation energy (Ea)showed variety of modulation below the temperature of the maximum conductivity (Tσmax), noanomaly was observed for the temperature dependence of χ at around Tσmax. The significant deviation

from the Pauli paramagnetism (inflection point in χ–T plot) was observed at lower temperature region
for each species.
Table 1 Composition, conductivity and magnetic properties of the radical cation salts of MeEDO and MeSEDO
MeEDO
*2

σ rt
(S cm-1)
26(M)

MeSEDO
σ rt(S cm-1)

Ea (eV)χ rt*3

2:1

5(S)0.018 (300-200K)
0.032 (200-100K)
0.040 (100-40K)

5.2

2.1

2:1

7(S)

0.011 (300-250K)
0.030 (200-100K)
0.035 (100-40K)

5.4

0.014 (250-150K)
0.022 (150-20K)

2.3

2:1

4(M ≥260 K)

0.014 (240-200K)
0.018 (200-100K)
0.022 (100-20K)

5.6

0.036 (300-50K)
0.024 (50-20K)

1.3

13 : 3

χ rt*3

D:A

4.2

-

0.100 (300-260K)
0.054 (250-100K)
0.022 (80-30K)

3.915(M ≥240 K)

0.023 (200-165K)
0.044 (160-100K)
0.028 (100-50K)

2:16(M ≥250 K)

4:1

6(S)

Anion

D:A

BF4

2:1

PF6

2:1

3(S)

AsF6

2:1

SbF6
HCTMM*1*2

Ea (eV)*1: Hexacyanotrimethylenemethanediide; *2: (M ≥ 240 K), Metallic down to 240 K; (S), Semiconductor. *3:
Room temperature magnetic susceptibility in the unit of 10-4 emu (mol of formula unit)-1.

The rest radical cation salts of (MeEDO)2PF6,(MeEDO)4(HCTMM),(MeSEDO)2PF6and(MeSEDO)2AsF6 showed semiconducting behaviors inwhole of the temperature region measured despite of
the high conductivities at rt.
The temperaturedependences of χ, however, resembled those of thesecond class of radical cation salts in the hightemperature region and inflection points were observedin χ−T plots in the case of latter two salts. Theseobservations suggest that these two radical cation salts
are metallic at around rt.
As concluding remarks, the physical properties ofMeEDO and MeSEDO radical cation salts are sensitive
to the counter anions like those of EDO salts, though
the same type of phase transition observed in(EDO)2PF6 could not be found. The variety of theconducting behavior of the salts examined here
indicate that MeEDO and MeSEDO, as conducting
component molecules, are similar to EDO rather than
to the self-assembling BO molecule.

References:Tσmax
Ea =0.028

0.044

250K
0.023

Figure 2 Temperature dependent resistivity (black)and static susceptibility (red) of MeEDO salts. In thelower pannel, Tσmax and the variation of Ea (eV) for
the transport property are also indicated.

1. S. Horiuchi et al., J. Am. Chem. Soc. 1996, 118, 86048622; H. Yamochi, in TTF Chemistry Fundamental and Application of Tetrathiafulvalene, Chapter 4, ed.
J. Yamada and T. Sugimoto, Kodansha Springer, Tokyo, 2004.
2. A. Ota et al., J. Mater. Chem. 2002, 12, 2600-2602; O. Drozdova et al., Phys. Rev. B, 2004,70, 075107.
3. M. Chollet et al., Science 2005, 307, 86-89.; N.Uchida et al., J. Phys. IV France, 2004, 114, 143-145.
4. X. F. Shao et al, Molecular Structure Meeting at Tokyo, Japan, 27-30th, Sept, 2005, 1P023_W