Synthesis and Electronic Properties of Bilayer Type Molecular Cconductors
Based on Asymmetrical Donor Molecules
Masashi Uebe, Hengbo Cui, Reizo Kato
Condensed Molecular Materials Laboratory, RIKEN

Many molecular conductors have beenreported because they can realize various
electronic properties different from inorganic
materials based on molecular design by usingsynthetic method. We synthesized newFig. 1 Asymmetric donor molecules.
asymmetrical donor molecules for searchinginteresting crystal structures and electronicproperties. For example, an asymmetrical donor
molecule TMET-STF is a hybrid type molecule
between HMTSF that is likely to take aone-dimensional
columnstructure
andBEDT-TTF that prefers a two-dimensional
molecular arrangement (Fig. 1). The BF4 salt of
this molecule, (TMET-STF)2BF4, has two
crystallographically independent cation radicallayers, which is called bilayer structure. Theband structure calculation indicates that eachFig. 2 The crystal structure and Fermi surface ofconducting layer has a quasi-one-dimensional(TMET-STF)2BF4.
Fermi surface and a cylindrical two-dimensionalFermi surface, respectively (Fig. 2) [1, 2].
(TMET-STF)2BF4 is an ambient pressure
superconductor at about 4.1 K. A small
resistance anomaly around 100 K and a
metal-insulator transition around 13 K are
observed above the superconducting transition
temperature, and this complicated behavior is
induced by the bilayer structure (Fig. 3).
Furthermore, the ClO4 salt also has a bilayer
structure like the BF4 salt. On the other hand, theAu(CN)2 salt has a one-dimensional column
structure similar to HMTSF salts and the PF6 salthas two-dimensional structure similar to(BEDT-TTF)2PF6. Depending on counter anions,Fig. 3 Temperature dependence of (TMET-STF)2BF4 atcation radical salts of asymmetrical donor exhibit
ambient pressure.
various crystal structures and show interesting
electronic states. This motivated us to develop molecular conductors and superconductors with the bilayer
structure induced by the asymmetry of molecules. We synthesized a donor TMET-TTF where sulfur atoms
substitute for selenium atoms within TMET-STF and prepared its cation radical salts (Fig. 1). In our previoussynthetic work for asymmetrical donor molecules, including TMET-STF, we employed the conventional
coupling reaction using trialkyl phosphate. In this coupling reaction, however, two symmetrical byproductsare formed along with the asymmetrical
donor molecule, and thus the yield of
the target compound is low. In addition,
when the selenocarbonyl group is used
for the coupling reaction as starting
material, it is necessary to use highly
toxic hydrogen selenide. To synthesize
efficiently only the aimed asymmetric
donor TMET-TTF, we used the reaction
developed by Yamada et al. which uses
a dibutyltin complex and an ester
compound of each donor unit in thepresence of a Lewis acid catalyst suchFig. 4 The crystal structures of (a) (TMET-TTF)2BF4 andas Me3Al. We then obtained cation(b) (TMET-TTF)2ClO4.
radical salts with various counter anions
using the electrochemical crystal
growth method. X-ray crystal structure
analysis revealed that the BF4 and ClO4salts have a bilayer structure similar to(TMET-STF)2BF4 as shown in Fig. 4.
(TMET-TTF)2BF4 has another type ofcrystal structure that is based onone-dimensional column formation. ThePF6 salt also has a one-dimensional
column structure. At ambient pressure,the BF4, ClO4 and PF6 salts ofTMET-TTF are all semiconductors.
Figure 5 shows temperature-dependent
resistivityof
thebilayer-type
Fig.5 Temperature and pressure dependence of (TMET-TTF)2BF4.
(TMET-TTF)2BF4 under pressure. The
resistivity decreased with the application of pressure. At 2.9 GPa, the system became metallic and kept themetal state down to ca. 100 K. Above 4.6 GPa, the system remained metallic down to the lowesttemperature.
In future, we plan to synthesis new asymmetrical donor molecules containing selenium atoms in order toenhance intermolecular interactions and prepare cation radical salts with various counter anions.
References[1] R. Kato K. Yamamoto, Y. Okano, H. Tajima, H. Sawa, Chem. Commun., 947 (1997).
[2] S. Uji, C. Terashima, Y. Okano, R. Kato, Phys. Rev. B, 64, 214517 (2001)[3] J. Yamada, S. Satoki, S. Mishima, N. Akashi, K. Takahashi, N. Matsuda, Y. Nishimoto, S. Takasaki, H.
Anzai, J. Org. Chem., 61, 3987 (1996)