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Physical properties of multicomponent molecular conductors with supramolecular assembly
ELSEVIER
Synthetic Metals 102 (1999) 1515-1516
Physical Properties of Multicomponent Molecular Conductors with Supramolecular Assembly Hiroshi
M. Yamamoto
*# , Jun-Ichi
The Institute for Solid State Physics, The University
Yamaura,
and Reizo Kate*
of Tokyo, Roppongi,
Minato-ku,
Tokyo 106-8666,
Japan.
Abstract Physical properties of molecular conductors that contain supramolecular assemblies are discussed. In these salts, neutral molecules such as diiodoacetylene (DIA), p-bis(iodoethynyl)benzene @BIB), and tetraiodoethylene (TIE) construct supramolecular assemblies with such anions as Cl-, Br-, I-, and/or AuBr,-. Transport measurements show that these salts range from metal to semiconductor. Several interesting properties such as unique band dispersion, highly anisotropic resistivity, and anisotropic thermal expansion are observed. Keywords:
X-ray diffraction,
Conductivity,
Organic
conductors
1. Introduction Applications of supramolecular chemistry, aiming at the development of new materials with novel physical properties, are one of the hottest current subjects in the field of material science. As far as molecular conductors are concerned, several groups have revealed that the functionalization of TTF derivatives with hydroxy- or iodo- groups is valid to construct supramolecular architectures in the conductive cation radical salts [ 11. Another approach to this subject has been made by us recently, in which several neutral molecules are utilized to construct supramolecular architectures [2]. In the presence of neutral molecules that contain several electron-deficient iodine atoms, cation radical salts of donor molecules such as BEDTTTF and EDT-TTF were prepared by electrochemical crystallization. In these cation radical salts, the Lewis acid-base interaction between the neutral molecules and the anions infinite supramolecular networks, or resulted in supramolecular assemblies, of which dimensionality ranges from one-dimensional (1D) to three- dimensional (3D). In terms of the physical properties of these salts, the effects of the supramolecular assemblies appeared in two points. At first, they influence the relative position of the donor molecules to produce novel donor arrangements, so that the novel band structures and conduction properties are developed. Secondly, the rigidity of the supramolecular structures resulted in anisotropic thermal expansions, so that the donor arrangement might vary gradually according to the temperature. In this paper, these two points of the supramolecular assemblies in the conductive cation radical salts will be reported. 2. Sample Preparation In the presence of neutral molecules, galvanostatic oxidation of donor molecules (BEDT-TTF, BETS, EDT-TTF, and EDTSTF) in 20 ml solvents (chlorobenzene or 1,1,2trichloroethane) with supporting electrolytes were performed under argon atmosphere. Constant currents were applied for several days. Crystals harvested on the electrodes or cell walls.
based on radical cation salts, supramolecule 3. Conductivity The conductivity of the salts was measured by standard fourprobe method, and the results are summarized in Table 1. The most interesting feature is the highly anisotropic conductivity of two salts, (EDT-TTF),BrI,(TIE), and (EDT-STF),I,(TIE),. The supramolecular assemblies in these salts exhibit threedimensional networks with one-dimensional cavities of which wall is made of halide anions and TIE molecules. In the cavities, the donor molecules are stacked to form one-dimensional array with orientational disorder [2(b), 2(c)]. The influence of the supramolecular structures is clearly seen in the conduction properties of these salts. The surrounding wall, or the supramolecular assembly of TIE and halide anions, prevents the inter-column interactions with the result that the conductivity perpendicular to the c axis (the stacking axis) is very small compared to the longitudinal one. Since the donor column is constructed by a regular stack of the dimerized donor molecules, the HOMO band split into two branches. Considering that the band filling is 5/8, and the overlap integrals between the donor molecules are large enough (26.7 and 13.6 X 10e3 for intra- and inter-dimer interactions, respectively, for the EDT-TTF salt), the system should have metallic characters. The temperature dependence of these salts is, however, semiconducting. The possibility of charge density wave was ruled out, because no satellite peak in X-ray oscillation photograph was observed. The semiconductive character (see Table I) of the salts might therefore be explained by the orientational disorder of the donor molecules. Another interesting phenomenon is the high RRR (residual resistivity ratio) values for (BEDT-TTF),X(DIA) and (BEDTTTF),X@BIB) (X.= Cl, Br). The temperature dependence of the resistivities for pBIB salts is shown in Fig. 1. Changes of the donor arrangements due to the anisotropic thermal expansion (see below) would be related to these phenomena. 4. Exotic band dispersion Due to the presence of the two-dimensional
’ Present address: Department of Physics, Faculty of Science, Gakushuin University, Mejiro, Toshima-ku, Tokyo 171-8588, Japan 0379-6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)00930-S
supramolecular
H.M. Yamamoto et al. I Synthetic Metals 102 (1999) 1515-1516
1516
assemblies made of AuBr,-, Bi, and TIE, the donor arrangement in (BEDT-TTF),(AuBr,),Br(TIE), is an exotic one [2b]. As a result, the calculated density of states at the Fermi level is high as shown in Fig. 2. Since this situation is favorable for itinerant ferromagnetism [3], we measured magnetic susceptibility of this salt. Preliminary result showed that the magnetic susceptibility increases significantly when the temperature decreases. The nature of this magnetic insulator will be reported elsewhere. Table I. Conduction Crystal Formula (BEDT-TTF),Cl(DIA) b (BEDT-TTFj;Br(DIAj b (BEDT-TTF)BrI,(DIA) (BEDT-TTF),CI@BIB) b (BEDT-TTF),Br@BIB) ’ (BEDT-TIF)Cl(BIBP)’ (BEDT-‘M’F),C1,(DIA)(TIE) (BEDT-TTF),Br,(DIA)(TIE)b (BEDT-TIF),(AuBr,),Br(TIE), (BEDT-TTF),Br,(TIE),(IB), (EDT-TTF)Br,I,(TIE) b (EDT-TTF),BrI,(TIE), b (EDT-STF),I,(TIE), (TMTSF)Br(TIE)” a: M = metallic, this work.
properties of the salts Temperature dependence a M down to 1.6 K M down to 1.6 K M down to 1.6 K M down to 1.6 K
b
S (Ea = 0.08 eV) S (E,= 0.15 eV) Mdownto 150K S (E, = 0.22 eV) S(E,=O.O27 eV) S (E, = 0.030 eV)
’
PO-.t.1 /Rem 5x10-z 6x10-l 8x10-l 1x10-2 2x10’ 7x102 1x10-1 7x100 1x10-1 (//c) 2xlO2(Ic) 1x10-1 (//c) 2xlO2(Ic)
5. Anisotropic Thermal Expansion Low temperature X-ray analyses revealed the rigidity of the supramolecular assemblies. The period (A and A’ in Fig. 3) of the one-dimensional supramolecular chain measured in the crystal of (BEDT-TTF),X(DIA) or (BEDT-TTF),X(DIA) (X = Cl, Br) show very little temperature dependence. On the other hand, the change in the inter-chain distance (B and B’ in Fig. 3) is relatively large. For instance, A(r. t.) and A(14 K) in (BEDT-TTF),Br(DIA) are 11.62 and 11.59 A, respectively. Corresponding inter-chain distances, i. e. B(r. t.) and B( 14 K), are 8.49 and 8.23 A. Therefore, AA (0.03 A; 0.26 %) value is about ten times smaller than the AB value (0.26 A; 3.1%). Similar trend is also observed in the crystal of (BEDTTTF),Br@BIB). A’(r. t.) and A’(90 K) are 18.62 and 18.63 A, while B’(r. t.) and B’(90 K) are 4.93 and 4.88 A, respectively. Therefore, normalized difference of A’ (U’IA’) is 0.05 % and is twenty times smaller than that of B’ (AB’lB’ = 1.O %). These anisotropic thermal expansion is due to the steep potential curve of the Br...I interaction compared to the weak van der Waals forces between the chains. Since this anisotropic expansion bring about an anisotropic change of the donor arrangement, it will change the band structure and the shape of the Fermi surface. The estimates of these changes by applying full structure analyses are now under progress. A
S (E,= 0.13 eV) 2xlb3 ’ S = semiconductive, I = insulator. b: ref [2b]. c:
Fig. 3. The one-dimensional
Fig. 1. Temperature (BEDT-TTF),X(pBIB)
-0.4
I
0
40
T/K dependence (X = Cl, Br).
80
Density of states/cub.
Fig. 2. Calculated (AuBr,),Br(TIE),
120
supramolecular
chains (see text).
Acknowledgement: One of the authors (H. M. Y.) wishes to thank the Nomura foundation for financial supports. We are grateful to Prof. Hiroyuki Tajima for the use of SQUID apparatus. of
the
resistivities
for References: [l] (a) P.Blanchard, G.Duguay, J.Cousseau, M.Sallo, M.Jubault, A.Gorgues, K.Boubekeur, P.Batail, Synrh. Metals. 56 (1993) 2113. (b) TImakubo, H.Sawa, R.Kato, J. Chem. Sot. Chem. Commun. 1995, 1097. (c) T. Imakubo, H.Sawa, R.Kato, Synth. Metals 73 (1995) 117-122. (d) T. Imakubo, H.Sawa, R.Kato, J. Chem. Sot. Chem. Commun. 1995, 1667. [2](a) H. M. Yamamoto, J. Yamaura, R. Kato, J. Materials Chem., 8 (1998) 15. (b) H. M. Yamamoto, J. Yamaura, R. Kato, J. Am. Chem. Sac., 120 (1998) 5905. (c) H. M. Yamamoto, J. Yamaura, R. Kato, Proceedings of ICSM ‘98. [3] (a) E. H. Lieb, Phys. Rev. Lett. 62 (1989) 1201. (b) A.Mielke, J. Phys. A 24 (1991) L73, 3311. (c) A. Mielke, Phys. Lett. A 174 (1993) 443. (d) H.Tasaki, Phys. Rev. Lett. 69 (1992) 1608.
160
unit
density of states for (BEDT-TTF), based on the tight-binding approximation.
Synthetic Metals 102 (1999) 1515-1516
Physical Properties of Multicomponent Molecular Conductors with Supramolecular Assembly Hiroshi
M. Yamamoto
*# , Jun-Ichi
The Institute for Solid State Physics, The University
Yamaura,
and Reizo Kate*
of Tokyo, Roppongi,
Minato-ku,
Tokyo 106-8666,
Japan.
Abstract Physical properties of molecular conductors that contain supramolecular assemblies are discussed. In these salts, neutral molecules such as diiodoacetylene (DIA), p-bis(iodoethynyl)benzene @BIB), and tetraiodoethylene (TIE) construct supramolecular assemblies with such anions as Cl-, Br-, I-, and/or AuBr,-. Transport measurements show that these salts range from metal to semiconductor. Several interesting properties such as unique band dispersion, highly anisotropic resistivity, and anisotropic thermal expansion are observed. Keywords:
X-ray diffraction,
Conductivity,
Organic
conductors
1. Introduction Applications of supramolecular chemistry, aiming at the development of new materials with novel physical properties, are one of the hottest current subjects in the field of material science. As far as molecular conductors are concerned, several groups have revealed that the functionalization of TTF derivatives with hydroxy- or iodo- groups is valid to construct supramolecular architectures in the conductive cation radical salts [ 11. Another approach to this subject has been made by us recently, in which several neutral molecules are utilized to construct supramolecular architectures [2]. In the presence of neutral molecules that contain several electron-deficient iodine atoms, cation radical salts of donor molecules such as BEDTTTF and EDT-TTF were prepared by electrochemical crystallization. In these cation radical salts, the Lewis acid-base interaction between the neutral molecules and the anions infinite supramolecular networks, or resulted in supramolecular assemblies, of which dimensionality ranges from one-dimensional (1D) to three- dimensional (3D). In terms of the physical properties of these salts, the effects of the supramolecular assemblies appeared in two points. At first, they influence the relative position of the donor molecules to produce novel donor arrangements, so that the novel band structures and conduction properties are developed. Secondly, the rigidity of the supramolecular structures resulted in anisotropic thermal expansions, so that the donor arrangement might vary gradually according to the temperature. In this paper, these two points of the supramolecular assemblies in the conductive cation radical salts will be reported. 2. Sample Preparation In the presence of neutral molecules, galvanostatic oxidation of donor molecules (BEDT-TTF, BETS, EDT-TTF, and EDTSTF) in 20 ml solvents (chlorobenzene or 1,1,2trichloroethane) with supporting electrolytes were performed under argon atmosphere. Constant currents were applied for several days. Crystals harvested on the electrodes or cell walls.
based on radical cation salts, supramolecule 3. Conductivity The conductivity of the salts was measured by standard fourprobe method, and the results are summarized in Table 1. The most interesting feature is the highly anisotropic conductivity of two salts, (EDT-TTF),BrI,(TIE), and (EDT-STF),I,(TIE),. The supramolecular assemblies in these salts exhibit threedimensional networks with one-dimensional cavities of which wall is made of halide anions and TIE molecules. In the cavities, the donor molecules are stacked to form one-dimensional array with orientational disorder [2(b), 2(c)]. The influence of the supramolecular structures is clearly seen in the conduction properties of these salts. The surrounding wall, or the supramolecular assembly of TIE and halide anions, prevents the inter-column interactions with the result that the conductivity perpendicular to the c axis (the stacking axis) is very small compared to the longitudinal one. Since the donor column is constructed by a regular stack of the dimerized donor molecules, the HOMO band split into two branches. Considering that the band filling is 5/8, and the overlap integrals between the donor molecules are large enough (26.7 and 13.6 X 10e3 for intra- and inter-dimer interactions, respectively, for the EDT-TTF salt), the system should have metallic characters. The temperature dependence of these salts is, however, semiconducting. The possibility of charge density wave was ruled out, because no satellite peak in X-ray oscillation photograph was observed. The semiconductive character (see Table I) of the salts might therefore be explained by the orientational disorder of the donor molecules. Another interesting phenomenon is the high RRR (residual resistivity ratio) values for (BEDT-TTF),X(DIA) and (BEDTTTF),X@BIB) (X.= Cl, Br). The temperature dependence of the resistivities for pBIB salts is shown in Fig. 1. Changes of the donor arrangements due to the anisotropic thermal expansion (see below) would be related to these phenomena. 4. Exotic band dispersion Due to the presence of the two-dimensional
’ Present address: Department of Physics, Faculty of Science, Gakushuin University, Mejiro, Toshima-ku, Tokyo 171-8588, Japan 0379-6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)00930-S
supramolecular
H.M. Yamamoto et al. I Synthetic Metals 102 (1999) 1515-1516
1516
assemblies made of AuBr,-, Bi, and TIE, the donor arrangement in (BEDT-TTF),(AuBr,),Br(TIE), is an exotic one [2b]. As a result, the calculated density of states at the Fermi level is high as shown in Fig. 2. Since this situation is favorable for itinerant ferromagnetism [3], we measured magnetic susceptibility of this salt. Preliminary result showed that the magnetic susceptibility increases significantly when the temperature decreases. The nature of this magnetic insulator will be reported elsewhere. Table I. Conduction Crystal Formula (BEDT-TTF),Cl(DIA) b (BEDT-TTFj;Br(DIAj b (BEDT-TTF)BrI,(DIA) (BEDT-TTF),CI@BIB) b (BEDT-TTF),Br@BIB) ’ (BEDT-TIF)Cl(BIBP)’ (BEDT-‘M’F),C1,(DIA)(TIE) (BEDT-TTF),Br,(DIA)(TIE)b (BEDT-TIF),(AuBr,),Br(TIE), (BEDT-TTF),Br,(TIE),(IB), (EDT-TTF)Br,I,(TIE) b (EDT-TTF),BrI,(TIE), b (EDT-STF),I,(TIE), (TMTSF)Br(TIE)” a: M = metallic, this work.
properties of the salts Temperature dependence a M down to 1.6 K M down to 1.6 K M down to 1.6 K M down to 1.6 K
b
S (Ea = 0.08 eV) S (E,= 0.15 eV) Mdownto 150K S (E, = 0.22 eV) S(E,=O.O27 eV) S (E, = 0.030 eV)
’
PO-.t.1 /Rem 5x10-z 6x10-l 8x10-l 1x10-2 2x10’ 7x102 1x10-1 7x100 1x10-1 (//c) 2xlO2(Ic) 1x10-1 (//c) 2xlO2(Ic)
5. Anisotropic Thermal Expansion Low temperature X-ray analyses revealed the rigidity of the supramolecular assemblies. The period (A and A’ in Fig. 3) of the one-dimensional supramolecular chain measured in the crystal of (BEDT-TTF),X(DIA) or (BEDT-TTF),X(DIA) (X = Cl, Br) show very little temperature dependence. On the other hand, the change in the inter-chain distance (B and B’ in Fig. 3) is relatively large. For instance, A(r. t.) and A(14 K) in (BEDT-TTF),Br(DIA) are 11.62 and 11.59 A, respectively. Corresponding inter-chain distances, i. e. B(r. t.) and B( 14 K), are 8.49 and 8.23 A. Therefore, AA (0.03 A; 0.26 %) value is about ten times smaller than the AB value (0.26 A; 3.1%). Similar trend is also observed in the crystal of (BEDTTTF),Br@BIB). A’(r. t.) and A’(90 K) are 18.62 and 18.63 A, while B’(r. t.) and B’(90 K) are 4.93 and 4.88 A, respectively. Therefore, normalized difference of A’ (U’IA’) is 0.05 % and is twenty times smaller than that of B’ (AB’lB’ = 1.O %). These anisotropic thermal expansion is due to the steep potential curve of the Br...I interaction compared to the weak van der Waals forces between the chains. Since this anisotropic expansion bring about an anisotropic change of the donor arrangement, it will change the band structure and the shape of the Fermi surface. The estimates of these changes by applying full structure analyses are now under progress. A
S (E,= 0.13 eV) 2xlb3 ’ S = semiconductive, I = insulator. b: ref [2b]. c:
Fig. 3. The one-dimensional
Fig. 1. Temperature (BEDT-TTF),X(pBIB)
-0.4
I
0
40
T/K dependence (X = Cl, Br).
80
Density of states/cub.
Fig. 2. Calculated (AuBr,),Br(TIE),
120
supramolecular
chains (see text).
Acknowledgement: One of the authors (H. M. Y.) wishes to thank the Nomura foundation for financial supports. We are grateful to Prof. Hiroyuki Tajima for the use of SQUID apparatus. of
the
resistivities
for References: [l] (a) P.Blanchard, G.Duguay, J.Cousseau, M.Sallo, M.Jubault, A.Gorgues, K.Boubekeur, P.Batail, Synrh. Metals. 56 (1993) 2113. (b) TImakubo, H.Sawa, R.Kato, J. Chem. Sot. Chem. Commun. 1995, 1097. (c) T. Imakubo, H.Sawa, R.Kato, Synth. Metals 73 (1995) 117-122. (d) T. Imakubo, H.Sawa, R.Kato, J. Chem. Sot. Chem. Commun. 1995, 1667. [2](a) H. M. Yamamoto, J. Yamaura, R. Kato, J. Materials Chem., 8 (1998) 15. (b) H. M. Yamamoto, J. Yamaura, R. Kato, J. Am. Chem. Sac., 120 (1998) 5905. (c) H. M. Yamamoto, J. Yamaura, R. Kato, Proceedings of ICSM ‘98. [3] (a) E. H. Lieb, Phys. Rev. Lett. 62 (1989) 1201. (b) A.Mielke, J. Phys. A 24 (1991) L73, 3311. (c) A. Mielke, Phys. Lett. A 174 (1993) 443. (d) H.Tasaki, Phys. Rev. Lett. 69 (1992) 1608.
160
unit
density of states for (BEDT-TTF), based on the tight-binding approximation.