Control of Molecular Packing in the ET Based Conductor by

Aug 5, 2009 - ... form a two-dimensional hydrogen-bonding double-sheet layer, and ET ... The ET arrangement in the sheet resembles that in α-ET2I3, w...
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DOI: 10.1021/cg900655v

Control of Molecular Packing in the ET Based Conductor by Supramolecular Mellitate Networks (ET = Bis(ethylenedithio)tetrathiafulvalene)

2009, Vol. 9 4830–4833

Hiromi Minemawari,† Toshio Naito, and Tamotsu Inabe* Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan. † Present address: Photonics Research Institute, AIST, Tsukuba 305-8562, Japan Received June 11, 2009; Revised Manuscript Received July 21, 2009

ABSTRACT: Electrochemical oxidation of bis(ethylenedithio)terathiafulvalene (ET) with mellitic acid and pyridine in methanol produces a partially oxidized salt of [ET]3[C6(COO)6H42-]2[C5H5NHþ]2 3 CH3OH 3 2H2O (1). In the crystal of 1, mellitate anions form a two-dimensional hydrogen-bonding double-sheet layer with pyridinium, methanol, and water. ET molecules are packed between the layers, forming a two-dimensional conducting sheet. The ET arrangement in the sheet resembles that in R-ET2I3, which has a metal-to-insulator transition. The transition into insulating states is completely suppressed in 1 due to the close packing of ET molecules in the supramolecular mellitate network.

1. Introduction The physical properties of molecular crystals are known to be dominated not only by the electronic states of components but also by molecular arrangements and intermolecular interactions in the crystalline state. Crystal engineering that utilizes supramolecular networks is one promising approach to control the structures and propertyies of molecular crystals.1-4 Hydrogen bonding is a representative intermolecular interaction that can form such networks. Recently, we have demonstrated that mellitate, a deprotonated form of mellitic acid (benzenehexacarboxylic acid; Chart 1), is a useful component to construct anionic networks, because strong hydrogen bonds are formed between the carboxy and carboxylate/carboxy groups.5-7 Furthermore, the anionic charge can be varied by the deprotonation number, and the hydrogen-bonding patterns are regulated to some extent by the deprotonation number. Using mellitate as an anionic component for the preparation of the tetrathiafulvalene (TTF) salt led to the formation of a helical column of the TTF cation radicals induced by the double helix network of mellitate.8 Although TTF is a well-known donor component that forms molecular conductors, the conductivity of this TTF salt was not high, due to the full oxidation state of TTF. Among the TTF derivatives, bis(ethylenedithio)tetrahiafulvalene (ET; Chart 1) has been widely investigated as a component for molecular conductors. Since the presence of periphery S atoms in ET makes it possible to interact along not only the π-π stacking direction but also the lateral direction of the molecular plane, the π-π interaction is extended two-dimensionally in many ET conductors. This feature is preferable to achieve a superconducting state, and a number of superconductors have been developed.9 The ET salts were categorized into groups according to the molecular arrangement pattern of ET in the crystal; the representatives are R-, β-, θ-, and κ-types.10 Salts in the same group exhibit generally similar properties at high temperature but in some cases fall into different ground states at lower temperature, *E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 08/05/2009

due to slight differences in the structural parameters. The R-type salts, in which the π-π interaction is two-dimensional, are a typical example. R-ET2I3 has a metal-to-insulator (M-I) transition at 135 K11 while R-ET2NH4Hg(SCN)4 exhibits a superconducting transition at 0.8 K.12 When ET is combined with mellitate and pyridinium, a partially oxidized salt of [ET]3[C6(COO)6H42-]2[C5H5NHþ]2 3 CH3OH 3 2H2O (1) with an R-type ET arrangement is obtained. The R-type arrangement in this salt is induced by the supramolecular network of mellitate; the electrical conductivity in this salt is quite insensitive to temperature change, which is in contrast to that observed for the other R-type ET salts. In this paper, the details of the preparation, crystal structure, electrical conductivity, and electronic structure of 1 are described. 2. Experimental Section Materials. Single crystals of 1 were obtained by electrochemical oxidation of ET. Mellitic acid (20 mg) was dissolved in methanol (10 mL) followed by the addition of one drop of pyridine. ET (15 mg) was separately dissolved in chloroform (20 mL). These solutions were mixed in an electrochemical cell and stirred well to obtain a clear solution. Black platelet crystals were obtained with a constant current of 1.0 μA for one week. X-ray Structure Analysis. A Rigaku R-AXIS rapid imaging plate diffractometer with graphite-monochromated Mo Ka radiation (λ = 0.71073 A˚) was used for data collection at 123 K. The measurement conditions and crystal data are summarized in Table 1. The structure was solved by a direct method (SHELX9713) and refined using the CrystalStructure program package.14 One of the terminal ethylene groups of ET is disordered, and the structure was refined with a disorder model of two overlapping configurations with occupancies of 0.7 and 0.3 (C-C distances are restrained). The positions of all hydrogen atoms not involved in hydrogen bonding, methanol, water, and the disordered ethylene group of ET were placed at the calculated ideal positions. The hydrogen atoms involved in hydrogen bonding were determined by applying the geometrical features6 of mellitate and pyridinium (pyridine) with cross-checking the consistency with Fourier maps. In paired hydrogen bonds, hydrogen atoms are disordered and placed at split two sites with occupancy of 0.5 for each O 3 3 3 H 3 3 3 O bond (O-H distance and thermal parameter of H are restrained). From the Fourier map, methanol and water molecules located near the r 2009 American Chemical Society

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Chart 1

Table 1. Crystal Data and Structural Refinement Parameters for 1 formula formula weight crystal system space group a/A˚ b/A˚ c/A˚ β/deg V/A˚3 Z Dx/(g cm-3) μ(Mo KR)/cm-1 2θmax temp of data collection/K no. of unique reflections Rint no. of variables R1 [I > 2.0σ(I)] Rw (all data) goodness of fit indicator

C65H52N2O27S24 2062.56 monoclinic P21/a (#14) 16.596(6) 9.602(4) 24.775(11) 91.26(3) 3947(3) 2 1.735 7.33 55.0 123 9002 0.037 542 0.050 0.133 0.867

Figure 1. Perspective view of the unit cell of 1 along the b axis. Red fragments are mellitate.

inversion center were found to be largely disordered, and three representative positions were assigned to overlapped C/O (positional parameters were constrained) and O with occupancy of 0.5 for each element (refined as isotropic, and hydrogen atoms were not determined). A full-matrix least-squares technique with anisotropic thermal parameters for non-hydrogen atoms, except for methanol and water, and isotropic thermal parameters for hydrogen atoms was employed for structure refinement. The temperature dependence of the lattice parameters was measured using the same diffractometer. Electrical Resistivity Measurements. Electrical resistivity measurements for single crystal specimens of 1 were carried out using a four-probe method in the temperature range of 5-300 K. The electrical contacts between the crystal surface and gold lead wires (φ = 20 μm) were prepared using gold paste. Measurements were performed along the b axis, because the crystals were primarily elongated along the b axis.

3. Results and Discussion

Figure 2. Two-dimensional hydrogen-bonding network of mellitate in the ab plane.

Crystal Structure. The overall crystal structure of 1, which is composed of ET sheets separated by mellitate double-sheet layers sandwiching pyridinium, methanol, and water, is shown in Figure 1. The anionic layer composed of mellitate, pyridinium, and methanol is first considered; the ab plane projection is shown in Figure 2. One mellitate anion is included in an asymmetric unit. Among the six carboxy/ carboxylate groups (A-F in Figure 2), B and D can be assigned as carboxy groups forming paired hydrogen bonds with the neighbors. A forms a strong hydrogen bond with E (O 3 3 3 O distance of 2.458(3) A˚). Similarly, C forms a strong hydrogen bond with F (O 3 3 3 O distance of 2.515(3) A˚). In these O-C-O groups, the ratios (r) of the two C-O bond lengths are in a range of 1.04-1.06. Such r values with short O 3 3 3 O distances are observed when a proton is almost equally shared by two carboxylate groups.4 As a result, mellitate can be assigned as a dianion. Pyridine is protonated and forms hydrogen bonds with three carboxylate groups. Two anionic sheets composed of mellitate and pyridinium are united to form a double-sheet layer incorporating methanol and water.

ET forms a conducting sheet between the double-sheet anionic layers. ET molecules are arranged so that the terminal ethylene groups fit in the hollows in the mellitate sheet as shown in Figure 3a. There are two crystallographically independent ET molecules; ET I is located on the inversion center, and ET II where the entire molecule with one disordered terminal ethylene group is crystallographically independent. Both terminal ethylene groups fit into the hollows for ET I, while one end fits into the hollow and the other end (disordered ethylene group) locates on the center of the benzene ring of mellitate for ET II (Figure 3b). The resultant ET arrangement becomes a herringbone type as shown in Figure 4. This arrangement is categorized as an Rtype ET salt.9 The formal charge of ET can be estimated from the bond lengths. According to the method proposed by Guionneau et al.,15 the formal charges of ET I and ET II are estimated to be 0.63 and 0.51, respectively. Based on the method proposed by Saito et al.,16 these values become 0.59 and 0.45, respectively. In both cases, the difference of the charge between ET I and ET II is small, although the total

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Figure 5. Calculated band structure of 1.

Figure 3. (a) Anionic sheet represented by a space-filling model, and (b) arrangement of ET molecules governed by the hollows in the anionic sheets.

Figure 6. Temperature dependence of the resistivity for 1 (along the b axis) and R-ET2I3.

Figure 4. Arrangement of ET molecules and the HOMO-HOMO overlap integrals between ET molecules in 1.

charge is somewhat short of the required value. Since, considering the standard deviation of each bond length, the error of the charge estimation may be greater than 0.1, it is likely that there is no obvious charge disproportionation in this salt. The HOMO-HOMO overlap integrals for this ET arrangement were calculated, and the resultant band structure is shown in Figure 5. The HOMO band splits into two parts with a very small energy gap, and the Fermi level is located just at this gap. This situation is typical of R-type ET salts, such as the recent attention received by R-ET2I3 as a zero-gap semiconductor.17-21 It is noteworthy that although the band filling of 1 (2/3-filled) is different from that of R-ET2I3 (3/4filled), both band structures are similar. Charge-Transport Properties. The temperature dependence of the electrical resistivity of 1 along the b axis is shown in Figure 6. The resistivity at room temperature is

relatively low (FRT = 0.2 Ω cm), and it only slightly increases with a decrease in temperature; F4K/FRT = 8. Some of the R-type ET salts are known to exhibit M-I transitions. For example, R-ET2I3 has a sharp M-I transition at 135 K (Figure 6). At this temperature, the uniform charge distribution in the ET sheet becomes disproportional;22-24 that is, charge-rich and charge-poor ET molecules can be distinguished (charge ordering). In contrast to R-ET2I3, such M-I transitions are completely suppressed in 1. Such insensitivity of charge transport to temperature change observed for 1 may be attributed to fixation of the ET arrangement by the supramolecular mellitate network. Six carboxy/carboxylate groups in mellitate are all connected with their neighbors by strong hydrogen bonds, as shown in Figure 3a. The lengths of hydrogen bonds change little with the temperature, compared with distances in the van der Waals contacts, so that the mellitate network does not contract with decreasing temperature. In addition, the terminal groups of ET are fixed at the hollows in the anionic network, and thus the ET sheet itself is considered to not contract with decreasing temperature. The temperature dependence of the ab plane area (corresponding to the area of the ET conducting sheet) of 1 is shown in Figure 7. The unit cell volume decreases with decreasing temperature, but the area of the ab plane is almost constant. The same plots for R-ET2I3 are also shown in Figure 7. Both the unit cell volume and the ab plane area (corresponding to the area of the ET conducting sheet in R-ET2I3) show significant decreases with decreasing temperature. The changes in molecular packing affect the balance between the degree of π-π interaction (transfer energy) and intersite Coulombic interaction. In the case of R-ET2I3, thermal contraction of the ET conducting sheet leads to

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and Grants-in-Aid for Scientific Research from the Japan Society for the Promotion of Science. Supporting Information Available: X-ray crystallographic information file (CIF) for compound 1, disorder model of methanol and water molecules in 1, table of bond lengths and estimated formal charge of ET in 1, and table of lattice constants at various temperatures for 1 and R-ET2I3. This material is available free of charge via the Internet at http://pubs.acs.org.

References

Figure 7. Temperature dependence of the ET conducting sheet area (A) and the volume (V) of the unit lattice for 1 and R-ET2I3. The values are normalized by those at 298 K. The solid lines indicate the trend.

charge ordering. However, this charge ordered insulating state was found to be very sensitive to uniaxial strain; both a and b axes strains recovered the conducting state, although the ground state (metal or narrow gap semiconductor) was dependent on the direction of uniaxial strain.17,19 In addition, a superconducting state was found to appear under a-axis strain.19 In contrast, thermal contraction of the ET conducting sheet in 1 is completely suppressed. This situation is considered to lead to insensitivity of transfer energy and electron correlation to temperature change in 1, and consequently to retention of the conducting state at low temperature. 4. Conclusion The ET conductor 1 was prepared by utilizing a supramolecular mellitate network. The molecular arrangement in the ET conducting sheet is fixed by fitting of the terminal groups into the hollows in the network. The area of the anionic network does not change with temperature, so that the ET conducting sheet does not exhibit thermal contraction. This leads to high temperature-independent conductivity, while R-ET2I3, which has a similar ET arrangement, has a M-I transition due to thermal contraction of the ET conducting sheet. The physical properties of molecular conductors are very sensitive to changes in the molecular packing due to thermal contraction; therefore, hydrogen-bonding supramolecular networks may be useful to maintain specific properties at low temperature. Acknowledgment. This work was supported in part by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan

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