J O U R N A L O F C H E M I S T R Y Materials Crystal structure and physical properties of (TTM-TTP)AuI2† Tadashi Kawamoto,*a Masanobu Aragaki,a Takehiko Mori,a Yohji Misakib and Tokio Yamabeb aDepartment of Organic and Polymeric Materials, T okyo Institute of T echnology, O-okayama, T okyo 152, Japan bDivision of Molecular Engineering, Graduate School of Engineering, Kyoto University, Yoshida, Kyoto 606–01, Japan Crystal structure analysis of new organic conductors (TTM-TTP)AuI2 and (TTM-TTP)AuBr2, where TTM-TTP is 2,5-bis[4,5- bis(methylthio)-1,3-dithiol-2-ylidene]-1,3,4,6-tetrathiapentalene, has been carried out. The donor to anion ratio is 151 and the donors form highly one-dimensional columns. These materials, however, show dimerization of the donor molecules along the stacking direction.Therefore the energy gap appears at the Fermi level, and these salts become band insulators. The observed temperature dependence of the conductivity is semiconducting, but the room-temperature conductivity is comparatively high, 10–40 S cm-1. Many bis-fused TTF donors, denoted TTP donors, have been Experimental synthesized and many radical-cation salts have been obtained.TTM-TTP was prepared as described in ref. 8. Crystals of the Most of the radical-cation salts of TTP series donors show linear anions were grown by electrochemical oxidation in THF metallic behavior down to liquid-He temperature and they are in the presence of the donor and the tetrabutylammonium quasi-two-dimensional conductors.1–3 Among many salts of salts of the corresponding anions under a constant current of TTP series donors, only (DTEDT)3Au(CN)2 shows supercon- 1 mA at 28°C.These conditions were the same as the case of ductivity below 4 K at ambient pressure.4 The radical-cation (TTM-TTP)I3. In the case of AuI2- and AuBr2-, single salts of TTM-TTP (see below), however, are quasi-one-dimencrystals were obtained. These crystals were in the form of sional conductors.5,6 Although most of the TTM-TTP salts elongated black thin plates with typical dimensions, are semiconductors below room temperature, (TTM-TTP)I3 2.0×0.14×0.08 mm.Their crystal structures were determined shows metallic behavior and a metal–insulator transition at by the X-ray single crystal structure analysis.‡ 160 K.6 It is surprising that an organic conductor with a half- All measurements were made on a Rigaku Raxis II area filled band, in other words with 151 composition, shows detector with graphite monochromated Mo-Ka radiation. The metallic conductivity, because in general 151 salts are insulators structure was solved by the direct method (SHELX86) and on account of the on-site Coulomb repulsion.Recently we was refined by the full-matrix least-squares procedure.9 Neutral have observed paramagnetic behavior which is fitted with the atom scattering factors were taken from ref. 10. Anisotropic one-dimensional Heisenberg model from room temperature to thermal parameters were adopted for all non-hydrogen atoms. 2 K, without showing any anomaly at the metal–insulator From the result of the X-ray crystal structure analysis, the transition temperature.So the low-temperature phase is attrielectronic structure was calculated on the basis of the extended buted to the Mott insulator.7 Such a perfect separation of the Hu� ckel method.11 spin and charge degrees of freedom has been predicted by the Electrical resistivity was measured by the four-probe method Luttinger liquid theory in the half-filling case.Although the using low-frequency ac current (usually 10 mA). metal–insulator transition temperature is lowered under pressure, the shift is so small that the transition still remains at 90 K even at 11.5 kbar. Results Crystal structures Crystallographic data are listed in Table 1. The lattice constants show that the AuI2- and the AuBr2- salts are isostructual.Although the structure analyses were carried out for both salts, the data of the AuBr2- salt are not very good owing to its S S S S S S S S H3CS H3CS SCH3 SCH3 TTM-TTP poor crystal quality. Then we will concentrate on the results of the AuI2- salt. The atomic numbering scheme is shown in Fig. 1(a). The In order to apply chemical pressure by the use of shorter length of the c axis of the AuI2- salt is almost twice as long anions, preparation of the linear-anion salts other than I3- as that of the I3- salt, indicating that the donor molecules has been attempted.In this paper, crystal structure, energy form a dimer along the stacking direction. The I3- salt has band structure, and transport properties of (TTM-TTP)AuI2 only one donor molecule in a unit cell, but one unit cell and (TTM-TTP)AuBr2 will be shown and discussed in comcontains two donor molecules in the present compounds.In parison with (TTM-TTP)I3. (TTM-TTP)I3 the donor molecule is located on an inversion ‡ Full crystallographic details, excluding structure factors, have been deposited at the Cambridge Crystallographic Data Centre (CCDC). * kawamoto@o.cc.titech.ac.jp † Presented at the 58th Okazaki Conference, Recent Development See Information for Authors, J.Mater. Chem., 1996, Issue 1. Any request to the CCDC for this material should quote the full literature and Future Prospects of Molecular Based Conductors, Okazaki, Japan, 7–9 March 1997. citation and the reference number 1145/56. J. Mater. Chem., 1998, 8(2), 285–288 285Table 1 Crystallographic data terminal methylthio groups [Fig. 1(b)]. The terminal CMS bonds also extend basically in the molecular plane except for (TTM-TTP)AuI2 (TTM-TTP)AuBr2 C(13); the deviation of C(13) from the least-squares plane is 1.16 A° . In the present donor, the terminal carbons of the chemical formula C14H12S12AuI2 C14H12S12AuBr2 neutral molecule are reported to be out of the molecular formula weight 1015.876 921.876 shape black plate black plate plane.8 In contrast, in (TTM-TTP)I3 with +1 charge the crystal system triclinic triclinic molecular structure is flat.6 Therefore the flat molecular strucspace group P19 P19 ture of the present salt is consistent with the expected +1 a/A ° 12.745(6) 12.49(5) charge.b/A ° 13.641(6) 13.39(4) Intramolecular bond lengths (averaged so as to have hypo- c/A ° 9.410(8) 9.41(4) thetical mmm symmetry) are listed in Table 2.Although the a/° 107.39(6) 107.3(3) b/° 104.54(8) 104.2(4) changes of the bond lengths, when the donor is oxidized from c/° 100.08(4) 100.2(2) neutral to +1, are comparable to the estimated standard V /A° 3 1454(2) 1402(12) deviations of the bond lengths, particularly in such cases as Z 2 2 the present compound that contains heavy atoms like Au and Dc/g cm-3 2.320 2.184 I, comparing the bond lengths after averaging under the mmm l/A ° 0.71070 0.71070 symmetry, we can make an approximate estimate of the degree temperature/K 298 298 m(Mo-Ka)/cm-1 79.86 89.24 of charge transfer, as exemplified in ref. 13. In comparison with R 0.071 0.100 the neutral TTM-TTP, most CNC bonds become longer, and Rw a 0.097 0.119 most CMS bonds become shorter.This is consistent with the reflections used 2237 1256 symmetry of the HOMO, which has nodes on all CMS bonds. The bond lengths in (TTM-TTP)AuI2 are very close to those aw=1/s(I)2. in (TTM-TTP)I3. This also demonstrates that the donor molecule of this salt has +1 charge. The diVerences of the bond lengths between D0 and D+ are, however, as small as 0.01–0.03 A ° ; these values are about half of the corresponding changes in the TTF series, 0.02–0.07 A° .12 This is reasonable because the HOMO of the TTM-TTP molecule spreads on the larger molecule as pointed out in ref. 6. The donor molecules are stacked along the c axis [Fig. 2(b)]. Fig. 3 shows the overlap modes in the stack. Because the donor molecules dimerize along the stacking direction, there are two overlap modes.Both modes are ring-over-bond type. In one overlap mode denoted c1 in this stack [Fig. 2(b) and Fig. 3(a)], the slip distance along the molecular long axis is 1.6 A ° , which corresponds to about half of the length of a 1,3-dithiole ring. The interplanar distance of the c1 mode i3.46 A ° , where the plane of the donor molecule is defined by the bis-fused TTF part, but the CH3S parts are excluded.The other overlap mode denoted c2 [Fig. 2(b) and Fig. 3(b)] is the same as the uniform overlapping mode in (TTM-TTP)I3; the slip along the molecular long direction of c2 is 4.8 A ° , which corresponds to one and a half 1,3-dithiole rings. The interplanar distance of c2 is 3.48 A ° .This is slightly larger than that of the c1 mode. These Fig. 1 (a) ORTEP drawing and atomic numbering scheme of the donor stacking overlap modes have many SMS contacts shorter than molecule of (TTM-TTP)AuI2 and (b) side view of the molecule of the van der Waals distance. (TTM-TTP)AuI2 center.6 In the AuI2- salt, however, the donor molecule is Energy band structures located on a general position.A unit cell contains two AuI2- Calculated intermolecular overlap integrals of the HOMO are anions on general equivalent positions [Fig. 2(a)]. In (TTMlisted in Table 3. The diVerence between c1 and c2 designates TTP)I3 there are short I I contacts of 4.235(1) A ° , while in the degree of dimerization along the stack. The ratio of c1 to the present compound the closest I I distances are 4.566(6) c2 is about 352, therefore the dimerization is not so strong.and 5.102(4) A ° . Since these values are larger than the van der The intrastack overlaps are 200 times as large as those of Waals distance of I-, 4.2 A ° , we can regard the anions as interstack overlaps. Along [1190], the donor molecules cannot discrete AuI2-. No anion deficiency was found from the approach close to each other because of the steric hindrance structure analysis; therefore this complex has exact 151 of the terminal methyl groups.So the side-by-side interaction composition. along the molecular short axis is very small. This situation has The donor molecule is almost planar; the deviations from the least-squares plane are less than 0.2 A° except for the a close resemblance to (TTM-TTP)I3.Therefore the electronic Table 2 Intramolecular bond lengths (A ° ) of TTM-TTP averaged by assuming mmm symmetry S(5)MC(7) S(5)MC(6) S(3)MC(5) S(3)–C(3) S(6)MC(8) S(6)MC(6) S(4)MC(5) S(4)MC(4) S(7)MC(7) S(7)MC(9) C(5)MC(6) S(9)MC(10) S(9)MC(11) C(3)MC(4) salt C(7)MC(8) S(8)MC(8) S(8)MC(9) C(9)MC(10) S(10)MC(10) S(10)MC(12) C(11)MC(12) neutral 1.346(7) 1.743(4) 1.764(4) 1.348(6) 1.752(4) 1.754(4) 1.339(4) I3 1.362(9) 1.732(9) 1.748(6) 1.38(1) 1.737(8) 1.752(8) 1.351(9) AuI2 1.37(4) 1.73(3) 1.76(3) 1.38(3) 1.74(3) 1.75(3) 1.33(4) 286 J.Mater. Chem., 1998, 8(2), 285–288Fig. 2 Crystal structure of (TTM-TTP)AuI2. (a) Projection onto the ab plane, (b) view along the molecular short axis, and (c) view along the molecular long axis.Table 3 Intermolecular overlap integrals, S (×103) of the HOMO of (TTM-TTP)AuI2 c1 27.4 c2 19.1 p1 -0.07 p2 -0.01 p3 0.19 q1 -0.17 q2 -0.03 q3 -0.10 q4 -0.05 structures of the present materials are regarded as highly onedimensional. Fig. 4 shows the energy band structure calculated on the basis of the extended Hu� ckel orbital calculation and the tightbinding method. Because a unit cell contains two donor molecules, there are two energy bands. This is diVerent from (TTM-TTP)I3.As a result of the 151 composition, the band is half-filled like the I3- salt. The energy gap, however, exists at the Fermi level, and thus no Fermi surface exists. This band structure predicts that the present salts are band insulators. Transport properties The conductivity at room temperature is about 10 and 40 S cm-1 for the AuI2- and AuBr2- salts, respectively.These conductivities are much lower than that of (TTM-TTP)I3.6 These values are, however, comparatively high for a band Fig. 3 Overlap modes of intrastack interactions in (TTM-TTP)AuI2 insulator with a dimerized structure. Fig. 5 shows the tempera- J. Mater. Chem., 1998, 8(2), 285–288 287Discussion The AuI2- and AuBr2- salts of TTM-TTP were obtained by the electrochemical crystal growth method similarly to (TTMTTP) I3.These salts have the same 151 composition as the I3- salt. These salts, however, have the dimerization of the donor molecules along the stacking direction. Therefore the energy gap appears at the Fermi level, and these salts become insulators.The observed conducting behaviors agree with this expectation from the crystal structure. The application of chemical pressure, namely the use of shorter anions, induces dimerization. In contrast, a naive prediction suggests that a uniform structure is generally more Fig. 4 Tight-binding energy band structure of (TTM-TTP)AuI2 calcupreferable under pressure, as most Peierls transitions are lated from the overlap of the HOMO obtained on the basis of the suppressed under pressure. Although the AuI2- anion (9.4 A ° ) extended Hu� ckel molecular orbital calculation is shorter than the I3- anion (10.1 A ° ),14 the lattice volume of the AuI2- salt is larger than twice that of the I3- salt.On the other hand the lattice constant c of the AuI2- salt is signifi- cantly shorter than twice that of the I3- salt.In these salts the linear anions are placed almost parallel to the intermolecular vector between the centers of dimerized molecules, and so are slightly inclined from the stacking direction. Therefore it is considered that the shrinkage of the linear anion makes crystal packing with a uniform stacking similar to the I3- salt impossible, and gives rise to the change of the stacking pattern, resulting in the dimerization.As a result the AuI2- salt becomes a band insulator. The lattice volume, however, increases owing to the change of the stacking pattern. Thus we cannot use shorter anions as a source of chemical pressure in the present case. The dimerization of the present compounds can be regarded as ‘chemically induced’ Peierls instability.One of the greatest mysteries in (TTM-TTP)I3 is the absence of the Peierls trans- Fig. 5 Temperature dependence of electrical resistance of (TTMition. Here we have encountered such an instability entirely TTP)AuI2 and (TTM-TTP)AuBr2 unexpectedly. References 1 Y. Misaki, H. Fujiwara, T. Yamabe, T. Mori, H. Mori and S. Tanaka, Chem. L ett., 1994, 1653. 2 T. Mori, Y. Misaki, H. Fujiwara and T. Yamabe, Bull. Chem. Soc. Jpn., 1994, 67, 2685. 3 T. Mori, T. Kawamoto, Y. Misaki, K. Kawakami, H. Fujiwara, T. Yamabe, H. Mori and S. Tanaka, Mol. Cryst. L iq. Cryst., 1996, 284, 271. 4 Y. Misaki, N. Higuchi, H. Fujiwara, T. Yamabe, T. Mori, H. Mori and S. Tanaka, Angew. Chem., Int. Ed. Engl., 1995, 34, 1222. 5 Y. Misaki, H. Nishikawa, T.Yamabe, T. Mori, H. Mori and S. Tanaka, Synth.Met., 1995, 70, 1153. 6 T. Mori, H. Inokuchi, Y. Misaki, T. Yamabe, H. Mori and S. Tanaka, Bull. Chem. Soc. Jpn., 1994, 67, 661. 7 T. Mori, T. Kawamoto, J. Yamaura, T. Enoki, Y. Misaki, Fig. 6 Temperature dependence of thermoelectric power of (TTMT. Yamabe, H. Mori and S. Tanaka, Phys. Rev. L ett., 1997, 79, TTP)AuI2 and (TTM-TTP)AuBr2 1702. 8 Y. Misaki, H. Nishikawa, K. Kawakami, S. Koyanagi, T. Yamabe ture dependence of electrical resistance of (TTM-TTP)AuI2 and M. Shiro, Chem. L ett., 1992, 2321. and (TTM-TTP)AuBr2 at ambient pressure. These salts were 9 G. M. Sheldrick, Crystallographic Computing 3, Oxford University semiconductors below room temperature. From the incli- Press, Oxford, 1985, pp. 175–189. 10 D.T. Cromer and J. T. Waber, International T ables for X-Ray nations of the straight lines in this figure, the activation energies Crystallography, Kynoch Press, Birmingham, 1974, vol. 4, are extracted to be 0.03 eV for the AuI2- salt, and 0.07 eV for Table 2.2 A. the AuBr2- salt. This semiconducting behavior is in agreement 11 T. Mori, A. Kobayashi, Y. Sasaki, H. Kobayashi, G. Saito and with the energy band calculation. H. Inokuchi, Bull. Chem. Soc. Jpn., 1984, 57, 627. Thermoelectric power (Seebeck coeYcient) of the AuI2- and 12 H. Kobayashi, R. Kato, T. Mori, A. Kobayashi, Y. Sasaki, AuBr2- salts were measured as shown in Fig. 6. The sign of G. Saito, T. Enoki and H. Inokuchi, Mol. Cryst. L iq. Cryst., 1984, 107, 33. the thermopower is negative in both salts. This means that the 13 P. M. Chaikin, R. L. Greene Etemad and E. Engler, Phys. Rev. mobility of electrons is higher than that of holes. The value of B., 1976, 13, 1627. |Q| is proportional to 1/T as the temperature decreases, as 14 J. M. Williams, J. R. Ferraro, R. J. Thorn, K. D. Carlson, U. Geiser, expected in a semiconductor.13 From Fig. 6 activation energies H. H. Wang, A. M. Kini and M.-H. Whangbo, Organic are estimated to be 0.08 eV for the AuI2- salt, and 0.05 eV for Superconductors (Including Fullerenes): Synthesis, Structure, the AuBr2- salt. Properties, and T heory, Prentice Hall, NJ, 1992. Paper 7/03200E; Received 9th May, 1997 288 J. Mater. Chem., 1998, 8(2), 285–288