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Crystal structure and physical properties of dication radical salts of cyclophane-shaped twin donor
~ )
0038-1098/93 $6.00+.00 Pergamon Press Ltd
Solid State Communications, Vol. 88, No. 3, pP. 207-209, 1993. Printed in Great Britain.
CRYSTAL STRUCTURE AND PHYSICAL PROPERTIES OF DICATION RADICAL SALTS OF CYCLOPHANE-SHAPED TWIN DONOR T. Tachikawa ~', A. Izuoka, and T. Sugawara* Department of Pure and Applied Sciences, College of Arts and Sciences, The University of Tokyo, Komaba 3-8-1, Meguro, Tokyo 153 (Japan) ( Received 22 July 1993 by T. Tsuzuki )
(acceptedfor publication 23 August 1993) Radical dication salts (2°(CIO4)2) was obtained by using twin donor (2). Conformation of the oxidized donor is a cyclophane-type with significant electronic interaction. Observation of a charge-transfer band (0.97 eV) in the absorption spectrum and detection of a thermally populated triplet signal (AEsT= 0.27 eV) by ESR spectroscopy enables to estimate electronic parameters of U and t to be 0.70 eV and 0.26 eV, respectively.
1. INTRODUCTION
band or an ESR signal assignable to the triplet exciton. Here we describe a unique molecular arrangement of cyclophanetype donors in radical ion salts and estimate of U and t parameter of the TTF-based conductor, by using AEcr and AEsr values determined experimentally.
Recently we have synthesized the twin donor l carrying two TTF units connected by a thiacrown ring, l, 4, 7, 10tetrathiacyclododeca-2,8-diene, and have succeeded in preparing metallic salts, 1-CIO4.1) In order to expand a variety of radical ion salts using twin-typed donors, a periphery of the donor was modified by introducing dimethylthio groups instead of an ethylenedithio group. The new twin donor 2 with dimethylthio groups turned out to afford radical cation salts 2.(CIO4)2 in which two oxidized donor moieties are strongly dimerized intramolecularly, forming a cyclophanetype conformation (i.e. two donor units are superimposed ). In some TCNQ complexes, for example, TCNQ anion radicals are dimerized strongly and energies of the charge transfer transition (AEcT) and of the triplet exciton (AEs.T) are determined)) These values make evaluation of on-site Coulombic repulsion (U) and transfer integral (t) possible. 3) Although a dimerized structure of donor-based conductors is also observed in some cation radical salts4) composed of TTF derivenives, such as (BEDT-TTF)X(THF)cL~(X=ReO 4, IO4), electronic parameters of U and t have not been evaluated by the experimental method aforementioned, because the degree of dimerization is not large enough to observe a charge transfer
2. EXPERIMENTAL Twin donor 2 was prepared according to the procedure published elsewhere 1). Instrumental and Mea,¢rcments Half-wave oxidation potentials of 2, Elfz vs. Ag/AgCI, were measured by cyclic voltammetry in dichloromethane in the presence of tetra-n-butylammonium perchlorate as the electrolyte with a scanning rate of 200 mVs1. Twin donor 2 showed reversible first and second oxidation waves at 0.60 and 0.86 V, respectively. The salt of 2.(CIO4)2 was prepared by galvanostatic electrocrystallization at the electric current of 0.5 ttA. The electrical conductivity was measured by the two-probe method at room temperature. ESR spectra were recorded in the temperature range between 150K and 400K on an ESR spectrometer (JES-RE2X JEOL) equipped with a variable temperature apparatus and a tem ~erature
MeSas S . _ . S ~ S ~ s s~SMe II ~-( II II )-~ II MeSaS S ~ S ~ S ~ S S~SMe 2 * To whom correspondence should be addressed Present address: Department of Applied Chemistry, Faculty
controller (ES-DVT2 JEOL). The powdered sample diluted with anthracene uniformly was used for the measurement. UV spectra were recorded by a Shimazu UV-3100 PC spectrometer using a KBr pellet.
of Engineering, Saitama University, Shimo-ohkubo 255, Urawa, Saitama 338. 207
CYCLOPHANE-SHAPED TWIN DONOR
208 X-ray crystal structure analysis of 2.(CIO4) z
Crystaldata : C20H20S16CI208, M = 972.24, C-centered monoclinic, space group C2, a = 23.189(6), b = 12.872(3), c = 16.278(4) L fl = 132.90(1) °, V = 3559(2) A ~, Z = 4, Dc = 1.815 g cm -3. Final conventional R factors: R = 4.7 % for 3434 observed reflections [ IFol > 3o(IFol) ] with 416 parameters. Data collection was measured at ambient temperature on a Rigaku AFC-5 four-circle diffractometer by using graphite monochromatized Mo K s radiation. Structure was solved by direct methods and refined by a block diagonal least-square refinement.
3. RESULTS AND DISCUSSION X-ray crystallographic analysis shows the stoichiometry of donor and anion of the salt, 2.(CIO4)2, is 1 : 2. Judging from the ratio of donor and anion, donor 2 exists as a dication radical, each donor unit of 2 being oxidized to the cation radical. Electrical conductivity of the salt was measured by
a)
Vol. 88, No. 3
the two probe method and the conductivity at room temperature was evaluated to be 2×10 .7 Scm -I. In the crystal the donor molecule constitutes a cyclophanelike conformation, in which the two donor units are overlapped in parallel (Figure I ). There exist many short S-S distances (3.3 ~ 3.6 A) within the cyclophane conformation, suggesting that the significant electronic interaction between two donor units. The averaged interplanar distance between two donor units is determined to be 3.32 ,~. The arrangement of the donors in crystals has following features. The cyclophane-typed donors do not stack in a column, but they form a chain structure, arranging their molecular long axes parallel to the crystallographic b axis (Figure 2). The donor molecules in the neighboring chains are shifted by one half of the molecular length. All intermolecular S-S distances are longer than 3.55 A which is the van der Waals distance between sulfur atoms. Counter anions, CIO4 are also linearly arranged along the donor chain. Conformation of the thiacrown moiety of the 2o(CIO4) ~ salt is substantially different from that observed in the neutral crystal of 1. 5~ There are strained torsional angles such as 35 ° or 90 ° in the conformation of the cyclophane-type structure. Therefore the donor conformation in the 2°(CIO4)2 salt is supposed to be govemed not by the conformational preference of the thiacrown moiety but by the attractive interaction (antiferromagnetic coupling) between oxidized donor units through unpaired electrons (vide infra ). The absorption spectrum of 2-(CIO~)? in a KBr pellet consists of absorption maxima at 7800 cm- and 11600 cm 1, the absorption not being extended over 5000 cm 1 (Figure 3). The tendency is consistent with the result of the conductivity measurement. The broad absorption maximum at 7800 cm t (AEcr = 0.97 eV) should be assigned to the intramolecular CT transition. 6) While the absorption maximum
b)
4 Me,, S t
s ~, ,,,~----:5
Me,,.,
Me Us
Figure 1. Molecular structure of 2 in 2°(CIO4)2, a) top view b) side view. : Black sticks represent C-C and C-S bonds of the donor unit situated below.
Figure 2. Crystal structure of 2o(CIO4) 2 , viewed normal to the ab plane.
Vol. 88, N o . 3
209
CYCLOPHANE-SHAPED TWIN DONOR
at 11600 cm" (1.44 eV) may be assigned to the local n-n* transition within the donor moiety of the open-shell electronic structure in terms of resemblance of the maximum position to that of radical ion salts of BEDT-TTF with 1:1 stoichiometry. 7) An ESR spectrum of 2.(CIO4)2 shows a thermally populated signal (g = 2.007, AH = 0.95 mT) at temperatures higher than 330 K. The temperature dependence of the signal intensity was interpreted by a S-T models) and the energy gap between the singlet and the excited triplet states was estimated tO be about 0.27 eV. The absence of a fine structure of the signal suggests that the thermally populated species exists as a triplet exciton of a Wannier type. This value (AEsT) is comparable with the activation energy of 0.36eV of the triplet exciton in morpholinium TCNQ2). The energy for the charge transfer transition of the salts is supposed to correspond to the sum of the on-site Coulomb repulsion (the energy difference between D ~', D+" and D2+, D) and the ST energy gap3k Thus the values of the on-site Coulomb repulsion (U) and the transfer integral (t) of the salts was evaluated to he U = 0.70 eV and It I = 0.26 eV, respectively, using experimentally determined values of AEcT = 0.97 eV and AEsT= 0.27 eV at the intradimer distances of 3.32 .~. This U value is almost one-half of that (1.56 eV) obtained for TIVIPD*C104 s).
4. CONCLUSION The twin donor 2 with methylthio groups turned out to afford dicationic salt in which the twin donor constitutes the cyclophane-type conformation. The strong intramolecular
2.
0.97 cV (D+'D+')
<
(D2+D)
/
44 eV +, (D+'D+') ~ (D " D+') ',
< i
0
I0
i
i
i
20 30 40 Wave Number / 10-3 cm-I
,
50
Figure 3. UV and NIR spectrum of 2.(C104) 2 measured in a KBr pellet.
interaction makes the estimation of electronic parameters, U and t , possible. The present data seems to be valuable in estimating transport property of organic conductors based on donors of the TTF family.
Acknowledgment This work was partly supported by Grants-in-Aid for New Program " Intelligent Molecular Systems with Controlled Functionality " (05NP0301) from the Ministry of Education, Science, and Culture, Japan. Authors wish to thank Prof. G. Saito for a stimulating discussion. Authors are also indebted to Dr. K. Hanawa of SHIMADZU CORPORATION for measurements of UV spectra.
REFERENCES 1.
2.
3.
4.
A. Izuoka, R. Kumai, T. Tachikawa, and T. Sugawara, Mol. Cryst. Liq. Cryst., 218, 213 (1992); T. Tachikawa, A. Izuoka, R. Kumai T. Sugawara, and Y. Sugawara, Solid State Communication, 8 2, 19 (1992). R. G. Kepler, J. Chem. Phys., 39, 3528 (1963); D. B. Chesnut and W. D. Phillips, J. Chem. Phys., 35, 1002 (1961); I. M. Brown and M. T. Jones, J. Chem. Phys., 51, 4687 (1969), Z. G. Soos, J. Chem. Phys., 46, 4284 (1967); P. L. Nordio, Z. G. Soos, and H. M. McConnell, Ann. Rev. Phys. Chem., 17, 237 (1966). Z. G. Soos, S. R. Bondeson, In Extended Linear Chain Compounds, pp. 193 - 261, J. S. Miller, Ed.; Plenum Press: New York, (1983). T. Mallah, C. Hollis, S. Bott, M. Kurmoo, P. Day, M. Allan, and R. H. Friend, J. Chem. Sot'. Dalton
5. 6. 7. 8.
Trans., 1990, 859; M. Tanaka, A. Kawamoto, J. Tanaka, M. Sano, T. Enoki, and H. Inokuchi, Bull. Chem. Soc. Jpn., 60, 2531(1987); H. Kobayashi, A. Kobayashi, Y. Sasaki, G. Saito, and H. Inokuchi, Chem. Lett., 1984, 183-186.; For reviews see; T. Mori, Kotai Buturi, 26, 149-162(1991). T. Tachikawa, A. Izuoka, and T. Sugawara, J. Chem. Soc., Chem. Commun., in press. A. Graja, Low-dimensional Organic Conductors, pp. 116 - 119, World Scientific, (1992). Private communication of M. Nakashima and G. Saito. D. D. Thomas, H. Keller, and H. M. McConnell, J. Chem. Phys., 39, 2321(1963); D. D. Thomas, A. W. Merkel, A. F. Hildebrandt, and H. M. McConnell, J. Chem. Phys., 40, 2588 (1964); T. Sakata, and S. Nagakura, Bull. Chem. Soc. Jpn., 42, 1497 (1969).
0038-1098/93 $6.00+.00 Pergamon Press Ltd
Solid State Communications, Vol. 88, No. 3, pP. 207-209, 1993. Printed in Great Britain.
CRYSTAL STRUCTURE AND PHYSICAL PROPERTIES OF DICATION RADICAL SALTS OF CYCLOPHANE-SHAPED TWIN DONOR T. Tachikawa ~', A. Izuoka, and T. Sugawara* Department of Pure and Applied Sciences, College of Arts and Sciences, The University of Tokyo, Komaba 3-8-1, Meguro, Tokyo 153 (Japan) ( Received 22 July 1993 by T. Tsuzuki )
(acceptedfor publication 23 August 1993) Radical dication salts (2°(CIO4)2) was obtained by using twin donor (2). Conformation of the oxidized donor is a cyclophane-type with significant electronic interaction. Observation of a charge-transfer band (0.97 eV) in the absorption spectrum and detection of a thermally populated triplet signal (AEsT= 0.27 eV) by ESR spectroscopy enables to estimate electronic parameters of U and t to be 0.70 eV and 0.26 eV, respectively.
1. INTRODUCTION
band or an ESR signal assignable to the triplet exciton. Here we describe a unique molecular arrangement of cyclophanetype donors in radical ion salts and estimate of U and t parameter of the TTF-based conductor, by using AEcr and AEsr values determined experimentally.
Recently we have synthesized the twin donor l carrying two TTF units connected by a thiacrown ring, l, 4, 7, 10tetrathiacyclododeca-2,8-diene, and have succeeded in preparing metallic salts, 1-CIO4.1) In order to expand a variety of radical ion salts using twin-typed donors, a periphery of the donor was modified by introducing dimethylthio groups instead of an ethylenedithio group. The new twin donor 2 with dimethylthio groups turned out to afford radical cation salts 2.(CIO4)2 in which two oxidized donor moieties are strongly dimerized intramolecularly, forming a cyclophanetype conformation (i.e. two donor units are superimposed ). In some TCNQ complexes, for example, TCNQ anion radicals are dimerized strongly and energies of the charge transfer transition (AEcT) and of the triplet exciton (AEs.T) are determined)) These values make evaluation of on-site Coulombic repulsion (U) and transfer integral (t) possible. 3) Although a dimerized structure of donor-based conductors is also observed in some cation radical salts4) composed of TTF derivenives, such as (BEDT-TTF)X(THF)cL~(X=ReO 4, IO4), electronic parameters of U and t have not been evaluated by the experimental method aforementioned, because the degree of dimerization is not large enough to observe a charge transfer
2. EXPERIMENTAL Twin donor 2 was prepared according to the procedure published elsewhere 1). Instrumental and Mea,¢rcments Half-wave oxidation potentials of 2, Elfz vs. Ag/AgCI, were measured by cyclic voltammetry in dichloromethane in the presence of tetra-n-butylammonium perchlorate as the electrolyte with a scanning rate of 200 mVs1. Twin donor 2 showed reversible first and second oxidation waves at 0.60 and 0.86 V, respectively. The salt of 2.(CIO4)2 was prepared by galvanostatic electrocrystallization at the electric current of 0.5 ttA. The electrical conductivity was measured by the two-probe method at room temperature. ESR spectra were recorded in the temperature range between 150K and 400K on an ESR spectrometer (JES-RE2X JEOL) equipped with a variable temperature apparatus and a tem ~erature
MeSas S . _ . S ~ S ~ s s~SMe II ~-( II II )-~ II MeSaS S ~ S ~ S ~ S S~SMe 2 * To whom correspondence should be addressed Present address: Department of Applied Chemistry, Faculty
controller (ES-DVT2 JEOL). The powdered sample diluted with anthracene uniformly was used for the measurement. UV spectra were recorded by a Shimazu UV-3100 PC spectrometer using a KBr pellet.
of Engineering, Saitama University, Shimo-ohkubo 255, Urawa, Saitama 338. 207
CYCLOPHANE-SHAPED TWIN DONOR
208 X-ray crystal structure analysis of 2.(CIO4) z
Crystaldata : C20H20S16CI208, M = 972.24, C-centered monoclinic, space group C2, a = 23.189(6), b = 12.872(3), c = 16.278(4) L fl = 132.90(1) °, V = 3559(2) A ~, Z = 4, Dc = 1.815 g cm -3. Final conventional R factors: R = 4.7 % for 3434 observed reflections [ IFol > 3o(IFol) ] with 416 parameters. Data collection was measured at ambient temperature on a Rigaku AFC-5 four-circle diffractometer by using graphite monochromatized Mo K s radiation. Structure was solved by direct methods and refined by a block diagonal least-square refinement.
3. RESULTS AND DISCUSSION X-ray crystallographic analysis shows the stoichiometry of donor and anion of the salt, 2.(CIO4)2, is 1 : 2. Judging from the ratio of donor and anion, donor 2 exists as a dication radical, each donor unit of 2 being oxidized to the cation radical. Electrical conductivity of the salt was measured by
a)
Vol. 88, No. 3
the two probe method and the conductivity at room temperature was evaluated to be 2×10 .7 Scm -I. In the crystal the donor molecule constitutes a cyclophanelike conformation, in which the two donor units are overlapped in parallel (Figure I ). There exist many short S-S distances (3.3 ~ 3.6 A) within the cyclophane conformation, suggesting that the significant electronic interaction between two donor units. The averaged interplanar distance between two donor units is determined to be 3.32 ,~. The arrangement of the donors in crystals has following features. The cyclophane-typed donors do not stack in a column, but they form a chain structure, arranging their molecular long axes parallel to the crystallographic b axis (Figure 2). The donor molecules in the neighboring chains are shifted by one half of the molecular length. All intermolecular S-S distances are longer than 3.55 A which is the van der Waals distance between sulfur atoms. Counter anions, CIO4 are also linearly arranged along the donor chain. Conformation of the thiacrown moiety of the 2o(CIO4) ~ salt is substantially different from that observed in the neutral crystal of 1. 5~ There are strained torsional angles such as 35 ° or 90 ° in the conformation of the cyclophane-type structure. Therefore the donor conformation in the 2°(CIO4)2 salt is supposed to be govemed not by the conformational preference of the thiacrown moiety but by the attractive interaction (antiferromagnetic coupling) between oxidized donor units through unpaired electrons (vide infra ). The absorption spectrum of 2-(CIO~)? in a KBr pellet consists of absorption maxima at 7800 cm- and 11600 cm 1, the absorption not being extended over 5000 cm 1 (Figure 3). The tendency is consistent with the result of the conductivity measurement. The broad absorption maximum at 7800 cm t (AEcr = 0.97 eV) should be assigned to the intramolecular CT transition. 6) While the absorption maximum
b)
4 Me,, S t
s ~, ,,,~----:5
Me,,.,
Me Us
Figure 1. Molecular structure of 2 in 2°(CIO4)2, a) top view b) side view. : Black sticks represent C-C and C-S bonds of the donor unit situated below.
Figure 2. Crystal structure of 2o(CIO4) 2 , viewed normal to the ab plane.
Vol. 88, N o . 3
209
CYCLOPHANE-SHAPED TWIN DONOR
at 11600 cm" (1.44 eV) may be assigned to the local n-n* transition within the donor moiety of the open-shell electronic structure in terms of resemblance of the maximum position to that of radical ion salts of BEDT-TTF with 1:1 stoichiometry. 7) An ESR spectrum of 2.(CIO4)2 shows a thermally populated signal (g = 2.007, AH = 0.95 mT) at temperatures higher than 330 K. The temperature dependence of the signal intensity was interpreted by a S-T models) and the energy gap between the singlet and the excited triplet states was estimated tO be about 0.27 eV. The absence of a fine structure of the signal suggests that the thermally populated species exists as a triplet exciton of a Wannier type. This value (AEsT) is comparable with the activation energy of 0.36eV of the triplet exciton in morpholinium TCNQ2). The energy for the charge transfer transition of the salts is supposed to correspond to the sum of the on-site Coulomb repulsion (the energy difference between D ~', D+" and D2+, D) and the ST energy gap3k Thus the values of the on-site Coulomb repulsion (U) and the transfer integral (t) of the salts was evaluated to he U = 0.70 eV and It I = 0.26 eV, respectively, using experimentally determined values of AEcT = 0.97 eV and AEsT= 0.27 eV at the intradimer distances of 3.32 .~. This U value is almost one-half of that (1.56 eV) obtained for TIVIPD*C104 s).
4. CONCLUSION The twin donor 2 with methylthio groups turned out to afford dicationic salt in which the twin donor constitutes the cyclophane-type conformation. The strong intramolecular
2.
0.97 cV (D+'D+')
<
(D2+D)
/
44 eV +, (D+'D+') ~ (D " D+') ',
< i
0
I0
i
i
i
20 30 40 Wave Number / 10-3 cm-I
,
50
Figure 3. UV and NIR spectrum of 2.(C104) 2 measured in a KBr pellet.
interaction makes the estimation of electronic parameters, U and t , possible. The present data seems to be valuable in estimating transport property of organic conductors based on donors of the TTF family.
Acknowledgment This work was partly supported by Grants-in-Aid for New Program " Intelligent Molecular Systems with Controlled Functionality " (05NP0301) from the Ministry of Education, Science, and Culture, Japan. Authors wish to thank Prof. G. Saito for a stimulating discussion. Authors are also indebted to Dr. K. Hanawa of SHIMADZU CORPORATION for measurements of UV spectra.
REFERENCES 1.
2.
3.
4.
A. Izuoka, R. Kumai, T. Tachikawa, and T. Sugawara, Mol. Cryst. Liq. Cryst., 218, 213 (1992); T. Tachikawa, A. Izuoka, R. Kumai T. Sugawara, and Y. Sugawara, Solid State Communication, 8 2, 19 (1992). R. G. Kepler, J. Chem. Phys., 39, 3528 (1963); D. B. Chesnut and W. D. Phillips, J. Chem. Phys., 35, 1002 (1961); I. M. Brown and M. T. Jones, J. Chem. Phys., 51, 4687 (1969), Z. G. Soos, J. Chem. Phys., 46, 4284 (1967); P. L. Nordio, Z. G. Soos, and H. M. McConnell, Ann. Rev. Phys. Chem., 17, 237 (1966). Z. G. Soos, S. R. Bondeson, In Extended Linear Chain Compounds, pp. 193 - 261, J. S. Miller, Ed.; Plenum Press: New York, (1983). T. Mallah, C. Hollis, S. Bott, M. Kurmoo, P. Day, M. Allan, and R. H. Friend, J. Chem. Sot'. Dalton
5. 6. 7. 8.
Trans., 1990, 859; M. Tanaka, A. Kawamoto, J. Tanaka, M. Sano, T. Enoki, and H. Inokuchi, Bull. Chem. Soc. Jpn., 60, 2531(1987); H. Kobayashi, A. Kobayashi, Y. Sasaki, G. Saito, and H. Inokuchi, Chem. Lett., 1984, 183-186.; For reviews see; T. Mori, Kotai Buturi, 26, 149-162(1991). T. Tachikawa, A. Izuoka, and T. Sugawara, J. Chem. Soc., Chem. Commun., in press. A. Graja, Low-dimensional Organic Conductors, pp. 116 - 119, World Scientific, (1992). Private communication of M. Nakashima and G. Saito. D. D. Thomas, H. Keller, and H. M. McConnell, J. Chem. Phys., 39, 2321(1963); D. D. Thomas, A. W. Merkel, A. F. Hildebrandt, and H. M. McConnell, J. Chem. Phys., 40, 2588 (1964); T. Sakata, and S. Nagakura, Bull. Chem. Soc. Jpn., 42, 1497 (1969).