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Extended donors based on tetrathiafulvalene
210 8
Synthetic Metals, 55-57 (1993) 2108--2112
EXTENDED DONORS BASED ON TETRATHIAFULVALENE
M. ADAM, U. SCHERER, Y.-J. SHEN and K. MI21LLEN* Max-Planck-Institut fi.ir Polymerforschung, Ackermannweg 10 D-W6500 Mainz, Germany
ABSTRACT The syntheses of dimeric and trimeric TTF-donor-systems derived from BEDT-TTF and T I ' C n - T r F is described. The investigation of the donor properties of the title structures have been carded out by cyclic voltammetry, EPR, and electrical conductivity measurements of the doped material. The crystal structures of the neutral compound of two TTCn-homologes and of several radical-cation salts derived from the BEDT-TTF dimers are given.
INTRODUCTION Organic metals based on radical cation salts of tetrathiafulvalene and its derivatives have been extensively investigated during the past two decades[l]. The highly anisotropic behavior of the electrical conductivity in these salts is caused by the seggregated stacking of donor and counter ion. The electrical conductivity is nearly one-dimensional and a phase transition can occur, which converts the metallic state to an insulating one upon cooling. In contrast a metal-superconductor transition takes place in radical cation salts, which possess a higher dimensionality. Such systems are characterized not only by short S-S contacts along the stacks, but also by short distances between neighboring donor stacks. For this reason, we attempt to incorporate those 'inter-stack' interactions in a single molecule by the linkage of TTF-subunits in full ~z-conjugation [2].
RESULTS The syntheses of the extended donor molecules I and 2 proceeds via the 1,3,5,7-tetrathia-sindacene-2,6-dithiones (4) [31, [4]. Dropwise addition of_4 in THF to two equivalents of a 4,5-bisalkylthio-2-1ithiomethylthio-1,3-dithiole (THF, -78"C) and subsequent addition of an excess of iodomethane yield the bis-hexathioorthooxalates 5a-c and 6. The TTF0379-6779/93/$6.00
© 1993- Elsevier Sequoia. All rights reserved
2109
OR R'S'~S~N/SCH3 R,S..I~" Li+ OR 4_.: R= C6H13;C2H4C1;C2H4OC2H5
ic /
R'=C6H13 R',R'=C2H4
x , ~ H3
OR R'S,-....~.HaCS. ~ H3CS,/-%,,.~SR' R ' S ~ U . ~ ~ I-.~SR , OR 5a: R',R'=C2H4; 519." R',R'=C2H4; 5c: R',R'=C2H4; 6 : R'=C6H~3;
OR S
R=C6H13 R=C2H4OC2H5 R=C2H4CI R=C6H13
8:R=C6H13
A I-H3CSSCI13 A ~I'H3CSSCH3
R'S..--.~
la: lb: .~: 2:
OR ~ . OR R',R'=C2H4; R',R'=C2H4; R',R'=C2H4; R'=C6H13;
OR .S....~ R' OR R=C6HI3 R=C2H4OC2Hg R=C2It4CI R=C6H13
Scheme 1
2
OR
OR
OR
OR ~;. R=C6HI3
2110
electrophores are generated under extrusion of dimethyldisufide by pyrolysis of the orthooxalates (C2H2CI 4, 70°C, ultrasound). ;3 is synthesized by a phosphit-induced coupling reaction of_7. Like ! and 2 the mono-TTF-thione _7is obtained by pyrolysis of the corresponding trithioorthoformate 8. The coupling reaction which leads to 5 is also used to prepare 8 by reacting only one equivalent of the 4,5-bisalkylthio-2-1ithiomethylthio-l,3dithiole with 4. As shown in Figure 1, the mode of stacking in the neutral compound of ~ leads to alternating layers of TTF-subunits and long alkyl-chains. The entire re-system is planar. S-S contacts on the lower limit of the sum of the van der Waals radii are found.A cyclic voltammetric investigation reveals that ! and 2 can be charged reversibly up to a tetracation, while 3 donates three charges reversibly. Further reversibility is inhibited due to adsorption phenomena at the surface of the electrode. All of these oligomeric systems exhibit an oxidation potential for the second redox step, which is significantly lower than that of the corresponding TTF-parent compound while the first oxidation potential is only slightly increased. Donor E ll/2/mV E21/2/mV E31/2/mV E41/2/mV E51/2./mV
la
!
(R=C2Hs) 380 389 590 570 950 983" 1160* --. . . . . . . . .
21t
3
410 560 900 1000
284 423 676 874* 1080*
BEDTTTF 360 780
Table 1: Half-wave-potentials of several donors(CH2C12, 3 = CH2CICHCI2, TBAPF 6, 100mVs- j, vs. SCE, ferrocene = 310 mV),*anodic peakpotential Chemical doping of the new donors with DDQ and TCNQ-F4 affords dark CT-complexes as microcrystals with a metallic luster. Their stoichiometries, determined by elemental analyses, are 1:1 (!) and 2:3 (2). Their powder conductivity is in the range of 10.3 to 10 -1 S cm q. Donor 1 (R=C2Hs) 1 (R=C6H13) 2a 3
DDQ (S cm -t) 2.10 .2 5.10 -3 3.10 .3 8.10 -3
J3(S cm -1) 9.10 -1 5.10 -2 3.10 -3 3.10 -2
Table 2: Powder conductivities of the donors synthesized
2111
Radical-cation salts of 1, 2. and 3 are obtained by electro-cyrstallization. 2 forms long fibrous crystals with ReO 4- as counter anion. The EPR-Signal of 2*ReO 4 shows a surprisingly small line width of 2.5 G at room temperature. This value is normally observed only at very low temperature in the insulating state [5]. On the other hand only a slight change in the line width is observed when lowering the temperature. Remarkably, the same crystal exhibits a high electrical conductivity of 8 S cm -1. So far, single crystals of Ia,PF6[6], lb,CIO4, l c , C I O 4, as well as of 3 , P F 6 and 3,SbF 6 were obtained. All the crystals of_l and 3 are semiconductors with a room-temperature conductivity in the range of 10 Scm -1 and a thermal activation energy E a of about 130 meV in case of/. and 220 meV in case of 3 is determined. The line width of the EPR signal of 3_*PF6 upon cooling is even smaller than that o f 2 * R e O 4 [7]. As shown in Figure 2, the single crystalline radical-cation salts o f l form seggregated stacks. Short S-S distances down to 3.3/~ are found. The packing pattern for the donors in I a , P F 6 is similar to that of c~-(BEDT-TTF)2*PF6 [8], while in l b * C l O 4 the donors are turned along the stacking axis, similar to (BEDT-TTF)z,CIO4(C4H802) [9]. As in many of the BEDTTTF salts, solvent molecules are incorporated into the crystal. Solvent inclusion may well be facilitated by the solubilizing groups in the central positions of the donors. This is avoided in the crystal of lc*C104, because of the short side-chains.
Figure 1 a: Donor overlap of 2
b: Unit cell of 2
2112
Figure 2 a: Unit cell of Ib.CIO~.
b: Unit cell of l e , C 1 0 4
ACKNOWLEDGEMENT Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.
REFERENCES 1
J.M. Williams, J. R. Ferraro, R. J. Thorn, K. D. Carlson, U. Geiser, H. H. Wang, A. M. Kini, M.-H.
2
H.J. Rader, U. Scherer, P. Wolf, K. M~illen, Synth. Met.. 32 (1989) 15
3
M. Adam, P. Wolf, H.J. R~ider, K. MiJllen; J. Chem. Soc.. Chem. Commun., (1990), 1624
Whangbo, Organic Superconductors, Prentice-Hall, Englewood Cliffs, New Jersey, 1992
4
U. Scherer, Y.-S. Shen, J. U. yon Schlitz, K. MiJllen, to be published
5
T. Sugano, H. Kuroda, Ch~m. Phys. Lett 47 (1977) 92
6
M.Adam, A. Bohnen, V. Enkelmann, K. M~illen, Adv. Mater. 3, (1991), 600
7
Special thanks are due to J.U. yon Schlitz, M. Schw0rer for Conductivity Mesaurements as well as J.U. yon S. and H.W. Spiess for Solid State EPR-measurements
8
H. Kobayashi, R. Kato T. Mori, A. Kobayashi, Y. Sasaki, G. Saito, H. Inokuchi, Chem. Lett. (1983) 759
9
H. Kobayashi, R. Kato, T. Moil, A. Kobayashi, Y. Sasaki, G. Saito, T. Enoki, H. lnokuchi; Mol. Crypt, Liq, Cryst. 107 (1984) 33-43
Synthetic Metals, 55-57 (1993) 2108--2112
EXTENDED DONORS BASED ON TETRATHIAFULVALENE
M. ADAM, U. SCHERER, Y.-J. SHEN and K. MI21LLEN* Max-Planck-Institut fi.ir Polymerforschung, Ackermannweg 10 D-W6500 Mainz, Germany
ABSTRACT The syntheses of dimeric and trimeric TTF-donor-systems derived from BEDT-TTF and T I ' C n - T r F is described. The investigation of the donor properties of the title structures have been carded out by cyclic voltammetry, EPR, and electrical conductivity measurements of the doped material. The crystal structures of the neutral compound of two TTCn-homologes and of several radical-cation salts derived from the BEDT-TTF dimers are given.
INTRODUCTION Organic metals based on radical cation salts of tetrathiafulvalene and its derivatives have been extensively investigated during the past two decades[l]. The highly anisotropic behavior of the electrical conductivity in these salts is caused by the seggregated stacking of donor and counter ion. The electrical conductivity is nearly one-dimensional and a phase transition can occur, which converts the metallic state to an insulating one upon cooling. In contrast a metal-superconductor transition takes place in radical cation salts, which possess a higher dimensionality. Such systems are characterized not only by short S-S contacts along the stacks, but also by short distances between neighboring donor stacks. For this reason, we attempt to incorporate those 'inter-stack' interactions in a single molecule by the linkage of TTF-subunits in full ~z-conjugation [2].
RESULTS The syntheses of the extended donor molecules I and 2 proceeds via the 1,3,5,7-tetrathia-sindacene-2,6-dithiones (4) [31, [4]. Dropwise addition of_4 in THF to two equivalents of a 4,5-bisalkylthio-2-1ithiomethylthio-1,3-dithiole (THF, -78"C) and subsequent addition of an excess of iodomethane yield the bis-hexathioorthooxalates 5a-c and 6. The TTF0379-6779/93/$6.00
© 1993- Elsevier Sequoia. All rights reserved
2109
OR R'S'~S~N/SCH3 R,S..I~" Li+ OR 4_.: R= C6H13;C2H4C1;C2H4OC2H5
ic /
R'=C6H13 R',R'=C2H4
x , ~ H3
OR R'S,-....~.HaCS. ~ H3CS,/-%,,.~SR' R ' S ~ U . ~ ~ I-.~SR , OR 5a: R',R'=C2H4; 519." R',R'=C2H4; 5c: R',R'=C2H4; 6 : R'=C6H~3;
OR S
R=C6H13 R=C2H4OC2H5 R=C2H4CI R=C6H13
8:R=C6H13
A I-H3CSSCI13 A ~I'H3CSSCH3
R'S..--.~
la: lb: .~: 2:
OR ~ . OR R',R'=C2H4; R',R'=C2H4; R',R'=C2H4; R'=C6H13;
OR .S....~ R' OR R=C6HI3 R=C2H4OC2Hg R=C2It4CI R=C6H13
Scheme 1
2
OR
OR
OR
OR ~;. R=C6HI3
2110
electrophores are generated under extrusion of dimethyldisufide by pyrolysis of the orthooxalates (C2H2CI 4, 70°C, ultrasound). ;3 is synthesized by a phosphit-induced coupling reaction of_7. Like ! and 2 the mono-TTF-thione _7is obtained by pyrolysis of the corresponding trithioorthoformate 8. The coupling reaction which leads to 5 is also used to prepare 8 by reacting only one equivalent of the 4,5-bisalkylthio-2-1ithiomethylthio-l,3dithiole with 4. As shown in Figure 1, the mode of stacking in the neutral compound of ~ leads to alternating layers of TTF-subunits and long alkyl-chains. The entire re-system is planar. S-S contacts on the lower limit of the sum of the van der Waals radii are found.A cyclic voltammetric investigation reveals that ! and 2 can be charged reversibly up to a tetracation, while 3 donates three charges reversibly. Further reversibility is inhibited due to adsorption phenomena at the surface of the electrode. All of these oligomeric systems exhibit an oxidation potential for the second redox step, which is significantly lower than that of the corresponding TTF-parent compound while the first oxidation potential is only slightly increased. Donor E ll/2/mV E21/2/mV E31/2/mV E41/2/mV E51/2./mV
la
!
(R=C2Hs) 380 389 590 570 950 983" 1160* --. . . . . . . . .
21t
3
410 560 900 1000
284 423 676 874* 1080*
BEDTTTF 360 780
Table 1: Half-wave-potentials of several donors(CH2C12, 3 = CH2CICHCI2, TBAPF 6, 100mVs- j, vs. SCE, ferrocene = 310 mV),*anodic peakpotential Chemical doping of the new donors with DDQ and TCNQ-F4 affords dark CT-complexes as microcrystals with a metallic luster. Their stoichiometries, determined by elemental analyses, are 1:1 (!) and 2:3 (2). Their powder conductivity is in the range of 10.3 to 10 -1 S cm q. Donor 1 (R=C2Hs) 1 (R=C6H13) 2a 3
DDQ (S cm -t) 2.10 .2 5.10 -3 3.10 .3 8.10 -3
J3(S cm -1) 9.10 -1 5.10 -2 3.10 -3 3.10 -2
Table 2: Powder conductivities of the donors synthesized
2111
Radical-cation salts of 1, 2. and 3 are obtained by electro-cyrstallization. 2 forms long fibrous crystals with ReO 4- as counter anion. The EPR-Signal of 2*ReO 4 shows a surprisingly small line width of 2.5 G at room temperature. This value is normally observed only at very low temperature in the insulating state [5]. On the other hand only a slight change in the line width is observed when lowering the temperature. Remarkably, the same crystal exhibits a high electrical conductivity of 8 S cm -1. So far, single crystals of Ia,PF6[6], lb,CIO4, l c , C I O 4, as well as of 3 , P F 6 and 3,SbF 6 were obtained. All the crystals of_l and 3 are semiconductors with a room-temperature conductivity in the range of 10 Scm -1 and a thermal activation energy E a of about 130 meV in case of/. and 220 meV in case of 3 is determined. The line width of the EPR signal of 3_*PF6 upon cooling is even smaller than that o f 2 * R e O 4 [7]. As shown in Figure 2, the single crystalline radical-cation salts o f l form seggregated stacks. Short S-S distances down to 3.3/~ are found. The packing pattern for the donors in I a , P F 6 is similar to that of c~-(BEDT-TTF)2*PF6 [8], while in l b * C l O 4 the donors are turned along the stacking axis, similar to (BEDT-TTF)z,CIO4(C4H802) [9]. As in many of the BEDTTTF salts, solvent molecules are incorporated into the crystal. Solvent inclusion may well be facilitated by the solubilizing groups in the central positions of the donors. This is avoided in the crystal of lc*C104, because of the short side-chains.
Figure 1 a: Donor overlap of 2
b: Unit cell of 2
2112
Figure 2 a: Unit cell of Ib.CIO~.
b: Unit cell of l e , C 1 0 4
ACKNOWLEDGEMENT Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.
REFERENCES 1
J.M. Williams, J. R. Ferraro, R. J. Thorn, K. D. Carlson, U. Geiser, H. H. Wang, A. M. Kini, M.-H.
2
H.J. Rader, U. Scherer, P. Wolf, K. M~illen, Synth. Met.. 32 (1989) 15
3
M. Adam, P. Wolf, H.J. R~ider, K. MiJllen; J. Chem. Soc.. Chem. Commun., (1990), 1624
Whangbo, Organic Superconductors, Prentice-Hall, Englewood Cliffs, New Jersey, 1992
4
U. Scherer, Y.-S. Shen, J. U. yon Schlitz, K. MiJllen, to be published
5
T. Sugano, H. Kuroda, Ch~m. Phys. Lett 47 (1977) 92
6
M.Adam, A. Bohnen, V. Enkelmann, K. M~illen, Adv. Mater. 3, (1991), 600
7
Special thanks are due to J.U. yon Schlitz, M. Schw0rer for Conductivity Mesaurements as well as J.U. yon S. and H.W. Spiess for Solid State EPR-measurements
8
H. Kobayashi, R. Kato T. Mori, A. Kobayashi, Y. Sasaki, G. Saito, H. Inokuchi, Chem. Lett. (1983) 759
9
H. Kobayashi, R. Kato, T. Moil, A. Kobayashi, Y. Sasaki, G. Saito, T. Enoki, H. lnokuchi; Mol. Crypt, Liq, Cryst. 107 (1984) 33-43