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Rod-like tetrathiafulvalene polymers with extended π-conjugation
Synthetic Metals 118 (2001) 97±103
Rod-like tetrathiafulvalene polymers with extended p-conjugation Stefan Frenzel, Martin Baumgarten, Klaus MuÈllen* Max-Planck-Institut fuÈr Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany Received 19 January 2000; received in revised form 12 May 2000; accepted 15 June 2000
Abstract Rigid-rod type tetrathiafulvalene (TTF) polymers with exocyclic double bonds were prepared via a precursor route. The processable precursor polymers which contain hexathio-ortho-oxalate repeat units undergo thermal extrusion of dimethyldisul®de and result in target structures with extended p-conjugation. The extrusion must be carefully monitored to obtain structurally de®ned products. Doping of the resulting polymers with iodine shows conductivities which compare favorably with those of other electrically conducting polymers such as, e.g. polyacetylene and polythiophene. # 2001 Published by Elsevier Science B.V. Keywords: Tetrathiafulvalene polymers; p-Conjugation; Hexathio-ortho-oxalate
1. Introduction Tetrathiafulvalene (TTF) is known as a donor in electrically conducting charge-transfer complexes [1]. The conductivity in the solid is attributed to two important structural features, namely, the formation of segregated stacks of donors and acceptors and a certain degree of charge transfer (CT) between the stacks [2]. Despite the inherent electronic advantages, CT-complexes of TTF tend to be brittle and unprocessable. Incorporation of TTF's in polymers which are known for their good processability and ®lm-forming properties may overcome this problem. There have been several attempts to link the TTF moiety to the backbone of polymers [3±8], however, the resulting materials were insoluble and therefore unprocessable. Only the incorporation of bis-TTF (1) into the main-chain of polymer 2 (Scheme 1) resulted in conducting and processable polymers [9] after doping with strong acceptors such as 2,3,5,6tetra¯uoro-7,7,8,8-tetracyanoquinodimethane (TCNQF4), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone(DDQ)oriodine. Due to the conformational freedom of the TTF-subunits in a main-chain polymer, as depicted in Fig. 1, the polymer backbone most likely adopts a random-coil conformation with less p±p-interaction between neighboring TTF units than expected for their stackwise arrangement. Since the charge-transfer complexes of the oligomeric TTF systems 3 and 4 (Scheme 2) form solid-state structures *
Corresponding author. Tel.: 49-6131-379150; fax: 49-6131-379350. E-mail address: [email protected] (K. MuÈllen). 0379-6779/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 2 8 3 - 6
[10] analogous to those of parent TTF, we hoped that the conduction process for the conjugated donor polymer 5 could be enhanced in the doped state [2,11]. This is expected to be due to the charge transport along the polymer backbone via p-conjugation and due to the stacking with p-orbital overlap. In the following, we describe the synthesis of conjugated polymers 5 and 6 which includes thermal conversion of soluble and processable precursor polymers. 2. Experimental 1 H-NMR spectra were recorded at 200 MHz on Varian Gemini 200, or at 500 MHz on Bruker AMX 500, or at 300 MHz on Bruker AC 300 and 13 C-NMR spectra at 50 MHz on Varian Gemini 200, or at 125 MHz on Bruker AMX 500, or at 75 MHz on Bruker AC 300. Mass spectra were recorded using a VG-Instrument ZAB-2. UV±VIS spectra were recorded using a Perkin-Elmer Lambda 15 and IR spectra were recorded using a Nicolet FT-IR-Spectrometer 320. Polymers were characterized by GPC using a Waters 150-C, DSC on Mettler TC 11, Mettler TC 10A, TGA using a Mettler TGA 500.
2.1. 4,8-Di-n-hexyloxy-1,3,5,7-tetrathia-s-indacen-2,6dithione (12) 1-Bromohexane (13.2 g, 80 mmol) was added to a suspension of 4,8-di-n-hydroxy-1,3,5,7-tetrathia-s-indacen2,6-dithione [19] (10 g, 31 mmol) and tetrabutylammonium
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S. Frenzel et al. / Synthetic Metals 118 (2001) 97±103
Scheme 1
¯uoride (24.4 g, 77 mmol) in DMF (200 ml). The reaction mixture was heated to 708C and maintained at this temperature for 6 h and cooled to room temperature. Then methanol (200 ml) was added. The precipitate was ®ltered, washed with methanol and recrystallized from ethyl acetate to afford 12 as yellow crystals. Yield: 11.2 g (74%), mp 1128C. Found: C, 49.08; H, 5.26; S, 38.88%. Requires: C, 48.95; H, 5.34; S, 39.19%. 1 H-NMR (200 MHz, CDCl3): d 4.03 (t, 4H), 1.75 (m, 4H), 1.65±1.25 (m, 12H), 0.92 (t, 6H). 13 C-NMR (50 MHz, CDCl3): d 14.1, 22.7, 25.8, 30.1, 31.9, 74.1, 133.5, 139.9, 209.7. m/z (FD): 490.3 (M). nmax/cmÿ1 (KBr): 1076.8 (C=S). 2.2. 5,11-Dihexyloxy-1,3,4,6,7,9,10,12-octathiabiscyclopenta[a,h]anthracene-2,8-dithione (14) 1-Bromohexane (8.2 g, 50 mmol) was added to a suspension of 5,11-dihydroxy-1,3,4,6,7,9,10,12-octathia-biscyclopenta[a,h]anthracene-2,8-dithione [9] (6 g, 12 mmol) and tetrabutylammonium ¯uoride (11.3 g, 37 mmol) in DMF (150 ml). The reaction mixture was heated to 708C, maintained at this temperature for 6 h and then cooled to room temperature. Then methanol (50 ml) was added. The precipitate was ®ltered, washed with methanol and recrystallized from 1,1,2,2-tetrachloroethane to afford 14 (4.9 g,
Scheme 2
61%) as yellow crystals, mp 2058C (decomp.). Found: C, 43.25; H, 4.01; S, 48.12%. Requires: C, 43.25; H, 3.93; S, 48.01%. 1 H-NMR (500 MHz, C2D2Cl4): d 3.97 (t, 4H), 1.84 (m, 4H), 1.50±1.36 (m, 12H), 0.92 (t, 6H). 13 C-NMR (50 MHz, C2D2Cl4): d 14.1, 22.7, 25.6, 30.2, 31.7, 75.7, 129.8, 130.3, 150.0, 211.5. m/z (FD): 666.1 (M). nmax/cmÿ1 (KBr): 1081 (C=S). 2.3. 4,8-Di-n-hexyloxy-2,6-bis(methylthio)-1,3,5,7tetrathia-s-indacenyl-bis(tetrafluoroborate) (20) Trimethyl-ortho-formate (6.4 ml, 58.5 mmol) was added dropwise to a solution of dichloromethane (30 ml) and BF3 etherate (8.6 ml, 69.3 mmol) at ÿ308C. After 15 min of stirring under an argon atmosphere the mixture was warmed to room temperature. 4,8-Di-n-hexyloxy-1,3,5,7-tetrathia-sindacen-2,6-dithione (12) (6 g, 12.2 mmol) dissolved in dichloromethane (120 ml) was added to the reaction mixture. After 48 h the solvent was removed. Compound 20 was puri®ed by precipitation from an acetonitrile solution by dropwise addition of dry diethyl ether. 3.1 g (36%), mp 1258C (decomp.). 1H-NMR (200 MHz, acetonitrile-d3): d 4.51 (t, 4H), 3.35 (s, 6H), 2.01±1.15 (m, 16H), 0.95 (t, 6H). 13 C-NMR (200 MHz, acetonitrile-d3): d 14.3, 23.2, 25.8, 25.9, 30.6, 75.4, 77.8, 135.7, 145.8, 215.3. nmax/cmÿ1 (KBr): 518 ([S±CSCH3±S]). 2.4. 5,11-Dihexyloxy-2,8-bis(methylthio)1,3,4,6,7,9,10,12-octathia-biscyclopenta[b,i]anthracenylbis(tetrafluoroborate) (15)
Fig. 1. Main-chain polymer with (a) random-coil conformation; (b) stackwise arrangement.
A solution of 14 (4 g, 6 mmol) and trimethyloxonium tetra¯uoroborate (2.7 g, 18 mmol) in 1,1,2,2-tetrachloro-
S. Frenzel et al. / Synthetic Metals 118 (2001) 97±103
99
ethane (50 ml) was heated to 708C under argon atmosphere, maintained at this temperature for 12 h and then cooled to room temperature. The solvent was evaporated in vacuo and ether (500 ml) was added to the oily residue. The resulting precipitate was ®ltered, washed with ether and dissolved in acetonitrile (10 ml). The solution was added dropwise to dry diethyl ether. The precipitate was again ®ltered and dried to afford 3.9 g (77%) of 15 as purple crystals. mp 738C (decomp.). Found: C, 35.75; H, 3.65; S, 35.56%. Requires: C, 35.86; H, 3.70; S, 36.82%. 1 H-NMR (200 MHz, acetonitrile-d3): d 3.73 (t, 4H), 2.79 (s, 6H), 1.63±1.04 (m, 16H), 0.64 (m, 6H). 13 C-NMR (50 MHz,): d 14.1, 23.7, 24.7, 28.6, 31.1, 32.7, 77.0, 130.8, 144.3, 151.5, 210.4. m/z (FD): 347.9 (M/2-2 BF4), 783.0 (M/2-2 BF4). nmax/cmÿ1 (KBr): 521 ([S±CSCH3±S]).
Table 1 Conductivity of polymer 5 according to the amount of dopand present
2.5. Synthesis of the precursor polymers 18 and 19
After thermolysis, 5 and 6 were exposed to a iodine atmosphere for several minutes (Table 1). The excess of absorbed iodine was removed in vacuo (20 mbar) for 10 min. The conductivity was measured by a four-probe setup (Tables 1 and 2).
The cations 20 or 15 were dissolved in dry acetonitrile, and THF was added (2 ml/8 ml) in the case of 20 and 5 ml/ 10 ml for 15. Zinc (0.7 g, 1 mmol) and a catalytic amount of bromine (0.02 ml) was added. The dark yellow solution became pale yellow after 10±30 min of vigorous stirring. The suspension was ®ltered and the solvent was removed in vacuo. The residue was dissolved in dichloromethane and precipitated in methanol (200 ml) and this process was repeated twice. The ®ltered zinc was extracted three times with 1,1,2,2-tetrachloroethane (5 ml) which was heated to 808C. The solutions were combined and precipitated three times in methanol to obtain pale yellow (18) and white (19) polymers in yields >80%. Polymer 18: 1 H-NMR (200 MHz, CD2Cl2): d 3.83 (b), 2.48 (b), 1.95±1.12 (m), 0.91 (b). 13 C-NMR (50 MHz, CDCl3): d 14.7, 18.6 (b), 23.4, 26.4, 30.9, 32.4, 71.3, 72.9 (b), 130.1 (b), 141.4. TGA: 1958C (18.1%). DSC: no phase transition until decomposition. UV±VIS (®lm) l (nm)259. Mw (GPC)52 500 g molÿ1. Mn (GPC)5800 g molÿ1 (Dn9). Polymer 19: 1 H-NMR (200 MHz, CD2Cl2): d 3.93 (b), 2.43 (b), 1.97±1.15 (b), 0.98 (b). 13 C-NMR (50 MHz, CD2Cl2): d 14.3, 18.3 (b), 23.1, 25.1, 30.4, 32.1, 73.3 (b), 75.3, 118.9, 130.4 (b), 148.9. TGA: 1608C (13.4%). DSC: no phase transition until thermolysis. UV±VIS (®lm) l (nm) 230. Mw (GPC)87 700 g molÿ1. Mn (GPC)7900 g molÿ1 (Dn11). 2.6. Preparation of the polymeric donors 5 and 6 The precursor polymers 18 and 19 were dissolved in dichloromethane (100 mg/2 ml) and casted on a special glass plate (2 cm4 cm) via spin coating. The glass substrate consisted of four indium-zinc oxide (ITO) contacts allowing a four-probe conductivity measurement. The resulting ®lms had an average thickness of 500±800 nm. The glass plate was heated to 1658C for 80 min. The thermolysis was monitored by UV±VIS spectroscopy which
Conductivity (S cmÿ1)
Exposure time (min)
Content of iodine (%)
0.36 0.53 0.55 0.48 0.13
2 3 4 5 10
35 45 60 65 90
showed the evolving absorption signals of the TTF-moiety at UV±VIS (®lm) l (nm)230, 315, 353. 2.7. Doping of 5 and 6
3. Results and discussion 3.1. Synthesis To date numerous TTF-derivatives have been synthesized [10±17]. The synthesis usually involves coupling of two building blocks in the ®nal step to form the TTF-double bond. Several coupling methods using various precursors are known [1,17±19]. In order to prepare TTF polymers with high molecular weight, the coupling method must meet requirements such as high yield and negligible formation of side products. Three different approaches to TTF polymers shall be considered which are based on TTF (11) formation itself (Scheme 3). Thereby 7, 8, and 9 serve as key starting compounds which are coupled using phosphites [19], bases [18] (via carbenes) [11], and reducing agents, respectively. Our ®rst attempt at synthesizing polymer 5 [19] proceeds by phosphite coupling of dithione (12) (Scheme 4). The resulting orange polymer is, despite its rigid structure, unexpectedly soluble in common organic solvents such as dichloromethane and THF. Table 2 Conductivity of polymer 6 according to the amount of dopand present Conductivity (S cmÿ1)
Time of exposure (min)
Content of iodine (%)
8.310ÿ3 9.510ÿ3 1.410ÿ2 1.510ÿ2 2.110ÿ3
2 3 4 5 10
20 35 45 55 70
100
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the end-cap of the polymer or part of the polymer backbone, in which 13 would interrupt the conjugation along the polymer and prevent strong p±p-interactions between adjacent polymer chains. This would also explain the high solubility of the polymer.
Scheme 3
Scheme 4
Gel-permeation chromatography (GPC) analysis of 5 reveals a molecular weight Mn4500 g molÿ1. 13 C-NMR spectroscopy exhibits two extra signals which are assigned to the ethyl groups of the diethoxythionylphosphane (13). The latter originates from the coupling reagent. This conclusion is supported by ®eld desorption (FD) mass spectrometry which shows signals for the mass of the n-mer and an additional mass being due to 13. Moiety 13 may be
Our second approach to semi-ribbon type TTF polymers, which we have reported previously [9], comprises the direct synthesis of 6, a rigid polymer with more sulfur atoms in the repeat unit than 5 (Scheme 5). Polymer 6 constitutes a highly polarizable species due to the high proportion of sulfur. Alkylation of the dithione (14) with the Meerwein salt trimethyloxonium tetra¯uoroborate affords the dication salt 15 in 77% yield, which is reduced with sodium borohydride to give 16. Treatment of 16 with tri¯uoromethanesulfonic acid gives rise to the monomer 17. The crucial step of the polymerization is deprotonation of 17 with diisopropylethylamine at 08C in acetonitrile. As in the case of the parent TTF, a carbene intermediate similar to 10 is most likely generated which polymerizes quantitatively to the orange rigid-rod type polymer 6 (Scheme 5). However, owing to the poor solubility, characterization methods are very limited and will be discussed later. Our third approach involves a reductive polymerization via radical cations [1] and gives the precursor polymers 18 and 19 in quantitative yields (Scheme 6). Polymers 18 and 19 allow for better solubility because of the ¯exible polymer backbone compared to the semi-ribbon type polymers 5 and 6. After processing the polymers 18 and 19, a thermally
Scheme 5
S. Frenzel et al. / Synthetic Metals 118 (2001) 97±103
101
Scheme 6
induced extrusion of dimethyldisul®de leads to the conjugated polymers 5 and 6. The synthesis of the precursor polymer 18 begins with the 4,8-dihexyloxy-1,3,5,7-tetrathia-s-indacene-2,6-dithione (12) [20] which is bis-alkylated to the bis-cation 20 with dimethoxycarbonium tetra¯uoroborate in 36% yield. A high proportion of monoalkylation is always observed even when using a 10-fold excess of the alkylating reagent. Treatment of 20 with zinc powder in acetonitrile/THF (1:4) affords precursor polymer 18 as a pale yellow material. As described above, 14 can be bis-alkylated to 15 [9] and polymerized with zinc powder in acetonitrile/THF (1:2) providing the precursor polymer 19 as a white powdery material. Both precursor polymers 18 and 19 are soluble in most organic solvents. From the two molecular weight analysis methods available, light scattering (LS) and GPC, only the latter can be applied by us due to the ¯uorescence of 18 and 19 which affects the detection method used in LS. GPC analysis of both polymers 18 and 19 in THF with polystyrene standards indicates that the weight average molecular weights are Mw52 500 and 87 700 g molÿ1, respectively, and the number average molecular weights are Mn5800 and 7900 g molÿ1. The polymers 18 and 19 constitute mixtures containing large amounts of oligomers hence the high polydispersity. This large distribution might be a result of ring formation for example dimerization of each of the cations 20 and 15. In fact, preliminary experiments have shown formation of a cage-type molecule 21 by treatment of 20 (Rethyl) with zinc according to FD
mass spectrometry.
Another possible reason for the high polydispersity of the precursor polymers 18 and 19 is that the growing polymer chain abstracts a hydrogen radical from the solvent which would then terminate the polymerization. Investigation of both polymers using FD mass spectrometry, again, reveals no possible end-groups. Fragmentation of the polymers 18 and 19 occurs under MALDI-TOF conditions due to the absorption of the laser energy. No thione groups, which would be the result of demethylation during the reduction with zinc, are detected using UV±VIS spectroscopy. Fractionation of the high molecular weight material by precipitation turns out to be successful. Other reduction conditions are applied in order to obtain polymers of higher molecular weight. The dications 20 and 15 are reduced electrochemically and chemically with samarium iodide or hydrazine. All these reduction conditions provide precursor polymers of low molecular weight. Electrochemical reductions of 20 and 15 are performed
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Scheme 7
galvanostatically with 20 mA at platin electrodes and in a 0.1 M acetonitrile solution of tetrabutylammonium tetra¯uoroborate yielding molecular weights according to GPC of Mn1800 and 1300 g molÿ1. The low molecular weights are possibly due to precipitation of the polymers 18 and 19 on the cathode. A homogeneous reduction using samarium iodide in THF increases the molecular weights to Mn2100 and 2600 g molÿ1. Using hydrazine, the dications 20 and 15 undergo polymerization (Mn4800 and 5600 g molÿ1), but elemental analysis shows 6% nitrogen content which is possibly the result of nucleophilic attack of hydrazine on the cationic monomer/oligomer under formation of 22b. Scheme 7 provides a tentative mechanism whereby it is important that this reaction terminates the polymerization. Upon heating, 22b looses methylthiol and results in 23. IR spectra of 18 and 19 prepared with hydrazine show absorption signals at 3500±3300 and 1650 cmÿ1 which are attributed to the =N±NH2 moiety. Reduction of 20 and 15 using zinc appears as the best method of polymerization and leads to high molecular weights and processable polymers. These polymers are soluble in common organic solvents such as THF or dichloromethane and are ®lm-forming which is a prerequisite for thermal conversion into the corresponding semi-ribbon type conjugated polymers 5 and 6 under extrusion of dimethyldisul®de. Thermogravimetric analysis (TGA) of polymer 18 exhibits the calculated mass-loss within 80 min on heat-
Fig. 2. Thermogravimetric analysis of polymer 18.
ing to 1958C (Fig. 2). The resulting polymer is stable up to 3008C. In the case of the precursor polymer 19 the ideal thermolysis temperature is 1608C. Heating to over 2008C causes further degradation of the conjugated polymer according to TGA. The above process of converting a processable precursor polymer into an insoluble target structure with extended pconjugation can also be monitored by UV±VIS spectroscopy. A thermolysis of 18 on a glass substrate (Fig. 2) diminishes the intensity of the precursor absorption at 259 nm and increases the intensity of the p±p-transition band of the TTF subunits at 280, 315 and 353 nm. The bold line in Fig. 3 is the absorption spectrum of the phosphitecoupled polymer 5 and is similar to the spectrum of the precursor polymer 18 after heating for 80 min. 4. Properties Precursor polymers 18 and 19 are ®lm forming and can be cast via spin coating on a glass substrate. The average thickness of the polymer ®lms determined by surface stepper is 500±800 mm. For conductivity measurements, the precursor polymers are cast on a special glass plate, which consists of four indium-zinc oxide (ITO) contacts, allowing a four-probe conductivity measurement. After exposure of the polymers to an iodine atmosphere and removal of the excess iodine in vacuo, a conductivity is measured which depends on the time of exposure and content of iodine (Tables 1 and 2). The content of iodine is determined by gravimetric analysis and therefore must be regarded as approximate. For polymer 5 the highest conductivity (0.55 S cmÿ1) is measured with an iodine content (I3ÿ) of 60% (Table 1) which is comparable to bromine doped polyacetylene or iodine doped polythiophene [21±23]. For polymer 6, the maximum conductivity of 1.510ÿ2 S cmÿ1 is detected at a 55% iodine content (Table 2). This value is low compared to polymer 5, but
Fig. 3. UV±VIS spectra of 18 during thermal extrusion of dimethyldisulfide and phosphite-coupled polymer 5 for comparison (bold line).
S. Frenzel et al. / Synthetic Metals 118 (2001) 97±103
is two orders of magnitude higher than that of non-conjugated TTF polymers [8]. Polymer 6 is also doped with iodine in the solid state. After 40 h exposure to iodine vapor, 67% iodine content is determined by elemental analysis. The IR spectrum of the doped material exhibits a strong absorption at 480 cmÿ1 which can be attributed to the radical cation of the TTF-moiety [10]. A lower electrical conductivity of 10ÿ3 S cmÿ1 can be measured compared to the conductivities in the precursor-route case. The same electrical conductivity upon doping is measured in the case of the phosphite-coupled 5 after doping with iodine in solution. The conductivity of charge-transfer complexes depends sensitively on the morphology of the donor and acceptor in the solid state. The lower conductivities may support the assumption that either the alignments of the backbones of the phosphite and base-coupled polymers 5 and 6 are poor or, as in the case of phosphite-coupled sample of 5, the polymer is structurally not well de®ned. As already mentioned above, the coupling reagent can be attached to the polymer backbone which diminishes the p±p-interaction between neighboring TTF subunits and therefore the conductivity. Further X-ray studies will reveal the exact morphology of these polymers in the solid state. 5. Conclusion Several ribbon-type TTF polymers with extended p-conjugation have been synthesized via three different routes. The most promising method involves the two processable precursor polymers 18 and 19. Complete conversion into the corresponding conjugated polymers 5 and 6 after thermally induced extrusion of dimethyldisul®de can be established by thermogravimetric analysis and UV±VIS spectroscopy. The conductivities of polymers 5 and 6 are several orders of magnitude higher than those of non-conjugated TTF-systems and are comparable to those of other electrically conducting polymers such as bromine doped polyacetylene [21,22] and iodine doped polythiophene [23].
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Acknowledgements Financial support by the Fonds der Chemie is gratefully acknowledged. References [1] F. Wudl, G.M. Smith, E.J. Hufnagel, J. Chem. Soc., Chem. Commun. (1970) 1453. [2] L. AlcaÂcer, The Physics and Chemistry of Low Dimensional Solids, Reidel, Dordrecht, Holland, 1980. [3] Y. Ueno, Y. Masuyama, M. Okawara, Chem. Lett. (1975) 603. [4] L. Van Hinh, G. Schukat, E.J. FanghaÈnel, Prakt. Chem. 3 (1979) 299. [5] G. Koûmehl, M. Rohde, Makromol. Chem. 183 (1982) 2084. [6] C.U. Pittman Jr., M. Narita, Y.F. Liang, Macromolecules 9 (1976) 360. [7] W.R. Hertler, J. Org. Chem. 41 (1976) 1412. [8] C.U. Pittman Jr., Y.F. Liang, M. Ueda, Macromolecules 12 (1979) 541. [9] S. Frenzel, S. Arndt, R.M. Gregorius, K. MuÈllen, J. Mater. Chem. 5 (1995) 1529. [10] M. Adam, K. MuÈllen, Adv. Mater. 6 (1994) 439. [11] R.R. Schumaker, E.M. Engler, J. Am. Chem. Soc. 102 (1980) 6652. [12] G. Kiesslich, Ph.D. Thesis, University of WuÈrzburg, 1968. [13] R. Zahradnik, P. Carsky, S. HuÈnig, G. Kiesslich, D. Scheutzow, Int. J. Sulfur Chem. 6 (1971) 9. [14] K. Bechgaard, C.S. Jacobsen, K. Mortensen, H.J. Peterson, N. Thorup, Solid State Commun. 33 (1980) 1119. [15] K. Bechgaard, K. Carneiro, F.G. Rasmussen, J. Am. Chem. Soc. 103 (1981) 2440. [16] M. Mizuno, A.F. Garito, M.P. Cava, J. Chem. Soc., Chem. Commun. (1978) 18. [17] M. Adam, P. Wolf, H.-J. RaÈder, K. MuÈllen, J. Chem. Soc., Chem. Commun. 22 (1990) 1624. [18] P.R. Moses, J.Q. Chambers, J. Am. Chem. Soc. 96 (1974) 945. [19] P. Wolf, K. MuÈllen, M. Przybylski, Chimia 40 (1986) 200. [20] N.G. Demtriadis, S.J. Huang, E.Z. Samulski, Tetrahedron Lett. 26 (1977) 2223. [21] S. Hotta, T. Hosaka, W. Shimotsuma, Synth. Met. 6 (1983) 69. [22] C.K. Chiang, A.J. Heeger, A.G. MacDiarmid, H. Shirakawa, J. Am. Chem. Soc. 100 (1978) 1013. [23] T. Yamamoto, K. Sanechik, A. Yamamoto, J. Polym. Sci., Polym. Lett. Ed. 18 (1980) 9.
Rod-like tetrathiafulvalene polymers with extended p-conjugation Stefan Frenzel, Martin Baumgarten, Klaus MuÈllen* Max-Planck-Institut fuÈr Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany Received 19 January 2000; received in revised form 12 May 2000; accepted 15 June 2000
Abstract Rigid-rod type tetrathiafulvalene (TTF) polymers with exocyclic double bonds were prepared via a precursor route. The processable precursor polymers which contain hexathio-ortho-oxalate repeat units undergo thermal extrusion of dimethyldisul®de and result in target structures with extended p-conjugation. The extrusion must be carefully monitored to obtain structurally de®ned products. Doping of the resulting polymers with iodine shows conductivities which compare favorably with those of other electrically conducting polymers such as, e.g. polyacetylene and polythiophene. # 2001 Published by Elsevier Science B.V. Keywords: Tetrathiafulvalene polymers; p-Conjugation; Hexathio-ortho-oxalate
1. Introduction Tetrathiafulvalene (TTF) is known as a donor in electrically conducting charge-transfer complexes [1]. The conductivity in the solid is attributed to two important structural features, namely, the formation of segregated stacks of donors and acceptors and a certain degree of charge transfer (CT) between the stacks [2]. Despite the inherent electronic advantages, CT-complexes of TTF tend to be brittle and unprocessable. Incorporation of TTF's in polymers which are known for their good processability and ®lm-forming properties may overcome this problem. There have been several attempts to link the TTF moiety to the backbone of polymers [3±8], however, the resulting materials were insoluble and therefore unprocessable. Only the incorporation of bis-TTF (1) into the main-chain of polymer 2 (Scheme 1) resulted in conducting and processable polymers [9] after doping with strong acceptors such as 2,3,5,6tetra¯uoro-7,7,8,8-tetracyanoquinodimethane (TCNQF4), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone(DDQ)oriodine. Due to the conformational freedom of the TTF-subunits in a main-chain polymer, as depicted in Fig. 1, the polymer backbone most likely adopts a random-coil conformation with less p±p-interaction between neighboring TTF units than expected for their stackwise arrangement. Since the charge-transfer complexes of the oligomeric TTF systems 3 and 4 (Scheme 2) form solid-state structures *
Corresponding author. Tel.: 49-6131-379150; fax: 49-6131-379350. E-mail address: [email protected] (K. MuÈllen). 0379-6779/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 2 8 3 - 6
[10] analogous to those of parent TTF, we hoped that the conduction process for the conjugated donor polymer 5 could be enhanced in the doped state [2,11]. This is expected to be due to the charge transport along the polymer backbone via p-conjugation and due to the stacking with p-orbital overlap. In the following, we describe the synthesis of conjugated polymers 5 and 6 which includes thermal conversion of soluble and processable precursor polymers. 2. Experimental 1 H-NMR spectra were recorded at 200 MHz on Varian Gemini 200, or at 500 MHz on Bruker AMX 500, or at 300 MHz on Bruker AC 300 and 13 C-NMR spectra at 50 MHz on Varian Gemini 200, or at 125 MHz on Bruker AMX 500, or at 75 MHz on Bruker AC 300. Mass spectra were recorded using a VG-Instrument ZAB-2. UV±VIS spectra were recorded using a Perkin-Elmer Lambda 15 and IR spectra were recorded using a Nicolet FT-IR-Spectrometer 320. Polymers were characterized by GPC using a Waters 150-C, DSC on Mettler TC 11, Mettler TC 10A, TGA using a Mettler TGA 500.
2.1. 4,8-Di-n-hexyloxy-1,3,5,7-tetrathia-s-indacen-2,6dithione (12) 1-Bromohexane (13.2 g, 80 mmol) was added to a suspension of 4,8-di-n-hydroxy-1,3,5,7-tetrathia-s-indacen2,6-dithione [19] (10 g, 31 mmol) and tetrabutylammonium
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S. Frenzel et al. / Synthetic Metals 118 (2001) 97±103
Scheme 1
¯uoride (24.4 g, 77 mmol) in DMF (200 ml). The reaction mixture was heated to 708C and maintained at this temperature for 6 h and cooled to room temperature. Then methanol (200 ml) was added. The precipitate was ®ltered, washed with methanol and recrystallized from ethyl acetate to afford 12 as yellow crystals. Yield: 11.2 g (74%), mp 1128C. Found: C, 49.08; H, 5.26; S, 38.88%. Requires: C, 48.95; H, 5.34; S, 39.19%. 1 H-NMR (200 MHz, CDCl3): d 4.03 (t, 4H), 1.75 (m, 4H), 1.65±1.25 (m, 12H), 0.92 (t, 6H). 13 C-NMR (50 MHz, CDCl3): d 14.1, 22.7, 25.8, 30.1, 31.9, 74.1, 133.5, 139.9, 209.7. m/z (FD): 490.3 (M). nmax/cmÿ1 (KBr): 1076.8 (C=S). 2.2. 5,11-Dihexyloxy-1,3,4,6,7,9,10,12-octathiabiscyclopenta[a,h]anthracene-2,8-dithione (14) 1-Bromohexane (8.2 g, 50 mmol) was added to a suspension of 5,11-dihydroxy-1,3,4,6,7,9,10,12-octathia-biscyclopenta[a,h]anthracene-2,8-dithione [9] (6 g, 12 mmol) and tetrabutylammonium ¯uoride (11.3 g, 37 mmol) in DMF (150 ml). The reaction mixture was heated to 708C, maintained at this temperature for 6 h and then cooled to room temperature. Then methanol (50 ml) was added. The precipitate was ®ltered, washed with methanol and recrystallized from 1,1,2,2-tetrachloroethane to afford 14 (4.9 g,
Scheme 2
61%) as yellow crystals, mp 2058C (decomp.). Found: C, 43.25; H, 4.01; S, 48.12%. Requires: C, 43.25; H, 3.93; S, 48.01%. 1 H-NMR (500 MHz, C2D2Cl4): d 3.97 (t, 4H), 1.84 (m, 4H), 1.50±1.36 (m, 12H), 0.92 (t, 6H). 13 C-NMR (50 MHz, C2D2Cl4): d 14.1, 22.7, 25.6, 30.2, 31.7, 75.7, 129.8, 130.3, 150.0, 211.5. m/z (FD): 666.1 (M). nmax/cmÿ1 (KBr): 1081 (C=S). 2.3. 4,8-Di-n-hexyloxy-2,6-bis(methylthio)-1,3,5,7tetrathia-s-indacenyl-bis(tetrafluoroborate) (20) Trimethyl-ortho-formate (6.4 ml, 58.5 mmol) was added dropwise to a solution of dichloromethane (30 ml) and BF3 etherate (8.6 ml, 69.3 mmol) at ÿ308C. After 15 min of stirring under an argon atmosphere the mixture was warmed to room temperature. 4,8-Di-n-hexyloxy-1,3,5,7-tetrathia-sindacen-2,6-dithione (12) (6 g, 12.2 mmol) dissolved in dichloromethane (120 ml) was added to the reaction mixture. After 48 h the solvent was removed. Compound 20 was puri®ed by precipitation from an acetonitrile solution by dropwise addition of dry diethyl ether. 3.1 g (36%), mp 1258C (decomp.). 1H-NMR (200 MHz, acetonitrile-d3): d 4.51 (t, 4H), 3.35 (s, 6H), 2.01±1.15 (m, 16H), 0.95 (t, 6H). 13 C-NMR (200 MHz, acetonitrile-d3): d 14.3, 23.2, 25.8, 25.9, 30.6, 75.4, 77.8, 135.7, 145.8, 215.3. nmax/cmÿ1 (KBr): 518 ([S±CSCH3±S]). 2.4. 5,11-Dihexyloxy-2,8-bis(methylthio)1,3,4,6,7,9,10,12-octathia-biscyclopenta[b,i]anthracenylbis(tetrafluoroborate) (15)
Fig. 1. Main-chain polymer with (a) random-coil conformation; (b) stackwise arrangement.
A solution of 14 (4 g, 6 mmol) and trimethyloxonium tetra¯uoroborate (2.7 g, 18 mmol) in 1,1,2,2-tetrachloro-
S. Frenzel et al. / Synthetic Metals 118 (2001) 97±103
99
ethane (50 ml) was heated to 708C under argon atmosphere, maintained at this temperature for 12 h and then cooled to room temperature. The solvent was evaporated in vacuo and ether (500 ml) was added to the oily residue. The resulting precipitate was ®ltered, washed with ether and dissolved in acetonitrile (10 ml). The solution was added dropwise to dry diethyl ether. The precipitate was again ®ltered and dried to afford 3.9 g (77%) of 15 as purple crystals. mp 738C (decomp.). Found: C, 35.75; H, 3.65; S, 35.56%. Requires: C, 35.86; H, 3.70; S, 36.82%. 1 H-NMR (200 MHz, acetonitrile-d3): d 3.73 (t, 4H), 2.79 (s, 6H), 1.63±1.04 (m, 16H), 0.64 (m, 6H). 13 C-NMR (50 MHz,): d 14.1, 23.7, 24.7, 28.6, 31.1, 32.7, 77.0, 130.8, 144.3, 151.5, 210.4. m/z (FD): 347.9 (M/2-2 BF4), 783.0 (M/2-2 BF4). nmax/cmÿ1 (KBr): 521 ([S±CSCH3±S]).
Table 1 Conductivity of polymer 5 according to the amount of dopand present
2.5. Synthesis of the precursor polymers 18 and 19
After thermolysis, 5 and 6 were exposed to a iodine atmosphere for several minutes (Table 1). The excess of absorbed iodine was removed in vacuo (20 mbar) for 10 min. The conductivity was measured by a four-probe setup (Tables 1 and 2).
The cations 20 or 15 were dissolved in dry acetonitrile, and THF was added (2 ml/8 ml) in the case of 20 and 5 ml/ 10 ml for 15. Zinc (0.7 g, 1 mmol) and a catalytic amount of bromine (0.02 ml) was added. The dark yellow solution became pale yellow after 10±30 min of vigorous stirring. The suspension was ®ltered and the solvent was removed in vacuo. The residue was dissolved in dichloromethane and precipitated in methanol (200 ml) and this process was repeated twice. The ®ltered zinc was extracted three times with 1,1,2,2-tetrachloroethane (5 ml) which was heated to 808C. The solutions were combined and precipitated three times in methanol to obtain pale yellow (18) and white (19) polymers in yields >80%. Polymer 18: 1 H-NMR (200 MHz, CD2Cl2): d 3.83 (b), 2.48 (b), 1.95±1.12 (m), 0.91 (b). 13 C-NMR (50 MHz, CDCl3): d 14.7, 18.6 (b), 23.4, 26.4, 30.9, 32.4, 71.3, 72.9 (b), 130.1 (b), 141.4. TGA: 1958C (18.1%). DSC: no phase transition until decomposition. UV±VIS (®lm) l (nm)259. Mw (GPC)52 500 g molÿ1. Mn (GPC)5800 g molÿ1 (Dn9). Polymer 19: 1 H-NMR (200 MHz, CD2Cl2): d 3.93 (b), 2.43 (b), 1.97±1.15 (b), 0.98 (b). 13 C-NMR (50 MHz, CD2Cl2): d 14.3, 18.3 (b), 23.1, 25.1, 30.4, 32.1, 73.3 (b), 75.3, 118.9, 130.4 (b), 148.9. TGA: 1608C (13.4%). DSC: no phase transition until thermolysis. UV±VIS (®lm) l (nm) 230. Mw (GPC)87 700 g molÿ1. Mn (GPC)7900 g molÿ1 (Dn11). 2.6. Preparation of the polymeric donors 5 and 6 The precursor polymers 18 and 19 were dissolved in dichloromethane (100 mg/2 ml) and casted on a special glass plate (2 cm4 cm) via spin coating. The glass substrate consisted of four indium-zinc oxide (ITO) contacts allowing a four-probe conductivity measurement. The resulting ®lms had an average thickness of 500±800 nm. The glass plate was heated to 1658C for 80 min. The thermolysis was monitored by UV±VIS spectroscopy which
Conductivity (S cmÿ1)
Exposure time (min)
Content of iodine (%)
0.36 0.53 0.55 0.48 0.13
2 3 4 5 10
35 45 60 65 90
showed the evolving absorption signals of the TTF-moiety at UV±VIS (®lm) l (nm)230, 315, 353. 2.7. Doping of 5 and 6
3. Results and discussion 3.1. Synthesis To date numerous TTF-derivatives have been synthesized [10±17]. The synthesis usually involves coupling of two building blocks in the ®nal step to form the TTF-double bond. Several coupling methods using various precursors are known [1,17±19]. In order to prepare TTF polymers with high molecular weight, the coupling method must meet requirements such as high yield and negligible formation of side products. Three different approaches to TTF polymers shall be considered which are based on TTF (11) formation itself (Scheme 3). Thereby 7, 8, and 9 serve as key starting compounds which are coupled using phosphites [19], bases [18] (via carbenes) [11], and reducing agents, respectively. Our ®rst attempt at synthesizing polymer 5 [19] proceeds by phosphite coupling of dithione (12) (Scheme 4). The resulting orange polymer is, despite its rigid structure, unexpectedly soluble in common organic solvents such as dichloromethane and THF. Table 2 Conductivity of polymer 6 according to the amount of dopand present Conductivity (S cmÿ1)
Time of exposure (min)
Content of iodine (%)
8.310ÿ3 9.510ÿ3 1.410ÿ2 1.510ÿ2 2.110ÿ3
2 3 4 5 10
20 35 45 55 70
100
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the end-cap of the polymer or part of the polymer backbone, in which 13 would interrupt the conjugation along the polymer and prevent strong p±p-interactions between adjacent polymer chains. This would also explain the high solubility of the polymer.
Scheme 3
Scheme 4
Gel-permeation chromatography (GPC) analysis of 5 reveals a molecular weight Mn4500 g molÿ1. 13 C-NMR spectroscopy exhibits two extra signals which are assigned to the ethyl groups of the diethoxythionylphosphane (13). The latter originates from the coupling reagent. This conclusion is supported by ®eld desorption (FD) mass spectrometry which shows signals for the mass of the n-mer and an additional mass being due to 13. Moiety 13 may be
Our second approach to semi-ribbon type TTF polymers, which we have reported previously [9], comprises the direct synthesis of 6, a rigid polymer with more sulfur atoms in the repeat unit than 5 (Scheme 5). Polymer 6 constitutes a highly polarizable species due to the high proportion of sulfur. Alkylation of the dithione (14) with the Meerwein salt trimethyloxonium tetra¯uoroborate affords the dication salt 15 in 77% yield, which is reduced with sodium borohydride to give 16. Treatment of 16 with tri¯uoromethanesulfonic acid gives rise to the monomer 17. The crucial step of the polymerization is deprotonation of 17 with diisopropylethylamine at 08C in acetonitrile. As in the case of the parent TTF, a carbene intermediate similar to 10 is most likely generated which polymerizes quantitatively to the orange rigid-rod type polymer 6 (Scheme 5). However, owing to the poor solubility, characterization methods are very limited and will be discussed later. Our third approach involves a reductive polymerization via radical cations [1] and gives the precursor polymers 18 and 19 in quantitative yields (Scheme 6). Polymers 18 and 19 allow for better solubility because of the ¯exible polymer backbone compared to the semi-ribbon type polymers 5 and 6. After processing the polymers 18 and 19, a thermally
Scheme 5
S. Frenzel et al. / Synthetic Metals 118 (2001) 97±103
101
Scheme 6
induced extrusion of dimethyldisul®de leads to the conjugated polymers 5 and 6. The synthesis of the precursor polymer 18 begins with the 4,8-dihexyloxy-1,3,5,7-tetrathia-s-indacene-2,6-dithione (12) [20] which is bis-alkylated to the bis-cation 20 with dimethoxycarbonium tetra¯uoroborate in 36% yield. A high proportion of monoalkylation is always observed even when using a 10-fold excess of the alkylating reagent. Treatment of 20 with zinc powder in acetonitrile/THF (1:4) affords precursor polymer 18 as a pale yellow material. As described above, 14 can be bis-alkylated to 15 [9] and polymerized with zinc powder in acetonitrile/THF (1:2) providing the precursor polymer 19 as a white powdery material. Both precursor polymers 18 and 19 are soluble in most organic solvents. From the two molecular weight analysis methods available, light scattering (LS) and GPC, only the latter can be applied by us due to the ¯uorescence of 18 and 19 which affects the detection method used in LS. GPC analysis of both polymers 18 and 19 in THF with polystyrene standards indicates that the weight average molecular weights are Mw52 500 and 87 700 g molÿ1, respectively, and the number average molecular weights are Mn5800 and 7900 g molÿ1. The polymers 18 and 19 constitute mixtures containing large amounts of oligomers hence the high polydispersity. This large distribution might be a result of ring formation for example dimerization of each of the cations 20 and 15. In fact, preliminary experiments have shown formation of a cage-type molecule 21 by treatment of 20 (Rethyl) with zinc according to FD
mass spectrometry.
Another possible reason for the high polydispersity of the precursor polymers 18 and 19 is that the growing polymer chain abstracts a hydrogen radical from the solvent which would then terminate the polymerization. Investigation of both polymers using FD mass spectrometry, again, reveals no possible end-groups. Fragmentation of the polymers 18 and 19 occurs under MALDI-TOF conditions due to the absorption of the laser energy. No thione groups, which would be the result of demethylation during the reduction with zinc, are detected using UV±VIS spectroscopy. Fractionation of the high molecular weight material by precipitation turns out to be successful. Other reduction conditions are applied in order to obtain polymers of higher molecular weight. The dications 20 and 15 are reduced electrochemically and chemically with samarium iodide or hydrazine. All these reduction conditions provide precursor polymers of low molecular weight. Electrochemical reductions of 20 and 15 are performed
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Scheme 7
galvanostatically with 20 mA at platin electrodes and in a 0.1 M acetonitrile solution of tetrabutylammonium tetra¯uoroborate yielding molecular weights according to GPC of Mn1800 and 1300 g molÿ1. The low molecular weights are possibly due to precipitation of the polymers 18 and 19 on the cathode. A homogeneous reduction using samarium iodide in THF increases the molecular weights to Mn2100 and 2600 g molÿ1. Using hydrazine, the dications 20 and 15 undergo polymerization (Mn4800 and 5600 g molÿ1), but elemental analysis shows 6% nitrogen content which is possibly the result of nucleophilic attack of hydrazine on the cationic monomer/oligomer under formation of 22b. Scheme 7 provides a tentative mechanism whereby it is important that this reaction terminates the polymerization. Upon heating, 22b looses methylthiol and results in 23. IR spectra of 18 and 19 prepared with hydrazine show absorption signals at 3500±3300 and 1650 cmÿ1 which are attributed to the =N±NH2 moiety. Reduction of 20 and 15 using zinc appears as the best method of polymerization and leads to high molecular weights and processable polymers. These polymers are soluble in common organic solvents such as THF or dichloromethane and are ®lm-forming which is a prerequisite for thermal conversion into the corresponding semi-ribbon type conjugated polymers 5 and 6 under extrusion of dimethyldisul®de. Thermogravimetric analysis (TGA) of polymer 18 exhibits the calculated mass-loss within 80 min on heat-
Fig. 2. Thermogravimetric analysis of polymer 18.
ing to 1958C (Fig. 2). The resulting polymer is stable up to 3008C. In the case of the precursor polymer 19 the ideal thermolysis temperature is 1608C. Heating to over 2008C causes further degradation of the conjugated polymer according to TGA. The above process of converting a processable precursor polymer into an insoluble target structure with extended pconjugation can also be monitored by UV±VIS spectroscopy. A thermolysis of 18 on a glass substrate (Fig. 2) diminishes the intensity of the precursor absorption at 259 nm and increases the intensity of the p±p-transition band of the TTF subunits at 280, 315 and 353 nm. The bold line in Fig. 3 is the absorption spectrum of the phosphitecoupled polymer 5 and is similar to the spectrum of the precursor polymer 18 after heating for 80 min. 4. Properties Precursor polymers 18 and 19 are ®lm forming and can be cast via spin coating on a glass substrate. The average thickness of the polymer ®lms determined by surface stepper is 500±800 mm. For conductivity measurements, the precursor polymers are cast on a special glass plate, which consists of four indium-zinc oxide (ITO) contacts, allowing a four-probe conductivity measurement. After exposure of the polymers to an iodine atmosphere and removal of the excess iodine in vacuo, a conductivity is measured which depends on the time of exposure and content of iodine (Tables 1 and 2). The content of iodine is determined by gravimetric analysis and therefore must be regarded as approximate. For polymer 5 the highest conductivity (0.55 S cmÿ1) is measured with an iodine content (I3ÿ) of 60% (Table 1) which is comparable to bromine doped polyacetylene or iodine doped polythiophene [21±23]. For polymer 6, the maximum conductivity of 1.510ÿ2 S cmÿ1 is detected at a 55% iodine content (Table 2). This value is low compared to polymer 5, but
Fig. 3. UV±VIS spectra of 18 during thermal extrusion of dimethyldisulfide and phosphite-coupled polymer 5 for comparison (bold line).
S. Frenzel et al. / Synthetic Metals 118 (2001) 97±103
is two orders of magnitude higher than that of non-conjugated TTF polymers [8]. Polymer 6 is also doped with iodine in the solid state. After 40 h exposure to iodine vapor, 67% iodine content is determined by elemental analysis. The IR spectrum of the doped material exhibits a strong absorption at 480 cmÿ1 which can be attributed to the radical cation of the TTF-moiety [10]. A lower electrical conductivity of 10ÿ3 S cmÿ1 can be measured compared to the conductivities in the precursor-route case. The same electrical conductivity upon doping is measured in the case of the phosphite-coupled 5 after doping with iodine in solution. The conductivity of charge-transfer complexes depends sensitively on the morphology of the donor and acceptor in the solid state. The lower conductivities may support the assumption that either the alignments of the backbones of the phosphite and base-coupled polymers 5 and 6 are poor or, as in the case of phosphite-coupled sample of 5, the polymer is structurally not well de®ned. As already mentioned above, the coupling reagent can be attached to the polymer backbone which diminishes the p±p-interaction between neighboring TTF subunits and therefore the conductivity. Further X-ray studies will reveal the exact morphology of these polymers in the solid state. 5. Conclusion Several ribbon-type TTF polymers with extended p-conjugation have been synthesized via three different routes. The most promising method involves the two processable precursor polymers 18 and 19. Complete conversion into the corresponding conjugated polymers 5 and 6 after thermally induced extrusion of dimethyldisul®de can be established by thermogravimetric analysis and UV±VIS spectroscopy. The conductivities of polymers 5 and 6 are several orders of magnitude higher than those of non-conjugated TTF-systems and are comparable to those of other electrically conducting polymers such as bromine doped polyacetylene [21,22] and iodine doped polythiophene [23].
103
Acknowledgements Financial support by the Fonds der Chemie is gratefully acknowledged. References [1] F. Wudl, G.M. Smith, E.J. Hufnagel, J. Chem. Soc., Chem. Commun. (1970) 1453. [2] L. AlcaÂcer, The Physics and Chemistry of Low Dimensional Solids, Reidel, Dordrecht, Holland, 1980. [3] Y. Ueno, Y. Masuyama, M. Okawara, Chem. Lett. (1975) 603. [4] L. Van Hinh, G. Schukat, E.J. FanghaÈnel, Prakt. Chem. 3 (1979) 299. [5] G. Koûmehl, M. Rohde, Makromol. Chem. 183 (1982) 2084. [6] C.U. Pittman Jr., M. Narita, Y.F. Liang, Macromolecules 9 (1976) 360. [7] W.R. Hertler, J. Org. Chem. 41 (1976) 1412. [8] C.U. Pittman Jr., Y.F. Liang, M. Ueda, Macromolecules 12 (1979) 541. [9] S. Frenzel, S. Arndt, R.M. Gregorius, K. MuÈllen, J. Mater. Chem. 5 (1995) 1529. [10] M. Adam, K. MuÈllen, Adv. Mater. 6 (1994) 439. [11] R.R. Schumaker, E.M. Engler, J. Am. Chem. Soc. 102 (1980) 6652. [12] G. Kiesslich, Ph.D. Thesis, University of WuÈrzburg, 1968. [13] R. Zahradnik, P. Carsky, S. HuÈnig, G. Kiesslich, D. Scheutzow, Int. J. Sulfur Chem. 6 (1971) 9. [14] K. Bechgaard, C.S. Jacobsen, K. Mortensen, H.J. Peterson, N. Thorup, Solid State Commun. 33 (1980) 1119. [15] K. Bechgaard, K. Carneiro, F.G. Rasmussen, J. Am. Chem. Soc. 103 (1981) 2440. [16] M. Mizuno, A.F. Garito, M.P. Cava, J. Chem. Soc., Chem. Commun. (1978) 18. [17] M. Adam, P. Wolf, H.-J. RaÈder, K. MuÈllen, J. Chem. Soc., Chem. Commun. 22 (1990) 1624. [18] P.R. Moses, J.Q. Chambers, J. Am. Chem. Soc. 96 (1974) 945. [19] P. Wolf, K. MuÈllen, M. Przybylski, Chimia 40 (1986) 200. [20] N.G. Demtriadis, S.J. Huang, E.Z. Samulski, Tetrahedron Lett. 26 (1977) 2223. [21] S. Hotta, T. Hosaka, W. Shimotsuma, Synth. Met. 6 (1983) 69. [22] C.K. Chiang, A.J. Heeger, A.G. MacDiarmid, H. Shirakawa, J. Am. Chem. Soc. 100 (1978) 1013. [23] T. Yamamoto, K. Sanechik, A. Yamamoto, J. Polym. Sci., Polym. Lett. Ed. 18 (1980) 9.