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Electrical properties of chemically synthesized polypyrrole-halogen charge transfer complexes
Solid State Communications, Printed in Great Britain. ELECTRICAL
Vol. 60, No. 5, pp. 457-459,
PROPERTIES
1986.
0038-1098/86 $3.00 + .OO 0 Pergamon Journals Ltd.
OF CHEMICALLY SYNTHESIZED POLYPYRROLE-HALOGEN CHARGE TRANSFER COMPLEXES E.T. Kang, K.G. Neoh and H.C. Ti
Department
of Chemical Engineering,
National University of Singapore, Kent Ridge 05 11, Singapore
(Received 18 June 1986 by W. Sasuki)
Simultaneous chemical polymerization and oxidation of pyrrole have been initiated by halogens, iodine (Iz) and bromine (Br,) at 0 to 4°C. The d.c. electrical conductivity (u) of this new family of polymeric charge transfer complexes is of the order of 30 ohm-’ cm-’ at room temperature. u was measured as a function of temperature between 300 and 150 K, using both standard four probe and voltage shorted compaction (VSC) techniques. A metallic temperature denendence of u was observed for both complexes in the VSC measurements. A
INTRODUCTION INTEREST IN THE electrical properties of highly conjugated polymers has increased rapidly during the past decade [l] . Indeed, conducting polymers has emerged as a new class of electrical/electronic materials [2] . Most of the attentions has been focused on three main types of conjugated polymers, poly(acetylene) and its derivatives [3], poly(phenylene) and its derivatives [4, 51, and poly(heterocyclic) polymers and cations [6-l l] . Unfortunately, the electrical properties for most of the conducting polymer with conjugated backbone degrade rapidly when exposed to the ambient air [12]. One of most promising systems appears to be polypyrrole (PPY) and related polyheterocyclic polymers prepared by electrochemical polymerization and oxidation [ 131 . In contrast to chemically oxidized conjugated polymers, PPY cations prepared by electrochemical methods are relatively stable and highly conductive [6, 131. The availability of stable conductive PPYs from electrochemical polymerization has encouraged us to search for alternative chemcial methods of synthesis and oxidation. Insulating and fairly oxidative PPY in film form has been synthesized chemically in the presence of an acid or peroxide initiators [ 14-161. These initially insulating films can be doped with halogens to achieve stable conductivity of the order of lo-’ ohm-’ cm-’ [15] . Oxidized PPY films with conductivity in the order of 10 to 10’ ohm-’ cm-’ have also been synthesized in the presence of a Lewis acid catalyst, such as RuCls or FeCls [17-l 93 . In this Communication, we report on a novel, one-step method for the simultaneous chemical polymerization and oxidation of pyrrole by halogens, such as bromine and iodine. The resulting PPY-halogen charge transfer (CT) complexes show remarkable stability in the atmosphere and in aqueous environment, and have
electrical conductivities comparable to that of oxidized PPYs produced electrochemically. The d.c. electrical conductivity of these PPY-halogen CT complexes are measured as a function of temperature (T) between 300 and 150 K. Possible conduction mechanisms involved are discussed. EXPERIMENTAL Pyrrole monomer (Merck, Reagent Grade) used in the present studies was purified by cold vacuum distillation. All solvents were of reagent grade and were used as received. All samples were exposed to the atmosphere after removal from the reaction mixture. The PPY-I2 CT complex was synthesized in aqueous medium as follows: 2% by weight of pulverized Iz was dispersed in deionized water in a glass reaction vessel. 1.5% by weight of monomeric pyrrole was introduced on to the water surface. The reaction vessel was closed and kept in the dark at 4’C for about 60 h. A thick, black spongy polymer layer was obtained at the bottom. The black material, which was later identified to be the PPY-I* CT complex, was extracted with copious amounts of carbon tetrachloride and absolute cold acetonitrile, ethanol in that order. It was then dried in dynamic vacuum. The yield was about 30-40%. In the synthesis of PPY-Bra CT complexes, 3% by weight of bromine was dissolved in acetonitrile in a glass vessel. The system was saturated with nitrogen and maintained at about 0 to 4°C. 2% by weight of monomeric pyrrole was then introduced into the reaction mixture. The system was stirred vigorously for about 6 h. A black precipitate was obtained in the reaction mixture and was subjected to the same washing and drying procedures described earlier. For conductivity measurements, the complexes were pressed into thin circular pellets of about 0.05-0.1
4.57
CHEMICALLY SYNTHESIZED
4.58
cm thick and 1.2 cm in diameter in a stainless steel press at a pressure of about 300 kg cme2. Electrical conductivities were measured using both standard four-probe and voltage shorted compaction (VSC) [20] techniques. The measuring circuit consisted of a Keithley 6 14 digital electrometer and a Hewlett-Packard Model 6212B d.c. power supply. Conductivities below room temperature were measured in situ in a liquid nitrogen cryostat. Samples were heated at a rate of about 0S”C min-‘. The uv-visible absorption spectra were measured using a Shimadzu UV-260 spectrophotometer. The infrared (IR) absorption spectra were measured using a Perkin-Elmer Model 682 spectrophotometer with the polymeric complexes dispersed in KBr.
The uv-visible absorption spectra of both the PPY-I2 and PPY-Br2 CT complexes reveal two relatively broad bands. One band centered at around 420nm and is characteristic of polypyrrole [ 16, 2 I] . The other intense broad band which appears in the red and extends well into the near-IR region probably results from the CT interaction between the polymer and the halogen, since undoped PPY does not have any appreciable absorption in the near-IR [ 161 The IR absorption spectra for both complexes are superimposable with those of oxidized PPY prepared electrochemically [21]. The PPY-I, CT complex prepared in the aqueous medium is sponge-like and somewhat rubbery in texture, while the PPY-Br, CT complex is granular in nature. The room temperature electrical conductivities of both complexes prepared by the present methods are in the order of 25-30ohm-i cm-‘. The conductivity is somewhat lower for samples prepared at higher temperatures. We wish to emphasise that both the physicochemical and electrical properties of the two complexes are stable in the atmosphere and in the presence of moisture. Typical chemical compositions such as Cal H 3.0N 1.00 1.0(I 2 ) 0.20_ 0.25 for the complex
and
C~.OH~.~NLOOO.~(B~~
Vol. 60, No. 5
o PPY - Br, l
PPY - I2
Fig. 1. Temperature dependence of the electrical conductivity of PPY-I2 and PPY-Br2 CT complexes in four probe conductivity measurements.
RESULTS AND DISCUSSION
ppy~-I,
POLYPYRROLE-HALOGEN
)0.22-0.24for
the PPY-Br, complex indicate that the C:H:N ratios in both complexes are rather close to the theoretical value of 4:3:1 for a perfectly linear chain of disubstituted pyrrole rings [14, 211 . The chemical compositions also suggest a pyrrole: acceptor ratio of about 4:l for both complexes. The ratio of about 3 to 4 pyrrole units to 1 anion has also been reported for a number of PPY-anion salt complexes obtained from electrochemical polymerization and oxidation [7, 211. The detailed chemical synthesis procedures and the physicochemical properties of various PPY-I2 and PPY-Br2 CT complexes will be reported in separate communications [22, 231 . Figure 1 shows the In u vs T-“4 plots of the PPYI2 and PPY-Br2 CT complexes near and below room
temperature obtained from the ordinary four probe conductivity measurements. An apparent linear fit of the experimental data was obtained for both complexes throughout the temperature range from 150 to 300K. The behavior has also been widely observed in electrochemically polymerized and oxidized PPY films [24-261, as well as in undoped PPY films prepared chemically in the presence of an acid catalyst [16]. Based on these result, the conduction mechanism in PPY has been interpreted in terms of Mott’s model [27] of variable range hopping between localized states near the Fermi surface. In fact, the l/4 power temperature dependence of conductivity has also been observed in many compressed pellets of polycrystalline molecular solids [28] . In doped polyacetylene films of granular and fibrillar morphology, ordinary four probe conductivity measurements also only indicate a thermally activated conduction characteristic of a semiconductor, although the metallic character of the polymer has been revealed by optical, magnetic susceptibility and thermoelectric power measurements [20b]. The discrepancy has been attributed to the contact resistance in the interfibrillar, intergranular or intercrystallitic regions of the polyacetylene films. By using the VSC method, which effectively short-circuits the intercrystallitic contact resistance, a true metallic temperature dependence of the conductivity has been observedin polyacetylene [20b] . More recently, the metallic temperature dependence of conductivity has also been demonstrated in electrochemically oxidized PPY films in VSC measurements [29]. In view of the reported inadequacy of the ordinary four probe conductivity measurements on compacted powder samples, we have also measured the temperature dependence of the conductivity in our PPY-I2 and PPY-Br2 samples using the VSC technique. The results are shown in Fig. 2. It is
Vol. 60, No. 5
CHEMICALLY SYNTHESIZED
POLYPYRROLE-HALOGEN
A.G. McDiarmid & A.J. Heeger, Synth. Met. 1, 101(1979/80). 4. J.F. Robolt, T.C. Clarke, K.K. Kanazawa, J.R. Reynolds & G.B. Street, J. Chem. Sot., Chem. Comm. 347 (1980). 5. L.W. Shacklette, R.R. Chance, D.M. Ivory, G.G. Miller & R.H. Bar&man, Synth. Met. 1, 307 (1980). 6. A.F. Biaz, K.K. Kanazawa & G.P. Gardini, JCS C’hem. Comm. 635 (1979). 7. K.K. Kanazawa, A.F. Diaz, R.H. Geiss, W.D. Gill, J.F. Kwak, J.A. Logan, J.F. Robolt & G.B. Street, JCS Chem. Comm. 854 (1979). 8. A.F. Diaz & J.1. Castillo, JCS Chem. Comm. 397 (1980). 9. A.F. Diaz, Chem. Ser. 17, 145 (1981). 10. G. Tourillon & F. Garnier, J. Electroanal. Chem. 135,173 (1982). 11. R.J. Waltman, J. Bargon & A.F. Diaz, J. Phys. Chem. 87,1459 (1983). 12. W. Deitz, P. Cukor, M. Rubner &J. Jopson, Synth. Met. 4, 189 (1982). 13. F. Gutmann, H. Keyzer & L.E. Lyons, Organic Semiconductors, Part B, p. 207, R.E. Krieger Pub. Co., Florida (1983). 14. G.P. Gardini, Adv. Heterocycl. Chem. 15, 67 (1973). 15. M. Salmon, K.K. Kanazawa, A.F. Diaz & M. Krounbi, J. PoZym. Sci. Lett. 20, 187 (1982). 16. H.S. Nalwa, L.R. Dalton, W.F. Schmidt & J.G. Robe, PoZym. Comm. 26,240 (1985). 17. M. Aizawa, H. Shinohara & H. Shirakawa, Polym. Prep. Japan 33,495 (1984). 18. K. Yoshino, S. Hayashi & R. Sugimoto, Japan J. Appl. Phys. 23, L899 (1984). 19. V. Bocchi & G.P Gardini, JCS Chem. Comm. 148 (1986). 20a. L.B. Coleman,Rev. Sci. Instr. 49,58 (1978). 20b. M. Wan, P. Wang, Y. Cao, R. Quan, F. Wang, X. Zhao 8~ Z. Gong, Solid State Commun. 47, 759 (1983). 21. G.B. Street, T.C. Clarke, M. Krounbi, K.K. Kanazawa, V. Lee, P. Pfluger, J.C. Scott & G. Weiser, Mol. Cyrst. Liq. Cryst. 83,253 (1982). 22. E.T. Kang, T.C. Tan, K.G. Neoh & Y.K. Ong, Polymer (In Press). 23. ET Kang, K.G. Neoh, T.C. Tan & Y.K. Ong, Submitted to J. Macro. Sci.-Chem. 24. K.K. Kanazawa, A.F. Diaz, W.D. Gill, P.M. Grant & G.B. Street, Synth. Metah 1,329 (1980). 25. A. Watanabe, M. Tanaka &J. Tanaka, Bull. Chem. Sot. Japan 54,2278 (1981). 26. P. Pfluger, G. Weiser, J.C. Scott & G.B. Street, A Handbook on Conducting Polymers Vol. I & II, (Edited by T. Skotheim), Marcel Dekker, N.Y. (1985). 27. N.F. Mott & E.A. Davis, Electronic Processes in Non-Crystalline Materials, p. 34, Clarendon Press, Oxford, 2nd. Ed., (1979). 28. C.S. Anithkumar & N. Umakantha, Phil. Mag. B44, 615 (1981). 29. X. Bi, Y. Yao, M. Wan, P. Wang, K. Xiao, Q. Yang & R. Qian, Makromol. Chem. 186,llOl (1985). 30. J.L. Bredas, B. Theman, J.M. Andre, R.R. Change, & R. Silbey, Synth. Met. 9,265 (1984). 3.
l
PPY-12
o PPY- Br,
TEMPERATURE , 1 , (K)
Fig. 2. Temperatures dependence of the electrical conductivity of PPY-I? and PPY-Br2 CT complexes in voltage shorted compaction measurements. (un.r. = room temperature conductivity). clear that both complexes show an increase in conductivity with decreasing temperature from 300 to 150K, a behavior characteristic of metallic conduction. Thus, the conduction behavior of the PPY-halogen CT complexes synthesized by the present chemical method is not unlike that of oxidized PPY prepared electrochemitally. Recently, a new conduction concept based on bipolarons hopping in the PPY system has also been introduced [30]. It therefore would be of great interest to elucidate the optical and magnetic properties of this new family of chemically synthesized PPY-halogen CT complexes. CONCLUSION A new family of PPY-halogen charge transfer complexes has been synthesized chemically via the simultaneous polymerization and doping technique. The PPY complexes so produced are stable in the atmosphere and have conductivity values and conduction behavior comparable to that of electrochemically polymerized and oxidized PPY. Acknowledgements - We would like to thank Prof. H.H. Lee and the Microanalysis Lab. of the Chemistry Department, NUS for conducting the chemical analysis of our samples. MS Sutini Suratman is most helpful in typing this manuscript.
REFERENCES 1.
2.
See for example, A Handbook on Conducting Polymers, Vol. I & II, (Edited by T. Skotheim), Marcel Dekker, N.Y. (1985). H.G. De Young, High Tech. 3 (l), 65 (1983).
459
Vol. 60, No. 5, pp. 457-459,
PROPERTIES
1986.
0038-1098/86 $3.00 + .OO 0 Pergamon Journals Ltd.
OF CHEMICALLY SYNTHESIZED POLYPYRROLE-HALOGEN CHARGE TRANSFER COMPLEXES E.T. Kang, K.G. Neoh and H.C. Ti
Department
of Chemical Engineering,
National University of Singapore, Kent Ridge 05 11, Singapore
(Received 18 June 1986 by W. Sasuki)
Simultaneous chemical polymerization and oxidation of pyrrole have been initiated by halogens, iodine (Iz) and bromine (Br,) at 0 to 4°C. The d.c. electrical conductivity (u) of this new family of polymeric charge transfer complexes is of the order of 30 ohm-’ cm-’ at room temperature. u was measured as a function of temperature between 300 and 150 K, using both standard four probe and voltage shorted compaction (VSC) techniques. A metallic temperature denendence of u was observed for both complexes in the VSC measurements. A
INTRODUCTION INTEREST IN THE electrical properties of highly conjugated polymers has increased rapidly during the past decade [l] . Indeed, conducting polymers has emerged as a new class of electrical/electronic materials [2] . Most of the attentions has been focused on three main types of conjugated polymers, poly(acetylene) and its derivatives [3], poly(phenylene) and its derivatives [4, 51, and poly(heterocyclic) polymers and cations [6-l l] . Unfortunately, the electrical properties for most of the conducting polymer with conjugated backbone degrade rapidly when exposed to the ambient air [12]. One of most promising systems appears to be polypyrrole (PPY) and related polyheterocyclic polymers prepared by electrochemical polymerization and oxidation [ 131 . In contrast to chemically oxidized conjugated polymers, PPY cations prepared by electrochemical methods are relatively stable and highly conductive [6, 131. The availability of stable conductive PPYs from electrochemical polymerization has encouraged us to search for alternative chemcial methods of synthesis and oxidation. Insulating and fairly oxidative PPY in film form has been synthesized chemically in the presence of an acid or peroxide initiators [ 14-161. These initially insulating films can be doped with halogens to achieve stable conductivity of the order of lo-’ ohm-’ cm-’ [15] . Oxidized PPY films with conductivity in the order of 10 to 10’ ohm-’ cm-’ have also been synthesized in the presence of a Lewis acid catalyst, such as RuCls or FeCls [17-l 93 . In this Communication, we report on a novel, one-step method for the simultaneous chemical polymerization and oxidation of pyrrole by halogens, such as bromine and iodine. The resulting PPY-halogen charge transfer (CT) complexes show remarkable stability in the atmosphere and in aqueous environment, and have
electrical conductivities comparable to that of oxidized PPYs produced electrochemically. The d.c. electrical conductivity of these PPY-halogen CT complexes are measured as a function of temperature (T) between 300 and 150 K. Possible conduction mechanisms involved are discussed. EXPERIMENTAL Pyrrole monomer (Merck, Reagent Grade) used in the present studies was purified by cold vacuum distillation. All solvents were of reagent grade and were used as received. All samples were exposed to the atmosphere after removal from the reaction mixture. The PPY-I2 CT complex was synthesized in aqueous medium as follows: 2% by weight of pulverized Iz was dispersed in deionized water in a glass reaction vessel. 1.5% by weight of monomeric pyrrole was introduced on to the water surface. The reaction vessel was closed and kept in the dark at 4’C for about 60 h. A thick, black spongy polymer layer was obtained at the bottom. The black material, which was later identified to be the PPY-I* CT complex, was extracted with copious amounts of carbon tetrachloride and absolute cold acetonitrile, ethanol in that order. It was then dried in dynamic vacuum. The yield was about 30-40%. In the synthesis of PPY-Bra CT complexes, 3% by weight of bromine was dissolved in acetonitrile in a glass vessel. The system was saturated with nitrogen and maintained at about 0 to 4°C. 2% by weight of monomeric pyrrole was then introduced into the reaction mixture. The system was stirred vigorously for about 6 h. A black precipitate was obtained in the reaction mixture and was subjected to the same washing and drying procedures described earlier. For conductivity measurements, the complexes were pressed into thin circular pellets of about 0.05-0.1
4.57
CHEMICALLY SYNTHESIZED
4.58
cm thick and 1.2 cm in diameter in a stainless steel press at a pressure of about 300 kg cme2. Electrical conductivities were measured using both standard four-probe and voltage shorted compaction (VSC) [20] techniques. The measuring circuit consisted of a Keithley 6 14 digital electrometer and a Hewlett-Packard Model 6212B d.c. power supply. Conductivities below room temperature were measured in situ in a liquid nitrogen cryostat. Samples were heated at a rate of about 0S”C min-‘. The uv-visible absorption spectra were measured using a Shimadzu UV-260 spectrophotometer. The infrared (IR) absorption spectra were measured using a Perkin-Elmer Model 682 spectrophotometer with the polymeric complexes dispersed in KBr.
The uv-visible absorption spectra of both the PPY-I2 and PPY-Br2 CT complexes reveal two relatively broad bands. One band centered at around 420nm and is characteristic of polypyrrole [ 16, 2 I] . The other intense broad band which appears in the red and extends well into the near-IR region probably results from the CT interaction between the polymer and the halogen, since undoped PPY does not have any appreciable absorption in the near-IR [ 161 The IR absorption spectra for both complexes are superimposable with those of oxidized PPY prepared electrochemically [21]. The PPY-I, CT complex prepared in the aqueous medium is sponge-like and somewhat rubbery in texture, while the PPY-Br, CT complex is granular in nature. The room temperature electrical conductivities of both complexes prepared by the present methods are in the order of 25-30ohm-i cm-‘. The conductivity is somewhat lower for samples prepared at higher temperatures. We wish to emphasise that both the physicochemical and electrical properties of the two complexes are stable in the atmosphere and in the presence of moisture. Typical chemical compositions such as Cal H 3.0N 1.00 1.0(I 2 ) 0.20_ 0.25 for the complex
and
C~.OH~.~NLOOO.~(B~~
Vol. 60, No. 5
o PPY - Br, l
PPY - I2
Fig. 1. Temperature dependence of the electrical conductivity of PPY-I2 and PPY-Br2 CT complexes in four probe conductivity measurements.
RESULTS AND DISCUSSION
ppy~-I,
POLYPYRROLE-HALOGEN
)0.22-0.24for
the PPY-Br, complex indicate that the C:H:N ratios in both complexes are rather close to the theoretical value of 4:3:1 for a perfectly linear chain of disubstituted pyrrole rings [14, 211 . The chemical compositions also suggest a pyrrole: acceptor ratio of about 4:l for both complexes. The ratio of about 3 to 4 pyrrole units to 1 anion has also been reported for a number of PPY-anion salt complexes obtained from electrochemical polymerization and oxidation [7, 211. The detailed chemical synthesis procedures and the physicochemical properties of various PPY-I2 and PPY-Br2 CT complexes will be reported in separate communications [22, 231 . Figure 1 shows the In u vs T-“4 plots of the PPYI2 and PPY-Br2 CT complexes near and below room
temperature obtained from the ordinary four probe conductivity measurements. An apparent linear fit of the experimental data was obtained for both complexes throughout the temperature range from 150 to 300K. The behavior has also been widely observed in electrochemically polymerized and oxidized PPY films [24-261, as well as in undoped PPY films prepared chemically in the presence of an acid catalyst [16]. Based on these result, the conduction mechanism in PPY has been interpreted in terms of Mott’s model [27] of variable range hopping between localized states near the Fermi surface. In fact, the l/4 power temperature dependence of conductivity has also been observed in many compressed pellets of polycrystalline molecular solids [28] . In doped polyacetylene films of granular and fibrillar morphology, ordinary four probe conductivity measurements also only indicate a thermally activated conduction characteristic of a semiconductor, although the metallic character of the polymer has been revealed by optical, magnetic susceptibility and thermoelectric power measurements [20b]. The discrepancy has been attributed to the contact resistance in the interfibrillar, intergranular or intercrystallitic regions of the polyacetylene films. By using the VSC method, which effectively short-circuits the intercrystallitic contact resistance, a true metallic temperature dependence of the conductivity has been observedin polyacetylene [20b] . More recently, the metallic temperature dependence of conductivity has also been demonstrated in electrochemically oxidized PPY films in VSC measurements [29]. In view of the reported inadequacy of the ordinary four probe conductivity measurements on compacted powder samples, we have also measured the temperature dependence of the conductivity in our PPY-I2 and PPY-Br2 samples using the VSC technique. The results are shown in Fig. 2. It is
Vol. 60, No. 5
CHEMICALLY SYNTHESIZED
POLYPYRROLE-HALOGEN
A.G. McDiarmid & A.J. Heeger, Synth. Met. 1, 101(1979/80). 4. J.F. Robolt, T.C. Clarke, K.K. Kanazawa, J.R. Reynolds & G.B. Street, J. Chem. Sot., Chem. Comm. 347 (1980). 5. L.W. Shacklette, R.R. Chance, D.M. Ivory, G.G. Miller & R.H. Bar&man, Synth. Met. 1, 307 (1980). 6. A.F. Biaz, K.K. Kanazawa & G.P. Gardini, JCS C’hem. Comm. 635 (1979). 7. K.K. Kanazawa, A.F. Diaz, R.H. Geiss, W.D. Gill, J.F. Kwak, J.A. Logan, J.F. Robolt & G.B. Street, JCS Chem. Comm. 854 (1979). 8. A.F. Diaz & J.1. Castillo, JCS Chem. Comm. 397 (1980). 9. A.F. Diaz, Chem. Ser. 17, 145 (1981). 10. G. Tourillon & F. Garnier, J. Electroanal. Chem. 135,173 (1982). 11. R.J. Waltman, J. Bargon & A.F. Diaz, J. Phys. Chem. 87,1459 (1983). 12. W. Deitz, P. Cukor, M. Rubner &J. Jopson, Synth. Met. 4, 189 (1982). 13. F. Gutmann, H. Keyzer & L.E. Lyons, Organic Semiconductors, Part B, p. 207, R.E. Krieger Pub. Co., Florida (1983). 14. G.P. Gardini, Adv. Heterocycl. Chem. 15, 67 (1973). 15. M. Salmon, K.K. Kanazawa, A.F. Diaz & M. Krounbi, J. PoZym. Sci. Lett. 20, 187 (1982). 16. H.S. Nalwa, L.R. Dalton, W.F. Schmidt & J.G. Robe, PoZym. Comm. 26,240 (1985). 17. M. Aizawa, H. Shinohara & H. Shirakawa, Polym. Prep. Japan 33,495 (1984). 18. K. Yoshino, S. Hayashi & R. Sugimoto, Japan J. Appl. Phys. 23, L899 (1984). 19. V. Bocchi & G.P Gardini, JCS Chem. Comm. 148 (1986). 20a. L.B. Coleman,Rev. Sci. Instr. 49,58 (1978). 20b. M. Wan, P. Wang, Y. Cao, R. Quan, F. Wang, X. Zhao 8~ Z. Gong, Solid State Commun. 47, 759 (1983). 21. G.B. Street, T.C. Clarke, M. Krounbi, K.K. Kanazawa, V. Lee, P. Pfluger, J.C. Scott & G. Weiser, Mol. Cyrst. Liq. Cryst. 83,253 (1982). 22. E.T. Kang, T.C. Tan, K.G. Neoh & Y.K. Ong, Polymer (In Press). 23. ET Kang, K.G. Neoh, T.C. Tan & Y.K. Ong, Submitted to J. Macro. Sci.-Chem. 24. K.K. Kanazawa, A.F. Diaz, W.D. Gill, P.M. Grant & G.B. Street, Synth. Metah 1,329 (1980). 25. A. Watanabe, M. Tanaka &J. Tanaka, Bull. Chem. Sot. Japan 54,2278 (1981). 26. P. Pfluger, G. Weiser, J.C. Scott & G.B. Street, A Handbook on Conducting Polymers Vol. I & II, (Edited by T. Skotheim), Marcel Dekker, N.Y. (1985). 27. N.F. Mott & E.A. Davis, Electronic Processes in Non-Crystalline Materials, p. 34, Clarendon Press, Oxford, 2nd. Ed., (1979). 28. C.S. Anithkumar & N. Umakantha, Phil. Mag. B44, 615 (1981). 29. X. Bi, Y. Yao, M. Wan, P. Wang, K. Xiao, Q. Yang & R. Qian, Makromol. Chem. 186,llOl (1985). 30. J.L. Bredas, B. Theman, J.M. Andre, R.R. Change, & R. Silbey, Synth. Met. 9,265 (1984). 3.
l
PPY-12
o PPY- Br,
TEMPERATURE , 1 , (K)
Fig. 2. Temperatures dependence of the electrical conductivity of PPY-I? and PPY-Br2 CT complexes in voltage shorted compaction measurements. (un.r. = room temperature conductivity). clear that both complexes show an increase in conductivity with decreasing temperature from 300 to 150K, a behavior characteristic of metallic conduction. Thus, the conduction behavior of the PPY-halogen CT complexes synthesized by the present chemical method is not unlike that of oxidized PPY prepared electrochemitally. Recently, a new conduction concept based on bipolarons hopping in the PPY system has also been introduced [30]. It therefore would be of great interest to elucidate the optical and magnetic properties of this new family of chemically synthesized PPY-halogen CT complexes. CONCLUSION A new family of PPY-halogen charge transfer complexes has been synthesized chemically via the simultaneous polymerization and doping technique. The PPY complexes so produced are stable in the atmosphere and have conductivity values and conduction behavior comparable to that of electrochemically polymerized and oxidized PPY. Acknowledgements - We would like to thank Prof. H.H. Lee and the Microanalysis Lab. of the Chemistry Department, NUS for conducting the chemical analysis of our samples. MS Sutini Suratman is most helpful in typing this manuscript.
REFERENCES 1.
2.
See for example, A Handbook on Conducting Polymers, Vol. I & II, (Edited by T. Skotheim), Marcel Dekker, N.Y. (1985). H.G. De Young, High Tech. 3 (l), 65 (1983).
459