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Be2+, Mg2+ and Al3+ N-dopants for polyacetylene
Synthetic Metals, 41-43 (1991) 101-105
101
B¢2 +, Mg 2 + AND AI3 + N-DOPANTS FOR POLYACETYLENE
Shiou-Mei Huang and Richard B. Kaner* Department of Chemistry and Biochemistry and Solid State Science Center, University of California, Los Angeles, Los Angeles, California 90024-1569, U.S.A.
ABSTRACT An electrolysis method has been developed which enables beryllium, magnesium and aluminum to dissolve in liquid ammonia with the creation of"solvated electrons". These solvated electrons give rise to highly colored solutions and provide the reducing power needed to transfer electrons to polyacetylene with the concomitant incorporation of Be +2, Mg +2 or AI +3 cations. These counterions serve to maintain electrical neutrality. The n-doping induces color changes in the polyacetylene films, causes dopant induced peaks in the infrared spectra and increases the electrical conductivity. The maximum conductivity observed, "0.6 O-I cm-t for Mg ÷2, is lower than those observed for the chemical n-doping of polyacetylene in metal-ammonia solutions with the heavier alkaline earth ions Ca ÷2, Sr ÷2 and Ba ÷2, which possess conductivities of-20 ~-1 cm-1. Polyacetylene n-doped from metal-ammonia solutions with the lanthanide ions Eu +2 and Yb +2 have conductivities o f " 20 and" 1 ~-1 cm-1, respectively. INTRODUCTION The discovery that doped polyacetylene films can conduct electricity [ 1] has given rise to extensive studies of many conjugated organic polymers [2]. In all cases dopant countedons are needed to maintain charge balance after partial reduction (n-doping) or oxidation (p-doping). Essentially all of the dopants which have been introduced into polyacetylene are monovalent ions. Recently, we reported that polyacetylene could be doped with the divalent cations Ca +2 , Sr +2, Ba +2, Eu +2 and Yb +2, by dissolving the metals in liquid ammonia solutions [3,4]. Here, we report a novel method for synthesizing polyacetylene doped with the small, highly-charged ions, Be +2, Mg +2 and A1+3. In addition, experiments on synthesizing the mixed metal dopant, Mg+2/Na+ are described. All these dopants lead to interesting changes in electrical and magnetic properties of the polyacetylene. We believe these highly charged dopant ions will improve our understandings of the conduction mechanism and the role of dopants in polyacetylene.
0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
102
EXPERIMENTAL Doping experiments were carried out in a glass and Teflon vacuum tight electrolytic cell. The cell consisted of two Pt mesh electrodes placed in a solution of an appropriate salt dissolved in liquid ammonia. The anode contained the pure metals Mg (Aesar, 99.99%), Be (Aesar, 99.5%) or A1 (Aesar, 99.99796). The salts used were Mg(CIO4)2 (Fisher), B¢I2 or AlI3, respectively. Anhydrous Mg(CIO4)2 was dried under vacuum at 120° C for 24 hours to removed any traces of moisture. BeI2 and AlI3 were synthesized from their elements in sealed evacuated tubes [5]. A polyacetylene film was cut and placed in the Pt mesh cathode. The two electrodes were separated by 0.2-0.7 cm. On a vacuum line pure ammonia (Mathson) was cryogenically distilled from Na into a side arm of the electrolysis cell. After warming to -43 ° C using an acetonitrile/iiquid nitrogen slush, the liquid ammonia was poured into the reaction chamber. Applying 5 mA to the cell initiallyproduced a" 5 V potential which decayed slowly with time. A dark blue solution, indicative of ammonia solvated electrons, appeared immediately at the cathode and over several minutes diffused out to color the entire solution. After 2 hours of doping the blue solution was poured into the side arm and pure ammonia was cryogenically distilled back in s i t , to wash the partially reduced polyacetylene. After many washings, the solution turned colorless. The films were washed one more time, evacuated and then brought into the drybox for physical measurements. Mg+2/Na + mixed metal doped polyacetylenes were prepared by dissolving sodium metal and magnesium perchlorate together in liquid ammonia solutions. After the polyacetylene films were immersed in the ammonia solutions for a desired amount of time, the doped films were washed as described above. RESULTS AND DISCUSSION Metal-ammonia solutions are capable of partially reducing polyacetylene with the incorporation of the large divalent dopant cations, Ca+2, Sr+2, Ba +2, Eu +2 and Yb +2. These cations serve to maintain charge balance in the partially reduced polymer. The metals,Ca, St, Ba, Eu and Yb, spontaneously dissolve in liquid ammonia to produce solvated electrons and solvated metal cations. This facilitates the chemical n-doping ofpolyacetylene. Unfortunately, the smaller alkaline earth metals, Be and Mg, as well as Al do not dissolve directly in liquid ammonia and therefore cannot easily be used as dopant counterions. We now report a new electrochemical process which allows the chemical doping ofpolyacetylene with Be +2, Mg +2 and AI +3 countercations. Very strong reducing solutions are produced at the surfaces ofa Pt cathode in an electrolytic cell which contains an appropriate salt dissolved in liquid ammonia and a metal anode [6,?]. The reactions can be represented by the following equations: M
........... >
M+n + n ¢"
n = 2 for M = Be or Mg n = 3 forM = AI
(1)
At the anode the metal is oxidized producing solvated cations, which diffuse away from the electrode and electrons which travel through the external circuit. At the cathode, solvated electrons are generated as is apparent by the blue color streaming away from the electrode's surface. This provides the reducing power needed to partially chemically reduce polyacetylene (equation 2). (CH)x +
xy e- -.......... > [(CH)'Y]x
(2)
103
The ions Be +2, Mg +2 or AI+3 then serve as counterions to maintain charge neutrality, giving the net reaction (equation 3). (CH)x + xy M
.......... >
[My(CH)lx
(3)
The n-doped polyacetylene films are extremely sensitive to air. All physical characterization were carded out using the anhydrous conditions of dryboxes and vacuum lines. The maximum room temperature conductivity averaged from four individual Mg +2 doped polyacetylene films is 0.6 f1-1 cm -1. The composition of each Mg +2 doped polyacetylene, based on atomic absorption results and weight uptake data, indicates the existence of some solvent coinsertion. Approximately 3 to 5 ammonia molecules associate with each Mg +2 dopant. Solvent coinsertion has also been reported after using other doping methods, for example, ammonia coordinates with Na + and Ca ÷2 in polyacetylene made from metal-ammonia solutions [3,4] and tetrahydrofuran (THF) coordinates with Li + and Na + ions in polyacetylene prepared from metal-naphthalide-THF solutions [81. The Be +2 and A1+3 counterions are more difficult to incorporate into polyacetylene and usually lead to some inhomogeniety in the doping process. This is likely due to the lower stability of the beryllium and aluminum electrolyte solutions as compared to the magnesium solution. The maximum conductivities of Be +2 and AI +3 doped polyacetylenes are 1.7x10 -3 and 2.1x10-3 f~-I cm-1 respectively. Approximately 5 to 7 ammonia molecules associate with each AI+3 ion. The conductivities of alkaline earth divalent cation and aluminum trivalent cation doped polyacetylenes are shown in Table 1. Be +2 and AI+3 doped films have the lowest conductivities which are about two orders of magnitude lower than that of Mg+2 doped films and four orders of magnitude lower than those of other larger divalent cation doped polyacetylenes. The results are likely related to the sizes of the ions. The smaller the ions, the stronger the bonding between the ions and the carbons in the polyacetylene backbone. This is consistent with the existence of a much larger number of Be +2 and A1+3 organometallic compounds compared to those of Ca +2, Sr+2 and Ba +2. In general, Be+2 and AI +3 have more covalent character, while Ca +2, Sr +2 and Ba+2 have more ionic character. It is likely that the smaller cations interact more strongly with the carbon backbone in TABLE 1 Composition and maximum conductivity for divalent and trivalent cation doped polyacetylene films Dopant
Be+2 Mg +2 Ca+2 Sr +2 Ba+2 AI+3
Formula*
[Be0.19(CH)]x [Mg0.16(CH)]x [Ca0.087(CH)]x [Sr0.06(CH)]x [Ba0.07(CH)]x [AI0.022(CH)]x
Conductivity (t~-I cm-I ) 0.002 0.6 20 30 20 0.002
* The formulas for Mg +2, Ca +2 and AI +3 doped polyacetylene are based on atomic absorption spectroscopy and weight uptake; the formulas for Be +2, Sr +2 and Ba+2 are based on weight uptake.
104
polyacetylene and possibly act like a trap for electrons through their covalent interactions. Because the 7r electron clouds in the polymer chain are more localized, the maximum conductivity is lower. Conductivity is not only influenced by the sizes of dopant cations but also the charges on the dopants. To compare conductivities with monovalent cation doped polyacetylene, Na + doped polyacetylene has been made from sodium-ammonia solutions. The maximum conductivity obtained for Na + doped films (- 100 ~-1 cm-1) is significantly higher than divalent cation and trivalent cation doped polyacetylenes. These results imply that conductivities decrease with an increase of the valence state of the dopants. In other words, dopant cations having a higher charge-to-radius ratio show lower conductivities. The FT-IR spectra of Mg +2 doped polyacetylene show doping induced peaks at around 1386 cm-1 and 800 cm -1. AI+3 cation doping also creates a new peak at approximately 1380 cm -1 and a broad peak centered at - 900 cm -1. These spectra are very similar to other doping induced peaks observed in polyacetylene IR spectra [9,10]. These two peaks have been attributed to the existence of solitons in the polyacetylene chains [ 11,12]. The magnetic susceptibility of [Mg0.16(CH)]x is 2.5x10 -8 emu/g, which is on the same order magnitude ofundoped trans-polyacetylene (3.8x10 -8 emu/g) [ 13]. It indicates that the number of free spins in [Mg0.16(CH)]x has not increased on doping when compared to pristine trans-polyacetylene. The peak-to-peak line width (AHpp) of this doped film is broadened to 2.2 G compared with thermally isomerized trans-polyacetylenewith a ~T-Ipp of 1.5 G. Mg+2/Na+ doped polyacetylenes can be prepared by immersion of polyacetylene films in sodiummagnesium perchlorate-ammonia solutions. Sodium in liquid ammonia provides the reducing power and allows both sodium and magnesium cations to diffuse into the polymer fibrils as counterions. The conductivity and composition data are shown in Table 2. Pure sodium doped and magnesium doped samples are prepared chemically from sodium-ammonia solutions and magnesium electrolysis, respectively. The electrical conducfivities of mixed metal doped films range from 100 to 0.6 ~-1 cm-1. The value is determined by the molar ratio of Mg +2 and Na + ions in the doped polyacetylene films. As the Mg+2/Na+ ratio increases, the conductivities of the samples decrease. This can be due to TABLE 2 Composition and conductivity for Mg+2/Na+ doped polyacetylene films Composition (by weight uptake)
Conductivity (£'1-1 cm- 1)
[Na0.17(CH)lx
100
Mg+ 2/Na+ ratio 0
[Mg0.018 Na0.049(CH)]x [Mg0.oa0Na0.059(CH)] x
62 27
0.37 0.51
[Mg0.032 Na0.037(CH)] x
11
0.86
[Mg0.047 Na0.046(CH)]x
3
1.02
[Mg0.16(CH)] x
0.6
co
105
stronger charge localization caused by replacing Na÷ with Mg +2. The dopant ratio in the films can be controlled by varying the doping time and the concentration of the two reagents, Mg(CIO4)2 and Na. Extending the doping time and increasing the Mg(CIO4)2 to Na ratio favor the formation of Mg +2 richdoped polyacetylenes. The data suggest that the doping process is essentially an ionic exchange process. The high mobility cations, Na +, are introduced first, while Ca+2 ions slowly diffuse in and substitute for Na + ions. CONCLUSIONS Novel synthetic methods have been developed for introducing small sized and highly charged cations into polyacetylene films. The solvated electrons created by electrolysis have reducing powers capable of n-doping polyacetylene with Be +2, Mg +2 and AI +3 countercations. The maximum conductivities of Be +2, Mg +2 and AI +3 doped polyacetylene films are lower than those of the larger alkaline earth cations, Ca +2, Sr+2 and Ba +2, which are lower than those of all the alkali ions such as Na +. It therefore appears that the maximum conductivities of metal cation doped polyacetylenes increase with decreasing charge to radius ratio of the dopant ions. Conductivities ofMg+~Na + mixed ion doped polyacetylenes decrease with increasing Mg +2 to Na + ratios. ACKNOWLEDGEMENT This work was support by the National Science Foundation Grant No. CHE 8657822 through the Presidential Young Investigator Award Program. REFERENCES 1
H. Shirakawa, E.J. Louis, A.G. MacDiarmid, C.K. Chiang and A.J. Heeger, ./. Chem. Soc. Chem. Comm., (1977) 578.
2
T.J. Skotheim, ed., "Handbook of Conducting Polymers", Vols. 1 and 2, ( Marcel Dekker, NY, 1986). S.-M. Huang and R.B. Kaner, Solid State lonic$, 32/33 (1989) 575. R.B. Kaner, S.-M. Huang, C.-H. Lin and J.H. Kaufman, S.Vnth. Met., 28 (1989) D115. G. Brauer, ed., "Handbook of Preparative Inorganic Chemistry", Vol. 1, 2nd edition, p 892 (Academic Press, NY, 1963).
3 4 5 6 7 8 9 10 11 12 13
A.D. McElroy, J. Kleinberg and A.W. Davidson, J. Am. Chem. Soc., 72 (1950) 5178. A.W. Davidson, J. Kleinberg, W.E. Bennett and A.D. McElroy, J. Am.Chem. Soc., 71 (1949) 377. B. Francois and C. Mathis, J. de Physique, C3 (1983) 21. C.R. Fincher, Jr., M. Ozaki, A.J. Heeger and A.G. MacDiarmid, Phys. Rev. B., 19 (1979) 4140. J.F. Rabolt, T.C. Clarke and G.B. Street, J. Chem. Phys., 71 (1979) 4614. E.J. Mele and M.J. Rice, Phys. Rev. Lett., 45 (1980) 926. W.P. Su, J.R. Schrieffer and A.J. Heeger, Phys. Rev. R , 22 (1980) 2099. T.-C. Chung, A. Feldblum, A.J. Heeger and A.G. MacDiarmid, J. Chem. Phys., 74 (1981) 5504.
101
B¢2 +, Mg 2 + AND AI3 + N-DOPANTS FOR POLYACETYLENE
Shiou-Mei Huang and Richard B. Kaner* Department of Chemistry and Biochemistry and Solid State Science Center, University of California, Los Angeles, Los Angeles, California 90024-1569, U.S.A.
ABSTRACT An electrolysis method has been developed which enables beryllium, magnesium and aluminum to dissolve in liquid ammonia with the creation of"solvated electrons". These solvated electrons give rise to highly colored solutions and provide the reducing power needed to transfer electrons to polyacetylene with the concomitant incorporation of Be +2, Mg +2 or AI +3 cations. These counterions serve to maintain electrical neutrality. The n-doping induces color changes in the polyacetylene films, causes dopant induced peaks in the infrared spectra and increases the electrical conductivity. The maximum conductivity observed, "0.6 O-I cm-t for Mg ÷2, is lower than those observed for the chemical n-doping of polyacetylene in metal-ammonia solutions with the heavier alkaline earth ions Ca ÷2, Sr ÷2 and Ba ÷2, which possess conductivities of-20 ~-1 cm-1. Polyacetylene n-doped from metal-ammonia solutions with the lanthanide ions Eu +2 and Yb +2 have conductivities o f " 20 and" 1 ~-1 cm-1, respectively. INTRODUCTION The discovery that doped polyacetylene films can conduct electricity [ 1] has given rise to extensive studies of many conjugated organic polymers [2]. In all cases dopant countedons are needed to maintain charge balance after partial reduction (n-doping) or oxidation (p-doping). Essentially all of the dopants which have been introduced into polyacetylene are monovalent ions. Recently, we reported that polyacetylene could be doped with the divalent cations Ca +2 , Sr +2, Ba +2, Eu +2 and Yb +2, by dissolving the metals in liquid ammonia solutions [3,4]. Here, we report a novel method for synthesizing polyacetylene doped with the small, highly-charged ions, Be +2, Mg +2 and A1+3. In addition, experiments on synthesizing the mixed metal dopant, Mg+2/Na+ are described. All these dopants lead to interesting changes in electrical and magnetic properties of the polyacetylene. We believe these highly charged dopant ions will improve our understandings of the conduction mechanism and the role of dopants in polyacetylene.
0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
102
EXPERIMENTAL Doping experiments were carried out in a glass and Teflon vacuum tight electrolytic cell. The cell consisted of two Pt mesh electrodes placed in a solution of an appropriate salt dissolved in liquid ammonia. The anode contained the pure metals Mg (Aesar, 99.99%), Be (Aesar, 99.5%) or A1 (Aesar, 99.99796). The salts used were Mg(CIO4)2 (Fisher), B¢I2 or AlI3, respectively. Anhydrous Mg(CIO4)2 was dried under vacuum at 120° C for 24 hours to removed any traces of moisture. BeI2 and AlI3 were synthesized from their elements in sealed evacuated tubes [5]. A polyacetylene film was cut and placed in the Pt mesh cathode. The two electrodes were separated by 0.2-0.7 cm. On a vacuum line pure ammonia (Mathson) was cryogenically distilled from Na into a side arm of the electrolysis cell. After warming to -43 ° C using an acetonitrile/iiquid nitrogen slush, the liquid ammonia was poured into the reaction chamber. Applying 5 mA to the cell initiallyproduced a" 5 V potential which decayed slowly with time. A dark blue solution, indicative of ammonia solvated electrons, appeared immediately at the cathode and over several minutes diffused out to color the entire solution. After 2 hours of doping the blue solution was poured into the side arm and pure ammonia was cryogenically distilled back in s i t , to wash the partially reduced polyacetylene. After many washings, the solution turned colorless. The films were washed one more time, evacuated and then brought into the drybox for physical measurements. Mg+2/Na + mixed metal doped polyacetylenes were prepared by dissolving sodium metal and magnesium perchlorate together in liquid ammonia solutions. After the polyacetylene films were immersed in the ammonia solutions for a desired amount of time, the doped films were washed as described above. RESULTS AND DISCUSSION Metal-ammonia solutions are capable of partially reducing polyacetylene with the incorporation of the large divalent dopant cations, Ca+2, Sr+2, Ba +2, Eu +2 and Yb +2. These cations serve to maintain charge balance in the partially reduced polymer. The metals,Ca, St, Ba, Eu and Yb, spontaneously dissolve in liquid ammonia to produce solvated electrons and solvated metal cations. This facilitates the chemical n-doping ofpolyacetylene. Unfortunately, the smaller alkaline earth metals, Be and Mg, as well as Al do not dissolve directly in liquid ammonia and therefore cannot easily be used as dopant counterions. We now report a new electrochemical process which allows the chemical doping ofpolyacetylene with Be +2, Mg +2 and AI +3 countercations. Very strong reducing solutions are produced at the surfaces ofa Pt cathode in an electrolytic cell which contains an appropriate salt dissolved in liquid ammonia and a metal anode [6,?]. The reactions can be represented by the following equations: M
........... >
M+n + n ¢"
n = 2 for M = Be or Mg n = 3 forM = AI
(1)
At the anode the metal is oxidized producing solvated cations, which diffuse away from the electrode and electrons which travel through the external circuit. At the cathode, solvated electrons are generated as is apparent by the blue color streaming away from the electrode's surface. This provides the reducing power needed to partially chemically reduce polyacetylene (equation 2). (CH)x +
xy e- -.......... > [(CH)'Y]x
(2)
103
The ions Be +2, Mg +2 or AI+3 then serve as counterions to maintain charge neutrality, giving the net reaction (equation 3). (CH)x + xy M
.......... >
[My(CH)lx
(3)
The n-doped polyacetylene films are extremely sensitive to air. All physical characterization were carded out using the anhydrous conditions of dryboxes and vacuum lines. The maximum room temperature conductivity averaged from four individual Mg +2 doped polyacetylene films is 0.6 f1-1 cm -1. The composition of each Mg +2 doped polyacetylene, based on atomic absorption results and weight uptake data, indicates the existence of some solvent coinsertion. Approximately 3 to 5 ammonia molecules associate with each Mg +2 dopant. Solvent coinsertion has also been reported after using other doping methods, for example, ammonia coordinates with Na + and Ca ÷2 in polyacetylene made from metal-ammonia solutions [3,4] and tetrahydrofuran (THF) coordinates with Li + and Na + ions in polyacetylene prepared from metal-naphthalide-THF solutions [81. The Be +2 and A1+3 counterions are more difficult to incorporate into polyacetylene and usually lead to some inhomogeniety in the doping process. This is likely due to the lower stability of the beryllium and aluminum electrolyte solutions as compared to the magnesium solution. The maximum conductivities of Be +2 and AI +3 doped polyacetylenes are 1.7x10 -3 and 2.1x10-3 f~-I cm-1 respectively. Approximately 5 to 7 ammonia molecules associate with each AI+3 ion. The conductivities of alkaline earth divalent cation and aluminum trivalent cation doped polyacetylenes are shown in Table 1. Be +2 and AI+3 doped films have the lowest conductivities which are about two orders of magnitude lower than that of Mg+2 doped films and four orders of magnitude lower than those of other larger divalent cation doped polyacetylenes. The results are likely related to the sizes of the ions. The smaller the ions, the stronger the bonding between the ions and the carbons in the polyacetylene backbone. This is consistent with the existence of a much larger number of Be +2 and A1+3 organometallic compounds compared to those of Ca +2, Sr+2 and Ba +2. In general, Be+2 and AI +3 have more covalent character, while Ca +2, Sr +2 and Ba+2 have more ionic character. It is likely that the smaller cations interact more strongly with the carbon backbone in TABLE 1 Composition and maximum conductivity for divalent and trivalent cation doped polyacetylene films Dopant
Be+2 Mg +2 Ca+2 Sr +2 Ba+2 AI+3
Formula*
[Be0.19(CH)]x [Mg0.16(CH)]x [Ca0.087(CH)]x [Sr0.06(CH)]x [Ba0.07(CH)]x [AI0.022(CH)]x
Conductivity (t~-I cm-I ) 0.002 0.6 20 30 20 0.002
* The formulas for Mg +2, Ca +2 and AI +3 doped polyacetylene are based on atomic absorption spectroscopy and weight uptake; the formulas for Be +2, Sr +2 and Ba+2 are based on weight uptake.
104
polyacetylene and possibly act like a trap for electrons through their covalent interactions. Because the 7r electron clouds in the polymer chain are more localized, the maximum conductivity is lower. Conductivity is not only influenced by the sizes of dopant cations but also the charges on the dopants. To compare conductivities with monovalent cation doped polyacetylene, Na + doped polyacetylene has been made from sodium-ammonia solutions. The maximum conductivity obtained for Na + doped films (- 100 ~-1 cm-1) is significantly higher than divalent cation and trivalent cation doped polyacetylenes. These results imply that conductivities decrease with an increase of the valence state of the dopants. In other words, dopant cations having a higher charge-to-radius ratio show lower conductivities. The FT-IR spectra of Mg +2 doped polyacetylene show doping induced peaks at around 1386 cm-1 and 800 cm -1. AI+3 cation doping also creates a new peak at approximately 1380 cm -1 and a broad peak centered at - 900 cm -1. These spectra are very similar to other doping induced peaks observed in polyacetylene IR spectra [9,10]. These two peaks have been attributed to the existence of solitons in the polyacetylene chains [ 11,12]. The magnetic susceptibility of [Mg0.16(CH)]x is 2.5x10 -8 emu/g, which is on the same order magnitude ofundoped trans-polyacetylene (3.8x10 -8 emu/g) [ 13]. It indicates that the number of free spins in [Mg0.16(CH)]x has not increased on doping when compared to pristine trans-polyacetylene. The peak-to-peak line width (AHpp) of this doped film is broadened to 2.2 G compared with thermally isomerized trans-polyacetylenewith a ~T-Ipp of 1.5 G. Mg+2/Na+ doped polyacetylenes can be prepared by immersion of polyacetylene films in sodiummagnesium perchlorate-ammonia solutions. Sodium in liquid ammonia provides the reducing power and allows both sodium and magnesium cations to diffuse into the polymer fibrils as counterions. The conductivity and composition data are shown in Table 2. Pure sodium doped and magnesium doped samples are prepared chemically from sodium-ammonia solutions and magnesium electrolysis, respectively. The electrical conducfivities of mixed metal doped films range from 100 to 0.6 ~-1 cm-1. The value is determined by the molar ratio of Mg +2 and Na + ions in the doped polyacetylene films. As the Mg+2/Na+ ratio increases, the conductivities of the samples decrease. This can be due to TABLE 2 Composition and conductivity for Mg+2/Na+ doped polyacetylene films Composition (by weight uptake)
Conductivity (£'1-1 cm- 1)
[Na0.17(CH)lx
100
Mg+ 2/Na+ ratio 0
[Mg0.018 Na0.049(CH)]x [Mg0.oa0Na0.059(CH)] x
62 27
0.37 0.51
[Mg0.032 Na0.037(CH)] x
11
0.86
[Mg0.047 Na0.046(CH)]x
3
1.02
[Mg0.16(CH)] x
0.6
co
105
stronger charge localization caused by replacing Na÷ with Mg +2. The dopant ratio in the films can be controlled by varying the doping time and the concentration of the two reagents, Mg(CIO4)2 and Na. Extending the doping time and increasing the Mg(CIO4)2 to Na ratio favor the formation of Mg +2 richdoped polyacetylenes. The data suggest that the doping process is essentially an ionic exchange process. The high mobility cations, Na +, are introduced first, while Ca+2 ions slowly diffuse in and substitute for Na + ions. CONCLUSIONS Novel synthetic methods have been developed for introducing small sized and highly charged cations into polyacetylene films. The solvated electrons created by electrolysis have reducing powers capable of n-doping polyacetylene with Be +2, Mg +2 and AI +3 countercations. The maximum conductivities of Be +2, Mg +2 and AI +3 doped polyacetylene films are lower than those of the larger alkaline earth cations, Ca +2, Sr+2 and Ba +2, which are lower than those of all the alkali ions such as Na +. It therefore appears that the maximum conductivities of metal cation doped polyacetylenes increase with decreasing charge to radius ratio of the dopant ions. Conductivities ofMg+~Na + mixed ion doped polyacetylenes decrease with increasing Mg +2 to Na + ratios. ACKNOWLEDGEMENT This work was support by the National Science Foundation Grant No. CHE 8657822 through the Presidential Young Investigator Award Program. REFERENCES 1
H. Shirakawa, E.J. Louis, A.G. MacDiarmid, C.K. Chiang and A.J. Heeger, ./. Chem. Soc. Chem. Comm., (1977) 578.
2
T.J. Skotheim, ed., "Handbook of Conducting Polymers", Vols. 1 and 2, ( Marcel Dekker, NY, 1986). S.-M. Huang and R.B. Kaner, Solid State lonic$, 32/33 (1989) 575. R.B. Kaner, S.-M. Huang, C.-H. Lin and J.H. Kaufman, S.Vnth. Met., 28 (1989) D115. G. Brauer, ed., "Handbook of Preparative Inorganic Chemistry", Vol. 1, 2nd edition, p 892 (Academic Press, NY, 1963).
3 4 5 6 7 8 9 10 11 12 13
A.D. McElroy, J. Kleinberg and A.W. Davidson, J. Am. Chem. Soc., 72 (1950) 5178. A.W. Davidson, J. Kleinberg, W.E. Bennett and A.D. McElroy, J. Am.Chem. Soc., 71 (1949) 377. B. Francois and C. Mathis, J. de Physique, C3 (1983) 21. C.R. Fincher, Jr., M. Ozaki, A.J. Heeger and A.G. MacDiarmid, Phys. Rev. B., 19 (1979) 4140. J.F. Rabolt, T.C. Clarke and G.B. Street, J. Chem. Phys., 71 (1979) 4614. E.J. Mele and M.J. Rice, Phys. Rev. Lett., 45 (1980) 926. W.P. Su, J.R. Schrieffer and A.J. Heeger, Phys. Rev. R , 22 (1980) 2099. T.-C. Chung, A. Feldblum, A.J. Heeger and A.G. MacDiarmid, J. Chem. Phys., 74 (1981) 5504.