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The preparation, characterization and physical properties of SbCl5-doped polyacetylene
Synthetic Metals, 10 (1985) 261 - 272
261
THE PREPARATION, CHARACTERIZATION AND PHYSICAL PROPERTIES OF SbCls-DOPED POLYACETYLENE
L. SODERHOLM*, C. MATHIS and B. FRANCOIS Centre de Recherches sur les Macromoldcules 6, rue Boussingault, 67083 Strasbourg Cddex (France)
J. M. FRIEDT Centre de Recherches Nucldaires, B.P. 20, 67037 Strasbourg Cddex (France)
(Received and accepted October 5, 1984)
Abstract T r a n s - p o l y a c e t y l e n e has been doped with SbC1s to molar concentrations varying between y = 0.005 and y = 0.12. Physical characterization was accomplished by employing 13C solid-state MAS n.m.r., d.c. conductivity, i.r. and 121Sb MSssbauer spectroscopy; all experiments being performed on a unique sample of the appropriate concentration. The results demonstrate two distinct concentration regions. For samples with y < 0.05, the conductivity increases with increasing y; 121Sb MSssbauer spectroscopy unequivocally demonstrates only the presence of SbC13. The charge transfer counterion is concluded to be CI-, since the presence of Sb (+5) (SbCls, SbC16-, Sb2Cllo ) as well as of SbC14- can be ruled out. For samples with y > 0.07, MSssbauer spectra show the presence of both Sb (+5) and Sb (+3); the conductivity decreases with increasing y. I.r. spectroscopy and ~3C n.m.r, consistently demonstrate C--C1 bond formation in this concentration range.
Introduction It has been demonstrated that the conductivity of polyacetylene may be increased by up to 12 orders of magnitude by reaction with n- or p-type dopants. Of the p dopants, AsF s has been the most widely studied, producing polymers with room temperature conductivities of the order of 103 ~-~ cm- 1 [ 1 ]. Although the exact chemical compositions of these doped polymers
*Present address: Chemistry Division, Argonne National Laboratories, Argonne, IL 60439, U.S.A. 0379-6779/85/$3.30
© Elsevier Sequoia/Printed in The Netherlands
262
are uncertain [2], it is thought that the enhancement in conductivity involves a dismutation reaction similar to the one proposed for the intercalation of AsFs into graphite [3]; that is (CH)x -
' (CH)~2+ + 2e-
(1) 3AsFs + 2e- ~
2AsF6- + AsF3
It has been proposed that such an oxidation of polyacetylene produces charged solitons, i.e., mobile e m p t y states at the Fermi level, and it is the enhancement in concentration of these charged carriers which produces the increase in conductivity. While fewer studies have been reported on the behaviour of polyacetylene doped with the Lewis acid SbFs, a maximum conductivity of 10 12-1 cm -1 has been reported [4], indicating that these materials behave in a manner comparable to those doped with AsFs. In the latter system, as well as in graphite interaction compounds, 121Sb MSssbauer spectroscopy has demonstrated the formation of both the Sb (+5) and Sb (+3) valence states of antimony, consistent with the occurrence of the above mentioned dismutation reaction (1) [5, 6, 7]. The work presented here involves the synthesis and characterization of polyacetylene doped with another antimony-based Lewis acid, SbC1 s. This system is suited for 12~Sb MSssbauer spectroscopy, from which the antimony oxidation state can be unequivocally assigned. It is also suitable for 13C solid state n.m.r, which, taken together with chemical analyses, electrical conductivities and i.r. spectroscopy, provides a determination of the physical and chemical transformations occurring during the doping of SbCI s into polyacetylene.
Experiments Sample preparation and characterization Polyacetylene films ( ~ 2 0 0 pm) were prepared following a method derived from that of Shirakawa et al. [8, 9]. The synthetic conditions were chosen to obtain films of low apparent density (~ 0.20) and high porosity [10]. The films were isomerized to the more stable trans form by heating under vacuum to 180 °C for 30 minutes. All results reported here were obtained on the trans isomer of polyacetylene. The SbC1 s (Merck) was doubly distilled under vacuum before use; a MSssbauer spectrum confirmed the presence of only Sb (+5). The polyacetylene and liquid SbCls were sealed under high vacuum into ampoules equipped with break seals. These ampoules were then attached to the allglass apparatus, which is also sealed under high vacuum. Doping was carried out by opening the break seals, bringing the polyacetylene film (room temperature) into contact with the SbCl s vapour, the vapour pressure being adjusted by the SbC1 s temperature (--20 °C to 20 °C). The doped films were
263 then cryogenically pumped for 24 hours before being investigated. Some SbC13 was removed by this procedure in the case of the heavily-doped films (y > 0.07). All ensuing manipulations of the doped films were carried out in an inert atmosphere (argon-filled glove box), since initial studies indicated their air sensitivity. Concentrations were determined by weight uptake, assuming SbC1 s as the doping species. Chemical analyses revealed a Sb:C1 ratio varying between 1:5 and 1:8 in the more lightly-doped region (y < 0.07). 121Sb Mdssbauer The source (0.66 mCi of 121mSn in the form of CaSnO3) is driven with the velocity changing as a sine function of time, in synchronization with a multichannel analyser operating in the time mode. The transmitted radiation was detected with a high-resolution germanium detector. The experimental data were least squares analysed by a computer. Solid-state 13C n.m.r. 13C magic angle spinning (MAS) solid-state n.m.r, spectra were obtained on a Bruker CXP 200 spectrometer operating at 50.3 MHz. A magic angle spinning rate between 3 and 5 kHz was used. There was no observable change in the spectra when the contact time was varied between 3 ms and 200 ps, so all spectra were run at 200 ps.
Results Conductivity measurements Room temperature d.c. electrical conductivities were obtained using a simple four-probe technique. Measurements were performed under an inert (argon) atmosphere, as a function of dopant concentration. The conductivity of the initial, undoped film, ~ 10 -9 ~2-1 cm -1, increases steadily as a function of dopant concentration, reaching a maximum of 40 ~-1 c m - ' (Fig. 1). Doping to higher concentrations results in samples that are too brittle to measure reliably with the above technique. This problem was circumvented by performing conductivity measurements in situ. Electrodes were attached to the polyacetylene film inside the sealed reaction vessel, and the conductivity monitored as a function of time (i.e., of increasing, but undetermined, dopant concentration). There is an initial, rapid rise in conductivity which levels off after about 100 minutes, as shown in Fig. 2. After the plateau has been attained, the conductivity remains stable over a short period, then begins to decrease rapidly, finally dropping to near the value of the initial polymer. However, the maximum value of the conductivity appears to depend on experimental conditions, i.e., the higher the temperature of SbC1s (--20 °C < T < 20 °C), the higher the resulting plateau.
264
1
.'~
-
O"
_o -1
-2
-4
-3
- 1
-2 Iog,oy
Fig. 1. Increase in conductivity (o) as a function of the SbCls molar concentration (y) for lightly (0.005 < y < 0.05) doped polyacetylene. la~o e ~ ~ o•eOOoo ..po-
++++++++++ +
?
+
E
-2 +
C
+
0
-4 +
-9
I
200
I
I
400
600 TIME (rain)
Fig. 2. In situ conductivity as a function of reaction time for SbCI 5 doped at +10 °C (o) and at --20 °C (+). Observe (1) the decrease in conductivity for heavily-doped samples and (2) th e effect of doping time on the conductivity.
265 lO
"\x
/' '1 f',,/'
~t iiI ~/~I
:]8
1S
MU
28 ZS
",....'l ,/i/
i
'! ?
)
r
Jl
2BI Z888
1588
1888
SOB
c,D 1
Fig. 3. Infrared spectrum of [CH(SbCls)0j1]x. The characteristic trans-(CH)x band at 1015 cm-1 has almost disappeared, being replaced by bands at 1270, 1200, 1180, 745 and 710 cm-1. These changes from pure trans-(CH)xare attributed to C--C1 bond formation, which destroys the extended conjugation. If the film is removed from the dopant vapour at the conductivity plateau and subsequently cryogenically pumped, the resulting film has a conductivity that is stable in an inert atmosphere over several days. On the other hand, exposure of conducting samples to air causes a drop in their conductivity (about 20% in the first hour). An infrared spectrum of the insulating material that remains after 500 minutes of doping as described above is shown in Fig. 3. The absorption characteristic of trans-CHx (1015 cm -1) has almost disappeared, being replaced by bands (1270, 1200, 1180, 745 and 710 cm -1) which reveal the presence of C--C1 bonding in this heavily-doped sample. Infrared spectroscopy of a more lightly doped, and therefore still conducting, sample was prohibited by the presence of a metallic absorption band covering the region of interest.
M6ssbauer spectroscopy A sufficient number of freshly prepared SbCls-doped polyacetylene discs (diameter = 13 mm) were superimposed in order to prepare MSssbauer samples with 1.5- 3.5 mg/cm 2 of Sb. They were transferred under a dry argon atmosphere to Teflon-lined, indium-sealed aluminium holders. All MSssbauer measurements were performed with the source and sample held at 4.2 K. The hyperfine parameters are least squares fitted to the data after diagonalization of the two-level nuclear quadrupole Hamiltonian:
e2qzQ ~J~Q -- 4 / ( 2 /
--
1)
3[z 2
+ ~-(I+
(2)
Twelve transitions occur between the sublevels of the nuclear ground (spin 5/2) and excited (spin 7/2) states, which are represented as Lorentzian line shapes, with intensities determined from the appropriate Clebsch-Gordon coefficients [ 11 ]. 121Sb MSssbauer spectra show widely different isomer shifts (81s) for Sb (+3) and Sb (+5), allowing the unambiguous assignment of the valence
266 I
o ~°
o
~ o °
~
o~°
y=5.2%
o!
7.2% o°
.~.
~,°
o
o
o
~° z o °o° o~o o°°~o°
o
=. Z
o
I-
tn m
~-~
-3o.o
I
o.o
11.0%
30.0
VELOCITY ( m m / s e c )
Fig. 4. Selected 121Sb M S s s b a u e r s p e c t r a at 4.2 K for d i f f e r e n t SbCls d o p a n t m o l a r conc e n t r a t i o n s . T h e r e s o n a n c e at ~IS = - - 1 4 . 5 (6) m m / s is assigned to t h e p r e s e n c e o f SbC13. T h e a p p e a r a n c e o f Sb (+ 5) (~Is = 2.50 (5)) for samples w i t h y > 0.07 is clearly seen.
state (or states) of antimony in polyacetylene. Spectra were obtained on thirteen samples with a range of doping levels 0.005 ~< y < 0.11, with the results falling into two main groups; y ~< 0.05 and y/> 0.07. The first group (y ~< 0.05) shows evidence of only the Sb (+3) valence state after the doping of polyacetylene with SbCls. This is concluded from the hyperfine parameters (5is = --14.5(6) mm/s; e 2 q Q = 13.8(9) mm/s; r/= 0.1) which are close to those reported for solid SbCla (6is = --14.3(5) mm/s; e 2 q Q = 12.5(5) mm/s; rt = 0.2) [12]. At higher dopant concentrations, i.e., y >~ 0.07, the samples present both the above-mentioned Sb (+3) absorption, plus a well-resolved component resulting from the presence of Sb (+5) (Fig. 4). The isomer shift of the new c o m p o n e n t (6IS = 2.50(5) mm/s) is similar to that found for S b C l ( [13], although the electric field gradient is non zero (e2Qq = 6(2) mm/s), indicating a distortion from octahedral symmetry. The MSssbauer hyperfine parameters (61s , e2qQ) of neither the Sb (+5) nor the Sb (+3) resonances change over the range of concentration studied; the abundance normalized to the initial polyacetylene thickness (T) is the only concentration-dependent parameter (Table 1). For samples with y ~<
267 /0
/ / A /
o
/ /
X
z 0 /
Ia.
/ /
0
u~
/ / /
e~ / I
!
I
0.02
I
'
0.04
y Fig. 5. Linear relationship between the intensity of the 121Sb Mbssbauer resonance (normalized to the original polyacetylene thickness) and the SbC15 molar concentration in the low y regime. For y > 0.07, the relationship breaks down, indicating the loss of an Sb-containing species from the polymer sample. TABLE 1 121Sb hyperfine results at 4.2 K in SbCls-doped polyacetylene of molar concentration y: percentage resonance area of the Sb (+3) line, isomer shift (5is) against CaSnO3 and quadrupole coupling constant (e2qQ) y 0.005 0.007 0.011 0.016 0.025 0.040 0.041 0.052 0.072 0.080 0.107 0.108 0.110
% Sb (+3) 100 100 100 100 100 100 100 100 91 74 64 63 60
5iS (+3) (+0.08) mm/s
e2qQ
6is(+ 5) (+0.08) mm/s
e2qQ
(+ 1.0) mm/s
--14.8 --15.0 --15.0 --14.9 --14.8 --14.8 --14.9 --14.8 --14.9 --14.5 --13.8 --13.6 --13.7
14.1 13.5 13.7 12.5 12.6 12.6 13.0 12.9 13.4 14.1 15.6 12.8 12.4
--2.53 --2.67 --2.54 --2.47 --2.45
--5.0 --5.0 --8.5 --8.4 --6.4
(+1.0) mm/s
0.05, T varies linearly with molar concentration. The y = 0.07 sample s h o w s e v i d e n c e o f a s m a l l q u a n t i t y o f S b (+ 5), w h i c h r e p r e s e n t s a b o u t 8% o f t h e t o t a l a n t i m o n y r e s o n a n c e . T h e r e l a t i v e p r o p o r t i o n o f S b ( + 5 ) rises rapidly with increasing y until it accounts for 40% of the total effect at y = 0 . 1 1 . W h i l e t h e r a t i o S b ( + 5 ) / S b ( + 3 ) is r e p r o d u c i b l e f o r a d o p a n t
268 concentration, the total 121Sb resonance area is a random function of y for the heavily-doped samples. This reveals that the antimony content does not scale with the weight gain, contrary to the situation occurring for y ~< 0.05, and indicates loss of an antimony-containing species in the concentration range considered. The presence of a white solid, analysed as SbC13, in the reaction vessel after doping supports this interpretation.
Nuclear magnetic resonance (n. m. r.) The samples were ground with powdered glass, and loaded into ceramic holders under an inert atmosphere. The 13C MAS n.m.r, spectra were run at room temperature. The contact time was varied between 3 ms and 200 ps, but since little difference was observed, all spectra were run at 200 ps. The frequency is expressed as a chemical shift, in ppm, from the tetramethylsilane (TMS) reference. The n.m.r, spectrum obtained for undoped trans-polyacetylene is a single line at 5 = 137 ppm. While the results fro the y = 0.02 sample show a slight broadening at the base of the peak (still centered at 5 = 137 ppm), the spectrum is essentially unchanged from that of the pure polymer. The spectrum obtained at y = 0.07, on the other hand, appears very different. While the original trans-polyacetylene peak remains at ~ 135 ppm, it is considerably broadened. This broadening coincides with the appearance of a new, intense peak at 62 ppm and assigned to a 13C--C1 resonance, which normally falls in the range 65 - 35 ppm. The spectra of samples with y > 0.07 are similar to the one just described, as shown in Fig. 6.
Discussion The results presented here reveal the occurrence of two distinctly different concentration regimes for the electronic behaviours of SbC1sdoped polyacetylene. At the lower doping levels (y < 0.05) the Lewis acid SbC1 s acts as a dopant for trans-polyacetylene, enhancing the electrical conductivity by several orders of magnitude, via an electron transfer involving the formation of SbC13. The n.m.r, single line resonance at 137 ppm (y = 0.02) shows a slight broadening at the base of the peak which, for AsF5, has been attributed to the presence of charged solitons [14]. While the n.m.r, spectra and conductivities reported here are similar to those found for AsFs-doped polyacetylene, the 121Sb Mbssbauer results are clearly inconsistent with the dismutation reaction (1). Instead these results reveal unambiguously that, for y < 0.05, the polymer is a strong enough reducing agent to convert all the Sb (+ 5) into Sb (+ 3) according to: SbC1 s + 2e- -----> SbC13 + 2C1(CH)x
) (CH) x 2+ + 2e-
(3)
In this case the polyacetylene is reduced according to eqn. (1); however,
269
.f
=lO.O
\
7.0%
2.0%
(CH-) x
I
200
i
I
i
I
I
I
l
L
100
i
I
0 ppm
Fig. 6. 13C MAS n.m.r, spectra for selected dopant concentrations. The peak appearing at 62 ppm for the heavily doped samples is attributed to 13C--C1bond formation (X indicates spinning sidebands). there are two important differences between this reaction and the one proposed for AsF s. The first is the proposed identity of the counterion. MSssbauer spectroscopy unequivocally demonstrates the absence of any Sb (+ 5) to within the experimental error (< 5%). Hence species of the type SbC1s, SbC16-, and (Sb2Cll0) 2- may all be ruled out. Furthermore, the hyperfine parameters of the Sb (+3) site are consistent with those of SbC13, and the formation of SbC14-, which has a MSssbauer resonance distinguishable from that of SbC13 [ 15 ], may also be ruled out. Since n.m.r, measurements show no evidence of C--C1 bond formation at low concentrations, the counterion species is concluded to be C1-. The second difference between the two dismutation reactions is the number of electrons transferred from (CH)x to the dopant molecule, i.e., there are two electrons transferred per SbCls, whereas there is only 2/3 of an electron donated to each AsF s. In other words, there should be three times as many carriers formed per SbCls molecule, yet a comparison of o vs. y for the two Lewis acids shows similar behaviours. However, the SbCls curve is displaced to lower conductivity for the same y. This might result, in part, from different secondary reactions occurring at low dopant concentrations, which would affect the charge carrier mobilities. Also, it has been demonstrated that the degree of dopant homogeneity plays a crucial role in the behaviour of these curves [16].
270 The physical properties of heavily-doped polyacetylene (y ~> 0.07) are very different from those just discussed. Instead of a doping process, it appears that there is substantial C--C1 bond formation, as demonstrated by i.r. spectroscopy. The appearance of a peak at 62 ppm in the 13C n.m.r. spectrum, characteristic of C--C1, supports this view. The broadening of the peak at 137 ppm is similar to that reported for AsFs-doped polyacetylene, where it has been attributed to an increase in the Pauli susceptibility as the polymer becomes metallic [17]. However, with heavy SbC1 s doping the broadening is attributed to a range of chemical shifts, arising from the slightly different 13C=C environments, caused by the addition of chlorine to the chain. A decrease in the conjugation length of the chain, by the introduction of an sp 3 carbon, also contributes to the broadening. This C--C1 bond formation interrupts the extended conjugation of the trans-polyacetylene chain, destroying some of the charges formed by the dismutation reaction (3), and trapping the remaining carriers. The result is a brittle, light blue material with low conductivity. Similar physical properties are seen for (CH)~ heavily doped with bromine [18] and have been attributed to C--Br bond formation. The chemical composition of the chlorinated polymer varies according to the specific samples. M5ssbauer spectra demonstrate that the weight gained in this region is the result of the incorporation of unreduced Sb (+5) (in addition to the reduced SbC13) and that the total antimony content varies in an apparently uncorrelated manner between samples. The product formed from the heavy doping of trans-polyacetylene by SbC1 s appears to be the result of the uncontrolled addition of C1- to C=C bonds. The results discussed here demonstrate an abrupt change in the chemical, and consequently, physical properties of SbCls-doped polyacetylene which occurs in the dopant concentration range y = 0.05-~ 0.07. Lightlyd o p e d films have properties similar to those previously reported for the Lewis acids AsFs and SbFs as dopants; notably a smooth increase in the conductivity as a function of y. The oxidation of the polyacetylene by the Lewis acid produces charge carriers, perhaps in the form of solitons, which are responsible for the augmentation in the conductivity. In this case, it is not SbC16 but instead CI- that acts as the counterion, held by coulombic attraction to the charged polymer chain. The positive charge can be delocalized over approximately 22 carbon atoms [19] and, for concentrations less than 5%, this stabilizes the cation/anion charge separation with respect to C--C1 bond formation. Increasing the carrier concentration, while augmenting the conductivity, also increases the positive charge on the chain. These data suggest that, at y -~ 0.05, there is a sharp increase in the number of C--C1 bonds which occurs over a relatively small range of concentration. Recent calculations, using a PPP model which explicitly includes coulombic interactions [19], predict an overlapping of the charged soliton tails at about y = 0.05. The coulombic repulsion which results from this overlapping destabilizes the solitonic state, thereby permitting C--C1 bond formation. Initial bonding further traps the remaining charges on the chain, which is further chlorinated, finally completely destroying the conjugation pathway.
271 While it appears that the three Lewis acids SbCls, SbF s and AsFs all act as reducing agents for trans-polyacetylene, the resulting materials exhibit different physical properties. Our preliminary work on SbFs-doped transpolyacetylene demonstrates the following: (1) there is no saturation in the conductivity at y < 0.09; (2) both Sb (+5) and Sb (+3) are present in the samples, their ratio apparently independent of doping concentration, depending instead on the doping conditions. (3) the exact nature of the Sb (+5) species could not be discerned, but the MSssbauer hyperfine parameters of the Sb (+3) site are consistent with those previously reported for SbF3. The data demonstrate the presence of Sb (+5) in these samples, but the chemical nature of the counterion is unclear, SbF6- , Sb2Fll- and/or Fall being possible candidates. It is not possible to use MSssbauer spectroscopy to determine the oxidation state (or states) of arsenic in AsFs-doped polyacetylene, but previously reported XPS data [20] indicate the presence of 'AsF 3' at low doping concentrations, with the appearance of an AsFs-like species in the more heavily-doped samples. Chemical analyses have revealed an As:F ratio of 1:5 to 1:6, which appears dependent on doping conditions, but not on y [2]. These results are similar to those found for SbC1 s doping and are not inconsistent with the presence of F- as a counterion. 13C n.m.r, on AsFsdoped samples produces a single very broad line [17] but the presence of C--F bonds may be masked by 13C coupling to 19F (spin 1/2). Conclusions At concentrations y < 0.05, SbC1 s is found to dope trans-polyacetylene. The oxidation of (CH)x, which loses two electrons per dopant molecule, results in an increase in the number of charge carriers, and hence in the conductivity. SbC1 s is quantitatively reduced to SbC13 in this concentration range, with the absence of pentavalent species such as SbCl6- and Sb2Clllclearly demonstrated; therefore the doping counterion is proposed to be C1-. At concentrations y > 0.05, there is clear evidence for C--C1 bond formation, with the conductivity decreasing as the dopant concentration is increased. Sb (+5) is also present, indicating that the dopant is no longer being totally reduced; the primary reaction is the chlorination of polyacetylene, rather than its oxidation, contrary to the low concentration regime (y < 0.05). Acknowledgements We wish to thank B. Meurer for providing the 13C n.m.r, and also for helpful discussion. One of the authors (L.S.) acknowledges financial support from NSERC (Canada) and NATO.
272
References 1 J. C. W. Chien, J. M. Warakomski, F. E. Karasz, W. L. Chia and C. P. Lillya, Phys. Rev. B, 28 (12) (1983) 6937. 2 A. Pron, A. G. MacDiarmid and A. J. Heeger, Synth. Met., 9 (1984) 115. 3 N. Bartlett, B. W. McQuillan and A. S. Robertson, Mater. Res. Bull., 13 (1978) 1259. 4 D. Davidov, S. Roth, W. Neumann and H. Sixl, Z. Phys. B, 51 (1983) 145. 5 J. M. Friedt, R. Poinsot and L. Soderholm, Solid State Commun., 49 (3) (1984) 223. 6 J. M. Friedt, L. Soderholm, R. Poinsot and R. Vang~listi, Synth. Met., 8 (1983) 99. 7 F. Godler, B. Perscheid, G. Kaindl, K. Menke and S. Roth, J. Phys. (Paris) Colloq., 44(b) (1983) C3- 233. 8 B. Franqois, M. Bernard and J. J. Andre, J. Chem. Phys., 75 (8) (1981) 4142. 9 T. Ito, H. Shirakawa and S. Ikeda, J. Polym. Sci., Polym. Chem. Ed., 12 (1974) 11. 10 B. Franqois, C. Mathis, R. Nuffer and A. Rudatsikira, Mol. Cry. Liq. Cryst. (Proc. ICSM 1984), in press. 11 See, e.g., N.N. Greenwood and T. C. Gibb, M6ssbauer Spectroscopy, Chapman and Hall, London, 1971, p. 612. 12 J. P. Devort, Thesis, University of Strasbourg, 1974. 13 J. M. Friedt, G. K. Shenoy and M. Burgard, J. Chem. Phys., 59 (8) (1973) 4468. 14 M. Sasai and H. Fukutome, Solid State Commun., 51 (8) (1984) 609. 15 J. Donaldson, J. T. Southern and M. J. Tricker, J. Chem. Soc., Dalton Trans., (1972) 2637. 16 C. R. Fincher, D. Moses, A. J. Heeger and A. G. MacDiarmid, Synth. Met., 6 (1983) 243. 17 T. Terao, S. Maeda, T. Yamabe, K. Akagi and H. Shirakawa, Solid State Commun., 49 (8) (1984) 829. 18 J. C. W. Chien, Polyacetylene, Academic Press, Florida, 1984, p. 286. 19 H. F u k u t o m e and M. Sasai, Prog. Theor. Phys., 69 (2) (1983) 373. 20 W. R. Salaneck, H. R. Thomas, C. B. Duke, A. Paton, E. W. Plummer, A. J. Heeger and A. G. MacDiarmid, J. Chem. Phys., 71 (5) (1979) 2044.
261
THE PREPARATION, CHARACTERIZATION AND PHYSICAL PROPERTIES OF SbCls-DOPED POLYACETYLENE
L. SODERHOLM*, C. MATHIS and B. FRANCOIS Centre de Recherches sur les Macromoldcules 6, rue Boussingault, 67083 Strasbourg Cddex (France)
J. M. FRIEDT Centre de Recherches Nucldaires, B.P. 20, 67037 Strasbourg Cddex (France)
(Received and accepted October 5, 1984)
Abstract T r a n s - p o l y a c e t y l e n e has been doped with SbC1s to molar concentrations varying between y = 0.005 and y = 0.12. Physical characterization was accomplished by employing 13C solid-state MAS n.m.r., d.c. conductivity, i.r. and 121Sb MSssbauer spectroscopy; all experiments being performed on a unique sample of the appropriate concentration. The results demonstrate two distinct concentration regions. For samples with y < 0.05, the conductivity increases with increasing y; 121Sb MSssbauer spectroscopy unequivocally demonstrates only the presence of SbC13. The charge transfer counterion is concluded to be CI-, since the presence of Sb (+5) (SbCls, SbC16-, Sb2Cllo ) as well as of SbC14- can be ruled out. For samples with y > 0.07, MSssbauer spectra show the presence of both Sb (+5) and Sb (+3); the conductivity decreases with increasing y. I.r. spectroscopy and ~3C n.m.r, consistently demonstrate C--C1 bond formation in this concentration range.
Introduction It has been demonstrated that the conductivity of polyacetylene may be increased by up to 12 orders of magnitude by reaction with n- or p-type dopants. Of the p dopants, AsF s has been the most widely studied, producing polymers with room temperature conductivities of the order of 103 ~-~ cm- 1 [ 1 ]. Although the exact chemical compositions of these doped polymers
*Present address: Chemistry Division, Argonne National Laboratories, Argonne, IL 60439, U.S.A. 0379-6779/85/$3.30
© Elsevier Sequoia/Printed in The Netherlands
262
are uncertain [2], it is thought that the enhancement in conductivity involves a dismutation reaction similar to the one proposed for the intercalation of AsFs into graphite [3]; that is (CH)x -
' (CH)~2+ + 2e-
(1) 3AsFs + 2e- ~
2AsF6- + AsF3
It has been proposed that such an oxidation of polyacetylene produces charged solitons, i.e., mobile e m p t y states at the Fermi level, and it is the enhancement in concentration of these charged carriers which produces the increase in conductivity. While fewer studies have been reported on the behaviour of polyacetylene doped with the Lewis acid SbFs, a maximum conductivity of 10 12-1 cm -1 has been reported [4], indicating that these materials behave in a manner comparable to those doped with AsFs. In the latter system, as well as in graphite interaction compounds, 121Sb MSssbauer spectroscopy has demonstrated the formation of both the Sb (+5) and Sb (+3) valence states of antimony, consistent with the occurrence of the above mentioned dismutation reaction (1) [5, 6, 7]. The work presented here involves the synthesis and characterization of polyacetylene doped with another antimony-based Lewis acid, SbC1 s. This system is suited for 12~Sb MSssbauer spectroscopy, from which the antimony oxidation state can be unequivocally assigned. It is also suitable for 13C solid state n.m.r, which, taken together with chemical analyses, electrical conductivities and i.r. spectroscopy, provides a determination of the physical and chemical transformations occurring during the doping of SbCI s into polyacetylene.
Experiments Sample preparation and characterization Polyacetylene films ( ~ 2 0 0 pm) were prepared following a method derived from that of Shirakawa et al. [8, 9]. The synthetic conditions were chosen to obtain films of low apparent density (~ 0.20) and high porosity [10]. The films were isomerized to the more stable trans form by heating under vacuum to 180 °C for 30 minutes. All results reported here were obtained on the trans isomer of polyacetylene. The SbC1 s (Merck) was doubly distilled under vacuum before use; a MSssbauer spectrum confirmed the presence of only Sb (+5). The polyacetylene and liquid SbCls were sealed under high vacuum into ampoules equipped with break seals. These ampoules were then attached to the allglass apparatus, which is also sealed under high vacuum. Doping was carried out by opening the break seals, bringing the polyacetylene film (room temperature) into contact with the SbCl s vapour, the vapour pressure being adjusted by the SbC1 s temperature (--20 °C to 20 °C). The doped films were
263 then cryogenically pumped for 24 hours before being investigated. Some SbC13 was removed by this procedure in the case of the heavily-doped films (y > 0.07). All ensuing manipulations of the doped films were carried out in an inert atmosphere (argon-filled glove box), since initial studies indicated their air sensitivity. Concentrations were determined by weight uptake, assuming SbC1 s as the doping species. Chemical analyses revealed a Sb:C1 ratio varying between 1:5 and 1:8 in the more lightly-doped region (y < 0.07). 121Sb Mdssbauer The source (0.66 mCi of 121mSn in the form of CaSnO3) is driven with the velocity changing as a sine function of time, in synchronization with a multichannel analyser operating in the time mode. The transmitted radiation was detected with a high-resolution germanium detector. The experimental data were least squares analysed by a computer. Solid-state 13C n.m.r. 13C magic angle spinning (MAS) solid-state n.m.r, spectra were obtained on a Bruker CXP 200 spectrometer operating at 50.3 MHz. A magic angle spinning rate between 3 and 5 kHz was used. There was no observable change in the spectra when the contact time was varied between 3 ms and 200 ps, so all spectra were run at 200 ps.
Results Conductivity measurements Room temperature d.c. electrical conductivities were obtained using a simple four-probe technique. Measurements were performed under an inert (argon) atmosphere, as a function of dopant concentration. The conductivity of the initial, undoped film, ~ 10 -9 ~2-1 cm -1, increases steadily as a function of dopant concentration, reaching a maximum of 40 ~-1 c m - ' (Fig. 1). Doping to higher concentrations results in samples that are too brittle to measure reliably with the above technique. This problem was circumvented by performing conductivity measurements in situ. Electrodes were attached to the polyacetylene film inside the sealed reaction vessel, and the conductivity monitored as a function of time (i.e., of increasing, but undetermined, dopant concentration). There is an initial, rapid rise in conductivity which levels off after about 100 minutes, as shown in Fig. 2. After the plateau has been attained, the conductivity remains stable over a short period, then begins to decrease rapidly, finally dropping to near the value of the initial polymer. However, the maximum value of the conductivity appears to depend on experimental conditions, i.e., the higher the temperature of SbC1s (--20 °C < T < 20 °C), the higher the resulting plateau.
264
1
.'~
-
O"
_o -1
-2
-4
-3
- 1
-2 Iog,oy
Fig. 1. Increase in conductivity (o) as a function of the SbCls molar concentration (y) for lightly (0.005 < y < 0.05) doped polyacetylene. la~o e ~ ~ o•eOOoo ..po-
++++++++++ +
?
+
E
-2 +
C
+
0
-4 +
-9
I
200
I
I
400
600 TIME (rain)
Fig. 2. In situ conductivity as a function of reaction time for SbCI 5 doped at +10 °C (o) and at --20 °C (+). Observe (1) the decrease in conductivity for heavily-doped samples and (2) th e effect of doping time on the conductivity.
265 lO
"\x
/' '1 f',,/'
~t iiI ~/~I
:]8
1S
MU
28 ZS
",....'l ,/i/
i
'! ?
)
r
Jl
2BI Z888
1588
1888
SOB
c,D 1
Fig. 3. Infrared spectrum of [CH(SbCls)0j1]x. The characteristic trans-(CH)x band at 1015 cm-1 has almost disappeared, being replaced by bands at 1270, 1200, 1180, 745 and 710 cm-1. These changes from pure trans-(CH)xare attributed to C--C1 bond formation, which destroys the extended conjugation. If the film is removed from the dopant vapour at the conductivity plateau and subsequently cryogenically pumped, the resulting film has a conductivity that is stable in an inert atmosphere over several days. On the other hand, exposure of conducting samples to air causes a drop in their conductivity (about 20% in the first hour). An infrared spectrum of the insulating material that remains after 500 minutes of doping as described above is shown in Fig. 3. The absorption characteristic of trans-CHx (1015 cm -1) has almost disappeared, being replaced by bands (1270, 1200, 1180, 745 and 710 cm -1) which reveal the presence of C--C1 bonding in this heavily-doped sample. Infrared spectroscopy of a more lightly doped, and therefore still conducting, sample was prohibited by the presence of a metallic absorption band covering the region of interest.
M6ssbauer spectroscopy A sufficient number of freshly prepared SbCls-doped polyacetylene discs (diameter = 13 mm) were superimposed in order to prepare MSssbauer samples with 1.5- 3.5 mg/cm 2 of Sb. They were transferred under a dry argon atmosphere to Teflon-lined, indium-sealed aluminium holders. All MSssbauer measurements were performed with the source and sample held at 4.2 K. The hyperfine parameters are least squares fitted to the data after diagonalization of the two-level nuclear quadrupole Hamiltonian:
e2qzQ ~J~Q -- 4 / ( 2 /
--
1)
3[z 2
+ ~-(I+
(2)
Twelve transitions occur between the sublevels of the nuclear ground (spin 5/2) and excited (spin 7/2) states, which are represented as Lorentzian line shapes, with intensities determined from the appropriate Clebsch-Gordon coefficients [ 11 ]. 121Sb MSssbauer spectra show widely different isomer shifts (81s) for Sb (+3) and Sb (+5), allowing the unambiguous assignment of the valence
266 I
o ~°
o
~ o °
~
o~°
y=5.2%
o!
7.2% o°
.~.
~,°
o
o
o
~° z o °o° o~o o°°~o°
o
=. Z
o
I-
tn m
~-~
-3o.o
I
o.o
11.0%
30.0
VELOCITY ( m m / s e c )
Fig. 4. Selected 121Sb M S s s b a u e r s p e c t r a at 4.2 K for d i f f e r e n t SbCls d o p a n t m o l a r conc e n t r a t i o n s . T h e r e s o n a n c e at ~IS = - - 1 4 . 5 (6) m m / s is assigned to t h e p r e s e n c e o f SbC13. T h e a p p e a r a n c e o f Sb (+ 5) (~Is = 2.50 (5)) for samples w i t h y > 0.07 is clearly seen.
state (or states) of antimony in polyacetylene. Spectra were obtained on thirteen samples with a range of doping levels 0.005 ~< y < 0.11, with the results falling into two main groups; y ~< 0.05 and y/> 0.07. The first group (y ~< 0.05) shows evidence of only the Sb (+3) valence state after the doping of polyacetylene with SbCls. This is concluded from the hyperfine parameters (5is = --14.5(6) mm/s; e 2 q Q = 13.8(9) mm/s; r/= 0.1) which are close to those reported for solid SbCla (6is = --14.3(5) mm/s; e 2 q Q = 12.5(5) mm/s; rt = 0.2) [12]. At higher dopant concentrations, i.e., y >~ 0.07, the samples present both the above-mentioned Sb (+3) absorption, plus a well-resolved component resulting from the presence of Sb (+5) (Fig. 4). The isomer shift of the new c o m p o n e n t (6IS = 2.50(5) mm/s) is similar to that found for S b C l ( [13], although the electric field gradient is non zero (e2Qq = 6(2) mm/s), indicating a distortion from octahedral symmetry. The MSssbauer hyperfine parameters (61s , e2qQ) of neither the Sb (+5) nor the Sb (+3) resonances change over the range of concentration studied; the abundance normalized to the initial polyacetylene thickness (T) is the only concentration-dependent parameter (Table 1). For samples with y ~<
267 /0
/ / A /
o
/ /
X
z 0 /
Ia.
/ /
0
u~
/ / /
e~ / I
!
I
0.02
I
'
0.04
y Fig. 5. Linear relationship between the intensity of the 121Sb Mbssbauer resonance (normalized to the original polyacetylene thickness) and the SbC15 molar concentration in the low y regime. For y > 0.07, the relationship breaks down, indicating the loss of an Sb-containing species from the polymer sample. TABLE 1 121Sb hyperfine results at 4.2 K in SbCls-doped polyacetylene of molar concentration y: percentage resonance area of the Sb (+3) line, isomer shift (5is) against CaSnO3 and quadrupole coupling constant (e2qQ) y 0.005 0.007 0.011 0.016 0.025 0.040 0.041 0.052 0.072 0.080 0.107 0.108 0.110
% Sb (+3) 100 100 100 100 100 100 100 100 91 74 64 63 60
5iS (+3) (+0.08) mm/s
e2qQ
6is(+ 5) (+0.08) mm/s
e2qQ
(+ 1.0) mm/s
--14.8 --15.0 --15.0 --14.9 --14.8 --14.8 --14.9 --14.8 --14.9 --14.5 --13.8 --13.6 --13.7
14.1 13.5 13.7 12.5 12.6 12.6 13.0 12.9 13.4 14.1 15.6 12.8 12.4
--2.53 --2.67 --2.54 --2.47 --2.45
--5.0 --5.0 --8.5 --8.4 --6.4
(+1.0) mm/s
0.05, T varies linearly with molar concentration. The y = 0.07 sample s h o w s e v i d e n c e o f a s m a l l q u a n t i t y o f S b (+ 5), w h i c h r e p r e s e n t s a b o u t 8% o f t h e t o t a l a n t i m o n y r e s o n a n c e . T h e r e l a t i v e p r o p o r t i o n o f S b ( + 5 ) rises rapidly with increasing y until it accounts for 40% of the total effect at y = 0 . 1 1 . W h i l e t h e r a t i o S b ( + 5 ) / S b ( + 3 ) is r e p r o d u c i b l e f o r a d o p a n t
268 concentration, the total 121Sb resonance area is a random function of y for the heavily-doped samples. This reveals that the antimony content does not scale with the weight gain, contrary to the situation occurring for y ~< 0.05, and indicates loss of an antimony-containing species in the concentration range considered. The presence of a white solid, analysed as SbC13, in the reaction vessel after doping supports this interpretation.
Nuclear magnetic resonance (n. m. r.) The samples were ground with powdered glass, and loaded into ceramic holders under an inert atmosphere. The 13C MAS n.m.r, spectra were run at room temperature. The contact time was varied between 3 ms and 200 ps, but since little difference was observed, all spectra were run at 200 ps. The frequency is expressed as a chemical shift, in ppm, from the tetramethylsilane (TMS) reference. The n.m.r, spectrum obtained for undoped trans-polyacetylene is a single line at 5 = 137 ppm. While the results fro the y = 0.02 sample show a slight broadening at the base of the peak (still centered at 5 = 137 ppm), the spectrum is essentially unchanged from that of the pure polymer. The spectrum obtained at y = 0.07, on the other hand, appears very different. While the original trans-polyacetylene peak remains at ~ 135 ppm, it is considerably broadened. This broadening coincides with the appearance of a new, intense peak at 62 ppm and assigned to a 13C--C1 resonance, which normally falls in the range 65 - 35 ppm. The spectra of samples with y > 0.07 are similar to the one just described, as shown in Fig. 6.
Discussion The results presented here reveal the occurrence of two distinctly different concentration regimes for the electronic behaviours of SbC1sdoped polyacetylene. At the lower doping levels (y < 0.05) the Lewis acid SbC1 s acts as a dopant for trans-polyacetylene, enhancing the electrical conductivity by several orders of magnitude, via an electron transfer involving the formation of SbC13. The n.m.r, single line resonance at 137 ppm (y = 0.02) shows a slight broadening at the base of the peak which, for AsF5, has been attributed to the presence of charged solitons [14]. While the n.m.r, spectra and conductivities reported here are similar to those found for AsFs-doped polyacetylene, the 121Sb Mbssbauer results are clearly inconsistent with the dismutation reaction (1). Instead these results reveal unambiguously that, for y < 0.05, the polymer is a strong enough reducing agent to convert all the Sb (+ 5) into Sb (+ 3) according to: SbC1 s + 2e- -----> SbC13 + 2C1(CH)x
) (CH) x 2+ + 2e-
(3)
In this case the polyacetylene is reduced according to eqn. (1); however,
269
.f
=lO.O
\
7.0%
2.0%
(CH-) x
I
200
i
I
i
I
I
I
l
L
100
i
I
0 ppm
Fig. 6. 13C MAS n.m.r, spectra for selected dopant concentrations. The peak appearing at 62 ppm for the heavily doped samples is attributed to 13C--C1bond formation (X indicates spinning sidebands). there are two important differences between this reaction and the one proposed for AsF s. The first is the proposed identity of the counterion. MSssbauer spectroscopy unequivocally demonstrates the absence of any Sb (+ 5) to within the experimental error (< 5%). Hence species of the type SbC1s, SbC16-, and (Sb2Cll0) 2- may all be ruled out. Furthermore, the hyperfine parameters of the Sb (+3) site are consistent with those of SbC13, and the formation of SbC14-, which has a MSssbauer resonance distinguishable from that of SbC13 [ 15 ], may also be ruled out. Since n.m.r, measurements show no evidence of C--C1 bond formation at low concentrations, the counterion species is concluded to be C1-. The second difference between the two dismutation reactions is the number of electrons transferred from (CH)x to the dopant molecule, i.e., there are two electrons transferred per SbCls, whereas there is only 2/3 of an electron donated to each AsF s. In other words, there should be three times as many carriers formed per SbCls molecule, yet a comparison of o vs. y for the two Lewis acids shows similar behaviours. However, the SbCls curve is displaced to lower conductivity for the same y. This might result, in part, from different secondary reactions occurring at low dopant concentrations, which would affect the charge carrier mobilities. Also, it has been demonstrated that the degree of dopant homogeneity plays a crucial role in the behaviour of these curves [16].
270 The physical properties of heavily-doped polyacetylene (y ~> 0.07) are very different from those just discussed. Instead of a doping process, it appears that there is substantial C--C1 bond formation, as demonstrated by i.r. spectroscopy. The appearance of a peak at 62 ppm in the 13C n.m.r. spectrum, characteristic of C--C1, supports this view. The broadening of the peak at 137 ppm is similar to that reported for AsFs-doped polyacetylene, where it has been attributed to an increase in the Pauli susceptibility as the polymer becomes metallic [17]. However, with heavy SbC1 s doping the broadening is attributed to a range of chemical shifts, arising from the slightly different 13C=C environments, caused by the addition of chlorine to the chain. A decrease in the conjugation length of the chain, by the introduction of an sp 3 carbon, also contributes to the broadening. This C--C1 bond formation interrupts the extended conjugation of the trans-polyacetylene chain, destroying some of the charges formed by the dismutation reaction (3), and trapping the remaining carriers. The result is a brittle, light blue material with low conductivity. Similar physical properties are seen for (CH)~ heavily doped with bromine [18] and have been attributed to C--Br bond formation. The chemical composition of the chlorinated polymer varies according to the specific samples. M5ssbauer spectra demonstrate that the weight gained in this region is the result of the incorporation of unreduced Sb (+5) (in addition to the reduced SbC13) and that the total antimony content varies in an apparently uncorrelated manner between samples. The product formed from the heavy doping of trans-polyacetylene by SbC1 s appears to be the result of the uncontrolled addition of C1- to C=C bonds. The results discussed here demonstrate an abrupt change in the chemical, and consequently, physical properties of SbCls-doped polyacetylene which occurs in the dopant concentration range y = 0.05-~ 0.07. Lightlyd o p e d films have properties similar to those previously reported for the Lewis acids AsFs and SbFs as dopants; notably a smooth increase in the conductivity as a function of y. The oxidation of the polyacetylene by the Lewis acid produces charge carriers, perhaps in the form of solitons, which are responsible for the augmentation in the conductivity. In this case, it is not SbC16 but instead CI- that acts as the counterion, held by coulombic attraction to the charged polymer chain. The positive charge can be delocalized over approximately 22 carbon atoms [19] and, for concentrations less than 5%, this stabilizes the cation/anion charge separation with respect to C--C1 bond formation. Increasing the carrier concentration, while augmenting the conductivity, also increases the positive charge on the chain. These data suggest that, at y -~ 0.05, there is a sharp increase in the number of C--C1 bonds which occurs over a relatively small range of concentration. Recent calculations, using a PPP model which explicitly includes coulombic interactions [19], predict an overlapping of the charged soliton tails at about y = 0.05. The coulombic repulsion which results from this overlapping destabilizes the solitonic state, thereby permitting C--C1 bond formation. Initial bonding further traps the remaining charges on the chain, which is further chlorinated, finally completely destroying the conjugation pathway.
271 While it appears that the three Lewis acids SbCls, SbF s and AsFs all act as reducing agents for trans-polyacetylene, the resulting materials exhibit different physical properties. Our preliminary work on SbFs-doped transpolyacetylene demonstrates the following: (1) there is no saturation in the conductivity at y < 0.09; (2) both Sb (+5) and Sb (+3) are present in the samples, their ratio apparently independent of doping concentration, depending instead on the doping conditions. (3) the exact nature of the Sb (+5) species could not be discerned, but the MSssbauer hyperfine parameters of the Sb (+3) site are consistent with those previously reported for SbF3. The data demonstrate the presence of Sb (+5) in these samples, but the chemical nature of the counterion is unclear, SbF6- , Sb2Fll- and/or Fall being possible candidates. It is not possible to use MSssbauer spectroscopy to determine the oxidation state (or states) of arsenic in AsFs-doped polyacetylene, but previously reported XPS data [20] indicate the presence of 'AsF 3' at low doping concentrations, with the appearance of an AsFs-like species in the more heavily-doped samples. Chemical analyses have revealed an As:F ratio of 1:5 to 1:6, which appears dependent on doping conditions, but not on y [2]. These results are similar to those found for SbC1 s doping and are not inconsistent with the presence of F- as a counterion. 13C n.m.r, on AsFsdoped samples produces a single very broad line [17] but the presence of C--F bonds may be masked by 13C coupling to 19F (spin 1/2). Conclusions At concentrations y < 0.05, SbC1 s is found to dope trans-polyacetylene. The oxidation of (CH)x, which loses two electrons per dopant molecule, results in an increase in the number of charge carriers, and hence in the conductivity. SbC1 s is quantitatively reduced to SbC13 in this concentration range, with the absence of pentavalent species such as SbCl6- and Sb2Clllclearly demonstrated; therefore the doping counterion is proposed to be C1-. At concentrations y > 0.05, there is clear evidence for C--C1 bond formation, with the conductivity decreasing as the dopant concentration is increased. Sb (+5) is also present, indicating that the dopant is no longer being totally reduced; the primary reaction is the chlorination of polyacetylene, rather than its oxidation, contrary to the low concentration regime (y < 0.05). Acknowledgements We wish to thank B. Meurer for providing the 13C n.m.r, and also for helpful discussion. One of the authors (L.S.) acknowledges financial support from NSERC (Canada) and NATO.
272
References 1 J. C. W. Chien, J. M. Warakomski, F. E. Karasz, W. L. Chia and C. P. Lillya, Phys. Rev. B, 28 (12) (1983) 6937. 2 A. Pron, A. G. MacDiarmid and A. J. Heeger, Synth. Met., 9 (1984) 115. 3 N. Bartlett, B. W. McQuillan and A. S. Robertson, Mater. Res. Bull., 13 (1978) 1259. 4 D. Davidov, S. Roth, W. Neumann and H. Sixl, Z. Phys. B, 51 (1983) 145. 5 J. M. Friedt, R. Poinsot and L. Soderholm, Solid State Commun., 49 (3) (1984) 223. 6 J. M. Friedt, L. Soderholm, R. Poinsot and R. Vang~listi, Synth. Met., 8 (1983) 99. 7 F. Godler, B. Perscheid, G. Kaindl, K. Menke and S. Roth, J. Phys. (Paris) Colloq., 44(b) (1983) C3- 233. 8 B. Franqois, M. Bernard and J. J. Andre, J. Chem. Phys., 75 (8) (1981) 4142. 9 T. Ito, H. Shirakawa and S. Ikeda, J. Polym. Sci., Polym. Chem. Ed., 12 (1974) 11. 10 B. Franqois, C. Mathis, R. Nuffer and A. Rudatsikira, Mol. Cry. Liq. Cryst. (Proc. ICSM 1984), in press. 11 See, e.g., N.N. Greenwood and T. C. Gibb, M6ssbauer Spectroscopy, Chapman and Hall, London, 1971, p. 612. 12 J. P. Devort, Thesis, University of Strasbourg, 1974. 13 J. M. Friedt, G. K. Shenoy and M. Burgard, J. Chem. Phys., 59 (8) (1973) 4468. 14 M. Sasai and H. Fukutome, Solid State Commun., 51 (8) (1984) 609. 15 J. Donaldson, J. T. Southern and M. J. Tricker, J. Chem. Soc., Dalton Trans., (1972) 2637. 16 C. R. Fincher, D. Moses, A. J. Heeger and A. G. MacDiarmid, Synth. Met., 6 (1983) 243. 17 T. Terao, S. Maeda, T. Yamabe, K. Akagi and H. Shirakawa, Solid State Commun., 49 (8) (1984) 829. 18 J. C. W. Chien, Polyacetylene, Academic Press, Florida, 1984, p. 286. 19 H. F u k u t o m e and M. Sasai, Prog. Theor. Phys., 69 (2) (1983) 373. 20 W. R. Salaneck, H. R. Thomas, C. B. Duke, A. Paton, E. W. Plummer, A. J. Heeger and A. G. MacDiarmid, J. Chem. Phys., 71 (5) (1979) 2044.