Synthesis and characterization of conducting polypyrrole-containing iron complexes

J. Phys. Chem. Solids Vol. 48, No. 7, pp. 635-640, 1987 Printed in Great Britain.

0022-3697/87 $3.00+ 0.130 Pergamon Journals Ltd.

SYNTHESIS A N D CHARACTERIZATION OF CONDUCTING POLYPYRROLE-CONTAINING IRON COMPLEXES J. PRZYEUSKI, M. ZAGbRSKA, Institute of Inorganic Technology, Warsaw University of Technology, 00-664 Warsaw, Noakowskiego 3, Poland A. PROIq, Institute for Polymer Research and Technology, Warsaw University of Technology, 00-664 Warsaw, Noakowskiego 3, Poland Z. KUCHARSKI a n d J. SUWALSKI Institute of Atomic Energy, Solid State Physics Department, 05-400 Otwock, Swierk, Poland (Received II June 1986; accepted in revised form 18 December 1986)

Abstract--Electrochemical or chemical oxidation of pyrrole-containing complex anions of iron cyanide or iron chloride results in the formation of films or powders of conducting polypyrroles. Freshly prepared films exhibit an additional IR band at ca 1630-1640 cm -t, slowly disappearing in air and not observed in previously studies polypyrrole-based systems. It is possible that this new band is associated with the existence of a C ~ N bond in dehydrogenated pyrrole tings which are transformed into regular pyrrole rings, probably due to the protonation reaction occurring in air and simultaneous bond rearrangement. The polypyrrole structure favours the presence of Fe(CN)]- over Fe(CN)]- since the former is the only iron species detected by M6ssbauer spectroscopy in electrochemically prepared samples. It is also the dominant iron species in the samples oxidized chemically. The polypyrrole-containing Fe(CN)~- is more ordered than those containing monovalent anions, as evidenced by X-ray diffraction studies. High-spin iron complexes can be inserted into polypyrrole during electrochemical oxidation of pyrrole in nonaqueous solutions containing LiCI/FeCI 3. The inserted species exhibit M6ssbauer parameters characteristic of slightly distorted FeCI4.

INTRODUCTION Electrochemical and chemical synthesis of heterocyclic conducting polymers has attracted significant interest in recent years [1]. These systems offer some advantages over polyenic and phenylene-based conducting polymers which are mainly associated with their improved environmental stability [2] and the feasibility of thin layer formation in order to modify electrode surfaces [3]. Electropolymerization of pyrrole in non-aqueous and aqueous electrolytes has been studied by several authors; however, the large majority of this work has been devoted to electrolyte systems containing common, monovalent or bivalent anions which are stable to oxidation [1]. The electrochemistry of such systems is limited to the redox reactions occurring in the polymer chains since the inserted anion is being neither oxidized nor reduced during the electrolysis. It is therefore interesting to introduce electroactive species between the polymer chains. Fe(CN)~-/Fe(CN)64- in an obvious choice for such studies. Some results concerning the polypyrrole iron cyanide complex anions system have already been reported. For example, Noufi et al. [4] have published cyclic voltammograms for polypyrrole films containing iron cyanides. The authors have not, however, carried out an spectroscopic studies of the

system since their goal was to study the possibility of the application of the polypyrrole film for the protection of semiconductor photoanodes. In the paper published by Daroux et al. [5] M6ssbauer spectra of polypyrrole-containing iron cyanide complex anions have been presented but no detailed M6ssbauer parameters have been reported. It is therefore evident that to date no systematic study of the preparation conditions and no detailed spectroscopic studies have been published for the polypyrrole-iron cyanide anions system. In our paper we discuss in detail the conditions of the synthesis and give elemental analysis data. In addition we report, for the first time, the calculated M6ssbauer parameters. In order to characterize better the obtained polymers we use additional spectroscopic methods like IR and EPR, which have not been used for the polypyrrole-iron cyanide system by previous authors. Finally, we report M6ssbauer and EPR studies of another polymer-containing M6ssbauer nucleus, namely the polypyrrole-iron chloride system. Electrochemically prepared polypyrrole containing iron chloride anions has previously been studied by Hahn et al. [6] using IR and Auger spectroscopy. However, no M6ssbauer and EPR data have been published to date.

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Table 1. Electrolyte compositions and electropolymerization conditions Sample 1 2 3 4

5

Electrolyte composition 0.15 M K4Fe(CN)6 0.01 M K3Fe(CN)6 0.05 M pyrrole 0.15 M K4Fe(CN)6 0.01 M K3Fe(CN)6 0.05 M pyrrole 0.20 M KNO3 0.01 M K3Fe(CN)~ 0.05 M pyrrole 0.20 M FeC13 0.20 M LiCI 0.05 M pyrrole nitromethane 0.10 M (Bu4N)3Fe(CN)6 0.05 M pyrrole nitromethane

Synthesis conditions (E vs S.C.E.) E~onst= 0.75 V Econst = 1.25 V Eco,,t = 0.70 V i~o,,t = 0.5 mA/cm2

iconst = 0.5 mA/cm2

EXPERIMENTAL

Pyrrole was polymerized either chemically or electrochemically from aqueous or non-aqueous solutions. Electrodepositions from deoxygenated aqueous electrolytes were carried out in a three-compartment cell with Pt foil as a working electrode, a Pt gauze as an auxiliary electrode and a saturated calomel electrode as a reference. For the electropolymerization in non-aqueous electrolytes standard high vacuum techniques were used. The electrochemical cell was equipped with two electrodes: Pt foil for the working and Pt gauze for the counter electrode. The exact conditions of electropolymerization are presented in Table 1. The electrolyte compositions used in the experiments were selected for the following reasons: (i) Macroscopically non-uniform films are obtained on a large electrode surface area if K4Fe(CN)6 is the sole ionic component of the electrolyte; (ii) In the case of K3Fe(CN)6, quick chemical polymerization occurs at salt concentrations higher than 0.01 M. The application of supporting electrolytes other than K 4Fe(CN)6 results in a decrease of Fe content in the samples studied probably due to co-insertion of iron-free anions.

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(iii) Of all the iron-free supporting electrolytes tested, KNO3 results in a relatively small decrease of iron content and therfore enables us to carry out the polymerization without the presence of the reduced form of iron in the solution. Chemical polymerization was achieved by mixing a 1.5 M solution of pyrrole in ethanol with a 0.15 M aqueous solution of K 3Fe(CN)6. Selected samples were subjected to elemental analysis. Fourier Transformed Infrared Spectra (FTIR) were recorded in air in pressed KBr pellets using a Bruker FTIR Spectrometer. For M6ssbauer effect (ME) studies, samples were transferred to the cryostat chamber in dry nitrogen and then the cryostat was evacuated. The absorber thickness was c a 0.1 mg/cm 2 of 57Fe isotope. The measurements were carried out in standard transmission geometry, in the temperature range 4.2-293 K. The spectra were recorded with statistics of several million counts per channel to become reliable even in the case of low absorption coefficients and then fitted with Lorentzian lines by the method of least-squares. ESR spectra were obtained on a Bruker X-band spectrometer and X-ray diffraction patterns were recorded on a DRON X-ray diffractometer at room temperature. RESULTS AND DISCUSSION The results of elemental analysis and conductivity measurements are given in Table 2. All samples contain significant amounts of oxygen. Since oxygen was determined by difference, its determination may included the errors of each independent determination. The chemical nature of oxygen is not clear, but it may originate from the solutions used for the polymerization. It is known that during oxidation, polypyrrole seems to preferentially incorporate some presently unidentified oxygen-based anions. It may also be associated with the formation of carbonyl or carbinol-type linkages in the polymer chain. Evidently the higher content of oxygen in sample 3 can be connected to the simultaneous incorporation of NO3 anions together with Fe(CN)~-.

Table 2. Composition from elemental analysis and conductivities of polypyrr01e films. The samples numbers of electrochemically prepared films correspond to the numbers in Table 1 Sample Chemically synthesized

Film composition

Conductivity (f~- icm - t )

C4N l.o2H3.fo[Fe(CN)6]o.oso Oo.537:~

3.0

C, No.99H3.so[Fe(CN)6]0.09000.697~ C4Nt.oIH3.64[Fe(CN)6]0.i0200.65 C4 NI,0tH3.2s[Fe(CN)6]0,070O i z27

1.0 1.0 1.0 7.5

Electrochemically synthesized 1

2 3

C4Nl.olH3.so(FeCls.t4)o.17Oo.5o t Traces of potassium were detected. Calculated assuming Fe(CN)~-. 4

Conducting polypyrrole-containing iron complexes

637

<

4000

3 5 0 0 3 0 0 0 2 5 0 0 2000 1800 1600 1400 1200 1000 800 600

Wavenumber (era-I) Fig I. IR spectra of conducting polypyrroles. (a) Chemically prepared polypyrrole; (b) electrochemically prepared polypyrrole in a solution containing 0.15 M K4Fe(CN) 6 and 0.01 M K 3Fe(CN)6 at E~o~t= 0.75 V vs S.C.E.; (c) electrochemically prepared polypyrrole in the same solution as (B), at Eco~t= 1.25 V vs S.C.E.; (d) electrochemically prepared polypyrrole in a solution containing 0.2 M FeCI3 and 0.2 M LiCI in nitromethane at i~o~,t= 0.5 mA/cm2. IR spectra of all polypyrroles studied exhibit features typical of other conducting polypyrroles, i.e. a decrease in absorption from 4000 to 1700 cm -~ [7, 8]. There are however some additional bands which were not observed previously (Fig. 1 (a)-(d)): a broad asymmetric peak at 3440 cm- ~ and peaks at 2920, 2026 and ca 1630-1640 cm-1, whereas in other oxidized polypyrroles featureless spectra between 4000 and 1600 cm -1 were recorded [7]. The mode at 2026cm -1 can be attributed to C ~ N stretching deformations in the inserted anions, since it matches the wavenumber range typically observed for this group in complex anions [9] and is absent in the spectra containing FeCI~- (cf. Fig. 1(b) and (d)). The absorption at 2920 cm -1 can be ascribed to a C - H stretching vibration in saturated carbon-ring systems. The existence of these weak peaks, corresponding to some degree of saturation of the polymer chain, may explain the lower conductivity of polypyrrole-iron cyanide as compared to polypyrroles synthesized previously [2] (Table 2). The presence of C - H stretching modes (ca 3100cm -1) characteristic of unsaturated systems is obscured by the tail of the broad 3440cm -1 peak. This latter peak may be associated either with N - H or O - H stretching deformations or with both. In neutral polypyrrole or pyrrole oligomers the maximum of the peak corresponding to N - H stretching is located around 3400cm -1 [7]. Since in our case we observed a shift

toward higher wavenumbers we postulate the presence of O - - H stretching modes associated either with carbinol-type defects or with water solvating the inserted anion (vide infra). The band in the range 1630-1640 cm-1 is within the region characteristic of C----C, C----O and ~ N . We favour ~ N for the following reasons: (i) C----O bands in polypyrrole-type systems are usually located at higher wavenumbers [8]; (ii) The extinction coefficient of the ~ N band is significantly higher than that of C----C, i.e. the band would be visible even for small concentrations of ~N defects; (iii) It is known from heterocyclic chemistry that pyrrole derivatives can be oxidized either with the use of KaFe(CN)6 [10] or electrochemically [11] to give monomeric or dimeric heterocycles containing ~N-groupings. The absorptions due to ~ N stretching observed in these compounds are within the range 1620-1640 cm-1, i.e. they match the wavenumbers of the absorption observed in polypyrrole. The band observed at ca 1630-1640cm -I disappears on exposure to air (Fig. 2(a) and (b)). Taking into account the high basicity of the > ~ N - grouping towards compounds providing protonic hydrogen, the explanation of the disappearance of this band shown in Fig. 3 seems plausible. In the proposed reaction (Fig. 3) the protonation of the

638

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1800

1600

1400

1200

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1000

800

600

(cm-t)

Fig. 2. IR spectra of chemicallypolymerized polypyrrole. (a) Fresh sample; (b) sample aged in air for 24 h. dehydrogenated pyrrole ring formed during the polymerization results in the formation of charged quinoid-like segments with the simultaneous disappearance of ~ N double bonds. To support this hypothesis ESR measurements were carried out. The ESR signal from polypyrrole polymerized chemically from KaFe(CN) 6 solution is very weak, comprising a single symmetrical line at g close to the free electron value and a line width AHpp = 1 G. No ESR signals from the iron were detected, implying that Fe(CN)~has been reduced to Fe(CN)~-, which is low-spin and diamagnetic, during the polymerization. Any residual Fe(CN)~- would have been observed as a very broad ESR signal around g = 2. The spectrum was recorded

I

at the same time as the IR spectrum to determine any correlation between the two. The ESR signal increased in intensity by about 50% during which time the IR band at 1630cm -~ completely disappeared. This 50% increase in the ESR signal is insignificant, since it is within the experimental error of repeated recordings of the ESR. This is because of the impossibility of replacing the sample exactly in the same position and orientation in the ESR cavity. Thus, the chemical process responsible for the disappearance of

I

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+ H+

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H

I

H

I

H

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--3

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H

I

H

I

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Fig. 3. Schematic explanation of the disappearance of the bands observed at c a 1630-1640cm -n in polypyrroles.

I

-2

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--1 +1 Velocity(nun/s)

I

+2

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+3

Fig. 4. Representative M6ssbauer spectra of non-dissolved electrolytes and polypyrroles obtained electrochemically. (a) Polypyrrole obtained from the electrolyte consisting of 0.1 M (Bu4N)3Fe(CN)6and 0.05 M pyrrole in nitromethane; (b) solid (Bu4N)3Fe(CN)6.

Conducting polypyrrole-eontaining iron complexes Table 3. M6ssbauer parameters of selected conducting polypyrroles prepared electrochemically and non-dissolved electrolyte salts used in the electropolymerization at 77 K (all isomer shifts (I.S.) are relative to natural iron at 300 K). The sample numbers of the polymers correspond to the

numbers in Table 1 I.S.

Sample 1 2 3 K 3Fe(CN)61" K4 Fe(CN)6 ? K4Fe(CN)~'3H20

(mm/s) +0.010 + 0.008 --0.008 + 0.015 +0.096 + 0.030 - 0.124 + 0.040, +0.070 _+ 0.009

Q.S.

(mm/s) 0.25 + 0.07 0.25 _ 0.04 0.26 + 0.09 0.469 0 0

t After Ref. [24]. :[:Measured at 138 K. the 1630 cm -t IR band does not result in the creation of spins. Polypyrrole prepared in a LiC1/FeC13 electrolyte gave a broad (AHpp = 980 G) ESR signal typical of high spin Fe(III) and very similar to that observed for polyacetylene-iron chloride [12] and polyparaphenylene-iron chloride [13]. M6ssbauer spectroscopy supports the ESR results and is in fair agreement with those reported previously by Daroux et al. [5]. Representative Mrssbauer spectra are shown in Fig. 4 and the M6ssbauer parameters obtained are collected in Table 3. The spectra of electrochemically prepared polypyrrole-iron cyanide complexes exhibit resonant lines attributable to only one iron site present in the system. The observed isomer shift values are close to zero as expected for all low-spin iron complexes. In addition, weak quadrupole interactions indicate that the iron is in the Fe(II) oxidation state, i.e. the Fe(CN) 4- entity is present in the polymer. In polypyrrole obtained by chemical oxidation, two nonequivalent iron sites are detected; the dominant one corresponding to low-spin Fe(II) (I.S. = - 0 . 0 0 4 , Q.S. = 0.290 mm/s at 78 K) and the second which is characteristic of low-spin Fe(III). It is not clear at the present time whether the lines corresponding to Fe(III) are associated with a second type of anion present in polypyrrole or with a side reaction product. The only high-spin iron complexes were detected in the polypyrrole-iron chloride system. The spectra are however complex. The dominant line with an I.S. =0.30mm/s is similar to that observed for polyacetylene-FeCl4 [14, 15] and poly(p-phenylene)FeC14 [16] and within the range typically observed for tetrahedral FeCI~-. A significantly larger quadrupole splitting (Q.S. = 0.43 mm/s) indicates a greater distortion of the FeCI~- tetrahedra in polypyrrole as compared to polyacetylene and poly(p-phenylene). In addition to the dominant feature, lines corresponding to other high-spin Fe species are present. Among them there exist lines which may be associated with degradation and/or contamination products

639

such as FeC12.nH20, as observed previously for partially degraded polyacetylene or partially degraded chemically polymerized polypyrrole [17]. The ESR spectrum supports this proposal. The resonant absorption increases significantly below room temperature, implying a rather low M6ssbauer lattice temperature. However, the expected 0M should be higher than in the case of other conducting polymers [18], The observation of low-spin Fe(II) with the M6ssbauer parameters of Fe(CN) 4- as the only type of iron in electrochemically prepared polypyrrole-iron cyanides is surprising. Given the potentials used for the electrolysis, no reduced form of iron is expected in the strongly oxidizing environment at the anode. However, it is always observed that even if the starting electrolyte contains only Fe(III) the resulting polymer exhibits the presence of low-spin Fe(II) (Fig. 4). It must therefore be postulated that a redistribution of charge takes place within the polymer after the anodic oxidation. This process occurs without any change of stoichiometry and involves the transfer of one electron from the polymer for each iron cyanide anion inserted. The main question to be answered is how the polymer ca i support the 4 - charge localized on each Fe(CN)~-. It has been suggested previously [5] that Fe(II) is inserted into polypyrrole in the form of KyFe(CN)~-~4-y) in order to lower the charge of the anion. Since elemental analysis shows that only traces of potassium are present in all samples of polypyrrole this hypothesis must be discarded. In addition, the M6ssbauer parameters of all electrochemically prepared polypyrrole--iron cyanides are essentially independent of the type of electrolyte used. If the co-insertion of some cations took place, one would expect different Mrssbauer spectra for the samples prepared in different electrolytes. The insertion of neutral electrolyte molecules is also highly improbable. To prove this hypothesis we have chosen tetrabutyloammonium hexacyanoferrate(III) [(Bu4N)3Fe(CN)6] as the electrolyte salt because it exhibits a significantly higher quadrupole splitting than other electrolyte salts investigated in the present study ( I . S . = + 0 . 0 2 2 m m / s , Q.S.=0.71mm/s). In case it is present in the deposited polymer it should be easily detected by ME spectroscopy. A comparison of the electrolyte salt spectrum with the resulting polymer spectrum shows no evidence of the presence of neutral reagent (Fig. 4). Moreover the resulting spectrum is very similar to the ones observed for polypyrrole obtained in different electrolytic systems (I.S. = 0.013 mm/s, Q.S. = 0.17 mm/s). Solvation is another possible way for additional charge screening. The hypothesis of solvation of Fe(CN)~- ions by solvent molecules cannot be unequivocally confirmed or discarded based on the analysis of ME spectra alone. Since in hydrated ferric cyanide, water does not belong to the first coordination sphere of the iron, its influence on the M6ss-

640

J. PRZYEUSKIet al.

bauer parameters is not very pronounced. The ME results obtained seem to favour the presence of hydration but given the possible error in and the small difference between the ME parameters of hydrated and non-hydrated anions (Table 3) this conclusion must be treated with caution. The X-ray diffraction pattern of ppy-Fe(CN)6 reveals the presence of a broad peak at 20 = 33 ° (FeK~ radiation) which corresponds to a d-spacing of about 3.4 A. It should be noted that this peak is hardly visible in polypyrrole--chloride obtained by chemical oxidation of pyrrole with F e C 1 3 . This well defined peak cannot be associated with the presence of a crystalline phase of electrolyte salt since a mechanical mixture of amorphous polypyrrole-chloride with K4Fe(CN)6.4H20 at concentrations corresponding to the Fe(CN) 4- doping level in the ppy-iron cyanide system does not give rise to any X-ray reflection in the vicinity of 33° . Fe(CN)64- is not the only tetravalent anion which can be inserted into the polypyrrole. Recently, Vellazques-Rosenthal et aL [19] reported the synthesis of polypyrrole containing tetrasulphonated cyanine anions of charge 4 - . Similarly as in the case of ppy-Fe(CN)6 the above polymer also exhibits a higher degree of crystallinity than polypyrrole containing monovalent anions. Evidently the introduction of polyvalent anions induces a better structural order in polypyrrole. CONCLUSIONS To summarize, we have studied conducting polypyrroles containing low- and high-spin iron complexes, namely iron cyanides and iron chlorides. It has been shown that the polypyrrole structure strongly favours the present of Fe(CN) 4- , since by ME studies, this complex anion was detected as the only iron-containing species in all the polypyrroles synthesized electrochemically, independently of the oxidation state of iron cyanides present in the electrolyte solutions. Similarly, in chemically polymerized polypyrroles, Fe(CN) 4- seems to be more ordered in comparison to polypyrrole containing monovalent anions as deduced from X-ray diffraction studies. However, the degree of crystallinity is still very low. High-spin iron chloride species, with M6ssbauer parameters characteristic of slightly distorted FeCI~-

tetrahedral anions, can be inserted into polypyrrole during electrochemical oxidation of pyrrole in nonaqueous electrolytes containing LiC1/FeC13.

Acknowledgements--One of the authors (A.P) wishes to

thank Dr. Joanna Glowczyk for helpful discussions and Dr. Patrick Bernier for the use of the ESR equipment.

REFERENCES

1. See e.g. Proceedings of the International Conference on Synthetic Metals, Mol. Cryst. Liq. Cryst., 117-121, (1985). 2. Kanazawa K. K., Diaz A. F., Geiss R. H., Gill W. D., Kwak J. F., Logan J. A., Rabolt J. F. and Street G. B., J. chem. Soc., Chem. Commun. 17, 854 (1979). 3. Diaz A. F., Kanazawa K. K. and Gardini G. P., J. chem. Soc., Chem. Commun. 14, 635 (1979). 4. Noufi R., Tench D. and Warren L. F. J. electrochem. Soc. 128, 2596 (1981). 5. Daroux M., Gerdes H., Scherson D., Eldridge J., Kordesch M. E. and Hoffman R. W., The Electrochemical Society Extended Abstracts, Vol. 83-1, San Francisco CA, May 8-13, 1983, Abstract 548, p. 829. 6. Hahn S. J., Gajda W. J., Vogelhut P. O. and Zeller, W. V., Synth. Met. 14, 89 (1986). 7. Street G. B., Clarke T. C., Krounbi M., Kanazawa K. K., Lee V., Pfluger P., Scott J. C. and Weiser G., Mol. Cryst. Liq. Cryst. 83, 253 (1982). 8. Hyodo K. and MacDiarmid A. G., Synth. Met. 11, 167 (1985). 9. Horak M. and Papousek D., Infracervena Spectra a Structura Molekul, p. 534. Academia, Praha (1976). 10. Rio G., Ranjon A., Pouchout U. and Scholl M-J., Bull. Soc. Chim. Ft. No. 5, 1667 (1969). 11. Libert M., Caullet C. and Longchamp S., Bull. Soc. Chim. Ft. No. 6, 1667 (1971). 12. Pron A., Bernier P., Billaud and Lefrant S., Solid St. Commun. 46, 587 (1983). 13. Kuivalainen P., Stubb H., Raatikainen P. and Holmstrom C., J. Phys. Col. C-3, 44, 757 (1983). 14. Pron A., Zagorska M., Kucharski Z., Lukasiak M. and Suwalski J., Mater. Res. Bull. 17, 1505 (1982). 15. Przyluski J., Zagorska M., Conder K. and Pron A., Polymer 23, 1872 (1982). 16. Pron A., Fatseas G. A., Krichene S., Lefrant S., Maurice F. and Froyer G., Phys. Rev. B32, 1839(1985). 17. Pron. A., Kucharski Z., Budrowski C., Zagorska M., Krichene S., Suwalski J., Dehe G. and Lefrant S., J. chem. Phys. 83, 5923 (1985). 18. Kucharski Z., Pron A., Suwalski J., Kulszewicz I., Billaud D. and Bernier P., Solid St. Commun. 50, 397 (1984). 19. Velazques Rosenthal M., Skotheim T. A. and Linkous C. A., Synth. Met. 15, 219 (1986).