- Email: [email protected]
Convenient synthesis of polybispyrrole system
Synthetic Metals 99 Ž1999. 181–189
Review
Convenient synthesis of polybispyrrole system Jadwiga Sołoducho
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Wrocław UniÕersity of Technology, Institute of Organic Chemistry, Biochemistry and Biotechnology, Wybrzeze 27, Wrocław, Poland ˙ Wyspianskiego ´ Dedicated to: Professor A.R. Katritzky in celebration of distinguished teaching and research Received 2 September 1998; revised 28 October 1998; accepted 24 November 1998
Abstract This review on polybispyrrole chemistry summarizes the recent progress in the chemistry of most attractive conducting polymers. Some details of the unique synthesis of bisŽpyrrole.arylenes, bisŽpyrrole.carbazoles, bisŽpyrrole.fluorenes, bisŽpyrrole.naphthalenes and their oxidative polymerization and electronic structures are also discussed. q 1999 Elsevier Science S.A. All rights reserved. Keywords: BisŽpyrrole.arylenes; BisŽpyrrole.carbazole; BisŽpyrrole.fluorene; Oxidative polymerization; Electronic structure
1. Introduction In recent years, polypyrrole and its derivatives have been widely investigated because of their easy electrosynthesis, good stability and excellent conductivity in the oxidized state w1x. The electropolymerization of pyrrole has made a significant impact in the synthesis of stable, highly conducting polymers via a relatively low oxidation potential monomer. Pyrrole oxidizes at significantly lower potentials than that of thiophene or furan, allowing for fewer defects in the polymer formed. The polymers redox processes are generally found between y0.5 and 0.0 V vs. SCE. With this low oxidation potential to form the conducting state, the polymer is quite stable as a conductor and can be subjected to a high number of redox switches with little degradation in charge response w2x. These properties suggest that polypyrrole may prove useful in a number of practical applications which include areas as diverse as solid cell lithium batteries, biocompatible electrodes and conducting textiles w3x. The electrochemical synthesis of conducting polymers from multi-ring aromatic monomers with electron-rich terminal heterocycles has attracted considerable attention as they have significantly lower oxidation potentials than the corresponding parent heterocycle due to the extended conjugation of the multi-ring system w4x. The presence of
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electron-rich heterocycles we.g., pyrrole, 3,4-Žethylenedioxy.thiophenex as terminal electropolymerizable moieties at the multi-ring conjugated monomers leads to stabilization of the cation radical intermediates allowing the electropolymerization to proceed at low potentials and with a minimum of side reactions w5x. These include b-coupling, cross-linking, and overoxidation of the resulting polymer leading to polymer degradation and defectcontaining materials w6x. It is well-known that the electronic and electrochemical properties of the polymer depend strongly on the molecular structure as has been demonstrated for several substituted derivatives of polyŽ p-phenylene. w7x, polypyrrole w8x, and polythiophene w9,10x and a series of polyw1,4-bisŽ2-heterocycle.-2,5-disubstituted phenylenesx w11,12x. The introduction of the alkyl chains has also been useful in inducing fusibility into polyŽ3-alkylthiophenes. w13x and polyŽ9,9-dialkylfluorenes. w14x.
2. Synthesis of aryl-bispyrroles The syntheses of 1,4-bisŽ2-thienyl.benzenes and 1,4bisŽ2-furanyl.benzenes were accomplished by reaction of 1,4-dihalogenated benzene with a 2-metallated heterocycle w15,16x. Such reaction is not possible with pyrrole due to the reactive N–H bond and it led us to investigate a series of protected pyrroles. The use of the vinyl, pirydyl, benzenesulfonyl, trimethylsilyl, methoxymethyl, t-butoxycarbonyl, and 2-Žtrimethylsilyl.etoxymethyl groups was
0379-6779r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 8 . 0 1 4 9 3 - 3
J. Sołoduchor Synthetic Metals 99 (1999) 181–189
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Scheme 1.
unsuccessful. The metallated pyrroles either did not couple with bromobenzenes, or the protecting group was cleaved from the pyrrole during reaction. Successful coupling of protected pyrroles was accomplished by the use of the dimethylamino protecting group as outlined in Scheme 1. The protected metallated pyrrole was obtained by reacting 1-Ždimethylamino.pyrrole Ž1. with butyllithium followed by treatment with ZnCl 2 to give zinc derivative 2. Successful coupling of 2 was accomplished with bromobenzene or 1,4-dibromobenzene using PdŽ0. catalyst to form 3 and 5, respectively. Deprotection of 3 to form
2-phenylpyrrole Ž4. was carried out by hydrogenation over Raney nickel. Identical deprotection conditions were attempted in the preparation of 1,4-bisŽpyrrol-2-yl.benzene and found to be unsuccessful. Other reductive conditions, including Raney nickel hydrogenation in THF and base Žalso at elevated temperatures and pressures., along with chromous acetate reduction w17x were attempted and provided no protecting group removal. Electrochemical studies of 4 indicated that, while it was relatively easy to oxidize, there was no evidence of polymerization to form a conducting polymer. Engel and Steglich w18x reported facile synthesis of 2-aryl and 2-heteroarylpyrroles from N-allylcarboxamides. The aryl-bis-pyrroles 6e–11e were prepared by modification of the method of Engel and Steglich in accordance with the general pathway set out in Scheme 2. Corresponding acid chlorides 6b–11b were reacted with allylamine to give aryl bisŽallylamides. 6c–11c in high yields. Subsequent treatment with phosgene furnished the aryl bisŽallylimino chlorides. 6d–11d, unstable intermediates, which were used without isolation or purification for cyclization under basic conditions to form 1,4-bisŽpyrrol-2-yl.benzene Ž6e, BPB., 1,4-bisŽpyrrol-2-yl.-2,5-dimethoxybenzene Ž7e, BPBŽOCH 3 . 2 ., 1,4-bisŽpyrrol-2-yl.-2,5diethoxybenzene Ž8e, PBBŽOC 2 H 5 . 2 ., 1,4-bisŽpyrrol-2yl.-2,5-didodecyloxybenzene Ž9e, BPBŽOC 12 H 25 . 2 ., 2,6bisŽpyrrol-2-yl.naphthalene Ž10e, BPN., or 4,4X-bisŽpyrrol2-yl.biphenyl Ž11e, BPBP. in moderate yields. The cyclization to the pyrrole ring can be interpreted in terms of the 1,5-dipolar ring closure of the nitrile ylide Ž12. w19x formed by elimination of HCl from Ž6d.. The primary product Ž13. subsequently isomerized to 6e w20x. In the case of N-allylimidochlorides of the aliphatic acids, bases promote elimination of HCl to give the ketenimines,
Scheme 2.
J. Sołoduchor Synthetic Metals 99 (1999) 181–189
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Scheme 3.
which subsequently rearrange into g,d-unsaturated nitriles w21x. This novel method for the synthesis of bis-pyrroles is therefore limited to allylamides of carboxylic acids which do not contain an H atom in the a-position ŽScheme 3..
4.2. Method B We also tried to obtain this compound by a different route, similar to method of synthesis of thiophene bis-pyrrole and phenylene bis-pyrrole reported by Lucchesini w25x.
3. Synthesis of 3,6-bis(pyrrol-2-yl)-9-ethylcarbazole (20) The synthesis of 3,6-bisŽpyrrol-2-yl.-9-ethylcarbazole 20 w22x is depicted in Scheme 4. This procedure is similar to the method we reported previously for the synthesis of aryl-bispyrroles w5x. The first step involves the Lewis acid-catalyzed amidation of N-ethylcarbazole Ž14) to give 3,6-bisŽdimethylcarbamido.9-ethylcarbazole Ž15). Then diamide Ž15. was hydrolyzed to the diacid Ž16., which was converted into the diacid chloride Ž17. by the use of SOCl 2 . Compound 17 was then reacted with allylamine to give the bisŽallylamide. 18. Subsequent treatment of 18 with phosgene gave carbazole bisŽallyliminochloride. Ž19.. Ring closure of 19 was carried out under basic conditions to give 3,6-bisŽpyrrol-2-yl.9-ethylcarbazole Ž20, BP-NEC..
4. Synthesis of 9,9-diethyl-2,7-bis(pyrrol-2-yl)fluorene (24) 4.1. Method A We also will report synthesis of 9,9-diethyl-2,7-bisŽpyrrol-2-yl.fluorene Ž24. w23x ŽScheme 5. from 9,9-diethylfluorene Ž21.. 9,9-Diethylfluorene was converted into 2,7dibromo-9,9-diethylfluorene Ž22. by treatment with bromine in the dark. CopperŽI. cyanide converted dibrom odiethylfluorene Ž 22 . into 9,9-diethyl-2,7-dicyanofluorene. 9,9-Diethyl-2,7-fluorenedicarboxylic acid Ž23. was obtained by heating dinitrile with phosphoric acid. The synthetic procedure from compound 23 into 24 is similar to our unique method w24x.
Scheme 4.
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5. Synthesis of indolo[7,6-g]indole derivative (33)
Scheme 5.
For synthesis of desired 5,10-dibutoxy-indolow7,6-g xindole Ž33. ŽScheme 7., we started from 1,5-dihydroxynaphthalene Ž27. which was alkylated by typical procedure w24x to give 1,5-dibutoxynaphthalene Ž28.. Compound 28 after nitration with fuming nitric acid in acetic acid gave the corresponding dinitronaphthalene Ž29.. Reduction of 29 with metallic Sn gave 4,8-diamino1,5-dibutoxynaphthalene Ž30.. Iodination of compound 30 by I 2 gave 4,8-diamino1,5-dibutoxy-3,7-diiodonaphthalene Ž31.. Compound 31 was reacted with TMSA and catalytic amounts of biswtriphenylphosphinexpalladium dichloride and copperŽI. iodide to give bis-Žtrimethylsilyldiamino.naphthalene Ž32.. The addition of two equivalents of CuI promotes heteroannulation of 32, providing an easy entry into ‘‘bisŽpyrrolo.naphthalene’’ Ž33..
We hoped that this synthesis will be convenient and alternative method for the preparation of 9,9-diethyl-2,7bisŽpyrrol-2-yl.fluorene Ž24. from 9,9-diethyl-2,7-dibromofluorene Ž22. ŽScheme 6.. In an alternative approach ŽScheme 6., 9,9-diethyl-2,7dibromofluorene Ž22. was reacted with n-BuLi and the product quench with aldehyde Ž25. to give carbinol 26. Compound 26 was directly oxidized to ketone with pyridinium chloroformate ŽPCC., and than directly cyclized by reaction with ammonium acetate to afford bis-pyrrole Ž24.. The yield of desired bis-pyrrole Ž24. in the method A was high Ž80%., but in method B low Ž25%., so we found that the use of the Method A is definitely more advantageous. More details about synthesis of 9,9-diethyl-2,7bisŽpyrrol-2-yl.fluorene Ž24. will be published in a forthcoming paper.
Scheme 6.
Scheme 7.
J. Sołoduchor Synthetic Metals 99 (1999) 181–189
185
Scheme 8.
Synthesis of 5,10-dibutoxyindolow7,6-g xindole Ž33. will be published in a forthcoming paper w26x.
6. Oxidative polymerization Scheme 8 shows the oxidative polymerization of described bisŽpyrrol-2-yl.arylenes and the subsequent reversible redox process of the as-made doped polymers using BPB Ž6e. as an example. The oxidative polymerization of bisŽpyrrol-2-yl.arylenes proceeds probably according to an electrochemically activated step-growth mechanism with the initial formation of a monomer cation radical followed by coupling polymerization. Multiple-scan cyclic voltammetric polymerization of the bisŽpyrrole. monomers was carried out to determine monomer oxidation onset and peak potentials, along with monitoring the rate of growth of the electroactive polymer films. The cyclic voltammetric polymerization of BPB Ž6e. as an example, is shown in Fig. 1. The monomer oxidation at the bare electrode begins at q0.25 V and peaks Ž Ep,m . at q0.35 V vs. AgrAgq. This is a very low monomer oxidation potential when compared to other electropolymerizable monomers. In comparison, pyrrole Ž Ep,m s q0.9 V., 3-methylthiophene Ž Ep,m s q1.5 V., 1,4-bisŽ2thienyl.benzene Ž Ep,m s q0.9 V., and 1,4-bisŽ2-thienyl.2,5-dimethoxybenzene Ž Ep,m s q0.7 V. all exhibit higher oxidation potentials. With repeated scanning, a reversible redox process quickly grows in width with an E1r2 at 0.0 V and a narrow peak to peak separation. It is evident that the polymer redox process develops quite rapidly and that the monomer polymerizes very efficiently to form a highly electroactive polymer. A comparison with a 10 mM solu-
tion of pyrrole, polymerized under similar scanning conditions, indicates that this monomer polymerizes about 40 times faster than pyrrole itself. The cyclic voltammetric polymerization of BPBŽOCH 3 . 2 Ž7e. is shown in Fig. 2 and is representative of the other monomers which were successfully electrochemically polymerized. In each case, during the first anodic scan, the monomer exhibited a low potential current onset
Fig. 1. Multiple scan electropolymerization of 10 mM 1,4-bisŽpyrrol-2yl.benzene Ž6e. in 0.1 M TBAPrCH 3 CN.
J. Sołoduchor Synthetic Metals 99 (1999) 181–189
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Fig. 2. Cyclic voltammetric scanning electropolymerization of 0.01 M BPBŽOCH 3 . 2 Ž7e. in 0.1 M TBAPrCH 3 CN cycled at 100 mVrs.
followed by a single peak due to the formation of the monomer radical cation as shown in Table 1. Fig. 3 illustrates the cyclic voltammetry of PBPBŽOCH 3 . 2 at various scan rates and is representative of all of the electroactive polymers. As can be seen, the potentials of the anodic and cathodic processes of the polymer film are relatively close Žabout 100 mV. indicating a reversible redox process. Both anodic and cathodic peak currents are linearly proportional to the scan rate, which is indicative of an electrode-supported electroactive film. Fig. 4 shows the cyclic voltammetric growth during electrosynthesis of the polymer from the oxidation of BPN. The monomer oxidation at the bare platinum electrode begins at q0.3 V vs. AgrAgq and peaks at Ž Ep,m . at 0.35 V. For comparative purposes, BPB has a monomer oxidation onset of q0.27 V vs. AgrAgq. Fig. 5 shows the repeated potential scan polymerization for 3,6-bisŽpyrrol-2-yl.-9-ethylcarbazole ŽBP-NEC.. The polymerization begins at Eonset,m of 0.09 V with Ep,m at
Fig. 3. Scan rate dependence of the cyclic voltammetry of PBPBŽOCH 3 . 2 in 0.1 M TBAPrCH 3 CN cycled at Ža. 50, Žb. 100, Žc. 150, Žd. 200, Že. 250. and Žf. 300 mVrs.
0.15 V. This bisheterocyclic carbazole electropolymerizes at much lower potentials and quite rapidly relative to the single-ring heterocyclic monomers, pyrrole and carbazole.
Table 1 Electrochemical properties for pyrrolearylene monomers and their polymers Monomer
Eonset,m ŽV.
Ep,m ŽV.
E1r2,p ŽV.
Eg ŽV.
BPB BPBŽOCH 3 . 2 BPBŽOC 12 H 25 . 2 BPN BPBP
0.30 0.12 0.13 0.30 0.40
0.35 0.15 0.17 0.35 0.45
y0.10 y0.05 y0.05 0.05 0.10
2.4 2.3 nra 2.3 nra
Fig. 4. Cyclic voltammetric polymerization of polyŽ2,7 bisŽpyrrol-2yl.naphthalene carried out in 10 mM BPNr0.1 M TBAPrCH 3 CN.
J. Sołoduchor Synthetic Metals 99 (1999) 181–189
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Fig. 7. Optoelectrochemical analysis of polyw1,4-bisŽpyrrol-2-yl.-p-phenylenex in 0.1 M TBAPrCH 3 CN. Spectra were run with polymer held at Ža. y0.8, Žb. y0.4, Žc. y0.0, Žd. q0.2, and Že. q0.5 V vs. Ag wire.
Fig. 5. Repeated potential scanning polymerization of 0.01 M BP-NEC Ž20. in 0.1 M TBAPrCH 3 CN at a scan rate of 100 mVrs.
For comparison purposes, the repeated potential scanning polymerization of carbazole is illustrated in Fig. 6. This monomer exhibits Ep,m at 0.93 V, 0.5–0.8 V higher than that for the pyrrole analogues. A series of electrochemical studies was carried out and confirmed the bipolaronic nature of the charge carriers as illustrated in Fig. 7. The neutral form of the polymer shows a distinct p to p ) transition with a band-gap onset of 2.4 eV Ž516 nm. and a peak at 2.7–2.8 eV Ž440–460 nm.. It is interesting that this is lower than the values for either polypyrrole or polyŽ p-phenylene.. This can be attributed to the highly delocalized nature of the neutral polymer. Stepwise oxidation of the polymer shows the
growth of bipolaron bands with peaks evolving at approximately 1 and 2.2 eV as the polymer dopes. Table 2 gives a compilation of the Eonset,m and Ep,m , BP-NEC Žcontaining unsubstituted external pyrrole rings. exhibits an Ep,m that is 0.26 V lower than that for 3,6bisŽ N-methylpyrrol-2-yl. N-ethylcarbazole, previously prepared and reported by Geibler and Hallnsleben w27x. This can be explained by steric hindrance induced between the pyrrole and carbazole moieties by the methyl groups.
7. Electronic structure Optoelectrochemical methods were used to elucidate the effect of the structure of the aryl core unit Ž p-phenylene and 2,6-naphthyl. and pendant substituents on the energy of the p electrons. Polymer films used for optoelectrochemical analysis were prepared potentiostatistically at the Ep,m of the monomer on ITO-coated glass plates and their spectra were obtained in a quartz cuvette containing electrolyte solution and counter and reference electrodes. The polymers were first fully reduced and the electronic spectrum Ž310–1600 nm. was obtained. The polymers were subsequently subjected to increasing potentials in 100 mV steps, and the spectrum was obtained at each potential after equilibration. The reversible nature of the redox doping process was confirmed by a similar stepwise reduction. An optoelectrochemical series for the dimethoxy-substituted polymer is shown in Fig. 8 and is similar to PBPB–ClO4 and PBPN–ClO4 . The optical band gap for each polymer was obtained from the onset of the p–p ) transition. The
Table 2 Electrochemical properties for carbazole-based monomers and their polymers Monomer
a Eonset,m
a Ep,m
a E1r2,p1
a E1r2,p2
Egb
BP-NEC c Carbazole
0.09 0.88
0.15 0.95
0.02 0.65
0.53 0.91
2.5 nra
a
Fig. 6. Repeated potential scanning polymerization of 0.01 M carbazole in 0.1 M TBAPrCH 3 CN at a scan rate of 100 mVrs.
Values listed in volts vs. AgrAgq reference electrode. Values listed in electronvolts as the onset of p – p ) transition. c Performed in 0.1 M TBAPrCH 3 CN, 10 mM monomer. b
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Fig. 8. Optoelectrochemical analysis of PBPBŽOCH 3 . 2 performed in 0.1 M TBAPrCH 3 CN. UV–visible–near-IR spectra taken at Ža. y0.6, Žb. y0.3, Žc. y0.2, Žd. y0.1, Že. 0.0, Žf. q0.1, Žg. q0.2, Žh. q0.3, and Ži. 0.4 V vs. AgrAgq.
unsubstituted BPB exhibits a band gap of 2.4 eV, which is lower than that of polypyrrole Ž2.7 eV. or polyŽ p-phenylene. Ž3.0 eV.. The band gap of PBPBŽOCH 3 . 2 is reduced further to 2.3 eV. We attribute this lowering of the band gap relative to polypyrrole to the increased structural regularity induced by the use of the three-ring-containing bisŽpyrrol-2-yl.benzene monomer. Additionally, the oxygen atoms of the methoxy substituents in PBPBŽOCH 3 . 2 are expected to form a hydrogen bond with the pyrrole hydrogen, causing the repeat unit to attain planarity more easily than for unsubstituted BPB. This compound was calculated to have its lowest energy between a torsional angle of 08 and 158, which increased only for 1 kcalrmol at 308. The optoelectrochemical study for the polymer grown from BPN and switched in 0.1 M TBAPrCH 3 CN is illustrated in Fig. 9. The Eg was found to be 2.3 eV with the growth of the bipolaronic bands upon doping evolving at approximately 2 eV and 1 eV.
Fig. 10. Long-term double-potential-stepping charge retention for PBPB and PBPBŽOCH 3 . 2 performed under an argon blanket in 0.1 M TBAPrCH 3 CN. Twenty seconds constitutes a double-potential step.
they must sustain a number of sequential oxidative and reductive processes while retaining their electroactivity w2,28x. Multiple redox switching experiments were carried out on PBPB–ClO4 and PBPBŽOCH 3 . 2 –ClO4 , prepared via constant-potential electropolymerization at the Ep,m until 2 mC of charge passed Ž400 mCrcm2 . and subsequently switched in monomer-free 0.1 M TBAPrCH 3 CN electrolyte. The films were first redox ‘broken in’ by repeated scanning to obtain a constant CV current response. Double potential steps were carried out by holding the potential at Ep,a q 0.15 V for 10 s, stepping to Ep,c y 0.15 V for 10 s, and then returning to the original potential. Fig. 10 shows the long-term double-potential-stepping results for the above polymers. After 2300 double potential steps, PBPB retained 50% of its original charge. PBPBŽOCH 3 . 2 was able to maintain 80% of its original charge after 3000 double-potential steps and 50% electroactivity after 6000 double-potential steps. For comparison purposes, polyŽpyrrole chloride., redox switched in 0.1 M LiClO4 Žaqueous., exhibits 65% retention after 900 double-potential steps.
8. Long-term redox switching stability In order for these polymers to be useful in practical devices where they are switched between charge states,
9. Concluding remarks As mentioned at the beginning, I tried to show the interested reader the fantastic diversity or the polybispyrrole chemistry. The synthetic methodology used to prepare this monomer and polymer opens up numerous pathway for the synthesis of derivatized polybispyrroles and is the subject of continued investigation.
References
Fig. 9. Optoelectrochemistry of polyŽ2,7-bisŽpyrrol-2-yl.naphthalene coated on an ITO coated quatz plate. Spectra taken at Ža. y1.0 V, Žb. y0.2 V, Žc. 0.0 V, Žd. 0.2 V, and Že. 0.4 V vs. AgrAgq.
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Review
Convenient synthesis of polybispyrrole system Jadwiga Sołoducho
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Wrocław UniÕersity of Technology, Institute of Organic Chemistry, Biochemistry and Biotechnology, Wybrzeze 27, Wrocław, Poland ˙ Wyspianskiego ´ Dedicated to: Professor A.R. Katritzky in celebration of distinguished teaching and research Received 2 September 1998; revised 28 October 1998; accepted 24 November 1998
Abstract This review on polybispyrrole chemistry summarizes the recent progress in the chemistry of most attractive conducting polymers. Some details of the unique synthesis of bisŽpyrrole.arylenes, bisŽpyrrole.carbazoles, bisŽpyrrole.fluorenes, bisŽpyrrole.naphthalenes and their oxidative polymerization and electronic structures are also discussed. q 1999 Elsevier Science S.A. All rights reserved. Keywords: BisŽpyrrole.arylenes; BisŽpyrrole.carbazole; BisŽpyrrole.fluorene; Oxidative polymerization; Electronic structure
1. Introduction In recent years, polypyrrole and its derivatives have been widely investigated because of their easy electrosynthesis, good stability and excellent conductivity in the oxidized state w1x. The electropolymerization of pyrrole has made a significant impact in the synthesis of stable, highly conducting polymers via a relatively low oxidation potential monomer. Pyrrole oxidizes at significantly lower potentials than that of thiophene or furan, allowing for fewer defects in the polymer formed. The polymers redox processes are generally found between y0.5 and 0.0 V vs. SCE. With this low oxidation potential to form the conducting state, the polymer is quite stable as a conductor and can be subjected to a high number of redox switches with little degradation in charge response w2x. These properties suggest that polypyrrole may prove useful in a number of practical applications which include areas as diverse as solid cell lithium batteries, biocompatible electrodes and conducting textiles w3x. The electrochemical synthesis of conducting polymers from multi-ring aromatic monomers with electron-rich terminal heterocycles has attracted considerable attention as they have significantly lower oxidation potentials than the corresponding parent heterocycle due to the extended conjugation of the multi-ring system w4x. The presence of
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electron-rich heterocycles we.g., pyrrole, 3,4-Žethylenedioxy.thiophenex as terminal electropolymerizable moieties at the multi-ring conjugated monomers leads to stabilization of the cation radical intermediates allowing the electropolymerization to proceed at low potentials and with a minimum of side reactions w5x. These include b-coupling, cross-linking, and overoxidation of the resulting polymer leading to polymer degradation and defectcontaining materials w6x. It is well-known that the electronic and electrochemical properties of the polymer depend strongly on the molecular structure as has been demonstrated for several substituted derivatives of polyŽ p-phenylene. w7x, polypyrrole w8x, and polythiophene w9,10x and a series of polyw1,4-bisŽ2-heterocycle.-2,5-disubstituted phenylenesx w11,12x. The introduction of the alkyl chains has also been useful in inducing fusibility into polyŽ3-alkylthiophenes. w13x and polyŽ9,9-dialkylfluorenes. w14x.
2. Synthesis of aryl-bispyrroles The syntheses of 1,4-bisŽ2-thienyl.benzenes and 1,4bisŽ2-furanyl.benzenes were accomplished by reaction of 1,4-dihalogenated benzene with a 2-metallated heterocycle w15,16x. Such reaction is not possible with pyrrole due to the reactive N–H bond and it led us to investigate a series of protected pyrroles. The use of the vinyl, pirydyl, benzenesulfonyl, trimethylsilyl, methoxymethyl, t-butoxycarbonyl, and 2-Žtrimethylsilyl.etoxymethyl groups was
0379-6779r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 8 . 0 1 4 9 3 - 3
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Scheme 1.
unsuccessful. The metallated pyrroles either did not couple with bromobenzenes, or the protecting group was cleaved from the pyrrole during reaction. Successful coupling of protected pyrroles was accomplished by the use of the dimethylamino protecting group as outlined in Scheme 1. The protected metallated pyrrole was obtained by reacting 1-Ždimethylamino.pyrrole Ž1. with butyllithium followed by treatment with ZnCl 2 to give zinc derivative 2. Successful coupling of 2 was accomplished with bromobenzene or 1,4-dibromobenzene using PdŽ0. catalyst to form 3 and 5, respectively. Deprotection of 3 to form
2-phenylpyrrole Ž4. was carried out by hydrogenation over Raney nickel. Identical deprotection conditions were attempted in the preparation of 1,4-bisŽpyrrol-2-yl.benzene and found to be unsuccessful. Other reductive conditions, including Raney nickel hydrogenation in THF and base Žalso at elevated temperatures and pressures., along with chromous acetate reduction w17x were attempted and provided no protecting group removal. Electrochemical studies of 4 indicated that, while it was relatively easy to oxidize, there was no evidence of polymerization to form a conducting polymer. Engel and Steglich w18x reported facile synthesis of 2-aryl and 2-heteroarylpyrroles from N-allylcarboxamides. The aryl-bis-pyrroles 6e–11e were prepared by modification of the method of Engel and Steglich in accordance with the general pathway set out in Scheme 2. Corresponding acid chlorides 6b–11b were reacted with allylamine to give aryl bisŽallylamides. 6c–11c in high yields. Subsequent treatment with phosgene furnished the aryl bisŽallylimino chlorides. 6d–11d, unstable intermediates, which were used without isolation or purification for cyclization under basic conditions to form 1,4-bisŽpyrrol-2-yl.benzene Ž6e, BPB., 1,4-bisŽpyrrol-2-yl.-2,5-dimethoxybenzene Ž7e, BPBŽOCH 3 . 2 ., 1,4-bisŽpyrrol-2-yl.-2,5diethoxybenzene Ž8e, PBBŽOC 2 H 5 . 2 ., 1,4-bisŽpyrrol-2yl.-2,5-didodecyloxybenzene Ž9e, BPBŽOC 12 H 25 . 2 ., 2,6bisŽpyrrol-2-yl.naphthalene Ž10e, BPN., or 4,4X-bisŽpyrrol2-yl.biphenyl Ž11e, BPBP. in moderate yields. The cyclization to the pyrrole ring can be interpreted in terms of the 1,5-dipolar ring closure of the nitrile ylide Ž12. w19x formed by elimination of HCl from Ž6d.. The primary product Ž13. subsequently isomerized to 6e w20x. In the case of N-allylimidochlorides of the aliphatic acids, bases promote elimination of HCl to give the ketenimines,
Scheme 2.
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183
Scheme 3.
which subsequently rearrange into g,d-unsaturated nitriles w21x. This novel method for the synthesis of bis-pyrroles is therefore limited to allylamides of carboxylic acids which do not contain an H atom in the a-position ŽScheme 3..
4.2. Method B We also tried to obtain this compound by a different route, similar to method of synthesis of thiophene bis-pyrrole and phenylene bis-pyrrole reported by Lucchesini w25x.
3. Synthesis of 3,6-bis(pyrrol-2-yl)-9-ethylcarbazole (20) The synthesis of 3,6-bisŽpyrrol-2-yl.-9-ethylcarbazole 20 w22x is depicted in Scheme 4. This procedure is similar to the method we reported previously for the synthesis of aryl-bispyrroles w5x. The first step involves the Lewis acid-catalyzed amidation of N-ethylcarbazole Ž14) to give 3,6-bisŽdimethylcarbamido.9-ethylcarbazole Ž15). Then diamide Ž15. was hydrolyzed to the diacid Ž16., which was converted into the diacid chloride Ž17. by the use of SOCl 2 . Compound 17 was then reacted with allylamine to give the bisŽallylamide. 18. Subsequent treatment of 18 with phosgene gave carbazole bisŽallyliminochloride. Ž19.. Ring closure of 19 was carried out under basic conditions to give 3,6-bisŽpyrrol-2-yl.9-ethylcarbazole Ž20, BP-NEC..
4. Synthesis of 9,9-diethyl-2,7-bis(pyrrol-2-yl)fluorene (24) 4.1. Method A We also will report synthesis of 9,9-diethyl-2,7-bisŽpyrrol-2-yl.fluorene Ž24. w23x ŽScheme 5. from 9,9-diethylfluorene Ž21.. 9,9-Diethylfluorene was converted into 2,7dibromo-9,9-diethylfluorene Ž22. by treatment with bromine in the dark. CopperŽI. cyanide converted dibrom odiethylfluorene Ž 22 . into 9,9-diethyl-2,7-dicyanofluorene. 9,9-Diethyl-2,7-fluorenedicarboxylic acid Ž23. was obtained by heating dinitrile with phosphoric acid. The synthetic procedure from compound 23 into 24 is similar to our unique method w24x.
Scheme 4.
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5. Synthesis of indolo[7,6-g]indole derivative (33)
Scheme 5.
For synthesis of desired 5,10-dibutoxy-indolow7,6-g xindole Ž33. ŽScheme 7., we started from 1,5-dihydroxynaphthalene Ž27. which was alkylated by typical procedure w24x to give 1,5-dibutoxynaphthalene Ž28.. Compound 28 after nitration with fuming nitric acid in acetic acid gave the corresponding dinitronaphthalene Ž29.. Reduction of 29 with metallic Sn gave 4,8-diamino1,5-dibutoxynaphthalene Ž30.. Iodination of compound 30 by I 2 gave 4,8-diamino1,5-dibutoxy-3,7-diiodonaphthalene Ž31.. Compound 31 was reacted with TMSA and catalytic amounts of biswtriphenylphosphinexpalladium dichloride and copperŽI. iodide to give bis-Žtrimethylsilyldiamino.naphthalene Ž32.. The addition of two equivalents of CuI promotes heteroannulation of 32, providing an easy entry into ‘‘bisŽpyrrolo.naphthalene’’ Ž33..
We hoped that this synthesis will be convenient and alternative method for the preparation of 9,9-diethyl-2,7bisŽpyrrol-2-yl.fluorene Ž24. from 9,9-diethyl-2,7-dibromofluorene Ž22. ŽScheme 6.. In an alternative approach ŽScheme 6., 9,9-diethyl-2,7dibromofluorene Ž22. was reacted with n-BuLi and the product quench with aldehyde Ž25. to give carbinol 26. Compound 26 was directly oxidized to ketone with pyridinium chloroformate ŽPCC., and than directly cyclized by reaction with ammonium acetate to afford bis-pyrrole Ž24.. The yield of desired bis-pyrrole Ž24. in the method A was high Ž80%., but in method B low Ž25%., so we found that the use of the Method A is definitely more advantageous. More details about synthesis of 9,9-diethyl-2,7bisŽpyrrol-2-yl.fluorene Ž24. will be published in a forthcoming paper.
Scheme 6.
Scheme 7.
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185
Scheme 8.
Synthesis of 5,10-dibutoxyindolow7,6-g xindole Ž33. will be published in a forthcoming paper w26x.
6. Oxidative polymerization Scheme 8 shows the oxidative polymerization of described bisŽpyrrol-2-yl.arylenes and the subsequent reversible redox process of the as-made doped polymers using BPB Ž6e. as an example. The oxidative polymerization of bisŽpyrrol-2-yl.arylenes proceeds probably according to an electrochemically activated step-growth mechanism with the initial formation of a monomer cation radical followed by coupling polymerization. Multiple-scan cyclic voltammetric polymerization of the bisŽpyrrole. monomers was carried out to determine monomer oxidation onset and peak potentials, along with monitoring the rate of growth of the electroactive polymer films. The cyclic voltammetric polymerization of BPB Ž6e. as an example, is shown in Fig. 1. The monomer oxidation at the bare electrode begins at q0.25 V and peaks Ž Ep,m . at q0.35 V vs. AgrAgq. This is a very low monomer oxidation potential when compared to other electropolymerizable monomers. In comparison, pyrrole Ž Ep,m s q0.9 V., 3-methylthiophene Ž Ep,m s q1.5 V., 1,4-bisŽ2thienyl.benzene Ž Ep,m s q0.9 V., and 1,4-bisŽ2-thienyl.2,5-dimethoxybenzene Ž Ep,m s q0.7 V. all exhibit higher oxidation potentials. With repeated scanning, a reversible redox process quickly grows in width with an E1r2 at 0.0 V and a narrow peak to peak separation. It is evident that the polymer redox process develops quite rapidly and that the monomer polymerizes very efficiently to form a highly electroactive polymer. A comparison with a 10 mM solu-
tion of pyrrole, polymerized under similar scanning conditions, indicates that this monomer polymerizes about 40 times faster than pyrrole itself. The cyclic voltammetric polymerization of BPBŽOCH 3 . 2 Ž7e. is shown in Fig. 2 and is representative of the other monomers which were successfully electrochemically polymerized. In each case, during the first anodic scan, the monomer exhibited a low potential current onset
Fig. 1. Multiple scan electropolymerization of 10 mM 1,4-bisŽpyrrol-2yl.benzene Ž6e. in 0.1 M TBAPrCH 3 CN.
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Fig. 2. Cyclic voltammetric scanning electropolymerization of 0.01 M BPBŽOCH 3 . 2 Ž7e. in 0.1 M TBAPrCH 3 CN cycled at 100 mVrs.
followed by a single peak due to the formation of the monomer radical cation as shown in Table 1. Fig. 3 illustrates the cyclic voltammetry of PBPBŽOCH 3 . 2 at various scan rates and is representative of all of the electroactive polymers. As can be seen, the potentials of the anodic and cathodic processes of the polymer film are relatively close Žabout 100 mV. indicating a reversible redox process. Both anodic and cathodic peak currents are linearly proportional to the scan rate, which is indicative of an electrode-supported electroactive film. Fig. 4 shows the cyclic voltammetric growth during electrosynthesis of the polymer from the oxidation of BPN. The monomer oxidation at the bare platinum electrode begins at q0.3 V vs. AgrAgq and peaks at Ž Ep,m . at 0.35 V. For comparative purposes, BPB has a monomer oxidation onset of q0.27 V vs. AgrAgq. Fig. 5 shows the repeated potential scan polymerization for 3,6-bisŽpyrrol-2-yl.-9-ethylcarbazole ŽBP-NEC.. The polymerization begins at Eonset,m of 0.09 V with Ep,m at
Fig. 3. Scan rate dependence of the cyclic voltammetry of PBPBŽOCH 3 . 2 in 0.1 M TBAPrCH 3 CN cycled at Ža. 50, Žb. 100, Žc. 150, Žd. 200, Že. 250. and Žf. 300 mVrs.
0.15 V. This bisheterocyclic carbazole electropolymerizes at much lower potentials and quite rapidly relative to the single-ring heterocyclic monomers, pyrrole and carbazole.
Table 1 Electrochemical properties for pyrrolearylene monomers and their polymers Monomer
Eonset,m ŽV.
Ep,m ŽV.
E1r2,p ŽV.
Eg ŽV.
BPB BPBŽOCH 3 . 2 BPBŽOC 12 H 25 . 2 BPN BPBP
0.30 0.12 0.13 0.30 0.40
0.35 0.15 0.17 0.35 0.45
y0.10 y0.05 y0.05 0.05 0.10
2.4 2.3 nra 2.3 nra
Fig. 4. Cyclic voltammetric polymerization of polyŽ2,7 bisŽpyrrol-2yl.naphthalene carried out in 10 mM BPNr0.1 M TBAPrCH 3 CN.
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Fig. 7. Optoelectrochemical analysis of polyw1,4-bisŽpyrrol-2-yl.-p-phenylenex in 0.1 M TBAPrCH 3 CN. Spectra were run with polymer held at Ža. y0.8, Žb. y0.4, Žc. y0.0, Žd. q0.2, and Že. q0.5 V vs. Ag wire.
Fig. 5. Repeated potential scanning polymerization of 0.01 M BP-NEC Ž20. in 0.1 M TBAPrCH 3 CN at a scan rate of 100 mVrs.
For comparison purposes, the repeated potential scanning polymerization of carbazole is illustrated in Fig. 6. This monomer exhibits Ep,m at 0.93 V, 0.5–0.8 V higher than that for the pyrrole analogues. A series of electrochemical studies was carried out and confirmed the bipolaronic nature of the charge carriers as illustrated in Fig. 7. The neutral form of the polymer shows a distinct p to p ) transition with a band-gap onset of 2.4 eV Ž516 nm. and a peak at 2.7–2.8 eV Ž440–460 nm.. It is interesting that this is lower than the values for either polypyrrole or polyŽ p-phenylene.. This can be attributed to the highly delocalized nature of the neutral polymer. Stepwise oxidation of the polymer shows the
growth of bipolaron bands with peaks evolving at approximately 1 and 2.2 eV as the polymer dopes. Table 2 gives a compilation of the Eonset,m and Ep,m , BP-NEC Žcontaining unsubstituted external pyrrole rings. exhibits an Ep,m that is 0.26 V lower than that for 3,6bisŽ N-methylpyrrol-2-yl. N-ethylcarbazole, previously prepared and reported by Geibler and Hallnsleben w27x. This can be explained by steric hindrance induced between the pyrrole and carbazole moieties by the methyl groups.
7. Electronic structure Optoelectrochemical methods were used to elucidate the effect of the structure of the aryl core unit Ž p-phenylene and 2,6-naphthyl. and pendant substituents on the energy of the p electrons. Polymer films used for optoelectrochemical analysis were prepared potentiostatistically at the Ep,m of the monomer on ITO-coated glass plates and their spectra were obtained in a quartz cuvette containing electrolyte solution and counter and reference electrodes. The polymers were first fully reduced and the electronic spectrum Ž310–1600 nm. was obtained. The polymers were subsequently subjected to increasing potentials in 100 mV steps, and the spectrum was obtained at each potential after equilibration. The reversible nature of the redox doping process was confirmed by a similar stepwise reduction. An optoelectrochemical series for the dimethoxy-substituted polymer is shown in Fig. 8 and is similar to PBPB–ClO4 and PBPN–ClO4 . The optical band gap for each polymer was obtained from the onset of the p–p ) transition. The
Table 2 Electrochemical properties for carbazole-based monomers and their polymers Monomer
a Eonset,m
a Ep,m
a E1r2,p1
a E1r2,p2
Egb
BP-NEC c Carbazole
0.09 0.88
0.15 0.95
0.02 0.65
0.53 0.91
2.5 nra
a
Fig. 6. Repeated potential scanning polymerization of 0.01 M carbazole in 0.1 M TBAPrCH 3 CN at a scan rate of 100 mVrs.
Values listed in volts vs. AgrAgq reference electrode. Values listed in electronvolts as the onset of p – p ) transition. c Performed in 0.1 M TBAPrCH 3 CN, 10 mM monomer. b
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Fig. 8. Optoelectrochemical analysis of PBPBŽOCH 3 . 2 performed in 0.1 M TBAPrCH 3 CN. UV–visible–near-IR spectra taken at Ža. y0.6, Žb. y0.3, Žc. y0.2, Žd. y0.1, Že. 0.0, Žf. q0.1, Žg. q0.2, Žh. q0.3, and Ži. 0.4 V vs. AgrAgq.
unsubstituted BPB exhibits a band gap of 2.4 eV, which is lower than that of polypyrrole Ž2.7 eV. or polyŽ p-phenylene. Ž3.0 eV.. The band gap of PBPBŽOCH 3 . 2 is reduced further to 2.3 eV. We attribute this lowering of the band gap relative to polypyrrole to the increased structural regularity induced by the use of the three-ring-containing bisŽpyrrol-2-yl.benzene monomer. Additionally, the oxygen atoms of the methoxy substituents in PBPBŽOCH 3 . 2 are expected to form a hydrogen bond with the pyrrole hydrogen, causing the repeat unit to attain planarity more easily than for unsubstituted BPB. This compound was calculated to have its lowest energy between a torsional angle of 08 and 158, which increased only for 1 kcalrmol at 308. The optoelectrochemical study for the polymer grown from BPN and switched in 0.1 M TBAPrCH 3 CN is illustrated in Fig. 9. The Eg was found to be 2.3 eV with the growth of the bipolaronic bands upon doping evolving at approximately 2 eV and 1 eV.
Fig. 10. Long-term double-potential-stepping charge retention for PBPB and PBPBŽOCH 3 . 2 performed under an argon blanket in 0.1 M TBAPrCH 3 CN. Twenty seconds constitutes a double-potential step.
they must sustain a number of sequential oxidative and reductive processes while retaining their electroactivity w2,28x. Multiple redox switching experiments were carried out on PBPB–ClO4 and PBPBŽOCH 3 . 2 –ClO4 , prepared via constant-potential electropolymerization at the Ep,m until 2 mC of charge passed Ž400 mCrcm2 . and subsequently switched in monomer-free 0.1 M TBAPrCH 3 CN electrolyte. The films were first redox ‘broken in’ by repeated scanning to obtain a constant CV current response. Double potential steps were carried out by holding the potential at Ep,a q 0.15 V for 10 s, stepping to Ep,c y 0.15 V for 10 s, and then returning to the original potential. Fig. 10 shows the long-term double-potential-stepping results for the above polymers. After 2300 double potential steps, PBPB retained 50% of its original charge. PBPBŽOCH 3 . 2 was able to maintain 80% of its original charge after 3000 double-potential steps and 50% electroactivity after 6000 double-potential steps. For comparison purposes, polyŽpyrrole chloride., redox switched in 0.1 M LiClO4 Žaqueous., exhibits 65% retention after 900 double-potential steps.
8. Long-term redox switching stability In order for these polymers to be useful in practical devices where they are switched between charge states,
9. Concluding remarks As mentioned at the beginning, I tried to show the interested reader the fantastic diversity or the polybispyrrole chemistry. The synthetic methodology used to prepare this monomer and polymer opens up numerous pathway for the synthesis of derivatized polybispyrroles and is the subject of continued investigation.
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Fig. 9. Optoelectrochemistry of polyŽ2,7-bisŽpyrrol-2-yl.naphthalene coated on an ITO coated quatz plate. Spectra taken at Ža. y1.0 V, Žb. y0.2 V, Žc. 0.0 V, Žd. 0.2 V, and Že. 0.4 V vs. AgrAgq.
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