Effect of sulphate ion on the electrochemical polymerization of pyrrole and N-methylpyrrole

Synthetic Metals, 11 (1985) 167 - 176

167

EFFECT OF SULPHATE ION ON THE ELECTROCHEMICAL POLYMERIZATION OF PYRROLE AND N-METHYLPYRROLE K. HYODO* and A. G. MACDIARMID

Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104 (U.S.A.) (Received February 26, 1985;accepted in revised form April 25, 1985)

Abstract

Pyrrole and N-methylpyrrole were electrochemically polymerized in an identical manner in aqueous electrolytes containing (Et4N)BF4, LiClO4 or Na2SO4 to give films of the corresponding p-doped polymers. The polymer obtained from the aqueous Na2SO4 electrolyte, [(polyN-methylpyrrole)+Y(SO4)y/22-]x, differed from the other polymers in that it exhibited a very low conductivity (~ 10 -v S/cm), contained a high concentration of ~>C=O groups and had an EPR linewidth, AHpp, more than an order of magnitude greater than that found in the corresponding polymers obtained from the (Et4N)BF4 and LiC104 electrolytes. A sample of [(poly N-methylpyrrole)+~(BF4)y-]x electrochemically synthesized in CH3CN and subsequently converted electrochemically to the corresponding sulphate in the Na2SO4 aqueous electrolyte exhibited normal behaviour. A mechanism is proposed for the introduction of > C=O groups into the poly(Nmethylpyrrole) polymer during its electrochemical polymerization in the Na2SO4 aqueous electrolyte.

Introduction In recent years, interest in poly(pyrrole) as a conducting polymer has been increasing rapidly [1 - 5]. This is due in large part to its ease of chemical or electrochemical synthesis and to its relatively good stability in its p-doped, i.e., oxidized, highly~onducting form in the presence of air [6]. Electrochemical synthesis can result in the formation of good freestanding films of the polymer [2]. Studies have also shown that the nature of the dopant anion employed in the electrochemical synthesis of poly(pyrrole) can significantly affect the conductivity of the resulting material [5]. For example, the conductivity of poly(pyrrole) formed in a solution of (Et4N)BF4 in CH3CN has been reported to be approximately three orders of magnitude greater than the conductivity of poly(pyrrole) formed from *Permanent address: Mitsubishi Paper Mills, Ltd., Research Laboratory, 4-1 HigashiKanamachi 1-Chome, Katsushika-Ku, Tokyo 125, Japan. 0379-6779/85/$3.30

© Elsevier Sequoia/Printed in The Netherlands

168

(Bu4N)HSO4 in CH3CN. From these studies it was not possible to determine whether the difference in conductivity was due to differences in the size, polarizability, solvation, etc. of the dopant ion, or to a difference in the form or chemical composition, of the poly(pyrrole). As shown by Andr~ et al., the conductivity and EPR spectra of n-doped (CH)x can be greatly affected under certain conditions by the above or related factors [7]. Several electrochemical syntheses and studies of polymeric N-alkyl derivatives of pyrrole have recently been reported [8, 9]. The p-doped Nmethyl derivative, [ (poly N-methylpyrrole) +y (BF4)y- ]x, i.e., ' [PNP(BF4) ]~ ', was found to have a conductivity of ~ 10 -3 S/cm. The present study was carried out in order to synthesize electrochemically other p
Experimental Pyrrole and N-methylpyrrole (Aldrich) were purified by distillation under argon. Other chemicals were used as received. Pyrrole and N-methylpyrrole were polymerized electrochemically in aqueous solutions of (Et4N)BF4, LiC104 or Na2SO4 and also in a CH3CN solution of (EtaN)BF4 and LiC104. Nitrogen was first bubbled through all solutions for ~ 10 minutes in order to remove dissolved air. In each case the m o n o m e r concentration was 0.05 M and that of the electrolyte was 0.10 M. The films were electrochemically deposited on a conducting glass anode (Practical Products Co.) ( 1 0 m m × 20 mm) separated from the counter platinum foil electrode (5 mm × 20 ram) by a distance of 2 cm. A constant applied current of 0.20 m A / c m 2 was used throughout each polymerization. The potential o f the anode at the beginning and end of each polymerization is given in Table 1. The time taken for polymerization was ~ 30 min. In each TABLE1 Anodepotentialduringelectrochemi~lpolymerizationofpy~oleandN-methylpy~ole ~e~ro~te

Anode potential (V) a Pyrrole

Na2SO 4 LiCIO4 (Et4N)BF4

N-methylpyrrole

Initial b

Fin al

Initial b

Final

0.71 0.88 0.73

0.59 0.64 0.56

0.81 0.79 0.78

0.67 0.65 0.54

avs. SSCE r e f e r e n c e electrode. bpotential measured ~ 1 - 2 s after commencement of electrochemical polymerization.

169 case the p o ten t i al fell during t he polymerization, presumably primarily because o f the p r o d u c t i o n of the p o l y m e r which had a greater effective surface area than the conducting glass on which it was deposited. After 350 m C / c m 2 had passed, t he film was judged to have a thickness o f ~ 875 n m based on the r e p o r t e d [3] thickness o f ~ 25 nm per 10 m C/ cm 2 for the deposition o f p o l y ( p y r r o l e ) f r om LiC104 and (Et~N)BF4 electrolytes. T he glass electrode was t h e n transferred to a beaker o f deaerated distilled water in which a razor blade was used to peel o f f a cohesive free-standing p o l y m e r film f rom the electrode. A glass electrode was used since it was f o u n d th at p o l y m e r films could be removed m o r e easily from glass t han from a platinum electrode. The films were t hen dried in a dynamic vacuum for ~ 12 hours. In one e x p e r i m e n t t he '[PNP(BF4)]x', synthesized in a (Et4N)BF4/ CH3CN electrolyte, was reduced to t he neutral form, (PNP) °, by holding it at a potential of 0.00 V for 30 minutes, at which time the current had d r o p p e d from an initial value of ~ 1 m A / c m 2 to a final value o f 0.01 mA/ cm 2. It was f o u nd in a separate cyclic v o l t a m m e t r y study o f this p o l y m e r that it was reduced to (PNP) ° at 0.0 V. The (PNP) ° p o l y m e r electrode was t hen washed briefly in CH3CN and transferred to a 0.10 M Na2SO4 electrolyte. It was held at a potential of 0.60 V for 30 minutes and t hen at a potential o f 0.00 V for 30 minutes, followed b y a second 30 m i nut e period at 0.60 V to convert it to [PNP(SO4)]x. The film was then removed from t he glass electrode, and was washed and dried as described above. An elemental analysis ( Schw a r zkopf Microanalytical L a b o r a t o r y , N.Y.) o f th e [PNP(SO4)]~ synthesized in t he aqueous Na2SO4 electrolyte was performed. Calculated: for CsHsN(SO4)0.023s(O)0.06ss, i.e., CsHsNS0.023sO0.159s : C = 65.18; H = 5.01; N = 16.21; S = 2.35; O = 11.25%. F o u n d : C = 65.90; H = 5.50; N = 15.37; S = 2.35; O = 10.88%. As will be shown below, t he additional o x y g en is associated with car bonyl groups in the polymer. F T - I R spectra were obtained with an IBM F T - I R 97 spectrophotome~ ter, and th e EPR spectra were obtained with a Brucker EPR spectrometer. Constant currents and constant potentials e m p l o y e d in the electrochemical experiments were obtained by means o f an EG and G PAR m odel 363 Galvanostat/Potentiostat. All potential measurements were made v e r s u s a sodium saturated calomel reference electrode (SSCE).

Results Th e F T - I R spectra of p o l y ( p y r r o l e ) s synthesized by t he electrochemical p o l y m e r i z a t i o n o f pyrrole in aqueous solutions o f (a) 0.10 M ( E t ~ ) B F 4 , (b) 0.10 M LiC104 and (c) 0.10 M Na~SO4 are given in Fig. 1 and are tabulated in Table 2. It is obvious t hat t her e is essentially no difference in the spectra regardless o f t he anion used; hence absorption m axi m a characteristic o f the d o p a n t anions, (BF4)-, (C104)- and (SO4) 2- [or (HSO4)- if present],

170

(a) [PP(BF4) ]x

f

(b) [PP(CI04)]x

(c) [PP(S04)]x

|

i

4000

i

3000

i

!

I

I

I

|

2000 1600 1200 NAVE ~ E R ( C:n-I )

I

I

I

800

Fig. 1. FT-IR spectra of p-doped poly(pyrrole)s synthesized by the electrochemical polymerization of pyrrole in aqueous solutions of (a) 0.10 M (Et4N)BF4, to produce [PP(BF4)]x, (b) 0.10 M LiCIO4 to produce [PP(C104)]x, (c) 0.10 M Na2SO4 to produce

[PP(SO4)]x. are n o t sufficiently intense to be observable. This is perhaps due to the low concentration of the anions or to their absorption being obscured by poly(pyrrole) bands or possibly to their replacement by some u n k n o w n oxygencontaining anion, as has been suggested by Street et al. [4]. Hotta and Shimotsuma have also reported a similar absence of dopant anion absorption in poly(thiophene), at least when lightly doped [ 10].

171

The F T - I R spectra of poly(N-methylpyrrole) synthesized b y the electrochemical polymerization of N-methylpyrrole in an aqueous solution of (a) 0.10 M (Et4N)BF4, (b) 0.10 M LiC104 and (c) 0.10 M Na2SO4 are given in Fig. 2 and are tabulated in Table 3. Essentially the same spectra were obtained for [PNP(BF4)]x and [PNP(C104)]x when they were synthesized electrochemically in acetonitrile solutions of the appropriate salts instead of in aqueous electrolytes. For [PNP(BF4)]x and [PNP(CIO4)]x characteristic absorptions, possibly due at least in part to the dopant anions, can be observed at 519 cm -1 and 1051 c m - 1 for ( B F 4 ) - , at 621 cm -l and 1091 cm -1 for (C104)- and at 611 cm -1 and 1087 cm -1 for (SO4) 2-. However, TABLE 2 I n f r a r e d b a n d p o s i t i o n s (cm -1 ) a n d a s s i g n m e n t s for p - d o p e d p o l y ( p y r r o l e ) s [PP(BF4)]x [PP(C104)]x a [PP(SO4)]x

-1700 (sh)

1546

1475

1299

1170

1093

1034

964

676

619

(m)

(w)

(m)

(s)

(w)

(m)

(w) (s)

900

(w)

(w)

1544 (s)

1483 (sh)

1298 (m)

1182 (s)

1093 (w)

1035 (s)

954 (w)

916 (s)

675 (w)

619 (w)

1703

1558

1479

1301

1184

1099

1041

964

906

680

611

(w)

(s)

(w)

(w)

(s)

(w)

(s)

(w)

(s)

(w)

(w)

aThis s p e c t r u m is q u a l i t a t i v e l y i d e n t i c a l t o t h a t o f [PP(CIO4)]x s y n t h e s i z e d e l e c t r o c h e m ically in a c e t o n i t r i l e [ 4 ] . A n e x a c t c o m p a r i s o n c a n n o t b e m a d e since values o f t h e b a n d p o s i t i o n s were n o t given in t h e p r e v i o u s p u b l i c a t i o n .

TABLE 3 I n f r a r e d b a n d p o s i t i o n s (cm -1 ) a n d a s s i g n m e n t s for p - d o p e d p o l y ( N - m e t h y l p y r r o l e ) s [PNP(BF4)]x [PNP(CIO4)]x

[PNP(SO4)]x [PNP(SO4)]xd

1705

1531

1446

1425

1303

(w)

(m)

(w)

(w)

(s)

1700

1533

1446

1427

1303

(vw)

(s)

(w)

(w)

(s)

1706 (vs)

1596 (m)

1440 (m)

1382 (m)

1303 (sh)

1703

1531

1446

1425

(w)

(s)

(w)

(w)

--

1051 a

761

723

519 a

(vs)

(w)

(w)

(w)

1091 b - -

769

723

621 b

(vs)

(w)

(w)

(m)

1087 c 1056 (w) (w)

767

717

611 c

(w)

(w)

(w)

1301

1099

1053 e

763

721

678

(s)

(w)

(m)

(w)

(w)

(w)

a T h e 1 0 5 1 c m -1 a n d 5 1 9 c m -1 p e a k s m a y b e d u e in p a r t t o BF4-, w h i c h is r e p o r t e d [ 1 0 ] t o h a v e c h a r a c t e r i s t i c a b s o r p t i o n s at 1 0 5 2 c m -1 (vs) a n d 520 c m - 1 (m). b T h e 1 0 9 1 c m -1 a n d 6 2 1 c m - 1 p e a k s m a y b e d u e in p a r t t o (C104)-, w h i c h is r e p o r t e d [ 1 0 ] t o h a v e c h a r a c t e r i s t i c a b s o r p t i o n s at 1 1 0 0 c m -1 (vs) a n d 6 2 5 c m -1 (m). CThe 1 0 8 7 c m -1 a n d 611 c m -1 p e a k s m a y b e d u e in p a r t t o (SO4) 2 - , w h i c h is r e p o r t e d [13] to have c h a r a c t e r i s t i c a b s o r p t i o n s at 1 1 0 5 c m -1 (vs) a n d 611 c m -1 (m). d S y n t h e s i z e d e l e c t r o c h e m i c a l l y as [ P N P ( B F 4 ) ] x a n d t h e n c o n v e r t e d e l e c t r o c h e m i c a l l y t o [ P N P ( S O 4 ) ] x (see t e x t ) . e T h e i n t e n s i t y o f this b a n d suggests t h e p r e s e n c e o f s o m e r e s i d u a l ( B F 4 ) - .

172

(a) [PNP(BF4)]x

(b) [PNP(CIO 4) ]x v

<< (c) [PNP(S04) ]x

, 4000

i 3000

|

I

2000 ~VE

i 1600

~I~R

!

i 1200

i

,

800

((m~-i)

Fig. 2. FT-IR spectra of p-doped poly(N-methylpyrrole)s synthesized by the electrochemical polymerization of N-methylpyrrole in aqueous solutions of (a) 0.10 M (Et4N)BF4 to produce [PNP(BF4)]x, (b) 0.10 M LiCIO4 to produce [PNP(C104)]x, (c) 0.10 M Na2SO4 to produce [PNP(SO4)]x.

the spectrum o f [PNP(SO4)]x synthesized in the Na2SO 4 electrolyte shows a very strong absorption at 1 7 0 6 cm -1 characteristic o f the carbonyl group; it also shows a C--H stretching vibration at 2 9 3 3 cm -1 for the CH3 group and a vibration at 3 0 5 0 cm -I for the ring CH groups. The C--H vibrations are ill defined in the spectra o f [PNP(BF4)]x and [PNP(C104)]x, presumably

173

because of the more metallic nature (higher conductivity) of these polymers. This results in more intense background absorption at higher wave numbers. The strong absorptions at 1531 cm -1 and 1533 cm -1 in [PNP(BF4)]x and [PNP(C104)]x respectively, which are absent in [PNP(SO4)]~ synthesized from Na2SO4, must be characteristic of a feature of polymer structure which has been significantly modified in the [PNP(SO4)]x. It should be noted that in the [PNP(SO4)]x obtained from [PNP(BF4)L there is a strong absorption at 1531 cm -1 similar to that present in the [PNP(BF4)]x. The conductivities of the p-doped poly(pyrrole) polymers, [PP(BF4)]~, [PP(C104)]~ and [PP(SO4)]x, were found to be 1 × 102, 5 X 101 and 1 × 10 ° S/cm respectively. These are in qualitative agreement with the values of 30 100, 6 0 - 200 and 0.3 S/cm (for (HSO4)- anion) respectively reported by Diaz et al. [3]. In the present study it was found that [PNP(BF4)]~ and [PNP(C104)]~ had conductivities of 2 × 10 -3 and 5 × 10:3 S/cm respectively, while [PNP(SO4)]~ had a much smaller conductivity of 1 × 10 -7 S/cm. The EPR spectra of [PP(BF4)]x, [PP(C104)]x and [PP(SO4)]x had half linewidths, AHpp, falling between 0.15 G and 0.30 G. The EPR spectra of

(a) [PNP(CIO4)]x

5G

Fig. 3. EPR spectra of p-doped poly(N-methylpyrrole)s synthesized by the electrochemical polymerization of N-methylpyrrole in aqueous solutions of (a) 0.10 M LiCIO4 to produce [PNP(CIO4)]x, and (b) 0.10 M Na2SO4 to produce [PNP(SO4)]x.

174 [PNP(BF4)]x from that of 3(a) and that width of 0.35

and [PNP(C104)]~ were identical, but differed significantly [PNP(SO4)]~. The spectrum of [PNP(C104)]~ is given by Fig. of [PNP(SO4)]x in Fig. 3(b). The former material has a lineG whereas [PNP(SO4)]x has a linewidth of 5.50 G.

Discussion

As shown in the previous section, the electrochemical polymerization of pyrrole in aqueous solutions using (Et4N)BF4, LiC104 or Na2SO4 results in polymers having identical IR spectra and conductivities ranging from 1 X 10 ° S/cm for [PP(SO4)]~, 5 X 101 S/cm for [PP(CIO4)]x and 1 X 102 S/cm for [PP(BF4)]~. It is therefore surprising to find that in the analogous study of [PNP(BF4)]x, [PNP(C104)]x and [PNP(SO4)]~ there is a major difference in IR and EPR spectra and conductivity between [PNP(SO4)]~ and the other two polymers, which are themselves identical, especially since they are all synthesized in an identical manner. Both the IR and EPR spectra of the [PNP(SO4)]~, together with its elemental analysis, clearly show that the polymer backbone has undergone considerable chemical modification. In view of the C=O stretching frequency (Table 3) in [PNP(SO4)]~, structures containing fragments derived from

CH3 [A] Pc=o -- 1675 cm-1

and

CH:5 [B] Pc=o = 1689 cm-1

are believed to be present in this polymer. Compounds [A] and [B] have been identified b y Smith and Jensen as being formed b y the oxidation of N-methylpyrrole b y gaseous oxygen [11]. It should be noted that these investigators also observed c o m p o u n d [C] amongst the products.

O~.E~-~O CH3 [C] Vc: o -- 1701 cm-1 Since the carbonyl peak in the [PNP(SO4)]x at 1706 cm -1 is so strong, we believe that there are carbonyl groups not only on the end rings b u t also on rings within the chain. This is consistent with the elemental analysis of the [PNP(SO4)]x. It is clearly apparent from this analysis that additional oxygen to that needed to satisfy the requirements of the observed percentage

175 of sulphur (as sulphate) is present. It therefore seems likely that [PNP(SO4)]x contains end groups derived from [A] and/or [B] together with a structure such as [D] within the chain, i.e.,

CH5

CH5

[D]

Such structures involve considerable interruption of the pi system due to the formation of sp 3 hybridized carbon linkages. This is expected to reduce the conductivity significantly, as is observed experimentally. It is clear that the introduction of carbonyl groups into the [PNP(SO4)]x occurs during the polymerization of the m o n o m e r since, as can be seen from Table 3, only a relatively weak carbonyl stretch is present in the sample o f [PNP(SO4)]x made b y electrochemically replacing the (BF4)ion in [PNP(BF4)]x with (SO4) 2- ion. Furthermore, the strong peak at 1531 and 1533 cm -1 that is characteristic of the polymer backbone in [PNP(BF4)]~ and [PNP(C104)]~ respectively is also present in the [PNP(SO4)]~ made from [PNP(BF4)]x but is absent in the [PNP(SO4)]~ synthesized in the Na~SO4 electrolyte. It is not clear w h y the (SO4) 2- ion as distinct from the (BF4)- or (ClO4)- ions causes the observed carbonyl formation. It should be noted that it cannot be associated with the electrochemical conversion o f the (SO4) 2- ion to more highly oxidized materials, which then react with the m o n o m e r or the polymer, since no electrochemical decomposition of any of the electrolytes above occurs within the potential range studied. The effectiveness of the (SO4) 2- ion in promoting the formation of carbonyl groups together with associated structural changes m a y be related to the observation b y Weinberg and Brown that the electrolysis of N-methylpyrrole presumably occurs via the intermediate dicarbocation [ 12];

H@H CH3 The divalent (804) 2 . anion may well facilitate the production of this dication as compared to the simultaneous interaction of the N-methyl m o n o m e r with two monovalent anions. The hydrolysis o f this dication to H H H0~N~0H CH5 followed b y electrochemical oxidation of the t y p e HH~ O H H CH5

H./'~ -2H + O J ~ O -2H+ -2~- ~ Hd"- N/-'O---T~CH3

CH5

176

together with the expected conventional electrochemical polymerization reactions are all quite reasonable.

Conclusions In conclusion, it can be seen that even apparently minor changes in synthesis of a conducting polymer involving an apparently unimportant change in dopant ion can lead to very considerable modification of the polymer backbone, with an attendant large change in conductivity and related properties. Such effects must be examined in detail in order to obtain a fundamental understanding of how minor variations in synthetic procedure may greatly affect the electronic and magnetic properties of the polymers.

Acknowledgements The authors wish to thank Mr. Jin-Chin Chiang for recording the infrared spectra and for many helpful discussions concerning their interpretation. They also wish to thank Mitsubishi Paper Mills Ltd. for financial support. Support of the National Science Foundation, grant No. DMR 80-22870, is also gratefully acknowledged.

References 1 A. Dall'Olio, Y. Dascola, V. Varacca and V. Bocchi, Compt. Rend., C6-267 (1968) 433. 2 A. F. Diaz, K. K. Kanazawa and G. P. Gardini, J. Chem. Soc., Chem. Commun., (1979) 635. 3 A. F. Diaz, J. I. Castillo, J. A. Logan and W. Y. Lee, J. Electroanal. Chem. Interracial Chem., 129 (1981) 115. 4 G. B. Street, T. C. Clarke, M. Krounbi, K. I~ Kanazawa, V. Lee, P. Pfluger, J. C. Scott and G. Weiser, Mol. Cryst. Liq. Cryst., 83 (1982) 253. 5 M. Salmon, A. F. Diaz, A. J. Logan, M. Krounbi and J. Bargon, Mol. Cryst. Liq. Cryst., 83 (1982) 265. 6 A. F. Diaz and B. Hall, IBMJ. Res. Develop., 27 (1983) 342. 7 J. J. Andre, M. Bernard, B. Francois and C. Mathis, J. Phys. (Paris) Colloq., 44 (1983) C3-199. 8 A. F. Diaz, J. I. Castillo, K. K. Kanazawa, J. A. Logan. M. Salmon and O. Fajardo, J. Electroanal. Chem. Interfacial Chem., 133 (1982)233. 9 S. Asavapiriyanont, G. IC Chandler, G. A. Gunawardena and D. Pletcher, J. Electroanal. Chem., 1 77 (1984) 245. 10 S. Hotta and W. Shimotsuma, Synth. Met., 10 (1984) 85. 11 E. B. Smith and H. B. Jensen, J. Am. Chem. Soc., 32 (1967) 3330. 12 N. L. Weinberg and E. A. Brown, J. Am. Chem. Soc., 31 (1966) 4054. 13 K. Nakanishi and P. H. Solomon, Infrared Absorption Spectroscopy, Holden-Day Inc., New York, 1977, p. 56.