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Characterization of doped polypyrroles by mass spectrometry
Synthetic Metals, 46 (1992) 45-51
45
Characterization of doped polypyrroles by mass spectrometry J. Barrie R a y n o r a n d W a l k i r i a S. Schlindwein Department of Chemistry, University of Leicester, Leicester LE1 7RH (UK)
(Received March 15, 1991; accepted in revised form July 12, 1991)
Abstract The technique of mass spectroscopy has been used to identify new polypyrroles made with [(C6H4CI)4B]-, [IC14]- and [Bra]- counter anions. Mass spectroscopy confirms the presence of Cl-, Br-, I-, (SO4) 2-, (HSO4)-, (FeCl4)-, (PF6)- and (CIO4)- as counter anions in other polypyrroles.
Introduction U n a m b i g u o u s c h a r a c t e r i z a t i o n of the d o p a n t a n i o n in electrically cond u c t i n g p o l y m e r s is n o t e a s y a n d o f t e n relies on e l e m e n t a l a n a l y s i s a n d the e x p e c t a t i o n of its p r e s e n c e b a s e d u p o n its use in the m e t h o d of p r e p a r a t i o n . Most studies have concentrated upon understanding conduction mechanisms o r a t t e m p t i n g to refine p r e p a r a t i v e p r o c e d u r e s in o r d e r to m a x i m i z e electrical c o n d u c t i v i t y . T h e v a s e m a j o r i t y of t h o s e p a p e r s w h i c h s e e k to i n v e s t i g a t e t h e m u t u a l i n t e r a c t i o n o f p o l y m e r c a t i o n a n d c o u n t e r a n i o n c o n s i d e r only t h e influence o f t h e c o u n t e r a n i o n u p o n the p r o p e r t i e s of the b u l k p o l y m e r . T h e inevitable influence of t h e p o l y m e r b a c k b o n e u p o n the s t r u c t u r e o f c o u n t e r a n i o n is r a r e l y m e n t i o n e d , let a l o n e investigated. In r e c e n t y e a r s , w e h a v e a p p l i e d a r a n g e of p h y s i c a l m e t h o d s to the s t u d y o f a v a r i e t y of d o p e d c o n d u c t i n g p o l y m e r s , in p a r t i c u l a r p o l y a c e t y l e n e a n d p o l y p y r r o l e . By u s i n g e l e c t r o n n u c l e a r d o u b l e r e s o n a n c e s p e c t r o s c o p y , w e h a v e d e t e r m i n e d t h e a v e r a g e length of the c o n j u g a t e d c h a i n in UF6- , ReF~- a n d SnC14-doped p o l y a c e t y l e n e s a n d identified t h e likely p o s i t i o n o f t h e c o m p l e x a n i o n s relative to t h e p o l y a c e t y l e n e b a c k b o n e [ 1 - 3 l . In t h e case of a [FeC14]--doped polypyrrole and a composite of polypyrrole and p o l y ( v i n y l alcohol), e l e c t r o n s p i n r e s o n a n c e a n d M S s s b a u e r s t u d i e s [4, 5] a n d EXAFS [6] m e a s u r e m e n t s h a v e h e l p e d to identify t h e c h e m i c a l s t r u c t u r e o f t h e d o p a n t . In o t h e r s t u d i e s in o u r l a b o r a t o r y , m e t a l h e x a f l u o r i d e s a n d m e t a l p e n t a f l u o r i d e s w e r e d o p e d into p o l y a c e t y l e n e [7, 8]. M a s s s p e c t r a l s t u d i e s o f CH(WF6) ~ a n d CH(TeF6) ~ s h o w e d t h e p r e s e n c e o f W-F4 + a n d TeF4 + ions [8]. In this p a p e r , w e m a k e u s e o f n e g a t i v e ion e l e c t r o n i m p a c t a n d n o r m a l e l e c t r o n i m p a c t m a s s s p e c t r o s c o p y in o r d e r to aid c h a r a c t e r i z a t i o n of t h e
0379-6779/92/$5.00
© 1992- Elsevier Sequoia. All rights reserved
46 dopant anion in a range of doped polypyrroles. We also tried to use fast atom b o m b a r d m e n t but were unsuccessful in obtaining any spectra. To our knowledge, no o t her mass spectroscopic studies of note have been published on electrically conducting polymers.
Experimental Preparation of polymers Electrochemical syntheses of polypyrrole were carried out in a onec o m p a r t m e n t cell with three electrodes: a platinum working electrode, a platinum secondary electrode and a saturated calomel reference electrode. The solvent used was dry acetonitrile and the electrolyte and pyrrole concentrations used were typically 0.1 M and 0.05 M, respectively. The following electrolyte salts were used: (Et4N)(BF4), (Et4N)(CIO4), K[B(C~H4CI)4], (Et4N)C1, (Et4N)Br, (Et4N)I, (Me4N)(PF0), (Et4N)(C104), (Bu4N)(HSO4) and (Et4N)(NO3). Solutions were purged with nitrogen gas and films grown at a constant potential of + 0.85 V versus SCE. Chemical preparation of polypyrrole was carried out by direct admixture of a solution of pyrrole in acetonitrile and a solution of the oxidant in acetonitrile. Oxidants used were (Bu4N)(ICI4), (Me4N)(Br3), (Me4N)(FeC14) and Ag2(SO4). Another polypyrrole was pr e p ared by passing ozonized oxygen through a solution of pyrrole in dilute sulfuric acid. In all preparations, the polymer was thoroughly washed with acetonitrile in a Soxhlet extractor for 3 h.
Mass spectra Negative ion electron impact spectra were recorded on an AEI MS 902 instrument at 70 eV electron energy and 7 kV ion energy at Nottingham University by Dr David Knight. Normal electron impact mass spect rom et ry was carried out using a VG analytical micromass 16B mass spectrometer. Samples were directly inserted into the probe as solids. Unless otherwise stated, the mass spectra described are of the latter type.
Results Polypyrrole halides (Cl-, Br-, I - ) (electrochemical preparation) Negative electron impact ( E I - ) spectra showed strong peaks in each case. For the chloride at 150 °C, two peaks at 36 and 38 m/z are assigned to HCI-. Similarly with bromide at 150 °C, two peaks at 80 and 82 m/z are assigned to H B r - . The iodide required a higher probe t em perat ure (200 °C) to give a peak at 128 m / z assigned to H I - . Not surprisingly, the expect ed halide anion was p r o t o n a t e d by the abundance of protons derived from fragmentation of the polypyrrole.
Polypyrrole tribromide (PPyBr3) (chemical preparation) E I - spectra of the sample anticipated to be PPyBr3 required a probe t e mp er atu r e of 300 °C before a sufficiently strong spectrum could be obtained.
47
It showed only a pair of peaks at 80 and 82 m / z attributable to H B r - . Since no spectrum could be obtained at the t em p erat ure (150 °C) at which PPyBr gave a spectrum, t hen the c om pound was different and was most likely PPyBr3 as anticipated. The known propensity for the Br3- anion to decom pose into Br2 and B r - , and of Br2 to split readily in the mass s p e c t r o m e t e r to Br atoms, accounts for seeing only H B r - in the mass spectrum. The normal mass s p ectr u m at 200 °C showed a pair of peaks at 79 and 81 m / z due to bromine isotopes and a pair at double the intensity of the other at 80 and 82 m / z attributed to Br ÷ and HBr +, respectively. During polymerization, the polymer cation is apparently more stable with a Br3- c o u n t e r anion from surplus Me4NBr3 present in solution than with B r - formed as the reduction pr oduc t of Br3-.
Polypyrrole tetrachloroiodate (PPyICIJ (chemical preparation) E I - s p ectr a of the sample anticipated to be PPyIC14 required a probe t e m p e r a t u r e of 300 °C for a good spectrum. This showed peaks at 36, 38 and 128 m / z which were assigned to HC1 and H I - . The intensity of HC1was 3.6 times that of the H I - line strongly suggesting that the ICL - unit was intact in the sample. The fact that no spectrum was obtained at 200 °C indicated that separate I - and Cl- were not present, nor a combination of IC12- and Cl2. Any free Cl2 would have fragmented to C1 atoms in the mass spectrometer. That the c o u n t e r anion in this polypyrrole is ICl4- is somewhat surprising bearing in mind its m e t h o d of preparation using (Bu4N)(IC14) as the oxidizing agent. The e x p e c t e d reduction products would be I and Cl- and the e x p e c t e d polymer polypyrrole iodide or chloride. The evidence suggests that surplus (ICl4)- takes p r e f e r e n c e over I - or Cl- as the counter ion during polymerization.
Polypyrrole sulfate ((PPy) 2S0 4 or PPyHSO ¢) The EI + spect r um of this polymer (prepared electrolytically) showed two strong peaks at 64 and 48 m / z with associated satellite peaks at 66 and 50 m / z o f intensity about 10% of the former peaks. These are assigned to SO2 +, SO +, H2SO2 + and H2SO +, respectively. A very weak peak at 80 m / z is probably SOa +. The absence of peaks at 243 and 185 m / z (strong in the mass s pe c t r um of pure (Bu4N)(HSO4)) showed that the source of sulfate was n o t residual electrolyte. The mass s p e c t r u m at 210 °C of the polymer prepared by bubbling ozone th r o u g h a solution of pyrrole in sulfuric acid showed the same peaks t og eth er with a strong peak at 67 m / z which is attributed to pyrrole. This might arise f r om some polypyrrole which has not been fully oxidized. The mass s p e c t r u m at 240 °C of the polymer prepared directly using Ag2SO4 as the oxidizing agent of pyrrole dissolved in acetonitrile was quite different. Strong peaks at 80 and 64 m / z were readily resolved but, in addition, a large n u m b e r of weaker peaks from 160 m / z downwards with mass intervals 12, 13, 14 suggested fragmentation of polypyrrole itself. A
48 peak at 67 m/z of the strength in between those due to SOa ÷ and SO2 + on the one hand and the fragmentation products of polypyrrole on the other may well be due to pyrrole (as was found with polypyrrole prepared using ozone). Since this third preparation of polypyrrole involved Ag2SO4 in a nonaqueous environment, then the expectation is that the anion in the polymer is SO42- . In contrast, the first preparation used (Bu4N)(HSO4) as carrier electrolyte suggesting the count er anion is [HSO4]-. The mass spectrum of the third polymer was quite different from the first two, suggesting that the backbone pyrrole polymer is different in the two cases. If the anion is [SO4[ 2-, then the polypyrrole unit has a double positive charge. This necessitates a different spatial and bonding relationship between the backbone polymer and counter anions c o m p a r e d with polypyrrole with a single charged anion.
Polypyrrole tetra(p-chlorophenyl) boronate (PPy (C6H4 CI) 4B) (electrolyticaUy prepared) The mass spectrum of this polymer is particularly well resolved above 100 m/ z with groups of peaks centred on 222, 186, 152 and 112 m/z. The 222 group comprises three strong peaks at 222, 224 and 226 with relative intensities 9:6:1 and attributed to p-dichlorobiphenyl +. A weaker but similar set of satellite peaks at 223, 225 and 227 m/z is prot onat ed p-dichlorobiphenyl +. The 186 group is weaker and consists of a pair of peaks at 186 and 188 m/z of relative intensities 3:1 and arising from chlorobiphenyl +. A weaker pair of lines at 186 and 189 m/z is the prot onat ed chlorobiphenyl + . A group of peaks from 1 4 9 - 1 5 3 m/z, with that at 152 m/z being strongest, arises from biphenyl ÷ with the loss of varying numbers of protons. The 112 group consists of a pair of peaks at 112 and 114 m /z with relative intensities of 3:1 arising from chlorobenzene + together with a similar but weaker pair at 111 and 113 m / z arising from chlorobenzene + with the loss of one proton. These results strongly suggest that the polymer does contain the anticipated tetra(p-chlorophenyl)boronate anion.
Polypyrrole tetrachloroferrate (PPy(FeCI¢)) (chemical preparation) The mass spectrum of this polymer showed poorly resolved peaks at 162, 164 and 165 m/z with relative intensities about 3:3:1 attributable to the FeC13H ÷ ion. Another group of lines at 127, 129 and 131 m/z with relative intensities about 9:6:1 are attributed to the FeCI2H + ion. There were many lines below 100 m / z arising from decomposition of the pyrrole polymer. This result corroborates the EXAFS results which provided evidence for retention of the FeCl4 unit in the polymer [6].
Polypyrrole hexafluorophosphate (PPY "PF6) (electrolytically prepared) The mass spectrum of this polymer showed many peaks from fragmentation of the polypyrrole cation as well as possible peaks from the PF6fragmentation. Amongst the many fragments with m /z < 150 arising from the
49 pyrrole polymer were peaks at 126, 107 and 88 m / z which could be assigned to PF5 +, PF4 + and PF3 ÷. Since their intensity was similar to that of the pyrrole fragments, we cannot be totally certain of their assignment.
Polypyrrole perchlorate (PPy . el04) (electrolyticaUy prepared) Fairly well-resolved pairs of peaks at 67, 69 and 83, 85 m / z can be assigned to chlorine-containing species and are probably ClO2 ÷ and ClO3 ~, respectively. Because of the abundance of pyrrole polymer fragments with re~z< 150, we cannot be totally certain of their assignment.
Polypyrrole tetrafiuoroborate (PPy .BF~) (electrolyticaUy prepared) Unambiguous assignment of peaks due to B - F species (BF4 ÷, BF3 +, BF2 +) could not be made amongst the fragmentation of pyrrole polymer fragments.
Polypyrrole nitrate (PPyNOs) (electrolytically prepared) Unambiguous assignment of peaks due to NOx species could not be made amongst the particularly well-resolved fragmentation pattern of pyrrole polymer peaks with re~z<240. A pattern of peaks repeated every 14 mass units suggested loss of CH2 or N. A strong peak at 112 m / z suggests the presence of pyiTole nitrite C4NH4-NO2 +.
Discussion There are a few reports of studies which have given structural information about the dopant anion in electrically conducting polymers. X-ray photoelectron spectroscopy has proved useful in elucidating the nature of the anion in iodine-doped polyethynylferrocene [9]. When doping is carried out in tetrahydrofuran solution, I5- ions are detected, whilst if doping is carried out in toluene, Is- is detected. Furthermore, if doping is carried out by mechanical grinding, the anion is I5- which converts to I3- when the polymer is subjected to mechanical stress. Our studies with bromine doping suggest the presence of Br3-; the Br~- unit has never been conclusively characterized in an isolated compound. The structure of I5- is known to be L shaped [10] and so it is not surprising that it breaks down to form I3- under mechanical stress. Another photoelectron study involving polyacetylene shows shifts in the spectrum upon doping with FeCl3, consistent with the charge transfer from C(ls) to the FeCla. Analysis of the Fe and Cl core levels shows that the F e / C1 atomic ratio is 1:4 and that the iron is in a high spin state, suggesting the dopant anion is [FeC14l- [11]. A MSssbauer study of [FeCl4]--doped polypyrrole shows that the quadrupole splitting is much greater than in doped polyacetylene, polyparaphenylene, polyparaphenylenesulfide and others [ 12 ]. This suggests that a larger distortion from Tu symmetry occurs, possibly due to hydrogen bonding via pyrrole N--H groups. Our mass spectral results on
50 FeCl3-doped p o l y p y r r o l e suggests the p r e s e n c e of FeCl3 (as FeCI3H ÷ in the m a s s s p e c t r u m ) , but no direct e v i d e n c e for the [FeCl4]-. A considerable n u m b e r of p a p e r s r e p o r t results using infrared and Raman s p e c t r o s c o p y in p e r c h l o r a t e - d o p e d polymers. Small c h a n g e s in the s p e c t r a are attributed to p o l y m e r f r e q u e n c i e s , but n o n e have b e e n o b s e r v e d to the C I - O stretching f r e q u e n c y a r o u n d 1100 c m - ' [13]. T h e r e is no d o u b t that the [ C 1 Q ] - unit is p r e s e n t in [ C 1 0 4 ] - - d o p e d polymers, but t h e r e are no r e p o r t s a b o u t deviations f r o m regular Td s y m m e t r y for the anion. Our mass s p e c t r a results provide indirect e v i d e n c e for the p r e s e n c e of [C104]-. Belanger has shown that w h e n p r o t o n a t e d poly(4-vinylpyridine) is loaded with [Fe(CN)6] 3-, the variation of the [Fe(CN)6]3-/[Fe(CN)~] 4- potential d e p e n d s o n pH and is a c c o u n t e d for b y p r o t o n a t i o n of [Fe(CN)6] 4-, w h e r e a s at p H > 4, the anion is not p r o t o n a t e d [14]. Our results with sulfate as c o u n t e r anion in p o l y p y r r o l e s h o w that b o t h [SO4] 2- and [HSO4]- exist in p o l y m e r s p r e p a r e d u n d e r different conditions. The different r e d o x b e h a v i o u r o f d o p a n t anions in a p o l y m e r c o m p a r e d with the anions in solution has b e e n highlighted b y Bidan e t a l . [15] and b y Kaye and Underhill [16]. W h e n p y r r o l e is e l e c t r o p o l y m e r i z e d in the p r e s e n c e of Keggin-type h e t e r o p o l y a c i d s , such as [SiW,2040] 4- o r [PW,2040] 4-, they yield p o l y m e r s having at c a t h o d e potentials several welldefined electronic transfers in c o n t r a s t to their electroactivity in solution [15]. P o l y p y r r o l e films having the r e d o x - a c t i v e anions [Ni(mnt)2]- and [ P d ( m n t ) 2 ] - (where m n t = m a l e o n i t r i l e d i t h i o l a t e ) s h o w e n h a n c e d stability. It is t h o u g h t that the anions b e h a v e as a solid state r e d o x buffer within the p o l y m e r film and maintain a potential in the solid b e t w e e n Eo and ER, the potentials at which t h e y d o n a t e or a c c e p t e l e c t r o n s [16]. It is clear that the p r o p e r t i e s a n d / o r s t r u c t u r e of the d o p a n t anion in electrically c o n d u c t i n g p o l y m e r s is influenced by the h o s t polymer, but few studies have p r o b e d the effect to any m a r k e d degree. W e believe that mass s p e c t r o s c o p y can be used fruitfully in this field.
Acknowledgement T h a n k s are due to the CNPq (Brazil) for the award o f a scholarship.
References 1 2 3 4
G. J. Baker, J. B. Raynor and A. Proh, J. Chem. Phys., 80 (1984) 5250. G. J. Baker, J. H. Holloway, J. B. Raynor and H. Selig, Synth. Met., 20 (1987) 323. J. B. Raynor, Mater. Sci. Forum, 21 (1987) 111. A. Profi, M. Zagorska, W. Fabianowski, J. B. Raynor and S. Lefrant, Polymer Commun., 28 (1987) 193. 5 A. Proh, W. Fabianowski, C. Budrowski, J. B. Raynor, Z. Kuchanski, J. Suwalski, S. Lefrant and G. Fatseas, Synth. Met., 18 (1987) 49. 6 J. B. Raynor and W. S. Schlindwein, in preparation.
51 7 H. Selig, J. H. Holloway, A. Profi and D. Billaud, J. Chem. Soc., Chem. Commun., (1983) 109. 8 H. Selig, J. H. HoUoway and A. Profi, J. Chem. Soc., Chem. Commun., (1982) 729. 9 G. Polzonetti, V. Faruffini, A. Furlani and M. V. Russo, Synth. Met., 29 (1989) E495. 10 F .W. Parrett and N. J. Taylor, J. Inorg. Nucl. Chem., 32 (1970) 2458. 11 G. Polzonetti, A. M. Cianciusi, A. Furlani and M. V. Russo, Synth. Met., 28 (1989) D413. 12 I. Kulszewicz-Bajer, A. Profi, J. Suwalski, Z. Kucharski and S. Lefrant, Synth. Met., 28 (1989) C225. 13 C. Zhong and K. Doblhofer, Synth. Met., 38 (1990) 117. 14 D. Belanger, Synth. Met., 29 (1989) E563. 15 G. Bidan, M. Lapkowski and J. P. Travers, Synth. Met., 29 ( 1 9 8 9 ) C l 1 3 . 16 B. Kaye and A. E. Underhill, Synth. Met., 28 (1989) C97.
45
Characterization of doped polypyrroles by mass spectrometry J. Barrie R a y n o r a n d W a l k i r i a S. Schlindwein Department of Chemistry, University of Leicester, Leicester LE1 7RH (UK)
(Received March 15, 1991; accepted in revised form July 12, 1991)
Abstract The technique of mass spectroscopy has been used to identify new polypyrroles made with [(C6H4CI)4B]-, [IC14]- and [Bra]- counter anions. Mass spectroscopy confirms the presence of Cl-, Br-, I-, (SO4) 2-, (HSO4)-, (FeCl4)-, (PF6)- and (CIO4)- as counter anions in other polypyrroles.
Introduction U n a m b i g u o u s c h a r a c t e r i z a t i o n of the d o p a n t a n i o n in electrically cond u c t i n g p o l y m e r s is n o t e a s y a n d o f t e n relies on e l e m e n t a l a n a l y s i s a n d the e x p e c t a t i o n of its p r e s e n c e b a s e d u p o n its use in the m e t h o d of p r e p a r a t i o n . Most studies have concentrated upon understanding conduction mechanisms o r a t t e m p t i n g to refine p r e p a r a t i v e p r o c e d u r e s in o r d e r to m a x i m i z e electrical c o n d u c t i v i t y . T h e v a s e m a j o r i t y of t h o s e p a p e r s w h i c h s e e k to i n v e s t i g a t e t h e m u t u a l i n t e r a c t i o n o f p o l y m e r c a t i o n a n d c o u n t e r a n i o n c o n s i d e r only t h e influence o f t h e c o u n t e r a n i o n u p o n the p r o p e r t i e s of the b u l k p o l y m e r . T h e inevitable influence of t h e p o l y m e r b a c k b o n e u p o n the s t r u c t u r e o f c o u n t e r a n i o n is r a r e l y m e n t i o n e d , let a l o n e investigated. In r e c e n t y e a r s , w e h a v e a p p l i e d a r a n g e of p h y s i c a l m e t h o d s to the s t u d y o f a v a r i e t y of d o p e d c o n d u c t i n g p o l y m e r s , in p a r t i c u l a r p o l y a c e t y l e n e a n d p o l y p y r r o l e . By u s i n g e l e c t r o n n u c l e a r d o u b l e r e s o n a n c e s p e c t r o s c o p y , w e h a v e d e t e r m i n e d t h e a v e r a g e length of the c o n j u g a t e d c h a i n in UF6- , ReF~- a n d SnC14-doped p o l y a c e t y l e n e s a n d identified t h e likely p o s i t i o n o f t h e c o m p l e x a n i o n s relative to t h e p o l y a c e t y l e n e b a c k b o n e [ 1 - 3 l . In t h e case of a [FeC14]--doped polypyrrole and a composite of polypyrrole and p o l y ( v i n y l alcohol), e l e c t r o n s p i n r e s o n a n c e a n d M S s s b a u e r s t u d i e s [4, 5] a n d EXAFS [6] m e a s u r e m e n t s h a v e h e l p e d to identify t h e c h e m i c a l s t r u c t u r e o f t h e d o p a n t . In o t h e r s t u d i e s in o u r l a b o r a t o r y , m e t a l h e x a f l u o r i d e s a n d m e t a l p e n t a f l u o r i d e s w e r e d o p e d into p o l y a c e t y l e n e [7, 8]. M a s s s p e c t r a l s t u d i e s o f CH(WF6) ~ a n d CH(TeF6) ~ s h o w e d t h e p r e s e n c e o f W-F4 + a n d TeF4 + ions [8]. In this p a p e r , w e m a k e u s e o f n e g a t i v e ion e l e c t r o n i m p a c t a n d n o r m a l e l e c t r o n i m p a c t m a s s s p e c t r o s c o p y in o r d e r to aid c h a r a c t e r i z a t i o n of t h e
0379-6779/92/$5.00
© 1992- Elsevier Sequoia. All rights reserved
46 dopant anion in a range of doped polypyrroles. We also tried to use fast atom b o m b a r d m e n t but were unsuccessful in obtaining any spectra. To our knowledge, no o t her mass spectroscopic studies of note have been published on electrically conducting polymers.
Experimental Preparation of polymers Electrochemical syntheses of polypyrrole were carried out in a onec o m p a r t m e n t cell with three electrodes: a platinum working electrode, a platinum secondary electrode and a saturated calomel reference electrode. The solvent used was dry acetonitrile and the electrolyte and pyrrole concentrations used were typically 0.1 M and 0.05 M, respectively. The following electrolyte salts were used: (Et4N)(BF4), (Et4N)(CIO4), K[B(C~H4CI)4], (Et4N)C1, (Et4N)Br, (Et4N)I, (Me4N)(PF0), (Et4N)(C104), (Bu4N)(HSO4) and (Et4N)(NO3). Solutions were purged with nitrogen gas and films grown at a constant potential of + 0.85 V versus SCE. Chemical preparation of polypyrrole was carried out by direct admixture of a solution of pyrrole in acetonitrile and a solution of the oxidant in acetonitrile. Oxidants used were (Bu4N)(ICI4), (Me4N)(Br3), (Me4N)(FeC14) and Ag2(SO4). Another polypyrrole was pr e p ared by passing ozonized oxygen through a solution of pyrrole in dilute sulfuric acid. In all preparations, the polymer was thoroughly washed with acetonitrile in a Soxhlet extractor for 3 h.
Mass spectra Negative ion electron impact spectra were recorded on an AEI MS 902 instrument at 70 eV electron energy and 7 kV ion energy at Nottingham University by Dr David Knight. Normal electron impact mass spect rom et ry was carried out using a VG analytical micromass 16B mass spectrometer. Samples were directly inserted into the probe as solids. Unless otherwise stated, the mass spectra described are of the latter type.
Results Polypyrrole halides (Cl-, Br-, I - ) (electrochemical preparation) Negative electron impact ( E I - ) spectra showed strong peaks in each case. For the chloride at 150 °C, two peaks at 36 and 38 m/z are assigned to HCI-. Similarly with bromide at 150 °C, two peaks at 80 and 82 m/z are assigned to H B r - . The iodide required a higher probe t em perat ure (200 °C) to give a peak at 128 m / z assigned to H I - . Not surprisingly, the expect ed halide anion was p r o t o n a t e d by the abundance of protons derived from fragmentation of the polypyrrole.
Polypyrrole tribromide (PPyBr3) (chemical preparation) E I - spectra of the sample anticipated to be PPyBr3 required a probe t e mp er atu r e of 300 °C before a sufficiently strong spectrum could be obtained.
47
It showed only a pair of peaks at 80 and 82 m / z attributable to H B r - . Since no spectrum could be obtained at the t em p erat ure (150 °C) at which PPyBr gave a spectrum, t hen the c om pound was different and was most likely PPyBr3 as anticipated. The known propensity for the Br3- anion to decom pose into Br2 and B r - , and of Br2 to split readily in the mass s p e c t r o m e t e r to Br atoms, accounts for seeing only H B r - in the mass spectrum. The normal mass s p ectr u m at 200 °C showed a pair of peaks at 79 and 81 m / z due to bromine isotopes and a pair at double the intensity of the other at 80 and 82 m / z attributed to Br ÷ and HBr +, respectively. During polymerization, the polymer cation is apparently more stable with a Br3- c o u n t e r anion from surplus Me4NBr3 present in solution than with B r - formed as the reduction pr oduc t of Br3-.
Polypyrrole tetrachloroiodate (PPyICIJ (chemical preparation) E I - s p ectr a of the sample anticipated to be PPyIC14 required a probe t e m p e r a t u r e of 300 °C for a good spectrum. This showed peaks at 36, 38 and 128 m / z which were assigned to HC1 and H I - . The intensity of HC1was 3.6 times that of the H I - line strongly suggesting that the ICL - unit was intact in the sample. The fact that no spectrum was obtained at 200 °C indicated that separate I - and Cl- were not present, nor a combination of IC12- and Cl2. Any free Cl2 would have fragmented to C1 atoms in the mass spectrometer. That the c o u n t e r anion in this polypyrrole is ICl4- is somewhat surprising bearing in mind its m e t h o d of preparation using (Bu4N)(IC14) as the oxidizing agent. The e x p e c t e d reduction products would be I and Cl- and the e x p e c t e d polymer polypyrrole iodide or chloride. The evidence suggests that surplus (ICl4)- takes p r e f e r e n c e over I - or Cl- as the counter ion during polymerization.
Polypyrrole sulfate ((PPy) 2S0 4 or PPyHSO ¢) The EI + spect r um of this polymer (prepared electrolytically) showed two strong peaks at 64 and 48 m / z with associated satellite peaks at 66 and 50 m / z o f intensity about 10% of the former peaks. These are assigned to SO2 +, SO +, H2SO2 + and H2SO +, respectively. A very weak peak at 80 m / z is probably SOa +. The absence of peaks at 243 and 185 m / z (strong in the mass s pe c t r um of pure (Bu4N)(HSO4)) showed that the source of sulfate was n o t residual electrolyte. The mass s p e c t r u m at 210 °C of the polymer prepared by bubbling ozone th r o u g h a solution of pyrrole in sulfuric acid showed the same peaks t og eth er with a strong peak at 67 m / z which is attributed to pyrrole. This might arise f r om some polypyrrole which has not been fully oxidized. The mass s p e c t r u m at 240 °C of the polymer prepared directly using Ag2SO4 as the oxidizing agent of pyrrole dissolved in acetonitrile was quite different. Strong peaks at 80 and 64 m / z were readily resolved but, in addition, a large n u m b e r of weaker peaks from 160 m / z downwards with mass intervals 12, 13, 14 suggested fragmentation of polypyrrole itself. A
48 peak at 67 m/z of the strength in between those due to SOa ÷ and SO2 + on the one hand and the fragmentation products of polypyrrole on the other may well be due to pyrrole (as was found with polypyrrole prepared using ozone). Since this third preparation of polypyrrole involved Ag2SO4 in a nonaqueous environment, then the expectation is that the anion in the polymer is SO42- . In contrast, the first preparation used (Bu4N)(HSO4) as carrier electrolyte suggesting the count er anion is [HSO4]-. The mass spectrum of the third polymer was quite different from the first two, suggesting that the backbone pyrrole polymer is different in the two cases. If the anion is [SO4[ 2-, then the polypyrrole unit has a double positive charge. This necessitates a different spatial and bonding relationship between the backbone polymer and counter anions c o m p a r e d with polypyrrole with a single charged anion.
Polypyrrole tetra(p-chlorophenyl) boronate (PPy (C6H4 CI) 4B) (electrolyticaUy prepared) The mass spectrum of this polymer is particularly well resolved above 100 m/ z with groups of peaks centred on 222, 186, 152 and 112 m/z. The 222 group comprises three strong peaks at 222, 224 and 226 with relative intensities 9:6:1 and attributed to p-dichlorobiphenyl +. A weaker but similar set of satellite peaks at 223, 225 and 227 m/z is prot onat ed p-dichlorobiphenyl +. The 186 group is weaker and consists of a pair of peaks at 186 and 188 m/z of relative intensities 3:1 and arising from chlorobiphenyl +. A weaker pair of lines at 186 and 189 m/z is the prot onat ed chlorobiphenyl + . A group of peaks from 1 4 9 - 1 5 3 m/z, with that at 152 m/z being strongest, arises from biphenyl ÷ with the loss of varying numbers of protons. The 112 group consists of a pair of peaks at 112 and 114 m /z with relative intensities of 3:1 arising from chlorobenzene + together with a similar but weaker pair at 111 and 113 m / z arising from chlorobenzene + with the loss of one proton. These results strongly suggest that the polymer does contain the anticipated tetra(p-chlorophenyl)boronate anion.
Polypyrrole tetrachloroferrate (PPy(FeCI¢)) (chemical preparation) The mass spectrum of this polymer showed poorly resolved peaks at 162, 164 and 165 m/z with relative intensities about 3:3:1 attributable to the FeC13H ÷ ion. Another group of lines at 127, 129 and 131 m/z with relative intensities about 9:6:1 are attributed to the FeCI2H + ion. There were many lines below 100 m / z arising from decomposition of the pyrrole polymer. This result corroborates the EXAFS results which provided evidence for retention of the FeCl4 unit in the polymer [6].
Polypyrrole hexafluorophosphate (PPY "PF6) (electrolytically prepared) The mass spectrum of this polymer showed many peaks from fragmentation of the polypyrrole cation as well as possible peaks from the PF6fragmentation. Amongst the many fragments with m /z < 150 arising from the
49 pyrrole polymer were peaks at 126, 107 and 88 m / z which could be assigned to PF5 +, PF4 + and PF3 ÷. Since their intensity was similar to that of the pyrrole fragments, we cannot be totally certain of their assignment.
Polypyrrole perchlorate (PPy . el04) (electrolyticaUy prepared) Fairly well-resolved pairs of peaks at 67, 69 and 83, 85 m / z can be assigned to chlorine-containing species and are probably ClO2 ÷ and ClO3 ~, respectively. Because of the abundance of pyrrole polymer fragments with re~z< 150, we cannot be totally certain of their assignment.
Polypyrrole tetrafiuoroborate (PPy .BF~) (electrolyticaUy prepared) Unambiguous assignment of peaks due to B - F species (BF4 ÷, BF3 +, BF2 +) could not be made amongst the fragmentation of pyrrole polymer fragments.
Polypyrrole nitrate (PPyNOs) (electrolytically prepared) Unambiguous assignment of peaks due to NOx species could not be made amongst the particularly well-resolved fragmentation pattern of pyrrole polymer peaks with re~z<240. A pattern of peaks repeated every 14 mass units suggested loss of CH2 or N. A strong peak at 112 m / z suggests the presence of pyiTole nitrite C4NH4-NO2 +.
Discussion There are a few reports of studies which have given structural information about the dopant anion in electrically conducting polymers. X-ray photoelectron spectroscopy has proved useful in elucidating the nature of the anion in iodine-doped polyethynylferrocene [9]. When doping is carried out in tetrahydrofuran solution, I5- ions are detected, whilst if doping is carried out in toluene, Is- is detected. Furthermore, if doping is carried out by mechanical grinding, the anion is I5- which converts to I3- when the polymer is subjected to mechanical stress. Our studies with bromine doping suggest the presence of Br3-; the Br~- unit has never been conclusively characterized in an isolated compound. The structure of I5- is known to be L shaped [10] and so it is not surprising that it breaks down to form I3- under mechanical stress. Another photoelectron study involving polyacetylene shows shifts in the spectrum upon doping with FeCl3, consistent with the charge transfer from C(ls) to the FeCla. Analysis of the Fe and Cl core levels shows that the F e / C1 atomic ratio is 1:4 and that the iron is in a high spin state, suggesting the dopant anion is [FeC14l- [11]. A MSssbauer study of [FeCl4]--doped polypyrrole shows that the quadrupole splitting is much greater than in doped polyacetylene, polyparaphenylene, polyparaphenylenesulfide and others [ 12 ]. This suggests that a larger distortion from Tu symmetry occurs, possibly due to hydrogen bonding via pyrrole N--H groups. Our mass spectral results on
50 FeCl3-doped p o l y p y r r o l e suggests the p r e s e n c e of FeCl3 (as FeCI3H ÷ in the m a s s s p e c t r u m ) , but no direct e v i d e n c e for the [FeCl4]-. A considerable n u m b e r of p a p e r s r e p o r t results using infrared and Raman s p e c t r o s c o p y in p e r c h l o r a t e - d o p e d polymers. Small c h a n g e s in the s p e c t r a are attributed to p o l y m e r f r e q u e n c i e s , but n o n e have b e e n o b s e r v e d to the C I - O stretching f r e q u e n c y a r o u n d 1100 c m - ' [13]. T h e r e is no d o u b t that the [ C 1 Q ] - unit is p r e s e n t in [ C 1 0 4 ] - - d o p e d polymers, but t h e r e are no r e p o r t s a b o u t deviations f r o m regular Td s y m m e t r y for the anion. Our mass s p e c t r a results provide indirect e v i d e n c e for the p r e s e n c e of [C104]-. Belanger has shown that w h e n p r o t o n a t e d poly(4-vinylpyridine) is loaded with [Fe(CN)6] 3-, the variation of the [Fe(CN)6]3-/[Fe(CN)~] 4- potential d e p e n d s o n pH and is a c c o u n t e d for b y p r o t o n a t i o n of [Fe(CN)6] 4-, w h e r e a s at p H > 4, the anion is not p r o t o n a t e d [14]. Our results with sulfate as c o u n t e r anion in p o l y p y r r o l e s h o w that b o t h [SO4] 2- and [HSO4]- exist in p o l y m e r s p r e p a r e d u n d e r different conditions. The different r e d o x b e h a v i o u r o f d o p a n t anions in a p o l y m e r c o m p a r e d with the anions in solution has b e e n highlighted b y Bidan e t a l . [15] and b y Kaye and Underhill [16]. W h e n p y r r o l e is e l e c t r o p o l y m e r i z e d in the p r e s e n c e of Keggin-type h e t e r o p o l y a c i d s , such as [SiW,2040] 4- o r [PW,2040] 4-, they yield p o l y m e r s having at c a t h o d e potentials several welldefined electronic transfers in c o n t r a s t to their electroactivity in solution [15]. P o l y p y r r o l e films having the r e d o x - a c t i v e anions [Ni(mnt)2]- and [ P d ( m n t ) 2 ] - (where m n t = m a l e o n i t r i l e d i t h i o l a t e ) s h o w e n h a n c e d stability. It is t h o u g h t that the anions b e h a v e as a solid state r e d o x buffer within the p o l y m e r film and maintain a potential in the solid b e t w e e n Eo and ER, the potentials at which t h e y d o n a t e or a c c e p t e l e c t r o n s [16]. It is clear that the p r o p e r t i e s a n d / o r s t r u c t u r e of the d o p a n t anion in electrically c o n d u c t i n g p o l y m e r s is influenced by the h o s t polymer, but few studies have p r o b e d the effect to any m a r k e d degree. W e believe that mass s p e c t r o s c o p y can be used fruitfully in this field.
Acknowledgement T h a n k s are due to the CNPq (Brazil) for the award o f a scholarship.
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