J. Phys. Chem. 1983,87,1459-1463
1459
Electrochemical Studies of Some Conducting Polythiophene Films Robert J. Wattman," Joachlm Bargon, and A. F. Diaz IBM Research Laboratoty, San Jose, California 95 193 (Received: September 29, 1982)
Polythiophene and @-substitutedpolythiophenes are prepared by electrochemicaloxidation and polymerization of their respective monomers. A property-structure correlation between monomers and their corresponding polymers is obtained by systematic variation of the chemical structure of the monomers. Film growth and conductivities are dependent on &substituents, with conductivity of the order of 1 ohm-' cm-' obtained for poly@-methylthiophene).
Introduction The application of electrochemical methods for the generation of some conducting organic polymers as free standing films is now ~ell-established.'-~In particular, recent studies with electroactive polypyrrole film obtained via electropolymerization of pyrrole and some N-substituted pyrrole derivatives have demonstrated the facility of the electrochemical approach to manufacturing of conducting A major advantage of the electrochemical approach over standard chemical methods include a clean, one-step production of polymeric material directly onto a Pt electrode surface, from which self-supporting films can be peeled off. More importantly, a wide choice of counteranions available from different electrolytes allows facile variation of polymer film properties. For example, polypyrroles with different counteranions show conductivities which vary over five orders of magnitude. Since polypyrrole is thought to form almost exclusively via a-a' linkages, it is of interest to alter the properties of the polymers via modification of the monomeric pyrroles with @-substituents. Appropriate @-substituentsshould induce a "push-pull" effect on the 7r-electronswhich may alter the electrochemical character of the corresponding polypyrrole films. However, @-substitutionof pyrroles is synthetically difficult and, therefore, with the exception of poly(3-methylpyrrole) reported herein, little data are available. In this regard, the pyrrole analogous heterocycle thiophene becomes particularly interesting as it also gives rise to conducting polymers: and @-substitutedthiophenes are much more readily obtained. Therefore, in order to investigate further the effects of chemical modification on the electrochemical character of conducting polymeric films derived from five-membered heterocycles, we have examined polythiophene along with some of its p-substituted forms with standard electrochemical methods.
Experimental Section All compounds used were obtained from commercial (1)A. F. Diaz, K. K. Kanazawa, and G. P. Gardini, J. Chem. SOC., Chem. Commun., 635 (1979). ( 2 ) A. F. Dim and J. A. Logan, J.Electroanul. Chem., 111,111 (1980). (3)A. F.Diaz and T. C. Clarke, J. Electroanal. Chem., 111,115(1980). (4)R. J. Waltman and J. Bargon, to be published. (5)K. K. Kanazawa, A. F. Diaz, R. H. Geiss, W. D. Gill, J. F. Kwak, J. A. Logan, J. F. Rabolt, and G. B. Street, J. Chem. SOC.,Chem. Commun., 854 (1979). (6)A. F.D i u and J. I. Castillo, J. Chem. SOC.,Chem. Commun., 397 (1980). (7)A. F.Diaz, J. I. Castillo, J. A. Logan, and W. Y. Lee,J.Electroanal. Chem., 129,115 (1981). (8)A. F. Diaz, Chem. Scr., 17, 145 (1981). (9)(a) T.Yamamoto, K. Sanechika, and A. Yamamoto, J.Polym. Sci., Polym. Lett. E d , 18,9(1980);(b)G.Kossmehl and G. Chatzitheodorou, Makromol. Chem., Rapid Commun.,2,551 (1981). 0022-3654/83/2087-1459$01.50/0
sources. Polythiophene and poly-p-substituted thiophene films were prepared by electrochemical oxidation of the appropriate monomers in a three-electrode, single-compartment cell. The films were grown on a platinum working electrode with a gold counterelectrode and a sodium chloride saturated calomel reference electrode (SSCE). The solutions used for the preparation of films typically contained M of the appropriate thiophene monomer with 0.1 M electrolyte (usually tetraethylammonium tetrafluoroborate, TEAFB) in acetonitrile. The acetonitrile (Burdick and Jackson) was used directly without further purification, and all systems were bubbled with argon for 5-10 min. All electrochemical measurements were made with an IBM EC225 voltammetric analyzer.
Results and Discussion Cyclic voltammetric data for the monomer and the corresponding polymer films obtained from thiophene and some @-substitutedthiophenes are given in Table I. All monomers show irreversible oxidation peaks suggesting the intermediate radical cation is unstable and extremely reactive, and, in several cases, a single sweep led to the formation of polymeric films onto the Pt electrode surface. The irreversible nature of the thiophene systems can be explained in terms of a reaction pathway as proposed previously for polypyrrole1° (Scheme I). When the peak oxidation potentials E,, of monomeric thiophene and thiophene derivatives are plotted vs. Hammett substituent constants, a linear correlation is obtained (Figure 1). The Hammet substituent constant u+i for para substituents for the reaction type Ro R+ was used." A linear correlation was obtained by using ud for para substituents also, even though this is pertinent for the reaction type Ro R-. The resultant data indicate that the substituents influence the oxidation reaction in a predictable manner. The position of the 3-thiophenecarboxylic acid in this plot suggests that the oxidation of this molecule results in the loss of an electron from the thiophene ring and not from the carboxylic acid group under these conditions. Not all of the thiophene monomers selected for this experiment give rise to polymeric films (Table I). However, for those which do, a plot of E,, monomer vs. E , polymer (Figure 2) is found to yield a linear correlation with a slope of 0.64, suggesting that the effect of @-substituentsis essentially similar for both the thiophene monomers and
-
-
(10)A. F. Diaz, A. Martinez, K. K. Kanazawa, and M. Salmon, J. Electroanal. Chem., 130,181 (1981). (11) C. D. Ritchie and W. F. Sager, "An Examination of StructureReactivity Relationship", in 'Progress in Physical Organic Chemistry", Vol. 2, S. G. Cohen, A. Streitwieser, and R. W. Taft, Ed., Interscience, New York, 1964.
0 1983 American Chemical Society
1460
The Journal of Physical Chemistry, Vol. 87, No. 8, 1983
Waltman et al.
TABLE I: Cyclic Voltammetric Data for 1 0 ~ 4 - 1 0M~ s Thiophenes Measured in 0.1 M TEAFB/CH,CN Using Pt vs. SSCE E,, monomer
compd
thiophene 2,2'-bithiophene 2-( 3-thieny1)pyridine 3-methylthiophene 3-thiopheneacetic acid 3-thiophenemalonic acid 3-bromothiophene 34hiopheneacetonitrile 3,4-dibromothiophene 3-thiophenecarboxylic 3-thiophenecarboxaldehyde
v polymer
2.06 1.32 1.69; 2.35
0.96 1.00
0.72
1.86 1.94 2.02 2.10 2.22
1.06 1.12
2.23
1.33
2.28 2.35
-0.4 -0.2
0
0.2
0.4
U
Scheme I
qx/J X = NH,S
-
Figure 1. Electrochemical oxldation of P-substttuted thiophene monomers, where p = Me, H, Br, and COOH from left to right, respectively.
qx7 ,Nucleophile
\
t
Soluble Products
their respective polymers where the latter are almost twice as sensitive to substituent effects. This result is important because it is a key example of a property-structure correlation between a conducting polymer and its corresponding monomer produced by a systematic variation in the chemical structure. A combination of Figures 1 and 2 leads to the conclusion that @-substitutedthiophene monomers as well as their respective polymers must contain a related system of a-electrons. This finding is most easily reconciled by assuming the naive polymer structure as expected to result from oxidative coupling of the heterocyclic monomers via their a-positions. Thus, prior assertions have suggested that the electrochemically induced polymerization of both pyrrole and thiophene proceeds via a two-electron process followed by loss of the two protons per monomer unit whereby the resulting linkages occur primarily in the a-positions.1° This is further substantiated by lack of any formation of polymeric films when electrochemically oxidizing a-substituted thiophenes, e.g., 2-methyl- and 2-iodothiophene, under otherwise identical conditions as for the @-substitutedthiophenes. Polythiophene films were grown on Pt electrode surfaces by electropolymerization of either thiophene or 2,2'-bithiophene at ca. 150 mV below their respective EPavalues. The cyclic voltammogram for the [Ptl-polythiophene-BF, (tetrafluoroborate anion, BF4-) film prepared from 2,2'bithiophene and measured in 0.1 M TEAFB/acetonitrile is shown in Figure 3. The [Ptl-polythiophene-BF, film oxidizes at ca. 1.0 V and is electroactive in this region. The i, values scale linearly with sweep rate between 20 and 90 mV s-l, as expected for a surface attached species. The film can be cycled repeatedly between the conducting (oxidized) and nonconducting (neutral) state with no significant decomposition of the material. The switching is also accompanied by a reversible color change from a pale yellow (neutral) to black (oxidized). ESCA studies reveal, however, the presence of bonded oxygen in the surface region of the thin films. Although the switching reaction of polythiophene is not as sensitive to oxygen as is polypyrrole, oxygen has to be hermetically excluded during the switching process in order to assure reproducibility. Preparation of films of a thickness of approximately 60
1
t
1
0.6
0.8 E,,
1.0
1.2
1.4
polvmer ( V i
Figure 2. Peak oxidation potentlals of thiophene monomers vs. their respective polymers in 0.1 M TEAFB/CH,CN solution. The slope was found to be 0.637 with a correlation of 0.948.
nm required ca. 24 mC cm-2. The film (flotation) density equaled 1.53 g/cm3, and n was assumed to equal 2.25.12 The oxidation reaction of [PtJ-polythiophene-BF, film involves a charge of ca. 1 mC cm-*, approximately 4% of the amount required to prepare the film. Electrochemical data suggest ca. 20% of the thiophene units (from the 2,2'-bithiophene monomers) are in the oxidized form (assuming n = 2.25). However, it should be noted that the degree of oxidation very much depends on the monomers, thus, polythiophene from thiophene monomer is oxidized to the extent of ca. 10%. In the region cathodic of 1.0 V, the [Ptl-polythiophene-BF4 is neutral and should be insulating. This is indeed demonstrated in the following: When using [Ptl-polythiophene-BF, as an electrode in a 0.1 M TEAFB/acetonitrile solution containing M each of nitrobenzene and of triphenylamine, the cyclic voltammogram shows the peaks for the oxidation reaction of triphenylamine (0.91 V vs. SSCE) since the f i i is oxidized and conducting in this region. When in the neutral form, however, the polymer is nonconducting and, accordingly, no reduction peak is seen for nitrobenzene which would otherwise have been expected to occur at -1.1 V vs. SCE (Figure 4). With the introduction of a &methyl substituent, the polymer becomes more easily oxidized. Thus, the cyclic (12) Polymerization of these heterocyclic compounds is thought to involve two electrons and the measured n value for polypyrrole is n = 2.25 (see ref 7), whereby the excess 0.25 charge is thought to be consumed in the partial oxidation of the film.
The Journal of Physical Chemistv, Vol. 87, No. 8, 1983
ElectrochemicalStudies of Conducting Polythiophene Films 90 mV/s 80
70
1461
90 mVls
A
60 50 40
1 0 0Af ~
T
30 20
0
5
e
d 0 .‘TI
5 d
1 -0.4 1
I 0
I e.4
I to.8
I t1.2
E/V
I 0
I -0.4
I t0.4
I t0.8
EIV
Figure 5. Cyclic voltammograms of a 0.1 bm thick [Pt]-poly(3methy1thiophene)-BF, film in 0.1 M TEAFB/CH,CN solution.
Figure 3. Cyclic voltammograms of a 60 nm thick [Pt]-polythiophene-BF, film in 0.1 M TEAFB/CH3CN solution.
A
I \
200pA
f,
.-U
e
I I
Start
a .-V
D 0
5
B I
I
r:
I -1.5
I I -1.0 -0.5
I 0
I I t0.5 t1.0
EN
Figure 6. Cyclic voltammograms of a 0.1 M TEAFB/CH,CN solution containing M nitrobenzene and M phenothiazine at 60 mV s-’ using a sodium chloride calomel reference electrode: (a) [Pt]poly(3-methylthiophene)-BF4and (b) Pt electrode.
Start _j
-1.0
-0.5
0
+0.5
+1.0
E/V Figure 4. Cyclic voltammograms of a 0.1 M TEAFB/CH,CN solution containing M nltrobenzene and M triphenylamine at 60 mV s-’ using sodium chloride calomel reference electrode: (a) [Ptlpolythiophene-BF, and (b) Pt electrode.
voltammogram for [Pt]-poly(3-methylthiophene)-BF4film in 0.1 M TEAFB/acetonitrile shows similar features, only the presence of the @-methylsubstituent shifts the oxidation potential ca. 300 mV cathodic of the E,, value of
the unsubstituted polymer film, and is therefore electroactive in a region considerably less anodic than the parent (Figure 5). The degree of oxidation of poly(3methylthiophene) is 0.12 charges per monomer unit as estimated from electrochemical data ( p = 1.46 g/cm3 for PFe film,n is assumed to equal 2.25), similar to the parent polythiophene. As with polythiophene, poly(3-methylthiophene) can be cycled repeatedly between the conducting and nonconducting states and exhibits electrochromic behavior (black and pale brown, respectively) (Figure 6). Thin films of 3-thiopheneacetonitrile and 3-bromo- and 3,4-dibromothiophenes adhered onto Pt could also be obtained (Table I). However, films are produced only when these monomers are oxidized at ca. 100 mV above their E,, values. Simultaneously, the solution became colored near the Pt electrode surface, and much diffusion into the solvent was observed. This suggests that a significant amount of short-chain oligomers (dimers) is being formed which is stable enough to diffuse away from the electrode. The bulky @-substituentsmay hinder chain propagation
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Wattman et al.
The Joumal of phvscal Chemistry, Vol. 87, No. 8, 1983
Figure 7. Scanning electron micrographs(SEM) of (a) polythiophene-BF,, (b) poly(3-bromothiophene)-BF,, and (c) poly(3-methytthiophene)-BF4 films grown in CH,CN.
by steric crowding, thereby giving rise to a more nonplanar polymer. The consequences of nonplanarity and lower molecular weight for the electrical conductivities of the corresponding films are not yet understood in detail, although one would naively expect that the conductivities would be lowered. Scanning electron micrographs (SEM) for polythiophene, poly(3-methylthiophene), and poly(3-bromothiophene) show no evidence of crystalline features (Figure 7), similar to polypyrrole.8 Free-Standing Films. Free-standing films peeled off from the Pt electrode of typical thickness 10-4-10-3 cm could be obtained only with thiophene, bithiophene, and 3-methylthiophene monomers under these conditions, and conductivity data as measured by four probe techniques are summarized in Table 11. Interpretation of the variation in the conductivities observed with films from different monomers with nonidentical counteranions is difficult. Variations in anions alone can produce conductivity differences of the order of 105ohm-l cm-' as was demonstrated with polypyrrole films incorporating different counteranions. With polythiophenes, we have found that self-supporting films are only obtained with certain monomers in specific electrolytes. Exchange of counteranions in the eventual polymeric films might represent a possibility to deviate this problem, but, so far, this concept has not been exploited in the systems discussed here due to the unknown effects of the original counterions upon the structure of the polymers. Since thiophene-derived polymer gives free-standing films with BF4- or PF6- counteranions, whereas bithiophene derived films are only obtained with HS04- counterions, comparison of the specific conductivities (a) of polythiophene-PF6 and po1y(bithiophene)-HSO4 suffer from this incompatibility. Intuitively, one would expect an increase in c for the bithiophene-derived polymer, which is expected to yield more regularly CY-CY' linked thiophene units. Bithiophene, however, may form a more stable radical cation which may hinder the overall formation of the polymeric chain. Since the respective molecular weights are unknown, these questions cannot be answered. However, in considering the effect of the @-methylgroup on the properties of the polymer, it is interesting to note that the conductivity of a PF6 film of poly(@-methylthiophene) is higher than that of the parent polythiophene-PF6 by a factor of lo2. This may be explained in part by the following reasons:
TABLE 11: Electrochemical Data for Some Conducting Polythiophene Films
1 Wn I I I I I I
r
i
I
I
-
I
I
0.02
I
I
I
I
0.02. 0.06
I
I
I
I
(1) The structure of poly(0-methylthiophene) could in principle be less regular because of the possibility of head-to-tail or head-to-head addition yielding @-methyl/ @-methylor O-methyl/@-hydrogenarrangements. These effects may, however, be offset by the @-methylgroup's ability to prevent any @-linkagebetween monomer units and perhaps thereby imparting a greater linearity to the overall structure of the polymer, which could result in improved conductivity; (2) The electron-releasing methyl group may improve carrier numbers and/or mobility. Nevertheless, the 100-fold increase in conductivity exhibited by the @-methylsubstitutent offers some interesting possibilities along with conventional doping techniques used to enhance the conductivity of polymers. Even though the improvement factor does not approach values obtained via doping with 12,Br2, C12,or AsF5 (for example, polyacetylene doped with AsF, gives rise to a conductivity enhancement of 107),13introduction of a @-methylsubstituent, as discussed for polythiophene here, provides for an alternative, avoiding the use of the highly toxic dopants mentioned above, and furthermore, allowing retention of film flexibility which is typically lost after doping.14 The conductivity data obtained here by electrochemical methods for the parent polythiophene (0.02 ohm-' cm-l) agree well with the values obtained by Yamamoto15 for (13)C. K. Chiang, C. R. Fincher, Jr., Y. W. Park,A. J. Heeger, H. Shirakawa, E. J. Louis, S. C. Gau, and A. G. MacDiarmid, Phys. Rev. Lett., 39, 1098 (1977). (14) G. B. Street and T. C. Clarke, IBM J. Res. Develop., 25,51 (1981).
Electrochemical Studles of Conducting Polythiophene Films
Scheme I1 (n
+ 2)[X]
-Z(n
+ 2)e-
X’-[X],-X”
The Journal of Physical Chemistry, Vol. 87, No. 8, 1983
1463
TABLE 111: Electrochemical Data for Some Conducting Polypyrrole Films
+ (2n + 2)H+
X = monomer chemically produced and I, doped polythiophene (10-3-10-2 ohm-‘ cm-’). Tourillon et a1.I6 have recently reported conductivities of 10-100 ohm-’ cm-’ for electrochemically polymerized polythiophene-C104, but we have not been able to reproduce their data. Thermal stability studies of polythiophene yield data which appear comparable to those otained for polypyrrole,8 Le., both of which were found to be stable to ca. 150 “C. Poly(3-methylthiophene), however, appears to be stable up to 200 “C.
Conclusions By now, a rather broad variety of aromatic monomers have been found to undergo electrochemically induced oxidative polymerization to yield electrically conductive films. This fact suggests that the polymerization mechanisms of these more or less related molecules may follow a rather general if not identical concept? More specifically, for the five-membered heterocyclic systems studied here, in particular polypyrrole and polythiophene, the overall polymerization process has been found to have electrochemical stoichiometry.8J0 Thus, the overall polymerization process for these five-membered heterocycles is readily symbolized by Scheme 11, whereby the linkages occur primarily at t h a-positions, as suggested above. Details of a polymerization process in polythiophene-AsF5 complexes has been forwarded by Kossmehl and Chatzitheod o r ~ uT .h~is~study is of interest because it is the chemical analogue to our electrochemical polymerization process. In addition, we have found that the fractional charge, for example, per thiophene unit, decreases as a function of @-substitutionfrom 0.22 for the parent polythiophene, to 0.12 for the @-methyl-substitutedform, to 0.001 for @-bromo-,and to 10-3-10-4 for the @-CH2CN-and @-p’dibromo-substituted thiophenes. Accordingly, the simple (15) T. Yamamoto, paper presented a t the 161st Meeting of the Electrochemical Society, 1982. (16) G. Tourillon and F. Garnier, J. Electroanal. Chem., 135, 173 (1982).
bl
EIJa
Compound Polymer (V)
,hm-ycm-, (BFi)
Charge Fraction
-0.150
100
0.25
-0.250
4
0.25
-0.500
0.25
notion of a-linked polythiophenes is incomplete: Fractions of counteranions are essential and they affect the electrical conductivity decisively. In turn, the fractional charge per monomer unit is not the only parameter to affect electrical conductivity: Poly(3-methylthiophene), for example, has half of the fractional charge (f) per monomer unit relative to the parent polythiophene (Table 11),however, it exhibits a 100-fold higher electrical conductivity. Methyl-substituted polypyrroles yield even more significant discrepancies between fractional charge and electrical conductivity. Thus, although all three polymers from pyrrole, N-methylpyrrole, and 3-methylpyrrole have the same fractional charge (Table 111), electrical conductivities vary by as much as lo5 ohm-’ cm-’. Efforts are currently underway to obtain more parameters characterizing the different structures of the above polymers in order to obtain a more meaningful correlation between polymer properties and polymer structures. Registry No. Poly(thiophene), 25233-34-5; poly(2,2’-bithiophene), 80029-99-8; poly(3-methylthiophene),84928-92-7; poly(3-bromothiophene),8492893-8;poly(3-thiopheneacetonitrile), 84928-94-9;poly(3,4-dibromothiophene),84928-95-0;thiophene, 110-02-1; 2,2’-bithiophene, 492-97-7; 2-(3-thienyl)pyridine, 21298-55-5; 3-methylthiophene, 616-44-4; 3-thiopheneacetic, 6964-21-2; 3-thiophenemalonic acid, 21080-92-2; 3-bromothiophene, 872-31-1; 3-thiopheneacetonitrile, 13781-53-8; 3,4dibromothiophene,3141-26-2;3-thiophenecarboxylicacid, E@-13-1; 3-thiophenecarboxaldehyde,498-62-4.
