Chemically Modified Surfaces in Catalysis and Electrocatalysis

5 Chemically Modified Conducting Polypyrrole Film Electrodes

Downloaded by UNIV OF GUELPH LIBRARY on October 3, 2012 | http://pubs.acs.org Publication Date: July 2, 1982 | doi: 10.1021/bk-1982-0192.ch005

M . SALMÓN, A . DIAZ, and J. GOITIA IBM Research Laboratory, San Jose, C A 95193

Various para substituted poly-N-arylpyrrole polymer films were prepared and their electrochemical properties were measured. Of particular interest are the poly-N-p-nitrophenylpyrrole films which can be oxidized to produce the polypyrrole cation and reduced to produce the nitrophenyl anion. The polymer films can be repeatedly switched between the neutral, cationic and anionic forms with coulombic reversibility and with little π-interaction between the pyrrole and the aryl ring.

Recognizing that the conducting polypyrrole films can be chemically modified (1,2), the phenyl substituent assumes a particularly important role because it provides a means of introducing a wide selection of functional groups into the polymer. With this objective in mind, we have prepared a series of N-arylpyrrole polymers and find the thin poly-N-(p-nitrophenyl)pyrrole films of particular interest because they combine the electroactive properties of nitrobenzene and polypyrrole. With this combination, the polymer can be switched electrochemically between the cationic, neutral, and anionic form. Thin films of the substituted polyphenylpyrrole were prepared on a platinum electrode by the electrooxidation of the corresponding monomer (3) in an acetonitrile solution containing 0 . 1 M tetraethylammonium tetrafluoroborate using the procedure described for the N-phenyl analog (4). Good films were produced in every case except in the electrooxidation of N,N-dimethylaminophenylpyrrole, which instead produces soluble products. The films used in this study were prepared using 20 m C / c m . These films were analyzed by cyclic voltammetry in a one compartment cell containing 0.1 M tetraethylammonium tetrafluoroborate in acetonitrile and a sodium chloride calomel reference electrode as before (4). The anodic region of the voltammograms show that peaks appear in the range 600-900 m V due to the redox reaction of the pyrrole units in the polymer backbone of each derivatized film (Table I). The reactions are coulombically reversible and the 2

0097-6156/82/0192-0065 $6.00/0 © 1982 American Chemical Society In Chemically Modified Surfaces in Catalysis and Electrocatalysis; Miller, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

66

CHEMICALLY MODIFIED SURFACES

TABLE I


Electrochemical Data for N-Substituted Pyrrole Polymer Films

Polymer Epc.mV E ,mV

N-Substituent

a

Monomer E ,mV p a

p a

phenyl

740

600

1800

p-tolyl

700

600

1500

p-anisyl

700

600

1360

p-dimethylaminophenyl

(E°)

p-nitrophenyl

900 -900

H methyl

-100 500

b

780 -1000

b

1600 -1110

(E°)

(E

1 / 2

)

-1140 730

E p versus SSCE measured in C H 3 C N using a Pt electrode. ^Values for the aryl substituent. Reference 11. Measured with Pt versus S C E electrodes in C H 3 C N containing E t N C 1 0 . a

c

4

b

b

1200 1200

-300 400 nitrobenzene dimethylaniline

1290 720

4

In Chemically Modified Surfaces in Catalysis and Electrocatalysis; Miller, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

c


5.

SALMON ET AL.

67

Polypyrrole Film Electrodes

ip values of the peaks scale linearly with sweep rate (?) in every case. Overall, the voltammograms resemble the one for the polyphenypyrrole 0 ) , except that the peaks produced by the poly-N-nitrophenylpyrrole films are shifted anodically by 200 mV. This is not unreasonable, considering the inductive and resonance effects of the aryl group. With these films an additional small peak of unknown origin appears at 600 mV. The reactions are accompanied by a color change from light yellow (neutral film) to dark brown (oxidized film). The poly-N-p-nitrophenylpyrrole films are of interest because the nitrophenyl group is independently electroactive. In the cathodic region of the voltammogram, the initial scans show double peaks for the reduction reaction of the pendent nitrophenyl group (Fig. (1)) at -1000 and -1150 m V ( E ^ ) plus the corresponding peak in the anodic scan at -900 m V ( E ) . The small peak at -1000 m V disappears after a few scans without changing the area under the peak. The reaction is coulombically reversible and the i values scale linearly with ?, where i^/Av equals 11 m A « s / V » c m . For comparison, the corresponding value for the oxidation reaction at 920 m V is 4 m A * s / V « c m . The position of these signals are close to those for the reduction reaction of nitrobenzene ( E ° at -1140 mV) and poly-p-nitrostyrene (E° at -1500 m V versus a silver electrode) (5). Thus the negative charge in the polymer must be localized on the nitrophenyl group and is not extensively delocalized throughout the polymer ir-electron structure. This result further suggests that the p-nitrophenyl and the pyrrole rings must remain orthogonal in this film and are poorly conjugated. The peak shapes and positions indicate that the reaction is not electrochemically reversible, which is not unexpected since the reaction of these films are known to involve slow ion diffusion (4,6,7). The charge density ratio of the pyrrole oxidation to the nitrophenyl reduction reaction in the film is 0.2. This low value indicates that the oxidation reaction involves 0.2 charges/pyrrole ring, since the nitrophenyl reaction involves one electron/nitro group. A similar value was found with poly-N-phenylpyrrole (0.16) (4). This low sensitivity of the degree of oxidation of the pyrrole polymer to changes in the nature of the attached aryl group supports the idea that the aryl group is poorly conjugated to the rest of the polymer. The E S C A spectra of the surface region shows peaks of approximately equal areas at 400.5 and 405.8 eV which are appropriate for the pyrrole nitrogen and the nitro nitrogen atoms, respectively (8). Therefore the p-nitrophenylpyrrole structure remains intact in the film. Peaks for carbon and oxygen are also present in the E S C A spectra. The scanning electron micrograph of the surface of the film shows that it is a continuous film with a fairly even surface as was observed with the polypyrrole films (2). The electroactive behavior of the nitrophenyl group is particularly intriguing. Although the fully-charged film appears to have the anionic charges localized on the nitrophenyl group, the electron transfer process between the platinum and the film for the reduction reaction may involve electron exchange between the unsaturated pyrrole backbone and the pendent p a

p

2

2





5.

SALMON ET AL.

69

Polypyrrole Film Electrodes

nitrophenyl groups rather than a hopping process between the groups. This proposed mechanism is reasonable since pyrrole is known to form tr-complexes with acceptor molecules (9), plus the fact that adjacent nitrophenyl groups along the chain are probably held apart in a near 180° orientation and are too far away to interact directly. A s expected, the nitro group is electrochemically reduced to the amine structure in the presence of water, which provides a convenient way to further modify the film. The accessibility of these amino groups for further modification of these films needs to be determined. As regards the electrooxidation of the corresponding monomers, they have less anodic E values than N-phenylpyrrole even with the nitro substituent and the reactions remain irreversible. The substituents influence the oxidation of these monomers much more than was observed with pentaphenylpyrrole. For example, substitution of a p-methoxy group in the N-phenyl of the latter produces a 20 m V cathodic shift in the E p value (10). The dimethylaminophenyl and nitrophenyl groups show reversible redox behavior and appear to behave independent of the pyrrole moiety in these derivatives. In summary, the phenyl group provides a practical way to chemically modify the polymer film. Polypyrrole films containing the N-phenyl group are as conducting as those containing the N-methyl group (ca. 10" (Bern)" ) and ca. 1 0 less conducting than the unsubstituted films (4). While good films can be prepared when the substituted phenyl group on the monomer is electroactive and easily reduced, we have not been able to prepare good films when there is a substituted phenyl group which is easily oxidized.


p a

a

3

1

5

LITERATURE CITED 1. 2. 3. 4. 5. 6. 7. 8.

Diaz, A. F.; Castillo, J.; Kanazawa, K. K.; Logan, J. A.; Salmón, M.; Fajardo, O. J. Electroanal. Chem. 1981, 0000. Diaz, A. F.; Kanazawa, K. "Extended Linear Chain Compounds"; Miller, J., Ed., Plenum Press, 1982, Vol. 3. Salmón, M.; Diaz, A. F., unpublished results. Diaz, A. F.; Castillo, J. I.; Logan, J. A.; Lee, W. Y. J. Electroanal. Chem. 1981, 0000. Van de Mark, M. R.; Miller, L. L. J. Amer. Chem. Soc. 1978, 100, 3223. Kerr, J. B.; Miller, L. L.; Van de Mark, M. R. J. Amer. Chem. Soc. 1980, 102, 3383. Kaufman, F. B.; Schroeder, A. H.; Engler, E. M.; Kramer, S. R.; Chambers, J. Q. J. Amer. Chem. 1980, 102, 483. Robinson, J. W., Ed.; "Handbook of Spectroscopy"; CRC Press, 1974, Vol. I.



70 9.

Jones, R. A. "Physiochemical Properties of Pyrroles"; Katritzky, A. R.; Boulton, A. J., Eds.; Acad. Press, 1970, Vol. 11, p. 383. 10. Cauquis, G.; Genies, M. Bull. Soc. Chim. Fr. 1967, 3220. 11. Weinberg, N. L. "Techniques of Electroorganic Synthesis"; John Wiley and Sons, 1975, Vol. V, Pt. II, p. 811.


RECEIVED November 12, 1981.


Chemically Modified Surfaces in Catalysis and Electrocatalysis

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