J. Phys. Chem. 1989,93,495-499
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Synthesis and Electrochemistry of Alkyl Ring-Substituted Polyanilines Yen Wei,’ Walter W. Focke: Gary E. Wnek? Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 021 39
Anjan Ray, and Alan G. MacDiarmid* Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104 (Received: April 25, 1988; In Final Form: July 25, 1988)
Poly(o-toluidine), poly(m-toluidine), and poly(o-ethylaniline) have been synthesized chemically and electrochemically. The polymers were characterized by elemental analysis, UV-vis spectroscopy, and cyclic voltammetry. Elemental analysis data suggest that the protonated polymers are derived from bases containing ca. 37-54% oxidized (imine) units. Upon treatment with 1 M HCl, conductivities of the polymers increase dramatically from ca. lo-* S/cm to ca. lo-’ S/cm for polytoluidines and to ca. lo-’ S/cm for poly(o-ethylaniline). The conductivities,UV-vis spectra, and electrochemicalreactions of the polymers are compared with those of polyaniline and are shown to be consistent with a reduction in ranjugation of the alkyl derivatives caused primarily by steric effects.
Introduction Polyaniline has recently been studied extensively as a rather unique member of the conducting polymer family because it can be doped to the metallic conducting regime by protonic acids and because of its interesting electrochemical b e h a ~ i o r . ~We , ~ are currently interested in alkyl-substituted polyanilines since these are anticipated to be more soluble. For instance, poly(3-butylthiophene) is known to be soluble in a range of organic solvents. Also, these materials provide opportunities to study the dependence of electrical properties on chain conformation, which in turn will depend, at least in part, on substituent size. Theoretical studies of polyaniline indicate that the ionization potential, bandgap, and band width are affected by the torsion angle (dihedral angle) between adjacent rings on the polymer Solid-state I3C N M R studies also suggested a noncoplanar conformation for the polyaniline backbone.6 It has been reported9 that the torsion angle influences the electronic properties of many conducting polymers with aromatic backbones. For example, single crystals of polypyrrole oligomers have been studied by X-ray diffraction.l0 The results indicate that polypyrrole has a coplanar conformation but that its 3,3’-dimethyl and N-methyl derivatives are significantly twisted. The maximum conductivities in doped polypyrrole are ca. 100 S/cm. They are ca. 10 S/cm in doped poly(3,3’-dimethylpyrrole) and are only ca. S/cm in doped poly(N-methylpyrrole).” However, it may not be appropriate to generalize the substituent effects on the conduc(1) Present address: Department of Chemistry, Drexel University, Philadelphia, PA 19104. (2) Present address: NIMR, CSIR, Box 395, Pretoria OOO1, South Africa. (3) Present address: Department of Chemistry, Rensselaer Polytechnic Institute, Troy, NY 12180-3590. (4) Huang, W.-S.; Humphrey, B. D.; MacDiarmid, A. G. J. Chem. SOC., Faraday Trans. 1 1986,82, 2385. (5) MacDiarmid, A. G.; Chiang, J. C.; Richter, A. F.; Somasiri, N. L.D. In Conducting Polymers; Alcacer, L., Ed.; Reidel: Dordrecht, Holland, 1987. (6) Chance, R. R.; Boudreaux, D. S.; Wolf, J. F.;Shacklette, L. W.; Silky, R.; Themans, B.; Andre, J. M.; Bredas, J. L. Synth. Met. 1986, 15, 105. (7) Euler, W. B. Solid State Commun. 1986, 57, 857. (8) Hjertberg, T.; Salaneck, W. R.; Lundstrom, I.; Somasiri, N. L. D.; MacDiarmid, A. G. J. Polym. Sci., Polym. Lett. Ed. 1985, 23, 503. (9) Bredas, J. L. In Electronic Properties of Polymers and Related Compounds; Kuzmany, H., Mehring, M., Roth, S., a s . ; Springer-Verlag: Berlin, 1985; Vol. 63. (10) Street, G. B.; Clarke, T. C.; Geiss, R. H.; Lee, V. Y.; Nazzal, A. I.; Pfluger, P.; Scott, J. C. J . Phys. (Les Ulis, Fr.) 1983, W C 3 , 599. (1 1) Pfluger, P.; Weiser, G.; Scott, J. C.; Street, G. B. In Handbook of Conducting Polymers; Skotheim, T. J., Ed.; Marcel Dekker: New York, 1985; Vol. 2, p 1369.
tivities of all conducting polymers. Poly( 3-alkylthienylenes) have been reported’* to have essentially the same electronic structure as and conductivities similar to unsubstituted polythienylene. It is therefore interesting to examine the effect of alkyl substituents in the phenyl rings of the polyaniline system. The first chemical synthesis of poly(o-toluidine) was achieved by Green and Woodhead13 in 1910 as an analogue of polyaniline. The conductivity of poly(m-toluidine) upon doping with 1 M HC1 has recently been reported to be 1.7 X lo-’ S/cm.14 Recently, we have briefly sumrnari~ed’~ both chemical and electrochemical syntheses of poly(o-toluidine) (R = O-CH,), poly(m-toluidine) (R = m-CH,), and poly(o-ethylaniline) (R = o-C2H5),which we believe are represented by the general formula
where y will have values characteristic of a given polymer when synthesized under given experimental conditions. It should be noted that the “emeraldine” oxidation state of polyaniline contains 50% oxidized units, Le., y = 0.5. The electrochemistry and the electronic properties of these polymers were compared with those of unsubstituted polyaniline (R = H). In the present article, we report full experimental details of these studies and interpret the results on the basis of electronic and steric effects of the alkyl substituents. Experimental Section Materials and Instrumentation. o-Toluidine, m-toluidine, and o-ethylaniline (Aldrich) were purified by vacuum distillation for the electrochemical syntheses. Electrochemical syntheses and cyclic voltammetry were performed on an EG & G PAR Model 273 potentiostat/galvanostat. A standard three-electrode configuration involving a saturated calomel electrode (SCE) was employed. Solution pH was measured with a Fisher-Acumet 230A pH/ion meter. Commercial buffer solutions were used for integral pH values. Intermediate pH values employed citric acid and HCI/KCl buffers made from freshly prepared solutions. The UV-vis-near I R spectra were recorded in N,N-dimethylformamide (DMF) solution by using a Perkin-Elmer (12) Hotta, S.; Rughooputh, S. D. D. V.; Heeger, A. J.; Wudl, F. Macromolecules 1987, 20, 212. (13) Green, A. G.; Woodhead, A. E.J. Chem. SOC.,Trans. 1910,97,2388. (14) Wang, S. L.; Wang, F. S.; Ge, X . H.Synth. Met. 1986, 16, 99. (15) Ray, A,; MacDiarmid, A. G.; Chiang, J. C.; Wei, Y.; Focke, W. W.; Wnek, G. E.; Epstein, A. J. Mol. Cryst. Liq. Cryst., in press.
0022-3654/89/2093-0495$01.50/00 1989 American Chemical Society
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The Journal of Physical Chemistry, Vol. 93, No. I , 1989
TABLE I: Elemental Analyses of Chemically Synthesized Alkyl Ring-Substituted Polyanitinesa compound CI/N* C, % H, % N, % C1, % total, % poly(o-ethyl- calcdC 80.96 7.24 11.80 0.00 100.00
Wei et al. TABLE II: Conductivities of Chemically Synthesized Polyaniline and Ring-Substituted Poly(alkylanilines)
polymer polyaniline
aniline) poly(o-toluidine)
base POIY(0-
found calcdc
81.09 7.51 11.65 (
