Langmuir 1996,11, 302-306
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Self-Assembled Monolayers of Pyrrole-Containing Alkanethiols on Gold Curtis N. Sayre and David M. Collard* School of Chemistry and Biochemistry and Polymer Education and Research Center, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 Received July 7,1994. In Final Form: September 23, 1994@ Electrooxidative polymerization of 3-ethylpyrroleon gold electrodes modifiedwith spontaneouslyadsorbed monolayers (SAMs)of alkanethiols bearing 3-substituted pyrroles such as 3-(1l-mercaptoundecyl)pyrrole, 4-(3-pyrrolyl)butyl 11-mercaptoundecanoate,and 11-mercaptoundecyl 3-pyrrolylacetate affords dense, robust, and extremely adhesive films of poly(3-ethylpyrrole). In the absence of monomer in contacting solution, the surface-confined monolayers are subject to irreversible electrooxidation. Scanning electron microscopy indicates that the presence of these monolayers enhances the morphological packing in films of poly(3-ethylpyrrole)compared to that of films deposited on unmodified electrodes. The conductivity of these films is enhanced as a consequence of this change in morphology.
Introduction
a quartz crystal microbalance study indicating that polymerization kinetics during early stages of deposition Electrooxidative polymerization constitutes the most depend on the solubility of the oligomers formed.8 More common method for the preparation of electronically recently, evidence for an electrochemically initiated chain conductive polymers. The polymers are formed directly polymerization reaction, rather than radical-radical on the electrode surface and can be removed to afford free coupling, has been s ~ g g e s t e d It . ~was further established standing films. Although numerous applications have that the rate-determining step for polypyrrolegrowth from been proposed for these materials,' difficulties encountered aqueous solutions is oxidative incorporation of monomers in processing, and poor material properties, remain as or oligomers into the growing polymer chains.1° severe limitations to their employment. These polymers are often brittle, insoluble in most common organic Studies of thiophene polymerization on gold anodes solvents, and amorphous. indicate formation of a monolayer of monomer,ll or initial Although there have been numerous investigations into formation of soluble oligomers followed by precipitation.12 the electrochemical properties and preparation of conducElectrochemical studies of the anodic polymerization of tive polymers, the mechanism for the deposition of these (3-thieny1)acetic acid on platinum indicate layer-by-layer films on electrode surfaces is not well-defined. Elecdeposition, Le., two-dimensional growth, rather than the trooxidative polymerization of pyrrole2 and N-methylpyrmore prevalent three-dimensional mechanism. This was role3 on Pt or vitreous carbon from aqueous solutions, or ascribed to the adsorption of the monomer, which provides pyrrole on I T 0 glass from acetonitrile: affords a rising two-dimensional nucleation centers which serve as sites current transient similar to the nucleation process obfor facile continued ~ 0 w t h . l ~ served during the initial deposition of a metal on a n From the above discussion, it is apparent that there is electrode surface. The presence of a preadsorbed monolayer of monomer on the electrode surface has been the a wide and complex variety of polymerization mechanisms subject of some controversy. The effect of monomer for the electrochemical deposition of heteroarenes. The concentrationon the polymerization of pyrrole on platinum polymerization kinetics and polymer structure depend on from aqueous solutions suggests formation of a monolayer concentration of monomer, electrochemical method used, of polypyrrole which nucleates further polymer g r ~ w t h , ~ solvent, electrolyte, and electrode material. A generally whereas the lack of dependence on substrate material for accepted mechanism for the anodic polymerization of the polymerization kinetics of poly(N-methylpyrrole)was heteroarenes is formation of soluble oligomers which interpreted as evidenceagainst monolayer preadsorption6 eventually precipitate on the electrode surface, forming Evidence for the formation of soluble oligomers includes nucleation sites for the polymeric film, and continued the effect of stirring on film deposition,' and results from growth. Determination of the number and location of nucleation sites would allow control over the final * Author to whom correspondence should be addressed at t h e disposition of the polymer film. A convenient method to School of Chemistry and Biochemistry. achieve this control would be through modification of the @Abstractpublished in Advance ACS Abstracts, December 1, electrode surface by formation of spontaneously adsorbed 1994. (1)Handbook of Conducting Polymers, Vols. 1 and 2;Skotheim, T. monolayers (SAMs). For example, pinhole-free, ionically A., Ed.; Marcel Dekker: New York, 1986. insulating monolayers of long-chain alkanethiols on (2)Asavapiriyanont, S.;Chandler, G. A.; Gunawardena, G. A.; coinage metals provide ideal surfaces for the study of Pletcher, D. J. Electroanal. Chem. 1984, 177,229. Downard, A. J.; Pletcher, D. J. Electroanal. Chem. 1986,206,139. (3)Asavapiriyanont, S.;Chandler, G. A.; Gunawardena, G. A,; Pletcher, D. J. Electroanal. Chem. 1984, 177,245. Downard, A. J.; Pletcher, D. J. Electroanal. Chem. 1986,206, 245. (4)Genies, E. M.; Bidan, G.; Diaz, A. F. J.Electroanal. Chem. 1983, 149. 101. ( 5 ) Marcos, M. L.; Rodriguez, I.; Gonzalez-Velasco, J. Electrochim. Acta 1987,32, 1453. (6)Jansson, R.; Arwin, H.; Bjorkland, R.; LundstrGm, I. Thin Solid Films 1986,125,205. (7)Miller, L.L.;Zinger, B.; Zhou, Q.-Z. J.Am. Chem. SOC.1987,109, 2267.
(8) Baker, C. K.; Reynolds, J. R.
J.Electroanal. Chem. 1988,251,
307.
(9)Qiu, Y.-J.; Reynolds, J. R. J.Polym. Sci., Part A: Polym. Chem. 1992,30,1315. (10)Scharifker,B. R.; Garda-Pastoriza, E.; Marino, W.J. Electroanal. Chem. 1991,300,85. (11)Hillman, A. R.; Mallen, E. F.J.Electroanal. Chem. 1987,220, 3.61
(12)Hamnet, A.; Hillman,A. R. J.Electrochem. SOC.1988,135,2517. Albery, W. J. Langmuir 1992,8,1645. (13)Li, F.-B.;
0743-746319512411-0302$09.00/00 1995 American Chemical Society
Langmuir, Vol.11, No.1, 1995 303
Pyrrole-ContainingAlkanethiols on Gold simple electrode processes such as heterogeneouselectron transfer,14J5ion transport,16 and double-layer phenomena.l7 These close-packed crystalline S A M s insulate modified electrodes15and can be used to immobilize redox-active18 and bioactivelggroups at surfaces, allowing for the control of surface properties and investigation of the chemistry of interfaces.20 There have been few reports of the effect of SAMs on the process of electrooxidative polymerization. A silyl-derivatized pyrrole21and a mercapto-substituted 2,5-bi~(2-pyrrolyl)thiophene~~ have been deposited on gold to enhance the adhesion of thick overlayers of polypyrrole. 4-Mercaptoaniline increases the uniformity of subsequently depositedp ~ l y a n i l i n e .None ~ ~ of these monomercontaining S A M s contain the long alkyl chain required for an ordered monolayer, and the electrochemical properties of the monolayers themselves have not been investigated. Covalent attachment of an appropriate heteroarene to a long chain alkanethiol should afford a monomer capable of forming a highly ordered monolayer on an electrode surface. This ordered layer should influence electrochemical polymerization kinetics and increase the adhesion of conductive polymers to the electrode. Here we report the effect of monolayers of alkanethiols bearing 3-substituted pyrroles, 3-(w-mecaptoalkyl)pyrroles(11, 4-( 3-pyrroly1)butylw-mercaptoalkanoates (2), and w-mercaptoalkyl 3-pyrrolylacetates (31, on polyheteroarene growth, morphology, and adhesion to gold anode surfaces. We chose pyrrole over thiophene as the monomeric unit due to its low oxidation potential (thereby avoiding oxidative desorption of the thiolz4) and to avoid interaction between the electrode and the sulfur of the thiophene.z5 The choice of 3-substituted pyrroles allows polymerization through the 2- and 5-positions. Poly(3-alkyl pyrrole)^ possess higher conductivities than poly(N-alkylpyrro1e)s and were chosen as initial targets for monolayer deposition. 0
H
H
1
2
H
3
Experimental Section Synthesis. Friedel-Crafts acylation of N-(t-butyldimethyl-
silyl)pyrrole26with o-bromoundecanoylchloride (Mc13,CH2C12, 45% yield), followed by reduction (BH~sTHF,74% yield), and ~~
(14)Miller, C.; Gratzel, M. J . Phys. Chem. 1991,95,5225.Beka, A.
M.;Miller, C. J. J. Phys. Chem. 1992,96,2657.
(15)Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. SOC.1987,109,3359. (16)Miller, C.; Cuendet, P.; Gratzel, M. J . Phys. Chem. 1991,95, 877. (17)Nuzzo, R. G.; Allara, D. L. J. Am. Chem. SOC.1983,105,4481. Li, T. T.-T.; Weaver, M. J. J . Am. Chem. SOC.1984,106,6107. (18)Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J . Am. Chem. SOC.1990,112,4301. Collard, D. M.; Fox, M. A. Langmuir 1991,7 , 1192.Bunding-Lee, K. A. Langmuir 1990,6,709. Rowe, G. K.; Creager, S. E. Langmuir 1991,7, 2307. (19)Collinson, M.;Bowden, E. E. Langmuir 1992,8,1247. Spinkle, J.; Liley, M.; Guder, H. J.; Angermaier, L.; Knoll, W. Langmuir 1993, 9,1821.Haeussling, L.;Ringsdorf, H.; Schmitt,F.-J.;Knoll, W. Langmuir 1991, 7 , 1837.DiMilla, P. A.;Folkers, J. P.; Biebuyck, H. A.; Harter, R.; Mpez, G. P.; Whitesides, G. M. J.Am. Chem. SOC.1994,116,2225. Prime, K. L.; Whitesides, G. M. Science 1991,252,1164. (20)Bain, C. D.;Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J . Am. Chem. SOC.1989,111, 321.Bain, C. D.; Bain, C. D.; Whitesides, G. M. Whitesides, G. M. Science 1988,240,62. Adu. Mater. 1989,28,506.Whitesides, G. M. Chimia 1990,44,310. (21)Simon, R. A.; Ricco, A. J.; Wrighton, M. S. J . Am. Chem. SOC. 1982,104,2031. (22)Kowalik, J.; Tolbert, L.; Ding, Y.; Bottomley, L.; Vogt, K.; Kohl, P. Synth. Met. 1993,55, 1171.
thiolation (N-acetylthiourea,EtOH, A, 82%yield)27afforded3-(omercaptoundecyl)pyrrole, 1. Flash chromatography (6:94 ethyl acetate/petroleum ether, on silica gel-60) gave 1(n = ll),as a colorless solid: mp 41.5-43.5 "C; 'H NMR (CDC13,300MHz) 6 1.4-1.1 (m, 14 H) 2.51 (m, 4 H, C-1 and C-11 CH2), 1.58 (m, 5 H, C-2 and C-10 CH2, and SH), 6.09 (m, 1 H, ArH), 6.57 (m, 1 H, ArH), 6.73 (m, 1H, ArH), 8.0 (br s, 1H, NH); IR (KBr) 3442, 2915, 2849, 1466 cm-l; MS (electron impact) 253 (M+). Anal. Calcd for C15H2,NS: C, 71.09; H, 10.74; N, 5.53; S, 12.65. Found: C, 71.18; H, 10.79; N, 5.44; S, 12.58. 4-(3-Pyrrolyl)butyl 11-mercaptoundecanoate was prepared from 4-(3-pyrrolyl)-4-oxobutyricacid.28 The acid was reduced (RedAl, THF, 60% yield) to afford 4-(3-pyrrolyl)-l-butanol. Esterification (DCC, DMAP, Et20)29with 11,ll'-dithiobis(undecanoic acid) (prepared by oxidation of the corresponding thiol with DMSO and catalytic iodine,3O82%yield)gave4-(3-pyrrolyl)butyl 11,ll'-dithiobis(undecan0ate) which was reduced (tributylphosphine, acetone, water, 72% yield overall for last two steps),31t o give 4-(3-pyrrolyl)butylll-mercaptoundecanoate, 2, as a colorless oil which was purified by flash chromatography (10:90 ethyl acetate/petroleumether): lH NMR (CDC13,300MHz) 6 1.2-1.5(m, 16H),1.65(m,8H,C-2',C-3', C-10,C-3,CH2),2.29 (t, J = 7.54 Hz, 2 H, C-2, CH2), 2.53 (m, 4 H, C-l', C-11, CHz), 4.09 (t,J = 6.29 Hz, 2 H, (2-4' CH2), 6.09 (9, 1H, ArH), 6.58 (s, 1H, ArH), 6.73 (t,J = 2.35 Hz, 1H, ArH), 8.0 (br s, 1 H, NH); IR (neat) 3400, 2918, 2850, 1732, 1463, 1176, 1064 cm-l; MS (electron impact) 339 (M+). Anal. Calcd for ClgH33N02S: C, 67.21; H, 9.80; N, 4.13; S, 9.44. Found: C, 67.30; H, 9.84; N, 4.15; S, 9.33. 11-Mercaptoundecyl 2-(3-pyrrolyl)acetate was prepared by esterification of (3-pyrroly1)acetic acidZ8 with ll-mercapto-lundecanoP2 (DCC, DMAP, Et20, 35% yield). Purification by column chromatography (10:90 ethyl acetate/petroleum ether) afforded the ester 3,as a colorless solid: mp 24-26 "C; lH NMR (CDC13,300MHz) 6 1.2-1.5 (m, 15 H), 1.61(m, 4 H, C-3, C-10, CH2), 2.53 (9, J = 7 Hz, 2 H, C-11, CH2), 3.62 ( ~ ,H, 2 C-2, CHd, 4.08(t,J=6.75H~,2H,C-l',CH2),6.18(t,J=2H~,lH,ArH), 6.72 (m, 2 H, ArH), 8.3 (br s, 1 H, NH); IR (neat) 3395, 2926, 2853, 1730, 1465, 1146 cm-l; MS (electron impact) 311 (M+). Anal. Calcd for Cl~H29N02S:C, 65.55;H, 9.38;N, 4.50; S, 10.29. Found: C, 65.29; H, 9.46; N, 4.27; S, 9.95. 3-Ethylpyrrole was synthesized according to the method of Wegner.33 lHNMR (CDC13)6 1.21(t,3 H, J = 7.69Hz, C2, CH3), 2.53 (9, 2 H, J = 7.39 Hz, C1, CHd, 6.12 (m, 1H, pyrrole C4), 6.58 (m, 1H, pyrrole C3), 6.72 (m, 1 H, pyrrole C5), 7.95 (br s, 1 H, pyrrole N-H). IR (neat) 3395,2930,2857, 1461 cm-I. Monolayer Deposition and Electrochemistry. Monolayers of pyrrole-containing monomers were prepared by repetitive cycles of immersing the gold electrode in a 20 mM solution of the thiol in ethanol or acetonitrile for 10-15 min, followed by rinsing with copious amounts of ethanol and air-drying. Electrochemistry was performed on a BAS-100B electrochemical analyzer. Working electrodes were typically 2.01 mm2polished gold or ca. 1 cm2 evaporated gold (1000 A) on titanium-coated glass; a platinum wire or gold film was used as a counter electrode; SCE or Ag/AgCl (saturated KC1)reference electrodes were used in all experiments. The presence of air had no effect on the reported electrochemistry;no precautions were taken to excludeair during monolayer deposition or electrochemical studies. Oxidations of modified electrodes were performed in 0.1 M aqueous HC104 or (23)Rubinstein, I.;Rishpon, J.; Sabatini, E.; Redondo, A.; Gottesfeld, S. J. Am. Chem. Soc. 1987,112,6135.
(24)Finklea, H. 0.;Avery, S.;Lynch, M.; Furtsch, T. Langmuir 1987, 3, 409. (25)Li, T. T.-T.; Weaver, M. J. J. Am. Chem. SOC.1984,106,1233. (26)Simchen, G.; Majchrzak, M. W. TetrahedronLett. 1985,26,5035. (27)Klayman, D. L.; Shine, R. J.; Bower, J. D. J . Org. Chem. 1972, 37, 1532. (28)Kakushima, M.; Hamel, P.; Frenette, R.; Rokach, J.J . Org. Chem. 1983,48,3214. (29)Hassner, A.; Alexanian, V. Tetrahedron Lett. 1978,4475. (30)Aida, T.; Akasaka, T.; Furukawa, N.; Oae, S. BUZZ. Chem. SOC. Jpn. 1976,49,1441. (31)Humphrey, R. E.; Potter, J. L. Anal. Chem. 1965,37, 164. (32)Miller, C.; Cuendet, P.; Gratzel, M. J . Phys. Chem. 1991,95, 877. (33)Riihe, J.; Ezquerra, T.; Wegner, G. MakromoZ. Chem., Rapid Ruhe, J.; Ezquerra, T.; Wegner, G. Synth. Met. Commun. 1989,10,103. 1989,28,177.
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304 Langmuir, Vol. 11, No. 1, 1995 E (mV versus SCE)
I
A
+goo
I
I
I
I
I
I
I
i500
I
1 0
B
W
0
C
0
v I +goo
I
I
50 100 150 200 250 300 350 v l mV s.1
Figure 2. Cyclic voltammograms of monolayers of ll-mercaptoundecyl2-(3-pyrrolyl)acetate, 3, in 0.1 M HClOd at a gold
working electrode (geometric area = 2.01 mm2) at different potential scan rates ( u ) a, 25; b, 50; c, 100; d, 200; e, 350 mV s-l. Inset: Plot ofpeak current, ipek,versus potential scan rate, I
1
I
I
+500
1
I
1
U.
0
E (mV v e w s SCE)
Figure 1. Cyclic voltammograms (100 mV
9-l) of pyrrolecontaining monolayers: A, 3-(o-mercaptoundecyl)pyrrole(1); B, 4(3-pyrrolyl)butyl o-mercaptoundecanoate (2); C, w-mercaptoundecyl 3-pyrrolylacetate (3). CVs obtained in 0.1 M aqueous HClO4 at a gold working electrode (geometric area = 2.01 mm2),referenced against SCE. The first two cycles are shown.
KC104, or 0.1 M LiC104 in anhydrous acetonitrile(distilled from calcium hydride under nitrogen immediately prior to use). Flotation densities were determined using ethanolkhloroform mixtures. Spectroscopy. Reflectance infrared spectra were obtained on a Nicolet 60SX FTIR spectrometer equipped with a MCT nitrogen-cooleddetector. A Spedratech FT-80 fixed grazing angle spectral reflectance accessory was used to introduce p-polarized light at 80" to the normal of the sample plane.
Results and Discussion Deposition of monolayers of pyrrole-containing alkanethiols l , 2, and 3on gold was performed by immersing the electrodes in organic solutions of the appropriate thiol.l6 These monolayer-modified electrodes gave rise to the appearance of a n irreversible oxidative wave during the first sweep to positive potential in the cyclic voltammogram in electrolyte solutions (0.1 M aqueous HC104 or 0.1 M LiC104 in acetonitrile), Figure 1. Monolayers of dodecanethiol fail to produce this electrochemical response. Subsequent cycles in either solvent display no redox activity over a wide potential range. The waves for 3-(o-mercaptoundecyl)pyrrole(1)and 4-(3-pyrrolyl)butyl o-mercaptoundecanoate (2) appear at approximately the same oxidation potential (Epe& a t +720 mV versus SCE, a t a scan rate of 100mV s-l in 0.1 M HC104). The oxidation peak for 1is much broader (Ef~hm= 168 mV) than that of 2 (Efwhm = 126 mV), Figure 1. o-Mercaptoundecyl 3-pyrrolylacetate, 3, affords a much sharper oxidation wave a t slightly higher potentials (Ef~hm= 105 mV, Epe* = +770 mV). These oxidation potentials correspond to those expected for the oxidation ofthe substituted pyrroles to the pyrrole radical cation. The higher oxidation potential for 3 is due to the proximity of the electronwithdrawing carbonyl to the pyrrole ring, thereby destabilizing the radical cation 3*+relative to 1*+ or 2'+.
The sharper redox waves observed for ester-containing
SAMs 2 and 3 might be explained by greater structural homogeneity in the monolayer by virtue of dipolar interactions between carbonyls on adjacent adsorbate chains providing a second anchor point,34or by a change in ion pairing.35 In the case of 3, the tether to the carbonyl is relatively short (one methylene unit), while that of 2 is relatively long (four methylene units). The flexibility of the butyl chain in 2 allowsthe pyrrole ring to adopt random orientations, placing them in different environments, resulting in a broad oxidative peak relative to that of 3. On the other hand, the pyrrole rings in 3 are all in approximately the same environment, resulting in a sharp oxidative peak. A plot of peak current, ipeak, versus scan rate, u , deviates only slightly from a linearity, Figure 2 inset. Although a linear dependence is expected for a fully reversible surface-confined redox couple, quasireversible couples should lead to nonlinearity. Integration of the charge beneath the oxidation wave is difficult due to the presence of a large background current at high positive potentials. Ifthe second potential cycle is considered as an appropriate background current and is subtracted from the first potential sweep, reproducible peak areas for the surface-confined oxidationwave, corrected for surface roughness, are as follows: 1 , 6 1pC cm-2;2,51pC cm-2;3,61pC cm-2, giving a total electronic transfer of 3.8 x 1014,3.1 x 1014,and 3.8 x 1014electrons cm-2, respectively (average of values obtained a t various scan rates in the range 25-350 mV s-l, f 1 5 % reproducibility). These charge densities are in good agreement with the surface coverage of alkanethiols on gold in a hexagonal close-packed arrangement (4.8 x lo1*molecules cm-2).36 The second cycle of the cyclic voltammogram is similar to that observedfor monolayers of dodecanethiolindicating that the monolayer is present afier oxidation but that the pyrrole rings have been rendered electrochemically inert. This is consistent with the observation of alkyl C-H stretching bands a t 2852 cm-' (CH2 v~,.,,,) and 2924 cm-l (34) Evans, S. D.; Urankar, E.;Ulman, A.; Ferris, N.; Eilers, J. E.; Shnidman, Y. J . Am. Chem. SOC.1992, 113, 4121. (35) Rowe, G. IC; Creager, S. E. Langmuir 1991, 7, 2307. (36) Chidsey, C. E.D.; Liu, G.-Y.;Rowntree, P.; Scoles, G. J . Chem. Phys. 1989,91, 4421.
Langmuir, Vol. 11, No. 1, 1995 305
Pyrrole-Containing Alkanethiols on Gold (CH2 vas-) in the reflectance infrared spectrum both prior to and after oxidation. In separate experiments we have observed oxidative desorption of alkanethiols above 1300 mV. Infrared analysis yields no information of the oxidation state of the pyrrole ring. We believe that the radical cation is formed but that it undergoes nucleophilic attack by water, producing an electrochemicallyinert layer on the electrode. We have not observed reduction of the radical cation in dry acetonitrile or at fast potential scan rates. In order to further investigate the utility of surfaceimmobilized monomers, we have studied the effect of the pyrrole-containing monolayers on the deposition of polyheteroarenes. The oxidative peak potential for pyrrole is +900 mV versus SCE.3 This potential is substantially higher than the oxidation potential of monomers 1,2,and 3,and cyclic voltammetry of pyrrole at modified electrodes indicates oxidation of the monolayer prior to, and independent of, oxidation of pyrrole from solution. 3-Ethylpyrrole possesses the same 3-alkyl substitution as adsorbates 1 and 2 and undergoes oxidation at a similar potential (Epeak = +740 mV versus SCE). Potentiostatic polymerization of 3-ethylpyrrole (30 mM) on unmodified gold from a stirred 0.1 M LiC104 solution in acetonitrile (+700 mV versus SCE) gives thick black films that are easily removed from the electrode surface by wiping the electrode with a paper towel. When the polymerization is carried out on electrodes modified with monolayers of 1 , 2 , or 3,the polymer appears smoother, and adheres strongly to the electrode surface. Deposition of poly(3-ethylpyrrole) on these monolayers produces films so strongly adherent to the electrode surface that the polymer must to be removed with coarse emery cloth, followed by extensive polishing with alumina. Polymerization of 3-ethylpyrrole on electrodes modifed with monolayers of undecanethiol or 11-mercapto-l-undecanol fails to give rise to this enhanced adhesion, discounting the possibility that the films are held in place by hydrophobic interactions or hydrogen bonding. Accordingly, we suggest that coupling of surface-confined electrochemically-generatedradical cations to 3-ethylpyrrole (radical cations) leads to a covalent bond between the monolayer and deposited film, thereby anchoring the polymer to the electrode surface. Polymerization kinetics were observed over the course of the deposition of thick films. A plot of charge versus time for potentiostatic polymerization of 3-ethylpyrrole (30 mM in 0.1 M LiC104 in acetonitrile, 2.01 mm2 Au electrode, stirred solution, +700 mVversus SCE)is shown in Figure 3. When the polymerization is carried out on monolayer-modified electrodes, the current remains low relative to that at unmodified electrodes, in which case the current (slope)increases dramatically. This increasing rate of polymerization is typical for the deposition of polypyrrole.2 The increase in current during polymerization of pyrrole suggests an increasing electroactive surface area at which further polymerization takes place. The lower current for polymer deposition on the modified electrode indicates that the surface area does not increase dramatically during the polymerization. This is consistent with the observed smoothness of films deposited on modified electrodes. To investigate these morphological differences, poly(3-ethylpyrrole) films were deposited (0.1 M LiC104 in acetonitrile at +900 mV versus Ag/AgCl) on bare and l-modified gold electrodes. Representative SEM images of relatively thick films (tens of micrometers) deposited by the passage of ca. 2.0 C cm-2 are shown in Figure 4. Films deposited on bare electrodes display the morpho-
-unmodified gold
-.-.-dodcanethiol-modified
+
gold
.....34 1 1-mercaptoundecyl)pyrrole-modified gold ; I
0
100
200
300
400
500
600
Time 1 s
Figure 3. Charge-time plot for bulk electrolysis of 3-ethylpyrrole (30 mM in 0.1 M LiC104 in acetonitrile) at an unmodified electrode, an electrode modifed with dodecanethiol, and an electrode modifiedwith 3-(cu-mercaptoundecyl)pyrrole, 1.Polymerizations were performed on stirred solutions at +700 mV versus SCE.
Figure 4. Scanning electron micrographs of poly(3-ethylpyrrole) (the side of the film facing solution) deposited on (A, top) gold and (B, bottom) an electrode modified with a monolayer of 3-(cu-mercaptoundecyl)pyrrole,1. Deposition from 30 mM 3-ethylpyrrole in 0.1 MLiC104in acetonitrile at +900 mVversus Ag/AgCl (saturated, deposition charge = 2.0 C cm-2).Scale bar = 9.9 pm.
logical appearance of p ~ l y p y r r o l e consisting ,~~ of 10-50 pm grains of polymer separated by large channels, Figure (37) Beck, F.;Oberst, M.Makromol. Chem., Macromol. Symp. 1987, 8,97.
Sayre and Collard
306 Langmuir, Vol. 11, No. I , 1995 4A. The films deposited on monolayer-modifiedelectrodes are smoother and less porous, consisting of a dense plane of 5-10 pm features, Figure 4B. Accordingly, the effect of the surface-confined monolayer is manifest in the morphology of the film tens of micrometers from the electrode-polymer interface. Films deposited on unmodified electrodes were peeled from the surface to examine the morphology of the side facing the electrode. These faces appear rough with 50-200 pm pits and depressions. Although films deposited on modified electrodes were generally extremely adhesive, one thick film (ca. 50 pm) which had been dried rapidly, buckled after several days, allowing examination of the back of the film which had a black mirror-like appearance and was featureless by SEM. The morphology of these films is apparent in the rate a t which thick films are deposited. The rates of deposition (per unit charge deposited) a t unmodified and modified electrodes are 23 and 7 pm per C cm-2, respectively. The two films (both with deposition charges of 2 C cm-9 have the same flotation density (1.42 g mL-l), implying that the pores and channels present in both films are accessible to solvent. Although the conductances of the films deposited with the same deposition charges are similar, the thickness corrected conductivities are different by a factor of approximately 3 (0.4 S cm-' for films on bare electrodes, 1.4 S cm-l for modified electrodes).
In conclusion, we have shown that the adsorption of a pyrrole-containing monolayer on a n electrode gives rise to an irreversible oxidative wave corresponding to formation of the pyrrole radical cation. Use of these monolayers as substrates for electrooxidative polymerization of 3-ethylpyrrole results in adhesive films of poly(3-ethylpyrrole). The presence of these monolayers enhances the morphological packing of poly(3-ethylpyrrole), affording a smooth, compact film displaying a higher electrical conductivity. The effects of monomer-modified SAMs on the kinetics of initial polymer growth and polymer morphology are under investigation in order to address the structure and nanoscale manipulation of these monolayer^.^^
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society; the Polymer Program Associates at Georgia Tech; and NASA-HiPPAC, for partial support of this research. D.M.C. gratefully acknowledgesthe Camille and Henry Dreyfus Foundation and Research Corporation for junior faculty awards. We thank Dr. X. Zheng for assistance in obtaining SEM images, and Professor R. McCarleyfor helpful discussions. LA940532R (38)Note added in proof: In the accompanyingpaper R. J. Willicut and R. L. McCarley report similareffectsof 1-(w-mercaptoalky1)pyrrole S A M s on the deposition of polypyrrole.
