Surface-Confined Monomers on Electrode Surfaces. 11

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Surface-Confined Monomers on Electrode Surfaces. 11. Electrochemical and Infrared Spectroscopic Characteristics of Aniline-Terminated Alkanethiol Monolayers on Au Electrochemically Treated in Nonaqueous Media K. Cory Schomburg† and Robin L. McCarley* Choppin Laboratories of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803-1804 Received July 18, 2000. In Final Form: January 9, 2001 The stability and electrochemical properties in nonaqueous media of self-assembled alkanethiol monolayers containing an alkoxyaniline tail group tethered to a gold surface via a hexyl or heptyl chain were investigated. It was previously found that upon electrochemical oxidation in 1 M H2SO4, 2-(6-mercaptohexan-1-oxy)aniline (AnC6SH) and 2-(7-mercaptoheptan-1-oxy)aniline (AnC7SH) monolayers on Au give rise to surface-confined dimeric and oligomeric/polymeric anilines as well as surface-confined hydrolyzed aniline dimer. In the study at hand, AnC6SH/Au and AnC7SH/Au were electrochemically oxidized in 0.1 M LiClO4/CH3CN to discourage the formation of surface-confined hydrolyzed aniline dimer. The resulting voltammetry was compared to that observed for monolayers oxidized in 1 M H2SO4 and that of dimeric model compounds in 0.1 M LiClO4/CH3CN. Reflection-absorption infrared spectroscopy and electrochemical data reveal that upon oxidation in a nonaqueous electrolyte both AnC6SH/Au and AnC7SH/ Au yield head-to-tail coupled, surface-confined oligomers and no detectable amounts of hydrolyzed products.

Introduction Several reports have recently been produced concerning the electrochemical polymerization of supported monolayers1 and self-assembled monolayers containing electroactive functionalities capable of forming surfaceconfined conducting polymers.2-14 It is believed that surface-confined conducting polymers could be used as “nano” wires, molecular switches, or data storage systems in the miniaturization of future devices.15,16 We have reported the electrochemical polymerization of pyrroleterminated self-assembled monolayers on Au.5 This selfassembled monolayer was found to form electroactive * To whom correspondence should be addressed. Phone: (225) 578-3239. Facsimile: (225) 578-3458. E-mail: [email protected]. † Current Address: ATMI, Buffalo Grove, IL 60089. (1) Mello, S. V.; Mattoso, L. H. C.; Oliveira, O. N.; Faria, R. M. Thin Solid Films 1996, 285, 187-190. (2) Hayes, W. A.; Shannon, C. Langmuir 1996, 12, 3688-3694. (3) Kuwabata, S.; Fukuzaki, R.; Nishizawa, M.; Martin, C. R.; Yoneyama, H. Langmuir 1999, 15, 6807-6812. (4) Lukkari, J.; Kleemola, K.; Meretoja, M.; Ollonqvist, T.; Kankare, J. Langmuir 1998, 14, 1705-1715. (5) Willicut, R. J.; McCarley, R. L. Langmuir 1994, 11, 296-301. (6) Sayre, C. N.; Collard, D. M. Langmuir 1995, 11, 302-306. (7) Michalitsch, R.; Lang, P.; Yassar, A.; Nauer, G.; Garnier, F. Adv. Mater. 1997, 9, 321-326. (8) Smela, E.; Kariis, H.; Yang, Z. P.; Mecklenburg, M.; Liedberg, B. Langmuir 1998, 14, 2984-2995. (9) Ng, S. C.; Miao, P.; Chen, Z. K.; Chan, H. S. O. Adv. Mater. 1998, 10, 782-786. (10) Inaoka, S.; Collard, D. M. Langmuir 1999, 15, 3752-3758. (11) Berlin, A.; Zotti, G. Macromol. Rapid Commun. 2000, 21, 301318. (12) Georgiadis, R.; Peterlinz, K. A.; Rahn, J. R.; Peterson, A. W.; Grassi, J. H. Langmuir 2000, 16, 6759-6762. (13) Sullivan, J. T.; Harrison, K. E.; Mizzell, J. P.; Kilbey, S. M. Langmuir 2000, 16, 9797-9803. (14) Turyan, I.; Mandler, D. J. Am. Chem. Soc. 1998, 120, 1073310742. (15) Skotheim, T. A.; Elsenbaumer, R. L.; Reynolds, J. R. Handbook of Conducting Polymers; 2nd ed.; Marcel Dekker: New York, 1998. (16) MacDiarmid, A. G.; Zhou, Y.; Feng, J. Synth. Met. 1999, 100, 131-140.

surface-confined poly(pyrrole) when the pyrrole monolayer was oxidized in a nonaqueous solution of tetrabutylammonium perchlorate. However, the applicability of this system was found to be hindered because of the facile oxidation of the poly(pyrrole) monolayer under ambient laboratory conditions.17 To form more robust surfaceconfined conducting polymer monolayers, we have turned to the polymerization of aniline-terminated self-assembled monolayers on Au. The surface-confined polymerization of 4-aminothiophenol monolayers on Au has been attempted in aqueous acidic media.2,4 Electrochemical oxidation of these monolayers has been found to produce a surface-confined aniline dimer. This aniline dimer has been observed to hydrolyze and yield a surface-confined benzoquinonemonoimine dimer. In the course of the work presented here, a report emerged discussing the voltammetric characteristics of an aniline-terminated ethanethiol monolayer on Au.3 The authors of that report present evidence supporting the possible electrochemical polymerization of the aniline terminus to form a surface-confined poly(aniline) monolayer on Au. Among the many available conducting polymers, poly(aniline) and poly(aniline) derivatives have received a great deal of attention.1,18-25 Poly(aniline) and its derivatives may be chemically or electrochemically formed, have (17) McCarley, R. L.; Willicut, R. J. J. Am. Chem. Soc. 1998, 120, 9296-9304. (18) D’Aprano, G.; Leclerc, M.; Zotti, G.; Schiavon, G. Chem. Mater. 1995, 7, 33-42. (19) Dhawan, S. K.; Trivedi, D. C. J. Appl. Polym. Sci. 1995, 58, 815-826. (20) Goncalves, D.; Mattoso, L. H. C.; Bulhoes, L. O. S. Electrochim. Acta 1994, 39, 2271-2275. (21) Jiang, Y.; Epstein, A. J. J. Am. Chem. Soc. 1990, 112, 28002801. (22) MacDiarmid, A. G.; Epstein, A. J. Faraday Discuss. Chem. Soc. 1989, 88, 333-349. (23) Mattoso, L. H. C.; Faria, R. M.; Bulhoes, L. O. S.; MacDiarmid, A. G. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 2147-2153.

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been shown to be quite stable under a variety of conditions, have good conductivity values, and may reside in one of several oxidation states, each containing its own unique electrochemical and optical properties.21,26-29 These physical properties have led to numerous applications and approaches to developing ion selective electrodes, artificial muscles, and electrochemical nanoswitches.15,16 Electrochemical formation of poly(aniline) and poly(aniline) derivatives in various protic solutions has been described.22,28-31 Aqueous solutions containing H2SO4, HCl, HNO3, and HBF4 have been used as electrolytes for electrochemical polymerization of anilines. Such polymerization procedures typically yield low conjugation length polymers that contain quinone-type degradation products. In the paper immediately preceding this one, we describe the synthesis and analysis of 2-(6-mercaptohexan-1-oxy)aniline and 2-(7-mercaptoheptan-1-oxy)aniline monolayers on gold (AnC6SH/Au and AnC7SH/Au). Electrochemical oxidation of these monolayers in 1 M H2SO4 leads to surface-confined products that exhibit voltammetry indicative of poly(aniline), dimeric aniline species, and hydrolysis products (quinonemonoimine dimers) similar to those produced in the bulk polymerization of aniline and aniline derivatives in aqueous media.32-36 To minimize the formation of hydrolysis products during the electrochemical production of bulk poly(aniline), nonaqueous solutions containing an inorganic electrolyte have been utilized. Formation of bulk poly(aniline) in nonaqueous solution has been shown to produce electroactive poly(aniline) films similar to polymers formed in acidic solutions but free of degradation products (quinones and quinonemonoimines).37-39 In an effort to prevent production of hydrolysis products and increase polymer formation during the oxidation of AnC6SH/Au and AnC7SH/Au, we report here the electrochemical oxidation of AnC6SH/Au and AnC7SH/Au in nonaqueous solution, referred to as ox(nonaq)-AnC6SH/ Au and ox(nonaq)-AnC7SH/Au. Cyclic voltammetry was performed on the pristine aniline monolayers in 0.1 M LiClO4/CH3CN, and the resulting voltammetry was compared to that of dimeric model compounds in 0.1 M LiClO4/ CH3CN. The “nonaqueous oxidized monolayers” are shown to be predominantly composed of surface-confined, headto-tail coupled oligomers; there is no electrochemical or (24) Mello, S. V.; Mattoso, L. H. C.; Faria, R. M.; Oliveira, O. N. Synth. Met. 1995, 71, 2039-2040. (25) Raposo, M.; Pontes, R. S.; Mattoso, L. H. C.; Oliveira, O. N. Macromolecules 1997, 30, 6095-6101. (26) Stilwell, D. E.; Park, S. M. J. Electrochem. Soc. 1989, 136, 427433. (27) Stilwell, D. E.; Park, S. M. J. Electrochem. Soc. 1988, 135, 24912496. (28) Stilwell, D. E.; Park, S. M. J. Electrochem. Soc. 1988, 135, 22542262. (29) Hirai, T.; Kuwabata, S.; Yoneyama, H. J. Chem. Soc., Faraday Trans. 1989, 85, 969-976. (30) Wei, X.-L.; Wang, Y. Z.; Long, S. M.; Bobeczko, C.; Epstein, A. J. J. Am. Chem. Soc. 1996, 118, 2545-2555. (31) Shim, Y. B.; Won, M. S.; Park, S. M. J. Electrochem. Soc. 1990, 137, 538-544. (32) Cui, C. Q.; Su, X. H.; Lee, J. Y. Polym. Degrad. Stab. 1993, 41, 69-76. (33) Fong, Y.; Schlenoff, J. B. Polymer 1995, 36, 639-643. (34) Kobayashi, T.; Yoneyama, H.; Tamura, H. J. Electroanal. Chem. 1984, 177, 293-297. (35) Stilwell, D. E.; Park, S. M. J. Electrochem. Soc. 1988, 135, 24972502. (36) Stilwell, D. E.; Park, S. M. J. Electrochem. Soc. 1989, 136, 688698. (37) Jiang, R. Z.; Dong, S. J.; Song, S. H. J. Chem. Soc., Faraday Trans. 1989, 85, 1575-1584. (38) Naudin, E.; Gouerec, P.; Belanger, D. J. Electroanal. Chem. 1998, 459, 1-7. (39) Yamada, K.; Teshima, K.; Kobayashi, N.; Hirohashi, R. J. Electroanal. Chem. 1995, 394, 71-79.

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Figure 1. Cyclic voltammetry of (A) AnC6SH/Au and (B) AnC7SH/Au in 0.1 M LiClO4/CH3CN. A scan rate of 0.1 V s-1 was used in all cases. The electrode area is 0.172 cm2.

IR evidence that supports the presence of hydrolysis products (quinonemonoimines). The nonaqueous oxidized monolayers were further analyzed by RAIRS at various potentials in order to shed light on the nature of the surface-confined aniline dimers. RAIR spectra observed for ox-AnC6SH/Au and ox-AnC7SH/Au produced in nonaqueous solution are similar to those reported for phenylcapped aniline dimers and tetramers. Experimental Section Chemicals. 2-Methoxy-N 4-phenyl-1,4-phenylenediamine (Aldrich, 95%), 3,3′-dimethoxybenzidine (Aldrich, 97%), azobenzene (Aldrich, 96%), and tetrabutylammonium fluoroborate, TBAFB (Sachem Inc., Electrolytic Grade), were used as received. Lithium perchlorate (Sigma, 96.9%) was vacuum-dried for 8 h before use. The aniline- and ferrocene-terminated thiols were from the paper immediately preceding this one and a previous study.40 Other Procedures. All methods used were the same as in the paper immediately preceding this one.

Results and Discussion Electrochemistry of AnC6SH/Au and AnC7SH/Au in Nonaqueous Media. When electrochemically oxidized in aqueous acidic media, AnC6SH/Au and AnC7SH/Au yield surface-confined oligomers and quinonemonoimine degradation products, as described in the paper immediately preceding this one. In an attempt to eliminate production of degradation products and enhance polymer formation, electrochemical oxidation of AnC6SH/Au and AnC7SH/Au was performed in 0.1 M LiClO4/CH3CN by scanning the potential of the working electrode from 0.0 to +0.9 V versus SSCE; see Figure 1. The first oxidative scan for both monolayers resulted in the observation of two irreversible oxidation waves at ca. +0.5 and ca. +0.850.9 V versus SSCE. Continued scanning of the potential in this range leads to voltammograms containing two reversible surface-confined waves at roughly +0.25 and +0.62 V; the intensity of these waves decreases after multiple potential cycles. The voltammetry displayed by the oxidized monolayers in this study is similar to the (40) Peanasky, J. S.; McCarley, R. L. Langmuir 1998, 14, 113-123.

Nonaqueous Electrochemistry of Alkanethiols on Au

Figure 2. Cyclic voltammetry of (A) ox(nonaq)-AnC6SH/Au and (B) ox(nonaq)-AnC7SH/Au in 1 M H2SO4. A scan rate of 0.1 V s-1 was used in all cases. The electrode area is 0.172 cm2.

voltammetry reported for phenyl-capped aniline oligomers in nonaqueous media.41 Phenyl-end-capped aniline dimers and tetramers were found to yield two reversible redox waves at roughly +0.3 and +0.7 V versus an Ag wire pseudo-reference for the dimer and at approximately +0.38 and +0.8 V versus an Ag wire pseudo-reference for the tetramer. As discussed in the paper immediately preceding this one, the voltammograms of ox-AnC6SH/Au and oxAnC7SH/Au obtained in aqueous acidic media (1 M H2SO4) were found to contain four surface-confined redox waves. Redox activity associated with the leucoemeraldine/ emeraldine (LE/EM) and emeraldine/pernigraniline (EM/ PN) transitions was observed at +0.2 and ca. +0.6 V versus SSCE, respectively, as well as that of the dimer near +0.45 V and that of the hydrolyzed dimer at roughly +0.38 V versus SSCE. To elucidate the nature of the products resulting from nonaqueous oxidation of AnC6SH/Au and AnC7SH/Au, ox(nonaq)-AnC6SH/Au and ox(nonaq)-AnC7SH/Au (prepared by cycling the potential between 0.0 and +0.9 V versus SSCE a minimum of five times in the nonaqueous electrolyte) were rinsed with water and dried with N2 gas, and then the voltammetry was obtained in 1 M H2SO4 between 0.0 and +0.9 V versus SSCE; see Figure 2. There is no voltammetric evidence for presence of the hydrolyzed dimer in either case (expected at ca. +0.38 V) on the first anodic scan, but the wave at roughly +0.45 V indicates the presence of dimers for both ox(nonaq)-AnC6SH/Au and ox(nonaq)-AnC7SH/Au. Because of the lack of voltammetric waves centered at +0.6 V for the EM/PN process in the case of ox(nonaq)-AnC6SH/Au, coupled with the presence of a set of waves at +0.25 V for the EM/LE process, only small oligomers (most likely trimers) are present.42 However, the voltammetric features associated with the LE/EM and EM/PN transitions are found in the first cyclic voltammogram of ox(nonaq)-AnC7SH/Au in 1 M H2SO4, pointing to the presence of larger oligomers (at least tetramers) or polymer. In addition, it is found for both ox(nonaq)-AnC6SH/Au and ox(nonaq)-AnC7SH/Au in the sulfuric acid electrolyte that the intensity of the dimer redox waves centered at approximately +0.45 V (41) Shacklette, L. W.; Wolf, J. F.; Gould, S.; Baughman, R. H. J. Chem. Phys. 1988, 88, 3955-3961. (42) Yang, H.; Bard, A. J. J. Electroanal. Chem. 1992, 339, 423-449.

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decreased after further potential cycling, whereas that of the waves associated with the hydrolyzed dimer (ca. +0.38 V) increased. During these potential excursions in the acid electrolyte, no increases in the amount of oligomeric anilines were noted (wave at ca. +0.6 V) for ox(nonaq)AnC7SH/Au nor was there any evidence for any changes in the length of the oligomers for ox(nonaq)-AnC6SH/Au (new voltammetric signal at ca. +0.6 V). To further support our hypothesis that head-to-tail coupled dimers are present in the aniline monolayers electrochemically oxidized in nonaqueous solution, the solution electrochemistry of three aniline dimers was investigated. The voltammetry of 2-methoxy-N4-phenyl1,4-phenylenediamine, 3,3′-dimethoxybenzidine, and azobenzene was obtained in CH3CN. The choice of 2-methoxy-N4-phenyl-1,4-phenylenediamine as a model compound is based on its structure (alkoxy substituent in 2-position) and the head-to-tail coupling mechanism proposed for aniline and aniline derivatives in mildly acidic solutions.43 In addition, it has been proposed that tailto-tail coupling can occur between the aniline rings in the bulk polymerization of aniline to yield benzidines if the concentration of acid in the electrolyte is relatively high (∼6 M).43 For this reason, the electrochemical characteristics of 3,3′-dimethoxybenzidine were obtained. Finally, azobenzenes (head-to-head coupling) have been shown to form during electrolysis of anilines in aqueous electrolytes at high pH or nonaqueous electrolytes containing a proton scavenger.43 In 0.1 M LiClO4/CH3CN, the benzidine and phenylenediamine model dimer compounds produce two reversible redox waves. Voltammograms of 3,3′-dimethoxybenzidine contain redox waves centered at E°2 ) +0.42 V and E°1 ) +0.62 V; see Figure 3A. 2-Methoxy-N4-phenyl-1,4-phenylenediamine exhibits two sets of waves at E°2 ) +0.22 V and E°1 ) +0.68 V; see Figure 3B. The voltammetry of 2-methoxy-N4-phenyl-1,4-phenylenediamine is very similar (E°2 ) +0.22 V and E°1 ) +0.68 V) to the voltammetry displayed by ox(nonaq)-AnC6SH/Au and ox(nonaq)AnC7SH/Au in nonaqueous solution (E°2 ) +0.25 V and E°1 ) +0.62 V), whereas that of 3,3′-dimethoxybenzidine is quite different (E°2 ) +0.42 V and E°1 ) +0.62 V). Although 2-methoxy-N4-phenyl-1,4-phenylenediamine does not have the additional alkoxy group on the second ring as would the AnC6SH/Au and AnC7SH/Au dimer, the similarity in the voltammetry of the aminodiphenylamine model compound and that of the oxidized monolayers is striking and further supports our hypothesis of dimer formation during the oxidation of aniline-terminated monolayers in nonaqueous media. To investigate whether the voltammetry exhibited by ox(nonaq)-AnC6SH/Au and ox(nonaq)-AnC7SH/Au between 0.0 and +0.9 V versus SSCE in Figure 1 is due to the presence of head-to-head coupled dimers (azobenzenes), AnC6SH/Au and AnC7SH/Au were oxidized in a nonaqueous solvent with pyridine added as a proton scavenger (ox(py)-AnC6SH/Au and ox(py)-AnC7SH/Au) so as to maximize43 the amount of possible azobenzenes present in the oxidized aniline monolayers. Pristinemonolayer-modified electrodes were immersed in a solution of 0.1 M tetrabutylammonium fluoroborate (TBAFB)/ 0.1 M pyridine/CH3CN, and the potential was scanned from 0.0 to +0.7 V and back to -0.4 V versus SSCE; see Figure 4. The first potential scan for both AnC6SH/Au and AnC7SH/Au produced an irreversible oxidation wave near +0.6 V that is attributed to radical cation formation. (43) Hand, R. L.; Nelson, R. F. J. Electrochem. Soc. 1978, 125, 10591069.

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Figure 4. Cyclic voltammetry of (A) AnC6SH/Au and (B) AnC7SH/Au in 0.1 M TBAFB/0.1 M pyridine/CH3CN. A scan rate of 0.1 V s-1 was used in all cases. The electrode area is 16.1 cm2. Figure 3. Cyclic voltammetry of 1 mM (A) 3,3′-dimethoxybenzidine and (B) 2-methoxy-N4-phenyl-1,4-phenylenediamine in 0.1 M LiClO4/CH3CN. A scan rate of 0.1 V s-1 was used in all cases. The electrode area is 0.0079 cm2.

Further scanning of the potential over this region for both aniline monolayers resulted in the observation of a set of quasi-reversible redox waves with an anodic peak potential at approximately +0.25 V and a cathodic peak potential at ca. -0.1 V. The voltammetry obtained for the two chain length aniline monolayers was compared to the solution voltammetry of the three model compounds, 2-methoxy-N4phenyl-1,4-phenylenediamine, 3,3′-dimethoxybenzidine, and azobenzene, in 0.1 M TBAFB/0.1 M pyridine/CH3CN. The 3,3′-dimethoxybenzidine (Figure 5A) yielded a cyclic voltammogram with only a broad, irreversible oxidation wave centered at approximately +0.5 V versus SSCE. The azobenzene did not give rise to any observable redox waves over the potential range investigated; see Figure 5B. (Because of the reductive desorption of thiol monolayers in nonaqueous solvents at potentials44 near that for the reduction reactions of azobenzene, it was not possible to directly probe the presence of azobenzenes in ox(py)AnC6SH/Au and ox(py)-AnC7SH/Au.) However, the 2-methoxy-N4-phenyl-1,4-phenylenediamine (Figure 5C) voltammogram is somewhat similar to that observed for ox(py)-AnC6SH/Au and ox(py)-AnC7SH/Au (Figure 4) (an oxidation wave at approximately +0.25 V and a reduction wave at roughly -0.15 V). From the electrochemical data presented here, we conclude that the preferred monomer coupling route for the two different surface-confined aniline monolayers, (44) Schneider, T. J.; Buttry, D. A. J. Am. Chem. Soc. 1993, 115, 12391-12397.

even under conditions that should promote head-to-head coupling (azobenzenes)43 or tail-to-tail coupling (benzidines, see paper immediately preceding this one), is such that head-to-tail species are produced. This selective coupling route may be caused by steric limitations imposed on the aniline tail group as a result of its being immobilized. In other words, the aniline rings cannot successfully couple together other than in a head-to-tail manner because of possible strain in the alkane chains. The observed difference in ring tilt (see previous paper) evidently does not lead to different coupling pathways. In addition, there is no evidence to support the presence of any hydrolyzed dimer (quinonemonoimine) in the aniline monolayers that have been electrochemically oxidized in 0.1 M LiClO4/ CH3CN. IR Analysis of Aniline-Terminated Monolayers after Nonaqueous Oxidation. To obtain further evidence for the presence of aniline oligomers in ox(nonaq)AnC6SH/Au and ox(nonaq)-AnC7SH/Au, reflectionabsorption infrared spectroscopy (RAIRS) was employed. AnC6SH/Au and AnC7SH/Au were electrochemically oxidized in 0.1 M LiClO4/CH3CN as discussed previously. After five potential cycles between 0.0 and +0.9 V versus SSCE, the electrochemical potential was held at 0.0 V versus SSCE and the electrode was then removed, rinsed with acetonitrile (to remove excess electrolyte from the electrode surface), and dried with N2, and an RAIR spectrum was obtained; this is designated as the 0.0 V spectrum. After RAIRS, the same electrode was then reimmersed in the nonaqueous electrolyte, and the potential was again cycled between 0.0 and +0.9 V and held at +0.9 V, at which point the electrode was removed, rinsed with acetonitrile, and dried with N2; the RAIR spectrum was then obtained. This process was again repeated to obtain another spectrum at 0.0 V to ensure

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Figure 6. Low-energy RAIR spectra of ox(nonaq)-AnC6SH/ Au removed at 0.0 and +0.9 V vs SSCE.

Figure 5. Cyclic voltammetry of 1 mM (A) 3,3′-dimethoxybenzidine, (B) azobenzene, and (C) 2-methoxy-N4-phenyl-1,4phenylenediamine in 0.1 M TBAFB/0.1 M pyridine/CH3CN. A scan rate of 0.1 V s-1 was used in all cases. The electrode area is 0.0079 cm2.

reproducibility. There were no differences noted for the 0 V spectra before and after obtaining the +0.9 V spectrum; thus, only the initial 0 V spectrum is shown. The RAIR spectra for ox(nonaq)-AnC6SH/Au removed at 0 V and +0.9 V are displayed in Figure 6. The spectra of ox(nonaq)-AnC7SH/Au at the two potentials are virtually identical to those of ox(nonaq)-AnC6SH/Au; thus, for brevity we show only the spectrum of ox(nonaq)-AnC6SH/ Au. In addition, the high-energy region (C-H stretches) of the spectra is extremely similar to that of spectra of the aniline monolayers oxidized in aqueous media. However, the spectra for the monolayers oxidized in nonaqueous electrolytes differ somewhat in the low-energy region; see Figure 6. For the +0.9 V spectra, the most notable difference is the lack of a band near 1660 cm-1 that would be due to CdO stretching of a quinonemonoimine, as discussed in the previous paper. Thus, it would seem that the use of a nonaqueous electrolyte indeed prevents any side-reactions with water. The 0 V spectra for the

nonaqueous electrolyte case exhibit intensities for the aromatic CdC stretch bands at roughly 1470 and 1455 cm-1 that are ∼60% less than those for the case where an aqueous electrolyte was used. In addition, a decrease of approximately 25% in the intensity of the benzoid bands at 1600 and 1510 cm-1 (b1 and b2) is also noted when comparing the nonaqueous spectra to the aqueous spectra. The remaining bands for the nonaqueous case are almost identical with respect to peak frequency and peak intensity when compared to those of the aqueous case. As discussed in the previous paper, certain characteristic bands exist in the IR spectra of oligo(anilines); the intensity of these bands is decreased, and new bands are present when the oligo(anilines) are in their fully oxidized state.41 In the fully reduced form of phenyl-capped aniline dimers and tetramers, transitions are observed at 1603 and 1515 cm-1 (b1 and b2) resulting from ring stretching associated with the benzoidal form of the oligomers. In addition, quinoid bands at about 1580 and ∼1485 cm-1 (q1 and q2) and much smaller intensity benzoid bands are observed for the fully oxidized phenyl-capped dimer and tetramer.41 Similar bands and trends in the intensities of the bands are noted in the spectra of ox(nonaq)-AnC6SH/Au and ox(nonaq)-AnC7SH/Au RAIR at 0 V and +0.9 V; see Figure 6. In particular, the b1 and b2 bands are located at 1600 and 1510 cm-1 for both ox(nonaq)-AnC6SH/Au and ox(nonaq)-AnC7SH/Au, and the intensity ratio of the b1 band to the b2 band is qualitatively similar to that of the phenylcapped aniline dimer.41 Also, the quinoidal bands for both ox(nonaq)-AnC6SH/Au and ox(nonaq)-AnC7SH/Au are located at 1590 and 1495 cm-1, and the intensity of the benzoid bands is decreased significantly when compared to those in the 0 V spectra. This is in agreement with the expected changes for a purely benzoidal system being converted to one with substantial quinoidal character. In addition, the spectra observed for ox(nonaq)-AnC6SH/ Au and ox(nonaq)-AnC7SH/Au at 0.0 V closely resemble, in general, those of phenyl-capped aniline dimers and tetramers in the 1900-500 cm-1 range, providing further support for our theory concerning the formation of headto-tail aniline oligomers in both chain length monolayers. As was discussed earlier, the voltammetry of ox(nonaq)AnC6SH/Au does not support the presence of oligo(anilines) with a repeat number greater than 2 or 3 (dimers and trimers), whereas the voltammetry of ox(nonaq)AnC7SH/Au indicates the presence of dimers and tetramers.

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Langmuir, Vol. 17, No. 6, 2001

Stability of Untreated and Electrochemically Treated Aniline Monolayers. Certain alkanethiol monolayers on gold surfaces have been shown to be displaced by solution-phase thiols when the coated Au surface is placed in solutions containing another thiol.45 The degree of displacement can be used to judge the stability of a given monolayer assembly. In this study, AnC6SH/Au, AnC7SH/Au, ox(nonaq)-AnC6SH/Au, and ox(nonaq)-AnC7SH/Au were placed in a solution of 0.2 mM 11-ferrocenoylundecane-1-thiol in ethanol for various amounts of time. The modified electrodes were then removed from the ferrocene solution, rinsed with ethanol, and dried with N2. The voltammetry of these electrodes in 0.1 M LiClO4/CH3CN was then inspected for the presence of ferrocene redox activity. This protocol allows for a qualitative assessment of the stability of the aniline monolayers. In the case of the untreated aniline monolayers for exposure times of up to 24 h (the longest time point), the voltammograms did not display any redox waves that can be attributed to ferrocenethiol but showed only that of the aniline monolayers themselves. Thus, AnC6SH/Au and AnC7SH/Au are quite stable to displacement by the solution-phase thiol. The ox(nonaq)-AnC6SH/Au and ox(nonaq)-AnC7SH/Au samples exhibited similar behavior at short exposure times, for no redox activity indicative of the presence of 11-ferrocenoylundecane-1-thiol was observed for 8 h of exposure to the ferrocenethiol solution. However, after 24 h of exposure to the ferrocenethiol solution a small amount of ferrocene redox activity was observed for ox(nonaq)-AnC7SH/Au but no ferrocene redox activity was observed for ox(nonaq)-AnC6SH/Au; see Figure 7. We believe this difference in monolayer stability is due to the fact that ox(nonaq)-AnC7SH/Au contains more hydrolyzed dimer and less oligomer (even though the oligomers are longer than in the ox(nonaq)-AnC6SH/ Au case) than does ox(nonaq)-AnC6SH/Au. The displacement data, combined with reductive desorption data reported in the paper immediately preceding this one, leads us to conclude that electrochemical oxidation of the aniline monolayers in either aqueous or nonaqueous electrolytes does not lead to desorption of AnC6SH or AnC7SH from the gold surface. Conclusions Electrochemical oxidation of AnC6SH/Au and AnC6SH/ Au in 0.1 M LiClO4/CH3CN was found to produce headto-tail coupled, surface-confined aniline oligomers but no hydrolyis products (quinonemonoimines). In an attempt to produce head-to-head coupled aniline dimers (azobenzenes), AnC6SH/Au and AnC6SH/Au were oxidized in nonaqueous media in the presence of pyridine. The voltammetry observed for ox(py)-AnC6SH/Au and ox(py)AnC7SH/Au supports the presence of only head-to-tail (45) Ulman, A. An Introduction to Ultrathin Organic Films From Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991.

Schomburg and McCarley

Figure 7. Cyclic voltammetry of (A) the 11-ferrocenoylundecane-1-thiol monolayer on gold in 0.1 M LiClO4/CH3CN. Voltammetry of (B) ox(nonaq)-AnC6SH/Au and (C) ox(nonaq)AnC7SH/Au in 0.1 M LiClO4/CH3CN after 24 h of immersion in 0.2 mM 11-ferrocenoylundecane-1-thiol solution. A scan rate of 0.1 V s-1 was used in all cases. The electrode area is 0.172 cm2.

aniline dimers and not the expected azobenzene. This result is rationalized by the steric limitations imposed on the aniline moiety of the surface-confined monolayers. The RAIR spectra of ox(nonaq)-AnC6SH/Au and ox(nonaq)AnC7SH/Au were found to be similar to the spectra of phenyl-capped aniline dimers and tetramers. In addition, the RAIR spectra of ox(nonaq)-AnC6SH/Au and ox(nonaq)AnC7SH/Au demonstrate that no hydrolysis products (quinonemonoimines) are formed during oxidation of the aniline monolayers in 0.1 M LiClO4/CH3CN. The stability of AnC6SH/Au, AnC7SH/Au, ox(nonaq)-AnC6SH/Au, and ox(nonaq)-AnC6SH/Au were evaluated by exposure to ethanolic solutions of 11-ferrocenoylundecane-1-thiol. Both pristine aniline monolayers and ox(nonaq)-AnC6SH/ Au showed no displacement from the Au surface in the presence of the competing ferrocenethiol adsorbate, but ox(nonaq)-AnC7SH/Au exhibited a small amount of displacement. Overall, the results from the study at hand and those from the preceding paper indicate that AnC6SH/ Au and AnC7SH/Au may be useful in the construction of poly(aniline) nanostructures on surfaces. Acknowledgment. We gratefully acknowledge financial support for this work from the National Science Foundation (CHE-9529770) and the Louisiana Education Quality Support Fund. LA0010222