Surface-Confined Monomers on Electrode Surfaces. 10

Langmuir 2001, 17, 1983-1992

1983

Surface-Confined Monomers on Electrode Surfaces. 10. Electrochemical and Infrared Spectroscopic Characteristics of Aniline-Terminated Alkanethiol Monolayers on Au Electrochemically Treated in Aqueous Solution 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 We have synthesized and conducted studies on two self-assembled monolayers which have an oxyaniline tail group that is tethered to a gold surface via an alkanethiol chain. Reflection-absorption infrared spectroscopy (RAIRS), electrochemical, and contact angle measurements were performed on pristine 2-(6mercaptohexan-1-oxy)aniline (AnC6SH) and 2-(7-mercaptoheptan-1-oxy)aniline (AnC7SH) monolayers on planar Au surfaces. The pristine monolayers were electrochemically oxidized in 1 M H2SO4, and the resulting voltammetry was compared to that of dimeric model compounds and electrochemically deposited poly(o-phenetidine) films on Au electrodes. RAIRS and electrochemical data reveal that electrochemical oxidation of AnC6SH and AnC7SH monolayers on Au results in the formation of surface-confined poly(aniline) as well as aniline dimers and hydrolyzed aniline dimers.

Introduction The initial synthesis of conducting poly(acetylene)1,2 led to considerable interest in the formation of conducting polymers that can potentially be used for battery technology, electrochromic and optical devices, and nanostructures.3-5 In the field of conducting polymer research, poly(aniline)s have received a great deal of attention because of their ease of synthesis as well as their environmental, optical, and electrochemical properties.6-9 Poly(aniline) derivatives can be synthesized by either chemical or electrochemical means in aqueous acidic or nonaqueous media.6-9 Synthesis of the polymer occurs through creation of the radical cation of the monomer by electrochemical or chemical oxidation, followed by its coupling with other activated monomer or oligomer units. The solubility of the polymer product in the solvent system used for the synthesis dictates the highest achievable molecular weight. Poly(aniline) and poly(aniline) derivatives have been shown to be stable in one of three oxidation states: the leucoemeraldine, the emeraldine, and the pernigraniline * 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) Ito, T.; Shirakawa, H.; Ikeda, S. J. Polym. Sci., Polym. Chem. Ed. 1974, 12, 11-20. (2) Chiang, C. K.; Park, Y. W.; Heeger, A. J.; Shirakawa, H.; Louis, E. J.; MacDiarmid, A. G. Phys. Rev. Lett. 1978, 39, 1098-1101. (3) Yang, L. S.; Shan, Z. Q.; Liu, Y. D. J. Power Sources 1991, 34, 141-145. (4) Duek, E. A. R.; Depaoli, M. A.; Mastragostino, M. Adv. Mater. 1992, 4, 287-291. (5) Barbero, C.; Kotz, R. J. Electrochem. Soc. 1994, 141, 859-865. (6) Mattoso, L. H. C.; Manohar, S. K.; MacDiarmid, A. G.; Epstein, A. J. J. Polym. Sci., Part A: Polym. Chem. 1995, 33, 1227-1234. (7) Stilwell, D.; Park, S.-M. J. Electrochem. Soc. 1988, 135, 22542262. (8) Osaka, T.; Nakajima, T.; Naoi, K.; Owens, B. B. J. Electrochem. Soc. 1990, 137, 2139-2142. (9) Stilwell, D. E.; Park, S.-M. J. Electrochem. Soc. 1988, 135, 24972502.

Figure 1. Oxidation states of poly(aniline) derivatives.

states (Figure 1).10,11 In the fully reduced form (leucoemeraldine, LE), the polymer is electronically insulating; this form contains a series of benzenoid rings linked by secondary amines. In acidic media, two-electron oxidation per four monomer repeats produces the conducting emeraldine salt form, EM. This cationic, polymeric form is composed of nitrogen groups (amines and imines) linked to benzenoid and quinoid rings. Protonic doping of the (10) Rannou, P.; Gawlicka, A.; Berner, D.; Pron, A.; Nechtschein, M. Macromolecules 1998, 31, 3007-3015. (11) MacDiarmid, A. G.; Epstein, A. J. Faraday Discuss. Chem. Soc. 1989, 88, 333-349.

10.1021/la001021+ CCC: $20.00 © 2001 American Chemical Society Published on Web 02/17/2001

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Langmuir, Vol. 17, No. 6, 2001

nitrogen linkages to yield the conducting emeraldine salt has been performed with acids such as CH3COOH, HBF4, HCl, and H2SO4 at various pHs so as to create polymers with tunable conductivities.11-13 The emeraldine form of the polymer may be further oxidized to produce the fully oxidized pernigraniline state (PN) that is, like the leucoemeraldine state, an electronically insulating form of the polymer. The PN form of poly(aniline) consists of a backbone of alternating benzenoid and quinoid rings linked by imines; the imines are highly susceptible to hydrolysis in aqueous acidic media. Hydrolysis of the imines can lead to polymer chain cleavage, formation of benzoquinonemonoimine functionalities at the ends of the polymer chain, and production of benzoquinone (hydrolysis followed by cleavage) in solution and trapped within the polymer films.14 Each of these poly(aniline) forms also possesses its own unique optical properties. For example, poly(aniline) films have been demonstrated as being effective in the fabrication of functional electrochromic displays.15 We are interested in creating a monolayer containing aniline tail groups, tethered to an electrode surface, which could undergo electrochemical polymerization to form a monolayer film of poly(aniline). This monomolecular polymer film could then be electrochemically switched to yield either an insulating or conducting monolayer film capable of being used in sensors and molecular electronic devices. Similar investigations aimed at forming monolayer films of conducting polymers from a variety of tethered monomers have been reported.16-24 Although ultrathin films of poly(aniline) have been created using Langmuir-Blodgett films15,25,26 and anilinium ions coordinated to sulfonic acid terminated thiol monolayers,24 the use of aniline-terminated alkanethiol monolayers in the study of polymerization reactions on surfaces would allow more facile evaluation of the effects of experimental variables on the resulting polymer properties. The formation of poly(aniline) monolayers from surfaceconfined monomers has previously been attempted through the use of 4-aminothiophenol (4-ATP) monolayers on Au. After oxidation of 4-ATP on Au in aqueous media, voltammetry indicative of a surface-confined dimer species was noted.27 Further potential cycling of the dimermodified electrode led to observation of a set of redox waves that was attributed to a surface-confined hydroquinone(12) Wei, X.-L.; Wang, Y. Z.; Long, S. M.; Bobeczko, C.; Epstein, A. J. J. Am. Chem. Soc. 1996, 118, 2545-2555. (13) Hirai, T.; Kuwabata, S.; Yoneyama, H. J. Chem. Soc., Faraday Trans. 1 1989, 85, 969-976. (14) Stilwell, D. E.; Park, S. M. J. Electrochem. Soc. 1989, 136, 688698. (15) Goldenberg, L. M.; Petty, M. C.; Monkman, A. P. J. Electrochem. Soc. 1994, 141, 1573-1576. (16) Sayre, C. N.; Collard, D. M. Langmuir 1995, 11, 302-306. (17) Michalitsch, R.; Lang, P.; Yassar, A.; Nauer, G.; Garnier, F. Adv. Mater. 1997, 9, 321-326. (18) Smela, E.; Kariis, H.; Yang, Z. P.; Mecklenburg, M.; Liedberg, B. Langmuir 1998, 14, 2984-2995. (19) Ng, S. C.; Miao, P.; Chen, Z. K.; Chan, H. S. O. Adv. Mater. 1998, 10, 782-786. (20) Inaoka, S.; Collard, D. M. Langmuir 1999, 15, 3752-3758. (21) Berlin, A.; Zotti, G. Macromol. Rapid Commun. 2000, 21, 301318. (22) Georgiadis, R.; Peterlinz, K. A.; Rahn, J. R.; Peterson, A. W.; Grassi, J. H. Langmuir 2000, 16, 6759-6762. (23) Sullivan, J. T.; Harrison, K. E.; Mizzell, J. P.; Kilbey, S. M. Langmuir 2000, 16, 9797-9803. (24) Turyan, I.; Mandler, D. J. Am. Chem. Soc. 1998, 120, 1073310742. (25) Dabke, R. B.; Dhanabalan, A.; Major, S.; Talwar, S. S.; Lal, R.; Contractor, A. Q. Thin Solid Films 1998, 335, 203-208. (26) Bodalia, R.; Manzanares, J.; Reiss, H.; Duran, R. Macromolecules 1994, 27, 2002-2007. (27) Hayes, W. A.; Shannon, C. Langmuir 1996, 12, 3688-3694.

Schomburg and McCarley

containing species that was formed by hydrolysis of a partially desorbed aniline dimer. Recently, elucidation of the degradation pathway of 4-ATP on Au in aqueous media was further explored.28 Radical-radical coupling was proposed to occur between adjacent aminothiophenol molecules, followed by desorption of one of the 4-ATP units to yield an electrode surface modified with 4′-mercapto4-aminodiphenylamine. Oxidation of the surface-confined aminodiphenylamine to the benzoquinonediimine followed by hydrolysis resulted in the formation of a benzoquinonemonoimine species, which then further hydrolyzed to produce 4-aminothiophenol on Au and benzoquinone in solution.28,29 Recently, the polymerization of 3-(2-mercaptoethane)aniline on Au substrates was reported.30 In that study, the authors report that electrochemical oxidation of 3-(2mercaptoethane)aniline on Au in aqueous HClO4 yields voltammetry suggestive of the possible presence of a surface-confined poly(aniline) monolayer that contains no degradation products (no products arising from hydrolysis reactions). In this paper, we present electrochemical and reflection-absorption infrared spectroscopy (RAIRS) studies of two self-assembled alkanethiol monolayers possessing an oxyaniline tail group. RAIRS analysis is performed on the self-assembled monolayers on Au, and the results are compared to isotropic infrared spectra of the monomers. These monolayers were also subjected to water contact angle and electrochemical blocking measurements for determination of their wettability characteristics and their ability to prevent electron-transfer reactions between the underlying Au surface and solution-phase redox species. In addition, the voltammetry of the aniline-terminated monolayers on Au in aqueous, acidic media was compared to that of a dimeric aniline model compound and poly(o-phenetidine) thin films on Au in a similar electrolyte. The oxidized aniline monolayers are shown to be composed of surface-confined polymer, dimer, and hydrolyzed dimer. The oxidized films were further analyzed by RAIRS at various potentials for characterization of the polymeric forms present in the films. Experimental Section Chemicals. All solvents were of chromatographic grade or better and were used without further purification. Dibromohexane (Aldrich, 96%), dibromoheptane (Aldrich, 97%), KOH (Aldrich, >85%), NaOH (Aldrich, 99%), thiourea (Aldrich, 99+%), o-phenetidine (Aldrich, 98%), and 2-methoxy-N4-phenyl-1,4phenylenediamine (Aldrich, 95%) were used as received. Distilled water was passed through a Barnstead reverse osmosis filter followed by a Nanopure water system to yield water with a resistivity of 18 MΩ cm. All other materials were reagent grade or better. Synthesis of Aniline-Terminated Alkanethiols. The 2-(6mercaptohexan-1-oxy)aniline (AnC6SH) and 2-(7-mercaptoheptan-1-oxy)aniline (AnC7SH) were both synthesized using a similar procedure. The potassium salt of o-aminophenol was first made by placing o-aminophenol (9.17 mmol) and potassium hydroxide (9.17 mmol) in an argon-purged round-bottom flask with dry ethylene glycol dimethyl ether (0.05 L). This solution was allowed to stir for 30 min to ensure phenoxide salt formation. The solution was cannulated to a second round-bottom flask, and a Williamson ether synthesis was performed using the o-aminophenoxide salt and excess dibromoalkane (22.93 mmol) under an argon atmosphere in degassed/dried ethylene glycol (28) Lukkari, J.; Kleemola, K.; Meretoja, M.; Ollonqvist, T.; Kankare, J. Langmuir 1998, 14, 1705-1715. (29) Lukkari, J.; Kleemola, K.; Meretoja, M.; Kankare, J. Chem. Commun. 1997, 1099-1100. (30) Kuwabata, S.; Fukuzaki, R.; Nishizawa, M.; Martin, C. R.; Yoneyama, H. Langmuir 1999, 15, 6807-6812.

Aqueous Electrochemistry of Alkanethiols on Au

Langmuir, Vol. 17, No. 6, 2001 1985

Table 1. Spectroscopic Assignments AnC6SH IR monomer AnC6SH/Au AnC7SH/Au (cm-1) RAIR (cm-1) RAIR (cm-1) 3468 3375 3047 2931 2855 1610 1602 1502 1466 1455 1439 1390 1342 1266 1218 1142 1075 1041 1020 905 733 a

3439 3379 3063 2931 2862 1616 1601 1508 1475 1455 n/oa 1396 1343 1278 1224 1146 1077 1048 1008 n/oa 735

3459 3377 3063 2929 2857 1616 1604 1507 1470 1459 n/oa 1392 1343 1277 1222 1144 1077 1048 n/oa n/oa 738

assignment νa(N-H) νs(N-H) ν(C-H)Ar νa(C-H) νs(C-H) δ(N-H) ν(CdC)Ar ν(CdC)Ar ν(CdC)Ar + δ(C-H)? ν(CdC)Ar + δ(C-H)? δ(C-H)? ν(C-N)? ν(C-N)? ν(Ar-O) ν(C-N) ωip(C-H)Ar ωip(C-H)Ar ν(C-O) ωip(C-H)Ar ωoop(C-H)Ar ωoop(C-H)Ar

Not observed.

Figure 2. RAIR spectra of AnC6SH/Au and AnC7SH/Au compared to the isotropic spectrum of AnC6SH. measurements were taken. Values reported are the average of five repetitions, and reported errors are given as ( one standard deviation.

Results and Discussion dimethyl ether. The resulting 2-(ω-bromoalkan-1-oxy)aniline (0.72 g, AnC6Br) was purified by column chromatography using 40 micron silica gel (Alltech Associates, Inc.) and a 10% hexane/ 90% ethyl acetate mixture. Conversion of the purified 2-(ωbromoalkan-1-oxy)aniline to the thiol was achieved by refluxing a mixture of the 2-(ω-bromoalkan-1-oxy)aniline (1.84 mmol) and thiourea (1.84 mmol) for 8 h in 0.020 L of argon-purged ethanol to form the thiouronium salt. The thiouronium salt was hydrolyzed by allowing the solution to come to room temperature, adding 1 equiv of sodium hydroxide, and refluxing for an additional 8 h or until thin-layer chromatography indicated complete conversion. After cooling to room temperature, the solution was next added to 0.150 L of water and then extracted using dichloromethane. The dichloromethane extracts were combined and concentrated by rotary evaporation to yield the pure 2-(ω-mercaptoalkan-1-oxy)aniline as an oil. All products were subjected to transmission infrared spectroscopy (IR), gas chromatography-mass spectroscopy (GC-MS), and high-resolution electron ionization mass spectroscopy analysis for characterization. GC-MS results indicated a purity of >97% for both monomers. Infrared band assignments are given in Table 1. For 2-(6-mercaptohexan-1-oxy)aniline: calcd, 225.1183; found MS(EI/HR), 225.1192 (δ ) 1.9 ppm). For 2-(7-mercaptoheptan-1oxy)aniline: calcd, 239.1339; found MS(EI/HR), 239.1344 (δ ) 0.1 ppm). Surface Derivatization. The pure 2-(ω-mercaptoalkan-1oxy)anilines were dissolved in ethanol to create 0.5 mM dosing solutions for self-assembled monolayer formation; these solutions were stored under nitrogen in a drybox or freezer until needed. Au surfaces were immersed in nitrogen-purged dosing solutions for a minimum of 8 h; however, longer dosing times did not lead to any noticeable differences in the electrochemical or infrared spectroscopic properties of the derivatized surfaces. Au electrodes for electrochemical analysis of poly(o-phenetidine) and 2-methoxy-N4-phenyl-1,4-phenylenediamine and substrates for reflection-absorption infrared spectroscopy and surface coverage analysis were prepared as previously described.31,32 Voltammetric Measurements and Infrared Spectroscopy. Electrochemistry and infrared spectroscopy were performed according to methods in the literature.32,33 Wettability Measurements. A VCA 2000 Contact Angle Instrument was used to obtain the contact angle of water on the various surfaces. Approximately 6 µL of 18 MΩ cm water was placed on the surfaces and allowed to sit for 30 s before (31) Willicut, R. J.; McCarley, R. L. Langmuir 1994, 11, 296-301. (32) Peanasky, J. S.; McCarley, R. L. Langmuir 1998, 14, 113-123. (33) McCarley, R. L.; Willicut, R. J. J. Am. Chem. Soc. 1998, 120, 9296-9304.

Infrared Spectroscopy of Pristine Aniline-Terminated Monolayers on Au. Transmission (isotropic) spectra and RAIR spectra were collected for both the AnC6SH and AnC7SH materials. For brevity, we show only the AnC6SH isotropic spectrum. Both molecules were observed to produce virtually the same isotropic spectra as well as nearly identical RAIR spectra (Figure 2). The following band analysis is thus for both the AnC6SH and AnC7SH monolayers on Au (AnC6SH/Au and AnC7SH/ Au) compared to the isotropic spectrum of AnC6SH. The band positions and their assignments are provided (Table 1). We noted no changes in the RAIR spectra of AnC6SH/ Au and AnC7SH/Au when exposed to the laboratory ambient for times up to 36 h. 4000-1700 cm-1 Region. In this spectral region, five infrared bands in the isotropic spectrum are observed at 3468, 3375, 3047, 2931, and 2855 cm-1. These bands are identified as the asymmetric (νa) and symmetric (νs) N-H stretches for the primary amine, the C-H stretch for the aromatic ring, and the asymmetric (νa) and symmetric (νs) C-H stretching modes for the methylene groups, respectively. The same transitions observed in the isotropic spectra are found at approximately the same energies in the RAIR spectra for both AnC6SH/Au and AnC7SH/Au. 1700-1300 cm-1 Region. In this region, several bands due to aromatic ring stretching, methylene scissoring and deformation, and N-H deformation are present. Aromatic ring stretching modes (ν(CdC)Ar) are observed in the RAIR spectra at roughly 1601, 1508, 1475, and 1455 cm-1. For o-substituted aromatic compounds, ring stretching bands occurring at 1600, 1600-1560, 1510-1460, and 1450 cm-1 are typical; however, these bands are sensitive to the type of ring substituent.34 Each of the four ν(CdC)Ar bands found here have nearby bands that are observed as shoulders. In the isotropic spectrum of AnC6SH, the first ν(CdC)Ar band observed when moving from high to low energy is at 1602 cm-1 and contains a shoulder at ∼1610 cm-1 which is attributable to an N-H deformation.35 In the RAIR spectra of AnC6SH/Au and AnC7SH/Au, these two transitions are somewhat better resolved but much (34) Bellamy, L. J. The Infra-Red Spectra of Complex Molecules; 3rd ed.; Chapman and Hall Ltd.: New York, 1975. (35) Evans, J. C. Spectrochim. Acta 1960, 16, 428-442.

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lower in intensity than in the isotropic spectrum. The band at ∼1508 cm-1 in the RAIR spectra (ring stretch, ν(CdC)Ar) has a nearby shoulder at 1515 cm-1; the shoulder at 1515 cm-1 is also observed in the literature spectra for o-anisidine and o-phenetidine, but there has been no assignment of this band. A third aromatic ring stretch is observed in the isotropic spectrum at 1455 cm-1 and contains two shoulders at 1466 cm-1 (fourth ν(CdC)Ar) and 1439 cm-1 (possible δ(CH2)).36 The third and fourth ring stretches are observed in the RAIR spectra at approximately 1475 and 1455 cm-1. Because of the expected δ(CH2) band in the 1450-1460 cm-1 range, it is possible that there is overlap between the methylene deformation band and the ν(CdC)Ar modes. Finally, two low-intensity bands at ∼1390 and ∼1342 cm-1 are found in all spectra; at this time, the nature of these two bands is not clear, but it is possible that they are C-N stretches. 1300-500 cm-1 Region. The transitions observed in this region are associated with C-N and C-O stretching and in-plane and out-of-plane C-H deformations. The 1266 cm-1 band in the isotropic spectrum (the ∼1278 cm-1 band in the RAIR spectra) is assigned as a C-N stretching vibration, and the band at ∼1220 cm-1 is associated with an aryl ether stretch.35 A small shoulder between these two bands is observed only in the isotropic spectrum, but the transition giving rise to this band is unclear. The next band at ∼1145 cm-1 is an in-plane C-H ring deformation and is followed by two other in-plane ring deformations at roughly 1075 and 1020 cm-1 as well as an alkyl ether vibration at about 1040 cm-1.36 Finally, an out-of-plane C-H deformation (ωoop(C-H)Ar) is observed at 905 cm-1 in the isotropic spectrum but not in the RAIR spectra, whereas another out-of-plane C-H deformation, characteristic of 1,2-ring substitution, is noted in both the isotropic and the RAIR spectra at ∼735 cm-1.36 Several qualitative and quantitative arguments concerning the monolayer structure can be made. Virtually all vibrations present in the isotropic spectra are observed in the RAIR spectra for both AnC6SH and AnC7SH. For both AnC6SH and AnC7SH on Au, the intensities of the aromatic ring, aromatic ether, and aromatic C-N stretch bands (transition dipole moments parallel to ring) and the out-of-plane ring C-H deformation band at ∼735 cm-1 (transition dipole moment perpendicular to ring) suggest that the phenoxy headgroup must be canted away from the surface normal somewhat, but this angle is not exceedingly large. Indeed, the plane of the ring is tilted away from surface normal 15-20° for AnC6SH and 2537° for AnC7SH, as determined from a simple geometric analysis and the band intensity ratios. Thus, the difference in chain length results in a measurable difference in the orientation of the ring in the two monolayers. The band positions of the methylene stretches for the aniline monolayers are 5-10 cm-1 higher than those observed for long-chain n-alkanethiols on Au, indicating that the alkane chains in the aniline monolayer are in a liquidlike (disordered) environment. Thus, if the alkane tethers are disordered and the aniline functionality has a preferred but different orientation for AnC6SH/Au and AnC7SH/ Au, the aniline group must control the overall structure of each monolayer. Finally, we find in the monolayer spectra that both amine N-H stretches are observed, thus leading us to conclude that the amine functionality must be oriented such that the transition dipole moment is directed along the surface normal (the transition dipole moment is parallel to the N-H bond). Two positions are (36) Socrates, G. Infrared Characteristic Group Frequencies; 2nd ed.; John Wiley & Sons: New York, 1994.

Schomburg and McCarley Table 2. Surface Coverage and Desorption Potential Data monolayer

surface coverage (Γ mol cm-2)

desorption potential (E (V) vs SSCE)

AnC6SH/Au AnC7SH/Au ox-AnC6SH/Au ox-AnC7SH/Au n-octadecanethiol

(9.3 ( 0.8) × 10-10 (9.9 ( 1.5) × 10-10 (8.8 ( 0.8) × 10-10 (11.8 ( 1.2) × 10-10 (8.8 ( 0.3) × 10-10

-1.10 ( 0.02 -1.06 ( 0.03 -1.12 ( 0.02 -1.09 ( 0.04 -1.24 ( 0.04

available to the amine when the aromatic ring is canted at some angle away from the surface normal: the amine may be pointing either down toward the Au substrate or up away from the Au surface. To gain some insight to the orientation of the amine functionality in the aniline monolayers, contact angle measurements were performed on pristine AnC6SH/Au and AnC7SH/Au using water as the probe liquid. Average contact angle values (5 repetitions) of 69 ( 2° and 71 ( 2° were found for AnC6SH/Au and AnC7SH/Au, respectively. Both monolayers appear to possess equivalent surface energies. When compared to literature values of ω-substituted alkanethiol monolayers on Au, the surface free energy is approximately equal to that of a methyl ether terminated surface (74°).37 Thus, we speculate that the amine functionality is most likely pointed down toward the Au substrate, because amine-terminated surfaces yield contact angle values of ∼40°.37 Preliminary amine-labeling experiments using fluorescein isothiocyanate (ethanol solvent) and quinones (acetonitrile solvent)29 also suggest this orientation of the amine group; we have been unable to label the amine groups in the aniline monolayers to date. However, we must stress that it is quite possible that AnC6SH/Au and AnC7SH/Au adopt a structure in a nitrogen ambient (infrared compartment atmosphere) or organic solvent that is much different from that found when such monolayers are in the presence of aqueous media. Surface Density of Aniline Monolayers on Au(111). To determine the surface coverage of AnC6SH and AnC7SH monolayers on Au(111)/mica, electrochemical desorption studies were conducted as described by Porter et al.38 In this technique, the potential of the working electrode is scanned cathodically in aqueous hydroxide under a nitrogen atmosphere so as to cause thiol desorption from the gold surface. The voltammetric signal associated with the desorption event is integrated to give the corresponding electrochemical charge, which is then divided by the geometric area of the electrode to yield the surface coverage, Γ (mol cm-2). n-Octadecane-1-thiol on Au was used as a control monolayer system to which to compare Γ values for AnC6SH and AnC7SH monolayers on Au. Surface coverage values and desorption potentials (five trials each) were obtained for AnC6SH/Au(111), AnC7SH/Au(111), and n-octadecane-1-thiol/Au(111) (Table 2). These Γ values for the aniline-terminated monolayers are virtually identical to that of the n-octadecane-1-thiol obtained here and those previously reported for long-chain n-alkanethiols on Au(111) (9.3 × 10-10 mol cm-2),38 indicating that the aniline functionality does not affect the packing density of the aniline-terminated alkanethiol on Au(111). However, the desorption potentials for pristine AnC6SH/Au and AnC7SH/Au are slightly lower than that for the wellordered n-octadecanethiol on Au. This information, in (37) Ulman, A. An Introduction to Ultrathin Organic Films From Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (38) Widrig, C. A.; Chung, C.; Porter, A. D. J. Electroanal. Chem. 1991, 310, 335-359.


Figure 3. Cyclic voltammetry of (A) bare gold electrode in 20 µM dimethylaminomethyl ferrocene/1 M H2SO4. Cyclic voltammetry of (B) AnC6SH/Au and (C) AnC7SH/Au in 20 µM dimethylaminomethyl ferrocene/1 M H2SO4. The scan rate was 0.1 V/s in all cases. The electrode area is 0.172 cm2.

conjunction with the previously discussed IR data on crystallinity of the aniline monolayers, allows us to conclude that the alkane chains are in a disordered environment. Blocking Capacity of Aniline Monolayers on Au to Solution-Phase Redox Probes. The blocking ability of pristine AnC6SH/Au and AnC7SH/Au to a solutionphase redox probe was evaluated (Figure 3). Cyclic voltammetry of the monolayers on Au(111) between 0.0 and +0.5 V versus SSCE provided for a featureless regime to examine the blocking properties of the monolayers. Dimethylaminomethylferrocene in 1 M H2SO4 at a bare Au electrode was found to produce a reversible redox wave centered at +0.35 V (Figure 3A) and was used for these blocking experiments. From Figure 3B,C, it is clear that electrochemical communication between the Au surface and the solution-phase redox probe has been diminished significantly for Au surfaces covered with a monolayer of AnC6SH, but this is much less so with AnC7SH. Greater than 99% blocking is achieved with AnC6SH, as judged by comparison of the current at +0.35 V for AnC6SH/Au versus bare Au. However, the ratio of the peak currents for the redox probe response at AnC7SH/Au and bare Au indicates that the aniline monolayer with seven methylene units in the tether yields only a 29% decrease in the response of the redox probe. This difference in blocking for the two different chain length aniline monolayers is in agreement with the slightly more positive reductive desorption potential observed for AnC7SH/Au (more facile electrolyte ion penetration) versus that of AnC6SH/Au. These differences in blocking ability are most likely due to the somewhat larger tilt of the aromatic ring for AnC7SH/Au (∼30°) versus that for AnC6SH/Au (roughly 20°). The more flat-lying ring for AnC7SH/Au layers may cause the monolayer structure to be somewhat more open than in the AnC6SH/Au case, thus leading to a higher likelihood of probe penetration. Electrochemistry of Aniline-Terminated Monolayers on Au in Aqueous Media. Electrochemical oxidation of AnC6SH/Au and AnC7SH/Au in 1 M H2SO4 was performed using cyclic voltammetry (CV). Upon oxidation, a large irreversible peak was observed at +0.7 V for both monolayer systems in the initial oxidative scan (Figure 4); this peak is attributed to aniline radical cation formation in the monolayer films. The current was found

Langmuir, Vol. 17, No. 6, 2001 1987

Figure 4. Cyclic voltammogram during the initial scan for (A) AnC6SH/Au and (B) AnC7SH/Au in 1 M H2SO4. The scan rate was 0.1 V/s in both cases. The electrode area is 0.172 cm2.

Figure 5. Cyclic voltammograms during scans 2-10 for (A) AnC6SH/Au and (B) AnC7SH/Au in 1 M H2SO4. The scan rate was 0.1 V/s in both cases. The electrode area is 0.172 cm2.

to vary linearly with scan rate (between 0.05 and 0.50 V s-1), indicating that the redox event is associated with a surface-confined species. Subsequent cycling of the potential between 0.0 and +0.9 V reveals a series of complex redox waves for both the oxidized AnC6SH/Au (ox-AnC6SH/Au) and AnC7SH/Au (ox-AnC7SH/Au), but the resulting voltammograms are substantially different for the two chain lengths, as seen in Figure 5. We hypothesize that the larger tilt of the aniline ring for AnC7SH/Au versus AnC6SH/Au leads to the differences in the observed voltammetric responses, but one must be cautious because of the fact that the RAIRS data were obtained for the monolayers when exposed to nitrogen (IR chamber), not the acidic aqueous solvent used for the electrochemical experiments. Four sets of redox waves are present in the ox-AnC6SH/ Au voltammograms. The first set of waves observed for ox-AnC6SH/Au occurs at +0.20 V, and the magnitude is initially small but was observed to increase with increased cycling time. The very small potential difference between Ep,a and Ep,c for the redox event at +0.20 V suggests that the redox couple is the result of a surface-confined wave.

1988


Such an observation is also noted for the remaining waves in voltammograms of ox-AnC6SH/Au and all of those in voltammograms of ox-AnC7SH/Au. The second set of waves for ox-AnC6SH/Au is centered at roughly +0.38 V, and the magnitude of these waves increases after several potential cycles, suggesting an electrochemical-chemical (EC) reaction is occurring wherein more of this species is being produced with further potential cycling. The third redox process observed in the ox-AnC6SH/Au voltammograms occurs at +0.47 V. These waves are seen to decrease in intensity as the potential is cycled, which indicates that this species is being consumed. A fourth set of waves is present in the ox-AnC6SH/Au voltammogram at +0.6 V; the intensity of these waves grows as the potential is cycled beyond 10 cycles. Finally, there is no observable wave at +0.7 V that can be ascribed to monomer oxidation in the film during the repeated potential cycling of the oxidized aniline monolayer. All redox waves present in ox-AnC6SH/Au voltammograms are present in those of ox-AnC7SH/Au, but the intensities of the waves for ox-AnC7SH/Au are much different than those of ox-AnC6SH/Au (Figure 5B). The first set of redox waves is centered at +0.20 V, and the current for the anodic and cathodic components is roughly 50% of that observed for ox-AnC6SH/Au. The second set of redox waves for ox-AnC7SH/Au occurs at +0.37 V and is much different in form than the corresponding set of waves in the ox-AnC6SH/Au voltammograms. This set of waves is readily apparent after the initial voltammetric scan (initial magnitude is much larger), and the magnitude does not increase as dramatically with scan number as for the same set of waves in the voltammograms of oxAnC6SH/Au. The third set of waves at +0.45 V for oxAnC7SH/Au is much smaller in peak intensity than in the case of ox-AnC6SH/Au (roughly one-third as large). The fourth set of redox waves present in the voltammetry of ox-AnC7SH/Au at +0.59 V is also very different than the corresponding waves at +0.60 V for ox-AnC6SH/Au; the intensity of the waves at +0.59 V is 50% less and is not observed to grow significantly upon additional potential scanning. Electrochemistry of Poly(o-phenetidine). To aid in the characterization of the ox-AnC6SH/Au and oxAnC7SH/Au voltammetry, o-phenetidine was chosen as a model compound for voltammetric studies. The structure of o-phenetidine is similar to AnC6SH and AnC7SH that we have synthesized, and its electrochemistry has been well documented in the literature.6,39 Polymerization of o-phenetidine in 1 M H2SO4 using a gold electrode by scanning the potential between 0.0 and +0.9 V was performed in order to form poly(o-phenetidine) films. Once the polymer film was deposited on the Au surface, the coated electrode was rinsed with water, and its voltammetry in 1 M HCl was obtained (Figure 6). Three sets of waves are observed in the voltammogram for the polymer film. The first and third sets occurring at +0.23 and +0.65 V are assigned to the leucoemeraldine/emeraldine and the emeraldine/pernigraniline transformations of the polymer backbone, respectively. These assignments are based on findings of Park and co-workers for poly(aniline) films and studies by D’Aprano and co-workers for polymerized substituted anilines.40,41 The second set of waves in the voltammogram at +0.40 V is due to the formation (39) Mattoso, L. H. C.; Mello, S. V.; Riul, J. A.; Oliveira, O. N.; Faria, R. M. Thin Solid Films 1994, 244, 714-717. (40) Stilwell, D. E.; Park, S.-M. J. Electrochem. Soc. 1988, 135, 24912496. (41) D’Aprano, G.; Leclerc, M.; Zotti, G.; Schiavon, G. Chem. Mater. 1995, 7, 33-42.


Figure 6. Cyclic voltammetry of poly(o-phenetidine) film on a bare Au electrode in 1 M HCl at a scan rate of 0.1 V/s. The electrode area is 16.1 cm2.

Figure 7. Oxidation pathways of anilines in aqueous media.

of “degradation products” (dimers and hydrolyzed dimers) in the polymer film.40,41 The oxidation pathway of aniline polymerization in acidic electrolyte solutions has been delineated, and it was shown that para-aminodiphenylamine (PADA) is initially produced (Figure 7).40,42 Oxidation of this intermediate leads to a para-quinonediimine species (QDI) that undergoes hydrolysis to form a quinonemonoimine (QMI). The quinonemonoimine and the quinonediimine have very similar redox potentials in aqueous acidic media; therefore, after several potential scans enough degradation product is formed in the polymer film so as to give rise to a single set of redox waves that (42) Yang, H.; Bard, A. J. J. Electroanal. Chem. 1992, 339, 423-449.


Figure 8. Cyclic voltammetry of 2-methoxy-N4-phenyl-1,4phenylenediamine in 1 M H2SO4 at a scan rate of 0.1 V/s. The electrode area is 0.0079 cm2.

is actually a combination of two sets. The oxidation pathways of N-alkylanilines have also been studied by Hand and co-workers, and similar degradation products were observed.43 Assignment of Features in Voltammograms of oxAnC6SH/Au and ox-AnC7SH/Au. On the basis of the previously discussed voltammetric behavior of the various anilines, some conclusions can be made concerning the identity of the species that give rise to the voltammetry of ox-AnC6SH/Au and ox-AnC7SH/Au. The first set of redox waves observed at +0.20 V for the oxidized films is attributed to the leucoemeraldine/emeraldine transformation of poly(aniline) in ox-AnC6SH/Au and ox-AnC7SH/ Au. The third set of redox waves at roughly +0.45 V is identified as that due to the dimer (PADA/QDI couple); this assignment will be discussed in further detail later. The second set at +0.38 V is assigned as that associated with the quinonemonoimine that is formed upon oxidation of AnC6SH/Au and AnC7SH/Au in aqueous acidic media. The ca. +0.6 V redox wave observed in voltammograms of ox-AnC6SH/Au and ox-AnC7SH/Au is attributed to the emeraldine/pernigraniline transformation of poly(aniline) in the oxidized aniline monolayers. Our rationale for identifying the redox transformations at +0.20 and ca. +0.6 V for ox-AnC6SH/Au and oxAnC7SH/Au as the emeraldine/leucoemeraldine and pernigraniline/emeraldine couples, respectively, and that at ca. +0.45 V as the dimer couple is based on previous literature data39,44 and electrochemistry of a dimeric aniline compound presented here. To investigate the possibility of dimer presence in ox-AnC6SH/Au and oxAnC7SH/Au, the electrochemistry of 2-methoxy-N4-phenyl-1,4-phenylenediamine was investigated; see Figure 8. This molecule produced a set of redox waves centered at +0.42 V when inspected in aqueous 1 M H2SO4, thus supporting the assignment of dimer formation in oxAnC6SH/Au and ox-AnC7SH/Au. In addition, the voltammogram of 2-methoxy-N4-phenyl-1,4-phenylenediamine exhibits a new set of waves at +0.34 V on the second potential scan; the product that gives rise to the new waves at +0.34 V is known to be the quinonemonoimine.28 Finally, aniline dimer formation has previously been reported in the oxidation of p-aminothiophenol monolayers (43) Hand, R. L.; Nelson, R. F. J. Am. Chem. Soc. 1974, 96, 850-859. (44) Stilwell, D. E.; Park, S. M. J. Electrochem. Soc. 1988, 135, 22542262.

Langmuir, Vol. 17, No. 6, 2001 1989

on Au.28 Radical-radical coupling of the surface-confined aniline group was found to produce a quinonediimine species; this quinonediimine gave rise to a set of surfaceconfined redox waves centered at ca. +0.5 V, and further cycling of the electrode potential yielded the hydrolysis product (the quinonemonoimine) as indicated by the presence of a redox couple at a slightly lower potential. It would appear that dimer formation is the preferred pathway for coupling of the radical cation monomers during oxidation of AnC6SH/Au and AnC7SH/Au, and further oxidation of the dimers leads to formation of polymer and hydrolyzed dimer. Comparison of the magnitude of the current for the two polymer waves (+0.20 and ca. +0.60 V) for ox-AnC6SH/Au and ox-AnC7SH/Au supports the proposal that there is a larger amount of polymer in ox-AnC6SH/Au than in ox-AnC7SH/Au. In oxAnC7SH/Au, the amount of dimer initially present (after the first potential scan) is much less and decreases more quickly with successive scans than in ox-AnC6SH/Au. In addition, further scanning of the potential leads to production of increased amounts of polymer, as evidenced by an increase in the current of the +0.2 and +0.6 V transformations, whereas the magnitude of the current for these waves in ox-AnC6SH/Au is much larger after the initial potential scan. Evidently, in aqueous acid solution the orientation of the aniline group of AnC6SH/ Au is such that a significant amount of polymer is formed, whereas the ring is oriented such that hydrolyzed dimer production occurs more readily for AnC7SH/Au. If the aniline rings retain their orientation when transferred from the nitrogen ambient (IR chamber) to the electrolyte solution, then the RAIRS data support this hypothesis. Although an alternative explanation for the four sets of redox waves observed in the voltammograms of oxAnC6SH/Au and ox-AnC7SH/Au could be proposed, the voltammetric data presented here and that in the paper immediately following this one45 support our dimer/ polymer argument. It could be proposed that the first set of redox waves observed at ca. +0.20 V is possibly that associated with a surface-confined 2-alkoxy-1,4-benzoquinone (resulting from a two-step hydrolysis of surfaceconfined aniline dimer) and not the leucoemeraldine/ emeraldine transition of a poly(aniline), and the set of waves at ca. +0.60 V is possibly associated with a surfaceconfined benzidine (tail-to-tail aniline dimer) and not the emeraldine/pernigraniline transformation of a poly(aniline). We note that the waves at ca. +0.60 V and those at roughly +0.20 V increase in magnitude, whereas the magnitude of the waves of the aminodiphenylamine decreases when the potential of ox-AnC6SH/Au and oxAnC7SH/Au is scanned multiple times between 0.0 and +0.90 V. From Figure 7, we see that hydrolysis of the head-to-tail aniline dimer (ADA) leads to the quinonemonoimine (hydrolyzed dimer) which could possibly decompose further to yield the original 2-alkoxyaniline monomer and a 2-alkoxy-1,4-benzoquinone. To form the surface-confined benzidine (tail-to-tail coupled product), the 2-alkoxyaniline produced from the two-step hydrolysis reaction would have to be oxidized. From voltammetric measurements in our laboratory, the potential of the 2-methoxy-1,4-hydroquinone/2-methoxy-1,4-benzoquinone couple in 1 M sulfuric acid is +0.36 V versus SSCE, or 0.16 V more positive than the set of waves attributed to the leucoemeraldine/emeraldine transition of poly(aniline) in ox-AnC6SH/Au and ox-AnC7SH/Au. We have also investigated the voltammetry of 3,3′-dimethoxybenzidine (45) Schomburg, K. C.; McCarley, R. L. Langmuir 2001, 17, 19931998.

1990


in 1 M sulfuric acid and found it to exhibit a couple at +0.59 V versus SSCE, which is very close to that set of waves tentatively identified as those associated with the emeraldine/pernigraniline transformation of poly(aniline) in ox-AnC6SH/Au and ox-AnC7SH/Au (ca. +0.60 V). In addition, the electrochemistry of 2-methoxy-N4-phenyl1,4-phenylenediamine (our ADA model compound) in sulfuric acid exhibits a set of waves at +0.42 V and another set due to its hydrolysis product28 at +0.34 V (Figure 8); the presence of the latter waves at +0.34 V could be rationalized as being due to the presence of benzoquinone resulting from hydrolysis of the ADA dimer (two-step hydrolysis product) instead of the presence of the quinonemonoimine (one-step hydrolysis product), but there is no wave at higher potentials that can be attributed to 2-alkoxyaniline (the other product that would result from the two-step hydrolysis of the ADA) oxidation. Similarly, we observe no wave that can be assigned to the oxidation of 2-alkoxyaniline (ca. +0.7 V) for ox-AnC6SH/Au or oxAnC7SH/Au during potential excursions that cause increases in the magnitude of the waves at +0.20 and ca. +0.60 V and decreases in the magnitude of the waves centered at ca. +0.45 V. Finally, virtually indistinguishable cyclic voltammograms are obtained for both AnC6SH/ Au and AnC7SH/Au when 1 or 6 M sulfuric acid is used as the electrolyte; it is known that the use of very low pH electrolytes during the oxidation of anilines leads to the formation of benzidines.43 On the basis of the voltammetric evidence presented here, we conclude that the features at +0.20 and ca. +0.60 V in the i-E plots of ox-AnC6SH/Au and ox-AnC7SH/Au are due to redox reactions associated with the leucoemeraldine/emeraldine and emeraldine/ pernigraniline transitions of surface-confined poly(aniline), and those at ca. +0.45 and ca. +0.38 V are due to redox transitions of the immobilized head-to-tail aniline dimer and its hydrolysis product, the quinonemonoimine, respectively. RAIRS of Aniline Monolayers upon Electrochemical Oxidation. To elucidate the structure of the oxidized aniline monolayer films, RAIRS was used to obtain the spectra of ox-AnC6SH/Au and ox-AnC7SH/Au at different electrochemical potentials. AnC6SH/Au and AnC7SH/Au were first oxidized in 1 M H2SO4 by scanning the potential between 0.0 and +0.9 V 10 times, removed from the sulfuric acid solution, rinsed with water, then immersed in 1 M HCl in order to prevent H2SO4 film formation. HCl is a volatile inorganic acid; thus, we are able to completely remove excess acid from the surface of the electrode (acid films form when H2SO4 is used, and their presence prevents RAIRS analysis). The potential of the electrode was then cycled between 0.0 and +0.65 V; chloride ions adsorb on gold and become oxidized at potentials just past +0.65 V, thus limiting the potential range that may be used. After five potential cycles, the electrode potential was returned to and held at 0.0 V, and the sample was removed, rinsed with 1 M HCl, dried with N2, and then analyzed by RAIRS to obtain a spectrum of the oxidized film at 0.0 V. Once the oxidized film was analyzed by RAIRS, the electrode was reimmersed in 1 M HCl and the potential was cycled to +0.65 V; the sample was again removed, rinsed, dried, and then analyzed to obtain a RAIR spectrum of the oxidized film at +0.65 V. The same process was repeated again, except the emersion potential was 0.0 V. It was found that the spectra at 0.0 V prior to and after the spectra obtained at +0.65 V were virtually identical, indicating that hysteresis (“memory”) effects are not a concern. Thus, for brevity and clarity we show only the initial 0.0 V spectrum and the +0.65 V spectrum.


Figure 9. RAIR spectra of pristine AnC6SH/Au and oxAnC6SH/Au obtained at emersion potentials of 0.0 and +0.65 V vs SSCE.

Figure 10. RAIR spectra of pristine AnC7SH/Au and oxAnC7SH/Au obtained at emersion potentials of 0.0 and +0.65 V vs SSCE.

We begin our discussion by comparing the ex situ spectra of the oxidized monolayers removed at 0.0 V versus SSCE to those of the pristine monolayers; see Figures 9 and 10. Except for slight differences in the band intensities, the spectra for the two different chain length aniline thiol monolayers are almost indistinguishable; thus, the discussion at hand for AnC6SH/Au applies to AnC7SH/Au. Upon a comparison of the spectrum of the aniline monolayer which has been oxidized and then removed at 0 V, referred to from here on as the 0 V spectrum, to that of pristine AnC6SH/Au, several significant differences come to light (Figure 9). The intensity of the methylene stretching bands in the ∼3000-2800 cm-1 range has increased by roughly 2-fold, indicating that the methylene chains have adopted a structure wherein they are tilted more from the surface normal (transition dipole moments are perpendicular to the alkane chain axis). Slight decreases in the band intensities for the CdC stretch at ∼1600 cm-1 and the N-H deformation at roughly 1610 cm-1 are also noted (∼20%), which may be the result of a different ring orientation and/or effects arising from a new ring substituent. The magnitude of the CdC transition at approximately 1510 cm-1 is found to be ∼50% less for the 0 V spectrum when compared to the pristine spectrum. The intensities of the aromatic ether stretch


and the C-N transition at roughly 1275 and 1225 cm-1, respectively, have decreased by about 50-60%, and these bands seem to have moved closer to each other to yield a broad single band at roughly 1250 cm-1. Once again, these decreases are due to either ring reorientation and/or new ring substituents. In addition, new bands are present in the 0 V spectrum at 1117, ∼1060, and 1015 cm-1. Finally, another new, somewhat broad band is found in the 0 V spectrum at roughly 825-840 cm-1, and the magnitude of the out-of-plane deformation transition originally present at 740 cm-1 has decreased by about 50%. All of these new bands in the spectrum of the oxidized aniline monolayer removed at 0 V support the presence of 1,2,4substituted aniline rings in the oxidized aniline monolayers,46 which in turn points to the fact that oxidation of the 1,2-substituted aniline rings (pristine monolayer) results in products that have rings which are substituted either with a new, small functional group (amine or hydroxyl) or with a substituent that is more substantial, such as an aniline ring. This latter possibility is supported by the fact that the voltammetry of the oxidized monolayers is characteristic of aniline dimers and polymers and not single aromatic rings with three simple substituents (amine or hydroxyl groups). In addition, the new band in the 0 V spectrum of ox-AnC6SH/Au and oxAnC7SH/Au centered at roughly 3150-3170 cm-1 is not due to the O-H stretch of a 2-alkoxyhydroquinone, for such a transition is known to occur in 2-methoxyhydroquinone at ∼3360 cm-1 (hydrogen bonding effects would not lead to a ∼200 cm-1 shift); this new band is most likely the weak band found in the isotropic spectrum of AnC6SH near 3160 cm-1 or an overtone/combination band of the CdC stretches. We are aware that if hydroquinones were present in the oxidized layers it is possible that the rings could be oriented such that the O-H stretch is not observed here because of the IR surface selection rules. The same could be true for hydroquinonemonoimines. Thus, the infrared spectra of both ox-AnC6SH/Au and ox-AnC7SH/Au at 0 V (aromatic species in their reduced states, i.e., leucoemeraldine poly(aniline) and the benzoidal state of aniline dimers/hydrolyzed dimers) support the presence of oligo(anilines) connected through the 1 and 4 positions of the aniline rings. Because of the similarity in the spectra of aniline oligomers/polymers47 and the fact that the IR end-group method for determining the lengths of poly(anilines) is based on a band intensity ratio protocol that would be affected by the IR surface selection rules,48 we are currently unable to ascertain with RAIRS the average length of oligo(anilines) present in ox-AnC6SH/Au and ox-AnC7SH/Au. We now turn our attention to the ex situ RAIR spectra of ox-AnC6SH/Au and ox-AnC7SH/Au in their fully oxidized state (emersion potential of +0.65 V vs SSCE, polymer/oligomer is in the pernigraniline form). Although attempts were made to obtain the ex situ spectra of oxAnC6SH/Au and ox-AnC7SH/Au at potentials that should give rise to the emeraldine form of the polymer/oligomer (+0.45 V), the spectra were irreproducible in nature; the spectra sometimes resembled the 0.0 V spectra and at other times resembled the +0.65 V spectra, possibly pointing to some sort of disproportionation reactions in the oxidized monolayers. Thus, the discussion at hand targets what species are present in ox-AnC6SH/Au and ox-AnC7SH/Au in their fully oxidized state. (46) Ping, Z.; Nauer, G. E.; Neugebauer, H.; Theiner, J.; Neckel, A. Electrochim. Acta 1997, 42, 1693-1700. (47) Shacklette, L. W.; Wolf, J. F.; Gould, S.; Baughman, R. H. J. Chem. Phys. 1988, 88, 3955-3961. (48) Honzl, J.; Tlustakova, M. J. Polym. Sci., Part C 1968, 451-462.

Langmuir, Vol. 17, No. 6, 2001 1991

The most significant differences in the +0.65 and 0 V spectra for both ox-AnC6SH/Au and ox-AnC7SH/Au occur between ∼1700 and 600 cm-1. For example, there is no observable change in either the peak frequency or peak intensity for the methylene stretches in the ∼3000-2800 cm-1 region when one moves from the 0 V spectrum to the +0.65 V spectrum, indicating that the structure of the alkane chains is not substantially different. However, what is very different in the spectra are the intensities of bands associated with the benzenoidal form of the aromatic rings and the presence of new bands that are the result of transitions for aromatic rings that are in the quinodal form. Taking ox-AnC6SH/Au as an example (Figure 9), the 0 V spectrum has two bands at roughly 1600 and 1510 cm-1 that have been previously assigned to the benzoid vibration for the fully reduced poly(aniline) structure, denoted as b1 and b2, respectively.46,49 Upon oxidation to the fully oxidized (pernigraniline) state of poly(aniline), the intensity of these bands has been shown to decrease significantly, and two new bands appear in this region at roughly 1590 cm-1 (q1) and 1505 cm-1 (q2) that have been assigned to the aromatic stretch of the quinoid species present in the polymer backbone in the fully oxidized, pernigraniline form of poly(aniline).50,51 The changes observed in the spectra of poly(aniline) are also found in the spectra of both ox-AnC6SH/Au and ox-AnC7SH/Au when moving from the 0 V to the +0.65 V spectra. In addition, another band at ∼845-860 cm-1 is observed in the +0.65 V spectra of both types of aniline monolayers and is consistent with that band due to the out-of-plane deformation of the quinoid ring in the pernigraniline form of poly(aniline);51 the out-of-plane deformation of the quinoid ring for 1,4-benzoquinone and 2-methyl-1,4benzoquinone is found at 880-890 cm-1. Finally, another band that presents support for the presence of poly(aniline) in the oxidized monolayers is one at ∼1623 cm-1; this band has been reported by Ping as a -CdC-CdNasymmetric vibration (imine mode) in highly oxidized polyaniline films.46 The infrared data presented above support the presence of poly(aniline) in the ox-AnC6SH/Au and ox-AnC7SH/ Au, but some of the spectral bands for the quinoid and benzenoid forms of other aromatic species (aminodiphenylamine/benzoquinonediimine dimer or its hydrolysis product, hydroquinonemonoamine/benzoquinonemonoimine or 2-alkoxyhydroquinone/2-alkoxybenzoquinone) may overlap with those of the poly(aniline) present in ox-AnC6SH/Au and ox-AnC7SH/Au. However, there is one notable band in the spectra that can be used to identify the dimeric degradation species. In the spectra presented for ox-AnC6SH/Au and ox-AnC7SH/Au, a small band at ∼1660 cm-1 appears in the +0.65 V spectra. This band does not increase significantly upon further potential scanning of ox-AnC6SH/Au and ox-AnC7SH/Au in aqueous acid. Studies of benzoquinone monolayers attached to Au substrates have been conducted and indicate that the carbonyl band of benzoquinone is at 1660 cm-1 and the transition is intense.52 Thus, a small quantity of carbonylcontaining material is present in ox-AnC6SH/Au and oxAnC7SH/Au; however, the carbonyl is most likely due to the presence of quinonemonoimines. If significant amounts of 2-alkoxybenzoquinone were produced, the reduced form (49) Quillard, S.; Louarn, G.; Lefrant, S.; MacDiarmid, A. G. Phys. Rev. B 1994, 50, 496-508. (50) Harada, I.; Furukawa, Y.; Ueda, F. Synth. Met. 1989, 29, E303E312. (51) Quillard, S.; Louarn, G.; Buisson, J. P.; Lefrant, S.; Masters, J.; MacDiarmid, A. G. Synth. Met. 1992, 49-50, 525-530. (52) Bae, I. T.; Sandifer, M.; Lee, Y. W.; Tryk, D. A.; Sukenik, C. N.; Scherson, D. A. Anal. Chem. 1995, 67, 4508-4513.

1992


of the quinone (hydroquinone) would produce a large O-H stretch at ∼3360 cm-1. This is not the case (vide supra). Of course, these conclusions are based on the assumption that if the quinone species were present the rings would not be perfectly parallel to the Au substrate. On the basis of the infrared data presented here, quinonemonoimines are the most likely degradation products produced when the aniline-terminated monolayers are electrochemically oxidized in aqueous media. To our knowledge, the literature is devoid of any infrared spectra of benzoquinonemonoimines that would allow us to readily identify surface-confined quinonemonoimines. If such species were present, we predict that a carbonyl band at ∼1660 cm-1 and an imine band at ∼1620-1640 cm-1 would be noted in the RAIR spectra. We have already noted a band at roughly 1660 cm-1 that would agree with the quinone functionality of the quinonemonoimine. In addition, a band occurring at 1622 cm-1 is observed in the 0 V spectra for ox-AnC7SH/Au ox-AnC6SH/Au and has been previously assigned as a -CdC-CdN- asymmetric vibration in highly oxidized polyaniline films.46 Thus, this band at 1622 cm-1 could be due to the imine stretch of the oxidized aniline dimer (aminodiphenylamine), the pernigraniline form of poly(aniline), or the imine moiety of quinonemonoimines possibly present in the oxidized aniline monolayers. In the paper immediately following this one, we demonstrate that it is possible to form surfaceconfined aniline dimers without any quinone-containing products being present if the aniline monolayers are electrochemically oxidized in nonaqueous electrolyte. In addition, we show that the aqueous voltammetry of such nonaqueous ozidized aniline monolayers exhibits waves that can be attributed not to quinoneimines but rather to aminodiphenylamines. Other Characteristics of ox-AnC6SH/Au and oxAnC7SH/Au. To test the environmental stability of the oxidized films, ox-AnC6SH/Au and ox-AnC7SH/Au were allowed to remain under ambient laboratory conditions for various amounts of time. Cyclic voltammetry and RAIRS were performed on the oxidized films every 4 h to monitor changes. For the longest period of time investigated, 36 h, no changes in the voltammetry or RAIR spectra were observed for the oxidized monolayers. Oxidized aniline monolayers were also subjected to electrochemical reductive desorption, as performed previously with the pristine monolayer films, to ensure that electrochemical oxidation of the pristine monolayer film does not desorb the thiol molecules from the Au surface. Surface coverages for ox-AnC6SH/Au and ox-AnC7SH/ Au and their respective desorption potentials are provided in Table 2. These values are within the standard deviation of values for pristine AnC6SH/Au and AnC7SH/Au and demonstrate that desorption of the molecules from the Au surface does not occur to any appreciable extent during oxidation of the aniline monolayers in aqueous media.


Summary AnC6SHAu and AnC7SH/Au have been analyzed by cyclic voltammetry and RAIR spectroscopy. The orientation of the aniline groups in the pristine form of both chain length monolayer films was found to be measurably different (tilt of ∼20° versus ∼30° for the C6 and C7 layers, respectively), and the order of the alkane tether was found to be disordered (liquidlike environment) in both cases. Electrochemical reductive desorption experiments yielded equivalent surface coverages for AnC6SH/Au and AnC7SH/ Au; these values are in accord with that of well-ordered long-chain alkanethiols on Au. It was determined that AnC6SH monolayers on Au give rise to a highly efficient barrier to electron transport, but AnC7SH films were significantly poorer barriers. Thus, it is concluded that the aniline group controls the spatial orientation of the tail groups in the monolayer (when the monolayers are exposed to nitrogen environments) but not the packing density of the monolayer. Electrochemical oxidation of the aniline monolayer films in 1 M H2SO4 resulted in the formation of surface-confined poly(aniline), aniline dimers, and hydrolyzed aniline dimer species, but the relative amount of each species is a function of the chain length used in the monolayer. This difference in behavior is attributed to the difference in aniline ring orientation in the two monolayers. Voltammetry of the oxidized monolayers displays waves due to the transformation of the leucoemeraldine (fully reduced polymer) to the emeraldine state at +0.20 V versus SSCE and the emeraldine to the pernigraniline (fully oxidized polymer) state at ca. +0.6 V versus SSCE. Aniline dimer redox activity was found to occur at roughly +0.45 V versus SSCE, and dimeric degradation products of quinonemonoimine showed redox activity near +0.38 V versus SSCE. RAIR spectra of the fully reduced and fully oxidized films contain bands which support the presence of species associated with the reduced and oxidized polymer states as well as those of the dimer and hydrolyzed dimer products. Finally, it would appear that these aniline-terminated monolayer systems may be suitable candidates for the fabrication and study of nanometer-scale polymer chains on surfaces. We base this conclusion on the observation that both the pristine monolayers and the polymeric monolayers are stable in air for a minimum of 36 h. Acknowledgment. We gratefully acknowledge financial support for this work from the National Science Foundation (CHE-9529770) and the Louisiana Education Quality Support Fund. LA001021+

Surface-Confined Monomers on Electrode Surfaces. 10

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