Thiol-Modified Pyrrole Monomers: 3. Electrochemistry of 1-(2-Thioethyl

The electrochemical oxidation of 1-(2-thioethyl)pyrrole (1-TEP) and ... The Journal of Physical Chemistry B 0 (proofing), ... Langmuir 2001 17 (6), 19...
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Langmuir 1998, 14, 2984-2995

Thiol-Modified Pyrrole Monomers: 3. Electrochemistry of 1-(2-Thioethyl)pyrrole and 3-(2-Thioethyl)pyrrole Monolayers in Propylene Carbonate Elisabeth Smela,* Hans Kariis, Zhongping Yang, Michael Mecklenburg, and Bo Liedberg Laboratory of Applied Physics, University of Linko¨ ping, S-581 83 Linko¨ ping, Sweden Received August 4, 1997. In Final Form: March 16, 1998 The electrochemical oxidation of 1-(2-thioethyl)pyrrole (1-TEP) and 3-(2-thioethyl)pyrrole (3-TEP) monolayers on gold was studied. Their cyclic voltammograms were different, with, for example, 1-TEP having three oxidation peaks and 3-TEP just one. Moreover, the cyclic voltammetry of both TEP monolayers was unusual and did not follow the expected behavior for simple electron transfer from an adsorbed monolayer. The chronoamperometry for 1-TEP was also surprising, showing nucleation-like peaks; this was not seen for 3-TEP. Air exposure left the 1-TEP, but not the 3-TEP, surface electrochemically inert. Spectroscopic data on the oxidation products of surface-bound pyrroles are presented for the first time. IRAS, XPS, and UV-vis-NIR showed that, after electrochemical cycling, all the pyrrole rings were absent. This demonstration that the molecules decompose calls into question the generality of the grafting mechanism of adhesion promotion for this type of compound.

1. Introduction This is part 3 of our report on (thioethyl)pyrrole (TEP) monolayers, and it should be read in conjunction with the other parts. The conjugated polymer polypyrrole (PPy) risks delamination from Au when it undergoes volume change during electrochemical cycling,1 which results in large shear stresses at the PPy/Au interface. The TEP monolayers were designed to improve the adherence of PPy films to gold surfaces, serving as a molecular glue by attaching to gold through the thiol and covalently binding to pyrrole in solution during electrochemical growth of PPy. A discussion of this approach, as well as the synthesis, characterization, and electropolymerization of the monomers, can be found in paper 1 of this series. In paper 2, a number of analytical techniques was used to examine freshly deposited monolayers of 1-(2-thioethyl)pyrrole (1-TEP) and 3-(2-thioethyl)pyrrole (3-TEP) adsorbed on gold, including infrared, UV-vis-NIR, mass spectroscopy, ellipsometry, wetting, and X-ray photoelectron spectroscopy. For both PPy deposition and device performance, it is crucial to know how the TEP monolayers behave electrochemically. It is of interest to know whether the surface-bound monomers polymerize with each other2-5 and at what voltage, whether the monolayer is desorbed in the potential window within which PPy is cycled,6-12 * Address for correspondence: Condensed Matter Chemistry and Physics Department, Risø National Laboratory, FYS-124 P.O. Box 49, DK-4000 Roskilde, Denmark. (1) Smela, E.; Ingana¨s, O.; Lundstro¨m, I. Science 1995, 268, 1735. (2) Willicut, R. J.; McCarley, R. L. J. Am. Chem. Soc. 1994, 116, 10823. (3) Willicut, R. J.; McCarley, R. L. Anal. Chim. Acta 1995, 307, 269. (4) Willicut, R. J.; McCarley, R. L. Adv. Mater. 1995, 7, 759. (5) Willicut, R. J.; McCarley, R. L. Langmuir 1995, 11, 296. (6) Collard, D. M.; Sayre, C. N. J. Electroanal. Chem. 1994, 375, 367. (7) Garrell, R. L.; Chadwick, J. E.; Severance, D. L.; McDonald, N. A.; Myles, D. C. J. Am. Chem. Soc. 1995, 117, 11563. (8) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 1252. (9) Everett, W. R.; Welch, T. L.; Reed, L.; Fritsch-Faules, I. Anal. Chem. 1995, 67, 292. (10) Everett, W. R.; Fritsch-Faules, I. Anal. Chim. Acta 1995, 307, 253.

and if the reactive 2 and 5 (R) sites on the pyrrole moiety are oxidized during electrochemical cycling.6,13,14 If, for example, the surface-confined monomers polymerize with each other at a lower potential than that at which PPy begins to polymerize from pyrrole in solution, or if they have reacted and become inert,15 then the surface-confined monomers cannot be incorporated into the overlying PPy film, and the thiol treatment will fail as an adhesionpromoting layer. Similarly, if the reactive positions on the pyrrole ring of TEP are already occupied by oxygen, as was shown to be the case for air-exposed monolayers in paper 2, then pyrrole cannot bind to the monolayer. Finally, should the monolayer be subject to oxidative attack or desorption in the voltage range used for electrochemical cycling of an overlying PPy film, it will degrade as an adhesion layer. A number of similar compounds have been presented in the literature (for a more complete discussion, refer to paper 1), and these did result in improved adhesion of conjugated polymers to the substrate4-6,13,15-18 or to other improvements in the films. This was attributed to the successful grafting of the overlying film to the surfacebound aromatic moiety or enhanced nucleation.5,6,13,15-21 During the first anodic potential scan in cyclic voltammetry, monolayer oxidation peaks have been observed.16 For short-chain alkanethiol 1-substituted pyrrole, this was (11) Schneider, T. W.; Buttry, D. A. J. Am. Chem. Soc. 1993, 115, 12391. (12) Miller, C.; Cuendet, P.; Gra¨tzel, M. J. Phys. Chem. 1991, 95, 877. (13) Collard, D. M.; Sayre, C. N. Synth. Met. 1995, 69, 459. (14) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (15) Sayre, C. N.; Collard, D. M. Langmuir 1995, 11, 302. (16) Simon, R. A.; Ricco, A. J.; Wrighton, M. S. J. Am. Chem. Soc. 1982, 104, 2031. (17) Lo, R.-K.; Ritchie, J. E.; Zhou, J.-P.; Zhao, J.; McDevitt, J. T.; Xu, F.; Mirkin, C. A. J. Am. Chem. Soc. 1996, 118, 11295. (18) Wu, C.-G.; Chen, J.-Y. 1997, 9, 399. (19) Mekhalif, Z.; Lang, P.; Garnier, F. J. Electroanal. Chem. 1995, 399, 61. (20) Rubinstein, I.; Rishpon, J.; Sabatani, E.; Redondo, A.; Gottesfeld, S. J. Am. Chem. Soc. 1990, 112, 6135. (21) Rubinstein, I.; Gottesfeld, S.; Sabatani, E. U.S. Patent 5,108,573, 1992.

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Electrochemistry of 1-TEP and 3-TEP Monolayers

attributed to polymerization of the adsorbed monolayer.2-4 Preliminary IR studies showed a change in the local order of the monolayer films, but further IR studies were needed to ascertain whether the changes were orientational or structural.4 For long-chain alkanethiol 3-substituted pyrroles, the irreversible electro-oxidation in monomerfree solution was accompanied by a loss of reactivity and attributed to nucleophilic attack by water after formation of the pyrrole radical cation.6,15 There have been no studies on the chemical nature of the oxidized products. Moreover, no spectroscopic evidence has ever been published that a surface-bound aromatic moiety still exists after anodic oxidation, although the CH2 stretches of the alkyl chain have been seen.6 In view of claims that compounds with an aromatic ring function as a molecular glue by binding with pyrrole in solution, this is a serious shortcoming. In this paper, we investigate the oxidation of the TEP monolayers using not only electrochemical methods, but also for the first time with a variety of spectroscopic methods. The purpose of this study was to further investigate the practicality and limitations of these surface-bound species as adhesion promoters in the electrochemical deposition of PPy on gold. In general, in the anodic oxidation of aromatic compounds the initial electron transfer can be represented by

Ar T Arn+ + newhere Ar is the aromatic species, Arn+ is an electrondeficient species, and n is the number of electrons e-.22 Arn+, usually a cation radical Ar•+, may be stable for a short time if there is a high degree of charge delocalization, the reactive sites are blocked, or it is stabilized by functional groups.22 This is of interest because the polymerization of pyrrole has been shown to proceed via the cation radical. (See, for example, refs 23 or 24). The stability of cation radicals depends on the solvent; propylene carbonate, for example, is a good solvent for their electrogeneration,25 but they are unstable in acetonitrile.22 Therefore, the solvent may affect the interaction of the bound monolayer with pyrrole in solution. This question is taken up in paper 4 of this series. Alternatively, Arn+ can undergo follow-up chemical reactions, particularly if the unpaired electron density is high at a few sites on the molecule, as is the case with the R positions of pyrrole (on either side of the N on the ring).22 A follow-up reaction which is of particular interest to us is one in which dimers are formed. This coupling occurs commonly with aromatic hydrocarbons, and it has been argued that surface-confined pyrroles polymerize with each other.2-4 Other reactions of interest are those with oxygen and the cases in which cleavage of the molecule occurs.26 Finally, there is the possibility of desorption of Arn+.22 This is also significant, since the TEP molecules must remain bound to the surface to be effective. In this third paper in the series, results on the electrochemical behavior of 1-TEP and 3-TEP monolayers will be reported. First, the basic experiment is described, (22) Adams, R. N. Electrochemistry at Solid Electrodes; Marcel Dekker: New York, 1969; pp 37, 152-156, 219, 305-311. (23) Diaz, A. F.; Bargon, J. In Handbook of Conducting Polymers; Skotheim, T. A., Ed.; Marcel Dekker: New York, 1986; Vol. 1, pp 81115. (24) Guyard, L.; Hapiot, P.; Neta, P. J. Phys. Chem. B 1997, 101, 5698-5706. (25) Sawyer, D. T.; J. L. Roberts, J. Experimental Electrochemistry for Chemists; John Wiley & Sons: New York, 1974; pp 208-209, 339341. (26) Weisshaar, D. E.; Walczak, M. M.; Porter, M. D. Langmuir 1993, 9, 323.

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of which the others in this paper are variations: cyclic voltammetry at positive potentials of monolayers in propylene carbonate (PC) with LiClO4 as the supporting electrolyte. The question we try to answer in the remainder of this paper is, what is the origin of the electrochemical peaks? Possibilities include desorption, decomposition, and polymerization of the pyrrole moieties. The results of infrared reflection-absorption spectroscopy (IRAS), UV-vis-NIR spectroscopy, and X-ray photoelectron spectroscopy (XPS) on these cycled monolayers are reported. The dependence of the cyclic voltammograms on scan rate, deposition time, and potential limits are also explored in this paper. Additional results, such as cycling in a dry electrolyte, the effect of aging in air, and replacement experiments, are included in the Supporting Information. In the next paper we shall discuss the growth of polypyrrole over the monolayers. Studies of the monolayers in an ultrahigh vacuum are presented elsewhere.27 Note on terminology: We do not refer to the adsorbed layers as self-assembled monolayers, or SAMs, since they are disorganized, as shown in paper 2. Although the TEPs spontaneously adsorb onto the gold surface, they do not form well-packed “assembled” structures. Therefore, the potentially misleading term is avoided. For brevity, we refer to the adsorbed layer formed when surfaces were exposed to 3-TEP as a 3-TEP monolayer, even though from paper 2 it is known that only ca. 30% of the thiolates are intact 3-TEP molecules. Similarly, the term monolayer is used to describe the adsorbed layer even after it has undergone some oxidation/desorption. 2. Experimental Section The procedures used to deposit the monolayers have been given in paper 2 of this series. The same procedures were followed here and the electrochemical measurements were done immediately afterward unless otherwise noted. Infrared, XPS, and UV-vis-NIR measurements were also performed as described previously. 2.1. Electrochemistry. In our experience, gold films evaporated under nominally the same conditions (with the same precleaning, evaporation rate, chamber pressure, temperature, film thickness, etc.) varied from batch to batch, especially in their surface morphology. Uosaki et al.28 also reported that, despite keeping preparation conditions as constant as possible, their anodic charge data had a large scatter; they attributed this to differences in the surface conditions, such as roughness, of the vacuum-deposited gold. Therefore, all direct comparisons in this study were made using gold surfaces evaporated at the same time. Clean gold surfaces with the same history (same evaporation batch, cleaning method, etc.) as those exposed to the thiols were used as references, without separate pretreatment, so that the response of surfaces identical but for the monolayer could be compared. Because sonication did not affect the cyclic voltammogram of clean gold surfaces, this step was omitted for the reference samples. All electrochemical experiments utilized these controls. The electrochemical work was done using an Autolab pgstat 10 from EcoChemie, computer-controlled by their General Purpose Electrochemical System software. Electrochemistry was performed in single-compartment, three-neck glass electrochemical cells. On each sample a line was lightly scribed at a distance of 1 cm from the end using a wafer scriber. The samples were held by miniature alligator clamps, and the area was controlled by the depth of immersion, which could be visually determined to a precision of ca. 5% by looking at the scribe line. A 1-mm thick gold wire several centimeters long was normally (27) Kariis, H.; Smela, E.; Uvdal, K.; Wirde, M.; Gelius, U.; Liedberg, B. J. Phys. Chem. B, submitted for publication. (28) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510.

2986 Langmuir, Vol. 14, No. 11, 1998 used as the counter electrode. To sustain the small currents encountered, this had a more than adequate surface area. An Ag/AgCl reference electrode (BAS) was usually employed. The Ag/AgCl wire was in a 3 N NaCl solution separated from the electrolyte in the electrochemical cell by a Vycor plug. For organic electrolytes, an Ag/Ag+ reference electrode was also sometimes used, as was a pseudoreference Ag/AgCl wire. For the former, the glass housing contained the same electrolyte that was used in the cell but with 0.1 M AgNO3 added, and it was again capped by a Vycor plug. The pseudoreference wire required caution: although the results were fairly reproducible, its zero voltage was approximately 230 mV above that of the actual Ag/AgCl zero and shifted slightly from day to day. In very dry PC/LiClO4, the shift was 120 mV. (The results obtained using an Ag wire pseudoreference were not reproducible from one day to the next, and consequently it was not employed.) All the measurements presented herein were corrected so that they are referenced to Ag/AgCl (saturated NaCl). The standard conditions that were used for cyclic voltammetry were a PC/LiClO4 (0.1 M) electrolyte, voltage limits between -0.2 or 0.0 and +1.0 V vs Ag/AgCl, and a scan rate of 10 or 20 mV/s. (Propylene carbonate is a common solvent for electrochemistry.25 It is aprotic, has a high dielectric constant and low background currents, is resistant to oxidation, and dissolves a variety of organic and inorganic compounds.) The electrolyte was not changed between samples run on the same day. Water and oxygen were not rigorously excluded, but the exclusion of water did not affect the results (see Supporting Information), and the presence of air has no effect on the electrochemistry of longchain 3-substituted pyrroles.15 The solubility of oxygen in organic solvents is typically on the order of 1 mM.29 2.2. Determination of Surface Roughness. The roughness of the evaporated gold surfaces was determined electrochemically by chronocoulometry, a standard procedure based on the Cottrell equation.30 Full details are provided in the Supporting Information. Deaerated aqueous solutions of 0.1 M KNO3, phosphate buffered to pH 7, were used at three concentrations of Fe(CN)63to obtain a good average value. The electrochemical area was found to be 1.3 times the geometrical area. These values are consistent with those determined by others.8,11,14,28 2.3. Competitive Adsorption. Monolayer affinity for the surface was examined by competitive adsorption. Two samples of each monolayer and two clean gold pieces were immersed halfway in the electrolyte and cycled electrochemically (two times at 100 mV/s between 0 and 1.2 V for 1-TEP and between 0 and 1.4 V for 3-TEP and clean gold). Samples were stored in air and analyzed within 24 h. Full details of the following replacement procedure are provided in the Supporting Information. Between each step, the samples were rinsed and blown dry under nitrogen. The long-chain thiol SH(CH2)15COOH, dissolved in ethanol, was placed on the surfaces and allowed to dry. The samples were exposed to an activation solution (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide ester) and then immediately placed in an amino-biotin solution and incubated. This enabled the amino-biotin to react with the long-chain thiol. The surfaces were placed face down on radio-labeled streptavidin and incubated. The streptavidin bound to the biotin, enabling the long-chain thiols to be imaged. The surfaces were then exposed to a Fuji phosphor-imaging film for 2 h and analyzed using the Fuji phosphor-imaging software. Uncycled gold-coated Si and mica surfaces underwent the same treatment and were used as references for the binding studies.

3. Results 3.1. Cyclic Voltammetry, First Potential Excursion. The first three scans of cyclic voltammograms of 1-TEP and 3-TEP monolayers are shown in Figure 1. For comparison, the cyclic voltammogram of a plain gold surface is included. The Au controls demonstrated that the Si substrate does not contribute to the electrochemical (29) Franco, C.; Olmsted, J. III Talanta 1990, 37, 905-909. (30) Heineman, W. R.; Kissinger, P. T. In Laboratory Techniques in Electroanalytical Chemistry; Kissinger, P. T., Heineman, W. R., Eds.; Marcel Dekker: New York, 1996; p 51.

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(a)

(b)

(c)

Figure 1. Cyclic voltammograms of (a) 1-TEP and (b) 3-TEP monolayers on gold and (c) clean gold in propylene carbonate/ LiClO4 at 10 mV/s. The first cycle is represented by a heavy solid line and the second and third by thinner solid lines. In (a) and (b) the first cycle of the clean gold surface is shown as a dashed line.

behavior and that the gold does not oxidize in this voltage range. Although for clean gold the current was somewhat larger in magnitude during the first cycle, it had the same shape in the second and subsequent cycles. In contrast, the first positive potential excursion of the surface-bound pyrrole monolayers yielded a characteristic curve that was not reproduced in subsequent scans. As discussed above, this has also been reported by a number of other groups.2,13,16 Below approximately +0.4 V versus Ag/AgCl, the current for both TEP monolayers was slightly lower than for plain gold and quite flat, without the small, broad peak at 0 V seen for clean gold. Above 0.4 V the current started to rise, developing into one anodic peak for 3-TEP (+0.56 V) and three anodic peaks for 1-TEP (a shoulder near 0.56 V on a larger peak at approximately 0.60 V and a small but distinct peak around 0.75 V). After the first potential excursion, the cyclic voltammograms strongly resembled those of clean gold, although they were not exactly the same, and underwent no further changes. These curves were reproducible and characteristic of each monomer. They were also irreversible, with no associated reduction peaks. Furthermore, no reduction or oxidation (redox) peaks that might be associated with polymerization of the monolayers were ever observed (the positions of the redox peaks for the TEP polymers, presented in paper 1, were ca. +0.5 and +0.4 V for 1-TEP and +0.35 and 0 V for 3-TEP at 50 mV/s). The cyclic voltammograms were the same for vapor and solution deposition methods irrespective of the solvent used in the deposition bath (ethanol, chloroform, and propylene carbonate (PC)). This is consistent with and supports the results on the freshly deposited monolayers (paper 2), which showed that the deposition method had no effect. The occurrence of three peaks for 1-TEP was unexpected. Willicut et al.2 found only one in PC/Bu4NClO4, a very similar electrolyte, and in ACN/Bu4NClO4 for analogous

Electrochemistry of 1-TEP and 3-TEP Monolayers

Figure 2. IRAS spectra of 1-TEP monolayers. (a) Uncycled and (b)-(d) electrochemically cycled at 200 mV/s in PC/LiClO4 three times to (b) below the peaks and (c) and (d) above the three peaks (two different samples). The positive going peaks (marked by *) are due to contamination of the gold reference.

1-substituted molecules with alkane chains one carbon longer. Simon et al.16 also reported only one peak at +1.3 V versus SCE at 50 mV/s in ACN/[n-Bu4N]BF4 for Pt electrodes treated with (CH2)3Si(OMe)3 1-substituted pyrrole. For other 3-substituted molecules, there was also only a single peak.15 However, three peaks have been observed due to the decomposition of another short-chain thiol, mercaptoethanol.26 The three peaks in the 1-TEP voltammogram could be indicative of three things. First, the oxidation of 1-TEP could be a multistep reaction whose intermediates are stable in PC/LiClO4; 3-TEP, however, does not have such stable intermediates. Second, different reactions between 1-TEP and its surroundings are dominant than those with 3-TEP, because for instance, 3-TEP may already be completely oxidized (see paper 2). Third, multiple peaks have been observed during reductive desorption of selfassembled monolayers (SAMs) and attributed to multiple crystal facets with different energetics of desorption;10 this might also influence the oxidation peak of 1-TEP but not 3-TEP if 3-TEP is adsorbed at preferred sites on the Au (see paper 2). It should also be noted that 1-TEP surfaces were hydrophobic before cycling, but hydrophilic afterward. However, even monolayers that were merely dipped into the electrolyte but not cycled became somewhat more hydrophilic. In the remainder of this paper and the next, measurements to ascertain the origin of these peaks are described. Some of the possibilities include desorption, the formation of oxides with the oxidized TEPs remaining on the surface, the decomposition of the TEP molecules, and polymerization of the pyrrole moieties. 3.1.1. IRAS. After electrochemical cycling, the samples were examined with various techniques to try to determine what had happened to the surface. Typical IRAS spectra for 1-TEP are shown in Figure 2. These samples were deposited from ethanolic solutions. To see the effect of the electrochemical oxidation, samples were cycled at 200 mV/s to just above the three peaks, to 1.2 V. To see the effect of cycling with minimal oxidation, as a comparison, samples were cycled three times to 0.7 V, just to the foot of the peaks. (Note that the peak positions differ from those in Figure 1 because of the difference in scan rates.) To obtain additional control surfaces that were treated

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the same way as the others, but without any electrochemical cycling, 1-TEP surfaces were merely dipped into the PC/LiClO4 electrolyte. After the dipping or cycling, they were rinsed and blown dry under nitrogen and placed in the vacuum chamber (10 Torr) within minutes, as quickly as possible to avoid prolonged contact with air. A clean gold surface was used as a reference. For the uncycled dipped surfaces (Figure 2a), the four largest 1-TEP peaks at 1497, 1280, 1090, and 728 cm-1 were easily visible. (For peak assignments and uncycled monolayer spectra, see papers 1 and 2 of this series.) This shows that the monolayers do not readily desorb in the electrolyte. The peaks were still present in the samples taken just prior to the start of the oxidation (Figure 2b), but were somewhat reduced in size. There was also a new peak at 1725 cm-1, which is near the carbonyl peak seen for the as-deposited 1-TEP monolayer left in air (see paper 2). Thus, surfaces taken to a potential at the foot of the oxidation peaks still had 1-TEP on the surface, but some had already started to oxidize to form a carbonyl. The samples cycled above the electrochemical oxidation potential (Figure 2c,d) were missing the characteristic 1-TEP peaks, instead showing a peak at 1725 cm-1 and an ensemble of peaks in the 1300-1100-cm-1 region. This part of the spectrum resembles that of 3-TEP exposed to air (see paper 2). Recall that those spectral features were of COOH containing species formed as oxidation products of 3-TEP. There are at least three possible reasons for the absence of a 1-TEP monolayer or 1-TEP polymer IRAS spectrum for the cycled monolayers. (If the surface-bound pyrrole moieties had polymerized with each other, we would have observed a spectrum similar to that of the bulk polymer, shown in paper 1). The simplest is that the monolayer desorbed. The second is that the molecules adopted a conformation parallel to the surface (with IRAS, only vibrations with oscillating dipole moments perpendicular to the plane of the substrate are detected). This was the reason that 3-TEP monolayers could not be seen with IRAS (paper 2). The third is that the molecules decomposed. Returning to the list of possibilities for the origin of the electrochemical peaks for 1-TEP, given the carboxylic acid signal, the peaks must be due at least in part to the formation of oxidized products. Whether this is accompanied by some rearrangement or desorption cannot be determined from these data. However, the possibility of polymerization of the surface-bound 1-TEP is not supported. Monolayers of 3-TEP were also cycled as described above. However, there was essentially no change in the IRAS spectrum compared to that of the as-deposited monolayer except for a small increase in the size of the double peak at 1415 and 1440 cm-1. Thus, no information on what happened to the formerly intact 3-TEP molecules was obtained, but it appears that short-chain carboxylic acids are still on the surface. 3.1.2. XPS. Because the IRAS results could not be used to unambiguously identify the oxidation products, or even to ascertain whether any intact 1-TEP and 3-TEP molecules remained on the surface, XPS measurements were performed in order to find which elements were present and what their oxidation states were. Different electrochemical treatments were investigated for both 1-TEP and 3-TEP. The gold surfaces, deposition solutions, scan rate, and other variables were varied. However, the basic results were the same. 1-TEP samples scanned past the three oxidation peaks (to 0.85 V at 10 mV/s) were compared to an uncycled 1-TEP monolayer prepared at the same time. There was a

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decrease in the total amount of sulfur to 60-80% of that for an uncycled sample, but an increase in the disulfideto-thiol ratio (163.2 eV for disulfide vs 162.1 eV for thiolate bound to gold).31 (For a spectrum refer to the Supporting Information; for full details and spectra, see ref 27.) The sulfur:nitrogen ratio (peak at 399.7 eV) decreased from 1:1 for uncycled samples to 1:0.77 to 0.66, for a total loss of nitrogen of 40-60%. Thus, there must have been some fragmentation of the 1-TEP in which pyrrole moieties were lost partially or entirely. The carbon:sulfur ratio, which should have been 6:1, varied because of carbonaceous contamination and could not be used to unambiguously confirm this conclusion. Oxygen was likewise difficult to quantify. Electrochemically induced bond cleavage has been seen before, for instance, in mercaptoethanol.26 As a control, we also examined 1-TEP samples cycled all the way to 2 V, above the potential at which gold is oxidized in PC/LiClO4. They had essentially no sulfur or nitrogen on the surface. The monolayers had thus completely desorbed, as expected. This also shows that the elements seen on the other surfaces were not airborne contaminants. A 3-TEP surface cycled above the oxidation peak was also found to have a decrease in the total amount of sulfur and an increase in the disulfide-to-thiolate ratio. Although, as for 1-TEP, the total amount of nitrogen decreased by more than 50%, the ratio of nitrogen to sulfur went up. The sulfur:nitrogen ratio was 2:1, compared with 3:1 for the uncycled, as-deposited monolayer. This suggests that some of the sulfur atoms or shorter thiolates (see paper 2) were preferentially removed. This is consistent with a change in the carbon:sulfur ratio, from 3:1 to 4:1. A more thorough report of the XPS results is given elsewhere.27 These results show that whereas the same elements were present after oxidation past the oxidation peaks as before, a considerable amount of material had been lost. Some of this lost material appears to be due to desorption of 1-TEP, as seen from the lower S and N content. The surface that was left had a lower N:S ratio than a pristine monolayer, so fragmentation must also have occurred. The remaining material does not fully desorb to leave a clean surface, however, until higher potentials are attained. Groups working with other thiol-modified pyrroles have also reported that the monolayer oxidation peak is not due to desorption.2,5,6,15 Thus, from IRAS and XPS the picture that emerges is that the TEPs undergo some reaction with oxygen, some degree of desorption, and some fragmentation. The questions of whether the decomposition left the pyrrole moieties intact and whether there was a conformational change remained to be answered. 3.1.3. UV-Vis-NIR. To ascertain whether the electrochemical cycling left any pyrrole rings intact, optical absorption spectra of cycled and uncycled monolayers were compared. In addition, if the cycling had resulted in a reorganization of the monolayer so that the molecules were parallel to the substrate, there would be no IRAS signal but the pyrrole absorbance peak above 5.2 eV would be clearly seen because optical absorption is strongest for a ring orientation parallel to the electric field vector.32 (This was used in paper 2 of this series to establish that there was some intact 3-TEP on the surface despite the absence of the IRAS signal.) We were also interested in using this technique to double check for evidence of oligomerization (31) March, J. Advanced Organic Chemistry, 3rd ed.; John Wiley & Sons: New York, 1992; p 1205. (32) Michl, J.; Thulstrup, E. W. Spectroscopy with Polarized Light. Solute Alignment by Photoselection, in Liquid Crystals, Polymers, and Membranes; VCH Publishers: New York, 1986; pp 20-21.

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Figure 3. UV-vis-NIR absorption. (a) Au films change absorption upon electrochemical cycling, as can be seen in the difference spectrum between a film cycled once and an uncycled film. There is less change between the second and first cycles. Thus, the Au was cycled once before the monolayers were adsorbed. Spectra of (b) 1-TEP and (c) 3-TEP monolayers before and after electrochemical cycling are shown. The absorption peak above 5.2 eV due to the pyrrole moiety disappears for both 1-TEP and 3-TEP.

or polymerization of the surface-confined species by comparison with previously obtained monomer and polymer UV-vis-NIR spectra (see paper 1 of this series). Unluckily, clean gold-coated quartz substrates changed their absorption properties after electrochemical cycling between 0 and 1 V in PC/LiClO4. The absorbance dropped overall by ca. 0.01 au, and a minimum appeared at 4.5 eV. The absorbance sloped upward from 4.5 to 6 eV, increasing by 0.005 au in this interval (see Figure 3a). Because this is the region in which pyrrole absorbs, and because the intensity of the absorption is also approximately 0.005 au, care had to be taken in finding an appropriate reference surface. Fortunately, the change in absorbance was greatest for the first scan. Therefore, clean gold substrates were electrochemically cycled once before immersion in the ethanolic TEP solutions. For some samples, a second optical absorbance spectrum was taken immediately after deposition, then they were cycled at 50 mV/s twice between 0 and 1 V, and another spectrum was taken. Keeping in mind the oxygen sensitivity of the monolayers, other samples were cycled immediately after removal from the deposition bath and then a spectrum was taken. (We knew that air exposure of the 1-TEP monolayers could result in loss of the cyclic voltammogram peaks; see below.) In both cases, the optical absorption peak usually seen for 1-TEP and 3-TEP monolayers (see paper 2) was completely missing after electrochemical cycling. (See

Electrochemistry of 1-TEP and 3-TEP Monolayers

Figure 3.) The formation of even short oligomers would have shifted the 5-eV peak to lower energy and led to an increase in intensity, but this did not occur. Polymerization would have resulted in optical absorption peaks typical of poly(1-TEP) or poly(3-TEP) (outside the energy range of Figure 3, see paper 1), but these were not observed, either. As a final check on the absence of monolayer polymerization, competitive adsorption experiments were done (see the Supporting Information for a full description). If the thiol-modified pyrrole monolayers polymerized during electrochemical oxidation, they would be stabilized and thus resistant to displacement by long-chain thiols. However, cycled and uncycled Au, 1-TEP, and 3-TEP surfaces all had the same affinity for the long-chain thiol. Thus, at this stage we can conclude from the UV-visNIR that the rings of both 1-TEP and 3-TEP were cleaved, either broken or lost, during oxidation. (The fact that a significant amount of nitrogen was measured with XPS shows that some of the rings were broken and not just severed.) There was no rearrangement that left intact pyrrole. On the basis of IRAS, UV-vis-NIR, and replacement measurements, we can also conclude that the adsorbed TEPs do not polymerize with each other. The results of the oxidation are therefore the formation of oxygen-containing compounds, some desorption, and most significantly, destruction of the pyrrole moiety. 3.2. Amount of Charge. Are the products seen in the previous sections the result of follow-up reactions set in motion by the initial formation of a cation radical (i.e., are they decomposition products of the radical cation) or do they consume charge (i.e., is there further electrochemistry after the formation of the radical cation)? The shape of the cyclic voltammogram suggests the latter for 1-TEP, but for 3-TEP it is unclear. To determine whether the reactions are electrochemically driven, one needs to know the total amount of charge. With an estimate of the surface coverage, this will provide the number of electrons involved in the oxidation process. However, the best way to calculate the charge consumed by oxidation of the monolayer requires some discussion. There are several possible methods, and several have been reported in the literature, but these give widely different results. Some methods are illustrated in Figure 4 using a cyclic voltammogram of 3-TEP as an example; the amount of charge one obtains is clearly dependent on the method. Extending the baseline before the peak (Figure 4d) is the recommended practice,2,22,25 but results can be misleading if there is a large background current. Taking the difference between the first and second scans (Figure 4a), which has also been done,15 or the difference between the first monolayer scan and a scan on clean gold (Figure 4b), might be problematic because of the differences in the capacitances of these surfaces. For the TEPs, calculating the charge is complicated by the fact that the current remains high, and more or less constant, right up to the point at which the gold oxidizes, even at very low scan rates. We do not know the origin of this current that persists after the peak, but it does not reappear in the second scan and we have also seen it in monolayers of long-chain alkanethiols with terthiophene end groups. Assuming that this charge is capacitive and subtracting a linear baseline from the peak (Figure 4c) is another means that has been employed to sum the charge.33 The biggest problem is deciding where to make the cutoff after the peak, which is not a textbook, sharp adsorbed-layer (33) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301.

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Figure 4. The amount of charge consumed in monolayer oxidation calculated by (a) subtracting the second scan from the first and counting only the charge under the peak, (b) subtracting the first gold scan from the first monolayer scan and assuming a t-1/2 decay, (c) counting the charge above a baseline connecting the bottom points of the peak, and (d) extending the baseline and counting the charge up to the minimum after the peak.

oxidation peak that returns to the baseline. One option is to integrate only to the end of the peak, determined by the eye (Figure 4a,d); another is to assume a t-1/2 decay (Figure 4b), as for a large-amplitude potential step.34 We therefore chose to use two different methods to calculate the charge. In the first, the baseline was subtracted from the first scan and the charge found by integration to the end of the peak(s) (Figure 4d). In the second, the difference between the anodic charge in the first and second cycles was integrated to the end of the peak(s) (Figure 4a). Using the second method gave values ca. 20% lower than the first. For 1-TEP, an average of 26 and 21 samples, respectively, yielded 273 ( 40 and 211 ( 37 µC/cm2 for these two methods. These values are uncorrected for roughness and were taken at 10 mV/s. For 3-TEP, the anodic charge above the baseline was 222 ( 84 µC/cm2 and the difference in charge between the first and second scans was 183 ( 74 µC/cm2 for 12 and 6 samples, respectively, from scans taken at 10 and 20 mV/ s. The smaller amount of charge for 3-TEP was due at least in part to halting the integration 50 mV lower, at 0.78 ( 0.04 mV instead of 0.83 ( 0.03 mV, and discarding the residual current after the peak. (Using Γs ) Q/FA, where Γs is the surface concentration in mol/cm2, Q is the charge in Coulombs, F is Faraday’s constant (96 485 C/mol), and A is the area, the values of 270 and 220 µC/ cm2 for the 1-TEP and 3-TEP monolayers correspond to 10.8 × 10-10 and 8.8 × 10-10 mol/cm2 if two electrons are transferred per molecule and the gold surface roughness factor of 1.3 is used. This is approximately equal to the surface coverage of perfectly packed SAMs of alkanethiols. However, the TEP monolayers are not perfectly packed (see part 2) and the number of electrons in the oxidation is unknown.) The charge is considerably higher than that found by other researchers for thiol-substituted pyrroles (75 µC/ cm2,2 61 µC/cm2 13), ferrocene alkanethiols (53 µC/cm2 28), and dodecanethiol (117 µC/cm2 11). For propanethiol on gold-coated mica, Widrig et al. found values of 90 µC/cm2 (34) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley & Sons: New York, 1980; pp 96, 220-221, 232, 525.

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Figure 6. Charge Q consumed during the first scan (calculated using a linear baseline) vs scan rate ν for 1-TEP (open triangles) and 3-TEP (filled circles). The curves show fits to 1/xν.

Figure 5. Forward potential excursions for 1-TEP at various scan rates between 2 and 1000 mV/s. Curves have been normalized to the height of the second oxidation peak. The first peak is marked with a circle, the second with a triangle, and the third with a diamond.

for reductive desorption, a one-electron reaction, and 280 µC/cm2 for oxidative desorption, a three-electron reaction.14 The lack of agreement, especially with the other thiolsubstituted pyrrole molecules, is puzzling because they would be expected to behave similarly. On the other hand, both TEP molecules are particularly reactive on the surface (see paper 2). The longer chain analogues must be substantially stabilized against decomposition. This instability of short-chain monolayers has been noticed before and attributed to their weaker barrier properties.26 Although one might attribute the excess charge for the TEPs to the presence of multilayers, rather than monolayers, this amount of charge was reproducible after ultrasonication, and multilayers were not observed in IRAS or XPS measurements (see paper 2). Thus, the high charge for the TEP monolayers must originate from multielectron reactions. From the amount of charge, and an estimated surface coverage of half a monolayer (paper 2), the number of electrons consumed per TEP should be around six, roughly. Initially, multiple sites on the molecule can be oxidized, including the 2 and 5 positions on the pyrrole ring and the thiolate. The XPS measurements showed that desorption of the entire monolayer does not occur, but that there is some loss of material, including sulfur, which must contribute additional charge. The creation of disulfide may also contribute to the current. Finally, there are decomposition reactions, such as ones that lead to breaking and loss of the pyrrole rings. Electrochemical oxidation of adsorbed pyrrole on Pt(111) has been shown to proceed completely to NO2 and CO2.35 3.3. Dependence on Scan Rate. To gain further information about the electrochemical reactions, cyclic voltammograms were taken at various scan rates between 2 and 1000 mV/s in PC/LiClO4 and are shown for 1-TEP with heights normalized to unity in Figure 5. (To see unnormalized curves, refer to the Supporting Information.) Above that rate, the capacitive currents became prohibi(35) Gui, J. Y.; Stern, D. A.; Lu, F.; Hubbard, A. T. J. Electroanal. Chem. 1991, 305, 37.

tively large and the upper voltage limit coincided with gold oxidation. For clarity, only the forward excursions have been shown. Extraordinarily, the data did not conform to the expected behavior. Looking at the figure, it is clear that each of the three peaks of 1-TEP had a different dependence on the scan rate; they did not maintain a constant position and height relationship with each other. The position of the first peak increased more slowly with scan speed than that of the second, so the two became increasingly separated. The third peak position was almost constant with scan rate, so it could no longer be distinguished when the much larger second peak merged into it at approximately 50 mV/s. The peaks also broadened with the scan rate. For 3-TEP, at higher scan rates the single oxidation wave split into two peaks (Supporting Information). The dependence of charge, peak position, and current on the scan rate will be described in turn. 3.3.1. Charge. For an adsorbed layer, the oxidation charge must be independent of the scan rate,6,15,33 even if the shape of the features changes, because the amount of material on the surface is fixed: only the molecules that are on the surface can be oxidized, so the speed at which this occurs should not matter. This linear relationship was found to be true for longer chain thiol-substituted pyrroles.15 However, there was a curious result for the TEP oxidations: the amount of charge was not constant with scan rate ν, but decreased strongly as shown in Figure 6 with an apparent 1/xν dependence. Although the apparent surface coverage of ferrocene sites measured by integrating the charge under the cathodic wave has been found to change as a function of scan rate, being 14% lower at 1000 mV/s than at 5 mV/s,36 this difference was small. For 1-TEP and 3-TEP the difference between those scan rates was approximately 300%. (A clean gold control surface did not behave in this way.) What is the true amount of charge involved in the oxidation? Perhaps there are different reaction pathways dominant at different scan rates, or perhaps prolonged times at the oxidizing potentials lead to more extensive reaction. We cannot easily explain these results. 3.3.2. Peak Position. The number of electrons in the rate-determining step of the electrochemical reaction can theoretically be found from the peak position. Irreversible reactions of adsorbed species should exhibit a scan rate dependence of the peak position, Ep, proportional to34

Ep ∼

( )

1 1 k0 ln RnR RnR ν

(1)

where R is the transfer coefficient, nR is the number of (36) Curtin, L. S.; Peck, S. R.; Tender, L. M.; Murray, R. W.; Rowe, G. K.; Creager, S. E. Anal. Chem. 1993, 65, 386.

Electrochemistry of 1-TEP and 3-TEP Monolayers

Figure 7. Peak positions vs ln(1/ν). Open and closed symbols represent data collected on different dates. For 1-TEP, the first peak obeys y ) 0.4 - 0.05x, the second y ) 0.4 - 0.08x, and the third y ) 0.7 - 0.02x; for 3-TEP, y ) 0.5 - 0.03x.

electrons involved in the rate-determining step, k0 is the rate constant, and ν is the scan rate. Peak positions are plotted in Figure 7 against ln(1/ν). There is an inherent inaccuracy in fixing the center of a peak that appears as a shoulder on a larger peak. Nevertheless, passable linear fits were obtained for all the peaks, especially for scan rates less than 200 mV/s (ln(1/ν) > -5.3). Because of peak broadening, at the highest scan rates the centers of all the peaks were difficult to pinpoint. Also, as noted above, the single peak for 3-TEP split into two as the scan rate increased, making the measurement ambiguous. Furthermore, another source of peak position dependence on scan rate is uncompensated resistance Ru which causes the potential of the working electrode to be E - iRu. Because of the dependence of current i on the scan rate, Ep is shifted in a positive direction with the increasing scan rate.34 To determine Ru, we performed impedance measurements which showed it to be approximately 2 kΩ. The peak current was on the order of 100 µA at 500 mV/s, which would introduce a shift in the applied voltage of 200 mV, enough to cause the deviations at a high scan rate in Figure 7. So, the scan rate dependence was essentially as expected for surfaceconfined irreversible reactions. However, each of the three peaks had a different slope with a ratio of 5:8:2. From eq 1, a difference in slope must be due to a difference in RnR for the three processes. Normally, R is between 0.3 and 0.7 and is usually assumed to be 0.5. Assuming R to be approximately the same for all three and a process that consumes a total of 7 electrons (based on the amount of charge, assuming half a monolayer coverage,27 considering the slope ratio, and requiring an integer number), one could roughly estimate a ratio of 2:4:1 for the number of electrons in each peak. If the same packing density found by other authors is used, 4 × 10-10 mol/cm2, then one obtains 260 µC/cm2 as the charge that should have been obtained, which it was at 10 mV/s. However, because the charge varied with the scan rate, this analysis does not present an accurate picture. Theoretically, one can also find RnR by the peak width, since the total width at the half-height of the anodic wave from an electroactive adsorbed species in an irreversible reaction should be 62.5/RnR mV at 25 °C regardless of the scan rate.34 However, from Figure 5 it is clear that the peak width increases with the scan rate. Using a commercial peak-fitting program, we found that at low

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scan rates, such as 2 mV/s, widths of 62.5 mV for both the first and second peaks were reasonable, which would give n ) 2 if R ) 0.5. The third peak was a better fit by a width of 125 mV, giving n ) 1. However, by 50 mV/s it was no longer possible to fit the data using peaks of those widths. Therefore, either R decreases with the scan rate, or n changes, or something else is occurring that is not accounted for in a simple model of irreversible surface oxidation. So, neither the charge nor the peak width have the dependence on the scan rate that they should for an adsorbed monolayer. Because the peaks do not have the expected behavior, it is difficult to extract the information we want, such as the total number of electrons or the number of electrons participating in each step. However, the shape of the 1-TEP oxidation peak suggests a threestep process. The independent behavior of the three peak positions versus the scan rate corroborates this. 3.3.3. Peak Current. Does the peak current behave as expected for a simple surface-confined, irreversible process? For a surface-confined reaction, both the Faradaic and charging contributions to the peak current ip vary linearly with the scan rate ν. Previous workers have found such a linear dependence.2,15 The recommended method to measure the peak current without interference from the charging current is to use an extrapolated forward baseline from the region in front of the peak as a reference. This will not be valid if the capacitance of the surface changes as a result of oxidizing the monolayer, however. A better method in this case might be to subtract the current from the second scan, which will consist only of charging current ic since the monolayer has been rendered inert. When bare gold was cycled at different scan rates the current had a linear dependence on the scan rate, as expected. Likewise, at the potential of the large middle peak of 1-TEP, a plot of ic from the second scan versus the scan rate was also linear. The determination of peak currents in the first scan, especially at high scan rates, was confounded by the high background current; this was not from gold oxidation, which does not occur until 1.5 V in this electrolyte. In addition, because the first and third peaks of 1-TEP were small and superimposed over the central peak, we could not determine their peak heights with any accuracy; curvefitting programs yielded a large number of possible peak heights and shapes in various combinations. Therefore, for 1-TEP only the peak current at the position of the largest, middle peak is plotted. Several methods to determine the peak current were compared: the absolute peak current at the peak position, the difference between the first and second scan at the peak position, the difference between the first TEP and first gold scans, the distance to a baseline connecting the bottom points of the peak, and the distance to a baseline projected from in front of the peak (see Figure 4). Peak currents are plotted as a function of the scan rate in Figure 8. It did not matter which method was used; all yielded a similar nonlinear scan rate dependence, as shown in Figure 8a for 1-TEP. The curves for 3-TEP looked similar. Although the current was roughly linear with the scan rate within the range 50-500 mV/s, at lower scan rates it clearly deviated. A plot against the square root of the scan rate provided better fits. The deviation at scan rates above 200 mV/s is most likely due to increasing inaccuracy introduced by the charging current. The nonlinearity is completely unexpected for a surfaceconfined species. Although one might reason that interference from the two shoulders could account for it in

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Figure 8. (a) Peak current vs scan rate (ν) in PC/LiClO4 for a 1-TEP monolayer measured using (i) an extrapolated baseline (filled circles), (ii) the absolute value of the current at the peak position (open circles), (iii) the difference between the first and second scans at the peak position (triangles), (iv) the difference between the first scans of 1-TEP and clean gold at the peak position (diamonds), and (v) the distance to a baseline connecting the starting and ending points of the peak (squares). (b) Peak currents from extrapolated baselines for 1-TEP (black circles) and 3-TEP (gray triangles) vs the square root of the scan rate.

1-TEP, the same behavior seen for the single peak of 3-TEP argues against this. We emphasize that it is definitely due to the monolayer: it cannot be due to electroactive species inadvertently in the electrolyte because neither the clean gold electrode nor the TEP electrodes in the second scan showed this behavior. We are unaware of other reports describing such behavior for monolayers. A square root dependence is normally observed in diffusion-controlled reactions. One can consider two possibilities for the diffusing species: first, that some of the TEP molecules desorb and diffuse. This cannot be the case, however, for at least three reasons. (1) Their concentration in the electrolyte would be so small that we would probably not be able to detect them. (2) The concentration profile would not be correct for this dependence, since the species would diffuse away from the electrode toward the lower concentration in the bulk, rather than toward the electrode from a higher concentration in the bulk. (3) The charge was lower for fast scan rates rather than that for slow ones, but for faster scan rates molecules would have less time to diffuse away and the charge should be greater in that case. The second possibility is that oxygen is the diffusing species and is necessary for the reactions to proceed. A typical concentration of dissolved oxygen in organic solvents is on the order of 1 mM,29 and no attempt was made to exclude oxygen. However, at this high concentration there should have been more than enough oxygen at the surface to react with the minute quantity of material in the monolayer without reaching a diffusion limit. To produce a depletion layer near the electrode, the diffusing species would have to be at quite a low concentration, indeed. Oxygen is therefore also unlikely to be the explanation. Water can also be ruled out as the culprit, since voltammograms produced under rigorously dry conditions were the same (see Supporting Information). The combination of results on charge, peak position, and peak current versus the scan rate clearly shows that

Smela et al.

the peaks are due to a more complicated process than simple electron transfer, such as that takes place in the formation of radical pyrrole cations (although this is probably the first step), which involves only electron transfer and has a large rate constant.34 Perhaps one or more steps in the monolayer oxidation require diffusion of some solution species to the surface. Another possibility is that some slow process, for instance, one involving conformational change or solvent incorporation, is generating this behavior. Complicated reactions involving significant molecular rearrangement upon electron transfer can be very sluggish and involve multistep mechanisms.34 These possibilities are investigated below. 3.4. Chronoamperometry. Because of the strange electrochemical behavior of the monolayers, such as the nonlinear increase of peak current and the decrease in consumed charge with scan rate, a slow process, like a conformational change, might be involved. Chronoamperometry is a tool that can be used to examine this possibility. In chronoamperometry the voltage is stepped from an initial value at which no oxidation takes place to one at which it does occur, and the resulting current is observed over time. This perspective can be used to supplement the information in the cyclic voltammograms. In general, for such a step, at time t ) 0 there is a current spike due to double-layer charging which decays exponentially with a time constant RuCd, where Ru is the uncompensated resistance and Cd the double-layer capacitance. This is normally on the order of milliseconds or less. There is also a decaying residual current resulting from extraneous processes associated with the electrode surface and from trace impurities of electroactive compounds in the solution.22 Finally, under diffusioncontrolled conditions the Faradaic current from the electrochemical reaction is superimposed over these currents with a longer decay time proportional to 1/xt. Surface-confined reactions, such as oxidation of an aromatic hydrocarbon to a cation radical, however, are kinetically controlled. These involve simple electron transfer and should be nearly instantaneous;34 the Faradaic current would therefore decay quickly. However, for processes involving significant molecular rearrangement upon electron transfer, the rate constant can be quite small. A larger applied potential will, of course, increase the rate of reaction. Chronoamperometry was done at a series of applied potentials spanning the oxidation peaks of the TEPs. After equilibration for several seconds at 0 V, the potential was stepped to a new value and held while the current was measured versus time. Some illustrative curves for 1-TEP are shown in Figure 9a, where one can clearly see the unexpected result that a distinct current peak was observed after the initial current decay. (When the current was plotted against t-1/2 or the charge against t1/2, the curves were not typical of either an adsorbed monolayer or diffusion.30 For plots, see the Supporting Information.) In addition, at potentials above 0.5 V two peaks could begin to be distinguished. At higher potentials, the peaks merged into the 1/xt current, so peak positions were imprecise. The peak time tp and peak width decreased as the applied potential increased, with tp proportional to e-V, as shown in Figure 9b, due to the dependence of the rate constant on the potential. The difference in slope between the first and second peaks is a factor of 2. The integrated charge, found by subtracting a second scan done directly after the first and integrating the peak area, was ca. 80 µC/cm2 for scans below ca. 0.7 V; above that

Electrochemistry of 1-TEP and 3-TEP Monolayers

Figure 9. (a) Chronoamperograms of 1-TEP monolayers in PC/LiClO4, shown offset for clarity. The dashed line shows a curve taken with 1-TEP monomers in solution. (b) The dependence of the position of the two peaks in the 1-TEP chronoamperograms on the applied potential: tp is proportional to e-V due to the potential dependence of the rate constant. Triangles and squares represent different experimental runs and open symbols the position of the second, later peak.

the peak blended into the Faradaic/capacitive current. This closely corresponds to the charge found by other researchers for two-electron charge transfers to a monolayer of this type (see above). Interestingly, 3-TEP monolayers did not show such peaks and neither did clean gold control surfaces. This type of i-t behavior is seen during nucleation and growth of PPy.37 It has also been observed previously in monolayers, for example, during the nucleation and growth of calomel islands on a mercury surface and the formation of surface oxides.38 The analysis of these results is outside the scope of this paper, but the procedures have been outlined in ref 38 (Briefly, the current density can be expressed as ia ) zFkaQ(1 - θt) exp(βVF/RT), where zF is the number of Faradays required per mole of product, ka is the rate constant, Q is the charge from monolayer formation, θt is the time-dependent coverage of the adsorbed species, β is between 0 and 1, usually 0.5, and V is the potential.) The appearance of different maxima corresponds to the nucleation and growth of successive monolayers. In the 1-TEP monolayers, there were two peaks and an amount of charge corresponding to two electrons, which is self-consistent. Nucleation could take place at potentials as low as 0.425 V. This result is consistent with the IRAS measurements on 1-TEP monolayers cycled to the foot of the oxidation peaks (Figure 2b) which showed that even low anodic potentials could cause the monolayer spectrum to change. The IRAS spectra indicated that this oxidation formed some carbonyl, perhaps on the pyrrole moiety or perhaps, through cleavage of the pyrrole, in the form of short-chain carboxylic acids. The reduction of the 1-TEP IRAS peak heights would also be consistent with some degree of conformational change. To further explore the i-t behavior of the 1-TEP monolayers, the potential was held at 0.5 V for 10 s, which was the time it took for the initial transient to decay but (37) Lyons, M. E. G. Transport and Kinetics in Electroactive Polymers; John Wiley & Sons: New York, 1996; Vol. 94, Chapter 4. (38) Conway, B. E. In Electrochemistry; Bockris, J. O. M., Ed.; University Park Press: Baltimore, MD, 1973; Vol. 6, pp 76-83.

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before the beginning of the peak. It was then either switched to open circuit, held at 0 V, or taken to -0.5 V for times as long as 90 s. After stepping back to 0.5 V, the peak occurred 10-15 s earlier than when no initial step to 0.5 V was performed. This reaction was therefore irreversible (i.e., it could not be undone with time or reversed potential). Even though no current peak was visible yet, part of the surface had been converted, and reapplication of the potential continued the process where it had left off. The experiments were repeated with initially clean gold surfaces immersed in PC/LiClO4 solutions containing 1-TEP monomers (ca. 20 mM). Of course, the 1-TEP immediately formed a monolayer on the clean surface. The results were striking: the peak times coincided exactly with those of the monolayer, as illustrated for 0.50 V by the dotted line in Figure 9a. The major difference with and without monomer in solution was that the current fell to zero after the monolayer peak without monomers in solution, but current continued to be consumed after the peak with monomers in solution because of the polymerization reaction. The coincidence of the peaks might suggest that the monolayer oxidizes in a similar manner even in the presence of a monomer in solution. In the next paper of this series, these results are followed up with further experiments on pyrrole monomercontaining solutions. These chronoamperometric peaks are consistent with monolayer nucleation. We do not know of another mechanism that can account for the shapes of the i-t curves. Nor do we know what may be nucleating on the surface, or why this might occur. It might involve a conformational change of the adsorbed 1-TEP, which might be nucleated, for example, at defect sites. A simple explanation involving nucleated sulfur desorption is inconsistent with the XPS data that showed ca. 70% of the sulfur remained after cyclic voltammetry, but it cannot be ruled out without further XPS studies; it is possible that the longer times the electrode was held at the anodic potentials in the chronoamperometric studies allowed more extensive desorption. It might be difficult to justify why this did not occur for 3-TEP surfaces, however, although the structures of the two monolayers are very different. The lack of the peaks for 3-TEP also means that a nucleation process cannot account for the abnormal scan rate dependence of the cyclic voltammograms; the reasons for that are still mysterious. Finally, the nucleation might involve the formation of carbonyl, which would account for its occurrence in 1-TEP and not 3-TEP, although why this reaction should be nucleated is unknown. 3.5. Monolayers Aged in Air. Finally, the effect of air exposure on electrochemistry was studied. It was shown in paper 2 that TEP monolayers were quite sensitive to oxygen: infrared spectroscopy (IRAS) showed carbonyl peaks for samples exposed to air. Willicut et al.3 observed that for 1-(6-mercaptohexyl)pyrrole monolayers, there was a very rapid decay in the height of the electrochemical oxidation peak; after only 15 min of exposure to air the peak was less than 20% of its original size. Likewise, Weisshaar et al.26 found that the oxidation waves for mercaptoethanol were dependent on the extent of exposure to air. To see the effect of air exposure on the first-scan oxidation peaks, 1-TEP monolayers were left in laboratory air for 2 h before beginning cyclic voltammetry. For these surfaces, there were no peaks during the first excursion. It is interesting that air oxidation leaves the surface completely inert, since many electrons are normally

2994 Langmuir, Vol. 14, No. 11, 1998

consumed in what appear to be three steps, and a large number of potentially current-consuming events take place, such as the formation of disulfide, the decomposition of the pyrrole moiety, and some degree of desorption. One can speculate that the formation of the radical cation is a necessary first step in the monolayer destruction, and that this cannot occur if there is carbonyl on the R positions of the pyrrole. Another good possibility is that the airoxidized species desorbed when they were placed in the electrolyte. Scott et al.39 have shown that exposure of alkanethiol SAMs to air converts the thiolates (S-) to sulfinates (SO2-) and sulfonates (SO3-), and these readily desorb in replacement experiments. However, thiolates containing aromatic rings are less prone to oxidation than alkanethiolates.7 In contrast, 3-TEP left in air for nearly 4 h retained the electrochemical oxidation peak. The results might be considered surprising since the 1-TEP was more stable than the 3-TEP in every other context and, in general, 1-alkylpyrroles react with oxygen more slowly than C-alkylpyrroles.40 It also contradicts the desorption hypothesis for the lack of a 1-TEP electrochemical wave because the thiolates on the 3-TEP-treated surface would also be expected to oxidize and desorb. More studies would be required to answer these question. 4. Discussion and Conclusions In paper 2 it was shown that the adsorbed 1-TEP and 3-TEP monolayers were very different from each other, despite the superficial similarity of their chemical structures. Neither molecule, however, was stable upon adsorption. These findings persisted in the electrochemistrysneither was stable during electrochemical cycling, but underwent decomposition and ring-opening. To summarize: During electrochemical cycling, irreversible oxidation peaks are observed during the first anodic potential excursion that must be attributed to the monolayers. These represent the destruction of the TEPs. For 1-TEP, there are no 1-TEP peaks in the IRAS spectrum after cycling, the XPS shows a loss of S (ca. 30%) and N (ca. 50%), and UV-vis-NIR shows the absence of pyrrole moieties. For 3-TEP, there was also a partial loss of S and a complete loss of the pyrrole moieties. This has not been seen in other substituted pyrroles or aromatics. However, mercaptoethanol (HOCH2CH2SH) has been shown to undergo reaction to mercaptoacetic acid followed by C-S bond cleavage during electrochemical oxidation.26 The TEP’s electrochemical behavior was also unusual. The charge decreased and peak widths increased with the scan rate rather than remaining constant, and the peak current had a nonlinear dependence on the scan rate. However, the peak positions did have the expected ln(1/ν) dependence. The chronoamperograms of 1-TEP had delayed peaks with a time dependence of e-V. Neither nonlinearity of peak current nor a decrease in consumed charge with the scan rate have been reported before for an adsorbed monolayer (although over the narrow range of scan rates sometimes used, a nonlinearity would not be apparent because a straight tangent can always be drawn). Likewise, nucleation has not been observed before, either. The reasons for this behavior are still unknown. It is likely that the electrochemical oxidation of the TEPs begins with the formation of the radical cation of pyrrole.13,40 This radical is not stable, since the process is (39) Scott, J. R.; Baker, L. S.; Everett, W. R.; Wilkins, C. L.; Fritsch, I. Anal. Chem. 1997, 69, 2636-2639. (40) The Chemistry of Pyrroles; Jones, R. A., Bean, G. P., Eds.; Academic Press: London, 1977; pp 210, 460-463.

Smela et al.

irreversible. Follow-up and/or further electrochemical reactions lead to redox-inactive products. However, given that 3-TEP monolayers have only one-third of the number of pyrrole rings that 1-TEP has (see paper 2), the difference in the amount of charge consumed by the two is not that large. Oxidations of the pyrrole therefore do not appear to be the primary electron-consuming reactions. For 1-TEP the oxidation is a three-step process, and twodimensional nucleation and growth could be involved. The electrochemical response of the two molecules is very different. Whereas 1-TEP has three oxidation peaks, 3-TEP has one. The amount of charged consumed by 1-TEP is larger. 1-TEP has nucleation-like peaks in chronoamperometry, 3-TEP does not. 1-TEP becomes inert after air exposure, 3-TEP does not. 1-TEP suffers a proportionally greater loss of nitrogen than 3-TEP. The primary oxidation pathways are clearly different. However, the IRAS spectra of the two surfaces after cycling do resemble each other, consisting predominantly of carboxylic acid peaks. Even though destruction of the monolayer by electrochemical redox is probably unique to thiolates with extremely short alkyl chains, no spectroscopic evidence has ever been published that the ring still exists after anodic oxidation in any other substituted aromatic compounds, although the CH2 stretches of the alkyl chain have been seen with IR.6 (Yet it is interesting to note that Φ-(CH2)n-SH compounds have been reported to improve the properties of conducting polymer films only for n ) 0 or 1.41) In view of claims that compounds with an aromatic ring function as a molecular glue by binding with pyrrole in solution, this is a serious shortcoming. Given these results with the TEPs, it should be confirmed for other thiol-substituted aromatics. Care should be taken in future studies of this type of compound to show that the monolayer is stable. More studies would be required to map out the various decomposition reactions, and this might lead to new insights about electrochemistry of adsorbed monolayers. But for practical applications, the salient point is that the TEP monolayers are electrochemically inert after cycling or air exposure and therefore may not function as intended. Others have also shown that the second scans of thiolsubstituted pyrroles usually show either little Faradaic current, indicating that all the pyrrole sites in the film have been completely oxidized,5,6,13 or redox peaks indicative of monolayer polymerization.2-4 In either case, the surface-bound pyrrole is potentially unavailable for reaction with the pyrrole in solution. The reason for doing this study was to investigate the feasibility of using the TEPs as adhesion promoters by doing careful studies of their behavior. These results are not encouraging. It is tempting to believe, as has been postulated, that in the presence of monomers the oxidation reactions are different and lead to coupling with radical cations in solution.5,15,16 This possibility is explored in the next paper of this series. Acknowledgment. We acknowledge the support of Dr. Olle Ingana¨s, in whose laboratory part of this work was performed. We would also like to acknowledge the financial support of Volvos Forskningsstiftelse & Volvos Utbildningsstiftelse and the Swedish Research Council for Engineering Sciences, TFR. Prof. U. Gelius and Mr. M. Wirde, at Uppsala University, Sweden, are thanked for help with the XPS measurements. We thank Dr. Guido Zuccarello for many useful discussions. (41) Lang, P.; Mekhalif, Z.; Garnier, F. J. Chim. Phys. 1992, 89, 1063.

Electrochemistry of 1-TEP and 3-TEP Monolayers

Supporting Information Available: Detailed experimental descriptions of the determination of surface roughness and competitive adsorption; results of replacement of cycled and uncycled monolayers by long-chain thiols; comparison of N and S peaks in the XPS spectra of cycled and uncycled 1-TEP; cyclic voltammograms at various scan rates, unnormalized, for 1-TEP and 3-TEP; excursion to more ((2 V) and less extreme poten-

Langmuir, Vol. 14, No. 11, 1998 2995 tials: cyclic voltammograms and IRAS; gradual increases in the upper potential limit; cycled samples returned to deposition solutions and cycled again (replacement); voltammetry in dry electrolyte; dependence of cyclic voltammograms on deposition time (21 pages). See any current masthead page for ordering information and Internet access instructions.

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