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Langmuir 1998, 14, 2970-2975
Thiol-Modified Pyrrole Monomers: 1. Synthesis, Characterization, and Polymerization of 1-(2-Thioethyl)pyrrole and 3-(2-Thioethyl)pyrrole Elisabeth Smela,* Guido Zuccarello, Hans Kariis, and Bo Liedberg Laboratory of Applied Physics, University of Linko¨ ping, S-581 83 Linko¨ ping, Sweden Received August 4, 1997. In Final Form: October 22, 1997 To try to solve the problem of delamination of polypyrrole from gold during electrochemical cycling, 1-(2-thioethyl)pyrrole and 3-(2-thioethyl)pyrrole were synthesized as adhesion promoters. These compounds were designed to bind to gold through the sulfur atom to form monolayers, and their alkyl chains were made short to permit electron transfer to and from the pyrrole moieties. It was hoped that during electropolymerization of unmodified pyrrole in solution, the pyrrole moieties of the monolayer would be incorporated into the growing film. In this first paper of the series, the synthesis and characterization of the monomers and their electrochemical polymerization to form thin films is presented. Nuclear magnetic resonance and infrared, Raman, UV-vis-NIR, and mass spectroscopy measurements form a basis for comparison with monolayers adsorbed on gold surfaces in the following papers.
1. Introduction Devices making use of conducting polymer films often require them to be in contact with metal electrodes. Although conducting polymers can be formed directly on the electrode by electrochemical polymerization, the two layers are not bonded together chemically. Stresses at the interface, such as those caused by volume changes induced during doping/undoping of the polymer, can cause the two layers to separate. We have found that delamination is the main cause of device failure in our micromachined polypyrrole (PPy) actuators, which undergo frequent electrochemical cycling.1-4 We therefore need a method to improve the adhesion of polypyrrole to gold. By using a specially synthesized adhesion-promoting monomer and, for the film, unsubstituted pyrrole together with the appropriate anion, the properties at the interface and in the bulk can be separately optimized for a particular application. A single monomer may not be able to achieve good adhesion in addition to high conductivity for example; the conductivity of films formed from substituted pyrroles is known to be lower than that of unsubstituted pyrrole. We chose to try chemically binding the two layers by using a molecule containing both a sulfhydryl and a pyrrole group as a link between the polymeric phase and the supporting substrate. This approach was based on the reports of several workers, including that of Wrighton et al., who synthesized N-[3-(trimethoxysilyl)propyl]pyrrole as a surface modification agent to improve adhesion of PPy films onto n-type Si by covalently anchoring the PPy to the surface.5,6 Its effectiveness was demonstrated using * 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) Smela, E.; Ingana¨s, O.; Lundstro¨m, I. In Molecular Manufacturing; Nicolini, C., Ed.; Plenum Press: New York, 1994; pp 189-214. (3) Smela, E.; Ingana¨s, O.; Lundstro¨m, I. J. Micromech. Microeng. 1993, 3, 203. (4) Smela, E.; Ingana¨s, O.; Pei, Q.; Lundstro¨m, I. Adv. Mater. 1993, 5, 630. (5) Simon, R. A.; Ricco, A. J.; Wrighton, M. S. J. Am. Chem. Soc. 1982, 104, 2031. (6) Wrighton, M. S. Science 1986, 231, 32.
a standard tape test. (They also showed that photopatterning of such self-assembled monolayers could be used to selectively deposit conducting polymers.7) In addition, a polymerizable thiol-modified monomer, N-(3sulfhydrylpropyl)bisthiophenepyrrole, was synthesized by Kowalik et al.:8 electropolymerized films showed improved adhesion to gold electrodes, but only in their undoped state, compared with films formed from the unsubstituted bisthiophenepyrrole. Rubinstein et al. found that films of polyaniline grown on SAMs of p-aminothiophenol were denser than those grown on bare gold.9 Lang and coworkers attached Φ-(CH2)n-SH molecules on Pt, where Φ was an aromatic cycle, and found that although the growth rates of poly(3-methylthiophene) were lower, the electrochemical properties were improved for n < 2, but not for longer chains.10 Modified alkanethiols had also been used to attach nonconducting polyethylene to gold, and these plasma-polymerized films also showed improved adhesion.11 On the other hand, electrodes treated with amorphous carbon showed increased adhesivity compared with bare gold, although the number of initiation sites was smaller; chronoamperometric data suggested that bonding had taken place between PPy and the amorphous carbon.12 To ensure that PPy could be electropolymerized over the monolayer, that the monolayer would be conducting rather than insulating, we wished to synthesize a molecule with a short alkyl chain. It is well-known that a tunneling current decreases exponentially with monolayer thickness.13-15 For this reason we were interested in 1-(2(7) Rozsnyai, L. F.; Wrighton, M. S. J. Am. Chem. Soc. 1994, 116, 5993. (8) Kowalik, J.; Tolbert, L.; Ding, Y.; Bottomley, L. A. Synth. Met. 1993, 55-57, 1171. (9) Rubinstein, I.; Rishpon, J.; Sabatani, E.; Redondo, A.; Gottesfeld, S. J. Am. Chem. Soc. 1990, 112, 6135. (10) Lang, P.; Mekhalif, Z.; Garnier, F. J. Chim. Phys. 1992, 89, 1063. (11) Stewart, K. R.; Whitesides, G. M.; Godfried, H. P.; Silvera, I. F. Rev. Sci. Instrum. 1986, 57, 138. (12) Helms, J. H.; Everson, M. P.; Plummer, H. K., Jr. Abstracts of Papers, 208th National Meeting of the American Chemical Society, Washington DC, Aug 21-25, 1994; American Chemical Society: Washington DC, 1994; COLL 214. (13) Miller, C.; Cuendet, P.; Gra¨tzel, M. J. Phys. Chem. 1991, 95, 877.
S0743-7463(97)00860-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/26/1998
Characterization of 1-TEP and 3-TEP
thioethyl)pyrrole (which will be abbreviated as 1-TEP) and 3-(2-thioethyl)pyrrole (3-TEP). Although the selfassembled monolayers would not be well-ordered because of the shortness of the alkyl chains, we hoped that the pyrrole moieties would be incorporated into the PPy film during electrochemical deposition. Rubinstein et al. listed 3-TEP as an adhesion-promoting compound in a patent for sandwich electrodes consisting of three layers: (1) a metal, (2) a self-assembling monolayer with one thio or halo functional group and a second functional group for bonding with a conducting polymer, and (3) a conducting polymer.16 However, no synthesis, characterization, or demonstration of efficacy was given. It was, however, claimed that a similar thiol-modified aniline, p-aminothiophenol, bound to gold and oriented on the metal surface to provide bonding sites for polyaniline, enabling the conducting film to grow in a denser layer than on untreated electrodes.9 Although it was postulated that this monolayer increased the number of nucleation sites during electropolymerization, they did not report testing for increased adhesion even though that was an objective for the surface treatment. As this work progressed, several other papers appeared that reported work with closely related compounds. Willicut and McCarley have synthesized a series of shortchain ω-(N-pyrrolyl)alkanethiol monomers, including 1-(3mercaptopropyl)pyrrole, which has an alkyl chain only one carbon longer than our 1-TEP.17-21 They found that these monomers form monolayers that enhance the nucleation, growth, smoothness, and adhesion of PPy. In addition, they suggested that the monolayers could be electrochemically polymerized. At the same time, Collard and Sayre had been working with 3-substituted pyrrole monomers with longer alkyl chains.22-25 They also reported enhanced nucleation and adhesion of poly(3ethylpyrrole) by these well-ordered monolayers, as well as an increase in conductivity. A long-chain 1,1′substituted disulfide has recently been synthesized by Zong et al.26 They plan to grow conducting films containing these long alkyl chains, hoping to achieve coupling of the surface-confined monomers with the monomers in solution. In related work on indium-tin oxide, Lukkari et al. used linking groups such as cyanuric chloride to attach thiophene to the surface, and they observed enhanced nucleation of 3-methylthiophene.27 (However, when the bridging agent was tin tetrachloride, there was no (14) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173. (15) Mekhalif, Z.; Lang, P.; Garnier, F. J. Electroanal. Chem. 1995, 399, 61. (16) Rubinstein, I.; Gottesfeld, S.; Sabatani, E. U.S. Patent 5,108,573, 1992. (17) Willicut, R. J.; McCarley, R. L. J. Am. Chem. Soc. 1994, 116, 10823. (18) Willicut, R. J.; McCarley, R. L. Langmuir 1995, 11, 296. (19) Willicut, R. J.; McCarley, R. L. Anal. Chim. Acta 1995, 307, 269. (20) Willicut, R. J.; McCarley, R. L. Adv. Mater. 1995, 7, 759. (21) Willicut, R. J.; McCarley, R. L. Abstracts of Papers, 210th National Meeting of the American Chemical Society, Aug 20-24, Chicago, IL, 1995; American Chemical Society: Washington DC, 1995; ANYL 052. (22) Collard, D. M.; Sayre, C. N. J. Electroanal. Chem. 1994, 375, 367. (23) Collard, D. M.; Sayre, C. N. Abstracts of Papers, 207th National Meeting of the American Chemical Society, March 13-18, San Diego, CA, 1994; American Chemical Society: Washington DC, 1994; POLY 14. (24) Collard, D. M.; Sayre, C. N. Synth. Met. 1995, 69, 459. (25) Sayre, C. N.; Collard, D. M. Langmuir 1995, 11, 302. (26) Zong, K.; Brittain, S. T.; Wurm, D. B.; Kim, Y.-T. Synth. Commun. 1997, 27, 157. (27) Lukkari, J.; Tuomala, R.; Ristima¨ki, S.; Kankare, J. Synth. Met. 1992, 47, 217.
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enhancement, possibly because of its greater instability.) Nishizawa et al. found that dodecylsulfonate amplified the promotion of lateral growth of PPy on n-alkylsilanetreated surfaces.28 They attributed the promotion by alkylsilanes to adsorption of pyrrole monomers and oligomers at the hydrophobic surfaces. Kupila and Kankare also found that pretreating platinum electrodes with allyl alcohol enhanced the nucleation process in the presence of sodium dodecylsulfonate (NaDS) and sodium toluenesulfonate (NaTs), but decanethiol, which had a positive effect with NaClO4, weakened adherence in the presence of NaDS.29 When 4-aminothiophenol or cysteamine were used, there was a big difference between PPy(DS) and PPy(Ts): cysteamine decreased the adherence of PPy(DS), but not PPy(Ts). With NaNO3, none of the pretreatments improved the film quality. These materials thus required different surface characteristics to obtain the best polymers. They also noted that initial adsorption of the counteranions on the electrode surface may affect the nucleation process significantly. Mekhalif et al.15 discovered that alkanethiols on platinum increased the conjugation length of electro-deposited polybithiophene films. When aromatic thiols were used, adhesion was improved relative to bare surfaces, the morphology was more rough and globular, the films were denser, and the conjugation length again increased. Lo et al.30 have used N-(3-aminopropyl)pyrrole to passivate active spots on YBa2Cu3O7-δ, which dominate the polymer growth dynamics at early polymerization times, allowing PPy to grow uniform films quickly with good contact to the substrate and improved morphology. The monolayer assisted nucleation of PPy on the superconductor by forming a surface with more uniform electrochemical characteristics. The synthesis and characterization of the 1-TEP and 3-TEP monomers, as well as the polymers formed from them electrochemically, have been studied using NMR, Fourier transform infrared spectroscopy (FTIR), infrared reflection-absorption spectroscopy (IRAS), Raman spectroscopy, ultraviolet-visible-near-infrared spectroscopy (UV-vis-NIR), and mass spectroscopy. These results will be described in this paper. (Additional information is provided in the Supporting Information.) They are important because complete spectral data on these compounds have not been presented previously, and because these data form the basis for evaluating the results of the TEP deposited as monolayers on gold that will be described in the remaining papers of this series. 2. Experimental Section 2.1. Synthesis. 1-TEP has been synthesized previously.31 For the present study, it was synthesized by reacting the commercially available 2-aminoethanethiol hydrochloride with dimethoxytetrahydrofuran in acetic acid containing sodium acetate32 to give 1-TEP directly in fair yield. The synthesis of 3-TEP was achieved starting from 3-(2hydroxyethyl)pyrrole,33,34 which was made in low yield by reacting the magnesium bromide salt of pyrrole with ethylene oxide. The (28) Nishizawa, M.; Miwa, Y.; Matsue, T.; Uchida, I. J. Electrochem. Soc. 1993, 140, 1650. (29) Kupila, E.-L.; Kankare, J. Synth. Met. 1995, 74, 241. (30) 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. (31) Pittit, A. O.; Pascale, J. V.; Hruza, D. E. U.S. Patent 3,980,089, 1976. (32) Josey, A. D. In Organic Synthesis, Collected Vol. V; Baumgarten, H. F., Ed.; John Wiley & Sons: New York, 1973; pp 716-717. (33) Castro, A. J.; Duncan, W. B.; Leong, A. K. J. Am. Chem. Soc. 1969, 91, 4304. (34) Moll, F.; Thoma, H. Arch. Pharm. (Weinheim, Ger.) 1968, 301, 872.
2972 Langmuir, Vol. 14, No. 11, 1998 alcohol formed was selectively converted to its tosylate, and this was reacted without purification with the potassium salt of thioacetic acid to give 3-(2-acetylthioethyl)pyrrole in good yield. Last, the acetyl group was removed in low yield with sodium methoxide in methanol. The low yield is due to instability of 3-TEP to air, heat, and the silica used in the chromatographic purification. To prevent polymerization of the intermediates, it was important to avoid the use of acids during workup. For complete details of the two syntheses and characterization of the compounds by NMR, see the Supporting Information. At room temperature, 1-TEP is a clear liquid. It is fairly stable and can be stored at -4 °C. If great care was taken during the purification of 3-TEP, keeping the temperature low during removal of the solvent, it could also be obtained as a clear liquid. However, 3-TEP was difficult to purify because it decomposed in the silica column and oxidized readily. C-alkylpyrroles are known to be less stable than 1-alkylpyrroles toward oxygen.35 Because of this instability, 3-TEP soon degraded if stored at -4 °C. Even pyrrole slowly turned yellow if kept for long periods of time at this temperature, so all the compounds were kept at -20 or -70 °C; at the latter they were solids and did not show signs of oxidation. In the batches used in the present work, the 3-TEP was a transparent orange-pink liquid at room temperature. 2.2. Electrochemistry. Working electrodes for the electrochemical growth of polymer films were made by evaporating 30 Å of Cr and then 3000 Å of Au onto Si wafers and dicing the wafers into strips of area 0.5 cm × 1.5 cm. Just prior to use, the strips were rinsed with acetone and ethanol to degrease and then cleaned in the standard RCA SC1 solution (a heated mixture of 25% ammonia, 30% H2O2, and water in a volume ratio of 1:1:5) to remove organic material. The electrochemical work was done using two instruments. The first was an EG&G PAR model 173 potentiostat/galvanostat with a model 179 digital coulometer, model 178 electrometer, and model 175 universal programmer. A Graphtec WX2400 x-y recorder was used to make a record of these scans. The second was an Autolab PGSTAT 10 from EcoChemie, computer controlled by their General Purpose Electrochemical System software. Electrochemistry was performed in single-compartment, threeneck glass electrochemical cells; the gold-covered Si substrates described above were used as working electrodes. These were partially immersed in the monomer-containing solution during electropolymerization, and the area of the resulting film was measured afterward. As a counter electrode, either a square of Pt foil approximately 1 × 1 cm2 or a thick gold wire was used. An Ag/AgCl reference electrode from BASF was employed: the Ag/AgCl wire was in an aqueous 3 N NaCl solution separated from the electrolyte in the electrochemical cell by a Vycor plug. 2.3. Infrared, Raman, and Mass Spectroscopy. The gold surfaces for the IRAS measurements of the polymer films were prepared by electron beam evaporation. Silicon slides 4.0 × 2.0 cm2 were first coated with a 25-Å thick titanium adhesion layer; on top of this 2000 Å of gold was evaporated at a rate of 5 Å/s. The base pressure in the chamber was 2 × 10-9 Torr, and the pressure during evaporation was held below 2 × 10-7 Torr. Just prior to use, the strips were cleaned in SC1 solution, described above. Infrared measurements on the polymer films were done on a Bruker IFS113v Fourier transform infrared spectrometer equipped with a grazing angle of incidence reflection accessory aligned at 83°. In the spectrometer there was a mild vacuum (10 Torr). The infrared radiation was polarized parallel to the plane of incidence. The spectra were recorded by averaging 500 interferograms at 4 cm-1 resolution taken with a deuterated triglycine sulfate (DTGS) detector. Infrared measurements on the monomer liquids were done in a Bruker IFS 48 spectrometer. Droplets of the pure monomers were placed between KBr windows after a reference spectrum had been taken of the clean windows. The spectra represent an average of 100 interferograms taken in transmission mode with a DTGS detector. (35) The Chemistry of Pyrroles; Jones, R. A., Bean, G. P., Eds.; Academic Press: London, 1977; pp 210, 460-463.
Smela et al. The Raman spectroscopic measurements were done on a Bruker FRA106 unit attached to a Bruker IFS66v spectrometer. The excitation source was a Nd:YAG-laser operating at 50 or 100 mW. The spectra were recorded in 180° scattering geometry with the scattered radiation focused on a liquid nitrogen cooled D-418 S solid-state detector. For each spectrum, 200 interferograms were averaged at 4-cm-1 resolution. Mass spectra were recorded in an ultrahigh vacuum system; the chamber held a pressure less than 2 × 10-9 Torr. The 1-TEP monomer was introduced as a vapor into the chamber and the mass spectrum recorded with a Hiden HAL2/301 quadrupole gas analyzer. Partial pressures of all masses between 2 and 130 amu were recorded. 2.4. UV-Vis-NIR Spectrophotometry. UV-vis-NIR measurements were made on a Perkin-Elmer Lambda 9 spectrophotometer in transmission mode. Spectra were taken of the neat monomers placed between quartz windows. Quartz slides 20 mm in diameter and 0.5-mm thick were used as windows because of their transparency between 0.5 and 6.7 eV (wavelengths between 2500 and 185 nm). In these measurements nothing was placed in the reference path: to correct each spectrum, that of the same pair of clean quartz substrates was subtracted. To obtain extinction coefficients, monomers were dissolved in spectroscopic-grade ethanol, which imposed an upper limit of 6.2 eV. The monomer droplets typically weighed 5 mg, which meant that they experienced rapid evaporation. To obtain accurate weights, the monomers were added directly to 3 g of ethanol on a high-resolution balance, and then an additional 76 g of ethanol was added to make 100 mL. This resulted in concentrations on the order of 500 µM. The solutions were measured in quartz cuvettes with a 1-cm depth. Scans were performed after a background correction was done using two cuvettes filled with ethanol, and a cuvette with ethanol was used as a reference during the scans. Higher concentration solutions were used to probe the low-absorbance region between 3 and 5 eV. For the polymer spectra, thin layers of Cr (3-10 Å) and Au (50-85 Å) were thermally evaporated onto the quartz substrates; at this thickness the metal is still semitransparent, but it is thick enough to be sufficiently conducting for the subsequent electropolymerization. Spectra of the metallized quartz pieces were taken first and then the films were deposited and their spectra recorded. The maximum energy range that could be used reliably was between approximately 1.6 and 6.4 eV: below that, results depended on film thickness because of reflection effects, and above it, the detector sensitivity dropped sharply. Film thicknesses were determined by profilometry using a Sloan Dektak 3030, which makes measurements mechanically using a stylus.
3. Results, Monomers Although similar compounds have been synthesized as adhesion-promoting agents, their full spectral characterization, especially in the infrared, has never been presented. A number of groups have relied on the CH2 peaks for identification. However, in light of the result that the monomers can disintegrate upon adsorption to gold (presented in paper 2 of this series), it is critical to establish the integrity of the molecule in the adsorbed monolayer. For that, the spectrum of the bulk monomer must be known. 3.1. FTIR and Raman Spectra. Normalized transmission FTIR spectra of pure liquid 1- and 3-TEP droplets between KBr windows are shown in Figure 1, along with the spectra of pyrrole and N-methylpyrrole (NmP) for comparison. The Raman spectra of the four monomer droplets are shown in Figure 2. (Detailed peak assignments36-39 are available in the Supporting Infor(36) Klots, T. D.; Chirico, R. D.; Steele, W. V. Spectrochim. Acta 1994, 50A, 765. (37) Handbook of Heterocyclic Chemistry; Katritzky, A. R., Ed.; Pergamon Press: Oxford, 1985; pp 66-72.
Characterization of 1-TEP and 3-TEP
Figure 1. Transmission FTIR spectra of pure liquid droplets of pyrrole, 3-TEP, 1-TEP, and N-methylpyrrole between KBr windows. The CO2 peaks between 2250 and 2500 in the 3-TEP spectrum have been omitted for clarity.
Figure 2. Raman absorption spectra of pure liquid droplets of pyrrole, 3-TEP, 1-TEP, and N-methylpyrrole. The vibrational peaks are superimposed on a fluorescent background.
mation.) In the infrared spectra, at the higher wavenumbers the NH vibration near 3400 cm-1 is seen in 3-TEP but is of course missing from 1-TEP. This is followed by in-plane vibrations of the C-H bonds in the pyrrole ring and other peaks typical of CH and CH2. The S-H peak appears at 2550 cm-1 for both molecules. There are no other important features until about 1700 cm-1. At 1710 cm-1 3-TEP has a carbonyl peak and at 1690 cm-1 a shoulder due to hydrogen-bonded carbonyl. The instability of this compound toward oxygen is apparent. The ring modes follow between approximately 1600 and 1300 cm-1, and at lower wavenumbers, C-H deformations on the ring. For 3-TEP and pyrrole the N-H deformation is also seen as a strong peak near 550 cm-1. 3.2. UV-Vis-NIR Spectra. Whereas the FTIR measurements above gave information on the vibrational levels, optical absorption provided information on the electronic transitions. Electrons in π orbitals, such as the double bonds of the pyrrole ring, and paired nonbonding electrons, such as those on N or S, are responsible for the majority of peaks in this part of the spectrum. Thus, this technique is particularly sensitive to the pyrrole moiety. Transmission spectra of the four monomers in ethanol are shown in Figure 3. Absorbance (A ) -log(38) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: Boston, 1991; pp 301-303. (39) The Aldrich Library of FTIR Spectra, 1st ed.; Pouchert, C. J., Ed.; Aldrich Chemical Co.: Milwaukee, WI, 1985; Vol. 2, p 565.
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Figure 3. UV-vis-NIR absorbance spectra of 1-TEP (black), 3-TEP (gray), pyrrole (thin black), and N-methylpyrrole (thindotted) in ethanol. The insert shows a close-up of the weaker bands at the base of the large peak for 1-TEP and 3-TEP.
(transmission)) was converted to molar extinction coefficient, , using Lambert Beer’s law. The maximum for all four monomers was ca. 7000 L/(mol cm). This figure was used (paper 2) to establish the presence and to estimate the surface coverage of the monolayers on gold. Up to approximately 5 eV (λ ∼ 250 nm), the spectra for pyrrole and N-methylpyrrole were featureless; at higher energies there was a strong absorption peak. The absorbance for NmP began at a lower energy due to a bathochromic shift from the substituent at the 1-position.35 The TEPs showed the same large absorption peak due to the pyrrole moiety, but they also both had a smaller, broad absorption peak starting at 4 eV. The fine structure is due to vibrational sublevels. The compound CH3-CH2SH begins to absorb around 4.6 eV (λ ∼ 267 nm), and CH3-S-S-CH3 begins at 4 eV (λ ∼ 315 eV),40 so the small foot in these spectra is due to the short, thiolated alkyl chains. 3.3. Mass Spectrum of 1-TEP. The vapor of 1-TEP was introduced into the mass spectrometer chamber, and the fragment ions with the highest relative abundance are shown in Table 1. Counts were normalized to the peak at 80 m/e. Assignments are based on Katritzky37 and Jones and Bean.35 There are a substantial number of intact molecules with a molecular weight of 127. Fragmentation begins at the tail, with the SH group expelled first. This is followed by the departure of the adjacent alkyl group, leaving the immonium ion with m/e ) 80, which can rearrange as shown to the pyridinium ion. For all substituted pyrroles, this is a major peak. Interestingly, the usually observed base peak from the N-methylpyrrole cation at m/e ) 81 is approximately 15% as much as m/e ) 80. From the six-membered pyridinium ring, HCN is expelled to leave a mass of 53. Alternatively, the last alkyl group may be lost from the immonium to leave pyrrole, m/e ) 67; pyrrole then fragments into smaller pieces by cleavage of the five-membered ring and expulsion of CHtCH and CHdNH groups. The observed fragments are consistent with what has been observed previously for other 1-substituted pyrroles. These data were used (paper 2) to evaluate the temperaturestimulated desorption of the TEP monolayers from gold. (40) Pretsch, E.; Clerc, T.; Seibl, J.; Simon, W. Tables of Spectral Data for the Structure Determination of Organic Compounds; Springer-Verlag: Berlin, 1983; pp U10, U15, U50, U70, U135.
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Smela et al.
Table 1. Fragment Ions from Mass Spectrometry of 1-TEP Vapor
4. Results, Polymers 4.1. Electrochemical Deposition of P(1-TEP) and P(3-TEP) in Propylene Carbonate. If the monomers are to function as an adhesion promotor when applied as monolayers on gold, then the pyrrole moieties must be able to polymerize with pyrrole in solution. In addition, it has been reported that adsorbed monolayers of similar compounds can be electrochemically polymerized.17 To ascertain whether the pyrrole moieties were able to polymerize, and to determine the potentials required, the electropolymerization of the bulk monomers was investigated. Most substituted pyrrole monomers can be electrochemically polymerized. Poly(N-methylpyrrole) resembles PPy closely, with similar density, appearance, and oxidation level, but if the N-substituent is larger, the quality, conductivity, and doping level decrease.41 Film properties are less sensitive to substitutions at the 3-position42 unless they are bulky.43 The main differences are that substituted polymers have lower conductivity than unsubstituted PPy and the oxidation potentials are shifted anodically.43-47 We deposited films on gold-covered silicon surfaces potentiodynamically and potentiostatically in several electrolytes (see also Supporting Information). Multiple potential sweeps at 50 mV/s are shown in Figure 4. Monomer concentrations were 0.1 M in propylene carbonate (PC) with 0.1 M LiClO4 as the supporting electrolyte. Polypyrrole and P(NmP) films were produced under identical conditions as controls and for comparison. The upper limits were chosen to give comparable peak (41) Diaz, A. F.; Castillo, J.; Kanazawa, K. K.; Logan, J. A.; Salmon, M.; Fajardo, O. J. Electroanal. Chem. 1982, 133, 233. (42) Merz, A.; Schwarz, R.; Schropp, R. Adv. Mater. 1992, 4, 409. (43) Street, G. B. In Handbook of Conducting Polymers; Skotheim, T. A., Ed.; Marcel Dekker: New York, 1986; Vol. 1; pp 265-291. (44) Diaz, A. F.; Bargon, J. In Handbook of Conducting Polymers; Skotheim, T. A., Ed.; Marcel Dekker: New York, 1986; Vol. 1; pp 81115. (45) Kaneko, T.; Matsui, N.; Kamiyama, M.; Yagihara, T. Synth. Met. 1993, 55-57, 1091. (46) Martina, S.; Enkelmann, V.; Schlu¨ter, A.-D.; Wegner, G.; Zotti, G.; Zerbi, G. Synth. Met. 1993, 55-57, 1096. (47) Schneider, O.; Schwitzgebel, G. Synth. Met. 1993, 55-57, 1406.
Figure 4. Potentiodynamic polymerization with 0.1 M monomer in PC/LiClO4 at 50 mV/s. For P(1-TEP) and P(3-TEP), the potential was swept between -0.8 and 1.2 V vs Ag/AgCl, for PPy and P(NmP) between -0.8 and 0.9 V. Every third scan of the first 10 is displayed, with the first shown as a dashed line.
polymerization currents; the lower limit did not affect the polymerization process and was chosen to be the same for all four. Whereas the current due to oxidation/reduction (redox) processes in PPy and P(NmP) increased strongly with each cycle and films grew quickly, the redox peaks in the P(TEP)s were significantly smaller. There are two reasons for this difference: high solubility of the TEP oligomers in the electrolyte and low conductivity of the TEP polymers. During oxidation, brown clouds formed at the working electrode surface that sank to the bottom of the electrochemical cell. For P(1-TEP) the peak polymerization current was constant or increased slightly during polymerization, as was also the case for PPy and P(NmP), and after polymerization the electrode was covered with a thin transparent brown film. Material was deposited steadily and films several thousand angstroms thick could be produced. However, P(3-TEP) was difficult to deposit, with the current at 1.2 V falling with each cycle. Lowering the potential to prevent possible overoxidation48 reduced the rate at which the current fell, but failed to stop it and also reduced the rate at which material was deposited. Raising the potential only caused the current to fall faster, without a significant increase in the size of the redox peaks. Moreover, after a number of scans, ca. 10 at 50 mV/s, the redox peaks not only stopped growing, but actually started decreasing in size. The first cycle for the monomers was distinct. Both 1-TEP and 3-TEP had a large oxidation peak before polymerization started. This feature is related to the adsorbed monomer layer and is discussed in detail in paper 3 of this series. Films were also deposited potentiostatically and in other electrolytes, including aqueous dodecylbenzenesulfonate, sodium salt. For these results and a further discussion (48) Ru¨he, J.; Ezquerra, T. A.; Wegner, G. Synth. Met. 1989, 28, C177.
Characterization of 1-TEP and 3-TEP
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Figure 6. UV-vis-NIR spectra of P(1-TEP) (black line), P(3TEP) (gray), P(NmP) (thin-dotted), and PPy (thin black) normalized to absorbance per 1000 Å.
Figure 5. IRAS spectra of thin films of P(3-TEP), P(1-TEP), P(NmP), and PPy polymerized potentiostatically in PC/LiClO4. The dashed lines show the positions of ClO4 peaks.
of the potentiodynamic growth in PC/LiClO4, please see the Supporting Information. 4.2. IRAS Spectra of P(TEP) Films. It has been reported that a monolayer of pyrrolylalkanethiol polymerizes upon electrochemical oxidation.17,20 To test whether this occurs in monolayers of 1- and 3-TEP (paper 3), we needed to know the spectral data for the polymers. Thin brown films approximately 1000-Å thick of P(1TEP), P(3-TEP), P(NmP), and PPy were prepared potentiostatically for IRAS and Raman spectroscopy. The films were deposited in PC/LiClO4 with monomer concentrations of approximately 20 mM and potentials of 0.7 V for PPy, 1.0 V for P(NmP), 1.2 V (to 0.07 C) followed by 1.0 V for P(1-TEP), and 1.2 V for P(3-TEP). Film deposition was halted when 0.1 C of charge was consumed, with the exception of the P(1-TEP) sample which was taken to 0.2 C because of the great number of soluble oligomers produced. In addition, a second P(1-TEP) sample was grown potentiodynamically between 0 and 1.2 V at 200 mV/s to 0.1 C. The IRAS spectra, normalized so that the peaks are approximately the same height, are shown in Figure 5. The film grown potentiodynamically gave the same results as the one grown at constant voltage, so that curve was omitted. Although it is not possible to see in the normalized figures, the TEP polymers were weaker infrared absorbers than the PPy by a factor of approximately 2, even though they had almost the same thickness; they were so weak as Raman scatterers that it was impossible to obtain good spectra, and these data are not presented. The spectrum of PPy was typical for ClO4-doped material,43,49,50 and the spectra of the other three polymers were quite similar. Therefore, the TEP deposits formed on the working electrode are polymers that resemble PPy. However, the absorption tail at higher wavenumbers seen in PPy, which shows that the material is conducting, is less pronounced in poly(N-methylpyrrole) and is entirely missing in the P(TEP)s. This is consistent with the electrochemical data. In addition, the N-H, C-H, and CH2 stretches between 2850 and 3500 cm-1, usually not seen in PPy, are clearly visible in the 3-TEP spectrum. (49) Gustafsson, G.; Lundstro¨m, I.; Liedberg, B.; Wu, C. R.; Ingana¨s, O.; Wennerstro¨m, O. Synth. Met. 1989, 31, 163. (50) Lei, J.; Liang, W.; Martin, C. R. Synth. Met. 1992, 48, 301.
4.3. UV-Vis-NIR Spectra of P(TEP) Films. Polymer films were prepared potentiostatically on metallized quartz substrates in 0.1 M PC/LiClO4 with a monomer concentration of 0.1 M. Films deposited at several potentials and of various thicknesses were examined. Figure 6 shows the spectra of films deposited at potentials of 1.2 vs Ag/AgCl for 1-TEP and 3-TEP, and 0.8 V for pyrrole and NmP; these results were typical. The spectra in the figure were normalized by film thickness after the spectra of the gold-coated substrates were subtracted. The absorbance of all four polymers increased steadily with energy. The spectra for P(1-TEP) and P(NmP) were quite similar. After the minima near 2 eV they both peaked at approximately 2.8 eV and again at 4.3 eV. The PPy spectrum had a minimum at 2.3 eV and a peak maximum at 2.8 eV. The P(3-TEP) spectra were the most complex, with peaks at 2.5, 3.3, and 4.7 eV. However, because of their insulating nature these films were thin, and the gold layer had an absorption peak at 4.35 eV, so the position of the last peak might not be accurate. Another point to note is that the quartz/Au/P(TEP) combination was actually less absorbing than quartz/Au at low energy, giving negative absorbance values. The polymer and monomer spectra are easy to distinguish, which will be valuable in the later papers in this series. 5. Conclusions Detailed FTIR, Raman, UV-vis-NIR, and mass spectroscopy measurements on thiol-substituted pyrroles, and the corresponding polymers, have been presented for the first time. These data are critical for the correct interpretation of the results obtained in the subsequent papers, forming a basis for evaluating the monolayers adsorbed on gold surfaces. The potential use of these monomers for surface modification is the subject of the remaining three papers in this series. Acknowledgment. We would like to 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. Supporting Information Available: Detailed description and schemes of the synthesis, table of infrared and Raman peaks and their assignments, and additional electropolymerization results, including potentiostatic polymerization and polymerization in other electrolytes (11 pages). See any current masthead page for ordering information and Internet access instructions. LA9708602
