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Langmuir 1999, 15, 6582-6586
Notes Electrochemical and Spectroscopic Characterization of Self-Assembled Monolayers of Unsymmetrical Ferrocenyl Dialkyl Sulfide Derivatives on Gold Montray C. Leavy, Sukanta Bhattacharyya, W. E. Cleland, Jr., and Charles L. Hussey* Department of Chemistry, University of Mississippi, University, Mississippi 38677 Received November 24, 1998. In Final Form: May 18, 1999
Introduction The formation of self-assembled monolayers (SAMs) of organosulfur compounds on metal surfaces has been a topic of great interest over the past decade. The majority of these investigations have been concerned with the self-assembly of various alkanethiols and dialkyl disulfides on vapor-deposited gold surfaces. It is generally accepted that the thiolate monolayers resulting from the selfassembly of these two classes of organosulfur compounds are indistinguishable.1-4 The identity of the surface-bound species that result from the self-assembly of dialkyl sulfide compounds is less clear and is the subject of controversy. This controversy stems from a 1994 report by Porter and Zhong,5 who claimed that the adsorption of aryl and alkyl sulfides on gold results in the cleavage of one of the S-C bonds with the formation of surface-bound species similar to those resulting from the adsorption of alkanethiols and dialkyl sulfides. Previous to this claim, it was reported that monolayers resulting from the adsorption of unsymmetrical dialkyl sulfides on gold are less densely packed and more poorly ordered than films originating from the adsorption of alkanethiols.6 More recently, Beulen et al.7 examined the adsorption of didecyl sulfide on gold by using TOF-SIMS and found evidence that this compound selfassembles without S-C bond cleavage. Grunze and coworkers8 used voltammetry and XPS to study the surfacebound species resulting from the adsorption of decanethiol, didecyl disulfide, and didecyl sulfide. They found that the films resulting from the adsorption of the latter compound were significantly different from those produced by adsorption of the corresponding alkanethiol and dialkyl disulfide, whereas S-C bond cleavage should result in films that are very similar. In this note we describe a unique electrochemical approach for investigating the adsorption of dialkyl (1) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723. (2) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 1766. (3) Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M. Langmuir 1994, 10, 1825. (4) He, Z.; Bhattacharyya, S.; Cleland, W. E., Jr.; Hussey, C. L. J. Electroanal. Chem. 1995, 397, 305. (5) Zhong, C.-J.; Porter, M. D. J. Am. Chem. Soc. 1994, 116, 11616. (6) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (7) Beulen, M. W. J.; Huisman, B.-H.; van der Heijden, P. A.; van Veggel, F. C. J. M.; Simons, M. G.; Biemond, E. M. E. F.; de Lange, P. J.; Reinhoudt, D. N. Langmuir 1996, 12, 6170. (8) Jung, C.; Dannenberger, O.; Xu, Y.; Buck, M.; Grunze, M. Langmuir 1998, 14, 1103.
sulfides on gold. Ferrocenyl dialkyl sulfides were synthesized so that one alkyl chain was terminated with a ferrocene group while the other was terminated with a ferrocenylcarbonyl group (Figure 1). The electron-withdrawing properties of the carbonyl moiety result in a positive shift in the half-wave potential of the latter redox centers relative to that of the terminal ferrocene group on the other alkyl chain of the dialkyl sulfide. Thus, the different ferrocenyl terminations serve as both electrochemical and spectroscopic markers that can be used to probe the mechanism associated with the adsorption of dialkyl sulfides on gold. Experimental Section Chemicals. Methylene chloride (CH2Cl2) (Optima Grade), perchloric acid (70%, HClO4), sulfuric acid (98%, H2SO4), hydrogen peroxide (30%, H2O2), and 2-propanol (Optima Grade) were purchased from Fisher Scientific and used as received. Acetonitrile (CH3CN) was purchased from Aldrich Chemical Co., dried by refluxing over CaH2 for at least 48 h, and then freshly distilled prior to use. Synthesis of Compounds Used To Prepare SAMs of 1-3. The preparation of compounds 1-3 involves a general one-pot synthetic approach typically used to prepare unsymmetrical dialkyl sulfides.9 The general synthetic sequence is outlined in Scheme 1. Reactions of ω-bromoalkanoylferrocenes with thiourea in absolute ethanol (1 mol equiv, 80 °C, 8 h) produced the corresponding isothiuronium salts, which on treatment with sodium ethoxide (2 mol equiv, 80 °C, 4 h) under an atmosphere of nitrogen afforded the corresponding thiolates. In situ trapping of these thiolates with ω-bromoalkylferrocenes (1 mol equiv, 80 °C, 4 h) furnished the dialkyl sulfides 1-3. The crude products were carefully purified by column chromatography on silica gel (eluent: hexanes-ethyl acetate, v/v ) 96:4) to afford analytically pure sulfides in 50-58% yields, which were characterized by spectral and elemental analysis. Compound 1: mp 71-72 °C; 1H NMR (300 MHz, CDCl3) δ [ppm] 4.77 (t, J ) 1.8 Hz, 2H), 4.47 (t, J ) 1.8 Hz, 2H), 4.18 (s, 5H), 4.08 (s, 5H), 4.03 (dd, J ) 1.7 and 1.4 Hz, 4H), 2.68 (t, J ) 7.3 Hz, 2H, FcCOCH2), 2.49 (t, J ) 7.4 Hz, 4H, H2CSCH2), 2.30 (t, J ) 7.4 Hz, 2H, FcCH2), 1.70 (t, J ) 7.3 Hz, 2H), 1.62-1.26 (m, 26 H); 13C NMR (75.5 MHz, CDCl3) δ [ppm] 204.4, 89.5, 79.2, 72.1, 69.7, 69.3, 68.4, 68.0, 66.9, 39.6, 32.2, 32.1, 31.0, 29.7, 29.6, 29.5, 29.4, 29.3, 29.2, 29.1, 28.9, 28.7, 24.5. Anal. Calcd for C39H54OSFe2: C, 68.62; H, 7.97; S, 4.70. Found: C, 68.47; H, 8.13; S, 4.51. Compound 2: mp 68-69 °C; 1H NMR (300 MHz, CDCl3) δ [ppm] 4.76 (t, J ) 1.8 Hz, 2H), 4.47 (t, J ) 1.8 Hz, 2H), 4.17 (s, 5H), 4.07 (s, 5H), 4.02 (dd, J ) 1.7 and 1.4 Hz, 4H), 2.67 (t, J ) 7.3 Hz, 2H, FcCOCH2), 2.48 (t, J ) 7.4 Hz, 4H, H2CSCH2), 2.30 (t, J ) 7.3 Hz, 2H, FcCH2), 1.69 (t, J ) 7.3 Hz, 2H), 1.61-1.25 (m, 26 H); 13C NMR (75.5 MHz, CDCl3) δ [ppm] 204.5, 89.3, 79.1, 71.9, 69.6, 69.2, 68.3, 67.9, 66.8, 39.6, 32.0, 30.9, 29.6, 29.5, 29.4, 29.3, 29.2, 29.1, 29.0, 28.4, 24.4. Anal. Calcd for C39H54OSFe2: C, 68.62; H, 7.97; S, 4.70. Found: C, 68.39; H, 8.23; S, 4.79. Compound 3: mp 56-57 °C; 1H NMR (300 MHz, CDCl3) δ [ppm] 4.77 (t, J ) 1.7 Hz, 2H), 4.48 (t, J ) 1.7 Hz, 2H), 4.19 (s, 5H), 4.08 (s, 5H), 4.03 (dd, J ) 1.6 and 1.4 Hz, 4H), 2.69 (t, J ) 7.3 Hz, 2H, FcCOCH2), 2.50 (t, J ) 7.4 Hz, 4H, H2CSCH2), 2.30 (t, J ) 7.5 Hz, 2H, FcCH2), 1.71 (t, J ) 7.4 Hz, 2H), 1.65-1.26 (m, 20 H); 13C NMR (75.5 MHz, CDCl3) δ [ppm] 204.6, 89.5, 79.1, 72.1, 69.7, 69.3, 68.4, 68.0, 66.9, 39.7, 32.2, 32.1, 30.7, 29.7, 29.5, 29.4, 29.3, 29.2, 29.1, 29.0, 28.9, 28.8, 28.7, 24.5. Anal. Calcd for (9) Luzzio, F. A. Synth. Commun. 1984, 14, 209.
10.1021/la981643j CCC: $18.00 © 1999 American Chemical Society Published on Web 07/07/1999
Notes
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Figure 1. Structures of unsymmetrical sulfides used to prepare SAMs on Au(111). Scheme 1
C36H48OSFe2: C, 67.51; H, 7.55; S, 5.01. Found: C, 67.79; H, 7.43; S, 4.84. The ω-bromoalkanoylferrocene starting materials were prepared by Friedel-Crafts monoacylation of ferrocene with the appropriate ω-bromoalkanoyl chloride in the presence of anhydrous aluminum chloride. The ω-bromoalkylferrocenes that were used to trap the thiolates (Scheme 1, step 3) were obtained in high yields by reductive deoxygenation of the appropriate acylated ferrocenes with ZnCl2 and Zn(BH4)2.10 Preparation of Gold Substrates and SAMs. Glass microscope slides (Fisher Scientific) served as substrates for the monolayers. They were cleaned by ultrasonication in successive baths of “piranha” solution (1:3 by volume 30% H2O2/concentrated H2SO4), deionized water, and 2-propanol, dried in an oven at 125 °C, and coated with ∼50-55 Å of a chromium (Alfa ÆSAR, 99.99%) adhesion layer at a deposition rate of 0.1-0.4 Å s-1 by thermal evaporation. The substrates were then coated with 14001550 Å of gold (Alfa ÆSAR, Premion, 99.9985%) at a deposition rate of 5-6 Å s-1 by using an Edwards Auto 306 vacuum chamber equipped with a Sycon Model STM-100/MF film thickness monitor. The deposition was carried out at a base pressure of 5 × 10-7 Torr. After vacuum deposition was completed, the vacuum chamber was backfilled with high-purity nitrogen gas. Gold films prepared by this method are known to exhibit strong Au(111) characteristics.4 The freshly coated slides were immediately immersed in a 1 mM methylene chloride solution of the appropriate sulfide for at least 24 h. After immersion in the sulfide solutions, the modified gold electrodes were rinsed with copious quantities of methylene chloride and deionized water. Electrochemical Measurements. All voltammetric measurements were carried out in a three-electrode cell using an EG&G Princeton Applied Research Corporation (PARC) Model 283 potentiostat/galvanostat employing PARC Model 270/250 Research Electrochemistry Analysis Software (v.4.23), running on an IBM-compatible Pentium computer. The gold-coated substrates, when clamped against the PTFE O-ring in an O-ring joint on the side of the electrochemical cell, served as the working (10) Bhattacharyya, S. Organometallics 1996, 15, 1065.
Figure 2. Representative cyclic voltammograms of SAMs prepared from compounds 1-3 recorded on gold electrodes: (a) compound 1; (b) compound 2; (c) compound 3. The experiments were carried out in 1.0 M HClO4 at ν ) 100 mV s-1. electrode. The O-ring provided a liquid-tight seal and defined the area of the working electrode, which was estimated to be about 1.54 cm2. The reference electrode was a saturated calomel electrode (SCE) placed in a Luggin capillary. All of the potentials reported in this article were measured with respect to this reference electrode. The counter electrode was a platinum wire spiral. All experiments were conducted in triply distilled water with HClO4 as the supporting electrolyte. Measurements were carried out at room temperature (ca. 24 °C), and the solution in the electrochemical cell was deaerated with high-purity nitrogen gas before each experiment. Spectroscopic Characterization of the SAMs. Infrared spectra were obtained using a Bruker Model IFS66 FTIR spectrometer equipped with a liquid N2-cooled MCT (Hg-CdTe) detector. A Plexiglass glovebox covered the spectrometer sample compartment. The glovebox and spectrometer were purged with high-purity N2 gas from the bleed-off of a liquid N2 tank. All spectra were acquired at 2 cm-1 resolution using a zero filling factor of 2. The spectra were obtained by employing 256 signal-averaged scans using p-polarized light incident at a grazing angle of 86° from the surface normal. XPS Characterization of the SAMs. High-resolution XPS spectra were obtained using a PHI 5600 MultiTechnique System Model 10-360 spherical capacitor electron analyzer (SCEA). The binding energies (BEs) were referenced to Au 4f7/2 at 84.0 eV. A pass energy of 25 eV was employed during the acquisition of the high-resolution XPS spectra with the sample tilted 30° off the axis of the SCEA.
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Notes
Table 1. Electrochemical Data for Self-Assembled Monolayers of Compounds 1-3 on Gold monolayer
∆Epa (mV)
∆Efwhmb (mV)
E°′ (V)
1010Γ (mol cm-2)
1 [Fc(CH2)11-] 1 [FcCO(CH2)7-] 2 [Fc(CH2)8-)] 2 [FcCO(CH2)10-] 3 [Fc(CH2)8-)] 3 [FcCO(CH2)7-] 11-ferrocenylundecyl disulfide bis[10-(ferrocenylcarbonyl)decyl] disulfide
16 29 6 25 24 18 30 22
180 133 153 153 120 127 74 98
0.175 ( 0.002 0.513 ( 0.003 0.203 ( 0.005 0.478 ( 0.003 0.257 ( 0.002 0.512 ( 0.001 0.284 ( 0.004 0.511 ( 0.002
1.8 ( 0.1 1.9 ( 0.1 1.3 ( 0.1 1.3 ( 0.1 1.4 ( 0.1 1.1 ( 0.1 4.7 ( 0.4 4.1 ( 0.1
a
Ideal: ∆Ep ) 0 mV. b Ideal: ∆Efwhm ) 90.3 mV. Note: statistical analysis based on 95% CI and a sample population of N ) 10-15.
Figure 3. Infrared spectra of self-assembled monolayers of organosulfur compounds in the 1300-3500 cm-1 region on gold: (a) compound 1; (b) compound 2; (c) compound 3; (d) bis[10-(ferrocenylcarbonyl)decyl] disulfide. Note the absence of an obvious CdO band around 1668 cm-1 in the spectra for compounds 1-3.
Results and Discussion Electrochemical Studies. Cyclic voltammetry is a powerful electrochemical technique for obtaining information about the chemical stability, electron transfer kinetics, and thermodynamics of surface-bound redox couples. In most cases, this information is readily acquired by observing the evolution of the shapes and positions of voltammetric waves as the potential scan rate is changed. Parts a-c of Figure 2 show representative cyclic voltammograms (CVs) of gold electrodes modified with compounds 1-3, respectively. Each of these voltammograms was recorded in 1.0 M aqueous HClO4. Two pairs of waves are evident in each voltammogram, signaling the presence of two independent redox centers. The peak oxidation current for both waves in each voltammogram varied linearly with changes in the scan rate, as expected for surface-bound species. The pair of waves appearing at less positive potentials in each of the voltammograms in Figure 2 must correspond to the oxidation and reduction
of the ferrocene group bonded directly to the alkyl chain, whereas the waves appearing at more positive potentials must arise from the ferrocene group adjacent to the electron-withdrawing carbonyl group. The formal potentials E°′ of monolayers prepared from compounds 1-3 are collected in Table 1. Also given in this table are E°′ values for monolayers prepared from 11-ferrocenylundecyl disulfide and bis[10-(ferrocenylcarbonyl)decyl] disulfide that illustrate the positive shift in the oxidation potential of the ferrocene redox center induced by an adjacent carbonyl group. Repetitive scanning for ∼50 cycles in and prolonged exposure to 1.0 M HClO4 did not lead to any observable changes in the voltammograms recorded for gold electrodes modified with compounds 1-3, demonstrating the very robust nature of these monolayers. The surface coverages Γ of monolayers prepared from compounds 1-3 were estimated from the charges corresponding to the ferrocene voltammetric oxidation waves and are shown in Table 1. The surface coverages given in
Notes
this table for compounds 1-3 are the average values from experiments conducted with a minimum of 10 individual electrodes. Note that there are two values of Γ for each sulfide monolayer because each of the oxidation waves (cf. Figure 2) was treated independently. In every case, the average surface coverages calculated from the two oxidation waves in each voltammogram are nearly identical within experimental error. The existence of two oxidation waves with comparable charges demonstrates that no S-C bond cleavage occurred during the chemisorption of these sulfide compounds on gold. The surface coverages given in Table 1 also give insight into the packing associated with each of the sulfide films. Clearly, monolayers prepared from compounds 1-3 exhibit much smaller surface coverages than those of films prepared from 11-ferrocenylundecyl disulfide or bis[10(ferrocenylcarbonyl)decyl] disulfide. These smaller surface coverages undoubtedly arise from the steric hindrance associated with the duplex chain of the surface-bound sulfides. This steric hindrance prevents the sulfides from obtaining the (x3 × x3)R30° superlattice that results from the chemisorption of alkanethiols and dialkyl disulfides on a gold (111) surface. The smaller values of Γ further emphasize that monolayers resulting from the chemisorption of sulfides are considerably different from those prepared by the chemisorption of disulfides and thiols. A stochastic process involving cleavage of one of the S-C bonds during the adsorption of compounds 1-3 could possibly lead to a random distribution of surfacebound species with ferrocenyl and ferrocenylcarbonyl terminations, but the surface coverages should be approximately the same as those for films prepared from 11-ferrocenylundecyl disulfide or bis[10-(ferrocenylcarbonyl)decyl] disulfide. Experimental values of the peak full-width at halfmaximum ∆Efwhm of the voltammetric waves for the oxidation of the ferrocene redox centers on each of the sulfide chains are also given in Table 1. This parameter gives a qualitative measure of the relative interactions taking place among redox centers in surface-bound species. For an ideal monolayer, ∆Efwhm ) 3.53RT/nF (90.3/n mV) is predicted at the temperature of this investigation (24 °C). However, as a general rule, the experimental values of ∆Efwhm given in Table 1 for the ferrocenylalkyl and ferrocenylcarbonylalkyl chains of the surface-bound sulfides greatly exceed this value. Large values of ∆Efwhm are usually attributed to multiple formal potentials arising from an ensemble of redox centers in different environments with a negligible rate of conversion between the environments.11 FT-IRRAS Studies. FT-IRRAS spectra for compounds 1-3 on gold substrates are shown in Figure 3. It is wellknown that the peak positions of the asymmetric and symmetric methylene stretching modes, νa(CH2) and νs(CH2), respectively, can provide information about the degree of disorder experienced by the alkyl chains in SAMs. For example, a classical study by Porter et al.12 demonstrated that νa(CH2) and νs(CH2) were approximately 4-6 cm-1 higher for polymethylene chains of n-alkanethiols in the liquid state (2924 and 2855 cm-1, respectively) compared to such chains in the crystalline state (2918 and 2851 cm-1, respectively) and that νa(CH2) and νs(CH2) for alkanethiolates on gold were very close to the values observed for the crystalline alkanethiols. In a related (11) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcell Dekker: New York, 1996; Vol. 19 and references therein. (12) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559.
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Figure 4. XPS spectra of self-assembled monolayers of organosulfur compounds in the O(1s) region: (a) bis[10(ferrocenylcarbonyl)decyl] disulfide; (b) compound 1.
investigation, νa(CH2) and νs(CH2) were found to be greater for ferrocenylcarboxylalkanethiolate monolayers (29232927 and 2853-2856 cm-1, respectively) than for simple alkanethiolate monolayers as a consequence of the structural disorder introduced by the ferrocene tail group.13 The average values of νa(CH2) and νs(CH2) measured for surface-bound sulfides prepared from compounds 1-3, 2927 and 2856 cm-1, respectively, suggest that these sulfide monolayers exist in a liquid-like state. Thus, our results are in good accord with those of Troughton et al.,6 who reported that disordered, liquid-like monolayers resulted from the chemisorption of unsymmetrical dialkyl sulfides on gold. This is to be expected in the present case because both the ferrocene tail groups and the duplex polymethylene chain of the sulfide promote disorder in the monolayer. The FT-IRRAS spectrum of a monolayer prepared from bis[10-(ferrocenylcarbonyl)decyl] disulfide (Figure 3d) indicates that the ν(CdO) band of the ferrocenylcarbonyl group terminating one end of the duplex polymethylene chain of the surface-bound sulfides should be observed in the proximity of 1668 cm-1. Although the sulfide spectra in Figure 3a-c clearly show the νa(CH2) and νs(CH2) bands, there is no obvious ν(CdO) band. This phenomenon has been observed for other films prepared from unsymmetrical sulfides containing carboxyl groups. For example, Troughton et al.6 were unable to detect the ν(CdO) band in surface-bound CH3(CH2)15S(CH2)10CO2H. They reconciled this result by invoking the surface selection rule, which states that only those vibrational modes that have a dipole change perpendicular to the substrate surface are active in the infrared region of the spectrum.14,15 However, they were able to detect a signal for the CdO group in the O(1s) XPS spectrum of this monolayer. (13) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687. (14) Pearce, H. A.; Cheppard, N. Surf. Sci. 1976, 59, 205. (15) Porter, M. D. Anal. Chem. 1988, 60, 1143A.
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Because the electrochemical results presented above provide unequivocal evidence that the carbonyl moiety is retained in the surface-bound sulfide, the inability to detect the ν(CdO) band in monolayers prepared from compounds 1-3 suggests that the carbonyl groups may be oriented parallel to the gold electrode surface. XPS Studies. The O(1s) regions of surface-bound bis[10-(ferrocenylcarbonyl)decyl] disulfide and sulfide compound 1 were investigated to verify the presence of carbonyl groups in these monolayers. The presence of carbonyl oxygen would indicate that the ferrocenylcarbonyl group is retained by the surface-bound species. The O(1s) BE range extends from 525 to 542 eV with the BE for CdO normally appearing at around 532-533 eV. Figure 4 shows XPS spectra recorded in the O(1s) region for the disulfide and compound 1. These spectra show strong signals at 532.05 and 532.15 eV, respectively, providing strong evidence that the ferrocenylcarbonyl groups remain intact in the monolayer films. These results support the supposition that the CdO groups in the sulfide monolayers are oriented parallel to the electrode surface. The S(2p) regions of the surface-bound bis[10-(ferrocenylcarbonyl)decyl] disulfide and sulfide compound 1 were investigated to explore the nature of the gold-sulfur bonds in these two different monolayers. As discussed in a recent article by Porter and co-workers,16 the S(2p3/2) BE for gold thiolates that result from the adsorption of thiol and disulfide compounds is normally located at 162.0 ( 0.2 eV. The S(2p3/2) BE for a monolayer prepared from (16) Zhong, C.-J.; Brush, R. C.; Anderegg, J.; Porter, M. D. Langmuir 1999, 15, 518 and references therein.
Notes
the disulfide compound was observed at 162.18 eV, in excellent agreement with this value. However, for monolayers prepared from the sulfide, the S(2p3/2) BE was found at 163.2 eV and is very close to the 163.4 eV BE observed for monolayers prepared from phenyl ethyl sulfide, which was recently determined to not undergo carbon-sulfur bond cleavage.16 In summary, the electrochemical, infrared spectroscopic, and XPS data that were obtained during this investigation strongly suggest that compounds 1-3 self-assemble on gold surfaces without S-C bond cleavage. The surface coverages of these compounds were substantially less than those for monolayers prepared from the related ferroceneterminated disulfides such as 11-ferrocenylundecyl disulfide and bis[10-(ferrocenylcarbonyl)decyl] disulfide. Analysis of the positions of the asymmetric and symmetric CH2 stretches indicated that the surface-bound sulfides exist in a disordered, liquid-like state. The absence of a detectable band for the CdO stretch in the infrared spectrum of these compounds and the obvious XPS evidence for this moiety in the monolayers suggest that the ferrocenylcarbonyl chain of these sulfides is probably oriented parallel to the electrode surface. In addition, XPS spectra indicate that the gold-sulfur bonds associated with monolayers prepared from sulfides and disulfides are considerably different. Acknowledgment. This research was supported by the Mississippi NSF EPSCoR Project (Grant No. RII-8902064). LA981643J
