Langmuir 1997, 13, 3157-3161
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Redox-Induced Orientation Change of a Self-Assembled Monolayer of 11-Ferrocenyl-1-undecanethiol on a Gold Electrode Studied by in Situ FT-IRRAS Shen Ye, Yukari Sato,† and Kohei Uosaki* Physical Chemistry Laboratory, Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060, Japan, and National Institute for Bioscience and Human Technology, 1-1 Higashi, Tsukuba, Ibaraki 305, Japan Received January 13, 1997X The potential dependent structure change of a self-assembled monolayer of 11-ferrocenyl-1-undecanethiol (FcC11SH) on a gold electrode surface during the redox reaction of the terminal ferrocene group was investigated in 0.1 M HClO4 solution by electrochemical in situ Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS). A number of bands were observed in the 3200-1200 cm-1 region with p-polarization measurement, but no band was observed by s-polarization measurement when the potential was kept more negative than +1.2 V. The intensity of these bands corresponded well to the degree of oxidation of the terminal ferrocene group in the monolayer. The potential dependent IRRAS behavior can be explained by considering an orientation change of the monolayer induced by the redox reaction of the terminal ferrocene moiety in the monolayer.
Introduction Self-assembled monolayers containing electrochemically or photochemically active functional groups have been widely used in studies of the intelligent modification of solid surface.1-20 Information about the structure and orientation of the monolayer is essential to develop new types of functional monolayers and to understand the fundamental behavior of electron transfer at interfaces. Thus, the structures of the monolayers exposed to air have been investigated by many research groups.21-38 Studies † X
National Institute for Bioscience and Human Technology. Abstract published in Advance ACS Abstracts, May 15, 1997.
(1) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87 and references therein. (2) Ulman, A. An introduction of ultra-thin organic films from Langmuir-Blodgett to self-assembly; Academic Press: San Diego, 1991. (3) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Chem. 1992, 43, 437. (4) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (5) Walczak, M. M.; Popenoe, D. D.; Denhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687. (6) Chidsey, C. D. E. Science 1991, 251, 919. (7) Long, H. C. D.; Donohue, J. J.; Buttry, D. A. Langmuir 1991, 7, 2196. (8) Long, H. C. D.; Buttry, D. A. Langmuir 1992, 8, 2491. (9) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510. (10) Uosaki, K.; Sato, Y.; Kita, H. Electrochim. Acta 1991, 36, 1799. (11) Shimazu, K.; Yagi, Y.; Sato, Y.; Uosaki, K. Langmuir 1992, 8, 1385. (12) Kondo, T.; Takechi, M.; Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1995, 381, 203. (13) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173. (14) Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1994, 10, 4380. (15) Bravo, B. G.; Mebrahtu, T.; Soriaga, M. P.; Zapien, D. C.; Hubbard, A. T.; Stickney, J. L. Langmuir 1987, 3, 595. (16) Hickman, J. J.; Offer, D.; Laibinis, P. E.; Whitesides, G. M.; Wrighton, M. S. Science 1991, 252, 688. (17) Zhang, L.; Lu, T.; Gokel, G. W.; Kaifer, A. E. Langmuir 1993, 9, 786. (18) Herr, B. R.; Mirkin, C. A. J. Am. Chem. Soc. 1994, 116, 1157. (19) Yu, H. Z.; Wang, Y. Q.; Cheng, J. Z.; Zhao, J. W.; Inokuchi, H.; Fujishima, A.; Liu, Z. F. J. Electroanal. Chem. 1995, 395, 327. (20) Wang, Y. Q.; Yu, H. Z.; Cheng, J. Z.; Zhao, J. W.; Liu, Z. F. Langmuir 1996, 12, 5466. (21) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52. (22) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (23) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (24) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2370.
S0743-7463(97)00043-7 CCC: $14.00
on the structure of the monolayers in solution are, however, rather limited, although many research groups are interested in the electrochemical characteristics of selfassembled monolayers. The structures of monolayers on electrode surfaces in electrolyte solutions should be quite different from those in air because of the interaction between the adsorbed molecules and species in solution, i.e., solvent molecules and ions. Furthermore, the structure may be affected by applied potential. In situ infrared reflection absorption spectroscopy (IRRAS) measurements have been successfully applied to the study of interfacial electrochemistry, and much valuable information about the structure of adsorbed species including ions, solvent molecules, reactants, products, and reaction intermediates has been obtained by this method.39-41 In situ IRRAS studies of selfassembled monolayers in electrolyte solutions are, however, still limited.42-46 Recently, we investigated by in situ FT-IRRAS the potential dependent structure change of a series of functional SAMs on gold electrodes, such as (25) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (26) Barner, B. J.; Corn, R. M. Langmuir 1990, 6, 1023. (27) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (28) Barner, B. J.; Green, M. J.; Saez, E. I.; Corn, R. M. Anal. Chem. 1991, 63, 55. (29) Creager, S. E.; Steiger, C. M. Langmuir 1995, 11, 1852. (30) Sinniah, K.; Cheng, J.; Terrettaz, S.; Reutt-Robey, J. E.; Miller, C. J. J. Phys. Chem. 1995, 99, 14500. (31) Sato, Y.; Frey, B. L.; Uosaki, K.; Corn, R. M. Bull. Chem. Soc. Jpn. 1994, 67, 21. (32) Frey, B. L.; Corn, R. M. Anal. Chem. 1996, 68, 3187. (33) Guyot-Sionnest, P.; Superfine, R.; Hunt, J. H.; Shen, Y. R. Chem. Phys. Lett. 1988, 144, 1. (34) Hines, M. A.; Todd, J. A.; Guyot-Sionnest, P. Langmuir 1993, 11, 493. (35) Du, Q.; Freysz, E.; Shen, Y. R. Science 1994, 264, 826. (36) Sefler, G. A.; Du, Q.; Miranda, P. B.; Shen, Y. R. Chem. Phys. Lett. 1995, 235, 347. (37) Ong, T. H.; Davies, P. B.; Bain, C. D. Langmuir 1993, 9, 1836. (38) Laibinis, P. E.; Bain, C. D.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1995, 99, 7663. (39) Beden, B.; Lamy, C. In Spectroelectrochemistry; Gale, R. J., Ed.; Plenum: New York, 1988; Chapter 5. (40) Ashley, K.; Pons, S. Chem. Rev. 1988, 88, 673. (41) Suetaka, W. Surface Infrared and Raman Spectroscopy; Plenum: New York, 1995. (42) Stole, S. M.; Porter, M. D. Langmuir 1990, 6, 1199. (43) Popenone, D. D.; Deinhammer, R. S.; Porter, M. D. Langmuir 1992, 8, 2521. (44) Anderson, M. R.; Gatin, M. Langmuir 1994, 10, 1638.
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mercaptoalkanenitrile (HSCnCN, n ) 2, 7)47 and the 2-(11mercaptoundecyl)hydroquinone monolayer (H2QC11SH).48 One of the most well studied self-assembled monolayer systems is the ferrocenylalkanethiol monolayer on a gold surface because the ferrocenyl group in the monolayer is expected to exchange electrons readily with gold and this can be considered as a model system for the formation of molecular electronic devices by self-assembly.4-6 We hve already studied the process of formation and electrochemical properties of ferrocenylalkanethiol-modified gold electrodes.9-12 Furthermore, we have investigated the order and orientation of ferrocenylalkanethiol monolayers on gold by using ex situ polarization-modulation FTIRRAS (Fourier transform infrared reflection adsorption spectroscopy) measurements31 and in situ ellipsometry.49 The first in situ IRRAS study on a ferrocene-terminated self-assembled monolayer was carried out by Popenoe et al. on an 11-mercaptoundecyl ferrocenecarboxylate (FcCOOC11SH) monolayer. We have also reported preliminary results of interfacial mass and structural changes during redox reaction of the self-assembled monolayer of 11-ferrocenyl-1-undecanethiol (FcC11SH) on a gold electrode.50 In this study, a novel FT-IRRAS/EQCM (electrochemical quartz crystal microbalance) combined system was used and suggested the possible orientation change of the monolayer during the redox reaction of the ferrocene moiety.50 In the present study, we have thoroughly investigated the potential dependence of IRRAS spectra of the FcC11SH monolayer on a gold electrode surface in a 0.1 M HClO4 solution and confirmed a redox-induced orientation change of the monolayer. The results are compared with those of a H2QC11SH monolayer which does not change orientation with redox reaction of the terminal quinone/hydroquinone group. Experimental Section Synthesis of FcC11SH. FcC11SH was synthesized following the procedure reported before.9 Chemicals used for the synthesis, i.e., 11-bromoundecanoic acid (Wako Pure Chemicals), ferrocene (Wako Pure Chemicals), AlCl3 (Merck), thiourea (Wako Pure Chemicals), and sodium hydroxide (Wako Pure Chemicals), were all reagent grade and were used as received. Thionyl chloride (Wako Pure Chemicals) was purified by distillation. The final product (FcC11SH) was characterized by 1H-NMR, IR, and mass spectra. Analysis data for FcC11SH are as follows: 1H-NMR (CDCl3) δ 4.08-4.23 (m, 9H), 2.52 (m, 2H), 2.30 (t, 2H), 1.61 (m, 2H), 1.48 (m, 2H), 1.30-1.24 (m, 15H); IR (neat) 3094, 2930, 2856, 2555, 1456, 1411, 1379, 1354, 1105 cm-1; MS m/z 372 (M+, base peak), 199, 186, 121. Electrochemical and IRRAS Measurements. The electrolyte solution was prepared by using suprapure HClO4 (Wako Pure Chemicals) and Mill-Q water. A polycrystalline gold disk (φ ) 8 mm) was used as an electrode both for electrochemical and IRRAS measurements. The electrode was polished with alumina (0.05 µm), rinsed with Mill-Q water, and electrochemically cleaned by cycling and potential between 0.05 and +1.5 V (vs RHE) in 0.1 M HClO4 with the sweep rate 0.05 V s-1 for ca. 10 min. The real surface area was determined from the charge for the reduction of the oxide layer of the Au electrode. After the surface state of the electrode was confirmed by the cyclic voltammogram, the electrode was dipped in a 1 mM FcC11SH hexane solution for 1 h to construct the monolayer. After the adsorption process, (45) Mielczarski, J. A.; Mielczarski, E.; Zachwieja, J.; Gases, J. M. Langmuir 1995, 11, 2787. (46) Bae, I. T.; Sandifer, M.; Lee, Y. W.; Tryk, D. A.; Sukenik, C. N.; Scherson, D. A. Anal. Chem. 1995, 67, 4508. (47) Sato, Y.; Ye, S.; Haba, T.; Uosaki, K. Langmuir 1996, 12, 2726. (48) Ye, S.; Yashiro, A.; Sato, Y.; Uosaki, K. J. Chem. Soc., Faraday Trans. 1996, 3813. (49) Ohtsuka, T.; Sato, Y.; Uosaki, K. Langmuir 1994, 10, 3658. (50) Shimazu, K.; Ye, S.; Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1994, 375, 409.
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Figure 1. Cyclic voltammogram of the FcC11SH-modified gold electrode in a 0.1 M HClO4 solution. Scan rate was 0.1 V s-1. the electrode was rinsed with hexane and Mill-Q water and, finally, dried with purified nitrogen. In situ FT-IRRAS measurements were performed using a BioRad FTS30 spectrometer equipped with a HgCdTe detector cooled with liquid nitrogen. A spectroelectrochemical cell was employed which allows the electrode to be pushed with a micrometer of the CaF2 infrared window without the rotation of the electrode.48 The incident angle was ca. 65° for both p- and s-polarization. The subtractively normalized interfacial Fourier transform infrared reflection-absorption spectroscopy (SNIFTIRS) method was used to improve the S/N ratio. The spectra were collected at the sample and reference potentials for 128 scans for 8 or 4 times with a resolution of 4 cm-1. Usually collection of the spectra was started 5 s after the potential was changed. The results are presented in the form of the normalized change of reflectance, i.e., ∆R/R, which is equal to (Rs - Rr)/Rs, where Rs and Rr are the reflectance at the sample and the reference potential, respectively. The upward and downward peaks in spectra mean weaker and stronger, respectively, absorption at the sample potential compared to the reference potential. A quasi-reversible hydrogen electrode was used as a reference electrode. All the potentials used in this paper are presented with respect to the reversible hydrogen electrode (RHE). A Pt wire was used as a counter electrode. A potentiostat (Hokuto Denko, HA151) and a function generator (Hokuto Denko, HBIII) were used to control the potential of the working electrode. For in situ IRRAS measurements, potential steps were provided by a personal computer (NEC, PC-8801 MH) via a 12 bit D/A converter, and the FTIR spectrometer was controlled by the same computer through a RS-232C interface. All the measurements were carried out at room temperature (23 °C) after the solution was deaerated by purging with purified nitrogen for at least 20 min.
Results and Discussion Figure 1 shows a cyclic voltammogram of the FcC11SH-modified gold electrode in 0.1 M HClO4 solution. The CV was recorded after a stable shape was reached. Oxidation and reduction peaks were observed at +0.67 and +0.64 V, corresponding to the oxidation and reduction of the terminal ferrocene group in the monolayer, respectively. The amount of adsorbed FcC11SH was estimated from the charge of the oxidation peak (45 µC/cm2) as 2.8 × 1014 molecules/cm2, which is in good agreement with the values reported before.5-7,43 Figure 2 shows in situ IRRAS spectra obtained by using p- and s-polarized infrared light in a 0.1 M HClO4 solution. The sample potential was +1.0 V, where the terminal group of the attachedFcC11SH monolayer was expected to exist as the ferricenium cation, and the reference potential was selected at +0.1 V, where the terminal group existed as ferrocene (cf. Figure 1). In the case of p-polarization (top curve), a number of well-defined bands were observed.
SAM of 11-Ferrocenyl-1-undecanethiol
Figure 2. In situ IRRAS spectra of a FcC11SH monolayer adsorbed on a gold electrode obtained by (a) p- and (b) s-polarization in a 0.1 M HClO4 solution. The sample potential was +1.0 V, and the reference potential was +0.1 V.
Figure 3. In situ IRRAS spectra of the FcC11SH monolayer adsorbed on a gold electrode at various sample potentials obtained by p-polarization in a 0.1 M HClO4 solution. The sample potentials are (a) 0.50 V, (b) 0.60 V, (c) 0.65 V, (d) 0.70 V, (e) 0.80 V, (f) 0.90 V, (g) 1.00 V, and (h) 1.10 V. The reference potential was +0.1 V.
Three downward bands at 3113, 1474, and 1419 cm-1 and two upward bands at 2923 and 2848 cm-1 were clearly observed. On the other hand, no band was observed in the spectrum of s-polarization (bottom curve). Thus, the bands observed with p-polarization in Figure 2 should be those of the attached FcC11SH monolayer. Figure 3 shows the IRRAS spectra with p-polarization in the region 3200-1200 cm-1 of the FcC11SH attached monolayer obtained at various sample potentials in 0.1 M HClO4 solution. The reference potential was +0.1 V. No band was found in the spectra when the potential was more negative than +0.5 V, where no oxidation of the ferrocene group took place. When the potential became more positive than +0.5 V, the IRRAS bands appeared as already shown in Figure 2. The band intensity increased as the electrode potential became more positive and reached a limited value around +0.8 V, where the oxidation of the terminal ferrocene group was completed. The position of these bands seemed to be independent of electrode potential. The downward band at 3113 cm-1 should be due to the C-H stretch absorption of the ferrocene ring.51,52 A similar band at 3112 cm-1 was observed by Popenoe et al.43 for (51) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to infrared and Raman spectroscopy, 3rd ed.; Academic Press: New York, 1990.
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the C-H stretch of the ferrocene ring in a FcCOOC11SH monolayer by in situ IRRAS measurements. The peak position of this band in the in situ spectrum of a FcC11SH monolayer was higher in frequency than that observed for the transmission spectrum of FcC11SH dispersed in fluorolube (3097 cm-1) and for the ex situ IRRAS spectra of a FcC11SH monolayer on gold (3100 cm-1).31 Similar trends were observed in the case of FcCOOC11SH.43 The frequency shift may reflect the difference in the environment around the ferrocene moiety. The two upward bands at 2923 and 2848 cm-1 are attributed respectively to the asymmetric and symmetric C-H stretch of the methylene group in the monolayer.51,52 The positions of these two bands are in good agreement with those observed in ex situ IRRAS spectra of the FcC11SH monolayer on gold (2923 and 2851 cm-1)31 and are close to those of crystalline samples,23,24 suggesting that an ordered monolayer was constructed in the present case. Although Popenoe et al. observed these bands at nearly the same position, the direction of the bands was the same as that of the C-H stretch of the ferrocene ring (3112 cm-1), i.e., in the reverse direction to the present result.43 Although Popenoe et al. reported two or three bands in the frequency region between 3000 and 2800 cm-1 in addition to the asymmetric and symmetric C-H stretch bands mentioned above, these additional bands were not found in the present study. The downward band at 1419 cm-1 can be assigned to the C-C stretch of the ferrocene ring.51,52 This band was not observed by Popenoe et al., although a weak band was found around 1410 cm-1 in their bulk IR transmission and ex situ IRRAS spectra.43 This difference may be caused by the different chemical structures of FcCOOC11SH and FcC11SH. The downward band at 1474 cm-1 should be due to two absorption bands, i.e., the bending mode of the methylene group and Fc-C stretch mode, which has been observed in substituted ferrocene.43,51,52 The IRRAS band intensity at 2923 cm-1 and oxidation electric charge passed were shown as a function of electrode potential in Figure 4.53 It is clear from Figure 4 that the IRRAS band intensity correlates well with the electric charge passed. Thus, it is concluded that the band intensities in the in situ IRRAS spectra reflect the degree of oxidation of the ferrocene group. As described in the Experimental Section, the upward and the downward bands in the IRRAS spectra mean weaker and stronger infrared absorption, respectively, at the sample potential than at the reference potential. Thus, the absorption due to the C-H stretch of the ferrocene ring (3113 cm-1) increased but those due to the asymmetric (νas) and symmetric (νs) C-H stretch of the methylene group decreased upon oxidation of the ferrocene group. To understand the origin of these changes, the “surface selection rule” of IRRAS measurement on a metal surface should be considered.54 As p-polarized light is used for measurement, the stronger IR absorption means the more perpendicular to the surface is the dipole moment change. From this consideration, one can conclude that the dipole (52) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric identification of organic compounds, 4th ed.; John Wiley & Sons, Inc.: New York, 1991. (53) The electric charges were estimated from the oxidation peak shown in Figure 1 after subtracting the charging current of the double layer. The oxidation charges normalized by the full charge of the oxidation peak should correspond to the degree of oxidation of the terminal ferrocene group. A similar potential dependence for the degree of oxidation of the terminal ferrocene group was obtained from the reduction electric charge as (1 - the normalized cathodic charge), suggesting the oxidation and reduction reaction of the terminal ferrocene group is reversible. (54) Greenler, R. G. J. Chem. Phys. 1966, 44, 310.
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Figure 4. Potential dependence of the IRRAS band intensity at 2923 cm-1 (O) normalized by the band intensity at +1.1 V in Figure 3, and the electric charge passed (4) normalized by the full charge of the oxidation peak of the cyclic voltammogram in Figure 2.
moments for the C-H stretch of the ferrocene ring and the methylene group become more and less, respectively, perpendicular to the surface upon oxidation. These results suggest that an orientation change in the FcC11SH monolayer takes placing during the redox reaction of the terminal ferrocene group as schematically shown in Figure 5. It is reasonable to assume that the FcC11SH monolayer is in the all-trans conformation, which is often formed in a closely packed monolayer. When the orientation of the alkyl chain changes from form 1 to form 2, as schematically shown in Figure 5, the alkyl chain becomes more perpendicualr to the electrode surface. With this change, the interaction between the external p-polarized electric field and the νas as well as νs modes of the methylene group becomes weaker and, therefore, the absorptions of these bands decrease, resulting in the two upward bands for the νas and νs modes in the IRRAS spectra (∆R/R), as shown in Figures 2 and 3. Although the lack of information for the initial conformation of the ferrocene ring makes it difficult to discuss the conformation or orientation change for the ferrocene moiety quantitatively, the downward band for the C-H stretch of the ferrocene ring in Figures 2 and 3 can be understood as a result of an orientation change of the alkyl chain if one assumes that the pentadienyl rings of the ferrocene moiety remain parallel to the C-C-C plane of the alkyl chain in the
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all-trans conformation. In this case, the interaction between the C-H stretch of the ferrocene ring and the external p-polarized field increases as the alkyl chain becomes more perpendicular to the electrode surface. The same considerations can also be made for the C-C stretch bands from the ferrocene ring and the CH2 bending mode or the Fc-C mode. Thus, in a few words, the alkyl chain becomes more perpendicular to the electrode surface when the potential becomes sufficiently positive enough to oxidize the terminal ferrocene group (Figure 5). One of the possible reasons for the potential dependent orientation changes in the FcC11SH monolayer should be the difference in the charge of the terminal group in the oxidized and reduced forms and, therefore, the difference in the lateral interactions among the terminal groups and between the terminal groups and the gold electrode surface. The terminal group of the FcC11SH monolayer is neutral in the reduced form but has a positive charge in the oxidized form. As the terminal ferrocene group is oxidized to ferricenium cation, an ion pair between the ferricenium cation and the perchlorate anion (ClO4-) is formed, as reported before.5-7 Considering the closely packed structure of the FcC11SH monolayer, the perchlorate anion should be located on the outside of the monolayer when the ion pair is formed. The gold surface is positively charged in the potential region where the ferrocene group is oxidized. Since the distance between the ferricenium cation and the gold electrode surface is smaller than that between the perchlorate anion and the gold electrode surface, a repulsive interaction between the terminal ferricenium cation and the gold electrode surface dominates when the terminal group is in the oxidized form. Thus, the distance between the terminal group and the gold electrode surface becomes larger, so that the monolayer changes its orientation to more perpendicular to the electrode surface as the ferrocene moiety is oxidized to ferricenium cation. The interaction among the positively charged ferricenium groups and perchlorate anions should also play an important role in the orientation change. This result is in contrast to that of the self-assembled monolayer of H2QC11SH on gold.48 The bands observed in the IRRAS spectra of H2QC11SH during the redox reaction of the terminal group were all due to the vibrational modes of the terminal group, i.e., hydroquinone in the reduced form and quinone in the oxidized form, and no indication of orientation change was observed in this monolayer.48 While the terminal group of the FcC11SH
Figure 5. Schematic model for the orientation change of the FcC11SH monolayer: left side, monolayer terminated with ferrocene; right side, monolayer terminated with oxidized ferrocene, i.e., ferricenium cation.
SAM of 11-Ferrocenyl-1-undecanethiol
monolayer is neutral in the reduced form but has a positive charge in the oxidized form, both the reduced and oxidized forms are neutral in the case of the hydroquinoneterminated monolayer (H2QC11SH). The size of the ions associated with the redox reaction of the terminal groups may be responsible for the difference between the two SAMs. While the relatively large anion (ClO4-) moves in and out during the oxidation and the reduction, respectively, of the ferrocene moiety, the much smaller proton is associated with the quinone/hydroquinone redox process. It is interesting to note that the potential dependence of the IRRAS band intensity exactly overlaps with that of the electric charge passed for the oxidation and reduction processes of the FcC11SH monolayer (Figure 4). This result suggests that the redox reaction of the FcC11SH monolayer is a reversible process, as expected from cyclic voltammetry (Figure 1). This result is also quite different from that of the H2QC11SH monolayer.48 In that case, the band intensity changed with time after the potential was stepped to a given value even after 5 min. Furthermore, a slower reaction rate was found for the reduction process than for the oxidation process.48 The present results confirm that a 5 s waiting time after the potential step is enough to acquire spectra if the electrochemical reaction is fast. The present results also demonstrate the usefulness of IRRAS for both structural and kinetic studies of SAMs. A more detailed study including the investigation of an anodic decomposition process of the FcC11SH monolayer
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by using the combined FT-IRRAS/EQCM technique is in progress. Conclusion Electrochemical in situ IRRAS measurement was carried out for a FcC11SH monolayer on a gold electrode in a 0.1 M HClO4 solution during the redox reaction of the terminal ferrocene group. Well defined upward bands and downward bands were observed in the IRRAS spectra (∆R/R) with a reference potential of +0.1 V, where the terminal group existed as ferrocene when the ferrocene moiety was oxidized. The experimental results show that a redox-induced orientation change in the monolayer took place and that the alkyl chain of the monolayer became more perpendicular to the electrode surface when the terminal ferrocene moiety was oxidized. These results contast with those of the hydroquinone-terminated SAM where no redox-induced orientation change occurs. Acknowledgment. This work was partially supported by Grant-in-Aids for Scientific Research (07750904) and Priority Area Research (05235201, 06226201, 07215202, 07241203, 08231203) and by the International Scientific Research Program (Joint Research 07044046) from the Ministry of Education, Science, Sports and Culture, Japan. Dr. A. J. McQuillan is acknowledged for the critical reading of the manuscript and for valuable discussions. Mr. T. Haba is acknowledged for drawing Figure 5. LA9700432
