Use of Alkanethiol-Coated Electrodes To Study the Importance of

On the other hand, in the presence of NH4PF6, the wet SAM of 1 showed sharp shoulder-reduction ... Rubinstein, I., Eds.; Marcel Dekker: New York, 1996...
0 downloads 0 Views 162KB Size
Download PDF
3816

Langmuir 1999, 15, 3816-3822

Use of Alkanethiol-Coated Electrodes To Study the Importance of Water Content on the Electrochemical Behavior of N-Ethyl-N′-octadecyl Viologen on a Gold Electrode Surface S. Abraham John, Fusao Kitamura, Noritoshi Nanbu, Koichi Tokuda, and Takeo Ohsaka* Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan Received January 14, 1999. In Final Form: March 15, 1999 The electrochemical behavior of an N-ethyl-N′-octadecyl viologen (1) assembly on Au and bound to alkanethiol-coated electrodes was studied. The wet and dry self-assembled monolayers (SAMs) of 1 on Au electrodes showed a single redox wave for the first reduction in the presence of KCl, NaNO3, Na2SO4, and NaClO4. On the other hand, in the presence of NH4PF6, the wet SAM of 1 showed sharp shoulder-reduction peaks at -0.50 and -0.57 V and two oxidation peaks at -0.50 and -0.42 V, and the dry SAM of 1 showed only a single redox wave. On the basis of in situ Fourier transform infrared reflection spectroscopy studies, we assigned the sharp reduction peak of -0.50 V to the reduction of strongly interacted dications of 1 surrounded by water molecules and the more negative potential reduction peak (-0.57 V) to the reduction of the dications of 1 ion-paired with PF6- ions. Among the two oxidation peaks, the oxidation peak of -0.50 V was assigned to the usual oxidation of the radical cation whereas the oxidation peak of -0.42 V was assigned to the oxidation of the radical cation dimer. Surprisingly, the assembly of 1 showed an irreversible response on long chain n-alkanethiols [CH3(CH2)nSH, n ) 11, 13, 15, and 17]-coated Au electrodes in the presence of 0.1 M NH4PF6. Meanwhile, transfer of the same electrode to other supporting electrolytes (typically, KCl, Na2SO4, and NaClO4) gave a clear redox wave. The observed irreversible response of 1 on long chain alkanethiol-coated Au electrodes is explained by the blocking effect of water molecules by the assembly of 1 on alkanethiol-coated Au electrodes in addition to the presence of very weakly hydrated PF6ions. It is proposed that because of the absence of water molecules in the assembly of 1 on long-chain alkanethiol-coated Au electrodes, the dications of 1 form an “insoluble salt” with PF6- ions at the electrode surface. Whereas the same electrode was transferred to the aqueous solution of hydrophilic anions such as Cl- and SO42-, the dications of 1 were well-solvated due to the ingress of water molecules along with the hydrophilic anions of Cl- or SO42- into the assembly of 1 on alkanethiol-coated electrodes, and thus, it showed a clear redox response. On the other hand, the assembly of 1 on short-chain n-alkanethiol (n < 9)-coated electrodes showed a well-resolved redox wave in the presence of 0.1 M NH4PF6. The ingress/ egress of water and anions into/from the assembly of 1 on Au and alkanethiol-coated Au electrodes during the redox reaction were studied by electrochemical quartz crystal microbalance. It is found that the ingress of water molecules into the assembly of 1 on alkanethiol-coated Au electrodes was less than that of 1 on Au electrodes. The ingress of water molecules also strongly depends on the identity of the anion that is present within the monolayer.

Introduction The past decade has witnessed much attention to the preparation and characterization of self-assembled monolayers (SAMs) of molecules bearing functional groups such as thiols, disulfides, and silane, as well as surfactant molecules lacking these functional groups on various metal electrode surfaces, mainly because of the ease in preparation and characterization.1-13 It is expected that such SAMs could allow for the fabrication of interfaces with * Corresponding author. E-mail: [email protected]. Fax: +81-45-924-5489. Telephone: +81-45-924-5404. (1) (a) Ulman, A. An Introduction to Ultrathin Organic Films-From Langmuir Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (b) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, p 109. (2) (a) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (b) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (c) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (d) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (3) (a) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (b) Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983, 99, 235. (c) Gun, J.; Iscovici, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 101, 201.

well-defined composition, structure, and thickness. It has been found that the SAMs of many molecules on different electrode surfaces exhibited interesting electrochemical properties.1-13 Also, these SAMs have been used for molecular recognition,14 for pH-dependent electrostatic (4) (a) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (b) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45. (c) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (d) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805. (5) (a) Finklea, H. O.; Avery, S.; Lynch, M.; Furtsch, T. Langmuir 1987, 3, 409. (b) Finklea, H. O.; Snider, D. A.; Fedyk, J. Langmuir 1990, 6, 371. (6) (a) Sabatani, E.; Rubinstein, I.; Maoz, R.; Sagiv, J. J. Electroanal. Chem. 1987, 219, 365. (b) Sabatani, E.; Rubinstein, I. J. Phys. Chem. 1987, 91, 6663. (7) De Long, H. C.; Buttry, D. A. Langmuir 1992, 8, 2491. (8) Tang, X.; Schneider, T. W.; Walker, J. W.; Buttry, D. A. Langmuir 1996, 12, 921. (9) Lee, C.-W.; Bard, A. J. J. Electroanal. Chem. 1988, 239, 441. (10) Widrig, C. A.; Majda, M. Langmuir 1989, 5, 689. (11) Gomez, M.; Li, J.; Kaifer, A. E. Langmuir 1991, 7, 1797. (12) Facci, J. S. Langmuir 1987, 3, 525. (13) Donohue, J. J.; Buttry, D. A. Langmuir 1989, 5, 671.

10.1021/la9900371 CCC: $18.00 © 1999 American Chemical Society Published on Web 04/30/1999

Use of Alkanethiol-Coated Electrodes

binding of ions,15 for fundamental studies of electrontransfer reaction,16 and as platforms for further surface chemical modification.17 Very recently, we reported the first successful electrochemical study on monomer-dimer equilibrium for the wet SAM of N-ethyl-N′-octadecyl viologen (1) on the electrode surface in the presence of a PF6- ion.18 We concluded that water molecules preexisting in the wet monolayer favor the dimerization of 1 on the electrode surface in the presence of a PF6- ion. Keeping the versatile properties of alkanethiol-coated electrodes in mind,1,2,4 such as the effective blocking of water molecules from the Au surface stability over a wide potential range, and a high degree of organization, we study the electrochemical behavior of 1 on alkanethiol-coated Au electrodes in the presence of PF6- ions to confirm the role of water molecules in the dimerization of SAM of 1 on the electrode surface. Furthermore, the ingress/egress of water molecule into/ from the assembly of 1 on Au and alkanethiol-coated Au electrodes are studied by electrochemical quartz crystal microbalance (EQCM) experiments. Experimental Section The Au electrodes were prepared by sealing 1 or 2 mm diameter Au rod into an insulating tube [poly(chlorotrifluroethylene)]. The exposed gold surface was polished with aqueous slurries of successively finer alumina powder (down to 0.06 µm) on a polishing microcloth, sonicated for 10 min in water, and rinsed with water. The Au electrode was then electropolished by potential cycling in 0.5 M H2SO4 in the potential range of -0.2 to +1.5 V at the potential scan rate of 100 mV s-1 for 20 min or until the cyclic voltametry (CV) characteristic for a clean Au electrode was obtained. Such an electrode was considered to be a bare Au electrode. The real surface area was calculated from the charge required to reduce the surface oxide layer using the previously established formula: 0.43mC cm-2.19 The geometric area of the Au electrode was calculated using the diameter of the employed Au electrode. The surface roughness was calculated as the ratio of the real surface area to the geometric area and was found to be 1.2. The asymmetric viologen, (bromide salt) N-ethyl-N′-octadecyl viologen (1) was synthesized and characterized by the reported procedure.10 Octadecanethiol (Aldrich), tetradecanethiol (TDT) (Tokyo Kasei Kogyo Co), and other alkanethiols (Analar grade) and ethanol (Analar grade) were purchased from Kanto Chemicals and used as received. The SAMs of 1 was prepared by immersing the cleaned Au electrode in a 10-100 µM aqueous solution of 1 for 30-90 min. Then, the electrode was dried at room temperature and transferred to the supporting electrolyte solution for electrochemical measurements. The thus prepared SAM is denoted as “dry SAM”. On the other hand, the electrode that was transferred to the supporting electrolyte solution immediately after soaking in the solution of 1 (without drying) is denoted as “wet SAM”. The alkanethiol monolayers were formed by immersing the Au electrode into a 1-10 mM ethanol solution of the respective alkanethiol for 6-12 h. The electrode was then (14) (a) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426. (b) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312. (c) Haeussling, L.; Ringsdorf, H.; Schmitt, F.-J.; Knoll, W. Langmuir 1991, 7, 1837. (15) (a) Sun, L.; Johnson, B. J.; Wade, T.; Crooks, R. M. J. Phys. Chem. 1990, 94, 8869. (b) Jones, T. A.; Perez, G. P.; Johnson, B. A.; Crooks, R. M. Langmuir 1995, 11, 1318. (c) Nahir, T. M.; Clark, R. A.; Bowden, E. F. Anal. Chem. 1994, 66, 2595. (16) Bard, A. J.; Abruna, H. D.; Chidsey, C. E.; Faulkner, L. R.; Feldberg, S. W.; Itaya, K.; Majda, M.; Melroy, O.; Murray, R. W.; Porter, M. D.; Soriaga, M. P.; White, H. S. J. Phys. Chem. 1993, 97, 7147. (17) (a) Ulman, A.; Tilman, N. Langmuir 1989, 5, 1418. (b) Sun, L.; Thomas, R. C.; Crooks, R. M.; Ricco, A. J. J. Am. Chem. Soc. 1991, 113, 8550. (c) Bent, S. F.; Schilling, M. L.; Wilson, H. E.; Katz, H. E.; Harris, A. L. Chem. Mater. 1994, 6, 122. (18) John, S. A.; Okajima, T.; Ohsaka, T. J. Electroanal. Chem., in press. (19) Gileadi, E.; Eisner, K. E.; Penciner, J. Interfacial Electrochemistry- An Experimental Approach; Addison-Wesley: Reading, MA, 1975.

Langmuir, Vol. 15, No. 11, 1999 3817

Figure 1. CV and frequency-potential curve for the SAM of 1 on the Au electrode in 0.1 M NaClO4. Scan rate ) 50 mV s-1. washed with ethanol and dried in air. Then, the electrode was immersed in an aqueous solution of 1 for 30-90 min and transferred to the supporting electrolyte for electrochemical measurements. All electrochemical experiments were performed at room temperature using a standard three-electrode, two-compartment configuration with a Au as the working electrode, a spiral platinum counter electrode, and an NaCl-saturated Ag/AgCl reference electrode. The cyclic voltammetric experiments were carried out with computer-controlled electrochemical systems (BAS 100 B/W). The surface coverage (Γ/mol cm-2) was estimated by graphically integrating the cyclic voltammogram recorded at various scan rates. All solutions were thoroughly deoxygenated by purging with nitrogen gas, and during the electrochemical experiments, nitrogen atmosphere was maintained above the solution. The EQCM principle and the instrumentation have been wellestablished.20 AT- cut quartz crystals of nominal fundamental frequency of 6 MHz were used in the present study. Signal averaging was employed to allow for the facile detection of frequency changes as small as 0.1 Hz. The Au quartz crystals were purchased from Hokuto Denko (Japan). The electrochemically active area of Au electrode was 1.33 cm2. The surface roughness of the electrode was determined to be 1.2 by the gold oxide method.19 Because both the electrochemistry and EQCM measurements were done on the same electrode, the Γ reported in the EQCM measurements do not take this roughness into account so as to allow direct comparison of Γ and frequency changes (which are influenced identically by surface roughness). The signal from the oscillator was sent to a Hokuto Denko EQCM controller HQ-101B. A potentiostat (PS-07, Toho Tech. Co.) and an X-Y-Y′ recorder (Graphtech Co.) were used to record the EQCM response. For in situ Fourier transform infrared reflection spectroscopy (FT-IRRAS) studies, the SAM of Au disk electrode of a 0.78 cm2 was used as a working electrode. A platinum wire and Ag/AgCl (NaCl sat) were used as the counter and the reference electrodes, respectively. The FT-IRRAS measurements were carried out in a mode of subtractively normalized interfacial FT-IR spectroscopy (SNIFTIRS) on an FTS-7T Fourier transform infrared spectrometer (Bio-Rad Laboratories) equipped with a wide-band MCT detector. For each spectrum, 320 scans were collected and averaged. The angle of incidence at the air/CaF2 window was set to 60°. Light polarization was controlled by a wire-grid polarizer. All measurements were performed at room temperature.

Results and Discussion CV and EQCM Studies of the SAM of 1 on Au and Alkanethiol-Coated Au Electrodes in the Presence of KCl, Na2SO4, and NaClO4. Figure 1 shows a set of CV and EQCM curve obtained for the SAM of 1 at a limiting surface coverage of 3.1 × 10-10 mol cm-2 in 0.1 M NaClO4 solution. A sharp redox peak was observed around -0.50 V (at a scan rate of 50 mV s-1), corresponding to the redox (20) Buttry, D. A. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1991; Vol. 17, pp 1-85.

3818 Langmuir, Vol. 15, No. 11, 1999

John et al.

Figure 2. CV and frequency-potential curve for the assembly of 1 bound to the ODT-coated electrode Au electrode in 0.1 M NaClO4. Scan rate ) 50 mV s-1. 2+

•+

reaction of the dication (V )/monocation radical (V ) couple of 1. The observed redox reaction of 1 is due to the surface confined species, which was confirmed from the linear dependence of peak current upon scan rate. Similarly, a single broad redox wave was observed for the SAM of 1 in the presence of 0.1 M KCl or Na2SO4. A total frequency change of 3.2 Hz was observed for the first reduction of the SAM of 1 on the Au electrode in the presence of a 0.1 M NaClO4 solution, which corresponds to a loss of mass of 250 g mol-1 from the electrode surface. The mass changes were calculated by using the Sauerbrey equation.21 A very similar loss of mass value was also obtained from the plot of frequency change against the charge consumed for the first redox process of 1. We assume that the observed mass loss is caused by the simultaneous loss of the anion previously present within the monolayer for charge compensation of the viologen dications and some water molecules. Such criteria have already been well established for a monolayer of several molecules on the electrode surface.20 Thus we found that ca. eight water molecules are lost from the monolayer for each ClO4- ion lost during the reduction and this compositional change is reversible on the CV time scale. In the case of Cl- and SO42-, ca. 17 and 15 water molecules are lost from the monolayer for each Cl- and SO42-, respectively, during the reduction. The water molecules lost from the monolayer should not be interpreted as constituting the solvation sphere of the anion.7 These water molecules are associated with both the V2+ group and the respective anion within the monolayer before its reduction. These water molecules are lost because of the transition induced by the first redox process from a hydrophilic species (V2+) to a very hydrophobic species (V•+). The observed loss of smaller number of water molecules from the monolayer in the presence of ClO4- indicates that the monolayer of 1 is less “leaky” toward the penetration of ClO4- ion than that of Cl- or SO42- ions. CV and EQCM curves for the assembly of 1 (Γ ) 3.2 × 10-10 mol cm-2) on the octadecanethiol (ODT)-coated Au electrode (prepared under conditions very similar to those on the Au electrode) in 0.1 M NaClO4 solution are shown in Figure 2. Several significant features were observed for the assembly of 1 on ODT-coated Au electrodes, compared with those on the Au electrode. The doublelayer capacitance (Cdl) calculated for the assembly of 1 on the ODT-coated Au electrode is much smaller than that on the Au electrode; for example, the Cdl values were 4.3 (21) Sauerbrey, G. Z. Z. Phys. Chem. 1959, 155, 206.

Figure 3. CVs obtained for the (a) dry and (b) wet SAMs of 1 on the Au electrode in the presence of 0.1 M NH4PF6. Scan rate ) 200 mV s-1.

and 38 µF cm-2 at 50 mV s-1 for the former and the latter, respectively. The observed small capacitance indicates that the assembly of 1 bound to the ODT-coated Au is more compact than on the assembly of 1 on an Au electrode. The observed redox behavior of 1 on the ODT-coated Au electrode was consistent with that expected for a surfaceconfined species, which was confirmed from the observation of the linear plot of peak current vs potential scan rate. The surface coverage of 1 on the ODT-coated Au electrode is almost similar to that on the Au electrode. It is likely that 1 assembles on the exterior of the mercaptan monolayer to form a bilayer structure on the ODT-coated electrode.22 The assembly of 1 also showed a very similar electrochemical behavior on other alkanethiol-coated electrodes. A total frequency change of 2 Hz was observed for the assembly of 1 on the ODT-coated Au electrode in the presence of a 0.1 M NaClO4 solution (Figure 2), which corresponds to the loss of mass of 152 g mol-1 from the electrode surface. Thus, ca. three water molecules are lost from the monolayer for each ClO4- ion lost during the reduction, and this compositional change is reversible on the CV time scale. In the case of Cl- anion, a loss of ca. six water molecules was observed on the ODT-coated Au electrode during the reduction. The observed ingress/ egress of smaller number of water molecules on the ODTcoated Au electrode suggests that the water molecules are more effectively blocked by the assembly of 1 on the ODT-coated Au electrode than on the Au electrode. Similar observations have already been made for the SAM of thiol derivatives of viologen in the presence of Cl- and ClO4anions.7 The ingress of water molecules increases with decreases in the chain length of alkanethiol-coated electrodes. CV and EQCM Studies of SAM of 1 on an Au Electrode in the Presence of NH4PF6. Figure 3 shows the CVs obtained for the wet and dry SAMs of 1 in the presence of 0.1 M NH4PF6. The dry SAM of 1 showed a single redox peak similar to the case of other supporting electrolytes with negative shift in the redox peak (Figure 3a). The observed negative shift may be due to the strong ion-pair formation between V2+ and PF6- ions when compared to those of other anions. On the other hand, the wet SAM of 1 exhibited sharp reduction and oxidation (22) Finklea, H. O.; Fedyk, J.; Schwab, J. ACS Symp. Ser. 1988, 378, 431.

Use of Alkanethiol-Coated Electrodes

peaks at less negative potentials in addition to the usual redox peak (Figure 3b). However, it showed only a single redox wave in the presence of other supporting electrolytes (vide supra). According to the potential difference SNIFTIR spectral studies for the SAM of 1 in 0.1 M NH4PF6, we assigned the more negative redox peak to the usual redox reaction of the V2+/V•+ couple and the less negative potential reduction and oxidation peaks to the reduction of the dications of 1 at a water-rich environment and oxidation of the radical cation dimer, respectively (vide infra). We consider that the preexisting water molecules in the wet SAM significantly influence the electrochemical behavior of 1 in the presence of PF6- ions when compared to the dry SAM. Upon continuous potential cycling in the range of 0.0 to about -0.65 V or when the electrode is kept at open circuit in the 0.1 M NH4PF6 solution for several minutes, the more negative redox peak increased while the less negative reduction and oxidation peaks decreased.18 This is due to the expulsion of preexisting water molecules from the monolayer by the entry of less hydrophilic PF6- ions. In the EQCM experiments, it is found that ca. five water molecules per one PF6- ion are lost from and enter into the monolayer during the reduction and oxidation, respectively in the presence of PF6- ions.18 This shows that the monolayer becomes more compact in the presence of PF6- ions than in the presence of ClO4- ions, where ca. eight water molecules per one ClO4- ion enter into the monolayer during the oxidation process (vide supra). SNIFTIR Spectral Studies of the SAM of 1 on an Au Electrode. The viologen cation radicals (V•+) are known to dimerize reversibly in aqueous media. In situ UV-vis,23 Raman,24,25 IR,26,27 and electron spin resonance28 spectral techniques have provided the evidence for the dimer formation. In the UV-vis spectroscopy studies, the formation of the dimer was studied by observing the different absorbance spectrum for the dimer from that for the monomer.23 On the other hand, in the FT-IR spectral studies,26,27 the dimerization was strongly supported by observing the abnormally strong intensities of the fundamental, totally symmetric ring modes (which should be forbidden in the IR). The strong intensity enhancement of these forbidden ring modes has been attributed to electron-molecular vibration (e-mv) interactions. Recently, Buttry and co-workers8 used the in situ Raman spectroscopy technique to study the multiple peaks observed for the first reduction of thiol derivatives of viologen on an Ag electrode. They demonstrated that the lateral interaction of a viologen cation radical in the monolayer results in the formation of π complex dimers. In the present study, we used potential difference SNIFTIR spectral technique to identify the species responsible for the multiple redox peaks observed for the wet SAM of 1 in the presence of 0.1 M NH4PF6. Figure 4 shows a series of potential difference SNIFTIR spectra obtained for the wet SAM of 1 on the Au electrode in the presence of 0.1 M NH4PF6. Because the observed less negative potential sharp reduction and oxidation peaks (Figure 3b) are decreased with continuous potential cycling or are only (23) (a) Kosower, E. M.; Cotter, J. L. J. Am. Chem. Soc. 1964, 86, 5524. (b) Bird, C. L.; Kuhn, A. T. Chem. Soc. Rev. 1981, 10, 49. (24) Hester, R. E.; Suzuki, S. J. Phys. Chem. 1982, 86, 4626. (25) (a) Lu, T.; Cotton, T. M.; Hurst, J. K.; Thompson, D. H. J. Phys. Chem. 1988, 92, 6978. (b) Ghoshal, S.; Lu, T.; Feng, Q.; Cotton, T. M. Spectrochim. Acta, Part A 1988, 44, 651. (26) Christensen, P. A.; Hamnett, A. J. Electroanal. Chem. 1989, 263, 49. (27) Bae, I. T.; Huang, H.; Yeager, E. B.; Scherson, D. A. Langmuir 1991, 7, 1558. (28) Evans, A. G.; Evans, J. C.; Baker, M. W. J. Chem. Soc., Perkin Trans. 2 1977, 1787.

Langmuir, Vol. 15, No. 11, 1999 3819

Figure 4. Potential difference SNIFTIR spectra obtained for the SAM of 1 on the Au electrode in 0.1 M NH4PF6. Sampling potentials are indicated in the figure. Reference potential ) 0 V vs Ag/AgCl (NaCl sat).

stable for 60 min, we used three sampling potentials (-0.20, -0.50, and -0.65 V vs Ag/AgCl) to measure the spectra. As indicated in the Figure 4, at an applied potential of -0.20 V, no clearly defined peaks were obtained where the 1 is in the oxidized state. At the sampling potential of -0.50 V, some of the less defined peaks were obtained in the region between 1100-1700 cm-1. On the other hand, when the sampling potential was switched to -0.65 V, several clearly defined negativegoing peaks were observed in the region of 1100 to 1700 cm-1. The spectrum shows bands at 1114, 1162, 1193, 1297, 1331, 1494, 1564, 1597, and 1640 cm-1. The spectral features observed at the sampling potential of -0.65 V are very similar to those reported by Christensen and Hamnett26 for solution-phase MV2+ in aqueous electrolytes and by Scherson and co-workers27 for the SAM of 1 prepared by potential cycling between 0.2 and -0.7 V vs SCE or by holding the potential at the most negative limit for a short period of time. The spectral features at 1640 and 1331 cm-1 may be attributed to B2u modes and those at 1597 and 1494 cm-1 to B3u modes.24,25 The intense bands observed at 1193 and 1162 cm-1 were assigned to Ag modes, dominated by C-N stretches.25b Scherson and co-workers27 proposed that the spectral bands observed in the reduced state originated from vibronically activated totally symmetric modes of the cationic radical of 1 in monomeric or dimeric form. The most intense band, at 1193 cm-1, and the bands observed at 1494 and 1597 cm-1 were assigned to the dimeric form of the monocation.26,27 The absence of similar features at the sampling potential of -0.50 V reveals that the observed sharp reduction peak of -0.50 V in Figure 3b is not due to the dimer of 1. On the other hand, appearance of strong dimeric features at the sampling potential of -0.65 V suggested that the oxidation peak of -0.42 observed in Figure 3b is due to the oxidation of the dimer of 1. Very recently, Reipa and co-workers29 also reported radical dimers for the first reduction of tetradecylmethyl viologen on a Au electrode using combined spectroscopic ellipsometry and voltammetry studies. (29) Reipa, V.; Monbouquette, H. G.; Vilker, V. L. Langmuir 1998, 14, 6563.

3820 Langmuir, Vol. 15, No. 11, 1999

John et al.

Figure 6. CVs obtained for the assembly of 1 bound to the ODT-coated Au electrode in the presence of (a) 0.1 M NH4PF6 and (b) the same electrode was transferred to 0.1 M KCl. Scan rate ) 500 mV s-1. Figure 5. Potential difference SNIFTIR spectra obtained for the SAM of 1 on the Au electrode in 0.1 M KCl. Sampling potentials are indicated in the figure. Reference potential ) 0 V vs Ag/AgCl (NaCl sat).

Figure 5 shows the series of potential difference SNIFTIR spectra obtained for the wet SAM of 1 on the Au electrode in the presence of a 0.1 M KCl solution. The spectral features observed in 0.1 M KCl are different from those observed in 0.1 M NH4PF6. The sharp bands at 1193 and 1640 cm-1 and the weak bands at 1564 and 1597 cm-1 observed for the SAM of 1 in 0.1 M NH4PF6 were absent in 0.1 M KCl. Instead we obtained sharp bands at 1495 and 1571 cm-1 and weak bands at 1147, 1222, 1301, and 1319 cm-1. Although the SAM of 1 exhibited a single broad redox response in KCl and Na2SO4, we could not observe the spectral features reported by Scherson and coworkers27 for the SAM of 1 in 0.1 M Na2SO4. The possible reason for the observed difference in spectral features may be associated with the different mode of preparation of SAM of 1 and the surface coverage of 1 on the electrode surface.27 Nevertheless, in the present investigation, the SAM of 1 prepared under identical conditions exhibited different spectral features in the presence of PF6- and Clions. CV and EQCM Studies of the Assembly of 1 on Alkanethiol-Coated Au Electrodes. The CV obtained for the assembly of 1 at the ODT-coated Au electrode in the presence of 0.1 M NH4PF6 is shown in Figure 6a. Surprisingly, an irreversible response was observed for 1 at the ODT-coated electrode when compared to the assembly of 1 on the Au electrode (Figure 3). On the other hand, it showed a clear redox wave in the presence of 0.1 M NaClO4 (Figure 2). A similar irreversible response was also observed for the assembly of 1 on hexadecanethiol (HDT)-, tetradecanethiol-, and dodecanethiol (DDT)coated Au electrodes in the presence of 0.1 M NH4PF6. The interfacial capacitance calculated for the assembly of 1 on the ODT-coated Au electrode is nearly 5 times less than that of 1 on the Au electrode in the presence of PF6ions. This shows that the assembly of 1 is much more compact on a ODT-coated Au electrode than the assembly of 1 on Au electrode. It can thus be presumed that the hydrophobic anions of PF6- actually do not associate with water molecules when they enter into the assembly of 1 on the ODT-coated Au electrode. Under this condition, we

strongly believe that because of the absence of water molecules in the assembly of 1 at the ODT-coated Au electrode the dications of 1 form an “insoluble salt” with PF6- ions on the electrode surface. Thus, the assembly of 1 on the ODT-coated Au electrode showed an irreversible redox response in the presence of 0.1 M NH4PF6 (Figure 6a). Mortimer and Anson30 earlier reported similar electrochemical behavior for the copolymer of methyl-(pvinylbenzyl)viologen coated on a graphite electrode in a 0.2 M KPF6 solution. In the presence of a KPF6 solution, small, unequal peak currents and larger peak separation were observed when compared to other supporting electrolytes.30 They thought that the observed behavior in KPF6 is due to the formation of insoluble salts of PF6anions with viologens.30 On the other hand, the reduction (or oxidation) of the assembly of 1 on the Au electrode in the presence of PF6- ions is accompanied by the egress (or ingress) of one PF6- ion and ca. five water molecules per redox site (vide supra) and thus the electrochemical behavior of 1 is actually unaffected. When the same electrode used in 0.1 M NH4PF6 (Figure 6a) was transferred into 0.1 M KCl, a clear redox wave was observed (Figure 6b). This shows that the insoluble salt of 1 formed on the ODT-coated Au electrode was well-solvated in the presence of Cl- ions because of the ingress of water molecules and became electroactive. The EQCM experiments also showed that ca. six water molecules per redox site go in and out of the assembly of 1 on the ODT-coated Au electrode during the redox process in the presence of Cl- ion (vide supra). The CV obtained for the assembly of 1 on the decanethiol (DT)-coated Au electrode in the presence of 0.1 M NH4PF6 is shown in Figure 7. It shows a reduction wave at -0.55 V and a oxidation wave at -0.43 V when compared to the irreversible redox response observed for the assembly of 1 on the ODT-coated Au electrode (Figure 6a). In this case, the value of Γred calculated for the assembly of 1 is higher than that of Γox. During the potential cycling (between 0 and -0.65 V), some of the radical cation of 1 may form an insoluble salt with PF6- anions, and thus, the Γox is less than that of Γred. This is confirmed from the observed increase in Γox when the same electrode used in 0.1 M NH4PF6 was transferred to the supporting elec(30) Mortimer, R. J.; Anson, F. C. J. Electroanal. Chem. 1982, 138, 325.

Use of Alkanethiol-Coated Electrodes

Langmuir, Vol. 15, No. 11, 1999 3821

Figure 7. CV obtained for the assembly of 1 bound to the DT-coated Au electrode in the presence of 0.1 M NH4PF6. Scan rate ) 200 mV s-1.

Figure 8. CVs obtained for the wet SAM of 1 on Au electrode in the presence of 0.5 M NH4PF6 after (a) 0 and (b) 10 min and (c) the same electrode used in part b was transferred to 0.5 M KCl. Scan rate ) 200 mV s-1.

Figure 9. (A) CVs obtained for (a) a bare Au electrode and (b) the assembly of 1 on the Au electrode in a solution of 0.5 mM Ru(NH3)6Cl3 in 0.1 M KCl. (B) Similar CVs for a bare Au electrode and (c) the assembly of 1 bound to TDT-coated Au electrode. The dotted line in shows the CVs of the assembly of 1 on the Au and bound to TDT-coated Au electrodes in the presence of 0.1 M KCl. Scan rate ) 200 mV s-1.

trolyte of 0.1 M KCl or Na2SO4 (not shown). In addition, the ratio of Γred to Γox decreases when the chain length of alkanethiols on an Au electrode, to which the assembly of 1 was bound, decreases. The CV obtained for the assembly of 1 at the DT-coated Au electrode was very stable upon repeated potential cycling. Effect of PF6- Concentration. The CV obtained for the wet SAM of 1 on the Au electrode in the presence of 0.5 M NH4PF6 at the first potential scan is shown in Figure 8a. It showed a electrochemical behavior very similar to that observed in 0.1 M NH4PF6 (Figure 3b), although the redox peaks shifted to the negative direction of potential. When the CV was recorded after 10 min, the redox peaks were decreased (Figure 8b). The observed electrochemical behavior of 1 at the first potential scan may be due to the presence of preexisting water molecules in the monolayer. While the SAM was immersed in 0.5 M NH4PF6 for 10 min, the preexisting water molecules around the redox moiety of 1 were expelled from the SAM by the ingress of hydrophobic PF6- ions to it via an exchange with the Brions that are the initial counterions of 1, which led to the insoluble salt as a result of the increase in concentration of PF6- anions around the dications of 1, as observed for the assembly of 1 on the ODT-coated Au electrode in the

presence of 0.1 M NH4PF6 (vide supra). On the other hand, the electrochemical behavior observed in 0.1 M NH4PF6 was highly stable for several hours. When the same electrode used in Figure 8b was transferred to 0.5 M KCl, a clear redox wave was observed at a less negative potential with increased surface coverage (Figure 8c). In addition, the interfacial capacitance increased. This again shows that the insoluble salt of 1 formed at the electrode surface is well-solvated because of the ingress of water molecules to the assembly of 1 on the Au electrode in the presence of the hydrophilic anion of Cl-, bringing about a clear redox response (Figure 8c). Electrochemical Behavior of Ru(NH3)63+ at the Assembly of 1 on Au and Bound to AlkanethiolCoated Electrodes. One way of assessing the compactness of the interfacial structure is to examine its blocking effects on the electrochemical behavior of the diffusing solution species. The CVs obtained for Ru(NH3)6Cl3 on clean Au and the assembly of 1 (Γ ) 3.0 × 10-10 mol cm-2) on Au electrodes are shown in Figure 9A. A well-diffusioncontrolled redox reaction for Ru(NH3)62+/3+ was observed on the Au electrode (line a, Figure 9A). Its redox reaction was also obtained at the assembly of 1 on the Au electrode (line b, Figure 9A). Therefore, we believe that the assembly

3822 Langmuir, Vol. 15, No. 11, 1999 Scheme 1. Schematic Representation of the Assembly of 1 on the ODT-Coated Au Electrode in the Presence of PF6- and Cl- Ions

John et al.

the assembly of 1 showed an irreversible response in the presence of 0.1 M NH4PF6. On the other hand, when the same electrode was transferred to the supporting electrolyte anions of Cl-, SO42- and ClO4-, the assembly 1 exhibited a clear redox wave due to the ingress of water molecules. Although PF6- and ClO4- ions do not differ much in their crystallographic radii (0.301 and 0.290 nm, respectively) or the Stokes radii calculated from the mobility data (0.233 and 0.232 nm, respectively), the electrochemical behavior observed in PF6- is very different from that observed in the ClO4- ion.32 This is due to the stronger interaction of 1 with a PF6- anion than with a ClO4- anion. In the case of the assembly of 1 bound to short-chain alkanethiol-coated Au electrodes, probably because of the easy ingress of water molecules, the formation of the above-mentioned insoluble salt is thought not to occur on the electrode surface in the presence of PF6- ions, and thus, the assembly of 1 showed a clear redox response. Conclusions

of 1 is not compact on the electrode surface, and thus, the electrode reaction of Ru(NH3)62+/3+ is unaffected. Similar experiments were also conducted at the assembly of 1 on the TDT-coated Au electrode and are shown in Figure 9B. The surface coverage calculated for the assembly of 1 on the TDT-coated Au electrode is very similar to that on Au electrode. In this case, the diffusion of Ru(NH3)63+ is significantly affected (line c, Figure 9B). The shape of the CV indicates that the reduction of Ru(NH3)63+ is mediated by 1 at the TDT-coated electrode.31 The observed blocking effect of Ru(NH3)63+ at the assembly of 1 bound to alkanethiol-coated electrodes clearly demonstrates that the assembly of 1 on alkanethiol-coated Au electrodes is more compact than on the Au electrode. Therefore, water molecules easily enter into the assembly of 1 on the Au electrode, which lead to a feasible redox reaction of 1 in the presence of 0.1 M NH4PF6. The ingress of water molecules also depends on the nature of the supporting electrolyte. This can be clearly inferred from the observed difference in the capacitance and EQCM measurements (vide supra). Arrangement of 1 at Alkanethiol-Coated Electrodes. According to the electrochemical behavior of 1 bound to long- and short-chain alkanethiol-coated electrodes in the presence of PF6- ions, we propose that the headgroup (redox moiety) of 1 is directed to the electrode surface rather than to the bulk solution, and this is shown in Scheme 1. If the headgroup of 1 is exposed to the solution side in the case of the alkanethiol coated-electrodes (n ) 11, 13, 15, and 17), then the headgroup of 1 is more highly solvated than at the electrode surface, and thus, the assembly of 1 should show the electrochemical response in the presence of PF6- ions. It is well-known that n-alkanethiols of n > 9 form a densely packed crystallinelike assembly on the electrode surface.2 As the chain length decreases (n < 9), the structure becomes increasingly disordered with lower packing density and coverage.2 Thus, at the densely packed alkanethiol-coated electrodes, (31) Creager, S. E.; Collard, D. M.; Fox, M. A. Langmuir 1990, 6, 1617.

The present work clearly demonstrates the important role played by the water molecule in the electrochemical behavior of the assembly of 1 on Au and alkanethiol-coated Au electrodes in the presence of PF6- ions. The ingress/ egress of water molecules to/from the self-assembled monolayer is controlled by both its structure and the anion of the supporting electrolyte. The assembly of 1 is less compact on the Au electrode, and thus, a higher number of water molecules enter into the monolayer. In the presence of PF6- ions, the wet SAM of 1 showed two couples of redox peaks for the first reduction of 1 on the Au electrode: one is due to the usual redox reaction of the V2+/V•+ couple, and the other corresponds to the oxidation and reduction of the dimer and dication (in the aqueous environment), respectively, (on the basis of the potential difference SNIFTIR spectral measurements). On the other hand, because of the absence of water molecules in the assembly of 1 bound to long-chain alkanethiols-coated Au electrodes, the dications of 1 form an insoluble salt with PF6- ions, and thus, an irreversible redox response was observed. However, when the same electrode was transferred to the supporting electrolyte of KCl, Na2SO4, or NaClO4 owing to the easy ingress of water molecules to the assembly, the insoluble salt of 1 became soluble and showed a clear redox response. We conclude that two couples of redox peaks observed for the assembly of 1 on the Au electrode in the presence of PF6- ion result from the presence of water molecules in the monolayer, which is now clearly established by using the alkanethiol-coated electrodes. Acknowledgment. The present work was financially supported by Grant-in-Aids for Scientific Research on Priority Areas of “New Polymers and Their NanoOrganized Systems” (No. 277/10126219) and “Scientific Research (A)” (No. 10305064) from the Ministry of Education, Science, Sports and Culture, Japan and the Katoh Science Foundation, Japan. We thank Mr. H. Kasahara for the preparation of the asymmetric viologen. S.A.J. thanks the Monbusho for the fellowship. LA9900371 (32) Visy, C. S.; Lukkari, J.; Pajunen, T.; Kankare, J. Synth. Met. 1989, 33, 289.