Electrochemical Characteristics of Hydroquinone-Terminated Self

A series of thiol-functionalized hydroquinone derivatives with different alkyl chain ... behavior of the hydroquinone (H2Q) SAMs has been investigated...
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Langmuir 2001, 17, 2485-2492

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Electrochemical Characteristics of Hydroquinone-Terminated Self-Assembled Monolayers on Gold Hun-Gi Hong* and Wonchoul Park Department of Chemistry, Sejong University, Seoul 143-747, Korea Received October 19, 2000. In Final Form: January 22, 2001 A series of thiol-functionalized hydroquinone derivatives with different alkyl chain lengths, 2-(nmercaptoalkyl)hydroquinone (abbreviated as H2Q(CH2)nSH, where n ) 0, 1, 4, 6, 8, 10, and 12), has been synthesized and used for preparation of self-assembled monolayers (SAMs) on gold electrodes. The compounds spontaneously adsorb onto a gold surface to form stable and reproducible monolayers. The voltammetric behavior of the hydroquinone (H2Q) SAMs has been investigated by cyclic voltammetry to study the effect of alkyl chain length on the electrochemical characteristics for H2Q-SAMs. The H2Q-SAMs show welldeveloped voltammetric responses in aqueous solution and pH dependence with a slope of 58.5 mV/pH of surface formal potential in the broad pH range from 1.3 to 12.1 corresponding to the two-electron twoproton redox reaction of a H2Q subunit. The redox response of SAMs at the same time scale was transformed from reversibility to irreversibility as the alkyl chain length became larger. The monolayers obtained from a 0.1 µM or lower thiol concentration solution show the most ideal current-potential features. From the dependence of logarithmic apparent electron-transfer rate constant on the alkyl chain length between the hydroquinone and the electrode, kinetic parameters and electron tunneling constants (β) were evaluated for H2Q(CH2)nSH SAMs. The obtained experimental value of β ) (1.04 ( 0.06)/CH2 unit is in good agreement with the values reported for reversible redox centers in solutions or tethered monolayer systems.

Introduction The self-assembled monolayers (SAMs)1 of alkanethiolates on gold have been used as a powerful method to prepare a chemical interface which is a stable and structurally well-defined monolayer with a controllable thickness and desirable function. These characteristics of a SAM based on a Au-sulfur interaction make them ideal model systems to study both fundamental and practical issues such as catalysis, electrooptic devices, sensors, corrosion, lubrication, adhesion, molecular recognition, and electron/energy transfer.2-4 SAMs consisting of redox molecules chemically bound to one terminal of an alkanethiol provide an ideal opportunity to study interfacial redox processes, e.g., electron-transfer kinetics between the redox center and the electrode, within a controlled chemical microenvironment. The electrochemical characteristics for SAMs containing redox couples such as pentaamine-cobalt(III) complexes,5 ferrocene,6,7 ruthenium(pyridine) complex,8 osmium bipyridyl complex,9 * To whom correspondence should be addressed. Fax: +82-23408-3217. E-mail: [email protected]. (1) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (2) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, p 109. (3) Murray, R. W. Molecular Design of Electrode surface; Techniques of Chemistry series; John Wiley & Sons: New York, 1992; Vol. XXII. (4) 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. (5) Weaver, M. J.; Li, Tomi T.-T. J. Am. Chem. Soc. 1984, 106, 6107. (6) (a) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (b) Chidsey, C. E. D. Science 1991, 51, 919. (7) Smalley, J. F.; Feldberg, S. W.; Chidsey, C. E. D.; Linford, M. R.; Newton, M. D.; Liu, Y.-P. J. Phys. Chem. 1995, 99, 13141. (8) (a) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173. (b) Ravenscroft, M. S.; Finklea, H. O. J. Phys. Chem. 1994, 98, 3843. (9) Foster, R. J.; Faulkner, L. R. J. Am. Chem. Soc. 1994, 116, 5444.

viologen,10 naphthoquinone,11 azobenzene,12,13 anthraquinone,14 and Ni(II)-azamacrocyclic complex15 have been widely studied by several groups. In their reports, the investigation of distance dependence of long-range electron transfer kinetics was based on the varying thickness of a thiol-functionalized SAM as a spacer between the electrode and a redox center. This strategy seems to work well because it substantially removes the possibilities of permeation or diffusion of dissolved redox molecules through defect sites within the monolayers. Using this approach, we expand the range of redox species to hydroquinone in this work. The electrochemical properties of the hydroquinone/quinone derivatives in solution have been extensively studied because of their important biological activities over a few decades.16 However, only a few works have been performed for the electrochemical behavior of hydroquinone SAMs. In a sense, SAMs based on 2,5-dihydroxythiophenol have been first studied by Hubbard and Soriaga17,18 before the concept of self-assembled monolayers became popular. Hickman19 et al. reported the preparation of a mixed SAM of hydroquinone- and ferrocene-terminated alkanethiol for (10) (a) Bunding-Lee, K. A. Langmuir 1990, 6, 709. (b) Delong, H. C.; Buttry, A. D. Langmuir 1992, 8, 2491. (c) Katz, E.; Itzhak, N.; Willner, I. Langmuir 1993, 9, 1392. (11) Mukae, F.; Takemura, H.; Takehara, K. Bull. Chem. Soc. Jpn. 1996, 69, 246l. (12) Yu, H. Z.; Shao, H. B.; Luo, Y.; Zhang, H. L.; Liu, Z. F. Langmuir 1997, 13, 5774. (13) Wang, Y. Q.; Yu, H. Z.; Cheng, J. Z.; Zhao, J. W.; Cai, S. M.; Liu, Z. F. Langmuir 1996, 12, 5466. (14) Zhang, L.; Lu, T.; Gokel, G. W.; Kaifer, A. E. Langmuir 1993, 9, 786. (15) Gobi, K. V.; Okajima, T.; Tokuda, K.; Ohsaka, T. Langmuir 1998, 14, 1108. (16) Chambers, J. Q. In The chemistry of the quinonoid comfounds; Patai, S., Ed.; Wiley: New York, 1974; pp 737-792 (17) Soriaga, M.; Hubbard, A. T. J. Am. Chem. Soc. 1982, 104, 3937. (18) Mebrahtu, T.; Berry, G. M.; Bravo, B. G.; Michelhaugh, S. L.; Soriaga, M. P. Langmuir 1988, 4, 1147. (19) Hickman, J. J.; Ofer, D.; Laibinis, P. E.; Whitesides, G. M.; Wrighton, M. S. Science 1991, 252, 688.

10.1021/la001466y CCC: $20.00 © 2001 American Chemical Society Published on Web 03/24/2001

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Hong and Park Scheme 1. Procedure for the Synthesis of 2-(4-Mercaptobutyl)hydroquinone, H2Q(CH2)4SH

Figure 1. Molecular structure of 2-(n-mercaptoalkyl)hydroquinone, H2Q(CH2)nSH, where, n ) 1, 4, 6, 8, 10, and 12.

the development of a pH sensor. Uosaki20,21 et al. studied the electrochemical properties of a mercaptohydroquinone monolayer and potential-dependent film structures of mercaptoundecylhydroquinone SAMs using in situ Fourier transfer infrared reflection absorption spectroscopy (FTIRRAS) techniques. However, there has been no experimental data for the effects of alkyl chain length on electrochemical behavior of a hydroquinone (H2Q)terminated self-assembled monolayer. In this work, we have synthesized a series of 2-(nmercaptoalkyl)hydroquinones with different alkyl chain lengths (shown in Figure 1), abbreviated as H2Q(CH2)nSH (n ) 0, 1, 4, 6, 8, 10, and 12), and prepared self-assembled monolayers from a series of hydroquinone derivatives. We report here the electrochemical characteristics of a hydroquinone self-assembled monolayer on a gold electrode to study the effects of alkyl chain length on heterogeneous electron transfer kinetics, differential capacitance, microscopic integrity, and surface concentration. In addition, the shape and the peak potential of the redox waves are strongly affected by the concentration of a thiol deposition solution. The electrochemical measurements show potential utility of hydroquinone SAMs as a model system for interfacial phenomena studies of distance dependence of heterogeneous proton-coupled electrontransfer reaction. Experimental Section Materials. Methylene chloride and chloroform were refluxed over P2O5 and distilled freshly before use. THF was distilled from CaH2. p-Dimethoxybenzene, boron tribromide, n-butyllithium, dibromoalkanes, and potasium thioacetate were used as received from Aldrich Chemical Co. All other materials were reagent grade and used without purification. 2,5-Dihydroxythiophenol (H2QSH)22 and 2-(1-mercaptomethyl)hydroquinone, H2Q(CH2)SH,23 were synthesized from the published procedures. Preparation of H2Q(CH2)4SH was performed using the procedure shown in Scheme 1. Synthesis of 2-(4-Bromobutyl)-1,4-dimethoxybenzene (1). A solution of n-butyllithium (2.5 M in hexane, 11.44 mL, 28.6 mmol) was slowly added to a dry THF (40 mL) solution of 1,4-dimethoxybenzene (3 g, 22 mmol) in a nitrogen-purged Schlenk flask. The mixture was stirred for 30 min at room temperature, transferred dropwise via a cannula to a second flask containing 1,4-dibromobutane (7.88 mL, 66 mmol) at 0 °C. The contents were allowed to stir for 30 min at 0 °C, and the reaction was allowed to warm to room temperature. The mixture was stirred for 2 h at room temperature and poured into 25 mL of saturated NH4Cl solution. The solution was extracted with CH2Cl2 (3 × 20 mL), and the organic solution was washed with water and dried over MgSO4. The solvent and unreacted dimethoxybenzene were evaporated in vacuo, and the residue was purified with column chromatography (silica gel, 230-400 mesh, hexane/ether (v/v) ) 95/5) to give colorless oil (3.7 g, 61%). (20) Sato, Y.; Fujita, M.; Mizutani, F.; Uosaki, K. J. Electroanal. Chem. 1996, 409, 145. (21) Ye, S.; Yashiro, A.; Sato, Y.; Uosaki, K. J. Chem. Soc., Faraday Trans. 1996, 92, 3813. (22) Alcolay, W. Helv. Chim. Acta 1947, 30, 578. (23) Bae, I. T.; Sandifer, M.; Lee, Y. W.; Tryk, D. A.; Sukenik, C. N.; Scherson, D. A. Anal. Chem. 1995, 67, 4508.

1H NMR(CDCl ): δ 1.71 (quintet, 2H), 1.87 (quintet, 2H), 2.59 3 (t, 2H), 3.41 (t, 2H), 3.74 (s, 3H), 3.75 (s, 3H), 6.69-6.77 (m, 3H). Synthesis of 2-(4-Bromobutyl)-1,4-dihydroxybenzene (2). Compound 1 (3.7 g, 13.6 mmol) was dissolved in the dry methylene chloride (30 mL) in a round flask immersed in the ice bath. A solution of BBr3 (1.0 M in CH2Cl2, 40 mL, 40.4 mmol) was added dropwise to the solution of 1. The mixture was stirred at 0 °C for 30 min and was stirred again at room temperature for 1 h. The solution was poured into 25 mL of saturated NH4Cl solution. The solution was extracted with CH2Cl2 (3 × 20 mL), and the organic solution was washed with water and dried over MgSO4. The solvent was removed in vacuo, and the residue was purified with column chromatography (silica gel, 230-400 mesh, hexane/ether (v/v) ) 70/30) to give powder. The recrystallization with chloroform/hexane gave a white solid of 2 (2.0 g, 60%). 1H NMR(CDCl3): δ 1.71 (quintet, 2H), 1.87 (quintet, 2H), 2.59 (t, 2H), 3.41 (t, 2H), 4.41 (s, 1H), 4.46 (s, 1H), 6.63-6.50 (m, 3H). Synthesis of 2-(4-Thioacetylbutyl)-1,4-dihydroxybenzene (3). A solution of 2 (1 g, 4.1 mmol) and potassium thioacetate (0.7 g, 6.1 mmol) in dry DMF (10 mL) was stirred at room temperature for 1 h. The solution was poured into water (10 mL). The organic phases were combined after extraction with ether (3 × 20 mL) and washed with water (50 mL) and dried over MgSO4. After removal of solvent, the residue was purified with column chromatography (silica gel, 230-400 mesh, hexane/ether (v/v) ) 50/50) to give a yellow liquid of 3 (0.65 g, 66%). 1H NMR(CDCl3): δ 1.58-1.65 (m, 4H), 2.32 (s, 3H), 2.59 (t, 2H), 2.89 (t, 2H), 4.41 (s, 1H), 4.46 (s, 1H), 6.63-6.51 (m, 3H). Synthesis of 2-(4-Mercaptobutyl)hydroquinone (4). A solution of 3 (0.65 g, 2.71 mmol), methanol (20 mL), and 5 mL of concentrated HCl (35 wt % in water) was heated at reflux for 3 h. The solution volume was decreased under vacuum and was extracted with methylene chloride (3 × 20 mL). The organic solution was dried over MgSO4. After removal of solvent, the crude product was purified by chromatography (silica gel, 230400 mesh, hexane/ether (v/v) ) 20/80). Recrystallization with hot hexane gave a white solid (0.45 g, yield 83%) of 2-(4mercaptobutyl)hydroquinone. 1H NMR (CDCl3): δ 1.33 (t, 1H), 1.65-1.69 (m, 4H), 2.49-2.59 (m, 4H), 4.41 (s, 1H), 4.46 (s, 1H), 6.51-6.63 (m, 3H). The other hydroquinonethiols of H2Q(CH2)nSH (n ) 6, 8, 10, and 12) were prepared from their dibromoalkanes according to

Hydroquinone-Terminated SAMs the same procedure as shown in Scheme 1 and were identified by 1H NMR (CDCl3), respectively. H2Q(CH2)6SH: 1.34 (t, 1H), 1.28-1.42 (m, 4H), 1.49-1.60 (m, 4H), 2.44-2.55 (m, 4H), 4.58(br, 2H), 6.49-6.64 (m, 3H). H2Q(CH2)8SH: 1.34 (t, 1H), 1.27-1.35 (m, 6H), 1.54-1.57 (m, 6H), 2.45-2.55 (m, 4H), 4.43 (br, 2H), 6.49-6.64 (m, 3H). H2Q(CH2)10SH: 1.31 (t, 1H), 1.27-1.36 (m, 10H), 1.54-1.57 (m, 6H), 2.45-2.55 (m, 4H), 4.32 (br, 2H), 6.50-6.64 (m, 3H). H2Q(CH2)12SH: 1.31 (t, 1H), 1.27-1.36 (m, 14H), 1.50-1.54 (m, 6H), 2.40-2.59 (m, 4H), 4.35 (s, 1H), 4.40 (s, 1H), 6.40-6.65 (m, 3H). Monolayer Preparation. Gold bead electrodes (ca. 2.1-2.4 mm diameter) were prepared by annealing the tip of a gold wire (99.999%, 0.5 mm diameter) in a gas-oxygen flame. This bead electrode was immersed for 10 min in a hot “piranha” solution (3:1 mixture of concentrated H2SO4 and 30% H2O2). After copious rinsing with deionized water, the Au electrode was electrochemically cleaned by potential cycling in 0.5 M H2SO4 in the potential range of -0.30 and 1.50 V vs SCE until the typical cyclic voltammogram of clean gold was obtained. After being rinsed with deionized water and ethanol and dried with nitrogen gas blowing, the gold electrode was immersed in 1 mM ethanol solution of H2Q(CH2)nSH for 2-5 h, unless otherwise specified. The electrode was rinsed with copious amounts of fresh ethanol and deionized water prior to use in electrochemical experiments. The roughness factor for these Au electrodes was measured to be in the 1.1-1.2 range, and the actual surface areas were obtained from the slope of the linear plot of cathodic peak current versus (scan rate)1/2 for the reversible reduction of Ru(NH3)63+. As previously reported,24 we have utilized for this purpose a diffusion coefficient of 7.5 × 10-6 cm2 s-1(at 25 °C in 0.1 M NaCl). Measurements. Electrochemical measurements were carried out in a single compartment cell with a homemade Au bead electrode as a working electrode, a platinum counter electrode, and a saturated calomel electrode (SCE) as a reference electrode. Voltammetric experiments were performed with the use of a BAS 100B/W potentiostat controlled by a HP 586 personal computer and a BAS 100B/W software package. The electrolyte solutions were prepared with deionized water purified to a resistivity of 18 MΩ/cm with a UHQ II system (Elga) and deaerated by purging with nitrogen. All measurements were carried out at room temperature. 1H NMR spectra were recorded on a 200 MHz FT-NMR spectrophotometer. Britton-Robinson (BR) buffer was used to study the pH dependence of the hydroquinone-modified electrode. The BR buffer used consisted of citric acid (0.03 M), KH2PO4 (0.03 M), and H3BO3 (0.03 M). The pH value was adjusted with 0.2 M NaOH and 1.0 M HClO4.

Results and Discussion Effect of the Alkyl Chain Length on Voltammetric Response of SAMs. Figure 2 shows the typical cyclic voltammograms (CVs) for the spontaneously adsorbed H2Q(CH2)nSH (n ) 1, 4, 6, 8, 10, and 12) self-assembled monolayers on Au electrodes, which were constructed by dipping in 1 mM H2Q(CH2)nSH in ethanol for 30 min, obtained in an aqueous 0.1 M HClO4 solution. These redox wave shapes are not changed to continuous potential cycling over a 2 h period at room temperature, demonstrating that H2Q-terminated monolayers are chemically and thermodynamically stable in acidic solution. As expected for the voltammetric behavior of the surfaceconfined redox center,25 the redox peak current is linearly proportional to scan rate (v) up to 10 V/s (used in this study), and the ratio of ipa to ipc at a given v is very close to unity for all of these H2Q monolayers. This indicates that the electroactive H2Q monolayers were prepared in reproducible form. In each voltammogram, a pair of redox peaks was observed due to the oxidation and reduction of hydroquinone which follows a two-electron two-protontransfer reaction mechanism. For the H2Q(CH2)1SH SAM (24) Gomez, M. E.; Li, J.; Kaifer, A. E. Langmuir 1991, 7, 1797. (25) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980

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Figure 2. Cyclic voltammograms of H2Q(CH2)nSH-SAMs on gold (for n ) (a) 1, (b) 4, (c) 6, (d) 8, (e) 10, and (f) 12, respectively) in 0.1 M HClO4. Scan rate was 100 mV/ s.

Figure 3. Plot of redox peak potentials of H2Q(CH2)nSHSAMs (n ) 1, 4, 6, 8, 10, 12) as a function of the number of methylene unit in the alkyl chain spacer. Anodic (9), cathodic(b), and the surface formal potential(2). Data were taken from Figure 2.

shown in Figure 2a, a symmetrical reversible redox wave was observed at 0.37 V and the peak-to-peak separation (∆Ep) value, although not zero, was typically very small (ca. 15 mV), suggesting a rapid electron transfer reaction kinetics in the strong acidic condition. On the other hand, the significantly large ∆Ep value (ca. 630 mV) was observed for H2Q(CH2)12SH SAM (shown in Figure 2f). As can be seen in more detail in Figure 3, the value of ∆Ep measured at the same scan rate remarkably increases as the number of methylenes in the alkyl chain spacer increase. When the alkyl chain length becomes longer, the electron transfer from the electrode to the redox center is forced to proceed at a larger distance, slowing the overall electron-transfer rate. This fact indicates that the heterogeneous electrontransfer kinetics of H2Q in SAM is gradually transformed from reversibility to irreversibility due to an increase in spacer length. It is worthwhile to note that the formal potential E°′ (estimated as the average of the anodic and cathodic peak potentials) in the CVs recorded for the H2QSAMs shifts toward negative potential direction with the slope of ca. 16 mV per the number of methylenes. This observation is obviously ascribed to the fact that the cathodic peak potential moves much faster to the negative

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Table 1. Surface Coverage (Γ) and Double-Layer Capacitance (Cd) Data for H2Q(CH2)nSH-SAMs (n ) 1, 4, 6, 8, 10, 12) on a Gold Electrode n

Γ (10-10 mol/cm2)

Cd (µF/cm2)

bare Au 1 4 6 8 10 12

3.22 4.70 5.02 5.40 5.65 5.75

101.7 32.50 15.05 9.85 7.05 5.60 4.75

direction than the anodic peak potential shifts to the positive direction as the alkyl chain length increases. The reason for this phenomenon results from the fact that benzoquinone generated in the oxidaton process of H2Q SAM is more hydrophobic than hydroquinone. Therefore, reduction of benzoquinone generated in acidic solution is less favorable, resulting in the greater negative shift of the cathodic peak than the anodic peak with increasing chain length. However, it should be noted that the E°′ of a H2Q moiety itself in the monolayer is independent of the alkyl chain length. In this sense, the observed regular potential shift in E°′ as the thermodynamic indicator might be due to a slight change in the solvation environment around the redox center for H2Q-SAM. The surface coverage (Γ) values of electroactive H2Q moiety in H2Q(CH2)nSH (n)1, 4, 6, 8, 10, and 12) SAMs on gold are listed in Table 1. These values were calculated from the integration of redox peak area of the corresponding CVs obtained in 0.1 M HClO4, taking a 2e-, 2H+ reaction mechanism. The amount of electroactive H2Q in the monolayer is increased with the spacer chain length increasing. This fact implies the packing structure of a hydroquinone monolayer on the electrode surface becomes more ordered with longer alkyl chain spacer, in accord with the result obtained in the long-chain alkanethiol SAM.26 In general, an adsorbate molecule having a longer alkyl chain is known to form more well-ordered and -packed monolayers. However, this trend is different from the phenomena observed by Mirkin27 and Liu12 in the longchain azobenzene SAMs on gold. In those cases, the longer the alkyl spacer chain length, the smaller the value of surface coverage. This is due to a gradual decrease in the amount of azobenzene groups electrochemically accessible in SAMs with the spacer chain length increasing. The salient contrast between hydroquinone and azobenzene SAMs is basically ascribed to the large difference in the heterogeneous rate constant of redox center in SAMs. The electrochemical capacitance measurements is an useful method to probe the packing structure of a monolayer on the electrode, using the Helmholtz theory that the electrical double layer at the interface can be treated as an ideal capacitor. The differential capacitance (Cd) of a monolayer interface is estimated from the capacitive charging current divided by the scan rate and electrode surface area. Each Cd value was calculated from the capacitive current measured at its surface formal potential of cyclic voltammogram recorded in 0.1 M HClO4 at a scan rate of 100 mV/s, and all Cd values for the H2QSAMs on gold are listed in Table 1. From Table 1, it is obvious that the Cd value is decreased as the number of CH2 group in the alkyl spacer is increased. In addition, the modification of a bare gold electrode with H2Q(CH2)SH monolayer caused a remarkable decrease in the double (26) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (27) Campbell, D. J.; Herr, B. R.; Hulteen, J. C.; Van Duyne, R. P.; Mirkin, C. A. J. Am. Chem. Soc. 1966, 118, 10211.

Figure 4. Plot of reciprocal of capacitance vs n for H2Q(CH2)nSH-SAMs (n ) 1, 4, 6, 8, 10, 12). The electrolyte is 0.1 M HClO4 and the scan rate is 100 mV/s.

layer capacitance at the electrode/solution interface. In Figure 4, the reciprocals of the differential capacitance are plotted as a function of n, the number of CH2 group in the hydrocarbon chain. The best straight line was obtained from the linear regression analysis of Cd-1 value for n ) 1-12 within a deviation range of 10%. This result clearly shows that the differential capacitance for the H2Q(CH2)nSH SAMs is inversely proportional to the length of the spacer alkyl chain. The slope and intercept of this line are 0.017 cm2/µF per CH2 group and 0.006 cm2/µF, respectively. The slope value in this study is relatively smaller than that of 0.055 cm2/µF for the n-alkanethiol monolayer on gold26 and that of 0.15 cm2/µF for the azobenzene monolayer.12 According to the Helmholtz theory which models the electrical double layer as an ideal capacitor, the reciprocal of the capacitance per unit area is

Cd-1 ) d/ o (1)

(1)

where d is the thickness of the dielectric medium separating the two parallel conducting plates, o is the dielectric constant of the separation medium, and o is the permittivity of free space. Here, the  for the hydroquinone SAM can be estimated from the slope value ()1/ o) in eq 1 and the assumption of chain length per CH2 group of 0.13 nm, demonstrating that the  is found to be 8.64. This value is larger than that of 2.6 for n-alkanethiol monolayer26 and that of 2.3 for the dielectric constant of polyethylene.28 Recently, Liu13 et al. reported the larger  value of 13 for azobenzene monolayer containing long alkoxy functionality as a tail group and suggested that the asymmetric π-conjugate system of an azobenzene group is responsible for a large dielectric constant. In this regard, the high  value obtained for the present system may be attributed to the existence of the π-electron system, i.e., the hydroquinone moiety in a self-assembled monolayer on a gold electrode. However, the clustering of the water molecules around the hydrophilic hydroquinone terminal group is unlikely to serve to increase the dielectric constant. Another method to examine the packing structure of a monolayer is an electrochemical measurement of a heterogeneous electron transfer using a redox probe in solution phase, which is highly sensitive to structural defects or pinholes in the monolayer. This method provides (28) Lanza, V. L.; Herrman, D. B. J. Polym. Sci. 1958, 28, 622.

Hydroquinone-Terminated SAMs

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Figure 5. Cyclic voltammograms for bare (a) and H2Q(CH2)12SH-SAM modified (b, c) Au electrodes. The solutions are 1 mM Ru(NH3)63+ in 1 M HCl (a, c) and only 1 M HCl (b). The scan rate is 100 mV/ s. The current density (S) is 37 µA/cm2.

additional information on the degree of structural integrity of the monolayer. As the degree of integrity increases, the current of electron transfer will be largely decreased due to the blocking the electron transfer to solution redox species. Figure 5a shows a voltammetric response for a clean, bare gold with 1 mM Ru(NH3)63+ as the electroactive species in 1 M HCl solution. Ru(NH3)63+ was selected as an electrochemical probe because it is a well-characterized one-electron outer-sphere redox couple with very fast heterogeneous electron-transfer kinetics. On bare gold the shape of the i-E curve is indicative of a diffusion-limited and electrochemically reversible one-electron redox process. Parts b and c of Figure 5 show the CVs for a H2Q(CH2)12SH monolayer on gold in only 1.0 M HCl (solid line) and 1.0 M HCl containing 1 mM Ru(NH3)63+ (dotted line), respectively. This modified electrode shows almost the same i-E response, including a pair of redox peaks due to surface-immobilized H2Q(CH2)12SH irrespective of the presence of a fast redox species in solution. There is a remarkable difference between the i-E responses for Ru(NH3)63+ at the modified and bare gold electrodes. The diffusion current at the formal potential (ca. -0.2 V vs SCE) of the solution redox couple is almost attenuated for the modified electrode, and most of the current is capacitative. Only a slow rise in current with no visible peak or plateau currents is observed in the negative potential region. In general, the absence of peak or plateau currents indicates that the pinhole area fraction is extremely small. This current rise is believed due to electron transfer at the small amount of defect site in the monolayer. In this regard, the H2Q(CH2)12SH monolayer exhibits a substantial degree of molecular organization and packing because of its effective blocking electron transfer of Ru(NH3)63+. Effect of the Coating Solution Concentration on the CV Profile. Figure 6 shows the CVs of gold electrodes that have been immersed in ethanol solution containing various concentrations of H2Q(CH2)6SH at room temperature for 2 h, measured in 0.1 M HClO4. To find out the influence of solution concentration on the electrochemical behavior for hydroquinone monolayer, each value of the surface coverage and of the redox peak potential obtained from the CV data in Figure 6 are respectively plotted as a function of the concentration of coating solution (Figure 7). In Figure 6, the profile of the CVs recorded for these monolayers is not greatly altered until the concentration of H2Q(CH2)6SH is decreased down from 100 to 1 µM in coating solution. In this micromolar concentration region, the surface coverage seems to reach almost the full

Figure 6. Cyclic voltammograms of gold electrodes immersed in ethanol solution containing various concentrations of H2Q(CH2)6SH at room temperature for 2 h, measured in 0.1 M HClO4. The concentrations of H2Q(CH2)6SH in ethanol and current density scale (S) were as follows: (a) 100 µM, S ) 27.8 µA/cm2; (b) 10 µM, S ) 26.2 µA/cm2; (c) 5 µM, S ) 29.2 µA/cm2; (d) 1 µM, S ) 31.3 µA/cm2; (e) 0.1 µM, S ) 16.7 µA/cm2; (f) 0.01 µM, S ) 9.7 µA/cm2. Scan rate was 100 mV/s.

monolayer coverage of 5.0 × 10-10 mol/cm2 (Figure 7a) and the peak splitting of ∆Ep is gradually decreased as the concentration is decreased to 1 µM (Figure 7b). However, the CVs for monolayers obtained from 0.1 and 0.01 µM solutions display almost ideal surface electrochemical wave features (Figure 6e,f). The full width at half-maximum of 51 mV, very close to the Nernstian value of 45.3 mV for the two-electron-transfer system, was observed for the hydroquinone redox peaks, and the values of ∆Ep measured at a scan rate of 100 mV/s were 0 mV within experimental uncertainty ((5 mV) as shown in Figure 7b. This zero value of ∆Ep observed in the extremely dilute deposition solution (0.01 µM) is unexpected. These CV characteristics clearly indicate that there are no lateral interactions between the hydroquinone groups in the monolayer and that a rapid equilibrium is established with the electrode on the time scale of the experiment. When the concentration of coating solution beomes smaller than 1 µM, the surface coverage is abruptly decreased in a logarythmic scale. It is worth noting that an Au electrode modified in the coating solution as dilute as 0.01 µM shows salient redox peaks whose area is equivalent to 0.5% (2.6 × 10-12 mol/cm2) of the full monolayer coverage. These observations imply that the control of solution concentration is a significantly important factor to be considered for the tailored electrochemical properties of a modified electrode. As stated above, the surface formal potential obtained from 0.1 and 0.01 µM solutions is constant at 0.31 V and the ∆Ep value is zero, as shown in Figure 7b. The reason for these characteristics is due to redox centers remoting each other, implying that the local environment around each H2Q is so uniform as to have the same formal potential for all of the redox centers. For example, the surface coverage value of H2Q measured from the CV in Figure 6e is about 2.5 × 10-11 mol/cm2, which is equivalent to occupying an area of 667 Å2 per redox molecule. This molecular area is significantly larger than 53.6 Å2 of

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Figure 8. Cyclic voltammograms (at 100 mV/s) of H2Q(CH2)4SH-SAM on a gold electrode at various BR buffer solution pH values: (a) 1.3; (b) 2.3; (c) 3.0; (d) 4.0; (e) 5.0; (f) 6.0; (g) 7.0; (h) 8.0; (i) 9.0; (j) 9.6; (k) 10.0; (l) 10.6; (m) 11.0; (n) 11.5; (o) 12.1.

Figure 7. Variations in the surface coverage (a) and peak potentials (b) as a function of the concentration of H2Q(CH2)6SH in ethanol solution: anodic (b), cathodic (2), and the formal potential (9).

hydroquinone adsorbed in the flat orientation on the welldefined Pt single-crystal surface, reported by Hubbard29 et al. At this point, it might be difficult to have interactions between the redox centers due to their long-distant location from each other. Effect of pH on the Peak Potential. As expected, the redox response of the H2Q(CH2)nSH-SAMs would be pH dependent due to their H2Q moiety. To investigate this, the voltammetric response of a modified gold electrode, which was immersed in 1 mM H2Q(CH2)4SH ethanol solution for 2 h, was obtained in Britton-Robinson buffer solutions of varing pH from 1.3 to 12.1 (Figure 8). All the solutions contained 0.1 M ClO4-, and the pH was adjusted by adding 0.2 M NaOH and 1.0 M HClO4. During the pH excursion to 12.1, each CV has shown a couple of redox waves and the peak potentials shifted toward negative potential direction. In this excursion, the surface coverage has gradually decreased by 7% from the full coverage monolayer value (5.1 × 10-10 mol/cm2) for H2Q(CH2)4SH-SAM on gold. The reason for this may be due to the displacement of thiol by complex anions consisting of the mixed buffer system used. In addition, with an increase in pH, some of the surface-attached hydroquinone in its reduced form will be present in its deprotonated form that would be expected to be more soluble due to its higher charge.30 As shown in Figure 9, the E°′ value of the surface redox couple was pH dependent with a slope of 58.5 mV per pH unit, which is very close to the Nernstian value of 59 mV for a two-electron two(29) Soriaga, M.; Hubbard, A. T. J. Am. Chem. Soc. 1982, 104, 2735. (30) Lorenzo, E.; Sanchez, L.; Pariente, F.; Tirado, J.; Abruna, H. D. Anal. Chim. Acta 1995, 309, 79.

Figure 9. pH dependence of the peak potential for a H2Q(CH2)4SH-SAM modified gold electrode at various BR buffer solution pH values: anodic (0), cathodic (O), and the formal potential ((). Data were taken from Figure 8.

proton process. Since two pKa values of hydroquinone are known to be 9.85 and 11.4 in solution,31 one would expect a change in the slope of E°′ vs pH plot around these values as reported by Flaig32 et al. However, no change was observed for a H2Q(CH2)4SH-modified electrode in a very broad pH range examined in this study. This linear relation in the plot of E°′ vs pH was also observed for other hydroquinone SAMs and was basically in accordance with the results for gold electrodes modified with hydroquinonethiol (H2QSH)20 and mercaptomethylhydroquinone.33 These observations indicate that the pKa values of the surface-attached hydroquinone derivatives are much higher than those of free H2Q in solution. Another point that should be mentioned is that the large values (ca. 100-200 mV) of ∆Ep of the CV profile observed in the acidic solution were dramatically decreased to smaller values (5-20 mV) for pH values above around 9.0. This demonstrates that the two-proton-coupled two-electrontransfer mechanism of H2Q in the monolayer is gradually changed to be reversible in the basic condition, which allows a much simpler redox mechanism under the condition of a highly deficient of proton ion. However, the (31) Bishop, C. A.; Tong, L. K. J. J. Am. Chem. Soc. 1965, 87, 501. (32) Flaig, W.; Beutelspacher, H.; Riemer, H.; Kalke, E. Justus Liebigs Ann. Chem. 1968, 719, 96. (33) Kim, H.; Hong, H.-G. Bull. Korean Chem. Soc. 1998, 19, 1385.

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large peak splitting observed in the neutral pH region might be related to multiple reaction sequences according to protonation and electron-transfer reactions. Laviron proposed the theoretical treatment for a 2e-, 2H+ electrode process34 and experimentally verified the heterogeneous electron/proton-transfer kinetics of the p-benzoquinone/ H2Q couple on a platinum electrode.35 On the basis of Laviron’s theoretical analysis, the possible reaction sequences for H2Q oxidation in the SAMs are considered as e-, H+, e-, H+ at low pH, as H+, e-, H+, e- at high pH, and as e-, H+, H+, e- or H+, e-, e-, H+ at intermediate pH values. Electrochemical Kinetics of H2Q(CH2)nSH SAMs. The electrochemical behavior for the H2Q redox center has been further studied to investigate the distance dependence of heterogeneous electron-transfer kinetics and evaluate the electron tunneling constant for longrange electron transfer in H2Q(CH2)nSH SAMs on gold. As stated above, Laviron34-36 presented a theoretical treatment of the 2e-, 2H+ reaction based on the ninemember square scheme under the assumption that the protonation reactions are at equilibrium in the absence of disproportionation and dimerization. In this case, the electron transfer reaction behaves as a bielectronic process with two successive apparent rate constants depending on the pH and the difference between the redox potentials of the individual reactions. At this moment, it is not possible to determine two correct apparent rate constants for two monoelectronic steps experimentally without information such as pKa values of surface-confined redox center, multistepped reaction sequence, redox potential differences in the elemental reaction, and the transition potential related to the thermodynamic and kinetic characteristics of the 2e- and 2H+ reaction system. As an alternative to extract the apparent rate constant for overall bielectronic reaction, we use Laviron’s procedure37 though Laviron showed that the definition of both the transfer coefficients and apparent rate constants in a 2e-, 2H+ surface electrochemical process is not straightforward. Takehara11 and Liu12 recently reported kinetic parameters in SAMs of naphthoquinone and azobenzene, respectively, which possesses a 2e-, 2H+ transfer mechanism as in the present hydroquinone-SAM. The standard rate constant (ks) and the electron-transfer coefficient (R) can be determined by measuring variation of peak potential splitting (∆Ep) with scan rate according to the well-known Laviron’s formalism.37 The kinetic parameters may be obtained from eqs 2 and 3

Epc ) Eco′ - (RT/RnF) ln[RnFνc/RTkapp]

(2)

Epa ) Eao′ - (RT/(1 - R)nF) ln[(1 - R)nFνa/RTkapp] (3) where the peak potential separation is larger than 200 mV/n. νc and νa are the critical scan rates obtained by extrapolating the linear portion of the Ep vs ln(ν) plots to the formal cathodic and anodic potentials Eco′ and Eao′. The slopes of the linear portion of the Ep vs ln(ν) curves are RT/RnF for the cathodic branch and RT/(1 - R)nF for the anodic branch. The values of Rn and (1 - R)n were obtained from the values of each slope and substituted back in eqs 2 and 3 to solve for kapp. The two apparant rate constants (kapp) obtained in this way were averaged, and their average value was denoted by kapp. Table 2 presents (34) Laviron, (35) Laviron, (36) Laviron, (37) Laviron,

E. E. E. E.

J. J. J. J.

Electroanal. Electroanal. Electroanal. Electroanal.

Chem. Chem. Chem. Chem.

1983, 1984, 1983, 1979,

146, 164, 146, 101,

15. 213. 1. 19.

Table 2. Electrochemical Kinetic Parameters of H2Q(CH2)nSH-SAMs (n ) 0, 1, 4, 6, 8, 10, and 12) on a Au Electrodea n

Rn

(1 - R)n

log kapp/s-1

0 1 4 6 8 10 12

0.89 1.00 0.81 0.62 0.70 0.70 0.64

1.05 1.01 1.20 1.08 0.99 1.04 0.90

1.90 1.67 0.70 -1.02 -1.77 -2.50 -3.44

a The values of Rn and (1 - R)n are transfer coefficients due to reduction and oxidation of hydroquinone group for overall reaction. The value of kapp is the average value of anodic and cathodic apparent rate constants for overall redox reaction.

Figure 10. Dependence of log(kapp/s-1) on the number of CH2 units in the alkyl chain spacer of H2Q(CH2)nSH-SAMs (n ) 0, 1, 4, 6, 8, 10, and 12) on the Au electrode.

the electrochemical kinetic parameters of H2Q(CH2)nSHSAMs (n ) 0, 1, 4, 6, 8, 10, 12) on a gold electrode. Unlike the anodic transfer coefficient values, the cathodic transfer coefficient value (Rn) is slightly smaller than unity in the hydroquinone monolayer with a relatively long alkyl chain spacer. It may be due to a small change in the cathodic formal potential (Eco′) of hydroquinone, which slightly changes with the alkyl chain length in the monolayer. However, the sum of the cathodic and the anodic transfer coefficient values is quite close to 2 (i.e., the total number of transferred electrons) in all of these hydroquinone SAMs. Generally, an increase in electron-transfer distance results in a decrease in the apparent rate constant. Figure 10 shows the plot of the logarithmic heterogeneous apparent rate constant as a function of the number of methylene groups of H2Q(CH2)nSH SAMs on gold. The dependence of the electron-transfer rate constant (kapp) on the distance (d) between the redox center and the electrode surface can be evaluated by eq 4

kapp(at d2) ) kapp(at d1) exp[-β(d2 - d1)]

(4)

where β is the electron tunneling constant. Equation 4 is a simplified equation derived from the Marcus theory38 under the assumption that the free energy and reorganization energy are similar to each other for these hydroquinone monolayers. The tunneling constant can be easily obtained from the slope of the plot of the log(kapp) vs d. The slope of the plot shown in Figure 10 gives the β value of 1.04 ( 0.06 per CH2 unit using the number of methylenes within the alkyl chain spacer instead of (38) Marcus, R. A.; Sutin, N.; Biochim. Biophys. Acta 1985, 811, 265.

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absolute distance (Å) in this system. Several reports on the determination of the tunneling constant are available for a few different redox-active self-assembled monolayer systems. The experimental β value for our hydroquinone SAMs is in good agreement with the reported β values of 1.06 for a ferrocene-SAM,6 1.06 ( 0.04 for a Ru(NH3)5Py-SAM,8 and 1.21 ( 0.05 for a short alkyl chain ferroceneSAM.7 The tunneling constants were also measured for nonelectroactive SAM systems using redox couples in solution. For example, SAMs of ω-hydroxyalkylthiol39 and alkanethiol40 gave the β values of 1.08 ( 0.2 and 1.02 ( 0.2, respectively. These measurements show that the tunneling constant is somewhat insensitive to the identity of a redox center for alkyl chain spacer. However, the β value (1.34 ( 0.2) for the azobenzene-SAM reported by Liu,16 a little larger than that in this work, indicates that the conformational changes associated with the conversion of the planar azobenzene in the rigid film structure should be considered to evaluate the distance-dependent electrontransfer rate. Takehara11 et al. reported a much smaller β value (0.42) for naphthoquinone-SAM on gold, which also undergoes the same number of electron and protontransfer reactions as in the hydroquinone monolayer system. The small β value observed in the naphthoquinone system may be ascribed to the structural defects and disorder introduced by the limitations in film preparation procedure in which the naphthoquinone molecule is anchored to amine-functionalized surface. Conclusions In this study we have prepared a series of ω-mercaptoalkylhydroquinones, i.e., H2Q(CH2)nSH (where n ) 1, 4, 6, 8, 10, and 12), and showed that these thiol-functionalized hydroquinone derivatives form stable and closely packed self-assembled monolayers on a gold electrode. With an increase in the alkyl chain length for hydroquinone-SAMs, the increase in the peak-to-peak separation (∆Ep) measured at the same scan rate clearly shows that the heterogeneous electron-transfer kinetics of H2Q is gradually transformed from reversibility to irreversibility. As (39) (a) Miller, C.; Cuendet, P.; Gratzel, M. J. Phys. Chem. 1991, 95, 877. (b) Becka, A. M.; Miller, C. J. Phys. Chem. 1992, 96, 2657. (40) Xu, J.; Li, H. L.; Zhang, Y. J. Phys. Chem. 1933, 97, 11497.

Hong and Park

expected, a longer chain introduces greater interchain interaction and results in a higher surface coverage. The electrochemical capacitance measurements show that the differential capacitance of a hydroquinone monolayer interface is reciprocally proportional to the alkyl chain length. The hydroquinone-SAMs look fairly impermeable to block a very fast electron transfer of solution redox couple although they contain a pretty low fraction of defective sites within the monolayer structure. We have also found that SAMs from deposition solution at a sublevel of µM concentration displayed excellent electrochemical characteristics. The monolayers obtained from a 0.1 µM or lower concentration solution show the most ideal i-E features, as determined by the full-width at half-maximum and ∆Ep of the redox peaks. The hydroquinone monolayer exhibits a well-behaved pH dependence of surface formal potentials in the broad pH range (pH 1.3-12.1). Furthermore, the dependence of logarithmic apparent electrontransfer rate constant on the number of CH2 groups in the alkyl chain spacer shows that the electron-transfer kinetics depends on the distance between the hydroquinone and the electrode. From the distance dependence of electrontransfer kinetics, the electron tunneling constant and kinetic parameters were evaluated for the hydroquinone self-assembled monolayers on a gold electrode. The experimental value of β ) (1.04 ( 0.06)/CH2 unit for our hydroquinone system lies in good agreement with the values reported for reversible redox centers in solutions or tethered monolayer systems. Taken together, the hydroquinone-SAM is potentially a good model system for fundamental studies of longrange heterogeneous proton-coupled electron transfer, double-layer phenomena, and ion transport occurring in a highly organized monolayer. Currently, we are investigating the effects of temperature, solvent, immersion time, and coadsorption with diluent thiols on the heterogeneous electron-transfer kinetics for the hydroquinone-SAM system. Acknowledgment. This work was supported by Grant No. 1999-2-121-003-4 from the interdisciplinary research program of the KOSEF. LA001466Y