Electrochemical and Vibrational Spectroscopic Characterization of

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Langmuir 2000, 16, 9493-9500

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Electrochemical and Vibrational Spectroscopic Characterization of Self-Assembled Monolayers of 1,1′-Disubstituted Ferrocene Derivatives on Gold Sang Woo Han, Hwimin Seo, Young Keun Chung, and Kwan Kim* School of Chemistry and Molecular Engineering and Center for Molecular Catalysis, Seoul National University, Seoul 151-742, Korea Received January 10, 2000. In Final Form: September 14, 2000 Self-assembled monolayers (SAMs) of 1,1′-disubstituted ferrocene derivatives, i.e., 1,1′-bis(11-mercaptoundecyl)ferrocene (1) and 1-decyl-1′-(11-mercaptoundecyl)ferrocene (2), on a gold surface were prepared, and their structural and electrochemical properties were characterized by reflection-absorption infrared spectroscopy (RAIRS), ellipsometry, cyclic voltammetry (CV), and subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS). From the RAIR spectral features, both molecules, 1 and 2, were found to chemisorb on gold as thiolates after deprotonation. The peak positions of the methylene stretching modes indicated that the alkyl chains of 1 and 2 assume disordered structures on the gold surface. The thicknesses of the monolayers 1 and 2 on gold were determined by ellipsometry to be 2.27 ( 0.10 and 2.30 ( 0.10 nm, respectively. In the CV experiments, symmetric redox peaks were identified at ca. 0.32 and 0.30 V versus saturated calomel electrode (SCE) for the SAMs of 1 and 2, respectively. The surface coverage values determined from the CV of 1 and 2 were 3.4 × 10-10 and 3.2 × 10-10 mol/cm2, respectively. Both SAMs at full-coverage limits were stable in neutral (0.2 M NaClO4) as well as in acidic (0.2 M HClO4) medium, suggesting that hardly any decomposition of the ferricinium cation occurred for the SAMs prepared from disubstituted ferrocene derivatives. In particular, the SAMs of 1 were stable irrespective of the surface coverage, displaying the bonding capability of dithiols to gold; the submonolayer of 2 was slightly unstable in neutral medium. The SNIFTIR spectral data suggested that the alkyl chain of the two SAMs takes a more upright orientation when the ferrocene moiety is oxidized to a ferricinium cation.

Introduction The self-assembly of organic compounds on gold and other surfaces, e.g., Ag and Pt, is a convenient route to customization of the molecular architecture of the electrode-solution interface. Usually, this customization has been achieved through the selective use of functionalized head and tail groups in the synthesis of adsorbates.1 Such a strategy has allowed the chemical and physical properties of interfaces to be manipulated to generate a variety of surfaces with well-defined composition and structure.2 The self-assembled monolayers (SAMs) prepared in this way have opened new dimensions for studying of electron transfer at surfaces.2a,3 Insights into the distance dependence of electron-transfer rates are now emerging, for instance, through the systematic variation of the chain length of the polymethylene spacer between the electrode surface and a redox couple.3b,4 The effects of the local * To whom all correspondence should be addressed. Fax, +822-8743704 and +82-2-8891568; e-mail, [email protected]. (1) (a) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (b) Tredgold, R. H. Order in Thin Organic Films; Cambridge University Press: New York, 1994; Chapter 6. (c) Ulman, A. Chem. Rev. 1996, 96, 1533 and references therein. (2) (a) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87 and references therein. (b) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (c) Zhong, C.-J.; Porter, M. D. Anal. Chem. 1995, 67, 709A. (3) (a) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (b) Li, T.-T.; Weaver, M. J. J. Am. Chem. Soc. 1984, 106, 6107. (c) Tarlov, M. J.; Bowden, E. F. J. Am. Chem. Soc. 1991, 113, 1847. (4) (a) De Long, H. C.; Donohue, J. J.; Buttry, D. A. Langmuir 1991, 7, 2196. (b) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (c) Li, T.-T.; Weaver, M. J. J. Am. Chem. Soc. 1984, 106, 1233. (d) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173.

environment on the thermodynamics of the redox reactions could also be examined by burying the reactive moiety within the matrix of a longer-chain monolayer.5 In these kinds of investigations, however, one must consider whether a chemical transformation of a reactive moiety within the monolayer leads to a structural transformation of the monolayer. This question is important because, for instance, dissociation reactions (e.g., ester hydrolysis) or redox chemistry of alkanethiol-derivatized SAMs can lead to a rearrangement of the polymethylene chain structure which may affect the targeted performance.4a,5a,6,7 Ferrocene is a prototypical redox group that has frequently been incorporated at the terminal site of alkanethiol-derivatized electroactive SAMs on various electrodes. In this light, a wealth of information has been gathered by several research groups on the reactivity and stability of ferrocene-terminated alkanethiol SAMs on gold.8-15 All of the ferrocene-containing SAMs investigated so far were, however, derived from monoalkylthiol or (5) (a) Rowe, G. K.; Creager, S. E. Langmuir 1991, 7, 2307. (b) Creager, S. E.; Rowe, G. K. Anal. Chim. Acta 1991, 246, 233. (6) Corrigan, D. S.; Weaver, M. J. Langmuir 1988, 4, 599. (7) Sasaki, T.; Bae, I. T.; Scherson, D. A.; Bravo, B. G.; Soriaga, M. P. Langmuir 1990, 6, 1234. (8) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510. (9) Ye, S.; Sato, Y.; Uosaki, K. Langmuir 1997, 13, 3157. (10) Popenoe, D. D.; Deinhammer, R. S.; Porter, M. D. Langmuir 1992, 8, 2521. (11) Zotti, G.; Gilberto, S.; Zecchin, S.; Berlin, A.; Pagani, G. Langmuir 1998, 14, 1728. (12) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687. (13) (a) Sato, Y.; Frey, B. L.; Corn, R. M.; Uosaki, K. Bull. Chem. Soc. Jpn. 1994, 67, 21. (b) Sabapathy, R. C.; Bhattacharyya, S.; Leavy, M. C.; Cleland, W. E., Jr.; Hussey, C. L. Langmuir 1998, 14, 124. (14) Lenhard, J. R.; Murray, R. W. J. Am. Chem. Soc. 1978, 100, 7870. (15) Chidsey, C. E. D. Science 1991, 251, 919.

10.1021/la000025b CCC: $19.00 © 2000 American Chemical Society Published on Web 10/31/2000

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Langmuir, Vol. 16, No. 24, 2000 Scheme 1

Scheme 2

monoalkylcarboxylic acid-derivatized ferrocene molecules. The primary purpose of the present work was to observe the effect of 1,1′-disubstitution of the ferrocene moiety with two alkyl chains on the reactivity and electrochemical stability of ferrocene-containing SAMs on metal substrates. In this light, we prepared two kinds of adsorbates, i.e., 1,1′-bis(11-mercaptoundecyl)ferrocene (1) and 1-decyl1′-(11-mercaptoundecyl)ferrocene (2), and investigated the electrochemical and infrared spectroscopic characteristics of their SAMs on gold. Following our previous work on simple aromatic dithiols,16 this investigation was also intended to see the possible structural and stability differences of the SAMs caused by the presence of different numbers of thiol groups in the adsorbate. Experimental Section General. All solvents were purified by standard methods. Reagent-grade chemicals were used without further purification. Triply distilled water, of resistivity greater than 18.0 MΩcm, was used to prepare aqueous solutions. The syntheses of the 1,1′-disubstituted ferrocene derivatives, compounds 1 and 2, are outlined in Schemes 1 and 2, respectively. Compounds 1,1′-bis(11-bromoundecyl)ferrocene (3)17 and 1-oxodecylferrocene (4)18 were prepared according to published procedures. Flash column chromatography was performed on Merck silica gel 60 (230-400 mesh). 1H NMR spectra were obtained using a Bruker DPX 300 instrument. Elemental analyses were performed at the Chemical Analytic Center, College of Engineering, Seoul National University. 1,1′-Bis(11-mercaptoundecyl)ferrocene (1). A solution of 3 (0.63 g, 0.96 mmol) and thiourea (0.74 g, 9.6 mmol) in THF (7 mL) and ethanol (15 mL) was heated to reflux for 20 h and cooled to room temperature. To the mixture were added K2CO3 (1.74 g, 12.5 mmol) and H2O (5 mL). The mixture was heated to reflux for 2.5 h and evaporated to dryness. Extraction with ether, drying with anhydrous magnesium sulfate, concentration, and column chromatography on a silica-gel column eluting with hexane gave 1 in 59% yield (0.32 g). 1H NMR (CDCl3) δ: 4.07 (br s, 8H), 2.52 (q, J ) 7.33 Hz, 4H), 2.23 (br s, 4H), 1.61∼1.27 (m, 38H). Anal. Calcd for C32H54S2Fe (558.75): C, 68.79; H, 9.74; S, 11.48. Found: C, 68.53; H, 9.94; S, 11.77. (16) (a) Lee, T. G.; Kim, K.; Kim, M. S. J. Phys. Chem. 1991, 95, 9950. (b) Cho, S. H.; Han, H. S.; Jang, D.-J.; Kim, K.; Kim, M. S. J. Phys. Chem. 1995, 99, 10594. (c) Lee, Y. J.; Jeon, I. C.; Paik, W.-K.; Kim, K. Langmuir 1996, 12, 5830. (d) Kim, C. H.; Han, S. W.; Ha, T. H.; Kim, K. Langmuir 1999, 15, 8399. (e) Joo, S. W.; Han, S. W.; Kim, K. J. Phys. Chem. B 1999, 103, 10831. (17) Bhatt, J.; Fung, B. M.; Nicholas, K. M. Liq. Cryst. 1992, 12, 263. (18) Wang, K.; Gokel, G. W. J. Phys. Org. Chem. 1997, 10, 323.

Han et al. 1-(11-Bromooxoundecyl)-1′-oxodecylferrocene (5). A solution of bromoundecanoic acid (2.35 g, 8.86 mmol) and SOCl2 (3 mL) was heated to reflux for 2 h. After all volatiles were removed, the residue was dissolved in dichloromethane (15 mL) and cooled to 0 °C. To the cold solution were added AlCl3 (1.18 g, 8.86 mmol) and a solution of 4 (1.94 g, 5.70 mmol) in dichloromethane (5 mL). The solution was allowed to warm to room temperature, stirred overnight, and quenched with ice water. To the aqueous solution was added an aqueous solution of K2CO3. Extraction with ether, drying with anhydrous magnesium sulfate, concentration, and column chromatography on a silica-gel column eluting with hexane/ether (v/v, 10:1) gave 5 in 43% yield (1.24 g). 1H NMR (CDCl3) δ: 4.77 (t, J ) 1.92 Hz, 4H), 4.48 (t, J ) 1.85 Hz, 4H), 3.41 (t, J ) 6.87 Hz, 2H), 2.65 (t, J ) 7.41 Hz, 4H), 1.85 (m, 2H), 1.68 (m, 3H), 1.42∼1.29 (m, 25H), 0.88 (t, J ) 6.68 Hz, 3H). Anal. Calcd for C31H47O2BrFe (587.45): C, 63.38; H, 8.06. Found: C, 62.94; H, 8.14. 1-(11-Bromoundecyl)-1′-decylferrocene (6). To a solution of 5 (0.68 g, 1.36 mmol) in 15 mL of THF at 0 °C were added NaBH3CN (1.25 g, 20 mmol) and BF3‚OEt2 (2.1 mL, 17 mmol). The solution was allowed to warm to room temperature, stirred for 6 h, and cooled to 0 °C. To the cold solution was added ammonia water. Extraction with ether, drying with anhydrous magnesium sulfate, concentration, and column chromatography on a silicagel column eluting with hexane gave 6 in 77% yield (0.49 g). 1H NMR (CDCl3) δ: 3.99 (s, 8H), 3.40 (t, J ) 6.85 Hz, 2H), 2.28 (t, J ) 7.64 Hz, 4H), 1.85 (m, 2H), 1.56∼1.28 (m, 32H), 0.88 (t, J ) 6.66 Hz, 3H). Anal. Calcd for C31H51BrFe (559.48): C, 66.55; H, 9.19. Found: C, 66.39; H, 8.89. 1-Decyl-1′-(11-mercaptoundecyl)ferrocene (2). A solution of 6 (0.67 g, 1.44 mmol) and thiourea (0.40 g, 7.20 mmol) in THF (8 mL) and ethanol (10 mL) was heated to reflux for 6.5 h. After the solution was cooled to room temperature, K2CO3 (1 g, 10 mmol) and water (5 mL) were added. The resulting solution was heated to reflux for 2 h and the solvent was removed under reduced pressure. Extraction with ether, drying with anhydrous magnesium sulfate, concentration, and column chromatography on a silica-gel column eluting with hexane gave 2 in 79% yield (0.48 g). 1H NMR (CDCl3) δ: 4.11 (br s, 8H), 2.52 (q, J ) 7.36 Hz, 2H), 2.20 (br s, 4H), 1.65∼1.27 (m, 35H), 0.88 (t, J ) 6.65 Hz, 3H). Anal. Calcd for C31H52SFe (512.67): C, 72.63; H, 10.22; S, 6.25. Found: C, 72.30; H, 10.54; S, 6.08. Self-Assembly of 1 and 2 on Gold. The gold substrates used for self-assembly of 1 and 2 were prepared by resistive evaporation of titanium (Aldrich, >99.99%) and gold (Aldrich, >99.99%) at ∼1 × 10-6 Torr onto freshly cleaved mica sheets (AshevilleSchoonmaker, for subtractively normalized interfacial Fourier transform infrared spectroscopy [SNIFTIRS] measurement) or glass slides (for reflection-absorption infrared spectroscopy [RAIRS], ellipsometry, and cyclic voltammetry [CV] measurements). Glass slides had been previously cleaned by sequential sonication in isopropyl alcohol, hot 1:3 H2O2 (30%)/H2SO4, and distilled, deionized H2O. Deposition of titanium was performed before that of gold to enhance adhesion of Au to the substrate. After deposition of approximately 200 nm of gold, the evaporator was backfilled with nitrogen. The gold substrates were subsequently immersed into a 1 mM solution of 1 or 2 in benzene for a predetermined period of time. After the substrates were removed, they were rinsed with excess benzene and then ethanol, and dried in a N2 gas stream. Reflection-Absorption Infrared Spectroscopy. All infrared spectra were obtained using a Bruker IFS 113v Fourier transform spectrometer equipped with a globar light source and a liquid N2-cooled, wide-band mercury cadmium telluride detector. The method for obtaining the RAIR spectra has been reported previously.19 Each spectrum was obtained by averaging 512 or 1024 interferograms at 4 cm-1 resolution with p-polarized light incident on the gold substrate at 80°. To reduce the effect of water-vapor rotational lines, we recorded the sample and reference interferograms alternately after every 32 scans. The Happ-Genzel apodization function was used in Fourier trans(19) (a) Son, D. H.; Ahn, S. J.; Lee, Y. J.; Kim, K. J. Phys. Chem. 1994, 98, 8488. (b) Kim, S. H.; Ahn, S. J.; Kim, K. J. Phys. Chem. 1996, 100, 7174. (c) Han, S. W.; Ha, T. H.; Kim, C. H.; Kim, K. Langmuir 1998, 14, 6113.

SAMs of 1,1’-Disubstituted Ferrocene Derivatives forming all the interferograms. The RAIR spectra are reported 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 of the sample and the bare clean metal substrate, respectively. Ellipsometry Measurement. The thicknesses of the 1 and 2 SAMs on gold were estimated using a Rudolph Auto EL II optical ellipsometer. The measurements were performed with the 632.8-nm line of an He/Ne laser incident on the sample at 70°. The ellipsometric parameters, ∆ and Ψ, were determined for both the bare clean substrate and the self-assembled film. The DafIBM program supplied by Rudolph Technologies was employed to determine the thickness values. At least seven different sampling points were considered in obtaining the averaged value. Voltammetry Measurement. All voltammetric measurements were carried out in a three-electrode cell using a CH Instrument Model 600A potentiostat, which employed CHI 600A Electrochemical Analyzer software (v. 2.03) running on an IBMcompatible Pentium computer. A gold-coated glass substrate onto which 1 or 2 was self-assembled served as a working electrode. The electrode was clamped against an O-ring in a joint on the side of the electrochemical cell. The O-ring provided a liquidtight seal and also defined the area of the working electrode (0.28 cm2). The reference electrode was a saturated calomel electrode (SCE); all potentials quoted in this work were relative to that of the SCE. A platinum spiral wire was used as the counter electrode. The electrolyte was HClO4 or NaClO4, and all experiments were carried out at room temperature. Before initiating any electrochemical measurement, we deaerated the electrolyte solution with high-purity N2 gas. Subtractively Normalized Interfacial Fourier Transform Infrared Spectroscopy. The experimental setup of SNIFTIRS has been reported previously.19a,20 The cell body was made from Teflon, and a triangular CaF2 prism was used as an optical window. A gold-coated mica substrate onto which 1 or 2 had been self-assembled was used as a working electrode. The electrode was pushed with a micrometer to the CaF2 window for contact. A Pt wire was used as a counter electrode. A silver wire was put into the cell as a quasi-reference electrode (AgQRE) and its potential was calibrated against the SCE; all potentials quoted in this work were thus relative to that of the SCE. The electrode potential was controlled by a Bioanalytical Systems CV-27 potentiostat. The SNIFTIR spectra were obtained by recording the reference and sample interferograms alternately after every 32 scans at two different potentials; the angle of incidence of p-polarized light was set at 60° with respect to the electrode surface normal when recording the spectra. The collection of interferograms was synchronized with the potential change such that the former started 5 s after the latter was made. The total number of scans at each specified potential was 1024 with 4 cm-1 resolution. The aqueous electrolyte solution was bubbled with high-purity N2 gas before being used to fill up the cell. The SNIFTIR spectra are reported as -∆R/R; the upward and downward peaks thus mean stronger and weaker absorption, respectively, at the sample potential compared to the reference potential.

Results and Discussion Ex Situ RAIR Spectroscopy of the Ferrocene Derivatives on Gold. The infrared spectra of the adsorbate precursors and their monolayers formed on gold are compared in Figure 1; parts a and b show the transmission infrared (TIR) spectra of 1 and 2 taken, respectively, and parts c and d show the RAIR spectra of 1 and 2, respectively, self-assembled on gold from a 1 mM benzene solution. The self-assembly times for the preparation of 1 and 2 SAMs were 6 and 24 h, respectively, to allow for the comparatively slower adsorption kinetics of monothiol versus dithiol on noble metals.21 Major peaks in Figure 1 are collectively summarized in Table 1, along (20) Son, D. H.; Kim, K. Bull. Korean Chem. Soc. 1994, 15, 5. (21) Zhang, L.; Lu, T.; Gokel, G. W.; Kaifer, A. E. Langmuir 1993, 9, 786.

Langmuir, Vol. 16, No. 24, 2000 9495

Figure 1. Transmission infrared spectra of 1 (a) and 2 (b) in neat state; RAIR spectra of 1 (c) and 2 (d) on Au. The SH grouprelated bands are marked with asterisks and the twistingrocking (Tx) and wagging progression (Wx) bands are marked with arrows. The inset shows the expanded illustration of the S-H stretching region. Table 1. Infrared Spectral Data for Compounds 1 and 2 Compound 1 neat (cm-1)a

RAIR (cm-1)b

3095, 3074 3085 2922

2926

2850 2563 1470 1464

2853 1467 1462

Compound 2 neat (cm-1)c 3086 2954 2922 2852 2570 1466

RAIR (cm-1)b 3086 2967 2925 2881 2854 1470

1459 1456 1433 1437 1439 1436 1106 1091 1099 1040, 1023 1039, 1007 1039, 1020 1041, 1028 914 922 822, 808 822, 802 850, 822, 804 822, 810 727, 719 721 721 721

Assignmentd ν(CH, Fce) νas(CH3) νas(CH2) νs(CH3) νs(CH2) ν(SH) ν(Fc-C) δ(CH2) γ(CH3) ν(CC, Fc) γ(CH, Fc) δ⊥(CH, Fc) δ(CSH) δ|(CH, Fc) ν(CS)

a Taken in KBr matrix. b Reflection-absorption infrared spectrum on gold film. c Drop-cast film on KBr crystal. d Assigned based on refs 10 and 22. e Fc: Ferrocene moiety.

with appropriate assignments made based on literature data.10,22 In the TIR spectra of 1 and 2, the S-H stretching (22) (a) Fritz, H. P. In Advances in Organometallic Chemistry; Stone, F. G. A., West, R., Eds.; Academic Press: New York, 1964; Vol. 1, pp 240-316. (b) Adams, D. M. Metal-Ligand and Related Vibrations: A Critical Survey of the Infrared and Raman Spectra of Metallic and Organometallic Compounds; St. Martin’s Press: New York, 1968; pp 189-234. (c) 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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peaks appeared at 2563 and 2570 cm-1, respectively, but their counterparts were completely absent in the RAIR spectra (see the asterisks and inset in Figure 1). The CSH bending bands identified at 914 and 922 cm-1 in the TIR spectra of 1 and 2, respectively, were also completely missing in the RAIR spectra. This observation indicates, as expected, that compounds 1 and 2 chemisorb on gold as thiolates. On the other hand, in the RAIR spectra one can readily identify several peaks associated with the ferrocene moiety as well as the C-H stretching peaks of the methyl and/or methylene groups. This observation implies that the species responsible for the RAIR spectra are in fact deprotonated forms of 1 and 2. Among the observed peaks related to the ferrocene moiety, the bands at 1039 and 1007 cm-1 in Figure 1c and at 1041 and 1028 cm-1 in Figure 1d can be assigned as the δ⊥(CH, Fc) modes having an out-of-plane character with respect to the plane of the cyclopentadienyl ring; their counterparts are observed at 1040 and 1023 cm-1 in Figure 1a and at 1039 and 1020 cm-1 in Figure 1b. Other modes of the ferrocene moiety can be rendered to possess in-plane character. One can notice that the relative intensities of the in-plane and out-of-plane modes of the ferrocene moiety in the RAIR spectra are a little different from those in the TIR spectra. Recalling the infrared surface-selection rule that vibrational modes whose transition dipole moments are directed normal to the metal surface are exclusively infrared active,23 the latter observation suggests that the ferrocene moieties of 1 and 2 have neither perpendicular nor parallel orientation with respect to the gold surface. It is well-known that the peak positions of the symmetric as well as the antisymmetric CH2 stretching vibrations can be used as a sensitive indicator of the ordering of the alkyl chains. Lower wavenumbers indicate highly ordered conformations with preferential all-trans characteristics;1a,3a for all-trans zigzag conformations, the νs(CH2) and νas(CH2) modes are usually observed below 2850 and 2920 cm-1, respectively. As can be seen in Figure 1 and Table 1, the νs(CH2) and νas(CH2) bands appeared at 2853 and 2926 cm-1 and at 2854 and 2925 cm-1 in the RAIR spectra of 1 and 2, respectively, on gold. This suggests that the alkyl chains of the SAMs of 1 and 2 on gold assume disordered structures.24 The disordering of the alkyl chains in the SAMs of 1 can also be inferred from the disappearance of the twisting-rocking (Tx) and wagging progression bands (Wx) in the RAIR spectrum (Figure 1c); the latter bands are clearly identified in the region of 1180 and 1380 cm-1 in the TIR spectrum (see the arrows in Figure 1a). The presence of those kinds of progressional bands as a series of well-resolved peaks has been known to be a strong indicator of crystallinity. For example, in the liquid-phase IR spectra of linear alkanes, such bands are weak and show broad features, whereas sharp features are seen for the crystalline phase.25 Considering that the SAMs of undecanethiolate on gold possess fairly wellordered alkyl chains, the disordering of the alkyl chains in the SAMs of 1 and 2 must be ascribed to the structural constraint imposed by the bulky ferrocene moiety. The SAMs of monosubstituted ferrocene derivatives such as 11-ferrocenyl-1-undecanethiol9 and 11-sulfidoundecyl ferrocenecarboxylate10,12 on gold have been reported to assume similarly disordered alkyl chains. (23) Greenler, R. G. J. Chem. Phys. 1966, 44, 310. (24) Because the positions of the νs(CH2) and νas(CH2) bands measured at 2 cm-1 resolution were the same as those measured at 4 cm-1 resolution, the mentioned peak position difference should be meaningful. (25) (a) Maroncelli, M.; Qi, S. P.; Strauss, H. L.; Snyder, R. G. J. Am. Chem. Soc. 1982, 104, 6273. (b) Snyder, R. G. J. Chem. Phys. 1967, 47, 1316.

Han et al.

Figure 2. Equilibrium geometries of deprotonated forms of 1 (a) and 2 (b) obtained from AM1 semiempirical calculations.

Semiempirical Calculation and Ellipsometry Measurement. The present work suggests that the two alkyl chains, i.e., two undecyl chains for 1 and one undecyl and the other decyl chain for 2, substituted at the 1 and 1′ positions of the ferrocene moiety do not come close to each other when forming two-dimensional SAMs on gold. To provide firmer evidence, it seemed necessary to perform a quantum mechanical calculation. To this end, we have carried out an AM1 semiempirical calculation to determine the equilibrium geometries of the deprotonated forms of 1 and 2.26 Although the level of calculation might not be extensive, the optimized geometries, shown in Figure 2, were hardly dependent on the initial geometries, and the two alkyl chains were always calculated to be directed away from the ferrocene moiety. (Ferrocene derivatives are usually more stable when the two cyclopentadienyl rings are staggered to each other. Intriguingly, the two rings in 1 and 2 are calculated to assume eclipsed conformation. Two long alkyl chains are supposed to impose structural constraint for the rings to have eclipsed conformation.) The disordering of the alkyl chains in the 1 and 2 SAMs on gold seems thus to be related to the long distance between the two alkyl chains; because they are apart from each other, a van der Waals-type interchain interaction will be nearly impractical. Using standard bond lengths, bond angles, and van der Waals atomic radii, one can predict the monolayers of molecules adsorbed on metal substrates with a specific binding geometry and orientation. Supposing that the 1 and 2 SAMs on gold assume geometries similar to those in Figure 2, the thicknesses of the two SAMs all are estimated to be around 2.5 nm. It would be desirable to compare this value with ellipsometric measurements. Because the ellipsometry equations did not yield independent estimates of the refractive index and thickness, a refractive index of 1.45 was assumed for the estimation of thickness.3a From a three-phase optical model, the ellipsometric parameters corresponded to thicknesses of 2.27 ( 0.10 and 2.30 ( 0.10 nm for the SAMs of 1 and 2, respectively, on Au. Although we could not rule out the growth of oxide overlayers and other ambient contamination, the values were reproducibly obtained. Considering that the thickness determined by ellipsometry is, in principle, a spatially averaged value and thus does not reflect the real molecular height, the present work strongly implies that the actual structures of the 1 and 2 SAMs on Au will be comparable to those depicted in Figure 2. Cyclic Voltammetry Measurement of the Ferrocene Derivatives on Gold. Typical CVs of the SAMs of 1 and 2 on Au electrodes recorded in an aqueous 0.2 M HClO4 solution are shown in parts a and b, respectively, (26) AM1 semiempirical calculations were performed with the PC Spartan Pro 1.0 program on an IBM-compatible computer.

SAMs of 1,1’-Disubstituted Ferrocene Derivatives

Figure 3. Representative cyclic voltammograms of SAMs prepared from compounds 1 (a) and 2 (b). The experiments were carried out in 0.2 M HClO4 at v ) 100 mV/s.

of Figure 3. The Au electrodes were the same substrates used in obtaining the RAIR spectra shown in Figure 1c,d. Symmetric redox peaks were identified at ca. 0.32 and 0.30 V (average of the anodic and cathodic peak potential, i.e., formal potential, E°′) for the SAMs of 1 and 2, respectively. Repeated scanning did not affect the CVs, demonstrating that the two SAMs were stable with respect to the potential cycling. The shape of the CV peaks was independent of the potential scan rate, v, and the ratio of the anodic and cathodic peak current, ipa/ipc, was unity at a given scan rate. In addition, the peak height varied linearly with v in the range v ) 0.05-1.5 V/s. These findings illustrate that the CV peaks are in fact derived from surface-confined species.27 Using the faradic charges in the CVs, we estimated the values of surface coverage to be 3.4 × 10-10 and 3.2 × 10-10 mol/cm2, respectively, for the SAMs of 1 and 2 on gold after accounting for surface roughness (roughness factor 1.128). Theoretically, if one treats the ferrocene group as a sphere with a diameter of 0.66 nm, the surface coverage of ferrocene derivatives on gold in a full-coverage limit is predicted to be 4.5 × 10-10 mol/cm2.4b,29 The low surface coverage actually measured is supposed to be related to the proposition that the two alkyl chains in the 1 and 2 SAMs are directed away from the ferrocene moiety as depicted in Figure 2. It is well-documented in the literature that the full width at half-maximum, ∆Efwhm, of the anodic voltammetric wave (27) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley & Sons: New York, 1980. (28) Roughness factor of gold substrates was determined using the roughness analysis routine applied to the AFM images of gold electrodes.

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provides a qualitative measure of the relative interaction taking place between the adsorbates within a monolayer. When the interaction between the electroactive tail groups of the adsorbates is negligibly weak, the width is expected to be ∆Efwhm ) 3.53RT/nF, i.e., 90.3/n mV at 24 °C.27 For the SAMs of 1, the value of ∆Efwhm was in fact close to the latter, implying that there was minimal lateral interaction between the redox centers. The CV feature of 1 in Figure 3a is comparable to those of other ferrocene-derivatized SAMs.8,9,12,13 However, for the SAMs of 2, the ∆Efwhm was ca. 30 mV, which is substantially narrower than the value expected for a simple faradic process. Similar spikelike peaks were observed for the poly(vinylferrocene)30 and aminophenylferrocene (∆Efwhm ) 28 mV)31 modified Pt electrode and the 6-ferrocenylhexanethiol (∆Efwhm ) 21 mV) modified Au electrode.8 Such sharp current peaks were suggested to imply that the ratio of the activities of the oxidized and reduced surface molecules was subjected to little change during exhaustive electrolysis of the surface layer.8,31 Intriguingly, non-faradic reorganization peaks were reported even for the cases in which no redox reaction was involved. For instance, Rusling and Couture32 observed current spikes at Hg and Ag electrodes in an aqueous micellar system containing hexadecyltrimethylammonium bromide (CTAB) and straight-chain alcohols. They attributed the peaks to the rapid structural reorganization of the mixed alcohol/CTAB layer on the electrode surface. Although the origin of different CV features of SAMs of 1 and 2 is not certain at the moment, the latter type of explanation seems not to be applicable to the present system, because the structure changes of the two SAMs upon oxidation/reduction of the ferrocene group appeared comparable to each other by in situ infrared spectroscopy (vide infra). Before evaluating the inherent reactivity and long-term stability of the 1 and 2 SAMs on gold, we must mention that a ferricinium cation (FeCp2+) can decompose through an exchange of the cyclopentadienyl anion (Cp-) with other nucleophiles such as OH-, Cl-, and NO3-.33 The rate of exchange increases as a function of the donor strength of the nucleophile, and the Cp- released can react with the undissociated FeCp2+ to produce a cyclopentadienyl radical. Much the same decomposition reaction has been reported to occur even for the SAMs of monosubstituted ferrocene derivatives on gold10,12 and platinum electrodes.14 For instance, the half-life of the Fc+COOC11S monolayers on gold was reported by Popenoe et al.10 to follow the donor strength dependence found in the solution-phase studies (e.g., Cl->Br->NO3->SO42-); in the latter medium at pH 6.5, the coverage of the ferrocene moiety decreased by more than 95% after 2-3 scans between the voltage limits of +0.2 and +0.85 V versus SCE. The loss of electroactivity of the Fc+COOC11S monolayer was markedly reduced in perchlorate solutions, however; in both 1.0 M HClO4 and 0.1 M LiClO4, the monolayer was stable even after +0.2 V was applied for a prolonged time. Nonetheless, when +0.70 V (∼0.15 V higher than the formal potential) was applied, about 30% of the ferricinium in the oxidized layer was lost after 50 min at pH 0, and nearly all of the ferricinium became lost at pH 4.0. At pH 6.5, the loss of ferricinium from the layer was essentially complete (>95%) within 20 min; the increase in the decomposition (29) Seiler, P.; Dunitz, J. D. Acta Crystallogr. 1979, B35, 1068. (30) Daum, P.; Murray, R. W. J. Electroanal. Chem. 1979, 103, 289. (31) Willman, K. W.; Rocklin, R. D.; Nowak, R.; Kuo, K.-N.; Schultz, F. A.; Murray, R. W. J. Am. Chem. Soc. 1980, 102, 7629. (32) Rusling, J. F.; Couture, E. C. Langmuir 1990, 6, 425. (33) (a) Prins, R.; Korswagen, A. R.; Kortbeek, A. G. T. G. J. Organomet. Chem. 1972, 39, 335. (b) Holecek, J.; Handlir, K.; Klikorka, J.; Dinh Bang, N. Collect. Czech. Chem. Commun. 1979, 44, 1379.

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rate was presumed to arise from the increase in the hydroxide ion concentration in solution. A decomposition of ferricinium cations has also been observed in airsaturated organic solvent by Zotti et al.11 In addition, they found that a thin layer of amorphous iron oxide formed on indium-tin-oxide (ITO) electrodes was able to give stable monolayers from carboxyl-terminated ferrocene derivatives such as 7-ferrocenylheptanoic acid (FcC6COOH). On the basis of the above information, we have attempted to compare the relative stability of monolayers formed by mono- and disubstituted ferrocene derivatives. To evaluate the stability of the 1 and 2 SAMs on gold, we generated a series of CV curves in acidic (0.2 M HClO4) and neutral (0.2 M NaClO4) media after holding the potential at +0.50 V for a prolonged time (up to 1 h); +0.50 V was about 0.2 V higher than the formal potential of the two SAMs. Surprisingly, even after these treatments, almost identical CV curves were obtained for the two SAMs, not only in the acidic medium but also in the neutral medium. This result is obviously in contrast with what is observed for the SAMs formed by monosubstituted ferrocene derivatives, illustrating that the disubstitution of the ferrocene moiety greatly enhances the stability of the ferricinium cation. We must mention, however, that the stability of the SAMs of 2 is somewhat dependent on the initial surface coverage of the monolayer, whereas the electrochemical activity of the SAMs of 1 is not affected at all down to the surface coverage of ∼0.5. This finding is evidenced by the CV curves of the SAMs of 2 on gold (the initial surface coverage was ∼0.7). These curves, shown in parts a and b of Figure 4 are generated in 0.2 M HClO4 and 0.2 M NaClO4, respectively; scans at a sweep rate of 100 mV/s were recorded sequentially after the potential was held at +0.50 V for the above-noted periods of time. A slight decrease in the oxidative and reductive charges after 40 min in 0.2 M HClO4 solution (Figure 4a) reflects the decomposition of the ferricinium moiety. The decomposition occurs more readily in 0.2 M NaClO4 solution. For instance, about 20% of the ferricinium becomes lost after 40 min in neutral medium (Figure 4b). The loss of ferricinium of 2 can also be confirmed from the ex situ RAIR spectra, shown in parts a and b of Figure 5, taken, respectively, before and after holding the SAM at +0.50 V in 0.2 M NaClO4 solution for 40 min. The intensities of the ν(CH, Fc), νas(CH3), and νs(CH3) peaks of 2, appearing at 3086, 2967, and 2881 cm-1, respectively, are lowered, indicating that both the ferricinium group and the methyl-terminated alkyl chain are partially lost upon such electrochemical treatment; the sulfurcontaining alkyl chain is, however, still intact on the gold surface. The above experimental data suggests that disubstitution of the ferrocene moiety should be beneficial in assembling stable monolayers of ferrocene derivatives. Disubstitution should also be effective for high surface coverages. For the systems considered in this work, Cpwill hardly exchange with other nucleophiles in the electrolyte solution, because the two alkyl chains are directed toward the gold substrate; owing to the interchain interaction, the release of the alkylated Cp- will not be favorable. In particular, using the bonding capability of dithiol to gold, a very robust SAM can be prepared. Along with the interchain interaction, the anchoring of both alkyl chains to gold via the Au-S bonds will preclude the Cpspecies from escaping. Modification of electroactive centers by disubstitution and/or dithiol derivatization seems thus to merit consideration as a new strategy to guarantee the

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Figure 4. Cyclic voltammetric curves of submonolayer from 2 in 0.2 M HClO4 (a) and in 0.2 M NaClO4 (b). Scans at a sweep rate of 100 mV/s were recorded sequentially after Eapp was held for the noted time periods at +0.50 V.

Figure 5. RAIR spectra of submonolayer from 2 before (a) and after (b) Eapp was held for 40 min at +0.50 V in aqueous 0.2 M NaClO4 solution.

stability of electroactive SAMs, so facilitating their use in various application devices. In Situ RAIR Spectroscopy Study; SNIFTIRS Measurement. Figures 6a,b shows the SNIFTIR spectra of the SAMs of 1 and 2 on gold, respectively, at various sample potentials obtained by p-polarization in a 0.2 M HClO4 solution; the reference potential was 0.0 V. Noticeable spectral changes that occurred with the potential variation are marked with arrows. With s-polarized light, no band was observed in the spectra, and therefore the bands in Figure 6 should arise exclusively from the 1 and

SAMs of 1,1’-Disubstituted Ferrocene Derivatives

Figure 6. SNIFTIR spectra of the 1 (a) and 2 (b) monolayers on Au, obtained by p-polarization at various sample potentials in a 0.2 M HClO4 solution. The sample potentials are (i) 0.10 V, (ii) 0.20 V, (iii) 0.30 V, (iv) 0.40 V, (v) 0.50 V, and (vi) 0.60 V. The reference potential was 0.0 V. The noticeable spectral changes are marked with arrows.

2 species intact on the gold substrates; this is consonant with the previous observation that both SAMs remain stable even after application of +0.50 V for a prolonged time. On the other hand, even with p-polarized light, no band appeared in the SNIFTIR spectra when the sample potential was below +0.20 V, but this simply indicated that no oxidation took place below 0.20 V. As can be seen in Figure 6a, however, a few peaks began to appear when the sample potential became more positive than +0.2 V. These bands grew upon increase in the electrode potential up to around +0.50 V, and then remained constant because the oxidation of the ferrocene moiety should have been completed by ∼0.50 V. These SNIFTIR spectral features were reversibly observed, independent of the direction of the potential change.34 In addition, the SNIFTIR band intensities correlated well with the electric charges passed, reflecting the degree of the oxidation of the ferrocene group. Similar SNIFTIR observations were already reported for other redox active group-containing SAMs.9,10,34-36 The two downward bands at 2925 and 2850 cm-1 in Figure 6a can be attributed to the νas(CH2) and νs(CH2) bands, respectively, of 1. On the other hand, the upward band at 1479 cm-1 can be assigned to the Fc-C stretching vibration, ν(Fc-C). The bands’ peak positions were quite insensitive to the potential variation. As described in the Experimental section, the upward and downward bands mean, respectively, stronger and weaker absorption at the sampling potential than at the reference potential. This explanation implies that the νas(CH2) and νs(CH2)

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absorption bands have actually been weakened while the absorption due to the ν(Fc-C) vibration has increased upon oxidization of the ferrocene moiety of the SAMs of 1. In fact, the peak frequencies of the νas(CH2), νs(CH2), and ν(Fc-C) bands in Figure 6a are different by 1, 3, and 12 cm-1, respectively, from their counterparts in the ex situ RAIR spectrum in Figure 1c. Because the former three bands in Figure 6a are not at all in a derivative shape, such frequency differences must be attributed mostly to the difference in the environment, i.e., in air and in electrolytic aqueous medium. This explanation also implies that the ν(Fc-C) frequency is barely dependent on the oxidation state of the ferrocene moiety. On these grounds, the changes in the band intensities of the νas(CH2), νs(CH2), and ν(Fc-C) bands in Figure 6a must be interpreted in terms of the potential-dependent orientational change of the adsorbate. Invoking the infrared surface selection rule,23 the stronger IR absorption should, on the other hand, mean a more perpendicular orientation to the surface of the corresponding transition dipole moment. Considering then that the transition dipoles of the νas(CH2) and νs(CH2) modes are aligned perpendicularly to the alkyl chains, the growth of these two bands downward with an increase in the applied potential suggests that the alkyl chains of 1 must take a more perpendicular orientation on the gold surface as the ferrocene moiety is oxidized. The increase in the upward band at 1479 cm-1 due to ν(Fc-C), along with the potential increase, can also be understood in terms of such a structural change. If the pentadienyl rings of 1 are aligned parallel to the C-C-C plane of the alkyl chain, the in-plane modes such as ν(CH, Fc) and ν(CC, Fc) may have to be intensified as the alkyl chains are oriented more perpendicularly to the electrode surface. It is intriguing that no such band is identified in Figure 6a, however. For the SAMs of 11ferrocenyl-1-undecanethiol on gold, Ye et al.9 were able to observe those bands in the SNIFTIR spectra. The absence of the ν(CH, Fc) and ν(CC, Fc) bands in Figure 6a may imply that the pentadienyl rings of 1 are not necessarily aligned parallel to the C-C-C plane of the alkyl chain and/or that the alkyl chains of 1 do not possess all-trans zigzag conformation, although the alkyl chains of the oxidized SAMs take a more perpendicular stance overall with respect to the electrode surface. In addition, the TIR spectrum of 1 in Figure 1a shows that the absence of those bands in Figure 6a may also be due to their intrinsically weak absorption characteristics. In any event, one must acknowledge that the structural change of the alkyl chains itself will be very limited for the SAMs of 1 because the adsorbate is originally anchored on gold by forming two Au-S bonds. In this light, it is not unexpected that the orientation change of the cyclopentadienyl rings is barely discernible. As can be seen in Figure 6b, the SNIFTIR spectral features of the SAMs of 2 are comparable to those seen for the SAMs of 1. That is, two downward bands and one upward band associated with the νas(CH2), νs(CH2), and ν(Fc-C) modes are identified at 2926, 2852, and 1477 cm-1, respectively, in Figure 6b. The peak positions of these bands were again independent of the applied potential. This result suggests that the alkyl chains of 2 must also take a more perpendicular orientation on the (34) Bae, I. T.; Huang, H.; Yeager, E. B.; Scherson, D. A. Langmuir 1991, 7, 1558. (35) Ye, S.; Yashiro, A.; Sato, Y.; Uosaki, K. J. Chem. Soc., Faraday Trans. 1996, 92, 3813. (36) Wang, R.; Iyoda, T.; Tyrk, D. A.; Hashimoto, K.; Fujishima, A. Langmuir 1997, 13, 4644.

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gold surface as the ferrocene moiety is oxidized. Once again, we could not identify the in-plane stretching bands of the ferrocene moiety in Figure 6b. Considering that the SAMs of 2 are anchored on gold by forming only one Au-S bond, the SAMs may, in principle, have to be subjected to a considerable structural change upon the oxidation of the terminal ferrocene group. The structural change in the SAMs of 2, however, looks quite comparable to that in the SAMs of 1. The observed potential-dependent orientation changes, although small, would be associated with the charge difference of the terminal groups in the oxidized and reduced forms, given that different charges should induce changes in the lateral interactions among the terminal groups as well as in the interaction between the terminal group and the electrode surface. Actually, the terminal groups of 1 and 2 are neutral in the reduced state but have positive charges in the oxidized state. One can then suppose that a ferricinium cation, produced by the oxidation of a ferrocene group, reacts subsequently with a perchlorate anion (ClO4-) used as an electrolyte to form an ion pair.4a,12,15 On the other hand, in the potential region where the ferrocene groups were oxidized, the gold surface should have been positively charged. On these grounds, if the repulsive interaction between the terminal ferricinium cation and the gold electrode surface dominates over the attractive interaction between ClO4- and the electrode surface, the distance between the terminal group and the gold electrode surface will become larger to reduce the repulsive interaction. In this case, upon oxidizing the terminal ferrocene group, the alkyl chains of the 1 and 2 SAMs will have to take a more perpendicular stance with respect to the electrode surface. The ClO4- ions will then reside at the outside of the monolayer even when forming an ion pair with the ferricinium cation. As such, the interaction between the electroactive moiety and the electrolyte ion can be presumed to play an important role in the response of the electroactive adsorbate on the electrode to the applied potential. Summary and Conclusion Two kinds of SAMs of 1,1′-disubstituted ferrocene derivatives, 1 and 2, were prepared on gold, and their

Han et al.

structural and electrochemical characteristics were investigated. The RAIR spectral features indicated that both SAMs anchored on gold as thiolates took disordered structures. Ellipsometry measurement and semiempirical calculation also suggested that the two alkyl chains did not come close to each other when forming two-dimensional SAMs on gold. This result could be ascribed to the structural constraint imposed by the bulky ferrocene moiety, as was the case for SAMs of monosubstituted ferrocene derivatives. The surface coverage values determined from CVs were thus somewhat smaller than those expected for close-packed SAMs. Otherwise, the CVs of the two SAMs were in all respects consistent with those anticipated for an electrochemically reversible reaction exclusively involving a surface-confined species. From the SNIFTIR spectral features, on the other hand, the alkyl chains of the two SAMs appeared to take a more upright orientation when the ferrocene moiety was oxidized to a ferricinium cation. The most noteworthy point observed in this work was that the two SAMs were stable in neutral as well as in acidic medium. That is, the decomposition of the ferricinium cation seemed hardly to take place for the SAMs prepared from disubstituted ferrocene derivatives. This result is in obvious contrast with the SAMs derived only from monosubstituted ferrocene derivatives. Invoking the fact that the stability should be the key factor for using the SAMs in an application device, modification of electroactive centers by disubstitution would provide a new strategy to guarantee the stability of electroactive SAMs on electrodes. In particular, dithiol derivatization seemed definitely to result in very robust SAMs, irrespective of the surface coverage. Acknowledgment. K.K. acknowledges the Korea Research Foundation (KRF, 042-D00073) and the Korea Science and Engineering Foundation (KOSEF, 1992-2121-001-5) for financial support. Y.K.C. also acknowledges the KOSEF for providing a research grant through the Center for Molecular Catalysis at Seoul National University. LA000025B