Self-Assembled Monolayers Formed using Zero Net Curvature

Sep 8, 2010 - Paul K. Eggers,†,‡ Paulo Da Silva,† Nadim A. Darwish,† Yi Zhang,‡ Yujin Tong,‡ Shen Ye,*,‡,§. Michael N. Paddon-Row,*,†...
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Self-Assembled Monolayers Formed using Zero Net Curvature Norbornylogous Bridges: The Influence of Potential on Molecular Orientation Paul K. Eggers,†,‡ Paulo Da Silva,† Nadim A. Darwish,† Yi Zhang,‡ Yujin Tong,‡ Shen Ye,*,‡,§ Michael N. Paddon-Row,*,† and J. Justin Gooding*,† †

School of Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia, ‡Catalysis Research Center, Hokkaido University, Sapporo 001-0021, Japan, and §PRESTO, Japan Science and Technology Agency (JST), Japan Received April 21, 2010. Revised Manuscript Received July 13, 2010

A new class of electroactive norbornylogous bridges, with no net curvature, that form self-assembled monolayers on gold electrodes were studied by electrochemistry and in situ infrared spectroscopy. The influence of the electrode potential on the structure and conformation of the self-assembled monolayers (SAMs) was investigated. This was performed using two different lengths of rigid norbornylogous bridges with terminal ferrocene moieties and ω-hydroxyalkanethiols. It was found that single component monolayers of the rigid norbornylogous bridges changed their tilt angle with their transition from the ferrocene to ferricinium. However, when the norbornylogous SAMs were diluted with ω-hydroxyalkanethiols the tilt angle remained unchanged upon oxidation of ferrocene to ferricinium. It was also observed that the tilt angle of the diluent, ω-hydroxyalkanethiols changed at potentials exceeding 500 mV.

Introduction Self-assembled monolayers (SAMs) have provided a robust construct for investigating long-range electron transfer through the components which comprise the SAM.1-7 The enabling power of SAMs for electron transfer studies is the well-defined spatial relationships between the electrode and the redox active species. This well-defined spatial relationship in a high-quality SAM, with minimal defects, arises from each molecule being held in place through the close packing of neighboring molecules. Using this feature, studies of long-range electron transfer through monolayer constructs have shown (1) that the distance dependence of electron transfer decays exponentially with distance at an attenuation factor (β value) between 0.9 and 1.3 A˚-1 as predicted by theory,8,9 (2) that the chain composition of the diluent influences electron transfer behavior,6 and (3) that immersion of the redox species within a SAM comprising diluent molecules significantly longer than the redox moiety bearing component will result in the suppression of redox electrochemistry.10 This latter study is significant, as it highlights the importance of how ion transfer is coupled to electron transfer and the effect of the environment around the redox active species. It does not, though, *Shen Ye ([email protected]), Michael Paddon-Row (m.paddonrow@ unsw.edu.au), Justin Gooding ([email protected]). (1) Chidsey, C. E. D. Science 1991, 251, 919–922. (2) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173–3181. (3) Ciampi, S.; Eggers, P. K.; Le Saux, G.; James, M.; Harper, J. B.; Gooding, J. J. Langmuir 2009, 25, 2530–2539. (4) Chou, A.; Eggers, P. K.; Paddon-Row, M. N.; Gooding, J. J. J. Phys. Chem. C 2009, 113, 3203–3211. (5) Weber, K.; Hockett, L.; Creager, S. J. Phys. Chem. B 1997, 101, 8286–8291. (6) Napper, A. M.; Liu, H. Y.; Waldeck, D. H. J. Phys. Chem. B 2001, 105, 7699–7707. (7) Sek, S.; Palys, B.; Bilewicz, R. J. Phys. Chem. B 2002, 106, 5907–5914. (8) Adams, D. M.; Brus, L.; Chidsey, C. E. D.; Creager, S.; Creutz, C.; Kagan, C. R.; Kamat, P. V.; Lieberman, M.; Lindsay, S.; Marcus, R. A.; Metzger, R. M.; Michel-Beyerle, M. E.; Miller, J. R.; Newton, M. D.; Rolison, D. R.; Sankey, O.; Schanze, K. S.; Yardley, J.; Zhu, X. Y. J. Phys. Chem. B 2003, 107, 6668–6697. (9) Paddon-Row, M. N. Aust. J. Chem. 2003, 56, 729–748. (10) Sumner, J. J.; Creager, S. E. J. Phys. Chem. B 2001, 105, 8739–8745.

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enlighten how the electrochemical double layer influences the redox moiety or how this influences electron transfer. Ion-dependent electron transfer, and how this is affected by the electrostatic interactions of ions with their local environment, was identified as one of the main general issues for future investigation by a Department of Energy (DOE) sponsored workshop on Charge Transfer on the Nanoscale.8 The importance of environmental effects on electron transfer is shown by studies which have demonstrated that the terminal group of the diluent (for example, substituting a methyl for a hydroxyl) can have an impact on the rate constant for electron transfer.11,12 These studies suggested that the terminal group of the diluent can affect the redox species in two ways. The first is that it changes how the electrochemical double layer forms on the surface.11 The second is that polar groups, such as a hydroxyl, induce destructive interference on the redox moiety’s bridge.6,11 Exploring the impact of the environment on the redox active species, and how this influences ion transfer and hence electron transfer, is limited by the current experimental systems. The conformational flexibility of aliphatic alkanethiols used in the standard SAM means that the redox active species position cannot be unambiguously known with respect to the distal surface of the diluent. In order to probe the electrical double layer, the position of the redox active species relative to the diluent component of the SAM must be known. This requires functional molecules that are conformationally rigid. Norbornylogous bridges have the qualities necessary to position a species above the distal surface of the diluent. These qualities are as follows: complete rigidity, symmetry, comparative synthetic ease by which the length can be altered, and the ability to attach a wide range of functional groups to their termini. We have previously designed a rigid self-assembled monolayer forming (11) Eggers, P. K.; Hibbert, D. B.; Paddon-Row, M. N.; Gooding, J. J. J. Phys. Chem. C 2009, 113, 8964–8971. (12) Eggers, P. K.; Zareie, H. M.; Paddon-Row, M. N.; Gooding, J. J. Langmuir 2009, 25, 11090–11096.

Published on Web 09/08/2010

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family of molecules based on norbornylogous bridges.12-15 One of us (M.P.R.) has used norbornylogous bridges extensively for fundamental studies into electron transfer,16 including demonstrating the important role played by superexchange (through-bond) coupling in mediating electron transfer over large distances17 and how the strength of this coupling depends on bridge length18 and bridge configuration.19 However, what is not known is how, or if, the orientation of a norbornylogous bridge varies as the potential changes. This is vitally important for measuring the properties within the electrical double layer, as even small changes in the height of the functional group with respect to the surface of the monolayer will result in a relatively large change in the electrostatic field surrounding the functional group. The purpose of this paper is to ascertain whether there is any significant change in position/orientation upon oxidation/reduction of the ferrocene species attached to the norbornylogous bridges incorporated within a SAM where the norbornylogous bridges are the minor component. The norbornylogous bridges are diluted with conventional alkanethiol molecules, primarily because such molecules take a considerable effort to synthesize, and incorporating bridges as diluents will prevent the determination of any change in tilt angle of the ferrocene bearing bridges. This will allow the use of norbornylogous bridges to position species at specific distances above the distal surface of the diluent.12 In situ infrared (IR) spectroscopy20-24 and surface-enhanced Raman scattering (SERS)25 have been used in the past to show that alkanethiol derived ferrocene species tilt with a change in oxidation state. The orientational changes have also been supported by other measurements of electrochemical quartz crystal microbalance (EQCM),22,26 surface plasmon resonance (SPR),27-29 and ellipsometry.30 It is interesting to note that the SAMs with quinone-derived and azobenzene-derived alkanethiols do not show such changes.31,32 Hence, it has been suggested that the reason for the change in orientation upon oxidation of the ferrocene-derived alkanethiols was a spatial requirement of the (13) Beebe, J. M.; Engelkes, V. B.; Liu, J.; Gooding, J. J.; Eggers, P. K.; Jun, Y.; Zhu, X.; Paddon-Row, M. N.; Frisbie, C. D. J. Phys. Chem. B 2005, 109, 5207– 5215. (14) Yang, W. R. R.; Jones, M. W.; Li, X. L.; Eggers, P. K.; Tao, N. J.; Gooding, J. J.; Paddon-Row, M. N. J. Phys. Chem. C 2008, 112, 9072–9080. (15) Kiani, A.; Alpuche-Aviles, M. A.; Eggers, P. K.; Jones, M.; Gooding, J. J.; Paddon-Row, M. N.; Bard, A. J. Langmuir 2008, 24, 2841–2849. (16) Paddon-Row, M. N. Acc. Chem. Res. 1994, 27, 18–25. (17) Hush, N. S.; Paddon-Row, M. N.; Cotsaris, E.; Oevering, H.; Verhoeven, J. W.; Heppener, M. Chem. Phys. Lett. 1985, 117, 8–11. (18) Seischab, M.; Lodenkemper, T.; Stockmann, A.; Schneider, S.; Koeberg, M.; Roest, M. R.; Verhoeven, J. W.; Lawson, J. M.; Paddon-Row, M. N. Phys. Chem. Chem. Phys. 2000, 2, 1889–1897. (19) Oliver, A. M.; Craig, D. C.; Paddon-Row, M. N.; Kroon, J.; Verhoeven, J. W. Chem. Phys. Lett. 1988, 150, 366–373. (20) Popenoe, D. D.; Deinhammer, R. S.; Porter, M. D. Langmuir 1992, 8, 2521– 2530. (21) Ye, S.; Sato, Y.; Uosaki, K. Langmuir 1997, 13, 3157–3161. (22) Ye, S.; Haba, T.; Sato, Y.; Shimazu, K.; Uosaki, K. Phys. Chem. Chem. Phys. 1999, 1, 3653–3659. (23) Han, S. W.; Seo, H.; Chung, Y. K.; Kim, K. Langmuir 2000, 16, 9493–9500. (24) Viana, A. S.; Jones, A. H.; Abrantes, L. M.; Kalaji, M. J. Electroanal. Chem. 2001, 500, 290–298. (25) Nishiyama, K.; Ueda, A.; Tanoue, S.; Koga, T.; Taniguchi, I. Chem. Lett. 2000, No.8, 930–931. (26) Shimazu, K.; Yagi, I.; Sato, Y.; Uosaki, K. Langmuir 1992, 8, 1385–1387. (27) Yao, X.; Wang, J.; Zhou, F.; Wang, J.; Tao, N. J. Phys. Chem. B. 2004, 108, 7206–7212. (28) Xiang, J.; Guo, J.; Zhou, F. M. Anal. Chem. 2006, 78, 1418–1424. (29) Norman, L. L.; Badia, A. J. Am. Chem. Soc. 2009, 131, 2328–2337. (30) Ohtsuka, T.; Sato, Y.; Uosaki, K. Langmuir 1994, 10, 3658–3662. (31) Ye, S.; Yashiro, A.; Sato, Y.; Uosaki, K. J. Chem. Soc., Faraday Trans. 1996, 92, 3813–3822. (32) Yu, H. Z.; Zhang, H. L.; Liu, Z. F.; Ye, S.; Uosaki, K. Langmuir 1998, 14, 619–624.

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Figure 1. Compounds used in this study.

ferricinium cation to bind to the perchlorate counteranion. In this paper, we will demonstrate that the norbornylogous bridges also change their tilt angle relative to the electrode surface. However, when the norbornylogous bridges are spaced apart by a diluent, there is no evidence that they tilt.

Experimental Section Compounds. The two structures used in this paper are shown in Figure 1. The synthesis of the tetrathiol bridges has been described previously.13 The general procedure for derivatization with ferrocene is as follows: The tetrathioacetate norbornylogous bridge (0.081 mmol) was dissolved in 10 mL of deoxygenated MeOH under argon. 100 mL of 32% HCl was added while stirring, and the mixture was refluxed for 18 h. The mixture was then reduced to dryness under reduced pressure. Subsequently, the solid was dissolved in dry deoxygenated dichloromethane (25 mL), and a round-bottom flask containing the mixture was placed in an ice/acetone bath and left for 15 min. Ferrocene carboxaldehyde (16.8 mg, 0.078 mmol) was then added followed by BF3 etherate (2 mL). The solution was stirred for 14 h. Milli-Q water (15 mL) was then added to the mixture, and it was allowed to stir for 15 min. The organic phase was next washed with Milli-Q water (3  20 mL). The resulting mixture was dried with Na2SO4, then concentrated under reduced pressure and chromatographed with a 3:7 light petroleum/ CH2Cl2 column. The result was a yellow solid at 60% yield. Electrochemical and IRRAS Measurements. All voltammetry experiments were carried out using a potentiostat PS-07 (TOHO Technical Research, Japan) at room temperature. The current and potential output from the potentiostat was collected by a multifunction data acquisition module (USB-6211, National Instruments) controlled by LabVIEW, which allows the collection of the electrochemical response with a sampling rate as high as 15 μs at a signal resolution of 0.3 mV. Reference electrode was a saturated KCl Ag|AgCl. All potentials in the paper are referred to relative to this reference. The counter electrode was a platinum gauze. The electrolyte solution was prepared by using suprapure HClO4 (Wako Pure Chemicals) and Milli-Q water. The electrolyte solution, 0.1 M HClO4 solution, was degassed with Ar (99.99%, Hokkaido Air Water Inc., Japan) for at least 15 min before each experiment.33,34 The details of the in situ surface-enhanced IR measurements with the Kretschmann ATR configuration have been described elsewhere.33,34 An approximately 50-nm-thick gold film, which was chemically deposited on the reflecting surface of a hemicylindrical Si prism, was used as a substrate for monolayer construction (33) Zhou, W.; Ye, S.; Abe, M.; Nishida, T.; Uosaki, K.; Osawa, M.; Sasaki, Y. Chem.;Eur. J. 2005, 11, 5040–5054. (34) Zhou, W.; Zhang, Y.; Abe, M.; Uosaki, K.; Osawa, M.; Sasaki, Y.; Ye, S. Langmuir 2008, 24, 8027–8035.

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and also served as the working electrode.35 Afterwards, the gold electrode surface was electrochemically cleaned by scanning the electrode potential between 0.0 V and þ1.5 V for 5-10 cycles to remove possible organic additives adsorbed on the surface during the gold chemical deposition. The gold surface area was determined from the charge for the reduction of the oxide layer of the Au electrode. After the electrochemical cleaning, the electrode surface was rinsed 3 times with Milli-Q water, and then ethyl acetate and immersed in a 1 mM solution of compound 1 or 2 for 10 min.12 In the case of mixed SAMs, the electrodes were then rinsed 3 times with ethyl acetate before immersing in an ethyl acetate solution with 1 mM of the diluents. The electrodes were then placed in an oven at 45 °C for 1 h. After removal from the oven, the electrodes were allowed to equilibrate to room temperature for 24 h. Prior to electrochemical analysis, the electrodes were rinsed 3 times with ethyl acetate, 3 times with ethanol, and 3 times with Milli-Q water.12 The in situ IR measurements were carried out using a BioRad FTS-60A/896 spectrometer equipped with a MCT detector.33,34 The IR spectra were recorded in the dynamic mode with a spectral resolution of 4 cm-1. Fifty interferograms were coadded to each spectrum. It takes 10 s to get one spectrum in the present work. All in situ IR spectra are shown as absorbance with respect to the IR spectrum recorded at 0 V in the same solution, where terminal ferrocene moiety exists as neutral ferrocene state and not the positively charged ferricinium. Each IR spectrum was recorded during a slow potential sweep with a scan rate of 2.5 mV/s, giving a spectrum every 25 mV. In order to calculate the coverage of the SAM, the CVs were also recorded at faster sweep rates for electrochemical characterization.

Results and Discussion Prior to being able to employ norbornylogous bridges 1 and 2 for exploring the electrical double layer, the SAMs formed from these molecules require characterization. Questions to be answered include the following: If these molecules form SAMs, what is the density of the molecules in the monolayer? Are the monolayers ordered or disordered? What is the tilt angle made by these molecules with respect to the electrode surface, and does it change with change in the oxidation state of the ferrocene? Do they give ideal Nernstian electrochemistry, and what is the distribution of the norbornylogous bridges in a mixed monolayer? Some of these questions have been answered in previous studies.11-14 For instance, electrochemical and scanning tunneling microscopy (STM) measurements have shown that the single component monolayers form well-ordered SAMs.12 STM, atomic force microscopy (AFM), and electrochemical techniques have shown that the mixed monolayers exhibit close to ideal electrochemical behavior and that the rigid norbornylogous bridges hold the ferrocene above the distal surface of the monolayer.12 X-ray photoelectron spectroscopy (XPS) and ellipsometry studies of both the curved13 and straight norbornylogous bridges14 have shown that these bridges sit approximately 30° from normal in SAMs composed of only norbornylogous bridges, which is the same as that observed for alkanethiols.36 Hence, in mixed monolayers of norbornylogous bridges and alkanethiols, it seems highly probable that the norbornylogous bridges will also project from the surface at ca. 30° to normal. This is an important aspect of these bridges; if the tilt angle was significantly different from the alkanethiol diluent, the norbornylogous bridges would create defects in the SAM. It has also been shown that the norbornylogous bridges occupy more area per molecule on a surface than (35) Miyake, H.; Ye, S.; Osawa, M. Electrochem. Commun. 2002, 4, 973–977. (36) Ulman, A. Chem. Rev. 1996, 96, 1533–1554.

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an alkanethiol12-14 and the area taken up by the norbornylogous bridge increases with the increase in the bridge length. Electrochemistry. For the chemically deposited gold electrodes used in the study for FTIR measurements, the area of the Faradaic peak for 2 gave a density of 6.2  1013 molecules per cm2. This is less than the value of 9.6  1013 molecules per cm2, previously published on the same norbornylogous backbone but lacking the ferrocene group.14 This is expected, as the decrease in the surface coverage with the addition of a ferrocene moiety has been observed previously. For instance, single component monolayers of 1 formed here (chemically deposited gold electrodes) and previously published (evaporated gold electrodes)12 had a density of 9.2  1013 molecules per cm2, while the corresponding norbornylogous bridge that did not possess a ferrocene had a coverage of 1.1  1014 molecules per cm2.14 The mixed monolayers formed from 1 and 2 have previously been shown to display ideal Nernstian electrochemistry on smooth surfaces.12 However, as shown by Figure 2 there is a separation between the oxidation and reduction peaks of both the single component monolayers and the mixed monolayers. This may be attributed to the influence of the large surface area of the chemically deposited gold electrode employed for subsequent in situ IR measurements. The large surface area of the working electrode results in a slow cell time constant, which, in combination with the increased resistance inherent with chemically deposited gold, causes the peak splitting. The increase in resistance will not alter the specific trends focused on herein. The similarity between the coverages of the chemically deposited gold and the evaporated gold electrodes enforce this point. As we have shown previously,12 the pure norbornylogous SAMs form dense, highly ordered monolayers. Hence, for the chemically deposited electrodes to have a similar surface coverage the monolayers would also have to be dense and ordered. In Situ IR Spectra: Single Component Monolayers. In situ IR characterization is used here as a primary method to observe a structural change in the SAMs formed from norbornylogous bridges during a redox reaction. Figure 3 shows an in situ IR spectra in 0.1 M perchloric acid for a monolayer of 1 (Figure 3c) and 2 (Figure 3a) with respect to the IR spectrum at 0 V. Hence, in the figures the upward or downward bands reflect either an increased or decreased absorbance compared with the IR spectrum at 0 V. The normalization potential of 0 V was chosen, as this potential is sufficiently negative for all the redox species in the SAMs to be found as the neutral ferrocene species and none as the ferricinium cation. As shown by Figure 3a and c, the major IR bands for the SAMs formed from either compound 1 or 2 are observed at almost identical positions. The upward band at 3114 cm-1 is attributed to the C-H stretching mode of the ferrocene cyclopentadiene (CP) ring. This band has previously been reported for the C-H stretch of the ferrocene moieties attached to an alkane chain via a methylene (3113 cm-1),21,22 via ester (3112 cm-1),20 and via carbonyl (3114 cm-1).24 The two downward bands at 2930 and 2854 cm-1 are attributed to the asymmetric and symmetric C-H stretch of the norbornylogous bridge, respectively. These bands, at 2930 and 2854 cm-1, are at higher wavenumbers than have been reported for single component monolayers of 11-ferrocenyl-1-undecanethiol in in situ IR measurements, 2923 and 2848 cm-1,21,22 and ex situ IRRAS measurements, 2923 and 2851 cm-1.37 (37) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A. J.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Akatsuji, H., et al. Gaussian 03, Revision E.01; Gaussian, Inc.: Pittsburgh PA, 2003.

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Figure 2. Cyclic voltammograms in 0.1 M perchloric acid with a single component monolayer of (a) 1 at a scan rate of 100 mV/s and (b) 2 at a scan rate of 100 mV/s and (c) a mixed monolayer of 2 in a 1 to 10 dilution ratio with 10-mercaptodecanol at a scan rate of 10 mV/s and formed on chemically deposited gold electrodes.

However, our DFT calculations using B3LYP/6-31G(d,p)// B3LYP/6-31G(d,p) harmonic vibrational frequency calculations37 carried out on 2 predict that the norbornylogous bridge asymmetric and symmetric C-H stretches discussed above occur at higher wavenumbers than monolayers composed of alkane bridges. The calculations, given that norbornylogous bridges have more structurally in common with cyclohexanes than alkanes, are supported by literature assigned asymmetric C-H stretches for cyclohexanes at 2930 cm-1.38 The intense upward band at 1104 cm-1 in Figure 3a and c is attributed to the perchlorate ion, indicating that the surface concentration of the perchlorate ion increases with the oxidation of ferrocene moiety.25 (38) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic Press, Inc.: San Diego, CA, 1990.

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Figure 3b and d shows the cumulative Faradaic charge passed plotted relative to the changes in normalized IR band area for the 3114, 2930, and 1104 cm-1 bands. It can be seen that the change in band areas closely follows the cumulative Faradaic charge and hence the change in oxidation state of the redox active species. In the present IR spectra, the upward bands indicate that the IR absorption is increasing with respect to the reference potential spectrum (observed at 0 V) and the downward bands indicate that the IR absorption is decreasing with respect to the reference spectrum. In situ IR measurements also allow the changes in the chemical structure to be tracked, as in the case of monolayers of quinone31 and azobenzene species.32 However, the monolayers constructed for this paper do not form a new chemical species (i.e., quinone to hydroquinone) under the conditions imposed during the potential window shown in Figure 3. Since p-polarized light was used, an increase in IR absorption is linked to the dipole moment shifting closer to perpendicular with respect to the electrode surface and closer to parallel to the surface for a decrease in the IR absorption. Thus, in the case of Figure 3, on oxidation the dipole moment of the ferrocene/ferricinium C-H stretch (3114 cm-1) shifts closer to perpendicular to the surface and the dipole moment of C-H stretch of the norbornylogous bridge (2930 and 2854 cm-1) shifts away from perpendicular to the surface. As shown by Figure 4, all of the CH groups of the norbornylogous bridge point perpendicular to its backbone. Hence, the ability of the IR measurement to detect the dipole moment of the C-H stretch will decrease as the backbone of the norbornylogous bridge shifts closer to perpendicular. Thus, the appearance and increase of C-H stretch as a downward band shows that the norbornylogous bridges decrease their tilt angle to closer to perpendicular to the surface upon oxidation of the distal ferrocene. This is the same result which Ye et al. discovered for single component monolayers of 11-ferrocenyl-1-undecanethiol.21,22 In all cases, where single component alkanethiol SAMs modified with ferrocene have been measured with in situ IR, the structure of the layers has changed on oxidation of terminal ferrocene groups.20-24 This has been reinforced by (EQCM),22,26 which confirmed the incorporation of anions on the monolayer surface, as well as plasmon resonance (SPR)27-29 and ellipsometry,30 which confirmed an increase in the thickness of the monolayer. Hence, it has been suggested that the change in the tilt angle of the alkyl chain is required to provide more space for the perchlorate ions to bind with the ferricinium cation to balance the charge.21,22 The close correlation between the cumulative Faradaic charge passed and the 2930 cm-1 band (which infers a change in the tilt angle of the bridge) in Figure 3b and d for SAMs of 1 and 2 supports this hypothesis. In Situ IR Spectra: Diluted Monolayers. The main purpose within this article was to ascertain whether mixed component SAMs change their orientation with respect to the distal surface of the diluent. The assignment of the asymmetric C-H stretch at 2930 cm-1 for single component monolayers allows us to analyze the mixed monolayers and differentiate between the change in the norbornylogous bridge tilt angle and that of the alkanethiol diluent. Figure 5a shows the in situ IR spectra of a mixed monolayer formed from a 1 mM solution of 2 and 10-mercaptodecanol in a ratio of 1:10. At 500 mV, bands at 2918 and 2851 cm-1 are observed. The areas of these bands increase with potential implying that, as with the single component monolayers, the orientation of the molecules on the surface is shifting closer to the perpendicular. The question is, are these bands due to the diluent or the norbornylogous bridges? The bands observed in Figure 5a are at the same positions reported for the asymmetric and Langmuir 2010, 26(19), 15665–15670

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Figure 3. (a) In situ IR spectra of a single component monolayer of 2. (b) Plot of the normalized in situ IRRAS band areas with the normalized Faradaic charge passed for 2. (c) In situ IR spectra of a single component monolayer of 1. (d) Plot of the normalized in situ IR band areas with the normalized Faradaic charge passed for 1. The meanings for symbols in (b) and (d) are given in the related figures.

Figure 4. B3LYP/6-31G(d,p) optimized structure of 2 shown in two perspectives.39

symmetric C-H stretches of alkane chains in the crystalline state, 2918 and 2851 cm-1,39 are at comparable positions to alkane chains in single component monolayers of 11-ferrocenyl-1-undecanethiol in in situ IR measurements, 2923 and 2848 cm-1,21,22 are at significantly lower positions than those for norbornylogous bridges alone (Figure 3a and c), 2930 cm-1 and 2854 cm-1, and are clearly apparent in in situ IR spectra of monolayers formed from 10-mercaptodecanol only (Figure 5c). Hence, the bands at 2918 and 2851 cm-1 are attributed to the tilting of the diluent 10-mercaptodecanol and not the norbornylogous bridges. (39) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559–3568.

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Consistent with the 2918 cm-1 band arising from the diluent is that the changes in this band with potential do not correlate with the Faradaic charge passed by 2 as depicted in Figure 5b. Therefore, the change in tilt angle of the diluent is not associated with the oxidation state of the surface redox species. The important difference between the in situ IR spectrum of a monolayer containing only norbornylogous bridge 2 and the spectrum for a SAM of 2 diluted by 10-mercaptodecanol is the absence of a band at 2930 cm-1 associated with the norbornylogous bridge asymmetric C-H stretch. The absence of this band (2930 cm-1) could be due to either low signal strength at these dilutions or that the orientation of the norbornylogous bridge is not changing with the oxidation of the ferrocene moiety to ferricinium. We dismiss the former possibility and suggest that the orientation of 2 does not change. The evidence to support this conclusion is as follows. First, although the 2918 and 2930 cm-1 bands will overlap to some extent, the onset of the 2918 cm-1 band occurs at much more negative potentials than that of 2930 cm-1 (see Figures 3 and 5). This is to such an extent that if the oxidation of the ferrocene moiety was causing a change in orientation of 2 then the 2930 cm-1 band would reach its maximum (all the ferrocene moieties have been oxidized to ferricinium) prior to the appearance of the 2918 cm-1 band (see Figures 3 and 5). Second, the coverage of 2 in the 1:10 monolayer of 2 and 10-mercaptodecanol (at 6.0  1012 norbornylogous bridges per cm2) is sufficient to provide a discernible peak. As expected, the coverage of 2 in the single component monolayer is an order of magnitude higher (9.2  1013 norbornylogous per cm2), but the bands in Figure 3a are so strong that an order of DOI: 10.1021/la101590b

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Article

Eggers et al.

visible band at 2930 cm-1. Hence, the exact ratio of redox moiety to diluent on the surface is unimportant as long as the spatial requirement is met for the binding of the perchlorate ion. The absence of a change in orientation of the norbornylogous bridge 2 in a SAM where it is diluted supports the hypothesis that the change in orientation of ferrocene upon oxidation to the ferricinium ion occurs due to the spatial requirement of ionpairing a perchlorate ion with ferricinium species. When mixed with a diluent, as in the case of Figure 5a and b, the distance between two norbornylogous bridges is greater than the diameter of a perchlorate ion and 2 will hold its ferrocene moiety above the distal surface of the diluent.12 These two properties mean that all of the spatial requirements for the binding of the perchlorate ion to the ferricinium are met and a change in the tilt angle of the norbornylogous bridge due to spatial requirements is unnecessary. This means that the ferrocene moiety does not change its position with respect to the distal surface of the diluent for the potential window where the Faradaic charge is passed. As shown by Figure 5, the distal surface of the diluent does change its position with respect to the ferrocene. However, this occurs after the Faradaic charge has essentially been passed. Thus, these rigid, zero net curvature norbornylogous bridges can allow us to probe at specific heights above the distal surface of the diluent and hence give us data at various positions within the electrochemical double layer.

Conclusion We have shown that the molecules within single component monolayers of both 1 and 2 shift their tilt angles toward perpendicular with respect to the surface on oxidation and away from perpendicular on reduction. The change in the tilt angle coincides with the cumulative Faradaic charge passed. These observations indicate that the change in tilt angle is due to a spatial requirement of the ferricinium ion to pair with perchlorate ions. No change in the tilt angles of 1 or 2 was observed in mixed SAMs with ω-hydroxyalkanethiol diluent molecules. However, the tilt angle of the ω-hydroxyalkanethiol diluent did change in mixed SAMs with 1 or 2 and in single component SAMs with only ω-hydroxyalkanethiol at potentials greater than 500 mV. All changes in tilt angles that were observed in SAMs composed of alkanethiols or norbornylogous bridges were reversible with reversal of applied potential.

Figure 5. (a) In situ IR spectra of a monolayer of 2 and 10-mercaptodecanol in a 1:20 ratio. (b) Plot of the normalized in situ IR band areas with the normalized Faradaic charge passed for a monolayer of 2, and 10-mercaptodecanol in a 1:20 ratio. (c) In situ IR spectraof a monolayer 10-mercaptodecanol.

magnitude decrease in intensity due to lower molecule coverage would only reduce the peak intensity to a similar magnitude that is observed for the 2918 cm-1 band shown in Figure 5a. Thus, it would be expected that the 2930 cm-1 band would be clearly apparent if the tilt angle of 2 in diluted monolayers was changing as much as in the single component monolayers. Furthermore, reducing the dilution ratio from 1:10 to 1:1 did not result in a

15670 DOI: 10.1021/la101590b

Acknowledgment. P.K.E. acknowledges a postdoctoral fellowship from Japan Science Promotion Society (JSPS). The Australian Research Council under the Discovery Projects funding scheme (Project Number DP0556397) is acknowledged. MNP-R acknowledges computing time from the Australian Partnership for Advanced Computing (APAC) awarded under the Merit Allocation Scheme. S.Y. acknowledges PRESTO, Japan Science and Technology Agency (JST) and a Grant-inAid for Exploratory Research 21655074 from MEXT, Japan. Supporting Information Available: The Cartesian coordinates of the B3LYP/6-31G(d,p) optimized structure of 2 and a table of calculated IR band assignments. This material is available free of charge via the Internet at http:// pubs.acs.org.

Langmuir 2010, 26(19), 15665–15670