J. Phys. Chem. B 2000, 104, 7449-7454
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Long-Range Heterogeneous Electron Transfer Between Ferrocene and Gold Mediated By n-Alkane and N-Alkyl-Carboxamide Bridges J. J. Sumner, K. S. Weber, L. A. Hockett, and S. E. Creager* Department of Chemistry, Clemson UniVersity, Clemson, South Carolina 29634 ReceiVed: March 15, 2000
Electron-transfer rates between ferrocene and gold were measured for two series of ferrocene-based alkanethiolate monolayers on gold electrodes. In one series, the bridge linking the ferrocene group to gold consisted of a simple alkane chain, and in the other, the bridge was modified such that the two methylene linkages immediately adjacent to the ferrocene group were replaced by a carboxamide linkage. Monolayers in both series were studied using a novel ac voltammetry method to measure the standard electron-transfer rate constants for oxidation/reduction of the ferrocene groups. Rate constants were nearly the same for pairs of molecules in the two series when the number of bonds in the direct pathway linking ferrocene to gold was the same, regardless of whether the group immediately adjacent to ferrocene was a pair of methylene units or a carboxamide unit. This observation suggests that the contribution of the carboxamide group to the overall bridge-mediated electronic coupling is not greatly different from that of a pair of methylene groups. An unusual “even-odd effect” in the diminution of rate with chain length for both monolayer series was also noted. Several factors that could cause such an alternation in rate are proposed, including an unusual quantum interference effect in the bridge-mediated coupling that has been predicted theoretically.
Introduction Long-range bridge-mediated electron transfer is a critical element of the mechanisms of action of many biological superstructures and future electronic technologies. Photosynthesis and cellular respiration are two biological phenomena that utilize bridge-mediated electron transfer in ways that are still not fully understood.1-3 The folding4 and function5 of certain proteins have also been linked to electron-transfer events. The developing field of “molecular-scale” electronics utilizes bridgemediated electron transfer in several ways, and an improved understanding of the nature of bridge-mediated electron-transfer reactions will be required for such technologies to advance.6,7 A broad range of experimental approaches, mostly involving photochemical and electrochemical techniques, have been used to study long-range electron transfer.7-9 In particular, electrochemical studies of redox-active monolayers on electrodes have contributed much to the understanding of long-range electron transfer between metal electrodes and redox molecules. Finklea recently presented an excellent review of the early work (prior to 1997) in this field for the case of alkanethiol-based monolayers on noble metal electrodes, particularly gold.9 Monolayers in which ferrocene groups are linked to gold electrodes by alkanethiol-based bridges have been particularly well studied,10-25 and from this body of work (and from related work on other redox-active monolayers) have emerged many of the present ideas about how electrons are transferred in molecular assemblies at electrode-solution interfaces. One aspect of the behavior of redox-active monolayer systems (and of bridge-mediated electron transfer in general) that is still not well understood involves the role of different functional groups in the bridge in promoting long-range electronic coupling. Some recent work has indicated that electronically conjugated bridges can promote much stronger electronic coupling between ferrocene and gold than that found for
saturated, hydrocarbon-based bridges.12,15 There have been some selected studies on monolayers in which selected functionalgroup substitutions have been made in hydrocarbon-based bridges;26 however in general it is the case that the effects of simple functional group substitutions in hydrocarbon-based bridges have not been systematically investigated and are not yet well understood. The present work considers the electrochemistry of two series of monolayers, one in which ferrocene groups are linked to gold electrodes by simple alkane chains and another in which the two methylene units in the bridge that are immediately adjacent to the ferrocene group have been replaced by a carboxamide group. The monolayer structures are illustrated in Figure 1. The carboxamide group is formally identical to a peptide bond, which makes these series a simplified model for studying long-range electronic coupling mediated by peptides in proteins. The series were designed to incorporate polymethylene chains of variable length (7-10 carbons) into the bridges to facilitate separation of the contribution to electronic coupling through the polymethylene chains from that due to coupling through the carboxamide group. Redox kinetics in these monolayers were studied using a recently described variant of ac voltammetry.27 As was expected from prior work,9,11,14 the standard electron-transfer rate constants (ko values) in both series decreased exponentially as the length of the polymethylene chains increased. Plots of ln(ko) versus the number of methylene groups in the chains exhibited the same slope in both series; however the rates for the carboxamide series were consistently lower than those in the alkane series. This difference is thought to reflect a decrease in electronic coupling caused by insertion of the carboxamide group into the chain in the former series. In contrast, the rate constants are nearly the same in the two series when they are compared according to the number of individual bonds in the direct pathway linking the ferrocene group to the gold electrode.
10.1021/jp000992v CCC: $19.00 © 2000 American Chemical Society Published on Web 07/13/2000
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Figure 1. Illustrative structures of (A) mixed monolayers of N-(ωmercaptoalkyl)-ferrocenecarboxamides with ω-mercapto-alkanols, and (B) mixed monolayers of ω-mercapto-alkylferrocenes with alkanethiols on gold.
This result may be interpreted as meaning that replacement of a pair of methylene units with a peptide unit has only a minimal effect on the rate. An unexpected alternation in the electron-transfer rate constants with alkane chain length was noted in both monolayer series. The cause of this alternation is not known for certain; however one intriguing possibility is that it might reflect a quantum interference phenomenon (i.e., a dephasing in the combination of bridge electronic states as the bridge length increases) in the bridge-mediated electronic coupling. The observed patterns of rate alternation in both monolayer series closely match the predictions of a recently described sequential coupling method as applied to the specific case of predicting electronic coupling in ferrocene-based monolayer assemblies on gold electrodes. 28,29 Experimental Section Materials. Gold wire for electrodes was obtained from Alfa (0.127 mm diam, Premion grade, >99.99% pure) and Johnson Matthey (0.05 mm diam, Purtronic grade, 99.998% pure). Ethanol (100%, from AAPER Alcohol and Chemical Co.) and perchloric acid (70% in water, reagent grade from Acros) were used as received. Water used in electrochemical experiments was purified with a NANOpure water filtration system (Barnstead) to a resistivity of at least 12 MΩ cm. All of the ferrocenyln-alkanethiols (Fc-(CH2)n-SH), ferrocene-N-mercaptoalkylcarboxamides (Fc-CONH-(CH2)n-SH), and ω-mercapto-nalcohols (HO-(CH2)n+1-SH) used in this work were synthesized as described previously.30 Purification was by flash or gravityfeed liquid column chromatography on silica gel using hexanes modified with acetone as the mobile phase. Structures were verified by IR and NMR spectroscopy. Details of the synthesis and of spectroscopic product identification are included in the Supporting Information. Electrode and Monolayer Preparation. Gold electrodes were prepared from gold wires by melting the tip of a length of wire in a gas-air Bunsen burner flame to form a sphere attached to the wire. The diameter of the sphere was in the range of 0.30-0.45 mm for the 0.127 mm diameter wire and 0.15-0.30 mm for the 0.050 mm diameter wire. Gold spheres were etched in dilute aqua regia (4:3:1 H2O/HCl/HNO3) for 1 min31 and
Sumner et al. then rinsed liberally with deionized water followed by 2-propanol prior to monolayer formation. Mixed monolayers were formed by soaking the wire-attached spheres in coating solutions containing both redox-active thiols and nonredox-active coadsorbate thiols in ethanol. In most cases, the coating solutions were 1 mM total thiol with a ratio of 1:40 redox-active thiol to coadsorbate (Fc-CO-NH-(CH2)n-SH/HO-(CH2)n+1-SH or Fc-(CH2)n-SH/H-(CH2)n-SH). The electrodes were soaked in coating solutions overnight (∼16 h) at room temperature to form the mixed monolayers. Electrochemical Monolayer Characterization. After being coated, the electrodes were thoroughly rinsed with 2-propanol and then with deionized water, after which they were placed in a three-electrode cell with a Ag/AgCl/saturated KCl reference electrode and a platinum wire counter electrode in an aqueous 1.0 M perchloric acid electrolyte. The gold sphere working electrode was positioned so that the sphere was poised just below the surface of the electrolyte, with as little of the contacting wire exposed to the solution as possible. The electrochemistry of ferrocene groups on the monolayer-coated electrodes was studied using dc cyclic voltammetry and a novel ac voltammetry method that was recently described by Creager and Wooster.27 Two different instrumentation configurations were used. Lowfrequency (1 Hz-10 kHz) ac voltammograms and slow-scanrate dc cyclic voltammograms were collected on a CH Instruments model 660a electrochemical workstation. High-frequency (10 kHz-1 MHz) ac voltammograms were collected using a Solartron model SI 1260 impedance/gain phase analyzer with an Elchema PS-1705 high-speed potentiostat. All ac voltammograms were acquired at a peak ac voltage amplitude of 25 mV (50 mV peak-to-peak). Both instrument configurations yielded similar responses in ac voltammetry in the frequency ranges in which they overlapped. Results Figure 2 presents two voltammograms, one a dc cyclic voltammogram and the other an ac voltammogram, that were acquired sequentially for a particular mixed N-(10-mercaptodecyl)ferrocenecarboxamide/11-mercaptoundecanol monolayer on a gold electrode. Both voltammograms show a feature near +0.5 V corresponding to ferrocene oxidation/reduction. (Note that this particular monolayer was formed from a coating solution in which the ratio of ferrocene-thiol to coadsorbate thiol was higher than usual (1:10) so as to increase the ferrocene coverage on the electrode and thereby enhance the voltammetric features for the ferrocene group.) The charge under the anodic peak in the dc voltammogram is approximately 60 nC, which corresponds to 6.2 × 10-13 mol of ferrocene and a ferrocene surface coverage of approximately 7.9 × 10-11 mol cm-2. This coverage is approximately 18% of that expected for a hexagonally close-packed monolayer of ferrocene molecules, if the ferrocene molecules are assumed to be spherical with a diameter of 0.66 nm.32 The peak current in the ac voltammogram may also be used to estimate the amount of ferrocene present on the electrode surface.33 The estimate may be obtained using eq 1, in which Iavg(Eo) is the average ac peak current in the voltammogram, n is the number of electrons transferred per redox event, F is the Faraday constant, R is the universal gas constant, T is the temperature, and Eac is the peak amplitude, and f is the frequency of the applied ac voltage perturbation. The equation may be solved for Ntot, the total number of moles of redox-active species giving rise to the peak. The peak current of 560 nA in the ac voltammogram in Figure 2 therefore corresponds to oxidation/
Electron Transfer Between Ferrocene and Gold
J. Phys. Chem. B, Vol. 104, No. 31, 2000 7451
Figure 2. Comparison of the dc and ac voltammetry of a mixed N-(10mercaptodecyl)ferrocenecarboxamide/11-mercaptoundecanol monolayer on gold. Integration of the anodic peak in the dc cyclic voltammogram gives a ferrocene surface coverage of 7.9 × 10-11 mol cm-2, and analysis of the ac voltammetry peak current gives a coverage of 8.1 × 10-11 mol cm-2.
reduction of 6.4 × 10-13 mol of ferrocene and a ferrocene surface coverage of approximately 8.1 × 10-11 mol cm-2, in good agreement with the value obtained by integration of the anodic peak in the dc voltammogram.
Iavg(Eo) ) 2nfFNtot
sinh(nFEac/RT) cosh(nFEac/RT) + 1
(1)
Redox kinetics in monolayers such as these have been widely studied using dc voltammetry;13,14,34-36 however there are problems associated with such studies. One such problem is evident in the dc voltammogram in Figure 2; e.g., the anodic peak in this voltammogram is not as symmetric or well-formed as the cathodic peak. To make matters worse, both the background current and the anodic peak change shape slightly on subsequent scans (not shown), which suggests that the overall response is a function of not just the redox reaction in the monolayer but possibly also of slow or irreversible structural changes in the monolayer associated with multiple scans or redox conversions. Multiple-scan artifacts are commonly observed in electrodes coated with redox-active monolayers, and they can produce anomalous features that may or may not be related to the rate of electron transfer between the redox molecule and the gold electrode. Interestingly, the ac voltammogram of the same monolayer shows no such anomaly; the peak and background are well formed and very reproducible,
which suggests that the redox sites giving rise to the ac signal are behaving more ideally. In fact, the ac method has a built-in bias against single-scan artifacts since the ac signal is inherently a steady-state signal that includes only those contributions that are periodic with respect to the fluctuating applied potential. Figure 3 presents a series of ac voltammograms acquired at different frequencies for an electrode coated with a monolayer similar to that in Figure 2, except that the ferrocene coverage is much lower. The voltammograms span a frequency range from 10 Hz to 10 kHz, over which the magnitude of the peak current relative to the background (baseline) is seen to diminish as frequency increases. This phenomenon has been explained in terms of the relative time scales of interfacial electron transfer and the potential fluctuation; i.e., as the frequency approaches a critical value above which electron transfer can no longer keep up with the rapidly oscillating potential, the peak diminishes relative to the background.15,27,33 Creager and Wooster recently showed that this phenomenon could be treated quantitatively using a Randles equivalent circuit model to fit a Bode plot of the ratio of the peak to the background current versus log(frequency) using the standard electron-transfer rate constant for the immobilized redox species as a fitting parameter.27 Figure 4 presents two such analyses, one for a mixed monolayer of N-(10-mercaptodecyl)ferrocenecarboxamide with 11-mercaptoundecanol (top) and the other for a mixed monolayer of N-(7-mercaptoheptyl)ferrocenecarboxamide with 8-mercaptooctanol (bottom). The fitted rate constant for the top data set is approximately 1300 s-1, and that for the bottom data set is approximately 110 000 s-1, corresponding to an increase in rate by a factor of approximately 85 for a decrease in chain length of three methylene units. Note that in both cases the observed diminution of peak current with frequency corresponds very well to the fitted curve over the full frequency range. This agreement indicates that the Randles equivalent circuit model is a good model for describing the redox behavior of the monolayers, and that the redox reaction is well described by a single value of the standard electron-transfer rate constant. Figure 5 presents the fitted electron-transfer rate constants for ferrocene oxidation/reduction for all members of the two monolayer series illustrated in Figure 1. Table 1 also lists the rate constants along with the ac voltammetric peak potentials for each monolayer acquired at a low frequency (10 Hz) for which the redox reaction is electrochemically reversible. The data are presented in Figure 5 as plots of ln(ko) versus the number of methylene groups in the chain linking the ferrocene (or ferrocenecarboxamide) to the electrode to emphasize the expected exponential decrease in rate with increasing chain length in both series. The slopes of the best-fit lines for both series are approximately -1.4 per CH2, which is in good agreement (though on the high side) with previously reported values (-1.3 per CH2) for related systems incorporating ferrocene and other redox molecules into similar monolayers.11,14 An important feature of the data in Figure 5 is the consistent offset between the curves for the two monolayer series. Specifically, for a given alkane chain length, the rate constants in monolayers with a carboxamide group interposed between the alkane chain and the ferrocene group are consistently lower relative to the rates in monolayers in which the alkane chain is directly linked to the ferrocene. This difference in rate undoubtedly reflects the decrease in bridge-mediated electronic coupling associated with inserting a carboxamide group between the ferrocene group and the alkane chain attached to the gold electrode.
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Figure 3. AC voltammograms of a mixed N-(10-mercaptodecyl)ferrocenecarboxamide/11-mercaptoundecanol monolayer on gold at different frequencies. As frequency increases, the peak current diminishes relative to the background current.
Another way of thinking about these data is to consider the number of covalent bonds in the direct pathway linking the ferrocene group to the electrode. For example, the number of bonds in the direct pathway between ferrocene and gold is the same for a dodecyl linkage (Fc-(CH2)12-S-) as for a decyl linkage with an interposed carboxamide group (Fc-CONH(CH2)10-S-). Inspection of the curves in Figure 5 reveals that the electron-transfer rate constants are nearly the same for each of the four pairs of monolayers for which such a comparison can be made. (A series of dashed lines linking the relevant pairs of monolayers has been included in the figure to facilitate comparison.) This fact indicates that, to first order, replacing the pair of methylene units immediately adjacent to the ferrocene with a carboxamide unit has only a relatively minor effect on the electron-transfer rate. A closer inspection of the two series in Figure 5 reveals an unusual but very reproducible alternation in the variation of rate with chain length. For example, in the carboxamide series, the rates for chains containing an odd number of methylenes are consistently higher than those for chains with an even number of methylenes. For the alkane-linked series, the trend is opposite; i.e. the rates are consistently higher when the chain contains an even number of methylenes than when it contains an odd number of methylenes. These “even-odd” effects, which are also evident in previous studies of related ferrocene-based monolayers using fast-scan dc cyclic voltammetry14 and laserinduced temperature jump methods,11 could be caused by several factors. One possible factor is an alternating dependence of the
monolayer surface structure on the length of the alkane chain. Prior work on nonredox-active alkanethiolate monolayers has noted “even-odd” effects on water contact angles and infrared spectral absorptions that are thought to reflect variations in the conformation of the terminal functional group(s) depending on whether the chain has an even or odd number of carbons.37-42 Similar variations in the surface structure or conformation of ferrocene groups in the redox-active monolayers could cause an alternation in rates through subtle variations in either the electronic coupling or the reorganization energies of the ferrocene groups. The fact that the peak potentials in Table 1 do not show an alternation pattern that correlates with the alternation in rate suggests that the local solvation environment (and therefore the reorganization energy) is probably not varying regularly with chain length. It is difficult to comment definitively regarding the possible role of ferrocene conformation, except perhaps to note that since the ferrocene group is positioned well above the surface of the monolayer, it can probably sample many different conformations on the time scale of the redox reaction. Another possible cause for the observed alternation in rates is suggested by recent theoretical work from Hsu and Marcus in which a similar alternation in electronic coupling factors between ferrocene groups and gold electrodes linked by alkane chains was predicted using a novel sequential formula developed for calculating electronic coupling factors in redox-active monolayers. The reader is referred to the original works by Hsu28 and Hsu and Marcus29 for details on the origin of the predicted
Electron Transfer Between Ferrocene and Gold
J. Phys. Chem. B, Vol. 104, No. 31, 2000 7453 TABLE 1: Peak Potentials and Standard Electron-Transfer Rate Constants for Ferrocene Monolayers monolayer components
Epeak (V)
k0 (s-1)
Fc-CO-NH-(CH2)7-SH/HO-(CH2)8-SH Fc-CO-NH-(CH2)8-SH/HO-(CH2)9-SH Fc-CO-NH-(CH2)9-SH/HO-(CH2)10-SH Fc-CO-NH-(CH2)10-SH/HO-(CH2)11-SH Fc-(CH2)9-SH/CH3-(CH2)8-SH Fc-(CH2)10-SH/CH3-(CH2)9-SH Fc-(CH2)11-SH/CH3-(CH2)10-SH Fc-(CH2)12-SH/CH3-(CH2)11-SH
+0.572 +0.550 +0.563 +0.568 +0.362 +0.365 +0.362 +0.341
1.1×105 1.5×104 6.9×103 1.3×103 9.0×104 4.0×104 4.7×103 1.8×103
strong phenomenological evidence in support of the quantum interference explanation for the alternating rates. Conclusions Electron-transfer rates were measured for two series of ferrocene-based monolayers on gold electrodes, one in which the bridge linking the ferrocene group to gold consisted of a simple alkane chain and another in which the bridge was modified to include a carboxamide linkage adjacent to ferrocene. Rate constants were found to be nearly the same in the two series when the number of bonds in the direct pathway between the ferrocene group and the gold electrode was the same. An unusual “even-odd effect” in the diminution of rate with chain length for both monolayer series was also noted, and both structural and electronic factors that could cause such an alternation in rate are proposed.
Figure 4. Ipeak/Ibackground vs log(frequency) plots for two representative mixed monolayers.
Acknowledgment. The authors gratefully acknowledge the National Science Foundation for financial support of this research, and Robert S. Clegg of the California Institute of Technology for helpful discussions regarding background material. Supporting Information Available: Details of the synthesis and characterization of the ferrocene derivatives used in this work are included in the Supporting Information (4 pages). References and Notes
Figure 5. Plots of ln(ko) vs the number of methylene units in the bridge for the two series of monolayers illustrated in Figure 1. The slopes of the fitted lines for the two data sets are nearly identical at -1.4 per methylene group.
even-odd effect. In brief, the effect arises from taking a linear combination of powers of the positive and negative eigenvalues corresponding to the various electronic states on the bridge. This results in a dephasing of the electronic coupling through the multiband bridge. The effect is sometimes also referred to as quantum interference. The fact that the pattern of alternation found experimentally in the present work agrees with that predicted by Hsu28 for both the alkane-linked series and the amide-linked series (with the caveat that Hsu presented predictions for esters and not amides in the theoretical work) provides
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