J. Phys. Chem. B 2000, 104, 5399-5402
5399
Effect of Interchain Hydrogen Bonding on Electron Transfer through Alkanethiol Monolayers Containing Amide Bonds Slawomir Sek, Aleksandra Misicka, and Renata Bilewicz* Department of Chemistry, UniVersity of Warsaw, Pasteura 1, 02-093 Warsaw, Poland ReceiVed: January 31, 2000; In Final Form: March 29, 2000
Two-component self-assembled monolayers (SAMs) were designed as model systems for the studies of the factors controlling long-range electron-transfer processes. The monolayers, which were attached to a gold electrode, consisted of two components: an alkanethiol with an amide moiety in the chain and an electroactive terminal ferrocene group and a nonelectroactive thiol acting as the diluent of the electroactive centers. The diluent was either a simple alkanethiol or an alkanethiol containing an amide group. The rates of electron transfer for the electroactive ferrocene group were calculated using the data obtained by voltammetry and chronoamperometry. Upon substitution of the diluent alkanethiol for an alkanethiol containing an amide group, the rate constant of the oxidation of ferrocene headgroups increased more than 3 times, while the reorganization energy remained unchanged. The increased electronic coupling between ferrocene and the electrode was ascribed as due to the contribution of the hydrogen bond network linking internal amide groups in the monolayer.
Introduction Carefully designed monolayers, formed either by the Langmuir-Blodgett technique or self-assembly on electrodes, are excellent model systems to study the effects of distance and medium on long-range electron-transfer processes.1-11 Purposely modified molecules used to form the monolayer assemblies allow one to change, in a controlled way, the structure and the properties of the medium separating the donor and acceptor, and in this way to change the electron-transfer rates. Such layers may be considered as very simple models of biological systems and may also be useful for practical applications in molecular electronics, electrocatalysis, sensor design, and protection against corrosion.12,13 Electrochemical methods are well suited for the studies of the effect of chemical bonds formed in the monolayer on the efficiency of electron tunneling across the layer.14,15 In particular, Chidsey reported that the electrodes covered with mixed monolayers containing alkanethiol with a terminal redox active group as one of the components can be used as model systems to study electron-transfer processes.10,11 Since then, monolayers of organic thiols with ferrocene redox centers bonded to the thiol were often used as the model systems.16-23 The molecules synthesized in the present work possess a ferrocene redox moiety, but they differ from simple ferrocenethiols by the amide groups purposely introduced into the alkyl chains. Clegg and Hutchison24-27 studied the effect of amide groups replacing the methylene units in the alkanethiols on the order and the properties of the monolayer. The contribution of the electronic coupling of an amide group was reported to be almost indistinguishable from that of two methylene groups.23 In the present work we show the effect of hydrogen bonds formed by amide groups in the monolayer on the electronic coupling between ferrocene and gold electrodes. To obtain the hydrogen bond network, two-component monolayers were * Corresponding author. Fax: (+48) 22 8225996. E-mail: bilewicz@ chem.uw.edu.pl.
prepared using functionalized alkanethiols. One component is an alkanethiol (compound 1) with an amide bond in the alkyl chain and a tethered ferrocene headgroup. The second component of the monolayer acts as the nonelectroactive diluent of the ferrocenethiols and is either a simple alkanethiol (SAM1) or a thiol with an amide group (SAM2) incorporated into the backbone (compound 2). In SAM1 only the electroactive ferrocene molecules contained amide bonds and the diluent thiol was a simple alkanethiol, while in SAM2 both ferrocenethiol and the diluent thiol have an amide moiety in the alkyl chains. Examination of electron transfer through these two types of monolayer assemblies revealed the important role of hydrogen bonding in the electron transport over larger distances. Experimental Section Materials and Syntheses. Cystamine and ferrocenemonocarboxylic acid were purchased from Sigma, di-tert-butyl dicarbonate was purchased from Fluka, and other starting materials were obtained from Aldrich. N-tert-Butyloxycarbonyl12-aminododecanoic acid was prepared according to procedures described in the literature.28 The syntheses of compounds forming monolayers were based on coupling reactions between the amine and carboxyl groups. Compound 1: (FcCONH(CH2)11CONH(CH2)2S)2 was synthesized in two steps. In the first step, the coupling reaction between BOC-12-aminododecanoic acid and cystamine was carried out in the presence of dicyclohexylcarbodiimide (DCC). In the second step (after removing BOC group), to attach ferrocenemonocarboxylic acid, O-(benzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate was used as a coupling reagent. Compound 2: (CH3(CH2)10CONH(CH2)2S)2 was synthesized by a coupling reaction between cystamine and dodecanoic acid in the presence of DCC. Confirmation of the desired structure was provided by NMR spectra. 1H NMR (CDCl3), δ (ppm): compound 1 1.20-1.60 (broad overlapping resonance, 36H), 2.24 (t, J ) 7 Hz, 4H), 2.88 (t, J ) 7 Hz, 4H), 3.60 (broad, 8H), 4.24 (s, 10H), 4.36 (s, 4H), 4.69 (s, 4H), 6.03 (broad s, 2H), 6.72 (broad s, 2H); compound 2 0.88 (t, J ) 7 Hz, 6H),
10.1021/jp000376z CCC: $19.00 © 2000 American Chemical Society Published on Web 05/13/2000
5400 J. Phys. Chem. B, Vol. 104, No. 22, 2000
Sek et al.
Figure 2. Cyclic voltammetric curve for the gold electrode modified by SAM2, recorded in 1 M HClO4, V ) 0.05 V/s.
ments were performed using a Princeton Applied Research model 273A potentiostat, connected with an IBM-compatible 486 computer. Results
Figure 1. Scheme of the monolayer assemblies SAM1 and SAM2.
1.20-1.40 (broad overlapping resonance, 32H), 1.61 (m, 4H), 2.21 (t, J ) 7 Hz, 4H), 2.83 (t, J ) 6.5 Hz, 4H), 3.55 (m, 4H), 6.60 (broad s, 2H). Monolayer Deposition. Monolayers were assembled on the evaporated gold electrodes. The Au working electrodes (A ) 0.55 cm2) were 75-100 nm thick films of gold vapor-deposited in a vacuum onto glass slides. Before depositing the gold, the glass slides were precoated with 5 nm thick layers of titanium. To obtain SAM1 (Figure 1), self-assembly was carried out in ethanol solutions containing 0.05 mM FcCONH(CH2)11CONH(CH2)2S)2 (1) and 0.9 mM hexadecanethiol, and for SAM2 (Figure 1), the solution contained 0.1 mM (FcCONH(CH2)11CONH(CH2)2S)2 (1) and 0.9 mM CH3(CH2)10CONH(CH2)2S)2 (2). The compounds employed to form the monolayers were in fact derivatives of disulfides, since they exhibited enhanced stability in the presence of oxygen as compared to simple thiols. However, upon chemisorption on gold, they were reported to form monolayers indistinguishable from those obtained by chemisorption of simple alkanethiols.29-31 The procedure for self-assembly was identical to those described earlier in the literature.10,11,16 Following 48 h of self-assembly in the solutions given above, the electrodes were placed in a 1 mM solution of a diluent nonelectroactive thiol (or disulfide) for 1 week. Electrochemistry. All electrochemical experiments were performed in a three-electrode arrangement with a saturated sodium chloride calomel electrode as the reference electrode, platinum foil as a counter electrode, and a gold working electrode. The supporting electrolyte was an aqueous 1 M HClO4. Cyclic voltammetry and chronoamperometry experi-
The two-component SAM1 and SAM2 monolayers were studied using voltammetry and chronoamperometry. A typical voltammogram obtained at low scan rates (10-100 mV/s) is shown in Figure 2. The peak-to-peak distance for SAM1 and SAM2 were never larger than 20 mV, and the width of the peak at half-height was ca. 90 mV; i.e., the values close to the theoretical for a reversible 1e electrode process were obtained.32 This is in contrast to the characteristics of the voltammetric curves obtained for single-component monolayers. Also, an almost reversible shape of the voltammetric curves at low scan rates points to a good organization of the monolayer and a similarity of the environment around each electroactive center. Additionally, the lack of peak broadening suggests no interactions between the ferrocenyl moieties. The capacitance for SAM1 (Table 1) is equal to that of a well-organized typical alkanethiol layer with methyl terminal groups.33 For SAM2, the capacitance is ca. 3-4 times larger than that and this value is in agreement with the values reported recently by Clegg and Hutchison26 for monolayers of nonelectroactive alkanethiols with similar length chains containing amide groups. The somewhat increased capacitance has been ascribed by these authors to a slight disorder in the hydrocarbon part of the layer and/or changes of the dielectric constant due to the presence of amides. Despite this disorder, the blocking efficiency remained 0.9988 and the authors concluded that the rate of electron transfer in these layers is controlled by throughchain electronic coupling. Results of these authors encourage us to use the monolayers as good model systems for the studies of long-range electron transfer. Despite somewhat larger permeability for solution species, SAM2 also gave more stable coverages of the electrodes than SAM1 or the monolayers without amide moieties. Enhanced time stability was noted earlier by Sabapathy and co-workers31 for single-component monolayers containing amide bonds in the molecules and was ascribed to the presence of hydrogen bonds between neighboring molecules. The apparent rate constants were calculated from chronoamperometry for selected overpotentials. In chronoamperometric
Electron Transfer through Alkanethiol Monolayers
J. Phys. Chem. B, Vol. 104, No. 22, 2000 5401
TABLE 1: Characteristics of the Monolayers components of the monolayera
formal potential (V) vs SSCE
surface coverage of ferroceneb (%)
Cdlc (µF/cm2)
k0app d (s-1)
λe (eV)
1 and C16SH 1 and 2
0.32 ( 0.01 0.33 ( 0.01
6(1 10 ( 3
1.40 ( 0.33 6.70 ( 0.44
4.5 ( 0.3 17 ( 1
0.85 ( 0.02 0.85 ( 0.01
a 1, (FcCONH(CH ) CONH(CH ) S) ; 2, CH (CH ) CONH(CH ) S) . b Surface coverage of ferrocene was in the range 5-8% for SAM1 and 2 11 2 2 2 3 2 10 2 2 2 7-13% for SAM2, and no dependence of rate constants on the fraction of ferrocenethiol was observed for these monolayers. All data depicted in this table are mean values of measurements at 10 monolayer-modified electrodes. c Double-layer capacitance measured at potential +0.05 V. d Standard rate constants were calculated using the Marcus theory approach as described in the text. e Reorganization energy.
Figure 3. Tafel plots for SAM2. The theoretical plot (full line) was calculated on the basis of eq 2 with k0app ) 17 s-1, λ ) 0.85 eV, and T ) 293 K. Points are experimental data.
experiments, the current transient, following a potential step, is measured. In this work, the potential step applied was symmetrical vs the formal potential of the ferrocene redox centers and the anodic and cathodic rate constants were determined in one experiment. At a given overpotential, η, the current, i, can be described by the following equation:
i ) kappQ exp(-kappt)
(1)
where Q is the charge associated with the redox process and kapp is the apparent rate constant. A plot of ln i vs t is linear, and the slope equals -kapp. The experimental data were used to find the relationship between ln kapp and η and to find the best fit to the theoretical Tafel plot. A typical Tafel plot with experimental data (for SAM2) is shown in Figure 3. The data are fitted to the theoretical Tafel line obtained from the following equation:10,17
kapp ) µFkBT
∫-∞+∞
exp{-(x - (λ + eη)/kBT)2(kBT/4λ)} 1 + exp(x)
(2)
where µ is the distance dependent electronic coupling between the electrode and the redox sites, F is the density of electronic states in the metal electrode, λ is the reorganization energy, η is the overpotential, kB is the Boltzmann constant, x is the electron energy relative to the Fermi level, T is temperature, and e is the charge of an electron. The integral in eq 2 was evaluated numerically and input parameters were the standard rate constant, reorganization energy, and temperature. As a result of fitting theoretical and experimental data, we obtained both the reorganization energy values and the standard rate constants for the systems studied. The standard rate constants were
obtained from Tafel plots for η ) 0 and were in agreement with data obtained from cyclic voltammetry using the Laviron treatment.34 The reorganization energies, λ, may be treated as a diagnostic parameter for the degree of penetration of the ferrocene group into the hydrophobic layer.35 In the case of penetration, a significant decrease of λ is expected, since the dielectric constant in the environment of the electroactive centers would decrease.36-38 However, in our case the values of reorganization energies are identical to those reported for normal alkanethiol monolayers with tethered ferrocene redox centers10,18 and are close to the theoretical value of 0.94 eV. It may be concluded, therefore, that despite a possibly slightly larger permeability for solution species (as indicated by larger capacitancies), the ferrocene groups are preferably located outside the hydrophobic layer, i.e., in the aqueous medium. Also linear ln i vs t (chronoamperometry) plots point to a kinetic uniformity in the layer, because identical rates with which the redox groups exchange the electrons suggest that they are located at the same distance from the electrode surface.10,38 The formal potentials, rate constants, and reorganization energies obtained for the two-component monolayers, SAM1 and SAM2, respectively, are presented in Table 1. Discussion The results presented in the previous section can be used for the comparison of two various types of monolayers: (1) SAM1 with an amide moiety situated only in the electroactive component and (2) SAM2 with amide groups both in the electroactive molecules and in the diluent thiol. In the case of SAM1, the results (Table 1) were in agreement with the literature data for monolayers of simple alkanethiols with tethered ferrocene groups. For example, a k0app value of 1.25 s-1 obtained by Chidsey10 and the value of 6.6 s-1 obtained by Creager18 are both close to our value of 4.5 s-1. This indicates that the introduction of a CONH bond exclusively into the electroactive molecules does not lead to an increase of the electron-transfer rate.39 Even smaller rate constants were reported earlier for thiols with a single ether moiety, double or triple bond inserted into the chain as compared to simple alkanethiols.6 In contrast, for SAM2 containing the CONH bonds both in the electroactive molecules and in the diluent thiol, the standard rate constants were over 3 times larger than for SAM1. Taking into account that SAM1 and SAM2 had similar lengths of the molecules, one would expect also similar rate constants. In SAM2 the redox centers are located even further from the electrode surface, since the alkanethiol chains are tilted by ca. 30° relative the electrode surface, while the amide-containing molecules are oriented closer to perpendicular.24 The higher rate constant for SAM2 than for SAM1 suggests that in the presence of the amide group a new internal interface is created, which increases the strength of electronic coupling, and larger values of rate constants are, therefore, observed.
5402 J. Phys. Chem. B, Vol. 104, No. 22, 2000 An increase of electron tunneling efficiency has been reported recently by Slowinski and co-workers in the case of a bilayer system based on nonelectroactive molecules containing amides and forming the tunneling junction between two mercury drops.40 On the basis of results presented above it can be suggested that the increase of coupling is due to the presence of a hydrogen bond network formed inside the layer. The destruction of this network, by introduction of a simple alkanethiol as the diluent into the monolayer (SAM1), leads to the decrease of the rate constant of the electron transfer through the layer. To explain this important observation, we have to consider, by analogy to the electron transfer in proteins,41 various possibilities for electron tunneling in the monolayer involving covalent bonds, hydrogen bonds, and van der Waals bridges (through a space tunneling mechanism). In the case of SAM1 containing alkanethiol as the diluent, we can expect that the predominant pathway of electron tunneling is through the covalent bonds with a small contribution of chain-to-chain coupling.9 The two-component model monolayer (SAM2) presented in this work were used to demonstrate that the contribution of the coadsorbate,23 or diluent, to the electronic coupling may be significant when additional electron delocalization is introduced by the lateral bonds formed by the diluent molecules. Such cooperativity effects confirm the importance of hydrogen bonding for long-range electron transfer, containing and encourage further modifications of monolayer assemblies designed to examine electron pathways resembling those responsible for electron transfer in biological systems. Acknowledgment. We thank K. Slowinski, J. Fuhrhop, and H. Elzanowska for helpful comments. References and Notes (1) Mann, B.; Kuhn, H. J. Appl. Phys. 1971, 42, 4398. (2) Kuhn, H. J. Photochem. 1972, 10, 11. (3) Polymeropoulos, E. E.; Mobius, D.; Kuhn, H. J. Chem. Phys. 1978, 68, 3918. (4) Polymeropoulos, E. E.; Mobius, D.; Kuhn, H. Thin Solid Films 1980, 68, 173. (5) Haran, A.; Waldeck, D. H.; Naaman, R.; Moons, E.; Cahen, D. Science 1994, 263, 948. (6) Cheng, J.; Saghi-Szabo, G.; Tossel, J. A.; Miller, C. J. J. Am. Chem. Soc. 1996, 118, 680. (7) . Sachs, S. B.; Dudek, S. P.; Hsung, R. P.; Sita, L.; Smalley, J. F.; Newton, M. D.; Feldberg, S. W.; Chidsey, C. E. D. J. Am. Chem. Soc. 1997, 119, 10563. (8) Slowinski, K.; Chamberlain, R. V.; Bilewicz, R.; Majda, M. J. Am. Chem. Soc. 1996, 118, 4709. (9) Slowinski, K.; Chamberlain, R. V.; Miller, C. J. K.; Majda, M. J. Am. Chem. Soc. 1997, 119, 11910.
Sek et al. (10) Chidsey, C. E. D. Science 1991, 251, 919. (11) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (12) Ulman, A. An Introduction to Ultrathin Organic Films. From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991; Chapter 5.4, pp 376-384. (13) Molecular Electronics; Jortner, J., Ratner, M., Eds.; IUPAC and Blackwell Science: Oxford, U.K., 1997; Chapter 8, pp 255-280. (14) Miller, C. J. Heterogeneous Electron-Transfer Kinetics at Metallic Electrodes. In Physical Electrochemistry: Principles, Methods, and Applications; Rubinstein, I.; Ed.; Marcel Dekker: New York, 1995; pp 2779. (15) Finklea, H. O. Electrochemistry of Organized Monolayers of Thiols and Related Molecules on Electrodes. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; pp 109-335. (16) Creager, S. E.; Rowe, G. K. Anal. Chim. Acta 1991, 246, 233. (17) Tender, L.; Carter, M. T.; Murray, R. W. Anal. Chem. 1994, 66, 3173. (18) Weber, K.; Creager, S. E. Anal. Chem. 1994, 66, 3164. (19) Rowe, G. K.; Creager, S. E. J. Phys. Chem. 1994, 98, 5500. (20) Richardson, J. N.; Peck, S. R.; Curtin, L. S.; Tender, L. M.; Terrill, R. H.; Carter, M. T.; Murray, R. W.; Rowe, G. K.; Creager, S. E. J. Phys. Chem. 1995, 99, 766. (21) Carter, M. T.; Rowe, G. K.; Richardson, J. N.; Tender, L. M.; Terrill, R. H.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 2896. (22) Smalley, J. F.; Feldberg, S. W.; Chidsey, C. E. D.; Linford, M. R.; Newton, M. D.; Liu, Y.-P. J. Phys. Chem. 1995, 99, 13141. (23) Creager, S.; Yu, C. J.; Bamdad, C.; O’Connor, S.; MacLean, T.; Lam, E.; Chong, Y.; Olsen, G. T.; Luo, J.; Gozin, M.; Kayyem, J. F. J. Am. Chem. Soc. 1999, 121, 1059. (24) Clegg, R. S.; Hutchison, J. E. Langmuir 1996, 12, 5239. (25) Clegg, R. S.; Reed, S. M.; Hutchison, J. E. J. Am. Chem. Soc. 1998, 120, 2486. (26) Clegg, R. S.; Hutchison, J. E. J. Am. Chem. Soc. 1999, 121, 5319. (27) Clegg, R. S.; Reed, S. M.; Smith, R. K.; Barron, B. L.; Rear, J. A.; Hutchison, J. E. Langmuir 1999, 15, 8876. (28) Stewart, J. M.; Young, J. D. Solid-Phase Peptide Synthesis, 2nd ed.; Pierce Chemical Co.: Rockford, IL, 1984; Chapter 2, pp 61. (29) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 1766. (30) Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M. Langmuir 1994, 10, 1825. (31) Sabapathy, R. C.; Bhattacharyya, S.; Leavy, M. C.; Cleland, W. E.; Hussey, C. L. Langmuir 1998, 14, 124. (32) For the reversible case with no interactions in the monolayer the theoretical width of the voltammetric peak at half-height is 3.53RT/nF. (33) Porter, M. D.; Bright, T. B.; Allara, D.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (34) Laviron, E. J. Electoanal. Chem. 1979, 101, 19. (35) Slowinski, K.; Slowinska, K. U.; Majda, M. J. Phys. Chem. 1999, 103, 8544. (36) Marcus, R. A. J. Phys. Chem. 1963, 67, 853. (37) Marcus, R. A. J. Chem. Phys. 1965, 43, 679. (38) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173. (39) In monolayers containing less than 15% ferrocenethiol we did not detect any dependence of electron-transfer rate on the content of electroactive component. (40) Slowinski, K.; Fong, H. K. Y.; Majda, M. J. Am. Chem. Soc. 1999, 121, 7257. (41) Regan, J. J.; Risser, S. M.; Beratan, D. N.; Onuchic, J. N. J. Phys. Chem. 1993, 97, 13083.
