J. Phys. Chem. 1995,99, 8458-8461
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Distance Dependence of Electron Transfer Rates in Bilayers of a Ferrocene Langmuir-Blodgett Monolayer and a Self-Assembled Monolayer on Gold Liang-Hong GUO,*~+ John S. Facci: and George McLendon*J NSF Center for Photoinduced Charge Transfer, Department of Chemistry, University of Rochester, Rochester, New York 14627, and Xerox Wilson Research and Technology Center, 114-390, 800 Phillips Road, Webster, New York 14580 Received: December 20, 1994; In Final Form: March 22, 1995@
Bilayers consisting of a self-assembled alkanethiol monolayer of variable chain length and Langmuir-Blodgett monolayers of 16-ferrocenylhexadecanoicacid were fabricated on Au electrodes. The hydrophobic ferrocene groups reside at the monolayer-monolayer interface and are variably spaced from the underlying electrode. Interfacial electron transfer rates k” for ferrocene electron transfer across the alkanethiol spacer monolayer were measured cyclic voltammetrically. The rates were found to fall off exponentially with increasing alkanethiol chain length, giving a decay constant of -0.96 per methylene. The electron transfer rate with a dodecanethiol spacer layer (k” = 12 s-l), which involves a non-bonded donor-acceptor couple, is almost 2 orders of magnitude lower than those for the analogous ferrocenes chemically bound on Au through alkane thiols of similar bond length. The higher rate in the latter may be attributed to a through-bond mechanism, as opposed to the former in which there exists a single “break” in the chemical bond network. The results have their implications on the electron transfer studies of biological systems.
We have sought to probe long-distance interfacial electron transfer’ (ET) by using designed electrochemical interfaces based on self-assembled monolayer2 (SAM) and LangmuirBlodgett (LB) films.3 In particular, we have fabricated on gold surfaces bilayer structures composed of an inner spacer monolayer of long-chain alkanethiols (CnH2,+1SH, n 2 10) and an outer electroactive LB monolayer, with electrochemically reactive head groups (ferrocene) positioned at the monolayermonolayer interface (Scheme la). The distance dependence of the heterogeneous ET rate constants can be investigated by varying the alkanethiol chain length. Furthermore, a comparison of these rates with those obtained at gold electrodes modified with ferrocene-terminated alkanethiol monolayers2c.f(Scheme lb) can address the question of the relative efficiency of “through-bond” versus “through-space’’ ET pathways. This is one of the key issues currently under investigation4 in the field of long range electron transfer, a reaction that is ubiquitous in biological systems and is involved in such important processes as oxidative phosphorylation and photosynthesis. In a biological electron transfer reaction, the electron donor and acceptor are separated by polypeptides organized into defined threedimensional structures mostly through noncovalent interactions such as hydrogen bonds and van der Waals contacts. The modulation of these noncovalent interactions on the overall electron tunneling efficiency through the polypeptide matrix is therefore well worth investigating. Experimental measurements5 as well as theoretical calculations6 have been carried out using metal-derivatized proteins and physiological protein-protein complexes to examine the effect of biological medium on electron transfer kinetics, although the ability to control the structural details of these systems has not been achieved yet. With the rapid advancement in the field of molecular selfassembly, we are now in a position to fabricate highly ordered monomolecular and multilayer films on solid surfaces with molecular level control over the structures and employ them in
‘University of Rochester.
* Xerox Wilson Research and Technology Center. @
Abstract published in Advance ACS Abstracts, May 15, 1995.
the study of long range electron transfer kinetics. It is with this particular issue in mind that we have designed the bilayer system and made an attempt to compare the relative efficiency of “through-bond” and “through-space’’ mechanisms. We have shown previously7 that the self-assembled monolayers we prepared on goldsilicon (AdSi) substrates possess similar structural properties as reported in a number of publications from other research groups. The monolayers were, as expected, densely packed, highly ordered, and with the alkyl chain tilted roughly 30” to the substrate normal. Virtually pinhole-free monolayers were routinely obtained as evidenced by the magnitude of the double-layer capacitance (ca. 1 pF1 cm2) and the inhibition of the reduction of dissolved ferricyanide in 1 M KCl. We have also successfully transferred* onto the alkanethiol-coated Au electrodes a single mixed LB monolayer of electroactive 16-ferrocenylhexadecanoic acid (FCAC) and hexadecanoic acid (HDAC). The electrochemical response of the ferrocene groups located at the monolayer-monolayer interface was greatly affected by the structural rigidity of the (outer) LB monolayer, which was in turn influenced strongly by the subphase composition. Well-behaved voltammetric waves for ferrocene were observed only when the outer monolayer was transferred at low surface pressures so that the monolayer was expanded and permeation of counter ions through the hydrophobic portion of the monolayer was not rate limiting. In this paper, we present measurements of the ET rates of SAM/LB bilayer electrodes modified with alkanethiols of variable chain length (n = 12, 14, 16, 18) and a comparison with ET rates in SAMs of ferrocene terminated alkanethiols. The latter system has a direct bond pathway from the ferrocene groups to the electrode substrate while the former has a single break. In a typical experiment, a AdSi substrate was first cleaned by short immersion in a hot “piranha” solution [1:3 H2S0d H202 (30%)] followed by electrochemical reduction’ in 1 M KC1. The Au electrode was subsequently immersed in an alkanethiol solution for about 2 days. The alkanethiol-modified electrodes were checked for the absence of pinholes (good
0022-365419512099-8458$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 21, 1995 8459
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SCHEME 1: Idealized structures of (a) the Bilayer Consisting of a Self-Assembled Monolayer (SAM) of Alkanethiols and a Langmuir-Blodgett Monolayer (LB) of 1:4 16-FerrocenylhexadecanoicAcid/Hexadecanoic Acid and (b) the Single Self-Assembled Monolayer of 12-FerrocenyldodecanethioYDodecanethiol
Au
SAM
blocking characteristics) as previously described by Porter et aL9 Those substrates which exhibited a cyclic voltammetric response (peaked or sigmoidal) characteristic of the presence of a large number of pinholes, and hence access of the electrolyte to the Au substrate, were discarded. LB transfer of mixed monolayers of 1:4 FCAC/HDAC to the alkanethiol-modified electrode was done by spreading and compressing the mixed film on an electrolytic subphase to a given pressure or area, annealing the monolayer film, and finally vertically immersing the alkanethiol-coated electrode slowly (2 d m i n ) into the subphase. Cyclic voltammograms were recorded immediately after film transfer when the substrate was still immersed in the trough. In a subphase containing 1 M NaC104, 0.5 mM CdClz, and 0.5 mM NaHCO3 (pH 6.5), the cyclic voltammetric response of the 1:4 mixed LB monolayer on an alkanethiol-modified Au electrode was sensitive to the way the LB film was transferred. Generally speaking, film annealing for longer than 20 min resulted in ill-defined or no cyclic voltammetric features in the potential range where electrochemistry of the ferrocene would normally appear (0-400 mV vs SCE), as reported before.* The transfer ratio and ex situ ellipsometric thickness indicated success of the LB transfer. We therefore attributed the lack of faradaic response to the following two factors: (1) the positive shift of the thermodynamic redox potential of the ferrocene in a more hydrophobic environment than its solvent-exposed analoguelo and (2) the redistribution of electrochemical potential across the double layer which decreases the effective potential imposed on the ferrocenes residing at the monolayer/monolayer interface." Both factors may share the same physical origin, i.e., blocking of counterion permeation to the redox active sites by the highly ordered LB monolayer. However, if the annealing time was shortened to about 5 min, cyclic voltammetric waves
were consistently observed independent of the inner alkanethiol thickness. We believe that short annealing time does not allow the outer monolayer to densify completely, permitting countenons to penetrate in some areas to the buried ferrocene sites. The dramatic effect of counterion permeation is shown explicitly in the following experiments. A mixed monolayer of 5 mol % 12-ferrocenyldodecanethioland 95 mol % tetradecanethiol normally exhibits well-defined cyclic voltammetric waves in a subphase electrolyte of 1 M NaC104 (pH 6.5, 0.2 mM Cd2+), with a midpoint potential of f 2 2 0 mV vs SCE, similar to those reported by Rowe and Creager.Io The coverage of ferrocene molecules calculated from integration of the faradaic current corresponds to approximately 5% of a monolayer. However, after an LB monolayer of 1:4 FCAC/HDAC was annealed for 20 min and then transferred (by vertical immersion into the subphase) onto the chemisorbed ferrocenyldodecanethiolmonolayer under the same conditions as used for the transfer onto alkanethiol modified Au, we observed that the ferrocene voltammetry was completely inhibited.8 Nevertheless, when the experiment was repeated with a film annealing time of 5 min, voltammetric waves were clearly seen again in the same potential region, although the calculated coverage was less than that before the LB monolayer transfer. We therefore assume that with short annealing a sufficient number of defects have been created in the outer LB monolayer that allow counterions to permeate to the buried ferrocene sites. While there are a number of possibilities for the origin of the defects, including "looping" of the amphiphile at the middle portion of its alkyl chain,I2 we think the most likely origin is that short annealing of the spread LB film produced a monolayer with a number of domains, and counterions permeated at domain edges, and activated the ferrocene groups in contact with the underlying S A M surface.
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8460 J. Phys. Chem., Vol. 99, No. 21, 1995
0.4
0.2
I
0.2 pAlcm2
1 9 c14
-2
~
10
12
14
16
18
20
Alkanethiol Carbon Number Figure 2. Plot of In ko vs methylene number of the alkanethiol, with the k" values calculated from the peak separations measured by cyclic voltammetry at 500 mV/s. The slope is -0.99 per methylene.
Figure 1. In-trough cyclic voltammograms at 500 mV/s of bilayers of a single Langmuir-Blodgett monolayer of 1:4 16-ferrocenylhexadecanoic acid (FCAC)/hexadecanoic acid spread on a subphase of 1 M NaC10d0.5 mM CdC12/0.5 mM NaHC03 (pH 6.5) and transferred at 16 mN/m (ca. 160 A2/FCAC) onto self-assembled alkanethiol monolayers on Au: (a) dodecanethiol, (b) tetradecanethiol, (c) hexadecanethiol, (d) octadecanethiol. Electrode area: 1 cm2.
Figure l a presents the cyclic voltammogram of 1:4 FCAC/ HDAC on a C12SWAu electrode. A single redox couple due to the oxidation and reduction of the ferrocenes is observed, with a formal potential (E"') of +0.23 V vs SCE, which is close to its thermodynamic redox potential of 0.19 V, estimated'O from 12-ferrocenyldodecanethiol in 1 M HC104. The ferrocene coverage obtained by integration of the area under the cathodic wave indicated a coverage of about 1% of the total number of FCAC molecules transferred to the electrode. The integrated charge was consistent from day to day and independent of inner layer thickness, demonstrating excellent reproducibility of the LB transfer. The separation between the anodic and cathodic peak (AEp = Ep,a- Ep,c)increased with the potential scan rate, indicating that the electrode reaction was controlled by ET kinetics. As the methylene number of the alkanethiol was increased, redox peak splitting at the same potential scan rate became larger as shown in Figure lb-d), indicating a decrease in ET rate with increasing thickness of the inner self-assembled monolayer. From those peak separations larger than 200 mV, the transfer coefficient can be extracted from the following equations derived by Laviron for the case of surface confined reagents.I3
EP,,- E"' = (RT/anfl ln(anFvIR7k")
(1)
Ep,c- E"' = [RT/(l - a>nF)] ln[(l - a)nFv/RTk"] (2) where a is the transfer coefficient and u is the sweep rate. Assuming that E"' is independent of sweep rate, a plot of (Ep - E"') as a function of ln(u) yielded two straight lines with a slope equal to (RTlmF) for the anodic current branch and [RT/ (1-a)nFI for the cathodic current branch. Transfer coefficients of a and (1 - a ) for FCAC/Cl&WAu were calculated as 0.68 and 0.41, respectively.
From the AE, values we estimated the standard heterogeneous rate constant (k") to be 12 s-l for FCAC/C12SWAu at a scan rate of 0.5 V/s. The rates for a given alkanethiol thickness were independent of the scan rate; therefore, there is no other complicated process involved in the heterogeneous reaction. Figure 2 shows a plot of In k" obtained at scan rates of 0.5 V/s as a function of the methylene number in the alkanethiol chain. The linearity of the plot demonstrates that the ET rate is exponentially dependent on the donor-acceptor distance even when there exists a break of chemical bonds between them. The slope of the plot corresponds to a value of the decay constant, /3, of -0.99 per methylene group. This /3 value agrees very well with those measured from electrochemical reactions across self-assembled long-chain alkanethiol monolayers on gold electrodes.2c-e In most of the previous reports on the ET rates across selfassembled alkanethiol spacer layers, the redox groups are linked to the alkanethiol through an unbroken linkage of chemical bondsksd from the donor to the electrode substrate. In covalently bound systems, donor and acceptor are thought to be coupled by through-bond as opposed to through-space interactions. On the other hand, a distinctive feature of the bilayer-modified gold electrode used in this work is that the electron donor (ferrocene) is contained in an adjacent monolayer, Le., physisorbed on the alkanethiol monolayer. Thus, the chemical bonding linkages between the donor and acceptor (Le., electrode) are broken between the ferrocene and the terminal methyl group of the SAM. A comparison of the rates we measured to those of the ferrocene-terminated alkanethiols would give us some idea of the influence of a single break in a through-bond electron transfer pathway on ET rate. Chidsey and Creagerl4~l5 have independently measured by electrochemistry the interfacial ET rate constants on Au electrodes for ferrocenes in self-assembled monolayer of HS -(CH2)1&00-Fc and HS - ( C H ~ ) ~ ~ - F C . respectively, and obtained rates of ca. 800 s-I in both cases. This allows us for the first time to make a straightforward comparison between two systems of similar "bridges", ET distance (about 14 A), electron donor and acceptor (ferrocene and Au), and driving force (AGO = 0). In the case of the nonbonded system (FCAC/C12SWAu), the ET rate (k" = 12 s-l) is slower by almost 2 orders of magnitude. Since the ferrocene in the bilayer film exhibits a midpoint potential similar to the value for its solvent-exposed monolayer equivalent, the electroactive sites in the bilayer film are presumably in contact with the electrolyte. Therefore, the slower ET rate is not a consequence of the possible redistribution of electrochemical potential which would reduce the effective potential sensed by
Letters the ferrocene groups and decrease the ET rates. We also believe ion transfer (IT) through the outer layer is not lowering the ET rate constants for the distances (C12 to Clg) we examined. Since in all the distance dependence experiments the outer LB monolayer was prepared following the same procedure and only the inner SAM thickness was varied, the IT rate should be the same regardless of the inner layer. Slow IT would set an upper limit for the apparent ET rate constant we could measure. We would then see a threshold effect; Le., the observed ET rate would be constant at short distances (limited by IT) and decrease exponentially at longer distances at which ET is slower than IT. Given the simple exponential relationship illustrated in Figure 2, it is believed that ion transfer was not the rate-limiting step. The present results and comparison suggest that the detailed nature of the bridge plays an important role in long distance electron transfer. Future work would include structural characterization of the interface between the SAM/LB monolayer and investigation of the effect of different interaction forces (e.g., ionic, hydrogen bonding) on the ET rates.
Acknowledgment. This work is supported by the National Science Foundation (Grant CHEM-9120001). The authors thank Prof. Stephen E. Creager of Indiana University for useful discussion and for providing the electron transfer rate of 12ferrocenyldodecanethiol. References and Notes (1) (a) Electron Transfer in Inorganic, Organic and Biological Systems; Advances in Chemistry Series 228; Bolton, J. R., Mataga, N., McLendon, G., Eds.; American Chemical Society: Washington, DC, 1991. (b) Marcus, R. A,; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265. (c) Simmons, J.
J. Phys. Chem., Vol. 99, No. 21, 1995 8461 G. J. Appl. Phys. 1963, 34, 1793. (d) Frese, K. W., Jr. J. Phys. Chem. 1981, 85, 3911. (e) Beratan, D. N.; Hopfield, J. J. J. Am. Chem. SOC. 1984, 106, 1584. (2) (a) Whitesides, G. M.; Laibinis, P. E. Lungmuir 1990, 6 , 87. (b) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: Boston, 1991. (c) Chidsey, C. E. D. Science 1991, 251, 919. (d) Finklea, H. 0.;Hanshew, D. D. J. Am. Chem. SOC.1992, 114, 3173. (e) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1992, 96, 2657. (f) Creager, S. E.; Weber, K. Langmuir 1993, 9, 844. ( 3 ) (a) Gaines, G. L. Insoluble Monolayers at Liquid-Gas Inteflaces. Intersciences: New York, 1966. (b) Robert, G. G. Langmuir-Blodgett Films; Plenum: New York, 1990. (c) Mobius, D. Acc. Chem. Res. 1981, 14, 63 and references therein. (d) Facci, J. S. In Techniques of Chemistry; Murray, R. W., Ed.; Wiley-Interscience: New York, 1993; Vol. XXII, Chapter 3. (e) Hsu, Y.; Penner, T. L.; Whitten, D. G. J. Phys. Chem. 1992, 96, 2790. (4) Moser, C. C.; Keske, J. M.; Wamcke, K.; Farid, R. S.; Dutton, P. L. Nature 1992, 355, 796 and references therein. ( 5 ) (a) McLendon, G. Acc. Chem. Res. 1988, 21, 160. (b) Liang, N.; Pielak, G.; Mauk, A. G.; Smith, M.; Hoffman, B. M. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 1249. (c) Therien, M. J.; Chang, J.; Raphael, A. L.; Bowler, B. E.; Gray, H. B. In Structure and Bonding; Palmer, A,, Ed.; SpringerVerlag: Berlin, 1991; Vol. 75, pp 109-129. (6) Onuchic, J. N.; Beratan, D. N.; Winkler, J. R.; Gray, H. B. Annu. Rev. Biophys. Biomol. Struct. 1992, 21, 349. (7) Guo, L.-H.; Facci, J. S.; McLendon, G. Langmuir 1994, 10,4588. ( 8 ) Guo, L.-H.; Facci, J. S.; McLendon, G. J. Phys. Chem. 1995, 99, 4106. (9) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. SOC.1987, 109, 3559. (10) Rowe, G. K.; Creager, S. E. Lungmuir 1991, 7, 2307. (11) Smith, C. P.; White, H. S. Anal. Chem. 1992, 64, 2398 (12) We thank one of our reviewers for pointing out this possibility. (13) Laviron, E. J. Electroanal. Chem. 1979, 101, 9. (14) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (15) Creager, S. E.; Hockett, L. A.; Alleman, K. S. Unpublished results.
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