Langmuir 1993,9, 1392-1396
1392
Electron Transfer in Self -Assembled Monolayers of N-Met hyl-N-carboxyalkyl-4,4’-bipyridinium Linked to Gold Electrodes Eugenii Katz,? Norbert Itzhak, and Itamar Willner’ Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Received April 17, 1992. In Final Form: March 1, 1993 Electron transfer processes in monolayer assemblies formed by covalent linkage of N-methyl-”carboxyalkyl-4,4’-bipyridinium(1) to cystamine, chemisorbed onto gold electrodes, are examined. The resulting monolayer exhibits nonordered structure being reflected by a similar rate constant to the bipyridiniumredox sites,that is independentof the bridging alkyl chain length,ket = 550 8-1. The nonordered structure is also confirmed by the low electron transfer coefficients a, and .a corresponding to 0.25 f 0.05 and 0.3 f 0.05, respectively. By treatment of the nonordered monolayer assembly with 1-hexadecanethiol, C1&3H,a more densely packed organized monolyer is formed, where the alkyl-substituted bipyridinium sites are stretched in the monolayer configuration. In these assemblies the electron transfer rate constants to the bipyridinium sites depend on the alkyl chain length bridging the redox site to the electrode. The electron transfer rate constants follow Marcus theory and the electron tunneling coefficient corresponds to 0 = 0.006 A-I.
Introduction Electron transfer processes play a fundamental role in chemistry’ and biology.2 The distance dependence and nature of chemical bonds linking a donor-acceptor pair on electron transfer rates has been the subject of extensive theoretical3 and experimentalP7 studies. For the characterization of spatial bonds spacing effects on electron transfer rates, well-organized assemblies are essential. Monolayers of alkylorganosulfur compounds such as thiols! sulfides? and disulfides10 are self-assemblingon gold electrodes. Previous studies have revealed long range electron transfer in alkylbipyridinium monolayers associated with Au electrodes via sulfur-containing anchor groups.11J2 Similarly, electron tunneling rates in other film structures of redox functionalized electrodes have been examined.13 Here we wish to report on electron transfer processes in self-assembledmonolayers of N-methyl-N’-carboxyalkyl-4,4’-bipyridiniumredox sites. These
* Author to whom correspondence should be addressed. +Postdoctoralfellow from the Institute of Soil Science and Photosynthesis, USSR Academy of Science, Pushchino, Moscow Region. (1) (a) Meyer, T.J. Acc. Chem. Res. 1989, 22, 163. (b) Willner, I.; Willner, B. Topics in Current Chemistry; Springer: Berlin, 1991; Vol. 159, p 153. (c)Wasielewaki, M. R. Photochem. Photobiol. 1988,47,923. (d) Gust, D.; Moore, T. A. Science 1989,244, 35. (2) (a) Therien, M. J.; Chang, J.; Raphael, A. L.; Bowler, B. E.; Gray, H. B. In Long-Range Electron Transfer in Biology, Structure and Bonding; Springer: Berlin, 1991; Vol. 75, p 109. (b) McLendon, G. Acc. Chem. Res. 1988, 21, 160. (c) Mayo, S. L.; Ellis, W. R., Jr.; Crutchley, R. J.; Gray, H. B. Science 1986,233,948. (d) Closs, G. L.; Miller, J. R. Science 1988,240,440. (e) Hoffman, B. M.; Natan, M. J.; Nocek, J. M.; Walling, S. A. Long-Range Electron Transfer in Biology, Structure and Bonding; Springer: Berlin, 1991; Vol. 75, p 85. (3) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985,811, 265. (4) MBbius, D. Ber. Bunsen-Ges. Phys. Chem. 1978,82, 848. (5) (a) Weaver, M. J.; Li, T. T.-T.J. Am. Chem. SOC.1984,106,6107. (b) Li, T. T.-T.; Weaver, M. J. J. Electroanal. Chem. 1985,188,121. (c) Weaver, M. J.; Li, T.T.-T. J. Phys. Chem. 1986, 90, 3823. (6) Closs, G. L.; Calcaterra, L. T.; Green, N. J., Penfield, K. W.; Miller, J. R. J. Phys. Chem. 1986, 90, 3673. (b) Beratan, D. N. J. Am. Chem. SOC.1986, 108,4321. (c) Oevering, H.; Paddon-Row, M. N.; Heppener, M.; Oliver, A. M.; Cotsaris, E.; Verhoeven, J. W.; Hush, N. S. J. Am. Chem. SOC.1987, 109, 3258. (d) Paddon-Row, M. N.; Oliver, A. M.; Warman, J. M.; Smith, K. J.; De Haas, M. P.; Oevering, H.; Verhoeven, J. W. J.Phys. Chem. 1988,92, 6958. (7) (a) Wasielewski, M. R. In Photoinduced Electron Transfer; Fox, M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part A, p 161. (b) Devering, H.; Verhoeven, J. W.; Paddon-Row, M. N.; Warman, J. M. Tetrahedron 1988, 45, 4751.
0743-7463193/2409-1392$04.O0/0
redox groups are covalently linked to amino-functionalized spacers immobilized on a gold electrode via a thiolate anchor group. We characterize the electron transfer processes between the electrode and bipyridinium redox units in nonorganized monolayer assemblies, where conformational flexibility of monolayer configuration exists, and in more densely packed well-orderedmonolayer films formed in the presence of added 1-hexadecanethiol. The latter monolayer configuration allows ‘us to correlate distance effects on electron transfer rates and to evaluate the contribution of the monolayer matrix thickness on electron tunneling processes. Experimental Section The bipyridinium salts were prepared by the following methods: Compound la was synthesized by the reaction of N-methyl-4-(4’-pyridyl)pyridiniumiodide with acrylic acid according to the 1iterat~re.l~~ Compounds l b and IC were synthesized by the reaction of N-methyl-4-(4’-pyridyl)pyridinium iodide with the appropriate bromocarboxylicacid by modification of the literature pr~cedure.~~b The N-methylpyridylpyridinium salt in DMF was heated (110 “C) with an excess of the (8) (a) Porter, M. D.; Bright, T.B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. SOC.1987,109,3559. (b) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.;Evall, J.;Whitesides, G. M.; Nuzzo, R. G.J.Am. Chem. SOC.1989, 111,321. (c) Bain, C. D.; Whitesides, G. M. J . Am. Chem. SOC.1989,111, 7164. (d) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. SOC. 1989,111,7155. (e) Bain, C. D.; Whitesides, G. M. Angew. Chem., Znt. Ed. Engl. 1989,101,522. (0 Nuzzo, R. G.; Duboie, L. H.; Allara, D. L. J.Am. Chem.SOC.1990,112,558. (g) Collard,D. M.;Fox,M. A.Langmuir 1991,7,1192. (h) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991,7,1510. (i) Rowe, G. K.; Creager, S. E. Langmuir 1991,7,2307. (j)Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. SOC.1991, 113, 2370. (9) (a) Strong, L.; Whitesides, G. M. Langmuir 1988, 4 , 546. (b) Rubinstein,I.; Steinberg, S.; Tor, Y.; Shanzer,A.; Sagiv, J. Nature 1988, 332, 426. (c) Steinberg, S.; Tor, Y.; Sabatani, E.; Rubinatein, I. J. Am. Chem.Soc. 1991,113,5176. (d)Katz,E.Yu.;Borovkov,V.V.;Evstigneeva, R. P. J. Electroanal. Chem. 1992, 326, 197. (10) (a) Hickman, J. J.; Ofer, D.; Zou, C.; Wrighton, M. S.; Laibinis, P. E.; Whitesides,G. M. J . Am. Chem. SOC.1991,113,1128. (b) Nuzzo, R. G.; Fusco, F. A,; Allara, D. L. J . Am. Chem. SOC.1987,109,2358. (c) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989,5, 723. (d) Katz, E. Yu.; Solov’ev,A. A. J. Electroanal. Chem. 1990, 291, 171. (11)De Long, H. C.; Buttry, D. A. Langmuir 1990,6, 1319. (12) Lee, K. A. B. Langmuir 1990,6,709. (13) (a) Hong, H.-G.; Mallouk, T. E. Langmuir 1991, 7 , 2362. (b) Chidsey, C. E. D. Science 1991,251,919. (14) (a) Delacroix,A. Bull. SOC.Chim.Fr. 1973,7-8,2404. (b) Widrig, C. A.; Majda, M. Anal. Chem. 1987,59,754.
0 1993 American Chemical Society
Langmuir, Vol. 9, No.5, 1993
Electron Transfer in Self-Assembled Monolayers
1393
Scheme 1. Sequence of processes for chemical modification of Au-electrodes by the bipyridinium salts (1) ,.**,
8
-.0.6
- 0.4
-0.2
0
Figure 1. Cyclic voltammograms of the Au electrode modified by Id: (a) before treatment with CleSH; (b) after treatment with CIGSH;scan rate, 200 mV/s; the electrolyte consists of 0.1 M phosphate buffer, pH 7.5. w-bromocarboxylic acid for 20 h. The resulting products after filtration were washed repeatedly with small volumes of DMF followedby diethyl ether. Compound Id was synthesized by the reaction of N-methyl-4-(4'-pyridyl)pyridiniumiodide with an excessof 15-bromopentadecanoicacid methyl ester in DMF (110 OC, 20 h). The resulting precipitate was washed with DMF and diethyl ether and was hydrolyzed in 16%aqueous HC1solution. The solution was evaporated and the product was dried. All compounds gave satisfactory lH NMR spectra.
la: n = 2 lb:n=5 lc: n = 10 Id: n = 14
Au foils, 0.2 mm thick, with geometrical area of ca. 0.2 cm2 were used as working electrodes. The electrodes were pretreated with boiling 2 M solution of KOH for 1 h, rinsed with water (Barstead Nanopure I1 system) and stored in cocentrated sulfuric acid, to obtain pure metallic surfaces. A cyclic voltammogram curve in 0.5 M HzS04 background was used to determine the purity of the electrode s~rface.'~ Prior to modification of the electrodes, they were soaked in concentrated nitric acid for 10 min and rinsed with water. Modification of the electrodes followed a sequential process: The electrodes were incubated in 0.01 M phosphate buffer, pH 7.3, containing 0.02 M cystamine (2,2'-diaminodiethyl disulfide, Fluka)lod for 3 h and rinsed thoroughly with water to remove physically adsorbed cystamine from the surfaces. The modified electrodes, containing surface amino groups, were subsequently modified with the respective bipyridinium salts (la-d): the amino-functionalized electrodes were incubated by 0.1 M HEPES buffer, pH 7.3, containing 0.01 M of the N-methyl-N'-carboxyalkyl-4,4'-bipyridinium salts (lad) and 0.01 M l-ethyl-3-(3-(dimethylamino)propyl)carbodiimide (EDC, Fluka) as coupling agent. For modification of the electrodes by lo and Id, the salts were solubilized in aqueous buffer/DMF (2:l) solution which was used as the medium for carbodiimidecoupling. After overnightincubation the electrodes were rinsed with ethanol and then with water to remove nonattached bipyridinium salts. Generation of densed monolayers included the treatment of the bipyridinium modified electrodes in a 5 X M 1-hexadecanethiol (CleSH) in CHC13 solution for 3 h followed by rinsing with CHCl3, ethanol, and water. Electrochemical measurements were performed using a BAS 1OObelectrochemicalanalyzerin a three-electrodecell, c6mprising the chemicallymodified electrode as working electrode, a glassy carbon auxiliary electrode, isolated by a frit, and a saturated calomel reference electrode. All potentials are reported with respect to this reference electrode. A positive feedback circuit was used for ca. 90 % ohmic drop compensation during the cyclic voltammetry study. The measurements were carried out at ambient temperature (24 2 OC) and the solution was deaerated by bubbling oxygen-free argon through the solution. The (16) Woodr~,R.InElectroanalyticaZChemistry;Bard, A. J., Ed.;Marcel Dekker: New York 1980; Vol. 9, p 1.
background solution in all measurements consists of 0.1 M phosphate buffer, pH = 7.5. Chronoamperometrymeasurements were performed with a PAR 273 potentiostat interfaced to a PC-AT computer. The potential step applied is from -0.2 V (vs SCE) to -0.58 V (vs SCE) and the potential program is identical to that given in ref 13b. The current transients observed are not dominated by the charging current.
Results and Discussion N-Methyl-N'-carboxyalkyl-4,4'-bipyridiniummodified electrodes were prepared according to the stepwise methodologyoutlined in SchemeI. Since splitting of S-S bonds is characteristic of disulfide chemisorption on a gold electrode,1°the primary modification step of the electrode involves adsorption of the thiolate anchor group by cleavage of the disulfide bond of cystamine (2,2'-diaminodiethyl disulfide). The cyclic voltammogram of Id modified Au electrode (obtained after step 2, Scheme I) is shown in Figure 1. One coupled cathodic-anodic peak pair, corresponding to a first redox process of the bipyridinium component is observed. The second peak pair for the reduction of the bipyridinium component can be observed at more negative potentials,12but in our experiments this process is masked by the cathodic discharge of the background solution. Similar cyclicvoltammograms are obtained for l b and ICmodified electrode surfaces.16 The formal potentials of lb-d immobilized onto the electrode surfaces are very similar, Eo = -0.58 V, and positively shifted by ca. 100 mV as compared to the potentials of the solubilizedbipyridinium salts lb-d prior to their immobilization. The surface concentration of the immobilized bipyridinium units was measured by integration of the cathodic (or anodic) peak of the cyclic voltammogram curve, after elimination of background current, and taking into account that a single electron reduction process is associated with the first electrochemical w8ve.l' The value obtained is ca. 2 X 10-lo mol-cm-2 on the basis of the geometrical data of the electrode (the roughness factor was estimated to be less than 1.2 by measuringthe double layer capacitance of Au nonmodified electrode).ls This surface density corresponds to an area of ca. 80 A2per bipyridinium component. This value is 4 times higher than the calculated area for a close-packed monolayer of bipyridinium molecules oriented perpendicular to the electrode surface.1s Thus, the surface concentration of the bipyridinium modified electrodes is similar to a monolayer coverage but less than for a
condensed monolayer. The surface density of the bipy(16) No electrochemical response of la modified Au electrode is detectable. Treatment of the la modified electrode with C&H reaulta in the characteristiccyclic voltammogramof the bipyridinium redox unit. (17) Bird, C. L.;Kyhn, A. T. Chem. SOC. Reu. 1981,10, 49. (18) Oeach, U.; Janata, J. Electrochim. Acta 1983,28, 1237. Uphaus, R. A. Microchem. J. 1990,42, (19) Cotton, T. M.; Kim, J.-H.; 44.
Katz et al.
1394 Langmuir, Vol. 9, No.5, 1993
ridinium redox units is comparable to the maximum coverages obtained recently for dicationic ruthenium complex monolayers.20 Charge repulsion of the bipyridinium units probably prevents higher coverages. All electrochemical properties of the bipyridinium modified electrodes are characteristic for immobilized redox compounds:21(a) the peak current densities j , and j c have the same values and vary linearly with the scan rate; (b) the peak-to-peak separation is small and independent of u at low scan rates (hE < 30 mV at u C 2 V/s); (c) at higher scah rates hE is proportional to log u. The dependencies of peak-to-peak separation vs log u are very similar for all bipyridinium modified electrodes, and independent of spacer length, linking the bipyridinium unit to the electrode (Figure 2A). Thus a similar rate constant of electron transfer between the electrode and anchored bipyridinium redox sites is estimated using Laviron's approach,22k, = 550 s-l. Hence, the electron transfer rate constants are independent of the length of the spacer group separating the redox site from the electrode. This result is attributed to the flexibility of the spacers in a relatively nonordered, nonrigid, monolayer assemblythat results in redox site conformationsof similar spatial distances in respect to the electrode. Furthermore, the electron transfer coefficients CY. and acfor anodic and cathodic processes are evaluated23as 0.25 f 0.05 and 0.3 f 0.05, respectively. These values differ considerablyfrom the expected value of 0.5, and their sum is not unity. These
low values of electrontransfer Coefficientsreflect, probably, the presence of a nonordered monolayer.l38 Finklea and c o - ~ o r k e rhave s ~ ~shown that low transfer coefficients are obtained when film defects carry most of the current at high overpotentials. Thus, the low transfer coefficients in the present system, would involve charge diffusion among bipyridinium sites, while the charge is collected by a single bipyridinium site close to the electrode surface. That is, conformational flexibility of all bridged bipyridiniummodifiedelectrodesallowsthe generation of charge collection sites of similar spatial orientation in respect to the electrode. These charge collection sites ultimately distribute the electrons in the monolayer array. An alternative explanation for the low transfer coefficients could be direct electron transfer to bipyridinium units at different distances and different local microenvironments in the nondeme monolayerconfiguration. Suchwide range of electron transfer rate constants would result in a low transfer coefficient. Incubation of the bipyridinium modified electrodes in a solution containing 1-hexadecanethiol,C~GSH, results in monolayer electrodes of new electrochemicalproperties. The cyclic voltammogram of Id modified Au electrodes after treatment with Cl&H (obtained after step 3, Scheme I) is also shown in Figure 1. It can be seen that the peaktu-peak separation is small. Also,the redox potential of the redox site and area under the peak, which reflects the surfaceconcentrationof electrochemicallyactive sites, are unaltered as compared to the electrode before treatment with C&H. But, treatment of the electrode with C&H results in a drastic decrease of the capacity current. The reduction in the capacity current might be attributed to the organization of a more densely packed monolayer assembly of the bipyridinium chains as a result of incorporation of alkanethiol long chains in the monolayer array adsorbed on the electrode. Such dense monolayer insulatesthe electrodein respect to the electrolytesolution, and thussolution inducedcapacity currents are eliminated. Further evidence for the formation of a more organized monolayer, upon treatment with Cl&H, is obtained by determination of the electron transfer coefficients a. and cyc. Both coefficients have the value of ca. 0.5 (in contrast to the values obtained for the modified electrodes before treatment with C16SH). In addition, it should be noted that while linkage of la to the cystamine modified Au electrode does not lead to a detectable electrochemical response, subsequent treatment of the la modified electrode with C&H yields a modified monolayer electrode that exhibits an electrochemical response for the bipyridinium units. The cyclic voltammogram of la modified Au electrode treated with Cl&H is similar to those obtained for all other long spacer bipyridinium modified electrodes,but greater values for peak-tu-peak separation are obtained for la modified electrode at low potential scan rate (Figure 2b). Although we do not have at present a reasonable explanation for the lack of electrochemical response of la modified electrode in the absence of C16SH, the large peak-to-peak separation of the organized monolayerafter treatment with C1&H might be attributed to microheterogeneity associated with the adsorption of la to the electrode. For this short spacer linked redox unit, the microheterogeneityof the electrode surfacecould reveal different redox properties of the electroactive component adsorbed to different electrode sites.
(20)Finklea, H. 0.;Hanshew, D. D. J. Am. Chem. SOC.,1992, 114, 3173. (21) Murray, R. W. In Electroanalytical Chemistry; Bard,A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, p 191. (22) Laviron, E. J. ElectroanaL Chem. 1979, 101, 19.
(23) (a) Sharp,M.; Petersson, M.; Edstrom, K. J.Electroanal. Chem. 1979,96,123. (b)Sharp,M.; Petersson, M.; Edstrom, K. J. Electroanal. Chem. 1980,109, 271. (24) Finklea, H. 0.;Avery, S.;Lynch, M.; Furtach, T. Langmuir 1987, 3,409.
A E , mV
1 0
-0.5
-2
1 logb , V / d
2.5
AE, mV
120
B
j
_I 40
-2
X
o x 0
m e
x
o
0
'
o
e
e '
2
O lo& ,V/s) Figure 2. Dependence of the peak-to-peak separation, AE, on the potential scan rate of (1) modified Au electrodes: (A) before treatmentwith C~GSH; (B)after treatment with Cl$H. Different plots correspond to various bipyridinium modified electrodes: la ( 0 ) ;l b (0); IC (XI; Id (0).
Electron Transfer in Self-Assembled Monolayers
Langmuir, Vol. 9, No. 5, 1993 1395
Table I. Electron Transfer Rate Constants to 1-Modified Au Electrodes in an Organized Monolayer using C&H. immobilized bipyridinium distance (A)* rate constant (s-')~ 2.4 800 6.0 580 12.0 400 16.8 300 Organization of the electrodes is performed by the sequence outlined in SchemeI. b The distanceof bipyridiniumunit is calculated for the variable alkyl chain linking the bipyridinium to cystamine and assuming 1.2A increase in the distanceper each methylene group and a tilt of 30° from the normal of the electrode of the monolayer assembly. The estimated error in rate constants is * 5 % .
5.5
1 5
0
10 Distance, A
15
20
Figure 4. Plot of the log k aa a function of a the alkyl chain length linking the bipyridinium redox unit to cystamine. The electrontransferrate constants (&) are determinedin the C1&H organized monolayer assembly.
0.5
transients is obtained. The derived electron transfer rate constant k (at the formal reduction potential) corresponds to 1500s-l, where k is given by eq 1 and kfand k b represent the forward and backward electron transfer reaction of the reversible couple,eq 2.13b Since at the formal reduction
1 0
1
2
3 4 5 Time (msec)
6
7
8
Figure 3. Plot of logarithmic values of current transients in potential step experiments: (a) electrode consists of la-modified electrodetreatedwith C14SH;(b) electrodeconsistsof lc-modified electrode treated with C14SH.
After treatment of the modified electrodeswith C&H, the electron transfer rates between the electrode and the bipyridinium redox sites differ substantially and strongly depend on the alkyl chain length bridging the bipyridinium site to the electrode. The plots of peak-tu-peak separation vs log u for the different bipyridinium modified electrodes treated with C16SH are shown in Figure 2B. The derived electron transfer rates using again Laviron's procedure are summarized in Table I. The table includes also calculated distances between the bipyridinium site and the amide group formed with cystamine associated with the electrode surface. The distances are estimated by assuming an all-trans well-ordered stretched alkyl chain conformation with an average tilt of 30' from the surface normal, being characteristic for Au electrodes modified by long alkanethiols and disulfides.8J0 This corresponds to an increase of ca. 1.2 A in the monolayer thickness in the alkyl chain length for each added methylene group. It is evident that the electron transfer rate constants are affected by the chain length bridging the bipyridinium sites to the electrode. As the chain length is increased, the electron transfer rate decreases. Further insight into organization of the bipyridinium monolayers and the electron transfer rate constants from the electrode to the bipyridinium redox sites in the absence and presence of the long chain thiol (C14SH) is obtained by potential step chronoamperometry. A potential step initiated at -0.2 V (vs SCE) to -0.58 V (the formal potential of the covalently linked bipyridinium sites) has been applied, and the current transients recorded.'3b Figure 3 shows the logarithmic plots of the current transients at different final time intervals of the potential step for two different monolayer assemblies: the short chain bridged bipyridinium functionalized monolayer (la) treated with C14SH, Figure 3a, and IC functionalized monolayer treated with C&H. For the short chain bipyridiium functionalized monolayer, a linear plot of the logarithmic current
V2+= bipyridinium unit potential kf = k b , the derived forward electron transfer rate constant is 750 s-l, a value that is in good agreement with that obtained by the Laviron analysis (800 8-1). Similarly,a linear plot of the logarithmictransient currents of IC modified electrode treated with C&H is obtained, Figure 3b. The derived forward electron transfer rate constant is 330 s-l, consistent with the value obtained by Laviron's method (400 s-l). The linear plots of the logarithmic transient currents indicate the structural homogeneityof the bipyridinium redox sites upon addition of the long chain thiol. The distance dependence of electron transfer rates is given by Marcus theory: eq 3, where d is the distance between the redox site and the electrode surface, AGO and X are the free energy change and reorganization energy associated with the transfer process, and B is the electron tunneling constant. The ket
oc e-B(d3)e-(ACo+X)2/4RTX
(3)
free energy change (AGO) associated with the electron transfer process to the various bipyridinium redox sites is similar as reflected by their identical redox potentials. Assuming that the reorganization energies (A) for all systems are similar,then ketshould exponentiallydepend on the distance of the redox site from the electrode surface. Figure 4 shows the plot of log ket as a function of the alkyl chain length bridging the bipyridinium site to cystamine previously attached to the electrode. We realize that a linear relationship is obtained, implying the distance control of electron transfer rates across the monolayer assemblies. The derived electron tunneling constant has the value of ca. 0.06 A-l. This value of electron tunneling constant is remarkably small when compared to other ~tudies3*"~J~ which find substantially higher values, 0.51.4A-I. Yet, it should be noted that closely related values have been observed in electron tunneling processes across fatty acid molecules in organizedLangmuir-Blodgett film assemblies4 (ca. 0.15 A-1). The analysis of the value
1396 Langmuir, Vol. 9, No. 5, 1993
should be considered with caution. Although our results indicate a linear dependence between electron transfer rates and the distance of the redox probe in the monolayer structure, the precise values of distances are not known. The provided values of distancesare a result of calculations that include the assumptiondetailed in Table I. However, the tilt angle of the monolayer might be higher than 30' due to the amide bonds that link the redox probes to the thiol spacer. Increase of the tilt angle will decrease the effective distances and will result in a higher @ value. We thus conclude that modification of Au electrodesby covalent linkage of carboxyalkylbipyridinium salts to cystamine spacers generates a nonordered monolayer assemblywhich is reflected in the low transfer coefficients and similar electron transfer rate constants for all alkyl bridging groups. In these nonordered arrays electron hopping between bipyridinium sites is feasible. The flexibility of the bipyridinium monomer chains allowsthe formationof similar electrode-relay distances for all chain lengths and charge collection sites for all attached redox units. Upon treatment of the bipyridinium modified electrodes with ClsSH, more organized monolayers are
Katz et al.
generated,wherethe alkyl bridges linking the bipyridinium components are stretched and preserved at confined distances relative to the electrode surface. In these organized monolayerselectron tranfer rates are controlled by the distance of the redox site from the electrodesurface. Electron transfer in these organized monolayer arrays proceeds across saturated methylene bonds exceeding distances of 20 A. This example complements other longrange electron transfer reactions in proteins and a-bonded donol-acceptor assemblies and reconfirmsthe importance of a-bondsin maintaining orbital overlap, enablingelectron tunneling. Further studies to construct similar monolayer configurations utilizing other redox groups and attempts to characterize the monolayer structures by other means are in progress in our laboratory.
Acknowledgment. This research is supported by the Bundesministerium fiir Forschung und Technologie (BMFT), Germany, and The Israeli Council for Research and Development, Ministry of Science and Technology, Israel.
