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J. Phys. Chem. B 2004, 108, 8622-8625
Electron Transport in Self-Assembled Bipyridinium Multilayers† Franc¸ isco M. Raymo,* Robert J. Alvarado, Eden J. Pacsial, and Daniel Alexander Center for Supramolecular Science, Department of Chemistry, UniVersity of Miami, 1301 Memorial DriVe, Coral Gables, Florida 33146-0431 ReceiVed: September 12, 2003; In Final Form: December 5, 2003
We have identified a simple experimental protocol to assemble electroactive films with attractive electron transport properties on gold electrodes. Their basic building block is a bipyridinium bisthiol, which adsorbs spontaneously on the electrode surface forming multiple electroactive layers. The resulting interfacial assemblies mediate the transfer of electrons from the electrode to redox probes in the electrolyte solution but prevent electron transfer in the opposite direction. After the insertion of electroactive anionic dopants in the polycationic bipyridinium matrix, the transfer of electrons from the redox probes to the electrode becomes possible. Under these conditions, the probe reduction accompanies that of the surface-confined bipyridinium dications, while the probe reoxidation follows the oxidation of the anionic dopants. This intriguing behavior imposes a large potential difference between the voltammetric reduction and oxidation peaks of the probe, which parallels the difference between the bipyridinium reduction and the dopant oxidation potentials. Thus, the careful selection of the electroactive dopant can be exploited to tune the electronic properties of the composite film. This chemical approach to interfacial assemblies with controlled dimensions and engineered properties can lead to electrode/organic film/electrode junctions with predefined current/voltage signatures.
Organic molecules are promising building blocks for the fabrication of future ultraminiaturized electronic devices.1 Indeed, the power of organic synthesis and the current understanding of molecular properties have already conspired in delivering rudimentary examples of diodes, memories, switches, and transistors based on functional molecular components. These prototypical devices incorporate thin films of organic compounds, or even single molecules, between pairs of electrodes. The current/voltage responses of the resulting electrode/molecules/ electrode junctions seem to reflect the stereoelectronic characteristics of the organic building blocks. At this stage of development, therefore, the adjustable design of the molecular components appears to be a convenient tool to engineer the overall device behavior. This fascinating opportunity is stimulating significant research efforts directed at exploring experimental strategies to (1) fabricate molecule-based devices with various configurations2 and (2) unravel the fundamental factors regulating electron transport across molecular building blocks.3 Most of the protocols developed for the fabrication of electrode/molecules/electrode junctions require the initial deposition of an electroactive film on one of the electrodes.2,4 The Langmuir-Blodgett technique, for example, has been employed routinely to transfer monolayers of electroactive amphiphiles from the air/water interface to metallic or semiconducting supports.2a,d,4,5 This method is fairly laborious but offers excellent control on the relative orientation of the amphiphilic building blocks and on the number of deposited molecular layers. Similarly, the interfacial polymerization of appropriate precursors or the casting of pre-assembled polymers can be also exploited to coat electrodes with electroactive films.4,6 These procedures are simple, efficient, and allow the regulation of the thickness from tenths of nanometers to few micrometers but lack structural control and can only produce films with randomly †
Part of the special issue “Alvin L. Kwiram Festschrift”. * Address correspondence to this author. E-mail:
[email protected].
oriented redox sites. An alternative, and perhaps more fascinating, approach to electroactive films relies on the spontaneous adsorption of redox-active thiols on gold.2b,4,7 The formation of gold-thiolate bonds and a myriad of secondary interactions between the adsorbing components encourage the self-assembly of monolayers on the supporting substrate. The beauty of this strategy is that it requires essentially no external intervention. The molecular building blocks are self-instructed and form spontaneously organized films on the electrode surface. However, a single molecular layer can only be obtained in most instances. Therefore, no control on the amount of adsorbed redox sites and the dimensions of the interfacial assembly is possible beyond monolayer surface coverages and thicknesses. In this article, we report the self-assembly of multiple electroactive layers on gold and the electron transport properties of the resulting films. Compound 1 (Figure 1) has thiol groups at its two ends and bipyridinium dications at its core.8 In agreement with previous reports on bipyridinium monothiols,9-14 we found that our bisthiol adsorbs on gold producing electroactive films. The corresponding cyclic voltammogram (a in Figure 1) reveals the characteristic response for the reversible reduction of the bipyridinium dications to their radical cations.15 It was recorded after the immersion of a gold electrode in a solution of 1 for only 1 h, followed by the copious rinsing of the electrode surface.16 We estimated the surface coverage (Γ) of this particular bipyridinium film to be ca. 0.7 nmol cm-2 (Table 1) from the integral of the redox waves measured at slow scan rates. This value includes a correction to account for surface roughness17 and is close to the limiting surface coverage of 0.4 nmol cm-2 expected for bipyridinium monolayers.18 Remarkably, we noticed that Γ grows to 2.5 nmol cm-2, when the exposure of the gold electrode to 1 is prolonged to 6 h (Table 1), and increases further to 8.4 nmol cm-2, after 48 h.16 Consistently, the cyclic voltammograms (a-c in Figure 1) reveal
10.1021/jp036730l CCC: $27.50 © 2004 American Chemical Society Published on Web 01/22/2004
Electron Transport
J. Phys. Chem. B, Vol. 108, No. 25, 2004 8623
Figure 1. Cyclic voltammograms (0.1 M KClaq, 100 mV s-1, V vs Ag/AgCl) recorded after the immersion of a polycrystalline gold electrode in a chloroform/methanol solution (2:1, v/v) of the tetrachloride salt of 1 (3 × 10-4 M) for 1 (a), 6 (b), or 48 h (c), followed by copious rinsing of the electrode surface. Scan rate dependence of the peak current in the cyclic voltammograms of electroactive films obtained after immersion times of 1 (d), 6 (e), or 48 h (f).
TABLE 1: Electrochemical Data for Films of the Bipyridinium Building Block 1 Adsorbed on Gold ta (h)
Γb (nmolcm-2)
E1/2c (V)
∆Epc (mV)
Ad
1 6 48
0.7 2.5 8.4
-0.36 -0.39 -0.42
19 57 90
0.90 0.73 0.68
a Exposure time (t) of the gold substrate to a solution of 1 (ref 16). Surface coverage (Γ) estimated from the cyclic voltammograms measured at a scan rate of 10 (t ) 1 and 6 h) and 1 mV s-1 (t ) 48 h) and corrected for surface roughness. c Half-wave potential (E1/2) and peak splitting (∆Ep) in the cyclic voltammograms measured at a scan rate of 100 mV s-1. d Slope (A) for the plot of the logarithm of the peak current in the cyclic voltammogram against the logarithm of the scan rate.
b
a dramatic enhancement of the peak current with the exposure time. Thus, our bipyridinium bisthiol keeps accumulating on the gold substrate even beyond monolayer surface coverages and remains there despite extensive washing with chloroform, methanol, and water. Presumably, gold-thiolate bonds at one end of the building block 1 encourage the initial assembly of a monolayer. Then, interlayer disulfide linkages at the other end or the interdigitation of the oligomethylene chains promote the subsequent adsorption of multiple overlayers. Indeed, the assembly of 1,n-alkanedithiols into disulfide multilayers19 and the interdigitation of bipyridinium amphiphiles in alkanethiolate monolayers20 have been both observed experimentally. The surface coverage increase has a modest influence on the half-wave potential (E1/2) for the bipyridinium reduction, which shifts slightly in the negative direction (Table 1, a-c in Figure 1). By contrast, the peak splitting (∆EP) varies from only 19 up to 90 mV (Table 1) and the scan rate dependence of the
Figure 2. Cyclic voltammograms (0.1 M KClaq, 100 mV s-1, V vs Ag/AgCl) of Ru(NH2)63+ (5 × 10-3 M) recorded without (a) or with electroactive films of 1 on the working electrode obtained after exposure times of 1 (b), 6 (d), or 48 h (f) and of the same films (c, e, and g) in the absence of Ru(NH3)63+.
peak current changes dramatically. At close to monolayer surface coverages, the current increases linearly (d in Figure 1) with the scan rate and the slope (A) of the corresponding logarithmic plot is close to unity (Table 1). At multilayer surface coverages, the current/scan rate correlation deviates from linearity (e and f in Figure 1) and A is ca. 0.7 (Table 1), indicating that the reduction process is not exhaustive at fast scan rates.4a In the absence of a bipyridinium film on the working electrode, the cyclic voltammogram of Ru(NH3)63+ (a in Figure 2) shows a reversible reduction with an E1/2 of -0.14 V and a ∆EP of 81 mV. When the electrode surface is coated with a film of 1, the response of the ruthenium probe changes dramatically. At close to monolayer surface coverage (0.7 nmol cm-2), the ruthenium reduction wave shifts from -0.18 to -0.34 V (a and b in Figure 2) and overlaps the bipyridinium reduction wave (c). A similar effect is observed at multilayer surface coverage (2.5 nmol cm-2). Once again, the ruthenium reduction wave moves in the negative direction (a and d in Figure 2) to overlap the bipyridinium reduction wave (e). The hindered access of the ruthenium probe to the electrode surface prevents its reduction at -0.18 V. When the gold voltage raises to match the bipyridinium reduction potential (a in Figure 3), electrons diffuse across the electroactive coating through the bipyridinium LUMOs to reach the ruthenium centers. In agreement with this interpretation, a significant increase in bipyridinium surface coverage (8.4 nmol cm-2) translates into the inefficient mediation of the ruthenium reduction at relatively fast scan rates. Under these conditions, the electrons do not have enough time to travel across the entire bipyridinium film to reach the
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Raymo et al.
Figure 3. The bipyridinium LUMOs mediate the electron transfer from the electrode to the Ru(NH2)63+ probe when a film of 1 without (a) or with (c) Fe(CN)64- anions is on the working electrode. No electron transfer can occur from the Ru(NH2)62+ probe to the electrode when the anionic components of the interfacial assembly are not electroactive (b). In the presence of Fe(CN)64- dopants, the accessible electronic states of the anions mediate the electron transfer from the Ru(NH2)62+ probe to the electrode (d).
ruthenium centers and the presence of Ru(NH3)63+ in the electrolyte solution has a negligible effect on the cyclic voltammogram (f and g in Figure 2). At low surface coverage (0.7 nmol cm-2), the reverse scan of the cyclic voltammogram (b in Figure 2) shows a significant broadening and current decrease for the ruthenium back oxidation compared to the Ru(NH3)63+ response at bare gold (a). At intermediate surface coverage (2.5 nmol cm-2), the ruthenium back oxidation is essentially suppressed (d in Figure 2). The larger amount of bipyridinium building blocks adsorbed on the gold support blocks more efficiently the access of the ruthenium probe to the electrode surface. Furthermore, the lower reduction potential of the ruthenium centers relative to that of the bipyridinium dications prevents the electron transfer from Ru(NH3)62+ back to the electroactive coating (b in Figure 3). Thus, only the bipyridinium dications are reoxidized after lowering the gold voltage. In fact, the cyclic voltammogram shows only the bipyridinium back oxidation wave in the reverse scan (d and e in Figure 2). Our results demonstrate that, at appropriate surface coverages, the interfacial assembly of bipyridinium building blocks imposes a sort of rectification on the ruthenium response.21 It mediates the transfer of electrons from the electrode to the ruthenium probe but prevents electron transfer in the opposite direction. The “addition” of accessible electronic states to the electroactive coating can reactivate the back electron transfer, if their energy is chosen to be lower than that of the occupied Ru(NH3)62+ orbitals. This condition can be satisfied by introducing electroactive anions with appropriate oxidation potentials in the bipyridinium film. In fact, the adsorption of the tetracationic building block 1 on the electrode surface is accompanied by the entrapment of chloride anions in the interfacial assembly. We found that these counterions can be exchanged with ferrocyanide anions by simply exposing a modified electrode to a dilute solution of K4Fe(CN)6. At low concentrations, the characteristic waves for the reversible oxidation of Fe(CN)64can be detected at a E1/2 of +0.19 V (∆EP ) 20 mV) only if the working electrode is coated with a film of 1 (a and b in Figure 4). This result indicates that the surface-confined polycationic bipyridinium matrix captures the ferrocyanide anions,22 enhancing their local concentration at the electrode surface. Consistently, the peak current for the ferrocyanide response increases linearly with the scan rate. The slope (A) of the corresponding logarithmic plot is 0.97, suggesting that the electroactive anions are, indeed, surface confined. We noticed also that the incorporation of ferrocyanide anions in place of the chloride counterions alters the redox response of the bipyridinium dications (b and c in Figure 4). Their E1/2 shifts from -0.39 to -0.46 V and their ∆EP changes from 57 to 109 mV. From the integrals of the corresponding redox waves, we
Figure 4. Cyclic voltammograms (0.1 M KClaq, 0.1 V s-1, V vs Ag/ AgCl) of Fe(CN)64- (4 × 10-5 M) recorded without (a) or with (b) a film of 1 on the working electrode obtained after an exposure times of 6 h, of the same films in the absence of Fe(CN)64- (c) and of Ru(NH2)63+ (5 × 10-3 M) and Fe(CN)64- (4 × 10-5 M) recorded with a film of 1 on the working electrode obtained after an exposure times of 6 h (d).
estimated the ratio between the number of bipyridinium dications and the number of trapped iron centers to be ca. 2. The incorporation of anionic and redox-active “dopants” in the bipyridinium film has a remarkable effect on the ability of the multilayer assembly to mediate the redox response of Ru(NH2)63+. The corresponding cyclic voltammogram (d in Figure 4) shows the reversible oxidation of the Fe(CN)64dopant with an E1/2 of +0.19 V (I and II), the reduction of the Ru(NH2)63+ probe together with that of the bipyridinium dications at -0.53 V (III), the back oxidation of the bipyridinium dications at -0.37 V (IV), the back oxidation of the Ru(NH2)62+ probe together with that of the Fe(CN)64- dopant at +0.17 V (V), and the reduction of the Fe(CN)63- dopant at +0.19 V (VI). Thus, the ruthenium probe is reduced at the onset of the bipyridinium reduction (III) and reoxidized at the onset of the iron dopant oxidation (V), with a ∆EP of as much as 0.70 V. This behavior can be rationalized by referring to the accessible electronic states of the bipyridinium dications and anionic dopants. When the gold voltage approaches the bipyridinium reduction potential (c in Figure 3), electrons are transferred from the electrode to the Ru(NH3)63+ probe through the bipyridinium
Electron Transport LUMOs. After lowering the gold voltage to match the dopant oxidation potential (d in Figure 3), electrons are transferred from the Ru(NH3)62+ probe to the electrode through the Fe(CN)64orbitals. Thus, the large energy difference between the electronic states involved in the electron-transfer mediation imposes a large splitting between the reduction and back oxidation waves of the ruthenium probe. In summary, our results demonstrate that a bipyridinium bisthiol forms stable multilayers on the surface of gold electrodes. The resulting electroactive films mediate the transfer of electrons from the electrode to a redox probe in the electrolyte solution but prevent electron transfer in the opposite direction. After the introduction of electroactive anions in the interfacial assembly, however, electrons can be transferred back to the electrode through the accessible electronic states of the anionic dopants. The energy difference between the bipyridinium reduction potential and the dopant oxidation potential regulates the gap between the reduction and oxidation peaks of the probe. Thus, the ability of the composite electroactive film to mediate electron transfer can be adjusted, at least in principle, by the careful choice of anionic dopant. This chemical approach to nanostructured films with pre-defined electrochemical properties can lead to the development of a general strategy to engineer the electron transport character of electroactive coatings based on cationic and anionic components and, perhaps, even the current/voltage signature of nanoelectronic junctions incorporating doped bipyridinium multilayers between pairs of electrodes. Acknowledgment. We thank the University of Miami for financial support. References and Notes (1) Molecular Nanoelectronics; Reed, M. A., Lee, T., Eds.; American Scientific Publishers: San Diego, CA, 2003. (2) (a) Metzger, R. M. Acc. Chem. Res. 1999, 32, 950-957. (b) Tour, J. M. Acc. Chem. Res. 2000, 33, 791-804. (c) Joachim, C.; Gimzewski, J. K.; Aviram, A. Nature 2000, 408, 541-548. (d) Pease, A. R.; Jeppesen, J. O.; Stoddart, J. F.; Luo, Y.; Collier, C. P.; Heath, J. R. Acc. Chem. Res. 2001, 34, 433-444. (e) Carroll, R. L.; Gorman, C. B. Angew. Chem., Int. Ed. 2002, 41, 4378-4400. (3) (a) Adams, D. M.; Brus, L.; Chidsey, C. E. D.; Creager, S.; Creutz, C.; Kagan, C. R.; Kamat, P. V.; Lieberman, M.; Lindsay, S.; Marcus, R. A.; Metzger, R. M.; Michel-Beyerle, M. E.; Miller, J. R.; Newton, M. D.; Rolison, D. R.; Sankey, O.; Schanze, K. S.; Yardley, J.; Zhu, X. Y. J. Phys. Chem. B 2003, 107, 6668-6697. (b) Nitzan, A.; Ratner, M. A. Science 2003, 300, 1384-1389. (4) (a) Molecular Design of Electrode Surfaces; Murray, R. W., Ed.; Wiley: New York, 1992. (b) Bard, A. J. Integrated Chemical Systems: a Chemical Approach to Nanotechnology; Wiley: New York, 1994. (c) Vos, J. G.; Forster, R. J.; Keyes, T. E. Interfacial Supramolecular Assemblies; Wiley: New York, 2003. (5) (a) Kuhn, H.; Mo¨bius, D. Angew. Chem., Int. Ed. Engl. 1971, 10, 620-637. (b) Facci, J. S. In Molecular Design of Electrode Surfaces;
J. Phys. Chem. B, Vol. 108, No. 25, 2004 8625 Murray, R. W., Ed.; Wiley: New York, 1992; p 119-158. (c) Fujihira, M. In Nanostructures Based on Molecular Materials; Go¨pel, W., Ziegler, C., Eds.; VCH: Weinheim, Germany, 1992; p 27-46. (6) (a) Chidsey, C. E. D.; Murray, R. W. Science 1986, 231, 25-31. (b) Wrighton, M. S. Science 1986, 231, 32-37. (c) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000, 100, 2537-2574. (7) (a) Finklea, H. O. Electroanal. Chem. 1996, 19, 109-335. (b) Chechik, V.; Stirling, C. J. M. In The Chemistry of Organic DeriVatiVes of Gold and SilVer; Patai, S., Rappoport, Z., Eds.; Wiley: Chichester, U.K., 1999; p 551-640. (8) We prepared this compound in four steps starting from 4,4′bipyridine and 1,10-diidodecane. The full experimental details of the synthetic procedures will be reported elsewhere. (9) (a) De Long, H.; Buttry, D. A. Langmuir 1990, 6, 1319-1322. (b) De Long, H. C.; Buttry, D. A. Langmuir 1992, 8, 2491-2496. (c) Hiley, S. L.; Buttry, D. A. Colloids Surf., A 1994, 84, 129-140. (10) Rojas, M. T.; Kaifer, A. E. J. Am. Chem. Soc. 1995, 117, 58835884. (11) (a) Yan, J.; Li, J.; Chen, W.; Dong, S. J. Chem. Soc., Faraday Trans. 2 1996, 92, 1001-1006. (b) Li, J.; Yan, J.; Deng, Q.; Cheng, G.; Dong, S. Electrochim. Acta 1997, 42, 961-967. (c) Ya, J.; Dong, S.; Li, J.; Chen, W. J. Electrochem. Soc. 1997, 144, 3858-3865. (d) Li, J.; Cheng, G.; Dong, S. Thin Solid Films 1997, 293, 200-205. (12) (a) Sagara, T.; Maeda, H.; Yuan, Y.; Nakashima, N. Langmuir 1999, 15, 3823-3830. (b) Sagara, T.; Tsuruta, H.; Nakashima, N. J. Electroanal. Chem. 2001, 500, 255-263. (13) Nakamura, N.; Huang, H. X.; Qian, D. J.; Miyake, J. Langmuir 2002, 18, 5804-5809. (14) (a) Terasaki, N.; Akiyama, T.; Yamada, S. Chem. Lett. 2000, 668669. (b) Kuwahara, Y.; Akiyama, T.; Yamada, S. Langmuir 2001, 17, 57145716. (c) Terasaki, N.; Akiyama, T.; Yamada, S. Langmuir 2002, 18, 86668671. (d) Akiyama, T.; Inoue, K.; Kuwahara, Y.; Terasaki, N.; Niidome, Y.; Yamada, S. J. Electroanal. Chem. 2003, 550, 303-307. (15) (a) Monk, P. M. S. The Viologens: Physicochemical Properties, Synthesis and Applications of the Salts of 4,4′-Bipyridine; Wiley: New York, 1998. (b) Raymo, F. M.; Alvarado, R. J.; Pacsial, E. J. J. Supramol. Chem. 2002, 2, 63-77. (16) A polycrystalline gold electrode was immersed in a chloroform/ methanol solution (2:1, v/v) of the tetrachloride salt of 1 (3 × 10-4 M) for 1, 6, or 48 h. After copious rinsing of the electrode surface, cyclic voltammograms were recorded in aqueous KCl (0.1 M) using a Ag/AgCl reference electrode and a platinum counter electrode. (17) Golan, Y.; Margulis, G.; Rubinstein, I. Surf. Sci. 1992, 264, 312326. (18) Widrig, C. A.; Majda, M. Langmuir 1989, 5, 689-695. (19) (a) Kohli, P.; Taylor, K. K.; Harris, J. J.; Blanchard, G. J. J. Am. Chem. Soc. 1998, 120, 11962-11968. (b) Joo, S. W.; Han, S. W.; Kim, K. Langmuir 2000, 16, 5391-5396. (c) Joo, S. W.; Han, S. W.; Kim, K. Mol. Cryst. Liq. Cryst. 2001, 371, 355-358. (20) (a) Finklea, H. O.; Fedyk, J.; Schwab, J. In Electrochemical Surface Science: Molecular Phenomena at Electrode Surfaces; Soriaga, M. P., Ed.; American Chemical Society: Washington, DC, 1988; p 431-437. (b) Lee, K. A. B.; Mowry, R.; McLennan, G.; Finklea, H. O. J. Electroanal. Chem. 1988, 246, 217-224. (21) A similar effect was observed for electroactive films composed of alternating layers of gold nanoparticles and bipyridinium dications. Gittins, D. I.; Bethell, D.; Nichols, R. J.; Schiffrin, D. J. J. Mater. Chem. 2000, 10, 79-83. (22) Our observations are consistent with the reported ability of bipyridinium polysiloxanes to bind preferentially ferrocyanide over chloride anions. Bruce, J. A.; Wrighton, M. S. J. Am. Chem. Soc. 1982, 104, 7482.
