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 under- standing 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 com- pounds, or even single molecules, between pairs of electrodes. The current/voltage responses of the resulting electrode/molecules/ electrode junctions seem to reflect the stereoelectronic charac- teristics 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 stimulat- ing significant research efforts directed at exploring experimental strategies to (1) fabricate molecule-based devices with various configurations 2 and (2) unravel the fundamental factors regulat- ing electron transport across molecular building blocks. 3 Most of the protocols developed for the fabrication of electrode/molecules/electrode junctions require the initial depo- sition 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 oriented redox sites. An alternative, and perhaps more fascinat- ing, 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. How- ever, 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 roughness 17 and is close to the limiting surface coverage of 0.4 nmol cm -2 expected for bipyridinium monolayers. 18 Remark- ably, 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 † Part of the special issue “Alvin L. Kwiram Festschrift”. * Address correspondence to this author. E-mail: fraymo@miami.edu. 8622 J. Phys. Chem. B 2004, 108, 8622-8625 10.1021/jp036730l CCC: $27.50 © 2004 American Chemical Society Published on Web 01/22/2004