The identification of strategies to assemble nanostructured films with engineered properties on solid supports can lead to the development of innovative functional materials. In particular, the self-assembly of electroactive multilayers from simple molecular building blocks on metallic electrodes can offer the opportunity to regulate the exchange of electrons between the underlying substrate and solution species. In this context, we designed an experimental protocol to prepare electroactive films from bipyridinium bisthiols. Specifically, we found that a compound incorporating two bipyridinium dications at its core and terminal thiol groups self-assembles into remarkably stable multilayers on polycrystalline gold. The surface coverage of the resulting films can be regulated by adjusting the exposure time of the gold substrate to the bipyridinium solution. Control experiments with appropriate model compounds demonstrate that both bipyridinium dications as well as both thiol groups must be present in the molecular skeleton to encourage multilayer growth. The resulting films transport electrons efficiently from the electrode surface to the film/solution interface. Indeed, they mediate the reduction of Ru(NH(3))(6)(3+) in the electrolyte solution but prevent the back oxidation of the resulting Ru(NH(3))(6)(2+). Furthermore, these polycationic bipyridinium films capture electrostatically Fe(CN)(6)(4-) tetraanions, which can also be exploited to transport electrons across the interfacial assembly. In fact, electrons can travel through the bipyridnium(2+/1+) couples to redox probes in solution and then back to the electrode through the Fe(CN)(6)(4/3-) couples. Thus, our original approach to self-assembling multilayers can produce stable electroactive films with unique electron transport properties, which can be regulated with a careful choice of the anionic components.

The supramolecular association of tetrathiafulvalene (TTF) donors and bipyridinium acceptors is employed routinely to direct the formation of host/guest complexes and interlocked molecules in bulk solution. We have reproduced these donor/acceptor interactions at electrode/solution interfaces and demonstrated their pronounced influence on heterogeneous electron transfer. Specifically, we have synthesized a TTF with an oligomethylene arm terminated by a thiol group and assembled monolayers of this compound on gold. We have observed that the cyclic voltammogram of the immobilized TTF donors varies significantly upon addition of benzyl viologen, tetracyanoquinodimethane (TCNQ), or tetracyanoethylene (TCNE) acceptors to the electrolyte solution. Indeed, the supramolecular association of the complementary donors and acceptors results in a pronounced current decrease for the TTF redox waves. Consistently, the electrochemical response of the acceptors changes dramatically in the presence of TTF donors on the electrode surface. Instead, hexadecanethiolate monolayers, lacking the TTF donors at the termini of the oligomethylene chains, have a marginal influence on the voltammograms of the acceptors. Impedance measurements indicate that the charge-transfer resistance (R CT) for the reduction of the acceptors increases from less than 0.3 kΩ, at bare gold, to 324, 24, and 43 kΩ for benzyl viologen, TCNQ, and TCNE, respectively, at TTF-coated electrodes. By contrast, the electrode coating has a negligible influence on the cyclic voltammogram and impedance response of ferrocene, which cannot sustain donor/acceptor interactions with the immobilized TTFs. Thus, our results demonstrate that the interfacial complexation of complementary donors and acceptors has a dramatic effect on the heterogeneous electron transfer to and from the associated components.

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.

Transparent, ultrathin Pt electrodes permit the simultaneous electrochemical and spectroscopic investigation of self‐assembled monolayers of electrochromic compounds. Voltage stimulations applied to the Pt substrate reversibly alter the redox state of the chemisorbed molecules and, hence, modulate the intensity of the light transmitted through the Pt/monolayer assembly (see Figure).

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