Electroactive films incorporating 4,4′-Bipyridinium building blocks

Electroactive Amphiphiles for Addressable Supramolecular Nanostructures

In this focus review we aim to highlight an exciting class of materials, electroactive amphiphiles (EAAs). This class of functional amphiphilic molecules has been the subject of sporadic investigations over the last few decades, but little attempt has been made to date to gather or organise these investigations into a logical fashion. Here we attempted to gather the most important contributions, provide a framework in which to discuss them, and, more importantly, point towards the areas where we believe these EAAs will contribute to solving wider scientific problems and open new opportunities. Our discussions cover materials based on low molecular weight ferrocenes, viologens and anilines, as well as examples of polymeric and supramolecular EAAs. With the advances of modern analytical techniques and new tools for modelling and understanding optoelectronic properties, we believe that this area of research is ready for further exploration and exploitation.

Redox-switching in a viologen-type adlayer: an electrochemical shell-isolated nanoparticle enhanced Raman spectroscopy study on Au(111)-(1×1) single crystal electrodes

We reported the first application of in situ shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) to an interfacial redox reaction under electrochemical conditions. We construct gap-mode sandwich structures composed of a thiol-terminated HS-6V6H viologen adlayer immobilized on a single crystal Au(111)-(1×1) electrode and covered by Au(60 nm)@SiO(2) core-shell nanoparticles acting as plasmonic antennas. We observed high-quality, potential-dependent Raman spectra of the three viologen species V(2+), V(+●), and V(0) on a well-defined Au(111) substrate surface and could map their potential-dependent evolution. Comparison with experiments on powder samples revealed an enhancement factor of the nonresonant Raman modes of ∼3 × 10(5), and up to 9 × 10(7) for the resonance modes. The study illustrates the unique capability of SHINERS and its potential in the entire field of electrochemical surface science to explore structures and reaction pathways on well-defined substrate surfaces, such as single crystals, for molecular, (electro-)catalytic, bioelectrochemical systems up to fundamental double layer studies at electrified solid/liquid interfaces.

Two-dimensional assembly and local redox-activity of molecular hybrid structures in an electrochemical environment

The self-assembly and redox-properties of two viologen derivatives, N-hexyl-N'-(6-thiohexyl)-4,4'-bipyridinium bromide (HS-6V6-H) and N,N'-bis(6-thiohexyl)-4,4'-bipyridinium bromide (HS-6V6-SH), immobilized on Au(lll)-(1 x 1) macro-electrodes were investigated by cyclic voltammetry, surface enhanced infrared spectroscopy (SEIRAS) and in situ scanning tunneling microscopy (STM). Depending on the assembly conditions one could distinguish three different types of adlayers for both viologens: a low coverage disordered and an ordered "striped" phase of flat oriented molecules as well as a high coverage monolayer composed of tilted viologen moieties. Both molecules, HS-6V6-H and HS-6V6-SH, were successfully immobilized on Au(poly) nano-electrodes, which gave a well-defined redox-response in the lower pA-current range. An in situ STM configuration was employed to explore electron transport properties of single molecule junctions Au(T)/HS-6V6-SH(HS-6V6-H)/Au(S). The observed sigmoidal potential dependence, measured at variable substrate potential E(S) and at constant bias voltage (E(T) - E(S)), was attributed to electronic structure changes of the viologen moiety during the one-electron reduction/re-oxidation process V2+ < -- > V+*. Tunneling experiments in asymmetric, STM-based junctions Au(T)-S-6V6-H/Au(S) revealed current (i(T))-voltage (E(T)) curves with a maximum located at the equilibrium potential of the redox-process V2+ < -- > V+*. The experimental i(T)--E(T) characteristics of the HS-6V6-H-modified tunneling junction were tentatively attributed to a sequential two-step electron transfer mechanism.

Electrochemical Switching of Chromogenic Monolayers Self-Assembled on Transparent Platinum Electrodes

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.

Self-Assembling Bipyridinium Multilayers

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.

Electron transport in bipyridinium films

Bipyridinium dications are versatile building blocks for the assembly of functional materials. In particular, their reliable electrochemical response has encouraged the design of electroactive films. Diverse and elegant experimental strategies to coat metallic and semiconducting electrodes with bipyridinium compounds have, in fact, emerged over the past two decades. The resulting interfacial assemblies span from a few nanometers to several micrometers in thickness. They incorporate from a single molecular layer to large collections of entangled polymer chains. They transport electrons efficiently from the electrode surface to the film/solution interface and vice versa. Electron self-exchange between and the physical diffusion of the bipyridinium building blocks conspire in defining the charge transport properties of these fascinating electroactive assemblies. Often, the matrix of electron-deficient bipyridinium dications can be exploited to entrap electron-rich analytes. Electrostatic interactions promote the supramolecular association of the guests with the surface-confined host matrix. Furthermore, chromophoric sites can be coupled to the bipyridinium dications to produce photosensitive arrays capable of harvesting light and generating current. Thus, thorough investigations on the fundamental properties of these functional molecule-based materials can lead to promising applications in electroanalysis and solar energy conversion, while contributing to advances in the basic understanding of electron transport in interfacial assemblies.