Interfacial Supramolecular Assemblies

Vos JG, Forster RJ, Keyes TE (2003) Interfacial supramolecular assemblies. Wiley, New York... 

Interfacial Supramolecular Assemblies. Johannes G. Vos, Robert J. Forster and Tia E. Keyes... 

Interfacial Supramolecular Assemblies comprise an electrochemically addressable solid surface functionalized with a film which incorporates molecular components that can be addressed electrochemically or photochemically. In these assemblies, specific bonding interactions exist between the surface and film and they are generally in contact with a solution. Typical of a supramolecular assembly, the individual building blocks retain much of their molecular character, but the overall assembly exhibits new properties, or is capable of performing a specific function beyond that possible when using the individual components. 

Figure 1.1 Schematic representations of (a) supramolecular and (b) interfacial supramolecular assemblies...

The intention is not to comprehensively review the literature that describes the multidisciplinary efforts of researchers to create interfacial supramolecular assemblies. The literature in this area is vast and involves research programs in chemistry, physics and biology, as well as analytical, materials and surface sciences. Rather, key examples of advances that have significantly influenced the field and will direct its future development are presented. In addition, some of the analytical methods, theoretical treatments and synthetic tools, which are being applied in the area of interfacial supramolecular chemistry and are driving its rapid development, will be highlighted. 

Election transfer remains one of the most important processes explored when using interfacial supramolecular assemblies and given the emerging area of molecular electronics, this trend is set to continue. Therefore, Chapter 2 outlines the fundamental theoretical principles behind the electiochemically and photochemi-cally induced processes that are important for interfacial supramolecular assemblies. In that chapter, homogeneous and heterogeneous electron transfer, photoinduced proton transfer and photoisomerizations are considered. 

Modern surface analytical tools make it possible to probe the physical structure as well as the chemical composition and reactivity of interfacial supramolecular assemblies with unprecedented precision and sensitivity. Therefore, Chapter 3 discusses the modern instrumental techniques used to probe the structure and reactivity of interfacial supramolecular assemblies. The discussion here is focused on techniques traditionally applied to the interrogation of interfaces, such as electrochemistry and scanning electron microscopy, as well as various microprobe techniques. In addition, some less common techniques, which will make an increasing contribution to supramolecular interfacial chemistry over the coming years, are considered. 

Electrochemical and photochemical processes are the most convenient inputs and outputs for interfacial supramolecular assemblies in terms of flexibility, speed and ease of detection. This chapter provides the theoretical background for understanding electrochemical and optically driven processes, both within supramolecular assemblies and at the ISA interface. The most important theories of electron and energy transfer, including the Marcus, Forster and Dexter models, are described. Moreover, the distance dependence of electron and energy transfer are considered and proton transfer, as well as photoisomerization, are discussed. 

Electron, energy and proton transfer or molecular rearrangements are the most important events that occur in interfacial supramolecular assemblies. In this chapter, the general theories of electron transfer, both within ISAs and across the film/electrode interface, are described. Moreover, photoinduced electron, energy and proton transfer processes are discussed. As this book focuses on supramolecular species, the treatment is restricted to intramolecular or interfacial processes without the requirement for prior diffusion of reactants. 

Figure 2.8 Schematic of a typical interfacial supramolecular assembly, A-L-B, when A is a metal electrode, illustrating the relative molecular band structures of A and B, respectively. Ef is the fermi level...

Heterogeneous electron transfers involving interfacial supramolecular assemblies are often assumed to be adiabatic because this simplifies the kinetic analysis. However, this assumption is often not justified and invalidates much of the subsequent analysis. 

The past decade has seen a dramatic improvement in the strategies and instrumentation available to characterize the structures of interfacial supramolecular assemblies. Current thrusts are towards in situ techniques that probe the structure of the interfacial supramolecular assembly with increasingly fine spatial and time resolution. The objective of this field is to assemble reaction centers around which the environment is purposefully structured at the molecular level, but extends over supramolecular domains. The properties of the assembly are controlled not only by the properties of the molecular building blocks but especially by the interface. Therefore, the focus is on both the interfacial and bulk properties of monolayers and thin films. Issues that need to be addressed include the film thickness, structural homogeneity and long-range order, as well as the electrochemical and... 

The SECM technique can image interfacial supramolecular assemblies so as to identify heterogeneities in their conductivity values. Information of this type is... 

Region where interfacial supramolecular assembly is formed. 

Figure 3.8 Kretchmann configuration of a metal-coated prism used for surface plasmon resonance investigations of interfacial supramolecular assemblies...

Figure 3.9 illustrates the electrochemical and mass transport events that can occur at an electrode modified with a interfacial supramolecular assembly [9]. For monolayers in contact with a supporting electrolyte, the principal process is heterogeneous electron transfer across the electrode/monolayer interface. However, as discussed later in Chapter 5, thin films of polymers [10] represent an important class of interfacial supramolecular assembly (ISA) in which the properties of the redox center are affected by the physico-chemical properties of the polymer backbone. To address the properties of these thin films, mass transfer and reaction kinetics have to be considered. In this section, the properties of an ideally responding ISA are considered. 

Voltammetry can provide a powerful insight into the thermodynamics and kinetics of electron transfer across the electrode/adsorbate interface and within the interfacial supramolecular assembly. Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) are the most commonly used techniques to study the equilibrium behavior and kinetics of redox-active interfacial supramolecular... 

Figure 3.10 Current-potential curves obtained from cyclic voltammetry measurements for the reduction and oxidation of an adsorbed interfacial supramolecular assembly under finite diffusion conditions...

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