Interfacial electron transfer, molecular

Chi Q, Zhang J, Jensen PS, Christensen HEM, Ulstrup J (2006) Long-range interfacial electron transfer of metalloproteins based on molecular wiring assemblies. Faraday Discuss 131 181-195... 

Interfacial electron transfer is the critical process occurring in all electrochemical cells in which molecular species are oxidized or reduced. While transfer of an electron between an electrode and a solvated molecule or ion is conceptually a simple reaction, rates of heterogeneous electron transfer processes depend on a multitude of factors and can vary over many orders of magnitude. Since control of interfacial electron transfer rates is usually essential for successful operation of electrochemical devices, understanding the kinetics of these reactions has been and remains a challenging and technologically important goal. 

Most of the work published to date on molecular dynamic studies of interfacial electron transfer involves the simplified assumption of a two-state model for the electronic degrees of freedom. Consider an ion of charge qj near a solution/metal interface. As a result of electron transfer between the ion and the metal surface, the charge of the ion changes to qj. We will consider both forward and backward electron transfer and assume that = <7 - = -1, so that the forward reaction corresponds to a single electron transfer from the metal to the ion, for example + e ... 

Motivating the research is the need for systematic, quantitative information about how different surfaces and solvents affect the structure, orientation, and reactivity of adsorbed solutes. In particular, the question of how the anisotropy imposed by surfaces alters solvent-solute interactions from their bulk solution limit will be explored. Answers to this question promise to affect our understanding of broad classes of interfacial phenomena including electron transfer, molecular recognition, and macromolecular self assembly. By combining surface sensitive, nonlinear optical techniques with methods developed for bulk solution studies, experiments will examine how the interfacial environment experienced by a solute changes as a function of solvent properties and surface composition. 

J. W. Halley and J. Hautmann, Phys. Rev. B 38 11704 (1988). First molecular dynamic simulation of interfacial electron transfer. 

The attention devoted to supramolecular sensitizers containing multifold chromophoric and electroactive centers arises from the construction of molecular devices based on nanometric and well-defined molecular architectures [4]. The use of these species for sensitization of titanium dioxide has provided fundamental insights into interfacial electron-transfer processes. 

Light-induced transformations over fluorinated titania (TiOi/F) cannot be initiated either by =Ti— 0 (OHads), due to the lack of =Ti—OH groups, or by SET from a surface complexed substrate, due to the fluoride competition. In addition to these major effects, the adsorption of molecular oxygen can be affected also and the surface charge is dramatically decreased. The last effect may be important particularly for charged substrates and intermediates and for the possibility of interfacial electron transfer. 

X < 380 nm) restores the slow current response, as shown by, curve (2). Thus, the chronoamperometric responses of the functionalized monolayer upon irradiation agree well with the shuttling of the Fc- -CD within the molecular network. In the presence of the trans-azobenzene unit, the Vc-f-CD is located close to the electrode surface, and a rapid interfacial electron transfer is observed. 

Burfeindt, B. Hannappel, T. Storck, W. Willig, F. Measurement of temperature-independent femtosecond interfacial electron transfer from an anchored molecular electron donor to a semiconductor as acceptor, J. Phys. Chem. 1996, 100, 16463. 

Sensitization of electrodes can be defined as the process by which interfacial electron transfers occurs as a result of selective light absorption by an entity called a photosensitizer, or simply a sensitizer [1]. The most common types of sensitizers are organic chromophores and inorganic coordination compounds, generically referred to as dyes. Interfacial electron transfer produces current or voltage response that can be measured in an external circuit. Thus sensitization provides a method for the conversion of a photon into an electrical signal that can be controlled at the molecular level. 

There are two possible excited state interfacial electron transfer processes that can occur from a molecular excited state, S, created at a metal surface (a) the metal accepts an electron from S to form S+ or (b) the metal donates an electron to S to form S . Neither of these processes has been directly observed. The two processes would be competitive and unless there is some preference, no net charge will cross the interface. In order to obtain a steady-state photoelectrochemical response, back interfacial electron transfer reactions of S+ (or S ) to yield ground-state products must also be eliminated. Energy transfer from an excited sensitizer to the metal is thermodynamically favorable and allowed by both Forster and Dexter mechanisms [20, 21]. There exists a theoretical [20] and experimental [21] literature describing energy transfer quenching of molecular excited states by metals. How-... 

The addition of electron donors or acceptors to the external electrolyte has allowed sustained photocurrents to be measured at sensitized metal interfaces, but the mechanism(s) often remain speculative. A photocurrent can be generated by excited state interfacial electron transfer like that shown in Figure 5, or by inter-molecular excited state electron transfer followed by dark redox reactions at the electrode. It can be experimentally difficult to distinguish between these distinct mechanisms and strong evidence exists only for the latter pathway which forms the basis of the photogalvanic cell. 

The long effective pathlength and high surface area afforded by these colloidal semiconductor materials allow spectroscopic characterization of interfacial electron transfer in molecular detail that was not previously possible. It is likely that within the next decade photoinduced interfacial electron transfer will be understood in the same detail now found only in homogeneous fluid solution. In many cases the sensitization mechanisms and theory developed for planar electrodes" are not applicable to the sensitized nanocrystalline films. Therefore, new models are necessary to describe the fascinating optical and electronic behavior of these materials. One such behavior is the recent identification of ultra-fast hot injection from molecular excited states. Furthermore, with these sensitized electrodes it is possible to probe ultra-fast processes using simple steady-state photocurrent action spectrum. 

Monolayer and multilayer thin films are technologically important materials that potentially provide well-defined molecular architectures for the detailed study of interfacial electron transfer. Perhaps the most important attribute of these heterogeneous systems is the ease with which their molecular architecture can be synthetically varied to tailor the properties of the ensemble. Assemblies incorporating specifically designed structures can, in principle, meet the needs of a variety of technological applications and be used as models for understanding fundamental interfacial reaction mechanisms. In fact, molecular assemblies are nearly ideal laboratories for the fundamental study of electron-transfer reactions at interfaces. In this chapter, the use of monolayer and multilayer assemblies to probe fundamental questions regarding electron transfer in surface-confined molecular assemblies will be addressed. 

In the sections that follow, we will first outline the approaches that have been taken to use molecular assemblies to understand interfacial electron transfer and the... 

These observations of an excitation wavelength dependence of the charge injection process show that photoinduced interfacial electron transfer from a molecular excited state to a continuum of acceptor levels can take place in competition with the relaxation from upper excited levels. The rather slow growth of the injection... 

Gaal D. A. and Hupp J. T. (2000), Thermally activated, inverted interfacial electron transfer kinetics high driving force reactions between tin oxide nanoparticles and electrostatically-bound molecular reactants , /. Am. Chem. Soc. 122, 10956-10963. 

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