Interfacial electron transfer sensitization

Upon absorption of light an electron from the HOMO of the adsorbed dye, D, is raised to the LUMO from where it is injected into the conduction band of the n-type semiconductor and transferred to a counter electrode where an oxidant, O, is reduced. From the reduced species, R, the electron is transferred to the HOMO of the adsorbed dye to fill the electron vacancy, so that after the overall photoelectro-chemical process the dye is in its original oxidation state. Vlachopoulos et al. (1987) have reported on Ti02 photoelectrodes that were sensitized to visible light with various dyes and that showed high quantum yields of interfacial electron transfer under visible irradiation. 

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. 

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. 

The discussion which follows is divided into two main sections. The first termed antenna sensitizers presents studies of polynuclear compounds with a surface bound unit that can accept energy from covalently linked chromophoric groups and inject electrons into the semiconductor from its excited state. The second describes supramolecular assemblies designed to promote intramolecular and interfacial electron transfer upon light absorption. 

Figure 3. Scheme for the electron transfer in dye sensitization process at the interfacial layer of Chi on SnO electrode... 

Our interest in SERS stemmed from our research activities concerned with establishing connections between the molecular structure of electrode interfaces and electrochemical reactivity. A current objective of our group is to employ SERS as a molecular probe of adsorbate-surface interactions to systems of relevance to electrochemical processes, and to examine the interfacial molecular changes brought about by electrochemical reactions. The combination of SERS and conventional electrochemical techniques can in principle yield a detailed picture of interfacial processes since the latter provides a sensitive monitor of the electron transfer and electronic redistributions associated with the surface molecular changes probed by the former. Although few such applications of SERS have been reported so far the approaches appear to have considerable promise. 

Nonetheless, sensitization by dyes held within the cores of microemulsions can be easily accomplished [69]. Such sensitization is an important component of photogalvanic effects, the magnitude of which are significantly enhanced in the non-homogeneous environment of a microemulsion [70], The hydrophilic core of an water-in-oil microemulsion can concentrate cation radicals formed via interfacial electron transfer and hence increase the yield of subsequent dimerization the dimethylnaphthalene cation radical exhibits a dimerization equilibrium constant of nearly 500 in a microemulsion [71]. For similar reasons, hexylviologen acts as a much more efficient relay than methyl viologen in a CTAB/hexanol microemulsion [72]. 

A predictive mechanistic treatment of dye-sensitized photoinduced interfacial electron transfer has been described by Gerischer [29]. According to this treatment, the rate of dye-sensitized electron transfer, pdye, can be described by the following ... 

Interfacial electron transfer between a metal and an excited sensitizer, A -L- B where B represents a metal electrode, may be reductive, whereby the electron transfers from the conduction band of the metal to the singly occupied HOMO state of the excited adsorbed molecules, thus resulting in A -L-B and a cathodic photocurrent at the electrode. Alternatively, it may be an oxidative process, wherein the electron is transferred from the adsorbate to the metal, so resulting in A+-L-B and an anodic photocurrent at the electrode. 

Khoudiakov M, Parise AR, Brunschwig BS. Interfacial electron transfer in [Fe(CN)6]4 - sensitized Ti02 nanoparticles a study of direct charge injection by electroabsorption spectroscopy. / Am Chem Soc 2003 125 4637-42. 

Rego LGC, Batista VS. Quantum dynamics simulations of interfacial electron transfer in sensitized Ti02 semiconductors. J Am Chem Soc 2003 125 7989-97. 

Since the reorganization term of the reactant Aj in Eq. (67) is also probably not very sensitive to the nature of the solvent, the total reorganization energy for interfacial and homogeneous electron transfer are approximately equal. The same conclusion can be inferred from Kharkat s treatment [177, 178],... 

Although photoelectrochemistry has been known as a field for over thirty years, its full impact on organic synthesis has yet to be revealed. This article has dealt with a variety of examples that show how chemical conversions can be induced by photo-electrochemical activation of light-sensitive semiconductor surfaces. Photoexcitation causes the promotion of an electron from the valence band to the conduction band, thus producing a surface-confined electron-hole pair. The charges represented by this pair are then trapped by interfacial electron transfer. The oxidized and reduced... 

Importantly, it was found [80-82, 311] that interfacial electron transfer from MLCT-excited Ru polypyridine complexes to Ti02 is an ultrafast process, completed in 25-150 fs This groundbreaking discovery implies that the search for new sensitizers need not to be limited to complexes with long-lived excited states. Indeed, [Fe(4,4 -(COOH)2-bpy)2(CN)2], whose MLCT excited state lifetime is only ca 330 ps, was found [304] to act as a sensitizer in a Ti02-based solar cell. In fact, even the classical Gratzel cell [36, 77, 78] would not operate as well as it does, were the interfacial electron transfer not ultrafast, since the [Ru(4,4 -(COOH)2-bpy)2-(NCS)2] sensitizer has an inherent excited state lifetime of only 50 ns. 

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. 

The sensitization of electrodes to visible light by dye molecules is an old area of science with a rich history [2]. A dye-sensitized photoefifect was measured at a semiconductor surface as early as 1887 in Vienna [3]. The accepted mechanisms for the dye sensitization of electrodes emerged from photoelectrochemical studies in the 1960s and 1970s [4-6]. These studies were motivated by a desire to quantify interfacial electron transfer processes and develop cells useful for solar energy conversion. The two most common approaches are shown schematically in Figure 1. 

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