Interfacial electron transfer sensitizer

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. 

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. 

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. 

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. 

Sensitization and interfacial electron transfer mechanisms have been described by Gerischer [4-6]. A basic assumption is that electron transfer, like light absorption, occurs under the restriction of the Franck-Condon principle. The time-scale for interfacial electron transfer is much shorter than that for nuclear motion. This means that the energy terms for electron transfer are different from the thermodynamic formal reduction potentials described above. Gerischer considered the appropriate energy levels and derived a distribution of energy levels when the sensi-... 

Gerischer s distribution curves can be interpreted as representing the energy dependence of the electron transfer rate constants involving the reduced and oxidized species. Only a few electrochemical studies have attempted to evaluate the model and quantify the distributions and reorganizational parameters [17, 18]. Nevertheless, it has become common practice to draw a pictorial representation of the distributions when discussing interfacial electron transfer kinetics relevant to dye sensitization. 

Equation 10 represents a simple relation useful for predicting interfacial electron transfer rates relevant to dye sensitization. At this time more detailed descriptions... 

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. 

A practical disadvantage of the sensitized planar electrodes described in the previous section was their low solar energy conversion efficiencies. An experimental drawback was that characterization was limited to photoelectrochemical or luminescence techniques. Both of these issues arose from the poor light harvesting by monolayers on flat surfaces or inefficient interfacial electron transfer yields from thick films or concentrated solutions. In 1990, Gratzel and co-workers reported experimental studies of dye sensitized colloidal Ti02 films that eliminated both of these problematic issues [112]. The effective surface area for sensitizer binding... 

Zaban and co-workers reported the use of chemical redox titrations to measure the potential of sensitizers bound to Ti02 [136], An unexpected result from these studies is that redox couples that are not pH sensitive in fluid solution become pH dependent when bound to the semiconductor surface. The magnitude of the pH-induced shift varied from 21 to 53 mV per pH unit depending on the physical location of the sensitizer. Sensitizers inside the semiconductor double layer track the 59 mV pH shift of the semiconductor. When sensitzers were outside the double layer, their potential was almost independent of the semiconductor. This finding has important implications for the determination of interfacial energetics for dye sensitization and interfacial electron transfer studies [136]. 

The accepted model for photocurrent generation in the dye-sensitized nanocrystalline solar cells involves excited state interfacial electron transfer. It is therefore desirable to have a mechanistic understanding of excited-state behavior of the sensitizers. Since the most successful sensitizers for this application are Ru poly-pyridyl compounds, we include a brief review of their excited-state properties. More thorough reviews are available in the literature [137]. 

Willig and co-workers used near-infrared spectroscopy to measure excited-state interfacial electron transfer rates after pulsed light excitation of cis-Ru(dcb)2(NCS)2-Ti02 in vacuum from 20 to 295 K [208]. They reported that excited-state electron injection occurred in less than 25 fs, prior to the redistribution of the excited-state vibrational energy, and that the classical Gerischer model for electron injection was inappropriate for this process. They concluded that the injection reaction is controlled by the electronic tunneling barrier and by the escape of the initially prepared wave packet describing the hot electron from the reaction distance of the oxidized dye molecule. It appears that some sensitizer decomposition occurred in these studies as the transient spectrum was reported to be similar to that of the thermal oxidation product of m-Ru(dcb)2(NCS)2. 

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