Photoinduced interfacial electron transfer

A microemulsion, Fig. 1, has a similar organization to that characteristic of a micelle but employs, rather than one, multiple surfactant components, allowing for introduction of other additives into the hydrophobic core [11], As with micelles, microemulsions are optically transparent and can be easily studied by standard spectroscopic methods. One important use of such microemulsions is in the photoinduced initiation of polymerization of monomers with low water solubility many such reactions involve a mechanism occurring through photoinduced interfacial electron transfer. 

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 ... 

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. 

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... 

Subsequently, photoinduced interfacial electron transfer from the conduction band of colloidal Ti02 to dimeric viologen, an electron acceptor, was also examined by picosecond time-resolved methods (Serpone et at., 1987). Electron transfer takes place in the picosecond time domain = 2.5 x 10 s at pH 7.8. The reaction involves consecutive one-electron transfer to give the mono-reduced initially, followed by formation of DV ". In acidic aqueous media (pH 3.5), transient absorption spectra showed that electron transfer does not proceed beyond the formation of the mono-reduced species. 

Photoinduced Interfacial Electron Transfer (Heterogeneous Electron Transfer)... 

We see therefore that photoactive semiconductor particles provide ideal environments for control of interfacial electron transfer. Photoinduced electron-hole pairs formed on irradiated semiconductor suspensions, as in photoelectrochemical cells, allow for reactivity control not available in homogeneous solution. This altered activity derives from controlled adsorption on a chemically manipula-ble surface, controlled potential afforded by the valence band edge positions, controlled kinetics by virtue of band bending effects, and controlled current flow by judicious choice of incident light intensity. 

Static electron transfer from photoexcited particles to adsorbed substrates has been observed for a wide range of semiconductor and organic materials respectively. For example, the photoinduced reduction of zwit-terionic viologen (ZV) by CdS conduction band electrons is found by time resolved transient absorption spectra to occur with a risetime for ZV -formation of less than 20 ps [116], consistent with a rate constant for interfacial electron transfer of >5 x 10los-1. Strong adsorption of the... 

Under potentiostatic conditions, photoinduced heterogeneous electron transfer between specifically adsorbed porphyrins and redox couples confined to the organic phase manifests itself by photocurrent responses. As in the case of dynamic photoelectrochemistry, these photoresponses provide information on the dynamics of heterogeneous electron transfer and recombination processes. In addition, we shall demonstrate that photocurrent measurements can be used to characterise the interfacial coverage of the specifically adsorbed porphyrins as well as their molecular orientation. 

As we mentioned earlier, the surface coverage of the ZnTPPS-ZnTMPyP complex is effectively independent of the Galvani potential difference. Estimations of the interfacial tension based on QELS studies have shown that the density number of ZnTPPS-ZnTMPyP is of the order of 10 " cm for a bulk concentration of 5x10 mol dm From the results illustrated in Figure 11.22b, this value corresponds to the maximum surface concentration at the water/DCE interface. According to Equation (11.45), the dependence of 7p, on AGg, is determined by the effective quantum yield (Qjq) for the photoinduced heterogeneous electron transfer ... 

Interfacial electron transfer at solid-liquid interfaces, photoinduced and/or in the presence of an applied potential bias, as in the case of water oxidation on semiconducting metal oxide electrodes involves, as will be discussed in the next section, multiple electron and proton transfer steps. The energy cost associated with charge transfer across the interface will translate into overpotentials for driving the (photo)electrochemical reactions. This is particularly significant in... 

Many nanoscale energy conversion devices, like dye-sensitized and pol5mer-based solar cells, rely on efficient interfacial electron transfer processes. Multiscale modelling of such devices ultimately involves length-and time-scales that scan many orders of magnitude, ranging from ultrafast molecular-scale quantum phenomena such as photoinduced electron transfer, via mesoscopic properties such as charge transport, to macroscopic device performance such as I-V characteristics. 

Almost all photoinduced surface electron transfer processes in sensitizer-semiconductor hetero-structures are most naturally characterized as two-step processes with initial excitation of the adsorbate, followed by interfacial electron transfer into a band of an acceptor state, as shown schematically in Figure 3.7. There is, however, also a more unusual case in which the lET is caused by a direct interfacial charge transfer excitation which is described in the... 

This review article attempts to summarize and discuss recent developments in the studies of photoinduced electron transfer in functionalized polyelectrolyte systems. The rates of photoinduced forward and thermal back electron transfers are dramatically changed when photoactive chromophores are incorporated into polyelectrolytes by covalent bonding. The origins of such changes are discussed in terms of the interfacial electrostatic potential on the molecular surface of the polyelectrolyte as well as the microphase structure formed by amphiphilic polyelectrolytes. The promise of tailored amphiphilic polyelectrolytes for designing efficient photoinduced charge separation systems is afso discussed. 

Photocyanations rely on photoinduced electron transfer [29]. This was demonstrated by monitoring cyanation yields as a function of the droplet size for oil-in-water emulsions. Hence increase in interfacial area is one driver for micro-channel processing. Typically, fluid systems with large specific interfacial areas tend to be difficult to separate and solutions for more facile separation are desired. 

Between 0.20 and 0.30 V, a decay of the initial photocurrent and a negative overshoot after interrupting the illumination are developed. This behavior resembles the responses observed at semiconductor-electrolyte interfaces in the presence of surface recombination of photoinduced charges [133-135] but at a longer time scale. These features are in fact related to the back-electron-transfer processes within the interfacial ion pair schematically depicted in Fig. 11. 

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