Photo-induced interfacial electron

Although the correlation between ket and the driving force determined by Eq. (14) has been confirmed by various experimental approaches, the effect of the Galvani potential difference remains to be fully understood. The elegant theoretical description by Schmickler seems to be in conflict with a great deal of experimental results. Even clearer evidence of the k t dependence on A 0 has been presented by Fermin et al. for photo-induced electron-transfer processes involving water-soluble porphyrins [50,83]. As discussed in the next section, the rationalization of the potential dependence of ket iti these systems is complicated by perturbations of the interfacial potential associated with the specific adsorption of the ionic dye. 

Fig. 3. Like a photoelectrochemical cell, such a powder includes sites for photo-induced oxidation and reduction, but no external current flow accompanies these transformations. Photoactivity is also maintained as the size of the particle decreases to the colloidal range although the absorption characteristics, the quantum efficiency of charge separation, and the kinetics of interfacial electron transfer may be influenced by the particle size. On sufficiently small particles, for example, the calculated space-charge width necessary for effective band bending may exceed the dimensions of the particle.

In order to account for such a mechanism, photochemical excitation of a semiconductor surface might induce the promotion of an electron from the valence band to the conduction band. Since relaxation of the high-energy electron is inhibited by the absence of intra-states, if the lifetime of this photo generated electron-hole pair is sufficiently long to allow the interfacial electron transfer from an accumulation site to an electron acceptor, as well as the interfacial electron transfer from an adsorbed organic donor to the valence-band hole, the irradiated semiconductor can simultaneously catalyze both oxidation and reduction reactions in a fashion similar to multifunctional enzymes reactions [232]. 

The separation of photoproducts formed in photosensitized electron transfer reactions is essential for efficient energy conversion and storage. The organization of the components involved in the photo-induced process in interfacial systems leads to efficient compartmentalization of the products. Several Interfaclal systems, e.g., lipid bllayer membranes (vesicles), water-in-oil mlcroemulslons and a solid SIO2 colloidal Interface, have been designed to accomplish this goal. 

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... 

Figure 6-3. Schematic view of photo-induced electron or hole transport through the DNA-based molecular chain. Reversible random walk of the charge along the chain is interrupted by irreversible chemical degradation ( quenching ) at given sites. Charge i.e. electron or hole) injection at the terminals corresponds to interfacial charge transfer in the in situ STM or nanogap electrode configurations to be discussed in Section 4.

The orientation and conformation of protein at the surface of electrodes represent key factors, as they influence the interfacial electron speed. A direct reversible fast electron transfer was observed for RC of Rhodobacter sphaeroides reconstituted in polycation sandwiched monolayer film. This layer-by-layer technique allows a such ordered orientation of RC that favours the electron transfer in film. Furthermore, RC in the film features a photo-induced redox-peak fluctuation, suggesting an intact and functional state redox peaks were also found dependent on pH, Implying a proton-coupled electron transfer. 

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