Photoactivated adsorption

Surface effects and adsorption equilibria thus will significantly influence the course of photoelectrochemical transformations since they will effectively control the movement of reagents from the electrolyte to the photoactivated surface as well as the desorption of products (avoiding overreaction or complete mineralization). The stability and accessibility toward intermolecular reaction of photogenerated intermediates will also be controlled by the photocatalyst surface. Since diffusion and mass transfer to and from the photocatalyst surface will also depend on the solvent and catalyst pretreatment, detailed quantitative descriptions will be difficult to transfer from one experiment to another, although qualitative principles governing these events can be easily recognized. 

The presence of an aromatic ring in the molecule favours the oxidation in the allylic position. By constrast with the selectivity pattern of photochemistry (see Fig. 1), the formation of 1-tetralone would be due to the mode of adsorption of tetralin at the surface of titania and/or to the mode of attack of photoactivated oxygen. 

We have shown how the band structure of photoexcited semiconductor particles makes them effective oxidation catalysts. Because of the heterogeneous nature of the photoactivation, selective chemistry can ensue from preferential adsorption, from directed reactivity between adsorbed reactive intermediates, and from the restriction of ECE processes to one electron routes. The extension of these experiments to catalyze chemical reductions and to address heterogeneous redox reactions of biologically important molecules should be straightforward. In fact, the use of surface-modified powders coated with chiral polymers has recently been reputed to cause asymmetric induction at prochiral redox centers. As more semiconductor powders become routinely available, the importance of these photocatalysts to organic chemistry is bound to increase. 

Adsorption of dyes onto a semiconductor surface allows for another mode of photoactivation [44-48]. The dye adsorbs a photon, generating an excited state in which a sufficiently higher-energy orbital is populated to allow direct injection of an electron into the conduction band edge [1]. The dye thus becomes oxidized and can either react chemically with nucleophiles by bond formation or can be restored to its original oxidation level by electron transfer. In the latter case, the reaction partner is oxidized, regenerating the ground state of the sensitizer ready to participate in... 

Most reactions on surfaces are complicated by variations in mass transfer and adsorption equilibrium [70], It is precisely these complexities, however, that afford an additional means of control in electrochemical or photoelectrochemical transformations. Not only does the surface assemble a nonstatistical distribution of reagents compared with the solution composition, but it also generally influences both the rates and course of chemical reactions [71-73]. These effects are particularly evident with photoactivated surfaces the intrinsic lifetimes of both excited states and photogenerated transients and the rates of bimolecular diffusion are particularly sensitive to the special environment afforded by a solid surface. Consequently, the understanding of surface effects is very important for applications that depend on chemical selectivity in photoelectrochemical transformation. 

A phenomenon closely related to photosensitization is that of photoactivated adsorption, in which non-stoicheiometric soUds show enhanced adsorption of molecules from solution during illumination. The... 

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