Photosensitized redox reaction

If a solution, being in contact with an electrode, contains photosensitive atoms or molecules, irradiation of such a system may lead to photoelectro-chemical reactions or, to be more exact, electrochemical reactions with excited particles involved. In such reactions the electrons pass either from an excited particle to the electrode (the anodic process) or from the electrode to an excited particle (the cathodic process). In this case, an elementary act of charge transfer has much in common with ordinary (dark) electrochemical redox reactions, which opens a possibility of interpreting certain aspects of photochemical processes under consideration with the use of concepts developed for general quantum mechanical description of electrode processes. 

A number of metal salts photosensitize radical polymerization (36). Most of them concuct photo redox reactions to produce radicals by processes similar to reactions (23), (24) and (25). Besides spontaneous photodecomposition of metal complexes, metal complexes may react directly with the monomer. 

This approach can be further extended to photoelectrochemical reactions at modified semiconductor electrodes. In such cases the immobilized substance(s) may serve several functions mediation of the redox process, photosensitization of the semiconductor, and photocorrosion protection. 

In related studies of metal ions, Krasnovski learned that the oxides of titanium, zinc, or tungsten possess high photosensitizing activity in redox reactions comparable to the activity of porphyrins and chlorophylls 15). 

For the systems with photoactive membranes discussed in the previous section the photosensitizer embedded into the vesicle membranes not only participated in photochemical and dark redox reactions with substances which are located in water phases on both sides of the membrane, but also served as the carrier of the electron across the membrane. In the presence of the appropriate electron carrier which is capable of penetrating through the membrane core it is also possible to perform electron transfer between membrane-separated water phases when photosensitizers are located in these phases rather than in the membrane. Membranes containing no photosensitizers can be called photopassive ones since no photophysical and photochemical processes occur in them, and their role is only to (i) provide electron transfer from one water phase to the other leading to the formation of spatially separated oxidant and reductant and (ii) to suppress recombination reactions. 

The chemistry of radical anions, generated by PET processes, is less developed than that of radical cations, possibly due to the nonavailability of suitable photosystems to initiate photosensitized one-electron redox reactions. 

Upon irradiation, redox dye photosensitizers adsorbed on the surface of wide-bandgap metal oxide semiconductors readily inject an electron in the conduction band of the solid. While charge injection has been found for numerous efficient systems to occur in the femtosecond time frame, the electron back transfer takes place much more slowly, typically in the microsecond-millisecond domain. This charge recombination process can be intercepted by reaction of a reducing mediator M with the oxidized dye (Eq. (43)). The overall efficiency of the light-induced charge separation then depends upon the kinetic competition between back electron transfer and dye regeneration processes. 

Cuprous ions (Cu+) also react with hydrogen peroxide to make hydroxyl radicals (Halliwell and Gutteridge, 1985). The presence of iron salts and cuprous ions in the formulation can lead to an accelerated photochemical degradation of the drug by these reaction mechanisms. Metal ions can also participate in redox reactions with the drug or excipients in the preparation, depending on the redox properties of the species involved. Such reactions may further influence photochemical stability of the product, e.g., by the formation of photosensitizers. 

Electron-rich polyaromatic compounds such as anthracene, pyrene, and pery-lene [107] are suitable as photosensitizers as they give redox reactions with DPI salts through exciplex to finally yield the initiating species for photoinduced cationic polymerizations. Scheme 11.28 demonstrates the mechanism of a polymerization followed via exciplex formation through the excited sensitizer with the ground-state onium salt. 

Analcite partially exchanged with Ag+ underwent a photosensitized redox reaction believed to involve Ag+ with H20 within the cavities, and giving rise to Ag°. The phenomenon was manifested first by the appearance of a yellow discoloration of the bulk material, suggesting the presence of color centers, and later by opaque metallic silver disseminated along grain boundaries within analcite crystallites. 

Brian D. McNicol (Koninklijke/Shell Laboratorium, Amsterdam, Netherlands) With respect to your comment on the formation of Ag° within the cavities by a photosensitized redox reaction, if the Ag° atoms are monatomically dispersed, then they could be identified by electron spin resonance. Ag°, of course, is paramagnetic. 

Photoactive Additives.—Ferric compounds, in particular, the chloride, continue to attract much interest as photosensitizers for thermoplastics. " From e.s.r. work the mechanism appears to involve a redox reaction resulting in the formation of active hydroxy-radicals. Photodegradable polyethylene film has been developed by doping it with radiation-modified atactic polypropylene and hydroxyethyl-ferrocene. Several workers have studied the dye-sensitized photo-oxidation of polyisoprene and di-n-butyl sulphide embedded in Augustyniak and... 

Unlike radical cations, the quantum of chemistry originating from PET-gener-ated radical anions is still limited possibly due to the impending development of a suitable photosystem to initiate photosensitized one-elecron redox reactions in wide array of functionalities. Nevertheless, the radical anion chemistry follows, more or less, the analogous pattern of bond dissociation and addition (electrophilic/radical) reactions as observed for the radical cations. As there are not many examples to describe the separate categories, this section is subdivided... 

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