Photoreduction separation

Photoinduced ET at liquid-liquid interfaces has been widely recognized as a model system for natural photosynthesis and heterogeneous photocatalysis [114-119]. One of the key aspects of photochemical reactions in these systems is that the efficiency of product separation can be enhanced by differences in solvation energy, diminishing the probability of a back electron-transfer process (see Fig. 11). For instance, Brugger and Gratzel reported that the efficiency of the photoreduction of the amphiphilic methyl viologen by Ru(bpy)3+ is effectively enhanced in the presence of cationic micelles formed by cetyltrimethylammonium chloride [120]. Flash photolysis studies indicated that while the kinetics of the photoinduced reaction,... 

FIG. 11 General mechanism for the heterogeneous photoreduction of a species Q located in the organic phase by the water-soluble sensitizer S. The electron-transfer step is in competition with the decay of the excited state, while a second competition involved the separation of the geminate ion-pair and back electron transfer. The latter process can be further affected by the presence of a redox couple able to regenerate the initial ground of the dye. This process is commonly referred to as supersensitization. (Reprinted with permission from Ref. 166. Copyright 1999 American Chemical Society.)... 

T. Hirai and I. Komasawa, Separation of Europium from Sm, Eu, Gd Mixture by Photoreductive Stripping in Solvent Extraction Process, Industrial Engineering Chemistry Research, Vol. 34, p. 237,1995. Titanium, MCP-18, Bureau of Mines, United States Department of Interior, August 1978. 

This reaction is the reverse of the initial ketyl radical formation by the benzophenone triplet and is therm Q4ynamically favorable. The experiments using optically active alcohols as source of hydrogen atoms show, however, that under normal conditions this reaction is unimportant. This is probably due to other, more efficient pathways for reaction of the ketyl radicals or perhaps to diffusion rates which separate the radicals before reverse transfer can occur. That this reaction can be important in some cases even without the presence of sulfur compounds was shown by studying the photoreduction of benzophenone in optically active ethers.<68) Although the reaction of benzophenone in methyl 2-octyl ether is only 0.17 times as fast as that in isopropanol, ethers can be used as sources of hydrogen atoms for photoreduction ... 

Several papers in the last two years attempted to separate the photoreceptor-flavin (and/or the associated b-type cytochrome) from the overwhelming amount of bulk-flavin which is involved in metabolism rather than photoreception. However, a clear-cut assay for the bluelight receptor has not yet been found. Specific photoaffinity labeling and the considerable selectivity in the photoreduction of a b-type cytochrome, as mediated by suitable dyes, are the most promising attempts. 

Nitroanilines and nitronaphthylamines (group III) show remarkable stability towards both photoreduction and photosubstitution. Photoreduction (including intramolecular hydrogen abstraction) occurs, however, after acylation of the amino group in nitroanilines 46-48). Xhe stability of liitroanilines towards photoreduction is evidently due to the chcirge transfer character of the lowest excited state. Another possibility could be the increased probability of radiationless deactivation due to smaller separation of the ground state and the lowest excited states. 

When G2-OH is mixed with a fourfold molar excess of Cu + ions the spectrum in Fig. 9b results. These data indicate that each G2-OH can sorb at least four Cu + ions. Moreover, the separation between adjacent copper adducts is 62.5, which indicates that the oxidation state of Cu inside dendrimer during the MALDI-MS experiments is -1-1. Reflectron-mode MS also confirms this assignment the mass differences between the monoisotopic peaks of protonated dendrimers, singlecopper adducts, and double-copper adducts are 61.96 and 61.93, respectively, which is consistent with the assignment of the adduct ions as [Mis + Cu(l)]+ and [Mis + 2Cu(I)-H] +. We speculate that the presence of Cu+ is a consequence of the photochemical reduction of Cu + during ionization. Such photoreduction in MALDI MS measurements has been observed previously when polymers or peptides are used as ligands for Cu + [117,118]. 

As expected, 140 can also be reduced photochemically when a solution in isopropanol was exposed to sunlight, yellow crystals (mp 160°C) separated which, on thermal treatment (200°C, 1 hr) in vacuum, yielded 138 (80%). Evidently, a hydroxyphthalan is formed first. When the photoreduction is conducted in the presence of acid, dimeric l,3-diphenylbenzo[c]-furan (144) is formed (Section IV,E). ... 

The photooxidation and photoreduction of water in separate reaction systems are demonstrated in Sections 7.3.1 and 7.3.2, respectively. Since the redox... 

Investigations of metals (Ag [28-30], Au [31, 33], Pt [18, 22, 30], Pd [11], Cu, Fig [41] etc.) photoreduction at surfaces of porous samples and colloidal particles of Ti02 shows, that in such systems metal is deposited on the semiconductor surface as separate particles of subnanometric - nanometric size. Such metal particles have ohmic contact with semiconductor surface [11, 24, 32] and developed electronic structure [24, 28-32], So, we concluded that photocatalytic nickel(II) reduction takes place at irradiation of suspensions, containing mesoporous Ti02, Ni2+ and ethanol, this process resulting in the... 

Bromobiphenyl undergoes photoreduction from the triplet state375. The dependence of the quantum yield upon the concentration of the substrate does indicate the formation of an excimer. Since cpisc = 0.98, it may be concluded that this excimer is formed via the triplet state. The linear solvation energy parameters indicate a weak polarization of the excimer, suggesting a weak radical anion and cation character in the two moieties. The charge separation is smaller than in the exciplex formed from 4-bromobiphenyl and tri-ethylamine. 

The application of the CIDEP technique to the kinetic isotope effect in the photoreduction of quinones was made by Adeleke and Wan (2). Photolyses of naphthaquinone in isopropanol and in isopropanol-dg were carried out under identical experimental conditions. Analysis of the polarization data indicates that the ratio kjj/kj = 2.6. However, in a separate series of experiments in which the naphthaquinone was photoreduced by substituted phenol and phenol-O,, the kinetic isotope effect drops to a value of kjj/kj = 1.6. 

Recently (93), a photochemical method for NO-separation from an gaseous mixture was proposed, in which the Fe substrate is generated and regenerated by photoreduction ... 

Organic compounds have the potential to abiotically reduce Fe(III) and Mn(IV) (Luther et al., 1992 Stone, 1987). Phenols and a variety of other aromatic compounds reduce Fe(III) rapidly at acidic pH, but slowly at circum neutral pH (LaKind and Stone, 1989). Humics can reduce Fe(III) effectively at circumneutral pH and they are abundant in soils and sediments. Because humics and other organic compounds often serve as electron shuttles between metal-reducing bacteria and metal oxides (Lovley et al., 1996a), it may be difficult to separate microbial and nonmicrobial sources of electrons. Finally, aerobic photoreduction of Fe(III) has been observed in freshwater and marine environments (Barbeau et al., 2001 Emmenegger et al., 2001), but it is unknown to what degree this process... 

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