Photoreduction and -oxidation

4- biradicals arising in the Norrish type II hydrogen abstraction process.This appears to be a consequence of the steric constraints imposed on the rotational motions of the 

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G.W. Aiken, G.R. Feder, G.L. Thom, K.A. (1992) Sorption of dissolved organic carbon by hydrous aluminum and iron oxides occurring at the confluence of Deer Creek with the Snake River, Summit County, Colorado. Environ. Sci. Techn. 26 1388-1396 McKnight, D.M. Kimball, B.A. Bencala, K.E. (1988) Iron photoreduction and oxidation in an acidic mountain stream. Science 240 637-640... 

McKnight, D.M., Kimball, B.A. Bencala, K.E. (1988) Iron photoreduction and oxidation in an acidic mountain stream. Science 240, 637-640. 

Certain azo-dyes were found to undergo both photoreductive and oxidative fading in alcohol solution, but in acetone only oxidation was observed. In another study, ultraviolet absorbers were found to inhibit the photofading of a basic cationic dye. In acetic acid, rhodamine B has been shown to fade by dealkylation, and certain fluorescent brightening agents have been shown to induce photoyellowing in silk. The luminescence properties of xanthene and acridine dyes have been quantified. 

Xirouchald C, Kirialddis G, Pedersen TF, Eritzsche H (1996) Photoreduction and oxidation of as-deposited microcrys-talline indimn oxide. J Appl Phys 79 9349-9352... 

The effects of uv radiation on V/-nitroso compounds depend on the pH and the medium. Under neutral conditions and ia the absence of radical scavengers, these compounds often appear chemically stable, although the E—Z equiUbrium, with respect to rotation around the N—N bond, can be affected (70). This apparent stabiUty is due to rapid recombination of aminyl radicals and nitric oxide [10102-43-9] formed duting photolysis. In the presence of radical scavengers nitrosamines decay rapidly (71). At lower pH, a variety of photoproducts are formed, including compounds attributed to photoelimination, photoreduction, and photo-oxidation (69). Low concentrations of most nitrosamines, even at neutral pH, can be eliminated by prolonged kradiation at 366 nm. This technique is used ki the identification of /V-nitrosamines that are present ki low concentrations ki complex mixtures (72). 

The continuous flow method is still necessary when one must use probe methods which respond only relatively slowly to concentration changes. These include pH, Oj-sensitive electrodes, metal-ion selective electrodes,thermistors and thermocouples, " epr and nmr detection. Resonance Raman and absorption spectra have been recorded in a flowing sample a few seconds after mixing horseradish peroxidase and oxidants. In this way spectra of transients (eompounds I and II) can be recorded, and the effext of any photoreduction by the laser minimized. ... 

The structure of these alkaloids was established on the basis of UV, IR, NMR, and MS, and proved by X-ray analysis. Further detailed studies (707) carried out on Sophora chrysophylla resulted in the isolation of one more new alkaloid that appears to be mamanine A -oxide (55). (-f)-Mamanine (16) was obtained by photoreduction and is easily oxidized to 55 by m-chlorobenzoic acid. 

Photoreduction of cobalt(III) complexes can occur under a variety of conditions. Irradition of the charge transfer bands of these systems results only in decomposition with production of cobaltous ion and oxidation of one of the ligands. In some instances photoreduction can be initiated by irradiation of the ligand field transitions. Irradiation of ion pairs formed by these complexes with iodide ion with ultraviolet light also leads to reduction of the complexes. Finally, irradiation of iodide ion in the presence of the complexes leads to reduction. 

The photochemistry of Eosin under both reductive and oxidative conditions has been studied by several groups [145-151], Photoreduction by amines such as tribenzylamine (R = CH2, R" = ) produces two leuco analogues, the dihydro derivative, and the cross-coupled product formed from the amine radical and the dye radical anion (2) [152], In addition, debromination of Eosin is reported during photobleaching with amines and phenols. The reader however is referred to the extensive studies of Rose Bengal dehalogenation by Paczkowski and Neckers [153]. Radiolysis of Eosin in methanol shows that debromination is a consequence of the photochemical decomposition of semireduced Eosin [154],... 

Almost all photoreactions of porphyrins and phthalocyanines occur via the triplet state whose lifetime (ca. 1 ms at T = 300K) and yields of formation are greatest for porphyrins. The redox potentials of porphyrins are also more amenable to water photoreduction via oxidative or reductive cycles and some selected values are found in Table 11. 

As indicated previously, it is frequently assumed that weakly bound oxygen leads to the complete combustion of hydrocarbons. However, another possible pathway to complete combustion, may involve the activation of an oxide ion (59). Kazanskii 60) studied in detail the elementary steps of the reduction, photoreduction, and reoxidation of the surfaces of oxide catalysts. He reported that upon exposure to light some semiconductor oxides undergo a charge transfer from the O2- ion to the cation, as shown ... 

The silver-loaded titania photocatalysts possess high activity in the photocatalytic decomposition of ozone [38], photoreduction of various thiols [39], photodestruction of 1,4-dichlorobenzene [40], dehydrogenation and oxidation of alcohols (e.g. of 2-propanole [41,42]), decoloration of textile waste water [43], photokilling of bacteria [44], and others. For additional detailed information on this topic, one can use a number of review articles and books [e.g. 45-48]. 

Photoredox processes include both photoreduction and photo-oxidation of the excited species. An electron transfer that results from an electronic state produced by the resonant interaction of electromagnetic radiation with matter is called photoinduced electron transfer (PET) [30-32]. This can be done by either a direct or a photosensitized process (see below and Figure 6.6). 

This activity is particularly useful for degradation of strongly hazardous substances or recalcitrant pollutants that are difficult to remove in chemical or biochemical processes. In this respect any pathway leading to abatement of chromate(VI) pollution arouses interest. One such pathway seems to be created by cooperation between iron and chromium photocatalytic cycles, which were reported as effective in conversion of chromate(VI) into Crm species [20-23,97]. A synergistic photoreduction of CrVI and Cu11 mediated by Ti02 [98], or photocatalytic reduction of Crvl and oxidation of organic matter by environmental polyoxometallates as photocatalysts [99], may constitute alternative possibilities. 

It is now realized that copper as metal next to iron and chromium participates in photoredox cycles and its role cannot be ignored. The most important part of the cycle is photoreduction of Cu(II) to Cu(I) induced by solar light and oxidation of ligands to their environmentally benign forms. Then Cu(I) is easily re-oxidized to Cu(II), which can coordinate the next ligand molecule, and thereby the Cu photocatalytic cycles contribute to continuous environmental cleaning. Besides oxida-tion/reduction, other critical processes relevant to the copper cycles are adsorption/desorption and precipitation/dissolution... 

An analysis of the time course of luminescence emission following selective laser excitation has enabled the forward and reverse rate constants to be obtained for the equilibrium between the Eu" complex of 1,2-diaminocyclohexanetetraacetate, [Eu(DCTA)], and iminodiacetate, IMDA, to form the ternary complex [Eu(DCTA)(IMDA)] . Some other systems in which chemical interconversion processes occur at rates much greater than, or much slower than, the reciprocal excited-state lifetime of the Eu" -containing species were also examined. A matrix isolation e.s.r. study has appeared of the photoreduction of the formato-complex of Eu " in a formic acid-sodium formate buffer. A three-step cyclic scheme has been suggested to explain the observations, namely photoreduction of by Eu, radical alternation from H to C02 , and oxidation of CO2 by Eu ". ... 

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