Electron-transfer mechanism photooxidations

We have recently found that this free radical oxidation of the methyl groups is in fact not a major pathway in the photooxidation of poly(phenylene oxide). Instead, the oxidation apparently occurs through an electron-transfer mechanism on the backbone of the polymer not chemically involving the methyl groups at all. In this paper, we present evidence inconsistent with the free radical mechanism and supporting this novel pathway for polymer photooxidation. 

An electron transfer mechanism appears to be operating in the photooxidation of the electron-rich alkene 61 which is quantitatively converted to the e t/o-peroxide when sensitised by Ceo deposited on alumina or silica <04SL971>. 

Silverman, S. K. and Foote, C. S., Singlet oxygen and electron-transfer mechanisms in the dicy-anoanthracene-sensitized photooxidation of 2,3-diphenyl-l,4-dioxene, /. Am. Chem. Soc., 113, 7672, 1991. 

In a number of cases ITIES can be used to separate the products of a photoinduced electron-transfer reaction. An early example is the work by Willner et al. [7] at the water/toluene interface, who studied the photooxidation of [Ru(bpy)3]2+ in the aqueous phase. The excited state was quenched by hexadecyl- 4,4 bipyridinium, which becomes hydrophobic on reduction and crosses to the toluene phase. There are other examples and mechanisms at the present time their main interest resides in their chemistry, and in the separation of products that can be achieved at the interface. 

PAHs photooxidized exclusively by the type I electron transfer-superoxide mechanism include naphthalene and 1-methylnaphthalene (Barbas et al., 1993), fluorene (Barbas et al., 1997), and acenaphthene (Re-... 

Fig. 20. Energy correlations for a mechanism of supersensitization by photoreduction (a) or photooxidation (b) of the excited dye prior to electron transfer...

A similar study of the photooxidation of some spiropyrans and spironaphthox-azines indicates that the spiro and open forms of these dyes are singlet oxygen quenchers and that the colored form does not act as a sensitizer. A mechanism is proposed that involves the formation of a superoxide radical anion by photoinduced electron transfer to oxygen from a merocyanine form of the dye, followed by nucleophilic attack of the radical anion on the radical cation of the dye.174... 

Sensitized photooxidation of sulfur-containing amino acids in neutral aqueous solution occurs via electron transfer from the sulfur to the triplet state of the sensitizer (the mechanism can be adapted to other dye-sensitized photoinitiated polymerization). Electron transfer quenching in the system is followed mainly by diffusion of the sulfur-centered radical cation and benzophenone radical anion. Laser... 

Before the chemical identity of the secondary electron acceptor and the reaction mechanism involved were known. Parson obtained some useful information indirectly from spectro-kinetic studies using a double-flash arrangement. Parson used a pair of laser flashes spaced a few microseconds apart to excite the chromatophores of Chromatium vinosum and found that while the first flash elicited photooxidation of P870, the second flash did not cause another photooxidation even though the photooxidized P870 " has been re-reduced by the endogenous, c-type cytochrome within -2 /js and presumably ready to undergo another photooxidation, provided there had been electron transfer from Qa Io Qb, i.e.,... 

Fig. 3. Top row formulation of the reaction sequence involved In the photochemical charge separation of the photosynthetic bacterial reaction center. (D is the excitation and charge separation. the electron transfer to the (secondary) acceptors, and the electron donation by a secondary donor, the cytochrome, to the photooxidized primary donor P. Figure adapted from RK Clayton (1980) Photosynthesis. Physical Mechanism and Chemical Patterns, p 91. Cambridge Univ Press.

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