Electron Transfer-Sensitized Photo-oxidation

As mentioned earlier in the discussion of exciplex formation, electron transfer between an excited state species and a ground state molecule (Equation 2.8 and Equation 2.9) is frequently observed in the photochemistry of systems containing an electron donor-acceptor combination. As a result, a pair of radical ions is formed that react with oxygen but with different rates. The reaction of ground state oxygen with radical anions occurs rapidly and yields superoxide anion (Equation 2.16). The superoxide then adds to the radical cation forming D02 (Equation 2.17). When D is an olefin, D02 is a dioxetan that is liable to cleave to yield ketones as products. 

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Figure 1. Schematic representation of electron transfer sensitization. 1 photo-oxidation of sensitizer 2 forward electron transfer (fluorescence quenching) 3 back electron transfer 4 product formation...

An electron transfer mechanism has been proposed to account for the formation of carbon dioxide on irradiation of alkyl pyruvates the yield of carbon dioxide is enhanced by the presence of electron acceptors such as methyl viologen. The dye-sensitized photo-oxidation of a-oxo-carboxylic acids and esters also leads to the production of carbon dioxide. An initial dye-substrate interaction rather than singlet oxygen appears to be responsible for this fragmentation. 

Electron or hydrogen transfer between a substrate and sensitizer is often responsible for initiation of a type II/ivRH photo-oxidation. Consequently, the type II/ivRH reaction can often be suppressed by physically separating (isolating)... 

Instead of MV2+ (in the photo-oxidation of leuco crystal violet (LCV)), a neutral species is sensitized by pyrene containing polymers and the Coulombic effect is not as drastic as in the case of MV2+. As shown in Figure 8, the cationic polymer is more effective than the neutral or anionic polymer. This is attributed to the Coulombic repulsion between LCV- and Py assisted by the cationic environment of the polycation. However, the Coulombic effect occurs only after forward electron transfer. 

The photo-induced electron transfer of l,4-bis(methylene)cyclohexane in acetonitrile-methanol solution with 1,4-dicyanobenzene (DCB) affords two products, both consistent with nucleophilic attack on the radical cation followed by reduction and protonation or by combination with DCB ).63 In the absence of a nucleophile, the product mixture is highly complex, as is the case under electro-oxidative conditions. Under UV irradiation, /nmv-stilbene undergoes dimerization and oxygenation (to benzaldehyde) by a single-electron mechanism in the presence of a sensitizer such as 2,4,6-triphenylpyrilium tetrafluoroborate (TPT).64 This reaction was found to yield a similar product mixture with the sulfur analogue of TPT and their relative merits as well as electrochemical and photophysical properties are discussed. 

The photo-oxidation of PE sensitized by DC A in homogeneous solution followed by reduction of the reaction mixture with sodium sulfite solution gave the ene product pinocarveol 14 and the non-ene products myrtenal 15, epoxide 16 and aldehyde 17, as shown in Fig. 21. The ene product and the non-ene products have been proposed to be derived from the energy-transfer and electron-transfer pathways, respectively [177-181], The product distributions in acetonitrile is given in Fig. 21. 

Although photoelectrochemistry has been known as a field for over thirty years, its full impact on organic synthesis has yet to be revealed. This article has dealt with a variety of examples that show how chemical conversions can be induced by photo-electrochemical activation of light-sensitive semiconductor surfaces. Photoexcitation causes the promotion of an electron from the valence band to the conduction band, thus producing a surface-confined electron-hole pair. The charges represented by this pair are then trapped by interfacial electron transfer. The oxidized and reduced... 

The possibility that an electron-transfer path is involved in photo-sensitized oxygenation has been considered on several occasions. This is relevant in several fields of application, from the biomimetic oxygenation of indole and flavin derivatives [106] to pollutant control. With reference to latter, it has been suggested that SET occurs in heterogeneous photosensitized oxidation by solid semiconductors, in which the adsorbed substrate donates an electron to the photogenerated hole and... 

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