Singlet oxygen by energy transfer

Several other compounds oeui quench singlet oxygen without chemical reaction,e.g.G-carotene,where only cis-trans isomerization occurs (50,jl) and metal complexes (52-55). It is probable that most of these compound quench singlet oxygen by energy transfer,but electron trcuisfer or oharge-treuisfer complex formation can be taken into consideration (44) ... 

For the past 15 years we and many others have been investigating the application of photosensitization to the treatment of a wide variety of cancers in man. Several reviews of the clinical results have been published [1-4]. While this treatment has been referred to by many descriptions, the most commonly employed is photodynamic therapy (PDT), although the involvement of the type II photodynamic process in vivo, that is, generation of singlet oxygen by energy transfer from the sensitizer to oxygen, is inferred rather than proven (see below). 

It was found that benzaldehyde [355] (its presence in the contaminated atmosphere is a result of the oxidation of abundant aromatic hydrocarbons contained in automobile exhaust) may also participate in the formation of singlet oxygen by the transfer of energy. 

The primary interaction of singlet oxygen, produced by energy transfer from the excited sensitizer, with the diene can give rise to an exciplet that then collapses to peroxide, to a 1,4-biradical or to a 1,4-zwitterion alternatively, the adduct is the result of a concerted action without the involvement of an intermediate. Detailed kinetic Diels-Alder investigations of singlet oxygen and furans indicate that the reactions proceed concertedly but are asynchronous with the involvement of an exciplex as the primary reaction intermediate [63]. 

As for the energy transfer to the subsurface layers of zinc oxide from the singlet oxygen molecules, the transfer should lead to an intn ease in the electrical conductivity of semiconductor either due to ejection of electrons into the conduction band h-om shallow traps [67], or due to the injection of electrons into zinc oxide by excited particles [68]. Effects of this kind were observed in the interaction between a ZnO surface and excited pairs of benzophenone [70], and also in adsorption of singlet oxygen on the surface of ZnO monocrystal in electrolyte [69]. 

Nitroimidazoles (such as metronidazole and misonidazole) can enhance the porphyrin sensitized Type I photooxidations83 that is, electron transfer from the sensitizer to the oxygen molecule is facilitated to give more of the superoxide ion (Scheme 6). The Type II mechanism operates by energy transfer from the sensitizer to afford the singlet oxygen84. 

Whereas several l-aryl-4-nitroimidazoles are found to be good sensitizers for superoxide ion formations85 (Type I photooxidation), only l-phenyl-2-methyl-4-nitroimidazole 140 is a photosensitizer for singlet oxygen, i.e. by energy transfer of type II photooxidation (equation 70). 

Type 1 intrazeolite photooxygenation of alkenes has been also reported to give mainly allylic hydroperoxides (Scheme 42). In this process, the charge transfer band of the alkene—O2 complex within Na-Y was irradiated to form the alkene radical cation and superoxide ion. The radical ion pair in turn gives the allylic hydroperoxides via an allylic radical intermediate. On the other hand, for the Type II pathway, singlet molecular oxygen ( O2) is produced by energy transfer from the triplet excited state of a photosensitizer to 02. 

Excited 0(21D) atoms are metastable and have very long life in the thin upper atmosphere. The atoms are deactivated by energy transfer on Collision with oxygen molecules generating singlet molecular oxygen,... 

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