Singlet oxygen quenching mechanism

Singlet Oxygen Quenching Mechanisms of Dithiolene Nickel Complexes in Solution... 

The ability of carotenoids to act as antioxidants is closely related to their long-chain conjugated polyene structures (see Section 2.2 in Chapter 2). Two main types of antioxidant actions can be distinguished singlet oxygen quenching and reactions with radicals. The first mechanism occurs in vivo in plants and has been extensively studied in vitro. Reactions with radicals of different types have also been extensively studied in vitro under different conditions but their occurrence in vivo is still a matter of discussion. 

Various aspects of the mechanistic efficiency of the hindered amines are discussed. Quenching of carbonyl excited states and UV absorption are not considered important modes of activity. Some effectiveness in singlet oxygen quenching and radical scavenging has been shown. Speculations on mechanistic efficiency include hydroperoxide decomposition and activity during processing to reduce sensitizer concentration. A comprehensive mechanism is not yet available. 

Finally, one unusual case of 3-Q2 quenching should be mentioned. In some studies It was shown that molecular oxygen can enhance the fluorescence of aromatic hydrocarbons In polymer matrices (63, 306-307). Bolton et al. (63) proposed a singlet oxygen feedback mechanism to account for this phenomenon In systems under conditions of steady-state excitation. In this mechanism the enhanced fluorescence Is attributed to a singlet oxygen-triplet organic molecule annihilation fluorescence ... 

Carotenoid chemical structure is usually not affected by the physical quenching. Another mechanism can occur in which a carotenoid chemically quenches singlet oxygen and is thus transformed in derived products. ... 

Farmillo, A. and F. Wilkinson. 1973. On the mechanism of quenching of singlet oxygen in solution. Photochem. Photobiol. 18 447-450. 

Foote, C. S., Y. C. Chang, and R. W. Denny. 1970. Chemistry of singlet oxygen. XI. Cis-trans isomerization of carotenoids by singlet oxygen and a probable quenching mechanism. J. Am. Chem. Soc. 92 5218-5219. 

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