Quenching efficiency, singlet-oxygen

Lebedkin, S., Kareev, I., Hennrich, F. and Kappes, M.M. (2008) Efficient quenching of singlet oxygen via energy transfer to semiconducting single-walled carbon nanotubes. Journal of Physical Chemistry C, 112 (42), 16236-16239. 

Competitive [4 + 2] and ene reactions of 2,4-dimethylpenta-1,3-diene (552) were suggested to occur by either a concerted or a perepoxide intermediate pathway (Scheme 6.265).1460 The major [4 + 2] product 553 is formed in nonpolar solvents nearly exclusively, whereas formation of the ene product (554) via a polar perepoxide intermediate 555 is enhanced in polar solvents, such as methanol. The less stable. v-cw-conformation of 552 is required for the concerted process. Another competing process, physical quenching of singlet oxygen, was found to be at least as efficient as the chemical processes. 

Various compounds have been found to function as stabilizers, /(-carotene -allegedly because of its quenching of singlet oxygen, ZnO and carbon black, 2,6-di-(2,2-dimethylethyl)-4-methyl phenol, and a flame retardant-decabromo-diphenyl ether.Two publications have appeared which report results for chemically bonded stabilizers. In both cases it was concluded that such a condition did not significantly improve the efficiency of stabilization. 

Di Mascio, R, Kaiser, S., and Sies, H., Lycopene as the most efficient biological carotenoid singlet oxygen quencher. Arch. Biochem. Biophys., 274, 532, 1989. Cantrell, A. et ah. Singlet oxygen quenching by dietary carotenoids in a model membrane environment. Arch. Biochem. Biophys., 412, 47, 2003. 

Moreover, carotenoids may quench electronically excited states and scavenge free radicals formed in the retina, and therefore protect biomolecules from oxidative damage. Due to the low energy level of the first excited triplet state ( Car), carotenoids (Car) can act as efficient acceptors of triplet state energy from photosensitizers (S) (Equation 15.1), such as all-tra .s-retinal, the photosensitizers of lipofuscin (Rozanowska et al., 1998), or singlet oxygen C02) (Equation 15.2) (Cantrell et al., 2003) ... 

A chemical actinometer especially designed for intensity measurements on high power lasers in the 330—520 nm range has been described.179 It consists of a closed 02-filled system containing methanolic [Ru(bipy)3]2+ and tetramethylethylene (TME). Photoexcited [Ru(bipy)3]2+ is quenched efficiently by dissolved Oz to generate singlet oxygen which then reacts with TME to form a nonvolatile hydroperoxide. The laser intensity is determined from the rate of 02 consumption monitored on a gas buret. 

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