Oxygen quenching of singlet excited states

Oxygen Quenching of Singlet Excited States OF Pyrene and DPA [27]... 

As can be seen in these reactions, carotenoids may protect photosynthetic bacteria at various levels by quenching the singlet-excited state of O2 or the tiiplet-excited state of chlorophyll. The ground states of oxygen would be 3O2 and for CHL the triplet state. The carotenoids may be the preferred substrates for oxidation or may act in quenching reactive species. ... 

It has been found that oxygen efficiently quenches the fluorescence of many organic molecules [Refs. 60, 68, 81, 92, 486, 534]. The following energetically allowed mechanisms for oxygen quenching of molecules excited to the singlet state can be written as... 

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. 

The unusually intense fluorescence of trithiophene derivatives has led to one patent on the use of such compounds as optical brighteners of fabrics 10). The lowest excited state of T1 and three derivatives must have n-n character, as their absorption maxima and fluorescence quantum yields are very similar in solvent systems of very different polarity (204). Oxygen does not quench the singlet excited state of T1 (209), which has a lifetime shorter than 1 x 10 s (104) and a quantum yield of fluorescence of ca. 0.06-0.08 (204,211). A more complete analysis of the fluorescence of T1 has been carried out in n-decane down to 4.2 K (27). 

Kawaoka, K., a. U. Khan, and D. R. Kearns Role of singlet excited states of molecular oxygen in the quenching of organic triplet states. J. Chem. Phys. 46, 1842 (1967). 

Photoreduction of m-dinitrobenzene is suppressed by nitric oxide and atmospheric oxygen Quenching by naphthalene, which also has been observed, does not necessarily imply a triplet excited reacting state, since quenching of the excited singlet by naphthalene would be an alternative possibility. 

The transient absorption formed from laser flash photolysis of 15 was not quenched in oxygen-saturated solutions, presumably because the photoenolization to form 16 takes place from a singlet excited state of 15. Furthermore, Schworer and Wirz did not observe any transient absorption that can be attributed to the formation of the triplet excited state of 15 from direct irradiation. In contrast. Hurley and Testa used energy transfer to estimate that the triplet excited state of 15 is formed in 67% yield,whereas Takezaki et al. have measured the yields for forming the triplet excited state of 15 to be slightly higher or above 80%. They estimated the lifetime of the lowest triplet excited state of 15 to be 350 ps in ethanol. 

B. Quenching of fluorescence by a08. The oxygen quenching of fluorescence of aromatic hydrocarbons both in solution and in vapour phase is in general diffusion-controlled. 1 he rate constants for the 08-qucnching of excited singlet and triplet states are given in Table 8.2... 

Most photodecarbonylation reactions of cyclic ketones, especially in the vapor phase, have been postulated to proceed from various vibrational levels of excited singlet states.321 However, the elimination reaction leading to unsaturated aldehydes has now been shown to occur largely via excited triplet states. In solution, where the lowest vibrational levels of the excited states are rapidly reached, to-alkenals are the major products observed in both photolysis and radiolysis of cyclopentanone and cyclohexanone. The reaction is quenched by oxygen and dienes,322-324 as well as by the alkenal produced in the reaction.325 The reaction is also sensitized by benzene triplets.322,323 With cyclopentanone, quenching by 1M piperylene occurs some 20 times as fast... 

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