Oxygen quenching of singlet excited

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

The quenching of singlet excited molecular oxygen ( g) l y ferrocene was also proposed to arise from the external heavy atom effect (258). The bimolecular rate constant for this... 

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... 

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. ... 

In solution photochemistry in the presence of acids, the primary process is also the same except that both NND and the aminyl radical are protonated the recombination of the aminium radical and NO to give 295A is too slow to compete with bond scissions174 (Scheme 11). The failure of oxygen to quench nitrosamine photoreactions in either solution (see below) or gas phases under various conditions must also mean a very short lifetime of singlet excited nitrosamines, in agreement with the fast dissociation159,160. 

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. 

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... 

Oxygen quenching of phenanthrene phosphorescence involves an exchange mechanism which also operates for triplet state energy transfer involving phenanthrene in biphenyl 2i. Radiationless processes of excited states of 5,6-diazaphenanthrene in hexane and hexafluoroisopropanol have been studied by both triplet and singlet lumi ne s cen ce 2 2 extremely fast radiationless transition in the... 

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