Oxygen, triplet state quencher

In this example, the triplet state of benzophenone is quenched by the added PAH. However oxygen is also known to be an effective triplet state quencher ... 

Since the first two processes are spin-forbidden, it can clearly be seen that in the absence of triplet quenchers (e.g., oxygen) the triplet will be long lived. Consequently the experimental determination of the lifetime of triplet states... 

In the first place, the apparent insensitivity of many (perhaps most) of the photolyses to oxygen would seem to limit consideration of intermediates to those species known to be insensitive to oxygen singlet states (whose lifetimes are short, about 10 9 sec, and whose steady-state concentrations are very low), or to other reactions which are too fast to be affected by oxygen, such as electron-transfer reactions. The reason for this conclusion is that oxygen is known to be a very efficient quencher for the longer-lived triplet states, and most reactions of triplet... 

Paramagnetic molecules K)a and 2NO are very efficient quenchers of singlet and triplet states. Oxygen may form CT complex and lead to peroxide formation in the triplet state. Paramagnetic ions of the transition series and lanthanides also quench the triplet states. 

Based on steady-state and time-resolved emission studies, Scaiano and coworkers have concluded that silicalite (a pentasil zeolite) provides at least two types of sites for guest molecules [234-236], The triplet states of several arylalkyl ketones and diaryl ketones (benzophenone, xanthone, and benzil) have been used as probes. Phosphorescence from each molecule included in silicalite was observed. With the help of time-resolved diffuse reflectance spectroscopy, it has been possible to show that these triplet decays follow complex kinetics and extend over long periods of time. Experiments with benzophenone and arylalkyl ketones demonstrate that some sites are more easily accessed by the small quencher molecule oxygen. Also, diffuse reflectance studies in Na + -X showed that diphenylmethyl radicals in various sites decay over time periods differing by seven orders of magnitude (t varies between 20/is and 30 min) [237]. 

Several flavonoids have given mixtures of products on irradiation for example, the 3-phenylflavone (380) in methanol is converted into the isocoumarins (381) and (382) the latter is the major product and is derived from the former (75CC241). Quercitin pentamethyl ether (383) in methanol and in the absence of oxygen yields four tetracyclic products. In deoxygenated benzene, the reaction rate is doubled but only (384) and (385) are formed. This photocyclization is not inhibited by triplet quenchers and the primary photointermediate is in the triplet state (67JA6213). 

Reaction in presence of oxygen The oxidation reaction proceeds from, the triplet state- of tli3 molecule as already postulated. Oxygen is kn to be a powerful quencher of fluorescence but for solutions in CO , except lot 9 ,/.l -diciiioro compouiio, me quenemug is practically coin... ... 

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