Triplet states, acid-base properties

One common complication in flash photolysis studies is the production from the excited state of radicals which may be confused with or inhibit observation of the triplet state. The radical species, which usually has a longer lifetime than that of the triplet state, may be identified by separate experiments (Jackson and Porter, 1961) but such species may themselves show interesting acid base properties (e.g. Lindqvist, 1960 Lindqvist and Kasche, 1965 Simic and Hoffman, 1972 Neta, 1975). Their optical density contributions may therefore change with acidity in the range studied. 

Because excited triplet states are less likely to have their unpaired electrons on the same atoms than are singlet states (see reference 24), the triplet states tend to have biradicaloid character. For a more detailed discussion of the acid-base properties of photoexcited organic molecules, including carbon acids and carbon bases, see Wan, P. Shukla, D. Chem. Rev. 1993, 93, 571. 

Table 14 also shows that triplet states have acid-base properties very similar to those of the ground state. This can be rationalized if we remember that triplet states can be pictured as having unpaired electrons... 

A laser study of the prototropic equilibrium of triplet benzophenone (BP) has been reported [111]. Acid-base properties in the triplet state of aromatic ketones in HjO-CHjCN (4 1) mixtures have been studied by means of nanosecond laser flash photolysis. The acidity constants p /T) in the triplet state are determined by means of the T T, absorbance titration curve, the Ware plot, and the Rayner-Wyatt plot, whose values agree well among them, showing that the acid-base equilibrium in the... 

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