Singlet states, acid-base properties

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. 

The origins of the enhanced acidity of hydroxyarenes and other photoacids are clearly due to the differences between the quantum-mechanical properties of the first electronic singlet state (the fluorescence emitting state) and the ground electronic state of the photoacid. Aside from the question whether acid or base is more important in determining the Kl of the excited photoacid, one faces a more fundamental question as to why photoacidity occurs at all. To answer this question one should deal with the electronic structure of... 

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