Acidity and Basicity in Excited States

The molecule A-H has an acidic proton as well as a r system that may absorb UV-vis radiation and remain intact after the deprotonation reaction. The 

A novel properly of an electronically excited molecule can be chirality that exists only because of photoexcitation. Miesen, F. W. A. M. Wollersheim, A. P. P. Meskers, S. C. J. Dekkers, H. P. J. M- Meijer, E. W. /. Am. Chem. Soc. 1994,116,5129 reported the synthesis of optically pure 3-( n,Ji )-(lS,6R)-bicyclo[4.4.0]decane-3,8-dione, which is chiral only in the excited state. The chirality was detected in the circular polarization of chemiluminescence associated with its synthesis from an optically active 1,2-dioxetane precursor. 

Fluorescence spectra of 2-naphthol in (A) 0.1 M HCIO4 and (B) 0.1 M NaOH solution. (Reproduced from reference 75.) 

The energy difference between the ground state and excited state of A-H can be determined from the 0,0 band of the electronic transition. Similarly, the energy difference between the grotmd and excited state of A can be deter- 

For a more detailed definition of these terms, see reference 1 Becker, H.-D. Pure Appl. Chem. 1982, 54,1589 Forster, T. Pure Appl. Chem. 1970, 24,443. 

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Among the earliest observations of this phenomenon was the demonstration by Forster [450]—following up an earlier observation by Weber—that acidic aqueous solutions of the dye fluorescein, which exhibit just a single species (the cation) in absorption, showed fluorescence from both the cation and an additional, dissociated species. It was concluded that the excited fluorescein cation was more acidic than its ground-state counterpart. Later work supported this conclusion and supplied additional examples of altered acidities and basicities in excited states [451-453]. 

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