Acidity and Basicity of Molecules in Excited States

Changes in the electron distribution on excitation are connected with changes in basicity and acidity. This becomes evident if one considers the indicator equilibrium 

When numerical values for constants are inserted, the following relation between the change in the value and the shift Av of the absorption maxi- 

Near 300 nm a spectral shift of 30 nm corresponds approximately to a change in wave number of Av = 3,300 cm. From Equation (1.53) this results in a pK value change of 7 units. Since shifts of this extent are quite common on protonation, changes in dissociation constants of protonated compounds by 6-10 orders of magnitude on excitation are often observed. 

Of special interest are compounds such as 3-hydroxyquinoline (11) for which it is expected that the heterocyclic nitrogen becomes more basic in the excited state whereas the hydroxyl group should become more acidic. 

At the extremes of the acidity range studied, the cation and the anion are the species found in both the ground and S states. At intermediate pH values, the hydroxy tautomer of the neutral molecule is the predominant species in the ground state. On excitation, however, the phenolic group becomes more acidic and the nitrogen more basic. The zwitterion is more stable, and only the fluorescence of this species is observed. 

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