Spectroscopy and photophysics of alloxazines studied in their ground and first excited singlet states

The acid–base properties of alloxazine (All) and its methyl derivatives have been studied in their ground and first excited singlet states. The concept of an effective electronic valence potential was applied to predict the changes in basicity and acidity of heteroatoms upon excitation and substitution. Changes in the acid–base properties of N(1) and N(10) nitrogen atoms are particularly important from the point of view of the excited state proton transfer in alloxazines from N(1) to N(10) to form isoalloxazinic structures. A good linear correlation was obtained between the calculated electronic potentials of N(1) and N(3) nitrogen atoms and the experimental pKa values for ground and excited state deprotonation.