Flavonols as metal-ion chelators: complex formation with Mg2+ and Ba2+ cations in the excited state

The excited state complexation of flavonol derivatives with alkali and alkali-earth metal ions were investigated. It was found that in the excited state chelating magnesium complexes are more stable than in the ground state, and they are produced both from normal and from phototautomer forms of flavonols, The 'external' barium complexes with flavonols containing electron donor groups in 4'-position behave similarly in the excited state. The 'external' barium complexes of unsubstituted and 4'-methoxy substituted flavonols dissociate in the excited state. Their disruption occurs as a result of the excited state intramolecular proton transfer (ESIPT) in flavonol molecules.