Site selectivity in excited-state intramolecular proton transfer in flavonols

Abstract

To investigate 4′-(diethylamino) (FET) and 4′- N-(15-azacrown-5) (FCR) derivatives of 3-hydroxyflavone in binary solvents and erythrocyte ghosts, we used the red-edge excitation spectroscopy (REES). The results obtained prove the existence of spectral heterogeneity of flavonols in the studied systems. The effect manifests itself in the dependence of the efficiency of excited-state intramolecular proton transfer (ESIPT) on the excitation frequency. The electro-optical absorption method (EOAM) was used to measure the dipole moments of the normal form of FET. The electric dipole moments in the ground ( μ g) and excited Franck–Condon ( μ e FC ) states have the values 22.7×10 −30 and 53.3×10 −30 C m , respectively. On optical excitation, the electric dipole moment increases by 34×10 −30 C m , and the angle between μ g and μ e FC is 25°. The results of the electro-optical and spectroscopic measurements enable us to describe more precisely the process of charge and proton transfer in 4′-amino-3-hydroxyflavones. Charge transfer and proton transfer occur alternately. The main stage of forward electron charge transfer takes place after excitation of the normal form (N ∗) and partly after ESIPT. In the phototautomer (T), only partial reverse charge transfer happens after photon emission. The second, more efficient stage of reverse electron charge transfer occurs after radiationless conversion of phototautomer into the normal form.