Internal Stark effect mediates intramolecular excited-state proton transfer in 3-hydroxyflavone derivatives

Internal Stark effect in electronic spectra is the effect that is observed when the electronic bands shift under the influence of proximal charges. In order to study the possible involvement of this effect in modulating the intramolecular proton transfer reactions in the excited state, we designed and studied several derivatives of 3-hydroxyflavone. They include the species containing neutral and positively charged substituents in 6 position of chromone ring. These compounds were studied in solvents of different polarities. In these experiments the shifts of both normal and tautomer fluorescence bands are clearly observed in a manner predicted by Stark effect theory. In addition, a dramatic effect of suppression by introduced charge of intramolecular excited-state proton transfer was observed.