Ultrafast excited state dynamics of two non-emissive flavonoids that undergo excited state intramolecular proton transfer in solution

Flavonoids have been proposed to be natural sunscreens that can provide protection against UV-induced damage in plant. In this study, excited state dynamics of luteolin and quercetin, two natural flavonoids with very low fluorescence quantum yields, were studied by combination of femtosecond transient absorption spectroscopy and quantum chemical calculations. We provide the first experimental demonstration of ultrafast ESIPT process in these two non-emissive flavonoids and proposed that rotation between B- and C-ring after ESIPT is the key to the ultrafast non-radiation decay channels in these two molecules, suggesting that they may indeed behave as natural sunscreens.