Excited-State Intramolecular Proton Transfer of the Natural Product Quercetin.

Intramolecular proton-transfer dynamics in the lowest excited state (ESIHT) were studied in the natural product quercetin. We found that in all seven solvents used in this study, the ESIHT rate is ultrafast. We estimate that the ESIHT rate is about 70 fs or less. We found that in deuterated protic solvents, such as methanol-d or ethanol-d, the ESIHT rate is slower and the proton-transfer time constant is about 110 fs. The tautomeric form fluorescence quantum yield of quercetin is very low, of the order of the normal form.