Excited-State Intramolecular Proton Transfer (ESIPT) of Fluorescent Flavonoid Dyes: A Close Look by Low Temperature Fluorescence.

Flavonoids have emerged to be an important molecular frame for chemical sensors, due to their capability to give dual emission and their compatibility to biological cells. Two flavonoid compounds with MeN- and EtN-substituents were examined by acquiring their fluorescence at different temperatures, in order to evaluate the impact of intramolecular charge transfer (ICT) on ESIPT. By freezing the sample solution in liquid N, the study detected the "locally excited" state at λ ≈ 460 nm, in which both ICT and ESIPT processes were not present. As temperature was warmed up (to about -90 °C), ICT process became gradually allowed and emission was shifted to λ ≈ 510 nm, which was attributed to the normal form N* of flavonoid. Emission from tautomeric form T* (λ ≈ 575 nm) could only be observed at a higher temperature, when ESIPT became allowed. With the aid of a model compound (without RN-substituent) and computational study, the study led to improved understanding on the photophysical properties of flavonoid materials in general.