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 Me2N- and Et2N-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 N2, the study detected the “locally excited” state at λem ≈ 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 λem ≈ 510 nm, which was attributed to the normal form N* of flavonoid. Emission from tautomeric form T* (λem ≈ 575 nm) could only be observed at a higher temperature, when ESIPT became allowed. With the aid of a model compound (without R2N-substituent) and computational study, the study led to improved understanding on the photophysical properties of flavonoid materials in general.