Solvent- and concentration-sensitive fluorescence of 2-(3,4,5,6-tetrafluoro-2-hydroxyphenyl)benzoxazole

2-(3,4,5,6-Tetrafluoro-2-hydroxyphenyl)benzoxazole (2) emits the long wavel ength fluorescence around 500 nm in nonpolar solvent via the intramolecular proton transfer process in the excited state of 2 (enol-form) and also emi ts the intermediate wavelength fluorescence around 440 nm in polar solvent, which is assumed to originate from the excited state of 2 (anion). The eas e of formation of 2 (anion), compared to 2-(2-hydroxyphenyl)benzoxazole (1) , is explained by the strongly inductive fluorine atoms. In a solvent with the intermediate polarity, 2 emits both fluorescences and their relative in tensity is dependent on the concentration of 2, which is supposed to be cau sed by the high sensitivity of the intermediate wavelength emission to the concentration quenching.