Temperature-dependent ratiometric fluorescence from an organic aggregates system

The aggregates of 2-(2'-hydroxyphenyl) benzoxazole (HBO), a typical molecule exhibiting excited-state intramolecular proton transfer (ESIPT), were prepared and the photophysical properties of the aqueous dispersion of aggregates were investigated. It is found that the aggregates and the solvated enols coexist in the aqueous dispersion system. Furthermore, the aggregates undergo ESIPT to give rise to keto for green emission, while the solvated enols give rise to blue emission. The temperature effects on the aqueous dispersion of the HBO aggregates system were also explored. It shows a fluorescent ratiometric change in a range of temperature from 15 to 60 degrees C. A mechanism of a temperature-dependent equilibrium between the aggregates and the solvated enols is proposed for the fluorescence change. The reversibility and robustness as well as the stability of the aqueous dispersion of aggregates show very good performances, which may be useful in the applications of molecular fluorescent temperature sensors or molecular thermometers.