Behavior of the solvatochromic probes Reichardt’s dye, pyrene, dansylamide, Nile Red and 1-pyrenecarbaldehyde within the room-temperature ionic liquid bmimPF6

Abstract

Recently discovered room-temperature ionic liquids (RTILs) show tremendous promise to replace volatile organic compounds (VOC). Investigation of these RTILs as solvents is in very early stages. Before the full potential of these RTILs are realized, much more information about them as solvent systems must be obtained. The dipolarity of one such RTIL, 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF₆) is investigated using both absorbance (Reichardt’s betaine dye) and fluorescence (pyrene, dansylamide, Nile Red, and 1-pyrenecarbaldehyde) solvatochromic probes. Results indicate that, in the case of pyrene and 1-pyrenecarbaldehyde the bmimPF₆ microenvironment immediately surrounding the probe is similar to the microenvironment sensed in acetonitrile and dimethyl sulfoxide. Dansylamide in bmimPF₆ sensed a microenvironment similar to that in acetonitrile. However, calculated E_T(30) values indicate the polarity of bmimPF₆ sensed by Reichardt’s betaine dye to be similar to ethanol. Nile Red showed that the polarity of the solvent in the immediate vicinity of the probe is similar to neat water and 90 wt% glycerol in water (a solvent with viscosity similar to bmimPF₆). The microenvironment sensed by a probe is dependent upon several factors besides polarity such as viscosity, polarizability, the ability to form hydrogen-bonds, etc. In light of this, the apparent discrepancies in the polarity of bmimPF₆ indicated in this study do not seem so vast.