Steady-state and time-resolved fluorescence behavior of C153 and PRODAN in room-temperature ionic liquids

Room temperature ionic liquids have emerged as nature-friendly media suitable for various applications. In this work, steady-state and time-resolved fluorescence behavior of coumarin 153 (C153) and 6-propionyl-2-dimethylaminonaphthalene (PRODAN) have been reported in two room-temperature ionic liquids based on substituted imidazolium salts. The steady-state fluorescence behavior of the two probe molecules suggests that these liquids are more polar than acetonitrile. The polarity of these media is found to be comparable to that of 2-propanol. No significant difference in the polarity of the two media could be observed although the cationic components of the two liquids contain alkyl substitutents of different chain length. Both the probe molecules display a wavelength-dependent fluorescence decay behavior. The decay profile at the red edge of the spectrum consists of an initial rise followed by a slow decay, while no rise could be observed at the blue edge of the spectrum. The solvation dynamics of the two probe molecules in the two room-temperature ionic liquids have been studied from the time-dependence of the spectral shift function constructed from the wavelength-dependent fluorescence decay profiles. The solvation of the probe molecules occurs on two different time scales with the absolute values strongly dependent on the cationic component of the ionic liquids. The average solvation time is found to be fairly long and dependent on the probe molecule used.