Dynamics of Solvation of the Fluorescent State of Some Electron Donor - Acceptor Molecules in Room Temperature Ionic Liquids, [BMIM](CF < inf > 3 < /inf > SO < inf > 2 < /inf > ) < inf > 2 < /inf > N] and [EMIM][(CF < inf > 3 < /inf > SO < inf > 2 < /inf > ) < inf > 2 < /inf > N]

Abstract

Room temperature ionic liquids, particularly those based on the imidazolium salts, have emerged in recent years as highly promising "green" media suitable for carrying out various reactions. In this paper, picosecond time-resolved fluorescence decay behavior of three electron donor - acceptor (EDA) molecules, coumarin 153 (C153), 6-propionyl-2-dimethylaminonaphthalene (PRODAN), and 4-aminophthalimide (AP), in two room temperature ionic liquids, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([EMIM][(CF3SO2) 2N]) and 1-butyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide ([BMIM][(CF3SO2)2N]), has been reported. The steady-state fluorescence behavior of the EDA molecules suggests that the polarity of the two ionic liquids in the ET(30) scale is around 47.6 for [EMIM][(CF3SO2)2N] and 47.1 for [BMIM][(CF3SO2)2N]. The fluorescence decay behavior of the systems in these media has been found to be dependent on the monitoring wavelength, suggesting solvent-induced relaxation of the fluorescent state of the systems in the picosecond time domain. Time-resolved fluorescence spectra of the systems, constructed from the wavelength-dependent decay profiles, show a Stokes shift of the fluorescence maxima with time. The time dependence of the spectral shift of the systems has been quantitatively analyzed to obtain information on the solvation dynamics in these media. The solvation dynamics has been found to be biphasic consisting of a short (∼0. 2 ns) and a long component (∼ 1.0-1.2 ns). The short component has been assigned to the diffusional motion of the anion, while the long component is attributed to the collective motion of the anion and the cation. The average solvation time in the present ionic liquids is found to be faster than that in [BF4]--based ionic liquids.