Solvation dynamics of coumarin-153 in a room-temperature ionic liquid

Abstract

Room-temperature ionic liquids are rapidly emerging as a new class of medium that is ideally suited for carrying out chemical reactions and various other applications. In this paper, steady-state and time-resolved fluorescence behavior of coumarin-153 (C153) have been reported in a room-temperature ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate or [BMIM][BF₄]. The polarity of the solvent, as estimated from the fluorescence maximum of C153, is found to be 48.9 on the E_T(30) polarity scale. The fluorescence decay behavior of C153 in [BMIM][BF₄] is found to be dependent on the monitoring wavelength. Although the time dependence of fluorescence in the blue region of the spectrum is represented by a multicomponent decay, that in the red region of the spectrum consists of clear growth followed by decay. The time-dependent evolution of the fluorescence spectrum of C153, constructed from the decay profiles at several wavelengths, indicates biphasic solvation dynamics similar to that observed in the case of the molten salts. In the present case, the solvation dynamics consists of two components with relaxation times of 278 ps and 3.98 ns, suggesting an average solvation time of 2.13 ns. The biphasic solvation dynamics in the room-temperature ionic liquid is proposed to be the result of a relatively fast initial response of the anion and a slow collective diffusion of the cations and the anions.