Fluorescence studies in environmentally benign solvents: solvation dynamics of Coumarin 102 in [BMIM][BF₄]

The steady-state and time-resolved fluorescence behavior of Coumarin 102 (C-102) has been investigated in a ionic liquid at room temperature, 1-buty1-3-methylimidazolium tetrafluoroborate, abbreviated here as [BMIM][BF₄]. From the steady-state fluorescence maximum of C-102 in [BMIM][BF₄], the polarity of this ionic liquid has been estimated to be 50.4 in the E_T (30) scale, suggesting that [BMIM][BF₄] is more polar than acetonitrile but less polar than methanol. In contrast to its time-resolved behavior in conventional polar solvents, C-102 exhibits wavelength-dependent fluorescence decay behavior in [BMIM][BF₄] because of slow solvation dynamics in this viscous medium. The solvation dynamics has been found to be biphasic in nature consisting of picosecond and nanosecond components. In addition to these slow resolvable components, evidence is found for a very fast component of solvation, which occurs within the time resolution of the set-up (25 ps).