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Characteristics of visible fluorescence from ionic liquids.

The observation of fluorescence in the visible spectral range in imidazolium-based ionic liquids, in which the peak of the fluorescence spectrum shifts with the change in the excitation wavelength by over 200 nm, was reported by Samanta and co-workers (Paul et al. J. Phys. Chem. B 2005, 109, 9148; Chem. Phys. Lett. 2005, 402, 375), and the aggregate structure in the bulk ionic liquid was suggested to explain this unique phenomenon. In this work, by employing 2D-scan fluorescence spectroscopy, we identified the long- and short-wavelength fluorescence components of the fluorescence spectrum of 1-butyl-3-methylimidazolium tetrafluoroborate ([C4MIM][BF4]), of which only the long-wavelength fluorescence component was found to be responsible for the reported fluorescence properties. The fluorescence intensity of the long-wavelength component decreased much faster upon dilution in aqueous mixtures than the short-wavelength component, supporting the conclusion that the long-wavelength fluorescence is from molecular aggregates in the bulk ionic liquid. Fluorescence correlation spectroscopy (FCS), which was used to accurately account for the number density of the long-wavelength fluorescent species in aqueous solutions of the ionic liquid, also suggested that the fluorescence came from aggregate structures of molecules in ionic liquids.