Characteristics of visible fluorescence from ionic liquids

Area of Science:

• Physical Chemistry
• Materials Science
• Spectroscopy

Background:

• Imidazolium-based ionic liquids exhibit unique fluorescence properties, with excitation-dependent spectral shifts.
• Previous work suggested aggregate structures in bulk ionic liquids explain these phenomena.
• The specific origin of fluorescence and its dependence on excitation wavelength remained unclear.

Purpose of the Study:

• To investigate the origin of fluorescence in 1-butyl-3-methylimidazolium tetrafluoroborate ([C4MIM][BF4]).
• To differentiate between molecular and aggregate contributions to the observed fluorescence spectrum.
• To elucidate the role of aggregate structures in the excitation-dependent fluorescence shifts.

Main Methods:

• 2D-scan fluorescence spectroscopy to resolve spectral components.
• Dilution experiments in aqueous mixtures to assess fluorescence intensity changes.
• Fluorescence Correlation Spectroscopy (FCS) to determine the number density of fluorescent species.

Main Results:

• Two fluorescence components, long- and short-wavelength, were identified in [C4MIM][BF4].
• Only the long-wavelength component exhibited properties consistent with previous observations, including large spectral shifts.
• The long-wavelength fluorescence intensity decreased significantly upon dilution, unlike the short-wavelength component.
• FCS confirmed that the long-wavelength fluorescent species exist as aggregates.

Conclusions:

• The observed excitation-dependent fluorescence in [C4MIM][BF4] originates from molecular aggregates.
• The short-wavelength fluorescence likely arises from individual molecules.
• Aggregate formation is crucial for understanding the unique photophysical properties of ionic liquids.