The microheterogeneous structure of ionic liquid mixtures with organic solvent determined by a cyanine-dye fluorescent probe

[Display omitted] ► A cyanine-dye probe was used in studying mixtures of DMSO with ionic liquids. ► A significant distinction between the mixtures was found in the fluorescence-decay. ► The existence of spatial heterogeneities in ionic liquid/DMSO mixtures was proposed. The photophysics of 3,3′-diethylthiacarbocyanine iodide (DTCI), as a fluorescence probe, in liquid mixtures of dimethyl sulfoxide (DMSO) with either trimethyl(butyl)ammonium bis((trifluoromethyl)sulfonyl)imide ([N(4)111] [Tf2N]) or 1-butyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide ([bmim] [Tf2N]) was studied by means of steady-state and time resolved fluorescence spectroscopy. The DTCI fluorescence decay was found to be double-exponential in mixtures with [N(4)111] [Tf2N] for DMSO volume fractions less than ca. 0.6 while that is single-exponential in mixtures with [bmim] [Tf2N] under the same conditions. These findings are explained in terms of the Hildebrand solubility model by assuming the existence of coursed-grained spatial micro-heterogeneities in ammonium-based ionic liquid/DMSO mixtures.