Fluorescence behavior and specific interactions of an ionic liquid in ethylene glycol derivatives

The excitation-wavelength- and concentration-dependent fluorescence response of an ionic liquid (IL), 1-octyl-3-methylimidazolium tetrafluoroborate [Omim][BF(4)], in the ethylene glycol derivatives, [CH(3)(OCH(2)CH(2))n-OH, n = 1-3], has been critically examined in the entire composition range. The pure IL exhibited a large red shift beyond an excitation wavelength of 390 nm, showing the heterogeneous nature of the liquid. Concentration dependence of the fluorescence spectra in the organic solvent-rich region favors the association of IL molecules into the aggregated structures. The maximum of the fluorescence spectra shifted toward blue with the increase of organic component concentration in all of the mixtures, reflecting an appreciable solute-solvent interaction. Very high concentrations of the organic liquid in the mixtures resulted in the inversion of the spectral shift toward red, indicating the dominance of pi-pi* transitions over the n-pi* transitions as a consequence of imidazolium ring stacking. (1)H NMR and FT-IR investigations over the whole composition range of the mixtures showed multiple hydrogen-bonded interactions of varying strengths between the unlike molecules and the existence of associated species of the IL in the dilute region. Both the specific interactions between unlike molecules and the tendency of aggregation of IL molecules in the dilute region reduced with the introduction of the -OC(2)H(4) group in the glycol derivative. A comparison of specific interactions with the volumetric properties of the similar mixtures shows that the packing efficiency depending on differences in the shape and size of the molecules mainly decides the overall magnitude of deviations from ideality.