Spectroscopic Evidence for Unusual Microviscosity in Imidazolium Ionic Liquid and Tetraethylene Glycol Dimethyl Ether Cosolvent Mixtures

The rotational dynamics of coumarin 153 (C153) in imidazolium-based ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and tetraethylene glycol dimethyl ether (TEGDME) mixtures across all mole fractions have been investigated to determine the local viscosity of the microenvironment surrounding the probe molecules. The excimer-to-monomer fluorescence emission intensity ratio (I-E/I-M) of a well-known microviscosity probe, 1,3-bis(1-pyrenyl)propane (BPP), is also employed to study the microviscosity of the mixtures as a complementary measurement. The rotational dynamics of C153 show that there are incompact and compact domains within the heterogeneous structural [bmim][PF6], resulting in fast and slow components of C153 rotational dynamics. The microviscosity in different structural domains of [bmim][PF6] before and after adding cosolvent TEGDME with different mole fractions is further investigated by studying the fluorescence anisotropy decay of probe molecules. The obtained average rotation time constants show that the microviscosity of [bmim] [PF6] is enhanced after mixing with a certain amount of TEGDME, although the bulk viscosity of TEGDME itself is much lower than that of the ionic liquid. This unusual behavior of microviscosity enhancement is further proven by the steady state fluorescence measurement with the microviscosity probe of BPP. The microviscosity enhancement is reasonably demonstrated by the fast time constant of C153 rotational dynamics and the departure between the experimentally observed and calculated ratio of I-E/I-M of BPP, which shows that this effect is most pronounced at intermediate mole fractions of the [bmim][PF6] and TEGDME mixtures. The strengthening effects caused by the molecular interactions between TEGDME and structural heterogeneous ionic liquid [bmim][PF6] are proposed to interpret the unusual microviscosity behaviors.