Spectroscopic Study of the Structural Heterogeneity and Microviscosity of [bmim][PF6] and [moemim][PF6] Ionic Liquids

Room temperature ionic liquids (RTILs), which have several special properties such as negligible vapor pressure, high thermal and chemical stability, good molecular structure and property designability, have received a great deal of attention, and have emerged as potential environmentally benign solvents. Therefore, a deep understanding of the solvent properties of RTILs, especially the microenvironment properties, is crucial to design new RTILs and extend their applications. The structural heterogeneities and local viscosities of the microenvironments of the ionic liquid [bmim][PF6] and the ether-functionalized ionic liquid [moemim][PF6] were investigated by the rotational dynamics of coumarin 153 (C153) and the excimer-to-monomer fluorescence emission intensity ratio (I-E/I-M) of 1,3-bis(1-pyrenyl)propane (BPP). The rotational dynamics of C153 shows that there are incompact and compact domains within the heterogeneous structure of [bmim][PF6], resulting in fast and slow components of C153 rotational dynamics. The rotational dynamics of C153 shows that there is mainly one type of microenvironment in [moemim][PF6]. The C153 rotation time constants show that the microviscosity of [moemim][PF6] is lower than that of [bmim][PF6], and this result is confirmed by steady-state fluorescence measurement with the BPP microviscosity probe. The side chain of [moemim][PF6] is more polar and more flexible than that of [bmim][PF6], and the oxygen of the ether group could act as a hydrogen bond acceptor and interact with the cations of the ionic liquid, which possibly reduces the electrostatic attraction between the cations and anions in the ionic liquid and leads to the lower structural heterogeneity and microviscosity of [moemim][PF6].