离子液体[bmim][PF6]与[moemim][PF6]结构微不均匀性以及微粘度的光谱探测

室温离子液体由于其极低的蒸汽压、比较好的热稳定性和化学稳定性、良好的分子结构与性能的可设计性等优点,作为一种新型的环境友好溶剂在很多领域有着广泛的应用.对于离子液体的微观结构和微观性能的研究是设计新型离子液体以及扩展离子液体应用的关键.本文通过荧光探针分子香豆素153(C153)的转动动力学和对微观环境敏感的荧光探针分子1, 3-二(1-芘基)丙烷(BPP)的稳态荧光光谱,探测和表征了烷基取代的离子液体1-丁基-3-甲基咪唑六氟磷酸盐(bmim]PF₆])和与其具有相似结构的醚键官能化的离子液体1-甲氧基乙基-3-甲基咪唑六氟磷酸盐(moemim]PF₆])的微观结构和微粘度. C153探针分子在离子液体bmim]PF₆]中的转动过程具有快、慢两个组分表明离子液体bmim]PF₆]内部存在松散和紧密的两种结构微区;而C153探针分子在离子液体moemim]PF₆]中的转动动力学只存在一种过程,说明醚键的引入使得moemim]PF₆]内部趋于一种类型的微环境.通过C153探针分子的转动时间研究发现,醚键官能化的离子液体moemim]PF₆]的微粘度小于烷基链取代的离子液体bmim]PF₆],同时通过BPP探针分子的二聚体激基复合物(excimer)与单体(monomer)荧光发射强度的比值(I_E/I_M)研究也证明这一结果.醚键的引入使得离子液体moemim]PF₆]相对于离子液体bmim]PF₆],侧链的极性更大、柔顺性更好,同时醚键有可能作为氢键的受体与阳离子形成氢键从而削弱离子液体中阴、阳离子间的相互作用,使得离子液体moemim]PF₆]的微观环境比离子液体bmim]PF₆]更为均一,同时具有更小的微粘度.

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].