Prabhu Sugosh R, Dutt G B
Radiation & Photochemistry Division, Bhabha Atomic Research Centre , Trombay, Mumbai 400 085, India.
J Phys Chem B. 2014 Nov 20;118(46):13244-51. doi: 10.1021/jp5079778. Epub 2014 Nov 5.
The rotational dynamics of 1-alkyl-3-methylimidazolium-based ionic liquids has been investigated by monitoring their inherent fluorescence with the intent to unravel the characteristics of the emitting species. For this purpose, temperature-dependent fluorescence anisotropies of 1-alkyl-3-methylimidazolium (alkyl = ethyl and hexyl) ionic liquids with anions such as tris(pentafluoroethyl)trifluorophosphate ([FAP]), bis(trifluoromethylsulfonyl)imide ([Tf2N]), tetrafluoroborate ([BF4]), and hexafluorophosphate ([PF6]) have been measured. It has been observed that the reorientation times (τr) of the ionic liquids with an ethyl chain scale linearly with viscosity and were found to be independent of the nature of the anion. The experimentally measured τr values are a factor of 3 longer than the ones calculated for 1-ethyl-3-methylimidazolium cation using the Stokes-Einstein-Debye (SED) hydrodynamic theory with stick boundary condition, which suggests that the emitting species is not the imidazolium moiety but some kind of associated species. The reorientation times of ionic liquids with a hexyl chain, in contrast, follow the trend τr([FAP]) > τr([Tf2N]) = τr([BF4]) > τr([PF6]) at a given viscosity (η) and temperature (T). The ability of the ionic liquids with longer alkyl chains to form the organized structure appears to be responsible for the observed behavior considering the fact that significant deviations from linearity have been noticed in the τr versus η/T plots for strongly associating anions [BF4] and [PF6], especially at ambient temperatures.
通过监测1-烷基-3-甲基咪唑鎓基离子液体的固有荧光来研究其旋转动力学，目的是揭示发光物种的特性。为此，测量了1-烷基-3-甲基咪唑鎓（烷基=乙基和己基）离子液体与三（五氟乙基）三氟磷酸根（[FAP]）、双（三氟甲基磺酰）亚胺（[Tf2N]）、四氟硼酸根（[BF4]）和六氟磷酸根（[PF6]）等阴离子的温度依赖性荧光各向异性。已观察到，具有乙基链的离子液体的重取向时间（τr）与粘度呈线性关系，且发现其与阴离子的性质无关。实验测量的τr值比使用具有粘性边界条件的斯托克斯-爱因斯坦-德拜（SED）流体动力学理论计算的1-乙基-3-甲基咪唑鎓阳离子的τr值长3倍，这表明发光物种不是咪唑鎓部分，而是某种缔合物种。相比之下，在给定的粘度（η）和温度（T）下，具有己基链的离子液体的重取向时间遵循τr（[FAP]）>τr（[Tf2N]）=τr（[BF4]）>τr（[PF6]）的趋势。考虑到在τr与η/T图中已注意到对于强缔合阴离子[BF4]和[PF6]存在显著的线性偏差，尤其是在环境温度下，具有较长烷基链的离子液体形成有序结构的能力似乎是观察到的行为的原因。
