Hunger Johannes, Roy Soham, Grechko Maksim, Bonn Mischa
Max Planck Institute for Polymer Research , Ackermannweg 10 , 55128 Mainz , Germany.
Graduate School Materials Science in Mainz , Staudingerweg 9 , 55128 Mainz , Germany.
J Phys Chem B. 2019 Feb 28;123(8):1831-1839. doi: 10.1021/acs.jpcb.8b10849. Epub 2019 Feb 14.
The dynamics of probe molecules is commonly used to investigate the structural dynamics of room-temperature ionic liquids; however, the extent to which this dynamics reflects the dynamics of the ionic liquids or is probe specific has remained debated. Here, we explore to what extent the vibrational and rotational dynamics of the dicyanamide anion, a common ionic liquid anion, correlates with the structural relaxation of ionic liquids. We use polarization-resolved, ultrafast infrared spectroscopy to probe the temperature- and probe-concentration-dependent dynamics of samples with small amounts of 1-ethyl-3-methylimidazolium ([emim]) dicyanamide ([DCA]) dissolved in four [emim]-based ionic liquids with tetrafluoroborate ([BF]), bis(trifluoromethylsulfonyl)imide ([NTf]), ethylsufate ([EtSO]), and triflate ([OTf]) as anions. The transient spectra after broad-band excitation at 2000-2300 cm, resonant with the symmetric and antisymmetric C≡N stretching vibrations, initially contain oscillatory signatures due to the vibrational coherence between both modes. Vibrational population relaxation occurs on two distinct time scales, ∼6-7 and ∼15-20 ps. The vibrational dynamics is rather insensitive to the details of the ionic liquid anion and temperature, except for the slow vibrational relaxation component. The decay of the excitation anisotropy, a measure of the rotational dynamics of [DCA], markedly depends on temperature, and the obtained decay time exhibits an activation energy of ∼15-21 kJ/mol. Remarkably, neither the rotation time nor the activation energy can be simply explained by the variation of the macroscopic viscosity. Hence, our results suggest that the dynamics of dicyanamide is only in part representative of the ionic liquid structural dynamics. Rather, the dynamics of the probe anion seems to be determined by the specific interaction of [DCA] with the ionic liquid's ions for the class of [emim]-based ionic liquids studied here.
探针分子的动力学常用于研究室温离子液体的结构动力学；然而，这种动力学在多大程度上反映离子液体的动力学或具有探针特异性一直存在争议。在此，我们探究双氰胺阴离子（一种常见的离子液体阴离子）的振动和转动动力学与离子液体结构弛豫的相关程度。我们使用偏振分辨的超快红外光谱来探测含有少量溶解于四种以1-乙基-3-甲基咪唑鎓（[emim]）为基础、分别以四氟硼酸根（[BF]）、双（三氟甲基磺酰）亚胺（[NTf]）、硫酸乙酯（[EtSO]）和三氟甲磺酸根（[OTf]）为阴离子的离子液体中的1-乙基-3-甲基咪唑鎓（[emim]）双氰胺（[DCA]）样品的温度和探针浓度依赖性动力学。在2000 - 2300 cm处宽带激发后，与对称和反对称C≡N伸缩振动共振的瞬态光谱最初包含由于两种模式之间的振动相干而产生的振荡特征。振动布居弛豫发生在两个不同的时间尺度上，约6 - 7皮秒和约15 - 20皮秒。除了缓慢的振动弛豫分量外，振动动力学对离子液体阴离子的细节和温度相当不敏感。激发各向异性的衰减，即[DCA]转动动力学的一种度量，明显取决于温度，并且所获得的衰减时间表现出约15 - 21 kJ/mol的活化能。值得注意的是，转动时间和活化能都不能简单地用宏观粘度的变化来解释。因此，我们的结果表明双氰胺的动力学仅部分代表离子液体的结构动力学。相反，对于此处研究的基于[emim]的离子液体类别，探针阴离子的动力学似乎由[DCA]与离子液体离子的特定相互作用所决定。
