Dynamics of Dicyanamide in Ionic Liquids is Dominated by Local Interactions

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 ([BF₄]^-), bis(trifluoromethylsulfonyl)imide ([NTf₂]^-), ethylsufate ([EtSO₄]^-), and triflate ([OTf]^-) as anions. The transient spectra after broad-band excitation at 2000-2300 cm^-1, 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.