Guchhait Biswajit, Tibbetts Clara A, Tracy Kathryn M, Luther Bradley M, Krummel Amber T
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, USA.
J Chem Phys. 2020 Apr 30;152(16):164501. doi: 10.1063/1.5141751.
A major impediment limiting the widespread application of ionic liquids (ILs) is their high shear viscosity. Incorporation of a tricyanomethanide (TCM) anion in ILs leads to low shear viscosity and improvement of several characteristics suitable for large scale applications. However, properties including interactions of TCM with the local environment and dynamics of TCM have not been thoroughly investigated. Herein, we have studied the ultrafast dynamics of TCM in several imidazolium ILs using linear IR and two-dimensional infrared spectroscopy techniques. The spectral diffusion dynamics of the CN stretching modes of TCM in all ILs exhibit a nonexponential behavior with a short time component of ∼2 ps and a long time component spanning ∼9 ps to 14 ps. The TCM vibrational probe reports a significantly faster relaxation of ILs compared to those observed previously using linear vibrational probes, such as thiocyanate and selenocyanate. Our results indicate a rapid relaxation of the local ion-cage structure embedding the vibrational probe in the ILs. The faster relaxation suggests that the lifetime of the local ion-cage structure decreases in the presence of TCM in the ILs. Linear IR spectroscopic results show that the hydrogen-bonding interaction between TCM and imidazolium cations in ILs is much weaker. Shorter ion-cage lifetimes together with weaker hydrogen-bonding interactions account for the low shear viscosity of TCM based ILs compared to commonly used ILs. In addition, this study demonstrates that TCM can be used as a potential vibrational reporter to study the structure and dynamics of ILs and other molecular systems.
限制离子液体（ILs）广泛应用的一个主要障碍是它们的高剪切粘度。在离子液体中引入三氰基甲烷化物（TCM）阴离子会导致低剪切粘度，并改善一些适用于大规模应用的特性。然而，包括TCM与局部环境的相互作用以及TCM的动力学等性质尚未得到充分研究。在此，我们使用线性红外光谱和二维红外光谱技术研究了几种咪唑鎓离子液体中TCM的超快动力学。在所有离子液体中，TCM的C≡N伸缩振动模式的光谱扩散动力学呈现非指数行为，具有约2皮秒的短时间成分和约9皮秒至14皮秒的长时间成分。与先前使用硫氰酸盐和硒氰酸盐等线性振动探针观察到的情况相比，TCM振动探针报告离子液体的弛豫速度明显更快。我们的结果表明，嵌入离子液体中振动探针的局部离子笼结构快速弛豫。更快的弛豫表明，在离子液体中存在TCM时，局部离子笼结构的寿命会缩短。线性红外光谱结果表明，离子液体中TCM与咪唑鎓阳离子之间的氢键相互作用要弱得多。与常用离子液体相比，较短的离子笼寿命以及较弱的氢键相互作用导致了基于TCM的离子液体的低剪切粘度。此外，这项研究表明，TCM可以用作研究离子液体和其他分子体系的结构与动力学的潜在振动报告分子。
