Ultrafast vibrational dynamics of a trigonal planar anionic probe in ionic liquids (ILs): A two-dimensional infrared (2DIR) spectroscopic investigation.

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.