Ionic liquids hold promise for applications in energy storage as the electrolyte in electric double-layer capacitors and other devices. Further optimizing device performance through their physical and chemical properties requires an improved understanding of the detailed interactions between cation and anion and how they reorganize in the presence of charge. Here we probe the dynamics of thiocyanate ions in several bulk imidazolium bis(trifluoromethylsulfonyl)amide ionic liquids from femtoseconds to 100 ps using ultrafast vibrational spectroscopy. Two-dimensional infrared (2D-IR) spectroscopy of thiocyanate ions detect both intertial motion (on the hundreds of femtosecond timescale) as well as slower, diffusive motions (on the tens of picosecond timescale). Both timescales are faster than the reorientational relaxation (on the hundreds of picoseconds timescale) determined from ultrafast vibrational pump-probe measurements, indicating that 2D-IR spectroscopy has valuable additional information on the structure and dynamics in these liquids. The 2D-IR experiments show that the rate of fluctuation of the electrostatic environment around the thiocyanate is sensitive to hydrogen bonding at the 2-position of the imidazolium ring, dependens mildly on water concentration, changes with counter-ion, and is independent of the thiocyanate concentration (up to 30 mM). The results are compared to ab initio simulations which predicted a 10 -- 15 picosecond hydrogen bond lifetime. These bulk studies open the door to studying the solvation dynamics in nanoconfined environments such as nanostructured electrodes and will provide detailed molecular information on time and length scales compatible with molecular simulations.

Ultrafast vibrational spectroscopy (2D-IR) of ionic liquids

247th National Meeting (2014)

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