Dielectric and optical Kerr-effect spectroscopy into the dynamics of ionic liquids
Final Report Abstract
In collaboration with the group of K. Wynne (Glasgow University, UK) the collective dynamics of room-temperature ionic liquids (RTILs) were studied by investigating their dielectric relaxation (DR) and time-resolved optical Kerr effect (OKE) in the frequency range of ~10 MHz to ~20 THz. Since DR spectroscopy is probing the dipole vector whereas OKE spectroscopy is sensitive to the polarizability anisotropy of the sample complementary information on the entire intermolecular dynamics of RTILs is obtained, spanning in time from ~100 femtoseconds up to several nanoseconds. It was found that at a first glance, DR and OKE spectra of all studied RTIL classes were rather similar in overall shape, with a dominating α relaxation roughly around 1 GHz, an intermediate “constant-loss” region, more or less prominent libration bands at ~1 to 5 THz, and finally a pronounced cut-off at ~10 THz. Rotational and translational motions of anions and cations are intimately coupled and all dynamics are highly collective so that the assignment of bands to individual species is generally problematic. Nevertheless, we may sketch the following rough scenario for all investigated RTILs: The fastest motions are dominated by librations (hindered rotations) of ions in the fluctuating cages formed by their neighbors, superimposed by cage rattling (inter-ionic vibrations). These motions may be modulated by H-bond interactions as. With increasing time, librations and cage rattling merge into the dynamics of cage formation and decay - the “constant-loss” region of the spectra - before the α relaxation associated with ion rotation emerges with relaxation times of the order of several tens to a few hundred picoseconds. Generally, DR and OKE “see” simultaneously anions and cations, albeit with different weights, so that ion-specific information is difficult to extract. The studied sulfonium and pyrrolidinium RTILs are an example. The situation is different for the protic ionic liquid ethylammonium nitrate, as here DR is specific to cation reorientation whereas the OKE α mode monitors the anion. Similar activation energies for anion and cation rotation suggest strong cooperativity in their dynamics and the comparison of the overall reorientation of ethylammonium ion (DR) with the reorientation of its N--H group (via time-resolved IR spectroscopy) indicates jump reorientation dictated by the hydrogen-bond network of this RTIL. For the studied imidazolium RTILs the α relaxation, both in DR and OKE spectra, is dominated by cation reorientation through large-angle jumps. Dipolar anions yield a further DR mode at ~5 to 10 GHz that is rather weak in amplitude whilst the α mode is enhanced compared to RTILs with nonpolar anions. This suggests strong static dipole-dipole correlations of cations and anions if both are polar. However, the most spectacular feature of imidazolium RTILs is the presence of a strong sub-α peak in the OKE signal indicative for the slow breathing motion of large (possibly α-stacked) aggregates.
