Transport properties, optical response and slow dynamics of ionic liquids

by Hu, Zhonghan.

In this thesis, we report on our studies of the transport properties, optical response and slow dynamical nature of novel room temperature ionic liquids. Using computer simulations we have demonstrated that the diffusive dynamics of these systems is in many ways analogous to that of other glassy or supercooled liquids. These solvents show non-Gaussian rotational and translational diffusion which have a temporal extent on the order of nanoseconds at room temperature. Our study of their response upon application of an external mechanical perturbation shows that even for systems with a box length as large as 0.03 microns the viscosities computed from perturbation wavenumbers compatible with this box size have not yet reached the hydrodynamic limit. We found these systems to behave in a non-Newtonian fashion and we also observe a clear break down of linear response theory on the nanoor sub-micrometer scale. Upon photoexcitation of an organic probe with lifetime shorter than the reorganization timescale in these ionic liquids, (which is quite long on the order of several nanoseconds at least), the emission spectrum is absorption wavelength dependent. Our computer simulations rationalized this observation in terms of local solvent environment around individual subensemble probe members. Excitation of different solute molecules in the liquid gives rise to site-specific optical responses. We revealed that the origin of this excitation wavelength dependence is the existence of persistent excited-state environments that do not get solvent averaged on a time scale relevant 2 to fluorescence. The computed time resolved fluorescence spectra show that the full loss of correlation between absorption and emission frequencies for probes in room temperature ionic liquids occur on a time scale of nanoseconds. One of the most interesting features of ionic liquids is their uncommonly large range of dynamical time scales which in turn makes some of their properties to be quite different from that of most other conventional solvents. We hope that our understanding of these phenomena will be useful in the future in the development of tools to harness their potential to control the outcome of chemical and photo-chemical reactions. Abstract Approved: Thesis Supervisor Title and Department Date