Time-resolved laser spectroscopy of reactive and nonreactive processes in ionic liquids and their mixtures with carbon dioxide

Ionic liquids (ILs) have emerged as unique fluid media with applications in a wide range of fields.
Over the recent years, considerable information regarding the structural complexity and molecular motions of ILs has been gathered by various techniques such as dielectric relaxation spectroscopy (DRS), optical Kerr effect spectroscopy (OKE), THz, ESR, NMR, neutron and x-ray scattering, as well as transient absorption of solvated electrons. Nonreactive and reactive processes crucially depend on the interactions of the embedded solute molecule(s) with the constituent ions of the IL. Therefore detailed information on Coulomb, dipolar, hydrogen-bonding and dispersion contributions to the solute-IL interaction is required, which is also the central topic of this project. In this respect, sensitive spectroscopic techniques following the response of photoexcited molecular probes are the methods of choice. Time-resolved fluorescence and absorption techniques have been used, which cover the timescale from the subpicosecond to the nanosecond regime. Regarding solvation dynamics, the findings from such experiments suggest that many ILs exhibit a biphasic solvation response which consists of a smaller amplitude ultrafast exponential component and a larger amplitude slow component which can be formally represented by a stretched-exponential. The assignment of motions of the IL to the individual components of the solvation response remains controversial, also from the viewpoint of theory. To solve this issue, more extended experimental data on a range of ILs and also IL-IL and IL-CO2 mixtures must be obtained by highly sensitive techniques, which cover the complete time range from the sub-100 fs to the several nanosecond range. The issue of IL polarity is also heavily debated. For instance, DRS provides extrapolated static dielectric constants in the range 9-15 for a range of imidazoliumbased ILs, with the exception of species with strongly bipolar anions such as ethylsulfates, where values around 30 are found. This is in contrast to the results of static solvatochromic and dynamic molecular probes, which find substantially higher polarities. The latter estimates might be more "at the heart of chemistry", because this "micropolarity" is likely more relevant when it comes to chemical reactivity. Recently, Weing?rtner found exceptionally high values up to 57 in his DRS experiments on protic-ILs, so it is clear that different classes of neat ILs and also IL mixtures should be sampled by dynamic polarity probes. Coming to reactive processes, experimental investigations of ultrafast reactions are still rather limited for ILs, and are mostly focused on charge transfer processes. We would like to investigate the specific influence of IL environments on ultrafast isomerization and electron transfer reactions, which are also of relevance for the understanding of the primary photophysical steps in dye-sensitized solar cells (DSSCs). In contrast, a variety of investigations is available on longer time-scales. For instance, an interesting effect has been found in the recombination of photogenerated lophyl radicals where the kinetics in three imidazolium-based ILs was faster than expected from the ILs' viscosity. The influence of viscosity on the kinetics of bimolecular reactions, specifically transient radical reactions in a number of ILs, and also IL-IL and IL-CO2 mixtures, is another central topic in this project.