Time-resolved laser spectroscopy of nonreactive processes in ionic liquids and their binary mixtures with organic solvents and CO2

Abstract

Nonreactive dynamics of molecules in ionic liquids (ILs) have been investigated in various environments, such as in neat aprotic and protic ILs, as well as in binary mixtures of ILs with polar organic solvents and CO2. Transient broadband absorption and fluorescence spectroscopic techniques on timescales from femtoseconds to milliseconds have been applied to elucidate the importance of different types of solute–solvent interactions in these media. Molecular probes such a 12′-apo-β-carotenoic-12′-acid and Coumarin C153 have been used as reporter molecules for ionic liquid solvation dynamics, micropolarity, hydrogen-bonding and collisional relaxation. With Coumarin C153 in ILs, the relationship between the complete solvation response and microwave-terahertz absorption measurements of the neat liquid was explored quantitatively. A reduced value for the conductivity plays a key role in such solvation experiments. Aprotic ILs exhibit a micropolarity larger than expected from their static dielectric constants, whereas the local polarity of protic alkylammonium formate ILs appears to be similar to that of alcohols having comparable dielectric constants. In IL mixtures with organic solvents or CO2, there is no indication for differences in the bulk composition and the local composition around the molecular probe. Collisional relaxation processes in ILs take place on similar timescales as in dipolar organic solvents.