Dynamics of a supercooled ionic liquid studied by optical and dielectric spectroscopy

We have measured the dynamics of solvation of a triplet state probe, quinoxaline, in the glass-forming ionic liquid dibutylammonium formate near its glass transition temperature Tg=153 K. The Stokes-shift correlation function displays a relaxation time dispersion of considerable magnitude and the optical line width changes systematically along the solvation coordinate. The solvent dynamics in the viscous regime is compared with the rotational behavior of the solute and with the dielectric relaxation of the ionic liquid. Among the different quantities derived from the dielectric experiments, the relaxation of the macroscopic electric field, i.e., the modulus Mt, matches best the solvent response Ct regarding time scale and stretching exponent. Many other properties are reminiscent of the behavior of polar molecular liquids which lack the ionic character.