Dielectric relaxation and ultrafast transient absorption spectroscopy of [C6mim]+[Tf2N]-/acetonitrile mixtures

Abstract

Mixts. of the ionic liq. (IL) [C6mim]+[Tf2N]- and acetonitrile have been investigated by a combination of dielec. relaxation spectroscopy (DRS) and ultrafast transient absorption techniques using the mol. probe 12'-apo-β-carotenoic-12'-acid (12'CA). Steady-state absorption spectra of the 12'CA mol. have also been recorded. The position of the probe's S0 → S2 absorption max. correlates linearly with the polarizability of the mixt., suggesting that the bulk compn. is a good approxn. to the local compn. The lifetime τ1 of the S1/ICT state of 12'CA varies rather smoothly with compn. between the value for pure acetonitrile (42 ps) and neat [C6mim]+[Tf2N]- (94 ps). At low IL contents there appears to be an influence of discrete ion pairs. Employing static dielec. consts. from the DRS expts., one finds that the lifetime of the probe in the IL mixts. is shorter than that in pure org. solvents with the same polarity parameter. This suggests an increased stabilization of the S1/ICT state in IL-contg. mixts., most likely due to IL-specific Coulombic interactions between the cation and the neg. end of the probe's dipole. An ultrafast solvation component is obsd. which is ca. 0.5 ps in pure acetonitrile, and approaches the value for the pure IL (2.0 ps) already around x(IL) = 0.3. This is interpreted in terms of an efficient perturbation of the cooperative solvation response of acetonitrile by the presence of small amts. of IL and possibly also the viscosity increase when adding IL. This view is also supported by the increase of the av. longitudinal relaxation time of acetonitrile upon addn. of small IL amts. extd. from the DRS expts.