Study on the Translational Diffusion of Transient Species in 1-Alkyl-3-methylimidazolium-Based Ionic Liquids by Transient Grating Spectroscopy

Translational diffusion coefficients of two transient species in 1-alkyl-3-methylimidazolium-based ionic liquids (Cnmim+) with different alkyl chain lengths (n = 4, 8, 12) and the bis(trifluoromethanesulfonyl)imide anion were measured by transient grating spectroscopy. The systems of interest are the benzophenone ketyl radical (BPK) produced by the hydrogen-abstraction reaction of benzophenone (BP) and the zwitterionic merocyanine dye (MC) produced by the photoisomerization of 1,3,3-trimethylindolino-6′-nitrobenzopyrylospiran (SP). The diffusion coefficients of BPK and MC closely obey the Stokes-Einstein (SE) relation in the ionic liquids regardless of the alkyl chain lengths of the cations, whereas the diffusion coefficients of BP are larger than those predicted by the SE equation. The temperature dependences of the translational diffusion coefficients of BP, BPK, and MC reveal that the diffusional activation energies of BPK and MC are larger than those of BP in all solvents. The origin of this difference is discussed in terms of intermolecular interactions within the ionic-liquid solvents.