Heavy Atom Substitution Effects in Non-Aromatic Ionic Liquids: Ultrafast Dynamics and Physical Properties

In this study, we have investigated the heavy atom substitution effects on the ultrafast dynamics in nonaromatic cation-based ionic liquids, as well as the static physical properties such as shear viscosity, surface tension, glass transition temperature, and melting point. Phosphonium-based ionic liquids show lower shear viscosities and lower glass transition temperatures than their corresponding ammonium-based ionic liquids. We have also examined the substitution of a (2-ethoxyethoxy)ethyl group for an octyl group in ammonium and phosphonium cations and found that the (2-ethoxyethoxy)ethyl group reduces the shear viscosity and increases the surface tension. From the results of the ultrafast dynamics, including intra- and interionic vibrations and reorientational relaxation in the ammonium- and phosphonium-based ionic liquids measured by means of femtosecond optically heterodyne-detected Raman-induced Kerr spectroscopy, we have found that the first moment of low-frequency Kerr spectrum, omitting the contributions of clear intraionic vibrational modes, correlates to the square root of surface tension divided by density. This fact indicates that heavy atom substitution in ionic liquids provides a weaker interionic interaction arising from the larger ionic volume. On the other hand, the ether group in the cations gives the stronger interionic interaction but with a more flexible and/or less segregated nature in the ILs than the alkyl group.