We report the data of liquid density, thermal properties including glass transition temperature, melting point, and decomposition temperature, surface tension, and shear viscosity for imidazolium-based dicationic ionic liquids with the anions of bis(trifluoromethylsulfonyl)amide, bis(pentafluoroethylsulfonyl)amide, tetrafluoroborate, and nitrate. To find the unique and general features of the dicationic ionic liquids, data of their corresponding monocationic ionic liquids are also summarized. The results of the dicationic ionic liquids showed that the density was high; the glass transition temperature and melting point were high, and they were thermally stable. Also the surface tension was large, and the shear viscosity was high in comparison with the reference monocationic ionic liquids. The data of the physical properties including liquid density, surface tension, and shear viscosity of the ionic liquids were also compared with that of alkanediols and alkyl alcohols to find the alkyene-linker and alkyl-group dependences in the ionic liquids and alcohols.

We have studied the ultrafast dynamics of forty aprotic molecular liquids by femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy. Some physical properties such as shear viscosity, density, and surface tension of the molecular liquids have also been measured. From the Fourier transform Kerr spectra in the frequency range of about 0–200 cm−1, we have found that the first moment of the low-frequency intermolecular vibrational spectrum is moderately correlated with the root of the value of surface tension divided by density. This fact indicates that the microscopic intermolecular interaction is related to the macroscopic physical property of intermolecular force in molecular liquids. On the other hand, a correlation between the first moment of the intermolecular vibrational spectrum and the interaction energy of two identical molecules is almost nonexistent. The difference between the two relations suggests that the many-body interaction effect takes a hand in the intermolecular vibrational dynamics in molecular liquids. We have also found that the shapes of the broad low-frequency vibrational spectra for aromatic molecular liquids show a clearer bimodal feature than those for non-aromatic molecular liquids. Picosecond Kerr transients for most of the molecular liquids are non-exponential. The slowest relaxation time is qualitatively explained by the Stokes–Einstein–Debye model.

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.

We investigated the ultrafast dynamics in 1-butyl-3-methylimidazolium-based ionic liquids with two series of anions, (1) cyano-group substituted anions (thiocyanate [SCN](-), dicyanamide [N(CN)(2)](-), and tricyanomethide [C(CN)(3)](-)) and (2) trifluoromethylsulfonyl-group substituted anions (trifluoromethanesulfonate [OTf](-), bis(trifluoromethylsulfonyl)amide [NTf(2)](-), and tris(trifluoromethylsulfonyl)methide [CTf(3)](-)). This was done by femtosecond Raman-induced Kerr effect spectroscopy. From the Fourier transform Kerr spectra of the ionic liquids, the low-frequency spectrum of 1-butyl-3-methylimidazolium tricyanomethide shows a low-frequency shift compared to the ILs with the other cyano-group substituted anions due to the planar structures of the cation and the anion. The relative amplitude of the low-frequency band at approximately 20 cm(-1) compared to the entire broad spectrum for the ionic liquids with trifluoromethylsulfonyl-group substituted anions becomes larger with the order [OTf](-) < [NTf(2)](-) ≈ [CTf(3)](-). This vibrational band can be attributed to the librational motion of anions and/or the coupling of the translational and reorientational motions.