Physical Properties and Low-Frequency Polarizability Anisotropy and Dipole Responses of Phosphonium Bis(fluorosulfonyl)amide Ionic Liquids with Pentyl, Ethoxyethyl, or 2-(Ethylthio)ethyl Group.

This study compared the physical properties, e.g., glass transition temperature, melting point, viscosity, density, surface tension, and electrical conductivity, and the low-frequency spectra under 200 cm -1 of three synthesized ionic liquids (ILs), triethylpentylphosphonium bis(fluorosulfonyl)amide ([P 2225 ][NF 2 ]), ethoxyethyltriethylphosphonium bis(fluorosulfonyl)amide ([P 222(2O2) ][NF 2 ]), and triethyl[2-(ethylthio)ethyl]phosphonium bis(fluorosulfonyl)amide ([P 222(2S2) ][NF 2 ]), at various temperatures using femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES) and terahertz time-domain spectroscopy (THz-TDS). The [P 222(2S2) ][NF 2 ] had the highest viscosity and glass transition temperature, whereas the [P 222(2O2) ][NF 2 ] had the lowest. Among the three ILs, the [P 222(2S2) ][NF 2 ] had the highest density and surface tension, and the [P 222(2O2) ][NF 2 ] had the highest electrical conductivity. The RIKES and THz-TDS spectral line shapes for the three ILs varied significantly. For the [P 2225 ][NF 2 ], molecular dynamics simulations successfully reproduced the line shapes of the experimental spectra and indicated that the RIKES spectrum was mainly due to the cation and cross-term and their rotational motions, whereas the THz-TDS spectrum was mainly due to the anion and its translational motion. This shows that it is desirable to utilize both fs-RIKES and THz-TDS methods to reveal molecular motions at the low-frequency domain. The [P 222(2S2) ][NF 2 ] had higher frequency peaks and broader bands in the low-frequency spectra via fs-RIKES and THz-TDS than those for the [P 2225 ][NF 2 ] and [P 222(2O2) ][NF 2 ].