Femtosecond Raman-Induced Kerr Effect Study of Temperature-Dependent Intermolecular Dynamics in Pyrrolidinium-Based Ionic Liquids: Effects of Anion Species

Comparison between Phosphonium Docusate Ionic Liquids and Their Equimolar Mixtures with Alkanes: Temperature-Dependent Viscosity, Glass Transition, and Fragility

In this study, we determined the temperature-dependent viscosities, glass transition temperatures, and fragilities of tetraalkylphosphonium docusate ionic liquids (ILs) and their equimolar mixtures with alkanes to elucidate the effects of the alkyl groups on the phosphonium cation. The target ILs were the docusate salts with tributylheptylphosphonium ([P4447][doc]), tributyltetradecylphosphonium ([P444,14][doc]), butyltrihexylphosphonium ([P4666][doc]), trihexylheptylphosphonium ([P6667][doc]), and trihexyltetradecylphosphonium cations ([P666,14][doc]). The comparable IL/alkane mixtures were equimolar mixtures of IL and alkane with the same carbon numbers of the target ILs: [P4447][doc]/hexane to [P6667][doc]; [P4447][doc]/heptane to [P444,14][doc]; [P444,14][doc]/hexane to [P666,14][doc]; [P4666][doc]/decane to [P666,14][doc]; and [P6667][doc]/heptane to [P666,14][doc]. The viscosities and glass transition temperatures of the neat ILs were higher than those of their respective IL/alkane mixtures. Based on the analysis of temperature-dependent viscosities, including a viscosity value of 1013 mPa·s at the glass transition temperature using the Vogel-Fulcher-Tammann equation, the neat ILs were stronger liquids than the corresponding IL/alkane mixtures. By comparing several combinations of the neat ILs and IL/alkane mixtures, we found that the larger the alkane, the more fragile the mixture.

Insights into structure-property relationships in ionic liquids using cyclic perfluoroalkylsulfonylimides

Room temperature ionic liquids of cyclic sulfonimide anions ncPFSI (ring size: n = 4-6) with the cations [EMIm]+ (1-ethyl-3-methylimidazolium), [BMIm]+ (1-butyl-3-methylimidazolium) and [BMPL]+ (BMPL = 1-butyl-1-methylpyrrolidinium) have been synthesized. Their solid-state structures have been elucidated by single-crystal X-ray diffraction and their physicochemical properties (thermal behaviour and stability, dynamic viscosity and specific conductivity) have been assessed. In addition, the ion diffusion was studied by pulsed field gradient stimulated echo (PFGSTE) NMR spectroscopy. The decisive influence of the ring size of the cyclic sulfonimide anions on the physicochemical properties of the ILs has been revealed. All ILs show different properties compared to those of the non-cyclic TFSI anion. While these differences are especially distinct for ILs with the very rigid 6cPFSI anion, the 5-membered ring anion 5cPFSI was found to result in ILs with relatively similar properties. The difference between the properties of the TFSI anion and the cyclic sulfonimide anions has been rationalized by the rigidity (conformational lock) of the cyclic sulfonimide anions. The comparison of selected IL properties was augmented by MD simulations. These highlight the importance of π+-π+ interactions between pairs of [EMIm]+ cations in the liquid phase. The π+-π+ interactions are evident for the solid state from the molecular structures of the [EMIm]+-ILs with the three cyclic imide anions determined by single-crystal X-ray diffraction.

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 ([P₂₂₂₅][NF₂]), ethoxyethyltriethylphosphonium bis(fluorosulfonyl)amide ([P_222(2O2)][NF₂]), and triethyl[2-(ethylthio)ethyl]phosphonium bis(fluorosulfonyl)amide ([P_222(2S2)][NF₂]), at various temperatures using femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES) and terahertz time-domain spectroscopy (THz-TDS). The [P_222(2S2)][NF₂] had the highest viscosity and glass transition temperature, whereas the [P_222(2O2)][NF₂] had the lowest. Among the three ILs, the [P_222(2S2)][NF₂] had the highest density and surface tension, and the [P_222(2O2)][NF₂] had the highest electrical conductivity. The RIKES and THz-TDS spectral line shapes for the three ILs varied significantly. For the [P₂₂₂₅][NF₂], 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)][NF₂] had higher frequency peaks and broader bands in the low-frequency spectra via fs-RIKES and THz-TDS than those for the [P₂₂₂₅][NF₂] and [P_222(2O2)][NF₂].

Low-Frequency Spectra of 1-Methyl-3-octylimidazolium Tetrafluoroborate Mixtures with Poly(ethylene glycol) by Femtosecond Raman-Induced Kerr Effect Spectroscopy

This is the first report on low-frequency spectra of ionic liquid (IL)/polymer mixtures using femtosecond Raman-induced Kerr effect spectroscopy. We studied mixtures of 1-methyl-3-octylimidazolium tetrafluoroborate ([MOIm][BF₄]) and poly(ethylene glycol) (PEG) with M_n = 400 (PEG400) at various concentrations. To elucidate the unique features of the IL/polymer mixture system, mixtures of PEG400 with a molecular liquid, 1-octhylimidazole (OIm), which is a neutral analog of the cation, were also studied. In addition, mixtures of [MOIm][BF₄] with ethylene glycol (EG) and poly(ethylene glycol) with M_n = 4000 (PEG4000) were also investigated. The first moments of broad low-frequency spectra, mainly due to intermolecular vibrations for the [MOIm][BF₄]/PEG400 and OIm/PEG400, increased slightly with increasing concentration of PEG400, indicating that microscopic intermolecular interactions, in general, are slightly enhanced. We also compared the [MOIm][BF₄] mixtures with EG, PEG400, and PEG4000 at concentrations of 5 and 10 wt % PEG or EG. The low-frequency spectra of samples with the same concentrations were quite similar, but a comparison of the normalized spectra showed that the spectral intensity in the low-frequency region below ∼50 cm^-1 of the [MOIm][BF₄] mixtures with PEG400 and PEG4000 is somewhat lower than that of the [MOIm][BF₄] mixtures with EG. Although the effect of the polymer is small compared to other polymer solution systems, this feature is attributed to a suppression of translational motion in the mixtures of [MOIm][BF₄] with PEG compared to the mixtures of [MOIm][BF₄] with EG due to the greater mass of PEG than EG. Density, surface tension, viscosity, and electrical conductivity were also estimated. From Walden plots, it was found that the [MOIm][BF₄]/PEG4000 system showed more ideal electrical conductive behavior than the [MOIm][BF₄]/PEG400 and [MOIm][BF₄]/EG systems.

Optical Kerr Effect of Liquid Acetonitrile Probed by Femtosecond Time-Resolved X-ray Liquidography

Optical Kerr effect (OKE) spectroscopy is a method that measures the time-dependent change of the birefringence induced by an optical laser pulse using another optical laser pulse and has been used often to study the ultrafast dynamics of molecular liquids. Here we demonstrate an alternative method, femtosecond time-resolved X-ray liquidography (fs-TRXL), where the microscopic structural motions related to the OKE response can be monitored using a different type of probe, i.e., X-ray solution scattering. By applying fs-TRXL to acetonitrile and a dye solution in acetonitrile, we demonstrate that different types of molecular motions around photoaligned molecules can be resolved selectively, even without any theoretical modeling, based on the anisotropy of two-dimensional scattering patterns and extra structural information contained in the q-space scattering data. Specifically, the dynamics of reorientational (libration and orientational diffusion) and translational (interaction-induced motion) motions are captured separately by anisotropic and isotropic scattering signals, respectively. Furthermore, the two different types of reorientational motions are distinguished from each other by their own characteristic scattering patterns and time scales. The measured time-resolved scattering signals are in excellent agreement with the simulated scattering signals based on a molecular dynamics simulation for plausible molecular configurations, providing the detailed structural description of the OKE response in liquid acetonitrile.

Understanding the Photoexcitation of Room Temperature Ionic Liquids

Photoexcitation of (neat) room temperature ionic liquids (RTILs) leads to the observation of transient species that are reminiscent of the composition of the RTILs themselves. In this minireview, we summarize state-of-the-art in the understanding of the underlying elementary processes. By varying the anion or cation, one aim is to generally predict radiation-induced chemistry and physics of RTILs. One major task is to address the fate of excess electrons (and holes) after photoexcitation, which implies an overview of various formation mechanisms considering structural and dynamical aspects. Therefore, transient studies on time scales from femtoseconds to microseconds can greatly help to elucidate the most relevant steps after photoexcitation. Sometimes, radiation may eventually result in destruction of the RTILs making photostability another important issue to be discussed. Finally, characteristic heterogeneities can be associated with specific physicochemical properties. Influencing these properties by adding conventional solvents, like water, can open a wide field of application, which is briefly summarized.