Hideaki Shirota

Deep eutectic solvents (DESs) are room-temperature liquid-state mixtures composed of two or more components that exhibit a significant depression in their melting points compared with those of their pure components. Owing to their facile preparation and broad range of component combinations, DESs are gaining attraction in materials chemistry and engineering. The intermolecular interactions in DESs are often more diverse than those in ionic liquids or hydrogen-bonding liquids, because they include different kinds of species, which are key to the strong eutectic effect. Thus, DESs are an attractive research target in chemical physics and physical chemistry from both dynamic and static perspectives. This article focuses on the intermolecular dynamics of DESs, particularly those explored using dynamic optical Kerr effect spectroscopy (OKES) and other spectroscopic techniques. OKES is a robust spectroscopic technique for observing molecular dynamics in the low-frequency (or terahertz to subterahertz) region, as it captures both intermolecular vibrations and collective orientational dynamics. Notably, this technique enables detecting dynamics without the need for a probe molecule. This article summarizes key findings from existing research, including characteristic vibrational modes and relaxation processes, as well as their dependence on DES constituents and the effects of water and temperature. Finally, potential directions for future research on DES dynamics are discussed. We hope that this Perspective will help not only to understand the current research situation in DES dynamics, but also to drive further progress and development of the DES and its related research area.

Synchrotron small- and wide-angle X-ray scattering and Raman spectroscopy were used to investigate the high-pressure (HP) liquid-liquid transitions (LLTs) in quaternary ammonium and phosphonium ionic liquids (ILs) at room temperature. Bis(trifluoromethylsulfonyl)imide ([TFSI]^-) was used as the anion, and triethyloctylammonium, (2-ethoxyethoxy)ethyltriethylammonium ([N_222(2O2O2)]⁺), triethyloctylphosphonium, and (2-ethoxyethoxy)ethyltriethylphosphonium ([P_222(2O2O2)]⁺) were used as cations. In diether-substituted [N_222(2O2O2)][TFSI] and [P_222(2O2O2)][TFSI], the polar/nonpolar nanodomains were not formed due to the flexible and curled alkyl chains. Despite no nanodomain, HP LLT occurred with changing the local structures. A series of quaternary ammonium and phosphonium ILs exhibited entirely different HP behavior, which implies that the ether bonds of the cations have an intrinsic property.

In this study, we investigated the intermolecular vibrational dynamics of five deep eutectic solvents (DESs) composed of a 4:1 molar ratio mixture of acetamide and lithium salts, namely, lithium nitrate, perchlorate, trifluoromethanesulfonate, bis(fluorosulfonyl)amide, and bis(trifluoromethylsulfonyl)amide, using femtosecond Raman-induced Kerr effect spectroscopy. The low-frequency spectra below ∼200 cm−1 revealed that the peak position and first moment of the intermolecular vibrational band depended on the anion species, while acetamide molecules mainly determined the spectral intensity in most cases. However, the librational motion associated with the planar nitrate ion resulted in an intense low-frequency spectrum. The first-moment value was larger for the nitrate DES than for the other four DESs, indicating stronger intermolecular interactions in the former. A similar trend was observed for surface tension values. Comparison with previous results for lithium bis(trifluoromethylsulfonyl)amide–organic amide DESs showed that variation in the first moment of the intermolecular vibrational band and in the bulk parameter (square root of surface tension divided by liquid density) is larger upon changing the anion species than upon changing the organic amide species. Accordingly, the microscopic intermolecular interactions probed by the intermolecular vibrational band are significantly influenced by the anion species. Furthermore, a plot of the first moment of the intermolecular vibrational band vs the bulk parameter displayed a linear correlation, indicating a strong relationship between microscopic intermolecular interactions and macroscopic properties in the DES systems.

In this study, we have investigated the concentration-dependent intermolecular dynamics of aqueous solutions of aniline hydrochloride, sodium phenoxide, and 4-methylpyridine using femtosecond Raman-induced Kerr effect spectroscopy at 298 K. The densities, viscosities, and surface tensions of the aqueous solutions were also measured at 298 K. Quantum chemistry calculations of the target aromatics and their clusters with water molecule(s) or a counterion were performed to obtain their optimized structures and cluster interaction energies. In the difference low-frequency Kerr spectra (-1) of the aqueous aromatic solutions and neat water, the first moment (M₁) of the intermolecular vibrational band, which mainly originated from the aromatic ring, showed that the librations of the anilinium cation and phenoxide anion were higher in frequency than that of 4-methylpydine. Furthermore, the libration of the phenoxide anion was also higher in frequency than that of the anilinium cation. Quantum chemistry calculations indicated that the strong hydrogen bonding and compact hydration structure resulting from the negatively charged aromatic ring led to higher-frequency libration of the phenoxide anion than the anilinium cation. In addition, the M₁ increased with increasing concentration. The concentration sensitivities were stronger in aqueous solutions of aniline hydrochloride and sodium phenoxide than in aqueous solutions of 4-methylpyridine. Based on the quantum chemistry calculation results, we conclude that strong aromatic-water and aromatic-counterion interactions lead to a higher-frequency libration of aromatics with charged side groups. The collective orientational relaxation times of the aqueous aromatic solutions showed the fractional Stokes-Einstein-Debye behavior.

This study reports the dimer formation behavior of 7-azaindole (AI) in trihexyltetradecylphosphonium ([P _666,14 ] ⁺ ) based ionic liquids (ILs) with seven anion species probed by ¹ H NMR.

The isomerization reaction of MBBA is accelerated at the interface between ionic liquids and sapphire.

Electrolyte solutions often show characteristic spectral features in their vibrational spectra, and a quantitative analysis of them is a key toward better understanding of the solvation structures and intermolecular interactions based on those spectral features. Here, such an analysis is carried out for the C≡N stretching mode of some electrolyte solutions of acetonitrile. It is found that a negative noncoincidence effect, in which the isotropic Raman component is located at a higher frequency than the anisotropic component, is observed for the band that newly appears upon solvation of salt, and based on a theoretical analysis that refers to quantum chemical calculations and molecular dynamics simulation results, it is shown that this phenomenon arises from the vibrational coupling among the C≡N bonds clustering around each metal ion (Li⁺, Mg²⁺, Ca²⁺, or Zn²⁺). The transition dipole coupling mechanism reasonably well explains the sign and magnitude of this intermolecular vibrational coupling. The overall high-frequency shift of this newly appearing band (compared to the band of neat liquid) is largely explained by an electrostatic interaction model that takes into account the spatially nonuniform nature of the electrostatic potentials and fields operating from each metal ion on the solvent molecules around it. On the basis of these results, the relation between the spectral features and the solvation structures is discussed.

In this study, we investigated the intermolecular dynamics, including intermolecular vibration and orientational dynamics, of five deep eutectic solvents (DESs) consisting of lithium bis(trifluoromethylsulfonyl)amide and organic amides, such as acetamide, propanamide, N-methylacetamide, butyramide, and urea, at a mole ratio of 1:3 using femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES) and subpicosecond optical Kerr effect spectroscopy (ps-OKES). The fs-RIKES results showed that the line shape of the low-frequency band of the N-methylacetamide was trapezoidal, while that of the other organic amide-based DESs was bimodal. The peak and first moment of the intermolecular vibrational band appearing in the frequency range less than 250 cm^-1 for the acetamide- and urea-based DESs were in a higher-frequency region compared to the other three DESs, indicating stronger intermolecular interactions. Furthermore, analysis of the intramolecular vibrational bands of the bis(trifluoromethylsulfonyl)amide anion showed that the population of the transoid conformer of the anion was slightly higher in the urea-based DES than in the other organic amide-based DESs, suggesting that urea solvate lithium cations more than the other organic amides. The slow relaxation dynamics of all five DESs were captured for up to 1 ns using ps-OKES. The slow relaxation dynamics also depended on the organic amide species. However, the slow relaxation time constant did not show a clear correlation with the viscosity; therefore, the relaxation dynamics of the DESs did not follow the Stokes-Einstein-Debye hydrodynamic model. The densities, viscosities, surface tensions, and electrical conductivities of the DESs were also measured for comparison with spectroscopic results.

This article presents experimental characterization information and synchrotron X-ray scattering measurements on a set of novel O- and S-substituted phosphonium-based ionic liquids (ILs) all coupled with the bis(fluorosulfonyl)imide (FSI^-) anion. The ILs include the ethoxyethyltriethylphosphonium (P_222(2O2)⁺) and triethyl[2-(ethylthio)ethyl]phosphonium (P_222(2S2)⁺) cations, and we contrast results on these with those for unsubstituted triethylpentylphosphonium (P₂₂₂₅⁺). The article also introduces a physics-based protocol that combines classical force field studies on larger simulation boxes with classical and first-principles studies on smaller boxes. The method produces significantly improved S(q) functions in the regime which in prior publications we have associated with inter- and intraionic adjacency correlations. By understanding which shorter-range structural changes improve S(q) in the q-regime of interest, we are also able to pinpoint specific deficiencies in the classical force field model. The approach we take should be quite general and could help study other complex liquids on different length scales.