Yusuke Imai

Pressure-induced phase transition of 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6]

We have investigated the pressure-induced Raman spectral change of 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) using Raman spectroscopy. The relative Raman intensity at 590 cm−1 of the CH2 rocking band assigned to the gauche conformer of the NCCC dihedral angle of the butyl group in the [bmim]+ cation increases when the pressure-induced liquid-crystalline phase transition occurs, while that at 610 cm−1 assigned to the trans conformer decreases. Our results show that the high-pressure phase transition of [bmim][PF6] causes the increase of the gauche conformer of the [bmim]+ cation.

Pressure- and temperature-induced Raman spectral changes of 1-butyl-3-methylimidazolium tetrafluoroborate

In the present study, we have investigated the pressure- and temperature-induced Raman spectral changes of 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]). [bmim][BF4] did not crystallize at high pressures below 1.4 GPa. Upon compression, spectral changes in the band due to the CH stretching mode of the alkyl chain are larger than those due to the imidazolium ring, indicating that the environment around the alkyl chain is significantly perturbed. On the other hand, the Raman spectral change of [bmim][BF4] upon cooling at ambient pressure is very small and no crystallization occurs down to 113 K.

Decompression-Induced Crystal Polymorphism in a Room-Temperature Ionic Liquid, N,N-Diethyl-N-methyl-N-(2-methoxyethyl) Ammonium Tetrafluoroborate

We explore the phase behavior of room-temperature ionic liquids (RTILs) compressed under high pressure to determine whether they crystallize or hold a liquid state. RTILs have attractive supercooling properties compared with ordinary molecular liquids, which easily become a glassy state without crystallizing at ambient pressure. Thus, phase behavior under extreme stress, such as pressure, might yield interesting results. Here, we show that N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate ([DEME][BF4]) could be crystallized upon compression, but it usually formed a superpressed liquid. Alternatively, unusual crystallization could be induced by releasing the pressure on the superpressed liquid. Notably, crystal polymorphism was observed in the decompression process. These facts along with visual observations indicate the possibility of [DEME][BF4] serving as a superpressurized glass. Our findings may facilitate the development of a new range of applications for RTILs that have undergone high-pressure recrystallization.

Existence of nearly-free hydrogen bonds in an ionic liquid, N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate-water at 77 K.

When making the glassy water-ionic liquid, N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate [DEME][BF(4)] solution, the water molecule existing as a single molecule without self-associated state is preserved even at 77 K and survives not to form the hydrogen bond network among themselves as in pure H(2)O liquid. The water molecule might loosely interact with BF(4)(-) anions. At water-rich concentration, H(2)O ice crystals and the water molecules in the ionic liquid coexist as a mixture, which may result from microheterogeneities in the solid aqueous ionic liquid solution.

Deuterated water effect in a room temperature ionic liquid: N,N-diethyl-N-methyl-N-2-methoxyethyl ammonium tetrafluoroborate.

A deuterated water effect in the room temperature ionic liquid (RTIL) N,N-diethyl-N-methyl-N-2-methoxyethyl ammonium tetrafluoroborate, [DEME][BF(4)], is observed in its crystal domain structures; crystallization temperature, T(c); crystal superstructures; and volume contractions. The above effect, seen in [DEME][BF(4)]-0.9 mol % H(2)O mixtures, is reduced in 1.0 mol % 0.75 H(2)O x 0.25 D(2)O and 0.9 mol % 0.5 H(2)O x 0.5 D(2)O mixtures and is completely suppressed in 1.3 mol % D(2)O mixtures. Interestingly, T(c) decreased systemically with D substitutions of water. In contrast to the crystal state, it was found that there is no difference between H(2)O and D(2)O mixtures in the liquid state, on the basis of X-ray diffraction patterns. At around 80-90 mol % H(2)O, the intermolecular correlation of [DEME][BF(4)] as a local structure changes to that of bulk water.

Photocell System Driven by Mechanoluminescence

A mechanoluminescence driven photocell system consisting of a mechanoluminescent (ML) material and a photocell was prepared. The ML material developed in our laboratory is the worlds first material developed for a practical use in the elastic deformation region. In this system, the ML composite (an epoxy pellet including europium-doped strontium aluminate (SAO:E), one of the most efficient ML materials) was used as a light source, and a silicon solar cell was used as the photoelectric converter. With the application of compressive stress to the ML composite pellet in the system, the photocurrent corresponding to the mechanoluminescence was successfully observed.

X-ray diffraction study of ionic liquid based mixtures.

Crystal structures in N,N-diethyl-N-methyl-N-2-methoxyethylammonium tetrafluoroborate, [DEME][BF(4)], are determined by X-ray diffraction method. In [DEME][BF(4)]-based mixtures, various kinds of crystal structures and superstructures are induced by additives, i.e., H(2)O, CH(3)OH, C(2)H(5)OH, and C(6)H(6). Most of the crystal structures in the mixtures are related to that of pure [DEME][BF(4)], though unit cells in 1.1 and 6.1 mol % C(6)H(6) are different from the pure one. Further, [DEME][BF(4)]-C(6)H(6) mixtures require the two kinds of superstructures, where volume per [DEME][BF(4)] is contracted by a small amount of additives. Also, volume contraction and the coexistence of two kinds of superstructures are seen in 0.9 mol % H(2)O. The superstructures are derived from orientational or displacive modulations of [DEME][BF(4)] molecules. In spite of a small amount of additives, molecular interactions of [DEME][BF(4)] are influenced extensively.

Orientational ordering of crystal domains in ionic liquid based mixtures.

By in situ observations using simultaneous X-ray diffraction and the DSC (differential scanning calorimetry) method, the effect of water, methanol, ethanol, and benzene on the crystallization has been observed in an ionic liquid (IL)-rich phase. The IL is a hydrophilic ionic liquid, N, N-diethyl-N-methyl-N-2-methoxyethyl ammonium tetrafluoroborate, [DEME][BF4]. At a small amount of the above additional molecules in the IL, the conventional preferred orientation on the Debye rings was seen by the X-ray diffraction. At 0.9 mol % H2O, twinlike crystal domains develop extraordinary on the microdomains. By the crystal-growth enhancement effect at a slight amount of water, a composite domain structure, which consists of the large domain and the weakly orientated microdomains, is formed without internal strains. Above 2.9 mol % H2O, the domain structure, however, disappears completely. It is remarkable that, in a thermal cycling experiment using pure [DEME][BF4], the similar composite domain structure appeared. This is also caused by an uptake of a slight amount of water.

Pressure-induced Raman spectral changes of N,N,diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate

We have measured Raman spectral changes in one of the aliphatic series of ionic liquids, N,N,diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate, [DEME][BF4], under high pressure. Although the [DEME] cation has mainly two stable conformers of TG +−tg + t and TG +−g + g − t in the liquid state, the population of the minor conformer, i.e. TG +−g + g − t, decreases with increasing pressure. The difference in the partial molar volume between the two conformers was determined from the pressure dependence of the relative Raman intensity ratio.

Conformational Analysis of Quaternary Ammonium-Type Ionic Liquid Cation, N,N-Diethyl-N-methyl-N-(2-methoxyethyl) Ammonium Cation

Conformational preference of N,N-diethyl- N-methyl- N-(2-methoxyethyl) ammonium cation ([DEME] +), which is one of the quaternary ammonium-based ionic liquid cation, in the gas phase has been investigated using a density functional theory (DFT) calculation. Eight candidates for the stable conformers of [DEME] + exist in the gas phase, and can it energetically classify into two groups. One is a five conformers group, which has the N + ⋯ O − intramolecular attractive interaction form (the folded form). The other is a three conformers group, which is the noninteraction form (the extended form). The transformation from the folded form to the extended form induces large changes in the dipole moment and partial charges of N and O atoms. Here we show that the difference in the dipole moment and partial charges of N and O atoms associated with the conformational change of [DEME] + are closely related to the molecular orientation of [DEME]-based ionic liquids in the liquid state.