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J. Phys. Chem. B 2007, 111, 4854-4859
Terahertz Time-Domain Spectroscopy of Imidazolium Ionic Liquids† Kohji Yamamoto,*,‡ Masahiko Tani,§ and Masanori Hangyo§ PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan, Institute of Laser Engineering, Osaka UniVersity, 2-6 Yamadaoka, Suita, Osaka 565-0871, Japan ReceiVed: October 31, 2006; In Final Form: March 22, 2007
We have measured the terahertz (THz) complex dielectric spectra of imidazolium ionic liquids by THz timedomain spectroscopy (THz-TDS) in the frequency range from 5 (0.15 THz) to 140 cm-1 (4.2 THz). The ionic liquids investigated are 1-ethyl-3-methylimidazolium (EMIm+)/trifluoromethanesulfonate (TfO-), EMIm+/ tetrafluoroborate (BF4-), 1-butyl-3-methylimidazolium (BMIm+)/TfO-, and BMIm+/BF4-. The dielectric values of the ionic liquids in the THz region are similar to those of short-chain alcohols. The THz dielectric values are related to subpicosecond-to-picosecond dynamics. The same trend has been observed in the empirical polarity ET(30) although it is related to the static characteristics of polarity and hydrogen bonding ability. A difference between the two types of liquids is observed in the THz dielectric spectral shapes: the ionic liquids show structured lineshapes but short-chain alcohols show much less structured ones. The structured lineshapes of the ionic liquids reflect the low-frequency motions of interion and/or intramolecular vibrations. When the ionic liquids composed of the different imidazolium cations contain the same anions as counterions, their density-normalized THz dielectric spectra above 20 cm-1 bear strong resemblance to each other in shape and magnitude. It shows clearly that the THz spectra do not originate from the intramolecular vibrations of the imodazolium cations. All of the intramolecular vibrations of the anions are located above 140 cm-1 except the CF3-SO3 torsion of TfO-, the band of which alone cannot explain the broad THz dielectric spectra of the ionic liquids. Therefore, we conclude that the interion vibrations rather than the intramolecular vibrations dominantly contribute to the THz dielectric spectra. The results strongly indicate that even in the liquid phase the ionic liquids have local structures similar to their solid-phase structures.
Introduction A group of organic salts which exist in the liquid state at room temperature are generally called ionic liquids. Air- and water-stable room-temperature ionic liquids composed of 1-ethyl3-methylimidazolium (EMI+) were synthesized in 1992 by Wilkes and Zaworotko.1 Over the past decade, room-temperature organic ionic liquids have received extensive attention as environmentally benign alternatives to conventional organic solvents and as innovative electrochemical solutions.2-4 Although these ionic liquids are “organic solvents”, they exhibit useful properties which have not been achieved in conventional organic solvents, including extremely low volatility and combustibility.5,6 These properties make it possible to recycle ionic liquids efficiently in organic syntheses when they are used as reaction solvents. In addition, ionic liquids provide new types of reaction media for material syntheses3,4,7 and for enzymatic reactions.8,9 One of the critical factors in regulating chemical reaction rates is the strength of the solvent polarity. Because the solvent polarity depends on all possible nonspecific and specific intermolecular interactions between solute and solvent molecules, the polarity scale is not so easy to define and to express quantitatively10 that all of the polarity scales presented up to now are obtained empirically. There are well-established correlations between the dielectric constant and other measures of †
Part of the special issue “Physical Chemistry of Ionic Liquids”. * To whom correspondence should be addressed. E-mail: kohji@ ile.osaka-u.ac.jp. ‡ PRESTO. § Osaka University.
polarity in molecular liquids10 although the relations of this kind have not yet been clearly established in ionic liquids. To investigate the polarity of ionic liquids, their scales have been determined in several ways,11-15 including the static dielectric constant16 and the ET(30) scale.17 The polarity parameter, ET(30), of a solvent is defined as the molar transition energy at the peak frequency of the intramolecular charge transfer absorption band of 4-(2,4,6-triphenyl-1-pyridinio)-2,6-diphenylphenolate, that is, Reichardt’s dye, dissolved in the solvent.10 For example, imidazolium ionic liquids have ET(30) values comparable with those of methanol and ethanol.17 Both the static dielectric constant and the ET(30) scale reflect static or quasistatic properties of the solvent polarity. In the course of a chemical reaction, however, interactions between solute and solvent molecules change over time. As a result, solute molecules are affected by the time-varying solvent polarity. Investigation of the dynamic polarity, that is, frequencydependent polarity, is the key to revealing the reaction characteristics in ionic liquids as reaction media. Solvation dynamics related to the dynamical polarity of ionic liquids has been investigated by dynamic Stokes shift experiments using a dye molecule dissolved in an ionic liquid.18-23 In these measurements, the solvation processes in the subnanosecond-to-nanosecond region have been extensively investigated. The observed solvation times in this region turn out to be dependent on dye molecules even if they are dissolved in the same ionic liquids.18d Thus, considerable caution must be exercised in interpreting the subnanosecond-to-nanosecond dynamics involving translational and rotational relaxation
10.1021/jp067171w CCC: $37.00 © 2007 American Chemical Society Published on Web 04/10/2007
THz-TDS of ionic liquids processes in ionic liquids from the dynamic Stokes shift experiments. In contrast, the solvation processes within several picoseconds have not fully been studied. For example, it was predicted that as much as 50% of the total amount of the solvation was completed within several picoseconds for imidazolium ionic liquids18,20 because unresolved relaxation dynamics was observed within instrumental time resolutions of picoseconds. In addition, it was implied that the unrevealed ultrafast relaxation process in this region was present only in imidazolium ionic liquids18b but not in other kinds of ionic liquids including ammonium and phosphonium ionic liquids. There is no doubt that the ultrafast dynamics plays an important role in the first stage of solvation dynamics in imidazolium ionic liquids. Ultrafast solvation dynamics as well as ultrafast ion motions of ionic liquids has been studied by molecular dynamic simulations.24-28 These studies indicated that the first stage of the solvation processes within several picoseconds is associated with ion monomer motions and/or collective cation-anion motions. Therefore, understandings of microscopic and dynamic pictures of ionic liquids can reveal the specific nature of ionic liquids. The dynamical polarity scale can be given by the complex dielectric spectra.29-31 This scale is determined without dissolving other molecules, enabling us to obtain the artifact-free dynamical polarity properties. Based on the continuum dielectric theory,32 the complex dielectric spectra can also theoretically provide temporal solvation behavior which is to be compared with that obtained by experiments such as dynamical Stokes shift measurements. Thus, the complex dielectric spectra of ionic liquids can give clear and broad pictures of ultrafast solvation dynamics. The dynamic behavior of ionic liquids slower than tens of picoseconds has been examined by conventional dielectric spectroscopy because the achieved frequencies of this method were less than several tens of GHz.29,30 From the difference of the dielectric constant between the GHz and optical regions, it has been predicted that an unrevealed dielectric response process is present in ionic liquids in the THz region.29,30 Therefore, higher frequency dielectric data up to the THz region are desired to investigate the ultrafast process of ionic liquids. The terahertz-time domain spectroscopy (THz-TDS) can be used to measure the dielectric spectra in the THz region. In the THz-TDS, we detect the electric field of a THz wave as a function of time, the Fourier transformation of which gives the amplitude and phase of the THz wave as a function of frequency. The analysis of the complex transmittance obtained from the reference and sample-transmitted THz waves gives the complex dielectric spectrum in the THz region.33-36 One early paper observed the THz dielectric spectra of 1-ethyl-3-methylimidazolium (EMIm+) trifluoromethanesulfonate (TfO-) using the THz-TDS.37 The measured frequency range of 5-50 cm-1 was rather limited, and the paper reported only one ionic liquid. In order to investigate the ultrafast dynamics of ionic liquids, it is necessary to study the THz dielectric spectra in a wider frequency range with changing the ion species systematically. This paper reports the research intended to investigate ultrafast dynamics and low-frequency modes of imidazolium ionic liquids using the THz-TDS. For systematic investigations of the dependence on ion species, we carried out the THz-TDS measurements of EMIm+/TfO-, EMIm+/tetrafluoroborate (BF4-), 1-butyl-3-methylimidazolium (BMIm+)/TfO-, and BMIm+/ BF4-, shown in Chart 1. We have obtained the THz dielectric spectra of the ionic liquids from 5 (0.15 THz) to 140 cm-1 (4.2 THz). We have found a strong anion dependence of the THz
J. Phys. Chem. B, Vol. 111, No. 18, 2007 4855 CHART 1: Molecular Structures of 1-Ethyl-3-methylimidazolium (EMIm+), 1-Butyl-3-methylimidazolium (BMIm+), Trifluoromethanesulfonate (TfO-), and Tetrafluoroborate (BF4-)
dielectric spectra in this region and discussed the interion motions of the ionic liquids. Terahertz spectra shown in this article are represented in complex dielectric constant, not in the set of refractive index and absorption coefficient. There is no difference in physical information included when one uses either representation (Experimental Section). However, the complex dielectric spectra are related more directly to dipole moment dynamics of a sample than the set of the refractive index and absorption coefficient spectra.36 For example, the absorption coefficient is defined as absorbance per unit length, but it does not contain the effect of the refractive index which changes the optical path length, that is, the effective interaction length between a sample and a THz wave. On the other hand, this effect is included in the complex dielectric spectra. Therefore, we consider the dielectric spectra to be more appropriate physical measures than the refractive index and absorption coefficient spectra to investigate dynamics of ionic liquids. Experimental Section We carried out the THz-TDS measurements with a standard system using emitter and detector photoconductive (PC) switches,33,34 each of which had an antenna structure fabricated on a low-temperature-grown gallium arsenide substrate. A modelocked Ti:sapphire laser (Tsunami, Spectra Physics), which generated 100-fs pulses at 800 nm in wavelength, pumped the PC switches to generate and detect pulsed THz waves. A hemispherical silicon lens was attached to each PC switch to raise the efficiencies of the THz waves outgoing from the emitter to air or incoming from air to the detector. The THz wave was propagated in air from the emitter to the detector using four off-axis parabolic mirrors. The first one made the THz wave collimated, the next two made it focused and collimated, and the last one made it focused onto the detector. The timedependent electric field of the THz wave was measured by scanning the relative delay times between the femtosecond pulses to the emitter and detector PC switches. An ionic liquid sample was put into an assembled cell when we conducted the THz-TDS measurement. We used two 3-mm thick silicon plates as cell windows. A sample thickness was controlled by a spacer sandwiched between the two windows. We used another pair of 3-mm thick silicon plates as a reference cell. We had their surfaces in contact when we measured a reference THz wave. After we computed Fourier transforms of the THz waves transmitted through the sample cell and the reference cell, E ˜ S(ν˜ )and E ˜ R(ν˜ ), respectively, we obtained the
4856 J. Phys. Chem. B, Vol. 111, No. 18, 2007
Figure 1. Complex dielectric spectra of the imidazolium ionic liquids in the THz region: (a) EMIm+/TfO- (solid lines) and BMIm+/TfO(dotted lines) and (b) EMIm+/BF4- (solid lines) and BMIm+/BF4(dotted lines).
complex refractive index n˜ (ν˜ ) from the complex transmittance of a sample, which was calculated by the ratio of the two waves (E ˜ S(ν˜ )/E ˜ R(ν˜ )). The complex dielectric constant of a sample was calculated using the relation ˜ (ν˜ ) ) [n˜ (ν˜ )]2. The precise method of the THz-TDS analysis for a sample contained in the cell will be given elsewhere.38 The ionic liquid samples investigated were EMIm+/TfO(Wako Pure Chemical Industry), EMIm+/BF4- (Tokyo Chemical Industry), BMIm+/TfO- (Lancaster Synthesis), and BMIm+/ BF4- (Kanto Chemical Co.). We conducted THz-TDS measurements of the ionic liquids under an ambient temperature (20.0 ( 0.5°C). We changed a sample thickness from 50 to 500 µm to obtain best spectra for each sample. All of the ionic samples purchased were dried for more than 6 h by being kept at 70 °C under a low pressure (
