ARTICLE pubs.acs.org/JPCB
Ultrafast Dynamics in 1-Butyl-3-methylimidazolium-Based Ionic Liquids: A Femtosecond Raman-Induced Kerr Effect Spectroscopic Study Hiroki Fukazawa,† Tateki Ishida,§ and Hideaki Shirota*,†,‡ †
Department of Nanomaterial Science, Graduate School of Advanced Integration Science, and ‡Department of Chemistry, Faculty of Science, Chiba University, 1-33 Yayoi, Inage-ku, Chiba 263-8522, Japan § Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, 38 Nishigo-Naka, Myodaiji, Okazaki 444-8585, Japan
bS Supporting Information ABSTRACT: We investigated the ultrafast dynamics in 1-butyl-3methylimidazolium-based ionic liquids with two series of anions, (1) cyano-group substituted anions (thiocyanate [SCN], dicyanamide [N(CN)2], and tricyanomethide [C(CN)3]) and (2) trifluoromethylsulfonyl-group substituted anions (trifluoromethanesulfonate [OTf], bis(trifluoromethylsulfonyl)amide [NTf2], and tris(trifluoromethylsulfonyl)methide [CTf3]). This was done by femtosecond Raman-induced Kerr effect spectroscopy. From the Fourier transform Kerr spectra of the ionic liquids, the low-frequency spectrum of 1-butyl-3-methylimidazolium tricyanomethide shows a low-frequency shift compared to the ILs with the other cyano-group substituted anions due to the planar structures of the cation and the anion. The relative amplitude of the low-frequency band at approximately 20 cm1 compared to the entire broad spectrum for the ionic liquids with trifluoromethylsulfonyl-group substituted anions becomes larger with the order [OTf] < [NTf2] ≈ [CTf3]. This vibrational band can be attributed to the librational motion of anions and/or the coupling of the translational and reorientational motions.
1. INTRODUCTION Ionic liquids (ILs) refer to molten salts at ambient conditions. Typical ILs consist purely of a cationanion pair. The physical and chemical properties of ILs strongly depend on the combination of cation and anion; therefore, it is possible to design and modify the liquid materials for desired properties and usage by choosing suitable combinations. Since the discovery of air- and water-stable ILs by Wilkes and Zaworotko in 1992,1 ILs have become a very popular research field for chemists, physicists, and material scientists and technologists.213 Variations in the physical properties of ILs are strongly affected by interionic interactions. Therefore, it is important to understand the molecular-level aspects of interionic interactions in ILs. Detailed understandings of the interionic interactions in ILs not only offer new knowledge for basic IL science but also help for precise control and design of materials. One method of evaluating the microscopic interionic interactions in ILs is the direct observation of interionic vibrations. The interionic vibrations in ILs are in the frequency range below 200 cm1, and this is not easy to access by conventional steadystate vibrational spectroscopic methods. The femtosecond Raman-induced Kerr effect spectroscopy (RIKES)1416 and terahertz time domain spectroscopy (THz-TDS)17 techniques allow the tracking of interionic vibration with a high signal-to-noise ratio. These time-resolved spectroscopic techniques complement each r 2011 American Chemical Society
other; RIKES observes the polarizability fluctuation, and THzTDS detects the dipole transition. One of the advantages of femtosecond RIKES is its wide observable frequency range. RIKES typically captures molecular motions within the frequency range of 0.2700 cm1, making it an ideal spectroscopic technique to study ultrafast dynamics in simple molecular liquids,14,15,1822 as well as complex condensed phases.2326 Studies on femtosecond RIKES have shown that anion species and cation’s skelton critically affect the spectral shape of the interionic vibrational bands in ILs.2733 Apart from these RIKES studies on choosing cation and anion species, Quitevis and coworkers focused on the microstructures of ILs32 as well as IL mixtures.33,34 They also examined the low-frequency Kerr spectra for the temperature dependence of 1-methyl-3-pentylimidazolium bis(trifluoromethylsulfonyl)amide, 35 the alkyl group dependence of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amides, 36 and the comparison between symmetrically and asymmetrically substituted 1,3dialkylimidazolium-based ILs. 37,38 Quitevis’s group also collaborated with Triolo’s group to explore both the dynamic and structural aspects of ILs by combining RIKES and X-ray Received: January 13, 2011 Revised: March 16, 2011 Published: April 04, 2011 4621
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motions in ILs are still limited, and further experimental and theoretical/computational studies are required for a broader and deeper understanding of the microscopic aspects in ILs. In this study, we focus on the structure of anions. We compared two series of anions, (1) cyano-group substituted anions (thiocyanate [SCN], dicyanamide [N(CN)2], and tricyanomethide [C(CN)3]) and (2) trifluoromethylsulfonyl-group substituted anions (trifluoromethanesulfonate [OTf], [NTf2], and tris(trifluoromethylsulfonyl)methide [CTf3]) (Figure 1). In this study, we choose [BMIm]þ for the countercation because it is one of the most typical cations in ILs.
Figure 1. Structures of anions and cation for the ILs used in this study.
scattering. 39,40 Wynne and co-workers recently compared the Kerr spectrum with the dielectric loss in several ILs. 41 Fayer and co-workers studied the crossover process between interionic vibrational relaxation and R-relaxation in some ILs. 42,43 The authors of the present paper also reported a series of heavy-atom substitution effects of cations and anions in aromatic and nonaromatic ILs. 31,4447 To reveal the unique nature of ILs, comparative studies such as IL versus neutral binary mixture48 and ILs versus concentrated electrolyte solutions30 were also carried out. Other than studies on femtosecond RIKES, some studies on the lowfrequency dipole transitions in ILs using THz-TDS were also reported in the literature. 4951 Theoretical and computational approaches have also been performed to elucidate the details of the interionic vibration in ILs. Urahata and Ribeiro compared a spectrum of the density of states for 1-butyl-3-methylimidazolium hexafluorophosphate52 with the experimentally obtained Kerr spectra by Giraud et al.27 Margulis and co-workers carried out a molecular dynamics (MD) simulation of 1-methoxyethylpyridinium dicyanamide and found that the decay of the collective polarizability anisotropy exhibits several different time scales originating from inter- and intraionic dynamics.53 Ishida et al. clarified the respective spectral contributions of cation and anion species in 1-butyl-3-methylimidazolium ([BMIm]þ)-based ILs with hexafluorophosphate, hexafluoroarsenate, and hexafluoroantimonate as anions; this was done by a MD simulation study, and the results were compared with the Kerr spectra to understand the atom substitution effects of octahedral anions.47 Ishida applied the polarizable model to his MD simulations in order to examine the polarizability effects on the interionic vibrational dynamics in these ILs.54 Balasubramanian and co-workers calculated the low-frequency vibrational spectra of the [BMIm]þ-based ILs with four anions, nitrate, tetrafluoroborate, hexafluorophosphate, and bis(trifluoromethylsulfonyl)amide ([NTf2]), based on the normal-mode analysis for MD trajectories. They found that low-frequency modes below 100 cm1 exhibit a red shift with an increase in anion size.55 The above-mentioned studies have led to a more in-depth understanding of the interionic vibrational dynamics in ILs; however, reports and spectral data on the low-frequency ionic
2. EXPERIMENTAL METHODS AND QUANTUM CHEMICAL CALCULATIONS 1-Butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIm][OTf]) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([BMIm][NTf2]) were synthesized according to the standard methods.5658 The synthesized ILs were confirmed by 1H NMR (solvent: DMSO-d6) and elemental analysis. Apart from the solvent, no extra peaks from organic compounds were found in the 1H NMR charts, and the elemental analysis data showed that the obtained values were in agreement with the calculated values within (0.4%. Details of the synthesis and assignments for the sample ILs are summarized in the Supporting Information. Water contents of the ILs were measured by Karl Fischer titration using a coulometer (Hiranuma, AQ-300), 66.7 ppm for [BMIm][SCN], 101.5 ppm for [BMIm][N(CN)2], 30.1 ppm for [BMIm][C(CN)3], 22.5 ppm for [BMIm][OTf], 74.7 ppm for [BMIm][NTf2], and 33.6 ppm for [BMIm][CTf3]. 1-Butyl-3-methylimidazolium tris(trifluoromethylsulfonyl)methide ([BMIm][CTf3]) was purchased from Covalent Associates, Inc., and used without further purification. 1-Butyl-3methylimidazolium thiocyanate ([BMIm][SCN], Merck), 1-butyl-3-methylimidazolium dicyanamide ([BMIm][N(CN)2], Aldrich), and 1-butyl-3-methylimidazolium tricyanomethide ([BMIm][C(CN)3], Merck) were first stirred in acetonitrile with activated charcoal and then purified by column chromatography (aluminum oxide and chloroformethanol mixture, 20:1 in volume). All of the ILs were dried in vacuo (∼103 Torr) at 308 K for over 36 h before measurements. Shear viscosities (η) of the ILs were measured by a reciprocating electromagnetic piston viscometer (Cambridge Viscosity, ViscoLab 4100) with a circulating water bath (Yamato, BB300) at 297.0 ( 0.2 K. Surface tensions (γ) of the ILs were measured with a du No€uy tensiometer (Yoshida Seisakusho) at 296.7 ( 0.5 K. The densities (d) were obtained using a volumetric flask at 297.0 ( 0.5 K. Details of the femtosecond optical heterodyne-detected RIKES setup used in this study have already been reported elsewhere.22,59 The light source was a lab-built titanium sapphire laser (laser kit, CDP Corp., TISSA-kit 20) pumped by a Nd:VO4 diode laser (Spectra Physics, Millennia Pro 5sJ). The output power of the titanium sapphire laser was approximately 350 mW. The typical temporal response, which was the cross-correlation between the pump and probe pulses measured using a 200 μm thick KDP crystal (type I), was 33 ( 3 fs (full width at halfmaximum). The scans with high time resolution of 2048 points at 0.5 μm/step were performed for a short time window (6.8 ps). Intermediate time window transients (33 ps) with a data acquisition of 5.0 μm/step and wide time window transients (300 ps) with a data acquisition of 50.0 μm/step were captured. 4622
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Pure heterodyne signals were achieved by combining the transient recording scans for both ∼þ1 and ∼1° rotations of the input polarizer to eliminate the residual homodyne signal. Prior to the femtosecond RIKES measurements, the ILs were injected into a 3 mm optical path length quartz cell (Tosoh Quartz) using a 0.2 μm Anotop filter (Whatman). All of the RIKES measurements were made at 297 ( 1 K.
Ab initio quantum chemical calculations based on the B3LYP/ 6-31þG(d,p) level of theory60,61 were carried out to find the optimized structures, polarizability tensor elements, and atomic charges for the ion species using the Gaussian 03 program suite.62 The obtained atom coordinates and atomic charges (CHelpG algorithm63) are summarized in the Supporting Information. The optimized structures of the pair of [BMIm]þ and [C(CN)3] were also obtained by ab initio quantum chemical calculations at the RHF/6-31þG(d,p) and the B3LYP/6-31þG(d,p) levels. The optimized atom coordinates are summarized in the Supporting Information.
3. RESULTS
Figure 2. Loglog plots of Kerr transients of (a) [BMIm][SCN], (b) [BMIm][N(CN)2], and (c) [BMIm][C(CN)3]. Triexponential fits to the transients are shown by blue lines.
3.1. Static Physical Properties. Data pertaining to the liquid densities, shear viscosities, and surface tensions of the ILs are summarized in Table 1. This table also contains the values of the formula weight and molar volume, estimated by the liquid density and van der Waals volume,64,65 respectively. Several other studies have already reported most of the data pertaining to shear viscosities, liquid densities, and surface tensions at ambient temperatures of the ILs studied here.58,6675 While there are some variations in the reported data, the values at 297 K obtained in this study are reasonably close to the already reported values at ambient temperatures. To the best of our knowledge, the value of the surface tension of [BMIm][C(CN)3], as obtained in this study, is being reported for the first time. As shown in Table 1, the variations in the shear viscosity and surface tension are not very large in each series of anions. When a fair comparison is made (e.g., for monofunctional-group substituted anions, [SCN] and [OTf]), the shear viscosity is lower for the ILs with the cyano-group substituted anions than that for the ILs with the trifluoromethylsulfonyl-group substituted anions. However, the surface tension is larger for the ILs with the cyano-group substituted anions than for the ILs with the trifluoromethylsulfonyl-group substituted anions. 3.2. Ultrafast Dynamics. Figures 2 and 3 show the loglog plots of the Kerr transients for the ILs with (1) the cyanogroup substituted anions ([BMIm][SCN], [BMIm][N(CN)2], and [BMIm][C(CN)3]) and (2) the trifluoromethylsulfonylgroup substituted anions ([BMIm][OTf], [BMIm][NTf2], and [BMIm][CTf3]), respectively. The Kerr transients are normalized by the intensity of electronic response at t = 0. The Kerr transients with strong beats are observed in the time region from 10 fs to several picoseconds. For the longest time scale in the Kerr transients, a triexponential function was used to fit the data within the range of 3270 ps. Although a quadruple exponential function was used to fit the transients, some fits are not reliable and result in parameters containing similar time constants. Apart
Table 1. Formula Weights FW, Liquid Densities d, Molar Volumes V, van der Waals Volumes VVDW, Shear Viscosities η, and Surface Tensions γ of the [BMIm]þ-Based ILsa FW
db (g/cm3)
V (Å3)
VVDW (Å3) (cation/anion)
ηc (cP)
γc (mN/m)
[BMIm][SCN]
197.1
1.067
306.8
195.1 (150.6/44.5)
56.0
48.0
[BMIm][N(CN)2]
205.2
1.058
322.1
206.5 (150.6/55.9)
28.2
46.4
[BMIm][C(CN)3]
229.2
1.046
364.0
228.7 (150.6/78.1)
27.2
48.8
[BMIm][OTf]
288.1
1.295
369.6
232.8 (150.6/82.2)
83.0
35.7
[BMIm][NTf2]
419.1
1.440
483.5
301.5 (150.6/150.9)
50.7
34.1
[BMIm][CTf3]
550.0
1.559
586.0
368.1 (150.6/217.5)
281.1
36.5
ILs
a
The physical properties are the data at 297 K. b (2%. c (3%. 4623
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from multiexponential function fits, a practical function based on the mode coupling theory (MCT) is often used to fit the overdamped decays in ILs.42,43 However, the time constant of the R-relaxation process in ILs is longer than the observed time range in this study, and such a case shows a large uncertainty for the fit parameters.29 Therefore, in this study, we use triexponential functions for data fitting. Triexponential fits are also shown in Figures 2 and 3, and the fit parameters are summarized in Table 2. As shown in the table, the errors are not small for the fit
Figure 3. Loglog plots of Kerr transients of (a) [BMIm][OTf], (b) [BMIm][NTf2], and (c) [BMIm][CTf3]. Triexponential fits to the transients are shown by blue lines.
parameters of the slowest components in the ILs with the trifluoromethylsulfonyl-group substituted anions. This is because the signal intensities for these ILs are smaller than those for the ILs with the cyano-group substituted anions. The Kerr transients were analyzed to produce the frequency domain Kerr spectra by the standard Fourier transform deconvolution analysis.76,77 Figures 4 and 5 show the Fourier transform
Figure 4. Fourier transform Kerr spectra within the frequency range of 0750 cm1 in the series of ILs with cyano-group substituted anions: (a) [BMIm][SCN], (b) [BMIm][N(CN)2], and (c) [BMIm][C(CN)3]. Black denotes the whole spectrum, blue denotes the component of the overdamped picosecond relaxation component, and red denotes the spectrum without the component of the overdamped picosecond relaxation.
Table 2. Triexponential Fit Parameters for Kerr Transients in [BMIm]þ-Based ILs ILs
a1
τ1 (ps)
a2
τ2 (ps)
a3
τ3 (ps) 200.6 ( 13.5
[BMIm][SCN]
0.00660 ( 0.00013
2.34 ( 0.05
0.00132 ( 0.00003
18.4 ( 1.0
0.00053 ( 0.00003
[BMIm][N(CN)2]
0.00958 ( 0.00030
2.00 ( 0.07
0.00323 ( 0.00009
12.1 ( 0.4
0.00183 ( 0.00003
158.9 ( 3.0
[BMIm][C(CN)3] [BMIm][OTf]
0.00721 ( 0.00016 0.00397 ( 0.00092
2.32 ( 0.08 2.20 ( 0.85
0.00237 ( 0.00010 0.00095 ( 0.00066
11.8 ( 0.6 10.2 ( 6.5
0.00151 ( 0.00002 0.00079 ( 0.00010
159.2 ( 3.2 131.9 ( 22.9
[BMIm][NTf2]
0.00507 ( 0.00086
2.07 ( 0.40
0.00168 ( 0.00027
13.1 ( 2.7
0.00096 ( 0.00008
198.4 ( 25.2
[BMIm][CTf3]
0.00268 ( 0.00033
3.02 ( 0.83
0.00095 ( 0.00032
16.8 ( 8.4
0.00031 ( 0.00014
176.8 ( 96.2
4624
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Figure 5. Fourier transform Kerr spectra within the frequency range of 0750 cm1 in the series of ILs with trifluoromethylsulfonyl-group substituted anions: (a) [BMIm][OTf], (b) [BMIm][NTf2], and (c) [BMIm][CTf3]. Black denotes the whole spectrum, blue denotes the component of the overdamped picosecond relaxation component, and red denotes the spectrum without the component of the overdamped picosecond relaxation.
Kerr spectra of the ILs with (1) the cyano-group substituted anions ([BMIm][SCN], [BMIm][N(CN)2], and [BMIm][C(CN)3]) and (2) the trifluoromethylsulfonyl-group substituted anions ([BMIm][OTf], [BMIm][NTf2], and [BMIm][CTf3]), respectively. The Kerr spectra without the contributions from the overdamped picosecond relaxation processes (second and third exponential components) were also obtained to elucidate the vibrational contributions; these spectra were used for the line shape analysis, according to the traditional RIKES experiments.14,15,1822 The contribution of the overdamped picosecond relaxation processes in each IL is also shown in Figures 4 and 5. The spectra are well-resolved up to approximately 750 cm1, as shown in the figures. The broad spectral bands below 200 cm1 for the ILs also underwent a line shape analysis to reproduce and characterize the broad and complicated spectral shape. The model fit function for
ARTICLE
Figure 6. Low-frequency Kerr spectra within the frequency range of 0250 cm1 and their fit functions for (a) [BMIm][SCN], (b) [BMIm][N(CN)2], and (c) [BMIm][C(CN)3]. Black dots denote the data, red solid lines denote the complete fits, orange solid lines denote the Ohmic functions (eq 1), blue solid lines denote the antisymmetrized Gaussian functions (eq 2), green solid lines denote the Lorentzian functions (eq 3) for the intraionic vibrational modes, and red broken lines denote the sums of eqs 1 and 2.
the low-frequency Kerr spectra used in this study is a sum of Ohmic (eq 1) and antisymmetrized Gaussian functions (eq 2)78 IO ðωÞ ¼ aO ω expðω=ωO Þ (
"
2ðω ωG, i Þ2 IG ðωÞ ¼ aG, i exp ΔωG, i 2 i¼1 " #) 2ðω þ ωG, i Þ2 aG, i exp ΔωG, i 2 3
∑
ð1Þ #
ð2Þ
where aO and ωO are the amplitude and characteristic frequency parameters of the Ohmic line shape, respectively; aG,i, ωG,i, and ΔωG,i are the amplitude, characteristic frequency, and bandwidth parameters for the ith antisymmetrized Gaussian function, respectively. The relative contributions of the fit functions to 4625
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assigned on the basis of the quantum chemical calculations. The bands at approximately 170 and 215 cm1 are due to the cation. Regarding the vibrational modes of the anions, ILs in which the anion consists of only one functional group, that is, either [BMIm][SCN] or [BMIm][OTf], did not show a clear anion vibrational band in this frequency region. Clear intraionic vibrational modes due to the anions with two or three functional groups were confirmed, and the mode shifts to the lower frequency with the larger anion in both of the series of cyanogroup substituted anions and trifluoromethylsulfonyl-group substituted anions (see Table 3). In contrast to the anions with two functional groups ([N(CN)2] and [NTf2]), the modes in the anions with three functional groups ([C(CN)3] and [CTf3]) are separated into two bands. The fit parameters are summarized in Table 3. In this table, the first spectral moments of the lowfrequency Kerr spectra without the contributions of intraionic vibrational modes and picosecond overdamped relaxation for the ionic liquids are defined as M1 Z Z M1 ¼ ωIðωÞ dω= IðωÞ dω ð4Þ
Figure 7. Low-frequency Kerr spectra within the frequency range of 0250 cm1 and their fit functions for (a) [BMIm][OTf], (b) [BMIm][NTf2], and (c) [BMIm][CTf3]. Black dots denote the data, red solid lines denote the complete fits, orange solid lines denote the Ohmic functions (eq 1), blue solid lines denote the antisymmetrized Gaussian functions (eq 2), green solid lines denote the Lorentzian functions (eq 3) for the intraionic vibrational modes, and red broken lines denote the sums of eqs 1 and 2.
R R the spectral areas were also estimated (An = In(ω) dω/ I(ω) dω: I(ω) = IO(ω) þ IG(ω)). A Lorentzian function (eq 3) was added when a clear intraionic vibrational band was observed in the Kerr spectra. IL ðωÞ ¼
aL ðω ωL Þ2 þ ΔωL 2
ð3Þ
where aL, ωL, and ΔωL are the amplitude, peak frequency, and bandwidth parameters for the Lorentzian function, respectively. Figures 6 and 7 show the results of the line shape analysis for the low-frequency broad spectral bands of the ILs with the cyanogroup substituted anions ([BMIm][SCN], [BMIm][N(CN)2], and [BMIm][C(CN)3]) and the trifluoromethylsulfonyl-group substituted anions ([BMIm][OTf], [BMIm][NTf2], and [BMIm][CTf3]), respectively. Clear intraionic bands were confirmed in the low-frequency region. The vibrational bands were
where I(ω) is the frequency-dependent spectral intensity. Note that the fit parameters of the model functions (eqs 1 and 2) are covariant, and the broad spectrum has no clear peak except for intraionic vibrational modes; thus, the fit parameters include rather large errors, as shown in Table 3. The entire fit (not each fit function component) and the first moment do not change much, though there are some variations in the fit parameters. The intermolecular vibrational spectra (and each motion or mode) in molecular liquids cannot be simply explained by the above model functions because the line shapes of each libration and interaction-induced motion do not conform to Ohmic, Gaussian, or Lorentzian line shapes.7984 This is also the case for ILs.47,53,55 Furthermore, the cross-term that is the coupling between the reorientational and translational motions cannot be expressed by the line shape analysis. Nevertheless, this analysis helps us to understand the qualitative or semiquantitative aspects of the entire intermolecular vibrational spectra and allows for the discussion of the spectral features in the low-, intermediate-, and high-frequency regions. Also note that the Kerr spectrum intensity is relative (not absolute) and internal. 3.3. Ab Initio Quantum Chemical Calculations. Ab initio quantum chemical calculations at the B3LYP/6-31þG(d, p) level of theory were performed to obtain the optimized structures, normal modes, and polarizability tensor elements of the ions. The atom coordinates, charges, and polarizability tensor elements of the optimized structures of the cation and anions are summarized in the Supporting Information. The calculations show that there is little change in the atomic charges among the mono-, di-, and trifunctional group substituted anions for both series of anions (Supporting Information). Figure 8 shows the computational Raman spectra of the two series of anions and the cation calculated on the basis of the optimized structures. The calculated Raman spectra are compared to the experimentally obtained Kerr spectra to reveal the contributions of intraionic vibrational modes in the broad low-frequency Kerr spectra. (This will be discussed later in the paper.) We also performed ab initio quantum chemical calculations for the ILs containing [BMIm]þ and [C(CN)3] to evaluate the stable structure of the pair (see Figure 9). The intermolecular vibrational spectrum for [BMIm][C(CN)3] is very different from those for [BMIm][SCN] and [BMIm][N(CN)2], 4626
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a
63.5 71.5
66.0
65.2
[C4mim][C(CN)3] [C4mim][OTf]
[C4mim][NTf2]
[C4mim][CTf3]
4627
0.295 ( 0.118 (0.100)
156.0 ( 0.0 170.3 ( 0.2 121.8 ( 0.0 115.5 ( 0.1
35.3 ( 1.1
25.7 ( 1.0
2.90 ( 0.08
2.93 ( 0.12
[C4mim][C(CN)3]
[C4mim][OTf]
[C4mim][NTf2]
[C4mim][CTf3]
14.4 ( 1.0
5.18 ( 0.07
5.94 ( 0.06
22.3 ( 0.4
8.5 ( 0.1
14.3 ( 0.0
7.40 ( 0.23
14.6 ( 0.3
18.1 ( 0.5
104.9 ( 2.1
17.4 ( 0.8
34.7 ( 2.3
aL2
24.7 ( 2.1
26.3 ( 1.9
26.8 ( 3.4 28.6 ( 3.6
26.9 ( 1.3
28.2 ( 1.9
(cm1)
123.7 ( 0.1
169.7 ( 0.1
214.3 ( 0.1
165.2 ( 0.1
219.7 ( 0.2
211.0 ( 0.4
(cm1)
aG2 (AG2)a
7.03 ( 0.08
14.8 ( 0.1
18.1 ( 0.2
13.9 ( 0.1
19.8 ( 0.5
30.7 ( 1.0
(cm1)
9.26 ( 0.18
28.8 ( 0.6
26.7 ( 1.9
aL3
0.280 ( 0.104 (0.363)
0.357 ( 0.148 (0.461)
0.541 ( 0.036 (0.537) 0.324 ( 0.011 (0.517)
0.735 ( 0.268 (0.455)
0.509 ( 0.185 (0.462)
(B) Intraionic Vibrations ΔωL2 ωL2
7.9 ( 3.8
8.2 ( 3.5
7.9 ( 3.8 8.3 ( 7.4
9.0 ( 2.3
8.8 ( 3.5
(cm1)
(A) Interionic Vibrations ΔωG1 ωG1
R R Area firaction of spectrum (An = In(ω) dω/ I(ω) dω: I(ω) = IO(ω) þ IG(ω)),∑An = 1.
176.2 ( 0.3 182.4 ( 0.1
3.63 ( 0.74
105.9 ( 0.5
[C4mim][N(CN)2]
(cm1)
(cm1)
0.265 ( 0.118 (0.102)
aL1
3.5 ( 0.1 3.2 ( 0.1
0.129 ( 0.001 (0.035)
0.118 ( 0.001 (0.031)
0.315 ( 0.205 (0.076) 0.233 ( 0.194 (0.080)
ΔωL1
3.2 ( 0.1 3.7 ( 0.1
0.185 ( 0.001 (0.031) 0.095 ( 0.001 (0.028)
0.282 ( 0.105 (0.073)
(AG1)a
aG1
0.333 ( 0.078 (0.064)
ωL1
3.3 ( 0.1 3.5 ( 0.1
0.169 ( 0.001 (0.029)
0.210 ( 0.001 (0.029)
[C4mim][SCN]
ILs
70.5
71.6
[C4mim][N(CN)2]
(cm1)
(AO)a
(cm1)
[C4mim][SCN]
ILs
aO
M1
ωO
Table 3. Fit Parameters and First Moments M1 for Low-Frequency Kerr Spectra in [BMIm]þ-Based ILs ωG2
179.4 ( 0.0
211.3 ( 0.1
217.9 ( 0.3
(cm1)
ωL3
28.5 ( 11.3
33.7 ( 15.8
38.6 ( 2.0 49.4 ( 0.5
30.0 ( 11.6
32.9 ( 10.4
(cm1)
ΔωG2
aG3 0.315 ( 0.047 (0.436)
(AG3)a
10.1 ( 0.1
23.4 ( 0.2 25.5 ( 1.3
aL4
0.261 ( 0.018 (0.504)
0.266 ( 0.039 (0.404)
ωG3
(cm1)
ωL4
93.8 ( 2.2
99.9 ( 2.2
99.2 ( 1.8 106.3 ( 1.1
103.3 ( 1.9
102.5 ( 4.6
(cm1)
219.2 ( 0.2
0.327 ( 0.029 (0.356) 0.267 ( 0.026 (0.375)
0.467 ( 0.033 (0.452)
26.1 ( 0.9
(cm1)
ΔωL3
61.8 ( 14.0
70.9 ( 21.7
63.4 ( 6.6 68.4 ( 7.0
73.8 ( 11.5
73.4 ( 15.4
(cm1)
ΔωG3
25.4 ( 0.6
(cm1)
ΔωL4
59.3 ( 1.3
55.0 ( 1.2
54.0 ( 1.1 51.2 ( 0.8
67.4 ( 1.0
70.9 ( 2.7
(cm1)
The Journal of Physical Chemistry B ARTICLE
dx.doi.org/10.1021/jp200370f |J. Phys. Chem. B 2011, 115, 4621–4631
The Journal of Physical Chemistry B
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Figure 10. Normalized low-frequency Kerr spectra with elimination of the contributions of intraionic vibrational modes and picosecond overdamped relaxations. (a) ILs with the cyano-group substituted anions: [BMIm][SCN] (red); [BMIm][N(CN)2] (blue); and [BMIm][C(CN)3] (green). (b) ILs with the trimethylsulfonyl-group substituted anions: [BMIm][OTf] (red); [BMIm][NTf2] (blue); and [BMIm][CTf3] (green).
Figure 8. Raman spectra calculated for gas-phase ions at the B3LYP/631þG(d,p) level of theory for (a) the cyano-group substituted anions, (b) the trifluoromethylsulfonyl-group substituted anions, and (c) the 1-butyl-3-metrhylimidazolium cation.
Figure 9. Optimized structures of [BMIm]þ and [C(CN)3]. The calculation is based on the B3LYP/6-31þG(d,p) level of theory.
and stable planar structures for [BMIm]þ and [C(CN)3] are expected from the calculations. A parallel structure of the cation and anion was confirmed using not only B3LYP/6-31þG(d,p) but also the RHF/6-31þG(d,p) level of theory (Supporting Information). The structure is considered in the discussion of the low-frequency Kerr spectrum of [BMIm][C(CN)3] (vide infra).
4. DISCUSSION The broad band within the frequency range of 0200 cm1 in most common organic molecular liquids is dominated by intermolecular vibrations.14,15,1822 This band is present in ILs, and compared to its reference neutral binary mixture,48 there is a slight high-frequency shift. Intermolecular interactions directly affect the intermolecular vibration in liquids; therefore, the intermolecular vibrational band is a measure of microscopic intermolecular interactions. It is well-known that interionic interactions promote unique features such as nonvolatility and low melting points in ILs. Therefore, it is essential to explore the interionic vibrational band to reveal the detailed molecular-level aspects in the interionic interactions and dynamic features of ILs. Here, we discuss the features of the interionic vibrational bands of the six ILs measured in this study. Figure 10 shows the spectral-area-normalized low-frequency Kerr spectra of the ILs with (a) cyano-group substituted anions and (b) trifluoromethylsulfonyl-group substituted anions. It should be noted that the clear intraionic vibrational modes and the contributions of the picosecond overdamped relaxations were removed from the Fourier transform Kerr spectra. The frequency-dependent spectral-area-normalized intensity IN(ω) is defined as Z IN ðωÞ ¼ IðωÞ= IðωÞ dω ð5Þ where I(ω) is the frequency-dependent intensity without the contribution of intraionic vibrational modes (sum of eqs 1 and 2) and the integral part is normalized to be unity. From Figure 10, two remarkable points are found in the normalized spectra. First, 4628
dx.doi.org/10.1021/jp200370f |J. Phys. Chem. B 2011, 115, 4621–4631
The Journal of Physical Chemistry B the [BMIm][C(CN)3] spectrum shows a low-frequency shift compared to the ILs with other cyano-group substituted anions. Second, in the trifluoromethylsulfonyl-group substituted anions, the relative amplitude in the low-frequency region less than 30 cm1 to the entire broad spectrum or the relative amplitude in the intermediate- to high-frequency region more than approximately 50 cm1 becomes larger with the order [BMIm][OTf] < [BMIm][NTf2] ≈ [BMIm][CTf3]. It is clear from a comparison of the low-frequency spectra of the three ILs with the cyano-group substituted anions that the first moment of the spectrum for the [BMIm][C(CN)3] exhibits a low-frequency shift compared to that of [BMIm][SCN] and [BMIm][N(CN)2]. The difference in the high-frequency region, more than 70 cm1, is obvious, and the result of the line shape analysis implies that the spectrum width of the highest-frequency component in [BMIm][C(CN)3] is narrower than that in the spectra of [BMIm][SCN] and [BMIm][N(CN)2]. Neutral aromatic liquids are known to show unique bimodal spectral shapes in the low-frequency Kerr spectra;22 further, the aromatic ring libration (or molecular reorientational motion) is dominant in the spectra, particularly in the high-frequency region (more than 50 cm1).22,79,80,8589 In both the experiments27,35,46,48 and simulations,47,52 the aromatic-cation-based ILs also showed similar features in low-frequency spectra. Thus, the unique spectral feature of the high-frequency region in the low-frequency Kerr spectrum in [BMIm][C(CN)3] can be accounted for by the smaller inhomogeneity of the interionic vibrations in the high-frequency region of [BMIm][C(CN)3] compared to [BMIm][N(CN)2] and [BMIm][SCN]. The result of the narrower vibrational band in the highest-frequency region leads to the lower shift of the first moment of the spectrum. The ab initio quantum chemical calculations for the optimization of the structure of [C(CN)3] show a planar structure (see the Supporting Information for the atom coordinates). The flat structure of the [C(CN)3] is indicative of the formation of a complex with the planar [BMIm]þ due to ππ interactions (mediated by the positive cation and negative anion) and/or geometric matching. This structure is for the specific pair of one cation and anion; it does not account for the interionic interactions in a real liquid system. However, we believe that the structure of the pair is reflected in the microscopic structure of the liquid, and thus, the discussion of the experimental spectral features together with quantum chemical calculations would support the discussion. As shown in Figure 9, [BMIm]þ and [C(CN)3] show a stable planar structure, and the liquid structure of the [BMIm][C(CN)3] is more compact and homogeneous than those of [BMIm][SCN] and [BMIm][N(CN)2]. Accordingly, the vibrational band of the intermolecular vibrational motion is narrower in [BMIm][C(CN)3] than that in [BMIm][SCN] and [BMIm][N(CN)2]. Thus, we observe the narrowing of the highest-frequency component in the Kerr spectrum for [BMIm][C(CN)3] compared to [BMIm][SCN] and [BMIm][N(CN)2]. [BMIm][C(CN)3] is liquid at room temperature, and its microscopic structure is not ordered unlike a crystalline,9093 but X-ray or neutron scattering studies might provide evidence for the microscopic-specific structure. The relative intensity in the low-frequency region below 30 cm1 for ILs with the trifluoromethylsulfonyl-group substituted anions is rather unusual. Most of the cases in polymer and oligomer solutions show that the intensity in the low-frequency region of the intermolecular vibrational spectrum is lower for heavier polymers and oligomers than that for lighter
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monomers.9496 It is also reported that the intensity in the low-frequency region below 20 cm1 of the low-frequency Kerr spectra of [BMIm]þ-based ILs with hexafluoropnictogenate anions becomes lower with the substitution from [PF6] to [AsF6] and [SbF6].46 MD simulations of the three ILs showed that the collision-induced motion was significant in [BMIm][PF6] but not active in the other two ILs with heavier anions.47 However, the trend observed in the ILs studied here is contrary to the above-reported cases. One reason for this unique trend may be the intraionic vibrational mode of the anions. As shown in Figure 8, the calculated Raman spectra showed that there were no strong Raman-active intraionic vibrational modes in the low-frequency region (below 50 cm1) for [OTf], [NTf2], and [CTf3]. Therefore, the band at ∼20 cm1 for ILs with trifluoromethylsulfonyl-group substituted anions was most likely not due to intraionic vibrational modes. Girud et al.27 and Rajian et al.35 investigated ILs with [NTf2] and also attributed the band at ∼20 cm1 to interionic vibrations. No low-frequency bands are observed in the spectra of the ILs with the cyano-group substituted anions (Figures 6 and 8a), again confirming that the low-frequency band at ∼20 cm1 in the ILs with the trifluoromethylsulfonyl-group substituted anions arises from the interionic vibrational motions of the anions. The band at ∼20 cm1 is thought to be mainly due to the libration of the anion, which is an alternative motion to the main intermolecular vibrational motions in molecular liquids because the relative amplitude of the low-frequency bands becomes larger compared to that of the high-frequency region, which is most likely due to the cation’s ring libration. The polarizability anisotropy volume of [OTf] (0.309 Å3) is much smaller than that of [NTf2] (4.065 Å3), and [CTf3] (3.970 Å3) and [NTf2] and [CTf3] show similar polarizability anisotropy volumes (see the Supporting Information). Therefore, the result of the low-frequency band intensities of the ILs with trifluoromethylsulfonyl-group substituted anions (Figure 10) is reasonable when the anion’s volume fractions in a unit space are taken into consideration. It should be noted again that the trend of increasing intensity in the low-frequency band for ILs with trifluoromethylsulfonyl-group substituted anions from [OTf] to [NTf2] to [CTf3] would be reversed if translational motions such as collision-induced and interaction-induced motions were dominant in this frequency region (see above discussion). In addition, we should consider a coupling (or cross-term) of the translational and reorientational motions. Several MD simulations have revealed that coupling between the translational and reorientational motions significantly influences the intermolecular vibrational spectrum in molecular liquids,79,82,83,97 particularly in the low-frequency region. In the case of conventional molecular liquids, MD simulations by Ishida et al. showed that the coupling motion also affects the interionic vibrational spectra in ILs.47 It is unclear whether the coupling in the present ILs has a positive or negative contribution to the interionic vibrational spectra; however, it is possible that the spectral difference in the low-frequency region below 30 cm1 in ILs with trifluoromethylsulfonyl-group substituted anions is partially caused by a difference in the coupling of the translational and reorientational motions. Further MD simulations could reveal this unique feature in the series of the ILs with the trifluoromethylsulfonylgroup substituted anions at the molecular level. Before providing a summary of this work, we would like to briefly mention the relationship between the interionic vibrational spectra and bulk properties. Previously, in 40 aprotic 4629
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The Journal of Physical Chemistry B molecular liquids including 20 aromatic liquids and 20 nonaromatic liquids, we reported a correlation between the first moment of the intermolecular vibrational band and the square root of the value of surface tension divided by liquid density.22 The correlations for aromatic ILs and nonaromatic ILs are, however, clearly different; the first spectral moments for aromatic ILs are only weakly dependent on the surface tension and liquid density, while those for nonaromatic ILs are strongly dependent.98 This difference in the correlation between aromatic and nonaromatic ILs could be due to the microheterogeneity in the ILs and the large contribution of the ring libration to the interionic vibrational spectra. Overall, the present results are consistent with the above interpretations. For the aromatic ILs, the data point of [BMIm][C(CN)3] that is at a distance from the correlation between the first spectral moment and the square root of the value of the surface tension divided by the liquid density is probably the result of the narrower libration mode due to the microscopic structure of the IL, that is, the planar structure between the cation’s ring and the flat anion. It is speculative to conclude the correlations in ILs at the moment. However, we believe that more data collection of the low-frequency spectra for ILs would be helpful in revealing deeper and broader microscopic and dynamic aspects in ILs.
5. CONCLUSIONS In this study, we explored the ultrafast dynamics in [BMIm]þbased ILs with two series of anions, (1) cyano-group substituted anions ([SCN], [N(CN)2], and [C(CN)3]) and (2) trifluoromethylsulfonyl-group substituted anions ([OTf], [NTf2], and [CTf3]), by means of femtosecond RIKES. We found from the low-frequency Kerr spectra of the ILs that the [BMIm][C(CN)3] spectrum shows a low-frequency shift when compared to the other cyano-group substituted anions, [BMIm][SCN] and [BMIm][N(CN)2]. From ab initio quantum chemical calculations for the optimized structure of [BMIm]þ and [C(CN)3], the low-frequency shift is most probably caused by low inhomogeneity due to the planar complex structure of the cation and the anion, compared to the other ILs with cyano-group substituted anions. For the ILs with trifluoromethylsulfonyl-group substituted anions, we found that in the low-frequency Kerr spectra, the relative intensity in the low-frequency band at ∼20 cm1compared to the entire broad spectrum (or the amplitude in the intermediate- to highfrequency region) becomes larger with the heavier anion, [OTf] < [NTf2] ≈ [CTf3]. This unique vibrational band in the series of ILs with the trifluoromethylsulfonyl-group substituted anions can be attributed to the librational motion of the anions and/or the coupling between the translational and reorientational motions. ’ ASSOCIATED CONTENT
bS
Supporting Information. Details of the synthesis procedures for the sample ILs and quantum chemistry calculation results (atom coordinates, atom charges (CHelpG algorithm), polarizability tensor elements, mean polarizability volumes, and polarizability anisotropy volumes). This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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’ ACKNOWLEDGMENT This work was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan (Grant-in-Aids for Young Scientists (A): 21685001). ’ REFERENCES (1) Wilkes, J. S.; Zaworotko, M. J. J. Chem. Soc., Chem. Commun. 1992, 965. (2) Ionic Liquids in Synthesis, 2nd ed.; Wasserscheid, P.; Welton, T., Eds.; Wiley-VCH: Weinheim, Germany, 2008. (3) Electrochemical Aspects of Ionic Liquids; Ohno, H., Ed.; WileyInterscience: Hoboken, NJ, 2005. (4) Ionic Liquids: Theory, Properties, New Approaches; Kokorin, A., Ed.; InTech: Rijeka, Croatia, 2011. (5) Welton, T. Chem. Rev. 1999, 99, 2071. (6) Wilkes, J. S. Green Chem. 2002, 4, 73. (7) Hough, W. L.; Rogers, R. D. Bull. Chem. Soc. Jpn. 2007, 80, 2262. (8) Rogers, R. D.; Voth, G. A. Acc. Chem. Res. 2007, 40, 1077. (9) Weingaertner, H. Angew. Chem., Int. Ed.. 2008, 47, 654. (10) Ohno, H.; Fukumoto, K. Electrochemistry 2008, 76, 16. (11) Plechkova, N. V.; Seddon, K. R. Chem. Soc. Rev. 2008, 37, 123. (12) Wishart, J. F. Energy Environ. Sci. 2009, 2, 967. (13) Castner, E. W., Jr.; Wishart, J. F. J. Chem. Phys. 2010, 132, 120901. (14) McMorrow, D.; Lotshaw, W. T.; Kenney-Wallace, G. A. IEEE J. Quantum Electron. 1988, 24, 443. (15) Lotshaw, W. T.; McMorrow, D.; Thantu, N.; Melinger, J. S.; Kitchenham, R. J. Raman Spectrosc. 1995, 26, 571. (16) Righini, R. Science 1993, 262, 1386. (17) Schmuttenmaer, C. A. Chem. Rev. 2004, 104, 1759. (18) Kinoshita, S.; Kai, Y.; Ariyoshi, T.; Shimada, Y. Int. J. Mod. Phys. B 1996, 10, 1229. (19) Castner, E. W., Jr.; Maroncelli, M. J. Mol. Liq. 1998, 77, 1. (20) Smith, N. A.; Meech, S. R. Int. Rev. Phys. Chem. 2002, 21, 75. (21) Zhong, Q.; Fourkas, J. T. J. Phys. Chem. B 2008, 112, 15529. (22) Shirota, H.; Fujisawa, T.; Fukazawa, H.; Nishikawa, K. Bull. Chem. Soc. Jpn. 2009, 82, 1347. (23) Loughnane, B. J.; Farrer, R. A.; Scodinu, A.; Reilly, T.; Fourkas, J. T. J. Phys. Chem. B 2000, 104, 5421. (24) Farrer, R. A.; Fourkas, J. T. Acc. Chem. Res. 2003, 36, 605. (25) Hunt, N. T.; Jaye, A. A.; Meech, S. R. Phys. Chem. Chem. Phys. 2007, 9, 2167. (26) Castner, E. W., Jr.; Wishart, J. F.; Shirota, H. Acc. Chem. Res. 2007, 40, 1217. (27) Giraud, G.; Gordon, C. M.; Dunkin, I. R.; Wynne, K. J. Chem. Phys. 2003, 119, 464. (28) Shirota, H.; Funston, A. M.; Wishart, J. F.; Castner, E. W., Jr. J. Chem. Phys. 2005, 122, 184512. (29) Shirota, H.; Wishart, J. F.; Castner, E. W., Jr. J. Phys. Chem. B 2007, 111, 4819. (30) Fujisawa, T.; Nishikawa, K.; Shirota, H. J. Chem. Phys. 2009, 131, 244519. (31) Shirota, H.; Fukazawa, H.; Fujisawa, T.; Wishart, J. F. J. Phys. Chem. B 2010, 114, 9400. (32) Xiao, D.; Rajian, J. R.; Cady, A.; Li, S.; Bartsch, R. A.; Quitevis, E. L. J. Phys. Chem. B 2007, 111, 4669. (33) Xiao, D.; Rajian, J. R.; Li, S. F.; Bartsch, R. A.; Quitevis, E. L. J. Phys. Chem. B 2006, 110, 16174. (34) Xiao, D.; Rajian, J. R.; Hines, L. G.; Li, S.; Bartsch, R. A.; Quitevis, E. L. J. Phys. Chem. B 2008, 112, 13316. (35) Rajian, J. R.; Li, S. F.; Bartsch, R. A.; Quitevis, E. L. Chem. Phys. Lett. 2004, 393, 372. (36) Hyun, B. R.; Dzyuba, S. V.; Bartsch, R. A.; Quitevis, E. L. J. Phys. Chem. A 2002, 106, 7579. (37) Xiao, D.; Larry G. Hines, J.; Li, S.; Bartsch, R. A.; Quitevis, E. L.; Russina, O.; Triolo, A. J. Phys. Chem. B 2009, 113, 6426. 4630
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