4724
J. Phys. Chem. B 2007, 111, 4724-4731
Solute Rotation and Solvation Dynamics in an Alcohol-Functionalized Room Temperature Ionic Liquid† Aniruddha Paul and Anunay Samanta* School of Chemistry, UniVersity of Hyderabad, Hyderabad 500 046, India ReceiVed: September 6, 2006; In Final Form: NoVember 22, 2006
Steady-state and time-resolved fluorescence behaviors of two dipolar solutes, coumarin 153 and 4-aminophthalimide, have been studied in an alcohol-functionalized room-temperature ionic liquid, 1-(hydroxyethyl)3-methylimidazolium bis(trifluoromethanesulfonyl)imide. The steady-state fluorescence parameters have been exploited for the estimation of the polarity of this ionic liquid and to obtain information on the hydrogen bonding interaction between the ionic liquid and the probe molecules. The time-resolved measurements have been focused on the dynamics of solvation by studying the dynamic Stokes shift in the ps-ns time scale and solute rotation by measuring the time dependence of the fluorescence anisotropy. The time-resolved anisotropy studies reveal a significant slow down of the rotational motion of one of the probe molecules. The timedependent fluorescence Stokes shift measurements suggest that the time-resolvable part of the dynamics is biphasic in nature, highly dependent on the probe molecule and the ultrafast component is comparatively less than that in other ionic liquids. The influence of the hydrogen bonding interaction between the probe molecules and the ionic liquids on the solute rotation and the various components of the solvation dynamics is carefully analyzed in an attempt to obtain further insight into the mechanism of solvation in these novel media.
1. Introduction Room-temperature ionic liquids (RTILs), which are salts comprising an organic cation and an inorganic anion with melting points below the room temperature, continue to receive great attention from researchers worldwide.1-4 This interest is primarily due to the potential of these substances serving as green alternatives to volatile organic solvents.1,4 The negligible vapor pressure at ambient conditions is perhaps the most attractive property of the RTILs from the point of view of their environment-friendly nature. Wide liquid range, high thermal stability, high ionic conductivity, miscibility with other solvents, and the nonreactive and recyclable nature of these substances are a few other properties that make these substances suitable as solvents for various applications. RTILs have already found use as solvents for separation process,2 organic and inorganic synthesis,1 electrochemical studies,3 and a few other applications. The fact that many of the properties of the RTILs can be tuned by proper choice of the cationic and anionic components makes these substances even more attractive. The most commonly used RTILs are based on nonsymmetrically substituted N,N′-dialkylimidazolium cations (such as the 1-butyl-3-methylimidazolium ion, usually abbreviated as [bmim]+ and the 1-ethyl-3-methylimidazolium ion, abbreviated as [emim]+) and noncoordinating bulky inorganic anions such as PF6-, BF4-, and [(CF3SO2)2N]- (commonly represented as Tf2N-) ions. Because these are polar liquids,5-7 solvation of the dipolar solutes in RTILs is an important aspect. Understanding the mechanism and dynamics of solvation in RTILs is a difficult and challenging problem because of its complex nature.8 Several studies in recent years have focused on this topic with the aim of obtaining insight into the mechanism of solvation in these * Corresponding author. Fax: +91-40-23011594. E-mail: assc@uohyd. ernet.in. † Part of the special issue “Physical Chemistry of Ionic Liquids”.
novel media.8-21 Because solvation leads to a substantial Stokes shift of the fluorescence spectrum of a dipolar molecule, the most commonly exploited procedure for studying the solvation dynamics is to follow the time-dependent shift of the fluorescence spectrum (dynamic Stokes shift) of a dipolar probe molecule following its electronic excitation using a short pulse of light.22 Ever since the early works of Karmakar and Samanta,9 several studies have been carried out in RTILs employing this technique.9-21 These studies have suggested that the timeresolvable component of the dynamics in RTILs is slow and biphasic or nonexponential in nature. These studies have also revealed a dependence of the average solvation time on the viscosity of the media and probe molecules.9,12,13 Interestingly, all these studies in imidazolium ionic liquids have also indicated that 40-50% of the solvation is too rapid to be time-resolved in experimental setups having a time resolution of 25 ps. The nature of the physical motion that contributes to this ultrafast (or missing) component, the exact time scale of this event, and the method of determination of the extent of the missing component are current topics of considerable speculation and debate.13-17 In this context, it is to be noted that, although the absence of the ultrafast component in ammonium and phosphonium ionic liquids suggests that this component in imidazolium ionic liquids may be due to the small amplitude motions of the polarizable and planar cationic ring system in the vicinity of the probe molecule,14 the presence of an ultrafast component in a pyrrolodinium RTIL, which is structurally similar to the ammonium salts, contradicts this assignment.12 In order to understand the solvation process in RTILs, a number of simulation studies have been performed.23-25 One of the early studies by Shim et al. attributes the fast component of the dynamics to the translational motion of the anion and the slow component to the overall diffusional motion of the cation and anion.23 Kobrak and Znamenskiy, on the other hand, assigned the ultrafast component of the dynamics to the
10.1021/jp065790z CCC: $37.00 © 2007 American Chemical Society Published on Web 01/06/2007
Rotation and Dynamics in a RTIL collective cation-anion motion.24 Later, Shim et al. suggested that ultrafast dynamics is dependent on the local density of the ions near the probe molecules.25 In the case of high local density of ions near the probe molecule at the time of excitation, it is shown that the ultrafast component is governed by the motion of a few ions close to the probe molecule. However, for low initial density, the ions from the further region contribute to the ultrafast dynamics. The solvation dynamics has also been studied in the mixtures of RTILs and conventional solvents and in ionic liquid-based microemulsions.19,20 Recent femtosecond time-resolved ultrafast dynamical studies based on the Kerr effect have also revealed the intermolecular dynamics and the time scales of the different relaxation processes in imidazolium and pyrrolidinium ionic liquids.26 Because functionalization of the RTILs by covalently tethering a functional group to the cation or anion (or both) of the imidazolium salts (or salts based on other cations such as pyridinium ion) imparts a particular capability to the ionic liquids, which in turn can enhance their utility for specific applications, there is considerable current interest in the functionalized ionic liquids, in particular, those based on the substituted imidazolium moiety.27-28 During the last 5 to 6 years, various types of functionalized ionic liquids, categorized as “task-specific” ionic liquids (TSILs), have been designed and developed for specific purposes such as catalysis, organic synthesis, separation of specific materials, as well as for the construction of nanostructure materials and ion conductive materials, etc.28 Taking into consideration the fact that a clear understanding of the solvation process in RTILs is possible only when the dynamical data are available for a variety of the RTILs, we have studied solvation dynamics in an alcohol-functionalized imidazolium ionic liquid, namely, 1-(hydroxyethyl)-3-methylimidazoilum bis(trifluoromethanesulfonyl)imide, abbreviated as [OH-emim][Tf2N] in this manuscript.29 In addition to the solvation process, we have studied the rotational dynamics of the probe molecules in this medium by measuring the timedependent rotational anisotropy. The factors that motivated us toward the selection of this particular RTIL are as follows. First, the presence of the OH group in the imidazolium cation makes it a better hydrogen bond donor. This is expected to enhance the hydrogen bonding interaction of the cation with the probe molecules, particularly those that are hydrogen bond acceptors. At the same time, the interactions between the cationic and anionic components of the RTIL may also be enhanced due to additional hydrogen bonding interaction. Clearly, the present RTIL provides an opportunity to study the influence of the local effect, which is expected to be dominated by the hydrogen bonding interaction between the [OH-emim] cation and the photoexcited fluoroprobe, and its consequence on the overall dynamics. Second, [OH-emim][Tf2N] is reported to be much more polar compared to most of the other imidazolium ionic liquids,29 presumably due to the presence of the alcoholfunctionalized side chain in the imidazolium moiety (Chart 1). Because the high polarity of [OH-emim][Tf2N] implies greater stabilization of the fluorescent state of the dipolar probe molecules, the time-dependent shift of any given probe molecule in this RTIL is expected to be larger than that observed in other RTILs. This should allow one to monitor the dynamics over a larger frequency domain. Furthermore, because the RTILs comprising the Tf2N- anion are in general less viscous compared to other RTILs, [OH-emim][Tf2N] is expected to be one of those low-viscosity ionic liquids. Two probes, C153 and AP (Chart 1), have been employed in this study. Although both the systems are well-known fluoroprobes for the study of
J. Phys. Chem. B, Vol. 111, No. 18, 2007 4725 CHART 1: Structure/Abbreviation of the Ionic Liquid and Probe Molecules Employed in the Present Study
solvation dynamics in various media, our choice has been guided by the fact that, between the two systems, AP, in particular, is well-known for its strong hydrogen bonding interaction with the hydroxylic solvents.30 Therefore, any difference in the rotational and solvation dynamics of the two probes may provide useful information on the influence of hydrogen bonding interactions and the proximity of the cationic component on these processes. 2. Experimental Section 2.1. Materials. AP was obtained from TCI and was recrystallized twice from ethanol. The purity of the sample was ascertained by thin layer chromatography (TLC) and spectral measurements. C153 (laser grade, Eastman Kodak) was used as received. 1-Methylimidazole and 2-bromoethanol were procured from Acro¨s, and lithium bis(trifluoromethanesulfonyl)imide (Li[Tf2N]) was obtained from Aldrich. 1-Methylimidazole was distilled under reduced pressure before the reaction, and the remaining reagents were used as received. 2.2. Synthesis of [OH-emim][Tf2N]. The ionic liquid was synthesized according to a published procedure.29 In brief, 1-methylimidazole and 2-bromoethanol were stirred for 24 h in an inert atmosphere under room temperature (or at 50-60 °C) to obtain 1-(2-hydroxyethyl)-3-methylimidazolium bromide ([OH-emim]Br) as a white solid at room temperature. This was then treated with Li[Tf2N] in water to yield the desired ionic liquid. The ionic liquid was washed 3-4 times with deionized distilled water until it was free from halide (confirmed by the AgNO3 test on the wash liquid). It was then dried under vacuum for several hours to minimize the water content. The purity of the ionic liquid was checked by NMR and IR techniques as well as by the examination of the UV-vis absorption and fluorescence behavior. 2.3. Instrumentation. The viscosity of the ionic liquid was measured by a LVDV-III Ultra Brookfield Cone and Plate viscometer (1% accuracy and 0.2% repeatability). The absorption and steady-state fluorescence spectra were recorded on a UV-vis spectrophotometer (Cary100, Varian) and a spectrofluorimeter (FluoroLog-3, Jobin Yvon), respectively. The fluorescence spectra were corrected for the instrumental response. Time-resolved fluorescence measurements were carried out using a time-correlated single-photon counting (TCSPC) spectrometer (5000, IBH). A diode laser (λexc ) 374 nm) was used as the excitation source and an MCP photomultiplier (Hamamatsu R3809U-50) as the detector (response time 40 ps). The width of the instrument function, which was limited by the fwhm of the exciting laser pulse, was 65 ps. The lamp profile was
4726 J. Phys. Chem. B, Vol. 111, No. 18, 2007
Paul and Samanta
Figure 1. Steady-state absorption and emission spectra of C153 (s) and AP (---) in [OH-emim][Tf2N]: excitation wavelength for emission (λexc) ) 374 nm. All spectra are normalized at the corresponding peak maximum.
recorded by placing a scatterer (dilute solution of Ludox in water) in place of the sample. Decay curves were analyzed by a nonlinear least-squares iteration procedure using IBH DAS6 (Version 2.2) decay analysis software. The quality of the fit was measured by the χ2 values, and the weighted deviation was obtained after fitting. The same setup was used for anisotropy measurements, and the same software was used to analyze anisotropy data. 2.4. Method. The time-resolved emission decay profiles were measured at a 5/10 nm interval across the entire steady-state emission spectra. The wavelength selection was made by a monochromator with a bandpass of 1 nm. The total number of measurements was 28-30 in each case. Each decay curve was then fitted to a triexponential decay function with an iterative reconvolution program (IBH). This procedure increased the effective time resolution of the experiment to ∼40 ps. The timeresolved emission spectra (TRES) were constructed according to procedures described earlier.9 The peak emission frequencies (in cm-1), νj(t), at various times were obtained by fitting each TRES to the following log-normal function.22c
I ) h exp[-ln 2 {ln(1 + R)/γ}2] )0
R e -1
R >-1 (1)
where R ) (2γ(νj - νjpeak))/∆, νjpeak) wavenumber corresponding to the peak, h ) peak height, ∆ ) fwhm, and γ corresponds to the asymmetry of the band shape. By optimizing these four parameters by a nonlinear least-squares iteration technique, we obtained the best fitted curve. The anisotropy measurements were performed at the respective fluorescence maxima of the probes using a monochromator with a bandpass of 4-8 nm. All experiments have been performed at 25 °C. 3. Results and Discussion 3.1. Steady-State Behavior. Recent reports have indicated that the absorption due to the imidazolium ionic liquids in the UV region is not negligible and that these liquids show emission that extends into the visible region.31 The UV-vis absorption and fluorescence behavior of the present ionic liquid is found to be similar to that of the other imidazolium ionic liquids reported previously.31 However, because the probe molecules were excited at 374 nm and they are highly fluorescent, the absorption and fluorescence due to the RTIL did not pose any problem in the studies described in this manuscript. The steadystate absorption and fluorescence spectra of C153 and AP in
[OH-emim][Tf2N] are shown in Figure 1. The spectral data of the two systems in this ionic liquid along with that in a few other imidazolium ionic liquids are collected in Table 1 for comparison. It can be seen that the emission maxima for both systems appear at higher wavelengths relative to their respective maxima in other RTILs. For example, the emission maximum of C153 is observed at 546 nm in [OH-emim][Tf2N]; that in [emim][Tf2N] appears at 530 nm. In the case of AP, which is highly prone to hydrogen bonding interactions with the hydroxylic solvents,30 this difference is much more pronounced (Table 1). This observation is clearly a reflection of the higher polarity of [OH-emim][Tf2N] compared to other imidazolium ionic liquids that lack the hydroxyl group and also specific hydrogen bonding interactions of the present ionic liquid with the probe molecules, in particular, with AP. An earlier measurement involving the betaine dye indicated a polarity of 60.8 for this RTIL in the ET(30) scale.29 However, the ET(30) values estimated for [OH-emim][Tf2N] from the observed fluorescence maxima (νjfluo max) of the two flouroprobes in this ionic liquid and in a series of normal solvents of known ET(30) values32 are 50.5 and 54.0 with C153 and AP, respectively (Table 2).12 Even though these values are significantly lower than the literature value,29 the present data confirm that [OHemim][Tf2N] is more polar than other imidazolium ionic liquids, which lack the hydroxyl group (Table 2). Another point of interest is the significant difference of the estimated ET(30) values with the two probe molecules. Interestingly, in other RTILs, where the hydrogen bonding interaction with the probe molecule is not significant, the estimated ET(30) values obtained using these two probe molecules are fairly similar (Table 2). 3.2. Time-Resolved Measurements. 3.2.1. Rotational Dynamics. Time-resolved fluorescence anisotropy, r(t), is calculated using the following equation:
r(t) )
I| (t) - GI⊥(t) I| (t) + 2GI⊥(t)
(2)
where G is the correction factor for the detector sensitivity to the polarization direction of the emission and I|(t) and I⊥(t) are the fluorescence decays polarized parallel and perpendicular to the polarization of the excitation light, respectively. The anisotropy results are collected in Table 3, and the anisotropy decay curves for the two probes, C153 and AP, are shown in Figure 2. The initial anisotropies, r0, are found to be 0.32 and 0.27 for C153 and AP, respectively. The anisotropy decay profiles were fitted to both bi- and single-exponential functions of time. Although the biexponential fits were found to be marginally better than the single-exponential fits, the average rotational correlation times, 〈τrot〉, obtained from the biexponential fits were found to be very similar to those obtained from the single-exponential fits. The τrot values obtained for C153 and AP are 3.7 and 4.3 ns, respectively. In the absence of any specific interaction, the solute rotation is primarily governed by the volume of the solute and the viscosity of the medium. That the average rotational time of C153 in [bmim][PF6] (12 ns) is higher than that for AP (8.7 ns at 25 °C15 is consistent with a larger size of the former molecule. Interestingly, in the present case, AP has a τrot value higher than that of C153 (Table 3). This certainly owes to a significant hydrogen bonding interaction between the ionic liquid and AP.30 The significant hydrogen bonding interaction between AP and [OH-emim][Tf2N] is also evident from the following consideration. According to the stick hydrodynamic prediction, the rotational time constant (τstk) of a nonspherical solute of volume
Rotation and Dynamics in a RTIL
J. Phys. Chem. B, Vol. 111, No. 18, 2007 4727
TABLE 1: Absorption and Fluorescence Properties of the Fluoroprobes in Ionic Liquids of Different Viscosities C153e viscosity at 20 °C (cP)
ionic liquids
λmax abs (nm)
154a 34b 71c (58)d
[bmim][BF4] [emim][Tf2N] [OH-emim][Tf2N]
APe λmax fluo (nm)
435 424 428c
λmax abs (nm)
536 530 546c
λmax fluo (nm)
371 361 364c
483 476 511c
a From ref 4. b From ref 7. c This study. d This study (at 25 °C). e Obtained from the values measured in previous experiments done in our lab. Error limit: 2% on measured viscosity and (1 nm in λmax values.
TABLE 2: Wavenumber Corresponding to the Fluorescence Maxima of the Systems and the Estimated ET(30) Values ET(30) (indicated by the νjfluo max) probe
a νjfluo max
C153 AP betaine dye a
(cm-1)
[OH-emim][Tf2N]a
[bmim][PF6]b
[bmim][BF4]b
[emim][Tf2N]b
50.5 54.0 60.8c
47.9 47.2 52.3d
49.1 50.1 52.7d
47.7 48.5 52.6d
18315 19555
This study. b Obtained from the values measured in previous experiments done in our lab. c From ref 29. d From ref 6.
TABLE 3: Rotational Relaxation Parameters for the Two Probes in [OH-emim][Tf2N]
a
probe
r0
τrot (ns)
V (Å3)a
fstka
Crot
C′rotb
C153 AP
0.32 0.27
3.7 ( 0.07 4.3 ( 0.07
243 134
1.5 1.6
0.7 1.4
0.1-0.7 (many solvents)c 2.7 ( 0.2 (alcohols)a 1.0 ( 0.1 (many aprotic)a
From ref 15. b C′rot are the literature values of rotational coupling constants of the probe molecules in conventional solvents. c From ref 33.
Figure 2. Decay of the fluorescence anisotropy, r(t), of C153 (0) and AP (() in [OH-emim][Tf2N].
V, rotating along the longest axis of the ellipsoid in a medium of viscosity η at temperature T is given by
τstk ) Vfstkη/kBT
(3)
where fstk is a factor accounting for the nonspherical shape of the solute and kB is the Boltzman constant. Using the literature value of V and fstk,15 we calculated the τstk values for two probes for the present ionic liquid at 25 °C. From the calculated τstk values, the rotational coupling constants, Crot, defined as Crot ) τrot/τstk, which are a measure of the extent of departure from normal hydrodynamic behavior of a solute due to specific interaction, are estimated. (Table 3) The estimated Crot values (0.7 for C153 and 1.4 for AP) for the probe molecules differ by a factor of 2 in [OH-emim][Tf2N], and the corresponding values for the probes in other RTILs, e.g., in [bmim][PF6], are 0.5 and 0.7, respectively.13 It can also be seen from Table 3,
although the Crot values for C153 are very similar in conventional solvents (both protic and aprotic), these values for AP are higher in protic solvents. The Crot value for AP in [OHemim][Tf2N], though lower than that observed in alcohols (Table 3), is considerably larger than those in aprotic media. Therefore, significant hydrogen bonding interactions between AP and the ionic liquid is indicative in the hydrodynamic behavior of AP. Thus, the steady-state fluorescence and rotational anisotropy data unambiguously establish that AP is strongly hydrogen bonded to the ionic liquid. That this hydrogen bonding is mediated by the interaction of the >CdO group of AP and the -OH moiety of the imidazolium cation is evident from the literature.30a 3.2.2. SolVation Dynamics. As stated in the Experimental Section, the fluorescence decay profiles of the systems have been measured at 28-30 different wavelengths covering the entire emission spectra. Wavelength-dependent decay profiles, which are a typical signature of slow solvation dynamics, have been observed for the systems. One representative wavelengthdependent decay behavior is illustrated in Figure 3. When monitored at the shorter wavelength region, only monotonous decay is observed, and at the longer wavelengths, the time profiles consist of a slow rise followed by the decay. TRES have been constructed by fitting the individual decay curves to a multiexponential function followed by normalization of the decay traces by steady-state spectra, a process described earlier.9 The TRES for C153 and AP at five different time intervals are shown in Figure 4. In both cases, a time-dependent shift of the emission spectra toward the lower energy, indicating solvent-mediated relaxation of the excited state of the fluorophore, is observable. The total shift of time-dependent emission (∆νj), calculated from the difference between the peak frequencies (in cm-1) of the measured spectra at zero time (νj(0)) and infinite time (νj(∞)), is found to be 1352 and 1834 cm-1 for C153 and AP, respectively (Table 4). The time constant for the observable part of the solvation dynamics is calculated from the peak frequencies at various times obtained by the log-normal fit to the TRES. Generally,
4728 J. Phys. Chem. B, Vol. 111, No. 18, 2007
Paul and Samanta is biphasic and similar to those observed in our earlier studies.9-12 The time constant of the short component of this dynamics is measured to be 180 ps with C153 and 435 ps with AP. The time constant for the long component is 1.14 ns with C153 and 1.77 ns with AP. The average solvation time, 〈τ〉, estimated with C153 is 535 ps; that with AP is 930 ps. (Table 4) In this context, we note that Maroncelli and co-workers treated the observable dynamics as nonexponential and fitted their data to a stretched exponential equation:13-15
νj(t) ) νj(∞) + ∆νj exp(-(t/τ0)β)
(5)
where 0 < β e 1 and the average time of solvation is obtained by Figure 3. Wavelength-dependent decay profiles of C153 in [OHemim][Tf2N]: (a) 480 nm, (b) 525 nm, (c) 580 nm, (d) 660 nm. The lamp profile is shown as a dotted line.
Figure 4. TRES of (a) C153 in [OH-emim][Tf2N] at (9) 0 ps, (O) 50 ps, (/) 250 ps, (4) 500 ps, and (1) 2.0 ns and (b) AP in [OHemim][Tf2N] at (9) 0 ps, (O) 100 ps, (/) 250 ps, (4) 500 ps, and (1) 2.0 ns. All spectra are normalized at the corresponding peak maximum. λexc ) 374 nm.
a correlation function, C(t), of the following form is constructed.
C(t) )
νj(t) - νj(∞) νj(0) - νj(∞)
(4)
These C(t) values are then plotted against time and fitted to a biexponential function of the following form: C(t) ) a1 exp(-t/τ1) + a2 exp(-t/τ2), where τ1 and τ2 are the solvent relaxation time constants. The representative plots for AP and C153 and the biexponential fit to the data are depicted in Figure 5. As can be seen, the observable part of the solvation dynamics
〈τsolv〉 )
1 ∆νj
τ
∫0∞ [νj(t) - νj(∞)] dt ) β0 Γ(β-1)
(6)
where Γ is the gamma function. A representative stretched exponential fit to our data according to eq 5 is shown in Figure 6, and the corresponding average solvation time 〈τsolv〉 and β-values are given in Table 4. As can be seen in Figure 6, the fit to the stretched exponential function is inferior to our biphasic fit to the C(t) data (Figure 4). However, the average solvation time obtained by these two methods is quite similar to each other (Table 4). The solvation dynamics in RTILs is fundamentally different from that in ordinary polar solvents. In molecular solvents, reorientation of the solvent dipoles around the probe molecule mainly contributes to solvation. However, the diffusional motion of the constituent ions is expected to contribute significantly to the relaxation process in RTILs. This could be the motion of the individual ions and the collective motion of the cation and anion. At the same time, small amplitude motions of the ions in the immediate vicinity of the probe molecule is likely to play an important role, especially in the early part of the solvation dynamics. In the case of dipolar ionic constituents, reorientation of the individual ions close to the probe molecule can also contribute to the early part of the dynamics. In the case of polarizable ionic constituents of the RTILs, electron redistribution in these ions influenced by the change in the dipole moment of the photoexcited probe molecule may also contribute to the short time domain of the dynamics. Further, because a significant fraction of the ionic constituents of the RTILs is known to exist as ion pairs,34 reorientation of these ion pairs around the photoexcited probe may also contribute to the early part of the dynamics. Clearly, understanding the mechanism of solvation dynamics in RTILs is a complex problem, and at this stage, it is impossible to pinpoint the various motions that contribute to the different components of the dynamics. Despite the complex nature of the dynamics in ionic liquids, recent literature suggests that the dielectric continuum model may work for ionic liquids,16c and also, orientation dynamics of ionic liquids can be similar to that in van der Waals liquids.26d We first focus on the time-resolvable slow component(s) of the dynamics. Because the biphasic nature of the solvation dynamics in imidazolium ionic liquids was found to be similar to that observed in molten salts by Huppert and co-workers,35 we attributed the short component of the observable dynamics to the translational motion of the anion, and taking into consideration the amplitudes associated with the two observable components, the relatively long component was assigned to the collective cation-anion diffusional motion without speculating what contributes to the missing or ultrafast component of the dynamics.9-12 Maroncelli and co-workers, on the other hand,
Rotation and Dynamics in a RTIL
J. Phys. Chem. B, Vol. 111, No. 18, 2007 4729
TABLE 4: Relaxation Parameters of Solvation and Observed Shift for the Two Probes in [OH-emim][Tf2N] stretched exponential fitc
from biexponential fita
c
probe
τ1 (ps)
τ2 (ns)
a1
a2
〈τ〉b (ps)
β
〈τsolv〉d (ps)
Observed shift [νj(0) - νj(∞)] (cm-1)
C153 AP
180 435
1.14 1.77
0.63 0.63
0.37 0.37
535 930
0.67 0.77
520 850
1352 1834
Missing component 25% 45%
a Using the eq C(t) ) a exp(-t/τ ) + a exp(-t/τ ). b Average solvation time 〈τ〉 ) a τ + a τ where a + a ) 1; experimental error is ( 5%. 1 1 2 2 1 1 2 2 1 2 Using eq 5. d By eq 6.
Figure 5. Decay of the spectral shift correlation function, C(t), of C153 (9) and AP (2) in [OH-emim][Tf2N]. In each case, the solid line denotes the biexponential fit to the data where R2 denotes the correlation coefficient.
Figure 6. Decay of the spectral shift correlation function, C(t), of C153 (O) and AP (0) in [OH-emim][Tf2N]. The solid line represents the stretched exponential fit to the data where χ2 is the standard deviation and R2 denotes the correlation coefficient.
treated the dynamics as nonexponential, and their interpretation was focused on the average solvation time, which was assigned to the large-scale diffusion of the constituent ions.13-15 The average solvation time, whether obtained from a biexponential or a stretched exponential fit, does correlate with the viscosity of the medium, at least for the RTILs that are not too viscous. However, one can argue whether the solvation dynamics can be viewed as biphasic9-12 or nonexponential with a distribution of solvation time.13-15 In the present case, judging by the quality of the fits to the spectral shift data (Figure 5 and 6), the biphasic description clearly appears to be more suitable than the nonexponential one, even though the nature of the motions responsible for these components may not be very clear. Let us attempt to see whether the probe dependence of the two resolvable components signifies something. As far as the shorter component of the observable dynamics is concerned, the dynamics is slower in the case of AP by a factor of ∼2.5.
Figure 7. Viscosity dependence of the average solvation time of C153 and AP in low viscosity ionic liquids (η < 100 cP). The data points shown in the plot are collected from our previous studies (refs 9 and 12) and the present one. The solid line represents the linear fit to all the data points except that for AP in [OH-emim][Tf2N]. bmpy ≡ N-butyl-N-methylpyrrolidinium.
Although probe dependence of the solvation dynamics in RTILs is a common observation, such a large difference of the magnitude of this component in the present case is most likely to be the direct consequence of a strong hydrogen bonding interaction of AP with the cation and, hence, could be due to local effects in the proximity of the probe molecule. We are however unable to comment on anything more at this point. Interestingly, because the longer component is not so different for the two probes (1.14 and 1.77 ns), this component is presumably due to the long-range collective cation-anion motion. It is also important to take note of the fact that the average solvation time obtained with AP does not correlate with the usual trend of the viscosity dependence (Figure 7). Because one important aspect of solvent relaxation dynamics in RTILs is the extent of the spectral shift missed due to the ultrafast nature of this component of relaxation, we now concentrate on the component missed in this study. In this context, we first discuss the literature information on the time scale and origin of this ultrafast component. Maroncelli and coworkers suggested a time scale of 5 ps or
