J. Phys. Chem. B 2004, 108, 5771-5777
5771
Solvation Dynamics and Rotation of Coumarin 153 in Alkylphosphonium Ionic Liquids N. Ito,† S. Arzhantsev,† M. Heitz,‡ and M. Maroncelli*,† Department of Chemistry, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802, and Department of Chemistry, The State UniVersity of New York at Brockport, Brockport, New York 14420 ReceiVed: January 5, 2004; In Final Form: February 24, 2004
Solvation and rotational dynamics of coumarin 153 (C153) were measured in a series of phosphonium ionic liquids consisting of the trihexyl(tetradecyl)phosphonium cation and the anions Br-, Cl-, dicyanamide-, bis(trifluoromethylsulfonyl)imide-, and BF4-. None of these liquids display a prominent ultrafast solvation component comparable to the one observed in imidizalium ionic liquids. The solvation dynamics observed on the nanosecond time scale are also 5-fold slower than those in imidazolium liquids of comparable viscosities. Both rotation and solvation are highly nonexponential functions of time and can be represented by stretchedexponential decays with exponents near 0.5. Characteristic times of these processes track solvent viscosity. A number of similarities between the dynamics observed in these ionic liquids and supercooled glass-forming liquids are noted.
I. Introduction By virtue of their potential as “designer solvents” and “green” replacements for volatile organic solvents,1 as well as for what they might reveal about fundamental aspects of solvation, a number of recent studies have explored solvation in ionic liquids.2-24 Comparison of spectral shifts of solvatochromic probes in ionic liquids with those found in dipolar organic solvents,2-6 as well as other types of measurements,7-9 have revealed that solvation energies in ionic liquids are often comparable to those found in highly polar organic solvents. Despite this similarity, the presence of ionic interactions provides a new solvation “mechanism”, and how such interactions lead to distinct behavior in ionic liquids is only beginning to be appreciated via computer simulation.20-23 Investigations of the dynamical aspects of solvation in ionic liquids10-19 as well as neat solvent dynamics24-27 are also starting to receive attention. The most widely studied of the ionic liquids are those based on alkyl imidazolium cations, for example, 1-methyl-3-butylimidazolium or “bmim+”. Measurements by several different groups10-15,17 have revealed that solvation dynamics in imidazolium ionic liquids has a markedly biphasic character, consisting of roughly equal contributions from an ultrafast (sub 5 ps10,11,17,28) component together with a much slower nonexponential component that relaxes on the nanosecond time scale. Simulations support the presence of a prominent subpicosecond response in imidazolium ionic liquids.29,30 But this remarkable feature of the dynamics is not characteristic of all ionic liquids. In a recent comparison of liquids made from imidazolium, alkylammonium, and alkylphosphonium cations, we found no comparable ultrafast component in liquids containing the latter two cations,10 suggesting that this rapid response is related to dynamics of local probe + imidazolium structures absent in other types of ionic liquids. In the aforementioned study we also noted that, with one exception, the slow component of the dynamics observed in all ionic liquids to date † ‡
The Pennsylvania State University. The State University of New York at Brockport.
Figure 1. Space-filling representations of the probe C153 and the ionic liquid components studied here. These models reflect geometries calculated with the AM1 semiempirical method and standard van der Waals radii.
seemed to follow a single correlation with solvent viscosity. The sole exception was the only phosphonium-based ionic liquid then available to us, which fell far from this correlation.10 Why this particular ionic liquid deviated from the correlation, or perhaps more importantly, why any simple correlation of the sort should exist, is unclear at this point. More data on a variety of liquids is necessary to form a clearer picture of solvation and solute dynamics in ionic liquids. The present work represents our attempt to add to the growing database in this area. Here we use the time-correlated singlephoton counting technique to examine solvation dynamics and rotation of the solvatochromic probe coumarin 153 (see Figure 1) in five alkylphosphonium ionic liquids. The liquids are comprised of a single cation, trihexyl(tetradecyl)phosphonium “[P(C6)3C14+]”, with the anions Br-, Cl-, BF4-, N(CN)2-
10.1021/jp0499575 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/14/2004
5772 J. Phys. Chem. B, Vol. 108, No. 18, 2004
Ito et al.
TABLE 1: Some Physical Properties of the Alkylphosphonium Ionic Liquids Studieda ionic liquid
water content wt/%
refractive index n
-dn/dT/ 10-3 K-1
viscosity η/cP
η0/10-3 cP
B/103 K
T0/K
std error
[P(C6)3C14+][Br-] [P(C6)3C14+][Cl-] [P(C6)3C14+][DCA-] [P(C6)3C14+][Tf2N-] [P(C6)3C14+][BF4-]
0.1 0.1 0.8 0.1 0.7
1.490 1.482 1.478 1.449 1.454
0.331 0.331 0.332 0.328 0.315
1370 1550 198 277 784
13.8 4.27 119 55.8 215
1.88 2.30 1.00 1.28 1.00
134 120 163 148 176
0.021 0.024 0.015 0.009 0.038
[dmpim+][Tf2N-]
0.1
1.433
0.246
80
229
0.866
172
Values of n ((.005) and η ((5%) are at 25 °C. Refractive indices and viscosities were measured over the temperature range 278-343 K. The temperature dependence of n is linear over this range and is characterized here by dn/dT. Viscosities were fit to the function ln(η/cP) ) ln(η0/cP) + B/(T - T0). “Std error” refers to the standard error of this logarithmic fit and these values provide estimates of the relative (statistical) uncertainties expected for η values calculated from this parametrization. a
(“DCA”), and (CF3SO2)2N- (“Tf2N”) illustrated in Figure 1. The results of this study corroborate the distinct solvation dynamics of ionic liquids based on this large hydrophobic cation and further highlight the similarities between the dynamics in ionic liquids and in glass-forming organic solvents.10,25,31 II. Experimental Section Coumarin 153 (laser grade) was purchased from Exciton and used as received. All alkylphosphonium ionic liquids (cation: trihexyl(tetradecyl)phosphonium+, (“P(C6)3C14+”); anions: Br-, Cl-, dicyanamide- (“DCA-”), bis(trifluoromethylsulfonyl)imide(“Tf2N-”), and BF4-) were obtained from Cytec Canada Inc.32 Samples of C153 were prepared in a N2-purged glovebag as detailed previously.10 The water content of the samples was determined by Karl Fischer titration. Approximately 0.5 g of ionic liquid was dissolved in 1 mL of anhydrous methanol and the water content of the mixture was determined via a condunctiometric titration with volumetric Karl Fischer titrant (Aldrich). The accuracy and minimum water content detectible via this method is ∼0.05 wt %. Viscosities of the ionic liquids were determined using a Brookfield cone-plate viscometer (Brookfield HBDV-III+, cone CPE-40). The overall uncertainty in the viscosity measurements is estimated to be about (5%. Refractive index values were measured with an Abbe refractrometer ((0.005). In both measurements, temperature was controlled over the range 5-70 °C, using a circulating water/methanol bath. Steady-state absorption spectra were measured with a Hitachi U-3000 UV/ vis spectrophotometer and corrected emission spectra with a Spex Fluorolog 212 fluorimeter with resolutions of 0.5 and 1 nm, respectively. Time-resolved emission decays were measured with a timecorrelated photon counting system previously described.33 The doubled output of a femtosecond mode-locked Ti:sapphire laser (Coherent Mira 900F) was used for excitation (∼427 nm) and the instrument had an overall response time of 25 ps (fwhm) as determined by a scattering solution. Emission was collected through a single monochromator (ISA H10) with an 8-nm bandpass over a time range of 20 ns. Emission decays were fit together with instrument response functions using an iterative reconvolution algorithm,34 which provided an effective time resolution of ∼5 ps. Time-resolved emission spectra were reconstructed from a series of 10-15 magic angle decays recorded at wavelengths covering the emission spectrum, as previously described.11,35 One aspect of the analysis of the spectra departs slightly from our previous practice and so deserves mention. Rather than using the observed (frequency domain) spectra F(ν) to characterize the solvent response, we herein adopt the more rigorous approach of analyzing the line shape or “susceptibility” func-
tions36 defined by f(ν) ≡ F(ν)/ν3. (The ν3 arises from the dependence of the radiative rate on emission frequency, whose presence leads to a time-dependent rate of overall intensity loss in Stokes-shifting spectra.37 Removing it provides a more proper representation of solvation effects on spectra.) However, except for causing slight changes in the frequencies and widths of the spectra, use of f(ν) in place of F(ν) has virtually no effect on the solvation dynamics (times or Stokes shifts) measured in the present work. The dynamical data reported here are therefore directly comparable to what we have reported in previous studies. The frequencies characteristic of f(ν) can be converted to equivalent frequencies of F(ν) spectra for C153 via the approximate relation VF/cm-1 = 782 + 0.979Vf/cm-1.38 Rotational correlation functions were determined from emission decays polarized parallel and perpendicular to the vertically polarized excitation light.34 These decays were tail-matched and simultaneously fit, together with a decay measured at magic angle polarization, to multiexponential population decay and multiexponential or stretched exponential anisotropy decay models. In some ionic liquids, polarized emission decays were measured at several different wavelengths. No significant wavelength dependence of the rotational dynamics was observed and in these cases average results are reported. Representative emission decay data are provided in Figures S1 and S2 in the Supporting Information. III. Results and Discussion A. Physical Properties. Some physical properties of the ionic liquids studied here are summarized in Table 1. For comparison, data for the imidazolium liquid [dmpim+][Tf2N-] (dmpim+ ) 1,2-dimethyl-3-propylimidazolium+) examined previously10 are also listed. As shown in this table, the phosphonium ionic liquids used in the present study are not completely dry. The liquids comprised of Cl-, Br-, and Tf2N- anions contained relatively little water, ∼0.1 wt % or 1-2 mol %, whereas the DCA- and BF4- liquids had water contents of nearly 1% w/w (∼11 mol %). We made attempts to dry these samples in a vacuum oven as is commonly done. Although vacuum drying for 10-15 h at 100 °C did decrease the water content significantly, this decrease was accompanied by a substantial increase in the amount of impurity fluorescence at the excitation wavelengths used. For this reason, we decided to work with the as-received samples. Fortunately, the presence of even substantial amounts of water did not seem to significantly affect the properties measured with the probe C153. Thus comparisons of several dried, as-received, and water-saturated solutions showed only minor changes in the steady-state emission spectra, apart from the increased impurity emission noted above. Water content does significantly alter the viscosity, by as much as a factor of 2 in the samples examined, and this change does alter the dynamics. But, to
Solvation and Rotational in Ionic Liquids within uncertainties, the changes in solvation and rotation times of interest here could be completely accounted for by the changes in solvent viscosity as a function of water content. We attribute this apparent insensitivity of C153 to specific interactions to the hydrophobic nature of this probe and note that this same insensitivity would not be expected of more water-sensitive or hydrophilic solutes.4,39 In this context, the recent study of the effects of water on solvation dynamics of the solute PRODAN (6-propionyl-2-(N,N-dimethylamino)naphthalene) in [bmim+][PF6-] by Bright and co-workers17 is noteworthy. In contrast to the situation with C153, they observed substantial shifts in the emission spectrum of PRODAN with water content over the range 50 ppm to 1.8 wt %. They also found a 40% decrease in the average solvation time measured at the highest water concentrations. Although Bright and co-workers ascribed the observed changes to specific water-probe interactions, we note that the decrease in solvation time is entirely consistent with the ∼50% decrease in solvent viscosity reported for this range of water contents.40 Thus, even in this case, influence of water on the dynamics may be primarily a nonspecific effect. Refractive index and viscosity data are also summarized in Table 1. The refractive indexes (nD) of the alkylphosphonium ionic liquids lie in the range 1.45-1.49 near room temperature. Such values are comparable to but slightly larger than typical long-chain organic solvents such as n-hexadecane (1.433) and n-dodecanol (1.441)41 as a result of the higher density of ionic liquids. Over the 5-70 °C range examined, nD varies linearly with temperature. The derivatives dnD/dT are all similar, -(0.32 - 0.33) × 10-3 K-1, and somewhat smaller than values found in typical organic solvents (for example, dnD/dT ) - 0.43 × 10-3 K-1 in dodecane41). The viscosities of these ionic liquids are all greater than 200 cP at room temperature. These values are larger than other ionic liquids we have studied,10 presumably as a result of the larger size of the P(C6)3C14+ cation. Viscosities vary with anion in the order DCA- < Tf2N- < BF4- < Br- ∼ Cl-. With the exception of DCA-, the viscosities increase with decreasing anion size. The ordering of DCA- probably reflects the greater water content of this sample, given the fact that water contamination is known to significantly reduce the viscosity of ionic liquids.32 The viscosities of all of these liquids are well represented by a Vogel-Fulcher-Tammann temperature dependence (5-70 °C) and such parametrizations, provided in Table 1, were used in subsequent analysis. We note that the large viscosity variations that can be effected by small temperature changes (where static properties change relatively little) make temperature a convenient control variable for viscosity in these systems. B. Steady-State Spectroscopy. The steady-state absorption and emission spectra of C153 in the alkylphosphonium ionic liquids studied here are similar to spectra observed in highly polar conventional solvents, as was noted previously in the case of the imidazolium ionic liquids.10 No significant structure is observed, and in all cases the spectra are reasonably represented by log-normal functions. (The spectral characteristics determined from log-normal fits are summarized in Table S1 of the Supporting Information.) The absorption frequency of C153 in [P(C6)3C14+][Tf2N-] is close to that observed in the imidazolium [Tf2N-] ionic liquids studied previously.10 A variation of ∼300 cm-1 in the absorption frequencies is found as a function of anion, and this variation tracks differences in refractive index in roughly the manner expected. But the magnitude of these variations is comparable to the accuracy of the absorption data, which are hindered by the significant absorption of many of
J. Phys. Chem. B, Vol. 108, No. 18, 2004 5773
Figure 2. (a) Representative time-resolved emission spectra of C153 in [P(C6)3C14+][BF4-] at 343 K. Points are reconstructed data at the times indicated and the solid curves are log-normal fits to these data. The dashed curve labeled “ss” is the steady-state emission spectrum and the dash-dot curve labeled ‘t ) 0’ is an estimate of the spectrum prior to solvent relaxation. (b) Temporal evolution of the peak emission frequencies of C153 in [P(C6)3C14+][BF4-] at 318 and 343 K. The dashed lines are stretched exponential fits to data.
these liquids at wavelengths below 400 nm. The steady-state emission frequencies, which can be measured with greater accuracy, and the steady-state Stokes shifts derived from them, do not show any clear-cut trends among the various liquids at room temperature. This inconsistency is due to the fact that, as shown below, the solvation time scale is often not rapid compared to the S1 lifetime (3-7 ns), which means that steadystate emission frequencies do not provide accurate measures of equilibrium solvation energetics in these systems. We note that this consequence of the unusually slow dynamics of ionic liquids is typically overlooked in solvatochromic comparisons. C. Solvation Dynamics. Wavelength-resolved emission decays of C153 in these ionic liquids are highly nonexponential, typically requiring 3-4 exponential components for a proper fit. (See Figure S1 in the Supporting Information for examples.) The longest decay constants, which correspond roughly to the S1 lifetime, are significantly shorter in the Cl- and Br- liquids (5 and 3 ns, respectively) compared to the other liquids (6-7 ns), indicating enhanced nonradiative decay in the former solvents. Time-resolved emission spectra, illustrated by the [P(C6)3C14+][BF4-] data in Figure 2a, display the continuous red shift with time characteristic of nonspecific solvation.42 As illustrated in this figure, the time-resolved spectra are accurately represented by a log-normal line shape function. The spectrum reconstructed at zero time lies close to the position of the timezero spectrum estimated from steady-state data,43 indicated by the dash-dot curve labeled ‘t ) 0’ in Figure 2a. In the data shown here, the steady-state spectrum (dashed curve labeled “ss”) is comparable to reconstructed spectra at times of ∼3 ns. Spectra recorded at later times are red-shifted relative to the steady-state spectrum, demonstrating that solvation is significantly slower than the population decay of the S1 state. The widths of the reconstructed spectra are nearly time independent and are comparable to those found in the ‘t ) 0’ and steady-
5774 J. Phys. Chem. B, Vol. 108, No. 18, 2004
Ito et al.
TABLE 2: Results of Stretched Exponential Fits to Temporal Peak Frequenciesa ionic liquid +
-
[P(C6)3C14 ][Br ] [P(C6)3C14+][Cl-] [P(C6)3C14+][DCA-] [P(C6)3C14+][ Tf2N-] [P(C6)3C14+][ BF4-]
T/K
η/cP
ν(∞)/103 cm-1
∆ν/103 cm-1
τsolv/ns
βsolv
〈τ〉solv/ns
∆νest/103 cm-1
fobs
343 343 343 343 318 298 343 318
116 133 31 39 102 277 87 251
18.28 18.30 18.51 18.67 18.61 (18.55)b 18.51 18.41
1.59 1.50 1.42 1.43 1.54 (1.6)b 1.37 1.57
1.31 ( 0.07 1.86 ( 0.09 0.36 ( 0.04 0.44 ( 0.02 1.31 ( 0.07 3.72 ( 0.19 1.01 ( 0.05 3.54 ( 0.18
0.38 0.46 0.48 0.47 0.44 0.42 0.51 0.41
5.05 ( 0.43 4.49 ( 0.30 0.78 ( 0.05 0.99 ( 0.06 3.52 ( 0.23 10.6 ( 0.63 1.98 ( 0.11 11.3 ( 0.76
1.61 1.61 1.52 1.47 1.50 1.51 1.44 1.52
0.99 0.93 0.93 0.97 1.03 1.06 0.95 1.03
a Fits are to the peak frequencies in the “line shape” spectra f(ν) ()F(ν)/ν3).38 ν(∞), ∆ν, τsolv, and βsolv are the parameters defined in eq 1 and 〈τ〉solv is the integral solvation time determined from the fit parameters according to eq 2. ∆νest is the difference between ν(∞) from the fits and the time-zero frequency estimated according to the methods of ref 43, and fobs ) ∆ν/∆νest. b Parameters fixed in the fitting.
state spectra (∼3500 cm-1). Comparable features are observed in all of the other ionic liquids examined here. The time evolution of the peak frequencies of C153 in [P(C6)3C14+][BF4-] at two temperatures is shown in Figure 2b. As discussed previously,11 we use a stretched exponential function,
ν(t) ) ν(∞) + ∆ν exp{- (t/τsolv)βsolv}
(0 < βsolv e 1) (1)
to characterize the spectral shift dynamics (dashed curves in Figure 1b). Table 2 summarizes the results obtained from such fits to the time-resolved spectra. In all cases, the magnitudes of the dynamic Stokes shifts (∆ν) are about 1500 cm-1 and are close to the values expected from steady-state estimates. Thus, as indicated by the column labeled “fobs” in Table 2, essentially all of the solvation dynamics is observed in these systems. This result contrasts with the behavior found in imidazolium ionic liquids, where roughly half of the solvation response is too rapid (>5 ps) to be observed with the instrumentation used here.10,11 As in the case of imidazolium ionic liquids, the observed dynamics is highly nonexponential, as indicated by the small values (0.4-0.5) of the stretching exponent βsolv listed in Table 2. The average (or integral) response time is determined from the fitted parameters via
〈τ〉solv )
1 ∆ν
τ
Γ(βsolv-1) ∫0∞ {ν(t) - ν(∞)} dt ) βsolv solv
Figure 3. Viscosity dependence of the solvation times of C153 in alkylphosphonium (open symbols) and other ionic liquids (solid symbols: [bmim+][Tf2N-] ) diamond, [dmpim+][Tf2N-] ) circles, and [N(C4)3C+][Tf2N-] ) triangle down) from ref 10. Representative uncertainties are shown for the [P(C6)3C14+][Tf2N-] data.
(2)
where Γ(x) is the gamma function. Generally, average times represent the overall relaxation process better than τsolv; however, errors in βsolv cause much larger errors in the average time, especially when βsolv is small. In Figure 3 the viscosity dependence of both solvation times is therefore shown. The solvation times in alkylphosphonium ionic liquids (open symbols) appear to follow a single correlation with solvent viscosity independent of anion. But this correlation is distinct from what was previously found for the observable response in imidizolium ionic liquids (smaller filled symbols). Both measures of solvation time show solvation to be about 5-fold slower in these phosphonium liquids than in imidazolium liquids of the same viscosity. Thus, the large size of the phosphonium ion not only produces larger viscosities, but it also leads to a different relationship between solvation times and viscosity. These observations suggest that cation motions are the primary determinants of solvation times in these systems. D. Rotational Dynamics. Figure 4 shows representative anisotropy decays r(t) of C153 in [P(C6)3C14+][Tf2N-] at three temperatures. As indicated by the single-exponential function shown there for reference (dashed curve labeled “exp”), the
Figure 4. Representative anisotropy decays of C153 in [P(C6)3C14+][Tf2N-] at 298, 318, and 343 K. Actual data (points) are shown only at 318 K for clarity. The solid curves are stretched exponential fits to data at all three temperatures. For comparison, the dashed curve is a single-exponential function having the same time constant as the 343 K fit.
rotation of C153 in phosphonium ionic liquids is not an exponential process. As in the case of solvation dynamics, the nonexponential character of the anisotropy decays is well represented by a stretched exponential time dependence,
r(t) ) r0 exp{ - (t/τrot)βrot}
(3)
where r0 is the initial value of anisotropy. The results of fits to this functional form are summarized in Table 3. In these
Solvation and Rotational in Ionic Liquids
J. Phys. Chem. B, Vol. 108, No. 18, 2004 5775
TABLE 3: Results of Anisotropy Decay Fitsa ionic liquid [P(C6)3C14+][Br-] [P(C6)3C14+][Cl-] [P(C6)3C14+][DCA-] [P(C6)3C14+][Tf2N-] [P(C6)3C14+][BF4-] [N (C4)3C1+][Tf2N-] [bmim+][Tf2N-] [dmpim+][Tf2N-]
T/K
η/cP
τrot/ns
βrot
〈τ〉rot/ns
343 318 343 331 343 343 318 298 343 318
116 396 133 230 31 39 102 277 87 251
1.1 3.2 1.3 1.3 0.91 1.0 ( 0.2 2.8 ( 1 7.5 ( 3 1.5 4.2
0.40 0.36 0.43 0.36 0.54 0.56 0.50 0.46 0.48 0.42
3.5 ( 2 15 ( 4 3.9 ( 2 6.0 ( 3 1.6 ( 1 1.7 ( 1 5.5 ( 3 18 ( 5 3.1 ( 2 12 ( 4
298 298 323 298 283
520 69 31 80 180
0.41 0.77 0.77 0.74 0.66
51 ( 20 2.6 ( 2 1.3 ( 0.8 4.3 ( 2 15 ( 4
17 ( 8 2.1 1.1 3.6 11
a Data fit to eq 3 with the initial anisotropy r0 fixed to 0.378.45 Uncertainties in βrot are typically less than (0.1. 〈τ〉rot is the integral response time obtained similarly to eq 2.
phosphonium ionic liquids, values of the exponent βrot are in the range 0.4-0.6, indicating large departures from exponential relaxation, i.e., a broad distribution of relaxation times. This behavior differs markedly from what was previously observed for rotation of the probe 4-aminophthalimide (4-AP) in imidazolium ionic liquids.11 In that system, anisotropy decays could be adequately fit to single-exponential functions of time (i.e. βrot ∼ 1). Measurements of C153 in other ionic liquids and 4-AP in some of these phosphonium solvents reveals that both solvent and solute differences contribute to this difference in behavior. Included in Table 3 are results for rotation of C153 in the ionic liquids[bmim+][Tf2N-],[dmpim+][Tf2N-],and[N(C4)3C+][Tf2N-]. Rotation of C153 is also nonexponential in these other liquids, but especially in the imidazolium cases, the departure from exponentiality is considerably smaller, i.e., βrot is closer to unity. In phosphonium ionic liquids the anisotropy of the probe 4-AP is also nonexponential, but not to the same extent as in C153. For example, 4-AP in [P(C6)3C14+][Tf2N-] exhibits values of βrot of 0.7-0.8. Thus, the character of the rotational dynamics depends on both the solvent and solute identity. Further investigation is needed to clarify the reasons for this solute dependence. Rotation times of C153 in alkylphosphonium (open symbols) and other ionic liquids (smaller filled symbols) are plotted versus η/T in Figure 5. The uncertainties in these data are large because the rotation times are comparable to the excited-state lifetime of C153 and because of the strong interdependence of τ and β for such small values of β. However, these time-resolved data do reproduce the steady-state anisotropies measured in these systems44 to within (15%. As shown in Figure 5, most of the rotation times fall within the region bounded by the predictions of hydrodynamic theory, using stick and slip boundary conditions and an ellipsoidal representation of C153.45 In contrast to the solvation times shown in Figure 3, the rotation times of C153 in these alkylphosphonium liquids are sensitive to the identity of the anion. The weakest rotational coupling (Crot ∝ τrot/(η/T)) is observed in liquids with Br- and Cl- anions and the strongest with the Tf2N- anion. This ordering is similar to the ordering of viscosities and refractive indices of these liquids. Curiously, the dynamic distinction between alkylphosphonium and imidazolium ionic liquids observed for solvation is not evident in the rotation times. E. Comparisons to Supercooled Liquids. The nonexponential character of the relaxation observed here is reminiscent of the type of dynamics often found in supercooled liquids and
Figure 5. Rotation times of C153 in alkylphosphonium (open symbols) and other (solid symbols; see Figure 3) ionic liquids. The dashed lines are predictions of slip (lower) and stick (upper line) hydrodynamics. Typical error bars are shown for the [P(C6)3C14+][Tf2N-] data.
Figure 6. (a) Relationship between the stretching exponent β and the time constant τ for solvation of C153 in alkylphosphonium (open symbols) and other ionic liquids (solid symbols, see Figure 3). (b) Comparison of the stretching exponents for rotation and solvation.
glasses.46 Figure 6 shows that the departure from exponential relaxation, as measured by the exponent β, is loosely correlated to the relaxation time, at least in the case of solvation. This plot, which incorporates data from a variety of ionic liquids each studied over a limited temperature range, resembles what is observed when a fragile glass-forming liquid is cooled from above the melting point to temperatures near to its glass transition (Tg). There has been a long debate over the homogeneous versus heterogeneous origins of such nonexponential relaxation in supercooled liquids, with most recent evidence tending to favor spatial heterogeneity as the primary cause for the distributed kinetics observed in such systems.47 It would seem reasonable to invoke the same mechanism in these ionic liquids; however, the near constancy of the widths observed in the time-evolving spectra is unexpected in this case. More study of time- and temperature-dependent emission line shapes of the
5776 J. Phys. Chem. B, Vol. 108, No. 18, 2004 sort performed in more conventional solvents48,49 is needed to explore this issue further. A second similarity involves the relationship between the fragility (a measure of the nonArrhenius character of the viscosity) of a glass-forming liquid and the nonexponentiality of its bulk relaxation dynamics.50 Although we observe probe dynamics here rather than neat solvent dynamics, the values of β (0.4-0.8) observed in these ionic liquids and the temperature dependence of their viscosities (B/T0 ) 6-20) fall within the correlations established by Bo¨hmer et al.50 for a variety of materials near Tg. Finally, we note that Yang and Richert51 reported measurements of probe solvation and rotation in organic glass-forming liquids near Tg that are quite similar in concept to the measurements reported here. In an extensive survey of 7 probe + solvent combinations, Yang and Richert found rotation times to be invariably slower than solvation times, and such that the ratio τrot/τsolv correlates linearly with the solute-to-solvent mass ratio mu/mv. They also found βrot > βsolv with βrot approaching unity (i.e. rotation becoming exponential) in the limit of large mu/mv. If one uses the cation mass for mv, the mass ratio mu/mv is ∼1 for the alkylphosphonium ionic liquids and ∼2 for the imidazolium liquids. The data in Tables 2 and 3 show that the rotation times of C153 are generally slower than, but within a factor of 2-3 of, solvation times in alkylphosphonium ionic liquids (where mu/mv is ∼1) but 8-10 times slower than solvation in imidazolium ionic liquids (mu/mv ∼ 2). Figure 6b illustrates the fact that there is little distinction between the stretching exponents for rotation and solvation in any of these ionic liquids. Here we find βrot ) βsolv to within uncertainties of (0.1. Both βrot and βsolv are larger in the imidazolium liquids where mu/mv is larger. These observations are qualitatively but not quantitatively consistent with correlations reported by Yang and Richert near the glass transition of molecular liquids.51 IV. Summary and Conclusions In this study we have examined solvation and rotation of the probe C153 in a series of five ionic liquids based on the trihexyl(tetradecyl)phosphonium cation. The main results are as follows. The “polarity” sensed by C153 in these liquids is similar to what is observed in other classes of ionic liquids.10 The nuclear polarizability, measured by the dynamic Stokes shift varies only slightly with anion. In all cases, the kinetics observed is highly nonexponential, and can be characterized by stretched exponential functions of time with exponents in the range 0.4-0.6. These kinetics are reminiscent of the behavior of supercooled liquids approaching the glass transition. Only a single, albeit a temporally disperse, solvation response is found: the ultrafast solvation component found in imidazolium ionic liquids is absent in phosphonium ionic liquids. Both rotation and solvation times track the solvent viscosity. Rotation times are approximately proportional to η/T and lie between the limits set by simple hydrodynamic theories. The correlation between rotation time and viscosity appears to vary slightly with the identity of the anion involved. Solvation times follow an approximate power-law relationship to viscosity τsolv ∝ ηp with a power p slightly greater than unity. Perhaps the most interesting result of the present work is that the solvation times of all of the alkylphosphonium ionic liquids studied here follow a single correlation with viscosity, independent of anion. The same is true of the imidazolium ionic liquids studied previously, but the correlations in the two liquids are clearly distinct, with the alkylphosphonium liquids being ∼5-fold slower for a given viscosity. The implication is that large-amplitude motions of the cation, which are expected to be much slower for the bulky
Ito et al. phosphonium ion, dictate the time scale of solvation in these liquids. The insensitivity to anion is surprising given the large range of sizes (Cl- to Tf2N-) studied here and the fact that both cations and anions must be involved in the solvation response. We conjecture that the solvation time may be set by the slowest moving species present, which just happens to be the cation in all of the systems studied to date. Acknowledgment. The authors gratefully acknowledge Cytec Canada Inc. for supplying samples of the alkylphosphonium ionic liquids used here, and the U.S. Department of Energy, Office of Basic Energy Sciences, for the support of this work. Supporting Information Available: Table giving characteristics of the steady-state spectra and figures showing representative emission decays and polarized decays of C153 [P(C6)3C14+][BF4-]. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) See, for example, the recent compilations: Ionic Liquids in Synthesis; Wasserscheid, P., Welton, T., Eds.; Wiley-VCH: Weinheim, Germany, 2003. Ionic Liquids, Industrial Applications for Green Chemistry; Rodgers, R., Seddon, K., Eds.; American Chemical Society: Washington, DC, 2002; Vol. 818. (2) Crowhurst, L.; Mawdsley, P. R.; Perez-Arlandis, J.; Salter, P. A.; Welton, T. Phys. Chem. Chem. Phys. 2003, 5, 2790. (3) Fletcher, K. A.; Storey, I. A.; Hendricks, A. E.; Pandey, S.; Pandey, S. Green Chem. 2001, 3, 210. (4) Baker, S. N.; Baker, G. A.; Bright, F. V. Green Chem. 2002, 4, 165. (5) Aki, S. N. V. K.; Brennecke, J.; Samanta, A. Chem. Commun. 2001, 2001, 413. (6) Muldoon, M. J.; Gordon, C. M.; Dunkin, I. R. J. Chem. Soc., Perkin Trans. 2 2001, 2001, 433. (7) Anderson, J. L.; Ding, J.; Welton, T.; Armstrong, D. W. J. Am. Chem. Soc. 2002, 124, 14247. (8) Huddleston, J. G.; Broker, G.; Willauer, H.; Rogers, R. D. FreeEnergy Relationships and Solvatochromic Properties of 1-Alkyl-3-methylimidazolium Ionic Liquids. In Ionic Liquids, Industrial Applications for Green Chemistry; Rogers, R. D., Seddon, K. R., Eds.; ACS Symp. Ser No. 818; American Chemical Society: Washington, DC, 2002; p 270. (9) Anthony, J. L.; Maginn, E. J.; Brennecke, J. F. J. Phys. Chem. B 2002, 106, 7315. (10) Arzhantsev, S.; Ito, N.; Heitz, M.; Maroncelli, M. Chem. Phys. Lett. 2003, 381, 278. (11) Ingram, J. A.; Moog, R. S.; Ito, N.; Biswas, R.; Maroncelli, M. J. Phys. Chem. B 2003, 107, 5926. (12) Karmakar, R.; Samanta, A. J. Phys. Chem. A 2003, 107, 7340. (13) Karmakar, R.; Samanta, A. J. Phys. Chem. A 2002, 106, 4447. (14) Karmakar, R.; Samanta, A. J. Phys. Chem. A 2002, 106, 6670. (15) Chowdhury, P. K.; Halder, M.; Sanders, L.; Calhoun, T.; Anderson, J. L.; Armstrong, D. W.; Song, X.; Petrich, J. J. Phys. Chem. Submitted for publication. (16) Chakrabarty, D.; Hazra, P.; Chakraborty, A.; Seth, D.; Sarkar, N. Chem. Phys. Lett. 2003, 381, 697. (17) Baker, S. N.; Baker, G. A.; Munson, C. A.; Chen, F.; Bukowski, E. J.; Cartwright, A. N.; Bright, F. V. Ind. Eng. Chem. Res. 2003, 42, 6457. (18) Bart, E.; Meltsin, A.; Huppert, D. J. Phys. Chem. 1994, 98, 3295. (19) Bart, E.; Meltsin, A.; Huppert, D. J. Phys. Chem. 1994, 98, 10819. (20) Znamensky, V.; Kobrak, M. N. J. Phys. Chem. B 2004. (21) Hanke, C. G.; Johansson, A.; Harper, J. B.; Lynden-Bell, R. M. Chem. Phys. Lett. 2003, 374, 85. (22) Hanke, C. G.; Atamas, N. A.; Lynden-Bell, R. M. Green Chem. 2002, 4, 107. (23) Lynden-Bell, R. M.; Atamas, N. A.; Vasilyuk, A.; Hanke, C. G. Mol. Phys. 2002, 100, 3225. (24) Hyun, B. R.; Dzyuba, S. V.; Bartsch, R. A.; Quitevis, E. L. J. Phys. Chem. A 2002, 106, 7579. (25) Cang, H.; Li, J.; Fayer, M. D. J. Chem. Phys. 2003, 119, 13017. (26) Giraud, G.; Gordon, C. M.; Dunkin, I. R.; Wynne, K. J. Chem. Phys. 2003, 119, 464. (27) Weingartner, H.; Knocks, A.; Schrader, W.; Kaatze, U. J. Phys. Chem. 2001, 105, 8646.
Solvation and Rotational in Ionic Liquids (28) Not all workers agree on the speed of the ultrafast component. Petrich and co-workers15 suggest that this component might be as slow as 50 ps. (29) Shim, Y.; Duan, J.; Choi, M. Y.; Kim, H. J. J. Chem. Phys. 2003, 119, 6441. (30) Reference 19 and M. Kobrak, personal communications. (31) Xu, W.; Cooper, E. I.; Angell, C. A. J. Phys. Chem. B 2003, 107, 6170. (32) Bradaric, C. J.; Downard, A.; Kennedy, C.; Robertson, A.; Zhou, Y. Green Chem. 2003, 5, 143. (33) Heitz, M. P.; Maroncelli, M. J. Phys. Chem. A 1997, 101, 5852. (34) Birch, D. J. S.; Imhof, R. E. Time-Domain Fluorescence Spectropscopy using Time-Correlated Single-Photon Counting. In Topics in Fluorescence Spectroscopy; Vol. 1, Techniques; Lakowicz, J. R., Ed.; Plenum: New York, 1991; p 1. (35) Maroncelli, M.; Fleming, G. R. J. Chem. Phys. 1987, 86, 6221. (36) Mukamel, S. Principles of Nonlinear Optical Spectroscopy; Oxford: New York, 1995. (37) Fee, R. S.; Milsom, J. A.; Maroncelli, M. J. Phys. Chem. 1991, 95, 5170. (38) The relationship between characteristics of emission spectra F(ν) and emission line shape functions f(ν) ≡ ν-3F(ν) was explored with use of spectral data for C153 in 36 solvents. Correlating six different measures of spectral frequency (216 points) provided the relation VF/cm - 1 = 782 + 0.979Vf/cm - 1 with a correlation coefficient of r2 ) 0.991 and a standard error of fit of σfit ) 104 cm-1. A similar correlation of two different measures of spectral width (Γ, fwhm) provided ΓF/cm - 1 = - 1870 +
J. Phys. Chem. B, Vol. 108, No. 18, 2004 5777 0.947Γf/cm - 1 with r2 ) 0.991 and σfit ) 32 cm-1 (72 points). These correlations are specific to C153 but should also provide reasonable estimates for other solutes whose emission spectrum is comparable in width (30003500 cm-1) and lies in the same frequency range (17000-22000 cm-1) as C153. (39) Fletcher, K. A.; Pandey, S. Appl. Spectrosc. 2002, 56, 266. (40) Zhang, J.; Wu, W.; Jiang, T.; Gao, H.; Liu, Z.; He, J.; Han, B. J. Chem. Eng. Data 2003, 48, 1315. (41) Marcus, Y. The Properties of SolVents; Wiley: New York, 1998. (42) Horng, M. L.; Gardecki, J. A.; Papazyan, A.; Maroncelli, M. J. Phys. Chem. 1995, 99, 17311. (43) Fee, R. S.; Maroncelli, M. Chem. Phys. 1994, 183, 235. (44) We recorded an extensive set of steady-state emission anisotropies of C153 in the phosphonium ionic liquids assuming that these data along with lifetime measurements could be used to accurately determine rotation times. However, the high degree of nonexponentiality of the anisotropy decays renders such measurements inaccurate (unless β is known a priori). (45) Horng, M.-L.; Gardecki, J.; Maroncelli, M. J. Phys. Chem. 1997, 101, 1030. (46) Ediger, M. D.; Angell, C. A.; Nagel, S. R. J. Phys. Chem. 1996, 100, 13200. (47) Ediger, M. D. Annu. ReV. Phys. Chem. 2000, 51, 99. (48) Yang, M.; Richert, R. J. Chem. Phys. 2001, 115, 2676. (49) Richert, R. J. Chem. Phys. 2001, 114, 7471. (50) Bo¨hmer, R.; Ngai, K. L.; Angell, C. A.; Plazek, D. J. J. Chem. Phys. 1993, 99, 4201. (51) Yang, M.; Richert, R. Chem. Phys. 2002, 284, 103.
