Dispersed Kinetics without Rate Heterogeneity in an Ionic Liquid

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Dispersed Kinetics without Rate Heterogeneity in an Ionic Liquid Measured with Multiple Population-Period Transient Spectroscopy Champak Khurmi and Mark A. Berg* Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208

ABSTRACT The kinetics of the electronic-state relaxation of the diphenylmethane dye auramine O are found to be much more dispersed (nonexponential) in a prototypical ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate, than in ordinary solvents. Recent theoretical and experimental evidence for structural heterogeneity in ionic liquids suggest that the dispersion is due to rate heterogeneity, that is, different relaxation rates for solute molecules in different spatial regions. This hypothesis is tested with a new multidimensional spectroscopy, MUPPETS (multiple population-period transient spectroscopy). Rate heterogeneity is not found to be the primary cause of the dispersed kinetics. Rather, a homogeneous mechanism is responsible for this unusual phenomenon. SECTION Kinetics, Spectroscopy

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auramine O in a typical ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6).18,19 The excited-state lifetime of auramine O (Chart 1) decreases dramatically as the rigidity of its local environment decreases.20-28 This phenomenon has been attributed to a twisting of the phenyl groups in the excited state, which leads first to a loss of transition strength and then, upon further twisting, to rapid internal conversion to the ground state.25,28 Thus, the excited-state lifetime has been widely used as an indicator of the local viscosity in micelles, membranes and polymers.26-28 This behavior is reproduced in the experiments shown in Figure 1A. Two-beam, pump-probe measurements of the absorption bleach are used to measure ground-state recovery in two solvents of differing viscosity: methanol (0.6 cP) and decanol (11.5 cP). The decays are nonexponential due to the complex time dependence of the twisting on the excited-state potential and the complex dependence of the relaxation rate on twist angle.25,29 Nonetheless, the ratio of the 1/e times in the solvents (11:1) is in general agreement with the ratio of viscosities (19:1). The sublinear viscosity dependence is typical.22 Because BMIM-PF6 has a large viscosity (196 cP),19 the lifetime in this solvent would be expected to be near 1 ns. In fact, our measurements show it to be much faster than expected. This unexpectedly fast lifetime is an important result, but it is not the focus of this Letter. The distinctive increase in the dispersion of the kinetics is. A careful

oom-temperature ionic liquids have attracted a great deal of attention because they have numerous properties that differ qualitatively from those of conventional solvents. Long-lived mesostructure is one unusual feature of ionic liquids that has been suggested by both simulations1-4 and experiments.5,6 This structure might be caused by microphase separation of ionic and nonpolar groups1,2 or it might be related to glass-transition phenomena.7 In either case, the question arises of whether this structural heterogeneity induces heterogeneity in the rates of fast chemical processes, i.e., if solutes in different spatial regions have different reaction rates. If so, the observed reaction kinetics should be dispersed, that is, not single exponential. Indeed, ionic liquids induce dispersion in the kinetics of many processes.8 For example, dispersed kinetics have been observed in the rotation of a solute.9,10 Solvation dynamics in ionic liquids are also much more dispersed than in conventional solvents.11 However, nonexponential relaxation can occur for other, homogeneous, reasons, so these results are necessary, but not sufficient, to prove the existence of rate heterogeneity. Jin et al. have made a more specific study of rate heterogeneity.12 They observed an excitation-wavelength dependence in the solvation rate of coumarin 153, in the quantum yields of two viscosity sensitive fluorophores and in the yield of the excited-state charge transfer reaction of crystal violet lactone. Simulations by Annapureddy and Margulis on the crystal violet lactone reaction support the idea of rate heterogeneity in this system.13 In this Letter, we report a more direct test for rate heterogeneity. A new two-dimensional spectroscopy named MUPPETS (multiple population-period transient spectroscopy)14-17 is applied to the ground-state recovery of

r 2009 American Chemical Society

Received Date: October 14, 2009 Accepted Date: November 3, 2009 Published on Web Date: November 11, 2009

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DOI: 10.1021/jz900141a |J. Phys. Chem. Lett. 2010, 1, 161–164

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Chart 1. Structures of Auramine O and BMIM-PF6

examination of the time-domain data in Figure 1A shows that the decay in BMIM-PF6 stretches over a broader range of times than the decays in either methanol or decanol. A more quantitative description of the dispersion is given by the rate spectra,16 which are shown in Figure 1B. These spectra are essentially multiexponential fits with continuous ranges for the time constants τ. The spectral intensity is the corresponding amplitude of each exponential. The rate spectra have been generated by the maximum-entropy method,30-32 which produces the broadest spectra consistent with the data. The time-domain fits corresponding to the rate spectra are shown in Figure 1A. The negative amplitude peaks in the rate spectra at small time constants is the result of an induction period,16 which is a well-known feature in auramine kinetics. These spectra not only show a broader range of rates in the ionic liquid than in the conventional solvents, they also show two peaks for the ionic liquid, whereas the conventional solvents have only one. These results immediately suggest the hypothesis that rate heterogeneity is being induced by mesoscale structure in the ionic liquid. Moreover, the presence of two peaks in the rate spectrum would be consistent with two types of local environment: ionic and nonpolar, as suggested by simulations.1,2 However, this hypothesis needs more rigorous support. Although rate heterogeneity will certainly cause dispersed kinetics, the converse is not necessarily true. Homogeneous mechanisms, in which every molecule has an inherently nonexponential decay, are also possible causes of dispersed kinetics.16 MUPPETS has the ability to distinguish between heterogeneous and homogeneous origins of dispersed kinetics. It is a two-dimensional technique in which three pairs of phasestabilized pulses interact with the sample.14-16 The first two pulse pairs are separated by a time t1 and excite the sample twice. After an additional time delay t2, the absorption change at the excitation wavelength is probed by the third pair of pulses in a differential heterodyne-detection scheme.17 The directions of all six beams are different, so the experiment is a high-order transient grating. If rate heterogeneity exists, the first time interval acts as a filter, differentiating between rapidly and slowly relaxing molecules. The second time interval measures the relaxation of the slower subset of molecules. In the presence of rate heterogeneity, as t1 is increased, rapidly decaying molecules are more effectively removed from the measured subensemble, and the decay in t2 becomes slower and less disperse. On the other hand, if rate heterogeneity is not present, changing t1 has no effect on the decay in t2. The MUPPETS results are easily predicted in the case in which all of the nonexponentiality in the one-dimensional decay is due to rate heterogeneity.16 These predictions are


Figure 1. One-dimensional, pump-probe measurements of the excited-state relaxation of auramine O in methanol (red, 0.6 cP), decanol (green, 11.5 cP), and BMIM-PF6 (blue, 196 cP). (A) Data (points) and maximum-entropy fits (lines) in the time-domain. (B) Rate spectra from the maximum-entropy fits. The strong dispersion of the kinetics is only found in the ionic liquid.

Figure 2. MUPPETS results for auramine O in BMIM-PF6. (A) Predicted signal, if the dispersion in the one-dimensional kinetics is only due to rate heterogeneity. The fraction of molecules in the measured subensemble is shown for each value of t1. (B) Experimental results.

shown in Figure 2A. The legend shows the fraction of molecules measured for each value of t1. This calculation is not completely applicable here, because it is known that the decay of auramine is inherently nonexponential. However, the inherent nonexponentiality is small compared to that induced by the ionic liquid, so these calculations are a good estimate of the size of the effect that should be observed. The range of t1's shown is sufficient to produce strong effects in the MUPPETS experiment, if rate heterogeneity is present.

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Figure 2B shows the experimental MUPPETS results. There is little change in the relaxation as t1 is increased, certainly not a change of the magnitude seen in the predictions. If there were no rate heterogeneity, all the curves in Figure 2B would fall on top of each other, whereas small systematic changes are observed, especially at early times. Effects of a similar magnitude are also seen for auramine in methanol.14,15 They can be attributed to two slightly different forms for the auramine, for example, conformers with the same and opposite twists on the phenyl groups. More precise measurements and analysis are underway to better characterize this small effect. However, at this point, it is already clear that rate heterogeneity is not the primary cause of the high ratedispersion found in this ionic liquid. Although these measurements alone cannot determine the specific mechanism responsible for the dispersed kinetics, it is useful to consider whether a plausible homogeneous mechanism exists. Most demonstrations of the viscosity sensitivity of auramine have been in the series of linear alcohols. However, several groups have noted that other solvents do not fit well into the same trend, leading to the suggestion that solvation times also play a role in the relaxation rate.24,26,27 It is well known that solvation in ionic liquids is unusually slow and disperse. In BMIM-PF6, the diffusive solvation has a 1/e time of 140 ps and a stretching parameter of β = 0.31.11 We speculate that the ground-state solvent configuration allows twisting into the configuration of most rapid internal conversion, but with sufficient stabilization of the excited state, the molecule moves to an angle with a slightly lower internal conversion rate. The slow tail of the relaxation would then represent those molecules that by chance survived long enough to become fully solvated. This mechanism would be a homogeneous one: the fact that a molecule survived long enough to become solvated and slowly relaxing in one excitation/relaxation cycle would not predict whether it would be slowly relaxing in another excitation/relaxation cycle. These results contrast with the results of Jin et al., who used different viscosity probes in a different ionic liquid.12 They found an excitation-wavelength dependence of the quantum yield of two probes whose fluorescence is quenched by conformational motion in the excited state. They also saw a similar excitation-wavelength dependence of the reactant/ product ratio in crystal violet lactone, which has an excitedstate reaction with a barrier. Both results are most obviously attributed to rate heterogeneity. We note that there are strict requirements for structural heterogeneity to be translated into rate heterogeneity. The reaction rate must be coupled to the solvent structure. However, if it is too strongly coupled, the reaction will slow to near the reorganization time of the solvent, which prevents rate heterogeneity. Thus, it is not clear if the differences between Jin et al.'s and our results are due to differences in the methods, differences in the ionic liquids, or differences in the probe reactions. More generally, the lack of rate heterogeneity in this reaction does not comment on the existence of structural heterogeneity in this, or any other, ionic liquid. In summary, we have shown that the kinetics of an environmentally sensitive reaction become much more


disperse in an ionic liquid than they are in conventional solvents. The results of standard, one-dimensional experiments are completely consistent with the hypothesis that this dispersion represents rate heterogeneity induced by a heterogeneous mesostructure in the ionic liquid. However, twodimensional MUPPETS experiments show that the dispersed kinetics are not due to rate heterogeneity. This result does not negate the importance of dispersed kinetics as an unusual characteristic of ionic liquids. It does show the importance of two-dimensional spectroscopy for determining the correct mechanism.

Experimental Methods All samples had an optical density of ∼0.4 at 400 nm, which is the wavelength of all the pulses used. Samples were flowed through a 1 mm cell. Samples of BMIM-PF6 (Covalent Associates) were handled under nitrogen to avoid the absorption of water. The excitation wavelength is on the high-frequency side of the absorption spectrum, so selection of molecules through a red-edge effect is unlikely. Measurements of the transient-absorption spectrum show that the dynamics at 400 nm are dominated by ground-state recovery.26,29 The ground-state recovery is slower than the fluorescence lifetime, due to intervening dark states, which are believed to be twisted conformations of the first excited state.25,29 The MUPPETS experimental setup has been described in detail elsewhere.14,15,17 The pump-probe measurements were done in the same set up by blocking unneeded beams. The pulses had widths of ∼200 fs, energies of ∼400 nJ each, and diameters of 250 μm at the sample. We estimate that 10% of the population is excited by each pulse pair. Decays were followed out to 2 ns to look for slow decay components. No decay was detectable, and the data from 1 to 2 ns was used to establish the signal baseline.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: berg@ mail.chem.sc.edu.

ACKNOWLEDGMENT This material is based upon work supported by the National Science Foundation under CHE0809306.

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Dispersed Kinetics without Rate Heterogeneity in an Ionic Liquid

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