Ultrafast Solvation Dynamics in Room Temperature Ionic Liquids

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Ultrafast Solvation Dynamics in Room Temperature Ionic Liquids Observed by Three-Pulse Photon Echo Peak Shift Measurements Masayasu Muramatsu, Yutaka Nagasawa,* and Hiroshi Miyasaka* Division of Frontier Materials Science, Graduate School of Engineering Science, Center for Quantum Science and Technology under Extreme Conditions, Osaka University and CREST, JST, Toyonaka, Osaka 560-8531, Japan

bS Supporting Information ABSTRACT: Three-pulse photon echo peak shift (3PEPS) measurement was applied to the investigation of the primary part (100 ps due to their high viscosity (see Table 1). The results of the analysis are also shown in Figure 3, and the parameters obtained are listed in Table 1. Details of the 3PEPS signal are plotted in Figures 4 and 5. The initial part of 3PEPS (Figures 4a and 5a) indicates that the signal in BmimCl clearly decays faster than those in other imidazolium ILs. In the picosecond time range (Figure 4b and 5b), the 3PEPS signal for BmimCl is nearly flat and no remarkable decay can be observed, while 3PEPS signals for other ILs continue to decay. However, correlation with the viscosity is not clear in the time range of e100 ps, as shown in Table 1. That is, the time constants for BmimBF4 with viscosity of 79 cP are close to those obtained for EmimTFSI with the lowest viscosity of 33 cP at 20 C. Nevertheless, the asymptotic peak shift of BmimBF4 (Aasy = 6.4 fs) is considerably larger than that of EmimTFSI (3.8 fs), indicating that the solvation process is not accomplished within e100 ps. Viscosity dependence is reported to be pronounced in the longer time range.20-26 The asymptotic peak shift of BmimCl (4.6 fs) is smaller than that of BmimBF4; however, this may be due to lower polarity of BmimCl and/or the red-shift of the absorption band. The amount of the peak shift depends not only on the solvation dynamics but also on the total value of reorganization energy. We will discuss this point on the basis of more detailed analysis using computer simulation in subsequent sections. 3.3. Solvation in Ethanol. Before the detailed discussion on ILs, in order to evaluate the validity of Ox4 as a probe of solvation dynamics, we compared 3PEPS data in ethanol (EtOH), the solvation dynamics of which is known to be completed within our experimental time window of ∼100 ps. The obtained M(t) was quite similar to the previously reported ones utilizing different types of probe molecules29,56-59 as can be seen in Figure 6. It is especially similar to that obtained by IR125,56 and also to the solvation correlation function, S(t), obtained from the DSS measurement of coumarin 153.57 Other M(t) curves show 3890

dx.doi.org/10.1021/jp108282v |J. Phys. Chem. A 2011, 115, 3886–3894

The Journal of Physical Chemistry A

Figure 6. (a) Ultrafast part (e500 fs) and (b) picosecond part of the fitting curve obtained for 3PEPS in ethanol. Our result for Ox4 (thick red solid curve) is compared with other results utilizing probe molecules such as IR144,29,58 IR125,56 rhodamine 6G (R6G),59 and Nile blue (NB).59 The asymptotic peak shift and intramolecular vibrations are ignored in all cases. Solvation correlation function, S(t) (red dotted curve), was obtained from the dynamic Stokes shift of coumarin 153.57.

more rapid decay in the range of e100 fs than those obtained by Ox4 or IR125, although the logarismic slope of the picosecond tail of IR144 is nearly identical.29,58 The initial femtosecond decay is usually interpreted as due to the small amplitude inertial motions of solvents in the vicinity of a solute, and it was suggested that librational motion of OH group contributes to this rapid response in the case of alcohols.60 Moreover, destructive interference between the coherent intramolecular vibrations also contributes to the ultrafast decay. The surplus ultrafast decay may be due to the higher intramolecular reorganization energy or to the excitation wavelength dependence of the initial decay.51,56,61 Park and Joo reported that the slower dynamics for IR125 was probably due to the multipolar nature of the large probe molecule.56 The M(t) obtained from rhodamine 6G and Nile blue also showed decays somewhat faster even in the picosecond range compared to other M(t), which may be due to the shorter scanning range.59 According to dynamic Stokes shift measurements, diffusive solvation in ethanol is accomplished within tens of picoseconds.57 However, 3PEPS of Ox4 in ethanol did not decay to baseline even at 100 ps with a residual peak shift of ∼4.7 fs. Such a nonzero peak shift in the time range much longer than the solvation times has been often observed in 3PEPS experiments.56,58,61-63 In the case of DTTCI in methanol and acetonitrile, such a residual peak shift was observed even at 1.0 ns, the result of which cannot be accounted for by a simple dielectric relaxation model.63 Joo and co-workers extensively studied the origin of the residual peak shift and concluded that conformational heterogeneity of the large and flexible probe molecule may be responsible for such a long-living inhomogeneity.56 Ox4 has a rather rigid structure compared to DTTCI or IR125, although

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Figure 7. (a) Anion dependence and (b) cation dependence of the ultrafast part (e1.0 ps) of the fitting curve obtained for 3PEPS in ionic liquids. Asymptotic peak shift and intramolecular vibrations are neglected.

there may be possibility of conformational heterogeneity due to rotation of the two amino groups (Scheme 1). Moreover, emission of Ox4 is reported to exhibit dual peaks in alcohols which may be due to formation of hydrogen bonded complex46 which could also contribute to an increase in the heterogeneity. (In ILs, only a single emission peak was observed disregarding the existence of such complex.) Nevertheless, the residual peak shift does not alter the observed solvation dynamics and it is often ignored or subtracted from the 3PEPS data by assuming that it is an artifact and/or a result of chromatic aberration.58,61,62 However, for ILs with high viscosity, the solvation process extends into a time range of nanoseconds and residual peak shift is expected in our experimental time window of 100 ps. 3.4. Solvation in Ionic Liquids. The ultrafast part of the M(t) obtained for ILs is shown in Figure 7, where signals with intramolecular vibrations and asymptotic component were excluded. The decay shows no remarkable dependence on the alkyl chain length of the cation in the time range of e200 fs (Figure 7b), while a correlation with the size of anion is suggested (Figure 7a), i.e., a smaller and lighter anion exhibits faster decay. For Bmimþ, the time constant for the fastest decay component increases in the order of Cl- (26 fs) < BF4- (110 fs) < PF6(150 fs) < TFSI- (210 fs). A similar trend has been reported for the optical Kerr effect (OKE) and DSS measurements. For the OKE signal of 1-heptyl3-methylimidazolium ILs, the time constant for the nondiffusive part of the orientational response increased in the order of Br(117 fs) < PF6- (215 fs) < TFSI- (354 fs).64 Solvation correlation functions, S(t), obtained from the dynamic Stokes shift of trans-4-dimethylamino-40 -cyanostilbene by Maroncelli and co-workers,25 are plotted also in Figure 8 for comparison. Time constants, τ1, reported for the initial ultrafast decay are 320, 330, and 740 fs for Bmimþ with BF4-, PF6-, and TFSI-, respectively. Although, BmimBF4 and BmimPF6 have similar 3891



Figure 8. (a) Ultrafast part (e1.0 ps) and (b) slower part of the complete solvation correlation function, S(t), of imidazolium ILs obtained from the dynamic Stokes shift of trans-4-dimethylamino-40 cyanostilbene by Maroncelli and co-workers.25.

values of τ1, BmimPF6 decays clearly faster than BmimBF4 in the total solvent correlation function with the inertial component combined with the stretched exponential function representing the slower diffusive solvation, as can be seen in Figure 8a. They also reported τ1 to be 100 fs for HmimCl which was distinctively shorter than other imidazolium ILs. Molecular dynamics computer simulation suggests that subpicosecond inertial regime of solvation is mainly governed by the motions of a few adjacent ions in the case of a dipolar solute with sufficiently high density of surrounding ions.47 Especially for EmimCl, the motion of Cl- in the first solvation shell dominates the ultrafast regime due to its small size which enables closer location to the solute. Maroncelli and co-workers concluded that the initial ultrafast decay is related to inertial ion motions, because it had a correlation with the factor, (μ((RþþR_)3)1/2, where μ( is the reduced mass of an ion pair and (Rþ þ R-) is the sum of their van der Waals radii.25 The Debye frequencies of alkali halide crystals are well represented by proportionality to a similar factor, because of a short-range repulsion and an electrostatic repulsion. On the other hand, in our results, τ1 had a better correlation simply with the square root of anion mass, m-1/2, as can be seen in Figure 9a. Because Ox4 is a cationic dye, it may be located in the microscopic polar domain of the IL and have a stronger interaction with the anion of IL. As can be seen in Figure 1, Ox4 seems to have a specific interaction with chloride anion. Therefore, translational motion of anion may contribute more strongly to the initial solvation process immediately after the photoexcitation. Interestingly, the second time constant, τ2, had a slightly better correlation with (μ((Rþ þ R_)3)1/2 rather than m-1/2 (Figure 9b), suggesting the beginning of more global solvation process including the motion of cations. It should be also noted that 3PEPS is excitation wavelength dependent. That is, shorter excitation wavelengths lead to faster initial decay. In the case of a similar dye Nile blue in acetonitrile, it

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Figure 9. The time constants (a) τ1 and (b) τ2 for the initial ultrafast and the second peak shifts, respectively, plotted against the square root of the anion mass and the inertial factor (μ((Rþ þ R_)3)1/2, respectively. The blue line is the result of the least-squares fit to the plots. The triangle is the point for PhosCl.

was reported that 72 fs decay reduced to 26 fs when excitation wavelength was changed from 660 to 630 nm.61 Also for IR125 in methanol, the ultrafast decay of 270 fs reduced to 229 fs by changing the excitation wavelength from 814 to 785 nm.56 In the preset systems, absorption spectrum in BmimCl is red-shifted by 615-695 cm-1 compared to those in other imidazolium ILs, as shown in Figure 1. Hence, the exposure at 630 nm corresponds to the excitation at shorter wavelengths. However, even if the 26 fs decay were to be elongated to ∼70 fs in the case of Ox4 in BmimCl, the correlation between τ1 and m-1/2 will be preserved. Moreover, the correlation between τ2 and (μ((Rþ þ R_)3)1/2 will be also retained even if the 540 fs decay were to be elongated to ∼1.0 ps. Therefore, we conclude that the faster inertial response is responsible for the ultrafast decay in BmimCl although the excitation wavelength dependence should be taken into account to some extent. Similar discussion is also valid for the result of PhosCl. 3.5. Computer Simulation of 3PEPS. In order to elucidate the amount of reorganization energies corresponding to each decay components, we applied numerical computer simulation on the basis of the method described previously.29,36 The simulated 3PEPS signals and electronic transition spectra for BmimCl and BmimBF4 are shown in Figure 10, and the solvent reorganization energies, λi, for each decay components are listed in Table 2. The fwhm of absorption and fluorescence spectra of Ox4 in imidazolium ILs is not much dependent on the solvent, indicating that total reorganization energy is also not very sensitive to the solvent. However, the vibrational structure of the absorption spectrum in BmimCl is somewhat clearer compared to those in other imidazolium ILs which may be due to rather smaller reorganization energy. Moreover in imidazolium ILs, the 3892



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Figure 10. (a) Experimental (markers) and simulated 3PEPS (solid curves) data of Ox4 in ILs. (b) Experimental (solid curves) and simulated (dotted curves) absorption (blue) and fluorescence spectra (red) of Ox4 in BmimBF4 and in BmimCl.

Table 2. Solvent Reorganization Energies, λi, Obtained from Computer Simulation for Each Decay Component in 3PEPS Data of Ox4 in ILs ethanol

BmimCla

BmimBF4

λ1/cm-1

60

170

95

λ2/cm-1

28

64

λ3/cm-1

46

λasy/cm-1

440

29 19

440

860

For BmimCl, a red shift of 615 cm-1 in the electronic transition spectra was also considered for the simulation, although the effect was rather minor. a

fluorescence spectra are always about 100-200 cm-1 narrower than absorption spectra indicating that harmonic linear coupling approximation48 may not be entirely valid for this chromophore. When the literature data of intramolecular vibrational reorganization energies obtained by a resonance Raman experiment46 was utilized as it is, the coherent oscillation in the simulated 3PEPS signal became excessively pronounced compared to the experimental one. To reproduce the amplitudes of the oscillations, it was necessary to reduce the reorganization energies for the modes at 570 and 586 cm-1 to 30% of their original values. The values for other vibrational modes were kept as they were, and the total intramolecular reorganization energy was set at 570 cm-1 for every case. As can be seen in Figure 10a, the experimental result of 3PEPS in BmimCl was well reproduced by the simulation, although

deviations were observed for BmimBF4 in the ultrashort time range of e60 fs. The deviation is attributable to the imperfect inclusion of intramolecular degrees of freedom.36,58,62,65 That is, resonance Raman scattering experiment may not cover all the modes with very small coupling to the electronic transition. The fwhm of the simulated spectra in BmimCl shown in Figure 10b are similar to the experimental ones, although the vibrational structure is not well reproduced due to the imperfect data on vibrational modes. The simulated spectra in BmimBF4 are somewhat broader than the experimental ones; thus the reorganization energies listed in Table 2 must be considered as the upper limit, especially the reorganization energy of 860 cm-1 for the asymptotic peak shift. The total reorganization energy in BmimBF4 can be reduced by introducing a rapid decay of 26 fs which represents the destructive interference of the missing intramolecular vibrational modes, the results of which are presented in Figure 4S of the Supporting Information. In the modified simulation, the reorganization energy for the asymptotic peak shift is reduced to 440 cm-1. Summarizing above results and discussion, we conclude that the simulation could qualitatively estimate the reorganization energies for each decay, although there are several deviations arising from imperfection of the current model, notably the lack of precise information about intramolecular vibrational modes. The contribution from the ultrafast inertial solvation in the e300 fs range for the present ILs is estimated to be 10-25% of the total solvation, which is in accordance with those obtained by the DSS measurements.25

4. CONCLUSIONS Three-pulse photon echo peak shift (3PEPS) measurement was employed for the elucidation of the ultrafast part (e100 ps) of the solvation dynamics in series of imidazolium room temperature ionic liquids (ILs). A cationic organic dye, oxazine 4 (Ox4) perchlorate with Stokes shift as small as e500 cm-1 was utilized as a probe. It was expected that Ox4 is sensitive to the ultrafast motion of a few adjacent ions and bring forth an important information about the primary process of solvation dynamics in ILs. The ultrafast initial solvent response in the range of e300 fs exhibited dependence on the square root of the anion mass, indicating its relation with the inertial motion of anions. The inertial response of ILs with chloride anion was the fastest among other ILs with heavier and larger anions. Because Ox4 is a cationic dye, it may have a stronger interaction with the anion of IL. The electronic spectra of Ox4 exhibited a peculiar interaction with chloride anion; i.e., the spectra were red-shifted and the Stokes shifts were noticeably smaller compared to other IL with larger anions. A similar anion effect observed in previously reported articles concerning ultrafast optical Kerr effect and dynamic Stokes shift measurements also supports our conclusion. Interestingly, the second time constant, τ2, in the range of e3.5 ps had a slightly better correlation with (μ((Rþ þ R_)3)1/2 rather than m-1/2, suggesting the beginning of more global solvation process including the motion of cation. Numerical computer simulation was also carried out and the solvent reorganization energies for each decay components were quantitatively estimate, although deviations were found between the simulation and experiment, due to nonlinearity and/or unharmonicity of the intramolecular oscillations. 3893



’ ASSOCIATED CONTENT

bS

Supporting Information. Figures of UV-vis absorption spectra of ionic liquids and time constants of peak shifts and table of specifications for EmimTFSI and BmimTFSI. This information is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] and miyasaka@chem. es.osaka-u.ac.jp.

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Ultrafast Solvation Dynamics in Room Temperature Ionic Liquids

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