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Dynamic Stokes Shift of 9,9′-Bianthryl in Ionic Liquids: A Temperature Dependence Study† Yutaka Nagasawa,* Akito Oishi, Tsuyoshi Itoh, Masakazu Yasuda, Masayasu Muramatsu, Yukihide Ishibashi, Syoji Ito, 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 ReceiVed: March 6, 2009; ReVised Manuscript ReceiVed: May 1, 2009
The temperature dependence of the fluorescence behaviors of 9,9′-bianthryl (BA) in the charge-transferred (CT) state in imidazolium ionic liquids (IL) was investigated by means of steady-state as well as timeresolved detections. At ambient and higher temperatures, the emission peak of BA in ILs shifted to longer wavelengths with decreasing temperature, which is a phenomenon commonly observed in normal polar solvents. On the other hand, the emission peak shifted toward shorter wavelengths with decreasing temperature below ca. 290 K. Time-resolved fluorescence (TRF) spectroscopy was carried out to investigate the dynamic Stokes shift, i.e., the time-dependent red-shift of the fluorescence peak. In highly viscous ILs, the time constant of the dynamic Stokes shift becomes comparable or longer than the lifetime of the CT state. In such a case, the ground state of BA is recovered before the completion of the solvation process. It is concluded that the origin of the blue-shifted emission at lower temperatures is due to the fluorescence from the unrelaxed CT state. It is confirmed that the charge separation process of BA in ILs occurs prior to the nanosecond fluorescence red-shift due to the slow solvation process in the CT state in the temperature range we have studied. 1. Introduction Ionic liquid (IL) is a molten organic salt at room temperature, which is anticipated to be applied as a safe and clean solvent for chemical synthesis and electrolytes in batteries and solar cells.1–4 Vast numbers of fundamental studies have been performed to understand the physical properties of ILs.5 Some of its peculiarities arise from its composition with freely mobile oppositely charged ions. Normal polar solvent molecules possess a constant electric dipole moment and act as dielectrics. Meanwhile, ions in ILs are mobile, and in principle they can move independently. Hence, IL is expected to form a solvation shell different from that of normal polar solvents surrounding a charged solute, leading to more effective stabilization of solutes with large dipole moments. Mobile charges are also interesting from the viewpoint of molecular dynamics. Normal solvent molecules are required to rotate in the process of forming a solvation shell, while translation of ions can also contribute largely in the case of ILs. This peculiar property has been attracting much attention, and a number of studies on the dynamic Stokes shift, the timedependent red-shift of a fluorescence spectrum induced by the solvation process, have been performed by time-resolved fluorescence (TRF) detection methods.6–15 From these studies, it was revealed that the dynamic Stokes shift in ILs generally takes place in a wide time range of subpicoseconds to nanoseconds. For the ultrafast subpicosecond time region, it was reported that the initial solvation could be correlated with the inertial characteristics of an IL.7,9,10 The slower picoseconds to nanosecond components are usually represented by stretchedexponential or multiexponential functions, and its averaged time constant is correlated well with solvent viscosity. This type of †
Part of the “Hiroshi Masuhara Festschrift”. * To whom correspondence should be addressed. E-mail: nagasawa@ chem.es.osaka-u.ac.jp,
[email protected].
kinetics is also found in viscous normal solvents16 and also in supercooled liquids near the glass transition.17,18 ILs are viscous and inhomogeneous liquids constructed from a pair consisting of a rather large organic cation and an inorganic/organic anion. A cation of imidazolium ILs comprises an alkyl chain with varying length. The nonpolar alkyl chains are considered to form weakly bounded micelle-like aggregates surrounded by polar domains comprised of ions.19–24 The highly inhomogeneous nature of ILs is the origin of the multiexponential solvent relaxation function commonly observed for ILs. An excitation wavelength dependence of the fluorescence spectrum known as the “red-edge effect” was observed for molecules with short excited state lifetime, which indicates the existence of static inhomogeneity in ILs,25–27 while MD simulation suggests an existence of dynamical inhomogeneity; that is, ions rapidly “rattle” in a long-lived cage while the orientational structure relaxes in a much longer time scale.22 The peculiar properties of ILs are expected to affect the charge transfer (CT) or electron transfer reactions and their related processes, because solvation dynamics and/or fluctuation of solvent molecules play a crucial role for these reactions in the condensed-phase. To unveil the nature of CT dynamics in ILs, we have investigated the intramolecular charge separation dynamics of 9,9′-bianthryl (BA) in imidazolium ILs.28 It was revealed that charge separation of BA taking place in the subpicosecond to subnanosecond time range was followed by a multiexponential dynamic Stokes shift of the CT fluorescence extending into the nanosecond time range. Formation of the CT state was observed by a femtosecond transient absorption measurement, revealing that the charge separation process is accomplished before the entire dynamic Stokes shift is completed. In addition, the steady-state fluorescence maximum of the CT state was much more red-shifted in ILs compared to that in polar solvents. The peak-to-peak Stokes shift between absorption and emission spectra in 1-butyl-3-methylimidazolium
10.1021/jp9020454 CCC: $40.75 2009 American Chemical Society Published on Web 05/18/2009
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CHART 1: Molecular Structure of the Samples
bis(trifluoromethylsulfonyl)imide (BmimTFSI) was ∼4390 cm-1 while that in acetonitrile was ∼4060 cm-1. It is known that the Stokes shift of BA scales rather well with the dielectric constant of the normal solvent following the Mataga-Lippert relation.29–32 The large Stokes shift in ILs indicates that a simple dielectric continuum model is not applicable for ILs. The empirical parameter of solvent polarity, ET(30), also indicates larger polarity of ILs compared to acetonitrile.6 By integrating the results of steady-state measurement with those obtained by the TRF and transient absorption measurements, we concluded that rapid local fluctuation of the IL induces symmetry breaking and subsequent charge separation while nonlocal large scale structural relaxation further stabilizes the resulting CT state. In the present work, we have investigated the temperature dependence of the fluorescence behaviors of BA in IL with steady-state and time-resolved detection in order to precisely elucidate the complex solvent responses of IL. Solvation times of ILs are known to correlate with viscosity, which is dependent on temperature. Solvent fluctuation will be suppressed at low temperatures, and the solvent structure of IL may stabilize, which is expected to affect the nature of the CT reaction. 2. Experimental Section The experimental setup for time-correlated single-photoncounting measurement was described previously.28 The light source for the picosecond time-resolved fluorescence (TRF) spectroscopy was the second harmonic (390 nm) of a Ti:sapphire laser (Spectra Physics, Tsunami) generated in a type I BBO crystal, and the repetition rate was generally reduced to 2-4 MHz with a power of 8-12 µW by an EO modulator (Conoptics). The emission was detected at the magic angle configuration utilizing a polarizer and a half-wave-plate. A cylindrical sample cell (1.0 cm diameter) with a stirrer was utilized. A photomultiplier-tube (Hamamatsu Photonics, R3809U50) with an amplifier (Hamamatsu Photonics, C5594) and a counting board (PicoQuanta, PicoHarp 300) were used for the signal detection. A monochromator (Newport, Oriel 77250) was placed in front of the photomultiplier-tube. The system response time was determined to be 32 ps full-width-at-half-maximum (fwhm) by a scattered light from a colloidal solution. The sample temperature was controlled by a high power Peltier temperature controller (Yamaki, KLT-2SS). The experiment was performed inside a box purged with dried air for low temperature measurements. The fluorescence time profile was measured at different wavelengths with intervals of 5 and 10 nm for below and above 550 nm, respectively. The reconstruction of the timeresolved fluorescence spectrum was based on a method similar to that reported by Tamai et al.33 It was assumed that the relative integrated intensity of the fluorescence decay at each wavelength corresponds to that of the steady state fluorescence spectrum. The number of photons was counted up to a delay of 200 ns with a step size of 4.0 ps, and it was averaged over certain
intervals (for example, between 1 and 5 ns, 200 ps interval with 50 points were averaged to obtain a single point) to obtain a TRF spectrum with a higher S/N ratio. The fluorescence peak shift function was obtained from a fitting method utilizing a log-normal function.34 9,9′-Bianthryl (BA) was synthesized according to the method described by Bell and Waring.35 Anthraquinone (10 g) and tin grains (40 g) were added into acetic acid (120 mL) and boiled under reflux. Fuming hydrochloric acid (60 mL) was slowly added over a period of 2 h, and the solution was refluxed for another 2 h. The obtained precipitate was filtered and corrected, followed by silica gel column chromatography with toluene and recrystallization in acetic anhydride. The purity of the product was checked by NMR spectroscopy and elemental analysis. 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (BmimTFSI) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (EmimTFSI) were prepared by Kanto Chemical as mutual samples for Research on Priority Area (452). 1-Butyl-3-methylimidazolium hexafluorophosphate (BmimPF6) and 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4) were also purchased from Kanto Chemical. 1-Hexyl3-methylimidazoliumbis(trifluoromethylsulfonyl)imide(HmimTFSI) was from Merck and used as received. The viscosity measurement was carried out with a viscometer (BrookField, LVDV-I+CP). The sample solution was always prepared in a dispensable glovebag filled with dried nitrogen gas to avoid moisture. Because of the high viscosity, dissolution of BA into IL was time-consuming. After adding BA into ILs, the solution was left in a tightly sealed container under vacuum for at least two days until dissolution of BA is completed. Modest heat (∼40 °C) was added for dehydration and to hasten the dissolution. Steady-state absorption and fluorescence spectra were measured with a Hitachi U-3500 spectrophotometer and a F850 spectrofluorometer, respectively, utilizing a 1.0 cm fused silica cell. 3. Results and Discussions The molecular structures of the samples are shown in Chart 1. BA is a symmetric molecule with two anthryl moieties (anthracenes) connected with a single bond at the center. In nonpolar solvents, the excited state of BA is an excitonic state which is shared equally by both anthryl moieties, although it is conventionally referred to as a locally excited (LE) state. The emission from the LE state peaks at shorter wavelengths; that is, the maximum emission wavelength in n-hexane is at ∼413 nm. Only in polar solvents, does the CT reaction take place in the excited state and is the emission red-shifted, i.e., to ∼465 nm in acetonitrile (ACN).30,36–41 Three types of imidazolium cations with different alkyl chain lengths (C2 to C6) were used in this study (also shown in Chart 1). Three types of anions, TFSI-, BF4-, and PF6-, were also used (only the molecular structure of TFSI- is shown in Chart 1).
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Figure 3. Temperature dependence of the fluorescence peak of BA in acetonitrile (filled circles). The solid line is the result of fitting with a slope of 7.2 cm-1/K.
Figure 1. Fluorescence spectra of 9,9′-bianthryl (BA) in (a) acetonitrile and (b) BmimPF6. Temperatures are (a) 253, 273, 293, 313, and 333 K and (b) 253, 258, 263, 268, 273, 278, 283, 293, 313, and 333 K. The arrow indicates the substantial direction of the peak shift when temperature is increased.
Figure 2. Fluorescence spectra of BA in BmimPF6 at 293 K (red dots) and the fitting curve (blue line) constructed from a log-normal function (dotted green curve) and a Gaussian function (green solid curve). The residue (black unfilled squares) between the experimental data and the fitting curve is also presented.
3.1. Steady State Fluorescence. The temperature dependences of the fluorescence spectra of BA in ACN and BmimPF6 are shown in Figure 1. In the temperature range of 253 to 333 K, the fluorescence spectra of BA in ACN simply shifted to higher frequencies with increasing temperature, as indicated by the arrow in Figure 1a. On the other hand, that in BmimPF6 shifts to lower frequency when the temperature is raised at 280 K, suggesting that the polarity experienced by BA in its CT state is not very dependent on the anion of IL. Figure 4b shows the temperature dependences in ILs with different alkyl chain length on the imidazolium cation and with the same anion of TFSI-. Comparing these three ILs, the peak frequency in HmimTFSI is the highest while that in EmimTFSI is the lowest, indicating that polarity is dependent on alkyl chain length. Nevertheless, all of the peak frequencies at >270 K are lower than that in ACN and that of EmimTFSI is the lowest among all the ILs investigated in this work. At ambient and higher temperatures, the peak frequencies decreased with decreasing temperature as observed in ACN and in Figure 4a. At lower temperatures, a
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Figure 5. Temperature dependence of the viscosity of imidazolium ionic liquids, BmimTFSI (red circle), BmimBF4 (black diamond), BmimPF6 (blue square), EmimTFSI (green triangle), and HmimTFSI (unfilled black square). The solid curves are the result of fitting with the Vogel-Tammann-Fulcher equation.
Tammann-Fulcher (VTF) equation, which was previously applied to IL.44 Figure 4. Temperature dependence of the fluorescence peak of BA in imidazolium ionic liquids. (a) Bmim+ with TFSI- (red circle), BF4(black diamond), and PF6- (blue square). (b) Bmim+ (red circle), Emim+ (green triangle), and Hmim+ (unfilled black square), with TFSI-.
clear increase of the frequency with decreasing temperature was also observed for BmimTFST and HmimTFST, while the increase was not very remarkable in EmimTFST. Among the five ILs presented in Figure 4, the degree of the increase of the peak frequency at lower temperatures is the largest in BmimPF6, while it is only modest for EmimTFSI. This behavior cannot be explained by the temperature dependence of the dielectric constant alone. As is mentioned next, BmimPF6 holds the highest viscosity among the ILs studied in the present investigation while that of EmimTFSI is the lowest. Hence, it is strongly suggested that the difference in the temperature dependence in these five ILs is closely related to that of the viscosity. To directly reveal the effect of viscosity, η, on the CT fluorescence of BA, we also measured the temperature dependence of η in the five ILs, as shown in Figure 5. It can be seen that ILs are much more viscous than normal solvents such as acetonitrile with a value of η of only 0.375 cP at 293 K. As shown in Figure 5, η is strongly dependent on temperature and that around 280 K is almost an order of magnitude larger than that at 340 K. In the temperature region examined here, the η values in EmimPF6, BmimTFSI, and HmimTFSI are not so different, while those among BmimBF4, BmimPF6, and BmimTFSI are large. This result indicates that the anionic part strongly affects the value of η more effectively than the length of the alkyl chain on the cation. According to MD simulations and other studies, it is argued that alkyl chains on anions with certain lengths aggregate and form nonpolar domains surrounded by ion-rich polar domains.20,21 However, interaction among these aggregated molecules seems not as strong as to affect the viscosity significantly. Solid lines for each experimental result in Figure 5 are the temperature dependence of η calculated by the Vogel-
ln η ) ln η0 +
DTc T - Tc
(1)
Here, D is the fragility parameter and Tc is a characteristic temperature for which η diverges. The obtained parameters for each IL are listed in Table 1. Our viscometer was not capable of measuring viscosity at low temperatures; thus, viscosities below 278 K were calculated from the VTF equation with the obtained fitting parameters. The temperature dependence of the viscosities of some of the ILs was already reported in the literature, which is in reasonable agreement with ours.45–47 Because the data shown in Figure 5 were obtained from the actual ILs used in our experiment and measured with an identical experimental apparatus and conditions, we will refer to these results throughout the present investigation. The temperature dependence of the steady state fluorescence of coumarin 153 (C153), a well established solvation probe, was measured in the range of 163-323 K in a IL, 1,2-dimethyl3-propylimidazolium bis(trifluoromethylsulfonyl)imide, and the fluorescence maximum exhibited a significant blue-shift of ∼1600 cm-1 in the temperature range prior to the glass transition at 191 K, where a significant increase of viscosity is expected.10 The time dependence of the fluorescence peak frequency was also reported, and a large amount of solvation was still not over in the time window of 20 ns at 238 K. A similar effect is also expected for the CT fluorescence of BA in an IL, and thus, time-resolved fluorescence (TRF) measurements were carried out. 3.2. Time-Resolved Fluorescence Spectroscopy. To obtain direct information on the temperature dependence of solvation dynamics in ILs and its effects on the charge separation process of BA, we applied time-resolved fluorescence (TRF) detection. Large viscosity at low temperature implies incomplete solvent relaxation during the lifetime of the CT state of BA. The time profiles of the fluorescence of BA in ILs were measured at various wavelengths. Strong and rapid decay was observed at shorter wavelengths while a weak rise was observed at longer wavelengths. In BmimTFSI at 293 K, the fluorescence decayed
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TABLE 1: Fitting Parameters of the Vogel-Tammann-Fulcher Equation for Ionic Liquids ln(η0) D Tc
Bmim PF6
Bmim BF4
Bmim TFSI
Hmim TFSI
Emim TFSI
-2.23 ( 0.48 7.2 ( 1.5 153 ( 11
-1.01 ( 0.32 3.9 ( 0.7 173 ( 9
-4.50 ( 0.78 23 ( 11 79 ( 22
-4.96 ( 0.88 20 ( 9.0 92 ( 23
-2.26 ( 0.48 7.4 ( 2.3 130 ( 16
multiexponentially and a component with a time constant of 92 ps occupied nearly 80% of the decay at 420 nm in the time region of picoseconds to nanoseconds. For the production of the CT state that was monitored with the rise of the transient absorption at 690 nm with femtosecond time-resolution, multiple rises were observed at ambient temperature.28 A tentative fit to a triple-exponential function gave time constants of 0.45 (27%), 7.5 (37%), and 140 ps (36%). Because the time resolution of our TRF measurement is limited to ca. 30 ps, subpicosecond dynamics is not observable. Nevertheless, a large portion of the 92 ps decay could be that of the long tail of the decay of the LE state, because it is rather close to the average of 7.5 and 140 ps rise components observed with transient absorption measurement, i.e., ∼73 ps. At 333 K, 84% of the fluorescence at 420 nm decays with a time constant of 59 ps while 110 ps (42%) and 600 ps (35%) decays were observed at 253 K, indicating temperature dependent CT. The rise of the transient absorption spectrum of the CT state of BA in BmimTFSI at 253 K was also monitored at 690 nm (see Supporting Information Figure 1S). The rise occurred within a similar time scale as that of the fluorescence decay of the LE state obtained by the picosecond single-photon counting, in addition to the rapid appearance within a few picoseconds. These results indicate that the CT process is much faster than the dynamic Stokes shift, as will be shown in the following. From fluorescence time profiles at various wavelengths and the steady state emission spectrum, TRF spectra were reconstructed by the method described in the Experimental Section. The TRF spectra of BA in BmimTFSI at 293 K from 18 ps to 53 ns are shown in Figure 6. The analysis of each TRF spectrum into those of the CT and LE states was achieved by introducing a log-normal and a Gaussian function. The CT fluorescence was fitted by a log-normal function with all the parameters set free. The LE state fluorescence was fitted by a Gaussian
Figure 6. Time-resolved fluorescence (TRF) spectra of BA in BmimTFSI at 293 K measured at various time delays: 18, 34, 82, and 170 ps and 0.5, 1.1, 10, and 53 ns, respectively.
function with peak frequency and fwhm set at the values obtained from steady state measurement at each temperature. The only variable for the Gaussian function was its intensity. As can be seen from the spectra at 18 and 34 ps, the CT and LE fluorescence cannot be separated sufficiently before 50 ps. The two peaks become separable only for the time range longer than 80 ps. The initial process of the charge separation of BA in alcohols has been measured by the subpicosecond fluorescence upconversion technique.48 Similar doublet peaks were observed at 418 and 434-438 nm in the time range of
