CHEM. RES. CHINESE UNIVERSITIES 2011, 27(4), 688—692
Study of Imidazolium Ionic Liquids: Temperature-dependent Fluorescence and Molecular Dynamics Simulation FU Hai-ying1, ZHU Guang-lai2, WU Guo-zhong1*, SHA Mao-lin1 and DOU Qiang1 1. Laboratory of Radiation Chemistry, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, P. R. China; 2. Institute of Atomic and Molecular Physics, Anhui Normal University, Wuhu 241000, P. R. China Abstract The steady-state fluorescence spectra and molecular dynamics simulations were explored to investigate the temperature dependent organization in some imidazolium ionic liquids: 1-butyl-3-methylimidazolium hexafluorophosphate([bmim][PF6]), 1-ethyl-3-methylimidazolium ethylsulfate([emim][EtSO4]) and 1-butyl-3-methylimidazolium tetrafluoroborate([bmim][BF4]). The pure room temperature ionic liquids(ILs) exhibit a large red shift at more than an excitation wavelength of around 340 nm, which demonstrates the heterogeneous nature of the liquids. Furthermore, the fluorescence spectra of the ionic liquids were found to be temperature-dependent. The emission intensity gradually decreased with increasing temperature for the neat ionic liquids and the mixed solutions of [bmim][BF4]-H2O, which was the special phenomena induced by not only the local structure but also the viscosity. The molecular dynamics simulation further confirms that the structures of ionic liquids are sensitive to the surrounding environment because of the aggregation degree of ILs. Keywords Temperature dependence; Fluorescence; Ionic liquid; Molecular dynamics simulation Article ID 1005-9040(2011)-04-688-05
1
Introduction
Room temperature(RT) ionic liquids(ILs) are organic salts that become molten at or near room temperature. These unique liquids are considered as “green solvents” for chemical reactions due to their desirable properties such as negligible vapor pressures, non-volatility, non-flammability, high thermal stability, and so on[1―7]. Imidazolium ionic liquids have been used as a medium for representative photochemical reactions in comparison with volatile organic solvents[3―7]. Steady-state and time-resolved fluorescence spectroscopy[8―13] was also performed to study the reaction processes in ILs. Researchers of the fluorescence behaviors of ILs have previously focused their attention on pure ILs and their molecular dipolarity. Their results clearly suggest that imidazolium ionic liquids, such as [bmim][BF4] and [bmim][PF6], possess very similar characteristics. Samanta and co-workers[6―13] found that the general emission characteristics of other imidazolium ionic liquids were quite similar. The target ionic liquids all exhibit excitation wavelength dependence behavior: for the short excitation wavelengths, an emission band around 340―360 nm is observed. However, the most interesting feature of the emission is the shift of the emission maximum toward longer wavelength with an increase in the excitation wavelength. The magnitude of the shift of the emission peak was observed to be significantly large. Samantha and co-workers[6―13] claimed that the excitation wavelength dependence of the long-wavelength emission band can be accounted for by the existence of energetically
different associated species. Very recently, we have also found that fluorescence intensity changes with an increase in the concentration of ionic liquid [bmim][PF6] in MeCN solution[14]. At the same time, many studies have described temperature effects on the physical properties of the ILs, such as density, viscosity, diffusion coefficient, and so on[15―17]. In particular, Whishart[18] used coumarin 153(C153) as a time-resolved fluorescence probe to study the temperature dependence of solvation dynamics and local orientational friction for a series of ionic liquids. The very long time scale orientational relaxation dynamics were observed in the C153 fluorescence anisotropy due to the long-lived local structures in the environment surrounding the C153 probe. The concept of the heterogeneity or the nanostructure of ILs has been also generally accepted based on molecular dynamics(MD) simulations. For [bmim][PF6], the three-dimensional ion network is formed by the C―H···F hydrogen-bond pertaining to the interaction of H atoms from the imidazolium cations with the anions[19]. Voth[20,21] also showed the unique heterogeneity of pure ionic liquids and the nanostructural organization in ionic liquid/water mixtures. Noticeably, the degree of aggregation of tail groups decreased with an increase in the side-chain length of the cation in IL. Lopes and Padua[22] confirmed that ILs with longer alkyl chains might be separated into two types of regions by MD simulation: one is the nonpolar region, which arises from the aggregation of the alkyl chain, and the other is the polar region, which arises from charge ordering of the anions and imidazolium rings of the cations. The polar regions are interconnected to
——————————— *Corresponding author. E-mail:
[email protected] Received August 13, 2010; accepted October 18, 2010 Supported by the National Natural Science Foundation of China(No.20973192, 11079007).
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form a three-dimensional ionic network that is permeated by nonpolar regions. To clearly understand the local structure of the imidazolium ionic liquids, the further exploration of their photophysical and photochemical properties is necessary. It was assumed that there is an inherent connection between the abnormal fluorescence behavior and the nanostructural organization in ILs. If this assumption holds true, the fluorescence at a short wavelength should be different from that at a longer wavelength with increasing temperature. For this purpose, a comparative temperature dependence investigation from room temperature to near 373 K on [bmim][PF6], [bmim][BF4] and [emim][EtSO4] have been made by means of steady-state fluorescence technique and molecular dynamics simulation in this work.
2 2.1
Experimental Materials and Equipment
Ionic liquids([bmim][PF6], [emim][EtSO4] and [bmim]· [BF4]) were purchased from Chenjie Co., China and specially treated according to the literature[7,17]. After purification, the water content in the ionic liquids was less than 150 mg/kg, as measured by Karl-Fischer titration, and no impurities were detected by NMR spectrometry analysis. The steady-state fluorescence spectra of [bmim][PF6], [bmim][BF4] and [emim][EtSO4] were performed on a Hitachi F-4500 fluorescence spectrophotometer in a quartz cuvette with an optical path length of 10 mm. The quartz cuvette was immediately sealed to avoid any contamination of moisture. Slits were set to provide widths of 5 nm for both the excitation and the emission monochromators in all the cases. The voltage applied to the photomultiplier tube(PMT) was 700 V. For all the variable-temperature experiments, a thermostatic cuvette holder connected to a constant-temperature water circulator was adapted to the spectrometer. For all the measurements the samples were equilibrated at each temperature for at least 15 min. The temperature was kept constant within a fluctuation of 1 K. The emission intensity ratios were determined by comparing fluorescence intensities at wavelengths ca. 290 and 350 nm. Viscosity measurement at different temperatures was carried out in steady shear between 50 mm diameter parallel plates on a Rheometric Scientific, Inc., ARES rheometer. Tests were performed at shear rates between 0.1 and 100 s–1, and none of the fluids tested showed any signs of non-Newtonian behavior. Small gaps of 0.5 mm or less were used to minimize the sample size and the rate of rotation needed to achieve higher shear rates. The relative standard uncertainty of the mean viscosity was less than 5%.
2.2
Simulation Details
In this work, [bmim][PF6] was chosen as the delegate of imidazolium-based ionic liquids due to the same cation structure and as the similar anion structure with hydrogen bond to those of the other two ionic liquids. The force field was used as a modified systematic all-atom force field developed by Lopes[23,24], which was based on the optimized potentials for
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liquid simulation(OPLS-AA) as suggested by Jorgensen et al.[25]. Parameters for PF6 were obtained from the work of Borodin[26]. The parameters of various atoms were used as in the reference [27]. In this potential model, reduced cation and anion charges were used with the total charges set at 0.8 e and –0.8 e on the cation and anion, respectively. The MD simulations were performed via Gromacs 3.2[28]. Prior work has validated that the size effects of the system do not affect the general trend of the radial distribution function(RDF) in a qualitative study. Therefore, the simulation systems containing 128 ion pairs of [bmim][PF6] were adopted in this work. The simulations were carried out at five different temperatures: 298, 323, 343, 363 and 383 K. These temperatures correspond to some of the temperatures used in the fluorescence experiment. In all the simulations, bond lengths were constrained with LINear Constraint Solver(LINCS)[29]. A cutoff was taken at 1 nm for Lennard-Jones interactions and the long-range coulomb interactions were handled by particle mesh ewald(PME) with a cutoff of 1.2 nm. All the configurations were equilibrated in an isothermal-isobaric simulation for 2.0 ns[24,30]. The Berendsen temperature coupling was used for the given temperature control. Finally, cations and anions were separated into two heat baths with a temperature coupling constant of 0.1 ps.
3
Results
3.1 Fluorescence Behaviors at Room Temperature The wavelength-dependent emission behavior of pure [emim][EtSO4], illustrated in Fig.1(A), shows that spectral behavior strongly depends on the excitation wavelength. For short excitation wavelengths below 320 nm, the emission band was observed around 360 nm. This band does not shift with the variation in excitation wavelength. However, when excited at longer wavelengths, which correspond to the long tail of the absorption band of the imidazolium ionic liquids(UV-spectra), a second emission band becomes prominent. The maximum of this second band, which appears as a shoulder for short excitation wavelengths, starts shifting towards red when the excitation wavelength is increased. As seen in Fig.1, the magnitude of the shift of the peak is unusually large(>100 nm) as Fig.1(A) inset. The fluorescence behavior of [bmim][BF4] and [bmim][PF6] shows very similar phenomena to that of [emim][EtSO4][Fig.1(B) and (C)]. This observation is consistent with the fluorescence behavior of imidazolium-based ionic liquids in which the red edge effect(REE) starts at short excitation wavelengths. According to the literature[7,9], the excitation wavelength dependence of the long wavelength emission band can be attributed to the existence of energetically different associated species. As the excitation wavelength is changed, a slightly different associated species is excited and an emission characteristic of this particular species is observed. The energy transfer between these energetically different species is inefficient because of the short fluorescence lifetime and the low concentration of these species. These features, along with high viscosity
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Fig.1
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Excitation-wavelength-dependent emission behavior of neat ionic liquids at 298 K
(A) [emim][EtSO4]; (B) [bmim][BF4]; (C) [bmim][PF6]. The excitation wavelengths for the spectra, marked from a to l, are 280, 290, 300, 310, 320, 330, 340, 350, 360, 380, 400 and 420 nm, respectively. Inset: the excitation wavelength to maximum emission wavelength.
and short fluorescence lifetime(which makes the relaxation of the photo-excited species inefficient), contribute to REE-like behavior in neat ionic liquids. Samanta and co-workers[6―13] also observed an excitation wavelength-dependent shift of the emission maximum of [bmim][PF6] at room temperature. In sum, in neat ionic liquids, the various associated forms of imidazolium cations exist and the short wavelength emission is due to the monomeric form of the imidazolium cation, while the long wavelength emission is due to its associated forms. In line with their conclusion for the neat [bmim][PF6], the fluorescence emission by excitation with a long wavelength(350 nm, for example) was assigned to the aggregates of imidazolium cations and the fluorescence emission by excitation with a short wavelength(for instance, 290 nm) was assigned to the monomer of cations.
3.2
Temperature Effect on Fluorescence Behavior Fig.2(A) and (B) show the intensity changes of the
Fig.2
fluorescence spectra for neat [bmim][BF4] as a function of temperature(from 298 K to 363 K). When [bmim][BF4] is excited at 290 or 350 nm, the emission peaks appear at longer wavelength and the fluorescence intensity always decreases regularly with increasing temperature. In all the cases, the emission peaks did not shift at the expected exciting wavelength, which indicates that there are different excited fluorescence species. When excited at each given wavelength, the fluorescence peaks did not shift with temperature. However, the temperature sensitivity at the two excitation wavelengths was different[Fig.2(C)]. Comparing both the fit lines, one can see the slopes were 11.3 at 290 nm, and 2.7 at 350 nm, respectively. The former shows higher sensitivity to temperature variation, which suggests that the trend of fluorescence behavior at shorter wavelengths is different from that at longer wavelengths and different species exist in ionic liquids. These results effectively support the conclusion that the mixed ILs-H2O solutions are inhomogeneous.
Emission spectra of neat ionic liquid [bmim][BF4]
(A) Excitation wavelength: 290 nm; (B) excitation wavelength: 350 nm. The measured temperature ranged from 298 K to 363 K; (C) dependence of the maximum of fluorescence intensity on temperature. Ì λexc=290 nm; { λexc=350 nm.
Like that of the neat ionic liquid, the emission intensity increases with an increase in the volume fraction of [bmim][BF4] mixed solution in the experimental temperature range. On the other hand, the density and viscosity of the bulk imidazolium-based ionic liquids decrease with temperature increasing, but the self-diffusion coefficient of the ions increases. That is, molecular migration becomes faster. Therefore, the probability of molecular collisions increases, which results in the increase of self-quenching and the decrease of emission intensity.
3.3
Molecular Dynamics Simulation Molecular dynamics simulation is a powerful method to
investigate the local structure of ionic liquids. The changes of the fluorescence of [bmim][PF6] observed by experiment should be related to the changes in their local structure. Hence, we explored the temperature effects on the microstructure of IL by MD simulation. Fig.3 shows the schematic representation of [bmim][PF6] with the atom labelled. The hydrogen atoms for the methyl and
Fig.3 Schematic representation of [bmim][PF6] The hydrogen atoms for the methyl and the butyl tail are not shown here.
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the butyl tail are not shown here. According to the literature[22,23], the cation of [bmim][PF6] is composed of a “head”, which includes cations, the imidazolium ring and adjacent atoms. The “tail”(the butyl) can be regarded as two domains: the “polar regions” or “charged regions” formed by the head of the cation and anion and the “nonpolar regions” formed by the tail of the cation. We ob-
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tained the snapshot of these two regions: the “charged region” as red and yellow and the nonpolar domain as green. Examples of the snapshots of 128 ion pairs containing “heads” and “tails” are shown in Fig.4. We investigated the change of the local structure of both the charged region and the nonpolar region to understand the fluorescence trend difference at different excitation wavelengths.
Fig.4 Schematic representation of snapshots of 128 ion pairs containing “heads” and “tails” The imidazolium ring and the adjacent atoms(red), anions(yellow) and tails(green) extracted from [bmim][PF6].
The changes of the local structure of the nonpolar regions can be attributed to the radial distribution function(RDF) of the terminal carbon(C4) of the alkyl side-chain, which indicates tail aggregation. As shown in Fig.5(A), the first peak of RDF of C4 decreases with temperature, which suggests that the degree of tail aggregation decreases. This trend was also found to be
similar to that of temperature effect on the charged region. The first peak in Fig.5(B) is assigned to the C―H···F hydrogen bond. With an increase in temperature, this peak dropped gradually which indicates that the degree of head aggregation also decreases.
Fig.5 Site-site intermolecular radial distribution function of the terminal carbon of the alkyl side-chain in [bmim][PF6](A) and site-site intermolecular radial distribution functions of fluorine atoms of PF6 around HA(B)
4
Discussion
As described previously in the literature, Samanta and coworkers[6―13] speculated that excitation wavelength dependent fluorescence behavior of RTILs might be due to the presence of various associated species that are energetically different. As the excitation wavelength is changed, a slightly different associated species is excited and an emission characteristic of this species is observed. Other experimental work and theoretical simulation studies attempted to obtain an insight into the structures of the imidazolium salts in their liquid states[15,20―23]. This work speculates that RTILs are not homogeneous, but are nanostructurally constructed with nonpolar regions and ionic networks. The former arises from the clustering of alkyl chains, while the latter arises from the charge ordering of anions and the imidazolium rings of the cations. Among the experimental studies, the results of the neutron scattering, NMR, X-ray scattering, and Raman spectroscopic
measurements are particularly important[31,32]. Pure RTILs contain a great deal of aggregation of the cation-anion and cation-cation pairs. These conclusions may be additionally supported by our experimental results. The fluorescence intensity of neat [emim][EtSO4] shows the similar excitation response at room temperature to those of [bmim][BF4] and [bmim][PF6]. With changes in the IL volume ratio of the mixtures, the emission peak does not shift and the fluorescence intensity gradually increases. To further interpret our fluorescence results, we measured the viscosities of the three ionic liquids(Table 1) and the mixed solutions(Table 2) at various temperatures. The viscosity of the Table 1
Viscosities of the three ionic liquids
ILs
283 K
298 K
323 K
333 K
[bmim][BF4] [bmim][PF6]
124 558
74.4 320
46.5 178
32.7 98.3
25.2 66.5
14.5 44.7
[Emim][EtSO4]
109
67.2
42.3
29.3
21.8
17.4
η/(mPa·s) 303 K 313 K
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Table 2
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Viscosities of the mixtures of [bmim][BF4]-H2O η/(mPa·s) 303 K 313 K
IL volume fraction(%)
283 K
298 K
0.2 0.5
2.72 4.53
1.77 3.09
1.53 2.65
0.8
12.5
7.94
7.20
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333 K
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5
Conclusions
In the present work, the temperature dependence of the nanostructure in imidazolium ionic liquids was studied by fluorescence and molecular dynamics simulation. This temperature dependence is consistent with the change in the local structure under different environmental conditions. This observation is attributed to the presence of energetically different associated forms of the constituent ions of the ionic liquids. The temperature dependences of the fluorescence spectra of ionic liquid may be explained in terms of the changes of the local structure, which is sensitive to the surrounding environment.
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