Fluorescence Behavior and Specific Interactions of an Ionic Liquid in

J. Phys. Chem. B 2008, 112, 4079-4086

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Fluorescence Behavior and Specific Interactions of an Ionic Liquid in Ethylene Glycol Derivatives Tejwant Singh and Arvind Kumar* Central Salt and Marine Chemicals Research Institute, BhaVnagar-364002, India ReceiVed: December 13, 2007; In Final Form: January 17, 2008

The excitation-wavelength- and concentration-dependent fluorescence response of an ionic liquid (IL), 1-octyl3-methylimidazolium tetrafluoroborate [Omim][BF4], in the ethylene glycol derivatives, [CH3(OCH2CH2)nOH, n ) 1-3], has been critically examined in the entire composition range. The pure IL exhibited a large red shift beyond an excitation wavelength of 390 nm, showing the heterogeneous nature of the liquid. Concentration dependence of the fluorescence spectra in the organic solvent-rich region favors the association of IL molecules into the aggregated structures. The maximum of the fluorescence spectra shifted toward blue with the increase of organic component concentration in all of the mixtures, reflecting an appreciable solutesolvent interaction. Very high concentrations of the organic liquid in the mixtures resulted in the inversion of the spectral shift toward red, indicating the dominance of π-π* transitions over the n-π* transitions as a consequence of imidazolium ring stacking. 1H NMR and FT-IR investigations over the whole composition range of the mixtures showed multiple hydrogen-bonded interactions of varying strengths between the unlike molecules and the existence of associated species of the IL in the dilute region. Both the specific interactions between unlike molecules and the tendency of aggregation of IL molecules in the dilute region reduced with the introduction of the -OC2H4 group in the glycol derivative. A comparison of specific interactions with the volumetric properties of the similar mixtures shows that the packing efficiency depending on differences in the shape and size of the molecules mainly decides the overall magnitude of deviations from ideality.

1. Introduction Room-temperature ionic liquids (ILs), due to their unique chemical and physical properties, are attracting a great attention from the scientific community.1-5 They are finding increasing uses in many areas of science and technology as a greener alternative to the conventional volatile organic solvents.6-15 Therefore, knowledge of the fundamental properties and molecular level details of these novel compounds are quite essential for utilizing and improving the performance of these materials in various chemical processes. In recent years, properties of imidazolium-based ionic liquids have been widely studied as representative liquids due to their unique spatial heterogeneity which results from the inherent polar/nonpolar segregation. Recent experimental and computational studies reveal the existence of nanostructural organization in pure 1-alkyl-3-methylimidazolium-based ILs with various alkyl chain lengths.16-20 These distinct features of the imidazolium-based ILs lead to a complex solvation behavior when they are mixed in water or organic solvents. Given the fact that the range of application and versatility of these materials can be enlarged by dissolving the ILs in water or organic liquids, it is of immense importance to understand molecular-level interactions in their binary mixtures. The solvation and mixing behavior of the ILs in water are extensively studied by thermodynamic, spectroscopic, and computational techniques.21-34 Studies on the binary mixtures of ILs with common organic solvents over the whole composition are also gaining momentum.35-56 However, the studies on * To whom correspondence should be addressed. Tel.: +91-2782567039.Fax: +91-278-2567562/2566970.E-mail: [email protected]; [email protected].

IL-organic solvent binary mixtures are not very systematic and mainly restricted to the measurement of excess properties or vapor-liquid equilibrium, which fail to provide the exact information about the nature of interactions at molecular level. When the entire composition range of a mixture between an ionic liquid and a molecular liquid is spanned, several different regimes are formed.57 These different regimes have consequences for the properties of the mixtures, and a molecularlevel understanding of these microscopic phases is very important for the use of ILs in different applications. Extending our earlier thermodynamic investigations,55,56 in the present work, we explored the nature of interactions in the binary mixtures of a representative ionic liquid, [Omim][BF4], in the ethylene glycol derivatives, [CH3(OCH2CH2)n-OH, n ) 1-3], using the fluorescence, 1H NMR, and FT-IR techniques (see Scheme 1). Fluorescence spectroscopy has proved to be an extremely useful technique in elucidating the structure and dynamics of the organic solutes and their solvation behavior in liquids. Samanta and co-workers,58-60 in their recent studies, have shown that the imidazolium-based ILs exhibit unusual fluorescence behavior, which covers a significant portion of the visible region. The interesting fluorescence behavior of ILs has been attributed to the presence of an imidazolium moiety and the existence of energetically different specific local structures. The effect of conventional solvents on the fluorescence properties of the ILs and the fluorescence of probe molecules in the ILs also shows the ion association or nano-heterogeneity in these liquids.61-64 It is therefore interesting to see how these local structures are effected with the systematic addition of a molecular liquid. Similar to other imidazolium-based ILs, [Omim][BF4] also

10.1021/jp711711z CCC: $40.75 © 2008 American Chemical Society Published on Web 03/07/2008

4080 J. Phys. Chem. B, Vol. 112, No. 13, 2008 SCHEME 1: (a) Constituent Ions of the Ionic Liquid 1-Octyl-3-methylimidazolium Tetrafluoroborate; (b) Structure of the Organic Liquids

showed fairly strong excitation-wavelength-dependent fluorescence spectra in neat solvent. Spectacular enhancement of the emission intensity of the spectra in an IL-dilute region of the binary mixtures of glycol derivatives indicated the formation of micelle-like organized assemblies of the IL molecules and positioning of the imidazolium cation that favored the ring stacking through π-π interaction. 1H NMR and FT-IR measurements provided additional evidence for the existence of aggregate-like structures of the IL in the dilute region and comparatively stronger cation-anion interactions in the IL-rich region of the mixture. In the recent past, 1H NMR, which is a direct molecular approach, has been also used for determining the solvation of imidazolium-based ionic liquids in conventional organic solvents. It is shown that ILs are capable of forming hydrogen bonds to basic solutes.65,66 The relative strengths of intermolecular interactions that different aromatic protons, side alkyl chain protons, and anions have with some of the commonly used organic solvents have been determined.67 However, systematic studies on the determination of the specific interactions and their relative strengths in the IL-organic solvent binary mixtures over the whole composition range are still lacking. In the present work, the excess chemical shifts provided important information about the relative hydrogen bond strengths of various protons of an IL with the glycol derivatives in the binary mixtures. Contrary to the volumetric results, the magnitude of the specific interaction between the IL and the glycol derivative decreased with the introduction of an -OC2H4 group in the organic component. In short, the aim of the present paper is to offer a systematic study to get finer details on solution structures and specific interactions of a representative imidazolium-based IL, [Omim][BF4], in the glycol derivatives. The investigations were carried out using spectroscopic techniques, focusing on solute-solvent interactions depending on the change in concentration and structure of the organic liquid.

Singh and Kumar 2. Experimental Section 2.1. Materials. Ionic liquid, 1-methyl-3-octylimidazolium tetrafluoroborate (>99.0 mol %), was purchased from Solvent Innovation, Germany. Prior to use, the IL was washed and decolorized in a column comprised of celite, silica, and charcoal using HPLC-grade dichloromethane as the solvent in a similar fashion as that described in the literature.68 After decolorization, the ionic liquid was dried and degassed under vacuum at 60 °C for 48 h. The process of decolorization and drying was repeated until concordant absorption spectra with no further reduction in the absorption were recorded. Karl Fischer analyses of the vacuum-dried purified samples indicated the water content to be less than 200 ppm. NMR and IR spectra did not change before and after decolorization of the IL. Analytical-grade ethylene glycol monomethyl ether (>99.5 mol %), diethylene glycol monomethyl ether (>98 mol %), and triethylene glycol monomethyl ether (>98 mol %) were obtained from MerckSchuchardt. All of the organic liquids were used after drying over the 0.4 nm molecular sieves and under vacuum at ambient conditions. Binary mixtures were prepared by mass using an analytical balance with a precision of (0.0001 g (Denver Instrument APX-200). The mole fraction of each mixture was obtained with an accuracy of 1 × 10-4 from the measured masses of the components. All samples were kept in tightly sealed, Teflon-covered bottles to minimize the absorption of atmospheric moisture and CO2. 2.2. Methods. 2.2.1. Absorption and Fluorescence Measurements. Absorption spectra of the neat ionic liquid were recorded on a UV-vis spectrophotometer (Cary 500, Varian). Steadystate fluorescence measurements at different concentrations of the binary mixtures and that of the pure ionic liquid were performed using a Fluorolog (Horiba Jobin Yvon) spectrometer. The fluorescence spectra were corrected for the instrumental response. Fluorescence spectra of the pure ionic liquid were recorded in the excitation wavelength range between 290 and 490 nm, and those of binary mixtures were recorded at an excitation wavelength of 290 nm. The emission and excitation slit width was kept at 2.5 nm for all of the measurements. Samples for recording the spectra were taken in a quartz cuvette which was immediately sealed after sampling to avoid contamination of moisture. 2.2.2. 1H NMR Measurements. The 1H NMR spectra of the IL-organic liquid mixtures in the dilute region and over the whole composition range were recorded using a Bru¨ker 500 and 200 MHz spectrophotometer, respectively, at 298 K. The proton chemical shifts were referenced with respect to external standard TMS (δ ) 0.000 ppm) in C6D6 (deuterated benzene). The chemical shifts of the peaks of interest were determined using the peak pick facility. 2.2.3. FT-IR Measurements. The Fourier transform infrared (FT-IR) spectra of the pure components and that of various binary mixtures over the complete composition range at 298 K were recorded using Spectrum GX Series 49387. 3. Results and Discussion 3.1. Absorption. The absorption characteristics of the vacuum-dried ionic liquid before and after repeated purification, as measured using a 1 cm path length cuvette, are shown in Figure 1. Absorbance at a wavelength of 350 nm decreased from ∼0.4 to ∼0.2 after following the purification procedure, and no further reduction in the absorbance by the ionic liquid was seen after the fourth purification run. The absorbance by the purified sample was ∼0.06 even at 400 nm, suggesting that more than 12% of the incident light was absorbed by the IL at this

An Ionic Liquid in Ethylene Glycol Derivatives

Figure 1. Comparison of absorption spectra of neat [Omim][BF4] as received and after repeated purification.

Figure 2. Excitation-wavelength-dependent [λexc (nm) ) 280-490 (am)] emission behavior of neat [Omim][BF4].

wavelength. The absorption tail of the spectra like other imidazolium-based ionic liquids58,59 extends beyond 400 nm even after repeated purification and decolorization of the sample. The long tail of the absorption spectra might be due to the presence of various associated species that are energetically different. The fact that the long tail of the absorption spectra is inherently due to the imidazolium moiety of the ionic liquid and not due to the impurities, which was unambiguously established from the controlled experiments of Samanta and coworkers,59 is reconfirmed by our optical measurements. 3.2. Fluorescence Behavior. The wavelength-dependent emission behavior of the pure [Omim][BF4] is shown in Figure 2. Fluorescence is observable even when the excitation wavelength is greater than 450 nm, where the absorption due to the IL is negligible. The fluorescence behavior of [Omim][BF4] seems very similar to that of short-chain imidazolium-based ILs, that is, [Emim][BF4] and [Bmim][BF4].58 The emission maximum does not shift upon the variation of the excitation wavelength from 290 to 390 nm. However, when excited at a wavelength of 390 nm and above, the emission maximum shifts toward red, with a progressive decrease of the emission intensity.

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Figure 3. Emission spectra at an excitation wavelength of 290 nm as a function of the mole fraction of the ionic liquid in the mixtures {[Omim][BF4] + CH3(OCH2CH2)-OH}.

Figure 4. Emission spectra at an excitation wavelength of 290 nm as a function of the mole fraction of the ionic liquid in the mixtures {[Omim][BF4] + CH3(OCH2CH2)2-OH}.

One noticeable feature in the fluorescence behavior of imidazolium-based ionic liquids is that the red edge effect (REE) starts at short excitation wavelengths in the case of the IL with a shorter side alkyl chain length, whereas in the IL with a longer alkyl chain, the red edge effect is observed at a comparatively longer excitation wavelength ([Emim][BF4] ≈ 280 < [Bmim][BF4] ≈ 310 < [Omim][BF4] ≈ 390). This behavior may be due to the higher viscosity of the longer alkyl chain IL or the more associated structure with enhanced polar/nonpolar segregation in [Omim][BF4]. Again, the rise of a long wavelength emission band at higher excitation wavelengths, which is comprised of a long absorption tail, can be attributed to the ground-state heterogeneity of the IL and slow excited-state relaxation of the photoexcited species.69 The fluorescence behavior of the [Omim][BF4]-[CH3(OCH2CH2)n-OH, n ) 1-3] mixtures over the complete composition range at an excitation wavelength of 290 nm is shown in Figures 3-5. A shoulder peak appeared in the spectra toward the shorter wavelength at a certain concentration of glycol derivatives in

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Figure 5. Emission spectra at an excitation wavelength of 290 nm as a function of the mole fraction of the ionic liquid in the mixtures {[Omim][BF4] + CH3(OCH2CH2)3-OH}.

the IL, and a further increase of the concentration of the organic liquid in the mixture resulted in a clear two-component spectrum. With the increase of the organic component concentration, the band at the shorter wavelength became more prominent, and the intensity of the longer wavelength band decreased gradually. In the course of dilution, formation of energetically different associated structures of the IL or existence of various solution regimes as a consequence of solute-solvent interactions may be responsible for such fluorescence behavior. Further, a large blue shift in the spectra was observed as a result of dilution of the IL in the mixtures. The blue shift in the spectra may be due to the decrease of electron density of the oxygens of the polyethylene glycol derivatives upon excitation via n-π* transitions. Specific interactions such as hydrogen-bonding interactions can also lead to dramatic changes in the fluorescence spectra, and the higher the strength of the hydrogen bonding, the larger the shift. Appreciable blue shifts indicate the significant intermolecular hydrogen bonding between [Omim][BF4] and ethylene glycol derivatives in the mixtures. We believe that the blue shift observed with the increase of organic liquid concentration in the mixtures may also be due to a decrease in the viscosity of the medium, which makes the relaxation of the photoexcited species more efficient. At very high concentrations of the glycol derivatives in the mixtures, that is, in the IL-dilute region, a red shift was observed with further increase of the glycol derivative concentration in the mixture, indicating a polarity-induced inversion of the n-π* to a π-π* transition as a consequence of aggregation of IL molecules through imidazolium ring stacking. Emission intensity versus concentration curves shown in the inset of Figures 3-5 also indicate the aggregation of IL molecules in the dilute region. Addition of the IL to the glycol derivatives increases the emission intensity of the fluorescence spectra; it attains a maximum value and then decreases with the further increase of the IL concentration in the solution. This manifold increase in the emission intensity in the dilute region as compared to that of the neat IL can be due to the dominance of the π-π* transition state, which is known to be more efficient in radiative emission than that from the n-π* state. For mole fractions xIL > 0.2, the emission intensity was more or less constant in all of the mixtures, indicating that the unlike components of the

Singh and Kumar

Figure 6. Variation of δobsd for the protons of the terminal methyl group of the imidazolium cation at C-14 against the reciprocal concentration of the IL in various binary mixtures in the IL-dilute region: (0) CH3(OCH2CH2)-OH; (O) CH3(OCH2CH2)2-OH; and (4) CH3(OCH2CH2)3-OH.

mixtures, though hydrogen bonded through intermolecular interactions, remain highly structured as in their pure states. 3.3. 1H NMR Studies. 3.3.1. Dilute Solution BehaVior. The non-obvious fluorescence behavior of the [Omim][BF4][CH3(OCH2CH2)n-OH, n ) 1-3] mixtures around a very low concentration of the IL in the solutions prompted us to pay special attention to the IL-dilute region of the mixture. NMR, which is a very sensitive technique for the study of the association of compounds having amphiphilic nature, was used for elucidating the aggregation behavior of the IL in the dilute region. The sudden change in the observed chemical shift for various protons of the IL as a function of concentration (∂δobsd/ ∂C) in different glycol derivatives corresponds to the association of IL molecules into the aggregated structures. Figure 6 shows observed chemical shifts for the terminal protons at C-14 as a function of the reciprocal concentration of the IL in the mixtures. Protons of the alkyl chain of the IL showed an upfield shift upon aggregation. Aromatic-ring-induced shielding of the methylene groups, the solvent effect, or structural changes may be the reason for the observed upfield shift. Since an upfield shift can be related to an increasing importance of the gauche conformation, it can be inferred from the concentration dependence of the chemical shift of various protons of the IL that the aggregation formation is accompanied by a partial changeover from the trans to gauche conformations in the alkyl chain of the IL. A small change in the chemical shift toward the upfield is also observed for the aromatic ring protons of the IL with the increase of concentration, which, upon aggregation, changes gradually in all of the glycol derivatives (see Supporting Information). There is a competition for the interaction of aromatic protons for the solvent and the anions. [BF4]- interacts strongly with the solvent molecules; thereby, its interaction with the aromatic protons is weakened, which causes an upfield shift for the aromatic protons in the mixtures. An upfield shift for the ring protons upon aggregation may also be due to ring stacking through π-π interactions, which become more significant as compared to the intermolecular interactions of the IL and self-associated glycol derivatives. A remarkable increase of the fluorescence emission intensity also favors the ring stacking in the IL-dilute region of the mixture. Also, the reduced

An Ionic Liquid in Ethylene Glycol Derivatives sharpness of the curve (Figure 6) for higher analogues of the glycol derivatives shows that the tendency of IL molecules to associate into the aggregated structures decreases with the introduction of an -OC2H4 group in the organic component of the mixture. 3.3.2. Solution BehaVior oVer the Complete Composition Range. The 1-alkyl-3-methylimidazolium cation forms multiple hydrogen bonds from the various protons to even weak hydrogen-bond-acceptor anions. A competition for the interaction of various protons of the imidazolium salt for the anions and solvent molecules is expected when the IL is mixed in an organic liquid. Addition of strong hydrogen-bond donors such as glycol derivatives can bring about significant changes in the magnitude of hydrogen-bonding interactions through formation of new hydrogen bonds or breaking/weakening of the existing hydrogen bonds in the pure IL. In order to examine the specific hydrogen-bonding-type interactions as a function of composition and structure of the organic liquid, we measured the 1H NMR chemical shift, δ, in the mixtures of [Omim][BF4] with ethylene glycol derivatives over the entire composition range at 298 K. When the mixtures were created, appreciable changes were noticed in the δ values of the hydroxyl protons of glycol derivatives as a consequence of hydrogen bonding with the anionic component of the IL and in the δ values of different protons of the IL, that is, aromatic ring protons or side alkyl chain protons due to the changes in the strength of hydrogen bonding with the anion or specific interactions with the oxygens of the glycol derivatives. A progressive downfield shift is observed for all of the protons of the IL with the addition of organic liquid, whereas an upfield chemical shift is observed for the hydroxyl proton of the organic liquid with the increase of the IL concentration in the mixture. The change in chemical shift for individual protons of the IL and organic liquid in the mixture as a function of composition was used to estimate the deviations from ideality in the 1H NMR chemical shift, ∆δ, from the additivity rule, that is

∆δ ) δ - x1δ∞1 - x2δ∞2 where δ is the chemical shift of different protons of the IL or organic solvent in the mixture and δ∞1 and δ∞2 are the chemical shifts of the protons of the IL and glycol derivative, respectively, at the lowest and highest mole fraction (values were obtained by extrapolating the curves from 0 to 1). Figure 7 shows a comparison of the deviations from ideality in the chemical shift ∆δ of various protons of [Omim][BF4] in the CH3(OCH2CH2)-OH. The negative deviation from ideality shows a comparatively stronger solute-solvent interaction than that of the cation-anion interaction over the whole composition range. The overall magnitude of the deviations from ideality in the chemical shift ∆δ for various protons is in the following order: the ring proton at C-2 > the ring proton at C-4,5 > the terminal methyl protons at C-14. Similar behavior was observed for the mixtures of [Omim][BF4] with CH3(OCH2CH2)2-OH and CH3(OCH2CH2)3-OH (see Supporting Information). This is because the proton at C-2 is bound to a carbon that is situated between two electronegative nitrogen atoms, and hence, it is most sensitive to the interactions with the anion and solvent molecules. Specific interactions of different protons of the IL, that is, at C-2, C-4,5 and the terminal methyl at C-14, are higher with CH3(OCH2CH2)-OH as compared to those of its higher analogues. For example, a comparison of the deviations from ideality in the chemical shift ∆δ for the ring proton at C-2 of the IL in different ethylene glycol derivatives is shown in Figure

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Figure 7. Comparison of the deviation in chemical shift ∆δ for various protons of the IL in CH3(OCH2CH2)-OH: (0) terminal methyl protons at C-14; (O) ring protons at C-4,5; and (4) ring proton at C-2.

8. In general, the magnitudes of the specific hydrogen-bonded interactions in all of the mixtures decrease with the introduction of an -OC2H4 group in the organic component. The nature and extent of the specific interactions in these mixtures will depend on many factors such as solvation ability and conformation of the IL, geometrical packing of the unlike molecules, and polarity and the stability of self-associated structures of the ethylene glycol derivatives. Due to the presence of -O- and -OH groups in the same molecule, dipole interactions become equally important in the mixtures of ethylene glycol derivatives. The effective dipole moment of the ethylene glycol derivatives decreases with the addition of an -OC2H4 group in the molecule. Therefore, a comparatively higher polarity and, maybe, the conformational compatibility of CH3(OCH2CH2)OH in the mixture lead to the higher magnitude of specific interactions between various protons of the IL and the organic molecule. A comparison of the specific interactions of the -OH group of different ethylene glycol derivatives with the IL molecules also shows a similar trend, wherein the deviations from ideality in the chemical shifts ∆δ are higher in the mixtures with CH3(OCH2CH2)-OH than those of the mixtures with the higher analogues (see Supporting Information). The -OH group of the ethylene glycol derivatives will mainly interact with the [BF4] anion of the IL through intermolecular hydrogen bonding -O-H‚‚‚[BF4], and the extent of intermolecular hydrogen bonding between the -OH group of the ethylene glycol derivatives and the anion of the IL will depend upon the breaking of self-associated structures of the ethylene glycol derivatives, which are produced via both the inter- and intramolecular H-bonds. Intramolecular hydrogen bonding increases with the number of oxygen atoms in the polyethylene glycol derivative and reduces the number of possible intermolecular H-bonds. Therefore, the magnitude of intermolecular hydrogen bonding between the -OH group of the ethylene glycol derivatives and the anion of the IL is more for the CH3(OCH2CH2)-OH as compared to that for the higher analogues. Further, a comparative analysis of the strengths of hydrogen bonding between the hydroxyl group of the glycol derivatives to the IL (mainly with the anion) and oxygens of the glycol derivatives to the different protons of the IL shows that the former interactions are weaker. This may be due to the fact that the

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Singh and Kumar

Figure 8. Comparison of the deviation in chemical shift ∆δ for the ring proton at C-2 of the IL in different glycol derivatives: (0) CH3(OCH2CH2)-OH; (O) CH3(OCH2CH2)2-OH; and (4) CH3(OCH2CH2)3-OH.

ethylene glycol derivatives predominantly exist in the cyclic gauche conformation, and the strong intramolecular hydrogenbonding tendency would reduce their ability to sustain the interaction connectivity to the anion. A strong electrostatic interaction between the cation and anion of the IL may be another reason for the comparatively weaker -O-H‚‚‚[BF4] interactions. The observations made from NMR measurements are contrary to excess molar volume results of the similar mixtures.56 The magnitude of the excess volume is more in the mixture of [Omim][BF4] with CH3(OCH2CH2)3-OH as compared to that in the mixtures with CH3(OCH2CH2)-OH and CH3(OCH2CH2)2-OH. The magnitude of the excess molar volume includes all of the effects, that is, physical, geometrical, and chemical interactions, when the mixture is created. Although the deviations from ideality in the above mixtures are negative for both the excess volume and the chemical shift, the opposite trend with a large difference in the magnitude indicates that the packing efficiency depending on the differences in the shape and size of the molecules dominates over the specific interactions. Since the specific interactions between glycol derivatives and the IL can give rise to the local packing effects and the weight of such effects depends on the probability that an organic liquid molecule interacts with the IL, the maxima in the deviations from ideality appears in the intermediate concentrations in both the volumes and NMR chemical shifts. 3.4. FT-IR Studies. Complementary information about the specific interactions in the IL-organic liquid mixtures over the whole composition range was obtained from FT-IR measurements. The shift in wavenumber of -O‚‚‚H stretching of different glycol derivatives with the addition of [Omim][BF4] showed prominent changes in the dilute region and are shown in Figure 9. The -O‚‚‚H stretching of the ethylene glycol derivatives could not be resolved in the IL-rich region of the mixtures. A sharp dip in wavenumber of the -O‚‚‚H stretching in the dilute IL region for CH3(OCH2CH2)-OH may correspond to the disruption of the associated structure of the organic

Figure 9. Comparison of the -OH stretching of the different glycol derivatives in the IL-dilute region of the mixtures: (0) CH3(OCH2CH2)-OH; (O) CH3(OCH2CH2)2-OH; and (4) CH3(OCH2CH2)3-OH. Solid lines represent the polynomial fits to the experimental data.

component and the formation of intermolecular hydrogen bonding between unlike components or due to the association of IL molecules in the mixture. Addition of an -OC2H4 group in the organic component reduced the tendency of disruption of the associated network of organic molecules and the association of the IL molecules in the mixture. Again, the increase of the wavenumber of the -O‚‚‚H stretching with the further increase of concentrations of the organic component in the mixture shows that the glycol derivatives regain their selfassociated network at higher concentrations. Figure 10 shows the change in the wavenumber of the -H‚‚‚C-2 stretching of the IL in the mixture over the whole concentration range. In the dilute region, the sharp decrease in the wavenumber shows

An Ionic Liquid in Ethylene Glycol Derivatives

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Figure 10. Comparison of the H-C-2 stretching of the IL in different glycol derivatives: (0) CH3(OCH2CH2)-OH; (O) CH3(OCH2CH2)2OH; and (4) CH3(OCH2CH2)3-OH.

the occurrence of comparatively stronger intermolecular hydrogen bonding between aromatic protons of the IL and the glycol derivatives. The gradual increase of the wavenumber of the -H‚‚‚C-2 stretching toward a value corresponding to neat IL is indicative of reduced intermolecular interactions. Furthermore, for the mole fractions xIL > 0.2, no significant changes were observed in the FT-IR spectra of BF4- stretching (see Supporting Information) in all of the glycol derivatives, showing that the cation and anion tend to strongly interact and that the IL behaves as a nonelectrolyte in the mixture. 4. Conclusions Fluorescence behavior and specific interactions of the IL [Omim][BF4] in the ethylene glycol derivatives have been investigated over the complete range of mixtures using the steady-state fluorescence, 1H NMR, and FT-IR spectroscopy. Different techniques favor the aggregation of IL molecules in the organic-liquid-rich region and significant intermolecular interactions between unlike molecules over the complete composition span. Addition of an -OC2H4 group in the glycol derivative reduces the tendency of aggregation of IL molecules in the organic-solvent-rich region and the magnitude of specific interactions between unlike molecules in the mixtures. 1H NMR results show that the chemical shifts of the protons of the IL are solvent-dependent and vary as a function of concentration, indicating that different protons of the IL interact with the solvent molecules to a different extent. A comparison of deviations in the chemical shifts and excess molar volumes shows that the overall deviations from ideality are mainly dominated by packing effects depending upon the shape and size of the unlike molecules. Acknowledgment. The authors are thankful to Dr. P.K. Ghosh, Director of the institute for his interest in the work. The authors are also thankful to Mr. Arvind Boricha for NMR measurements. Financial support for this work was provided by the Department of Science and Technology (DST), Government of India (Project No. SR/S1/PC-36/2005). Supporting Information Available: NMR chemical shift data and figures are provided. This information is available free of charge via the Internet at http://pubs.acs.org.

(1) Earle, M. J.; Esperanca, J. M. S. S.; Gilea, M. A.; Canongia Lopes, J. N.; Rebelo, L. P. N.; Magee, J. W.; Seddon, K. R.; Widegren, J. A. Nature 2006, 439, 831. (2) Paulechka, Y. U.; Zaitsau, Dz. H.; Kabo, G. J.; Strechan, A. A. Thermochim. Acta. 2005, 439, 158. (3) Rebelo, L. P. N.; Canongia Lopes, J. N.; Esperanca, J. M. S. S.; Filipe, E. J. Phys. Chem. B 2005, 109, 6040. (4) Welton, T. Chem ReV. 1999, 99, 2071. (5) Seddon, K. R. Chem. Commun. 2001, 2399. (6) Anderson, J. L.; Ding, J.; Welton, T.; Armstrong, D. W. J. Am. Chem. Soc. 2002, 124, 14247. (7) Visser, E.; Swatloski, R. P.; Rogers, R. D. Green Chem. 2000, 2, 1. (8) Hussey, C. L. In Chemistry of Nonaqueous Solutions; Mamantov, G., Popov, A. I., Eds.; VCH: Weinheim, Germany, 1994; p 227. (9) Endres, F.; El Abedin, S. Z. Phys. Chem. Chem. Phys. 2006, 8, 2101. (10) Wang, P.; Zakeeruddin, S. M.; Shaik, M.; Humphry-Baker, R.; Gra¨tzel, M. Chem. Mater. 2004, 16, 2694. (11) Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.; Visser, A. E.; Rogers, R. D. Chem. Commun. 1998, 1765. (12) Gordon, C. M.; Holbrey, J. D.; Kennedy, A. R.; Seddon, K. R. J. Mater. Chem. 1998, 8, 2627. (13) Itoh, H.; Naka, K.; Chujo, Y. J. Am. Chem. Soc. 2004, 126, 3026. (14) Zhou, Y.; Antonietti, M. Chem. Mater. 2004, 16, 544. (15) Dattelbaum, A. M.; Baker, S. N.; Baker, G. A. Chem. Commun. 2005, 939. (16) Bowers, J.; Vergara-Gutierrez, M. C. Langmuir 2004, 20, 309. (17) Lynden-Bell, R. M. Mol. Phys. 2003, 101, 2625. (18) Baldelli, S. J. J. Phys. Chem. B 2003, 107, 6148. (19) Wang, Y.; Voth, G. A. J. Am. Chem. Soc. 2005, 127, 12192. (20) Canongia Lopes, J. N.; Padua, A. A. H. J. Phys. Chem. B 2006, 110, 3330. (21) Seddon, K. R.; Annegret, S.; Maria-Jose, T. Pure Appl. Chem. 2000, 72, 2275. (22) Aki, S. N. V. K.; Brennecke, J.; Samanta, A. Chem. Commun. 2001, 413. (23) Wasserscheid, P., Welton, T., Eds.; Ionic Liquids in Synthesis; Wiley-VCH: Weinheim, Germany, 2003. (24) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. ReV. 2002, 102, 3667. (25) Antonietti, M.; Kuang, D.; Smarsly, B.; Zhou, Y. Angew. Chem., Int. Ed. 2004, 43, 4988. (26) Rebelo, L. P. N.; Najdanovic-Visak, V.; Visak, Z. P.; Nunes da Ponte, M.; Szydlowski, J.; Cerdeirina˜, C. A.; Troncoso, J.; Romanı´, L. Green Chem. 2004, 6, 369. (27) Katayanagi, H.; Nishikawa, K.; Shimozak, H.; Miki, K.; Westh, P.; Koga, Y. J. Phys. Chem. B 2004, 108, 19451. (28) Miki, K.; Westh, P.; Nishikawa, K.; Koga, Y. J. Phys. Chem. B 2005, 109, 9014. (29) Saha, S.; Hamaguchi, H. J. Phys. Chem. B 2006, 110, 2770. (30) Dominguez-Vidal, A.; Kaun, N.; Ayora-Canˇada, M. J.; Lendl, B. J. Phys. Chem. B 2007, 111, 4446. (31) Singh, T.; Kumar, A. J. Phys. Chem. B 2007, 111, 7843. (32) Hanke, C. G.; Lynden-Bell, R. M. J. Phys. Chem. B 2003, 107, 10873. (33) Canongia Lopes, J. N.; Costa Gomes, M. F.; Padua, A. A. H. J. Phys. Chem. B 2006, 110, 16816. (34) Jiang, W.; Wang, Y.; Voth, G. A. J. Phys. Chem. B 2007, 111, 4812. (35) Wang, J.; Tian, Y.; Zhuo, K. Green Chem. 2003, 5, 618. (36) Heintz, A.; Lehmann, J. K.; Wertz, C. J. Chem. Eng. Data 2003, 48, 472. (37) Zhang, J.; Wu, W.; Jiang, T.; Gao, H.; Liu, Z.; He, J.; Han, B. J. Chem. Eng. Data 2003, 48, 1315. (38) Zhang, S.; Li, X.; Chen, H.; Wang, J.; Zhang, J.; Zhang, M. J. Chem. Eng. Data 2004, 49, 760. (39) Kim, K. S.; Shin, B. K.; Lee, H.; Ziegler, F. Fluid Phase Equilib. 2004, 218, 215. (40) Rebelo, L. P. N.; Najdanovic-Visak, V.; Visak, Z. P.; Nunes da Ponte, M.; Szydlowski, J.; Cerdeirina, C. A.; Troncoso, J.; Romani, L.; Esperanca, J. M. S. S.; Guedes, H. J. R.; de Sousa, H. C. Green Chem. 2004, 6, 369. (41) Xu, H.; Zhao, D.; Xu, P.; Liu, F.; Gao, G. J. Chem. Eng. Data 2005, 50, 133. (42) Vasiltsova, T. V.; Verevkin, S. P.; Bich, E.; Heintz, A.; BogelLukasik, R.; Domanska, U. J. Chem. Eng. Data 2005, 50, 142. (43) Heintz, A.; Klasen, D.; Lehmann, J. K.; Wertz, C. J. Solution Chem. 2005, 34, 1135. (44) Zafarani-Moattar, M. T.; Shekaari, H. J. Chem. Eng. Data 2005, 50, 1694.

4086 J. Phys. Chem. B, Vol. 112, No. 13, 2008 (45) Harris, K. R.; Kanakubo, M.; Woolf, L. A. J. Chem. Eng. Data 2005, 50, 1777. (46) Wang, J.; Zhu, A.; Zhao, Y.; Zhuo, K. J. Solution Chem. 2005, 34, 585. (47) Pereiro, A. B.; Tojo, E.; Rodriguez, A.; Canosa, J.; Tojo, J. J. Chem. Thermodyn. 2006, 38, 651. (48) Pereiro, A. B.; Tojo, E.; Rodriguez, A.; Canosa, J.; Tojo, J. Green Chem. 2006, 8, 307. (49) Arce, A.; Rodil, E.; Soto, A. J. Solution Chem. 2006, 35, 63. (50) Zafarani-Moattar, M. T.; Shekaari, H. J. Chem. Thermodyn. 2006, 38, 624. (51) Pereiro, A. B.; Rodrı´guez, A. J. Chem. Eng. Data 2007, 52, 600. (52) Pereiro, A. B.; Rodrı´guez, A. J. Chem. Thermodyn. 2007, 39, 978. (53) Zhong, Y.; Wang, H.; Diao, K. J. Chem. Thermodyn. 2007, 39, 291. (54) Aliotta, F.; Ponterio, R. C.; Saija, F.; Salvato, G.; Triolo, A. J. Phys. Chem. B 2007, 111, 10202. (55) Kumar, A.; Singh, T.; Gardas, R. L.; Coutinho, J. A. P. J. Chem. Thermodyn. 2008, 40, 32. (56) Singh, T.; Kumar, A. J. Chem. Thermodyn. 2008, 40, 417. (57) Padua, A. A. H.; Costa Gomes, M. F.; Canongia Lopes, J. N. A. Acc. Chem. Res. 2007, 40, 1087.

Singh and Kumar (58) Paul, A.; Mandal, P. K.; Samanta, A. J. Phys. Chem. B 2005, 109, 9148. (59) Paul, A.; Mandal, P. K.; Samanta, A. Chem. Phys. Lett. 2005, 402, 375. (60) Paul, A.; Samanta, A. J. Chem. Sci. 2006, 118, 335. (61) Mandal, P. K.; Sarkar, M.; Samanta, A. J. Phys. Chem. A 2004, 108, 9048. (62) Paul, A.; Mandal, P. K.; Samanta, A. J. Photochem. Photobiol., A 2006, 182, 113. (63) Samanta, A. J. Phys. Chem. B 2006, 110, 13704. (64) Chakrabarty, D.; Chakraborty, A.; Seth, D.; Sarkar, N. J. Phys. Chem. A 2005, 109, 1764. (65) Wilkes, J. S.; Zaworotko, M. J. J. Chem. Soc., Chem. Commun. 1992, 965. (66) Huang, J. F.; Chen, P. Y.; Sun, I. W.; Wang, S. P. Spectrosc. Lett. 2001, 34, 591. (67) Headley, A. D.; Kotti, S. R. S. S.; Nam, J.; Li, K. J. Phys. Org. Chem. 2005, 18, 1018. (68) Earle, M. J.; Gordon, C. M.; Plechkova, N. V.; Seddon, K. R.; Welton, T. Anal. Chem. 2007, 79, 758. (69) Demchenko, A. P. Luminescence 2002, 17, 19.