Solvatochromic Probe Response within Ionic ... - ACS Publications

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Solvatochromic Probe Response within Ionic Liquids and Their Equimolar Mixtures with Tetraethylene Glycol Rewa Rai and Siddharth Pandey* Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi -110016, India S Supporting Information *

ABSTRACT: Synergism in a probe response within a mixture hints at the presence of strong interactions involving the solvent constituents of the mixture and possibly the probe. Unusual and rare “hyperpolarity” resulting from the synergism in probe response exhibited by ionic liquid (IL) mixtures with glycol family solvents is investigated in detail for equimolar mixtures of tetraethylene glycol (TEG) with many structurally different ILs using several UV−vis absorbance and fluorescence solvatochromic probes. Thirteen different ILs, of the same cation 1-butyl-3-methylimidazolium and different anions, of the same anion bis(trifluoromethylsulfonyl)imide and different cations, and of C2 methyl-substituted imidazolium cations, are used to assess the structural dependence of the IL on synergism exhibited by (IL + TEG) mixture. Responses from UV−vis absorbance probes are used to obtain ET [dipolarity/ polarizability and/or H-bond donating (HBD) acidity] and Kamlet−Taft parameters [π* (dipolarity/polarizability), α (HBD acidity), and β (HB accepting basicity)] within (IL + TEG) mixtures. The band I-to-band III fluorescence intensity ratio of dipolarity probe pyrene along with the lowest energy fluorescence band maxima of pyrene-1-carboxaldehyde (PyCHO, a probe for the permittivity of the medium), coumarin-153 and N,N-dimethyl-6-propionyl-2-naphthylamine PRODAN (neutral photoinduced charge-transfer fluorescence probes), and 6-p-toluidine-2-naphthalenesulfonic acid (TNS) and l-anilinonaphthalene-8-sulfonate (ANS) (ionic photoinduced charge-transfer fluorescence probes) are used to assess whether synergism is exhibited by (IL + TEG) equimolar mixtures. Probe responses within TEG equimolar mixtures with ILs are compared to those with common organic solvents. An attempt is made to establish a correlation between the synergism observed in the probe response within an (IL + TEG) mixture and the structural features of the cation and anion of the IL, such as acidity of the protons of the cation, aromaticity of the cation, and size, shape, and coordinating ability of the anion. It is established that the solvatochromism exhibited by the probes within (IL + TEG) mixtures is due to complex coupling of several different interactions and dynamical processes involving the probe as well as IL and TEG within the mixture.

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INTRODUCTION Room temperature ionic liquids (ILs) have become an integral part of research in chemistry and related areas.1−11 The fact that ILs are composed of ions alone and are still in liquid state at room temperature has opened the possibilities of novel and unusual properties associated with these substances. Structural modifications/alterations in cation or anion result in widely varying physicochemical properties of ILs (and hence the title “designer” solvents) and subsequently produces an enormous number of ILs that are possible. The appropriate combination of constituents of ILs turns them into a solvent milieu of anomalous behavior that is remarkably different from that of the common molecular solvents or aqueous electrolytes. Apart from using synthetic methodology to tweak the chemical structure of cation/anion of an IL, the physicochemical properties of ILs may also be effectively modified by simple addition of a cosolvent. In this context, environmentally benign cosolvents, such as water, ethanol, fluorous solvents, supercritical (sc) CO2, and glycol family solvents, among others, are obviously the preferred choices. While water and ethanol © 2014 American Chemical Society

exhibit limited miscibility in several common and popular ILs,12−15 the use of scCO2 is cumbersome, and fluorous solvents are fairly limited in their scope. The glycol family solvents, especially tetraethylene glycol (TEG) and polyethylene glycols (PEGs) of different average molecular weights, are not only inexpensive, easily available, and easy to handle, but they have also been shown to have complete or near complete miscibility with common and popular ILs.16−19 Glycol family solvents as cosolvents to ILs have shown the potential to impart favorable and unusual properties to a solubilizing milieu. Tetraethylene glycol (TEG), one of the members of the glycol family of solvents, is an odorless, nonvolatile, hydrophilic, and environmentally benign liquid which has several industrial applications especially as an intermediate and/or ingredient in polyester resins, as a component of antifreezes/ Received: April 29, 2014 Revised: August 27, 2014 Published: August 27, 2014 11259

dx.doi.org/10.1021/jp504165a | J. Phys. Chem. B 2014, 118, 11259−11270

The Journal of Physical Chemistry B

Article

coolants, as plasticizers for nitrocellulose finishes, as humectants in tobacco and textiles, as a lubricant for rubber, as heat transfer fluids, and in gas dehydration and treatment.20,21 Due to its attractive properties and applications combined with Hbond donating/accepting capabilities, TEG clearly has the potential to form a hybrid system with ILs that possesses attractive physicochemical properties. Apart from physical properties, solute solvation is of utmost importance as far as characterizing a complex solubilizing milieu is concerned. Information regarding the dependence of the solvation process on molecular architecture and functionalities of the solute is essential to establish any milieu as acceptable solubilizing medium in the chemical sciences. Solvatochromic solutes (or probes), in this respect, may provide systematic information regarding the dipolarity afforded by a complex milieu. The response of a solvatochromic probe (i.e., the UV−vis molecular absorption or fluorescence emission) strongly depends on the polarity of the medium and changes systematically as the dipolarity of the medium is changed. Apart from the obvious solute−solvent interactions, solvent−solvent interactions resulting in a changed solubilizing milieu may also get reflected in a solvatochromic probe response. The unusual and rare synergism demonstrated by various solvatochromic probes dissolved in certain mixtures of IL with TEG leading to the suggestion of remarkable “hyperpolarity” is systematically investigated in detail in the work presented here. Previously, we have reported that solvatochromic probes dissolved in mixtures of TEG with IL 1-butyl-3-methylimidazolium hexafluorophosphate ([C4C1Im][PF6]) exhibit unusual and rare synergism.17 Many different absorbance- and fluorescence-based solvatochromic probes were used in that investigation. A solvent ordering via formation of HBD/HBA complexes involving the C-2 proton of the [C4C1Im+] cation and oxygen atoms of TEG, as well as interactions between [PF6−] and the terminal hydroxyl groups of TEG, were proposed to account for the observed behavior. Further spectroscopic evidence of strong intersolvent interactions occurring within the ([C4C1Im][PF6] + TEG) mixture was provided based on substantial frequency shifts in the [PF6−] asymmetric stretching mode observed in the infrared spectra as TEG was incrementally added to [C4C1Im][PF6]. The observations contributed to the growing literature advocating the notion that ILs and certain organic solvents form ordered, nanostructured, or microsegregated phases upon mixing. Similar “hyperpolarity” shown by the probes was also reported for the mixtures of the same IL with PEGs of different average molecular weights.18 To afford insight into the reason for such anomalous probe behavior, responses of several solvatochromic solutes were investigated within equimolar mixtures of TEG with several different ILs. To assert the separate roles of the cation and/or the anion of the IL in producing the “hyperpolarity” within the (IL + TEG) mixture, ILs of the same cation 1-butyl-3-methylimidazolium [C4C1Im+] with different anion and ILs of the same anion bis(trifluoromethylsulfonyl)imide [(CF3SO2)2N−] with different cations along with other judiciously selected ILs were used in this investigation. Further, to assess the dependence of the chemical structure of the solvatochromic probe on the extent of “hyperpolarity” exhibited by the (IL + TEG) mixture, several different UV−vis absorbance- and fluorescence-based probes were used. Our results clearly indicate the complexity of the coupling of many different interactions and dynamical

processes associated with the solvatochromic probes and the (IL + TEG) mixtures.

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EXPERIMENTAL SECTION Materials. HPLC grade ILs of 1-butyl-3-methylimidazolium cation, [C4C1Im][PF6] (≥99%), [C4C1Im][BF4] (ultra pure), and [C4C1Im][CF3SO3] (≥98%), and IL of 1-butyl-1methylpyrrolidinium cation, [C4C1Pyrr][(CF3SO2)2N] (ultra pure), were purchased from Merck. IL [C4C1Im][CF3COO] (97%) and ILs of 1-butyl-1-methylpiperidinium cation, [C4C1Pip][(CF3SO2)2N] (99%), N-butylpyridinium cation, [C 4 Pyr][(CF 3 SO 2 ) 2 N], and choline cation, [N 1,1,1,2OH ] [(CF3SO2)2N] (99%), were purchased from Iolitec. IL [C4C1Im][(CF3SO2)2N], IL of 1-ethyl-3-methylimidazolium cation, [C2C1Im][(CF3SO2)2N], and IL of 1,2-dimethyl-3propylimidazolium cation, [C1C1C3Im][(CF3SO2)2N], were purchased from Covalent Associates, Inc., and they were of electrochemical grade (>99.9%). IL of 1,2-dimethyl-3-butylimidazolium cation, [C1C 1 C 4 Im][BF 4 ] (high purity), was purchased from Fluka. IL of tributylmethylammonium cation, [N1,4,4,4][(CF3SO2)2N] (high purity), was synthesized using the method described in the literature.22 Structures of all ILs are provided in Scheme 1. Before use, all ILs were dried overnight Scheme 1. Structures of the ILs Used

under vacuum to have water content 18.0 MΩ·cm resistivity was obtained from Millipore Milli-Q Academic water purification system. Methods. Stock solutions of all probes were prepared by dissolving them in ethanol in precleaned amber vials and stored at 4 ± 1 °C. The required amount of probes was weighed using a Mettler-Toledo AB104-S balance with a precision of ±0.1 mg. An appropriate amount of the probe solution from the stock was transferred to a 1 cm2 quartz cuvette. Ethanol was evaporated using a gentle stream of high purity nitrogen gas. The final concentrations of the fluorescence probes in the sample were ≤10 μM and that of absorbance probes were ≤100 μM. ILs selected and TEG show complete miscibility, and all (IL + TEG) equimolar mixtures form one phase. A PerkinElmer Lambda Bio 35 double-beam spectrophotometer with variable bandwidth was used for acquisition of UV−vis molecular absorbance data. Steady-state fluorescence spectra were acquired on a Jobin-Yvon Fluorolog-3 (model FL3-11) modular spectrofluorometer equipped with a 450 W Xe arc lamp as the excitation source and single-grating monochromators as wavelength selection devices and a photomultiplier tube as the detector. All emission spectra were corrected for the detector response. All absorbance and fluorescence data were acquired using 1 cm2 quartz cuvettes at 25 °C. All spectroscopic measurements were performed in triplicate starting from sample preparation, and the results were

=

[E T(30)solvent − E T(30)TMS ] [E T(30)water − E T(30)TMS ] [E T(30)solvent − 30.7] 32.4

(2)

Here TMS stands for tetramethylsilane, and ET(30)water = 63.1 kcal·mol−1 and ET(30)TMS = 30.7 kcal·mol−1 are experimentally observed values. ENT is easier to conceive, as it is dimensionless and varies between 0 for TMS (extreme nonpolar) and 1 for water (extreme polar). From the absorbance spectra of betaine dye 33 dissolved in different ILs (having same cation [C4C1Im+] with different anions and same anion [(CF3SO2)2N−] with different cations, Scheme 1), relevant organic solvents, TEG, and equimolar mixtures of ILs and organic solvents with TEG, ENT were estimated and are reported in Table S1, Supporting Information (representative absorbance spectra of equimolar mixture of ([C4C1Im][PF6] + TEG) and ([C4C1Pyrr][(CF3SO2)2N] + TEG) are presented in Figure 1, panels A and B). In accordance with that reported previously,17 λabs max in [C4C1Im][PF6] is hypsochromically shifted from that in TEG, indicating the higher dipolarity/polarizability and/or HBD acidity associated with [C4C1Im][PF6] as compared to TEG. Further, the λabs max in the equimolar mixture of ([C4C1Im][PF6] + TEG) is even lower than that found in [C4C1Im][PF6]. The “hyperpolarity” implied by this interesting synergism is proposed to be due to the HBD/HBA association taking place between the C-2 proton of [C4C1Im+] and ether oxygens of TEG as well as to coupling between [PF6−] and hydroxyl termini on TEG.17 It is interesting to note, however, that λabs max in [C4C1Pyrr][(CF3SO2)2N] (cation has no appreciable HBD acidity) is also hypsochromically shifted (though not to the same extent) from that in TEG, highlighting relatively higher dipolarity/polarizability and/or HBD acidity of [C4C1Pyrr][(CF3SO2)2N]. A careful examination of λabs max for the equimolar mixture of ([C4C1Pyrr][(CF3SO2)2N] + TEG) reveals slight abs hypsochromic shift from λ max found for [C 4 C 1 Pyrr][(CF3SO2)2N]. This highlights an interesting fact that the equimolar mixture of TEG with ILs having no obvious acidic 11261



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Scheme 2. Structures of the Absorbance and Fluorescence Probes Used

proton may also exhibit synergism, albeit to a significantly lesser extent, proposing the unusual “hyperpolarity” of the medium. To elucidate further the role of cation and/or anion of ILs in this context, ENT for ILs having [C4C1Im+] with different anions + TEG and for ILs having [(CF3SO2)2N−] with different cations + TEG in equimolar mixtures are tabulated (Table S1, Supporting Information) and presented in Figure 1 (panels C and D). Whether the synergism (and hence the “hyperpolarity”) is shown by a mixture or not is highlighted in Table 2. It is clear from the data that ENT are higher in ILs (except [C4C1Im][CF3COO]) and in polar-protic solvents as compared to that in TEG, indicating the higher dipolarity/ polarizability and/or HBD acidity of ILs and polar-protic solvents over TEG. The ENT of equimolar mixtures of TEG with ILs containing [C4C1Im+] with different anions (except in [C4C1Im][(CF3SO2)2N]) are even greater than those found for the respective ILs showing “hyperpolarity” exhibited by these mixtures due to synergism in the probe response. For convenience, we have defined the extent of synergism (Δ) in terms of probe response, R, as

Δ=

R mix − R avg |RIL − RTEG|

,

where

R avg =

1 [RIL + RTEG] 2 (3)

where RIL, RTEG, and Rmix correspond to probe responses in neat IL, in neat TEG, and in an equimolar mixture of (IL + TEG), respectively. Clearly, Δ = 0 indicates ideal additive behavior, Δ > 0.5 implies “hyperpolarity”, and Δ < −0.5 may be considered to represent “hypopolarity”. Though Δ (Table S1, Supporting Information) are different for TEG equimolar mixtures with different [C4C1Im+] ILs, they are observed to be >0.5 for almost all these mixtures, representing synergism in the betaine dye response thus suggesting “hyperpolarity” (Table 2). For ENT within equimolar mixtures of TEG with ILs having cations other than [C 4 C 1 Im + ] with the same anion [(CF3SO2)2N], the extent of synergism (i.e., “hyperpolarity”) is found to be significant when the system is composed of IL [C4Pyr][(CF3SO2)2N] followed by [C2C1Im][(CF3SO2)2N] and to a smaller extent for systems with ILs [C1C1C3Im][(CF3SO2)2N] and [C4C1Pyrr][(CF3SO2)2N] (Table S1, Supporting Information, and Figure 1D). Interestingly, for equimolar TEG mixtures with ILs [N1,4,4,4][(CF3SO2)2N], [C4C1Pip][(CF3SO2)2N], and [N1,1,1,2OH][(CF3SO2)2N], the 11262



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([C1C1C4Im][BF4] + TEG) mixture also exhibits significant “hyperpolarity” (both of these ILs do not have acidic C-2 protons). It is important to mention here that the protons of the methyl group on C-2 also show some acidic character.25 This may contribute to the synergism by the betaine dye within TEG mixtures of ILs with a C-2-methyl-substituted imidazolium cation. Synergism, albeit to a smaller extent, within the equimolar mixture of ([C4C1Pyrr][(CF3SO2)2N] + TEG) could be tentatively attributed to the extra strain associated with the five-membered pyrrolidinium cation as opposed to the six-membered piperidinium or acyclic ammonium cation. It is well-established that ENT is a combined manifestation of the dipolarity/polarizability and the HBD acidity of the milieu, with the latter having larger contribution. Except for [N1,1,1,2OH][(CF3SO2)2N], which may have a relatively larger static dielectric constant (ε) (we could not find ε of [N1,1,1,2OH][(CF3SO2)2N] in the literature; however, ε of ILs with choline cation and acetate, nitrate, formate, and lactate anions are reported to be 58.3, 60.9, 61.0, and 85.6, respectively), the ε of the ILs used in this investigation along with other structurally similar ILs are not too high and vary in a narrow range between 12.9 to 15.7.26−28 The ε for TEG is 20.4.29 The relatively high ENT within [N1,1,1,2OH][(CF3SO2)2N] could be due to its inherently high ε, and it is clear that any explanation of trends in probe response based on only one parameter, ε, is completely inadequate. Similarly, the refractive index (n) cannot be evoked, as for most of the ILs used in the investigation, n varies between 1.41 and 1.44 while it is 1.46 for TEG. Among organic solvents investigated, only the equimolar mixture of acetonitrile with TEG exhibits synergism; surprisingly, none of the TEG mixtures of polar-protic solvents showed any “hyperpolarity”. Due to the presence of the C-2 acidic proton combined with the ionic nature, initial polarity

Figure 1. Normalized absorbance spectra of betaine dye 33 in TEG and its equimolar mixture with IL [C4C1Im][PF6] (panel A) and [C4C1Pyrr][(CF3SO2)2N] (panel B). Experimental ENT within ILs of the same cation [C4C1Im+] and different anions (panel C) and within ILs of the same anion [(CF3SO2)2N−] and different cations (panel D) along with ENT within TEG and corresponding equimolar (IL + TEG) mixtures.

synergism is not observed. These outcomes hint at the possible role of aromaticity of the IL cation that results in the associated acidity of the proton(s) in imparting “hyperpolarity” to the equimolar TEG mixtures. It is clear that the presence of an acidic C-2 proton on the imidazolium cation appears to be unnecessary for the existence of synergism in the betaine dye response within equimolar TEG-IL mixtures. Apart from the synergism exhibited by the betaine dye in the equimolar ([C 1 C 1 C 3 Im][(CF 3 SO 2 ) 2 N] + TEG) mixture, the

Table 2. Observation of Synergism within Equimolar Mixture of Different Solvents with TEG Using Several Absorbance and Fluorescence Probes whether synergism (Δ > 0.5) is observed PyCHO, λfluoro max (nm)

TNS, λfluoro max (nm)

ANS, λfluoro max (nm)

PRODAN, λfluoro max (nm)

C-153, λfluoro max (nm)

solvents

ENT

π*

α

β

Py, I1/ I3

[C4C1Im][PF6] [C4C1Im][BF4] [C4C1Im][CF3SO3] [C4C1Im][CF3COO] [C4C1Im][(CF3SO2)2N] [C1C1C3Im] [(CF3SO2)2N] [C2C1Im][(CF3SO2)2N] [C4Pyr][(CF3SO2)2N] [N1,4,4,4][(CF3SO2)2N] [C4C1Pyrr] [(CF3SO2)2N] [C4C1Pip][(CF3SO2)2N] [N1,1,1,2OH] [(CF3SO2)2N] [C1C1C4Im][BF4] ethanol ethylene glycol glycerol acetonitrile dimethyl sulfoxide dichloromethane

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investigations compared dialkylimidazolium ILs with short chain alcohols. Although this observation indicates the superior ability of ILs in affording “hyperpolarity” when combined with appropriate cosolvent, it again casts doubts over the proposition that HBD acidity of the cation of ILs is primarily responsible for the synergism. Further, it appears that the “hyperpolarity” shown by the equimolar mixtures of IL and TEG is controlled more by the cation than by the anion of the IL. Empirical Kamlet−Taft (K-T) Parameters. To deconvolve the roles of dipolarity/polarizability and HBD acidity on the rare synergism exhibited by certain (IL + TEG) mixtures, we estimated empirical K-T parameters30−32 within the same mixtures under identical conditions. The dipolarity/polarizability (π*) was estimated from the absorption maxima of N,N-diethyl-4-nitroaniline (DENA) (ν̅DENA in kK, where kK = 103 cm−1), a non-HBD solute, using the following expression. π ∗ = 8.649 − 0.314νDENA ̅

(4)

The HBD acidity (α) was determined from ET(30) and π* using ⎡ E (30) − 14.6(π ∗ − 0.23δ) − 30.31 ⎤ α=⎢ T ⎥ ⎦ ⎣ 16.5

(5)

(6)

Figure 2. Empirical Kamlet−Taft parameters, π* (panels A and B), α (panels C and D), and β (panels E and F), within ILs of the same cation [C4C1Im+] and different anions and within ILs of the same anion [(CF3SO2)2N−] and different cations, respectively, along with these parameters within TEG and corresponding equimolar (IL + TEG) mixtures.

K-T parameters (π*, α, and β) estimated using eqs 4, 5, and 6 for different ILs and organic solvents and their respective equimolar mixtures with TEG and the extent of synergism (Δ), if any, in (IL + TEG) equimolar mixtures are given in Table S1, Supporting Information [for comparison purposes, π*, α, and β, respectively, of TEG, ILs, and an equimolar mixture of (ILs + TEG) are presented as bar charts in Figure 2]. The parameters obtained for neat solvents and selected ILs are in good agreement with those reported previously in the literature.17,33 The π* of all ILs investigated are higher than the π* of TEG. It is obvious that π* is not a manifestation of only the ε of the medium, as ε of TEG is higher than most ILs investigated (except for [N1,1,1,2OH][(CF3SO2)2N]). Among [C4C1Im+] ILs, the “hyperpolarity” (or the synergism in π*) is only observed for the equimolar TEG mixture of [C4C1Im][PF6], implying that the symmetric anion [PF6−] plays an important role in the unusually higher dipolarity/polarizability of the mixture. Among the equimolar TEG mixtures of ILs with the same anion [(CF 3 SO 2 ) 2 N], “hyperpolarity” is observed for [C4C1Pyrr][(CF3SO2)2N] and for [C4C1Pip][(CF3SO2)2N] only (Figure 2 and Table 2). It appears that interactions involving localized charges on the nitrogen of the strained saturated cyclic cations [C4C1Pyrr+]/[C4C1Pip+] with ether and hydroxyl functionalities of TEG play an important role in affording unusually enhanced π* within the mixtures. It is important to mention that no “hyperpolarity” is observed for the equimolar TEG mixture of any of the organic solvents investigated, reasserting the superiority of ILs over organic solvents in this context. An important outcome is that, although ε of most ILs investigated are not too high, the dipolarity/ polarizability of these ILs manifested through the response of absorbance probe DENA appears to be significant. Macroscopic

bulk properties, such as ε and n, do not influence the probe responses; instead, the fact that ILs are composed of ions alone that impart inherent polarity to ILs perhaps gets manifested in the probe responses more. Inspection of HBD acidity, α (Figure 2, Table S1, Supporting Information, and Table 2), obtained within equimolar TEG mixtures of ILs and organic solvents, confirms the fact that HBD acidity contributes significantly to ENT . It is clear that except for the TEG mixtures containing [C4C1Im][CF3COO] (shows the lowest ETN among all ILs investigated) and [C4C1Im][(CF3SO2)2N] (ENT of the mixture with TEG not showing synergism), all other equimolar TEG mixtures with ILs composed of an aromatic cation having acidic proton(s) exhibit synergism in α; even [C4Pyr][(CF3SO2)2N], [C1C1C3Im][(CF3 SO2 )2 N], and [C 1C 1C4 Im][BF4] containing TEG mixtures show unusually high α. According to Handy et al.,25 [C1C1C3Im+] and [C1C1C4Im+] show acidic behavior, though to lesser extents, even after the replacement of C-2 hydrogen with a methyl group, whereas in [C4Pyr+], it appears that the electron-deficient pyridinium ring makes the α-hydrogen of the butyl group somewhat acidic, which may take part in H-bond interaction with TEG moieties as described previously.34 None of the TEG mixtures of ILs with nonaromatic cations show synergism in α. It is interesting to note that, among organic solvents, only the (acetonitrile + TEG) equimolar mixture exhibits synergism in α. The HBA basicities (β) of all ILs (except for [C4C1Im][CF3COO]) are expectedly lower than that of TEG, which is in agreement with that reported previously, as the anions of most ILs investigated do not possess prominent HBA sites ([CF3COO−] possesses appreciable basicity).35 A careful

and the H-bond accepting (HBA) basicity (β) was estimated from the enhanced solvatochromic shift of 4-nitroaniline (NA) relative to its homomorph DENA, −Δν̅(DENA − NA)/kK, according to ∗ β = −0.357νNA ̅ − 1.176π + 11.12

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examination of β within equimolar mixtures of (TEG + ILs) (Figure 2, Table S1, Supporting Information, and Table 2) indicates no mixture showing synergism in β. The lack of the presence of HBA functionalities on ILs investigated is the reason for this observation (the experimental β are not much deviated from those predicted from ideal-additive behavior). Appreciable polarity as well as HBD acidity inherent to ILs results in unusually high ENT , π*, and α for certain ILs when mixed with TEG in an equimolar amount. When organic solvent−TEG mixtures are compared, it is found that equimolar mixtures of ethanol and DCM, respectively, with TEG do show unusually high β. Solvatochromic Fluorescence Polarity Probe Behavior. Dipolarity Probes Pyrene and Pyrene-1-carboxaldehyde. The pyrene solvent polarity scale (Py I1/I3) is defined by the ratio of its emission intensities, I1/I3, where I1 is the intensity of the solvent-sensitive band and corresponds to the S1(v = 0) → S0(v = 0) transition and I3 corresponds to the solventinsensitive S1(v = 0) → S0(v = 1) transition.36−38 The I1/I3 ratio increases with increasing solvent dipolarity and is a function of both the solvent dielectric (ε) and the refractive index (n) via the dielectric cross-term, f(ε,n2).37 Py I1/I3 in ILs are expectedly higher than that in TEG and in common organic solvents used in this investigation due perhaps to the fact that pyrene solvation within ILs is governed by the fact that ILs are composed of ions alone (Figure 3 and Table

[except in ([N1,1,1,2OH][(CF3SO2)2N] + TEG)] than those predicted ideally [(I1/I3)calcd] from the procedure provided by Acree and co-workers (eq 7).39 ⎛I ⎞ I1,IL + I1,TEG = ⎜ 1⎟ I3,IL + I3,TEG ⎝ I3 ⎠calcd

(7)

The higher experimental Py I1/I3 as compared to the idealadditive Py I1/I3 could either manifest interactions between IL and TEG or could be due to the pronounced interaction of cations of ILs with the pyrene π-cloud (i.e., preferential solvation of excited pyrene by or interaction with ILs as opposed to TEG). On the basis of the synergism exhibited through the response of betaine dye 33, dipolarity/polarizability, and/or the HBD ability probe, the solvent−solvent interactions comprising IL and TEG appear to be more probable. To further corroborate this, similar to ENT and α, the Py I1/I3 in the equimolar mixture containing acetonitrile and TEG also shows pronounced synergism in the system. Fluorescence of pyrene-1-carboxaldehyde (PyCHO) can originate from either or both of the closely lying excited singlet states, n−π* and π−π*, in fluidic medium. The π−π* is more stabilized and is brought below n−π* upon increasing the polarity of the surrounding thus rendering π−π* the emitting state in more polar media. The emission from π−π* is manifested by a broad, reasonably intense emission that red shifts with increasing solvent dielectric. Experimentally obtained fluorescence emission spectra and lowest energy fluorescence emission maxima (λfluoro max ) of PyCHO provided in Figure 4 and Table S2, Supporting Information (normalized PyCHO probe responses, estimated according to eq 2 using nheptane instead of TMS for the highly nonpolar solvent, are fluoro also presented) indicate PyCHO λmax in ILs (except [N1,1,1,2OH][(CF3SO2)2N]) to be lower than that in TEG. This is easily attributed to the lower ε of ILs as compared to

Figure 3. Normalized fluorescence emission spectra of pyrene within TEG and its equimolar mixture with IL [C4C1Im][PF6] (panel A) and [C4C1Pyrr][(CF3SO2)2N] (panel B). Band I-to-band III emission intensities ratio of pyrene (Py I1/I3) within ILs of the same cation [C4C1Im+] and different anions (panel C) and within ILs of the same anion [(CF3SO2)2N−] and different cations (panel D) along with Py I1/I3 within TEG and corresponding equimolar (IL + TEG) mixtures.

S2, Supporting Information; both ε and n of most ILs used here are less than that of TEG). Normalized pyrene probe responses, estimated according to eq 2, using DMSO and cyclohexane instead of water and TMS for highly polar and highly nonpolar solvents, respectively, are also presented. In all (IL + TEG) equimolar mixtures, Py I1/I3 are found to be lower than their corresponding values in neat ILs. However, the experimental Py I1/I3 in such (IL + TEG) mixtures are higher

Figure 4. Normalized fluorescence emission spectra of PyCHO within TEG and its equimolar mixture with IL [C4C1Im][PF6] (panel A) and [C4C1Pyrr][(CF3SO2)2N] (panel B). PyCHO λfluoro max within ILs of the same cation [C4C1Im+] and different anions (panel C) and within ILs of the same anion [(CF3SO2)2N−] and different cations (panel D) along with PyCHO λfluoro max within TEG and corresponding equimolar (IL + TEG) mixtures. 11265



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TEG, as the PyCHO response is reflective of the ε of the fluoro in [N1,1,1,2OH]solubilizing milieu.17,40,41 The higher λmax [(CF3SO2)2N] is due to the higher ε of this IL. Though we could not find ε for [N1,1,1,2OH][(CF3SO2)2N] in the literature, according to Huang et al., ε of ILs with aprotic cations increases when the alkyl chain is replaced with an OH-terminated alkyl chain (vide supra).26 PyCHO λfluoro max in equimolar mixtures of (IL + TEG) follow similar trends as shown by ENT . Experimental PyCHO λfluoro max for TEG equimolar mixtures with ILs or common solvents are higher than that predicted from ideal additive behavior estimated using fluoro fluoro fluoro (λmax,calcd )−1 = 0.5 × [(λmax,solvent )−1 + (λmax,TEG )−1]

(8)

fluoro However, interestingly, PyCHO λmax in (IL + TEG) equimolar mixtures with ILs containing imidazolium and pyrrolidinium cations clearly demonstrate synergism thus hinting at “hyperpolarity” in the medium. The systems with ILs composed of cations with no clear acidic proton (i.e., [N1,4,4,4+], [C4C1Pip+], and [N1,1,1,2OH+]), on the other hand, do not show “hyperpolarity” (Table 2). These observations of PyCHO λfluoro are similar to those obtained from ENT and max manifest the fact that perhaps cations play a more important role than the anions in modulating the physicochemical properties of (IL + TEG) mixtures, reflecting the solvent− solvent interactions present within the system in the process. However, the synergism in the PyCHO response within equimolar mixtures of TEG with IL [C4C1Pyrr][(CF3SO2)2N] and aprotic polar solvents acetonitrile (“hyperpolarity” in acetonitrile is observed from responses of other probes) and dichloromethane reiterates that the HBD acidity of the cation of the IL does not have to be very high to afford “hyperpolarity”. The aromatic nature of the IL cation (except for [C4C1Pyrr+]) appears to play an important role in synergism shown by the PyCHO response. It is important to mention that IL [C4Pyr][(CF3SO2)2N] could not be used for any fluorescence probe investigation due to the inherent ability of pyridinium to effectively quench the fluorescence. Ionic Photoinduced Charge-Transfer Probes TNS and ANS. The major reason for the bathochromic shift in the fluorescence emission band of probes p-toluidinyl-6-naphthalenesulfonate (TNS) and 1-anilino-8-naphthalenesulfonate (ANS) with the increase in polarity of the cybotactic region is the change in the intramolecular charge-transfer process within the probe.42−44 However, along with solvent polarity, specific solute−solvent interactions, change in molecular conformation, intersystem crossing to the triplet state, and monophotonic photoionization might also take part in changing the fluorescence behavior of these probes.42−44 fluoro The λmax of TNS and ANS, respectively, obtained experimentally in ILs, common solvents, and their equimolar mixtures with TEG along with normalized values (normalized TNS and ANS probe responses are estimated according to eq 2 using dichloromethane and ethanol, respectively, instead of TMS for highly nonpolar solvent) and the extent of synergism are presented in Table S3, Supporting Information, Figure 5, and Figure 6. In comparison to the dipolarity of TEG, the dipolarity of ILs manifested through TNS and ANS responses are higher, which is in good agreement with that observed from Py I1/I3 and π*. A careful examination of TNS λfluoro max within (IL + TEG) equimolar mixtures reveals the behavior of TNS to be nearly similar to that of PyCHO; TNS λfluoro max in equimolar

Figure 5. Normalized fluorescence emission spectra of TNS within TEG and its equimolar mixture with IL [C4C1Im][PF6] (panel A) and [C4C1Pyrr][(CF3SO2)2N] (panel B). TNS λfluoro max within ILs of the same cation [C4C1Im+] and different anions (panel C) and within ILs of the same anion [(CF3SO2)2N−] and different cations (panel D) along with TNS λfluoro max within TEG and corresponding equimolar (IL + TEG) mixtures.

Figure 6. Normalized fluorescence emission spectra of ANS within TEG and its equimolar mixture with IL [C4C1Im][PF6] (panel A) and [C4C1Pyrr][(CF3SO2)2N] (panel B). ANS λfluoro max within ILs of the same cation [C4C1Im+] and different anions (panel C) and within ILs of the same anion [(CF3SO2)2N−] and different cations (panel D) along with ANS λfluoro max within TEG and corresponding equimolar (IL + TEG) mixtures.

mixtures of TEG with each IL (except for CF3COO− and N1,1,1,2OH+ containing) show synergism thus hinting at the “hyperpolarity” of the mixture (Table 2). It appears that the complexing ability of CF3COO− imparts significantly pronounced dipolarity to IL [C4C1Im][CF3COO] through the TNS response that points toward the presence of strong solute−solvent interactions. Similarly, TNS λfluoro max shows the dipolarity to be significant higher within [N 1,1,1,2OH ] [(CF3SO2)2N] (the high polarity of choline ILs were reported previously).26 Except for these two cases, the response of TNS 11266



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in (IL + TEG) equimolar mixtures, in general, appears to substantiate the presence of solvent−solvent interactions involving IL and TEG giving rise to synergism. The ANS, on the other hand, is found to behave somewhat differently than TNS. Examination of ANS λfluoro reveals max prominent synergism to exist in mixtures containing ILs with [C4C1Im+] and symmetrical anions [PF6−] and [BF4−] (Table 2, Table S3, Supporting Information, and Figure 6). The synergism is not exhibited by TEG mixtures of [C4C1Im+] IL with asymmetric anions [CF3SO3−], [(CF3SO2)2N−], and [CF3COO−]. These observations, somewhat similar to those for π* (where Δ ≥ 0.5 in a mixture containing [C4C1Im][PF6], Δ = 0 in a mixture containing [C4C1Im][BF4], and Δ < 0.5 for a mixture containing ILs of the same cation with anions [CF3SO3−], [(CF3SO2)2N−], and [CF3COO−]) attribute the role of symmetrical anions (where the charges are delocalized) in exhibiting “hyperpolarity”. It is possible that relatively higher coordinating ability of asymmetric [CF3SO3−], [(CF3SO2)2N−], and [CF3COO−] helps preferentially solvate the probe in equimolar (IL + TEG) mixtures. ANS λfluoro max within equimolar mixtures having ILs with different cations but the same asymmetric anion [(CF3SO2)2N−] show “hyperpolarity” for ILs with [C4C1Pyrr+], [C4C1Pip+], and [N1,4,4,4+]; ILs with [C2C1Im+] and [N1,1,1,2OH+] do not show any such “hyperpolarity”. Neutral Photoinduced Charge-Transfer Probes PRODAN and Coumarin-153. PRODAN undergoes substantial change in its dipole moment upon excitation with approximately no conformational change owing to intramolecular charge-transfer from an electron-donating dimethylamino group to an electronwithdrawing propionyl group connected through an aromatic spacer.44 Because PRODAN has no permanent charge it readily avoids contributions from ionic interactions.44−46 PRODAN λfluoro max for neat ILs and common solvents and their equimolar mixtures with TEG along with normalized values (normalized PRODAN probe responses, estimated according to eq 2 using cyclohexane instead of TMS for highly nonpolar solvent), and the extent of synergism, if any, are reported in Table S4, Supporting Information, and Figure 7. PRODAN λfluoro max in ILs (except for [N1,1,1,2OH][(CF3SO2)2N]) are lower than that in TEG. This is expected, as ILs (except for [N 1,1,1,2OH ][(CF3SO2)2N]) possess lower ε and n as compared to TEG (Table 1). While the experimental PRODAN λfluoro max are higher than the ideal-additive ones, synergism in probe response is not observed in any (IL + TEG) equimolar mixture. These outcomes echo those of Py I1/I3 (Table S2, Supporting Information). It may be inferred that the behavior of PRODAN is highly influenced by the ε of the solvation milieu. In concert with the responses of other probes, it may be suggested that for (IL + TEG) mixtures investigated, the physical properties ε and/or n is/are not unusually high. The fact that experimental PRODAN λfluoro max are higher than those estimated ideally for every (IL + TEG) mixture hints at the possible presence of significant IL−TEG (solvent−solvent) interactions that may render ε and/or n higher than that expected from ideal-additive behavior. The λfluoro max of coumarin-153 are also, in general, lower for ILs as compared to that for TEG (Table S4, Supporting Information, Figure 8; normalized coumarin-153 probe responses, estimated according to eq 2 using cyclohexane instead of TMS for highly nonpolar solvent, are also presented). This could again be due to the lower ε associated with ILs as compared to TEG (again, coumarin λfluoro max for higher

Figure 7. Normalized fluorescence emission spectra of PRODAN within TEG and its equimolar mixture with IL [C4C1Im][PF6] (panel A) and [C4C1Pyrr][(CF3SO2)2N] (panel B). PRODAN λfluoro max within ILs of the same cation [C4C1Im+] and different anions (panel C) and within ILs of the same anion [(CF3SO2)2N−] and different cations (panel D) along with PRODAN λfluoro max within TEG and corresponding equimolar (IL + TEG) mixtures.

Figure 8. Normalized fluorescence emission spectra of C-153 within TEG and its equimolar mixture with IL [C4C1Im][PF6] (panel A) and [C4C1Pyrr][(CF3SO2)2N] (panel B). C-153 λfluoro max within ILs of the same cation [C4C1Im+] and different anions (panel C) and within ILs of the same anion [(CF3SO2)2N−] and different cations (panel D) along with C-153 λfluoro max within TEG and corresponding equimolar (IL + TEG) mixtures.

ε IL [N1,1,1,2OH][(CF3SO2)2N] only is higher than that for TEG). However, in contrast to the response of PRODAN, the coumarin-153 λfluoro response shows synergism within equimax molar mixtures of TEG with ILs [C4C1Im][PF6], [C4C1Im][BF4], [C4C1Im][CF3SO3], and [C4C1Pyrr][(CF3SO2)2N] (Table 2). Thus, coumarin-153 λfluoro max within IL-TEG equimolar mixtures further corroborate the fact that the “hyperpolarity” is a combined effect of cation and anion of the constituent ILs. Although the role of solute−solvent interactions may never be completely ruled out, the presence of interactions between 11267



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Any correlation with the properties of organic solvents to that of ILs in the context of synergism observed within equimolar mixtures with TEG was difficult to establish. This further highlights the inherent complexity associated with the mixtures of IL with TEG as far as the interactions are concerned.

cation and/or anion of ILs with TEG appear to be the major reason for “hyperpolarity” or the existence of synergism in the probe response within the (IL + TEG) system.

■

DISCUSSION The betaine dye response shows synergism in all but one of the (IL + TEG) equimolar mixtures with ILs possessing imidazolium cation irrespective of whether it is 1,3dialkylimidazolium or 1,2,3-trialkylimidazolium cation. The acidity of the C2 proton (appreciable acidity) or that of protons of the methyl group on C2 (relatively lower acidity) is proposed to be responsible for this. Equimolar mixtures of TEG with ILs having ammonium or piperidinium cations, respectively, do not show synergism, as these cations do not possess such acidic protons. However, the synergism exhibited by the betaine dye appears not to be a consequence of the unusually elevated dipolarity/polarizability of the medium alone albeit it is due in major part to the significantly increased HBD acidity of the medium. This is amply reflected in synergism in the α parameter shown by almost all the (IL + TEG) equimolar mixtures that exhibited synergism through ENT . On the contrary, the π* parameter is not observed to be unusually high for many of these (IL + TEG) equimolar mixtures. Due to the very weak HBA basicity associated with most of the anions of the ILs used, none of the (IL + TEG) equimolar mixtures investigated showed any synergism through its β parameter. The outcome that the contribution of dipolarity/polarizability of the mixture might be minor toward synergism exhibited by the probe is further revealed through the responses of the fluorescence dipolarity probes pyrene and PRODAN. These are the most trusted dipolarity probes perhaps because pyrene has no functionality other than the four fused benzene rings and PRODAN does not bear a charge and fulfills most of the photophysical requirements for a dipolarity probe.47 However, the solute−solvent interactions affecting the synergism exhibited through a probe response are amply demonstrated by fluorescence probes PyCHO, TNS, ANS, and coumarin-153. As the H-bonding interactions with the milieu starts to play a possible role with probes PyCHO, TNS, and ANS, the synergism is exhibited through these probes for several (IL + TEG) equimolar mixtures; PyCHO shows synergism for all TEG mixtures with imidazolium ILs, TNS shows synergism for all but one, and ANS and coumarin-153 show synergism for some. It is also important to notice that the possibility of solute−solvent interactions arise due to the fact that TNS and ANS bear negative charges, PyCHO bears an aldehyde functionality, and coumarin-153 possesses a CF3 group. The fact that neat choline IL possess relatively high static dielectric constant is amply and effectively reflected through the responses of all the solvatochromic probes used. This leads us to infer that perhaps the static dielectric constants (and the polarity that relates to it) of the equimolar mixtures of (IL + TEG) are not unusually high; the H-bonding between the IL and the TEG (i.e., the solvent−solvent interaction) and that between the probe and the (IL + TEG) mixture (i.e., the solute−solvent interaction) results in the observed synergism in a probe response. The “hyperpolarity” exhibited by the (IL + TEG) mixtures as manifested through the probe responses is largely due to the presence of extensive intercomponent Hbonding within the mixture. Such H-bonding also results in interactions with the solute probes that possess functionalities that get affected by the H-bonding present within the milieu.

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CONCLUSIONS The unusual and rare synergism in a solvatochromic probe response (the optical response of the probe in the mixture is in excess of its response in any of the components constituting the mixture) is indicative of strong interactions/restructuring present within the mixture where the interactions can be either solute−solvent or solvent−solvent or a combination of the two.48 Probe responses within (IL + TEG) mixture highlight the complexity associated with the interactions present within the media. It is clear that synergism exhibited by a probe within (IL + TEG) mixture is governed by the functionalities present on the probe. In other words, apart from manifesting obvious solvent−solvent interactions, probe responses also reflect the presence of solute−solvent interactions within these hybrid systems. It appears that probes sensitive to the HBD ability of the milieu, in general, exhibit synergism when dissolved in (IL + TEG) equimolar mixture. Specifically, ENT and α show synergism for many (IL + TEG) mixtures followed by PyCHO, ANS, and TNS fluorescence emission maxima. While ENT and α clearly manifest HBD acidity of the medium, the three fluorescence probes have functionalities that render them sensitive to the HBD ability of the (IL + TEG) milieu. All-in-all, complexity of the interactions and dynamics present within IL mixtures with TEG are amply highlighted.

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ASSOCIATED CONTENT

S Supporting Information *

Tables S1−S4 provide information regarding probe responses and estimated normalized values using eq 2 in neat solvents and in their equimolar mixture with TEG and the extent of synergism that exists within equimolar mixtures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +91-11-26596503. Fax: +91-11-26581102. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was generously funded by a grant to SP from the Council of Scientific and Industrial Research (CSIR), Government of India [grant no. 01(2767)/13/EMR-II]. R.R. also thanks CSIR for her fellowship.

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