Introducing the Bipolar Solvent Media Using the Aqueous Mixtures of

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Introducing the Bipolar Solvent Media Using Aqueous Mixtures of Amino-Acid Anion Based Ionic Liquids Vijay Beniwal, and Anil Kumar J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b08240 • Publication Date (Web): 05 Dec 2017 Downloaded from http://pubs.acs.org on December 6, 2017

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The Journal of Physical Chemistry

Introducing the Bipolar Solvent Media Using the Aqueous Mixtures of Amino-acid Anion Based Ionic Liquids Vijay Beniwal and Anil Kumar* Physical & Materials Chemistry Division, CSIR-National Chemical Laboratory, Pune 411008, India. Corresponding Author *E-mail: [email protected].

ABSTRACT To carry out a chemical reaction between the reactants with largely different polarities it becomes important to have a reaction medium which possesses both the polar and non-polar solvation environments. In an attempt to explore the reaction media with such unique polarity properties, present study provides a thorough understanding of the bipolar solvent media using the aqueous mixtures of amino-acid anion based ionic liquids. Highly polar behaviour of the binary mixtures used in the study has been ascribed to the pure ionic liquid state. However the less polar solvation shells have been attributed to the presence of neutral form of the anions. Addition of water in the amino-acid anion based ionic liquids causes the protonation of a certain fraction of the anions of the ionic liquids, resulting into the formation of less polar non-ionic protonated form along with the highly polar natural anionic form. This results into the formation of two solvation spheres with different polarities which can be seen very clearly from the

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presence of two absorption bands (LWAB and HWAB) in the UV-vis absorption spectrum of Reichardt’s ET(30) dye and two emission bands (LWEB and HWEB) in the fluorescence emission spectrum of C481 dye. The values of the ETN polarity parameter corresponding to the two solvation shells having different polarities have been calculated from the deconvoluted absorption spectra of the Reichardt’s ET(30) and were analyzed in three amino acid anion based ionic liquids. Generation of the neutral form of anions in the aqueous mixtures formed via protonation transfer reaction has been confirmed by the 1HNMR spectroscopy and UV-vis absorption spectrum of 18DHAQ dye. The study also establish that the Reichardt’s ET(30) dye can be used as a valid polarity probe to study the solvatochromic behaviour of the binary mixtures of amino-acid based ionic liquids.


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INTRODUCTION The present study is an attempt to explore the unique concept of bipolar solvents in the form of aqueous mixtures of amino-acid anion based ionic liquids. Bipolar solvents can prove to be a class of reaction media of great significance in the future owing to their ability to solubilize the reactants with different polarities. The solubility of the reactants is one of the most important parameters to be considered while choosing a solvent. It becomes particularly significant when the reactants possess largely varying polarities. Though, the application of phase transfer catalysts and the heterogeneous reaction systems has been proposed to address the solubility concerns of the reactants, it remains a challenge to solubilize the reactants having largely different polarities in a single solvent medium. In view of this, the development of bipolar solvent media can prove to be of great significance. The polarity of the solvents is determined in the form of various polarity parameters using different solvent sensitive reference molecules based on the specific and non-specific interactions provided by the solvent. The microscopic polarity of the solvents in general is determined in the form of electronic transition normalized parameter, the ETN parameter, which is obtained with the help of solvatochromic probe molecule Reichardt’s ET(30) dye.1-4 In pursuit of designing alternate solvent media with specialized characteristics, the development of ionic liquid media has seen a growing interest of the chemists in recent times for different chemical processes.5-8 Ionic liquids provide a highly specialized form of the chemical compounds which can be designed as per the requirement of the chemical or physical processes, owing to their tunable physico-chemical properties.9 Ionic liquids provide greener alternatives to the volatile organic compounds attributed to their negligible vapour pressure, non-toxic nature, high chemical and thermal stability and recyclability.10,11 Along with the pure form of these



special materials, improved solvent properties have also been reported using them in association with the molecular solvents.12 However the additions of molecular solvents are generally accompanied with drastic changes in the nature of solute-solvent interactions provided by the ionic liquids.13 The solute-solvent interactions of the ionic liquids in their pure form4,14,15 as well as in the mixed solvent state13,16-19 are studied in the form of solvatochromic polarity parameters. In the mixtures of ionic liquids with molecular solvents, some very interesting polarity phenomena have been reported for the ETN polarity parameter, of them particularly important are the synergistic polarity behaviour of the alcoholic mixtures of imidazolium cation based ionic liquids13,19-22 and negative deviations of the ETN values from the linear behaviour in the aqueous mixtures of ionic liquids13,19. Though a number of polarity studies of the ionic liquids and their binary mixtures are available in the literature, the studies related to the polarity behaviour of amino acid anion based ionic liquids have been very limited so far.23-25 Amino-acid based ionic liquids can be designed with tunable hydrophilicity and lipophilicity to solubilize a large range of chemical compounds having different polarities using different structural features of the constituent ions.26 This amphiphilic nature of these ionic liquids has been exploited to improve the solubility of drugs and drug delivery systems. The solvation behaviour of carbon nanosystems viz., fullerenes, carbon nano-tubes and graphene has also been studied theoretically and the presence of internal solvation shells made of ionic liquid ions has been confirmed inside the fullerenes.27 The present study is intended to understand the solvatochromic polarity behaviour of the aqueous mixtures of the amino acid anion based ionic liquids. It has been seen in general that the solvation shells of solvatochromic probe molecules have a unique structure in any given liquid or liquid mixture and the solvation behaviour of the solvatochromic probe molecules indicates the microscopic nature of the interactions provided by the solvent molecules. However,


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the aqueous mixtures of amino-acid anion based ionic liquids as can be seen in the present study display extremely unusual solvatochromic solvation behaviour, “the existence of dual solvation environments possessing different polarities”. An important contribution regarding this unusual solvatochromic behaviour of the amino-acid based ionic liquids was made by Harifi-Mood et. al. and it was reported that the solvatochromic polarity behaviour of the binary mixtures of aminoacid anion based ionic liquids with molecular solvents results in the generation of two absorption bands in the UV-vis spectra of Reichardt’s ET(30) dye.25 It was assumed in the study that the peculiar behaviour of the binary mixtures of amino-acid anion based ionic liquids arises from two microsphere solvation structures, one comprising only ionic liquid and other containing molecular solvent with a small amount of ionic liquid. However the authors did not made any conclusive statement regarding the origin of such a peculiar polarity behaviour, rather it was suggested that the Reichardt’s ET(30) dye is not a proper solvatochromic probe to study the polarity behaviour of amino-acid based ionic liquids. It is important to note here that the absorption spectra of the Reichardt’s ET(30) dye in the pure amino-acid based ionic liquids are in accordance with the general polarity behaviour of the dye in other ionic liquids and molecular solvents, which rules out the possibility of any chemical change associated with the dye in these ionic liquids. In such a scenario it becomes extremely important to study the origin of this unique solvatochromic behaviour of the binary mixtures of amino-acid based ionic liquids. According to the theory of two microstructures as described by Harifi-Mood et. al. the second solvation shell mostly comprises the molecular solvent having a small amount of ionic liquid. However the positioning of second absorption peak of the Reichardt’s ET(30) dye in the non-polar region in the binary mixtures of amino-acid based ionic liquids raises questions on the applicability of the theory, since the polarities of the molecular



solvents (as obtained in the pure molecular solvents) are much higher compared to the polarity values corresponding to the absorption band observed at higher wavelengths, which according to the authors arises from the solvation shell composed of molecular solvents. This disagreement regarding the positioning of the absorption peak becomes much more clear when we compare the absorption peaks in the pure water state (λmax = 454 nm) and the peak arising in the aqueous mixtures of amino-acid anion based ionic liquids (λmax = 590 to 665 nm), as observed in the present study. Such a large bathochromic shift in the absorption spectrum of the Reichardt’s ET(30) dye cannot be explained on the basis of two microsphere solvation structures theory as described above. In the present study we determine that the unique polarity behaviour of the aqueous mixtures of amino-acid anion based ionic liquids arises because of the presence of two forms of the ionic liquid anions i.e., anionic and neutral forms. In order to determine the appropriate reasoning for the unique polarity behaviour of the aqueous mixtures of amino-acid anion based ionic liquids and to establish the presence of unique bipolar solvents we have studied the solvation behaviour of aqueous mixtures of three amino acid anion based ionic liquids, viz., triethylmethylammonium phenylalanate [N2221]PhAla, tetrabutylposphonium alanate [P4444]Ala, and tetrabutylposphonium valanate [P4444]Val. The solvatochromic behaviour of all the three ionic liquids has been investigated with the help of Reichardt’s ET(30) dye. The unique polarity phenomenon in the aqueous mixtures of [N2221]PhAla has also been studied using the emission spectra of a fluorescence polarity probe, coumarin-481 (C481) dye.28-30 The structures of the ionic liquids and dye molecules used in the study are provided in Figures 1 and 2, respectively. The presence of two forms of the anions i.e., anionic and neutral forms, in the aqueous mixtures of the ionic liquids has been confirmed using 1

H-NMR and UV-vis absorption (of 1,8 dihydroxyanthraquinone, 18DHAQ dye) spectroscopies.


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Figure 1. Structures of the cations and anions of the ionic liquids (a) [N2221]+, (b) [P4444]+, (c) PhAla -, (d) Ala - and (e) Val -.

Figure 2. Structures of the dyes used in the study (a) Reichardt’s ET(30) dye, (b) C481 dye and (c) 18DHAQ dye.

METHODS Materials Triethylmethylammonium hydroxide solution (20 Wt. % in water) and tetrabutylphosphonium hydroxide solution (40 Wt. % in water) were used as obtained from M/s Sigma Aldrich. Lphenylalanine with 99 % purity, L-alanine with 99.5 % purity and L-valine with 99 % purity



were purchased from M/s Sigma Aldrich and were used as obtained. Reichardt’s ET(30) (2,6diphenyl-4-(2,4,6-triphenyl-pyridinium-1-yl)phenolate dye was obtained from M/s Sigma Aldrich

and

laser

grade

C481

(7-N,N-diethylamino-4-trifluoromethyl-1,2-benzopyrone;

coumarin-481) dye was purchased from Exciton. Both the dyes were used without further purification. The 18DHAQ dye was purchased from TCI, Japan and was purified through repeated crystallization via cyclohexane. D2O with 99.9% purity and DMSO-D6 with 99.9% purity used for the 1H NMR measurements were obtained from M/s Sigma Aldrich. Synthesis and characterization of the ionic liquids Ionic liquids were synthesized as per the procedures reported previously, via the simple acidbase neutralization reaction between the triethylmethylammonium or tetrabutylphosphonium hydroxide solutions and amino-acids.24,31 To carry out the reaction aqueous solutions of the triethylmethylammonium hydroxide and tetrabutylphosphonium hydroxide solutions were mixed with the required amino-acids in 1: 1 ratio. The reaction mixture was allowed to stir for 24 h at the room temperature. After completion of the reaction, water was evaporated using rota-vapour. The left over water was removed using the high vacuum at 60°C for 6 h. The water contents of the purified ionic liquids were measured using Karl Fischer coulometer and were found to be less than 50 ppm for all the ionic liquids. The characterization of the ionic liquids has been done using the 1H NMR spectrophotometer from Bruker India Pvt. Ltd. The 1H NMR spectra of the [P4444]Ala and [P4444]Val in DMSO-D6 as solvent at 200 MHz are provided in Figures S1 and S2 and the 1H NMR spectrum of the [N2221]PhAla is given in the Figure 6. Spectral measurements To record the UV-vis and fluorescence spectra of the dyes first the stock solutions of the dyes were prepared in dichloromethane. The stock solutions of the dyes were added drop wise to the


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ionic liquids and the dichloromethane present in the samples was removed using ultrahigh vacuum at 50°C. The aqueous solutions of the ionic liquids were prepared by the addition of the required amount of water into ionic liquids already equipped with the dyes. The amount of the dyes in the samples is taken in the range of 10-6 M and has not been considered in the mole fraction values. Hence, the xwater = 0.0 represents the pure ionic liquid state whereas xwater = 1.0 represents the pure water state, containing a very small amount of dye. The mixtures thus prepared were transferred to the cuvette to record the UV-vis and fluorescence spectra. The UVvis spectra of the dyes in the aqueous mixtures of ionic liquids were recorded using Cary-50 UVvis spectrophotometer and the steady state fluorescence spectra were measure with the help of quanta master-400 fluorescence spectrometer. The temperature of the sample shells were kept constant at 25°C using a Peltier temperature controller with an accuracy of ± 0.01°C. The samples for the UV-vis and fluorescence spectroscopic measurements were prepared under the inert atmosphere of the nitrogen gas in the glove box. Calculation of the ETN polarity parameter Calculation of the ETN parameter was done with the help of Reichardt’s ET(30) dye, Figure 2(a), a UV-vis solvatochromic probe e.3,4 The wavelength corresponding to the maximum absorption, λmax was used to calculated the energy (in kcal mol-1) required for the electronic transition (also known as ET(30) polarity parameter) of the dye from ground state to excited state via the following relation. ET(30) = 28591/λmax

(1)

The other polarity parameter, ETN parameter is the normalized form of ET(30) parameter and can be calculated from equation (2). ETN = [ET(solvent) – 30.7]/32.4


(2)


RESULTS AND DISCUSSION The bipolar solvation behaviour of the aqueous mixtures of amino acid anion based ionic liquids was first studied via the UV-vis absorption spectroscopy using the Reichardt’s ET(30) dye in the mixtures of [N2221]PhAla with varying compositions of water. The normalized UV-vis spectra obtained for the Reichardt’s ET(30) dye in different [N2221]PhAla-water mixtures are provided in Figure 3.

Figure 3. Steady state UV-vis spectra of the Reichardt’s ET(30) dye obtained in the aqueous mixtures of [N2221]PhAla, with xwater = 0.0 (▬), xwater = 0.2 (▬), xwater = 0.4 (▬), xwater = 0.6 (▬), xwater = 0.8 (▬), xwater = 0.9 (▬), and xwater = 1.0 (▬). It can be seen from Figure 3 that the UV-vis spectrum of the dye in pure [N2221]PhAla (black line) shows a regular peak with a λmax value at 504 nm. The λmax value at 504 nm suggests highly polar nature of the ionic liquid with its ETN value as high as 0.80. Highly polar nature of the amino-acid anion based ionic liquids such as [N2221]PhAla, facilitates the complete miscibility and well dispersed nature of amino-acid based ionic liquids in water (a highly polar solvent) through entire composition range.25,26 However the addition of water into the amino-acid anion


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based ionic liquids generates a special solvatochromic behaviour. As soon as water is introduced into the ionic liquid, a new peak in the higher wavelength region becomes visible. The peak gradually intensifies with the increasing fraction of water in the mixtures and at xwater = 0.6, two distinct peaks become clearly visible in the spectrum with the λmax values at 502 nm and 602 nm. These observation are compatible with the results reported earlier for the binary mixtures of two amino-acid anion based ionic liquids.25 The absorption bands corresponding to these two λmax values can be termed as the lower wavelength absorption band (LWAB) and higher wavelength absorption band (HWAB), respectively. Further addition of the water causes a decrease in the intensity of LWAB, whereas an increase in the intensity of HWAB. It is important to note here that the peak at around 602 nm represents a very less polar solvation environment around the solvatochromic probe. Given the fact that both ionic liquid and water are highly polar species and show the λmax values in their pure states at around 504 and 454 nm, respectively, such a less polar solvation environment around the probe molecule is a very unusual. The absorption peak at 602 nm cannot be described using the solvation sphere composed of molecular solvent as suggested earlier,25 since water has the λmax value at about 454 nm. The presence of second absorption bands in the UV-vis spectrum of Reichardt’s ET(30) dye in the higher wavelength region, 590-665 nm is really strange since none of the original components i.e., water and ionic liquid has the absorption maximum in the vicinity of higher wavelength absorption band. For the absorption spectrum of Reichardt’s ET(30) dye, LWAB belongs to highly polar solvation environment whereas HWAB belongs to very less polar solvation environment around the probe molecule. The presence of two absorption bands in the absorption spectra of Reichardt’s ET(30) dye indicates the existence of two solvation environments of different polarities in the aqueous mixtures of [N2221]PhAla, which is very important phenomenon to be



understood as it enables the formation a solvent having two solvation environments with different polarities, simultaneously. In order to analyse these spectra and to get clear positioning and relative intensities of the two absorption bands we have deconvoluted the spectra obtained for these mixtures. A representative deconvoluted absorption spectrum of the Reichardt’s ET(30) dye for the [N2221]PhAla-water mixture with xwater = 0.6 is provided in Figure 4 with a value of fitting parameter, R2 = 0.996.

Figure 4. Deconvolution of steady state UV-vis absorption spectra of the Reichardt’s ET(30) dye obtained in the aqueous mixtures of [N2221]PhAla at xwater = 0.6. Experimental spectrum (▬), peak 1 (▬), peak 2 (▬), reconvoluted spectrum (▬). The deconvoluted spectra for the aqueous mixtures of [N2221]PhAla at xwater = 0.6 comprises two absorption bands with absorption maxima (λmax) at 500 and 635 nm. The two absorption maxima belong to the LWAB and HWAB, respectively and indicate the presence of two solvation environments for the Reichardt’s ET(30) dye molecules with widely different polarities. The deconvoluted absorption spectra of the Reichardt’s ET(30) dye were obtained in the similar manner for all the binary aqueous mixtures of [N2221]PhAla. The absorption maxima (λmax)


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corresponding to both the LWAB and HWAB for the mixtures are provided in Table 1 along with their intensity ratios HWAB/LWAB (IHWAB/ILWAB). Table 1. Absorption maxima (λmax) and the intensity ratios, IHWAB/ILWAB corresponding to the HWEB and LWEB obtained for the aqueous mixtures of [N2221]PhAla. xwater

λmax(HWAB) in nm

λmax(LWAB) in nm

IHWAB/ILWAB

0.0

*

504

*

0.2

665

498

0.062

0.4

652

501

0.185

0.6

635

500

0.644

0.8

600

*

*

0.9

564

*

*

1.0

*

454

*

* Values could not be obtained. The deconvolution of the spectra reveals the presence of two distinct absorption bands in the aqueous mixtures of [N2221]PhAla in the range xwater = 0.2 to xwater = 0.6. It is important to mention here that the mixtures comprised of higher water fractions exhibit only one absorption band at higher wavelength, HWAB. It can be seen from Table 1 that increasing the water content in the mixtures causes a hypsochromic shift of the peak corresponding to HWAB, a standard solvatochromic behaviour observed when the polarity of the medium increases.32,33 This observation suggest that increasing the water content of ionic liquid mixtures the polarity of the medium increases and the λmax value shifts towards pure water phase, whereas the λmax value corresponding to LWAB does not show any significant change with the increasing fraction of water in the mixtures. This is quite similar to the preferential solvation behaviour of the



Reichardt’s ET(30) dye, a usual phenomenon obtained in the aqueous mixtures of ionic liquids.13,17,19 From the values of the absorption maxima, λmax corresponding to both the bands we have calculated the values of ETN parameter. The ETN values thus obtained are plotted in Figure 5 as a function of xwater.

Figure 5. Change in the ETN values for the aqueous mixtures of [N2221]PhAla corresponding to LWAB (■) and HWAB (●) as a function of xwater. Since the ETN values follow the trend of λmax value, no significant change can be observed in the value of ETN parameter for LWAB in the mixtures up to xwater = 0.6. However for HWAB the value of ETN parameter increases gradually from 0.38 for xwater = 0.2 to 0.62 for xwater = 0.9. Very minimal change in the value of ETN parameter for LWAB can be ascribed to the preferential solvation of the Reichardt’s ET(30) dye via the ionic liquid, a polarity behaviour of the dye in the aqueous mixtures of most of the ionic liquids. The very important observation to be analysed here, in the given set of mixtures is the presence of two solvation environments around Reichardt’s ET(30) dye with two different polarities. Both the parent solvents viz., water and [N2221]PhAla are highly polar in nature with the value of ETN parameter 1.00 and 0.80, respectively. In such a case the solvation environment belonging to


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very less polar nature with ETN = 0.38 is highly unlikely and is only possible as a result of certain chemical transformation in the structure of one of the components of the mixture. In this regard, it becomes important to analyze the structural features of the ionic liquids and water and the possible chemical transformations in the mixed solvent state. The structure of the ionic liquid as shown in Figure 1 consists of an anion possessing amine and carboxylate groups which are capable of providing their lone pair to a proton in the case a suitable proton donor is available.34 In the case of pure ionic liquids no such protons are available and the ionic liquids remain in their native anionic form (A). However in the presence of a protic solvent this might not be the case. The proton from the protic solvent can effectively be transferred to the anion of the ionic liquid enabling a protonated form of the anion (P), a neutral species. The possible proton transfer from the solvent (here for example, H2O) to aminoacid anion can be represented as Scheme 1. Scheme 1. Protonation of the amino-acid anion by water.

The protonated form of the anion can exist either in a neutral or a zwitterionic form as shown by P in Scheme 1.34 The protonated form of the ionic liquid anion is expected to be very less polar in nature in comparison to its native anionic form. Thus the generation of very less polar solvation shells in the aqueous mixtures of [N2221]PhAla can be understood with the help of a possible proton transfer from water to the anion of the ionic liquid. Now the simultaneous existence of two solvation shells with two different polarities in the mixtures can be ascribed to



the protonation of the certain number of phenylalanate anions, while others being in their natural anionic form. For convenience we can refer this process as the partial protonation of the anions. The anions present in their native form create a highly polar solvation shell around the probe dye with the λmax value of about 500 nm. On the other hand, the anions in protonated form gives rise to the solvation shells with significantly less polarities and bathochromically shifted HWAB. The presence of two absorption bands in these mixtures make it clear that the two type of anions create two different types of solvation shells which belong to very different polarities. These mixtures present a highly unique solvent system with two different polarities, which can be used to explore the possibilities with highly innovative methods to dissolve the substances with different polarities in a single solvent medium. In order to ascertain the above described proton transfer phenomenon in these mixtures, we have studied the UV-vis absorption spectrum of 18DHAQ in [N2221]PhAla. 18DHAQ dye possesses two hydrogen atoms in its native form (structure of the dye is given in Figure 2(c)) and displays the UV-vis absorption maximum at around 430 nm in almost all the molecular solvents and ionic liquids irrespective of their polarity.35-37 However, if the dye is dissolved into a basic medium there is a possibility of proton transfer from the dye molecule to solvent. This proton transfer might significantly alter the absorption maximum of the dye. In view of this we have recorded the UV-vis absorption spectrum of the dye in [N2221]PhAla, which has shown an absorption maximum at 500 nm. Such a large shift in the absorption maximum of 18DHAQ can only be explained by certain chemical change in the structure of dye, possibly the proton transfer reaction from the dye molecule to the solvent anion as shown in Scheme 1. To obtain further evidence for the possible proton transfer phenomenon in the water mixtures of [N2221]PhAla we have recorded the 1H-NMR spectra of the ionic liquid using its solutions in


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DMSO-D6 and D2O. Since DMSO-D6 is a non-protic solvent and there is no possibility of the protonation of the anion via solvent. However in the presence of D2O there can be a possible deuterium transfer from D2O to the anion of ionic liquid. Given the non-protic and protic nature of the DMSO-D6 and D2O respectively, the 1H NMR spectra of the ionic liquid are expected to exhibit significant changes in the chemical shift values of the anionic protons, which can be seen very clearly in Figure 6, showing the 1H NMR spectra of [N2221]PhAla in the two solvents.

Figure 6. 1H NMR spectra of the [N2221]PhAla at 200 MHz in (I) DMSO-D6 and (II) D2O. The 1H NMR spectrum of the ionic liquid is expected to be of the natural anionic form in DMSO-D6 whereas of the protonated form in D2O. Transfer of the deuterium from D2O to phenylalanate anion results in the deuterization of either of the amine group or of carboxylate anion. Since, the zwitterionic form of the amino acids is considered to be a more stable form than the neutral form, the chemical shift values of the protons in 1H NMR spectrum of [N2221]PhAla are expected to be most affected for the protons present at the amine group and its neighbouring



carbon. A significant downfield shift in the chemical shift value of the protons at positions b and f of the phenylalanate anion can be seen in the 1H NMR spectrum of [N2221]PhAla in D2O as compared to that in DMSO-D6, suggesting a decreased electron density in the anion of the ionic liquid. Thus the 1H NMR spectra of the [N2221]PhAla in DMSO-D6 and D2O helps us to understand a definitive proton transfer reaction occurring from water to the amino acid anion of the ionic liquid. It is however important to note here that a similar dual absorption band has also been reported in the UV-vis absorption spectrum of Reichardt’s ET(30) dye, when amino-acid based ionic liquids are mixed with acetonitrile.25 The presence of dual absorption spectrum in these mixtures suggests a possible proton transfer from acetonitrile to the amino-acid anion which can be attributed to the acidic nature of acetonitrile (pKa = 25).38 Acetonitrile is considered as significantly more acidic solvent in comparison to other common organic compounds having only C-H bonds such as DMSO and acetone. This difference in the acidic nature of acetonitrile can be accounted for in terms of its resonance stabilized conjugate base, [CH2CN]-.39,40 Since the earlier study by Harifi-Mood et. al. has concluded that the Reichardt’s ET(30) dye is not a good solvatochromic choice to study the polarities of the binary mixtures of amino-acid based ionic liquids,25 we have studied the solvatochromic behaviour of [N2221]PhAla-water mixtures using another solvatochromic probe molecule, the C481 dye, which is a fluorescence polarity probe30. The steady-state fluorescence emission spectra of the C481 dye in the aqueous mixtures of [N2221]PhAla using an excitation wavelength of 390 nm are depicted in Figure 7. The steady state fluorescence spectra of C481 dye, again for our surprise have shown the presence of two emission bands, pretty much in accordance with the UV-vis absorption spectra


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of the Reichardt’s ET(30) dye. A careful examination of the spectra reveals that the dye molecule in the pure ionic liquid state exhibits an emission peak at 461 nm, whereas in pure water at about

Figure 7. Steady state fluorescence spectra of the C481 dye obtained in the aqueous mixtures of [N2221]PhAla, with xwater = 0.0 (▬), xwater = 0.2 (▬), xwater = 0.4 (▬), xwater = 0.6 (▬), xwater = 0.8 (▬), xwater = 0.9 (▬), and xwater = 1.0 (▬). 447 nm. However, the addition of water in the ionic liquid causes the generation of another emission peak at around 500-540 nm. The two emission bands can be termed as lower wavelength emission band (LWEB) and higher wavelength emission band (HWEB), respectively. As the fraction of water in the mixtures increases the relative intensity of the peak at HWEB increases gradually, indicating the increasing amount of protonated form of the ionic liquid anions in the mixtures. Similar to the shift observed in the absorption maxima of Reichardt’s ET(30) dye, the increase in the amount of water in these mixtures results in a hypsochromic shift of the emission maxima, particularly visible in the mixtures with higher fraction of water, xwater = 0.8 to xwater = 1.0. Since C481 dye is a polarity probe, the presence of two emission bands can be ascribed to the presence of two solvation environments possessing



different polarities. The solvatochromic behaviour of the C481 dye in the aqueous mixtures of [N2221]PhAla ionic liquid suggest that the strange polarity behaviour of these mixtures observed using the Reichardt’s ET(30) dye is not the property of the probe molecule, instead is the nature of the mixed solvent comprising the amino-acid anion based ionic liquids, which determines the outcome of solvatochromic polarity behaviour. The fluorescence emission results of the C481 dye also specifies that there is nothing wrong in the solvatochromic behaviour recorded by the Reichardt’s ET(30) dye and the dye can be used to study the solvatochromic behaviour of the binary mixtures of the amino-acid based ionic liquids. In order to find out the possibility of bipolar solvation shells in other amino-acid anion based ionic liquids we have recorded the UVvis absorption spectra of Reichardt’s ET(30) dye in the aqueous mixtures of two more amino-acid anion based ionic liquid, [P4444]Ala and [P4444]Val. The UV-vis absorption spectra of the Reichardt’s ET(30) dye in the aqueous mixtures of [P4444]Ala and [P4444]Val are provided in the Figures 8(a) and 8(b), respectively. It can be seen from Figures 8(a) and 8(b) that the absorption spectra of both the ionic liquids in their pure form are composed of a single absorption band with the absorption maxima at 511 and 516 nm, respectively. The addition of water in the ionic liquids enables the generation of, initially a hump, than a shoulder and later a distinct absorption band in the higher wavelength region. With increasing amount of water, the relative peak intensity of the peak at HWAB increases in comparison to the LWAB. After the addition of a certain amount of water viz., xwater = 0.9 in [P4444]Ala and xwater = 0.8 in [P4444]Val only the peak corresponding to the HWAB can be observed in the absorption spectra of the Reichardt’s ET(30) dye. The peak corresponding to these mixtures describes the polarities of the neutral form of the anions of the ionic liquids mixed with water. The presence of only the higher wavelength absorption bands corresponding to very


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small ETN values, Figure 9 in these mixtures indicates that the polarity of the neutral form of the ionic liquid anions are very less compared to the anionic form.

(a)

(b)

Figure 8. Steady state UV-vis spectra of the Reichardt’s ET(30) dye obtained in the aqueous mixtures of (a) [P4444]Ala and (b) [P4444]Val, with xwater = 0.0 (▬), xwater = 0.2 (▬), xwater = 0.4 (▬), xwater = 0.6 (▬), xwater = 0.8 (▬), xwater = 0.9 (▬), and xwater = 1.0 (▬). The absorption maxima corresponding to the LWAB and HWAB of the Reichardt’s ET(30) dye and their relative intensity ratios, IHWAB/ILWAB in the aqueous mixtures of [P4444]Ala and



[P4444]Val obtained after deconvolution are provided in Table 2. The binary mixture of the [P4444]Ala at xwater = 0.8 display two absorption bands whereas other two ionic liquids display

(a)

(b)

Figure 9: Change in the ETN values for the aqueous mixtures of (a) [P4444]Ala and (b) [P4444]Val corresponding to LWAB (■) and HWAB (●) as a function of xwater. only one absorption band i.e., HWAB at xwater = 0.8. Such an observation is suggestive of the relatively higher contribution of the neutral form of the anion in [P4444]Ala ionic liquid than other two ionic liquids. This can also be seen from the comparison of relative intensity ratio of the two bands IHWAB/ILWAB from Tables 1 and 2. The aqueous mixtures of [P4444]Ala in the range of xwater


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= 0.2 to xwater = 0.6 display significantly smaller IHWAB/ILWAB values in comparison to [N2221]PhAla and [P4444]Val, indicating the smaller amount of protonated form of the anions in Table 2. Absorption maxima (λmax) and the intensity ratios, IHWAB/ILWAB corresponding to the HWEB and LWEB obtained for the aqueous mixtures of I) [P4444]Ala and II) [P4444]Val. λmax(HWAB) in nm

λmax(LWAB) in nm

IHWAB/ILWAB

0.0

*

511

*

0.2

684

500

0.034

0.4

677

509

0.131

0.6

664

509

0.399

0.8

631

495

0.953

0.9

584

*

*

1.0

*

454

*

0.0

*

516

*

0.2

671

507

0.090

0.4

665

510

0.365

0.6

660

522

0.623

0.8

609

*

*

0.9

575

*

*

1.0

*

454

*

xwater I) [P4444]Ala

II) [P4444]Val

*Values could not be obtained.



the water mixtures of [N2221]PhAla. The absorption maxima in both the ionic liquids corresponding to HWAB, shift towards the lower wavelength region with an increase in the amount of water. The hypsochromic shift of the absorption maxima in these mixtures is in accordance with the results obtained in [N2221]PhAla and follows a similar trend. However the absorption maxima related to the LWAB remain more or less similar. The hypsochromic shift of the HWAB can be described with the help of increasing polarity of the medium with increasing water fraction. On the other hand, very small change in the λmax value of LWAB can be understood via the preferential solubilisation of the Reichardt’s ET(30) dye from ionic liquids. The presence of two distinct absorption bands in the aqueous mixtures of [P4444]Ala and [P4444]Val again confirms the existence of unique bipolar solvation behavior in these mixtures. The values of the ETN polarity parameter calculated from the λmax value corresponding to two absorption bands in the aqueous mixtures of [P4444]Ala and [P4444]Val are plotted in Figures 9(a) and 9(b), respectively. The values of the ETN parameter in the mixtures changes in a similar manner to that of [N2221]PhAla. However both the [P4444]Ala and [P4444]Val were found to be comparatively less polar than the [N2221]PhAla, possibly because of the large size of the [P4444]+ cation in comparison to the [N2221]+ cation. The smaller polarity of the [P4444]Val in comparison to that of [P4444]Ala can be understood by the larger size of valanate anion. The two type of ETN value obtained in the aqueous mixtures of [P4444]Ala and [P4444]Val ionic liquids validate our finding of creating bipolar solvent media with the help of water and amino acid anion based ionic liquids.

CONCLUSIONS The present study is aimed to explore the new class of solvent media in the form of aqueous mixtures of the amino-acid anion based ionic liquids, which possesses two solvation


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environments having different polarities in a single, homogeneous phase. The amino acid ionic liquids used in the study are highly polar species in their pure form with ETN values as high as 0.76-0.80. However the addition of water (another highly polar species with ETN = 1.00) to these ionic liquids results in the formation of solvation shells with significantly reduced polarity, with ETN values as low as 0.34-0.37. The solvation shells with the significantly reduced polarities are not composed of the molecular solvents as suggested earlier, instead are composed of the neutral form of the anions of the ionic liquids. The neutral form of the anions arises via the protonation reaction of the anions of amino acid based ionic liquids. The protonation of the anions causes the diminished charge densities and hence the decreased ETN values. This protonation of the anion is assisted by the basic nature of the amine group present in the structure of the anions of these ionic liquids. Dual polar solvation behaviour of the aqueous mixtures of amino acid anion based ionic liquids have been confirmed by the UV-vis absorption spectra of the Reichardt’s ET(30) dye and the fluorescence emission spectra of the C481 dye, whereas the generation of the neutral form of the ionic liquid anion through proton transfer reaction has been confirmed by the 1H NMR spectroscopy and UV-vis absorption spectrum of 18DHAQ dye in [N2221]PhAla. The study also concludes that the Reichardt’s ET(30) dye can be used as a valid solvatochromic probe molecule to study the polarity behaviour of binary mixtures of amino-acid based ionic liquids as opposed to reported earlier. The study puts forward a strategy to create the bipolar solvent media using a simple molecular switch reaction which can generate two species having different polarities. The preparation of the novel solvents using such type of switch reactions may provide new directions to the generation of variety of homogeneous bipolar reaction media for the reactants having largely different polarities and thus might play a huge role in the future for wide range of chemical processes.



ASSOCIATED CONTENT Supporting Information. 1H NMR spectra of the [P4444]Ala and [P4444]Val recorded at 200 MHz in DMSO-d6 as solvent are provided in the supporting information. “This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author *Email: [email protected] ACKNOWLEDGMENT VB thanks CSIR, New Delhi, for awarding SPM fellowship. AK thanks Department of Science and Technology, New Delhi for the award of a JC Bose National Fellowship (SR/S2/JCB-26/2009). Authors are grateful to an anonymous reviewer for his/her invaluable suggestions. REFERENCES (1)

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Introducing the Bipolar Solvent Media Using the Aqueous Mixtures of

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