PCCP View Article Online

Published on 18 November 2013. Downloaded by Aston University on 16/01/2014 09:21:15.

PAPER

Cite this: Phys. Chem. Chem. Phys., 2014, 16, 1559

View Journal | View Issue

How polar are choline chloride-based deep eutectic solvents? Ashish Pandey, Rewa Rai, Mahi Pal and Siddharth Pandey* Developing and characterizing green solvents with low toxicity and cost is one of the most important issues in chemistry. Deep Eutectic Solvents (DESs), in this regard, have shown tremendous promise. Compared to popular organic solvents, DESs possess negligible VOCs and are non-flammable. Compared to ionic liquids, which share many characteristics but are ionic compounds and not ionic mixtures, DESs are cheaper to make, much less toxic and mostly biodegradable. An estimate of the polarity associated with DESs is essential if they are to be used as green alternatives to common organic solvents in industries and academia. As no one physical parameter can satisfactorily represent solute–solvent interactions within a medium, polarity of DESs is assessed through solvatochromic optical spectroscopic responses of several UV-vis absorbance and molecular fluorescence probes. Information on the local microenvironment (i.e., the cybotactic region) that surrounds several solvatochromic probes [betaine dye, pyrene, pyrene-1-carboxaldehyde, 1-anilino-8-naphthalene sulfonate (ANS), p-toluidinyl-6-naphthalene sulfonate (TNS), 6-propionyl-2-(dimethylaminonaphthalene) (PRODAN), coumarin-153, and Nile Red] for four common and popular DESs formed from choline chloride combined with 1,2-ethanediol, glycerol, urea, and malonic acid, respectively, in 1 : 2 molar ratios termed ethaline, glyceline, reline, and maline is obtained and used to assess the effective polarity afforded by each of these DESs. The four DESs as indicated by these probe responses are found to be fairly dipolar in nature. Absorbance probe betaine dye and fluorescence probes ANS, TNS, PRODAN, coumarin-153, and Nile Red, whose solvatochromic responses are based on photoinduced charge-transfer, imply ethaline and glyceline, DESs formed using alcohol-based H-bond donors, to be relatively more dipolar in nature as compared to reline and maline. The pyrene polarity scale, which is based on polarity-induced changes in vibronic bands, indicates reline, the DES composed of urea as the hydrogen bond donor, to be significantly more dipolar than the other three DESs. Response of pyrene-1-

Received 14th August 2013, Accepted 12th November 2013 DOI: 10.1039/c3cp53456a

carboxaldehyde, a polarity probe based on inversion of n–p* and p–p* states, hints at maline to be the most dipolar of the four DESs. The molecular structure of the H-bond donor in a DES clearly controls the dipolarity afforded by the DES. H-bonding and other specific solute–solvent interactions are found to play an important role in solvatochromic probe behavior for the four DESs. The cybotactic region of a probe dissolved in a DES

www.rsc.org/pccp

affords information on the polarity of the DES towards solutes of similar nature and functionality.

Introduction Deep eutectic solvents (DESs) aptly fulfill many requirements of alternative environmentally-benign media and are academically interesting due to their enormous potential in a variety of applications.1–6 DESs, which are also called ionic liquid analogues, are fairly easy to make (most of them just need two compounds to be mixed) – can be prepared using cheap and non-toxic materials [an example is choline (2-hydroxyethyl-trimethylammonium) chloride (this is vitamin B4) mixed with urea].7–9 DESs are documented to be environmentally-benign, and are capable of dissolving a Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi – 110016, India. E-mail: [email protected]; Fax: +91-11-26581102; Tel: +91-11-26596503

This journal is © the Owner Societies 2014

plethora of solutes.1–11 Although DESs are touted as one of the most versatile and environmentally-benign alternate media with enormous potential in a variety of applications, detailed knowledge of the dipolarity afforded by DESs is clearly lacking. Due to the fact that polarity/polarizability is the key indicator of the solvent strength, most solvents are often classified based on their ability to dissolve polar and/or charged species.12 Solvatochromic optical spectroscopic probes are well-suited to obtain this knowledge in an easy and effective manner. In this paper, we present results of our systematic approach to address the lack of physicochemical information on DESs by exploring the behavior of well-established solvent polarity probes dissolved in four common and popular DESs. The DESs used are ethaline, glyceline, maline, and reline, prepared by mixing 1 mol of choline chloride with 2 moles of a hydrogen

Phys. Chem. Chem. Phys., 2014, 16, 1559--1568 | 1559

View Article Online

PCCP

Published on 18 November 2013. Downloaded by Aston University on 16/01/2014 09:21:15.

Paper

Scheme 1

Molecular structures of the solvatochromic probes used.

bond donor – 1,2-ethanediol, glycerol, malonic acid, and urea, respectively. In order to obtain a general and broader view of the polarity/dipolarity associated with these DESs we have used several different optical spectroscopic probes – Reichardt’s betaine dye (no. 30) and one of its analogs (no. 33) as absorbance solvatochromic probes, and pyrene (Py), pyrene-1-carboxaldehyde (PyCHO), 1-anilino-8-naphthalene sulfonate (ANS), p-toluidinyl6-naphthalene sulfonate (TNS), 6-propionyl-2-(dimethylaminonaphthalene) (PRODAN), coumarin-153, and Nile Red as fluorescence solvatochromic probes. The molecular structures of all these solvatochromic probes are presented in Scheme 1. To compare the results obtained for each solvatochromic probe for the DESs, the behavior of the probe in other judiciously

1560 | Phys. Chem. Chem. Phys., 2014, 16, 1559--1568

selected solvents of varying physicochemical properties is also included. For comparison sake, responses of these probes in several common and popular conventional ionic liquids are also presented.

Experimental section Materials 2,6-Dichloro-4-(2,4,6-triphenyl-N-pyridino)phenolate (betaine dye 33) was purchased in the highest available purity from Fluka (Z99%, HPLC). Pyrene [Z99.0% (GC), puriss for fluorescence], 1-pyrenecarboxaldehyde (99%), Nile Red [Z98% (HPLC),

This journal is © the Owner Societies 2014

View Article Online

Published on 18 November 2013. Downloaded by Aston University on 16/01/2014 09:21:15.

PCCP

bioreagent suitable for fluorescence], and coumarin-153 (dye content 99%) were obtained in the highest purities from Sigma-Aldrich and were used as received. Probes 1-anilino-8naphthalene sulfonate (99%) and p-toluidinyl-6-naphthalene sulfonate (99%) were obtained in the highest purities from Acros Organics and were used as received. 6-Propionyl-2(dimethylaminonaphthalene) [Z98% (HPLC)] was obtained from Biochemika in the highest purity and used as received. All four deep eutectic solvents (DES) were purchased in highest purity from Scionix Ltd. and were stored in an inert environment before their use. Alternatively, in order to obtain ethaline, glyceline, maline, and reline, choline chloride (Z99% from Sigma-Aldrich) was mixed with 1,2-ethanediol (99.8%, anhydrous from Sigma-Aldrich), glycerol (Z99.5%, spectrophotometric grade from Sigma-Aldrich), malonic acid (99%, reagent plus from SigmaAldrich), and urea (Z99% from Sigma-Aldrich), respectively, in a molar ratio of 1 : 2 and stirred under heating (B80 1C) until a homogeneous, colorless liquid was formed. All spectroscopic measurements on DESs purchased from Scionix Ltd. and those prepared by mixing choline chloride with the corresponding H-bond donor were found to be statistically similar. Absolute ethanol was used to prepare stock solutions. Doubly distilled deionized water with Z18.0 MO cm resistivity was obtained from the Millipore Milli-Q Academic water purification system. Methods Stock solutions of all probes were prepared by dissolving in ethanol in pre-cleaned amber glass vials and storing at 4  1 1C. The required amount of probes was weighed using a MettlerToledo AB104-S balance with a precision of 0.1 mg. An appropriate amount of the probe solution from the stock was transferred to the 1 cm2 quartz cuvette. Ethanol was evaporated using a gentle stream of high purity nitrogen gas. A pre-calculated amount of the DES is directly added to the cuvette and the solution is thoroughly mixed. The solubility of a probe in a DES is checked using the linearity of the absorbance and/or the fluorescence intensity versus the concentration plot(s). A Perkin-Elmer Lambda 35 double beam spectrophotometer with variable bandwidth and a Peltier-temperature controller are used for acquisition of the UV-vis molecular absorbance data. Steady-state fluorescence spectra were acquired on a Jobin-Yvon Fluorolog-3 (model FL-3-11) modular spectrofluorometer equipped with a 450 W Xe arc lamp as the excitation source and single-grating monochromators as wavelength selection devices with a photomultiplier tube as the detector. All absorbance and fluorescence data were acquired using 1 cm2 quartz cuvettes. The refractive index of each DES was measured using a digital A. Kruss Optronic GmbH refractometer with a precision of 0.0002. The calibration of the refractometer was performed by measuring the refractive indices of distilled water and acetone at 298.15 K which was found to be consistent with the values reported in literature. All spectroscopic measurements were performed in triplicate starting from sample preparation, and the results were averaged. All spectra were duly corrected by measuring the spectral responses from suitable blanks prior to data analysis and statistical treatment.

This journal is © the Owner Societies 2014

Paper

The UV-vis absorbance and molecular fluorescence spectra were normalized to clearly depict any shifts in the band maxima on going from one DES to the other. Except for pyrene fluorescence emission spectra where the normalization was carried out with respect to band I, in all other cases the normalization was carried out with respect to the lowest energy spectral band maxima. All fluorescence probes used were found to have adequate fluorescence quantum yields for DESs under investigation.

Results and discussion Response of the absorbance probe: betaine dye It is well-established that 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate (Reichardt’s betaine dye 30) exhibits an unusually high solvatochromic band shift.12 The lowest energy intramolecular charge-transfer absorption band of betaine dye 30 is hypsochromically shifted by ca. 357 nm on going from relatively nonpolar diphenyl ether (lmax B 810 nm) to water (lmax B 453 nm). The negative solvatochromism of betaine dye 30 originates from the differential solvation of its highly polar equilibrium ground state and the less polar first Franck–Condon excited state with increasing solvent polarity. There is a considerable charge transfer from the phenolate to the pyridinium part of the zwitterionic molecule. Because of its zwitterionic nature the solvatochromic probe behavior of betaine dye 30 is strongly affected by the hydrogen-bond donating (HBD) acidity of the solvent; hydrogen-bond donating solvents stabilize the ground state more than the excited state. Betaine dye 30 is one of the most widely used probes of its kind; the empirical scale of solvent ‘polarity’, ET(30), is defined as the molar transition energy of the dye traditionally in kcal mol1 at room temperature and normal pressure according to the expression ET(30) = 28 591.5/lmax in nm. However, due to solubility restrictions of betaine dye 30 in our DESs, in the present work a derivative of this dye, 2,6-dichloro-4-(2,4,6-triphenylpyridinium1-yl)phenolate (henceforth named betaine dye 33, Scheme 1), is used in our investigation.12 The protonation and solubility restrictions of betaine dye 30 in our DESs renders it unsuitable for our investigations. Betaine dye 33, on the other hand, has no such problems due to inherent structural differences between the two betaine dyes (the diphenyl group in betaine dye 30 is replaced by dichloro in betaine dye 33). For historical reasons, it has been related to number 33, and the lowest energy absorbance transition of this dye [i.e., ET(33)] is calculated in the same way as ET(30) is calculated.12 Absorbance spectra of betaine dye 33 are collected in ethaline, glyceline, maline, and reline solutions at 30 1C (Fig. 1), the corresponding ET(33) values are converted into EN T using eqn (1) and (3): ET(30) = 0.9953(0.0287)  ET(33)  8.1132(1.6546)

(1)

R = 0.9926, standard error of estimate = 0.8320, n = 20

Phys. Chem. Chem. Phys., 2014, 16, 1559--1568 | 1561

View Article Online

Published on 18 November 2013. Downloaded by Aston University on 16/01/2014 09:21:15.

Paper

PCCP

Fig. 1 The lowest energy intramolecular charge-transfer absorption band of Reichardt’s betaine dye 33 dissolved in four DESs at 30 1C.

[To obtain ET(30) from ET(33), both were determined for 20 different solvents, and a linear regression analysis between the two afforded the above relationship]   ET ð30ÞSOLVENT  ET ð30ÞTMS N  ET ¼  (2) ET ð30ÞWATER  E T ð30ÞTMS Here, TMS stands for tetramethylsilane. From ET(30)WATER = 63.1 kcal mol1 and ET(30)TMS = 30.7 kcal mol1, we obtain ETN ¼

½ET ð30ÞSOLVENT  30:7 32:4

(3)

EN T is easier to determine as it is dimensionless and varies between 0 for TMS (extremely non-polar) and 1 for water (extremely polar). EN T values for ethaline, glyceline, and reline solutions thus obtained are listed in Table 1. We could not obtain ET(33) Table 1 UV-vis absorbance maxima (labs max) of betaine dye 33, ET(33), ET(30) and EN T for three DESs, some common ionic liquids and certain relevant organic solvents under ambient conditions. Standard deviation in EN T is r 0.005

Solvent

labs ET(30) max of betaine ET(33) dye 33 (nm) (kcal mol1) (kcal mol1) EN T

Ethaline (1,2-Ethanediol) Glyceline (Glycerol) Reline

435 (436) 430 (431) 437

65.7 (65.5) 66.4 (66.3) 65.4

57.3 (57.1) 58.0 (57.9) 57.0

0.82 (0.82) 0.84 (0.84) 0.81

[bmim][BF4]a [bmim][PF6]a [N1,4,4,4][Tf2N]a [bmim][OTf]a [bmpyrr][Tf2N]a

464 464 439 460 440

61.6 61.6 65.1 62.1 64.9

53.2 53.2 56.7 53.7 56.4

0.69 0.69 0.80 0.71 0.79

Dimethylsulfoxide Acetonitrile Ethanol Methanol Water 2,2,2-Trifluoroethanol

530 521 472 447 399 418

53.9 54.8 60.5 63.9 71.5 68.4

45.5 46.5 52.1 55.5 63.1 59.9

0.46 0.49 0.66 0.76 1.00 0.90

a [bmim][BF4]: 1-butyl-3-methylimidazolium tetrafluoroborate; [bmim][PF6]: 1-butyl-3-methylimidazolium hexafluorophosphate; [N1,4,4,4][Tf2N]: methyltributylammonium bis(trifluoromethylsulfonyl)imide; [bmim][OTf]: 1-butyl3-ethylimidazolium trifluoromethanesulfonate; [bmpyrr][Tf2N]: 1-butyl-1methylpyrrolidinium bis(trifluoromethylsulfonyl)imide.

1562 | Phys. Chem. Chem. Phys., 2014, 16, 1559--1568

for maline as the low energy intramolecular charge-transfer band in the UV-vis absorbance spectrum of betaine dye 33 dissolved in maline was hard to observe as it appeared to be buried within the high-energy more prominent band (Fig. 1). It is important to mention that our EN T values for the other three DESs are similar, if not exact, to those reported by Abbott et al.4 and by Harris.11 The minor discrepancies between the two values are attributed to the use of two different betaine dyes. A careful examination of our EN T data reveals several interesting outcomes. Dipolarity/polarizability and/or HBD ability of ethaline, glyceline, and reline as reported using betaine dye 33 are found to be fairly high – they are significantly higher than even those of short chain alcohols. Apart from water, only few fluorinated alcohols, 2,2,2-trifluoroethanol, 2,2,3,3-tetrafluoro1-propanol, and 1,1,1,3,3,3-hexafluoro-2-propanol, show higher EN T values than those observed for the three DESs. In this context, these DESs also show significantly higher EN T values than those observed for most of the common and popular pyridinium, imidazolium, and pyrrolidinium cation-based ionic liquids (Table 1). Only ethylammonium nitrate shows a higher 13,14 EN T value (B0.95) than those for the three DESs examined. According to the response of betaine dye 33 for the three DESs, glyceline appears to have the highest dipolarity/polarizability and/or the HBD ability followed by ethaline and then by reline. We tentatively attribute this to three hydrogen bond donation sites in glycerol as opposed to two in 1,2-ethanediol and urea. Indeed, the HBD acidity parameter of Kamlet and Taft (a) is higher for glycerol (0.882) than 1,2-ethanediol (0.877).11 Further, the relatively low EN T value for reline could also be due to the presence of the carbonyl group adjacent to the two –NH2 groups in urea that may decrease the H-bond donating acidity of the DES. Finally, it is interesting to note that the absorbance wavelength maxima (and the ET values) obtained from betaine dye 33 for glycerol and 1,2-ethanediol are similar to that observed for the corresponding DESs, glyceline and ethaline, respectively, hinting at the similar dipolarity/polarizability and/or HBD ability of these two DESs with their H-bond donating components. As a major part of the betaine dye response depends on the H-bonding ability of the milieu,12–14 the substantial H-bonding ability of neat glycerol and 1,2-ethanediol renders their EN T values fairly high. We believe the gain in dipolarity/ polarizability upon DES formation between choline chloride and glycerol and 1,2-ethanediol, respectively, is compensated by the loss in H-bonding ability. Responses of fluorescence dipolarity probes: pyrene and pyrene-1-carboxaldehyde Molecular fluorescence from an appropriate fluorophore is well-suited to furnish information regarding complex systems owing to the higher sensitivity and orthogonality of information inherent to fluorescence techniques.15,16 In order to assess the dipolarity of the four DESs via probe behavior, we have utilized common and valuable fluorescence polarity probes, pyrene and pyrene-1-carboxaldehyde. Pyrene (Scheme 1) is one of the most widely used neutral fluorescence probes for polarity studies.17,18 The pyrene solvent polarity scale (Py I1/I3) is defined by its I1/I3

This journal is © the Owner Societies 2014

View Article Online

PCCP

Paper

Published on 18 November 2013. Downloaded by Aston University on 16/01/2014 09:21:15.

Table 2 The ratio of band I-to-band III emission intensities of pyrene (Py I1/I3) and fluorescence emission maxima of 1-pyrenecarboxaldehyde (PyCHO lem max) in four DESs, some common ionic liquids, and certain relevant organic solvents under ambient conditions. Standard deviation in Py I1/I3 r 0.01, in PyCHO lem max r 1 nm (for pyrene, lex = 337 nm, slits: excitation/emission = 1/1 nms; for PyCHO, lex = 365 nm)

Solvent

Py I1/I3

PyCHO lem max (nm)

Ethaline (1,2-Ethanediol) Glyceline (Glycerol) Maline Reline

2.14 (1.78) 2.21 (1.70) 2.18 2.51

452 (459) 452 (462) 460 453

[bmim][BF4]a [bmim][PF6]a [N1,4,4,4][Tf2N]a [bmpyrr][Tf2N]a

2.01 1.92 1.92 2.10

433 429 425 424

Water Dimethylsulfoxide Propylene carbonate N,N-Dimethylformamide Acetonitrile Tetraethylene glycol Polyethylene glycol 400 Polyethylene glycol 200 Polyethylene glycol 600 Acetone Ethanol Butanol Cyclohexane

— 1.98 1.92 1.81 1.79 1.74 1.71 1.70 1.70 1.69 1.24 1.05 0.56

475 418 — — 417 443 — — — — 447 — —

a

[bmim][BF4]: 1-butyl-3-methylimidazolium tetrafluoroborate; [bmim][PF6]: 1-butyl-3-methylimidazolium hexafluorophosphate; [N1,4,4,4][Tf2N]: methyltributylammonium bis(trifluoromethylsulfonyl)imide; [bmpyrr][Tf2N]: 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide.

emission intensity ratio, where I1 is the intensity of the solventsensitive band arising from the S1(v = 0) - S0(v = 0) transition and I3 corresponds to the solvent-insensitive S1(v = 0) - S0(v = 1) transition.18 The I1/I3 ratio increases with increasing solvent dipolarity and is a function of both the solvent dielectric (e) and the refractive index (n) via the dielectric cross term, f (e,n2).18 Measured Py I1/I3 for ethaline, glyceline, maline, and reline, respectively, is reported in Table 2, whereas representative pyrene fluorescence emission spectra when dissolved in four DESs are shown in Fig. 2A. It is interesting to note that similar to the observation for EN T , Py I1/I3 values also imply the four DESs to be significantly dipolar in nature as compared to other common and popular solvents. As a matter of fact, Py I1/I3 values for the four DESs are found to be the highest among most common solvents including common and popular ionic liquids. The differences in Py I1/I3 between ethaline and its constituent 1,2-ethanediol and that between glyceline and its constituent glycerol are 0.36 and 0.51, respectively, indicating the enormous gain in dipolarity of the medium upon mixing 1,2-ethanediol and glycerol, respectively, with choline chloride to form corresponding DESs. Further, while common and popular ionic liquids have Py I1/I3 values higher than most common solvents, based on Py I1/I3, each of the four DESs affords even higher dipolarity than common ionic liquids (Table 2). Interestingly, in contrast to EN T , the dipolarity as reported for pyrene

This journal is © the Owner Societies 2014

Fig. 2 Normalized fluorescence emission spectra of pyrene (Py, 1 mM, lex = 337 nm; panel A) and pyrene-1-carboxaldehyde (PyCHO, 10 mM, lex = 365 nm; panel B) dissolved in four DESs under ambient conditions.

among the four DESs decrease in the order reline >> glyceline > maline > ethaline; with reline showing exceptionally high Py I1/I3. The contrast in the trend for Py I1/I3 when compared to that for EN T is attributed to the fact that while the response of the betaine dye is affected in a major way by the H-bond donating acidity of the milieu (B68% of the contribution) along with dipolarity/polarizability,12–14 it is not so with the response of pyrene. As mentioned earlier, pyrene response is affected only by the static dielectric constant (e) and the refractive index (or the high-frequency dielectric constant, n2) of the solubilizing milieu. While glycerol, malonic acid, and 1,2-ethanediol possess –OH functionalities along with one or more saturated carbons, the –NH2 groups of urea may impart higher dipolarity when combined with choline chloride in a 2 : 1 molar ratio as reflected in the pyrene polarity scale. While urea does not have any saturated carbons, the carbonyl functionality may also contribute to the very high dipolarity manifested through the Py response for reline. Behavior of another fluorescence probe pyrene-1-carboxaldehyde (PyCHO), a pyrene probe analog containing an aldehyde functionality (Scheme 1), in the four DESs is found to be different from that of pyrene. PyCHO has two types of closely-lying excited singlet states (n–p* and p–p*), both of which show emission in fluid solution. In nonpolar solvents, the emission from PyCHO is highly structured and weak (fF o 103 in hexane), arising exclusively from the n–p* state. On increasing the polarity of the medium, however, the p–p* state is brought below the n–p* state via solvent relaxation to become the emitting state, manifested by broad, moderately intense emission (fF E 0.15 in MeOH) that usually red-shifts with increasing solvent dielectric.19 Fig. 2B presents fluorescence emission spectra of PyCHO dissolved in four DESs, respectively. For comparison, emission

Phys. Chem. Chem. Phys., 2014, 16, 1559--1568 | 1563

View Article Online

Published on 18 November 2013. Downloaded by Aston University on 16/01/2014 09:21:15.

Paper

behavior of PyCHO in certain other solvents is also included. Estimated fluorescence emission maxima (lem max) for four DESs and several other solvents are reported in Table 2. A careful examination of the experimental data reveals that although the emission spectral shape and intensity of PyCHO clearly suggest the four DESs to possess substantial dipolarity, interestingly, PyCHO lem max for 1,2-ethanediol and glycerol are bathochromicallyshifted from PyCHO lem max of the corresponding DESs, ethaline and glyceline, respectively. Further, while the solvent dielectrics of the four DESs as implied by the PyCHO response are lower than that of water, they are significantly higher than those observed for common and popular imidazolium, ammonium, and pyrrolidinium cation-based ionic liquids. Among the four DESs, it is interesting to note that PyCHO lem max for maline is at the lowest energy indicating it to be the most dipolar in nature; the other three DESs appear to have similar values. The structural dissimilarities and the difference in probe mechanism of PyCHO over pyrene could be the reason for these observations. We envision that the two carboxylic acid groups present on malonic acid may interact with the aldehyde functionality of PyCHO resulting in the most bathochromically-shifted fluorescence emission band of this probe in maline. This could also explain the slightly higher PyCHO lem max for reline as compared to those for ethaline and glyceline as urea possesses a carbonyl group. Responses of charged photoinduced charge-transfer fluorescence probes: ANS and TNS Two of the most common photoinduced charge transfer (PCT) fluorescence probes are 1-anilino-8-naphthalene sulfonate (ANS, Scheme 1) and p-toluidinyl-6-naphthalene sulfonate (TNS, Scheme 1). The primary reason why these probes undergo large variations in their fluorescence responses with solvent polarity is linked to an intramolecular charge transfer process.20,21 However, specific solute–solvent interactions, change in molecular conformation, intersystem crossing to the triplet state and monophotonic photoionization might also contribute to the change in fluorescence responses of these probes with change in solvent dipolarity. It is well-established, nonetheless, that fluorescence emission of these two probes shifts bathochromically as the polarity of their cybotactic region is increased.20,21 Fluorescence emission spectra of ANS and estimated fluorescence emission maxima (lem max) for four DESs are presented in Fig. 3A and Table 3, respectively. Response of ANS in other common solvents is also included for comparison. In em accordance with EN T , Py I1/I3, and PyCHO lmax, the presence of em ANS lmax at rather lower energies (as compared to many common solvents) clearly implies relatively higher dipolarities associated with the four DESs. The ANS lem max values for four DESs are considerably bathochromically shifted from those for dimethylsulfoxide, dichloromethane, ethanol, or acetonitrile. The cybotactic region dipolarity as reported by ANS decreases in the order: ethaline > glyceline > reline > maline. However, it is interesting to note that while for ethaline ANS lem max appears at 497  1 nm and for glyceline at 493  1 nm, the corresponding H-bond donating components in the two DESs, 1,2-ethanediol and glycerol, show ANS lem max at 495  1 nm and 492  1 nm,

1564 | Phys. Chem. Chem. Phys., 2014, 16, 1559--1568

PCCP

Fig. 3 Normalized fluorescence emission spectra of 1-anilino-8naphthalenesulfonate (ANS, 10 mM, lex = 346; panel A) and p-toluidinyl6-naphthalene sulfonate (TNS, 10 mM, lex = 320 nm; panel B) in four DESs under ambient conditions.

Table 3 Fluorescence emission maxima (lem max) of ANS and TNS dissolved in four DESs, some common ionic liquids and certain relevant organic solvents under ambient conditions. Standard deviation in lem max r 1 nm (for ANS, lex = 346 nm; for TNS, lex = 320 nm)

Solvent

ANS lem max (nm)

TNS lem max (nm)

Ethaline (1,2-Ethanediol) Glyceline (Glycerol) Maline Reline

497 (495) 492 (492) 483 490

473 (459) 461 (460) 455 458

[bmim][BF4]a [bmim][PF6]a [N1,4,4,4][Tf2N]a [bmpyrr][Tf2N]a

488 488 480 486

446 447 436 441

Dimethylsulfoxide Dichloromethane Ethanol Acetonitrile

478 478 474 476

440 375 429 429

a [bmim][BF4]: 1-butyl-3-methylimidazolium tetrafluoroborate; [bmim][PF6]: 1-butyl-3-methylimidazolium hexafluorophosphate; [N1,4,4,4][Tf2N]: methyltributylammonium bis(trifluoromethylsulfonyl)imide; [bmpyrr][Tf2N]: 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide.

respectively, implying the strong similarities in the cybotactic region dipolarity between the two DESs and their corresponding H-bond donating components. We believe the higher dipolarity/polarizability of the DES formed is compensated by the strong H-bonding ability of neat 1,2-ethanediol and glycerol, and similar to the betaine dye response, this is amply manifested in the ANS fluorescence response as well. While relatively higher ANS lem max values represent much higher

This journal is © the Owner Societies 2014

View Article Online

Published on 18 November 2013. Downloaded by Aston University on 16/01/2014 09:21:15.

PCCP

dipolarities of DESs, the higher ANS lem max in 1,2-ethanediol and glycerol, respectively, is due to the strong H-bonding ability of the two. In this respect, the responses from ANS for DESs and their H-bond donating components, respectively, parallel the responses from betaine dye. In order to further substantiate this hypothesis and afford generalization, if any, to our observation, we used another popular fluorescence probe TNS. TNS, a derivative of ANS, has stark similarities with the structure of ANS and the reason for solvatochromism by the two can also be considered to be similar (Scheme 1). Fluorescence emission spectra of TNS in four DESs are presented in Fig. 3B and the estimated lem max values are listed in Table 3. It is clear that TNS lem max shows the em same trend as that observed for ANS lem max. TNS lmax also reveals ethaline to be the most dipolar followed by glyceline, reline, and maline in that order. Responses of neutral photoinduced charge-transfer fluorescence probes: PRODAN, coumarin-153, and Nile Red In contrast to ANS and TNS, 6-propionyl-2-(dimethylaminonaphthalene) (PRODAN, Scheme 1) is a neutral photoinduced charge transfer fluorescence probe. PRODAN possesses properties that render it a highly favorable fluorescence probe.22 Fluorescence emission spectra of PRODAN dissolved in four DESs are presented in Fig. 4A and the corresponding emission maxima (lem max) are reported in Table 4. A careful examination of the data reveals that PRODAN lem max has the lowest value for maline indicating this to be the least dipolar DES among the four. Reline is reported by PRODAN to be significantly more dipolar than maline; ethaline and glyceline are more dipolar

Paper Table 4 Fluorescence emission maxima (lem max) of PRODAN and coumarin153 dissolved in four DESs, some common ionic liquids and certain relevant solvents under ambient conditions. Standard deviation in lem max r 1 nm (for PRODAN, lex = 351 nm; for coumarin-153, lex = 394 nm)

Solvent

PRODAN lem max (nm)

Coumarin-153 lem max (nm)

Ethaline (1,2-Ethanediol) Glyceline (Glycerol) Maline Reline

499 (509) 501 (508) 460 489

540 (543) 540 (536) 524 532

[bmim][BF4]a [bmim][PF6]a [N1,4,4,4][Tf2N]a [bmpyrr][Tf2N]a

475 471 436 458

531 529 516 519

Dimethylsulfoxide Dichloromethane Ethanol Acetonitrile

403 440 490 455

529 499 527 517

a [bmim][BF4]: 1-butyl-3-methylimidazolium tetrafluoroborate; [bmim][PF6]: 1-butyl-3-methylimidazolium hexafluorophosphate; [N1,4,4,4][Tf2N]: methyltributylammonium bis(trifluoromethylsulfonyl)imide; [bmpyrr][Tf2N]: 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide.

than reline with glyceline showing a slightly higher dipolarity than ethaline. Clearly, according to PRODAN fluorescence response the alcohol-based choline chloride DESs are significantly more dipolar than the urea- and malonic acid-based DESs. The trend in dipolarity for the four DESs observed from PRODAN fluorescence response, in this respect, is not too different from that observed from ANS and TNS fluorescence responses, respectively. However, in contrast, the PRODAN lem max values in 1,2-ethanediol and glycerol are 7 to 10 nm higher than those in corresponding DESs, ethaline and glyceline, respectively. PRODAN response is known to get affected by the H-bonding ability of the milieu,22 and the strong H-bonding ability of 1,2-ethanediol and glycerol, respectively, appears to be manifested through PRODAN lem max values. Stokes shifts ( n A  nF, where nA and nF are the absorbance and fluorescence emission band maxima, respectively) of PRODAN in several solvents can be plotted against the orientational polarizability (Df) of the milieu according to the empirical formulation proposed originally by Lippert and Mataga:15,16 ðnA  nF Þ ¼

  2 e1 n2  1 ðmE  mG Þ2  2 þ constant (4) hc 2e þ 1 2n þ 1 a3

where  Df ¼

Fig. 4 Normalized fluorescence emission spectra of 6-propionyl-2dimethylaminonaphthalene (PRODAN, 1 mM, lem = 351 nm; panel A) and coumarin-153 (1 mM, lem = 394 nm; panel B) dissolved in four DESs under ambient conditions.

This journal is © the Owner Societies 2014

e1 n2  1  2e þ 1 2n2 þ 1

 (5)

In this expression, h is Planck’s constant, c is the speed of light, a is the radius of the cavity in which the fluorophore resides, mE and mG are the excited- and the ground-state dipole moments, respectively, for the probe PRODAN. The Lippert–Mataga equation is only an approximation with several assumptions. Although most of these assumptions can be considered valid in our case,

Phys. Chem. Chem. Phys., 2014, 16, 1559--1568 | 1565

View Article Online

Paper

PCCP

Published on 18 November 2013. Downloaded by Aston University on 16/01/2014 09:21:15.

a modified form that assumes the polarizability of the fluorophore to be the same as that of the solvent, and that mE and mG point in the different directions with a being the angle between mE and mG:16   2  2 e1 n2  1 2n þ 1 ðnA  nF Þ ¼  hc 2e þ 1 2n2 þ 1 n2 þ 2 (6)  mG2  mE2  mG mE cos a  þ constant a3 with Df 0 ¼



 2  e1 n2  1 2n þ 1  2 2e þ 1 2n þ 1 n2 þ 2

(7)

turns out to be more reasonable in its treatment of a solvatochromic probe behavior in different solvents. Due to structural similarities, most of the restrictions can be eased in our case. Fig. 5 presents Stokes shifts of PRODAN in several different solvents plotted against Df 0 of those solvents. Linear regression analysis of the data reveals reasonable linear behavior between Stokes’ shifts and Df 0 values for these types of empirical analyses [R = 0.9204, slope = 3881(522) cm1, intercept = 4087(345) cm1 and standard error of estimate = 631; Fig. 5]. From the experimentally measured labs max of PRODAN combined with lem max in Table 4, respective Stokes’ shifts were estimated for the four DESs. These Stokes’ shifts are then used to estimate Df 0 for the four DESs from the parameters recovered from linear regression analysis (Fig. 5). We next measured the refractive index (n) of the four DESs. We were not able to find refractive index values of maline and reline in literature; however, it is important to mention that our experimentallymeasured refractive index values for ethaline and glyceline are in good agreement with those reported earlier.23,24 From Df 0 and refractive index values, ‘‘apparent’’ static dielectric constants for four DESs are estimated. The e thus obtained is 32 for ethaline, 22 for glyceline, 4 for maline, and 12 for reline. It is noteworthy that e decreases in the order ethaline > glyceline > reline > maline. Further, e values for ethaline and glyceline turn out to be fairly high, it is moderate for reline, and is relatively low for maline. The e values again imply the DESs

Fig. 6 Normalized fluorescence excitation and emission spectra (10 mM; panel A) and estimated Stokes’ shifts (panel B) of Nile Red dissolved in four DESs under ambient conditions.

(at least the three with e > 10) to possess significant dipolarity. In the absence of the reported static dielectric constant for any of these DESs in literature, it was not possible for us to compare the e values estimated from solvatochromic shifts of PRODAN with those obtained using more established experimental techniques. In order to corroborate fluorescence responses from PRODAN observed for DESs, another popular fluorescence probe coumarin-153 (Scheme 1) was used next. Fluorescence emission spectra of coumarin-153 dissolved in four DESs are presented in Fig. 4B and the corresponding fluorescence emission maxima (lem max) are reported in Table 4. It is interesting

Table 5 Fluorescence emission maxima (lem max), excitation maxima (lex max) and Stokes’ shifts of Nile Red dissolved in four DESs and other relevant solvents at ambient conditions. Standard deviation in lem max and lex max r 1 nm

Nile Red

Fig. 5 The Lippert–Mataga plot of PRODAN dissolved in a number of solvents under ambient conditions (1: cyclohexane; 2: benzene; 3: triethylamine; 4: chlorobenzene; 5: chloroform; 6: acetone; 7: N,N-dimethylformamide; 8: acetonitrile; 9: propylene glycol; 10: ethanol; 11: methanol and 12: water). Stokes’ shift of the four DESs is also included.

1566 | Phys. Chem. Chem. Phys., 2014, 16, 1559--1568

Solvent

lem max (nm)

lex max (nm)

Stokes’ shift (cm1)

Ethaline (1,2-Ethanediol) Glyceline (Glycerol) Maline Reline

650 (655) 649 (650) 658 642

566 (557) 567 (578) 574 565

2283 (2690) 2228 (1916) 2224 2122

[bmim][PF6]a Dimethylsulfoxide Dichloromethane Ethanol Acetonitrile

667 640 606 640 620

580 544 523 539 520

2249 2730 2617 2907 3075

a

[bmim][PF6]: 1-butyl-3-methylimidazolium hexafluorophosphate.

This journal is © the Owner Societies 2014

View Article Online

Published on 18 November 2013. Downloaded by Aston University on 16/01/2014 09:21:15.

PCCP

to note that as reported by PRODAN, according to coumarin153 lem max maline again turns out to be the least dipolar of the four DESs followed by reline. While glyceline and ethaline are again shown to be significantly more dipolar than reline; coumarin-153 response implies these two DESs to have similar dipolarity. Nile Red (Scheme 1) is another of the common and popular solvatochromic probes that is used extensively to obtain information about various biological systems.25 The fluorescence excitation and emission spectra of Nile Red dissolved in four DESs are presented in Fig. 6A. The estimated Stokes shifts for the four DESs are shown in Fig. 6B. Nile Red fluorescence emission maxima (lem max) and fluorescence excitation maxima (lex max) along with Stokes’ shifts in four DESs and several other common and popular solvents are listed in Table 5. A careful examination of the data reveals that, among the four DESs, ex while lem max and lmax, respectively, of Nile Red have the highest value in maline, the Stokes’ shift is the largest again in ethaline ex followed by glyceline. While lem max and lmax of Nile Red do vary with overall properties of the solubilizing milieu, Stokes’ shift is the one measure that truly reflects the dipolarity of the medium.25 In this regard, ethaline again is shown by Nile Red to be the most dipolar among the four DESs; the least dipolar, however, is found to be reline.

Conclusions Polarity of a solvent is a term used commonly in the context of the capacity of a solvent for solvating dissolved charged or neutral (dipolar or apolar) species. This concept, however, is difficult to define precisely and impossible to express quantitatively. No single macroscopic physical parameter could possibly account for the multitude of solute–solvent interactions at the molecular level. Inadequacy to define solvent polarity in terms of single physical constants leads to assessment of polarity through the responses of solutes having a wide variety of functionalities. The optical spectroscopic responses of several different UV-vis absorbance and molecular fluorescence probes are found to be somewhat in agreement with the polarities of DESs ethaline, glyceline, maline, and reline. All photoinduced charge-transfer solvatochromic probes appear to be affected in a similar manner by the H-bonding ability and other solute– solvent interactions present within a DES as their responses indicate ethaline and glyceline to be more dipolar out of the four DESs. This is attributed to the presence of alcohol functionalities on 1,2-ethanediol and glycerol; the H-bond donors used to prepare ethaline and glyceline, respectively, whereas the other two H-bond donors, malonic acid and urea, possess carboxylic acid and amide-type groups, respectively. This results in appreciably higher hydrogen bonding capability of ethaline/glyceline versus reline/maline that gets reflected in the charge-transfer absorbance and fluorescence transitions of the probes. Responses of pyrene and pyrene-1-carboxaldehyde, respectively, which are not affected as much by the H-bonding ability of the milieu, hint at reline and maline, respectively,

This journal is © the Owner Societies 2014

Paper

to be the more dipolar of the DESs investigated as they possess more polar carboxylic acid and amide-type functionalities. Although the molecular architecture of the probe does control the dipolarity manifested by the solubilizing DES, the outcomes can be easily generalized for a broad class of solutes possessing similar functionalities (e.g., functionalities involved in photoinduced charge transfer). The H-bond donor used to form the DES clearly controls the probe reported dipolarity of the DES. Fair-to-significant dipolarities afforded by these choline chloride-based DESs are amply established nonetheless. The outcomes of these investigations will help open new avenues for potential applications of these DESs in chemical and material sciences.

Acknowledgements This work is generously supported by the Department of Science and Technology (DST), Government of India through a grant to Siddharth Pandey (grant number SB/S1/PC-80/2012). AP, RR, and MP would like to thank CSIR/UGC, Government of India, for their fellowships.

Notes and references 1 Q. Zhang, K. D. O. Vigier, S. Royer and F. Jerome, Chem. Soc. Rev., 2012, 41, 7108. 2 A. P. Abbott, D. Boothby, G. Capper, D. L. Davies and R. K. Rasheed, J. Am. Chem. Soc., 2004, 126, 9142. 3 A. P. Abbott, P. M. Cullis, M. J. Gibson, R. C. Harris and E. Raven, Green Chem., 2007, 9, 868. 4 A. P. Abbott, R. C. Harris, K. S. Ryder, C. D’Agostino, L. F. Gladden and M. D. Mantle, Green Chem., 2011, 13, 82. 5 T. Gorke, F. Srienc and R. J. Kazlauskas, Chem. Commun., 2008, 1235. 6 K. D. Weaver, H. J. Kim, J. Sun, D. R. MacFarlane and G. D. Elliott, Green Chem., 2010, 12, 507. 7 W. G. McGavock, J. M. Bryant and W. W. Wendlandt, Science, 1956, 123, 897. 8 P. S. Gentile and L. H. Talley, J. Am. Chem. Soc., 1957, 79, 4296. 9 A. P. Abbott, G. Capper, D. L. Davies, R. K. Rasheed and V. Tambyrajah, Chem. Commun., 2003, 70. 10 A. P. Abbott, G. Frisch, J. Hartley and K. S. Ryder, Green Chem., 2011, 13, 471. 11 R. M. Harris, Physical Properties of Alcohol Based Deep Eutectic Solvents (Doctoral Thesis), University of Leicester, 2008. 12 C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, Wiley-VCH, Weinheim, Germany, 2003; C. Reichardt ¨rnert, Liebigs Ann. Chem., 1983, 721. and E. Harbusch-Go 13 S. K. Poole, P. H. Shetty and C. F. Poole, Anal. Chim. Acta, 1989, 218, 241. 14 C. Reichardt, Green Chem., 2005, 7, 339. 15 P. Suppan and N. Ghoneim, Solvatochromism, RSC Publishing, Cambridge, 1997.

Phys. Chem. Chem. Phys., 2014, 16, 1559--1568 | 1567

View Article Online

Paper

21 E. M. Kosower, H. Dodiuk, K. Tanizawa, M. Ottolenghi and N. Orbach, J. Am. Chem. Soc., 1975, 97, 2167. 22 S. N. Baker, G. A. Baker, M. A. Kane and F. V. Bright, J. Phys. Chem. B, 2001, 105, 9663. 23 R. B. Leron, A. N. Soriano and M.-H. Li, J. Taiwan Inst. Chem. Eng., 2012, 43, 551. 24 K. Shahbaz, F. S. Ghareh Bagh, F. S. Mjalli, I. M. AlNashef and M. A. Hashim, Fluid Phase Equilib., 2013, 354, 304. 25 J. F. Deye, T. A. Berger and A. G. Anderson, Anal. Chem., 1990, 62, 615.

Published on 18 November 2013. Downloaded by Aston University on 16/01/2014 09:21:15.

16 B. Valeur and M. N. Berberan-Santos, Molecular Fluorescence Principles and applications, Wiley-VCH, Weinheim, Germany, 2nd edn, 2012. 17 K. W. Street Jr. and W. E. Acree Jr., Analyst, 1986, 111, 1197. 18 D. S. Karpovich and G. J. Blanchard, J. Phys. Chem., 1995, 99, 3951. 19 R. Warris, W. E. Acree Jr. and K. W. Street Jr., Analyst, 1988, 113, 1465. 20 L. Stryer, Science, 1968, 162, 526.

PCCP

1568 | Phys. Chem. Chem. Phys., 2014, 16, 1559--1568

This journal is © the Owner Societies 2014

How polar are choline chloride-based deep eutectic solvents?

Developing and characterizing green solvents with low toxicity and cost is one of the most important issues in chemistry. Deep Eutectic Solvents (DESs...
2MB Sizes 0 Downloads 0 Views