Structure and Reactivity of the Ionic Liquid 1-Allyl-3

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B: Liquids, Chemical and Dynamical Processes in Solution, Spectroscopy in Solution

Structure and Reactivity of the Ionic Liquid 1-Allyl-3Methylimidazolium Iodide Under High Pressure Luiz F. O. Faria, Marcelo Medre Nobrega, Naomi Falsini, Samuele Fanetti, Marcia L.A. Temperini, Roberto Bini, and Mauro C. C. Ribeiro J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b10669 • Publication Date (Web): 07 Feb 2019 Downloaded from http://pubs.acs.org on February 7, 2019

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The Journal of Physical Chemistry

Structure and Reactivity of the Ionic Liquid 1-allyl-3methylimidazolium Iodide under High Pressure Luiz F. O. Fariaa*, Marcelo M. Nobregaa,b, Naomi Falsinib, Samuele Fanettib,c, Marcia L. A. Temperinia, Roberto Binib,c, Mauro C. C. Ribeiroa*

a

Laboratório de Espectroscopia Molecular, Departamento de Química

Fundamental, Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes 748, 05508-000, Brazil b

LENS, European Laboratory for Nonlinear Spectroscopy, Via Nello Carrara 1,

50019 Sesto Fiorentino (FI), Italy c

Dipartimento di Chimica“Ugo Schiff”dell’Università degli Studi di Firenze, Via

della Lastruccia 3, 50019 Sesto Fiorentino (FI), Italy

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

*

E-mail

address:

[email protected]

(L.F.O.F.);

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[email protected]

(M.C.C.R.)

ABSTRACT

Poly(ionic liquid)s are an interesting class of compounds due to their unique chemical and physical properties gathering the characteristics of ionic liquids and polymers. Pressure and temperature have been demonstrated to be an alternative method to obtain polymers from monomeric species using only physical tools. In this work we investigate the reaction under high pressure and room temperature of the ionic liquid 1-allyl-3-methylimidazolium iodide by using the


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diamond anvil cell technique in combination with synchrotron X-ray diffraction and electronic and vibrational spectroscopies. The results indicate a chemical reaction happening through the terminal double bond of the allyl group both in crystalline and glassy phases with the onset of the reaction around ~ 7 GPa. Vibrational spectra present evidence for an oligomerization reaction in both phases. The reaction occurring both in glassy and crystal phases indicate a mechanism not driven by collective motions and likely related to local topological arrangements. The results presented herein extend our understanding of ionic liquid instability boundaries under high pressure and contribute to the development of alternative synthetic routes to achieve poly(ionic liquids).

I. INTRODUCTION

Polymeric or polymerized ionic liquids, so-called poly(ionic liquid)s, are a new family of functional polymers which are receiving a growing attention because of their unique chemical and physical properties related to the retention of the ionic liquid properties combined with those intrinsic to



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polymers.1-2 Some advantages of poly(ionic liquid)s related to conventional ionic liquids

are

the

enhanced

mechanical

stability,

durability,

improved

processability, and designed architecture.1-2 The major potential application of poly(ionic liquid)s is on electrochemical devices as conductive polymeric solid electrolytes3 and as a multifunctional material, for example, dispersant to stabilize nanomaterials and sorbent for gases.4-5 In particular, the use of poly(ionic liquid)s in solar cell devices is a very prominent research field.6-7 Two strategies are used to synthetize poly(ionic liquid)s: direct polymerization of ionic liquid monomers or chemical modification of some conventional polymer.12

The first strategy is the most common using free radical polymerization of ionic

liquid monomers. The drawback of this method is the necessity to use a reaction initiator and the undesired reaction products and impurities formed during the polymerization reaction.1-2 An alternative method to obtain polymers from monomeric species is based only on physical tools as pressure, temperature and photo-irradiation.8-11 The polymerization induced by pressure of molecules containing unsaturated bonds was observed in many molecular compounds, for example, ethylene, 4 ACS Paragon Plus Environment

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acetylene, and isoprene.12-15 The appeal of this alternative polymerization method is that it is not necessary to use a solvent or reaction initiator, which makes it a “green” method. Moreover, the reaction product might be obtained with a highly ordered structure without or with fewer impurities due to the high selectivity of the reaction through distinct topochemical paths. Recently, carbon nanothreads were derived from crystalline benzene,16 aniline17 and pyridine18 by application of high pressures and eventually temperature. These one dimensional materials present high degree of structural organization and very distinct features, e.g. they are probably the thinnest materials with highest hardness known until now,19 prompting they to many applications.

Recently, we studied the instability under high pressure of an ionic liquid having the dicyanamide anion as counter ion, [allylmim][N(CN)2].20 Raman and IR spectroscopies supported by quantum chemical calculations were used to probe the reaction product formed from glassy phase above approximately 8 GPa.20 A polymerization reaction was inferred to happen through the terminal double bond of the cation allyl group giving rise to an oligomeric product.20 In this work, we report the instability and reaction under high pressure at room temperature of an ionic liquid based on the same 1-allyl-3-methylimidazolium cation with iodide as counter ion, [allylmim]I (see molecular structure on Figure 1). The aim is to extend the understanding of the reaction occurring under high pressure in [allylmim]+ based ionic liquids. Differently 5 ACS Paragon Plus Environment


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from [allylmim][N(CN)2], that always becomes a glass with compression at room temperature, the [allylmim]I may also crystallize. Also, it was recently showed that iodide based ionic liquid could present polyiodide species even at room conditions.2122

Herein, we have studied the phase transitions and reactivity of [allylmim]I up to ~

15 GPa and room temperature conditions by using the diamond anvil cell (DAC) technique in combination with synchrotron X-ray diffraction and vibrational and electronic spectroscopies. DFT calculations were also used to compute the structure and the vibrational and electronic properties.

H2C

N

N

CH3

Figure 1. Molecular structure of 1-allyl-3-methylimidazolium cation, [allylmim]+.

II. METHODS

The [allylmim]I sample (> 98% purity) was purchased from Iolitec and the liquid was dried under high vacuum (below 10−5 mbar) for 24 h prior the measurements. The [allylmim]I is a solid sample at room conditions that


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promptly melts over handling retaining the metastable liquid state. It is noteworthy that all experiments performed in this work started with the metastable liquid sample and always keeping the condition of constant room temperature. X-ray diffraction (XRD) measurements were performed as a function of pressure at the Brazilian Synchrotron Light Laboratory (LNLS) using a homemade DAC1 having a diamond culet size of 600 μm. The MicroElectrical Discharge Machine driller (Hylozoic Products) was used to drill a 260 μm hole in a 250 μm thick stainless steel gasket preindented to ∼95 μm. The beam energy was set to 20 keV for a wavelength of λ = 0.6199 Å using a sagittal Si(111) monochromator, and the beam size on horizontal and vertical directions was 200 and 150 μm, respectively. The CCD detector used was a RAYONYX SXS165. The two-dimensional images obtained were integrated to provide intensity

as

a

scattering

angle

function (θ)

in

of degrees

2θ

using was

the

converted

software to

FIT2D.23

wavevector

The

(Q)

in

reciprocal angstroms (Å-1) using Q = (4π /λ)·sinθ. FTIR experiments were performed by using a MDAC2 (membrane diamond anvil cell) equipped with a stainless steel gasket with a 150 μm hole. In order to reduce the strong sample absorption the optical path was reduced by pressing KBr in the sample chamber producing a pellet whose surface was successively scratched. Afterwards, the ionic liquid (sample thickness of about 15 m) and a ruby chip were added above the pellet. FTIR absorption



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measurements were recorded with an instrumental resolution of 1 cm-1 using a Bruker-IFS 120 HR spectrometer modified for high-pressure measurements.24 Raman spectra were recorded with a Jobin-Yvon T64000 triple monochromator spectrometer having a coupled Olympus BX41microscope equipped with CCD. Raman spectra were excited with the 647.1 nm line of a mixed argon-krypton laser. The spectra were obtained in the 180° scattering geometry with no polarization selection of the scattered radiation. Spectral resolution was kept at 2.0 cm-1. High pressure at room temperature was achieved with a DAC3 from EasyLab Technologies Ltd., model Diacell LeverDAC Maxi, having a diamond culet size of 500 μm. The Boehler microDriller (EasyLab) was used to drill a 200 μm hole in a 250 μm thick stainless steel gasket preindented to ∼150 μm. The near-UV−visible spectra were measured with a spectrometer expressely designed for UV−Vis absorption measurements in the MDAC2.25 We deposited the ionic liquid sample on a previously scratched KBr pellet as for FTIR experiments. The source was a xenon lamp (Hamamatsu L10725) attenuated by UV-fused silica neutral density filters with varying optical densities and filtered by a short-pass filter with a cutoff wavelength at 500 nm to reject light out of the desired spectral range. The light was focused onto the sample and collected by a couple of Al coated 90° off- axis parabolic mirrors (with a reflected focal lens of 50.8 mm), and then the light was focused by Al coated 8 ACS Paragon Plus Environment

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90° off-axis parabolic mirrors into a 1/8m monochromator (Newport 77250), with an F/number = F/3.7. The light was dispersed by a ruled grating (Newport 77304) with 600 lines/mm blazed at 200 nm and collected on a CCD (Hamamatsu S9971− 1006UV). The resulting spectral resolution was 3 nm. A ruby chip was placed in the DAC sample chamber for in situ pressure calibration in all experiments and no pressure transmitting medium besides KBr was used. Pressure has been obtained by the ruby fluorescence method using as source of excitation few mWs of a laser radiation at 647 or 532 nm .26-27 Quantum chemistry calculations were performed using the Gaussian09 package28 to obtain optimized [allylmim]I ionic pair structure and the characteristic vibrational frequencies. Density functional theory (DFT) using the Becke’s three-parameter hybrid exchange functional and Lee−Yang−Parr correlation functional (B3LYP) with 6-311++G(d,p) basis set for C, N and H atoms of [allylmim]+ cation and LanL2DZ base set for iodide anion were used. No imaginary vibrational frequencies were obtained indicating that the vacuum geometry was at the minimum of the potential surface. The theoretical absorption UV-Vis spectrum was calculated using the TD-DFT protocol.29



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III. RESULTS AND DISCUSSION

Synchrotron X-ray diffraction

The [allylmim]I has a phase behavior depending on the compression rate as previously verified for other imidazolium based ionic liquids.30-31 Recently, it was showed that other factors as the size of DAC sample chamber and the roughness of the chamber walls formed during the gasket drill process might also influence the glass transition or the crystallization of the ionic liquid.30 In the case of [allylmim]I, as the experiments started from the metastable liquid, the presence of crystalline nuclei might also contribute to observation of crystallization or glass transition under compression. In Figures 2 e 3 are inserted the XRD pattern obtained in distinct experiments with the formation of a glassy or crystalline phases of [allylmim]I, respectively, during compression and decompression processes. The glass formation on compressing the sample is inferable by the absence of Bragg peaks in the XRD patterns (Figure 2) which are solely composed by the typical broad peak related to the pair distribution functions of


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amorphous samples, i.e. liquid and glass. Many studies have been performed to investigate the liquid structure and X-ray scattering features of ionic liquids.32-34 The broad peak with maximum wavevector, QMAX, at ~ 1.65 Å-1 observed in XRD pattern of the sample at room conditions is assigned to the cation-anion distances.32-34 It is observed that the peak becomes broader and shifts to higher wavevectors (shorter distances) during the compression indicating the reduction of the cation-anion distances. This peak was fitted with a Voigt function for different pressures and the QMAX as a function of pressure is also showed in Figure 2. We can observe that the peak profile and the initial QMAX value are not recovered after the compression-decompression cycle of sample. The broader peak and longer average cation-anion distances (lower QMAX) of recovered sample is an evidence of some structural change happening under high pressure.



0.6 GPa 1.3 GPa

13.5 GPa 10.4 GPa

Compression Decompression

2.0 1.9

1.3 GPa

1.8

Intensity

8.0 GPa

2.1

QMAX / Å-1

5.1 GPa

Pressure variation

2.5 GPa

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.7

7.5 GPa

1.5

1.5 0

2

1.1 GPa

1.5

1.8 Q / Å-1

2.1

13.5 GPa

1.6

3.0 GPa

1.2

experimental fitted

4

6

1.8

8

2.1 Q / Å-1

10

2.4

2.7

12

14

P / GPa

2.4

Figure 2. (Left) XRD patterns of [allylmim]I during compression up to 13.5 GPa leading to glass formation, followed by decompression down to 1.1 GPa. The blue arrow indicates the peak shift to higher Q values during compression. (Right) The maximum wavevector, QMAX, during compression and decompression processes is showed as a function of pressure. As inset is also presented the experimental data and the fitted curves using a Voigt function for two different pressures.

The crystallization of [allylmim]I has been observed in the compression of another sample. Here, Bragg peaks of the crystalline form appeared at 1.1 GPa in the XRD patterns during the compression as reported in Figure 3. The XRD pattern showed small changes after 2.2 GPa up to 8.4 GPa. Major changes in peak intensities and some new peaks are observed at 10.5 GPa. It might be an evidence of structural changes happening with compression. It is important to mentioning that data were not of sufficient quality for structure refinement. The Bragg peaks become broader at higher 12 ACS Paragon Plus Environment

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pressures, but their persistence indicates the lack of amorphization which has been previously reported for other ionic liquid.30 During sample decompression, the XRD pattern is nearly the same until 4.1 GPa. Below ~ 2 GPa, some Bragg peaks disappear and others become sharper, being the complete melting observed at ~ 0.5 GPa. It is noteworthy that the initial XRD pattern of the crystal is not recovered on decompression. This fact is also in contrast with the previous work that showed the recovering of XRD pattern of crystal on decompression after complete crystal amorphization.30 Based solely on XRD analysis we could not infer about any chemical change happening on crystal phase of [allylmim]I after compression-decompression cycle.

2.2 GPa

14.4 GPa

4.5 GPa

12.3 GPa

6.1 GPa

1

2

3 Q/Å

Intensity

*

8.7 GPa

6.9 GPa

6.4 GPa

8.41 GPa

4.1 GPa

10.5 GPa

1.9 GPa

14.7 GPa

0.7 GPa

4

1

-1

2 Q/Å

-1

3

4

Figure 3. XRD patterns of [allylmim]I during compression up to 14.7 GPa (left panel) and sequential decompression down to 0.7 GPa. The asterisk in XRD pattern at 2.2 GPa 13 ACS Paragon Plus Environment

Pressure variation

14.7 GPa

1.1 GPa

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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indicates a broad peak around ~ 3.1 A-1 that is due to steel gasket scattering being this region disregarded in the analysis.

Morphological analysis

Contrary to the formation of the glassy phase which cannot be visually detected, the crystallization and the following morphological changes induced by pressure are visually observable by direct inspection of the sample inside the DAC3. In Figure 4 photographs of the DAC3 sample chamber taken at different pressures in other experiment are shown: the liquid sample at 0.1 MPa, the crystallization process at ~1.2 GPa and crystal compression-decompression cycle until the recovered sample. The crystal growth starts from the gasket as observed in the first photograph obtained at ~1.2 GPa. The complete crystallization of the sample takes less than one minute (second photograph). The crystal aspect starts to change at ~7.0 GPa with the sample becoming more opaque with compression. On decompression, the initial crystal or liquid aspect is not recovered at atmospheric pressure. Interestingly, the XRD experiments indicate the melting at ~0.5 GPa, but in the photographs the liquid morphology is not recovered at room conditions. This is an important indication that chemical changes happen when the crystal [allylmim]I is submitted to high pressures.


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Liquid 0.1 MPa

Crystallization 1.2 GPa

7.0 GPa

9.5 GPa

2.0 GPa

4.1 GPa

1.2 GPa

Recovered sample 0.1 MPa

Figure 4. DAC3 sample chamber photographs showing the crystallization process at 1.2 GPa (upper) and the morphologic changes during the compression (middle) and decompression (bottom) until the recovered sample at room conditions.

Ruby luminescence bandwidth

The ruby luminescence bandwidth, Γ(P), can act as a microscopic probe of the stress in the sample, probing differences in the local environment due to phase transitions and structural modifications. The method of measuring Γ(P) is commonly used to determine the glass transition pressure, Pg, of the samples in the DAC.35-36 In a previous work we also used Γ(P) to probe the chemical instability of [allylmim][N(CN)2].20 In this work, the Γ(P) was used to determine the glass transition



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pressure (Pg) and to estimate the instability boundary (Pr) in which the reaction starts in both glass and crystal phases of [allylmim]I. Figure 5 presents Γ(P) related to atmospheric pressure value, Γ(0), of [allylmim]I during compression-decompression cycle for glassy and crystal phases. The Pg is identified at ~ 1.5 GPa as indicated by the change of slope of Γ(P). This value of Pg is in the same pressure range reported for other ionic liquids.37 In [allylmim][N(CN)2] the reaction onset was shown to coincide with a sudden change and oscillation of Γ(P).20 Here, a sudden change and oscillations of Γ(P) started after ~ 4 GPa also suggesting that a reaction occurs. As we will discuss in the following using FTIR results, the reaction effectively starts around ~ 6.6 GPa. Figure 5 also shows the Γ(P) for crystal phase until 15 GPa. The crystallization pressure (Pc) is not identified by the Γ(P) evolution and a further increase in pressure shows a small discontinuity in the Γ(P) evolution at ~ 3 GPa. It is interesting that after this pressure value the XRD pattern does not present major changes in peak intensities and positions as discussed in Figure 3. XRD data are also in line with Raman spectroscopy results, one of the most used techniques to probe these phase transitions,38 that not show any indication of a solid-solid phase transition as later discussed. It is worth noting the large values of Γ(P) for the crystalline phase compared to the glassy one, probably due to the highly anisotropic local environment of the crystal phase that causes larger stress on the ruby probe. Differently from the glassy phase, it is not observed an oscillation in Γ(P) values after the supposed instability boundary. The Γ(P) shows a discontinuity around ~ 8 GPa, close to the pressure in which morphological changes were visually detected, suggesting the onset of sample instability. Meanwhile, in both cases the initial value of Γ(P) is not recovered after the compressiondecompression cycle.


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Pg

0.2

(P) - (0) / cm-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pr

Pc

Pr

1.5

0.1

1.0

0.0 0.5 compression decompression

-0.1

0

2

4

6

8

10

0.0 12

0

P / GPa

2

4

6

8 10 12 14 16

P / GPa

Figure 5. Pressure dependence of the ruby luminescence linewidth, Γ(P), with respect to the atmospheric pressure value, Γ(0), for the sample showing the crystallization (right) or glass transition (left) events. The yellow areas indicate the pressure range in which the crystallization (right, Pc) and the glass transition (left, Pg) occur. Blue areas are indicating the pressure range where the chemical reaction (Pr) takes place as derived by different techniques.

Electronic and vibrational properties

The X-ray diffraction and ruby luminescence results as well as direct sample observation provided some hints about the phase transitions and instability boundaries under high pressure and room temperature of [allylmim]I. Electronic and vibrational spectroscopies were also used to probe the [allylmim]I sample during compressiondecompression cycles. Unfortunately, the optical path and the sample thickness needed to be reduced, by employing KBr substrates, in order to obtain the UVVis and FTIR spectra and after several attempts we could not generate a crystal phase under these experimental conditions, as sample size and container boundaries are 17 ACS Paragon Plus Environment


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important factors for crystallization as mentioned before. On the other hand, the high fluorescence detected for [allylmim]I in liquid phase is reduced in crystal phase and we could perform Raman spectroscopy experiments to investigate structural changes on the crystalline phase. In the following the UV-Vis and FTIR spectra for glassy phase and Raman spectra for crystalline phase of [allylmim]I are presented and discussed. The experimental UV-Vis spectrum at room conditions and the calculated spectrum by TD-DFT for an ionic pair of [allylmim]I are reported in Figure 6. The calculated HOMO-LUMO orbitals, that indicate an anion-cation charge transfer (CT) at 381 nm close to the experimental value at max ~ 375 nm, are also showed in the inset. In fact, it was reported the CT transition for [C4C1im]I ([C4C1im]+ = 1-butyl-3methylimidazolium) at ~378 nm and close to this value for different ionic liquid solutions in dichloromethane and acetonitrile having I- anion.39-40 Other two electronic transitions with small oscillator strength are expected at higher  in the calculated spectrum of ionic pair, which justifies the asymmetric shape of the experimental UVVis spectrum with a long tail until ~ 500 nm. This result explains the brown color of [allylmim]I sample. Figure 6 also presents the UV-Vis spectra of [allylmim]I as a function of pressure as well as the absorption maximum (max) dependence on pressure. During the compression, a red shift is observed up to ~ 4 GPa, and then, further compression leads to a blue shift. This pressure range corresponds to the region where the ruby luminescence bandwidth starts changing in a random way thus strengthening the possibility that these changes are effectively related to structural changes due to a chemical transformation occurring after 4 GPa. No max changes are evident around Pg since the liquid and glassy phases are structurally similar not producing any implication in electronic transitions. 18 ACS Paragon Plus Environment

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Absorbance / arb. unit.

4

3

 = 381 nm

2

Experimental Calculated 1

0

320

360

400  / nm

440

480

392 388 384

Absorbance / arb. units

max  nm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


380 376 372 368 0

2

4

3

0.1 GPa 0.8 GPa 1.5 GPa 4.0 GPa 6.3 GPa 11.5 GPa

2 1 0

320 360 400 440 480  / nm

6

P / GPa

8

10

12

Figure 6. (Upper) Comparison between the experimental UV-Vis spectrum of [allylmim]I measured at room conditions and the TD-DFT calculated spectrum for the optimized ion pair. The calculated spectrum was plotted using gaussian functions having a peak width at half height of 0.333 eV for the oscillator strength associated to each transition (red lines). As inset is illustrated the HOMO-LUMO orbitals responsible for the most intense electronic transition band at 381 nm (experimental value at ~ 375 nm). (Bottom) UV-Vis spectra as function of pressure up to 11.5 GPa (inset) and pressure evolution of the absorption maximum (max).



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Comparison of selected spectral regions of the FTIR spectra of [allylmim]I acquired before and after a compression-decompression cycle up to ~ 11 GPa are showed in Figure 7. It should be recalled that in these experiments we always obtained the glassy phase. The spectra present subtle differences ascribable to structural changes due to some incomplete chemical reaction. The consistent reduction of the peak assigned to the (C=C) stretching mode at ~1650 cm-1 is suggestive of a reaction involving the double bond of cation allyl group with the formation of an oligomeric product. This hypothesis is also supported by the appearance in the recovered sample spectrum of new bands indicated by green arrows in Figure 7. Particularly interesting is the broad band forming at ~ 1460 cm-1 where the CH2 scissoring modes are expected.41 Moreover, the high frequency region containing the (C-H) modes also supports a reaction occurring through the allyl group since a consistent intensity reduction is observed for the band related to this group. The emergence of new bands below 3000 cm-1, characteristic of C-H stretching modes of saturated carbon atoms, agree with the linkage of oligomeric products through the alkyl chain. The intensity reduction of the (C-H) band assigned to the most acidic hydrogen between nitrogen atoms on imidazolium ring (C2), might be related to a change in anion-imidazolium ring configuration after the chemical reaction involving the allyl group, which could also explain the change in the electronic spectrum.


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1.5

Absorbance

Initial Recovered 1.0

(C=C) 0.5

0.0 1400

1500 1600 wavenumber / cm-1

1700

1800

1.5

as(C-H)allyl

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(C-H)Im-C2

1.0

0.5

0.0 2700

2800

2900 3000 3100 wavenumber / cm-1

3200

3300

Figure 7. Comparison of the FTIR spectra of [allylmIm]I measured in the 1360-1800 cm-1 (upper) and 2700-3300 cm-1 (bottom) spectral regions at ~ 0.5 GPa during compression (black curve) and decompression (red curve) after a compressiondecompression cycle of up to ~ 11 GPa. The black and green arrows indicate, respectively, characteristic IR bands that had a decreased intensity and the regions with a change of the spectral pattern. Vibrational frequencies and relative intensities calculated for ν(C-H) modes of [allylmim]+ in the optimized ion pair structure are indicated by green and blue bars for ν(C-H) modes assigned to allyl group and other ν(C-H) modes, respectively. A scaling factor of 0.95 was used for theoretical frequencies. 21 ACS Paragon Plus Environment


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In Figures 8 and 9 the Raman spectra acquired in different spectral regions both for liquid and crystalline [allylmIm]I during compression-decompression cycle up to ~ 11 GPa are shown. The fluorescence reduction after sample crystallization can be readily observed from the spectra reported in Figure 8. The low frequency spectral region containing the intermolecular vibrational modes has been used to study the phase transitions of ionic liquids.38, 42-43 It is clear the distinction of Raman spectra in this low frequency region for crystals and liquid or amorphous phases.38, 42-43 The inset in Figure 8 upper panel presents the liquid spectrum at 0.5 GPa characterized by the quasi-elastic scattering due to the translation and rotation relaxation dynamics of ions. After sample crystallization, new bands characteristic of lattice vibrational modes appeared in the crystal Raman spectrum. After compression-decompression most of the peaks related to the lattice modes disappeared but two broad bands still survive in full agreement with the XRD results discussed previously: the reaction produces a crystalline product or partly destroys the lattice framework. Interestingly, the band surviving in the recovered sample at ~110 cm-1 is assigned to the imidazolium ring librational mode

in

imidazolium based ionic liquids,38 thus qualitatively supporting the missed participation of this cation moiety to the reaction. Insight about the chemical changes is gained through the analysis of the medium and high frequency spectrum regions. The imidazolium ring deformation band (breathing) at ~ 1021 cm-1 38 becomes sharper in the crystal phase and, after the reaction under high pressure, the band becomes broader according to the increased degree of disorder (breakage of the translational symmetry). Despite the intensity reduction of the whole spectrum, the intensity of (C=C) band at ~1650 cm-1 is even more reduced confirming the reaction occurring through the allyl group. The analysis of (C-H) 22 ACS Paragon Plus Environment

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modes in Figure 9 also supports this interpretation with the intensity reduction of bands

Raman Intensity / arb. unit.

480

400

Crystal phase: compression decompression

320


related to allyl group, mainly the ~ 3040 cm-1 band.

800

liquid (0.5 GPa)

700 600 500 50

100

150

200

250

wavenumber / cm-1

imidazolium ring libration

240

160 50

100


250

800


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


imidazolium ring deformation

700 600

(C=C)

500 400 300 200 900


1620

1680

Figure 8. Raman spectra of [allylmim]I in the low frequency region with intermolecular vibrational modes (upper) and fingerprint region with intramolecular vibrational modes (bottom) for crystal (1.5 GPa, blue and red curves during compression and decompression, respectively) and liquid (0.5 GPa, black curve) phases during compression-decompression cycle up to ~ 11 GPa.



Crystal phase: compression decompression

s(C-H)allyl

Raman Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2800

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as(C-H)allyl

2900

3000

3100

3200

-1

wavenumber / cm

Figure 9. Raman spectra in the high frequency region of [allylmIm]I crystal phase at 2 GPa during compression (blue curve) and decompression (red curve) after a compression-decompression cycle up to ~ 11 GPa. The black arrows indicate characteristic Raman bands weakening or disappearing after the reaction. Vibrational frequencies and relative Raman intensities calculated for ν(C-H) modes of [allylmim]+ in the optimized ion pair structure are indicated by black and green bars. The latter indicates the ν(C-H) modes related to the allyl group. A scaling factor of 0.95 was used for theoretical frequencies.

FTIR spectroscopy permitted to infer more accurately about the chemical reaction onset under high pressure in the glassy phase of [allylmim]I. A plot of (C=C) band intensity as a function of pressure and some of the corresponding spectra are reported in Figure 10. It can be observed a suddenly decreases in band intensity above 6.6 GPa where the band becomes also strongly asymmetric. A similar analysis is not possible for the Raman spectra, the only available for the crystal, because of the strong fluorescence background and for the overlapping of new bands. However, the reaction 24 ACS Paragon Plus Environment

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threshold for glassy phase obtained using FTIR spectra is closer to that for the crystalline form extracted using the ruby luminescence bandwidth method. It is worth mentioning that in both FTIR and Raman spectroscopy experiments the spectrum pattern does not change keeping the sample for hours at the same pressure.

Pr

1.0 0.8 Absorbance

Normalized intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6 0.4

0.8 0.7 0.6 0.5 0.4 0.3 0.2

P / GPa 2.1 4.2 5.1 6.6 7.9 8.7 9.6

1640 1660 1680 wavenumber / cm-1

0.2 1

2

3

4

5

6

7

8

9

10

P / GPa Figure 10. FTIR intensity of (C=C) vibrational mode as function of pressure during compression for glassy phase of [allylmim]I. The FTIR spectra were reproduced using Voight functions, above 6 GPa more than one band was required to fit the experimental spectra. The inset shows the FTIR spectra measured at different pressure values.

Phase transitions and reactivity

The main results concerning the phase transitions and the reactivity of [allylmim]I obtained in this work are summarized in Figure 11. The 2D XRD patterns obtained at different pressures provide qualitative information about the crystalline or glassy phases formation inside the DAC1. The crystallization is evidenced by the appearance of characteristic spots in the XRD pattern contrary to the smoothed and diffused pattern proper of an amorphous sample. The XRD pattern of the crystal is 25 ACS Paragon Plus Environment


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composed by spots instead of Debye-Scherrer concentric rings thus suggesting the formation of a highly oriented crystal inside the DAC1. The glassy phase maintained the smoothed X-ray scattering pattern in all the accessed pressure range during compression-decompression cycle. Altogether, the experimental results showed an irreversible chemical transformation in [allylmim]I occurring at high pressures with the formation of an oligomeric product. The observation of a pressure induced reaction in both crystal and glassy phases of [allylmim]I is intriguing since a lack of highpressure reactivity has been reported for the glassy phases of different molecules.44-45 compression Crystallization ~ 1.0 GPa

Reactivity on crystal phase

0.5 GPa

Crystal instability boundary > 7 GPa

3.1 GPa

14.7 GPa

decompression Recovered sample

Melting ~ 0.5 GPa

0.2 GPa

0.7 GPa

compression Glass transition ~ 1.5 GPa

Reactivity on glassy phase

Glass instability boundary > 6.6 GPa

0.1 GPa

decompression Recovered sample

14.0 GPa

Figure 11. Scheme summarizing the phase transitions and instability limits of [allylmim]I under high pressure at room temperature. 2D XRD patterns obtained at different pressures to illustrate the crystal or glass formation are also shown.


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The reaction occurring in both crystal and glassy phases of [allylmim]I can be reasoned comparing with reactivity studies for pyridine under high pressure.44 It was showed that pyridine might crystallize or become glass during compression, although the chemical reaction was only observed for crystalline phase.44 The authors argued that the chemical reaction does not occur in glassy phase due to the absence of cooperative dynamics essential to the formation of reaction seeds. This conclusion was also used to explain relevant anomalies in the instability boundary of propene.45 In this way, we can argue that the chemical reaction observed for both glass and crystal phases of [allylmim]I has a mechanism which results from the combination of the lattice dynamics and the local topological arrangement of the ions.

IV. CONCLUSIONS

In this report we have characterized by different experimental techniques the room temperature high-pressure behavior of an ionic liquid based in 1-allyl3-methylimidazolium. Synchrotron X-ray diffraction was used to probe the structure of [allylmim]I with the results indicating the formation of both glass or crystal phases in the DAC. The ruby luminescence analysis was used to estimate the onset pressure for glass transition and gain some intriguing hints about the reaction occurrence. UV-Vis and FTIR spectra for glassy phase indicates the polymerization reaction occurring selectively through the allyl group of [allylmim]+. As previously verified for [allylmim][N(CN)2], the results evidence that the reaction is not 27 ACS Paragon Plus Environment


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complete. Raman spectra analysis of the [allylmim]I crystalline phase also showed the reaction occurring through the allyl group. The comparison of the reactivity in the two solids provides some useful hints about the oligomerization reaction mechanism which seems to be mainly related to local topological arrangements. This work extends our understanding of ionic liquid instability boundaries under high pressure and evidences the possibility to obtain poly(ionic liquids) from ionic liquid monomers using only pressure. The role played by other factors, such as temperature and distinct functional groups on ion structures, e.g. longer alkyl chain, deserves further investigations.

AUTHOR INFORMATION

Corresponding author *E-mail adress: [email protected]; [email protected]; Tel.: +55(11) 30912057

Notes


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. No competing financial interests have been declared.

ACKNOWLEDGMENT The authors acknowledge the Brazilian agencies CNPq and FAPESP (Grant Nos.

2016/21070-5,

2015/09763-2,

2015/05803-0,

2014/15107-8,

and

2012/13119-3) for fellowships and financial support. The authors also thank the LNLS (Brazilian Synchrotron Light Laboratory) for synchrotron X-ray diffraction experiments in the W09A-XDS beamline (proposal XDS 20170104)

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TOC GRAPHIC


Structure and Reactivity of the Ionic Liquid 1-Allyl-3

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