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TABLE OF CONTENTS

harden ionic liquids. Soft materials obtained (ionogels) have good thermal stability, electric conductivity and an enhanced ability of self-healing.

Soft Matter Accepted Manuscript

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Diimidazolium and dipyrrolidium organic salts were able to

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Soft Matter

RSC

Dicationic Organic Salts: Gelators for Ionic Liquids Francesca D’Anna,a* Carla Rizzo,a Paola Vitale, a Giuseppe Lazzara b and Renato Notoa*

Diimidazolium and dipyrrolidinium organic salts were tested for their ability to gel both organic solvents and ionic liquids. Organic salts containing the 1-(1-imidazolylmethyl)-3,5-di-(3’octylimidazolylmethyl)-benzene and the 1-(N-pyrrolidylmethyl)-3,5-di-(N,Noctylpyrrolidylmethyl)-benzene cations were used. In addition to the simple bromide anion, also dianions having a naphthalene core such as 1,5- and 2,6-naphthalenedisulfonate and 2,6naphthalenedicarboxylate were taken into account. Gelation tests demonstrated that organic salts used were able to harden ionic liquids. Materials obtained were investigated for their thermal stability but also for electric conductivity properties using micro-DSC and dielectric spectroscopy investigation. Furthermore, opacity of some gel phases was monitored using UV -vis measurements. To have information about gelation mechanism, gel phase formation was studied as a function of time by means of resonance light scattering investigation. Finally, ability of materials to respond to external stimuli such as magnetic stirring or ultrasound irradiation was also analyzed. Data collected show that different relationships operate among gelator and ionic liqu id structure, determining properties of materials and their possible applications.

Introduction The study of self-assembly given by organic salts has been recently the topic of several papers.1-4 The main reason of this growing interest seems to lie in the possibility of obtaining them from easy synthetic procedures and in the wide range of available cations and anions that allows a huge number of different combinations. From an applicative point of view, selfassembly of organic salts gives rise to conductive materials that can find different applications. 5-9 In this context, in the last few years, a certain attention has been paid to the design of low molecular weight gelators (LMWGs) able to entrap in the selfassembled fibrillary network ionic solvents such as ionic liquids (ILs).10-21 Properties of ILs are continuously investigated; it is well known that they possess low vapor pressure and flammability in addition to a high thermal stability and conductivity. Materials obtained from gelation of ILs are called ionogels22-27 and they are hybrid materials that can show wide electrochemical window and could be used as solid electrolytes

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for lithium batteries, 28, 29 dye-sensitized solar cells (DSSCs), 3032 capacitors33 and electrochemical actuators. 34, 35 On this subject, inexpensive ionogels having intriguing and good properties have been obtained also from the aqueous gelation of amphiphilic ammonium oligoether-based ILs.36, 37 In addition to the high conductivity, these materials also give the possibility of being used at temperatures much higher than ambient. This property could prove to be very useful if development of DSSCs is taken into account. Indeed, substitution of organic solvents with ILs could be the solution to problem of solvent evaporation. It is well known that properties of ILs can be tuned up by small changes in the cation or anion structure. Then, materials obtained from their gelation might show peculiar performance as a consequence of the different features of ions constituting the solvent. This peculiarity plays a more significant role if the gelation of organic salts in ILs is taken into account. On this subject, recently Zhou et al. have reported data about the gelation of a benzimidazolium based low molecular weight gelator in ILs solution. 38 The obtained materials had good

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conductivity and anticorrosion properties. These latter, together with thixotropic character, made them potential candidates for their use as semisolid lubricants. In the light of this and in the framework of our interest in studying ILs properties and applications 39, 40 and gelation ability of diimidazolium salts, 41-43 we investigated the behavior of some task specific diimidazolium and dipyrrolidinium salts (Chart 1) both in organic solvents and in ILs solution. In particular, we took into account organic salts containing the 1(1-imidazolylmethyl)-3,5-di-(3’-octylimidazolylmethyl)benzene and the 1-(N-pyrrolidylmethyl)-3,5-di-(N,Noctylpyrrolidylmethyl)-benzene cations. The comparison between the above cationic units should contribute to a better understanding of the role played by  interactions and hydrogen bond donor ability of the cation in favoring the occurrence of self-assembly processes. Furthermore, in both cases, a neutral imidazole or pyrrolidine unit took up the third Gelators structures:

position of substitution in the central core. In future, the presence of a basic functionality on the cation structure could allow the use of materials obtained as catalytic and confined reaction media. On this purpose, a recent review by Escuder et al. sheds light on advantages in carrying out chemical reactions in the fibrous network of a gel phase. 44 As for anions, mono- and divalent species were taken into account. In particular, in addition to bromide chosen for its symmetry and coordination ability, we also used dianions having aromatic spacers, such as 1,5- and 2,6naphthalenedisulfonate anions ([1,5-NDS] and [2,6-NDS]). Indeed, the use of isomeric anions differing in dipole moment might affect properties of materials, also thanks to their ability to give hydrogen bond along different directions. Furthermore, we tried to evaluate the effect deriving from changes in the anion coordination ability using a 2,6-naphthalenedicarboxylate ([2,6-NDC]) salt.

N N N

C8H17 N

2X

N

C8H17 N

X2

X2

2X N

N

N C8H17

1

[Im][X]2 or [Im][X]

C8H17 2

[Pyrr][X]2 or [Pyrr][X]

SO3 COO

SO3

X = Br a

OOC

O 3S SO3 b

X = [1,5-NDS]

c

X = [2,6-NDS]

d

X = [2,6-NDC]

Ionic Liquids structures: X Me

N

N

N Bu

X = BF4 [bmim][BF4] PF6 [bmim][PF6] NTf2 [bmim][NTf2]

R = Me Bu

NTf2 N R [Bzmim][NTf2] [Bzbim][NTf2]

NTf2 Et N Bu Et Et [bEt3N][NTf2]

NTf2 Me

NTf2 N

N Bu

[bmpyrr][NTf2]

Me

Bu

[bmpip][NTf2]

Chart 1. Gelators and Ionic Liquids structure. The gelling behavior of our salts was investigated in conventional organic solvents differing in polarity and hydrogen bond donor ability. The possibility of hardening ILs was evaluated using some of these neoteric solvents differing in cation and anion structure. In particular, both aromatic and aliphatic ILs were taken into account. We used imidazolium based ILs, such as [bmim][NTf2], [Bzbim][NTf2] and [Bzmim][NTf2] to evaluate the effect due to the extension of the -surface area of the cation (on going from [bmim+] to [Bzmim+]), but also the relevance of the van der Waals interactions (see the comparison between [Bzmim+] and [Bzbim+]). Among aliphatic cations, the change from ammonium to

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piperidinium based ILs, through pyrrolidinium based IL, allowed us to check the role played by the increase in the order degree of the cation (on going from ammonium to pyrrolidinium cation). Furthermore, for cyclic structures, we tried to evaluate the effect exerted by a different size and geometry of the ring (see the comparison between pyrrolidinium and piperidinium cations). As for the ILs anions, they were chosen on the grounds of their size, shape and coordination ability. After preliminary gelation tests, materials obtained were investigated for their thermal stability also using both the lead-ball method and micro-DSC (-DSC) measurements. Gel phases formation was studied as a function of

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time by using resonance light scattering (RLS) and UV-vis measurements. Owing to the wide range of possible applications of the obtained materials and taking into account the possibility they would be subjected to external stimuli, we tested their self-repairing ability. In particular, we analyzed the ability of our ionogels to repair after mechanical stress or ultrasound irradiation. Finally, the ionogel conductivity was investigated by using dielectric spectroscopy measurements.

Results and Discussion Synthesis of pyrrolidinium salts and thermal properties investigation We synthesized all diimidazolium salts according to the procedure previously reported. 40 As far as the 1-(Npyrrolidylmethyl)-3,5-di-(N,N-octylpyrrolidylmethyl)-benzene bromide ([Pyrr][Br] 2) is concerned, we used a two steps procedure (Scheme 1) Synthesis of dipyrrolidinium bromide salts: Br

N +

Br

6

ACN r. t.

Br

Br

+ 3

N

NH

N H

H

N

Br N N

+ 2

C8H17Br

ACN 90 °C, 48 h

N

C8H17 N

Br

N

N C8H17 2a

Scheme 1. Schematic representation of the synthetic procedure of N-octylpyrrolidinium bromide salts. In the first step using the 1,3,5-tribromomethylbenzene as starting material, the neutral precursor 1,3,5trispyrrolidylbenzene was obtained. The following alkylation of precursor in acetonitrile solution was carried out adding dropwise a stoichiometric amount of alkyl bromide in the same solvent (see Experimental Section). The crude solid obtained by solvent evaporation was washed with diethyl ether under ultrasound irradiation. This procedure allowed solubilization of the very low amount of monocationic salt in the organic phase. The brown solid, afterwards identified as the pure dibromide salt [Pyrr][Br]2, was analysed using NMR and ESI-MS investigations that didn’t show the presence even in traces of the tricationic derivative. To carry out the anion exchange, we didn’t use the classical metathesis reaction,45 but a previously reported exchange protocol on resin. 46 This allowed converting the bromide in the corresponding hydroxide that subsequently was neutralized in the presence of conjugate acids of the anions. According to previous reports, this procedure gives organic salts with high yields and avoids the use of large volumes of organic solvents (details of synthetic procedures are reported in the Experimental Section). Pyrrolidinium salts were investigated for their melting points and thermal stabilities. Analysis of melting points, reported in the Experimental Section, shows that among dipyrrolidinium salts, [Pyrr][Br] 2 was the only one having a melting point lower than 100 °C and behaving as IL. Differently, as previously reported, all diimidazolium salts, on the grounds of their melting points fell within the class of ILs. 40

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In Figure S1 of ESI TGA tracks for dipyrrolidinium salts 2a-c are presented, whereas in Table S1 of ESI we report decomposition temperatures and percentage of loss in weight. The knowledge of the transformation induced by a temperature increase on the structure of organic salts used might be important in application of gel phases that they are able to form. Indeed, this information may give insights into the temperature ranges in which materials or solutions obtained from their melting can be used. We have recently investigated the thermal behavior of diimidazolium salts 1a-d and data collected evidenced that the different nature of the anion played a significant role in determining their thermal stability. 40 These salts showed multi-step degradation processes (two steps at least) and we considered the temperature corresponding to the first step (Td1) as the operational limit for the salts used. For these salts, this parameter changed according to the following trend: [Im][Br] 2 (235.4 °C) < [Im][2,6-NDS] (236.2 °C) < [Im][2,6-NDC] (250.4 °C) < [Im][1,5-NDS] (289.0 °C).40 However, according to previous reports by Seddon et al. the minimum and the maximum values should be lowered by 75 and 100 °C respectively. 47, 48 As far as pyrrolidinium salts are concerned, their operational limit ranged from 246 up to 307 °C. Comparison with thermal stabilities of dipyrrolidinium salts having aliphatic spacers previously reported by Armstrong et al., 49 shows that decomposition of our salts occurred at lower temperatures. We detected a similar trend also for diimidazolium salts and we ascribed the result not only to the different nature of the spacer, but also to the presence of different conformers deriving from the high substitution degree on the aromatic spacer. On the other hand, according to previous reports, 49 thermal stability increased on going from diimidazolium (1a-d) to dipyrrolidinium (2a-c) salts. Gelation Tests in Organic Solvents Firstly, we analyzed the gelling behavior of organic salts, testing the ability of some of them to harden conventional organic solvents. The results are collected in Table S2 of ESI and perusal of data evidences that, independently of the concentration used, both imidazolium and pyrrolidinium salts were not able to act as organogelators. In particular, the [Im][Br]2 showed a high solubility in protic as well as in aprotic polar solvents. In general, the corresponding [NDS] salts were soluble in protic polar solvents, such as alcohols, glycols and glycerol, but they were insoluble also at low percentage in weight in aprotic solvents. Among tested salts, pyrrolidinium derivatives showed the lowest solubility. Indeed, they were insoluble or only partially soluble in most of solvents used and showed a good solubility in highly polar solvents such as glycols, DMSO and DMF. Finally, we observed gel-like precipitate formation in glycerol, but attempts to harden it were unsuccessful. Gelation Tests in Ionic Liquids As above said, we tested our organic salts for their ability to gel ILs. Results of gelation tests relevant to diimidazolium and dipyrrolidinium salts are reported in Tables 1 and 2. In all cases in which a gel phase was obtained, the CGC (i.e., the critical gelation concentration allowing gel phase formation) and Tgel (i.e., the gel melting temperature) are indicated.

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Table 1. Gelation tests for diimidazolium salts 1b-d in different IL solutions.a [Im][1,5-NDS] Rangeb

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Solvent

CGCc

[Im][2,6-NDS]

Tgel

Cp

(K)d

(J g-1

CGCc

[Im][2,6-NDC]

Tgel

Cp

(K)d

(J g-1

K-1)

CGCc

OGe

10.6

296

GP

GP

[bmim][BF4]

2.0-15.0

OGe

14.8

307

S

S

[bmim][PF6]

4.0-15.0

S

S

GP

[bEt3N][NTf2]

8.0-21.0

OG

[bmpip][NTf2]

4.6-11.0

(K)d

(J g-1 K-1)

4.0-16.1

310 304f

Cp

K-1)

[bmim][NTf2]

16.1

Tgel

OG

16.1

307 297f

0.079

GP

OG

11.6

295f

0.055 OGe,g

GP

SM

8.7e/3.2g

0.102

SM ns

[bmpyrr][NTf2]

4.0-12.0

GP

OGe

GP

8.3

SM ns

[Bzbim][NTf2]

4.0-13.0

GP

S

GP

[Bzmim][NTf2]

4.0-13.0

GP

S

GP

S = soluble; OG = opaque gel; GP = gel-like precipitate; SM = soft material; ns = no DSC signal. bInvestigated organogelator percentage range. c(%, w/w, organogelator/solvent). dTgel values were reproducible within 1 K. e The gel phase was obtained after that the cold solution was magnetically stirred for 10 minutes at 1000 rpm. fDetermined by DSC investigation. gThe gel phase was obtained after ultrasound irradiation of the cold solution for 5 minutes at 200 W and 45 kHz and magnetic stirring for 10 minutes at 1000 rpm. a

Table 2. Gelation tests for dipyrrolidinium salts 2b-c in different IL solutions.a [Pyrr][1,5-NDS] Solvent

Rangeb

[bmim][NTf2]

8.0-15.0

OG

CGCc

Tgel (K)d

15.0

346

[Pyrr][2,6-NDS] CGCc

Tgel (K)d

14.0

331

Cp (J g-1 K-1)

S

ns [bmim][BF4]

7.0-15.0

OGe

10.0

308

OG

301f [bmim][PF6]

8.0-15.0

OGe

[bEt3N][NTf2]

4.0-10.0

S

S

[bmpip][NTf2]

1.0-8.0

S

S

[bmpyrr][NTf2]

1.0-9.0

OGe

[Bzbim][NTf2]

7.0-17.0

PG

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11.6

12.6

316

SM

OGe

10.5

SM

16.8

SM

0.075

S OGe

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ARTICLE [Bzmim][NTf2]

8.0-19.0

OGe

16.1

386

OGe

18.1

SM

S = soluble; OG = opaque gel; GP = gel-like precipitate; SM = soft material; ns = no DSC signal. bInvestigated organogelator percentage range. c(%, w/w, organogelator/solvent). dTgel values were reproducible within 1 K. eThe gel phase was obtained after that the cold solution was magnetically stirred for 10 minutes at 1000 rpm. f Determined by DSC investigation.

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a

It is noteworthy the fact that gel phase formation was performed by using different protocols. Indeed, in some cases the gel was obtained simply cooling (at 4 °C) the clear hot solution. However, in some other cases, the cold solution was opaque and the gel phase was obtained after magnetic stirring at constant rate, or a combined action of ultrasound irradiation and magnetic stirring. Probably, the above actions favored the entanglement of pre-formed fibers, giving rise to the gel phase formation.50

(a)

The stability of gel phases obtained in different conditions (cooling, stirring and/or sonication of the cold solution) was checked by using the “tube inversion” test 51 and in this condition our gel phases were stable for almost six months at 20 °C. Materials obtained were brown (for pyrrolidinium salts) or lightly yellow (for imidazolium salts) opaque gel phases (Figure 1).

(b)

Figure 1. Gel phases obtained: (a) [Im][Br]2/[bEt3N][NTf2] at 20% (w/w); (b) [Pyrr][1,5-NDS]/[bmim][NTf2] at 16 % (w/w). Firstly, gelation tests performed for bromide salts did not give ionogel phases, with the only exception of [Im][Br]2 in [bEt3N][NTf2] at CGC value of 20.1% (Tgel = 348 and 288 K obtained with lead-ball method and -DSC measurements respectively, Cp = 0.114 J g-1 K-1). The low ability of these salts to form gel phases is evidenced also by the high solubility that they exhibit in almost all organic solvents. Moreover, this result perfectly agrees with what previously reported by us about the poor gelling ability of bromide salts in respect to the corresponding naphtalenedisulfonate ones.42 Analysis of data reported in Table 1 shows that diimidazolium salts 1a-d preferably hardened aliphatic ILs. Indeed, only [Im][1,5-NDS] was able to gel [bmim][NTf2] and [bmim][BF4]. In these cases, the CGC increased on going from [bmim][NTf2] to [bmim][BF4], evidencing a negative effect deriving from the increase in the IL anion coordination ability ( = 0.376 and 0.243 for [bmim][BF4] and [bmim][NTf2] respectively)52 and symmetry. Better results were obtained in aliphatic ILs. Indeed, all imidazolium salts were able to gel [bEt3N][NTf2]. In the class of dianions, the anion basicity seems to affect the CGC values, as accounted for the significant decrease detected in this parameter on going from naphthalenedisulfonate salts (1b-c) to naphthalenedicarboxylate salt (1d). Indeed, this trend perfectly parallels that of pKa of corresponding conjugated acids (pKa= 0.60, 0.41 and 3.60 for 1,5-NDS, 2,6-NDS and 2,6-NDC acid respectively).53 Furthermore, small differences detected in CGC values corresponding to gel phases formed by [Im][1,5-NDS] and

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[Im][2,6-NDS] in [bEt3N][NTf2] seem to be due to differences in pKa values of NDS acids. On the other hand, the experimental trend also recalls the one previously reported about the hydrogen bond affinities of carboxylate and sulfonate anions.54, 55 Nevertheless, for diimidazolium ionogelators, CGC values were barely affected by the different isomeric substitution on the anion. Indeed, this parameter stayed constant on going from [Im][1,5-NDS] to [Im][2,6-NDS], underlining that gelation process was not affect by changes in dipole moment of ionogelator anion. In order to evaluate the effect due to a different IL cation structure, the gelling ability of task specific diimidazolium salts (1b-d) was also tested in [bmpip][NTf2] and in [bmpyrr][NTf2]. In general, naphthaledisulfonate salts, regardless of the concentration used, gave rise to the formation of gel-like precipitates. On the other hand, in the case of naphthalenedicarboxylate salt [Im][2,6-NDC], stable opaque gels were obtained. Analysis of data collected in Table 1 indicates a sort of correlation between the cation structure of the solvent and the CGC value. Indeed, this parameter decreased on going from [bEt3N][NTf2] to [bmpyrr][NTf2]. Then, the arrangement of the solvent cation in a cyclic structure seems to make easier the ionogel fibers growth and entanglement. It is noteworthy that in [bmpip][NTf2], the combined action of magnetic stirring and ultrasound irradiation of the cold solution induced a significant decrease in the CGC value.

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Soft Matter Tgel values of gel phases obtained were firstly determined by using the lead-ball method.56 In some cases, a consequence of soft texture of gel phases, lead-ball slid downward before gel melting hampering Tgel determination using the above method. In Tables 1 and 2 these materials are indicated by SM. Analysis of values reported in Table 1 shows that thermal stability at the CGC increased with the anion coordination ability of the IL, as accounted for by values collected for [Im][1,5-NDS] (1b) in [bmim][BF4] and [bmim][NTf2] solution. As far as values collected in [bEt3N][NTf2] are concerned, Tgel decreased on going from [Im][Br]2 (1a) to [Im][2,6-NDC] (1d), evidencing that thermal stability of materials obtained was negatively affected by the increase in the anion coordination ability of the ionogelator used. Differently from diimidazolium salts 1a-d, dipirrolydinium ones 2bc were able to harden aromatic ionic liquids. Indeed, data reported in Table 2 evidence that, with the only exception of [Pyrr][1,5-NDS] in [bmpyrr][NTf2] solution, gel phase formation was mainly observed in solution of imidazolium based ILs. It is noteworthy that, in most cases, the fibers entanglement of these aliphatic ionogelators required the action of magnetic stirring on the cold solution. Probably, this indicates a less pronounced gelation ability of dipyrrolidinium salts respect to the one detected for diimidazolium salts (1a-d). This trend could be rationalized, taking into account changes in the ionogelator cation ability to give hydrogen bond and - interactions. Indeed, on going from diimidazolium to dipyrrolidinium salts a decrease in both the above-mentioned abilities should be detected. Data collected seem to indicate that ionogel phases formation is favored by a higher hydrogen bond donor ability, as accounted for more pronounced gelling ability of diimidazolium salts. On the other hand, on the grounds of structural complementarity detected between solvent and gelator (aromatic ionogelators preferably gel aliphatic ILs and vice versa), cation- interactions rather than - interactions seem to play a more significant role in determining gelation process. Once again the comparison between data collected for [Pyrr][1,5-NDS] and [Pyrr][2,6-NDS] does not allow clearly understanding the role played by the isomeric substitution on the naphthalene core. Quite interestingly, data collected allow shedding light on the effect deriving from different properties of the solvent cation and anion. On this purpose, data collected for [Pyrr][1,5-NDS] evidence that the CGC value was affected by the anion symmetry and cross-linking ability, rather than its coordination ability. Indeed, significant differences detected for this parameter in [bmim][NTf2] and [bmim][PF6] cannot be ascribed to differences in the anion coordination abilities ( = 0.243 and 0.207 for [bmim][NTf2] and [bmim][PF6] respectively),52 but rather to the capability of the [PF6-] anion to favor the formation of a more reticulated three-dimensional network. This hypothesis is also confirmed by data corresponding to [Pyrr][2,6-NDS]. Indeed, in this case, once more the lowest CGC value was detected in [bmim][PF6] solution, namely in the solvent system having the lowest  value. Structure of IL cation can play a significant role in determining gel phase formation. On this subject, taking into account data collected for aliphatic ionogelators in aromatic ILs, main structural features that could affect ionogel phase formation are the substitution on the imidazolium ion of a butyl chain with a benzyl group (on going from [bmim][NTf2] to [Bzmim][NTf2]) and the increase in alkyl chain length (on going from [Bzmim][NTf2] to [Bzbim][NTf2]). In the first case, as a consequence of the structural modification, an increase in the cation ability to give - interactions should be detected. On the other hand, the presence of a longer alkyl chain should enhance the ability of the cation to establish van der Waals interactions. Analysis

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ARTICLE of data collected in Table 2 for [Pyrr][1,5-NDS] (2b) indicates that the first factor negatively affects ionogel phase formation, as accounted for the increase in CGC value. Probably, as a consequence of the aliphatic nature of the ionogelator used, van der Waals interactions play a more significant role compared to - interactions, in the growth of the three-dimensional network needed for gel phase formation. This hypothesis is also well supported by data collected for [Pyrr][2,6-NDS] (2c). Indeed, in this case, the CGC value decreased on going from [Bzmim][NTf2] to [Bzbim][NTf2], according to the increase in the alkyl chain length on the cation structure and consequently in the possible van der Waals interactions. Analysis of Tgel collected at the CGC shows that thermal stability of gel phases formed by dipyrrolidinium salts was higher than that detected for diimidazolium salts. It ranges from 308 up to 386 K. This parameter decreased with the increase in the anion symmetry of IL used, as accounted for by values collected for [Pyrr][1,5-NDS] (2b) in [bmim][BF4] and [bmim][NTf2]. At this stage, a preliminary evaluation of results collected from gelation tests could be interesting as a consequence of the fact that in most cases materials obtained show a distinctive feature being formed by gelation of dicationic ILs in IL solution. This feature, as far as diimidazolium salts are concerned, seems to be due to the presence of neutral imidazole unit on the cation. Indeed, data previously collected by us using corresponding m-diimidazolium salts shed light on the lack of gelling ability.42 Analysis of previous reports in literature evidences that notwithstanding gelation of organic salts, above all monocationic ones, frequently occurs both in organic solvents and water solution, 3, 57-59 only very few examples deal with their gelation in IL solution. Among these latter, to the best of our knowledge, the only case in which gelation of monocationic ILs in IL solution has been observed, was reported few years ago.38 Similarly to our results, gelation of diimidazolium salts was affected by the nature of imidazolium IL used as solvent system. In particular, the length of alkyl chain proved to play a significant role. As far as dicationic organic salts are concerned, no evidences have been reported about their gelation in IL solution. In one of the few examples, Dötz et al. synthesized pyridine-bridged bis(benzoimidazolylidene)-palladium complexes able to gel not only different protic and aprotic organic solvents, but also different ILs. However, in this case, gel phase formation could also be a result of metal coordination. 60  In all cases where the gel was obtained upon heating and cooling,  measurements for gel phases obtained after magnetic stirring, but no transition was detected. A representative thermogram is showed in Figure 2. The thermograms showed a step change in the heat capacity that proves the occurrence of a second order phase transition in the systems. It is notable that the thermally induced process is reversible, in fact the signal is reproducible in a heating cooling cycle for 3 cycles at least. The onset temperature and heat capacity changes are reported in Tables 1 and 2. Whenever possible, it was systematically evidenced lower Tgel values from calorimetry than those from leadball method. Such a difference is understandable by considering that by calorimetry the onset of the process was monitored while the other method evidenced the full loss of gel consistency. Finally, in

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Soft Matter

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RLS and UV-vis investigation

Heat flow (a.u.)

To study the ionogel formation, we performed RLS and UV-vis measurements. RLS could be a useful tool for this purpose. Indeed, it is well known that this technique allows detecting the presence in solution of aggregates formed by different systems.61, 62 Furthermore, it also provides the size of aggregates as a consequence of the relationship working between this parameter and RLS intensity.63 In general, RLS intensity parallel increases in size of

aggregates. On the other hand, UV-vis measurements are frequently used to evaluate the opacity of a gel phase, accounting for its cristallinity.64 In the case of diimidazolium salts (1a-d), we observed the ionogels formation also at 288 K (the lowest operational temperature of our instruments), this allowed studying the above process as a function of time. Differently, in the case of dipyrrolidinium salts the ionogels formation at the above temperature was too slow and did not allow having reproducible results. In Figure 3 plots of RLS intensity (IRLS) as a function of time for ionogels formed by diimidazolium salts (1a-d) in aliphatic ILs are reported.

third heating ramp second heating ramp first heating ramp 0

20

40 60 Temperature (°C)

80

100

Figure 2. Thermogram of ionogel [Im][2,6-NDS]/[bEt3N][NTf2].

300

670

500

(b)

(a)

660

250

400

[Im][Br]2/[bEt3N][NTf2]

300

[Im][2,6-NDS]/[bEt3N][NTf2]

150 200

I (u. a.)

[Im][2,6-NDC]/[bmpip][NTf2]

I (a. u.)

650

[Im][1,5-NDS]/[bEt3N][NTf2]

200

I (a.u.)

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all cases the heat capacity change (Cp) for the thermally induced gel rupturing is positive. This result evidences that some degree of freedom are gained into the system above the Tgel in agreement with the formation of a fluid mixture.

640 [Im][2,6-NDC]/[bEt3N][NTf2]

630

100

620 100

50

610

0 0

500

1000

1500

2000

2500

3000

3500

0 4000

Time (s)

600 0

500

1000

1500

2000

2500

3000

3500

4000

Time (s)

Figure 3. Plots of RLS intensity (I RLS) as a function of time for ionogels formed by diimidazolium salts (1a-d) in aliphatic ILs at 17%(w/w). gel formation,44 we supposed that the presence of [2,6-NDS] Data corresponding to diimidazolium salts highlight that anion favors previous formation of large aggregates that gelation processes in [bEt 3N][NTf2] solution were significantly subsequently rearranged in smaller and more stable ones affected by the anion nature. In the case of [Im][Br] 2, [Im][1,5- present in the gel network. 42 Our hypothesis seems to be NDS] (Figure 3a) and [Im][2,6-NDC] (Figure 3b) gel phase confirmed also in this case evidencing that a different isomeric formation occurred by a single step mechanism. In the latter substitution in the anion plays a significant role in determining case, the process was very fast and we were able to observe the mechanism of gel phase formation. On the other hand, only the last part. On the other hand, in the case of [Im][2,6- comparison between tracks obtained for [Im][2,6-NDC] in NDS] a two-step mechanism was detected. In that case, bearing [bEt3N][NTf2] and [bmpip][NTf2] seems to indicate that the in mind the hierarchical organization that allows gel phase outcome of gelation process was also affected by the nature of formation through a self-assembly process giving fibers before the IL cation. Indeed, on going from acyclic to cyclic aliphatic

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ARTICLE

cation a change from a single to a two-step process was observed. Data corresponding to time and intensity of intermediate formation (tm, Im) and gelation (te, Ie) are reported in Table S3 of ESI. Analysis of data indicates that the rate of gelation processes was affected by the nature of the ionogelator anion. Indeed, gelation time (t e) increased along the series: [Im][1,5NDS] < [Im][2,6-NDC] < [Im][Br] 2 < [Im][2,6-NDS] with the highest time detected for the ionogelator giving rise to the material through a two-step mechanism. On the other hand, analysis of RLS intensity corresponding to ionogel phases indicates that materials formed by ionogelators having 2,6-

naphthalene based anions showed the presence of more extended aggregates. In the case of [Im][2,6-NDC], change in the IL cation structure induced not only a change in the gelation mechanism, but also significant differences in the gelation time. Indeed, gel phase in [bmpip][NTf 2] was formed more slowly and was characterized by the presence of less extended aggregates. As previously stated, we analyzed changes in the absorbance at 568 nm as a function of time to have information about the opacity of ionogels obtained. In Figure 4 plots corresponding to ionogels formation in [bEt 3N][NTf2] from diimidazolium salts are reported.

4 (a)

[Im][Br]2

3.5

5

[Im][1,5-NDS]

(b)

[Im][2,6-NDS]

3

4 [Im][2,6-NDC]

2.5 Abs (568 nm)

Abs (568 nm)

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Soft Matter

2 1.5

3

2

1

1 0.5

0

0 0

500

1000

1500

2000

2500

3000

3500

0

4000

20

40

60

80

100

Time (s)

Time (s)

Figure 4. Plots of absorbance values at 568 nm as a function of time corresponding to gelation processes of diimidazolium salts (1a-d) in [bEt3N][NTf2]. Analysis of data reported in Figure 4 shows that opacity of gel phases increases along the following trend: [Im][Br] 2 ≈ [Im][1,5NDS] < [Im][2,6-NDS] < [Im][2,6-NDC]. Interestingly, UV-vis measurements evidence that the above trend agrees with the one showing the size of the aggregates characterizing gel phases (RLS measurements).

investigated in the case of organogels.51, 65-67 Differently, in the case of ionogels, to the best of our knowledge, there are only two cases in which thixotropic behavior has been investigated. In these cases, their self-healing ability proved to be an advantage for quasi-solidstate DSSCs performances31 and for the obtainment of solid-like lubricants.38

Self-healing ability

In Table 3, results relevant to thixotropic or sonotropic behavior of the gel phases obtained are reported.

We also investigated the gel phases obtained for their ability to selfrepair after the action of external stimuli. On this purpose, we took into account the effects of magnetic stirring and ultrasound irradiation. It is noteworthy that these abilities have been largely

Table 3. Thixotropic and sonotropic behavior of gel phases obtained.a Ionogel

Thixotropy

Sonotropy

[Im][Br]2/[bEt3N][NTf2]

Y

N

[Im][1,5-NDS]/[bEt3N][NTf2]

Y

Y

[Im][1,5-NDS]/[bmim][NTf2]

Y

N

[Im][1,5-NDS]/[bmim][BF4]

Y

S

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Soft Matter

ARTICLE [Im][2,6-NDS]/[bEt3N][NTf2]

Y

Y

[Im][2,6-NDC]/[bEt3N][NTf2]

Y

Y

[Im][2,6-NDC]/[bmpyrr][NTf2]

Y

Y

[Im][2,6-NDC]/[bmpip][NTf2]

Y

Y

[Pyrr][1,5-NDS]/[bmim][NTf2]

N

N

[Pyrr][1,5-NDS]/[bmim][PF6]

Y

Y

[Pyrr][1,5-NDS]/[bmim][BF4]

Y

S

[Pyrr][1,5-NDS]/[bmpyrr][NTf2]

Y

S

[Pyrr][1,5-NDS]/[Bzmim][NTf2]

Y

Y

[Pyrr][2,6-NDS]/[bmim][BF4]

N

N

[Pyrr][2,6-NDS]/[Bzmim][NTf2]

Y

Y

[Pyrr][2,6-NDS]/[Bzbim][NTf2]

Y

N

[Pyrr][2,6-NDS]/[bmim][PF6]

N

Y

a

Y = the gel phase was able to self-repair after the action of external stimulus. N = the gel phase was not able to selfrepair after the action of external stimulus. S = the gel phase was stable to the action of external stimulus.

Analysis of data reported in the Table shows that, with the exception of gel phase formed by [Pyrr][1,5-NDS] in [bmim][NTf2] and those formed by [Pyrr][2,6-NDS] in [bmim][BF4] and [bmim][PF6], all materials obtained showed a thixotropic behavior, namely they were able to self-repair after the action of magnetic stirring. Different results were obtained as far as the response to the ultrasound irradiation was concerned. Indeed, among all cases considered, gel phases formed by [Im][1,5-NDS] and [Pyrr][1,5NDS] in [bmim][BF4] and by [Pyrr][1,5-NDS] in [bmpyrr][NTf2] proved to be stable to ultrasound irradiation. On the other hand, in some other cases (5 out of 16), the ionogels were totally destroyed under ultrasound irradiation. Finally, in remaining cases (8 out of 16), they showed sonotropic behavior, self-repairing after the action of ultrasound irradiation. Comparison between data collected for pyrrolidinium and imidazolium salts shows that aromatic salts exhibited a better response to the action of external stimuli. Indeed, in six of the eight cases analyzed, the ionogels obtained show both thixotropic and sonotropic behavior. Dielectric spectroscopy

performed measurements at the CGC value. All ILs presented a monotonic change of the dissipation factor vs  that indicates the absence of relaxation phenomena in the investigated range (examples are in Figure S2 of ESI). A similar result was found for the ionogel that presented dielectric spectra almost coincident in shape with those of the solvent. As concerns the absolute value of the dissipation factor (D), as a general trend the curve for the gels diverge from those of the corresponding IL solvent in the high frequency range. Table 4 shows that in the case other than [bEt3N][NTf2] based gels, the dissipation factor of the gel is improved with compared to the corresponding IL. An interesting feature was found for the systems that provided a soft material (SM in Tables 1 and 2). In the latter cases, a peak in the dissipation factor vs was evidenced (Figure 5). The  at the maximum as well as the maximum value of conductivity are given in Table 4. It should be noted that the relaxation phenomenon is due to the gelator as the solvent did not show any peak in the same frequency range. Moreover the absence of relaxation in the more rigid gels indicates that the gelator motions are strongly reduced in these systems.

The dielectric properties of the gels were monitored at 298 K as function of frequency (). In all cases we

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Figure 5. Dissipation factor (D) vs  for ionogels that present a maximum value of conductivity. Table 4. Dielectric properties of ionogels at 25 °C.

IL

Gelator

max (kHz)

[bEt3N][NTf2]

[Im][2,6-NDS]

360

[bmpip][NTf2] [bmpyrr][NTf2] [Bzbim][NTf2]

[Im][2,6-NDC] [Im][2,6-NDC] [Pyrr][2,6-NDS]

420 1.06 360 245

Conductivity at maximum (-1 m-1) 1.35×10-2

Dgel/DILa 1.05

-2

2.61

-4

6.69

-2

3.04

-2

5.55

1.24×10 3.05×10 3.60×10

[Bzmim][NTf2]

[Pyrr][2,6-NDS]

5.59×10

[bmim][BF4]

[Im][1,5-NDS]

3.78

[bmim][BF4]

[Pyrr][2,6-NDS]

1.78

[bmim][BF4]

[Pyrr][1,5-NDS]

1.00

[bEt3N][NTf2]

[Im][Br]2

0.96

[bEt3N][NTf2]

[Im][1,5-NDS]

0.27

[bEt3N][NTf2]

[Im][2,6-NDC]

0.27

[bmim][PF6]

[Pyrr][2,6-NDS]

1.11

[bmim][PF6]

[Pyrr][1,5-NDS]

1.16

[bmim][NTf2]

[Im][1,5-NDS]

2.06

[bmim][NTf2]

[Pyrr][1,5-NDS]

5.98

[bmpyrr][NTf2]

[Pyrr][1,5-NDS]

3.28

[Bzmim][NTf2]

[Pyrr][1,5-NDS]

8.15

a

Ratio of the gel dissipation factor with respect to those of IL solvent measured at 10 kHz.

Although a strict comparison is not possible due to frequency, temperature and concentration dependence of conductivity, we can state that the conductivity values found in the investigated gels are in the same order of magnitude of those reported in the literature (roughly 10-2 -1 m-1).23, 24, 36, 68 Notwithstanding, it is interesting that most of the gels presented in the manuscript showed an increase of conductivity compared to the

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corresponding IL. This is surprisingly as generally an almost constant24 or a slight decrease23 of conductivity with ionogelator addition is observed.

Experimental Section Materials and Measurements

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Soft Matter 1,3,5-tris(bromomethyl) benzene, 1-bromooctane, imidazole, sodium hydroxide, Amberlite IRA 400 resin (chloride form), 1,5naphthalenedisulfonic · 4 H2O acid, 2,6-naphthalenedicarboxilic acid, Amberlite IR 120 PLUS resin (sodium form), sodium 2,6naphthalenedisulfonate and all organic solvents were analytical reagents purchased from commercial sources and used as received. Diimidazolium salts 1a-d were synthesized as previously reported.40 ILs as [bmim][NTf2], [bmim][BF4], [bmim][PF6], [bmpyrr][NTf2], [bmpip][NTf2], [bEt3N][NTf2], [Bzmim][NTf2] and [Bzbim][NTf2] were prepared and purified according to the reported procedures.45, 69 Pyrrolidine and dichloromethane were distilled before use. 1

H-NMR and 13C-NMR spectra were recorded on 250, 300 and 400 MHz nuclear magnetic resonance spectrometer. Chemical shifts were reported relative to Me4Si. Solvents used were DMSO or CD3OD. All compounds were also characterized using HPLC measurements and ESI-MS spectra. The first ones were recorded on a LC-10AD VP Shimadzu Liquid Chromatograph instrument with a C18 inverse phase column and a SPD-M10AUP Shimadzu Diode Array Detector. The latter ones were recorded on a Waters LCT Premier ES-MS instrument with an Advion Nanomate infusion system. Decomposition temperatures were measured using a TGA Q5000IR (TA instrument) thermogravimetric analyzer, at a heating rate of 5 °C min-1. The maximum values of the DTG curves of each thermogram were used as a measure of the decomposition temperature. Melting points of compounds 2b and 2c were determined with a Kofler. General procedure for the synthesis of the precursor 1,3,5-tris(pyrrolidinemethyl)-benzene. The compound was obtained modifying the procedure reported in literature. 70 Pyrrolidine (5.98 g; 0.084 mol) was dissolved in CH3CN (360 mL) at room temperature. Then 1,3,5-tris(bromomethyl)benzene (5 g, 0.014 mol) was added to the solution. The reaction mixture was stirred ad room temperature for 3 h under Ar atmosphere. The mixture was concentrated in vacuo and the residue was dissolved in CHCl3 (1 L) and it was washed with water (6 x 100 mL) until the neutrality of the pH. The organic layer was dried with Na2SO4 and it was concentrated in vacuo. Yield 72%. Brown oil. 1H-NMR (300 MHz; CD 3OD);  (ppm): 1.81 (dd, J1H-H = 3.3 Hz, J2H-H = 6.6 Hz, 12H); 2.55 (t, JH-H = 5.1 Hz, 12H); 3.64 (s, 6H); 7.25 (s, 3H). 13C-NMR (250 MHz; CD3OD);  (ppm): 24.1; 54.9; 61.3; 130.5; 139.7. Elemental Anal. Calcd (%) for C 21H33N3 (327.51): C, 77.01; H, 10.16; N, 12.83. Found: C, 76.82; H, 10.19; N, 12.87 %. General procedure for the synthesis of the dicationic bromide salt (2a) To a stirred solution of 1,3,5-tris(pyrrolidinemethyl)-benzene (3.29 g; 0.01 mol), in CH 3CN (150 mL) a solution of 1bromooctane (3.5 mL; 0.02 mol) in CH 3CN (30 mL) was added dropwise. The reaction mixture was stirred at 90 °C for 48 h under Ar atmosphere. Then the solvent was concentrated in vacuo. The residue was washed with diethyl ether (4x100 mL) under ultrasound irradiation.

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1-(N-pyrrolidylmethyl)-3,5-di-(N,N-octylpyrrolidylmethyl)benzene dibromide (2a). Yield 97%. Brown solid. m. p.: 54.1 °C. 1H-NMR (300 MHz; DMSO [d 6]);  (ppm): 0.87 (m, 6H); 1.30 (m, 18H); 1.96 (m, 8H); 2.28 (m, 6H); 2.69 (m, 4H), 3.22 (m, 6H); 3.66 (m, 6H); 3.88 (m, 2H); 4.67 (m, 4H); 4.92 (m, 4H); 7.67 (m, 2H); 7.94 (m, 1H). 13C-NMR (400 MHz; DMSO [d6]);  (ppm): 13.9; 20.9; 22.0; 22.4; 22.5; 22.9; 23.1; 25.9; 26.1; 28.5; 31.2; 53.4; 58.4; 60.3; 60.7; 116.2; 129.6; 130.2; 138.2. ESI-MS: m/z (+): 553; m/z (-): 79, 81. Elemental Anal. Calcd (%) for C37H67Br2N3 (713.76): C, 62.26; H, 9.46; N, 5.89. Found: C, 62.15; H, 9.43; N, 5.92 %. General procedure for anionic exchange on basic resin used to obtain dicationic salts (2b-c) The compounds were obtained modifying the procedure reported in literature. 46 A column packed with anion-exchange Amberlite resin IRA400 (15.9 g) was used. To convert the chloride form of the resin into the hydroxide form, it was first washed with an aqueous solution of NaOH (804 mL, 10% w/v) and subsequently with water until the eluate was neutral. A binary mixture of methanol/ water (70:30, v/v) was used as eluent. Bromide salt (0.037 mol) was dissolved in the binary mixture (25 mL) and eluted. The eluate, containing the salt in the hydroxide form, was collected in a flask containing a solution of the naphthalenedisulfonic acid in stoichiometric amount. The final product was washed with acetone (3 x 30 mL) under ultrasound irradiation. 1-(N-pyrrolidylmethyl)-3,5-di-(N,N-octylpyrrolidylmethyl)benzene 1,5 naphthalenedisulfonate (2b). Yield 97%. Orange solid. m. p.: 128.7-131.4 °C. 1 H-NMR (300 MHz; DMSO [d6]);  (ppm): 0.94 (t, JH-H = 6.3 Hz, 6H); 1.34 (m, 20H); 1.79 (m, 8H); 2.09 (m, 10H); 2.61 (m, 4H); 3.07 (m, 4H); 3.48 (m, 8H); 3.76 (m, 2H); 4.53 (m, 4H); 7.44 (d, JH-H = 7.2 Hz, 2H); 7.69 (m, 3H); 7.98 (d, JH-H = 7.2 Hz, 2H); 8.93 (d, JH-H = 8.4 Hz, 2H). 13C-NMR (250 MHz; DMSO [d6]);  (ppm): 14.3; 21.2; 22.4; 22.7; 23.4; 26.2; 26.4; 29.0; 31.1; 31.6; 53.8; 60.7; 61.1; 71.2; 116.2; 122.9; 123.2; 123.3; 123.4; 124.2; 124.3; 129.4; 129.9. ESI-MS: m/z (+): 840; total mass = 840.5034; calculated mass = 840.5019. Elemental Anal. Calcd (%) for C 47H63N3O6S2 (840.23): C, 67.18; H, 8.76; N, 5.00. Found: C, 67.02; H, 8.73; N, 5.03 %. 1-(N-pyrrolidylmethyl)-3,5-di-(N,N-octylpyrrolidylmethyl)benzene 2,6 naphthalenedisulfonate. (2c) The 2,6naphthalenedisulfonic acid was obtained by elution of a water solution of the corresponding sodium salt through an Amberlite IR 120 plus column. Yield 98%. Orange solid. m. p.: 135.3-138.4 °C. 1H-NMR (300 MHz; DMSO [d6]);  (ppm): 0.92 (t, JH-H = 4.8 Hz, 6H); 1.31 (m, 20H); 1.82 (m, 6H); 2.12 (m, 8H); 2.61 (m, 4H); 3.11 (m, 4H); 3.41 (m, 8H); 3.55 (m, 2H); 4.59 (m, 4H); 7.74 (m, 5H); 7.76 (d, JH-H = 8.4 Hz, 2H); 7.93 (d, JH-H = 8.7 Hz, 2H); 8.15 (s, 2H). 13C-NMR (250 MHz; DMSO [d 6]);  (ppm): 14.3; 21.3; 22.4; 22.7; 23.4; 26.3; 28.9; 29.0; 31.0; 31.6; 53.8; 58.7; 60.6; 61.0; 116.2; 122.9; 123.3; 123.5; 124.1; 124.6; 128.4; 132.2; 146.3. ESI-MS: m/z (+): 840; m/2z (-): 143; total mass = 840.5021; calculated mass = 840.5019. Elemental Anal. Calcd (%) for C47H63N3O6S2 (840.23): C, 67.18; H, 8.76; N, 5.00. Found: C, 67.05; H, 8.74; N, 5.05 %. Preparation of gels and Tgel determination

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Soft Matter Gels were prepared by weighing into a screw-capped sample vial (diameter 1 cm) the amount of salt and solvent (~ 250 mg). The sample vial was heated in an oil bath at 130 °C until a clear solution was obtained. The vial was then cooled and stored at 4 °C overnight. In some cases the gelation process occurred after stirring the mixture at 1000 rpm for 5 or 10 minutes. The tube inversion test method was used to examine gel formation in different solvents. To determine the Tgel values, a lead ball (weighing 46.23 mg) was placed on the top of the gel and the vial was put into a water bath. The bath temperature was gradually increased until the gel melted (Tgel) and the lead ball started to move downward. Tgel values were reproducible within 1 K. -DSC measurements In all cases in which gel phases were formed simply cooling the hot solution, the sol-gel transitions were also recorded on a micro-DSC III (SETARAM) under nitrogen flow in the range from 0 to 70 °C with a scan rate of 0.3 K min-1. The stainless steel (1 cm3) sample cell was filled with ca. 250 mg of ionogel and the reference cell with the corresponding amount of IL. The calibration was performed by using naphthalene. RLS measurements RLS measurements were carried out at 288 K on a spectrofluorophotometer (JASCO FP-777W) using a synchronous scanning mode in which the emission and excitation monochromators were preset to identical wavelengths. The RLS spectrum was recorded from 300 to 600 nm with both the excitation and emission slit widths set at 1.5 nm. We chose as working wavelength the one corresponding to the intensity maximum of the spectrum obtained. Samples for a typical kinetic measurement were prepared injecting in a quartz cuvette (light path 0.2 cm) the limpid hot solution of salt. Spectra were recorded until gel formation. The gel phase obtained at the end of the measure resulted stable after the tube inversion test.

ARTICLE stand for 12 h. After this time the stability of the obtained material was tested by means of the tube inversion test. Dielectric spectroscopy A Hewlett-Packard impedance analyzer (HP 4294A) equipped with HP 16451B dielectric test fixtures was used at 298 K. The dielectric constant and the dispersion factor were measured as function of frequency.

Conclusions All of data collected shed light on the ability of organic salts used to harden ILs, giving rise to stable and thermoreversible ionogel phases. In particular, results collected by means of gelation tests evidence that a sort of structural complementarity between gelator and IL cation structure operates to determine the ionogel phase formation. Indeed, diimidazolium organic salts preferably harden aliphatic ILs, whereas dipyrrolidinium derivatives better work in the presence of aromatic ILs. Gelator being the same, CGC value and thermal stability of ionogels depend on both cation and anion structure of IL. Furthermore, these parameters also significantly affect the mechanism of formation and size of the aggregates characterizing gel phases. Results collected, using dielectric spectroscopy, evidence that in some cases a peak in the dissipation factor as a function of frequency can be detected. In these cases, a conductivity value significantly higher than that corresponding to IL solvent system can be measured. This should be of significant importance when application as electrochemical materials is taken into account. On the other hand, our ionogels show self-healing ability having both thixotropic and sonotropic behaviour. These properties, important for applications in quasi-solid-state DSSCs and as solid-like lubricants, are in our case significantly affected by the nature of gelator. Indeed, aromatic organic salts exhibit a better response to the action of external stimuli than the corresponding aliphatic ones.

Opacity Measurements Opacity measurements were recorded with a spectrophotometer. The opacity of the gel phases was determined with UV-Vis measurements as a function of time, at a wavelength of 568 nm and a temperature of 288 K. As described for RLS measurements, samples for a typical measurement were prepared by injecting into a quartz cuvette (light path 0.2 cm) the limpid hot solution of salt. Spectra were recorded until gel formation. The gel phase obtained at the end of the measurement was stable after the tube inversion test.

Acknowledgements

Thixotropy

Notes and references

A magnetic stirring bar (length 8 mm, height 3 mm) was added to a preformed gel in a screw-capped vial. The sample was stirred for 5 min at 1000 rpm and left to stand at room temperature for 12 h. The stability of the phase obtained was investigated by the tube inversion test.

*Corresponding Authors. E-mail: [email protected] (F. D’Anna), [email protected] (R. Noto) Phone: +3909123897540.

Sonotropy A gel, preformed in a screw-capped vial, was placed in an ultrasonic cleaning bath and irradiated for 5 min at the frequency of 45 kHz and with a power of 200 W. The suspension obtained was left to

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This work was supported by University of Palermo (2012-ATE0405: Catalytic Systems for the development of eco-sustainable processes) and MIUR (FIRB 2010 RBFR10BF5V: Multifunctional hybrid materials for the development of ecosustainable catalytic processes).

a

Dipartimento STEBICEF, Sezione di Chimica, Università degli Studi di Palermo, Viale delle Scienze-Parco d’Orleans II, 90128 Palermo (Italy). b Dipartimento di Fisica e Chimica, Università degli Studi di Palermo, Viale delle Scienze-Parco d’Orleans II, 90128 Palermo (Italy).

Electronic Supplementary Information (ESI) available: TGA tracks, decomposition temperatures, percentage of loss in weight, gelation test in organic solvents and figures of dissipation factor vs frequency for ionogels. See DOI: 10.1039/b000000x/

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