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Visualization of the complexation between chloride and anion receptors usingvolume change of ionomer gels in organic solvents†

Kazuya Iseda,a Yohei Haketa,b Kenta Kokado,ac Hiromitsu Maeda,b Hiroyuki Furutad and Kazuki Sada*ac

Received 27th April 2012, Accepted 7th June 2012

DOI: 10.1039/c2sm25994j

Stimuli-responsive gels that can change their volumes drastically in response to various external

physical and chemical stimuli have been of much interest due to their various applications. Herein,

novel stimuli-responsive gels were achieved through the complexation between anion receptors and the

chloride anion of polystyrene ionomer gels in aprotic organic solvents. The volume expansions induced

by the addition of the receptors originated from breaking the aggregation of ionic groups and

enhancing the dissociation of the ion-pairs, accompanied by their colour changes.

Introduction

The design and synthesis of molecular receptors for selective

recognition and sensing has been thoroughly investigated over

the last three decades.1–6 In particular, anion recognition

processes by hydrogen bonding have been found in many

proteins and have been of considerable interest due to their

importance in the biological, medical, environmental and

chemical sciences.7–11 Many efforts in anion receptors and their

molecular recognitions are currently being directed towards the

implementation of functional materials, resulting in smart or

intelligent soft materials, such as supramolecular organogels12–14

and supramolecular hydrogels,15 chemosensors,16–20 molecular

capsules,21–24 liquid crystals,25,26 and artificial ion channels.27–32

However, the utility of anion binding properties as essential

construction principles for stimuli-responsive materials has

remained the great challenge.33

Ionomer gels are a kind of ionic polymer gel and have cova-

lently cross-linked hydrophobic polymer chains attached with

small amounts (less than 10 mol%) of ionic groups with free

counter ions. Generally, in non or less polar media, they collapse

because the ionic groups act as tightly bound ion-pair clusters or

highly aggregated species, although the main part of the polymer

chain has a good compatibility with the media.34,35 Thus, the

aGraduate School of Chemical Sciences and Engineering, HokkaidoUniversity, Kita 10, Nishi 8, Kita, Sapporo, 060-0810, Japan. E-mail:sadatcm@mail.sci.hokudai.ac.jp; Fax: +81-11-706-3473; Tel: +81-11-706-3473bCollege of Pharmaceutical Sciences, Ritsumeikan University, Kusatsu,525-8577, JapancDepartment of Chemistry, Graduate School of Science, HokkaidoUniversity, Kita 10, Nishi 8, Kita, Sapporo, 060-0810, JapandDepartment of Chemistry and Biochemistry, Graduate School ofEngineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan

† Electronic supplementary information (ESI) available. See DOI:10.1039/c2sm25994j

7490 | Soft Matter, 2012, 8, 7490–7494

complexation of the counter ions along the polymer chain with

the hydrophobic ion receptors should break the aggregation of

ion-pairs clusters and increase the osmotic pressure by the

promotion of the dissociation of the ion-pairs, which should

trigger a transition between a swollen and collapsed state in the

ionomer gels as schematically shown in Fig. 1. Herein, we report

that anion receptors triggered the macroscopic volume and

colour changes of ionomer gels with chloride anions. Our

molecular design for new stimuli-responsive gels is based on

anion receptors (R136 and R237) for chloride anions and a poly-

styrene-based ionomer gel with quaternary alkylammonium

chloride as the ionic group (EG-Cl) as shown in Fig. 2.

Experimental

Materials and measurements

Tri-(n-hexyl)amine, 4-(chloromethyl)styrene, azobisisobutyro-

nitrile (AIBN), styrene and divinylbenzene were purchased from

Tokyo Chemical Ind. Co. Styrene was purified by distillation

before being used. All solvents were purchased from commercial

suppliers and were used without further purification.

Compounds 1, 2 and sodium tetrakis[3,5-bis(trifluoromethyl)

phenyl]borate were synthesized according to reported methods.38

1H NMR spectra were measured on a Bruker AV300 and a JEOL

JNM-AL300 apparatus. Mass spectral data were obtained using

Fig. 1 Schematic illustration of a stimuli-responsive gel induced by an

anion receptor.

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Fig. 2 Molecular structures of (a) the ionic gel (EG-Cl and EG-TFPB)

and non-ionic gel (NG), and (b) the anion receptors (R1 and R2).

Scheme 1 Preparation of ionic monomers (1 and 2) and polymer gels

(EG-Cl, EG-TFPB and NG).

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a Perceptive Voyager RPMALDI TOF mass spectrometer and a

Bruker Daltonics Autoflex III.

Preparation of polymer gels

The polymerization conditions are summarized in Table 1, and

polymer gels (EG-Cl, EG-TFPB and NG) were prepared by

radical polymerization as shown in Scheme 1. A typical protocol

is as follows; 211 mg (0.50 mmol) of 1, 469 mg (4.50 mmol) of

styrene, 6.50 mg (0.05 mmol) of divinylbenzene (DVB), and

8.21 mg (0.05 mmol) of AIBN were placed in a glass tube and

dissolved in DMF 500 ml. The solution was degassed by three

freeze–thaw cycles and polymerized by heating at 65 �C for 24

hours. The feed ratio was adjusted to 1 (or 2) : styrene : DVB ¼10 : 90 : 1 and the obtained gels are denoted as EG-Cl and EG-

TFPB.38 As a reference, a non ionic gel (NG) was prepared under

the same copolymerization conditions (styrene : DVB ¼ 100 : 1)

without an ionic monomer. The formed gels, EG-X (X ¼ Cl or

TFPB) andNGwere washed by swelling in THF, and air-dried at

room temperature, then the samples were dried in vacuo at 45 �Cfor 24 hours.

Measurement of the degree of swelling

A capillary gel with a diameter of ca. 0.6 mm and length of ca.

2.5 mm was placed in the following typical organic solvents with

various polarities; toluene (dielectric constant 3 ¼ 2.4), chloro-

form (3 ¼ 4.8), tetrahydrofuran (THF) (3 ¼ 7.6), dichloro-

methane (3 ¼ 8.9), cyclopentanone (3 ¼ 13.6), acetone (3 ¼ 20.6),

N,N-dimethylformamide (DMF) (3 ¼ 36.7), and dimethyl sulf-

oxide (DMSO) (3 ¼ 46.5). After immersion for 6 hours, we

measured the lengths of the cylindrical gels to calculate the

degree of swelling (Q) of the gels defined with the following

equation;

Q ¼ Lwet/Ldry(mm/mm) (1)

where Lwet and Ldry are the lengths of the wet and dry gel. The

effect of the anion receptors on the degree of swelling was

Table 1 The polymerization conditionsa

SampleIonic monomer(mol L�1)

Sty(mo

EG-CI 1 (1.00) 9.0EG-TFPB 2 (1.00) 9.0NG 0 10.0

a Reaction temperature ¼ 65 �C. Reaction time ¼ 24 h. Solvent ¼ DMF.

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evaluated by the relative degree of swelling (Q0) as per the

following equation;

Q0 ¼ Q/Q0 (2)

where Q and Q0 are the degrees of swelling in the presence

(10�5 to 10�3 M) and absence of the anion receptors, respectively.

Results and discussion

Colour change of EG-Cl upon the addition of anion receptors in

acetone

We investigated the colour changes upon the addition of anion

receptors (R1 and R2) in acetone. The gels were soaked in

acetone for 6 hours at room temperature for equilibrium swelling

and transferred into solutions of R1 and R2 (1 mM) and were

kept in a similar way. The colourless wet gels turned orange and

deep green after immersion for 6 hours in the receptor solutions

as shown in Fig. 3. After sufficient washing with acetone, EG-Cl

was still stained with R1 and R2, whereas NG was completely

decolourized. This result indicated that the complexations

between the anion receptors and the chloride anions in the

polymer gels occurred in the ionomer gel of EG-Cl.

Change of the swelling abilities of EG-Cl upon the addition of

anion receptors in acetone

The swelling behaviours of NG and EG-Cl in the presence or

absence of the anion receptor in acetone were investigated as

shown in Fig. 4. Firstly,NG hardly absorbed acetone (Q¼ 1.16),

indicating the poor compatibility of the polystyrene polymer

backbone in acetone. The addition of the anion receptors

increased the degree of swelling: Q ¼ 1.16 and 1.19 for R1 and

R2, respectively. The anion receptors should not have the ability

to improve the compatibility and osmotic pressure of the

renel L�1)

DVB(mol L�1)

AIBN(mol L�1)

0 0.10 0.100 0.10 0.10

0.10 0.10

Soft Matter, 2012, 8, 7490–7494 | 7491

Fig. 3 Photographic images of (a and b) EG-Cl and (c and d) NG under different conditions: (1) dried gel, (2) swollen gel in acetone, (3) swollen gel in

acetone in the presence of an anion receptor (R1 or R2), and (4) swollen gel after sufficient washing with acetone.

Fig. 4 The degree of swelling (Q) of NG and EG-Cl in the absence and

presence of R1 and R2 in acetone.

Fig. 5 The log plot of anion receptor concentrations against the relative

degree of swelling (Q0) in acetone; (A) R1 and (-) R2.

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polymer gels in acetone, when the polymer gel does not have

chloride anions. For EG-Cl, the degree of swelling increased (to

1.80) in acetone compared to that of NG (Q ¼ 1.16). The

enhanced degree of swelling should be attributed to the genera-

tion of osmotic pressure and electrostatic repulsion due to the

dissociation of quaternary tetraalkylammonium chloride in the

polymer gel. In the case of EG-Cl, addition of the anion receptors

clearly affected the swelling abilities. The degree of swelling of

EG-Cl rose to 2.31 and 2.40 in the acetone solution of R1 andR2,

respectively. This is a good contrast to the effect of these recep-

tors on the degree of swelling for NG (NG, NG + R1, and NG +

7492 | Soft Matter, 2012, 8, 7490–7494

R2 as shown in Fig. 4). Therefore, these results clearly indicated

that the swelling ability of EG-Cl was enhanced by R1 and R2.

Then, we further investigated the anion receptor concentration

dependence of the degree of swelling, and the enhancement of the

swelling abilities was estimated by relative swelling ratios (Q0) asshown in Fig. 5. The relative degree of swelling (Q0) increasedwith increasing anion receptor concentration in each case. In the

case of R1, the relative degree of swelling abruptly increased at

2.5 � 10�5 M, and was saturated above 1.0 � 10�4 M. A similar

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Fig. 7 The relative degree of swelling (Q0) of EG-Cl, EG-TFPB, andNG

in the presence of R1 and R2 in acetone and CH2Cl2.

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concentration dependence was observed for R2, but Q0 sharplyincreased at 1.0� 10�5 M and was saturated above 5.0� 10�5 M.

Therefore, both of the anion receptors provided a similar

concentration dependence on Q0, and R2 affected the swelling

abilities at much smaller concentrations than R1. Thus, R2 was

more capable of stimulating a volume change of the ionomer gel.

This should be attributed to the difference of their anion binding

abilities.

As the model system, the anion-binding properties of R1 and

R2 for the chloride anion in acetone were studied by the UV/vis

absorption spectral changes upon the addition of tetra(n-butyl)

ammonium chloride (TBACl). In the UV/vis titrations, the

absorption maximum at 513 nm was decreased gradually for R1,

and the absorption maximum at 446 nm was shifted and

enhanced for R2 as shown in Fig. S1†. According to reported

procedures,39 the binding constants (Ka, M�1) of R1 and R2

toward the chloride anion were estimated as 226 000 and

1,420 000 M�1, respectively, which are comparable with those in

CH2Cl2.36,37 This result indicated that both the anion receptors

have enough binding abilities for the chloride anion in them, and

the complexation should promote the dissociation of quaternary

ammonium chloride or the breaking of the aggregation of the

ionic groups to enhance swelling abilities.

In order to clarify the role of complexation, we tried the

removal of the anion receptor from the swollen gels in acetone by

soaking them in a 10 mM acetone solution of TBACl. After

immersion for 6 hours at room temperature, the coloured gels

collapsed and turned nearly colourless as shown in Fig. S2†.

Further cycles of immersion in the solution of R1 or R2, TBACl

and fresh acetone provided cyclic swelling behaviours as shown

in Fig. 6. This result clearly indicated that the volumes of EG-Cl

were controlled by the addition of the anion receptors, and they

should be regarded as a stimuli-responsive materials on the basis

of anion recognition.

Counter anion effect on the swelling behaviours upon the addition

of anion receptors in acetone and dichloromethane

To demonstrate the essentiality of the complexation for the

enhancement of the swelling abilities, we compared the swelling

behaviours of EG-TFPB with EG-Cl in acetone and CH2Cl2

Fig. 6 Changes of the relative degree of swelling (Q0) of EG-Cl upon the

addition of anion receptors and TBACl in acetone.

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upon the addition of anion receptors. As shown in Fig. 7, the

relative degree of swelling (Q0) of EG-TFPB didn’t change on

addition both of R1 and R2, and the values in acetone and

CH2Cl2 were 1.01 and 1.00 for R1, and 1.00 and 0.99 for R2,

respectively, which are comparable with those of NG. This result

clearly indicated that the degree of swelling of EG-Cl was

enhanced by the complexation between the chloride anions in the

ionomer gel and anion receptors.

Solvent dependence of the swelling behaviours of EG-Cl

For the evaluation of the solvent dependence of the swelling

behaviours of EG-Cl, the relative degree of swelling (Q0) in

various organic solvents was investigated as shown in Fig. 8.

Starting from the non-polar solvents, the relative degree of

swelling was almost unchanged in toluene (dielectric constant 3¼2.4, Q0 ¼ 1.00 for R1, 1.02 for R2) and chloroform (3 ¼ 4.8, Q0 ¼1.04 for R1, 1.05 for R2). In the aprotic non-polar media, the

anion binding of these anion receptors should have a high affinity

to chloride anions but it didn’t affect the swelling ability. Thus,

even in the presence of the anion receptors, the dissociation of

quaternary alkylammonium chloride in EG-Cl was completely

suppressed owing to their low polarity, and the ionic groups in

the polymer gels would be tightly bound ion-pairs. The receptors

should recognize the chloride anion as an ion-pair. On the other

hand, the degree of swelling increased with increasing polarity,

i.e., dielectric constants, and the maximum Q0 was observed in

acetone as shown in Fig. 8. This increase was explained simply by

Fig. 8 Swelling behaviours of EG-Cl in various organic solvents that

possess different dielectric constants (3). The dielectric constants of these

solvents are shown in parentheses.

Soft Matter, 2012, 8, 7490–7494 | 7493

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the promotion of ionic dissociation due to the increased polarity

of the media. Finally, in the highly polar solvents such as DMF

(3 ¼ 36.7) and DMSO (3 ¼ 46.5), the relative degree of swelling

decreased to 1.0 (Q0 ¼ 1.02 in DMF with R1, 1.00 in DMSO with

R1, 1.05 in DMF with R2, and 1.00 in DMSO with R2, respec-

tively), indicating that the receptor had no effect on the increase

of the degree of swelling. It is mainly because the ionic groups of

EG-Cl should be dissociated fully in these media, and thus,

the swelling abilities were not enhanced by the addition of the

receptors, although the receptors had enough affinity to the

chloride anions. Therefore, the degree of swelling was controlled

mainly by the dissociation of the ion-pairs induced by the

complexation of the hydrophobic receptors.

Conclusions

In conclusion, we demonstrated the macroscopic volume and

colour changes of the polystyrene-based ionomer gels triggered

by the addition of anion receptors as stimuli. The swelling and

deswelling of the gels should be induced by the result of the

complexation and regarded as the visualization of the molecular

recognition processes of the polymer chains.40 This should open

new perspectives in the utility of anion receptors in materials

science and should suggest new molecular designs as chemo-

mechanical systems.41,42 Reliable molecular recognitions that

have been well documented in the field of supramolecular

chemistry should provide various stimuli-responsive materials.

Acknowledgements

Financial support for this research was provided by the Grant-in-

Aid (B) no. 23354801, Nanotechnology Network Project

(Kyushu-area Nanotechnology Network) and the Grant-in-Aid

for the Global COE Program, ‘‘Catalysis as the Basis for the

Innovation in Materials Science’’ from the Ministry of Educa-

tion, Culture, Sports, Science and Technology, Japan (MEXT),

and PRESTO/JST (2007–2011).

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Addition and correction Note from RSC Publishing This article was originally published with incorrect page numbers. This is the corrected, final version.

The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.

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