Protic Ionic Liquids for the Belousov–Zhabotinsky ... - ACS Publications

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Protic Ionic Liquids for the Belousov−Zhabotinsky Reaction: Aspects of the BZ Reaction in Protic Ionic Liquids and Its Use for the Autonomous Coil−Globule Oscillation of a Linear Polymer Takeshi Ueki,*,†,§ Ko Matsukawa,‡,§ Tsukuru Masuda,‡ and Ryo Yoshida*,‡ †

National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Department of Materials Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

‡

S Supporting Information *

ABSTRACT: Herein, we describe the physicochemical aspects of the Belousov−Zhabotinsky (BZ) reaction in hydrated protic ionic liquids (PILs) combined with different cations with a hydrogen sulfate ([HSO4−]) anion. PILs were prepared from the neutralization reactions of sulfuric acid with 13 different aliphatic amines. The amine structure was selected to investigate the effect of the number of active protons, alkyl chain length (hydrophilicity), and cationic linearity on the mechanism of the BZ reaction. The pKa values of PILs were significantly higher (pKa = 1.0−2.5) than those of inorganic acids (H2SO4 = −3.0; HNO3 = −1.4), the conventional proton source in a BZ reaction. A periodic redox oscillation was observed in Ru(bpy)3 when appropriate amounts of BZ reaction substrates (NaBrO3 oxidant; malonic acid reductant) were added to the hydrated PILs. A long-lasting BZ oscillation was realized when hydrophilic cations (ammonium, ethylammonium, and dimethylethylammonium) were employed. Interestingly, a large ΔA5000 (oscillation amplitude of absorbance for the scale of the oscillation stability observed after 5000 s from the initiation of the BZ reaction) was achieved for PILs possessing less than four carbon atoms in their cationic structure. The apparent BZ oscillation activation energy (Ea) in the hydrated PILs was estimated to be ∼40 kJ mol−1, which is 30 kJ mol−1 less than that observed in a conventional system. The catalytic reaction in the BZ reaction subprocess suppresses the total activation energy of the reaction. To realize long-lasting self-oscillating polymeric materials acting under milder condition, we demonstrated an autonomous coil−globule polymer chain transition (BZ-driven) that directly converts chemical energy to mechanical motion in hydrated PILs without freely diffusing Ru(bpy)3 metal catalyst. Ethylammonium hydrogen sulfate ([ea-H+][HSO4−]) is selected as the suitable proton source for the BZ reaction. A well-defined self-oscillating polymer that incorporated Ru(bpy)3 metal catalyst into the polymer backbone accompanied by good compatibility in hydrated [ea-H+][HSO4−] is successfully prepared by ATRP, followed by postmodification of the metal catalyst. The rhythmic solubility changes of the polymer under milder conditions, realized by combination with PILs, will expand the potential applications of PILs to novel functional materials.

1. INTRODUCTION

Meanwhile, we have developed self-oscillating polymeric materials that can express spatiotemporal structures as dissipative structures, far from equilibrium, under constant conditions, for example, autonomous swelling−shrinking oscillation or peristaltic motion of gels,26−28 autonomous unimer−micelle29 (or vesicle) oscillation of block copolymers,30,31 colloidosomes32 undergoing cell-like autonomous shape oscillation with buckling, and autonomous viscosity oscillation of polymer solutions.33−35 Self-oscillating polymeric materials are designed by copolymerization of the metal catalyst via the Belousov−Zhabotinsky (BZ) reaction. It is well known

Ionic liquids (ILs) are being recognized as unique solvents for a variety of applications, such as battery electrolytes,1 solvents for organic synthesis2,3 or polymerization,4,5 gas separation,6 functional membranes,7−10 and smart soft materials.11,12 Recently, ILs involving active proton(s) (protic ionic liquids; PILs) have attracted much attention13,14 as promising candidates in material science applications. These include proton-conducting media for nonhumidified fuel cells15−18 and (bio)polymer processing or preservation solvents for biorelated polymers (proteins, DNA, and enzymes).19−22 With a view to realizing efficient (P)ILs with a designed functionality, a number of synthetic methodologies of producing (P)ILs from biorelated chemicals are also available.23−25 © 2017 American Chemical Society

Received: February 9, 2017 Revised: April 13, 2017 Published: April 14, 2017 4592

DOI: 10.1021/acs.jpcb.7b01309 J. Phys. Chem. B 2017, 121, 4592−4599

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The Journal of Physical Chemistry B

Figure 1. Chemical structures and abbreviations of the PIL cations used in this study.

polymeric materials that can act under milder conditions than those that have been used to date.

as the oscillatory reaction that accompanies an autonomous periodic change of the redox state of metal catalysts having a relatively higher redox potential, such as ferroin, cerium, and Ru(bpy)3.36−38 The BZ reaction typically occurs in a strong acidic aqueous solution with an appropriate concentration of oxidant (NaBrO3) and an organic acid acting as the reductant (malonic acid; MA). On the other hand, we have recently reported that certain hydrated PILs, involving high proton activity comparable to acidic aqueous solution,14 show good potential as an alternative medium for BZ reactions, replacing strong acids, such as H2SO4 and HNO3, which are common proton source. Importantly, by using hydrated PILs possessing hydrogen sulfate ([HSO 4 − ]) or methane sulfonate ([CH3SO3−]), a stable and long-lasting oscillation with a shorter period was achieved under milder (higher pH) conditions relative to the traditional BZ reaction medium.39 In this study, we investigated the effect of the PIL cation on the BZ oscillation behavior, that is, oscillation stability, BZ substrate concentration, and temperature dependence of the period of oscillation, which is defined as the time interval from one absorbance minimum peak of oxidation to the next minimum peak. A total of 13 different kinds of PILs, including a [HSO4−] anion, have been systematically prepared by straightforward neutralization reactions of sulfuric acid with the corresponding amines (selected from aliphatic amines or ammonia, Figure 1). The effects of hydrophilicity (alkyl chain length), the number of protons, and the linearity of the alkyl chain in the resulting cationic structure on the BZ oscillation mechanism were also investigated. Furthermore, the apparent activation energy (Ea) of the BZ reaction, depending on the PIL chemical structure, has been successfully estimated from the Arrhenius analysis. Finally, we have prepared a well-defined, ILlike polymer consisting of N-isopropylacrylamide (NIPAAm), N-3-aminopropylmethacrylamide (NAPMAm), and Ru(bpy)3 (P(NIPAAm-r-NAPMAm-r-Ru(bpy)3NAPMAm) by atom transfer radical polymerization (ATRP), followed by postmodification of Ru(bpy)3 having an activated ester group to P(NIPAAm-r-NAPMAm) precursor.33,40,41 By using a random copolymer, the periodic coil−globule transition driven by the redox changes of the Ru(bpy)3 catalyst has been demonstrated. This achievement expands the development of self-oscillating

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. The PILs17 and Ru(bpy)3 comprising an activated ester group, (bis(2,2′-bipyridine)[1(4′-methyl-2,2′-bipyridine-4-carbonyloxy)2,5-pyrrolidinedione]-ruthenium(II) bis(hexafluorophosphate)) ([Ru(bpy)3NHS),42 were synthesized and characterized according to the procedures reported in the literature. The Ru(bpy)3Cl2 metal catalyst was purchased from Sigma-Aldrich. NIPAAm was donated by Kojin Corporation and purified by recrystallization using a toluene/hexane (1:10 by volume) mixed solvent. NAPMAm was purchased from Polyscience. All other chemical reagents were purchased from Tokyo Chemical Industry Co., Ltd and were used as received without further purification unless otherwise noted. The abbreviation expressions of the amine precursor of cations are as follows: ammonia [a], methylamine [ma], ethylamine [ea], butylamine [ba], hexylamine [ha], diethylamine [dea], dibutylamine [dba], piperidine [pp], ethylpiperidine [epp], dimethylamine [dmea], diethylmethylamine [dema], triethylamine [tea], and tributylamine [tba]. The corresponding protonated ammonium cation is indicated as [X-H+] (Figure 1), and [HSO4−] was employed as the counteranion for all of the PILs. 2.2. Synthesis of P(NIPAAm-r-NAPMAm-r-Ru(bpy)3NAPMAm). Self-oscillating polymer P(NIPAAm-rNAPMAm-r-Ru(bpy)3NAPMAm) was synthesized and characterized according to the procedure reported earlier.40,41 The P(NIPAAm-r-NAPMAm) precursor was first obtained by ATRP. NIPAAm (50.4 mmol) and NAPMAm (5.6 mmol) were dissolved in 40 mL of water/dimethyl formamide (DMF) (1:1 by volume) solution, which was previously purged by dry argon gas. CuBr (0.56 mmol), tris[2-(dimethylamino)ethyl] amine (Me6TREN) (0.56 mmol) as a catalyst for ATRP, and ethyl-2-bromoisobutyrate (0.56 mmol) as an initiator were then added to the solution, and the polymerization reaction was carried out under argon atmosphere for 24 h at 25 °C. After the polymerization reaction, the reaction mixture was dialyzed against 0.1 M Na2CO3 aqueous solution for 1 day and deionized water at 25 °C; then, the polymers were dried by 4593

DOI: 10.1021/acs.jpcb.7b01309 J. Phys. Chem. B 2017, 121, 4592−4599

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The Journal of Physical Chemistry B

Figure 2. BZ oscillation profiles obtained at 25 °C for (a) [ea-H+][HSO4−], (b) [ba-H+][HSO4−], and (c) [pp-H+][HSO4−]. BZ substrate conditions for these measurements are as follows: [MA] = 62 mM, [NaBrO3] = 84 mM, and [Ru(bpy)3] = 1.7 mM. The concentration of PIL in each measurement is fixed to be 1.3 M.

Figure 3. BZ oscillation stability (ΔA5000) of hydrated PILs as functions of (a) pKa and (b) the number of carbon atoms in the cationic structure. Crosses in (a) indicate the results for conventional strong acids (H2SO4 or HNO3) as proton sources. The inset shows the magnified view of the PIL results. Plots for stable and unstable oscillations are indicated by the red circles and blue squares, respectively. The ΔA5000 of [dea-H+][HSO4−] exhibiting intermediate stable oscillations is indicated by the green triangles.

wavelength employed to monitor the periodic solubility changes of the self-oscillating polymer.

lyophilization. Subsequently, postmodification of Ru(bpy)3 group was carried out. P(NIPAAm-r-NAPMAm) (100 mg) was dissolved in 500 μL of dimethyl sulfoxide (DMSO) and added to a DMSO solution (700 μL) containing Ru(bpy)3NHS (85 mg). Triethylamine (100 μL) was added to the solution and stirred for 24 h at 25 °C. The polymer solution was purified by dialysis against water for 3 days at 25 °C and finally dried by lyophilization. The copolymer compositions were determined by proton nuclear magnetic resonance (JEOL, JNM-LA400WB) using D2O as a solvent. The amount of Ru(bpy)3 conjugated to the polymer chain was measured by an ultraviolet−visible (UV−vis) spectrometer (Shimadzu, UV2500 PC). The number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (Mw/Mn) of P(NIPAAm-r-NAPMAm) were determined by gel permeation chromatography (GPC) (Tosoh, Tokyo, Japan). DMF containing 50 mM LiCl was used as an eluent. The calibration curve for the average molecular weight was obtained by poly(ethylene glycol) standard. 2.3. Observation of the Redox Oscillation of Ru(bpy)3 during BZ Reaction. The self-oscillation profile (absorbance at 455 nm) of the stirred aqueous solution comprising appropriate amounts of MA, NaBrO3, proton source (HNO3, H2SO4, or PILs), and Ru(bpy)3Cl2 (17 mM) was measured with a UV−vis spectrometer (Shimadzu, UV-2500 PC) at different temperatures (10−40 °C). The isosbestic point of Ru(bpy)3 (583.5 nm) was selected as the measurement

3. RESULTS AND DISCUSSION 3.1. Relationship between the PIL Structure and BZ Oscillation Stability. The self-oscillation profiles for three representative hydrated PILs, [ea-H+][HSO4−], [ba-H+][HSO4−], and [pp-H+][HSO4−], are shown in Figure 2. The results clearly indicate that the oscillation stability is strongly influenced by the chemical structure of the cations. A longlasting and stable oscillation was achieved when relatively shorter alkyl (hydrophilic) cations, such as [ea-H+][HSO4−], were utilized. In contrast, an unstable and short-lasting oscillation, decaying within 1000 s, was observed when a PIL with a longer alkyl (hydrophobic) cation ([ba-H+][HSO4−]) or a cyclic alkyl chain ([pp-H+][HSO4−]) was employed. Herein, to discuss the correlation between the chemical structure of the cation and the oscillation stability, the oscillation stability (ΔA5000) is defined as the amplitude of oscillatory absorbance after 5000 s of the BZ reaction. Figure 3a illustrates ΔA5000 as a function of PIL pKa values together with the results for inorganic strong acid (HNO3 and H2SO4) aqueous solutions, which are conventional BZ reaction media. The PIL pKa values (1.0−2.5) are higher than those of strong acids, such as H2SO4 and HNO3 (−3.0 and −1.4, respectively), indicating a weaker proton-releasing capability of PILs. Notably, not all of the PILs provide a stable oscillation 4594

DOI: 10.1021/acs.jpcb.7b01309 J. Phys. Chem. B 2017, 121, 4592−4599

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The Journal of Physical Chemistry B

Figure 4. BZ oscillation profiles using (a, b) [ma-H+][HSO4−] and (c, d) [ha-H+][HSO4−] as proton sources. The measurements were conducted at 15 °C (a, c) and 30 °C (b, d). BZ substrate conditions for these measurements are as follows: [MA] = 62 mM, [NaBrO3] = 84 mM, and [Ru(bpy)3] = 1.7 mM. The concentration of PIL in each measurement is fixed to be 1.3 M.

profile. PILs with hydrophilic cations, such as [a-H+][HSO4−], [ma-H+][HSO4−], [ea-H+][HSO4−], and [dmea-H+][HSO4−], afford long-lasting and stable oscillations, independent of pKa values. For instance, although the pKa values of [ea-H+][HSO4−] and [ba-H+][HSO4−] are comparable (1.4 and 1.8, respectively), only the former provides a stable oscillation. Figure 3b is a plot of ΔA5000 against the number of carbon atoms attached to the ammonium center as an index of the hydrophilicity of the cation. The oscillation stability depends on the number of carbons rather than the PIL pKa values. Interestingly, there is a critical threshold at C = 4. This is termed the “magic number” and determines the oscillation stability. When the number of carbons is less than four, a stable oscillation is achieved, whereas the oscillation stability is immediately lost if the carbon number exceeds four. This suggests that steric hindrance, based on the hydrophobic interaction via a longer alkyl chain of the cation with the hydrophobic Ru(bpy)3 center, prevents a favorable redox reaction. This consideration is collaboratively supported by the comparison of the temperature dependence on the oscillation profile for two PILs with different cationic hydrophilicities (Figure 4). At a lower temperature (15 °C), stable oscillation profiles are afforded for both the PIL solutions. However, at 30 °C, a stable BZ oscillation is achieved only for [maH+][HSO4−] (shorter alkyl group), whereas no oscillation occurs for the more hydrophobic [ha-H+][HSO4−]. These observations are not limited to [ha-H+][HSO4−]. Redox oscillations with PILs comprising longer alkyl chains become

unstable at a higher temperature (∼30 °C), even when a stable oscillation is observed at lower temperatures. PILs possessing more hydrophobic alkyl chains than [ha-H+], such as [dba-H+], [pp-H+], [epp-H+], and [tba-H+], do not provide oscillation even if the temperature is reduced to 15 °C. The temperature at which inhibition effect occurred depends on the chemical structure of the PILs. For example, [tea-H+][HSO4−] can induce redox oscillation of Ru(bpy)3 below 30 °C, but no oscillation can be observed above that temperature. This suggests that the hydrophobic interaction between a nonpolar (longer) alkyl group and Ru(bpy)3 becomes stronger when the temperature increases, leading to an unfavorable state for the redox reaction. On the other hand, in the case of hydrophilic PILs with shorter alkyl chains, a significant inhibition effect on the redox reaction will not occur, even at higher temperatures. Notably, there are three PILs, namely, [dmea-H+][HSO4−], [deaH+][HSO4−], and [ba-H+][HSO4−]) that possess the critical number of four carbon atoms within their cationic structure. Their oscillation profiles are different so that the oscillations for [dmea-H+][HSO4−] and [ba-H+][HSO4−] are stable and unstable, respectively, whereas [dea-H+][HSO4−] exhibits an intermediate state; oscillations with a smaller amplitude continue over 5000 s. This can also be predicted from the effect of steric hindrance based on the attractive hydrophobic interaction between the cation and Ru(bpy)3 moiety. In the case of [ba-H+][HSO4−], the nonpolar alkyl group is localized in one part of the cationic structure. This leads to a stronger hydrophobic interaction, resulting in an unfavorable state for 4595

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and other inorganic acids on the induction period, such as kinetic study on the reaction rate of bromination of the MA by means of UV spectroscopy,44 will be reported elsewhere. 3.2. Dependence of Activation Energy of the BZ Reaction on the Chemical Structure of Proton Source. It is well known that the oscillation frequency of the BZ reaction against temperature can be fitted to the Arrhenius relation.45 A number of efforts confirmed that the apparent activation energy (Ea) of the BZ reaction can be estimated from the Arrhenius analysis. We have employed this analysis to obtain further insight into the BZ reaction using different hydrated PILs (Figure 6). The Ea values calculated from the Arrhenius plot for

the redox reaction. In contrast, it is speculated that effective redox changes occur with [dmea-H+][HSO4−] because of dispersing four carbons around the cationic structure. The reason for the critical number of carbon atoms being four is still unclear and under investigation. Furthermore, we focused on the relationship between the period of oscillation (T) at 25 °C and the initial molar concentration of the BZ substrates when [ea-H+][HSO4−], categorized with the PILs that provide stable oscillation, was

Figure 5. Plot of period against the initial concentration of the BZ component. [ea-H+][HSO4−] is used as a proton source. Figure 6. Arrhenius plots for the BZ reaction using various PILs, [eaH+][HSO4−], [ba-H+][HSO4−], [dmea-H+][HSO4−], [dea-H+][HSO4−], [tea-H+][HSO4−] as proton sources, together with the results from conventional inorganic strong acids.

employed (Figure 5). The following empirical relation was obtained T = 0.627{[ea‐H+][HSO4 −]}−1.21[NaBrO3]−1.17 [MA]−0.158 (1)

H2SO4 and HNO3 as the proton source in the BZ reaction are 74.2 and 69.9 kJ mol−1, respectively. These values agree with the values reported in the literature.45 Although activation energy from the Arrhenius analysis of the BZ reaction depends on the concentration of the metal catalyst and BZ substrates, activation energies reported using H2SO4 as a proton source are 44.6 kJ mol−1 (for iron phenanthroline ([Fe(phen)32+] = 2 mM)), 71.6 kJ mol−1 (for iron bipyridyl ([Fe(bpy)32+] = 0.3 mM)), 62.4 kJ mol−1 (for Ce3+ = 2 mM), 64.9 kJ mol−1 (for Ce4+ = 5 mM), and 64.1 kJ mol−1 (for Mn2+ = 2 mM), where indicated with parentheses after each activation energy are catalysts and their concentration in the BZ reaction employed. On the other hand, the Ea values of hydrated PILs are independent of the chemical structure and are estimated to be ∼40 kJ mol−1, a lower value than that afforded from conventional BZ solutions. This implies that there are specific elementary chemical reaction processes only in PIL systems. We have reported that the BZ reaction using hydrated PILs exhibits a shorter oscillation period than that using a strong acid (HNO3) as the proton source. The waveform analysis of the BZ oscillation profiles confirm that the re-reduction process from Ru(bpy) 3 3+ to Ru(bpy) 3 2+ accompanying proton production becomes shorter by replacing HNO3 with a PIL [dema-H+][HSO4−].39 We have suggested that the free amine derived from the PIL ammonium cation acts as a proton acceptor that captures the proton produced in the re-reduction process of Ru(bpy)33+. This results in the acceleration of the rereduction kinetics. Thus, the neutral amine from the PIL acts as

This result indicates that the oscillation period strongly depends on the concentrations of [ea-H+][HSO4−] and NaBrO3, compared with the previously reported self-oscillation using HNO3 as the proton source (exponents of [HNO3], [NaBrO3], and [MA] are 0.743, 0.796, and 0.414, respectively).43 The specific strong concentration dependency can be explained by analyzing the waveform of the BZ reaction on the basis of the discussion on the Field−Kôrös−Neyes (FKN) mechanism (Figures S1−S3). The overall BZ reaction can be divided into the following three subprocesses: the consumption of the bromide ion (process A), the autocatalytic reaction of bromous acid with the oxidation of Ru(bpy)32+ (process B), and the production of bromomalonic acid and the proton accompanying the re-reduction of Ru(bpy)33+ to Ru(bpy)32+ (process C). Briefly, there are excessive amounts of active protons conserved in the form of [ea-H+] in addition to the freely released proton in the [ea-H+][HSO4−] solution. The stored proton appears to significantly accelerate the subprocess of the BZ reaction (process A rather than processes B and C) under the higher-concentration condition of [ea-H+][HSO4−] and NaBrO3, in terms of equilibrium (Supporting Information). We did not observe any induction period before the BZ oscillation started. It would be interesting to compare the effects of the chemical structure of PILs on the induction period during the accumulation of a crucial amount of bromomalonic acid necessary to induce the oscillation. Detailed investigations on the effect of the chemical structure of PILs 4596

DOI: 10.1021/acs.jpcb.7b01309 J. Phys. Chem. B 2017, 121, 4592−4599

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The Journal of Physical Chemistry B Table 1. Characterization of the Self-Oscillating Polymer [NIPAAm]/[NAPMAm]/[Ru(bpy)3 NAPMAm]

Mw/Mn

Mn (kDa)

90.9:2.6:6.5

1.13

17.9

P(NIPAAm-r-NAPMAm-r-Ru(bpy)3NAPMAm)

Figure 7. Periodic coil−globule oscillation profiles of the P(NIPAAm-r-NAPMAm-r-Ru(bpy)3NAPMAm) self-oscillating random copolymer in (a) 0.6 M, (b) 0.8 M, and (c) 1.3 M [ea-H+][HSO4−] solution. (d) Relationship between coil−globule oscillation period and the initial concentration of the BZ substrates when [ea-H+][HSO4−] is the proton source.

the main monomer with a small amount (typically several mol %) of Ru(bpy)3 group (P(NIPAAm-r-Ru(bpy)3). The LCSTtype phase separation temperature (Tc) of PNIPAAm alters depending on the redox state of the Ru(bpy)3 comonomer. When Ru(bpy)3 is reduced, the self-oscillating polymer becomes more hydrophobic, leading to a lower LCST-type Tc. Conversely, the Tc increases when Ru(bpy)3 is oxidized. However, we confirmed that the conventional self-oscillating polymer, P(NIPAAm-r-Ru(bpy)3), was not soluble in hydrated [ea-H+][HSO4−]. Byrne et al. have recently reported that the Tc of PNIPAAm is drastically changed by adding PILs in the polymer solution.46 They pointed out that the shift in Tc was consistently explained in terms of the Hofmeister series and also demonstrated that the Tc of PNIPAAm remarkably decreased in the presence of a certain amount of [teaH+][HSO4−] in solution. To improve the solubility of the selfoscillating polymer in hydrated PILs, we prepared P(NIPAAmr-NAPMAm-r-Ru(bpy)3NAPMAm) by a postmodification strategy. The polymer has a similar structure to PILs resulting from an amino residue of an NAPMAm precursor of the Ru(bpy)3 group.33 The characterization results of the selfoscillating polymer is summarized in Table 1. By utilizing the chemical structure of a self-oscillating random copolymer similar to that of ILs, the compatibility of the polymer with [eaH+][HSO4−] was remarkably improved.

a catalyst for process C. We therefore concluded that the catalytic reaction involving a free amine significantly suppresses the activation energy of process C, thereby contributing to a lower Ea in the BZ reaction. 3.3. Autonomous Coil−Globule Oscillation of the SelfOscillating Polymer in PILs. As previously mentioned, selfoscillating polymeric materials can be realized from coupling of the BZ reaction. The copolymerization of Ru(bpy)3 into PNIAAm chains changes the lower critical solution temperature (LCST)-type phase-transition temperature, depending on the redox state of Ru(bpy)3. The phase-transition temperature shifts to a higher temperature at the oxidized state than that at the reduced state. As a result, because of coil−globule changes, the self-oscillation of transmittance of the solution occurs at an appropriate constant temperature in the bistable region. Thus, the chemical energy of the BZ reaction is directly converted into mechanical motion of the polymers. This mechanism is now being recognized as a strategy to establish unprecedented biomimetic materials. To establish long-lasting self-oscillating polymeric materials acting under milder conditions, we next aimed at realizing an autonomous coil−globule oscillation of a linear polymer using a hydrated PIL; [ea-H+][HSO4−] was selected for the selfoscillating polymer system. A self-oscillating linear polymer is a random copolymer consisting of thermosensitive NIPAAm as 4597

DOI: 10.1021/acs.jpcb.7b01309 J. Phys. Chem. B 2017, 121, 4592−4599

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The Journal of Physical Chemistry B *E-mail: [email protected] (R.Y.).

Figure 7a−c illustrates the periodic coil−globule transition monitored as turbidity of the BZ solution, including hydrated [ea-H+][HSO4−], without the application of any on−off switching from the external environment. Autonomous transmittance oscillation could be clearly observed when the [eaH+][HSO4−] concentration was optimized properly at a fixed concentration for other BZ substrates. We also deduced the following empirical relation for period (T) as a function of the initial molar concentration of the BZ substrates (Figure 7d).

ORCID

Takeshi Ueki: 0000-0001-9317-6280 Ryo Yoshida: 0000-0002-0558-2922 Author Contributions §

T.U. and K.M. equally contributed to this work

Notes

The authors declare no competing financial interest.

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T = 3.93([ea‐H+][HSO4 −])−1.10 [NaBrO3]−0.900 [MA]−0.523

ACKNOWLEDGMENTS This study was supported in part by Grants-in-Aid for Scientific Research (Nos. 15H05495 and 26620164 to T.U., and Nos. 15H05758 and 15H02198 to R.Y.) from the Ministry of Education, Culture, Sports, Science and Technology of Japan, the Japan Society for the Promotion of Science (No. 14J09902 to T.M.), and a research fellowship to K.M. from the Japan Society for the Promotion of Science through the Program for Leading Graduate Schools (MERIT).

(2)

This correlation is similar to that obtained from the redox oscillation of Ru(bpy)3 without any polymer conjugation,43 except for a slightly stronger dependence on [MA]. The autonomous solubility oscillation of the random copolymer under milder conditions, achieved by combination with PILs, will expand the potential applications of PILs to novel functional materials.

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4. CONCLUSIONS In this study, we describe the BZ reaction using hydrated PILs comprising various cationic structures. The oscillation stability and Ea estimated from the Arrhenius analysis were systematically compared with the cationic structure of PILs. The number of carbon atoms attached to the ammonium center (hydrophilicity) of the cation governed the oscillation stability defined as ΔA5000. Interestingly, ΔA5000 drastically changed at the critical threshold number of carbon atoms (C = 4). The steric hindrance, based on the hydrophobic interaction between a nonpolar alkyl group and the Ru(bpy)3 center, is a possible factor that contributes to the loss of the oscillation stability. The value of Ea in hydrated PILs was determined to be ∼30 kJ mol−1 lower than that in HNO3 and H2SO4 aqueous solutions. This suggests the presence of a catalytic process that suppresses the activation energy of the elementary process of the BZ reaction in the PIL system. Finally, we successfully demonstrated the periodic coil−globule changes of the self-oscillating linear polymer in hydrated [ea-H+][HSO4−]. To date, H2SO4 and HNO3 have been typically used as the proton source for these reactions. It is noteworthy that neutralizing a strong acid by an amine base, such as [ea-H+][HSO4−], provides a longlasting, stable oscillation rather than the BZ oscillation using H2SO4 as the conventional proton source. Thus, in this study, we tried to expand the possibility of the chemical structure designability of the proton source. The structure designability of (P)ILs provides useful progress, challenges, and opportunities for designing functional materials that can express spatiotemporal structures. Moreover, a deeper insight into the complex BZ reaction mechanism is provided.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b01309. Discussion on the concentration dependency of the oscillation period in PIL system based on the consideration of the FKN mechanism (PDF)

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REFERENCES

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*E-mail: [email protected] (T.U.). 4598

DOI: 10.1021/acs.jpcb.7b01309 J. Phys. Chem. B 2017, 121, 4592−4599

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DOI: 10.1021/acs.jpcb.7b01309 J. Phys. Chem. B 2017, 121, 4592−4599