Kinetic Study on Achiral-to-Chiral Transformation of Achiral Poly

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Kinetic Study on Achiral-to-Chiral Transformation of Achiral Poly(diphenylacetylene)s via Thermal Annealing in Chiral Solvent: Molecular Design Guideline for Conformational Change toward Optically Dissymmetric Structures Kyo-Un Seo,† Young-Jae Jin,† Hyojin Kim,‡ Toshikazu Sakaguchi,§ and Giseop Kwak*,† †

Department of Polymer Science & Engineering, Polymeric Nanomaterials Laboratory, School of Applied Chemical Engineering, Kyungpook National University, 1370 Sankyuk-dong, Buk-ku, Daegu 702-701, Korea ‡ Daegu Technopark Nano Convergence Practical Application Center, 891-5 Daecheon-dong, Dalseo-ku, Daegu 704-801, Korea § Department of Materials Science and Engineering, Graduate School of Engineering, University of Fukui, Bunkyo 3-9-1, Fukui 910-8507, Japan S Supporting Information *

ABSTRACT: Achiral poly(diphenylacetylene)s (PDPAs: pMe3, pEt3, piPr3, pMe2O1, pMe2OD1, and mMe3) with different alkyl side chains at the para or meta position of the side phenyl ring were prepared to examine achiral-to-chiral transformations upon thermal annealing in a chiral solvent. All the para-substituted PDPAs showed significant circular dichroism (CD) enhancement upon annealing, whereas the meta-substituted polymer, mMe3, was inert to the same treatment. To investigate the kinetics, the asymmetric conformational change was monitored by observing the changes in the magnitude of circular polarization (gCD) or optical rotation. PDPAs with bulkier, round-shaped side groups (pEt3 and piPr3) had greater gCD values at equilibrium than pMe3 with a smaller side group. Moreover, the activation energy for the forward reaction (Eaf) was lower in the bulkier polymers than in pMe3 owing to enhanced miscibility with the chiral solvent. Similarly, the long alkyl chains of pMe2O1 and pMe2OD1 acted as internal plasticizers to lower their Eaf values relative to that of pMe3, whereas their gCD values at equilibrium were smaller than that of pMe3. The kinetics of the achiral-to-chiral transformation is discussed in detail based on the spectroscopic changes observed during the annealing process.

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INTRODUCTION

into chiral ones simply by dissolving them in appropriate chiral solvents and then heating the solutions. The solvent-to-polymer chirality transfer has been ascribed to asymmetric conformational changes in the intramolecular stack structure of the side phenyl rings, and the transformation mechanism is now very well established. Achiral-to-chiral transformations have been achieved under either hydrodynamic or thermodynamic control, with the optical activities of the resulting chiral polymers maintained semipermanently around room temperature, even in achiral solvents.19 This transformation method is also applicable to similar conjugated polymers with intramolecular stack structures and is expected to be very useful and universal for the development of novel OAPs. In general, PDPAs have extremely large free volumes owing to the semipermanent, glassy-state network structure of

Optically active polymers (OAPs) have a practical use as column chromatography packing materials or membranes for chiral resolution of organic racemic compounds.1−6 In particular, conjugated OAPs have additional potential applications as highly advanced optoelectronic device materials.7−10 There are several synthetic strategies to prepare OAPs. Polymers with optically dissymmetric structures of one-handed helicity and axial chirality are usually obtained from the corresponding chiral monomers. However, this synthetic approach is often labor intensive and expensive. To overcome these drawbacks, various methods for asymmetric polymerization of achiral monomers have recently been developed to produce highly stable OAPs.11−13 One unique method to obtain OAPs, which is very different from previous methods, is to transform achiral conjugated polymers into chiral ones via thermal annealing or aggregation (gelation) in chiral solvents.14−18 Especially, achiral poly(diphenylacetylene)s (PDPAs) have readily been transformed © XXXX American Chemical Society

Received: November 1, 2017 Revised: December 9, 2017

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randomly arranged rigid backbones in the amorphous phase; thus, PDPA membranes are highly gas and vapor permeable.20 High molecular weights can be readily achieved by group V transition metal catalysts, such as TaCl5 and NbCl5, with the aid of Lewis bases, such as organic tin or silane compounds.21,22 PDPAs are very stable either thermally or (photo)chemically owing to the protective effect of the bulky aromatic side groups and are also mechanically robust owing to the stiffness of the main chain.23 These excellent features guarantee that these polymers can be used practically for chiral resolution and optoelectronic device applications without any significant attenuation in function.24−27 PDPA-based OAPs have been synthesized from the corresponding chiral monomers, but this synthetic route is very labor intensive and expensive.28−36 In this respect, the above-mentioned achiral-to-chiral transformation approach has a considerable advantage over conventional methods because it uses various achiral polymers that have already been developed. However, the kinetics of the conformational change has not been studied to date; hence, the most suitable polymer for realizing conformational changes toward an energy-minimized, optically dissymmetric structure has not yet been identified. To address this issue, we prepared several achiral PDPAs (pMe3, pEt3, piPr3, pPh3, pMe2O1, pMe2OD1, and mMe3; Figure 1) with different alkyl (or aryl) side chains at the para or

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EXPERIMENTAL SECTION

Materials. All the polymers used in this work were synthesized following a previously reported method.37,38 All polymerization were performed under the same conditions by heating the reaction mixture at 80 °C for 1 h. All the polymers existed in the solid state at room temperature, although individual differences in brittleness were observed. The polymerization yield, weight-average molecular weight (Mw), number-average molecular weight (Mn), and polydispersity index (PDI) of pMe3, pEt3, piPr3, pPh3, pMe2O1, pMe2OD1, and mMe3 are summarized in Table S1. Chiral (S)-(−)-limonene ([α]20D: −108.0° to −118.0° (c = 10, EtOH), purity (GC): >95.0%) and racemic limonene ([α]20D: −1.0° to +1.0° (neat), purity (GC): >95.0%) were purchased from TCI (Tokyo, Japan) and used as received. Solution Preparation and Annealing Procedure for Spectroscopic Analysis. A 1.0 wt % solution was obtained by completely dissolving the polymer in chiral limonene at room temperature with stirring overnight. The solution was separated into individual vials, which were capped and heated at different temperatures for different times and then slowly cooled to room temperature. The solutions were precipitated into ethanol to eliminate chiral limonene and then dried under vacuum overnight. Subsequently, the annealed polymer was dissolved in toluene and diluted to a concentration of 5.0 × 10−4 mol L−1, and ultraviolet−visible (UV−vis), CD, and fluorescence (FL) emission spectra were measured at room temperature. For the [α]D measurements, the annealed polymer was dissolved in CHCl3 and diluted to a concentration of 2.0 × 10−4 g mL−1. Chiroptical Analysis. The magnitude of the circular polarization in the ground state is defined as gCD = 2 × ((εL − εR)/(εL + εR)), where εL and εR denote the extinction coefficients for left and right circularly polarized light, respectively. Experimentally, the value of gCD is determined by Δε/ε at the wavelength of the CD extremum. In order to obtain the Δε and ε, the CD and UV−vis spectra were actually measured with several hundred samples annealed at different temperatures in different time duration. The CD and UV−vis spectra measured with each polymer annealed at 80 °C in different time duration are represented in Figures S1−S5 to show which original spectral data were actually acquired. Measurements. The weight-average molecular weight (Mw) and the number-average molecular weight (Mn) of the polymers were determined by gel permeation chromatography (GPC) calibrated with a polystyrene standard using a JASCO liquid chromatography system consisting of PU-2080, DG-2080-53, CO-2060, and UV-2070 units with two polystyrene gel columns (Shodex KF-806L × 2, eluent: THF). UV−vis absorption and FL emission spectra of the solutions (SQ-grade cuvette with a path length of 10 mm) were recorded at room temperature on a JASCO V-650 spectrophotometer and a JASCO FP-6500 spectrofluorometer, respectively. The CD spectra of the solutions (SQ-grade cuvette with a path length of 1 mm) were recorded at 25 °C on a JASCO J-815 spectropolarimeter (in a single accumulation with a scanning rate of 500 nm min−1, bandwidth of 2 nm, and response time of 1 s) equipped with a temperature control unit. The [α]D values were measured at 20 °C using a KRUSS P3000 polarimeter.

Figure 1. Chemical structures of the PDPA derivatives used in this study.

meta position of the side phenyl ring. The achiral polymers were subjected to thermal annealing treatment in a chiral solvent, and the achiral-to-chiral transformation behavior of each polymer was compared in terms of kinetics. The conformational change could be considered as a first-order reaction including only one reactant and monitored by the changes in the magnitude of circular polarization (gCD) and optical rotation ([α]D) in chiroptical spectroscopy. The rate constant (k) and activation energy (Ea) for the conformational change were determined by the regression and Arrhenius analyses. The para-substituted polymers showed significant circular dichroism (CD) enhancement upon annealing, whereas the meta-substituted polymer did not show any CD enhancement. The gCD values at equilibrium were higher for PDPAs with bulkier round-shaped side groups (pEt3 and piPr3) than for pMe3 with a smaller side group, whereas the Ea values for forward reaction (Eaf) were lower for the former than for the latter. The long alkyl chains of pMe2O1 and pMe2OD1 acted as internal plasticizers to loosen the stacked structure of the side phenyl rings. This phenomenon made the asymmetric conformational change more efficient kinetically but less efficient thermodynamically, leading to relatively low Eaf and gCD values at equilibrium. The details of the achiral-to-chiral transformation will be discussed based on the observed spectroscopic changes during the annealing process.

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RESULTS AND DISCUSSION The present achiral PDPAs were synthesized according to a previously reported method.37,38 All polymerization reactions for the preparation of the PDPAs were conducted very carefully under the same conditions, e.g., temperature and time, to exclude any possible influence of the polymerization conditions on the degree of stacking between the side phenyl rings.36 This approach guarantees an exact comparative analysis of the annealing effect on the chiroptical properties of the polymers. All the polymers, except pPh3, were easily dissolved in chiral limonene, and then the solutions were annealed for various times at various temperatures. On the other hand, pPh3 was absolutely insoluble in chiral limonene probably owing to a B

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Figure 2. CD enhancement of (a) pMe3, (b) pEt3, (c) piPr3, (d) pMe2O1, and (e) pMe2OD1 solutions (c = 5.0 × 10−4 mol L−1 in toluene) with annealing time at different temperatures (40, 60, 80, 100, and 120 °C) in (S)-(−)-limonene.

Table 1. gCD and Kinetic Parameters of PDPA Derivatives for Achiral-to-Chiral Transformation during Thermal Annealing in Chiral Limonene reaction rate constant, kfb polymer pMe3 pEt3 piPr3 pMe2O1 pMe2OD1

gCD, ×10 a

3.7 4.8 5.0 3.1 3.0

−3

40 °C 3.51 7.02 6.45 2.83 2.54

× × × × ×

10−3 10−3 10−3 10−3 10−3

60 °C 19.8 13.3 10.0 7.01 6.22

× × × × ×

80 °C

10−3 10−3 10−3 10−3 10−3

60.6 49.5 52.8 22.8 21.7

× × × × ×

10−3 10−3 10−3 10−3 10−3

Eaf,c kJ mol−1

Eab,d,e kJ mol−1

ΔEa,f kJ mol−1

64 44 47 44 48

104 129 129 130 112

40 85 82 86 64

a gCD values at equilibrium, determined for c = 5.0 × 10−4 mol L−1 in toluene. bRate constant for the forward reaction. cActivation energy for forward reaction. dActivation energy for backward reaction. eDetermined from the data shown in Figure 5 and Table S3. fEab − Eaf.

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at equilibrium from the best-fit plots. However, when annealed at 100 and 120 °C, the polymers showed much smaller gCD values at equilibrium. Noticeably, the gCD value of pMe3 at equilibrium was smaller than those of pEt3 and piPr3 but larger than those of pMe2O1 and pMe2OD1. This observation indicates that pEt3 and piPr3 have higher probabilities than the other polymers of achieving a chiral conformation at equilibrium. This behavior may be explained as follows: Once the side phenyl rings are rearranged asymmetrically during the annealing process, the intramolecular stack structure will be tightly locked by the steric hindrance of the round-shaped, bulkier side groups, such as triisopropyl and triethyl groups. In contrast, the longer alkyl chains, such as octyl and octadecyl groups, will serve as plasticizers to loosen the intramolecular stack structure, making the conformational change incomplete thermodynamically. Thus, the asymmetric conformational change is more favorable for the two polymers with roundshaped, bulkier side groups (pEt3 and piPr3) than for the two polymers with long alkyl side chains (pMe2O1 and pMe2OD1). Unlike the para-substituted polymers, mMe3 showed no CD change (Figure S6), indicating that this polymer did not undergo an achiral-to-chiral transformation. The achiral conformation of mMe3 is already locked tightly owing to the static structure of the side phenyl rings continuously stacked in a helical manner.39 Effect of Intramolecular Stacking on gCD at Equilibrium. In the present PDPA derivatives, the wavelength of the FL emission band is a critical indicator of the degree of intramolecular stacking of the side phenyl rings.40 The FL band should appear at shorter wavelengths as the degree of intramolecular stacking decreases because the FL originates from intramolecular excimer emission based on the stacked structure. As described previously, all the polymers, except for mMe3, showed CD enhancement after annealing. Notably, polymers with shorter maximum FL wavelengths (λmax,FL) before annealing exhibited higher gCD values at equilibrium after annealing (Figure 3 and Table 1). For instance, piPr3 had the shortest λmax,FL of 485 nm but exhibited the highest gCD value at equilibrium after annealing. This trend probably occurs because the bulkier round-shaped side group facilitates the asymmetric rearrangement of the side phenyl rings owing to the lower degree of intramolecular stacking, but once equilibrium is reached, the chiral conformation is locked more tightly owing to the steric hindrance of the bulky side groups. This finding indicates that the gCD value at equilibrium is closely related to the degree of intramolecular stacking of the side phenyl rings before annealing. In other words, the gCD values of the PDPAs at equilibrium can be predicted by measuring the λmax,FL before annealing. As expected, mMe3, which did not exhibit a conformational change during the annealing process, had the longest λmax,FL. This observation supports the idea that the meta-substituted polymer has a highly static stacked structure, whereas the para-substituted polymers have conformationvariable stacked structures. Effect of Annealing Temperature on gCD at Equilibrium (Forward vs Backward Reaction). As mentioned previously, the gCD at equilibrium was highly dependent on the annealing temperature. For instance, the gCD of pMe3 reached equilibrium at about 3.7 × 10−3 when annealed at a temperature below 80 °C, whereas the gCD reached equilibrium at much smaller values when annealed at temperatures above 100 °C. This behavior indicates that the backward reaction, i.e., chiral-to-achiral transformation, began to occur in earnest

Figure 3. gCD values of the PDPAs at equilibrium after annealing as a function of λmax,FL before annealing.

Figure 4. Distributions of kinetic energy for the fractions of molecules participating in forward and backward reactions at different annealing temperatures.

higher aromaticity. Detailed information about the polymers and chiral limonene used in this study, the solution sample preparation, and the annealing procedure for spectroscopic analysis are described in the Experimental Section. CD Enhancement upon Annealing. In each of the parasubstituted polymers, the gCD value increased as the annealing time increased until reaching an equilibrium, whereas the reaction rate of CD enhancement showed a considerably dependence on the heating temperature. In the case of pMe3 (Figure 2a and Table 1), when annealed at 60 and 80 °C, gCD reached equilibrium at approximately 3.7 × 10−3. When annealed at 40 °C, gCD did not reach equilibrium within the annealing time tested in this study, but the best-fit plot indicated that the same value would be reached. In contrast, when annealed at 100 and 120 °C, gCD increased very quickly, but unexpectedly, reached equilibrium at much lower values of 1.9 × 10−3 and 1.0 × 10−3, respectively. Namely, these values are much lower than those achieved below 80 °C, even though the reaction rate was considerably faster at temperatures over 100 °C. This finding indicates that the achiral-to-chiral forward reaction primarily occurs below 80 °C, but the chiral-to-achiral backward reaction begins to occur above 100 °C, even in the chiral solvent. The same tendency was observed for the other polymers (Figure 2b−e and Table 1). When annealed below 80 °C, pEt3, piPr3, pMe2O1, and pMe2OD1 showed gCD values of 4.8 × 10−3, 5.0 × 10−3, 3.1 × 10−3, and 3.0 × 10−3, respectively, D

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Figure 5. CD attenuation for (a) pMe3, (b) pEt3, (c) piPr3, (d) pMe2O1, and (e) pMe2OD1 (fully annealed at 80 °C in chiral limonene, c = 5.0 × 10−4 mol L−1 in toluene) with annealing time at different temperatures (80, 100, and 120 °C) in racemic limonene.

between 80 and 100 °C, even in the chiral solvent. Accordingly, the activation energy for the forward reaction (Eaf) and the backward reaction (Eab) can be depicted as shown in Figure 4. Determination of Rate Constants (kf) for the Forward Reaction. Because the CD enhancement should result from an achiral-to-chiral conformational change, kf for the forward reaction can be obtained from eq 1 via regression analysis. As mentioned previously, when annealed below 80 °C, the achiralto-chiral forward reaction is predominant. Accordingly, kf was determined at three different annealing temperatures (40, 60, and 80 °C) (Table 1). In addition, kf can also be determined from the changes in [α]D, and the obtained values are similar to those determined from gCD (Table S2).

gCD (or [α]D ) = gCD,max (or [α]D,max )(1 − e−tk )

(1)

where gCD,max is the value of gCD at equilibrium and t is the annealing time. Determination of Eaf. Eaf was determined from the Arrhenius equation via regression analysis (eq 2). The Eaf values of all polymers based on gCD are summarized in Table 1. For pMe3, Eaf was determined to be 64 kJ mol−1, which is almost the same as that based on [α]D (60 kJ mol−1; Table S2). The Eaf value of pMe3 is higher than those of the other polymers (44−48 kJ mol−1), which suggests that a bulkier or longer alkyl group decreases the energy barrier for the achiralto-chiral transformation. E

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decrease at temperatures of over 100 °C because the solubility parameter has a significant dependence on temperature. Namely, the Eab values of the PDPA derivatives depend on the steric hindrance of the side group rather than on the miscibility with the solvent at high temperatures. Consequently, the bulkier polymers transform more easily into the chiral conformation, but their reversion to the achiral conformation is more difficult. Indeed, the differences between Eab and Eaf (ΔEa = Eab − Eaf) for the bulkier polymers were determined to be 64−86 kJ mol−1, which are much larger than that of pMe3 (40 kJ mol−1; Table 1). Among the bulkier polymers, as described previously, the asymmetric conformational change is thermodynamically more favorable for the two polymers with round-shaped side groups (pEt3 and piPr3) than for the two polymers with longer alkyl side chains (pMe2O1 and pMe2OD1). Moreover, the asymmetric conformational change is both kinetically and thermodynamically more favorable for the two polymers with bulkier round-shaped side groups than for pMe3 with a short alkyl chain. Accordingly, an energy diagram showing the possible pathways for the conformational changes is depicted in Figure 6.

Figure 6. Energy diagram showing the possible pathways for the conformational change in PDPAs.

k = Ae−Ea/ RT

(2)

where k, A, R, and T are the rate constant, frequency factor, gas constant, and absolute temperature, respectively. CD Attenuation upon Annealing in Racemic Solvent. To determine Eab for the chiral-to-achiral conformational change, the polymers fully annealed at 80 °C in chiral limonene were reannealed in racemic limonene at different temperatures for different times. The CD signals were attenuated with increasing annealing time, and as a result, equilibrium was reached at a gCD value of almost zero (Figure 5). Determination of Rate Constants (kb) for the Backward Reaction. The rate of decrease in gCD, i.e., kb, was obtained from eq 1 via regression analysis at different annealing temperatures (80, 100, and 120 °C) (Table S3). Determination of Eab. Eab was obtained from the Arrhenius equation via regression analysis eq 2. The Eab values are summarized in Table 1. For pMe3, Eab was determined to be 104 kJ mol−1, which is lower than those of the other polymers (112−130 kJ mol−1). This finding suggests that a bulkier or longer alkyl group increases the energy barrier for the chiral-to-achiral transformation. Effect of Side Group on Activation Energy. The gCD, Eaf, and Eab values of the present polymers are summarized in Table 1. The activation energy for the asymmetric conformational change in the intramolecular stack structure was expected to increase as the steric hindrance of side groups increases. Contrary to our expectation, however, pMe3 with a shorter alkyl chain had a higher Eaf but a lower Eab than the other polymers with longer alkyl chains (pMe2O1 and pMe2OD1) and bulkier side groups (pEt3 and piPr3). Most likely, the higher Eaf of pMe3 can be ascribed to a lower miscibility with chiral limonene relative to the other polymers. That is the bulkier or longer alkyl groups may serve as internal plasticizers to enhance miscibility with the chiral solvent. The solubility parameter of chiral limonene (δ = 16.5) is more similar to those of the polymers with bulkier or longer alkyl groups (e.g., δ = 16.8 for pMe2OD1) than to that of pMe3 (δ = 15.6). As a result, the intramolecular stack structure of the side phenyl rings will be loosened complementarily, despite the higher steric hindrance, and the energy barrier for the asymmetric conformational change will be decreased by the added molecular mobility. In contrast, the lower Eab of pMe3 should mainly result from the side phenyl rings of pMe3 being sterically less crowded than those of the other polymers. Further, the difference between the miscibilities with chiral limonene of pMe3 and the other polymers will significantly

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CONCLUSIONS In this study, thermal annealing in a chiral solvent was used to realize the achiral-to-chiral transformation of several achiral PDPAs with different alkyl side chains at the para or meta position of the side phenyl ring. The para-substituted polymers showed remarkable CD enhancement, but no CD enhancement was observed for the meta-substituted polymer. The kinetics of the asymmetric conformational change was discussed in detail based on the spectroscopic analysis of gCD and [α]D, and the most suitable polymers for revealing optically dissymmetric structures were identified. The asymmetric conformation change is kinetically and thermally more favorable for pEt3 and piPr3 with bulkier round-shaped side groups than for pMe2O1 and pMe2OD1 with longer alkyl side chains or for pMe3 with a shorter alkyl chain. Our findings are expected to be a very useful guideline for the molecular design of PDPA derivatives suitable for realizing asymmetric conformational changes through thermal annealing in a chiral solvent. In addition, these results will be helpful for developing conjugated OAPs for advanced applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02328. Figures S1−S6 and Tables S1−S3 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (G.K.). ORCID

Young-Jae Jin: 0000-0002-4670-4199 Giseop Kwak: 0000-0003-3111-0918 Notes

The authors declare no competing financial interest. F

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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) grants funded by the Korean government (MEST) (2017R1A2B4007348 and 2017R1A6A3A11034225). We thank Prof. Chang-Woo Cho (Kyungpook National University) for the use of a polarimeter.

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DOI: 10.1021/acs.macromol.7b02328 Macromolecules XXXX, XXX, XXX−XXX