Interchain Helically π-Stacked Assembly of Cationic Chiral Poly(para

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Interchain Helically π‑Stacked Assembly of Cationic Chiral Poly(paraphenylene) Derivatives Enforced by Anionic π‑Conjugated Molecules through Both Electrostatic and π−π Interactions Kazuyoshi Watanabe, Zemeng Sun, and Kazuo Akagi* Department of Polymer Chemistry, Kyoto University, Katsura, Kyoto 615-8510, Japan S Supporting Information *

ABSTRACT: Herein, a novel approach for preparing ionic, helically π-stacked polymer assemblies with high dissymmetry factors is presented. We synthesized cationic chiral poly(para-biphenylene) [PPP1] and poly(para-terphenylene) [PPP2] derivatives by introducing alkyl substituents containing both asymmetric centers and quaternary ammonium cations into the side chains of the polymers. We discovered that these polymers have an intrachain helical structure bearing one-handed twisted main chains in solution; however, there was only a slight preference for one twisted conformation over its opposite. Subsequently, by adding an anionic π-conjugated molecule to the cationic polymers, we prepared polymer assemblies bearing an interchain helically π-stacked structure between the polymers and the molecules. Additionally, we clarified that the anionic molecule functioned as the binding species that closed the cationic π-conjugated polymers through both electrostatic and π−π interactions. Although the polymer assembly of PPP1 was dissociated upon thermal heating, the formation of the polymer assembly of PPP2 was even developed due to thermal treatment of the polymer chains. The helically π-stacked polymer assembly showed circular dichroism in both absorption and luminescence, yielding high dissymmetry factors of 10−2−10−1. This approach using cooperative intermolecular electrostatic and π−π interactions should be useful for preparing various types of electrically and chiroptically active polymer assemblies.

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INTRODUCTION Helical conjugated polymers (HCPs) have recently attracted interest because of their peculiar optoelectronic functionalities such as induced solenoid magnetism,1 circularly polarized luminescence (CPL),2−5 and nonlinear second harmonic optics.6 Therefore, they are regarded as next-generation plastic materials. Several approaches for synthesizing HCPs have recently been reported, and among them, the introduction of a chiral moiety into the side chain of the polymer is the most simple and straightforward method. Various types of HCPs based on polyacetylene [PA],7 poly(para-phenylene) [PPP],8 poly(meta-phenylene) [PMP],9 polypyrrole,10 polythiophene [PT],10 and polyfluorene [PF]5,11 derivatives, all bearing chiral substituents in their side chains, have been synthesized. These HCPs form helically associated species through supramolecular assemblies and exhibit chiroptical properties. Meanwhile, polymers with zigzag main chains such as PMP and poly(meta-phenylene ethynylene) derivatives are known to have an intrachain helicene-type helical structure resembling a spring in the shape of a coil.2 Although it has been assumed that polymers with linear conjugated skeletons, such as the PPP derivatives, have an intrachain helically twisted main chain with a certain degree of dihedral angle between the phenylene units, no direct evidence has yet been presented. © 2015 American Chemical Society

The degree of helicity is evaluated with the dissymmetry factor (expressed by g), and the dissymmetry factor of a chiral molecule in an isolated state is very small, on the order of 10−3−10−4. Hence, it is desirable for molecules to have supramolecular structure or to become a molecular assembly to attain large dissymmetry factors simply because chirality amplification is expected in a molecular assembly. In the case of conjugated polymers, spontaneous alignment and helical arrangement using liquid crystallinity are promising ways to attain an amplified dissymmetry factor.12,13 High dissymmetry factors can be attained when the conjugated polymers are helically arranged in thermotropic or lyotropic chiral liquid crystal phases. However, aligned polymer films often encounter artifacts in circular dichroism (CD) and CPL spectra causing inevitable difficulty and uncertainty in evaluating the intrinsic dissymmetry factor through chiral spectroscopic measurements. Herein, we present a novel approach for preparing helically π-stacked assemblies of ionic conjugated polymers with high dissymmetry factors. The assemblies are enforced in stability by cooperatively functioning electrostatic and π−π interactions between cationic π-conjugated polymers and anionic aromatic Received: January 12, 2015 Revised: March 25, 2015 Published: March 27, 2015 2895

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Scheme 1. Cationic Chiral Poly(para-phenylene) Derivatives (PPP1 and PPP2) and Anionic Additives (SNap and A1−A4)

Figure 1. UV−vis absorption spectra, CD spectra, and gabs factors of (a) the PPP1s and (b) the PPP2s in methanol (c = 5.0 × 10−5 M). (c) Schematic model of a poly(para-phenylene) derivative with transition dipole moments.

molecules. We synthesized cationic chiral poly(para-biphenylene) [PPP1] and poly(para-terphenylene) [PPP2] derivatives where cationic and asymmetric center containing chiral moieties were incorporated in the side chains. CD measurements indicate that these polymers have an intrachain helically

twisted structure where one helical sense is predominant. This report is the first to demonstrate intrachain helicity due to the one-handed twisting of the conjugated backbone in the PPP derivatives. Furthermore, when the anionic achiral naphthalene derivative was added to the polymer solution, a polymer 2896

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wavelengths of absorption and CD bands, the molar circular dichroism (Δε), and the absorption dissymmetry factors (gabs) are summarized in Tables S1−S4 of the Supporting Information. Both PPP1 and PPP2 showed three CD bands with a Δε of approximately 2−13 M−1cm−1 in the wavelength region of 220−300 nm, which correspond to the absorption of phenylene rings (green arrow in Figure 1c). In fact, previous studies revealed that the axially chiral biphenyl derivatives showed three CD bands in the 200−300 nm region.14−18 Mori et al.,14 who reported one of the studies about chiral biphenyls, demonstrated theoretically and experimentally that axial chirality was induced in biphenyls by introducing asymmetric centers at the ortho-position of the phenyl−phenyl bond. They also demonstrated that most of the CD signals were canceled out between the rotamers because their axial chirality easily switched between P- and M-conformations via axial rotation (Figure 2a) due to the small steric hindrance of the chiral substituent. Therefore, these chiral biphenyl derivatives

assembly was formed. Although the polymer assembly of PPP1 was dissociated upon thermal heating, that of PPP2 was enhanced due to thermal arrangement of the polymer main chains. By investigating their chiroptical properties, the intrachain and interchain helical structures coexisting in the polymer assembly were unambiguously identified. The polymer assemblies exhibited high dissymmetry factors of 10−2−10−1 that were increased by 10−100 times by chirality amplification upon formation of the polymer assembly. Additionally, the annealing of a disperse system such as a solution of the PPP2 polymer assembly is also an effective method for enhancing the dissymmetry factor. This enhancement occurs because annealing can improve the higher-ordered structure of conjugated polymers, similar to the heating effect in the liquid crystal state.

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RESULTS AND DISCUSSION Synthesis of PPP1 and PPP2. Cationic chiral poly(paraphenylene) derivatives were synthesized by introducing both chiral carbons and tetraalkylammonium cations into the side chains. The polymers containing biphenylene and terphenylene moieties as repeating units are abbreviated as PPP1 and PPP2, respectively (Scheme 1). The synthesis routes for PPP1s and PPP2s are shown in Scheme S1 of the Supporting Information. The counteranion was purified by an ion-exchange process. Low molecular weight oligomers and salts were removed by dialysis with a regenerated cellulose dialysis membrane (Spectra/Por 6, molecular weight cutoff: 8000). Polar solvents such as water and methanol were good solvents for the polymers, whereas typical organic solvents such as acetone, chloroform, and tetrahydrofuran (THF) were poor. Chiroptical Properties of PPP1 and PPP2. Figure 1, panels a and b depict UV−vis absorption spectra, CD spectra, and absorption dissymmetric factors (gabs) of the PPP1s and PPP2s in methanol. The concentrations were kept constant at 5.0 × 10−5 M (expressed in terms of the repeating unit). In the UV−vis absorption spectra, all polymers showed absorption bands in the 200−380 nm wavelength region, which originates from the π−π* transition of the π-conjugated backbone. The absorption bands near 340 nm correspond to the transition dipole moment parallel to the main chains (orange arrow in Figure 1c), whereas the absorption bands ranging from 200− 300 nm correspond to those on the phenylene moieties (green arrow in Figure 1c).14−18 Even when dissolved in water or a methanol−water mixture as an alternative solvent, the polymer exhibited nearly the same absorption spectrum as in methanol (see Figure S1 of the Supporting Information). Moreover, the shapes and intensities of the absorption bands remained unchanged even when the polymer solutions were heated to 60−95 °C (see Figures S2−S5), which indicates that the absorption band is attributable to the isolated polymer chain but not the aggregated one. Interestingly, PPP2 even in dilute solution of water showed an absorption band located at a longer wavelength (350 nm) that was blue-shifted with increasing temperature (Figures S4e and S5e). This shift implies that PPP2 partially aggregates in water, which may be because the conjugated backbone is highly hydrophobic, and the nonsubstituted phenylene rings impart easy π-stackability between the main chains. The CD spectra of the (R,R)- and (S,S)-enantiomers of the PPP1s and PPP2s showed Cotton effects with mirror images in sign, which imply that the optical activity of the polymers strictly depends on the chirality of the side chains. The

Figure 2. (a) Switching of axial chirality on a biphenyl moiety between the P- and M-conformations through internal rotation. Models of intrachain helicity in the (b) PPP1 and (c) PPP2 main chains. The chiral side chains containing the asymmetric center are located on the ortho-positions of all phenylene−phenylene (Ph−Ph) linkages of the PPP1 main chain. Meanwhile, the PPP2 main chain has racemic bonds between the nonsubstituted phenylenes (dot circles) where no axial chirality is induced due to the absence of chiral side chains. 2897

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Figure 3. (a) UV−vis absorption spectra, CD spectra, and gabs factors of (R,R)-PPP1−SNap (1:4 mol/mol) in water at various temperatures. The concentration of (R,R)-PPP1 is 2.0 × 10−5 M. (b, c) SEM images of (R,R)-PPP1−SNap (1:4 mol/mol) cast on a sample stage.

exhibited weak CD bands with a Δε of 2−8 M−1cm−1. From the aforementioned results, the weak CD bands of PPP1 and PPP2 indicate axial chirality on the neighboring phenylene rings in the polymer main chain, that is, intrachain helicity due to the chiral side chains containing asymmetric centers. The induction of axial chirality is also supported by the fact that PPP1 and PPP2 exhibited CD couplets in the wavelength region of 200−260 nm, whereas monomer 6 exhibited a monosignate CD band (Figure S6). According to the gabs values on the order of 10−4 (see also Tables S1−S4), it should be emphasized that although intrachain helicity exists in the PPP main chains, there is only a slight preference for one twisted conformation over the reverse. The small gabs value agrees with the argument that the asymmetric-center-induced axial chirality on the biphenyl moieties easily switches between P- and Mconformations. Furthermore, the signs of the CD couplets indicate the predominant helical sense of the intrachain helicity according to the exciton chirality theory.19 For example, in the 200−260 nm region, (R,R)-PPP1 showed a negative bisignate Cotton effect, that is, a CD couplet consisting of positive and negative Cotton effects at shorter and longer wavelengths, respectively. This means that the neighboring phenylenes in the (R,R)-PPP1 main chain prefer the M-conformation, and therefore the polymer has intrachain helicity with predominant left-handedness. Furthermore, PPP1 showed a weak CD band at approximately 340 nm corresponding to the main chain absorption, whereas PPP2 showed no CD band. In PPP1, the chiral side chains are located on the ortho-positions of all phenylene− phenylene (Ph−Ph) linkages of the main chain; hence, the main chain absorption also exhibited a CD signal due to the slight predominance of a twisted conformation of the πconjugated backbone (Figure 2b). However, PPP2 has a racemic bond between nonsubstituted phenylenes where no axial chirality is induced due to the absence of nearby chiral side chains. As a result, the axial chirality of the biphenylene moieties was unable to extend over the main chain (Figure 2c). Except for PPP2 in water, where the polymer tends to aggregate, the Cotton effects of PPP1 and PPP2 remained unchanged in both shape and intensity even for high

temperatures such as 60−95 °C, and they were also independent of the mixing ratio of the solvent (Figures S1− S5). These results indicate that the observed Cotton effect is not due to chiral aggregation but instead due to the intrachain helicity of the conjugated polymer backbone in an isolated chain. The results herein are the first to demonstrate intrachain helicity in the π-conjugated backbone of the PPP derivatives bearing the chiral side chains. It can be argued that the main chain conformations of PPP1 and PPP2 are sensitive to and depend on the bulkiness of chiral side chains. Chiral Assembly of PPP1−SNap. We investigated the structural changes of PPP1 and PPP2 upon addition of various anionic compounds (see Scheme 1). The synthetic routes for the anionic compounds are shown in Scheme S2 of the Supporting Information. We found that the polymers form a chiral assembly with an anionic sulfonate-substituted naphthalene derivative (abbreviated as SNap). Figure 3, panel a shows the UV−vis absorption spectra, the CD spectra, and the gabs factor of a mixture of (R,R)-PPP1 and SNap [abbreviated as (R,R)-PPP1−SNap] (1:4 mol/mol) in water. In the UV−vis absorption spectrum, (R,R)-PPP1−SNap displayed a new absorption band at a longer wavelength (λ = 349 nm) than the inherent band of PPP1, which suggests the formation of a polymer assembly with a π-stacked structure. Concurrently, in the CD spectrum, a new negative bisignate Cotton effect was observed in the 300−400 nm region, which corresponds to the polymer main chain. This effect may be due to the exciton coupling between the π-conjugated main chains of the polymers. Specifically, PPP1 forms a chiral assembly with an interchain helically π-stacked structure with the aid of SNap. According to the exciton chirality theory,19 the helical sense of the chiral assembly is left-handed (M-helicity). The new absorption band at 349 nm and the corresponding CD band both decreased in intensity with increasing temperature and ultimately disappeared (Figure 3a). This change indicates that these new bands are attributable to the polymer assembly formed with the aid of SNap. The bisignate Cotton effect in (S,S)-PPP1−SNap showed the opposite sign as the effect seen with (R,R)-PPP1−SNap (Figure S7), which indicates that the helicity of the polymer assembly depends on the chirality of the 2898

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Figure 4. Models of the hierarchical structure of the (R,R)-PPP1 main chains in the chiral assembly. The yellow transparent belts represent twisted π-conjugated backbones of the polymer. (a) Right-handed intrachain twisted structure of the (R,R)-PPP1 main chain. (b) The π-stacked structure among the three twisted main chains of (R,R)-PPP1. (c) The lef t-handed interchain helically π-stacked structure consisting of the twisted (R,R)PPP1 main chains.

polymer side chains. The gabs value was +3.59 × 10−3 (at 344 nm) for (R,R)-PPP1−SNap (1:4 mol/mol), which is approximately 10 times larger than the value for the isolated chain of (R,R)-PPP1 (see Table S1). This increase implies that chiral amplification occurred upon formation of the polymer assembly. This polymer assembly was also confirmed using scanning electron microscopy (SEM). As seen in Figure 3, panels b and c, SEM images of PPP1−SNap show entangled morphologies of the polymer bundles that are several hundred nanometers in length. In addition, we measured the average particle sizes of the polymer and the assemblies in water using dynamic light scattering (DLS). The results are summarized in Table S5. Although the average particle size of (R,R)-PPP1 was evaluated to be 334.9 ± 112.5 nm, those of the assemblies, (R,R)-PPP1−SNap = 1:2 and 1:4 (mol/mol) were evaluated to be 765.1 ± 216.5 and 865.9 ± 236.4 nm, respectively. It is evident that the average particle size of the polymer increases with increasing the amount of SNap. Importantly, (R,R)-PPP1 showed two types of bisignate Cotton effects at approximately 230 and 360 nm in the CD spectra, which corresponded to the intrachain and interchain helicity, respectively. The CD bands at approximately 230 nm increased in intensity, and the sign was reversed upon formation of the polymer assembly. Specifically, (R,R)-PPP1 showed a negative bisignate Cotton effect in an isolated chain state and a positive bisignate one in an aggregated state. This indicates that (R,R)-PPP1 in an aggregated state prefers to form right-handed intrachain helicity. The increase in the intensity of the CD bands near 230 nm may be from suppression of the internal rotation around the Ph−Ph linkage upon formation of the assembly, leading to an enhancement in intrachain helicity. Interestingly, recent work demonstrated that

the interchain helically π-stacked structure in the PPP derivatives is favorably formed when the intra- and interchain helical senses are opposite one another.20 This corroborates the present experimental result that (R,R)-PPP1 with right-handed intrachain helicity formed the interchain helically π-stacked structure with left-handedness (Figure 4). The helical sense of the intrachain helicity changes depending on the interchain helicity in the chiral assembly because the internal chirality of the aromatic main chain easily switches between P- and Mconformations through axial rotation around the Ph−Ph linkage, which results in the inversion of the intrachain helical sense. Consequently, intra- and interchain helicity coexist in the assembly of (R,R)-PPP1 with SNap, and the polymer assembly has the hierarchical structure that is shown in Figure 4. Notably, the CD band near 230 nm is attributed to (R,R)-PPP1 but not SNap. In the UV−vis spectra, the absorption band of SNap at 270 nm increased in intensity with an increasing amount of SNap (see Figure S8), but no notable change was observed in the CD spectra. This spectral trend indicates that the CD band near 230 nm is not from the induced CD of SNap but instead from the inherent CD of (R,R)-PPP1. We also found that both electrostatic and π−π interactions between the cationic PPP derivative and the anionic additive are indispensable for the formation of a polymer assembly with a helically π-stacked structure. Figure S9 of the Supporting Information shows the UV−vis absorption and CD spectra of the mixtures of (R,R)-PPP1 with aliphatic (A1 and A2) or aromatic (A3 and A4) additives in aqueous solution. Although the aliphatic additives gave no remarkable change in the UV− vis absorption and the CD spectra of the polymer (Figure S9a,b), the aromatic additives caused a red-shift in the absorption band corresponding to the conjugated main chain 2899

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Figure 5. (a) UV−vis absorption spectra, CD spectra, and gabs factors of (R,R)-PPP2−SNap (1:10 mol/mol) in water before and after annealing treatment. When adding SNap to the aqueous solution of PPP2, part of the polymer precipitated (see Figure S10); hence, absorbance (Abs.) and ellipticity are used for the Y-axis labels instead of ε and Δε to exclude the factor of concentration. (b) PL spectra, CPL spectra, and glum factors of annealed solutions of (R,R)-PPP2−SNap (1:10 mol/mol) and (S,S)-PPP2−SNap (1:10 mol/mol) in water. The initial concentration of PPP2 is 2.0 × 10−5 M.

chirality theory,19 the interchain helicity of the (R,R)-PPP2 assembly has a right-handed helical sense, or P-helicity. Arguably, the helical sense of the PPP-based assembly depends not only on the chirality of the side chains, but also on the number of nonsubstituted phenylenes in the repeating unit. The gabs values after annealing were −2.68 × 10−2 (at 341 nm) and +4.70 × 10−2 (at 380 nm) for (R,R)-PPP2−SNap (1:10 mol/mol); these gabs values are approximately 100 times greater than those of the nonassembled (R,R)-PPP2 (see Table S3). Therefore, the helicity of the PPP derivatives is enhanced to give an amplified g factor through the formation of an assembly with SNap. Thus, SNap plays a role in bringing the polymers close to one another by means of both electrostatic and π−π interactions. The degree of amplification for the gabs factor in (R,R)-PPP2−SNap is 10 times greater than in (R,R)-PPP1− SNap. This difference implies that the terphenylene repeating unit of PPP2 is more favorable for π−π interaction between the PPP main chains than the biphenylene repeating unit of PPP1, resulting in a stronger helically π-stacked structure in the assembly. The formation of the polymer assembly was also confirmed by DLS measurements (see Table S5). Similar to (R,R)-PPP1, the average particle size of (R,R)-PPP2 was 223.7 ± 65.2 nm, whereas those of (R,R)-PPP2−SNap were approximately 400 nm. Interestingly, the average particle size of (R,R)-PPP2− SNap = 1:10 (mol/mol) decreased from 463.9 to 281.7 nm after annealed at 70 °C for 30 min, resulting in disappearance of the polymer precipitate. This implies that the annealing treatment contributes to the improvement of dispersibility of the polymer assembly. In contrast, when THF was added as a poor solvent to an aqueous solution of the polymer, the assembly was also formed but showed less reproducible CD bands. Therefore, SNap is more useful for the generation of a polymer assembly with a highly helical π-stacked structure.

and produced a new CD band in the wavelength region (Figure S9c,d). The changes in the UV−vis absorption and CD spectra result from the formation of a polymer assembly with interchain helicity. Thus, we demonstrated that not only the anionic moieties, but also the aromatic rings are required for the anionic additive compound. These results are supported by our previous report3 in which a cationic achiral PPP derivative formed a chiral spherulite with interchain helical structure when mixed with an anionic chiral binaphthyl derivative by virtue of both electrostatic and π−π interactions between them. Chiral Assembly of PPP2−SNap. Figure 5, panel a shows the UV−vis absorption spectra, the CD spectra, and the gabs factors of a mixture of (R,R)-PPP2 and SNap [(R,R)-PPP2− SNap] (1:10 mol/mol) in water. As mentioned previously, PPP2 partially aggregates in water; therefore, the addition of SNap leads to enhancement of the aggregation but not the interchain helicity (Figure 5a, blue solid line). However, after the mixture was annealed at 70 °C, the positive bisignate Cotton effect was observed in the wavelength region corresponding to the absorption of the polymer main chain (Figure 5a, red solid line). This result indicates that PPP2 forms an assembly bearing an interchain helically π-stacked structure upon thermal heating. Interestingly, the induced CD band remained unchanged even after cooling from 70 to 20 °C (Figure 5a, light blue dashed line). This is because PPP2, unlike PPP1, partially maintains the assembly at high temperature (70 °C) because of its highly π-stacked structure, and thermal arrangement of the polymer chains occurs during the annealing process. Liquid crystalline (LC) conjugated polymers assemble strongly when annealed at the LC phase temperature, and the formation of the interchain helical structure is consequently developed.13,21 However, to our knowledge, such an arrangement of conjugated polymers occurring in solution or in a dispersion has not yet been reported. The (R,R)-PPP2 assembly exhibited an induced CD band with a sign opposite that of the (R,R)-PPP1 assembly. According to the exciton 2900

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Figure 6. (a) UV−vis absorption spectra, CD spectra, and gabs factors and (b) PL spectra, CPL spectra, and glum factors of cast films of PPP1−SNap = 1:4 (mol/mol). Excitation wavelength: 331 nm for PL spectra and 300 nm for CPL spectra.

Chiroptical Properties of Chiral Assembly in Thin Film. We prepared thin films of PPP1−SNap = 1:4 (mol/mol) by casting the corresponding aqueous solution on a quartz plate and examined their chiroptical properties. Figure 6, panel a shows the UV−vis absorption and CD spectra. Similarly to the corresponding solutions, both of the (R,R)- and (S,S)-PPP1− SNap films showed an absorption band at longer wavelength (355 nm) than the inherent band of PPP1, which indicates the presence of the polymer assembly in the thin films. Moreover, the PPP1−SNap films showed bisignate Cotton effects in their absorption wavelength, the signs of which were in good agreement with those of the corresponding solutions. Namely, the (R,R)- and (S,S)-PPP1−SNap films showed negative and positive bisignate Cotton effects, respectively. These results indicate that the chiral polymer assembly formed in the aqueous solution is maintained even in the cast film. In addition, the cast films exhibited blue CPL as shown in Figure 6, panel b, although the corresponding solutions did not. The CPL spectra of the (R,R)- and (S,S)-PPP1−SNap films showed negative and positive monosignate Cotton effects, respectively, which were almost mirror images to each other in sign. The glum values were evaluated to be −2.66 × 10−3 (at 414 nm) and +2.50 × 10−3 (at 414 nm) for the (R,R)- and (S,S)-PPP1− SNap films, respectively. The small glum value of 10−3 may be the reason why the CPL of PPP1−SNap was observed only in the cast films but not in the diluted aqueous solutions. On the other hand, no Cotton effect was observed in the cast film of PPP2−SNap = 1:10 (mol/mol). This is because PPP2−SNap tends to form granular aggregates when cast on a quartz plate, which prevented us to measure the chiroptical properties accurately.

Circularly Polarized Luminescence of Chiral Assembly. PPP2 exhibited CPL when it was mixed with SNap (1:10 mol/mol in PPP2−SNap) and subsequently annealed (Figure 5b). (R,R)- and (S,S)-PPP2−SNap showed CPL bands with mirror images, which indicate that the sign of the Cotton effect in CPL depends on the interchain helicity of the chiral assembly. (R,R)-PPP2−SNap exhibited a positive monosignate Cotton effect (ΔI = IL − IR > 0) in the CPL spectrum, which indicates the emission of lef t-handed circularly polarized light (IL > IR) with P-helicity. Moreover, IL and IR are luminescent intensity with left- and right-handed circular polarization, respectively. This result is consistent with the fact that the chiral assembly of (R,R)-PPP2−SNap has a helically π-stacked structure with P-helicity, which was confirmed by the positive bisignate Cotton effect observed in the CD spectrum. Meanwhile, (S,S)-PPP2−SNap exhibited a negative monosignate Cotton effect in the CPL spectrum, which indicates the emission of right-handed circularly polarized light with Mhelicity. (S,S)-PPP2−SNap also showed a negative bisignate Cotton effect in the CD spectrum; hence, the assembly of (S,S)-PPP2−SNap has a helically π-stacked structure with Mhelicity.3 The dissymmetry factors in luminescence (glum) are +1.1 × 10−1 (at 424 nm) for (R,R)-PPP2−SNap (1:10 mol/ mol) and −7.9 × 10−2 (at 423 nm) for (S,S)-PPP2−SNap (1:10 mol/mol). Previously, the glum factor of 10−1 in conjugated polymers was achieved only when thermotropic or lyotropic LC conjugated polymers were helically selforganized in the LC phase.5,11−13 The large glum value of 10−1 in PPP2−SNap may be attributed to a helical arrangement of the polymer main chains in solution during annealing. Hence, the annealing treatment is effective for arranging the conjugated polymers into a higher-ordered structure even in a dispersion, such as an aqueous solution, as well as in solid films. In contrast, PPP1−SNap showed no CPL because PPP1 may have been unfavorably arranged in the polymer assembly due to rapid aggregation. In addition, the assembly easily dissociates upon thermal heating without forming a higher-ordered structure, which is in contrast to PPP2.

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CONCLUSION We presented a novel approach for preparing interchain helically π-stacked assemblies of ionic conjugated polymers with high dissymmetry factors of 10−2−10−1 for absorption and luminescence by utilizing both intermolecular electrostatic and π−π interactions. We designed and synthesized cationic chiral 2901

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(9) Suda, K.; Akagi, K. Macromolecules 2011, 44, 9473−9488. (10) Kane-Maguire, L. A. P.; Wallace, G. G. Chem. Soc. Rev. 2010, 39, 2545−2576. (11) Oda, M.; Nothofer, H. G.; Scherf, U.; Šunjić, V.; Richter, D.; Regenstein, W.; Neher, D. Macromolecules 2002, 35, 6792−6798. (12) San Jose, B. A.; Matsushita, S.; Akagi, K. J. Am. Chem. Soc. 2012, 134, 19795−19807. (13) Watanabe, K.; Osaka, I.; Yorozuya, S.; Akagi, K. Chem. Mater. 2012, 24, 1011−1024. (14) Mori, T.; Inoue, Y.; Grimme, S. J. Phys. Chem. A 2007, 111, 4222−4234. (15) Lindsten, G.; Wennerström, O.; Isaksson, R. J. Org. Chem. 1987, 52, 547−554. (16) Mazaleyrat, J. P.; Wright, K.; Gaucher, A.; Toulemonde, N.; Wakselman, M.; Oancea, S.; Peggion, C.; Formaggio, F.; Setnicka, V.; Keiderling, T. A.; Toniolo, C. J. Am. Chem. Soc. 2004, 126, 12874− 12879. (17) Superchi, S.; Casarini, D.; Laurita, A.; Bavoso, A.; Rosini, C. Angew. Chem., Int. Ed. 2001, 40, 451−454. (18) Superchi, S.; Bisaccia, R.; Casarini, D.; Laurita, A.; Rosini, C. J. Am. Chem. Soc. 2006, 128, 6893−6902. (19) Berova, N.; Nakanishi, K. Exciton Chirality Method: Principles and Applications. In Circular Dichroism: Principles and Applications, 2nd ed.; Berova, N., Nakanishi, K., Woody, R. W., Eds.; Wiley-VCH: New York, 2000; pp 337−382. (20) Mori, T.; Akagi, K. Macromolecules 2013, 46, 6699−6711. (21) Satrijo, A.; Meskers, S. C. J.; Swager, T. M. J. Am. Chem. Soc. 2006, 128, 9030−9031.

PPP derivatives, PPP1 and PPP2, with ammonium moieties linked to asymmetric centers in their side chains. In the CD spectra, both PPP1 and PPP2 showed a bisignate Cotton effect corresponding to the exciton coupling between neighboring phenylene rings in the conjugated backbone, which indicates the presence of intrachain helicity. This is the first case in which aromatic conjugated polymers with a linear main chain, such as the PPP derivatives, are verified to exhibit intrachain helicity in solution. Upon addition of an anionic aromatic additive, SNap, into an aqueous solution of PPP1 or PPP2, chiral assemblies with an interchain helically π-stacked structure were formed through electrostatic and π−π interactions between the cationic polymer and the anionic additive. Although the chiral assembly of PPP1 was dissociated upon thermal heating, the helically πstacked structure of the PPP2 assembly was thermally reinforced. The annealed assembly of PPP2−SNap showed a large glum value of |10−1| even in a dispersion such as an aqueous solution, which is the same magnitude as the values achieved in the LC conjugated polymers. The present approach based on cooperative intermolecular electrostatic and π−π interactions is versatile and applicable for the preparation of other types of polymer assemblies with chiroptical functions.

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ASSOCIATED CONTENT

S Supporting Information *

Materials and Methods. Synthetic routes of cationic chiral poly(para-phenylene) derivatives and anionic additives. Optical properties of (R,R)-PPP1, (S,S)-PPP1, (R,R)-PPP2, and (S,S)PPP2 in water, methanol, and the mixtures. Average particle size evaluated by DLS measurements. UV−vis absorption spectra, CD spectra, and gabs factors. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Mr. Kensuke Notsu for his helpful cooperation in DLS measurements. This work was supported by Grants-in-Aid for Science Research (A) (No. 25246002) and (No. 25620098) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

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REFERENCES

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DOI: 10.1021/acs.chemmater.5b00121 Chem. Mater. 2015, 27, 2895−2902