Controlled Synthesis of a Helical Conjugated Polythiophene

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Controlled Synthesis of a Helical Conjugated Polythiophene Pieter Leysen,† Joan Teyssandier,‡ Steven De Feyter,‡ and Guy Koeckelberghs*,† †

Laboratory for Polymer Synthesis, Department of Chemistry, and ‡Division of Molecular Imaging and Photonics, Department of Chemistry, KU Leuven, Celestijnenlaan 200F, B-3001 Heverlee, Belgium S Supporting Information *

ABSTRACT: Two new polymer systems, poly(3-phenylenevinylene)thiophene (P3PVT) and poly(3-phenyl)thiophene (P3PT), were designed with the aim of obtaining a helical conjugated polymer via a living polymerization. The polymerization proceeded without transfer and termination reactions via the Kumada catalyst transfer condensative polymerization (KCTCP) mechanism, confirming the living nature of the polymerization. Solvatochroism and circular dichroism (CD) experiments showed the helical nature of P3PVT and the stacking behavior of P3PT in poor solvent conditions. Block copolymers of 3-alkyl-substituted polythiophenes and helical P3PVT were prepared to determine the aggregation behavior of such systems. Solvatochroism, CD, and AFM measurements showed that the blocks influence each other’s behavior. If the P3AT block stacks before the helical P3PVT block organizes, one-handed helix formation is hindered. If helix formation occurs first, the stacking behavior is not influenced.

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INTRODUCTION Helical polymers have been intensively investigated in the past few decades due to their interesting chiral properties and their abundance in nature. They find applications in molecular recognition, as molecular scaffolds and in chiral sensing.1−4 Several synthetic helical conjugated polymers have already been prepared, including poly(acetylenes),5−15 oligo(m-phenylene ethynylene)s,16−19 oxazoline functionalized polythiophenes,20 poly(3,6-carbazoles),21 poly(3,6-phenanthrenes),22 and gallic acid-functionalized poly(dithienopyrroles).23,24 However, among these polymers, only poly(acetylenes) can be synthesized using a living polymerization via insertion polymerization. As a result, poly(acetylene) is the only conjugated helical polymer that can be made via a living polymerization.25 However, block copolymers of poly(acetylene) and other conjugated polymers cannot be prepared easily, since other conjugated polymers cannot be polymerized via insertion polymerization. In addition, poly(acetylene) is poorly conjugated. A common way of inducing helicity in polymers is the use of self-assembling polymers with sterically hindered side groups that force the polymer into a helical conformation.26 The polymers described in this paper are designed with this concept in mind. They consist of poly(thiophene) backbones substituted with bulky gallic acid derivatives. Poly(thiophene)s are wellknown for their self-assembly character, arising from the conjugated system, which stretches out over the polymer chain. The gallic acid substituents are known to induce helicity in several systems due to their bulkiness, π-stacking, and van der Waals interactions.1,23,27 Chirality is easily introduced into gallic acidbased systems using chiral alkyl chains. © XXXX American Chemical Society

An advantage of using 3-substituted thiophenes is that they can be polymerized using the living KCTCP-polymerization. This polymerization results in regioregular polymers of controlled molar mass, low dispersity, and with defined end groups. During this polymerization, the Ni catalyst remains complexed to the polymer chain after reductive elimination and does not diffuse away. It then quickly inserts oxidatively into the terminal C−Br bond, and the polymerization continues. As a result, the polymerization proceeds via a chain-growth mechanism without transfer or termination reactions.28−31 Living polymerizations open up possibilities for more complex structures such as block copolymers, graft copolymers, etc.32 The combination of the gallic acid-based monomers with the living KCTCP polymerization results in a novel strategy for the controlled synthesis of helical conjugated polymers. It also opens up possibilities for more advanced architectures in which helical segments can be combined with aggregating segments in the same polymer molecule, a hitherto unexplored class of polymers.

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RESULTS AND DISCUSSION KCTCP requires the introduction of a Grignard function on the monomer. Most commonly this is done via a Grignard metathesis (GRIM) reaction between an aryl iodide function and a sterically hindered alkyl Grignard reagent such as i-PrMgCl. An alternative is direct C−H functionalization of the aryl group by a Knochel−Hauser base.33 Received: March 12, 2018 Revised: April 23, 2018

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DOI: 10.1021/acs.macromol.8b00527 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of Precursor Monomer 3a,b and Compound 4

polymerization was attempted, but this did not result in polymer. Analysis of the GRIM reaction by 1H NMR after the addition of water showed the formation of several unidentifiable side products which probably prevented polymerization (see Supporting Information, Figure S1). Monomer 8 was synthesized by the Suzuki reaction of 614,36 and 3-thienylboronic acid. Bromination was done with N-bromosuccinimide. This resulted not only in bromination on the 2-position of the thiophene ring but also on the phenyl ring. This side product could easily be separated by flash column chromatography. Because of the electron-rich character of the trialkoxy-substituted phenyl ring, electrophilic substitutions can take place both on the phenyl ring and on the 2-position of the thiophene ring. Iodination of 8 using I2 and PhI(OAc)2 was tried, but this exclusively iodinated the phenyl ring. This can be explained using the lower reactivity of the 5-position of the thiophene ring compared to the 2-position. Because neither monomer could be synthesized by GRIM reaction, C−H functionalization with 2,2,6,6-tetramethylpiperidinylmagnesium chloride (TMPMgCl·LiCl) was done to obtain the desired monomers 5a,b and 9a,b. Water was added after C−H functionalization to verify whether the reaction did not yield unwanted side products. 1H NMR showed the re-formation of the starting compound, while no side products were formed, so the reaction was deemed successful (see Figures S2 and S3).

Scheme 2. Synthesis of Monomer 8a,b

Table 1. GPC Results of Polymers P1−P4 P1 P2 P3 P4

M̅ n (kg mol−1)

Đ

DP (1H NMR)

6.2 10.0 4.6 8.9

1.1 1.1 1.1 1.1

17 28 10 24

The synthesis of precursor monomer 4 is shown in Scheme 1. The monomer was made by the Horner−Wadsworth−Emmons reaction of 134 and 2,35 followed by iodination to obtain the precursor monomer 4a. After GRIM reaction of this monomer, Scheme 3. Synthesis of Polymers P1−P4

B


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Figure 1. (a) M̅ n as a function of [M]/[I] for 5a; a linear plot shows that no transfer reactions occur. (b) GPC traces of the chain extension experiment for 5a. Black trace is of the initial polymerization; red trace is after second monomer addition.

Figure 2. (a) M̅ n as a function of [M]/[I] for 9a; a linear plot shows that no transfer reactions occur. (b) GPC traces of the chain extension experiment for 9a. Black trace is of the initial polymerization; red trace is after second monomer addition.

Polymer Synthesis. An external initiator was used for the synthesis of the polymers. Because external initiators are soluble in THF, they cause a more efficient initiation and, as a result, a lower dispersity. Also, if such an initiator is not used, the catalyst can insert in both chain ends via “catalyst walking”, resulting in BAB block copolymers. If an external initiator is used, only AB block copolymers are formed.37 Initiator 1037 was transformed to the actual initiator 11 via ligand exchange with dppp prior to polymerization.38 2 equiv of dppp with respect to 10 were added to ensure a fast and complete ligand exchange.31 A completely regioregular polymer is then obtained. The dispersity of the obtained polymers can be measured by gel permeation chromatography (GPC). GPC also gives a rough idea of the molar mass, since the obtained results are compared to polystyrene calibration standards. Because of the higher rigidity of polythiophenes compared to polystyrene, this leads to an overestimation of the molar mass of the polymers by GPC. The number-average molar mass and dispersity for P1−P4 are shown in Table 1. An estimate of the actual DP can be made using 1H NMR spectroscopy. The signal arising from the methyl group on the o-tolyl initiator can be compared to the signal arising from the α-CH2 group in the alkoxy side chain. The values should be seen as estimates, since there is no certainty every chain contains this o-tolyl initiator. However, since we have a living polymerization with no transfer reactions, this is a safe assumption to make. The values are also shown in Table 1. Analysis of the Living Character of the Polymerization. Phenyl-substituted thiophenes (P3PT) have already been prepared via noncontrolled oxidative polymerizations39−45 and direct arylation polycondensation.46 Recently, a controlled synthesis of sterically hindered 3-(2,5-dioctylphenyl)thiophene was also reported.47 Also, 3-phenylenevinylene-substituted polythiophenes (P3PVT) have been extensively described in the literature. Polymerizations

Figure 3. UV−vis spectra of polymer P1. Solvatochroism (a) and dilution (b) experiments.

were done using either Stille couplings or CTCP.48−50 But the living character of these polymerizations has never been investigated. C


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Figure 4. UV−vis and CD spectra of polymer P2. Solvatochroism (a, c) and dilution (b, d) experiments.

Two experiments were done to determine the living character of the polymerizations of 5a and 9a. First, six polymers were made with several different monomer over initiator ratios. If no transfer reactions occur, there exists a linear relationship between the molar mass and this ratio. This method is preferred over monitoring the molar mass in function of the conversion, since it requires less manipulations that can introduce moisture or air into the polymerization vessel. The second experiment is a chain extension experiment to examine if termination reactions are absent. [M]

Termination is often determined by monitoring ln [M]0 as a funct

tion of time. However, this method only works for CTCP-type polymerizations if the transmetalation step is the rate-determining step. The rate-determining step of our polymerization was not determined and is unknown; as a result this method was not used. Monomer 5a was polymerized with different monomer over initiator ratios, and its molar mass was measured via GPC. The relation between [M] is clearly linear, indicating that no transfer [I]

reactions occur (Figure 1). It should be noted that for [M] = 100 a [I]

bimodal GPC trace and a higher dispersity are observed (Figure S45), indicating that the polymerization suffers from termination reactions if the degree of polymerization (DP) is that high. A chain extension experiment, for two blocks of 20 units, was performed by adding another batch of monomer after complete consumption of the first batch of monomer, to verify whether the polymerization suffers from termination reactions (Figure 1). GPC shows that all polymer chains reacted further, indicating no termination occurs if the polymer chains do not grow too long. Neither transfer nor termination occurs, indicating that the polymerization of 5a via KCTCP is living.

Figure 5. UV−vis spectra of polymer P3. Solvatochroism (a) and dilution (b) experiments. D


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Figure 6. UV−vis and CD spectra of polymer P4. Solvatochroism (a, c) and dilution (b, d) experiments.

The same experiments were repeated for the polymerization of monomer 9a. Again, a relatively linear correlation between M̅ n and [M] was found (Figure 2). In this case the correlation [I]

phenylenevinylene side chain and at 450 nm corresponding to a transition in the thiophene backbone (see Figures 3 and 4). Polymers P1 and P2 only differ in the side chains on the gallic acid moiety, and as such they have the same UV−vis spectrum. Addition of methanol, a nonsolvent, leads to a red-shift of the backbone transition to 510 nm. This is caused by an increase in conjugation length due to the planarization of the polymer backbone. This planarization can be caused by the formation of supramolecular structures such as an intramolecular helix or a supramolecular stack of polymer chains. Supramolecular stacks of aggregated poly(thiophene)s show fine structure in the UV−vis spectrum due to long-range order.51 The lack of fine structure in the UV−vis spectra of P1 and P2 is an indication that no supramolecular aggregates form but that the polymers organize as helical structures. For P2, CD spectra can also be recorded. A signal starts appearing at 50% of nonsolvent, indicating chiral organization. This signal appears on the absorption band of the phenylenevinylene side chain, indicating a chiral organization of the side chains. No signal (or a very weak signal) is observed for the backbone absorption. If the formed helix has a large radius, the chiral organization of the thiophene backbone might not be visible in the CD spectrum. Helical structures and supramolecular aggregates are respectively independent and dependent on concentration. Upon dilution, helical structures will remain, but aggregates will break up. In order to verify if the polymers organize helically, a dilution series was made. For P1 and P2, this series was made in a 50/50 mixture of chloroform and methanol. In this mixture, the UV−vis spectra are red-shifted, and a CD signal is present for P2. At this ratio of solvent/nonsolvent there is an equilibrium between

appears to have a lower degree of linearity. This might be explained by the additional sterical hindrance of the phenyl group directly in ortho to the C−Br bond. However, the correlation is still linear, so we can conclude that transfer reactions are mostly absent. Addition of an extra batch of monomer after polymerization results in chain extension (Figure 2) as was the case for 5a. In conclusion, both monomer 5a and 9a polymerize via a living polymerization. Determination of Supramolecular Structure. The supramolecular structure of the polymers can easily be determined by studying their optical properties. In good solvents, polythiophenes are present as semiflexible wormlike chains. Addition of nonsolvent leads to the formation of supra- or macromolecular structures, accompanied by a red-shift in the UV−vis spectrum and by the appearance of circular dichroism in chiral polymers. Most conjugated polymers form supramolecular stacks of aggregated polymers in poor solvent conditions. For chiral conjugated polymers these stacks are twisted, resulting in a chiral supramolecular structure. In achiral helical polymers, both left- and right-handed helices will be present in the same amount. If a chiral polymer is used, one helix sense will favorably be formed and will dominate, resulting in a CD signal. To determine their supramolecular organization, the polymers were all dissolved in chloroform, and increasing amounts of methanol were added to induce organization. The UV−vis spectra of polymers P1 and P2 show two absorption maxima, at 320 nm corresponding to a transition in the E


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Macromolecules Scheme 4. Scheme of the Block Copolymerization

Also for these polymers a dilution series was made, for P3 in 60% nonsolvent and for P4 in 55% nonsolvent. In this ratio of solvent/nonsolvent, there is an equilibrium between organized and “free” polymers chains. Upon dilution, the UV−vis spectrum of P3 and P4 is blue-shifted and similar to the spectrum obtained in 100% chloroform. In the UV−vis spectrum of P4, the extra absorption band at 525 nm also disappears upon dilution. This is not surprising, since this band is associated with long-range order in aggregated chains and a strong indication of aggregate formation. Dilution breaks up these aggregates, resulting in the disappearance of this absorption band. Because the UV−vis spectrum of these polymers clearly depends on their concentration and fine structure is visible in the UV−vis spectrum, we can conclude that these polymers do not form an intramolecular helix, but supramolecular aggregates. These results are confirmed by the CD spectrum of P4. The spectrum shows two CD signals, one for each absorption band, upon addition of nonsolvent, indicating that both the polymer backbone and the phenyl side chain organize in a chiral way. Upon diluting, the CD signal disappears, as is expected for supramolecular aggregates, conforming the conclusion of the solvatochroism experiment that the formed supramolecular structure is an aggregate. In conclusion, of the two designed polymer systems, only P3PVT (P1 and P2) shows a helical structure in poor solvent conditions. P3PT (P3 and P4) behaves similarly to P3AT and forms supramolecular aggregates in poor solvent conditions.

Table 2. GPC of Block Copolymers P5b and P6b −1

P5a P5b P6a P6b

M̅ n (kg mol )

Đ

DP

3.7 7.9 3.5 8.0

1.2 1.1 1.2 1.2

16 27 16 26

organized chains and “free” polymer chains (see Figures S29, S31, and S39). It is clear from the dilution series that the UV−vis spectrum of both polymers does not significantly alter upon dilution. For P2 the CD signal also remains present. This means the chiral organization is still present even at low concentrations. These results are as expected for intramolecular helices, since the formation of these structures is independent of concentration. The same experiments were repeated for P3 and P4 (see Figures 5 and 6). These polymers also show two absorption maxima, one at 265 nm corresponding to a transition in the phenyl side chain. The lower λmax compared to P1 and P2 can be explained by a shorter conjugation length due to the lack of a double bond in the side chain. The second absorption maximum at 460 nm corresponds to a transition in the thiophene backbone. Addition of nonsolvent leads to a shift of the backbone absorption to 480 nm. This shift can also be explained by an increased conjugation length due to planarization. For P4, addition of nonsolvent also leads to the appearance of an extra absorption band at 525 nm. This extra band is caused by long-range order in aggregated polymer stacks.51 F


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Figure 7. Assignment of 1H NMR signals for determination of DP for the first block (a) and for the second block (b); R can be any alkyl chain.

Block Copolymer Synthesis. As the polymerization of P1 and P2 is living, block copolymers with poly(3-alkylthiophene) (P3AT) monomers can also be synthesized. This allows to investigate the chiral properties of block copolymers consisting of a helical block and a supramolecular aggregating block. Specifically, we can investigate the influence of (chiral) aggregation on (chiral) helix formation and vice versa. The block copolymerizations were all done starting from an external initiator to obtain only AB block copolymers. Both the synthesis of P3AT−P3PVT and P3PVT−P3AT block copolymers was attempted. Only P3AT−P3PVT block copolymers could be successfully prepared. 2-Bromo-3-hexyl-5chloromagnesiothiophene was barely built in after polymerization of 5a (see Figure S4). Two block copolymers (P5b and P6b) consisting of a chiral and an achiral block were then prepared. Chirality was either introduced into the P3AT block (P5b) using (S)-2-bromo-5iodo-3-(3,7-dimethyloctyl)thiophene (12b) as the monomer or into the P3PVT block (P6b) using monomer 3b (see Scheme 4). The characteristics of the polymers are listed in Table 2. The length of both blocks can be determined via 1H NMR spectroscopy (see Figure 7 and eq 1). If the signal of the internal α-methylenes (a) is compared to the signals of the terminal α-methylene (b) and the o-tolyl unit from the initiator (c), an idea of the DP of the first P3AT block (P5a and P6a) is obtained, according to eq 1. The polymerization of P3AT via KCTCP is living, so it predominately results in H-terminated polymer, although some other end groups such as -Br and -iPr can occur. This method of determining the DP takes into account all these possible end groups. The DP of the second P3PVT block and as such of the whole polymer is obtained by comparing the signal of the α-methylenes of the thiophene backbone (a) to the α-methylenes in the gallic acid-derived substituent (d).

DP = 2 ×

a 2 b 2

+ +

b 2 c 3

(1)

This results in a DP of 16 for P5a and a DP of 27 for the entire block copolymer P5b. The homopolymer P6a has a chain length of 16, and block copolymer P6b consists of 26 units. The integration values of the peaks can be found in the Supporting Information (Figures S27 and S28). The optical and chiral properties in solution and in poor solvent were also studied. Upon addition of nonsolvent to P5b, a red-shift is observed in the UV−vis spectrum at 60/40 and 50/50 CHCl3/MeOH (see Figure 8). At these ratios of solvent/ nonsolvent, no CD signal is observed. Higher amounts of nonsolvent lead to a stronger red-shift and the appearance of an extra signal at 610 nm. This absorption band is characteristic for the aggregation of P3AT’s. With the appearance of this extra absorption band, a CD signal is also observed. This signal is situated at the absorption band of the polymer backbone and is similar in intensity to the signal obtained for regular poly(3-((S)3,7-dimethyloctyl)thiophene).52 No signal is seen at the absorption band of the phenylenevinylene unit. The achiral helical block does not organize in a chiral fashion. Addition of nonsolvent to P6b leads to the appearance of an extra transition at 610 nm (see Figure 9). Further addition of nonsolvent also leads to a red-shift of the absorption band of the thiophene backbone. The poly(3-hexylthiophene) block starts to aggregate, while the chiral P3PVT block is still solubilized. No CD signal is seen in this case. Higher concentrations of nonsolvent lead to a red-shift of the absorption band of the thiophene backbone. The block takes on a helical conformation. This is also visible in the CD spectrum. However, the intensity of this CD signal is lower than previously seen for P2. The aggregation of the first block thus prevents the formation of a proper helix and lowers the intensity of the CD signal. G


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Figure 9. UV−vis (a) and CD spectra (b) of P6b.

Figure 8. UV−vis (a) and CD spectra (b) of P5b.

start to π-aggregate. Because of the lateral resolution of ambient AFM, individual molecules cannot be resolved. As a consequence, no comments can be made on the chiral or achiral nature of the fibers. Extra experiments (see Supporting Information) performed at the same concentration in toluene also show that P6b can form fibers even without addition of MeOH, a feature never observed for P5b. The width of the fibers observed in all those samples (18 ± 4 nm for P5b, 17 ± 2 nm for P6b) is in good agreement with the DP of the first block. A possible explanation is that the P3PVT block may not participate in the π-stacking (or very partially) and that only the P3AT is driving the fiber formation. However, since no internal structure could be imaged within the lamellae, it is impossible to conclude on the relative role of each of the blocks in the assembly process.

These findings were corroborated by atomic force microscopy (AFM). AFM imaging was performed on films obtained by dropcasting on graphite relatively diluted (0.1 mg mL−1) solutions of the copolymers in a 50/50 toluene/MeOH mixture. For both P5b and P6b, films with varying thicknesses were formed (large scale images where domains of different thicknesses coexist can be found in the Supporting Information). Thicker regions (i.e., multilayers) of the films (on average up to 30 nm for P5b and 15 nm for P6b) are shown in Figure 10a,c. They exhibit intertwined fibers in both cases, even if the density of organized stripes appears higher for P6b. Other parts of the surface are covered with an ≈2 nm thick layer, which is compatible with a monolayer of poly(3-hexylthiophene).53 As opposed to multilayers, the behavior of both polymers differs in these thinner regions; P5b exhibits films with many holes and no apparent organization. The holes result from an incomplete surface coverage. Their presence in the layer could indicate an amorphous character for the polymers, which is also confirmed by the lack of long-range order in the layer. P6b, on the other hand, self-assembles into lamellae similar to those of the multilayers, but with a much higher order (single lamellae can extend over several micrometers). This type of morphology, common for poly(3-hexylthiophene),53−57 is caused by π-stacking of the polymer backbone perpendicular to the axis of the fibers. Therefore, the presence of a chiral P3PVT block does not prevent the aggregation of the P3AT block for any of the block copolymers, confirming the conclusions of the CD measurements. Besides, it appears that P6b has a higher tendency than P5b to self-assemble into lamellae in these conditions, which qualitatively confirms solution phase experiments where it was shown that P6b needs a smaller amount of nonsolvent to

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CONCLUSION New monomer systems were developed to obtain a helical polymer via a living polymerization. Both systems were polymerized using the KCTCP mechanism after C−H functionalization of the monomers. The living character of the polymerization was proven by varying the ratio of monomer to initiator. It was established that this ratio was linearly correlated to the molar mass, proving that no transfer reactions took place. The absence of termination was proven by performing a chain extension experiment. The helical nature of the polymers was investigated by combined UV−vis and CD experiments. These experiments showed that one of the polymer systems adopts a helical structure in poor solvent conditions. This system was then used to synthesize block copolymers with P3AT. Two block copolymers were made: one with a chiral P3AT block and an achiral P3PVT block H


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Figure 10. AFM images of dry films of P5b (a, b) and P6b (c, d) drop-casted on graphite. (a) and (c) correspond to multilayer regions of the films and (b) and (d) to regions compatible with a monolayer thickness. (d) is a phase image, while the other ones are all topography images.

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ACKNOWLEDGMENTS We are grateful to the Onderzoeksfonds KU Leuven/Research Fund KU Leuven and the Fund for Scientific Research (FWOVlaanderen) for financial support. J.T. and S.D.F. are grateful to the Belgian Federal Science Policy Office (IAP-7/05).

and one with an achiral P3AT block and a chiral P3PVT block. When the aggregation behavior of these block copolymers was investigated, it was found that if aggregation of the P3AT block occurs first, helix formation of the P3PVT block is hampered. However, if the helical P3PVT block organizes first, aggregation of the P3AT block is not hindered.

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ABBREVIATIONS (K)CTCP, (Kumada) catalyst transfer condensative polymerization; iPr, isopropyl; TMP, 2,2,6,6-tetramethylpiperidinyl; THF, tetrahydrofuran; DCM, dichloromethane; GPC, gel permeation chromatography; DP, degree of polymerization; P3PT, poly(3-phenylthiophene); P3PVT, poly(3-phenylenevinylene thiophene); P3AT, poly(3-alkylthiophene); CD, circular dichroism.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00527. Reagents and instrumentation, synthetic procedures of the monomers, polymerization procedures, NMR spectra, GPC traces, UV−vis absorption and CD spectra recorded for the polymers under different solvent conditions, additional AFM images (PDF)

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REFERENCES

(1) van Gorp, J. J.; Vekemans, J. a J. M.; Meijer, E. W. Facile Synthesis of a Chiral Polymeric Helix; Folding by Intramolecular Hydrogen Bonding. Chem. Commun. 2004, 2 (1), 60−61. (2) Nakano, T.; Okamoto, Y. Synthetic Helical Polymers: Conformation and Function. Chem. Rev. 2001, 101 (12), 4013−4038. (3) Yashima, E.; Maeda, K.; Nishimura, T. Detection and Amplification of Chirality by Helical Polymers. Chem. - Eur. J. 2004, 10 (1), 42−51. (4) Yashima, E.; Maeda, K. Chirality-Responsive Helical Polymers. Macromolecules 2008, 41 (1), 3−12. (5) Maeda, K.; Morino, K.; Yashima, E. Macromolecular Helicity Inversion of Poly(phenylacetylene) Derivatives Induced by Various External Stimuli. Macromol. Symp. 2003, 201, 135−142.

AUTHOR INFORMATION

Corresponding Author

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

Joan Teyssandier: 0000-0003-4369-0542 Steven De Feyter: 0000-0002-0909-9292 Guy Koeckelberghs: 0000-0003-1412-8454 Notes

The authors declare no competing financial interest. I


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