Chiral Amplification in π-Conjugated Helical Polymers with Circularly

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

Chiral Amplification in π‑Conjugated Helical Polymers with Circularly Polarized Luminescence Tomoyuki Ikai,*,† Sho Shimizu,† Seiya Awata,† and Ken-ichi Shinohara‡ †

Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan School of Materials Science, Japan Advanced Institute of Science and Technology (JAIST), 1-1 Asahi-dai, Nomi 923-1292, Japan

‡

S Supporting Information *

ABSTRACT: A series of D-glucose-bound optically active πconjugated polymers (poly-Tr) were synthesized by ternary copolymerization of 2,5-diiodothiophene with diethynyl monomers containing a chiral and an achiral biphenyl unit using the Sonogashira coupling reaction. The effect of the chiral and achiral biphenyl contents on the chiral amplification in the preferred-handed helix formation (“the sergeants and soldiers effect”) was investigated by comparing the circular dichroism (CD) and circularly polarized luminescence (CPL) intensities of poly-Tr to that of the corresponding helical polymer (poly-T) without an achiral biphenyl unit. We observed that even when the chiral biphenyl content in the copolymer was 50 mol % (poly-T0.50), its CD and CPL intensities were almost comparable to that of poly-T, demonstrating the amplification of the helicity.

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INTRODUCTION Appropriately designed synthetic oligomers and polymers can fold into a specific ordered conformation thanks to some sort of noncovalent intramolecular interactions, including solvophobic and coordination interactions and hydrogen bonding. These compounds are called “foldamers” and have unique characteristics, such as inclusion complexation with other molecules and a functional switching capability triggered by a conformational change.1,2 Since the pioneering works carried out by Moore and co-workers,3−6 foldamers have been recognized as one of the most attractive research subjects in terms of both fundamental study and practical applications, particularly in the fields of polymer chemistry and supramolecular chemistry. Thus far, a large number of foldamers, especially containing π-conjugated units in the main chain, have been reported.7−16 To develop novel foldamer-based materials by taking advantage of unique structural elements in chiral natural products, we recently synthesized a series of chiral π-conjugated polymers bearing optically active glucose-linked biphenyl (GLB) units in the main chain, whose molecular design was inspired by naturally occurring saccharides, ellagitannins.17−20 We observed that poly-T and poly-2T (Chart 1) containing (oligo)thiophenetype comonomer units underwent a conformational transition between the random-coil and helix in solution and/or the solid state in response to the external solvent environment. We also demonstrated that when the backbone was folded into the helical structure, the GLB-based polymers showed remarkable circularly polarized luminescence (CPL) performance.17,18 Chiral amplification is an attractive phenomenon and can occur in a system in which a small chiral bias is significantly enhanced through covalent or noncovalent bonding inter© XXXX American Chemical Society

Chart 1. Poly-T and Poly-2T

actions.14,21−28 In 1989, Green and co-workers demonstrated that optically active helical polymers with a greater excess of preferred-handedness can be prepared through copolymerization of achiral monomers with a small amount of optically active monomers.29−32 This is a representative example of the chiral amplification observed in synthetic polymers, and its concept was described as “the sergeants and soldiers effect”. In this study, we synthesized a series of optically active πconjugated polymers (poly-Tr and poly-2Tr in Scheme 1), in which a part of the GLB unit in poly-T and poly-2T was replaced with an achiral biphenyl unit. The effect of the chiral and achiral biphenyl contents on the chiral amplification in the preferred-handed helical folding (“the sergeants and soldiers effect”) was investigated by a combination of circular dichroism Received: January 31, 2018 Revised: March 1, 2018

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DOI: 10.1021/acs.macromol.8b00229 Macromolecules XXXX, XXX, XXX−XXX

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1724 (CO). Calcd for C39.5H41O13S·1·3H2O: C, 60.89; H, 5.64. Found: C, 60.66; H, 5.44. Spectroscopic Data for Poly-T0.20. 1H NMR (400 MHz, CDCl3, 55 °C): δ 8.04 (d, J = 7.8 Hz, 1.6H, ArH), 6.99−7.67 (m, 6.4H, ArH), 5.64 (t, J = 9.4 Hz, 0.2H, CH), 5.31−5.39 (m, 0.4H, CH), 4.96−5.00 (m, 0.2H, CH), 4.08−4.39 (m, 4.6H, CH, CH2), 3.17−3.75 (m, 25.8H, CH2, OCH3). IR (KBr, cm−1): 2200 (CC), 1753 (CO), 1723 (CO). Calcd for C37.4H39.2O11.2S·0·8H2O: C, 62.88; H, 5.76. Found: C, 62.71; H, 5.48. Spectroscopic Data for Poly-2T0.90. 1H NMR (600 MHz, CDCl3, 55 °C): δ 8.03 (d, J = 8.2 Hz, 0.2H, ArH), 7.00−7.66 (m, 9.8H, ArH), 5.64 (t, J = 9.5 Hz, 0.9H, CH), 5.15−5.37 (m, 1.8H, CH), 4.88−4.99 (m, 0.9H, CH), 4.08−4.39 (m, 6.7H, CH, CH2), 3.23−3.75 (m, 25.1H, CH2, OCH3). IR (KBr, cm−1): 2197 (CC), 1755 (CO). Calcd for C46.3H45.4O15.4S2·0·9H2O: C, 59.89; H, 5.12. Found: C, 59.68; H, 4.90. Spectroscopic Data for Poly-2T0.75. 1H NMR (400 MHz, CDCl3, 55 °C): δ 8.04 (d, J = 7.3 Hz, 0.5H, ArH), 7.01−7.65 (m, 9.5H, ArH), 5.63 (t, J = 9.6 Hz, 0.75H, CH), 5.16−5.39 (m, 1.5H, CH), 4.88−4.99 (m, 0.75H, CH), 3.94−4.39 (m, 6.25H, CH, CH2), 3.17−3.75 (m, 25.25H, CH2, OCH3). IR (KBr, cm−1): 2197 (CC), 1754 (CO). Calcd for C45.25H44.5O14.5S2·1·1H2O: C, 60.10; H, 5.21. Found: C, 59.99; H, 5.07. Spectroscopic Data for Poly-2T0.50. 1H NMR (500 MHz, CDCl3, 55 °C): δ 8.03 (d, J = 8.0 Hz, 1H, ArH), 7.01−7.66 (m, 9H, ArH), 5.64 (t, J = 9.7 Hz, 0.5H, CH), 5.19−5.38 (m, 1H, CH), 4.88−4.99 (m, 0.5H, CH), 3.94−4.39 (m, 5.5H, CH, CH2), 3.20−3.75 (m, 25.5H, CH2, OCH3). IR (KBr, cm−1): 2197 (CC), 1755 (CO), 1724 (CO). Calcd for C43.5H43O13S2·0·8H2O: C, 61.30; H, 5.27. Found: C, 61.21; H, 5.08. Molecular Modeling. An all-atom MD simulation was carried out using the Forcite module of the BIOVIA Materials Studio 2017 (Dassault Systèmes BIOVIA, San Diego, CA) on the supercomputer system (PRIMERGY CX250, Fujitsu, Tokyo, Japan). A initial model of poly-T0.50 was built at the torsion angle across the ethyne axis of −134°, based on a previous report.34 The MD cell was built by means of usual procedure of the Amorphous Cell module. Here, a single polymer chain was put in the center of the cell, and the solvent molecules of acetonitrile/chloroform (70/30, v/v ≈ 78/22, mol/mol) were packed in the cell at density of 0.9807 g cm−3. Sequentially, the geometry of the MD cell was optimized. After the equilibration at 298 or 400 K, simulation in the NVE ensemble (constant number of atoms, volume, and energy) was conducted for 1000 ps (time step of 1.0 fs, 1 000 000 steps) as the production run. The COMPASS II (ver. 1.2) force field was used, and the charges were assigned by the force field. See the Supporting Information for details. Instruments. NMR spectra were taken on a JNM-ECS 400 (JEOL, Tokyo, Japan) (400 MHz for 1H) or a JNM-ECA 500 (JEOL) (500 MHz for 1H, 125 MHz for 13C) or a JNM-ECA 600 (JEOL) (600 MHz for 1H) spectrometer in CDCl3 and DMSO-d6 using tetramethylsilane or a solvent residual peak as the internal standard. Melting points were measured on a Yanako melting point apparatus and were uncorrected. IR spectra were obtained using a JASCO (Hachioji, Japan) Fourier transform IR-4700 spectrophotometer with a KBr pellet. The molecular weights (Mn) and distributions (PDI) of the polymers were estimated using size-exclusion chromatography (SEC) equipped with a TSKgel MultiporeHXL-M column (Tosoh, Tokyo, Japan), a JASCO PU-2080 Plus high-performance liquid chromatography pump, and a JASCO UV-970 UV/vis detector at 254 nm, where chloroform was used as the eluent. The molecular weight calibration curve was obtained with polystyrene standards (Tosoh). Absorption and CD spectra were measured using a JASCO V-570 (a scanning rate of 100 nm min−1 and a bandwidth of 0.5 nm) and a JASCO J-725 (a scanning rate of 200 nm min−1 and a bandwidth of 1.0 nm) spectrometers, respectively, with a quartz cell of 10 mm path length. The temperature was controlled using a JASCO ETC-505T (absorption spectroscopy) and a JASCO PTC-348WI apparatus (CD spectroscopy). Fluorescence quantum yields were measured on a JASCO FP-8500 using quinine sulfate in 0.5 M sulfuric acid aqueous solution as a standard material. CPL spectra were recorded at room

Scheme 1. Synthesis of Poly-Tr and Poly-2Tr

(CD) measurements and all-atom molecular dynamics (MD) simulations. We also investigated the CPL features of the resulting polymers.

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EXPERIMENTAL SECTION

Materials. Anhydrous solvents (tetrahydrofuran (THF), chloroform, acetonitrile, dichloromethane, and N,N-dimethylformamide) and common organic solvents were purchased from Kanto Kagaku (Tokyo, Japan). Copper(I) iodide (CuI), 5,5′-diiodo-2,2′-bithiophene, triethylamine, diisopropylamine (DIPA), potassium fluoride, and tetran-butylammonium fluoride (1.0 M in THF, 0.32 mL, 0.32 mmol) were from Sigma-Aldrich (St. Louis, MO). Triethylene glycol monomethyl ether was from Tokyo Kasei Kogyo (Tokyo, Japan). 1-Ethyl-3-(3(dimethylamino)propyl)carbodiimide hydrochloride, N,N-dimethyl-4aminopyridine, trans-dichlorobis(triphenylphosphine)palladium(II), 2,5-diiodothiophene, and trimethylsilylacetylene were purchased from Wako Pure Chemical Industries (Osaka, Japan). Tetrakis(triphenylphosphine)palladium(0) and 2-[(tert-butyldimethylsilyl)oxy]ethanol were purchased from Nacalai (Kyoto, Japan). 5,5′Dibromo-(1,1′-biphenyl)-2,2′-dicarboxylic acid,33 poly-T,18 and poly2T17 were prepared according to a literature procedure. Details on the synthesis of achiral diethynyl compounds (2 and 3) are described in the Supporting Information (Scheme S1). Polymerization. Polymerization was carried out in a dry Schlenk flask under a nitrogen atmosphere in a similar way to that previously reported.17 The copolymerizations results are summarized in Table S1. A typical polymerization procedure is described below. Poly-T0.75. GLB-1 (77 mg, 0.10 mmol), 2 (20 mg, 0.034 mmol), 2,5diiodothiophene (44 mg, 0.13 mmol), CuI (6 mg, 0.03 mmol), and Pd(PPh3)4 (7 mg, 6 μmol) were placed in a Schlenk flask under a nitrogen atmosphere. Degassed THF/DIPA (5/1, v/v) (3 mL) was added with a syringe, and the mixture was stirred at 60 °C for 12 h. After cooling to room temperature, the reaction mixture was poured into a large amount of hexane, and the resulting polymer was collected by centrifugation, washed with ethanol, and dried in vacuo to yield poly-T0.75 as a yellow solid (33 mg, 31%). 1H NMR (400 MHz, CDCl3, 55 °C): δ 8.20−6.60 (br, 8H, ArH), 6.40−4.70 (br, 3H, CH), 4.60−3.90 (br, 6.25H, CH, CH2), 3.90−3.00 (br, 25.25H, CH2, OCH3). IR (KBr, cm−1): 2201 (CC), 1754 (CO). Calcd for C41.25H42.5O14.5S·0·6H2O: C, 60.93; H, 5.42. Found: C, 60.80; H, 5.20. Spectroscopic Data for Poly-T0.50. 1H NMR (400 MHz, CDCl3, 55 °C): δ 8.03 (d, J = 7.8 Hz, 1H, ArH), 7.90−6.60 (br, 7H, ArH), 6.40− 4.70 (br, 2H, CH), 4.60−3.90 (br, 5.5H, CH, CH2), 3.90−3.00 (br, 25.5H, CH2, OCH3). IR (KBr, cm−1): 2201 (CC), 1755 (CO), B

DOI: 10.1021/acs.macromol.8b00229 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules temperature on a JASCO CPL-300 with 10 mm path length quartz cell (GL Sciences, UV-grade). A scanning rate of 50 nm min−1, an excitation bandwidth of 3000 μm, a monitoring bandwidth of 3000 μm, a response time of 8 s, and 4 times accumulation were employed. Elemental analyses were performed by the Research Institute for Instrumental Analysis of Advanced Science Research Center, Kanazawa University, Kanazawa, Japan.

polymerization solvent was synthesized and used as the achiral comonomer instead of 2 (Scheme S1B). Ternary copolymerizations of GLB-1, 3, and 4 were carried out in the same way as described above, in which the GLB-1/3 molar feed ratios were 75/25, 50/50, and 20/80 (Scheme 1). Each copolymerization proceeded homogeneously. A series of optically active poly-Tr with the Mn values exceeding 0.8 × 104 g mol−1, which were estimated by SEC, were obtained (Table S1). The compositional ratios of the GLB-1 and 3 units in the copolymers were confirmed to be almost consistent with the feed ratios, according to their 1H NMR and elemental analyses. For comparison, the bithiophene-type copolymers, poly-2Tr, containing chiral and achiral biphenyl units with varying ratios were also prepared. Chiroptical Properties. In our previous report,17,18 we demonstrated that the chiroptical properties of the GLB-based polymers, including poly-T and poly-2T, were greatly affected by their molecular weights. Therefore, we investigated the molecular weight dependence of the chiroptical properties of the resulting copolymers. Three samples of poly-Tr and poly2Tr with different molecular weights were individually prepared by SEC fractionation (Figures S1−S6), and their CD and absorption spectra were measured in an acetonitrile/chloroform mixture (Figure 1A−C), which is a typical solvent system for poly-T and poly-2T to fold into a helical conformation (Figures S7 and S8).17,18 Spectral measurements of poly-Tr and poly-2Tr were performed in acetonitrile/chloroform mixtures (70/30 and 40/60, v/v, respectively) to account for their solubility. The CD intensity of poly-Tr tended to gradually increase with increasing molecular weight, and the values of poly-T0.75 and poly-T0.50 became almost constant when the molecular weight was more than 1.4 × 104 and 2.5 × 104 g mol−1, respectively (Figure 1A,B). In the case of poly-T, even a

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RESULTS AND DISCUSSION Synthesis. To investigate the chiral amplification behavior of the GLB-based polymers through a sergeants and soldiers experiment, a diethynyl compound (2 in Chart 2) was

Chart 2. Achiral Diethynyl Monomers (2 and 3)

synthesized for use as an achiral monomer according to Scheme S1A, which has the same ten-membered ring structure containing two ester groups as the corresponding chiral monomer (GLB-1). However, the resulting 2 had limited solubility in THF and toluene, which are typical cosolvents used for polymerization by Sonogashira−Hagihara crosscoupling. Thus, when a ternary copolymerization of GLB-1, 2, and 2,5-diiodothiophene (4) ([GLB-1]/[2]/[4] = 1/1/2) was performed in a THF/DIPA mixture (5/1, v/v) at 60 °C according to our previously reported procedure,17 the reaction did not proceed homogeneously and only gave a trace amount of oligomers. Therefore, a novel diethynyl compound bearing tri(ethylene glycol) residues (3) with good solubility in a

Figure 1. Molecular weight dependences of the CD and absorption spectra of poly-T0.75 (A), poly-T0.50 (B), and poly-T0.20 (C) in acetonitrile/ chloroform (70/30, v/v) at 25 °C. (D) CD and absorption spectra of the fractionated high molecular weight components of poly-T and poly-Tr in acetonitrile/chloroform (70/30, v/v) at 25 °C. To allow comparison of the CD intensities, the CD spectra were normalized with respect to the absorbance at the absorption maximum wavelength. C

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Figure 2. CD and absorption spectra of the fractionated high molecular weight components (Mn ≥ 3.6 × 104 g mol−1) of poly-2T and poly-2Tr in acetonitrile/chloroform (40/60, v/v) (A) and chloroform (B) at 25 °C. To allow comparison of the CD intensities, the CD spectra were normalized with respect to the absorbance at the absorption maximum wavelength.

Figure 3. (A) Structure of the poly-T0.50 model used for the computational study. (B, D) Top view and (C, E) side view of the molecular model of the helically folded poly-T0.50 in acetonitrile/chloroform (70/30, v/v) at 0 ps (B, C) and 1000 ps (D, E) in an all-atom MD simulation after equilibration at 298 K represented by space-filling (backbone) and stick (side chain) models. The poly-T0.50 backbone is highlighted in purple. The acetonitrile and chloroform solvent molecules are represented by line models, and their hydrogen atoms are omitted to simplify the view. All scale bars represent 1 nm. Plots of the torsion angles θi (F) and ϕi (G) as a function of the calculation time.

low molecular weight fraction (0.7 × 104 g mol−1) showed a saturation level of the CD intensity (Figure S7), indicating that the minimum molecular weights to sufficiently stabilize the helically folded state seem to decrease with increasing content of the GLB unit. Conversely, the CD intensity of poly-T0.20 was not saturated even when using the polymer with a molecular weight of 2.7 × 104 g mol−1 (Figure 1C). When compared with the results of each highest molecular weight fraction, poly-T0.75 and poly-T0.50 exhibited almost the same degree of CD intensity as poly-T without an achiral biphenyl unit (Figure

1D). These results indicate that the chiral information on the GLB unit as a sergeant was effectively transferred to the neighboring achiral soldier moieties, which resulted in the preferred-handed helix formation of poly-Tr through chiral amplification. In addition, 50 mol % of the chiral sergeant unit was sufficient to induce the whole polymer backbone to adopt the helically folded state, although a high molecular weight poly-T0.50 of more than 2.5 × 104 g mol−1 was required.35 We observed that the degree of chiral amplification decreased in chloroform compared with the case in the acetonitrile-rich D

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ϕ̅ i) and their standard deviations (SD) are summarized in Table 1. Each θ̅i value in the chiral GLB units was only varied

solvent system (Figure S10). This is probably because π−π interactions between adjacent helical turns, which is the primary driving force for promoting helix formation, did not efficiently work in chloroform owing to its moderate affinity for the π-conjugated backbone. In contrast to poly-Tr, the CD intensities of poly-2Tr in acetonitrile/chloroform (40/60, v/v) were gradually decreased with an increase in the molar fractions of the achiral biphenyl unit, even though the high molecular-weight components (≥3.6 × 104 g mol−1) were used for comparison (Figure 2A). Although the helical conformation was more or less constructed in poly-2T0.75 and poly-2T0.90, poly-2T0.50 is expected to have a totally random-coil conformation in the acetonitrile/chloroform mixture. This reasoning is based on the following facts: (1) poly-2T cannot fold into a helical conformation in chloroform, as demonstrated previously;17 (2) thus, the Cotton effects of poly-2Tr in chloroform (Figure 2B) are also considered to be exclusively derived from the chirality of the GLB units rather than from the chirality of the higher order structure; and (3) the CD intensity of poly-2T0.50 in acetonitrile/chloroform (40/60, v/v) was almost the same as the result in chloroform. Because the CD intensity of poly-T0.50 in chloroform (Figure S10) was much higher than that of poly2T0.50 in acetonitrile/chloroform (40/60, v/v) (Figure 2A), the difference in the acetonitrile content between the poly-Tr and poly-2Tr systems would not be a key factor to provide a difference in the chiral amplification effect. In the case of poly2Tr containing bithiophene units in the main chain, the number of C−C single bonds capable of free internal rotation was increased compared with that of poly-Tr with monothiophene units. This caused more structural irregularity in poly-2Tr and would likely be the main reason for the difficulty in the preferred-handed helix induction in the entire poly-2Tr chains containing the achiral segments through a “sergeants and soldiers” chiral amplification mechanism. Computational Study. To obtain further information about the chiral amplification in the preferred-handed helix formation, an all-atom MD simulation of poly-T0.50 in acetonitrile/chloroform (70/30, v/v) was conducted with a corresponding poly-T0.50 model containing 20 biphenyl units (Figure 3A and Movie S1). The initial polymer structure with a left-handed helical main chain was constructed using a molecular model composed of alternating sequences of chiral and achiral biphenyl-based π-conjugated units, according to the findings of a previous report.34 After equilibration at 298 K (see Supporting Information), the simulation in the NVE ensemble was carried out for 1000 ps as the production run. The molecular models in the initial (0 ps) and final (1000 ps) states are presented in Figure 3B−E. The corresponding molecular models in the middle (250, 500, and 750 ps) stages are also shown in Figure S11. This simulation demonstrates that the helically folded structure was maintained over the calculation period and was likely to be the preferred conformation for polyT0.50 in acetonitrile/chloroform (70/30, v/v) (Movie S2 and Movie S3), which is in good agreement with the experimental result obtained from the CD analysis. The time-dependent changes of the torsion angles (θi and ϕi, see Figure 3A) between the two benzene planes in the ith GLB and 3 units from the terminal acetylene, respectively, are plotted as a function of time in Figures 3F and 3G, respectively. To reduce the influence of the chain ends, only the results of the six biphenyl units (i = 3−8) at the center of the poly-T0.50 model were used for the discussion. The average torsion angles (θ̅i and

Table 1. Average Torsion Angles and Their SD Valuesa θi

av torsion angleb (deg)

SDc

ϕi

av torsion angleb (deg)

SDc

θ3 θ4 θ5 θ6 θ7 θ8

56 56 58 55 57 60

7.42 7.28 7.11 6.47 7.18 7.70

ϕ3 ϕ4 ϕ5 ϕ6 ϕ7 ϕ8

73 75 75 75 71 71

9.93 9.89 8.24 9.41 9.90 9.00

a

Simulation results after equilibration at 298 K. bAverage value during the whole calculation period (0−1000 ps). cThe number of samples is 100.

within the range from +55° to +60°, suggesting that the axial chirality of the GLB units was maintained in the (S)configuration (cisoid). This is understandable because a pair of benzene planes in the GLB unit was not allowed to rotate freely owing to the asymmetric ten-membered ring structure. Interestingly, the same tendency was observed in the achiral 3 units (Figure 3G); the variations in the ϕ̅ i values fell within the range from +71° to +75° (Table 1). These results indicate that the axial chirality of all the biphenyl units contained in the polyT0.50 model was perfectly adjusted to the (S)-configurations during the whole calculation period. The inconsistency in the θ̅i and ϕ̅ i values in the GLB and 3 units probably reflects the difference in their rotational barriers based on the absence or presence of the ten-membered ring structure. The simulation after equilibration at 400 K also gave a similar result (Figure S12 and Table S2, Movie S4). These computational studies demonstrated that when poly-T0.50 was dissolved in acetonitrile/chloroform (70/30, v/v), not only the one-handed macromolecular helicity but also the one-handed axially twisted conformations were most likely induced in the polymer backbone and the inherently achiral 3 units, respectively. Circularly Polarized Luminescence Properties. Photographs of the polymers in acetonitrile/chloroform (70/30, v/v) under irradiation at 365 nm are shown in Figure 4B. Poly-Tr and poly-T exhibited an apparent blue emission in the fluorescence region of the π-conjugated main chain. Their fluorescence quantum yields were estimated to be in the range of 4−6%. Figure 4A shows the photoluminescence (PL) and dissymmetry factor (glum) spectra of the high molecular weight fraction of poly-Tr and poly-T measured in acetonitrile/ chloroform (70/30, v/v). Here, glum = 2 (IL − IR)/(IL + IR), where IL and IR are the PL intensities of the left- and righthanded circularly polarized light, respectively. Because the PL emission bands seem to overlap with the absorption bands, we performed a correction for the PL spectra according to a method reported in the literature (Figure S13).36,37 It was found that the corrected PL spectra were mostly overlapped with the corresponding unprocessed data, which was due to a sufficiently low polymer concentration (ca. 10−6 M). This indicates that the influence of the reabsorption on the PL properties was almost negligible. As expected from the result obtained in the CD spectral analysis, poly-Tr clearly showed a positive nonlinear relationship between the molar ratios of the chiral and achiral biphenyl units and the resulting CPL intensity; poly-T0.50 containing 50 mol % of the achiral biphenyl units had a remarkable CPL performance, and its E

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Movie S3 (MPG) Movie S4 (MPG)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.I.). ORCID

Tomoyuki Ikai: 0000-0002-5211-2421 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grants-in-Aid for Scientific Research (C), Grant 17K05875. Computation time was provided by the supercomputer system, Research Center for Advanced Computing Infrastructure, JAIST.

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Figure 4. (A) PL (bottom) and glum (top) spectra of poly-T and polyTr in acetonitrile/chloroform (70/30, v/v) at room temperature. λex = 292 nm. (B) Photographs of the corresponding poly-T and poly-Tr solutions under irradiation at 365 nm.

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glum (6.0 × 10−3) was totally comparable to that of poly-T, which did not have an achiral biphenyl unit. Again, poly-2Tr showed an apparent negative nonlinear response in the CPL performance, which is consistent with the result of the CD spectroscopic study. Thus, poly-2T0.50 did not give an apparent CPL signal (glum ≈ 10−4), in contrast to the result of poly-2T, which showed a glum value of 1.0 × 10−2 (Figure S14).

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CONCLUSIONS We demonstrated the explicit example of chiral amplification in π-conjugated helical polymers with circularly polarized luminescence. Chiral amplification in the preferred-handed helix formation was significantly influenced by the frameworks of the incorporated comonomer units. Poly-Tr containing 2,5thienylene units in the main chain exhibited “the sergeants and soldiers effect”, and its macromolecular helicity was significantly amplified by the chirality transfer from the chiral GLB moieties to the neighboring achiral ones. Thanks to the chiral amplification behavior, poly-T0.50 containing 50 mol % of the achiral biphenyl units had a remarkable CPL performance, and its glum (6.0 × 10−3) was the same as that of poly-T, which did not have an achiral biphenyl unit. The GLB-based polymers can be easily functionalized through the appropriate design of achiral comonomer units. Thus, we believe that the further development of chiral functional materials is quite possible using the framework of the GLB-based polymers in combination with the chiral amplification effect. Related work is currently underway in our laboratory and will be reported in due course.

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REFERENCES

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00229. Experimental procedures, characterizations of monomers, and additional spectroscopic, chromatographic, and computational data (PDF) Movie S1 (AVI) Movie S2 (MPG) F

DOI: 10.1021/acs.macromol.8b00229 Macromolecules XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.macromol.8b00229 Macromolecules XXXX, XXX, XXX−XXX