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Synthesis of Optically Active Conjugated Polymers Bearing m‑Terphenylene Moieties by Acetylenic Coupling Polymerization: Chiral Aggregation and Optical Properties of the Product Polymers Yu Miyagi,† Hiromitsu Sogawa,‡ Masashi Shiotsuki,§ and Fumio Sanda⊥,* †

Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University Katsura Campus, Nishikyo-ku, Kyoto 615-8510, Japan ‡ Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro, Tokyo 152-8552, Japan § Molecular Engineering Institute, Kinki University, Kayanomori, Iizuka, Fukuoka 820-8555, Japan ⊥ Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering, Kansai University, 3-3-35 Yamate-cho, Suita, Osaka 564-8680, Japan S Supporting Information *

ABSTRACT: The acetylenic coupling polymerization of Dhydroxyphenylglycine-derived m-terphenylene diynes 1−5 using Pd/Cu catalyst gave the corresponding polymers [poly(1)−poly(5)] with Mn = 12 000−60 000 in 53−89% yields. The polymers were soluble in THF and DMF. CD and UV−vis spectroscopic analysis revealed that p,p′-phenyleneethynylene-linked poly(1), poly(3), and poly(5) formed chiral higher-order structures in THF/H2O mixtures, while m,m′-phenyleneethynylene-linked poly(2) and poly(4) did not. The sign of CD signal of poly(1) was reasonably predicted by time-dependent density functional calculations of the model system. The polymers emitted fluorescence with quantum yields ranging from 0.2−14.8%.

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INTRODUCTION Conjugated polymers such as poly(acetylene)s, 1 poly(phenylene)s, 2 poly(phenylenevinylene)s, 3 and poly(phenyleneethynylene)s4 attract considerable attention because of their useful properties: electrical conductivity, paramagnetic susceptibility, optical nonlinearity, photoconductivity, and fluorescence emission. Conjugated polymers substituted with optically active groups tend to form chiral higher order structures, i.e., predominantly one-handed helices or chiral aggregates, due to the small mobility of the conjugated main chains. In some cases, the higher order structures are further stabilized by intra- and/or intermolecular interactions, such as hydrogen bonding and π-stacking.5 The acetylenic coupling reaction is a useful method for synthesizing diacetylene compounds.6 Since the first report on cupper (Cu)-mediated oxidative acetylenic coupling developed by Glaser in the 19th century, various modifications have been reported, including the Eglington coupling using excess Cu(OAc)27 and Hay coupling using a catalytic amount of CuCl−N,N,N′,N′-tetramethylethylenediamine.8 A milestone work on acetylenic coupling was the development of palladium (Pd)-catalyzed coupling reactions reported by Rossi et al.9 In recent years, variations of Pd-catalyzed coupling reactions of terminal alkynes have been utilized for synthesizing diaryl and dialkyl diynes, and applied to the synthesis of natural products and polymers. Pd-catalyzed coupling polymerization is a © 2014 American Chemical Society

powerful tool for synthesizing conjugated diyne polymers that show useful photoelectrical properties. The p-terphenylene-linkage is often used as a key unit for constructing molecules with rigid conformation, including αhelix mimic-, 10 pillar-,11 triangle-,12 and cage-shaped13 structures, due to the rod-like rigid nature. p-Terphenylenecontaining molecules have also been examined as photoelectrically functional materials.14 In a similar fashion, the mterphenylene-linkage is used as a building block for assembling bent-shaped molecules.15 Various p- and m-linked terphenylene-based polymers have been synthesized, primarily for preparation of photoelectrically functional polymers with controlled secondary and assembled structures.16 In this study, we report the synthesis of novel hydroxyphenylglycine-derived conjugated polymers bearing terphenylene moieties by acetylenic coupling polymerization and an investigation of influence of the phenyleneethynylene moieties on the higher-order structures. We analyzed the aggregation of the polymers by dynamic light scattering (DLS) and proposed a mechanism using density functional theory (DFT) calculations of the model compounds. Received: October 15, 2013 Revised: February 12, 2014 Published: February 21, 2014 1594

dx.doi.org/10.1021/ma402123w | Macromolecules 2014, 47, 1594−1603

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3′,5′-Di(4″-ethynylphenyl)-4′-hydroxy-N-α-tert-butoxycarbonylHexylamide (1). 4-[(Trimethylsilyl)ethynyl]phenylboronic acid pinacol ester (5.41 g, 18.0 mmol), Cs2CO3 (7.82 g, 24.0 mmol) and Pd2(dba)3 (166 mg, 0.182 mmol) were added to a solution of 9 (3.61 g, 6.01 mmol) in acetone/H2O (60 mL/18 mL) under nitrogen, and the resulting mixture was heated with refluxing for 18 h at 65 °C. The resulting solution was extracted with ethyl acetate, and the organic layer was washed with 0.5 M HCl, dried over anhydrous MgSO4, and then filtered. The filtrate was concentrated, and the residual mass was purified by silica gel column chromatography with CHCl3/hexane/ethyl acetate =38/1/1 (v/v/v) and preparative HPLC to obtain 1 as a white powder in 39% yield. Mp: 111 °C. [α]D: −82° (c = 0.10 g/dL, THF). IR (in CHCl3): 3546, 3294, 2930, 2858, 2107, 1657, 1165 cm−1. 1H NMR (400 MHz, CDCl3): δ 0.83 (t, J = 6.3 Hz, 3H, −CH2CH3), 1.20−1.41 [m, 17H, −C(CH3)3, (CH2)4], 3.13 (s, 2H, −CCH), 3.20−3.25 (m, 2H, −NCH2), 5.16 (br, 1H, C*H), 5.38 (br, 1H, CONH), 5.87 (br, 1H, NHCH2), 5.98 (br, 1H, OH), 7.27−7.57 (10H, Ar). 13C NMR (100 MHz, CDCl3): δ 13.9 (−CH2CH3), 22.4, 26.4, 29.4, 31.4, 39.8 [(CH2)4], 28.3 [−C(CH3)3], 57.8 (C*H), 78.0 (−CCH), 80.1 [−C(CH3)3], 83.3 (−CCH), 121.7, 128.6, 128.8, 129.2, 131.3, 132.5, 137.4, 149.2 (Ar), 155.3 (CONH), 170.1 (OCONH). HRMS. (m/z): [M + H]+ calcd for C35H39N2O4, 551.2904; found, 551.2901. 3′,5′-Di(3″-ethynylphenyl)-4′-hydroxy-N-α-tert-butoxycarbonylD-phenylglycine Hexylamide (2). This compound was synthesized from 9 and 3-[(trimethylsilyl)ethynyl]phenylboronic acid pinacol ester in a manner similar to 1. Yield: 28% (white powder). Mp: 77−78 °C. [α]D: −87° (c = 0.10 g/dL, THF). IR (KBr): 3298, 2928, 2858, 2107, 1656, 1163 cm−1. 1H NMR (400 MHz, CDCl3): δ 0.79 (t, J = 6.3 Hz, 3H, −CH2CH3), 1.15−1.37 [m, 17H, −C(CH3)3, (CH2)4], 3.06 (s, 2H, −CCH), 3.22−3.17 (m, 2H, −NCH2), 5.39 (br, 1H, C*H), 5.62 (br, 1H, CONH), 6.11 (br, 1H, NHCH2), 6.77 (br, 1H, OH), 7.29−7.57 (10H, Ar). 13C NMR (100 MHz, CDCl3): δ 13.9 (−CH2CH3), 22.4, 26.3, 29.3, 31.3, 39.7 [(CH2)4], 28.2 [−C(CH3)3], 57.4 (C*H), 77.7 (−CCH), 80.0 [−C(CH3)3], 83.2 (−CCH), 122.6, 128.2, 128.6, 129.6, 131.1, 131.2, 132.8, 137.2, 149.2 (Ar), 155.4 (CONH), 170.3 (OCONH). HRMS. (m/z): [M + H]+ calcd for C35H39N2O4, 551.2904; found, 551.2899. 3′,5′-Di(4″-ethynylphenyl)-4′-hydroxy-N-α-tert-butoxycarbonylD-phenylglycine N-Hexylmethylamide (3). This compound was synthesized from 10 and 4-[(trimethylsilyl)ethynyl]phenylboronic acid pinacol ester in a manner similar to 1. Yield: 39% (white solid). Mp: 168−169 °C. [α]D: −107° (c = 0.10 g/dL, THF). IR (KBr): 3410, 3292, 2929, 2860, 2107, 1640, 1166 cm−1. 1H NMR (400 MHz, CDCl3): δ 0.83 (t, J = 6.3 Hz, 3H, −CH2CH3), 1.21−1.42 [m, 17H, −C(CH3)3, (CH2)4], 2.92−2.93 (ds, 3H, −NCH3), 3.13 (s, 2H, −C CH), 3.23−3.56 (m, 2H, −NCH2), 5.39 (br, 1H, C*H), 5.51−5.59 (br, 1H, CONH), 6.06 (br, 1H, OH), 7.29−7.57 (10H, Ar). 13C NMR (100 MHz, CDCl3): δ 13.9 (−CH2CH3), 22.4, 26.4, 31.5, 33.6, 35.1 [(CH2)4], 28.4 [−C(CH3)3], 48.4 (NCH3), 54.3 (C*H), 78.0 (−C CH), 79.7 [−C(CH3)3], 83.3 (−CCH), 121.7, 128.6, 129.3, 130.8, 131.4, 132.6, 137.4, 149.1 (Ar), 155.0 (CONH), 169.7 (OCONH). HRMS. (m/z): [M + H]+ calcd for C36H41N2O4, 565.3061; found, 565.3056. 3′,5′-Di(3″-ethynylphenyl)-4′-hydroxy-N-α-tert-butoxycarbonylD-phenylglycine N-Hexylmethylamide (4). This compound was synthesized from 10 and 3-[(trimethylsilyl)ethynyl]phenylboronic acid pinacol ester in a manner similar to 2. Yield: 16% (white solid). Mp: 73−74 °C. [α]D: −113.8° (c = 0.10 g/dL, THF). IR (KBr): 3408, 3292, 2957, 2929, 2859, 2372, 2109, 1707, 1638 cm−1. 1H NMR (400 MHz, CDCl3): δ 0.82 (t, J = 6.3 Hz, 3H, −CH2CH3), 1.20−1.47 [m, 17H, −C(CH3)3, (CH2)4], 2.91−2.92 (ds, 3H, −NCH3), 3.10 (s, 2H, −CCH), 3.20−3.57 (m, 2H, −NCH2), 5.50 (br, 1H, C*H), 5.59 (br, 1H, CONH), 6.04 (br, 1H, OH), 7.27−7.65 (10H, Ar). 13C NMR (100 MHz, CDCl3): δ 14.0 (−CH2CH3), 22.5, 26.4, 31.5, 33.7, 35.1 [(CH2)4], 28.4 [−C(CH3)3], 48.4 (NCH3), 54.6 (C*H), 77.7 (−C CH), 79.6 [−C(CH3)3], 83.1 (−CCH), 122.7, 128.3, 128.7, 129.2, 129.6, 130.5, 131.3, 132.8, 137.0, 149.1 (Ar), 154.8 (CONH), 169.6 (OCONH). HRMS. (m/z): [M + H]+ calcd for C36H41N2O4, 565.3061; found, 565.3057.

EXPERIMENTAL SECTION 1

D -phenylglycine

13

Measurements. H (400 MHz) and C (100 MHz) NMR spectra were recorded on a JEOL EX-400 or a JEOL AL-400 spectrometer. IR spectra were measured on a JASCO FT/IR-4100 spectrophotometer. Melting points (mp) were measured on a Yanaco micro melting point apparatus. Mass spectra were measured on a Thermo Scientific Exactive mass spectrometer. Specific rotations ([α]D) were measured on a JASCO DIP-1000 digital polarimeter. Number- and weightaverage molecular weights (Mn and Mw) of polymers were determined by GPC (TSK gel α-3000) using a solution of LiBr (10 mM) in N,Ndimethylformamide (DMF) as the eluent with polystyrene standards at 40 °C. CD and UV−vis absorption spectra were recorded on a JASCO J-820 spectropolarimeter. DLS measurements were performed using a Malvern Instruments Zetasizer Nano ZS at 25 °C. The measured autocorrelation function was analyzed using a cumulant method. The Z-average values of the polymers were calculated from the Stokes−Einstein equations. Materials. 3′,5′-Diiodo-4′-hydroxy-N-α-tert-butoxycarbonyl-D-phenylglycine hexylamide (9) was prepared according to the literature.17 Reagents, including di-tert-butyl dicarbonate [(Boc)2O, Tokuyama], 4(4,6-dimethoxy-1,3,5-triazine-2-yl)-4-methylmorpholinium chloride (TRIAZIMOCH, Tokuyama), anhydrous THF (Wako, water max. 0.001%), Pd(OAc)2 (Aldrich, assay 99.9%), CuI (Wako, 99.5%), and 1,4-diazabicyclooctane (DABCO) (Wako, 95.0%) were used as received. Monomer Synthesis. 3′,5′-Diiodo-4′-hydroxy-N-α-tert-butoxycarbonyl-D-phenylglycine N-Hexylmethylamide (10). (Boc)2O (82.2 g, 377 mmol) and Na2CO3 (38.1 g, 359 mmol) were added to a solution of D-4-hydroxyphenylglycine (50.0 g, 299 mmol) in 1,4dioxane/H2O (500 mL/500 mL) at 0 °C, and the resulting mixture was stirred at room temperature overnight. 1,4-Dioxane was removed by evaporation, and the residual solution was carefully acidified with citric acid to pH = 3 and extracted with ethyl acetate. The organic layer was washed with water and saturated NaCl(aq), dried over anhydrous MgSO4, and then filtered. The filtrate was concentrated to obtain 4hydroxy-N-α-tert-butoxycarbonyl-D-phenylglycine (6) as a viscous liquid. After that, TRIAZIMOCH (12.9 g, 46.8 mmol) and Nhexylmethylamine (6.10 mL, 40.0 mmol) were added to a solution of 6 in ethyl acetate (200 mL) at 0 °C, and the resulting mixture was stirred at room temperature overnight. It was washed with 0.5 M HCl, saturated NaHCO3(aq), and saturated NaCl(aq), dried over anhydrous MgSO4, and then filtered. The filtrate was concentrated to obtain 4-hydroxy-N-α-tert-butoxycarbonyl-D-phenylglycine N-hexylmethylamide (8) as a yellow solid. After that, NaIO4 (9.66 g, 45.1 mmol) and NaCl (10.6 g, 181 mmol) were added to a solution of 8 in acetic acid/H2O (135 mL/15 mL) at room temperature, and the resulting mixture was stirred at room temperature for 15 min. KI (22.5 g, 135 mmol) was added to the solution at 0 °C, and then the resulting mixture was stirred at room temperature overnight. It was washed with water, 1 M Na2S2O3(aq), and saturated NaCl(aq), dried over anhydrous MgSO4, and then filtered. The filtrate was concentrated, and the residual mass was purified by silica gel column chromatography with CHCl3/hexane = 4/1 (v/v) as the eluent to obtain 10 as a yellow powder in 44% yield. 1H NMR (400 MHz, CDCl3): δ 0.87 [t, J = 1.6 Hz, 3H, −N(CH2)5CH3], 1.24−1.42 [m, 17H, −C(CH3)3, (CH2)4], 2.86 and 2.94 (s, 3H, −NCH3), 3.25−3.48 (m, 2H, −NCH2−), 5.35−5.40 (m, 1H, NHCO), 5.86 (s, 1H, C*H), 6.05 (m, 1H, −OH), 7.69 (s, 2H, Ar). 3′,5′-Diiodo-4′-methoxy-N-α-tert-butoxycarbonyl-D-phenylglycine Hexylamide (11). Compound 9 (1.20 g, 2.0 mmol) and K2CO3 (0.41 g, 3.0 mmol) were dissolved in acetone (7.5 mL). MeI (0.19 mL, 3.0 mmol) was added to the solution at 0 °C, and the resulting mixture was refluxed for 3 h. The mixture was poured into iced water, and it was extracted with CHCl3. The organic layer was washed with saturated aqueous NaCl, dried over anhydrous MgSO4, filtered and then concentrated to obtain 11 as a white solid. Yield: 97%. 1H NMR (400 MHz, CDCl3): δ 0.87 (t, J = 6.4 Hz, 3H, −CH2CH3), 1.24−1.42 [m, 17H, −C(CH3)3, (CH2)4], 2.85, 2.94 (ds, 3H, −NCH3), 3.23− 3.53 (dq, J = 6.8 Hz, 2H, −NCH2−), 5.36−5.40 (m, 1H, NHCO), 5.87 (s, 1H, C*H), 6.05 (m, 1H, −OH), 7.69 (s, 2H, Ar). 1595


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Scheme 1. Synthesis of Monomers 1−5

Scheme 2. Acetylenic Coupling Polymerization of Monomers 1−5

using a membrane filter (ADVANTEC H100A047A) and dried under reduced pressure. Spectroscopic Data of the Polymers. Poly(1). IR (KBr): 3530, 3297, 2927, 2856, 2372, 2209, 1656, 1162 cm−1. 1H NMR (400 MHz, CDCl3): δ 0.80 (br, 3H, −CH2CH3), 1.13−1.35 [m, 17H, −C(CH3)3, (CH2)4], 7.40−8.06 (br, 10H, NHCO, NHCH2, OH, Ar). Poly(2). IR (KBr): 3530, 3413, 2929, 2858, 2369, 2217, 1671, 1163 cm−1. 1H NMR (400 MHz, CDCl3): δ 0.84 (br, 3H, −CH2CH3), 1.23−1.40 [m, 17H, −C(CH3)3, (CH2)4], 7.35−7.78 (br, 10H, NHCO, NHCH2, OH, Ar). Poly(3). IR (KBr): 3530, 2409, 2928, 2869, 2369, 2202, 1648, 1166 cm−1. 1H NMR (400 MHz, CD3OD): δ 0.85 (br, 3H, −CH2CH3), 1.23−1.40 [m, 17H, −C(CH3)3, (CH2)4], 2.94−2.95 (ds, 3H, −NCH3), 7.35−7.62 (br, 10H, NHCO, OH, Ar). Poly(4). IR (KBr): 3422, 2927, 2370, 2212, 1655, 1165 cm−1. 1H NMR (400 MHz, CDCl3): δ 0.84 (br, 3H, −CH2CH3), 1.23−1.39 [m, 17H, −C(CH3)3, (CH2)4], 2.88−2.95 (ds, 3H, −NCH3), 7.34−7.78 (br, 10H, NHCO, OH, Ar). Poly(5). IR (in CHCl3): 3301, 2928, 2857, 2371, 2208, 1656, 1163 cm−1. 1H NMR (400 MHz, CDCl3): δ 0.84 (br, 3H, −CH2CH3), 1.23−1.39 [m, 17H, −C(CH3)3, (CH2)4], 3.11 (br, 3H, −OCH3), 7.62−7.91 (br, 10H, NHCO, NHCH2, Ar). Computation. All calculations were performed with the GAUSSIAN 09 program,18 EM64L-G09 Rev C.01, running on the

3′,5′-Di(4″-ethynylphenyl)-4′-methoxy-N-α-tert-butoxycarbonylHexylamide (5). This compound was synthesized from 11 and 4-[(trimethylsilyl)ethynyl]phenylboronic acid pinacol ester in a manner similar to 1. Yield: 41% (white solid). Mp: 112−113 °C. [α]D: −68.1° (c = 0.10 g/dL, THF). IR (KBr): 3296, 2957, 2931, 2371, 2107, 1655, 1508 cm−1. 1H NMR (400 MHz, CDCl3): δ 0.81 (t, J = 5.2 Hz, 3H, −CH2CH3), 1.16−1.39 [m, 17H, −C(CH3)3, (CH2)4], 3.08 (s, 3H, −OCH3), 3.12 (s, 2H, −CCH), 3.17−3.22 (m, 2H, −NCH2), 5.42 (br, 1H, C*H), 6.11 (br, 1H, CONH), 6.53 (br, 1H, NHCH2), 7.30−7.45 (10H, Ar). 13C NMR (100 MHz, CDCl3): δ 14.0 (−CH2CH3), 22.5, 26.4, 29.4, 31.4 [(CH2)4], 28.3 [−C(CH3)3], 57.0 (C*H), 60.4 (−OCH3), 77.7 (−CCH), 80.2 [−C(CH3)3], 83.5 (−CCH), 121.0, 128.9, 129.1, 131.8, 134.7, 135.1, 138.4 (Ar), 154.5 (CONH), 170.0 (OCONH). HRMS. (m/z): [M + H]+ calcd for C36H41N2O4, 565.3061; found, 565.3056. Polymerization. All polymerizations were carried out in a glass tube equipped with a three-way stopcock under oxygen. In a typical experiment, a solution of Pd(OAc)2 (0.9 mg, 2 μmol) in THF (1 mL) and a solution of CuI (0.76 mg, 2 μmol) in THF (1 mL) were added to a mixture of a monomer (0.2 mmol) and 1,4-diazabicyclooctane (0.3 mmol) under oxygen, and the resulting solution was kept at 0 °C for 6−48 h. Then, the reaction mixture was poured into a large volume of MeOH to precipitate the polymer. It was separated by filtration D-phenylglycine

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Table 1. Acetylenic Coupling Polymerization of Poly(1)−Poly(5)a run

monomer

solvent

[M]0 (M)

temp (°C)

time (h)

yieldb (%)

Mnc

Mw/Mnc

1 2 3 4 5 6 7 8 9 10 11

1 1 1 1 1 1 1 2 3 4 5

CH2Cl2 DMF DMF DMF DMF THF THF THF THF THF THF

0.05 0.05 0.05 0.05 0.10 0.10 0.10 0.10 0.10 0.10 0.10

30 30 0 0 0 0 0 0 0 0 0

24 3 2 24 24 24 48 48 6 24 4

36 57 8 8 e 70 59 75 53 89 86

d d 17 000 34 000 d 12 000 22 000 60 000 18 000 43 000 20 000

d d 2.3 4.0 d 3.2 2.6 5.4 3.6 4.2 4.6

a Conditions: Pd(OAc)2:CuI:1,4-diazabicyclooctane =1:1:150, [M]0/[Pd(OAc)2] = 50. bMeOH-insoluble part. cEstimated by SEC (10 mM LiBr solution in DMF, polystyrene standards). dInsoluble in common organic solvents. eNot determined.

supercomputer system, Academic Center for Computing and Media Studies, Kyoto University. The energies were calculated by the integrated molecular orbital and molecular mechanics method (ONIOM),19 in which density functional theory (DFT)20 with the M06-2X functional21 in conjunction with the 6-31G* basis set and the molecular mechanics (MM) method with universal force filed (UFF)22 were employed for higher and lower layers, respectively. Transition electric dipole moment and excitation energies were calculated by the time-dependent (TD) DFT method23 with M06-2X/6-31G*. The spectra were simulated by calculating 20 excited states, using Gaussian band shapes with a half bandwidth of 20 nm.

polymeric mass precipitated at the late stage of the polymerization. In this case, the polymer yield decreased to 59%, while the Mn increased to 22,000. Monomers 2−5 were polymerized in THF at 0 °C as well, wherein the polymerization times were adjusted to obtain DMF- and THF-soluble polymers in good yields (runs 8−11). The IR and 1H NMR spectra of the polymers were very consistent with the structures linked by diacetylene moieties. The IR spectral patterns of the solventsoluble and insoluble polymers were almost identical. It seems that the solvent-insoluble parts are high molecular weight polymers, and there is no significant structural difference between the solvent-soluble and insoluble polymers. Chiroptical Properties of Poly(1)−Poly(5). CD and UV−vis spectra of the polymer samples (runs 7−11) were measured to obtain information on the chiroptical properties. Poly(1) exhibited intense CD signals at the absorption region of the main chain chromophore around 340−380 nm in THF− H2O mixtures, while monomer 1 did not (Figure 1). This result

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RESULTS AND DISCUSSION Monomer Synthesis and Polymerization. Scheme 1 illustrates the synthetic procedures for monomers 1−4. First, compounds 9 and 10 were synthesized by protection of the amino group of 4-hydroxy-D-phenylglycine with a tertbutoxycarbonyl group, amidation of the carboxy group and 3,5-diiodination of the phenyl group. Then, monomers 1−4 were synthesized by the Suzuki−Miyaura coupling of 9 and 10 with 3- and 4-[(trimethylsilyl)ethynyl]phenylboronic acid pinacol esters. Monomer 5 was synthesized via 11, which was obtained by methyl etheration of the phenol group of 9. The monomer structures were confirmed by IR, 1H and 13C NMR spectroscopy, high-resolution mass spectrometry, and elemental analysis. The acetylenic coupling polymerization of 1−5 was carried out using Pd and Cu catalysts in the presence of DABCO in CH2Cl2, DMF and THF at 0 and 30 °C under oxygen for 4−48 h (Scheme 2). The corresponding polymers [poly(1)− poly(5)] with Mn’s ranging from 12 000 to 60 000 were obtained in 8−89% yields as listed in Table 1. First, the polymerization of 1 was carried out in CH2Cl2 at 30 °C for 24 h, [M]0 = 0.05 M (run 1). The polymer was obtained in 36% yield, but it was insoluble in common organic solvents including DMF and THF. When DMF was used as a solvent, the polymer yield increased to 57%, but the obtained polymer was still solvent-insoluble (run 2). A DMF- and THF-soluble polymer was obtained by decreasing the temperature and time to 0 °C and 2 h (Run 3), although the yield was low (8%). Extension of polymerization time to 24 h did not improve the polymer yield (run 4). Increase of initial monomer concentration to 0.10 M resulted in formation of a solvent-insoluble polymer (run 5). On the other hand, the polymerization of 1 in THF with [M]0 = 0.10 M at 0 °C for 24 h gave a DMF- and THF-soluble polymer with Mn = 12,000 in 70% yield (run 6). When the polymerization time was extended to 48 h (run 7), a

Figure 1. CD and UV−vis spectra of poly(1) and monomer 1 measured in THF−H2O mixture (H2O content 50%, c = 0.04 mM) at room temperature.

indicates that poly(1) forms a chiral higher-order structure in the solvent. Poly(1), poly(3), and poly(5) exhibited strong intensity CD signals, while the signals for poly(2) and poly(4) were less intense (Figures S1 and S2, Supporting Information). The Kuhn dissymmetry factor24 (g = Δε/ε) of poly(1), poly(3), and poly(5) at the ε maxima increased with increasing H2O content in THF−H2O mixtures to reach a maximum at 1597


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H2O concentrations of 50%, 80% and 50%, respectively (Figure 2). The g-values decreased with further increase in H2O

Figure 3. CD and UV−vis spectra of poly(1) measured in THF−H2O mixture (H2O content 50%, c = 0.04 mM) at various temperatures.

Figure 2. g-values of poly(1)−poly(5) at the ε maxima measured in THF−H2O mixtures with various compositions (c = 0.04 mM) at room temperature.

(H2O content 50%) (Figure 4). The CD intensity around 360 nm increased with increasing concentration of poly(1). The

content. On the other hand, the g-values of poly(2) and poly(4) remained small irrespective of H2O content. These results suggest that p-linked poly(1), poly(3), and poly(5) form chiral higher-order structures possibly aggregation in THF− H2O mixture, and the chiral intensities depend on the solvent composition, while m-linked poly(2) and poly(4) do not form such chiral structures. It is considered that the m- and plinkages of the phenyleneethynylene moieties remarkably influence the formation of higher-order structures, in a manner similar to m- and p-phenylene-linked polyfluorene.25 In THF, poly(1) and poly(3) exhibited negligibly small CD signals. On the other hand, poly(5) exhibited intense CD signals, which were still observed after filtration with a membrane filter (Figure S3). It seems that poly(5) adopts a unimolecularly chiral higher order structure such as helix in THF. The CD intensity (g-value) of poly(5) became high by the addition of H2O mixture as shown in Figure 2, presumably due to the formation of aggregates as mentioned above. It is likely that the aggregation only partly contributes to the CD signal, and therefore the UV signal does not show the typical feature of an aggregation band. Poly(5) exhibited a positive CD signal at 371 nm, 6 nm longer wavelength than that of poly(1), and a negative CD signal at 332 nm in THF/H2O = 1/1 (Figure S1), which was not observed for poly(1). The way of aggregation poly(5) seems to be somewhat different from that of poly(1) in the solvent, definitely due to the difference between −OMe and −OH but the concrete reason is unclear. Poly(3) having −(CO)NCH3− group showed a weak CD signal compared with poly(1) having −(CO)NH− group. It indicates that the amide hydrogen bond plays a truly major role in forming a higher order structure, as opposed to the carbamate. Next, the CD and UV−vis spectra of poly(1) were measured in THF/H2O solvent (H2O content 50%) at various temperatures (Figure 3). The intensity of the CD signal around 360 nm gradually decreased as temperature increased from 0 to 60 °C, while the UV−vis signal maintained almost the same pattern. Once the temperature was raised to 60 °C, the CD signals did not return to the original values when the samples were cooled to 20 °C. This result indicates that the higherorder structure of poly(1) irreversibly changes when the temperature is raised. The CD and UV−vis spectra of poly(1) were further measured at various concentrations in THF−H2O mixture

Figure 4. CD and UV−vis spectra of poly(1) measured in THF−H2O mixture (H2O content 50%) with various concentrations.

CD and UV−vis signals of poly(1) disappeared after filtration using a membrane filter with a pore size of 0.45 μm (Figure 5).

Figure 5. CD and UV−vis spectra of poly(1) measured in THF−H2O mixture (H2O content 50%, c = 0.04 mM) before and after filtration using a membrane filter with a pore size of 0.45 μm. 1598


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Table 3. On the other hand, m-linked poly(2) and poly(4) emitted almost no fluorescence. Similar trends were observed in

These results indicate that the polymers do not adopt a unimolecularly chiral higher-order conformation, such as a predominantly one-handed helix, but that they form chiral aggregates. Previous researchers have reported that m-phenyleneethynylene oligomers carrying nonpolar chiral side chains maintain the same conformation and aggregation state in nonpolar solvents.26 Aggregation plays an important role in the twist sense bias that coincides with the occurrence of optical activity, and hydrophobic interactions are used for controlling the ordering of the oligomers. In the present study, DLS measurements were performed to obtain information on aggregation of the polymers. No particle was determined in a poly(1) solution in THF, indicating the absence of aggregates. On the contrary, the presence of particles with Z-average =113 nm was confirmed in a poly(1) solution in THF−H2O mixture (H2O content 50%). In a similar fashion, particles with Zaverages ranging from 70 to 421 nm were observed in the cases of poly(2)−poly(4), as summarized in Table 2. The Z-values of

Table 3. Optical Data of Poly(1)−Poly(5)a

H2O content of solvent (THF−H2O mixture, %)

Z-average (nm)

PDI

0 50 20 80 20 20

b 113 235 70 421 c

b 0.101 0.069 0.040 0.141 c

poly(1) poly(2) poly(3) poly(4) poly(5) a

c = 0.04 mM. determined.

b

No particle was determined.

c

λabs (nm)

λemib (nm)

Φemic (%)

poly(1) poly(2) poly(3) poly(4) poly(5)

337.5 335.5 336.0 334.0 332.0

383, 404 − 383, 404 − 373, 490

14.8 0.4 12.5 0.2 8.1

Measured in THF. bExcited at the λabs in the left column. cMeasured using anthracene as a standard (Φemi = 0.27) in EtOH. The values were corrected using the refractive indices of THF and EtOH.

a

the CD and UV−vis spectra of poly(1)−poly(5) as mentioned above. The fluorescence quantum yields of aggregated conjugated polymers tend to be small compared with those of less-aggregated states28 and monomeric model compounds, due to radiationless deactivation and/or intersystem crossing.29 In the present study, poly(5) chirally aggregated in THF and emitted fluorescence, but the intensity was lower than that of poly(1) and poly(3), neither of which forms chiral aggregates in THF. This is also supported by the fluorescence spectra of monomer 1 measured in THF and THF/H2O = 1/1 (Figure S5). In THF, monomer 1 showed a single fluorescence peak at 386 nm. In THF/H2O = 1/1, the peak intensity decreased and another peak assignable to aggregate-derived fluorescence appeared at 510 nm. Conformational Analysis. In order to obtain information on aggregation of the polymers, DFT calculations of aggregates of monomers 1 and 2 were carried out as model systems at the M06-2X/6-31G* level. M06-2X functional is superior compared with commonly used B3LYP functional in estimating noncovalent interactions including π-stacking and hydrogen bonding. Two-layer ONIOM calculations were employed to save CPU time; tert-Bu and hexyl groups were placed in the low layer assigned to MM calculations. Chart 1 shows how aggregation of the monomers was modeled. Dummy atoms were placed at the center of the tetrasubstituted benzene rings

Table 2. DLS Measurement of Poly(1)−poly(5)a polymer

polymer

Could not be

m-linked polymers [poly(2), poly(4)] were larger than those of the p-linked counterparts [poly(1), poly(3)], presumably because aggregate formation is favorable for the largely bent structures of the m-linked polymers. m-Linked poly(2) and poly(4) possibly adopt multiple conformations in contrast to the p-linked counterparts [poly(1) and poly(3)].27 The former two polymers showed no CD signal in THF/H2O upon raising the H2O content, while the latter two polymers showed CD signals assignable to chiral aggregation as summarized in Figure 2. It is apparent that the former two polymers also formed aggregates judging from the DLS data (Table 2), but the aggregation is not chirally regulated. The p-linked polymers poly(1), poly(3), and poly(5) emitted blue colored fluorescence in THF (Figure 6), with fluorescence quantum yields of 8.1% to 14.8%, as listed in

Chart 1. Aggregation of 1 (p-,p′-Diethynyl) or 2 (m-,m′Diethynyl) Moleculesa

a

X: dummy atom placed at the center of the tetrasubstituted benzene ring. Green and blue dotted lines represent π-stacking and hydrogen bonding, respectively. The intermolecular distance between the two molecules is illustrated longer than the starting geometries of the DFT calculations for clarity.

Figure 6. Fluorescence spectra of poly(1)−poly(5) measured in THF (c = 0.004 mM) at room temperature, excited at the λmax. 1599


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Figure 7. Initial and optimized geometries of tetrameric aggregate of 1, top and side views. Dummy atoms are indicated in violet. In the initial geometry, adjacent molecules are separated by 3.73 Å and rotated by +18.0° at the dummy atoms. The high and low layers for the ONIOM are indicated with tube and wireframe models, respectively.

bonding between amide/carbamate moieties, and π-stacking between the benzene rings. DFT calculations were also carried out from starting geometries rotated by −18°. As shown in Figure 8, the optimized geometries were less stable compared to the counterparts started from +18° for both 1 and 2. This result is explained by the distance between the molecules. As summarized in Table 4, the average interatomic distance of N−H···OC of amides and carbamate moieties ranged from 1.82 to 2.09 Å, indicating the presence of hydrogen bonding. There was no clear difference of N−H···OC distances of the optimized geometries between the conformers initially with X− X torsions of +18.0° and −18.0° in the series of 1 and 2. On the other hand, the X···X distances between the conformers initialized at +18.0° torsion were around 3.8 Å, shorter than those of −18.0° counterparts (X···X distances >4.0 Å in most cases). The X···X distances (3.8 Å) of the former aggregates were comparable to the values observed for π-stacking between aromatic rings.31 Consequently, the conformers initialized at +18.0° torsion are more efficiently stabilized by π-stacking between the benzene rings compared to the conformers initialized at −18.0°. It is likely that this kind of aggregation also occurs between the polymer molecules. The relative energies of 2−6-mers of 2 are slightly smaller than those of the corresponding 2−6-mers of 1 in Figure 8. This may explain the larger aggregation tendency, as well as the smaller fluorescence quantum yield of poly(2) compared to poly(1). The TD-DFT method is a cost-efficient and accurate method to simulate electronic spectra of optically active molecules.32

to construct the initial geometry accurately. The carbonyl groups of amides and carbamates were located vertical to the tetrasubstituted benzene rings in an antiparallel manner to form regulated hydrogen-bonding strands. The left two pictures in Figure 7 are the starting geometries of the tetrameric aggregate of 1, top and side views, in which adjacent molecules are separated by 3.73 Å (X···X distance) and rotated by +18.0° (torsional angle at X−X) at the dummy atom placed at the center of the tetrasubstituted benzene ring.30 Thus, the aggregate forms a columnar shaped helically stacked structure. After geometry optimization, the average distance between the dummy atoms became 3.94 Å (the right two pictures in Figure 7, top and side views). It is considered that πstacking contributes to association. The average rotation became +19.4°, almost the same as the starting geometry (+18.0°). The >N−H···OC< distance at the amide groups between adjacent molecules was 1.86 Å on average after geometry optimization, indicating the presence of intermolecular hydrogen bonding between the amide groups. In a similar fashion, the >N−H···OC< distance at the carbamate groups between adjacent molecules became 1.97 Å on average, indicating the presence of hydrogen bonding between the carbamate groups as well. The specific data of the dimeric− hexameric aggregates are listed in Table 4. Figure 8 depicts the relationship between the energy per molecule and number (n) of molecules of 1. The energy decreased with increasing n, and is likely to be saturated around n = 6. The molecules were stabilized by association based on intermolecular hydrogen 1600


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Figure 9 shows the CD and UV−vis spectra of a stacked 4-mer of 1 simulated by the TD-DFT (M06-2X/6-31G*). The

Table 4. Data of Geometries of Dimeric (n = 2)−Hexameric (n = 6) Aggregates of 1 and 2 Optimized by M06-2X/6-31G* average interatomic distance (Å) N−H···OC compound

n

amide

carbamate

X···Xa

torsional angle of X···Xa (deg)

1 (+18.0)

2 3 4 5 6 2 3 4 5 6 2 3 4 5 6 2 3 4 5 6

2.02 1.87 1.86 1.83 1.85 2.03 2.02 1.89 1.91 1.82 1.99 1.86 1.82 1.86 1.90 1.98 1.85 1.86 b b

2.00 1.91 1.97 1.90 1.96 2.04 1.89 1.87 1.88 1.99 2.09 2.03 1.87 1.97 1.95 1.89 1.94 2.00 b b

3.81 3.83 3.94 3.87 3.86 3.85 4.27 4.02 4.70 4.14 3.66 3.88 3.75 3.90 4.03 3.81 4.21 4.07 b b

+13.7 +22.6 +19.4 +29.3 +25.6 +14.6 −38.0 −34.0 −28.8 −29.5 +5.6 +29.7 +36.2 +27.5 +21.0 −0.6 −29.6 −16.8 b b

1 (−18.0)

2 (+18.0)

2 (−18.0)

a b

X is a dummy atom positioned at the center of the benzene ring. Not determined.

Figure 9. TD-DFT (M06-2X/6-31G*) simulated CD and UV−vis spectra of tetrameric aggregates of 1 (Rvel and f are divided by 4 as the values per monomer unit, nstates = 20, half-width wavelength of Gaussian distribution =20 nm). (a) Aggregate with a right-handed twist. (b) Aggregate with a left-handed twist. Figure 8. Relationship between the energy per molecule and number (n) of molecules in aggregates of 1 (□) and 2 (●). Energy standard: n = 1. Top: Optimized from geometries with +18° rotation. Bottom: Optimized from geometries with −18° rotation.

conformers initiated from +18° (right-handed) and −18° (lefthanded) are predicted to show CD signals with positive and negative signs at 220−350 nm, respectively. As depicted in Figure 1, poly(1) exhibits a positive CD signal around 350 nm. As shown in Figure 10, the π-electrons are delocalized over the phenyleneethynylene moieties of the four molecules of 1, which 1601


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the CD simulation of the model system for chirally aggregated polymer molecules. The structural differences between the pand m-linkages of the phenyleneethynylene moieties remarkably influenced the formation of higher-order structures. While m-linked poly(2) and poly(4) emitted negligibly small fluorescence, p-linked poly(1), poly(3), and poly(5) emitted fluorescence with quantum yields ranging from 8.1−14.8%. The DLS and fluorescence spectra in conjunction with DFT calculations indicate a much greater aggregation tendency for the more largely bent structures of m-linked poly(2) and poly(4), compared with the structures of p-linked poly(1), poly(3) and poly(5).

■

ASSOCIATED CONTENT

S Supporting Information *

Additional CD and UV−vis spectra of poly(1)−poly(5) (Figures S1−S3) and monomers 1 and 2 (Figure S4), UV− vis and fluorescence spectra of 1−5 (Figures S5−S7), optical data of 1−5 (Table S1), and Cartesian coordinates for 1, 2 and the dimeric−hexameric aggregates, initial and M06-2X/631G*-optimized conformers. This material is available free of charge via the Internet at http://pubs.acs.org.

■

Figure 10. Shapes of MO 586 (the third highest occupied MO) of a right-handed twist tetrameric aggregate, corresponding to Figure 7, right. Hydrogen atoms are omitted for clarity.

AUTHOR INFORMATION

Corresponding Author

*(F.S.) E-mail: [email protected].

supports the presence of π-stacking. This MO 586 is the third highest occupied MO, which strongly affects the excited state corresponding to the absorption peak at 286 nm (Rvel = 319, f = 0.0912) shown in Figure 9 (a). The simulated positive CD signal around 290 nm is assignable to the absorption due to the phenyleneethynylene moieties stacked with a right-handed twist. It is therefore likely that poly(1) aggregates also have a right-handed twist in a fashion similar to the model system. The simulated positive CD peak assignable to the phenyleneethynylene moieties is positioned at ca. 80 nm shorter wavelength compared with poly(1). This is explained by the short conjugation of the tetrameric aggregate compared with the polymer. In fact, monomer 1 exhibits λmax at a wavelength 80 nm lower than that of poly(1) as shown in Figure 1, which agrees well with the value of the difference simulated by the TD-DFT.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was supported by Research Fellowships of Young Scientists and a Grant-in-Aid for Science Research (B) (22350049) from the Japan Society for the Promotion of Science, and by a Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on ElementBlocks (No. 2401)” from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. The authors are grateful to Prof. Kazunari Akiyoshi and Prof. Shinichi Sawada for DLS measurement and Prof. Kenneth B. Wagener and Dr. Kathryn R. Williams at the University of Florida for their helpful suggestions and comments.

■

■

CONCLUSIONS The present paper demonstrated the synthesis of D hydroxyphenylglycine-derived novel optically active bisethynylene-terphenylene polymers, poly(1)−poly(5), by the acetylenic coupling polymerization of the corresponding diethynylm-terphenyl monomers 1−5 using Pd−Cu catalysts. mPhenyleneethynylene-linked poly(2) and poly(4) exhibited only weak CD signals. On the other hand, p-phenyleneethynylene-linked poly(1), poly(3), and poly(5) exhibited intense CD signals in the absorption region of the conjugated main chain in THF/H2O mixtures, and the chiral intensities increased to some extent as the percentage of H2O content increased. The strong chiroptical property originates from the formation of chirally ordered aggregates, as supported by DLS measurements and DFT calculations. The conformational analysis indicated that these aggregated structures are stabilized by intermolecular hydrogen bonding and π-stacking between the benzene rings. The TD-DFT calculations successfully predicted the direction of molecular twisting in aggregation. As far as we know, the present study is the first report concerning

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