Synthesis of Poly(3-substituted thiophene)s of Remarkably High

Article pubs.acs.org/Macromolecules

Synthesis of Poly(3-substituted thiophene)s of Remarkably High Solubility in Hydrocarbon via Nickel-Catalyzed Deprotonative CrossCoupling Polycondensation Keisuke Fujita,† Yugo Sumino,† Kenji Ide,† Shunsuke Tamba,† Keisuke Shono,† Jian Shen,† Takashi Nishino,† Atsunori Mori,*,† and Takeshi Yasuda‡ †

Department of Chemical Science and Engineering, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan Organic Thin-Film Solar Cells Group, Photovoltaic Materials Unit, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

‡

S Supporting Information *

ABSTRACT: Polythiophenes bearing a siloxane moiety in a substituent at the 3-position are prepared by deprotonative polycondensation of 2-bromo-3-substituted-thiophene with a bulky magnesium amide chloromagnesium 2,2,6,6-tetramethylpiperidine-1-yl lithium chloride salt (TMPMgCl·LiCl) catalyzed by a nickel complex. Deprotonation takes place at 60 °C for 1 h to form the corresponding thiophene magnesium species, which is subjected to the polymerization by addition of 0.1−5 mol % NiCl2(PPh3)IPr (IPr: 1,3-bis(2,6-diisopropylphenyl)imidazole-2-yl). Polymerization proceeds in a highly regioregular manner, and the molecular weight of the thus-obtained polymer is controllable by the ratio of monomer feed/ catalyst loading to indicate Mn of up to 280 000 with narrow molecular weight distribution. Chlorothiophenes are also found to induce polymerization in a deprotonative manner with TMPMgCl·LiCl or nBuLi (the Murahashi coupling polymerization). The obtained polymers bearing a siloxane moiety in the substituent is revealed to be dissolved in a hydrocarbon allowing formation of thin film from hexane.

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details.7 Herein, we describe that the molecular weight of polythiophene is controllable and highly dependent toward catalyst loading with narrow polydispersities in the nickelcatalyzed polymerization. Thus, the polythiophene with molecular weight of >280 000 is successfully prepared, and the polymer is shown to be still soluble in hexane. Synthesis of additional polythiophene derivatives bearing different structures at the polythiophene substituent to estimate the solubility in hydrocarbon and polythiophene syntheses with other polymerization protocols are also described.

INTRODUCTION Siloxanes that involve Si−O−Si bond incorporated into organic and polymer framework show remarkable change in their characteristics such as thermal behaviors, chemical stabilities, mechanical properties, and miscibility in solvents.1 Introduction of a siloxane moiety into organic and polymer molecules has thereby been studied to date to induce a new class of chemical and physical properties particularly improving processability as materials.2 Since we have been engaged in developing practical synthetic protocols in polythiophene synthesis,3−5 which greatly contributes to the design of highly conjugated molecules, it is intriguing to prepare highly regioregular poly(3-substituted thiophene)s bearing a siloxane moiety, which potentially show unprecedented characteristics in polythiophene derivatives. However, there have been few studies on such preparation of polymers as well as monomers as their precursors due to the synthetic difficulties of thiophene derivative bearing both a siloxane moiety and polymerizable functionality.2b,6 We have recently shown as a preliminary communication that polythiophene derivatives bearing a pentamethyldisiloxy substituent at the ω-alkene on the 3position of thiophene can be prepared by deprotonative polymerization with a nickel catalyst, and the obtained polymer of molecular weight ca. >10 000 is easily dissolved in hydrocarbon whereas the reaction has not been studied in © 2016 American Chemical Society

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EXPERIMENTAL SECTION

General. Unless otherwise specified, organic and polymerization reactions were carried out under nitrogen with standard Schlenk technique. Purification of 2,2,6,6-tetramethylpiperidine (TMPH) was performed by distillation and stored under a nitrogen atmosphere. Knochel−Hauser base (chloromagnesium 2,2,6,6-tetramethylpiperidine-1-yl lithium chloride salt: TMPMgCl·LiCl) was prepared by the reaction of 2,2,6,6-tetramethylpiperidine with isopropylmagnesium chloride lithium chloride salt in THF at room temperature for 24 h, and the resulting solution was stored in the freezer as a 1.0 M THF solution under nitrogen.8 THF and cyclopentyl methyl ether Received: November 22, 2015 Revised: January 26, 2016 Published: February 5, 2016 1259

DOI: 10.1021/acs.macromol.5b02524 Macromolecules 2016, 49, 1259−1269

Article

Macromolecules

= 7.0, 10.2, 17.4 Hz, 1H), 5.05 (d, J = 17.4 Hz, 1H), 5.00 (d, J = 10.2 Hz, 1H), 2.67 (t, J = 8.7 Hz, 2H), 2.34 (dt, J = 7.0, 7.0 Hz, 2H). 13C NMR (125 MHz): δ 141.1, 137.7, 128.4, 125.4, 115.5, 109.3, 33.8, 29.0. IR (ATR): 3077, 2978, 2925, 2854, 1639, 1409, 991, 913 cm−1. The obtained 3 was directly employed for the next reaction without further purification. 2-Bromo-3-(4-pentamethyldisiloxybutan-1-yl)thiophene (5). To a solution of 3 (1.57 g, 7.26 mmol) were added pentamethyldisiloxane (1.71 mL, 8.7 mmol) and one drop of (1,1,3,3-tetramethyl-1,3divinyldisiloxane)platinum(0) (2 wt % xylene solution). After stirring the mixture at room temperature for 5 min, the resulting reaction mixture was subjected to column chromatography on silica gel to afford 2.50 g of 5 (94%). 1H NMR (300 MHz): δ 7.18 (d, J = 5.7 Hz, 1H), 6.79 (d, J = 5.7 Hz, 1H), 2.56 (t, J = 7.5 Hz, 2H), 1.54−1.65 (m, 2H), 1.29−1.42 (m, 2H), 0.51−0.59 (m, 2H), 0.05 (s, 9H), 0.04 (s, 6H). 13C NMR (125 MHz): δ 142.1, 128.4, 125.3, 108.9, 33.5, 29.3, 23.1, 18.3, 2.1, 0.5. IR (ATR): 2955, 2858, 1409, 1252, 840, 753 cm−1. HRMS (DART-ESI+, M + H) calcd for C13H2679BrOSSi2: 365.0426; found m/z 365.0427. Synthesis of 2-Bromo-3-(4-pentamethyldisiloxypropan-1-yl)thiophene (6). The solution of 2 (0.78 mL, 6.0 mmol) in 12.0 mL of acetone was treated with sodium iodide (1.8 g, 12.0 mmol) at 40 °C for 1.5 h. After filtration of the solid material, concentration of the filtrate under reduced pressure left a crude oil, which was dissolved in THF (12 mL). The solution was subjected to the reaction of vinylmagnesium bromide (6.0 mL, 6.0 mmol) in the presence of CuI (114 mg, 0.6 mmol) and 2,2′-bipyridyl (95 mg, 0.6 mmol) at room temperature for 1 h. The mixture was poured into water to separate into two phases. Aqueous was extracted with diethyl ether, and the combined organic layer was dried over anhydrous sodium sulfate. Removal of the solvent left an oil, which was passed through a short path silica gel column to afford crude 4 (0.49 g, 2.66 mmol). Pentamethyldisiloxane (0.62 mL, 3.19 mmol) and one drop of (1,1,3,3-tetramethyl-1,3-divinyldisiloxane)platinum(0) (2 wt % xylene solution) were added to 4, and the mixture was stirred at 60 °C for 3 h. After cooling to room temperature, purification by column chromatography on silica gel afforded 0.85 g of 6 (89%). 4: 1H NMR (300 MHz): δ 7.19 (d, J = 4.5 Hz, 1H), 6.80 (d, J = 4.5 Hz, 1H), 5.83−5.97 (m, 1H), 5.05−5.10 (m, 2H), 3.33 (d, J = 6.0 Hz, 2H). 13C NMR (125 MHz): δ 139.2, 135.3, 128.5, 125.5, 116.3, 109.6, 33.9. 6: 1 H NMR (300 MHz): δ 7.19 (d, J = 4.5 Hz, 1H), 6.79 (d, J = 4.5 Hz, 1H), 2.58 (t, J = 7.5, 2H), 1.57−1.65 (m, 2H), 0.51−0.61 (m, 2H), 0.06 (s, 9H), 0.05 (s, 6H). 13C NMR (125 MHz): δ 142.0, 128.4, 125.3, 109.1, 33.1, 23.8, 18.3, 2.2, 0.5. IR (ATR): 2956, 1408, 1340, 1252, 1172, 1054, 995, 839, 809, 753, 687, 634, 623 cm−1. HRMS (DART-ESI+, M + H) calcd for C12H2479BrOSSi2: 351.0270; found m/ z 351.0273. 2-Chloro-3-(4-pentamethyldisiloxybutan-1-yl)thiophene (7). To a 20 mL Schlenk tube equipped with a Teflon-coated magnetic stirring bar was added 5 (124 mg, 0.34 mmol) under a nitrogen atmosphere. The mixture was cooled to 0 °C, and ethylmagnesium chloride (0.43 mL of 0.95 M THF solution, 0.41 mmol) was added dropwise to the mixture. After stirring the mixture at 60 °C for 1.5 h, Nchlorosuccinimide (NCS) (45 mg, 0.34 mmol) and THF (0.43 mL) were added to the resulting solution, and stirring was continued at room temperature overnight. The reaction mixture was cooled to room temperature and poured into saturated aqueous solution of sodium thiosulfate to result in separation into two phases. The aqueous phase was extracted with diethyl ether twice, and the combined organic layer was washed with brine and then with water. The solvent was removed under reduced pressure to leave a crude oil. Column chromatography on silica gel using hexanes as an eluent afforded 109 mg of 7 (>99%). 1H NMR (500 MHz): δ 7.02 (d, J = 7.5 Hz, 1H), 6.80 (d, J = 7.5 Hz, 1H), 2.59 (t, J = 7.5 Hz, 2H), 1.57−1.66 (m, 2H), 1.32−1.42 (m, 2H), 0.53−0.59 (m, 2H), 0.07 (s, 9H), 0.05 (s, 6H). 13C NMR (125 MHz): δ 139.3, 128.0, 124.6, 122.1, 33.4, 27.8, 23.1, 18.3, 2.1, 0.5. IR (ATR): 2956, 2856, 1413, 1252, 1054, 840, 807, 783, 753, 712, 686, 638 cm−1. Anal. Calcd for C13H25ClOSSi2: C, 48.60; H, 7.79%. Found: C, 48.74; H, 7.82%.

(CPME) employed as a solvent (anhydrous grade) for the polymerization were purchased and used without further purification. 1 H NMR (500, 400, and 300 MHz) and 13C NMR (125, 100, and 75 MHz) spectra were measured on BRUKER Avance-500, JEOL ECZ400, and Varian Gemini 300 as a CDCl3 solution. The chemical shifts were expressed in ppm with CHCl3 (7.26 ppm for 1H) or CDCl3 (77.16 ppm for 13C) as internal standards. SEC analyses were carried out by standard HPLC system equipped with a UV detector at 30 °C using CHCl3 as an eluent with Shodex GPC KF-404HQ and KF402HQ. Molecular weights and molecular weight distributions were estimated on the basis of the calibration curve obtained by eight standard polystyrenes. MALDI-TOF mass spectra were measured by Bruker Daltonics Flexscan ultrafleXtreme. Elemental analysis was carried out at Quantum Beam Science Center Japan Atomic Energy Agency, Harima, Japan, with vario MICRO cube. Measurements of HRMS were carried out with JEOL JMS-T100LP AccuTOF LC-Plus (ESI) with a JEOL MS-5414DART attachment. IR spectra were recorded on Bruker Alpha with an ATR attachment (Ge). Purification by HPLC with preparative SEC column (JAI-GEL-2H) was performed by JAI LC-9201. For thin-layer chromatography (TLC) analyses throughout this work, Merck precoated TLC plates (silica gel 60 F254) were used. HOMO energy levels of the polymer films were measured by photoelectron yield spectroscopy (PYS) using an AC-3 spectrometer (Riken Keiki). The UV−vis spectrum was measured by a Hitachi U-3010 UV−vis spectrometer. The photoluminescences (PL) spectrum was recorded by a Jasco FP-6500 spectrofluorometer. XRD analysis was carried out with Rigaku RINT-2500. Observation of atomic force microscope (AFM) was performed with SII Nanocute. Regioregular poly(3-hexylthiophene) (P3HT) that was purchased from the Luminescence Technology Corp. was used as a reference material in the film state. Allylmagnesium chloride was purchased from Sigma-Aldrich Co. Ltd. as a 2 M THF solution. A platinum catalyst for hydrosilylation (1,1,3,3-tetramethyl-1,3-divinyldisiloxane)platinum(0) (2 wt % xylene solution) was purchased from Sigma-Aldrich Co. Ltd. and used as received. Although 2-bromo-3-bromomethylthiophene (2) is commercially available, preparation of 2 was carried out by the reaction of 2-bromo-3-methylthiophene (1) with N-bromosuccinimide/AIBN, and the obtained crude 2 was directly employed for further allylation and hydrosilylation reactions without isolation, leading to 4 to avoid technical loss of volatile 2 and 3 during isolation. 2-Bromo-3-bromomethylthiophene (2). To a solution of 2-bromo3-methylthiophene (1: 4.92 mL, 44.0 mmol) in CCl4 (44.0 mL) was added AIBN (0.22 g, 1.3 mmol). N-Bromosuccinimide (NBS) (3.13 g, 17.6 mmol) was added to the resulting solution, and stirring was continued at 80 °C for 1 h. AIBN (0.07 g, 0.44 mmol) and NBS (2.35 g, 13.2 mmol) were further added to the reaction mixture, and the solution was stirred at 80 °C for 1 h. The above procedure was repeated, and further stirring was continued for 2 h. The reaction mixture was cooled to room temperature and poured into water to result in separation into two phases. Aqueous was extracted with hexanes, and the combined organic phase was dried over anhydrous sodium sulfate. Removal of the solvent under reduced pressure left a crude oil, which was purified by column chromatography on silica gel using hexanes as an eluent to afford 6.94 g of 2 (62%), whose spectroscopic properties were identical with those of the authentic sample.9 1H NMR (300 MHz): δ 7.26 (d, J = 5.7 Hz, 1H), 7.00 (d, J = 5.7 Hz, 1H), 4.46 (s, 2H). 2-Bromo-3-(3-buten-1-yl)thiophene (3). To a 100 mL Schlenk tube equipped with a Teflon-coated magnetic stirring bar under a nitrogen atmosphere were added 2 (2.24 mL, 17.2 mmol) and THF (34.4 mL). The mixture was cooled to 0 °C, and allylmagnesium chloride (11.2 mL of 2.0 M THF solution, 22.4 mmol) was added dropwise to the mixture. After stirring the mixture for 1.5 h, the solution was poured into water (50 mL) to result in phase separation. The aqueous phase was extracted with diethyl ether twice (30 × 2 mL), and the combined organic layer was washed with brine and then with water. The solvent was removed under reduced pressure to leave a colorless oil, which was passed through silica gel column using hexanes as an eluent to afford 3.61 g of crude 3 (97%). 1H NMR (300 MHz): δ 7.19 (d, J = 5.7 Hz, 1H), 6.81 (d, J = 5.7 Hz, 1H), 5.85 (ddt, J 1260


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Macromolecules Scheme 1. Preparation of Thiophene Monomers (Precursors) Bearing Siloxane

2-Bromo-3-(4-triethylsilylbutan-1-yl)thiophene (8). Synthesis of 8 was carried out in a similar manner to the synthesis of 5 from 3 (0.13 g, 0.60 mmol), triethylsilane (84 mg, 0.72 mmol), and one drop of (1,1,3,3-tetramethyl-1,3-divinyldisiloxane)platinum(0) (2 wt % xylene solution) at room temperature for 2.5 h (>99%). 1H NMR (400 MHz): δ 7.18 (d, J = 5.5 Hz, 1H), 6.79 (d, J = 5.5 Hz, 1H), 2.58 (t, J = 7.5 Hz, 2H), 1.66−1.56 (m, 2H), 1.41−1.30 (m, 2H), 0.93 (t, J = 8.0 Hz, 9H), 0.59−0.47 (m, 8H). 13C NMR (100 MHz): δ 142.1, 128.3, 125.3, 108.9, 33.9, 29.2, 23.7, 11.3, 7.6, 3.5. IR (ATR): 2952, 2910, 2874, 1749, 1413, 1376, 1168, 1012, 915, 828, 724, 685, 635 cm−1. HRMS (DART-ESI+, M + H) calcd for C14H2679BrSSi: 333.0708; found m/z 333.0710. 2-Bromo-3-(4-bistrimethylsiloxysilylbutan-1-yl)thiophene (9). Synthesis of 9 was carried out in a similar manner to the synthesis of 5 from 3 (0.14 g, 0.64 mmol), 1,1,1,3,5,5,5-heptamethyltrisiloxane (0.21 mL, 0.77 mmol), and one drop of (1,1,3,3-tetramethyl-1,3divinyldisiloxane)platinum(0) (2 wt % xylene solution) at room temperature for 1 h (96%). 1H NMR (300 MHz): δ 7.18 (d, J = 6.0 Hz, 1H), 6.79 (d, J = 6.0 Hz, 1H), 2.56 (t, J = 7.5 Hz, 2H), 1.54−1.64 (m, 2H), 1.30−1.41 (m, 2H), 0.45−0.52 (m, 2H), 0.08 (s, 18H), −0.01 (s, 3H). 13C NMR (125 MHz): δ 142.0, 128.4, 125.3, 109.0, 33.3, 29.3, 22.9, 17.6, 2.0, −0.1. IR (ATR): 2957, 1409, 1256, 1045, 993, 839, 798, 782, 754, 714, 686, 652, 633. HRMS (DART-ESI+, M + H) calcd for C15H32O279BrSSi3: 439.0614; found m/z 439.0625. Typical Synthetic Procedure for the Polymerization of 2-Halo-3Substituted Thiophenes Bearing Siloxane Moiety Representative as the Case with 2-Bromo-3-(4-Pentamethyldisiloxybutan-1-yl)thiophene (5). To 2-bromo-3-(4-pentamethyldisiloxybutan-1-yl)thiophene 5 (0.18 g, 0.50 mmol) under a nitrogen atmosphere was added a 1.0 M THF solution of TMPMgCl·LiCl (0.55 mL, 0.55 mmol) at room temperature. After stirring at 60 °C for 1 h, THF (5 mL) and NiCl2(PPh3)IPr (3.9 mg, 0.5 μmol) were added to the resulting mixture, which was further stirred at 60 °C for 24 h. Hydrochloric acid (0.1 M, 2 mL) and methanol were added to the mixture to quench the reaction, the formed precipitate was filtered off, and the residue was washed with methanol repeatedly to leave dark purple solid, which was dried under reduced pressure to afford 70.9 mg of poly(3-(4-pentamethyldisiloxybutan-1-yl)thiophen-2,5-diyl) (10) (77% yield). Molecular weight and the molecular weight distribution were estimated by SEC analysis to show Mn = 28 100, Mw/Mn = 1.07. Analysis by 1H NMR (thienyl−CH2− signals) at δ 2.80 (H−T) and δ 2.60 (T−T), respectively, suggested HT (head-to-tail) regioregularity as >99%. 1H NMR (300 MHz): δ 6.98 (brs, 1H), 2.72−2.87 (br, 2H), 1.64−1.80 (br 2H), 1.38−1.53 (br, 2H), 0.55−0.64 (m, 2H), 0.06 (brs, 15H). 13C NMR (125 MHz): δ 140.0, 133.9, 130.7, 128.8, 34.4, 29.4, 23.6, 18.5, 2.2, 0.6.

Preparation of 10 was also carried out in a larger scale with 0.66 g of bromothiophene 5 (1.8 mmol) and TMPMgCl·LiCl (2.0 mL, 2.0 mmol) at 60 °C for 1 h (deprotonation) and polymerization at 60 °C for 24 h with NiCl2(PPh3)IPr (0.15 mol %, 14 mg, 1.8 μmol) in THF (18 mL) to afford 10 (0.49 g, 74%). Mn = 296 000; Mw/Mn = 1.26. HT regularity >99%. Poly(3-(4-triethylsilylbutan-1-yl)thiophen-2,5-diyl) (11). Synthesis of 11 was carried out in a similar manner to the synthesis of 10 from 2bromo-3-(4-triethylsilylbutan-1-yl)thiophene (8, 183 mg, 0.55 mmol), TMPMgCl·LiCl (0.66 mL, 0.66 mmol, 1 M in THF), and NiCl2(PPh3)IPr (4.3 mg, 5.5 μmol) in THF (5.5 mL) at 60 °C for 24 h (70%) (HT regularity >99%, Mn = 14 400, Mw/Mn = 1.05). 1H NMR (400 MHz): δ 6.97 (brs, 1H), 2.76−2.85 (br, 2H), 1.77−1.67 (br, 2H), 1.49−1.39 (br, 2H), 0.96−0.90 (br, 9H), 0.63−0.45 (m, 8H). 13C NMR (100 MHz): δ 139.9, 133.9, 130.6, 128.7, 34.9, 29.4, 24.1, 11.4, 7.7, 3.5. Poly(3-(4-bistrimethylsiloxysilylbutan-1-yl)thiophen-2,5-diyl) (12). Synthesis of 12 was carried out in a similar manner to the synthesis of 10 from 2-bromo-3-(4-bistrimethylsiloxysilylbutan-1yl)thiophene (9, 220 mg, 0.50 mmol), TMPMgCl·LiCl (0.6 mL, 0.6 mmol, 1 M in THF), and NiCl2(PPh3)IPr (3.9 mg, 5 μmol) in THF (5.0 mL) at 60 °C for 24 h (64%) (HT > 99%, Mn = 37 200, Mw/Mn = 1.09). 1H NMR (400 MHz): δ 6.97 (brs, 1H), 2.76−2.85 (br, 2H), 1.67−1.77 (br, 2H), 1.51−1.39 (br, 2H), 0.49−0.58 (br, 2H), 0.08 (brs, 18H), 0.02 (brs, 3H). 13C NMR (100 MHz): δ 139.9, 133.9, 130.7, 128.7, 34.3, 29.4, 23.4, 17.7, 2.0, −0.1. Poly(3-(4-pentamethyldisiloxypropan-1-yl)thiophen-2,5-diyl) (13). Synthesis of 13 was carried out in a similar manner to the synthesis of 10 from 2-bromo-3-(4-pentamethyldisiloxypropan-1yl)thiophene (6, 175 mg, 0.50 mmol), TMPMgCl·LiCl (0.6 mL, 0.6 mmol, 1 M in THF), and NiCl2(PPh3)IPr (3.9 mg, 5 μmol) in THF (5.0 mL) at 60 °C for 24 h (99%) (HT > 99%, Mn = 32 200, Mw/Mn = 1.18). 1H NMR (400 MHz): δ 6.98 (brs, 1H), 2.78−2.88 (br, 2H), 1.76−1.71 (br, 2H), 0.62−0.72 (br, 2H), 0.08 (brs, 6H), 0.07 (brs, 9H). 13C NMR (100 MHz): δ 139.9, 133.9, 130.8, 128.8, 33.2, 24.7, 18.8, 2.2, 0.6. Polymerization of chlorothiophene 7 (91.2 mg, 0.28 mmol) was carried out with 1 M THF solution of TMPMgCl·LiCl (0.31 mL, 0.31 mmol) and NiCl2(PPh3)IPr (2.2 mg, 2.8 μmol) under the abovementioned conditions to afford 72.7 mg of 10 (90%) (HT > 99%, Mn = 29 800, Mw/Mn = 1.17). Murahashi Coupling Polymerization of Chlorothiophene (7). To a solution of 2-chloro-3-(4-pentamethyldisiloxybutan-1-yl)thiophene (7, 116 mg, 0.36 mmol) in 5.4 mL of CPME was added a hexane solution of 1.6 M n-butyllithium (0.23 mL, 0.36 mmol) at −78 °C, and the resulting mixture was stirred with raising the temperature to 0 °C over 30 min. Then, NiCl2(PPh3)IPr (2.8 mg, 0.36 μmol) was added, 1261


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Macromolecules Table 1. Estimation of the Deprotonation of 5 at the 5-Position of Thiophenea

temp (°C)

timeb (min)

conversionc (%)

temp (°C)

timeb (min)

conversionc (%)

25

10 30 60 180 300

54 72 81 82 81

60

10 30 45 60 120 180

38 61 77 97 93 91

a

The reaction was carried out with thiophene 5 (0.2 mmol) and TMPMgCl·LiCl (0.22 mmol) in THF (0.22 mL). The reaction mixture was quenched with DMF (3.0 mmol). bThe reaction period of deprotonation. cConversion was estimated by 1H NMR analysis. (See also Supporting Information).

Scheme 2. Deprotonative Polymerization of 5 with TMPMgCl·LiCl and a Nickel(II) Catalyst

Figure 1. MALDI-TOF MS of polythiophene 10 bearing disiloxane moiety in the side chain. and further stirring was continued for 30 min. The reaction mixture was poured into a mixture of hydrochloric acid (1.0 M, 2 mL) and methanol to form a precipitate, which was filtered off to leave a dark purple solid. After washing with methanol repeatedly, the solid was dried under reduced pressure to afford 62 mg of 10 (61% isolated yield). SEC analysis showed Mn = 28 200, Mw/Mn = 1.29. HT regioregularity >99%. Studies on Deprotonation of 5 with TMPMgCl·LiCl. To a solution of 2-bromo-3-(4-pentamethyldisiloxybutan-1-yl)thiophene (5, 73 mg, 0.2 mmol) was added a 1.0 M THF solution of TMPMgCl·LiCl (0.22 mmol, 0.22 mL). After stirring at 60 °C for 60 min, N,Ndimethylformamide (DMF) (3.0 mmol, 0.23 mL) was added in one portion, and stirring was continued at room temperature overnight. The resulting mixture was poured into a mixture of hydrochloric acid, and aqueous was extracted with diethyl ether twice. The combined

organic layer was concentrated under reduced pressure to leave a crude oil, which was subjected to 1H NMR analysis to confirm formation of 2-bromo-3-(4-pentamethyldisiloxybutan-1-yl)-5-formylthiophene comparing proton signals of formylated product and the remaining 5: 1H NMR (300 MHz) δ 7.45 (s, 0.97 H) and 6.79 ppm (d, J = 5.7 Hz, 0.03 H), respectively (corresponding to the 4-position of thiophene rings). Fabrication and Characterization of Organic Field-Effect Transistors (OFETs). To estimate the hole mobilities of the siloxanebearing polythiophene, OFETs with a top-contact geometry were fabricated and characterized as follows. A glass/Au gate electrode/ Parylene-C insulator substrate was prepared. The polymer 10 (Mn = 12 200) was spin-coated from hexane solution onto the Parylene-C layer. The coated substrate was then transferred to a N2-filled glovebox where it was dried for 10 min at 110 °C. Au (40 nm) source-drain 1262


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Macromolecules electrodes were thermally evaporated onto the substrates through shadow masks. The channel length and width were fixed at 75 μm and 5 mm, respectively. The OFET measurements were conducted using a Keithley 2636A System source meter under vacuum. The average values with standard deviations of OFET parameters were calculated from the results of four OFETs.

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RESULTS AND DISCUSSION Synthesis of several thiophene monomers bearing siloxane was carried out as summarized in Scheme 1 starting from 2-bromoTable 2. Relationship in the Polymerization of 5 with the Reaction Perioda time, h

conversion,b %

Mn × 10−3 c

Mw/Mnc

0.5 1 3 9 24

70 79 83 86 86

12.5 25.5 26.7 28.2 28.1

1.26 1.17 1.17 1.09 1.07

Figure 2. Relationship of the molecular weight (Mn) and molecular weight distribution (Mw/Mn) of polythiophene 10 with the ratio of monomer feed/catalyst loading (20−1000). ●: Mn; ▲: Mw/Mn. The dotted line indicates theoretical value based on the monomer feed/ catalyst loading.

a

The reaction was carried out with 1 mol % nickel catalyst, bromothiophene 5 (0.3 mmol), and TMPMgCl·LiCl (0.33 mmol) in 3 mL of THF at 60 °C. bConversion was estimated by 1H NMR analysis of the filtrate after isolation of polymer 10 with pentachlorobenzene as an internal standard. cMn and Mw/Mn were measured by SEC analysis.

Table 3. Relationship of the Molecular Weight of Polythiophene 10 with the Ratio of Catalyst Loading/ Monomer Feeda catalyst loading/monomer feed (mol/mol)

conversionb (%)

Mn × 10−3 c

Mw/Mnc

20 50 100 200 500 1000

73 81 86 77 83 82

4.1 12.1 28.1 42.4 126.6 280.8

1.12 1.24 1.07 1.26 1.44 1.19

a

Figure 3. SEC profile of 10 with different monomer feed/catalyst loading ratio.

3-methylthiophene (1). Radical bromination with NBS in the presence of AIBN afforded bromomethylated intermediate 2, which was subjected to allylation with a Grignard reagent to give ω-olefinic product 3 in an excellent yield. Hydrosilylation of 3 with a platinum catalyst10 led to the silylation product 5 in a quantitative yield. By contrast, the related vinylation with vinylmagnesium bromide did not proceed smoothly to result in giving 4 in ca. 25% yield. Transformation of 2 into the corresponding iodide followed by the reaction with vinyl Grignard reagent proceeded smoothly to afford 4,11 which was subjected to hydrosilylation in a similar manner to that of 3 to afford 6 in excellent yields (overall yield: 89%). Chlorothiophene 7 bearing a siloxane moiety was obtained by halogen− metal exchange of bromide 5 with EtMgCl followed by treatment of N-chlorosuccinimide (NCS). We first examined deprotonation conditions in the reaction of halothiophene bearing siloxane 5 with TMPMgCl·LiCl,8 and the progress of deprotonation was estimated by the reaction with DMF leading to the corresponding aldehyde. As

summarized in Table 1, it was found that deprotonation of 5 at room temperature for 3 h resulted in 82% conversion. Compared with the deprotonation of 2-bromo-3-hexylthiophene, which completed within 3 h at room temperature,3c,h the reaction of 5 was found to proceed slightly slower. Further stirring for 5 h did not improve the conversion. Deprotonation was revealed to be induced more efficiently when the reaction was carried out at 60 °C for 1 h to show 97% conversion whereas shorter reaction period for 10, 30, and 45 min resulted in 38%, 61%, and 77% conversions, respectively. With the optimized deprotonation conditions in hand, halothiophene 5 was subjected to polymerization with a nickel catalyst. After treatment of 5 with TMPMgCl·LiCl at 60 °C for 1 h, nickel(II) catalyst NiCl2(PPh3)IPr12 was added to the mixture to initiate polymerization. When 1 mol % nickel catalyst was employed, the corresponding polymer bearing disiloxane moiety was obtained in 77% isolated yield (86% conversion) after stirring 60 °C for 24 h. SEC analysis of the thus-obtained polymer revealed to show Mn = 28 100 and Mw/ Mn = 1.07. 1H NMR analysis of the obtained polymer 10 showed that the head-to-tail regioregularity was >99% as well as the presence of pentamethyldisiloxy group in the side chain (Scheme 2). Figure 1 shows MALDI-TOF MS of the siloxane-

The reaction was carried out with 5 (0.5 mmol) and TMPMgCl·LiCl (0.55 mmol) (deprotonation at 60 °C for 1 h) and polymerization with NiCl2(PPh3)IPr in 5.0 mL THF (at 60 °C for 24 h). bConversion was estimated by 1H NMR analysis of the filtrate after isolation of polymer 10 with pentachlorobenzene as an internal standard. cMn and Mw/Mn were measured by SEC analysis.

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Macromolecules Scheme 3. Polymerization of Thiophene Derivatives Bearing Organosilicon Moiety in the Alkyl Substituent

Scheme 4. Polymerization of Chlorothiophene 7 by Deprotonation with TMPMgCl·LiCl and nBuLi

ization with catalyst loading of 0.1 mol % afforded polythiophene of ca. 281 000 (Mn).3a,d,14 These results suggest that deprotonative polymerization of a thiophene derivative bearing a siloxane moiety in the substituent at the 3-position proceeds in a catalyst transfer manner15 to result in chain growth polymerization despite polycondensation (Figures 2 and 3). Thiophene derivatives bearing different silicon moieties were then studied. Preparation of other monomer precursors was performed as shown in Scheme 3, and thiophene derivatives were subjected to polymerization under similar conditions at 60 °C (deprotonation at 60 °C for 1 h) with 1.0 mol % nickel catalyst. In addition to the formation of 5 and 6, formation of thiophenes bearing triethylsilyl group 8 and branched siloxane moiety 9 also proceeded in a similar manner. The reaction of the thus-obtained bromothiophenes 6, 8, and 9 was performed with TMPMgCl·LiCl (deprotonation) and NiCl2(PPh3)IPr (polymerization) to afford the corresponding polymers (13, 11, and 12, respectively) with good to excellent conversions (81− 99%). The Mn values of polymers bearing siloxane moiety agreed with the theoretical molecular weight based on the monomer/catalyst ratio whereas the molecular weight of 11 bearing triethylsilyl group was relatively inferior to those of 10, 12, and 13.

containing polymer 10 (Mn = ca. 4000), indicating the repeating interval of ca. 285 Da, which reasonably agreed with the corresponding monomer unit (284.6 Da). Peaks observed as 3131.9 and 3210.8 Da also showed correspondence with the polymer bearing end groups as H-(thienyl)11-H and Br-(thienyl)11-H (theoretical M = 3132.1 and 3210.8 Da, respectively). The deprotonative cross-coupling polymerization was studied under several conditions to exhibit that the molecular weight of the polymer was increased as the progress of the reaction period as shown in Table 2. Polymerization was mostly complete in 9 h (86% conversion) at 60 °C, whereas the reaction reached to ca. 70% within 1 h. The molecular weight distribution was found constant to ca. 1.0−1.3 during the polymerization. We also examined the reaction with different monomer feed/catalyst loading as summarized in Table 3. When 1 mol % of the nickel catalyst was employed, Mn was 28 100, whose value support that the molecular weight was controlled by the concentration of the nickel catalyst.5 Increase of catalyst loading produced the polymers with lower molecular weights, which also showed reasonable correspondence with the monomer feed/catalyst loading ratio.13 The reaction with much lower catalyst loading such as 0.5−0.1 mol % also afforded 10 with much higher molecular weight and relatively small Mw/Mn values. It should be pointed out that polymer1264


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Table 4. Optical Properties and Energy Levels of Polymers in Film States polymer P3HTa 10 (12.2 kDa) 10 (280 kDa) 13

abs λmax (nm)

Egopt b (eV)

PL λmax (nm)

HOMOc (eV)

LUMOd (eV)

554, 601 519

1.94 2.00

726 645, 706

−4.74 −4.96

−2.80 −2.96

527

1.99

655, 716

−4.98

−2.99

519

2.02

646, 682

−4.96

−2.94

a

Spin-coated from o-dichlorobenzene solution. bEstimated from the absorption onset in the film state. cEstimated from photoelectron yield spectroscopy. dELUMO = Egopt + EHOMO.

the reaction was performed with 1 mol % catalyst loading, the corresponding polythiophene 10 was obtained (Mn = 29 800; Mw/Mn = 1.17; 84% conversion). It was also found that polymerization proceeded by Murahashi coupling,16 which is the cross-coupling reaction with organolithium,17 using nBuLi as a deprotonating agent resulting in giving 75% conversion in 30 min at 0 °C with Mn = 28 200 and Mw/Mn = 1.29. The obtained polythiophenes bearing a silyl group in the side chain were found to be highly soluble in various organic solvents. Figure 4 shows the deprotonative polymerization mixture of 2-bromo-3-hexylthiophene to give poly(3-hexylthiophene): P3HT3i (a) and the related thiophene bearing pentamethyldisiloxy group in the side chain 7 (b) with a nickel catalyst in THF suggesting that (a) showed color change to dark purple along with progress of the polymerization caused by aggregation of polymer chains. On the other hand, the mixture of (b) maintained clear orange solution throughout polymerization to show excellent solubility of polymer 10 in THF. It was also found that the isolated polymer 10 was soluble in a hydrocarbon such as hexane. Figure 4 shows the result of solubility test of polythiophenes in hexane (1 mg/mL) at room temperature indicating excellent solubility of 10 (c) in hexane, whereas P3HT was hardly soluble under similar conditions (d) and even poly(3-dodecylthiophene) bearing a longer alkylene chain (e) only showed much inferior solubility. The polymer with higher molecular weight (ca. 280 000) was also found to be soluble in hexane (f). Worthy of note is that polythiophene 10 was dissolved in 1-butanol. Although the solubility of 10 was much lower at room temperature, heating the mixture to 100 °C resulted in complete dissolution (g and h). Polythiophene bearing branched siloxane 12 and that with shorter alkylene chain 13 similarly showed excellent solubilities in organic solvents THF and hexane. On the other hand, polythiophene bearing a triethylsilyl group in the side chain 11 was found much less soluble in hexane, whereas 11 was dissolved in THF (Figure 5). These results suggest that siloxane group plays a key role2 to dissolve polythiophene in hydrocarbon regardless of its molecular weight; however, considering the result on the effect of triethylsilyl group the presence of silicon atom does not necessarily improve the solubility of the polymer albeit steric bulkiness. Optical properties of siloxane-bearing polythiophenes in solution and film states were examined. As described in the preliminary communication, measurement of UV−vis absorption spectrum of a hexane solution of 10 (Mn = 12 200) exhibited the λmax of 446 nm, and that in the film spin coated from hexane solution followed by annealing at 110 °C for 10 min exhibited λmax = 519 nm to reveal red-shift of 73 nm.7

Figure 4. Photographs of the polymerization mixture of (a) for the synthesis of P3HT and (b) 10 bearing siloxane moiety in the side chain performed by the reaction with TMPMgCl·LiCl and NiCl2(PPh3)IPr. Solubility test of polythiophenes in organic solvents (1 mg/mL). (c) Polythiophene 10 (Mn = 12 500) bearing a disiloxane moiety in the side chain in hexane. (d) P3HT (Mn = 12 500) in hexane. (e) P3DT (Mn = 14 900) in hexane. (f) 10 (Mn = 280 000) in hexane. (g) 10 (Mn = 12 500) dissolved in 1-butanol (1 mg/mL). (h) The mixture (e) after heating to 100 °C.

Figure 5. Solubility tests of polythiophene poly-3-(CH2)nSi-thiophen2,5-diyl) (11: Si = SiEt3, n = 4; 12: Si = SiMe(OSiMe3)2, n = 4; 13: Si = Si(CH3)2OSi(CH3)3, n = 3).

Figure 6. UV−vis spectra (normalized) of (a) 10 in the film state (Mn = 12 200: solid; Mn = 280 000: dotted), (b) 13 (hexane solution: dotted; thin film: solid).

Synthesis of polythiophene bearing a siloxane substituent was also found to be achieved with the corresponding chlorothiophene 7,3h which was prepared by halogen exchange with bromothiophene 5. Deprotonation of 7 was carried out in a similar manner to the case of 5 (60 °C, 1 h) and following addition of nickel catalyst induced the polymerization. When 1265


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Figure 7. AFM images (5 × 5 μm region) of siloxane-bearing polythiophenes 10 ((a) Mn = 12 200 and (b) 280 000), (c) 13, and (d) P3HT.

Figure 8. XRD profiles of siloxane-bearing polythiophenes 10 ((a) Mn = 12 200 and (b) 280 000), (c) 13, and (d) P3HT.

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Figure 9. (a) Output and (b) transfer characteristics (at a drain voltage of −100 V) of OFET of 10 (Mn = 12 200).

in the 10 (Mn = 280 000) film probably due to the formation of random orientation which is also observed in the film of P3HT with high Mn.20 To estimate a hole mobility as electric properties of the siloxane-bearing polythiophene, we fabricated and characterized OFETs. Figure 9 shows the output and transfer characteristics (at a drain voltage of −100 V) of the OFETs based on 10 (Mn = 12 200). Above the threshold voltage, the field-effect hole mobility can be calculated from the slope of the plot of the square root of the drain current versus gate voltage. From the plot, the field-effect hole mobility for the OFET based on 10 (Mn = 12 200) was calculated to be (1.1 ± 0.1) × 10−5 cm2 V−1 s−1, with a threshold voltage of −22 ± 3 V and an on/off ratio of (2.4 ± 0.5) × 102. Field-effect hole mobilities of P3HT were reported to be ranging from 0.12 × 10−4 to 6.2 × 10−4 cm2 V−1 s−1, which were dependent on solvents used for spin-coating.21 Additionally, field-effect hole mobilities of regioregular poly(3alkylthiophene)s (P3ATs) were reported to be ranging from 0.19 × 10−4 to 5.2 × 10−4 cm2 V−1 s−1, which were dependent on the alkyl chain length.22 Even though the hole mobility of the siloxane-bearing polythiophene is several orders of magnitude lower than that of P3ATs due to the steric hindrance on the top of their alkyl chains, the use of the siloxane side chain for polythiophenes offers an advantage of high solubility even in hexane. P3HTs with lower Mn may also be soluble in hexane. However, the field-effect hole mobility of P3HT with the low Mn (= 2200) was reported to be quite low mobility of 5.5 × 10−7 cm2 V−1 s−1.23 Thus, our experimental results will provide one of strategies for molecular design of highly soluble conjugated polymers compatible with increasing hole mobilities.

Siloxane-baring polythiophene 10 of higher molecular weight (Mn = 280 000) showed λmax of 527 nm, which slightly redshifted ca. 8 nm compared with that of lower Mn as shown in Figure 6a. On the other hand, an analogous siloxane-bearing polymer of shorter alkylene length 13 exhibited mostly similar λmax (519 nm in the film and 444 nm in hexane) (Figure 6b). A comparison of optical properties, HOMO−LUMO energy levels, and morphology for thin films based on siloxane-bearing polythiophenes with those for regioregular P3HT is a major concern since the siloxane-bearing polythiophenes have a steric hindrance on the top of their alkyl chains. The absorbance spectra of P3HT displayed a shoulder peak on red-shifted side of the maximum absorbance peak.18 This feature is known to be enhanced by the coplanarization of thiophene main chains due to the ease of π−π stacking in P3HTs with normal alkyl chains,19 leading to the long effective conjugation length and the narrow HOMO−LUMO energy gap (red-shifted absorption and photoluminescence spectra) in P3HTs compared to siloxane-bearing polythiophenes. The lack of fine shoulder peaks in the absorption spectra of siloxane-bearing polythiophene films is most likely due to the large steric hindrance on the top of their alkyl chains disturbing the π−π stacking. These optical properties and HOMO−LUMO energy levels are summarized in Table 4 (see also Supporting Information Figure S2). AFM image for the P3HT film in Figure 7d shows a significant roughness (the root-mean-square (RMS) roughness of 6.33 nm), indicating the presence of P3HT aggregates due to the ease of crystallization in P3HT. The out-of-plane XRD diffraction patterns (diffraction angle 2θ = 5.4°, 10.8°, and 16.1° for the primary (100), secondary (200), and tertiary (300) peaks in Figure 8d) indicate highly crystalline states in the P3HT film. In contrast, homogeneous surfaces (RMS roughness ranging from 0.93 to 1.45 nm) were observed for the films of siloxane-bearing polythiophenes as shown in Figures 8a−c. Even though the layer distance of (100), which is a interchain spacing between thiophene main chains through the long side chains, is observed to some extent in XRD diffraction patterns of 10 (Mn = 12 200) and 13 films, the lack of fine shoulder peaks in absorption spectra and the homogeneous surfaces observed in AFM images for the films based on siloxane-bearing polythiophenes indicate a weak π−π interaction between neighboring polymer chains. XRD analysis of siloxane-bearing polymer 13 with shorter alkylene length indicated a peak at 2θ = 4.42° (100) suggesting layer distance of 19.97 Å, which was shorter than that of 20.48 Å in the 10 (Mn = 12 200) film, while a remarkable peak was not observed

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CONCLUSION In conclusion, we have shown that polythiophene derivatives bearing a siloxane moiety in a substituent of the thiophene ring at the 3-position was successfully synthesized by nickel(II)catalyzed deprotonative polymerization of 2-bromo-3-substituted thiophenes with bulky magnesium amid Knochel−Hauser base. The reaction was revealed to proceed to afford regioregular head-to-tail-type polythiophene, in which the molecular weight is controllable by the monomer feed/catalyst loading ratio. The deprotonative polymerization was also shown to proceed with the related chlorothiophene with Knochel−Hauser base or butyllithium (Murahashi coupling polymerization). The obtained siloxane-containing polythiophenes were shown to be dissolved in a variety of organic solvents, particularly in hexanes. Characterization of thin films 1267


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of siloxane containing polythiophenes by AFM and XRD was performed, and the hole mobility of the thin film was studied. Further researches on physical and mechanical properties of the obtained polythiophenes are in progress, which will be described in due course.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02524.

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Experimental details and analytical data (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (A.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by KAKENHI B (No. 25288049), Special Coordination Funds for Promoting Science and Technology, Creation of Innovation Centers for Advanced Interdisciplinary Research Areas (Innovative Bioproduction Kobe) by MEXT, Japan, and ASTEP by JST, MEXT, Japan. S.T. thanks JSPS for the Research Fellowship for Young Scientists.

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REFERENCES

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