Facile Syntheses of Dioxythiophene-Based Conjugated Polymers by

Sep 25, 2012 - The molecular weight (Mn, GPC) and molecular weight distribution ... was allowed to stir for 1 h to ensure the consumption of 1-hexanol...
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Facile Syntheses of Dioxythiophene-Based Conjugated Polymers by Direct C−H Arylation Haichao Zhao,† Ching-Yuan Liu,† Shyh-Chyang Luo,† Bo Zhu,† Tsai-Hui Wang,‡ Hsiu-Fu Hsu,‡ and Hsiao-hua Yu*,† †

Yu Initiative Research Unit, RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan Department of Chemistry, Tamkang University, Tamsui, Taiwan 25137

‡

S Supporting Information *

ABSTRACT: Various substituted dioxythiophenes bearing 3,4propylenedioxythiophenes (ProDOT) and 3,4-ethylenedioxythiophene (EDOT) moieties successfully undergo Pd-catalyzed direct C−H arylation to yield π-conjugated polymers. The effects of palladium catalysts, phosphine ligands or additives, and functional groups on this facile polycondensation approach are investigated. Polymers from alkoxy-substituted ProDOT are synthesized with reasonable molecular weight (Mn = 6100−9600) and low PDI (1.3−1.9). Four substituted EDOT with alkoxy or protected functional groups also undergo direct C−H arylation polycondensation to yield corresponding polymers. The obtained polydioxythiophenes exhibit UV−vis absorptions ranging from 480 to 590 nm, and these conjugated polymers are electroactive and reversibly switched between the oxidized and neutral states upon applying potentials.

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INTRODUCTION Interest in design and synthesis of π-conjugated polymers and oligomers has recently increased considerably due to its variety of applications.1−5 Among all π-conjugated polymers, the most commercially successful one is poly(3,4-ethylenedioxythiophene) (PEDOT).6−8 Composite PEDOT−polystyrenesulfonate has been mass-produced by Bayer AG and demonstrated as a gold standard for hole-transporting layer in a variety of organic electronic devices, including polymer light-emitting diodes and photovoltaics. Furthermore, PEDOT and its derivatives have shown promises for applications such as antistatic coatings, electrochromic displays, supercapacitors, solid state ion sensors, biosensors, fuel cells, and biomedical devices. Typically, PEDOTs are synthesized by chemical or electrochemical oxidation polymerization. However, this method requires excess oxidants and yields corresponding polymers with a broad molecular weight distribution. Therefore, development of efficient synthetic approaches to functionalized 3,4-ethylenedioxythiophene (EDOT)-based conjugated polymers remains an important and challenging task. Conventionally, π-conjugated polymers are often synthesized by a variety of cross-coupling reactions9 through organometallic compounds whose metal centers are magnesium (Kumada− Tamao−Corriu),10 zinc (Negishi),11 boron (Suzuki−Miyaura),12 and tin (Migita−Kosugi−Stille).13 Unfortunately, these methods usually suffer from disadvantageous features including the preparation of bifunctional organometallic monomers, air sensitivity, toxic side products, and extra synthetic steps. Recently, cross-coupling reactions of heteroarenes with aryl halides by direct C−H arylation emerges as facile approaches to the cross-coupling © XXXX American Chemical Society

reactions mentioned above due to their advantages of fewer reaction steps (step-economic) and reduced waste of toxic metal salt byproducts (green chemistry).14−17 Although direct C−H arylation has been widely applied for the synthesis of small aromatic compounds, only a few publications discussed this approach for the synthesis of π-conjugated polymers.18−24 Herein, we report a general method to synthesize various functionalized dioxythiophene-based π-conjugated polymers, mainly from 3,4propylenedioxythiophene (ProDOT) and EDOT, by C−H arylation polycondensation. The roles of palladium catalysts, phosphine ligands, and additives are also investigated.

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EXPERIMENTAL SECTION

Measurements. 1H NMR and 13C NMR spectra were recorded with a VARIAN 500 at 500 MHz (1H) and at 125 MHz (13C) spectrometers in deuteriochloroform. The molecular weight (Mn, GPC) and molecular weight distribution (Mw/Mn) of the polymers were measured on a Waters GPC system, which was equipped with a Waters 1515 HPLC solvent pump, a Waters 2414 refractive index detector, and two Waters Styragel High Resolution columns, at 40 °C using HPLC grade THF as eluent at a flow rate of 0.35 mL/min. Monodispersed polystyrenes were used to generate the calibration curve. MALDI-TOF MS was performed using Ultraflex (Bruker Daltonics, Bremen, Germany) mass spectrometer, and UV−vis spectra were measured on a Jasco V-630 spectrophotometer. Mass spectra were recorded on JMS-700 V (JEOL). All electrochemical measurements were performed with an Autolab PGSTAT 128N potentiostat Received: April 7, 2012 Revised: August 27, 2012

A

dx.doi.org/10.1021/ma300719n | Macromolecules XXXX, XXX, XXX−XXX

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Table 1. Synthesis of Alkyl Group Containing Poly(1−2) by Direct C−H Arylation Polycondensationa entry 1 2 3 4 5 6 7f 8 9 10 11 12 13 14f

polymer poly(1) poly(1) poly(1) poly(1) poly(1) poly(1) poly(1) poly(2) poly(2) poly(2) poly(2) poly(2) poly(2) poly(2)

Pd catalyst Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)(o-Tol) Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)(o-Tol)

ligand e

none PPh3 P(p-Tol)3 P(o-PhOMe)3 PCy3-HBF4 PtBu2Me-HBF4 P(o-PhOMe)3 nonee PPh3 P(p-Tol)3 P(o-PhOMe)3 PCy3-HBF4 PtBu2Me-HBF4 P(o-PhOMe)3

yieldb (%)

Mnc

PDI

DPd

60 92 78 63 85 56 91 92 85 65 −g −g 40 85

8800 7400 6400 6100 9600 6800 8400 3800 3900 3800

1.8 1.4 1.3 1.9 1.6 1.7 1.5 1.1 1.1 1.1

25 21 18 17 27 19 22 11 12 11

2700 4800

1.1 1.1

8 14

a The polycondensation was carried out at 100 °C for 48 h in DMF solution containing 5 mol % Pd catalyst, 10 mol % phosphine ligand, and 2 equiv of Cs2CO3 as base. bThe products were collected by reprecipitation from methanol−water with the reaction mixture. cDetermined by gel permeation chromatography (GPC) calibrated with polystyrene standards. dThe average degree of polymerization were calculated from molecular weights of the polymer and repeating unit. eNo phosphine ligand was used, and instead, tetra(n-butyl)ammonium bromide (TBAB) was added as phase transfer reagent. fThe polycondensation was carried out in THF solution containing 2 mol % Pd catalyst and 4 mol % phosphine ligand in the presence of 2 equiv of Cs2CO3 as base. gThe obtained product was a highly viscous liquid.

(s, 4H). 13C NMR (125 MHz, CDCl3) δ: 14.1, 22.6, 25.8, 29.5, 31.7, 47.8, 69.4, 71.7, 74.3, 91.0, 147.1. HR-MS (EI) calculated for C21H34Br2O4S 540.0545 [M+]; found 540.0553. 2-(Dodecyloxymethyl)-2,3-dihydrothieno[3,4-b][1,4]dioxine (2). To a dry 250 mL three-neck round-bottom flask charged with hydroxymethyl-EDOT (3.40 g, 20 mmol) and anhydrous THF (200 mL), NaH (1.2 g, 50 mmol) was added slowly under N2. The solution was allowed to stir for 1 h to ensure complete consumption of hydroxylmethyl EDOT. 1-Bromododecane (7.50 g 30 mmol) was added, and the reaction was conducted at 60 °C for 24 h. The flask was cooled, and the mixture was then poured into saturated NaCl solution and extracted with ethyl acetate. The organic layer was dried over anhydrous MgSO4 and filtered, and the solvent was removed by rotary evaporation under reduced pressure. The crude compound was purified by column chromatography on silica gel (eluent: ethyl acetate/ hexane, v/v, 1:5) and dried under vacuum to yield white powder (4.96 g,73%). 1H NMR (500 MHz, CDCl3) δ: 0.88 (t, 3H, J = 6.5 Hz), 1.25− 1.29 (m, 18H), 1.54−1.60 (m, 2H), 3.48 (t, 2H, J = 6.5 Hz), 3.59 (dd, 1H, J = 6.0, 10.5 Hz), 3.68 (dd, 1H, J = 5.0, 11.5 Hz), 4.06 (dd, 1H, J = 7.5, 11.5 Hz), 4.26 (dd, 1H, J = 2.0, 13.5, Hz), 4.29−4.31 (m, 1H), 6.31 (d, 1H, J = 3.5 Hz), 6.33 (d, 1H, J = 3.5 Hz). 13C NMR (125 MHz, CDCl3) δ: 14.1, 22.7, 26.0, 29.3, 29.4, 29.5, 29.5, 29.6, 29.6, 29.7, 31.9, 66.3, 69.1, 72.1, 72.6, 99.5, 99.7 141.6,141.6. HR-MS (EI) calculated for C19H32O3S 340.2072 [M+]; found 340.2074. 5,7-Dibromo-2-(dodecyloxymethyl)-2,3-dihydrothieno[3,4b][1,4]dioxine (2-Br). A dry 100 mL three-neck round-bottom flask was flushed with N2 and charged with 2 (3.40 g, 10 mmol) and dry THF (50 mL). The reaction mixture was cooled down to 0 °C, and Nbromosuccinimide (NBS) (4.40 g, 25 mmol) was added. After the resulting solution was stirred for 1 h at room temperature, it was added to saturated NaCl solution and extracted with ethyl acetate (100 mL). The combined organic layer was washed with saturated NaCl solution, dried over anhydrous MgSO4, filtered, and concentrated at a reduced pressure. The crude compound was purified by column chromatography on silica gel by using ethyl acetate as eluent. The compound was dried under vacuum to yield white solid (3.05 g, 61%). 1H NMR (500 MHz, CDCl3) δ: 0.87 (t, 3H, J = 6.5 Hz), 1.25−1.30 (m, 18H), 1.55− 1.58 (m, 2H), 3.50 (t, 2H, J = 6.5 Hz), 3.61 (dd, 1H, J = 6.5, 10.5 Hz), 3.73 (dd, 1H, J = 4.5, 10.5 Hz), 4.12 (dd, 1H, J = 8.0, 12.0 Hz), 4.32− 4.34 (m, 2H). 13C NMR (125 MHz, CDCl3) δ: 14.1, 22.7, 26.0, 29.4, 29.4, 29.5, 29.6, 29.6, 29.6, 29.7, 31.9, 66.6, 68.4, 72.2, 73.1, 85.3, 85.4, 139.6,139.6. HRMS (EI) calculated for C19H30Br2O3S 496.0282 [M+]; found 496.0284.

(Metrohm). Cyclic voltammetry was performed in a three-electrode cell versus a quasi-internal Ag wire reference electrode submersed in 0.01 M AgNO3/0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in CH3CN. Typical cyclic voltammograms were recorded by using platinum button as the working electrode and a platinum coil counter electrode. Chemicals. All reagents were purchased from Tokyo Chemical Industry (TCI), Sigma-Aldrich, Wako Chemicals, and used without further purification unless otherwise noted. Dibromomethyl-ProDOT and aminomethyl-EDOT were synthesized following the literature procedure.25,26 3,3-Bis(hexyloxymethyl)-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine (1). To a dry 250 mL three-neck round-bottom flask charged with 1-hexanol (8.25 g, 50 mmol) and anhydrous DMF (200 mL), NaH (1.80 g, 75 mmol) was added slowly under N2. The solution was allowed to stir for 1 h to ensure the consumption of 1hexanol. Dibromomethyl-ProDOT (5.10 g, 15 mmol) was added, and the reaction was conducted at 90 °C for 24 h. The flask was cooled, and the mixture was then poured into saturated NaCl solution and extracted with ethyl acetate. The organic layer was dried over anhydrous MgSO4 and filtered, and the solvent was removed by rotary evaporation under reduced pressure. The crude compound was purified by column chromatography on silica gel (eluent:ethyl acetate/ hexane, v/v, 1:5) and dried under vacuum to yield colorless oil (4.15 g, 73%). 1H NMR (500 MHz, CDCl3) δ: 0.90 (t, 6H, J = 5.5 Hz), 1.24− 1.34 (m, 12H), 1.51−1.56 (m, 4H), 3.41 (t, 4H, J = 6.5 Hz), 3.48 (m, 4H), 4.00 (s, 4H), 6.44 (s, 2H). 13C NMR (125 MHz, CDCl3) δ: 14.1, 22.6, 25.8, 29.5, 31.7, 47.8, 69.5, 71.7, 73.7, 105.04, 149.7. HR-MS (EI) calculated for C21H36O4S 384.2334 [M+]; found 384.2339. 6,8-Dibromo-3,3-bis(hexyloxymethyl)-3,4-dihydro-2Hthieno[3,4-b][1,4]dioxepine (1-Br). A dry 100 mL three-neck round-bottom flask was flushed with N2 and charged with 1 (2.34 g, 6 mmol) and dry THF (50 mL). The reaction mixture was cooled down to 0 °C, and N-bromosuccinimide (NBS) (2.76 g, 15 mmol) was added. After the resulting solution was stirred for 1 h at room temperature, it was added to saturated NaCl (100 mL) and extracted with ethyl acetate (100 mL). The combined organic layer was washed with saturated NaCl solution, dried over anhydrous MgSO4, filtered, and concentrated at a reduced pressure. The crude compound was purified by column chromatography on silica gel using (eluent:ethyl acetate/ hexane, v/v, 1:5). The compound was dried under vacuum to yield light yellow oil (2.84 g, 88%) and was stored in the freezer. 1H NMR (500 MHz, CDCl3) δ: 0.87 (t, 6H, J = 6.5 Hz), 1.22−1.33 (m, 12H), 1.49−1.55 (m, 4H), 3.40 (t, 4H, J = 6.5 Hz), 3.49 (m, 4H), 4.01 B

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tert-Butyl((2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl)methoxy)dimethylsilane (3). To a dry 100 mL three-neck roundbottom flask charged with hydroxymethyl-EDOT (2.50 g, 15 mmol), imidazole (1.03 g, 15 mmol), and anhydrous CH2Cl2 (100 mL), TBDMSCl (3.00 g, 20 mmol) was added slowly. The solution was allowed to stir for 6 h, and the mixture was then poured into saturated NaCl solution and extracted with ethyl acetate. The organic layer was dried over anhydrous MgSO4 and filtered, and the solvent was removed by rotary evaporation under reduced pressure. The crude compound was purified by column chromatography on silica gel by using ethyl acetate as eluent and dried under vacuum to yield light oil (3.95 g, 92%). 1H NMR (500 MHz, CDCl3) δ: 0.03 (s, 6H), 1.29 (m, 18H), 0.82 (s, 9H), 3.67 (dd, 1H, J = 2.0, 11.0 Hz), 3.79 (dd, 1H, J = 4.5, 11.0 Hz), 3.98 (dd, 1H, J = 7.5, 11.5 Hz), 4.09−4.11 (m, 1H), 4.18 (dd, 1H, J = 2.0, 11.5, Hz),6.22 (d, 1H, J = 3.5 Hz), 6.23 (d, 1H, J = 4 Hz). 13C NMR (125 MHz, CDCl3) δ: −5.4, −5.4, 25.8, 25.8, 25,8, 61.7, 66.1, 73.8, 99.3, 99.5, 141.7, 141.7. HRMS (EI) calculated for C13H22O3SSi 286.1059 [M+]; found 286.1054. tert-Butyl((5,7-dibromo-2,3-dihydrothieno[3,4-b][1,4]dioxin2-yl)methoxy)dimethylsilane (3-Br). A dry 100 mL three-neck round-bottom flask was flushed with N 2 and charged with hydroxymethyl-EDOT (4.30 g, 25 mmol) and dry THF (50 mL). The reaction mixture was cooled down to 0 °C, and Nbromosuccinimide (NBS) (8.90 g, 50 mmol) was added. After the resulting solution was stirred for 1 h at room temperature, it was added to saturated NaCl solution and extracted with ethyl acetate (100 mL). The combined organic layer was washed with saturated NaCl solution, dried over anhydrous MgSO4, filtered, and concentrated at a reduced pressure. The crude compound was purified by column chromatography on silica gel by using ethyl acetate as eluent. The compound was dried under vacuum to yield light yellow oil (6.63 g, 81%). 1H NMR (500 MHz, CDCl3) δ: 2.05 (s, 1H), 3.87 (dd, 1H, J = 4.5, 11.0 Hz), 3.94 (dd, 1H, J = 2.0, 11.0 Hz), 4.12 (dd, 1H, J = 4.5, 11.0 Hz), 4.16 (dd, 1H, J = 7.5, 11.5 Hz), 4.24−4.30 (m, 1H), 4.33 (dd, 1H, J = 2.0, 11.5 Hz). 13C NMR (125 MHz, CDCl3) δ: 61.1, 66.1, 74.6, 85.6, 85.6, 139.4, 139.5. To a dry 100 mL three-neck round-bottom flask charged with the above-obtained bromominated hydroxymethyl EDOT (6.46 g, 15 mmol), imidazole (1.02 g, 15 mmol), and anhydrous CH2Cl2 (100 mL), TBDMSCl (2.25 g, 15 mmol) was added slowly. The solution was allowed to stir for 6 h, and the mixture was then poured into saturated NaCl solution and extracted with ethyl acetate. The organic layer was dried over anhydrous MgSO4 and filtered, and the solvent was removed by rotary evaporation under reduced pressure. The crude compound was purified by column chromatography on silica gel by using ethyl acetate as eluent and dried under vacuum to yield colorless oil (7.50 g, 85%). 1H NMR (500 MHz, CDCl3) δ: 0.09 (s, 6H), 0.91 (s, 9H), 3.81 (dd, 1H, J = 4.0, 7.0 Hz), 3.94 (dd, 1H, J = 4.5, 10.5 Hz), 4.13 (dd, 1H, J = 7.5, 11.5 Hz), 4.21−4.13 (m, 1H), 4.35 (dd, 1H, J = 7.0, 11.5, Hz). 13C NMR (125 MHz, CDCl3) δ: −5.4, −5.4, 25.6, 25.6, 25,8, 61.4, 66.5, 74.2, 85.1, 85.3, 139.7, 139.7. HRMS (EI) calculated for C13H22Br2O3SSi 441.9269.2334 [M+]; found 441.9267. tert-Butyl 2-((2,3-Dihydrothieno[3,4-b][1,4]dioxin-2-yl)methoxy)acetate (4). To a dry 250 mL three-neck round-bottom flask charged with hydroxylmethyl EDOT (1.72 g, 10 mmol) and anhydrous THF (100 mL), NaH (300 mg, 15 mmol) was added slowly under N2. The solution was allowed to stir for 1 h to ensure complete consumption of hydroxylmethyl EDOT. tert-Butyl bromoacetate (1.95 g, 10 mmol) was added, and the reaction was conducted at 90 °C for 24 h. The flask was cooled, and the mixture was then poured into saturated NaCl solution and extracted with ethyl acetate. The organic layer was dried over anhydrous MgSO4 and filtered, and the solvent was removed by rotary evaporation under reduced pressure. The crude compound was purified by column chromatography on silica gel by using ethyl acetate as eluent and dried under vacuum to yield light yellow oil (1.52 g, 53%). 1H NMR (500 MHz, CDCl3) δ: 1.47 (s, 9H), 3.35−3.40 (m, 1 H), 3.74 (dd, 1H, J = 5.5, 10.5 Hz), 3.81 (dd, 1H, J = 5.0, 10.5 Hz), 4.03 (s, 2H), 4.11 (dd, 1H, J = 3.5, 7.5 Hz), 4.28 (dd, 1H, J = 3.5, 12.0 Hz),4.32−4.36 (m, 1H), 6.32 (d, 1H, J = 3.5 Hz), 6.33 (d, 1H, J = 3.5 Hz). 13C NMR (125

MHz, CDCl3) δ: 28.1, 28.1, 28.1, 66.0, 69.2, 69.7, 72.6, 81.9, 99.6, 99.8, 141.4, 141.5, 169.2. HRMS (EI) calculated for C13H18O5S 286.0874 [M+]; found 286.0872. tert-Butyl 2-((5,7-Dibromo-2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl)methoxy)acetate (4-Br). A dry 100 mL three-neck round-bottom flask was flushed with N2 and charged with 4 (2.86 g, 10 mmol) and dry THF (50 mL). The reaction mixture was cooled down to 0 °C, and N-bromosuccinimide (NBS) (3.56 g, 20 mmol) was added. After the resulting solution was stirred for 1 h at room temperature, it was added to 100 mL of water and extracted with ethyl acetate (100 mL). The combined organic layer was washed with saturated NaCl solution, dried over anhydrous MgSO4, filtered, and concentrated at a reduced pressure. The crude compound was purified by column chromatography on silica gel using ethyl acetate as eluent. The compound was dried under vacuum to yield viscous oil (3.17 g, 72%). 1H NMR (500 MHz, CDCl3) δ: 1.47 (s, 9H), 3.35−3.40 (m, 1 H), 3.72 (dd, 1H, J = 3.5, 10.5 Hz), 3.76 (dd, 1H, J = 6.0, 10.5 Hz), 3.85 (dd, 1H, J = 4.5, 10.5 Hz), 4.01 (s, 2H), 4.16 (dd, 1H, J = 7.5, 12.0 Hz),4.35−4.38 (m, 1H). 13C NMR (125 MHz, CDCl3) δ: 28.1, 28.1, 28.1, 66.4, 69.0, 69.2, 73.1, 82.1, 85.4, 85.4, 139.4.4, 139.5, 169.2. HRMS (EI) calculated for C13H16Br2O5S 441.9085 [M+]; found 441.9096. tert-Butyl(2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl)methylcarbamate (5). To a dry 100 mL three-neck round-bottom flask charged with EDOT−CH2NH2 (2.0 g, 12 mmol) and anhydrous THF (50 mL), Boc2O (2.62 g, 12 mmol) and triethylamine (1 mL) were added slowly. The solution was allowed to stir overnight, and the mixture was then poured into saturated NaCl solution and extracted with ethyl acetate. The organic layer was dried over anhydrous MgSO4 and filtered, and the solvent was removed by rotary evaporation under reduced pressure. The crude compound was purified by column chromatography on silica gel using ethyl acetate as eluent and dried under vacuum to yield white solids (2.65 g, 82%). 1H NMR (500 MHz, CDCl3) δ: 1.48 (s, 9H), 3.35−3.40 (m, 1 H), 3.47 (d, 1H, J = 14.0 Hz), 3.93 (dd, 1H, J = 3.0, 12.0 Hz), 4.12 (dd, 1H, J = 7.0, 14.0 Hz), 4.19−4.22 (m, 1H), 4.96 (s, 1H,), 6.31 (d, 1H, J = 4.0 Hz), 6.32 (d, 1H, J = 4.0 Hz). 13C NMR (125 MHz, CDCl3) δ: 28.3, 28.3, 28.3, 40.8, 66.2, 73.0, 79.9, 99.8, 99.7, 141.3, 141.3, 155.9. HRMS (EI) calculated for C12H17NO3S 271.0878 [M+]; found 271.0875. tert-Butyl(5,7-dibromo-2,3-dihydrothieno[3,4-b][1,4]dioxin2-yl)methylcarbamate (5-Br). A dry 100 mL three-neck roundbottom flask was flushed with N2 and charged with 5 (3.00 g, 11 mmol) and dry THF (50 mL). The reaction mixture was cooled down to 0 °C, and N-bromosuccinimide (NBS) (3.92 g, 22 mmol) was added. After the resulting solution was stirred for 1 h at room temperature; it was added to saturated NaCl and extracted with EtOAc (100 mL). The combined organic layer was washed with saturated NaCl solution, dried over anhydrous MgSO 4 , filtered, and concentrated at a reduced pressure. The crude compound was purified by column chromatography on silica gel using ethyl acetate as eluent. The compound was dried under vacuum to yield wax solid (3.60 g,76%) and was stored in a freezer. 1H NMR (500 MHz, CDCl3) δ: 1.42 (s, 9H), 3.46 (s, 1 H), 3.72 (t, 1H, J = 7.0 Hz), 3.96 (dd, 1H, J = 8.5, 12.0 Hz), 4.24 (d, 1H, J = 5.5 Hz), 4.30 (d, 1H, J = 7.0 Hz), 5.01 (s, 1H). 13C NMR (125 MHz, CDCl3) δ: 28.3, 28.3, 28.3, 40.5, 66.9, 73.8, 74.02, 85.5, 85.2, 139.4, 139.36, 155.9. HRMS (EI) calculated for C12H15Br2NO4S 426.9088 [M+]; found 426.9081. Syntheses of Poly(1−5) by Direct C−H Arylation Polycondensation. A typical procedure for C−H arylation (entry 2, Table 1) is as follows. A N2-filled gastight Schlenk tube was charged with 1 (192 mg, 0.5 mmmol), 1-Br (270 mg, 0.5 mmol), Pd(OAc)2 (0.05 mmol), anhydrous triphenylphosphine (15.6 mg, 0.10 mmol), Cs2CO3 (326 mg, 1.0 mmol), and anhydrous DMF (2.0 mL). The mixture was gradually heated up to 100 °C and kept stirring at this temperature for 48 h. The mixture was quenched by addition of 1 M HCl solution (1 mL) followed by pouring into a mixture solution of MeOH (100 mL) and H2O (100 mL). The polymer was filtered and dried under vacuum to yield ferric red solid (353 mg, 92%). The spectroscopic data of all polymers are listed below. C

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Poly(1). 1H NMR (500 MHz, CDCl3) δ: 0.89 (bm, 6H), 1.254− 1.72 (bm, 16H), 3.42 (bm, 4 Hz), 4.00 (bm, 4H), 6.44 (bm, endcapped proton). 13C NMR (125 MHz, CDCl3) δ: 14.0, 19.7, 22.6, 29.7, 32.0, 47.7, 67.1, 69.5, 69.8, 113.6, 144.8. Poly(2). 1H NMR (500 MHz, CDCl3) δ: 0.88 (bm, 3H), 1.45 (bm, 18H), 1.54−1.60 (m, 2H), 2.95−3.64 (bm, 4H), 4.06−4.30 (bm, 3H). 13 C NMR (125 MHz, CDCl3) δ: 14.1, 22.7, 26.0, 29.3, 29.4, 29.5, 29.5, 29.6, 29.6, 29.7, 32.0, 66.6, 69.1, 72.2, 73.1, 112.3, 112.3, 141.6,141.6.

Poly(3). 1H NMR (500 MHz, CDCl3) δ: 0.00 (bm, 6H), 0.91 (s, 9H), 0.85 (bm, 9H), 3.81−4.34 (bm, 5H), 6.32 (bm, end-capped proton). 13C NMR (125 MHz, CDCl3) δ: −3.0, 18.1, 25.4, 65.8, 71.0, 74.3, 112.3, 112.3, 142.2, 142.2. Poly(4). 1H NMR (500 MHz, CDCl3) δ: 1.48 (bm, 9H), 3.40−4.45 (bm, 7 H), 6.33 (bm, end-capped proton). 13C NMR of poly(4) was not available. It is most likely due to limited solubility of poly(4) in the majority of solvents, including CDCl3 and d7-N,N-dimethylformamide. Poly(5). 1H NMR (500 MHz, CDCl3) δ: 1.45 (bm, 9H), 3.36−3.57 (bm, 2 H), 4.11 (bm, 3H), 4.96 (bm, 1H,), 6.44 (bm, end-capped proton). 13C NMR (125 MHz, CDCl3) δ: 27.6, 35.0, 66.9, 73.8, 81.7, 108.5, 108.7, 142.1, 142.5, 155.9.

Scheme 1. Syntheses of All Monomers

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RESULTS AND DISCUSSION Syntheses and Characterizations of Monomers and Polymers. Dihexyloxy ProDOT monomer (1) was synthesized by a two-step process from 3,4-dimethoxythiophene,25a and dodecyloxymethyl-substituted EDOT (2) was synthesized by Williamson etherification of hydroxymethyl-functionalized EDOT as shown in Scheme 1. Bromination by N-bromosuccinimide (NBS) of 1 and 2 yielded dibromo adducts 1-Br and 2-Br, respectively. EDOT bearing deprotetable tert-butyldimethylsiloxy (−OTBDMS), tert-butyl ester (−COOtBu), and N-tert-butoxycarbonyl (−NHtBOC) groups were also synthesized as shown in Scheme 1. The molecular structures of the monomers were confirmed by 1H and 13C nuclear magnetic resonance spectra and mass spectroscopic measurements. The typical C−H arylation polycondensation was as carried out as shown in Scheme 2. We first explored the polymerization of alkylated dioxythiophenes. The polycondensation to yield poly(1) and poly(2) was first examined in the presence of palladium acetate (Pd(OAc)2, 5 mol %), phosphine ligand (10 mol %), and Cs2CO3 (2 equiv). The reaction was performed in a Schlenk tube in DMF at 100 °C for 48 h. We chose Cs2CO3 as base because our previous studies on the diarylation of EDOT showed that Cs2CO3 resulted in the best yields.27 The experiment results are summarized in Table 1. Previous studies by the Ozawa group19 and Leclerc group22 demonstrated that arylphosphines (e.g., P(o-PhOMe)3) were more effective than alkylphosphines (e.g., PtBu3 and PCy3) to couple alkylthiophene monomers. On the other hand, Kanbara and co-workers used PCy3-HBF4 and PtBu2Me-HBF4 to couple dibromofluorene with bithiophene and tetrafluorobenzene.21 Therefore, we decided to screen all these phosphine ligands to determine the appropriate reaction condition. Polycondensation of 1 and 1-Br in the presence of phosphine ligand (entries 2−6, Table 1) yielded poly(1) with reasonable molecular weight (Mn = 6100−9600), low molecular Scheme 2. Synthesis of Polydioxythiophenes by Direct C−H Arylations

D

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Table 2. Synthesis of Protected Functional Groups Containing Poly(3−5) by Direct C−H Arylation Polycondensationa entry 1 2 3 4 5 6 7f 8 9 10 11 12 13 14f 15 16f

polymer poly(3) poly(3) poly(3) poly(3) poly(3) poly(3) poly(3) poly(4) poly(4) poly(4) poly(4) poly(4) poly(4) poly(4) poly(5) poly(5)

Pd catalyst Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)(o-Tol) Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)(o-Tol) Pd(OAc)2 Pd(OAc)(o-Tol)

ligand e

none PPh3 P(p-Tol)3 P(o-PhOMe)3 PCy3-HBF4 PtBu2Me-HBF4 P(o-PhOMe)3 Nonee PPh3 P(p-Tol)3 P(o-PhOMe)3 PCy3-HBF4 PtBu2Me-HBF4 P(o-PhOMe)3 nonee P(o-PhOMe)3

yieldb (%)

Mnc

PDI

DPd

69 58 62 41 45 28 68 93 0 89 11 0 0 0 69 67

3500 2100 3300 3100 −g −g 3400 2700

1.1 1.2 1.1 1.1

12 7 12 11

1.1 1.2

12 10

2000 2100

1.1 1.2

7 7

3900 3200

1.2 1.2

15 12

The polycondensation was carried out at 100 °C for 48 h in DMF solution containing 5 mol % Pd catalyst, 10 mol % phosphine ligand, and 2 equiv of Cs2CO3 or sodium acetate (for poly(4) synthesis) as base. bThe products were collected by reprecipitation from methanol−water with the reaction mixture. cDetermined by gel permeation chromatography (GPC) calibrated with polystyrene standards. dThe average degree of polymerization were calculated from molecular weights of the polymer and repeating unit. eNo phosphine ligand was used, and instead, tetra(nbutyl)ammonium bromide (TBAB) was added as phase transfer reagent. fThe polycondensation was carried out in THF solution containing 2 mol % Pd catalyst and 4 mol % phosphine ligand in the presence of 2 equiv of Cs2CO3 as base. gThe obtained product was insoluble, and this could be due to the deprotection of silane groups by the fluoride ions in the catalyst. a

weight distribution (PDI = 1.3−1.8), and reasonable yields (56−92%). Poly(1) with similar molecular weight was also obtained under phosphine ligand-free environment in the presence of tetrabutylammonium bromide (TBAB) as phase transfer reagent (entry 1, Table 1). The phosphine-free reaction condition is favorable because it allowed easier work-up with less toxic waste. Although similar phosphine-free reaction condition was described for synthesis of polyalkylthiophene before,18 herein we reported the case with highest molecular weight and degree of polymerization (DP). N,N-Dimethylforamide (DMF) was used as solvent in the condensation polymerization. Several recent papers suggested that the use of N,N-dimethylacetamide (DMAc) increased the solubility of the polymer and resulted in polymers with higher molecular weight.20,21 However, polycondensation of 1 and 1-Br did not proceed in DMAc. Therefore, the reaction conditions are substrate-dependent, and we only used DMF for the following studies. When similar screening (entries 8−13, Table 1) was performed on the polycondensation between 2 and 2-Br, a drastic ligand dependence was observed. We obtained poly(2) with comparably lower molecular weight (Mn = 2700−3900) in the presence of PPh3, P(p-Tol)3, PtBu2Me-HBF4, or TBAB as phase transfer reagent. In the presence of P(o-PhOMe)3 and PCy3-HBF4, only a viscous oligomeric liquid was yielded. Although 1 and 2 presented similar molecular formula and side-chain hydrophobicity, they yielded polymer with big molecular weight difference. This might be due to the differences of planarized enolate formations between fused six-membered ring and seven-membered ring. However, the small reaction yield difference (96% and 93%) of single C−H arylation reaction based on the degree of polymerization in a polycondensation did not allow us to conduct meaningful molecular simulation for further insights of the reaction. In addition to a typical procedure using Pd(OAc)2 as Pd source, recent reports indicated that Hermann’s catalyst (Pd(OAc)(o-Tol)) with tri-o-anisole phosphine (P(o-PhOMe)3) was a promising candidate to yield π-conjugated polymers with high molecular weight.19,22 Under similar reaction conditions, poly(1)

Figure 1. 1H NMR spectra of poly(1−5) measured in CDCl3.

was synthesized in comparable yields and molecular weight (entry 7, Table 1). On the other hand, poly(2) with higher molecular weight was obtained using similar reaction condition (entry 14, Table 1). We then screened the formation of functional group containing polydioxythiophenes utilizing the same direct C−H arylation polycondensation. EDOT monomers with protected −OTBDMS, −COOtBu, and −NHtBOC functional groups were synthesized. These groups could be deprotected to yield corresponding polymers containing reactive −OH, −COOH, and −NH2 groups which could be applied for bioconjugation or attachment of other molecular moieties. The results of the polycondensation are summarized in Table 2. 3 and 3-Br underwent direct C−H arylation to yield poly(3) in E

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All other phosphine ligands lead to polycondensation with low yields. In the case of tBOC-protected aminomethyl-EDOT (5), a slightly higher degree of polymerization was achieved under phosphine ligand-free condition (entry 15, Table 2). Unlike the previous cases on alkyl-functionalized EDOT and ProDOT, utilization of Hermann’s catalyst did not improve the performance on direct C−H arylation polymerization (entries 7, 14, and 16, Table 2). Although the polymers obtained exhibited low to moderate molecular weight, direct C−H arylation polycondensation could provide access to functionalized polymers which were unlikely to be obtained by traditional cross-coupling methods, e.g., 4-Br and 5-Br. Characterization and Physical Characteristics of the Polymers. Proton nuclear magnetic resonance (NMR) spectra of poly(1−5) in CDCl3 are summarized in Figure 1. Besides poly(1), other polymers displayed significant peak broadening comparing to monomer spectra. The is most likely due to the nonregioregularity of the polymer. In our C−H arylation polycondensation approach, there was no control of the headto-head and head-to-tail coupling because we coupled diprotonated (1−5) and dibrominated (1−5-Br) monomers. Therefore, we could not control the regioregularity of the polymers. Poly(1) displayed sharper peaks because the repeating unit is symmetrical so there was not stereoisomers. Most polymers also exhibit a weak signal at around 6.3 ppm, which is corresponding to the end-capped proton of the polymers. Carbon NMR spectra of the polymers were also collected, and they were in agreement with the molecular

the presence of most phosphine ligands and additive (entries 1−4 and 7, Table 2). However, we could only obtain poly(3) with

Figure 2. UV−vis spectra of drop-casted poly(1) (black solid line), poly(2) (red solid line), poly(3) (blue dashed line), poly(4) (blue solid line), and poly(5) (red dashed line) films on glass slides.

molecular weight (Mn) between 2100 and 3500 and narrow molecular weight distribution (PDI = 1.1−1.2). Phosphine ligands with tetrafluoroborate resulted in insoluble polymers. This is most likely due to the deprotection of silane group. PEDOT bearing ester functional groups (poly(4)) was more synthetically challenging. Polymers with reasonable yield and moderate molecular weight were only obtained in the presence of TBAB phase transfer agent without phosphine (entry 8, Table 2) or P(p-Tol)3 ligand.

Figure 3. Cyclic voltammograms of drop-casted films of the polyoxythiophenes on Pt button in acetonitrile solution containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) electrolyte: (A) poly(1), (B) poly(2), (C) poly(3), (D) poly(4), and (E) poly(5). The measurements were done using a Ag/Ag+ reference electrode. Cyclic voltammetry is carried out at a Pt button electrode with a scan rate of 50 mV/s. F

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Notes

structures of the polymers. Unfortunately, the limited solubility in poly(4) hindered the possibility of collecting any meaningful carbon NMR spectrum of it. Besides GPC, molecular weight analyses of the purified poly(1) were also carried out by MALDI-TOF mass spectroscopy (as shown in Supporting Information). The data confirmed the presence of polymer chains from 5000 to 10 000 g/mol, and the dominant peaks were separated by 382 amu, which is corresponding to the mass of monomer repeating unit. However, the molecular weight obtained from MALDI-TOF could not tell the real average molecular weight because polymers with longer chains were less likely to be volatized. The preliminary optical and electrochemical characterization of poly(1−5) were also carried out with drop-casted films. All polymer films showed broad absorption in the UV−visible region as shown in Figure 2. Poly(1) displayed dual absorption peak at 545 and 587 nm. This is similar to other dialkylated ProDOT previously reported.25a,b The electrochemical properties of poly(1−5) were examined by cyclic voltammetry in acetonitrile solution containing 0.1 M nBu4NPF6 electrolyte. As shown in Figure 3, all polymers displayed high capacitance of the polymer film upon oxidative charging. Except poly(1), other polymers displayed cathodic peak (Ep,c) at 0.1−0.2 V (vs Ag/Ag+) without significant peak upon reduction. The results are similar to electropolymerized PEDOT films reported previously.28 In the case of poly(1), it displayed significant redox peaks with half-wave peak potential (E1/2) at 0.3 V (vs Ag/Ag+). The previous reports all indicated similar E1/2 in functionalized poly(ProDOT)s.25a,b Noticeably, the onset oxidation potential was lower than that of polythiophenes and polyoxythiophenes due to the oxygen donor on the thiophene rings.29

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by RIKEN Advanced Science Institute and Grant-in-Aid for Young Scientist (No. 22681016 and 23700565) from JSPS/MEXT, Japan. Dr. B. Zhu thanks RIKEN for a Special Postdoctoral Researcher Fellowship. We thank Dr. Hiroyuki Koshino at Molecular Characterization Team, RIKEN, for assistance on carbon NMR measurements. We also thank Dr. Takehiro Suzuki at Chemical Biology Department of RIKEN for MALDI-TOF mass spectroscopic measurements.

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CONCLUSIONS In conclusion, we successfully synthesized a variety of functionalized polydioxythiophenes with a facile direct C−H arylation polycondensation approach. This approach is step economical because it does not require the preparation of organometallic intermediates. It is also more environmentalfriendly due to the reduction of toxic byproduct waste. On the basis of the studies of poly(1−5) syntheses, we concluded that the utilization of phase transfer reagent TBAB was generally most effective. On the other hand, the catalytic behaviors of Pd(OAc)2 and Hermann’s catalyst for direct C−H arylation polycondensation of dioxythiophene were substrate-dependent, and it required screening to decide the optimized reaction conditions. As these materials are promising for applications including organic electronics and conductive nanobiointerface with cells, this synthetic approach will enable facile access to the materials. Ongoing research of applying these materials for above-mentioned applications is currently underway.

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ASSOCIATED CONTENT

* Supporting Information S

Figure S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. G

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