Fluorinated Thiophene-Based Synthons - ACS Publications

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Fluorinated Thiophene-Based Synthons: Polymerization of 1,4Dialkoxybenzene and Fluorinated Dithieno-2,1,3-benzothiadiazole by Direct Heteroarylation Carl Roy,† Thomas Bura,† Serge Beaupré,† Marc-André Légaré,‡ Jon-Paul Sun,§ Ian G. Hill,§ and Mario Leclerc*,† †

Canada Research Chair on Electroactive and Photoactive Polymers, Department of Chemistry, Université Laval, Quebec City, Quebec G1V 0A6, Canada ‡ Institut für Anorganische Chemie, Julius-Maximilians Universität Würzburg, Am Hubland, 97074 Würzburg, Germany § Department of Physics, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada S Supporting Information *

ABSTRACT: The incorporation of fluorine atoms along the conjugated polymer backbone is an effective strategy to tune the electro-optical properties of donor−acceptor based copolymers. We report here an efficient way to synthesize and purify new mono-fluorinated thiophene derivatives for the synthesis of fluorinated dithienobenzothiadiazole (DTBT) comonomers. It was observed that the reactivity and regioselectivity of the direct (hetero)arylation polymerization (DHAP) were modified upon the amount and the positioning of the fluorine atoms on the DTBT moiety. Indeed, the polymerization time went from 66 h (for non-fluorinated DTBT, M1) to only 11 min (for tetrafluorinated DTBT, M2). In addition to enhanced reactivity, the degree of fluorination of the flanking thiophene of the DTBT moiety also modulates the electro-optical properties, lowering both the bandgap and stabilizing the ionization energy level. Indeed, P1 (with non-fluorinated flanking thiophene) has a bandgap of 1.73 eV and an ionization energy (IE) of 4.96 eV while a bandgap of 1.65 eV and a IE of 5.20 eV were obtained for P4 (with the fluorine atom facing the 1,4-alkoxyphenylene moiety). may otherwise perturb the backbone planarity.9,10 In addition, changes in crystallinity, internal polarization, and modification of the morphology of the active layer have been attributed to fluorination. For fluorinated PTB7 derivatives, the increase of PCE is mainly due to a deeper HOMO energy level that leads to higher Voc and less charge recombination. On the other hand, D−A copolymers using DTF2BT as electron-accepting moiety (instead of 4,7-dithieno-2,1,3-benzothiadiazole (DTBT)) show higher crystallinity and hole mobility, allowing PSCs with thick active layer (up to 300 nm), high fill factor (FF), and PCE.8 Nielsen et al. have reported that DTF2BT chromophore has higher molar absorptivity than DTBT and shows more prominent intra- and intermolecular interactions than DTBT that can contribute to a better charge transport, two important parameters for photovoltaic applications.10 As discussed by Heeney and co-workers, the flanking thiophenes found on the DTBT moiety prevent torsional disorder along the polymer backbone. On the downside due to the its electron-rich nature, the incorporation of the thiophene unit

1. INTRODUCTION Over the past decade, the performance of polymer solar cells (PSCs) has steadily increased owing to the development of new and well-defined electron-donor/electron-acceptor (D−A) copolymers.1 This approach is convenient since it allows efficient tuning of the electro-optical properties through hybridization of the HOMO and LUMO energy levels that gives copolymers with a broad absorption spectrum. Enhancement of the processing of the bulk heterojunction combined with better interfaces engineering now leads to single junction polymer solar cells with power conversion efficiencies exceeding 10%.2−4 Recently, fluorination of conjugated backbone of D−A copolymers has proven to be effective to enhance the performance of such PSCs.5 Indeed, among the most efficient D−A copolymers reported so far in the literature, all of them have fluorinated moieties. PTB7 and PTB7-Th6,7 have a 3fluoro-2(2-ethylhexylcarbonyl)thieno[3,4-b]thienophenediyl unit while PPDT2FBT and PffBT4T-2OD have a 5,6-difluoro4,7-dithieno-2,1,3-benzothiadiazole (DTF2BT) moiety (see Figure 1).3,8 The strong electronegativity of fluorine atom effectively lowers both the HOMO and LUMO energy levels of the fluorinated copolymers, and owing to its small van der Waals radius (r = 1.35 Å), it minimizes steric interactions which © 2017 American Chemical Society

Received: May 2, 2017 Revised: May 25, 2017 Published: June 5, 2017 4658

DOI: 10.1021/acs.macromol.7b00905 Macromolecules 2017, 50, 4658−4667

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method for the development of low-cost, well-defined, and efficient materials for plastic electronics. Along these lines, we report here, for the first time, pivotal fluorinated thiophene synthons for the synthesis of new fluorinated dithienobenzothiadiazole comonomers (M3 and M4, Scheme 1). Four D−A copolymers based on 2,5-di(2octyldodecyloxy)-1,4-phenylene and fluorinated DTBT (P1− P4; Scheme 1) were synthesized by direct (hetero)arylation polymerization (DHAP) in order to investigate the effect of the amount and positioning of the fluorine atom on the flanking thiophene on the physical and electro-optical properties of the polymers. Recently, Livi et al. have shown that an analogue of P1 (also synthesized by DHAP) outperformed the one obtained by Stille cross-coupling polymerization in PSCs, confirming that DHAP is a reliable and valuable polymerization method that can contribute to the implementation of plastic electronics.19

2. EXPERIMENTAL SECTION 2.1. Methods. 1H, 13C, and 19F NMR spectra were recorded on a Varian AS400 or Agilent DD2 500 MHz apparatus in deuterated solvents. Chemical shifts were reported as δ values (ppm) relative to the residual protic solvent. High-resolution mass spectra (HRMS) were recorded with an Agilent 6210 time-of-flight (TOF) LC-MS apparatus equipped with an APPI ion source using a 0.2 mL/min flow of isopropanol as eluent. Number-average (M̅ n) and weight-average (M̅ w) molecular weights were determined by size exclusion chromatography (SEC) using a high-temperature Varian Polymer Laboratories GPC120 equipped with an RI detector and a PL BV400 HT Bridge Viscometer. The column set consists of two PL gel Mixed C (300 × 7.5 mm) columns and a PL gel Mixed C guard column. The flow rate was fixed at 1 mL/min using 1,2,4-trichlorobenzene (TCB) (with 0.0125% BHT w/v) as eluent. The temperature of the system was set to 110 °C. All the samples were prepared at concentrations of nominally 1.0 mg/mL in TCB. Dissolution was performed using a Varian Polymer Laboratories PL-SP 260VC sample preparation system. The sample vials were held at 110 °C with shaking for 1 h for complete dissolution. The solutions were filtered through a 2 mm porous stainless steel filter used with the 0.40 μm glass filter into a 2 mL chromatography vial. The calibration method used to generate the reported data was the classical polystyrene method using polystyrene standards Easi-Vials PS-M from Varian Polymer Laboratories which were dissolved in TCB. UV−vis absorption spectra were obtained by using a Hewlett-Packard (8452A) diode array spectrophotometer with 1 cm path length quartz cells. For solid state measurements, polymer solution was spun-cast on glass plates. Optical bandgaps were calculated from the onset of the absorption band. The ionization energies and electron affinities were measured by performing

Figure 1. Fluorinated materials studied in PSCs.

often leads to a reduction of the ionization potential which can be detrimental for the open-circuit voltage (Voc) in PSCs. To prevent this adverse effect, Heeney and co-workers reported the synthesis of a new dithienogermole D−A copolymer imbedding ((3,4-difluorothiophen-2-yl)-2,1,3-benzothiadiazole) (TFDTBT) as electron-accepting unit (Figure 1).9 They reported that the incorporation of the fluorinated thiophene spacers gives a strong electron-accepting effect while stabilizing both HOMO and LUMO energy levels. The fluorinated copolymer has higher charge mobility and stronger tendency to aggregate. Despite significant improvement of the PSC performance, to the best of our knowledge, TFDTBT electron-accepting moiety has not been used in any other D− A copolymers reported so far in the open literature. One can think that this electron-accepting moiety could be used to modulate the electro-optical properties of known D−A copolymers based on 1,4-dialkoxyphenylenes.11−16 Moreover, this TFDTBT is well suited for direct heteroarylation polymerization,17,18 a valuable and eco-friendly polymerization Scheme 1. Synthesis of P1−P4 by DHAP

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phases were combined and washed with water and brine, dried over MgSO4, and concentrated under vacuum. Purification was achieved using column chromatography (silica gel; hexane), affording the desired compound as colorless oil (Y = 65%). Traces of 3dimethyloctylsilylthiophene (dehalogenation side reaction of 2) was observed as a side product, but it can be easily removed after the bromination step (synthesis of 4 or 5) by reverse phase column chromatography. 1H NMR 500 MHz (acetone-d6) δ (ppm): 7.47 (t, J = 3.4 Hz, 1H), 6.95 (dd, J = 3.3, 0.7 Hz, 1H), 1.37−1.23 (m, 12H), 0.89 (t, J = 7 Hz, 3H), 0.82−0.76 (m, 2H), 0.27 (d, J = 0.6 Hz, 6H). 19 F NMR 376 MHz (acetone-d6) δ (ppm): 126.31 (d, J = 3.4 Hz). 13C NMR 101 MHz (acetone-d6) δ (ppm): 162.49 (d, J = 252.9 Hz), 131.88 (dd, J = 13.4, 4.6 Hz), 130.36 (d, J = 34.6 Hz), 104.55 (dd, J = 25.8, 2.3 Hz), 33.14, 32.64, 29.96, 24.42, 23.33, 16.13, 14.36, −2.73. HRMS calculated for C14H25FSSi (M*+): 272.1425; found: 272.1441 (difference: 5.87 ppm). 2.3.3. Synthesis of 2-Bromo-3-fluoro-4-(dimethyloctylsilyl)thiophene (4). Compound 3 (2.68 g, 9.85 mmol, 1 equiv) was placed in a round-bottom flask with a magnetic stirrer and dissolved in 35 mL of DMF. The solution was cooled to 0 °C using an ice bath and was sheltered from light. Recrystallized N-bromosuccinimide (2.11 g, 11.8 mmol, 1.2 equiv) was added in small portions in solid form. Then, the mixture was warmed to room temperature and left to react overnight, after which it was quenched with water and extracted three times using Et2O. The organic phases were combined and washed with water and brine, dried over MgSO4, and concentrated under reduced pressure. The crude product was purified by column chromatography (reverse phase C18; ethanol, 95%), affording the titled compound as colorless oil (Y = 65%). 1H NMR (400 MHz, acetone-d6) δ 7.62 (d, J = 4.6 Hz, 1H), 1.37−1.24 (m, 12H), 0.90−0.84 (m, 3H), 0.83−0.76 (m, 2H), 0.28 (s, 6H). 19F NMR (376 MHz, acetone-d6) δ −124.46 (d, J = 4.5 Hz). 13C NMR (101 MHz, acetone-d6) δ 160.70 (d, J = 255.0 Hz), 132.84 (d, J = 12.9 Hz), 131.35 (d, J = 33.4 Hz), 91.86 (d, J = 26.6 Hz), 34.07, 32.62, 29.95, 29.92, 24.30, 23.32, 15.80, 14.37, −3.02 (d, J = 0.9 Hz). HRMS calculated for C14H24BrFSSi (M*+): 350.0530; found: 350.0547 (difference: 4.83 ppm). 2.3.4. Synthesis of 2,5-Dibromo-3-fluoro-4-(dimethyloctylsilyl)thiophene (5). Compound 5 was obtained as described for compound 4 with 6.34 g of 3 (23.3 mmol, 1 equiv) dissolved in 90 mL of DMF and 10.0 g (55.9 mmol, 2.4 equiv) of recrystallized N-bromosuccinimide. The crude product was purified by column chromatography (reverse phase C18; ethanol 95%), affording the desired compound as colorless oil (Y = 65%). 1H NMR (400 MHz, acetone-d6) δ 1.37−1.25 (m, 12H), 0.94−0.84 (m, 5H), 0.39 (d, J = 1.6 Hz, 6H). 19F NMR (376 MHz, acetone-d6) δ −116.36. 13C NMR (101 MHz, acetone-d6) δ 159.18 (d, J = 260.1 Hz), 131.78 (d, J = 32.6 Hz), 117.68 (d, J = 15.7 Hz), 90.96 (d, J = 27.6 Hz), 33.98, 32.62, 29.94, 29.89, 24.25, 23.33, 15.87 (d, J = 1.4 Hz), 14.39, −2.12 (d, J = 2.9 Hz). HRMS calculated for C14H23Br2FSSi (M*+): 427.9635; found: 427.9641 (difference: 1.3 ppm). 2.3.5. Synthesis of 5-Bromo-3-fluoro-4-(dimethyloctylsilyl)thiophene (6). Compound 5 (1.145 g, 2.66 mmol, 1 equiv) was placed in a dried round-bottom flask with magnetic stirrer and was purged on a Schlenk line. Anhydrous THF (5 mL) was added, and the solution was cooled at 0 °C with ice bath. Then a solution of iPrMgCl.LiCl (1.3 M in THF, 1.95 mL, 2.58 mmol, 0.97 equiv) was added dropwise. The mixture was brought back to room temperature and was reacted for an additional 45 min. The reaction was then quenched with methanol and stirred for 10 min. Water was added to the reaction, and the mixture was extracted with Et2O three times. The organic phases were combined and washed with water and brine, dried over MgSO4, and concentrated under reduced pressure. The crude product was purified by column chromatography (reverse phase C18; ethanol 95%), affording the desired compound as colorless oil (Y = 90%). 1H NMR (400 MHz, acetone-d6) δ 7.00 (d, J = 2.0 Hz, 1H), 1.38−1.21 (m, 12H), 0.94−0.83 (m, 5H), 0.37 (d, J = 1.6 Hz, 6H). 19F NMR (376 MHz, acetone-d6) δ −117.34. 13C NMR (101 MHz, acetone-d6) δ 161.38 (d, J = 258.4 Hz), 130.26 (d, J = 33.4 Hz), 118.91 (d, J = 15.8 Hz), 106.40 (d, J = 26.5 Hz), 34.01, 32.61, 29.93, 29.91, 24.32, 23.32, 15.99 (d, J = 1.7 Hz), 14.36, −2.05 (d, J = 2.9 Hz).

ultraviolet photoelectron spectroscopy (UPS) and inverse photoelectron spectroscopy (IPES) of thin films on indium tin oxide (ITO) substrates (2 mg/mL solutions in chlorobenzene spin-cast at 4000 rpm). The analysis chamber was equipped with a hemispherical energy analyzer (Specs Phoibos 150) for UPS studies. The UPS measurements were carried out using a He I (hν = 21.22 eV) source. IPES measurements were performed in the isochromat mode using a homemade spectrometer located in the PES analysis chamber, with a resolution of approximately 0.6 eV as determined by the width of the Fermi edge of clean polycrystalline silver. The positions of the Fermi edge were used to align the UPS and IPES energy scales. Thermogravimetric analysis (TGA) measurements were carried out with a Mettler Toledo TGA SDTA 851e apparatus at a heating rate of 10 °C/min under a nitrogen atmosphere. The temperature of degradation (Td) corresponds to a 5% weight loss. Differential scanning calorimetry (DSC) analysis was performed on a PerkinElmer DSC-7 instrument calibrated with ultrapure indium at a scanning rate of 10 °C/min under a nitrogen flow. The WAXS diffraction (powder) measurements were done with a Krytalloflex 760 generator (40 kV, 30 mA), a goniometer, and a two-dimensional Hi-Star detector. A sealed tube emitting at 1.5418 Å (copper Kα) nickel-filtered was used as the source. GADDS software was used to control and to do analysis of all experiments. Melting temperatures were recorded using a Stanford Research Systems Optimelt automated melting point system at a heating rate of 2 °C/min. 2.2. Materials. 3,4-Dibromothiophene (1) was purchased from Combiblocks, and n-octyldimethylchlorosilane was purchased from Gelest. 2,1,3-Benzothiadiazole-4,7-bis(boronic acid pinacol ester) (7) was purchased from Aldrich. 4,7-Di(2-thienyl)-2,1,3-benzothiadiazole (M1) and 4,7-di(3,4-difluorothien-2-yl)-2,1,3-benzothiadiazole (M2) were prepared according to the literature9,20 (Scheme 1). NFluorobenzenesulfonimide (NFSI) was recrystallized in diethyl ether prior to use.21 2.3. Synthesis of Monomers. 2.3.1. Synthesis of 3-Bromo-4(dimethyloctylsilyl)thiophene (2). Compound 1 (4.26 g, 17.6 mmol, 1 equiv) was placed in a dried round-bottom flask with a magnetic stirrer and purged on a Schlenk line. Anhydrous diethyl ether (25 mL) was added, and the solution was cooled to −100 °C using an Et2O/N2 ice bath. Then, a solution of n-BuLi (2.5 M in hexanes 7.4 mL, 18.5 mmol, 1.05 equiv) was added dropwise, and the mixture was left to react for 20 min at −100 °C. Then n-octyldimethylchlorosilane (4.6 mL, 19.4 mmol, 1.1 equiv) was rapidly added, and the reaction mixture was allowed to warm to room temperature and was left to react overnight. The reaction was quenched with a saturated solution of NH4Cl and extracted three times with diethyl ether. The organic phases were combined and washed with water and brine, dried over MgSO4, and concentrated under vacuum. Purification was achieved by vacuum distillation (bp 130−135 °C at 0.50 mmHg), affording the desired compound as colorless oil (Y = 75%). 1H NMR 400 MHz (acetoned6) δ (ppm): 7.62 (d, J = 3.1 Hz, 1H), 7.56 (d, J = 3.0 Hz, 1H), 1.36− 1.22 (m, 12H), 0.94−0.84 (m, 5H), 0.34 (s, 6H). 13C NMR 101 MHz (acetone-d6) δ (ppm): 141.06, 135.56, 125.37, 115.81, 34.11, 32.63, 29.94, 24.50, 23.32, 15.83, 14.38, −2.40. HRMS calculated for C14H25BrSSi (M*+): 332.0624; found: 332.0626 (difference: 0.44 ppm). 2.3.2. Synthesis of 3-Fluoro-4-(dimethyloctylsilyl)thiophene (3). 3-Bromo-4-dimethyloctylsilane (2) (2.41 g, 7.24 mmol, 1 equiv) was placed in a dried round-bottom flask with a magnetic stirrer and was purged on a Schlenk line. Anhydrous THF (30 mL) was added, and the solution was cooled to −100 °C in a Et2O/N2 bath. Freshly recrystallized N-fluorobenzenesulfonimide (NFSI) (2.52 g, 7.96 mmol, 1.1 equiv) was solubilized in 25 mL of anhydrous THF in another round-bottom flask under argon and was also cooled to −100 °C. nBuLi (2.5 M in hexane, 7.6 mmol, 1.05 equiv) was added dropwise into the solution of 2, which was left to react for 25 min. The solution with NFSI was then added dropwise to the organolithium mixture via a cannula while keeping both flasks at −100 °C. The reaction mixture was allowed to warm to room temperature and was left to react overnight. The reaction was quenched with a saturated solution of NH4Cl and extracted three times with diethyl ether. The organic 4660


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recrystallized in a mixture of Et 2O/CHCl3/MeOH by slow evaporation. 1H NMR (400 MHz, chloroform-d) δ 7.90 (d, J = 1.7 Hz, 2H), 7.81 (s, 2H), 6.83 (dd, J = 1.5 Hz, 2H). 19F NMR (376 MHz, chloroform-d) δ −126.59 (d, J = 0.9 Hz). 13C NMR (101 MHz, chloroform-d) δ 158.38 (d, J = 258.6 Hz), 152.35, 137.72 (d, J = 8.8 Hz), 125.93 (d, J = 2.4 Hz), 125.07, 117.67 (d, J = 27.5 Hz), 104.89 (d, J = 20.4 Hz). HRMS calculated for C14H6F2N2S3 (M*+): 335.9656; found: 335.9660 (difference: 1.25 ppm). Tm: 119.0−120.0 °C. 2.3.10. Synthesis of 1,4-Dibromo-2,5-di(2-octyldodecyloxy)benzene (M5). In a round-bottom flask fitted with a condenser 2,5dibromohydroquinone (12.3 g, 46 mmol, 1 equiv) was dissolved in 180 mL of toluene. Then, a solution of KOH (50% m/m, 11 mL, 4 equiv) was added, and the mixture was vigorously stirred and heated at 110 °C. 9-(Bromomethyl)nonadecane (42.7 g, 115 mmol, 2.5 equiv) was added and was left to react overnight. After cooling to room temperature, an aqueous solution of HCl 1M was added, and the mixture was extracted with hexane. The organic phases were combined and washed with water and brine, dried over MgSO4, and concentrated under reduced pressure. The crude product was purified by column chromatography (silica gel; hexane), affording the desired compound as colorless oil (Y = 70%). 1H and 13C NMR data were identical as those previously reported in the literature.22 HRMS calculated for C46H84Br2O2 (M*+): 826.4833; found: 828.4833 (difference: 0.01 ppm). 2.4. General Procedure for the Synthesis of P1−P4 by DHAP. 2.4.1. P1. M5 (0.495 mmol, 1 equiv), M2 (0.495 mmol, 1 equiv), Pd2dba3 (2% mol), (o-OMeC6H4)3P (8 mol %), Cs2CO3 (3 equiv), and pivalic acid (1 equiv) were put in a microwave vial with a magnetic stirring bar. The vial was sealed with a cap and then purged with nitrogen to remove oxygen (3 times). Oxygen-free and anhydrous THF was added (C = 0.25 mol L−1, 2 mL), and the reaction was heated with an oil bath preheated at 120 °C (reaction under pressure) until gelation of the reaction mixture. The reaction was cooled to 65 °C, and then 1 mL of TCB was added. The mixture was poured in methanol/acidified water (10% HCl) (9:1), and the solid was recovered by filtration using a 0.45 μm nylon filter. The polymer was washed using a Soxhlet apparatus with acetone, hexane, dichloromethane, and chlorobenzene (when needed). The dichloromethane fraction was reduced to 5−10 mL and poured in methanol. The polymer was recovered by filtration over a 0.45 μm nylon filter and dry under vacuum (Y = 96%) (dichloromethane fraction). 1H NMR 500 MHz (TCE, 90 °C) δ (ppm): 8.28 (br, s), 8.02 (br, s), 7.77 (br, s), 7.49 (br, s), 4.20 (br, s), 2.10 (br, s), 1.76 (br, s), 1.65 (br, s), 1.55 (br, s), 1.42−1.34 (br, m), 0.95 (br, s). 2.4.2. P2. With a concentration of monomers in THF at C = 0.15 mol L−1 (Y = 93%; chlorobenzene fraction). 1H NMR (TCE 90 °C) δ (ppm): 8.33 (br, s), 7.67 (br, s), 4.20 (br, s), 2.13 (br, s), 1.77 (br, s), 1.68 (br, s), 1.53 (br, s), 1.32 (br, s), 0.95 (br, s). 19F NMR 470 MHz (TCE 90 °C) δ (ppm): −132.3 (br, s), −134.1 (br, s). 2.4.3. P3. With a concentration of monomers in THF at C = 0.15 mol L−1 (Y = 90%; hexane fraction). 1H NMR (TCE 90 °C) δ (ppm): 8.32 (br, s), 7.56 (br, s), 7.42 (br, s), 4.21 (br, s), 2.12 (br, s), 1.80− 1.75 (br, m), 1.71−1.64 (br, m), 1.44−1.33 (br, m), 0.95 (br, s). 19F NMR 470 MHz (TCE 90 °C) δ (ppm): −122.3 (br, s), −124.1 (residual peak, s). 2.4.4. P4. With a concentration of monomers in THF at C = 0.15 mol L−1 (Y = 94%; chlorobenzene fraction). 1H NMR (TCE 90 °C) δ (ppm): 8.09 (br, s), 7.98 (br, s), 7.67 (br, s), 4.17 (br, s), 2.08 (br, s), 1.74 (br, s), 1.63 (br, s), 1.52 (br, s), 1.32 (br, s), 0.94 (br, s). 19F NMR 470 MHz (TCE 90 °C) δ (ppm): −123.6 (br, s), −124.3 (residual peak, s), −126.1 (residual peak, s).

HRMS calculated for C14H24BrFSSi (M*+): 350.0530; found: 350.0529 (difference: −0.19 ppm). 2.3.6. Synthesis of 4,7-Bis(3-fluoro-4-dimethyloctylsilylthien-2-yl)2,1,3-benzothiadiazole (8). Compound 4 (0.7 g, 2 mmol, 2.25 equiv), 2,1,3-benzothiadiazole-4,7-bis(boronic acid pinacol ester) (7) (0.345 g, 0.88 mmol, 1 equiv) were added to a 20 mL microwave tube with magnetic stirrer and was sealed with a cap and then purged with nitrogen to remove the oxygen (3 times). Toluene (oxygen-free, 7.5 mL), K2CO3 (oxygen-free, 2M in water 1.1 mL), Pd(PPh3)4 (0.05 g, 0.04 mmol), and 2 drops of Aliquot 336 were added, and the mixture was stirred and heated to 120 °C for 24 h. After cooling to room temperature, water was added to the mixture and extracted with CHCl3. The organic phases were combined and washed with water and brine, dried over MgSO4, and concentrated under reduced pressure. The crude product was purified by column chromatography (silica gel; CHCl3/hexane 30/70), affording desired compound as orange oil (Y = 56%). 1H NMR (400 MHz, chloroform-d) δ (ppm): 8.19 (s, 2H), 7.39 (d, J = 4.1 Hz, 2H), 1.39−1.23 (m, 24H), 0.91−0.78 (m, 10H), 0.33 (s, 12H). 19F NMR (376 MHz, chloroform-d) δ (ppm): −118.11 (d, J = 4.2 Hz). 13C NMR (101 MHz, chloroform-d) δ 160.71 (d, J = 261.8 Hz), 152.55, 132.41 (d, J = 15.3 Hz), 130.76 (d, J = 35.6 Hz), 127.37 (d, J = 13.0 Hz), 123.23 (dd, J = 5.4, 1.9 Hz), 117.26 (d, J = 15.0 Hz), 33.64, 32.08, 29.42, 23.86, 22.83, 15.58, 14.27, −2.77. HRMS calculated for C34H50S3Si2F2N2 ((M + H)+): 677.2716; found: 677.2701 (difference: −2.15 ppm). 2.3.7. Synthesis of 4,7-Bis(4-fluoro-3-dimethyloctylsilylthien-2-yl)2,1,3-benzothiadiazole (9). Compound 9 was obtained and purified as described for compound 8 (Y = 63%). 1H NMR (400 MHz, chloroform-d) δ 7.61 (s, 2H), 6.89 (d, J = 1.2 Hz, 2H), 1.29−1.15 (m, 24H), 0.87 (t, J = 7.0 Hz, 6H), 0.63−0.55 (m, 4H), −0.06 (d, J = 1.0 Hz, 12H). 19F NMR (376 MHz, chloroform-d) δ −118.03. 13C NMR (101 MHz, chloroform-d) δ 162.48 (d, J = 257.1 Hz), 154.55, 142.82 (d, J = 12.7 Hz), 130.34 (d, J = 1.6 Hz), 130.18, 129.52 (d, J = 32.1 Hz), 104.59 (d, J = 25.3 Hz), 33.59, 32.06, 29.40, 29.38, 23.79, 22.82, 16.09 (d, J = 1.7 Hz), 14.27, −1.97 (d, J = 2.0 Hz). HRMS calculated for C34H50S3Si2F2N2 (M*+): 676.2637; found: 676.2647 (difference: 1.43 ppm). 2.3.8. Synthesis of 4,7-Bis(3-fluorothien-2-yl)-2,1,3-benzothiadiazole (M3). In a round-bottom flask fitted with a condenser, compound 8 (0.273 g, 0.4 mmol, 1 equiv) and trichloroacetic acid (0.712 g, 4 mmol, 10 equiv) were dissolved in 1.6 mL of toluene. The reaction mixture was heated at 135 °C until complete consumption of starting material. The reaction was monitored by TLC (3 h). At the end of reaction and after cooling at room temperature, water was added, and the mixture was extracted with diethyl ether. The organic phases were combined and washed with water and brine, dried over MgSO4, and concentrated under reduced pressure. The crude product was purified by column chromatography (silica gel; CHCl3/hexane 30/70), affording the desired compound as an orange solid (Y = 90%). The compound was recrystallized in a mixture of Et2O/CHCl3/MeOH by slow evaporation. 1H NMR (400 MHz, chloroform-d) δ 8.17 (s, 2H), 7.38 (dd, J = 5.6, 4.0 Hz, 2H), 6.97 (d, J = 5.6 Hz, 2H). 19F NMR (376 MHz, chloroform-d) δ −124.35 (d, J = 4.0 Hz). 13C NMR (101 MHz, chloroform-d) δ 156.48 (d, J = 264.7 Hz), 152.35, 127.41 (d, J = 12.6 Hz), 126.06 (d, J = 10.7 Hz), 123.13 (dd, J = 5.2, 2.2 Hz), 117.85 (d, J = 27.3 Hz), 116.89 (d, J = 11.1 Hz). HRMS calculated for C14H6F2N2S3 (M*+): 335.9656; found: 335.9661 (difference: 1.61 ppm). Tm: 182.2−183.5 °C. 2.3.9. Synthesis of 4,7-Bis(4-fluorothien-2-yl)-2,1,3-benzothiadiazole (M4). In a round-bottom flask, compound 9 (0.455 g, 0.672 mmol, 1 equiv) was dissolved in 3 mL of DMF, and then K2CO3 (0.653 g, 4.7 mmol, 7 equiv) was added. The reaction mixture was stirred at room temperature until complete consumption of starting material. The reaction was monitored by TLC (48 h). At the end of reaction a saturated solution of NH4Cl was added, and the mixture was extracted with diethyl ether. The organic phases were combined and washed with water and brine, dried over MgSO4, and concentrated under reduced pressure. The crude product was purified by column chromatography (silica gel; CHCl3/hexane 30/70), affording the desired compound as orange needles (Y = 90%). The product was

3. RESULTS AND DISCUSSION 3.1. Synthesis of Monomers. As reported by Turkman, viable synthetic procedures for effective and regioselective fluorination of thiophene ring are scarce.23 The few reports shed light on important limitations such lengthy precursor syntheses, harsh fluorinated agent, low overall yields, and, most 4661


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Macromolecules Scheme 2. Synthesis of Pivotal Compounds 3, 4, and 6

identify each signals (due to H−F coupling), TOCSY and NOESY 1H NMR experiments were done (see Supporting Information). The resonance at 7.49 ppm corresponds to Ha (5-position) while the resonance at 6.97 ppm corresponds to Hb (2-position). The 19F NMR spectrum shows a doublet at −126.7 ppm with a constant coupling of 2.9 Hz (inset of Figure 2). Bromination of 3 with N-bromosuccinimide (1.2 equiv) yielded pure 2-bromo-3-fluoro-4-(dimethyloctylsilyl)thiophene (4) in 65% after purification by reverse phase chromatography. The 1H NMR and 19F spectra of 4 (Figure 2) show only a doublet at 7.59 ppm with a coupling constant J = 4.6 Hz and a doublet at −124.5 ppm with the same coupling constant as the one observed in 1H NMR (JHF = 4.5 Hz). The NMR data confirm that the monobromination reaction with NBS occurs exclusively at the 2-position. The regioselectivity can be explained by the mesomeric effect of fluorine atom and/or by a less inductive effect of the silane group compared to alkyl chains. To obtain the other mono-fluorinated isomer 5-bromo3-fluoro-4-(dimethyloctylsilyl)thiophene (6), we used the same synthetic approach reported for the fluorination of poly(3hexylthiophene). Bromination of 3 with an excess of Nbromosuccinimide (2.4 equiv) yielded pure 2,5-dibromo-4fluoro-3-dimethyloctylsilane−thiophene (5) in 70% after purification by reverse phase chromatography. 5-Bromo-3fluoro-4-(dimethyloctylsilyl)-thiophene (6) was obtained in 90% by using i-PrMgCl.LiCl (Turbo Grignard) on 5 and quenching with methanol. The selectivity of the reaction was confirmed by 1H NMR as shown in Figure 2. 4,7-Bis(3-fluoro4-dimethyloctylsilylthien-2-yl)-2,1,3-benzothiadiazole (8) and 4,7-bis(4-fluoro-3-dimethyloctylsilylthien-2-yl)-2,1,3-benzothiadiazole (9) were obtained in 56% and 63%, respectively (Scheme 3). Unlike the synthesis of 4,7-di(3,4-difluorothien-2yl)-2,1,3-benzothiadiazole (M2) reported by Fei et al.,9 we did not observed any cleavage of silyl side chain during the Suzuki cross-coupling reaction. Indeed, 1H NMR spectra of compounds 8 and 9 show that the dimethyloctylsilane protecting group remain on the compounds (see Supporting Information). Compound 8 was treated with an excess of trichloroacetic acid (TCA) in toluene at reflux to afford M3 in 90% yield. Unlike compound 8, treatment of compound 9 with the same synthetic procedure led to the degradation of the starting material. Treatment of compound 9 with K2CO3 in DMF led to 4,7-bis(4-fluorothien-2-yl)-2,1,3-benzothiadiazole (M4) in 90% yield. M5 was prepared according to the literature using longer 2-octyldodecyloxy side chains (compared to 2-hexyldecyloxy) to promote the solubility of the resulting fluorinated copolymers.22 Monomers M2−M4 were recrystallized by

importantly, the lack of selectivity that leads to purification problems. Turkman has reported a procedure for the synthesis of highly volatile 3-fluorothiophene (bp 30−32 °C) with an overall yield of 50% and a purity of 97% estimated by 1H NMR spectroscopy. While Turkman successfully obtained pure 2,5dibromo-3-fluorothiophene, the synthesis of either 2-bromo-3fluorothiophene or 2-bromo-4-fluorothiophene that can be useful for step-efficient synthesis of M3 and M4 was not investigated. By combining our previous work on fluorinated poly(3alkylthiophene)21 that led to the synthesis of regioselective and pure 2-bromo-4-fluoro-3-hexylthiophene and the work of Krebs et al. on polydimethyloctylsilylthiophene,24 we synthesized 3bromo-4-(dimethyloctylsilyl)thiophene (2) from lithiation of 3,4-dibromothiophene (1) (at −100 °C) with n-BuLi followed by treatment with n-octyldimethylchlorosilane (Scheme 2). 3Bromo-4-(dimethyloctylsilyl)thiophene (2) was purified by vacuum distillation and obtained in 90% yield. Then, lithiation of 2 at −100 °C in THF followed by careful addition of Nfluorobenzenesulfonimide (NFSI) via a cannula provided 3fluoro-4-(dimethyloctylsilyl)thiophene (3) in 65% after purification by column chromatography (silica gel). It is worth noting that freshly recrystallized NFSI and monitoring of the reaction temperature (−100 °C) are mandatory to obtain 3 and minimize the formation of traces of 3-dimethyloctylsilylthiophene (dehalogenation side reaction of 2) which cannot be separated from (3). As shown in Figure 2, the 1H NMR spectrum of compound 3 shows a triplet at 7.49 ppm and a doublet of doublet at 6.97 ppm. In order to unambiguously

Figure 2. 1H NMR spectra of compounds (a) 3, (b) 4, and (c) 6 in acetone-d6. 4662


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Macromolecules Scheme 3. Synthetic Pathway for Monomers M3 and M4

Table 1. Molecular Weight and Thermal Properties of P1−P4 polymer

M̅ n (kg mol−1)

M̅ w (kg mol−1)

Đ

yield (%)

P1 P2 P3 P4

52 85 25 97

114 183 50 296

2.2 2.1 2.0 3.1

96 93 90 94

time 66 11 46 40

slow evaporation of diethyl ether−chloroform-methanol mixture to obtain monocrystal. Crystallographic data (see Supporting Information) confirm the positioning of the fluorine atom on the flanking thiophene and show that each monomer is coplanar. 3.2. Synthesis and Characterization of Polymers. Polymerization of M5 with M1−M4 (Scheme 1) was carried out by direct (hetero)arylation polymerization using synthetic conditions known to lead to well-defined materials: equimolar amount of comonomers [ ] = 0.2 mol L−1, Pd2dba3 (2 mol %), (o-OMeC6H4)3P (8 mol %), Cs2CO3 (3 equiv), and pivalic acid (1 equiv) in superheated THF at 120 °C.17,18,25 The reaction was stopped upon gelation of the reaction mixture. After precipitation in methanol, the polymers were purified by successive Soxhlet extraction using acetone, hexane, dichloromethane, and chlorobenzene. The effect of the amount and positioning of the fluorine installed on the flanking thiophene of the DTBT moiety on the reactivity of the DHAP is clear. Indeed, gelation of the reaction mixture for P2 and P4 occurs rapidly (11 and 40 min, respectively) while the polymerization of P1 and P3 was slower. While P2 and P4 are soluble in hot chlorobenzene, P1 and P3 were soluble in dichloromethane at room temperature. As reported in Table 1, high molecular weight was obtained for most polymers. With a number-average molecular weight (M̅ n) of 52 kg mol−1 and molar dispersity of 2.2, P1 has higher molecular weight than its analogue reported in the literature (also synthesized by DHAP; M̅ n = 15 kg mol−1, Đ = 2.1).19 Moreover, P1 has similar M̅ n compared to the highest ones reported for its analogue synthesized by Stille cross-coupling polymerization (M̅ n = 59 kg mol−1, Đ = 3.3).19 As shown in Table 1, the polymerization time for P2 and P4 was short and led to higher molecular weight compared to P1. On the other hand, the polymerization time of P3 was longer and molecular weights lower than those obtained for P2 and P4. Thermal properties were evaluated by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). The polymers exhibited good thermal stability with 5% weight loss at temperature higher than 390 °C. Heating and cooling traces are shown in Figure 3, and data are summarized in Table

h min h min

Tm (°C)

Tc (°C)

ΔH(Tc) (J/g)

Td (°C)

221 254 201 279

213 246 205 268

13.0 12.5 6.0 25

404 394 405 396

Figure 3. DSC traces for heating and cooling for P1−P4 under nitrogen at scanning speed scanning of 10 °C/min. Endothermic transitions point downward.

1. P1, P2, and P4 display sharp melting peaks (221, 254, and 279 °C) and crystallization peaks (213, 246, and 268 °C) while P3 shows broad thermal transition. It is worth noting that melting and crystallization temperatures observed for P1 are lower than those reported by Livi et al. for an analogue polymer using smaller 2-hexyldecyloxy side chains on the phenylene moiety, evidencing the effect of the longer side chain (2octyldodecyloxy) on the interactions between polymer chains. Livi et al. also showed the influence of the molecular weights on the thermal properties of PPDTBT.19 Indeed, lower molecular weight PPDTBTx (M̅ n = 16 kg mol−1, Đ = 2.1) led to Tm of 236 °C and Tc of 220 °C compared to Tm of 265 °C and Tc of 247 °C for PPDTBTy (M̅ n = 59 kg mol−1). Melting temperature (Tm), crystallization temperature (Tc), and enthalpy of crystallization (ΔH(Tc)) were determined by differential scanning calorimetry (DSC) under nitrogen at a scanning speed of 10 °C/min. The degradation temperature (Td) was determined at 5% weight loss via thermogravimetric analysis (TGA). 4663


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Figure 4. X-ray diffractograms of P1−P4.

Since the molecular weights of P2 are higher than P1, it obfuscates the effect of the di-fluorination of the flanking thiophene on BT moiety on the thermal properties. Indeed, we cannot determine if the higher melting temperature observed for P2 (254 °C compared to 221 °C) is due to fluorination or to higher molecular weights. However, since P2 and P4 have similar molecular weights, one can assume that both the higher melting temperature (279 °C) and enthalpy of crystallization (ΔH(Tc) 25 J g−1) observed for P4 compared to P2 are mainly due to better interchain interactions for the polymer with the mono-fluorinated flanking thiophene. On the other hand, the lower melting temperature and enthalpy of crystallization observed for P3 compared to the other polymers are in good agreement with the molecular weight reported in Table 1. To confirm the structural order observed by DSC measurements, X-ray diffraction analyses on powders were performed (Figure 4). P1, P2, and P4 show a d-spacing diffraction peak at 2θ = 4.3 (P1), 4.5 (P2), 3.7 (P3), and 4.4 (P4), corresponding to lamellar distance of 20.55, 19.64, 23.88, and 20.08 Å. Since d1 values correspond to the distance between π-conjugated main chain separated by the long and branched 2-octydecyloxy side chains installed on the phenylene moiety, one can think that the broader peak and longer d1 distance observed for P3 means poor chain packing, which is in good agreement with the DSC traces observed for P3 (Figure 3). Since clear additional diffraction peaks can be found for P1 and P4, a good lamellar organization is expected for these two polymers. A π−π stacking corresponding to the 2θ = 23.6 peak is calculated to be 3.77 Å for P3 and P4. P1 and P2 present a flat band centered near 4.44 Å. According to the DSC traces and X-ray diffractograms, P4 has the most regular structure. In order to explain why the polymerization of M1, M2, and M4 proceeds smoothly while M3 was less efficient, we used DFT calculations to estimate the activation energy of each C− H bond available on the monomers. We have recently demonstrated that these calculations can be useful to rationalize and predict regioselectivity of the DHAP.17 As shown in Figure 5, for M1, a difference in the activation energy (ΔEa) between Hα (23.6 kcal mol−1) and Hβ (29.6 kcal mol−1) is found to be 6 kcal mol−1. Using Arrhenius’s law, it is possible to tentatively estimate a selectivity ratio of the α-position at 120 °C (the temperature of polymerization). For this system, a ratio of

Figure 5. Gibbs free energy of the CMD transition state associated with the activation of α and β hydrogen atoms calculated by DFT.

about 2000/1 favoring Hα can indeed be calculated for the M1 unit. On the other hand, a difference in the activation energy (ΔEa) between Hα and Hγ for M1 is found to be at 7.2 kcal mol−1 (23.6 kcal mol−1 vs 30.8 kcal mol−1), giving a higher selectivity in favor of Hα (about 10000/1). From previous work reported in the literature on the synthesis of benzothiadiazole derivatives by direct (hetero)arylation, the protons on BT are found to be inactive.26,27 For 4,7-di(3,4-difluorothien-2-yl)2,1,3-benzothiadiazole (M2), only one C−H bond is available for the concerted metalation−deprotonation (CMD) step in DHAP with an activation energy calculated at 18.8 kcal mol−1. This lower activation energy value, compared to the activation energy of Hα of M1 (23.6 kcal mol−1), shows the effect of electron-withdrawing fluorine atom on the adjacent C−H bond. For 4,7-bis(3-fluorothien-2-yl)-2,1,3-benzothiadiazole (M3) where the fluorine atoms point toward the BT core, a difference in the activation energy (ΔEa) between Hα (23.7 kcal mol−1) and Hβ (26.4 kcal mol−1) is 2.7 kcal mol−1. Here, theoretical calculations show that the electron-withdrawing fluorine atom strongly modifies the activation energy of the adjacent C−H bond. Indeed, while the activation energy of Hα is 23.7 kcal mol−1 for M3, the activation energy of Hβ is lowered to 26.4 kcal mol−1 compared to 29.6 kcal mol−1 for M1, which leads to a lower selectivity in favor of Hα (20/1) compared to 2000/1 for M1. This estimated lower selectivity may lead to believe that branching could occur on M3 during 4664


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Figure 6. 1H and 19F NMR spectra of P1−P4 in TCE-d4 at 90 °C.

P3 and −123.6 ppm for P4. For P3, in addition to peaks related to end-groups, the broad and ill-defined major peaks could involve some β-branching, as predicted by theoretical calculations and supported by SEC and DSC measurements. Finally, for P4, additional signals (weak intensity) can be observed in both 1H and 19F NMR spectra and as proposed for P1 could be related to end-groups. 3.3. Electro-Optical Properties. The absorption spectra of the polymers in dilute chlorobenzene solution and in the solid state are shown in Figure 7. The data are summarized in Table

polymerization. This is in agreement with the lower molecular weight and crystallinity of P3. Finally, for 4,7-bis(4-fluorothien2-yl)-2,1,3-benzothiadiazole (M4), an activation energy of 18.6 kcal mol−1 for Hα and 27.0 kcal mol−1 for Hγ were calculated. Here, the position of the fluorine atom on the flanking thiophene influences both Hα and Hγ. Indeed, when compare to M1, activation energies are lowered (Hα 18.6 vs 23.6 kcal mol−1; Hγ = 27.0 vs 30.8 kcal mol−1). Although the activation energy of Hγ is decreased by 3.8 kcal mol−1 compared to M1, a selectivity of 45000/1 in favor of Hα over Hγ was calculated, meaning that the polymerization reaction will likely proceed at the α-position and lead to well-defined copolymer. Polymer regioregularity was studied by 1H NMR analysis in deuterated tetrachloroethane (TCE) at 90 °C. 1H NMR spectra of P1−P4 are shown in Figure 6 with emphasis on the aromatic section. It is worth noting that longer alkyl chain (2octyldodecyloxy) allowed well-defined 1H NMR spectra while polymer with smaller 2-hexyldecyloxy side chains on phenylene moiety showed broad signals, even at high temperature due to possible aggregate in solution.19 For P1, the main peaks 1−4 correspond to the protons of the main chain (see Figure 6 for annotation). One can see some extra peaks (low intensities) which can be attributed to end-groups since the apparent degree of polymerization (calculated for SEC measurements) is about 50. However, since no model compounds were available, the assignment of the residual peaks cannot be done accurately. As expected for P2, only the signals for the proton of the phenylene (7.67 ppm) and of the BT core (8.33 ppm) were observed, the weak peak around 7.45 ppm being attributed to one end-group. The inset (19F NMR of P2) shows two signals at −132.3 and −134.1 ppm due to two fluorine atoms on the flanking thiophene. For P3 and P4, the corresponding proton of the thiophene unit appears at 7.42 and 7.98 ppm, respectively. The 19F NMR spectra of these polymers show the presence of mains peaks of fluorine atom at −122.3 ppm for

Figure 7. UV−vis spectra of P1−P4 (in solution, solid line; thin film, dashed line).

2. Each polymer shows a broad absorption spectrum along with a medium bandgap (1.65−1.77 eV). A thin film of P1 shows an important bathochromic shift of the absorption band (68 nm) which indicates a strong intermolecular stacking in the solid state. P1 has a maximum of absorption at 646 nm in the solid state which is lower than its analogue (2-hexyldecyloxy side 4665


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P4 has the highest EA at 3.0 eV. P2 and P3 have intermediate EA values of 2.7 and 2.8 eV, respectively. We should note that IPES is a lower resolution technique than UPS, leading to greater uncertainty in the absolute position of the conduction band minimum (ECBM). However, the values of ECBM listed in Table 2 follow the same trend as a number of distinct features in the total IPES spectra as shown in the upper panel of Figure 8. The calculated electronic transport bandgaps (Eg,PES = IE − EA) also follow the same trend as the optical bandgaps, with P4 having the smallest Eg,PES of 2.2 eV and P2 having the largest Eg,PES of 2.6 eV. P1 and P3 have intermediate Eg,PES values of 2.4 and 2.3 eV, respectively. Eg,PES is expected to be larger than Eg,opt due to exciton binding energies. One can think that the stabilization of the IE observed along with the modulation of the bandgap of the polymers upon fluorination of the flanking thiophene on benzothiadiazole moiety would lead to better photovoltaic performance (higher Voc and Jsc) compared to the non-fluorinated analogues. Fabrication and characterization of polymer solar cells are underway and will be the subject of a future publication.

Table 2. Electro-Optical Properties of P1−P4 λmax (nm) polymer

solution

film

Eg,opt (eV)

IEa (± 0.05 eV)

EAb (± 0.3 eV)

Eg,PES (± 0.3 eV)

P1 P2 P3 P4

582 624 576 652

646 628 592 656

1.73 1.77 1.71 1.65

4.96 5.33 5.07 5.20

2.6 2.7 2.8 3.0

2.4 2.6 2.3 2.2

a IE = hν − (Eonset − EVBM). hν = 21.22 eV. bEA = hν − (Eonset − ECBM). hν = 21.22 eV.

chain on phenylene) synthesized either by Stille cross-coupling (659 nm) or by DHAP (657 nm).19 P2 has a maximum of absorption at a shorter wavelength (628 nm) and a higher bandgap (1.77 eV) than P1. Heeney and co-workers have observed the same trend (hypsochromic shift of the absorption spectrum) for dithienogermole copolymerized with highly fluorinated DTBT comonomer.9 P3 exhibits a maximum of absorption at 592 nm and a clear shoulder near 640 nm, indicating that the fluorine atom facing the BT core does not hinder backbone planarization and might even improve aggregation of the main chain. However, it is worth noting that this polymer has the lowest molecular weight. Finally, P4 (fluorine atom facing the phenylene moiety) has a maximum of absorption at 656 nm and a bathochromic shift of 64 nm compared to P3. With a bandgap of 1.65 eV and a broad absorption spectrum, this polymer has promising optical properties for polymer solar cells. UPS and IPES were used to measure the ionization energies (IEs) and electron affinities (EAs) of thin-film samples of P1− P4, with spectra shown in Figure 8. All four compounds exhibit

4. CONCLUSION In this study, we have shown an efficient way to synthesize and purify different mono-fluorinated thiophene synthons (4, 5, and 6 in Scheme 2) that creates an opportunity for new thiophenebased electron-deficient monomers. Here, the new synthons were utilized for the synthesis of new fluorinated dithienobenzothiadiazole (dTBT) comonomers. Then, the DTBT comonomers (M1−M4) were polymerized with 2,5-di(2octyldodecyloxy)-1,4-dibromophenylene by direct (hetero)arylation polymerization (DHAP) to investigate the effect of the amount and positioning of the fluorine on the physical and electro-optical properties of the D−A copolymers. We found that both the amount and the position of the fluorine atoms also change the monomer reactivity for DHAP. An enhanced reactivity led to shorter polymerization times for P2 (11 min) and P4 (40 min), higher molecular weights (M̅ n = 85 kg mol−1, Đ = 2.1 and M̅ n = 97 kg mol−1, Đ = 3.1) and more regioregular structures. Furthermore, UPS, IPES, and UV−vis absorption experiments confirmed the effect of the fluorination of the flanking thiophene on the electro-optical properties. Indeed, the stronger effect was observed for P4 with EA = 3.0 ± 0.3 eV, Eg,PESS = 2.2 ± 0.3 eV, and Eg,opt = 1.65 eV. The monofluorinated thiophene synthons open up new opportunities for the synthesis of well-defined fluorinated materials with enhanced electro-optical properties for application in organic electronics.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 8. UPS (positive binding energy) and IPES (negative binding energy) spectra of P1−P4. Vertical lines on bottom row of panels demark onset on secondary electrons (Eonset), valence band maximum (EVBM), and conduction band minimum (ECBM). IPES intensity is measured in counts per incident electron.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00905. 1 H, 19F, and 13C NMR spectra; crystallographic data; computational details (PDF)

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similar features; however, the effect of fluorination is evident in the calculated IEs and EAs listed in Table 2. P1, with nonfluorinated thiophene units, has the lowest IE of 4.96 eV, while P2 with di-fluorinated thiophenes has the largest IE of 5.33 eV. P3 and P4, with mono-fluorinated thiophenes, have IEs in between, at 5.07 and 5.20 eV, respectively. The EAs follow a slightly different trend. P1 has the lowest EA at 2.6 eV, while

AUTHOR INFORMATION

Corresponding Author

*(M.L.) E-mail: [email protected]. ORCID

Mario Leclerc: 0000-0003-2458-9633 4666


Article

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Properties of Organic Solar Cells Based on Dithienylbenzothiadiazolealt-phenylene Conjugated Polymers. Macromolecules 2015, 48, 3481− 3492. (16) Heuvel, R.; van Franeker, J. J.; Janssen, R. A. J. Energy Level Tuning of Poly(phenylene-alt- dithienobenzothiadiazole)s for Low Photon Energy Loss Solar Cells. Macromol. Chem. Phys. 2017, 218, 1600502. (17) Bura, T.; Beaupré, S.; Légaré, M. A.; Quinn, J.; Rochette, E.; Blaskovits, J. T.; Fontaine, F. G.; Pron, A.; Li, Y.; Leclerc, M. Direct heteroarylation polymerization: guidelines for defect-free conjugated polymers. Chem. Sci. 2017, 8, 3913−3925. (18) Pouliot, J. R.; Grenier, F.; Blaskovits, J. T.; Beaupré, S.; Leclerc, M. Direct (Hetero)arylation Polymerization: Simplicity for Conjugated Polymer Synthesis. Chem. Rev. 2016, 116, 14225−14274. (19) Livi, F.; Gobalasingham, N. S.; Thompson, B. C.; Bundgaard, E. Analysis of diverse direct arylation polymerization (DArP) conditions toward the efficient synthesis of polymers converging with stille polymers in organic solar cells. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 2907−2918. (20) Blouin, N.; Michaud, A.; Leclerc, M. A Low-Bandgap Poly(2,7Carbazole) Derivative for Use in High-Performance Solar Cells. Adv. Mater. 2007, 19, 2295−2300. (21) Blaskovits, J. T.; Bura, T.; Beaupré, S.; Lopez, S.; Roy, C.; de Goes Soares, J.; Oh, A.; Quinn, J.; Li, Y.; Aspuru-Guzik, A.; Leclerc, M. A Study of the Degree of Fluorination in Regioregular Poly(3hexylthiophene). Macromolecules 2017, 50, 162−174. (22) Jellison, J. L.; Lee, C. H.; Zhu, X.; Wood, J. D.; Plunkett, K. N. Electron Acceptors Based on an All-Carbon Donor−Acceptor Copolymer. Angew. Chem., Int. Ed. 2012, 51, 12321−12324. (23) Turkman, N. PhD Thesis, The University of Texas at Arlington, 2008. (24) Bundgaard, E.; Hagemann, O.; Bjerring, M.; Nielsen, N. C.; Andreasen, J. W.; Andreasen, B.; Krebs, F. C. Removal of Solubilizing Side Chains at Low Temperature: A New Route to Native Poly(thiophene). Macromolecules 2012, 45, 3644−3646. (25) Morin, P. O.; Bura, T.; Sun, B.; Gorelsky, S. I.; Li, Y.; Leclerc, M. Conjugated Polymers à la Carte from Time-Controlled Direct (Hetero)Arylation Polymerization. ACS Macro Lett. 2015, 4, 21−24. (26) Wang, X.; Wang, K.; Wang, M. Synthesis of conjugated polymers via an exclusive direct-arylation coupling reaction: a facile and straightforward way to synthesize thiophene-flanked benzothiadiazole derivatives and their copolymers. Polym. Chem. 2015, 6, 1846− 1855. (27) Matsidik, R.; Martin, J.; Schmidt, S.; Obermayer, J.; Lombeck, F.; Nübling, F.; Komber, H.; Fazzi, D.; Sommer, M. C−H Arylation of Unsubstituted Furan and Thiophene with Acceptor Bromides: Access to Donor−Acceptor−Donor-Type Building Blocks for Organic Electronics. J. Org. Chem. 2015, 80, 980−987.

C. Roy, T. Bura, and S. Beaupré contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC) for their support. The authors thank J. T. Blaskovits for his help in the synthesis of some synthetic precursors.

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