Synthesis and Characterization of All-Conjugated Graft Copolymers

Feb 26, 2013 - (1-5) Poly(3-hexylthiophene) (P3HT) is one of the most studied ... In this study, the externally initiated KCTP method was employed to ...
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Synthesis and Characterization of All-Conjugated Graft Copolymers Comprised of n‑Type or p‑Type Backbones and Poly(3hexylthiophene) Side Chains Jin Wang,†,∥ Chien Lu,‡,∥ Tetsunari Mizobe,† Mitsuru Ueda,† Wen-Chang Chen,*,‡ and Tomoya Higashihara*,†,§ †

Department of Organic and Polymeric Materials, Graduated School of Science and Engineering, Tokyo Institute of Technology, 2-12-1-H120, O-okayama, Meguro-ku, Tokyo, 152-8552, Japan ‡ Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan 10617 § Japan Science and Technology Agency (JST), 4-1-8, Honcho, Kawaguchi, Saitama 332-0012, Japan S Supporting Information *

ABSTRACT: All-conjugated graft copolymers containing poly(3-hexylthiophene) (P3HT) side chains and both of p-type and n-type backbones that are connected with all π-conjugated linkages were synthesized via a two-step method involving the Stille coupling reaction and Kumada catalyst-transfer polycondensation (KCTP). A series of naphthalene diimide copolymers with different compositions of 3-(4′-chloro-3′tolyl)thiophene (CTT) units (PNDICTT) were designed as ntype backbones, while the poly(3-(4′-chloro-3′-tolyl)thiophenealt-thiophene) (PCTT) was designed as a p-type backbone which were converted into n-type or p-type macroinitiators, and P3HT side chains were then in situ grafted from the macroinitiators via an externally initiated KCTP at room temperature. By using this newly developed two-step method for the synthesis of all-conjugated graft copolymers, the number of P3HT side chains in the graft copolymers can be simply controlled by varying the composition of the CTT units in PNDICTT. Meanwhile, the chain length of P3HT was controllable by varying the feed molar ratio of the thiophene monomer to CTT unit during the KCTP. The optical and electrochemical properties of the all-conjugated graft copolymers were investigated by UV−vis, cyclic voltammetry (CV), and organic field-effect transistor (OFET) measurements. Moreover, the differential scanning calorimetry (DSC) and grazing incident wide-angle X-ray scattering (GIWAXS) results revealed that there were two distinguished crystalline domains in the thin films of the graft copolymer. The morphology of the graft copolymers was first observed by transmission electron microscopy (TEM), in which there was a microphase separation between the PNDICTT and P3HT domains, and the P3HT domains showed partial nanofibril structures with a width of 10−20 nm.

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P3HT-b-poly(methyl acrylate),12 P3HT-b-poly(2-vinylpyridine),13 and PS-b-P3HT-b-PS.14 Moreover, by using the sequential monomer addition technique in the quasi-living Grignard metathesis (GRIM) polymerization, also known as the Kumada catalyst-transfer polycondensation (KCTP),15−17 a variety of all-conjugated block copolymers including P3HT-bpoly(dodecylthiophene),9 P3HT-b-poly(3-(2-(2methoxyethoxy)ethoxy)methylthiophene),18 P3HT-b-poly(3phenoxymethylthiophene),19 P3HT-b-poly(3-(2-ethylhexyl)thiophene), 20 P3HT-b-poly(3-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)methylthiophene),21 poly(3-butylthiophene)-bpoly(3-octylthiophene) (P3BT-b-P3OT),22 and P3BT-bP3HT23 have been prepared.

INTRODUCTION Bulk-heterojunction (BHJ) photovoltaic devices, using the blending of donor−acceptor conjugated polymers in their active layers based on a solution-processable fabrication, have been widely investigated because of their promising advantages, such as light weight, cost-effective manufacturing, and the possibility of flexible large-area devices for future industrial applications.1−5 Poly(3-hexylthiophene) (P3HT) is one of the most studied polymers due to its high mobility, stability, and relatively efficient light absorption in the visible range of the solar spectrum.6,7 Recently, P3HT and related materials have received further attention especially after the independent discovery of the chain-growth polycondensation of the regioregular P3HT with a well-defined molecular weight and low polydispersity by McCullough et al.8,9 and Yokozawa et al.10,11 Previously, various block copolymers containing P3HT segments were reported, such as P3HT-b-polystyrene (PS),12 © 2013 American Chemical Society

Received: January 7, 2013 Revised: February 15, 2013 Published: February 26, 2013 1783

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Interestingly, the externally initiated KCTP has been developed for synthesizing regioregular P3HTs with welldefined α-end groups, such as ethynylene, alcohol, tosylate, azide, amine, and carboxylic acid.24 By using hexaphenylbenzene and V-shaped and Y-shaped core compounds as initiators, 6-arm25, 2- and 3-arm26 all-conjugated branched P3HTs were also successfully prepared. Most recently, all-conjugated block copolymers comprising both the n-type and p-type have been reported, in which the Br-end-functional P3HT was first prepared by KCTP, and then it was reacted with n-type monomers or chain-end-functional n-type polymers via the Stille coupling reaction.27−32 Nevertheless, to the best of our knowledge, there have never been reports concerning the allconjugated P3HT graft polymers with a conjugated backbone and controlled length of the P3HT side chains. Furthermore, the preparation of the all-conjugated graft copolymers with both n-type and p-type segments is a big challenge, especially when aiming at the single-component organic photovoltaic (OPV) application, in which the n-type and p-type domains may possibly be phase-separated via ideal distributing lengths of 10−20 nm which correspond to the exciton diffusion length. In this study, the externally initiated KCTP method was employed to synthesize a series of all-conjugated graft copolymers with P3HT side chains and both p-type and ntype backbones at ambient temperature via a one-pot grafting from process for the first time. As presented in Schemes 1 and 2, both the n-type and p-type polymers with different numbers of initiation sites (CTT units) for the KCTP were prepared via the Stille coupling reaction, and then the P3HT side chains were polymerized from the precursors via the externally initiated KCTP. The structures, optical and electrochemical properties, and crystalline and phase-separated morphologies of these newly developed all-conjugated graft copolymers were investigated in detail.

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Scheme 1. Synthetic Process of (a) CTT Monomer, (b) nType Macromoinitiator PNDICTT, and (c) p-Type Macroinitiator PCTT

EXPERIMENTAL SECTION

Materials and Characterization. All reagents were purchased from commercial sources and used without further purification unless otherwise noted. N,N′-Bis(2-decyltetradecyl)-2,6-dibromonaphthalene-1,4,5,8-bis(dicarboximide) (NDI-Br2) (monomer 3) and 2,5bis(trimethylstannyl)thiophene (monomer 4) were prepared according to literature procedures.1,2 1 H and 13C NMR spectra were recorded on a Bruker DPX (300 MHz) in DMSO-d6 (40 °C) or CDCl3 (25 °C) using tetramethylsilane as the internal standard. Size exclusion chromatography (SEC) measurements were carried out on a Jasco GULLIVER 1500 equipped with a pump and an absorbance detector (UV, λ = 254 nm) using polystyrene standards. THF was used as an eluent at a flow rate of 1.0 mL/min at 40 °C. UV−vis absorption spectra of the polymer solutions in chloroform and films were recorded on a Jasco V-560 spectrometer over a wavelength range of 200−1000 nm. Matrix-assisted laser desorption ionization with time-of-flight (MALDI-TOF) mass spectra were recorded on a Kratos Kompact MALDI instrument operated in a linear detection mode to generate positive ion spectra using 2,2′:5′,2″terthiophene as the matrix and THF as a solvent. Elemental analyses were performed on a Yanaco MT-6 CHN recorder elemental analysis instrument. DSC analysis was performed on a Seiko EXSTAR 6000 DTA 6300 thermal analyzer at a TA Instruments Q-100 connected to a cooling system at a heating rate of 10 °C min−1. CV was performed with the use of a three-electrode cell in which ITO was used as a working electrode, and the polymer film was coated on it in 0.5 × 0.7 cm2. A platinum wire was used as an auxiliary electrode. All cell potentials were taken in a 0.1 mol/L acetonitrile solution of tetrabutylammonium perchlorate at a scan rate of 0.1 V/s with the use of a homemade Ag/AgCl, KCl(sat.) reference electrode. GIWAXS experiments were conducted at the SPring-8 on beamline BL19B2.

The samples were irradiated at a fixed incident angle on the order of 0.12° through a Huber diffractometer, and the GIWAXS patterns were recorded with a 2-D image detector (Pilatus 300 K). GIWAXS patterns were recorded with an X-ray energy of 12.39 keV (λ = 1 Å). The samples for GIWAXS were prepared by drop-casting the polymer solution on the Si/SiO2 substrate. TEM was performed using a JEOL 1230 operated at an acceleration voltage of 100 kV; the samples were prepared from the polymer solution (10 mg/3 mL in chloroform) via slow evaporation over 1 week. After drying, the films were annealed at 240 °C for 10 min and at 100 °C for 6 h. Synthesis of 3-(4′-Chloro-3′-tolyl)thiophene (CTT) (1). The mixture of 3-thiopheneboronic acid (0.281 g, 2.2 mmol), 2-chloro-5iodotoluene (0.505 g, 2.0 mmol), Pd(PPh3)4 (57.8 mg, 0.050 mmol), toluene (20 mL), and 2 M K2CO3 aqueous solution (5 mL) was stirred at 100 °C for 20 h under a nitrogen atmosphere. After cooling to room temperature, the mixture was washed with water and the organic layer was dried over MgSO4 and evaporated. The residue was purified with column chromatography (hexane) to yield 1 as a white powder (0.364 g, 87%). 1H NMR (300 MHz, DMSO-d6, ppm, 40 °C): δ = 7.85−7.87 (H, q, Ar−H), 7.71−7.72 (H, d, J = 2.1 Hz, Ar−H), 7.61−7.64 (H, q, Ar−H), 7.53−7.57 (2H, m, Ar−H), 7.40−7.43 (H, d, J = 3.6 Hz, Ar−H), 2.38 (3H, s, CH3). 13C NMR (300 MHz, DMSOd6, ppm, 40 °C): δ = 140.7, 136.2, 134.4, 132.2, 129.5, 129.1, 127.5, 126.5, 125.5, 121.7, and 19.9; mp 74.8−76.0 °C. Anal. Calcd for C11H9ClS: C, 63.27%; H, 4.35%. Found: C, 63.21%; H, 4.31%. Synthesis of 2,5-Dibromo-3-(4′-chloro-3′-tolyl)thiophene (DBCTT) (2). N-Bromosuccinimide (0.427 g, 2.4 mmol) was added to a stirred solution of 1 (0.200 g, 0.096 mmol) in 20 mL of CHCl3/ 1784

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Scheme 2. Synthetic Schemes of (a) Model Reaction of KCTP Initiated by CTT, (b) All-Conjugated Grafting P3HT, and (c) All-Conjugated Graft Copolymers Initiated by n-Type Precursor

precipitated solid was collected by filtration under reduced pressure. Poly(3-(4′-chloro-3′-tolyl)thiophene-alt-thiophene) (PCTT) was washed with methanol, and poly(N,N′-bis(2-decyltetradecyl)-1,4,5,8naphthalene diimide-alt-3-(4′-chloro-3′-tolyl)thiophene) (PNDICTTxy: x and y indicate the feed molar ratio of 3 to 2) was subjected to sequential Soxhlet extraction with methanol, acetone, and chloroform. Finally, the polymer solution in chloroform was passed through a silica gel column. The polymers are finally freeze-dried from dehydrated benzene before use. For preparing PNDICTTxy series, the feed molar ratio (x:y) of 3 to 2 is 9:1, 7:3, and 5:5, respectively. PCTT: deep red solid (44.0 mg, 56%). 1H NMR (CDCl3, 300 MHz, ppm): δ = 6.67−7.33 (6nH, m, Ar−H), 2.26−2.47 (3H, br, t, J = 31.5 Hz, CH3). PNDICTT91: deep blue solid (284 mg, 86%). 1H NMR (CDCl3, 300 MHz, ppm): δ = 8.97 (2xH, s, Ar−H), 7.45 (2xH, s, Ar− H), 7.35 (6yH, s, Ar−H), 4.14 (4xH, br, N−CH2−C(C)2), 2.42−2.44 (3yH, d, J = 6.0 Hz, CH3 in CTT), 2.03 (2xH, br, C−CH(C)2), 1.19− 1.34 (80xH, br, −CH2−), 0.81−0.85(12xH, m, CH3). PNDICTT73: blue solid (234 mg, 72%). 1H NMR (CDCl3, 300 MHz, ppm): δ = 8.84−8.98 (2xH, m, Ar−H), 7.44 (2xH, s, Ar−H), 7.34 (6yH, s, Ar− H), 4.13 (4xH, br, N−CH2−C(C)2), 2.39−2.43 (3yH, d, J = 12.0 Hz, CH3 in CTT), 2.01 (2xH, br, C−CH(C)2), 1.19−1.32 (80xH, br, −CH2−), 0.81−0.85 (12xH, m, CH3). PNDICTT55: dark green solid (209 mg, 88%). 1H NMR (CDCl3, 300 MHz, ppm): δ = 8.73−8.96

CH3COOH (1:1 in volume) at room temperature under a nitrogen atmosphere. The mixture was then stirred for 24 h and quenched by water. It was washed with distilled water, the organic layer was dried with Na2CO3 and filtrated, and the solvents were removed by rotary evaporation. The residue was purified with column chromatography (hexane) to yield 2 as a white solid (0.197 g, 56%). 1H NMR (300 MHz, DMSO-d6, ppm, 40 °C): δ = 7.52−7.53 (H, d, J = 2.1 Hz, Ar− H), 7.48−7.51 (H, d, J = 8.4 Hz, Ar−H), 7.37−7.41 (H, q, Ar−H), 7.37 (H, s, Ar−H), 2.38 (3H, s, CH3). 13C NMR (300 MHz, DMSOd6, ppm, 40 °C): δ = 141.21, 136.15, 133.63, 132.61, 132.58, 131.52, 129.34, 127.85, 111.46, 108.05, and 19.89; mp 66.2−67.8 °C. Anal. Calcd For C11H7ClBr2S: C, 36.06%; H, 1.93%. Found: C, 36.11%; H, 2.08%. General Procedure for Polymerization of Precursors. To a flask, 1 equiv of 2 and 1 equiv of monomer 4 were placed under argon followed by adding 0.025 equiv of Pd(PPh3)2Cl2. The flask and its contents were subjected to three pump/purge cycles with argon followed by addition of anhydrous and degassed toluene (10 mL) via a syringe. The reaction mixture was stirred at 90 °C for 2 days, followed by adding 2-(tributylstannyl)thiophene and 2-bromothiophene in sequence. After cooling to room temperature, the deeply colored mixture was dripped into 100 mL of vigorously stirred methanol (containing 5 mL of 12 M hydrochloric acid). After stirring for 3 h, the 1785

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Table 1. Compositions, Mns, PDIs, and Yields of PNDICTT and PCTT molar ratio of 3:2

a

entry

feed

founda

Mn (NMR)a (kDa)

Mn (SEC)b (kDa)

PDI (SEC)b (kDa)

no. of CTTa

yield (%)

PNDICTT91 PNDICTT73 PNDICTT55 PCTT

9:1 7:3 1:1

8.8:1 7.1:3 1.1:1

15.8 6.9 5.0

21.7 10.4 7.39 2.10

2.04 2.01 2.08 2.30

1.7 2.5 3.4 7.2b

86 72 88 56

Calculated by 1H NMR. bDetermined by SEC using THF as eluent at 40 °C.

(2xH, m, Ar−H), 7.16−7.43 (2xH + 6yH, m, Ar−H), 4.13 (4xH, br, N−CH2−C(C)2), 2.39−2.42 (3yH, d, J = 9.0 Hz, CH3 in CTT), 2.00 (2xH, br, C−CH(C)2), 1.19−1.32 (80xH, br, −CH2−), 0.81−0.85 (12xH, m, CH3). Model Reaction of P3HT Initiated by CTT via KCTP. KCTP was carried out in a glovebox under a purified nitrogen atmosphere. To a flask, 1 (6.3 mg, 0.030 mmol), Ni(COD)2 (8.3 mg, 0.030 mmol), and PPh3 (23.6 mg, 0.090 mmol) were placed and dissolved in 2 mL of toluene. The mixture was stirred for 24 h at room temperature, and the color was changed from dark red to yellow. After adding dppp (24.7 mg, 0.060 mmol), the mixture was stirred for another 2 h, and the mixture became light orange and transparent. In the meantime, 2,5dibromo-3-hexylthiophene (294 mg, 0.90 mmol) was dissolved in 40 mL of THF containing LiCl (38.0 mg, 0.90 mmol). Isopropylmagnesium chloride (0.495 mL, 0.99 mmol) was then added at room temperature and stirred for 30 min. Then, the solution was transferred into the 1/Ni mixture. After 5 h, 5 mL of 5 M HCl was added to quench the polymerization. The reaction mixture was stirred for 15 min and poured into 300 mL of methanol/water (2:1 in volume) mixture. The materials were isolated by filtration, washed with methanol and acetone, and dried in vacuo (119.9 mg, 80%). 1H NMR (CDCl3, 300 MHz, ppm): δ = 7.40−7.51 (6H, m, Ar−H in CTT), 6.98 (nH, s, Ar−H in P3HT), 2.78−2.83 (2nH, t, J = 7.5 Hz, Ar− CH2−C−), 2.55 (3H, s, CH3 in CTT), 1.71 (2nH, m, Ar−C−CH2− C−), 1.34−1.44 (6nH, m, Ar−C2−(CH2)3−C), 0.89−0.93 (3nH, t, J = 6.0 Hz, CH3 in P3HT). Synthesis of PCTT-g-P3HT. To a flask containing PCTT (24.0 mg, 1.14 × 10−5 mol), Ni(COD)2 (22.0 mg, 8.02 × 10−5 mol), and PPh3 (63.0 mg, 24.1 × 10−5 mol), 4 mL of toluene was added at ambient temperature in a glovebox. The mixture was stirred for 24 h. After adding dppp (66.0 mg, 0.16 mmol), a freshly prepared a 2bromo-5-chloromagnesio-3-hexylthiophene (5) (20 equiv to Ni(COD)2) solution in 40 mL of THF was added to the suspension. The solution was then stirred for 5 h at room temperature. 5 mL of 5 M HCl was added to quench the polymerization. The reaction mixture was stirred for 15 min and poured into 300 mL of methanol/water (2:1 in volume). The materials were isolated by filtration and subjected to sequential Soxhlet extraction with methanol, acetone, and chloroform. The polymer solution in chloroform was passed through silica gel column. After evaporation, the polymer was finally freezedried from the dehydrated benzene solution. (252 mg, 88%). 1H NMR (CDCl3, 300 MHz, ppm): δ = 7.41−7.71 (6xH, m, Ar−H in PCTT), 6.98 (nH, s, Ar−H in P3HT), 2.80 (2nH, br, Ar−CH2−C−), 2.37 (3xH, br, CH3 in PCTT), 1.71 (2nH, m, Ar−C−CH2−C−), 1.34− 1.44 (6nH, m, Ar−C2−(CH2)3−C), 0.91 (3nH, br, CH3 in P3HT). Synthesis of PNDICTTxy-g-P3HT. The synthetic procedure of the graft copolymers is the same as the model reaction. The typical experiment of PNDICTT91-g-P3HT is as follows: To a flask presented in a glovebox, PNDICTT91 (143 mg, 0.65 × 10−5 mol), Ni(COD)2 (4.2 mg, 1.54 × 10−5 mol), and PPh3 (12.1 mg, 4.62 × 10−5 mol) were dissolved in 2 mL of toluene and stirred for 24 h; the color of the mixture was dark purple. After addition of dppp (12.7 mg, 3.08 × 10−5 mol) and stirring for 2 h, the color of the mixture became blue. Then freshly prepared 5 (64 equiv to Ni(COD)2) solution in 40 mL of THF was added into the mixture, and the solution was stirred for 5 h at room temperature. 5 mL of 5 M HCl was then added to quench the polymerization. The reaction mixture was stirred for 15 min and poured into 300 mL of methanol/water (2:1 in volume). The materials were isolated by filtration and subjected to sequential Soxhlet

extraction with methanol, acetone, and chloroform. The polymer solution in chloroform was passed through silica gel column, followed by freeze-drying from the dehydrated benzene solution. PNDICTT91g-P3HT: deep purple solid (282 mg, 95%). 1H NMR (CDCl3, 300 MHz, ppm): δ = 8.97 (2xH, s, Ar−H), 7.45 (2xH−2yH, s, Ar−H), 7.35 (8yH, br, Ar−H), 6.98 (nH, s, Ar−H in P3HT), 4.13 (4xH, br, N−CH2−C(C)2), 2.78−2.83 (2nH, t, J = 7.5 Hz, Ar−CH2−C−), 2.43 (3yH, br, CH3 in CTT), 2.02 (2xH, br, C−CH(C)2), 1.71 (2nH, m, Ar−C−CH2−C−), 1.34−1.44 (6nH, m, Ar−C2−(CH2)3−C in P3HT), 1.19−1.30 (80xH, br, −CH2− in PNDICTT), 0.91 (3nH, br, CH3 in P3HT), 0.85 (12xH, m, CH3 PNDICTT). PNDICTT73-gP3HTa: deep purple solid (248 mg, 93%). 1H NMR (CDCl3, 300 MHz, ppm): δ = 8.95 (2xH, br, Ar−H), 7.35 (2xH+8yH, br, Ar−H), 6.98 (nH, s, Ar−H in P3HT), 4.12 (4xH, br, N−CH2−C(C)2), 2.78− 2.83 (2nH, t, J = 7.5 Hz, Ar−CH2−C−), 2.43 (3yH, br, CH3 in CTT), 2.0 (2xH, br, C−CH(C)2), 1.70 (2nH, br, Ar−C−CH2−C−), 1.34− 1.44 (6nH, m, Ar−C2−(CH2)3−C in P3HT), 1.22 (80xH, br, −CH2− in PNDICTT), 0.85−0.93 (3nH +12xH, br, −CH3 in P3HT and −CH3 in PNDICTT). PNDICTT55-g-P3HT: deep purple solid (241 mg, 90%). 1H NMR (CDCl3, 300 MHz, ppm): δ = 8.81−8.95 (2xH, d, br, Ar−H), 7.34 (2xH+8yH, br, Ar−H), 6.98 (nH, s, Ar−H in P3HT), 4.10 (4xH, br, N−CH2−C(C)2), 2.78−2.83 (2nH, t, J = 7.5 Hz, Ar− CH2−C−), 2.43 (3yH, br, CH3 in CTT), 2.02 (2xH, br, C−CH(C)2), 1.70 (2nH, m, Ar−C−CH2−C−), 1.33−1.43 (6nH, m, Ar−C2− (CH2)3−C in P3HT), 1.21 (80xH, br, −CH2− in PNDICTT), 0.91 (3nH + 12xH, br, −CH3 in P3HT and −CH3 in PNDICTT). Fabrication and Characterization of Thin Film Transistors (TFT). Highly doped n-type Si (100) wafers were used as substrates. A 300 nm SiO2 layer (capacitance per unit area C0 = 10 nF cm−1) as a gate dielectric was thermally grown onto the Si substrates. These wafers were cleaned in piranha solution, a 7:3 (v/v) mixture of H2SO4 and H2O2, rinsed with deionized water, and then dried by nitrogen. The octadecyltrichlorosilane (OTS)-treated surfaces on SiO2/Si substrates were obtained by the following procedure: a clean SiO2/ Si substrate was immersed into a 10 mM solution of octadecyltrichlorosilane in toluene at 80 °C for 2 h. Then the substrates were rinsed with toluene and dried with a steam of nitrogen. FET devices were deposited by spin-coating from chloroform/chlorbenzene (7−10 mg/mL) at a spin rate of 1000 rpm for 60 s and annealing at 140 °C for 60 min. The top-contact source and drain electrodes were defined by 100 nm thick gold through a regular shadow mask, and the channel length (L) and width (W) were 50 and 1000 μm, respectively. FET transfer and output characteristics were recorded in a nitrogen-filled glovebox by using a Keithley 4200 semiconductor parametric analyzer.

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RESULTS AND DISCUSSION Preparation of n-Type (PNDICTTxy) and p-Type (PCTT) Macroinitiators. Recently, the externally initiated KCTP has been developed to synthesize various regioregular P3HTs with specific initiators.33 Because of the living nature of the KCTP, well-defined all-conjugated star-shaped P3HT25,26 and P3HT brushes34,35 have been synthesized. Interestingly, Luscombe and co-workers reported that the thiophene monomers were successfully polymerized by the externally initiated KCTP from o-chlorotoluene with a quantitative initiation efficiency.36 Therefore, it would be possible to prepare the all-conjugated block copolymers containing a regioregular P3HT segment by 1786

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Figure 1. MALDI-TOF mass spectrum of CTT-P3HT in model reaction.

the externally initiated KCTP, in which a conjugated polymer was first prepared with functional groups, such as Ar−Br or Ar−Cl, at the chain end(s) that can subsequently be used as a macroinitiator for the KCTP of thiophene monomers. More interestingly, if the Ar−Br or Ar−Cl groups were introduced on the backbone of a conjugated polymer, the all-conjugated graft copolymers bearing P3HT side chains could be prepared. Therefore, we proposed a two-step method to synthesize a series of all-conjugated graft copolymers comprised of the ntype or p-type backbone and P3HT side chains. As shown in Schemes 1 and 2, a series of n-type naphthalene diimide (NDI)-based and p-type thiophene-based backbone polymers were synthesized via the Stille coupling reaction under the similar conditions previously reported,37−39 followed by grafting from the initiation sites via the KCTP. First, 2,5-dibromo-3-(4′-chloro-3′-tolyl)thiophene (DBCTT) (2) was designed as a key compound and synthesized as illustrated in Scheme 1a. By varying the feed molar ratio of 3 to 2, PNDICTTxy with different compositions of CTT units were prepared via the Stille coupling reaction (Scheme 1b). As there is an o-chlorotoluene group in the CTT unit, the all-conjugated graft copolymers with P3HT side chains could be prepared via the KCTP initiated by PNDICTT. Based on the assumption of one P3HT side chain is initiated by one CTT unit, the number of P3HT side chains can be controlled by incorporating a different number of CTT units into the PNDICTT. As a control experiment, PCTT, which has one CTT group in each repeat unit, was prepared (Scheme 1c). The yields, compositions, and molecular weights of the products are summarized in Table 1. The structures of the products were confirmed by 1H NMR (Figures S1−4 in Supporting Information), and the number-

Figure 2. 1H NMR spectra of (A) PCTT, (B) PCTT-g-P3HT, and (C) SEC traces of PCTT and PCTT-g-P3HT.

Figure 3. 1H NMR spectra of PNDICTT73 and PNDICTT73-gP3HTb.

Table 2. Yields and Mn Values of the All-Conjugated Graft Copolymers

a

entry

feed ratio 5:Ni cat.

Mn (theory)a (kDa)

Mn (NMR)b (kDa)

Mn (SEC)c (kDa)

PDI (SEC)

PNDICTT91-g-P3HT PNDICTT73-g-P3HTa PNDICTT73-g-P3HTb PNDICTT55-g-P3HT PCTT-g-P3HT

64:1 25:1 60:1 25:1 20:1

26.4 11.2 16.9 9.2 25.9

31.2 16.8 26.4 15.0 35.7

27.6 11.5 18.8 12.1 33.7

1.44 1.52 1.33 1.40 2.20

weight ratiob P3HT:PNDI 0.92:1 1.5:1 4.5:1 2.7:1

yield (%) 95 93 87 90 89

Theoretical values based on 100% conversion. bCalculated by 1H NMR results. cDetermined by SEC using THF as eluent. 1787

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aromatic proton peak (8.96 ppm) from the NDI. Based on the molar ratio of the CTT unit to NDI unit in PNDICTTxy, the Mn of these n-type polymers were calculated by comparing the integral area of the aromatic proton peak (7.44 ppm) from the thiophene unit (adjacent to NDI) and that from the aromatic proton peak (7.34 ppm) from the CTT and the chain ends. The Mn of PCTT cannot be determined by 1H NMR as the peaks of the chain end are overlapped with the aromatic protons from the CTT units (Figure S4). As can be seen in Table 1, the found molar ratios of the NDI and CTT units in PNDICTTxy are almost the same as the feed molar ratios, which indicates that the content of the CTT units in the PNDICTTxy is controllable. Consequently, the number of P3HT side chains in the graft copolymers could also be predetermined. The Mn of PNDICTT91 determined by SEC is around 22 kDa, which is similar to the reported results for typical n-type polymers.37−39 However, with the increasing feed molar ratio of 2 to 3, the Mn of the products gradually decreased. For the PCTT, in which 100% of 2 was used (without 3), only an Mn of ca. 2 kDa can be obtained due to the poor solubility of PCTT, which precipitated out during the polymerization. Thus, the decreasing Mn values of the PNDICTT series with a higher content of the CTT unit possibly resulted from a poorer solubility of the product containing the higher content of the CTT unit. It is worth pointing out that the Mn values of PNDICTTxy determined by SEC are higher than that determined from the 1H NMR. The reason should be the overestimated of Mn values by SEC due to the rigid-rod-like structure as well as aggregation of PNDICTTxy, as have been reported elsewhere.40,41 Therefore, the average number of CTT units in the PNDICTT series is estimated based on the Mn values determined by 1H NMR and the found molar ratio of CTT and NDI units. As illustrated in Table 1, the average number of CTT unit in PNDICTT91 is 1.7, while that in PNDICTT55 is increased to 3.4. Model Reaction of KCTP Polymerization of P3HT Initiated by CTT. The CTT units in the PNDICTT series, which could potentially initiate the polymerization of P3HT via KCTP, is not exactly the same as o-chlorotoluene and any other reported initiators.33,36 Therefore, a model reaction was first carried out using 1 as the initiator. As described in Scheme 2a, 1 equiv of Ni(COD)2 and 3 equiv of PPh3 were added to 1 equiv of 1 in toluene, and then the mixture was stirred for 24 h. The color of the solution changed from dark red to yellow upon the addition of Ni(COD)2 and PPh3 similar to the literature,26,36 which indicated that the Ni complex had been formed by incorporating Ni species into the CTT unit.24 Then, 2 equiv of 1,3-diphenylphosphinopropane (dppp) was added for the ligand exchange, and the reaction mixture was allowed to stand for 2 h. Subsequently, the thiophene monomer 5, which was prepared in situ by the Grignard exchange reaction of 2,5dibromo-3-hexylthiophene and isopropylmagnesium chloride in THF, was added to the initiator solution containing the 1/Ni complex. The 1H NMR spectra of the product shown in Figures S5 and S6 (Supporting Information) indicate that the P3HT chain was initiated from the CTT group, as signals of the protons from the aromatic rings (7.41−7.50 ppm) and the methyl group (2.55 ppm) of CTT, and that from P3HT (6.98 ppm for the proton at 4-position) can be finely ascribed. Moreover, the signal of the methyl protons of the CTT group in the product shows a clear downfield shift compared to that of 1. In addition, the MALDI-TOF mass spectrum of the product presented in

Figure 4. UV−vis spectra of all-conjugated graft copolymers and PNDICTT91 in the film states.

Table 3. LUMO, HOMO, and Energy Band Gap Values of PNDICTT91 and Graft Copolymers entry PNDICTT91 PNDICTT91-g-P3HT PNDICTT73-gP3HTa PNDICTT55-g-P3HT PCTT-g-P3HT

LUMOa (eV)

HOMOb (eV)

c Eele g (eV)

d Eopt g (eV)

−4.08 −2.58 −2.61

−5.89 −4.95 −4.92

1.81 2.37 2.31

1.74 1.89 1.89

−2.72 −2.97

−4.78 −4.76

2.06 1.79

1.89 1.92

a

Calculated from the onset potential of reduction by cyclic voltammetry. bCalculated from the onset potential of oxidation by cyclic voltammetry. cCalculated from the equation Eele g = LUMO (eV) − HOMO (eV) based on cyclic voltammetry. dCalculated from the onset wavelengths in UV−vis spectroscopy.

Figure 5. Cyclic voltammograms of (A) PCTT-g-P3HT, (B) PNDICTT91, (C) PNDICTT91-g-P3HT, (D) PNDICTT73-g-P3HTa, and (E) PNDICTT55-g-P3HT.

average molecular weight (Mn) values of the products were determined by SEC and 1H NMR. The contents of the CTT (or molar ratio of CTT to NDI) in PNDICTTxy can be determined by comparing the integral area of the methyl proton peak (2.44 ppm) from the CTT and that of the 1788

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Table 4. FET Characteristics of PNDICTT91 and All-Conjugated Graft Copolymersa annealedb

nonannealed entry PNDICTT91 PCTT-g-P3HT PNDICTT91-g-P3HT PNDICTT73-g-P3HTa PNDICTT55-g-P3HT

−1 −1

μ (cm V s ) c

2

6.10 2.46 6.88 3.57 8.28

× × × × ×

−3

10 10−3 10−5 10−5 10−4

Ion/Ioff 4.18 1.14 5.41 3.73 6.36

× × × × ×

Vth 5

10 102 100 100 100

28.40 −2.07 47.55 115.16 69.27

μ (cm V c

2

1.73 1.88 5.17 2.60 1.20

× × × × ×

−1

−1

s ) −3

10 10−3 10−4 10−5 10−4

Ion/Ioff 4.19 1.85 8.40 2.63 2.86

× × × × ×

Vth 7

10 103 100 100 100

23.51 −30.26 33.74 103.44 130.49

Devices were fabricated from the chloroform solutions with a concentration of 7.5 mg/mL. bAnnealed at 140 °C for 1 h. cPNDICTT91 shows only n-type charge mobility while the other samples show only p-type charge mobility.

a

2C) shows an obvious decrease in the elution time compared to that of the pure PCTT, which also reveals the successful transformation of PCTT to PCTT-g-P3HT. Furthermore, the molecular weights of PCTT-g-P3HT determined by SEC and 1 H NMR are 33.7 and 35.7 kDa, respectively (Table 2), which are remarkably increased from 2 kDa of PCTT. Preparation of All-Conjugated Graft Copolymers Comprised of n-Type Backbone and P3HT Side Chains. Based on the successful results of the model polymerization and the synthesis of PCTT-g-P3HT, the all-conjugated graft copolymers initiated by the n-type backbone PNDICTTxy were synthesized by the externally initiated KCTP method as illustrated in Scheme 2c. The compositions and PDI values of the products are summarized in Table 2. The yields of all the graft copolymers were relatively high (87−95%), indicating that the thiophene monomer 5 was successfully initiated and polymerized. The PDIs of all the products significantly decreased (for instance, the PDI of PNDICTT91 is 2.08, while that of PNDICTT91-g-P3HT decreased to 1.43) as compared to their corresponding macroinitiators, possibly due to the living nature of the KCTP of 5. The 1H NMR spectra of the graft copolymers and their corresponding precursors are presented in Figures S7−S9. All the peaks corresponding to the protons of the P3HT and PNDICTT segments can be clearly assigned. Besides, the peak of the aromatic protons in the NDI unit (a in Figures S7−S9) was broadened and split into multimodal peaks, and that of the methyl group in the CTT units (d) was strengthened with the increase of the CTT unit in the PNDICTTxy series. The peaks corresponding to the protons of the thiophene groups in PNDICTTxy were clearly identified as e and f; however, in the graft copolymers, these peaks were broadened and overlapped. As shown in Figure 3, the peak indicated by a in PNDICTT73 is assigned to the thiophene protons next to the NDI units, while b corresponds to the protons of the aromatic rings in CTT as well as that of the thiophene units next to the CTT unit. The two peaks, a and b, were also identified in the other PNDICTTxy series. Interestingly, the two peaks were broadened and formed a large broad peak in the graft copolymers, possibly due to the fact that P3HT was grafted from the CTT units, considerably lowering the conformational flexibility around the CTT units. This phenomenon is also seen in the case of PCTT-g-P3HT. The SEC traces of PNDICTT91g-P3HT, PNDICTT73-g-P3HTa, and PNDICTT55-g-P3HT are presented in Figure S10, all of which present sharp and nearly unimodal peaks, although there is a small side peak on the higher molecular weight side which might be resulted from minor side reactions. All these results thus demonstrate that the P3HT chains are successfully initiated from the n-type backbones.

Figure 6. DSC curves of (A) CTT-P3HT, (B) PNDICTT91, and (C) PNDICTT91-g-P3HT.

Figure 1 shows that all P3HT bears the CTT unit at the α-end group and H or Br unit at the ω-end group, indicating a 100% initiation efficiency from the CTT units. This result is crucial for the successful preparation of the all-conjugated graft copolymers in the following sections because the contamination of the homo-P3HT byproduct should be absent in this system. The Mn and PDI of the product determined by SEC are 13 kDa and 1.4, respectively, while the Mn calculated by 1H NMR (Figure S5) is 4.96 kDa, close to the theoretical value of 5.20 kDa, which indicates the relatively controlled manner of the polycondensation process, as compared to similar initiators.33 Synthesis of All-Conjugated Grafting P3HT via KCTP Initiated by PCTT. PCTT was used as a macroinitiator to synthesize the all-conjugated grafting P3HT (PCTT-g-P3HT). The process (Scheme 2b) is the same as described in the model reaction with the exception of using PCTT instead of 1. The amount of the Ni(COD)2 catalyst was 1 equiv to the CTT unit in the PCTT in order to suppress the exclusion of free Ni species. 5 was then added to start the polymerization via the grafting-from method. The yield and Mn values of the product are presented in Table 2. As can be seen in the 1H NMR spectrum of PCTT-g-P3HT (Figure 2B), all the resonance signals from the P3HT and PCTT protons can be identified. Moreover, the downfield shift of the aromatic protons of the PCTT units in PCTT-g-P3HT (indicated as i in Figure 2B) is observed by a comparison with PCTT (Figure 2A). These results indicate that P3HT was successfully initiated from the PCTT backbone, and the allconjugated grafting P3HT has been synthesized. To calculate the Mn value of the product, the integral areas of the methyl proton peak located at 2.44 ppm (a in Figure 2B) and that of the aromatic proton peak of P3HT located at 6.98 ppm (b in Figure 2B) are applied. The SEC curve of the product (Figure 1789

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Figure 7. 2-D GIWAXS images of (a) PNDICTT91 and (b) PNDICTT91-g-P3HT after annealing at 150 °C for 30 min. Out-of-plane and in-plane GIWAXS diffraction profiles of (a1 and a2) PNDICTT91 and (b1 and b2) PNDICTT91-g-P3HT, respectively.

values of all the graft copolymers determined by 1H NMR are higher than those by SEC. The number of P3HT side chains in these all-conjugated graft copolymers prepared via the combination of the Stille coupling reaction and KCTP can be predetermined by controlling the feed molar ratio of 3 and 2 in the preparation of PNDICTTxy. For instance, the average number of P3HT side chains is 1.7 in PNDICTT91-g-P3HT, while that in PNDICTT55-g-P3HT is 3.4. In addition, the chain length of P3HT in these graft copolymers prepared by the proposed twostep method can also be tailored by varying the feed molar ratio of PNDICTTxy and 5 in the second step. As shown in Table 2, PNDICTT73-g-P3HTa and PNDICTT73-g-P3HTb were syn-

It is noteworthy that the Mn values of the graft copolymers determined from SEC (Table 2) only slightly increased when compared to their precursors (except for PNDICTT73-gP3HTb, which showed a remarkable increase in Mn compared to the precursor PNDICTT73). This may be due to the fact that the PNDICTT series are aggregated in THF (eluent for SEC) to provide overestimated Mn values. After grafting the P3HT side chains, the aggregation of PNDICTT may be suppressed due to the introduction of well-soluble P3HT chains. In addition, the branched structure of a graft copolymer present a smaller hydrodynamic volume than the linear analogue, so that the Mn values of the graft polymer determined by SEC might be underestimated. Indeed, as can be seen from Table 2, the Mn 1790

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shoulder at ca. 580 nm, which is related to the absorption of the PNDICTT backbone. The λmax values of the polymer in the film state red-shift from 455 to 558 nm, which indicates a more ordered morphology and extended π-conjugation length of these graft copolymers in the film state. Electrochemical Properties of Graft Copolymers. Cyclic voltammetry experiments were performed in order to determine the HOMO and LUMO energy levels of the allconjugated graft copolymers. Figure 5 shows the cyclic voltammograms of all the graft copolymers as well as PNDICTT91, and the associated numerical results are summarized in Table 3. PCTT-g-P3HT (Figure 5A) possesses a HOMO energy level of −4.76 eV. The n-type PNDICTT91 shows the expected low-lying HOMO energy level of −5.89 eV. The HOMO energy levels of the graft copolymers based on the PNDICTT backbones ranged from −4.78 to −4.95 eV, which are lower than that of PCTT-g-P3HT based on a p-type backbone. Moreover, with the increasing amount of PNDICTT, the HOMO values of the graft copolymers are reduced accordingly. These results indicate that the electrochemical properties of the graft copolymers can be controlled by varying the weight ratios of the PNDICTT backbone and P3HT side chains. FETs and Charge Transport Properties. The effects of the solvents and thermal annealing on the charge transporting characteristics were investigated using field effect transistor (FET) devices. The FET mobility was calculated in the saturation regime by using the plot of the square root of the drain-to-source current (Ids)1/2 versus the gate voltage (Vg). The correlation between the Ids and other parameters uses the equation42

Figure 8. TEM images of film morphology of (A) PNDICTT91-gP3HT and (B) PNDICTT73-g-P3HTa, annealed at 240 °C for 10 min and at 100 °C for another 6 h under nitrogen.

thesized by using the same PNDICTT73 batch as the macroinitiator. By increasing the ratio of [5]/PNDICTT73 from 25 to 60 equiv, the molecular weight of the corresponding graft polymer increased from 11.5 kDa (PNDICTT73-gP3HTa) to 18.8 kDa (PNDICTT73-g-P3HTb). It should be noted that the characterizations in the following sections will focus on PNDICTT73-g-P3HTa rather than PNDICTT73-gP3HTb in the case of PNDICTT73-g-P3HT series. All the results confirmed that the all-conjugated graft polymers were, for the first time, successfully synthesized via a two-step method involving the Stille coupling reaction and KCTP. In addition, it was found that not only the number of P3HT side chains but also the chain length of each P3HT was controllable. Optical Properties of Graft Copolymers. The optical properties of PNDICTTxy-g-P3HT and PCTT-g-P3HT were investigated by measuring their absorption spectra in the thin film state. The UV−vis spectra of the graft polymer are shown in Figure 4, and the energy band gaps (Eopt g ) determined by UV−vis spectroscopy are presented in Table 3. As can be seen, the λmax values of the graft copolymers are in the range 524− 558 nm. Furthermore, all the graft copolymers revealed a shoulder at 607 nm, which is related to a vibronic absorption of the P3HT segment. However, the shoulder of the graft copolymers with the PNDICTT backbone would be related to both the vibronic absorption of P3HT as well as the absorption of the backbone because PNDICTT shows a strong absorption around 607 nm. This might be the reason for the weaker shoulder of PCTT-g-P3HT as compared to the PNDICTTxy-gP3HT at 607 nm. Because of the similar structure of these graft copolymers, the Eopt g values of the graft copolymers were nearly equal, around 1.9 eV, as shown in Table 3. Figure 4 also presents the spectrum of the blend film of PNDICTT73 and P3HT with the similar weight ratio to PNDICTT73-g-P3HT (1:1.5 w/w). Different from the graft copolymers, the blend system exhibits a clear optical feature of PNDICTT domains, where the absorption at ca. 370 nm is stronger than that of graft copolymers, and the onset absorption is red-shifted to 760 nm. In the case of graft copolymers, the optical property of PNDICTT backbone is not obvious, possibly due to the fact that the PNDICTT backbone was covered by P3HT side chains. For a comparison, the optical properties of a selected graft copolymer, the n-type precursor, and linear P3HT prepared in the model reaction were investigated in CHCl3, as shown in Figure S11. As can be seen clearly, the spectrum of PNDICTT91-g-P3HT shows the λmax value at ca. 455 nm, corresponding to the absorption of the P3HT segment, and a

Ids =

WCiμ (Vg − Vt)2 2L

where Ids is the drain-to-source current in the saturated region, W and L are the channel width and length, respectively, μ is the field-effect mobility, Ci is the capacitance per unit area of the insulation layer, and Vg and Vth are the gate and threshold voltages, respectively. The charge mobility values and Ion/Ioff ratios of the devices fabricated from chloroform are presented in Table 4, while those of the devices fabricated from chloroform/chlorobenzene (95:5) are given in Table S1 of the Supporting Information. Representative transfer characteristics of the OFETs are given in Figure S12. PNDICTT91 shows a moderate electron mobility of 6.10 × 10−3 cm2 V−1 s−1 and relatively high Ion/Ioff ratio on the order of 105 and a high value of 4.19 × 107 after annealing at 140 °C for 1 h. The allconjugated grafting P3HT, PCTT-g-P3HT, which has a p-type backbone, shows a moderate hole mobility in the scale of 10−3 cm2 V−1 s−1 both before and after annealing. The Ion/Ioff ratio of PCTT-g-P3HT increased more than 16 times after annealing. The graft copolymers based on PNDICTT were expected to show both n-type and p-type charge transfer characteristics as they contain both n-type and p-type blocks. However, all the graft copolymers show only the p-type charge transfer characteristics. In addition, these graft copolymers present very low Ion/Ioff ratios in the order of 100. These unexpected values of the Ion/Ioff ratio would possibly result from the allconjugated structure, in which the n-type backbone and P3HT side chains are connected by π-conjugated linkage. Therefore, the quenching of the generated charges may significantly occur in these systems to show only ohmic behavior in the FET characteristics. To confirm this effect, an FET device was 1791

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and P3HT side chains, in which the P3HT crystalline domains align in the edge-on rich structure and the PNDICTT crystalline domains are isotropically dispersed. TEM Observation. The microphase separation between the P3HT side chains and PNDICTT backbone was further investigated by TEM observations. Figure 8 exhibits the TEM images of the thin films of PNDICTT 91-g-P3HT and PNDICTT73-g-P3HTa. It can be seen clearly that P3HT forms a partial nanofibril structure (dark region) in the range 10−20 nm. Moreover, with the increasing amount of P3HT, the area corresponding to the P3HT domains also shows a significant increase (from A to B in Figure 8). It should be mentioned that no large phase separation was observed or aggregations in the entire images, which suggests that microphase separation occurred between P3HT and PDNICTT segments in the graft copolymers due to the chemical linkage between the PNDICTT and P3HT chains.

fabricated based on the physical blends of PCTT-g-P3HT and PNDICTT91 under the same conditions and device structure (Figure S13). Though both PCTT-g-P3HT and PNDICTT91 show a relatively good charge mobility and Ion/Ioff ratios, those of the device based on the blend are extremely low, as shown in Table S2; the Ion/Ioff ratio is 3.8 and the hole mobility is 4.74 × 10−7 cm2 V−1 s−1, which indicate that the charge of PCTT-gP3HT was significantly quenched by PNDICTT91. After annealing, the Ion/Ioff ratio increased more than 24 times, which indicated that a large phase separation may occur, so that the quenching of the generated charges is suppressed. In addition, the blend system also shows an n-type charge transfer characteristic. The Ion/Ioff ratios of all the graft copolymers, however, were not improved at all after annealing, probably due to the microphase-separated structures between the p-type and n-type domains, resulting in the frequent recombination of the separated charges. The properties of the FET devices based on the graft copolymers fabricated from chloroform/cholorobenzene are also shown in Table S1. There is a slight increase in the charge mobility of the graft copolymers with the elevated boiling point of the mixing solvents; however, the Ion/Ioff ratio was not improved. The Ion/Ioff ratio is expected to be improved by introducing a nonconjugated linkage between the p- and n-type segments, reducing the recombination of generated charges. The synthesis and FET device characterization of newly designed such block copolymers with p- and n-type segments via the nonconjugated linkage at the junction point are now under investigation. Crystalline Morphology. Figure 6 shows the DSC curves of the linear P3HT (A), PNDICTT91 (B), and PNDICTT91-gP3HT (C). As can be seen, both the linear P3HT (synthesized in the model reaction) and PNDICTT91 are crystalline, with a relatively high melting temperature (Tm) at ca. 233 and 224 °C, respectively. The curve of the selected graft copolymer (PNDICTT91-g-P3HT) exhibits two melting peaks at 174 and 210 °C, which could be attributed to the melting of crystals formed by the P3HT side chains and PNDICTT91 backbone, respectively. This result indicates that P3HT and the backbone of the graft copolymer were phase separated and formed two distinct crystalline domains. Similar results have been reported for the all-conjugated block copolymers.31 The crystalline structures of the graft copolymer were further confirmed by the GIWAXS characterization. As an example, the crystalline morphology in the thin films of PNDICTT91 and PNDICTT91-g-P3HT was investigated by GIWAXS. Figure 7 shows the 2D WAXS images of the thin film and the diffraction profiles of the out-of-plane and in-plane directions. As can be seen in Figures 7a, 7-a1, and 7-a2, there is a series of strong diffraction signals (PNDICTT (100) at qxy = 0.248 (2.5 nm)) in both the out-of-plane and in-plane directions of the PNDICTT91 thin films. Regarding the PNDICTT91-g-P3HT thin film, the strong P3HT (h00) diffractions at qz = 0.381 (1.6 nm) and 0.769 (0.82 nm) were observed in the out-of-plane direction (Figure 7b1) in addition to the diffraction of PNDICTT (100) at qz = 0.273 (2.3 nm). In the in-plane direction profile, there are two distinct P3HT (100) and PNDICTT (100) diffractions at qxy = 0.381 (1.6 nm) and 0.273 (2.3 nm), respectively (Figure 7b2). The P3HT (010) diffraction was also observed at qxy = 1.675 (0.37 nm) in the in-plane direction. These results indicate that the PNDICTT91-g-P3HT thin film is composed of two distinct crystalline domains corresponding to the PNDICTT backbone

■

CONCLUSION A series of all-conjugated graft copolymers comprised of an ntype or p-type backbone and P3HT side chains have been synthesized via a newly developed two-step method. In the first step, a series of n-type conjugated polymers, PNDICTTxy, with different compositions of the CTT units were prepared via the Stille coupling polycondensation. In the second step, P3HT chains were grafted from the CTT group of PNDICTTxy via an externally initiated KCTP using PNDICTTxy as macroinitiators. The proposed method could produce the first examples of allconjugated graft copolymers comprising both n-type and p-type semiconductors. The advantage of the method is that the number of P3HT sides chains as well as the chain length of P3HT in the graft copolymers are controllable. The UV−vis spectra and CV results of the graft copolymers indicated that the HOMO values decreased with the increasing number of P3HT side chains, although the band gap values of all the graft copolymers were almost the same. OFET devices based on the graft copolymers were fabricated, and the results indicated that all the graft copolymers showed a p-type characteristic with a moderate hole mobility on the order of 10−4 cm2 V−1 s−1. DSC curves revealed that there were two distinguished crystalline domains in the graft copolymers corresponding to the PNDICTT and P3HT segments. Moreover, the GIWAXS results confirmed that the PNDICTT91-g-P3HT thin film possesses two different crystalline domains of edge-on P3HT domains and isotropically dispersed PNDICTT domains. The TEM images of the thin film of the graft copolymers revealed a microphase separation between the P3HT and PNDICTT segments, in which the P3HT domains show a partial nanofibril-like morphology with a width of 10−20 nm.

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ASSOCIATED CONTENT

S Supporting Information *

1 H NMR spectra of PNDICTT series, PCTT, CTT-P3HT, CTT, and graft copolymers; SEC traces of graft copolymers. UV−vis absorption of the product in chloroform; FET results of the devices. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.H.); chenwc@ ntu.edu.tw (W.-C.C.). 1792

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Author Contributions ∥

(24) Smeets, A.; Willot, P.; Winter, J. D.; Gerbaux, P.; Verbiest, T.; Koeckelberghs, G. Macromolecules 2011, 44, 6017−6025. (25) Senkovskyy, V.; Beryozkina, T.; Bocharova, V.; Tkachov, R.; Komber, H.; Lederer, A.; Stamm, M.; Severin, N.; Rabe, J.; Kiriy, A. Macromol. Symp. 2010, 291−292, 17−25. (26) Yuan, M.; Okamoto, K.; Bronstein, H. A.; Luscombe, C. K. ASC Macro Lett. 2012, 1, 392−395. (27) Mulherin, R.; Jung, S.; Huettner, S.; Johnson, K.; Kohn, P.; Sommer, M.; Allard, S.; Scherf, U.; Greenham, N. C. Nano Lett. 2011, 11, 4846−4851. (28) Woody, K. B.; Leever, B.; Durstock, M. F.; Collard, D. M. Macromolecules 2011, 44, 4690−4698. (29) Izuhara, D.; Swager, T. Macromolecules 2011, 44, 2678−2684. (30) Sommer, M.; Komber, H.; Huettner, S.; Mulherin, R.; Kohn, P.; Greenham, N. C.; Huck, W. T. S. Macromolecules 2012, 45, 4142− 4151. (31) Ku, S. Y.; Brady, M. A.; Treat, N. D.; Cochran, J. E.; Robb, M. J.; Kramer, E. J.; Chabinyc, M. L.; Hawker, C. J. J. Am. Chem. Soc. 2012, 134, 16040−16046. (32) Nakabayashi, K.; Mori, H. Macromolecules 2012, 45, 9618− 9625. (33) Doubina, N.; Ho, A.; Jen, A.; Luscombe, C. K. Macromolecules 2009, 42, 7670−7677. (34) Sontag, S. K.; Marshall, N.; Locklin, J. Chem. Commun. 2009, 3354−3356. (35) Khanduyeva, N.; Senkovskyy, V.; Beryozkina, T.; Horecha, M.; Stamm, M.; Uhrich, C.; Riede, M.; Leo, K.; Kiriy, A. J. Am. Chem. Soc. 2009, 131, 153−161. (36) Bronstein, H. A.; Luscombe, C. K. J. Am. Chem. Soc. 2009, 131, 12894−12895. (37) Durban, M. M.; Kazarinoff, P. D.; Luscombe, C. K. Macromolecules 2010, 43, 6348−6352. (38) Guo, X. G.; Watson, M. D. Org. Lett. 2008, 10, 5333−5336. (39) Chen, Z. H.; Zheng, Y.; Yan, H.; Facchetti, A. J. Am. Chem. Soc. 2009, 131, 8−9. (40) Durban, M. M.; Kazarunoff, P. D.; Segawa, Y.; Luscombe, C. K. Macromolecules 2011, 44, 4721−4728. (41) Senkovskyy, V.; Tkachov, R.; Komber, H.; Sommer, M.; Heuken, M.; Voit, B.; Huck, W. T. S.; Kataev, V.; Peter, A.; Kiriy, A. J. Am. Chem. Soc. 2011, 133, 19966−19970. (42) Dimitrakopoulos, C. D.; Malenfant, P. R. L. Adv. Mater. 2002, 14, 99−117.

J.W. and C.L. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Japan Science and Technology Agency (JST), PRESTO program (JY 220176). GIWAXS experiments were performed at the BL19B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2012B1728). We thank Prof. Itaru Osaka (Hiroshima University) and Dr. Tomoyuki Koganezawa (Japan Synchrotron Radiation Research Institute (JASRI)) for operating the GIWAXS experiments. The technical support of Mr. Ryohei Kikuchi, Material Analysis O-okayama Center, Tokyo Institute of Technology, for the TEM operation is also gratefully acknowledged. Work at NTU was supported by National Science Council of Taiwan (Grant NSC 101-2823-E-002-014-MY2).

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dx.doi.org/10.1021/ma400043s | Macromolecules 2013, 46, 1783−1793