difuran–Diketopyrrolopyrrole-Containing Copolymers for High

Jan 6, 2014 - The highest hole mobilities reached to 0.24 and 0.11 cm2 V–1 s–1 for ... 160 °C. These features in the present polymers offer great...
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Synthesis and Characterization of Angular-Shaped Naphtho[1,2‑b;5,6‑b′]difuran−Diketopyrrolopyrrole-Containing Copolymers for High-Performance Organic Field-Effect Transistors Shaowei Shi,† Xiaodong Xie,‡ Chen Gao,† Keli Shi,†,‡ Song Chen,§ Gui Yu,*,‡ Longhai Guo,† Xiaoyu Li,*,† and Haiqiao Wang†,* †

State Key Laboratory of Organic−Inorganic Composite, Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China ‡ CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China § China Textile Academy, Beijing, 100025, China ABSTRACT: We reported the synthesis, characterization, and fieldeffect transistor properties of two diketopyrrolopyrrole (DPP)-based π-conjugated copolymers PNDF3-T-DPP and PNDF3-BT-DPP by introducing naphtho[1,2-b;5,6-b′]difuran (NDF3) or NDF3 bridged with alkylthienyl as the donor unit. Compared with PNDF3-T-DPP, the incorporation of a short π-conjugated thiophene spacer into PNDF3-BT-DPP resulted in a “wave” shape molecular backbone, leading to a poorer ordered structure and lower charge carrier transport of the polymer in the thin film, though improved the solubility and processability. On the other hand, by replacing NDF3 with its sulfur analogues, naphtho[1,2-b;5,6-b′]dithiophene (NDT3), the resulting NDT3-based polymers possessed poor solubility and twisty spatial structure, which lead to lower hole mobilities. In contrast, PNDF3-T-DPP and PNDF3-BT-DPP exhibited excellent hole mobility when used as the active layer in organic field-effect transistors (OFETs) devices. The highest hole mobilities reached to 0.24 and 0.11 cm2 V−1 s−1 for PNDF3-T-DPP and PNDF3-BT-DPP respectively, even without thermal annealing. Higher hole mobilities of up to 0.56 and 0.35 cm2 V−1 s−1 were obtained when annealed at 160 °C. These features in the present polymers offer great interest of using NDF3 moiety as the building block for semiconducting polymers and give new insight into the design of a new class of semiconducting polymers.

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INTRODUCTION Organic field-effect transistors (OFETs) consisting of conjugated polymers have attracted considerable attention in recent years, owing to the promise of low cost, lightweight, and potential use of flexible substrates.1−6 Among the numerous reported semiconducting materials for OFETs, donor−acceptor (D−A) alternating copolymers containing diketopyrrolopyrrole (DPP), which was usually used as the electron-deficient unit, showed an attractive performance due to the strong intermolecular overlap through π−π stacking. Excellent holetransport property with high hole mobility of over 1 cm2 V−1 s−1 (the highest value even over 10 cm2 V−1 s−1) has been obtained from some copolymers based on DPP-thiophene (e.g., oligothiophene or fused thiophene) system (see Figure 1).7−12 On the other hand, acenedithiophenes (AcDTs), in which two thiophene rings fused at both ends of acene cores, have been the representative thienoacene structures and widely utilized as building blocks in the development of organic semiconductors.13−18 In particular, as a family of thiophenefused heteroarenes, naphthodithiophenes (NDTs) have been of great interest as a core unit for building many semiconducting © XXXX American Chemical Society

materials with high performance OFETs, and mobilities of over 0.7 cm2 V−1 s−1 have been achieved in semiconducting polymers containing angular-shaped naphtho[1,2-b;5,6-b′]dithiophene (NDT3).19 However, compared with NDTs, their oxygen analogue, NDFs, has received far less attention unless a few reports of NDFs-based small molecule OFETs.20 Very recently, Nakamura et al. reported a new p-type semiconductor based on naphtho[2,1-b:6,5-b′]difuran (NDF4) derivatives, which was applied for solution-processed single-crystal OFETs and hole mobilities of up to 3.6 cm2 V−1 s−1 were obtained, indicated of the potentials of naphthodifurans (NDFs) as promising building blocks in semiconducting materials.21 Like naphthodithiophenes (NDTs), naphthodifuran (NDFs) also possess two class of isomers, including linearand angular-configurations.22 Considering that angular-configuration is more likely to pack into a higher ordered structure, in this paper, we chose angular-shaped NDF3 as a donor unit, Received: October 12, 2013 Revised: December 16, 2013

A

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Figure 1. Chemical structures of DPP−thiophen-based polymer semiconductors reported previously.

Figure 2. Chemical structures of isomeric angular-shaped NDF/NDT units and the resulting copolymers.

polymers can not be used for further characterization. Conversely, PNDF3-T-DPP and PNDF3-BT-DPP possess better solubility and can be processed to form smooth and pinhole-free films upon spin-coating. The poor solubility of NDT3-based polymers can be attributed to the fact that the higher aromatic NDT3 unit could induce the relative polymers with higher crystallinity and denser crystallite packing. Both NDF3-based polymers were purified by sequential Soxhlet extraction with methanol, hexanes, and CHCl3. The CHCl3 fraction was then reduced in volume, precipitated into methanol, and collected by filtration, yielding shiny, bronzecolored solids. Compared with PNDF3-T-DPP, PNDF3-BTDPP can be easier dissolved in common solvents, such as chloroform, toluene, and chlorobenzene. PNDF3-T-DPP shows slight poor solubility but also can be dissolved in hot solvents (the solubility of PNDF3-T-DPP and PNDF3-BTDPP in CHCl3 is ∼10 mg/mL and more than 20 mg/mL at room temperature, respectively). It is worth mentioning that during our preparation of this article, Lee et al. reported on a soluble copolymer PTNDTT-DPP based on NDT3 flanked with two 4-hexadecylthiophen as donor unit and DPP as acceptor unit, and a hole mobility of 2.8 × 10−4 cm2 V−1 s−1 was obtained (see Figure 2),18 which gave us some guides in applying the NDT3 for designing higher performance semiconducting polymers. The molecular weights of the polymers were determined by GPC in THF solution relative to polystyrene standards and the detailed GPC data are listed in Table 1. Both PNDF3-T-DPP

combining with above-mentioned DPP acceptor unit, designed and synthesized two new D−A copolymers PNDF3-T-DPP and PNDF3-BT-DPP to explore their OFETs performance in detail. Also, we tried to synthesize PNDT3-T-DPP and PNDT3-BT-DPP as references by replacing NDF3 with NDT3 to further investigate the effects of oxygen atom (O) and sulfur atom (S) on electronic structure, spatial structure and physical-chemical properties of the resulting copolymers.

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RESULTS AND DISCUSSION Synthesis and Thermal Stability. NDF3 and NDT3 were starting from commercially available 1,5-dihydroxynaphthalene or 2,6-dihydroxynaphthalene followed by several steps and the final ring-closing reactions that construct fused-furan or -thiophene rings were easily accomplished in reasonable yields,19c,22 and then after two steps to give monomer 3 or 7 by attaching 2-bromo-3-dodecylthiophen at the 2- and 7position of the NDF3/NDT3 core. Monomer 3 and 7 were reacted with n-butyllithium and then with Me3SnCl to give compounds 4 and 8 in a good yield. All polymers were synthesized by palladium-catalyzed Stille-coupling polymerization. However, it is surprising that even a simple replace of oxygen atom with sulfur atom, the solubility of NDF3/NDT3based copolymers is dramatically different. Both reaction systems in PNDT3-T-DPP and PNDT3-BT-DPP formed insoluble residues quickly during the course of polymerization even two dodecylthiophens were attached to the NDT3 in PNDT3-BT-DPP for increasing solubility, leading to the two B

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Scheme 1. Synthetic Route of the Monomers and Copolymers

Table 1. Molecular Weights and Thermal Properties of the Copolymers polymer

Mna

Mwa

PDI (Mw/Mn)a

Td (°C)b

PNDF3-T-DPP PNDF3-BT-DPP

67.3K 29.1K

395.5K 160.2K

5.88 5.51

404 404

a

Mn, Mw, and PDI of the polymers were determined by gel permeation chromatography using polystyrene standards in THF. bThe 5% weight-loss temperatures in the air.

and PNDF3-BT-DPP show large PDI which could be attributed to the high tendency to aggregate in the solution, resulting in overestimation of Mw. Compared with PNDF3-TDPP, PNDF3-BT-DPP shows lower molecular weight because the high molecular weight fraction is insoluble in THF and can be filtered out during GPC measurements. The thermal properties of the polymers were determined by thermogravimetric analysis (TGA) under nitrogen atmosphere at a heating rate of 10 °C/min, as shown in Figure 3. Both polymers have good thermal stability with onset decomposition temperatures with 5% weight loss at ca. 404 °C. Obviously, the thermal stability of these polymers is adequate for their applications in OFETs, and other optoelectronic devices. Electronic Structure of the Polymers. Optical and electrochemical properties of the polymers were characterized by UV−vis absorption spectroscopy, cyclic voltammetry (CV), and theoretical calculation, and these data are summarized in Table 3.

Figure 3. TGA plots of the polymers with a heating rate of 10 °C/min under an inert atmosphere.

We first compared the CV and UV−vis absorption spectra of the monomer units NDT3/NDF3 and NDT3-2T/NDF3-2T in chloroform, as shown in Table 2 and Figure 4. As reported by Takimiya, both the oxidation waves of NDT3 and NDF3 are irreversible, and the oxidation potential of NDF3 is lower than that of NDT3 as a result of the chrysene-like structure for NDT3 is dominant over the substituted naphthalene-like structure for NDF3 due to higher aromaticity of thiophene than furan.22 After attaching two 3-dodecylthiophens at the 2and 7-position of NDT3/NDF3, both the highest occupied molecular orbital (HOMO) energy levels of NDT3-2T and C

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between the acceptor and donor units.23 Also, well-defined vibronic splitting peaks at ca. 650 nm and long tails extending to the infrared region were observed, suggesting a high tendency to aggregate even in diluted solution. This tendency to aggregation is in agreement with the results made upon measuring molecular weights as discussed above.24 In films, the absorption spectra of PNDF3-T-DPP and PNDF3-BT-DPP are similar to those in solution, with slight red-shifts (6−10 nm) of their absorption maxima as shown in Figure 6b, which is a common phenomenon for conjugated polymers, owing to the aggregation of the conjugated polymer main chains in the solid films.25 In addition, both NDF3-T-DPP and PNDF3-BT-DPP display enhanced vibronic shoulders at shorter wavelengths, implying an enhanced arrangement in their solid films. Compared with PNDF3-T-DPP, PNDF3-BT-DPP shows a slight red-shifted absorption maxima and absorption edge due to the π-extension of NDF3-2T donor unit relative to NDF3. CVs of the polymer films are shown in Figure 7. The onset oxidation potential (Eox)/onset reduction potential (Ered) of PNDF3-T-DPP and PNDF3-BT-DPP are +0.47/−1.14 and +0.46/−1.17 V vs Ag/Ag+ respectively. The HOMO and LUMO energy levels of the polymers were calculated from the onset oxidation potential and the onset reduction potential according to the equations:26

Table 2. Optical and Electrochemical Properties of the Monomers compound NDT3 NDT3-2T NDF3 NDF3-2T

Eox (V) EHOMO (eV)a +1.13 +0.78 +0.99 +0.69

−5.84 −5.49 −5.70 −5.40

λonset in solution (nm)b Egopt (eV)c 364 408 354 404

3.41 3.04 3.50 3.07

a The HOMO energy levels were calculated from: EHOMO = −e (Eox + 4.71) (eV). bMeasured in chloroform solution. cBandgap estimated from the onset wavelength (λonset) of the optical absorption: Egopt= 1240 nm*eV/λonset.

E HOMO = −IP = − e(Eox + 4.71) (eV)

(1)

E LUMO = −EA = − e(Ered + 4.71) (eV)

(2)

The EHOMO, ELUMO and electrochemical bandgaps (Egec) of PNDF3-T-DPP and PNDF3-BT-DPP, calculated from the value of Eox and Ered of the polymers, are −5.18/−3.57/+1.61 eV and −5.17/−3.54/+1.63 eV, respectively (see Table 3). For the convenience of comparison, the optical and electrochemical parameters of the PTNDTT-DPP (see ref 18) were also summarized synchronously in Table 3. The electrochemical band gaps of the polymers match well with their optical band gaps within the experimental error. It is worth noting that when incorporating DPP into the polymer main chain, PTNDTTDPP shows blue-shifted absorption maxima and absorption edge compared with PNDF3-BT-DPP, which is in contrast with the monomers NDT3 vs NDF3 or NDT3-2T vs NDF3-2T. To further study the fundamentals of molecular architecture, theoretical calculations using the density functional theory (DFT) methods were carried out for PNDT3-T-DPP/PNDT3BT-DPP and PNDF3-T-DPP/PNDF3-BT-DPP to investigate the similarity/difference of the energy levels of the frontier orbital as well as the molecular electronic structures with a molecular main chain length n = 1, at the b3lyp/6-31g(d, p) level with the Gamess program package.27 The alkyl chains of thiophen and DPP were replaced by methyl group in the calculation to avoid excessive computation demand. The wave functions of the frontier molecular orbital are depicted in Figure 8. As can be observed, the HOMO is delocalized along the whole π-conjugated backbone while the LUMO is mostly concentrated on the DPP-based acceptor groups. These images provide further evidence of the formation of well-defined D−A structure and the ICT behavior of the material (i.e., the HOMO to LUMO transition is a donor to acceptor intramolecular charge transfer). Also, from the calculated energy levels, it is clear that the LUMO levels of the four polymers are mainly dominated by the DPP unit and nearly the same. Since NDF3based copolymers show higher HOMO levels, smaller bandgaps

Figure 4. Cyclic voltammograms (in dichloromethane) (a and b) and absorption spectra (in chloroform) (c) of NDT3/NDF3 and NDT32T/NDF3-2T.

NDF3-2T show significant rise due to the extension of πconjugation and smaller bandgaps are observed (see Figure 4, parts b and c). The energy gaps estimated from the absorption edges are almost constant for NDT3/NDF3 or NDT3-2T/ NDF3-2T, resulting in higher-lying the lowest unoccupied molecular orbital (LUMO) energy levels for NDF3/NDF3-2T than for NDT3/NDT3-2T as shown in Figure 5. Figure 6 shows the absorption spectra of PNDF3-T-DPP and PNDF3-BT-DPP in solution and films. In Figure 6a, both of the absorption spectra recorded from dilute chloroform solutions display π−π* transitions at short wavelength, and large charge transfer features at long wavelength range which can be attributed to the intramolecular charge transfer (ICT)

Figure 5. Schematic representation of the frontier orbitals energy levels of NDF3/NDF3-2T and NDT3/NDT3-2T. The HOMO energy levels and HOMO−LUMO energy gaps (Egopt) were estimated from oxidation onset in cyclic voltammograms and absorption edges in the UV−vis absorption spectra. The LUMO energy levels were calculated from: ELUMO = EHOMO + Egopt. D

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Figure 6. UV−Vis absorption spectra of copolymers in CHCl3 solutions (a) and in thin films (b).

Figure 7. Cyclic voltammograms of PNDF3-T-DPP and PNDF3-BTDPP films on a platinum electrode in 0.1 mol L−1 Bu4NPF6 acetonitrile solution at a scan rate of 100 mV s−1.

Figure 8. Frontier molecular orbital (LUMO, top; HOMO, bottom) obtained from density functional theory (DFT) calculations on the polymers with a chain length n = 1 at the b3lyp/6-31g(d, p) level of theory.

can be obtained and this can explain the blue-shifted absorption of PTNDTT-DPP compared with PNDF3-BT-DPP. Field-Effect Transistor Properties. The performances of polymer field-effect transistors based on PNDF3-T-DPP and PNDF3-BT-DPP were investigated using a bottom-gate, bottom-contact configuration. The gold (Au) source-drain electrodes were prepared by photolithography and the polymer films were deposited onto octadecyltrichlorosilane (OTS)modified Si/SiO2 (300 nm) substrates by spin-coating. Different annealing temperature and different channel length (L) of the OFET devices (L = 5, 10, 20, 30, 40, and 50 μm) were used to optimize the device performance. Devices containing PNDF3-T-DPP with L = 5 μm showed the highest hole mobility while devices containing PNDF3-BT-DPP with L = 10 μm showed the highest hole mobility at different annealing temperature. As shown in Figure 9 and Table 4, for the two polymers the annealing temperature tends to affect the hole mobility. All devices based on polymers were annealed at 80, 120, 160, 200, 240, and 280 °C, and both polymers showed thermal stability on each device. Figure 10 shows the output curves and transfer characteristics of the OFETs based on the polymers at optimal annealing temperature. It is worth noting that even without annealing, the hole mobilities of PNDF3-TDPP or PNDF3-BT-DPP-based OFET devices could reach to 0.24 or 0.11 cm2 V−1 s−1, respectively, which are among the

Figure 9. Evolution of hole mobilities of OFETs based on PNDF3-TDPP and PNDF3-BT-DPP films with different annealing temperatures.

high results of reported OFETs based on polymeric semiconductors without annealing. With a rise of annealing temperature, the hole mobility tends to increase, implying the improved crystallinity of the films and ordered lamellar structure of π−π close-packing for polymer main chain at higher temperatures. High current modulation is observed with on/off current ratio of 106−107 for PNDF3-T-DPP while PNDF3-BT-DPP shows relatively lower on/off current ratio of

Table 3. Optical and Electrochemical Properties of the Polymers polymer

λmax in solution (nm)a

λmax in film (nm)b

λonset in film (nm)

Egopt (eV)c

EHOMO/Eox (eV/V)

ELUMO/Ered (eV)/(V)

Egec (eV)d

PNDF3-T-DPP PNDF3-BT-DPP PTNDTT-DPP18

380, 727 434, 731 415, 659

393, 733 439, 741 444, 663

827 830 819

1.50 1.49 1.51

−5.18/+0.47 −5.17/+0.46 −5.22/+0.82

−3.57/−1.14 −3.54/−1.17 −3.5/−0.9

1.61 1.63 1.72

a Measured in chloroform solution. bCast from chlorobenzene solution. cBandgap estimated from the onset wavelength (λonset) of the optical absorption: Egopt= 1240 nm*eV/λonset. dEgec= e(Eox − Ered) (eV).

E

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Table 4. Optimized Electrical Parameters of Field-Effect Transistors at Different Annealing Temperatures PNDF3-T-DPP

PNDF3-BT-DPP

annealing temperature/°C

μh (cm2 V−1 s−1)

Ion/Ioff

Vth (V)

μh (cm2 V−1 s−1)

Ion/Ioff

Vth (V)

25 80 120 160 200 240 280

0.24 0.30 0.40 0.56 0.2 0.074 0.0051

104−105 106−107 106−107 106−107 106−107 104−105 104−105

+36 +24 +25 +17 +9 −7 +12

0.11 0.25 0.31 0.35 0.15 0.11 0.0041

104−105 104−105 104−105 104−105 104−105 105−106 104−105

+31 +8 +35 +23 +30 −8 −10

Figure 10. Output (left) and transfer (right) characteristics of the spin-coated film of polymer transistors based on OTS-modified Si/SiO2 substrate: (a, b) PNDF3-T-DPP with annealing at 160 °C for 5 min; (c, d) PNDF3-BT-DPP with annealing at 160 °C for 5 min.

Figure 11. Top view (top) and side view (bottom) of optimized structures of the copolymers backbone units with a chain length n = 1.

104−105. The resulting optimal p-channel transistors demonstrated high hole mobilities of 0.56 cm2 V−1 s−1 for PNDF3-TDPP and 0.35 cm2 V−1 s−1 for PNDF3-BT-DPP. Compared with the reported copolymer PTNDTT-DDP with a mobility of 2.8 × 10−4 cm2 V−1 s−1, the hole mobilities of

PNDF3-T-DPP and PNDF3-BT-DPP increased 3 orders of magnitude. In order to explore the origin of this difference, DFT calculations were performed to verify stationary points as stable states for the optimized conformations and single point energies, with a molecular main chain length n = 1, at b3lyp/6F

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can be observed, indicating some degree of crystallinity has formed in the films of PNDF3-T-DPP and PNDF3-BT-DPP at room temperature. When the films were subjected to thermal annealing at 160 °C, the diffraction patterns became sharper and multiple diffraction peaks were more obvious, which demonstrated that higher crystalline films were formed in the annealing process. The out-of-plane patterns for both PNDF3T-DPP and PNDF3-BT-DPP thin films annealed showed sharp peaks assigned to a-axis (h00) direction according to the lamellar structure, where (100) peaks appear at 2θ = 4.5° and 4.2°, which is corresponding to the interlayer distances of 19.61 and 21.01 Å, respectively. The observed interlayer distances are much shorter than that of fully extended alkyl side chain (ca. 36 Å), indicating that the side chains are closely interdigitated with the other side chains in adjacent layers.10 We attribute the larger interlayer distance of PNDF3-BT-DPP to the zigzag polymer backbones as shown in Figure 13. As reported, the backbone curvature shows an important effect on the OFETs performance.28 With a better linear backbone, PNDF3-T-DPP is more likely to form the pattern of “molecular docking” and therefore a higher hole-mobility of up to 0.56 cm2 V−1 s−1 was obtained.29,30 Two-dimensional grazing incidence X-ray diffraction (2DGIXD) was also used to compare the ordering structure between PNDF3-T-DPP and PNDF3-BT-DPP. As shown in Figure 14, both PNDF3-T-DPP and PNDF3-BT-DPP show

31+g(2d, p) level of theory in vacuum using the GAMESS program. Optimized geometries of the NDF3/NDT3-based copolymers in ground state are given in Figure 11. For PNDT3-T-DPP, we can find that there has been a torsion angle of 13.57° between NDT3 unit and DPP unit. After incorporating an alkylthienyl group between NDT3 and DPP, the dihedral angles between alkylthienyl group and NDT3/ DPP units are 33.76° and 14.72°, respectively owing to the steric hindrance of alkyl substituents on the thiophene. Compared with NDT3-based copolymers, both PNDF3-TDPP and PNDF3-BT-DPP show very planar conformation with the dihedral angles of nearly 0°,which is benefical for the delocalizing of the HOMO wave function and increasing the charge transfer along the polymer chain. On the other hand, we can note that PNDF3-BT-DPP shows a “wave” shape due to the introducing of alkylthienyl, while PNDF3-T-DPP gives a pseudostraight shape and is likely to afford the more highly ordered structure in the thin film, which is good for facilitating the charge carrier transport. In order to deeply study the variation of the performances of OFETs based on PNDF3-T-DPP and PNDF3-BT-DPP after annealing, the thin-film microstructures and topographies were investigated by grazing incident X-ray scattering diffraction (GIXRD) and atomic force microscopy (AFM). Figure 12

Figure 12. Out-of-plane XRD pattern of the polymer thin films with or without annealing. Figure 14. 2D-GIXD images of the polymer thin films with or without annealing.

shows GIXRD patterns of the polymer thin films deposited on OTS-modified Si/SiO2 substrates. Before annealing, both PNDF3-T-DPP and PNDF3-BT-DPP show first diffraction peaks (100) in X-ray scattering patterns (out-of-plane) at 2θ = 4.3° and 4.2°, respectively. Also, weak diffraction peak (200)

two out-of-plane diffraction peaks, which are attributed to (100) and (200) diffractions. After the films were thermally

Figure 13. Cartoon representation of copolymers PNDF3-T-DPP/PNDF3-BT-DPP and their film packings. G

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annealed at 160 °C for 5 min, the intensity of diffraction peaks increased, and diffraction points became more centered. Both PNDF3-T-DPP and PNDF3-BT-DPP show relatively large arcing of diffractions and no clear diffraction corresponding to the π−π stacking structure is observed, indicating that there is no preferential orientation throughout the film.31 Compared with PNDF3-T-DPP, PNDF3-BT-DPP shows larger arcing diffraction corresponding to the lamellar structure, indicative of the less crystalline nature of PNDF3-BT-DPP in the thin film, and this is in good agreement with the fact that the OFET performances of PNDF3-BT-DPP is inferior to that of PNDF3T-DPP. AFM images were performed on polymer thin films on OTSmodified SiO2 /Si substrates as shown in Figure 15. The highly

PNDF3-T-DPP and PNDF3-BT-DPP exhibited high carrier mobilities of 0.56 and 0.35 cm2 V−1 s−1, respectively, showing the very promising application of NDF3-based polymers in the field of OFETs. Further modifications to the polymer structure or the device structure are under progress in an effort to achieve better performances.

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EXPERIMENTAL SECTION

Materials and Synthetic Procedures. Naphtho[1,2-b:5,6-b′]difuran (NDF3),22 naphtho[1,2-b:5,6-b′]dithiophene (NDT3),19c and 3,6-bis(5-bromothiophene-2-yl)-2,5-bis(2-octyl-1dodecyl)pyrrolo[3,4c]pyrrole-1,4(2H,5H)-dione (DPP)11b were synthesized according to literature procedures. Tetrahydrofuran (THF) was dried over Na/ benzophenone ketyl and freshly distilled prior to use. Other reagents and solvents were commercial grade and used as received without further purification. All reactions were performed under nitrogen atmosphere. 2,7-Bis(trimethylstannyl)naphtho[1,2-b:5,6-b′]difuran. n-Butyllithium (2.4 M solution in hexane, 1.46 mL, 3.50 mmol) was added dropwise to a solution of NDF3 (208.21 mg, 1 mmol) in dry THF (50 mL) at −78 °C. The mixture was stirred at this temperature for 30 min and then at room temperature for 1 h. After cooling down to −78 °C, Me3SnCl (1 M solution in hexane, 4 mL, 4 mmol) was added dropwise. After being stirred for 30 min at this temperature, the reaction was allowed to warm up to room temperature and stirred overnight. The reaction was quenched with 100 mL of water and the compound was extracted with dichloromethane. The residue obtained after removing the solvent was recrystallized from ethanol to yield 458 mg (86%) 1H NMR (600 MHz, CDCl3): δ (ppm) 8.17 (d, 2H), 7.73 (d, 2H), 7.00 (s, 2H), 0.46 (s, 18H). 2,7-Bis(trimethylstannyl)naphtho[1,2-b:5,6-b′]dithiophene. n-Butyllithium (2.4 M solution in hexane, 3.03 mL, 7.28 mmol) was added dropwise to a solution of NDT3 (500 mg, 2.08 mmol) in dry THF (50 mL) at −78 °C. The mixture was stirred at this temperature for 30 min and then at room temperature for 1 h. After cooling down to −78 °C, Me3SnCl (1 M solution in hexane, 8.32 mL, 8.32 mmol) was added dropwise. After stirring for 30 min at this temperature, the reaction was allowed to warm up to room temperature and stirred overnight. The reaction was quenched with 100 mL of water and the compound was extracted with dichloromethane. The residue obtained after removing the solvent was recrystallized from acetone to yield 940 mg (80%) 1H NMR (600 MHz, CDCl3): δ (ppm) 8.04 (d, 2H), 7.92 (d, 2H), 7.57 (s, 2H), 0.47 (s, 18H). 2,7-Bis(3-dodecylthiophene-2-yl)naphtho[1,2-b:5,6-b′]difuran. To a solution of 2 (213 mg, 0.4 mmol) in chlorobenzene (50 mL) were added 2-bromo-3-dodecylthiophene (0.4 g, 1.2 mmol) and dichlorobis(triphenylphosphine)palladium(II) (14 mg, 0.02 mmol) and the mixture then flushed with nitrogen for 10 min.The reaction mixture was heated to reflux for 24 h and was monitored by TLC. The reaction was quenched with water and extracted with dichloromethane (3 × 50 mL), then dried over MgSO4, filtered, and concentrated in vacuo. Purification by silica column chromatography (hexanes) to afford the crude product and then recrystallized from ethanol to afford the title compound as a pale yellow solid (134 mg, 47%). 1H NMR (600 MHz, CDCl3): δ (ppm) 8.19 (d, 2H), 7.76 (d, 2H), 7.29 (d, 2H), 7.00 (d, 2H), 6.95 (s, 2H), 2.97 (t, 4H), 1.77 (m, 4H), 1.46−1.24 (m, 36H), 0.87 (t, 6H). 13C NMR (150 MHz, CDCl3): δ (ppm) 150.70, 150.29, 140.95, 130.28, 127.17, 124.67, 124.44, 119.87, 118.54, 115.95, 103.75,31.91, 30.24, 29.81, 29.71, 29.67, 29.64, 29.56, 29.34, 22.67, 14.10. 2,7-Bis(3-dodecylthiophene-2-yl)naphtho[1,2-b:5,6-b′]dithiophene. To a solution of 6 (850 mg, 1.5 mmol) in chlorobenzene (50 mL) were added 2-bromo-3-dodecylthiophene (1.49 g, 4.5 mmol) and dichlorobis(triphenylphosphine)palladium(II) (53 mg, 0.075 mmol) and the mixture then flushed with nitrogen for 10 min.The reaction mixture was heated to reflux for 24 h and was monitored by TLC. The reaction was quenched with water and extracted with dichloromethane (3 × 50 mL), then dried over MgSO4, filtered, and concentrated in vacuo. Purification by silica column chromatography

Figure 15. . AFM topography images (5 × 5 μm2) of polymer films on OTS-modified SiO2/Si substrates. The PNDF3-T-DPP thin films: (a) without annealing and (b) with annealing at 160 °C. The PNDF3-BTDPP thin films: (c) without annealing and (d) with annealing at 160 °C.

uniform intertwined polymer fibers are observed for both of the two polymers. With film annealing treatments, the films formed larger crystal grains and better film interconnectivity, which are favorable for the charge transport and lead to an increase of the mobility.

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CONCLUSION In conclusion, two new D−A copolymers based on a fused furan skeleton NDF3 and DPP, namely PNDF3-T-DPP and PNDF3-BT-DPP, have been synthesized by the Pd-catalyzed Stille-coupling reaction and well characterized. Both the copolymers possess acceptable solubility and thermal stability as well as good film-forming ability, and the electronic structure and field-effect transistor properties are well characterized. Also, a deep look on the differences between NDT3-DPP- and NDF3-DPP-containging copolymers to study the structure− property relationship is taken on. Both PNDF3-T-DPP and PNDF3-BT-DPP exhibited excellent p-channel charge transport characteristics when used as the active semiconductor layer in organic thin-film transistor devices. OFETs devices based on H

dx.doi.org/10.1021/ma402107n | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

Article

3.12−2.71 (br,4H), 2.50−0.44 (m,124H). Anal. Calcd for (C100H144N2O4S4)n, C, 76.67; H, 9.27; N, 1.79. Found: C, 76.68; H, 9.37; N, 1.75. Measurements and Characterization. The molecular weight of the polymer was measured using gel permeation chromatography (GPC). The GPC measurements were performed on Waters 515− 2410 with polystyrenes as reference standard and THF as an eluent. All new compounds were characterized by nuclear magnetic resonance spectra (NMR). The NMRs were recorded on a Bruker AV 600 spectrometer in CDCl3 at room temperature. 1H NMR chemical shifts were referenced to internal tetramethylsilane (TMS, 0 ppm). Splitting patterns were designated as s (singlet), t (triplet), d (doublet), m (multiplet), and br (broad). Matrix assisted laser desorption/ ionization time-of-flight (MALDI-TOF) mass spectra were made on a Shimadzu KOMPACT MALDI II using CHCA as a matrix. Elemental analyses were performed on a Flash EA 1112 analyzer or Elementar vario EL III. Thermal gravimetric analysis (TGA, Netzsch TG209C) measurements were carried out under a nitrogen atmosphere at a heating rate of 10 °C/min. UV−vis−NIR absorption spectra were measured on polymer solutions in chloroform and polymer films cast onto quartz glass using a Shimadzu spectrometer model UV-3150. The electrochemical cyclic voltammetry was conducted on a Zahner IM6e Electrochemical Workstation with Pt disk coated with the polymer film, Pt plate, and Ag/Ag+ electrode as working electrode, counter electrode and reference electrode respectively in a 0.1 mol/L tetrabutylammonium hexafluorophosphate (Bu4NPF6) acetonitrile or CH2Cl2 solution. For grazing incidence Xray scatting (GIXS), the drop-coated films were illuminated at a constant incidence angle of 0.2° (λ = 2d sin θ =1.54 Å). Atomic force microscopy (AFM) measurements were carried out on a Nanoscope V instrument, and operated in a tapping mode. AFM images were performed on polymer thin films on OTS-modified SiO2 /Si substrates. Molecular simulation was carried out for copolymers, with a chain length of n = 1 at the b3lyp/6-31g(d,p) level with the Gamess program package. Fabrication of Field-Effect Transistor Devices. Polymer fieldeffect transistors were fabricated in a bottom-gate bottom-contact (BGBC) configuration (gold electrode on Si/SiO2 substrates). The gold (Au) source-drain electrodes were prepared by a photolithography technique to control the channel length (L) and channel width (W = 1400 μm). Before the deposition of polymer semiconductors, octadecyltrichlorosilane (OTS) treatment was performed on the gate dielectrics which were placed in a vacuum oven with OTS to form an OTS self-assembled monolayer. Then the polymer thin films were spin-coated on the OTS modified SiO2/Si substrates from the solutions in hot o-dichlorobenzene (5 mg/mL) at a speed of 2500 rpm for 60s. The OFETs devices were annealed on a hot plate for 5 min in air directly and measured in air by using a Keithley 4200 Semiconductor Characterization System. The mobility of the devices was calculated in the saturation regime. The equation is listed as follows:

(hexanes) to afford the crude product and then recrystallized from ethanol to afford the title compound as a yellow solid (683 mg, 62%). 1 H NMR (600 MHz, CDCl3): δ (ppm) 8.01 (d, 2H), 7.88 (d, 2H), 7.47 (s, 2H), 7.27 (d, 2H), 7.01 (d, 2H), 2.90 (t, 4H), 1.71 (m, 4H), 1.41−1.23 (m, 36H), 0.86 (t, 6H). 13C NMR (150 MHz, CDCl3): δ (ppm) 140.72, 138.37, 137.54, 135.73, 130.53, 130.22, 125.59, 124.67, 123.44, 122.49, 121.31, 31.91, 30.73, 29.67, 29.64, 29.60, 29.51, 29.46, 29.34, 29.30, 22.68, 14.11. 2,7-Bis(3-dodecylthiophene-2-yl)naphtho[1,2-b:5,6-b′]difuran Distannane. n-Butyllithium (2.4 M solution in hexane, 0.25 mL, 0.6 mmol) was added dropwise to a solution of NDF3-2T (120 mg, 0.17 mmol) in dry THF (30 mL) at −78 °C. The mixture was stirred at this temperature for 30 min and then at room temperature for 1h. After cooling down to −78 °C, Me3SnCl (1 M solution in hexane, 0.67 mL, 0.67 mmol) was added dropwise. After stirring for 30 min at this temperature, the reaction was allowed to warm up to room temperature and stirred overnight. The reaction was quenched with 50 mL of water and the compound was extracted with dichloromethane. The residue obtained after removing the solvent was recrystallized from acetone to yield 150 mg (86%) 1H NMR (600 MHz, CDCl3): δ (ppm) 8.18 (d, 2H), 7.76 (d, 2H), 7.00 (s, 2H), 6.92 (s, 2H), 2.97 (t, 4H), 1.78 (m, 4H), 1.48−1.25 (m, 36H), 0.87 (t, 6H), 0.42 (s, 18H). 2,7-Bis(3-dodecylthiophene-2-yl)naphtho[1,2-b:5,6-b′]dithiophene distannane. n-Butyllithium (2.4 M solution in hexane, 0.88 mL, 2.1 mmol) was added dropwise to a solution of NDT3-2T (445 mg, 0.6 mmol) in dry THF (30 mL) at −78 °C. The mixture was stirred at this temperature for 30 min and then at room temperature for 1 h. After cooling down to −78 °C, Me3SnCl (1 M solution in hexane, 2.4 mL, 2.4 mmol) was added dropwise. After being stirred for 30 min at this temperature, the reaction was allowed to warm up to room temperature and stirred overnight. The reaction was quenched with 50 mL of water and the compound was extracted with dichloromethane. The residue obtained after removing the solvent was recrystallized from acetone to yield 577 mg (90%) 1H NMR (600 MHz, CDCl3): δ (ppm) 8.00 (d, 2H), 7.98 (d, 2H), 7.44 (s, 2H), 7.07 (s, 2H), 2.91 (t, 4H), 1.73 (m, 4H), 1.43−1.24 (m, 36H), 0.87 (t, 6H), 0.41 (s, 18H). Synthesis of PNDF3-T-DPP. To a 25 mL two-necked flask were added monomer 6 (53.4 mg, 0.10 mmol), DPP (101.9 mg, 0.1 mmol), and chlorobenzene (6 mL). The mixture was purged with nitrogen for 15 min and then Pd2(dba)3 (5 mg, 0.005 mmol) and P(o-tol)3 (12.5 mg, 0.04 mmol) were added. After being purged for 15 min, the reaction mixture was heated at 135 °C for 24 h. After cooled to room temperature, the reaction mixture was added dropwise to 200 mL methanol and then collected by filtration and washed with methanol. Then the solid was subjected to Soxhlet extraction with methanol, hexane and chloroform. Subsequently, the fraction that was extracted by chloroform was evaporated under reduced pressure, then precipitated in methanol, filtered and finally dried under vacuum to obtain a shiny, bronze-colored solid (50 mg, yield 47%, Mn = 67.3 kDa, Mw = 395.5 kDa, PDI = 5.88). 1H NMR (600 MHz, CDCl3): δ (ppm) 9.66−8.48 (br, 4H), 8.11−5.99 (br,6H), 4.45−3.44 (br,4H), 2.92−030 (m,78H). Anal. Calcd for (C68H92N2O4S2)n, C, 76.64; H, 8.70; N, 2.63. Found: C, 76.70; H, 8.67; N, 2.65. Synthesis of PNDF3-BT-DPP. . To a 25 mL two-necked flask were added monomer 8 (103.4 mg, 0.10 mmol), DPP (101.9 mg, 0.1 mmol), and chlorobenzene (6 mL). The mixture was purged with nitrogen for 15 min and then Pd2(dba)3 (5 mg, 0.005 mmol) and P(otol)3 (12.5 mg, 0.04 mmol) were added. After being purged for 15 min, the reaction mixture was heated at 135 °C for 24 h. After cooled to room temperature, the reaction mixture was added dropwise to 200 mL of methanol and then collected by filtration and washed with methanol. Then the solid was subjected to Soxhlet extraction with methanol, hexane and chloroform. Subsequently, the fraction that was extracted by chloroform was evaporated under reduced pressure, then precipitated in methanol, filtered and finally dried under vacuum to obtain a shiny, bronze-colored solid (100 mg, yield 64%, Mn = 29.1 kDa, Mw = 160.2 kDa, PDI = 5.51). 1H NMR (600 MHz, CDCl3): δ (ppm) 9.40−8.42 (br, 4H), 8.09−5.99 (br,8H), 4.34−3.68 (br,4H),

IDS = (W /2L)Ciμ(VGS − Vth)2 where W/L is the channel width/length, Ci is the insulator capacitance per unit area, and VGS and Vth are the gate voltage and threshold voltage, respectively.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.W.). *E-mail: [email protected] (G.Y.). *E-mail:[email protected] (X.L). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Beijing Natural Science Foundation (2122047) and Specialized Research Fund for the I

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(20) Nakano, M.; Mori, H.; Shinamura, S.; Takimiya, K. Chem. Mater. 2012, 24, 190. (21) Mitsui, C.; Soeda, J.; Miwa, K.; Tsuji, H.; Takeya, J.; Nakamura, E. J. Am. Chem. Soc. 2012, 134, 5448. (22) Nakano, M.; Shinamura, S.; Houchin, Y.; Osaka, I.; Miyazaki, E.; Takimiya, K. Chem. Commun. 2012, 48, 5671. (23) (a) Ha, J. S.; Kim, K. H.; Choi, D. H. J. Am. Chem. Soc. 2011, 133, 10364. (b) Wang, E.; Ma, Z.; Zhang, Z.; Vandewal, K.; Henriksson, P.; Inganäs, O.; Zhang, F.; Andersson, M. R. J. Am. Chem. Soc. 2011, 133, 14244. (24) Guo, X.; Puniredd, S. R.; Baumgarten, M.; Pisula, W.; Müllen, K. Adv. Mater. 2013, 25, 5467. (25) (a) Bronstein, H.; Chen, Z.; Ashraf, R. S.; Zhang, W.; Du, J.; Durrant, J. R.; Tuladhar, P. S.; Song, K.; Watkins, S. E.; Geerts, Y.; Wienk, M. M.; Janssen, R. A. J.; Anthopoulos, T.; Sirringhaus, H.; Heeney, M.; McCulloch, I. J. Am. Chem. Soc. 2011, 133, 3272. (b) Lei, T.; Dou, J.-H.; Cao, X.-Y.; Wang, J.-Y.; Pei, J. J. Am. Chem. Soc. 2013, 135, 12168. (26) (a) Li, Y.; Cao, Y.; Gao, J.; Wang, D.; Yu, G.; Heeger, A. J. Synth. Met. 1999, 99, 243. (b) Sun, Q.; Wang, H.; Yang, C.; Li, Y. J. Mater. Chem. 2003, 13, 800. (27) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. J. Comput. Chem. 1993, 14, 1347. (28) (a) Schroeder, B. C.; Nielsen, C. B.; Kim, Y. J.; Smith, J.; Huang, Z.; Durrant, J.; Watkins, S. E.; Song, K.; Anthopoulos, T. D.; McCulloch, I. Chem. Mater. 2011, 23, 4025. (b) Rieger, R.; Beckmann, D.; Mavrinskiy, A.; Kastler, M.; Müllen, K. Chem. Mater. 2010, 22, 5314. (c) Osaka, I.; Shimawaki, M.; Mori, H.; Doi, I.; Miyazaki, E.; Koganezawa, T.; Takimiya, K. J. Am. Chem. Soc. 2012, 134, 3498. (29) Lei, T.; Cao, Y.; Zhou, X.; Peng, Y.; Bian, J.; Pei, J. Chem. Mater. 2012, 24, 1762. (30) Osaka, I.; Takimiya, K.; McCullough, R. D. Adv. Mater. 2010, 22, 4993. (31) Osaka, I.; Akita, M.; Koganezawa, T.; Takimiya, K. Chem. Mater. 2012, 24, 1235.

Doctoral Program of Higher Education (20130010110006). The GIXRD data was obtained at 1W1A, Beijing Synchrotron Radiation Facility. The authors gratefully acknowledge the assistance of scientists of the Diffuse X-ray Scattering Station during the experiments.

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dx.doi.org/10.1021/ma402107n | Macromolecules XXXX, XXX, XXX−XXX