High Mobility Ambipolar Diketopyrrolopyrrole-Based Conjugated

Oct 26, 2018 - Yao Gao†‡ , Junhua Bai† , Ying Sui† , Yang Han† , Yunfeng Deng*† , Hongkun Tian‡ , Yanhou Geng*†‡§ , and Fosong Wang...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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High Mobility Ambipolar Diketopyrrolopyrrole-Based Conjugated Polymers Synthesized via Direct Arylation Polycondensation: Influence of Thiophene Moieties and Side Chains Yao Gao,†,‡ Junhua Bai,† Ying Sui,† Yang Han,† Yunfeng Deng,*,† Hongkun Tian,‡ Yanhou Geng,*,†,‡,§ and Fosong Wang‡

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School of Materials Science and Engineering and Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072, P. R. China ‡ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China § Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, P. R. China S Supporting Information *

ABSTRACT: Seven diketopyrrolopyrrole (DPP)-based donor− acceptor (D−A) conjugated polymers, i.e., poly[2,5-bis(4octadecyldocosyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-alt5,5′-di(thiophen-2-yl)-2,2′-3,3′,4,4′-tetrafluoro-2,2′-bithiophene] (P4F2T-C40), poly[2,5-bis(4-octadecyldocosyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-alt-5,5′-di(thiophen-2-yl)-2,2′1,2-bis(3,4-difluorothiophen-2-yl)ethyne] (P4FTAT-C40), poly[2,5-bis(4-octadecyldocosyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-alt-5,5′-di(thiophen-2-yl)-2,2′-(E)-1,2-bis(3,4difluorothien-2-yl)ethene] (P4FTVT-C40), poly[2,5-bis(4tetradecyloctadecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-alt-5,5′-di(thiophen-2-yl)-2,2′-(E)-1,2-bis(3,4-difluorothien-2-yl)ethene] (P4FTVT-C32), poly[2,5-bis(4-decyltetradecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-alt-5,5′-di(thiophen-2-yl)2,2′-(E)-1,2-bis(3,4-difluorothien-2-yl)ethene] (P4FTVT-C24), poly[2,5-bis(2-decyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-alt-5,5′-di(thiophen-2-yl)-2,2′-(E)-1,2-bis(3,4-difluorothien-2-yl)ethene] (P4FTVT-C22), and poly[2,5-bis(2decyltetradecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-alt-5,5′-di(thiophen-2-yl)-2,2′-(E)-1,2-bis(3,4-difluorothien-2-yl)ethene] (P4FTVT-C10C12), were synthesized by direct arylation polycondensation (DArP) with multi-fluorinated thiophene derivatives 3,3′,4,4′-tetrafluoro-2,2′-bithiophene (4F2T), (E)-1,2-bis(3,4-difluorothien-2-yl)ethene (4FTVT), or 1,2-bis(3,4difluorothiophen-2-yl) acetylene (4FTAT) as the comonomer. The structures of the multi-fluorinated thiophene derivatives have a noticeable influence on the optical properties and self-assembly properties of the polymers. Compared to P4F2T-C40, P4FTVT-C40 showed an ∼30 nm red-shift while P4FTAT-C40 exhibited an ∼60 nm blue-shift of the absorption spectrum. Top-gate and bottom-contact (TGBC) organic field-effect transistors (OFETs) of all the polymers exhibited ambipolar transport behavior. The devices based on P4FTAT-C40 displayed poor device performance since its film was almost amorphous. In contrast, the polymer based on 4FTVT with the same alkyl side chains delivered much better device performance due to its crystalline nature, favorable molecular orientations, and appropriate film morphology. With optimized side chains, P4FTVT-C32 exhibited the highest hole (μh) and electron mobilities (μe) of ca. 2.6 and ca. 8.0 cm2 V−1 s−1 in air, respectively.

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INTRODUCTION High mobility conjugated polymers (CPs) have drawn much attention these years due to their applications in solutionprocessed organic field-effect transistors (OFETs).1−7 A prevailing strategy to design high mobility CPs is alternatively linking electron-rich and electron-deficient conjugated units to form donor−acceptor (D−A) conjugated polymers.3,4,8,9 Over the past several years, diketopyrrolopyrrole (DPP) has emerged as one of the most popular A building blocks for high mobility D−A CPs because of its highly electron-deficient nature, planar structure, and ease of structural modification.1,10−16 Moreover, not only p-type but also ambipolar and © XXXX American Chemical Society

n-type high mobility D−A conjugated polymers can be obtained by tuning the chemical structures of comonomers.17−19 To date, DPP-based conjugated polymers are mainly synthesized by Pd-catalyzed Stille or Suzuki polycondensations. These methods must go through tedious steps to get high-purity bifunctional organometallic reagents and may also involve toxic organotin compounds and byproducts. Certainly, Received: May 27, 2018 Revised: October 19, 2018

A

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Figure 1. Chemical structures of the DPP-based conjugated polymers.

Scheme 1. Synthetic Route to the Polymers

we focus on unveiling the impact of these two aspects on the properties of the DPP-based CPs made by DArP. The synthesized seven CPs as shown in Figure 1 can be divided into two groups. Group I: P4F2T-C40, P4FTAT-C40, and P4FTVT-C40, which carry the same alkyl side chains, 4octadecyldocosyl, but vary by the thiophene moiety, 3,3′,4,4′tetrafluoro-2,2′-bithiophene (4F2T), 4FTVT, or 1,2-bis(3,4difluorothiophen-2-yl)acetylene (4FTAT). Group II: P4FTVT-C40, P4FTVT-C32, P4FTVT-C24, P4FTVT-C22, and P4FTVT-C10C12, which have the same thiophene moietiy but contain different alkyl side chains, 4-octadecyldocosyl (C40), 4-tetradecyloctadecyl (C32), 4-decyltetradecyl (C24), 2-decyldodecyl (C22), and 2-decyltetradecyl (C10C12), respectively. Therefore, we could readily and comprehensively study the impact of thiophene moieties and alkyl side chains on the optical, electrochemical, and chargetransport properties of the polymers.

environmentally benign reagents and greener and more atomeconomical protocols would be highly desirable for the synthesis of high mobility DPP-based CPs. Compared to Pdcatalyzed Stille or Suzuki polycondensations, direct arylation polycondensation (DArP) is more straightforward, atomeconomical, and eco-friendly in the synthesis of CPs.20−27 Several DPP-based CPs have been synthesized via DArP by using thiophene-flanked DPP as C−H monomers.20,28−32 C− Br/C−Br and/or C−H/C−H homocouplings were often observed in DArP, leading to low-molecular-weight products.28,30,31,33,34 Branching and cross-linking were also observed in some cases,30,35−38 especially when the polymerization was performed in polar aprotic solvents.30,37,38 Recently, we found that C−H bonds in (E)-1,2-bis(3,4difluorothien-2-yl)ethene (4FTVT) were highly reactive in DArP without any selectivity issues.20,21 A high-molecularweight DPP-based CP, i.e., poly[2,5-bis(2-decyltetradecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-alt-5,5′-di(thiophen2-yl)-2,2′-(E)-1,2-bis(3,4-difluorothien-2-yl)ethene] (P4FTVT-C10C12), was successfully synthesized via DArP.20 The incorporation of F atoms can reduce frontier molecular orbital (FMO) energy levels and improve the planarity of the backbones, attributed to the emergence of noncovalent intramolecular interactions. In consequence, the polymer exhibited promising ambipolar transport properties. It is wellknown that alkyl side chains and comonomers have a pronounced impact on the semiconducting properties of DPP-based CPs which were synthesized by conventional cross-coupling polycondensations.39−42 In the current paper,

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RESULTS AND DISCUSSION Synthesis. The DPP-based polymers P4F2T-C40, P4FTAT-C40, P4FTVT-C40, P4FTVT-C32, P4FTVT-C24, and P4FTVT-C22 were all synthesized in high yields by DArP according to our previously established procedure (Scheme 1).20,21 The synthesis of P4FTVT-C10C12 (Mn: 59.9 kDa) was depicted in the previous report.20 Two P4FTVT-C32 samples with different number-average molecular weights (Mn), named as P4FTVT-C32H and P4FTVT-C32L, were obtained by adjusting the polymerization time. A long branched alkyl chain, 4-octadecyldocosyl, was used as the B

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Macromolecules Table 1. Molecular Weights and Optical and Electrochemical Properties of the Polymers polymer

Mn (kDa)

Đ

Td (°C)

P4F2T-C40 P4FTAT-C40 P4FTVT-C40 P4FTVT-C32H P4FTVT-C32L P4FTVT-C24 P4FTVT-C22

44.6 70.5 34.8 49.8 23.0 52.9 46.0

1.85 5.50 2.21 1.94 2.54 2.73 3.42

406 395 396 369 372 373 369

λsol max (nm) 430, 426, 460, 457, 457, 463, 442,

718, 675, 738, 739, 739, 732, 731,

793 728 824 824 824 823 823

λfilm max (nm) 436, 429, 470, 466, 465, 466, 467,

728, 677, 741, 740, 739, 738, 739,

799 742 831 832 832 833 833

a Eopt g (eV)

Eox onset/EHOMO (V)/(eV)b

Ereonset/ELUMO (V)/(eV)b

c Ecv g (eV)

1.43 1.52 1.40 1.41 1.41 1.40 1.40

0.70/−5.50 0.66/−5.46 0.59/−5.39 0.60/−5.40 0.60/−5.40 0.60/−5.40 0.61/−5.41

−1.22/−3.58 −1.23/−3.57 −1.29/−3.51 −1.30/−3.50 −1.29/−3.51 −1.28/−3.52 −1.27/−3.53

1.92 1.89 1.88 1.90 1.89 1.88 1.88

b The optical bandgaps (Eopt g ) calculated from the film absorption onsets. HOMO and LUMO energy levels were calculated according to EHOMO = ox re ox −(4.80 + Eonset) eV and ELUMO = −(4.80 + Eonset) eV, in which Eonset and Ereonsetrepresent oxidation and reduction onset potentials of the polymers versus Fc/Fc+, respectively. cCalculated according to Ecv g = LUMO − HOMO. a

Figure 2. Film UV−vis−NIR absorption spectra (a, b) and HOMO and LUMO energy levels diagrams (c) of the polymers.

4FTVT unit relative to 4F2T.21 On the other hand, the insertion of an acetylene spacer between the two fluorinated thiophene rings induced a ca. 60 nm blue-shift in the absorption spectrum of P4FTAT-C40. A similar blue-shift in the absorption spectrum was also observed for the conjugated polymer with acetylene linkage43 because of the “push−pull” effect between D and A moieties that is somewhat disrupted by the acetylene linkage because of the electron-withdrawing nature of the acetylene unit.46,47 As shown in Figure 2b, P4FTVT-C40, P4FTVT-C32L, P4FTVT-C24, and P4FTVTC22 (P4FTVT-C10C12 as well20) exhibited almost identical thin film λmax, indicating that these polymers have similar packing structures in solid state. P4FTVT-C40, P4FTVTC32L, P4FTVT-C24, and P4FTVT-C22 also had similar solution λmax at room temperature (Figure S10). For all these polymers, the solution λmax was blue-shifted and the 0−0 absorption band was weakened when the solutions were heated to 90 °C (Figure S11), indicative of polymer aggregation in solution. The optical bandgaps (Egopt) of the polymers estimated from film absorption onsets are 1.43, 1.52, 1.40, 1.41, 1.40, and 1.40 eV for P4F2T-C40, P4FTAT-C40, P4FTVT-C40, P4FTVT-C32L, P4FTVT-C24, and P4FTVTC22, respectively (Table 1). To estimate the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels, film cyclic voltammograms (CV) of the polymers were recorded (Figure S12). The redox potentials and calculated HOMO and LUMO levels are summarized in Table 1, and the energy levels of FMOs are also schematically depicted in Figure 2c. The HOMO and LUMO energy levels of the three polymers containing different thiophene moieties are −5.50 and −3.58 eV for P4F2T-C40, −5.46 and −3.57 eV for P4FTAT-C40, and −5.39 and −3.51 eV for P4FTVT-C40. Compared to P4FTVT-C40, P4FTATC40 has slightly deeper HOMO and LUMO levels because of

side chains for group I polymers, since the solubility of 4FTAT-based polymer was very poor due to the high rigidity of 1,2-bis(thiophen-2-yl)acetylene (TAT) skelton.43 After the polymer purification by successive Soxhlet extractions with ethanol, acetone, and hexane, the polymers were dissolved in odichlorobenzene (o-DCB) and precipitated into methanol. All the polymers were characterized with 1H NMR and elemental analysis. However, well-resolved 1H NMR spectra could not be obtained even at 120 °C in 1,1,2,2-tetrachloroethane-d2 (C2D2Cl4) (Figures S1−S7) due to the strong aggregation of the polymers in solution.12,44 The molecular weights of the resulting polymers were measured by high temperature gel permeation chromatography (GPC) with polystyrene as standard and 1,2,4-trichlorobenzene as eluent at 150 °C, and the results are listed in Table 1. The large molar mass dispersity (Đ) of P4FTAT-C40 can be attributed to its low solubility, leading to strong aggregation in solution.20,27 These polymers show good thermal stability with 5% weight loss temperature above 360 °C (Figure S8). Thermal transitions at ∼4 °C caused by the melting of the long alkyl chains45 were observed for P4F2T-C40, P4FTAT-C40, P4FTVT-C40, and P4FTVT-C32 in the differential scanning calorimetry (DSC) profiles, while no phase transition can be detected for P4FTVT-C24 and P4FTVT-C22 (Figure S9). Photophysical and Electrochemical Properties. The UV−vis−NIR absorption spectra of the polymers were recorded in both solution (Figure S10) and film state (Figure 2a,b), and the related data are summarized in Table 1. Because the optical and electrochemical properties of P4FTVT-C32H and P4FTVT-C32L are identical (Table 1, Figures S10 and S12), only the data of P4FTVT-C32L are included in Figure 2. P4F2T-C40 showed an absorption maximum (λmax) at 793 nm in solution and 799 nm in thin film. In comparison with P4F2T-C40, the absorption spectrum of P4FTVT-C40 redshifted by ca. 30 nm due to the more electron-rich nature of C

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Macromolecules Table 2. OFET Device Performance of the Polymers p-channel polymera P4F2T-C40 P4FTAT-C40 P4FTVT-C40 P4FTVT-C32H P4FTVT-C32L P4FTVT-C24 P4FTVT-C22 P4FTVT-C10C12

μlin (cm2 V−1 s−1)b 0.10 0.009 0.59 1.68 1.30 0.32 0.36 0.28

(0.08) (0.007) (0.42) (1.43) (0.96) (0.28) (0.30) (0.22)

μsat (cm2 V−1 s−1)b 0.69 0.013 1.54 2.66 2.63 0.61 0.74 0.44

(0.50) (0.011) (0.97) (2.35) (1.49) (0.40) (0.64) (0.34)

n-channel VT (V)c

Ion/Ioffd

∼−22 ∼−50 ∼−40 ∼−50 ∼−40 ∼−40 ∼−55 ∼−35

10 −10 104−105 104−105 103−104 103−104 103−104 103−104 103−104 3

4

μlin (cm2 V−1 s−1)b 0.66 0.008 1.13 3.80 4.00 0.89 0.47 0.80

(0.60) (0.006) (0.88) (3.02) (2.28) (0.69) (0.35) (0.77)

μsat (cm2 V−1 s−1)b 1.21 0.024 3.22 7.90 8.11 2.10 1.24 1.51

(1.14) (0.015) (2.96) (6.72) (5.68) (1.78) (1.00) (1.30)

VT (V)c

Ion/Ioffd

∼40 ∼60 ∼55 ∼45 ∼50 ∼50 ∼55 ∼37

103−104 104−105 104−105 103−104 103−104 103−104 103−104 103−104

All polymer films were annealed at 200 °C for 15 min. bMobilities are measured under ambient conditions. The average values are in parentheses. Threshold voltage. dCurrent on/off ratio.

a c

Figure 3. Typical output (a−d) and transfer (e−h) characteristics of the OFET devices based on P4FTVT-C40 (a, e), P4FTVT-C32L (b, f), P4FTVT-C24 (c, g), and P4FTVT-C10C12 (d, h). Red and blue lines correspond to data measured under p- and n-channel operations, respectively.

bifurcation point of the side alkyl chains have little influence on the energy levels of these polymers. Note that the electroopt chemical bandgaps (Ecv g ) are ca. 0.5 eV larger than Eg . This phenomenon is often observed and can be explained by the

the electron-withdrawing nature of sp-hybridized acetylene linkages.43,48,49 All the polymers in group II have identical HOMO of ∼−5.40 eV and LUMO of ∼3.50 eV, which indicates that both the chain length and the position of the D

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above 20 kDa is enough to achieve high mobility for this polymer. Compared to P4FTVT-C40 and P4FTVT-C32, P4FTVT-C24, P4FTVT-C22, and P4FTVT-C10C12 all showed lower μh and μe, as shown in Table 2. Above results indicate that adjusting the commoner structure and optimizing the alkyl side chain length are equally important to target the high OFET performance for this type of DPP-based CPs. Notably, the undesirable kink phenomenon in the IDS1/2 versus VG plots was not observed in all OFET devices, which is often shown in OFETs of high mobility D−A copolymers.55 This means that the mobility values extracted from the transfer curves in the saturation regime are reliable. Also, the device performance data in linear regime are summarized in Table 2 and Figure S15. Linear regime μh and μe of P4FTVT-C32L were still up to 1.30 and 4.00 cm2 V−1 s−1, respectively. These values are comparable to or even higher than those of the reported DPP-based polymers.1,10 Film Morphology and Microstructures. To understand the correlation between the polymer structures and device performance, the morphology and microstructures of the thin films of the polymers on Si/SiO2 substrates were investigated by the tapping-mode atomic force microscopy (AFM) and grazing incidence X-ray diffraction (XRD). Figure 4 presents

exciton binding energy of the conjugated polymers, which is in the range ∼0.4−1.0 eV.50,51 Figure S13 depicts the energy levels and the optimized geometries of the three kinds of conjugated backbones obtained by density functional theory (DFT) calculations. The calculated energy levels show the same trend as the CV results. All the polymers exhibited highly planar main chains, and their HOMO and LUMO are highly delocalized along the backbones. This would facilitate both hole and electron transport, thus leading to an ambipolar transport behavior in OFETs.52,53 Semiconducting Properties. OFETs were fabricated to evaluate the semiconducting properties of the polymers. Because the bottom-gate devices often exhibit kink phenomenon in the IDS1/2 versus VG plots due to the interfacial disorder or traps,5,54 accurate or reliable mobility values cannot be extracted from the transfer curves in a saturation regime.55,56 To avoid this problem, top-gate and bottomcontact (TGBC) OFETs of the polymers were fabricated. Au source and drain electrodes (∼25 nm) were first deposited on the bare Si/SiO2 wafer through a shadow mask. The polymer film (∼30 nm) was then deposited on the substrate by spincasting the preheated (100 °C) polymer solution in o-DCB. The film was then thermally annealed in N2 at 200 °C for 15 min. After the poly(methyl methacrylate) (PMMA) dielectric layer (∼600 nm, Ci ≈ 4.6 nF/cm2) was deposited by spincasting and then annealed at 100 °C for 1 h, the device was finished by vacuum-depositing an Au gate electrode (∼80 nm). The channel length and width of the resulting devices were 80 μm and 5.6 mm, respectively. The devices were tested in the ambient conditions, and the performance data are outlined in Table 2. Figure 3 and Figure S14 show the typical output and transfer characteristics of TGBC OFETs based on these polymers. The output curves are characterized by the superposition of standard saturation behavior for one carrier at high VG and a superlinear current increase at low VG and high VD due to the injection of the opposite carrier. The transfer curves exhibited a V-shape. These are typical characteristics for ambipolar OFETs. Among the three polymers with different thiophene moieties (group I), P4FTVT-C40 showed the best device performance with hole mobility (μh) and electron mobility (μe) up to 1.54 and 3.22 cm2 V−1 s−1, respectively. P4F2T-C40 exhibited a lower device performance with μh and μe of 0.69 and 1.21 cm2 V−1 s−1, respectively. This phenomenon is consistent with the previous study of DPP-based p-type CPs. When bithiophene was replaced with the more planar (E)-1,2bis(thien-2-yl)ethene unit, the μh of the DPP-based polymer was dramatically enhanced.57 Surprisingly, very poor performance was observed for P4FTAT-C40-based devices, both μh and μe (0.013 and 0.024 cm2 V−1 s−1, respectively) were 2 orders of magnitude lower than those of P4FTVT-C40. Because the backbones of these polymers are all highly planar, this discrepancy in charge transport properties may be related to the different molecular packing and/or morphology in thin films (as discussed below). To further improve the device performance of 4FTVT-based polymer, we synthesized P4FTVT-C32, P4FTVT-C24, and P4FTVT-C22 that contain different alkyl side chains. With shorter alkyl side chains compared with P4FTVT-C40, P4FTVT-C32 exhibited much higher μh and μe, which were up to ca. 2.6 and ca. 8.0 cm2 V−1 s−1, respectively. This electron mobility is among the highest value observed to date for OFETs.58 P4FTVT-C32H and P4FTVT-C32L had similar μh and μe, suggesting that Mn of

Figure 4. Out-of-plane XRD profiles of the thermally annealed films of the polymers. The thermal annealing was done at 200 °C for 15 min.

the out-of-plane XRD profiles of these films. P4F2T-C40, P4FTVT-C40, P4FTVT-C32L, P4FTVT-C24, P4FTVTC22, and P4FTVT-C10C12 all show well-defined (h00) diffraction peaks, with up to the fourth order for P4F2T-C40, P4FTVT-C40, and P4FTVT-C32L and the third order for P4FTVT-C24, P4FTVT-C22, and P4FTVT-C10C12, indicative of the lamellar packing of the polymer chains. In addition, P4FTVT-C40 and P4FTVT-C32L showed sharper (100) diffraction peaks, implying that their films comprised larger ordered domains. P4FTVT-C32H showed the XRD patterns very similar to that of P4FTVT-C32L (Figure S17). The lamellar distances calculated from (100) peaks are 31.53, 32.70, 39.23, 27.59, 23.54, 21.80, and 21.80 Å for P4F2T-C40, P4FTVT-C40, P4FTVT-C32L, P4FTVT-C24, P4FTVTC22, and P4FTVT-C10C12, respectively. This trend of the lamellar distances is consistent with the varied lengths of the side chains. Surprisingly, P4FTAT-C40 only displayed a broad and weak (100) diffraction peak although it has a planar and quasi-linear conjugated backbone (Figure S13). This means that the film of P4FTAT-C40 was almost amorphous, consistent with its low mobility. We suspect that the highly stiff backbone of P4FTAT-C40, because of the rigid nature of TAT unit, endows the polymer with very strong aggregation E

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Figure 5. The 2 μm scale AFM height images of the thermally annealed films of P4F2T-C40 (a), P4FTAT-C40 (b), P4FTVT-C40 (c), P4FTVTC32H (d), P4FTVT-C32L (e), P4FTVT-C24 (f), P4FTVT-C22 (g), and P4FTVT-C10C12 (h). The thermal annealing was done at 200 °C for 15 min.

larger fiber-like nanostructures. Larger domains mean less grain boundaries and are favorable for charge carrier transport. This result further supports the fact that P4FTVT-C32 presented high mobilities.

tendency in organic solvents. Thereby, the polymer chains aggregated too fast either in the polymerization course or when the solution was cooled from high temperature. This behavior prohibits the polymer from forming the ordered nanostructures in the aggregates via self-assembly of polymer chains. For P4F2T-C40, one could observe two more diffraction peaks at 2θ = 20.55° and 25.10°, the former of which is associated with the alkyl chains packing,59−62 while the latter one can be attributed to (010) diffraction and corresponds to a π−π distance of 3.54 Å. The appearance of the (010) peak in the out-of-plane diffraction patterns indicated that partial P4F2TC40 chains might be packed in a face-on mode. This packing behavior of P4F2T-C40 is not favorable for charge transport in the OFET devices. The other polymers all likely packed in an edge-on mode since no (010) diffraction was observed in their out-of-plane XRD profiles. In the in-plane diffraction patterns (Figure S18), no (010) peaks were observed for P4F2T-C40, P4FTAT-C40, and P4FTVT-C40, while weak (010) diffraction peaks with π−π stacking distances of ∼3.5 Å appeared for other polymers. Both P4FTVT-C32H and P4FTVT-C32L displayed stronger in-plane (010) diffraction peaks relative to P4FTVT-C24, P4FTVT-C22, and P4FTVT-C10C12. Overall, P4FTAT-C40 is characterized by low crystallinity and therefore showed lower mobilities while P4FTVT-C32 exhibited the highest packing order and clean edge-on orientation, delivering remarkably high mobilities. Figure 5 and Figure S19 show the AFM height images of the annealed polymer thin films. All polymers formed intercalating fibrillary networks except for P4FTAT-C40, which only showed some aggregates of up to hundreds of nanometers on the top of amorphous surface. This is consistent with its poor crystalline properties and low mobilities. Recently, Pei et al. found that the film morphology and device properties of the D−A conjugated polymers are closely related to the aggregation of the polymer chains in solution.63 We suspect that P4FTAT-C40, which shows very poor solubility, formed disorderly packed aggregates in solution. This led to the formation of the film with disordered aggregates and low crystallinity. Among P4F2T-C40 and the polymers based on 4FTVT, P4FTVT-C32H and P4FTVT-C32L film showed

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CONCLUSIONS

A series of DPP-based ambipolar conjugated copolymers, i.e., P4F2T-C40, P4FTAT-C40, P4FTVT-C40, P4FTVT-C32, P4FTVT-C24, P4FTVT-C22, and P4FTVT-C10C12, were synthesized via DArP. The structures of the multi-fluorinated thiophene segments (i.e. 4F2T, 4FTAT, and 4FTVT) have a noticeable impact on the optical properties, energy levels, and film microstructures of the polymers. Compared with P4FTVT-C40, P4F2T-C40 and P4FTAT-C40 exhibited blue-shifted absorption spectra and slightly lower FMO energy levels due to the less electron-rich nature of 4F2T and 4FTAT relative to the 4FTVT unit. Both P4FTVT-C40 and P4F2TC40 formed ordered films, in which the polymer chains adopted edge-on packing mode for P4FTVT-C40, whereas face-on packing feature was also observed for P4F2T-C40. In contrast, the films of P4FTAT-C40 were almost amorphous. Accordingly, the charge carrier mobilities (μh and μe) of OFETs based on these three polymers were in the order of P4FTVT-C40 > P4F2T-C40 > P4FTAT-C40. All the polymers based on 4FTVT with different alkyl chains showed similar optical properties and FMO energy levels. However, the alkyl side chains are helpful for finely tuning the film morphology and microstructures of the polymers. P4FTVTC32, with optimized alkyl side chains, exhibited the highest packing order, favorable molecular orientations, and appropriate film morphology. Therefore, OFETs based on this polymer showed the best device performance with μh and μe of ca. 2.6 and ca. 8.0 cm2 V−1 s−1, respectively. Our study demonstrates that high mobility DPP-based conjugated polymers can be synthesized by DArP, an atom-economical and eco-friendly protocol, via selecting the appropriate comonomer and carefully optimizing the solubilizing side chains. F

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Macromolecules

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Notes

EXPERIMENTAL SECTION

The authors declare no competing financial interest.

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Materials. Toluene, triethylamine (Et3N), and tetrahydrofuran (THF) were dried and distilled prior to use. 4F2T, 4FTVT, and P4FTVT-C10C12 were prepared according to the literature.20,21 The synthesis of 4FTAT is outlined in the Supporting Information. Other chemicals or reagents were used as received. General Procedure for the Synthesis of the Polymers. Fluorinated thiophene monomer (1 equiv), DPP monomer (1 equiv), Herrmann’s catalyst (2 mol %), P(o-MeOPh)3 (4 mol %), pivalic acid (1 equiv), and Cs2CO3 (3 equiv) were added in a pressure-proof tube with a magnetic stirring bar. The tube was sealed with a cap after toluene was added in a glovebox (monomer concentration: 0.1 mol/ L). The mixture was heated to 120 °C and stirred for 6−16 h (see below). After cooling to room temperature, the mixture was precipitated in methanol. The crude polymer was collected by filtration and purified by Soxhlet extraction with ethanol, acetone, and hexane in succession. The remaining solid dissolved in o-DCB and filtered by a 0.45 μm filter. Polymer was obtained by precipitating the o-DCB solution in methanol, filtrated, and dried in a vacuum. P4FTVT-C22. Polymerization time: 6 h. Yield: 91%. GPC: Mn = 46.0 kDa, Đ = 3.42. Elemental Anal. Calcd for (C68H96F4N2O2S4)n: C, 69.23; H, 8.37; N, 2.37; S, 10.87. Found: C, 68.99; H, 8.29; N, 2.41; S, 9.15. P4FTVT-C24. Polymerization time: 6 h. Yield: 87%. GPC: Mn = 52.9 kDa, Đ = 2.73. Elemental Anal. Calcd for (C72H104F4N2O2S4)n: C, 69.97; H, 8.64; N, 2.27; S, 10.38. Found: C, 68.81; H, 8.16; N, 1.93; S, 9.62. P4FTVT-C32H. Polymerization time: 16 h. Yield: 94%. GPC: Mn = 49.8 kDa, Đ = 1.94. Elemental Anal. Calcd for (C88H136N2F4O2S4)n: C, 72.38; H, 9.53; N, 1.92; S, 8.78. Found: C, 70.82; H, 10.31; N, 2.27; S, 8.62. P4FTVT-C32L. Polymerization time: 12 h. Yield: 90%. GPC: Mn = 23.0 kDa, Đ = 2.54. Elemental Anal. Calcd for (C88H136N2F4O2S4)n: C, 72.38; H, 9.53; N, 1.92; S, 8.78. Found: C, 72.11; H, 9.29; N, 1.88; S, 8.95. P4FTVT-C40. Polymerization time: 12 h. Yield: 94%. GPC: Mn = 34.8 kDa, Đ = 2.21. Elemental Anal. Calcd for (C104H168N2F4O2S4)n: C, 74.23; H, 10.06; N, 1.66; S, 7.62. Found: C, 74.12; H, 9.97; N, 1.56; S, 7.70. P4FTAT-C40. Polymerization time: 12 h. Yield: 92%. GPC: Mn = 70.5 kDa, Đ = 5.50. Elemental Anal. Calcd for (C104H166N2F4O2S4)n: C, 74.32; H, 9.96; N, 1.67; S, 7.63. Found: C, 74.18; H, 9.93; N, 1.59; S, 7.72. P4F2T-C40. Polymerization time: 12 h. Yield: 92%. GPC: Mn = 44.6 kDa, Đ = 1.85. Elemental Anal. Calcd for (C102H166N2F4O2S4)n: C, 73.95; H, 10.10; N, 1.69; S, 7.74. Found: C, 73.88; H, 10.04; N, 1.57; S 7.80.

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ACKNOWLEDGMENTS This work is supported by the National Key R & D Program of “Strategic Advanced Electronic Materials” (No. 2016YFB0401100) of Chinese Ministry of Science and Technology, the National Natural Science Foundation of China (No. 51333006), and the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB12010300).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01112.

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REFERENCES

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Instruments, experimental details for preparation and characterization of OFET devices, preparation of intermediates and monomers, TGA and DSC traces of the polymers, and other device data (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.H.G.). *E-mail: [email protected] (Y.F.D.). ORCID

Yao Gao: 0000-0003-0172-8151 Yanhou Geng: 0000-0002-4997-3925 G

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