Ï-Extended Isoindigo-Based Derivative: A Promising Electron

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#-Extended Isoindigo-based Derivative: a Promising Electrondeficient Building Block for Polymer Semiconductors Long Xu, Zhiyuan Zhao, Mingchao Xiao, Jie Yang, Jian Xiao, Zhengran Yi, Shuai Wang, and Yunqi Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13570 • Publication Date (Web): 19 Oct 2017 Downloaded from http://pubs.acs.org on October 20, 2017

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ACS Applied Materials & Interfaces

π-Extended Isoindigo-based Derivative: a Promising Electron-deficient Building Block for Polymer Semiconductors Long Xu,†,§ Zhiyuan Zhao,‡,§Mingchao Xiao,† Jie Yang,† Jian Xiao,† Zhengran Yi*,† Shuai Wang,*,† Yunqi Liu.*,‡ †

College of Chemistry and Chemical Engineering, and Key Laboratory for Large-format Battery

Materials and System, Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and Technology, Wuhan 430074, China. ‡

Key Laboratory of Organic Solids, Beijing National Laboratory for Molecular Sciences,

Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. KEYWORDS: organic thin-film transistors, electron-deficient building blocks, donor−acceptor copolymers, isoindigo, molecular packing.

ABSTRACT: The exploration of novel electron-deficient building blocks is a key task for developing high-performance polymer semiconductors in organic thin film transistors (OTFTs). In view of the situation of the lack of strong electron-deficient building blocks, we designed two novel π-extended isoindigo-based electron-deficient building blocks, IVI and

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F4IVI. The strong electron-deficient nature and the extended π-conjugated system for the two acceptor units endow their copolymers containing 2,2ʹ-bithiophene donor units, PIVI2T and PF4IVI2T, deep-lying HOMO/LUMO energy levels and the strong intermolecular interactions. In comparison to PIVI2T, the fluorinated PF4IVI2T exhibits stronger intra- and intermolecular interactions, lower HOMO/LUMO energy levels up to –5.74/–4.17 eV, more ordered molecular packing with smaller π−π stacking distance up to 3.53 Å, resulting in excellent ambipolar transporting behavior and promising application in logic circuits for PF4IVI2T in ambient with hole and electron mobilities of up to 1.03 and 1.82 cm2V−1s−1, respectively. The results reveal that F4IVI is a promising and strong electron-deficient building unit to construct highperformance semiconducting polymers, which provides an insight into the structure-property relationships for the exploration and molecular engineering of excellent electron-deficient building blocks in the field of organic electronics.

1. Introduction Conjugated polymers have drawn great attention for their potential applications in low-cost, solution-processed and flexible organic thin film transistors (OTFTs).1−7 Despite charge carrier mobilities of polymer semiconductors frequently exceeded 1 cm2V−1s−1,8−13 their development still lags far behind inorganic semiconductors with carrier mobility of up to 100 cm2V−1s−1, hindering potential use in wearable electronic and organic light-emitting diode, etc.

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recent years, great efforts have been made to develop high-mobility polymer semiconductors via tuning their electronic structures, including energy levels [highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)], crystallinity, molecular packing,


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and so forth. The electronic structures determined by molecular structures can greatly influence oxidative stability, efficient charge carrier hopping, stabilized charge carrier transport of the polymers. Therefore, the exploration of novel building blocks in the polymer backbone, especially for electron-deficient building blocks (acceptors), is one of the most key tasks in developing high-mobility polymer semiconductors. Recently, imide-functionalized fused aromatic rings such as naphthalene diimide (NDI),16,17 isoindigo (II),18,19 diketopyrrolopyrrole (DPP),20,21 bithiopheneimide (BTI),22,23 have received significant attention in the field of OTFTs due to the strong electron-withdrawing property of imide groups. Among them, II is a very promising acceptor unit to construct donor-acceptor (D– A) copolymers, facilitating the profound development of OTFTs. However, the high LUMO levels of many II-based copolymers limit their application in ambipolar and n-type OTFTs, promoting us to develop stronger electron-deficient acceptor by molecular engineering of II core. Herein, we designed a new acceptor unit, IVI (Figure 1), in which two II molecules are bridged by a vinyl group. This choreographed acceptor unit IVI is expected to possess an improved electron-withdrawing property, which contributes to lower the HOMO and LUMO energy levels and thereby stabilize charge carrier transport and enhance oxidative stability of the corresponding copolymer. Moreover, the extended π-conjugated system for IVI may be favorable to enhancing molecular packing and crystallinity of the polymer, facilitating the charge transport.24 Additionally, in order to further enhance the electron-deficient nature of acceptor unit, four electron-withdrawing fluorine atoms were introduced into IVI to form F4IVI (Figure 1), which can provide the enhanced intermolecular interactions and the lower HOMO and LUMO energy levels. Meanwhile, the introduced fluorine atoms could “lock” the vinyl group and neighboring units to form more intramolecular hydrogen bonds,11,25 leading to the enhanced


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planarity of polymer backbone and thereby the effective charge intrachain transport. Using the two acceptors, we prepared two D–A copolymers, PIVI2T and PF4IVI2T, in which a 2,2′bithiophene unit was used as the donor. PIVI2T displayed p-type transport behavior with hole mobility of 0.43 cm2V−1s−1, while fluorinated PF4IVI2T showed typical ambipolar transporting behavior under ambient conditions with high electron mobility of up to 1.82 cm2V−1s−1 and significantly enhanced hole mobilities of up to 1.03 cm2V−1s−1, which is among the best performance in ambipolar polymer semiconductors tested in ambient. 2. Experimental details 2.1. Materials and instrumentation. All the chemicals were purchased form chemical companies. NMR spectra and mass spectra were performed on Mercury-VX300n (400 MHz) and Ion Spec 4.7 T FTMS instrument, respectively. Thermal gravimetric analysis (TGA) differential scanning calorimetry (DSC) analyses were carried out on Netzsch STA 449C and PerkinElmer Pyris 1, respectively. Absorption spectra were conducted on a Uv6100pc double-beam spectrophotometer. Gel permeation chromatography (GPC) was recorded on an Agilent PL-GPC220 instrument at 150 °C with trichlorobenzene as eluent. Cyclic voltammetry (CV) were carried out on a CHI 760E electrochemical workstation, and measured in acetonitrile solution with 0.1 M Bu4NPF6 as electrolyte. Ag/AgCl(saturated), platinum wire, Glassy carbon electrode were used as reference electrode, auxiliary electrode, working electrode, respectively. Ferrocene (EHOMO = −4.8 eV) was used as a reference. Tapping atomic force microscopy (AFM) was conducted on a Nanoscope V AFM. The grazing-incidence X-ray diffraction (GIXD) was taken on a Xenocs-SAXS/WAXS system at 1W1A, Beijing Synchrotron Radiation Facility, China.


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2.2. Fabrication and measurement of OTFTs. Top-gate and bottom-contact (TGBC) OTFTs with n+-Si/SiO2 used as substrates were fabricated. The source/drain electrodes (Au) were patterned by photolithography on the substrates, followed by the cleaning of the substrates with deionized water, iso-propanol and acetone. Subsequently, PIVI2T and PF4IVI2T thin films were deposited on the substrate by spincoating PIVI2T and PF4IVI2T solutions in dichlorobenzene (10 mg/mL) at 4000 rpm for 1 min, respectively. After annealing at 160 °C, a PMMA solution (80 mg/mL) was spin-coated on the semiconducting films. Then, 50 nm layer of Al was thermally evaporated by a shadow mask on the substrate to form gate electrodes. All the OTFT devices were tested under ambient conditions. Channel length (L) and channel width (W) are 10 and 1400 µm, respectively. The OTFT performance was measured using a Keithley 4200 parameter analyzer. The carrier mobilities of the polymers were calculated according to the equation Ids = Ciµ(W/2L)(Vgs − VT)2 , where Ids, Ci, Vgs, and VT are the drain current in the saturated regime, the capacitance per unit area of the SiO2 dielectric layer, gate voltage and threshold voltage, respectively. The fabrication procedures of complementary-like inverters were similar with those of TGBC FETs. The inverters were fabricated on corning glass substrate. Then the substrates were combined with two identical ambipolar TGBC OTFTs based on one-component ambipolar polymer (PF4IVI2T). The two ambipolar transistors had the same channel widths (W = 1400 µm) and different channel lengths (20 and 40 µm, respectively). A common gate was used as the input voltage (VIN) and a common drain was used as the output voltage (VOUT). The inverters were also measured under the ambient conditions. 2.3. Synthetic procedures.


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Synthesis of 6-bromo-1-(4-decyltetradecyl)indoline-2-dione (2a). To a solution of 6-bromo1-(4-decyltetradecyl)indoline-2,3-dione (1a, 500 mg, 0.889 mmol) in 1,4-dioxane (20 mL) was added 5 mL of hydrazine hydrate. The reaction mixture was refluxed for 24 h under argon atmosphere. After cooling to room temperature, the mixture was poured into water and then extracted with dichloromethane. The collected organic phase was dried over anhydrous Na2SO4, concentrated and finally purified by chromatography with silica gel column (PE/CH2Cl2 = 2/1) to afford 2a as a yellow oil (405 mg, 82.1%). 1H NMR (CDCl3, 400 MHz, ppm): δ 7.15 (dd, J1 = 7.84 Hz, J2 = 1.56 Hz, 1H), 7.09 (d, J = 7.8 Hz,1H), 6.95 (d, J = 1.28Hz,1H), 3.63 (t, J = 7.4Hz, 2H), 3.45 (s, 2H), 1.62 (t, J = 7.4 Hz, 2H), 1.26 (m, 39H), 0.88 (t, J = 7.4 Hz, 6H).

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C NMR

(CDCl3, 100 MHz, ppm): δ 174.63, 146.04, 125.62, 124.82, 123.43, 121.34, 111.71, 63.56, 40.54, 37.10, 35.36, 33.51, 31.92, 30.70, 30.09, 29.69, 29.65, 29.35, 26.65, 24.40, 22.69, 14.11. HR-MALDI-TOF: [M+H]+ calcd for C32H54BrNO : 549.68, found: 549.34. Synthesis of 6-bromo-7-fluoro-1-(4-decyltetradecyl)indoline-2-one (2b). the synthetic method is similar to 2a (79.3%). 1H NMR (400 MHz, CDCl3, ppm): δ 7.18 (m, 1H), 6.90 (d, J = 8.0 Hz, 1H), 3.80 (t, J = 7.6 Hz, 2H), 3.62 (t, J = 8.0 Hz, 2H), 3.50 (s, 2H), 1.64 (m, 2H), 1.32−1.02 (m, 39H), 0.88 (m, 4H). 13C NMR (CDCl3, 100 MHz, ppm): δ 174.08, 145.13, 142.71, 132.49, 132.40, 126.50, 126.46, 126.10, 120.97, 120.93, 109.35, 109.15, 63.50, 42.41, 42.37, 37.24, 37.08, 35.77, 33.61, 33.50, 31.93, 30.42, 30.13, 30.09, 29.98, 29.71, 29.66, 29.59, 29.36, 26.67, 26.62, 26.19, 26.16, 22.69, 14.10. HR-MALDI-TOF: [M+Na]+ calcd for C32H53BrFNO : 589.67, found: 589.32. Synthesis of 2,2'-(1E)-1,2-ethenediyl-bis[1-(4-decyltetradecyl)indoline-2,3-dione] (3a). 1a (430 mg, 0.764 mmol), (Z)-1,2-bis-trimethylstannylethyne (244 mg, 0.40 mmol), Pd2(dba)3 (18 mg, 0.02 mmol), P(o-tol)3 (24 mg, 0.08 mmol), and anhydrous chlorobenzene (20 ml) were


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added to a Schlenk tube under argon protection. Subsequently, the mixture was stirred for 48 h at 130 °C. After cooling to room temperature, the mixture was poured into water and then extracted with chloroform. The collected organic phase was dried over anhydrous Na2SO4, concentrated and finally purified by chromatography with silica gel column (PE/CH2Cl2 = 2/3) to afford 3a as a red solid (289 mg, 72.8%). 1H NMR (C6D6, 400 MHz, ppm: δ 7.30 (d, J = 7.72 Hz, 2H), 6.83 (s, 2H), 6.67 (d, J = 7.76 Hz, 2H), 6.57(s, 2H), 3.42 (t, J = 6.84 Hz, 4H), 1.55 (s, 4H), 1.32 (m, 78H), 0.91 (d, J = 6.92 Hz, 12H). 13C NMR (CDCl3, 100 MHz, ppm): δ 182.54, 158.45, 151.60, 145.88, 132.01, 125.97, 122.46, 117.57, 107.82, 40.70, 37.10, 33.48, 31.91, 30.78, 30.12, 29.70, 29.64, 29.35, 26.64, 24.52, 22.68, 14.11. HR-MALDI-TOF: [M+Na]+ calcd for C66H106N2O4: 1014.56 , found: 1014.80. Synthesis of 2,2'-(1E)-1,2-ethenediyl-bis[7-fluoro-1-(4-decyltetradecyl)indoline-2-dione] (3b). The synthetic method is similar to 3a (68.1%). 1H NMR (CDCl3, 400 MHz, ppm): δ 7.52(s, 2H), 7.47 (d, J = 8.16 Hz, 2H), 7.39 (t, J = 5.52 Hz, 2H), 3.89 (t, J = 7.04 Hz, 4H), 1.71 (s, 4H), 1.24 (m, 78H), 0.86 (d, J = 6.36 Hz, 12H). 13C NMR (CDCl3, 100 MHz, ppm): δ 181.98, 158.08, 147.11, 144.61, 137.87, 137.78, 134.90, 134.79, 126.08, 121.81, 121.31, 119.80, 43.15, 37.09, 33.47, 31.91, 30.47, 30.09, 29.70, 29.65, 29.35, 26.61, 26.01, 22.68, 14.11. HR-MALDI-TOF: [M+Na]+ calcd for C66H104F2N2O4: 1050.54, found: 1050.78. Synthesis of 2,2'-(1E)-1,2-ethenediyl-bis[6-bromo-N,N-(4-decyltetradecyl)-isoindigo] (4a, IVI). To a solution of acetic acid (10 mL) were added 3a (174 mg, 0.175 mmol) and 2a (211.8 mg, 0.386 mmol), followed by the addition of 5 mL of concentrated hydrochloric acid. Subsequently, the mixture was refluxed for 24 h under argon atmosphere. After cooling to room temperature, the precipitated solid was washed with methanol, and then purified by chromatography with silica gel column (PE/CH2Cl2 = 1/1) to afford a black solid (12.08 mg,


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62.4%). 1H NMR (CDCl3, 400 MHz, ppm): δ 9.20 (d, J = 8.36 Hz, 2H), 9.08(d, J = 8.6 Hz, 2H), 7.21 (s, 2H), 7.19 (s, 2H), 7.17 (dd, J1 = 8.64 Hz, J2 = 6.84 Hz, 2H), 6.93 (s, 4H), 3.81 (t, J = 6.64 Hz, 4H), 3.73 (t, J = 7.04 Hz, 4H), 1.70 (t, J = 6.92 Hz, 8H), 1.23(m, 156H), 0.86 (d, J = 6.92 Hz, 24H).

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C NMR (CDCl3, 100 MHz, ppm): δ 168.10, 167.77, 145.57, 145.36, 140.97,

133.14, 131.50, 131.07, 130.47, 130.37, 126.29, 124.95, 121.89, 121.44, 120.64, 111.16, 105.21, 40.61, 40.47, 37.16, 33.54, 31.93, 30.95, 30.88, 30.15, 30.12, 29.73, 29.66, 29.37, 26.69, 24.72, 24.56, 22.70, 14.13. HR-MALDI-TOF: [M+Na]+ calcd for C130H210Br2N4O4 : 2075.89, found: 2075.46. Synthesis of 2,2'-(1E)-1,2-ethenediyl-bis[6-bromo-7,7-difluoro-N, N-(4-decyltetradecyl)isoindigo (4b, F4IVI). The synthetic method is similar to 4a (70.4%). 1H NMR (CDCl3, 400 MHz, ppm): δ 8.88 (d, J =8.52 Hz, 2H), 8.77 (d, J = 8.64 Hz, 2H), 7.30 (s, 2H), 7.13 (d, J = 2.28Hz, 2H), 7.05 (d, J = 2.24 Hz, 2H), 3.85 (t, J = 7.36 Hz, 8H), 1.62 (t, J = 7.04 Hz, 8H), 1.16 (m, 156H), 0.79 (d, J = 7.56Hz, 24 H).

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C NMR (CDCl3, 100 MHz, ppm): δ 166.22, 165.90,

144.88, 143.63, 142.42, 141.22, 132.06, 131.16, 131.07, 130.92, 128.52, 128.42, 125.42, 125.01, 124.45, 123.35, 122.84, 122.28, 118.43, 113.46, 113.27, 41.72, 36.20, 36.14, 36.09, 32.54, 30.92, 29.75, 29.64, 29.13, 29.03, 28.72, 28.66, 28.36, 26.07, 25.67, 25.26, 25.14, 21.67, 13.08. HRMALDI-TOF: [M+H]+ calcd for C130H206Br2F4N4O4: 2125.85, found: 2125.44. Synthesis of PIVI2T. 4a (120 mg, 0.05845 mmol), 5,5'-bis(trimethylstannyl)-2,2'-bithiophene (28.75 mg, 0.05845 mmol), Pd2(dba)3 (1 mg, 1.169 µmol), P(o-tol)3 (1.4 mg, 4.676 µmol) was added to a dried Schlenk tube. The Schlenk tube was degassed and filled with argon three times, followed by the addition of 6 mL of anhydrous chlorobenzene. The mixture was stirred for 3 days at 130 °C and Subsequently dropped into 100 mL of methanol. The precipitated solid was then purified by Soxhlet extraction with methanol, acetone, ethyl acetate, and chloroform. The


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collected chloroform fraction was concentrated, and then precipitated from methanol to get a dark solid in a yield of 64%. Mw/Mn (GPC) = 213509/46294. Synthesis of PF4IVI2T. The synthetic method is similar to PIVI2T (70.4%). Mw/Mn (GPC) =75052/32775. 3. Results and Discussion Scheme 1 shows the synthetic route of PIVI2T and PF4IVI2T. The target polymers were started from N-alkylated isatins (1a, 1b), which were synthesized according to the reported literature.26 The isatins were subjected to a Wolff-Kishner-Huang reduction reaction and a Stille coupling reaction with (E)-1,2-bis-(tributylstannyl)ethane, giving N-alkylated indolin-2-one (2a, 2b) and doubly isatins (3a, 3b), respectively. The acceptor units (IVI, 4a; F4IVI, 4b) were obtained by the condensation of indolin-2-one and doubly isatins in acidic solution, followed by Stille coupling polymerization using 5,5′-bis(trimethylstannyl)-2,2′-bithiophene to afford PIVI2T and PF4IVI2T, respectively. The two polymers were purified by Soxhlet extraction with methanol, acetone and hexane to remove oligomers, and finally chloroform to get polymer products. The number-average molecular weight (Mn) evaluated by high-temperature gel permeation chromatography (GPC) at 150 °C was 46.3 kDa and 36.8 kDa for PIVI2T and PF4IVI2T, respectively (Figure S1, Supporting Information). Both the polymers displayed good thermal stability with decomposition temperatures exceeding 380 °C (Figure S2, Supporting Information). Differential scanning calorimetry (DSC) analysis of the polymers showed that no phase transition occurred in the range of 25 to 300 °C (Figure S3, Supporting Information). The UV–vis absorption spectra of PIVI2T and PF4IVI2T in chloroform solution and thin film are displayed in Figure 2a and summarized in Table 1. It can be observed that the absorption


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maximum (λmax) of PIVI2T in thin film showed no obvious red-shift compared to that in solution, whereas λmax of PF4IVI2T exhibited about 9 nm red-shifted absorption from solution to film, likely due to the enhanced solid-state packing effects with the introduction of F atoms. In comparison to PIVI2T, absorption spectra of PF4IVI2T both in solution and film exhibited distinct red-shifts and increased 0−0 vibrational peaks, implying that the introduction of F atoms endows PF4IVI2T with a planar polymer backbone. The optical band gaps (Egopt) of PIVI2T and PF4IVI2T are estimated to be 1.61 and 1.57 eV, respectively, which are estimated from the absorption onset (λmaxfilm). The smaller Egopt for PF4IVI2T could reveal the existence of stronger intramolecular and intermolecular interactions in PF4IVI2T backbone. Computational analysis of methyl-substituted PIVI2T and PF4IVI2T fragments shows that both polymers have idential phenyl-vinyl dihedral angles (0.5°) and clearly different phenyl-thienyl dihedral angles (20.7° for PIVI2T vs 17.5° for PF4IVI2T, Figure 2b and 2c). The results indicates that the phenyl-thienyl dihedral angles can be greatly influenced by intramolecular F−H interaction, leading to the enhanced planarity of PF4IVI2T backbone. The cyclic voltammetry (CV) measurements show that both polymers have stronger oxidative peaks relative to their reductive ones (Figure 3a), which are similar to many reported II-based polymers.27,28 Compared to PII2T containing the same donor and side chain,28 PIVI2T exhibited lower HOMO/LUMO levels (–5.60/–3.99 eV vs –5.52/–3.74 eV), which is ascribed to the stronger electron-withdrawing property of IVI accepter unit. Moreover, fluorination of IVI core results in a further decrease of HOMO/LUMO levels to –5.74/–4.17 eV, due to the enhanced electron-deficient nature of F4IVI. The decreasing trend of HOMO/LUMO levels is consistent with density functional theory (DFT) calculations (–5.00/–3.05 eV for PIVI2T and –5.16/–3.27 eV for PF4IVI2T, Figure 3b−3e).


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Top-gate/bottom-contact (TGBC) OTFTs with gold used as source/drain electrodes were fabricated to evaluate the charge transport properties of the polymers. PIVI2T and PF4IVI2T thin films were deposited on the heavily doped n+-Si/SiO2 substrate by spin-coating PIVI2T and PF4IVI2T solutions in dichlorobenzene (10 mg/mL), respectively. Then, the thin films were annealed at 160 °C (optimizing temperature), and followed by the spin-coating of a PMMA solution on the semiconducting films. All the OTFT devices were tested under ambient conditions. The corresponding output and transfer curves of OTFT devices based on PIVI2T and PF4IVI2T are displayed in Figure S4 (Supporting Information) and Figure 4, respectively. The PIVI2T devices exihibited p-channel behavior with hole mobility of up to 0.32 cm2V−1s−1, while PF4IVI2T devices showed clear ambipolar transport characteristics with hole and electron mobilities of up 1.03 and 1.82 cm2V−1s−1 (Table S1, Supporting Information), respectively, to best of our knowledge, which is one of the best performance for ambipolar polymer semiconductors tested in ambient. The high hole and electron mobilities for PF4IVI2T can be attributed to the extended π-conjugated system and strong electron-deficient nature of F4IVI acceptor unit in polymer backbone, leading to strong intra- and intermolecular interactions as well as low LUMO energy levels. Furthermore, the excellent amibipolar characteristic of PF4IVI2T indicates that it could have a promising application in organic complementary circuits. Figure 4e shows a schematic device configuration of organic complementary-like inverter based on PF4IVI2T. Notably, unlike the conventional and complex inverters in need of deposition of two p-type and n-type polymer semiconductors, we fabricated two identical ambipolar TGBC OTFTs based on one-component ambipolar polymer PF4IVI2T in this inverter. To solve the problem of the unbalanced hole and electron mobilities of PF4IVI2T, two ambipolar transistors with different channel lengths (20 and 40 µm, respectively) were designed. The inverter revealed


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a high gain value of up to 75, which is among the highest of II-based polymers (Figure 4f). The fast switching property with low power consumption indicates that PF4IVI2T is a promising polymer semiconductor applied in logic circuits. In order to in-depth understand the relationship between molecular structures and OTFT performance, the thin-film microstructures and morphologies of the polymers were researched by 2D grazing incidence X-ray diffraction (2D−GIXRD) (Figure 5a, 5b, 5d and 5e) and atomic force microscopy (AFM) (Figure 5c and 5f). Both of polymers showed four strong and distinct out-of-plane diffraction peaks (100, 200, 300 and 400), revealing the existence of significant crystallization for the polymer films. The (010) diffraction peaks at 2θ = 24.8° and 25.2° are assigned to the π−π stacking distance of 3.59 Å for PIVI2T and 3.53 Å for PF4IVI2T, respectively. The π−π stacking distances are among the smallest of conjugated polymers, implying the strong intermolecular interactions of both polymers. Note that the smaller π−π distance of PF4IVI2T compared with PIVI2T is attributed to the introduction of F atoms, thereby inducing the stronger interchain π−π interaction. The (010) diffractions are found in both inplane and out-of-plane diffraction, revealing the formation of both edge-on and face-on molecular packing motif for PIVI2T and PF4IVI2T thin films. Except for the difference PIVI2T tends to form the face-on molecular packing, while PF4IVI2T tends to form edge-on molecular packing, which is reflected by the different intensity of the (010) peak in in-plane and out-ofplane diffraction for PIVI2T and PF4IVI2T, and the observable (100) and (200) diffraction peaks in in-plane diffraction for PIVI2T. The edge-on molecular orientation in PF4IVI2T thin film may contribute to charge transport, which accords with most of the reported high-mobility polymer semiconductors.29−32 The AFM images of both PIVI2T and PF4IVI2T films show dense-packed fibril networks, likely owing to strong interchain π−π interactions for the two polymers (Figure


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5c and 5f). In comparison, the PF4IVI2T film exhibits better connectivity with more ordered structure relative to PIVI2T, which is favorable to constructing efficient pathways of charge carrier transport and thereby results in higher charge mobility for PF4IVI2T.33,34 4. Conclusion In summary, we have rationally designed two novel electron-deficient building block, IVI and F4IVI, and their copolymers, PIVI2T and PF4IVI2T. Both polymers exhibited deep lying HOMO/LUMO energy levels with the strong intermolecular interactions, due to the strong electron-deficient nature and the extended π-conjugated system of IVI and F4IVI. PIVI2T displayed p-type transport behavior with hole mobility of 0.43 cm2V−1s−1, while PF4IVI2T showed typical ambipolar transporting behavior under ambient conditions with high electron mobility of up to 1.82 cm2V−1s−1 and significantly enhanced hole mobilities of up to 1.03 cm2V−1s−1, exhibiting a promising application in logic circuits. The better OTFT performance for PF4IVI2T can be ascribed to stronger intra- and intermolecular interactions, more ordered molecular packing as well as lower LUMO energy levels of the polymer with the introduction of F atoms, leading to more efficient charge carrier hopping and transport. Our results unambiguously demonstrate that F4IVI is a promising electron-deficient building unit to construct high-performance semiconducting polymers. We believe that the molecular engineering strategy could have guidance significance to the exploitation of excellent building blocks in the field of organic electronics.


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R N

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O

II O R N

N R

O

R N

O

IVI O

F

N R

N R

O H-bond

R N

O H

F

R N

O

F4IVI O

N R

F

H O

N R

F

Figure 1. Molecular engineering strategy of II-based electron-deficient building units.

Figure 2. (a) UV−vis absorption spectra of the polymers in chloroform and thin film; Molecular models of (b) and (c) for methyl-substituted PIVI2T and PF4IVI2T fragments, respectively (the


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optimization of geometries were carried out through using Gaussian 09 at B3LYP/6-31G(d) level).

Figure 3. (a) Cyclic voltammograms of polymers in thin film. (b−e) Calculated molecular orbitals of the dimers of polymers. b and d for PIVI2T and PF4IVI2T HOMO, respectively; c and e for PIVI2T and PF4IVI2T LUMO, respectively.


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Figure 4. Output (a and b) and transfer (c and d) characteristics of PF4IVI2T annealed at 160 °C (L = 10 µm; W = 1400 µm). Schematic device configuration (e) and gain (f) of an inverter based on PF4IVI2T.

Figure 5. 2D−GIXRD patterns of PIVI2T (a) and PF4IVI2T (d) thin films annealed at 160 °C; the out-of-plane and in-plane cuts of the corresponding 2D−GIXRD patterns for PIVI2T (b) and PF4IVI2T(e); AFM height images of PIVI2T (c) and PF4IVI2T (f) thin films annealed at 160 °C. Scheme 1. Synthetic route of electron-deficient building units (IVI and F4IVI) and the corresponding polymers (PIVI2T and PF4IVI2T).


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Table 1. Molecular weights, optical and electrochemical properties of polymers. Polymers

Mn (kDa)

λmaxsol (nm)

λonesetsol (nm)

λmaxfilm (nm)

PIVI2T

46.3

710

765

710,648

772

1.61

–5.60

–3.99

PF4IVI2T

32.8

721,667

785

728,669

788

1.57

–5.74

–4.17

a

λonsetfilm Egopt EHOMO ELUMO (nm) (eV)a (eV) (eV)b

Egopt = 1240/λonsetfilm; bELUMO = EHOMO + Egopt.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge. GPC chromatograms, TGA plots, DSC curves and OTFT Performance of PIVI2T and PF4IVI2T; 1H NMR, 13C NMR and spectra of some intermediates. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]; [email protected] Author Contributions §

Mr. L. Xu and Dr. Z. Zhao contributed equally.


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Project No. 51572016 and 21504026), the China Postdoctoral Science Foundation (No. 2013M542009 and 2016T90686), the National Program on Key Basic Research Project (973 Program, Grant No. 2013CBA01600). We acknowledge the technical support from the Institute of High Energy Physics Chinese Academy of Sciences and Analytical and Testing Center of Huazhong University of Science and Technology. REFERENCES (1) Beaujuge, P. M.; Frechet, J. M. J. Molecular Design and Ordering Effects in π-Functional Materials for Transistor and Solar Cell Applications. J. Am. Chem. Soc. 2011, 133, 20009– 20029. (2) Forrest, S. R. The path to ubiquitous and low-cost organic electronic appliances on plastic. Nature 2004, 428, 911–918. (3) Dimitrakopoulos, C. D.; Malenfant, P. R. L. Organic Thin Film Transistors for Large Area Electronics. Adv. Mater. 2002, 14, 99–117. (4) Wang, C.; Dong, H.; Hu, W.; Liu, Y.; Zhu, D. Semiconducting π-Conjugated Systems in Field-Effect Transistors: A Material Odyssey of Organic Electronics. Chem. Rev. 2012, 112, 2208–2267. (5) Sirringhaus, H.; Tessler, N.; Friend, R. H. Integrated Optoelectronic Devices Based on Conjugated Polymers. Science 1998, 280, 1741–1744.


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(34) Yi, Z.; Ma, L.; Li, P.; Xu, L.; Zhan, X.; Qin, J.; Chen, X.; Liu, Y.; Wang. S. Enhancing the organic thin-film transistor performance of diketopyrrolopyrrole–benzodithiophene copolymers via the modification of both conjugated backbone and side chain. Polym. Chem. 2015, 6, 5369–5375. Briefs: Two promising π-extended electron-deficient building blocks (IVI and F4IVI) are developed to construct semiconducting polymers for organic thin film transistors. Synopsis TOC.


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Ï-Extended Isoindigo-Based Derivative: A Promising Electron

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