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Jan 18, 2018 - (35) Besides, the introduction of fluorine atoms could also fine-tune the frontier energy levels, leading to an inversion of charge tra...
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Article Cite This: Macromolecules 2018, 51, 966−976

Cyanostyrylthiophene-Based Ambipolar Conjugated Polymers: Synthesis, Properties, and Analyses of Backbone Fluorination Effect Zuzhang Lin,†,‡ Xiaotong Liu,† Weifeng Zhang,*,† Jianyao Huang,† Qiang Wang,†,‡ Keli Shi,† Zhihui Chen,† Yankai Zhou,†,‡ Liping Wang,*,‡ and Gui Yu*,†,§ †

Organic Solids Laboratory, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, P. R. China § University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: Herein, we report the synthesis and characterization of a series of ambipolar cyanostyrylthiophene (CST)based conjugated polymers without or with fluorine substituents, namely PD-CST, PD-3F-CST, PD-23F-CST, and PD-25F-CST. With the change of the number or incorporating position of fluorine substitutes on the novel CST unit, the electron mobilities of field-effect transistors based on these polymers dramatically increased from 0.0015 cm2 V−1 s−1 for PD-CST, 0.13 cm2 V−1 s−1 for PD-3F-CST, and 0.25 cm2 V−1 s−1 for PD-23F-CST to 0.53 cm2 V−1 s−1 for PD-25F-CST. AFM and GIXRD investigation indicated that the annealed PD-25F-CST thin film possesses stronger crystallization tendency and highly ordered lamellar packing than those of the other polymers. Further theoretical calculations and single-crystal X-ray diffraction analysis deeply revealed the remarkable influences of the number and incorporating position of fluorine substitutes on the frontier orbital energy levels, backbone conformation, and charge transport properties of polymer semiconductors.

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INTRODUCTION Over the past decades, solution-processable π-conjugated polymers have received considerable attention owing to their tunable optoelectronic properties, mechanical robustness, and high compatibility with heat-sensitive substrates. Such properties endow conjugated polymers and polymer field-effect transistors (PFETs) with great application prospect in nextgeneration flexible electronics such as radio-frequency identification, displays, chemical sensors, artificial skin, etc.1−5 In recent years, the device performance of PFETs has been significantly improved by combined efforts in rational designing molecular structures, improving thin film microstructures, optimizing device fabrication techniques, and understanding charge transport physics.6−10 For materials design, a powerful motif to achieve high charge carrier transport is to develop novel highly planar and linear donor−acceptor (D−A) alternating π-conjugated polymers. The interactions between D and A units in the neighboring polymer chains are beneficial to forming a long-range ordered molecular packing with close π−π stacking, thus increasing the charge hopping between polymer chains and enhancing interchain carrier transport.11−13 In line with this consideration, a wide variety of electronaccepting and electron-donating aromatics, such as naphthalenediimide (NDI),7,14−16 diketopyrrolopyrrole (DPP),17−20 indigo (II),10,21,22 isoindigo,23,24 3,7-bis[(E)-2-oxoindolin-3© 2018 American Chemical Society

ylidene]-3,7-dihydrobenzo[1,2-b:4,5-b′]difuran-2,6-dione (BDOPV),25 thieno[3,2-b]thiophene,22 2,2′-bithiophene,14,26 and dithiophethene,6 and their derivatives have been developed and/or used to construct high-mobility polymer semiconductors. It is necessary to point out that, however, the state-of-the-art device performance of conjugated polymers still lags behind the requirements of PFETs for practical applications, embodying in not only their harsh thin film annealing temperatures but also unsatisfied air stability and performance durability of PFETs, especially ambipolar and n-channel PFETs.26,27 Therefore, the exploitation of new design principle for developing highperformance polymer semiconductors has very strong realistic meanings. Recently, an approach of loading suitable heteroatoms/heteroatom-substituted groups on polymeric conjugated backbones or alkylated side chains has been extensively utilized to improve the order of molecular aggregation or solution processability.7,28,29 When the substituents are incorporated on polymeric conjugated backbones, some effective intramolecular noncovalent interactions might be produced. These noncovalent interactions could promote the Received: November 11, 2017 Revised: January 8, 2018 Published: January 18, 2018 966

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frontier orbital energy levels, backbone conformation, and charge transport properties of polymer semiconductors.

planarity of conjugated polymers and decrease the conformational freedom of polymeric conjugated backbones, even leading to forming single backbone conformation for polymer semiconductors.30 Under the circumstances, these noncovalent interactions can also be called as “conformational locks”. The commonly used heteroatoms include fluorine, oxygen, and nitrogen atoms.31−34 Among them, the fluorine atom is the most frequently adopted because of its strong electron-deficient property and small atomic radius. Fluorine atoms could induce several kind of conformational locks such as F···H, F···S, etc.35 Besides, the introduction of fluorine atoms could also fine-tune the frontier energy levels, leading to an inversion of charge transporting type from unique p-type to ambipolar, even to ntype.9,10,36 This indicates that the fluorine atom is one of the most important substitutes for high-mobility polymer semiconductors. Therefore, the systematical investigation of backbone fluorination effect is of significance. With the aim of widening the understanding of backbone fluorination effect, we herein designed and synthesized a series of cyanostyrylthiophene (CST)-based conjugated polymers, namely PD-CST, PD-3F-CST, PD-23F-CST, and PD-25FCST. The CST unit is a type of diarylethene, which is one of important building blocks for constructing high mobility polymer semiconductors. Different to symmetric diarylethenes such as dithiophethene, the CST unit is easy to synthesize and can supply several functionalizable sites, allowing easy loading of fluorine atoms or other heteroatoms/heteroatom-substituted groups to form varied of noncovalent interactions on conjugated backbone. These noncovalent interactions are helpful to lock the polymeric conjugated backbone to form single conjugated backbone conformation, which is in favor of obtaining high charge carrier transport properties.30,35 In addition, fluorine atoms or other heteroatoms/heteroatomsubstituted groups along with the cyano group of CST unit can increase the electron-deficiency property of conjugated backbones, which is also in favor of electron injection and transport, thus constructing n-type and ambipolar polymer semiconductors. Therefore, the CST unit provides an excellent research platform for not only discovering structure−property relationships of polymeric semiconductors but also developing ambipolar or n-type polymer semiconductors. All the resulting polymers have high thermal stability and possess broad absorption spectra extending to ca. 900 nm. With the change of the number or incorporating position of fluorine substitutes on the CST unit, the lowest unoccupied molecular orbital (LUMO) energy levels of these polymers progressively lowered from PD-CST, PD-3F-CST, and PD-23F-CST to PD-25FCST. The charge carrier transport properties were examined by fabricating PFET devices with top-gate/bottom-contact (TGBC) configuration. Ambipolar transport properties with dramatically increasing electron mobilities of 0.0015, 0.13, 0.25, and 0.53 cm2 V−1 s−1 were obtained for PD-CST, PD-3F-CST, PD-23F-CST, and PD-25F-CST, respectively. Thin film microstructures were investigated by grazing incidence X-ray diffraction (GIXRD) and tapping-mode atomic force microscopy (AFM) techniques, suggesting that the annealed PD-25FCST thin film possesses stronger crystallization tendency and more ordered lamellar packing than those of the other three polymers. The backbone fluorination effect was further studied through theoretical calculations and single-crystal X-ray diffraction analysis, revealing that the number or incorporating position of fluorine atoms have remarkable influences on the

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EXPERIMENTAL SECTION

General Measurements and Characterization. All chemicals were purchased from Acros, Sigma-Aldrich, J&K Scientific, or other commercial sources and used directly without further purifications. All solvents were freshly distilled according to standard laboratory processes. 1H and 13C NMR spectra were recorded using a Bruker Fourier 300 NMR spectrometer or 400 NMR. High-resolution mass spectroscopy (HRMS) analyses were carried out with electron-impact mass spectra on a Bruker BIFlEXII mass spectrometer. Hightemperature gel permeation chromatography (GPC) was performed on Polymer Laboratories PL-GPC220 at 150 °C with 1,2,4trichlorobenzene (TCB) as the eluent and polystyrene (PS) as the standard. To examine the thermal properties, thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) measurements were performed on a PerkinElmer series 7 thermal analysis system and PerkinElmer TA-Q2000 instrument, respectively. The UV−vis−NIR absorption spectra were measured on a Hitachi U-3010 spectrophotometer. Cyclic voltammograms (CV) experiments were conducted on an electrochemistry workstation with a three-electrode cell in a solution containing n-Bu4NPF6 (0.1 M) in dry acetonitrile under an argon atmosphere. The glassy carbon electrode coated with the thin polymer films, Pt wire, and Ag/AgCl electrodes was used as the working electrode, counter electrode, and reference electrode, respectively. The surface morphologies of polymer thin films were obtained by using a Digital Instruments Nanoscope V atomic force microscope operated in tapping mode. The molecular packing modes of polymer thin films were studied by employing grazing incidence Xray diffraction (GIXRD) technique, illuminating at a constant incidence angle of 0.2° (λ = 2d sin θ = 1.24 Å). (E)-3-(4-Bromophenyl)-2-(5-bromothiophen-2-yl)acrylonitrile (3a). To a round-bottom flask containing a solution of sodium methoxide (0.29 g, 5.44 mmol) in 50 mL of methanol were added 2(5-bromothiophen-2-yl)acetonitrile (1) (1.10 g, 5.44 mmol) and 4bromobenzaldehyde (2a) (1.00 g, 5.44 mmol). After the reaction mixture was stirred at room temperature for 24 h, the resulting solid was filtered and further purified by recrystallization from chloroform/ methanol to give the title compound as a yellow needle (1.02 g, 51%). 1 H NMR (400 MHz, CDCl3) δ: 7.68 (d, J = 7.7 Hz, 2H), 7.56 (d, J = 7.7 Hz, 2H), 7.13 (d, J = 4.2 Hz, 2H), 7.03 (d, J = 3.5 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ: 140.05, 138.17, 132.35, 131.96, 131.11, 130.46, 127.73, 125.18, 115.94, 113.83, 106.11. HRMS: Calcd for C13H7Br2NS, 366.8666, 368.8645, 370.8625. Found: 366.8673, 368.8634, 370.8633. (E)-3-(4-Bromo-3-fluorophenyl)-2-(5-bromothiophen-2-yl)acrylonitrile (3b). The same synthetic procedure as that described for 3a was followed using sodium methoxide (0.27 g, 4.93 mmol), 1 (1.00 g, 4.93 mmol), and 4-bromo-3-fluorobenzaldehyde (2b) (1.00 g, 4.93 mmol) to give the title compound as a yellow solid (1.18 g, 62%). 1H NMR (300 MHz, CD2Cl2) δ: 7.71−7.61 (m, 2H), 7.50 (dd, J = 8.4, 2.1 Hz, 1H), 7.18 (d, J = 3.6 Hz, 2H), 7.09 (d, J = 4.0 Hz, 1H). 13C NMR (75 MHz, CD2Cl2) δ: 159.19 (d, JF−C = 264.75 Hz), 139.79, 136.83 (d, JF−C = 2.25 Hz), 134.42 (d, JF−C = 7.5 Hz), 134.15, 131.29, 128.19, 136.06 (d, JF−C = 3.00 Hz), 116.24 (d, JF−C = 24 Hz), 115.58, 114.26, 111.42 (d, JF−C = 21 Hz), 107.0. HRMS: Calcd for C13H6Br2FNS, 384.8572, 386.8551, 388.8531. Found: 384.8574, 386.8543, 388.8535. (E)-3-(4-Bromo-2,3-difluorophenyl)-2-(5-bromothiophen-2-yl)acrylonitrile (3c). The same synthetic procedure as that described for 3a was followed using sodium methoxide (0.27 g, 4.93 mmol), 1 (1.00 g, 4.93 mmol), and 4-bromo-2,3-difluorobenzaldehyde (2c) (1.08 g, 4.93 mmol) affording the title compound as a yellow solid (1.22 g, 61%). 1H NMR (300 MHz, CD2Cl2) δ: 7.86 (ddd, J = 8.5, 6.9, 1.6 Hz, 1H), 7.46 (ddd, J = 8.5, 6.3, 2.0 Hz, 1H), 7.37 (s, 1H), 7.22 (d, J = 4.0 Hz, 1H), 7.11 (d, J = 4.0 Hz, 1H). 13C NMR (75 MHz, CD2Cl2) δ: 149 (dd, JF−C = 258.56 Hz, JF−C = 14.14 Hz), 148.33 (dd, JF−C = 248.97 Hz, JF−C = 14.14 Hz), 139.62, 131.33, 128.65, 128.50 (dd, JF−C 967

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Scheme 1. Synthetic Routes of CST-Based Monomers and Polymers, PD-CST, PD-3F-CST, PD-23F-CST, and PD-25F-CST

= 6.06 Hz, JF−C = 3.03 Hz), 128.32 (d, JF−C = 4.04 Hz), 123.17 (d, JF−C = 4.04 Hz), 122.96 (d, JF−C = 10.10 Hz), 115.31, 114.96, 112.48 (d, JF−C = 18.18 Hz), 109.11 (d, JF−C = 3.03 Hz). HRMS: Calcd for C13H5Br2F2NS, 402.8477, 404.8457, 406.8437. Found: 402.8470, 404.8450, 406.8447. (E)-3-(4-Bromo-2,5-difluorophenyl)-2-(5-bromothiophen-2-yl)acrylonitrile (3d). The same synthetic procedure as that described for 3a was followed using sodium methoxide (0.27 g, 4.93 mmol), 1 (1.00 g, 4.93 mmol), and 4-bromo-2,5-difluorobenzaldehyde (2d) (1.08 g, 4.93 mmol) affording the title compound as a yellow solid (1.29 g, 65%). 1H NMR (300 MHz, CD2Cl2) δ: 7.98 (dd, J = 9.2, 6.4 Hz, 1H), 7.43 (dd, J = 9.3, 5.7 Hz, 1H), 7.35 (s, 1H), 7.22 (d, J = 4.0 Hz, 1H), 7.10 (d, J = 4.0 Hz, 1H). 13C NMR (75 MHz, CD2Cl2) δ: 156.30 (dd, JF−C = 249.75 Hz, JF−C = 2.25 Hz), 155.54 (dd, JF−C = 240.00 Hz, JF−C = 3.00 Hz), 169.60, 131.35, 128.69, 128.48 (dd, JF−C = 6 Hz, JF−C = 2.25 Hz), 121.95 (dd, JF−C = 13.50 Hz, JF−C = 6.00 Hz), 120.82 (d, JF−C = 27 Hz,), 115.23, 114.96, 114.53 (dd, JF−C = 26 Hz, JF−C = 2.25 Hz), 111.79 (dd, JF−C = 24 Hz, JF−C = 10.50 Hz), 108.80 (d, JF−C = 2.25 Hz). HRMS: Calcd for C13H5Br2F2NS, 402.8477, 404.8457, 406.8437. Found: 402.8475, 404.8446, 406.8445. General Procedures for Polymerization and Purification. To a Schlenk tube were added the DPP-based monomer (0.20 mmol), the CST-based monomer (0.20 mmol), and chlorobenzene (8.00 mL) under a nitrogen atmosphere. Then, Pd2(dba)3 (6 mg) and P(o-tol)3 (16.2 mg) were added in one portion. The tube was charged with nitrogen through a freeze−pump−thaw cycle for three times and stirred for at 130 °C under a nitrogen atmosphere 24 h. After cooled to room temperature, the resulting sticky solution was poured into methanol (100 mL) containing 6 M HCl (3 mL) and stirred for 4 h. The crude polymer was collected by filtration, washed with methanol, and further purified by Soxhlet extraction using methanol, acetone, and hexane to remove the low-molecular-weight fraction of the starting materials and residual catalytic metal. The residue was finally extracted with chloroform and dried under vacuum to give the desired polymer materials. PD-CST (138.5 mg, 58%). 1H NMR (400 MHz, CDCl3) δ: 9.16− 7.52 (br, 11H), 4.01 (br, 4H), 1.32−1.22 (br, 82H), 0.87 (br, 12H). GPC: Mn = 13.7 kDa, Mw = 22.6 kDa, PDI = 1.63. Elemental Anal. Calcd for C75H109N3O2S3: C 76.28, H 9.30, N 3.56. Found: C 75.83, H 9.37, N 3.79. PD-3F-CST (148.5 mg, 62%). 1H NMR (300 MHz, CDCl3) δ: 9.12−8.66 (br, 10H), 3.96 (br, 4H), 1.27−1.13 (br, 82H), 0.77 (br, 12H). GPC: Mn = 10.7 kDa, Mw = 22.9 kDa, PDI = 2.14. Elemental Anal. Calcd for C75H108FN3O2S3: C 75.14, H 9.08, N 3.51. Found: C 74.68, H 9.08, N 3.56. PD-23F-CST (141.6 mg, 58%). 1H NMR (300 MHz, CDCl3) δ: 9.11−8.82 (br, 9H), 4.04 (br, 4H), 1.35−1.22 (br, 82H), 0.85 (br,

12H). GPC: Mn = 11.1 kDa, Mw = 24.4 kDa, PDI = 2.22. Elemental Anal. Calcd for C75H107F2N3O2S3: C 74.03, H 8.56, N 3.45. Found: C 73.38, H 8.65, N 3.49. PD-25F-CST (137.2 mg, 56%). 1H NMR (300 MHz, CDCl3) δ: 8.89−8.81 (br, 9H), 4.04 (br, 4H), 1.25−1.21 (br, 82H), 0.87 (br, 12H). GPC: Mn = 11.0 kDa, Mw = 24.9 kDa, PDI = 2.27. Elemental Anal. Calcd for C75H107F2N3O2S3: C 74.03, H 8.56, N 3.45. Found: C 73.46, H 8.78, N 3.51. X-ray Crystallography. X-ray crystallographic data of monomer 3d were collected with a Bruker Smart CCD diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å). The data were collected at 173.15 K using ω scan in a θ range of 0.736°− 27.450°. A total of 6072 reflections were measured, of which 2633 were independent reflections [R(int) = 0.0496]. The structure was resolved by the direct method and refined by full-matrix least-squares on F2 using the SHELXL-97 program. All non-hydrogen atoms were refined anisotropically, and all hydrogen atoms were placed using AFIX instructions. Crystallographic data for single crystal of monomer 3d: C13H5Br2F2NS; FM = 405.06; crystal size 0.576 × 0.136 × 0.032 mm3; monoclinic; P1211; a = 3.963(3) Å; b = 5.849(5) Å; c = 27.67(3) Å; α = 90°; β = 91.654(12)°; γ = 90°; Z = 2; ρcalculated = 2.099 g/cm3. The refinement was converged to R1 = 0.0370 for I > 2σ(I), wR2 = 0.0916, GOF = 1.083 for all data. CCDC 1581899 contains the supplementary crystallographic data. Device Fabrication and Characterization. Top gate/bottom contact PFET and complementary-like inverters were fabricated using Corning glass as substrates. The Corning glass was fully cleaned by ultrasonication in deionized water, ethanol, and acetone. After that, the Au source and drain bottom electrodes (ca. 30 nm) were deposited by thermal deposition at vacuum conditions. Then, the semiconducting layer was fabricated by the spin-coating method with 2000 rpm for 60 s. After thermal annealing, a PMMA thin film (∼960 nm thickness) was spin-coated on the top of the semiconducting layer and then dried at 80 °C for 30 min. At last, an aluminum layer was thermally evaporated as the gate electrode. All devices were fabricated in glovebox and measured under ambient condition by using a Keithley 4200 semiconductor characterization system. The charge mobilities of the devices were calculated in the saturation regime according to the equation

IDS = CiμW (VGS − Vth)2 /2L where IDS is the source-drain current, Ci is the capacitance of the dielectric layer, μ is the mobility of the devices, W/L is the width/ length of channel, VGS is the gate voltage, and Vth is the threshold voltage of the devices. The fabrication procedure of complementary-like inverters was similar to those of PFET devices. The inverters consisted of two 968

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Figure 1. UV−vis−NIR absorption spectra of CST-based polymers (a) in chloroform solution and (b) as thin film spin-coated on quartz glass substrates.

Table 1. Optical and Electrochemical Properties of the CST-Based Polymers λabs max (nm) polymer

sola

PD-CST PD-3F-CST PD-23F-CST PD-25F-CST

646 754 708 710

filmb 672, 694, 700, 706,

750 764 768 778

Tdec (°C)c

d Eopt g (eV)

Eonset (eV)e ox

e Eonset red (eV)

HOMO (eV)e

LUMO (eV)e

355 351 392 393

1.49 1.38 1.42 1.36

0.89 1.06 1.12 1.08

−1.02 −0.98 −0.89 −0.87

−5.29 −5.46 −5.52 −5.48

−3.38 −3.42 −3.51 −3.53

a

e In chloroform solution. bCoated on quartz glass. cMeasured by TGA. dCalculated from the absorption edges (Eopt g = 1240/λabs,onset). Determined by CV.

attributed to π−π* transition, while the low-energy bands correspond to intramolecular charge transfer (ICT) arising from the interactions between donor and acceptor moieties.38 In chloroform solution, PD-CST owns the low-energy absorption band in a range of ca. 500−800 nm, whereas PD3F-CST, PD-23F-CST, and PD-25F-CST show similar absorption behavior with the low-energy absorption band in a range of ca. 500−900 nm. This result implies that with the introduction of fluorine atoms on polymeric conjugated backbone, the UV−vis−NIR absorption spectra of the resulting polymers obviously red-shift. PD-CST owns the maximum absorbance (λabs max) at wavelengths of 646 nm while PD-3F-CST, PD-23F-CST, and PD-25F-CST show their λabs max at 754, 708, of PD-23F-CST and 710 nm, respectively. Note that the λabs max and PD-25F-CST has an apparent blue-shift compared to that PD-3F-CST, indicating that slight aggregation occurs in the two solutions. In comparison with the absorption spectra in solution, the thin film absorption of these polymers become broader, and the corresponding major absorption either divided into two sections. Such changes suggest the aggregation or orderly π−π stacking happens in these polymer thin films.39 It must be pointed out that all thin film absorption spectra have a tiny and clear red-shift from PD-CST, PD-3F-CST, and PD23F-CST to PD-25F-CST, demonstrating that both the number and incorporating position of fluorine atoms have influence on molecular packing mode in solid state. On the basis of their thin film absorption edges, the optical bandgaps (Eopt g ) of the four polymers were estimated to be in a range of 1.36−1.49 eV. The related optical data are summarized in Table 1. The electrochemical properties of CST-based polymers were investigated by thin film cyclic voltammetry (CV) under an argon atmosphere by using Ag/AgCl as reference electrode and polymer thin film coated glassy carbon electrode as working electrode (Figure S3 and Table 1). The HOMO and LUMO energy levels of the polymers were determined according to equation EHOMO = −(4.40 + Eox onset) eV and ELUMO = −(4.40 + Ered onset) eV, respectively. The onset oxidation potentials of PD-

ambipolar transistors on Corning glass substrates, with a common drain as the output voltage (VOUT) and a common gate as the input voltage (VIN). The inverters were measured in air 40−60% air humidity.

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RESULTS AND DISCUSSION Synthesis and Thermal Properties. The synthetic routes and molecular structures of CST-based monomers and polymers are depicted in Scheme 1. Under basic conditions, 2-(5-bromothiophen-2-yl)acetonitrile (1) reacted with aldehydes 2a−2d to give the desired monomers 3a−3d, respectively. Stille coupling polymerizations of monomers 3a−3d and tin monomer 4 using Pd2(dba)3 and P(o-tol)3 as the catalyst affording the target polymer materials. The chemical structures of these polymers were characterized by 1 H NMR, elemental analysis, and high-temperature GPC at 150 °C using polystyrene as standard and 1,2,4-trichlorobenzene as eluent. The number-average molecular weights (Mn) of the four polymers were estimated to be in a range of 10.7−13.7 kDa with a polydispersity index (PDI) of 1.63−2.27. The low molecular weight could be attributed to the low reactivity of these CST-based monomers originated from their enhanced steric effect or electron-deficient properties. The thermal properties of all polymers were investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The TGA measurements demonstrate that PD-CST, PD-3F-CST, PD-23F-CST, and PD-25F-CST possess good thermal stabilities with 5% weight loss temperatures (Tdec) of 355, 351, 392, and 393 °C, respectively (Figure S1 in the Supporting Information). For the DSC analyses, no obvious endothermic transitions and glass ones observed for all polymers in the temperatures ranging from 30 to 250 °C (Figure S2). Optical and Electrochemical Properties. The UV−vis− NIR absorption spectra of the CST-based polymers were recorded both in dilute CHCl3 solution and as thin films spincoated on quartz glass (Figure 1). All the polymers exhibit typically dual band absorption: the higher-energy bands are 969

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Figure 2. Typical transfer (IDS − VGS) and output (IDS − VDS) characteristics of PFET devices based on PD-25F-CST.

Table 2. TGBC FET Device Performance of CST-Based Polymers n-channel

a

p-channel

polymer

μmaxa

μavea,b

Vth (V)

Ion/Ioff

μmax

μavea,b

Vth (V)

Ion/Ioff

d−d (Å)c

π−π (Å)c

PD-CST PD-3F-CST PD-23F-CST PD-25F-CST

0.0015 0.13 0.25 0.53

0.0013 0.12 0.22 0.49

57−78 43−55 33−44 39−51

102−103 103−104 103−104 103−104

0.0089 0.039 0.030 0.033

0.0055 0.036 0.025 0.029

−35 to −47 −51 to −43 −51to −34 −51 to −33

103−104 102−103 102−103 102−103

22.18 21.13 21.89 22.13

3.58 3.57 3.58 3.58

In cm2 V−1 s−1. bThe values were calculated from more than 10 devices. cThe d−d and π−π stacking distances.

CST, PD-3F-CST, PD-23F-CST, and PD-25F-CST locate at 0.89, 1.06, 1.12, and 1.08 V, respectively. Based on these values, the HOMO energy levels of these copolymers were estimated to be −5.29, −5.46, −5.52, and −5.48 eV, respectively. Similarly, the LUMO energy levels of PD-CST, PD-3F-CST, PD-23F-CST, and PD-25F-CST were estimated to be −3.38, −3.42, −3.51, and −3.53 eV, respectively, based on their onset reduction potentials of −1.02, −0.98, −0.89, and −0.87 eV. Apparently, with the increasing number of the fluorine substituents, the LUMO energy levels of PD-23F-CST and PD-25F-CST have been efficiently decreased to −3.52 and −3.56 eV, respectively. Apparently, the bandgaps calculated from CV data for these polymers are higher than their optical bandgaps estimated from thin film absorption spectra. This differences in bandgap are attributable to the exciton binding energy of the polymers. To well understand the electron state density distributions in the HOMOs and LUMOs of these polymers, we performed theoretical modeling using density functional theory at the B3LYP/6-31G(d) level with replacement of alkyl chains by methyl groups for simplicity. In view of the two different linking methods between the thiopheneflanked DPP and the CST-based units, we carried out the

theoretical modeling of two different structures in one polymer repeating unit for each polymer for simplification (Figure S4). The modeling studies reveal that the LUMOs of these polymers are delocalized over the conjugated backbone, whereas the HOMOs are mainly localized on DPP core. Charge Transport Properties. Charge transport properties of the four CST-based polymers were studied by fabricating PFET devices with TGBC configuration and using PMMA as a dielectric layer. The TGBC configuration has been proved to be favorable for ambipolar and n-type organic semiconductors because of the encapsulation effect of dielectric layer. Gold source/drain bottom electrodes (with W = 1400 μm and L = 50 μm) were deposited on Corning glass substrate by thermally evaporated under vacuum condition, and then the active polymer-based semiconducting layer was fabricated through spin-coating a polymer solution in chloroform. As the thermal annealing always plays an important influence on the device performance, several thermal annealings at temperatures ranging from 120, 160, and 200 to 240 °C were performed for the resulting PFET devices. The optimal annealing temperature of all these PFET devices was 200 °C. When thermal annealed at a temperature lower or higher than 200 °C, 970

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Figure 3. AFM topography images of the CST-based polymer thin films before and after annealing at 200 °C on Corning glass substrates. (a, e) PDCST, (b, f) PD-3F-CST, (c, g) PD-23F-CST, and (d, h) PD-25F-CST. All images are 5 μm × 5 μm in size.

Figure 4. 2D-GIXRD patterns of the CST-based polymer thin films before and after annealing at 200 °C of (a, e) PD-CST, (b, f) PD-3F-CST, (c, g) PD-23F-CST, and (d, h) PD-25F-CST.

PD-23F-CST- and PD-25F-CST-based inverters, respectively. The results suggest the potential applications of the two CSTbased fluorinated polymers in complex logic circuits. Thin Film Microstructural Characterization. It is well established that the charge carrier mobilities in conjugated polymer films are very sensitive to the intermolecular overlap integral,11−13 so further studies on molecular aggregation are needed to interpret the remarkable backbone fluorination effect on charge transport properties. To investigate thin film surface morphology and molecular packing, we performed GIXRD and tapping-mode AFM experiments on thin-film samples cast onto Corning glass substrates. Figure 3 and Figure S8 present the thin film surface morphology images of the four CST-based polymers before and after annealing at the optimal temperature of 200 °C. In comparison with the as-cast thin films, the annealed thin films exhibited improved crystalline fibrillary intercalating networks. In addition, the crystalline domains become denser and bigger from PD-CST, PD-3F-CST, and PD-23F-CST to PD-25F-CST. The thin film morphology change agrees well with the obtained mobilities of the PFET devices. Figure 4 depicts the two-dimensional GIXRD (2DGIXRD) images of these polymer thin films before and after annealing at 200 °C. Similarly, the annealed thin film of all

these PFET devices afforded lower mobilities (Figure S5). All polymers showed typical ambipolar charge transport properties under ambient conditions. The typical transfer (IDS−VGS) and output (IDS−VDS) curves of the optimized PFET devices are shown in Figure 2 and Figure S6. The maximum electron/hole mobilities of PD-CST, PD-3F-CST, PD-23F-CST, and PD25F-CST are 0.0015/0.0089, 0.13/0.038, 0.25/0.030, and 0.53/ 0.033 cm2 V−1 s−1, respectively. Apparently, with the number of fluorine atoms changes from 0 to 2, the electron mobilities of related polymer materials dramatically improved, and the maximum electron mobility of PD-25F-CST is 2 times higher than that of PD-23F-CST. Such results imply that the incorporating number and position of fluorine atoms on CST unit both have remarkable influence on the charge transport properties of polymer semiconductors. The performances of the devices based on these polymers are summarized in Table 2. Moreover, we also fabricated the complementary-like inverters based on PD-23F-CST and PD-25F-CST, respectively, with two identical PFETs with a common gate as the input voltage (VIN) and a common drain as the output voltage (VOUT). The static transfer characteristics and corresponding output voltage gains of the complementary inverters are shown in Figure S7. Gain values of 37/33 and 44/42 were obtained in 971

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Macromolecules Table 3. FWHM and Coherence Length of the CST-Based Polymer Films FWHM (Δq, nm−1) polymer PD-CST PD-3F-CST PD-23F-CST PD-25F-CST a

(100) a

coherence length (nm) (010)

b

0.118 /0.087 0.286a/0.186b 0.656a/0.257b 0.337a/0.219b

a

(100) b

1.106 /0.872 1.872a/1.036b 2.094a/1.217b 1.432a/1.154b

a

(010) b

47.390 /64.276 19.553a/30.065b 8.524a/21.759b 16.594a/25.534b

5.056a/6.413b 2.987a/5.398b 2.670a/4.595b 3.905a/4.846b

As-fabricated thin films. bThin films after annealing at 200 °C.

Figure 5. Theoretical calculations and single crystal of the CST-based molecules. (a) Frontier orbital energy levels. (b) Top-view and side-view of single crystal structure of monomer 3d. (c) Gas-phase torsional potential energy distribution of CST-based molecules.

their continuously enhanced molecular stacking order and lowered LUMO energy levels, meaning that electron injection would be facilitated. The d−d and π−π stacking distances and the FWHM and coherence length of (100) and (010) diffraction peaks of all polymer thin films were also estimated and listed in Tables 2 and 3, respectively. Analyses of Backbone Fluorination Effect. As discussed above, the dramatically increasing electron mobilities from PDCST, PD-3F-CST, and PD-23F-CST to PD-25F-CST could be attributed to their progressively lowered LUMO energy levels and improved thin film microstructures. We herein study how these fluorine atoms affect these inherent properties of the series of polymers by means of theoretical calculations and single-crystal X-ray diffraction analysis. Figure 5a shows the frontier orbital energy levels of (E)-3-phenyl-2-(thiophen-2yl)acrylonitrile (CST), (E)-3-(3-fluorophenyl)-2-(thiophen-2yl)acrylonitrile (3F-CST), (E)-3-(2,3-difluorophenyl)-2-(thiophen-2-yl)acrylonitrile (23F-CST), and (E)-3-(2,5-difluorophenyl)-2-(thiophen-2-yl)acrylonitrile (25F-CST). The calculated HOMO and LUMO energy levels are −5.97 and −2.37 eV for 3F-CST, respectively, which is stabilized by 0.15 and 0.17 eV compared with those of CST, respectively. Further introduction of fluorine atoms stabilizes both HOMO and LUMO energy level of the other two monomers: 23F-CST possesses the HOMO/LUMO energy levels of −6.02/−2.45 eV, respectively, and 25F-CST owns the narrowest bandgap of 3.53 eV compared to the other three monomers due to more extended π-orbitals. The change tendency agrees with the HOMO/LUMO energy levels of their polymer derivatives,

polymers exhibited strong diffraction peaks compared to their respective as-cast thin films, implying that the molecular arrangement becomes more orderly in these annealed thin films.40,41 The annealed PD-25F-CST thin film owns a strong (100) peak in out-of-plane (qxy) diffraction orientation, corresponding to d−d stacking distance of 22.13 Å. The other two orders of diffraction peaks, indexed as (200) and (300), were also observed. In addition, an obvious (010) peak in in-plane diffraction orientation, corresponding to the π−π stacking distance of 3.58 Å, could also be observed, indicating that the polymer takes a lamellar molecular packing mode with predominantly edge-on oriented respective to Corning glass substrates. Nevertheless, the annealed PD-23F-CST thin film has arc-shape (010) diffraction peak and similar (h00) diffraction peaks, suggesting that the polymer adopts a lamellar molecular packing mode with random oriented respective to the substrates. In addition, both the annealed PD-CST and PD3F-CST thin films demonstrate weaker out-of-plane and inplane diffraction peaks. To well elucidate the crystallinity and packing structure of these polymer films, we also calculated the full width at half-maximum (fwhm) and coherence length from their one-dimensional GIXRD (1D-GIXRD) profiles in out-ofplane and in-plane direction (Figures S9 and S10).42 Upon annealing at 200 °C, the FWHM and coherence length of (100) and (010) diffraction peaks of all polymer thin films become smaller or greater in comparison with those of asfabricated thin films, respectively. The changes indicate that the relative crystallinity of the films increased. Therefore, the remarkably different electron mobilities of PD-CST, PD-3FCST, PD-23F-CST, and PD-25F-CST could be attributed to 972

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Macromolecules

Figure 6. Chemical structures and dihedral torsional potentials of CST−thiophene molecules. All geometries were calculated at the the B3LYP/631G(d) level, and single-point energies were calculated at the MP2/cc-pVDZ level.

indicating significant effect of backbone fluorination on the frontier orbital energy levels of conjugated polymers. Recently, theoretical and experimental results have suggested that structural and conformational freedom of conjugated polymers has great influence on molecular packing in solid state and thus interchain carrier transport.11−13 For understanding the influence of fluorine atoms on backbone conformations, we successfully prepared the single crystal of monomer 3d, and the X-ray crystallographic data are presented in Table S1. As depicted in Figure 5b, the molecule has almost planar molecular conformation. Note that the F···H−C distance of 2.338 Å is apparently smaller than 2.66 Å for the sum of van der Waals radii.43 In combination of the fluorine atom-induced weak F···N and F···C interactions, the 25F-CST molecule possesses multiple noncovalent interactions, which are helpful for forming a “locked” backbone conformation as revealed by previous literatures.30 Since no suitable single crystals of the other three monomers were obtained for X-ray crystallography, we therefore performed the relaxed potential energy scan using density functional theory (DFT) at the B3LYP/6-31G(d) level, and the single-point energy of each geometric structure was further calculated at the MP2/cc-pVDZ level.22 There are two flexible single bonds that are capable of forming conformers for each monomer. In consideration of the fact that CST moiety shows a preferential trans conformation with respect to adjacent thiophene−ethene subunits,8 we fixed the conformation of thiophene−ethene subunits and focused on the torsional potentials of the ethene−benzene subunits (Figure 5c). The torsional potentials of the four monomers exhibit different energetic preferences. For CST and 3F-CST, the torsional barriers are 14.4 and 14.5 kJ/mol with negligible energy differences between the two planar conformers, indicating the coexistence of two conformers in their respective polymer chains. When the fluorine atom is substituted at the

position adjacent to the vinylene linkage, as is the case in 23FCST or 25F-CST, the steric and electrostatic repulsions between fluorine and cyano groups greatly raise the torsional barrier to ca. 21 kJ/mol, demonstrating that only one conformer is energetically stable for both 23F-CST and 25FCST. Such repulsion-induced conformational locks may allow the more ordered orientations of polymer backbones, leading to high charge transport characteristics. Compared with 23F-CST, 25F-CST demonstrates a more preferential 0° conformation and a more unstable 180° conformation. This is due to the different electron-pulling strength of the other fluorine atom resulting from the different substitution positions. On the basis of the optimized geometries, we further investigated the dihedral torsion between benzene−thiophene subunit of (E)-2-(thiophen-2-yl)-3-(4-(thiophen-2-yl)phenyl)acrylonitrile (CST-T), (E)-3-(3-fluoro-4-(thiophen-2-yl)phenyl)-2-(thiophen-2-yl)acrylonitrile (3F-CST-T), (E)-3(2,3-difluoro-4-(thiophen-2-yl)phenyl)-2-(thiophen-2-yl)acrylonitrile (23F-CST-T), and (E)-3-(2,5-difluoro-4-(thiophen-2-yl)phenyl)-2-(thiophen-2-yl)acrylonitrile (25F-CST-T) to study whether there is fluorine-induced interactions (Figure 6). Consistent with previously reported structures, both noncovalent S···F and H···F interactions exist and contribute to the conformational control. Such interactions lead to varying energy differences between the conformers. For example, the higher dihedral torsional potentials of 3F-CST-T than those of CST-T reveal the presence of noncovalent S···F and H···F interactions. The H···F interactions are likely preferable in the polymer backbones because the corresponding structures are more stable. Both interactions can planarize the backbone, enhance the orbital overlaps, and favor intermolecular packing. On the basis of these the X-ray diffraction crystallography and theoretical simulation results, we proposed the optimized backbone conformations of the four CST-based polymers as 973

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Macromolecules shown in Figure 7. We can also conclude that 23F-CST and 25-CST are more favorable for constructing semiconducting

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sponding output voltage gains of the complementary inverters based on PD-23F-CST and PD-25F-CST, AFM height images of the CST-based polymer thin films after annealing at 120, 160, and 240 °C on Corning glass; 1D-GIXRD profiles of the as-fabricated and annealed CST-based polymer films; NMR spectra of the CSTbased monomers and polymers (PDF) Crystallographic data of monomer 3d (CIF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.Y.). *E-mail: [email protected] (W.Z.). *E-mail: [email protected] (L.W.). ORCID

Jianyao Huang: 0000-0003-4177-6393 Gui Yu: 0000-0001-8324-397X Author Contributions

Z.L. and X.L. contributed equally to this work. Notes

The authors declare no competing financial interest.

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Figure 7. Optimized backbone conformations of (a) PD-CST, (b) PD-3F-CST, (c) PD-23F-CST, and (d) PD-25F-CST.

ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (Grants 51773016, 21774134, 51473021, 21474116, and 21673258), the National Key R&D Program of China (2016YFB0401100 and 2017YFA0204703), and the Strategic Priority Research Program of the Chinese Academy of Sciences (CAS) (XDB12030100). The high temperature GPC measurements are supported by Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, CAS. The GIXRD analyses were performed at the BL14B1 Station of Shanghai Synchrotron Radiation Facility (SSRF). The authors are very grateful to the assistance of scientists from the station during the experiments.

materials than CST and 3F-CST do. Polymers containing 25CST are likely to possess highest electrical properties because of the suitable energy levels, large torsional barrier, and reduced conformational freedom.

■

CONCLUSION In summary, we have developed a series of cyanostyrylthiophene-based conjugated polymers and investigated how the number or incorporating position of fluorine atoms affect the frontier orbital energy levels, backbone conformation, and charge transport properties of these conjugated polymers. An enhanced electron mobility of 0.53 cm2 V−1 s−1 was achieved for PD-25F-CST in comparison with that of the other three polymers. Further thin film investigations and theoretical calculations indicated that PD-25F-CST possesses a more planar conjugated backbone conformation and owns stronger crystallization tendency and highly ordered lamellar packing than those of the other three polymers. Our combination of experimental and theoretical studies would supply better understanding of the remarkable significance of backbone fluorination in D−A conjugated polymers that promise high performance for not only PFETs but also other organic electronic devices.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02401. TGA traces, DSC curves, and CV traces of the CSTbased polymers, molecular frontier orbitals of the CSTbased polymers in one polymer unit, annealing temperature-dependent mobilities of PFETs based on the CSTbased polymers, typical transfer and output characteristics of PFETs based on the CST-based polymers except PD-25F-CST; static transfer characteristics and corre974

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