Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Donor−Acceptor Conjugated Copolymers Containing Difluorothienylethylene-Bridged Methyleneoxindole or Methyleneazaoxindole Acceptor Units: Synthesis, Properties, and Their Application in Field-Effect Transistors Yankai Zhou,†,‡ Shiying Zhang,†,‡ Weifeng Zhang,*,‡ Jianyao Huang,‡ Congyuan Wei,‡ Hao Li,†,‡ Liping Wang,*,† and Gui Yu*,‡,§ Macromolecules Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/07/18. For personal use only.
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School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, P. R. China 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 Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049, P. R. China ‡
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ABSTRACT: We report the synthesis and characterization of two novel difluorothienylethylene-bridged methyleneoxindole (FVTI) and methyleneazaoxindole (FVTAI) acceptor units and six FVTI- or FVTAI-based donor−acceptor (D−A) copolymers P1−P3F. On the basis of the optimized conjugated backbone conformations, we put forward the intramolecular noncovalent interactions existing in these copolymers and suggested that the fluorine and pyridinenitrogen atoms could improve the polymeric backbone planarity by forming multiple conformation locks such as N···H, F···H, and F···S interactions, etc. Thin film microstructure investigations indicate the copolymers with more fluorine or pyridine-nitrogen atoms in their respective single D−A pair generally formed more orderly thin films. All the six copolymers exhibited p-type charge transport characteristics with increasing mobilities from P1 to P1F and P2 to P2F. On the contrary, P3F showed relatively lower mobility than that of P3, which was attributed to the higher hole injection barrier for the P3F-based field-effect transistor device.
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INTRODUCTION Over the past decades, π-conjugated polymers have been heavy studied with aims of achieving highly efficient organic electronics such as organic field-effect transistor (OFET) and photovoltaic (OPV) devices.1−6 In the process of exploration, conjugated backbones with alternating electron-donating (D) and electron-withdrawing (A) units become a popular approach in the design of new polymer semiconductors. Upon careful consideration of D and A units, the frontier molecular orbital (FMO) energy levels, conjugated backbone conformation, self-assembly, crystallinity, and solubility of D− A conjugated polymers could be well fine-tuned.7 In the field of OFETs, most of polymer semiconductors with record high hole and/or electron mobilities have been developed on the basis of the concept. To date, naphthalenediimide (NDI),1,8,9 diketopyrrolopyrrole (DPP),10−16 isoindigo (II),17,18 (3E,7E)3,7-bis(2-oxoindolin-3-ylidene)benzo[1,2-b:4,5-b′]difuran-2,6(3H,7H)-dione,19−21 benzothiadiazole (BT),22 benzobisthiadiazole (BBT),23 2,2′-bithiazolothienyl-4,4′,10,10′-tetracarboxydiimide (DTzTI),24 7,15-diazadiazuleno[1,2,3-ef:1′,2′,3′-kl]heptalene-6,8,14,16(7H,15H)-tetraone (TBAzDI),25 and their derivatives are some representative electron-withdrawing units. © XXXX American Chemical Society
Compared with the well-investigated DPP units, the II unit is of particular interest due to its more electron-deficient nature, endowing the II-based conjugated polymers with stronger electron push−pull effect between the D and A units along their polymeric conjugated backbones. In consequence, the IIbased conjugated polymers always form good lamellar packing mode in the solid state.18 Nevertheless, the II unit is a nonplanar conjugated structure, in which a torsion angle of about 15° stemming from the C−H···OC steric interactions exists inside its structure; even torsion angles of around 20°− 40° rooting from the C−H···H−C steric interactions often take place between the benzene ring of II units and the aryl ring of neighboring copolymerization units.26,27 Because planar conjugated structures are considered to be favorable for efficient charge transport, elaborate efforts were made to obtain new II analogues with planar conjugated backbones and use them as electron-withdrawing building blocks in constructing high performance D−A polymer semiconductors. One Received: June 18, 2018 Revised: August 23, 2018
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DOI: 10.1021/acs.macromol.8b01297 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules important class of examples include thienoisoindigo,26,28 azaisoindigo,27 thiazoloisoindigo,29 thieno[3,2-b]thiophene,30 thieno[3,2-b][1]benzothiophene,31 and their halogen-substituted derivatives32,33 These II analogues and their polymer derivatives generally possess enhanced conjugated backbone planarity. As a result, expressively high mobilities up to 14.4 cm2 V−1 s−1 were obtained for these kinds of polymer derivative-based PFET devices.34 Another class of famous examples include 2,2′-bithiophene- or (E)-1,2-di(thiophen-2yl)ethene (DTE)-bridged methyleneoxindole, which could also be regarded as II analogues.35,36 In these conjugated units, the two methyleneoxindoles are separated by large π-extended structures, endowing these II analogues with planar conjugated backbones. Their polymer derivatives were also used in PFET and OPV devices, exhibiting promising power conversion efficiencies or charge carrier mobilities. The introduction of intramolecular noncovalent interactions, i.e., “conformation locks”, is also a useful approach of constructing high-performance polymer semiconductors. Using a computational method, Ratner et al. evaluated the binding energies of a variety of noncovalent interactions occurring between various heterocyclic and pendant atoms taken from a group of representative π-conjugated systems.37 They proposed that their binding capabilities reducing from C−H···N (2.20 kcal/mol), C−H···O (1.86 kcal/mol), C−H··· F (0.94 kcal/mol), C−H···S (0.74 kcal/mol), S···S (0.72 kcal/ mol), O···S (0.51 kcal/mol), N···S (0.46 kcal/mol), and F···S interactions (0.44 kcal/mol), while no significant binding capabilities were found in N···F, O···F, and O···N interactions. In addition, several conjugated polymers containing heteroatom-induced conformation locks have been developed toward PFET devices in recent years, and most of them also afforded impressively high charge transport properties.38−42 Obviously, further developing new polymer semiconductors containing conformation locks and well understanding their structure and property relationship are of great significance for polymer electronics. Therefore, we herein designed and synthesized two novel difluorothienylethylene-bridged methyleneoxindole (FVTI) and methyleneazaoxindole (FVTAI) acceptor units, and six FVTI- or FVTAI-based donor−acceptor copolymers P1−P3F with fluorine- and/or pyridine-nitrogeninduced conformation locks. The conjugated backbone planarity, thermal and optoelectronic properties, and thin film microstructures of these copolymers were investigated using theoretical calculations or experimental tests. Their semiconducting properties were studied by fabricating PFET devices. The influences of fluorine- and pyridine-nitrogeninduced conformation locks on the conjugated backbone planarity and charge transport properties of these copolymers were explored in detail. Our results highlight that the FVTI and FVTAI units are promising building blocks for polymers semiconductors toward high-performance PFETs.
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Measurements and Characterization. The chemical structures of all the compounds were confirmed by 1H NMR and/or 13C NMR spectra on Bruker-300 and Bruker-400 NMR spectrometers. The high-resolution matrix-assisted laser desorption/ionization time-offlight (MALDI-TOF) mass spectra were collected on a 9.4T Solarix FT-ICR mass spectrometer. The molecular weights and polydispersity indexes (PDIs) of the six newly synthetic copolymers were determined by high-temperature gel permeation chromatography (GPC) analyses on a Polymer Laboratories PL 220 system at 150 °C using 1,2,4-trichlorobenzene as the eluent and narrow polydispersity polystyrene as standards, respectively. The UV−vis−NIR absorption spectra of all copolymers were acquired on a Hitachi U-3010 spectrophotometer. The cyclic voltammetry (CV) curves of all copolymers were obtained on a CHI660c electrochemistry workstation with a scan rate of 50 mV s−1 under an argon atmosphere. In the measuring process, 0.1 M n-Bu4NPF6 in dry acetonitrile, platinum stick, Ag/AgCl, and platinum wire were adopted as supporting electrolyte, working electrode, reference electrode, and counter electrode, respectively. The thermogravimetric analyses (TGA) traces of all copolymers were recorded on a PerkinElmer series 7 thermal analysis system. The thin film microstructures of all copolymers were investigated using a Digital Instruments Nanoscope V atomic force microscope operated in tapping mode and the two-dimensional grazing incidence X-ray diffraction (2D-GIXRD) technique. (3Z,3′Z)-3,3′-((5,5′-((E)-Ethene-1,2-diyl)bis(4-fluorothiophene5,2-diyl))bis(methanylylidene))bis(6-bromo-1-(2-decyltetradecyl)indolin-2-one) (4). To a 100 mL two-necked round-bottom flask were added 6-bromo-1-(2-decyltetradecyl)indolin-2-one, 2 (2.81 g, 5.13 mmol), and (E)-5,5′-(ethene-1,2-diyl)bis(4-fluorothiophene-2-carbaldehyde), 3 (0.51 g, 1.79 mmol), and absolute ethyl alcohol (60 mL). After the solution was stirred at room temperature (rt) for 10 min, piperidine (2 mL) was added dropwise. The resulting mixture was heated to 100 °C and stirred for 24 h under an argon atmosphere. The reaction mixture was cooled to room temperature, extracted with dichloromethane, washed with brine, and dried over anhydrous sodium sulfate. After the solvent was removed under reduced pressure, the solid residue was purified by silica gel chromatography eluting with dichloromethane and hexane (1:3) to give the target compound 4 as a purple solid (1.19 g, 48%). 1H NMR (300 MHz, CD2Cl2): δ 7.44 (s, 1H), 7.18 (s, 1H), 7.15 (d, J = 8.1 Hz, 1H), 7.01 (d, J = 7.8 Hz, 1H), 6.94 (s, 1H), 6.79 (s, 1H), 3.48 (d, J = 7.1 Hz, 2H), 1.77 (s, 1H), 1.16 (s, 50H), 0.78 (t, J = 6.5 Hz, 7H). 13C NMR (75 MHz, CD2Cl2): δ 165.96, 155.68, 143.19 (s), 134.04, 128.45, 126.75, 125.19, 124.86, 124.23, 122.33, 121.98, 119.98, 117.95, 111.74, 54.15, 53.79, 53.43, 53.07, 52.71, 36.23, 31.95, 31.54, 30.02, 29.59, 26.40, 22.71, 13.91. HRMS (MALDI-TOF): Calcd for C76H110Br2F2N2O2S2: 1342.6338. Found: 1342.6335. 3,3,6-Tribromo-1-(2-decyltetradecyl)-1H-pyrrolo[2,3-b]pyridin2(3H)-one (6). To a solution of 6-bromo-1-(2-decyltetradecyl)-1Hpyrrolo[2,3-b]pyridine, 5 (5.10 g, 9.56 mmol), resolved in 1,4-dioxane (60 mL) was added pyridinium hydrobromide perbromide (PBPB, 9.17 g, 28.7 mmol) in one portion. The resulting mixture was stirred at rt overnight. After the solvent was removed under reduced pressure, the residue was dissolved in dichloromethane, washed with water, and dried over anhydrous sodium sulfate. After the removal of the solvent, the residue was purified by silica gel chromatography eluting with dichloromethane and petroleum ether (1:4) to give the target compound 6 as an orange liquid (14.6 g, 72%). 1H NMR (300 MHz, CDCl3): δ 7.57 (d, J = 7.8 Hz, 1H), 7.19 (d, J = 7.8 Hz, 1H), 3.63 (d, J = 7.3 Hz, 2H), 1.97 (s, 1H), 1.17 (s, 48H), 0.80 (t, J = 7.0 Hz, 12H). 13 C NMR (75 MHz, CDCl3): δ 169.30, 152.84, 142.81, 134.77, 124.45, 123.03, 44.28, 42.14, 35.73, 31.95, 31.33, 30.04, 29.22, 26.12, 22.71, 14.15. HRMS (MALDI-TOF): Calcd for C31H52Br3N2O: 705.1624. Found: 705.1625. 6-Bromo-1-(2-decyltetradecyl)-1H-pyrrolo[2,3-b]pyridin-2(3H)one (7). To a mixture of 6 (3.00 g, 4.27 mmol), THF (30 mL), and saturated ammonium chloride solution (30 mL) was added zinc powder (5.59 g, 85.40 mmol) in portions. The resulting solution was stirred at rt until the orange color of the solution disappears. After filtering off the excess zinc powder, the filtrate was extracted with
EXPERIMENTAL SECTION
General Procedures and Requirements. All chemicals were purchased from Acros, Alfa, and Innochem and used as received unless otherwise specified. The air- or water-sensitive reactions were performed using the Schlenk technique under an argon atmosphere with anhydrous solvents. N,N-Dimethylformamide (DMF) and tetrahydrofuran (THF) were dried according to their respective standard procedure and freshly distilled before use. The intermediates 1, 2, 5, 9a, 9b, 10a, and 10b were synthesized according to the reported literatures.18,27,42 B
DOI: 10.1021/acs.macromol.8b01297 Macromolecules XXXX, XXX, XXX−XXX
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Scheme 1. Chemical Structures and Synthetic Routes of FVTI- and FVTAI-Based Monomers 4 and 8 and D−A Copolymers P1−P3F
General Procedure of Stille Polymerization and Purification. P1. To a 25 mL Schlenk tube were added 4 (264 mg 0.20 mmol) and 5,5′-bis(trimethylstannyl)-2,2′-bithiophene, 9a (97.9 mg, 0.20 mmol), Pd 2(dba) 3 (6.0 mg), P(o-tol)3 (16 mg), and chlorobenzene (10 mL). After the tube was conducted with a freeze−pump−thaw cycle three times under argon at −78 °C, the reaction mixture was naturally warmed to room temperature, then heated to 120 °C, and stirred for 15 min under an argon atmosphere. The resulting sticky solution was cooled by ice water and then poured into 200 mL of methanol containing 6 M hydrochloric acid (10 mL) and stirred for 2 h. The formed blue-purple solid was collected by filtration and subjected to Soxhlet extraction for 2 days in methanol, acetone, and hexane for the removal of the low molecular weight fraction of the material and residual catalytic impurities. The final solid residue was collected with o-dichlorobenzene. After removing the organic solvent under reduced pressure, the desired polymer material P1 was obtained as a black filmlike solid (250 mg, 92%). 1H NMR (300 MHz, d2-CDCl2CDCl2, δ): 7.75−6.50 (br), 3.71 (br), 2.65 (br), 2.00 (br), 1.75−1.10 (br), 1.08−0.75 (br). GPC: Mn = 52.3 kDa, Mw = 96.7 kDa, PDI = 1.85. Elemental Anal. Calcd for C84H114F2N2O2S4: C 74.73, H 8.51, N 2.07. Found: C 74.09, H 8.46, N 2.10. P1F. The synthetic procedure is similar to that of P1 using 4 (266 mg, 0.20 mmol), (3,3′-difluoro-[2,2′-bithiophene]-5,5′-diyl)bis(trimethylstannane), 9b (106 mg, 0.20 mmol), Pd2(dba)3 (6.0 mg), P(o-tol)3 (16 mg), and chlorobenzene (20 mL) affording the desired polymer (258 mg, 90%). 1H NMR (300 MHz, d2-CDCl2CDCl2, δ): 7.78−6.45 (br), 3.71 (br), 2.61 (br), 2.00 (br), 1.80−1.10 (br), 1.00− 0.75 (br). GPC: Mn = 28.2 kDa, Mw = 42.3 kDa, PDI = 1.50. Elemental Anal. Calcd for C84H112F4N2O2S4: C 72.79, H 8.14, N 2.02. Found: C 71.84, H 8.05, N 2.03.
DCM and dried anhydrous sodium sulfate. The volatile solvent was removed under reduced pressure, and then the residue was purified by silica gel chromatography eluting with dichloromethane and petroleum ether (1:4) to give the target compound 7 as a colorless liquid (2.18 g, 93%). 1H NMR (300 MHz, CDCl3): δ 7.28 (d, J = 7.6 Hz, 1H), 7.11 (d, J = 7.5 Hz, 1H), 3.68 (d, J = 7.3 Hz, 2H), 3.47 (s, 2H), 1.99 (s, 1H), 1.25 (s, 42H), 0.88 (t, J = 6.7 Hz, 6H). 13C NMR (75 MHz, CDCl3): δ 169.30, 152.84, 142.81, 134.77, 124.45, 123.03, 44.28, 42.14, 35.00, 31.95, 31.33, 29.89, 29.71, 29.67, 29.58, 29.39, 29.37, 26.12, 22.71, 14.15. HRMS (MALDI-TOF): Calcd for C31H52BrN2O: 547.3268. Found: 547.3271. (3Z,3′Z)-3,3′-((5,5′-((E)-Ethene-1,2-diyl)bis(4-fluorothiophene5,2-diyl))bis(methanylylidene))bis(6-bromo-1-(2-decyltetradecyl)1H-pyrrolo[2,3-b]pyridin-2(3H)-one) (8). To a mixture of 7 (355 mg, 0.65 mmol), 3 (85.2 mg 0.30 mmol), and methanol (10 mL) was added piperidine (2 mL) slowly by springe. The resulting mixture was heated to 120 °C and stirred overnight under an argon atmosphere. The reaction system was cooled to rt, extracted with dichloromethane, washed with water, and dried over anhydrous sodium sulfate. After the solvent was removed under reduced pressure, the solid residue was purified by silica gel chromatography eluting with dichloromethane and petroleum ether (1:2) giving the target compound 8 as a dark purple solid (120 mg, 30%). 1H NMR (400 MHz, CDCl3): δ 7.61 (s, 1H), 7.51 (d, J = 7.6 Hz, 1H), 7.43 (s, 1H), 7.23 (s, 1H), 7.14 (d, J = 7.9 Hz, 1H), 3.80 (d, J = 7.3 Hz, 2H), 2.09 (s, 1H), 1.26 (d, J = 20.8 Hz, 43H), 0.87 (t, J = 6.2 Hz, 8H). 13C NMR (75 MHz, CDCl3): δ 165.94, 155.23, 139.80, 133.76, 129.37, 128.76, 127.25, 125.98, 125.66, 120.78, 120.19, 118.67, 116.55, 77.45, 77.02, 76.60, 43.63, 36.11, 31.95, 31.41, 30.03, 29.84, 29.19, 26.19, 22.71, 14.14. HRMS (MALDI-TOF): Calcd for C74H108Br2F2N4O2S2: 1344.6254. Found: 1344.6261. C
DOI: 10.1021/acs.macromol.8b01297 Macromolecules XXXX, XXX, XXX−XXX
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P2. The synthetic procedure is similar to that of P1 using 4 (200 mg, 0.15 mmol), (E)-1,2-bis(5-(trimethylstannyl)thiophen-2-yl)ethene, 10a (77.7 mg, 0.15 mmol), Pd2(dba)3 (4.7 mg), P(o-tol)3 (13 mg), and chlorobenzene (20 mL) affording the desired polymer (196 mg, 95%). 1H NMR (300 MHz, d2-CDCl2CDCl2, δ): 7.80−6.30 (br), 3.71 (br), 2.61 (br), 2.00 (br), 1.75−1.10 (br), 0.95−0.75 (br). GPC: Mn = 47.7 kDa, Mw = 134.0 kDa, PDI = 2.81. Elemental Anal. Calcd for C86H116F2N2O2S4: C 75.06, H 8.50, N 2.04. Found: C 74.37, H 8.56, N 2.05. P2F. The synthetic procedure is similar to that of P1 using 4 (198 mg, 0.15 mmol), (E)-1,2-bis(3-fluoro-5-(trimethylstannyl)thiophen2-yl)ethene, 10b (83.1 mg 0.15 mmol), Pd2(dba)3 (4.5 mg), P(o-tol)3 (13 mg), and chlorobenzene (20 mL) affording the desired polymer (190 mg, 91%). 1H NMR (300 MHz, d2-CDCl2CDCl2, δ): 7.80−6.35 (br), 3.72 (br), 2.61 (br), 1.99 (br), 1.70−1.05 (br), 0.95−0.85 (br). GPC: Mn = 33.5 kDa, Mw = 49.9 kDa, PDI = 1.49. Elemental Anal. Calcd for C86H114F4N2O2S4: C 73.15, H 8.14, N 1.98. Found: C 72.47, H 8.05, N 1.94. P3. The synthetic procedure is similar to that of P1 using 8 (116 mg, 0.086 mmol), (E)-1,2-bis(5-(trimethylstannyl)thiophen-2-yl)ethene, 10a (44.5 mg, 0.086 mmol), Pd2(dba)3 (2.6 mg), P(o-tol)3 (7.5 mg), and chlorobenzene (10 mL) affording the desired polymer (105 mg, 89%). 1H NMR (300 MHz, CDCl2CDCl2, δ): 7.28−6.50 (br), 3.84 (br), 2.13 (br), 1.40−1.31 (br), 0.90 (br). GPC: Mn = 36.6 kDa, Mw = 59.6 kDa, PDI = 1.63. Elemental Anal. Calcd for C84H114F2N4O2S4: C 73.21, H 8.34, N 4.07. Found: C 72.72, H 8.30, N 3.98. P3F. The synthetic procedure is similar to that of P1 using 8 (116 mg 0.086 mmol), (E)-1,2-bis(3-fluoro-5-(trimethylstannyl)thiophen2-yl)ethene, 10b (47.8 mg 0.086 mmol), Pd2(dba)3 (2.6 mg), P(otol)3 (7.5 mg) and chlorobenzene (10 mL) affording the desired polymer (110 mg, 90%). 1H NMR (300 MHz, CDCl2CDCl2, δ): 7.38−6.61 (br), 3.81 (br), 2.13 (br), 1.40−1.31 (br), 0.90 (br). GPC: Mn = 22.9 kDa, Mw = 38.6 kDa, PDI = 1.69. Elemental Anal.: Calcd for C84H112 F4N4O2S4: C 71.35, H 7.98, N 3.96; Found: C 70.72, H 7.92, N 3.90. Device Fabrication and Characterization. Bottom-contact/ bottom-gate field-effect transistor devices were fabricated to study the charge transport properties of the six newly synthetic copolymers. Heavily doped silicon wafers covered with 300 nm thick silicon dioxide layers were used as the gate electrode and gate dielectric, respectively. Gold was used as source-drain electrodes, which were prepatterned by photolithography on the silicon dioxide surface. The substrates were fully cleaned using ultrasonication in acetone, deionized water, and isopropanol. After being dried under vacuum at 80 °C, the substrates were treated with UV-ozone cleaner for 20 min, followed by concentrated H2SO4 and H2O2 (3:1 in volume) to form a layer of hydroxyl. After washed successively with deionized water, ethanol, and acetone, the cleaned substrates were modified with octadecyltrichlorosilane (OTS) to form an OTS self-assembled monolayer on the silicon dioxide surface. The polymer thin layers were deposited by spin-coating a polymer solution (7 mg/mL) in odichlorobenzene at 2500 rpm for 60 s. The thermal annealing processes of P1, P1F, P2, and P2F-based PFET devices were performed at 80, 120, 160, and 200 °C for 10 min in ambient conditions while 80, 120, 160, 200, and 240 °C for 5 min for P3- and P3F-based devices. The evaluations of the PFET devices were accomplished using a Keithley 4200 SCS semiconductor parameter analyzer on a probe stage. The charge carrier mobility, μ, was calculated from the data in the saturated regime according to the equation
RESULTS AND DISCUSSION Synthesis and Thermal Properties. The chemical structures and synthetic routes of the FVTI- and FVTAIbased monomers 4 and 8 and D−A copolymers P1−P3F are shown in Scheme 1. 6-Bromo-1-(2-decyltetradecyl)indoline2,3-dione, 1, was reduced by hydrazine hydrate to give 6bromo-1-(2-decyltetradecyl)indolin-2-one, 2, in high yield. Under weak basic conditions, a Knoevenagel condensation reaction of 2 and (E)-5,5′-(ethene-1,2-diyl)bis(4-fluorothiophene-2-carbaldehyde), 3, was performed, affording the desired FVTI-based monomer, 4, as a purple solid in moderate yield. 6-Bromo-1-(2-decyltetradecyl)-1H-pyrrolo[2,3-b]pyridine, 5, was first converted to 6 by reacting with pyridinium hydrobromide perbromide, PBPB. Then intermediate 6 reacted with zinc powder in THF in the presence of ammonium chloride(aq) to give 7 in 92% yield. A similar Knoevenagel condensation reaction of 7 and 3 afforded the FVTAI-based monomer 8 as a dark purple solid. Stille copolymerization reactions of monomers 4 and 8 with 5,5′bis(trimethylstannyl)-2,2′-bithiophene, 9a, (3,3′-difluoro[2,2′-bithiophene]-5,5′-diyl)bis(trimethylstannane), 9b, (E)1,2-bis(5-(trimethylstannyl)thiophen-2-yl)ethene, 10a, or (E)1,2-bis(3-fluoro-5-(trimethylstannyl)thiophen-2-yl)ethene, 10b, gave the crude polymer products P1−P3F, respectively, followed by further purification by Soxhlet extraction with methanol, acetone, n-hexane, and o-dichlorobenzene successively. The chemical structures of these copolymers were confirmed by NMR spectra and elemental analyses, while their molecular weights were evaluated by high-temperature gel permeation chromatography (GPC) analyses (Figure S1). The number-average molar mass (Mn) and polydispersity indexes (PDI) of these copolymers are located in the ranges of 28.2− 52.0 kDa and 1.63−2.81, respectively (Table 1). The thermal Table 1. Molecular Weight, Thermal, Optical and Electrochemical Properties of FVTI- and FVTAI-Based D− A Copolymers P1−P3F polymer
Mna (kDa)
PDIa
Tdecb (°C)
λmax (nm)
e Eopt g (eV)
EHOMOf (eV)
ELUMOf (eV)
P1
52.0
1.85
409.8
1.63
−5.25
−3.36
P1F
28.2
1.50
401.3
1.64
−5.35
−3.39
P2
47.5
2.81
396.9
1.60
−5.20
−3.36
P2F
33.5
1.49
366.7
1.63
−5.27
−3.43
P3
36.6
1.63
379.2
1.58
−5.82
−3.89
P3F
22.9
1.69
360.1
628c/ 628d 624c/ 630d 624c/ 632d 616c/ 628d 646c/ 648d 632c/ 642d
1.60
−5.99
−3.93
Determined by GPC at 150 °C. bOnset decomposition temperature (5% weight loss) measured by TGA. cIn chlorobenzene solutions. dIn thin films. eCalculated from thin-film absorption edges (Eopt g = 1240/ λabs,onset). fDetermined by CV. a
properties of all polymer materials were characterized by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) (Figures S2 and S3). The high decomposition temperatures (Tdec) with 5% weight loss of P1−P3F lie in the range 360−409 °C, revealing that all the six copolymers have high thermal stabilities. However, no apparent thermal transitions were observed in the six
IDS = (W /2L)Ciμ(VGS − Vth)2 where ISD is the drain current in the saturated regime. W/L is the channel width/length, Ci is the gate dielectric layer capacitance per unit area, and VGS and Vth are the gate and threshold voltages, respectively. The W/L of FET devices in this work are 50 μm/1400 μm. D
DOI: 10.1021/acs.macromol.8b01297 Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. Optimized conjugated backbone conformations obtained by DFT calculation (in trimer) (a) and proposed intramolecular noncovalent interactions (b) of FVTI- and FVTAI-based D−A copolymers P1−P3F (in dimer with alkyl chains replaced with methyl groups for simplicity).
Figure 2. Normalized absorption spectra of FVTI- and FVTAI-based D−A copolymers P1−P3F in o-dichlorobenzene solution (a) and as-spun thin film on quartz plates (b).
copolymers except in P3, and the Tg of P3 was estimated to be 143 °C. DFT Calculations and Conjugated Backbone Conformation Analyses. For well understanding the molecular geometry and electronic structure of P1−P3F, theoretical calculations were performed using density functional theory (DFT) at the B3LYP/6-31G(d) level on a trimeric system.
The optimized conjugated backbone conformations of these copolymers are shown in Figure 1a, and their electron state density distributions in the HOMOs and LUMOs will be discussed in the electrochemical properties section. The side views of these trimers reveal that the six copolymers have almost planar and linear conjugated backbones. In specific, the torsional angles of P1 are 23.07°, 24.13°, and 12.54°, while E
DOI: 10.1021/acs.macromol.8b01297 Macromolecules XXXX, XXX, XXX−XXX
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Figure 3. CV curves of the FVTI- and FVTAI-based D−A copolymers, P1−P3F.
their respective as-spun ones. The absorption changes suggest that orderly π−π stacking occurs in these annealed films. Based on their absorption edges, the optical bandgaps of P1−P3F were estimated to be in the range 1.58−1.63 eV (Table 1). The electrochemical properties of these copolymers were investigated by the cyclic voltammetry (CV) technique. The CV curves of all copolymers feature with clear oxidation peaks and reduction peaks (Figure 3). The highest occupied orbital (HOMO) and the lowest unoccupied orbital (LUMO) energy levels of these polymers were calculated from their respective oxidative onset potentials or the reductive onset potentials according to the equations EHOMO = −(4.40 + Eox onset) eV and ELOMO = −(4.40 + Ered onset) eV. The HOMO and LUMO energy levels of P1−P2F were estimated to be −5.20 to −5.35 eV and −3.36 to −3.43 eV, respectively. However, the HOMO/LUMO energy levels of −5.82 eV/−3.89 eV for P3 and −5.99 eV/−3.93 eV for P3F are apparently lower than those of the other four copolymers, indicating the strong electron-withdrawing capacity of pyridine-nitrogen atoms. In addition, P2 and P2F have higher HOMO energy levels than those of P1 and P1F, respectviely. The results show the introduction of internal double bonds elevates the HOMOs of both P2 and P2F. For gaining more insight into the electronic properties of these copolymers, theoretical modeling was performed using DFT at the B3LYP/6-31G(d) level on a trimeric system as aforementioned (Figure S5). Computational results show that all the six copolymers have evenly distributed HOMO electron densities, suggesting good delocalization along polymeric conjugated backbones. On the other hand, the LUMOs of P1−P2F are localized on their electron-withdrawing FVTI units. For P3 and P3F, their LUMOs have increased delocalization over the electron-donating FDTE or DTE units, though their LUMOs are still mainly located the electron-withdrawing FVTAI units. DFT predicted the HOMO and LUMO energy levels of these copolymers are collected in Table S1. The introduction of fluorine and pyridine-nitrogen atoms lowers the both HOMO and LUMO energy levels, which matches well with the CV measurement results. PFET Device Performance. Bottom-gate bottom-contact PFET devices were fabricated to investigate the charge carrier transport properties of these copolymers. Heavily doped silicon wafers covered with 300 nm thick silicon dioxide layers were used as the gate electrode and gate dielectric, respectively. Gold source and drain electrodes were prepatterned by photolithography technique, and the octadecyltrichlorosilane (OTS) treatment on the gate dielectrics was performed to
those of P1F are 23.25°, 23.14°, and 0.44°, suggesting that the conjugated backbone of P1F is more planar than that of P1. The similar changes could also be observed in P2 (22.76° and 22.76°) and P2F (22.51° and 22.36°). Such observations suggest that the loading of fluorine atoms is generally beneficial for obtaining high planar polymeric conjugated backbones. Comparing the torsional angles in the polymer pairs of P2/P3 and P2F/P3F, moreover, another conclusion might be also obtained that the incorporation of nitrogen flattens the related polymeric conjugated backbones as fluorine atoms do. On the basis of the above results in combination with the conjugated structures of (E)-1,2-bis(3-fluorothiophen-2-yl)ethene (FDTE), 3,3′-difluoro-2,2′-bithiophene, 7,7′-azaindigo, and DTE-bridged methyleneoxindole unit, we purposed the intramolecular noncovalent interactions existing in these copolymers on a dimeric system (Figure 1b). The F···H or F···S conformation locks appear in P1, P1F, P2, and P2F. For P3 and P3F, the presence of pyridine-nitrogen-induced N···H conformation locks endows them with further reduced torsional angles close to 0°. Recently, both theoretical modeling and experimental results have demonstrated that conjugated backbone conformations of polymer semiconductors exert important influences on carrier transport through adopting various interchain molecular packings.40,43 Thus, the planar conjugated backbones of these copolymers, especially P3 and P3F, might be in favor of acquiring efficient charge carrier transport in PFET devices. Optical and Electrochemical Properties. The normalized absorption spectra of the six copolymers in dilute odichlorobenzene solution and as-spun thin film on quartz plates are depicted in Figure 2. All copolymers exhibit wide absorption profiles in the whole UV−vis zone, indicating strong D−A interactions in these polymeric π-systems.44 It is worth to note that the absorption spectrum of P3 in solution has an more obvious red-shift than those of the other five copolymers, which might grow out of the stronger D−A interactions along its conjugated backbone than those of the other copolymers. Compared to those in dilute solutions, the absorption spectra of as-spun thin films broaden and red-shift in different extents. In addition, shoulder peaks (ca. 580 and 700 nm) appear in these thin film absorption spectra, illustrating that there are enhanced interchain stacking and conjugated backbone planarity in these as-spun thin films.45 In addition, the absorption spectra of the thin films after thermal annealing at 120 °C for P1, P1F, P2, and P2F and at 160 and 240 °C for P3 and P3F were also recorded. As depicted in Figure S4, the annealed thin films show red-shifted absorption spectra except similar absorption profiles compared to those of F
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Figure 4. Typical transfer (IDS−VGS) and output (IDS−VDS) characteristics of PFETs based on P1 (a), P1F (b), P2 (c), P2F (d), P3 (e), and P3F (f) after annealing at their respective optimal temperatures.
Table 2. Device Performance of P1−P3F-Based PFETs polymer
tempa (°C)
μmaxb (cm2 V−1 s−1)
μavc (cm2 V−1 s−1)
Ion/Ioffd
Vthe (V)
P1 P1F P2 P2F P3 P3F
120 120 120 120 160 240
0.33 0.39 0.30 0.48 0.47 0.31
0.24 0.26 0.21 0.31 0.37 0.19
104−106 105−107 104−106 104−106 105−108 104−107
1.36−17.39 −10.65 to 7.44 3.39−12.95 2.19−11.70 2.78−9.75 ∼5.97 to 3.78
a
Optimal annealing temperature. bThe maximum mobilities. cThe average mobilities. dOn/off current ratio. eThreshold voltage. The values were summarized from more than 10 devices.
form an OTS self-assembled monolayer. The polymer thin films were deposited by spin-coating a solution of polymer material in o-dichlorobenzene. Detailed fabrication and characterization processes are described in the Experimental Section. The thin film annealing temperature investigations demonstrated that the optimal annealing temperatures of P1, P1F, P2, and P2F were 120 °C, while 160 and 240 °C for P3 and P3F, respectively. All copolymers exhibited typical hole charge transport characteristics. Their transfer (IDS−VGS) and output (IDS−VDS) characteristics of these PFETs are shown in Figure 4, and the corresponding device parameters are listed in
Table 2. The maximum and average hole mobilities are 0.33 and 0.24 cm2 V−1 s−1 for P1, 0.39 and 0.26 cm2 V−1 s−1 for P1F, 0.30 and 0.21 cm2 V−1 s−1 for P2, 0.48 and 0.31 cm2 V−1 s−1 for P2F, 0.47 and 0.37 cm2 V−1 s−1 for P3, and 0.31 and 0.19 cm2 V−1 s−1 for P3F, respectively. The results illustrate that the mobilities of P1 to P1F and P2 to P2F increase with the loading of fluorine atoms onto polymeric conjugated backbones. Note that P2 showed a lower hole mobility than that of P1. This might not be true because P2 has a larger PDI than that of P1, which is not in favor of forming ordered molecular packing in P2 thin film, thus leading to a slightly G
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Figure 5. Tapping-mode AFM height images of the P1−P3F thin films after annealing at their respective optimal temperatures: P1 (a), P1F (b), P2 (c), P2F (d), P3 (e), and P3F (f).
Figure 6. 2D-GIXRD patterns of the P1−P3F thin films after annealing at their respective optimal temperatures: P1 (a), P1F (b), P2 (c), P2F (d), P3 (e), and P3F (f).
Table 3. Molecular Aggregation Parameters of the P1−P3F As-Spun and Annealed Thin Films polymer
thin film
q1a
fwhm (Δq, nm−1)
d−db (Å)
coherence lengthc (nm)
q2d
π−πe (Å)
P1
as-spun annealed as-spun annealed as-spun annealed as-spun annealed as-spun annealed as-spun annealed
3.22 3.18 3.25 3.16 3.24 2.53 3.05 2.54 3.41 3.29 3.29 3.30
0.485 0.281 0.54 0.19 0.588 0.307 1.331 0.429 0.516 0.212 0.531 0.191
19.5 19.7 19.3 19.9 19.4 24.8 20.6 24.8 18.4 19.1 19.1 19.1
11.53 19.90 10.36 29.43 9.51 18.19 4.20 13.04 10.84 26.38 10.53 29.28
17.87 17.87 17.76 17.89 17.78 17.95 17.85 17.72 18.06 17.97 18.01 17.97
3.52 3.52 3.54 3.51 3.53 3.50 3.52 3.55 3.48 3.50 3.49 3.50
P1F P2 P2F P3 P3F
The scattering vector of (100) Bragg peaks. bThe distances of d-stacking, d−d (Å) = 20π/q1. cCoherence lengths, λ (nm) = 2πk/fwhm, k = 0.89. The scattering vector of (010) Bragg peaks. eThe distances of π-stacking, π−π (Å) = 20π/q2.
a
d
H
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CONCLUSION Two novel difluorothienylethylene-bridged methyleneoxindole (FVTI) and methyleneazaoxindole (FVTAI) acceptor units and six FVTI- or FVTAI-based D−A copolymers, P1−P3F, were developed. On the basis of the optimized conjugated backbone conformations, we put forward the intramolecular noncovalent interactions existing in these copolymers and suggested that the fluorine substituents and pyridine-nitrogen atoms improve the polymeric backbone planarity by forming multiple conformation locks such as N···H, F···H, and F···S, etc. Thin film microstructures investigations indicated that the copolymers with more fluorine or pyridine-nitrogen atoms in their respective single D−A pairs generally formed more ordered thin films. PFET devices based on these copolymers were fabricated and increasing charge mobilities were obtained from P1 to P1F- and P2 to P2F-based devices. Compared to P3, the P3F-based PFET devices exhibited lower mobilities. We attributed the low mobility of P3F to its higher hole injection barrier between polymer semiconducting thin film and Au source-drain electrodes. Our results demonstrate the important influences of fluorine- and pyridine-nitrogeninduced conformation locks on the conjugated backbone planarity and charge transport properties of these copolymers and highlight that the FVTI and FVTAI units are promising building blocks for polymers semiconductors toward highperformance PFETs.
lowered carrier mobility. Compared to P3, the P3F-based PFET devices exhibited lower mobilities, which might be partly attributed to its low-lying HOMO energy level, inducing an increased hole injection barrier between its polymer semiconducting thin film and Au source-drain electrodes. Besides frontier orbital energy levels, the molecular aggregation state of polymer semiconductors could also exert important influences on charge transport properties; thus, the thin film microstructure investigation is necessary, and the corresponding results will be discussed in the following content. Thin Film Microstructure Analyses. Atomic force microscopy (AFM) operated in tapping mode and the twodimensional grazing incidence X-ray diffraction (2D-GIXRD) technique were used to study the thin film microstructures of these copolymers. The AFM height images of the as-spun and annealed P1−P3F thin films are displayed in Figures S6 and S5, respectively. All the thin films own uniform fibrillar intercalating networks with small crystalline domains, which might result from strong intermolecular π−π interactions. The 2D-GIXRD patterns of the as-spun and annealed P1−P3F thin films are shown in Figures S7 and 6, respectively. Apparently, all the annealed thin films showed more (h00) Bragg peaks such as (200), (300), and even (400) than those of their respective as-spun thin films in out-of-plane (qz) orientation, implying that enhanced lamellar packings formed in P1−P3F thin films after annealing at their respective optimal temperatures. Meanwhile, the crystallinity of both the as-spun and annealed thin films were also investigated by calculating coherence length and full width at half-maximum (fwhm) from their scattering vectors (q1) of (100) Bragg peaks (Table 3). Both coherence length and fwhm are important parameters in evaluating crystallinity.46 In general, the longer coherence length and the smaller fwhm imply the higher crystallinity of polymer thin films. For P1, for example, the coherence length and fwhm of its annealed thin film are 19.90 nm and 0.281 nm−1, respectively, which are apparently higher than 11.53 nm and smaller than 0.485 nm−1 of its as-spun one. And the annealed thin films of the other five copolymers also own higher coherence length and smaller fwhm than those of their respective as-spun ones. These results imply that all these annealed thin films have improved crystallinities. The (010) Bragg peaks only appeared in qxy orientation, so we can judge that the six copolymers take predominantly edge-on oriented respective to the substrates in these annealed thin films. Depended on the scattering vectors (q1 or q2) of (100) or (010) Bragg peaks, the d−d and π−π distances of the annealed P1−P3F thin films were also estimated, and the data are listed in Table 3. Comparing the 2D-GIXRD patterns of the annealed films of P1/P1F and P3/P3F, it is obvious that the molecular packing of P1F and P3F containing more fluorine atoms in their respective single D−A pairs are more ordered than those of P1 and P3, respectively. Similarly, the molecular packing of P3F with pyridine-nitrogen atoms is more ordered than that of P3, too. The results suggest that the incorporation of fluorine and pyridine-nitrogen atoms onto polymeric conjugated backbones induced relatively stronger intermolecular interactions. Many previous studies demonstrated that crystalline, highly ordered, and good lamellar edge-on packing mode are helpful for obtaining high efficient charge transport;10−16 thus, the molecular packing patterns of P1−P3F are in accordance with their PFET device performances.
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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.8b01297. GPC distribution plots, TGA traces, DSC curves; UV− vis absorption spectra of polymer thin films before and after thermal annealing; electron state density distributions in the HOMOs and LUMOs; tapping-mode AFM height images and 2D-GIXRD patterns of the P1−P3F as-spun thin films, NMR spectra of intermediates 3, 4, 6, and 7, and the FVTI- and FVTAI-based monomers 4 and 8, and copolymers P1−P3F (DOCX)
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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 Congyuan Wei: 0000-0003-3554-8001 Gui Yu: 0000-0001-8324-397X Author Contributions
Y.Z. and S.Z. contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 51473021, 51773016, 21673258, 21774134, and 21474116), the National Key Research and Development Program of China (2016YFB0401100 and 2017YFA0204703), and the Strategic Priority Research I
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Program of the Chinese Academy of Sciences (CAS) (XDB12030100). 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.
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DOI: 10.1021/acs.macromol.8b01297 Macromolecules XXXX, XXX, XXX−XXX
