Fluorinated Dithienylethene–Naphthalenediimide ... - ACS Publications

Jul 23, 2017 - School of Material Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China ... toward high-mobilit...
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Fluorinated Dithienylethene−Naphthalenediimide Copolymers for High-Mobility n‑Channel Field-Effect Transistors Zhihui Chen,†,‡ Weifeng Zhang,*,† Jianyao Huang,† Dong Gao,†,‡ Congyuan Wei,† Zuzhang Lin,†,§ 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, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § School of Material Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China S Supporting Information *

ABSTRACT: We develop two donor−acceptor copolymers based on a fluorinated dithienylethene building block, namely PNFDTE1 and PNFDTE2, in which naphthalenediimide (NDI) acts as an acceptor unit. Thermogravimetric analysis displayed both copolymers having good thermal stability with high decomposition temperatures over 400 °C. Broad absorption spectra were observed in the UV−vis−NIR region, with the absorption maxima being 720 and 724 nm for PNFDTE1 and PNFDTE2, respectively. Cyclic voltammetry tests exhibited deep-lying lowest unoccupied molecular orbital energy levels of ca. −4.0 eV. Twodimensional grazing incidence X-ray diffraction patterns showed that different packing modes for two polymers result in the variation in charge transport properties. Backbone fluorination effectively decreases electron injection barrier, thereby facilitating electron mobility. An impressive electron mobility of 3.20 cm2 V−1 s−1 was achieved in air for PNFDTE1-based polymer field-effect transistors fabricated on the poly(ethylene terephthalate) substrate. The mobility value is almost the highest for NDI-containing polymers on the flexible substrate. This work provides a guideline for design and synthesis of fluorinated semiconductors that enables control of chargetransport polarity.

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INTRODUCTION Backbone fluorination has evolved as a strategic molecular design method in organic electronics.1−16 Since the early 2000s, many efforts have been made to develop high-mobility semiconducting materials with fluorine substituents for organic field-effect transistors (FETs).17−21 Diverse fluorinated materials have been synthesized to realize the substituent effects on electronic structures and crystallization behaviors. The extreme electronegativity of fluorine atoms pulls electrons from the backbone to stabilize frontier molecular orbitals, facilitating electron transporting characteristics and resulting in a change in intermolecular interactions. For example, perfluorination has proven to be an effective way to convert a p-type semiconductor to an n-type or ambipolar one by greatly lowering the lowest unoccupied molecular orbital (LUMO) energy level.18 The quadropole moment of perfluorinated acene is opposite in sign compared with that of acene, thus significantly tuning the orbital energetics and intermolecular overlaps.22 In terms of crystal engineering, the stacking of fluorinated molecules differs from that of unsubstituted ones, which is experimentally confirmed by single crystal characterization and theoretically assessed by energy surface potential analysis.23 The hydrophobic nature of fluorinated compounds can also © XXXX American Chemical Society

impart better air stability of devices by resisting the penetration of oxygen or water.24,25 Inspired by the success of fluorination in small-molecule semiconductors, further investigations on fluorine-containing polymers have been a hot research topic. Recent studies have revealed that fluorine-induced intramolecular interactions including S···F and H···F can promote backbone planarity, enhance the frontier orbital overlaps, tune the solubility, and change the packing motif of chains.1,4,23,26−30 Therefore, controlling the conjugated backbone conformations of polymer chains with such noncovalent interactions serves as a useful tool to better understand the charge transport mechanism, thereby providing new rational design strategies toward high-mobility materials.31−35 In this regard, it is worthwhile to note that the fluorination position is one of the key factors to conformational control. In our early studies, we designed and synthesized two fluorinated diphenylethenebased copolymers.26 Their optical and electrical properties suggest that different positions of fluorine can induce diverse conformational locks, thus predominantly affecting the Received: June 3, 2017 Revised: July 23, 2017

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DOI: 10.1021/acs.macromol.7b01169 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthetic Routes to FDTE-Based Copolymers, PNFDTE1 and PNFDTE2

molecular aggregation in solid state. Bazan and co-workers also systematically studied the influence of fluorine substitution on benzothiadiazole-based polymers, indicating that different substitution positions and regioregularities contribute greatly to the electrical properties.28,36 In addition, the degree of fluorination also enables the modulation of frontier orbital energy levels and bandgaps.3 Generally, taking into account the position and degree of fluorination is of great significance to obtain high-performance polymer semiconductors. Vinylene unit is a versatile linker to construct highly planar building blocks.37−39 The vinylene linker minimizes the torsional energy and extends the π-conjugation, which planarizes the polymer backbone and leads to enhanced intrachain charge transport properties. A good example is dithienylethene (DTE). DTE has been used as the electrondonating building block in many imide- or amide-containing high-mobility semiconducting polymers.40−42 The naphthalenediimide (NDI)−DTE-derived copolymers exhibited good ambipolar charge transport behaviors with electron mobility of 1.57 cm2 V−1 s−1 and hole mobility of 0.3 cm2 V−1 s−1.37 To obtain better electron transport properties, we herein design and synthesize two fluorinated DTE (FDTE)-based copolymers, namely PNFDTE1 and PNFDTE2, in which the FDTE unit acts as the donor unit and NDI acts as the acceptor unit. Fluorination of DTE at the 3,3′-positions provides three major advantages: (1) fluorination slightly stabilizes both the highest occupied molecular orbital (HOMO) and LUMO and minimally changes the packing motif because of the small radius of fluorine; (2) it could induce H···F intramolecular interactions, thus affording enhanced planarity and backbone rigidity; (3) it would not introduce new steric hindrance between thiophene−NDI subunits. (If the fluorine atoms were introduced into the 4,4′-positions of DTE unit, electrostatic repulsion would occur between the fluorine atom and exocyclic carbonyl group.) As expected, the two FDTE-based copolymers show the dual-band absorption profiles in the UV−vis−NIR

region. Both copolymers have deep-lying LUMO energy levels of ca. −4.0 eV. PNFDTE1-based top-gate/bottom-contact (TGBC) polymer field-effect transistors (PFETs) on the poly(ethylene terephthalate) substrate exhibited enhanced electron mobility of up to 3.20 cm2 V−1 s−1 in air; meanwhile, PNFDTE2-based devices only showed lower electron mobility of 0.266 cm2 V−1 s−1. Further thin film microstructure characterization reveals that the difference in mobility might originate from their different molecular aggregations in thin films. We have also discussed how fluorination affects the optical and electrical properties compared with unfluorinated polymers. Our results demonstrate the great potential of FDTE unit in constructing high-performance polymer semiconductors and offer insights into the tuning of molecular aggregation and field-effect property.

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RESULTS AND DISCUSSION Synthesis of PNFDTE. The synthetic routes of FDTEbased monomer and copolymers are shown in Scheme 1, and synthetic procedures are provided in the Expermental Section. Owing to the requirement of selective fluorination, the synthesis of FDTE involved more steps than those of (E)1,2-bis(3,4-difluorothien-2-yl)ethene.43 Thiophene-2-carboxylic acid (1) reacted with 2 equiv of n-BuLi and was then quenched with (PhSO2)2NF (NSFI) to give 3-fluorothiophene-2carboxylic acid (2) in a moderate yield.44 After esterification, reduction reaction with DIBAL-H, and oxidation reaction with MnO2, intermediate 2 was transformed to 3-fluorothiophene-2carbaldehyde (5). The reductive coupling reaction of aldehyde 5 under McMurry conditions afforded the desired FDTE as a white crystalline solid.45,46 According to a conventional method, FDTE was converted to FDTE-based ditin monomer (7) in 76% yield.47,48 The two FDTE-based donor−acceptor copolymers were then readily prepared by Stille-coupling polymerization reaction. The crude polymer materials were purified by Soxhlet extraction and recrystallized from CHCl3/ B

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chains should be the major contribution to electrical transport. Therefore, a highly self-assembled crystalline structure with tight packing could facilitate the charge transport of NDI-based copolymers.51 The effect of fluorination can also be precisely illustrated in frontier molecular orbital distributions. The energy levels of frontier molecular orbitals dropped by 0.1− 0.2 eV through fluorination according to DFT calculations. Notably, the fluorine atoms mainly give contribution to the HOMO, thereby slightly lowering the HOMO energy level and resulting in an increased HOMO−LUMO gap. As is consistent from the optical bandgap, fluorination of the donor can finetune the energy gaps.37 Optical Properties. Figure 2 exhibits the UV−vis−NIR absorption spectra of the two polymers in dilute chlorobenzene solution and in thin film. The corresponding optical data of two FDTE-based copolymers are listed in Table 1. Both copolymers show typical dual-band absorption profiles. The low-energy absorption maxima in solutions are 716 and 710 nm for PNFDTE1 and PNFDTE2, respectively. As a result of the planarization effect on polymer backbones in thin films, the absorption maxima increase to 720 and 724 nm. Moreover, the low-energy bands broaden significantly in thin film, with the absorption edges red-shifting to ca. 850 nm. The low-energy bands exhibit significant 0−0 and 0−1 vibronic features caused by aggregation.51,52 It is worth noting that the oscillator strengths of the low-energy bands and the relative strength of 0−0 and 0−1 features are different for PNFDTE1 and PNFDTE2. The variation is related to the strength of intermolecular coupling of aggregates, which is caused by the different packing modes.52,53 PNFDTE1 tends to form highly aggregated structures in both solution and thin film, as is evident from the strong vibronic coupling peaks. The calculated optical bandgaps are 1.41 and 1.44 eV for PNFDTE1 and PNFDTE2, respectively. We notice that the optical bandgaps are slightly larger compared with those of previously reported PNVT polymers without fluorine atoms.37 The result is in good agreement with theoretical modeling that fluorination on the donor units has a more significant effect on the HOMO than the LUMO. As shown in Figure S2, temperature-dependent UV−vis−NIR absorption spectra were performed to illustrate the aggregation tendencies. The shoulder peak becomes weak as the temperature increases, with a slight blue-shift occurring. Such phenomena are originated from the free rotation of flexible single bonds and partial dissociation of aggregates. We emphasize that there exists a certain extent of aggregation in hot solution of PNFDTE1, revealing an intense self-aggregation feature of this polymer that may promote solid-state packing.

MeOH to give the desired copolymers in high yields. The two FDTE-based copolymers have good solubility in warm chlorinated solvents such as chlorobenzene, o-dichlorobenzene, and even chloroform (ca. 10 mg mL−1). Their chemical structures and degree of polymerization were examined by high-temperature NMR and high-temperature gel permeation chromatography, respectively. The 1H NMR spectra of both copolymers are depicted in the Supporting Information and match well with their respective chemical structures. Both polymers exhibit high degree of polymerization with the number-average molecular weight (Mn) and polydispersity indices (PDIs) of 38.9 kDa and 2.31 for PNFDTE1 and 38.2 kDa and 3.54 for PNFDTE2. The thermal behaviors of both copolymers were evaluated by thermogravimetric analysis (TGA). The resulting TGA traces are collected in Figures S1. Both copolymers have good thermal stability with high decomposition temperatures of 420 and 409 °C for PNFDTE1 and PNFDTE2, respectively. Theoretical Modeling. We performed density functional theory (DFT) calculations to illustrate the backbone planarity and frontier molecular orbitals of PNFDTE polymers. As shown in Figure 1, HOMO and LUMO are exclusively

Figure 1. Frontier molecular orbitals of PNFDTE trimers.

localized on the FDTE and NDI units, respectively, demonstrating the significant charge transfer between the FDTE and NDI units in the HOMO−LUMO transition.49,50 The dihedral angle between the NDI and FDTE is ca. 45°, indicating the presence of steric hindrance. The extent of frontier molecular orbital delocalization is one of the primary parameters to judge the possibility of intramolecular charge transport. Conformational traps within the chains hinder the efficient charge hopping from one end of a polymer chain to the other. Such significant π-electron localization infers that the charge hopping between closely packed NDI units of adjacent

Figure 2. Normalized UV−vis−NIR absorption spectra of PNFDTE1 and PNFDTE2 in solution (a) and in thin film (b). C

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Macromolecules Table 1. Optical and Electrochemical Properties of PNFDTE1 and PNFDTE2 λabs (nm) polymer

soln

film

Egopt (eV)

ELUMO (eV)

EHOMO (eV)

PNFDTE1 PNFDTE2

398, 680, 716 394, 666, 710

401, 676, 720 402, 687, 724

1.44 1.46

−4.00 −4.00

−5.93 −5.89

Figure 3. Transfer and output characteristics of the TGBC FETs based on polymer films: (a, b) PNFDTE1 and (c, d) PNFDTE2 annealed at 120 °C.

Table 2. Electrical Properties of PNFDTE1 and PNFDTE2 Based Thin Film Transistorsa polymer

TGBC devices

μav (cm2 V−1 s−1)

μmax (cm2 V−1 s−1)

Ion/Ioff

PNFDTE1

n-channel p-channel n-channel p-channel

2.27 4.13 × 10−3 0.255 1.47 × 10−2

3.20 6.42 × 10−3 0.266 1.72 × 10−2

103−104 101−102 102−103 101−102

PNFDTE2

Vth (V) 42 −55 40 −65

± ± ± ±

15 5 5 15

The average values were obtained from 10 devices. Devices with default L = 30 μm and W = 1400 μm were used. The average thicknesses were ca. 40 and 50 nm for PNFDTE1 and PNFDTE2 thin films, respectively. a

Electrochemical Properties. The electrochemical properties of PNFDTE1 and PNFDTE2 were investigated by cyclic voltammetry (CV) (Figure S3). Both copolymers show strong reduction and weaker oxidation during negative and positive scans, suggesting that the two copolymers are more readily reduced than oxidized. Ferrocene was used as the internal standard to calibrate the energy levels by the equation E = [Eonset(Fc/Fc+ vs Ag/AgCl) − Eonset − 4.80] eV, where 4.80 eV is the energy level of ferrocene below the vacuum level and the potential Eonset(Fc/Fc+ vs Ag/AgCl) is 0.37 V during the test. On the basis of the onset reductive potentials (Ered onset) of ca. − 0.43 V versus Ag/AgCl, both LUMO energy levels were estimated to be ca. −4.00 eV for PNFDTE1 and PNFDTE2. Meanwhile, the HOMO energy levels of PNFDTE1 and PNFDTE2 were also estimated to be ca. −5.93 and −5.89 eV, respectively, according to the onset oxidation potentials (Eox onset) of 1.50 and 1.46 V versus Ag/AgCl. The deep-lying LUMO energy level would effectively enhance electron injection behavior. Accordingly, the energy gaps of the two copolymers

were calculated to be 1.93 and 1.89 eV, respectively, which were higher than the optical band gaps (1.44 and 1.41 eV) obtained from the UV−vis−NIR absorption spectra of their thin films. The differences in energy gaps are reasonable in consideration of the exciton binding energy of polymer materials.54 Compared with unfluorinated NDI-based copolymers PNVTs, the resulting HOMO energy levels were ca. 0.3 eV lower, whereas the LUMO energy levels only reduced by ca. 0.1 eV.37 The observations are in accordance with the theoretical modeling as aforementioned that fluorine atoms mainly give contribution to the HOMO, thereby slightly lowering the HOMO energy level and resulting in an increased HOMO− LUMO gap. Field-Effect Properties. To investigate the charge transport properties of the two copolymers, thin film PFET devices were fabricated with TGBC configuration. We fabricated the devices on poly(ethylene terephthalate) (PET) substrate by spin-coating a polymer solution (5 mg/mL) in chloroform. Gold source/drain and aluminum gate electrodes were D

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Figure 4. 2D-GIXRD patterns and pole figures of PNFDTE1 (a, b) and PNFDTE2 (c, d) before and after annealing treatment. (e) Schematic view of packing motif for polymers. (f) The theoretical length of a repeat unit.

(Figure S8). Additionally, the difference in mobility is sometimes observable in homologous polymer semiconductors with similar alkyl chains. The side-chain engineering strategy has demonstrated that such a phenomenon is attributed to the variation in crystalline lattice and packing mode, forming different extents of aggregation that alter the pathway of chargecarrier transport.55 Note also that higher electron mobilities and a lower hole ones are achieved compared with unfluorinated PNVT polymers.37 Our results demonstrate the control over charge-carrier injection processes by incorporation of fluorine substitution. Thin Film Microstructure. To further quantify the crystallinities and molecular orientations of the thin films, we employed two-dimensional grazing incidence X-ray diffraction (2D-GIXRD) and tapping-mode atomic force microscopy (AFM) measurements to study polymer packings and film morphologies. The 2D-GIXRD patterns of both polymer thin films show typical bimodal lamellar packing motifs (Figure 4). The arc types of diffraction patterns indicate broad orientation distributions.53 In the out-of-plane directions (qz vector), both PNFDTE1 and PNFDTE2 exhibit (h00) diffraction peaks up to the third order, indicative of moderate lamellar packing. The d-spacings are calculated to be 23.1 and 24.7 Å for as-spun films of PNFDTE1 and PNFDTE2, respectively. The values marginally raise to 23.7 and 25.4 Å for annealed films. We note that the d-spacing distances are comparable to that of P(NDI2OD-T2) (or N2200, Polyera Activink), confirming that the alkyl chains are highly interdigitated in the solid state.56 A

thermally evaporated via a shadow mask. Poly(methyl methacrylate) (PMMA) was used as a dielectric layer and encapsulation, leading to stable device performance under ambient condition. We first explored the optimal annealing temperatures (Figure S4). Annealing the film is a general procedure to optimize the device performance. After the annealing process, the self-assembly of side chains and the intermolecular interactions between backbones would change. An optimal annealing temperature provides a balance of good crystallization behaviors and film morphologies. Both copolymers showed the highest mobilities after thermal treatment at 120 °C for 5 min, meaning that both polymer thin films have highly ordered molecular aggregations. Figure 3 presents the typical transfer and output characteristics of PNFDTE1- and PNFDTE2-based PFET devices. The mobilities, on/off current ratios, and threshold voltages extracted from transfer curves in saturation regimes are summarized in Table 2. Both copolymers exhibited ambipolar charge transport properties with high electron mobilities and low hole ones. For PNFDTE1, the highest electron and hole mobilities significantly are 3.20 and 6.42 × 10−3 cm2 V−1 s−1, respectively. To the best of our knowledge, the electron mobility is one of the highest values for NDI-based polymer semiconductors on the flexible substrates. PNFDTE2 only afforded lower electron mobility of 0.266 cm2 V−1 s−1, which is about a twelfth that of PNFDTE1. Both polymer thin films exhibited moderate air stability after exposure to the humid air (relative humidity of about 60%) for 2 weeks with the mobilities slightly decreasing to about 80% E

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Macromolecules Table 3. Microstructure Characterization of PNFDTE1 and PNFDTE2 Thin Films d-spacing (Å) polymer

mode

PNFDTE1

edge-on face-on edge-on face-on

PNFDTE2 a

(100) 23.7 24.3 25.4 26.5

(23.1) (24.0) (24.7) (25.8)

coherence length (Å)

(010) 3.71 3.77 3.73 3.89

(3.72) (3.81) (3.75) (3.90)

(001)

(100)

NA 16.0 (16.0) NA 15.9 (15.9)

147 (106) 95.7 (75.8) 66.8 (52.4) 152 (100)

(010) 23.0 22.6 27.0 23.0

(19.4) (22.7) (22.2) (22.6)

(001) NA 127 (107) NA 105 (93)

The values are related to annealed thin films, and the values in parentheses are related to the as-spun ones.

significant (010) signal at qz ≈ 1.69 Å−1 was observed for PNFDTE2, corresponding to a π−π stacking of ca. 3.73 Å. PNFDTE1 thin films exhibit weak (010) signals in the out-ofplane direction, indicative of preferential edge-on packing modes. The calculated π−π stacking distance for PNFDTE1 is 3.71 Å, which is smaller than that of PNFDTE2, exhibiting the tight interchain packing in the thin film. As indicated by theoretical modeling, a small π−π stacking distance may greatly facilitate electron transport. In the in-plane directions (qxy vector), complex diffraction patterns are visible. The in-plane intensity-corrected wedge cuts exhibit that the peaks can be assigned to (h00), (010), and (00l) reflections. These reflections are indicative of the spacing along the alkyl, π−π, and backbone directions, respectively (Figure 4e). The presence of (00l) diffraction peaks indicates the high extent of crystallinity along the ordered backbone, which is generally observed in NDI-based polymers. The (00l) diffraction peaks are observed up to the third order, corresponding to d-spacing of ca. 16.0 Å. The value matches well with the theoretical length of a repeat unit (16.3 Å), as shown in Figure 4f. We further calculated the coherence length along each direction to exhibit the improved crystallinity upon annealing treatment. Each interlamellar spacing slightly increases, each π-stacking distance decreases, and the coherence lengths also increase, suggesting that the structural and energetic disorder is reduced. For example, after annealing treatment, the coherence length of (001) peak corresponds to 7−8 ordered repeating units. The crystallite orientation distributions of (100) Bragg reflections are illustrated using pole figures.57 The areas integrated with polar angle χ ranges of −45° to 45° correspond to the fraction of edge-on crystallites, while the other areas indicate the ratio of face-on crystallites. The face-on/edge-on ratio of PNFDTE1 remained unchanged after thermal annealing treatment, whereas PNFDTE2 thin film tended to form face-on crystallites under the same conditions. The variation in crystallites’ orientation distributions is one possible explanation for the difference of mobility between the two copolymers. The surface morphologies of PNFDTE1 and PNFDTE2 thin films spin-coated on PET substrates and SiO2 substrates are shown in Figure 5 and Figure S7, respectively. For PNFDTE1, the thin films prepared on PET substrates and SiO2 substrates showed typical granular features. For the as-spun PNFDTE1 films, the root-mean-square (RMS) values were 0.894 and 2.00 nm on SiO2 and PET substrates, respectively. The relative rougher surfaces are inherited from intrinsic roughness of PET substrates. After the PNFDTE1 thin film was thermal-annealed at 120 °C for 5 min, larger grains and similar grain boundaries were observed in comparison with its as-spun thin film, whereas the annealed PNFDTE2 thin film on PET substrates displayed similar domains and fewer grain boundaries compared with its as-spun thin film. The change in surface morphology illustrates that strong molecular self-aggregation occurs on both substrates during the thermal annealing process. These results agree well

Figure 5. AFM height images of as-spun and annealed polymer thin films of PNFDTE1 (a, b) and PNFDTE2 (c, d) on PET substrate.

with the temperature dependence of transfer characteristics. Unlike the thin films of PNFDTE1, those of PNFDTE2 fabricated on PET substrates and SiO2 substrates both exhibit smooth surfaces with smaller grains. It is well recognized that large granular domains and continuous thin film morphologies are favorable to charge transport while deep grain boundaries result in trapping of charge carriers. Thus, the different thin film morphologies of the two copolymers contribute to interpreting their charge transport mobilities.

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EXPERIMENTAL SECTION

3-Fluorothiophene-2-carboxylic Acid (2). To a solution of thiophene-2-carboxylic acid (1) (17.0 g, 132.7 mmol) in anhydrous THF (300 mL) was added n-BuLi (120 mL, 2.5 M in THF, 300.0 mmol) dropwise at −78 °C under an argon atmosphere. The reaction mixture was stirred at the same temperature for 30 min. Then a solution of N-fluorobenzenesulfonimide (50.00 g, 158.6 mmol) in THF (300 mL) was added while keeping the temperature at −78 °C. The mixture was stirred at the same temperature for another 3 h and then warmed to ambient temperature and stirred for 16 h. To the reaction mixture was added aqueous HCl (1 M, 300 mL), extracted with ethyl acetate (600 mL × 3). The combined organic phase was dried over Na2SO4 and concentrated to afford a solid residue, which was subjected to silica gel column chromatography with petroleum ether/ethyl acetate as eluent to afford the crude product (8.50 g). The crude product was further crystallized in petroleum ether/ethyl acetate to give the desired compound (4.80 g, 24.8%). 1H NMR (300 MHz, d6-DMSO, δ): 13.3 (s, 1H), 7.90 (dd, J = 6.0 Hz, J = 1.5 Hz, 1H), 7.13 (d, J = 6.0 Hz, 1H). 13C NMR (75 MHz, d6-DMSO, δ): 161.6, 161.3, 157.6, 132.1, 132.0, 119.4, 119.4, 113.8, 113.5. HRMS: Calcd for [C5H3FO2S]+: 145.9838. Found: 145.9838. F

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2H), 6.70 (s, 2H), 0.22 (s, 18H). 13C NMR (75 MHz, CDCl3, δ): 158.6, 154.9, 136.7, 125.9, 125.7, 125.2, 124.9, 115.9, 115.8, −8.65. HRMS: Calcd for [C16H22F2S2Sn2]+: 551.9170, 553.9169, 555.9201. Found: 551.9162, 553.9167, 555.9194. General Procedures for Polymer Synthesis and Purification. To a Schlenk tube were added (E)-1,2-bis(3-fluoro-5-(trimethylstannyl)thiophen-2-yl)ethene, 7 (0.20 mmol), and NDI-based monomer, 8a or 8b (0.20 mmol), a catalytic amount of Pd2(dba)3 (6.0 mg), P(o-tol)3 (17.0 mg), and chlorobenzene (5.0 mL). The tube was charged with argon through a freeze−pump−thaw cycle. The reaction mixture was heated to 115 °C and stirred for 3 days. The resulting gel was cooled to room temperature and then precipitated in 200 mL of methanol containing 5 mL of HCl(aq) (6 M). The precipitate was purified using Soxhlet extraction with methanol, acetone, and hexane for 24 h. The residue was extracted with odichlorobenzene and precipitated in methanol to afford the desired polymer materials. PNFDTE1 (179 mg, 85%). 1H NMR (300 MHz, C2D2Cl4, δ): 8.84 (s, 2H), 7.25 (s, 2H), 7.16 (s, 2H), 4.20 (br, 4H), 2.08 (br, 2H), 1.46− 1.33 (br, 64H), 0.93 (m, 12H). GPC: Mn = 38.9 kDa, Mw = 90.3 kDa, PDI = 2.31. Anal. Calcd for C64H90F2N2O4S2: C, 72.96; H, 8.61; N, 2.66. Found: C 72.94, H 8.52, N 2.58. PNFDTE2 (191 mg, 82%). 1H NMR (300 MHz, C2D2Cl4, δ): 8.83 (s, 2H), 7.22 (s, 2H), 7.18 (s, 2H), 4.19 (br, 4H), 2.09 (br, 2H), 1.44− 1.33 (br, 80H), 0.93 (m, 12H). GPC: Mn = 38.2 kDa, Mw = 135.5 kDa, PDI = 3.54. Anal. Calcd for C72H106F2N2O4S2: C, 74.18; H, 9.17; N, 2.40. Found: C 74.23, H 9.04, N 2.32. General Procedures for Thin Film PFETs Fabrication. Topgate bottom-contact (TGBC) PFET devices were fabricated on PET substrates, and gold source−drain electrodes were prepared by thermal evaporation by a shadow mask with the channel length/width of 30/ 1400 μm. The substrate surfaces were cleaned by ultrasonication in acetone, deionized water, and ethanol. Then the polymer solution in chloroform (concentration: 5 mg/mL) was spin-coated onto the PET interfaces at the speed of 2000 rpm for 1 min. Polymer films were annealed in nitrogen at 120 °C for 5 min. PMMA (Mw = 1000 kDa) dissolved in anhydrous n-butyl acetate (Aldrich, concentration: 65 mg/ mL) was heated to 80 °C and stirred for 3 h. The PMMA solution was spin-coated to form a dielectric film on the semiconductor layer with a thickness of about 900 nm. An 80 nm aluminum gate electrode were then evaporated on top of the samples dried at 90 °C for 20 min. The gate leakage currents were on the order of 1−10 nA (Figure S9). Generally, the relatively low gate leakage current allows the accurate estimation of carrier mobilities from the transfer curves. A Keithley 4200 SCS semiconductor parameter analyzer was utilized to demonstrate FET parameters. TGBC FETs were measured under ambient conditions. The mobilities were extracted according to the equation IDS = (W/2L)Ciμ(VGS − Vth)2 in saturated regions, where IDS is source−drain saturation current, L the channel length, W the channel width, Vth the threshold voltage, Ci the capacitance per unit area of the insulator, VGS the gate voltage, and μ the mobility.

Ethyl 3-Fluorothiophene-2-carboxylate (3). A mixture of carboxylic acid, 2 (34.0 g, 232.7 mmol), potassium carbonate (64.3 g, 465.9 mmol), and ethyl iodide (43.6 g, 279.5 mmol) in DMF (400 mL) was stirred at 50 °C for 16 h. To the reaction mixture was added water (1.5 L). The mixture was extracted with petroleum ether/ethyl acetate (1.0 L × 3). The organic layers were dried over Na2SO4 and concentrated to give the desired compound as a colorless oil (36.0 g, 89%). The mixture was used in next step without further purification. 1 H NMR (300 MHz, CDCl3, δ): 7.42 (d, J = 9.0 Hz, 1H), 6.86 (d, J = 6.0 Hz, 1H), 4.79 (q, J = 7.5 Hz, 2H), 1.40 (t, J = 7.5 Hz, 1H). 13C NMR (75 MHz, CDCl3, δ): 161.9, 160.7, 158.3, 129.9, 129.7, 118.6, 118.3, 61.2, 14.3. HRMS: Calcd for [C7H7FO2S]+: 174.0151. Found: 174.0150. (3-Fluorothiophen-2-yl)methanol (4). To a solution of intermediate 3 (33.0 g, 189.4 mmol) in DCM (500 mL) was slowly added a solution of DIBAL-H (1 M in toluene, 480 mL, 480 mmol) at −78 °C under an argon atmosphere. The reaction mixture was stirred at −78 °C for another 2 h at the same temperature, and then a solution of NaOH (10%, 300 mL) was added dropwise. The resulting mixture was extracted with DCM (500 mL × 3). The combined organic layers were dried over Na2SO4 and concentrated to give crude product (20.06 g). The crude product was further purified by silica gel column chromatography with petroleum ether/ethyl acetate as eluent to afford the desired compound as a colorless oil (14.0 g, 56%). 1H NMR (300 MHz, CDCl3, δ): 7.14 (t, J = 6.0 Hz, 1H), 6.78 (d, J = 6.0 Hz, 1H), 4.76 (s, 1H), 1.74 (m, 1H). 13C NMR (75 MHz, CDCl3, δ): 156.6, 153.1, 123.6, 123.5, 120.6, 120.4, 117.4, 117.1, 55.1. HRMS: Calcd for [C5H5FOS]+: 132.0045. Found: 132.0044. 3-Fluorothiophene-2-carbaldehyde (5). To a solution of intermediate 4 (14.0 g, 105.9 mmol) in DCE (500 mL) was added MnO2 (47.50 g, 546.0 mmol). The mixture was refluxed for 16 h until the disappearance of intermediate 4 monitored by TLC. The mixture was cooled to room temperature and filtered. The filtrate was concentrated to afford a residue (12.50 g). The residue was purified by silica gel column chromatography with petroleum ether/ethyl acetate as eluent to give the desired compound as a colorless oil (8.0 g, 58%). 1 H NMR (300 MHz, CDCl3, δ): 10.03 (s, 1H), 7.66 (t, J = 4.5 Hz, 1H), 6.91 (t, J = 1.5 Hz, 1H). 13C NMR (75 MHz, CDCl3, δ): 179.9, 152.7, 149.9, 134.2, 134.1, 130.9, 128.8, 118.3, 117.9. HRMS: Calcd for [C5H3FOS]+: 129.9889. Found: no visible. (E)-1,2-Bis(3-fluorothiophen-2-yl)ethene (6). To a stirred suspension of zinc powder (10.2 g, 156.1 mmol) in dry THF (500 mL), TiCl4 (9.02 mL, 97.60 mmol) was added slowly at 0 °C under an argon atmosphere. The resulting mixture was heated to reflux and stirred for 2 h under Ar. Then pyridine (6.6 mL, 182 mmol) and intermediate, 5 (8.00 g, 61.47 mmol) were added successively. The reaction system was heated to reflux and stirred for 16 h. After the mixture was cooled to room temperature, water (500 mL) was added. The resulting mixture was extracted by DCM (500 mL × 3), dried over Na2SO4, and concentrated to give crude product. Further purification with silica gel column chromatography with petroleum ether/ethyl acetate as eluent and recrystallization with methanol afforded the desired compound as a white solid (4.71 g, 67.3%). 1H NMR (300 MHz, CD2Cl2, δ): 7.00 (dd, J = 6 Hz, J = 0.3 Hz, 2H), 6.81 (s, 2H), 6.89 (d, J = 6 Hz, 2H). 13C NMR (75 MHz, CD2Cl2, δ): 157.1, 153.6, 122.6, 122.4, 120.5, 120.3, 118.1, 117.7, 116.08, 116.05, 116.02, 115.99. HRMS: Calcd for [C10H6F2S2]+: 227.9879. Found: 227.9876. (E)-1,2-Bis(3-fluoro-5-(trimethylstannyl)thiophen-2-yl)ethene (7). To a solution of intermediate 6 (1.14 g, 5.0 mmol) in THF (50 mL) was added dropwise 2.5 M solution of n-BuLi in hexane (4.8 mL, 12.0 mmol) at −78 °C. The mixture was stirred for 1 h at the temperature, then slowly warmed to −40 °C, and stirred for 30 min. After the suspension was cooled to −78 °C again, a 1.0 M solution of Me3SnCl in THF (12.0 mL, 12.0 mmol) was added. The reaction solution was stirred overnight at room temperature, followed by quenching with H2O and extraction with diethyl ether (60 mL × 3). After the removal of the solvent in vacuo, the crude solid was recrystallized from hexane twice to give the desired compound as a colorless crystal (2.1 g, 76%). 1H NMR (300 MHz, CDCl3, δ): 6.72 (s,

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CONCLUSION In this contribution, we have synthesized two fluorinated dithienylethene (FDTE)-based copolymers, PNFDTE1 and PNFDTE2, and investigated their charge transport properties by fabricating polymer field-effect transistors on PET substrates with TGBC configuration. Both copolymers have good thermal stability, a broad absorption in UV−vis−NIR region, and deeplying LUMO energy levels. The highest electron mobility in air for PNFDTE1-based PFET devices is 3.20 cm2 V−1 s−1, which is among the highest for NDI-containing polymers. By contrast, a lower electron mobility of 0.266 cm2 V−1 s−1 was afforded for PNFDTE2-based counterpart. Thin film microstructure of the two copolymers demonstrates that PNFDTE1 thin films exhibit crystalline granular intercalating networks and preferential edge-on crystallites, whereas PNFDTE2 thin films have smooth surfaces and smaller crystallites with mainly face-on packing G

DOI: 10.1021/acs.macromol.7b01169 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Influence of Backbone Fluorination in Regioregular Poly(3-alkyl-4fluoro)thiophenes. J. Am. Chem. Soc. 2015, 137, 6866−6879. (5) Zhang, A.; Xiao, C.; Wu, Y.; Li, C.; Ji, Y.; Li, L.; Hu, W.; Wang, Z.; Ma, W.; Li, W. Effect of Fluorination on Molecular Orientation of Conjugated Polymers in High Performance Field-Effect Transistors. Macromolecules 2016, 49, 6431−6438. (6) Leclerc, N.; Chávez, P.; Ibraikulov, O.; Heiser, T.; Lévêque, P. Impact of Backbone Fluorination on π-Conjugated Polymers in Organic Photovoltaic Devices: A Review. Polymers 2016, 8, 11. (7) Wood, S.; Kim, J.-H.; Hwang, D.-H.; Kim, J.-S. Effects of Fluorination and Side Chain Branching on Molecular Conformation and Photovoltaic Performance of Donor−Acceptor Copolymers. Chem. Mater. 2015, 27, 4196−4204. (8) Bronstein, H.; Frost, J. M.; Hadipour, A.; Kim, Y.; Nielsen, C. B.; Ashraf, R. S.; Rand, B. P.; Watkins, S.; McCulloch, I. Effect of Fluorination on the Properties of a Donor−Acceptor Copolymer for Use in Photovoltaic Cells and Transistors. Chem. Mater. 2013, 25, 277−285. (9) Wang, C.; Mueller, C. J.; Gann, E.; Liu, A. C. Y.; Thelakkat, M.; McNeill, C. R. Influence of Fluorination on The Microstructure and Performance of Diketopyrrolopyrrole-Based Polymer Solar Cells. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 49−59. (10) Fan, Q.; Liu, Y.; Jiang, H.; Su, W.; Duan, L.; Tan, H.; Li, Y.; Deng, J.; Yang, R.; Zhu, W. Fluorination as An Effective Tool to Increase The Photovoltaic Performance of Indacenodithiophene-altQuinoxaline Based Wide-Bandgap Copolymers. Org. Electron. 2016, 33, 128−134. (11) Jo, J. W.; Kim, J. H.; Jung, J. W. Isoindigo-Based Fluorinated Low Band Gap Polymers for Environmentally Stable Field Effect Transistor. Dyes Pigm. 2016, 133, 333−338. (12) Qiao, X.; Wu, Q.; Wu, H.; Zhang, J.; Li, H. Bithienopyrroledione-Based Copolymers, Versatile Semiconductors for Balanced Ambipolar Thin-Film Transistors and Organic Solar Cells with Voc > 1 V. Adv. Funct. Mater. 2017, 27, 1604286. (13) Yi, Z.; Wang, S.; Liu, Y. Design of High-Mobility Diketopyrrolopyrrole-Based π-Conjugated Copolymers for Organic Thin-Film Transistors. Adv. Mater. 2015, 27, 3589−3606. (14) Li, P.; Xu, L.; Shen, H.; Duan, X.; Zhang, J.; Wei, Z.; Yi, Z.; Di, C. A.; Wang, S. D-A1-D-A2 Copolymer Based on Pyridine-Capped Diketopyrrolopyrrole with Fluorinated Benzothiadiazole for HighPerformance Ambipolar Organic Thin-Film Transistors. ACS Appl. Mater. Interfaces 2016, 8, 8620−8626. (15) Uddin, M. A.; Lee, T. H.; Xu, S.; Park, S. Y.; Kim, T.; Song, S.; Nguyen, T. L.; Ko, S.-j.; Hwang, S.; Kim, J. Y.; Woo, H. Y. Interplay of Intramolecular Noncovalent Coulomb Interactions for Semicrystalline Photovoltaic Polymers. Chem. Mater. 2015, 27, 5997−6007. (16) Zhang, X.; Wang, Z.; Chen, S.; Zhao, Z.; Yuan, W.; Wang, H.; Gao, X. Tuning the Charge Transport Property of Naphthalene Diimide Derivatives by Changing the Substituted Position of Fluorine Atom on Molecular Backbone. Chin. J. Chem. 2014, 32, 1057−1064. (17) Facchetti, A.; Yoon, M.-H.; Stern, C. L.; Hutchison, G. R.; Ratner, M. A.; Marks, T. J. Building Blocks for N-Type Molecular and Polymeric Electronics. Perfluoroalkyl- versus Alkyl-Functionalized Oligothiophenes (nTs; n = 2−6). Systematic Synthesis, Spectroscopy, Electrochemistry, and Solid-State Organization. J. Am. Chem. Soc. 2004, 126, 13480−13501. (18) Sakamoto, Y.; Suzuki, T.; Kobayashi, M.; Gao, Y.; Fukai, Y.; Inoue, Y.; Sato, F.; Tokito, S. Perfluoropentacene: High-Performance p-n Junctions and Complementary Circuits with Pentacene. J. Am. Chem. Soc. 2004, 126, 8138−8140. (19) Ando, S.; Murakami, R.; Nishida, J.; Tada, H.; Inoue, Y.; Tokito, S.; Yamashita, Y. n-Type Organic Field-Effect Transistors with Very High Electron Mobility Based on Thiazole Oligomers with Trifluoromethylphenyl Groups. J. Am. Chem. Soc. 2005, 127, 14996−14997. (20) Ando, S.; Nishida, J.; Tada, H.; Inoue, Y.; Tokito, S.; Yamashita, Y. High Performance n-Type Organic Field-Effect Transistors Based on π-Electronic Systems with Trifluoromethylphenyl Groups. J. Am. Chem. Soc. 2005, 127, 5336−5337.

mode. Such results match well with their different charge transport properties. Our work highlights the promising prospect of FDTE unit in developing high-performance polymer semiconductors and the important influences of intramolecular interaction on conformational control of organic semiconductors. The efforts toward the use of high-mobility fluorinated polymers for flexible electronics applications are currently underway in our lab.

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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.7b01169. TGA curves of the polymers, temperature-dependent absorption spectra, CV, GIXRD data, NMR spectra, AFM images, and other device characteristics and data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (Grants 21474116, 51473021, 21673258, and 51233006), the National Key Research and Development Program of China (2016YFB0401100), and the Strategic Priority Research Program of the Chinese Academy of Sciences (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, Chinese Academy of Sciences. The GIXRD analyses were performed at the BL14B1 Station of Shanghai Synchrotron Radiation Facility (SSRF), 23A1 Station of National Synchrotron Radiation Research Centre (NSRRC, Taiwan), and 1W1A station of Beijing Synchrotron Radiation Facility (BSRF). The authors are very grateful for the assistance of scientists from the stations during the experiments.

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REFERENCES

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DOI: 10.1021/acs.macromol.7b01169 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b01169 Macromolecules XXXX, XXX, XXX−XXX