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Diazaisoindigo Based Polymers with High-Performance Charge Transport Properties: From Computational Screening to Experimental Characterization Jianyao Huang, Zupan Mao, Zhihui Chen, Dong Gao, Congyuan Wei, Weifeng Zhang, and Gui Yu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00154 • Publication Date (Web): 14 Mar 2016 Downloaded from http://pubs.acs.org on March 19, 2016

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Chemistry of Materials

Diazaisoindigo Based Polymers with High-Performance Charge Transport Properties: From Computational Screening to Experimental Characterization Jianyao Huang,‡ Zupan Mao,‡ Zhihui Chen, Dong Gao, Congyuan Wei, Weifeng Zhang, and Gui Yu* Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. ABSTRACT: One of the major challenges confronting the organic electronics is the development of high-mobility semiconducting materials, especially the n-channel and ambipolar semiconductors. Solution-processable semiconducting polymers have attracted much attention for their tunable properties and are suitable for the fabrication of large-scale devices. Aza-substitution has been proven effective in electron transport small-molecule semiconductors, however, high performance polymeric semiconductors via aza-substitution are still lacking. We started from a computational screening procedure to introduce nitrogen atoms into isoindigo based polymers, followed by synthesis and fabrication of field-effect transistors. The resulting 7,7’-diazaisoindigo based polymers exhibit extensive π-conjugation and high crystallinity with hole mobilities exceeding 7 cm2 V–1 s–1 with bottom-gate/bottom-contact configuration and ambipolar transport properties with top-gate/bottom-contact configuration in air. These properties make diazaisoindigo a promising building block for polymeric semiconductors.

Introduction The design of electroactive materials for organic electronics, including organic photovoltaics (OPVs) and organic field-effect transistors (OFETs), has occupied the attention of the chemical sciences.1-5 In the field of OFETs, great efforts toward the enhancement of carrier mobilities have led to the discovery of numerous high-performance p-type, n-type, or ambipolar polymeric semiconducting materials. Charge carrier mobilities of these materials are higher than 1 cm2 V–1 s–1, which corresponds to that of the presently used amorphous silicon. A major trend in the development of such polymeric materials is the judicious functionalization of electron-donor and electron-acceptor (D-A) subunits to obtain tailorable frontier molecular orbital (FMO) energy levels,6, 7 solubility, self-assembly,8-12 and crystallinity.13, 14 To date, the selection of optimum building block is still a challenge. For instance, the introduction of heteroatoms into the π-system brings about variations in FMO energy levels, which could stabilize the π-system against oxidation or doping.15-17 However, it may result in unpredictable intra- and intermolecular interactions that largely change the packing arrangement, and, further influence the charge transport properties.18 There is also a trade-off between crystallinity and solution processability in the introduction of solubilizing groups.13 Consequently, careful adjustment of functional groups, combined with computational predictions, is required in the rational design of polymeric semiconductors.19-21 Symmetrical bislactam cores continue to develop thanks to their suitable energy levels, coplanar structures,

strong intermolecular interactions, and ready synthetic derivatizations. Among state-of-the-art polymeric materials for OFETs, diketopyrrolopyrrole and isoindigo based transistors, both of which contain bislactam structures, afforded record high hole and/or electron mobilities over 5 cm2 V–1 s–1.14, 22-28 Compared with well-investigated diketopyrrolopyrrole, isoindigo derivatives are of particular interest due to their synthetic challenges and more electron-deficient nature to fabricate n-type or ambipolar semiconductors. According to the previously established structure-property-device performance correlations, a viable improvement for isoindigo is to enhance planarity of the backbone. A torsion angle of c.a. 20 ~ 40° attributed to C−H···H−C steric interactions generally occurs between the benzene ring and the neighboring moiety. Replacing the benzene ring of isoindigo with thiophene or thienothiophene gives a more planar molecule, which is considered to be favorable for efficient charge transport.29-31 Fused thiophene or thienothiophene isoindigos were also studied as promising candidates for OFETs and OPVs, demonstrating the potential for further investigation.32 Another method to promote the coplanarity of the backbone is to introduce “conformational locks”.2, 6, 20, 33, 34 Molecules with fluorine substituents contain possible S···F through-space interactions and intramolecular hydrogen bonding, which are capable of lowering the degree of conformational freedom, thus eventually enhancing the coplanarity and π-conjugation. Compared with extensively exploited thiophene flanked or fused bislactams, chemical modification with nitrogen substitution in polymer backbone is not intensively stud-

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ied. Generally, there are three kinds of nitrogen substitution/doping: pyridinic, pyrrolic, and graphitic nitrogens. Pyridinic-N acts as an electron-deficient substituent (due to the electronegative nature of nitrogen atoms) while the latter two are electron-rich (owing to the unpaired electrons are conjugated into the aromatic heterocyclic ring). As a matter of fact, pyridinic-nitrogen-containing oligoacences were proven to be good ambipolar smallmolecule semiconductors.35-37 However, intentional introduction of pyridinic-N into the polymer backbone did not always achieve desirable results and is still unexplored.38 With these concepts in mind, our interests arise from the replacing of benzene ring with pyridine to investigate whether aza-semiconducting polymers are suitable for excellent charge transport efficiency. Herein, we report the diazaisoindigo (AIID) based polymers for use in highperforming transistors (Figure 1). The substitution position of nitrogen atom in isoindigo units was finely screened by computational predictions. The theoretical simulation suggests the incorporation of nitrogen atom not only increases the electron-deficient capability, but also eliminates undesirable dihedral angles between corepeating units. Previously, these azaisoindigo derivatives were studied as antiproliferative drugs in medicinal chemistry.39, 40 Our results suggest that they could also be excellent organic semiconductors with hole mobilities of over 7 cm2 V–1 s–1 and ambipolar mobilities of 2.33/0.78 cm2 V–1 s–1 in ambient.

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on an Autoflex III (Bruker Daltonics Inc.) MALDI-TOF spectrometer. UV-vis spectra were recorded on a Jasco– 570 spectrophotometer. Cyclic voltammetric measurements were carried out on a CHI660c electrochemical workstation. Molecular weights were determined with gel permeation chromatography (GPC) at 160°C on a PL-220 system using 1,2,4-trichlorobenzene as eluent. 2D-GIXRD data were performed at BL14B1 Station of Shanghai Synchrotron Radiation Facility (SSRF). Scheme 1. Synthetic Approach towads the Azaisoindigo Based Polymers

Synthesis. The reagents and starting materials employed were commercially available and used without any further purification otherwise indicated. Anhydrous tetrahydrofuran was purified with a standard distillation procedure prior to use. 6-Bromo-1-methyl-1H-pyrrolo[2,3b]pyridine was synthesized according to the reported procedures.41, 42

Figure 1. Molecular structures of isoindigo, theinoisoindigo, fluorinated isoindigo and azaisoindigo.

Experimental Section Instruments and Measurement. The reagents and starting materials employed were commercially available and used without any further purification otherwise indicated. Anhydrous solvent was purified with a standard distillation procedure prior to use. 1HNMR and 13C NMR spectra were recorded on a Bruker DMX 300 spectrometer. Chemical shifts are reported as δ values [ppm] relative to internal tetramethylsilane (TMS). Electron-impact (EI) mass spectra were collected on a GCI-MS micromass (UK) spectrometer. MALDI-TOF mass spectra were collected

6-bromo-1-(2-decyltetradecyl)-1H-pyrrolo[2,3b]pyridine. To a suspension of NaH (60% in mineral oil, 235 mg, 5.88 mmol) in DMF (10 mL) was added 6-bromo1H-pyrrolo[2,3-b]pyridine (1.00 g, 5.08 mmol) in batches at 0 °C. The mixture was stirred at room temperature for 15 min and 2-decyl-1-tetradecyl iodide (2.59 g, 5.58 mmol) was added. The mixture was stirred at room temperature overnight prior to quenching by addition of water and then extracted with ethyl acetate. The organic layers were concentrated and the residue was subjected to column chromatography (eluent: PE) to give 6-bromo-1-(2decyltetradecyl)-1H-pyrrolo[2,3-b]pyridine as a colorless oil (2.15 g, yield: 79.4%). 1H NMR (300 MHz, CD2Cl2) δ 7.75 (d, J = 8.1 Hz, 1H), 7.23 (d, J = 3.6 Hz, 1H), 7.20 (d, J = 8.1 Hz, 1H), 6.45 (d, J = 3.6 Hz, 1H), 4.13 (d, J = 7.2 Hz, 2H), 2.19~1.74 (m, 1H), 1.33~1.06 (m, 40H), 0.88 (t, J = 6.7 Hz, 6H). 13C NMR (75 MHz, CD2Cl2) δ 147.86, 134.70, 131.11, 129.08, 119.48, 119.32, 99.99, 48.82, 39.22, 32.37, 31.80, 30.33, 30.10, 30.06, 29.98, 29.80, 26.67, 23.13, 14.32. HR-MALDITOF: [M+H]+ calcd for C31H54BrN2: 533.346488, found: 533.346830. 6-bromo-1-(2-decyltetradecyl)-1H-pyrrolo[2,3b]pyridine-2,3-dione. To a suspension of grounded mix-


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ture of PCC and silica gel (2.0 g PCC, 9.28 mmol, 2.0 g silica gel) in anhydrous 1,2-dichloroethane (10 mL) and acetonitrile (15 mL) was added a solution of 6-bromo-1-(2decyltetradecyl)-1H-pyrrolo[2,3-b]pyridine (1.86 g, 3.49 mmol) in 1,2-dichloroethane (5 mL) while stirring at room temperature. A catalytic amount of AlCl3 (8 mg) was added, and the mixture was heated to reflux for 3 h. After completion of the reaction, the solvent was evaporated and the residue was purified on column chromatography (eluent: PE/toluene, 8:1, v/v ~ PE/EA, 9:1, v/v) to afford N,N’-bis(2-decyltetradecyl)-6,6’-dibromo-7,7’diazaisoindigo as a crimson oil (yield: ~15%) and 6bromo-1-(2-decyltetradecyl)-1H-pyrrolo[2,3-b]pyridine2,3-dione as an orange solid. (1.25 g yield: 63.5 %). 1H NMR (300 MHz, CD2Cl2) δ 7.64 (d, J = 7.7 Hz, 1H), 7.27 (d, J = 7.7 Hz, 1H), 3.69 (d, J = 7.1 Hz, 2H), 1.97 (m, 1H), 1.23~1.17 (m, 40H), 0.88 (t, J = 6.5 Hz, 6H).13C NMR (75 MHz, CD2Cl2) δ 181.45, 164.96, 158.93, 150.28, 134.32, 123.62, 110.77, 44.07, 36.57, 32.37, 31.82, 30.35, 30.10, 29.80, 26.57, 23.13, 14.32. HR-MALDI-TOF: [M+Na]+ calcd for C31H51BrN2NaO2: 585.302612, found: 585.302615. N,N’-bis(2-decyltetradecyl)-6,6’-dibromo-7,7’diazaisoindigo (C1-AIID). To a solution of 6-bromo-1-(2decyltetradecyl)-1H-pyrrolo[2,3-b]pyridine-2,3-dione (0.676 g, 1.2 mmol) in anhydrous toluene (5 mL) was added tris(dimethylamino) phosphine (0.196 g, 1.2 mmol) dropwise at 0 °C. The reaction mixture was warmed up to room temperature and stirred for 1h to give a firebrick solution. The reaction was quenched with water, dried over magnesium sulfate and concentrated in vacuo. The residue was purified on column chromatography using PE/toluene (8:1, v/v) as eluent to yield 150 mg 6,6’dibromo-7,7’-diazaisoindigo as a crimson oil, which gradually solidified at room temperature. (yield:22.8%). 1H NMR (300 MHz, CD2Cl2) δ 9.28 (d, J = 8.3 Hz, 2H), 7.18 (d, J = 8.3 Hz, 2H), 3.74 (d, J = 7.0 Hz, 4H), 2.02 (m, 2H), 1.53 ~ 0.98 (m, 80H), 0.88 (t, J = 6.1 Hz, 12H). 13C NMR (75 MHz, CD2Cl2) δ 168.15, 158.73, 144.02, 139.34, 131.71, 122.11, 114.91, 44.14, 36.54, 32.38, 31.90, 30.39, 30.14, 30.12, 30.06, 29.81, 26.62, 23.14, 14.33. HR-MALDI-TOF: [M+H]+ calcd for C62H103Br2N4O2: 1093.644229, found: 1093.644307. 6-bromo-1-(4-decyltetradecyl)-1H-pyrrolo[2,3b]pyridine. The procedure is similar as described above. 1 H NMR (300 MHz, CD2Cl2) δ 7.75 (d, J = 8.1 Hz, 1H), 7.23 (d, J = 3.5 Hz, 1H), 7.17 (d, J = 8.1 Hz, 1H), 6.45 (d, J = 3.5 Hz, 1H), 4.21 (t, J = 7.2 Hz, 2H), 1.83 (dt, J = 14.9, 7.5 Hz, 2H), 1.39 ~ 1.12 (m, 39H), 0.88 (t, J = 6.7 Hz, 6H). 13C NMR (75 MHz, CD2Cl2) δ 147.48, 134.74, 131.16, 128.66, 119.67, 119.37, 100.00, 45.43, 37.46, 33.94, 32.38, 30.99, 30.51, 30.12, 29.81, 27.73, 27.02, 23.14, 14.32. HR-MALDI-TOF: [M+H]+ calcd for C31H54BrN2: 533.346488, found: 533.346889. 6-bromo-1-(4-decyltetradecyl)-1H-pyrrolo[2,3b]pyridine-2,3-dione. The procedure is similar as described above. 1H NMR (300 MHz, CD2Cl2) δ 7.64 (d, J = 7.7 Hz, 1H), 7.28 (d, J = 7.7 Hz, 1H), 3.78 (t, J = 7.3 Hz, 2H), 1.88 ~ 1.59 (m, 2H), 1.53 ~ 1.12 (m, 39H), 0.88 (t, J = 6.7 Hz, 6H). 13C NMR (75 MHz, CD2Cl2) δ 181.49, 164.63, 158.58, 150.30, 134.39, 123.64, 110.85, 40.34, 37.41, 33.89, 32.37, 31.08, 30.53, 30.13, 30.10, 29.80, 27.02, 24.92, 23.13, 14.32. HR-

MALDI-TOF: [M+Na]+ calcd 585.302612, found: 585.303131.

for

C31H51BrN2NaO2:

N,N’-bis(4-decyltetradecyl)-6,6’-dibromo-7,7’diazaisoindigo (C3-AIID). The procedure is similar as described above. 1H NMR (300 MHz, CD2Cl2) δ 9.29 (d, J = 8.3 Hz, 2H), 7.18 (d, J = 8.3 Hz, 2H), 3.82 (t, J = 7.2 Hz, 4H), 1.74 (m, 4H), 1.58 – 1.11 (m, 78H), 0.87 (t, J = 6.6 Hz, 6H). 13 C NMR (75 MHz, CD2Cl2) δ 167.81, 158.40, 144.05, 139.42, 131.76, 122.15, 115.02, 40.34, 37.42, 33.93, 32.38, 31.08, 30.54, 30.14, 30.11, 29.81, 27.05, 24.97, 23.14, 14.32. HR-MALDITOF: [M+H]+ calcd for C62H103Br2N4O2: 1093.644229, found: 1093.644482. General Procedures for Stille Polymerization: To a 25 mL Schlenk tube was added 5,5'bis(trimethylstannyl)-2,2'-bithiophene (0.2 mmol), AIID (0.2 mmol), a certain amount of Pd2(dba)3 (9 mg), P(o-tol)3 (24.6 mg), and chlorobenzene (6 mL). The tube was charged with argon through a freezepump-thaw cycle for three times. The mixture was stirred for 24 h at 130 °C under argon atmosphere prior to pouring into 100 mL of methanol. The mixture was stirred for 2 h, filtered, and the residue was then further purified by Soxhlet extraction with methanol, acetone and hexane to remove the low-molecularweight fraction and residual catalyst. The residue was finally extracted with chloroform to afford the polymer. PAIID-BT-C1. GPC: Mn = 61.1 kDa, PDI = 3.00. 1H NMR (300 MHz, CDCl3) δ 9.03 (m, 2H), 7.80-7.20 (m, 6H), 3.6 (m, 2H), 2.0-0.7 (m, 84H). PAIID-BT-C3. GPC: Mn = 50.6 kDa, PDI = 3.87. 1H NMR (300 MHz, CDCl3) δ 9.00 (m, 2H), 7.80-7.20 (br, 6H), 3.5 (br, 2H), 2.0-0.7 (m, 84H). General Procedures for FET Device Fabrication. FETs with a bottom-gate bottom-contact (BGBC) configuration were fabricated on an n-doped silicon wafer with 300 nm silicon dioxide insulator. Silicon was utilized as the bottom gate electrode. The source−drain gold electrodes were prepared by photolithography. The substrates were then cleaned by ultrasonication in acetone, deionized water, and ethanol. Octadecyltrichlorosilane (OTS) treatment was performed on the gate dielectrics in a vacuum to form an OTS self-assembly monolayer. Polymer film (∼40 nm) was deposited on the OTS-treated substrates by spin-coating a polymer solution in odichlorobenzene (5 mg/mL for PAIID-BT-C1, and 8 mg/mL for PAIID-BT-C3) at a speed of 2000 rpm for 60 s. All the samples were further annealed on a hot plate at 180 °C for 5 min in ambient before cooling down to room temperature. Top-gate/bottom-contact (TGBC) FET devices were fabricated on silicon dioxide substrates. Bottom-contact electrodes were also prepared by photolithography. The same procedures of cleaning and spin-coating were used to fabricate the devices. Polymer film was annealed on a hot plate at 180 °C for 5 min in nitrogen box. Polymethyl-



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Figure 2. Computational screening of optimal azaisoindigo monomer : (a) Optimized structures according to DFT calculation at 1 B3LYP/6-31G* level; (b) Comparison of calculated energy levels of azaisoindigo and isoindigo; (c) H NMR spectra of azaisoindigo and azaisatin; (d) Gas-phase torsional potential energy distribution of azaisoindigo-thiophene subunits in the polymer backbone.

methacrylate (PMMA) (Mw = 1000 KDa) solution in anhydrous n-butyl acetate (60 mg/mL) was then spin-coated onto the surface of the polymer films. PMMA thickness is ∼900 nm. The samples were dried at 80 °C for 30 min in nitrogen box before evaporating an aluminum gate electrode (thickness ∼100 nm). FETs were determined by using a Keithley 4200 SCS semiconductor parameter analyzer. Channel lengths (L = 50 μm) of the FET devices and channel widths (W) of 1400 μm were used to test device performance. Both BGBC and TGBC FETs were measured in ambient air with relative humidity of 20−40%.

Results and Discussion Molecular Design and Theoretical Modeling. Our molecular design started from a computational screening procedure to find the best position for nitrogen substitution. We noticed that polymers bearing thiazole units possessed lower mobilities than thiophene-containing counterparts.43, 44 We deduce that the short C=N double bonds in the conjugation pathway is inferior to efficient intramolecular charge transport. Our first concern, thereby, is sufficient π-conjugation. Previous paradigm also confirmed the coplanarity of the polymer backbone is one of the crucial factors to obtain high mobilities, given that the intersubunit coupling is a function of the dihedral angles.45 In view of the rational design concepts discussed

above, the screen procedure involves the following steps: (1) design the potential candidates with molecular modification of isoindigo to replace one methine group (=CH-) with a nitrogen atom and ensure the substitution will not break the conjugation; (2) simulate the molecules to find whether the modification could promote the coplanarity or not; (3) consider the synthesis of remaining candidates. There are three possible candidates that would not break the conjugation of the polymer backbone: (a) 4,4’diazaisoindigo, (b) 5,5’-diazaisoindigo, and (c) 7,7’diazaisoindigo. Molecular simulations were performed using density functional theory (DFT) at B3LYP/6-31G* level. Molecular formula and simulated structures of isoindigo and its aza-derivatives are shown in Figure 2. The results indicate that the tuning of the nitrogen substituent would significantly change the planarity and electronic structures of the molecules. We calculated the dihedral angles between the two indolone subunits. Isoindigo exhibits a moderate dihedral angle of 15.8° due to the formation of C−H···H−C steric repulsion. 4,4’-Diazaisoindigo shows a large dihedral angle of 27.9°, most likely as a result of the coulomb repulsion of lone pair electrons between the neighboring nitrogen and oxygen atoms. As for the π-conjugation, this structure is obviously inferior to the original isoindigo, therefore, it is excluded from the candidates. Both 5,5’- and 7,7’-diazaisoindigo display en-


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hanced planarity. According to the calculation, the dihedral angles between aza-indolone subunits are 12.6° and 0° for 5,5’-diazaisoindigo and 7,7’-diazaisoindigo, respectively. 7,7’-Diazaisoindigo notably shows planar structure, which is favorable for efficient conjugation. The starting materials to obtain 7,7’-diazaisoindigo are commercially available, therefore making it the optimal candidate among these azaisoindigos. The highly planar structure of 7,7’-diazaisoindigo is attributed to the short carbon-nitrogen double bonds. Compared with the original C=C double bonds, the shorter carbon-nitrogen double bonds pulled the hydrogen atom at 4-position away from the spatially neighboring carbonyl group, thus making the C-O distance more closer to the C-H ··· O equilibrium distance (ca. 2.2 Å in 7,7’diazaisoindigo) and consequently minimizing the steric hindrance. According to B3LYP geometry optimizations, the distance of H-O is calculated to be 2.02 Å and the C−O distance (2.92 Å ) is shorter than the combined van der Waals radii (ca. 3.3 Å ), indicative of the existence of intramolecular C-H ··· O hydrogen bonding. Given that the similar hydrogen bonding in thiophene-flanked diketopyrrolopyrrole (2.167 Å ) exerts intensive conformational controls, this hydrogen bond has the potential to further immobilize the conformation and form an additional driving force for the planarization of AIID. Another evidence of the hydrogen bond was from 1H NMR spectra (Figure 2c), showing that the signal of Ha significantly shifted from 7.64 ppm (in azaisatin) to 9.29 ppm (in azaisoindigo), arising from the deshielding effect of the carbonyl group. Enhanced planarity could not only give rise to the effective conjugation, but also follow the inclination of ordered packing. The symmetric transformation from C2 to C2i (if alkyl chains are introduced, isoindigo should be asymmetric, while AIID could still maintain Ci symmetry) is also a promotion for long-range ordering in stacked polymer strands, as fewer degrees of freedom exist when dihedral angles between flexible single-bonded subunits rotate. Theoretical simulation also suggests, the replacement of aromatic benzene ring with pyridine reduces the energy levels of the monomer. As shown in Figure 2b, both HOMO and LUMO energy levels of azaisoindigo drop by about 0.3 ~ 0.4 eV in comparison with isoindigo according to DFT calculations. The low-lying LUMO energy level is considered as an essential prerequisite to circumvent chemical reactions that create electron traps for n-channel or ambipolar charge carrier transport. The coplanarity of the polymer backbone is also important for predicting efficient intramolecular carrier transport. A preliminary gas-phase dihedral potential energy scan was carried out to determine the energyminimum conformation of the co-repeating unit of AIIDthiophene copolymer. Relaxed potential energy scans were performed for azaisoindigo-thiophene subunits at 10° intervals. The dihedral angle was constrained and all other degrees of freedom were allowed to relax to their energy minima using a B3LYP geometry optimization. Single point energies were also performed at mp2/cc-pvdz level for comparison because DFT calculations were not relia-

ble for nonbonding interactions. Two nearly isoenergetic local conformational energy minima were found at 0° and ±180°, exhibiting both planar conformations owing to the elimination of steric interactions. Note that the conformation containing intramolecular S-N short contacts (ca. 2.93 Å ) was calculated to be the most stable, which exemplified an use of conformational control. Notably, the dihedral barrier is about 25.4 kJ/mol, which is significantly larger than the value of recently reported S-F conformation locks in poly(3-alkyl-4-fluoro)thiophene.46 This result is consistent with reported binding energies that nonbonding N−S interaction (0.46 kcal/mol) is stronger than F−S interaction (0.44 kcal/mol).47 Based on the optimal conformation, we performed a geometry optimization of the trimmer to further elucidate the conjugation and coplanarity of the polymer backbone (Figure S1, SI). The polymer backbone showed approximately planar conformation, with small dihedral angles between adjacent thiophene units. Both HOMO and LUMO spread out over the entire molecule, suggesting the possibility of ambipolar intramolecular charge transport. Synthesis. Isoindigos have been conventionally prepared by an acid-catalyzed reaction of isatin and oxindole.48 A reductive dimerization reaction using isatin as starting material in the presence of phosphoruscontaining reagents were also proved to be efficient, especially for those cases that isatins were readily available. Considering the various procedures for preparing isatins (including Sandmeyer methodology, Stolle procedure, and oxidation of indole),49 the latter route is noteworthy because fewer steps are involved. Herein, our procedure started from 6-bromo-7-azaindole. We introduced the alkyl chain first to ensure the solubility, because the direct oxidation of nonalkylated 6-bromo-7-azaindole led to poor yields, probably due to the limited solubility of the reactants. Note that the typical alkylation of isoindigo usually undergoes in alkaline conditions, where nucleophilic attacks facilely proceed at ortho-position of fused pyridine in this system, it is also necessary to alkylate first. Alkylated 7-azaindole was readily prepared using 6bromo-7-azaindole and alkyl iodide as the starting reactants in the presence of NaH, which was subsequently oxidized by PCC to yield alkylated 6-bromo-7-azaisatin.50 Partially oxidized final target product AIID was also detected in the reaction mixture. Intentional use of polar solvent acetonitrile could improve the yield of AIID, probably due to the poorer solubility of alkylated intermediates in polar solvents and the presence of catalytic Lewis acid. To obtain a second batch of AIID, alkylated 6bromo-7-azaisatin was treated with hexamethylphosphorous triamide to undergo a reductive dimerization in anhydrous toluene. Previous work also suggested the use of Lawesson’s reagent to achieve the same goal,30 however, in our tentative experiments, the dimerization was not successful as a result of less reactive carbonyl group at 3position. It should be mentioned that a susceptible aromatic nucleophilic substitution acted as the major side reaction by replacing bromine with dimethylamine. Careful control of the reaction temperature is required,



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Table 1. Summary of Optical, Electrochemical Properties and GIXRD Results of Azaisoindigo Based Copolymers λmax (nm) Polymer

opt

Eg

(eV)

ELUMO (eV)

EHOMO (eV)

Eg (eV)

d-d (Å )

- (Å )

cv

soln.

film

PAIID-BT-C1

745

740

1.54

–3.62

–5.66

2.04

23.2

3.80

PAIID-BT-C3

742

756

1.50

–3.64

–5.67

2.03

24.5

3.56

together with a TLC monitoring to confirm the optimal yield. Two alkyl chains, 2-decyltetradecyl and 4decyltetradecyl, were introduced in this work for comparison of the side chain effect. Optical and Electrochemical Properties. UV-vis-NIR absorption spectra of both polymers exhibit typical spectral shapes in analogous to other D-A polymers (Figure 3). Intramolecular charge transfer absorptions are observed at 500 ~ 800 nm. Intensive vibrational coupling shows strong aggregate tendencies in both solution and solid state. Compared with isoindigo analogues, the absorption edge of AIID-based polymers subtly extend to 820 nm, exhibiting the enhancement of the conjugation derived from the planarization of the backbone. For PAIID-BT-C1, the maximum absorption peak in the film is blue-shifted from 745 nm to 740 nm in comparison with the spectrum in solution. Previous studies also observed this hypochromatic shift in thienoisoindigo based polymers, probably due to the formation of H-aggregation. In contrast, PAIID-BT-C3 exhibits slight red shift (from 742 nm to 756 nm), most likely as a result of stronger chain-to-chain interactions. This phenomenon can be attributed to the competition of H-aggregation and J-aggregation as a result of the alternation of substituent side chains. The optical bandgap is estimated to be 1.54 and 1.50 eV from the absorption edge of the film for PAIID-BT-C1 and PAIIDBT-C3, respectively. Compared with the isoindigo analogues IIDDT and IIDDT-C3, the optical gaps of AIIDbased polymers are smaller (1.60 eV for IIDDT and 1.58 eV for IIDDT-C3),51 suggesting a higher degree of πconjugation and enhanced molecular orbital hybridization. This phenomenon could be attributed to the better coplanarity of the polymer backbone, which is in good agreement with our design concept.

Figure 3. UV-vis-NIR absorption spectra of (a) PAIID-BT-C1 –3 and (b) PAIID-BT-C3 in chloroform solution (3 × 10 mg/mL) and in the film.

Cyclic voltammetry (CV) measurements were carried out to calculate FMO energy levels of polymers under a standard three-electrode cell electrochemical workstation. The corresponding energy levels are calculated using the

equation EFMO = – (Eonset + 4.40 eV) and are listed in Table 1. Both PAIID-BT-C1 and PAIID-BT-C3 exhibit low-lying HOMO levels of –5.66 eV and –5.67 eV, respectively, with an energy gap of about 2.0 eV (Figure S2, SI). The LUMO energy levels are estimated to be –3.62 and –3.64 eV, respectively. It should be noted that the existence of intense oxidation peaks suggest both polymers could operate as typical p-type materials, in line with the following discussed field-effect characteristics. The relatively weak reduction peaks suggest these materials could also act as n-channel semiconductors, although the electron mobility might be lower than the hole mobility. Charge Transport Properties. OFET devices were initially fabricated by spin-coating of polymers on octadecyltrichlorosilane (OTS)-treated SiO2/n++-Si (300 nm) substrates with BGBC configuration. Because the TGBC device configuration has better encapsulation effect, which could enhance ambipolar carrier transport, we subsequently used this configuration to test ambipolar charge transport properties. I-V curves of FET devices are shown in Figure 4. When measured in ambient conditions using the BGBC configuration, all devices exhibited only unipolar hole transporting behaviors with relatively high mobilities and current on/off ratios. Ambipolar FET characteristics were observed for both polymers tested in air with a TGBC configuration. Mobilities of both polymers extracted from the saturation region are calculated using the equation IDS = (W/2L)Ciμ(VGS-Vth)2. All mobility values reported here were conservatively estimated. The lack of electron transporting in the air is ascribed to the relatively high LUMO energy levels compared with state-of-theart air-stable n-type semiconducting materials (below −4.0 eV), presumably as a consequence of doping by oxygen or water. BGBC devices show highest hole mobilities of 3.63 cm2 V–1 s–1 and 7.28 cm2 V–1 s–1 for PAIID-BT-C1 and PAIID-BT-C3, respectively. These values are higher than isoindigo analogues (1.06 cm2 V–1 s–1 for IIDDT and 3.62 cm2 V–1 s–1 for IIDDT-C3),51 in agreement with our initial design concept. For TGBC devices, the highest hole/electron mobilities reach 0.45/0.47 cm2 V–1 s–1 for PAIID-BT-C1 and 2.33/0.78 cm2 V–1 s–1 for PAIID-BT-C3, respectively. Having the branching point remote from the polymer backbone significantly promote charge carrier transport properties.51-54 To our knowledge, these values are among the highest reported ambipolar isoindigo or its derivative based semiconductors. For example, our results are comparable to ambipolar fluorinated isoindigo copolymers with hole/electron mobilities of 1.25/0.51 cm2 V–1 s–1 in glovebox,55 and are much higher than those of original isoindigo copolymers.48


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Figure 4. Device configurations and current−voltage output and transfer curves of polymer based FETs: (a) Bottomgate/bottom-contact (BGBC) and top-gate/bottom-contact (TGBC) configurations; (b) I-V characteristics of FETs tested in ambient with BGBC configuration; (c) I-V characteristics of FETs tested in ambient with TGBC configuration.

Table 2. FET Characteristicsa of Aazisoindigo Based Copolymers compd

Bottom-gate/bottom-contact 2

–1 –1

Top-gate/bottom-contact 2

–1 –1

2

–1 –1

μh,aver/μh,max (cm V s )

μh,aver/μh,max (cm V s )

μe,aver/μe,max (cm V s )

PAIID-BT-C1

2.86 (3.63)

0.28 (0.45)

0.30 (0.47)

PAIID-BT-C3

6.43 (7.28)

1.89 (2.33)

0.48 (0.78)

a

Devices with default L = 50 µm and W = 1400 µm fabricated on Si/SiO2 substrates modified with octyltrichlorosilane (OTS) self-assembled monolayer (SAM) were used. Mobilities were extracted from the saturation regime. Average device characteristics were obtained from 20 devices for each polymer.



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Figure 5. 2D-GIXRD patterns of (a) as-spun PAIID-BT-C1, (b) annealed PAIID-BT-C1, (c) as-spun PAIID-BT-C3, and (d) annealed PAIID-BT-C3 thin films. Crystallinities and Morphologies. In order to explore the crystallinity and microstructure of the PAIID thin films, we employed 2D grazing incidence X-ray diffraction (2D-GIXRD) and tapping-mode atomic force microscopy (AFM). 2D-GIXRD images of as-spun and thermally annealed films are shown in Figure 5. Bimodal distributions of face-on and edge-on crystallites were observed, demonstrating a mixed stacking structure. Thermal annealing technique greatly promoted the crystallinity of the films. Both polymers exhibit mainly edge-on features, as evident from the observation of Bragg reflections (h00) up to fifth order along the out-of-plane direction and intense (010) peaks along the in-plane direction, indicating that both polymers have a highly crystalline tendency in films (Figure S4, SI). This orientation is in accordance with other high performing semiconductors previously reported. Moving the branching point away from the backbone definitely enhances the π-π stacks. PAIID-BT-C1 shows a π-π distance of 3.80 Å , while PAIID-BT-C3 displays a very close π-π distance of 3.56 Å . The closer π-π stacking is favorable for improving intermolecular charge carrier transport properties on the premise of almost identical backbones. Note that the previously reported isoindigo analogue, IIDDT-C3, reveals a π-π stacking of 3.57 Å ,51 which is consistent with our results. AFM morphologies in Figure 6 show well-interconnected wormlike fibrils in both polymer films. The as-spun thin film displays relatively small domains. Once after annealing, the root-mean-square-roughness (RRMS) values of the films raised and the domains became larger, indicative of better crystallinity. PAIID-BT-C3 shows larger crystalline domains than PAIID-BT-C1, which minimizes the intergranular charge hopping barriers. Both XRD and AFM results demonstrate that PAIID-BT-C3 has higher degree of crystalline tendency that might be favorable for better carrier transport properties.

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Figure 6. AFM images of (a) as-spun PAIID-BT-C1, (b) annealed PAIID-BT-C1, (c) as-spun PAIID-BT-C3, and (d) annealed PAIID-BT-C3 films.

Conclusion In summary, we designed, synthesized, and characterized diazaisoindigo based polymers. Computational screening suggests that 7,7’-substitution with nitrogen atoms is the optimal candidate. This monomer not only enhances the planarity, but also minimizes dihedral angles between neighboring moieties, thus greatly promoting the intramolecular carrier transport. We fabricated FETs based on azaisoindigo copolymers with BGBC and TGBC configurations, exhibiting hole mobilities as high as 7.28 cm2 V–1 s–1 and hole/electron mobilities of 2.33/0.78 cm2 V–1 s–1 in ambient. These values are among the highest yet reported for isoindigo derivatives-based ambipolar OFETs. Strong crystallinity tendencies of the polymer films were confirmed by 2D grazing incidence X-ray diffraction. These values are comparable to well-investigated diketopyrrolopyrrole based polymeric semiconductors. Our results could be extended for the rational design of other high performance OFET materials. Further introducing other comonomers to achieve air-stable n-channel or ambipolar polymeric semiconductors and device applications are underway.

ASSOCIATED CONTENT SUPPORTING INFORMATION Experimental details, analysis of the new compounds, and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected].

Author Contributions ‡These authors contributed equally.


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Notes

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The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was financially support by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12030100) and the National Science Foundation of China (21474116 and 51233006). The GIXRD results were tested at BL14B1 Station of Shanghai Synchrotron Radiation Facility (SSRF), 23A1 Station of National Synchrotron Radiation Research Center (NSRRC, Taiwan), and 1W1A Station of Beijing Synchrotron Radiation Facility. The authors gratefully thank the assistance of scientists from three stations during the experiments.

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