Pyrrolo[3,2-b]pyrrole Based Quinoidal Compounds For High

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Pyrrolo[3,2-b]pyrrole Based Quinoidal Compounds For High Performance n-Channel Organic Field-Effect Transistor Hongzhuo Wu, Yang Wang, Xiaolan Qiao, Deliang Wang, Xiaodi Yang, and Hongxiang Li Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01791 • Publication Date (Web): 19 Sep 2018 Downloaded from http://pubs.acs.org on September 19, 2018

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Pyrrolo[3,2-b]pyrrole Based Quinoidal Compounds For High Performance n-Channel Organic Field-Effect Transistor Hongzhuo Wu,†,‡ Yang Wang,†,‡ Xiaolan Qiao,†,* Deliang Wang,†,‡ Xiaodi Yang,§,* Hongxiang Li †,* †Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Materials, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, CAS, Shanghai, 200032, China ‡ University of Chinese Academy of Sciences, Beijing, 100049, P. R. China

§Experiment Center for Science and Technology, Shanghai University of Traditional Chinese Medicine, Shanghai, 201203, China

ABSTRACT: It is a very important and challenging topic to design high performance n-channel organic semiconductors with new type conjugated skeleton. Herein, a new class of n- channel organic semiconductors, pyrrolo[3,2-b]pyrrole based quinoidal molecules QBPBP and QFBPBP, are designed and synthesized. Both compounds have low-lying LUMO energy level and high thermal stability. Single crystal structures reveal they adopt unusual two-dimensional layer type packing structures in single crystals. Multiple π-π and CN⋅⋅⋅H interactions are observed in the layer, and alkyl-alkyl chain interactions exist between layers. Transistors based on single ribbons displayed gate voltage-dependent electron mobility with an average of 4.0 cm2V-1s-1 and a peak over 6.0 cm2V-1s-1 under N2. These results demonstrate pyrrolo[3,2-b]pyrrole based quinoidal molecules are new type of prototype for high performance n-channel organic semiconductors.

INTRODUCTION

The potential applications of organic field-effect transistors (OFETs) have driven researchers to explore high performance organic semiconductors (OSCs).[1-4] Currently, p-channel OSCs have made great progress in terms of hole-mobility and stability.[5] Comparing with p-channel OSCs, the development of n-channel OSCs lags far behind their p-channel counterparts. Recently, some promising and ambient stable n-channel OSCs,[6-13] such as PQDPP-2FT, P2FIID-2FBT, F4BDOPV-2T, P(NDI2OD-T2), NDI3HU-DTYM2, and 2DQTT (chemical structures see Figure S1), have been reported. Nevertheless, n-channel OSCs with high electron mobility are still rare, and most of them are based on the well-known conjugated units of DPP, IID and BDOPV. Considering the important roles of n-channel OSCs in organic p-n junctions and complementary logic circuits,[14-15] the exploration of novel n-channel OSCs with high mobility is still a challenging and significant topic in organic transistors. ACS Paragon Plus Environment

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Designing new class of conjugated backbone is not only an important and effective strategy to obtain high performance n- channel organic semiconductors, but also crucial to understand the structure-property relationship of organic semiconductors.[16-17] Thieno[3,2-b]thiophene (TT) is a good module to construct conjugated backbone and has been extensively studied.[18-20] Many high performance organic semiconductors, such as p- channel molecule BTBT and n- channel quinoidal compound QDTBDT (Chemical structures see Figure S1), consist of TT modules.[21-23] Pyrrolo[3,2-b]pyrrole (PP) is an analogue of TT. Though it retains the central symmetric planar structure as TT and has the merits to be easily modified on the N atom,[24-26] PP based organic semiconductors are rarely reported. On the other hand, dicyanomethylene-substituted quinoidal compounds have low LUMO energy levels and planar structures, and are potential n-channel organic semiconductors.[27-29] Currently, the reported dicyanomethylene-substituted quinoidal semiconductors are mainly thiophene based ones.[30-33] Considering the merits of PP unit and the unique properties of quinoidal compounds, in this manuscript, pyrrole[3,2-b]pyrrole contained quinoidal compounds QBPBP and its fluorine - substituted derivative QFBPBP (chemical structure see Scheme 1) are firstly designed and synthesized. X-ray diffraction results reveal QBPBP and QFBPBP adopt unusual two-dimensional layer type packing structures in single crystals. Transistors based on QFBPBP show high, though gate voltage-dependent electron mobility with an average of 4.0 cm2V-1s-1 and a peak over 6.0 cm2V-1s-1. These results demonstrate PP-contained quinoidal compounds are new type of prototype for high performance n-channel organic transistors. Scheme 1. Chemical structures of QBPBP and QFBPBP

RESULTS AND DISSCUSSION Syntheses, theoretical calculation and physicochemical properties. The synthetic route of QBPBP and QFBPBP is shown in Scheme 2. The introduction of fluorine substituents can further lower the LUMO energy level of QFBPBP and enhance intermolecular interactions.

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[8-9]

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QBPBP and QFBPBP were synthesized through a Pd-catalyzed Takahashi reaction, following oxidation with excess DDQ. They are soluble in common organic solvents such as CH2Cl2, CHCl3, THF and toluene, and fully characterized by 1HNMR, MS, and elemental analysis. Thermogravimetric analysis (TGA) results revealed QBPBP and QFBPBP have good thermal stability, and their decomposition temperatures at 5% weight loss are above 350 °C (Figure S2).

Scheme 2. Synthetic route of QBPBP and QFBPBP

To estimate the frontier orbitals of QBPBP and QFBPBP, density functional theory (DFT) calculation was carried out at the B3LYP/6-31G(d) level using the Gaussian 09 program package. The contour plots and energies of the HOMOs and LUMOs are displayed in Figure 1. Theoretical calculation predicated the LUMO energy level is -4.217 eV for QBPBP and -4.371 eV for QFBPBP, indicating they are potential n- channel OSCs. Both the HOMO and LUMO energy levels of QFBPBP were about 0.1 eV lower than those of QBPBP, being ascribed to the electron withdrawing property of fluorine substituents. QBPBP and QFBPBP show the same electron density distributions of HOMO and LUMO orbitals, and the orbital densities are fully delocalized onto the quinoidal backbones.

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Figure 1. The frontier molecular orbitals and energy levels of QBPBP and QFBPBP.

QBPBP and QFBPBP exhibit nearly identical solution absorptions in CH2Cl2 solutions with a strong absorbance in the range of 550 nm to 850 nm (Figure 2a). The film absorptions of QBPBP and QFBPBP are largely red-shifted and reach the IR region when being compared with solution absorptions. The peak absorption is 784 nm for QBPBP film and 1080 nm for QFBPBP film. The large bathochromic shift indicates QBPBP and QFBPBP films are highly ordered. The cyclic voltammetry curves of QBPBP and QFBPBP are shown in Figure 2b. Both compounds display reversible oxidation and reduction behavior. The HOMO/LUMO energy levels estimated from electrochemical results are -5.56/-4.29 eV for QBPBP and -5.68/-4.41 eV for QFBPBP, close to those of theoretical calculations.

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Figure 2. a) Normalized absorption spectra of QBPBP and QFBPBP in CH2Cl2 solutions and thin films (on quartz substrate). b) Cyclic voltammogram of QBPBP and QFBPBP in CH2Cl2 solutions. The CV was measured by using 0.1 M n-Bu4NPF6 as supporting electrolyte, SCE as reference electrode, Pt disk as working electrode, and Pt wire as counter electrode, and ferrocene was used as internal standard. The scan rate was 50 mV s-1.

Single crystal structures analysis. Single crystals of QBPBP and QFBPBP were successfully grown from chlorobenzene solution at ambient atmosphere through solvent-evaporation method. The crystal structure of QBPBP belongs to a triclinic space group (P-1) with a =10.9536(12) Å, b =12.6964(12) Å, c = 12.7800(11) Å, α = 116.804(4)°, β = 92.354(6)° and γ =98.939(6)°. The crystal structure of QFBPBP belongs to a monoclinic space group (P21/n) with a =15.4387(6) Å, b =12.7404(4) Å, c = 20.8005(8) Å, α =90°, β =106.901(2)° and γ =90°. Single crystal X-ray diffraction analyses show QBPBP and QFBPBP have planar conjugated backbones, and their side chains are turned out at the same side of backbones (Figure 3a, b, c, d). In the crystals, QBPBP and QFBPBP adopt unusual layer type structures (Figure 3f, h). In the layer, two molecules form dimmers with π-π interactions, and the π-π stacking distance is 3.383 Å for QBPBP and 3.252 Å for QFBPBP. Among dimmers, multiple CN···H (benzene ring) interactions are observed (Figure 3e, g). QFBPBP have stronger CN···H (benzene ring) interactions (with shorter distance) than QBPBP. The shorter π-π and CN···H (benzene ring) distances suggest QFBPBP is more favorable for charge carriers transport into the layers. Alkyl-alkyl interactions are observed between layers, and the distance (measured between the conjugated backbone) is 7.064 Å for QBPBP and 10.954 Å for QFBPBP.

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Figure 3. Single crystal structures, packing structures and intermolecular interactions of QBPBP and QFBPBP. (a, b, e, f ) QBPBP; (c, d, g, h)QFBPBP. Alkyl side chains were omitted for clarification in (e, g).

Charge transport properties, morphology and microstructures. The charge transport properties of QBPBP and QFBPBP were first investigated with bottom-gate top-contact thin film transistors. QBPBP films were deposited with vacuum evaporation, and QFBPBP films were deposited through spin-coating technique (5 mg/mL in THF solution). The devices had a channel length of 31 µm and a channel width of 273 µm. The field effect mobility (µ) was extracted from the saturation regime. Figure S3 illustrates the typical output and transfer curves of QBPBP and QFBPBP transistors. Both devices exhibited typical n-channel field-effect behavior. QBPBP devices showed a moderate electron mobility of 0.09 cm2 V-1 s-1. The as spun films of QFBPBP displayed an electron mobility of 0.55 cm2 V-1 s-1, and an electron mobility of 0.75 cm2 V-1 s-1 was observed after thermal annealing at 80 °C (Table S1). The morphologies and microstructures structures of QBPBP and QFBPBP films were investigated with atomic force microscopy (AFM) and XRD (Figure S4). AFM results showed the grain size of QBPBP was much smaller than that of QFBPBP, though the continuity of QBPBP film was better than that of QFBPBP film. After thermal annealing, the grains in QFBPBP film did not increase gradually but the film continuity became better, which was consistent with the device performance. QBPBP film showed a weak X-ray diffraction peak at 2θ=7.81 degree, this peak could be assigned to the (001) diffraction pattern, referred to its single crystal structure. The as spun QFBPBP film displayed a strong diffraction peak at 2θ=6.34 degree, suggesting its higher ACS Paragon Plus Environment

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crystalline character. This peak could also be assigned to the (001) diffraction pattern based on the single crystal structure. After thermal annealing, the intensity of (001) diffraction pattern further increased, indicating the increased order of QFBPBP films. The larger grain size and enhanced crystallinity of QFBPBP films should be responsible for their higher electron mobility. To eliminate the effect of grain boundaries on device performance and further explore the intrinsic charge carrier transport property of QBPBP and QFBPBP, transistors based on their micro-sized structures were studied. The micro-sized structures were prepared by drop-casting chlorobenzene solutions (0.5 mg/mL) of QBPBP and QFBPBP onto the octadecyltrichlorosilane (OTS)-modified SiO2/Si substrates. The micro-sized structures of QBPBP are too thick to fabricated transistor devices (Figure S5). QFBPBP forms two different shapes of micro-sized structures, that is parallelogram type ribbons and hexagon type flakes (Figure 4a,b). Both shapes of micro-sized structures have a length of tens micrometers and a width of several micrometers, and their thicknesses are more than 100 nm (Figure 4e, f). XRD results showed a reflection at 2θ = 6.34 degree for parallelogram type ribbons and a peak at 2θ=7.57 degree for hexagon type flakes, indicating QFBPBP adopts different packing structures (crystal phases) in these two micro-sized structures (Figure 4c, d). Herein we name the crystal phase of ribbons as α-phase and that of flakes as β-phase. The diffraction pattern of α-phase ribbon is the same as that of spin-coated films and single crystals, suggesting α-phase ribbons have the same molecular packing as single crystals, whereas the β-phase flakes adopt a new crystal phase. Interestingly, the formation of ribbons and flakes can be controlled by changing the temperature of SiO2 /Si substrates. When the substrate temperature was higher than 26 ℃, the ribbons were the major products, and when the substrate temperature was at 20 ℃, the flakes were the only products. Moreover, β-phase flakes can transfer to α–phase ribbons with solvent annealing (dichloromethane as solvent, room temperature 18 ~ 27℃) (Figure S6). This transformation was proved by XRD results, that is the typical diffraction pattern of α –phase ribbons at 2θ=6.34 degree appeared. Currently, the formation and transformation mechanisms of α-phase ribbons and β-phase flakes are still unclear. The attempt to get the selected area electron diffractions (SAED) of ribbons and flakes failed. Transistors of single ribbon and flake were fabricated with bottom-gate top-contact structure. Au source and drain electrodes were deposited by the “Au stripe mask” method. All transistors show typical n-channel field-effect behavior (Figure 5). More than fifteen devices based on each of micro-sized structures were measured. The α- phase ribbon transistor exhibited non-ideal beACS Paragon Plus Environment

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havior in the IDS-VGS characteristics. Its mobility as a function of VGS as point-wise extracted from the saturation curve in forward and backward sweeps was plotted in Figure S7. Besides peak mobilities over 6.0 cm2V−1s−1 for both forward and backward sweeps, electron mobilities of 4.0 cm2V−1s−1 were observed while leveling out the mobility - VGS curves for both forward and backward sweeps. This leveling out mobility is much higher than that of polymer semiconductor F4BDOPV-2T. [9] The non-ideal behavior in the IDS-VGS characteristics and the appearance of peak mobility in mobility – VGS curve is likely due to the varying contact resistance at low ǀVGSǀ. [34-35] The non-negligible hysteresis of transfer and output characteristics indicated that large numbers of traps exist in the microstructures or the microstructures / insulator interface (Figure S7a and b). The low on / off current ratio of devices, resulting from the high off-state current, is likely ascribed to the nature of the quinoidal molecules. [2, 13, 30] Moreover, devices with low off-state current were also observed (Figure S8). Hence, it is speculated that the high off-state current is also related to the quality of the microstructures. Comparing with α- phase ribbons, the β- phase flake transistors displayed lower device performance with a maximum electron mobility of 0.04 cm2V−1s−1 and an average electron mobility of 0.024 ± 0.01 cm2V−1s−1. It is no doubt, their different molecular packing structures should be responsible for the distinct device performance.

Figure 4. Micro-sized structures of QFBPBP. (a, b) Optical microscope image of α –phase ribbons and β-phase flakes; (c, d) XRD patterns of ribbons and flakes;(e, f) AFM images of ribbon and flake.

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Figure 5. Typical transfer and output curves of QFBPBP transistors based on single ribbon and flake. (a, b) ribbon; (c, d) flake. Insert: the optical image of the devices.

CONCLUSION In summary, a new class of quinoidal compounds QBPBP and QFBPBP are designed and synthesized. These compounds have low LUMO energy levels and high thermal stability. Single crystal X-ray diffraction reveals both compounds have planar conjugated backbone and adopt unusual layer type packing structures. Multiple interactions including π-π and CN···H (benzene ring) interactions are observed into the layer, and alkyl-alkyl interactions exist between layers. Both QBPBP and QFBPBP show typical n-channel field-effect behavior, the electron mobility is 0.09 cm2V-1s-1 for QBPBP films and 0.75 cm2V-1 s-1 for QFBPBP films. Two different types of microsized structures of QFBPBP, namely α-phase ribbons and β-phase flakes, are synthesized and characterized. Transistors based on single ribbons displayed gate voltage-dependent electron mobility with an average of 4.0 cm2V-1s-1 and a peak over 6.0 cm2V-1s-1. Our results demonstrate PP contained quinoidal compounds are good candidates for high performance n-channel OSCs. And we believe, higher performance PP contained quinoidal n-channel OSCs will be developed with further chemical structure modification.

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EXPERIMENTAL SECTION

Synthesis of QBPBP: A two-neck flask was charged with NaH (0.4g, 10 mmol) and anhydrous DME (glycol dimethyl ether, 30 mL). CH2(CN)2 (0.33g, 5 mmol) was added under ice-bath. The reaction mixture was stirred for 30minutes at room temperature, and then 2a (0.29 g, 0.5 mmol) and Pd(PPh3)4(0.057 g, 0.05 mmol) were added. The reaction mixture was heated to reflux for 5 hours. After being cooled down, the reaction mixture was poured into brine (300 mL), and extracted with CH2Cl2. The combined organic solution was added excess DDQ and stirred at room temperature for 30 minutes. The solvent was removed and the crude product was purified by flash column chromatography (PE:CH2Cl2=1:2), affording QBPBP as green solid (0.18 g, 64.7%). 1

H NMR (300 MHz, CDCl3) δ 7.17 (d, J = 9.1 Hz, 2H), 7.04 (d, J = 9.2 Hz, 2H), 6.82 (s, 2H), 3.83 (d,

J = 7.4 Hz, 4H), 1.88 (s, 2H), 1.49-1.26 (m, 16H), 1.06-0.81 (m, 12H). HRMS (MALDI-TOF) calcd. For C36H41N6 ([M+H]+): 557.33807; Found: 557.33826. Elemental analysis calculated for (C36H40N6) (%): C, 77.66; H, 7.24; N, 15.09. Found: C, 77.77; H,7.32; N, 15.10. Synthesis of QFBPBP: Compound QFBPBP was synthesized according to the similar procedure of QBPBP. Yield, 62.9%. 1H NMR (300 MHz, CDCl3) δ 6.73 (dd, 4H), 3.74 (d, 4H), 1.78 (s, 2H), 1.33 (m, 16H), 0.94 (dt, J = 24.1 Hz, 12H). 19F NMR (CDCl3, 300 MHz, ppm) δ -117.17. HRMS (MALDI-TOF) calcd. For C36H39F2N6 ([M+H]+): 593.31988; Found: 593.31837. Elemental analysis calculated for (C36H38F2N6) (%): C, 72.95; H, 6.46; N, 14.18. Found: C, 72.76; H,6.42; N, 14.11. Thin film transistor fabrication: The thin-film transistors were bottom-gate top-contact structure. n-Doped Si was used as the gate electrode and 300 nm SiO2 was the dielectric layer. The OTS modification was done according to a previously reported procedure.

[24]

The QBPBP

film was prepared by vacuum deposition and spin-coating. QFBPBP films were deposited through spin-coating a THF solution (5 mg mL-1) at 3000 rpm min−1 under ambient conditions. Au source and drain electrodes were deposited under vacuum through shadow masks. Thermal annealing of films was conduct at given temperatures for 30 min under N2. The transistors were characterized in glove box by using a Keithley4200-SCS semiconductor analyzer. Single micro-sized structure Transistor fabrication: A solution of QFBPBP in chlorobenzene solution (0.5 mg/mL) was drop-cast onto an OTS-modified SiO2 substrate. The substrate was put into a glass bottle with a grinding plug. After the evaporation of the solvent, micro-sized ribbons and flakes were obtained. The Au source and drain electrodes were fabricated through the “Au stripe mask.” The channel width (W) and channel length (L) of the devices were meas-

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ured through optical microscopy and AFM images. The transistors were characterized in a glove box by using a Keithley4200-SCS semiconductor analyzer. ASSOCIATED CONTENT Synthetic procedures and characterization data of all new compounds, TGA, electric characteristics of TFTs characterization. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION

Corresponding Author E-mail: [email protected]; [email protected]; [email protected] ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundation of China (Grant Nos. 21672252, 21472116 and 21790362), the “Strategic Priority Research Program” of the Chinese Academy of Sciences (XDB12010100), Shanghai Rising-Star Program (18QA1405000) and Youth Innovation Promotion Association of Chinese Academy of Sciences (2018290).

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