Epindolidione-Based Conjugated Polymers: Synthesis, Electronic

Development of new electron-deficient building blocks is essential to donor–acceptor conjugated polymers. Herein, epindolidione (EPD) as electron-de...
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Epindolidione-Based Conjugated Polymers: Synthesis, Electronic Structures, and Charge Transport Properties Chi-Yuan Yang, Ke Shi, Ting Lei, Jue Wang, Xiao-Ye Wang, Fang-Dong Zhuang, Jie-Yu Wang,* and Jian Pei* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, Center for Soft Matter Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: Development of new electron-deficient building blocks is essential to donor−acceptor conjugated polymers. Herein, epindolidione (EPD) as electron-deficient unit was integrated into conjugated polymers for the investigation of field-effect transistors for the first time. We systematically studied the electronic structures and charge transport properties of the EPD-based donor−acceptor polymers. They exhibit ptype transport characteristics with the highest mobility of up to 0.40 cm2 V−1 s−1, thus demonstrating its great potential as a building block for polymer field-effect transistors and photovoltaics. KEYWORDS: epindolidione, electron-deficient unit, conjugated polymer, field-effect transistor, side chain effect y virtue of low-cost fabrication, mechanical flexibility, and tunable optoelectronic properties, conjugated polymers are promising materials for next-generation optoelectronic devices.1 They have been incorporated into various electronic devices such as photovoltaics,2 field-effect transistors,3,4 and light-emitting diodes.5 The past few years have witnessed significant progress in the development of polymer semiconductors with FET mobility over 1 cm2 V−1 s−1 and photovoltaic efficiency over 10%.6,7 This significant progress of polymer semiconductors is not only due to the improvement of fabrication processes but more importantly attributed to the development of novel building blocks, in particular, electrondeficient units such as benzothiadiazole (BT),8,9 benzobisthiadiazole (BBT),10−12 perylenedicarboximide (NDI),13−15 diketopyrrolopyrole (DPP),16−18 isoindigo (IID),19,20 and benzodifurandione-based oligo(p-phenylenevinylene) (BDOPV).21−23 The field-effect transistors (FETs) performance of these polymers has been surpassing their amorphous silicon counterparts, with mobilities over 1 cm2 V−1 s−1. However, to further improve the device performance and investigate the structure−property relationships, new electrondeficient building blocks are desired. Epindolidione (EPD), a structural isomer of electrondeficient unit isoindigo (IID), was first reported by Sir Robingson in 1934.24 It contains four six-membered fused rings analogous to tetracene (Figure 1). Recently, IID has attracted increasing attention in the development of conjugated polymers for photovoltaics and FETs.20,25 However, EPD is less

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© XXXX American Chemical Society

Figure 1. Chemical structures of IID, EPD, and their conjugated polymers.

investigated for electronics. Compared with IID, EPD reveals similar nontoxicity but remarkable higher thermal stability.26,27 With a planar and electron-deficient conjugated backbone, EPD is a promising building block for organic semiconductors. In 2013, Głowacki et al. reported the first small-molecule FET based on EPD and the device fabricated by vacuum deposition exhibited a hole mobility of 1.5 cm2 V−1 s−1 with good air Special Issue: Applied Materials and Interfaces in China Received: August 19, 2015 Accepted: September 28, 2015

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DOI: 10.1021/acsami.5b07715 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces stability.28 Further study on fluoride and chloride of EPD revealed that EPD derivatives could also exhibit electron transporting properties by using low work function metals as electrodes.29 However, there are several critical problems restricting its extensive applications. First, EPD is insoluble in common organic solvents without extra solubilizing groups. Besides, due to the electron-deficient feature of lactam structure, EPD is difficult to be functionalized through electrophilic halogenation. Therefore, it is an intriguing challenge to obtain solution-processable conjugated polymers based on EPD. Herein, we report the synthesis, electronic structures, and charge transport properties of EPD-based conjugated polymers. To functionalize EPD, an efficient approach was developed to synthesize 2,8-dibromo-epindolidione. By using 2,2′-bithiophene as a donor unit, two EPD-based donor−acceptor conjugated polymers were synthesized. Different types of branched side chains were introduced into EPD to investigate the side chain effect on the polymer properties.30 Solutionprocessable polymer FETs based on farther branched alkyl side chains exhibit high hole mobilities of up to 0.40 cm2 V−1 s−1 with almost ideal transfer characteristics, thus demonstrating promising applications of EPD-based conjugated polymers in organic electronics. Scheme 1 illustrates the synthetic route to the EPD-based conjugated polymers PEPD2T-1 and PEPD2T-2. First, a

(iodomethyl)-nonadecane or 11-(3-iodopropyl)henicosane to give polymerization precursors 5. Stille-coupling polymerization between 5,5′-bis(trimethylstannyl)-2,2′-bithiophene and 5 gave EPD-based polymers PEPD2T-1 and PEPD2T-2. PEPD2T-1 shows a Mn of 12.0 kDa, and PEPD2T-2 shows a similar Mn of 10.7 kDa (Supporting Information (SI) Figure S1). Both PEPD2T-1 and PEPD2T-2 show good solubility in common chlorinated solvents such as chloroform, trichloroethylene, and chlorobenzene. Both polymers show excellent thermal stability with decomposition temperatures above 350 °C under nitrogen atmosphere, and no phase transition was observed before decomposition (SI Figure S2). For comparison, monomer 6 was also synthesized through direct alkylation of EPD in 51% yield. The optical and electrochemical properties of EPD-based polymers were explored by UV−vis absorption spectroscopy and cyclic voltammetry. Figure 2a displays the absorption spectra of monomer 6 in dilute chloroform and the EPD-based

Scheme 1. Synthetic Route to the EPD-Based Polymersa

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Reagents and conditions: (i) 4-bromoaniline, HCl/H2O, MeOH, reflux, 4 h, 87%; (ii) Dowtherm A, 150−250 °C, 2 h, 94%; (iii) AlCl3, NaCl, 160 °C, 2 h, 79%; (iv) 9-(iodomethyl)nonadecane or 11-(3iodopropyl)henicosane, Cs2CO3, DMF, 130 °C, 48 h, 35%−37%; (v) 5,5′-bis(trimethylstannyl)-2,2′-bithiophene, Pd2(dba)3, P(o-tol)3, chlorobenzene, 140 °C, 72 h, 67%−71%; (vi) 1-bromohexane, Cs2CO3, DMF, 130 °C, 48 h, 51%.

condensation reaction of commercial available 4-bromoaniline and dimethyl-2,3-dihydroxyfumarate were used in the presence of acid catalyst to afford 2 in 87% yield. After cis−trans isomerization and Conrad−Limpach cyclization, 3 was obtained in one port with a high yield of 94% by employing a gradually increasing temperature method. After an intramolecular Friedel−Crafts reaction of 3 under the catalysis of aluminum trichloride, 2,8-dibromo-epindolidione (4) was synthesized in 79% yield. 4 was then alkylated with 9-

Figure 2. (a) Normalized UV−vis absorption spectra of monomer 6 and EPD-based polymers in chloroform (1 × 10−5 M) and in thin films. (b) Cyclic voltammograms of EPD-based polymer films dropcasted on a glassy carbon electrode. (c) Calculated molecular orbitals of the trimer of PEPD2T-1 (or PEPD2T-2) (B3LYP/6-311+G(d,p)). Long alkyl chains were replaced with methyl groups to simplify calculation. B

DOI: 10.1021/acsami.5b07715 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces polymers in dilute chloroform and thin films. The absorption spectrum of monomer 6 shows an absorption band in lowenergy region with maximum at 470 nm. The 0−0 vibrational absorption intensity is much stronger than that of 0−1 vibrational absorption due to the rigidity of EPD backbone. Both PEPD2T-1 and PEPD2T-2 show two absorption bands, with one from the absorption of EPD core (320−460 nm) and another from the charge-transfer absorption from bithiophene unit to EPD unit (460−580 nm). The absorption spectrum of PEPD2T-1 in thin film shows obvious red-shift in low-energy region compared to that in dilute solution, indicating that the polymer backbone becomes more planar in thin film. However, the absorption spectrum of PEPD2T-2 film shows only a small red-shift in low-energy region with increased 0−0 vibrational absorption intensity compared to that in dilute solution. It is worth noticing that although PEPD2T-1 and PEPD2T-2 exhibit similar absorption band at high-energy region, the absorption bands in low-energy region are quite different in solutions. PEPD2T-2 shows a 30 nm red-shift compared to PEPD2T-1, which can be attributed to their different aggregation behaviors caused by different side chains. Moreover, from the onsets of the thin films absorption spectra, the optical band gaps of PEPD2T-1 and PEPD2T-2 are calculated to be 2.14 and 2.08 eV, respectively. Figure 2b illustrates the cyclic voltammetry of the thin films of EPD-based polymers. The HOMO/LUMO levels of PEPD2T-1 and PEPD2T-2 were calculated to be −5.57/− 3.40 eV and −5.50/−3.42 eV from CV measurement. The HOMO levels of PEPD2T-1 and PEPD2T-2 were also evaluated by photoelectron spectroscopy and were determined to be −5.53 eV for PEPD2T-1 and −5.47 eV for PEPD2T-2 (SI Figure S3), consistent with HOMO levels estimated from the CV measurement. Thus, after introducing different alkyl side chains, the HOMO level of PEPD2T-2 is clearly higher than that of PEPD2T-1, whereas the LUMO level of PEPD2T2 is close to that of PEPD2T-1. This is largely due to the strong aggregation behavior of PEPD2T-2 because aggregation will lead to more planar backbone and better conjugation. To better understand the electronic structures of EPD-based polymers, we performed density functional theory (DFT) calculation for the epindolidione-alt-bithiophene trimer. The calculated molecular orbitals are shown in Figure 2c. The DFT calculation shows that the LUMOs of the polymers are localized on EPD unit, while the HOMOs are distributed along the polymer chains. This result agrees well with CV results that the HOMO levels are more easily affected by backbone planarization but the LUMO levels are less affected. Bottom-gate/top-contact FET devices were fabricated to evaluate the charge transport properties of EPD-based polymers. The semiconducting layer was deposited by spincoating a polymer solution (4 mg/mL in trichloroethylene) on octadecyltrichlorosilane-treated SiO2/n++-Si substrates. After thermal annealing and vacuum evaporation of Au as the source and drain electrodes, the FET devices were tested under ambient conditions (RH = 50−60%). Both PEPD2T-1 and PEPD2T-2 show typical p-type transfer and output characteristics (Figure 3). For PEPD2T-1, the spin-coated film without thermal annealing showed a low hole mobility of 10−3 cm2 V−1 s−1. After thermal annealing at 140 °C, the hole mobility of PEPD2T-1 increases to a maximum of 0.081 cm2 V−1 s−1 (average 0.057 cm2 V−1 s−1), with an on/off ratio above 104 which did not show further enhancement at higher annealing temperature. For PEPD2T-2, its as-deposited film showed a

Figure 3. Transfer (a,c) and output characteristics (b,d) of FET device based on PEPD2T-1 (a,b) or PEPD2T-2 (c,d).

mobility of 0.021 cm2 V−1 s−1. After thermal annealing at 240 °C, the hole mobility of PEPD2T-2 film increased to a maximum of 0.40 cm2 V−1 s−1 (average 0.31 cm2 V−1 s−1) with an on/off ratio of 107. Our results indicate that the branching positions of solubilizing alkyl chains have a significant influence on the carrier mobilities of EPD-based polymers. To understand the mobility difference of the two EPD-based polymers, X-ray diffraction (XRD) and atomic force microscopy (AFM) were employed to analyze the microstructure and film morphology of each polymer film. X-ray diffraction patterns of as-cast films of PEPD2T-1 and PEPD2T-2 are shown in Figure 4a. The as-cast film of PEPD2T-1 showed only one weak diffraction at 2θ = 4.85° and that of PEPD2T-2 showed two diffractions at 3.42 and 6.84°, indicating that the as-cast films are relatively amorphous. In sharp contrast, after thermal annealing, PEPD2T-1 film showed three peaks and PEPD2T-2 showed stronger five diffraction peaks (Figure 4b), demonstrating more ordered packing in the annealed polymer films. The results are consistent with the 2D grazing incidence X-ray diffraction (2D-GIXD) patterns illustrated in SI Figure S4. In the 2D-GIXD patterns, PEPD2T-1 shows three diffraction peaks, (100), (200), and (300), and PEPD2T-2 shows four stronger diffraction peaks after thermal annealing, indicating that both polymers have an ordered edge-on lamellar packing in the annealed films. Thus, thermal annealing increased the crystallinity of the polymer films and enhanced the hole mobilities. Moreover, PEPD2T-2 film shows a more ordered lamellar packing compared to PEPD2T-1. From the XRD data, the lamellar distances of PEPD2T-1 and PEPD2T-2 films were calculated to be 18.2 and 25.8 Å, respectively. The different lamellar distances of PEPD2T-1 and PEPD2T-2 films could be attributed to the different lengths of alkyl chains. The cartoon representations of thin film microstructures before and after annealing are shown in Figure 4c,d. The AFM images of the two polymer thin films are shown in Figure 4e,f. The rootC

DOI: 10.1021/acsami.5b07715 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b07715. Monomers/polymers synthesis, characterization, and device fabrication details (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*For J.Y.W: E-mail, [email protected]. *For J.P.: phone/fax, 86-10-62758145; E-mail, jianpei@pku. edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Major State Basic Research Development Program (2013CB933501) from the Ministry of Science and Technology, the National Natural Science Foundation of China, the Beijing Natural Science Foundation (2144049), and the Doctoral Program Foundation from the Ministry of Education of China (20130001120018). We are grateful for the beam time at beamline BL14B1 (Shanghai Synchrotron Radiation Facility). We thank Xinyue Zhang for the help with AFM measurements.

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Figure 4. XRD patterns of (a) as-cast polymer film and (b) annealed polymer film. Molecular packing models of (c) as-cast polymer film and (d) annealed polymer film. Tapping mode AFM height images of (e) PEPD2T-1 and (f) PEPD2T-2 annealed films.

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DOI: 10.1021/acsami.5b07715 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX