Letter pubs.acs.org/OrgLett
Thiadiazoloquinoxaline-Fused Naphthalenediimides for n‑Type Organic Field-Effect Transistors (OFETs) Ben-Lin Hu,† Ke Zhang,† Cunbin An,† Wojciech Pisula,†,‡ and Martin Baumgarten*,† †
Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128, Mainz, Germany Department of Molecular Physics, Faculty of Chemistry, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz, Poland
‡
S Supporting Information *
ABSTRACT: Thiadiazoloquinoxaline-fused naphthalenediimides (TQ-f-NDIs) are designed and synthesized. They show high electron affinities (EAs) of ∼4.5 eV. Organic field-effect transistor (OFET) devices, fabricated by dip-coating, provided maximum high electron mobilities of 0.03 cm2/(V·s) with an on/off ratio of 2 × 105.
he development of electron-deficient π-building blocks (electron acceptors) has been a hot topic in the past decade, since they are necessary and indispensable elements to develop n-type and ambipolar organic field-effect transistors (OFETs).1,2 Numerous donor units have been reported with low ionization potential (IP) and good device performance.3 However, the development of acceptors, especially the electrondeficient ones with high electron affinities (EAs), e.g. strong electron acceptors, still lags compared to their donor counterparts. Therefore, the development and design of high performance n-type organic semiconductors based on strong acceptor units remain a focal issue in the field of functional πmaterials.4−8 Naphthalenediimide (NDI) is no-doubt the most important kind of electron-deficient π-building block and serves as a promising candidate for organic electronics applications, such as photovoltaic devices and flexible displays, due to their high EAs, high charge carrier mobility, and excellent thermal stability.9−13 Even though NDIs possess high EA values, some of their derivatives are still unstable under air. When they were applied in optoelectronic devices, for example, a high electron mobility of 6.2 cm2/(V·s) has been reported from the crystals of a core-unsubstituted NDI. However, significant degradation was observed in the OFET devices.14 To improve the air stability of the NDIs, higher electron-deficient units have been generated by grafting N-heteroacene, cyano, perfluoroalkyl, and chloro groups onto the NDI cores.10,15−26 Furthermore, no report has shown the possibility of developing another electrondeficient unit annulated to the NDI core with high EA. Herein, thiadiazoloquinoxaline, as a strong electron-deficient unit, is annulated on the core position of naphthalenediimide to decrease the LUMO levels. As shown in Figure 1, thiadiazoloquinoxaline-fused naphthalenediimides (TQ-f-NDI), combining naphthalenediimide (NDI), thiadiazoloquinoxaline (TQ), and N-heteroacene (NHA) in one molecule, are first reported. As expected, the DFT calculated LUMO energy level of TQ-fNDI is as low as −3.96 eV, which is much lower than that of unsubstituted NDI (−3.41 eV). A series of TQ-f-NDIs are
T
© 2017 American Chemical Society
Figure 1. Chemical structures of [1,2,5]thiadiazolo[3,4-f ]quinoxaline, N-heteroacene, NDI, TQ-f-NDI, and the LUMO and HOMO distribution of NDI and TQ-f-NDI (DFT calculations by B3LYP/631G(d)).
synthesized by the nucleophilic substitution of tetrabromo-NDI with benzothiadiazole diamine, followed by oxidation with PbO2. Thereby strong acceptors with high EA of ∼4.5 eV are obtained. OFETs based on the TQ-f-NDI were fabricated by solution process, revealing a good n-channel field-effect response and good stability under ambient conditions. The synthesis of diamine 1a is shown in Scheme S1, starting from 4,7-dibromobenzothiadiazole by a four-step procedure, and benzothiadiazole diamine 1b was synthesized according to a reported procedure.27 The tetrabromo NDIs 2 were synthesized according to reported procedures with little modification.28,29 To balance the crystallinity and solubility, Received: September 28, 2017 Published: November 14, 2017 6300
DOI: 10.1021/acs.orglett.7b03041 Org. Lett. 2017, 19, 6300−6303
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Organic Letters Scheme 1. Synthesis of P1−P3
all five compounds are stronger acceptors than coreunsubstituted NDIs. Notably, the EA of P1 is as low as 4.51 eV due to the electron-withdrawing effect of Br atoms, pointing toward ultrastrong acceptors. The charge carrier transport properties of the compounds P1−P3 were evaluated from “top-contact bottom gate” fabricated FETs. When we tried to use the solution of P1 to prepare FET devices, dewetting occurred (a schematic image of the dewetting process is shown in Figure S28), which may be caused by the bromo atoms in the molecules,32resulting in the poor interface between films and substrates. The reductive debromination was employed to remove the bromo atoms in P1 followed by oxidization to P2 and P3. As expected, no dewetting was observed in the film of P3a (Figure S29). However, not enough crystals of P3a were formed to fabricate the OFET devices. Therefore, the films were deposited by dipcoating. Solutions of compounds P1−P3 (0.5 mg/mL) were blended with PMMA (0.5 mg/mL) in chloroform and were dip-coated on a silicon substrate with a speed of 20 μm/s. As shown in Figures S30 and S31, crystal fibers were obtained by P1a:PMMA and P3a:PMMA. The length of P1a fiber is less than 15 μm, which is not long enough to bridge the source and drain electrodes. However, the nanofibers of P3a are formed with a length of more than 1 mm. The width of the nanofibers is 700 ± 100 nm, and the height is 65 ± 15 nm, as shown in Figure S31. The heavily doped n-type Si wafers were used as a gate electrode, and 300 nm thick SiO2 was adopted as a gate dielectric layer.33 While P2 and P3b did not a show good charge transport due to poor crystallinity caused by a long branched side chain, FETs based on P3a revealed an average electron mobility of around 0.024 cm2/(V·s), and the corresponding transfer and output curves are shown in Figure 3. The maximum electron mobility of P3a is 0.03 cm2/(V·s), with an on/off ratio of 2 × 105 and a threshold voltage (Vth) of
different side chains were employed: pentyl and 3-decylpentadecyl alkyl chains at the imide position, and hydrogen and TIPS acetylene at the ortho-positon of benzothiadiazole, respectively. The intermediate monosubstituted 3 can be easily obtained by the nucleophilic substitution of diamines 1 and tetrabromo NDIs 2 with a yield of ∼90% (Scheme 1). Trace bisubstituted compounds were separated as two isomers (see the chemical structures (Chart 1) in the Supporting Information). Subsequently, strong acceptors P1a and P1b were obtained by oxidation of 3 in PbO2 at room temperature with yields of higher than 90%. Reductive debromination was employed to remove the Br atoms in P1 with formic acid/ trimethylamine by the catalyst Pd/C. Monobromo 4 and nonbromo 5 intermediate products were obtained and then were easily oxidized to P2 and P3, respectively. Compounds 4 and P2 are a mixture of two isomers with a ratio of ∼7:1 from the 1H NMR spectra. The above compounds P1a, P1b, P2, P3a, and P3b were characterized unambiguously by 1H NMR, 13 C NMR, and HRMS (see the synthetic details and Figures S1−S27). The UV−vis absorption spectra of P1−P3 in diluted dichloromethane (DCM) are shown in Figure 2a. Because the TIPS acetylene group in P1a extends the conjugation, the absorption edge of P1a (∼566 nm) is longer than that of the other four compounds (∼521−532 nm). Two main absorption bands at 300−380 nm and 420−590 nm are observed in a DCM solution of P1−P3. Cyclic voltammetry was performed for the TQ-f-NDI in DCM with ferrocene (Fc) as the inner standard. The onset oxidation of Fc is ∼0.32 V in our setup. The CV curves of P1−P3 in dichloromethane using a 0.1 M solution of tetrabutylammonium hexafluorophosphate (nBu4PF6) at 100 mV·s−1 are shown in Figure 2b, and the electrochemical data are summarized in Table 1. All four compounds exhibit two reversible reduction waves. Obviously, 6301
DOI: 10.1021/acs.orglett.7b03041 Org. Lett. 2017, 19, 6300−6303
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Figure 3. Transfer (a) (measured in air and in the glovebox) and output (b) (measured in the glovebox) characteristics of FET devices fabricated by dip-coating of P3a.
the NDI unit is an effective way to extend the core and obtain strong acceptors for high performance OFETs.
Figure 2. (a) UV−vis absorption of P1−P3 in DCM (10−5 M) and (b) the cyclic voltammograms of P1−P3 in DCM with ferrocene as an internal standard (ferrocene peaks are marked with purple stars and occur at positive potential) and adjusted for Ag/Ag+.
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S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b03041. Experimental details, characterization data, NMR spectra of all new compounds, and the preparation of OFET devices (PDF)
Table 1. Optical and Electrochemical Properties of P1−P3 P1a P1b P2 P3a P3b
Ered,onset (V)a
Egopt (eV)b
EA (eV)c
IP (eV)d
0.03 0.03 0 −0.03 −0.10
2.19 2.33 2.37 2.38 2.38
4.51 4.51 4.48 4.45 4.38
6.70 6.84 6.85 6.83 6.76
ASSOCIATED CONTENT
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a
Measured in n-Bu4NPF6 solution in CH2Cl2 with a scan rate of 100 mV/s, and ferrocene as an internal standard. bEstimated from absorption onset. cThe energy of Fc/Fc+ is assumed at −4.8 eV relative to vacuum.30,31 dCalculated with the formula IP = EA − Egopt.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
15 V in the glovebox. The electron mobility of P3a measured in air was around 0.01 cm2/(V·s), with an on/off ratio of about 2 × 104, and a threshold voltage of 35 V (the output curves in air are shown in Figure S32). In conclusion, strong acceptors of TQ-f-NDIs (P1−P3) with high electron affinity (∼4.50 eV) have been synthesized for airstable n-channel organic field effect transistors by a facile procedure. A maximum electron mobility of 0.03 cm2/(V s) with and on/off ratio of 2 × 105 was obtained. Even though the electron mobility can be further improved by optimizing the preparation of a device, our results show that the annulation of another electron-withdrawing unit, thiadiazoloquinoxaline, to
Wojciech Pisula: 0000-0002-5853-1889 Martin Baumgarten: 0000-0002-9564-4559 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by SFB-TR49. We thank ChengCheng Yan for the AFM measuring. B.-L.H. gratefully acknowledges the Alexander von Humboldt Stiftung for granting a research fellowship. K.Z. thanks the China Scholarship Council (CSC) for financial support. 6302
DOI: 10.1021/acs.orglett.7b03041 Org. Lett. 2017, 19, 6300−6303
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