Thiadiazole-fused Quinoxalineimide as an Electron-deficient Building

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Thiadiazole-fused Quinoxalineimide as an Electron-deficient Building Block for N‑type Organic Semiconductors Tsukasa Hasegawa,† Minoru Ashizawa,*,† Koutarou Aoyagi,† Hiroyasu Masunaga,‡ Takaaki Hikima,§ and Hidetoshi Matsumoto*,† †

Department of Materials Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan ‡ Japan Synchrotron Radiation Research Institute (JASRI)/SPring-8, 1-1-1 Kouto, Sayo, Sayo 679-5198, Japan § RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo, Sayo 679-5148, Japan S Supporting Information *

ABSTRACT: A strong electron-accepting planar π-conjugated framework, thiadiazole-fused quinoxalineimide (TQI), was designed and synthesized. Three TQI-based small molecules exhibit deep lowest-unoccupied molecular orbital (LUMO) levels, which require air stable n-channel conduction (∼−4.0 eV). Among these molecules, Hex-TQI-Br exhibits air-stable n-channel charge transport with a moderate mobility of 0.044 cm2 V−1 s−1.

N

-type organic semiconductors are promising materials for a wide variety of potential applications including n-channel organic field-effect transistors (OFETs).1 To date, tremendous efforts have been devoted to exploring n-type small molecules and polymers, which lag behind their p-type counterparts because n-type materials that stably operate under air conditions are still rare. An effective approach for designing n-type materials is to incorporate strong electron-withdrawing groups, such as halogens,2a cyano groups,2b diimide groups,2c and thiadiazole groups,2d onto the π-framework since the electron-withdrawing groups pull electrons out of the π-backbone, thereby lowering their LUMO energy levels. Additionally, the design of novel ntype building blocks with synthetic diversity remains an intriguing area of research. Thiadiazoloquinoxaline units (TQs) are very strong electron acceptors with deep LUMO levels (∼−3.83 eV), and TQ-based small molecules and polymers possess small energy gaps.2d,3 We previously reported an n-type quinoxalineimide (QI) building block and demonstrated that this unit is a potential candidate for preparing n-type materials (Figure 1a).4 Along with this line, we have designed and synthesized thiadiazole-fused quinoxalineimide, i.e., thiadiazoloquinoxalineimide (TQI) derivatives Hex-TQI-Br (1), EH-TQI-Br (2), and EH-TQI-Th (3) (Figure 1b). Thiadiazole and imide groups located at molecular termini result in robust molecular planarity. Additionally, the solubilizing N-substituted alkyl chain not only improves the solubility of the TQI unit but also provides opportunities for side-chain engineering to tighten the molecular packing, as demonstrated by the designed linear chain of 1 and branched chains of 2 and 3. The TQI-Br framework contains bromines at the 4 and 10 positions because electron-withdrawing bromine is expected to be favorable for lowering the LUMO energy level and enables synthetic functionalization, such as © 2017 American Chemical Society

Figure 1. Chemical structures of (a) QI and TQI units and (b) target TQI small molecules.

lateral π-expansion based on the TQI unit. TQI-Th, which consists of TQI and thiophene wings, is regarded as part of D−A alternation, which is known to finely tune electronic structures and facilitate π−π stacking.5 Based on the TQI unit, we report the synthesis, optical and electrochemical properties, single-crystal structures, and carrier transport properties of TQI-based molecules 1−3 to explore the structure−function correlation. The syntheses of 1−3 are illustrated in Scheme 1. The key intermediate compound 4 was obtained via the condensation of 4,7-dibromobenzo[c][1,2,5]thiadiazole-5,6-diamine6 with diisopropyl 2,3-dioxosuccinate.7 The hydrolysis of 4 with sodium hydroxide afforded 5, and then, 5 was treated with acetyl chloride to yield the cyclized acid anhydrate 6. The direct imidization of 6 failed using a conventional procedure with heating in DMF or an AcOH solution with alkylamine.8 Alternatively, we adopted an in Received: May 10, 2017 Published: May 31, 2017 3275

DOI: 10.1021/acs.orglett.7b01424 Org. Lett. 2017, 19, 3275−3278

Letter

Organic Letters

relatively lower LUMO levels of the TQI derivatives are beneficial to the air-stable electron transport property. These results confirm that incorporating a thiadiazole ring into the QI unit effectively achieves molecular planarity and a deep LUMO level. The optical and electrochemical properties of 1−3 were evaluated using UV−vis absorption spectroscopy and cyclic voltammetry (CV), and the data are listed in Table 1. Molecules

Scheme 1. Synthetic Route of TQI Derivatives

Table 1. Optical Properties of 1−3 1 2 3

λmaxsola (εmaxa) [nm] ([× 104 M−1 cm−1])

λmaxfilmb [nm]

Egoptc [eV]

318(4.1), 495(0.48), 526(0.43) 321(3.9), 495(0.49), 527(0.44) 354(4.5), 699(0.90)

390, 511, 545 387, 508, 544 341, 734

2.21 2.21 1.47

a In CHCl3 solution. bSpin-coated thin film. cEstimated from the solution absorption onset.

1 and 2 exhibited nearly the same absorption spectra in both solutions and thin films. In solutions, molecules 1 and 2 exhibited multiple absorption peaks from the UV region to the visible light region. However, molecule 3 exhibited two main peaks reaching from the UV region to the near IR region (Figure 2a). Relative to

situ stepwise approach under mild conditions. Molecules 1 and 2 were successfully obtained by amidation with the corresponding alkylamine and successive imidization by acid-chloride-mediated cyclization using oxalyl chloride. A Stille coupling reaction between 2 and 2-(tributylstannyl)thiophene using a Pd catalyst afforded 3. The detailed synthetic procedures and characterizations are described in the Supporting Information. The decomposition temperature, which was defined by a weight loss of 5%, was estimated from thermal gravimetric analysis (TGA) results to be 314 °C for 1, 248 °C for 2, and 337 °C for 3 (Figure S2). Therefore, these compounds exhibit adequate thermal resistance for use in electronic devices. Obvious melting points below the decomposition temperatures were not observed for any of the molecules under ambient conditions. In differential scanning calorimetry (DSC) profiles under nitrogen, the only molecule 2 showed a couple of exotherm and endotherm peaks corresponding to crystallization and melting, respectively (Figure S2). The crystallization and melting points of 2 determined by the onset peak are 160 and 164 °C. To evaluate the electronic structures and optimized geometries of 1−3 at the vacuum level, density functional theory (DFT) calculations using the Gaussian 09 program at the B3LYP/6-31G*(d,p) level were performed on methyl-substituted molecules TQI-Br and TQI-Th. Figure S3 shows the energy-minimized structures and the frontier molecular orbitals of TQI-Br and TQI-Th. The optimized molecular geometry of TQI-Th is a completely planar structure, while that of the corresponding QI derivative (QI-Th) is slightly twisted (Figure S3). Fusing a thiadiazole ring into the QI core can prevent steric hindrance between the protons of the QI core and the adjacent thiophene. Therefore, the TQI unit is favorable for maintaining the high planarity when extending the π-conjugation to oligomeric and polymeric systems. The π-electron density distributions of the LUMO levels are widely delocalized over the π-framework, while those of the HOMO levels are slightly localized around the central phenyl ring of the TQI core. The calculated HOMO and LUMO levels are −6.85 and −4.26 eV, respectively, for TQI-Br, and −5.66 and −3.96 eV, respectively, for TQI-Th. The LUMO levels of the TQI derivatives are approximately 0.8 eV lower than the calculated LUMO levels of the corresponding QI derivatives (QI-Br and QI-Th). The

Figure 2. UV−vis absorption spectra of 1−3 for (a) 10−5 M CHCl3 solutions and (b) thin films spin-coated from CHCl3 solutions.

the low-energy absorption peaks of 1 and 2, the peaks of 3 were remarkably red-shifted and exhibited an enhanced molar extinction coefficient (εmax), suggesting a donor−acceptor interaction between the flanking thiophenes and the TQI core. The absorption spectra of the thin films of 1−3 (Figure 2b) were significantly red-shifted relative to those of the solutions (Figure 2a). The significant red shifts in the spectra of the thin films could be attributed to the strong π−π intermolecular aggregation in the solid state, resulting from the highly planar π-framework. The optical energy gaps (Egopt) estimated from the solution absorption onset were 2.21 eV for 1, 2.21 eV for 2, and 1.47 eV for 3. The electrochemical potentials of EHOMO and ELUMO were calculated from the onset oxidation and reduction potential curves, which were calibrated by the ferrocene/ferrocenium (Fc/ Fc+) couple with a redox potential that was assumed to be −4.8 eV at the vacuum level.9 As shown in Figure S4, 1−3 exhibited two separate reversible reduction waves and one irreversible oxidation wave. These distinguishing reduction profiles indicate the highly stable reductant structures of the derivatives. ELUMO and EHOMO were −4.01 and −5.94 eV for 1, − 4.00 and −5.96 eV for 2, and −3.82 and TQI − 5.38 eV for 3, respectively. The ELUMO of the TQI derivatives were approximately 0.3−0.5 eV lower than those of the corresponding QI derivatives (QI-Br and 3276

DOI: 10.1021/acs.orglett.7b01424 Org. Lett. 2017, 19, 3275−3278

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Organic Letters QI-Th).4 The deep LUMO levels (i.e., below −4.0 eV) of 1 and 2 are desirable for efficient electron transport under ambient conditions.10 In molecule 3, flanking thiophenes contribute to a simultaneous increase in the HOMO and LUMO levels, and these energy levels satisfy the ambipolar characteristics required for OFET device.10 The HOMO−LUMO energy gaps (EgCV) were 1.93 eV for 1, 1.96 eV for 2, and 1.56 eV for 3, which are consistent with the theoretical and optical energy gap trends. X-ray single-crystal structure analyses of 1−3 were performed using single crystals grown via the slow diffusion of chloroform into a toluene solution of 1 and hexane into chloroform solutions of 2 and 3. The crystallographic data are listed in Table S1, and the molecular and crystal structures are shown in Figures 3 and

Bottom-gate/top-contact FET devices with a channel width of 1000 nm and a length of 50 μm were fabricated to evaluate the charge transport properties of 1−3. The thin films (45 nm) were thermally deposited on tetratetracontane (C44H90, TTC)modified Si/SiO2 substrates,12 and then, Au drain/source electrodes were vacuum deposited to afford OFET devices. Figures S8−S10 show the transfer and output curves, and the corresponding FET data are summarized in Table S2. The devices based on TQI-Br (1 and 2) exhibited typical n-type carrier transport, which is good agreement with the low-lying LUMO levels below −4.0 eV. The device based on 1 showed the highest electron mobility (μe,max) of 9.0 × 10−2 cm2 V−1 s−1 and Vth of 10 V under vacuum. Notably, the device based on 1 preserved the moderate n-channel transport mobilities of μe,max = 4.4 × 10−2 cm2 V−1 s−1 and μe,max = 3.8 × 10−2 cm2 V−1 s−1 (over 1 month) under ambient conditions. In contrast, the device based on TQI-Th (3) exhibited slightly n-channel-dominant ambipolar characteristics due to the electron-rich thiophenes, resulting in an elevated HOMO level (Figure S10). The electron mobility (μe,max = 8.5 × 10−4 cm2 V−1 s−1) of 3 was slightly higher than that of 2 bearing identical ethylhexyl side chains. This finding implies that the lateral π-extension of the TQI unit is pivotal for enhancing the intermolecular electronic couplings, as shown in the transfer integral estimation. All molecules 1−3 form nearly the same two-dimensional carrier transport system,13 as shown in the crystal structure analyses. Therefore, evaluating the microstructure and surface morphology is important for clarifying the structure−function relationship related to carrier transport. To evaluate the microstructures and surface morphology, we conducted grazing-incidence wide-angle X-ray scattering (GIWAXS) and atomic force microscopy (AFM) measurements (Figures 4 and

Figure 3. Overlap molecular structure and molecular packings of (a,b) 1, (c,d) 2, and (e,f) 3. For molecular packings (b), (d), and (f), alkyl chains are omitted for clarity, and the chloroform of 3 also is deleted.

S5−S7, respectively. Molecules 1 and 2 crystallize in the triclinic system with the P(1̅) space group, and the entire molecule is crystallographically independent. Meanwhile, molecule 3 crystallizes in a monoclinic system with the P21/c space group, and molecule 3 and one chloroform molecule are crystallographically independent (Figure S7). As expected from the DFT calculations, the π-platforms of 1−3 adopt an entirely planar geometry, and the alkyl side chains extend out of the molecular plane. Molecule 1 forms intermolecular S···N, S···Br, N···N, and O···Br short contacts with the adjacent molecules, while only the O···Br interaction was observed in molecule 2, which is presumably due to the disordered ethylhexyl chains arising from the presence of racemic forms at C22 atom. Therefore, molecule 1 is expected to form more compact molecular packing than molecule 2. In particular, the planarity of 3 was achieved by intramolecular short S···N contacts between thiophene and the pyrazine rings (Figure S7c). All molecules are packed into twodimensional slipped stacks along the a axis with diagonal electronic couplings that enable two-dimensional carrier transport (Figures 3b,d,f). The estimated interplanar spacings are 3.29 Å for 1, 3.28 Å for 2, and 3.28 Å for 3, and the slip distances along the molecular long and short axes are 1.56 and 3.65 Å for 1, 2.71 and 2.99 Å for 2, and 2.89 and 3.35 Å for 3, respectively. Assuming the tight binding method,11 the calculated LUMO transfer integrals along the stacks are tp = 11 meV and tq = 24 meV for 1, tp = 18 meV and tq = 19 meV for 2, and tp = 17 meV and tq = 23 meV for 3, and the HOMO transfer integrals are tp = 5 meV and tq = 23 meV for 1, tp = 18 meV and tq = 3 meV for 2, and tp = 20 meV and tq = 12 meV for 3. The calculated transfer integral values are indicative of a two-dimensional carrier transport path.

Figure 4. GIWAXS patterns and AFM images of thin films (45 nm) of (a,b) 1, (c,d) 2, and (e,f) 3 thermally evaporated on TTC (20 nm)modified substrates.

S11 and Table S3). In the GIWAXS patterns, molecules 1−3 exhibited intense crystalline peaks, and multiscattering peaks were observed for 1, implying higher crystallinity than those of 2 and 3. Additionally, two different out-of-plane peaks were observed for 2. The corresponding d-spacings along the out-ofplane (100) diffraction were 15.5 Å (qz = 0.41 Å−1) for 1, 23.6 Å (qz = 0.26 Å−1) and 17.5 Å (qz = 0.36 Å−1) for 2, and 22.1 Å (qz = 0.28 Å−1) for 3. In the in-plane direction, additional peaks due to π−π stacking distances of 3.44 Å (qxy = 1.82 Å−1) for 2 and 3.44 Å (qxy = 1.83 Å−1) for 3 were observed. The bulk crystal lattice constants estimated from the single-crystal X-ray structure 3277

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Organic Letters analyses indicate that the molecular π-frameworks of 1−3 are tilted with their side chains extended on the substrate. The tilt angles were roughly estimated to be 30° for 1, 6° for 2, and 13° for 3. In general, molecular arrangements with small tilt angles normal to the substrate favor carrier transport since the channel is parallel to the substrate in FETs. In contrast, the molecules in this study exhibit the opposite behavior. The thin film of 1, which exhibited the highest mobility, has a more highly tilted molecular orientation than 2 and 3. The carrier transport properties are remarkably limited by the crystalline crystal grains. In the AFM images, the obtained thin films of 2 and 3 consisted of discontinuous, small grains. The poor, granular surface morphologies of 2 and 3 are less favorable for carrier transport than that of 1. In contrast, molecule 1 formed large, dense, surface-interconnected rod-like crystalline grains. Therefore, the higher carrier transport of 1 arises from the dense interconnected surface morphology. This result is closely associated with the side-chain selection, which significantly affects the crystal-growth mechanism.14 The linear side chains of 1 would better facilitate crystalline domain growth than the branched side chains of 2 and 3. Chemical modifications to the TQI unit, such as side-chain selection and π-framework expansion utilizing the 4,10-dibromo sites of the TQI unit, could further improve the carrier transport. In conclusion, a planar and highly strong electron-accepting unit, TQI, has been designed and successfully synthesized via a stepwise cyclization approach to afford imide functionality. TQIbased molecules 1−3 have deep LUMO levels that allow for airstable n-channel conduction. In the single crystals, molecules 1− 3 adopt fully planar geometries, resulting in two-dimensional carrier transport. The thin films based on 1−3 exhibit FET performance, i.e., n-channel transport for 1 and 2 and ambipolar transport for 3. The device based on 1 exhibit a moderate airstable n-channel mobility of 4.4 × 10−2 cm2 V−1 s−1. We believe that the newly developed TQI unit has versatile synthetic functionality for use as a promising electron-accepting unit to create high-performance semiconducting materials.

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Aid for Scientific Research (C) (No. 26410087) from the Ministry of Education, Culture, Sports, Science and Technology (for M.A.). Authors are thankful to Dr. Hiroyasu Sato, Rigaku Corporation, for X-ray single crystal structure analyses. The synchrotron radiation experiments were performed at BL45XU in SPring-8 with the approval of JASRI (Proposal No. 2015B1690).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b01424. Experimental details, characterization data, NMR, EI-MS, and FT-IR spectra of all new compounds (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Tsukasa Hasegawa: 0000-0002-6311-2988 Hidetoshi Matsumoto: 0000-0002-4949-1184 Notes

The authors declare no competing financial interest. Crystallographic data for 1−3 can be found at CCDC 1548527− 1548529.

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ACKNOWLEDGMENTS This study was partially supported by funding the Development of Human Resources in Science and Technology of the Japan Science and Technology Agency, JST (for H.M.) and a Grant-in3278

DOI: 10.1021/acs.orglett.7b01424 Org. Lett. 2017, 19, 3275−3278