Critical Role of Molecular Symmetry for Charge Transport Properties

Article pubs.acs.org/cm

Critical Role of Molecular Symmetry for Charge Transport Properties: A Paradigm Learned from Quinoidal Bithieno[3,4‑b]thiophenes Longbin Ren,†,∥ Dafei Yuan,†,∥ Eliot Gann,‡,§ Yuan Guo,†,∥ Lars Thomsen,‡ Christopher R. McNeill,§ Chong-an Di,† Yuanping Yi,† Xiaozhang Zhu,*,†,∥ and Daoben Zhu†,∥ †

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia § Department of Materials Science and Engineering, Monash University, Clayton, Victoria 3800, Australia ∥ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: High molecular symmetry is always observed in highperformance organic semiconductors. However, whether it is an essential factor for molecular design is unclear. In this work, we designed and synthesized three quinoidal isomers, QBTT-o, QBTT-i, and QBTT-s, with different sulfur orientations and a stable E configuration to investigate the relationship between the structure symmetry and organic thin-film transistor performance. We found that QBTT-o and QBTT-i with high C2h symmetry exhibit electron mobilities of 0.02 and 0.15 cm2 V−1 s−1, respectively, while QBTT-s exhibits an unexpectedly high electron mobility of 0.32 cm2 V−1 s−1 with Ion/Ioff ratios of ≤106. The enhanced electron mobilities from QBTT-o and QBTT-i to QBTT-s can be attributed to the different sulfur orientations, especially, molecular symmetry. The thin-film microstructures of three QBTTs were systematically investigated by grazing incidence wide-angle Xray scattering, near-edge X-ray absorption fine structure spectroscopy, atomic force microscopy, and molecular dynamic simulations. The crystallinities are critically dependent on sulfur orientations and increase from QBTT-o to QBTT-i to QBTT-s, which agrees well with the organic thin-film transistor (OTFT) performance. The poor OTFT performance of QBTT-o compared to that of QBTT-i with the same C2h symmetry can be attributed to the different sulfur orientations; meanwhile, we speculate that the strongest crystallinity of QBTT-s might be attributed to the weak dipole moment that originated from the asymmetric molecular structure. Therefore, molecular symmetry is an important issue that needs to be carefully considered for the design of high-performance organic semiconductors.

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INTRODUCTION With important features such as low cost, solution processability, and flexibility, organic thin-film transistors (OTFTs)1,2 have demonstrated great potential for versatile applications such as organic integrated circuits, identification tags, chemical/ biological sensors, memory devices, and flexible displays.3−9 Over the past few decades, the development of high-mobility small-molecule semiconductors10,11 has allowed a deep understanding of the relationship between molecular structure and charge transport properties because of their well-defined molecular structures. To date, substantial progress has been made on p-type semiconductors, and high hole mobilities have been achieved.12−15 However, only a few solution-processed nchannel materials have achieved >1.0 cm2 V−1 s−1 in OTFTs.16−21 As is widely recognized, the development of innovative electron-deficient molecular frameworks has played a pivotal role in the development of high-performance n-type organic semiconductors22 such as N-heteropentacenes,23 BDOPVs,24 NDI derivatives,25 and 2DQTTs.26 Recently, it © 2017 American Chemical Society

has been found that the regiochemistry of conjugated frameworks27 containing asymmetric π-building blocks can be very important in determining electronic performance through tuning of solid-state packing, charge distribution, frontier orbital energy levels, and solubility. To date, most molecular frameworks of n-type semiconductors are symmetric.16 Few researchers have studied the effect of broken symmetry on OTFT performance, which may be due to the scarcity of suitable molecular models. Molecular symmetry is closely connected to regiochemistry. Bazan et al. have speculated that the net dipole moment caused by the relative orientation of two [1,2,5]thiadiazolo[3,4-c]pyridine acceptor units within the framework of a donor molecule has an influence on the self-assembling behavior of this molecule and the resulting p-type OTFT performance.28 Yamaguchi et al. Received: April 14, 2017 Revised: May 11, 2017 Published: May 11, 2017 4999

DOI: 10.1021/acs.chemmater.7b01551 Chem. Mater. 2017, 29, 4999−5008

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Chemistry of Materials

to investigate the effect of molecular regiochemistry on OTFT properties and their origin. These three compounds share almost the same molecular framework except the sulfur orientation. Although, compared with the parent QBT, all the compounds exhibit electron mobilities dramatically increased by 3−4 orders of magnitude, it is more interesting that the electron mobilities are highly dependent on the sulfur orientations. OTFTs based on QBTT-o, QBTT-i, and QBTT-s displayed electron mobilities of ≤0.02, ≤0.15, and ≤0.32 cm2 V−1 s−1, respectively, with current on/off ratios (Ion/ Ioff) of ≤108. Despite the fact that the two symmetric molecules exhibit differing performances, it is worth noting that the best performance was obtained for QBTT-s with the lowest symmetry. Differing from the zero dipole moment of centrosymmetric QBTT-o and QBTT-i, a dipole moment of 1.035 D (Figure 1b) was observed for only QBTT-s based on density functional theory (DFT) calculations, which can be attributed to the asymmetric molecular alignment. We deduce that the weak dipole moment of QBTT-s can induce a favorable molecular layout and promote crystallinity by dipole− dipole interactions. According to detailed film morphology analysis, we found that the different sulfur orientations have a crucial impact on the microstructure of QBTT films.

implied that the dipole moment associated with an asymmetric molecular structure promotes a different molecular packing resulting in isotropic transfer integrals higher than those of pentacene and rubrene, both representative p-type organic semiconductors.29 Additionally, Würthner et al. revealed a highly dipolar merocyanine dye for single-crystal FETs with a high hole mobility, which explicitly demonstrated that the high dipolarity of asymmetric molecules is not necessarily unfavorable for organic semiconductors.30 However, it is unclear which kind of symmetry is best for achieving highperformance OTFTs. In light of these observations, the creation of a molecular platform for elucidating relationships between molecular symmetry and device performance is highly desirable. In spite of the scarcity of examples, asymmetric diketopyrrolopyrrole31 and benzodithiophene building blocks32 were recently employed in OTFTs or organic photovoltaic devices that even surpassed the homologous building blocks in some aspects such as favorable morphology and better processability with a nonchlorinated solvent. Quinoidal oligothiophenes (QOTs) have been recognized as model n-type semiconductors because of their intrinsically high electron affinity.33−37 Quinoidal bithiophene (QBT) was first synthesized by Ogura et al. in 198738 as E/Z isomers39 and was used for n-channel OTFTs with a low electron mobility (μe) of 4.1 × 10−5 cm2 V−1 s−1 as reported by Kunugi et al. in 2004.40 Frisbie et al. studied the OTFT performance of quinoidal terthiophene (QTT) with an extended π-conjugation, giving a high μe of 0.2 cm2 V−1 s−1.41 Recently, we have developed a new molecular framework, namely, two-dimensional expanded quinoidal terthiophene, 2DQTT,26 and found that the sulfur orientation can significantly affect electron mobility.42 However, E,E- and E,Z-stereoisomers were observed in 2DQTT-o-B that had the best performance. We report herein the design and synthesis of quinoidal bithieno[3,4-b]thiophenes with different regiochemistry, central symmetric QBTT-o and QBTT-i and asymmetric QBTT-s (Figure 1b), with a stable E-configuration

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RESULTS AND DISCUSSION Synthesis of QBTTs. The general procedure for the synthesis of the QBTTs is shown in Scheme 1. Key intermediates 2−5 were prepared from 2-(2-hexyldecyl)thieno[3,4-b]thiophene (TbT) according to our previously reported procedures.26,43,44 TbT dimers 6−8 were synthesized from 2 Scheme 1. Synthesis of QBTT-o-HD, QBTT-i-HD, and QBTT-s-HDa

Figure 1. (a) Molecular structures of QBT isomers. (b) Molecular structures and electronic surface potentials of QBTT-o, QBTT-i, and QBTT-s (the dipole moment of QBTT-s is indicated by the arrow). Calculations were conducted at the DFT//B3LYP/6-31G** level. Alkyl substituents were replaced with methyl groups to simplify the calculations.

a Reagents and conditions: (a) NIS, CH2Cl2, room temperature (rt); (b) (i) n-BuLi, THF, −78 °C; (ii) tributylchlorostannane; (c) (i) nBuLi, THF, −78 °C; (ii) diiodoethane; (d) Pd(PPh3)4, toluene/DMF, 100 °C; (e) (i) malonitrile, NaH, Pd(PPh3)4, THF, 60 °C; (ii) HCl, DDQ, rt.

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Chemistry of Materials and 3, 2 and 4, and 4 and 5 by Stille coupling reaction in 90, 70, and 56% yields, respectively. The diiodide intermediates were prepared by deprotonation of compounds 6−8 with nbutyllithium in THF at −78 °C and iodization with diiodoethane in approximately 100% yields, which were used in the next step without further purification. QBTT-o was synthesized by Pd-catalyzed coupling between compound 6 and sodium dicyanomethanide followed by DDQ oxidation in 30% yield as a deep gray solid. Following similar procedures, QBTTi and QBTT-s were obtained as a golden yellow solid and a green solid, respectively. All QBTTs were unambiguously confirmed by NMR, high-resolution mass, and elemental analysis and show almost the same thermal stability with a high decomposition temperature of ∼330 °C according to thermogravimetric analysis (TGA) (Table 1 and Figure S1). Table 1. Photophysical, Electrochemical, and Thermal Properties of QBTT-o, QBTT-i, and QBTT-s compd

λmaxabsa (nm)

εb (cm−1 M−1)

λmaxabsc (nm)

Φ (%)

ELUMOd (eV)

Tde (°C)

QBTT-o QBTT-i QBTT-s

542 577 558

5.00 5.04 4.95

582 661 603

0.1 1.2 0.4

−4.05 −4.09 −4.10

331 332 330

a

Measured in dichloromethane. bLogarithmic values of absorption coefficients. cMeasured in the thin-film state. dThe LUMO energy levels were determined by the equation ELUMO = −[4.80 + Ered1/2 − E(Fc/Fc+)1/2] eV. eThe Td (decomposition temperature) was the temperature at a 5 wt % loss.

QBTT-o and QBTT-i have a high C2h symmetry with a stable and coplanar E-configuration (Figure 1b) according to their simple NMR spectra that do not evolve with time [δ 7.25 (QBTT-o) and 7.54 (QBTT-i) in the aromatic region]. By contrast, two singlet peaks were observed for QBTT-s, δ 7.56 and 7.20 which can be attributed to an asymmetric molecular structure instead of the isomerization process. The solubility of QBTTs is sufficiently high in chloroform to facilitate the formation of thin films. Photophysical and Electrochemical Properties of QBTTs. The photophysical and electrochemical properties of QBTTs are summarized in Table 1. The absorption spectra of QBTTs in diluted dichloromethane solution and thin films are shown in Figure 2a. QBTTs show structured absorption features in the visible region, 542 nm (QBTT-o), 577 nm (QBTT-i), and 558 nm (QBTT-s), with similarly high absorption coefficients (ε) of ∼105 cm−1 M−1. The absorption maximum of QBTT-s lies between those of the symmetric QBTT-o and QBTT-i. Compared with the absorption in solution, there are different bathochromic shifts in thin films. The maximal absorption of QBTT-i is bathochromically shifted to 661 nm (2202 cm−1), which suggests strong J aggregation,45 while the absorption maximum is less shifted for QBTT-o and QBTT-s by 40 nm (1268 cm−1) and 45 nm (1337 cm−1), respectively. In the cyclic voltammograms calibrated by ferrocene (Figure 2b), QBTTs exhibit two typical reversible reduction peaks, which demonstrates similar LUMO energy levels below −4.05 eV. Asymmetric QBTT-s possesses the lowest LUMO energy level of −4.10 eV. The fluorescence spectra of QBTTs in dichloromethane are shown in Figure 2c, and the fluorescence quantum yields (Φ) are listed in Table 1. QBTT-i with a strong J aggregation possesses the highest fluorescence quantum yield.

Figure 2. (a) Ultraviolet−visible−near infrared absorption spectra in dilute dichloromethane (S) and in thin film (F), (b) cyclic voltammograms, and (c) fluorescence spectra of QBTT-o, QBTT-i, and QBTT-s.

OTFT Performance of QBTTs. To investigate the effect of the sulfur orientation on the charge transport properties of QBTTs, bottom-gate top-contact (BGTC) OTFTs devices based on QBTT-o, QBTT-i, and QBTT-s were fabricated by spin coating from chloroform solutions and characterized under nitrogen. The thin films were deposited on octadecyltrichlorosilane (OTS)-modified SiO2 (300 nm)/Si substrates with Au used as the source-drain electrodes. The ISD − VG curves with fluctuation in Figure S3 at −40 to 0 V segments of VGS display a unipolar n-channel feature. The field effect mobility (μe) was extracted from the saturation regime of transfer curves and calculated by the linear fitting of (IDS)1/2 versus VG curves. The OTFT performances were examined with thermal annealing at 80, 140, and 160 °C. Figure 3 shows the typical transfer and output characteristics of QBTT-o, QBTT-i, and QBTT-s annealing at 140 °C, respectively. The corresponding device parameters are outlined in Table 2. For QBTT-o, the electron mobility is enhanced by annealing at higher temperatures, increasing from 4 × 10−5 cm2 V−1 s−1 (80 °C) to 0.02 cm2 V−1 s−1 (160 °C). For QBTT-i and QBTT-s, the electron mobility considerably improves with annealing up to 140 °C but then decreases with annealing above 140 °C. The OTFT performance of QBTT-i exhibits a maximal μe of 0.15 cm2 V−1 s−1, while QBTT-s exhibits a maximal mobility of 0.32 cm2 V−1 s−1, with outstanding Ion/Ioff values of ≤106. In comparison with that of QBTT-o, the OTFT performance of QBTT-i with the same C2h symmetry is better, indicating that sulfur orientation 5001


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Figure 4. AFM images of thin films of (a−c) QBTT-o, (d−f) QBTT-i, and (g−i) QBTT-s after thermal annealing at (a, d, and g) 80 °C, (b, e, and h) 140 °C, and (c, f, and i) 160 °C.

temperature. Compared with that of QBTT-o, QBTT-i and QBTT-s exhibit relatively large and well-defined domains with increasing RMS values of ∼0.58 and ∼1.41 Å (80 °C), ∼1.50 and ∼2.57 Å (140 °C), and ∼2.32 and ∼4.07 Å (160 °C), respectively, which suggests an increased crystallinity from QBTT-o to QBTT-s. The cracks on the thin-film surface become wider and deeper at a high temperature, 160 °C, which could be the reason for the decrease in mobility observed for this annealing temperature. Compared with QBTT-i and QBTT-o, QBTT-s displayed the largest domain size and best film continuity with the fewest cracks. The different film features observed from this AFM analysis are in conformity with the device performance of QBTTs. Two-dimensional grazing incidence wide-angle X-ray scattering (2D GIWAXS) was performed on QBTT thin films to gain insight into the effect of chemical structure on molecular packing. The GIWAXS patterns of thin films annealed at 80 and 140 °C are presented in Figure 5. In general, GIWAXS patterns are characterized by multiple orders of lamellar stacking peaks along the qz axis and a series of vertical Bragg rods. Via comparison of the patterns of films annealed at 80 and 140 °C, similar diffraction features are observed at both temperatures for a given molecule but with peaks becoming more well-defined at 140 °C indicating an increase in crystallinity. The orientation of the lamellar stacking peaks vertically along the qz axis indicates that the molecules in

Figure 3. Transfer and output characteristics of BGTC OTFTs based on (a and b) QBTT-o, (c and d) QBTT-i, and (e and f) QBTT-s with a channel length of 100 μm.

has an influence on the electron transport property. By contrast to those of symmetric QBTT-o and QBTT-i, the OTFT performance of asymmetric QBTT-s with a dipole moment of 1.035 D is the best, suggesting that the dipole moment might have a notable positive influence. The electron mobilites of QBTTs decreased slightly under ambient conditions, which can be ascribed to the relatively high LUMO energy levels of more than −4.1 eV.46−49 Thin-Film Morphology Analysis. To understand the influence of regiochemistry on OTFT performance, thin films of QBTTs as prepared in OTFTs were characterized by atomic force microscopy (AFM) annealed at 80, 140, and 160 °C as shown in Figure 4. QBTT-o films exhibit small domain sizes with root-mean-square (RMS) values of ∼0.93 Å (80 °C), ∼2.04 Å (140 °C), and ∼2.90 Å (160 °C). The domain size of QBTT-o slightly grows with an increase in annealing

Table 2. OTFT Performance of QBTTs with a BGTC Device Structure compd

annealing temperature (°C)

electron mobility (μe)a (cm2 V−1 s−1) [max (avg)]

threshold voltage (VTh) (V) [min (avg)]

QBTT-o

80 140 160 80 140 160 80 140 160

0.000040 (0.000029) 0.014 (0.010) 0.020 (0.015) 0.015 (0.011) 0.15 (0.10) 0.07 (0.05) 0.022 (0.020) 0.32 (0.20) 0.21 (0.12)

15 (20) 20 (25) 21 (25) 8.0 (10) 14 (15) 14 (15) 9.0 (10) 7.0 (12) 10 (11)

QBTT-i

QBTT-s

a

Ion/Ioff [max (avg)] 105 106 108 108 107 107 106 106 107

(105) (105) (107) (107) (107) (106) (105) (105) (106)

Typical device characteristics obtained from more than 30 devices. 5002


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Figure 5. GIWAXS patterns for thin films of (a and b) QBTT-o, (c and d) QBTT-i, and (e and f) QBTT-s at thermal annealing temperatures of (a, c, and e) 80 °C and (b, d, and f) 140 °C.

general exhibit an edge-on stacking of the planar π-conjugated framework with respect to the substrate. Interestingly, QBTT-o shows a mixed orientation at 80 °C with a second population of crystallites oriented with the lamellar stacking direction at 45° to the substrate. With annealing to 140 °C, the additional lamellar stacking peaks at this different azimuthal orientation disappear with a single population of edge-on crystallites remaining. Via comparison of the lamellar stacking distances, d spacings of 2.51, 2.32, and 2.34 nm (80 °C) and 2.49, 2.33, and 2.34 nm (140 °C), respectively, are found corresponding to the spacing of 2D layers of the π-conjugated framework separated by the alkyl side chains. QBTT-o has a lamellar d spacing notably larger than those of QBTT-i and QBTT-s, suggesting a distinct molecular packing. Via examination of the position of the vertical Bragg rods, QBTT-o also has Bragg rods located at very different locations compared to those of QBTT-i, and QBTT-s, which have Bragg rods that are similarly located. In particular, QBTT-o has a prominent low-q Bragg rod located at qxy ∼ 7 nm−1 compared to a spacing of qxy ∼ 6.5 nm−1 for the prominent low-q Bragg rod in QBTT-i and QBTT-s. Furthermore, QBTT-i and QBTT-s have prominent scattering features located at qxy ∼ 16 nm−1 that are completely absent in QBTT-o. Another major difference between QBTT-o and the other two molecules is the smearing of the Bragg rods along the qz direction for QBTT-o compared to the more well-defined scattering peaks seen in the Bragg rods for QBTT-i and QBTT-s. This observation suggests that while there is longrange translational order within the molecular planes of QBTTo there is only one-dimensional translational order between molecular planes of QBTT-o vertically. We have attempted to index the unit cells corresponding to observed scattering features, with the proposed unit cells summarized in Table 3 based on the scattering patterns observed at 140 °C (see also Figure S4). In the case of QBTT-s, the clarity of the scattering pattern has permitted a refined fitting of the unit cell, providing confidence in the unit cell obtained. In this case, a triclinic unit

Table 3. Proposed Unit Cell Parameters for QBTT Films Annealed at 140 °C

a

material

a (Å)

b (Å)

c (Å)

α (deg)

β (deg)

γ (deg)

volume (Å3)

QBTT-o QBTT-ia QBTT-ib QBTT-s

26.0 24.1 24.0 23.4

9.1 9.7 9.8 9.8

5.7 5.4 5.7 5.5

93.5 81.6 77.4 80.7

91.4 102.3 102.7 91.5

105 89.1 89.2 93

1305 1263 1323 1275

Polymorph 1. bPolymorph 2.

cell is obtained with a single molecule per unit cell. Triclinic cells are proposed for the other molecules consistent with the scattering features observed. Interestingly, QBTT-i has more vertical Bragg rods than QBTT-s does, which is interpreted in terms of two polymorphs. These two polymorphs have similar parameters, although the second polymorph has a notably larger c spacing and a slightly larger volume. For QBTT-o, the larger lamellar spacing along the a axis is compensated by a shorter spacing along the b axis. Interestingly, relative to QBTT-s and the dominant polymorph of QBTT-i, QBTT-o actually has a larger spacing along the c axis, suggesting a larger π-stacking of molecules. Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy has also been performed to characterize the molecular orientation within the thin films. Angle-resolved spectra were recorded with the incident X-ray angle varied from 20° (glancing) to 90° (normal to the substrate) (see Figure 6). The low-energy peaks around 285 eV are associated with transitions from the 1s core state to π* antibonding orbitals, with the dichroism observed providing information about the average tilt angle of the planar π-conjugated framework with respect to the substrate. From analysis of how the intensity of the π* manifold varies with X-ray angle (see Experimental Section), the average tilting of the planar core relative to the substrate, ⟨γ⟩, was determined. ⟨γ⟩ values of 52.0°, 65.4°, and 5003


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Figure 6. NEXAFS spectra of optimized thin films for (a and b) QBTT-o, (c and d) QBTT-i, and (e and f) QBTT-s at thermal annealing temperatures of (a, c, and e) 80 °C and (b, d, and f) 140 °C.

64.8° for QBTT-o, QBTT-i, and QBTT-s, respectively, were calculated for films annealed at 80 °C, compared to values of 52.6°, 65.5°, and 64.6°, respectively, for films annealed at 140 °C. These values in general indicate an edge-on tilting of the planar framework with a ⟨γ⟩ value of 90° corresponding to a perfectly edge-on orientation and a ⟨γ⟩ value of 0° corresponding to a perfectly face-on orientation. In general, annealing is found to have little effect on the average tilt angle.50−52 However, a distinctly lower tilt angle of ∼52° is observed for QBTT-o compared to a value of ∼65° for both QBTT-i and QBTT-s. This observation further confirms a distinct molecular packing geometry for QBTT-o compared to those of the other two molecules. Interestingly, the tilt angles observed for QBTT-i and QBTT-s are similar to the optimal tilt angles found for NDI3HU-DTYM2 (⟨γ⟩ = 66°)17 and 2DQTT-o-B (⟨γ⟩ = 68.2°).42 To gain deeper insight into molecular packing, we have performed molecular dynamics simulations to mimic the molecular self-assembly with solvent evaporation following thermal annealing treatment (see calculation details in the Supporting Information). The bulk stacking natures can be quantified by the center-of-mass radial distribution functions (RDF) of thieno[3,4-b]thiophene moieties of the QBTT molecules (see Figure 7). Here, we note that the first peak at a distance of ∼0.39 nm can approximately represent π−π stacking interaction between two adjacent molecules. Obviously, the first RDF peak of QBTT-s exhibits the strongest intensity and is located at the shortest distance among all three types of molecules, indicating the most compact packing of QBTT-s. Relatively, the peak intensity is much weaker for QBTT-o than for QBTT-i and QBTT-s. These RDF results are quite consistent with the calculated densities for the three types of molecules: QBTT-s (1.040 g/cm3) > QBTT-i (1.039 g/cm3) > QBTT-o (1.036 g/cm3). Discussion. On the basis of the selective functionalization of TbT at positions 4 and 6, we prepared three QBTT isomers with different sulfur orientations. Unlike the parent QBT bearing mixed and tautomeric E- and Z-configurations, QBTTs

Figure 7. Center-of-mass radial distribution functions of thieno[3,4b]thiophene moieties of the QBTT molecules at an optimized annealing temperature of 140 °C.

exhibit the E-configuration exclusively, which is thermodynamically stable. QBTTs can be applied for n-channel OTFT applications because of the high electron affinity. Upon device optimization, we found that symmetric QBTT-o and QBTT-i exhibited electron mobilities of 0.02 and 0.15 cm2 V−1 s−1, respectively, while QBTT-s exhibited an unexpectedly high electron mobility of 0.32 cm2 V−1 s−1 with superior Ion/Ioff ratios of ≤106. The different device performance can be closely related to the sulfur orientations. Differing from those of the polymers, the semiconducting property of small molecules is mainly dependent on intermolecular charge transport. In the AFM images at the optimal annealing temperature, QBTT-o showed an unfavorably small domain size, which is different from those of QBTT-i and QBTT-s. Simultaneously, QBTT-s possessed a larger crystalline domain and better continuity compared to those of QBTT-i. Compared with the other two molecules, QBTT-o showed the largest d spacing, a different location of the smearing vertical Bragg rods, disparate triclinic 5004


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dissolved in anhydrous THF (8 mL) under a nitrogen atmosphere and cooled to −78 °C, and n-butyllithium (0.84 mL, 1.34 mmol, 1.60 M in hexane) was then added via a syringe. After the mixture had been stirred at −78 °C for 0.5 h, 1,2-diiodoethane (0.378 g, 1.34 mmol) was added with continued stirring at −78 °C for 0.5 h, before the reaction solution was warmed to room temperature over 0.5 h. The reaction was quenched with a few drops of a saturated NH4Cl solution, and the mixture was extracted with hexane and washed with a saturated NaHSO3 solution. The organic layer was combined, dried over MgSO4, and concentrated under reduced pressure to give 0.595 g of compound 5 in 95% yield as a yellow oil without further purification: 1 H NMR (400 MHz, CDCl3) δ (s, 1H), 6.69 (s, 1H), 2.68−2.66 (d, 3J = 8.0 Hz, 2H), 1.66 (br, 1H), 1.30−1.26 (m, 24H), 0.89−0.86 (t, 3J = 8.0 Hz, 6H). Tributyl[2-(2-hexyldecyl)thieno[3,4-b]thiophen-4-yl]stannane (3). Compound 2 (0.213 g, 0.43 mmol) was dissolved in anhydrous THF (4 mL) under a nitrogen atmosphere and cooled to −78 °C, and nbutyllithium (0.28 mL, 0.45 mmol, 1.60 M in hexane) was then added via a syringe. After the mixture had been stirred at −78 °C for 0.5 h, tributylchlorostannane (0.146 g, 0.45 mmol) was added with continued stirring at −78 °C for 0.5 h, before the reaction solution was warmed to room temperature over 0.5 h. The reaction was quenched with a few drops of a saturated NH4Cl solution, and the mixture was extracted with hexane. The organic layer was combined, dried over MgSO4, and concentrated under reduced pressure to give 0.28 g of compound 3 in 98% yield as a yellow oil without further purification: 1H NMR (400 MHz, CDCl3) δ 7.41 (s, 1H), 6.51 (s, 1H), 2.70−2.68 (d, 3J = 8.0 Hz, 2H), 1.65 (br, 1H), 1.62−1.56 (m, 6H), 1.37−1.26 (m, 30H), 0.92−0.86 (m, 15H). Tributyl[2-(2-hexyldecyl)thieno[3,4-b]thiophen-6-yl]stannane (4). Yellow oil in 98% yield without further purification: 1H NMR (400 MHz, CDCl3) δ 7.39 (s, 1H), 6.62 (s, 1H), 2.69−2.67 (d, 3J = 8.0 Hz, 2H), 1.62 (br, 1H), 1.60−1.55 (m, 6H), 1.31−1.26 (m, 30H), 0.91− 0.88 (m, 15H). 2,2′-Bis(2-hexyldecyl)-4,4′-bithieno[3,4-b]thiophene (6). Compounds 2 (0.213 g, 0.43 mmol) and 3 (0.301 g, 0.43 mmol) and Pd(PPh3)4 (25 mg, 0.022 mmol) were added to an oven-dried roundbottom flask in dry DMF (1 mL) and toluene (1 mL) under a nitrogen atmosphere. The mixture was stirred and refluxed overnight in the dark. The reaction mixture was then cooled to room temperature, the reaction quenched with a saturated KF solution, and the mixture extracted with hexane. The organic layer was combined, dried over MgSO4, and concentrated under reduced pressure. The crude product was purified using silica-gel column chromatography (hexane) to give 0.291 g of products in 90% yield as a brown oil: 1H NMR (400 MHz, CDCl3) δ 7.09 (s, 2H), 6.91 (s, 2H), 2.73−2.71 (d, 3J = 8.0 Hz, 4H), 1.69 (br, 2H), 1.32−1.26 (m, 48H), 0.88−0.85 (t, 3J = 8.0 Hz, 6H); 13 C NMR (100 MHz, CDCl3) δ 153.0, 143.5, 139.9, 122.1, 115.3, 109.2, 39.3, 37.0, 33.5, 32.1, 30.2, 29.8, 29.6, 26.8, 22.9, 14.3; HRMS (MALDI-TOF) calcd for C 44H70S 4 [M]+ 726.435488, found 726.435309. 2,2′-Bis(2-hexyldecyl)-4,6′-bithieno[3,4-b]thiophene (7). Brown oil in 70% yield: 1H NMR (300 MHz, CDCl3) δ 7.13 (s, 1H), 7.10 (s, 1H), 6.99 (s, 1H), 6.60 (s, 1H), 2.75−2.71 (m, 4H), 1.71 (br, 2H), 1.31−1.26 (m, 48H), 0.89−0.87 (m, 12H); HRMS (MALDI-TOF) calcd for C44H70S4 [M]+ 726.435488, found 726.435404. 2,2′-Bis(2-hexyldecyl)-6,6′-bithieno[3,4-b]thiophene (8). Brown oil in 56% yield: 1H NMR (400 MHz, CDCl3) δ 7.17 (s, 2H), 6.61 (s, 2H), 2.74−2.72 (d, 3J = 8.0 Hz, 4H), 1.72 (br, 2H), 1.32−1.25 (m, 48H), 0.87−0.86 (t, 3J = 4.0 Hz, 12H); 13C NMR (75 MHz, CDCl3) δ 153.0, 148.0, 134.4, 122.3, 115.0, 109.2, 39.2, 36.9, 33.5, 32.1, 31.8, 30.2, 29.8, 29.6, 27.2, 26.8, 22.9, 14.3; HRMS (MALDI-TOF) calcd for C44H70S4 [M]+ 726.435488, found 726.435868. (E)-2,2′-[2,2′-Bis(2-hexyldecyl)-6H,6′H-(4,4′-bithieno[3,4-b]thiophenylidene)-6,6′-diylidene]dimalononitrile (QBTT-o). Compound 6 (0.484 g, 0.66 mmol) was dissolved in anhydrous THF (12 mL) under a nitrogen atmosphere and cooled to −78 °C, and nbutyllithium (0.96 mL, 1.53 mmol, 1.60 M in hexane) was then added via a syringe. After the mixture had been stirred at −78 °C for 0.5 h, 1,2-diiodoethane (0.413 g, 1.46 mmol) was added with continued

unit cell parameters, and the smallest tilt angles according to the GIWAXS and NEXAFS measurements. These observations indicated that QBTT-o had the lowest crystallinity and an unfavorable molecular layout, which is detrimental for effective charge transport even though QBTT-o has the same C2h symmetry as QBTT-i. While QBTT-i and QBTT-s apparently possessed similar microstructures, QBTT-s exhibits the highest mobility. The GIWAXS patterns of QBTT-s showed peaks that were sharper and more well-defined than those of QBTT-i at both annealing temperatures, indicating a higher degree of crystallinity. Furthermore, two distinct polymorphs are observed in QBTT-i, which will result in an increased level of energetic disorder within the film. The highest crystallinity of QBTT-s is further disclosed by the molecular dynamics simulations, which showed the strongest intensity of π−π stacking and the largest bulk molecular density. We consider the highest degree of order in the QBTT-s films might be attributed to the dipole moment deriving from its asymmetric structure. Thus, from this work, QBTT-s exhibits the best performance of all the isomers, and we regard the dipole− dipole interactions to be a persuasive argument for rationalizing this issue.

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CONCLUSION We have successfully designed and synthesized a series of quinoidal isomers with different sulfur orientations, QBTT-o, QBTT-i, and QBTT-s, which exhibit very different n-type charge transport properties. QBTT-s displayed the highest electron mobility of ≤0.32 cm2 V−1 s−1 and an Ion/Ioff of ≤106, which is among the highest level for n-type solution-processed OTFTs based on asymmetric small molecules.53−57 GIWAXS, NEXAFS spectroscopy, AFM, and molecular dynamics simulations have been utilized to investigate the relationship between the regiochemistry, including the molecular symmetry, and the performance of QBTT-based transistors, which reveals that the crystallinity of QBTTs is critically dependent on both sulfur orientation and molecular symmetry. QBTT-o was inferior to QBTT-i with the same C2h symmetry, which is attributed to the different sulfur orientation. On the basis of further analysis of QBTT-i and QBTT-s, we speculate that the dipole moment caused by asymmetry may be the origin of the significantly enhanced performance of QBTT-s-based OTFTs. The further exploitation of molecular symmetry is therefore a promising strategy for obtaining high-performance OTFTs.

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EXPERIMENTAL SECTION

Synthesis of Materials. 2-(2-Hexyldecyl)-4-iodothieno[3,4-b]thiophene (2). 2-(2-Hexyldecyl)thieno[3,4-b]thiophene (0.291 g, 0.80 mmol) was dissolved in CH2Cl2 (10 mL), and 1-iodopyrrolidine-2,5dione (NIS) (0.182 g, 0.81 mmol) was then added in the dark. After the mixture had been stirred at room temperature for 1.5 h, the reaction was quenched with a few drops of a saturated NaHSO3 solution, and the mixture was extracted with hexane and washed with a saturated NaHSO3 solution. The organic layer was combined, dried over MgSO4, and concentrated under reduced pressure. The crude product was purified using silica-gel column chromatography (hexane) to give 0.284 g of compound 2 in 72% yield as a yellow oil: 1H NMR (400 MHz, CDCl3) δ 7.26 (s, 1H), 6.41 (s, 1H), 2.68−2.67 (d, 3J = 4.0 Hz, 2H), 1.66 (br, 1H), 1.30−1.26 (m, 24H), 0.89−0.86 (t, 3J = 8.0 Hz, 6H); 13C NMR (100 MHz, CDCl3) δ 154.0, 152.7, 137.9, 116.7, 115.4, 56.6, 39.2, 36.9, 33.5, 32.1, 32.1, 30.1, 29.8, 29.5, 26.8, 26.7, 22.9, 14.4. 2-(2-Hexyldecyl)-4-iodothieno[3,4-b]thiophene (5). 2-(2Hexyldecyl)thieno[3,4-b]thiophene (0.474 g, 1.30 mmol) was 5005


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Chemistry of Materials stirring at −78 °C for 0.5 h, before the reaction solution was warmed to room temperature over 0.5 h. The reaction was quenched with a few drops of a saturated NH4Cl solution, and the mixture was extracted with hexane and washed with a saturated NaHSO3 solution. The organic layer was combined, dried over MgSO4, and concentrated under reduced pressure to give 0.649 g of diiodates in 98% yield as a yellow oil without further purification. Then sodium hydride (0.213 g, 60%, 5.33 mmol) was added to a suspension of malononitrile (0.132 g, 2.00 mmol) in anhydrous THF (11.0 mL) under a nitrogen atmosphere, and the mixture was stirred for 15 min at room temperature. The diiodates (0.649 g, 0.66 mmol) and Pd(PPh3)4 (77 mg, 0.066 mmol) were added to this mixture, which was then heated under reflux. After 6 h, the reaction mixture was cooled to room temperature, diluted hydrochloric acid (1.0 M, 10 mL) was added slowly followed by 1,2-dichloro-4,5-dicyanobenzoquinone (0.302 g, 1.33 mmol), and the mixture was stirred at room temperature for 30 min. The resulting mixture was extracted with CH2Cl2, washed with brine, and dried over MgSO4. After evaporation of the solvent, the residue was purified using silica-gel column chromatography followed by recrystallization twice to give 0.095 g of QBTT-o in 26% yield as a deep gray solid: 1H NMR (300 MHz, CDCl3) δ 7.25 (s, 2H), 2.96− 2.94 (d, 3J = 8.0 Hz, 4H), 1.78 (br, 2H), 1.32−1.26 (m, 48H), 0.88− 0.86 (t, 3J = 3.0 Hz, 12H); 13C NMR (75 MHz, CDCl3) δ 164.9, 161.5, 151.1, 138.9, 126.9, 120.7, 113.5, 113.3, 66.3, 40.7, 37.1, 33.4, 32.1, 32.0, 30.1, 29.8, 29.7, 29.5, 26.64, 26.60, 22.9, 22.8, 14.3, 14.3; HRMS (MALDI-TOF) calcd for C50H68N4S4[M]− 852.433231, found 852.433309. Anal. Calcd for C50H68N4S4 (%): C, 70.37; H, 8.03; N, 6.57. Found: C, 70.27; H, 7.99; N, 6.61. (E)-2,2′-[2,2′-Bis(2-hexyldecyl)-4H,4′H-(6,6′-bithieno[3,4-b]thiophenylidene)-4,4′-diylidene]dimalononitrile (QBTT-i). Golden yellow solid in 30% yield: 1H NMR (400 MHz, CDCl3) δ 7.54 (s, 2H), 2.89−2.87 (d, 3J = 8.0 Hz, 4H), 1.75 (br, 2H), 1.31−1.25 (m, 48H), 0.88−0.85 (t, 3J = 8.0 Hz, 12H); 13C NMR (75 MHz, CDCl3) δ 162.8, 160.8, 147.5, 143.9, 124.8, 120.0, 113.5, 113.1, 69.0, 40.6, 36.8, 33.4, 32.1, 32.0, 30.1, 29.8, 29.5, 26.7, 26.6, 22.9, 22.8, 14.31, 14.29; HRMS (MALDI-TOF) calcd for C50H68N4S4[M]+ 852.432134, found 852.432088. Anal. Calcd for C50H68N4S4 (%): C, 70.37; H, 8.03; N, 6.57. Found: C, 70.14; H, 7.98; N, 6.66. (E)-2,2′-[2,2′-Bis(2-hexyldecyl)-4′H,6H-(4,6′-bithieno[3,4-b]thiophenylidene)-4′,6-diylidene]dimalononitrile (QBTT-s). Green solid in 24% yield: 1H NMR (400 MHz, CDCl3) δ 7.56 (s, 1H), 7.20 (s, 1H), 2.95−2.93 (d, 3J = 8.0 Hz, 2H), 2.90−2.88 (d, 3J = 8.0 Hz, 2H), 1.77 (br, 2H), 1.33−1.27 (m, 48H), 0.88−0.86 (t, 3J = 4.0 Hz, 12H); 13C NMR (75 MHz, CDCl3) δ 165.0, 163.0, 161.4, 160.9, 150.9, 147.7, 143.7, 138.9, 125.9, 125.7, 120.3, 119.9, 113.5, 113.4, 113.3, 113.1, 76.8, 68.7, 66.7, 40.7, 40.6, 37.1, 36.8, 33.4, 32.1, 32.0, 30.1, 29.9, 29.8, 29.7, 29.5, 26.7, 26.64, 26.60, 22.9, 22.8, 14.31, 14.29; HRMS (MALDI-TOF) calcd for C50H68N4S4 [M]− 852.433231, found 852.433933. Anal. Calcd for C50H68N4S4 (%): C, 70.37; H, 8.03; N, 6.57. Found: C, 70.17; H, 7.91; N, 6.50.

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Xiaozhang Zhu: 0000-0002-6812-0856 Daoben Zhu: 0000-0002-6354-940X Author Contributions

L.R. and D.Y. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Basic Research Program of China (973 Program) (2014CB643502), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12010200), and the National Natural Science Foundation of China (91333113 and 21572234) for financial support. C.R.M. acknowledges support from the Australian Research Council (DP130102616). This work was performed in part at the SAXS/WAXS and Soft X-ray beamlines at the Australian Synchrotron.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01551. Experimental methods, thermogravimetric analysis (TGA) curves, gate-dependent mobility distribution curves, GIWAXS images with indexed peaks, and NMR spectra of QBTTs (PDF)

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REFERENCES

(1) Wang, C.; Dong, H.; Hu, W.; Liu, Y.; Zhu, D. Semiconducting πConjugated Systems in Field-Effect Transistors: A Material Odyssey of Organic Electronics. Chem. Rev. 2012, 112, 2208−2267. (2) Sirringhaus, H. 25th Anniversary Article: Organic Field-Effect Transistors: The Path Beyond Amorphous Silicon. Adv. Mater. 2014, 26, 1319−1335. (3) Guo, Y.; Yu, G.; Liu, Y. Functional Organic Field-Effect Transistors. Adv. Mater. 2010, 22, 4427−4447. (4) Di, C. A.; Zhang, F.; Zhu, D. Multi-Functional Integration of Organic Field-Effect Transistors (OFETs): Advances and Perspectives. Adv. Mater. 2013, 25, 313−330. (5) Arias, A. C.; MacKenzie, J. D.; McCulloch, I.; Rivnay, J.; Salleo, A. Materials and Applications for Large Area Electronics: Solution-Based Approaches. Chem. Rev. 2010, 110, 3−24. (6) Sokolov, A. N.; Tee, B. C.-K.; Bettinger, C. J.; Tok, J. B.-H.; Bao, Z. Chemical and Engineering Approaches To Enable Organic FieldEffect Transistors for Electronic Skin Applications. Acc. Chem. Res. 2012, 45, 361−371. (7) Gelinck, G. H.; Huitema, H. E. A.; van Veenendaal, E.; Cantatore, E.; Schrijnemakers, L.; van der Putten, J. B. P. H.; Geuns, T. C. T.; Beenhakkers, M.; Giesbers, J. B.; Huisman, B.-H.; Meijer, E. J.; Benito, E. M.; Touwslager, F. J.; Marsman, A. W.; van Rens, B. J. E.; de Leeuw, D. M. Flexible Active-matrix Displays and Shift Registers Based on Solution-processed Organic Transistors. Nat. Mater. 2004, 3, 106− 110. (8) Yan, H.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J. R.; Dötz, F.; Kastler, M.; Facchetti, A. A High-mobility Electron-transporting Polymer for Printed Transistors. Nature 2009, 457, 679−686. (9) Anthony, J. E.; Facchetti, A.; Heeney, M.; Marder, S. R.; Zhan, X. n-Type Organic Semiconductors in Organic Electronics. Adv. Mater. 2010, 22, 3876−3892. (10) Mei, J.; Diao, Y.; Appleton, A. L.; Fang, L.; Bao, Z. Integrated Materials Design of Organic Semiconductors for Field-Effect Transistors. J. Am. Chem. Soc. 2013, 135, 6724−6746. (11) Usta, H.; Facchetti, A.; Marks, T. n-Channel Semiconductor Materials Design for Organic Complementary Circuits. Acc. Chem. Res. 2011, 44, 501−510. (12) Yuan, Y.; Giri, G.; Ayzner, A. L.; Zoombelt, A. P.; Mannsfeld, S. C.; Chen, J.; Nordlund, D.; Toney, M. F.; Huang, J.; Bao, Z. Ultra-high Mobility Transparent Organic Thin Film Transistors Grown by an Off-centre Spin-coating Method. Nat. Commun. 2014, 5, 3005−3013. (13) Park, J.-I.; Chung, J. W.; Kim, J.-Y.; Lee, J.; Jung, J. Y.; Koo, B.; Lee, B.-L.; Lee, S. W.; Jin, Y. W.; Lee, S. Y. Dibenzothiopheno[6,5b:6′,5′-f]thieno[3,2-b]thiophene (DBTTT): High-Performance SmallMolecule Organic Semiconductor for Field-Effect Transistors. J. Am. Chem. Soc. 2015, 137, 12175−12178.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Christopher R. McNeill: 0000-0001-5221-878X Yuanping Yi: 0000-0002-0052-9364 5006


Article

Chemistry of Materials (14) Kim, G.; Kang, S.; Dutta, G. K.; Han, Y.; Shin, T. J.; Noh, Y.; Yang, C. A Thienoisoindigo-Naphthalene Polymer with Ultrahigh Mobility of 14.4 cm2/V·s That Substantially Exceeds Benchmark Values for Amorphous Silicon Semiconductors. J. Am. Chem. Soc. 2014, 136, 9477−9483. (15) Yao, J.; Yu, C.; Liu, Z.; Luo, H.; Yang, Y.; Zhang, G.; Zhang, D. Significant Improvement of Semiconducting Performance of the Diketopyrrolopyrrole−Quaterthiophene Conjugated Polymer through Side-Chain Engineering via Hydrogen-Bonding. J. Am. Chem. Soc. 2016, 138, 173−185. (16) Zhao, Y.; Guo, Y.; Liu, Y. 25th Anniversary Article: Recent Advances in n-Type and Ambipolar Organic Field-Effect Transistors. Adv. Mater. 2013, 25, 5372−5391. (17) Zhang, F.; Hu, Y.; Schuettfort, T.; Di, C.-a.; Gao, X.; McNeill, C. R.; Thomsen, L.; Mannsfeld, S. C.; Yuan, B. W.; Sirringhaus, H.; Zhu, D. Critical Role of Alkyl Chain Branching of Organic Semiconductors in Enabling Solution-Processed N-Channel Organic Thin-Film Transistors with Mobility of up to 3.50 cm2 V−1 s−1. J. Am. Chem. Soc. 2013, 135, 2338−2349. (18) Kang, B.; Kim, R.; Lee, S. B.; Kwon, S.-K.; Kim, Y.-H.; Cho, K. Side-Chain-Induced Rigid Backbone Organization of Polymer Semiconductors through Semifluoroalkyl Side Chains. J. Am. Chem. Soc. 2016, 138, 3679−3686. (19) Lei, T.; Xia, X.; Wang, J.-Y.; Liu, C.-J.; Pei, J. Conformation Locked” Strong Electron-Deficient Poly(p-Phenylene Vinylene) Derivatives for Ambient-Stable n-Type Field-Effect Transistors: Synthesis, Properties, and Effects of Fluorine Substitution Position. J. Am. Chem. Soc. 2014, 136, 2135−2141. (20) Bucella, S. G.; Luzio, A.; Gann, E.; Thomsen, L.; McNeill, C. R.; Pace, G.; Perinot, A.; Chen, Z.; Facchetti, A.; Caironi, M. Macroscopic and High-throughput Printing of Aligned Nanostructured Polymer Semiconductors for MHz Large-area Electronics. Nat. Commun. 2015, 6, 8394−8403. (21) Park, J. H.; Jung, E. H.; Jung, J. W.; Jo, W. H. A Fluorinated Phenylene Unit as a Building Block for High-Performance n-Type Semiconducting Polymer. Adv. Mater. 2013, 25, 2583−2588. (22) Sun, B.; Hong, W.; Yan, Z.; Aziz, H.; Li, Y. Record High Electron Mobility of 6.3 cm2 V−1 s−1 Achieved for Polymer Semiconductors Using a New Building Block. Adv. Mater. 2014, 26, 2636−2642. (23) Liang, Z.; Tang, Q.; Xu, J.; Miao, Q. Soluble and Stable N -Heteropentacenes with High Field-Effect Mobility. Adv. Mater. 2011, 23, 1535−1539. (24) Dou, J.; Zheng, Y.; Yao, Z.; Yu, Z.; Lei, T.; Shen, X.; Luo, X.; Sun, J.; Zhang, S.; Ding, Y.; Han, G.; Yi, Y.; Wang, J.; Pei, J. FineTuning of Crystal Packing and Charge Transport Properties of BDOPV Derivatives through Fluorine Substitution. J. Am. Chem. Soc. 2015, 137, 15947−15956. (25) Gao, X.; Di, C.-a.; Hu, Y.; Yang, X.; Fan, H.; Zhang, F.; Liu, Y.; Li, H.; Zhu, D. Core-Expanded Naphthalene Diimides Fused with 2(1,3-Dithiol-2-Ylidene)Malonitrile Groups for High-Performance, Ambient-Stable, Solution-Processed n-Channel Organic Thin Film Transistors. J. Am. Chem. Soc. 2010, 132, 3697−3699. (26) Zhang, C.; Zang, Y.; Gann, E.; McNeill, C. R.; Zhu, X.; Di, C.; Zhu, D. Two-Dimensional π-Expanded Quinoidal Terthiophenes Terminated with Dicyanomethylenes as n-Type Semiconductors for High-Performance Organic Thin-Film Transistors. J. Am. Chem. Soc. 2014, 136, 16176−16184. (27) Zhang, C.; Zhu, X. The Impact of Regiochemistry of Conjugated Molecules on The Performance of Organic Electronic Devices. Chin. Chem. Lett. 2016, 27, 1357−1366. (28) Takacs, C. J.; Sun, Y.; Welch, G. C.; Perez, L. A.; Liu, X.; Wen, W.; Bazan, G. C.; Heeger, A. J. Solar Cell Efficiency, Self-Assembly, and Dipole−Dipole Interactions of Isomorphic Narrow-Band-Gap Molecules. J. Am. Chem. Soc. 2012, 134, 16597−16606. (29) Fukazawa, A.; Kishi, D.; Tanaka, Y.; Seki, S.; Yamaguchi, S. Diarylated Bi(thieno[2,3-c]thiophene)s: A Ring-Fusing Strategy for Controlling the Molecular Alignment of Oligoarenes. Angew. Chem., Int. Ed. 2013, 52, 12091−12095.

(30) Liess, A.; Stolte, M.; He, T.; Würthner, F. Single-crystal FieldEffect Transistors of a Highly Dipolar Merocyanine Dye. Mater. Horiz. 2016, 3, 72−77. (31) Ji, Y.; Xiao, C.; Wang, Q.; Zhang, J.; Li, C.; Wu, Y.; Wei, Z.; Zhan, X.; Hu, W.; Wang, Z.; Janssen, R. A. J.; Li, W. Asymmetric Diketopyrrolopyrrole Conjugated Polymers for Field-Effect Transistors and Polymer Solar Cells Processed from a Nonchlorinated Solvent. Adv. Mater. 2016, 28, 943−950. (32) Liu, D.; Zhu, Q.; Gu, C.; Wang, J.; Qiu, M.; Chen, W.; Bao, X.; Sun, M.; Yang, R. High-Performance Photovoltaic Polymers Employing Symmetry-Breaking Building Blocks. Adv. Mater. 2016, 28, 8490− 8498. (33) Casado, J.; Ponce Ortiz, R.; López Navarrete, J. T. Quinoidal Oligothiophenes: New Properties Behind an Unconventional Electronic Structure. Chem. Soc. Rev. 2012, 41, 5672−5686. (34) Pappenfus, T. M.; Chesterfield, R. J.; Frisbie, C. D.; Mann, K. R.; Casado, J.; Raff, J. D.; Miller, L. L. A π-Stacking TerthiopheneBased Quinodimethane Is an n-Channel Conductor in a Thin Film Transistor. J. Am. Chem. Soc. 2002, 124, 4184−4185. (35) Qiao, Y.; Guo, Y.; Yu, C.; Zhang, F.; Xu, W.; Liu, Y.; Zhu, D. Diketopyrrolopyrrole-Containing Quinoidal Small Molecules for High-Performance, Air-Stable, and Solution-Processable n-Channel Organic Field-Effect Transistors. J. Am. Chem. Soc. 2012, 134, 4084− 4087. (36) Xiong, Y.; Tao, J.; Wang, R.; Qiao, X.; Yang, X.; Wang, D.; Wu, H.; Li, H. A Furan−Thiophene-Based Quinoidal Compound: A New Class of Solution-Processable High-Performance n-Type Organic Semiconductor. Adv. Mater. 2016, 28, 5949−5953. (37) Handa, S.; Miyazaki, E.; Takimiya, K.; Kunugi, Y. SolutionProcessible n-Channel Organic Field-Effect Transistors Based on Dicyanomethylene-Substituted Terthienoquinoid Derivative. J. Am. Chem. Soc. 2007, 129, 11684−11685. (38) Yui, K.; Aso, Y.; Otsubo, T.; Ogura, F. New Electron Acceptors for Organic Metals: Extensively Conjugated Homologues of Thiophene-7,7,8,8-Tetracyanoquinodimethane (TCNQ). J. Chem. Soc., Chem. Commun. 1987, 1816−1817. (39) Higuchi, H.; Yoshida, S.; Uraki, Y.; Ojima, J. Synthesis and Properties of Dihexylbithienoquinonoid Derivatives with Head-tohead, Head-to-tail, and Tail-to-tail Orientations. Bull. Chem. Soc. Jpn. 1998, 71, 2229−2237. (40) Kunugi, Y.; Takimiya, K.; Toyoshima, Y.; Yamashita, K.; Aso, Y.; Otsubo, T. Vapour Deposited Films of Quinoidal Biselenophene and Bithiophene Derivatives As Active Layers of N-Channel Organic FieldEffect Transistors. J. Mater. Chem. 2004, 14, 1367−1369. (41) Chesterfield, R. J.; Newman, C. R.; Pappenfus, T. M.; Ewbank, P. C.; Haukaas, M. H.; Mann, K. R.; Miller, L. L.; Frisbie, C. D. High Electron Mobility and Ambipolar Transport in Organic Thin-Film Transistors Based on a π-Stacking Quinoidal Terthiophene. Adv. Mater. 2003, 15, 1278−1282. (42) Zhang, C.; Zang, Y.; Zhang, F.; Diao, Y.; McNeill, C. R.; Di, C.; Zhu, X.; Zhu, D. Pursuing High-Mobility n-Type Organic Semiconductors by Combination of “Molecule-Framework” and “SideChain” Engineering. Adv. Mater. 2016, 28, 8456−8462. (43) Ren, L.; Liu, F.; Shen, X.; Zhang, C.; Yi, Y.; Zhu, X. Developing Quinoidal Fluorophores with Unusually Strong Red/Near-Infrared Emission. J. Am. Chem. Soc. 2015, 137, 11294−11302. (44) Liu, F.; Espejo, G. L.; Qiu, S.; Oliva, M. M.; Pina, J.; Seixas de Melo, J. S.; Casado, J.; Zhu, X. Multifaceted Regioregular Oligo(thieno[3,4-b]thiophene)s Enabled by Tunable Quinoidization and Reduced Energy Band Gap. J. Am. Chem. Soc. 2015, 137, 10357− 10366. (45) Würthner, F.; Kaiser, T. E.; Saha-Möller, C. R. J-Aggregates: From Serendipitous Discovery to Supramolecular Engineering of Functional Dye Materials. Angew. Chem., Int. Ed. 2011, 50, 3376− 3410. (46) Zhou, K.; Dong, H.; Zhang, H. L.; Hu, W. High Performance NType and Ambipolar Small Organic Semiconductors for Organic Thin Film Transistors. Phys. Chem. Chem. Phys. 2014, 16, 22448−22457. 5007


Article

Chemistry of Materials (47) Gao, J.; Xiao, C.; Jiang, W.; Wang, Z. Cyano-substituted Perylene Diimides With Linearly Correlated LUMO Levels. Org. Lett. 2014, 16, 394−397. (48) Weitz, R. T.; Amsharov, K.; Zschieschang, U.; Villas, E. B.; Goswami, D. K.; Burghard, M.; Dosch, H.; Jansen, M.; Kern, K.; Klauk, H. Organic n-channel Transistors Based on Core-cyanated Perylene Carboxylic Diimide Derivatives. J. Am. Chem. Soc. 2008, 130, 4637− 4645. (49) Jones, B. A.; Ahrens, M. J.; Yoon, M. H.; Facchetti, A.; Marks, T. J.; Wasielewski, M. R. High-mobility Air-stable n-type Semiconductors with Processing Versatility: Dicyanoperylene-3,4:9,10-bis(dicarboximides). Angew. Chem., Int. Ed. 2004, 43, 6363−6366. (50) Schuettfort, T.; Thomsen, L.; McNeill, C. R. Observation of a Distinct Surface Molecular Orientation in Films of a High Mobility Conjugated Polymer. J. Am. Chem. Soc. 2013, 135, 1092−1101. (51) Rivnay, J.; Noriega, R.; Northrup, J. E.; Kline, R. J.; Toney, M. F.; Salleo, A. Structural Origin of Gap States in Semicrystalline Polymers and the Implications for Charge Transport. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 121306. (52) Gann, E.; McNeill, C. R.; Szumilo, M.; Sirringhaus, H.; Sommer, M.; Maniam, S.; Langford, S. J.; Thomsen, L. Near-edge X-ray Absorption Fine-structure Spectroscopy of Naphthalene Diimidethiophene Co-polymers. J. Chem. Phys. 2014, 140, 164710−1−8. (53) Tang, M.; Reichardt, A. D.; Miyaki, N.; Stoltenberg, R. M.; Bao, Z. Ambipolar, High Performance, Acene-Based Organic Thin Film Transistors. J. Am. Chem. Soc. 2008, 130, 6064−6065. (54) Ortiz, R. P.; Herrera, H.; Blanco, R.; Huang, H.; Facchetti, A.; Marks, T. J.; Zheng, Y.; Segura, J. L. Organic n-Channel Field-Effect Transistors Based on Arylenediimide-Thiophene Derivatives. J. Am. Chem. Soc. 2010, 132, 8440−8452. (55) Wang, H.; Wen, Y.; Yang, X.; Wang, Y.; Zhou, W.; Zhang, S.; Zhan, X.; Liu, Y.; Shuai, Z.; Zhu, D. Fused-Ring Pyrazine Derivatives for n-Type Field-Effect Transistors. ACS Appl. Mater. Interfaces 2009, 1, 1122−1129. (56) Chen, M. C.; Chiang, Y. J.; Kim, C.; Guo, Y. J.; Chen, S. Y.; Liang, Y. J.; Huang, Y. W.; Hu, T. S.; Lee, G. H.; Facchetti, A.; Marks, T. J. One-pot [1 + 1+1] Synthesis of Dithieno[2,3-b:3′,2′-d]thiophene (DTT) and Their Functionalized Derivatives for Organic Thin-film Transistors. Chem. Commun. 2009, 1846−1848. (57) Jones, B. A.; Facchetti, A.; Marks, T. J.; Wasielewski, M. R. Cyanonaphthalene Diimide Semiconductors for Air-Stable, Flexible, and Optically Transparent n-Channel Field-Effect Transistors. Chem. Mater. 2007, 19, 2703−2705.

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