Charge-Transfer Complexes of Benzothienobenzothiophene with

Article pubs.acs.org/JPCC

Charge-Transfer Complexes of Benzothienobenzothiophene with Tetracyanoquinodimethane and the n‑Channel Organic Field-Effect Transistors Ryonosuke Sato,† Masaki Dogishi,†,§ Toshiki Higashino,†,∥ Tomofumi Kadoya,†,⊥ Tadashi Kawamoto,† and Takehiko Mori*,†,‡ †

Department of Materials Science and Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan ACT-C, JST, Honcho, Kawaguchi, Saitama 332-0012, Japan

‡

S Supporting Information *

ABSTRACT: n-Channel organic transistors with excellent air stability are realized on the basis of charge-transfer complexes, (BTBT)(TCNQ), (BTBT)(F2TCNQ), (BSBS)(F2TCNQ), and (BTBT)(F4TCNQ), where BTBT is benzothieno[3,2b]benzothiophene, BSBS is benzoseleno[3,2-b]benzoselenophene, and FnTCNQ (n = 0, 2, and 4) are fluorinated 7,7,8,8-tetracyanoquinodimethanes. These complexes consist of mixed stacks of essentially neutral molecules, and the transistors are air stable even after several-month storage in ambient conditions.

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INTRODUCTION The first stable synthetic metal, (TTF)(TCNQ) (TTF = tetrathiafulvalene and TCNQ = 7,7,8,8-tetracyanoquinodimethane), shows high and metallic conductivity down to 54 K,1,2 and similarly several TTF derivatives such as tetramethylTTF (TMTTF) and hexamethylene-TTF (HMTTF) form highly conducting charge-transfer (CT) complexes with TCNQ.3,4 These metallic complexes have segregated stacks, but mixed-stack complexes are basically insulating.5 For example, bis(ethylenedithio)−TTF (BEDT-TTF) constitutes three different phases with TCNQ: two triclinic complexes consisting of segregated stacks show high conductivity, and a monoclinic complex with mixed stacks exhibits 107 times higher resistivity than the triclinic phases.6−10 The TCNQ complex of dibenzo-TTF (DBTTF) also has mixed stacks, whereas the single-crystal transistor shows electron, hole, and ambipolar transport depending on the source and drain (S/D) electrode materials.11,12 In (anthracene)(TCNQ) crystals, reorientational motion of anthracene molecules drastically changes the transistor characteristics at low temperatures.13 Recently, perylene has been reported to form not only 1:1 but also 2:1 and 3:1 TCNQ complexes, and these mixed-stack complexes show respectively electron, ambipolar, and hole-transporting transistor properties.14 Accordingly, similar CT complexes have been attracting considerable attention.15−22 It is well-known that benzothieno[3,2-b]benzothiophene (BTBT) derivatives show excellent p-channel transistor properties.23 The herringbone packing is related to the transistor properties, and especially the transistors of the octyl derivative, © XXXX American Chemical Society

C8-BTBT, exhibit quite high mobility even in the solution process.24,25 Organic CT complexes of BTBT analogs have been reported recently; the complex (BTBT)2XF6 (X = P, As, Sb, and Ta) exhibits high conductivity of 4100 S cm−1 at room temperature,26,27 and the selenium analog, benzoseleno[3,2b]benzoselenophene (BSBS), forms a similar high conducting salt, (BSBS)2TaF6, with one-dimensional columns.28 Cn-BTBT (n = 4, 8, and 12) form CT complexes with TCNQ derivatives,29,30 and the single-crystal transistors attain maximum mobility of 0.4 cm2 V−1 s−1, although these CT complexes have mixed-stack structures. We have reported that (DMeO-BTBT)(FnTCNQ) shows n-channel transistor characteristics with extreme air stability (DMeO-BTBT: 3,8dimethoxy-BTBT, and n = 0, 2, and 4), where the electron mobilities are in the 10−2 cm2 V−1 s−1 order for the singlecrystal transistors.31 When thiophene units of BTBT are substituted by pyrrole moieties, the resulting dibenzopyrrolo[3,2-b]pyrrole (DBPP) is a much stronger electron donor. DBPP forms a CT complex with 2,5-dimethyl-N,N′-dicyano-pquinonediimine (DMDCNQI), and this complex shows airstable ambipolar transistor properties.32 These findings have prompted us to investigate the most fundamental combination: unsubstituted BTBT and TCNQ. In this paper, we report crystal structures and transistor properties of (BTBT)(FnTCNQ) (S0, S2, and S4 for n = 0, 2, and 4) and Received: January 28, 2017 Revised: March 3, 2017 Published: March 7, 2017 A

DOI: 10.1021/acs.jpcc.7b00902 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

for S0, S2, and Se2, and the b axis for S4. Because the thin crystals were placed on the substrate in the single-crystal transistors, the substrate planes were perpendicular to the crystallographic b axis for S0, S2, and Se2 and the a axis for S4. The transistor properties were measured under the vacuum of 10−3 Pa and in air conditions by using a Keithley 4200 semiconductor parameter analyzer. The mobility was estimated from the saturated-region transfer characteristics.

(BSBS)(F2TCNQ) (Se2). Because these transistors are very stable in air, the long-term stability in air is investigated.

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EXPERIMENTAL SECTION BTBT and BSBS were prepared by following the reported methods.33−35 FnTCNQ (n = 0, 2, and 4) were purchased from Tokyo Chemical Industry and used as purchased. Crystals of charge-transfer complexes of BTBT and BSBS with FnTCNQ were grown by the diffusion method in the acetonitrile solutions. Transistors were fabricated onto n-doped Si substrates with a thermally grown SiO2 dielectric layer (300 nm, C = 11.5 nF/ cm 2 ). For the thin-film device, the passivation layer, tetratetracontane (C44H90, TTC, ε = 2.5) was evaporated under a vacuum of 10−4 Pa on the substrates with a thickness of 20 nm,36,37 where the calculated overall capacitance of the gate dielectrics was 10.4 nF/cm2.38 Then the CT complex layer with a thickness of 50 nm was vacuum evaporated. The top-contact electrodes were patterned by thermal deposition of (TTF)(TCNQ) powder using a shadow mask; the channel length (L) and width (W) were 100 and 1000 μm, respectively.39−45 For the single-crystal device, the passivation layer polystyrene (PS, ε = 2.5) was deposited by spin coating (3000 rpm, and 30 s) a solution of PS (20 mg) in toluene (1 mL) on the substrates with a thickness of 100 nm,42,46 where the calculated overall capacitance was 7.6 nF/cm2.38 Needle-like black crystals were put on the PS layer using ethanol.47 The S/D electrodes were fabricated using carbon paste (DOTITE, XC-12). Carbon paste was deposited on two ends of the single crystal to make the source-drain electrodes, and the crystal long axis was oriented in the channel direction. The single-crystal X-ray diffraction indicates that the crystal long axis is parallel to the molecular stacking axis, which corresponds to the crystallographic a axis

RESULTS AND DISCUSSION

Crystal structures. Crystal data of S0, S2, Se2, and S4 are summarized in Table 1. S0, S2, and Se2 are isostructural with the space group P1̅, but S4 forms a different structure with the space group P21/c. Each donor and acceptor molecule is placed on an inversion center, and the half molecules are crystallographically independent. Donor and acceptor molecules stack alternately in a ring-over-bond manner, where the molecular long axes are approximately parallel (Figure 1). The donor− acceptor interplanar spacings are 3.40 Å for S0, 3.36 Å for S2, 3.38 Å for Se2, and 3.33 Å for S4. The ab lattice constants of the triclinic crystals (7.2 Å × 8.0 Å) are very close to the ab lattice constants of (C8-BTBT)(FnTCNQ) and the ca lattice constants of (DMeO-BTBT)(FnTCNQ),30,31 reflecting the similarity of the core stacking structures. In the triclinic crystals, all molecules are parallel to each other, but the adjacent columns are tilted alternately in S4 (Figure 1c). However, the lattice constants are very similar after the replacement: a → b, b → a, and c → 2c, because the structure of each column is essentially the same. There are hydrogen bonds between the H−N and H−F atoms of the donors and the acceptors (Figures S1−S4). In addition, the S4 crystal has a short contact (3.288 Å) between the S atom of BTBT and the N atom of F4TCNQ. The degrees of charge transfer evaluated from the TCNQ bond distances are close to zero (Table S1), indicating that the present complexes are basically neutral. The intermoleculartransfer integrals are estimated from overlaps of donor highest occupied molecular orbitals (HOMO) and acceptor lowest unoccupied molecular orbitals (LUMO). All crystals have

Table 1. Crystallographic data of S0, S2, Se2, and S4

formula formula weight crystal system space group Z a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) ρ (g/cm3) no. of total reflns no. of unique reflns (Rint) R1 (F2 > 2σ(F2)) wR2 (all reflns) GOF temp (K)

S0

S2

Se2

S4

C26H12N4S2 444.53 triclinic P1̅ 1 7.1933(9) 8.0044(10) 9.0183(12) 81.181(6) 89.310(6) 87.721(5) 512.70(11) 1.440 5534 1663 (0.1140) 0.0966 0.2708 0.960 279

C26H10F2N4S2 480.51 triclinic P1̅ 1 7.1608(7) 7.9489(12) 9.1084(12) 80.928(11) 89.567(11) 87.878(12) 511.62(12) 1.559 3558 2987 (0.0277) 0.0534 0.1467 0.969 298

C26H10F2N4Se2 574.31 triclinic P1̅ 1 7.1911(13) 8.039(3) 9.156(3) 80.96(3) 89.226(19) 88.727(19) 522.5(3) 1.825 2946 2400 (0.0450) 0.0555 0.1417 0.965 295

C26H8F4N4S2 516.49 monoclinic P21/c 2 7.9628(14) 7.062(4) 19.111(4)

B

99.328(14) 1060.4(7) 1.617 3935 3087 (0.0487) 0.0505 0.1443 0.991 298 DOI: 10.1021/acs.jpcc.7b00902 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article


Figure 1. Crystal structure of S2 projected (a) along the c axis and (b) along the a axis. Crystal structure of S4 projected (c) along the c axis and (d) along the b axis.

Figure 2. (a) XRD patterns and AFM images of (b) S0, (c) S2, (d) Se2, and (e) S4 thin films.

β)/2), suggesting a possibility that the S4 molecules are partly arranged in a tail-on manner. The atomic force microscopy (AFM) images show that in S2, Se2, and S4, the substrates are densely covered by needle-like microcrystals (Figure 2b−e). In S0, however, the substrate is not entirely covered by the microcrystals. This is in agreement with the absence of the XRD peaks. The thin film of S4 shows larger roughness than those of S2 and Se2, probably due to the presence of the tail-on orientation. Transistor Properties. Transfer and output transistor characteristics are shown in Figure 3 for the single-crystal transistors, and the transistor properties are summarized in Table 2. For the thin-film transistors, the electron mobilities are in the 10−4 cm2 V−1 s−1 order for S2 and Se2, and in the 10−3 cm2 V−1 s−1 order for S4 (Figure S6). These values are comparable to those of the (DMeO-BTBT)(FnTCNQ) transistors.31 In air, the threshold voltages (Vth) increase, but the mobilities are almost unchanged. S4 shows the maximum mobility of 0.019 cm2 V−1 s−1 even in air. The mobility tends to increase with increasing the acceptor ability (S2 ∼ Se2 < S4), and in particular, S0 does not show the transistor characteristics in the thin films. After three-months storage in a vacuum

donor−donor interactions as large as or larger than the intracolumnar donor−acceptor interactions (Table S2−S3). Thin-Film Properties. X-ray diffraction (XRD) patterns of the thin films deposited on TTC-modified Si/SiO2 substrates show comparatively small diffraction peaks at around 2θ = 13.5° (Figure 2a). The resulting d = 6.4−6.6 Å are contrasting to (DMeO-BTBT)(FnTCNQ), which afford sharp peaks at d = 11.1−11.4 Å corresponding to the molecular long axes.31 In the present complexes, it is likely that the molecules are arranged so as to make the molecular short axes (|b + c| /2 = 6.5 Å) perpendicular to the substrate, and the crystal (011) planes parallel to the substrate. Accordingly, the molecules are arranged in a side-on manner (Figure S5). Such an arrangement is occasionally observed; an example is tetramethyltetrathiafulvalene (TMTTF) and the TCNQ salt,48 and actually the (TMTTF)(TCNQ) film exhibits a very similar XRD pattern to Figure 2a.42 Note that additional small peaks attributed to TTC appear around 2θ = 6°, 8°, and 21°.49 In S0, only TTC peaks are observed. S4 shows essentially the same d value, indicating the side-on arrangement as well. S4 affords another small peak at 2θ = 9.3° (d = 9.5 Å). This is attributed to the half of c ((c sin C


Article


Figure 3. Transfer and output characteristics of single-crystal transistors of (a and b) S0, (c and d) S2, (e and f) Se2, and (g and h) S4. The solid curves are measurements under a vacuum, and the dotted curves are observed in air.

estimated from the off current (0.8 × 10−8 A in Figure 3g) amounts to 3 × 10−7 S cm−1. It is notable that high mobility tends to appear associated with the relatively high conductivity. All single-crystal transistors show n-channel operation even after three-month storage in a vacuum desiccator without any reduction of the performance. The threshold voltage tends to increase slightly due to the change of trap states, and as a result the mobility tends to increase after storage. After an additional eight-months storage in air, only a few S4 transistors work under a vacuum but S0, S2, and Se2 show n-channel operation without any reduction of the performance even in air. This demonstrates the particularly robust nature of the present transistors under ambient conditions. It is a general tendency that thin-film transistors of mix-stack 1:1 TCNQ complexes show only n-channel transport,50,51 though p-channel transport is sometimes observed in singlecrystal transistors.9,10,12−14,17,20,52 The charge polarity of donor−acceptor polymers is a complicated issue,53 but carriers of mix-stacked crystals are sometimes a great mystery as well.54−56 Although the present materials have considerable

desiccator, S2 and Se2 show n-channel operation without any reduction of the performance, demonstrating the very air-stable n-channel transistor characteristics. By contrast, it is characteristic of S4 that the performance is lost after three months. After an additional eight-months storage in air, S2 and Se2 also show n-channel operation. For the single-crystal transistors, the electron mobilities are in the 10−3 cm2 V−1 s−1 order for S0, S2, and Se2, and in the 10−2 cm2 V−1 s−1 order for S4. Accordingly, the mobility increases approximately by 1 order compared to mobilities of the thin-film transistors. S4 exhibits a particularly high maximum mobility of 0.19 cm2 V−1 s−1. In addition, S1−S4 show very small threshold voltages less than 10 V. The shift of the threshold voltages in air is smaller than the thin-film transistors, probably due to the reduction of the air-exposed areas. However, the on−off ratio decreases with increasing the acceptor strength as S0 < S2 < S4. This comes from the increasing off current from less than 10−10 A to nearly 10−8 A (Figure 3), because the bulk conductivity increases according to the increasing acceptor ability. The bulk conductivity of S4 D


Article

The Journal of Physical Chemistry C Table 2. Transistor Properties for Thin-Film and Single-Crystal Devices of S0, S2, Se2, and S4 devices thin film

complexes S2

conditions pristine after 3 months under vacuum after additional 8 months in air

Se2

pristine after 3 months under vacuum after additional 8 months in air

S4


single crystal

S0


S2


Se2


S4


measurements under in air under in air under in air under in air under in air under in air under in air under in air under in air under in air under in air under in air under in air under in air under in air under in air under in air under in air under in air under in air under in air

a vacuum a vacuum a vacuum a vacuum a vacuum a vacuum a vacuum a vacuum

μav [μmax] (cm2 V−1 s−1) 3.6 × 10−4 3.3 × 10−4 4.1 × 10−4 3.3 × 10−4 3.5 × 10−4 1.6 × 10−4 8.1 × 10−4 8.0 × 10−4 1.5 × 10−3 1.4 × 10−3 1.4 × 10−3 9.9 × 10−4 7.5 × 10−3 8.0 × 10−3 no FETs

[8.1 × 10−4] [5.1 × 10−4] [6.2 × 10−4] [5.7 × 10−4] [7.0 × 10−4] [3.1 × 10−4] [1.5 × 10−3] [1.2 × 10−3] [2.5 × 10−3] [2.4 × 10−3] [2.7 × 10−3] [1.6 × 10−3] [0.018] [0.019]

Vth (V)

on/off

23 45 13 31 8 23 31 49 15 25 5 16 10 20

6 4 3 3 3 1 6 1 3 3 4 4 7 3

× × × × × × × × × × × × × ×

103 103 103 103 103 103 103 104 104 104 103 103 103 103

16 30 22 23 27 27 −1 3 −2 −1 9 6 −7 1 −4 −2 3 9 −8 −6 −9 −11 −5

7 2 7 8 1 1 8 9 4 1 1 2 1 1 2 3 1 5 2 2 1 1 1

× × × × × × × × × × × × × × × × × × × × × × ×

102 103 102 102 103 103 101 101 101 102 102 102 102 102 102 102 103 102 101 101 101 101 101

a vacuum a vacuum a vacuum a vacuum a vacuum a vacuum a vacuum a vacuum a vacuum a vacuum a vacuum a vacuum a vacuum

7.7 × 10−3 [0.021] 9.7 × 10−3 [0.021] 0.011 [0.040] 0.011 [0.044] 0.011 [0.044] 9.1 × 10−3 [0.035] 5.9 × 10−3 [0.022] 4.9 × 10−3 [0.018] 7.8 × 10−3 [0.023] 7.2 × 10−3 [0.019] 0.012 [0.026] 7.1 × 10−3 [0.021] 4.9 × 10−3 [0.013] 4.6 × 10−3 [0.012] 7.2 × 10−3 [0.022] 6.5 × 10−3 [0.020] 9.7 × 10−3 [0.029] 0.012 [0.027] 0.054 [0.19] 0.043 [0.11] 0.049 [0.087] 0.047 [0.081] 0.050 [0.056] no FETs

5.8°, 8.3°, 8.2°, and 8.1° from the normal to the substrate (approximately the (011) plane for S0, S2, and Se2, and the (102̅) plane for S4). The S4 molecules are still tilted from the substrate, but the alternately tilted columns attain the perpendicular crystal lattice. Advantageous transistor performance associated with the perpendicular molecular arrangement is sometimes attributed to the misfit at the domain boundaries. However, the improved transistor performance of S4 is mainly observed in the single-crystal transistors rather than the thinfilm transistors. We have to suppose that the perpendicular lattice is more intrinsically preferable even in single-crystal transistors.

intercolumnar donor−donor interactions (Supporting Information), this does not seem to realize the hole transport. In the present case, however, TCNQ is a strong acceptor with the LUMO level of −4.6 eV,57 but BTBT is a weak donor with the HOMO level of −5.6 eV.23,26 It may be appropriate to naively attribute the observation of the n-channel transport to the difference of the donor and acceptor strengths. Owing to the monoclinic symmetry, the ac plane of the S4 crystal is exactly perpendicular to the b axis. It has been observed that transistor performance is maximized when the molecules are perpendicular to the substrate.58,59 The improved mobility of S4 is ascribed to this preferable molecular packing. In the single-crystal transistors, the molecular planes in S0, S2, Se2, and S4 are respectively tilted by 16.3°, 18.5°, 18.1°, and 17.8° from the normal to the substrate (approximately the ac plane for S0, S2, and Se2, and the bc plane for S4). In the thinfilm transistors, the molecular planes are respectively tilted by

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CONCLUSION We have investigated crystal structures and transistor characteristics of (BTBT)(F n TCNQ) (n = 0, 2, and 4) and (BSBS)(F2TCNQ). In contrast to the tail-on molecular E


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trathiofulvalenium - Tetracyano–Quinodimethanide. Tetrahedron Lett. 1973, 14, 2553−2556. (4) Greene, R. L.; Mayerle, J. J.; Schumaker, R.; Castro, G.; Chaikin, P. M.; Etemad, S.; LaPlaca, S. J. The Structure, Conductivity, and Thermopower of HMTTF-TCNQ. Solid State Commun. 1976, 20, 943−946. (5) Goetz, K. P.; Vermeulen, D.; Payne, M. E.; Kloc, C.; McNeil, L. E.; Jurchescu, O. D. Charge-Transfer Complexes: New Perspectives on an Old Class of Compounds. J. Mater. Chem. C 2014, 2, 3065−3076. (6) Mori, T.; Inokuchi, H. Structural and Electrical Properties of (BEDT-TTF) (TCNQ). Solid State Commun. 1986, 59, 355−359. (7) Mori, T.; Inokuchi, H. Crystal Structure of the Mixed-Stacked Salt of Bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) and Tetracyanoquinodimethane (TCNQ). Bull. Chem. Soc. Jpn. 1987, 60, 402−404. (8) Yamamoto, H. M.; Hagiwara, M.; Kato, R. New Phase of (BEDTTTF) (TCNQ). Synth. Met. 2003, 133−134, 449−451. (9) Sakai, M.; Sakuma, H.; Ito, Y.; Saito, A.; Nakamura, M.; Kudo, K. Ambipolar Field-Effect Transistor Characteristics of (BEDT-TTF) (TCNQ) Crystals and Metal-like Conduction Induced by a Gate Electric Field. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, 045111. (10) Hasegawa, T.; Mattenberger, K.; Takeya, J.; Batlogg, B. Ambipolar Field-Effect Carrier Injections in Organic Mott Insulators. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 69, 245115. (11) Takahashi, Y.; Hasegawa, T.; Abe, Y.; Tokura, Y.; Nishimura, K.; Saito, G. Tuning of Electron Injections for N-Type Organic Transistor Based on Charge-Transfer Compounds. Appl. Phys. Lett. 2005, 86, 063504. (12) Takahashi, Y.; Hasegawa, T.; Abe, Y.; Tokura, Y.; Saito, G. Organic Metal Electrodes for Controlled P- and N-Type Carrier Injections in Organic Field-Effect Transistors. Appl. Phys. Lett. 2006, 88, 073504. (13) Yokokura, S.; Takahashi, Y.; Nonaka, H.; Hasegawa, H.; Harada, J.; Inabe, T.; Kumai, R.; Okamoto, H.; Matsushita, M. M.; Awaga, K. Switching of Transfer Characteristics of an Organic Field-Effect Transistor by Phase Transitions: Sensitive Response to Molecular Dynamics and Charge Fluctuation. Chem. Mater. 2015, 27, 4441− 4449. (14) Vermeulen, D.; Zhu, L. Y.; Goetz, K. P.; Hu, P.; Jiang, H.; Day, C. S.; Jurchescu, O. D.; Coropceanu, V.; Kloc, C.; McNeil, L. E. Charge Transport Properties of Perylene−TCNQ Crystals: The Effect of Stoichiometry. J. Phys. Chem. C 2014, 118, 24688−24696. (15) Hu, P.; Ma, L.; Tan, K. J.; Jiang, H.; Wei, F.; Yu, C.; Goetz, K. P.; Jurchescu, O. D.; McNeil, L. E.; Gurzadyan, G. G.; et al. SolventDependent Stoichiometry in Perylene−7,7,8,8-Tetracyanoquinodimethane Charge Transfer Compound Single Crystals. Cryst. Growth Des. 2014, 14, 6376−6382. (16) Hu, P.; Du, K.; Wei, F.; Jiang, H.; Kloc, C. Crystal Growth, HOMO−LUMO Engineering, and Charge Transfer Degree in Perylene-F X TCNQ (X = 1, 2, 4) Organic Charge Transfer Binary Compounds. Cryst. Growth Des. 2016, 16, 3019−3027. (17) Goetz, K. P.; Tsutsumi, J.; Pookpanratana, S.; Chen, J.; Corbin, N. S.; Behera, R. K.; Coropceanu, V.; Richter, C. A.; Hacker, C. A.; Hasegawa, T.; et al. Polymorphism in the 1:1 Charge-Transfer Complex DBTTF-TCNQ and Its Effects on Optical and Electronic Properties. Adv. Electron. Mater. 2016, 2, 1600203. (18) Salzillo, T.; Masino, M.; Kociok-Köhn, G.; Di Nuzzo, D.; Venuti, E.; Della Valle, R. G.; Vanossi, D.; Fontanesi, C.; Girlando, A.; Brillante, A.; et al. Structure, Stoichiometry, and Charge Transfer in Cocrystals of Perylene with TCNQ-F X. Cryst. Growth Des. 2016, 16, 3028−3036. (19) Sun, H.; Wang, M.; Wei, X.; Zhang, R.; Wang, S.; Khan, A.; Usman, R.; Feng, Q.; Du, M.; Yu, F.; et al. Understanding ChargeTransfer Interaction Mode in Cocrystals and Solvates of 1-Phenyl-3(Pyren-1-Yl) Prop-2-En-1-One and TCNQ. Cryst. Growth Des. 2015, 15, 4032−4038. (20) Qin, Y.; Cheng, C.; Geng, H.; Wang, C.; Hu, W.; Xu, W.; Shuai, Z.; Zhu, D. Efficient Ambipolar Transport Properties in Alternate

arrangement of (DMeO-BTBT)(FnTCNQ), these complexes have the side-on arrangement in the thin films, which is evident from the clearly different XRD. Although the alternate stacking structure is quite similar, this is attributable to the absence of the substituents in the donor molecule. In the single-crystal transistors, all CT complexes show long-term air stability, and even the thin-film transistors of the F2TCNQ complexes maintain the air stable n-channel transistor properties for a long time. (BTBT)(F4TCNQ) shows the highest mobility, which is potentially related to the monoclinic structure with alternately tilted mix-stacked columns. Transistors of CT complexes are not only an analogy of donor−acceptor polymers but also a strategy to achieve very stable n-channel transistors with reasonable performance by using a strong acceptor unit such as TCNQ. However, the present results demonstrate the considerable variety: side-on molecular arrangement in the thin films and alternately tilted molecular packing, even though the mixed stack structure in a column is basically the same.

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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.jpcc.7b00902. Description of the single-crystal structure, hydrogen bonds, degree of charge transfer, transfer integrals, molecular orientation in the thin-film transistors, and thin-film transistors (PDF) Crystal data (CIF)

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AUTHOR INFORMATION

Corresponding Author

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

Tomofumi Kadoya: 0000-0002-9490-0629 Takehiko Mori: 0000-0002-0578-5885 Present Addresses §

Imaging Science and Engineering Research Center, Tokyo Institute of Technology, Yokohama, Kanagawa 226-8503, Japan. ∥ The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan. ⊥ Department of Material Science, University of Hyogo, Akogun, Hyogo 678-1297, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Tokyo Institute of Technology Center for Advanced Materials Analysis for X-ray diffraction measurements and scanning atomic force microscope observations.

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REFERENCES

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