Chem. Mater. 2011, 23, 795–804 795 DOI:10.1021/cm102109p
Quinoidal Oligothiophenes with (Acyl)cyanomethylene Termini: Synthesis, Characterization, Properties, and Solution Processed n-Channel Organic Field-Effect Transistors† Yuki Suzuki,‡ Masafumi Shimawaki,‡ Eigo Miyazaki,‡ Itaru Osaka,‡ and Kazuo Takimiya*,‡,§ ‡
Department of Applied Chemistry, Graduate School of Engineering and §Institute for Advanced Materials Research, Hiroshima University, Higashi-Hiroshima 739-8530, Japan Received July 28, 2010. Revised Manuscript Received September 2, 2010
A new class of oligothienoquinoidal derivatives with newly employed (acyl)cyanomethylene termini are reported. For the synthesis of (acyl)cyanomethylene-substituted thienoquinoidals, similar methods successfully employed for the synthesis of related dicyanomethylene- or ((alkyloxy)carbonyl)cyanomethylene-substituted thienoquinoidals were not applicable, and thus a new synthetic route was developed. The introduced (acyl)cyanomethylene terminal groups act both as a solublizing group to facilitate solution processability and an electron-withdrawing group to keep the LUMO energy levels sufficiently low for n-channel organic semiconductors. The LUMO energy levels estimated from their reduction potential are ∼4.2 eV below the vacuum level, which just falls in between those for the corresponding dicyanomethylene- and ((alkyloxy)carbonyl)cyanomethylene-substituted thienoquinoidals. This qualitatively agrees with the electron-withdrawing nature of the terminal groups; the order of the electron withdrawing nature is cyano- > acyl- > (alkyloxy)carbonyl- groups. Spin-coating chloroform solutions of (acyl)cyanomethylene-substituted thienoquinoidals gave homogeneous thin films on the Si/SiO2 substrates, and the thin films based on the terthienoquinoidal derivatives became highly crystalline upon thermal annealing. The annealed film acted as the active semiconducting channel in the FET devices under ambient conditions, and the electron mobilities extracted from the saturation regime were ∼0.06 cm2 V-1 s-1. These n-channel FET characteristics are nearly the same or slightly better than those of the FETs based on the related ((alkyloxy)carbonyl)cyanomethylene-terminated terthienoquinoidal, indicating that the (acyl)cyanomethylene moiety is a useful terminal group on the thienoquinoidals for the development of soluble n-channel organic semiconductors. Introduction 7,7,8,8-Tetracyanoquinodimethane (TCNQ, Figure 1), the archetypical electron-accepting molecule, has played important roles in the field of conducting charge-transfer (CT) complexes since the discovery of the first metallic organic material, terathiafulvalene-TCNQ CT salt, reported by Cowan et al. in 1970s.1 Since then, a variety of TCNQ derivatives2 as well as its related quinoidal compounds3 have been developed. Dicyanomethylene-terminated quinoidalthiophenes or thiophene TCNQs are among such compounds4 and were intensively studied by Ogura et al. in the Accepted as part of the “Special Issue on π-Functional Materials”. *Corresponding author. E-mail:
[email protected].
†
(1) (a) Ferraris, J.; Cowan, D. O.; Walatka, V.; Perlstein, J. H. J. Am. Chem. Soc. 1973, 95, 948–949. (b) Martin, N.; Segura, J. L.; Seoane, C. J. Mater. Chem. 1997, 7, 1661–1676. (2) Wheland, R. C.; Martin, E. L. J. Org. Chem. 1975, 40, 3101–3109. (3) (a) Iwatsuki, S.; Itoh, T.; Nishihara, K.; Furuhashi, H. Chem. Lett. 1982, 11, 517–520. (b) Iwatsuki, S.; Itoh, T.; Iwai, T.; Sawada, H. Macromolecules 1985, 18, 2726–2732. (4) Gronowitz, S.; Uppstr€ om, B. Acta Chem. Scand. 1974, B28, 981–985. (5) (a) Yui, K.; Aso, Y.; Otsubo, T.; Ogura, F. J. Chem. Soc., Chem. Commun. 1987, 1816–1817. (b) Yui, K.; Aso, Y.; Otsubo, T.; Ogura, F. Bull. Chem. Soc. Jpn. 1989, 62, 1539–1546. (c) Yui, K.; Ishida, H.; Aso, Y.; Otsubo, T.; Ogura, F.; Kawamoto, A.; Tanaka, J. Bull. Chem. Soc. Jpn. 1989, 62, 1547–1555. r 2010 American Chemical Society
1980s as a promising electron accepting molecules for conducting CT salts.5 Recent rising interests in the field of organic electronics has renewed the interests in the TCNQ-type compounds as n-channel organic semiconductors.6 In fact, the parent TCNQ7 and its π-extended analogue, 9,9,10,10-tetracyanonaphtho-2,6-quinodimethane (TNAP) (Figure 1),8 were examined as an active material in the field-effect transistors (FETs) showing relatively good field-effect electron mobility. Several dicyanomethylene-terminated thienoquinoidals have also been examined as active n-channel materials in OFET devices.9 However, the parent bithienoquinoidal- (1a) or terthienoquinoidal (1b)-based FETs showed poor device characteristics with low mobility (6) (a) Newman, C. R.; Frisbie, C. D.; daSilvaFilho, D. A.; Bredas, J. L.; Ewbank, P. C.; Mann, K. R. Chem. Mater. 2004, 16, 4436– 4451. (b) Facchetti, A. Mater. Today 2007, 10, 28–37. (c) Wen, Y.; Liu, Y. Adv. Mater. 2010, 22, 1331–1345. (7) (a) Brown, A. R.; de Leeuw, D. M.; Lous, E. J.; Havinga, E. E. Synth. Met. 1994, 66, 257–261. (b) Yamagishi, M.; Tominari, Y.; Uemura, T.; Takeya, J. Appl. Phys. Lett. 2009, 94, 053305. (c) Uemura, T.; Yamagishi, M.; Ono, S.; Takeya, J. Appl. Phys. Lett. 2009, 95, 103301. (8) (a) Diekmann, J.; Hertler, W. R.; Benson, R. E. J. Org. Chem. 1963, 28, 2719–2724. (b) Laquindanum, J. G.; Katz, H. E.; Dodabalapur, A.; Lovinger, A. J. J. Am. Chem. Soc. 1996, 118, 11331–11332.
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Figure 2. Thienoqunioidal derivatives with soluble terminal groups.
Figure 1. Structures of representative dicyanomethylene-substituted benzoquinoidal and thienoquinoidal compounds.
(∼1 10-4 cm2 V-1 s-1) because of their poor molecular ordering in the thin film state.9c,10 In sharp contrast, the derivatives of 1b with substituents on the thienoquinoidal core (2) act as superior n-channel organic semiconductors showing mobility up to 0.2 cm2 V-1 s-1.9a,b Furthermore, a proper modification with a solubilizing group affords a solution processable derivative (3) with retaining high mobility.9d,e These fruitful results on 2 and 3 imply that the molecular modification on the thienoquinoidal core would be a key not only for solubilizing the materials but also enabling molecular ordering in the thin-film state. However, the core substitution often requires elaborated synthesis,9d which makes the material less practical, even though the given material provides superior semiconductor characteristics. On the other hand, we have pursued another approach for developing superior new n-channel organic semiconductors based on the quinoidal thiophene structure rather than the molecular modification on the quinoidal core. Actually, we have recently found that a newly employed terminal group containing solublizing long alkyl group, namely the ((alkyloxy)carbonyl)cyanomethylene terminus instead of the conventional dicyanomethylene terminus is quite effective to develop a series of new thienoquinoidals. Thus developed ((alkyloxy)carbonyl)cyanomethylene-substituted thienoquinoidals (Figure 2, 4) can act as soluble n-channel organic semiconductors that afford solution-processed OTFTs with electron mobility up to 0.015 cm2 V-1 s-1.10 The devices based on 4 can operate under ambient conditions, indicating (9) (a) Pappenfus, T. M.; Chesterfield, R. J.; Frisbie, C. D.; Mann, K. R.; Casado, J.; Raff, J. D.; Miller, L. L. J. Am. Chem. Soc. 2002, 124, 4184–4185. (b) Chesterfield, R. J.; Newman, C. R.; Pappenfus, T. M.; Ewbank, P. C.; Haukaas, M. H.; Mann, K. R.; Miller, L. L.; Frisbie, C. D. Adv. Mater. 2003, 15, 1278–1282. (c) Kunugi, Y.; Takimiya, K.; Toyoshima, Y.; Yamashita, K.; Aso, Y.; Otsubo, T. J. Mater. Chem. 2004, 14, 1367–1369. (d) Handa, S.; Miyazaki, E.; Takimiya, K.; Kunugi, Y. J. Am. Chem. Soc. 2007, 129, 11684– 11685. (e) Handa, S.; Miyazaki, E.; Takimiya, K. Chem. Commun. 2009, 3919–3921. (f) Ribierre, J. C.; Fujihara, T.; Watanabe, S.; Matsumoto, M.; Muto, T.; Nakao, A.; Aoyama, T. Adv. Mater. 2010, 22, 1722–1726. (g) Kashiki, T.; Miyazaki, E.; Takimiya, K. Chem. Lett. 2009, 38, 568–569. (h) Ribierre, J. C.; Watanabe, S.; Matsumoto, M.; Muto, T.; Nakao, A.; Aoyama, T. Adv. Mater. 2010, 22, No. 10.1002/adma.201001170. (10) Suzuki, Y.; Miyazaki, E.; Takimiya, K. J. Am. Chem. Soc. 2010, 132, 10453–10466.
Figure 3. Calculated HOMO and LUMO of terthienoquinoidal derivatives with different terminal groups (1b, 4b, and 5b).
good air stability due to their low-lying LUMO energy level ( acyl- > (alkyloxy)carbonyl groups. The present (acyl)cyanomethylene-substituted thienoquinoidals (5) gave homogeneous thin films on the Si/SiO2 substrates by spin coating, and the thin films based on the terthienoquinoidal derivatives (5b) became highly crystalline upon thermal annealing. The annealed film acted as the active semiconducting channel in the FET devices, and the mobilities extracted from the saturation regime are ∼0.06 cm2 V-1 s-1. These n-channel FET characteristics are nearly the same as or slightly higher than those of 4-based FETs. These results indicate that the (acyl)cyanomethylene moiety is a useful terminal group on the thienoquinoidals for the development of soluble n-channel organic semiconductors. Further optimization of the molecular structure of the present thienoquinoidal series is now underway in our group. In this relation, although the present synthetic method is useful for the synthesis of oligothienoquinoidals, the method can not be applicable to the synthesis of the fused thienoquinoidals. Thus, new versatile and efficient methods for the synthesis of this class of thienoquinoidals are also intensively investigated.
Synthesis. General. All chemicals and solvents are of reagent grade unless otherwise indicated. THF was purified with a standard distillation procedure prior to use. Melting points were uncorrected. All reactions were carried out under a nitrogen atmosphere. Nuclear magnetic resonance spectra were obtained in deuterated chloroform (CDCl3) with TMS as internal reference; chemical shifts (δ) are reported in parts per million. IR spectra were recorded as a neat spectrum for oil samples or using a KBr pellet for solid samples. EI-MS spectra were obtained using an electron impact ionization procedure (70 eV). MALDITOF spectra were obtained using dithranol matrix. FAB-MS spectra were recorded using 3-nitrobenzyl alcohol as a matrix. Solubilities of 5a and 5b in chloroform were determined as the saturated concentration (g L-1) at room temperature. Synthesis of 2-Thienylethanol Derivative (General Procedure)14. 1-(2-Thienyl)butane-2-ol (9, R = C2H5). To a solution of thiophene (5 mL, 63 mmol) in THF (100 mL) was slowly added n-butyllithium (1.59 M solution in hexane, 45 mL, 69 mmol) at -78 °C. After stirring for 1 h at room temperature, 1,2epoxybutane (11 mL, 126 mmol) was slowly added at -78 °C, and the mixture was stirred overnight at room temperature. To the mixture was added hydrochloric acid (1 M, 100 mL), and the resulting mixture was extracted with ethyl acetate (100 mL 3). The combined extracts were washed with water (100 mL) and brine (100 mL), dried over magnesium sulfate (anhydrous), and concentrated in vacuo. The residue was purified by column chromatography on silica gel eluted with dichloromethane (Rf = 0.45) to give 9 (R = C2H5) as a pale yellow oil (9.0 g, 91%). 1H NMR (270 MHz) δ 7.18 (d, J = 4.0 Hz, 1H), 6.96 (dd, J = 4.0 Hz, J = 3.3 Hz, 1H), 6.87 (d, J = 3.3 Hz, 1H), 3.74 (m, 1H), 3.00 (m, 2H), 1.56 (m, 2H), 1.00 (t, J = 7.4 Hz, 3H). 13C NMR (67.8 MHz) δ 10.1, 29.4, 37.5, 73.9, 124.3, 126.1, 127.1, 140.8. MS (EI) m/z = 156 (Mþ). IR (neat) ν = 3381 cm-1 (OH). Anal. Calcd for C8H12OS: C, 61.50; H, 7.74%. Found: C, 61.16; H, 7.54%. 1-(2-Thienyl)octan-2-ol (9, R = n-C6H13). Yellow oil, 93% isolated yield. 1H NMR (400 MHz) δ 7.18 (d, J = 5.2 Hz, 1H), 6.96 (dd, J = 5.2 Hz, J = 3.3 Hz, 1H), 6.86 (d, J = 3.3 Hz, 1H), 3.80 (m, 1H), 3.01 (m, 2H), 1.55-1.29 (m, 10H), 0.89 (t, J = 6.8 Hz, 3H). 13C NMR (67.8 MHz) δ 14.2, 22.7, 25.8, 29.4, 31.9, 36.6, 38.0, 72.5, 124.3, 126.1, 127.1, 140.8. MS (EI) m/z = 212 (Mþ). IR (neat) ν = 3381 cm-1 (OH). Anal. Calcd for C14H24OS: C, 67.87; H, 9.49%. Found: C, 67.78; H, 9.64%. 1-(2-Thienyl)tetradecan-2-ol (9, R = n-C12H25). Waxy white solid (mp 2σ(I)) and 271 variable parameters, wR2 = 0.290 for all data. Fabrication and Evaluation of FET Devices. Si/SiO2 substrates were treated with octadecyltrichlorosilane (ODTS) by immersing the silicon wafers to ODTS solution in dry toluene at room temperature under nitrogen for 12 h.23 OFETs were fabricated in a “top-contact” configuration on a heavily doped nþ-Si (100) wafer with a 200 nm thermally grown SiO2 (Ci = 17.3 nF cm-2). Thin film of 5a or 5b as an active layer was spin-coated on the Si/SiO2 substrate. On top of the organic thin film, gold films (80 nm) as drain and source electrodes were deposited through a shadow mask. For a typical device, the drain-source channel length (L) and width (W) are 50 μm and 1.5 mm, respectively. Characteristics of the OFET devices were measured at room temperature under ambient conditions with a Keithley 4200 semiconducting parameter analyzer. Field-effect mobility (μFET) was calculated in the saturation regime (Vd = 60 V) of the Id using the following equation, Id ¼ ðWCi =2LÞμFET ðVg - Vth Þ2 where Ci is the capacitance of the SiO2 insulator, and Vg and Vth are the gate and threshold voltages, respectively. Current on/off ratio (Ion/Ioff) was determined from the Id at Vg = 0 V (Ioff) and Vg = 60 V (Ion). The μFET data reported are typical values from more than five different devices.
Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research (20350088) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and the Seeds Excavation Program from Japan Science and Technology Agency (JST) of Japan. Combustion elemental analyses and measurements of HRMS were carried out at the Natural Science Center for Basic Research and Development (N-BARD), Hiroshima University. Supporting Information Available: Instrumentation for the characterization of compounds and physicochemical evaluation, crystallographic information file (CIF) for 14, complete entry for ref 12, NMR spectra of compounds (PDF). This material is available free of charge via the Internet at http:// pubs.acs.org. (22) teXsan: Single Crystal Structure Analysis Software, Version 1.2; Molecular Structure Corporation and Rigaku Corporation: The Woodlands, TX, 2000. (23) Ong, B. S.; Wu, Y.; Liu, P.; Gardner, S. J. Am. Chem. Soc. 2004, 126, 3378–3379.
