Comprehensive Evaluation of Electron Mobility for a Trifluoroacetyl

J. Phys. Chem. C 2009, 113, 17189–17193

17189

Comprehensive Evaluation of Electron Mobility for a Trifluoroacetyl-Terminated Electronegative Conjugated Oligomer Yutaka Ie,† Masashi Nitani,† Takafumi Uemura,‡ Yukihiro Tominari,‡ Jun Takeya,‡,§ Yoshihito Honsho,| Akinori Saeki,† Shu Seki,|,§ and Yoshio Aso*,† The Institute of Scientific and Industrial Research, Osaka UniVersity, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan, Graduate School of Science, Osaka UniVersity, Machikaneyama, Toyonaka 560-0043, Japan, PRESTO-JST, Kawaguchi 333-0012, Japan, and Department of Applied Chemistry, Graduate School of Engineering, Osaka UniVersity, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan ReceiVed: May 28, 2009

We have synthesized the electronegative oligomer having trifluoroacetyl groups at the terminal positions as new organic semiconductors and revealed that the presence of trifluoroacetyl groups is effective in lowering the lowest unoccupied molecular orbital energy level and arranging the molecules in crystals. The oligomer exhibited good n-type characteristics on thin-film field-effect transistor (FET) devices with field-effect electron mobility of 0.08 cm2 V-1 s-1. Further evaluation of electron mobility for this compound by a single-crystal FET device and time-resolved microwave conductivity measurements has been accomplished, and the electron mobilities are calculated to be ∼0.2 and >2.4 cm2 V-1 s-1, respectively. Direct comparison of electron mobilities estimated from these methods shows the importance of not only the molecular properties and stacking structure but also the interface with gate dielectrics and metal electrodes. Introduction π-Conjugated systems for application to organic field-effect transistors (OFETs) have been significantly developed over the past few decades, and some p-type OFET materials have outperformed amorphous silicon in terms of hole mobility.1 However, superior n-type OFET materials, which are essential for the fabrication of complementary circuits, are still limited because of the insufficiency of certain guidelines for material design,2 although only a few π-conjugated systems such as perfluoroalkyl-substituted oligomers,3-6 acene carboxylic diimides,7-13 and dicyanovinylene-substituted compounds14,15 have been reported as active materials for this purpose. To accelerate the development of new π-conjugated systems for n-type OFET materials, we find experimental evaluation of intrinsic electron mobility is important. For this purpose, single-crystal OFET devices16 and time-resolved microwave conductivity (TRMC) measurements17 have an advantage over thin-film OFET devices.18 However, only a limited number of studies have been reported for the evaluation of n-type materials with the use of single-crystal OFET devices19 and TRMC measurements.20 Recently, oligothiophenes bearing carbonyl groups have been exploited as n-type OFET materials,21 and we have revealed that the presence of trifluoroacetyl groups at the terminal positions of the π-conjugated backbone is effective in lowering the lowest unoccupied molecular orbital (LUMO) energy level and arranging the molecules in crystals.22 Motivated by these results, we have newly designed 2T-FAc as a simple and ideal compound for the present study (Figure 1). In this article, we * To whom correspondence should be addressed. E-mail: aso@ sanken.osaka-u.ac.jp. Telephone: +81-(0)6-6879-8475. Fax: +81-(0)6-68798479. † The Institute of Scientific and Industrial Research, Osaka University. ‡ Graduate School of Science, Osaka University. § PRESTO-JST. | Graduate School of Engineering, Osaka University.

Figure 1. (a) Chemical structure and (b) calculated HOMO and LUMO energies of 2T-FAc.

report on the synthesis, properties, structure, and comprehensive evaluation of electron mobility for 2T-FAc. Experimental Section General Information. 1H NMR and 13C NMR spectra were recorded on a Bruker ARX-400 in CDCl3 with tetramethylsilane as an internal standard. Data are reported as follows: chemical shift in ppm (δ), multiplicity (d ) doublet), coupling constant (Hz), and integration. Mass spectra were obtained on a Shimadzu GCMS-QP-5050. UV-visible spectra were recorded on a Shimadzu UV-3100PC. Fluorescence spectra were recorded using a Fluoromax-2 spectrometer in the photocounting mode equipped with a Hamamatsu R928 photomultiplier. The bandpass for the emission spectra was 1.0 nm. All spectra were obtained in spectrograde solvents. Fluorescence quantum efficiencies are measured by using diphenylanthracene (Φf ) 0.90 in cyclohexane) as a standard. The concentrations of solutions were adjusted to yield an absorptivity of A < 0.1 in the absorption spectrum for any fluorescence experiments. Cyclic voltammetry was carried out on a BAS CV-50W voltammetric analyzer. Elemental analysis was performed on PerkinElmer LS50B. The surface structure of the deposited organic film was

10.1021/jp9077322 CCC: $40.75  2009 American Chemical Society Published on Web 09/02/2009

17190

J. Phys. Chem. C, Vol. 113, No. 39, 2009

observed by atomic force microscopy (Shimadzu, SPM9600), and the film crystallinity was evaluated by X-ray diffractometer (Rigaku, RINT2500). Materials. All reactions were carried out under a nitrogen atmosphere. Solvents of the highest purity grade were used as received. Unless stated otherwise, all reagents were purchased from commercial sources and used without purification. 5,5′Bis(tributhylstannyl)-2,2′-bithiopene (1) was prepared by the reported procedure.23 1H NMR data of 1 were in agreement with that previously reported. Synthesis. 1 (1.49 g, 2.00 mmol), 4′-bromo-2,2,2-trifluoroacetophenone (1.27 mg, 5.00 mmol), and tetrakis(triphenylphosphine)palladium(0) (115 mg, 0.050 mmol) were placed in a test tube and dissolved with toluene (20 mL). The reaction mixture was stirred at 120 °C for 19 h. After being cooled at room temperature, the red solid was collected and washed several times with methanol and Et2O. The red solid was purified by gradient sublimation to give 2T-FAc (863 mg, 85%). Reddish orange solid; mp 236-237 °C; 1H NMR (CDCl3) δ 8.10 (d, 4H, J ) 7.1 Hz), 7.77 (d, 4H, J ) 7.1 Hz), 7.47 (d, 2H, J ) 3.9 Hz), 7.30 (d, 2H, J ) 3.9 Hz); MS (EI) m/z 510 (M+); UV/vis (THF) λ maxabs 428 nm (ε 92000), λ maxems 510 nm (Φf 0.39). Anal. Calcd for C24H12F6O2S2: C, 56.47; H, 2.37; N, 0.00. Found: C, 56.27; H, 2.32; N, 0.00. Device Fabrication of Thin-Film Transistors. The fieldeffect mobility of 2T-FAc was measured using top-contact fieldeffect transistor (FET) geometry. The p-doped silicon substrate functions as the gate electrode. A 300 nm thick silicon oxide dielectric layer with a capacitance of 10.0 nF cm-2 was thermally grown on the gate substrate. The silicon oxide surface was first washed with acetone and 2-propanol. The silicon oxide surface was then activated by ozone treatment and pretreated with HMDS. The substrate was washed again with toluene, acetone, and 2-propanol. The semiconductor layer was vacuum deposited on the Si/SiO2 at a rate of 1 Å/s under a pressure of 10-6 Pa to a thickness of 10 nm determined by a quartz crystal monitor. During the deposition of 2T-FAc, we held the substrate at a temperature of 90 °C. On the top of semiconductor layer, gold films (20 nm) as source and drain electrodes were deposited by using shadow masks with a channel width (5 mm) and channel length (100 µm). The characteristics of the OFET devices were measured at room temperature under a pressure of 10-3 Pa. The field-effect mobility (µ) was calculated in the saturated region at the source-drain voltage (VDS) of 100 V by the following equation of IDsat ) W/(2L) · Ci µsat(VG-Vth)2, where Ci is the capacitance of the SiO2 dielectric layer and Vth is the threshold gate voltage. Current on/off ratio was determined from the source-drain current (IDS) at the gate voltage (VGS) of 0 V and 100 V. Device Fabrication of Single-Crystal Transistors. Highly doped silicon with a 500 nm thermally oxidized SiO2 dielectric layer was used as a base substrate. The dielectric surface was covered with a polymethyl methacrylate (PMMA) buffer layer by spin coating in order to prevent electron-trapping influences of hydroxyl groups on the SiO2 surface, which were reported for thin films and single crystals.24,25 The 2T-FAc single-crystals, typically less than 1 µm in thickness, were grown by physical vapor transport in a stream of Ar gas. The temperatures for subliming the compound in the upstream and crystallizing zone in the downstream were kept at 235 °C and at 180 °C typically for 70 h, respectively. The single-crystals were transferred into a glovebox without exposing them to air and laminated onto the PMMA-coated substrate. The electrodes of gold and calcium were thermally evaporated in sequence from different angles

Ie et al. through a mask in a vacuum chamber that is connected to the glovebox with a gate valve. The transistor performances were measured using a Agilent Technology E5270 semiconductor parameter analyzer and probe station in the globe box. We note that the devices were fabricated and measured in a glovebox of Ar atmosphere, keeping the content of oxygen and water below 0.5 ppm. Time-Resolved Microwave Conductivity Measurement. The nanosecond laser pulses from a Nd:YAG laser [third harmonic generation, THG (355 nm) from Spectra Physics, INDY-HG, fwhm 5-8 ns] have been used as excitation sources. The power density of the laser was set at 0.14-4.8 mJ/cm2. For a time-resolved microwave conductivity (TRMC) measurement, the microwave frequency and power were set at ∼ 9.1 GHz and 3 mW, respectively, so that the motion of charge carriers cannot be disturbed by the low electric field of the microwave. The TRMC signal picked up by a diode (risetime < 1 ns) is monitored by a digital oscilloscope. All of the above experiments were carried out at room temperature. The transient photoconductivity (∆σ) of the samples is related to the reflected microwave power (∆Pr/Pr) and sum of the mobilities of charge carriers via

1 ∆Pr A Pr

(1)

∑ µφN

(2)

〈∆σ〉 ) ∆σ ) e

where A, e, φ, N, and Σµ are the sensitivity factor, elementary charge of the electron, photo carrier generation yield (quantum efficiency), number of absorbed photons per unit volume, and sum of mobilities for negative and positive carriers, respectively. Polarization of the laser pulses is isotropic. The measurement was carried out at room temperature under an Ar, air, and SF6 atmosphere. The single crystal of the compounds was placed on a quartz substrate. The number of photons absorbed by the film is estimated by calculation on the basis of the steady state absorption spectrum of the thin plane-like crystal and/or direct measurement of the transmitted power of laser pulses with a Opher NOVA-display power meter. The values of φ in the films were determined by conventional direct current (DC) integration in a vacuum chamber (2.4 cm2 V-1 s-1. The value of mobility gives the estimate of the oscillating displacement of electrons induced by the microwave electric field (∼102 V cm-1) in the cavity under the TRMC measurements as ∼2 nm, which is much larger than the molecular size of 2T-FAc along the c-axis in Figure 5. The local motion of electrons, hence, ranges over 5-6 molecular stacks, suggesting the presence of paths of highly mobile electrons along the stacking axes. Conclusion

Figure 6. (a) Schematic illustration of a 2T-FAc single-crystal transistor with asymmetric electrodes of gold and calcium and topview picture of a prepared top-contact device. In this device, the channel width (W) and length (L) are 145 and 95 µm, respectively. The thickness of PMMA is ∼80 nm. (b) Transfer characteristics of an ambipolar 2TFAc single-crystal FET in electron enhancement mode. (c) Output characteristics of a 2T-FAc single-crystal FET in electron enhancement mode.

dielectric layers and Vth is the threshold gate voltage.33 We consider that the higher performance of single-crystal FET is mainly associated with the minimization of the extrinsic effects of grain boundaries, which can be obstacles for carrier transport in microcrystalline thin-film devices. To clarify the influence of the interface with the gate dielectrics and metal electrode, we applied the electrodeless TRMC method. Figure 7a shows the photoconductivity transients observed for the 2T-FAc single crystal under an excitation of 355 nm. The initial decay kinetics observed within ∼5 µs obey the pseudo-first-order ones in the range of excitation density, suggesting that the predominant charge carrier species are inactivated within the time range by recombination with some impurities such as defects in the materials other than

In summary, the electronegative oligomer bearing trifluoroacetyl groups at the terminal positions was synthesized, and its electronic properties and structures were investigated by spectroscopic and electrochemical measurements and X-ray analysis. We have successfully evaluated the electron mobility of 2TFAc by thin-film FET devices, single-crystal FET devices, and TRMC measurements. To the best of our knowledge, direct comparison of electron mobilities estimated from these methods for n-type organic semiconductor materials has never been reported. Among the three methods, electrodeless measurements gave the highest electron mobility, indicating again the importance of not only the molecular properties and stacking structure but also the interface with gate dielectrics and metal electrodes.34,35 Further studies on the evaluation of electron mobilities for other n-type materials and on the improvement of the performance of thin-film OFET devices are underway. Acknowledgment. Thanks are given to the CAC, The Institute of Scientific and Industrial Research (ISIR), for assistance in obtaining elemental analysis. This work was supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan, the Industrial Technology Research Grant Program in 2008 from the New Energy and Industrial Technology Developing Organization (NEDO) of Japan, and by the Cooperative Research with Sumitomo Chemical Co., Ltd.

Trifluoroacetyl-Terminated Electronegative Oligomer Supporting Information Available: Crystallographic CIF file of 2T-FAc. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Anthony, J. E. Chem. ReV. 2006, 106, 5028–5048. (b) Murphy, A. R.; Fre´chet, J. M. J. Chem. ReV. 2007, 107, 1066–1096. (c) Takimiya, K.; Kunugi, Y.; Otsubo, T. Chem. Lett. 2007, 36, 578–583. (d) Anthony, J. E. Angew. Chem., Int. Ed. 2008, 47, 452–483. (2) (a) Newman, C. R.; Frisbie, C. D.; da Silva Filho, D. A.; Bre´das, J.-L.; Ewbank, P. C.; Mann, K. R. Chem. Mater. 2004, 16, 4436–4451. (b) Facchetti, A.; Yoon, M.-H.; Marks, T. J. AdV. Mater. 2005, 17, 1705–1725. (3) (a) Facchetti, A.; Deng, Y.; Wang, A.; Koide, Y.; Sirringhaus, H.; Marks, T. J.; Friend, R. H. Angew. Chem., Int. Ed. 2000, 39, 4547–4551. (b) Facchetti, A.; Mushrush, M.; Katz, H. E.; Marks, T. J. AdV. Mater. 2003, 15, 33–38. (c) Li, L.; Counts, K. E.; Kurosawa, S.; Teja, A. S.; Collard, D. M. AdV. Mater. 2004, 16, 180–183. (d) Dholakia, G. R.; Meyyappan, M.; Facchetti, A.; Marks, T. J. Nano. Lett. 2006, 6, 2447–2455. (4) (a) Facchetti, A.; Yoon, M.-H.; Stern, C. L.; Hutchison, G. R.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 13480–13501. (b) Facchetti, A.; Mushrush, M.; Yoon, M.-H.; Hutchison, G. R.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 13859–13874. (5) (a) Ando, S.; Nishida, J.; Tada, H.; Inoue, Y.; Tokito, S.; Yamashita, Y. J. Am. Chem. Soc. 2005, 127, 5336–5337. (b) Ando, S.; Murakami, R.; Nishida, J.; Tada, H.; Inoue, Y.; Tokito, S.; Yamashita, Y. J. Am. Chem. Soc. 2005, 127, 14996–14997. (6) (a) Ie, Y.; Umemoto, Y.; Kaneda, T.; Aso, Y. Org. Lett. 2006, 8, 5381–5384. (b) Ie, Y.; Nitani, M.; Ishikawa, M.; Nakayama, K.-i.; Tada, H.; Kaneda, T.; Aso, Y. Org. Lett. 2007, 9, 2115–2118. (c) Ie, Y.; Umemoto, Y.; Nitani, M.; Aso, Y. Pure. Appl. Chem. 2008, 80, 589–597. (7) (a) Katz, H. E.; Lovinger, A. J.; Johnson, J.; Kloc, C.; Siegrist, T.; Li, W.; Lin, Y.-Y.; Dodabalapur, A. Nature 2000, 404, 478–481. (b) Katz, H. E.; Johnson, J.; Lovinger, A. J.; Li, W. J. Am. Chem. Soc. 2000, 122, 7787–7792. (8) (a) Jones, B. A.; Ahrens, M. J.; Yoon, M.-H.; Facchetti, A.; Marks, T. J.; Wasielewski, M. R. Angew. Chem., Int. Ed. 2004, 43, 6363–6366. (b) Wang, Z.; Kim, C.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2007, 129, 13362–13363. (c) Jones, B. A.; Facchetti, A.; Wasielewski, M. R.; Marks, T. J. J. Am. Chem. Soc. 2007, 129, 15259–15278. (9) (a) Ling, M.-M.; Erk, P.; Gomez, M.; Koenemann, M.; Locklin, J.; Bao, Z. AdV. Mater. 2007, 19, 1123–1127. (b) Chen, H. Z.; Ling, M.M.; Mo, X.; Shi, M. M.; Wang, M.; Bao, Z. Chem. Mater. 2007, 19, 816– 824. (c) Ling, M.-M.; Bao, Z.; Erk, P.; Koenemann, M.; Gomez, M. Appl. Phys. Lett. 2007, 90, 093508-1-3. (d) Schmidt, R.; Ling, M. M.; Oh, J. H.; Winkler, M.; Ko¨nemann, M.; Bao, Z.; Wu¨rthner, F. AdV. Mater. 2007, 19, 3692–3695. (10) Hosoi, Y.; Tsunami, D.; Ishii, H.; Furukawa, Y. Chem. Phys. Lett. 2007, 436, 139–143. (11) Kao, C.-C.; Lin, P.; Lee, C.-C.; Wang, Y.-K.; Ho, J.-C.; Shen, Y.Y. Appl. Phys. Lett. 2007, 90, 212101-1-3. (12) Weitz, R. T.; Amsharov, K.; Zschieschang, U.; Villas, E. B.; Goswami, D. K.; Burghard, M.; Dosch, H.; Jansen, M.; Kern, K.; Klauk, H. J. Am. Chem. Soc. 2008, 130, 4637–4645. (13) (a) Yan, H.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J. R.; Do¨tz, F.; Kastler, M.; Facchetti, A. Nature 2009, 457, 679–687. (b) Chen, Z.; Zheng, Y.; Yan, H.; Facchetti, A. J. Am. Chem. Soc. 2009, 131, 8–9. (14) (a) Handa, S.; Miyazaki, E.; Takimiya, K.; Kunugi, Y. J. Am. Chem. Soc. 2007, 129, 11684–11685. (b) Kashiki, T.; Miyazaki, E.; Takimiya, K. Chem. Lett. 2009, 38, 568–569. (15) (a) Usta, H.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 8580–8581. (b) Usta, H.; Risko, C.; Wang, Z.; Huang, H.; Deliomeroglu, M. K.; Zhukhovitskiy, A.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2009, 131, 5586–5608. (16) de Boer, R. W. I.; Gershenson, M. E.; Morpurgo, A. F.; Podzorov, V. Phys. Status Solidi A 2004, 201, 1302–1331. (17) (a) Grozema, F. C.; Siebbeles, L. D. A.; Warman, J. M.; Seki, S.; Tagawa, S.; Scherf, U. AdV. Mater. 2002, 14, 228–231. (b) Acharya, A.; Seki, S.; Saeki, A.; Koizumi, Y.; Tagawa, S. Chem. Phys. Lett. 2005, 404, 356–360. (c) Warman, J. M.; Piris, J.; Pisula, W.; Kastler, M.; Wasserfallen,

J. Phys. Chem. C, Vol. 113, No. 39, 2009 17193 D.; Mu¨llen, K. J. Am. Chem. Soc. 2005, 127, 14257–14262. (d) Saeki, A.; Seki, S.; Tagawa, S. J. Appl. Phys. 2006, 100, 023703-1-6. (e) Saeki, A.; Seki, S.; Takenobu, T.; Iwasa, Y.; Tagawa, S. AdV. Mater. 2008, 20, 920– 923. (18) Coropceanu, V.; Cornil, J.; da Silva Filho, D. A.; Olivier, Y.; Silbey, R.; Bre´das, J.-L. Chem. ReV. 2007, 107, 926–952. (19) (a) Menard, E.; Podzorov, V.; Hur, S. H.; Gaur, A.; Gershenson, M. E.; Rogers, J. A. AdV. Mater. 2004, 16, 2097–2101. (b) Sakai, K.-i.; Hasegawa, T.; Ichikawa, M.; Taniguchi, M. Chem. Lett. 2006, 35, 302– 303. (c) Briseno, A. L.; Tseng, R. J.; Li, S.-H.; Chu, C.-W.; Yang, Y.; Falcao, E. H. L.; Wudl, F.; Ling, M. M.; Chen, H. Z.; Bao, Z.; Meng, H.; Kloc, C. Appl. Phys. Lett. 2006, 89, 222111-1-3. (d) Tang, Q.; Tong, Y.; Li, H.; Hu, W. Appl. Phys. Lett. 2008, 92, 083309-1-3. (e) Yamada, K.; Takeya, J.; Takenobu, T.; Iwasa, Y. Appl. Phys. Lett. 2008, 92, 2533111-3. (20) (a) Umemoto, Y.; Ie, Saeki, A.; Seki, S.; Tagawa, S.; Aso, Y. Org. Lett. 2008, 10, 1095–1098. (b) Amaya, T.; Seki, S.; Moriuchi, T.; Nakamoto, K.; Nakata, T.; Sakane, H.; Saeki, A.; Tagawa, S.; Hirao, T. J. Am. Chem. Soc. 2009, 131, 408–409. (21) (a) Yoon, M.-H.; DiBenedetto, S. A.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2005, 127, 1348–1349. (b) Letizia, J. A.; Facchetti, A.; Stern, C. L.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 2005, 127, 13476–13477. (c) Yoon, M.-H.; DiBenedetto, S. A.; Russel, M. T.; Facchetti, A.; Marks, T. J. Chem. Mater. 2007, 19, 4864–4881. (d) Cai, X.; Gerlach, C. P.; Frisbie, C. D. J. Phys. Chem. C. 2007, 111, 452–456. (e) Ie, Y.; Umemoto, Y.; Okabe, M.; Kusunoki, T.; Nakayama, K.-i.; Pu, Y.-J.; Kido, J.; Tada, H.; Aso, Y. Org. Lett. 2008, 10, 833–837. (f) Lee, T.; Landis, C. A.; Dhar, B. M.; Jung, B. J.; Sun, J.; Sarjeant, A.; Lee, H.-J.; Katz, H. E. J. Am. Chem. Soc. 2009, 131, 1692–1705. (g) Ie, Y.; Okabe, M.; Umemoto, Y.; Tada, H.; Aso, Y. Chem. Lett. 2009, 38, 460–461. (22) Ie, Y.; Nitani, M.; Aso, Y. Chem. Lett. 2007, 36, 1326–1327. (23) Hucke, A.; Cava, M. P. J. Org. Chem. 1998, 63, 7413–7417. (24) Chua, L. L.; Zaumseil, J.; Chang, J. F.; Ou, E. C. W.; Ho, P. K. H.; Sirringhaus, H.; Friend, R. H. Nature 2005, 434, 194–199. (25) Takahashi, T.; Takenobu, T.; Takeya, J.; Iwasa, Y. Appl. Phys. Lett. 2006, 88, 033505-1-3. (26) Mori, T.; Kobayashi, A.; Sasaki, Y.; Kobayashi, H.; Saito, G.; Inokuchi, H. Bull. Chem. Soc. Jpn. 1984, 57, 627–633. (27) The calculation was conducted using the Gaussian 03 program. The geometry was optimized with the restricted Becke Hybrid (B3LYP) at 6-31 G(d, p) level. (28) Bard, A. J.; Faulkner, L. R. Electrochemical Methods-Fundamentals and Applications: Wiley: New York, 1984; The LUMO and HOMO energy levels are calculated under the premise that the energy level of Fc/Fc+ is -4.8 eV below the vacuum level. (29) The diffraction data of 2T-FAc were collected on a Rigaku Mercury CCD with monochromated Mo KR (λ ) 0.71075 Å) radiation. The structure was determined by direct method (SIR 97). The non-hydrogen atoms were refined anisotropically. Crystal data for 2T-FAc: C24H12O2S2F6, M ) 510.47, orthorhombic, space group Cmca(64), a ) 6.844(6) Å, b ) 22.211(19) Å, c ) 13.877(10) Å, V ) 2109.0(3) Å3, Z ) 4, Dcalc ) 1.607 gcm-3, F(000) ) 1032.00, µ ) 3.262 cm-1 (Mo KR, λ ) 0.71075 Å), 4558 reflections measured, 1304 unique, R ) 0.0405 for I > 2σ(I), and wR ) 0.0754 for all data. (30) Mori, T.; Kobayashi, A.; Sasaki, Y.; Kobayashi, H.; Saito, G.; Inokuchi, H. Bull. Chem. Soc. Jpn. 1984, 57, 627–633. (31) Takeya, J.; Goldmann, C.; Haas, S.; Pernstich, K. P.; Ketterer, B.; Batlogg, B. J. Appl. Phys. 2003, 94, 5800–5804. (32) Haddon, R. C.; Perel, A. S.; Morris, R. C.; Palstra, T. T. M.; Hebard, A. F.; Fleming, R. M. Appl. Phys. Lett. 1995, 67, 121–123. (33) The hole mobility is estimated as µhsat ∼ 4.3 × 10-3 cm2 V-1 s-1. (34) (a) Hulea, I. N.; Fratini, S.; Xie, H.; Mulder, C. L.; Iossad, N. N.; Rastelli, G.; Ciuchi, S.; Morpurgo, A. F. Nat. Mater. 2006, 5, 982–986. (b) Takeya, J.; Kato, J.; Hara, K.; Yamagishi, M.; Hirahara, R.; Yamada, K.; Nakazawa, Y.; Ikehata, S.; Tsukagoshi, K.; Aoyagi, Y.; Takenobu, T.; Iwasa, Y. Phys. ReV. Lett. 2007, 98, 196804-1-4. (35) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. AdV. Mater. 1999, 11, 605–625.

JP9077322