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Synthesis, Solid State Properties, and Semiconductor Measurements of 5,6,11,12-Tetrachlorotetracene Elisey Yagodkin,†,‡ Yu Xia,‡ Vivek Kalihari,‡ C. Daniel Frisbie,*,†,‡ and Christopher J. Douglas*,† Department of Chemistry, UniVersity of Minnesota-Twin Cities, Minneapolis, MN 55455, and Department of Chemical Engineering and Material Science, UniVersity of Minnesota-Twin Cities, Minneapolis, MN 55455 ReceiVed: May 13, 2009; ReVised Manuscript ReceiVed: July 29, 2009
Tetrachlorotetracene is an organic semiconductor and has possible applications in flexible organic devices. We have synthesized tetrachlorotetracene in multigram quantities in three steps from commercially available substances, with an overall yield of 52%. X-ray crystallographic analysis of tetrachlorotetracene and its precursor dihydroxytetracenedione showed similar packing structures, with better pitch stacking and roll stacking angles observed for tetrachlorotetracene. Single crystals of tetrachlorotetracene are semiconducting with field effect hole mobility values up to 0.2 cm2/V s. The hole mobility has been measured in the temperature range of 230-290 K, and Arrhenius behavior was observed, with an activation energy of nearly 200 meV. Such a large activation energy suggests significant carrier trapping. Air stability studies showed slow degradation of the crystal surface by atomic force microscopy, along with degradation of the semiconducting properties. We hypothesize that the instability of tetrachlorotetracene to air is the result of decomposition to a quinone species, a degradation previously observed in solution phase studies. 1. Introduction Modern organic semiconducting materials have potential application in flexible electronic devices because they have charge carrier mobility values comparable to amorphous silicon (a-Si, µFET ) 10-1 - 1 cm2/V s).1,2 Tetracenes, along with pentacenes, are well-known semiconducting materials.3 Although tetracene and several of its derivatives pack in crystals with a herringbone motif,4 some tetracene analogs such as 5,6,11,12-tetraphenyltetracene (rubrene) 1 have face-to-face stacking with much better π-π orbital overlap, resulting in exceptional mobility values of up to 20 cm2/V s.5 The high mobility values are obtained in field effect transistors (FETs) based on single crystals of rubrene in the direction of tetracene core stacking.6 To better understand the exceptional behavior of rubrene single crystal devices, we are undertaking the synthesis and characterization of rubrene and its analogs such as tetrachlorotetracene 2. Tetrachlorotetracene was first synthesized in 1985,7 and there was an initial study of its electronic properties.8 In this paper, we describe a synthesis of tetrachlorotetracene 2, crystal structures of 2 and its synthetic precursor dihydroxytetracenedione 3, and FET properties and atomic force microscopy (AFM) measurements of 2. 2. Experimental Methods 2A. Synthesis. All reagents and catalysts were purchased from Aldrich and used as received. N,N′-Dimethylformamide (DMF) was vacuum distilled from 3 Å molecular sieves, and nitrobenzene was distilled from CaH2. 1H and 13C NMR spectra were recorded on a Varian 300 MHz instrument. Chemical shifts are reported as δ values in parts per million (ppm) relative to chloroform or tetramethylsilane. 1H NMR coupling constants * To whom correspondence should be addressed. E-mail: frisbie@ cems.umn.edu (C.D.F.),
[email protected] (C.J.D.). † Department of Chemistry. ‡ Department of Chemical Engineering and Material Science.
are reported in Hz. Multiplicity is indicated as follows: s (singlet), d (doublet), and dd (doublet of doublets). 6,11-Dihydroxy-5,12-tetracenedione 3. A flame-dried 3-neck round-bottomed flask was equipped with an overhead mechanical stirrer, dropping funnel, and internal temperature thermometer. AlCl3 (60.0 g, 0.44 mol) and nitrobenzene (35 mL) were added and stirred at 70 °C for 15 min under nitrogen. Additional nitrobenzene (approximately 15 mL) was added until the AlCl3 was dissolved. 1,4-Dihydroxynaphthalene9 (20.0 g, 0.124 mol) and phthaloyl chloride (26.0 g, 0.128 mol) were combined and added over 25 min by dropping funnel to the reaction flask, and the funnel was washed with nitrobenzene (15 mL). The solution was stirred for 12 h at 70 °C. Water (100 mL) and concentrated HCl (1000 mL) were added slowly and sequentially by dropping funnel, and the reaction mixture was stirred vigorously at 90 °C for 2 h. The reaction mixture was allowed to cool to room temperature and filtered through a fritted glass filter. The solids were washed sequentially with concentrated HCl and acetone. The filtrate was transferred into a beaker containing a solution of sodium potassium tartrate tetrahydrate (180 g, 0.6 mol) in H2O (1500 mL), and the mixture was stirred for 5 h at 70 °C. The mixture was filtered though a fritted glass filter, and the solids were washed sequentially with H2O and acetone. The solids were dried in a vacuum desiccator containing P2O5 to give 3 as a red powder (28.0 g, 0.096 mol, 78%): mp 349-351 °C. 1H NMR (300 MHz, CDCl3): δ 15.19 (s, 2H), 8.51 (dd, J ) 3.3, 6.0, 4H), 7.84 (dd, J ) 3.0, 6.0, 4H). Crystals for X-ray crystallographic analysis were obtained by slow evaporation of 3 in a saturated chloroform solution. 5,5,6,11,12,12-Hexachloro-5,12-dihydrotetracene 4. POCl3 (120 mL, 210 g, 1.4 mol) was added slowly by dropping funnel to a mixture of PCl5 (50 g, 0.35 mol) and 3 (10 g, 0.034 mol) in a flame-dried round-bottomed flask. The red reaction mixture was heated to reflux (approximately 110 °C) and maintained for 5 h until the red color had dissipated. The reaction mixture was cooled to room temperature and filtered through a fritted
10.1021/jp904476z CCC: $40.75 2009 American Chemical Society Published on Web 08/20/2009
Synthesis and Properties of Tetrachlorotetracene
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SCHEME 1: Synthesis of Tetrachlorotetracene 2
glass filter. The precipitate was washed sequentially with glacial acetic acid and hexanes. The residue was collected and dried under vacuum to give 4 as a white powder (12.27 g, 0.028 mol, 84%): mp 280-283 °C. 1H NMR (300 MHz, CDCl3): δ 8.69 (dd, J ) 3.3, 6.6, 2H), 8.14 (dd, J ) 3.3, 6.3, 2H), 7.80 (dd, J ) 3.3, 6.6, 2H), 7.55 (dd, J ) 3.3, 6.3, 2H). 5,6,11,12-Tetrachlorotetracene 2. DMF (200 mL) was added to 4 (15 g, 0.034 mol) and NaI (22 g, 0.015 mol). The reaction mixture was heated to reflux under nitrogen for 30 min, cooled to 80 °C, and filtered warm through a fritted glass filter. The precipitate was washed sequentially with ethanol, water, and acetone. The residue was dried under vacuum to give 2 as a red solid (10.33 g, 0.028 mol, 79%): mp 215-217 °C. 1H NMR (300 MHz, CDCl3): δ 8.60 (dd, J ) 3.3, 6.9, 4H), 7.62 (dd, J ) 3.3, 6.9, 4H). Crystals for X-ray crystallographic analysis were obtained by slow evaporation of a saturated chloroform solution. 2B. X-ray Crystallography. Single crystal X-ray crystallographic data were collected on a Siemens CCD diffractometer with graphite monochromated Mo KR radiation (λ ) 0.71073 Å) operated at 50 kV and 40 mA. The structures were solved by direct methods and refined on F2 using the SHELX-97 software package. Absorption corrections were applied using SADABS. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in idealized positions and refined with a riding model. 2C. Crystal Sublimation, OFET Preparation and Measurements. Device quality single crystals of tetrachlorotetracene 2 were grown in a quartz tube through horizontal physical vapor transport10 using Ar as the carrier gas. Tetrachlorotetracene powder was placed at the source end of the tube with a temperature of 190 °C, allowing the powder to sublime. The tetrachlorotetracene vapor recrystallized at the downstream of the tube at a lower temperature of around 140 °C. The growth process took approximately 3 weeks and yielded needle-like crystals approximately 5-10 mm in length and up to 0.2 mm in width and thickness (Figure 5b). A subsequent sublimation was performed on the obtained crystals to increase the quality of the final product. Field effect transistors were fabricated by laminating the long axis of a single crystal along the gap of the metal-coated poly(dimethylsiloxane) PDMS stamp similar to that first reported by Menard, et. al (Figure 5a).11 The gap feature served as the gate dielectric layer, and the metal-coated areas functioned as source, drain, and gate electrodes, respectively. Channel length L varied from 100 to 300 µm. Channel width W was determined by the width of the crystal. Dielectric thickness t was 4.9 µm measured by profilometry. Electrical characterization of the devices was carried out using a Desert Cryogenics vacuum probe station with Keithley 237 and 6517A electrometers. 2D. AFM Characterization. AFM characterization was done using contact mode on a Veeco Nanoscope IIIA multimode atomic force microscope under ambient conditions. The probes used for AFM measurements were silicon nitride V-shaped cantilevers with integrated contact mode tips fabricated by Veeco Metrology (model DNP and force constant ∼0.58 N/m).
The image was obtained at a nominal load of ∼2 nN, and the scan rate for a 10 µm X10 µm image size was 1.5 Hz. 3. Results and Discussion 3A. Synthesis. We have modified the known synthetic route to tetrachlorotetracene 27 to be amenable to multigram scale (Scheme 1). Our synthesis begins with a Friedel-Crafts acylation of phthaloyl chloride and 1,4-dihydroxynaphthalene. While formation of the red dihydroxytetracenedione 3 was readily observed, isolation of 3 using the previous methods12 was incompatible with our much larger scale. By dissolving the crude product with its aluminum-containing impurities in HCl/H2O and precipitating only the desired product from solution using a sodium potassium tartrate mixture, we were able to isolate dihydroxytetracenedione in good yield (78%). Material prepared by this method was typically higher in quality as judged by 1H NMR and thin-layer chromatography compared to material available at relatively high price from typical fine chemical vendors (Sigma-Aldrich, 1 g, $59.10). Chlorination of 3 using phosphorus oxychloride and phosphorus pentachloride (84%), followed by dechlorination with sodium iodide in DMF (79%), proceeded largely according to the established procedure,7 with only minor modifications to improve yield and purity. 3B. X-ray Crystallography. Crystals of dihydroxytetracenedione 3 were obtained by slow evaporation from nitrobenzene solution.13 In the solid state, dihydroxytetracenedione 3 forms stacks along the b-axis with the interplanar distance (R) of 3.41 Å, pitch stacking angle (Φp) of 66.03° (Figure 2a) and a roll stacking angle (Φr) of 88.78° (Figure 2b). The pitch angle is the molecular slip along the long axis of the acene in a stack, while the roll angle is the molecular slip along the short molecular axis.14 Crystals of tetrachlorotetracene 2 were obtained by slow evaporation from chloroform and are crystallographically similar to the crystals obtained by sublimation (Section 2C). Crystallography was performed on the larger crystals of 2 available from sublimation to determine the structure and packing arrangement. Crystals of 2 harvested from the solution growth
Figure 1. Rubrene (1) and targeted rubrene analogs (2-4).
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Figure 2. (A) Stacks of dihydroxytetracenedione 3 along the b-axis with pitch stacking angle (Φp) of 66.03°. (B) Roll stacking angle (Φr) of 88.78°.
Figure 3. (A) Stacks of tetrachlorotetracene 2 along the b-axis with pitch stacking angle (Φp) of 71.53°. (B) Roll stacking angle (Φr) of 82.67°.
were also mounted on the defractometer, and the unit cell collection indicated an identical unit cell to the sample of 2 prepared from vapor phase growth. Tetrachlorotetracene 2 has similar pi-face stacking in the solid state compared to the dihydroxytetracenedione 3, with stacking angles closer to 90°. The pitch stacking angle (Φp) is 71.53° (Figure 3a), and the roll stacking angle (Φr) is 82.67° (Figure 3b). Tetrachlorotetracene 2 showed a larger interplanar distance (R) of 3.56 Å compared to that of 3, likely due to the presence of more sterically demanding chlorine atoms. 3C. AFM of Single Crystals. Figure 4a presents a typical topography image of a tetrachlorotetracene single crystal. It is clear from the height image that the single crystal has steps that are evenly distributed on the surface. The observed step density is about one step per 2 µm. The observed steps were oriented perpendicular to the long axis of the needle-shaped crystal. The measured step height is ∼12 Å (Figure 4c), which indicates that the molecules are standing up along the c-axis with some tilt with respect to the surface normal. It was also observed that the single crystal surface deteriorated after air exposure for 1 day. Figure 4b clearly demonstrates formation of pits on the single crystal surface when it was imaged under AFM after a day of air exposure. 3D. Electrical Characterization. As shown in Figure 5a, tetrachlorotetracene single crystal field effect transistors were fabricated using a vacuum as a gate dielectric. This approach minimized the interface defects between the crystal and gate dielectric and allowed us to study the charge transport on the pristine surface of the single crystals. Figure 5c shows the transfer characteristics (channel current versus gate voltage, ID-VG) of a typical FET, which were acquired with a drain-source voltage (VD) of -30 V. As is shown, the transistor turned on near 0 V, with an on-off current ratio of over 104 and relatively small hysteresis. The output characteristics (ID-VD, Figure 5d) also show typical FET behavior. We have calculated the linear mobility of the FETs using the standard equation µlin ) (eLID)/(WC′(VG - Vth)VD), where C′ is the measured sheet capacitance of the vacuum dielectric, and L and W are the channel length and width, respectively.15 Our tetrachlorotetracene single crystal FETs have a mobility range of 0.02-0.07 cm2/V s. It is worth noting that these values were considerably smaller than those recently reported by Chi, et al.8
From the transfer characteristics in Figure 5c, one may notice that the slope of the ID-VG characteristic increased with decreasing VG. This phenomenon is commonly observed when there are contact barriers for charge injection and release (i.e., contacts between single crystal and source-drain electrode are not ohmic contacts) or when there is a large density of carrier traps in the semiconductor channel.16 To compensate for the contact resistance, we adopted the four-probe measurement,17 which yielded a corrected mobility as high as 0.2 cm2/V s. In addition, we investigated the FET characteristics over the temperature range of 230-290 K, and the mobility exhibited approximate Arrhenius behavior, with a rather large activation energy of nearly 200 meV (Supporting Information). Such a large activation energy is usually indicative of deep trap states. The low capacitance of the vacuum dielectric means that the injected carrier density in these FETs is much smaller compared to that in transistors with conventional dielectrics such as SiO2 (1011 versus 1013 cm-2); hence, the carrier trapping effects are exacerbated (fewer traps are filled). Still, it is noteworthy that rubrene crystals examined in the same vacuum gap test bed can exhibit hole mobilities as high as 20 cm2/V s. After the single-crystal FET based on 2 was exposed to air for several days, no conductance or semiconductor behavior was observed. This is consistent with the surface degradation observed with time using AFM. Tetrachlorotetracene is known to react with oxygen and give peroxide 5, which can continue to undergo further and irreversible decomposition to form quinone 6 (Scheme 2).7 Our proposed degradation pathway is similar to the reaction of rubrene 1 with oxygen to form peroxide 7, although rubrene 1 can be regenerated from the peroxide upon heating or decreasing pressure.18 To confirm that quinone formation via reaction with oxygen was the cause of the surface degradation rather than other pathways (such as acene dimerization), we sought to identify the formation of 6 in solid samples of 2. Crystals of 2 were allowed to stand in a beaker exposed to ambient air and light for two days. The crystals were then dissolved, and the sample was analyzed by 1H NMR. Very weak singles were observed that matched an independently synthesized sample of 6. We increased the surface area on samples of 2 by pulverizing crystals and repeating the exposure to ambient air and light for 2 days. These samples showed substantial decomposition to quinone 6 upon dissolution and analysis by 1H NMR. This mixture now contained approximately 7 mol % of quinone 6. To determine that quinone 6 was forming on the surface of samples of 2, we recorded the reflection infrared spectra (IR) of freshly prepared thin films of 2 and 6. After allowing the sample of 2 to age in ambient light and air, the reflection IR spectrum was again recorded and singles for 2 and 6 were observed. Taken in combination with the NMR studies above, we conclude that the formation of 6 occurs readily upon prolonged exposure of 2 to air and light.
Synthesis and Properties of Tetrachlorotetracene
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Figure 4. (A) Topography image of a freshly grown tetrachlorotetracene single crystal surface showing steps on the crystal surface. Step density is about one step per 2 µm. (B) Topography of the single crystal surface after it was exposed to air for 1 day. (C) Measured step height is ∼12 Å. (D) Indexed crystal and schematic of molecules on the surface.
stances, with an overall yield of 52%. X-ray crystallographic analysis of tetrachlorotetracene 2 and its precursor dihydroxytetracenedione 3 showed similar packing structures, with better pitch stacking and roll stacking angles observed for tetrachlorotetracene 2. Single crystals of tetrachlorotetracene 2 are semiconducting with field effect hole mobility values up to 0.2 cm2/V s. The hole mobility has been measured in the temperature range of 230-290 K, and Arrhenius behavior was observed, with an activation energy of nearly 200 meV. Such a large activation energy suggests significant carrier trapping. Air stability studies showed degradation of the surface of the crystals by AFM, along with degradation of the semiconducting properties. We hypothesize that the instability of 2 to air is the result of decomposition to a quinone species, a degradation previously observed in solution phase studies. Figure 5. Tetrachlorotetracene single crystal field effect transistors. (A) Cross-section schematic of a vacuum-gated single-crystal field effect transistor. (B) Optical image of a tetrachlorotetracene single crystal under polarized light. (C) Transfer characteristics and (D) output characteristics of a typical transistor.
SCHEME 2: Reactions of Tetrachlorotetracene 2 and Rubrene 1 with oxygen
Acknowledgment. We thank Drs. Letitia Yao and Victor Young for assistance with NMR spectroscopy and X-ray crystallography, respectively. The National Science Foundation (NSF) Materials Research Science and Engineering Centers program under DMR-0212302 primarily supported this work. Partial support for facilities and supplies was provided from the University of Minnesota startup funds (C.J.D.). Part of this work was carried out at the Institute of Technology Characterization Facility, University of Minnesota, which received partial support from NSF through the National Nanotechnology Infrastructure Network Program. Supporting Information Available: NMR spectra for compounds 2, 3, 4, and 6. NMR spectra of 2 before and after standing in ambient air and light as well as reflection IR spectra for thin films of 2 and 6 and an aged thin film of 2. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
4. Conclusions Tetrachlorotetracene 2 was synthesized in multigram quantities in three steps from commercially available sub-
(1) Horowitz, G. AdV. Mater. 1998, 10, 365–377. (2) (a) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99–117. (b) Lee Comber, P. G.; Spear, W. E. Phys. ReV. Lett. 1970, 25, 509. (3) Anthony, J. E. Angew. Chem., Int. Ed. 2007, 46, 2–34. (4) Desiraju, G. R.; Gavezzotti, A. J. Chem. Soc., Chem. Commun. 1989, 621–623. (5) Podzorov, V.; Menard, E.; Borissov, A.; Kiryukhin, V.; Rogers, J. A.; Gershenson, M. E. Phys. ReV. Lett. 2004, 93, 086602. (6) Filho, D. A. S.; Kim, E. G.; Bredas, J. L. AdV. Mater. 2005, 17, 1072–1076.
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(7) Balodis, K. A.; Medne, R. S.; Neiland, O. Y.; Kozlova, L. M.; Klyavinya, Z. P.; Mazheika, I. B.; Gaukhman, A. P. Zh. Org. Khim. 1985, 21, 2216–2219. (8) Chi, X.; Li, D.; Zhang, H.; Chen, Y.; Garcia, V.; Garcia, C.; Siegrist, T. Organic Electronics 2008, 9, 234–240. (9) Oatis, E. J.; Walle, T.; Daniell, H. B.; Gaffney, T. E.; Knapp, D. R. J. Med. Chem. 1985, 28, 822. (10) Laudise, R. A.; Kloc, C.; Simpkins, P. G.; Siegrist, T. J. Cryst. Growth 1998, 187, 449. (11) Menard, E.; Podzorov, V.; Hur, S.-H.; Gaur, A.; Gershenson, Rogers, M. E. AdV. Mater. 2004, 16, 2097. (12) (a) Sartori, G.; Casnati, G.; Bigi, F.; Robles, P. Tetrahedron Lett. 1987, 28, 1533–1536. (b) Dhananjeyan, M. R.; Milev, Y. P.; Kron, M. A.; Nair, M. G. J. Med. Chem. 2005, 48, 2822.
Yagodkin et al. (13) While our study was underway, a report of the crystal structure of 2 appeared elsewhere Tomura, M.; Yamaguchi, H.; Ono, K.; Saito, K. Acta Crystallogr., Sect. E: Struct. Rep. 2008, E64, o172. (14) Curtis, D. M.; Cao, L.; Kampf, J. W. J. Am. Chem. Soc. 2004, 126, 4318. (15) Sze, S. M. In Physics of Semiconductor DeVices 2nd ed.; John Wiley & Sons: New York, 1981. (16) Horowitz, G. J. Mater. Res. 2004, 19, 1946. (17) Pesavento, P. V.; Chesterfield, R. J.; Newman, C. R.; Frisbie, C. D. J. Appl. Phys. 2004, 96, 7312. (18) Wittig, G.; Waldi, D. J. Prakt. Chem. 1942, 160, 242.
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