J. Phys. Chem. C 2008, 112, 7939–7945
7939
Syntheses, Solid State Structures, and Electrical Properties of Oxadiazole-Based Oligomers with Perfluorinated Endgroups† Chad A. Landis, Bal Mukund Dhar, Taegweon Lee, Amy Sarjeant, and Howard E. Katz* Departments of Materials Science and Engineering and Chemistry, Johns Hopkins UniVersity, 3400 North Charles Street, Baltimore, Maryland 21218 ReceiVed: December 17, 2007; ReVised Manuscript ReceiVed: January 26, 2008
A series of four new oligomers with oxadiazole rings and perfluorinated endgroups was synthesized, utilizing ring closure and coupling reaction procedures that we developed to be compatible with this family of compounds. Three shorter oligomers (three and four rings) crystallize readily and show rigid well-defined structures with close π interactions as determined by X-ray crystallography and thus are a suitable basis for further development as organic semiconductors. Reduction potentials (E1/2) are at or slightly more negative than –1.1 V versus Ag/AgCl. The longest of the oligomers, with a terthiophene core, showed hole mobility in a field-effect transistor of 0.002 cm2/Vs and a threshold voltage well into accumulation. On the basis of electrochemical redox potentials, we expect that elaboration of this oligomer motif with further electronwithdrawing groups will ultimately lead to n-channel organic semiconductors. Introduction Electronics based wholly on organic conductors,1 dielectrics,2 and semiconductors is a rapidly growing field of academic research and the target of considerable investment in the commercial sector. The vast majority of the semiconductors are p-channel semiconductors (p-OSCs) and they include a large family of highly conjugated molecules ranging from single fused-ring systems3 to oligomers of a variety of aromatic systems.4 The synthetic strategies have been used to alter the solubility, self-assembly, air-stability, and HOMO-LUMO levels of the resulting derivatives, mostly through the action of side chains, but occasionally via ring design. While p-OSCs have dominated the field, the number of n-channel organic semiconductors (n-OSCs) has been considerably fewer.5 Certain classes of semiconductor devices require pn junctions, and there are cases where having devices based on both p- and n-OSCs would result in a more efficient circuit.6 The problem with n-OSCs is that electrons are generally much less stable on organic semiconductors than are positive charges (holes). These electrons are easily quenched by environmental agents such as oxygen and water. Stable n-OSCs usually incorporate highly fluorinated groups either directly into the aromatic structure or as side chains at the end of the molecule to act as barriers against these quenching effects, and in some cases to lower the LUMO levels.7 There are a few examples of air-stable n-OSCs; these are derivatives that have highly electron-withdrawing groups present, such as tetracarboxylic diimides and perfluoroacyl substituted thiophene oligomers.8 There are few examples of relatively electron-deficient subunits being used as segments of the oligomer chains (as alternatives to thiophene rings, for example) in the construction of high-threshold voltage (Vth)/high on/off ratio p-OSCs and intrinsically n-OSCs. We consider 1,3,4-oxadiazoles to be particularly attractive for these purposes. Oxadiazoles are already in use as electron-transporting materials for OLEDs.9 Oxadiazoles would be expected to maintain the planarity and conjuga† Part of the “Larry Dalton Festschrift”. * To whom correspondence should be addressed.
Figure 1. Structures of oxadiazole derivatives.
tion of the oligomer while having a higher electron affinity than other heterocycles. In one electrochemical study, a poly(phenyloxadiazolyl) showed a 0.6 V increase in electron affinity (which would presumably be accompanied by an increase in ionization potential) versus polyphenylene.10 In the present manuscript, we report preliminary findings of our investigation of oxadiazole-based oligomers 1-4 (Figure 1). Although we have not yet reached a level of electron deficiency combined with morphology that would support efficient electron transport, we do demonstrate the electron deficiency via electrochemistry, useful solid state packing motifs, and sufficient intermolecular overlap to support accumulation-only hole transport in an OFET film. Experimental Section General. 1H and 13C NMR spectra were recorded on Bruker Avance (300 MHz/400 MHz) spectrometers. Mass spectral analyses were performed on either a GC/MS or an EI mass spectrometer at the University of Kentucky Mass Spectrometry Facility. DSC measurements were carried out by using a TA DSC Q20 modulated instrument at a heating rate of 10 °C/min under a nitrogen atmosphere. All of the electrochemical measurements were carried out in acetonitrile solutions for 1–3 and in dichloromethane solution for 4 containing 1 mM oligomer and 0.1 M tetrabutylammonium hexafluorophosphate (NBu4PF6)
10.1021/jp711836b CCC: $40.75 2008 American Chemical Society Published on Web 04/23/2008
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SCHEME 1: Syntheses of Short Oxadiazole Oligomers
SCHEME 2: Synthesis of Oxadiazole-Terthiophene Oligomer
as supporting electrolyte at room temperature using a Potentiostat/Galvanostat 263A, EG & G Instrument. The cyclic voltammograms were obtained at a scan rate of 50 mV/s. A platinum disk and platinum wire were used as a working and counter electrode, respectively. The reference electrode used was Ag/AgCl. 4-(Trifluoromethyl)benzoyl chloride and perfluorobenzoyl chloride were purchased from SynQuest Laboratories Inc. All other chemicals were supplied by Aldrich.
N,N′-Bis(4-trifluoromethylbenzoyl) hydrazide. Under dry conditions, 0.6 g (12 mmol) of hydrazine hydrate was dissolved in 50 mL of benzene and 0.2 mL of pyridine. Two equivalents (5 g, 24 mmol) of 4-(trifluoromethyl)benzoyl chloride were added dropwise. After stirring for 1 h, a white precipitate formed, which was collected by filtration (3.3 g, 8.8 mmol, 73% yield). The product was carried on without further purification. 1H NMR (DMSO-d 300 MHz) δ 7.89 (d, J ) 6.4 Hz, 4H), 6
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Figure 2. X-ray crystal packing structures and molecular structure of 1-3.
8.14 (d, J ) 7 Hz, 4H), 10.89 (s, 2H). 13C (DMSO-d6, 100 MHz) δ 121.5, 125.1 (q), 128.0, 131.4, 135.7, 164.3. MS (EI, 70 eV) m/z 376 (5%, M+), 357 (7%, M+ - F), 173 (100%, M+ - C8H6N2OF3). Anal. Calcd for C16H10F6N2O2: C, 51.07; H, 2.68; N, 7.45. Found: C, 51.01; H, 2.63; N, 7.34. 2,5-Bis(4-trifluoromethylphenyl)-1,3,4-oxadiazole (1). A total of 0.9 g (2.4 mmol) of the hydrazide was dissolved in 20 mL of acetonitrile, and 8 equiv of POCl3 were added. The solution was warmed to 80 °C and stirred overnight. The reaction mixture was poured onto ice, and the white precipitate was collected by filtration and purified by recrystallization from ethanol, giving 1 (0.65 g, 1.8 mmol, 75% yield) as white needles. 1H NMR (DMSO-d , 400 MHz) δ 7.98 (d, J ) 8.0 Hz, 4H), 6 8.34 (d, J ) 8.0 Hz, 4H). 13C (DMSO-d6, 100 MHz) δ 122.3, 126.3 (q), 126.9, 127.7, 131.7, 163.5. MS (EI, 70 eV) m/z 358 (30%, M+), 339 (10%, M+ -F), 233 (50%, M+ - C6F2), 173 (100%, CF3C6H4CO), 145 (80%, C6H4CF3). Anal. Calcd for C16H8F6N2O: C, 53.64; H, 2.25; N, 7.81; F, 31.81. Found: C, 53.08; H, 2.23; N, 7.61; F, 30.7.
N,N′-Bis(4-trifluoromethylbenzoyl)oxalic hydrazide. A suspension of 1.4 g (12 mmol) of oxalic dihydrazide in 50 mL of NMP was heated to 80 °C to dissolve the hydrazide, followed by dropwise addition of 4-(trifluoromethyl)benzoyl chloride (5 g, 24 mmol). The solution was stirred for 1 h at 80 °C and poured into water and the white precipitate was collected by filtration and washed twice with water. The white solid product (5.2 g, 11.3 mmol, 94% yield) was sufficiently pure to be carried on without further purification. 1H NMR (DMSO-d6, 300 MHz) δ 7.87 (d, J ) 8.3 Hz, 4H), 8.05 (d, J ) 8.1 Hz, 4H), 10.76 (s, 2H), 10.95 (s, 2H). MS (EI, 70 eV) m/z 462 (20%, M+), 173 (100%, M+ - C10H8N4O3F3). Anal. Calcd for C18H12F6N4O4: C, 46.76; H, 2.61; N, 12.11. Found: C, 46.85; H, 2.55; N, 11.88. Bis[2-(4-trifluoromethylphenyl)-1,3,4-oxadiazolyl-5] (2). A total of 5.2 g (11.3 mmol) of the above bishydrazide was suspended in 30 mL of acetonitrile. A total of 13.9 g (90.4 mmol) of POCl3 was added, and the mixture was set to reflux overnight. The reaction mixture was poured into water, and a dark brown solid was collected by filtration and applied to a
7942 J. Phys. Chem. C, Vol. 112, No. 21, 2008 silica gel pad. The product was eluted with 1:1 hexanes/ethyl acetate, and then 1.1 g (2.58 mmol, 23% yield) of tan solid was obtained. Further purification was done by recrystallization from ethanol ans gave 0.66 g of white solid. 1H NMR (DMSOd6, 400 MHz) δ 8.06 (d, J ) 8.1 Hz, 4H), 8.38 (d, J ) 8.1 Hz, 4H). MS (EI, 70 eV) m/z 426 (80%, M+), 407 (20%, M+ - F), 173 (50%, M+ - C8H4OF3). Anal. Calcd for C18H8F6N4O2: C, 50.71; H, 1.89; N, 13.14; F, 26.74. Found: C, 51.25; H, 2.18; N, 12.69; F, 27.6. N,N′-Bis(pentafluorobenzoyl)oxalic hydrazide. A total of 2 g (16.9 mmol) of oxalic dihydrazide was suspended in 20 mL of NMP, increasing the temperature to 80 °C to dissolve the dihydrazide. Then, 7.8 g (33.9 mmol) of perfluorobenzoyl chloride was added dropwise. The mixture was stirred at 80 °C for 1 h, cooled to room temperature, and stirred overnight. The reaction mixture was poured into water, and the white solid was collected by filtration, washing with water and ethanol. The product, 3.1 g (6.12 mmol, 36% yield) of a white solid, was sufficiently pure to carry on. 1H NMR (DMSO-d6, 400 MHz) δ 11.03 (s, 2H), 11.27 (s, 2H). Anal. Calcd for C16H4N4O2F10: C, 37.96; H, 0.79; N, 11.06. Found: C, 37.69; H, 0.71; N, 11.29. Bis[2-(pentafluorophenyl)-1,3,4-oxadiazolyl-5] (3). A total of 1.7 g (3.36 mmol) of the above bishydrazide was suspended in 30 mL of acetonitrile. Next, 1.1 g (7.4 mmol) of POCl3 was added to the mixture, and the temperature was increased to reflux. Stirring continued at reflux overnight followed by cooling to room temperature. The reaction mixture was poured into water, and the white solid was collected by filtration. The product was recrystallized from DMSO and collected as white needles (0.4 g, 0.85 mmol, 25% yield). MS (EI, 70 eV) m/z 470 (45%, M+), 235 (10%, M+ - C6F5C2N2O), 193 (100%, M+ - C9H2F5N4O). Anal. Calcd for C16F10N4O2: C, 40.87; N, 11.92; F, 40.41. Found: C, 40.84; N, 11.77; F, 36.0. N-(Pentafluorobenzoyl)-N′-(2-thiophenecarbonyl)hydrazide. A total of 2.13 g (15 mmol) of 2-thiophenecarboxylic acid hydrazide was dissolved in 20 mL of NMP, and then 2.16 mL (3.46 g, 15 mmol) of perfluorobenzoyl chloride was added dropwise at room temperature. The mixture was stirred overnight at that temperature. The reaction mixture was poured into water and followed by the addition of aqueous sodium bicarbonate slowly and carefully. The precipitated white solid was collected by filtration, washing with water. The product, 4.7 g (15 mmol, 93% yield) of a white solid, was sufficiently pure to carry on. 1H NMR (DMSO-d , 400 MHz) δ 7.12 (dd, 1H), 7.87 (d, J ) 6 4.5 Hz, 1H), 7.91 (d, J ) 3.6 Hz, 1H), 10.94 (s, 2H). Anal. Calcd for C12H5F5N2O2S: C, 42.87; H, 1.50; N, 8.33. Found: C, 42.68; H, 1.41; N, 8.05. 2-(2-Thienyl)-5-(perfluorophenyl)-1,3,4-oxadiazole. A total of 15 mL of POCl3 was added to 4.65 g (13.8 mmol) of the hydrazide and stirred 2 h at 110 °C. After cooling, the reaction mixture was poured onto ice, and the white precipitate was collected by filtration. The raw products were purified by recrystallization from ethanol, giving product (4.17 g, 13.1 mmol, 93% yield) as white plates. 1H NMR (CDCl3, 400 MHz) δ 7.21 (dd, 1H), 7.62 (d, J ) 4.8 Hz, 1H), 7.85 (d, J ) 4.0 Hz, 1H). Anal. Calcd for C12H3F5N2OS: C, 45.29; H, 0.95; N, 8.80. Found: C, 45.51; H, 0.83; N, 8.59. 2-(5-Bromo-2-thienyl)-5-(perfluorophenyl)-1,3,4-oxadiazole. Oxadiazole (3 g, 9.43 mmol) and NBS (1.68 g, 9.43 mmol) were dissolved in 8 mL of dichloromethane and 24 mL of trifluoroacetic acid mixed solvent. Without light, the solution was stirred for 1 day at ambient temperature. After 50 mL of water and the same amount of dichloromethane were added, the organic layer was washed 2 times with water. The collected
Landis et al. TABLE 1: Spectroscopic and Electrochemical Data for Oxadiazole Oligomers
1 2 3 4
mp (°C)
λmax (nm)a
1/2 Ered (V)b
LUMO (eV)
Eg (eV)d
HOMO (eV)
169.6 186.7 241.2 283.5
284 298 282 410
-1.1 -1.1 -1.0c -1.2c
3.3 3.3
4.01 3.84 3.97 2.55
7.3 7.1
a In dichloromethane. b In 0.1 M solution of Bu4NPF6 in acetonitrile for 1–3 and in dichloromethane for 4, Pt electrode, ferrocene as internal standard (set to 0.4 versus Ag/AgCl). c Irreversible. d Calculated from the optical absorption onset.
Figure 3. (a) Cyclic voltammograms of 1 (solid black), 2 (dashed red), and 3 (dashed blue). (b) Cyclic voltammogram of 4. The feature at +0.9 V is an artifact.
solution was dried with anhydrous MgSO4 and evaporated. The product was recrystallized from EtOH and acquired as an offwhite solid (2.35 g, 5.92 mmol, 63% yield). 1H NMR (CDCl3, 400 MHz) δ 7.17 (d, J ) 4 Hz, 1H), 7.59 (d, J ) 4 Hz, 1H). Anal. Calcd for C12H2BrF5N2OS: C, 36.29; H, 0.51; N, 7.05. Found: C, 36.19; H, 0.69; N, 6.97. 2,5′′-Bis(5-perfluorophenyl-1,3,4-oxadiazole-2-yl)-2,2′:5′,2′′terthiophene (4). After degassing with N2 blowing for 30 min, Pd(PPh3)4 (0.11 g, 0.1 mmol) was added to a stirred DMF solution (20 mL) of bromoxadiazole (1.59 g, 4 mmol) and 2,5bis(tributylstannyl)thiophene (1.32 g, 2 mmol). The reaction mixture was heated at 90 °C for 2 days. After cooling, the precipitated red solid was collected by filtration then washed with ethyl acetate and hot chlorobenzene. The highly insoluble crude product was purified by sublimation to yield 0.58 g (0.81 mmol, 41%) of red solid. Anal. Calcd for C28H6F10N4O2S3: C, 46.93; H, 0.84; N, 7.82. Found: C, 47.01; H, 1.05; N, 7.72. Mass Spectrum (MS): 716 (M+, 75%), 509 (bp, -C6F5CNN). High resolution MS calcd for C28H6F10N4O2S3: 715.55. Found: 715.95. UV-vis (CH2Cl2) 410 nm (max), 485 nm (onset).
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Figure 4. p-Channel OFET behavior of 4 on Cytop with Au electrodes.
Results and Discussion Synthesis. Oxadiazole-based oligomers cannot be constructed with the standard aryl-aryl coupling methods generally used for electron-rich heterocycles such as thiophenes. Most organometallic compounds, especially organolithium and Grignard reagents, have a tendency to reduce oxadiazoles rings rather than couple to them. There is no standard synthesis of 2-halo-1,3,4oxadiazoles to act as coupling partners. Instead, substituted oxadiazoles are typically formed by the ring closure of appropriately substituted hydrazides, and for mono-oxadiazoles with electron-rich substituents, these methods are standard and effective.11 Although a few papers reported the synthesis of perfluoroalkyl substituted bis oxadiazoles and diphenyl bis oxadiazoles using the reaction of oxalyl chloride with tetrazole derivatives, the yield is very low.12 With electron-deficient substituents and bis oxadiazoles, standard methods require modifications developed in the course of the present work to overcome the diminished nucleophilicity of the hydrazide oxygens and lower solubility of bis hydrazides containing four hydrogen-bonded NH groups. These modifications included the use of more polar solvents than typical for these electrophilic transformations and extended heating at elevated temperatures. The shorter oxadiazole oligomers were prepared through the corresponding hydrazides or bis hydrazides by the known dehydrating cyclization reaction utilizing POCl3 as the dehydrating reagent (Scheme 1). The final oxadiazole products were purified by recrystallization from solvent, forming very fine needles that for 1-3 were suitable for X-ray diffraction. Differential scanning calorimetry of the derivatives all showed a reversible phase change due to melting (Table 1). However, it should be noted that for 1, when the DSC scan was carried out to 250 °C or higher, the return peak was no longer evident. This suggests that either the material was decomposing or sublimation was occurring.
A longer oligomer was constructed using a Stille coupling reaction of an unsymmetrical oxadiazole to 2,5-bis(tributylstannyl)thiophene (Scheme 2). In other work not discussed in detail here, we have found the Ullman coupling to be compatible with oxadiazole rings, and have used it to prepare the bithiophene analogue of 4. Thus, by avoiding strongly reducing organometallics in the presence of oxadiazole rings, a methodology for building up oxadiazole oligomers with a variety of lengths and sequences is in hand. Structures and Redox Properties of Short Oxadiazole Oligomers. These oxadiazole derivatives are extremely planar as determined by X-ray diffraction (XRD) on single crystals (Figure 2). The oligomers contain torsion angles of no more than 7.08°, which is more planar than the thiophene analogs.13 Oligomer 1 packs in an orthorhombic space group Pcba. This derivative packs in a herringbone arrangement with a dihedral angle between nonparallel molecules of 41.9° and an intramolecular root-mean-square deviation from planarity of the ring atoms of just 0.07 Å. The closest intermolecular edge-to-face π interaction is 3.1 Å, and the closest approach of an H atom to a ring is 2.9 Å. Oligomer 2 forms crystals with a P-1 space group and has two unique half-molecules in the unit cell. The root-mean-square deviations from planarity are only 0.04 Å, and all molecules are essentially parallel. This derivative forms what is close to being a slip-stack array, a good sign for possible long-range order in future derivatives. Many perpendicular π-π distances between ring planes of neighboring molecules are in the range of 2.8-3.3 Å. Derivative 3 packs in a monoclinic P2(1)/c space group. This derivative is unique and curious among these crystal structures in that the individual molecules are in an unexpected syn geometry. The reason for this change in orientation is uncertain; the X-ray data show no extraneous atoms which might affect the orientation of the oxadiazole oligomer. The oligomer packs in a paired herringbone arrange-
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Figure 6. AFM image and line scan across a terrace of a film of 4 on Si/SiO2. The terrace height of 20 ( 3 Å is in good agreement with the XRD layer spacing of 17.3 Å.
Figure 5. XRD scans of films of 4. (a) Cytop with O2 plasma treatment and (b) SiO2 with O2 plasma treatment.
ment. Root mean square deviation from planarity of all atoms is 0.27 Å. The closest intermolecular distances are not associated with pi interactions. The reduction potentials were taken from solution for these derivatives in acetonitrile at room temperature at a scan rate of 50 mV/s. All of the derivatives showed a single reduction event, occurring near -1.1 V versus Ag/AgCl (setting internal ferrocene to +0.4 V14) with the perfluorophenyl derivatives 3 and 4 being nearly irreversible (Table 1 and Figure 3). These potentials are approximately 0.4 V less negative than for analogous oligomers without fluorinated end groups.12c From the midpoints of the reduction signals,15 the LUMO levels were determined to be 3.3 eV for 1 and 2. Based on a recently published listing of n-channel semiconductors, it appears that a LUMO level at least 0.5 V farther from vacuum (0.5 V less negative reduction potential) is necessary for stable n-channel transistor activity in air.16 UV-vis spectra of the derivatives were taken in dichloromethane, with all of the shorter derivatives showing similar absorbances in the range of 295 nm. It was then possible to determine the optical HOMO-LUMO gaps for the three derivatives from the onset of absorbance of the UV-vis spectra. The HOMO-LUMO gap for 1 is 4.01 eV, with 2 and 3 having slightly smaller HOMO-LUMO gaps of 3.84 and 3.97 eV, respectively. The gap for 4 is 2.55 eV. Thin Film Properties. The shorter oligomers were very volatile, and films of 1 were not obtainable by vacuum deposition because of desorption from the substrate. Films of 2
and 3 were indeed prepared by vacuum deposition but showed no crystallinity by XRD. Attempts to prepare top-contact OFETs from these materials with gold contacts showed no transistor behavior under nitrogen. OFETs from solution grown films also showed no transistor behavior. The materials tended to form discontinuous films containing fine needles from solution. However, compound 4 formed a crystalline film when vapordeposited, and initial charge transport measurements on thin film transistors were made. Transistors were fabricated by vacuum depositing the films in a thermal evaporator on different dielectric surfaces and using different metals as S/D contacts. Cytop and SiO2 were used as dielectrics and were treated with O2 plasma (10 W power, 0.3 torr pressure for 30 s) prior to semiconductor deposition in order to make the surface slightly hydrophilic, so as to achieve better film crystallinity. A 9% Cytop solution (CTL-809M, Asahi Glass company) was spin coated on the Si/SiO2 (400 nm) substrate (2000 rpm for 60 s) and then cured for 1 h at 130 °C in a vacuum oven to make Cytop dielectric samples. The resulting film thickness was 2 µm. Au, Al, and Ca were tried as the source drain contacts in order to understand the Fermi level position with regard to the energy levels in the metal. 100-nm-thick metal films were deposited in a thermal evaporator at a vacuum of 10-6 torr. The Ca electrodes were prevented from oxidation by subsequently depositing a thin layer (∼20 nm) of Al on top. Figure 4 shows characteristics of the best performing transistor type, having Au as the S/D electrodes and Cytop as the dielectric. The transistor shows p-channel behavior with a hole mobility of 2.44 × 10-3 cm2/Vs, threshold voltage of -10 V, and an on/off ratio of 440 for a channel length of 250 µm. There is no significant degradation in the performance of the transistors
Oxadiazole-Based Oligomers over a period of 3 months. Transistors having Al and Ca electrodes also show p-type behavior, but the current is severely contact limited. All the measurements were done in air. Also, there is no change in the transistor response on repeating the measurements under a vacuum of 10-3 torr. The transistors show some n-channel and ambipolar behavior at high potential difference between drain and gate electrodes (∼200 V, VG ) 100 V, VD ) -100 V). However, the gate leakage at such high voltages is of the order of the drain current (1 nA) and hence the electron mobility cannot be conclusively extracted from the graphs. The films pack in a similar fashion on both Cytop and SiO2 surfaces with the first-order peak appearing at 5.1°. This corresponds to a layer spacing of 17.3 Å, considerably less than a modeled molecular length of between 25 and 30 Å, indicating that the molecules are significantly tilted with respect to the substrate. The films are more crystalline on the SiO2 surface (Figure 5), and the morphology on the SiO2 surface appears typical for rigid oligomers (Figure 6). However, there is no transistor behavior observed for the films on the SiO2 substrate. This can be attributed to the presence of H+ and OH- traps on the SiO2 surface which are absent on the highly hydrophobic Cytop surface. This is in accordance with the findings of the Chua et al.17 in which use of hydrophobic BCB dielectric eliminates silanol traps responsible for trapping electrons at the interface in the established p-channel polymer systems. For our semiconductor compound, the hydrophilic SiO2 surface appears to trap holes. In conclusion, we have prepared the new series of oligomers based on the oxadiazole framework, devising ring closure and coupling reaction procedures compatible with this family of compounds. These oligomers crystallize readily from solvent and show rigid well-defined structures with close π interactions and, thus, are a suitable basis for further development as organic semiconductors. Elaboration of the longer oligomer motif with further electron-withdrawing groups, acyl end groups in particular, would be a logical approach to ultimately yield highmobility, air stable n-channel organic semiconductors. Acknowledgment. We are grateful to AFOSR (Award No. FA9550-06-01-0076) and the JHU MRSEC (NSF Division of Materials Research) for support of this work.
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