Organometallic Complexes as Hole-Transporting Materials in Organic

Inorg. Chem. 2004, 43, 1697−1707

Organometallic Complexes as Hole-Transporting Materials in Organic Light-Emitting Diodes Xiaofan Ren, Bert D. Alleyne, Peter I. Djurovich, Chihaya Adachi,† Irina Tsyba, Robert Bau, and Mark E. Thompson* Department of Chemistry, UniVersity of Southern California, Los Angeles, California 90089 Received October 13, 2003

The use of metal complexes fac-tris(1-phenylpyrazolato-N,C2′)cobalt(III) [fac-Co(ppz)3], fac-tris(2-phenylpyridinatoN,C2′)cobalt(III) [fac-Co(ppy)3], and {tris[2-((pyrrole-2-ylmethylidene)amino)ethyl]amine}gallium(III) [Ga(pma)] as materials for hole-transporting layers (HTL) in organic light-emitting diodes (OLEDs) is reported. Co(ppz)3 and Co(ppy)3 were prepared by following literature procedures and isolated as mixtures of facial (fac) and meridional (mer) isomers. The more stable fac isomers were separated from the unstable mer forms via column chromatography and thermal gradient sublimation. Crystals of fac-Co(ppz)3 are monoclinic, space group P21/c, with a ) 13.6121(12) Å, b ) 15.5600(12) Å, c ) 22.9603(17) Å, β ) 100.5°, V ) 4781.3(7) Å3, and Z ) 8. {Tris[2((pyrrol-2-ylmethylidene)amino)ethyl]amine}gallium [Ga(pma)] was prepared by the reaction of gallium(III) nitrate with the pmaH3 ligand precursor in methanol. Ga(pma) crystallizes in the cubic space group I4h3d with cell parameters a ) 20.2377(4) Å, b ) 20.2377(4) Å, c ) 20.2377(4) Å, β ) 90.0°, V ) 8288.6(3) Å3, and Z ) 16. These cobalt and gallium complexes are pale colored to colorless solids, with optical energy gaps ranging 2.6−3.36 eV. A two-layer HTL/ETL (ETL ) electron-transporting layer) device structure using fac-Co(ppz)3 and fac-Co(ppy)3 as the HTL does not give efficient electroluminescence. However, the introduction of a thin layer of a hole-transporting material (N,N′-bis(1-naphthyl)-N,N′-diphenylbenzidine, NPD) as an energy “stair-step” and electron/exciton-blocker dramatically improves the device performance. Both fac-Co(ppz)3 and fac-Co(ppy)3 devices give external quantum efficiencies higher than 1.0%, with brightness 5000 and 7000 Cd/m2 at 10 V, respectively. Ga(pma) also functions as an efficient interface layer, giving device performances very similar to those of analogous devices using NPD as the interface layer. Stability tests have been carried out for Co(ppz)3/NPD/Alq3 and Co(ppy)3/NPD/Alq3 devices. While fac-Co(ppy)3 gave stable OLEDs, the fac-Co(ppz)3-based devices had very short lifetimes. On the basis of the experimental results of chemical oxidation of fac-Co(ppz)3, the major cause for the fast decay of the fac-Co(ppz)3 device is proposed to be the decomposition of fac-Co(ppz)3+ in the HTL layer during the device operation.

Introduction There has been a growing interest in the use of organic compounds as charge-transporting materials in electronic devices. One type of device that exploits the chargetransporting ability of organic compounds is the organic lightemitting diode (OLED).1,2 A common OLED structure consists of an ITO (indium tin oxide) anode on glass, a holetransporting layer (HTL), an electron-transporting layer (ETL) (which often also serves as the emissive layer), and * To whom correspondence should be addressed. E-mail: [email protected]. † Present address: Department of Photonics Materials Science, Chitose Institute of Science & Technology, 758-65 Bibi, Chitose 066-8655, Japan. (1) Tang, C. W.; Van Slyke, S. A. Appl. Phys. Lett. 1987, 51, 913.

10.1021/ic035183f CCC: $27.50 Published on Web 02/07/2004

© 2004 American Chemical Society

a metal cathode (i.e. glass/ITO anode/HTL/ETL/cathode).1,2 Holes and electrons are injected into the carrier-transporting materials from the anode and cathode, respectively, and migrate in the presence of the applied electric field to the HTL/ETL interface. The holes and electrons recombine at (2) (a) Rothberg, L. J.; Lovinger, A. J. J. Mater. Res. 1996, 11, 3174. (b) Tang, C. W. Inf. Disp. 1996, 10, 16. (c) Sibley, S.; Thompson, M. E.; Burrows, P. E.; Forrest, S. R. In Optoelectronic Properties of Inorganic Complexes”; Roundhill, D. M., Fakler, J., Eds.; Plenum Press: New York. (d) Shoustikov, A.; You, Y.; Thompson, M. E. IEEE J. Sel. Top. Quantum Electron. 1998, 4, 3. (e) Burrows, P. E.; Gu, G.; Bulovic, V.; Forrest, S. R.; Thompson, M. E. IEEE Trans. Electron DeV. 1997, 44, 1188. (f) Forrest, S. R.; Burrows, P. E.; Thompson, M. E. In Organic Electroluminescent Materials and DeVices; Miyata, S., Nalwa, H. S., Eds.; Gordon and Breach: Langhorne, PA, 1996.

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Ren et al. or near the HTL/ETL interface to give excited molecules, or excitons, which give rise to the electroluminescent (EL) emission. A number of materials, both molecular and polymeric, have been used to prepare OLEDs; however, they all share the same basic mechanism for electroluminescence. Research efforts have been focused on the design of new molecular materials for use as charge-transporting materials in OLEDs.3 Although a variety of inorganic complexes have been used in the fabrication of OLEDs, the most extensively studied metal complex has been aluminum tris(8-hydroquinolate) (Alq3).4,5 Alq3 has been used as an electrontransporting, electron-emitting, and host material in doped systems. Homoleptic metal quinolates, with metals such as Zn2+, Be2+, and Ga3+, also exhibit good performance in OLEDs.6 These metal complexes, however, have been used solely as electron-transporting materials in OLEDs and not as hole transporters. While metal complexes such as metallophthalocyanines and metalloporphyrins have been used as hole-injecting materials,7 the intrinsically high molar absorptivity ( > 104 M-1 cm-1 throughout the visible spectrum) of these materials leads to excessive absorbance of the OLED emission and, thus, hinders their ability to act as bulk hole transporters. Therefore, the most common holetransporting materials used for small-molecule-based OLEDs have been organic triarylamine compounds (e.g. N,N′-bis(1-naphthyl)-N,N′-diphenylbenzidine (NPD), N,N′-bis(metatolyl)-N,N′-diphenylbenzidine (TPD), and 4,4′,4′′-tris((3methylphenyl)phenylamino)triphenylamine (m-MTDATA)).8 Triarylamines were originally developed for charge-transporting layers in xerography and have proven to be excellent (3) (a) Strukelj, M.; Papadimitrakopoulos, F.; Miller, T. M.; Rothberg, L. J. Science 1995, 267, 1969. (b) Noda, T.; Shirota, Y. J. Am. Chem. Soc 1998, 120, 9714. (c) Murata, H.; Kafafi, Z. H.; Uchida, M. Appl. Phys. Lett. 2002, 80, 189. (d) Ma, D.; Wang, G.; Hu, Y.; Zhang, Y.; Wang, L.; Jing, X.; Wang, F.; Lee, C. S.; Lee, S. T. Appl. Phys. Lett. 2003, 82, 1296. (4) (a) Tang, C. W.; Van Slyke, S. A.; Chen, C. H. J. Appl. Phys. 1998, 65, 3610. (b) Shi, J.; Tang, C. W. Appl. Phys. Lett. 1997, 70, 1665. (c) Tang, C. W.; Van Slyke, S. A.; Chen, C. H. Chen J. Appl. Phys. 1989, 65, 3610. (d) Van Slyke, S. A.; Chen, C. H.; Tang, C. W. Appl. Phys. Lett. 1996, 69, 2160. (e) Shoustikov, A.; You, Y.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R. Synth. Met. 1997, 91, 217. (5) (a) Anderson, J. D.; McDonald, E. M.; Lee, P. A.; Anderson, M. L.; Ritchie, E. L.; Hall, H. K.; Hopkins, T.; Mash, E. A.; Wang, J.; Thayumanavan, S.; Barlow, S.; Marder, S. R.; Jabbour, G. E.; Shaheen, S.; Kippelen, B.; Peyghambarian, N.; Wightman, R. M.; Armstrong, N. R. J. Am. Chem. Soc. 1998, 120, 9646. (b) Burrows, P. E.; Shen, Z.; Bulovic, V.; McCarty, D. M.; Forrest, S. R.; Cronin, J. A.; Thompson, M. E. J. Appl. Phys. 1996, 79, 7991. (6) (a) Hamada, Y. IEEE Trans. Electron DeV. 1997, 44, 1208. (b) Hamada, Y.; Kanno, H.; Sano, T.; Fujii, H.; Nishio, Y.; Takahashi, H.; Usuki, T.; Shibata, K. Appl. Phys. Lett. 1998, 72, 1939. (c) Donze, D.; Pechy, P.; Gratzel, M.; Schaer, M.; Zuppiroli, L. Chem. Phys. Lett. 1999, 315, 405. (d) Tokito, S.; Noda, K.; Tanaka, H.; Taga, Y.; Tsutsui, T. Synth. Met. 2000, 111-112, 393. (e) Sano, T.; Nishio, Y.; Takahashi, Y.; Usuki, T.; Shibata, K. J. Mater. Chem. 2000, 10, 157. (7) Zhu, L.; Tang, H.; Harima, Y.; Yamashita, K.; Ohshita, J.; Kunai, A. Synth. Met. 2002, 126, 331. (8) (a) Borsenberger, P. M.; Mey, W.; Chowdry, A. J. Appl. Phys. 1978, 49, 273. (b) Van Slyke, S. A.; Tang, T. W. U.S. 5,061,569, 1991. (c) Tokito, S.; Tanaka, H.; Okada, A.; Taga, Y. Appl. Phys. Lett. 1996, 69, 878. (d) Shirota, Y.; Kuwabara, Y.; Inada, H.; Wakimoto, T.; Nakada, H.; Yonemoto, Y.; Kawami, S.; Imai, K. Appl. Phys. Lett. 1994, 65, 807. (e) O’Brien, D. F.; Burrows, P. E.; Forrest, S. R.; Koene, B. E.; Loy, D. E.; Thompson, M. E. AdV. Mater. 1998, 10, 1108. (f) Koene, B. E.; Loy, D. E.; Thompson, M. E. Chem. Mater. 1998, 10, 2235. (g) Hu, N.-X.; Xie, S.; Popovic, Z.; Ong, B.; Hor, A.-M.; Wang, S. J. Am. Chem. Soc. 1999, 121, 5097.

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OLEDs.9

hole-transporting materials for These compounds have good thermal stabilities and wide optical gaps, making them readily processable and transparent to OLED emission. Metal complexes are promising candidates to consider for use as hole-transporting materials, since metal ions can both assume multiple oxidation states and have low kinetic barriers for self-exchange reactions.10 The cationic complex Ru(bpy)32+ (bpy ) 2,2′-bipyridyl) and its derivatives have been used in single-layer device structures.11 In these devices the Ru(II) complex, in addition to acting as the luminescent material, transports both holes and electrons.12 Unfortunately, the low optical energy gap for Ru(bpy)32+ cation makes these complexes practical only for orange to red emitting devices. Also, the ionic nature of the complex makes fabrication of efficient multilayer devices difficult, if not impossible. Our goal is to prepare metal complexes that can function as efficient hole-transporting materials. The complexes should have energy gaps wide enough to be applicable in blue to red OLEDs and physical characteristics (thermal stability, volatility) that are amenable to deposition using standard vacuum deposition techniques. Recently, cyclometalated complexes of Ir(III) have been shown to be effective as both emissive and charge-transporting materials in vapor-deposited OLEDs.13 However, the high cost associated with iridium makes these complexes more appropriate for use as luminescent dopants and less attractive as bulk charge-transporting materials. Thus, it would be worthwhile to identify similar electroactive complexes that would serve the same purpose as a charge carrier and yet, which would also incorporate less expensive metals into their structures. In this study, we have chosen to investigate tris-cyclometalated Co(III) and Ga(III) complexes tris(1-phenylpyrazolato-N,C2′)cobalt(III) [Co(ppz)3], (tris(2-phenylpyridinato-N,C2′)cobalt(III) [Co(ppy)3], and {tris[2-((pyrrole-2-yl)methylidene)amino)ethyl]amine}gallium(III) [Ga(pma)] as HTL materials (Figure 1). We have discovered several characteristics inherent in the cobalt complexes that affect their ability to function as effective charge transport materials and have identified some solutions that allow them to be successfully employed as HTL materials in OLEDs. We have also (9) (a) Klupfel, K.-W.; Sus, O.; Behmenburg, H.; Neugebauer, W. U.S. 3,180,730, 1965. (b) Brantly, T. B.; Contois, I. E.; Fox, C. J. U.S. 3,567,450, 1970. (c) Brantly, T. B.; Contois, I. E.; Fox, C. J. U.S. 3,658,520, 1972. (10) (a) Marcus, R. A. J. Chem. Phys. 1965, 43, 679. (b) Newton, M. D.; Sutin, N. Annu. ReV. Phys. Chem. 1984, 35, 437. (c) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 81, 265. (d) Sakanoue, K.; Motoda, M.; Sugimoto, M.; Sakaki, S. J. Phys. Chem. A 1999, 103, 5551. (11) Rudmann, H.; Rubner, M. F. J. Appl. Phys. 2001, 90, 4338-4345. Gao, F. G.; Bard, A. J. J. Am. Chem. Soc. 2000, 122, 7426. Slinker, J.; Bernards, D.; Houston, P. L.; Abrun˜a, H. D.; Bernhard, S.; Malliaras, G. G. J. Chem. Soc., Chem. Commun., in press. (12) While the Ru(bpy)3X2-based devices are actually light-emitting electrochemical cells rather than OLEDs, they conduct holes in much the same way that OLED HTLs do. (13) Adamovich, V. I.; Cordero, S. R.; Djurovich, P. I.; Tamayo, A.; Thompson, M. E.; D’Andrade, B. W.; Forrest, S. R. Org. Electron. 2003, 4, 77. The devices reported in the following paper utilized single layers of organoiridium complexes; thus, for the OLED to generate light the Ir complex must both conduct charge and emit: Grushin, V. V.; Herron, N.; LeClooux, D. D.; Marshall, W. J.; Petrov, V. A.; Wang, Y. Chem. Commun. 2001, 16, 1494.

Organometallic Complexes as Materials in OLEDs

Materials. All starting reagents were purchased from Aldrich Chemical Co., and all syntheses were carried out under an argon or nitrogen atmosphere. Tetrahydrofuran was freshly distilled over sodium/benzophenone prior to use. NPD14 and aluminum tris(8hydroquinolate)15 (Alq3) were synthesized according to the literature procedures. All materials to be used in device fabrications are further purified by thermal gradient sublimation before being loaded into the chamber, to ensure at least 99% purity. Equipment. Mass spectra were recorded on an HP 5973 mass spectrometer, using electron ionization at 70 eV. NMR spectra were recorded on Bruker AMX 250, 360, or 500 MHz spectrometers. Absorption spectra were recorded on an Aviv model 14DS UVvis-IR spectrophotometer and corrected for background solvent absorption. Emission spectra were recorded on a Photon Technology International QuantaMaster model C-60SE spectrofluorometer, equipped with a 928 PMT detector and corrected for detector response. Elemental analyses (C, H, N) were performed at the Microanalysis Lab at the University of Illinois, Urbana-Champaign, IL. The HOMO energies for Co(ppz)3 and Ga(pma) were determined by photoelectron spectroscopy, using an AC-1 (Riken Keiki Co., Japan) UV photoelectron spectrometer. Electrochemistry. Cyclic voltammetry was carried out in nitrogen-purged anhydrous DMF (Aldrich) at room temperature with an EG&G Princeton potientiostat/galvanostat model 283 interfaced to a PC. The working electrode was a glassy carbon electrode, a Pt wire was used as the counter electrode, and an Ag wire was used as the pseudoreference electrode with ferrocene Fc+/ Fc as the internal standard. Tetrabutylammonium hexafluorophosphate (0.1 M) was used as supporting electrolyte while the concentration of the analyte was 0.1 mM. Cyclic voltammograms were obtained at scan rate of 50 mV. Formal potentials are calculated as the average of cyclic voltammetric anodic and cathodic peaks. Synthesis of Tris(2-phenylpyrazolato-N,C2′)cobalt(III) [facCo(ppz)3].16 Ethylmagnesium bromide (7.6 mL, 7.6 mmol) was

added to a solution of 1-phenylpyrazole (1.00 g, 6.93 mmol) in THF (3 mL). The reaction mixture was refluxed for 2 h, under an argon atmosphere, and then cooled in a dry ice/acetone bath. A solution of cobalt(II) bromide (0.76 g, 3.47 mmol) in THF (8 mL) was then added slowly to the Grignard reagent. The bath was removed, and the black mixture was stirred overnight. The reaction mixture was added to aqueous NH4Cl (10 g/L, 75 mL) and CH2Cl2 (75. mL). The resulting thick emulsion was filtered and transferred to a separatory funnel. The organic layer was isolated, and the aqueous layer was extracted twice with CH2Cl2. All organic portions were combined, dried over anhydrous MgSO4, filtered, and concentrated under reduced pressure. Addition of hexanes to the yellow/brown concentrate precipitated a dark yellow solid. 1H NMR indicated that the crude product was a mixture of facial and meridional isomers. The two isomers were separated by column chromatography with 1:1 CH2Cl2-toluene eluent.17 Yield: 80.5 mg, 7% (facial isomer). 1H NMR (500 MHz, CDCl3): δ 8.08 (d, 1H, J ) 2.6 Hz), 7.15 (dd, 1H, J ) 7.7, 1.4 Hz), 6.96 (ddd, 1H, J ) 7.5, 7.5, 1.7 Hz), 6.84-6.80 (m, 2H), 6.62 (d, 1H, J ) 7.7 Hz), 6.38 (dd, 1H, J ) 2.5, 2.5 Hz). MS: m/e 488 (M+). Anal. Calcd for CoC27H21N6: C, 66.40; H, 4.30; N, 17.21. Found: C, 66.38; H, 4.26; N, 17.00. Synthesis of Tris(1-phenylpyridinato-N,C2′)cobalt(III) [facCo(ppy)3].18 Magnesium turnings (0.31 g, 12.8 mmol), bromomesitylene (2.50 g, 12.6 mmol), THF (15.0 mL), and a drop of dibromoethane were combined and refluxed gently under argon atmosphere until the magnesium was consumed. The solution was cooled to ca. -30 °C, and a solution of CoBr2 (0.92 g, 4.20 mmol) in THF (7 mL) was then added slowly. Throughout the addition the color of the reaction mixture changed from clear yellow to opaque ochre and finally to yellow/black. The cooling bath was removed, and the mixture was stirred for 1 h. 2-Phenylpyridine (2.18 g, 14.1 mmol) was then added, and the reaction was brought to a gentle reflux for another 1 h. Workup and isolation of the crude product was performed in a manner analogous to that for Co(ppz)3. The crude product was column chromatographed in silica gel using CH2Cl2 as eluent. 1H NMR spectroscopy indicated that the sample was a mixture of facial and meridional isomers.17 Pure facial isomer was obtained by gradient sublimation. 1H NMR (500 MHz, CDCl3): δ 7.86 (d, 1H, J ) 7.9 Hz), 7.66-7.61 (m, 2H), 7.28 (d, 1H, J ) 7.5 Hz), 6.92 (dd, 1H, J ) 7.2, 7.2 Hz), 6.85 (dd, 1H, J ) 6.2, 6.2 Hz), 6.81 (dd, 1H, J ) 7.2, 7.2 Hz), 6.52 (d, 1H, J ) 7.5 Hz). MS: m/ e 521 (M+). Anal. Calcd for CoC33H24N3: C, 76.00; H, 4.64; N, 806. Found: C, 75.89; H, 4.55; N, 8.09. Synthesis of {Tris[2-((pyrrol-2-ylmethylidene)amino)ethyl]amine}gallium(III) [Ga(pma)]. The ligand [((pyrrol-2-ylmethylidene)amino)ethyl]amine was prepared by adding a methanolic solution of pyrrole-2-carboxaldehyde (1.43 g, 15.0 mmol, 100 mL) to a methanolic solution of tris(2-aminoethyl)amine (0.73 g, 5.0 mmol, 10 mL). The resulting yellow solution was stirred at room temperature for 30 min. A methanolic solution of gallium(III) nitrate hydrate (1.28 g, 5.0 mmol, 150 mL) was added to the ligand solution and stirred at room temperature for 30 min. The solution was filtered and left to stand at ambient temperature until crystallization occurred. The crude material was then sublimed at 235 °C. 1H NMR (350 MHz, CDCl3): δ 7.93 (s, 3H), 6.64 (dd, 3H, J ) 1.1, 3.3 Hz), 6.19 (dd, 3H, J ) 1.7, 3.4 Hz), 6.17 (br s, 3H), 3.45 (ddd, 3H, J ) 3.7, 12.7, 12.7), 3.21 (dd, 3H, J ) 3.7, 23.93 Hz), 3.17

(14) (a) Drive, M. S.; Hartwig, J. F. J. Am. Chem. Soc. 1996, 118, 7217. (b) Wolre, J. P.; Wagaw, S.; Buchwald, S. L. J. Am. Chem. Soc. 1996, 118, 7215. (15) Schmidbauer, H.; Letterbauer, J.; Kumberger, O.; Lachmann, L.; Muller, G. Z. Naturforsch, 1991, 466, 1065.

(16) (a) Drevs, H. Z. Chem. 1976, 16, 493. (b) Marxer, A.; Siegrist, M. HelV. Chim. Acta 1974, 57, 1988. (17) Dedeian, K. Ph.D. Dissertation, University of California, Santa Barbara, CA, 1992. (18) Drevs, H. Z. Anorg. Allg. Chem. 1991, 605, 145.

Figure 1. Chemical structures of the compounds used to make OLEDs.

identified a possible chemical origin for the oxidative instability of the fac-Co(ppz)3-based HTL films. Experimental Section

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Ren et al. Table 1. Crystal Data and Structure Refinement for fac-Co(ppz)3‚0‚5CH2Cl2 (C27H20N6Co‚0‚5CH2Cl2), Ga(pma) (C21H24N7Ga), and Co(ppz2)Cl2 (C18H14ClN4Co) param empirical formula fw temp, K wavelength (Å) cryst system space group unit cell dimens (Å) V (Å3) Z density, calcd (Mg/cm3) abs coeff (mm-1) F(000) indpndt reflcns refinement method data/restraints/params goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data)

fac-Co(ppz)3‚0‚5CH2Cl2

Ga(pma)

Co(ppz2)Cl2

C27H20N6Co‚0‚5CH2Cl2 530.89 85(2) 0.710 73 monoclinic P21/c a ) 13.6121(12) b ) 15.5600(12) c ) 22.9603(17) 4781.3(7) 8 1.475 0.859 2184 7587 [R(int) ) 0.0667] full-matrix least squares on F2 7587/0/641 1.042 0.0685 0.1062

C21H24N7Ga 444.1 296(2) 0.710 73 cubic I4h3d a ) 20.2377(4) b ) 20.2377(4) c ) 20.2377(4) 8288.6(3) 16 1.424 1.350 3680 1586 [R(int) ) 0.0392] full-matrix least squares on F2 1586/0/89 0.964 0.0250 0.0288

C18H14ClN4Co 416.16 273(2) 0.710 73 monoclinic Pn a ) 7.866(5) b ) 8.854(6) c ) 13.619(9) 944.9(11) 2 1.463 1.198 422 3410 [R(int) ) 0.0522] full-matrix least squares on F2 3410/2/226 1.015 0.0298 0.0312

(dd, 3H, J ) 3.4, 21.97 Hz), 2.70 (ddd, 3H, J ) 3.7, 13.7, 13.7 Hz). MS: m/e 443 (M+). Anal. Calcd for GaC21H24N7: C, 56.73; H, 5.40; N, 22.06. Found: C, 56.75; H, 5.50; N, 21.80. Synthesis of Co[(ppz)2]Cl2. fac-Co(ppz)3 (0.2 mmol, 0.098 g) was dissolved in 10 mL of CH2Cl2. A 1 equiv amount of (4-BrPh)3N-SbCl6 (0.2 mmol, 0.163 g) was added, and the solution was stirred for 30 min at room temperature under N2 atmosphere. Concentration of the solution followed by filtration afforded deep blue microcrystals of Co[(ppz)2]Cl2 (yield: ∼50%). Crystals suitable for X-ray crystallography were grown by slow evaporation from a CH2Cl2 solution at room temperature. Density Functional Calculations. DFT calculations were performed using Titan software package (Wavefunction, Inc.) at the B3LYP/LACVP** level. The HOMO and LUMO energies were determined using minimized singlet geometries to approximate the ground state. The minimized singlet geometries were used to calculate the triplet molecular orbitals and approximate the triplet HSOMO (HSOMO ) highest singly occupied molecular orbital). X-ray Crystallography. Diffraction data for the fac-Co(ppz)3‚ 0.5CH2Cl2, Ga(pma), and Co(ppz2)Cl2 complexes were collected on SMART APEX CCD diffractometer with graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å). The cell parameters for the compounds were obtained from the least-squares refinement of the spots (from 60 collected frames) using SMART program. A hemisphere of the crystal data was collected up to a resolution of 0.75 Å, and the intensity data were processed using the Saint Plus program. All calculations for structure determination were carried out using the SHELXTL package (version 5.1).19 Initial atomic positions were located by Patterson and direct methods using XS, and the structure was refined by least-squares methods using SHELX. Absorption corrections were applied by using SADABS.20 Calculated hydrogen positions were input and refined in a riding manner along with the attached carbons. A summary of the refinement details and the resulting factors are given in Table 1. Device Fabrication and Testing. Prior to device fabrication, ITO with a resistivity of 20 Ω/0 on glass was patterned as 2 mm wide strips. The substrates were cleaned by sonication in soap solution, rinsed with deionized water, boiled in trichloroethylene, acetone, and ethanol for 3-4 min in each solvent, and dried with (19) Sheldrick, G. M. SHELXTL, version5.1; Bruker Analytical X-ray Systems, Inc.: Madison, WI, 1997. (20) Blessing, R. H. Acta Crystallogr. 1995, A51, 33.

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nitrogen. Finally, the substrates were treated with UV/ozone for 10 min. Organic layers were deposited sequentially by thermal evaporation from resistively heated tantalum boats onto the substrate at a rate of 2.0-3.0 Å/s. The base pressure at room temperature was (3-4) × 10-6 Torr. The deposition rate was controlled using a crystal monitor that was located near the substrate. After organic film deposition, the vacuum chamber was vented and a shadow mask with a 2 mm wide stripe was placed onto the substrate and oriented 90° with respect to the ITO stripes. The metal cathode of magnesium silver alloy (Mg0.9Ag0.1) was deposited at a rate of 2.2 Å/s capped with a protection Ag layer with the thickness of 400 Å. The devices were tested in air within 2 h of fabrication. Currentvoltage measurements were made using a Keithley source meter (model 2400). Light intensity was measured using a Newport model 1835 optical power meter with an 818-UV detector. Only light emitting from the front face of the OLED was collected and used in subsequent efficiency calculations. EL spectra were measured with a Photon Technology International fluorometer. Device Encapsulation. Devices selected for stability testing were immediately transferred to a N2-filled glovebox upon fabrication. A thin bead of epoxy adhesive (Epotek, Billerica, MA, No. 730 Epoxy) was applied from a syringe around the edge of the glass substrates, taking care not to make contact between the epoxy and the organic layers. To complete the package, a clean cover glass was placed on top of the device. The epoxy was cured first under intense UV radiation (OAI Collimated UV Lightsources) for 2 min and then was left to fully cure at room temperature for 3 h.

Results and Discussion Synthesis and Characterization. The cyclometalated cobalt complexes were synthesized using two different routes. The synthesis of fac-Co(ppz)3 involved the reaction of anhydrous CoCl2 with (2-pyrazolylphenyl)magnesium bromide, ppzMgBr, which gave fac-Co(ppz)3 as well as a number of side products.16,17 The 1-phenylpyrazolyl Grignard reagent was prepared by orthometalating 1-phenylpyrazole (ppzH) with ethylmagnesium bromide. The fac-Co(ppy)3 complex was prepared by a method involving cyclometalation of 2-phenylpyridine (ppyH) with the reactive MgCo(mesitylene)3Br reagent.18 Here, the three mesitylene anions assist in the cyclometalation reaction of the incoming ligand

Organometallic Complexes as Materials in OLEDs

Figure 2. Molecular structures of (a) fac-Co(ppz)3 and (b) Ga(pma).

precursors. It has been suggested that both processes involve a disproportion reaction to give the final Co(III) products.16 Both reactions give low yields (