Highly Phosphorescent Bis-Cyclometalated Iridium Complexes

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Chem. Mater. 2004, 16, 2480-2488

Highly Phosphorescent Bis-Cyclometalated Iridium Complexes Containing Benzoimidazole-Based Ligands Wei-Sheng Huang,† Jiann T. Lin,*,†,‡ Chin-Hsing Chien,‡ Yu-Tai Tao,*,‡ Shih-Sheng Sun,‡ and Yuh-Sheng Wen‡ Department of Chemistry, National Central University, Chungli, Taiwan 320, Republic of China, and Institute of Chemistry, Academia Sinica, Taipei, Taiwan 115, Republic of China Received January 18, 2004. Revised Manuscript Received April 5, 2004

New benzoimidazoles (bi) have been synthesized. These compounds readily undergo cyclometalation with iridium trichloride, and bis-cyclometalated iridium complexes, (bi)2Ir(acac) (bi ) cyclometalated benzoimidazole; acac ) acetylacetonate), can be isolated. One of the complexes, (fbi)2Ir(acac) (fbi ) 2-(9,9-diethyl-9H-fluoren-2-yl-1H-benzoimidazole), was also characterized by single-crystal X-ray structural determination. Some of the complexes, (bi)2Ir(acac), are highly phosphorescent at ambient condition. Light-emitting devices using these complexes as dopants were fabricated, and the emission colors range from green to red. Some green- and yellow-emitting devices exhibit very high efficiencies.

Introduction Organic electroluminescent materials have attracted considerable interest among academic and industrial communities since the seminal reports on molecular and polymeric organic light-emitting diodes (OLEDs) by Tang et al.1 and Burroughes et al.,2 respectively. In most OLEDs, triplet states constitute 75% of electrogenerated excited states. These triplet states are generally nonemissive due to their long lifetime (commonly from milliseconds to minutes) as well as their spin-forbidden nature for radiative relaxation to the ground states.3 Consequently, the maximum internal quantum efficiency of OLEDs is normally limited to 25%. To remove such constraint, efforts have been directed to using transition metal complexes, particularly 4d and 5d metals.4 The strong spin-orbit coupling caused by heavy metal ions in these complexes results in efficient intersystem crossing from the singlet to the triplet excited state. Mixing of the singlet and triplet excited states not only removes the spin-forbidden nature of the radiative relaxation of the triplet state, but also significantly shortens the triplet state lifetime. Triplet-triplet annihilation is more effectively suppressed because of the shorter lifetime of the triplet excited state. Therefore, higher phosphorescence efficiencies can be achieved. * Authors to whom correspondence should be addressed. Fax: 8862-27831237. E-mail: [email protected]. † National Central University. ‡ Academia Sinica. (1) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (2) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Mackay, R. N.; Marks, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539. (3) (a) Turro, N. J. Modern Molecular Photochemistry; Benjamin/ Cummings: Menlo Park, NJ, 1978. (b) Valeur, B. Molecular Fluorescence; Wiley-VCH: Weinheim, 2002. (4) (a) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Pure Appl. Chem. 1999, 71, 2095. (b) Thompson, M. E.; Burrows, P. E.; Forrest, S. R. Curr. Opin. Solid State Mater. Sci. 1999, 4, 369. (c) Ko¨hler, A.; Wilson, J. S.; Friend, R. H. Adv. Mater. 2002, 14, 701.

Numerous organometallic d6, d8, and d10 complexes are luminescent in solution or solid state.5,6 Among these, Os(II),6a Cu(I),6b and cyclometalated Ir(III)6e-6j and Pt(II) complexes6k-6l have been fabricated into lightemitting devices. Cyclometalated iridium(III) complexes receive the most extensive study partly due to their ease of preparation from iridium precursors with the corresponding imines capable of cyclometalation.7 Prototype imines used include 2-phenylpyridine, benzoquinoline, and 2-phenylbenzothiazole. A commonly used electron transporter and hole blocker, 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI),8 is a derivative of ben(5) (a) Lees, A. J. Chem. Rev. 1987, 87, 711. (b) Dixon, I. M.; Collin, J.-P.; Sauvage, J.-P.; Flamigni, L.; Encinas, S.; Barigelletti, F. Chem. Soc. Rev. 2000, 29, 385. (c) Yam, V. W.-W.; Lo, K. K.-W. Chem. Soc. Rev. 1999, 28, 323. (d) Tyson, D. S.; Bialecki, J.; Castellano, F. N. Chem. Commun. 2000, 2355. (e) Yam, V. W.-W.; Choi, W.-K.; Cheung, K.-K. Organometallics 1996, 15, 1734. (f) Yam, V. W.-W.; Yu, K.-L.; Wong, K. M.-C.; Cheung, K.-K. Organometallics 2001, 20, 721. (g) Yang, Q.Z.; Wu, L.-Z.; Wu, Z.-X.; Zhang, L.-P.; Tung, C.-H. Inorg. Chem. 2002, 41, 5653. (6) (a) Ma, Y.; Zhang, H.; Shen, J.; Che, C.-M. Synth. Met. 1998, 94, 245. (b) Ma, Y.; Che, C.-M.; Chao, H.-Y.; Zhou, X.; Chan, W.-H.; Shen, J. Adv. Mater. 1999, 11, 852. (c) Laamnsky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee, H.-E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E. J. Am. Chem. Soc. 2001, 123, 4304. (d) Wang, Y.; Herron, N.; Grushin, V. V.; LeCloux, D.; Petrov, V. Appl. Phys. Lett. 2001, 79, 449. (e) Xie, H. Z.; Liu, M. W.; Wang, O. Y.; Zhang, X. H.; Lee, C. S.; Hung, L. S.; Lee, S. T.; Teng, P. F.; Kwong, H. L.; Zheng, H.; Che, C. M. Adv. Mater. 2001, 13, 1245. (f) Grushin, V. V.; Herron, N.; LeCloux, D. D.; Marshall, W. J.; Petrov, V. A.; Wang, Y. Chem. Commun. 2001, 1494. (g) Ostrowski, J.; Robinson, M. R.; Heeger, A. J.; Bazan, G. C. Chem. Commun. 2002, 784. (h) Duan, J.P.; Sun, P.-P.; Cheng, C.-H. Adv. Mater. 2003, 15, 24. (i) Su, Y.-J.; Huang, H.-L.; Li, C.-L.; Chien, C.-H.; Tao, Y.-T.; Chou, P.-T.; Datta, S.; Liu, R.-S. Adv. Mater. 2003, 15, 884. (j) Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba, I.; Ho, N. N.; Bau, R.; Thompson, M. E. J. Am. Chem. Soc. 2003, 125, 7377. (k) Lu, W.; Mi, B.-X.; Chan, M. C. W.; Hui, Z.; Zhu, N.; Lee, S.-T.; Che, C.-M. Chem. Commun. 2002, 206. (l) D’Andrade, B. W.; Brooks, J.; Adamovich, V.; Thompson, M. E.; Forrest, S. R. Adv. Mater. 2002, 14, 1032. (m) Brooks, J.; Babayan, Y.; Lamansky, S.; Djurovich, P. I.; Tsyba, I.; Bau, R.; Thompson, M. E. Inorg. Chem. 2002, 41, 3055. (7) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Kwong, R.; Tsyba, I.; Bortz, M.; Mmui, B.; Bau, R.; Thompson, M. E. Inorg. Chem. 2001, 40, 1704. (8) (a) Chen, C. H.; Shi, J. Coord. Chem. Rev. 1998, 171, 161. (b) Shi, J.; Tang, C. W.; Chen, C. H. U.S. Patent 5,645,948, 1997.

10.1021/cm0498943 CCC: $27.50 © 2004 American Chemical Society Published on Web 05/21/2004

Highly Phosphorescent Benzoimidazole-Iridium Complexes

zoimidazole compounds. However, there have not yet been any reports of imidazole-based cyclometalated iridium complexes. Herein we describe the synthesis, and spectroscopic and structural studies, of new benzoimidazole-based cyclometalated iridium complexes. Experimental Section General Information. All reactions and manipulations were carried out under N2 with the use of standard inertatmosphere and Schlenk techniques. Solvents were dried by standard procedures. All column chromatography was performed under N2 with the use of silica gel (230-400 mesh, Macherey-Nagel GmbH & Co.) as the stationary phase in a column of 30 cm long and 2.0 cm diam. The 1H NMR spectra were measured by using Bruker AC300 spectrometers. Mass spectra (FAB) were recorded on a JMS-700 double focusing mass spectrometer (JEOL, Tokyo, Japan). Elemental analyses were performed on a Perkin-Elmer 2400 CHN analyzer. Cyclic voltammetry experiments were performed with a BAS-100 electrochemical analyzer. All measurements were carried out at room temperature with a conventional three-electrode configuration consisting of a platinum working electrode, an auxiliary electrode, and a nonaqueous Ag/AgNO3 reference electrode. The E1/2 values were determined as 1/2(Epa + Epc), where Epa and Epc are the anodic and cathodic peak potentials, respectively. All potentials reported are not corrected for the junction potential. The solvent in all experiments was DMF and the supporting electrolyte was 0.1 M tetrabutylammonium hexafluorophosphate. TGA measurements were performed on a TA-7 series thermogravimetric analyzer at a heating rate of 10 °C/min under a flow of nitrogen. Electronic absorption spectra were obtained on a Cary 50 Probe UV-visible spectrometer. Emission spectra were recorded in deoxygenated solution at 298 K with an SLM 48000S lifetime fluorescence spectrophotometer equipped with a red-sensitive Hamamatsu R928 photomultiplier tube. The emission spectra were collected on samples with o.d. ∼0.1 at the excitation wavelength. UVvisible spectra were checked before and after irradiation to monitor possible sample degradation. Emission maxima were reproducible to within 2 nm. Luminescence quantum yields (Φem) were calculated relative to (ppy)3Ir (Φem ) 0.40 in toluene).9 Luminescence quantum yields were taken as the average of three separate determinations and were reproducible to within 10%. Luminescence lifetimes were determined on a PRA System 3000 time-correlated pulsed single-photon counting apparatus.10 Samples were excited with light from a PRA 510 nitrogen flash lamp transmitted through an Instruments SA Inc. H-10 monochromator, and emission was detected at 90° via a second Hamamatsu R995 photomultiplier tube. The resulting photon counts were stored on a Tracer Northern 7200 microprocessorbased multichannel analyzer. The instrument response function was then deconvoluted from the emission data to yield an undisturbed decay which was fitted by an iterative leastsquares procedure on an IBM PC. The reported lifetimes were found to be within 10% over at least three measurements. Luminescence decays were measured on a Photon Technologies International LS-1 single-photon counting apparatus with a gated nitrogen arc lamp using a scatter solution to profile the instrument response function. The samples were excited at 337 nm. Nonlinear least-squares fitting of the decay curves was performed with the Levenburg-Marquardt algorithm11 and implemented by the Photon Technologies International Timemaster (version 1.2) software. General Procedure for the Synthesis of Benzoimidazole Ligands. Benzoimidazole ligands (abbreviated with the (9) Tsuboyama, A.; Iwawaki, H.; Furugori, M.; Mukaide, T.; Kamatani, J.; Iawa, S.; Moriyama, T.; Miura, S.; Takiguchi, T.; Okada, S.; Hoshino, M.; Ueno, K. J. Am. Chem. Soc. 2003, 125, 12971. (10) O’Connor, D. V.; Phillips, D. Time-Correlated Single Photon Counting; Academic Press: London, 1984. (11) James, D. R.; Siemiarczuk, A.; Ware, W. R. Rev. Sci. Instrum. 1992, 63, 1710.

Chem. Mater., Vol. 16, No. 12, 2004 2481 general formula bi) 1,2-diphenyl-1H-benzoimidazole (pbi), 2-naphthalen-1-yl-phenyl-1H-benzoimidazole (nbi), 2-phenanthren-9-yl-1-phenyl-1H-benzoimidazole (pnbi), 9-ethyl-3-(1phenyl-1H-benzoimidazole-2-yl)-9H-carbazole (cbi), 2-(9,9diethyl-9H-fluoren-2-yl)-1-phenyl-1H-benzoimidazole (fbi), 1-phenyl-2-thiophen-3-yl-1H-benzoimidazole (tbi), diphenyl{5-[4-(1-phenyl-1H-benzoimidazol-2-yl)-phenyl]-thiophen-2-yl}amine (tabi), and 2-[4-(2,6-diphenyl-pyridin-4-yl)-phenyl]-1phenyl-1H-benzoimidazole (pybi) were obtained from the reaction of N-phenyl-o-phenylenediamine and the corresponding aldehydes by a similar procedure, so that a detailed description is provided only for fbi. Synthesis of fbi. N-phenyl-o-phenylenediamine (1.55 g, 8.54 mmol), 9,9-diethyl-9H-fluorene-2-carbaldehyde (1.92 g, 7.76 mmol), and 2-methoxyethanol (20 mL) were charged sequentially in a two-neck flask under nitrogen atmosphere and heated to reflux for 48 h. The volatiles were removed under vacuum and the resulting solid was extracted into diethyl ether. The organic extract was washed with brine solution, dried over anhydrous MgSO4, filtered, and pumped dry. The residue was chromatographed using CH2Cl2/hexane (1:1) as eluent to afford the reddish brown fbi in 54% yield (1.71 g). Anal. Calcd for C30H26N2: C, 86.92; H, 6.32; N, 6.76. Found: C, 86.55; H, 6.38; N, 6.80. 1H NMR (CDCl3): δ 0.14 (t, J ) 7.5 Hz, 6 H), 1.69-1.82 (m, 2 H), 1.84-1.91 (m, 2 H), 7.24-7.39 (m, 9 H), 7.46-7.52 (m, 3 H), 7.66-7.69 (m, 2 H), 7.76 (dd, J ) 7.8, 1.8 Hz, 1 H), 7.96 (d, J ) 7.8 Hz, 1 H). MS (FAB): m/e 415.1 ((M + H)+). pbi. Brown powders. Yield: 63%. Anal. Calcd for C19H14N2: C, 84.42; H, 5.22; N, 10.36. Found: C, 84.21; H, 5.26; N, 10.34. 1H NMR (CDCl3): δ 7.21 (d, J ) 8.0 Hz, 1 H), 7.30-7.34 (m, 5 H), 7.38 (d, J ) 7.6 Hz, 1 H), 7.41 (d, J ) 7.6 Hz, 1 H), 7.51-7.55 (m, 3 H), 7.62 (d, J ) 7.6 Hz, 2 H), 8.03 (d, J ) 8.0 Hz, 1 H). MS (FAB): m/e 271.1 (M+). nbi. Light brown powders. Yield: 33%. Anal. Calcd for C23H16N2: C, 86.22; H, 5.03; N, 8.74. Found: C, 85.82; H, 4.87; N, 8.60. 1H NMR (CDCl3): δ 7.17 (d, J ) 7.2 Hz, 1 H), 7.18 (d, J ) 7.8 Hz, 1 H), 7.25-7.29 (m, 2 H), 7.33-7.52 (m, 8 H), 7.82-7.84 (m, 1 H), 7.86 (d, J ) 9.3 Hz, 1 H), 8.00 (dd, J ) 7.8, 1.8 Hz, 1 H), 8.05 (d, J ) 6.9 Hz, 1 H). MS (FAB): m/e 321.2 (M+). pnbi. Yellow powders. Yield: 75%. Anal. Calcd for C27H18N2: C, 87.54; H, 4.90; N, 7.56. Found: C, 87.11; H, 4.85; N, 7.34. 1H NMR (CDCl3): δ 6.45∼7.38 (m, 3 H), 7.25-7.30 (m, 4 H), 7.50 (d, J ) 7.8 Hz, 2 H), 7.55-7.63 (m, 2 H), 7.69 (t, J ) 7.5 Hz, 1 H), 7.80 (d, J ) 7.8 Hz, 1 H), 7.88-7.90 (m, 1 H), 7.90 (s, 1 H), 8.09 (d, J ) 7.8 Hz, 1H), 8.65 (dd, J ) 8.8, 3.3 Hz, 2 H). MS (FAB): m/e 371.1 ((M + H)+). cbi. Brown powders. Yield: 48%. Anal. Calcd for C27H21N2: C, 83.69; H, 5.46; N, 10.84. Found: C, 83.46; H, 5.34; N, 10.57. 1H NMR (CDCl ): δ 1.40 (t, J ) 7.2 Hz, 3 H, CH ), 4.03 (q, J 3 3 ) 7.2 Hz, 2 H, CH2), 7.20 (t J ) 7.5 Hz, 1 H), 7.24-7.30 (m, 3 H), 7.35-7.41 (m, 4 H), 7.43 (d, J ) 7.5 Hz, 1 H), 7.50-7.53 (m, 3 H), 7.63 (dd, J ) 7.8, 1.2 Hz, 1 H), 7.93 (d, J ) 7.8 Hz, 2 H), 8.37 (d, J ) 1.2 Hz, 1 H). MS (FAB): m/e 388.2 ((M + H)+). tbi. Brown powders. Yield: 33%. Anal. Calcd for C17H12N2: C, 73.88; H, 4.38; N, 10.14. Found: C, 73.66; H, 4.24; N, 10.05. 1H NMR (CDCl ): δ 7.12 (d, J ) 8.1 Hz, 1 H), 7.25-7.27 (m, 3 2 H), 7.32 (d, J ) 8.4 Hz, 2 H), 7.35-7.40 (m, 3 H), 7.57 (d, J ) 2.7 Hz, 2 H), 7.59 (s, 1 H), 7.89 (d, J ) 8.1 Hz, 1 H). MS (FAB): m/e 277.1 ((M + H)+). tabi. Yellow powders. Yield: 50%. Anal. Calcd for C35H25N2: C, 80.89; H, 4.85; N, 8.09. Found: C, 81.25; H, 4.77; N, 7.81. 1H NMR (CDCl3): δ 6.60 (d, J ) 3.9 Hz, 1 H), 7.01 (t, J ) 8.4 Hz, 2 H), 7.11-7.30 (m, 10 H), 7.30-7.35 (m, 4 H), 7.40-7.48 (m, 2 H), 7.49-7.53 (m, 3 H), 7.53 (d, J ) 8.1 Hz, 2 H), 7.54 (d, J ) 8.1 Hz, 2 H), 7.92 (d, J ) 8.1 Hz, 1 H). MS (FAB): m/e 519.6 (M+). pybi. Yellow powders. Yield: 46%. Anal. Calcd for C36H25N3: C, 86.55; H, 5.04; N, 8.41. Found: C, 86.32; H, 5.24; N, 8.53. 1H NMR (CDCl3): δ 7.24-7.27(m, 3 H), 7.31 (t, J ) 7.2 Hz, 1 H), 7.38 (d, J ) 8.4 Hz, 2 H), 7.41-7.44 (m, 2 H), 7.49 (t, J ) 7.6 Hz, 4 H), 7.56-7.59 (m, 2 H), 7.70 (d, J ) 8.4

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Hz, 2 H), 7.77 (d, J ) 8.4 Hz, 2 H), 7.85 (s, 2 H), 7.97 (d, J ) 8.4 Hz, 1 H), 8.17 (d, J ) 7.2 Hz, 4H). MS (FAB): m/e 500.2 ((M + H)+). General Procedure for the Synthesis of (bi)2Ir(acac). Cyclometalated Ir(III) µ-chloro-bridged dimers of general formula (bi)2Ir(µ-Cl)2Ir(bi)2, where bi represents the cyclometalated imidazole, were synthesized by the same method reported by Nonoyama.12 The crude products of these dimers were used for subsequent preparation of (bi)2Ir(acac) (acac ) acetylacetonate) following a similar procedure. Only the synthesis of (fbi)2Ir(acac) will be described in detail. Synthesis of (fbi)2Ir(acac). To a flask containing IrCl3‚ 3H2O (243 mg, 0.68 mmol) and fbi (710 mg, 1.36 mmol) was added a 3:1 mixture of 2-ethoxyethanol and water (25 mL). The mixture was refluxed for 48 h and cooled to room temperature. A small quantity of water was added to precipitate an orange solid. The solid was collected by filtration and pumped dry to give crude (fbi)2Ir(µ-Cl)2Ir(fbi)2. Crude (fbi)2Ir(µ-Cl)2Ir(fbi)2 was mixed with Na2CO3 (216 mg, 2.04 mmol) in a two-neck flask. 2-Methoxyethanol (25 mL) and 2,4pentanedione (204 mg, 2.04 mmol) were added and the mixture was refluxed for 16 h. The solution was cooled to room temperature and a small quantity of water was added. The orange red solid was collected by filtration. The crude product was chromatographed using CH2Cl2/hexane (1:3) to afford orange powdery (fbi)2Ir(acac) in 54% yield (412 mg). Anal. Calcd for C65H59N4O2Ir: C, 69.81; H, 5.14; N, 5.01. Found: C, 69.69; H, 5.04; N, 4.94. 1H NMR (CDCl3): δ -0.05 (t, J ) 7.2 Hz, 3 H, CH3), 0.25 (t, J ) 7.2 Hz, 3 H, CH3), 1.36 (m, 2 H), 1.38 (m, 2 H), 2.02 (s, 6 H), 5.37 (s, 1 H), 6.42 (s, 1 H), 6.71 (s, 1 H), 7.06 (dd, J ) 4.2, 2.8 Hz, 8 H), 7.30 (d, J ) 5.6 Hz, 2 H), 7.32 (m, 4 H), 7.66 (m, 6 H), 7.70 (m, 4 H), 7.76 (dd, J ) 6.4, 2.0 Hz, 2 H). MS (FAB): m/e 1118.5 (M+). (pbi)2Ir(acac). Yellow green powders. Yield: 55%. Anal. Calcd for C43H33N4O2Ir: C, 62.23; H, 4.01; N, 6.75. Found: C, 62.00; H, 4.40; N, 6.25. 1H NMR (CDCl3): δ 1.83 (s, 6 H), 5.23 (s, 1 H), 6.35-6.44 (m, 4 H), 6.50-6.56 9m, 4 H), 7.09-7.12 (m, 2 H), 7.26-7.28 (m, 4 H), 7.56-7.58 (m, 2 H), 7.63-7.67 (m, 8 H), 7.70-7.72 (m, 2 H). MS (FAB): m/e 830.2 (M+). (nbi)2Ir(acac). Yellow orange powders. Yield: 40%. Anal. Calcd for C51H37N4O2Ir: C, 65.86; H, 4.01; N, 6.02. Found: C, 65.80; H, 4.35; N, 5.96. 1H NMR (CDCl3): δ 1.81 (s, 6 H), 5.25 (s, 1 H), 6.49 (d, J ) 8.4 Hz, 2 H), 6.64 (t, J ) 7.2 Hz, 2 H), 6.89-6.94 (m, 2 H), 6.94 (d, J ) 8.7 Hz, 2 H), 7.29-7.40 (m, 18 H), 7.50 (d, J ) 8.7 Hz, 2 H), 7.71 (d, J ) 8.7 Hz, 2 H). MS (FAB): m/e 930.2 (M+). (pnbi)2Ir(acac). Red powders. Yield: 25%. Anal. Calcd for C59H41N4O2Ir: C, 68.79; H, 4.01; N, 5.44. Found: C, 68.41; H, 4.23; N, 5.46. 1H NMR (CDCl3): δ 1.37 (s, 6 H), 5.27 (s, 1 H), 6.63 (d, J ) 8.0 Hz, 2 H), 6.71-6.75 (m, 6 H), 6.89 (d, J ) 6.4 Hz, 2 H), 7.00-7.05 (m, 4 H), 7.24-7.25 (m, 2 H), 7.27 (d, J ) 8.0 Hz, 2 H), 7.39 (t, J ) 7.2 Hz, 2 H), 7.50 (t, J ) 7.6 Hz, 2 H), 7.43 (s, 2 H), 7.56 (t, J ) 7.2 Hz, 2 H), 7.78 (t, J ) 7.2 Hz, 2 H), 7.88(d, J ) 7.6 Hz, 2 H), 8.13 (d, J ) 8.0 Hz, 2 H), 8.65 (d, 8.0 Hz, 2 H). MS (FAB): m/e 1030.4 (M+). (cbi)2Ir(acac). Yellow powders. Yield: 71%. Anal. Calcd for C59H48N4O2Ir: C, 66.58; H, 4.45; N, 7.90. Found: C, 66.29; H, 4.42; N, 7.54. 1H NMR (CDCl3): δ 0.93 (t, J ) 2.8 Hz, 6 H), 1.88 (s, 6 H), 3.67-3.73 (m, 4 H), 5.27 (s, 1 H), 6.34 (s, 2 H), 6.90 (t, J ) 7.6 Hz, 2 H), 7.03 (d, J ) 8.0 Hz, 2 H), 7.16 (d, J ) 7.6 Hz, 2 H), 7.18 (t, J ) 4.8 Hz, 2 H), 7.20 (s, 2 H), 7.287.31 (m, 4 H), 7.34 (d, J ) 6.8 Hz, 2 H), 7.56 (d, J ) 8.0 Hz, 2 H), 7.68 (t, J ) 6.4 Hz, 2 H), 7.75 (d, J ) 7.2 Hz, 2 H), 7.797.82 (m, 6 H). MS (FAB): m/e 1064.2 (M+). (tbi)2Ir(acac). Yellow powders. Yield: 71%. Anal. Calcd for C39H29N4O2Ir: C, 55.63; H, 3.47; N, 6.65. Found: C, 55.66; H, 3.44; N, 6.32. 1H NMR (CDCl3): δ 1.91 (s, 6 H), 5.27 (s, 6 H), 6.28 (d, J ) 5.2 Hz, 2 H), 6.70 (d, J ) 5.2 Hz, 2 H), 7.17 (t, J ) 7.6 Hz, 3 H), 7.21-7.28 (m, 3 H), 7.56-7.64 (m, 12 H). MS (FAB): m/e 842 (M+). (tabi)2Ir(acac). Orange powders. Yield: 50%. Anal. Calcd for C73H55N4O2Ir: C, 67.93; H, 4.35; N, 6.25. Found: C, 67.70; (12) Nonoyama, K. Bull. Chem. Soc. Jpn. 1974, 47, 467.

Huang et al. H, 4.21; N, 6.02. 1H NMR (d6-acetone): δ 1.81 (s, 6 H), 5.39 (s, 6 H), 6.39 (d, J ) 2.0 Hz, 2 H), 6.40 (d, J ) 4.0 Hz, 2 H), 6.43 (d, J ) 8.0 Hz, 2 H), 6.71 (dt, J ) 8.0, 0.8 Hz, 2 H), 6.78 (d, J ) 4.0 Hz, 2 H), 6.99 (d, J ) 8.0 Hz, 10 H), 7.04 (t, J ) 7.6 Hz, 2 H), 7.24 (t, J ) 7.6 Hz, 10 H), 7.32 (m, 4 H), 7.64 (t, J ) 7.6 Hz, 2 H), 7.70 (d, J ) 8.0 Hz, 2 H), 7.77 (t, J ) 7.6 Hz, 2 H), 7.80-7.83 (m, 4 H). MS (FAB): m/e 1328.2 (M+). (pybi)2Ir(acac). Orange powders. Yield: 55%. Anal. Calcd for C77H55N6O2Ir: C, 71.77; H, 4.30; N, 6.52. Found: C, 71.40; H, 4.50; N, 6.72. 1H NMR (CDCl3): δ 1.94 (s, 6 H), 5.34 (s, 1 H), 6.67 (d, J ) 8.0 Hz, 2 H), 6.89 (s, 2 H), 6.91 (d, J ) 8.0 Hz, 2 H), 7.20 (d, J ) 8.0 Hz, 2 H), 7.24-7.26 (m, 2 H), 7.35-7.41 (m, 14 H), 7.44 (s, 4 H), 7.54 (d, J ) 7.2 Hz, 2 H), 7.61-7.64 (m, 2 H), 7.69 (d, J ) 4.4 Hz, 4 H), 7.80 (d, J ) 6.4 Hz, 2 H), 7.87 (d, J ) 6.4 Hz, 8 H), 7.92 (d, J ) 7.6 Hz, 2 H). MS (FAB): m/e 1288.4 (M+). Structural Determination of (fbi)2Ir(acac). An orange prismatic crystal of (fbi)2Ir(acac) (dimensions 0.25 × 0.30 × 0.30 mm3) was grown from a dichloromethane solution layered with hexane at room temperature. Relevant crystal data are summarized in Table S1 (Supporting Information). The monoclinic space group C2/c was determined from systematic absence of specific reflections; successful refinement of the structure confirmed the space group assignment. Direct methods were used to locate the Ir atom, whereas subsequent cycles of least-squares refinements and difference Fourier map were used to locate the remaining nonhydrogen atoms. Hydrogen atoms were placed at calculated positions. All calculations were performed using the SHELX software package. LEDs Fabrication and Measurements. Compound BCP (2.9-dimethyl-4,7-diphenyl-1,10-phenanthroline) was purchased from Aldrich and used as received. Compounds Alq3 (tris(8-hydroxyquinoline) aluminum),8a NPB (4,4′-bis{N-(1naphthyl-N-phenylamino)biphenyl}),13 CBP (4,4′-N,N′′-dicarbazolebiphenyl),13 and BPAPF (9,9-bis{4-[di(p-biphenyl)aminophenyl}}fluorene)14 were synthesized according to literature procedures, and were sublimed twice prior to use. Prepatterned ITO substrates with an effective individual device area of 3.14 mm2 were cleaned as described in a previous report.15 A 40nm-thick film of BPAPF or NPB was deposited first as the hole transport layer (HTL). The light-emitting layer (30 nm) was then deposited by coevaporating a CBP host and a phosphorescent dopant (∼6% dopant concentration), with both deposition rates being controlled with two independent quartz crystal oscillators. A 10-nm-thick BCP as a hole and exciton blocking layer (HBL) and 30-nm-thick Alq3 as an electron transport layer were then deposited sequentially. Finally, an alloy of magnesium and silver (ca. 10:1, 500 Å) was deposited as the cathode, which was capped with 1000 Å of silver. I-V curve was measured on a Keithley 2400 Source meter in ambient environment. Light intensity was measured with a Newport 1835 optical meter. Lifetime Test. Freshly prepared devices were taken out of the vacuum chamber and encapsulated with UV-cure epoxy resin. The device was then driven at a constant current to give an initial luminance of 10000 candela and then followed for the decay of luminance with time.

Results and Discussion Synthesis of the Materials. Standard procedure16 was followed to synthesize 2-aryl-substituted benzoimidazoles (abbreviated with the general formula bi) from N-phenyl-o-phenylenediamine and appropriate aromatic aldehydes, as illustrated in (a) of Scheme 1. The syntheses of the bis-cyclometalated iridium complexes of imidazoles involved two steps. These are shown in (b) (13) Koene, B. E.; Loy, D. E.; Thompson, M. E. Chem. Mater. 1998, 10, 2235. (14) Ko, C.-W.; Tao, Y.-T. Synth. Met. 2002, 126, 37. (15) Balasubramaniam, E.; Tao, Y. T.; Danel, A.; Tomasik, P. Chem. Mater. 2000, 12, 2788. (16) Neuse, E. W.; Loonat, M. S. Macromolecules 1983, 16, 128.

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Scheme 1

Chart 1

and (c) of Scheme 1, using fbi as the representative example: (b) reaction of IrCl3‚3H2O with benzoimidazole ligand to form a chloride-bridged dimer, (bi)2Ir(µ-Cl)2Ir(bi)2 (bi ) cyclometalated benzoimidazole); and (c) replacement of bridging chlorides with bidentate β-diketonate ligands to give the desired products, (bi)2Ir(acac) (acac ) acetylacetonate). Chart 1 shows the complexes (bi)2Ir(acac) synthesized in this study. (bi)2Ir(µ-Cl)2Ir(bi)2 complexes exhibit negligible emission at room temperature. In contrast, most (bi)2Ir(acac)

complexes possess strong emissive character at room temperature (vide infra). Therefore, only (bi)2Ir(acac) complexes were subjected to further studies. Although (bi)2Ir(acac) decompose before melting, they are stable up to g300 °C by TGA (thermal gravimetric analysis) measurements. Cyclic Voltammetric Studies. Most (bi)2Ir(acac) complexes exhibit a quasi-reversible one-electron oxidation wave ranging from 700 to 1200 mV vs Ag/AgCl, which can be attributed to the oxidation of the iridium-

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Table 1. Photophysical and Electrochemical Data for (bi)2Ir(acac) and bi bi or bi

Eoxa mV

λabs (log )b nm

λemc nm

τc µs

Φemc

τrd µs

pbi nbi pnbi cbi fbi tbi tabi pybi pbi nbi pnbi cbi fbi tbi tabi pybi

903 (81) 938 (90) 700 (100) 758 (75) 841 (73) 973(i) 776 (83), 1183 (i) 1032 (91)

300 (4.6), 315 (4.6), 346 (3.5), 388 (3.2), 414 (3.1), 453 (3.0) 307 (4.0), 423 (3.1), 463 (3.0), 501 (3.0) 261 (5.0), 318 (4.7), 438 (3.2), 503 (3.2) 322 (4.7), 378 (3.6), 430 (3.0) 327 (4.8), 341 (4.8), 407 (3.3), 450 (3.0), 477 (3.0) 300 (4.9), 350 (3.0), 409 (3.4) 293 (4.9), 327 (4.6), 409 (4.0), 471 (3.0) 267 (5.1), 319 (5.0), 406 (3.1) 249 (4.3) 296 (4.0) 300 (4.0) 300 (4.5), 320 (3.0) 323 (4.7) 300 (4.0) 300 (4.0), 384 (4.0) 315 (4.4)

523 604 651 520 563 507 620 566 360 386 382 385 380 366 484 398

1.82 0.32 2.37 0.18 1.51 1.23 0.91 0.45

0.73 0.14 0.0089 0.52 0.51 0.0085 0.023 0.22

2.5 2.3 266 0.35 3.0 145 40 2.0

a Measured in DMF at a concentration of 10-3 M and the scan rate was 80 mV s-1. The potentials are reported relative to Ag/AgCl. The numbers in parentheses are the cathodic and anionic peak separation; i ) irreversible. b Measured in CH2Cl2 solution. c Measured in toluene at 298 K. Excitation wavelength was 400 nm for all complexes. (ppy)3Ir was used as the reference for the quantum yield (Φem) measurement. d τr) τ/Φem.

Figure 1. Absorption spectra of selected (bi)2Ir(acac) complexes in CH2Cl2. Those of the corresponding bi ligands are shown in the inset.

(II). These values fall within those of (ppy)2Ir(acac) (ppy ) 2-phenylpyridine),7 (ppy)3Ir,6f,6j and analogues, and appear to be significantly lower than those of (ppy)2Ir(L)+ (L ) 2,2′-bipyridine) and analogues.17 The highest occupied molecular orbitals (HOMOs) of (ppy)2Ir(acac) and (ppy)3Ir were demonstrated to be 5d of Ir with substantial mixing with the π orbital of ppy.18 Variations of cyclometalated ligands, including replacement of pyridine by other heteroaromatic rings, replacement of phenyl ring by other aromatic rings, and incorporation of substituents on the rings, were shown to affect the oxidation potential of the iridium ion in congeners of (ppy)2Ir(acac).7 Consistent with this, the oxidation potentials of benzoimidazole complexes were found to be sensitive to the 2-substituent of the benzoimidazole ligand. The oxidation potential of (pbi)2Ir(acac) appears to be slightly higher (∆E ≈ 30 mV) than that of (ppy)2Ir(acac). More electron-rich aromatic rings, such as phenanthrene and carbazole, significantly lower the oxidation potential of the iridium ion, whereas the electron-deficient pyridinyl ring has the opposite effect. The extra oxidation wave at lower potential for (tabi)2Ir(acac) is attributed to the oxidation of diphenylthienylamine entity,19 and the irreversible oxidation of (tbi)2Ir(acac) likely stems from polymerization of the thiophene moiety.20 No reduction waves were detected in the complexes up to -2.0 V.

Figure 2. Phosphorescence spectra of selected complexes in toluene at 298 K: (a) (pbi)2Ir(acac); (b) (fbi)2Ir(acac); (c) (cbi)2Ir(acac); and (d) (nbi)2Ir(acac).

Photophysical Properties. The photophysical data of bi ligands and (bi)2Ir(acac) complexes are collected in Table 1. Absorption spectra for selected free ligands and (bi)2Ir(acac) complexes are shown in Figure 1. The ligands have a characteristic absorption at ∼300 nm due to the π-π* transition of benzoimidazolyl moiety. Absorption at ∼380 nm in tabi can be attributed to (17) (a) Ohsawa, Y.; Sprouse, S.; King, K. A.; DeArmond, M. K.; Hanck, K. W.; Watts, R. J. J. Phys. Chem. 1987, 91, 1047. (b) Garces, F. O.; King, K. A.; Watts, R. J. Inorg. Chem. 1988, 27, 3464. (18) Hay, P. J. J. Phys. Chem. A 2002, 106, 1634. (19) Wu, I.-Y.; Lin, J. T.; Tao, Y.-T.; Balasubramaniam, E. Adv. Mater. 2000, 12, 668. (20) Reynolds, J. R., Skotheim, T., Elsenbaumer, R. L., Eds. Handbook of Conductive Polymers, 2nd ed.; Marcel Dekker: New York, 1997.

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Table 2. Electrophophorescence Data for (bi)2Ir(acac) Complexes bi pbia pbib nbia nbib fbia fbib cbia cbib tbia tbib

brightness cd/m2

ηext %

ηc cd/A

ηp lm/W

voltage V

VON V

λem (fwhm) nm

CIE x,y

12116c 46393d 148552e 6987c 27640d 112446e 1208c 4562d 12874e 746c 3119d 11598e 6006c 21105d 31072e 4760c 17336d 40794e 8880c 35332d 67844e 4299c 17738d 55272e 2587c 10122d 19184e 1630c 6811d 14013e

16.7 12.7 4.2 9.7 7.7 2.5 4.8 3.6 2.6 2.9 2.4 1.7 10.4 7.3 3.1 8.2 6.0 2.2 12.1 9.7 7.7 5.9 4.9 3.5 4.0 3.2 0.67 2.6 2.2 0.5

61 46 30 35 28 18 6.1 4.6 3.3 3.7 3.1 2.2 30 21 18 24 17 13 44 36 28 22 18 13 13 10 4.4 8.1 6.8 3.0

20 12 6.4 12 7.5 3.8 1.7 1.1 0.7 1.2 0.8 0.5 7.8 4.7 3.8 6.8 4.2 2.7 13 8.6 5.9 7.0 4.7 2.7 5.0 3.1 1.1 3.2 2.1 0.7

9.8 12 15 9.5 12 15 11 13 15 10 12 15 12 14 15 11 13 15 11 14 15 10 12 15 8.2 10 13 7.9 10 13

3.6

530 (78)

0.36, 0.60

3.6

528 (74)

0.35, 0.59

5.1

608 (84)

0.63, 0.35

4.8

608 (84)

0.58, 0.34

4.0

568 (68)

0.51, 0.48

3.7

570 (70)

0.51, 0.48

4.2

530 (80)

0.35, 0.61

4.2

526 (76)

0.33, 0.61

3.8

518 (78)

0.27, 0.59

3.6

516 (78)

0.26, 0.55

a Device I. b Device II. c At 20 mA/cm2. d At 100 mA/cm2. e Maximum value; fwhm, full width at half-maximum; η , external quantum ext efficiency; ηc, current efficiency; ηp, power efficiency; VON, turn-on voltage.

Figure 3. ORTEP diagram of (fbi)2Ir(acac).

diphenylthienylaminefbenzoimidazole charge-transfer transition. The π-π* bands of (bi)2Ir(acac) resemble those of bi and appear in the ultraviolet region (λem ∼300 nm) with extinction coefficients ∼104-105. According to the previous reports on (ppy)2Ir(acac), (ppy)3Ir, and analogues,6c,6i,18,21,22 weaker bands located at longer wavelengths can be attributed to the S0f1MLCT and S0f3MLCT transitions. The spin-forbidden metal-toligand charge-transfer absorption band (3MLCT) is redshifted by ∼50-100 nm compared to the spin-allowed 1MLCT band in (bi) Ir(acac) complexes. The 1MLCT 2 bands fall in the range of ∼350-450 nm with extinction coefficients ∼3000-1000. The 3MLCT bands appear at ∼400-500 nm with extinction coefficients ∼2000-1000. The comparable intensities of 1MLCT and 3MLCT imply

Figure 4. Schematic diagram of EL device configurations and the molecular structures of the compounds used.

the presence of significant singlet-triplet coupling due to spin-orbital coupling. At 298 K in toluene, the (bi)2Ir(acac) complexes emit characteristic phosphorescent light, as evidenced by the

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lifetime lying in submicron/micro seconds region. The phosphorescence spectra of selected complexes are shown in Figure 2. The emitted colors range from green to red. In contrast, most benzoimidazole ligands emit in the violet-purple in CH2Cl2 except for tabi which emits blue-green light. Both 3MLCT and ligand-based 3π-π* transitions can contribute to the phosphorescence emission in complexes (C∧N)2Ir(acac) and (C∧N)3Ir (where C∧N ) cyclometalated ligand containing nitrogen donors), depending on the variation of ligands.6c,21,23 The relative contribution of 3MLCT vs. ligand-based 3π-π* transitions may affect the Stokes shifts between the 3MLCT and emission bands6c as well as the phosphorescence spectral feature.22 For instance, ligandbased 3π-π* transitions result in a larger Stokes shift and better resolved vibronic progression. The Stokes shifts of (bi)2Ir(acac) are larger than that of (ppy)2Ir(acac) (∼2200 cm-1).6c Among (bi)2Ir(acac) complexes, (pnbi)2Ir(acac) (4567 cm-1), (tbi)2Ir(acac) (4726 cm-1), (tabi)2Ir(acac) (5102 cm-1), and (pybi)2Ir(acac) (6938 cm-1) have larger Stokes shifts than others the (29554025 cm-1). Some of the (bi)2Ir(acac) complexes (where bi ) pbi, nbi, fbi, tabi, and pybi) exhibit more prominent vibronic peaks in the phosphorescence spectra, others are somewhat broader. Complexes containing ligands with better π-conjugation such as (pnbi)2Ir(acac), and (nbi)2Ir(acac), or ligands with strong intramolecular charge-transfer character such as (tabih)2Ir(acac), appear to have more red-shifted phosphorescence spectra. This is consistent with the trend observed in (C∧N)3Ir.22 Strong spin-orbit coupling in (bi)2Ir(acac) complexes leads to efficient phosphorescence. Except for (pnbi)2Ir(acac), (tbi)2Ir(acac), and (tabi)2Ir(acac), most of the complexes exhibit high solution phosphorescence quantum yields (ΦP ) 0.14-0.73) at room temperature under air free condition. These values appear to be high among (C∧N)2Ir(acac) complexes.6c,6i,7 Another significant feature of the highly emitting (bi)2Ir(acac) complexes is the short lifetime of the triplet excited state which ranges from 0.35 to 3.0 µs (Table 1). The radiative lifetime of the triplet excited state was calculated from τr ) τ/Φem assuming the intersystem-crossing yield to be 1.0. The short excited state lifetime together with the high phosphorescence yield should be advantageous to highly efficient devices. Molecular Structure of (fbi)2Ir(acac) Complex. The ORTEP drawing of (fbi)2Ir(acac) is shown in Figure 3. Crystallographic data are listed in Table S1, and important bond angles and bond distances are collected in Table S2 (Supporting Information). The iridium resides in an approximately octahedral environment and the two nitrogen atoms of fbi ligands exhibit cisC-C and trans-N,N chelate dispositions. The two fbi ligands are mirror images to each other with respect to the symmetry plane passing the iridium metal and bisecting the acac ligand. The Ir-C bonds (2.006(4) Å) are shorter than the Ir-N bonds (2.038(3) Å). These (21) Micro, G. C.; Hauser, A.; Gu¨del, H. U. Inorg. Chem. 1993, 32, 3088. (22) Tsuboyama, A.; Iwawaki, H.; Furugori, M.; Mukaide, T.; Kamatami, J.; Igawa, S.; Moriyama, T.; Miura, S.; Takiguchi, T.; Okada, S.; Hoshino, M.; Ueno, K. J. Am. Chem. Soc. 2003, 125, 12971. (23) Colombo, M. G.; Brunold, T. C.; Riedener, T.; Gu¨del, H. U.; Fo¨rtsch, M.; Bu¨rgi, H.-B. Inorg. Chem. 1994, 33, 545.

Huang et al.

Figure 5. EL spectra of the devices I (a) and II (b) for selected complexes.

values are larger than those in (ppy)2Ir(acac) (Ir-C, 2.003(9) Å; Ir-N:, 2.010(9) Å) and (tpy)2Ir(acac) (Ir-C, 1.985(7) Å; Ir-N, 2.023(5) Å)7 possibly due to the more steric congestion of fbi. The Ir-O bonds (2.145 Å) are similar to those of (ppy)2Ir(acac) (2.146(6) Å) and (tpy)2Ir(acac) (2.161(4) Å).7 Other features appear to be normal: the benzoimidazolyl ring is approximately coplanar with the fluorenyl ring (the dihedral angle between the two planes ) 5.0(0.1)°), and the N-phenyl ring tilts 70.6(0.1)° from the benzoimidazolyl plane. OLEDs with (bi)2Ir(acac) as Emissive Dopant. The iridium complexes which exhibit higher solution quantum yields at room temperature were selected for device fabrication. The device structure used is similar to that developed by Forrest and Thompson: a layer (40 nm) of 9,9-bis{4-[di(p-biphenyl)aminophenyl}}fluorene (BPAPF)14 (device I) or 4,4′-bis{N-(1-naphthyl-N-phenylamino)biphenyl} (NPB) (device II) for hole transport, a layer (30 nm) of 4,4′-N,N′′-dicarbazolebiphenyl (CBP) doped with 6% of (bi)2Ir(acac) as the emitter, a thin layer (10 nm) of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) for hole and exciton blocking (HBL), and a layer (30 nm) of tris(8-hydroxyquinoline) aluminum (Alq3) for electron transport.24 Figure 4 shows the (24) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 1999, 75, 4.

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Figure 6. C-V-L characteristics for the devices I (a and b) and II (c and d).

configurations of the devices and the molecular structures of the compounds used in these devices. Important device performance characteristics are collected in Table 2. All devices exhibit low turn-on voltages: 3.6-5.0 V for device I and 3.6-4.8 V for device II. The EL spectra of these devices are shown in Figure 5. Devices I emit light characteristic of (bi)2Ir(acac) (Figure 5a). Light was emitted mainly from (bi)2Ir(acac) in device II, too. However, minor emission from NPB (at ∼450 nm)25 was found in some devices (Figure 5b). Such an outcome may be attributed to the higher hole-transport rate of BPAPF than of NPB26 which leads to a charge recombination area well inside the CBP/(bi)2Ir(acac) layer in view of the similarity of both HOMO energy level (NPB, 5.2 eV; BPAPF, 5.3 eV) and LUMO energy level (NPB, 2.2 eV; BPAPF, 2.2 eV) between NPB27 and BPAPF.14 The current-voltage-luminance characteristics are shown in Figure 6. Though efficiencies of the devices appear to drop rapidly as the applied voltage increases (Figure 7), it is worth noting that the performances of greenemitting devices I for (pbi)2Ir(acac) (λem ) 530 nm) and (25) Ko, C.-W.; Tao, Y.-T.; Lin, J. T.; Justin Thomas, K. R. Chem. Mater. 2002, 14, 357. (26) The hole-transporting rates measured by the time-of-flight method were found to be 3.0 × 10-4 and 3.0 × 10-3 cm2V-1s-1 for NPB and BPAPF, respectively. (27) (a) Tao, Y. T.; Balasubramaniam, E.; Danel, A.; Tomasik, P. Appl. Phys. Lett. 2000, 77, 933. (b) Ko, C.-W.; Tao, Y.-T. Chem. Mater. 2001, 13, 2441.

Figure 7. Current and power efficiencies of devices I for selected complexes.

(cbi)2Ir(acac) (λem ) 528 nm), and yellow-emitting (λem ) 568 nm) device I for (fbi)2Ir(acac) appear to be very promising. For instance, the performance parameters for (pbi)2Ir(acac) (brightness, 46393 cd/m2; external quantum efficiency, 12.7%; current efficiency, 46 cd/A; power efficiency, 12 lm/W at a current density of 100 mA/cm2), (cbi)2Ir(acac) (brightness, 37217 cd/m2; external quantum efficiency, 9.7%; current efficiency, 37 cd/A; power efficiency, 8.6 lm/W at a current density of 100 mA/cm2), and (fbi)2Ir(acac) (brightness, 21105 cd/ m2; external quantum efficiency, 7.3%; current efficiency, 21 cd/A; power efficiency, 4.7 lm/W at a current

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strate, encapsulation, andmaterial stability, we nevertheless compared the “relative” stability of devices I and II to elucidate the effect of different HTL. The stability of devices I and II of (pbi)2Ir(acac) was examined by operating the device at an initial luminance of 10000 cd/m2 and retaining a constant current density (Figure 8). The lifetimes (the time taken for the brightness to drop to 50% of the initial value) were found to be 1360 and 595 min for I and II, respectively. The device with BPAPF exhibits a longer lifetime. Presumably the better device efficiency is beneficial to a longer lifetime. Conclusions

Figure 8. Stability test of the devices for (pbi)2Ir(acac).

density of 100 mA/cm2) compare favorably with those of the green-emitting (λem ) 525 nm) (ppy)2Ir(acac) and yellow-emitting (bt)2Ir(acac) reported by Thompson.6c On the other hand, green-emitting (tbi)2Ir(acac) and red-emitting (nbi)2Ir(acac) devices have efficiencies not much different from normal fluorescence devices. The better performance of the devices based on (pbi)2Ir(acac), (fbi)2Ir(acac), and (cbi)2Ir(acac) than (tbi)2Ir(acac) and (nbi)2Ir(acac) may be partly attributed to the higher solution quantum yields of the former. The EL spectra of these devices were found to be independent of the applied voltage (6-12 V). Although the device stability is a complex problem relating to sub-

In summary, we have synthesized new bis-cyclometalated iridium complexes containing benzoimidazolebased ligands. These complexes are strongly phosphorescent at ambient condition. High-performance greenand yellow-emitting devices were also fabricated. Acknowledgment. We thank Academia Sinica and the National Science Council (grant NSC-92-2113-M001-020) for financial support. Supporting Information Available: Crystallographic information files (CIF) for (fbi)2Ir(acac); table of crystal data and structure refinement for (fbi)2Ir(acac) (Table S1), and table of selected bond distances (Å) and angles (deg) for Ir(fbi)2(acac) (Table S2) (pdf). These materials are available free of charge via the Internet at http://pubs.acs.org. CM0498943