Green and Yellow Electroluminescent Dipolar Carbazole Derivatives

Novel Star-shaped Hole-transporting Materials Based on Triphenylamine Cores End-capped with Carbazole and Triarylamine Derivatives for use in OLEDs...
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Chem. Mater. 2002, 14, 3852-3859

Green and Yellow Electroluminescent Dipolar Carbazole Derivatives: Features and Benefits of Electron-Withdrawing Segments K. R. Justin Thomas,† Jiann T. Lin,*,†,‡ Yu-Tai Tao,*,† and Chang-Hao Chuen† Institute of Chemistry, Academia Sinica, 115 Nankang, Taipei, Taiwan, and Department of Chemistry, National Central University, 320 Chungli, Taiwan Received March 7, 2002. Revised Manuscript Received June 26, 2002

New multiply substituted carbazole derivatives containing fluorene or phenylene conjugated oxadiazole segments and quinoxaline units were obtained by palladium-catalyzed C-N coupling reactions. They are amorphous with the glass transition temperature (Tg) in the range 104-176 °C. The emission color of the materials varies from blue to yellow and is dependent on the nature of the electron-withdrawing segments and solvents. Two reversible one-electron oxidations were observed for these molecules in cyclic voltammograms, which originate from the peripheral 3,6-diarylamino units in the 3,6,9-trisubstituted derivatives and diarylamine and carbazole segments in the 3,9-disubstituted compounds. Reductions originating from quinoxaline segments were also located for the molecules incorporating quinoxaline moieties. The double-layer organic light-emitting diodes fabricated using these compounds as hole-transporting/emitting layers and TPBI or Alq3 as an electron-transporting layer emit bluish green to yellow colors. The recombination zone is restricted in the HTL layer for the quinoxaline-containing molecules irrespective of the electron-transporting layer used and emission occurs from them. However, for the oxadiazole derivatives emission in the Alq3-based devices is either red-shifted or resembles that of Alq3. Cyclic voltammetric and spectroscopic data support more pronounced electron affinity for the quinoxalineincorporated carbazole derivatives than for the oxadiazole-tethered carbazole materials.

Introduction Fabrication of light-emitting diodes (LED) from organic and polymeric thin films attracts wide attention owing to their application in flat panel displays.1 It demands sufficient hole/electron transport, adequate emission characteristics, and stable morphology to result in efficient performance in OLED.2-4 Holetransporting and electron-transporting properties are opposing functions associated with different structural features and often used independently. However, attempts have also been made to incorporate all the above functions in the same molecule or polymer in an effort to attain a maximum performance.5-8 It is believed that this approach will help to make the fabrication process * To whom correspondence should be addressed. Fax: Int. code +(2)27831237. E-mail: [email protected]. †Academia Sinica. ‡National Central University. (1) Sheats, J. R.; Antoniadis, H.; Hueschen, M.; Leonard, W.; Miller, J.; Moon, R.; Roitman, D.; Stocking, A. Science 1996, 273, 884. Gross, M.; Muller, D. C.; Nothofer, H. G.; Scherf, U.; Neher, D.; Brauchle, C.; Meerholz, K. Nature 2000, 405, 661. (2) Shirota, Y. J. Mater. Chem. 2000, 10, 1. (3) Mitschke, U.; Ba¨uerle, P. J. Mater. Chem. 2000, 10, 1471. (4) Segura, J. L. Acta Polym. 1998, 49, 319. (5) Lee, Y. Z.; Chen, X. W.; Chen, S. A.; Wei, P. K.; Fann, W. S. J. Am. Chem. Soc. 2001, 123, 2296. Jiang, X. Z.; Register, R. A.; Killeen, K. A.; Thompson, M. E.; Pschenitzka, F.; Sturm, J. C. Chem. Mater. 2000, 12, 2542. Song, S. Y.; Jang, M. S.; Shim, H. K.; Hwang, D. H.; Zyung, T. Macromolecules 1999, 32, 1482. (6) Tian, H.; Zhu, W. H.; Elschner, A. Synth. Met. 2000, 111, 481. Ng, S. C.; Lu, H. F.; Chan, H. S. O.; Fujii, A.; Laga, T.; Yoshino, K. Adv. Mater. 2000, 12, 1122.

simpler and cost-effective. The dipolar materials featuring combinations such as triarylamine-oxadiazole9-12 and triarylamine-pyridine/quinoline13,14 were synthesized and applied in LEDs. Adachi, in his pioneering work9,10 on amine-oxadiazole dyads, found that the incorporation of amino substituents benefits the injection of holes into the materials while retaining its electron-transporting capability. Even though these materials were employed in single-layer devices successfully, it was observed that better balance of hole/ electron flow is achieved if a hole-transporting layer with low ionization potential is also used. We report herein a series of dipolar compounds incorporating oxadiazole/quinoxaline segments and carbazole core that function as efficient hole-transporting material in electroluminescent devices. (7) Kwok, C. C.; Wong, M. S. Macromolecules 2001, 34, 6821. Peng, Z. H.; Pan, Y. C.; Xu, B. B.; Zhang, J. H. Macromol. Symp. 2000, 154, 245. (8) Hong, Y. R.; Lee, D. W.; Kim, K.; Jin, J. I.; Lee, C. E.; Lee, H. M.; Park, Y.; Shon, B. H.; Park, J. K. Macromol. Symp. 2001, 175, 169. (9) Mochizuki, H.; Hasui, T.; Kawamoto, M.; Shiono, T.; Ikeda, T.; Adachi, C.; Taniguchi, Y.; Shirota, Y. Chem. Commun. 2000, 1923. (10) Tamoto, N.; Adachi, C.; Nagai, K. Chem. Mater. 1997, 9, 1077. (11) Peng, Z. H.; Bao, Z. N.; Galvin, M. E. Chem. Mater. 1998, 10, 2086. (12) Zhu, W. H.; Tian, H.; Elschner, A. Chem. Lett. 1999, 501. (13) Wang, Y. Z.; Epstein, A. J. Acc. Chem. Res. 1999, 32, 217 and references therein. (14) Jenekhe, S. A.; Lu, L. D.; Alam, M. M. Macromolecules 2001, 34, 7315.

10.1021/cm0202512 CCC: $22.00 © 2002 American Chemical Society Published on Web 08/08/2002

Electroluminescent Dipolar Carbazole Derivatives

Chem. Mater., Vol. 14, No. 9, 2002 3853 Scheme 1

We have recently reported a series of hole-transporting materials that is built on carbazole.15-18 We felt that the incorporation of electron-withdrawing segments onto the carbazole may lead to improved electron injection into the material and to make the materials bifunctional. We also uncover that emission color can be tuned by adjusting the donor and acceptor strengths in these molecules. To the best of our knowledge, quinoxaline-incorporated carbazole derivatives are not yet reported. Experimental Section General Information. Unless otherwise specified, all the reactions were performed under a nitrogen atmosphere using standard Shlenk techniques. Toluene was distilled from sodium and benzophenone under a nitrogen atmosphere. All chromatographic separations were carried out on silica gel (60 M, 230-400 mesh). Dichloromethane and dimethylformamide were distilled from calcium hydride under a nitrogen atmosphere. 1H NMR spectra were recorded on a Bruker 300-MHz spectrometer operating at 300.135 MHz. Emission spectra were recorded on a Perkin-Elmer spectrofluorometer. 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 glassy carbon working electrode, a platinum 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 CH2Cl2 and the supporting electrolyte was 0.1 M tetrabu(15) Justin Thomas, K. R.; Lin, J. T.; Tao, Y.-T.; Ko, C.-W. Adv. Mater. 2000, 12, 1949. (16) Justin Thomas, K. R.; Lin, J. T.; Tao, Y.-T.; Ko, C.-W. J. Am. Chem. Soc. 2001, 123, 9404. (17) Ko, C.-W.; Tao, Y.-T.; Lin, J. T.; Justin Thomas, K. R. Chem. Mater. 2002, 14, 357. (18) Justin Thomas, K. R.; Lin, J. T.; Tao, Y.-T.; Ko, C.-W. Chem. Mater. 2002, 14, 1354.

tylammonium perchlorate. DSC measurements were carried out on a Perkin-Elmer differential scanning calorimeter at a heating rate of 10 °C/min under a nitrogen atmosphere. TGA measurements were performed on a TA-7 series thermogravimetric analyzer at a heating rate of 10 °C/min under a flow of air. Carbazole, fluorene, and Pd(dba)2 were procured from commercial sources and used as received. Preparation and characterization of the secondary amines (1 and 2), brominated oxadiazole precursors, and quinoxaline derivative (14) are provided in the Supporting Information. The C-N coupling reactions were performed in a similar fashion to obtain the target molecules so only a representative procedure is described below for 5. The molecular structures for compounds 1-11 and 12-16 are found in Schemes 1 and 2, respectively. {4-[5-(4-tert-Butyl-phenyl)-[1,3,4]oxadiazol-2-yl]-phenyl}-(9-ethyl-9H-carbazol-3-yl)-phenyl-amine (5). A doublenecked 250-mL flask was charged with the secondary amine (9-ethyl-9H-carbazol-3-yl)-phenyl-amine (1) (2.86 g, 10 mmol), 2-(4-bromo-phenyl)-5-(4-tert-butyl-phenyl)-[1,3,4]oxadiazole (3.57 g, 10 mmol), Pd(dba)2 (0.11 g, 0.2 mmol), tri(tert-butyl)phosphine (0.04-0.06 g, 0.2-0.3 mmol), sodium tert-butoxide (1.44 g, 15 mmol), and toluene (25 mL) and heated at 80 °C for 8 h. After cooling, the mixture was quenched with water and the solid extracted with diethyl ether (3 × 20 mL). The combined ethereal extract was washed with brine solution and dried over anhydrous MgSO4. Evaporation of volatiles left a yellow solid, which on column chromatography produced the title compound in 79% yield as a pale yellow solid. 1H NMR (CDCl3): δ 1.34 (s, 9 H, tert-butyl), 1.46 (t, J ) 7.3 Hz, 3 H, CH3), 4.37 (q, J ) 7.3 Hz, 2 H, CH2), 7.04-7.09 (m, 3 H), 7.157.23 (m, 3 H), 7.27-7.32 (m, 3 H), 7.37-7.50 (m, 3 H), 7.517.53 (m, 2 H), 7.87-8.03 (m, 6 H). FAB MS (m/e): 563 (M+). Anal. Calcd for C38H34N4O: C, 81.11; H, 6.09; N, 9.96. Found: C, 81.04; H, 6.13; N, 9.81. N,N′-Bis-{4-[5-(4-tert-butyl-phenyl)-[1,3,4]oxadiazol-2yl]-phenyl}-9-ethyl-N,N′-diphenyl-9H-carbazole-3,6-diamine (6). 1H NMR (CDCl3): δ 1.34 (s, 18 H, tert-butyl), 1.52 (t, J ) 7.3 Hz, 3 H, CH3), 4.39 (q, J ) 7.3 Hz, 2 H, CH2), 7.027.08 (m, 12 H), 7.17(d, J ) 7.5 Hz, 4 H), 7.87 (d, J ) 8.7 Hz, 8 H), 8.02 (d, J ) 8.7 Hz, 8 H). FAB MS (m/e): 930 (M+). Anal. Calcd for C62H55N7O2: C, 80.06; H, 5.96; N, 10.54. Found: C, 79.87; H, 5.91; N, 10.46.

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Justin Thomas et al. Scheme 2

{4-[5-(9,9-Diethyl-9H-fluoren-2-yl)-[1,3,4]oxadiazol-2yl]-phenyl}-(9-ethyl-9H-carbazol-3-yl)-phenyl-amine (7). 1 H NMR (CDCl3): δ 0.52-0.71 (m, 10 H, CH2CH3), 1.00-1.12 (m, CH2, 4 H), 1.46 (t, J ) 7.3 Hz, 3 H, CH3), 1.98-2.10 (m, 4 H, CH2), 4.37 (q, J ) 7.3 Hz, 2 H, CH2), 7.08-7.12 (m 3 H), 7.20-7.22 (m, 3 H), 7.28-7.46 (m, 9H), 7.73-7.76 (m, 1 H), 7.79 (d, J ) 7.8 Hz, 1 H), 7.93-7.99 (m, 4 H), 8.04-8.08 (m, 2 H). FAB MS (m/z): 707 (M+). Anal. Calcd for C49H46N4O: C, 83.25; H, 6.56; N, 7.93. Found: C, 83.07; H, 6.48; N, 7.69. N,N′-Bis-{4-[5-(9,9-diethyl-9H-fluoren-2-yl)-[1,3,4]oxadiazol-2-yl]-phenyl}-9-ethyl-N,N′-diphenyl-9H-carbazole3,6-diamine (8). 1H NMR (CDCl3): δ 0.53-0.66 (m, 20 H, CH2CH3), 0.99-1.11 (m, 8 H, CH2), 1.50 (t, J ) 7.3 Hz, 3 H, CH3), 1.98-2.05 (m, CH2, 8 H), 4.39 (q, J ) 7.3 Hz, 2 H, CH2), 7.04-7.08 (m, 6 H), 7.19 (d, J ) 7.6 Hz, 4 H), 7.27 (d, J ) 7.6 Hz, 4 H), 7.35 (s, 8 H), 7.42 (d, 2 H), 7.71-7.75 (m, 2 H), 7.79 (d, J ) 7.8 Hz, 2 H), 7.83 (d, J ) 1.2 Hz, 2 H), 7.92 (d, J ) 8.8 Hz, 4 H), 8.02-8.07 (m, 4 H). FAB MS (m/e): 1219 (M+). Anal. Calcd for C84H79N7O2: C, 82.79; H, 6.53; N, 8.05. Found: C, 82.64; H, 46.44; N, 7.91. N-{4-[5-(9,9-Diethyl-9H-fluoren-2-yl)-[1,3,4]oxadiazol2-yl]-phenyl}-9-ethyl-N′-naphthalen-1-yl-N,N′-diphenyl9H-carbazole-3,6-diamine (9). 1H NMR (CDCl3): δ 0.550.63 (m, 10 H, CH2CH3), 1.02-1.12 (m, 4 H, CH2), 1.44 (t, J ) 7.3 Hz, 3 H, CH3), 2.06-2.16 (m, 4 H, CH2), 4.49 (q, J ) 7.3

Hz, 2 H, CH2), 6.76-6.81 (m, 3 H), 6.98-7.02 (m, 2 H), 7.067.09 (m, 3 H), 7.11-7.14 (m, 2 H), 7.29-7.54 (m, 11 H), 7.56 (d, J ) 8.8 Hz, 1 H), 7.62 (d, J ) 8.8 Hz, 1H), 7.78 (d, J ) 8. 1 Hz, 1 H), 7.88-7.94 (m, 6 H), 7.99 (d, J ) 8.1 Hz, 1 H), 8.088.13 (m, 2 H), 8.19 (d, J ) 1.2 Hz, 1 H). FAB MS (m/e): 924 (M+). Anal. Calcd for C66H57N5O: C, 84.47; H, 6.22; N, 7.58. Found: C, 84.31; H, 6.04; N, 7.34. N-{4-[5-(9,9-Diethyl-9H-fluoren-2-yl)-[1,3,4]oxadiazol2-yl]-phenyl}-9-ethyl-N,N′-diphenyl-N′-pyren-1-yl-9H-carbazole-3,6-diamine (10). 1H NMR (CDCl3): δ 0.52-0.65 (m, 10 H, CH2CH3), 1.00-1.11 (m, 4 H, CH2), 1.43 (t, J ) 7.3 Hz, 3 H, CH3), 2.06-2.14 (m, 4 H, CH2), 4.47 (q, J ) 7.3 Hz, 2 H, CH2), 6.79-6.86 (m, 3 H), 6.93-6.97 (m, 2 H), 7.01-7.15 (m, 5 H), 7.24-7.31 (m, 3 H), 7.36-7.47 (m, 4 H), 7.58 (t, J ) 8.8 Hz, 2 H), 7.83-7.91 (m, 5 H), 7.94-8.00 (m, 4 H), 8.04-8.10 (m, 3 H), 8.14-8.28 (m, 5 H). FAB MS (m/e): 998 (M+). Anal. Calcd for C71H59N5O: C, 85.42; H, 5.96; N, 7.02. Found: C, 85.56; H, 5.81; N, 6.93. (9-Ethyl-9H-carbazol-3-yl)-phenyl-[4-(5-pyren-1-yl-[1,3,4]oxadiazol-2-yl)-phenyl]-amine (11). 1H NMR (acetone-d6): δ 1.46 (t, J ) 7.3 Hz, 3 H, CH3), 3. 98 (s, 2 H), 4.37 (q, 2 H), 7.06-7.11 (m, 3 H), 7.16-7.22 (m, 2 H), 7.28-7.35 (m, 4 H), 7.37-7.49 (m, 5 H), 7.57 (d, J ) 7.4 Hz, 1 H), 7.81-7.99 (m, 6 H), 8.11 (dd, J ) 8.8, 1.2 Hz, 1 H), 8.27 (d, J ) 1.2 Hz, 1 H). FAB MS (m/e): 631 (M+). Anal. Calcd for C44H30N4O: C, 83.79;

Electroluminescent Dipolar Carbazole Derivatives

Chem. Mater., Vol. 14, No. 9, 2002 3855

Table 1. Physical Data for the Compounds compound parameter

5

6

7

8

9

10

11

15

16

367 364 467 (22) 523 (7) 490

364 364 461 (37) 521 (15) 496

376, 301 374, 303 469 (47) 526 (15) 494

373, 319 374, 306 464 (44) 526 (17) 512

371 372 475 (34) 542 (7) 502

380, 319 381, 317 498 (20) 548 (10) 536

374, 305 372, 302 470 (44) 527 (14) 500

409, 315 408, 314 536 (13) 548

564

Tg/Td, °C

106/415

164/417

104/415

157/405

132/420

149/474

124/415

176/460

170/460

Eox (∆Ep)b, mV

+322 (88), +851 (81) 5.12 2.17 2.95

+279 (69), +579 (67) 5.08 2.12 2.96

+321 (72), +851 (66) 5.12 2.17 2.95

+283 (66), +574 (69) 5.08 2.16 2.92

+200 (70), +555 (65) 5.00 2.07 2.93

+188 (70), +519 (68) 4.99 2.21 2.78

+322 (75), +851 (68) 5.12 2.20 2.92

+158 (71), +490 (66)c 4.96 2.69 2.27

+204 (74), +532 (60)c 5.00 2.65 2.35

λmax, nm λem, nm (Φf, %)a

HOMO,d eV LUMO,d eV band gap,d eV

toluene CH2Cl2 toluene CH2Cl2 film

408, 349 406, 343 531 (5)

a Quantum yield was measured relative to coumarin I (99% in ethyl acetate) or coumarin-6 (90%). Corrections due to the change in solvent refractive indices were applied. b Measured in CH2Cl2. All the potentials are reported relative to ferrocene, which was used as the internal standard in each experiment. Ferrocene oxidation potential was located at +35 mV relative to the Ag/AgNO3 nonaqueous reference electrode. The concentration of the compound was 2.5 × 10-4 M and the scan rate was 100 mV/s. c In addition quasi-reversible reductions at potentials -2111 (112) and -2154 (91) mV were also observed for 8 and 9, respectively. d HOMO energy was calculated with reference to ferrocene (4.8 eV). Solvent to vacuum correction was not applied. Band gap was derived from the observed optical edge and LUMO energy was derived from the relation, band gap ) HOMO - LUMO except for 8 and 9 where it was deduced from the reduction potentials.

H, 4.79; N, 8.88. Found: C, 83.61; H, 4.81; N, 8.83. N-{4-[3-(9,9-Diethyl-9H-fluoren-2-yl)-quinoxalin-2-yl]phenyl}-9-ethyl-N,N′-diphenyl-N′-pyren-1-yl-9H-carbazole3,6-diamine (15). 1H NMR (acetone-d6): δ 0.13 (t, J ) 7.3 Hz, 6 H, CH3), 1.40 (t, J ) 7.3 Hz, 3 H, CH3), 1.79-1.88 (m, 4 H, CH2), 4.44 (q, J ) 7.3 Hz, 2 H, CH2), 6.81-6.93 (m, 6 H), 7.01-7.05 (m, 2 H), 7.11-7.44 (m, 12 H), 7.54 (d, J ) 7.8 Hz, 2 H), 7.78-7.88 (m, 7 H), 7.96-8.08 (m, 5 H), 8.10 (s, 2 H), 8.15 (d, J ) 7.8 Hz, 1 H), 8.21-8.29 (m, 3 H). FAB MS (m/e): 1002 (M+). Anal. Calcd for C73H55N5: C, 87.48; H, 5.53; N, 6.99. Found: C, 87.33; H, 5.25; N, 6.78. N,N′-Bis-{4-[3-(9,9-diethyl-9H-fluoren-2-yl)-quinoxalin2-yl]-phenyl}-9-ethyl-N,N′-diphenyl-9H-carbazole-3,6-diamine (16). 1H NMR (CDCl3): δ 0.19 (t, J ) 7.3 Hz, 12 H, CH3), 1.44 (t, J ) 7.3 Hz, 3 H, CH3), 1.75-1.96 (m, 8 H, CH2), 4.29 (q, J ) 7.3 Hz, 2 H, CH2), 6.89-6.93 (m, 6 H), 7.06 (d, J ) 7.8 Hz, 4 H), 7.15 (d, J ) 7.8 Hz, 4 H), 7.21-7.30 (m, 11 H), 7.39 (d, J ) 8.7 Hz, 4 H), 7.68-7.82 (m, 13 H), 8.10-8.14 (m, 4 H). FAB MS (m/e): 1226 (M+). Anal. Calcd for C88H71N7: C, 86.17; H, 5.83; N, 7.99. Found: C, 86.50; H, 5.60; N, 7.78. OLED Fabrication and Measurements. Electron-transporting materials TPBI (1,3,5-tris(N-phenylbezimidazol-2-yl)benzene)19 and Alq3 (tris(8-hydroxyquinoline) aluminum)20 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.21 Double-layer EL devices using carbazole derivatives as the hole-transport layer and TPBI or Alq3 as the electron-transport layer were fabricated. For comparison, a typical device using NPD (1,4-bis(1-naphthylphenylamino) biphenyl) as the hole-transporting layer was also prepared. All devices were prepared by vacuum deposition of 400 Å of the hole-transporting layer, followed by 400 Å of TPBI or Alq3. An alloy of magnesium and silver (ca. 10:1, 500 Å) was deposited as the cathode, which was capped with 1000 Å of silver. The I-V curve was measured on a Keithley 2400 Source Meter in an ambient environment. Light intensity was measured with a Newport 1835 Optical Meter.

Results and Discussion Syntheses. The syntheses of the oxadiazole-incorporated triarylamines were accomplished by the pal(19) Shi, J.; Tang, C. W.; Chen, C. H. U.S. Patent, 5,645,948, 1997. Sonsale, A. Y.; Gopinathan, S.; Gopinathan, C. Indian J. Chem. 1976, 14, 408. (20) Chen, C. H.; Shi, J.; Tang, C. W. Coord. Chem. Rev. 1998, 171, 161. (21) Wu, I.-Y.; Lin, J. T.; Tao, Y.-T.; Balasubramaniam, E.; Su, Y.Z.; Ko, C.-W. Chem. Mater. 2001, 13, 2626.

ladium-catalyzed C-N coupling strategy developed by Hartwig and co-workers.22 These reactions took place without any base-induced oxadiazole cleavage reactions and produced the final products in more than 70% yield. It is interesting to note that oxadiazoles can be readily attacked by stronger bases to yield ring-opened side products. The mild condition in addition to the use of palladium catalyst may be beneficial to the success of these reactions. To the best of our knowledge, amination (Ullmann or Hartwig) reactions have not been demonstrated for oxadiazole-containing substrates. Photophysical Characteristics. The absorption and emission spectra of the compounds were investigated in dichloromethane and toluene solutions (Table 1) to ascertain the polarity of the ground and excited states. The absorption maxima in the oxadiazole derivatives undergo bathochromic shift on extending the conjugation with fluorene or pyrene. However, no alteration in absorption maxima is observed between the monosubstituted (5 and 7) and disubstituted (6 and 8) derivatives. Nonetheless, the molar absorptivity increases, which may reflect the increase in the chromophoric population (Figure 1). This suggests that the longer wavelength absorption in these materials is mainly associated with the peripheral substituents and the central carbazole is ineffectively conjugated with the side chains.23 There is no notable solvent effect on this band. Shorter wavelength absorptions attributable to the carbazole core are also realized in most derivatives. Quinoxaline derivatives (15 and 16) display the most red-shifted absorption maxima (Figure 1), probably arising from the charge-transfer transition. A bright blue emission is observed for the oxadiazole derivatives in toluene except 11, which exhibits a redshifted emission close to a greenish blue color. A pronounced bathochromic shift is noticed in CH2Cl2 for all the compounds (Figure 2). This is contrary to the behavior observed in the absorption spectra of the compounds. The quantum yield also drops substantially (22) Hartwig, J. F. Acc. Chem. Res. 1998, 31, 852-860. Hartwig, J. F.; Kawatsura, M.; Hauck, S. I.; Shaughnessy, K. H.; Alcazar-Roman, L. M. J. Org. Chem. 1999, 64, 5575-5580. (23) Morin, J.-F.; Leclerc, M. Macromolecules 2001, 34, 4680.

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Figure 1. Absorption spectra of the compounds 5, 6, and 16 recorded in dichloromethane solution.

Figure 2. Emission spectra of the compounds 6 and 8 in (a) toluene, (b) film, and (c) dichloromethane solutions.

when measured in CH2Cl2. These solvent-dependent emission characteristics more likely stem from the dipolar interactions with the polar solvents.24 The Stokes shift for the compounds are generally high and the quinoxaline derivatives display larger Stokes shift. This observation may be attributed to the following reasons: a more planar conformation of the excited state, π-stacking of the molecules, and dipolar interactions cooperate with the electronic effect of the quinoxaline group.25 It is also corroborative of the fact that the emission in quinoxaline derivatives is completely quenched in CH2Cl2. However, in the film state the emission for all the derivatives are located between that observed for toluene and CH2Cl2 solutions (Figure 2). Electrochemistry. The redox propensity of these materials was determined by cyclic voltammetry studies. All the dipolar derivatives undergo two reversible oxidations at potentials higher than that observed for ferrocene (Figure 3). In the monosubstituted derivatives (24) Brinkmann, M.; Gadret, G.; Muccini, M.; Taliani, C.; Masciocchi, N.; Sironi, A. J. Am. Chem. Soc. 2000, 122, 5147. Baldo, M. A.; Soos, Z. G.; Forrest, S. R. Chem. Phys. Lett. 2001, 347, 297. (25) Gill, R. E.; Hilberer, A.; vanHutten, P. F.; Berentschot, G.; Werts, M. P. L.; Meetsma, A.; Wittmann, J. C.; Hadziioannou, G. Synth. Met. 1997, 84, 637. Diez-Barra, E.; Garcia-Martinez, J. C.; Merino, S.; del Rey, R.; Rodriguez-Lopez, J.; Sanchez-Verdu, P.; Tejeda, J. J. Org. Chem. 2001, 66, 5664.

Justin Thomas et al.

Figure 3. Cyclic voltammograms of the compounds 7 and 8 (Assignments: A, peripheral amine; Fc, ferrocene; C, carbazole core).

they originate from the peripheral diarylamine and central carbazole units, respectively. However, the latter oxidation is not observed for the 3,6-disubstituted derivatives. In these derivatives the two diarylamine segments undergo stepwise oxidation. The first oxidation potential for the monosubstituted derivatives is more positive than that of the disubstituted derivatives. This is because the second diarylamine substitution increases the donor strength of the carbazole and the effect is manifested on the first oxidation potential. The absence of central carbazole oxidation in the disubstitued derivatives is due to the destabilization introduced by the formation of a dication after the initial oxidation of the diarylamine segments.15-18 This effect will be less pronounced in the monosubstituted derivatives. In the compounds 9 and 10 the first oxidation preferentially occurs at the naphthyl- or pyrenylcontaining diarylamine segment; consequently, they possess the lowest first oxidation potentials in the oxadiazole series (5-11). Thermal Properties. All the compounds are amorphous in nature. They form stable glass that persists even if the heating is performed above the glass transition temperature (Tg). It is interesting to note here that most oxadiazole derivatives (including Adachi’s) reported to date tend to form a crystalline state if heated above Tg.9-12 The glass transition occurs in all the compounds at moderately high temperatures and it reaches 176 °C in the case of compound 15. The disubstituted carbazole derivatives (6 and 8) display higher Tg’s when compared to their monosubstituted analogues (5 and 7). Similarly, incorporation of pyrene in the structure leads to a significant enhancement in the Tg. For instance, in the series 5, 7, and 11 the role of pyrene is clearly evident. Upon comparison of the phenylene (5 and 6) and fluorene derivatives (7 and 8), observation of lower Tg’s for the fluorene derivatives, despite the rigidity and elongated conjugation, is attributed to the flexible n-butyl moieties.26 The decomposition of these materials starts only above 400 °C in air. In general, it appears that the quinoxaline deriva(26) Kreger, K.; Jandke, M.; Strohriegl, P. Synth. Met. 2001, 119, 163.

Electroluminescent Dipolar Carbazole Derivatives

Chem. Mater., Vol. 14, No. 9, 2002 3857

Figure 4. EL and film PL spectra of 5.

tives possess improved thermal stability when compared to the oxadiazole derivatives. Electroluminescent Devices. Since these molecules possess both oxadiazole/quinoxaline and amino groups, they might be employed in OLED as electron-transporting, hole-transporting, or bifunctional materials. We have tried to emulate all three possibilities in our device structures. Initially, three types of devices (ITO/NPB/ 5/Mg:Ag; ITO/5/TPBI/Mg:Ag, and ITO/5/Mg:Ag) were fabricated with compound 5. The effective organic layer thickness was maintained at 800 Å in all three devices. The devices ITO/NPB/5/Mg:Ag (external quantum efficiency, 0.02; power efficiency, 0.01 lm/W; luminance 29 cd/m2 at 100 mA of current density) and ITO/5/Mg: Ag (external quantum efficiency, 0.46; power efficiency, 0.31 lm/W; luminance 983 cd/m2 at 100 mA of current density) failed to produce reasonable light output. However, the device ITO/5/TPBI/Mg:Ag produced a bright bluish green light (Figure 4) that peaked at 482 nm, very close to that observed for the film PL (490 nm). This observation clearly suggests that these materials transport holes better than they carry electrons. This is contrary to Adachi’s oxadiazole-amino dyads which function as electron transporters.9 This difference may be attributed to the high-lying LUMO in the present molecules. A fourth device of configuration ITO/5/Alq3/ Mg:Ag was also fabricated that led to a typical green OLED with an emission maximum resembling that of Alq3. This can be explained by invoking the interfacial energy barriers (Figure 5) that exist for the transport of holes and electrons into ETL and HTL, respectively. For the TPBI-based device the barrier for the holes (1.08 eV) to cross into TPBI is double the barrier for the electrons (0.53 eV) to enter 5. So in the device ITO/5/ TPBI/Mg:Ag the holes are effectively deterred from entering TPBI. However, for the device ITO/5/Alq3/Mg: Ag there is no significant difference in the barrier height for the holes to enter Alq3 (0.97 eV) and electrons to enter 5 (0.78 eV). Therefore, the holes overcome the barrier and the recombination takes place inside the Alq3 layer. As the emission profile of the oxadiazole derivatives overlap with that of Alq3 in most other cases, it is difficult to determine their individual contributions to the EL. However, the fact that EL spectra display larger fwhm (full-width at half-maximum) for the Alq3 devices, mixing of profiles cannot be excluded. A simi-

Figure 5. Energy alignments in the four devices of 5.

larity in behavior is observed for the other oxadiazole derivatives also.10 The carbazole derivatives (5 and 7) that contain only one oxadiazole moiety exhibit better performance (Table 2) when compared with those containing two oxadiazole groups (6 and 8). It suggests that the interruption of the hole-transport (hopping) path by the hole-blocking oxadiazole is more pronounced in the disubstituted compounds (6 and 8). This could eventually diminish the hole current and decrease the probability of recombination with electrons before they decay. In contrast, for the quinoxaline derivatives 15 and 16, emission occurs from the HTL layer for both TPBI- and Alq3-based devices. The confinement of exciton formation in the HTL layer in these devices is intriguing. It is noted earlier that quinoxaline-based polymers display better hole-blocking properties when compared to the oxadiazole analogues and help to achieve optimized performance.27 In the present cases where they were used as HTLs, the rather lower LUMO may particularly facilitate the transportation of electrons to them so that emission from these was observed irrespective of the ETL used. Recently, Scherf found that photo- or electro-oxidation of mono- and dialkylated fluorene derivatives leads to keto compounds that act as energy transfer and trap sites.28 This eventually red shifts the emission profile at high operating voltages and long runs. The fluorenefunctionalized compounds 7 and 8 do not display any evidence attributable to such degradation over the operating voltages. However, the possibility of ketoinduced quenching in compound 8 cannot be ruled out at this stage, which displays comparatively low brightness at high operating voltages (Table 2). It is interesting to note that several green-emitting devices display exceptional performance in standard (27) Dailey, S.; Feast, W. J.; Peace, R. J.; Sage, A. C.; Till, S.; Wood, E. L. J. Mater. Chem. 2001, 11, 2238. (28) List, E. J. W.; Guentner, R.; de Freitas, P. S.; Scherf, U. Adv. Mater. 2002, 14, 374.

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Table 2. Electroluminescence Characteristics

turn-on voltage, V max. brightness, cd/m2 max. external quantum effic., % max. power effic., lm/W λem (fwhm), nm CIE, x, y

5 6 Alq3/TPBI Alq3/TPBI

7 Alq3/TPBI

8 9 10 11 15 16 Alq3/TPBI Alq3/TPBI Alq3/TPBI Alq3/TPBI Alq3/TPBI Alq3/TPBI

3.0/3.0

3.0/3.0

3.0/3.0

5.0/5.4

3.5/2.8

3.2/2.9

3.5/3.4

2.5/2.6

3.0/3.0

30400/ 40300 1.8/5.2

24230/ 21700 1.5/2.6

34200/ 34050 3.3/4.2

6480/ 2740 0.4/0.3

49800/ 46750 2.7/3.5

29130/ 21250 1.4/1.2

23750/ 16300 1.1/0.9

26800/ 22600 1.2/0.9

24650/ 16150 1.1/0.8

1.7/6.3

1.6/2.4

4.5/4.7

0.4/0.2

2.9/4.7

2.0/2.2

1.1/0.9

2.2/1.4

2.7/1.5

518 (92)/ 482 (74) 0.28, 0.52/ 0.16, 0.29 9.9/8.5

512 (100)/ 492 (84) 0.26, 0.49/ 0.20, 0.39 9.9/10.0

496 (82)/ 492 (78) 0.20, 0.43/ 0.17, 0.40 9.5/9.0

528 (112)/ 518 (130) 0.35, 0.53/ 0.33, 0.48 11.6/11.9

508 (92)/ 496 (78) 0.26, 0.50/ 0.20, 0.42 8.9/7.9

526 (84)/ 524 (88) 0.30, 0.58/ 0.30, 0.57 8.8/8.2

508 (102)/ 484 (94) 0.27, 0.49/ 0.22, 0.36 9.5/9.3

552 (90)/ 552 (94) 0.41, 0.55/ 0.41, 0.55 6.8/7.1

556 (98)/ 560 (98) 0.41, 0.54/ 0.44, 0.53 8.4/7.2

voltage at 100 mA/cm2, V brightness at 100 5270/9630 4380/5940 8550/10430 1340/750 mA/cm2, cd/m2 external quantum effic. 1.7/4.8 1.5/2.4 3.2/4.2 0.4/0.3 at 100 mA/cm2, % power effic. at 100 1.7/3.6 1.4/1.9 2.8/3.7 0.4/0.2 mA/cm2, lm/W

Figure 6. I-V-L curves of the compounds 5, 7, and 9 for the TPBI and Alq3 devices.

operating conditions (see Table 2, Figures 6 and 7). Particularly the green-emitting TPBI-based devices incorporating 5, 7, and 9 exhibit promising efficiencies. Yellow-emitting Alq3-based devices derived from 15 and 16 also display brighter light output. They are comparable to the compounds recently reported by Lee29 and others.30,31 For the compounds 5-9 the TPBI devices (29) Lin, X. Q.; Chen, B. J.; Zhang, X. H.; Lee, C. S.; Kwong, H. L.; Lee, S. T. Chem. Mater. 2001, 13, 456.

7820/8960 4800/3865 3140/2185 4130/2980 3580/2420 2.6/3.5

1.4/1.2

1.1/0.9

1.2/0.9

1.1/0.8

2.8/4.6

1.7/1.5

1.0/0.9

1.9/1.3

1.4/1.1

Figure 7. I-V-L characteristics of the compounds 15 and 16 for the TPBI and Alq3 devices.

display better performance (brightness and efficiency) when compared with the Alq3 devices. However, for the compounds 10, 11, 15, and 16 a reversal is noticed where the Alq3 devices show comparatively better characteristics. A notable difference between these two groups is the position of LUMO. The LUMO for the first (30) Hamada, Y.; Matsusue, N.; Kanno, H.; Fujii, H.; Tsujioka, T.; Takahashi, H. Jpn. J. Appl. Phys. Part 2 2001, 40, L753. (31) Matsumura, M.; Furukawa, T. Jpn. J. Appl. Phys. Part 1 2001, 40, 3211.

Electroluminescent Dipolar Carbazole Derivatives

set (5-9) resides above 2.2 eV while the latter compounds (10, 11, 15, and 16) possess LUMO lower than 2.2 eV. Thus, when the LUMO is lowered, the electron injection from Alq3 improves, which in turn enhances the device performance. Obviously, there exists a critical barrier for the electron injection from ETL to HTL that is instrumental in deciding the recombination zone and efficiency of the devices. Thompson and co-workers recently observed the formation of excited-state complexes (exciplexes and electroplexes) in single-layer LEDs that employ either PVK:PBD blend or a statistical copolymer containing carbazole and oxadiazole pendant groups.32 Blends lead to the formation of exciplexes while the copolymers display electroplexes. It is argued that the discrepancy is mainly due to the topological constraints in the position of carbazole and oxadiazole units in the polymer. The nonobservation of excited-state complexes in the present compounds can also be explained in a similar fashion. The incorporation of an oxadiazole group as a part of an amino moiety gives rise to steric (32) Jiang, X.; Register, R. A.; Killeen, K. A.; Thompson, M. E.; Pschenitzka, F.; Hebner, T. R.; Sturm, J. C. J. Appl. Phys. 2002, 91, 6717.

Chem. Mater., Vol. 14, No. 9, 2002 3859

inhibition that prohibits the close intermolecular contact between the carbazole and oxadiazole units. In conclusion, a series of bipolar materials were prepared by joining triarylamines and electronegative quinoxaline/oxadiazole around a carbazole unit. Much improved morphology stability was observed. The materials served as better HTL rather than ETL or bifunctional materials in the EL device fabricated. Obviously, there is an imbalance of electron-transporting and hole-transporting in these materials. Efficient green and yellow devices were obtained in a two-layer configuration in conjunction with ETL such as Alq3 and TPBI. Further work in designing materials with balanced electron-transporting and hole-transporting properties are vigorously pursued. Acknowledgment. This work was supported by Academia Sinica and National Science Council. Supporting Information Available: Data are presented for some key intermediates (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM0202512