Chem. Mater. 2004, 16, 1285-1291
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High-Efficiency Organic Electrophosphorescent Diodes Using 1,3,5-Triazine Electron Transport Materials Hiroko Inomata,† Kenichi Goushi,†,§ Takuma Masuko,† Tohru Konno,† Toshiro Imai,† Hiroyuki Sasabe,†,§ Julie J. Brown,‡ and Chihaya Adachi*,†,§ Department of Photonics Materials Science, Chitose Institute of Science and Technology, 758-65 Bibi, Chitose, Hokkaido 066-8655, Japan, Universal Display Co., 375 Phillips Boulevard, Ewing, New Jersey 08618, and CREST Program, Japan Science and Technology Co. (JST) Received July 25, 2003. Revised Manuscript Received January 13, 2004
This paper reports on novel electron transport materials, 1,3,5-triazine derivatives (TRZ), which have been useful in high-efficiency electrophosphorescent (EP) organic light-emitting diodes (OLEDs). We synthesized four 2,4,6-tris(diarylamino)-1,3,5-triazine derivatives (TRZ1-TRZ4) that had electron-donating substituents and examined their OLED characteristics. Of these, we found that TRZ 2, 3, and 4 function as a host for tris(2-phenylpyridine)iridium (Ir(ppy)3) and, in particular, 2,4,6-tris(carbazolo)-1,3,5-triazine (TRZ2) demonstrated a very high external electroluminescent (EL) quantum efficiency (ηext) of ∼10.2 ( 1.0% and an energy conversion efficiency (ηenergy) of 14.0 ( 2.0 lm/W. Detailed transient photoluminescent (PL) measurement revealed that the triplet energy level of TRZ2 (ET1 ) 2.81 eV) is higher than that of conventional 4,4′-N,N′-dicarbazol-biphenyl (CBP) (ET1 ) 2.56 eV), suggesting that TRZ2 has excellent capabilities in confining Ir(ppy)3 triplet excitons.
Introduction Recently, various functional devices using organic materials have been developed in a wide variety of fields because of their unique electrical and optical properties.1 After organic light-emitting diodes (OLEDs) based on anthracene single crystals were first reported in 1960, OLEDs that aimed at high conversion efficiency from electricity to light were developed.2,3 Other extensive studies on device structures and material developments have been reported,4,5 particularly after Tang reported on his multilayer OLED structure in 1987.6 The very high efficiency of OLEDs has been recently demonstrated by adopting organic phosphors, especially in the course of developing emitter materials, where heavymetal effects have led to 100% intersystem crossing from singlets to triplets and very strong phosphorescence even at room temperature. RGB electrophosphorescence has been successfully achieved using specific iridium and platinum based metal complexes. The red phosphor, tris(2-(2′benzo[4,5-a]thienyl)pyridinato-N,C3′)iridium (acethylace-tonate) (Btp2Ir(acac)), has produced a maximum electroluminescent (EL) external quantum * To whom correspondence should be addressed. E-mail: c-adachi@ photon.chitose.ac.jp. † Chitose Institute of Science and Technology. ‡ Universal Display Co. § CREST Program, Japan Science and Technology Co. (1) Pope, M.; Swenberg, C. Electronic Processes in Organic Crystals and Polymers; Oxford Science Publications: New York, 1999. (2) Helfrich, W.; Schneider, W. G. Phys. Rev. Lett. 1965, 14, 229. (3) Helfrich, W.; Schneider, W. G. J. Chem. Phys. 1966, 44, 2902. (4) Adachi, C.; Tokito, S.; Tsutsui, T.; Saito, S. Jpn. J. Appl. Phys. 1988, 27, L269. (5) Adachi, C.; Tokito, S.; Tsutsui, T.; Saito, S. Jpn. J. Appl. Phys. 1988, 27, L713. (6) Tang, C. W.; Van Slyke, S. A. Appl. Phys. Lett. 1986, 51, 913.
efficiency (ηext) of ∼7%.7 Also, a ηext of 8∼19% has successfully been demonstrated8-10 using the green phosphor, tris(2-phenylpyridine)iridium (Ir(ppy)3). Further, a high-efficiency ηext of ∼7.5% using the blue phosphor, iridium(III)bis(4,6-difluorophenyl)-pyridinatoN,C2)picolinate (FIrpic), has been reported.11,12 Host materials need to be selected that have a higher triplet energy level than that of the guest molecules, which is the key to causing efficient radiative decay of guest triplet excitons.13 Until now, although 4,4′-N,N′-dicarbazole biphenyl (CBP), 2,9-dimethyl-4,7-diphenyl, -9,10phenanthroline (BCP), triazole (TAZ), and oxadiazole (OXD) derivatives have been revealed to be excellent hosts,10 the number of host materials available for a high-efficiency electrophosphorescent emitter layer (EML) is still limited. In this study, we synthesized new 1,3,5-triazine (TRZ) derivatives and evaluated their electrophosphorescent properties. Thus far, there have been some reports on OLEDs using TRZ derivatives as an electron transport layer (ETL).14-18 However, most compounds form exci(7) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Lamansky, S.; Thompson, M. E.; Kwong, R. C. Appl. Phys. Lett. 2001, 78, 1622. (8) Baldo, M. A.; Lamannsky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 1999, 75, 4. (9) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. J. Appl. Phys. 2000, 90, 5048. (10) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E. Appl. Phys. Lett. 2001, 77, 904. (11) Adachi, C.; Kwong, R. C.; Djurovich, P.; Adamovich, V.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 2001, 79, 2082. (12) Holmes, R. J.; Forrest, S. R.; Tung, Y. J.; Kwong, R. C.; Browm, J. J.; Garon, S.; M. E. Appl. Phys. Lett. 2003, 82, 2422. (13) Adachi, C.; Thompson, M. E.; Forrest, R. IEEE J. Sel. Top. Quantum Electron. 2002, 8, 372. (14) Bacher, A.; Fink, R.; Posch, P.; Schmit, H. W.; Thelakkat, M. Inorganic and Organic Electroluminescence/EL 96 Berlin; 1996; Abstract, p 109.
10.1021/cm034689t CCC: $27.50 © 2004 American Chemical Society Published on Web 03/03/2004
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Figure 1. Chemical structures of TRZ1-TRZ4 used in this study.
plexes or charge transfer (CT) complexes between TRZ and the adjacent organic layers, demonstrating that the triazine core has strong electron-accepting characteristics. Also, it seems probable that the triazine ring is so strong in terms of electron affinity that it prevents fast movement of electrons through the layers. In our observations, fast charge carrier mobilities have been provided by rather neutral chemical structures. In general, polarized molecular structures have resulted in slow carrier mobilities. On the basis of the previous work, we expect that modifying the triazine’s electron affinity by introducing electron-donating substitutes will provide a good chance of finding compounds with the well-tuned electronic properties required for electron transport and host layers that are required for electrophosphorescence. Only a limited number of compounds have been reported for tris(diarylamino)-substituted sym-triazines. The parent 2,4,6-tris(diphenylamino)-1,3,5-triazine was discovered more than a century ago,19,20 and since then several of its chemical behaviors19-21 and thermochemical properties21,22 have been studied. After we started our present study, we learned that this class of com(15) Fink, R.; Frenz, C.; Thelakkat, M.; Schmidt, H.-W. Macromolecules 1997, 30, 8177. (16) Fink, R.; Heischkel, Y.; Thelakkat, M.; Schmidt, H.-W. J. Mater. Chem. 1998, 10, 3620. (17) Lupton, J. M.; Hemingway, L. R.; Samuel, W. I. D.; Burn, P. L. J. Mater. Chem. 2000, 10, 876. (18) Pang, J.; Tao, Y.; Freiberg, S.; Yang, X. P.; D’Iorio, M.; Wang, S. J. Mater. Chem. 2002, 12, 206. (19) Weith, Ber. Dtsch. Chem. Ges. 1874, 7, 843. (20) Hofmann, A. W. Ber. Dtsch. Chem. Ges. 1885, 18, 3217. (21) Levedev, B. V.; Bykova, T. A.; Kiparisova, E. G.; Pankratov, V. A.; Korshak, V. V.; Laktionov, V. M. J. Gen. Chem., USSR (Engl. Transl.) 1984, 54, 372.
pounds has attracted renewed interest because of its application to OLED’s18,23 and because of the magnetic properties of its radical cations.24 Here, we describe three new compounds of this class as well as the parent tris(diphenylamino) derivative.19,20 Nucleophilic substitution of cyanuric chloride with three equivalents of diarylamine seems to be the most straightforward way of preparing these compounds. Some of the previous reports,20,21 however, suggested that the conventional method using diarylamines might result in incomplete substitution, taking into account the relatively low nucleophilicity and relatively high steric requirements of diarylamines. We therefore adopted here a more reliable method of utilizing lithium diarylamide (Figure 1), although this may not always be required.18 In the present study, 1H NMR spectroscopy played only a limited role in determining the structure of these products, because they did not have any protons in the core triazine. Here, mass spectroscopy played a decisive role in confirming whether 3-fold substitution had been achieved or not. In fact, we experienced somewhat surprising results in reactions with seven-membered cyclic amines such as iminostilbene and iminodibenzyl as they yielded only 2-fold substitution products, i.e., corresponding to 2-chloro-4,6-bis(diarylamino)-1,3,5-triazines, even when three equivalents of lithium diarylamide were used with hours of reflux. It seems likely (22) Kiparisova, E. G.; Levedev, B. V. Russ. J. Phys. Chem (Engl. Transl.) 1999, 73, 602. (23) It has been pointed out that for a few tris(diarylmino)triazine derivatives, an amorphous glass state can be achieved, although no details have been given. Shirota, Y. J. Mater. Chem. 2000, 10, 1. (24) Selby, T. D.; Stickley, K. R.; Blackstock, S. C. Org. Lett. 2000, 2, 171.
High-Efficiency Organic Electrophosphorescent Diodes
that, with these diarylamino of higher steric requirement particularly in the front side, substitution by the third group may become difficult. In this study, we demonstrate novel TRZ derivatives that function both as a host layer and an ETL. On the basis of the OLED characteristics attained with the three device structures, we discuss the carrier transport properties of the TRZ derivatives. Triplet energy levels of TRZ derivatives and the effect of confining the guest triplet are also discussed on the basis of the phosphorescent spectra of organic layers. Experimental Section 1. Synthesis of Organic Materials. Melting points (uncorrected) were measured in a glass capillary by Laboratory Devices MEL-TEMP II equipped with a metal heating block. Melting points were also obtained using a differential scanning calorimetric (DSC) measurement, as will be discussed later. The IR spectra were measured as neat thin films between NaCl plates for the liquid samples or as KBr disks for the solid samples with the JASCO IR Report-100 or the Shimadzu FTIR-8700 spectrophotometer. The 400 MHz 1H NMR spectra were measured in a CDCl3 solution with the JEOL ECP400. The UV/VIS absorption spectra and the emission spectra were recorded as chloroform solution with the Shimadzu UV2400PC UV-Vis recording spectrophotometer and RF-5300PC spectrofluorophotometer, respectively. The morphological properties (Tg, glass transition temperature; Tc, crystallization temperature; and Tm, melting point) of these compounds were evaluated by DSC analysis with a Shimadzu DSC-60 instrument. Preparation and Properties of 2,4,6-Tris(diphenylamino)1,3,5-triazine (with the Standard Preparative Procedure) (TRZ1). Diphenylamine (5.58 g, 33 mmol) was placed in a 100-mL sidearmed round-bottomed flask equipped with a stirring bar, a septum inlet, and a three-way stopcock. The flask was replaced with nitrogen by applying cycles of evacuation and nitrogen gas release, and the nitrogen atmosphere was maintained throughout the reaction. Dry ether (33 mL) was introduced through the septum with a syringe to make a solution of the amine. Then a 1.6 M n-butyllithium/hexane solution (20.6 mL, 33 mmol) was added, and the mixture was cooled in an ice bath and stirred. After this had been added, the cold bath was removed, and the mixture was stirred for a further 15 min. A slurry of cyanuric trichloride (1.84 g, 10 mmol) in dry ether (33 mL), again under a nitrogen atmosphere, was prepared in another 100-mL sidearmed roundbottomed flask equipped with a stirring bar, a septum inlet, and a Liebig condenser, which was fitted with a three-way stopcock. We added the prepared lithium diphenylamide solution, in ca. 15 min via a double-ended needle, to this while stirring. Mild exothermicity was noted during addition, and a large amount of solid product was precipitated. The mixture was then heated under reflux for 4 h while stirring. The mixture was poured into water (100 mL), and the product remaining as a solid was filtered and washed with water and ether (50 mL). The solid was first dried in air and then in a vacuum desiccator over calcium chloride. The thus-obtained crude product (5.5 g) was purified by extractive filtration with hot chlorobenzene through a short silica gel bed by utilizing a modified Soxhlet extractor followed by repeated recrystallizations from the same solvent. The resulting colorless crystalline solid was finally dried under a vacuum at ca. 200 °C to remove traces of chlorobenzene, 3.8 g (65%), mp 294-296 °C (uncorrected, cf. lit. mp 292 [1a], 300 [1b], and >300 °C [2]). IR (KBr disk) 3015, 2950, 1685(s), 1595, 1490, 1450, 1420, 1365, 1265, 1160, 1080, 1040, 1000, 990, 940, 785, 755, 730, 705, 690, and 620 cm-1. 1H NMR (400 MHz JEOL ECP400, CDCl3) δ 6.987.07 (m, 6H) and 7.08-7.14 (narrow m, 24H) ppm. HRMS (FD) m/z 582.2511 (cf. 582.2535 calculated for C39H30N6). Absorption λmax 280 nm ( 52 000) and emission λmax 450 nm with the
Chem. Mater., Vol. 16, No. 7, 2004 1287 excitation at 280 nm. DSC Analysis. (1) When the solid was heated from 100 to 400 °C at a rate of 10 °C/min, we observed a small endothermic peak at 280 °C prior to Tm at 300.5 °C, suggesting the existence of polymorphism. (2) When the sample was cooled at the same rate to 100 °C and the above heat treatment was repeated, a small exothermic peak could be observed at 197 °C followed by a Tm at 302.5 °C, which consisted of a sharper peak than that obtained in the first scan. The results suggest that, while a small part of the sample may have remained in an amorphous state, most of it might have existed as crystal melting at 302.5 °C. 2,4,6-Tricarbazolo-1,3,5-triazine (TRZ2). We obtained 5.6 g of solid as the crude product from carbazole (5.52 g, 33 mmol) and cyanuric chloride (1.84 g, 10 mmol), using the standard procedure. Pure material was obtained by extractive filtering with hot chlorobenzene through a short silica gel bed, recrystallizing repeatedly from the same solvent, and drying under a vacuum at 100 °C for 1 h to remove chlorobenzene, which was 3.8 g (65%) of faint yellow microcrystalline solid. IR 3060, 3030, 1600, 1595, 1545 (s), 1490, 1450 (s), 1440 (s), 1415 (s), 1335 (s), 1315, 1305, 1265, 1230, 1205, 1165, 1155, 1032,921, 800, 750, 740 (s), 730, 715 (s), 712, 660, 625 cm-1. 1H NMR (CDCl3, 400 MHz JEOL ECP400) δ 7.42-7.52 (m with further fine splitting, 12 H), 8.13 (d, J ) 6.4 Hz, with further fine splitting, 6 H), 9.02 (d, J ) 6.4 Hz, with further fine splitting, 6 H) ppm. HRMS (FD) m/z 576.2058 (cf. 576.2065 calculated for C39H24N6). Absorption λmax 281 nm ( 27 000) and 329 nm ( 38 000) and emission λmax 429 nm with the excitation at 329 nm. DSC analysis. (1) When the above solid was heated from 200 to 500 °C at a rate of 10 °C/min, Tm was observed at 452 °C as a sharp endothermic peak. (2) When the sample was cooled at the same rate to 200 °C and the above heat treatment was repeated, a small endothermic peak could be observed at 430 °C prior to Tm at 448 °C which consisted of a broader and much smaller peak than that of the first scan. The results suggest that the cooled sample existed in an amorphous glass state, and the preceding endothermic peak may be attributed to Tg; above that partial crystallization might have been promoted. 2,4,6-Tris(N-phenyl-2-naphthylamino)-1,3,5-triazine (TRZ3). We obtained 6.8 g of solid as the crude product from N-phenyl2-naphthylamine 7.24 g (33 mmol) and cyanuric chloride 1.84 g (10 mmol), using the standard procedure. Pure material was obtained by passing through a short silica gel column with chloroform as the eluant, extractive filtering with hot toluene through a short silica gel bed, recrystallizing repeatedly from toluene, and drying under a vacuum at 160 °C for 1.5 h, as 4.4 g (61%) of light yellow microcrystalline solid, mp 271-272 °C. IR 3055, 1595, 1535(s), 1495, 1455, 1385(s), 1350, 1288, 1130, 1025, 965, 855, 805, 745, 775, 730, 690, 670 cm-1. 1H NMR (CDCl3, 400 MHz JEOL ECP400) δ 6.86-6.94 (m, 3H), 6.94-7.05 (m, 6H), 7.15 (d, J ) 7.6 Hz, 6H), 7.2-7.29 (most likely d with the left line overlapping with CHCl3 resonance line, 2H estimated from the light line), 7.3-7.42 (m, 9H), 7.477.55 (m, 6H), 7.62 (d, J ) 7.7 Hz, 3H) ppm. HRMS (FD) m/z 732.2984 (cf. 732.2994 calculated for C51H36N6). Absorption λmax 265 nm ( 71 000) and emission λmax 448 nm with the excitation at 265 nm. DSC analysis. (1) When the above solid was heated from 100 to 400 °C at a rate of 10 °C/min, Tm was observed at 268.5 °C as a sharp endothermic peak. (2) When the sample was cooled at the same rate to 100 °C and the above heating program was repeated, two endothermic peaks were observed at 121.5 and 190.5 °C prior to Tm at 267.5 °C which maintained nearly the same sharpness as that of the first scan. (3) Rapid cooling of the sample resulted in essentially the same DSC curve as that for (2);. The results suggest that this compound exhibited polymorphism and existed mostly in some crystalline state. 2,4,6-Tris(N-phenyl-1-naphthylamino)-1,3,5-triazine (TRZ4). We obtained 6.4 g of solid as the crude product from N-phenyl1-naphthylamine (7.67 g, 35 mmol) and cyanuric chloride (1.84 g, 10 mmol) using the standard procedure. Pure material was obtained by passing the crude through a short silica gel column (4:1 mixture of dichloromethane and hexane), recrystallizing
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it successively from a 2:1 toluene/hexane mixture and then from 1,2-dimethoxyethane, and drying it under a vacuum at 100 °C for 2 h, as 4.5 g (62%) of colorless microcrystalline solid, mp 226-233 °C. IR 3060, 1600, 1590, 1540 (s), 1510, 1495, 1440, 1385 (s), 1295, 1280, 810, 805, 780, 725, 695, 640 cm-1. 1H NMR(CDCl3, 400 MHz JEOL ECP400) δ 6.6-7.9 (m, 36H) ppm. HRMS (FD) m/z 732.3007 (cf. 732.2994 calculated for C51H36N6). Absorption λmax 279 nm ( 89 000) and emission λmax 382 nm with the excitation at 279 nm. DSC analysis. (1) When the solid was heated from 40 to 400 °C at a rate of 10 °C/min, Tm was observed at 222.5 °C as a sharp endothermic peak. (2) When the sample was cooled at the same rate to 40 °C and the above heat treatment was repeated, Tg was observed at 113 °C as a small endothermic peak, after which no appreciable peak was observed. (3) Rapid cooling of the above sample resulted in essentially the same DSC curve as that for (2). The results indicate that this compound, after melting, exists in a very stable amorphous glass state. 2. Optical and Thermal Properties of TRZ Derivatives. The optical characteristics of TRZ deposited films were estimated through photoluminescence and absorption spectra. We also measured the transient fluorescence and phosphorescence spectra using a streak camera (Hamamatsu C4334) with a nitrogen gas laser (λ ) 337 nm) as an excitation source to determine singlet and triplet energy levels. The UV/VIS absorption spectra and emission spectra of these compounds were recorded with a Shimadzu UV-2400PC UV-Vis spectrophotometer and RF-5300PC spectrofluorophotometer, respectively. The TRZ films were deposited by high vacuum (
