Hydroxynaphthyridine-Derived Group III Metal Chelates

Hydroxynaphthyridine-Derived Group III Metal Chelates -...

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Hydroxynaphthyridine-Derived Group III Metal Chelates: Wide Band Gap and Deep Blue Analogues of Green Alq 3 (Tris(8-hydroxyquinolate)aluminum) and Their Versatile Applications for Organic Light-Emitting Diodes Szu-Hung Liao, †,‡ Jin-Ruei Shiu, †,§ Shun-Wei Liu, †, | Shi-Jay Yeh, Yu-Hung Chen, | Chin-Ti Chen,* ,†, Tahsin J. Chow,* ,† and Chih-I Wu* , | Institute of Chemistry, Academia Sinica, Taipei, Taiwan 11529, R.O.C., Department of Chemistry, National Taiwan Normal Uni V ersity, Taipei, Taiwan 11677, R.O.C., Department of Chemistry and Biochemistry, National Chung Cheng Uni V ersity, Chia-Yi, Taiwan 62102, R.O.C., Graduate Institute of Photonics and Optoelectronics and Department of Electrical Engineering, National Taiwan Uni V ersity, Taipei, Taiwan 10617, R.O.C., and Department of Applied Chemistry, National Chiao Tung Uni V ersity, Hsinchu, Taiwan 30050, R.O.C. Received September 20, 2008; E-mail: cchen@chem.sinica.edu.tw; tjchow@chem.sinica.edu.tw Abstract: A series of group III metal chelates have been synthesized and characterized for the versatile application of organic light-emitting diodes (OLEDs). These metal chelates are based on 4-hydroxy-1,5- naphthyridine derivates as chelating ligands, and they are the blue version analogues of well-known green Fuorophore Alq 3 (tris(8-hydroxyquinolinato)aluminum). These chelating ligands and their metal chelates were easily prepared with an improved synthetic method, and they were facially puri±ed by a sublimation process, which enables the materials to be readily available in bulk quantity and facilitates their usage in OLEDs. Unlike most currently known blue analogues of Alq 3 or other deep blue materials, metal chelates of 4-hydroxy-1,5-naphthyridine exhibit very deep blue Fuorescence, wide band gap energy, high charge carrier mobility, and superior thermal stability. Using a vacuum-thermal-deposition process in the fabrication of OLEDs, we have successfully demonstrated that the application of these unusal hydroxynaphthyridine metal chelates can be very versatile and effective. ²irst, we have solved or alleviated the problem of exciplex formation that took place between the hole-transporting layer and hydroxynaphthyridine metal chelates, of which OLED application has been prohibited to date. Second, these deep blue materials can play various roles in OLED application. They can be a highly ef±cient nondopant deep blue emitter: maximum external quantum ef±ciency η ext of 4.2%; Commision Internationale de L’Eclairage x , y coordinates, CIE x,y ) 0.15, 0.07. Compared with Alq 3 , Bebq 2 (beryllium bis(benzoquinolin-10-olate)), or TPBI (2,2 ,2 ′′ -(1,3,5-phenyle- ne)tris(1-phenyl-1 H -benzimidazole), they are a good electron-transporting material: low HOMO energy level of 6.4 - 6.5 eV and not so high LUMO energy level of 3.0 - 3.3 eV. They can be ambipolar and possess a high electron mobility of 10 - 4 cm 2 /V s at an electric ±eld of 6.4 × 10 5 V/cm. They are a quali±ed wide band gap host material for ef±cient blue perylene (CIE x , y ) 0.14, 0.17 and maximum η ext 3.8%) or deep blue 9,10-diphenylanthracene (CIE x,y ) 0.15, 0.06 and maximum η ext 2.8%). ²or solid state lighting application,
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Hydroxynaphthyridine-Derived Group III Metal Chelates -...

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