Tunable emission of polymer light emitting diodes bearing green-emitting Ir(III) complexes: The structural role of 9-((6-(4-fluorophenyl)pyridin-3-yl) methyl)-9H-carbazole ligands Min Ju Cho a , Jung-Il Jin a , Dong Hoon Choi a, * , Jung Hee Yoon a , Chang Seop Hong a , Young Min Kim b , Young Wook Park b , Byeong-Kwon Ju b a Department of Chemistry, Advanced Materials Chemistry Research Center, Korea University, 5 Anam-dong, Sungbuk-gu, Seoul 136-701, Republic of Korea b Display and Nanosystem Laboratory, College of Engineering, Korea University, 5 Anam-dong, Sungbuk-gu, Seoul 136-701, Republic of Korea article info Article history: Received 14 September 2009 Received in revised form 24 October 2009 Accepted 28 October 2009 Available online 31 October 2009 PACS: 78.60.Fi 78.66-w 78.55 Keywords: Iridium(III) complex Ligand Solubility Photoluminescence Triplet energy transfer Electrophosphorescence abstract 9-((6-Phenylpyridin-3-yl)methyl)-9H-carbazole and 9-((6-(4-fluorophenyl)pyridin-3-yl)methyl)-9H-car- bazole were synthesized as ligands by attaching a carbazolyl group to the pyridine in 2-phenylpyridine and 2-(4-fluorophenyl)pyridine, respectively. Four different Ir(III) complexes were prepared using a simple procedure, the solubility of which was significantly greater than that of conventional green- emitting Ir(ppy) 3 . Among the many devices fabricated, that which contained 10% of Ir(Cz-ppy) 1 (Cz- Fppy) 2 in PVK/PBD (70:30 wt%), exhibited an external quantum efficiency of 6.80%, luminous efficiency of 20.23 cd/A, and maximum brightness of 17520 cd/m 2 . In particular, the electronic property of the new 9-((6-(4-fluorophenyl)pyridin-3-yl)methyl)-9H-carbazole ligand can manipulate the triplet energy level of the Ir(III) complex to finely tune the emission spectra. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Of the various types of light-emitting organic materials avail- able, phosphorescent materials have been recognized as superior candidates because both singlet and triplet excitons can generate unique light emissions, with a theoretical internal quantum effi- ciency of 100% [1e3]. In particular, cyclometalated Ir(III) complexes show high phosphorescent efficiencies and are one of the most promising classes of phosphorescent dyes used in organic light- emitting diodes (OLEDs) [4e11]. Along with advancements in processing solutions for the fabrication of OLEDs, novel, highly soluble materials displaying phosphorescence have been reported recently [12e14]. An extensively used technique for enhancing the solubility of Ir(III) complexes is via the attachment of lengthy alkyl moieties or their introduction into dendrimers [15e18]. A dendritic architecture improves both solubility and the site-isolation effect by employing various peripheral groups around the emission center [19,20]. In addition to the above methods, a very promising Ir(III) complex containing the tetraphenylsilyl group has been reported to exhibit excellent luminous efficiency using a conventional poly- mer-based [poly(N-vinyl carbazole), PVK] OLED. The design and synthesis of a highly phosphorescent tris-cyclometallated homo- leptic Ir(III) complex [Ir(TPSppy) 3 ] (TPSppy ¼ 2-(4 0 -(triphenylsilyl) biphenyl-3-yl)pyridine) with a silane-based dendritic substituent have been demonstrated. It has also been suggested that aryl silane was quite effective at emitting UV light for improving device effi- ciency compared to a long alkyl substituent [21]. Liu et al. reported PVK based light-emitting diodes fabricated with pinene-substituted Ir(III) phosphorescent dopants; three different Ir(III) complex dyes were demonstrated for which a pho- toluminescent spectral shift was observed with ligand modifica- tion. In addition, the pinene substitution induced steric hindrance to molecular structure of the dopant that aided suppression of * Corresponding author. Tel.: þ82 2 3290 3140; fax: þ82 2 924 3141. E-mail address: dhchoi8803@korea.ac.kr (D.H. Choi). Contents lists available at ScienceDirect Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig 0143-7208/$ e see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.dyepig.2009.10.017 Dyes and Pigments 85 (2010) 143e151