Blue-Emitting Cationic Iridium Complexes with 2-(1H-Pyrazol-1-yl)pyridine as the Ancillary Ligand for Efficient Light-Emitting Electrochemical Cells

Authors

  • Lei He,

    1. Key Lab of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Department of Chemistry, Tsinghua University Beijing 100084 (P. R. China)
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  • Lian Duan,

    1. Key Lab of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Department of Chemistry, Tsinghua University Beijing 100084 (P. R. China)
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  • Juan Qiao,

    1. Key Lab of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Department of Chemistry, Tsinghua University Beijing 100084 (P. R. China)
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  • Ruji Wang,

    1. Key Lab of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Department of Chemistry, Tsinghua University Beijing 100084 (P. R. China)
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  • Peng Wei,

    1. Key Lab of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Department of Chemistry, Tsinghua University Beijing 100084 (P. R. China)
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  • Liduo Wang,

    1. Key Lab of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Department of Chemistry, Tsinghua University Beijing 100084 (P. R. China)
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  • Yong Qiu

    Corresponding author
    1. Key Lab of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Department of Chemistry, Tsinghua University Beijing 100084 (P. R. China)
    • Key Lab of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Department of Chemistry, Tsinghua University Beijing 100084 (P. R. China).
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  • The research was supported by the National Natural Science Foundation of China (Grant No. 50403001 and 50325310) and the National Basic Research Program of China (Grant No. 2006CB806200).

Abstract

Two blue-emitting cationic iridium complexes with 2-(1H-pyrazol-1-yl)pyridine (pzpy) as the ancillary ligands, namely, [Ir(ppy)2(pzpy)]PF6 and [Ir(dfppy)2(pzpy)]PF6 (ppy is 2-phenylpyridine, dfppy is 2-(2,4-difluorophenyl) pyridine, and PF6 is hexafluorophosphate), have been prepared, and their photophysical and electrochemical properties have been investigated. In CH3CN solutions, [Ir(ppy)2(pzpy)]PF6 emits blue-green light (475 nm), which is blue-shifted by more than 100 nm with respect to the typical cationic iridium complex [Ir(ppy)2(dtb-bpy)]PF6 (dtb-bpy is 4,4′-di-tert-butyl-2,2′-bipyridine); [Ir(dfppy)2(pzpy)]PF6 with fluorine-substituted cyclometalated ligands shows further blue-shifted light emission (451 nm). Quantum chemical calculations reveal that the emissions are mainly from the ligand-centered 3ππ* states of the cyclometalated ligands (ppy or dfppy). Light-emitting electrochemical cells (LECs) based on [Ir(ppy)2(pzpy)]PF6 gave green-blue electroluminescence (486 nm) and had a relatively high efficiency of 4.3 cd A−1 when an ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate was added into the light-emitting layer. LECs based on [Ir(dfppy)2(pzpy)]PF6 gave blue electroluminescence (460 nm) with CIE (Commission Internationale de L'Eclairage) coordinates of (0.20, 0.28), which is the bluest light emission for iTMCs-based LECs reported so far. Our work suggests that using diimine ancillary ligands involving electron-donating nitrogen atoms (like pzpy) is an efficient strategy to turn the light emission of cationic iridium complexes to the blue region.

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