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Issue 45, 2011
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Solution-processable ambipolar host oligomers with high triplet energies for phosphorescent green emitters

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Abstract

Four carbazole–diaryloxadiazole oligomers 14–17 based on 9,9′-dioctyl-[3,3′]bicarbazolyl and 2,5-diaryl-[1,3,4]-oxadiazole monomer units have been synthesised by Suzuki cross-coupling reactions. The molecular weights of the oligomers were estimated using gel permeation chromatography: Mw 3130–4266 Da; polydispersity indices Mw/Mn 1.41–1.76 (i.e. 3–4 repeat units). These oligomers are, therefore, more like low molar mass polymers than monodisperse molecular materials. Thermal gravimetric analysis (TGA) indicated their good thermal stability with decomposition temperatures Td5% 381–440 °C. The photophysical properties have been investigated in detail in solution and as thin films. The data are consistent with the existence of an intramolecular charge transfer state (ICT) in all of these oligomers, which is enhanced by the more electron withdrawing pyridyl-oxadiazole substituent in 15. The triplet energy is sufficiently high (ET = 2.57 eV for λmax of the phosphorescence) for oligomers 14–17 to host a soluble green phosphorescent iridium guest emitter, as demonstrated in electroluminescence studies which showed emission exclusively from the guest complex. Single-emitting-layer organic light-emitting diodes (OLEDs) were constructed with the architecture glass/ITO/PEDOT:PSS/oligomer host + Ir guest (10% w/w)/Ba/Al. Devices with 16 as host demonstrated the best results reaching maximum current efficiency, ηc 3.93 cd A−1, external quantum efficiency (EQE) ηext 1.1% and good stability. The solution processability of the oligomers, their ambipolar structure, and the simplicity of the device architecture are attractive for further development.

Graphical abstract: Solution-processable ambipolar host oligomers with high triplet energies for phosphorescent green emitters

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Publication details

The article was received on 05 Aug 2011, accepted on 02 Oct 2011 and first published on 19 Oct 2011


Article type: Paper
DOI: 10.1039/C1JM13781F
Citation: J. Mater. Chem., 2011,21, 18439-18446
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    Solution-processable ambipolar host oligomers with high triplet energies for phosphorescent green emitters

    S. U. Pandya, H. A. Al Attar, V. Jankus, Y. Zheng, M. R. Bryce and A. P. Monkman, J. Mater. Chem., 2011, 21, 18439
    DOI: 10.1039/C1JM13781F

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