16 October 2007 Origin of the different emission wavelengths in Alq3 analyzed by solid-state NMR
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Tris(8-hydroxyquinoline) aluminum(III) (Alq3) is one of the most widely used materials in organic light-emitting diodes (OLEDs), and the relationship between the structures and the luminescent wavelengths is of recent interest; yellowish-green emissions are observed for the α- and amorphous Alq3, whereas blue emissions are found for the γ- and δ-Alq3. In order to clarify the relationship between the structures and the emission wavelengths, we carried out solid-state nuclear magnetic resonance (NMR) experiments on the different polymorphs of Alq3 and the amorphous state. Based on 27Al and 13C magic angle spinning (MAS) NMR experiments, it is found that the isomeric state of the amorphous Alq3 is the same as that of α-Alq3 and is different from those of γ- and δ-Alq3. Not only for the amorphous, but also for α-Alq3, the local structures are found to be disordered. We also obtained clear evidence that γ-Alq3 is in the facial isomeric state. It is suggested that δ-Alq3 is also facial. The difference between γ- and δ-Alq3 is the intermolecular packing; the effect of intermolecular packing is found only for δ-Alq3. A further confirmation of the isomeric states of these Alq3 samples is obtained from temperature-dependent X-ray diffraction experiments.
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Hironori Kaji, Hironori Kaji, Tatsuya Fukushima, Tatsuya Fukushima, Kousuke Takami, Kousuke Takami, Yasunari Kusaka, Yasunari Kusaka, } "Origin of the different emission wavelengths in Alq3 analyzed by solid-state NMR", Proc. SPIE 6655, Organic Light Emitting Materials and Devices XI, 665504 (16 October 2007); doi: 10.1117/12.735504; https://doi.org/10.1117/12.735504

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