Origin of the different emission wavelengths in Alq 3 analyzed by solid-state NMR

Abstract

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.