Relationships between light-emitting properties and different isomers in polymorphs of tris(8-hydroxyquinoline) aluminum(III) (Alq3) analyzed by solid-state 27Al NMR and density functional theory (DFT) calculations

The structures of tris(8-hydroxyquinoline) aluminum(III) (Alq(3)) in the different polymorphs, alpha-, gamma-, and delta-Alq(3), and in the amorphous state, amo-Alq(3), have been analyzed by solid-state Al-27 nuclear magnetic resonance (NMR). The local structures of alpha- and amo-Alq(3) are found to be similar; both samples are composed of the meridional isomer and are locally disordered. No evidence of the existence of the facial isomer is found even for amo-Alq(3). In contrast, the isomeric states of gamma- and delta-Alq(3) are facial. The Al-27 NMR spectrum of delta-Alq(3) is influenced by intermolecular interactions, whereas that of gamma-Alq(3) is determined only by a single facial Alq(3) molecule, suggesting that intermolecular interactions are negligible for gamma-Alq(3). This result is closely related to the experimentally observed good solubility of gamma-Alq(3). Density functional theory (DFT) calculations support the identification of the isomeric state and the effect of the intermolecular interactions. A clear correlation between the isomeric state and. the fluorescence wavelength is found, indicating that the isomeric state of Alq(3) is a crucial factor for the light-emitting properties.