Characterization of local structures in amorphous and crystalline tris(8-hydroxyquinoline) aluminum(III) (Alq 3) by solid-state 27Al MQMAS NMR spectroscopy

Abstract

The local structure of tris(8-hydroxyquinoline) aluminum(III) (Alq 3) in three different crystalline polymorphs, α-, γ-, and δ-Alq 3, and amorphous Alq 3, are investigated by 27Al NMR experiments, which include two-dimensional multi-quantum magic-angle spinning (MQMAS) experiments and one-dimensional variable B 0 field MAS experiments. The quadrupolar and chemical shift parameters of these Alq 3 samples are determined, including the distributions. These parameters reflect the local structure of respective samples and it is found that the γ- and δ-Alq 3 consist of the facial isomer with well-defined crystal structure, whereas the α- and amorphous Alq 3 consist of the meridional isomer with distributions of quadrupolar coupling constants. It is also found that two distinct components exist in α-Alq 3.