Characterization of local structures in amorphous and crystalline tris(8-hydroxyquinoline) aluminum(III) (Alq3) by solid-state 27Al MQMAS NMR spectroscopy

The local structure of tris(8-hydroxyquinoline) aluminum(III) (Alq3) in three different crystalline polymorphs, α-, γ-, and δ-Alq3, and amorphous Alq3, are investigated by 27Al NMR experiments, which include two-dimensional multi-quantum magic-angle spinning (MQMAS) experiments and one-dimensional variable B0 field MAS experiments. The quadrupolar and chemical shift parameters of these Alq3 samples are determined, including the distributions. These parameters reflect the local structure of respective samples and it is found that the γ- and δ-Alq3 consist of the facial isomer with well-defined crystal structure, whereas the α- and amorphous Alq3 consist of the meridional isomer with distributions of quadrupolar coupling constants. It is also found that two distinct components exist in α-Alq3.