Characterization of isomers in aluminum tris(quinoline-8-olate) by one-dimensional 27Al nuclear magnetic resonance under magic-angle spinning

Solid-state 27Al NMR spectra under magic-angle spinning of different forms of aluminum tris(quinoline-8-olate) (Alq3) are presented. Alq3 is an organometallic complex of great importance in the context of organic light-emitting diodes. Our results demonstrate a strong difference in the asymmetry of the electric field gradient (EFG) tensor at the aluminum site between the α and the recently discovered δ polymorph of Alq3. While the EFG is nearly planar (η≈1) in the α phase, it is nearly axially symmetric (η≈0) for the δ phase. This result provides strong support to the hypothesis that the δ phase contains the facial isomer of Alq3. While the spectra of both the α and the δ polymorphs exhibit sharp features, highly disordered forms of Alq3 obtained from rapid vapor deposition onto a cold substrate, yield broadened spectra, indicating substantial structural disorder in the local geometry of different Alq3 molecules.