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

Solid-state Al-27 NMR spectra under magic-angle spinning of different forms of aluminum tris(quinoline-8-olate) (Alq(3)) are presented. Alq(3) 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 alpha and the recently discovered delta polymorph of Alq(3). While the EFG is nearly planar (etaapproximate to1) in the alpha phase, it is nearly axially symmetric (etaapproximate to0) for the delta phase. This result provides strong support to the hypothesis that the delta phase contains the facial isomer of Alq(3). While the spectra of both the alpha and the delta polymorphs exhibit sharp features, highly disordered forms of Alq(3) obtained from rapid vapor deposition onto a cold substrate, yield broadened spectra, indicating substantial structural disorder in the local geometry of different Alq(3) molecules. (C) 2003 American Institute of Physics.