Ligand exchange dynamics in aluminum tris-(quinoline-8-olate): A solution state NMR study

The exchange kinetics between the three symmetry-inequivalent ligands in the meridianal isomer of aluminum tris-(quinoline-8-olate) (Alq₃), a widely used electron-transporting and light-emitting material in the field of organic light emitting diodes, have been studied using two-dimensional exchange NMR spectroscopy in solution. The three inequivalent ligands were found to exchange on a time scale of about 5 s^-1 at room temperature. A simple first-order mechanism based on consecutive 180° flips of the ligands is sufficient to quantitatively explain the experimental data. Activation enthalpies between 83 and 106 kJ mol^-1 were found for the flips of the three inequivalent ligands. The activation entropies are positive, suggesting a highly disordered transition state. These findings elucidate the internally mobile nature of the Alq₃ complex, and may have important implications for the morphology of vapor deposited thin films of Alq₃ as well as for crystallization-assisted device failures.