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.