Ligand Exchange Dynamics in Aluminum Tris-(Quinoline-8-olate): A Solution State NMR Study Marcel Utz, †,‡ Changqing Chen, †,§,| Martha Morton, | and Fotios Papadimitrakopoulos †,§,| Contribution from the Institute of Materials Science, the Department of Physics, the Department of Chemistry, and the Nanomaterials Optoelectronics Laboratory, UniVersity of Connecticut, Storrs, Connecticut 06269 Received July 23, 2002 ; E-mail: marcel.utz@uconn.edu Abstract: 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 5s - 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. I. Introduction Organic light emitting diode (OLED) devices have received considerable interest over the past fifteen years. 1 Aside from their low operational voltage, high brightness, efficiency, and tunable emission, their simple fabrication that is amenable to large area deposition renders them a formidable candidate for flat panel display applications. Aluminum tris-(quinoline-8-olate) (Alq 3 ) has been used as an electron transport and electron- hole recombination and emissive layer in the earliest working OLED devices, 2 and has remained a workhorse material for this purpose ever since. Alq 3 can be sublimed under mild conditions, which simplifies both purification and the deposition of thin films from the vapor phase. Its electron-transport capacity relies on the stability of its radical anion, which is not prone to oxidative attack by O 2 . A great deal of effort has been invested into optimization of Alq 3 -based devices in terms of quantum efficiency and life- time, 3-5 and the tuning of their luminescence color. 6,7 By contrast, most fundamental studies directed toward the prop- erties of the Alq 3 molecule, its electronic structure, 8,9 dynam- ics, 12,11,10 and crystal polymorphs, 13,14 have only appeared recently. The present work focuses on the internal dynamics of the Alq 3 molecule. Two-dimensional exchange NMR spectroscopy has been used in order to quantify the interchange of quinoli- nolate ligands in chloroform solution at various temperatures. Current OLED devices use Alq 3 in the form of amorphous thin films. Both the deposition of these films from the vapor phase and their gradual physical aging are kinetically controlled processes. Knowledge of the kinetic properties of the Alq 3 molecule constitutes an important step toward understanding both the processing and the long-time behavior of OLED devices. In particular, profound effects of the deposition rate and conditions on the performance of Alq 3 -based devices have been demonstrated recently. 15-18 † Institute of Materials Science, University of Connecticut. ‡ Department of Physics, University of Connecticut. § Department of Chemistry, University of Connecticut. | Nanomaterials Optoelectronics Laboratory, University of Connecticut. (1) Farchioni, R., Grosso, G., Eds. 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Appl. Phys. Lett. 1999, 75, 4010-4012. Published on Web 01/14/2003 10.1021/ja027825o CCC: $25.00 © 2003 American Chemical Society J. AM. CHEM. SOC. 2003, 125, 1371-1375 9 1371