Computational study of novel tris-(5-aryl-8-hydroxyquinolato) aluminum (III) complexes with use in molecular electronics

Organic light emitting diodes (OLED) are multilayer devices arranged in a "sandwich" form, in which each layer performs a particular function. Different functions are provided by a hole-transporting layer (HTL) and an electron-transport/emitting layer (ETL/EL), whose nature is in essence organic. A density functional study was performed on a set of novel AlQ3-derivative electroluminescent compounds whose ligands were substituted in the C-5 position with several aryl moieties that exert electron-withdrawing and electron-donating effects. For comparison, the prototypical complex AlQ3 was also taken into account. All of the geometries were optimized with the hybrid functional B3-LYP and the 6-31G(d) gaussian basis set. In this chapter, we have computed and compared the electronic structures and the reactivity trends of these novel complexes via electronic properties and DFT-based reactivity descriptors calculated at B3-LYP/6-31G(d) level. Descriptors calculated comprise the energy of the frontier molecular orbitals HOMO and LUMO, the energy of the gap HOMO-LUMO, atomic charges via natural population analysis, electronegativity, electron affinity, electrophilicity index, chemical hardness, global softness, condensed Fukui function and local softness.The geometries around the aluminum center are shown to be distorted octahedral, in which the metal center presents an oxidation state of +3. However, the ionicity of the aluminum-ligand bonds is decreased as revealed by the NPA charges of Al+3, which show that the theoretical formal value is less than +3, indicating an important covalency degree in such bonds. On the other hand, the inner coordination sphere bond lengths remain unchanged when they are compared with those of the pristine AlQ3. The analysis of the electronic structure reveals that the frontier molecular orbitals HOMO and LUMO are split in energy (triplets) and are strongly localized on specific regions in the ligands. Specifically, the HOMO set is mainly located in the phenoxide ring, whereas the LUMO density is located at the pyridyl side. The substituents do not exert an appreciable effect on the orbital shapes, but do so on the orbital energies. Interrelationships between some reactivity descriptors are also demonstrated, and their correlation with the Hammett parameter are studied. The local reactivity analysis was performed on the basis of DFT-based descriptors and revealed that the atoms prone to nucleophilic and electrophilic attack are atoms C4 (ligand 3) and C5 (ligand 1), respectively. © 2009 by Nova Science Publishers, Inc. All rights reserved.