Electronic Properties, Spectroscopic Properties and Monomolecular Isomerization Processes of Prototype OLED Compound Aluminum Tris(Quinolin-8-Olate) Facial and Meridianal Isomers

Abstract

Aluminum tris(quinolin-8-olate) (Alq3 in the following) represents a prototype compound for studies in the field of organic LEDs (OLEDs) and it is the chosen substance in many OLEDs which have hit the market. Alq3 meridianal and facial isomers ground state and spectroscopic properties have been studied by DFT and TD-DFT computations. The differences between the two isomers have been compared with the differences between δ-phase crystals and meridianal Alq3 crystals. In this way, the possibility that the facial isomer is contained in the Alq3 δ-phase has been discussed. This work has allowed the assignment of the experimental absorption bands of meridianal Alq3 up to about 255nm and the theoretical prediction of facial Alq3 absorption spectrum. The comparison between the absorption and emission characteristics of the two isomers has suggested the spectroscopic properties to be used to distinguish the facial isomer from the meridianal isomer. Indications have been obtained about the excited state kinetics of the more studied meridianal Alq3. The evaluation of the reaction surface of the isolated Alq3 allowed the determination of its monomolecolar isomerization paths and their energetics. Thus, it has been possible to associate the known high rate of the Alq3 isomerizations to the dissociative processes which involve the Al-N bond. Furthermore, new suggestions about the Alq3 chemistry have been introduced. This represents a first glance at the understanding of undesirable processes as atmospheric-induced decomposition and other degradation paths which limit the Alq3 technological applications.