Luminescent Compounds fac- and mer-Aluminum Tris(quinolin-8-olate). A Pure and Hybrid Density Functional Theory and Time-Dependent Density Functional Theory Investigation of Their Electronic and Spectroscopic Properties

Density functional theory and time-dependent density functional theory have been applied to describe the ground and excited states and the spectral characteristics of the meridianal and facial isomers of aluminum tris(quinolin-8-olate) (Alq 3 ). Vertical absorption wavelengths and oscillator strengths of both meridianal Alq 3 and facial Alq 3 have been computed up to about 255 nm (4.86 eV) and compared with experimental data. Experimental meridianal Alq 3 absorption band locations are well reproduced by the computations, allowing easy assignment of the absorption bands. The facial Alq 3 absorption spectrum has been computed, and a detailed comparison of the excited state characteristics of the two isomers has been addressed for pointing out differences in absorption and emission properties. This work suggests that the facial Alq 3 may be the constituent of the recently reported Alq 3 crystalline phase known as the δ-phase. This is an important new material for understanding the Alq 3 solid-state properties and for a possible organic light-emitting diode fabrication. Some suggestions to experimentally distinguish the two isomers have been presented. Furthermore, information about the excited-state kinetics of both the isomers and properties of their emissive excited states has been gained. Both the hybrid B3LYP and the pure BLYP, LSDA, BPW91, and LB94 exchange-correlation functionals have been tested, and the B3LYP functional clearly seems to be the best choice for this class of molecules.