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 Alq3 and facial Alq3 have been computed up to about 255 nm (4.86 eV) and compared with experimental data. Experimental meridianal Alq3 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 Alq3 may be the constituent of the recently reported Alq3 crystalline phase known as the delta-phase. This is an important new material for understanding the Alq3 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.