Molecular orbital study of the first excited state of the OLED material tris(8-hydroxyquinoline)aluminum(III)

Tris(8-hydroxyquinoline)aluminum(III), Alq3, is used in organic light-emitting diodes (OLEDs) as an electron transport material and emitting layer. The lowest singlet excited state (S-1) of Alq3 has been studied by the singles configuration interaction (CIS) method and time-dependent density functional theory (TD-DFT) using a hybrid functional, B3-LYP, and the 3-21+G** basis set. For comparison and calibration, 8-hydroxyquinoline has also been examined with these methods using the 3-21+G** and larger basis sets. The lowest singlet electronic transition (S-0 --> S-1) of Alq3 is primarily localized on one of the quinolate ligands. Comparison of the CIS optimized excited-state structure and the Hartree-Fock ground-state structure indicates that the geometric shift is mainly confined to the a-quinolate. Very similar changes are found for the S-1 state of 8-hydroxyquinoline, and these changes can be easily understood in terms of the nodal patterns of the highest occupied and lowest unoccupied molecular orbitals. The structural relaxation upon excitation, when expressed in terms of ground-state normal modes of vibration, corresponds to a quinolate skeletal vibrational mode at 534 cm(-1) and serves to assign the vibronic structure observed in the low-temperature emission spectra. On the basis of the CIS-optimized structure of the excited state, TD-B3-LYP calculations predict an emission wavelength of 538 nm, which is comparable to 514 nm observed experimentally for solution phase photoluminescence. The Stokes shift calculated by TD-B3-LYP is 123 nm, in excellent agreement with the observed value of 126 nm.