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-emitt ing diodes (OLEDs) as an electron transport material and emitting layer. Th e lowest singlet excited state (S-1) of Alq3 has been studied by the single s configuration interaction (CIS) method and time-dependent density functio nal theory (TD-DFT) using a hybrid functional, B3-LYP, and the 3-21+G** bas is set. For comparison and calibration, 8-hydroxyquinoline has also been ex amined with these methods using the 3-21+G** and larger basis sets. The low est singlet electronic transition (S-0 --> S-1) of Alq3 is primarily locali zed on one of the quinolate ligands. Comparison of the CIS optimized excite d-state structure and the Hartree-Fock ground-state structure indicates tha t the geometric shift is mainly confined to the a-quinolate. Very similar c hanges 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 occ upied and lowest unoccupied molecular orbitals. The structural relaxation u pon excitation, when expressed in terms of ground-state normal modes of vib ration, 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 excit ed state, TD-B3-LYP calculations predict an emission wavelength of 538 nm, which is comparable to 514 nm observed experimentally for solution phase ph otoluminescence. The Stokes shift calculated by TD-B3-LYP is 123 nm, in exc ellent agreement with the observed value of 126 nm.