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 (S1) 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 (S0 → S1) 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 S1 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...

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