Electronic Structure and Molecular Orbital Study of the First Excited State of the High-Efficiency Blue OLED Material Bis(2-Methyl-8-Quinolinolato)Aluminum(III) Hydroxide Complex from Ab Initio and TD-B3LYP

摘要

Bis(2-methyl-8-quinolinolato)aluminum(III) hydroxide complex (AlMq_2OH) is used in organic light-emitting diodes (OLEDs) as an electron transport material and emitting layer. By means of ab initio Hartree-Fock (HF) and density functional theory (DFT) B3LYP methods, the structure of AlMq_2OH was optimized. The frontier molecular orbital characteristics and energy levels of AlMq_2OH have been analyzed systematically to study the electronic transition mechanism in AlMq_2OH. For comparison and calibration, bis(8-quinolinolato)aluminum(III) hydroxide complex (Alq_2OH) has also been examined with these methods using the same basis sets. The lowest singlet excited state (S_1) of AlMq_2OH has been studied by the singles configuration interaction (CIS) method and time-dependent DFT (TD-DFT) using a hybrid functional, B3-LYP, and the 6-31G basis set. The lowest singlet electronic transition (S_0 -> S_1) of AlMq_2OH is pi -> pi electronic transitions and primarily localized on the different quinolate ligands. The emission of AlMq_2OH is due to the electron transitions from a phenoxide donor to a pyridyl acceptor from another quinolate ligand including C -> C and O->N transference. Two possible electron transfer pathways are presented, one by carbon, oxygen, and nitrogen atoms and the other via metal cation Al~(3+) . The comparison between the CIS-optimized excited-state structure with the HF ground-state structure indicates that the geometric shift is mainly confined to the one quinolate and these changes can be easily understood in terms of the nodal patterns of the highest occupied and lowest unoccupied molecular orbitals. On the basis of the CIS-optimized structure of the excited state, TD-B3-LYP calculations predict an emission wavelength of 499.78 nm. An absorption wavelength at 380.79 nm on the optimized structure of B3LYP/6-31G was predicted. They are comparable to AlMq2OH 485 and 390 nm observed experimentally for photoluminescence and UV-vis absorption spectra of AlMq_2OH solid thin film on quartz, respectively. Lending theoretical corroboration to recent experimental observations and supposition, the reasons for the blue-shift of AlMq2OH were revealed.