Density functional theory calculation of spectrum and excitation properties of mer-Alq3

Abstract

Meridional tris(8-hydroxyquinoline)aluminum (III) (mer-Alq₃) is an organometallic semiconductor material with phenomenal photo-electric properties. In order to understand the molecular luminescence properties of mer-Alq₃, the density functional theoretical (B3LYP) method with 6-31G^* basis set is employed to calculate its structure, infrared spectrum and Raman spectrum and the frontier molecular orbital of its ground state. The UV-vis absorption and the excited state characteristics are investigated by the time-dependent density functional theory (TD-DFT) method. The results show that the calculated spectral characteristics are in good agreement with the experimental data. The electron cloud of the highest occupied molecular orbital (HOMO) is located mostly on the phenoxide ring, whereas that of the lowest unoccupied molecular orbital (LUMO) sits on the pyridine ring. The absorption peaks of the UV-visible absorption spectrum are located in the visible and ultraviolet region. S0→S2 is attributed to the superposition of the π-π^* local excitation in the direction from benzene ring to pyridine ring and the n-π^* local excitation in the direction from oxygen atom to pyridine ring. The π-π^* local excitation from benzene ring to pyridine ring is S0→S4. The superposition of π-n local excitation from benzene to carbon and n-n local excitation from oxygen to carbon are excited by S0→S11. S0→S14 is charge-transfer excitation and contributed by the superposition of π-π^* in the direction from benzene ring to pyridine ring and n-π^* in the direction from oxygen atom to pyridine ring. This work is significant for understanding the basic properties of mer-Alq₃ and the mechanisms of electron excitations. It provides a deeper insight into the luminescence mechanism of mer-Alq₃, thus playing a guidance role in further improving the luminescence efficiency and regulating the spectral range of the light-emitting mer-Alq₃.