The triplet state in tris-(8-hydroxyquinoline)aluminum

This paper presents the characterization of the triplet state in tris-(8-hydroxyquinoline)aluminum (Alq(3)). An emission spectrum of Alq(3), namely, the phosphorescence, is presented for crystalline and amorphous Alq(3), which enables direct investigation of the T-1-->S-0 transition. The assignment of this spectrum to the phosphorescence is further confirmed by temperature-dependent measurements and comparison with the delayed fluorescence. The triplet energies of the meridional and facial isomer (in alpha- and delta-Alq(3)) are determined from the well-resolved vibronic progressions of the phosphorescence as 2.11+/-0.1 and 2.16+/-0.1 eV, respectively. Furthermore, the lifetime of the triplet state is measured for a temperature range from 6 to 150 K. These temperature-dependent measurements also identify a so far unknown phase transition of Alq(3) at about 50 K. Optically detected magnetic resonance at a zero field is used to measure the characteristic zero-field splitting parameters (\E\=0.0114 cm(-1) and \D\=0.0630 cm(-1)), and these are discussed in terms of a molecular symmetry and contribution of different ligands and suggest a mini-exciton-like behavior of the triplet state on the three ligands of the Alq(3) molecule. All these measurements are performed on different crystalline phases (alpha- and delta-Al-q3) and on evaporated amorphous films. The differences observed for the delta-phase are consistent with the reduced intersystem crossing and thus with the facial isomer in this phase. (C) 2004 American Institute of Physics.