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

2004; American Institute of Physics; Volume: 96; Issue: 11 Linguagem: Inglês

10.1063/1.1808249

1520-8850

Michael Cölle, C. Gärditz, M. Braun,

Organic Electronics and Photovoltaics

This paper presents the characterization of the triplet state in tris-(8-hydroxyquinoline)aluminum (Alq3). An emission spectrum of Alq3, namely, the phosphorescence, is presented for crystalline and amorphous Alq3, which enables direct investigation of the T1→S0 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 α- and δ-Alq3) are determined from the well-resolved vibronic progressions of the phosphorescence as 2.11±0.1 and 2.16±0.1eV, respectively. Furthermore, the lifetime of the triplet state is measured for a temperature range from 6to150K. These temperature-dependent measurements also identify a so far unknown phase transition of Alq3 at about 50K. Optically detected magnetic resonance at a zero field is used to measure the characteristic zero-field splitting parameters (∣E∣=0.0114cm−1 and ∣D∣=0.0630cm−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 Alq3 molecule. All these measurements are performed on different crystalline phases (α- and δ-Alq3) and on evaporated amorphous films. The differences observed for the δ-phase are consistent with the reduced intersystem crossing and thus with the facial isomer in this phase.