The enhanced phosphorescence from Alq₃ fluorescent materials by phosphor sensitization

The green emission accompanied with intense red phosphorescence is observed from the tris(8-hydroxyquinoline) aluminum (Alq₃) in the heavily doped tris(2-phenylpyridine) iridium [Ir(ppy)₃] films. This photoluminescence (PL) emission mechanism is studied by the PL excitation (PLE) spectra for the red and green emissions. From the PLE spectrum for the red emission, the energy transfer from Ir(ppy)₃ to Alq₃ is confirmed. The increase of the green emission with increasing temperature from 12 K is explained by the endothermic back Förster energy transfer from the triplet T₁ state of Ir(ppy)₃ to the singlet S₁ state of Alq₃. It is suggested that this back transfer is partially responsible for the decrease of the red emission intensity corresponding to the increase of the green emission. Unlike the Förster energy transfer, the Dexter energy transfer from the T₁ state of Ir(ppy)₃ to the T₁ state of Alq₃ is highly efficient. The explanation for this process is presented here. The green emission is not attributed to Ir(ppy)₃ but to Alq₃, because not only the green emission profile but also the red emission profile is quite similar to the δ-phase polycrystalline Alq₃. From these results, it is concluded that the Alq₃ forms a δ-phase polycrystalline state to accept the phosphor sensitization. The unusual enhancement of the red phosphorescence from the Alq₃ is explained by the mixing of the T₁ state of Ir(ppy)₃ with the T₁ state of Alq₃ under the Dexter energy transfer of short range process, by taking into account that the T₁ state of Ir(ppy)₃ contains the singlet state by the strong spin-orbit coupling.