Optical and ESR studies of Tris(8-hydroxyquinoline)aluminum

Tris(8-hydroxyquinoline)aluminum (AlQ) powder has been characterized using photoluminescence and electron spin resonance (ESR). The broad asymmetric excitation band is centered at ∼425 nm, while the emission band centered at ∼505 nm is similar to those reported in films. A significant ESR signal with a linewidth of 10 G and g-value of 2.0038 has been observed in as synthesized AlQ powder at room temperature. The signal at same position is present in AlQ powder as-received from Aldrich, as synthesized AlQ powder, and AlQ powder purified by vacuum sublimation. The appearance of the ESR signal is attributed to organic free radicals. Moreover, exposure of AlQ powder to humid air enhances the ESR signal intensity which increases linearly with exposure duration. This is in agreement with recently proposed degradation mechanism in AlQ based organic light emitting diodes, where degradation was attributed to unstable cationic AlQ species. The discovery of natural free radicals in freshly synthesized AlQ may be useful to prevent the degradation of AlQ and other analogous compounds in organic light emitting diodes (OLEDs).