Paramagnetic defect centres in crystalline Alq3

X- and Q-band electron paramagnetic resonance (EPR) investigation of different crystalline Alq3 (tris(8-hydroxyquinoline)aluminium (III)) fractions formed by a train sublimation method are reported. Several paramagnetic defect centres corresponding to 1/2, 1, and 3/2 spin are observed at room temperature. Their intensity is dependent on the temperature, nature of the crystalline phase, and preparation conditions. Spectra simulation and analysis based on the spin Hamiltonian appropriate to a high spin system (S1) suggest the existence of randomly oriented triplets and quartets in annealed Alq3 fractions. The crystalline Alq3 phases responsible for the EPR powder spectra have been identified by transmission electron microscopy measurements performed on these sample fractions.