Electroabsorption study of excited states in tris 8-hydroxyquinoline aluminum complex

Electric field-modulated absorption spectra, from 280–490 nm, of vacuum-evaporated films of tris (8-hydroxyquinoline) aluminum (Alq3) have been studied to determine the polarity of the electronic excited states. Semiempirical molecular modelling has been employed to relate the energetics and molecular electrical dipole moments of the isolated molecule to those resulting from the electroabsorption experiment, interpreted in terms of the second-order Stark effect. The results indicate the lowest excited singlets with relatively small dipole moments to be well localized on individual quinolate ligands, and those with higher energy and large dipole moments to show delocalization by charge transfer to the nearest-neighbour ligands of different molecules. The implications for the electroluminescent behaviour of Alq3 are discussed.