A new crystalline phase of the electroluminescent material tris(8-hydroxyquinoline) aluminum exhibiting blueshifted fluorescence

We report on two different crystalline phases of tris(8-hydroxyquinoline) aluminum (Alq3) which were obtained by thermal sublimation in a horizontal glass tube. These phases are investigated by x-ray powder diffraction, Raman and infrared spectroscopy, and low temperature photoluminescence measurements. Apart from the already known α phase we could identify a new crystalline phase of Alq3 (δ-Alq3) showing blueshifted fluorescence. As compared to the α phase this new phase is characterized by a larger unit cell volume, a reduced number of Raman lines in the energy range between 70 and 700 cm−1, a blueshift of the photoluminescence maximum by about 0.2 eV, and a decreased intersystem crossing to the triplet state. These differences are interpreted in terms of the isomery of the Alq3 molecule. It is supposed that the new phase contains the facial isomer, whereas in the other phases only the meridianal isomer was reported. Low temperature photoluminescence spectra show a well-resolved vibronic progression with about the same spacing of 550 cm−1 for both crystalline phases of Alq3. Site-selective photoluminescence measurements reveal the existence of an additional redshifted featureless emission, which is ascribed to energy relaxation into low-lying states.