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) a luminum (Alq(3)) which were obtained by thermal sublimation in a horizontal glass tube. These phases are investigated by x-ray powder diffraction, Ram an and infrared spectroscopy, and low temperature photoluminescence measure ments. Apart from the already known alpha phase we could identify a new cry stalline phase of Alq(3) (delta -Alq(3)) showing blueshifted fluorescence. As compared to the alpha phase this new phase is characterized by a larger unit cell volume, a reduced number of Raman lines in the energy range betwe en 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 d ifferences are interpreted in terms of the isomery of the Alq(3) molecule. It is supposed that the new phase contains the facial isomer, whereas in th e other phases only the meridianal isomer was reported. Low temperature pho toluminescence spectra show a well-resolved vibronic progression with about the same spacing of 550 cm(-1) for both crystalline phases of Alq(3). Site -selective photoluminescence measurements reveal the existence of an additi onal redshifted featureless emission, which is ascribed to energy relaxatio n into low-lying states. (C) 2001 American Institute of Physics.