Interplay between optical properties and molecular packing in novel isolated crystalline phases of tris-(8-hydroxyquinoline) Aluminium (III)

Two novel unsolvated polymorphs of tris-(8-hydroxyquinoline) Aluminium (IIl) (Alq3) have been synthesized and their crystalline structure determined from X-ray diffraction data on powders and single crystals. Both these crystal structures are triclinic, consist of a racemic mixture of the mer isomer, and are characterized by different molecular packings involving short contacts between quinoline ligands belonging to symmetry related Alq3 molecules (inter-ligand spacings in the 3.5-3.9 Å range). Investigations of the different crystal phases, as well as of vacuum sublimed thin films by spectroscopic techniques, allowed to highlight the effect of the molecular packing on the optical emission properties, and to clarify the nature of the photoexcitations.