Structure and Spectroscopic Properties of the Crystalline Structures Containing Meridional and Facial Isomers of Tris(8-hydroxyquinoline) Gallium(III)

The various polymorphs of tris(8-hydroxyquinoline) gallium(III) (GaQ 3 ) have been prepared and identified in powders and thin films. Three nonsolvated polymorphs have been identified, and their structures have been obtained from powder X-ray diffraction (PXRD). GaQ 3 polymorphs were found to be isomorphous to the α, β, and δ structures reported for the aluminum analogue tris(8-hydroxyquinoline) aluminum(III) (AlQ 3 ). The α, β, and the solvated structures are racemates of the meridional isomer, whereas the δ polymorph, formed by annealing the α form to 380 °C under vacuum, contains the facial isomer as demonstrated by FT-IR spectroscopy. The α→δ phase transformation can be described as a thermal isomerization reaction in the solid state that largely preserves the unit cell parameters of the initial α structure. Transmission electron microscopy (TEM) and electron diffraction demonstrate that the α structure is formed in oriented films on poly(tetrafluoroethylene) substrates for a substrate temperature T s = 100 °C. Evidence is found for a high level of structural defects within needles of α GaQ 3 . The fluorescence at 293 K of the crystal structures containing mer GaQ 3 exhibits the same structural dependence as observed in the case of AlQ 3: the maximum of fluorescence is red-shifted with increasing density of the crystal structure.