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) (GaQ3) 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). GaQ3 polymorphs were found to be isomorphous to the (alpha),(beta), and (delta)structures reported for the aluminum analogue tris(8-hydroxyquinoline) aluminum(III) (AlQ3). The (alpha),(beta), and the solvated structures are racemates of the meridional isomer, whereas the (delta)polymorph, formed by annealing the (alpha)form to 380 °C under vacuum, contains the facial isomer as demonstrated by FT-IR spectroscopy. The (alpha)(right arrow)(delta)phase transformation can be described as a thermal isomerization reaction in the solid state that largely preserves the unit cell parameters of the initial (alpha)structure. Transmission electron microscopy (TEM) and electron diffraction demonstrate that the (alpha) structure is formed in oriented films on poly(tetrafluoroethylene) substrates for a substrate temperature Ts = 100 °C. Evidence is found for a high level of structural defects within needles of mer GaQ3. The fluorescence at 293 K of the crystal structures containing mer GaQ3 exhibits the same structural dependence as observed in the case of AlQ3: the maximum of fluorescence is red-shifted with increasing density of the crystal structure.