Systematic study of the photoluminescent and electroluminescent properties of pentacoordinate carboxylate and chloro bis(8-hydroxyquinaldine) complexes of gallium(III)

A detailed analysis of the structural, optical absorption, photoluminescent, and electroluminescent properties of a series of pentacoordinate bis(8-hydroxyquinaldine)gallium(III) complexes, i.e. q'(2)GaX with X = acetate, dimethylpropionate, benzoate, and chloro, is presented. These materials are compared with tris(8-hydroxyquinaldine)Ga and the tris(8-hydroxyquinolate) complexes of aluminum (Alq(3)) and gallium. Structural studies show that the pentacoordinate complexes have significantly less steric congestion than their hexacoordinate analogues, leading to comparatively smaller Franck-Condon shifts in their absorption and emission spectra, Relatively short pi-pi stacking distances are observed between the quinolate or quinaldine ligands of adjacent complexes (i.e., 3.3-3.5 Angstrom), which facilitates electron transport in the materials. All of the pentacoordinate complexes had high electroluminescent efficiencies at low drive currents. The benzoate derivative exhibits light output saturation at high drive currents when used in electroluminescent devices, while the other derivatives had characteristics comparable to that of Alq(3) at all drive currents.