A new insight into the temperature induced molecular aggregations in tris(8-hydroxyquinoline) metals

Annealing of tris(8-hydroxyquinoline) gallium (Gaq3) film at various temperatures in dry N2 atmosphere has shown the existences of four different phases of molecular aggregations before the burning out of the film at about 310 °C. The first three phases, up to 235 °C, are amorphous molecular aggregations, while the fourth one at 255 °C is a crystalline structure, very likely α-polymorph. The photoluminescence (PL) intensity was increased to about five times greater than that of the pristine film at 235 °C, while the PL peak was blue shifted consistently. Although a small contribution of Rayleigh scattering cannot be excluded at high temperatures when crystallites appear, the PL blueshift was mainly attributed to the nanostructured molecular aggregations followed by enhanced PL intensity. These new findings can be a common characteristic of organometallic complexes at varied annealing temperatures. The presented results open a new route of fabricating highly emissive thin films of amorphous nanostructure, which are specifically important for organic light emitting diode (OLED) based displays.