Effects of systematic methyl substitution of metal (III) tris(n-methyl-8-quinolinolato) chelates on material properties for optimum electroluminescence device performance.
We relate the chemical structure of a series of methyl (Me) substituted group III metal tris(8-quinolinolato) chelates (nMeq(3)M: n = 0, 3, 4, 5; M = Al(3+), Ga(3+)) to their photoluminescence (PL), electroluminescence, and thermal properties. Methylation of the 8-quinolinol ligand at the 3 or 4 position (pyridyl ring) results in a factor of 1.4 and 3.0 enhancement of PL quantum efficiency (phi(PL)), respectively, whereas methylation at the 5 position (phenoxide ring) results in a factor of approximately 3.0 decrease in phi(PL) relative to the unsubstituted analogue. Electroluminescent quantum efficiencies of undoped organic light-emitting devices using the aluminum tris(8-quinolinolato) chelates are 1, 0.45, 1.4, and 0.80% for unsubstituted 5-, 4-, and 3-methyl-8-quinolinol ligands, respectively. Devices made with the latter two ligands have a higher operating voltage to generate the same current density. Similar trends were observed for methylation of gallium tris(8-quinolinolato) chelates. We relate these results to the thermal properties of the compounds measured by simultaneous differential scanning calorimetry and thermal gravimetric analysis. The C-4 methylated derivatives exhibit approximately 60 degrees C lower crystalline melting points than all other derivatives, indicating the weakest cohesive forces between molecules. Unlike Alq(3), both the C-4 and C-5 methylated derivatives show no recrystallization of the glassy state below 500 degrees C and exhibit approximately 20-25 degrees C higher glass transition temperatures. We infer that methylation of the 8-quinolinol ligand reduces intermolecular interactions and consequently impedes charge transport through the film.