Substituent effect on the electroluminescence efficiency of amidinate-ligated bis(pyridylphenyl) iridium(III) complexes

This paper reports the synthesis, structure, and photophysical and electrophosphorescence properties of heteroleptic amidinate/bis(pyridylphenyl) iridium(III) complexes having different substituents on the nitrogen atoms of the amidinate ancillary ligands. The reaction of bis(pyridylphenyl) iridium(m) chloride [(ppy)(2)Ir(mu-Cl)](2) with the lithium salt of various amidinate ligands Li{(NR)(NR')CPh} at 80 degrees C gave in 60-80% yields the corresponding heteroleptic bis(pyridylphenyl)/amidinate iridium(m) complexes having a general formula [(ppy)(2)Ir{(NR)(NR')CPh}], where R = R' = 'Pr (1), R = R' = t-Bu (2), R = Et, R' = t-Bu (3), and R = Et, R' = (CH2)(3)N(CH3)(2) (4). These heteroleptic iridium(m) complexes exhibited bright yellowish-green phosphorescence emission with moderate photoluminescence (PL) quantum yields (Phi(PL) = 0.16-0.34) and short phosphorescence lifetimes of 0.98-1.18 mu s in toluene solution at room temperature. Organic light-emitting diodes (OLEDs) were fabricated by the use of these complexes as phosphorescent dopants in various concentrations (x = 5-100 wt%) in the 4,4'-N,N'-dicarbazolylbiphenyl (CBP) host. Because of the steric hindrance of the amidinate ligands, no significant intermolecular interaction was observed in these complexes, thus leading to the reduction of self-quenching and triple-triplet annihilation at high currents/luminance. A significant influence of the substituents in the amidinate ligands on the electroluminescence efficiency was observed. Among these complexes, complex (2), which contains the bulky t-butyl group on the amidinate nitrogen atoms, showed the highest current efficiency (eta(c): up to 116 cd A(-1)), power efficiency (eta(p): up to 72.2 lm W-1) and external quantum efficiency (eta(ext); up to 16.3%).