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

Abstract

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(III) chloride [(ppy)₂Ir(μ-Cl)]₂ with the lithium salt of various amidinate ligands Li{(NR)(NR′)CPh} at 80 °C gave in 60–80% yields the corresponding heteroleptic bis(pyridylphenyl)/amidinate iridium(III) complexes having a general formula [(ppy)₂Ir{(NR)(NR′)CPh}], where R = R′ = ⁱPr (1), R = R′ = t-Bu (2), R = Et, R′ = t-Bu (3), and R = Et, R′ = (CH₂)₃N(CH₃)₂ (4). These heteroleptic iridium(III) complexes exhibited bright yellowish-green phosphorescence emission with moderate photoluminescence (PL) quantum yields (Φ_PL = 0.16–0.34) and short phosphorescence lifetimes of 0.98–1.18 μs 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 (η_c: up to 116 cd A⁻¹), power efficiency (η_p: up to 72.2 lm W⁻¹) and external quantum efficiency (η_ext; up to 16.3%).