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

• Published in: Journal of materials chemistry. C, Materials for optical and electronic devices Vol. 2; no. 27; pp. 5317 - 5326
• Main Authors: Rai, Virendra Kumar, Nishiura, Masayoshi, Takimoto, Masanori, Hou, Zhaomin
• Format: Journal Article
• Language: English
• Published: 21.07.2014
• Subjects: Chlorides; Ligands; Lithium; Luminance; Nitrogen atoms; Phosphorescence; Reduction
• ISSN: 2050-7526; 2050-7534
• DOI: 10.1039/C3TC32553A

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) 2 Ir(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) 2 Ir{(NR)(NR′)CPh}], where R = R′ = i Pr ( 1 ), R = R′ = t -Bu ( 2 ), R = Et, R′ = t -Bu ( 3 ), and R = Et, R′ = (CH 2 ) 3 N(CH 3 ) 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%).