Positional Isomeric Cyano-Substituted Bis(2-phenylpyridine)(acetylacetonate)iridium Complexes for Efficient Organic Light-Emitting Diodes with Extended Color Range

Bis(2-phenylpyridine)(acetylacetonate)iridium,
Ir(ppy)2(acac), is a benchmark green emitter for phosphorescent
organic light-emitting
diodes (PhOLEDs). In this work, we reported three positional isomeric
cyano-substituted Ir(ppy)2(acac) complexes, i.e., Ir(3-CN),
Ir(4-CN), and Ir(10-CN), with the emission in the yellow to red region
(544–625 nm). Through theoretical investigation and single-crystal
analysis, it was found that the introduction of cyano substitution
at various positions of the ppy ligand allows for tuning the electron
distribution and coordination bond length of Ir complexes. Therefore,
the charge transfer property of Ir complexes is enhanced such that
the energy gap of the cyano-substituted Ir(ppy)2(acac)
complexes was reduced. In addition, Ir(3-CN), Ir(4-CN), and Ir(10-CN)
exhibited high PLQYs of 83, 54, and 75%, respectively, with the phosphorescence
lifetime in the range of 0.79–2.08 μs. Notably, the device
utilizing Ir(3-CN) as the emitter exhibited a maximum external quantum
efficiency (EQE) of 25.4%, current efficiency of 56.9 cd A–1, power efficiency of 68.7 lm W–1, and brightness
of 61,340 cd m–2 at 8 V. The EQE of this device
remained 24.3 and 19.9% at luminances of 1,000 and 10,000 cd m–2, corresponding to the efficiency roll-off of 4.3
and 21.7%, respectively. Comparing to the Ir complexes using the ligand
with an extended conjugated structure, our results demonstrated a
simple molecular design strategy for phosphorescence emitters with
reduced molecular weight for efficient PhOLEDs in the yellow to red
color region.