Pyrimidine-Based Mononuclear and Dinuclear Iridium(III) Complexes for High Performance Organic Light-Emitting Diodes

Containing two nitrogen
atoms, the electron-deficient pyrimidine ring has excellent coordinating
capability with transition metal ions. However, compared with the
widely used pyridine ring, applications of the pyrimidine ring in
phosphorescent Ir­(III) complexes are rare. In this research, two highly
emissive pyrimidine-based mononuclear Ir­(III) complexes and their
corresponding dinuclear Ir­(III) complexes were prepared with a simple
one-pot reaction. The incorporation of the second Ir­(III) center can
lead to dramatic differences of both photophysical and electrochemical
properties between the mono- and dinuclear complexes. Besides, these
properties can also be fine-tuned with different substituents. Theoretical
calculations have also been performed to understand their photophysical
behaviors. The electroluminescent investigations demonstrate that
the pyrimidine-based mono- and dinuclear Ir­(III) complexes could show
impressive device performance. The vacuum-deposited organic light-emitting
diode (OLED) based on the mononuclear Ir­(III) complex exhibited an
external quantum efficiency (EQE) of 16.1% with almost no efficiency
roll-off even at 10 000 cd m–2. More encouragingly,
the solution-processed OLED based on the dinuclear Ir­(III) complex
achieved the outstanding EQE, current efficiency (CE), and power efficiency
(PE) of 17.9%, 52.5 cd A–1, and 51.2 lm W–1, respectively, representing the highest efficiencies ever achieved
by OLEDs based on dinuclear Ir­(III) complexes.