Molecular Engineering of Iridium Blue Emitters Using Aryl N-Heterocyclic Carbene Ligands

The synthesis of a new series of neutral bis[2-(2,4-difluorophen-2-yl)pyridine][1-(2-aryl)-3-methylimidazol-2-ylidene]iridium(III) complexes is reported. Each complex has been characterized by NMR spectroscopy, UV/Vis spectrophotometry, and cyclic voltammetry, and the photophysical properties examined in depth. Furthermore, two of the complexes have been characterized by single-crystal X-ray diffraction analysis. By systematically modifying the cyclometalating aryl group on the N-heterocyclic carbene (NHC) ligand from 2,4-dimethoxyphenyl to 6-methoxy-2-methyl-3-pyridyl, the energy levels of the Ir complexes were modified to produce new blue emitters with increased HOMO and triplet-state energies. OLED devices fabricated with these emitters showed external quantum efficiencies (EQEs) in the range of 2.3-3.2% with low turn-on values of 2.7-2.9 V and efficacies of up to 6.3%.