Red-light-emitting iridium complexes with hole-transporting 9-arylcarbazole moieties for electrophosphorescence efficiency/color purity trade-off optimizationE

The synthesis, structures, photophysics, electrochemistry and electrophosphorescent properties of new red phosphorescent cyclometalated iridium(III) isoquinoline complexes, bearing 9-arylcarbazolyl chromophores, are reported. The functional properties of these red phosphors correlate well with the results of density functional theory calculations. The highest occupied molecular orbital levels of these complexes are raised by the integration of a carbazole unit to the iridium isoquinoline core so that the hole-transporting ability is improved in the resulting complexes relative to those with I-phenylisoquinoline ligands. All of the complexes are highly thermally stable and emit an intense red light at room temperature with relatively short lifetimes that are beneficial for highly efficient organic light-emitting diodes (OLEDs). Saturated red OLEDs, fabricated using these dyes as the phosphorescent dopants both as vacuum-evaporated and spin-coated emissive layers, have been achieved in a multi-layer configuration with outstanding red color purity at Commission International de L'Eclairage (CIE) coordinates of (0.67,0.33) to (0.68,0.32). Some of the devices can show very high efficiencies with a maximum external quantum efficiency of up to 12 % photons per electron. The excellent performance of these red emitters indicates the advantage of the carbazole module in the ligand framework; demonstrated by an improved hole-transporting ability that facilitates exciton transport. These materials could thus provide a new avenue for the rational design of heavy-metal electrophosphors that reveal a superior device efficiency/color purity trade-off necessary for pure red-light generation.