Three Types of Charged Ligands Based Carboxyl-Containing Iridium(III) Complexes: Structures, Photophysics, and Solution Processed OLED Application

A novel family of three types of charged (0, -1, -2) ligands based phosphorescent iridium(III) complexes with different carboxyl-containing dianionic (-2) ligands have been synthesized. Their single-crystal structures show that all neutral complexes (Ir1, Ir2, and Ir3) show a trans-N<^>N configuration between dianionic (-2) and monoanionic (-1) ligands, which is in contrast with the trans-N<^>C configuration in cationic complex Ir4, which has an interesting hydrogen bond in the solid state. Notably, Ir4 shows higher luminescence efficiency and an obvious blue shift emission relative to those in Ir1, Ir2, and Ir3. DFT calculations demonstrate that all neutral complexes (Ir1, Ir2, and Ir3) exhibit ligand-to-ligand charge transfer (LLCT) excited state character from the dianionic (-2) ligand to the neutral (0) ligand, which are completely different from the cationic complex Ir4 that exhibits an LLCT excited state from the monoanionic (-1) ligand to the neutral (0) ligand. Considering better solubility, Ir1 was eventually used in solution-processed OLED and achieved moderate efficiency (6.6%, 14.3 cd A(-1), 2.8 lm W-1) with an orange light displaying CIEx,y coordinates of (0.53, 0.46). This work provides a new strategy to construct three types of charged (0, -1, -2) ligands based phosphorescent iridium(III) complexes and extends the range of iridium complex luminescent materials.