Inter-Ligand Energy Transfer of Heteroleptic Tris-Cyclometalated Iridium Complexes and Their Applications to OLEDs

Abstract

Novel red phosphorescent heteroleptic tris-cyclometalated iridium complex, Ir(ppy)2(dpq-3F) based on 2-phenylpyridine (ppy) and 2-(3-fluorophenyl)-4-phenylquinoline (dpq-3F) ligands have been synthesized and characterized for the application in organic light-emitting diodes (OLEDs). The heteroleptic tris-cyclometalated iridium complex leads to a significant improvement in a luminous efficiency at high currents to avoid the T–T annihilation by the exciton transfer from two ppy ligands to one luminescent dpq-3F ligand decreasing the number of the luminescent ligand. This inter-ligand energy transfer (ILET) can occur because the dpq-3F-centered 3MLCT state is lower than that of the ppy-centered 3MLCT state and because the ILET time from the ppy-centered 3MLCT state to the dpq-3F-centered 3MLCT state is shorter than the radiative lifetime of Ir(ppy)3. A maximum luminous efficiency of ηc = 13.70 cd/A and power efficiency of ηp = 10.80 lm/W are achieved at a current density of J = 0.07 mA/cm2. At a higher current density of J = 100 mA/cm2, ηc = 9.17 cd/A and ηp = 2.42 lm/W are obtained.