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)- 4phenylquinoline ( dpq- 3F) ligands have been synthesized and characterized for the application in organic light- emitting diodes ( OLEDs). The heteroleptic triscyclometalated 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 (MLCT)-M-3 state is lower than that of the ppy- centered (MLCT)-M-3 state and because the ILET time from the ppy- centered (MLCT)-M-3 state to the dpq- 3F-centered (MLCT)-M-3 state is shorter than the radiative lifetime of Ir( ppy)(3). A maximum luminous efficiency of eta(c) =13.70 cd/A and power efficiency of eta(p) =10.80lm/W are achieved at a current density of J=0.07mA/cm(2). At a higher current density of J=100mA/cm(2), eta(c) =9.17 cd/A and eta(p) =2.42 lm/W are obtained.