Iridium(III) complexes adopting thienylpyridine derivatives for yellow-to-deep red OLEDs with low efficiency roll-off

Abstract

By introduction of trifluoromethyl and phenyl groups to 2-(2-thienyl)pyridine (thp), four new phosphorescent bis-cyclometalated iridium(III) complexes, (thp)2Ir(tpip), (cf3thp)2Ir(tpip), (cf3btp)2Ir(tpip) and (3-cf3btp)2Ir(tpip) (cf3thp = 2-(thiophen-2-yl)-4-(trifluoromethyl)pyridine, cf3btp = 2-(benzo[b]thiophen-2-yl)-4- (trifluoromethyl)pyridine, 3-cf3btp = 2-(benzo[b]thiophen-3-yl)-4-(trifluoromethyl)pyridine, tpip = tetraphenylimidodiphosphinate), were synthesized and fully characterized. The density functional theory and time-dependent DFT calculations show that the frontier orbitals are mainly localized in the Ir(III) ion and the cyclometalated ligands. Thus, the photophysical properties were dominated by the cyclometalated C∧N ligand, attributed to 3MLCT and 3ILCT transition. These Ir(III) complexes emit in the yellow-to-deep red region with photoluminescence quantum yields in the range 40.5–86.4% in CH2Cl2 solutions at 298 K. The organic light-emitting diodes (OLEDs) using (thp)2Ir(tpip), (cf3thp)2Ir(tpip) and (cf3btp)2Ir(tpip) as yellow, orange and deep red emitters display good electroluminescent performance with low efficiency roll-off. Notably, the device based on (cf3thp)2Ir(tpip) possesses very high EL efficiencies with the maximum luminance efficiency and external quantum efficiency (EQE) of 53.9 cd A−1 and 17.9%, respectively. Furthermore, the EQE for this complex could be still retained as 15.4% at a luminance of 1000 cd m−2.