Controlling emitting dipole orientations by N^O-ancillary electronic effects of [Ir(C^N)₂(N^O)]-heteroleptic Ir(iii)-complexes towards efficient near-infrared (NIR) polymer light-emitting diodes (PLEDs)

Despite the domination of conventional [Ir(C^N)2(L^X)]-bis-heteroleptic iridium(iii)-complexes with NIR-phosphorescence (NIR = near-infrared) as dopants for reliable NIR-OLEDs/PLEDs (organic/polymer light-emitting diodes), the achievement of high performance remains a real challenge. In this study, taking Hiqbt (1-(benzo[b]-thiophen-2-yl)-isoquinoline) as the C^N ligand and one of the asymmetric Schiff-base ligands with different electronic effects as the N^O-ancillary ligand, three novel C1-symmetric [Ir(C^N)2(N^O)]-bis-heteroleptic iridium(iii)-complexes 1-3 were developed. With contributions from the TDM (transition dipole moment) and the emitting dipole orientation induced by the N^O-ancillary electronic effects, the doped NIR-PLEDs-1-3 exhibited attractive high efficiency (ηmaxEQE = 3.3-5.6%; λem = 708-712 nm) apart from the almost negligible (<5%) efficiency roll-off. This finding shows that for typical [Ir(C^N)2(L^X)]-bis-heteroleptic iridium(iii)-complexes, apart from the (L^X)-incorporated electronic effects on the HOMO-LUMO gap, the L^X-induced TDM vector and the molecular orientation effects on the ΦPL and the ηout (25-29%) should be further taken into account, especially in challenging the intrinsic low NIR-phosphorescent efficiency of their [Ir(C^N)2(L^X)]-bis-heteroleptic iridium(iii)-complexes. This journal is