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)

Abstract

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 (η^max_EQE = 3.3–5.6%; λ_em = 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 Φ_PL and the η_out (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.