C1-Symmetric [Ir(C^N1)(C^N2)(N^O)]-tris-heteroleptic Ir(iii)-complexes with a horizontal orientation for efficient near-infrared (NIR) polymer light-emitting diodes (PLEDs)

Abstract

Using conventional fac-[Ir(C^N)₃]-homoleptic or [Ir(C^N)₂(L^X)]-bis-heteroleptic iridium(III)-complexes with NIR-phosphorescence (NIR = near infrared) as dopants, the realization of reliable NIR-OLEDs/PLEDs (organic/polymer light-emitting diodes) with high performance remains a real challenge. In this study, taking Hqibt (1-(benzo[b]-thiophen-2-yl)-isoquinoline) as the HC^N¹ ligand, Hppy (phenyl-4-yl)pyridine) as the HC^N² ligand and Br-Hpic (5-Br-picolinic acid) or Hpic (picolinic acid) as the N^O-ancillary, two novel C₁-symmetric [Ir(C^N¹)(C^N²)(N^O)]-tris-heteroleptic iridium(III)-complexes [Ir(iqbt)(ppy)(pic)] (1) and [Ir(iqbt)(ppy)(Br-pic)] (2) are molecularly designed, respectively, where large TDM (transition dipole moment) and strengthened ³MLCT (metal-to-ligand charge transfer) effects are established to afford their good NIR-phosphorescent efficiency (Φ_PL = 0.27 for 1 (λ_em = 698 nm); 0.21 for 2 (λ_em = 696 nm)). Moreover, in their doped EMLs (emitting layers), a preferentially horizontal orientation of the TDMs is promoted, due to which, the NIR-PLEDs-1–2 exhibit an attractively high efficiency (η^max_EQE = 3.1–4.7%; λ_em = 698 nm) as well as an almost negligible (<5%) efficiency roll-off. This finding means the C₁-symmetric [Ir(C^N¹)(C^N²)(N^O)]-tris-heteroleptic Ir(III)-complexes show potential as low-cost, large-area and scalable NIR-PLEDs.