Asymmetric [Ir(C1^N1)(C2^N2)(L^X)]-tris-heteroleptic iridium(iii) complexes enable deep blue phosphorescent emission

Abstract

Three novel blue iridium(III) complexes, namely Ir(medfppy)₂(fptz) (Ir-mm), Ir(dfbpy)(medfppy)(fptz) (Ir-dm), and Ir(medfppy)(dfbpy)(fptz) (Ir-md), were prepared and characterized by adopting a new [Ir(C₁^N₁)(C₂^N₂)(LX)]-tris-heteroleptic Ir(III) configuration, where C₁^N₁ and C₂^N₂ denote two different main ligands and LX denotes a third ancillary ligand. These three complexes exhibited exclusively blue emission peaks from 446 to 454 nm, with high photoluminescence quantum yields (PLQYs) of up to 48% and short decay lifetimes ranging from 0.42 to 2.63 μs in degassed CH₂Cl₂ solutions. Density function theory (DFT) calculations showed that the resultant asymmetric isomers, Ir-dm and Ir-md, possessed quite different frontier molecular orbitals when the ligand medfppy in the symmetric complex Ir-mm was replaced by the stronger electron-withdrawing ligand dfbpy. Time-dependent DFT (TD-DFT) calculations suggested that the lowest triplet excited states of the isomers were dominated by the hole-particle transitions and the contributions of ligand-centered transitions (LC) were all higher than 78% for the three complexes. Phosphorescent blue OLEDs based on these complexes exhibited the maximum external quantum efficiencies (EQE_max) of up to 10.2% with the Commission Internationale de l’Eclairage (CIE) coordinates of (0.17, 0.24). This study presents the creative application of tris-heteroleptic configuration to blue iridium complexes for efficient OLEDs.