Unsymmetric Heteroleptic Ir(III) Complexes with 2‑Phenylquinoline and Coumarin-Based Ligand Isomers for Tuning Character of Triplet Excited States and Achieving High Electroluminescent Efficiencies

2-Phenylquinoline (PQ) and four coumarin-based ligand isomers with ease of synthesis have been selected to construct the unsymmetric heteroleptic [Ir­(C₁^∧N)­(C₂^∧N)­(acac)]-type complex phosphors for organic light-emitting diodes (OLEDs). Six unsymmetric heteroleptic Ir­(III) complexes have been obtained by employing four coumarin-based ligand isomers (L-C5/L-C6/L-C7/L-C8) in the [Ir­(PQ)­(C^∧N)­(acac)] structure due to two different coordinating carbon atoms in ligands L-C6 and L-C7 to form C–Ir bond. Through adopting unsymmetric heteroleptic [Ir­(C₁^∧N)­(C₂^∧N)­(acac)] structure, these Ir­(III) complexes can not only achieve impressive absolute quantum yield Φ_p (ca. 0.5–1.0), higher than that of complex [Ir­(PQ)₂(acac)] (ca. 0.4), but also realize a dual modulation of both emission color from orange (AIrC6out, λ = 578 nm) to red (AIrC5, λ = 622 nm) and the character of the lowest triplet excited states (T₁), showing both ³MLCT character and ³ILCT (intraligand charge transfer) character in their T₁ states. AIrC5, AIrC7out, and AIrC7in show MLCT character from Ir­(III) center to ligand L-C5 or L-C7 and ILCT character in ligand L-C5 or L-C7 in their T₁ states, while AIrC6out, AIrC6in, and AIrC8 show MLCT character from Ir­(III) center to ligand PQ and ILCT character in ligand PQ in their T₁ states. Moreover, the color-tuning mechanism and the lowest triplet state characters are investigated in detail. AIrC6in and AIrC8 were selected as emitters to evaluate the electroluminescent (EL) performance due to their high Φ_P of nearly up to unity. Optimal orange-emitting device B2 based on AIrC8 can give a maximum external quantum efficiency (η_ext) of 23.9%, a maximum current efficiency (η_L) of 70.9 cd A^–1, and a maximum power efficiency (η_P) of 60.7 lm W^–1. All these impressive results can definitely demonstrate the effectiveness of our simple approach for tuning character of the triplet excited states and achieving high-performance Ir-based phosphors in OLEDs.