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(C 1 ∧ N)(C 2 ∧ 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(C 1 ∧ N)(C 2 ∧ 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 (T 1 ), showing both 3 MLCT character and 3 ILCT (intraligand charge transfer) character in their T 1 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 T 1 states, while AIrC6out, AIrC6in, and AIrC8 show MLCT character from Ir(III) center to ligand PQ and ILCT character in ligand PQ in their T 1 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.