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 Φp (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 ΦP of nearly up to unity. Optimal orange-emitting device B2 based on AIrC8 can give a maximum external quantum efficiency (ηext) of 23.9%, a maximum current efficiency (ηL) of 70.9 cd A-1, and a maximum power efficiency (ηP) 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.