Isomers of Coumarin-Based Cyclometalated Ir(III) Complexes with Easily Tuned Phosphorescent Color and Features for Highly Efficient Organic Light-Emitting Diodes

Abstract

Three Ir(C^∧N)₂(acac)-type and one Ir(C₁^∧N)(C₂^∧N)(acac)-type coumarin-based cyclometalated Ir(III) complex isomers (IrC5, IrC7, IrC7-A, and IrC8) have been obtained using three coumarin-based isomers of 2-phenylpyridine (ppy)-type cyclometalating ligands (L-C5, L-C7, and L-C8). Two coordination isomers emerging as principal products (IrC7 and IrC7-A) are obtained in the synthesis of corresponding coumarin-based cyclometalated Ir(III) complexes because of two different coordination sites in ligand L-C7 to form a C–Ir bond. To the best of our knowledge, there are no such isomers reported to date. Interestingly, a broad range of phosphorescent color tuning from green (IrC8, λ = 516 nm) to red (IrC5, λ = 608 nm) has been realized through variation of the pyridyl substitution positions on the fused phenyl ring of the coumarin skeleton. In addition, based on natural transition orbital (NTO) analyses, features of the lowest triplet excited states (T₁) from these coumarin-based cyclometalated Ir(III) complex isomers can be tuned easily by these ligand isomers as well. IrC5, IrC7, and IrC7-A show prevailing ³MLCT character associated with their T₁ states which emit the phosphorescent signals, while the T₁ state of IrC8 exhibits the dominant ligand-centered π–π* transition feature. Importantly, owing to the strong rigidity of the coumarin skeleton, all the coumarin-based cyclometalated Ir(III) complex isomers can show high phosphorescent quantum yields Φ_p (ca. 0.4–1). Together with the improved electron-injection/electron-transport (EI/ET) ability, all the phosphorescent emitters display impressive electroluminescence (EL) performance. The device based on IrC8 gives the highest EL efficiencies of external quantum efficiency (η_ext) 22.7%, current efficiency (η_L) 79.7 cd A^–1, and power efficiency (η_P) 58.2 lm W^–1, representing the most state-of-the-art EL ability ever achieved by coumarin-based phosphorescent emitters. All these encouraging data definitely suggest the great potential of the coumarin skeleton in both easy tuning of the photophysical properties of ppy-type Ir(III) phosphorescent complexes and developing high-performance phosphorescent emitters.