Theoretical study on phosphorescence efficiency and color tuning from orange to blue-green of Ir(III) complexes based on substituted 2-phenylimidazo[1,2-a]pyridine ligand

The geometrical structures, phosphorescence quantum yields, and electroluminescence (EL) efficiency of six iridium(III) complexes containing 2-phenylimidazo[1,2-a]pyridine ligand are investigated by density functional theory (DFT), which show a wide color tuning of photoluminescence from orange (λ_em = 550 nm) to blue-green (λ_em = 490 nm). The calculated results shed some light on the reasons of the remarkably manipulated excited-state and EL properties through substitution effect. The Mulliken charge calculation reveals that attached -CF₃ groups on phenyl and imidazo[1,2-a]pyridine (impy) moieties (4) can make both of them as electron-deficient region, which will lead to the contraction of the whole coordination sphere and strengthen the metal-ligand interaction. While attaching two -CF₃ groups on phenyl ring can make it more electron-deficient, which will induce electron transferring from acac and impy fragment to phenyl ring, and also result in the contracted structure. The largest metal-to-ligand charge transfer (³MLCT) character and the smaller S ₁-T₁ energy gap (ΔE_S1-T1) value increase the emission quantum yields of 4 and 6 than other complexes. For EL efficiency, because of the similar highest occupied molecular orbital (HOMO) levels of 4 and 6 to that of holes injection material poly(N-vinylcarbazole) (PVK) and the larger dipole moments, majority hole will be accumulated on the HOMO of 4 and 6. Combination with the lower lowest unoccupied molecular orbital energy levels compared with PVK, the recombination zones of 4 and 6 can be well confined within emitting material layer (EML) and lead to the higher EL efficiency.