Tuning the Photophysical Properties of Homoleptic Tris-Cyclometalated Ir(III) Complexes by Facile Modification of the Imidazo-Phenanthridine and Their Application to Phosphorescent Organic Light-Emitting Diodes

To explore the excited-state electronic structure of the blue-emitting Ir(dmp)₃ dopant material (dmp = 3-(2, 6-dimethylphen-yl)-7-methylimidazo[1, 2-f]phenanthridine), which is notable for durable blue phosphorescent organic light-emitting diode (PhOLED), a series of homoleptic dmp-based Ir(III) complexes (DMP−R, tris[3-(2, 6dimethylphenyl)-7-R-imidazo[1, 2-f]phenanthridin-12-yl-κC¹², κN¹]iridium, R = H, CH₃, F, and CF₃) were prepared by introducing an electron-donating group (EDG; −CH₃) or an electron-withdrawing group (EWG; −F and −CF₃) at the 7-position of the imidazo-phenanthridine ligand. The photophysical analysis demonstrated that the alteration from EDG to EWGs led to redshifted structureless emission profiles, which were correlated with variations in the ³MLCT/³ILCT ratio in the T₁ excited state. From electrochemical studies and density functional theory calculations, it turned out that the excited-state nature of the dmp-based Ir(III) complexes was significantly affected by the inductive effect of the 7-substituent of the cyclometalating dmp ligand. As a result of the lowest unoccupied molecular orbital energy stabilization by the EWGs that suppressed the non-radiative pathway from the emissive triplet excited state to the ³d−d state, the F- and CF₃-modified Ir(dmp)₃ complexes (DMP−F and DMP−CF₃) showed quantum yields of 27−30% in the solution state, which were at least 4- or 5-fold higher than those shown by DMP−H and DMP−CH₃. A PhOLED device based on DMP−CF₃ [CIE chromaticity (0.17, 0.39)], which demonstrated a distinct ³MLCT characteristic, exhibited better electroluminescent efficiencies with an external quantum efficiency of 13.5%