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-dimethylphenyl)-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-(2,6-dimethylphenyl)-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% than that based on DMP–CH3.