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

Abstract

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-kappa C-12,kappa N-1]-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 imidazophenanthridine 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 (MLCT)-M-3/(ILCT)-I-3 ratio in the T-1 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 cydometalating 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 (3)d-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 (MLCT)-M-3 characteristic, exhibited better electroluminescent efficiencies with an external quantum efficiency of 13.5% than that based on DMP-CH3.