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-κ C 12 ,κ N 1 ]iridium, R = H, CH 3 , F, and CF 3 ) were prepared by introducing an electron-donating group (EDG; -CH 3 ) or an electron-withdrawing group (EWG; -F and -CF 3 ) 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 3 MLCT/ 3 ILCT 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 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 3 d - d state, the F- and CF 3 -modified Ir(dmp) 3 complexes ( DMP-F and DMP-CF 3 ) 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-CH 3 . A PhOLED device based on DMP-CF 3 [CIE chromaticity (0.17, 0.39)], which demonstrated a distinct 3 MLCT characteristic, exhibited better electroluminescent efficiencies with an external quantum efficiency of 13.5% than that based on DMP-CH 3 .