Probing photophysical properties of isomeric N-heterocyclic carbene Ir(III) complexes and their applications to deep-blue phosphorescent organic light-emitting diodes

Two isomeric N-heterocyclic carbene Ir(III) complexes, f-Ir-(dbfmi)(3) and m-Ir-(dbfmi)(3), were isolated and systematic investigation of their photophysical and electrochemical properties was carried out. Among them, the crystal structure of the meridional isomer, m-Ir-(dbfmi)(3), is determined. The origin of the phosphorescence emissions differs depending on the configurations of the two isomers. Both isomers, f-Ir-(dbfmi)(3) and m-Ir-(dbfmi)(3), showed efficient emission in dichloromethane solution with quantum yields of 68% and 53%, respectively, due to unusual properties of the N-heterocyclic carbene (NHC) ligand; -that is, the complexes possess a strong metal-ligand bond that destabilizes the nonradiative metal centred ligand-field states. Depending on their configurations, the two isomers showed slightly different excited states. Even though both isomers possess a largely (LC)-L-3 characteristic mixed with the (MLCT)-M-3 characteristic in their excited states, m-Ir-(dbfmi)(3) showed a higher (MLCT)-M-3 characteristic as evidenced by the solvent polarity dependence emission spectra and emission lifetime. DFT calculations clearly support this different photophysical property. Finally, both isomeric complexes were used as dopant materials for the blue PHOLED device. Among the two devices, the f-Ir-(dbfmi) 3 based device (device I) showed relatively high efficiencies with an external quantum efficiency (EQE) of up to 18.5%. Most importantly, a deep-blue Commission Internationale de l'Eclairage (CIE) chromaticity diagram (0.14, 0.11) was obtained when f-Ir-(dbfmi)(3) is used as a dopant.