The effect of CN-substitution on the electronic and photophysical properties of bis(carbene) Ir(III) complexes containing 2-(1H-pyrazol-5-yl)pyridinato ancillary ligand: A theoretical perspective

The electronic structures and photophysical properties of four heteroleptic Ir(III) complexes 1 [(Mepmi)2Ir(pypz)], 2 [(CNpmi)(Mepmi)Ir(pypz)], 3 [(Mepmi)(CNpmi)Ir(pypz)] and 4 [(CNpmi)2Ir(pypz)] (where Mepmi = 1-(4-methyl-phenyl)-3-methylimdazolin-2-ylidene-C,C2′; CNpmi = 1-(4-cyano-phenyl)-3-methylimdazolin-2-ylidene-C,C2′; pypz = 2-(1H-pyrazol-5-yl)pyridinato) are investigated using the density functional method. The influence of different positions and numbers of CN-substitution has been explored. The calculated results reveal that the introduction of CN substituent leads to lowered HOMO and LUMO energy levels, increased HOMO–LUMO energy gaps, and the enhanced intensity of the absorption spectra. Meanwhile, the CN-substitution in carbene ligand is found to be an efficient approach of tuning the emitting color. The one CN substituent in 2 leads to an obvious blue shift of emission spectra, while a slight red shift is observed for the 3 and 4 in comparison with 1. The qualitative analysis on the quantum yields of four complexes has been carried out with the assistance of Δ E S 1 − T 1 (singlet–triplet energy splitting), MLCT% (metal-based charge transfer contribution in T1 state) and μ S 1 (S0 → S1 transition electric dipole moment). It showed that the designed complex 2 and 3 with one CN substituent in carbene ligand may be the potential blue-emitting materials and possess high quantum efficiency.