Theoretical study on the effect of N-substitution on the electronic structures and photophysical properties of phosphorescent Ir(III) complexes

DFT/TDDFT calculations were carried out to investigate the electronic structures, absorption and emission spectra, and phosphorescence efficiency of recent synthesized Ir(III) complexes [Ir(tfmppy)2(tpip)] (1), [Ir(dfppy)2(tpip)] (2) (tfmppy = 4-trifluoromethylphenylpyridine; dfppy = 4,6-difluorophenylpyridine; tpip = tetraphenylimidodiphosphinate). The calculated absorption and emission wavelengths are in agreement with experimental data. The electron transition properties have been analyzed. Compared with 2, the higher quantum yield of experimental observation for 1 was explained by its larger separation between 3MLCT/π→π* and 3MC d–d states. Based on these experimental structures, a series of cyclometalated Ir(III) complexes have been designed by substituting “CH” groups with the N atom at 1-, 2-, 3-, and 4-positions on the pyridine moiety in C^N ligands of 1 and 2, respectively. The calculated results reveal that the N substitution not only tunes the emitting color of 1 and 2 but also enhances photoluminescence quantum efficiency. It is hoped that the designed 1a, 2a and 2c could be potential candidates as blue-emitting materials with high quantum efficiency. Moreover, the effects of different functionals on emission properties were also investigated.