DFT and TD-DFT study on the electronic structures and phosphorescent properties of a series of heteroleptic iridium(III) complexes

The electronic structures and phosphorescent properties of a series of heteroleptic iridium(III) complexes (mpmi)₂Ir(dmpypz) (1; mpmi = 1-(4-tolyl)-3-methylimidazolium, dmpypz = 3,5-dimethyl-2-(pyrazol-3-yl)pyridine) , (bpmi)₂Ir(dmpypz) (2; bpmi = 1-biphenyl-4-yl-3-methylimidazole), (dfmi)₂Ir(dmpypz) (3; dfmi = 1-(2,6-difluorobiphenyl)-3- methylimidazole), (mtmi)₂Ir(dmpypz) (4; mtmi = 1-methyl-3-(4′- (trifluoromethyl)biphenyl-4-yl)imidazole), (fmmi)₂Ir(dmpypz) (5; fmmi = 1-(fluoren-2-yl)-3-methylimidazole), and (mhmi)₂Ir(dmpypz) (6; mhmi = 1-methyl-3-phenanthren-2-ylimidazole) have been investigated by using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. The influence of different substituent groups and π-conjugation degrees on the optical and electronic properties of Ir(III) complexes was also explored by introducing phenyl, fluorophenyl, (trifluoromethyl)phenyl, and rigid construction on the phenylimidazole moiety of a cyclometalated ligand (C^̂C) in complex 1. The calculated results show that the lowest energy absorption wavelengths of complexes 1-6 are 387, 380, 378, 375, 391, and 384 nm, respectively. The introduction of different substituent groups leads to different degrees of red shift for complexes 2-6 in emission spectra in comparison with that of complex 1. It is believed that the highest triplet metal to ligand charge transfer ³MLCT (%) contribution, smallest ΔE_S1-T1 and higher μ_S1 values, and larger ³MC-³MLCT energy gap for 3 ensure its higher quantum yield in comparison with that of other complexes.