DFT/TDDFT investigation on the electronic structures and photophysical properties of phosphorescent iridium(III) complexes with 2-(pyridin-2-yl)- benzo[d]imidazole ligand

We have reported a theoretical analysis of a series of heteroleptic iridium(III) complexes (mpmi)₂Ir(pybi) [mpmi=1-(4-tolyl)-3-methyl- imidazole, pybi=2-(pyridin-2-yl)-benzo[d]imidazole] (1a), (fpmi) ₂Ir(pybi) [fpmi=1-(4-fluoro-phenyl)-3-methyl-imidazole] (1b), (tfpmi)₂Ir(pybi) [tfpmi=1-methyl-3-(4-trifluoromethyl-phenyl)- imidazole] (1c), (pypmi)₂Ir(pybi) [pypmi=3-(3-methyl-imidazol)- pyrazole] (2a), (phpymi)₂Ir(pybi) [phpymi=3-(3-methyl-imidazol)-5- phenyl-pyrazole] (2b), and (inpymi)₂Ir(pybi) [inpymi=3-(3-methyl- imidazol)-indeno[1,2-c]pyrazole] (2c) by using the density functional theory (DFT) method to investigate their electronic structures and photophysical properties and obtain further insights into the phosphorescent efficiency mechanism. By changing cyclometalated ligands, the conjugation length, and substituents of the cyclometalated ligands, one can tune the emission color from green (λ_em=520 nm) to orange (λ_em=592 nm). Complexes 1a, 1b, 2a, and 2b have the almost identical emission wavelength about 550 nm, while 592 nm for 1c and 520 nm for 2c are red shifted and blue shifted, respectively, relative to 1a. The calculated results indicate that, for 1b and 1c, the substituents of -F and -CF₃ at the phenyl moiety cause a poor hole-injection ability compared with that of 1a. For all these complexes studied, the hole-transporting performances are better than the electron-transporting ones. The difference between reorganization energies for hole transport (λ_ih) and reorganization energies for electron transport (λ_ie) for complex 1c are relatively smaller, indicating that the hole and electron transfer balance could be achieved more easily in the emitting layer. The alteration of cyclometalated ligands with different conjugation lengths and substituents has an impact on the optoelectronic properties of these complexes. It is believed that the larger metal to ligand charge transfer (MLCT) character, the highest μ _S1 and E_T1 values, and the smallest ΔE_S1-T1 value could result in the larger phosphorescent quantum efficiency for 2c than other complexes.