Density functional theory investigation on iridium(iii) complexes for efficient blue electrophosphorescence

Abstract

The geometrical structures, electronic structures, optoelectronic properties and phosphorescence efficiencies of four blue-emitting phosphors [Ir(fpmi)₂(pyim)] (1), [Ir(pyim)₂(fpmi)] (2), [Ir(fpmi)₂(fptz)] (3), [Ir(tfmppz)₂(pyim)] (4), [fpmi = 1-(4-fluorophenyl)-3-methylimdazolin-2-ylidene-C,C²′; pyim = 2-(1H-imidazol-2-yl)pyridinato; fptz = 5-(trifluoromethyl-2H-1,2,4-triazol-3-yl)pyridine; tfmppz = 1-(4-trifluoromethylphenyl)pyrazolyl] were investigated by DFT and TDDFT methods. We first optimized geometrical structures in the ground and lowest triplet states, and computed the absorption and emission spectra of 1 and 5 [Ir(fpmi)₂(pypz)] [pypz = 2-(1H-pyrazol-5-yl)pyridinato], which have been synthesized and characterized in a laboratory, using three functionals, B3LYP, CAM-B3LYP, and M062X. The calculation results were compared with relevant experimental data to assess the performance of the functionals. The suitable methods and functionals were then applied to study properties of the three other complexes. The HOMOs of 1–3 are composed of d(Ir) and π(cyclometalated ligands), however, the HOMO of 4 resides on the pyim ligand, while the LUMOs of all four complexes are dominantly localized on the chelating ligands. The calculated absorption results show that the corresponding absorption peaks for the four mainly studied complexes are almost at the same positions, however, the absorption intensities of the bands differ largely from each other. The lowest energy emissions of the four complexes are localized at 507, 512, 468, and 513 nm, respectively. In order to estimate their efficiencies, we carried out simplified radiative rate constant calculations. It turns out that complex 3, which possesses the shortest emission wavelength and the largest radiative rate constant (k_r) value, can be considered as a highly efficient blue-emitting iridium(III) complex.