Effects of N‑Substitution on Phosphorescence Efficiency and Color Tuning of a Series of Ir(III) Complexes with a Phosphite Tripod Ligand: A DFT/TDDFT Study

A DFT/TDDFT investigation was applied to understand the unusual properties of the recently synthesized blue-emitting Ir­(III) complexes [Ir­(PMe₂Ph)­(dppit)­(py2pz)] (1) [PMe₂Ph = dimethylphenylphosphine; dppit = diphenyl phenylphosphonite; py2pz = 3,5-di­(2-pyridyl)­pyrazole] and [Ir­(PMe₂Ph)­(dppit)­(bptz)] (2) [bptz =3-tert-butyl-5-(2-pyridyl)­triazolate], which are successfully used as emitters in organic light-emitting diodes (OLEDs). The influence of N-substitution on optical and electronic properties of Ir­(III) complexes was also explored by introducing a N atom on the pyridine moiety of N^∧N ligands for 1 and 2. The calculated results reveal that introduction of N substitution leads to a blue shift for 1a, 1b, 1c, and 1d (a, b, c, d indicate different positions for N substitution) and slightly red shift for 2a–2d in absorption spectra compared with that of 1 and 2, respectively. The N substitution at different positions on N^∧N ligands may also be an efficient approach of tuning emitting color for 1 and 2. The 1-position substituent (1a and 2a) leads to an obvious blue shift of emission spectra compared with 1 and 2, while a significant red shift is observed for the 3-substituted derivatives 1c and 2c. It is believed that the larger ³MLCT–³MC energy gap and higher μ_S1 value, as well as the smaller ΔE_S1–T1 for 1a/2a, are good indications for the higher quantum efficiency compared with that of experimental structures 1/2. These new structure–property relationships can provide improved design and optimization of OLED devices based on blue-emitting phosphorescent Ir­(III) complexes.