Theoretical characterization and design of highly efficient iridium (III) complexes bearing guanidinate ancillary ligand

• One synthesized and two designed guanidinate-based iridium (III) complexes are investigated.
• Phosphorescence quantum efficiency is evaluated with the aid of kr and knr values.
• Different cyclometalated ligands can greatly affect the photophysical properties of complexes.
• Changing the pyridine ring with diazole groups can significantly enhance the SOC effects.
• The designed complex 3 is expected to be a promising candidate for highly efficient guanidinate-based phosphorescence emitter.

A density functional theory/time-depended density functional theory was used to investigate the synthesized guanidinate-based iridium(III) complex [(ppy)2Ir{(NiPr)2C(NPh2)}] (1) and two designed derivatives (2 and 3) to determine the influences of different cyclometalated ligands on photophysical properties. Except the conventional discussions on geometric relaxations, absorption and emission properties, many relevant parameters, including spin-orbital coupling (SOC) matrix elements, zero-field-splitting parameters, radiative rate constants (kr) and so on were quantitatively evaluated. The results reveal that the replacement of the pyridine ring in the 2-phenylpyridine ligand with different diazole rings cannot only enlarge the frontier molecular orbital energy gaps, resulting in a blue-shift of the absorption spectra for 2 and 3, but also enhance the absorption intensity of 3 in the lower-energy region. Furthermore, it is intriguing to note that the photoluminescence quantum efficiency (ΦPL) of 3 is significantly higher than that of 1. This can be explained by its large SOC value  (n = 3–4) and large transition electric dipole moment (μS3), which could significantly contribute to a larger kr. Besides, compared with 1, the higher emitting energy (ET1) and smaller 2 value for 3 may lead to a smaller non-radiative decay rate. Additionally, the detailed results also indicate that compared to 1 with pyridine ring, 3 with imidazole ring performs a better hole injection ability. Therefore, the designed complex 3 can be expected as a promising candidate for highly efficient guanidinate-based phosphorescence emitter for OLEDs applications.

Calculation results indicate that upon changing the main ligand of guanidinate-based complex 1 from 2-phenylpyridine to different phenyl-diazoles, significant SOC effects can be observed, leading to higher kr values compared to 1. The designed complex 3 would be considered as a promising candidate for highly efficient guanidinate-based phosphorescence emitter in OLEDs.Figure optionsDownload high-quality image (242 K)Download as PowerPoint slide