Shedding Light on the Photophysical Properties of Iridium(III) Complexes with N‑Heterocyclic Carbene Ligands from a Theoretical Viewpoint

The phosphorescent efficiencies of the Ir­(III) carbene complexes 1–3 with wide-range color tuning were focused on in this work. A DFT/TDDFT (density functional theory/time-dependent density functional theory) investigation on the geometries in the ground and lowest triplet excited states, the frontier molecular orbitals, the absorption spectra, and d-orbital splittings of 1–3 were provided to get a better understanding of structure–property relationships. Importantly, to shed light on the difference in phosphorescent quantum yields for 1–3, radiative decay constants as well as zero-field-splitting parameters were calculated based on the estimation of spin–orbit coupling (SOC) matrix elements denoted as ⟨T₁^α|H_SOC|S_n⟩. The results show that, for any complex, the radiative decay rates in the three substates (namely, T_x, T_y, and T_z) are not equal, and the largest radiative rates of 1–3 are all located in x substates with values of 1.0764 × 10⁴, 0.8231 × 10⁴, and 1.9596 × 10⁴ s^–1, respectively. Moreover, for 3 with the highest quantum efficiency, we make efforts to modify it through varying substituents and substituent positions not only to achieve blue shift in the emission but also to obtain improved triplet energy.