A theoretical study on the electronic structures and phosphorescence properties of light-emitting electrochemical cell champions reported in early 2022

Abstract

The electronic structure, absorption properties, phosphorescence quantum efficiency and emission spectra, of a series of iridium(III) complexes containing phenyltriazole-type cyclometallated ligands (1–4) were studied using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The simulated electronic/photophysical properties of the iridium complexes are in good agreement with the experimental observations. The results of the calculations show that the lowest lying singlet absorptions for 1–4 are located at 3.83, 3.91, 3.01, and 2.98 eV, respectively. All complexes show emissions characterised as a mixture of triplet ligand-to-ligand charge transfer (3LLCT) and metal-to-ligand charge transfer (3MLCT) states. The fluorination of the attached phenyl groups (complex 2) and the introduction of –CF3 moieties (complex 3) in the phenyltriazole cyclometallated ligands results in a strong interaction in the T1 states, due to the contracted Ir–Ns bonds with the bpy ancillary ligand. Therefore, both complexes show a moderate contribution of the metal character (%Mc), a lower energy gap between the T1 and the S1 states (ΔET1-S1) together with the largest transition dipole moment (μS1), resulting in an enhanced phosphorescence quantum efficiency. The introduction of –CH3 groups (complex 4) in the phenyltriazole cyclometallated ligands stabilises the triplet metal-centred (3MC) state, resulting in a fast nonradiative decay along with a lower quantum yield. The vertical emission energies calculated at the B3LYP level for the 1–4 complexes are located at 2.60, 2.38, 2.68, and 2.62 eV, respectively. Based on these results, the theoretical calculations can provide an accurate prediction of the electronic and the photophysical properties of iridium complexes, and then can be used to guide the synthesis of new phosphorescent materials.