A Detailed Evaluation for the Nonradiative Processes in Highly Phosphorescent Iridium(III) Complexes

To understand the intrinsic nature of nonradiative processes in heteroleptic cyclometalated Ir­(III) complexes, highly phosphorescent Ir³⁺ complexes containing 2-(3-sulfonylfluorophenyl)­pyridine (ppySO₂F) as the cyclometalated ligand were newly synthesized. Three ancillary ligands, acetylacetonate (acac), picolinate (pic), and <i>tetrakis</i>-pyrazolyl borate (bor), were employed for the preparation of the Ir­(III) complexes [Ir­(ppySO₂F)₂(acac)] (<b>Ir-acac</b>), [Ir­(ppySO₂F)₂(pic)] (<b>Ir-pic</b>), and [Ir­(ppySO₂F)₂(bor)] (<b>Ir-bor</b>). The molecular structures were characterized by X-ray crystallography. Blue phosphorescence maxima were observed at 458, 467, and 478 nm for <b>Ir-bor</b>, <b>Ir-pic</b>, and <b>Ir-acac</b>, respectively, at 77 K, and the corresponding emission quantum yields were determined to be 0.79, 0.80, and 0.98 in anaerobic CH₂Cl₂ at 300 K. Additionally, the phosphorescence decay times were measured to be 3.58, 1.94, and 1.44<i>μ</i>s for <b>Ir-bor</b>, <b>Ir-pic</b>, and <b>Ir-acac</b>, respectively. No temperature dependence was observed for the emission lifetimes in 298–338 K. These results indicate that there is no activation barrier to crossing to a nonradiative state like metal-centered (MC, d–d) state. The radiative rate constants (<i>k</i>_r) are within a narrow range of 3.0–5.5 × 10^–5 s^–1. However, the nonradiative rate constants (<i>k</i>_nr) are within a wide range of 14.2–0.52 × 10^–4 s^–1. The <i>k</i>_nr values showed exponetial correlation with the energy gap. We carried out <i>ab</i> <i>initio</i> calculations to evaluate the energy states and their corresponding orbitals. The nonemissive MC states lie at higher energies than the emissive metal-to-ligand charge transfer (MLCT) state, and hence, the MC states can be excluded from the nonradiative pathway