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 tetrakis-pyrazolyl borate (bor), were employed for the preparation of the Ir(III) complexes [Ir(ppySO₂F)₂(acac)] (Ir-acac), [Ir(ppySO₂F)₂(pic)] (Ir-pic), and [Ir(ppySO₂F)₂(bor)] (Ir-bor). The molecular structures were characterized by X-ray crystallography. Blue phosphorescence maxima were observed at 458, 467, and 478 nm for Ir-bor, Ir-pic, and Ir-acac, 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 1.44μs for Ir-bor, Ir-pic, and Ir-acac, 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 (k_r) are within a narrow range of 3.0-5.5 × 10^-5 s^-1. However, the nonradiative rate constants (k_nr) are within a wide range of 14.2-0.52 × 10^-4 s^-1. The k_nr values showed exponetial correlation with the energy gap. We carried out ab initio 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.