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 (kr) are within a narrow range of 3.0–5.5 ×
10–5 s–1. However, the nonradiative
rate constants (knr) are within a wide
range of 14.2–0.52 × 10–4 s–1. The knr 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.