Trinuclear Cyclometalated Iridium(III) Complex Exhibiting Intense Phosphorescence of an Unprecedented Rate

Herein, we present two novel cyclometalated Ir(III) complexes of dinuclear and trinuclear design, Ir(dppm)(acac) and Ir(dppm)(acac), respectively, where dppm is 4,6-di(4-tert-butylphenyl)pyrimidine ligand and acac is acetylacetonate ligand. In both cases, rac-diastereomers were isolated during the synthesis. The materials show intense phosphorescence of outstanding rates (k_r = Φ_PL/τ) with corresponding radiative decay times of only τ_r = 1/k_r = 0.36 μs for dinuclear Ir(dppm)(acac) and still shorter τ_r = 0.30 μs for trinuclear Ir(dppm)(acac), as measured for doped polystyrene film samples under ambient temperature. Measured under cryogenic conditions, radiative decay times of the three T₁ substates (I, III, and III) and substate energy separations are τ_I = 11.8 μs, τ_II = 7.1 μs, τ_III = 0.06 μs, ΔE(II-I) = 7 cm^-1, and ΔE(III-I) = 175 cm^-1 for dinuclear Ir(dppm)(acac) and τ_I = 3.1 μs, τ_II = 3.5 μs, τ_III = 0.03 μs, ΔE(II-I) ≈ 1 cm^-1, and ΔE(III-I) = 180 cm^-1 for trinuclear Ir(dppm)(acac). The determined T₁ state ZFS values (ΔE(III-I)) are smaller compared to that of mononuclear analogue Ir(dppm)(acac) (ZFS = 210^-1 cm). Theoretical analysis suggests that the high phosphorescence rates in multinuclear materials can be associated with the increased number of singlet states lending oscillator strength to the T₁ → S₀ transition.