Aligning π-Extended π-Deficient Ligands to Afford Submicrosecond Phosphorescence Radiative Decay Time of Mononuclear Ir(III) Complexes

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, Ir2(dppm)3(acac)2 and Ir3(dppm)4(acac)3, 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 (kr = ΦPL/τ) with corresponding radiative decay times of only τr = 1/kr = 0.36 μs for dinuclear Ir2(dppm)3(acac)2 and still shorter τr = 0.30 μs for trinuclear Ir3(dppm)4(acac)3, 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 τI = 11.8 μs, τII = 7.1 μs, τIII = 0.06 μs, ΔE(II-I) = 7 cm-1, and ΔE(III-I) = 175 cm-1 for dinuclear Ir2(dppm)3(acac)2 and τI = 3.1 μs, τII = 3.5 μs, τIII = 0.03 μs, ΔE(II-I) ≈ 1 cm-1, and ΔE(III-I) = 180 cm-1 for trinuclear Ir3(dppm)4(acac)3. The determined T1 state ZFS values (ΔE(III-I)) are smaller compared to that of mononuclear analogue Ir(dppm)2(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.