Aligning π-Extended π-Deficient Ligands to Afford Submicrosecond Phosphorescence Radiative Decay Time of Mononuclear...

Herein, we report a profound investigation of the photophysical properties of three mononuclear Ir(III) complexes (Hdppm-4,6-bis(4-(tert-butyl)phenyl)pyrimidine), Ir(dppm)(acac) (acac-acetylacetonate), and Ir(ppy)(acac) (Hppy-phenylpyridine). The heteroleptic Ir(dppm)(acac) is found to emit with efficiency above 80% and feature a remarkably high rate of emission. As measured under ambient temperature, Ir(dppm)(acac) emits with the unusually short (sub-μs) radiative decay time of τ_r = τ_em/Φ_PL = 1/k_r = 0.91 μs in degassed toluene and τ_r = 0.73 μs in a doped polystyrene film under nitrogen. Investigations at cryogenic temperatures in glassy toluene showed that the emission stems from the T₁ state and thus represents T₁ → S₀ phosphorescence with individual decay times of the T₁ substates of T_1,I = 66 μs, T_1,II = 7.3 μs, T_1,III = 0.19 μs, and energy gaps between the substates of ΔE(T_1,II-T_1,I) = 14 cm^-1 and ΔE(T_1,III-T_1,I) = 210 cm^-1. Analysis of the electronic structure of Ir(dppm)(acac) showed that such a high rate of phosphorescence may stem from the two dppm ligands, with extended π-conjugation system and π-deficient character due to the pyrimidine ring, being serially aligned along one axis. Such alignment, along with the quasi-symmetric character of Jahn-Teller distortions in the T₁ state, affords a large chromophore, comprising four (het)aryl rings of the two dppm ligands. This affords an exceptionally large oscillator strength of the MLCT-character singlet state spin-orbit coupled with the T₁ state and thus brings about enhancement of the phosphorescence rate. These findings reveal molecular design principles paving the way to new phosphors of enhanced emission rates.