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

Herein, we report a profound investigation of the photophysical properties of three mononuclear Ir(III) complexes fac-Ir(dppm)3 (Hdppm-4,6-bis(4-(tert-butyl)phenyl)pyrimidine), Ir(dppm)2(acac) (acac-acetylacetonate), and Ir(ppy)2(acac) (Hppy-phenylpyridine). The heteroleptic Ir(dppm)2(acac) is found to emit with efficiency above 80% and feature a remarkably high rate of emission. As measured under ambient temperature, Ir(dppm)2(acac) emits with the unusually short (sub-μs) radiative decay time of τr = τem/ΦPL = 1/kr = 0.91 μs in degassed toluene and τr = 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 T1,I = 66 μs, T1,II = 7.3 μs, T1,III = 0.19 μs, and energy gaps between the substates of ΔE(T1,II–T1,I) = 14 cm–1 and ΔE(T1,III–T1,I) = 210 cm–1. Analysis of the electronic structure of Ir(dppm)2(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.