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 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 T 1 state and thus represents T 1 → S 0 phosphorescence with individual decay times of the T 1 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 Δ E (T 1,II -T 1,I ) = 14 cm -1 and Δ E (T 1,III -T 1,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 T 1 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 T 1 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.