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

Abstract

Herein, we report a profound investigation of the photophysical properties of three mononuclear Ir(III) complexes fac-Ir(dppm) 3 phenyl)pyrimidine), Ir(dppm) 2 (acac) (acacacetylacetonate), 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 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) 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.

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

“…In both cases, the emission spectrum strongly overlaps with the lowest energy absorption band (Figure 1). Tentatively assigning the observed emission to T 1 → S 0 phosphorescence, similarly to that in mononuclear Ir-(dppm) 2 (acac), 21 such an spectral overlap is evident of a relatively small energy separation of states S 1 and T 1 in Ir 2 (dppm) 3 (acac) 2 and Ir 3 (dppm) 4 (acac) 3 . With the help of TD-DFT calculations (see below), we substantiate that this is a The absorption spectrum was measured in toluene under ambient conditions.…”

“…Chart 1. Chemical Structures of Ir(dppm) 2 (acac), Reported Previously,21 and of the Dinuclear Ir 2 (dppm) 3 (acac) 2 and Trinuclear Ir 3 (dppm) 4 (acac) 3 ,…”

“…The emission decay time constants and quantum yields, measured for degassed toluene solutions (c ≈ 10 The obtained radiative decay time value of the dinuclear Ir 2 (dppm) 3 (acac) 2 is in the range of the fastest emitting dinuclear Ir(III) complexes reported so far and about two times shorter than that of the mononuclear Ir(dppm) 2 (acac) at the same conditions. 21 Meanwhile, the radiative decay time of trinuclear Ir 3 (dppm) 4 (acac) 3 of τ r = 0.33 μs is unprecedentedly short for T 1 → S 0 phosphorescence, making the complex the fastest phosphorescing material known so far. At T = 77 K in frozen toluene glass, phosphorescence efficiency increases to unity for Ir 2 (dppm) 3 (acac) 2 and reaches 0.9 (90%) for Ir 3 (dppm) 4 (acac) 3 (Table 1), which is explained by the significantly reduced rate of nonradiative relaxation processes in rigid media at low temperatures.…”

“…τ(I) = τ(1.6 K) = 11.8 μs suggests the following parameter values for the best fit: τ(II) = 7.1 μs, τ(III) = 0.06 μs (60 ns), ΔE(II−I) = 7 cm −1 and ΔE(III−I) = 175 cm −1 . According to these data, dinuclear Ir 2 (dppm) 3 (acac) 2 has notably faster emitting T 1 substates I and III, compared to those of mononuclear Ir(dppm) 2 (acac) (τ(I) = τ(1.6 K) = 66 μs, τ(II) = 7.3 μs, τ(III) = 0.19 μs, ΔE(II−I) = 14 cm −1 and ΔE(III−I) = 210 cm −1) 21. A similar study of trinuclear Ir 3 (dppm) 4 (acac) 3 is shown in Figure3b.…”

Theoretical investigation of the influence of heterocycles on the radiative and non-radiative decay processes of iridium(iii) complexes

“…Because of their good color purities, high quantum efficiencies, relatively short phosphorescence lifetimes and high thermal stabilities, iridium(III) complexes are being applied as phosphorescent materials for OLEDs. [1][2][3][4][5][6][7][8][9][10][11] For Ir(III)cyclometalate complexes, the metal-to-ligand charge transfer ( 3 MLCT) involved in the emissive triplet excited state and the rigid octahedral structure can effectively facilitate the radiative decay process and suppress the non-radiative decay route. In addition, the color purities of iridium(III) complexes can be tuned via the addition of various substituents.…”