Tuning the electronic properties and quantum efficiency of blue Ir(iii) carbene complexes via different azole-pyridine-based N^N′ ligands

Abstract

The electronic structures and photophysical properties of a series of heteroleptic Ir(III) carbene complexes (fpmi)_3−xIr(N^N′)_x (x = 1, 2) [fpmi = 1-(4-fluorophenyl)-3-methylimidazolin-2-ylidene-C,C^2′, N^N′ = 2-(1H-pyrrol-2-yl)pyridinato (1a(x = 1)/1a′(x = 2)); 2-(1H-pyrazol-5-yl)pyridinato (2a/2a′); 2-(1H-1,2,4-triazol-5-yl)pyridinato (3a/3a′); 2-(1H-tetrazol-5-yl)pyridinato (4a/4a′)] with different azole-pyridine-based N^N′ ligands as ancillary and main chelate, respectively, are investigated using the density functional method. It is found that, with the systematic addition of N substitution in N^N′ ligands, the HOMO–LUMO energy gap follows a decreasing order from 1a to 4a with ancillary N^N′ ligands, but increasing order for 1a′–4a′ with main N^N′ ligands. Besides, the emission spectra are blue shifted with the addition of N substitution and the emissive state of all of the studied complexes is predominantly controlled by the N^N′ ligand, regardless of being ancillary or the main chelate. Furthermore, the evaluation of the radiative (k_r) and nonradiative (k_nr) rate constants for all of the complexes are also investigated based on the calculated results. It's found that the quantum yield (Φ_PL) shows an apparent dependence on the selection of different N^N′ ligands and the N^N′ ligands as main chelate present a small k_r and large k_nr, which is not beneficial for efficient materials. While the pyrazole- and 1,2,4-triazole-pyridine-based N^N′ ligands as ancillary chelate is believed to be more favorable for highly efficient phosphorescence emitters in OLEDs.