Highly luminescent 2-phenylpyridine-free diiridium complexes with bulky 1,2-diarylimidazole cyclometalating ligands

Abstract

While a number of highly emissive dinuclear Ir(III) complexes have been reported, they have generally been restricted to structures based on 2-phenylpyridine (Hppy) cyclometalates. We now present a series of new hydrazide-bridged diiridium complexes (5–8) which incorporate bulky 1,2-diarylimidazole cyclometalating ligands in the place of Hppy. Complexes 6–8 are strongly emissive when doped into poly(methyl methacrylate) (PMMA), displaying the highest PLQYs yet reported for ppy-free diiridium emitters (Φ_PL = 47–55 ± 10%). Notably, complex 8 has an emission peak at 452 nm and CIE_xy colour coordinates in the sky-blue region (0.18, 0.27), which is competitive with state-of-the-art monoiridium analogues. X-ray crystallography and solution-state ¹⁹F NMR spectra reveal the presence of rigidifying intramolecular π–π interactions for complexes 6–8, which explains their improved photophysical performance compared to 5 which does not have these interactions. Structure–property relationships are further rationalised through density functional theory (DFT) and cyclic voltammetry (CV) data. All the complexes studied in this work display aggregation induced phosphorescent emission (AIPE). This series of compounds increases the structural diversity of highly luminescent dinuclear Ir(III) complexes to include luminophoric ligands that are not restricted to Hppy-type fragments. The colour range accessible to AIPE-active diiridum complexes has also been substantially broadened.