Design and Synthesis of Iridium Bis(carbene) Complexes for Efficient Blue Electrophosphorescence

Abstract

Five iridium bis(carbene) complexes, [Ir(pmi)(2)(pypz)] (1), [Ir(mpmi)(2)(pypz)] (2), [Ir(fpmi)(2)(pypz)] (3), [Ir(fpmi)(2)(pyim)] (4), and [Ir(fpmi)(2)(tfpypz)] (5) (pmi=1-phenyl-3-methylimdazolin-2-ylidene-C,C(2'); fpmi=1-(4-fluorophenyl)-3-methylimdazolin-2-ylidene-C,C(2'); mpmi=1-(4-methyl-phenyl)-3-methylimdazolin-2-ylidene-C,C(2'); pypz=2-(1H-pyrazol-5-yl)pyridinato; pyim=2-(1H-imidazol-2-yl)pyridinato; and tfpypz=2-(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyridinato), were synthesized and their structures were characterized by NMR spectroscopy, mass spectroscopy and X-ray diffraction. These complexes showed phosphorescent emission with the emission maxima between 453 and 490 nm. Various spectrophotometric measurements, cyclic voltammetric studies, and density functional theory (DFT) calculations show that, unlike most of the phosphorescent cyclometalated iridium complexes, the lowest unoccupied molecular orbital (LUMO) energy and the emissive state of these iridium complexes are mainly controlled by the N,N'-heteroaromatic (N^N) ligand. Despite the fact that the LUMO levels of these complexes are mainly on the N^N ligands, the efficiencies of the electroluminescent (EL) devices are very high. For example, the EL devices using [Ir(mpmi)(2)(pypz)], [Ir(fpmi)(2)(pypz)], and [Ir(fpmi)(2)(tfpypz)] as the dopant emitters exhibited light- to deep-blue electrophosphorescence with external quantum efficiencies of 15.2, 14.1, and 7.6% and Commission Internationale d'Énclairage (x,y) coordinates (CIE(x,y)) of (0.14, 0.27), (0.14, 0.18) and (0.14, 0.10), respectively.

An exploration of N -heterocyclic carbene-based Ir(III) complexes for phosphorescent organic light-emitting diode applications

“…Compounds 1, 3, and 4 were synthesized according to the reported procedures [22,26]. Compound 1 was reacted with iridium trichloride hydrate (IrCl 3 ·3H 2 O) to yield the cyclometalated Ir(III) µ-chloride bridged dimer 2 with quantitative yield.…”

Wide‐Range Color Tuning of Iridium Biscarbene Complexes from Blue to Red by Different N⁁N Ligands: an Alternative Route for Adjusting the Emission Colors

“…Kido et al have reported a triscarbene iridium complex‐based device with a high external quantum efficiency of 18.6% with CIE coordinates of (0.15, 0.19) 16. Earlier, we reported the synthesis of blue iridium biscarbene complexes with an N , N ′‐heteroaromatic ( N ⁁ N ) as the third ligand 17. We found that the lowest unoccupied molecular orbitals (LUMOs) and the emissive states of these iridium complexes are mainly controlled by the N ⁁ N ligand, which is in agreement with the theoretical calculation by Su et al18 The results suggest the possibility of altering the emission color of iridium biscarbene complexes over a wide range by using different N ⁁ N ligands.…”

Design Rules for Charge-Transport Efficient Host Materials for Phosphorescent Organic Light-Emitting Diodes

“…However, due to the large band gap of the deep-blue emitter and the resulting high triplet energy, the number of promising compatible host materials is limited, since an even higher triplet energy is necessary for the host to ensure trapping of the exciton on the emitter. 10,11 Among possible charge-transporting units fulfilling this criterion are dibenzofurans and N-phenylcarbazoles. 12 Substituents must be attached to these materials to prevent crystallization, 13 suppress emitter aggregation, and optimize their molecular weight for vacuum deposition.…”