Naphthalene Benzimidazole Based Neutral Ir(III) Emitters for Deep Red Organic Light-Emitting Diodes

Rigid naphthalene benzimidazole (NBI) based ligands (L¹ and L²) are synthesized and utilized to make deep red phosphorescent cyclometalated iridium(III) complexes ([Ir(NBI)₂(PyPz^CF3)] (1) and [Ir(DPANBI)₂(PyPz^CF3)] (2)). Complexes 1 and 2 are prepared from the reaction of L¹/L² with the aid of ancillary ligands (PyPz^CF3_, 2-(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyridine) in a two step method. The complexes are characterized by analytical and spectroscopic methods, as well as X-ray diffraction for 1. These complexes show a strong emission in the range of 635-700 nm that extends up to the near-infrared region (800 nm). The introduction of the diphenylamino (DPA) donor group on the naphthalene unit leads to a further red-shift in the emission. The complexes exhibit radiative quantum efficiency (Φ_PL) of 0.27-0.29 in poly(methylmethacrylate) film and relatively short phosphorescence decay lifetimes (τ = 1.1-3.5 μs). The structural, electronic, and optical properties are investigated with the support of density functional theory (DFT) and time-dependent-DFT calculations. The calculation results indicate that the lowest-lying triplet (T₁) excited state of 1 has a mixed metal-to-ligand charge transfer (³MLCT) and ligand-centered (³LC) character, while 2 shows a dominant ³LC character. Deep red-emitting organic light-emitting diodes fabricated using 1 as a dopant display a maximum external quantum efficiency of 10.9% with the CIE color coordinates of (0.690, 0.294), with an emission centered at 644 and 700 nm. Similarly, the emitter 2 also shows a maximum external quantum efficiency of 6.9% with emissions at 657 and 722 nm.