Highly efficient phosphorescence from cyclometallated iridium(III) compounds: Improved syntheses of picolinate complexes and quantum chemical studies of their electronic structures

We report a new method to make heteroleptic cyclometallated iridium(III) complexes that employs 1,2-dimethoxyethane (DME) instead of the commonly used 2-ethoxyethanol for the addition of ancillary ligands to chloro-bridged dimeric iridium species. Thus, picolinic acid was quickly and cleanly added to [(F₂ppy)₂Ir(Cl)]₂ (where F₂ppy = 2-(4′,6′-difluorophenyl)pyridinato) with base under DME reflux to afford (F₂ppy)₂Ir(picolinate) (FIrpic) in excellent yield. The high purity of the products obviates the need for further purification, an improvement over the common route in which column chromatography is required. We prepared eleven picolinate complexes by this route, eight of which were characterized by single crystal x-ray diffraction. The complexes possess the same distorted octahedral geometry with two bidentate phenylpyridine ligands and one bidentate 2-picolinate ligand. All of the compounds exhibited efficient phosphorescence with colors ranging from blue to orange; photophysical measurements revealed emission quantum yields as high as 0.92, while most surpassed 0.5. Time-dependent density functional theory calculations, coupled with the use of natural transition orbitals (NTOs), allowed a detailed interpretation of the electronic structures for the complexes. The nature of the acceptor orbital for the lowest-energy triplet state NTO was found to be an important predictor for the emission spectra of FIrpic and its congeners.