Deep-Red/Near-Infrared to Blue-Green Phosphorescent Iridium(III) Complexes Featuring Three Differently Charged (0, −1, and −2) Ligands: Structures, Photophysics, and Organic Light-Emitting Diode Application

We have designed and synthesized a new family of neutral phosphorescent iridium(III) complexes (Ir1–Ir6) featuring three differently charged (0, −1, and −2) ligands, in which biphenyl (bp) is used as a dianionic (−2) ligand, 4,6-difluorophenylpyridine (dfppy) or 1-phenylisoquinoline (piq) is used as a monoanionic (−1) ligand, and 2,2′-bipyridyl (bpy), 1,10-phenanthroline (phen), 1,2-bis(diphenylphosphanyl)benzene (dppb), or 1,2-bis(diphenylphosphanyl)ethane (dppe) is used as a neutral (0) ligand. The X-ray structures confirm that three coordination carbon atoms of all complexes assume a facial geometry, which can be beneficial to the stability of the structure. More importantly, the emitting color of the complexes can be tuned from deep red/near-infrared (NIR) (680–710 nm) to blue-green (466–496 nm) with different monoanionic (−1) ligands and neutral (0) ligands. Interestingly, the complex Ir5 shows a significant aggregation-induced phosphorescent emission effect, while Ir6 with a similar structure shows an opposite aggregation-caused quenching effect, mainly due to slight differences in the neutral (0) ligand structure. Notably, all deep red/NIR-emitting complexes (Ir1–Ir4) exhibit a distinct charge transfer (CT) excited state from the dianionic (−2) ligand to the neutral (0) ligand according to density functional theory calculations, whereas the excited state of blue-green-emitting complexes (Ir5–Ir6) displays the CT from the dianionic (−2) ligand to the monoanionic (−1) ligand. Considering better stability and optical performance, the deep red-emitting complexes (Ir2 and Ir4) with a simple structure are used as emitting layers of organic light-emitting diode devices and achieved good maximum external quantum efficiency (4.9 and 5.8%) peaking at 676 and 655 nm, respectively, with a very low turn-on voltage (2.5 V). This research provides a good strategy for the design of phosphorescent iridium complexes based on three differently charged (0, −1, and −2) ligands and their optoelectric applications.