Ir(III)-Based Phosphors with Bipyrazolate Ancillaries; Rational Design, Photophysics, and Applications in Organic Light-Emitting Diodes

A series of three charge-neutral Ir­(III) complexes bearing both neutral chelating ligands 4,4′-di-t-butyl-2,2′-bipyridine (dtbbpy) and monoanionic cyclometalated ligands derived from 2-phenylpyridine (ppyH), together with either two monoanionic ligands (i.e., chloride and monodentate pyrazolate) or a single dianionic chelate derived from 5,5′-di­(trifluoromethyl)-3,3′-bipyrazole (bipzH2) or 5,5′-(1-methylethylidene)-bis-(3-trifluoromethyl-1H-pyrazole) (mepzH2), was successfully synthesized. These complexes are derived from a common, structurally characterized, Ir­(III) intermediate complex [Ir­(dtbbpy) (ppy)­Cl2] (1), from treatment of IrCl3·3H2O with equal amount of the diimine (N^N) and precursor of the cyclometalated (C^N) ligands in a form of one-pot reaction. Treatment of 1 with various functional pyrazoles afforded [Ir­(dtbbpy) (ppy) (pz)­Cl] (2), [Ir­(dtbbpy) (ppy) (bipz)] (3), and [Ir­(dtbbpy) (ppy) (mepz)] (4), which display intense room-temperature emission with λmax spanning the region between 532 and 593 nm in both fluid and solid states. The Ir­(III) complexes, 3 and 4, showcase rare examples of three distinctive chelates (i.e., neutral, anionic, and dianionic) assembled around the central Ir­(III) cation. Hybrid density functional theory (DFT; B3LYP) electronic structure calculations on 1–4 reveal the lowest unoccupied molecular orbital to be π*­(bpy) in character for all complexes and highest occupied molecular orbital (HOMO) offering d­(Ir)−π­(phenyl) character for 1, 2, and 4 and π­(bipz) character for 3. The different HOMO composition of 3 and 4 is also predicted by calculations using pure DFT (BLYP) and wave function (MP2) methods. On the basis of time-dependent DFT calculations, the emissive processes are dominated by the phenyl group-to-bipyridine, ligand(ppy)-to-ligand(bpy) charge transfer admixed with metal-to-ligand transition for all Ir­(III) complexes. Organic light emitting diodes were successfully fabricated. A double emitting layer design was adopted in the device architecture using Ir­(III) metal complexes 3 and 4, attaining peak external quantum efficiencies, luminance efficiencies, and power efficiencies of 18.1% (59.0 cd/A and 38.6 lm/W) and 16.6% (53.3 cd/A and 33.5 lm/W), respectively.