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

摘要

A series of three charge-neutral Ir(III) complexes bearing both neutral chelating ligands 4,4'-di-t-butyl2,2'-bipyridine (dtbbpy) and monoanionic cydometalated ligands derived from 2-phenylpyridine (ppyH), together with either two monoanionic ligands (i.e., chloride and monodentate p-yrazolate) or a single dianionic chelate derived from 5,5'-di(trifluoromethyl)-3,3'-bipyrazole (bipzH(2)) or 5,5'-(1-methylethylidene)-bis-(3-trifluoromethyl-1H-pyrazole) (mepzH(2)), was successfully synthesized. These complexes are derived from a common, structurally characterized, Ir(Ea) intermediate complex [Ir(dtbbpy) (ppy)Cl-2] (1), from treatment of IrCl3 center dot 3H(2)O with equal amount of the diimine ((NN)-N-<^>) and precursor of the cyclometalated ((CN)-N-<^>) ligands in a form of one-pot reaction. Treatment of 1 with various functional pyrazoles afforded [Ir(dtbbpy) (ppy) (pz)C1] (2), [Ir(dtbbpy) (ppy) (bipz)] (3), and [Ir(dtbbpy) (ppy) (mepz)] (4), which display intense room-temperature emission with lambda(max), 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 pi*(bpy) in character for all complexes and highest occupied molecular orbital (HOMO) offering d(Ir)-pi(phenyl) character for 1, 2, and 4 and pi(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, ligancl((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 1m/VV) and 16.6% (53.3 cd/A and 33.5 lm/W), respectively.