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.