Highly Efficient Red-Emitting Bis-Cyclometalated Iridium Complexes

Bis-cyclometalated
iridium complexes with enhanced phosphorescence quantum yields in
the red region of the visible spectrum are described. Here, we demonstrate
that incorporating strongly π-donating, nitrogen-containing
β-ketoiminate (acNac), β-diketiminate (NacNac), and N,N′-diisopropylbenzamidinate (dipba)
ancillary ligands can demonstrably perturb the excited-state kinetics,
leading to enhanced photoluminescence quantum yields (ΦPL) for red-emitting compounds. A comprehensive study of the
quantum yields and lifetimes for these complexes reveals that for
the compounds with the highest quantum yields, the radiative rate
constant (kr) is significantly higher
than that of related complexes, and contributes substantially to the
increase in ΦPL. Experimental and computational evidence
is consistent with the notion that an increase in spin–orbit
coupling, caused by an enhancement of the metal-to-ligand charge transfer
(MLCT) character of the excited state via destabilization of the HOMO,
is mainly responsible for the faster radiative rates. One of the compounds
was shown to be effective as the emissive dopant in an organic light-emitting
diode device.