Enhanced Deep-Red Phosphorescence in Cyclometalated Iridium Complexes with Quinoline-Based Ancillary Ligands

Compounds with good photoluminescence
quantum yields
(ΦPL) in the deep-red to near-infrared parts of the
spectrum
are desired for a variety of applications in optoelectronics, imaging,
and sensing. However, in this region of the spectrum, quantum yields
are usually modest, which is explained by the energy gap law and the
inherently slower radiative decay rates for low-energy emitters according
to the second-order perturbation theory. In this work, we outline
a new direction in deep-red luminescence, introducing a new suite
of bis-cyclometalated iridium complexes with efficient luminescence
beyond 650 nm. Seven new complexes are prepared using two different
cyclometalating (C^N) ligands with four quinoline-derived ancillary
ligands (L^X). The chosen cyclometalating ligands are well-established
to produce deep-red phosphorescence and include a metalated phenyl
ring appended to a conjugated heterocycle. The ancillary ligands combine
a rigid quinoline or benzoquinoline “L” donor with a
variable anionic “X” donor comprised of an O-donor aryloxy or carboxylate or an N-donor amidate.
These complexes phosphoresce in the deep-red region with wavelengths
between 650 and 700 nm and solution quantum yields between 0.018 and
0.42.