Excited-State Lifetime Modulation by Twisted and Tilted Molecular Design in Carbene-Metal-Amide Photoemitters

Carbene–metal–amides (CMAs) are an emerging
class
of photoemitters based on a linear donor–linker–acceptor
arrangement. They exhibit high flexibility about the carbene–metal
and metal–amide bonds, leading to a conformational freedom
which has a strong influence on their photophysical properties. Herein
we report CMA complexes with (1) nearly coplanar, (2) twisted, (3)
tilted, and (4) tilt-twisted orientations between donor and acceptor
ligands and illustrate the influence of preferred ground-state conformations
on both the luminescence quantum yields and excited-state lifetimes.
The performance is found to be optimum for structures with partially
twisted and/or tilted conformations, resulting in radiative rates
exceeding 1 × 106 s–1. Although
the metal atoms make only small contributions to HOMOs and LUMOs,
they provide sufficient spin–orbit coupling between the low-lying
excited states to reduce the excited-state lifetimes down to 500 ns.
At the same time, high photoluminescence quantum yields are maintained
for a strongly tilted emitter in a host matrix. Proof-of-concept organic
light-emitting diodes (OLEDs) based on these new emitter designs were
fabricated, with a maximum external quantum efficiency (EQE) of 19.1%
with low device roll-off efficiency. Transient electroluminescence
studies indicate that molecular design concepts for new CMA emitters
can be successfully translated into the OLED device.