Intramolecular π–π Interactions with a Chiral Auxiliary Ligand Control Diastereoselectivity in a Cyclometalated Ir(III) Complex

The application of a chiral auxiliary
ligand to control the diastereoselectivity in the synthesis of a cyclometalated
iridium­(III) complex is presented. The diastereomeric iridium­(III)
complexes 1a and 1b are reported, in which
a phenoxyoxazoline auxiliary ligand incorporates a chiral center functionalized
with a pendant pentafluorophenyl group. The diastereomers were readily
separated, and their structural, electrochemical and photophysical
properties are discussed. Solution-state NMR data and X-ray crystal
structures establish that the pentafluorophenyl group engages in intramolecular
π–π interactions. The X-ray analysis reveals that
the two diastereomers display very different modes of intramolecular
stacking. The variable-temperature 19F NMR data indicate
that rotation of the pendant pentafluorophenyl rings in 1b and 1a is a temperature-dependent process and that
there is a smaller energy barrier to rotation in 1b in
comparison to 1a. This correlates with variable-temperature
photoluminescence data, which show that upon heating the integrated
emission intensity is reduced substantially more for 1b than for 1a, which is ascribed to the enhanced rotation
in 1b, providing a more easily populated nonradiative
pathway in comparison to 1a. These experimental data
are supported by computational calculations. Phosphorescent organic
light-emitting devices (PhOLEDs) using 1a as the dopant
complex give blue-green emission with a high maximum external quantum
efficiency (EQEmax) of 25.8% (at ca. 270 cd m–2) and with a low efficiency roll-off to 24.9% at 1000 cd m–2. Our results extend the scope of ligand design for cyclometalated
iridium complexes which possess interesting structural and emission
properties.