Synthesis, Characterization, and Photophysical and Electroluminescent Properties of Blue-Emitting Cationic Iridium(III) Complexes Bearing Nonconjugated Ligands

The development of pure-blue-to-deep-blue-emitting
ionic phosphors is an ultimate challenge for full-color displays and
white-light sources. Herein we report two series of short-wavelength
light-emitting cationic iridium­(III) complexes with nonconjugated
ancillary and cyclometalating ligands, respectively. In the first
series, nonconjugated 1-[(diphenylphosphino)­methyl]-3-methylimidazolin-2-ylidene-C,C2′ (dppmmi) is used as the ancillary ligand and 2-phenylpyridine
(ppy), 2-(2,4-difluorophenyl)­pyridine (dfppy), and 1-(2,4-difluorophenyl)-1H-pyrazole (dfppz) are used as cyclometalating ligands.
In the second one, nonconjugated 2,4-difluorobenzyl-N-pyrazole (dfbpz) is used as the cyclometalating ligand and 3-methyl-1-(2-pyridyl)­benzimidazolin-2-ylidene-C,C2′ (pymbi) as the ancillary ligand. The synthesis and
photophysical and electrochemical properties, together with the X-ray
crystal structures of these complexes, have been investigated. At
room temperature, blue-emitting complexes [Ir­(ppy)2(dppmmi)]­PF6 (1) and [Ir­(dfppy)2(dppmmi)]­PF6 (2; PF6– is hexafluorophosphate)
show much larger photoluminescence quantum yields of 24% and 46%,
respectively. On the contrary, for complexes [Ir­(dfppz)2(dppmmi)]­PF6 (3) and [Ir­(dfbpz)2(pymbi)]­PF6 (4), deep-blue luminescence is
only observed at low temperature (77 K). Density functional theory
calculations are used to rationalize the differences in the photophysical
behavior observed upon changes of the ligands. It is shown that the
electronic transition dipoles of cationic iridium complexes 1 and 2 are mainly confined to cyclometalated
ligands (3MLCT and LC 3π–π*)
and those of complex 3 are confined to all of the ligands
(3MLCT, LC 3π–π*, and 3LLCT) because of the high LUMO energy level of dfppz. The
emission of 4 mainly originates from the central iridium­(III)
ion and cyclometalated ligand to ancillary ligand charge transfer
(3MLCT and 3LLCT), in contrast to commonly designed
cationic complexes using carbene-type ancillary ligands, where emission
originates from the cyclometalated main ligands. Solution-processed
organic light-emitting diodes based on complexes 1 and 2 gave blue-green (498 nm) and blue (478 nm) electroluminescence
with maximum current efficiencies of 3.8 and 3.4 cd A–1, respectively. The results indicate that introducing nonconjugated
ligands into cationic iridium complexes is an effective means of achieving
short-wavelength light-emitting phosphors.