Charged Bis-Cyclometalated Iridium(III) Complexes with Carbene-Based Ancillary Ligands

Charged
cyclometalated (C∧N) iridium­(III) complexes
with carbene-based ancillary ligands are a promising family of deep-blue
phosphorescent compounds. Their emission properties are controlled
primarily by the main C∧N ligands, in contrast to
the classical design of charged complexes where N∧N ancillary ligands with low-energy π* orbitals, such as 2,2'-bipyridine,
are generally used for this purpose. Herein we report two series of
charged iridium complexes with various carbene-based ancillary ligands.
In the first series the C∧N ligand is 2-phenylpyridine,
whereas in the second one it is 2-(2,4-difluorophenyl)-pyridine. One
bis-carbene (:C∧C:) and four different pyridine–carbene
(N∧C:) chelators are used as bidentate ancillary
ligands in each series. Synthesis, X-ray crystal structures, and photophysical
and electrochemical properties of the two series of complexes are
described. At room temperature, the :C∧C: complexes
show much larger photoluminescence quantum yields (ΦPL) of ca. 30%, compared to the N∧C: analogues (around
1%). On the contrary, all of the investigated complexes are bright
emitters in the solid state both at room temperature (1% poly­(methyl
methacrylate) matrix, ΦPL 30–60%) and at 77
K. Density functional theory calculations are used to rationalize
the differences in the photophysical behavior observed upon change
of the ancillary ligands. The N∧C:-type complexes
possess a low-lying triplet metal-centered (3MC) state
mainly deactivating the excited state through nonradiative processes;
in contrast, no such state is present for the :C∧C: analogues. This finding is supported by temperature-dependent
excited-state lifetime measurements made on representative N∧C: and :C∧C: complexes.