.
Angewandte
Communications
potentials follow the order of fac-o > fac-m > fac-p > 1 and
mer-o > mer-m > mer-p > 2, consistent with the order of
electron-withdrawing ability of the carborane isomers. The
results reflect that carborane isomers can increase oxidation
potential and reduce the energy level of HOMO (Table 1), in
[
8a]
agreement with our previous report.
To elucidate photophysical and electrochemical proper-
ties of these complexes, DFT calculations were performed to
gain insight into structure–property relationships (Supporting
Information, Figure S6–S10 and Tables S5–S7). In each case
the lowest-energy absorption is mainly characterized by
a HOMO!LUMO transition, which is the same as in the
[
11]
reported iridium(III) analogues. The HOMO is located on
the iridium(III) center and the phenyl ring of the cyclo-
metalated ligand, but the LUMO is located on the pyridyl,
phenyl, and partial carboranyl units, indicating that carbor-
anes are involved in the excited states of all of the complexes.
To explain why fac-o exhibits special photophysical properties
that are dependent on solvents, structural optimizations were
also conducted in benzene (e = 2.27), toluene (e = 2.38),
diethyl ether (e = 4.30), CH Cl2 (e = 9.10), methanol (e =
2
3
3.0), and acetonitrile (e = 37.5) with differing dielectric
constants. The result showed that the distribution of electron
cloud and energy levels of HOMO and LUMO are almost
same in these solvents, thus the emission wavelength is
independent on solvents. The frequency calculations with
different solvents demonstrated that the vibration intensity of
the CꢀC bond of o-carborane which is involved in excited
state increases with the increased dielectric constant of
solvent (Supporting Information, Figures S9, S10). However,
the CꢀC vibration dissipates energy. Thus now it is easy to
understand that fac-o exhibits strong emissions in solvents
with low dielectric constants and weak or quenched emissions
in solvents with high dielectric constants. Finally, the HOMO
and LUMO energy levels descend owing to incorporation of
carboranes, and follow the order of fac-o > fac-m > fac-p > 1
and mer-o > mer-m > mer-p > 2, which is in agreement with
the tendency as revealed by experimental data (Supporting
Information, Figure S6 and S7).
Figure 2. a) PL spectra of fac-o in eleven solvents with different
dielectric constants at the same concentration (3ꢀ10 m). Bottom:
corresponding luminescence photographs. b) PL spectra of fac-o in PS
ꢀ5
(
2 wt%, black), PMMA (2 wt%, red), and a neat film (not visible).
Inset: corresponding luminescence photographs.
phorescence efficiency and lifetime of fac-o in solution were
observed to closely correlate to dielectric constants (e) of
solvents (Table 2). This phenomenon is also applicable to
solid films (Figure 2b). For example, fac-o exhibits very
In an attempt to explore potential applications of
carborane-functionalized iridium(III) complexes, we synthe-
sized a hydrophilic complex containing nido-o-carborane
from fac-o in boiling ethanol in an isolated yield of 90%,
strong emission in polystyrene film (e = 2, F = 0.70, 2 wt%)
P
1
and weaker emission in PMMA film (e = 4, F = 0.33,
denoted as nido-fac-o (Figure 3a). The H NMR showed the
P
2
wt%), in sharp contrast to no emission in the neat film
characteristic broad BꢀHꢀB resonance of a nido-o-carbor-
1
1
(F < 0.001); thus fac-o become strongly emissive by doping it
ane. B NMR showed less overlapping signals in a wider
P
in suitable host materials. These observed luminescence
range for typical nido-o-carborane cage. The ESI-MS clearly
ꢀ
2ꢀ
phenomena might be attributed to the special CꢀC bond of
demonstrated three peaks of (M-K) , (M-2K) and (M-
3
ꢀ
the o-carboranyl unit, which is sensitive to the outer environ-
ment. In the case of mer-o, the solvent effect is not evident, as
luminescence is nearly quenched in all solvents tested. This is
determined by the meridional structure, which feasibly leads
3K) . Its emission spectra in different solvents (Supporting
Information, Figure S3) exhibited 10–40 nm red shifts in
comparison to fac-o. To our delight, nido-fac-o has high
phosphorescence efficiency (absolute F = 0.36) in aqueous
P
[
11]
to nonradiative decay.
solution, the highest among the reported water-soluble
[12]
Electrochemical studies reveal that all complexes have
reversible oxidation waves with potentials in the range of
iridium(III) complexes.
In addition, nido-fac-o is also
soluble in other polar solvents but shows differing phosphor-
0
.14–0.52 eV (Supporting Information, Figure S4). The oxi-
escence efficiency, for example, absolute F = 0.53 in DMSO,
P
dation potentials of facial complexes are positively shifted
relative to those of meridional analogues, demonstrating that
the latter are more easily oxidized. The values of oxidation
absolute F = 0.33 in DMFand absolute F = 0.06 in CH OH.
P
P
3
The maximum emission wavelength also shifts from 526 nm in
CH OH to 543 nm in H O (Supporting Information,
3
2
1
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 13434 –13438