switch (Fig. 4.29) [69]. The interconversion between the ligand and zinc-complexed
states was analyzed via first-principles calculations, which highlighted the change
from π-πà transitions in the organic ligand to charge transfer transitions in the
Zn complex. Overall, this system behaved as a chiroptical switch offering multioutput readout (UV-vis, ECD, luminescence, and CPL). Furthermore, the switching
process triggered conformational changes and molecular motion around the Zn
center, from a clear trans (W-shape) conformation in the free ligand to a cis
(U-shape) one in the Zn-complex 73 (Fig. 4.29, Table 4.13). Recently, we have
prepared a novel enantiopure bis-helicenic 2,2
0 -bipyridine system ligand, 74
[70]. Thanks to the bipyridine unit, the coordination to 75 with Zn
II and protonation
processes to 74.2H
+ were studied revealing efficient tuning of photophysical
(UV-vis and emission) and chiroptical properties (ECD and CPL) of the system
(Fig. 4.29, Table 4.13). The coordination-decoordination and protonation/
deprotonation processes appeared reversible thus constituting novel chiroptical
switches.
4.9.4 CPL-Active Iridium Complexes from Helicene-NHC
Ligands
In the last two decades, octahedral cyclometalated iridium(III) complexes have
attracted attention due to their appealing properties as phosphors in high-efficiency
organic light-emitting devices (OLEDs) [71]. In 2017, the first fused π-helical
NHC system was prepared and examined through its diastereoisomerically pure
cyclometalated complexes mer-(P,Λ Ir )-77a
1 and mer-(P,Δ Ir )-77a
2 from pentahelical
imidazolium 76 (Fig. 4.30) [49]. These chiral organometallic species displayed
light-green phosphorescence with (i) circular polarization that depends on both the -
helical-NHC (P)/(M) stereochemistry and the iridium (Δ)/(Λ) one (Fig. 4.30 and
Table 4.14) and (ii) unusually long lifetimes (up to 250 μs as compared to 530 ns
for model mer-78). The unprecedented features of 77a
1,2 can be attributed to extended
π-conjugation within helical carbenic ligand. A similar cycloiridiated complex, namely,
mer-(Δ Ir /Λ Ir )-79, bearing a NHC N-substituted with a carbo[4]helicene unit was recently
obtained and also displayed long-lived mirror-image circularly polarized phosphorescence (Table 4.14) [72].
4.10 Applications in Optoelectronics
There is an obvious interest to develop the use of helicenes and helicenoids as
chiral molecular materials in chiral OLEDs, in chiral sensors, in chiral bioimaging
agents, chiroptical switching activity [73, 74] applications that directly take benefit
from circularly polarized emission. For instance, there is a strong potential of CP
88
J. Crassous
states was analyzed via first-principles calculations, which highlighted the change
from π-πà transitions in the organic ligand to charge transfer transitions in the
Zn complex. Overall, this system behaved as a chiroptical switch offering multioutput readout (UV-vis, ECD, luminescence, and CPL). Furthermore, the switching
process triggered conformational changes and molecular motion around the Zn
center, from a clear trans (W-shape) conformation in the free ligand to a cis
(U-shape) one in the Zn-complex 73 (Fig. 4.29, Table 4.13). Recently, we have
prepared a novel enantiopure bis-helicenic 2,2
0 -bipyridine system ligand, 74
[70]. Thanks to the bipyridine unit, the coordination to 75 with Zn
II and protonation
processes to 74.2H
+ were studied revealing efficient tuning of photophysical
(UV-vis and emission) and chiroptical properties (ECD and CPL) of the system
(Fig. 4.29, Table 4.13). The coordination-decoordination and protonation/
deprotonation processes appeared reversible thus constituting novel chiroptical
switches.
4.9.4 CPL-Active Iridium Complexes from Helicene-NHC
Ligands
In the last two decades, octahedral cyclometalated iridium(III) complexes have
attracted attention due to their appealing properties as phosphors in high-efficiency
organic light-emitting devices (OLEDs) [71]. In 2017, the first fused π-helical
NHC system was prepared and examined through its diastereoisomerically pure
cyclometalated complexes mer-(P,Λ Ir )-77a
1 and mer-(P,Δ Ir )-77a
2 from pentahelical
imidazolium 76 (Fig. 4.30) [49]. These chiral organometallic species displayed
light-green phosphorescence with (i) circular polarization that depends on both the -
helical-NHC (P)/(M) stereochemistry and the iridium (Δ)/(Λ) one (Fig. 4.30 and
Table 4.14) and (ii) unusually long lifetimes (up to 250 μs as compared to 530 ns
for model mer-78). The unprecedented features of 77a
1,2 can be attributed to extended
π-conjugation within helical carbenic ligand. A similar cycloiridiated complex, namely,
mer-(Δ Ir /Λ Ir )-79, bearing a NHC N-substituted with a carbo[4]helicene unit was recently
obtained and also displayed long-lived mirror-image circularly polarized phosphorescence (Table 4.14) [72].
4.10 Applications in Optoelectronics
There is an obvious interest to develop the use of helicenes and helicenoids as
chiral molecular materials in chiral OLEDs, in chiral sensors, in chiral bioimaging
agents, chiroptical switching activity [73, 74] applications that directly take benefit
from circularly polarized emission. For instance, there is a strong potential of CP
88
J. Crassous