phosphorescence, and CPL were achieved, thus yielding the first acid-based
CPL switch (see the increase of g lum upon protonation in Fig. 4.27). Furthermore,
we showed that organic helicene ligand (P)- and (M)-68 was also an efficient
chiroptical switch since, after double protonation, it displayed a strong bathochromic
shift in emission wavelength while keeping strong CPL fluorescence signal
(g lum ¼ Æ2 Â 10
À3 in CH 2 Cl 2 ). TDDFT calculations showed that, upon protonation,
the HOMO-to-LUMO transition changed from a π–πÃ-type to a charge transfer-type
transition.
Rhenium(I)-chloro-tricarbonyl complexes bearing a bipy ligand are known
to display efficient luminescence, usually a
3 CT emission from an excited
state based on the bis-imine ligand. In this context, organic helicene-bipy ligand
(P)- and (M )-68 was used as N^N chelate to prepare enantioenriched CPL-active
helicene-bipyridine-rhenium complexes 70 (Fig. 4.28) [68]. Starting from (M)-68
ligand, two diastereomeric complexes, i.e., (M,A Re )-70a
1 and (P,C Re )-70a
2 ,
were formed, since the Re(I) atom is also a stereogenic center. These
stereoisomers were separated by regular silica gel column chromatography and
their chiroptical and emissive properties were studied. They revealed strong
ECD spectra in CH 2 Cl 2 (whose intensity depends on the rhenium stereochemistry;
see Fig. 4.28), accompanied by substantial phosphorescence and CPL activity.
Indeed (M,A Re )-70a
1 and (M,C Re )-70a
2 displayed phosphorescence emission
(λ max
phos
¼ 673–680 nm, ϕ ¼ 0.13–0.16%, τ ¼ 27–33 ns) and good g lum
values ((M,C Re )-70a
2 : g lum ~ À3 Â 10
À3 around 670 nm). Upon reaction
with AgOTf and 2,6-dimethylphenyl isocyanide in the presence of NH 4 PF 6 ,
(M,C Re ) and (P,A Re )-71a
2 were transformed to cationic complexes (P)- and
(M)-71a
1,2 , respectively (see Fig. 4.28). The latter displayed stronger phosphorescence (λ max
phos
¼ 598 nm, ϕ ¼ 6%, τ ¼ 79 μs) and still good CPL activity
(g lum ~ Æ1.5 Â 10
À3 ). However, the stereochemical information at the Re
(I) center was lost (epimerization to 50:50 mixture). Nevertheless, the ECD spectrum
of (P)-71a
1,2 displayed an additional positive ECD-active band around 450 nm as
compared to (P,C Re )-114a
1 and (P,A Re )-70a
2 . According to TDDFT calculations,
this band does not involve the Re center but corresponds to the HOMO-to-LUMO
transition with strong intra-ligand charge transfer from the π-helicene to the bipy
moiety [68]. We have thus shown that the incorporation of a rhenium atom within
an extended helical π-conjugated bipyridine system can impact the chiroptical
and photophysical properties of the resulting neutral or cationic complexes, leading
to the first rhenium-based circularly polarized phosphors.
4.9.3 Coordination Chemistry of Bis-Helicene-Terpyridine
and Bis-Helicene-Bipy Ligands
In 2016, our group also prepared the bis-helical terpyridine (terpy) ligand 72 which
acted as a chiroptical switch upon reversible coordination-decoordination to zinc(II).
The strong conformational changes induced led to a multi-responsive chiroptical
86
J. Crassous
CPL switch (see the increase of g lum upon protonation in Fig. 4.27). Furthermore,
we showed that organic helicene ligand (P)- and (M)-68 was also an efficient
chiroptical switch since, after double protonation, it displayed a strong bathochromic
shift in emission wavelength while keeping strong CPL fluorescence signal
(g lum ¼ Æ2 Â 10
À3 in CH 2 Cl 2 ). TDDFT calculations showed that, upon protonation,
the HOMO-to-LUMO transition changed from a π–πÃ-type to a charge transfer-type
transition.
Rhenium(I)-chloro-tricarbonyl complexes bearing a bipy ligand are known
to display efficient luminescence, usually a
3 CT emission from an excited
state based on the bis-imine ligand. In this context, organic helicene-bipy ligand
(P)- and (M )-68 was used as N^N chelate to prepare enantioenriched CPL-active
helicene-bipyridine-rhenium complexes 70 (Fig. 4.28) [68]. Starting from (M)-68
ligand, two diastereomeric complexes, i.e., (M,A Re )-70a
1 and (P,C Re )-70a
2 ,
were formed, since the Re(I) atom is also a stereogenic center. These
stereoisomers were separated by regular silica gel column chromatography and
their chiroptical and emissive properties were studied. They revealed strong
ECD spectra in CH 2 Cl 2 (whose intensity depends on the rhenium stereochemistry;
see Fig. 4.28), accompanied by substantial phosphorescence and CPL activity.
Indeed (M,A Re )-70a
1 and (M,C Re )-70a
2 displayed phosphorescence emission
(λ max
phos
¼ 673–680 nm, ϕ ¼ 0.13–0.16%, τ ¼ 27–33 ns) and good g lum
values ((M,C Re )-70a
2 : g lum ~ À3 Â 10
À3 around 670 nm). Upon reaction
with AgOTf and 2,6-dimethylphenyl isocyanide in the presence of NH 4 PF 6 ,
(M,C Re ) and (P,A Re )-71a
2 were transformed to cationic complexes (P)- and
(M)-71a
1,2 , respectively (see Fig. 4.28). The latter displayed stronger phosphorescence (λ max
phos
¼ 598 nm, ϕ ¼ 6%, τ ¼ 79 μs) and still good CPL activity
(g lum ~ Æ1.5 Â 10
À3 ). However, the stereochemical information at the Re
(I) center was lost (epimerization to 50:50 mixture). Nevertheless, the ECD spectrum
of (P)-71a
1,2 displayed an additional positive ECD-active band around 450 nm as
compared to (P,C Re )-114a
1 and (P,A Re )-70a
2 . According to TDDFT calculations,
this band does not involve the Re center but corresponds to the HOMO-to-LUMO
transition with strong intra-ligand charge transfer from the π-helicene to the bipy
moiety [68]. We have thus shown that the incorporation of a rhenium atom within
an extended helical π-conjugated bipyridine system can impact the chiroptical
and photophysical properties of the resulting neutral or cationic complexes, leading
to the first rhenium-based circularly polarized phosphors.
4.9.3 Coordination Chemistry of Bis-Helicene-Terpyridine
and Bis-Helicene-Bipy Ligands
In 2016, our group also prepared the bis-helical terpyridine (terpy) ligand 72 which
acted as a chiroptical switch upon reversible coordination-decoordination to zinc(II).
The strong conformational changes induced led to a multi-responsive chiroptical
86
J. Crassous