groups [26]. Compared to pristine [5]carbohelicene 23, the introduction of an
electron-withdrawing maleimide group onto a [5]carbohelicene core contributes to
the stabilization of the LUMO level in 21a, whereas the energy level of HOMO level
in MeO-substituted 21b increases due to the electron donation. As a result, the
HOMO-LUMO gap of 21b is smaller than that of 21a and of carbo[5]helicene
23, giving bathochromic shift of absorption and emission bands. The absolute
fluorescence quantum yield of 21a was found higher (0.37) as compared to [5]
carbohelicene 23 (0.04), whereas Φ F of 21b was slightly smaller (0.22). The chirality
of these [5]carbohelicene derivatives 21a,b was evidenced by their ECD and CPL
activities. In particular, 21a and 21b gave good CPL activity with anisotropy factors
g lum estimated to be Æ2.4 Â 10
À3 and Æ 2.3 Â 10
À3 in THF. This example was the
first observation of CPL in [5]carbohelicene derivatives which are usually thought to
be configurationally unstable. Here, the authors verified that the CPL signal was
stable with time in solution. Note that this negative CPL signal for the (P)-(+) isomer
is a S-type one (vide supra and [22]), i.e. corresponding to the small negative ECD
signal at 435 nm with g abs values of À4.8/À5.7 Â 10
À3 for (P)-(+)-21a/b. The same
authors also reported a carbo[5]helicene 22 bearing a fused benzimidazole and its
protonated form 22.H
+ (see Fig. 4.7 and Table 4.2) [27].
In 2016, Chen and coworkers reported the preparation of configurationally stable
helical aromatic imides displaying full-color CPL responses [28]. For this purpose,
they prepared enantiomers (P)-(À)- and (M)-(+)-24a-e with high ee’s (98.4–99%).
Each pair of enantiomers displayed mirror-image ECD spectra of moderate magnitude and absolute configurations opposite to those of classical heterohelicenes.
(P)-(À)- and (M )-(+)-24a-e exhibited full color fluorescence emission (from
445 to 617 nm) and mirror-image CPL signals in THF (Fig. 4.8). The g abs values
of the enantiomers fell within the range of Æ1.5 Â 10
À3 to Æ3.5 Â 10
À4 and the g lum
values between Æ0.2 Â 10
À3 and Æ 1.5 Â 10
À3 (Table 4.2).
4.2.7 Carbocationic Azahelicenes
Lacour and coworkers prepared functionalized carbocationic [4]helicene (25–30)
and [6]helicene (31–33) derivatives and studied their photophysical and
chiroptical properties (Figs. 4.9 and 4.10 and Table 4.3) [29–31]. Interestingly,
these compounds exhibit fluorescence emissions in the red to near infrared region
(with Φ F ranging from 0.01 to 0.445), which corresponds to an unusual spectral
range for helicene-based chromophores, especially for fully organic ones. As a
result, these helical derivatives may be interesting for chiral bioimaging. The
same authors investigated different diaza[4]helicenium chiral dyes functionalized
with different donor and acceptor groups in order to tune their chiroptical properties
in terms of ECD and CPL responses (Fig. 4.9) [29]. These helical derivatives
present ECD signatures up to 750 nm with moderate intensity in the visible-red
region (Δε ~ 10 M
À1 cm
À1 ), resulting from partial charge-transfer transitions
involving the nitrogen atoms and the central carbocation. Furthermore, CPL
62
J. Crassous
electron-withdrawing maleimide group onto a [5]carbohelicene core contributes to
the stabilization of the LUMO level in 21a, whereas the energy level of HOMO level
in MeO-substituted 21b increases due to the electron donation. As a result, the
HOMO-LUMO gap of 21b is smaller than that of 21a and of carbo[5]helicene
23, giving bathochromic shift of absorption and emission bands. The absolute
fluorescence quantum yield of 21a was found higher (0.37) as compared to [5]
carbohelicene 23 (0.04), whereas Φ F of 21b was slightly smaller (0.22). The chirality
of these [5]carbohelicene derivatives 21a,b was evidenced by their ECD and CPL
activities. In particular, 21a and 21b gave good CPL activity with anisotropy factors
g lum estimated to be Æ2.4 Â 10
À3 and Æ 2.3 Â 10
À3 in THF. This example was the
first observation of CPL in [5]carbohelicene derivatives which are usually thought to
be configurationally unstable. Here, the authors verified that the CPL signal was
stable with time in solution. Note that this negative CPL signal for the (P)-(+) isomer
is a S-type one (vide supra and [22]), i.e. corresponding to the small negative ECD
signal at 435 nm with g abs values of À4.8/À5.7 Â 10
À3 for (P)-(+)-21a/b. The same
authors also reported a carbo[5]helicene 22 bearing a fused benzimidazole and its
protonated form 22.H
+ (see Fig. 4.7 and Table 4.2) [27].
In 2016, Chen and coworkers reported the preparation of configurationally stable
helical aromatic imides displaying full-color CPL responses [28]. For this purpose,
they prepared enantiomers (P)-(À)- and (M)-(+)-24a-e with high ee’s (98.4–99%).
Each pair of enantiomers displayed mirror-image ECD spectra of moderate magnitude and absolute configurations opposite to those of classical heterohelicenes.
(P)-(À)- and (M )-(+)-24a-e exhibited full color fluorescence emission (from
445 to 617 nm) and mirror-image CPL signals in THF (Fig. 4.8). The g abs values
of the enantiomers fell within the range of Æ1.5 Â 10
À3 to Æ3.5 Â 10
À4 and the g lum
values between Æ0.2 Â 10
À3 and Æ 1.5 Â 10
À3 (Table 4.2).
4.2.7 Carbocationic Azahelicenes
Lacour and coworkers prepared functionalized carbocationic [4]helicene (25–30)
and [6]helicene (31–33) derivatives and studied their photophysical and
chiroptical properties (Figs. 4.9 and 4.10 and Table 4.3) [29–31]. Interestingly,
these compounds exhibit fluorescence emissions in the red to near infrared region
(with Φ F ranging from 0.01 to 0.445), which corresponds to an unusual spectral
range for helicene-based chromophores, especially for fully organic ones. As a
result, these helical derivatives may be interesting for chiral bioimaging. The
same authors investigated different diaza[4]helicenium chiral dyes functionalized
with different donor and acceptor groups in order to tune their chiroptical properties
in terms of ECD and CPL responses (Fig. 4.9) [29]. These helical derivatives
present ECD signatures up to 750 nm with moderate intensity in the visible-red
region (Δε ~ 10 M
À1 cm
À1 ), resulting from partial charge-transfer transitions
involving the nitrogen atoms and the central carbocation. Furthermore, CPL
62
J. Crassous