4.8.3 CPL-Active Heptahelicenic Structures
In 2012, Tanaka et al. reported the preparation of helically chiral 1,1
0 -bitriphenylenes
displaying a central fluorenyl cycle. These compounds correspond to heptahelicenic
structures and exhibit among the strongest CPL activity [59]. Indeed, compounds
62 and 63 (Fig. 4.24) obtained with high enantiomeric excesses (93% and 91% ee,
respectively) exhibited mirror-imaged CPL spectra with particularly high fluorescence
dissymmetry factors (g lum ¼ À0.030 at 428 nm for (M)-(À)-62 and g lum ¼ À0.032 at
449 nm for (M)-(À)-63 in chloroform, Table 4.11), which are comparable to
phthalhydrazide-functionalized [7]helicene-like molecule 34 (g lum ¼ À0.035 at
476 nm for the assembly state and À0.021 for the molecularly dispersed state)
and significantly larger than those for helically chiral molecules reported to date.
However, the g lum values are found very high compared to the g abs values, which
are around 2.5–2.9 Â 10
À3
, i.e., one order of magnitude lower. This is clearly seen
in the ECD shape, which displays the typically large bands of (P)-helicenes (a large
positive band at ~320 nm, and two large negative bands at ~260 and 285 nm,
Fig. 4.25) but small bands between 340 and 400 nm which correspond to the lowest
energy transitions. Later on in 2016, Nozaki and coworkers reported the synthesis
and photophysical properties of enantiopure [7]helicene-like fluorenyl systems 64a,b
[60]. These compounds showed similar absorption spectra with the longest absorption
maximum at ~400 nm slightly blue-shifted compared to that of carbo[7]helicene, while
they are significantly red-shifted compared to those of fluorene and phenanthrene.
Indeed, the π-conjugation is well extended over the whole molecule despite their
helically twisted structures. In addition, the longest absorption maximum is slightly
red-shifted compared to that of compound 62 despite the fact that 62 possesses
triphenylene units [57]. This phenomenon may be attributed to a smaller twist of
compounds 64 than compound 62, resulting in more effective π-conjugation
along the helical structure. [7]Helicene-like compounds 64a,b exhibited an emission
maximum at around 420 nm with a very small Stokes shift and high fluorescence
quantum yields (up to 40%) among the highest reported helicenes at that time and
slightly higher than those of [7]helicene-like compounds [59]. This makes helicenes
incorporating a fluorene unit very appealing for highly emissive chiral molecular
materials. ECD spectra of 64a,b enantiomers display a similar shape as for 62 and
(M)-(-)-62 (93% ee)
(M)-(-)-63 (91% ee)
(P)-65
N
N
(P)-(+)-64a R=H
(P)-(+)-64b R=Ph
R
R
Fig. 4.24 Chemical structures of CPL-active heptahelicenic derivatives
4 Circularly Polarized Luminescence in Helicene and Helicenoid Derivatives
81
In 2012, Tanaka et al. reported the preparation of helically chiral 1,1
0 -bitriphenylenes
displaying a central fluorenyl cycle. These compounds correspond to heptahelicenic
structures and exhibit among the strongest CPL activity [59]. Indeed, compounds
62 and 63 (Fig. 4.24) obtained with high enantiomeric excesses (93% and 91% ee,
respectively) exhibited mirror-imaged CPL spectra with particularly high fluorescence
dissymmetry factors (g lum ¼ À0.030 at 428 nm for (M)-(À)-62 and g lum ¼ À0.032 at
449 nm for (M)-(À)-63 in chloroform, Table 4.11), which are comparable to
phthalhydrazide-functionalized [7]helicene-like molecule 34 (g lum ¼ À0.035 at
476 nm for the assembly state and À0.021 for the molecularly dispersed state)
and significantly larger than those for helically chiral molecules reported to date.
However, the g lum values are found very high compared to the g abs values, which
are around 2.5–2.9 Â 10
À3
, i.e., one order of magnitude lower. This is clearly seen
in the ECD shape, which displays the typically large bands of (P)-helicenes (a large
positive band at ~320 nm, and two large negative bands at ~260 and 285 nm,
Fig. 4.25) but small bands between 340 and 400 nm which correspond to the lowest
energy transitions. Later on in 2016, Nozaki and coworkers reported the synthesis
and photophysical properties of enantiopure [7]helicene-like fluorenyl systems 64a,b
[60]. These compounds showed similar absorption spectra with the longest absorption
maximum at ~400 nm slightly blue-shifted compared to that of carbo[7]helicene, while
they are significantly red-shifted compared to those of fluorene and phenanthrene.
Indeed, the π-conjugation is well extended over the whole molecule despite their
helically twisted structures. In addition, the longest absorption maximum is slightly
red-shifted compared to that of compound 62 despite the fact that 62 possesses
triphenylene units [57]. This phenomenon may be attributed to a smaller twist of
compounds 64 than compound 62, resulting in more effective π-conjugation
along the helical structure. [7]Helicene-like compounds 64a,b exhibited an emission
maximum at around 420 nm with a very small Stokes shift and high fluorescence
quantum yields (up to 40%) among the highest reported helicenes at that time and
slightly higher than those of [7]helicene-like compounds [59]. This makes helicenes
incorporating a fluorene unit very appealing for highly emissive chiral molecular
materials. ECD spectra of 64a,b enantiomers display a similar shape as for 62 and
(M)-(-)-62 (93% ee)
(M)-(-)-63 (91% ee)
(P)-65
N
N
(P)-(+)-64a R=H
(P)-(+)-64b R=Ph
R
R
Fig. 4.24 Chemical structures of CPL-active heptahelicenic derivatives
4 Circularly Polarized Luminescence in Helicene and Helicenoid Derivatives
81