addition of an appropriate combination of tetraarylcyclopentadienone and
diarylacetylenes, followed by the aromatization via decarbonylation. Note that the
point-chiral alkyl group (R
à ) employed is the simplest, i.e., (R)-1-methylpropyl.
Although the chiroptical behavior of oligomeric solid materials containing HAB unit
has been reported [53], chiral HABs are still relatively rare and little is known about
the chiroptical properties of such molecules.
Circular dichroism (CD) spectra of chiral HABs with varying number of the
chiral auxiliary at the para-position of radial phenyls (H1–H6) are compared in
Fig. 7.6. The intense trisignate Cotton effects (with a negative-positive-negative
pattern for the R-configuration) observed are assignable to the
1 L b ,
1 L a , and
1 B b
transitions, respectively, from low to high excitation energy. The CD spectrum of
H6 shows no concentration dependence, indicating the absence of aggregation in
this system, which is in sharp contrast to the dimer formation of boron
dipyrromethenes (BODIPY) derivative (vide infra) [29]; probably the aggregation
or stacking is infeasible for such a highly substituted, less polarized compound as
H6. Crucially, the overall spectral shapes are practically indistinguishable for all
the HABs, suggesting that similar propeller geometries are evoked irrespective of
the number of introduced chiral auxiliary. It is to note that alkoxybenzene and
4,4
0 -dialkoxylbiphenyl carrying the same chiral auxiliary as reference compounds
afford molar CD values (Δε) as low as À0.4 and À0.5 M
À1 cm
À1 , respectively,
for the lowest-energy
1 L b band [13], which however seems reasonable if the
remote chiral modification at a position far from the chromophore is taken into
account. In this context, it would be surprising that the single chiral auxiliary
introduced to HAB at a remote para-position enhances the Δε value of the
1 L b
band of H1 up to À7.7 M
À1 cm
À1 . This sudden increase of molar CD, though
difficult to explain as a direct influence of the peripheral chiral auxiliary alone, is
not unexpected but is rather taken as experimental evidence supporting the formation of the hypothetical chiral propeller driven by the synchronized unidirectional “domino” twisting of six radial phenyls. The CD intensity increases with
increasing number of the chiral auxiliary to afford nearly doubled Δε values for
H2 and H2P and 4.6-fold larger À35.1 M
À1 cm
À1 for H6. This progressive CD
Fig. 7.6 Comparison of CD
spectra of HABs H1–H6 in
methylcyclohexane at 25
C
156
T. Mori
diarylacetylenes, followed by the aromatization via decarbonylation. Note that the
point-chiral alkyl group (R
à ) employed is the simplest, i.e., (R)-1-methylpropyl.
Although the chiroptical behavior of oligomeric solid materials containing HAB unit
has been reported [53], chiral HABs are still relatively rare and little is known about
the chiroptical properties of such molecules.
Circular dichroism (CD) spectra of chiral HABs with varying number of the
chiral auxiliary at the para-position of radial phenyls (H1–H6) are compared in
Fig. 7.6. The intense trisignate Cotton effects (with a negative-positive-negative
pattern for the R-configuration) observed are assignable to the
1 L b ,
1 L a , and
1 B b
transitions, respectively, from low to high excitation energy. The CD spectrum of
H6 shows no concentration dependence, indicating the absence of aggregation in
this system, which is in sharp contrast to the dimer formation of boron
dipyrromethenes (BODIPY) derivative (vide infra) [29]; probably the aggregation
or stacking is infeasible for such a highly substituted, less polarized compound as
H6. Crucially, the overall spectral shapes are practically indistinguishable for all
the HABs, suggesting that similar propeller geometries are evoked irrespective of
the number of introduced chiral auxiliary. It is to note that alkoxybenzene and
4,4
0 -dialkoxylbiphenyl carrying the same chiral auxiliary as reference compounds
afford molar CD values (Δε) as low as À0.4 and À0.5 M
À1 cm
À1 , respectively,
for the lowest-energy
1 L b band [13], which however seems reasonable if the
remote chiral modification at a position far from the chromophore is taken into
account. In this context, it would be surprising that the single chiral auxiliary
introduced to HAB at a remote para-position enhances the Δε value of the
1 L b
band of H1 up to À7.7 M
À1 cm
À1 . This sudden increase of molar CD, though
difficult to explain as a direct influence of the peripheral chiral auxiliary alone, is
not unexpected but is rather taken as experimental evidence supporting the formation of the hypothetical chiral propeller driven by the synchronized unidirectional “domino” twisting of six radial phenyls. The CD intensity increases with
increasing number of the chiral auxiliary to afford nearly doubled Δε values for
H2 and H2P and 4.6-fold larger À35.1 M
À1 cm
À1 for H6. This progressive CD
Fig. 7.6 Comparison of CD
spectra of HABs H1–H6 in
methylcyclohexane at 25
C
156
T. Mori