augmentation, gradually saturating at higher substitutions, may indicate existence
of the equilibrium between right- and left-handed (or C-CC) propellers in H6.
Because the UV-vis spectra of these HABs slightly differ from each other
(presumably due to the different degrees of interaction between the blades), a
quantitative comparison of the CD intensities of H1–H6 (as a function of the number
of chiral unit) is more accurately performed by using their dissymmetry factors (g ¼
Δε/ε). As can be seen from Fig. 7.7 (left), the g factors for all the extrema at
229, 266, and 292 nm almost linearly increase with increasing number (n) of the
chiral unit until n reaches two (i.e., H1, H2, and H2P), but gradually saturate at
n ¼ 4–6 (H4 and H6). It is interesting that the CD spectra of H2 and H2P do not
appreciably differ in shape and intensity (in methylcyclohexane), revealing that the
relative position has almost no effect and only the number of chiral auxiliary is
important in forming the chiral propeller. These results reveal that the impact of
chiral modification at the periphery of propeller blade is not restricted to the adjacent
blade but further propagates to more distant blades, being driven by the domino
effect. In this system, the C and CC propellers are in equilibrium and the energy
difference between them is incrementally enhanced with increasing number of the
chiral modification to shift the equilibrium to the favored propeller with accompanying augmentation of CD intensity.
The specific rotations ([α] D ) of H1–H6 (in chloroform at 25
C) progressively but
somewhat irregularly increases from À19
for H1 to À120
for H6, all of which are
larger than À13
, that of (R)-1-methylpropanol, introduced as the chiral unit. As
shown in Fig. 7.7 (right), the changing pattern of the [α] D value substantially differs
Fig. 7.7 Plots of the dissymmetry (g) factor (left) and the specific rotation (right) against the
number of chiral unit in HABs
7 Propeller Chirality: Circular Dichroism and Circularly Polarized Luminescence
157
of the equilibrium between right- and left-handed (or C-CC) propellers in H6.
Because the UV-vis spectra of these HABs slightly differ from each other
(presumably due to the different degrees of interaction between the blades), a
quantitative comparison of the CD intensities of H1–H6 (as a function of the number
of chiral unit) is more accurately performed by using their dissymmetry factors (g ¼
Δε/ε). As can be seen from Fig. 7.7 (left), the g factors for all the extrema at
229, 266, and 292 nm almost linearly increase with increasing number (n) of the
chiral unit until n reaches two (i.e., H1, H2, and H2P), but gradually saturate at
n ¼ 4–6 (H4 and H6). It is interesting that the CD spectra of H2 and H2P do not
appreciably differ in shape and intensity (in methylcyclohexane), revealing that the
relative position has almost no effect and only the number of chiral auxiliary is
important in forming the chiral propeller. These results reveal that the impact of
chiral modification at the periphery of propeller blade is not restricted to the adjacent
blade but further propagates to more distant blades, being driven by the domino
effect. In this system, the C and CC propellers are in equilibrium and the energy
difference between them is incrementally enhanced with increasing number of the
chiral modification to shift the equilibrium to the favored propeller with accompanying augmentation of CD intensity.
The specific rotations ([α] D ) of H1–H6 (in chloroform at 25
C) progressively but
somewhat irregularly increases from À19
for H1 to À120
for H6, all of which are
larger than À13
, that of (R)-1-methylpropanol, introduced as the chiral unit. As
shown in Fig. 7.7 (right), the changing pattern of the [α] D value substantially differs
Fig. 7.7 Plots of the dissymmetry (g) factor (left) and the specific rotation (right) against the
number of chiral unit in HABs
7 Propeller Chirality: Circular Dichroism and Circularly Polarized Luminescence
157