HAB propellers. This speculation is definitely supported by the theoretical calculations. The molar CD (Δε) values for B7 in methylcyclohexane at 25
C are À5.2 and
À4.3 M
À1 cm
À1 at 229 and 281 nm, respectively, both of which are roughly ten
times smaller than those of H6 under the comparable conditions, but still much
larger than those of the reference compounds, i.e., chirally modified aryl ether and
biphenyl (À0.4 and À0.5 M
À1 cm
À1 ) [13]. The following structural and electronic
differences between B7 and H6 would be responsible for the tenfold smaller molar
CD observed for the
1 L b and
1 L a transitions of B7. Although the radial aromatic rings
interact with each other in B7, the domino effect is less efficient in B7 than in H6, as
a consequence of the larger separation between two adjacent radial aromatic rings,
which measures 3.1 Å at the ipso carbon for B7 but 2.9 Å for H6. The smaller twist
angles (%55
on average) of radial aromatic rings should also diminish the CD
intensity. More importantly, an interplane interaction is partially missing in B7 at the
boron atom, which should lead to a significant loss of CD intensity, as demonstrated
in a study to elucidate how the toroidal interaction is affected by losing one or more
blades from HAB [52].
The temperature dependence of the CD spectra of B7 has been analyzed quantitatively. By decreasing the temperature, the Cotton effects at the
1 L b and
1 L a band
region of radial aromatic rings (below 300 nm) gradually grows in intensity (until the
transition temperature of T d ¼ À70
C is reached) to give the extremal Δε of
ca. À14 M
À1 cm
À1 at T d and then decreases. A closer examination reveals an
additional inflection point at T c ¼ +10
C in the positive-slope region, disclosing
the existence of three different temperature domains for B7 (instead of two for
HABs). The variable temperature UV spectra of B7 behave similarly to show three
distinct temperature domains for the temperature-dependence profiles of both excitation wavelength and absorptivity (Fig. 7.15, right). The BODIPY’s long-axis
transition in the low-energy region exhibits more dramatic variable temperature CD
spectral changes as shown in Fig. 7.15 (right). Crucially, this band is essentially CD
silent at higher temperatures but starts to develop a positive excitonic couplet near
Fig. 7.15 Left: variable temperature CD spectra of B7 in methylcyclohexane. Right: plots of the
CD couplet amplitude A (solid circle) and the excitation energy E max (open circle) evaluated from
the absorption maximum of the long-axis transition of the BODIPY chromophore of B7
166
T. Mori
C are À5.2 and
À4.3 M
À1 cm
À1 at 229 and 281 nm, respectively, both of which are roughly ten
times smaller than those of H6 under the comparable conditions, but still much
larger than those of the reference compounds, i.e., chirally modified aryl ether and
biphenyl (À0.4 and À0.5 M
À1 cm
À1 ) [13]. The following structural and electronic
differences between B7 and H6 would be responsible for the tenfold smaller molar
CD observed for the
1 L b and
1 L a transitions of B7. Although the radial aromatic rings
interact with each other in B7, the domino effect is less efficient in B7 than in H6, as
a consequence of the larger separation between two adjacent radial aromatic rings,
which measures 3.1 Å at the ipso carbon for B7 but 2.9 Å for H6. The smaller twist
angles (%55
on average) of radial aromatic rings should also diminish the CD
intensity. More importantly, an interplane interaction is partially missing in B7 at the
boron atom, which should lead to a significant loss of CD intensity, as demonstrated
in a study to elucidate how the toroidal interaction is affected by losing one or more
blades from HAB [52].
The temperature dependence of the CD spectra of B7 has been analyzed quantitatively. By decreasing the temperature, the Cotton effects at the
1 L b and
1 L a band
region of radial aromatic rings (below 300 nm) gradually grows in intensity (until the
transition temperature of T d ¼ À70
C is reached) to give the extremal Δε of
ca. À14 M
À1 cm
À1 at T d and then decreases. A closer examination reveals an
additional inflection point at T c ¼ +10
C in the positive-slope region, disclosing
the existence of three different temperature domains for B7 (instead of two for
HABs). The variable temperature UV spectra of B7 behave similarly to show three
distinct temperature domains for the temperature-dependence profiles of both excitation wavelength and absorptivity (Fig. 7.15, right). The BODIPY’s long-axis
transition in the low-energy region exhibits more dramatic variable temperature CD
spectral changes as shown in Fig. 7.15 (right). Crucially, this band is essentially CD
silent at higher temperatures but starts to develop a positive excitonic couplet near
Fig. 7.15 Left: variable temperature CD spectra of B7 in methylcyclohexane. Right: plots of the
CD couplet amplitude A (solid circle) and the excitation energy E max (open circle) evaluated from
the absorption maximum of the long-axis transition of the BODIPY chromophore of B7
166
T. Mori