precisely controlling the emission wavelength as well as the luminescence efficiency
through the rational design and sophisticated structural modification. So far, organic
CPL molecules have shown relatively low luminescence dissymmetry factors (g lum )
in the order 10
À2 to 10
À5 (g lum ¼ 2 Â (I L À I R ) / (I L + I R ), where I L and I R refer to the
intensities of left- and right-handed circularly polarized luminescence observed upon
excitation by unpolarized light).
Boron dipyrromethenes (BODIPYs), being highly fluorescent (Φ lum ¼ 0.93 for
parent BODIPY in methanol [76]), have been successfully employed as scaffolds for
superior CPL materials [77]. The straightforward approach to introduce point-chiral
auxiliary/ies to the parent structure does not appear to work due to the intrinsically
small chiroptical responses of such BODIPY derivatives. Incorporating axial chirality in the BODIPY core to induce a helical twist to the chromophore has been proven
more successful [77–79], but the fluorescence efficiency is significantly altered,
mostly reduced, due to the deformation of the planar chromophore. In our approach
to obtain less-distorted chiral BODIPYs, fully substituted BODIPY derivative B7
was prepared by the condensation of chiral tetraarylcycopentadienes and the subsequent reaction with boron trifluoride in the presence of amine (Fig. 7.14). Indeed, the
propeller chirality is attained in B7, but the magnitudes of chiroptical responses
induced and their temperature-dependence behaviors are quite different from those
of HAB H6.
The variable temperature CD spectra of boron heptaaryldipyrromethene B7 in
methylcyclohexane are shown in Fig. 7.15 (left). The bands at around 600 and
430 nm are assigned to the long- and short-axis transitions of the BODIPY chromophore, respectively, while that at 300 nm to a mixture of the
1
L b and
1 L a transitions of
the radial aromatic rings. At 25
C, practically no CD signals are seen for the
BODIPY transitions at longer wavelengths, while two negative Cotton effects of
similar strengths are observed at 230–340 nm. The Cotton effect pattern analogous
to those reported for various chiral HABs suggests that the clockwise chirality is
favored by this BODIPY-based propeller molecule incorporating the (R)-point chiral
auxiliaries at the periphery of radial aromatic rings, as is the case with the chiral
B7
R
R* =
O
N B
N
F F
R*O
R*O
OR*
OR*
OR*
OR*
R*O
Fig. 7.14 Propeller chiral
boron
heptaaryldipyrromethene
(heptaaryl-BODIPY)
7 Propeller Chirality: Circular Dichroism and Circularly Polarized Luminescence
165
through the rational design and sophisticated structural modification. So far, organic
CPL molecules have shown relatively low luminescence dissymmetry factors (g lum )
in the order 10
À2 to 10
À5 (g lum ¼ 2 Â (I L À I R ) / (I L + I R ), where I L and I R refer to the
intensities of left- and right-handed circularly polarized luminescence observed upon
excitation by unpolarized light).
Boron dipyrromethenes (BODIPYs), being highly fluorescent (Φ lum ¼ 0.93 for
parent BODIPY in methanol [76]), have been successfully employed as scaffolds for
superior CPL materials [77]. The straightforward approach to introduce point-chiral
auxiliary/ies to the parent structure does not appear to work due to the intrinsically
small chiroptical responses of such BODIPY derivatives. Incorporating axial chirality in the BODIPY core to induce a helical twist to the chromophore has been proven
more successful [77–79], but the fluorescence efficiency is significantly altered,
mostly reduced, due to the deformation of the planar chromophore. In our approach
to obtain less-distorted chiral BODIPYs, fully substituted BODIPY derivative B7
was prepared by the condensation of chiral tetraarylcycopentadienes and the subsequent reaction with boron trifluoride in the presence of amine (Fig. 7.14). Indeed, the
propeller chirality is attained in B7, but the magnitudes of chiroptical responses
induced and their temperature-dependence behaviors are quite different from those
of HAB H6.
The variable temperature CD spectra of boron heptaaryldipyrromethene B7 in
methylcyclohexane are shown in Fig. 7.15 (left). The bands at around 600 and
430 nm are assigned to the long- and short-axis transitions of the BODIPY chromophore, respectively, while that at 300 nm to a mixture of the
1
L b and
1 L a transitions of
the radial aromatic rings. At 25
C, practically no CD signals are seen for the
BODIPY transitions at longer wavelengths, while two negative Cotton effects of
similar strengths are observed at 230–340 nm. The Cotton effect pattern analogous
to those reported for various chiral HABs suggests that the clockwise chirality is
favored by this BODIPY-based propeller molecule incorporating the (R)-point chiral
auxiliaries at the periphery of radial aromatic rings, as is the case with the chiral
B7
R
R* =
O
N B
N
F F
R*O
R*O
OR*
OR*
OR*
OR*
R*O
Fig. 7.14 Propeller chiral
boron
heptaaryldipyrromethene
(heptaaryl-BODIPY)
7 Propeller Chirality: Circular Dichroism and Circularly Polarized Luminescence
165