less fluorescent [80–85]. In this regard, the moderate fluorescence observed for B7 at
lower temperatures seems reasonable, as the H-type association is naturally avoided
by the steric crowding of radial aromatic rings.
At 25
C, where the contribution of whizzing toroids is substantial, nearly
negligible CPL is observed for both H6 and B7 (Fig. 7.17), a behavior parallel to
the CD spectra that only exhibit the weak Cotton effects (Figs. 7.11 and 7.15). This
situation is remarkably improved in fully chiral HAB H6 when the clockwise
propeller chirality becomes dominant by reducing the temperature to À120
C,
affording positive CPL with a reasonably large g lum of À6.0 Â 10
À4 at 340 nm.
On the other hand, having the same clockwise propeller chirality, fully chiral
heptaaryl-BODIPY B7 gives the oppositely signed, positive CPL with a much larger
g lum of 2.0 Â 10
À3 at 650 nm under the comparable conditions, which is rationalized
by the formation of right-twisted supramolecular dimer (vide supra).
7.10 Perspective
Thanks to the sterical congestion of the adjacent aromatic rings, the conceptually
new propeller chirality is induced in highly substituted benzenes and related aromatics. In this new type of chirality, an array of axially chiral aromatic blades
attached to the hub behaves rather cooperatively to achieve very strong chiroptical
responses, which are accordingly not static but highly dynamic and hence feasibly
manipulatable in nature. We showed above such extraordinary features of the
propeller chirality using the circular dichroism and circularly polarized luminescence of HABs and heptaaryl-BODIPY. By attaching a small chiral auxiliary at a
periphery of radial aromatic blades, the whole propeller-shaped molecule of rich
conformational diversity becomes highly susceptive to various environmental
Fig. 7.17 Circularly polarized luminescence and intensity-normalized total luminescence spectra
of HAB H6 and heptaaryl-BODIPY B7 in methylcyclohexane at 25
C (dotted lines) and À120
C
(solid lines). Excitation wavelengths: 260 and 440 nm for H6 and B7, respectively
7 Propeller Chirality: Circular Dichroism and Circularly Polarized Luminescence
169
lower temperatures seems reasonable, as the H-type association is naturally avoided
by the steric crowding of radial aromatic rings.
At 25
C, where the contribution of whizzing toroids is substantial, nearly
negligible CPL is observed for both H6 and B7 (Fig. 7.17), a behavior parallel to
the CD spectra that only exhibit the weak Cotton effects (Figs. 7.11 and 7.15). This
situation is remarkably improved in fully chiral HAB H6 when the clockwise
propeller chirality becomes dominant by reducing the temperature to À120
C,
affording positive CPL with a reasonably large g lum of À6.0 Â 10
À4 at 340 nm.
On the other hand, having the same clockwise propeller chirality, fully chiral
heptaaryl-BODIPY B7 gives the oppositely signed, positive CPL with a much larger
g lum of 2.0 Â 10
À3 at 650 nm under the comparable conditions, which is rationalized
by the formation of right-twisted supramolecular dimer (vide supra).
7.10 Perspective
Thanks to the sterical congestion of the adjacent aromatic rings, the conceptually
new propeller chirality is induced in highly substituted benzenes and related aromatics. In this new type of chirality, an array of axially chiral aromatic blades
attached to the hub behaves rather cooperatively to achieve very strong chiroptical
responses, which are accordingly not static but highly dynamic and hence feasibly
manipulatable in nature. We showed above such extraordinary features of the
propeller chirality using the circular dichroism and circularly polarized luminescence of HABs and heptaaryl-BODIPY. By attaching a small chiral auxiliary at a
periphery of radial aromatic blades, the whole propeller-shaped molecule of rich
conformational diversity becomes highly susceptive to various environmental
Fig. 7.17 Circularly polarized luminescence and intensity-normalized total luminescence spectra
of HAB H6 and heptaaryl-BODIPY B7 in methylcyclohexane at 25
C (dotted lines) and À120
C
(solid lines). Excitation wavelengths: 260 and 440 nm for H6 and B7, respectively
7 Propeller Chirality: Circular Dichroism and Circularly Polarized Luminescence
169