294
T. Mori
providing overall propeller-shaped geometries. More importantly, the dynamics of
toroidal interaction in HABs have been less explored and mostly ignored in many
situations. We have been independently attracted in the chirality of HABs. By small
modification at the periphery of aromatic rings (i.e., attaching chiral auxiliaries),
clockwise or counter-clockwise propeller shape is induced in HABs, exhibiting
unique chiroptical responses [45–48]. Remarkably, the propeller chirality is not a
simple summation of multiple axial chirality, but the peripheral aromatic rings show
cooperative or synergetic dynamism in a domino-like movement. Theoretical calculation as well as condensed phase structural study revealed that the rotation of propeller
blades is quite feasible, but the overall propeller conformation is easily developed
in non-polar solvent at low temperature. Because the propeller chirality is dynamic
chirality, the responses were found quite sensible to the environmental variant such as
solvent polarity, temperature, as well as hydrostatic pressure. Very recently, propeller
chiral HAB composed of sterically congested 3,6-dimethoxycarbazole units has been
also documented and the enantiomers were separated by chiral HPLC [49].
A detailed study of environmental variants such as solvent, temperature, as well as
pressure, on the propeller chirality of HABs revealed the dynamic nature of conformational equilibria of HABs. Briefly, the clockwise and counter-clockwise propeller
conformations are dominant under the low temperature in less polar solvent, but
the contribution of whizzing toroids becomes substantial at increased temperature.
The solvation to the peripheral aromatic rings becomes more significant in polar
solvents. In addition, solvent molecules may intervene the void space between the
peripheral aromatic rings that increase the effective tilt angle of the blades against the
central benzene ring. All of these factors influence the conformational equilibrium
between clockwise and counter-clockwise propeller as well as toroidal structure.
Such a unique feature may be explored in advanced functional chiroptical materials and the process of energy and/or exciton transfer and migration. Indeed, we
successfully demonstrated that the CPL response from the propeller chiral HABs
can be switched on and off, by controlling the dynamism of propeller and toroidal
structures (Fig. 17.5).
17.4 Synergetic TTA Through the Toroidal Interaction
in HABs
A significant feature shared by photosynthetic organisms is that they capture photons
in the form of excitons delocalized over tens of pigment molecules embedded in
protein environments of light-harvesting complexes, typically in cylindrical manners.
Delocalized excitons created remain well protected and are successfully delivered
to the reaction center. It is also known that the conversion of photon into chemical
energy is driven by electronic couplings between the pigments that ensure the efficient transport of energy from light-capturing antenna pigments to the destinations
(Fig. 17.6). The role of delocalized excitons and tubular arrangement in achieving
T. Mori
providing overall propeller-shaped geometries. More importantly, the dynamics of
toroidal interaction in HABs have been less explored and mostly ignored in many
situations. We have been independently attracted in the chirality of HABs. By small
modification at the periphery of aromatic rings (i.e., attaching chiral auxiliaries),
clockwise or counter-clockwise propeller shape is induced in HABs, exhibiting
unique chiroptical responses [45–48]. Remarkably, the propeller chirality is not a
simple summation of multiple axial chirality, but the peripheral aromatic rings show
cooperative or synergetic dynamism in a domino-like movement. Theoretical calculation as well as condensed phase structural study revealed that the rotation of propeller
blades is quite feasible, but the overall propeller conformation is easily developed
in non-polar solvent at low temperature. Because the propeller chirality is dynamic
chirality, the responses were found quite sensible to the environmental variant such as
solvent polarity, temperature, as well as hydrostatic pressure. Very recently, propeller
chiral HAB composed of sterically congested 3,6-dimethoxycarbazole units has been
also documented and the enantiomers were separated by chiral HPLC [49].
A detailed study of environmental variants such as solvent, temperature, as well as
pressure, on the propeller chirality of HABs revealed the dynamic nature of conformational equilibria of HABs. Briefly, the clockwise and counter-clockwise propeller
conformations are dominant under the low temperature in less polar solvent, but
the contribution of whizzing toroids becomes substantial at increased temperature.
The solvation to the peripheral aromatic rings becomes more significant in polar
solvents. In addition, solvent molecules may intervene the void space between the
peripheral aromatic rings that increase the effective tilt angle of the blades against the
central benzene ring. All of these factors influence the conformational equilibrium
between clockwise and counter-clockwise propeller as well as toroidal structure.
Such a unique feature may be explored in advanced functional chiroptical materials and the process of energy and/or exciton transfer and migration. Indeed, we
successfully demonstrated that the CPL response from the propeller chiral HABs
can be switched on and off, by controlling the dynamism of propeller and toroidal
structures (Fig. 17.5).
17.4 Synergetic TTA Through the Toroidal Interaction
in HABs
A significant feature shared by photosynthetic organisms is that they capture photons
in the form of excitons delocalized over tens of pigment molecules embedded in
protein environments of light-harvesting complexes, typically in cylindrical manners.
Delocalized excitons created remain well protected and are successfully delivered
to the reaction center. It is also known that the conversion of photon into chemical
energy is driven by electronic couplings between the pigments that ensure the efficient transport of energy from light-capturing antenna pigments to the destinations
(Fig. 17.6). The role of delocalized excitons and tubular arrangement in achieving