incorporating the whizzing toroids in the equilibrium of the C and CC propellers and
indicate that their relative contribution as well as the interconversion dynamics play
essential roles in determining the (chir)optical properties.
7.6 Propeller Inversion Dynamics
The energy profile for the propeller-chirality inversion of fully chiral HAB H6 has
been theoretically evaluated at the SCS-MP2/def2-TZVPP level incorporating the
COSMO solvation model for methylcyclohexane. This level of theory has been
proven to be quite accurate in describing the energy profiles of a variety of systems,
especially where the weak interactions play critical roles [69–72]. A small energy
difference of 0.8 kcal mol
À1 found for the C versus CC propeller geometries of H6
corresponds to a C:CC ratio of 97:3, or 94% enantiomeric excess, at À150
C.
Careful kinetic studies on the blade inversion of substituted HABs have already
demonstrated that the blades do not rotate simultaneously but rather one by one and
the experimental free energy of activation for single inversion varies from 17 to
33 kcal mol
À1 , depending on the substituent introduced [73]. In these studies,
although the possible motion and distortion caused by the single blade rotation
have been described, the cooperative effect has not been discussed any further.
Our theoretical investigation has revealed that the domino inversion is the most
likely mechanism operative in propeller-inversion of HABs. Thus, the synchronous
inversion, where all six radial aromatic blades rotate simultaneously, turned out to
proceed with a fairly large rotation barrier of 8.4 kcal mol
À1 , which is much larger
than the barrier (4.2 kcal mol
À1 ) calculated for the single blade rotation assuming a
fixed geometry for the remaining blades. Although this result clearly demonstrates
the high preference for the single-blade rotation model (even without optimizing the
geometries of the neighboring blades), it seems more probable that when one blade
starts to rotate, the adjacent blades have to change the tilt angles to minimize the
energy and such conformational adjustments propagate in a domino manner to the
rest of the blades to eventually achieve the whole propeller inversion. This domino
inversion model requires an activation energy as low as 2.1 kcal mol
À1
(by theoretical calculation) for the C-to-CC conversion in fully chiral HAB H6.
Such a shallow potential surface well explains the diversity of the twist angles found
in the crystal structures (vide supra) and the whizzing nature of aromatic blades in
various HAB derivatives discussed in the earlier sections.
7.7 Propeller Chirality in Boron Heptaaryldipyrromethene
Recently, much attention has been paid to the preparation, analysis, and development of efficient circularly polarized luminescence (CPL) materials of small or
simple organic molecules [74, 75]. Such molecules should be advantageous for
164
T. Mori
indicate that their relative contribution as well as the interconversion dynamics play
essential roles in determining the (chir)optical properties.
7.6 Propeller Inversion Dynamics
The energy profile for the propeller-chirality inversion of fully chiral HAB H6 has
been theoretically evaluated at the SCS-MP2/def2-TZVPP level incorporating the
COSMO solvation model for methylcyclohexane. This level of theory has been
proven to be quite accurate in describing the energy profiles of a variety of systems,
especially where the weak interactions play critical roles [69–72]. A small energy
difference of 0.8 kcal mol
À1 found for the C versus CC propeller geometries of H6
corresponds to a C:CC ratio of 97:3, or 94% enantiomeric excess, at À150
C.
Careful kinetic studies on the blade inversion of substituted HABs have already
demonstrated that the blades do not rotate simultaneously but rather one by one and
the experimental free energy of activation for single inversion varies from 17 to
33 kcal mol
À1 , depending on the substituent introduced [73]. In these studies,
although the possible motion and distortion caused by the single blade rotation
have been described, the cooperative effect has not been discussed any further.
Our theoretical investigation has revealed that the domino inversion is the most
likely mechanism operative in propeller-inversion of HABs. Thus, the synchronous
inversion, where all six radial aromatic blades rotate simultaneously, turned out to
proceed with a fairly large rotation barrier of 8.4 kcal mol
À1 , which is much larger
than the barrier (4.2 kcal mol
À1 ) calculated for the single blade rotation assuming a
fixed geometry for the remaining blades. Although this result clearly demonstrates
the high preference for the single-blade rotation model (even without optimizing the
geometries of the neighboring blades), it seems more probable that when one blade
starts to rotate, the adjacent blades have to change the tilt angles to minimize the
energy and such conformational adjustments propagate in a domino manner to the
rest of the blades to eventually achieve the whole propeller inversion. This domino
inversion model requires an activation energy as low as 2.1 kcal mol
À1
(by theoretical calculation) for the C-to-CC conversion in fully chiral HAB H6.
Such a shallow potential surface well explains the diversity of the twist angles found
in the crystal structures (vide supra) and the whizzing nature of aromatic blades in
various HAB derivatives discussed in the earlier sections.
7.7 Propeller Chirality in Boron Heptaaryldipyrromethene
Recently, much attention has been paid to the preparation, analysis, and development of efficient circularly polarized luminescence (CPL) materials of small or
simple organic molecules [74, 75]. Such molecules should be advantageous for
164
T. Mori