six biaryl units share the central benzene ring. Each two aromatic rings in the biaryl
unit is in general not perpendicular to each other, due to the π-conjugation with the
central benzene, which is counterbalanced by the steric conflict with the orthohydrogens (or substituents) of adjacent aromatic blades [13, 35–40]. This gives
rise to the atropisomerism, the origin of axial chirality. The adjacent radial aromatic
rings in HABs are separated by ca. 2.9 Å at the ipso positions, which is appreciably
shorter than the van der Waals contact. Accordingly, the six aromatic blades
synchronously tilt in one direction to give a propeller geometry, which is enantiomeric, being twisted in either clockwise (C) or counterclockwise (CC) manner (Fig. 7.3). The short inter-blade distance practically inhibits the independent
rotation of a single blade but allows synchronized rotation of all the blades, which
enables us to modulate the inter-blade interaction and thus the chiroptical responses.
The syntheses [41, 42] and unique properties of HABs and related compounds
have attracted much attention for the practical applications in organic light-emitting
diodes, photochemical switches, redox materials, molecular receptors, and liquid
crystalline materials [34]. It is believed that the global electron and/or exciton
delocalization over the entire radial aromatic rings occurs in the doughnut-shaped
scaffold (Fig. 7.4), which is often referred to as “toroidal interaction” [43]. This
unique interaction has been extensively studied in conjunction with the sequential
donor-acceptor interaction, the electron/energy migration, and the π-delocalization
by modulating the global interaction of radial aromatic rings [44–52]. Highlighting
the toroidal interaction, all the radial aromatic rings are assumed to be orthogonally
arranged in these studies, where the energetically more realistic propeller structure
Clockwise (C)
Counter-Clockwise (CC)
Fig. 7.3 Clockwise (C) and
counterclockwise (CC)
propeller chirality in
hexaarylbenzene (HAB)
Fig. 7.4 Schematic
drawing of the toroidal
interaction in HAB
154
T. Mori
unit is in general not perpendicular to each other, due to the π-conjugation with the
central benzene, which is counterbalanced by the steric conflict with the orthohydrogens (or substituents) of adjacent aromatic blades [13, 35–40]. This gives
rise to the atropisomerism, the origin of axial chirality. The adjacent radial aromatic
rings in HABs are separated by ca. 2.9 Å at the ipso positions, which is appreciably
shorter than the van der Waals contact. Accordingly, the six aromatic blades
synchronously tilt in one direction to give a propeller geometry, which is enantiomeric, being twisted in either clockwise (C) or counterclockwise (CC) manner (Fig. 7.3). The short inter-blade distance practically inhibits the independent
rotation of a single blade but allows synchronized rotation of all the blades, which
enables us to modulate the inter-blade interaction and thus the chiroptical responses.
The syntheses [41, 42] and unique properties of HABs and related compounds
have attracted much attention for the practical applications in organic light-emitting
diodes, photochemical switches, redox materials, molecular receptors, and liquid
crystalline materials [34]. It is believed that the global electron and/or exciton
delocalization over the entire radial aromatic rings occurs in the doughnut-shaped
scaffold (Fig. 7.4), which is often referred to as “toroidal interaction” [43]. This
unique interaction has been extensively studied in conjunction with the sequential
donor-acceptor interaction, the electron/energy migration, and the π-delocalization
by modulating the global interaction of radial aromatic rings [44–52]. Highlighting
the toroidal interaction, all the radial aromatic rings are assumed to be orthogonally
arranged in these studies, where the energetically more realistic propeller structure
Clockwise (C)
Counter-Clockwise (CC)
Fig. 7.3 Clockwise (C) and
counterclockwise (CC)
propeller chirality in
hexaarylbenzene (HAB)
Fig. 7.4 Schematic
drawing of the toroidal
interaction in HAB
154
T. Mori