of multiple chromophores (or fluorophores) could be an important source of optical
chirality including CD and CPL activities. On the basis of the theory, the chiroptical
switch can be achieved if the external stimuli modulate the chiral chromophoric
arrangement in molecular systems. Figure 8.4 depicts a schematic design of possible
CPL-photo-switch. The photo-responsive unit with a chirally controlled geometry
should carry multiple fluorophores, arranging them in a chiral manner so that
the exciton coupling interaction operates on them. The photo-irradiation induces
the structural change in the photo-responsive unit, leading to a change in the
fluorophoric arrangement to modulate the inter-fluorophore exciton coupling.
Chiral photochromic scaffold with a helical secondary structure was employed as
a platform for modulating CPL activity through a control of chiral fluorophore
arrangement in an intramolecular manner. The central ethene part of diarylethene
was replaced with a heteroaromatic ring to form a tetra-arylene scaffold with keeping
the photochromic capability [29]. The use of oligo-heteroarylene framework enables
the molecule to take advantage of inter-heteroaryl-unit interactions involving heteroatoms so that they can adopt the molecular conformation suitable for the electrocyclization reaction [30–33]. The extension of heteroaromatic units led to the
formation of helical foldamers with photo-reactivity [34–36]. Tetra-arylenes composed of 2-phenylthiazole unit with a cis-cisoid-like connectivity form a one-turn
helical secondary structure as the most stable conformation, which is also suitable
for the stereo-specific electro-cyclization reaction (Fig. 8.5) [34–36]. The incorporation of chiral groups as a side-chain unit readily controlled the handedness of
helicity. For the design of CPL photo-switch, a pyrene group was introduced at each
Fig. 8.4 Schematic design of a CPL-photo-switch
N
S
S
N
S
N
N
S
N
S
S
N
folding
vis
S
N
N
S
N
S
S
N
N
S
N
S
UV
Fig. 8.5 Conformational folding and photoreactions of tetra(2-phenylthiazole)
180
T. Nakashima and T. Kawai
chirality including CD and CPL activities. On the basis of the theory, the chiroptical
switch can be achieved if the external stimuli modulate the chiral chromophoric
arrangement in molecular systems. Figure 8.4 depicts a schematic design of possible
CPL-photo-switch. The photo-responsive unit with a chirally controlled geometry
should carry multiple fluorophores, arranging them in a chiral manner so that
the exciton coupling interaction operates on them. The photo-irradiation induces
the structural change in the photo-responsive unit, leading to a change in the
fluorophoric arrangement to modulate the inter-fluorophore exciton coupling.
Chiral photochromic scaffold with a helical secondary structure was employed as
a platform for modulating CPL activity through a control of chiral fluorophore
arrangement in an intramolecular manner. The central ethene part of diarylethene
was replaced with a heteroaromatic ring to form a tetra-arylene scaffold with keeping
the photochromic capability [29]. The use of oligo-heteroarylene framework enables
the molecule to take advantage of inter-heteroaryl-unit interactions involving heteroatoms so that they can adopt the molecular conformation suitable for the electrocyclization reaction [30–33]. The extension of heteroaromatic units led to the
formation of helical foldamers with photo-reactivity [34–36]. Tetra-arylenes composed of 2-phenylthiazole unit with a cis-cisoid-like connectivity form a one-turn
helical secondary structure as the most stable conformation, which is also suitable
for the stereo-specific electro-cyclization reaction (Fig. 8.5) [34–36]. The incorporation of chiral groups as a side-chain unit readily controlled the handedness of
helicity. For the design of CPL photo-switch, a pyrene group was introduced at each
Fig. 8.4 Schematic design of a CPL-photo-switch
N
S
S
N
S
N
N
S
N
S
S
N
folding
vis
S
N
N
S
N
S
S
N
N
S
N
S
UV
Fig. 8.5 Conformational folding and photoreactions of tetra(2-phenylthiazole)
180
T. Nakashima and T. Kawai