assembled structures in highly ordered supramolecular assemblies [2]. The helix
is one of the most frequently appearing motifs in chiral supramolecular systems.
The right-handed helix is a mirror-image of the left-handed one, and thus, the
helix is chiral. Various types of helical assemblies have been developed: chiral
and achiral small molecules, oligomers, and polymers form helical assemblies
[36]. Considering the CPL properties, helically stacked assemblies of planar
π-conjugated luminophores are optimal to balance the highly assembled structure
with the chiral orientation of a luminophore.
To form an assembly of a small π-conjugated molecule, the appropriate molecular
design is indispensable. If a π-conjugated molecule forms a helical stacked
assembly, a racemic mixture of P-helix and M-helix would be obtained without
any chiral source. However, the introduction of a chiral source makes the P-helix
and M-helix diastereomers. The diastereomers have differences in their Gibbs
free-energy, and thus one (P or M ) is more stable than the other (M or P)
(Fig. 9.8). Therefore, the chiral source, a chiral alkyl side-chain in many cases,
must be introduced into the molecule.
The assembled structure of a small π-conjugated molecule is stabilized by π–π
stacking interactions, but other intermolecular interactions are needed to form a
stable stacked assembly. The hydrogen-bonding interactions of amide moieties
are often employed to form stacked assemblies. The hydrogen bonds are formed
between hydrogen atoms attached to nitrogen and carbonyl oxygen. The consecutive
hydrogen bonds of amide moieties form a one-dimensional network of hydrogen
bonds (Fig. 9.9). When the amide moiety is attached to the aromatic rings of
a π-conjugated luminophore, the amide moiety takes a twisted conformation to
the π-conjugated plane of the aromatic ring. Thus, one-dimensional consecutive
Fig. 9.8 CPL produced by a helical supramolecular assembly of an achiral luminophore
N
O
H
N
O
H
N
O
H
N
O
H
Fig. 9.9 One-dimensional hydrogen bond network of amide moieties
206
T. Ikeda and T. Haino
is one of the most frequently appearing motifs in chiral supramolecular systems.
The right-handed helix is a mirror-image of the left-handed one, and thus, the
helix is chiral. Various types of helical assemblies have been developed: chiral
and achiral small molecules, oligomers, and polymers form helical assemblies
[36]. Considering the CPL properties, helically stacked assemblies of planar
π-conjugated luminophores are optimal to balance the highly assembled structure
with the chiral orientation of a luminophore.
To form an assembly of a small π-conjugated molecule, the appropriate molecular
design is indispensable. If a π-conjugated molecule forms a helical stacked
assembly, a racemic mixture of P-helix and M-helix would be obtained without
any chiral source. However, the introduction of a chiral source makes the P-helix
and M-helix diastereomers. The diastereomers have differences in their Gibbs
free-energy, and thus one (P or M ) is more stable than the other (M or P)
(Fig. 9.8). Therefore, the chiral source, a chiral alkyl side-chain in many cases,
must be introduced into the molecule.
The assembled structure of a small π-conjugated molecule is stabilized by π–π
stacking interactions, but other intermolecular interactions are needed to form a
stable stacked assembly. The hydrogen-bonding interactions of amide moieties
are often employed to form stacked assemblies. The hydrogen bonds are formed
between hydrogen atoms attached to nitrogen and carbonyl oxygen. The consecutive
hydrogen bonds of amide moieties form a one-dimensional network of hydrogen
bonds (Fig. 9.9). When the amide moiety is attached to the aromatic rings of
a π-conjugated luminophore, the amide moiety takes a twisted conformation to
the π-conjugated plane of the aromatic ring. Thus, one-dimensional consecutive
Fig. 9.8 CPL produced by a helical supramolecular assembly of an achiral luminophore
N
O
H
N
O
H
N
O
H
N
O
H
Fig. 9.9 One-dimensional hydrogen bond network of amide moieties
206
T. Ikeda and T. Haino