compared to those in THF and MCH. The assemblies in chloroform, THF, and MCH
exhibited good fluorescence quantum yields (42%, 31%, and 29%, respectively).
A helical assembly can be formed via intermolecular interactions other than
hydrogen-bonding interactions. Dipole–dipole interactions are fruitful to form a
helical assembly since C 3 symmetrically arranged dipole moments tend to form
a helical stacked assembly [39]. Haino and coworkers have reported the helical
assembly behavior of C 3 symmetric 1,3,5-tris(4-alkoxyphenylisoxazolyl)benzene
27, which possesses three isoxazoles that provide the local dipole moment
(Fig. 9.12) [40, 41]. The directional circular arrangement of the isoxazole rings
is directed by the head-to-tail dipole–dipole interaction of the local dipole of
isoxazoles. Molecular modeling studies for the hexameric assemblies of 1,3,5-tris
(phenylisoxazolyl)benzene revealed that the hexameric assembly has two major
geometries, helical and eclipsed. In the former geometry, the local dipoles
of isoxazole align in a head-to-tail fashion, whereas they take an antiparallel
conformation in the latter geometry. The optical and chiroptical properties of
27 in MCH were considered by using UV-vis and CD spectroscopy techniques.
Monomeric 27 in the diluted MCH solution exhibited a monomeric absorption band
at 278 nm, whereas the assembly of 27 in the concentrated MCH solution displayed
the absorption maximum at 310 nm. The redshift of the absorption suggests the
formation of the J-type aggregate. The monomeric 27 displayed no CD signals,
indicating that the chiral side-chain of 27 does not perturb the π ! πà transition.
On the other hand, the assembly of (S)- and (R)-27 formed in the concentrated
MCH solution exhibited CD spectra that have a mirror-image relationship. This
suggests that not an antiparallel but helical assembly is formed, and the helicity was
determined by the chirality of the side-chain. By using exciton coupling theory,
the helical sense of the assembly of (S)-27 was determined to be right-handedness.
From these results, the dipole–dipole interaction of the isoxazole rings drives the
helical assembly of small molecules. Unfortunately, the CPL properties of 27 were
not reported, but the authors have reported the CPL properties of some luminophores
possessing phenylisoxazoles.
Haino and coworkers have reported the helical assembly of PBI possessing
phenylisoxazoles 28 and their optical and chiroptical properties (Fig. 9.13)
[42]. The formation of the supramolecular assembly of PBI perturbs its π ! πÃ
absorption and πà ! π emission. The introduction of tris(phenylisoxazolyl)benzene
onto a nitrogen atom of PBI resulted in the formation of a helical assembly, in which
the π ! πà and πà ! π transitions are chirally perturbed to give chiroptical
N
H
N
H
H
N
H
N
R
O
O
R
O
R
O
R
(S)-26: R=
(R)-26: R=
Fig. 9.11 CPL-active
helical assembly of pyrene
derivatives
208
T. Ikeda and T. Haino
exhibited good fluorescence quantum yields (42%, 31%, and 29%, respectively).
A helical assembly can be formed via intermolecular interactions other than
hydrogen-bonding interactions. Dipole–dipole interactions are fruitful to form a
helical assembly since C 3 symmetrically arranged dipole moments tend to form
a helical stacked assembly [39]. Haino and coworkers have reported the helical
assembly behavior of C 3 symmetric 1,3,5-tris(4-alkoxyphenylisoxazolyl)benzene
27, which possesses three isoxazoles that provide the local dipole moment
(Fig. 9.12) [40, 41]. The directional circular arrangement of the isoxazole rings
is directed by the head-to-tail dipole–dipole interaction of the local dipole of
isoxazoles. Molecular modeling studies for the hexameric assemblies of 1,3,5-tris
(phenylisoxazolyl)benzene revealed that the hexameric assembly has two major
geometries, helical and eclipsed. In the former geometry, the local dipoles
of isoxazole align in a head-to-tail fashion, whereas they take an antiparallel
conformation in the latter geometry. The optical and chiroptical properties of
27 in MCH were considered by using UV-vis and CD spectroscopy techniques.
Monomeric 27 in the diluted MCH solution exhibited a monomeric absorption band
at 278 nm, whereas the assembly of 27 in the concentrated MCH solution displayed
the absorption maximum at 310 nm. The redshift of the absorption suggests the
formation of the J-type aggregate. The monomeric 27 displayed no CD signals,
indicating that the chiral side-chain of 27 does not perturb the π ! πà transition.
On the other hand, the assembly of (S)- and (R)-27 formed in the concentrated
MCH solution exhibited CD spectra that have a mirror-image relationship. This
suggests that not an antiparallel but helical assembly is formed, and the helicity was
determined by the chirality of the side-chain. By using exciton coupling theory,
the helical sense of the assembly of (S)-27 was determined to be right-handedness.
From these results, the dipole–dipole interaction of the isoxazole rings drives the
helical assembly of small molecules. Unfortunately, the CPL properties of 27 were
not reported, but the authors have reported the CPL properties of some luminophores
possessing phenylisoxazoles.
Haino and coworkers have reported the helical assembly of PBI possessing
phenylisoxazoles 28 and their optical and chiroptical properties (Fig. 9.13)
[42]. The formation of the supramolecular assembly of PBI perturbs its π ! πÃ
absorption and πà ! π emission. The introduction of tris(phenylisoxazolyl)benzene
onto a nitrogen atom of PBI resulted in the formation of a helical assembly, in which
the π ! πà and πà ! π transitions are chirally perturbed to give chiroptical
N
H
N
H
H
N
H
N
R
O
O
R
O
R
O
R
(S)-26: R=
(R)-26: R=
Fig. 9.11 CPL-active
helical assembly of pyrene
derivatives
208
T. Ikeda and T. Haino