properties. The self-assembly behavior of 28 was considered by using
1 H NMR,
UV-vis, and CD spectroscopy techniques. 28 formed a J-type assembly that was
confirmed by the redshift of the absorption band of the assembly. The assembly of 28
in decalin solution displayed strong CD with a g abs of 1.4 Â 10
À3 . The righthandedness of the helix was assigned by the plus-to-minus patterns observed in
the ascending energy in the CD spectrum of 27. In the emission spectrum of the
assembly of 28 in decalin, a broad and redshifted band of the assembly was observed
in addition to the sharp emission band of the monomeric species. CPL was observed
on the emission of the assembly with a g lum of 7 Â 10
À3 , but no CPL was observed
on the monomeric emission. Furthermore, CPL was not observed on the emission of
28 in chloroform, in which most 28 does not form an assembly. (S)- and (R)-28
displayed mirror-image CPL spectra. These results clearly suggest that the formation
of a helical assembly leads to the CPL activity of 28.
A square planar Pt(II) complex tends to form a stacked assembly creating
a one-dimensional metal array through metallophilic (Pt–Pt) interactions [43]. The
Pt(II) phenylbipyridine complex is known as a luminophore exhibiting
phosphorescence that comes from a triplet metal-to-ligand charge transfer (
3
MLCT)
transition. The phosphorescence property of the Pt(II) phenylbipyridine complex
is perturbed by the formation of a stacked assembly via Pt–Pt interactions to
exhibit a triplet metal-metal-to-ligand charge transfer (
3
MMLCT) transition
[44]. The Pt(II) phenylbipyridine complex does not form a helical assembly without
any assistance of other intermolecular interactions, but the complex formed a
helical assembly when the phenylisoxazole moiety was introduced onto the ligand.
Haino and coworkers have reported the optical and chiroptical properties of a
Pt(II) phenylbipyridine complex possessing a 3,5-bis(phenylisoxazolyl)phenylethynyl
ligand ((S)- and (R)-29, Fig. 9.14) [45]. 29 effectively formed a stacked assembly via
Pt–Pt, dipole–dipole, and π–π stacking interactions. Interestingly, the self-assembly
behavior of 29 drastically changed depending on the solvent effect. In chloroform, 29
formed a stacked assembly exhibiting
3
MMLCT absorption and emission bands, but
the assembly displayed no CD and CPL. It turns out that the assembly of 29 formed in
chloroform is not helical, most likely due to the strong solvation that prevents the
O
N
N
N
O
N O
OR 1
R 1 O
O
O
N
O
O
R 2
(S)-28: R 1 =
(R)-28: R 1 =
right-CPL
left-CPL
R 2 =
C6H13
C6H13
M-helical
self-assembly
P-helical
self-assembly
Fig. 9.13 CPL-active helical assembly of PBI derivatives possessing tris(phenylisoxazolyl)benzene
28. Adapted with permission from Satrijo et al. [5]. Copyright 2012 The Royal Society of Chemistry
210
T. Ikeda and T. Haino
1 H NMR,
UV-vis, and CD spectroscopy techniques. 28 formed a J-type assembly that was
confirmed by the redshift of the absorption band of the assembly. The assembly of 28
in decalin solution displayed strong CD with a g abs of 1.4 Â 10
À3 . The righthandedness of the helix was assigned by the plus-to-minus patterns observed in
the ascending energy in the CD spectrum of 27. In the emission spectrum of the
assembly of 28 in decalin, a broad and redshifted band of the assembly was observed
in addition to the sharp emission band of the monomeric species. CPL was observed
on the emission of the assembly with a g lum of 7 Â 10
À3 , but no CPL was observed
on the monomeric emission. Furthermore, CPL was not observed on the emission of
28 in chloroform, in which most 28 does not form an assembly. (S)- and (R)-28
displayed mirror-image CPL spectra. These results clearly suggest that the formation
of a helical assembly leads to the CPL activity of 28.
A square planar Pt(II) complex tends to form a stacked assembly creating
a one-dimensional metal array through metallophilic (Pt–Pt) interactions [43]. The
Pt(II) phenylbipyridine complex is known as a luminophore exhibiting
phosphorescence that comes from a triplet metal-to-ligand charge transfer (
3
MLCT)
transition. The phosphorescence property of the Pt(II) phenylbipyridine complex
is perturbed by the formation of a stacked assembly via Pt–Pt interactions to
exhibit a triplet metal-metal-to-ligand charge transfer (
3
MMLCT) transition
[44]. The Pt(II) phenylbipyridine complex does not form a helical assembly without
any assistance of other intermolecular interactions, but the complex formed a
helical assembly when the phenylisoxazole moiety was introduced onto the ligand.
Haino and coworkers have reported the optical and chiroptical properties of a
Pt(II) phenylbipyridine complex possessing a 3,5-bis(phenylisoxazolyl)phenylethynyl
ligand ((S)- and (R)-29, Fig. 9.14) [45]. 29 effectively formed a stacked assembly via
Pt–Pt, dipole–dipole, and π–π stacking interactions. Interestingly, the self-assembly
behavior of 29 drastically changed depending on the solvent effect. In chloroform, 29
formed a stacked assembly exhibiting
3
MMLCT absorption and emission bands, but
the assembly displayed no CD and CPL. It turns out that the assembly of 29 formed in
chloroform is not helical, most likely due to the strong solvation that prevents the
O
N
N
N
O
N O
OR 1
R 1 O
O
O
N
O
O
R 2
(S)-28: R 1 =
(R)-28: R 1 =
right-CPL
left-CPL
R 2 =
C6H13
C6H13
M-helical
self-assembly
P-helical
self-assembly
Fig. 9.13 CPL-active helical assembly of PBI derivatives possessing tris(phenylisoxazolyl)benzene
28. Adapted with permission from Satrijo et al. [5]. Copyright 2012 The Royal Society of Chemistry
210
T. Ikeda and T. Haino