accounts and reviews are available; thus, this review will not highlight such a case.
We will focus on the recent developments in the field of chiral gelators and chiral
supramolecular gels [21].
11.3.2 CPL from Chiral Gelators
11.3.2.1 CPL from Amino Acid Derivative Gelators
In supramolecular gel system, amino acids are excellent gelaton candidates because
of its natural chirality, easy accessibility, and wide diversity [19]. For the CPL-active
amino acid derivatives, the gelator molecule usually does not show CPL because
the spacer between the chromophore and the chiral center is too long. However,
upon gelation, the chirality can be transferred to the assemblies which show intense
CPL activity. For example, Ihara et al. designed a glutamic acid gelaton-linked
anthracene gelator 4 (Fig. 11.5) [22]. Interestingly, no CPL signal was detected
for the monomeric state in tetrahydrofuran (THF). However, when 4 formed
supramolecular gel through self-assembly in n-hexane/THF (50:1), the excimer of
anthracene formed and emitted CPL at 25
C. It was found that the formation
of assembly state of 4 could be controlled by temperature. At low temperature,
Assembly
a
b
Sandwich type excimer
4
25 °C
O
O
*
*
*
O
O
O
O
O
O
O
HN
HN
C12H25
C12H25
H
N
NH
10
0
–10
300
400
500
600
700
30
20
10
0
–10
–20
–30
Wavelength (nm)
CPL intensity (mdeg)
CD intensity (mdeg)
NH
NH
HN
HN
O
O
NH
O
O
Fig. 11.5 (a) Schematic
illustration of the excimer
formed by molecule 4 in an
n-hexane/THF (50:1) mixed
solution at 25
C. (b) CD
(blue line) and CPL (red
line) spectra of the 4 gel in
an n-hexane/THF (50:1)
mixed solution (solid line)
and in THF (dash line) at
25
C. Reproduced with
permission [22]. Copyright
2015, The Royal Society of
Chemistry
254
T. Zhao et al.