Biomedical Engineering Reference
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OR
(a)
(b)
HO
OR
′
R
OR
O
H
d
RO
H
c
R
′
O
OH
N
H
b
RO
H
a
24
: R, R
=
n
-C
12
H
25
20
: R =
n
-C
12
H
25
, R
′
O
N
′
=
*
(
S
)
O
N
O
R
O
R
(c)
H
N
H
N
O
CH
3
O
22
: R =
CH
3
n
9
O
O
CH
3
R
-
23
: R =
R
-
25: n = 5
R
-
26: n = 7
27
S
-
23
:
R =
Figure 1.19
(a) Molecular structures
of tris(phenylisoxazolyl)benzene-based,
(Reproduced from Ref. [81c] with per-
mission of The Royal Society of Chem-
istry.
http://dx.doi.org/10.1039/B715871H
.)
(b) OPV-based,(Reprinted with permission
from Ref. [81a]. Copyright 2006 Wiley.) and
(c) pyrene-based chiral and achiral gelators
for induced CD. (Reprinted with permission
from Ref. [81e]. Copyright 2010 American
Chemical Society.)
In some systems, a small amount of chiral dopant can be added to an achiral
gelator to induce chirality through the so-called ''sergeant-and-soldiers'' effect.
This effect was first discovered in poly(alkyl isocyanates) where monomeric units
are covalently bonded [80], and was later applied to organogel systems where the
building blocks are connected through intermolecular interactions [81].
As an example, the addition of only 0.01 mol% of chiral
R
-
23
or
S
-
23
to achiral
22
(Figure 1.19a) induced a remarkable Cotton effect by forming helical columnar
structures;
R
-
, alone at the same concentration, exhibited a very weak
Cotton effect [81c]. Similar chiral amplification effects were also observed from the
co-assembly of OPV-based [81a] or pyrene-based [81e] achiral and chiral gelators
(Figure 1.19b,c, respectively).
Most sergeant-and-soldiers systems have structurally similar chiral dopant and
achiral gelators. If the two components are structurally dissimilar, they will usually
remain segregated in the fibers, and no co-assembly will be observed. However,
Hong
et al
. have demonstrated that the sergeant-and-soldiers effect can also be
achieved from chiral and achiral molecules of very different structures [81d].
An achiral molecule containing an aromatic ring and two alkyl amides (
23
or
S
-
23
28
)and
chiral molecules containing two d-orl-alanine residues (
29
or
30
, respectively)
(Figure 1.20a) formed gels in toluene [81d]. Xerogels from
in toluene formed both
left- and right-handed helical ribbons. However, no helical structures were observed
from the xerogels of
28
28
induced enantio-pure helices with mirror image Cotton effects (characterized from
xerogels), while xerogels of
29
and
30
showed CD spectra corresponding to the
intrinsic chirality of the gelators (as opposed to a helical structure) (Figure 1.20b).
28
and
29
. The addition of 1% of chiral
29
or
30
to achiral
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