Control of macroscopic helicity by using the sergeants-and-soldiers principle in organogels

Article abstract of DOI:10.1002/chem.200800702

The creation of well-defined homochiral helical ribbon structures in the gel phase was demonstrated. The achiral gelator 1 consists of a central aromatic group for aromatic stacking and alkylamide groups that can participate in inter-molecular hydrogen bonds and van der Waals interactions. The gelators were synthesized by simple amide coupling in moderate yields. NMR experiments were performed to obtain the evidence for π-π interaction and hydrogen-bonding interaction in organogelator 1. The solvent polarity was varied by changing the methanol and chloroform composition. The gelation behavior of the gelators was tested in various organic solvents. The SEM images of xerogels exhibited the macroscopic aggregation modes of the gelators. The CD experiments reveal that the chiral property of gelators can be reflected within the aggregates of achiral 1 without changing the helical structure derived from the aggregates of 1.

Full text of DOI:10.1002/chem.200800702

DOI: 10.1002/chem.200800702
Control of Macroscopic Helicity by Usingthe Sergeants-and-Soldiers
Principle in Organogels
[a]
SeongRyongNam, Ho YongLee, and Jong-In Hong*
Helical structures of biomolecules (e.g., proteins, nucleic
acids) play a significant role in many natural systems. Be-
cause self-assembly is a powerful tool for constructing vari-
ous types of nano- and microstructures, many artificial heli-
ces have been constructed for mimicking natural helical
structures using self-assembly via hydrogen bonding, aro-
matic stacking and/or metal–ligand coordination interac-
tions.[1] Since Green and co-workers set up a system based
on the “majority” rule and the “sergeants-and-soldiers”
principle,[2] various induced helical structures have been de-
veloped using Greenꢀs methods.[3] It turned out that the for-
mation of homochiral helical structures only requires either
the addition of a small portion of chiral units (sergeants) to
the achiral units (soldiers) or having a slight majority of R
over S units (or vice versa) in the supramolecules, because
noncovalent interactions between chiral and achiral units
can lead to chiral supramolecular structures by co-assembly
of achiral and chiral monomers.
Figure 1. SEM images of xerogels, a) chiral gelator 2, b) chiral gelator 3,
c) achiral organogelator 1, d) and e) enlarged images of each P and M
helix of 1 (scale bar: a)–c)=10 mm, d) and e)=5 mm).
Recently, while considerable attention has been paid to
the development of
a
low-molecular-weight gelator
(LMWG) for use in organic solvents, there are more and
more reports on nano- and microstructures being developed
in the gelation process.[4] One of the structures of interest is
a helical structure.[5] Although the control or amplification
of chirality in gels becomes a matter of primary concern,[6]
complete control of macroscopic helicity has never been re-
ported in the gelation process (Figure 1.).
This is the first paper that reports on the control of mac-
roscopic helical structures, which are self-assembled via hy-
drogen bonding, aromatic stacking, and van der Waals inter-
actions between chiral and achiral organogelators, as well as
on the unidirectional helicity of gels which are induced by
the chiral organogelators. Although most sergeants-and-sol-
diers systems consist of chiral and achiral units that are
structurally almost the identical, we exploit chiral and achi-
ral units with different structures (see below). The achiral
gelator 1 consists of a central aromatic group for aromatic
stacking and alkylamide groups that can participate in inter-
molecular hydrogen bonds and van der Waals interactions.
The chiral gelators 2 and 3 have alanine residues between
the dodecanoyl and p-phenylenediamine groups as the
chiral trigger unit. The gelators were synthesized by simple
amide coupling in moderate yields (see Supporting Informa-
tion).
NMR experiments were performed to obtain the evidence
for p–p interaction and hydrogen-bonding interaction in or-
ganogelator 1. The solvent polarity was varied by changing
the methanol and chloroform composition (Figure 2a).
NMR spectroscopy showed that the aromatic protons of or-
ganogelator 1 shifted upfield (d=À0.95 ppm) with increas-
ing methanol ratio. These phenomena were attributed to an
increase in the population of the p–p stacked, oligomeric or
[a] S. R. Nam, H. Y. Lee, Prof. Dr. J.-I. Hong
Department of Chemistry, College of Natural Sciences
Seoul National University, Seoul 151-747 (Korea)
Fax : (+82)2-889-1568
E-mail: jihong@snu.ac.kr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800702.
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COMMUNICATION
tors 2 and 3 also formed organogels in toluene, but no heli-
cal fibers were observed when only a chiral gelator was
present (Figure 1a, b). Various ratios (1:1, 2:1, 5:1, 10:1,
97:3, 98:2, and 99:1) of the achiral and chiral gelators were
tested. When the chiral portion increased too much, the
amount of the helical structure was not clear (Figure 3). Be-
cause the chiral methyl groups of 2 and 3 presumably inhibit
the stacking interactions, an excess of either 2 or 3 probably
obstructs the formation of helical structures. When the
amount of 2 or 3 is above 33%, helical structures are clearly
not formed (see Supporting Information).
The SEM images of xerogels exhibited the macroscopic
aggregation modes of the gelators. Figure 4 shows that the
xerogels obtained from complexes consist mainly of helical
structures. Figure 4a and b shows that all helices formed by
1 and 2, in the ratio of 99:1, reveal the characteristic left-
handed helical ribbon structures (M helices). In the case of
1 and 3, right-handed helical ribbon structures (P helices)
are exclusively formed (Figure 4c and d). Because the width,
length and average helical pitch length of helices are similar
to those of the helices formed by 1 alone, chiral triggers 2
and 3 did nothing but play a role in inducing the unidirec-
tional helicity.
CD experiments were performed to investigate the self-
assembly of 1 and chiral triggers from a microscopic view-
point. Gelator 1 was almost CD-inactive because organogels
were constructed by the random placement of nearly equal
numbers of P and M helices. In contrast, 2 and 3 show com-
plementary CD spectra because of the presence of the in-
polymeric structures of organogelator 1. This implies that
the p–p interaction in organogelator 1 exerted a strong in-
fluence on self-association and gelation. The H NMR sig-
1
nals of 1 are concentration-dependent in [D8]toluene (Fig-
ure 2b). The amide proton signal shifted downfield (d=
+0.15 ppm) upon increasing the concentration, whereas the
aromatic proton signals showed insignificant shift. This indi-
cated that hydrogen bonding is a crucial factor of self-associ-
ation rather than aromatic stacking in toluene.
The gelation behavior of the gelators was tested in various
organic solvents. Gelation occurred in aromatic solvents
such as toluene and p-xylene. The xerogel obtained from
achiral organogelator 1 in toluene (1.14% w/w) shows very
interesting features in which remarkably thick ribbon struc-
tures are twisted in both left- and right-handed helical struc-
tures (Figure 1c, d and e). The
width of the ribbons varies
from 4 to 5 mm and they are a
few hundred micrometers in
length, with an average helical
pitch length of 7–8.5 mm. The
aggregated structures were sta-
bilized presumably by coopera-
tive aromatic stacking, hydro-
gen bonding, and van der Waals
interactions. While aromatic
stacking between the phenyl
groups induces one-dimensional
aggregates, hydrogen bonds
among the amide groups propa-
gate along the aggregate axis
and enforce a helical mode of
the aggregate.
Figure 2. a) Stacked 1H NMR spectra of 17 mm achiral gelator 1 at 508C. The solvent ratio of CDCl3/CD3OD
(v/v) from bottom to top: 10:1; 5:1; 2:1; 1:1. b) Stacked 1H NMR spectra of 1 are concentration-dependent in
&
*
[D8]toluene at 808C. The concentration of 1 from bottom to top: 2, 4, 10, 20 mm. (Ha = , Hb = ).
For the purpose of control-
ling P and M helicity, chiral
triggers 2 and 3 were used. It
turned out that the chiral
methyl group of the d- or l-ala-
nine residue in 2 and 3 induced
the formation of homochiral
helical structures in the aggre-
gates consisting of the achiral
gelator and chiral gelator. Gela- Figure 3. SEM images of xerogels a) 1/3 1:1, b) 1/3 10:1 and c) 1/3 98:2 (scale bar a)–c)=5 mm).
Chem. Eur. J. 2008, 14, 6040 – 6043
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J.-I. Hong et al.
from the CD experiments reveal that the chiral property of
2 and 3 can be reflected within the aggregates of achiral 1
without changing the helical structure derived from the ag-
gregates of 1. Macroscopic SEM observations also proclaim
that the chiral property of the microscopic structure is pre-
served within the macroscopic structures without changing
the original helical structure derived from 1. This work re-
veals that a composition of only 1% of a chiral trigger can
induce completely the helicity not only from the microscopic
viewpoint, but also from the macroscopic structural view-
point, in the gelation process.
In conclusion, we have demonstrated the creation of well-
defined homochiral helical ribbon structures in the gel
phase. This is the first example of direct and complete con-
trol of microscopic and macroscopic helicity in gel phase, by
the application of the sergeants-and-soldiers principle by
using achiral gelator 1 and chiral gelators 2 and 3. It is a
unique system in that most of the sergeants-and-soldiers sys-
tems consisted of almost the same structure of chiral and
achiral units, but we exploited chiral and achiral units with
different structures. We expect that our results can be ap-
plied to the preparation of various types of homochiral
nano- and microstructures by using different units of ser-
geants-and-soldiers in the gelation process.
Figure 4. SEM images of xerogels a) and b) 1/2 99:1, M helices, c) and d)
1/3 99:1, P helices (scale bar a), c)=20 mm and b), d)=5 mm).
trinsic chirality in enantiomeric 2 and 3. The fascinating fact
is that assembly with only 1% of 2 or 3 provides a driving
force for the complete induction of homochiral helices. As
shown in Figure 5, addition of 1% of 2 induces the first pos-
Experimental Section
General method for the preparation of gels: Gelation tests were per-
formed by solubilization of a weighed amount of gelator mixtures in a
measured volume of selected organic solvent. The mixtures were heated
until clear and cooled to room temperature. All the sample images were
monitored using 10 mg gelator per 1 mL toluene (1.14% w/w). Samples
for the xerogel images were dried in the air before examining SEM
images. Organogels in the vial were carefully picked up and applied to
the polymer or stainless steel stubs by a carbon tape.
Circular dichroism spectra techniques: Gel samples were sandwiched by
two quartz plates and dried in the air for removal of toluene, because tol-
uene is the UV-active solvent. Only xerogel phase CD experiments were
performed. In order to rule out the possibility of the birefringency phe-
nomena and intensity difference influenced by the sample thickness and
measurement direction, the thickness of the samples were kept approxi-
mately the same and CD measurements were done several times in dif-
ferent directions by turning the sample.
~
~
&
Figure 5. CD spectra of 1 (c), 2 ( ), 3 ( ), 1/2 99:1 ( ) and 1/3 99:1
(&). UV spectra of 1/2 99:1 (a) and 2 (c) in xerogel phase.
itive Cotton effect (at long wavelength) and a second nega-
tive one (at short wavelength) whereas addition of 1% of 3
induces the first negative and second positive Cotton effects,
nearly symmetrically to the Cotton effect upon the addition
of 2. The 99:1 complexes show complementary Cotton ef-
fects at the same wavelength, and the opposite sign for each
enantiomer, clearly indicating that predetermination of the
helicity by a chiral trigger was successful. The CD spectra of
99:1 complexes and pure chiral gelators show Cotton effects
and absorption maxima at different wavelengths, which
clearly show that the Cotton effect of 99:1 complex is
caused by induced helicity of an achiral gelator. The Cotton
effects were shifted in the composite compared with the
pure chiral compounds. This indicates that the packing
mode is probably different in the composite and chiral gela-
tors. There is a remarkable correspondence between the CD
experiments and SEM observations. Microscopic insights
Acknowledgements
We thank the KRF (Grant No. 2007–314-C00172) for financial support.
This work was supported in part by the Seoul R&BD. H.Y.L. is grateful
to the Ministry of Education for a BK 21 postdoctoral fellowship.
Keywords: gels
· helical structures · nanostructures ·
self-assembly · sergeants-and-soldiers principle
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Received: April 14, 2008
Published online: May 26, 2008
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