Foldamer organogels: A circular dichroism study of glucose-mediated dynamic helicity induction and amplification

Article abstract of DOI:10.1021/ja8043322

This paper reports a systematic study of the dynamic process for the self-assembly of chiral organogels from achiral hydrogen bonded hydrazide foldamers by induction of chiral glucose. Six foldamers incorporated with six decyl chains and two benzene, naphthalene, anthracene, or pyrene units at the ends are revealed to strongly gelate apolar and polar solvents, including alkanes, arenes, esters, alcohols, and 1,4-dioxane. The gels are characterized by UV-vis, fluorescent, XRD, SEM, and AFM methods, based on which a dislocated tail-to-tail stacking pattern is proposed. Addition of octylated glucose considerably enhances the capacity of the foldamers to gelate apolar solvents due to strong complexation. The complexation also causes unique dynamic helicity induction in the gels, which is studied systematically by circular dichroism. The results are treated with the Avrami theory according to a reported method (J. Am. Chem. Soc. 2005, 127, 4336), which suggests that the gelation involves a nucleation-elongation mechanism. In addition, the Sergeants and Soldiers effect in the gel phase is also revealed.

Full text of DOI:10.1021/ja8043322

Published on Web 09/13/2008
Foldamer Organogels: A Circular Dichroism Study of
Glucose-Mediated Dynamic Helicity Induction and
Amplification
Wei Cai, Gui-Tao Wang, Ping Du, Ren-Xiao Wang, Xi-Kui Jiang, and Zhan-Ting Li*
State Key Laboratory of Bio-Organic and Natural Products Chemistry, Shanghai Institute of
Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu, Shanghai 200032, China
Received June 8, 2008; E-mail: ztli@mail.sioc.ac.cn
Abstract: This paper reports a systematic study of the dynamic process for the self-assembly of chiral
organogels from achiral hydrogen bonded hydrazide foldamers by induction of chiral glucose. Six foldamers
incorporated with six decyl chains and two benzene, naphthalene, anthracene, or pyrene units at the ends
are revealed to strongly gelate apolar and polar solvents, including alkanes, arenes, esters, alcohols, and
1,4-dioxane. The gels are characterized by UV-vis, fluorescent, XRD, SEM, and AFM methods, based on
which a dislocated “tail-to-tail” stacking pattern is proposed. Addition of octylated glucose considerably
enhances the capacity of the foldamers to gelate apolar solvents due to strong complexation. The
complexation also causes unique dynamic helicity induction in the gels, which is studied systematically by
circular dichroism. The results are treated with the Avrami theory according to a reported method (J. Am.
Chem. Soc. 2005, 127, 4336), which suggests that the gelation involves a nucleation-elongation mechanism.
In addition, the “Sergeants and Soldiers” effect in the gel phase is also revealed.
Introduction
dimensional, entangled network of fibers formed by a gelator
through various noncovalent forces. On the other hand, chirality
Self-assembly provides new possibilities in both biology and
materials sciences because it enables rapid formation of
complicated, functionalized architectures.1 One family of self-
assembled architectures is organogels, which consist of an
organic liquid and a low molecular mass organic gelator.2-4 In
an organogel, the liquid is immobilized by a continuous, three-
is a basic feature of nature that appears hierarchically at both
molecular and supramolecular levels.5 At the supramolecular
level, chirality is strongly related to intermolecular interactions.
Therefore, achiral molecules can be induced by chiral molecules
to form various chiral architectures. Currently, this process can
be readily realized in the solid and solution phases and
membranes, as well as on surfaces or at interfaces.6 In many
cases, chirality amplification can be exhibited, giving rise to
the so-called “Sergeants and Soldiers” effect.7 In recent years,
much effort has been devoted to developing chiral organogels
from single chiral molecules, owing to their potentials as
advanced materials and constrained media for chiral synthesis
and separation.8 Because gelation is a thermoreversible process
of phase separation, systematic understanding of this dynamic
process is of importance fundamentally and for practical
applications. Pioneering studies by Feringa and Weiss and co-
workers have focused on several chiral bisurea and cholesterol-
derived gelators.9,10 In this paper, we present a novel study of
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9
13450 J. AM. CHEM. SOC. 2008, 130, 13450–13459
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2008 American Chemical Society

Foldamer Organogels
A R T I C L E S
recognition-mediated dynamic helicity induction and amplifica-
tion in the gel phase,11 which is based on the complexation of
achiral hydrazide foldamer gelators toward chiral glucoses.
Discotic molecules represent a class of structurally unique
organic gelators consisting of a rigid aromatic core and aliphatic
chains of suitable length.12 They are also useful scaffolds for
investigating discrete secondary interactions and chiral ampli-
fication in helical aggregates.13 In recent years, a variety of
hydrogen bonding-mediated aromatic amide, urea, and hy-
drazide-based foldamers have been developed,14-19 some of
which are found to complex chiral guests of matched sizes,17-19
leading to interesting supramolecular helicity.18,19 It was
envisioned that suitably modified foldamers of this family might
resemble discotic structures to gelate liquids, because they could
fit well within the “rigid core/flexible tail” motif.2a We therefore
have designed a new series of hydrazide-based foldamers 1a-f
for exploiting their tunable gelation properties. We herein report
(i) their robust capacity to gelate both polar and apolar organic
liquids,20,21 which can be further enhanced through complexing
with glucose, and (ii) a systematic circular dichroism investiga-
tion of their dynamic helicity transfer and amplification and
“Sergeants and Soldiers” effect in the gel phase.
Results and Discussion
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Design and Synthesis. For hydrazide-based foldamers, the
backbone of a heptamer can fold to give rise to a helical
conformation. Such a helix would facilitate a chiral differentia-
tion upon complexing a chiral guest.22 Therefore, compounds
1a-f were synthesized, which consist of a heptameric frame-
work and six n-decyl groups. The two arene units of varying
size were incorporated at the ends to impose influence on
intermolecular aggregation of the folded frameworks, owing to
different stacking tendencies, whereas the long decyl chains were
expected to induce phase separation and thus to facilitate
gelation. Pentamer 2 was synthesized as a reference compound.
(9) De Loos, M.; Van Esch, J.; Kellogg, R. M.; Feringa, B. L. Angew.
Chem., Int. Ed. 2001, 40, 613–616.
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Soc. 2005, 127, 4336–4344. (b) Huang, X.; Raghavan, S. R.; Terech,
P.; Weiss, R. G. J. Am. Chem. Soc. 2006, 128, 15341–15352.
(11) An example of helicity induction of polymeric gels has been reported,
see: (a) Goto, H.; Zhang, H. Q.; Yashima, E. J. J. Am. Chem. Soc.
2003, 125, 2516–2523.
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The synthetic routes for these oligomers are similar, and the
route for 1f is provided in Scheme 1 as an example. Compound
3
23 was first reacted with n-decyl bromide to give 4, which was
(15) Hecht, S., Huc, I., Eds. Foldamers: Structure, Properties, and
Applications; Wiley-VCH: Weinheim, 2007; p 434.
then treated with excessive hydrazine to afford 5. This inter-
mediate was coupled with 6 to produce diester 7, and its
hydrolysis with lithium hydroxide yielded diacid 8, which was
then reacted with pentafluorophenol to give 9. With intermediate
9 available, compound 4 was treated with potassium hydroxide
to give 10. This acid was then coupled with 11 to yield 12,
which was further reacted with hydrazine to generate 13. Finally,
compound 13 was coupled with 9 in hot DMF to afford 1f.
Details for the synthesis of 1a-e and 2 are provided in the
Supporting Information.
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2004, 17–29. (b) Sanford, A.; Yamato, K.; Yang, X. W.; Yuan, L. H.;
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Z.-T.; Hou, J.-L.; Li, C.; Yi, H.-P. Chem.-Asian J. 2006, 1, 766–778.
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March 25, http://dx.doi.org/10.1021/ar700219m. (e) Gong, B. Acc.
Chem. Res., published online May 7, http://dx.doi.org/10.1021/
ar700266f.
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A R T I C L E S
Cai et al.
Scheme 1
Molecular Modeling. Molecular mechanics calculations were
performed to explore the possible conformations of methyl-
substituted analogues of compounds 1a-f. The results show
that their low-energy conformations form a helical structure with
more than one turn, in which the appended arene units do not
stack with each other. In principle, the appended naphthalene,
anthracene, or pyrene units of 1b-f may adopt three different
arrangements, that is, the “in-and-in”, “in-and-out”, and “out-
and-out” arrangements. Molecular mechanics calculations of the
“in-and-in” and “out-and-out” conformations indicate that the
“in-and-in” arrangement is more stable, and the energy differ-
ence between the two conformers of 1d-f is larger than the
counterparts of 1b or 1c. Such an energetically favorable “in-
and-in” conformation is expected to promote intermolecular
dislocated “head-to-head” stacking (Figure 4, vide infra) because
it allows for a larger stacking area, as demonstrated by 1e in
Figure 1. Details of the computational methods and results are
provided in the Supporting Information.
Glucose Complexation in Chloroform. Previous studies have
revealed that hydrazide-based foldamers complex alkylated
saccharides in chloroform through multiple intermolecular
hydrogen bonds.19 By using the fluorescent titration method,19
we evaluated the association constants of the complexes between
1a-f and D- or L-glucose 14 in chloroform to be 1.6 × 104,
1.8 × 104, 1.6 × 104, 1.3 × 103, 7.7 × 103, and 9.6 × 103
M-1, respectively. These values, except that of 1d, are
comparable to that of a heptamer whose appended benzene units
are intramolecularly hydrogen bonded.19a Because 1a-f possess
the identical backbone, the difference of their capacity to bind
14 should be mainly affected by the appended arene units. The
weaker binding capacity of 1d relative to other foldamers may
be attributed to its small cavity size and large energy of
conversion from the “in-and-in” state to the “out-and-out” state,
which should be more favorable for the binding. Although
quantitative measurements could not be performed in hydro-
carbon solvents due to gelation, the above result supports that,
in these solvents of less polarity, similar complexes of even
larger stability should be formed.
Gelation Behavior. The gelation capacity of 1a-f and 2 was
evaluated using the “inverse flow” method.24 At room temper-
(18) (a) Li, C.; Ren, S.-F.; Hou, J.-L.; Yi, H.-P.; Zhu, S.-Z.; Jiang, X.-K.;
Li, Z.-T. Angew. Chem., Int. Ed. 2005, 44, 5725–5729. (b) Yi, H.-P.;
Chuang Li, C.; Hou, J.-L.; Jiang, X.-K.; Li, Z.-T. Tetrahedron 2005,
61, 7974–7980.
(19) (a) Hou, J.-L.; Shao, X.-B.; Chen, G.-J.; Zhou, Y.-X.; Jiang, X.-K.;
Li, Z.-T. J. Am. Chem. Soc. 2004, 126, 12386–12394. (b) Li, C.; Wang,
G.-T.; Yi, H.-P.; Jiang, X.-K.; Li, Z.-T.; Wang, R.-X. Org. Lett. 2007,
9, 1797–1800.
(20) Several examples of foldamer-based gelators have been reported; see:
(a) Cussia, L. A.; Ruiz, E.; Lehn, J.-M.; Homo, J.-C.; Schmutz, M.
Chem.sEur. J. 2002, 8, 3448–3457. (b) Bradford, V. J.; Iverson, B. L.
J. Am. Chem. Soc. 2008, 130, 1517–1524. (c) Cai, W.; Wang, G.-T.;
Xu, Y.-X.; Jiang, X.-K.; Li, Z.-T. J. Am. Chem. Soc. 2008, 130, 6936–
3937.
ature, these compounds were soluble in chloroform and DMF
but insoluble in lower alcohols and esters, such as methanol,
(22) (a) Prince, R. B.; Barnes, S. A.; Moore, J. S. J. Am. Chem. Soc. 2000,
122, 2758–2762. (b) Inouye, M.; Waki, M.; Abe, H. J. Am. Chem.
Soc. 2004, 126, 2022–2027. (c) Maurizot, V.; Dolain, C.; Huc, I. Eur.
J. Org. Chem. 2005, 1293–1301.
(21) Aggregation of ꢀ-peptide foldamers has been reported; see: (a)
Pomerantz, W. C.; Abbott, N. L.; Gellman, S. H. J. Am. Chem. Soc.
2006, 128, 8730–8731. (b) Pomerantz, W. C.; Yuwono, V. M.; Pizzey,
C. L.; Hartgerink, J. D.; Abbott, N. L.; Gellman, S. H. Angew. Chem.,
Int. Ed. 2008, 47, 1241–1244.
(23) Zeng, H.; Miller, R. S.; Flowers, R. A.; Gong, B. J. Am. Chem. Soc.
2000, 122, 2635–2644.
(24) Eldridge, J. E.; Ferry, J. D. J. Phys. Chem. 1954, 58, 992–995.
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Foldamer Organogels
A R T I C L E S
propose that, in the gel phases of toluene or p-xylene, the
foldamers strongly complexed D-14. Considering that D-14
should also aggregate through intermolecular hydrogen bonding,
this gelation enhancement by D-14 may be attributed to the
capacity of the glucose to promote the stacking of the foldamers
by forming a hydrogen-bonded glucose chain in the hole of the
column of the stacked foldamers (Figure 4, vide infra). Ad-
ditional evidence for this mechanism comes from the observation
that a similar enhancement effect did not occur for the gels of
polar 1,4-dioxane and n-butanol, in which conplexation should
not occur. Furthermore, the circular dichroism study of the two-
component gels also supports this mechanism (vide infra).
UV-vis Spectroscopy. To ascertain how the foldamers self-
assembled, UV-vis spectra of the gels (4 mM) in solvents of
different polarity were recorded and compared with those of
their corresponding dilute solutions (0.01 mM), in which the
foldamers were assumed to exist in a single molecular state. It
was found that the peaks assignable to the absorption of the
appended arene units in the gel phases all red-shifted notably
(2-7 nm). As examples, the spectra of 1a, 1b, and 1e are
provided in Figure 2. This result indicates that intermolecular
stacking occurred in the gel phase26 but does not provide useful
information about the stacking pattern of the foldamers. Because
the spectra were recorded at varying concentrations and the
transparencies in solution and gel phases are different, they could
not either be used to determine whether gelation caused
hyperchromism or hypochromism.20b
Fluorescent Spectroscopy. Fluorescent spectra provided in-
formation on the stacking pattern of the foldamers. Although
the spectra of 1b and 1c in chloroform exhibited an emission
band at λmax ) ca. 420 nm, which may be attributed to the
excimer band of the appended naphthalene units27 or the
emission of the benzene units of the backbones,19a the spectra
of 1d-f in chloroform did not exhibit the excimer band of the
anthracene or pyrene units, which typically appears in the long-
wavelength area of ca. 490 or 505 nm.28 The results showed
that the appended arene units of the foldamers did not stack
intramolecularly, as indicated by the dynamic modeling (Figure
1). To detect whether intermolecular stacking occurred for the
appended arene units, fluorescent spectra of the alkane gels of
1a-1f were also recorded. Once again, the spectra of 1d-f did
not exhibit the excimer band of the anthracene or pyrene units
in the expected area (Figure 3). This observation supports that
in the gel phase the foldamers aggregated through the dislocated
“head-to-head” stacking but not the “face-to-face” stacking of
the appended arene units (Figure 4). A similar “head-to-head”
stacking pattern has been revealed for the aggregation of a
conjugated helicene.29 The appended arenes should mainly adopt
the “in-and-in” arrangement (Figure 1), because it could give
rise to the maximum stacking area. Considering that “face-to-
face” stacking of the appended arene units would prevent the
other part of the frameworks from stacking intermolecularly
Figure 1. Minimized helical conformations of 1e (left, the “in-and-in”
arrangement; right, the “out-and-out” arrangement). The former is energeti-
cally more favorable because the anthracene units allow for a larger stacking
area.
Table 1. Minimum Gelation Concentrations (wt %) of Hydrazide
Foldamers in Different Solventsa,b
1a
1b
1c
1d
1e
1f
2
decalin
tetralin
0.57
2.01
0.25c
2.60
0.30c
1.48
0.28c
0.67
0.56 0.54
0.23 0.98
0.26c
0.27 PS
0.24 2.11
1.32c
0.32
P
0.11 0.06
0.20 0.06
0.17 0.07
toluene
1.71 2.05
0.27 1.26
0.37c
0.62c
p-xylene
0.36 1.38
0.24c
0.17 2.15
0.72c
butyl acetate
isobutyl acetate 1.69
ethyl acrylate
n-butanol
0.59 0.26
0.63 0.27
0.59 0.62
0.07 0.21
0.20c
0.24 0.45
0.21 0.49
0.27 1.08
0.31 0.32
Pc
0.34
1.17
1.19
P
P
P
P
P
0.49
0.74
PSc
n-octanol
1,4-dioxane
0.10
0.96
0.98c
0.06 0.08
0.06 0.36
0.06
0.16
P
P
P
0.56
P
0.19
0.54c
0.18c
a The incubation temperature is 25 °C. b The data are average values
of two experiments. c In the presence of D-14 (1 equiv).
precipitation, PS ) self-supporting precipitation.
P )
ethanol, and ethyl acetate, or linear aliphatic hydrocarbons, such
as n-hexane and n-decane, even with heating. However, the
foldamers were found to gelate many solvents of varying
polarity, including cyclic hydrocarbons, arenes, higher alcohols
and esters, and 1,4-dioxane. The results are summarized in Table
1. Generally, introduction of the two arene units at the ends
enhances the capacity of gelating polar solvents, even though
2 exhibits a surprisingly high capacity of immobilizing less polar
tetralin, toluene, and p-xylene. Among the investigated systems,
approximately 66% has a minimum gelation concentration of
less than 1%. Notably, eight of them have a value of less than
0.1%, making them rank among the very limited “supergela-
tors”.25 The possibility of shorter molecules, such as 7, as
gelators was also evaluated for all the solvents shown in Table
1. However, no gels were found to form even at the saturation
concentration. This result indicates that the rigid framework of
the longer foldamers, which should also possess increased
stacking tendency,20a,c is crucial for gelating the solvents.
It was also revealed that addition of alkylated sugars
considerably enhanced the capacity of the foldamers to gelate
less polar solvents (Table 1), even though the sugars themselves
could not gelate any solvent. For example, in the presence of
D-14 (1 equiv), the minimum gelation concentration of 1a for
toluene decreased from 2.60% to 0.30%. It is reasonable to
(26) (a) Kunitake, T. Angew. Chem., Int. Ed. Engl. 1992, 31, 709–726. (b)
Bao, C.; Lu, R.; Jin, M.; Xue, P.; Tan, C.; Xu, T.; Liu, G.; Zhao, Y.
Chem.sEur. J. 2006, 12, 3287–3294.
(27) Samain, F.; Malinovskii, V. K.; Langenegger, S. M.; Ha¨ner, R. Bioorg.
Med. Chem. 2008, 16, 27–33.
(28) (a) Zhang, X.; Sadaki, K.; Kuroda, Y. J. Org. Chem. 2006, 71, 4872–
4877. (b) Shiraishi, Y.; Tokitoh, Y.; Nishimura, G.; Hirai, T. Org.
Lett. 2005, 7, 2611–2615.
(25) (a) Gronwald, O.; Shinkai, S. Chem.sEur. J. 2001, 7, 4328–4334.
(b) Mukhopadhyay, S.; Maitra, U.; Ira, Krishnamoorthy, G.; Schmidt,
J.; Talmon, Y. J. Am. Chem. Soc. 2004, 126, 15905–15914. (c)
Desvergne, J.-P.; Brotin, T.; Meerschaut, D.; Clavier, G.; Placin, F.;
Pozzo, J.-L.; Bouas-Laurent, H. New J. Chem. 2004, 28, 234–243.
(29) (a) Nuckolls, C.; Katz, T. J.; Castellanos, L. J. Am. Chem. Soc. 1996,
118, 3767–3768. (b) Lovinger, A. J.; Nuckolls, C. J.; Katz, T. J. J. Am.
Chem. Soc. 1998, 120, 264–268. (c) Nuckolls, C.; Katz, T. J.; Katz,
G.; Collings, P. J.; Castellanos, L. J. Am. Chem. Soc. 1999, 121, 79–
88.
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Figure 2. (Time-dependent) UV-vis spectra of compounds (a) 1a, (b) 1b, and (c) 1e in sols and gels at 25 °C. The thickness of the sample cell was 1 cm
for solution samples. The thickness of the gel samples was 0.1 mm.
Figure 3. Fluorescent emission of the solution and gel of (a) 1d in decalin (λex ) 360 nm), (b) 1e in n-octanol (λex ) 360 nm) and (c) 1f in decalin (λex
) 350 nm) at 25 °C. The thickness of the sample cell was 1 cm for solution samples. The gel samples were sandwiched between two quartz plates.
(Figure 4), this result is not surprising. Because the aggregation
was a dynamic process, the less stable “in-and-out” or even
“out-and-out” arrangement might also exist, even though it
should be of a minor percentage. XRD diffraction study and
SEM images (Figures 5 and 6, vide infra) also support such
cylindrical column assemblies of the folded frameworks. It is
expected that such cylindrical aggregates were driven by van
der Waals forces of the long decyl chains to further assemble
into bundle fibrils, which finally immobilized the solvents.2a
X-Ray Diffraction (XRD). XRD measurements were carried
out for the gels of 1a-f and 2. The spectra of 1a, 1d, and 1f in
n-octanol are shown in Figure 5. The broad peak at 2θ ) ca.
19.2° was generated by the solvents.10b All the samples gave
rise to a sharp peak in the range of 2.36-2.59 nm. Molecular
modeling suggested that the helical framework of the foldamers
had a diameter of approximately 2.1 nm. Considering the
contribution of the flexible side chains, this result supports that
this ordered structure was generated through stacking of the
folded frameworks (Figure 4). The spectra of the n-octanol gel
of 1d and the p-xylene gel of 2 also exhibited a peak at 0.36
(Figure 5b) and a peak at 0.35 nm (not shown), respectively,
which reflect the average distance between the stacked
foldamers12d and also further support the “head-to-head” stack-
ing pattern (Figure 4). Similar shoulder peaks were not observed
in the profiles of other investigated gels, which we consider
might also exist but be buried in the strong peak of the solvents.
Morphology Study. SEM images were recorded for the gels
of 1a-f and 2 in all the solvents shown in Table 1. The images
of the n-ocatanol gels showed fibrils of micrometers in length
and hundreds of nanometers in width, supporting that the
cylindrical column aggregates of the foldamers further as-
sembled to form bundle structures (Figure 4). Representative
results are provided in Figures 6. Similar fibrous structures could
also be observed for the gels of other solvents including tetralin,
toluene, and p-xylene. For the gels of polar solvents such as
1,4-dioxane and esters, fibers of wider and thicker cotton
wadding were exhibited. The fibrous structures of the gels could
also be evidenced by AFM, as shown by the images of 1d and
1f in Figure 6.
Induced Circular Dichroism. It has been found that in
chloroform hydrazide foldamers are induced by chiral saccha-
rides to produce supramolecular helicity, as evidenced by
formation of induced circular dichroism (ICD).19 Upon addition
of D- or L-14, the solutions of foldamers 1a-f in chloroform
also exhibited ICD signals (see the Supporting Information),
which well suggested that helicity induction might occur in the
gel phases. To test this possibility, ICD spectra of the p-xylene
and/or toluene gels of 1a-f in the presence of D- or L-14 were
measured. These two solvents were chosen mainly because of
their low polarity, which guaranteed strong complexation
occurred between 1a-f and 14.
The ICD spectra of all the gels of 1a-f and D- or L-14 were
recorded in a cell of 0.1 mm thickness (Figure 7). As expected,
addition of D- or L-14 (1 equiv) caused all the foldamers to
produce ICDs of mirror symmetry. In contrast, their gels in polar
solvents, including butyl acetate, isobutyl acetate, ethyl acrylate,
n-butanol, n-octanol, and 1,4-dioxane, did not give rise to any
detectable signals. For compound 2, even excessive D- or L-14
did not induce its toluene or p-xylene gels or chloroform solution
to produce observable ICD, implying that the appended arene
units in 1a-f played a key role in amplifying the helicity
differentiation of their frameworks.
Because the orientation of gelators might be affected by
macroscopic anisotropy and observed CD spectra might hence
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Foldamer Organogels
A R T I C L E S
Figure 4. Schematic representation of the self-assembly of foldamer gels and glucose-initiated helicity induction. It is assumed that the stacked columns of
the foldamers were driven by van der Waals forces of the decyl chains of the foldamers to aggregate into bundle fibrils, which gelated the solvent molecules.
The decyl chains are not shown in the model for clarity. In the two-component system, there should also be free foldamer and glucose molecules, which are
not presented for clarity.
Figure 5. XRD spectra of the n-octanol gels of (a) 1a, (b) 1d, and (c) 1f (0.1 g/mL) at 25 °C. The samples were prepared by drop casting of the gels on
a silica plate and then measured immediately.
include components arising from linear dichroism (LD),30 we
also recorded the LD spectra under the same measurement
conditions. The spectra, together with the related UV-vis
spectra, are shown in Figure 7. It can be found that the optical
densities of the gels of 1a, 1c, and 1e were very weak (<0.001).
Their contribution to the CD spectra can be neglected.30b The
optical densities of the LD spectra of 1b, 1d, and 1f were
considerably stronger. Thus, their contributions to the CDs were
not negligible. Control experiments showed that the gels of 1a-f
themselves were all LD-silent. Therefore, these LDs should be
produced though the induction of the glucose. Because the LDs
could not be easily subtracted from the CDs, we may more
reasonably regard the observed CDs of 1b, 1d and 1f as
“apparent” ones, which implies including the contribution of
the LDs.
The toluene gel of 1a and D-14 featured a strong positive
band (λmax ) 322 nm) and a weak negative band (λmax ) 357
nm). A small shoulder peak was also exhibited at λ ) ca. 300
nm. A similar shoulder peak was also observed in the spectra
of the p-xylene gel of 1c. These results suggest that the ICDs
were generated by superimposition of signals of discrete
aromatic units. Conformers 1b and 1c were induced by chiral
14 to form a strong positive CD band at λmax ) 322 and 324
nm, respectively. However, only 1b exhibited a strong negative
band above 350 nm, indicating that the geometry of the
naphthalene units remarkably affected their helical conforma-
(30) (a) Shindo, Y.; Ohmi, Y. J. Am. Chem. Soc. 1985, 107, 91–97. (b)
Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komri,
T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994, 116,
6664–6676.
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Figure 6. SEM images of the dried gels of (a) 1a, (b) 1c, (c) 1d, and (d) 1f obtained from n-octanol and AFM images of the dried gels of (e) 1d and (f)
1f from toluene (0.1 mM).
tions. The LD of 1b in the area was positive. Therefore, its
exact role for the formation of this negative band is not clear
yet. For anthracene-bearing conformers 1d and 1e, in addition
to a strong band below 350 nm, several weak bands were also
exhibited in the region of longer wavelength, which should be
produced by the anthracene units with a negligible LD com-
ponent (Figure 7d and e). The gels of 1f exhibited two strong
CDs which were severalfold stronger than those of other
foldamers. Notably, the strength of the bands at above 350 nm,
produced by the pyrene units, was comparable to that of the
band of the shorter wavelength. This result should reflect the
increased capacity of its pyrene units in helicity induction
relative to that of its smaller counterparts in 1a-e. It also
indicates that the helicity differentiation was not linearly related
to the stability of their single complex. Because 1a-f contain
different aromatic units, the signals of the ICD spectra could
not be used to assign their P or M helicity.31,32
The temperature-dependent CD spectra of the p-xylene gels
of the mixtures were also recorded, and those of 1a-c and D-14
are shown in Figure 8. For all the investigated systems, the
intensity of the main signals remained unchanged within a range
of lower temperature but began to decrease after a turning point
and vanished at high enough temperature (Figure 8, insets). This
result can be easily explained by considering gradual deaggre-
gation of both helical stacking architectures and chiral molecular
complexes. Interestingly, the negative signal of 1b first under-
went an enhancement before turning weaker, which further
supports that the signals were formed by a combination of those
of discrete aromatic units that were nonlinearly temperature-
dependent.
Most reported organogels form very quickly once their
solution phases are cooled to below gelation temperature.
Recently, Weiss et al. found that the transformation of two chiral
cholesteryl N-(2-naphthyl) carbamates in n-alkanes from sols
to gels occurred on a time scale of minutes to hours, allowing
for a CD investigation of the gelation kinetics.10 By using a
similar method, we have followed the transformation of our two-
component systems from sols to gels. It was found that the ICD
intensities of the toluene or p-xylene gels of 1d and 1f in the
presence of D- or L-14 reached a maximum in less than one
minute and then became stable for days. This result implied
that their appended arenes were well orientated for intermo-
lecular stacking. In contrast, those of other foldamers needed
much longer time (from 0.5 to 10 h) to reach equilibrium (Figure
7). Furthermore, the ICDs were weak at the early stage, implying
existence of an induction or incubation period. The processes
could be repeated. However, the ICDs recorded for the gel
phases were different in shape from those of the solutions of
the same samples in chloroform, indicating that the ICDs of
the gels were produced by the helical assemblies of chiral
complexes (Figure 4). The remarkable time-dependent enhance-
ment of the ICDs of the systems of 1a, 1c, and 1e should be
attributed to stacking-resulted elongation of the cylindrical
column architectures, which was further driven by van der Waals
forces of the decyl chains to lead to the formation of the gels,
as shown in Figure 4. The stacking process was expected to
have several outcomes. First, induced helicity of the single
folded molecule was transferred to supramolecular helicity of
cylindrical column architectures. Second, it gave rise to a tube-
(31) Tanatani, A.; Yokoyama, A.; Azumaya, I.; Takakura, Y.; Mitsui, C.;
Shiro, M.; Uchiyama, M.; Muranaka, A.; Kobayashi, N.; Yokozawa,
T. J. Am. Chem. Soc. 2005, 127, 8553–8561.
(32) Dolain, C.; Jiang, H.; Le´ger, J.-M.; Guionneau, P.; Huc, I. J. Am. Chem.
Soc. 2005, 127, 12943–12951.
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A R T I C L E S
Figure 7. UV-vis, LD, and (time-dependent) CD spectra of (a) 1a in toluene, (b) 1b in p-xylene, (c) 1c in p-xylene, (d) 1d in p-xylene, (e) 1e in toluene,
and (f) 1f in p-xylene in the presence of D- or L-14 ([1] ) [14] ) 4 mM) at 25 °C. The UV-vis and LD spectra were recorded after the intensity became
unchanged. The thickness of the sample cell for the optical path is 0.1 mm.
Figure 8. Temperature-dependent ICD spectra of the p-xylene gels of (a) 1a, (b) 1b, and (c) 1c and D-14 (1:1, 4 mM) (inset: plots of the CD intensity at
the defined wavelength against temperature). The thickness of the sample cell for the optical path is 0.1 mm.
styled structure, which as a whole forced complexed glucose
molecules to form a chiral hydrogen-bonded glucose chain. It
is expected that the foldamer column and the chiral glucose
chain in it would stabilize each other because the density of
the hydrogen bonding sites of both aggregates were increased
in the compact structure. Third, the stacking also decreased the
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A R T I C L E S
Cai et al.
growth leading to phase separation, and t is time. For the
cholesterol-based single molecular systems, the chirality of the
gels is generated exclusively by the gelators themselves. For
the present two-component systems, while gelation is mainly
driven by stacking of the foldamers, the helicity is generated
through chirality transfer from the complexed glucoses to the
stacked foldamers. Therefore, the n values were extracted on
the assumption that the helicity transfer is a statistical outcome
of the whole system. The n values of 1a, 1c, and 1e were derived
to be 1.44 (322 nm), 1.57 (325 nm), and 1.87 (326 nm),
respectively (Figure 9).34,35 The result suggests that gelation of
the two-component systems underwent a nucleation-elongation
mechanism (Figure 4),10b,36 which involves two distinct phases
of self-assembly.37 The first was the slow nucleation phase. At
this stage, cylindrical column assemblies were not formed yet,
and the helicity was mainly expressed by the single complex
species, as reflected by the small and slow change of the ICD
intensity. Once stable nuclei were formed, the system entered
the second elongation phase. Further addition of helical
complexes onto the ends of the cylindrical assemblies became
favorable, leading to quick enhancement of the helicity. The
cylindrical structures further aggregated into long fibril networks,
immobilizing the solvents.
Figure 9. ICD intensities of the gels of 1a (toluene), 1c (p-xylene), and
1e (toluene) and L- or D-14 (1:1, 4 mM), which were obtained from the
spectra in Figure 7, plotted against time according to eq 1. The time period
covered is from 20 to 600 min. Each spectrum required 40 s to record.
conformational flexibility of the backbones of the foldamers,
especially the appended arene units, facilitating amplification
of the helicity of the single complexes. The fact that the sol-gel
transformation of 1d and 1f took a very short time might suggest
that they possessed even larger stacking capacity and hence did
not have the induction period. In accordance with the ICD
spectra, UV-vis spectra of the investigated systems also
exhibited a similar time-dependent decrease (Figure 2), which
may be ascribed to enhancement of intermolecular stacking but
may also result from a transparency decrease. The gelation of
1a-f in the absence of 14 should also undergo a similar two-
stage mechanism, even although the process is CD-silent and
cannot be investigated by the CD method.
The dynamic ICD data of 1a, 1c, and 1e (Figure 7), which
covered a time scale of hours and could be repeated with
acceptable precision, were also treated with the Avrami equation
(eq 1)33 according to the method established by Weiss and co-
workers for chiral cholesterol-based organogels,10 where I0, It,
and Imax are the CD intensities at times ) 0, t, and ∞,
respectively, K is a temperature-dependent parameter that is like
a rate constant, n is the Avrami exponent reflecting the type of
ln ln I -I / I -I ) ln K + n ln t
t) ( )]}
0
(1)
{
[(
max
max
One of the unique behaviors of chiral supramolecular
assemblies is the so-called “Sergeant and Soldiers” effectschiral
amplification occurring through the induction of chiral compo-
nents for similar achiral components to follow their helicity.38
Such a nonlinear effect has also been revealed between chiral
folded m-phenylene ethynylene oligomers and their achiral
counterparts in solution.39 To test whether a similar effect existed
in the gel phases,40,41 ICDs of the p-xylene gels of the foldamers
in the presence of varying amounts of D-14 were recorded.
Deviations from linearity were observed for all the investigated
systems, but the patterns were different (Figure 10a-e). The
Figure 10. “Sergeants-and-Soldiers” measurements of the p-xylene gels of (a) 1a, (b) 1b, (c) 1c, (d) 1d, and (e) 1f in the presence of D-14 (0 to 1 equiv),
and (f) 1a + D-14 (1:1, in toluene) in the presence of L-14 (0 to 1 equiv) at 25 °C ([1a-f] ) 4 mM). The thickness of the sample cell for the optical path
is 0.1 mm.
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A R T I C L E S
plots of the ICD intensity of 1a-c against mole percent of D-14
showed positive nonlinearity. Thus, addition of approximately
50, 35, and 30% of D-14 induced them to produce signals
comparable in strength to that induced by 1 equiv of D-14. This
phenomenon is similar to the common “Sergeant and Soldiers”
effect in the context of helicity amplification, except that the
new “Sergeant” and “Soldiers” are completely different in shape.
The plots of 1d and 1f exhibited interesting sigmoidal curvessthe
intensity changed from negative to positive nonlinearity with
the increase of the amount of D-14. That is, the “Sergeants and
Soldiers” effect occurred only after a considerable amount of
D-14 was added. A possible explanation for this observation is
that the large appended anthracene or pyrene units of 1d and
1f have even larger stacking capacity. As a result, more glucose
molecules are needed to “trigger” the helicity differentiation.
This result is also consistent with the formation of the hydrogen-
bonded glucose chain in the cylindrical assemblies, because it
is reasonable to propose that the more the amount of the glucose
was, the easier the chain would be formed. It can also be found
that, for the foldamer series from 1a to 1f, the tendency of
producing positive nonlinearity is gradually decreased. Once
again, this can be ascribed to the increasing stacking capacity
of the appended arene units. The CD spectra of the toluene gels
of 1a and D-14 (1:1) with varying amounts of L-14 were also
measured, which also revealed a sigmoidal curve and a weak
“Sergeants and Soldiers” effect (Figure 10f).
Conclusions
We have described the self-assembly of a new family of
foldamer gelators that can immobilize a broad range of organic
solvents. Intramolecular hydrogen bonding stabilizes the rigid
helical conformations of the foldamers, whereas large arene units
at the ends of the backbones facilitate intermolecular dislocated
“tail-to-tail” stacking. As a result, cylindrical column aggregates
are formed, which are held together by van der Waals forces of
long aliphatic side chains to give rise to fibrous networks,
leading to a new class of organogels with minimal gelation
concentrations as low as 0.06% in weight.
One unique feature of the new foldamer gelators is their
capacity to complex chiral glucoses. As a result, helicity
induction can occur in both sols and gels. A systematic circular
dichroism investigation has shown that this helicity induction
in the gel phases is dynamic and, in several cases, helicity
amplification occurs during the sol-gel transfer. The fact that
addition of glucoses notably raises the gelation capacity of the
foldamers shows that, through doping or mixing, we may endow
organogels with new tunable or required properties.
(33) (a) Avrami, M. J. Chem. Phys. 1939, 7, 1103–1112. (b) Avrami, M.
J. Chem. Phys. 1940, 8, 212–224.
(34) The Avrami theory is a statistical model that describes the crystal-
lization of polymer melts and has been applied to gelation processes
(see refs 10 and 34).
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Academic Press: New York, 1976; Vol. 2; p16-52.
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Jonkheijm, P.; van der Schoot, P.; Schenning, A. P. H. J.; Meijer,
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Kooijman, H.; Spek, A. L.; Sijbesma, R. P.; Meijer, E. W. J. Am.
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Acknowledgment. We thank the National Science Founda-
tion of China (Nos. 20732007, 20621062, 20425208, 20572126,
20672137), the National Basic Research Program (2007CB808000),
and the Chinese Academy of Sciences (KJCX2-YW-H13) for
financial support.
(38) Green, M. M.; Reidy, M. P.; Johnson, R. J.; Darling, G.; O’Leary,
D. J.; Willson, G. J. Am. Chem. Soc. 1989, 111, 6452–6454.
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(40) Helicity induction of achiral oligo(p-phenylenevinylene)- and discotic
triazine triamide-based organogelators by their chiral analogues has
been reported; see: (a) Ajayaghosh, A.; Varghese, R.; George, S. J.;
Vijayakumar, C. Angew. Chem., Int. Ed. 2006, 45, 1141–1144. (b)
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Supporting Information Available: Experimental procedures
1
and characterizations, H NMR of 1a-f and 2, and additional
UV-vis, fluorescent, CD, and XRD spectra and SEM and AFM
images. This material is available free of charge via the Internet
at http://pubs.acs.org.
(41) Because the ICD intensity was time-dependent, all the spectra were
recorded after the equilibrium time.
JA8043322
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