Tuning the helicity of self-assembled structure of a sugar-based organogelator by the proper choice of cooling rate

Article abstract of DOI:10.1021/la903064n

A novel sugar-appended low-molecular-mass gelator, 4″-butoxy-4- hydroxy-p-terphenyl-β-D-glucoside (BHTG), was synthesized. It formed thermally reversible gels in a variety of aqueous and organic solvents. Three-dimensional networks made up of helical ribbons were observed in the mixture of H₂O/1,4-dioxane (40/60 v/v). The handedness of the ribbons depended on the rate of gel formation. Fast-cooling process led to right-handed ribbons, while slow-cooling process led to left-handed ones. A combinatory analyses of microscopic, spectroscopic, and diffraction techniques revealed that BHTG formed a twisted interdigitated bilayer structure with a d spacing of 3.1 nm in gels through a kinetically controlled nucleation-growth process. There were two kinds of molecular orientations of BHTG in the nuclei, clockwise and anticlockwise, which dictated the growth of ribbons. One was metastable and formed first during the cooling process of gel formation. It was able to gradually transform into the more stable latter one with further decreasing temperature. Fast-cooling process did not leave enough time for the nuclei to evolve from metastable to stable state and the ribbons grown from them exhibited right-handedness. However, the metastable nuclei transformed into the stable one when cooled slowly and directed the molecules of BHTG to grow into left-handed aggregates.

Full text of DOI:10.1021/la903064n

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© 2009 American Chemical Society
Tuning the Helicity of Self-Assembled Structure of a Sugar-Based
Organogelator by the Proper Choice of Cooling Rate
Jiaxi Cui, Anhua Liu, Yan Guan, Jia Zheng, Zhihao Shen, and Xinhua Wan*
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of
Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871,
China
Received August 17, 2009. Revised Manuscript Received October 28, 2009
A novel sugar-appended low-molecular-mass gelator, 400-butoxy-4-hydroxy-p-terphenyl-β-
D-glucoside (BHTG), was
synthesized. It formed thermally reversible gels in a variety of aqueous and organic solvents. Three-dimensional
networks made up of helical ribbons were observed in the mixture of H2O/1,4-dioxane (40/60 v/v). The handedness of
the ribbons depended on the rate of gel formation. Fast-cooling process led to right-handed ribbons, while slow-cooling
process led to left-handed ones. A combinatory analyses of microscopic, spectroscopic, and diffraction techniques
revealed that BHTG formed a twisted interdigitated bilayer structure with a d spacing of 3.1 nm in gels through a
kinetically controlled nucleation-growth process. There were two kinds of molecular orientations of BHTG in the
nuclei, clockwise and anticlockwise, which dictated the growth of ribbons. One was metastable and formed first during
the cooling process of gel formation. It was able to gradually transform into the more stable latter one with further
decreasing temperature. Fast-cooling process did not leave enough time for the nuclei to evolve from metastable to
stable state and the ribbons grown from them exhibited right-handedness. However, the metastable nuclei transformed
into the stable one when cooled slowly and directed the molecules of BHTG to grow into left-handed aggregates.
Introduction
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these molecules are asymmetric and chiral aggregates such as
The self-assembly of small molecules and macromolecules into
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Fax: 86-10-62751708. E-mail: xhwan@pku.edu.cn.
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Langmuir 2010, 26(5), 3615–3622
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DOI: 10.1021/la903064n 3615

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Cui et al.
twisting fibrils and helical ribbons are therefore generated.6 In
general, the handedness of a chiral superstructure reflects the
underlying chirality of the subclass building blocks.18 One enantio-
mer gives a right-handed aggregate, while the other gives a left-
handed one.19 However, several conflicting examples have been
reported recently. Thomas and co-workers found the simultaneous
formation of right- and left-handed helical ribbons by an enantio-
merically pure phosphonate analogue of 1,2-bis(10,12-tricosa-
diynoyl)-sn-glycero-3-phosphocholine (DC(8,9)PC) and the related
compounds.20 Zastavker et al. obtained similar results in a variety of
multicomponent enantiomerically pure systems containing a bile
salt or a nonionic detergent, a phosphatidylcholine or a fatty acid,
and a steroid analogue of cholesterol.21 Moreover, Ho, Hsu, and
their co-workers observed the dependence of helical twisting power
on the length of achiral alkyl tail in a series of sugar-appended Schiff
base rod-coil amphiphiles.22 These interesting findings demon-
strate, on the one hand, the complexity in the generation, transfer,
and expression of chirality during the formation of hierarchical
supramolecular structures. It enables us, on the other hand, to tune
the handedness of the chiral aggregates without changing the
chemistry of the building blocks.
A gel is usually prepared by first dissolving LMMG in a certain
solvent at high temperature and then cooling the hot solution to a
lower temperature.23 In this sense, thermal control might repre-
sent one of the most convenient methods to tune the substructures
of the gel from LMMG. Indeed, Weiss et al. noticed a correlation
between the cooling rate and the length and thickness of the fibers
from anthraquinone-appended steroid-based gelators.24 Shinkai
and co-workers found thatthe sign inversion of circulardichroism
(CD) effects of the gels from some azobenzene-linked cholesterol
derivatives depended on the rate of network formation.25 Espe-
cially, in a chiral optical molecular switch system reported by van
Esch and Feringa,26 reversible transcription of supramolecular
chirality to molecular chirality with nearly complete stereoselec-
tivity was achievedthrough locking metastable chiral assembles in
gel by a combination of thermal and light control. The molecular
switch had open and closed forms. Irradiation of the open form in
the gel phase yielded metastable assembly of closed form, which
was regulated to be a stable state. The stable state of the closed
form produced a metastable state of the open form by irradiation.
Thus, tetra-state cycle can be carried out conveniently by regulat-
ing the metastable state of the open form to be a stable one.
Sugar-appended gelators are a large family of LMMGs display-
ing remarkable diversity in gelation ability and supramolecular
structures.13-15 The preparation and structures of hollow glycolipid
Chart 1. Chemical Structure of BHTG
nanotubules and their precursors, helical fibrous ribbons, have been
systematically investigated.27 However, no work on the thermal
control of supramolecular chirality of sugar-appended gelator has
been known.
In this study, we report the synthesis, gelling behavior, and
cooling rate dependence of supramolecular chirality of a novel
sugar-based gelator, 400-butoxy-4-hydroxy-p-terphenyl-β-
D-glu-
coside (BHTG), in a mixture of H2O/1,4-dioxane (40/60 v/v).
BHTG was able to immobilize a variety of aqueous and organic
solvents at concentration down to 1 mg/mL. In the mixed solvent,
three-dimensional networks made up of helical ribbons form. The
handedness of the ribbons depends on the rate of gel formation
from hot solution. The formation of right-handed helical ribbons
is kinetically favored, and that of left-handed helical ribbons is
thermodynamically favored. A combination of spectroscopic,
diffraction, and microscopic methods is used to approach the
molecular ordering of BHTG in the gel and the formation and
transition mechanism of chiral aggregates.
Results and Discussion
Synthesis. The gelator BHTG (Chart 1) was prepared via a
multistep synthetic route (Scheme S1 in the Supporting Infor-
mation). The glycosylation of 40-hydroxy-4-bromobiphenyl with
D
-glucose pentaacetate, catalyzed by BF3•OEt2, led to 40-bromo-
biphenyl-β-O-D-tetraacetylglucoside with good stereoselectiv-
ity.28 The subsequent Suzuki cross-coupling reaction with
4-butoxyphenylboronic acid yielded 400-butoxy-4-hydroxy-p-ter-
phenyl-β-O-D-tetraacetylglucoside, which gave the target mole-
cule after the deacetylation with freshly prepared CH3ONa in the
mixture of methanol and CHCl3 (1/1 v/v). The target molecule
and all of the intermediates were characterized by H and 13C
1
NMR, elementary analysis, and mass spectroscopy. All data
agreed well with the expected structures. The presence of only
one doublet with J = 7.3 Hz at δ = 4.91-4.94 ppm for the
anomeric proton in the 1H NMR spectrum of BHTG indicated a
100% β-glucosidic linkage (Figure S1).27a The four small peaks in
the 4.5-5.5 ppm region were ascribed to the hydroxyl protons of
the sugar head,28b which disappeared when D2O was added to
exchange out the protons (as evidenced in Figure S5). The specific
(17) (a) Hirst, A. R.; Smith, D. K. Top. Curr. Chem. 2005, 256, 237-273.(b) Hill,
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Tetrahedron 2007, 7375–7385. (f) Kundu, S. K.; Osaka, N.; Matsunaga, T.; Yoshida,
M.; Shibayama, M. J. Phys. Chem. B 2008, 112, 16469–16477.
optical rotation [R]20 of BHTG was -34 (c = 10 mg/mL in
365
DMF).
Gelation Test. To test the gelation ability of BHTG, a
weighed amount of the sample and solvent were put into a
septum-capped bottle and heated until all the solids dissolved.
The solution was then allowed to cool in air (16 °C). Afterward,
the bottle was inversed. If the mixture did not fall, it was
recognized as a gel (Figure S2). Table 1 summarizes the testing
results. BHTG gelled glycol, acetonitrile, acetic acid, 1,4-dioxane,
tetrahydrofuran (THF), and toluene while dissolved in dimethyl
sulfoxide (DMSO), dimethylformamide (DMF), and pyridine.
(18) Maeda, K.; Yashima, E. Top. Curr. Chem. 2006, 265, 47-88.
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Kirsch, J. E.; Persichini, P. J. J. Am. Chem. Soc. 1998, 120, 12178–12186. (b)
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Schnur, J. M.; Benedek, G. B. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 7883–7887.
(22) Lin, T.; Ho, R.; Sung, C.; Hsu, C. Chem. Mater. 2008, 20, 1404–1409.
(23) Lin, Y. C.; Kachar, B.; Weiss, R. G. J. Am. Chem. Soc. 1989, 111, 5542–
5551.
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3616 DOI: 10.1021/la903064n
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Cui et al.
Article
Table 1. Gelation Experimental Resultsa
DMF pyridine
solvent
H2O
DMSO
alcohol
acetone
ethyl acetate
statea
solvent
I
ethanediol
G
O
124
4.2
S
acetonitrile
G
T
89
2.7
S
S
I
I
toluene
G
T
109
1
I
benzene
I
/
/
/
acetic acidb
1,4-dioxaneb
THFb
statea
G
O
68
6.4
G
O
51
9.5
G
T
74
8
appearance
Tgel/°C
Cgel/mg/mL
a Samples were prepared by fast-cooling procedure; concentrations = 5 mg/mL; G, gel; S, solubilization; I, insolubilization; T, translucent; some small
crystallites or slightly opaque regions; O, opaque. b Measurement concentrations = 10 mg/mL. c Tgel: gel-sol transition temperature; the concentration
is 10 mg/mL for acetic acid, THF, and 1,4-dioxane, while 5 mg/mL for other solvents. d Cgel: critical gelation concentration.
Information), which reflected the influence of solvent nature on
the suprastructure of BHTG in gels.15a The morphology of the gel
fiber in other solvents was not studied and was beyond the scope
of the present work. One of the interesting morphological
observations was that the handedness of the ribbons formed in
1,4-dioxane and its mixture with water depended on the rate of
network formation. Figure 1a showed the TEM picture of BHTG
network formed in the mixture of water and 1,4-dioxane (40/60 v/v),
which consisted of helical ribbons with 20-150 nm diameters.
Moreover, as demonstrated by SEM images, slow-cooled gels
displayed left-handed helical ribbons (Figure 1b), while fast-
cooled gel exhibited right-handed helical ribbons (Figure 1c).
The pitch tilt was about 45° for the right-handed helices but about
60° for the left-handed ones. Similar results could be obtained at
various concentrations (2-10 mg/mL) and compositions of
mixed solvent of H2O/1,4-dioxane (60/40-0/100 v/v) but were
not obtained from other solvents. The reason is not known at the
moment. In order to monitor the gelling process with UV-vis and
circular dichroic spectroscopy, low critical gelation concentration
(Cgel) was preferred. Therefore, the gelation behavior of BHTG
was investigated in the mixture of H2O and 1,4-dioxane at a fixed
volume ratio of 40/60 in the subsequent study. The role of water
addition was to reduce Cgel. The formation of right- and left-
handed helical ribbons from enantiomerically pure BHTG sug-
gested that the molecular chirality was not the only determining
Figure 1. TEM picture of slow-cooled gel of BHTG (a), SEM
images of the gels of BHTG obtained through slow-cooling
factor in helix formation of the present system.
Thomas, Clark, and co-workers have done a very nice work on
process (b), and fast-cooling process (c). Solvent: H2O/1,4-dioxane
the self-assemblying of diacetylenic phospholipids.20b Under
(40/60 v/v); concentration = 5 mg/mL.
differential phase contrast microscopy, they examined in situ
the nucleation and growth of DC(8,9)PC at a very slow cooling
Similar to its biphenyl analogues, 4-(40-butoxyphenyl)phenyl-β-
rate from high enough temperature. It was found that roughly
O-D
-glucoside,15a it was concluded that the self-assembly of
equal numbers of left- and right-handed helices were generated in
the first seconds of tubule formation from spherical multilamellar
vesicles of a pure enantiomer, followed by the formation of
multilamellar tubules with single handedness exterior helical
ridges. Unfortunately, the aggregates formed by BHTG are not
as large as that of DC(8,9)PC and are not suitable for real-time
optical microscopic observation. We therefore turn to other
methods for help.
To exclude the possibility that the cooling rate causes the
stereostructure variation of BHTG, Fourier transform infrared
(FT-IR) and1H NMR spectroscopies were employed to characterize
xerogels of both fast-cooled gels and slow-cooled gels. FT-IR is
sensitive to the conformation of the glucoside group. For exam-
ple, a typical peak around 890 cm-1 originates from the vibration
of C1-H in β-form linkage, while the peak around 844 cm-1
arises from that in R-form linkage.29 The spectra of fast-cooled
and slow-cooled xerogels were similar and both exhibited peaks at
896 cm-1, implying that no β- to R-form configurational change
occurred (Figure S4). In addition, the two samples showed
BHTG was mainly driven by hydrogen bonding among sugar
moieties, π-π stacking interaction among p-terphenyl segments,
and hydrophobic interaction of butoxy tails. These interactions
were so strong that the resultant gels were quite stable. For
example, a gel of BHTG formed in glycol at a concentration of
30mg/mLexhibits a very highgel-sol transition temperature (Tg)
of 170 °C and was kept unchanged for more than 2 years at room
temperature. The gelationexperiments of BHTG in the mixture of
H2O/1,4-dioxane were carried out with two methods. “Fast-
cooled gels” were obtained by allowing hot isotropic solution of
BHTG to cool to room temperature (16 °C) immediately, while
“slow-cooled gels” were obtained by putting the same hot
isotropic solution into a water bath mediated with a PolyScience
programmable temperature controller and allowed to cool from
70 °C to room temperature at a rate of 1 °C/min.
Electron Microscopy. Transmission electron microscopy
(TEM) and scanning electron microscopy (SEM) were used to
investigate the aggregate morphologies of BHTG in acetonitrile,
toluene, 1,4-dioxane, and the mixture of 1,4-dioxane and water.
Sheet aggregates were observed in the first solvent, while helical
ribbons were found in the latter three (Figure S3 in the Supporting
(29) Barker, S. A.; Bourne, E. J.; Whiffen, D. H. Methods of Biochemical
Analysis; Interscience: New York, 1956; Vol. 3.
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Cui et al.
Figure 2. Temperature-variable absorption (a) and emission (b) spectra of BHTG in H2O/1,4-dioxane (40/60 v/v) with a concentration of
2 mg/mL. The inset shows the absorption spectra of the sample at 60 °C and BHTG in DMF at 20 °C.
shows absorption and emission bands similar to that in DMF,
suggesting that BHTG was molecularly dispersed in the mixed
solvent at 60 °C. When the temperature was lowered to 20 °C,
where the gel was generated, not only the maximal absorption
blueshifts14 nm but alsothe intensity around 340 nm enhances. It
is known that p-terphenyl in the crystal state reveals the absorp-
tion of a low-lying 1Lb state at around 340 nm, which is usually
hidden or superimposed on the long-wavelength side of the strong
1La band in solution.30b Here, the increased intensity at 340 nm on
the absorption spectra of BHTG suggested that the p-terphenyl of
BHTG tended to take planar conformation in the aggregate state.
Besides, the considerable blue shift indicated BHTG took an “H”
type aggregate in gel.30a,31 Accordingly, red shifts in emission spectra
were observed: at low temperature, the gel of BHTG exhibits a
structured fluorescence with bands at 343, 371, 387, and 410 nm,
while at 60 °C, only one peak centered at 363 nm was shown
Figure 3. WAXD patterns of the xerogel obtained from the
fast-cooled gel of BHTG in H2O/1,4-dioxane and (40/60 v/v) at
5 mg/mL.
(Figure 2b). Fast-cooled sample gave similar results (Figure S6).
WAXD. The packings of BHTG ingels were further addressed
by WAXD. Figure 3 shows the powder WAXD pattern of the
fast-cooled xerogel of BHTG from H2O/1,4-dioxane. There were
three diffraction peaks corresponding to the d spacings of 3.09,
1.48, and 0.74 nm in the low-angle region. The d spacing ratio of
1:1/2:1/4 was consistent with a lamellar structure. The length of
3.09 nm was larger than the extended molecular length of BHTG
(2.1 nm estimated by CPK molecular modeling) but smaller than
twice the length suggesting an interdigitated bilayer structure
with thickness of 3.1 nm. Similar result was observed for the
slow-cooled xerogel (Figure S7).
1
identical H NMR spectra and equal specific optical rotations
with the original gelator molecule, indicating again the chemical
structure of gelator did not change during the gelling process.
Wet gels were also characterized in situ by 1H NMR technique
(Figure S5). However, the 1H NMR spectra of BHTG recorded in
D2O/1,4-dioxane-d6 (40/60 v/v) showed surprisingly little change
between the dissolved state (65 °C) and the two gel states. The
peaks in the gel samples might represent the fraction of molecules
that were still dissolved at room temperature or in equilibrium
with those adhered to the outside of gel fibers, while the signals
from the molecules in the fibers became so broadened that they
could no longer be observed. The increased area of the solvent
peaks (water and 1,4-dioxane) relative to the gelator peaks in the
gel samples seemed consistent with this speculation.
Several models based on chiral elastic properties have been
advised to depict the formation of self-assembled tubule and
helical ribbons.32 It is believed that chiral molecules form bilayer
membranes with some favored tilt, and therefore, the only way for
molecules to twist with respect to their neighbors is for the entire
membrane to twist. The favored tilt dictates a stable state, while
the opposite direction is a metastable one. Increasing the tem-
perature may have the effect to cause metastable- to stable-state
transition as photoinduced conformation transition of the azo
compound. In the interdigitated bilayer structure of BHTG,
it was proposed that sugar heads were exposed outside and the
p-terphenyl core and butoxy tail were shielded from water due to
amphiphilic interactions, as shown in Scheme 1. It has been
suggested in these models that the molecular tilt direction in the
Spectroscopy. UV-vis and fluorescence spectroscopies were
used to investigate the local ordering of BHTG in gel. Figure 2
shows the absorption and emission spectra of BHTG under
various conditions. In the absorption spectra, BHTG shows a
single broad structureless band with its maximum at 292 nm in
DMF (a good solvent for BHTG). It was a typical absorption
band of p-terphenyl and was assigned to 1La.30 In the fluorescence
spectra, on the other hand, BHTG shows an emission peak at
363 nm along with a shoulder at 382 nm in DMF, which probably
originated from the ring stretching vibration of p-terphenyl.30a In
the mixture of H2O/1,4-dioxane (40/60 v/v) at 60 °C, BHTG
(30) (a) Momicchioli, F.; Bruni, M. C.; Baraldi, I. J. Phys. Chem. 1972, 76, 3983–
3990. (b) Dutta, A. K.; Misra, T. N.; Pal, A. J. J. Phys. Chem. 1994, 98, 12844–12848.
(c) Geiger, H. C.; Perlstein, J.; Lachicotte, R. J.; Wyrozebski, K.; Whitten, D. G.
Langmuir 1999, 15, 5606–5616.
(31) Kemnitz, K.; Tamai, N.; Yamazaki, I.; Nakashima, N.; Yoshihara, K.
J. Phys. Chem. 1986, 90, 5094–5101.
(32) Selinger, J. V.; Spector, M. S.; Schnur, J. M. J. Phys. Chem. B 2001, 105,
7157–7169.
3618 DOI: 10.1021/la903064n
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Cui et al.
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Scheme 1. Proposed Packing Model of BHTG
Figure 4. CD spectra of BHTG in H2O/1,4-dioxane (40/60 v/v)
with a concentration of 2 mg/mL in the solution state at 60 °C (9),
fast-cooled gel state (b), and slow-cooled gel state (2) at 20 °C.
Figure 5. The ln tg-l/(Δμ/kT)2 plots for the mixture of BHTG in
H2O/1,4-dioxane (40/60 v/v).
bilayer membrane varied with molecular condensed phase: the
molecular membranes with crystal structure have uniform tilt,
while those with fluid structures (e.g., liquid crystal state) take
various tilts.32 In the former case, the orientation of the molecular
tilt is represented by the angle (j) between the projection of the
three-dimensional molecular director into the local tangent plane
and the section of the ribbons and the local normal height.33
Because the xerogels of BHTG from H2O/1,4-dioxane exhibited
sharp diffraction peaks, which indicated a crystal-like order, it
was believed that BHTG arranged with a uniform tilt angle in
bilayer membranes. Therefore, a possible molecular packing
model was hypothesized, as shown in Scheme 1 in this file. The
n represents assembled dimer in the membrane, which was
depicted by two angles θ and j. The θ depicted the tilt of n,
which could be estimated by the thickness of 3.1 nm (local normal
height of one bilayer) and approximate length of dimer n, and it
was about 57° in both xerogels. The j can be estimated by the
pitch tilt of helical ribbons,33 as mentioned above, 45° for right-
handed ribbons and 60° for left-handed ribbons. Unfortunately,
although the angles were reasonable in energy for these models,
no more evidence can be obtained to further prove the accurate
location. In the bilayer structure, the dimer can tilt with two
different orientations: clockwise and anticlockwise, which was
considered to clarify the opposite helical ribbons. However, the
exact relationship between molecular orientation and supramo-
lecular helicity was unclear.
Tuning the Helicity. The helical ribbons with opposite sense
in the microscopic images of fast- and slow-cooled gels illustrated
chiral structures at the fiber levels. In order to probe the chiral
(33) (a) Helfrich, W.; Prost, J. Phys. ReV. A 1988, 38, 3065–3068. (b) Ou-Yang, Z.
C.; Liu, J. X. Phys. Rev. A 1991, 43, 6826–6836. (c) Chung, D. S.; Benedek, G. B.;
Konikoff, F. M.; Donovan, J. M. Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 11341–11345.
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Cui et al.
Figure 6. Temperature-variable CD spectra of the gel of BHTG in H2O/1,4-dioxane (40/60 v/v) at 2 mg/mL (a) and the variety of the peak of
the band centered at 285 (9) and 268 (b) nm.
concept that the diacetylene group in the long tail acts as a kink
to cause the rod-like molecules to pack stably with P-twist or
M-twist orientation.32 Here the p-terphenyl segments which
exhibited strong π-π interaction are similar to the diacetylene
group in the nature of packing kink action. It had been mentioned
above that in the interdigitated bilayer structure BHTG can break
into two packing orientations: clockwise and anticlockwise, as
show in Scheme 1. The opposite orientation of molecular packing
was considered to clarify the different helical sense. However, on
the principle of biased chiral symmetry-breaking theory,34 the
stabilities of the two molecular tilted orientations were different
due to the chirality of the glycosyl part: one was stable, while
another was metastable.
The formation of crystal fibers of BHTG was essentially
controlled by a kinetic nucleation-growth process.36 The nuclea-
Figure 7. Time-variable CD spectra of the gel of BHTG in H2O/
1,4-dioxane (40/60 v/v) at 52.5 °C with a concentration of2 mg/mL.
tion process can be verified by the linear relationship between ln tg
and l/(Δμ/kT)2 (tg is the induction time of nucleation, Δμ denotes
the chemical potential difference between BHTG molecules in the
fiber and in the liquid phase, k is Boltzmann’s constant, T is
temperature; the equations used to get the relationship are shown
in Supporting Information). Because the absorption bands of
BHTG and its aggregates were far away from 500 nm, the
transparence at 500 nm was used to monitor the self-assembled
process. The values of tg with various concentrations were recorded
(Figure S8), and the relationship of ln tg and l/(Δμ/kT)2 is shown in
Figure 5. The ln tg increased linearly with l/(Δμ/kT)2, indica-
ting that the self-assembly of BHTG in H2O/1,4-dioxane was a
nucleation-growth process.
CD spectroscopy was used to investigate the evolution of
supramolecular chirality in the nucleation-growth process. The
process of BHTG in H2O/1,4-dioxane (40/60 v/v) from 60 to
20 °C with a cooling rate of 1 °C/min was monitored via CD
spectroscopy (Figure S9). At 37.5 °C, the mixture exhibited a
negative band at 305 nm and a positive band at 280 nm. The two
bands intersected the abscissa at 292 nm, the position of absorp-
tion maximum, indicating the appearance of chiral molecular
clusters. When the temperature decreased to 35 °C, the negative
band disappeared and was replaced by a positive band centered at
294 nm. The M-P transition suggested that, during the slow-
cooling process, metastable molecular clusters (“nuclei”) formed
at first and then transferred into the stable state.
packing at the molecular level, CD spectroscopy, which is
extremely sensitive to the intermolecular chiral order at the
molecular level,6 was used. Figure 4 shows the CD spectra of
BHTG in H2O/1,4-dioxane at different conditions. At 60 °C,
where molecules were dispersive, the mixture exhibits negligible
CD effect. With decreasing the temperature of the mixture, the
resultant gels show intense CD effect in the absorption region of
p-terphenyl, suggesting the formation of chiral self-assemblies.
The fast-cooled gel shows an intensive negative CD band at
286 nm and two small positive bands centered at 266 and 230 nm,
while the slow-cooled gel exhibits a positive band centered at
214 nm, a negative band centered at 236 nm, and a intensive
positive band centered at 282 nm along with a shoulder band
centered at 264 nm. The opposite CD effect paralleled the helicity
of the ribbons obtained from different cooling rate, implying that
the reversed helicity of the ribbons resulted from the different
molecular orientation at the molecular level.
In chiral or biased chiral symmetry-breaking theory, which
develops from these models mentioned above,32,34 it is suggested
that the chirality of the membrane contributes to a collective tilt of
the molecules with respect to the membrane normal rather than
chiral intermolecular interaction. Within the framework of chiral
symmetry-breaking theory, chiral aggregates may result even
from achiral molecules.17b,35 The theories have been successfully
used to explain diacetylenic phospholipid systems where minority
handedness was observed during the kinetic process of tubule
formation at high lipid concentration.20 This is an important
Because of the high degree of supersaturation of the solution
during nucleation, the appearance of the transition from metastable
(34) Seifert, U.; Shillcock, J.; Nelson, P. Phys. Rev. Lett. 1996, 77, 5237–5240.
(35) Ribo, J. M.; Crusats, J.; Sagues, F.; Claret, J.; Rubires, R. Science 2001, 292,
2063–2066.
(36) (a) Liu, X. Y. Langmuir 2000, 16, 7337–734. (b) Liu, X. Y.; Sawant, P. D.; Tan,
W. B.; Noor, I. B. M.; Pramesti, C.; Chen, B. H. J. Am. Chem. Soc. 2002, 124, 15055–
15063.
3620 DOI: 10.1021/la903064n
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Cui et al.
Article
Figure 8. Time-variableCDspectraofBHTGinH2O/1,4-dioxane (40/60v/v)at40°Cwithaconcentrationof2mg/mLafterinjecting1wt%
fast-cooled (a) or slow-cooled (b) gel of BHTG in H2O/1,4-dioxane (40/60 v/v) with a concentration of 2 mg/mL as seed crystal. The gels as
seed crystal were located in a water bath of 40 °C before injection.
to stable state was transitory and, therefore, it was difficult to
further investigate their properties in the cooling process. In
contrast, in the heating process, it was possible to get the
metastable state longer via subtly controlling temperature.
Figure 6a shows the CD spectra of the fast-cooled gels of
BHTG in H2O/1,4-dioxane (40/60 v/v) at 2 mg/mL during a
heating-cooling process. It was found that the transition of a
negative CD signal to a positive one appeared at 52.5 °C as the
fast-cooled gel did not dissolve completely (generally, if the fast-
cooled sample with negative CD signals was heated to dissolve
completely and then cooled slowly, they always exhibit positive
CD signals, as that in Figure 4, but the CD signal disappeared
when gel dissolved completely). The intensities of CD spectra
at 285 and 268 nm at different temperature are plotted with
temperature as abscissa to track the transition, as shown in
Figure 6b. Though the signal at 285 nm exhibits M-P transition
while that at 268 nm does not, the signals at 285 and 268 nm both
show a transition region of 50-52.5-50 °C (heated and then
cooled process). The M-P transition of the CD signal suggested
that molecules modulated stacking orientation from the meta-
stable state to the stable one in this temperature region. To further
confirm this proposal, the fast-cooled gel was directly annealed
at 52.5 °C, as that shown in Figure 7. When heated to 52.5 °C,
the fast-cooled gel exhibits a negative band centered at 305 nm at
first. With increasing annealing time, the band became small
and turned to be a positive band centered at 293 nm, which was
stable for further annealing, indicating the formation of the
thermodynamically controlled molecular cluster of BHTG
(when annealed gel was heated to dissolve completely and then
cooled rapidly to 20 °C, a negative CD signal appeared again).
Furthermore, the M-P transition did not occur when the fast-
cooled gel was annealed at 47.5 °C (Figure S10), suggesting
that molecular nucleation and rearrangement occurred in
the region of 50-52.5-50 °C. Consequently, the molecular
clusters with opposite molecular arrangements could be ob-
tained by subtly controlling temperature, supplying the possi-
bility to probe the chiral inducing ability of nuclei to grown-up
fibers.
transition, positive CD signals were observed regardless of the
cooling rates. These results further confirmed the inducing ability
of the molecular cluster acted as nuclei to the supramolecular
chirality.
On the basis of the results above, the self-assembling mecha-
nism and process of BHTG with cooling rate dependence of
supramolecular chirality was hypothesized. At first, BHTG was
dispersed in the solution at high temperatures. With decreasing
temperature, molecules self-assembled into clusters as nuclei. The
nuclei with metastable supermolecular chirality formed at first,
and they would evolve into stable ones as annealed at the
temperature of nucleation for a time long enough. In the fast-
cooling process, the metastable nuclei would directly grow up to
right-handed ribbons, while in the slow-cooling process, the
metastable nuclei evolved into stable nuclei and then further
aggregated to be left-handed ribbons. In the gel state, the right-
handed ribbons could not evolveto the left-handed ribbons due to
the kinetically controlling network structures. Consequently, the
right-handed ribbons were obtained via kinetically controlling
process, while left-handed ribbons were obtained via a thermo-
dynamically controlling process.
Under the framework of the mechanism above, it was expected
that, in a supersaturation solution of BHTG in H2O/1,4-dioxane,
well-defined chiral ribbons would be obtained by adding prepared
fast-cooled or slow-cooled gel as “seed crystal”. It was known
that, when cooled slowly from the solution phase, the mixture of
BHTG in H2O/1,4-dioxane (40/60 v/v) at a concentration of
2 mg/mL did not gelate until the temperature of the mixture fell to
37.5 °C, while the M-P transition occurred at a temperature
higher than 47.5 °C (Figure S9 in the Supporting Information and
Figure 7). On the other hand, the supersaturated solution of
BHTG in H2O/1,4-dioxane (40/60 v/v) at a concentration of
2 mg/mL did not gel at 40 °C even for more than 1 h (the result
was not shown), and therefore, it was a good candidate to probe
the inducing ability of added “seed crystal”. The fast-cooled and
slow-cooled gels were prepared beforehand as “seed crystal” and
were located in a water bath of 40 °C before being injected into the
supersaturated solution. Figure 8 shows the CD spectra of BHTG
in H2O/1,4-dioxane (40/60 v/v) at a concentration of 2 mg/mL at
40 °C after 1 wt% gel as seed crystal was injected. The solution
containing the seed crystal at 0 min exhibits no discernible CD
signals due to the small amount of seed crystal. As expected, with
increasing growth time, the mixture containing fast seed crystal
shows negative CD signals as that of fast-cooled gel, while the
mixture containing slow seed crystal shows positive signals as that
of slow-cooled gel. The opposite CD signals of supersaturated
At first, the fast-cooled sample, which had remainded at
47.5 °C for 30 min, was cooled to room temperature slowly
(Figure S10). The resultant gel reveals negative Cotton effect at
290 nm, contrary to the positive Cotton effect of slow-cooled gel
(Figure 4). The opposite sense of two gels obtained from the slow-
cooling process indicated that the helix of ribbons was determined
in nucleation. On the other hand, when the fast-cooled sample
had remained at 52.5 °C for a long enough time for M-P
Langmuir 2010, 26(5), 3615–3622
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Cui et al.
solutions containing seed crystals prepared via different cooled
rates showed that the supramolecular chirality of added gel
determined the helical senses of ribbons in the gels obtained from
supersaturated solutions.
alkoxyphenyl)phenyl-β-O-
D
-glucoside (n = 1-12), in the mixture
of water and 1,4-dioxane.37 Among them, 4-(40-pentyloxyphe-
nyl)phenyl-β-O-
D
-glucoside and 4-(40-hexyloxyphenyl)phenyl-β-
O-D-glucoside were found to show obvious cooling-rate-depen-
dent supramolecular chirality. Therefore, this work provides a
convenient way to tune the chirality of supramolecular structure
for some gelators with delicately designed structures. We will
introduce vinyl group(s) to the molecule of BHTG and try to
stabilize the resultant chiral aggregates by polymerization or
cross-linking, targeting for the novel chiral template of nanoma-
terials and the chiral carrier of asymmetric catalysts.
Conclusion
A new LMMG, BHTG, was synthesized and employed to
efficiently immobilize a variety of solvents. In the mixture of H2O
and 1,4-dioxane, three-dimensional networks made up of helical
ribbons were generated. The handedness of helical ribbons could
be mediated by the rate of gelation: the fast-cooling process
yielded right-handed helical ribbons, while the slow-cooling
process yielded left-handed counterparts. Microscopic, spectro-
scopic, and diffraction analyses suggested that BHTG aggregated
into a twisted interdigitated bilayer structure with a thickness of
3.1 nm through a kinetically controlled nucleation-growth
process. After being cooled from the hot solution, the molecularly
dispersed BHTG self-assembled to form chiral associates, which
acted as nuclei for the ribbons to grow. The molecules of BHTG
had two kinds of arrangements in the nuclei, clockwise and
anticlockwise orientations. One was metastable and was formed
at high temperature immediately after the self-assembly of BHTG
occurred and gradually transformed into the stable one with
decreasing temperature. Fast-cooling process did not leave
enough time for the nuclei to evolve from the metastable to the
stable state. As a result, the ribbons grown from the metastable
nuclei had right handedness. During slow-cooling process,
however, the metastable nuclei transformed into the stable one
and dictated the molecules of BHTG to packin a left-handed way.
Such a method has been used to tune the helicities of twisting
ribbons generated by the analogues of BHTG, that is, 4-(40-
Acknowledgment. We thank the National Natural Science
Foundation of China through the General Program (No.
20674001), Key Program (No. 20834001), and the National
Science Fund for Distinguished Young Scholars (No.
20325415) for the financial support.
Note Added after ASAP Publication. This article was
published ASAP on November 18, 2009. Reference 13(f)
has been modified. The correct version was published on
December 1, 2009.
Supporting Information Available: Synthesis and charac-
terization of BHTG, gel pictures, FT-IR spectra of BHTG
and UV-vis and fluorescence spectra of 1 in H2O and 1,4-
dioxane, and turbidity measurement for nucleation process
and the equations of dynamic calculation. This material is
available free of charge via the Internet at http://pubs.acs.org.
(37) Details will be reported.
3622 DOI: 10.1021/la903064n
Langmuir 2010, 26(5), 3615–3622