Structural optimization of super-gelators derived from naturally-occurring mannose and their morphological diversity

Article abstract of DOI:10.1039/c4ra03096f

Mannose derivatives were synthesized as low molecular-weight gelators with various alkoxy substituents on the aromatic ring of methyl-4,6-O-benzilidene- α-d-mannopyranoside. Most of these mannose derivatives could gel in various solvents, such as octane, cyclohexane, toluene, ethylene glycol and ethanol solutions, at concentrations lower than 2.0 wt%. In particular, the critical gelation concentration (CGC) of methyl-4,6-O-(4-butoxybenzylidene)- α-d-mannopyranoside (2) for squalane was only 0.025 wt%, one of the lowest CGCs we have ever experienced. The observations of xerogels by FE-SEM, TEM and AFM revealed that the length of the alkoxy chains of the mannose derivatives influences the gel morphologies. Moreover, the toluene gels formed from the mannose derivatives 1-6 functionalized by a linear alkoxy group exhibited thixotropic properties. Interestingly, the gels of various solvents formed from methyl-4,6-O-(4-dodecyloxybenzylidene)-α-d-mannopyranoside (6) (with the longest alkoxy chain on the aromatic ring in this paper) exhibited thixotropic properties. Thus, we confirmed that alkoxy groups on the aromatic ring exert noticeable effects on the gelation properties of these mannose derivatives. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra03096f

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Structural optimization of super-gelators derived
from naturally-occurring mannose and their
morphological diversity†
Cite this: RSC Adv., 2014, 4, 25940
a
Fumiyasu Ono, Hisayuki Watanabeab and Seiji Shinkaic
*
Mannose derivatives were synthesized as low molecular-weight gelators with various alkoxy substituents on
the aromatic ring of methyl-4,6-O-benzilidene-a-D-mannopyranoside . Most of these mannose derivatives
could gel in various solvents, such as octane, cyclohexane, toluene, ethylene glycol and ethanol solutions,
at concentrations lower than 2.0 wt%. In particular, the critical gelation concentration (CGC) of methyl-4,6-
O-(4-butoxybenzylidene)-a-D-mannopyranoside (2) for squalane was only 0.025 wt%, one of the lowest
CGCs we have ever experienced. The observations of xerogels by FE-SEM, TEM and AFM revealed that
the length of the alkoxy chains of the mannose derivatives influences the gel morphologies. Moreover,
the toluene gels formed from the mannose derivatives 1–6 functionalized by a linear alkoxy group
exhibited thixotropic properties. Interestingly, the gels of various solvents formed from methyl-4,6-O-(4-
dodecyloxybenzylidene)-a-D-mannopyranoside (6) (with the longest alkoxy chain on the aromatic ring in
this paper) exhibited thixotropic properties. Thus, we confirmed that alkoxy groups on the aromatic ring
exert noticeable effects on the gelation properties of these mannose derivatives.
Received 3rd February 2014
Accepted 28th April 2014
DOI: 10.1039/c4ra03096f
www.rsc.org/advances
A variety of the physical properties inherent to LMWGs can
be applied to integrate unique functions within the gels. One of
Introduction
the most fundamental properties is the phase transition
between the sol phase and the gel phase, which can be used as a
stimulus to control the imparted functions. It is possible to
induce phase transition by heat, mechanical means, light, etc.
In 1998, Shinkai’s group reported that sugars are useful as basic
skeletons to design LMWGs, where the gels are stabilized owing
to the hydrogen-bonding interactions among the OH groups.13
As sugars are naturally-occurring materials, one can expect to
apply them to those purposes that require environmentally-
friendly properties and functions.14–19 For example, Jadhav et al.
prepared gels using sorbitol or mannitol derivatives, which can
selectively remove oil from an oil–water mixture,15 whereas
Vidyasagar et al. reported that mannitol derivatives can form
transparent self-standing oil gels applicable to lm
processing.17
We previously reported the gelation ability of methyl-4,6-O-
benzilidene-a-D-mannopyranoside (referred to as ‘Bn’ in this
paper).13,20–23 It was found that this mannose derivative can form
gels accommodating various solvents: in particular, the ability
to gelate both water and certain organic solvents is
unique.13,20–36 However, the gels formed from this mannose
skeleton could be either clear or slightly opaque (e.g., the
toluene-loaded gel was slightly opaque; Fig. 1, le). We have
newly synthesized a mannose derivative functionalized with a
methoxy group at the para position of the aromatic ring and
evaluated its gelation ability for toluene. Interestingly, using
this derivative resulted in a transparent toluene-loaded gel
Gelators are materials capable of forming a gel structure, which
can contain uid in its three-dimensional network. Specically,
a hydrogel accommodates water, whereas an oil gel or an
organogel accommodates oil or organic solvent, respectively.
Polymeric gels supported by a cross-linked three-dimensional
network have been
a primary class of studied gelator
compounds. Recently, there has been a marked increase in
studies on low molecular-weight gelators (LMWGs) that are
constructed from self-assemblies of low molecular-weight
compounds. Because of the synthetic ease and the diversity of
self-assemblies, one can integrate various additional functions
within the LMW supramolecular gels. These advantages are
characteristics of LMWGs that are not available with polymeric
gels. The development of LMWGs has proceeded with both
aqueous and organic solvents.1–12 One can expect that the
continuing endeavour to develop LMWGs will eventually result
in their implementation in industrial, cosmetic, electronic,
optical device, oil spill and medicinal applications.
aAdvanced Materials Research Laboratory, Collaborative Research Division, Art,
Science and Technology Center for Cooperative Research, Kyushu University, 4-1,
Kyudai-Shinmachi, Nishi-ku, Fukuoka, Fukuoka 819-0388, Japan
bNissan Chemical Industries, Ltd., 10-1 Tsuboi-Nishi 2-chome, Funabashi, Chiba 274-
8507, Japan
cInstitute of Systems, Information Technologies and Nanotechnologies (ISIT), 4-1
Kyudai-Shinmachi, Nishi-ku, Fukuoka, Fukuoka 819-0388, Japan
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra03096f
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Fig. 3 Synthetic scheme of the mannose-based gelators.
orthoformate in the presence of a Cu(BF4)2 catalyst in methanol
at room temperature (RT). The nal acetal compounds were
derived by the reaction of aldehydes and methyl-a-D-man-
nopyranoside, the reaction being catalysed by p-TsOH in DMF at
RT under reduced pressure. The yields in this reaction were up
to 55%. These products were characterized by 1H NMR, 13C
NMR and HRMS.
Fig. 1 Images of toluene-filled gel formed from methyl-4,6-O-ben-
zylidene-a-D-mannopyranoside (Bn) in a previous work (left) and 1
(right).
Gelation abilities
The gelation abilities of the mannose-based derivatives were
determined for a range of 19 different solvents and 6 aqueous
mixtures; in each case, 2.0–20 mg of mannose derivative was
submerged in 1.0–8.0 mL of the desired solvent in a sealed vial
while being heated (Table 1). The sample vial was cooled to RT,
then the sample vial was inverted aer 1 h. We classied the
obtained results as follows: a transparent gel, a translucent gel,
an opaque gel, a precipitate and a clear solution or a suspen-
sion. The results are summarized in Table 1.
The abbreviations used for each mannose derivative are as
follows: linear alkoxy groups, 1–6; linear alkoxy group with a
terminal olen, 7; branched alkoxy group at the para position
on the aromatic ring, 8; cyclic alkoxy group para-substituted on
the aromatic ring, 9; and methoxy group at the meta and para
positions on the aromatic ring, 10. Derivatives 1–8 served as
gelators for linear and cyclic alkanes (octane, cyclohexane,
squalane and squalene), an aromatic solvent (toluene) and
decamethylcyclopentasiloxane (DOW CORNING TORAY SH 245
FLUID, referred to as ‘SH245’ in this paper). Surprisingly, the
critical gelation concentration (CGC) of 2 for squalane was 0.025
wt%, one of the lowest CGCs we have ever experienced. In other
words, this gel consisted of 99.975 wt% squalane. In addition,
1–6 and 7 were able to gelate olive oil, isopropyl myristate (IPM)
and even ethylene glycol (in spite of the protic solvent). Both 2
and 7 gelated water. On the other hand, 9 and 10 exhibited no
gelation ability for any of these solvents.
Fig. 2 Structures of the mannose derivatives investigated in this study.
showing thixotropic behaviour (Fig. 1, right). The nding
implies that the slight modication of the gelator’s structure
greatly changes the gelation ability.
Stimulated by this nding, we extensively studied the
possible correlation between the structures of mannose deriv-
atives as LMWGs and their physical gelation properties, such as
critical gelation concentration, gel–sol phase transition
temperature, thixotropy, etc. Specically, we investigated the
inuence of the structures of the alkoxy groups, such as linear
versus branched, saturated versus unsaturated, and para-
substituted versus meta-substituted on the aromatic ring, etc.
(Fig. 2).
As demonstrated in Fig. 4, an inverse correlation of CGC and
solvent polarity was demonstrated with six analogous gelators.
We estimated the CGCs of 2 and 6 to be 0.5 wt% (14.1 mM) and
2.0 wt% (42.9 mM) in toluene, respectively. In contrast, the
CGCs of 2 and 6 were estimated to be 2.0 wt% (56.4 mM) and
Results and discussion
Synthesis of mannose-based gelators
The mannose derivatives used as LMWGs were synthesized 0.25 wt% (5.4 mM), respectively, in ethylene glycol. These
from the corresponding aromatic aldehydes in two steps apparently anomalous tendencies are rationalized in terms of
(Fig. 3). Several O-substituted aromatic aldehydes were synthe- the inversion of the aggregation modes of nonpolar solvents
sized from 4-hydroxybenzaldehyde and the corresponding and polar solvents: that is, in toluene, the sugar moiety should
alkylbromides using K2CO3 as a base in DMF at 80 ꢀC or 150 ꢀC. occupy the core and the alkoxy group should cover the shell
The acetals (mannose derivative precursors) were synthesized surface, whereas the core–shell relationship should be inverted.
by the reaction of the corresponding aldehydes and trimethyl The gelation abilities of the mannose derivatives in DMSO–H2O
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Table 1 The observed critical gelation concentration (CGC) values (wt%) and qualitative notea (qual. only if CGC could not be determined, and
CGC values only if the gel was transparent)
Entry
Solvent
1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
7
8
Octane
Cyclohexane
Squalane
Squalene
Toluene
SH245
Olive oil
IPM
Chloroform
Ethyl acetate
Acetone
Cyclohexanone
Acetonitrile
DMSO
EtOH
MeOH
Ethylene glycol
Glycerol
Water
0.1
0.25
0.05
0.1
0.5
0.05
0.5
0.5
S
S
S
S
S
S
S
S
S
S
P
S
S
0.05
0.05
0.025c
0.05
0.5
0.05
0.5
0.5
S
S
S
S
0.1
0.05
0.05
0.1
1.0
0.05
1.0
1.0
S
S
S
S
S
S
S
S
2.0 TL
P
P
2.0 OG
0.1 TL
0.1 TL
S
0.5 OG
0.1 TL
0.1
0.1
0.05
0.25
1.0
0.1
1.0
1.0
S
S
S
S
S
S
S
S
0.1
0.1
0.05
0.25
2.0
0.25
1.0
1.0
S
S
S
S
S
S
S
S
0.5 TL
P
IS
0.5 TL
0.1 TL
P
2.0 OG
0.25 OG
P
0.25
0.1
0.1
0.25
2.0
0.5
1.0
1.0
S
S
S
S
S
S
S
S
0.25 TL
P
IS
0.5 TL
0.25 TL
P
2.0 OG
0.25 TL
P
0.1 PG
0.1
0.05
0.1
0.5
0.05
0.5
0.5
S
S
S
S
S
S
S
S
0.05
0.25
0.1
0.25
S
0.1
S
S
S
S
S
S
S
S
S
S
P
P
IS
S
P
IS
IS
IS
P
IS
IS
IS
IS
S
S
S
S
P
S
S
S
S
P
IS
S
P
IS
S
IS
IS
IS
P
IS
IS
P
IS
S
S
S
S
S
S
S
S
S
S
P
S
S
S
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
S
S
S
S
2.0 OG
2.0 TL
0.1 OG
S
0.5 TL
0.1 TL
S
1.0 TL
P
IS
S
P
0.1 TL
S
1.0 OG
0.25 TL
S
DMSO–H2O (75/25)b
DMSO–H2O (50/50)b
DMSO–H2O (25/75)b
EtOH–H2O (75/25)b
EtOH–H2O (50/50)b
EtOH–H2O (25/75)b
1.0
0.05
0.25 TL
S
0.5 OG
0.05 TL
S
S
S
P
IS
S
P
S
S
S
1.0 OG
0.1 TL
2.0
0.25 OG
S
IS
IS
a
TL, translucent gel; OG, opaque gel; S, solution; IS, insoluble; P, precipitation; PG, partial gel. b Numbers in parentheses are the ratios of the mixed
solutions (vol/vol). c Aer 12 h.
and 2 were higher than that of Bn without alkoxyl group func-
tionalization on the aromatic ring. Additionally, since the Tgel of
7 was consistently higher than those of the other mannose
derivatives, the thermal stabilization of toluene gel is likely due
to the p–p interaction occasionally observed between terminal
olens.37
Morphology of xerogels
We investigated the morphologies of several organogels
prepared from 2 and 6 with toluene, cyclohexane and a 50/50
Fig. 4 The CGCs of mannose derivatives 2–6, in toluene and ethylene
(vol/vol) mixture of ethanol and a few hydrogels, using eld
glycol (the numbers in parentheses are the lengths of the alkoxy
chains).
and EtOH–H2O mixtures were also investigated using deriva-
tives 1–6 and 7. As demonstrated in Fig. 5, the higher water-
content mixtures are gelated more easily with shortening of the
lengths of the linear alkoxy chains.
Gel–sol phase transitions
In Fig. 6, the gel–sol phase transition temperatures (Tgel) of Bn,
1–6 and 7 in toluene are plotted against the gelator concentra-
tions in order to compare their concentration-dependent gela-
tion properties. When compared at the same concentration, the
Tgel values always maintained the following order from the
highest to the lowest: 7, 1, 2, Bn, 3, 4, 5, 6. The Tgel values of 7, 1 (the numbers in parentheses are the lengths of the alkoxy chains).
Fig. 5 The CGCs of mannose derivatives 2–6, in DMSO–H2O mixtures
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Fig. 6 Tgel vs. gelator concentration in toluene.
emission scanning electron microscopy (FE-SEM), transmission
electron microscopy (TEM) and atomic force microscopy (AFM)
(Fig. 7). For the FE-SEM measurements, the xerogels were
obtained by freezing and pumping the gel for 5–14 h. For the
TEM measurements, the samples were obtained by dipping a
carbon-coated copper grid into the gel and negatively staining it
with 1% phosphotungstic acid (adjusted to pH 7). For the AFM
measurements, a droplet of a toluene solution containing 2 or 6
at 0.02 wt% below the CGC value was placed on highly-ordered
pyrolytic graphite (HOPG). Aer 1 h, the substrates were dried
under vacuum for more than 12 h.
The FE-SEM image of the xerogel prepared from 2 and
cyclohexane displayed a three-dimensional entangled brous
network 10–20 nm in width, whereas that of 6 displayed a sheet-
like structure. The FE-SEM images of the xerogels formed from
both 2 and 6 with toluene displayed sheet-like structures. For
the TEM images of gels prepared with toluene, 2 displayed a
brous network 140–200 nm in width, whereas 6 displayed a
owing stream-like structure. Interestingly, the AFM image of
the dried sample for a toluene solution of 2 at 0.02 wt% below
the CGC value also depicted the brous networks when
observed by TEM. Similarly, the AFM image of 6 in toluene
showed the owing stream-like structure. These phenomena
reveal that the rates of the self-assembly processes involving
mannose derivatives 2 and 6 are faster than the evaporation rate
of toluene. In the previous work, the SEM image of the xerogel
formed from Bn with toluene displayed the brous network,19
whereas those of 2 and 6 displayed the sheet-like structures
because of bundled bers created by the van der Waals inter-
actions between alkoxy chains. Furthermore, the difference in
the TEM images obtained from 2 and 6 is attributable to the
difference in the strength of the van der Waals interactions
induced by the length of the alkoxy chains: that is, the TEM and
FE-SEM images of the xerogels prepared from the cyclohexane
gel and the toluene gel of 6 showed sheet-like and stream-like
structures because of the stronger van der Waals interactions of
6, which arises from having a longer alkoxy chain than 2. For the
xerogels prepared with the ethanol–water mixture, the FE-SEM
image of 2 displayed both short and long, thick brous
networks 130–180 nm in width, whereas that of 6 displayed
right-handed helical brous structures 200–400 nm in width.
These images were also observed for their corresponding TEM
images. The FE-SEM and TEM images of the xerogel prepared
from the hydrogel of 2 displayed both short and long, bundled
Fig. 7 FE-SEM images of xerogels: cyclohexane with (a) 2 and (b) 6;
toluene with (c) 2 and (d) 6; EtOH–H2O (50/50, vol/vol) with (g) 2 and
(h) 6; and (k) hydrogel of 2. TEM images of gels as follows: toluene with
(e) 2 and (f) 6; EtOH–H2O (50/50, vol/vol) with (i) 2 and (j) 6; and (l)
hydrogel of 2. The AFM images of dried toluene solutions of (m) 2 and
(n) 6 at 0.02 wt% on HOPG.
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thick brous networks 150–550 nm in width. These character-
istic xerogel morphologies prepared from the hydrogel of 2 are
created by the periodic structure corresponding to the hexag-
onal organization, which is evidenced by XRD measurements
described in the next section.
XRD measurements
Fig. 9 The molecular length of 4 by CPK molecular modelling on the
basis of PM3 calculation and the proposed molecular packing of 4 in
hydrogel and toluene gel.
The X-ray diffraction (XRD) patterns of the xerogels prepared
from 2 and water show periodical reection peaks (Fig. 8),
which indicate that 2 indeed assembles into a hexagonal orga-
nization. The obtained long spacing distances (d) were 2.27 nm,
1.31 nm, 1.14 nm, 0.85 nm, 0.75 nm and 0.65 nm, which
pffiffiffi
pffiffiffi
Thixotropic behaviour
correspond to the ratios of 1 : 1, 3 : 1, 2 : 1, 7 : 1, 3 : 1 and
pffiffiffiffiffi
12 : 1, respectively. The 2.27 nm length is shorter than twice
that of the extended molecular length of 2 (1.78 nm by CPK
molecular modelling) but longer than the length of one mole-
cule. The aqueous gels prepared from 2, therefore, should
maintain an interdigitated bilayer structure with a thickness of
2.27 nm (Fig. 9). This value is compatible with a bilayer struc-
ture that has the alkoxy chain tilted with respect to a normal
vector of the layer's plane. In addition, the wide-angle region of
the X-ray diagram for the hydrogel of 2 revealed a series of sharp
reection peaks. This nding supports the view that the long
alkoxy chain groups form the highly ordered interdigitated layer
packing stabilized by hydrophobic interactions.
Thixotropy is a phenomenon characteristically observed for
LMWGs and has garnered considerable interest from gel
scientists because of its signicant relationship with some
biological self-healing processes, such as those of muscular
bers and injured neural bers, and with industrial applica-
tions, for materials such as cosmetics, paints, biomaterials and
foodstuffs.38–47 We examined this behaviour for derivatives Bn,
1–6 and 7 at their CGCs (Fig. 11 and 12). The toluene gels were
permitted to rest for 1 h at RT aer the mixtures of mannose
derivatives were heated, and then shear stress was introduced
via vortex mixing. Aer 1 h at RT, the sample vials were inverted.
The samples prepared from 1–6 and 7 showed the regeneration
of the gels, indicating that they possess thixotropic properties,
whereas the sample prepared from Bn does not.
Furthermore, the thixotropic properties of several gels
prepared from mannose derivatives 2 and 6 were examined in
more detail (Fig. 13). For 2, the gels of toluene, olive oil, IPM and
ethylene glycol indicated thixotropic properties, whereas those
of octane, cyclohexane and SH245 did not. Interestingly, the
gels prepared from 6 exhibited thixotropic properties with
octane, cyclohexane, toluene, SH245, olive oil, IPM and ethylene
glycol. Thixotropic processes involve the self-assembly of small
pieces of bers to form bundled bers.44 These results reveal
that the self-assembly process was induced by the van der Waals
interactions between the alkoxy chains on the gelators. In
particular, the van der Waals interactions between the alkoxy
chains on gelator 6 (with the longest alkoxy chain among the
In contrast, the XRD diagram of the xerogel of 2 and toluene
is remarkably different from that obtained in water; it has two
broad peaks appearing at d ¼ 4.26 and 2.03 nm in the small-
angle region and one broad reection peak in the wide-angle
region. The main peak at 2.03 nm suggests an ordered
arrangement of gelators in the brous structure. The weak
peaks appearing at 4.26 and 0.62 nm are attributed to the
bundled main bers and the mannose moiety (self-organized by
hydrogen bonding), respectively.
Next, we used XRD to measure the long spacing distances of
2, 4 and 6 in the xerogels prepared from toluene (Fig. 10). The
peaks appeared at 2.27 nm, 2.35 nm and 2.96 nm, respectively,
indicating that the ber diameter becomes thicker with an
increase in the length of the alkoxy chain attached to the
aromatic ring.
Fig. 8 XRD measurements of the xerogels of organogel (toluene) and Fig. 10 XRD measurements of the xerogels prepared from toluene
hydrogel prepared from 2.
with 2, 4 and 6.
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Fig. 11 Thixotropic behaviour of organogels prepared from 3 with toluene (left), after shear stress (middle) and after 1 h at RT (right).
Experimental section
Apparatus for spectroscopic measurements
1H and 13C NMR spectra were acquired on a JEOL JNM-ECA 600
instrument. Chemical shis were reported in ppm from tetra-
methylsilane for 1H NMR spectra and CDCl3 for 13C NMR
spectra as an internal standard. EI and FAB mass spectra were
measured on a JEOL JMS-700 instrument.
Fig. 12 Thixotropic behaviour of organogels prepared from Bn and
1–7 with toluene (left) and after applying shear stress, then resting for
1 h at RT (right).
Synthesis of methyl-4,6-O-(4-butoxybenzylidene)-a-D-
mannopyranoside (2)
Copper(II) tetrauoroborate (48 mg, 0.2 mmol) was added to a
mixture of 4-butoxybenzaldehyde (3.6 g, 20 mmol) and trimethyl
orthoformate (4.4 mL, 40 mmol) in anhydrous methanol (8 mL)
under nitrogen atmosphere. The reaction mixture was stirred
for 1 h at RT. Aer 1 h, the reaction mixture was quenched with
saturated NaHCO3 and extracted with ethyl acetate. The extracts
were washed with brine, dried over Na2SO4, ltered and
concentrated under reduced pressure. The obtained clear oil
Fig. 13 Thixotropic behaviour of several gels formed from several
solvents and 2 (left) or 6 (right).
was afforded to the next reaction without further purication.
A solution of crude dimethylacetal in DMF (10 mL) was
added dropwise to a suspension of methyl-a-D-mannopyrano-
side (4.3 g, 22 mmol) and p-toluenesulfonic acid (98 mg,
mannose derivatives in this paper) are stronger than those of 2. 0.5 mmol) in anhydrous DMF (20 mL) under a nitrogen atmo-
Therefore, 6 induced thixotropic properties in more of the gels sphere. The reaction mixture was stirred for 10 min at RT and
formed from various solvents than 2.
for 2 h under reduced pressure at RT. Aer 2 h, the reaction
mixture was quenched with saturated NaHCO3 and extracted
with ethyl acetate. The extracts were washed with brine, dried
over Na2SO4, ltered and concentrated under reduced pressure.
The crude product was puried by column chromatography
(SiO2; hexane–ethyl acetate from 70/30 to 50/50, vol/vol). The
desired product, 2, was obtained as a white solid (3.2 g, 45%).
Methyl-4,6-O-(4-butoxybenzylidene)-a-D-mannopyranoside
(2). dH (600 MHz, CDCl3): 7.40 (2H, d, J ¼ 8.5 Hz), 6.89 (2H, d, J ¼
8.8 Hz), 5.52 (1H, s), 4.77 (1H, s), 4.31–4.22 (1H, m), 4.10–4.02
(2H, m), 3.96 (2H, t, J ¼ 6.6 Hz), 3.93–3.86 (1H, m), 3.85–3.76
(2H, m), 3.40 (3H, s), 2.66–2.60 (2H, br), 1.80–1.71 (2H, m), 1.53–
1.43 (2H, m), 0.97 (3H, t, J ¼ 7.6 Hz). dC (150 MHz, CDCl3):
159.86, 129.46, 127.51, 114.32, 102.29, 101.21, 78.81, 70.82,
68.82, 68.73, 67.75, 62.89, 55.10, 31.23, 19.20, 13.82. HRMS
(EI+): m/z calcd for C18H26O7 (M+): 354.1679; found: 354.1682.
The other mannose derivatives are described in the ESI.†
Conclusions
We demonstrated that mannose derivatives with various alkoxy
substituents are able to gelate organic solvents and protic
solvents. Through this study, we have established that as a
general tendency, the mannose derivatives bearing a short
linear alkoxy chain tend to gelate the nonpolar solvents such as
toluene and cyclohexane at a low concentration. On the other
hand, those bearing a long linear alkoxy chain tend to gelate
polar and protic solvents (e.g., ethylene glycol) at a low
concentration. Furthermore, LMWGs 2 and 6 exhibited thixo-
tropic properties with a wide range of solvent groups. It has thus
become clear that the proper alkoxy chain at the para position of
the aromatic ring imparts transparency, stability and thixo-
tropic behavior to the gels. We are now extending our research
toward the functional application of these gels possessing
Gelation tests of organic solvents, mixed solvents and water
unique physical properties. In the future, we will report on the The gelator and the solvent were put in a sealed-capped vial and
results obtained from these gels.
heated in a dry bath until the solid was dissolved. The solution
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was cooled at room temperature. If the stable gel was observed
at this stage, it was classied with CGC values (wt%) in Table 1.
4 N. M. Sangeetha and U. Maitra, Chem. Soc. Rev., 2005, 34,
821–836.
5 A. Ajayaghosh, V. K. Praveen and C. Vijayakumar, Chem. Soc.
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Gel–sol phase transition temperatures
6 A. R. Hirst, B. Escuder, J. F. Miravet and D. K. Smith, Angew.
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7 D. K. Smith, Chem. Soc. Rev., 2009, 38, 684–694.
8 M.-O. M. Piepenbrock, G. O. Lloyd, N. Clarke and J. W. Steed,
Chem. Rev., 2010, 110, 1960–2004.
A sealed vial containing the gel was immersed in a thermostatic
dry bath. In 5 ꢀC increments, the sample vial was inverted and
Tgel was dened as the temperature at which gel–sol consistency
was observed.
9 A. Dawn, T. Shiraki, S. Haraguchi, S. Tamaru and S. Shinkai,
Chem.–Asian. J., 2011, 6, 266–282.
10 L. E. Buerkle and S. J. Rowan, Chem. Soc. Rev., 2012, 41,
6089–6102.
11 J. Raeburn, A. Z. Cardoso and D. J. Adams, Chem. Soc. Rev.,
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12 G. Yu, X. Yan, C. Han and F. Huang, Chem. Soc. Rev., 2013,
42, 6697–6722.
TEM observations
TEM imaging was performed on a JEOL JEM-2010HCKM
instrument. The gel was prepared in a sample tube. For the TEM
measurements, the samples were obtained by dipping a carbon-
coated copper grid in the gel and dried under vacuum for more
than 12 h. The dried grid was negatively stained with 1%
phosphotungstic acid adjusted to pH 7 for 5 or 10 min. The
grids were washed with water and dried under vacuum for more
than 12 h. The TEM accelerating voltage was 200 kV.
13 K. Yoza, Y. Ono, K. Yoshihara, T. Akao, H. Shinmori,
M. Takeuchi, S. Shinkai and D. N. Reinhoudt, Chem.
Commun., 1998, 907–908.
SEM observations
14 J. Cui, A. Liu, Y. Guan, J. Zheng, Z. Shen and X. H. Wan,
Langmuir, 2010, 26, 3615–3622.
15 S. R. Jadhav, P. K. Vemula, R. Kumar, S. R. Raghavan and
G. John, Angew. Chem., Int. Ed., 2010, 49, 7695–7698.
16 G. Wang, H. Yang, S. Cheuk and S. Coleman, Beilstein J. Org.
Chem., 2011, 7, 234–242.
FE-SEM imaging was performed on a Hitachi SU-8000 instru-
ment at the Center of Advanced Instrumental Analysis, Kyushu
University. The xerogel of 1 was obtained by freezing and
pumping a gel of 1 for 5–14 h. The obtained xerogel was thus not
coated with metal. The SEM accelerating voltage was less than
0.5 kV.
17 A. Vidyasagar, K. Handore and K. M. Sureshan, Angew.
Chem., Int. Ed., 2011, 50, 8021–8024.
18 M. F. Abreu, V. T. Salvador, L. Vitorazi, C. E. N. Gatts,
D. R. dos Santos, R. Giacomini, S. L. Cardoso and
P. C. M. L. Miranda, Carbohydr. Res., 2012, 353, 69–78.
19 E. Bedini, L. Cirillo and M. Parrilli, Tetrahedron, 2013, 69,
1285–1296.
20 K. Yoza, N. Amanokura, Y. Ono, T. Akao, H. Shinmori,
M. Takeuchi, S. Shinkai and D. N. Reinhoudt, Chem.–Eur.
J., 1999, 5, 2722–2729.
AFM observations
AFM was performed on an SII NanoTechnology Inc. Nanonavi/
Nanocute instrument (now Hitachi High-Tech Science Corp.).
For the AFM measurements, a droplet of the solution of 2 or 6 at
0.02 wt% in toluene was placed on highly ordered pyrolytic
graphite (HOPG). Aer 1 h, the substrates were dried under
vacuum for more than 12 h.
21 O. Gronwald, K. Sakurai, R. Luboradzki, T. Kimura and
S. Shinkai, Carbohydr. Res., 2001, 331, 307–318.
22 O. Gronwald and S. Shinkai, J. Chem. Soc., Perkin Trans. 2,
2001, 1933–1937.
23 O. Gronwald and S. Shinkai, Chem.–Eur. J., 2001, 7, 4328–
4334.
XRD measurements
Powder XRD patterns were measured on a Rigaku RINT-TTR III
X-ray diffractometer equipped withꢁC1 uKa radiation (50 kV,
300 mA) at a scanning rate of 1ꢀ min
.
24 M. Mukai, H. Minamikawa, M. Aoyagi, T. Shimizu and
M. Hogiso, J. Colloid Interface Sci., 2000, 224, 154–160.
25 J. H. Jung, G. John, M. Masuda, K. Yoshida, S. Shinkai and
T. Shimizu, Langmuir, 2001, 17, 7229–7232.
26 J. H. Jung, S. Shinkai and T. Shimizu, Chem.–Eur. J., 2002, 8,
2684–2690.
Acknowledgements
We thank Dr Yumi Fukunaga for the support and valuable
advice regarding TEM observations and Mr Taisuke Matsumoto
for the support with XRD measurements. We thank Dr Osamu
Hirata and Dr Akihiro Tanaka for their valuable comments and
discussions.
27 S. Kiyonaka, S. Shinkai and I. Hamachi, Chem.–Eur. J., 2003,
9, 976–983.
28 M. Suzuki, M. Yumoto, M. Kimura, H. Shirai and
K. Hanabusa, Helv. Chim. Acta, 2004, 87, 1–10.
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