Construction of two- or three-component low molecular weight gel systems

Article abstract of DOI:10.1246/bcsj.20100136

Pyridyl group-bound tris-urea 2, whose structure issimilar to the tris-urea low molecular weight gelator (LMWG) 1, was synthesized. Addition of a trace amount of 2 and Pd(OAc)₂ to 1 resulted in a reduction in the critical gelation concentration (CGC) in acetone. Metal-ligand interaction between Pd(OAc)₂ and the pyridyl group of 2 assist the gelation. Assistance by intermolecular hydrogen bonding was also effective for gelation of acetone. An acetone gel of 1 was formed at half the value of the CGC of 1 alone by adding a trace amount of 2 and isophthalicacid. Scanning electron microscopy (SEM) measurements of these aggregates showed structural alterations caused by the additive. A mixture of 1 and a trace amount of 2 in EtOAc gave a gel. Addition of 2 was essential to form the gel, because 1 alone did not gel EtOAc. SEM measurements indicated that 2 produced extension of the fibrous aggregates.

Full text of DOI:10.1246/bcsj.20100136

© 2010 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 83, No. 9, 1127–1131 (2010) 1127
Construction of Two- or Three-Component Low Molecular
Weight Gel Systems
Masamichi Yamanaka* and Ryohei Aoyama
Department of Chemistry, Faculty of Science, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529
Received May 7, 2010; E-mail: smyaman@ipc.shizuoka.ac.jp
Pyridyl group-bound tris-urea 2, whose structure is similar to the tris-urea low molecular weight gelator (LMWG) 1,
was synthesized. Addition of a trace amount of 2 and Pd(OAc)2 to 1 resulted in a reduction in the critical gelation
concentration (CGC) in acetone. Metal-ligand interaction between Pd(OAc)2 and the pyridyl group of 2 assist the
gelation. Assistance by intermolecular hydrogen bonding was also effective for gelation of acetone. An acetone gel of 1
was formed at half the value of the CGC of 1 alone by adding a trace amount of 2 and isophthalic acid. Scanning electron
microscopy (SEM) measurements of these aggregates showed structural alterations caused by the additive. A mixture of 1
and a trace amount of 2 in EtOAc gave a gel. Addition of 2 was essential to form the gel, because 1 alone did not gel
EtOAc. SEM measurements indicated that 2 produced extension of the fibrous aggregates.
Directional regulated self-assembly of preorganized small
molecules enables construction of supramolecular polymers.1
One of the attractive manifestations of these supramolecular
polymers is the supramolecular gel.2 Principally, a fibrous
aggregate composed of a one-dimensional assembly of small
molecules, called a low molecular weight gelator (LMWG),
is the unit constituent. Several types of single-component
LMWGs have been reported, their gelations being achieved
through a variety of interactions, such as hydrogen bonding,3
³-³,4 dipole-dipole,5 etc. Some of them showed a gel-sol
phase transition responsive to photo-,6 chemo-,7 or other8
stimuli in addition to the thermo-responsiveness that is
potentially provided in supramolecular gels. Supramolecular
gel formation as a result of multicomponent assembly of small
molecules has also been achieved.9 Formation of a fibrous
aggregate via complementary hydrogen bonding pairs,10 metal-
ligand coordination bond,11 and other interactions12 has been
reported. As a pioneering work, Hanabusa et al., reported a
two-component gelling system using an equimolar mixture
of 5-alkyl-2,4,6-triaminopyrimidine and 5,5-dialkylbarbituric
acid.10a Beck and Rowan developed a metallo-supramolecular
gelling system that was based on a metal-ligand interaction
between a bis-ligand monomer and a combination of metal ions
(transition-metal ion and lanthanoid metal ion).11b Multicom-
ponent supramolecular gels usually mix each component in
the ratio of appropriate whole numbers. In contrast, a supra-
molecular gel composed of a main ingredient and a small
quantity of accessory ingredient is hardly known. A supra-
molecular gel formed from a small quantity of additive has the
potential to be a smart material, because its rheology enables it
to be converted by a trace amount of chemical. As a unique
conception, Lee and co-worker reported that addition of a small
quantity of rod-coil-rod molecule to a cylindrical object that
was composed of a coil-rod-coil molecule induced gelation.13
Partial introduction of ligand or hydrogen bonding acceptor to
a fibrous aggregate of LMWG would alter the gelation ability
by adding a multidentate metal ion or hydrogen bonding donor,
depending on the type of interaction between the fibrous
aggregates (Figure 1). In this paper, we report multicomponent
supramolecular gel formation involving increase in gelation
ability by addition of trace amounts of molecule(s). Previously,
we found that C3-symmetric tris-urea molecule 1 and deriva-
tives act as LMWGs for a variety of organic solvents.14 For the
purpose of making part of a multicomponent supramolecular
gel, we designed tris-urea 2 with 4-pyridyl groups as the ligand
or hydrogen-bonding unit (Figure 2). The gelation ability of
tris-urea 1 was dramatically improved by the addition of a trace
amount of 2 with or without metal salt or dicarboxylic acid.
Results and Discussion
Synthesis of Pyridyl-Substituted Tris-Urea 2. Tris-urea 2
was synthesized by a condensation reaction of triamine 3 and
4-pyridyl isocyanate (Scheme 1). Triamine 3 was prepared
from 1,3,5-tris(bromomethyl)-2,4,6-triethylbenzene by etheri-
fication with 3-nitrophenol and reduction of the nitro groups
in the presence of tin(II) chloride.14 The obtained triamine 3
was reacted with in situ prepared 4-pyridyl isocyanate;
64% yield of the desired tris-urea 2 being obtained after
1
purification. The structure of tris-urea 2 was confirmed by H
and 13C NMR spectroscopy and electrospray ionization (ESI)
mass spectrometry.
Three-Component LMWG Systems Using Metal-Ligand
Interaction.
Tris-urea 1 gelled acetone upon ultrasound
irradiation, the critical gelation concentration (CGC) being
1.5 wt % (Table 1, Entry 1 and Figure 3a). The gelation of
acetone was not complete at a lower concentration of 1 than the
CGC. For example, a 0.75 wt % mixture of 1 in acetone gave a
partial gel (Table 1, Entry 2 and Figure 3b). Scanning electron
microscopy (SEM) images of xerogels prepared from the
acetone gel of 1 and the partial gel of acetone and 1 showed
structures resembling nanosized fibers (Figures 3a and 3b).
These results indicate that conduction of intertwining of the
Published on the web August 26, 2010; doi:10.1246/bcsj.20100136

1128 Bull. Chem. Soc. Jpn. Vol. 83, No. 9 (2010)
Low Molecular Weight Gelator
Figure 1. Schematic representation of three-component gelation system.
Table 1. Gelation Properties of Mixtures of 1, 2, and
Pd(OAc)2 in Acetone
H
H
N
N
R
O
Components
Entry
2/mol %
for 1
Pd(OAc)2/mol %
for 1
Statea)
O
1/wt %
Et
Et
1
2
3
4
5
6
7
8
1.5
®
®
®
®
G
PG
G
I
I
I
R
0.75
0.75
0.75
0.75
0.75
0.75b)
0.75
HN
O
O
1.0
1.0
®
1.5
®
HN
O
Et
1.5
10
1.5
0.75
1.0
10
0.5
O
R
NH
NH
G
G
a) G: gel, PG: partial gel, I: insoluble suspension. b) Total
amount of 1 and 2.
1 : R =
2 : R =
opaque gel after ultrasound irradiation, and the CGC of 1 + 2
was estimated at 0.75 wt % (Table 1, Entry 3 and Figure 3c).
The gelation completed with half the CGC by adding just trace
amounts of 2 and Pd(OAc)2. Neither 0.75 wt % mixtures of 1
and 2 (1.0 mol % for 1) nor 1 and Pd(OAc)2 (1.5 mol % for 1) in
acetone formed a gel (Table 1, Entries 4 and 5, and Figures 3d
and 3e). This means that both 2 and Pd(OAc)2 worked
cooperatively to produce gelation. An SEM image of the
xerogel prepared from 0.75 wt % acetone gel of 1, 2, and
Pd(OAc)2 (Table 1, Entry 3) showed moderately flexible and
intertwined nanosized fibers (Figure 3c). In contrast, the SEM
images of insoluble suspensions (Table 1, Entries 4 and 5)
displayed different morphologies (Figures 3d and 3e). SEM
observations of a sample prepared from a mixture of 1 and a
small quantity of 2 showed more upright fibrous structures than
the Pd(OAc)2-containing three-component gel (Figure 3c vs.
3d). Short curved objects were observed by the SEM measure-
ment of a mixture of 1 and Pd(OAc)2 in comparison with the
three-component gel (Figure 3c vs. 3e). Homogeneous nano-
sized fibers, as shown in Figure 3d, would indicate that partial
mutation of the fibrous aggregate of 1 was attained by adding a
trace amount of 2. Crosslinking of the pyridyl group of 2 and
Pd(OAc)2 might help the intertwinement of fibers and promote
gelation. Pd(OAc)2 itself would obstruct the one-dimensional
extension of 1 (and 2) and proper connection between fibers.
The following result supports this idea; a 0.75 wt % mixture
of 1, 2 (1.0 mol % for 1), and Pd(OAc)2 (10 mol % for 1) in
acetone gave an insoluble suspension and more curled fibers
than the xerogel shown in Figure 3c, as observed by SEM
(Table 1 Entry 6 and Figure 3f). The amount of 2 was not
critical for gelation, and a mixture of 1, 2 (10 mol % for 1), and
N
Figure 2. Chemical structures of tris-urea 1 and 2.
NH2
O
N
NCO
Et
Et
2
CH2Cl2
rt
O
H2N
O
Et
NH2
3
Scheme 1. Synthesis of tris-urea 2.
fibers of 1 may enable formation of a gel at a lower
concentration than the present CGC. Partial variation of the
fibrous aggregate of 1 with coordination sites should promote
an intertwining of the fibers in the presence of an appropriate
metal complex. Pyridyl groups introduced in tris-urea 2, which
resembles 1 in structure, seemed an adequate analog for partial
mutation of the fibrous aggregate of 1. A transition-metal
complex, which is able to interact bidentately with pyridyl
groups, should act as a glue for the fibers. A broad survey to
find a suitable metal salt was carried out and Pd(OAc)2 was
identified as an effective salt. A mixture of 1, 2 (1.0 mol % for
1), and Pd(OAc)2 (1.5 mol % for 1) in acetone formed an

M. Yamanaka et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 9 (2010) 1129
Table 2. Gelation Properties of Mixtures of 1, 2, and
Carboxylic Acid in Acetone
Components
Entry
2/mol %
for 1
1/wt %
Additive/mol % for 1
Statea)
1
2
3
4
5
6
7
8
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
1.0
®
isophthalic acid (1.5)
isophthalic acid (1.5)
phthalic acid (1.5)
terephthalic acid (1.5)
terephthalic acid (3.0)
dimethyl isophthalate (1.5)
benzoic acid (1.5)
G
I
I
PG
G
I
1.0
1.0
1.0
1.0
1.0
1.0
I
I
benzoic acid (3.0)
a) G: gel, PG: partial gel, I: insoluble suspension.
Figure 3. SEM images of dried samples and photographs of
samples (inset): (a) 1 (1.5 wt %) in acetone; (b)
1
(0.75 wt %) in acetone; (c) 1 (0.75 wt %), 2 (1.0 mol %
for 1), and Pd(OAc)2 (1.5 mol % for 1) in acetone; (d) 1
(0.75 wt %) and 2 (1.0 mol % for 1) in acetone; (e) 1
(0.75 wt %) and Pd(OAc)2 (1.5 mol % for 1) in acetone; and
(f) 1 (0.75 wt %), 2 (1.0 mol % for 1), and Pd(OAc)2
(10 mol % for 1) in acetone.
Figure 4. SEM images of dried samples and photographs of
samples (inset): (a) 1 (0.75 wt %), 2 (1.0 mol % for 1), and
isophthalic acid (1.5 mol % for 1) in acetone; (b) 1
(0.75 wt %), 2 (1.0 mol % for 1), and phthalic acid
(1.5 mol % for 1) in acetone; and (c) 1 (0.75 wt %), 2
(1.0 mol % for 1), and terephthalic acid (3.0 mol % for 1) in
acetone.
Pd(OAc)2 (1.5 mol % for 1) in acetone afforded an opaque gel
after ultrasound irradiation (Table 1, Entry 7). The amounts of
2 and Pd(OAc)2 could be reduced to 0.5 and 0.75 mol %,
respectively, maintaining the CGC of 1 + 2 (Table 1, Entry 8).
Three-Component LMWG Systems Using Hydrogen-
Bonding Interaction. A bidentate hydrogen bonding donor
can be employed instead of a metal complex in this gelling
system; therefore, it is possible for the pyridyl group to act as a
hydrogen bonding acceptor. A dicarboxylic acid should be
suitable as the bidentate hydrogen bonding donor. A mixture of
1, 2 (1.0 mol % for 1), and isophthalic acid (1.5 mol % for 1) in
acetone formed an opaque gel after sonication, the CGC being
0.75 wt % (Table 2, Entry 1 and Figure 4a). The SEM image of
this xerogel showed intertwined nanosized fibers (Figure 4a).
Crosslinking hydrogen bonds between the pyridyl groups of 2
and the carboxy groups of isophthalic acid were essential to
improve the gelation ability. A 0.75 wt % mixture of 1 and
isophthalic acid (1.5 mol % for 1) in acetone gave an insoluble
suspension (Table 2, Entry 2). Phthalic acid was not effective
for gelation; a mixture of 1, 2 (1.0 mol % for 1), and phthalic
acid (1.5 mol % for 1) in acetone gave an insoluble suspension
(Table 2, Entry 3 and Figure 4b). A sterically crowded
dicarboxylic acid might cause excess cohesion of fibrous
aggregates. The insoluble suspension containing 1, 2, and
phthalic acid showed thin curled fibers (Figure 4b). A mixture
of 1, 2 (1.0 mol % for 1), and terephthalic acid (1.5 mol % for 1)
in acetone (0.75 wt %) afforded a partial gel (Table 2, Entry 4).
Crosslinking with terephthalic acid might cause a decrease
in entropy because of the remote arrangement of the two
hydrogen bonding carboxy groups. In fact, a 0.75 wt % mixture
of 1, 2 (1.0 mol % for 1), and terephthalic acid (3.0 mol %
for 1) in acetone formed an opaque gel (Table 2, Entry 5 and
Figure 4c). Sheet-shaped structures were intermingled with
fibrous structures on SEM observation of the gel composed
of 1, 2, and terephthalic acid (Figure 4c). Crosslink between
fibrous aggregates (1 and 2) and para disubstituted terephthalic
acid might promote two-dimensional assembly to form sheet-
shaped structures. A gelation experiment of acetone with 1, 2
(1.0 mol % for 1), and dimethyl isophthalate (1.5 mol % for 1),
the dimethyl ester of isophthalic acid, did not succeed, and an
insoluble suspension was obtained (Table 2, Entry 6). These
results indicate that the hydrogen-bonding interaction between

1130 Bull. Chem. Soc. Jpn. Vol. 83, No. 9 (2010)
Low Molecular Weight Gelator
Table 3. Gelation Properties of Mixtures of 1 and 2 in
MeOH and EtOAc
the MeOH gel of 1 and 2 (0.50 wt %) showed similar
morphologies (Figures 5a and 5b). However, the three-compo-
nent systems, i.e., 1, 2, and Pd(OAc)2 or isophthalic acid, were
not effective for gelation of MeOH (Table 3, Entries 4 and 5).
A dramatic change was observed on gelation of EtOAc with
1 and a trace amount of 2. An insoluble suspension was
obtained by mixing 1 and EtOAc in any concentration (Table 3,
Entry 6 and Figure 5c). A mixture of 1 and 2 (1.0 mol % for 1)
in EtOAc formed an opaque gel, the CGC being estimated
at 1.5 wt % (Table 3, Entry 7 and Figure 5d). A suspended
mixture of 1 and EtOAc showed fragmentary objects on SEM
measurement (Figure 5c). In contrast, the EtOAc gel of 1 and 2
was composed of intertwined nanosized fibers (Figure 5d). A
trace amount of 2 assists the one-dimensional extension of 1 in
some manner. The three-component systems were also not
effective for gelation of EtOAc (Table 3, Entries 8 and 9).
Components
Entry Solvent
2/mol %
1/wt %
Additive/mol % for 1 Statea)
for 1
1
2
3
4
5
6
7
8
9
MeOH
MeOH
MeOH
MeOH
MeOH
EtOAc
EtOAc
EtOAc
EtOAc
2.0
®
®
®
®
G
PG
G
PG
I
0.50
0.50
0.50
0.50
5.0
1.5
1.5
1.5
1.0
1.0
1.0
®
®
Pd(OAc)2 (1.5)
isophthalic acid (1.5)
®
I
G
1.0
1.0
1.0
®
Pd(OAc)2 (1.5)
isophthalic acid (1.5)
PG
I
a) G: gel, PG: partial gel, I: insoluble suspension.
Conclusion
In conclusion, we have designed and synthesized a pyridyl-
substituted tris-urea 2 as a structurally similar analog of tris-
urea LMWG 1. The gelation ability of 1 in acetone was
dramatically improved by the presence of trace amounts of 2
and Pd(OAc)2. Crosslinking of nanosized fibers composed of 1
and 2 through the metal-ligand coordination bond of 2 and
Pd(OAc)2 plays an important role in the gelation. Isophthalic
acid was also an effective crosslinker of the fibers in acetone.
The hydrogen-bonding interaction between 2 and isophthalic
acid was an essential force to achieve the gelation. In MeOH
and EtOAc, addition of a trace amount of 2 was effective
in improving the gelation ability of 1. A one-dimensional
extension of 1 would be caused by 2.
Experimental
1H and 13C NMR spectra were recorded on a JEOL JNM-
ECA600 spectrometer. Mass spectra were measured on a JEOL
JMS-T100LC AccTOF spectrometer. Ultrasound irradiation
was performed using a BRANSON B2510J ultrasonic cleaner.
SEM studies were carried out on a JEOL JSM-6300 spec-
trometer.
Figure 5. SEM images of dried samples and photographs
of samples (inset): (a) 1 (2.0 wt %) in MeOH, (b) 1 (0.50
wt %) and 2 (1.0 mol % for 1) in MeOH, (c) 1 (5.0 wt %) in
EtOAc, and (d) 1 (1.5 wt %) and 2 (1.0 mol % for 1) in
EtOAc.
Synthesis of 1,3,5-Triethyl-2,4,6-tris[3-(4-pyridylureido)-
phenoxymethyl]benzene (2). To a solution of 4-pyridylamine
(72 mg, 0.77 mmol) in CH2Cl2 (7.6 mL) was added triphosgene
(75 mg, 0.25 mmol) and Et3N (0.21 mL, 1.5 mmol) under argon
atmosphere at 0 °C. The mixture was stirred at room temper-
ature for 30 min (CH2Cl2 solution of 4-pyridyl isocyanate).
Triamine 3 (100 mg, 0.19 mmol) was added to the solution and
stirred at room temperature for 21 h. Saturated sodium hydro-
gen carbonate solution was poured into the reaction mixture,
and the precipitate was filtered off. The precipitate was added
to a CH2Cl2 solution of 4-pyridyl isocyanate which was
prepared from the same procedure and amounts with mentioned
above. The mixture was stirred at room temperature for 12 h.
Saturated sodium hydrogen carbonate solution was poured into
the reaction mixture, and the precipitate was filtered off. The
crude product was purified by reprecipitation from THF-Et2O
and MeOH-Et2O successively to give 2 as a white solid
the pyridyl groups of 2 and the carboxy groups of isophthalic
acid is indispensable to form a gel. The bidentate nature of the
hydrogen bonding donor was essential in this gelation, and
consequently a mixture of 1, 2 (1.0 mol % for 1), and benzoic
acid (1.5 or 3.0 mol % for 1), a monodentate hydrogen bonding
donor, in acetone gave only an insoluble suspension (Table 2,
Entries 7 and 8).
Two-Component LMWG System.
In the course of
investigating the three-component gelling system (vide ante),
it was clear that the gelation ability of 1 was dramatically
improved by adding a trace amount of 2 in some solvents. Tris-
urea 1 itself gelled MeOH upon ultrasound irradiation, the
CGC being 2.0 wt % (Table 3, Entry 1 and Figure 5a).14
A
partial gel was observed for a 0.50 wt % mixture of 1 in MeOH
(Table 3, Entry 2). A mixture of 1 and 2 (1.0 mol % for 1)
gelled MeOH at 0.50 wt % after ultrasound irradiation (Table 3,
Entry 3 and Figure 5b). The CGC in MeOH decreased by a
factor of four upon adding 1.0 mol % of 2. Both SEM images of
the xerogels prepared from the MeOH gel of 1 (2.0 wt %) and
1
(107 mg, 64%): mp >250 °C; H NMR (600 MHz, DMSO-d6):
¤ 1.20 (t, J = 7.6 Hz, 9H), 2.77 (q, J = 7.6 Hz, 6H), 5.08 (s,
6H), 6.79 (dd, J = 8.2, 2.1 Hz, 3H), 7.05 (dd, J = 8.2, 1.4 Hz,

M. Yamanaka et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 9 (2010) 1131
Matsumoto, S. Yamaguchi, A. Wada, T. Matsui, M. Ikeda, I.
Hamachi, Chem. Commun. 2008, 1545. e) T. Muraoka, C.-Y. Koh,
H. Cui, S. I. Stupp, Angew. Chem., Int. Ed. 2009, 48, 5946.
3H), 7.25 (t, J = 2.1 Hz, 3H), 7.26 (t, J = 8.2 Hz, 3H), 7.43 (d,
J = 6.2 Hz, 6H), 8.35 (d, J = 6.2 Hz, 6H), 8.91 (s, 3H), 9.12 (s,
3H); 13C NMR (150 MHz, DMSO-d6): ¤ 16.3, 22.5, 64.0,
104.9, 108.3, 111.3, 112.3, 129.8, 130.8, 140.4, 145.5, 146.4,
150.1, 152.0, 159.1; HRMS (ESI, M + Na+) calcd for
C51H51N9NaO6 908.3860, found 908.3837.
General Procedure of Gelation Experiments. A weighed
amount of tris-ureas 1 and 2 were placed in test tube, and
acetone was added. The closed test tube was sonicated for
a few minutes. An acetone solution of crosslinker (e.g.,
Pd(OAc)2 or isophthalic acid) was added to the test tube. The
mixture was sonicated for 20 min, and the gelation was
confirmed by the inverted test tube method.
7
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Supporting Information
8
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