Gel sculpture: Moldable, load-bearing and self-healing non-polymeric supramolecular gel derived from a simple organic salt

Article abstract of DOI:10.1002/chem.201200986

An easy access to a library of simple organic salts derived from tert-butoxycarbonyl (Boc)-protected L-amino acids and two secondary amines (dicyclohexyl- and dibenzyl amine) are synthesized following a supramolecular synthon rationale to generate a new series of low molecular weight gelators (LMWGs). Out of the 12 salts that we prepared, the nitrobenzene gel of dicyclohexylammonium Boc-glycinate (GLY.1) displayed remarkable load-bearing, moldable and self-healing properties. These remarkable properties displayed by GLY.1 and the inability to display such properties by its dibenzylammonium counterpart (GLY.2) were explained using microscopic and rheological data. Single crystal structures of eight salts displayed the presence of a 1D hydrogen-bonded network (HBN) that is believed to be important in gelation. Powder X-ray diffraction in combination with the single crystal X-ray structure of GLY.1 clearly established the presence of a 1D hydrogen-bonded network in the xerogel of the nitrobenzene gel of GLY.1. The fact that such remarkable properties arising from an easily accessible (salt formation) small molecule are due to supramolecular (non-covalent) interactions is quite intriguing and such easily synthesizable materials may be useful in stress-bearing and other applications. Copyright

Full text of DOI:10.1002/chem.201200986

FULL PAPER
DOI: 10.1002/chem.201200986
Gel Sculpture: Moldable, Load-Bearing and Self-Healing Non-Polymeric
Supramolecular Gel Derived from a Simple Organic Salt
Pathik Sahoo,[a] Ravish Sankolli,[b] Hee-Young Lee,[c] Srinivasa R. Raghavan,[c] and
Parthasarathi Dastidar*[a]
Abstract: An easy access to a library of
simple organic salts derived from tert-
butoxycarbonyl (Boc)-protected l-
amino acids and two secondary amines
(dicyclohexyl- and dibenzyl amine) are
synthesized following a supramolecular
synthon rationale to generate a new
series of low molecular weight gelators
(LMWGs). Out of the 12 salts that we
prepared, the nitrobenzene gel of dicy-
ties displayed by GLY.1 and the inabili-
ty to display such properties by its di-
benzylammonium counterpart (GLY.2)
were explained using microscopic and
rheological data. Single crystal struc-
tures of eight salts displayed the pres-
ence of a 1D hydrogen-bonded net-
work (HBN) that is believed to be im-
portant in gelation. Powder X-ray dif-
fraction in combination with the single
crystal X-ray structure of GLY.1 clearly
established the presence of a 1D hy-
drogen-bonded network in the xerogel
of the nitrobenzene gel of GLY.1. The
fact that such remarkable properties
arising from an easily accessible (salt
formation) small molecule are due to
supramolecular (non-covalent) interac-
tions is quite intriguing and such easily
synthesizable materials may be useful
in stress-bearing and other applica-
tions.
clohexylammonium
Boc-glycinate
Keywords: crystal engineering
·
(GLY.1) displayed remarkable load-
bearing, moldable and self-healing
properties. These remarkable proper-
gels · gelators · X-ray diffraction ·
supramolecular chemistry
Introduction
Self-healing requires an energy dissipation process that in-
volves reversibly breakable cross-links with finite lifetimes.[4]
This can be achieved through non-covalent interactions
rather than through permanent covalent cross-links.[5] Al-
though moldability or plasticity is a well-studied property of
thermoplastic polymers, synthetic materials that possess
both moldablity and self-healing properties are not so
common. Most of the materials displaying both moldablity
and self-healing properties reported thus far are hydrogels
derived from polymers and nanoparticles such as clays.[1,5]
To our knowledge, supramolecular organogels derived
from low molecular weight gelators (LMWGs) displaying
both moldablity and self-healing are extremely rare.[6]
LMWGs[7,8] are small organic compounds (typically with
molecular weights <3000) capable of immobilizing (gelling)
large amounts of solvent. The resultant solid-like materials
(gels) are being investigated with vigor owing to their vari-
ous possibilities for application.[9–12] However, designing ge-
lators is a daunting task. Serendipity plays a major role in
gel science and most gelators have been found by an acci-
dent rather than by design. Nevertheless, there are some ef-
forts to design gelators.[13] In all these attempts, the target
gelator molecule is obtained through time-consuming and
complex organic synthesis, which often leads to frustration
in the quest for new LMWGs. Our approach towards the
design of LMWGs, on the other hand, has been based on
the supramolecular synthon.[14] By exploiting this strategy, it
is possible to generate a supramolecular assembly of various
dimensions (1D, 2D or 3D). Microscopic (optical, scanning
electron, transmission electron, atomic force, confocal etc.)
Materials that can be molded to form self-supporting,
shape-persistent objects are of great importance in materials
science and technology.[1] Self-healing is another highly
sought-after material property that is usually seen in natu-
rally occurring materials such as bone, wood, biological tis-
sues.[2] Self-healing materials are of great importance as they
offer longer lifetime, low replacement cost and greater prod-
uct safety and can be used in any stress-bearing applications.
Chemically cross-linked synthetic hydrogels[3] do not possess
self-healing properties and are often found to be brittle.
[a] P. Sahoo, Dr. P. Dastidar
Department of Organic Chemsitry
Indian Association for the Cultivation of Science (IACS)
2A and 2B, Raja S. C. Mullick Road
Jadavpur, Kolkata - 700032, West Bengal (India)
Fax : (+91)33-2473-2805
E-mail: ocpd@iacs.res.in
parthod123@rediffmail.com
[b] R. Sankolli
SSCU, Indian Institute of Science
Bangalore - 560012 (India)
[c] Dr. H.-Y. Lee, Prof. Dr. S. R. Raghavan
Department of Chemical & Biomolecular Engineering
University of Maryland, College Park
MD 20742-2111 (USA)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200986.
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studies on gel/xerogel (dried gel) samples reveal the exis-
tence of networks of highly branched and/or entangled 1D
fibers, which are known as self-assembled fibrillar networks
(SAFINs).[15] The solvent molecules are immobilized within
the SAFINs, resulting in gels. The entangled 1D fiber or
SAFIN formation may be attributed to some anisotropic in-
teractions that help the gelator molecules preferentially self-
assemble in 1D, whereas the lack of such interactions in the
other two dimensions prevents lateral growth. Thus, mole-
cules that have self-complementary, reasonably strong, and
directional hydrogen bonding site(s) are expected to form
1D fibers which ultimately might lead to SAFINs. This hy-
pothesis originally proposed by Shinkai and co-workers[16]
has been most explicitly demonstrated by us—we used the
supramolecular synthon concept to identify a number of or-
ganic salts that have the ability to undergo 1D growth, and
we were able to confirm that these molecules acted as gel-
ling agents.[7,17]
Scheme 2. Boc-protected l-amino acids were treated with two secondary
amines to generate the SAM salts.
Among the various organic-salt-based supramolecular
synthons that we identified and exploited in designing
LMWGs, the secondary ammonium monocarboxylate
(SAM) synthon is particularly interesting. SAM salts display
both 1D and 0D (discrete) synthons (synthon A and B, re-
spectively, Scheme 1). The dimensionality of the SAM
In this article, we present a series of simple SAM salts
(Scheme 2), some of which displayed organogelation proper-
ties. Particularly remarkable is the salt GLY.1; the nitroben-
zene gel of GLY.1 showed moldable, load-bearing, and self-
healing properties. The moldablity of nitrobenzene gel of
GLY.1 was so impressive that it allowed us to create a sculp-
ture of “mother and child” (ꢀ4.0 inches in height, weighing
ꢀ158.0 g) that has remained stable for months (see below).
To our knowledge, such properties have not been previously
observed for an organogel derived from such a simple or-
ganic salt.
Results and Discussion
Synthesis: Commercially obtained tert-butoxycarbonyl
(Boc)-protected l-amino acids were treated with the corre-
sponding amines, namely dicyclohexylamine (DCHA) or di-
benzylamine (DBA) in a 1:1 molar ratio in MeOH at room
temperature. After evaporation of the solvent in a rotary
evaporator, the white precipitate obtained in near quantita-
tive yield was washed with hexane to remove unreacted
Scheme 1. 1D HBN formation of synthon A, and 0D HBN formation of
synthon B.
supramolecular synthon was shown to be dependent on both
the cation and the anion. For example, the cinnamate salts
of dicyclohexylamine corresponded to a 1D synthon (syn-
thon A), whereas the corresponding benzoate salts resulted
in a 0D synthon (synthon B) HBN in the single crystal struc-
tures.[17] However, when a dibenzylammonium cation was
used, most of the cinnamate and benzoate salts displayed
a 1D synthon A.[18] Hydrogen bond isomerism (0D to 1D)
was also observed in the dicyclohexylammonium salts of
alkyl monocarboxylic acids.[19] About 34% of the SAM salts
that we have reported thus far[17–22] showed moderate to
good gelation properties; all the 18 gelator salts for which
single crystal structures could be obtained showed 1D syn-
thon A exclusively, whereas among the 33 single crystal
structures of the non-gelator SAM salts reported by us, 23
salts showed 0D synthon B, 8 salts displayed 1D synthon A
and 2 salts showed a halogen···halogen interaction assisted
2D network. These results demonstrate the efficiency of the
SAM synthon and the importance of 1D HBN in gelation.
1
amine. H NMR spectroscopy and elemental analysis clearly
established the stoichiometry of the products as 1:1 (acid/
amine). The presence of a strong band within the range of
1612–1651 cmÀ1 in the FTIR spectra indicated salt formation
(see the Supporting Information, pages S2 and S3).
Single crystal X-ray diffraction
Supramolecular synthons: To assess the robustness of the
SAM synthon in these salts, it was necessary to determine
the single crystal structures of the salts. We were able to
crystallize 8 salts namely GLY.1, LEU.1, PHE.1, LEU.2,
ILE.2, GLY.2, ALA.2, and PHE.2, out of the 12 salts syn-
thesized. Single crystal X-ray diffraction studies (see the
Supporting Information, Table S1) revealed the presence of
an overall 1D hydrogen-bonded network in the crystal struc-
tures of these salts. More detailed analyses of the crystal
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Non-Polymeric Supramolecular Gel
FULL PAPER
structures indicated the presence of three different types of
1D HBNs.
Crystal structures of dicyclohexyammonium Boc-glycinate
(GLY.1)—double-stranded 1D SAM synthon: The salt
GLY.1 crystallizes in the centrosymmetric triclinic space
¯
group P1. One ion pair is located in the asymmetric unit.
FTIR data (1645 cmÀ1 attributed to COOÀ) and the C–O
distances of the carboxylic moiety [1.2514(17)–1.2554(17) ꢁ]
indicate salt formation. A typical 1D SAM synthon is ob-
Figure 2. Left: 1D hydrogen-bonded network in the crystal structures of
LEU.1 and PHE.1. Right: existence of 1D SAM synthon A in these
structures (the side chain of the amino acid and the ammonium cation
are hidden for better clarity).
À
served, involves N H···O interactions (N···O=2.6998(15)–
À
2.7081(16) ꢁ; ] N H···O=171.9–174.08). However, such 1D
chains are self-assembled to form a 1D double-stranded
À
tape sustained by N H···O interactions (N···O=
À
2.9289(16) ꢁ; ] N H···O=154.48; Figure 1 and the Sup-
bonded tape involving SAM synthon A: The salts LEU.2
and ILE.2 are isostructural as is evident from their different
cell dimensions and identical space group (noncentric mono-
clinic P21). The asymmetric unit contains one ion pair. A
strong band at 1647 and 1649 cmÀ1 and C–O distances of
1.252(3)–1.254(3) ꢁ and 1.249(3)–1.263(3) ꢁ for LEU.2 and
ILE.2, respectively, which clearly supports salt formation.
porting Information, Table S2 and Figure S1).
À
The ion pairs are involved in hydrogen bonding through N
À
H···O interactions (N···O=2.678(3)–2.797(4) ꢁ;
]
N
H···O=165.6–175.38 in LEU.2; N···O=2.688(3)–2.698(3) ꢁ;
À
]N H···O=147.9–161.18 in ILE.2) resulting in the forma-
tion of a 1D infinite chain akin to 1D SAM synthon A; the
cations and anions are assembled on either side of the chain
and the Boc-protected NH is found to be involved in
a rather weak hydrogen-bonding interaction with one of the
Figure 1. Left: 1D hydrogen-bonded network in the crystal structures of
GLY.1. Right: existence of 1D double stranded SAM synthon A in
GLY.1 and its schematic representation (the side chain of the amino acid
and the ammonium cation are hidden for better clarity).
À
carboxylate O atoms (N···O=3.085(4) ꢁ; ] N H···O=
À
166.68 in LEU.2 and N···O=3.058(3) ꢁ; ]N H···O=160.88
in ILE.2]; thus the overall HBN may be best described as
1D tape running parallel to the a-axis (Figure 3). The 1D
tapes are packed in a parallel fashion apparently stabilized
Crystal structures of dicyclohexyammonium Boc-leucinate
(LEU.1) and dicyclohexylammonium Boc-phenylalaninate
(PHE.1)—A typical 1D SAM synthon: The salts LEU.1 and
PHE.1 are isomorphous and have the same noncentrosym-
metric orthorhombic space group P212121 and near identical
cell dimensions. The asymmetric unit is comprised of one
ion pair. Strong bands at 1634 and 1624 cmÀ1 in FTIR and
C–O distances of 1.236(4)–1.251(4) ꢁ and 1.246(3)–
1.262(3) ꢁ in LEU.1 and PHE.1, respectively, provide evi-
dence of salt formation. In the crystal structures, the cation
À
and the anion are hydrogen bonded through N H···O inter-
À
actions (N···O=2.713(4)–2.743(4) ꢁ;
]
N H···O=167.4–
À
177.28 for LEU.1; N···O=2.700(3)–2.748(3) ꢁ; ]N H···O=
157.6–173.18 for PHE.1); this results in a 1D infinite chain
(synthon A, Scheme 1; Figure 2). The chains are further
packed in a parallel fashion and are apparently stabilized by
À
dispersion forces in LEU.1 and by C H···p (3.849 ꢁ) inter-
actions involving one of the methylene CH groups of the cy-
clohexyl moiety and the phenyl ring of the amino acid side
chain in PHE.1 (see the Supporting Information, Tables S5
and S8 and Figures S4 and S7).
Figure 3. Left: 1D hydrogen-bonded tape in the crystal structures of
LEU.2, ILE.2, and GLY.2. Right: details of 1D hydrogen-bonded tape,
Crystal structures of dibenzylammonium Boc-leucinate
(LEU.2), dibenzylammonium Boc-isoleucinate (ILE.2) and
dibenzylammonium Boc-glycinate (GLY.2)—1D hydrogen-
À
wherein Boc-protected N H takes part in hydrogen bonding; the exis-
tence of 1D SAM synthon A is marked (the side chain of the amino acid
and the ammonium cation are hidden for better clarity).
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P. Dastidar et al.
by dispersion forces (see the Supporting Information, Ta-
bles S6 and S7 and Figures S5 and S6).
Glycine being an achiral molecule, the salt GLY.2 crystal-
lized in the centrosymmetric monoclinic space group P21/n.
The asymmetric unit contains one amine and one acid
moiety. The C–O distances of 1.253(2)–1.260(2) ꢁ clearly in-
dicate the carboxylate nature of the acid moiety revealing
salt formation. The FTIR band at 1629 cmÀ1 also supports
salt formation. In the crystal structure, each ammonium
cation is hydrogen bonded with the adjacent two carboxyl-
À
À
ate anions by N H···O (N···O=2.692(2)–2.729(2) ꢁ; ] N
H···O=165.2–165.78) interactions resulting in an infinite 1D
chain, as expected for 1D SAM synthon A. However, the
Boc-protected NH also participates in hydrogen-bonding in-
teractions with one of the O atoms of the adjacent carboxyl-
Figure 4. Left: 1D hydrogen-bonded tape in the crystal structures of
ALA.2 and PHE.2. Right: details of 1D hydrogen-bonded tape, wherein
À
ate anion (N···O=2.853(2) ꢁ; ]N H···O=149.58), resulting
À
Boc-protected N H takes part in hydrogen bonding; the existence of dis-
crete SAM synthon B is marked; (the side chain of the amino acid and
in a 1D tape-type HBN (see the Supporting Information,
Table S3 and Figure S2). Thus, the HBN present in this
structure is almost identical to that observed in LEU.2 and
ILE.2 except that the anionic moieties are arranged in an up
and down fashion in an alternating manner, whereas the car-
boxylate moietes in LEU.2 and ILE.2 are located on the
same side of the propagating axis of the 1D hydrogen-
bonded tape (Figure 3). The 1D tapes are further packed in
the ammonium cation are hidden for better clarity).
Thus, it is clear that despite the presence of a hydrogen-
À
bond donor such as Boc-protected N H in the amino acid
moiety, all the salts displayed 1D HBNs; a typical 1D SAM
synthon A is present in two cases (LEU.1 and PHE.1),
À
whereas Boc-protected N H participates in hydrogen bond-
À
a parallel fashion apparently stabilized by C H···p interac-
tions involving the C H of the methylene group of the
amino acid moiety and one of the phenyl rings of the benzyl
moiety of the cation (C H···pcentroid =3.640 ꢁ; see the Sup-
porting Information, Tables S6 and S7 and Figures S5 and
S6).
ing in three cases (LEU.2, ILE.2, and GLY.2) resulting in
a 1D tape with 1D SAM synthon A. 1D tape HBNs are also
observed in two cases (ALA.2 and PHE.2), wherein the dis-
crete SAM synthon B is propagated through complementary
À
À
À
dimeric N H···O interactions involving the Boc-protected
À
N H and carboxylate O atoms. It may be important to note
that in all the cases, the powder X-ray diffraction (PXRD)
patterns (simulated from the single-crystal X-ray data and
that of as-synthesized solids) are nearly superimposable,
which indicates high crystalline phase purity of the salts
except in LEU.1 (see the Supporting Information, Fig-
ure S9).
A Cambridge structural database (CSD; version 1.13,
Nov. 2010) search using a Boc-protected a-amino acid car-
boxylate fragment yielded only nine hits of which seven are
organic salts of various amines; interestingly, the amine used
in two of these seven salts (REFCODE: JAFMOS and
SEZLOZ) is dicyclohexylamine, which is also used in the
present study; both these structures display a 1D HBN. Al-
though SEZLOZ (dicyclohexylammonium Boc-threoninate)
showed a 1D tape involving synthon A, JAFMOS (dicyclo-
hexylammonium Boc-b-methyl phenulalaninate) displayed
a typical 1D SAM synthon A (see the Supporting Informa-
tion, Figure S10).
Crystal structure of dibenzylammonium Boc-alaninate
(ALA.2) and dibenzylammonium Boc-phenylalaninate
(PHE.2)—1D hydrogen bonded tape involving synthon B:
The single crystals of ALA.2 and PHE.2 belong to the non-
centrosymmetric space groups P1 (triclinic) and P212121 (or-
thorhombic), respectively. In the asymmetric units of both
the salts, two cations and two anions are located. The C–O
distances of 1.206(9)–1.242(9) ꢁ in ALA.2 and 1.203(3)–
1.253(10) ꢁ in PHE.2 indicate the deprotonation of the acid
group, and this was also supported by FTIR (1612 and
1625 cmÀ1 for ALA.2 and PHE.2, respectively). The ion
À
pairs are involved in N H···O hydrogen-bonding interac-
tions involving the ammonium NH and carboxylate O atoms
resulting in the discrete assembly akin to the SAM synthon
B; in ALA.2, one of the ammonium H atoms participates in
bifurcated hydrogen-bonding interactions with the carboxyl-
ate O atoms, that are not present in PHE.2. Such discrete
HBNs are further self-assembled to form a 1D hydrogen-
À
bonded tape stabilized by N H···O interactions (N···O=
Gelation studies: Since all these salts showed 1D HBN in-
cluding the typical SAM synthon A that is implicated in gel
formation (see above), we evaluated the gelation properties
of the salts. The salts were scanned for gelation in 13 differ-
ent organic solvents (see the Supporting Information,
Table S10). Most of the salts turned out to be highly soluble
in the solvents studied and produced well-shaped crystals.
Many of the salts produced gelatinous precipitates. The salts
À
2.660(9)–2.901(9) ꢁ; ]N H···O=130.8–165.58 in ALA.2;
À
N···O=2.902(3)–2.995(2) ꢁ;
]
N H···O=153.3–156.08 in
PHE.2) involving the Boc-protected NH and carboxylate
moieties in a complementary fashion (Figure 4). Such 1D
tapes are packed in a parallel fashion (see the Supporting
Information, Tables S4 and S9 and Figures S3 and S8).
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Non-Polymeric Supramolecular Gel
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GLY.1, GLY.2, and ALA.1 showed reasonable gelation abili-
ties with a few solvents. GLY.1 gelled only nitrobenzene,
with a minimum gelator concentration (MGC) of 2.0 wt%
and a gel melting temperature Tgel of 698C. GLY.2 gelled
both nitrobenzene (MGC=4.0 wt%, Tgel =528C) and meth-
ylsalicylate (MGC=4.0 wt%, Tgel =678C). ALA.1 gelled
structure of GLY.1 is 1D and believed to be responsible for
gel network formation, keeping two pieces of gel together
for a minute appears to be good enough to reconstruct the
HBN in the interface and thus leads to self-healing. In com-
parison to GLY.1, when the cation is changed to dibenzy-
lammonium as in GLY.2, the corresponding gel was found
to be unstable (Figure 5E). These results clearly emphasize
the role of gel-network-solvent interactions, which are
poorly understood. Changing the alicyclic cation to an aro-
matic cation in going from GLY.1 to GLY.2, the gel-net-
work-solvent interactions must not favor the formation of
a strong gel with nitrobenzene.
a much less polar solvent, o-xylene (MGC=8 wt%, Tgel
488C).
=
Mouldablity, load-bearing and self-healing properties of
GLY.1: A nitrobenzene gel (7 wt%) of GLY.1 was found to
be a free-standing gel, meaning that a piece of gel could be
left as such without any confinement within a container
(Figure 5A). It also had a remarkable load-bearing capacity,
as demonstrated in Figure 5B, where we show that 13
Microscopy: We carried out both optical microscopy (OM)
and scanning electron microscopy (SEM) of nitrobenzene
gels of GLY.1 and GLY.2. A highly entangled gel network
comprised of thin fibrils is seen in the nitrobenzene gel of
GLY.1, whereas micro-thin needle-shaped crystals could be
observed in the corresponding gel of GLY.2. Similar mor-
phological features are also seen in the SEM micrographs of
the corresponding gels. The lack of a network on the micro-
scale in the images of the GLY.2 gel may explain its weaker
mechanical properties compared with that of the GLY.1 gel
(Figure 6).
Figure 5. A) Free-standing gel (7.0 wt%, nitrobenzene) of GLY.1; B) the
free-standing gel of GLY.1 withstanding the pressure of 13 Indian 5
Rupee coins (ꢀ117 g); C) a gel sculpture of “mother and child” made
from a 7.0 wt% nitrobenzene gel of GLY.1; D) self-healing of five gel
blocks of GLY.1; E) room temperature gel–sol transition of 5.0 wt% ni-
trobenzene gel of GLY.2.
Indian 5 Rupee coins (ꢀ117 g) can be placed on a 0.17 in2
area of a piece of free-standing gel (2 mL volume), which
means that it can withstand a pressure of 1.4 psi (9.65 kPa).
Such strength is remarkable for a non-polymeric supra-
molecular organogel and it encouraged us to attempt to
create a sculpture using the gel. A solution of this gel was
poured into a template of the sculpture, which depicts
a mother and child. After cooling, the gel sculpture (Fig-
ure 5C) was taken out with care and it was found to be
stable under ambient conditions for months (see the Sup-
porting Information, Figure S11). The same gel also dis-
played self-healing behavior (Figure 5D). For this experi-
ment, a block of a gel (3.0ꢂ0.8ꢂ0.6 cm) was cut into five
pieces. Alternating pieces were dyed with Rhodamine B and
the pieces were put together and allowed to self-heal. As
shown by Figure 5D, the pieces of gel healed within
a minute to form a single structure, and the whole structure
is strong enough to hold its weight when placed on two col-
umns separated by 2.0 cm. Since the HBN in the crystal
Figure 6. Nitrobenzene gel of A) OM of 2.0 wt% nitrobenzene gel of
GLY.1; B) SEM of 1.0 wt% nitrobenzene gel of GLY.1; C) OM of
4.0 wt% nitrobenzene gel of GLY.2; D) SEM of 2.0 wt% nitrobenzene
gel of GLY.2.
Rheology: A Tgel versus [gelator] plot of nitrobenzene gels
derived from GLY.1 and GLY.2 displayed a steady increase
in Tgel with the increase in [gelator]; this indicates that the
gel networks are governed by noncovalent interactions such
as hydrogen bonding; the greater strength of the GLY.1 gel
compared with the GLY.2 gel is also clear from this plot
(Figure 7). It is interesting to note that the PXRD of the ni-
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P. Dastidar et al.
than that of G’’ (in turn, tan w=G’’/G’ꢀ0.1), which implies
that the response is strongly elastic at short timescales. On
the other hand, at low w, G’ and G’’ become much closer
and both moduli show a weak frequency dependence (in
turn, tan w=G’’/G’ꢀ0.6 to 0.8). This means that at long
timescales, the sample shows some viscous character and ex-
hibits a very slow relaxation process. A similar shape of the
frequency spectrum was observed at all concentrations of
GLY.1 tested. We surmise that the weak relaxation at long
timescales is conducive to the self-healing character of this
gel.
Figure 8b shows a dynamic stress sweep, which was con-
ducted at a constant frequency of 10 radsÀ1. Here the
moduli are plotted as functions of the stress amplitude
s and we see the typical behavior of a shear thinning or
thixotropic material. The moduli are both relatively constant
at low stress amplitudes, with G’ being considerably higher
than G’’, consistent with the frequency data (Figure 8a) at
this frequency. However, when the stress is increased
beyond about 300 Pa, the moduli rapidly plummet and G’
falls below G’’. This type of response indicates the yielding
of the sample at high deformations, that is, the disruption of
bonds in the network. The stress at which the moduli begin
to plummet can be termed the yield stress sy of the gel and
its value here is about 200 Pa (this is marked by the vertical
line on Figure 8b). This is a high value of the yield stress,
and it reflects the free standing and load bearing nature of
the sample.
Figure 7. Tgel versus [gelator] plot of GLY.1 and GLY.2.
trobenzene xerogel of GLY.1 matched reasonably well with
that of the experimental bulk solid and simulated pattern
obtained from the single-crystal structure of GLY.1 which
means that the 1D HBN observed in the crystal structure of
GLY.1 is also present in the gel network of the xerogel (see
the Supporting Information, Figure S9).
We conducted rheological experiments on a 7 wt% GLY.1
gel in nitrobenzene. Figure 8a shows data from a dynamic
frequency spectrum, which was conducted at a constant
stress within the linear viscoelastic region of the sample.
The data show that the elastic modulus G’ exceeds the vis-
cous modulus G’’ over the entire range of frequencies w. At
high w, G’ approaches a plateau modulus (Gp ꢀ300 kPa).
Also, note that at high w, the value of G’ is much higher
Interestingly, this gel also shows rapid recovery from
shearing (Figure 9). For this experiment, we sheared the
sample at a high oscillatory stress of 500 Pa (beyond sy) for
a period of 2 min and then switched to a low stress within
the linear viscoelastic region. At high stress, the sample ex-
hibited a viscous response (G’’>G’) with low values of G’
(ꢀ10 Pa), which indicates a disruption of its network struc-
ture. When switched to low stress, the sample instantaneous-
Figure 9. Cycling of a 7 wt% GLY.1 gel in nitrobenzene between high
and low extents of oscillatory shear. First, the sample is vigorously
sheared for 2 min at an oscillatory stress amplitude of 500 Pa, which is
well beyond its yield stress sy. For the next 4 min, the sample is switched
to a low stress amplitude of 5 Pa, which is within its linear viscoelastic
region. This cycle is then repeated twice more. The frequency is fixed at
10 radsÀ1 for all these experiments. Note that the sample instantaneously
recovers to its gel-like state as soon as the vigorous shear is switched off.
Figure 8. Dynamic rheological data on a 7 wt% GLY.1 gel in nitroben-
zene. A) Frequency sweep showing the elastic modulus G’ and the vis-
cous modulus G’’ as functions of angular frequency w. B) Stress sweep
showing G’ and G’’ as functions of stress amplitude s at constant w=
10 radsÀ1. A sharp decrease in G’ and G’’ occurs at a value of s marked
by the vertical line, which is the yield stress sy of the sample.
8062
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 8057 – 8063

Non-Polymeric Supramolecular Gel
FULL PAPER
Crystal structures: Data were collected using MoKa (l=0.7107 ꢁ) radia-
tion on a Bruker APEX II diffractometer equipped with a CCD area de-
tector. Data collection, data reduction, structure solution/refinement
were carried out using the software package of SMART APEX. All
ly reverted to an elastic response (G’>G’’) with high values
of G’ (ꢀ100 kPa), which indicated that the bonds in the net-
work are quickly re-formed. The above cycle was repeated
twice and the same behavior was observed. In summary, the
noteworthy rheological properties of GLY.1 gels are: 1) a
high plateau modulus (Gp ꢀ300 kPa); 2) a high yield stress
(sy ꢀ200 Pa); 3) a weak decay of G’ and G’’ at low w indi-
cating slow dissipative dynamics; and 4) quick recovery after
disruptive shearing.
structures were solved by direct methods and refined in
a routine
manner. Non-hydrogen atoms were treated anisotropically. All the hydro-
gen atoms were geometrically fixed. CCDC-844914 (GLY.1), CCDC-
811915 (GLY.2), CCDC-827781 (ALA.2), CCDC-811916 (LEU.1),
CCDC-811917 (LEU.2), CCDC-811918 (ILE.2), CCDC-811919 (PHE.1)
and CCDC-811920 (PHE.2) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_-
request/cif.
Conclusion
A supramolecular synthon approach led to the discovery of
an organogel derived from an easily accessible organic salt
(GLY.1); the nitrobenzene gel of GLY.1 displayed remark-
able moldable, load-bearing, and self-healing properties that
are thus far confined to clay- and polymer-based gels with
a few exceptions.[6] The strength of the GLY.1 gel is so much
so that a small piece of free standing gel (2.0 mL volume;
0.17 in2 area) can withstand a pressure of 9.65 kPa and also
allowed us to create a relatively large sculpture of “mother
and child” that did not deform for months. These remark-
able properties displayed by GLY.1 and the inability of its
dibenzylammonium counterpart (GLY.2) to display such
properties were explained using microscopic and rheological
data. The fact that such remarkable properties arising from
an easily accessible (by salt formation) small molecule (the
molecular weight of GLY.1 is only 256.38) are due to nonco-
valent supramolecular interactions is quite intriguing. The
results presented herein are expected to attract the attention
of research groups interested in developing materials dis-
playing such intriguing mechanical properties.[23]
Acknowledgements
P.D. thanks the CSIR (New Delhi, India) for financial support. P.S.
thanks the IACS for a research fellowship. H.Y.L. was partially funded
by a grant from the Chemistry division of the National Science Founda-
tion.
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Materials and methods: All of the chemicals (Aldrich) and solvents
(A.R. grade, commercially available, India) were used without any fur-
ther purification. Petrol used in the gelation experiments was purchased
from the local market. Microanalyses were performed on a Perkin–
Elmer elemental analyzer 2400, Series II. FTIR spectra are recorded
using Perkin–Elmer Spectrum GX. Powder X-ray patterns were recorded
on an XPERT Philips diffractometer (CuKa radiation, l=1.5418 ꢁ).
Scanning electron microscopy (FTSEM) was performed on a JEOL;
JSM-6700F. Single-crystal X-ray diffraction was done with a Bruker axs,
SMART APEX II. Rheological experiments were performed on an
AR2000 stress-controlled rheometer (TA Instruments). Samples were
run on cone and plate geometries (20 mm diameter/28 cone angle or
40 mm diameter/48 cone angle). The plates were equipped with Peltier-
based temperature control and all samples were studied at 25Æ0.18C. A
solvent trap was used to minimize evaporation of solvent.
[22] D. R. Trivedi, A. Ballabh, P. Dastidar, J. Mater. Chem. 2005, 15,
2606.
[23] Note added in proof: While this paper was being reviewed, we came
across two publications (T. Kishida, N. Fujita, K. Sada, S. Shinkai, J.
Am. Chem. Soc. 2005, 127, 7298; S. Samai, K. Biradha, Chem.
Mater. 2012, 24, 1165), wherein the mechanical strength of a gel was
demonstrated by putting a number of coins on a small piece of gel
as demonstrated in Figure 5b of this paper.
Preparation of salts: Salts were prepared by mixing Boc-protected amino
acids, namely glycine, alanine, valine, leucine, isoleucine, and phenylala-
nine with the corresponding dicyclohexylamine and dibenzylamine ac-
cording to Scheme 2 in 1:1 molar ratio in methanolic medium. The resul-
tant mixture was subjected to sonication for a few minutes to ensure the
homogenous mixing of the two components. A white precipitate was ob-
tained after complete removal of MeOH by rotary evaporator, which was
subjected to various physico-chemical analyses and a gelation test.
Received: March 22, 2012
Published online: May 24, 2012
Chem. Eur. J. 2012, 18, 8057 – 8063
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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