Anion binding inhibition of the formation of a helical organogel

Article abstract of DOI:10.1039/b606373j

A chiral tris(urea) organogelator gels dmso-water and methanol-water mixtures at low weight percent. The formation of the helical gel fibres is partially inhibited by addition of chloride, which is bound by the gelator, resulting in fully crystalline mate

Full text of DOI:10.1039/b606373j

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Anion binding inhibition of the formation of a helical organogel{
Claire E. Stanley, Nigel Clarke, Kirsty M. Anderson, Judith A. Elder, Joseph T. Lenthall and
Jonathan W. Steed*
Received (in Austin, TX, USA) 5th May 2006, Accepted 1st June 2006
First published as an Advance Article on the web 22nd June 2006
DOI: 10.1039/b606373j
aqueous DMSO (1 : 1 v/v) over a period of ca. 5 min above
0.3 wt% (Fig. 1(a)). This contrasts to a range of achiral tris(urea)s
that do not gel DMSO or methanol but do gel other solvents.23
The initiation of gel formation in the methanol system may be
markedly accelerated by sonication,27,28 however gelation does not
go to completion in the ultrasonic bath, presumably because of
A chiral tris(urea) organogelator gels dmso–water and metha-
nol–water mixtures at low weight percent. The formation of the
helical gel fibres is partially inhibited by addition of chloride,
which is bound by the gelator, resulting in fully crystalline
material characterised by X-ray crystallography
The phenomenon of gelation may be understood as a form of
arrested or partial crystallisation.1 Recently, there has been a surge
of interest in low molecular weight gelators (LMWGs), a class of
organic small molecules that form gels via supramolecular
interactions.2–9 Supramolecular two-component LMWG systems4
and metallogels based on coordination complexes5,10–12 have also
been reported. In addition chiral LMWGs are known in which
the molecular chirality results in nanoscale helicity in the
gel fibres.2,13–19 Bis- and tris(urea) LMWGs, particularly bearing
long alkyl substituents, have proved to be highly effective
gelators.8,9,20–23 Unrelated to gel formation, the tris(urea) motif
has also been used in a number of effective supramolecular hosts
for anions.24–26 We now report a system in which LMWG self-
association and LMWG–anion interactions are in competition
with consequent effects on the behaviour and properties of the
gel phase.
The enantiomerically pure tris(urea) (1) was prepared by
reaction of tris(2-aminoethyl)amine with (S)-(2)-a-methylbenzyl
isocyanate. A chiral gelator was chosen in the anticipation that the
chirality of the LMWG would be expressed in the gel structure
yielding information about inter-gelator interactions. Compound 1
forms stiff, opaque thermoreversible gels in aqueous methanol and
Department of Chemistry, University of Durham, South Road, Durham,
UK DH1 3LE. E-mail: jon.steed@durham.ac.uk;
Fax: +44 (0)191 384 4737; Tel: +44 (0)191 334 2085
{ Electronic supplementary information (ESI) available: Experimental
details and crystal structure information for compound 1. See DOI:
10.1039/b606373j
Fig. 1 (a) Hydrogel formed from a solution of 1 in DMSO–water
(1 : 1 v/v), (b) mixed gel and single crystal formed from water–methanol
with NaCl, (c) ESEM image of the partially dried gel, (d) SEM of the
xerogel showing the helical fibrous structure.
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Chem. Commun., 2006, 3199–3201 | 3199

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shear stresses on the developing supramolecular gel network. The
sol–gel transition temperature, Tgel, was measured using the
‘tilted tube’ method and increases regularly with concentration
of 1 reaching a limit at 116 uC from a 0.0170 M gel of 1 in
DMSO–water.
macroscopic mechanical behaviour of semi-crystalline polymers.
The scaling of the yield stress with concentration according to the
mechanism of failure of the cellular material has been studied.32,33
If the struts buckle during deformation (elastic collapse), then
dÃel # 0.05ES(c/rS)2, whereas if plastic collapse occurs (i.e., the
struts exhibit plastic deformation), then dÃpl # 0.3dS(c/rS),1.5
,
Frequency sweep rheometry measurements, with a small
amplitude stress, indicate that the storage modulus, G9, remains
constant with increasing angular frequency. Such linear viscoelastic
behaviour is associated with classical gels, and supports the belief
that these materials undergo a transition to a true gel state. The
where dS is the plastic yield stress of the individual struts. By
comparing the two scaling laws with our results, and by noting
that our values for the yield stress are roughly an order of
magnitude less than the predicted yield stress for elastic collapse, it
seems that the mechanism of plastic collapse agrees well with our
observations and hence dS y1.4 MPa.
storage modulus scales with concentration, c, as G9 3 c1.9¡0.1
,
in good agreement with previous work.29,30 The non-linear
rheological response was probed using large amplitude stress
sweep experiments, during which an oscillatory stress is imposed
with a fixed frequency (1 Hz) over a range of shear stress
amplitudes. Below a critical value of shear stress, the yield stress,
d*, the storage modulus remains constant. Above the yield
stress, G9 decreases rapidly, suggesting that the gel has collapsed.
The yield stress scales with concentration as d* 3 c1.4¡0.03, again
in agreement with the work of Terech et al.29
The partially dried gels were imaged by environmental SEM
which shows a well-defined dendritic, feathery type structure
(Fig. 1(c)). The dried xerogel was also examined by SEM which
shows a network of monodisperse left handed helical fibres ca.
30 nm wide, some grouped into larger bundles, Fig. 1(d). The
helical pitch is variable in the larger scale bundles although there is
some sign of very small pitch helicity within the smallest fibres. It is
unclear whether the helical twisting arises from the molecular
chirality or an anisotropy of the interfacial energy.21 Low-
temperature DSC measurements on the methanol–water gel
showed a glass transition at 246 uC. Two distinct freezing
exotherms at 2122 and 281 uC were observed during the cooling
cycle which might be attributable to free solvent (sharp peak at
2122 uC) and solvent included within the gel matrix (broader peak
at 281 uC ) (see ESI{).
Sangeetha et al.30 discussed two quantitative theoretical frame-
works for the mechanical properties of molecular gels, based on
concepts developed to describe either colloidal gels or cellular
solids. A colloidal gel is modelled as comprised of flocs with a
fractal dimension, df, which reflects how the mass of colloidal
material within a floc scales with floc size, connected by interfloc
links.31 The dependence of the mechanical properties on
concentration is determined by the relative strength of interactions
between flocs and within flocs. In the strong-link regime, the
former dominate and the following is predicted,
Anion binding by 1 was probed by 1H NMR titrations with the
tetrabutylammonium salt of Cl2 in DMSO-d6 solution. The
binding isotherm was fitted readily to a 1 : 1 binding stoichiometry.
Compound 1 binds Cl2, Kassoc = 1154 M21, in line with previously
reported related compounds.24,34 The effect of anion binding on
the gelation process was monitored by repeating the gelation
experiments in the presence of stoichiometric amounts of NaCl at
various concentrations, and in excess. The NaCl markedly
retarded the rate of the gelation process and decreased the amount
of the solvent incorporated into the gel phase. Surprisingly
concomitant formation of gel and crystalline material was
invariably observed (Fig. 1(b)). The crystalline material was
characterised by X-ray crystallography which revealed it to
comprise solely ligand 1. Thus the Cl2 binds to the gelator 1 (as
indicated by the NMR experiments) and promotes crystallisation
but the material does not crystallise as the NaCl complex under
these conditions.
G9 3 c(3+x)/(32d ); d* 3 c2(1+x)/(32d )
(1)
f
f
where x is the fractal dimension of the elastically effective number
of interactions within a floc. In the weak-link regime, intrafloc
interactions dominate and the following is predicted,
G9 3 c1/(32d ); d* 3 c1/(32d )
(2)
f
f
From eqn (1) and (2), it is clear that the strong-link model permits
agreement with the experimentally observed scaling of the shear
modulus, but only if x = df = 1.30 Both these latter conditions are
inconsistent with the model of an interconnected fractal colloidal
gel,31 which requires that (i) x be less than the fractal dimension, df,
and (ii) in order to ensure connectivity between flocs, x must be
greater than unity. Furthermore, neither model is able to correctly
predict the scaling of the yield stress, regardless of the fractal
dimension. Hence, we believe that the mechanical properties of small
molecule gelators cannot be described by analogy with colloidal gels.
The cellular solid model32 appears a more promising frame-
work. For open-cell cellular materials, which consist of, load-
bearing struts interconnected via crosslinks or junction points,
which deform by bending, this model predicts G9 3 c2, in good
agreement with our observations. For a wide range of materials
with a wide range of cellular concentrations, G9 # 3ES(c/rS)2/8,33
where ES is the Young’s modulus of the struts and rS the density
within the struts. From the crystallographic data (vide infra), rS =
1.171 g cm23. The Young’s modulus of the individual fibres is
ES y1.2 GPa, comparable to values associated with the
The X-ray structure of 1 is shown in Fig. 2.{ Compound 1
adopts a needle morphology, crystallising in the chiral space group
P212121 forming polar stacks of molecules along the crystal-
lographic b direction, without any molecular symmetry. Molecules
within the stacks are linked by two six-membered hydrogen
bonded ring interactions involving two pairs of urea groups,
reminiscent of the urea tape motif. The third urea group is
involved in an intramolecular hydrogen bonded ring of the same
type, aligned in the same direction. This type of interaction has
been postulated to be responsible for gel fibre formation in
bis(urea)s.21 If the X-ray crystal structure is taken as a starting
model for the gel fibres then the fibres of width 30 nm would
comprise collinear bundles of ca. 15–25 stacks of unit cell cross
section 1.25 6 2.0 nm. The fact that the chirality of the individual
stacks is reflected in the left handed helicity of the fibres (Fig. 1(d))
3200 | Chem. Commun., 2006, 3199–3201
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Notes and references
{ See ESI for crystallographic data. CCDC 604537. For crystallographic
data in CIF or other electronic format see DOI: 10.1039/b606373j
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suggests that all of the stacks are orientated in the same direction.
It is thus possible to propose a model for gelation in which gel fibre
growth along the stack axis, promoted by strong urea–urea
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long enough to entangle in the time taken to reach the optimum
fibre width by van der Waals packing. Fibre branching to produce
a dendritic 3D network (Fig. 1(c)) is possible by occasional cross-
linking involving breaking the intramolecular urea–urea hydrogen
bond to form a second intermolecular interaction. Dynamic
chloride binding to the three urea groups would significantly retard
the rate of stack growth along the fibre axis resulting in the
observed needle crystals with a much lower length : diameter ratio.
Face indexing of the crystal confirmed that the b axis along which
the urea hydrogen bonding is orientated corresponds to the long
axis of the needle shaped sample.
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gelation behaviour of LMWG compound 1. Addition of Cl2 tips
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close correspondence in the gel fibre and single-crystal structures.
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paves the way for tailored gels with novel properties.
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We are grateful to the EPSRC for a post-doctoral grant
(K. M. A.).
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