Balancing Hydrogen Bonding and van der Waals Interactions in Cyclohexane-Based Bisamide and Bisurea Organogelators

Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands.
Langmuir (Impact Factor: 4.46). 08/2009; 25(15):8802-9. DOI: 10.1021/la9004714
Source: PubMed
ABSTRACT
The solvent dependence of the gelation properties, the thermotropic behavior, and the melting enthalpy of a series of enantiomerically pure cyclohexane-based bisamide and bisurea compounds are reported. The two series of gelators examined are related structurally with the intermolecular interactions responsible for gelation differing in a systematic manner through varying the length of the alkyl tail and the number of hydrogen bonding units present. The gelation properties of the compounds in decalin, DMSO, and 1-propanol were studied by FTIR spectroscopy and by comparison of the thermal stability of their gels as determined by dropping ball experiments and by differential scanning calorimetry (DSC). FTIR spectroscopy, supported by the single-crystal X-ray diffraction of a3, indicates that the gelator molecules are aggregated through intermolecular hydrogen bonding in all of the solvents examined. The thermal stability of the gels in apolar and polar solvents was found to be dependent primarily on the relative strength of intermolecular hydrogen bonding and van der Waals interactions, respectively, compared with the strength of solvent-gelator interactions. The results of DSC indicated that the contribution of the difference in intergelator van der Waals interactions, compared with the gelator-solvent van der Waals and hydrogen bonding interactions, provided by the alkyl tail to the stability of the gel has a linear relationship with the number of methylene units in alkyl chains of length greater than six. In polar solvents, this contribution lies between 3.5 and 4.2 kJ mol(-1) per methylene unit, and in apolar solvents, it is 2.2 kJ mol(-1). The hydrogen bonding interactions were weaker in polar solvents and hence gelation occurred only where sufficient compensation was provided by intergelator van der Waals interactions. The results show that the direct relation of gelation strength to changes in solvent properties is not possible and more complex relationships should be considered. Furthermore, it is apparent that the development of design rules for the construction of LMWG molecules incorporating more than one anisotropic growth element must take into consideration the role of the solvent in determining which of the contributions is dominant.

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8802 DOI: 10.1021/la9004714 Langmuir 2009, 25(15), 8802–8809Published on Web 05/19/2009
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© 2009 American Chemical Society
Balancing Hydrogen Bonding and van der Waals Interactions in
Cyclohexane-Based Bisamide and Bisurea Organogelators
Niek Zweep,
Andrew Hopkinson,
§
Auke Meetsma,
Wesley R. Browne,
Ben L. Feringa,
and Jan H. van Esch*
,
)
Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Nether-
lands.,
§
Unilever Research and Development Port Sunlight, Quarry Road East, Wirral CH63 3JW, U.K., and
)
TU Delft, DelftChemTech, Julianalaan 136, 2628BL Delft, The Netherlands,
Received February 6, 2009. Revised Manuscript Received April 4, 2009
The solvent dependence of the gelation properties, the thermotropic behavior, and the melting enthalpy of a series of
enantiomerically pure cyclohexane-based bisamide and bisurea compounds are reported. The two series of gelators
examined are related structurally with the intermolecular interactions responsible for gelation differing in a systematic
manner through varying the length of the alkyl tail and the number of hydrogen bonding units present. The gelation
properties of the compounds in decalin, DMSO, and 1-propanol were studied by FTIR spectroscopy and by comparison
of the thermal stability of their gels as determined by dropping ball experiments and by differential scanning calorimetry
(DSC). FTIR spectroscopy, supported by the single-crystal X-ray diffraction of a3, indicates that the gelator molecules
are aggregated through intermolecular hydrogen bonding in all of the solvents examined. The thermal stability of the
gels in apolar and polar solvents was found to be dependent primarily on the relative strength of intermolecular
hydrogen bonding and van der Waals interactions, respectively, compared with the strength of solvent-gelator
interactions. The results of DSC indicated that the contribution of the difference in intergelator van der Waals
interactions, compared with the gelator-solvent van der Waals and hydrogen bonding interactions, provided by the
alkyl tail to the stability of the gel has a linear relationship with the number of methylene units in alkyl chains of length
greater than six. In polar solvents, this contribution lies between 3.5 and 4.2 kJ mol
-1
per methylene unit, and in apolar
solvents, it is 2.2 kJ mol
-1
. The hydrogen bonding interactions were weaker in polar solvents and hence gelation
occurred only where sufficient compensation was provided by intergelator van der Waals interactions. The results show
that the direct relation of gelation strength to changes in solvent properties is not possible and more complex
relationships should be considered. Furthermore, it is apparent that the development of design rules for the construction
of LMWG molecules incorporating more than one anisotropic growth element must take into consideration the role of
the solvent in determining which of the contributions is dominant.
Introduction
Gelation, the trapping of a liquid by a network of fibers,
1
is a
remarkable phenomenon, not least in the case of low-molecular-
weight gelators (LMWGs). These LMWGs typically form a
network of supramolecular fibers through the anisotropic aggre-
gation of individual molecules; therefore, the formation of gels by
LMWGs is an excellent phenomenon for furthering the under-
standing of the mechanisms involved in supramolecular self-
assembly, which is one of the major challenges in modern science.
2
The number of polymeric, inorganic, and LMWG systems cap-
able of gelating both polar and apolar solvents is increasing
steadily;
3-7
concomitantly, our understanding of the forces
that drive gelation (e.g., ion-ion, dipole-dipole, magnetic
dipole-dipole,
8
hydrogen bonding, van der Waals, and π-π
stacking interactions) is increasing.
9-13
There are several ways that these LMWGs form gels: via
platelets,
14,15
via colloids,
16
or via fibrillar material.
4
In the case
of LMWGs that form a fiber network, a central aspect that has
been deemed essential in the formation is the anisotropy of the
intermolecular interactions responsible for the growth of aggre-
gates in one dimension.
3,4,17
Once formed, these fibers interact to
form part of a 3D network that traps the solvent and leads to what
is recognized at the macroscopic level as a gel. A key challenge in
the rational design of LMWGs is to understand the level and
nature of (anisotropic) intermolecular interactions that are re-
quired to achieve gelation and the relative contributions that are
made by the various intermolecular interactions and how the
solvent itself affects these contributions and hence the strength of
Part of the Molecular and Polymer Gels; Materials with Self-Assembled
Fibrillar Networks special issue.
*Corresponding author. E-mail: j.h.vanesch@tudelft.nl.
(1) Cohen Addad, J. P. Physical Properties of Polymeric Gels; Wiley: New York,
1995.
(2) Service, R. F. Science 2005, 309, 95.
(3) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133–3159.
(4) Van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263–2266.
(5) Weiss, R. G.; Terech, P. Molecular Gels: Materials with Self-Assembled
Fibrillar Networks; Springer: Dordrecht, The Netherlands, 2005.
(6) Estroff, L.; Hamilton, A. D. Chem. Rev. 2004, 104, 1201–1217.
(7) Fages, F. Ed.; Low Molecular Mass Gelators: Design, Self-Assembly,
Function; Topics in Current Chemistry; Springer: New York, 2005; Vol. 256.
(8) Gao, J.; Zhang, B.; Zhang, X.; Xu, B Angew. Chem., Int. Ed. 2006, 45, 1220–
1223.
(9) Van Esch, J.; De Feyter, S.; Kellogg, R. M.; De Schryver, F.; Feringa, B. L.
Chem.;Eur. J. 1997, 3, 1238–1243.
(10) Oda, R.; Huc, I.; Candau, S. J. Angew. Chem., Int. Ed. 1998, 37, 2689–2691.
(11) Yoza, K.; Amanokura, N.; Ono, Y.; Akao, T.; Shinmori, H.; Takeuchi, M.;
Shinkai, S.; Reinhoudt, D. N. Chem.;Eur. J. 1999, 5, 2722–2729.
(12) Chen, J.; McNeil, A. J. J. Am. Chem. Soc. 2008, 130, 16496–16497.
(13) Dastidar, P. Chem. Soc. Rev. 2008, 37, 2699–2715.
(14) Ashbaugh, H. S.; Radulescu, A.; Prud’homme, R. K.; Schwahn, D.;
Richter, D.; Fetters, L. J. Macromolecules 2002, 35, 7033–7053.
(15) Hirst, A. R.; Smith, D. K.; Harrington, J. P. Chem.;Eur. J. 2005, 11, 6552–
6559.
(16) Weng, W.; Li, Z.; Jamieson, A. M.; Rowan, S. J. Macromolecules 2009, 42,
236–246.
(17) Gronwald, O.; Shinkai, S. Chem.;Eur. J. 2001, 7, 4328–4343.
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Zweep et al. Article
the gel fibers formed.
12
Already some attempts have been made to
further the understanding of gelators by X-ray crystal data
analysis,
12,18-20
and elaborate studies have been done to design
nanoscaled materials with these self-assembled systems.
21
Here, we focus on the relative contribution of two distinct
supramolecular interactions involved in the gelation of solvents
by two classes of LMWGs (bisamide- and bisurea cyclohexyl-
based gelators (Figure 1)) and on how the solvent influences the
contribution of each of these interactions. The molecular scaffold
in both series of LMWGs is comparable, hence they provide an
excellent handle for understanding the relative contribution of the
supramolecular interactions involved in the gelation process.
Both systems consist of a cyclohexane framework on which two
hydrogen bonding moieties are connected at the 1,2 position in a
transoid fashion. The hydrogen bonding motifs (i.e., bisamide
and bisurea) are substituted with a linear alkyl tail. These motifs
are quite common in the design of LMWGs.
22,23
The syntheses of bisamide a11
24
and bisureas u4, u12,andu18
25
were first reported by Hanabusa et al. in 1996. The ability of
bisamides a7 and a11 and bisureas u4, u12,andu18 to gelate a
number of solvents has also been demonstrated.
19,24-26
For both
classes, the gelation is presumed to be driven by two different
types of intermolecular interactions: van der Waals and hydrogen
bonding interactions. The overall sum of these interactions is
assumed to lead to an anisotropic aggregation orthogonal to the
cyclohexane ring. In the present contribution, we will describe the
results of gelation for a series of these systems with respect to their
structure, thermal stability, and melting enthalpy. The anisotropy
in the system is varied systematically through the length of the
n-alkyl unit and hence also the contributions of van der Waals
interactions and the number of hydrogen bonds (i.e., the bisamide
(2 H-bonds) and bisurea gelators (4 H-bonds). The strength of gels
in different solvents for each series of LMWGs allows for the role
of the solvent, in determining which intermolecular interaction is
primarily responsible for the gel fiber formation, to be identified.
Experimental Sectio n
All solvents and reagents for synthesis were reagent grade or
better and used as received. Solvents for gelation and spectro-
scopy were of HPLC or spectrophotometric grade. Bisamide
gelators a2-a17 were synthesized as described by Hanabusa et
al. for the preparation of bisamide a11.
24
One equivalent of
(1R,2R)-(-)-1,2-diaminocyclohexane is coupled with 2 equiv of
an alkyl acid chloride using excess of triethylamine in THF at
0 °C. Upon addition of the acid chloride, the reaction mixture
becomes viscous, and by heating the reaction mixture at reflux, the
aggregates break up. The product is purified by multiple washing
steps (the product is only sparely soluble in most organic solvents)
and after drying is isolated as a white solid (yields between 32%
and 93%). The lower isolated yields are due primarily to the
purification procedure.
The synthesis of bisurea gelators u4-u18 was carried out
according to the procedure reported earlier for the synthesis of
u12.
26
One equivalent of (1R,2R)-(-)-1,2-diaminocyclohexane was
coupled to 2 equiv of an alkyl isocyanate in toluene. Immediately
after addition of the isocyanate, the reaction mixture became
viscous, and the mixture was heated at reflux to break up the
aggregates. Purification of the product was performed by multiple
washing steps yielding 24-93% of the product as a white powder
after drying. Full experimental details and analyses are provided
as Supporting Information.
Critical Gelation Concentration (cgc) Determinatio n.
The
compounds are insoluble at room temperature in most of the
solvents examined. Above the critical gelation concentration
(cgc), upon heating, they dissolve, and subsequent cooling to
room temperature results in the formation of gels. To determine
the cgc values for the different compounds, the gels were diluted,
heated, and then cooled to room temperature repeatedly until
either gels did not form upon cooling or the gels were too weak to
withstand gravity. The gels formed were examined after 1 day and
1 week to ensure that aging did not affect the results.
Gel Melting Temperature Determination.
Gels were pre-
pared at least 1 day prior to melting experiments. A stainless steel
ball with a diameter of 2.5 mm (6.238 mg) was placed on the gels
and held at 5 °Ch
-1
, during which its position was monitored
27
via a video camera. The gel was considered to be melted when the
ball had reached the bottom of the vial. Dropping-ball experi-
ments were carried out at least in duplicate, and the melting
temperatures obtained were reproducible to within (1 °C.
Single-Crystal X-ray Analysis of a3.
Colorless thin platelet-
shaped crystals of a3 were obtained by recrystallization from
1-propanol with slow evaporation of the solvent. A crystal of
0.41 0.29 0.04 mm
3
, although providing only weak X-ray
scattering, was sufficient to give a final refinement on F
2
by full-
matrix least-squares techniques converged at wR(F
2
) =0.1033
for 1574 reflections and R(F ) = 0.0425 for 1324 reflections with
F
o
g 4.0 σ(F
o
) and 267 parameters and 1 restraint. Because of the
lack of anomalous scatters, absolute structures could not be
determined reliably, although on the basis of the starting ma-
terials used to prepare a3 the configuration of C6 and C10 is
known to be R. For details, see Supporting Information, CCCD
reference number CCDC 717396.
Figure 1.
Cyclohexane-based bisamide organogelators a2 to a17
and bisurea organogelators u4 to u18 employed in this study.
(18) Menger, F.; Yamasaki, Y.; Catlin, K.; Nishimi, T. Angew. Chem., Int. Ed.
1995, 34, 585–586.
(19) van Esch, J.; Schoonbeek, F.; De Loos, M.; Kooijman, H.; Spek, A. L.;
Kellogg, R. M.; Feringa, B. L. Chem.;Eur. J. 1999, 5, 937–950.
(20) Luboradzki, R.; Gronwald, O.; Ikeda, M.; Shinkai, S.; Reinhoudt, D. N.
Tetrahedron 2000, 56, 9595–9599.
(21) Hirst, A. R.; Miravet, J. F.; Escuder, B.; Noirez, L.; Castelletto, V.;
Hamley, I. W.; Smith, D. K. Chem.;Eur. J. 2009, 15, 372–379.
(22) Fages, F.; Vogtle, F.;
Zini
c, M. Top. Curr. Chem. 2005, 256, 77–131.
(23) Hardy, J. G.; Hirst, A. R.; Ashworth, I. Tetrahedron 2007, 63, 7397–7406.
(24) Hanabusa, K.; Yamada, M.; Kimura, M.; Shirai, H. Angew. Chem., Int. Ed.
1996, 35, 1949–1951.
(25) Hanabusa, K; Shimura, K; Hirose, K.; Kimura, M.; Shirai, H. Chem. Lett.
1996, 10, 885–886.
(26) Brinksma, J.; Feringa, B. L.; Kellogg, R. M.; Vreeker, R.; Van Esch, J.
Langmuir 2000, 16, 9249–9255.
(27) Tan, H. M.; Moet, A.; Hiltner, A.; Baer, E. Macromolecules 1983, 16, 28–34.
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8804 DOI: 10.1021/la9004714 Langmuir 2009, 25(15), 8802–8809
Article Zweep et al.
FTIR Spectroscopy.
Spectra were recorded on a Nicolet
Nexus FTIR instrument. Solid samples were recorded as an
intimate mixture with powdered KBr. Liquid and gel samples
were recorded in a liquid cell equipped with CaF
2
windows and a
0.2 mm lead spacer.
Differential Scanning Calorimetry.
DSC measurements
were carried out on a Perkin-Elmer DSC7. A known quantity
of gelator together with a known amount of solvent was placed in
a60μL stainless steel sample cup, which was sealed immediately.
The resulting concentrations of gelator are 20-40 mg/mL of
solvent. The sample cup was placed in the DSC apparatus
together with an empty sample cup as a reference. The cups were
heated for 15 min at 20 °C above the melting temperature of the
corresponding gel, as determined by the dropping ball technique.
Subsequently, the cups were cooled at a rate of 5 °Cmin
-1
to
10 °C, where 1-propanol or decalin was employed as the solvent,
and to 25 °C, where DMSO was employed as the solvent, at which
point the gels formed were allowed to age for 1 h. Heating and
cooling scans were recorded at a scan rate of 5 °Cmin
-1
. Repeated
heating and cooling of the samples were ( 1kJmol
-1
,anda
prolonged aging time of 8 h did not affect the results obtained. It
was confirmed that gels were formed by opening the cups after the
measurements.
Results
The gelation behavior for bisamide gelators a2-a17 and
bisurea gelators u4-u18 were determined in a range of apolar
and polar solvents, and for those cases where gelation was
observed, the critical gelation concentration (cgc) was deter-
mined.
19,24-26
The cgc values for bisamide gelators a2-a17 and
bisurea gelators u4-u18 are listed in Tables 1 and 2, respectively.
It is apparent from Table 1 that a2, the bisamide compound
with the shortest tail (ethyl), is already a good gelator for
many apolar solvents. In solvents consisting of linear or cyclic
hydrocarbons, the cgc values are between 1.5 and 2.0 mg mL
-1
,
which corresponds to gelation at only 1.5 wt % of gelator. In more
polar solvents, the cgc of a2 increases; however, in solvents such as
DMSO, gelation is inhibited, and precipitation or solubilization is
observed.
For a2, the formation of intermolecular hydrogen bonds and
interactions of the cyclohexyl rings are expected to be the major
driving force for gelation, as the van der Waals interactions of the
short alkyl tails are unlikely to contribute significantly. In solvents
that interfere with hydrogen bond formation (e.g. alcohols),
gelation is not observed, and a2 is soluble at concentrations
>20 mg mL
-1
. Elongation of the alkyl tail by one methylene unit
(i.e., a3) improves gelation (i.e., lower cgc values) in apolar
solvents compared with a2; however, the gelation of polar
solvents is again not observed. The same trends in gelation
behavior are observed for remaining amide-based gelators a5 to
a13, with the exception of compound a4, which forms micro-
crystals in many of the apolar solvent examined. Compound a4
has, as with a2, an odd number of methylene units in the alkyl tail
in contrast to gelators a3 to a17, which have an even number of
methylene units. It might be related to an odd-even effect (i.e.,
there is a different orientation of the molecules in the packing for
the compounds with an odd number of carbon atoms compared
to those with an even number).
29-32
Compounds a5-a13 are
potent gelators of the apolar solvents and polar solvents tested
that do not compete or compete only weakly for hydrogen
Table 1. Gelation Behavior of Bisamide Gelators with Selected Solvents
a
solvents
b
a3 a5 a7 a11 a13 a15 a17 a2 a4
cyclohexane tg (1.5) tg (1.1) tg (2.0) tg (1.0) tg (4.5) tg (0.5) tg (2.5) tg (1.5) c
decalin cg (2.1) cg (1.0) cg (6.0) cg (4.0) cg (1.6) cg (5.0) cg (4.0) cg (2.0) cg (5.0)
di-n-butylether cg (1.9) cg (3.0) cg (1.5) cg (1.5) cg (2.3) cg (0.7) cg (1.5) cg (2.0) c
toluene cg (3.0) cg (9.0) cg (5.0) cg (2.0) cg (6.0) cg (4.0) cg (1.5) tg (9.0) tg (3.0)
n-butylacetate og (15.0) og (10.0) tg (6.0) tg (1.5) tg (3.0) tg (1.0) tg (1.5) p og (10.0)
chloroform s s sssog(5.0) og (5.0) s s
1,2-dichloroethane c s s tg (4.0) tg (5.0) tg (1.5) tg (2.5) s s
dimethylsulfoxide s s tg (5.0) tg (2.0) tg (2.0) tg (3.0) tg (1.5) s s
1-propanol s s p s
c
p tg (2.5) tg (2.5) s s
ethanol s s p s
c
p og (2.5) og (2.5) s s
a
Abbreviations used: c, formation of microcrystals; cg, clear gel (critical gelation concentration in milligrams of compound per milliliter of solvent);
og, opaque gel; tg, turbid gel; i, insoluble at solvent reflux temperature; p, precipitate; s, soluble at room temperature (solubility >20 mg of compound/
mL of solvent).
b
The solvents used are listed according to the commonly used E
T
(30) scale for solvent polarity.
28 c
a11 is soluble up to at least 20 mg/mL,
but ref 24 reported gel formation at 33 and 40 mg/mL. However, at the cgc’s reported in ref 24, in the present report a precipitate was observed and
not a gel.
Table 2. Gelation Behavior of Bisurea Gelators with Selected Solvents
a
solvents
b
u4 u6 u8 u10 u12 u16 u18
cyclohexane i tg (15.0) tg (15.0) tg (2.0) tg (2.0) tg (5.0) tg (1.0)
decalin c cg (15.0) cg (12.0) cg (10.0) cg (3.0) cg (5.0) cg (7.0)
di-n-butylether i og (15.0) tg (10.0) tg (10.0) tg (10.0) tg (5.0) tg (4.5)
toluene cg (10.0) cg (10.0) cg (10.0) cg (5.0) cg (4.0) cg (4.0) cg (4.5)
n-butylacetate c c p og (10.0) og (10.0) og (10.0) og (10.0)
chloroform c p p p og (10.0) og (10.0) og (10.0)
1,2-dichloroethane p p tg (15.0) tg (10.0) tg (1.0) tg (5.0) tg (5.0)
dimethylsulfoxide og (20.0) og (17.5) og (7.5) og (2.5) og (2.0) og (0.5) og (1.0)
1-propanol p c p og (10.0) og (2.0) og (1.5) og (6.0)
ethanol c c p og (10.0) og (3.0) og (0.8) og (4.5)
a
For abbreviations, see Table 1.
b
The solvents used are listed according to the commonly used E
T
(30) scale for solvent polarity.
28
(28) Dimroth, K.; Reichardt, C.; Siepmann, T.; Bohlmann, F. Liebigs. Ann.
Chem. 1963, 661, 1–37.
(29) Suzuki, M.; Nanbu, M.; Yumoto, M.; Shirai, H.; Hanabusa, K. New J.
Chem. 2005, 29, 1439–1444.
(30) Piepenbrock, M-O,M.; Loyd, G. O.; Clarck, N.; Steed, J. W. Chem.
Commun. 2008, 2644–2646.
(31) Fujita, N.; Sakamoto, Y.; Shirakawa, M.; Ojima, M.; Fujii, A.; Ozaki, M.;
Shinkai, S. J. Am. Chem. Soc. 2007, 129, 4134–4152.
(32) Aoki, K; Kudo, M; Tamaoki, N. Org. Lett. 2004, 6, 4009–4012.
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Zweep et al. Article
bonding (e.g., 1,2-dichloroethane and n-butylacetate, Table 1).
33
In polar solvents that can compete strongly for hydrogen bonding
(e.g., alcohols), gelation does not occur with the exception of a11
in 1-propanol, which forms a gel at high gelator concentrations
(>44 mg mL
-1
), as demonstrated by Hanabusa et al.
24
Where the alkyl tails of the bisamide gelators are longer (e.g., in
a15 and a17), gelation occurs in all apolar and in hydro-
gen bonding polar solvents (e.g., 1-propanol) at a low concen-
tration of 2.5 mg mL
-1
. In aliphatic and aromatic solvents,
these compounds are potent gelators, and the cgc values are
sufficiently low to classify them as supergelators (<0.5 wt %).
34
Overall, it is apparent that the elongation of the alkyl tail on
the bisamide gelators leads to lower cgc, especially in polar
solvents; however, the effect is much less pronounced in apolar
solvents.
35-37
The solubility of the bisurea-based compounds (e.g., u4)is
much less than that observed for the bisamide gelators (e.g., a4),
which is also substituted with two n-butyl tails. Compound u4 is
insoluble in most of the apolar solvents examined (Table 2). An
increase in the alkyl tail length increases solubility, and com-
pounds u6 and u8 are soluble at elevated temperatures in apolar
solvents with gelation occurring upon cooling. However, in polar
solvents these compounds precipitate upon cooling rather than
form gels. For compounds u10-u18, in which the length of the
alkyl tail is further increased, the gelation of polar solvents upon
cooling is also observed, and these compounds form gels in all of
the solvents examined.
Of the bisurea compounds with long alkyl chains tested,
compound u16 has, overall, the lowest cgc values, and it appears
that this alkyl chain length is optimal. Indeed, the cgc values
determined are sufficiently low enough to classify u16 as a
supergelator (0.5 wt %).
34
X-ray Crystallographic Analysis of a3.
As a consequence of
the high anisotropy of the growth of crystals of the compounds
examined in the present study, the needle-shaped crystals
obtained were typically unsuitable for X-ray crystallographic
analysis. However, suitable crystals were obtained for bisamide
a3 (Figure 2) by dissolving a small amount of a3 in 1-propanol
and allowing the solvent to evaporate slowly. The unit cell of
a3 has P2
1
symmetry and contains two molecules of a3
(a = 11.776(2) A
˚
, b =4.777(1)A
˚
, c = 13.250(3) A
˚
and
β=99.185(3)°). In the crystalline state, the compounds arrange
to form an infinite hydrogen-bonded chain along the b axis via the
amide groups. The orientation of both amide groups in the
molecule is antiparallel, and the two intermolecular hydrogen
bonds are of differing lengths: 2.845(3) A
˚
(N1-H21-O1, 170(3)°)
and 2.855(3) A
˚
(N2-H22-O2, 164(3)°).
The alkyl chains do not adopt an all-trans configuration, as
shown by the torsion angles from C1-N1-C7 to C8-C9-C10
of 179.4(2)°,128.5(3)°,and-64.8(4)° for one chain and from
C6-N2-C11 to C12-C13-C14 of 175.4(2)°, -122.8(3)°,and
63.6(4)° for the second chain. Presumably, the chains are too short
for an all-trans configuration to be energetically favorable. In
addition, Figure 2 shows that there is a screw axis along the b axis,
which leads to the formation of sheets.
Unfortunately, it was not possible to obtain crystals large
enough for single-crystal X-ray analysis for any of the bisurea-
based gelators. From crystals grown from u6 in ethanol, it was
possible to determine only, with considerable uncertainty, the unit
cell symmetry: P2
1
2
1
2
1
. From modeling studies on bisurea
gelators
19
and the single-crystal structure of a3, it is apparent
that both classes of gelators require hydrogen bonding units to be
in an antiparallel conformation to achieve gelation.
The validity of the extrapolation of intermolecular structure in
the crystalline state and the gel state was assessed by FTIR
spectroscopy. FTIR spectroscopy is a powerful probe of hydro-
gen bonding interactions because several absorptions in the
infrared region display distinct shifts upon hydrogen bonding.
38
In particular, the NH (stretch), amide I, and amide II vibrations
are sensitive to changes in hydrogen bonding and hence aggrega-
tion. FTIR spectra of the gelators in solution were recorded to
compare the position of the bands with those in the gel and
crystalline state. In Tables 3 and 4, the positions of these bands are
listed for the bisamide and bisurea gelators in different states.
Unfortunately, because of the strong aggregation of the bisurea
gelators only the solution spectra of compounds u4-u12 could be
obtained.
Figure 2.
Drawing of the asymmetric unit cell of a3 along the crystal lattice 010, showing the presence of intermolecular hydrogen bonding
between the amide groups, positioned in an antiparallel fashion and a projection along the unit cell b axis.
(33) The involvement of two distinct intermolecular interaction types with
differing solvent dependence precludes a facile quantitative correlation to simpli-
fied solvent parameters such as solvent acceptor and donor numbers.
(34) Luboradzki, R.; Gronwald, O.; Ikeda, A.; Shinkai, S. Chem. Lett. 2000, 10,
1148–1149.
(35) Abdallah, D.; Weiss, R. G. Langmuir 2000, 16, 352–355.
(36) Zhu, G.; Dordick, J. S. Chem. Mater. 2006, 18, 5988–5995.
(37) Hanabusa, K.; Matsumoto, M.; Kimura, M.; Kakehi, A.; Shirai, H. J.
Colloid Interface Sci. 2000, 224, 231–244.
(38) Mido, Y. Spectrochim. Acta 1973, 29A , 431–438.
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Page 4
8806 DOI: 10.1021/la9004714 Langmuir 2009, 25(15), 8802–8809
Article Zweep et al.
From the FTIR spectroscopic data, shown in Table 3, it is
apparent that in CH
2
Cl
2
solution the bisamide gelators do not
aggregate because the NH stretch absorption (3429-3430 cm
-1
)
and the amide I absorption (1661-1662 cm
-1
)appearatwave-
numbers typical of non-hydrogen-bonded systems.
38
In the gel
state, the bisamides are hydrogen bonded in apolar as well as
polar solvents with the position of the bands shifted to values
typical of hydrogen-bonded systems.
38
There are no large differ-
ences observed between the absorption band positions for the
gelators of differing chain lengths, indicating similar hydrogen
bonding in the gelators examined. Furthermore, the FTIR
spectrum of a3 in the crystalline state compares closely to the
spectrum of a3 in the gel state, indicating that in both states the
compound is hydrogen bonded in a similar arrangement, justify-
ing the comparison of the results of the single-crystal X-ray
studies with those of the gels. There is a minor difference between
the position of the amide II band in the crystalline state compared
to that in the gel state, possibly because of the alkyl tail being more
ordered in the crystalline state because this band is due to a
combination of N-H bending and C-N stretching vibrations.
The position of the amide I band in methanol shows that these
bisurea compounds are not aggregated in this solvent (Table 4).
38
However, in this solvent for bisureas with longer alkyl tails (e.g.,
u16 and u18), precipitation was observed, showing the aggregative
tendency of these compounds. The position of the absorption
bands in the gels formed in the different solvents shows the
presence of hydrogen bonds. Furthermore, in the crystalline state
the molecules are aggregated via hydrogen bonding as was
suggested by molecular modeling.
19
However, the relative con-
tributions of hydrogen bonding and van der Waals interactions to
the formation of the gel state could not be inferred from the FTIR
spectra.
Thermal Stability.
The effects of changing the relative con-
tribution of hydrogen bonding and van der Waals interactions,
together with the effect of the solvent on the thermal stability of
the gel, were studied by the dropping ball method.
27
The T
gs
of the
gel was considered to be reached when the ball had reached the
bottom of the vial. It should be noted that the gelator may not
have been dissolved completely at this temperature.
39
Never-
theless, this technique provides a good indication of the thermal
stability of the gel. By determining the T
gs
of gels over a range of
concentrations, the gel-sol phase-transitions curve of the gelator
in a particular solvent can be plotted (e.g., Figure 3).
26
An example of a plot of a gel-sol phase-transition curve for
gelators a2-a17 is provided in Figure 3. At low gelator concen-
trations, an increase in T
gs
is observed with an increase in the
concentration of gelator; however, this increase holds up only to a
specific concentration.
26
In this solvent (decalin), all of the
bisamide gelators tested were capable of forming gels. The gels
of the bisamide gelator with the shortest alkyl chain, a2,havethe
highest gel-sol phase-transition temperature (>140 °Catcon-
centrations of >70 mmol). The high thermal stability of gels of a2
indicates that the high polarity is a major driving force for gelation
in this solvent. Notably, the T
gs
values for bisamide gelators at the
same concentration decrease with increasing alkyl tail length. This
decrease in T
gs
reaches a minimum for a13, and for the bisamide
gelator with the longest alkyl tail tested, a17, essentially similar T
gs
values were found. The exception to this trend is a4, possibly due
to an odd-even effect. The leveling off of the gel stability is
counterintuitive because it would be expected that an increase in
the length of the alkyl tail leads to an increase in the thermal
stability due to larger van der Waals interactions. Most likely, the
increased LMWG interactions achieved with longer alkyl chains
are less important than the increase in molecular solubility in
organic solvents. That is, the longer-tailed bisamide gelators are
more soluble in decalin, and hence the decrease in the T
gs
is
observed.
In decalin, the T
gs
values of the bisurea gels could not be
determined reliably because they are >130 °C. Above these
temperatures, the urea group is unstable and decomposes.
40
Nevertheless, the high gel-sol phase-transition temperatures
Table 3. Selected FTIR Absorption Bands (ν,cm
-1
) of Bisamide
Gelators in Crystalline, Gel, and Solution States
a,b
bisamide state NH (stretch) amide I amide II
a3 solution in CH
2
Cl
2
c
3430 1662 1514
a17 solution in CH
2
Cl
2
c
3429 1661 1514
a3 crystal
d
3287 1639 1551
a3 decalin
c
3280 1635 1549
a17 decalin
c
3280 1637 1546
a7 DMSO
c
3284 1636 1545
a17 DMSO
c
3287 1637 1545
a15 1-propanol
d
3279 1638 1546
a17 1-propanol
d
3279 1638 1546
a
For a complete overview, see Supporting Information.
b
Uncer-
tainty ((2cm
-1
).
c
Recorded in a CaF
2
cell with 0.1 mm spacing and
concentrations of <1 mg/mL for solutions.
d
Recorded as an intimate
mixture with KBr.
Table 4. Selected FTIR Absorption Bands (ν,cm
-1
) of Bisurea
Gelators in Crystalline, Gel, and Solution States
a,b
bisurea state NH (stretch) amide I amide II
u4 solution in MeOH
ce
1656 1575
u12 solution in MeOH
ce
1652 1574
u6 crystal
d
3331 1633 1603
u6 decalin
c
3323 1632 1601
u18 decalin
c
3327 1631 1595
u4 DMSO
c
3332 1630 1604
u18 DMSO
c
3337 1629 1611
u10 1-propanol
d
3324 1633 1591
u18 1-propanol
d
3329 1633 1590
a
For a complete overview. see Supporting Information.
b
Uncer-
tainty ((2cm
-1
).
c
Recorded in a CaF
2
cell with 0.1 mm spacing.
d
Recorded as an intimate mixture with KBr.
e
Region blocked by solvent
absorption bands.
Figure 3.
Gel-sol phase-transition curves of bisamide gelators
a2-a17 in decalin.
(39) Hirst, A. R.; Coates, I. A.; Boucheteau, T. R.; Miravet, J,F.; Escuder, B.;
Castalletto, V.; Hamley, I. W.; Smith, D. K. J. Am. Chem. Soc. 2008, 130, 9113–
9121.
(40) Schaber, P. M.; Colson, J.; Higgins, S.; Thielen, D.; Anspach, B; Brauer, J.
Thermochem. Acta 2004, 424, 131–142.
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Page 5
DOI: 10.1021/la9004714 8807Langmuir 2009, 25(15), 8802–8809
Zweep et al. Article
confirm that in apolar solvents the thermal stability of the bisurea
gels is higher than that of the bisamide gels. Most likely, the
introduction of extra hydrogen bonding units leads to stronger
gelator-gelator interactions within the fibers, together with less-
favorable solvation in apolar solvents.
The gel-sol phase-transition curves of bisamides a7-a17 and
bisurea gelators u4-u18 in DMSO are displayed in Figure 4. The
T
gs
of the gels shows an opposite trend to that observed in decalin.
The melting temperatures (T
gs
values) of the gels increase with
increasing length of the alkyl tail of the gelator. The only
exception to this trend is the melting temperature of u4,which
is higher than that of u6.TheT
gs
values of the gels formed by a15
and a17 as well as u16 and u18 are essentially identical and show
that the elongation of the alkyl substituents has a limited effect.
The gel-sol phase-transition curves of bisamide gelators a15
and a17 and bisurea gelators u10-u18 in 1-propanol are shown in
Figure 5. The other compounds examined do not form stable gels
in this solvent (Tables 1 and 2). In 1-propanol, as was observed for
DMSO, the stability of the gels increases with an increase in the
length of the alkyl tail. The bisurea gelators behave in a more
complex manner compared with the bisamide gelators. Com-
pounds u10 and u12 have similar T
gs
values; however, the thermal
stability increases with the elongation of the alkyl chain, as
observed for u16 and u18. In both of these polar solvents, the
increase in the thermal stability with elongation of the alkyl
substituents is most likely due to increasing van der Waals
interactions between gelator molecules together with less-favor-
able solvation in polar solvents.
More quantitative information regarding the melting of the
gels was obtained from DSC measurements on native gels. In the
heating traces, most of the bisurea gels exhibit multiple transitions
in all three solvents investigated in this work (SOI). These multiple
transitions are due to thermotropic polymorphism
41
as has been
reported before for u12.
9
Because of this thermotropic poly-
morphism, a direct correlation between the DSC phase-transition
temperatures and T
gs
obtained from the dropping ball experi-
ments was not observed. Nevertheless, the melting enthalpy
(ΔH
m
) of the gels can be obtained from the combined areas of
the different phase transitions in the DSC traces. Figure 6 shows
the melting enthalpies for the gels in the three investigated
solvents as a function of the length of the alkyl chain (Figure 6).
From these data, it can be concluded that for both the bisamide
and bisurea gelators the ΔH
m
of the gels increases with increasing
molecular mass (i.e., increasing alkyl chain length). Furthermore,
the ΔH
m
of the gels of bisamide gelators in the polar solvents
examined are much higher compared to those in apolar solvent
decalin (5-25 kJ mol
-1
, see also Supporting Information). Also,
Figure 4.
Gel-sol phase-transition curves of gel formed from
bisamide gelators a7-a17 and bisurea gelators u4-u18 in DMSO.
Figure 5.
Gel-sol phase-transition curves of the gel formed from
bisamide gelators a15-a17 and bisurea gelators u10-u18 in 1-
propanol.
(41) The thermotropic behavior of the bisurea gelators will be reported
separately. Zweep, N.; Browne, , W. R.; Feringa, , B. L.; van Esch, , J. H.,
manuscript in preparation.
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Page 6
8808 DOI: 10.1021/la9004714 Langmuir 2009, 25(15), 8802–8809
Article Zweep et al.
in DMSO the melting enthalpies of the gels are higher compared
to those in 1-propanol. The high melting temperatures of bisurea
gelators in apolar solvents precluded the determination of their
melting enthalpies. The low ΔH
m
of the gels of bisamide gelators
in decalin compared to ΔH
m
of gels formed in the polar solvents
examined is unexpected because one would expect that the
hydrogen-bonding interactions in apolar solvents would be
stronger. This points to a much lower relative contribution of
the van der Waals interaction by the alkyl chain as a result of
favorable solvation in apolar solvents.
In the plots of melting enthalpy against the length of the alkyl
tail, a striking effect is observed; the melting enthalpy of the
gelators is related linearly to the length of the alkyl tail. By fitting a
linear function to the melting enthalpy, the increase in stability per
increase in methylene unit can be determined (Supporting In-
formation). In decalin, it was found that, for the bisamide, it
increases by 2.2 kJ mol
-1
((0.1) per methylene unit. In the polar
solvents examined, the alkyl tail provides the same contribution to
the stability in both the bisamide and bisurea gelator systems,
albeit with the contribution differing depending on the solvent: ca.
3.6 kJ mol
-1
in DMSO and ca. 4.2 kJ mol
-1
in 1-propanol. This
indicates that the relative difference in the strength of the inter-
gelator van der Waals interactions provided by the alkyl tails and
the solvent-gelator van der Waals interactions is more pro-
nounced in more polar solvents.
In 1-propanol and DMSO, the x-axis intercept of the linear fit
provides an estimate of the minimum length of the alkyl tail
required for aggregation in these solvents (i.e., n = 6 for
1-propanol and n = 2 for DMSO). These lengths are in good
agreement with the results of the gelation experiments (Tables 1
and 2). Also, by extrapolation a more negative melting enthalpy
for the core is found in 1-propanol compared to that in DMSO.
For the bisamide core, it is found to be -52.6 kJ mol
-1
in
1-propanol and -16.1 kJ mol
-1
in DMSO. For the bisurea core, it
is found to be -46.3 kJ mol
-1
in 1-propanol and -12.7 kJ mol
-1
in DMSO. This indicates that in 1-propanol the core is more
soluble than in DMSO, most likely because 1-propanol acts as a
hydrogen bond donor and acceptor whereas DMSO is only a
hydrogen bond acceptor.
42
Furthermore, from these data the
difference in stability between bisamide and bisurea gelators
in these solvents is obtained, 6.3 kJ mol
-1
in 1-propanol and
3.4 kJ mol
-1
in DMSO in favor of the bisurea gelator. This shows
that the ability of the urea group to form four hydrogen bonds
compared to the two of the amide group provides for better
thermal stability.
In decalin, bisamide gelators a2-a5 do not hold the linear trend
found in melting enthalpy for bisamide gelators. This suggest that
the contribution of the alkyl chains to the stability of the gel via
van der Waals interactions requires a length longer than n =6.
Most likely, below n = 6 the alkyl chains do not adopt an all-trans
configuration in the assembled state as seen in the X-ray structure
of a3 and previously for lipids.
43
The comparable melting
enthalpies of these gelators (ca. 16 kJ mol
-1
) shows that the
stability of the gels is governed solely by the bisamide core.
Discussion
For the gelation of solvents by low-molecular-weight molecules
to occur, a combination of intermolecular interactions, which
results in the anisotropic growth of supramolecular structures, is
required. The specific contribution of each type of interaction as
well as the influence of the solvent itself on gelation is still not fully
understood, however.
3,4,17
In the present report, the gelation
behavior of a series of bisamide- and bisurea-based low-molecu-
lar-weight gelators, built into a transoid cyclohexyl-diamine core
with systematic variation in the length of the two linear alkyl
moieties, is examined in a range of apolar and polar solvents in an
attempt to understand the relative importance of the several
contributions to anisotropic fiber growth and stability.
Figure 6.
ΔH
m
of the gel formed from the bisurea (9) and bisamide (2) gelators in decalin, DMSO, and 1-propanol plotted against the length
of their alkyl tails. The dashed line represents a linear fit.
(42) Marcus, Y. The Properties of Solvents; Wiley: Chichester, U.K., 1998; pp
133-202, ISBN 0-471-98369-1.
(43) Oakes, R. E.; Beattie, J. R.; Moss, B.; Bell, S. E. J. J. Mol. Struct. 2002, 586,
91–110.
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Page 7
DOI: 10.1021/la9004714 8809Langmuir 2009, 25(15), 8802–8809
Zweep et al. Article
The combination of single-crystal X-ray analysis, dropping
ball, DSC, and FTIR spectroscopy indicates that the relative
contributions of the van der Waals and hydrogen-bonding inter-
actions to gelation by the bisamide and bisurea LMWG are
similar but not equal. The influence of solvent on the gelation
behavior is apparent from the cgc values of the different gelators.
It is tempting to correlate gelation properties such as the cgc
values of these organogelators with known solvent parameters
(e.g., solvent acidity and basicity). However, such a correlation
with potentially relevant solvent parameters could not be identi-
fied, in contrast to other classes of low-molecular-weight gela-
tors.
44
Most likely, this is due to the presence of two distinct motifs
for anisotropic aggregation (i.e., hydrogen bonding and van der
Waals interactions). The relative contribution of each motif
change depends on the solvents employed; however, the two
aggregation driving interactions are essentially independent of
each other.
In comparison to the bisamide-based organogelators, the
increased hydrogen-bonding ability in the bisurea gelator system
reduces the solubility of the compounds in apolar solvents. The
hydrogen-bonding ability is presumed to lead to too rapid an
aggregation of the molecules in solution to allow for the forma-
tion of the 3D networks required for gelation. This effect is most
pronounced for compound u4, where crystallization rather than
gelation occurred in some of the apolar solvents examined. The
increased gelation ability of the bisurea compounds in polar
solvents shows that the contribution from the additional hydro-
gen bonds to gelator-gelator interactions in the gel fibers is more
important than the solvation of these polar groups in polar
solvents.
In polar solvents, however, van der Waals interactions pro-
vided by the alkyl chains are essential because gelation occurs only
above a critical length of the alkyl chain formation in order to
compensate for the favorable solvation of the polar bisamide and
bisurea moieties. Elongation of the alkyl tail of the gelators leads
to the further improvement of gelation properties in both apolar
and polar solvents, albeit for different reasons. In polar solvents,
the cgc, T
gs
and ΔH
m
values improve with increasing alkyl chain
length, which is most likely due to both an increase in gelator-
gelator interactions together with decreasing interactions of the
apolar chains with polar solvent molecules. In apolar solvents,
however, we observed an improvement in the cgc and ΔH
m
but a
decrease in the T
gs
with increasing alkyl chain length. Most likely,
the increase in gelator-gelator interactions with increasing alkyl
chain length is compensated for by the increasingly favorable
solvation of these apolar moieties in apolar solvents. These
conclusions are consistent with the lower slopes of the ΔH
m
alkyl
chain length regressions in apolar decalin compared to those in
polar DMSO and 1-propanol.
It is apparent that an increase in anisotropy through an
increase in the length of the alkyl tail does not result in an
improvement in the thermal stability of the gels of the apolar
solvent (i.e., the melting enthalpy increased only marginally with
increasing alkyl tail length). An increase in the hydrogen bonding
strength, through replacement of the bisamide with a bisurea
moiety, resulted in a substantial increase in the melting tempera-
tures. In polar solvents, however, the increase in thermal stability
and melting enthalpy with an increase in the length of the alkyl tail
was dominant with the effect of variation of the hydrogen
bonding unit being only marginal.
Conclusions
In the present study, the solvent dependence of the gelation
properties of two series of bifunctional organogelators is de-
scribed. We have demonstrated that for organogel-forming
compounds in polar solvents van der Waals interactions play a
dominant role whereas in apolar solvents hydrogen-bonding
interactions dominate. The results demonstrate the key point that
in understanding the dependence of gelation strength on both
solvent properties and the anisotropy of intermolecular interac-
tions it is essential to recognize that the primary interaction
driving the assembly of the gel fiber is itself solvent-dependent.
Hence, the development of design rules for the construction of
LMWG molecules incorporating more than one anisotropic
growth element must take into consideration the role of the
solvent in determining which of the contributions is dominant.
The corollary of this conclusion is that the direct relation of
gelation strength to changes in solvent properties is not possible
and more complex relationships should be considered.
Acknowledgment.
This research was supported by the Dutch
Foundation for Scientific Research (NWO) in the Softlink
program. We thank Dr. M. White and Dr. K. Franklin of
Unilever Research, Port Sunlight, for discussion and Unilever
Research, Port Sunlight, for financial support.
Supporting Information Available: Details of syntheses
and experimental details, DSC and FTIR spectroscopic data,
and single-crystal X-ray data for a3. This material is available
free of charge via the Internet at http://pubs.acs.org.
(44) Hirst, A. R.; Smith, D. K. Langmuir 2004, 20, 10851–10857.
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Page 8
  • Source
    [Show abstract] [Hide abstract] ABSTRACT: The recent application of supramolecular principles to the design of molecular gelators has led to an enormous variety of new gelators and functional gels but has contributed relatively little to the understanding of molecular gelation phenomena. How do we progress from here?
    Full-text · Article · Jul 2009 · Langmuir
  • Source
    [Show abstract] [Hide abstract] ABSTRACT: Bis(amino acid)- and bis(amino alcohol)oxalamide gelators represent the class of versatile gelators whose gelation ability is a consequence of strong and directional intermolecular hydrogen bonding provided by oxalamide units and lack of molecular symmetry due to the presence of two chiral centres. Bis(amino acid)oxalamides exhibit ambidextrous gelation properties, being capable to form gels with apolar and also highly polar solvent systems and tend to organise into bilayers or inverse bilayers in hydrogel or organic solvent gel assemblies, respectively. (1)H NMR and FTIR studies of gels revealed the importance of the equilibrium between the assembled network and smaller dissolved gelator assemblies. The organisation in gel assemblies deduced from spectroscopic structural studies are in certain cases closely related to organisations found in the crystal structures of selected gelators, confirming similar organisations in gel assemblies and in the solid state. The pure enantiomer/racemate gelation controversy is addressed and the evidence provided that rac-16 forms a stable toluene gel due to resolution into enantiomeric bilayers, which then interact giving gel fibres and a network of different morphology compared to its (S,S)-enantiomer gel. The TEM investigation of both gels confirmed distinctly different gel morphologies, which allowed the relationship between the stereochemical form of the gelator, the fibre and the network morphology and the network solvent immobilisation capacity to be proposed. Mixing of the constitutionally different bis(amino acid) and bis(amino alcohol)oxalamide gelators resulted in some cases in highly improved gelation efficiency denoted as synergic gelation effect (SGE), being highly dependent also on the stereochemistry of the component gelators. Examples of photo-induced gelation based on closely related bis(amino acid)-maleic acid amide and -fumaramide and stilbene derived oxalamides where gels form by irradiation of the solution of a non-gelling isomer and its photo-isomerisation into gelling isomer are provided, as well as examples of luminescent gels, gel-based fluoride sensors, LC-gels and nanoparticle-hydrogel composites.
    Full-text · Article · Jan 2010 · Chemical Communications
  • Source
    [Show abstract] [Hide abstract] ABSTRACT: Low molecular weight gelator molecules consisting of aliphatic acid, amino acid (phenylglycine), and omega-aminoaliphatic acid units have been designed. By varying the number of methylene units in the aliphatic and omega-aminoaliphatic acid chains, as defined by descriptors m and n, respectively, a series of positionally isomeric gelators having different positions of the peptidic hydrogen-bonding unit within the gelator molecule has been obtained. The gelation properties of the positional isomers have been determined in relation to a defined set of twenty solvents of different structure and polarity and analyzed in terms of gelator versatility (G(ver)) and effectiveness (G(eff)). The results of gelation tests have shown that simple synthetic optimizations of a "lead gelator molecule" by variation of m and n, end-group polarity (carboxylic acid versus sodium carboxylate), and stereochemistry (racemate versus optically pure form) allowed the identification of gelators with tremendously improved versatility (G(ver)) and effectiveness (G(eff)). Dramatic differences in G(eff) values of up to 70 times could be observed between pure racemate/enantiomer pairs of some gelators, which were manifested even in the gelation of very similar solvents such as isomeric xylenes. The combined results of spectroscopic ((1)H NMR, FTIR), electron microscopy (TEM), and X-ray diffraction studies suggest similar organization of the positionally isomeric gelators at the molecular level, comprising parallel beta-sheet hydrogen-bonded primary assemblies that form inversed bilayers at a higher organizational level. Differential scanning calorimetry (DSC) studies of selected enantiomer/racemate gelator pairs and their o- and p-xylene gels revealed the simultaneous presence of different polymorphs in the racemate gels. The increased gelation effectiveness of the racemate compared to that of the single enantiomer is most likely a consequence of its spontaneous resolution into enantiomeric bilayers and their subsequent organization into polymorphic aggregates of different energy. The latter determine the gel fiber thickness and solvent immobilization capacity of the formed gel network.
    Full-text · Article · Mar 2010 · Chemistry - A European Journal
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