C3-symmetrical supramolecular architectures: Fibers and organic gels from discotic trisamides and trisureas

Article abstract of DOI:10.1021/ja020984n

Hydrogen bonded C₃-symmetrical molecules that associate into supramolecular stacks are described. Structural mutation on these molecules has been performed to elucidate the contribution of the different secondary interactions (hydrogen bonding,

Full text of DOI:10.1021/ja020984n

Published on Web 11/16/2002
C3-Symmetrical Supramolecular Architectures: Fibers and
Organic Gels from Discotic Trisamides and Trisureas
Judith J. van Gorp, Jef A. J. M. Vekemans, and E. W. Meijer*
Contribution from the Laboratory of Macromolecular and Organic Chemistry,
EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB, EindhoVen, The Netherlands
Received July 18, 2002
Abstract: Hydrogen bonded C3-symmetrical molecules that associate into supramolecular stacks are
described. Structural mutation on these molecules has been performed to elucidate the contribution of the
different secondary interactions (hydrogen bonding, π-π stacking) to the self-assembly of the disks into
chiral stacks. Twelve C3-symmetrical molecules have been investigated, six of which contain three central
amide functionalities (1a-f) and six of which contain three central urea groups (2a-f). Peripheral groups
of the disks are “small”, “medium”, or “large”, half of them being achiral and the other half being chiral, to
enable investigation of the supramolecular architectures with CD spectroscopy. In all of the cases, elongated,
helical stacks are formed in apolar solution, except for the “medium” amide disks 1c/d. The elongated
stacks of the C3-symmetrical disks form gels, which are visualized by AFM and SANS, and this confirms
the directionality of the interactions. For the “large” urea disk, 2f, fibers with a length of up to 2 µm are
observed. Temperature dependent and “sergeants-and-soldiers” CD measurements reveal that the urea
stacks are much more rigid than the corresponding amide ones. In case of the “medium” urea disks, 2c/d,
a true rigid rod, is formed. Where amide disks immediately reach their thermodynamic equilibrium, kinetic
factors seem to govern urea aggregation. In a number of experiments aimed at reversibility with the urea
stacks, hysteresis is observed, implying that these urea disks initially form a poorly defined stack, which
subsequently transforms slowly into a well-defined, chiral architecture.
Introduction
factors (concentration, temperature, solvent, and stoichiometry)
can prevent gel formation.6 Irreversible precipitation of fibers
Self-assembly may open new possibilities in both the fields
of biology and materials science because it enables the rapid
formation of nanosized, complex architectures, adopting stable
and compact conformations, despite their noncovalent, reversible
nature.1a-g A vast number of these structures are known, which
contain strong (self-)complementary and unidirectional inter-
molecular interactions to enforce one-, two-, or three-dimen-
sional self-assembled architectures.2a-j The self-assembled
systems have been applied in so-called organogels,3a-n for
example, those of the well-known cyanuric acid/melamine
motifs.2a-e,3a-c In an organogel, the liquid is prevented from
flowing by a continuous, three-dimensional, entangled network
of fibers of low molecular weight organogelators,4a-b held
together solely by noncovalent forces, including hydrogen
bonding, π-π stacking, and solvophobic interactions. Much
effort is being put into the design of new organogelators,5a-e
but even when the basic requirements of gel formation are met
(supramolecular aggregation and cross-link formation), external
is induced, similar to the irreversible aggregation of proteins in
case of neuro-degenerative diseases, such as Alzheimer’s and
Creutzfeldt-Jakob’s disease.7a-b To increase insight into the
supramolecular aggregation phenomena, the exact nature and
relative importance of the secondary interactions were studied.
For this purpose, discotic molecules were selected as highly
suitable building blocks for the formation of cylindrically shaped
fibers.8 Both solvophobic effects and π-π stacking,9a-c as well
as hydrogen bonding,9d,e are used to assemble the disklike
entities. Remarkably, hydrogen bonding is also used to preor-
ganize the core of the disk,9f,g or even to form the disk by
bringing together multiple precursors.9h-l Introduction of chiral-
ity in the discotics may lead to helical fibers.9m-o Next to the
(2) Some examples of self-assembled architectures: cyanuric acid/melamine
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P.; Bonazzi, S.; Garbesi, A.; Ciuchi, F.; Mariani, P. Biopolymers 1997, 42,
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* To whom correspondence should be addressed. E-mail: E.W.Meijer@
tue.nl.
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Hughes, T. S.; Moore, J. S. Chem. ReV. 2001, 101, 3893-4011. (c)
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9
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J. AM. CHEM. SOC. 2002, 124, 14759-14769
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A R T I C L E S
van Gorp et al.
Figure 1. First bypiridine based C3-symmetrical disks and a cartoon representing their helical supramolecular stacking.
possibility of creating cholesteric and ferrroelectric liquid
crystals, chirality was recognized as a new tool in studying the
self-assembly of the disks. The discotic structures cover a broad
range of rotational symmetries (e. g., ranging from C2 to C6),
but from an esthetic and practical point of view, C3-symmetrical
compounds were found to be highly attractive,9e,g,k,l,p-r especially
those containing a 1,3,5-benzenetricarboxamide unit.10a-c,11
Recently, we reported on various C3-symmetrical columnar
supramolecular architectures relying on hydrogen bonding in
combination with π-π stacking.11-13 The first representatives
of this class of C3-symmetrical self-assembled structures were
built up convergently by the stepwise acylation of 2,2′-
bipyridine-3,3′-diamine (Figure 1).12a-d The bipyridinyl parts
are planar and preorganized due to their intramolecular hydrogen
bonds; the gallic moieties are equipped with alkyl groups,
inducing phase separation, whereas the assemblies feature a
3-fold array of intermolecular hydrogen bonds. Order is not only
observed in the solid or liquid crystalline phase,12a the lyotropic
organic gel phase,12b but also in dilute solution.12c Chirality in
the solubilizing side chain of the constituting molecules allowed
investigation of the three-dimensional structure of the aggregates
using CD spectroscopy. The self-assembled stacks undergo a
huge amplification of chirality,14 with 1 chiral molecule capable
of organizing as many as 80 achiral ones in either a right- or a
left-handed helical stack in dilute solution. Although the
cooperativity is high, the aggregation (K ) 108 l/mol) is still
completely reversible without displaying any hysteresis. How-
ever, enlargement of the central aromatic core with methoxy
groups or pyrazine rings gives stronger but less ordered π-π
stacks.12d
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C -Symmetrical Supramolecular Architectures
3
A R T I C L E S
Figure 2. Overview of twelve C3-symmetrical disks, containing central amides (1a-f) or ureas (2a-f). The peripheral groups of the disks are “small” (1a/b
and 2a/b), “medium” (1c/d and 2c/d) or “large” (1e/f and 2e/f). Half of the disks are achiral (1a/c/e and 2a/c/e); the other half is chiral (1b/d/f and 2b/d/f).
To what extent both intermolecular hydrogen bonding and
π-π interactions contribute to the cooperative, helical self-
assembly is an intriguing question. That both secondary interac-
tions play a role was elucidated by the stepwise growth of
analogous disks equipped with oligo(ethylene oxide) tails into
chiral columns in polar media.13 First, achiral, small aggregates
are generated by the hydrophobicity of the core. Second, the
expression of the more sensitive hydrogen bonding interactions
gives rise to large, chiral, self-assembled structures. In apolar
media, with hydrophobic peripheral tails, such a stepwise
aggregation does not occur and both secondary interactions are
proposed coming to expression simultaneously.
symmetrical disks. Previously, the supramolecular properties of
1a/b10a-c, 11 and 1e/f12a-d have been described, all of the other
members of the library are novel.
Results
Synthesis. Trisamides 1a-f are formed by 3-fold reaction
of trimesyl chloride with amines 3a-f, of which the chiral ones
3b,15 3d,16 and 3f12a-d are based on the (S)-3,7-dimethyloctyl
unit (Scheme 1). The syntheses of compounds 1a/b and 1e/f
have been described before.11,12a-d Trisamides 1c and 1d are
obtained in reasonable yield after column chromatography (86%
and 70% for 1c and 1d, respectively). The trisureas 2a-f are
obtained by 3-fold reaction of amines 3a-f with 1,3,5-
benzenetriisocyanate (5). The latter is synthesized in 90% yield
by treatment of trimesyl chloride with sodium azide, and
subsequent thermolysis of trimesyl azide (4)17 to induce the
Curtius rearrangement.18 The urea disks are obtained in satisfac-
To unravel the features governing self-assembly in apolar
media, several π-π interacting groups and hydrogen bonding
units have been combined to afford a series of C3-symmetrical
disks (Figure 2). The π-π interactions were increased by
replacing “small” alkyl tails (1a/b, 2a/b) by “medium” gallic
groups (1c/d, 2c/d) or “large” bipyridinyl-gallic groups (1e/f,
2e/f). The intermolecular hydrogen bonding amide moieties in
1a-f were substituted by urea ones in 2a-f. Because a urea
has two protons to form hydrogen bonds, whereas an amide
has only one, the urea bond is considered stronger and more
rigid. In this paper, we compare the behavior of twelve C3-
(15) Fontana, M.; Chanzy, H.; Caseri, W. R.; Smith, P.; Schenning, A. P. H. J.;
Meijer, E. W.; Gro¨hn, F. Chem. Mater. 2002, 14, 1730-1735.
(16) Pieterse, K. Thesis: Electron deficient materials based on azaheterocycles;
Technische Universiteit Eindhoven: Eindhoven, 2001.
(17) Trimesyl azide (4) is explosive under heat or pressure in the solid state.
(18) Ambade, A. V.; Kumar, A. J. Polym. Sci., Part A: Polym. Chem. 2001,
39, 1295-1304.
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A R T I C L E S
van Gorp et al.
Scheme 1. Synthesis of Disk Shaped Amides 1a-f and Ureas 2a-f
tory yield after washing with the appropriate solvent (yields
Table 1. Transition Temperatures T [°C] and Corresponding
Enthalpies ∆H [kJ/mol] of Disk Shaped Amides 1a-fa
for 2a, 70%; 2b, 68%; 2c, 87%; 2d, 78%; 2e, 75%; and 2f,
1
87%). All of the compounds are characterized with H NMR,
compd
K
T (∆H)
M
T (∆H)
I
13C NMR, and IR spectroscopy, MALDI-TOF mass spectrom-
etry and/or elemental analysis. Details on synthesis and char-
acterization are given in the Supporting Information.
1ab
1bc
1c
•
102 (19)
119 (16)
-4 (44)
96 (1.8)
9 (56)
•
•
•
•
Dho
Dho
204 (17)
236 (21)
178 (27)
113 (15)
355 (27)f
373 (28)f
•
•
•
•
•
•
•
•
1d
•
Behavior of the Pure Compounds. Optical Polarizing
Microscopy and Differential Scanning Calorimetry. All of
the C3-symmetrical amide disks, 1a-f, show liquid crystallinity,
most of them over a broad temperature range (Table 1). Optical
microscopy and DSC of the achiral “medium” compound 1c
shows that the temperature range of liquid crystallinity expands
from -4 °C (44 kJ/mol) to 178 °C (27 kJ/mol). Several phases
are found, indicated by the small ordered/disordered transitions
at 61 °C (4.2 kJ/mol) and 96 °C (2.1 kJ/mol). In case of the
chiral compound 1d a small (<1 kJ/mol) crystalline-crystalline
transition is found at lower temperatures (0 °C). Compound 1d
enters the liquid crystalline phase at 96 °C (1.8 kJ/mol) and
becomes already isotropic at 113 °C (15 kJ/mol). Typical focal
conic structures are found for the amide disks 1a-f, pointing
to a helical columnar structure (Figure 3).
The disk shaped ureas 2a-f are difficult to study in the pure
form, due to degradation of these compounds at temperatures
around 190 °C, which is lower than the isotropization temper-
ature. However, infrared spectroscopy shows that hydrogen
bonding occurs in the ordered (liquid) crystalline phases (Table
2), indicated by the typically low positions of the carbonyl
vibrations.10a/b
1ed
1fd
•
e
-
-
a • ) phase is observed; - ) phase is not observed; K ) crystalline
phase; M ) unidentified mesophase; Dho ) hexagonally ordered columnar
phase; I ) isotropic phase. b Reference 10c. c Reference 11. d Reference
19. e Cooling the sample down to -80 °C did not show any transition.
f The clearing is accompanied by some decomposition of the sample making
the calculated enthalpies less reliable.
in an apolar solvent by heating and subsequent cooling. When
the jar could be inverted without product movement, the
substance was judged a gel. Minimal gel concentrations were
determined by this method (Table 3). “Medium” amide disk 1c
is not able to form a gel in apolar solvents. The trisureas 2
generally need lower minimal concentrations than the corre-
sponding amides 1; the “small” urea compound 2a performs
especially well in this respect.
Macroscopic order of the “large” disk shaped molecules 1e/f
and 2e/f in the gel was proven by their birefringence in optical
microscopy. In birefringent solutions of “large” amide disks 1e
in concentrations varying from 56 mg/mL (2.9 × 10-2 M) to
207 mg/mL (0.11 M) an Nc phase is present. Concentrated
samples (407 mg/mL, 0.21 M) show hexagonal packing of the
columns as well as a helical superstructure, indicating a Dho
phase.19 For the urea disks, both in case of the achiral compound
2e (17 mg/mL, 6.2 × 10-3 M) and chiral compound 2f (57
Gelation. A property inherent to the presence of elongated,
columnar stacks is the possibility of forming macroscopic
organogels. These gels were made by dissolving the compound
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C -Symmetrical Supramolecular Architectures
3
A R T I C L E S
Figure 3. Typical focal conic structure found in the liquid crystalline phase
for achiral “medium” amide disk 1c.
Table 2. Wavenumbers σ [cm-1] of the Carbonyl Stretch
Vibrations of Amides 1a-f and Ureas 2a-f in the Solid State
compd
1aa
1c
1eb
1640
1682
1664
1668
solid state
compd
2a
2c
2e
1632
1641
1670
1623
solid state
a Reference 20. b Reference 19.
Table 3. Minimal Gel Concentrations (concentration [mg/mL];
concentration [M]; solvent) of Amides 1a-f and Ureas 2a-f
amide (1)
urea (2)
“small” (a)
“medium” (c)
“large” (e)
55 mg/mLa
8.8 × 10-2
hexane
1.2 mg/mL
2.0 × 10-3
toluene
174 mg/mL
8.1 × 10-2
heptane
17 mg/mL
6.2 × 10-3
dodecane
M
M
M
M
M
37 mg/mLb
1.4 × 10-2
dodecane
Figure 4. CPK models of “medium” 2d (upper model) and “large” 2f
(lower model) urea disks. The blue triangle represents the aromatic core of
the disks (the area looking gray (carbon), red (oxygen) and blue (nitrogen)
in the CPK models); the orange line represents the radius of this aromatic
core.
a 1b in reference 10b. b Reference 19.
mg/mL, 2.3 × 10-2 M), focal conic structures were found in
urea disks 2d and 2f, a CPK model of the molecule was used
to derive the radius of the aromatic core (Figure 4). A radius of
9.8 Å was calculated for the “medium” urea disk and of 16 Å
for the “large” urea disk. Reasonably, the total cross section of
the molecule is somewhat smaller than twice this radius. On
the other hand, some extra space has to be accounted for the
alkyl tails. In summary, with both AFM and SANS, cross
sections of about 20 Å for “medium” urea 2d and 32-38 Å for
“large” urea 2f should be found.
Atomic Force Microscopy. For the “medium” amide 1d, no
network was found (Figure 5). This was expected, in view of
the absence of a viscous solution. By contrast, the trisurea
molecules 2d and 2f did show gel formation, and a network of
fibers was observed in the AFM scans, indeed. In case of the
“large” urea 2f, the lengths of the fibers are as large as 2 µm.
With an interdisk distance of 3.5 Å, this would correspond to
columns consisting of as much as 5700 molecules!
dodecane, indicative of a columnar phase.
Macroscopic gelation is supposed to be the result of a network
of fibers present in apolar solutions of amides 1a/b and 1e/f or
ureas 2a-f. For the “medium” and “large” molecules 1/2c-f
this network can be visualized by atomic force microscopy
(AFM) and small angle neutron scattering (SANS). With these
techniques, the dimensions of the aggregates can also be verified.
The cross sections of the amide and urea molecules should be
almost equal. For the “large” amides 1f an interdisk distance21
of 3.5 Å and an intercolumn distance of 38 Å were calculated
from X-ray diffraction.19 In case of the “medium” and “large”
(19) Palmans, A. R. A. Thesis: Supramolecular structures based on the
intramolecular H-bonding in the 3,3′-di(acylamino)-2,2′-bipyridine unit;
Technische Universiteit Eindhoven: Eindhoven, 1997.
(20) Brunsveld, L. Thesis: Supramolecular chirality, from molecules to helical
assemblies in polar media; Technische Universiteit Eindhoven: Eindhoven,
2001.
(21) Interdisk distance: distance between two superimposed central aryl units
of two disks in one column; intercolumn distance: lateral distance between
the centers of two coplanar columns.
Formation of the network occurs via physical cross-link
points. At these points, several strands that are lying next to
9
J. AM. CHEM. SOC. VOL. 124, NO. 49, 2002 14763

A R T I C L E S
van Gorp et al.
Figure 5. AFM pictures and height profiles of “medium” amide disk 1d
(top left), “medium” urea disk 2d (top right) and “large” urea disk 2f (bottom
left) as well as a physical cross-link point of “large” urea disk 2f (bottom
right).
Figure 6. SANS data of “medium” urea disk 2d (red line), “large” urea
disk 2f (top black line) and “medium” amide disk 1d (bottom black line).
Table 4. Cross Sections [Å] of “medium” Urea 2d, “large” Urea 2f
and “large” Amide 1f Determined with Different Techniques
intermediate angle area) with a symmetrical side maximum (in
the large angle area) indicates the presence of cylindrical
aggregates. Indeed, this situation applies to 6 mg/mL solutions
of the “medium” and “large” urea disks 2d and 2f in dodecane-
d26, in which case the length of the aggregate is at least 150
nm (Figure 6). At very low scattering vectors, the curve of
“medium” urea 2d positively deviates from linearity. Because
these systems reveal thermo-reversible gel formation, this
deviation must be attributed to the presence of columnar clusters
that represent the physical cross-links in the gel. From the
position of the side maximum, the radius of the cross section
“Rc” can be determined using the formula “Rc ≈ 4.48/qmax”.
The cross section of the “large” urea disk 2f (39 Å) is indeed
larger than that of the “medium” one 2d (33 Å).
Finally, “medium” amide disk 1d is investigated as a
reference (Figure 6). In contrast to the “medium” urea disk 2d
no linear curve with a slope of -1, and no side maximum are
observed. Obviously, in this case, no elongated stacks are
formed.
Infrared Spectroscopy. For “small” and “large” amide disks
1a and 1e, it is obvious from different techniques, such as IR
and NMR spectroscopy, that intermolecular hydrogen bondings
and hence aggregationsremains present in (dilute) apolar
solutions, whereas for “medium” amide disks 1c, this is not
AFM
(height)
AFM
(width)
CPK
SANS
“medium” (2d)
“large” (2f)
20
32
22
32
33
39
42
X-ray
38
“large” (1f)a
28
a Reference 19.
each other split up and continue separately, to rejoin some other
strands later on (Figure 5). In an area where several strands are
aligned in a parallel fashion, the width of one strand can be
estimated by dividing the total width by the number of strands.
In this way, a width of 4.2 nm is calculated (Table 4).
However, the height profile of the structures can also be
considered (Figure 5). For the “medium” amide 1d, the height
of the plateaus (3.1 nm) is constant over the sample, although
no special architecture is formed. The heights of the “medium”
and “large” urea fibers 2d and 2f are 2.2 and 3.2 nm. As
expected, the smaller molecule displays the smallest height.
Small Angle Neutron Scattering. A technique for investi-
gating the shape and dimensions of aggregates in the actual
solution is small angle neutron scattering (SANS). In the plot
of the scattering vector against the scattering intensity, the
combination of a linear curve with a slope of -1 (in the
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C -Symmetrical Supramolecular Architectures
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A R T I C L E S
Figure 7. CD spectra of “small” 2b (left), “medium” 2d (middle) and “large” 2f (right) urea disks (2b: 10-4 M in heptane, 2d: 10-5 M in dodecane, 2f:
10-5 M in heptane).
Table 5. Wavenumbers σ [cm-1] of the Carbonyl Stretch
Vibrations of Amides 1a-f and Ureas 2a-f in 10-4 M Heptane
Solution
Table 6. UV and CD Data of Chiral Amides 1b/d/f and Ureas
2b/d/f; λMax [nm] ) the Absorption Band Most toward the Red and
ꢀ/∆ꢀ [L/Mol.cm] ) the Intensity of the Signala
compd
1aa
1c
1e
UV−Vis
CD
σ
1640
1670
1650
1669
compd
λmax
ꢀ
λmax
∆ꢀ
10-4 M heptane
1bb
1d
<210
308
364
9.2 × 103
2.1 × 104
3.2 × 104
2.3 × 104
224
-25.1
compd
2a
2c
2e
1fc
369
387
-23.6
-35.6
σ
1736
1641
1670
1624
384
10-4 M heptane
UV−Vis
CD
a Reference 20.
compd
λmax
ꢀ
λmax
∆ꢀ
2b
2d
2f
224
256
357
375
7.4 × 103
6.0 × 104
3.1 × 104
2.4 × 104
228
275
371
384
+3.70
-51.5
+52.5
+57.4
the case. IR gave two carbonyl vibrations in both the solid state
(1682 and 1664 cm-1, Table 2) and in heptane solution (1670
and 1650 cm-1, Table 5), indicating the presence of both
hydrogen-bonded and mono-molecular species. On the other
hand, for urea disks 2c and 2e, the positions of the single
carbonyl vibration were similar in the solid state and in 10-4
M heptane solution, indicating hydrogen bonding and aggrega-
tion in solution. “Small” urea 2a is only sparingly soluble in
heptane (10-3 M, 40 °C) and shows a carbonyl vibration that is
significantly higher in heptane than in the solid state (1736 vs
1632, respectively). However, in toluene, a 10-4 M solution
gives a similar NH stretch vibration as that found in the solid
state (3285 vs 3295 cm-1, respectively).
a 1b, 1d and 2f measured in heptane at ∼10-5 M; 1f and 2d measured
in dodecane at ∼10-5 M; 2b measured in heptane at ∼10-4 M. b Reference
11 and 20. c Reference 19.
Table 7. Temperature-Dependent CD Data of Amides 1b/d/f and
Ureas 2b/d/f; T [°C] ) Temperature of Disappearance of the
Cotton Effect; Hysteresis Indicates Whether Kinetic Factors
Influence the System during Sample Preparation and
Measurementsa
compd
1bb
1d
1fc
T
90
no
110
no
hysteresis
UV-Vis and Circular Dichroism Studies. To investigate
the supramolecular chirality of the aggregates, the chiral disks
were studied in dilute, apolar solution with CD spectroscopy.
Where both “small” and “large” chiral amide disks 1b and 1f
show significant Cotton effects, no Cotton effects were found
in 10-4 and 10-5 M solutions of 1d (or mixtures of 1c and 1d)
in heptane. Similarly, the “small” (2b), “medium” (2d) and
“large” (2f) urea disks were investigated (Figure 7). In contrast
to the substantial Cotton effects shown by “medium” and “large”
urea disks 2d and 2f, the effect produced by the “small” urea
2b is only marginal (Table 6). Notable is the sign inversion
between the amide and urea molecules 1f and 2f, despite the
fact that they are based on the same side chain, with the same
(S)-configuration of the (S)-3,7-dimethyloctyl group.
Temperature-Dependent Circular Dichroism Studies. To
better understand the behavior of the C3-symmetrical amide
(1a-f) and urea (2a-f) molecules, temperature-dependent
measurements are performed emphasizing the different maxima
of the Cotton effects. The Cotton effects of “small” amide 1b,
“large” amide 1f and “small” urea 2b gradually decrease upon
raising the temperature until they disappear at approximately
compd
2b
2d
2f
T
100
no
135
yes
80
yes
hysteresis
a,b,c See the footnotes of Table 6.
100 °C (Table 7). These systems also respond immediately to
changes in temperature. Upon cooling, the signal is restored at
once. The “medium” urea 2d shows a different behavior (Figure
8). The structure is retained up to very high temperatures (135
°C). At 120 °C, still one-third of the original Cotton effect is
present, after which the rest is lost in a sharp transition. In case
of the “large” urea disk 2f, an even sharper transition is observed
at 75 °C. The “medium” urea disk 2d shows strong hysteresis.
Upon cooling, it takes 2 days before the original signal is
retrieved, while the hysteresis only takes 1.5 h for 2f.
Comparing and Mixing Compounds. Need for C3-Sym-
metry: Mono-, Di-, and Tri-Urea. From the measurements
described above, we assume that the chiral aggregates of the
urea disks are helical stacks, similar to the amide stacks. In a
helical stack, all three wedges of the molecule are involved in
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J. AM. CHEM. SOC. VOL. 124, NO. 49, 2002 14765

A R T I C L E S
van Gorp et al.
Figure 8. Temperature-dependent CD spectra of “small” 2b (left), “medium” 2d (middle) and “large” 2f (right) urea disks at λmax ) 228, 275 and 295 nm,
respectively (2b: 10-4 M in heptane, 2d: 10-5 M in dodecane, 2f: 10-5 M in heptane).
Figure 9. Chemical structures and CD spectra of “medium” mono-urea 6 (left), di-urea 7 (middle) and tri-urea 2d (right) (10-5 M in heptane for 6 and 7
and in dodecane for 2d).
formation of the supramolecular structure. To prove that this is
indeed the case, we investigated a mono-urea (6) and a di-urea
(7) as reference compounds for the “medium” urea disk 2d.
The CD spectrum of the mono-urea 6 shows no Cotton effect
(Figure 9). The di-urea 7 does show a nice Cotton effect, but it
is much smaller than that of the tri-urea 2d and differs in shape,
sign and maxima.
More differences between the di-urea 7 and tri-urea 2d
become clear when inspecting the temperature-dependent CD
spectra (Figure 10). In the first place, the di-urea (80 °C) is
less stable upon raising temperature than the tri-urea (135 °C).
Furthermore, upon heating, the loss of signal occurs more
gradually, whereas upon cooling, the signal of the di-urea is
restored immediately, in contrast with that of the tri-urea, which
displays a hysteresis of 2 days.
stacking and of the columns of achiral disks being inherently
chiral. The insertion of a seed of chiral disk to equal amounts
of P and M helix of the achiral disks induces a strong preference
of one helicity over the other.
In the “large” ureas 2e/f no “sergeants-and-soldiers” effect
could be observed (Table 8). Several methods of sample
preparation were applied (mixing of two solutions, heating and
cooling of samples, premixing of two solids), but in all cases,
a linear relationship between the intensity of the Cotton effect
and the percentage of chiral molecules was observed. “Medium”
ureas 2c/d did give an amplification of 2 when the sample was
prepared by premixing the two solids in chloroform, evaporating
the solvent and, finally, dissolving the mixed solids in heptane
at 10-5 M (Figure 11).
Upon mixing 10-4 M solutions of achiral “small” urea disk
2a and chiral “small” urea disk 2b a remarkable aggregate
(Figure 11, squares) was formed just above room temperature
(30-40 °C). Cotton effects opposite in sign and with intensities
up to 5 times that of the pure chiral compound 2b were
observed. However, these initial aggregates (Figure 11, squares)
proved not stable in time. After approximately 1 h, the Cotton
“Sergeants-and-Soldiers” Measurements. Both “small” and
“large” amide disks 1a/b and 1e/f show a strong amplification
of chirality, with one chiral molecule capable of organizing as
much as 200 and 80 achiral ones, respectively, in either a right-
or a left-handed helical stack. These results are interpreted as
being the result of both a strong cooperative effect in the
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C -Symmetrical Supramolecular Architectures
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A R T I C L E S
urea), or 1e and 2f (achiral amide and chiral urea), a linear
relationship existed between the percentage of chiral compound
and the intensity of the Cotton effect, implying that also “large”
amide and urea disks are not compatible.
Discussion
From the results above, it is clear that all investigated disks
form helical columnar structures in the solid (liquid crystalline)
phase. Most of these C3-symmetrical supramolecular architec-
tures are retained in gels in apolar solution. For “medium” urea
2d and “large” urea 2f, respectively, the value for the cross
section Rc of the moleculesand hence the supramolecular fibers
found in the AFM height profile (22 and 32 Å, respectively)
corresponds well to the CPK estimate (20 and 32 Å, respec-
tively). The values for the cross section Rc found with SANS,
X-ray or “AFM-width” also correspond (Table 4, 33 and 40 Å,
respectively), but are substantially larger than those found using
AFM height profiles or CPK models. This can be explained
because the latter two techniques are underestimating the space
occupied by the aliphatic tails. In the case of CPK modeling,
this was done consciously by only taking into account the
aromatic core; in case of AFM height profiles, one can imagine
that the AFM tip compresses the flexible outer shell of the fiber.
Strikingly, the difference between the values found with CPK/
AFM-height and AFM-width/X-ray/SANS is 10 Å, which
corresponds to the space taken up by one extended chiral alkyl
chain ((S)-3,4-dimethyloctyl: 8 × 1.25 Å ) 10 Å). X-ray studies
on systems as different as “large” amide 1f19 and sugar appended
organogelators3n, have shown that solvent molecules can widen
the intercolumn distance by taking part in the supramolecular
ordering. But in view of this striking difference of 10 Å, the
influence of the solvent is most likely small in case of “medium”
and “large” ureas 2d and 2f. This is confirmed by the value for
the cross section Rc of 2f found with SANS in the lyotropic
phase (39 Å), that corresponds closely to that of 1f found with
X-ray for the neat compound (38 Å).
Surprisingly, “medium” amide disks 1c/d do not retain their
helical columnar structure in apolar solution, in contast to all
the other amide and urea disks (1a/b-e/f, 2a-f). The small
liquid crystalline phase found for 1d and the good solubility of
1c/d can be indications for this behavior. “Medium” amide disks
with inversed (nitrogen centered) amide groups do not form
chiral aggregates either,22 confirming the behavior found for
“medium” amide disks 1c/d. Generally, it depends on several
factors whether aggregates are formed, among which, the
strength of the hydrogen bonding, the strength of the π-π and
van der Waals interactions and the mutual proportionality of
the two. Apparently, the combination of π-π stacking between
single aromatic rings and hydrogen bonding between amide
groups is not strong enough to form helical stacks. In the “small”
amide disks 1a/b, strong van der Waals interactions probably
account for a crystalline like packing of the alkyl tails. Moreover,
chirality is easily expressed, because the distance of the chiral
center to the chromophore (center of the molecule) is small. In
the “large” amide disks 1e/f strong π-π interactions are present
between the planarized and preorganizing bipyridinyl rings,
which both stabilize the aggregate and cause chirality to be
transferred a long way.
Figure 10. Temperature-dependent CD spectra of aggregates of “medium”
di-urea 7 (upper graph) and tri-urea 2d (lower graph) at λmax ) 264 and
275 nm, respectively (10-5 M in heptane for 7 and in dodecane for 2d).
Table 8. Amplification of Chirality of Amides 1a-f and Ureas
2a-f; Hysteresis Indicates Whether Kinetic Factors Influence the
System during Sample Preparation and Measurementsa
compd
1a/bb
1c/d
1e/fc
amplification of chirality
hysteresis
40
no
40
no
compd
2a/b
2c/d
2e/f
amplification of chirality
hysteresis
5
yes
2
yes
no
a 2a/b: 10-4 M, all others: 10-5 M in heptane. b,c See the footnotes of
Table 6.
effect changed completely (Figure 11, dots). More than 75%
chiral compound gave the Cotton effect of the pure chiral
compound 2b, while less chiral compound gave no substantial
Cotton effect.
Mixing Amides and Ureas. The above findings were
confirmed when amide disks 1a-f were mixed with urea disks
2a-f. If amplification of chirality occurs upon mixing of a chiral
amide with an achiral urea (or an achiral amide with a chiral
urea), this would imply that amide and urea molecules can be
accommodated in one single aggregate. Upon mixing in solution
5-20 mol % of “small” chiral amide 1b with “small” achiral
urea 2a a chiral amplification of 4 was observed (Figure 12).
However, within 1 h, the effect disappeared, and a linear
relationship was observed similar to that found in the remainder
of the concentration range (20-100 mol % 1b). When solutions
of “small” achiral amide 1a and “small” chiral urea 2b were
mixed, no Cotton effect appeared.
Mixing “medium” disks 1d and 2c (chiral amide and achiral
urea) or 1c and 2d (achiral amide and chiral urea) did not give
rise to any Cotton effect, independent of the type of mixing.
Upon mixing “large” disks 1f and 2e (chiral amide and achiral
Strikingly, and in contrast to the “medium” amide disk 1d,
the “medium” urea disk 2d does show supramolecular aggrega-
(22) To be published.
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J. AM. CHEM. SOC. VOL. 124, NO. 49, 2002 14767

A R T I C L E S
van Gorp et al.
Figure 11. “Sergeants-and-soldiers” measurements of “small” 2a/b (left), “medium” 2c/d (middle) and “large” 2e/f (right) urea disks at λmax ) 228, 275
and 295 nm, respectively (2a/b: 10-4 M, 2c/d and 2e/f: 10-5 M in heptane).
configuration. The additional NH group in urea 2f could cause
an “odd-even” effect, inducing a sign inversion with every atom
placed between the chiral center and (a part of) the chromophore.
This odd-even effect has been observed before in the optical
rotation of a series of poly(isocyanides)23a and polythiophenes.23b
Alternatively, the additional NH group in urea 2f could also
slightly alter the conformation of the disk in the stack, inducing
a small change in the pitch of the helix, which could suffice to
change the sign of the Cotton effect produced by the helical
aggregate.
Differences between amide and urea stacks become more
evident when temperature-dependent CD behavior is studied.
The temperature at which the Cotton effect disappears is
significantly higher for “medium” urea 2d (135 °C) than for
“small” and “large” amides 1b (90 °C) and 1f (110 °C),
indicating that the three additonal hydrogen bonding protons
of the urea indeed can strengthen the supramolecular aggregate.
Notably, the “large” urea disk 2f (80 °C) cannot do so, implying
that the aggregate of the “large” disk is less stable upon heating
than that of the “medium” one 2d. This can be rationalized by
the fact that the intermolecular hydrogen bonds of the “large”
urea 2f might be weaker due to competitive intramolecular
hydrogen bonding to the bipyridinyl groups. Furthermore, the
reversible behavior of the amide molecules is in contrast with
the hysteresis of 2 days for urea 2d and 1.5 hours for urea 2f.
The amide molecules reach thermodynamical equilibrium im-
mediately, whereas kinetics intervenes before the urea disks
Figure 12. “Sergeants-and-soldiers” measurements with “small” chiral
reach their thermodynamically controlled aggregation state.
These findings were confirmed by comparing temperature-
dependent measurements of mono-urea 6, di-urea 7 and tri-urea
amide 1b and “small” achiral urea 2a (5 × 10-5 M in heptane). The
schematic representation shows the mixing of “small” disks. Mixing 15%
of chiral amide 1b (red) and 85% of achiral urea 2a (blue) gives aggregates
2d. The one urea group of 6 does not have enough hydrogen
bonding capacity to form chiral superstructures in solution. The
reversible temperature-dependent behavior of di-urea 7 (80 °C)
implies that the aggregate formed by its four hydrogen bonding
protons is less rigid than the typical aggregate formed by tri-
urea 2d in which all six hydrogen-bonding protons are involved.
containing both kinds of disks (red and blue). However, in time the disks
rearrange into two different stacks, containing only chiral amide 1b (red)
or only achiral urea 2a (blue).
tion in solution. Apparently, in this case ‘stronger’ intermo-
lecular hydrogen bonding increases the tendency to form chiral
aggregates. The CD effect produced by “small” urea 2b is only
marginal, also compared to that of “small” amide 1b. This is in
congruence with IR and gelation experiments, which show that
toluene is clearly a better solvent for aggregation of urea 2b
than heptane, probably due to the very low solubility of 2b in
the latter solvent. However, CD measurements have to be
performed in heptane, because of the overlap in absorption of
toluene and “small” urea 2b. The Cotton effect of “large” urea
2f is comparable in intensity and shape to that of “large” amide
1f, but opposite in sign, although both molecules are based on
the same (S)-3,7-dimethyloctyl side chain, with the same (S)-
Knowing that the amide aggregates 1 behave much more
flexibly and reversibly than the rigid urea stacks 2, it is not
surprising that the urea disks do not show the pronounced
“sergeants-and-soldiers” effect, as that found in case of the
amides. These “sergeants-and-soldiers” measurements are able
to show clear differences between “formally” reversible archi-
(23) (a) Amabilino, D. B.; Ramos, E.; Serrano, J.-L.; Sierra, T.; Veciana, J. J.
Am. Chem. Soc. 1998, 120, 9126-9134. (b) Ramos Lermo, E.; Langeveld-
Voss, B. M. W.; Janssen, R. A. J.; Meijer, E. W. Chem. Commun. 1999,
791-792.
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C -Symmetrical Supramolecular Architectures
3
A R T I C L E S
tectures and systems with a nonequilibrium character. Further-
more, “sergeants-and-soldiers” measurements give valuable
information about the compatibility of the constituting compo-
nents (which might be largely different chiral and achiral
molecules19,9o), especially when the (mixing) behavior of the
ureas 2 is hard to study in the solid state (vide supra).
Where stacks of achiral and chiral molecules cannot be
brought to compatibility in case of “large” ureas 2e/f, “medium”
urea disks 2c/d can only be premixed in a good solvent.
However, “small” urea molecules 2a/b do mix in apolar solution,
although, here also, kinetics plays an important role. The mixed
stacks, consisting of achiral and chiral molecules, undergo
microphase separation within an hour, giving aggregates
containing either only chiral or achiral molecules. Therefore,
in a solution containing both 2a and 2b, probably three types
of aggregates are present (Figure 11, circles): aggregates
consisting of chiral compound 2b (giving a small, positive
Cotton effect), of achiral compound 2a (giving no Cotton effect),
and aggregates consisting of both 2a and 2b (giving a negative
Cotton effect), the latter being kinetically favored, the former
two, thermodynamically favored. It is believed that the three
methyl groups in chiral 2b destabilize π-π interactions between
the disks in such a way that no ideal packing, such as the one
in achiral 2a, can be achieved.
Similar results and explanations apply to situations where
“small” chiral amide 1b and achiral urea 2a are brought together
in one solution. Also in this case, an aggregate in which both
chiral and achiral disks are present is not thermodynamically
stable and after microphase separation, two different columnar
stacks are present containing either only chiral amide 1b or
achiral urea 2a (Figure 12).
In the reverse case (mixing “small” chiral urea 2b and achiral
amide 1a) the absence of a Cotton effect is rationalized by de
marginal Cotton effect of pure chiral urea 2b. Because
“medium” amides 1c/d are incapable of forming aggregates,
let alone chiral ones, no amplification of chirality is to be
expected upon mixing “medium” amide disks 1c/d and ureas
2d/c. Also the immiscibility of “large” amides 1e/f and ureas
2f/e, shows that amide and urea molecules are virtually
immiscible. Obviously, a minor structural variation in the
molecule (an extra methyl group or NH group) may dramatically
disturb the subtle interactions between molecules building a
helical columnar aggregate.
of molecules (2 µm for 2f), displaying the enormous direction-
ality of the hydrogen bonding interactions. These strands are
held together by physical cross-link points, giving rise to gels
with minimal gel concentrations as low as 1.2 mg/mL for 2a.
However, choosing a different combination of secondary
interactions may drastically affect the behavior and properties
of the supramolecular aggregates. Substituting amide by urea
results in a replacement of thermodynamics by kinetics. Tem-
perature-dependent CD measurements show that, unlike the
reversible behavior of the amide disks, distinct hysteresis takes
place upon heating and cooling samples of ureas. Furthermore,
the urea compounds do not display overwhelming “sergeants-
and-soldiers” effects. This indicates that the initially formed,
less-defined urea stacks only slowly transform into the well-
defined, thermodynamically most favorable state. Once this
optimal structure is found, the stacks have rigid rod character,
which prevents the molecules from easy rearrangement and
intermixing, as occurs in the amide stacks. In case of “medium”
urea 2d, a true rigid rod is formed, of which the constituting
disks almost have to be destroyed (135 °C), before breaking up
of the aggregates occurs. Changes in the van der Waals and
π-π interactions in the periphery of the molecule may strongly
influence the capacity for aggregation (in case of 1c/d).
However, these influences are still hard to assess, because other
factors, like the solubility and the distance of the chiral center
to the chromophore, are altered simultaneously. The distinct
influence of minor structural variations also accounts for the
immiscibility of the amide and urea compounds, both forming
similar C3-symmetical supramolecular architectures, but in their
own distinct way.
The exciting observation of intermediate stages within the
formation of the urea helical aggregates can be interpreted as
the folding of a supramolecular polymer, using its self-healing
properties to reach or retain its most favorable conformation.1b-c,6
The mobility of the disks in the helical architectures, leading
to continuous reorganization and therefore the possibility of self-
repair, are currently investigated in more detail.
Acknowledgment. We highly appreciate the support from
the Institut fu¨r Werkstoffforschung at the GKSS Forschung-
szentrum in Geesthacht where Ralf Kleppinger performed SANS
measurements. We thank Joost van Dongen and Xianwen Lou
for MALDI-TOF/MS measurements and Henk Eding for
elemental analyses. Furthermore, Daan Wouters is acknowl-
edged for AFM pictures and Jolanda Spiering for the synthesis
of various gallic acid derivatives.
Conclusions
All C3-symmetrical discotics investigated form helical, elon-
gated, columnar aggregates, both in the solid (liquid crystalline)
state and in solution (except for “medium” amide disks 1c/d).
This shows that a broad range of combinations of hydrogen
bonding groups (amide and urea) and peripheral groups (alkyl,
gallic, and bipyridinyl-gallic) is applicable wherein the formation
of helical columns appears. AFM and SANS made it possible
to visualize micrometer long strands consisting of thousands
Supporting Information Available: Experimental details
concerning the techniques used (AFM, SANS), synthetic
procedures and characterization data of all new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA020984N
9
J. AM. CHEM. SOC. VOL. 124, NO. 49, 2002 14769