Structure-function studies of modular aromatics that form molecular organogels

Article abstract of DOI:10.1021/jo071159y

(Figure Presented) Three urea-based aromatics 1-3, each with distinct steric and electronic characteristics, have been synthesized and their ability to undergo hierarchical assembly and gel organic solvents investigated. We have found that compound 1 prom

Full text of DOI:10.1021/jo071159y

Structure-Function Studies of Modular Aromatics That Form
Molecular Organogels
Christopher Baddeley, Zhiqing Yan, Graham King, Patrick M. Woodward, and
Jovica D. Badjic´*
Department of Chemistry, The Ohio State UniVersity, 100 West 18th AVenue, Columbus, Ohio 43210
badjic@chemistry.ohio-state.edu
ReceiVed June 1, 2007
Three urea-based aromatics 1-3, each with distinct steric and electronic characteristics, have been
synthesized and their ability to undergo hierarchical assembly and gel organic solvents investigated. We
have found that compound 1 promotes the sol-gel phase transition in primary alcohols (from CH3OH to
C10H21OH; CGC < 15 mg/mL), while 2 and 3 do not. IR spectroscopy, X-ray powder diffraction (XRPD),
and transmission electron microscopy (TEM) measurements show that 1 organizes into “cylinders” in
primary alcohols, using three-centered hydrogen bonds and π-π aromatic interactions. The cylinders
further organize into pairs through interdigitation of the methyl groups of the adjacent aromatic rings.
Importantly, the lateral packing of the cylinders is enhanced as the bulk solvent polarity increases providing
a mechanism for controlling the material’s morphology via the solvophobic effect. Molecular mechanics
(Amber) and semiempirical (AM1) calculations suggested similar conformational behavior but distinct
electronic properties of 1-3. Thus, π-deficient 2 without the methyl groups and π-rich 3 without aromatic
nitrogen fail to promote the sol-gel phase transition in alcohols due to their inability to undergo effective
hierarchical assembly, which is necessary for the formation of a 3D fibrillar network. In addition, we
have found that the ultrasonication of a supersaturated solution of 1 is necessary for the gelation.
Presumably, a fast exchange of the aggregates of 1 is assisted with sonic waves to allow for the effective
and “error free” assembly wherein an entangled net of fibers capable of encapsulating solvent molecules
is formed.
Introduction
molecular weigh compounds (LMWGs) that can gel organic
solvents.1 The relationships between the structure of such
molecules, their propensity to organize into self-assembled
fibrillar networks (SAFINs), and the rheological (and other
useful) properties of these gels are not fully developed.2 The
fundamental structure-function studies,3 following the prin-
Nowadays, there exists a strong technological and scientific
drive to develop predictive tools for the rational design of low
* Address correspondence to this author. Phone: (614) 247-8342. Fax: (614)
292-1685.
10.1021/jo071159y CCC: $37.00 © 2007 American Chemical Society
Published on Web 08/23/2007
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J. Org. Chem. 2007, 72, 7270-7278

Modular Aromatics That Form Molecular Organogels
FIGURE 1. Chemical structures of 1-3, and energy minimized (molecular mechanics, MM2) longitudinal (a) and lateral (b) assembly of 1.
ciples of self-assembly and molecular engineering,4 can trans-
form the contemporary design of organogels from a somewhat
serendipitous approach to a more rational one. In that vein, the
study presented herein addresses both the structural and
electronic requirements needed for small functional compounds
to effectively assemble5 and trigger the microscopic phase
separation needed to gel organic solvents, including the role of
solvent in such processes.6
Flory, early on,7 referred to gels as materials with continuous
structure and macroscopic appearance persistent on the time
scale of our day-to-day activities, which are viscoelastic and
solid-like below a certain stress limit. For the physical orga-
nogels of interest in this study, LMWGs are known to assemble
into fibrillar 3D networks that in supersaturated solutions (sols)
cause a phase transition to macroscopically immobilize the
solvent molecules via surface tension and capillary forces.1b
Such solid-like materials (gels) are composed largely of the
liquid phase (typically g98%) and a rather small quantity of
the gelator.
(1) (a) DeRossi, D.; Kajiwara, K.; Osada, Y.; Yamauchi, A., Eds. Polymer
Gels. Fundamentals and Biomedical Applications; Plenum: New York,
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R-Y.; Liu, X-Y.; Narayanan, J.; Xiong, J-Y.; Li, J-L. J. Phys. Chem. B
2006, 110, 25797. (h) Weiss, R. G.; Terech P., Eds. Molecular Gels:
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It has been recognized that anisotropic intermolecular interac-
tions are a prerequisite for enhancing the linear aggregation of
LMWGs while attenuating the growth along other dimensions,
which allows for the gel formation.8 On the basis of this
principle, we have designed a series of divalent urea-based
aromatics (Figure 1), all capable of interacting via hydrogen
bonding, and examined their propensity to gel organic solvents.
These molecules were chosen so as to explore the impact of
systematic structural and electronic perturbations on their
propensity to organize and form gels. Intriguingly, we discov-
ered that a delicate balance between both the molecular structure
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J. Org. Chem, Vol. 72, No. 19, 2007 7271

Baddeley et al.
FIGURE 2. (A) Calculated global minimum energy conformers of 1-3, obtained with Monte Carlo conformational search (Amber*). (B) Electrostatic
potential surface maps of energy minimized 1a-3a (AM1, Spartan Software).
and electronic properties of our LMWGs, as well as the solvent
polarity, is important to achieve gelation and control the
morphology of the assembled fibers. We also revealed that
ultrasonication of supersaturated solutions is necessary to
complete the sol-gel transition with our gelators.
of the multivalent monomers for assembly on account of the
multivalency effect12sthe entropic gain is attained through the
multiplication of monovalent into multivalent units. With that
in mind we designed compounds 1-3, each containing two urea
functionalities bridged with an aromatic linker, to create a
situation in which a strong anisotropic ordering can take place
(Figure 1).
Results and Discussion
The longitudinal assembly (Figure 1a) portrays the urea
functional units in 1-3 stacked on top of one another, directing
the chain propagation orthogonally to the plane of the aromatic
scaffolds. In contrast, the lateral assembly (Figure 1b) incor-
porates the self-complementary ureas organizing into a two-
dimensional plane, with a molecule linking four neighboring
molecules together so that the hydrogen-bonding sites are
entirely exploited. This arrangement disqualifies the aromatic
stacking and is entropically more expensive. In that regard, the
morphology of the supramolecular array is expected to favor
the longitudinal arrangement. Even so, the nature of the
surrounding solvent and the scaffold holding the urea function-
alities can play a determining role for the assembly thermody-
namics and mechanism.
Design Logic, Hypothesis, and Motivation. It has long been
known that molecules carrying urea functionality(ies) organize
into hydrogen-bonded networks.9 Intermolecular three-center
1
hydrogen bonds, encoded as R2 (6),10 are usually formed
between the carbonyl group of one urea molecule and the N-H
groups of the adjacent one. N,N′-Disubstituted ureas have thus
been shown to associate into oligomeric one-dimensional chains
in both solution and the solid state.11 Bouteiller et al. have, in
particular, studied the morphology and cooperativity for the
assembly of low molecular weight bis-ureas forming organogels
in nonpolar solvents.11b-d Typically observed formation of short
chains in solution results from a weak interaction between the
repeating monovalent units. Therefore, increasing the number
of urea functionalities within a single unit5 augments the avidity
Compounds 1-3 incorporate rigid aromatic scaffolds, each
with different steric and electronic properties (Figure 2).
Molecular mechanics calculations have, in the past, been shown
to be important for examining the assembly preference in
medium-sized systems.13 In the case of 1-3, it indeed affords
insight into the assembly pattern (Figure 1). However, hardly
any information is available to develop a sound relationship
between the compounds with slightly different molecular
structures (1-3) and their genuine propensity for ordering and
thereby triggering the sol-to-gel phase transition in liquids. What
are the actual electronic and structural requirements for a
molecule to assemble and gel organic solvents? Supposedly,
the modular architectures of 1-3 (Figure 1) will allow us to
explore this relationship. In continuation, the principles of
(8) (a) van Esch, J.; De Feyter, S.; Kellogg, R. M.; De Schryver, F.;
Feringa, B. L. Chem. Eur. J. 1997, 3, 1238. (b) Van Esch, J.; Schoonbeek,
F.; De Loos, M.; Kooijman, H.; Spek, A. L.; Kellogg, R. M.; Feringa, B.
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Hughes, A. D.; Smith, M. D.; Davis, M. J.; Zhang, B. P.; Zur Loye, H-C.;
Shimizu, K. D. J. Am. Chem. Soc. 2003, 125, 14972. (d) Laliberte, D.;
Maris, T.; Wuest, J. D. Can. J. Chem. 2004, 82, 386.
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(11) (a) Jadzyn, J.; Stockhausen, M.; Zywucki, B. J. Phys. Chem. 1987,
91, 754. (b) Lortie, F.; Boileau, S.; Bouteiller, L. Chem. Eur. J. 2003, 9,
3008. (c) Simic´, V.; Bouteiller, L.; Jalabert, M. J. Am. Chem. Soc. 2003,
125, 13148. (d) Bouteiller, L.; Colombani, O.; Lortie, F.; Terech, P. J. Am.
Chem. Soc. 2005, 127, 8893.
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J. F. Acc. Chem. Res. 2005, 38, 723.
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Modular Aromatics That Form Molecular Organogels
TABLE 1. Minimum Amount of Compound 1 (CGC) Necessary
for Gelation (Compounds 2 and 3 Were Incapable of Hardening the
Alcoholic Solvents) and Dimroth-Reichardt ET(30) Solvent Polarity
Parameters38
SCHEME 1. Synthesis of Compounds 1-3
solvent
CGC (g/L)
ET(30)
methanol
ethanol
15
12
8
N/G
6
N/G
5
6
55.4
51.9
50.7
48.4
49.7
43.3
49.1
48.8
47.2
48.5
48.1
n-propanol
isopropanol
n-butanol
tert-butanol
n-pentanol
n-hexanol
cyclohexanol
n-heptanol
n-octanol
N/G
6
6
template-directed organic synthesis can be applied to engineer
nanoscale structures, comprising 1-3 or related compounds,
with intriguing macroscopic properties.
isooctanol
n-decanol
N/G
6
47.7
Synthesis. The preparation of 1-3 is outlined in Scheme 1.
In a nucleophilic displacement reaction, (bis)chloromethyl aryl
aromatics 4-6 were each reacted with ammonia to give
diamines 7-9. Compounds 4-6 were prepared in accord with
already described literature procedures.14 Following, the con-
densation of diamines 7-9 with ethylisocyanate gave rise to
desired divalent ureas 1-3 in satisfactory high yields
(Scheme 1).
Gelation Studies. At room temperature, compounds 1 and 3
were sparingly soluble in almost any solvent that we tried (<7
mg/mL). Compound 2, however, showed better solubility
behavior, particularly in polar alcoholic solvents (>70 mg/mL).
Upon heating an alcoholic solution of 1 (Table 1), the compound
gradually dissolved and at elevated temperatures (typically at
the solvent boiling point) became completely solubilized.
Interestingly, the subsequent cooling of such prepared solution
did not lead to the gelation, unless sonication was applied
(Figure 3).15 The ultrasound evidently assists the microphase
separation and formation of a 3D-network, which is necessary
for solvent encapsulation. In fact, without ultrasonication,
flocculation, and in some instances precipitation, of 1 was
observed. The collapse of cavitation bubbles created by the
ultrasound can be hypothesized to produce shear forces that
mediate a more effective entanglement of the aggregates of 1,
thus partitioning the solvent molecules. Sound waves, on the
other hand, have been known to stimulate rapid translational
and rotational molecular movements which shorten the lifetime
of noncovalent complexes and promote the disruption of
aggregates.16 Consequently, the observed ultrasound-promoted
phase transition can be qualitatively rationalized by presuming
that the lifetime of the aggregates of 1 is indeed a function of
the ultrasound, under the experimental conditions. Accordingly,
a statistically distributed mixture of aggregates of 1 in the
supersaturated solution (sol) undergoes a more facile intercon-
version of the ensemble members when aided by ultrasonication.
The ultrasound-promoted combinatorial exchange of the ther-
modynamic states in the system17 thus “proofreads” the ag-
gregation, eliminates possible “errors”, and allows for the
FIGURE 3. Photograph showing inverted vials with each containing
the gel phase of compound 1 formed in (a) ethanol, (b) 1-propanol, (c)
1-butanol, (d) 1-pentanol, (e) 1-hexanol, (f) 1-heptanol, and (g)
1-octanol. The gels have been shown to be persistent, without changing
appearance for at least 6 months at ambient temperature.
effective gelation. In the absence of the ultrasonication the
exchange became slow, leading to the “kinetic” aggregation that
results in precipitation or flocculation.
Compounds 2 and 3 would not harden any of the examined
alcohols (Table 1), regardless of the procedure or their initial
concentration. Notably, the electronic and structural information
incorporated in these two molecules, subtly different from that
in 1, mirrored into considerably different behavior at the
macroscopic scale!
A careful inspection of Table 1 shows that only primary
alcohols are gelated with 1, suggesting a possible role of the
solvent molecules in the assembly.6 As 1 incorporates a pyridine
ring, the molecules of primary alcohols can be expected to
hydrogen bond to it by sharing their proton with the pyridine
nitrogen. With that in mind, branching a linear chain in the
alcohol can introduce the van der Waals strain in the assembly
that upsets the packing, lowers the stability, and deters the
formation of aggregates. It is also evident from Table 1 that
compound 1 gels alcoholic solvents of different polarity.
Infrared Spectroscopy (IR): Hydrogen Bonding-Driven
Assembly. The premise of our work was based on the prior
understanding of the aggregation behavior of ureas and their
ability to form three-centered hydrogen bonds. Verifying the
(14) (a) Tsuda, K.; Ikekawa, N.; Mishima, H.; Ilino, A.; Morishage, T.
Pharm. Bull. Jpn. 1953, 1, 122. (b) Mathia, J.; Seto, C.; Simanek, E.;
Whitesides, G. M. J. Am. Chem. Soc. 1994, 116, 1725. (c) Starnes, S. D.;
Rudkevich, D. M.; Rebek, J., Jr. J. Am. Chem. Soc. 2001, 123, 4659. (d)
Collman, J. P.; Decreau, R. A.; Constanzo, S. Org. Lett. 2004, 6, 1033. (e)
Tsuge, A.; Matsubara, A.; Moriguchi, T. Tetrahedron Lett. 2006, 37, 6607.
(15) For an example of the ultrasound promoted sol-gel phase transition,
see: Naota, T.; Koori, H. J. Am. Chem. Soc. 2005, 127, 9324.
(16) (a) Paulusse, J. M. J.; Sijbesma, R. P. Angew. Chem., Int. Ed. 2006,
45, 2334. (b) Paulusse, J. M. J.; Huijbers, J. P. J.; Sijbesma, R. P. Chem.
Eur. J. 2006, 12, 4928.
(17) The concept of dynamic combinatorial chemistry is based on the
fast exchange of various host-guest thermodynamic states. See: (a) Rowan,
S. J.; Cantrill, S. J.; Cousins, G. R. L.; Sanders, J. K. M.; Stoddart, J. F.
Angew. Chem., Int. Ed. 2002, 41, 899. (b) Corbett, P. T.; Leclaire, J.; Vial,
L.; West, K. R.; Wietor, J-L.; Sanders, J. K. M.; Otto, S. Chem. ReV. 2006,
106, 3652.
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Baddeley et al.
(CD3OD) evaporated over time (40 min), the position of the
band remained unchanged. The FT-IR spectrum of a hot
supersaturated CD3OD solution of 1 (Figure 4B) revealed three
major CdO stretching vibrationssa weak and broad vibration
at 1645 cm-1, a strong and sharp vibration at 1604 cm-1, and
a shoulder-like vibration at 1610 cm-1. As the solution cooled
and the flocculation occurred, the relative intensity of the
sharper peak increased while the broad signal remained almost
unaffected.
The xerogel formation (Figure 4A), caused by the solvent
evaporation of the gel sample, had no apparent effect on the
carbonyl stretch frequency, indicating a uniform environment
for this functional group during the phase transition. This points
to the absence of hydrogen bonding between the solvent
molecules and the carbonyl oxygens in 1, in the gel phase. That
is to say, the urea moieties in 1 are likely to be themselves
engaged in forming uniform hydrogen bonds in the gel, to give
rise to the sharp carbonyl stretch at 1604 cm-1 (Figure 4A).
The analogous carbonyl vibration (1604 cm-1) also developed
with the maturation of the supersaturated solution of 1 (Figure
4B). Evidently, the aggregates 1[n] occurred before the mi-
crophase separation was even completed. Interestingly, this
suggests the prospect of the spontaneous nucleation mechanism
operating in the gelation.18 A more detailed study is, however,
necessary to provide information about the assembly mechanism.
The ineffective gel formation, due to the absence of ultrasoni-
cation, is also noticeable (Figure 4B). Namely, the appearance
of (a) a broad 1645 cm-1 signal likely corresponding to the
carbonyl groups in 1 forming weak hydrogen bond(s) with CD3-
OD molecules and (b) a shoulder-like vibration at 1610 cm-1
likely corresponding to the carbonyl group in 1 forming ill-
defined and small aggregates, point to it. The broadness also
implicates the existence of a fraction-weighted mixture of the
conformers of 1 (Figure 2).
When the concentration of 1 was kept below the CGC (CD3-
OD, 7.0 mM), the FT-IR analysis showed two broad bands
centered at 1640 and 1610 cm-1, respectively (Figure 4C). In
accord with the prior discussion, the higher frequency stretch
would correspond to the solvated, and the lower frequency
stretch to the “imperfectly” aggregated molecules of 1.
X-ray Powder Diffraction: Real Space Structure Solution.
Since traditional structure determination techniques, such as
single-crystal X-ray diffraction, do not lend themselves to
organogels we have used X-ray powder diffraction (XRPD) to
gain insight into the molecular packing of 1. We obtained high-
resolution XRPD patterns of the dried xerogel of 1 and applied
real space structure solution methods to determine the molecular
organization in the organogel. The strong well-resolved peaks
observed in the XRPD pattern indicate that the molecular
packing is well ordered. To extract structural information from
the XRPD data, we first indexed the XRPD pattern to determine
the crystal system and unit cell dimensions. Peak positions were
obtained by analytically fitting the peak profiles. These positions
served as the input for the autoindexing programs.19 Autoin-
dexing produced several possible unit cells with good figures
of merit, all possessing triclinic symmetry. A total of five unique
unit cells with distinct unit cell volumes were obtained. The
FIGURE 4. CdO stretch region of infrared spectra of 1 obtained from
(A) a thin film of its gel sample (18.0 mM, CD3OD) deposited on the
transmitting crystal (ATR-IR) and acquired after 0, 5, 15, and 40 min,
at ambient temperature, (B) a 18.0 mM supersaturated solution of 1
(CD3OD) at 70 °C deposited on the transmitting cryatel (ATR-IR) and
acquired after 1, 4, 6, and 12 min at room temperature, and (C) a 7.0
mM CD3OD solution of 1 (below CGC), contained in a long path, 10
mm, liquid cell.
existence of N-HsOdC noncovalent interactions, in addition
to the prospect of obtaining a mechanistic insight into the
assembly of 1, prompted systematic and thorough IR spectro-
scopic studies of the gelation. We, therefore, conducted a
detailed FT-IR study (attenuated total reflectance method) of a
gel sample of 1 in CD3OD (Figure 4A). Also, we obtained IR
spectra of a supersaturated solution of 1 in CD3OD, with real-
time FT-IR “snapshots” of the gelation process (Figure 4B). In
addition, a CD3OD solution 1 below the compound’s critical
gelation concentration (CGC) was examined (Figure 4C). The
FT-IR spectrum of the gel (Figure 4A) revealed a strong and
rather sharp CdO stretching band at 1604 cm-1. As the solvent
(18) Huang, X.; Terech, P.; Raghavan, S. R.; Weiss, R. G. J. Am. Chem.
Soc. 2005, 127, 4336.
(19) Coelho, A. A. Topas Academic, General Profile and Structure
Analysis Software for Powder Diffraction Data; Bruker AXS: Karlsruhe,
Germany, 2004.
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Modular Aromatics That Form Molecular Organogels
FIGURE 5. (A) High-resolution X-ray diffraction image of the xerogel of 1. The material was obtained by drying a gel sample of 1 (22.0 mM,
1-butanol) at 80 °C for 1 h, and the data were collected from a thin film (1 mm) of the material deposited onto a glass substrate. (B) Computer-
generated image of the X-ray diffraction spectrum of 1, obtained via the algorithm of simulated annealing. (C and D) Ball and stick representations
of the solid-state structure of 1 and its packing in the xerogel phase.
small peak at approximately 7.1° 2θ was excluded from the
indexing process since all attempts to include it resulted in
unreasonably large unit cells with poor figures of merit. This
peak is thought to be due to the presence of a small impurity.
Next, real space structure solution was carried out for each
unique unit cell that emerged from the autoindexing process.
This structure solution process was performed by using a
software19-20 that utilizes a simulated annealing algorithm to
find the location, orientation, and torsion angles of the molecule
that give the best agreement between the experimental and
calculated diffraction patterns. Space group P1h was used in all
cases and 30 runs per trail were performed. One unit cell
provided a distinctly superior fit to the data. This unit cell was
also the only one produced by both autoindexing programs
independently. The dimensions of the unit cell were therefore
determined to be a ) 4.4570(11) Å, b ) 11.0920(12) Å, c )
16.9610(19) Å, R ) 95.67(1)°, â ) 88.77(2)°, γ ) 78.37(2)°,
V ) 815.78, Z ) 2.
Initial simulated annealing runs consistently underestimated
the intensity of the hkl peak located at 25.8° 2θ. This peak results
from the overlap of two closely spaced peaks with hkl values
of 0,-3,2 and 0,3,1. This raised the possibility that preferred
orientation effects were present. Preferred orientation terms were
introduced about the 0,-3,2, 0,3,1, and 0,1,0 axes, with the 0,-
3,2 correction resulting in the greatest improvement to the fit.
The value of the preferred orientation term was consistent with
a needle-like crystal habit. This finding is supported by the TEM
studies discussed in the next section. Since for chemical reasons
the urea moiety is suspected of being nearly planar it was
restricted to be so to help reduce the number of degrees of
freedom.
columns of pyridine rings stacked in a slipped mode with an
antiparallel orientation. The distance between the rings is about
4.2 Å, which is slightly longer than is typical for optimal
attractive π-π interactions.22 Apparently, steric interactions
limit the close approach of the planar aromatic rings. The
simulation is much less sensitive to the orientation of the urea
moieties. Several otherwise similar solutions were obtained with
similar figures of merit, but with different orientations of the
urea group. Nevertheless, the solution suggests the possible
formation of bifurcated hydrogen bonds orienting perpendicular
to the planes or the aromatic rings. The simulation did prove
sensitive to the angle between the pyridine ring and the linear
chains with the connecting bond angle being approximately 53°
relative to the plane of the pyridine ring.
A careful inspection of the ordering of 1 also revealed that
the molecular columns further organize in pairs through
interdigitation of the methyl groups of the adjacent aromatic
rings (Figure 5D). This feature of the structure, which could
not be anticipated a priori, denotes another level of the
hierarchical organization. Consequently, the observed inability
of 2 to gel organic solvents can, to a degree, be rationalized by
the absence of the methyl group in its structure.
Due to the possibility of a phase transition and molecular
transposition during the xerogel formation, the presented results
need to be taken with some caution.1g We cannot exclude the
possibility that the molecular assembly could undergo a reor-
ganization during the preparation of a sample for X-ray studies.
In particular, the results of the IR spectroscopic measurements
(Figure 4A) suggest that such reorganization is not likely to
occur.
Transmission Electron Microscopy (TEM). Transmission
electron micrograph images of organogels of 1, with different
alcoholic solvents, display a web of entangled fibers (Figure
The final structure solution is shown in Figure 5. The resulting
crystal structure shows that the molecules adopt a longitudinal
assembly (see Figure 1), as anticipated. The structure contains
(21) David, W. I. F.; Shankland, K.; van de Streek, J.; Pidcock, E.;
Motherwell, W. D. S.; Cole, J. C. J. Appl. Crystallogr. 2006, 39, 910-
915.
(20) David, W. I. F.; Shankland, K.; Shankland, N. Chem. Commun.
1998, 931-932.
J. Org. Chem, Vol. 72, No. 19, 2007 7275

Baddeley et al.
FIGURE 6. Electron microscope images (TEM) of dried gels of 1, prepared in (A) 1-butanol, (B) 1-hexanol, (C) 1-heptanol, and (d) 1-decanol and
deposited on a carbon coated Formvar grid. The diameter of the assembled fibers decreases in the series (from A to D), suggesting a role of the
solvophobic effect in the hierarchical ordering.
SCHEME 2. Conformational Isomers of Symmetrical
N,N′-Disubstituted Ureas, Created by Rotation about Their
(Od)C-N Bonds
6). The high aspect ratio of the fibers indicates highly anisotropic
interactions between the gelator molecules, in agreement with
a one-dimensional, hydrogen bonded, and π-π stacked as-
sembly of 1 (Figure 5). The fibers are several micrometers long,
rod-like in n-butanol while intertwined and bundled in n-decanol
(Figure 6A-D). Interestingly, the diameter of the fibers appears
to be smaller in less polar n-decanol (ET(30) ) 47.7; the
distribution is centered on 50 nm), than in more polar n-butanol
(ET(30) ) 49.7; the distribution is centered on 700 nm). The
thickness is related to the lateral ordering of the hydrogen-
bonded columns of 1, capable of developing van der Walls
attractive interactions. These noncovalent forces, operating at
the periphery of the assembled 1, are weak and multiple,
however, solvophobic in origin. As a result, changing the
environmental polarity should have an effect on the free energy
of the columnar assemblies, via the solvophobic effect.23
Namely, a more polar solvent promotes the lateral column
ordering and makes the fibers thicker (therefore long and
straight). This hypothesis is indeed supported with the experi-
mental results (Figure 6A-D).
structured material is about to be formed. These details have
prompted us to research the conformational and electronic
behavior of N,N′-dialkyl ureas 1-3 (Figure 2).
N,N′-Disubstituted ureas adopt three (or four) conformations
with the rotation of the substituents about the (Od)C-N bonds
(Scheme 2). Calculations at both low and high level of theory,28
IR and NMR spectroscopic measurements,29,30 and X-ray solid-
state data9,10 all indicate that symmetrical dialkylureas prefer-
entially assume trans-trans and cis-trans, but not cis-cis
conformations. The rotation about the (Od)C-N bond in ureas
is hindered, which has been explained by delocalization of the
nitrogen lone-electron pair into the carbonyl group to increase
the (Od)C-N bond order.31 Depending on the level of theory,
the activation barrier for the rotation about the (Od)C-N bond
in variously substituted ureas has been calculated to be in the
range of 7-10 kcal/mol.32 The experimentally determined
barriers are below 11.5 kcal/mol, and also solvent dependent.33
With this in mind, we completed a conformational analysis
of 1-3 using molecular mechanics (Amber)33 and the Monte
Carlo method for stochastic conformational sampling (Figure
2A). As implemented in MacroModel, the results of the
calculated free energy differences suggested “trans” 1a, 2a, or
3a (Figure 2A) as the most favorable conformers for each series
of compounds. The Boltzmann population distribution, however,
Molecular Mechanics. Our molecular mechanics (MM2)24
calculations of the energy minimized octomers of 1 suggested
that two columns pack in the antiparallel orientation with respect
to the aromatic nitrogens. The steric energy for such ordering
is, interestingly, lower than that for other possible arrangements
(see the Supporting Information for more details), which is in
agreement with the refined xerogel structure of 1 (Figure 5).
Preorganization, Steric, and Electronic Effects. The ef-
fective assembly of LMWGs is important for the formation of
a 3D molecular network that encapsulates solvent molecules
and causes gel formation. Molecular recognition, in which
functional molecules organize into ordered assemblies, requires
molecular components to desolvate, conformationally preorga-
nize, and interact using reversible noncovalent interactions.25
As a result, the formation of molecular organogels may be
conditioned with the level of preorganization26 of the consisting
molecular components, particularly if they sample rotational
isomers within a narrow energy window.27 Furthermore, the
height of the energy barrier for the conversion of rotamers is
fundamental for the conformational sampling, which in turn
allows “error checking” and “proof reading” whereby a well-
(28) (a) Kontoyianni, M.; Bowen, J. P. J. Comput. Chem. 1992, 13, 657.
(b) Galabov, B.; Ilieva, S.; Hadjieva, B.; Dudev, T. J. Mol. Struct. 1997,
407, 47-51. (c) Lecomte, F.; Lucas, B.; Gregoire, G.; Schermann, J. P.;
Desfrancois, C. Phys. Chem. Chem. Phys. 2003, 5, 3120. (d) Custelcean,
R.; Gorbunova, M. G.; Bonnesen, P. V. Chem. Eur. J. 2005, 11, 1459.
(29) (a) Rao, C. N. R.; Rao, K. G.; Goel, A.; Balasubramanian, D. J.
Chem. Soc. A 1971, 19, 3077. (b) Mido, Y. Spectrochim. Acta 1973, 29.
(c) Mido, Y. Bull. Chem. Soc. Jpn. 1974, 47, 1833. (d) Mido, Y.; Gohda,
T. Bull. Chem. Soc. Jpn. 1975, 48, 2704.
(30) (a) Filleux-Blanchard, M. L.; Durand, A. Org. Magn. Reson. 1971,
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1971, 27, 4583. (c) Hartman, J. S.; Schrobilgen, G. J. Can. J. Chem. 1973,
51, 99.
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(32) (a) Haushalter, K. A.; Lau, J.; Roberts, J. D. J. Am. Chem. Soc.
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Ferguson, D. M.; Spellmeyer, D. C.; Fox, T.; Caldwell, J. W.; Kollman, P.
A. J. Am. Chem. Soc. 1995, 117, 5179.
(22) (a) Hunter, C. A.; Lawson, K. R.; Perkins, J.; Urch, C. J. J. Chem.
Soc., Perkin Trans. 2 2001, 5, 651. (b) Meyer, E. A.; Castellano, R. K.;
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(23) Feiters, M. C.; Nolte, R. J. M. AdV. Supramol. Chem. 2000, 6, 41.
(24) (a) Allinger, N. L. J. Am. Chem. Soc. 1977, 99, 8127. (b) Allinger,
N. L.; Kok, R. A.; Imam, M. R. J. Comput. Chem. 1988, 9, 591.
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J. Am. Chem. Soc. 1993, 115, 879.
7276 J. Org. Chem., Vol. 72, No. 19, 2007

Modular Aromatics That Form Molecular Organogels
is insignificantly weighted on the side of the “trans” structured
molecules, with considerable amounts of other conformers
created by the rotation about (Od)C-N bonds. On the basis of
their similar thermodynamic stabilities, and prior studies, the
low activation barriers for the interconversion of the conformers
of 1-3 can also be anticipated.
solvent gelation. In the case of more substituted and less solvated
1, the gelation process was not impeded.
Conclusion
The anisotropy of noncovalent interactions is an important
and yet insufficient prerequisite for a compound to gel liquids.
The structural and electronic properties of small functional
molecules, in addition to the solvent characteristics, must be
taken into account for a rational design of soft gel materials.
Our current understanding of modularly architectured 1-3
shows that the electronic and structural features of their aromatic
scaffolds, in addition to their propensity to hydrogen bond
solvent molecules, all need to be finely balanced to achieve the
desired hierarchical assembly and microphase separation. More-
over, the solvent polarity plays an important role in directing
the ordering of molecules via the solvophobic effect. This and
other solvent parameters necessitate a careful consideration if
one is to attempt to engineer a particular morphology in the
soft material. The sol-gel phase transition can, interestingly,
be promoted with ultrasonication. Presumably, a fast exchange
of the assembled aggregates in a supersaturated sol is assisted
with sonic waves to allow for the effective and “error-free”
organization toward formation of a fully developed 3D network
of fibers. The protocol of direct space structure solution of X-ray
powder diffraction data, applied here, has been of great
assistance in revealing the assembly pattern in the xerogel.37 It
is indeed appealing to use this strategy in the future for directly
observing the molecular packing in the intact gel phase.
The electrostatic potential maps of 1-3, obtained by semiem-
pirical (AM1) energy minimizations,34 are shown in Figure 2B.
They reveal comparable and slightly electron-deficient aromatic
surfaces of 1 and 2. Compound 3, however, exhibits a larger
negative electrostatic potential at the center of its aromatic core.
Ambiguities in Developing the Predicting Powers. What
grants compound 1, but not 2 and 3, the ability to gel primary
alcohols? The computed conformational behavior of 1-3 was
shown to be almost identical: all three compounds have many
vibrational degrees of freedom and yet show a certain level of
preorganization (Figure 2A) enabling them to engage in forming
hydrogen bonds and to undergo the longitudinal ordering. The
electronic properties of 1-3 are, however, distinctive (Figure
2B). According to our findings (Figure 5), the ordering of 1
necessitates π-π aromatic interactions (Figure 5). This leads
to a presumption that the assembly of electron-deficient 1 or 2
would be thermodynamically more favorable than that of more
electron-rich 3. In fact, the electrostatic Hunter/Sanders model,35
in combination with Siegel and Waters experimental studies,36
supports the notion of the slipped and stacked π-deficient
aromatics as thermodynamically more stable than the corre-
sponding π-rich compounds.
Along with its electron-rich character, the low solubility of
3 in alcoholic solvents (Table S1, Supporting Information)
furthermore disqualifies this compound as a gelator. The
solubility is likely to result from the absence of the aromatic
nitrogen that presumably allows the formation of hydrogen
bonds with the alcohols. Actually, the same hydrogen bonding
lends 1 and 2 a different solubility behavior (vide supra).
Intriguingly, both 1 and 2 portray almost indistinguishable
conformational and electronic properties, and similar solubility
behavior. Yet, compound 1 incorporates two additional methyl
groups and is structurally different from 2. On the basis of the
X-ray studies of the xerogel of 1 (Figure 5), one of the methyl
groups aids the lateral ordering of the columnar assemblies,
comprised of stacked and hydrogen bonded aromatics. The role
of the methyl is thus prominent in increasing the capacity of
the constituent molecules to engage in the hierarchical assembly
of 1, and thereby allowing the effective packing and recognition.
Compound 2 has no such capacity, and therefore fails in
mediating the gelation. Furthermore, we find that the nature of
the surrounding solvent molecules is also critical for the
assembly: the gelation was only evidenced for 1 but not 2
suspended in primary alcohols (Table 1). The alcohols are
expected to form hydrogen bonds with both 1 and 2. However,
sterically less hindered 2 should have a greater affinity for
interacting with the protic solvents. This higher propensity of
2 for binding alcohols increases its solvation energy, which
likely obstructs the microphase separation and therefore the
It is a part of our research program to additionally modify
the structure of the aromatics examined herein. The objective
is to provide further insight into the mechanism of the gelation,
and at the same time allow the preparation of nanostructured
soft materials with intriguing properties.
Experimental Section
General Procedure for the Synthesis of Compounds 7-9. A
solution of 4-6 (1.47 mmol) in methanol (80.0 mL) in a 150-mL
high-pressure reaction vessel was treated with ammonia gas for
approximately 25 min at room temperature. The vessel was capped,
and the solution was heated at 55 °C for 3 h. The reaction mixture
was left to cool to room temperature for 12 h, and the solvent was
evaporated under reduced pressure to yield a solid residue. The
solid was dissolved in 20 mL of distilled water and the solution
titrated with NaOH (6.0 M in water) until it became basic (pH ∼9).
It was then extracted with dichloromethane (3 × 30 mL), and the
organic layer was subsequently dried with anhydrous Na2SO4,
filtered, and evaporated under reduced pressure to yield pure 7-9.
Data for (2,6-dimethylpyridine-3,5-diyl)dimethanamine (7):
Yield 84%. 1H NMR (CDCl3) δ 7.54 (s, 1H), 3.85 (s, 4H), 2.49 (s,
6H), 1.4 (br s, 4H); 13C NMR (CDCl3) δ 153.4, 135.2, 133.9, 43.1,
21.6; HRMS (ESI) m/z calcd for C9H16N3 166.1344 [M + H]+,
found 166.1347.
Data for pyridine-3,5-diyldimethanamine (8): Yield 56%. 1H
NMR (CDCl3) δ 8.46 (d, 1H), 7.67 (t, 2H), 3.92 (s, 4H), 1.42 (br
s, 4H); 13C NMR (CDCl3) δ 147.7, 138.2, 134.0, 44.0; HRMS (ESI)
m/z calcd for C7H11N3Na 160.0851 [M + Na]+, found 160.0852.
(34) (a) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart. J. J. P.
J. Am. Chem. Soc. 1985, 107, 3902. (b) Kamieth, M.; Kla¨rner, F.-G.;
Diederich, F. Angew. Chem., Int. Ed. Engl. 1998, 37, 3303.
(35) (a) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112,
5525. (b) Cockroft, S. L.; Hunter, C. A.; Lawson, K. R.; Perkins, J.; Urch,
C. J. J. Am. Chem. Soc. 2005, 127, 8594.
(37) (a) Terech, P.; Ostuni, E.; Weiss, R. G. J. Phys. Chem. 1996, 100,
3759. (b) Ostuni, E.; Kamaras, P.; Weiss, R. Angew. Chem., Int. Ed. Engl.
1996, 35, 1324. (c) Sakurai, K.; Jeong, Y.; Koumoto, K.; Friggeri, A.;
Gronwald, O.; Sakurai, S.; Okamoto, S.; Inoue, K.; Shinkai, S. Langmuir
2003, 19, 8211.
(36) (a) Cozzi, F.; Siegel, J. S. Pure Appl. Chem. 1995, 67, 693. (b)
Rashkin, M. J.; Waters, M. L. J. Am. Chem. Soc. 2002, 124, 1860.
(38) Reichardt, C. Chem. ReV. 1994, 94, 2319.
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Baddeley et al.
Data for (4,6-dimethyl-1,3-phenylene)dimethanamine (9):
were obtained with 4 cm-1 resolution. A demountable IR cell
equipped with an attenuated total reflectance ZnSe window was
utilized for obtaining the ATR spectra (Figure 4A/B). A long path,
10 mm, liquid cell with KBr windows was used for obtaining the
spectrum shown in Figure 4C.
1
Yield 75%. Mp 118-119 °C; H NMR (CDCl3) δ 7.24 (s, 1H),
6.97 (s, 1H) 3.83 (s, 4H), 2.29 (s, 6H), 1.42 (br s, 4H); 13C NMR
(CDCl3) δ 153.4, 135.2, 133.9, 43.1, 21.6; HRMS (ESI) m/z calcd
for C10H17N2 165.1391 [M + H]+, found 165.1389.
General Procedure for the Synthesis of Compounds 1-3. To
a solution of 7-9 (1.31 mmol) in dichloromethane (100 mL), at
room temperature and under an atmosphere of argon, was added
dropwise neat ethyl isocyanate (2.88 mmol). The solution was left
to stir for 12 h, during which a solid precipitate appeared. The solid
was filtered, washed with methylene chloride (2 × 10 mL), and
dried in air and then under vacuum for 2 h to yield pure 1-3.
X-ray Powder Diffraction and Simulated Annealing. The
X-ray powder diffraction spectrum of 1 was collected in Bragg-
Brentano geometry on a diffractometer (40 kV, 50 mA, Cu KR1
radiation) equipped with a Ge 111 incident beam monochromator
and a linear position sensitive detector. The data were taken from
3 to 50° 2θ with a step size of 0.007168° 2θ and a dwell time
of 3 s.
Transmission Electron Microscopy. TEM measurements were
conducted with a CM 12 Transmission Electron Microscope
(STEM). A small quantity of gel was placed onto a carbon-coated
TEM grid to form a thin layer. The solvent was removed under
vacuum, and the sample was placed directly into the microscope
without negative staining. Images were recorded at selected areas
of the grid, at magnifications ranging from ×6800 to ×150000.
Molecular Modeling. Molecular modeling (Figure 2A) was
carried out by molecular mechanics calculations. The calculations
were performed by employing the Amber force field as implemented
in the Maestro software. A Monte Carlo conformational search
(torsional sampling 100 000 steps, energy window for saving
structures 20 kJ/mol) generated conformers whose distribution
population was analyzed by the Boltzmann equation. The global
minimum conformers (1a-3a, Figure 2A) were further optimized
by using the semiempirical AM1 method to generate the corre-
sponding electrostatic potential maps.
1
Data for compound 1: Yield 66%. Mp 242 °C dec; H NMR
(CD3SOCD3) δ 7.33 (s, 1H), 6.17 (t, 2H, J ) 5.8 Hz), 5.81 (t, 2H,
J ) 5.8 Hz), 4.11 (d, 4H, J ) 5.5 Hz), 3.02 (dq, 4H, J1 ) 5.8 Hz,
J2 ) 7.0 Hz), 2.37 (s, 6H), 0.99 (t, 6H, J ) 7.3 Hz); 13C NMR
(CDCl3) δ 157.8, 153.0, 134.9, 130.7, 40.2, 34.1, 21.1, 15.7; HRMS
(ESI) m/z calcd for C15H25N5O2Na 330.1906 [M + Na]+, found
330.1902.
Data for compound 2: Yield 64%. Mp 210 °C; 1H NMR
(CDCl3) δ 8.31 (d, 2H, J ) 2.0 Hz), 7.49 (t, 1H), 6.35 (t, 2H, J )
5.8 Hz), 5.90 (t, 2H, J ) 6.2 Hz), 4.19 (d, 4H, J ) 6.0 Hz), 3.02
(dq, 4H, J1 ) 5.6 Hz, J2 ) 7.0 Hz), 0.99 (t, 6H, J ) 7.1 Hz); 13C
NMR (CDCl3) δ 157.9, 146.9, 136.0, 133.6, 40.5, 34.2, 15.6; HRMS
(ESI) m/z calcd for C13H21N5O2Na 302.1593 [M + Na]+, found
302.1592.
1
Data for compound 3: Yield 54%. Mp 215 °C dec; H NMR
(CDCl3) δ 8.31 (s, 1H) 7.49 (s, 1H), 6.01 (t, 2H), 5.75 (t, 2H),
4.11 (d, 4H, J ) 5.6 Hz), 3.02 (dq, 4H, J1 ) 6.0 Hz, J2 ) 7.2 Hz),
2.19 (s, 6H), 0.98 (t, 6H, J ) 7.2 Hz); 13C NMR (CDCl3) δ 157.8,
135.4, 133.9, 131.7, 127.8, 41.0, 34.1, 18.0, 15.7; HRMS (ESI)
m/z calcd for C16H26N4O2Na 329.1953 [M + Na]+, found 329.1952.
General Procedure for the Preparation of Gels. Compound 1
was placed in a glass vial, and 0.5 mL of solvent was added. The
vial was capped, and the cap was pierced with a needle. The solution
was heated with a heat gun until the solid completely dissolved,
after which it was sonicated in an ultrasound bath to allow for the
gel formation.
Acknowledgment. We thank Professor Christopher M.
Hadad of the Ohio State University and Dr. Sasha Stankovich
of Northwestern University for useful suggestions. This work
was financially supported with funds obtained from the Ohio
State University.
Supporting Information Available: Additional experimental
results and spectra for all new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
IR Experiments. IR spectrophotometers, used in the study, were
equipped with a DTGS KBr detector. FT-IR spectra of 32 scans
JO071159Y
7278 J. Org. Chem., Vol. 72, No. 19, 2007