Self-assembly of dendron rodcoil molecules into nanoribbons

Full text of DOI:10.1021/ja015653+

J. Am. Chem. Soc. 2001, 123, 4105-4106
4105
Self-Assembly of Dendron Rodcoil Molecules into
Nanoribbons
Eugene R. Zubarev, Martin U. Pralle, Eli D. Sone, and
Samuel I. Stupp*
Department of Materials Science and Engineering
Department of Chemistry, Medical School
Northwestern UniVersity, EVanston, Illinois 60208
ReceiVed February 9, 2001
One of the expectations in science is the discovery of materials
with surprising properties or functionality based on designed
molecules that self-order or fold. The focus on self-assembly
throughout the 90s has generated very useful knowledge toward
this expectation and many prospects are on the horizon.1-6
Learning how to control the dimensionality and shape of self-
assembled structures through molecular design remains a chal-
lenge.
We report here on the self-assembly of molecules 1 which we
refer to as dendron rodcoils (DRC) because of their blocked
covalent architecture consisting of coil-like, rodlike, and dendritic
segments. These molecules are synthesized in 15 steps with an
Figure 1. Bright-field TEM micrograph of unstained DRC nanoribbons
formed in dichloromethane.
µm, and thus their aspect ratio can be as high as 1000. The fully
extended length of an average-size DRC molecule is ∼6.5 nm,
and thus the 10 nm width is consistent with a bimolecular packing
of DRC molecules. A head-to-head packing of molecules could
generate the narrow structures observed by TEM. Examination
of the one-dimensional structures by atomic force microscopy
(AFM) reveals their uniform thickness of 2 nm, clearly indicating
a ribbonlike shape (see Supporting Information). Therefore, the
self-assembly of DRC molecules results in the formation of
nanoribbons 10 × 2 nm and several µm long, which we conclude
lie flat on the carbon substrates used for imaging. Small-angle
X-ray scattering (SAXS) experiments on the gels did not reveal
peaks even when a synchrotron source was used, suggesting that
the gels contain one-dimensional structures that are not highly
aggregated and therefore lack the necessary structural coherence
to generate X-ray diffraction.
overall yield of 40% (see Supporting Information). Extremely
dilute solutions (as low as 0.2 wt %) of 1 in various organic
solvents undergo spontaneous gelation, producing birefringent soft
solids with a blue-violet hue. Formation of a birefringent gel
strongly suggests self-assembling behavior of molecules 1 in
organic solvents.
The bulky geometry of the dendron relative to the rod could
frustrate the formation of two-dimensional assemblies. Nonethe-
less, the identical aromatic rod-dendron segments of molecules
We synthesized a series of molecules analogous to 1 to probe
the role played by hydrogen bonding in self-assembly. In addition
to 1, three other structures were synthesized that differed only in
the number of hydroxyl groups present in the dendron segment.
Molecules 2 do not contain hydroxyls at all, whereas 3 and 4
contain two and six hydroxyl groups, respectively (Table 1).
Molecules 2 form isotropic solutions when dissolved in organic
solvents, and gelation was never observed in this system (see
Supporting Information). When only two hydroxyl groups are
present (material 3), gelation is still not observed when molecules
are dissolved at elevated temperature. Once these solutions are
cooled to room-temperature precipitation occurs. Therefore, the
presence of at least four hydroxyl groups per molecule is necessary
for solvent gelation by the network of self-assembled nanoribbons.
As expected, the same gelation behavior described for 1 was
observed for molecules 4 which contain six hydroxyl groups in
their dendron. A second series of molecules (5-7) was synthe-
sized in order to probe the role of aromatic interactions in self-
assembly. In this series all of the molecules have identical dendron
segments with four hydroxyl groups and therefore retain the same
capacity to form hydrogen bonds. However, the molecules differ
in the number of biphenyl ester units forming their rod segment.
Interestingly, results from these studies indicate that aromatic
interactions play an important synergistic role together with
hydrogen bonds in triggering self-assembly. As shown in Table
1, molecules 5 (with one biphenyl-ester unit) do not gel organic
1
should be strongly driven to aggregate in one dimension through
noncovalent interactions. These could involve hydrogen bonding
among hydroxyl groups in the periphery of the dendron as well
as aromatic π-π stacking of biphenyl units. We used transmission
electron microscopy (TEM) to study self-assembly of molecules
1
in organic solvents. Figure 1 shows a micrograph obtained from
a 0.004 wt % solution of the DRC in dichloromethane cast onto
a TEM grid. The unstained sample clearly shows one-dimensional
objects with a strikingly uniform width of 10 nm. Most of the
strands shown in Figure 1 have lengths on the order of
micrometers. We also observed isolated strands as long as 10
(
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Keser, M.; Amstutz, A. Science 1997, 276, 384-389. (b) Zubarev, E. R.;
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0.1021/ja015653+ CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/04/2001

4106 J. Am. Chem. Soc., Vol. 123, No. 17, 2001
Communications to the Editor
Table 1. Structure and Behavior in Dichloromethane of DRC
Analogs and Model Compounds
Figure 2. Side view of the ribbonlike structure taken directly from the
crystal structure of 14 shows color-coded hydrogen-bonded tetramers
solvents or produce the characteristic blue-violet hue. In contrast,
the addition of a second biphenyl-ester unit in molecules 6 results
in the formation of a gel but one that is mechanically weak relative
to that formed by 1. Self-assembly is enhanced in molecules 7
containing 4 biphenyl ester units (in comparison with DRC 1),
as indicated by the fact that gelation is now observed in a larger
variety of solvents. For example, birefringent gels are obtained
when molecules 7 are dissolved in the monomeric polar solvents
methyl methacrylate and butyl methacrylate at 1 wt %, whereas
molecules 1 only form clear solutions in these same solvents at
this concentration.
We have also synthesized DRC molecules containing the same
rod-dendron part, but different coils. The presence of a diblock
coil in molecules 9 and 10 significantly increases their solubility
and gelation does not occur. On the other hand, the absence of a
coil (compound 13) resulted in formation of highly insoluble
material. Compound 12 containing a fairly short dodecanyl coil
was still insoluble, whereas molecule 11 with a larger coil segment
(upper right) stacked on top of each other along the [100] direction of
the crystal (top view is shown in the middle right). Schematic (left and
bottom right) representation of the proposed structure for a DRC
nanoribbon.
segments and are stacked on top of each other along the a axis
of the crystal. The crystal structure reveals eight hydrogen bonds
that stitch the tetramers along the axis of the ribbon. Two OH- -
OH and two OH- -OdC hydrogen bonds form the cyclic tetramer,
whereas four OH- -OH and four OH- -OdC hydrogen bonds
connect adjacent tetramers along the ribbon axis (hydrogen bonds
are shown in yellow). The biphenyl units are stacked 4.93 Å apart,
a distance that should allow for π-π stacking interactions among
them. The thickness of the tetrameric cycles from the crystal
structure is about 2 nm, which is in excellent agreement with the
thickness of the DRC nanoribbons, as determined by AFM.
Twisting of the nanoribbons into helical structures has also been
observed by TEM (see Supporting Information). Thus, on the basis
of these data as well as the crystal structure of the model
compound 14, we envision the structure of the ribbons as depicted
in Figure 2 (bottom right, and left).
We have demonstrated that dendron rodcoil molecules 1 self-
assemble via hydrogen bonding and π-π stacking interactions
into well-defined nanoribbons with uniform width and thickness
(10 × 2 nm) and lengths that can be 1000 times greater. These
structures can build networks that cause solvents to gel at self-
assembler contents as low as 0.2 wt %. The type of self-assembly
described here offers a synthetic route to well-defined one-
dimensional organic nanostructures.
(2-octyl-1-dodecanyl) was found to form gels similar to those of
DRC 1. Finally, molecules 15-18 (see Supporting Information)
containing the same coil and rod as those in DRC 1 but different
dendrons (generation 2, 3, 4, and 5) were prepared in order to
understand the influence of the dendron’s size. These four
molecules were soluble in organic solvents, and none of them
led to the formation of gels. Thus, at least three biphenyl-ester
units in the rod and four hydroxyls in the dendron segment are
required to form a robust gel, which is a signature of extensive
self-assembly given the large number of molecules in one
nanoribbon. Also the dendritic block must be of generation 1,
and a coil of length comparable to the rod must be present in the
structure (see Supporting Information). On the basis of these
findings, and also the disappearance of all aromatic and hydroxyl
peaks in the NMR spectra of gels, we conclude that self-assembly
involves the aggregation of DRC molecules 1 through hydrogen
bonding and aromatic interactions. The oligoisoprene segments,
on the other hand, are found by NMR to retain their rotational
freedom after gelation (see Supporting Information).
Acknowledgment. This work was supported by the U.S. Army
Research Office, the National Science Foundation, and the Department
of Energy. E.D.S. thanks NSERC of Canada for a graduate scholarship.
Supporting Information Available: Synthesis of 1, AFM image and
height profiles of ribbons, NMR of DRC gel and solution, TEM of twisted
nanoribbons (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
To gain insight into the internal structure of the nanoribbons,
we synthesized model compound 14 containing a dendron
identical to that present in DRC 1 but covalently attached to only
one biphenyl. Figure 2 shows side and top views of the crystal
structure of this compound revealing the presence of a ribbonlike
JA015653+
(
22
7) Crystal data for compound 14: C33H O10, M ) 578.51, triclinic, P 1h ,
7
structure composed of tetrameric cycles (upper and middle right).
a ) 4.9341 (5) Å, b ) 19.219 (2) Å, c ) 33.022 (3) Å, R) 85.805 (2)°, â
3
The tetramers are formed by hydrogen bonds among dendritic
) 88.850 (2)°, γ ) 88.669 (2)°, V ) 3121.6 (6) Å , Z ) 4.