Mirror image nanostructures

Article abstract of DOI:10.1021/ja051183y

Chiral molecules that self-assemble to form chiral supramolecular structures exhibit interesting structural features reminiscent of tertiary and quaternary structures of proteins and have applications in catalysis and nonlinear optics. Often, these structures are hierarchical, with their chiral structure difficult to interpret on the molecular scale. In this communication, we observe chiral assembling molecules that form well-defined helices with a pitch of 28 nm. We observe the behavior in both R- and S-enantiomers of the molecule, forming mirror image nanostructures. The molecular chirality is determined by the dimethyloctyl alkyl coil of the molecule and is located more than 4 nm from the hydrogen-bonding segment. The nanostructures observed are not hierarchical, which could be a result of the significant separation between the stereocenter and hydrogen-bonding dendron. The subtle structural modification at the periphery of the molecule biases the supramolecular assembly, which is driven primarily by strong hydrogen-bonding and π-π stacking interactions. Copyright

Full text of DOI:10.1021/ja051183y

Published on Web 05/17/2005
Mirror Image Nanostructures
Benjamin W. Messmore, Preeti A. Sukerkar, and Samuel I. Stupp*
Departments of Chemistry, Department of Materials Science & Engineering, and Feinberg School of Medicine,
Northwestern UniVersity, EVanston, Illinois 60208
Received February 24, 2005; E-mail: s-stupp@northwestern.edu
Supramolecular organic nanostructures are formed through
concerted weak interactions among self-assembling molecules.
Hydrogen-bonding, π-π stacking, and metal-ligand interactions
play central roles in controlling the form and function of these
nanostructures,1 including one-dimensional assemblies with co-
lumnar,2 ribbon,3-5 and cylindrical6 architectures. Several groups
have incorporated chiral moieties in self-assembling organic
molecules with the objective of forming chiral supramolecular
assemblies.7,8 In one case, a chiral assembly was mineralized with
silica, transcribing the chirality of the molecule onto the inorganic
Figure 1. Chemical structures of R-, S- and racemic DRCs. Chirality located
at the indicated stereocenter.
nanostructure to create a catalyst that showed high enantioselec-
tivity.9 Supramolecular nanostructures with appropriately sized
cavities10 and well-defined chiral structure on the scale of their
constituent molecules will enable the rational design of enantio-
selective catalysts. We report here the use of enantiomerically
enriched self-assembling molecules to create such one-dimensional
nanostructures with well-defined chiral structure.
Previously, our laboratory has developed triblock molecules
known as dendron rodcoils (DRCs) that self-assemble in organic
solvents to form high aspect ratio ribbonlike nanostructures that
are 10 nm wide and µm long.3,4 Self-assembly of this class of
molecules is driven by head-to-head hydrogen-bonding interactions
among their dendritic segments and also by π-π stacking interac-
tions among their rodlike segments. We showed previously that
the coil segment in DRC molecules imparts solubility to these high
aspect ratio, self-assembling nanostructures; however, thus far, only
achiral or racemic coil segments have been explored for this
purpose.
Figure 2. Circular dichroism spectra of R-, S-, and racemic DRC.
The DRC molecules synthesized and studied in this work have
a triblock structure consisting of a generation one 3,5-dihydroxy-
(20.0 µL at 0.25 wt %) of R-, S-, and racemic DRC were dispersed
benzoic ester dendron,
a ter(biphenyl ester) rod, and a
in 0.50 mL of acetonitrile and analyzed by CD. The CD spectra of
self-assembled R- and S-DRC each show Cotton effects in the
biphenyl group absorbance due to exciton coupling of the chromo-
phore (Figure 2). The differences in the two spectra demonstrate
the influence of the chiral coil segment on handedness of the π-π
stacked biphenyl rod segments of the molecule. As expected,
similarly prepared samples in THF do not show any exciton
coupling in the CD spectrum, thus linking the source of the observed
handedness to self-assembly (see Supporting Information). Fur-
thermore, racemic DRC does not have a CD signal (Figure 2),
despite the ability of these molecules to form a self-supporting gel.
DRC nanostructures cast from dilute solution onto freshly cleaved
mica were imaged by AFM. Images reveal collections of micron-
long, one-dimensional nanostructures with a height that varies
periodically from 6 to 9.3 nm above the mica surface and with a
regular pitch of 28 ( 3 nm along their length. Images of isolated
nanostructures formed by R- and S-DRC molecules are clearly
mirror images of each other (Figure 3), forming left- and right-
handed helices, respectively. Interestingly, AFM studies of self-
assembled racemic DRC molecules show similar helical features
with left- and right-handed ribbonlike structures with the same
3,7-dimethyloctyl coil (Figure 1). The stereocenter at carbon 3 of
the dimethyloctyl coil is enantiomerically enriched.11 The structures
were synthesized by carbodiimide esterifications12 using tertiary
butyl13 and benzyl ester14 protection/deprotection chemistry similar
to that described previously by our laboratory3 (details can be found
in the Supporting Information). The macroscopic indicator of self-
assembly is an increase in solution viscosity and ultimately gelation
due to formation of an interconnected network of high aspect ratio
ribbonlike nanostructures. All three compounds, R-, S-, and racemic
DRC, were found to form birefringent gels in acetonitrile at
0.25 wt %, indicative of molecular assembly.3,4,15,16 Gelation was
found to be solvent dependent and did not occur in tetrahydrofuran
(THF) (NMR spectra of R-, S-, and racemic DRC show well-
resolved peaks in deuterated THF). In deuterated acetonitrile,
however, aromatic peaks are unresolved, while aliphatic peaks are
broadened (see Supporting Information). This behavior is consistent
with the ribbonlike packing of DRC molecules observed previ-
ously.3,4
We used circular dichroism (CD) spectroscopy as a tool to probe
handedness of the self-assembled nanostructures.17 Preformed gels
9
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J. AM. CHEM. SOC. 2005, 127, 7992-7993
10.1021/ja051183y CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Supporting Information Available: Full synthetic details, absorb-
ance spectra, control CD in THF, and additional AFM. This material
is available free of charge via the Internet at http://pubs.acs.org.
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Acknowledgment. This work was funded by the U. S. Depart-
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