Synthesis, self-assembly, and switching of one-dimensional nanostructures from new crowded aromatics

Article abstract of DOI:10.1021/ja034783a

This study outlines a versatile and expeditious synthesis of the first derivatives of a new class of benzene that is substituted with both three amide and three alkyne substituents. Sparsely covered monolayer films, made through spin-casting, reveal one-d

Full text of DOI:10.1021/ja034783a

Published on Web 06/11/2003
Synthesis, Self-Assembly, and Switching of One-Dimensional
Nanostructures from New Crowded Aromatics
Mark L. Bushey, Thuc-Quyen Nguyen, and Colin Nuckolls*
Contribution from the Department of Chemistry, Columbia UniVersity,
New York, New York 10027
Received February 20, 2003; E-mail: cn37@columbia.edu
Abstract: This study outlines a versatile and expeditious synthesis of the first derivatives of a new class
of benzene that is substituted with both three amide and three alkyne substituents. Sparsely covered
monolayer films, made through spin-casting, reveal one-dimensional nanostructures that can be visualized
with atomic force microscopy. In bulk, synchrotron X-ray diffraction and polarized light microscopy show
that these nanostructured columns assemble further into a two-dimensional liquid crystalline phase. The
birefringence of this phase can be switched by application of an electric field. The half-time for the liquid
crystalline phase to switch is very fast and proportional to the applied voltage.
nonlinear optics.7 Our efforts,8 as well as others’,9 are in
controlling the intermolecular forces that bring the aromatic
Introduction
Discotic liquid crystals1 have a number of useful applications
that are a consequence of stacking aromatic molecules in a face-
to-face geometry including semiconductors,2 ionic conductors,3
light emitting diodes,4 photoconductors,5 electrooptics,6 and
subunits together by using a combination of hydrogen bonds
and π-stacking. This study describes the first experiments on a
new class of hexasubstituted aromatic molecules 1 shown in
Figure 1A. These structures have a central benzene ring with
both amide and alkyne substituents. The three alkynes are
conjugated to the central aromatic ring, meaning substituents
on the alkynes can alter the reduction/oxidation potentials of
the central aromatic ring and provide a path for charge injection
into the stack. Shown below is a general method to synthesize
these new structures and their assembly into columnar super-
structures. Individual one-dimensional superstructures that are
only a few nanometers wide can be visualized with atomic force
microscopy (AFM). In bulk, the columns form a two-
dimensional liquid crystalline phase that can be switched with
the application of an electric field. The rates of this process are
fast and proportional to the applied electric field strength.
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S. Rep. Prog. Phys. 1990, 53, 57-84. (d) Boden, N.; Bushby, R. J.;
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(2) (a) Boden, N.; Movaghar, B. Handbook of Liquid Crystals; Wiley-VCH:
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Mater. 1999, 11, 1469-1472. (c) Chandrasekhar, S.; Prasad, S. K. Contemp.
Phys. 1999, 40, 237-245. (d) Boden, N.; Bushby, R. J.; Clements, J.;
Movaghar, B. J. Mater. Chem. 1999, 9, 2081-2086. (e) Percec, V.; Glodde,
M.; Bera, T. K.; Miura, Y.; Shiyanovskaya, I.; Singer, K. D.; Balagurusamy,
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Soc., Perkin Trans. 1 1993, 1411-1420. (b) Ungar, G.; Batty, S. V.; Percec,
V.; Heck, J.; Johansson, G. AdV. Mater. Opt. Electron. 1994, 4, 303-313.
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W.; Nuckolls, C. Angew. Chem., Int. Ed. 2002, 41, 2828-2831. (c) Nguyen,
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(4) Christ, T.; Gluesen, B.; Greiner, A.; Kettner, A.; Sander, R.; Stuempflen,
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10.1021/ja034783a CCC: $25.00 © 2003 American Chemical Society

Synthesis of 1D Nanostructures from Crowded Aromatics
A R T I C L E S
4.12 Unfortunately, the Sonogashira couplings13 with 4 were
sluggish and after extended reaction times gave complex
mixtures from which the desired trisubstituted product could
not be isolated. The key to synthesizing these molecules was
employing a Farina-modified14 Stille coupling15 between 4 and
the alkynyl stannanes.16 Considering the demanding nature of
this coupling and that the reaction is occurring three times in
the same molecule, the yields of 88% for 5a and 48% for 5b
are remarkably good. These esters can be converted to the target
amides 1a-c by saponification, conversion to the acid chloride,
and reaction with appropriate primary amines. 1c was synthe-
sized as the levorotatory isomer from the enantiomerically pure
(S)-amine. Another route that was explored to synthesize 1a-c
involved first the conversion of the tris-acid chloride 3 to the
desired amides followed then by the palladium-mediated
couplings. Although this approach also yielded 1a-c, the yields
for the Stille couplings were lower than those with 4, and the
desired products were extremely difficult to separate from the
reaction mixture due to the polarity of the amide substituents.
AFM Imaging. By spin-casting from solution, conditions
were found that produced films with monolayer thickness but
sparse coverage on highly ordered pyrolytic graphite (HOPG).17
The morphology of these films could be directly imaged with
atomic force microscopy as shown in Figure 2a. For 1c, the
overwhelming feature of the micrograph is that of long and thin
superstructures. The width of these structures varies over a
range, with the lower limit being around one molecule wide.
This dispersity in widths of the strands is indicative of the
mechanism for their formation where the molecules self-
associate to form isolated columns in solution which are then
deposited by spin-casting.18 Within wider samples, striations
can be seen running collinear with the long axis that correlate
for the diameter of an individual stack of molecules. From the
height profile in Figure 2b, each feature is ca. 1.9 nm high,
meaning the films are one molecule high with column axes
parallel to the surface. The height of these features is ca. 3 Å
higher than similar structures observed for 2, probably due to
the core of 1 being more rigid than that of 2. These one-
dimensional structures, which are only nanometers in width and
microns long, are not typically seen for discotic liquid crystals.19
These structures likely form here because the association in the
stacking direction far outweighs the van der Waals forces
holding the columns next to each other. Similar results were
seen with 1a and 1b. As was seen in earlier work with 2,8c the
orientation of these mesogens on the surface appears to be
governed by the steric encumbrance when stacking. The alkynes
Figure 1. (a) Mesogens investigated; (b) previously studied mesogens;
(c) energy minimized molecular model of 1, side view, the modeling was
performed with methyl groups on the alkynes and amides; (d) top view of
the model.
Scheme 1a
a Reagents and conditions: (a) MeOH, pyr, 82%; (b) Pd(PPh3)4 (cat),
AsPh3 (cat), 48-88%; (c) (i) KOH, i-PrOH; (ii) SOCl2; (iii) R-NH2, 39-
66%.
The design for 1 is based on recent studies of molecules
having the general structure 2. These structures were recently
shown to create very regular and stable columnar superstruc-
tures.10 One interesting consequence of substituting the alkoxyls
of 2 for the alkynes of 1 is that the six ring-substituents, by
virtue of the linearity of the alkynes, cannot adopt an up/down
gear-like conformation like the substituents of 2. Shown in
Figure 1c,d is an energy minimized model11 of a dimer of 1.
This model indicates there is a synergy between hydrogen bonds
and π-π interactions as the central aromatic rings are stacked
within their van der Waals radii (ca. 3.5 Å apart) and the amides
are able to form three strong hydrogen bonds (N to O distance
of ca. 2.8 Å). These models in comparison with similar ones
from 2 indicate that the distance between the aromatic rings is
less with alkyne substituents rather than alkoxyl substituents
due to the relative size of these two groups. Moreover, the
alkyne is linear and therefore should have less steric encum-
brance relative to the dodecyloxy side chains.
(12) The preparation of 3 is contained in the Supporting Information and
produced by methods similar to: (a) Wallenfels, K.; Witzler, F.; Friedrich,
K. Tetrahedron 1967, 23, 1845-1855. (b) Anthony, J. E.; Khan, S. I.;
Rubin, Y. Tetrahedron Lett. 1997, 38, 3499-3502. (c) Hennrich, G.; Lynch,
V. M.; Anslyn, E. V. Chem.-Eur. J. 2002, 8, 2274-2278.
(13) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis 1980,
8, 627-630.
Results and Discussion
(14) Farina, V.; Krishman, B. J. Am. Chem. Soc. 1991, 113, 9585-9595.
(15) Rudisill, D. E.; Stille, J. K. J. Org. Chem. 1989, 54, 5856-5866.
(16) The preparation of tributyl(1-dodcynyl)tin is contained in the Supporting
Information by a procedure similar to: Ito, Y.; Inouye, M.; Yokota, H.;
Murakami, M. J. Org. Chem. 1990, 55, 2567-2568.
Synthesis. Our strategy in synthesizing these molecules was
to employ a palladium-mediated coupling between an alkynyl
subunit and the trisbromo/triester 4. The starting material in
Scheme 1, 3, can be made easily in 37 g batches and then
converted on a multiple-gram scale into the versatile trisester
(17) Conditions to achieve a sparsely covered monolayer were found by varying
the spin-rate, solvent, and concentration during spin-casting of 1c on the
basal plane of HOPG by the methods used in ref 8c.
(18) Nguyen, T.-Q., Bushey, M.; Martel, R.; Avouris, P.; Brus, L. E.; Nuckolls,
C., unpublished results.
(10) See ref 8 above.
(11) Molecular modeling was performed using MacroModel v. 7.0 and the
Amber* forcefield: Mohamadi, F.; Richards, N. G. J.; Guida, W. C.;
Liskamp, R.; Lipton, M.; Caufield, C.; Chang, G.; Hendrickson, T.; Still,
W. C. J. Comput. Chem. 1990, 11, 440-467.
(19) Micron-wide crystalline domains have been seen for discotic liquid
crystals: (a) Lovinger, A. J.; Nuckolls, C.; Katz, T. J. J. Am. Chem. Soc.
1998, 120, 264-268. (b) van Gorp, J. J.; Vekemans, J. A. J. M.; Meijer,
E. W. J. Am. Chem. Soc. 2002, 124, 14759-14769.
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A R T I C L E S
Bushey et al.
Figure 3. Polarized light micrograph for 1c at 161 °C.
Figure 2. (a) AFM image of a sparsely covered monolayer film of 1c on
the basal plane of freshly cleaved HOPG. (b) Height profile of the film in
Figure 2a.
Table 1. Phase Transition Temperatures and Enthalpies for 1a-c
heating cycle T/°C
(∆H/kJ mol-1
cooling cycle T/°C
(∆H/kJ mol-1
)
)
Figure 4. Synchrotron X-ray diffraction (λ ) 1.15 Å) from a 2 mm
Lindemann capillary tube filled with 1c at 150 °C.
1a
1b
1c
141 (9.0)
96 (4.5)
110 (69)
225 (dec)
225 (dec)
207 (15.2)
Table 2. Core-to-Core Distances for 1a-c and the Columnar
Arrangement
185 (-3.6)
42 (-4.7)
core-to-core
distances (nm)
arrangement of
columns
are sterically less demanding groups than the alkoxyls in 2 and
therefore promote assembly parallel to the substrate.
1a
1b
1c
2.14
2.02
2.09
hexagonal
hexagonal
hexagonal
Differential Scanning Calorimetry. In bulk, each of the
derivatives of 1 displays similar polymorphism. The enthalpies
and heats of transition for 1a-c are shown in Table 1. For all
three, upon heating, an initial endothermic transition is observed,
indicating a transition from solid phase to a liquid crystalline
phase (vide infra). The values are similar to those measured
for other discotic liquid crystals.20 For 1a and 1b, the transition
temperature from a liquid crystalline phase to an isotropic liquid
was above 225 °C where an exothermic decomposition occurred.
Between the lowest endothermic transition and below the
decomposition temperature, 1a and 1b were stable when viewed
under the polarized light microscope, and the material did not
show any significant discoloration. For 1c, the transition to an
isotropic liquid was at a low enough (ca. 207 °C) temperature
that no exothermic decomposition occurred upon heating, and
the liquid crystalline phase returned after cooling the sample.
This process was stable for at least five cycles without
degradation of the sample.
mesophases.23 Thinner regions of these films are uniaxial and
negatively birefringent, a strong indication that the mesophase
is columnar in origin and that the column axes are aligned
parallel to the substrate.24
Powder X-ray Diffraction. The synchrotron X-ray diffraction
pattern for bulk samples of 1c is shown in Figure 4. Equivalent
diffraction patterns for 1a,b are contained in the Supporting
Information, each showing a columnar assembly whose primary
reflection is between 18.1 and 19.1 Å. The core-to-core distances
computed from these diffractograms are shown in Table 2. In
each case, higher-order reflections allow the lattices to be
indexed to a two-dimensional hexagonal arrangement of col-
umns for 1a-c. The small number of higher-order peaks in the
diffractograms for 1a-c and the broadness of the reflections in
the alkyl region (ca. 4.5 Å) are indications that the columns are
Polarized Light Microscopy. Shown in Figure 3 is a
polarized light micrograph (at 161 °C) from a film of 1c as it
is cooled from its isotropic phase into its mesophase. It is similar
to the ones formed from liquid crystalline derivatives of 221
and other columnar liquid crystals,22 particularly pyramidic
(22) (a) Destrade, C.; Foucher, P.; Gasparoux, H.; Nguyen, H. T.; Levelut, A.
M.; Malthete, J. Mol. Cryst. Liq. Cryst. 1984, 106, 121-146. (b) Billard,
J.; Dubois, J. C.; Nguyen H. T.; Zann, A. NouV. J. Chim. 1978, 2, 535-
540.
(23) Zimmermann, H.; Bader, V.; Poupko, R.; Wachtel, E. J.; Luz, Z. J. Am.
Chem. Soc. 2002, 124, 15286-15301.
(24) (a) Hartshorne, N. H.; Stuart, A. Crystals and the Polarising Light
Microscope, 3rd ed.; Edward Arnold Ltd.: London, 1960; p 290ff. (b)
Lovinger, A. J.; Nuckolls, C.; Katz, T. J. J. Am. Chem. Soc. 1998, 120,
264-268.
(20) For similar calorimetry values, see the references cited in ref 1.
(21) For similar micrographs from derivatives of 2, see ref 8a,b.
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Synthesis of 1D Nanostructures from Crowded Aromatics
A R T I C L E S
Figure 5. 1c sandwiched between two sheets of ITO coated glass separated
by 5 µm at 160 °C: (a) 0 V/µm field; (b) 20 V/µm field.
loosely correlated in a liquid crystalline phase.25 The distances
measured for 1 are similar to the 18.4-18.8 Å measured for
derivatives of 2.
Switching. Further evidence for the liquid crystallinity is seen
when an electric field is applied to samples of 1c that are
sandwiched between indium tin oxide coated glass plates
separated by 5 µm. A polarized light micrograph between the
electrodes is displayed in Figure 5a. As compared to the samples
on glass in Figure 3, the domains on ITO are much smaller.
Moreover, the samples in Figure 5a are much thinner than the
samples in Figure 3. Analysis of the polarization in each of the
domains of Figure 5a shows that they are uniaxial and negatively
birefringent (like thin regions of Figure 3), implying that these
domains have their column axes parallel to the surface.24 Some
areas that are dark in Figure 5a appear bright when the sample
stage is rotated, implying that the columnar axes are parallel to
the polarizer or analyzer axes in the dark areas.
Figure 6. (a) Decrease in intensity of transmitted plane polarized light for
1c (150 °C), as an electric field is triggered at t ) 0 s, red ) 12.8 V/µm,
orange ) 14.8 V/µm, black ) 16.6 V/µm, green ) 18.6 V/µm, blue )
20.4 V/µm; the symbols are 1/50 of the measured data, and the color-coded
solid lines are the curve fit. The y-axis is logarithmic. (b) A plot of the
half-times for switching measured from Figure 6a versus the applied electric
field. The axes are logarithmic. The solid line is a power function fit to the
data.
When an electric field is then applied to the sample, the areas
that were bright now turn dark (shown in Figure 5b). This is
likely due to a homeotropic alignment. This switching process
is reversible, and the material shows little discoloration for many
cycles at 160 °C. When the voltage across the cell is removed,
the material slowly relaxes back to the parallel alignment shown
in Figure 5a. Because 1a and 1b decompose at the temperature
they became isotropic liquids, they could not be loaded into
the liquid crystalline cells, and therefore the analogous experi-
ments failed.
Therefore,
τ1/2
E
where E is the applied electric field strength, and τ1/2 is the
half-time that characterizes the rate of the switching.
What is left unclear is whether macroscopic polarity exists
in the two switching states in this system. Nematic discotics
that consist of short, self-assembled columns in solution were
recently shown to switch with half-times that vary as the inverse
square of the applied electric fieldsa dielectric response.6f Given
the difference in the power dependence, the switching mecha-
nism in these samples is not the same as these lyotropic
discotics. Possibly the packing of polar columns in 1c cannot
proceed with an all antiparallel orientation due to the inability
to pack dipoles antiparallel into a hexagonal lattice.26 Ongoing
experiments are aimed at determining if this process is ferro-
electric.
Shown in Figure 6a is the time that it takes for a bright,
birefringent sample of 1c (in Figure 5a) to switch to a dark
state (i.e., optically isotropic, Figure 5b) at several electric field
strengths. The light intensity is measured over the entire field
of view. The zero time value is when the voltage is triggered
across the two ITO electrodes. All of the switching processes
with field strengths between 13 and 20 V/µm were complete
within 1 ms. These are fast switching times for columnar liquid
crystals, comparable to the fastest measured for a columnar
mesophase6c,g,h and even faster than discotic samples that are
diluted to lower their viscosity.6f From the curve-fit of the log-
normal plot of the data in Figure 6a, it is evident that the decay
is exponential. The rate of decay (from Figure 6a) of the
birefringence and therefore the half-time (τ1/2) to go from bright
to dark were calculated for each field strength. The half-times
are plotted as a function of the applied electric field in Figure
6b. The data can be fit to a power function, and from this fit
the power dependence is determined to be 1.04 ( 0.03.
Conclusion
In summary, the study outlined above has shown that
hexasubstituted aromatics alternatingly substituted with amides
and alkynes self-assemble into columnar superstructures. In films
with sparse coverage that are a monolayer in thickness, the
columns can be directly visualized packing parallel to the
substrate. The strands are nanometers in width and microns in
length. Bulk samples display a liquid crystalline phase that can
be switched with the application of an electric field. The
(25) For many examples of these characteristic diffraction patterns for columnar
discotics, see: (a) Levelut, A. M. J. Chim. Phys. Phys.-Chim. Biol. 1983,
80, 149-161 and (b) the citations in ref 1.
(26) (a) Zimmermann, H.; Poupko, R.; Luz, Z.; Billard, J. Z. Naturforsch., A
1985, 40A, 149-160. (b) See also ref 1g.
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A R T I C L E S
Bushey et al.
and phenylethynylstannane (Aldrich) were used as received. The
preparation of tributyl(1-dodcynyl)tin is contained in the Supporting
Information.
switching process is fast, and its rate is proportional to the
applied field strength.
Experimental Section
2,4,6-Tribromo-1,3,5-trisbenzoyl Chloride (3). To a 1 L flask that
was equipped with a reflux condenser and a stir bar were added 1,3,5-
trisacetoxymethyl-2,4,6-tribromobenzyl ester12 (37.0 g, 69.7 mmol), 3
M NaOH (175 mL, aq), KMnO4 (49.6 g, 314 mmol), and H2O (500
mL). The reaction mixture was boiled under reflux for 12 h and then
cooled to room temperature. i-PrOH (50 mL) was added to the reaction
mixture, and the solids were removed by filtration. The filter cake was
washed with several portions of water, and the combined filtrate
solutions were reduced in volume. The pH of the combined solutions
was adjusted to ca. 1 with a concentrated HCl solution (aq). The mixture
was reduced to dryness, and the flask was then equipped with a reflux
condenser and a stir bar and boiled under reflux with SOCl2 (200 mL)
for 12 h. After being cooled to room temperature, the reaction mixture
was filtered with CH2Cl2 washings. Distillation, under reduced pressure,
yielded a white solid, 3 (30.3 g, 87%). 13C NMR (75 MHz, CDCl3):
Atomic Force Microscopy. Solutions (between 1 mM and 1 µM)
for 1 were prepared in anhydrous methylene chloride and then spun
onto the basal plane of freshly cleaved HOPG substrates immediately
prior to use. The films were dried in air for ∼20 min before imaging.
Images of the films were obtained using a Nanoscope IIIa/MultiMode
Scanning Probe Microscope from Digital Instruments. Etched silicon
tips with a typical spring constant of 1-5 N/m and a resonant frequency
of 50-80 kHz were used. Images were collected using tapping mode
and in air under ambient conditions.
Molecular Modeling. Modeling and visualization was performed
on a VA Linux 420 workstation (733 MHz Intel Pentium III processor)
outfitted with the Maestro Interface (Schrodinger Inc., MacroModel v.
7.0, Linux version). All models were minimized using the Amber*
molecular force field.
Polarized Light Microscopy. Polarized light microscopy was
performed on a Leica DMLP polarized light microscope with a 10×,
20×, or 40× objective, and an Optronics MagnaFire model S99802
digital camera system and power supply. Digital images where collected
and analyzed with Image-Pro Express software version 4.0 by Media
Cybernetics. Samples were placed between a cover slip and glass slide
or loaded into ITO coated glass slides spaced by 5 µm (from Linkam
Scientific). Photographic images were captured as the samples were
cooled from their clearing temperature. The temperature was varied
with a Linkam TMS 350E heating and cooling stage, with a Linkam
TMS 94 heat controller and a Linkam LNP cooling system.
Switching. Experiments were performed using the microscope and
camera systems described above with the addition of a Tektronix TDS
224 oscilloscope, a Tektronics CFG253 function generator, a Kepco
bipolar power supply, and a Hamamatsu PMT detector. Switching
experiments were performed on samples between ITO coated glass
slides spaced by 5 µm that were heated in a Linkam TMS 350E heating
stage equipped with electrode leads.
δ 165.4, 142.1, 115.8. IR (film KBr): 1773, 1540, 1356, 1015 cm-1
.
HRMS (FAB) m/z: Calcd for C9Br3O3Cl2 (M - Cl)+ 462.1936, found
462.6780.
2,4,6-Tribromobenzene-1,3,5-tricarboxylic Acid Trimethyl Ester
(4). To a 200 mL flask that was equipped with a reflux condenser and
a stir bar were added 3 (3.0 g, 6.00 mmol), MeOH (50 mL), and
pyridine (2 mL). The reaction mixture was boiled under reflux for 12
h. After being cooled to room temperature, H2O (50 mL) and the
mixture were transferred to a separatory funnel. The two phases were
separated after the addition of an HCl solution (1 M, 50 mL) and CH2-
Cl2 (100 mL). The aqueous layer was extracted with methylene chloride
(3 × 100 mL), and the combined organic extracts were dried over
MgSO4 and concentrated under reduced pressure. Chromatography on
silica gel yielded 4 as a white solid (2.41 g, 82%). 1H NMR (300 MHz,
CDCl3): δ 3.95 (s). 13C NMR (75 MHz, CDCl3): δ 165.3, 138.7, 118.2,
53.9. IR (film KBr): 3010, 2960, 1730, 1550, 1460, 1440, 1370, 1350,
1240, 1180, 990 cm-1. HRMS (FAB) m/z: Calcd for C12H10Br3O6
(M + H)+ 486.8028, found 486.8026.
2,4,6-Trisphenylethynylbenzene-1,3,5-tricarboxylic Acid Tri-
methyl Ester (5a). An oven-dried 200 mL flask that was equipped
with a reflux condenser and a stir bar was cooled under a stream of
argon. Into this flask were added 4 (1.0 g, 2.05 mmol), tetrakis-
(triphenylphosphine)palladium (0.59 g, 5.13 mmol), triphenylarsine
(0.314 g, 1.03 mmol), and tributyl(phenylethynyl)tin (2.81 g, 7.18
mmol). After the flask was evacuated and back filled with argon several
times, toluene (40 mL) was added. The reaction mixture was boiled at
reflux under a slight positive argon pressure for 2 days. After being
cooled to room temperature, the mixture was filtered through a plug
of silica gel, and combined washings were evaporated under reduced
pressure. Chromatography on silica gel yielded 5a a white solid (0.99
g, 88% yield). 1H NMR (300 MHz, CDCl3): δ 7.52 (m, 6H), 7.40 (m,
9H), 4.05 (s, 9H). 13C NMR (75 MHz, CDCl3): δ 166.8, 138.8, 132.3,
130.0, 129.0, 122.2, 121.3, 99.3, 83.3, 53.4. IR (film KBr): 3060, 3010,
2960, 2220, 1730, 1550, 1490, 1440, 1340, 1270, 1220, 1130, 1070,
1020, 960 cm-1. HRMS (FAB) m/z: Calcd for C36H25O6 (M + H)+
553.1652, found 553.1675.
Synchrotron Powder X-ray Diffraction. The samples were loaded
into Lindemann capillary tubes (1, 1.5, 2, and 2.5 mm). They were
heated in a thermostatically controlled furnace equipped with Kapton
windows. Samples were rotated 3° around the long axis of the capillary
to average out any effects of preferential orientation. The X-ray
diffraction measurements were performed at Beamline X3B1 of the
Brookhaven National Synchrotron Light Source. X-rays of wavelength
0.702 Å and 1.151 Å, selected by a double crystal Si(111) monochro-
mator, were incident on the sample. X-rays diffracted in the vertical
plane passed through a So¨ller collimator of 0.03° fwhm and were
detected by a conventional NaI scintillation counter. Counts were
normalized to the incident flux via an ionization chamber in the incident
beam.
Differential Scanning Calorimetry. DSC was performed on a
Perkin-Elmer Pyris 1 Calorimeter. The samples (between 1 and 3 mg)
were weighed into aluminum calorimetry pans. The pans were then
sealed and analyzed. The scan rate was 20 °C/min.
1
General Synthetic. H NMR (300 MHz) and 13C NMR (75 MHz)
2,4,6-Tri(dodec-1-ynyl)benzene-1,3,5-tricarboxylic Acid Tri-
methyl Ester (5b). The reaction was performed similar to that of
5a. 4 (1.0 g, 2.05 mmol) and tributyl(1-dodcynyl)tin (3.08 g, 6.77 mmol)
yielded 5b a pale yellow oil (0.73 g, 0.99 mmol, 48%) after
chromatography on silica gel. 1H NMR (300 MHz, CDCl3): δ 3.90 (s,
9H), 2.39 (t, J ) 6.98 Hz, 6H), 1.56 (p, J ) 7.2 Hz, 6H), 1.39 (m,
6H), 1.28 (m, 36H), 0.89 (t, J ) 6.7 Hz, 9H). 13C NMR (75 MHz,
CDCl3): δ 167.2, 138.8, 120.9, 100.6, 74.7, 53.0, 32.3, 30.0, 29.9, 29.7,
29.6, 29.2, 28.8, 23.1, 20.0, 14.5. IR (film KBr): 2930, 2850, 2230,
spectra were recorded on a Bruker DRX 300 spectrometer. Infrared
spectra were recorded on a BioRad FTS 7000 FT-IR spectrophotometer
from films evaporated from solution onto a single crystal of KBr.
Solvents were degassed in 20 L drums and passed through two
sequential purification columns (activated alumina column followed
by a supported copper catalyst column) under a positive nitrogen
atmosphere.27 Column chromatography was performed on a CombiFlash
Sg100c system using RediSep normal phase silica columns (ISCO, Inc.,
Lincoln, NE). Tetrakis(triphenylphosphine)palladium (Strem Chemicals)
1750, 1550, 1470, 1470, 1430, 1380, 1340, 1230, 1130, 1000 cm-1
.
HRMS (FAB) m/z: Calcd for C48H73O6 (M + H)+ 745.5408, found
(27) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers,
F. J. Organometallics 1996, 15, 1518-1520.
745.5408.
9
8268 J. AM. CHEM. SOC. VOL. 125, NO. 27, 2003

Synthesis of 1D Nanostructures from Crowded Aromatics
A R T I C L E S
2,4,6-Trisphenylethynylbenzene-1,3,5-tricarboxylic Acid, Tris-
dodecylamide (1a). To a 50 mL flask that was equipped with a reflux
condenser and a stir bar were added 5a (0.29 g, 0.53 mmol), i-PrOH
(16 mL), NaOH (2.12 g, 53.1 mmol), and H2O (8 mL). The mixture
was heated under reflux for 2 h. After the mixture was cooled to room
temperature, the i-PrOH distilled from the reaction mixture under
reduced pressure, and then acidified to ca. pH 1 with concentrated HCl.
The solution was extracted with Et2O (3 × 5 mL), and the combined
organic extracts were dried over MgSO4, filtered, and concentrated to
dryness under reduced pressure. To this flask were then added a stir
bar, reflux condenser, and SOCl2 (2 mL). The reaction mixture was
heated under reflux for 2 h and cooled to room temperature. After
removal of the excess thionyl chloride, a viscous oil was left. To this
oil were added CH2Cl2 (2 mL), 1-dodecylamine (0.33 g, 1.75 mmol),
and pyridine (140 µL). After being stirred for 12 h, the reaction mixture
was diluted with CH2Cl2 (5 mL) and H2O (5 mL). The two phases
were separated, and the aqueous phase was extracted with CH2Cl2
(3 × 10 mL). The organic extracts were combined, dried over MgSO4,
filtered, and concentrated to dryness. Chromatography on silica gel
3250, 3190, 3140, 3080, 2960, 2920, 2850, 2230, 1660, 1600, 1550,
1500, 1440, 1320, 1260 cm-1. HRMS (FAB) m/z: Calcd for C63H82O3N3
(M + H)+ 928.6357, found 928.6328.
2,4,6-Tri(dodec-1-ynyl)benzene-1,3,5-tricarboxylic Acid Tris[(2-
(S)-phenylpropyl)amide] (1c). 1c was synthesized by the same
procedure as 1b. 5b (0.10 g, 0.13 mmol) and (S)-2-phenyl-1-
propylamine (60 mg, 0.44 mmol, 64 µL) yielded 1b as a waxy solid
1
(74 mg, 0.07 mmol, 52%). [R]23D) - 29.5° (c ) 1.0, CH2Cl2). H
NMR (300 MHz, CDCl3): δ 7.33-7.23 (m, 15H), 5.58 (t, J ) 6.0 Hz,
3H), 3.72 (p, J ) 6.9 Hz, 3H), 3.47 (m, 3H), 3.02 (h, J ) 6.9 Hz, 3H),
2.34 (t, J ) 6.9 Hz, 6H), 1.54 (m, 6H), 1.35 (d, J ) 6.9 Hz, 9H), 1.27
(s, 42H), 0.90 (t, J ) 6.6 Hz, 9H). 13C NMR (75 MHz, CDCl3): δ
167.0, 144.2, 142.2, 129.2, 127.4, 127.2, 120.2, 100.0, 75.1, 47.0, 40.2,
32.3, 30.1, 30.0, 29.7, 29.4, 28.9, 23.1, 20.2, 19.4, 14.5. IR (film
KBr): 3280, 3080, 3060, 3030, 2960, 2920, 2850, 2230, 1650, 1550,
1490, 1470, 1380, 1330, 1280, 1130, 1020 cm-1. HRMS (FAB) m/z:
Calcd for C72H100O3N3 (M + H)+ 1054.7766, found 1054.7756.
Acknowledgment. We thank Libor Vylicky (Columbia
University) for help with the electrooptic switching experiments.
We acknowledge financial support from the Chemical Sciences,
Geosciences and Biosciences Division, Office of Basic Energy
Sciences, US D.O.E. (#DE-FG02-01ER15264) and National
Science Foundation, Nanoscale Exploratory Research Grant
(#DMR-01-02467). This work is supported in part by the
Nanoscale Science and Engineering Initiative of the National
Science Foundation under NSF Award Number CHE-0117752.
This work has used the shared experimental facilities that are
supported primarily by the MRSEC Program of the National
Science Foundation under Award Number DMR-0213574. C.N.
thanks the Beckman Young Investigator Program (2002) and
the Dupont Young Investigator Program (2002) for financial
support. M.L.B. thanks the ACS Division of Organic Chemistry
for a graduate fellowship sponsored by Bristol-Myers Squibb.
The Brookhaven National Laboratory NSLS is supported by
the D.O.E., Division of Chemical Sciences and Division of
Materials Sciences. The SUNY X3 Beamline at the NSLS is
supported by the Division of Basic Energy Sciences of the
D.O.E. (DE-FG02-86ER45231). We thank Prof. Peter Stephens
(SUNY Stonybrook) for help with the powder diffraction
experiments.
1
yielded 1a as a waxy solid (0.34 g, 0.33 mmol, 66%). H NMR (300
MHz, CDCl3): δ 7.34-7.09 (m, 18H), 3.31 (bm, 6H), 1.48 (m, 6H),
1.3-1.1 (m, 54H), 0.91 (t, J ) 6.2 Hz, 9H). 13C NMR (75 MHz,
CDCl3): δ 166.6, 142.5, 132.3, 129.0, 128.4, 123.0, 119.4, 97.3, 84.2,
40.7, 32.4, 30.1, 30.0, 29.9, 29.8, 27.7, 23.1, 14.6. IR (film KBr): 3270,
3080, 2960, 2920, 2850, 2220, 1650, 1600, 1550, 1490, 1470, 1440,
1370, 1290, 1240, 1150, 1070, 1020 cm-1. HRMS (FAB) m/z: Calcd
for C69H93O3N3 (M + H)+ 1012.7296, found 1012.7272.
2,4,6-Tri(dodec-1-ynyl)benzene-1,3,5-tricarboxylic Acid Tris-
phenylamide (1b). To a 25 mL flask that was equipped with a reflux
condenser and a stir bar were added 5b (0.11 g, 0.15 mmol), i-PrOH
(4 mL), NaOH (0.580 g, 14.5 mmol), and water (2 mL). The mixture
was boiled under reflux for 12 h. After the mixture was cooled to room
temperature, the i-PrOH distilled from the reaction mixture under
reduced pressure, and then acidified to ca. pH 1 with concentrated HCl.
The solution was extracted with Et2O (3 × 5 mL), and the combined
organic extracts were dried over MgSO4, filtered, and concentrated to
dryness under reduced pressure. To this flask were then added a stir
bar, reflux condenser, and SOCl2 (2 mL). The reaction mixture was
heated under reflux for 2 h and cooled to room temperature. After
removal of the excess thionyl chloride, a viscous oil was left. To this
oil were added CH2Cl2 (2 mL), aniline (45 mg, 0.48 mmol, 44 µL),
and pyridine (38 mg, 0.48 mmol, 40 µL). After being stirred for 12 h,
the reaction mixture was diluted with CH2Cl2 (5 mL) and H2O (5 mL).
The two phases were separated, and the aqueous phase was extracted
with CH2Cl2 (3 × 10 mL). The organic extracts were combined, dried
over MgSO4, filtered, and concentrated to dryness. Chromatography
on silica gel yielded 1b as a white, waxy solid (53 mg, 0.057 mmol,
39%). 1H NMR (300 MHz, CDCl3): δ 9.03 (bs, 3H), 7.76 (d, J ) 7.8
Hz, 6H), 7.28 (t, J ) 7.9 Hz, 6H), 7.07 (t, J ) 7.2 Hz, 3H), 1.94 (t,
J ) 7.4 Hz, 6H), 1.15-0.91 (m, 57H). 13C NMR (75 MHz, CDCl3):
δ 165.3, 141.1, 139.0, 129.0, 124.6, 120.9, 120.8, 101.6, 75.0, 32.4,
30.1, 29.8, 29.8, 29.5, 28.9, 23.1, 20.1, 14.6. IR (film KBr): 3290,
Supporting Information Available: Data used to characterize
1-5 (13C NMR and 1H NMR spectra). Experimental procedures
for the preparation of ethynyltributylstannane and the materials
for the preparation of 3. Powder X-ray diffraction patterns for
1a and 1b (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
JA034783A
9
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