Synthesis, self-assembly, and characterization of supramolecular polymers from electroactive dendron rodcoil molecules

Article abstract of DOI:10.1021/ja049325w

We report here the synthesis and self-assembly of a series of three molecules with dendron rodcoil architecture that contain conjugated segments of oligo(thiophene), oligo(phenylene-vinylene), and oligo(phenylene). Despite their structural differences, all three molecules yield similar self-assembled structures. Electron and atomic force microscopy reveals the self-assembly of the molecules into high aspect ratio ribbon-like nanostructures which at low concentrations induce gelation in nonpolar solvent. Self-assembly results in a blue-shifted absorption spectrum and a red-shifted, quenched fluorescence spectrum, indicating aggregation of the conjugated segments within the ribbon-like structures. The assembly of these molecules into one-dimensional nanostructures is a route to π-π stacked supramolecular polymers for organic electronic functions. In the oligo(thiophene) derivative, self-assembly leads to a 3 orders of magnitude increase in the conductivity of iodine-doped films due to self-assembly. We also found that electric field alignment of these supramolecular assemblies can be used to create arrays of self-assembled nanowires on a device substrate.

Full text of DOI:10.1021/ja049325w

Published on Web 10/19/2004
Synthesis, Self-Assembly, and Characterization of
Supramolecular Polymers from Electroactive Dendron Rodcoil
Molecules
Benjamin W. Messmore, James F. Hulvat, Eli D. Sone, and Samuel I. Stupp*
Contribution from the Department of Chemistry, the Department of Materials Science &
Engineering, and the Feinberg School of Medicine, Northwestern UniVersity, EVanston, Illinois
60208
Received February 6, 2004; E-mail: s-stupp@northwestern.edu
Abstract: We report here the synthesis and self-assembly of a series of three molecules with dendron
rodcoil architecture that contain conjugated segments of oligo(thiophene), oligo(phenylene-vinylene), and
oligo(phenylene). Despite their structural differences, all three molecules yield similar self-assembled
structures. Electron and atomic force microscopy reveals the self-assembly of the molecules into high aspect
ratio ribbon-like nanostructures which at low concentrations induce gelation in nonpolar solvent. Self-
assembly results in a blue-shifted absorption spectrum and a red-shifted, quenched fluorescence spectrum,
indicating aggregation of the conjugated segments within the ribbon-like structures. The assembly of these
molecules into one-dimensional nanostructures is a route to π-π stacked supramolecular polymers for
organic electronic functions. In the oligo(thiophene) derivative, self-assembly leads to a 3 orders of magnitude
increase in the conductivity of iodine-doped films due to self-assembly. We also found that electric field
alignment of these supramolecular assemblies can be used to create arrays of self-assembled nanowires
on a device substrate.
Introduction
ester dendritic segments, rigid rod-like segments, and various
flexible coil-like segments, referred to as dendron rodcoil (DRC)
Electronically active organic molecules are of great interest
for electronic devices because of low processing costs and
desirable mechanical properties as compared to their inorganic
counterparts.1-3 Conductive polymers have desirable mechanical
properties, but often lack long-range molecular order due to
defects arising during polymerization, thus lowering device
performance.4 Molecular order can be achieved through vapor
deposition of small oligomers, resulting in crystalline films, but
with higher processing costs.5 The recent strategies of self-
assembly and supramolecular chemistry are interesting alterna-
tives for the bottom-up design of organic electronic devices.6-12
Our group reported recently on a family of self-assembling
triblock molecules with generation one 3,5 dihydroxy-benzoic
molecules.13-15 In certain solvents and at low weight percent-
ages, DRC molecules form self-supporting gels that result from
a 3-D network of high aspect ratio, ribbon-like nanostructures.
Due to their unique triblock architecture, DRC molecules aggre-
gate into one-dimensional nanostructures through specific and
nonspecific interactions. Hydrogen bonding between dendrons
and π-π stacking between rods enable assembly in one di-
mension, while the conformationally flexible coil solubilizes
the nanostructure and frustrates further crystallization. The
ribbons are ∼10 nm wide, 1.5 nm thick, and micrometers long.
Head-to-head packing of the hydrogen-bonded dendrons has
been inferred from X-ray crystallography of a model com-
pound.13
(1) Katz, H. E.; Bao, Z. N.; Gilat, S. L. Acc. Chem. Res. 2001, 34, 359-369.
(2) Dimitrakopoulos, C. D.; Mascaro, D. J. IBM J. Res. DeV. 2001, 45, 11-
27.
(3) Martin, R. E.; Diederich, F. Angew. Chem., Int. Ed. 1999, 38, 1350-1377.
(4) Bao, Z. N.; Rogers, J. A.; Katz, H. E. J. Mater. Chem. 1999, 9, 1895-
1904.
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B.; Ries, S.; Alnot, P. J. Am. Chem. Soc. 1993, 115, 8716-8721.
(6) Lehn, J. M. Supramolecular Chemistry; VCH Press: New York, 1995.
(7) Stupp, S. I.; LeBonheur, V.; Walker, K.; Li, L. S.; Huggins, K. E.; Keser,
M.; Amstutz, A. Science 1997, 276, 384-389.
(8) Hulvat, J. F.; Stupp, S. I. Angew. Chem., Int. Ed. 2003, 42, 778-781.
(9) Yoshio, M.; Mukai, T.; Ohno, H.; Kato, T. J. Am. Chem. Soc. 2004, 126,
994-995.
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H. J.; Derrick, P. J.; Feast, W. J.; Lazzaroni, R.; Leclere, P.; McDonell, L.
A.; Meijer, E. W.; Meskers, S. C. J. J. Am. Chem. Soc. 2002, 124, 1269-
1275.
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T.; Ito, K.; Hashizume, T.; Ishii, N.; Aida, T. Science 2004, 304, 1481-
1483.
(12) Samori, P.; Francke, V.; Mullen, K.; Rabe, J. P. Chem.-Eur. J. 1999, 5,
2312-2317.
In this work, we have designed and synthesized DRC mole-
cules containing phenyl-quater(thiophene), ter(phenylene vi-
nylene), and quater(phenylene) segments as part of the rod seg-
ment of the DRC structure. We have characterized the self-
assembling behavior and electronic properties in those contain-
ing phenyl quater(thiophene) segments.
Results and Discussion
Synthesis. The synthesis of the dendron-biphenyl segment
(14) is based on orthogonal protection/deprotection chemistry
(13) Zubarev, E. R.; Pralle, M. U.; Sone, E. D.; Stupp, S. I. J. Am. Chem. Soc.
2001, 123, 4105-4106.
(14) Zubarev, E. R.; Pralle, M. U.; Sone, E. D.; Stupp, S. I. AdV. Mater. 2002,
14, 198-203.
(15) Lecommandoux, S.; Klok, H. A.; Sayar, M.; Stupp, S. I. J. Polym. Sci.,
Part A 2003, 41, 3501-3518.
9
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Polymers from Electroactive Dendron Rodcoil Molecules
A R T I C L E S
Scheme 1. Chemical Structures of DRCs
Scheme 2. Synthesis of Dendrona
a (a) BnCl, K2CO3, DMF, room temperature; (b) isobutylene, H2SO4, CHCl3, room temperature; (c) LiOH, THF, H2O, room temperature; (d) 7, DPTS,
DIPC, CH2Cl2, room temperature; (e) H2, Pd/C, MeOH, room temperature; (f) 12, 10, DPTS, DIPC, CH2Cl2, room temperature; (e) 13, H2, Pd/C, MeOH,
room temperature.
and carbodiimide esterifications16 (Scheme 2). The tertiary
butyl17 and benzyl groups18 were selected because of their near
quantitative cleavages (using trifluoroacetic acid or hydrogen
chloride and hydrogen over Pd/C, respectively) and relative ease
of introduction at multigram scales. The synthesis of compound
14 allows for the facile synthesis of DRCs with various rodcoils.
The key factor in the synthesis of the conjugated segments 23,
32, and 39 is the coupling of the branched alkyl alcohol to the
asymmetric aromatic handle through an ester or ether linkage
prior to carbon-carbon bond formation (Schemes 3a, 4e, 5c).
This linkage increases the solubility of the aromatic rod in the
subsequent synthetic steps.19,20 The branched alkane was selected
as the coil segment because of its excellent solubility charac-
teristics13 and stability to all subsequent reaction conditions.
The synthesis of each conjugated rod segment in the DRC
series studied here is accomplished using a different carbon-
carbon bond forming reaction (Stille,21,22 Horner-Emmons,20,23
(19) Malenfant, P. R. L.; Jayaraman, M.; Frechet, J. M. J. Chem. Mater. 1999,
11, 3420-3422.
(20) Tew, G. N.; Pralle, M. U.; Stupp, S. I. J. Am. Chem. Soc. 1999, 121, 9852-
9866.
(16) Moore, J. S.; Stupp, S. I. Macromolecules 1990, 23, 65-70.
(17) Alexakis, A.; Gardette, M.; Colin, S. Tetrahedron Lett. 1988, 29, 2951-
2954.
(18) Denieul, M. P.; Laursen, B.; Hazell, R.; Skrydstrup, T. J. Org. Chem. 2000,
65, 6052-6060.
(21) Scott, W. J.; Crisp, G. T.; Stille, J. K. J. Am. Chem. Soc. 1984, 106, 4630-
4632.
(22) Facchetti, A.; Deng, Y.; Wang, A. C.; Koide, Y.; Sirringhaus, H.; Marks,
T. J.; Friend, R. H. Angew. Chem., Int. Ed. 2000, 39, 4547-4551.
(23) Boutagy, J.; Thomas, R. Chem. ReV. 1974, 74, 87-99.
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A R T I C L E S
Messmore et al.
Scheme 3. Synthesis of Quaterthiophene Phenyl Rodcoila
a (a) DIPC, DPTS, CH2Cl2, room temperature; (b) 15, Bu3Sn bithiophene, Pd(PPh3)4, DMF, 90 °C; (c) NBS, DMF, 0 °C to room temperature; (d)
isobutylene, H2SO4, CHCl3, room temperature; (e) Bu3Sn thiophene, Pd(PPh3)4, DMF, 90 °C; (f) nBuLi, Bu3SnCl, THF, -78 °C to room temperature; (g)
17, 21, Pd(PPh3)4, DMF, 110 °C; (h) TFA, CH2Cl2, room temperature; (i) 23, 14, DPTS, DIPC, CH2Cl2, room temperature; (j) TFA, CH2Cl2, room temperature.
Scheme 4. Synthesis of Terphenylene Vinylene Rodcoila
a (a) TDSCl, imidazole, CH2Cl2, room temperature; (b) triethyl phosphite, 130 °C; (c) 26, LDA, 25, THF, -78 °C to room temperature; (d) DIBALH,
Et2O, -78 to 0 °C; (e) 2-octyl-dodecan-1-ol and R-bromotoluic acid, DPTS, DIPC, CH2Cl2, room temperature; (f) triethyl phosphite, 130 °C; (g) 30, LDA,
28, THF, -78 °C to room temperature; (h) TBAF, THF, -78 °C; (i) 14, 32, DPTS, DIPC, CH2Cl2, room temperature; (j) HCl, dioxanes, room temperature.
and Suzuki24,25 couplings). The synthesis of phenyl-quater-
(thiophene) rodcoil (23) utilized Stille cross-coupling reactions.
Bromination conditions (Scheme 3c) were adapted from the
literature.26 4-Bromo phenol was utilized as an aromatic handle
for facile incorporation of the thiophene rod into the ester-based
DRC architecture due to tautomerization of 3-hydroxythio-
phenes.27 The synthesis of the hydroxyphenyl quater(thiophene)
rodcoil was convergent to avoid solubility problems (Scheme
3g). The synthesis of the ter(phenylene-vinylene) rodcoil (32)
utilized Horner-Emmons couplings to form exclusively trans
olefins in the phenylene vinylene rod.20 Using lithium diiso-
propyl amide as a base at -78 °C in the Horner-Emmons cou-
plings avoided ester and silyl ether cleavage (Scheme 4 c and
g). The dimethylthexyl silyl protecting group was cleaved with
tetrabutylammonium fluoride at -78 °C to avoid ester cleavage.
Suzuki coupling 4′-bromo-[1,1′-biphenyl]-4-carboxylic acid was
not readily available. The 4′-bromo-[1,1′-biphenyl]-4-ol was
reacted with the branched alkyl mesylate under standard
(24) Watanabe, T.; Miyaura, N.; Suzuki, A. Synlett 1992, 207-210.
(25) Ohe, T.; Miyaura, N.; Suzuki, A. J. Org. Chem. 1993, 58, 2201-2208.
(26) Bauerle, P.; Wurthner, F.; Gotz, G.; Effenberger, F. Synthesis 1993, 1099-
1103.
(27) Capon, B.; Kwok, F.-C. Tetrahedron Lett. 1986, 27, 3275-3278.
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Polymers from Electroactive Dendron Rodcoil Molecules
A R T I C L E S
Scheme 5. Synthesis of Quaterphenylene Rodcoila
a (a) MsCl, TEA, CH2Cl2, room temperature; (b) isobutylene, H2SO4, CHCl3, room temperature; (c) 34, K2CO3, 18-crown-6, acetone, reflux; (d) nBuLi,
B(OMe)3, HCl; (e) 37, 35, Pd(PPh3)4, Na2CO3, toluene, EtOH, H2O, reflux; (f) TFA, CH2Cl2, room temperature; (g) 14, DPTS, DIPC, CH2Cl2, room
temperature; (h) TFA, CH2Cl2.
conditions28 (Scheme 5c), and the boronic acid29 was synthesized
from the solubilized 4-bromo-4′-alkyloxy-biphenyl (Scheme 5d).
The 4′-bromo-[1,1′-biphenyl]-4-ol was tertiary butyl protected
using isobutylene and catalytic H2SO4 and was reacted under
Suzuki conditions with the boronic acid.
Self-Assembly. Our initial attempts at designing the conju-
gated DRC series involved the complete replacement of the three
biphenyl-ester units of the original DRC molecule13 for a shorter
conjugated segment. For example, we synthesized molecules 4
and 6 in Scheme 1. These molecules can be dissolved at high
temperatures in toluene. Upon cooling, however, we observed
precipitation rather than gelation, suggesting that extensive one-
dimensional self-assembly does not occur. Molecule 5 does
exhibit gelation without the added length of a biphenyl segment,
pointing out how subtle are the required noncovalent interactions
between rod segments to trigger the assembly of high aspect
ratio nanostructures. As prepared in tetrahydrofuran (THF), DRC
samples absorb and fluoresce at wavelengths corresponding to
the fully solvated chromophores30 (Figure 1a, Table 1). At the
same concentration, 30:1 toluene:THF, 4T-DRC, 3PV-DRC,
and 4P-DRC form self-supporting, birefringent gels after
dissolution by heating to 110 °C and subsequent cooling to
room temperature (Figure 1b). The self-assembly behaviors of
4T-DRC, 3PV-DRC, and 4P-DRC are remarkably similar
despite their structural variations. The added length of the
biphenyl segment increases the ability of rod segments to π-π
stack, but also adds additional rotational degrees of freedom
about the ester bond.
Figure 1. (a) Absorbance and fluorescence emission spectra of 4T-DRC
gel dissolved in toluene and THF. (b) White light illuminated photographs
of 4T-DRC in toluene (left) and THF (right). (c) 365 nm light illuminated
photographs of 4T-DRC in toluene (left) and THF (right).
Table 1. Optical Behavior of DRCs
λ
max (nm)a
λ
max (nm)b
toluene
THF
toluene
THF
The formation of a gel at low concentrations (1 wt %) sug-
gests self-assembly of DRC molecules into high aspect ratio
nanostructures.11,13,31,32 Upon gelation of the DRC series, ab-
sorbance is blue-shifted and fluorescence emission red-shifts.
The shifts in absorbance and fluorescence maxima are consistent
with exciton coupling of the aromatic units due to the formation
of H aggregates.33 The absorption and fluorescence intensity
of the DRCs decreases in the assembled state, typical of other
aggregated systems due to increased scattering as well as fluor-
4T-DRC
3PV-DRC
4P-DRC
398
337
296
424
367
305
566
444
424
510
473
418
a Absorption. b Emission.
escence quenching in the aggregated state10,34,35 (Figure 1, Table
1, Supporting Information). Solution NMR reveals further
evidence of DRC aggregation. The aromatic peaks of 4T-DRC
in THF-d8 are unresolved in toluene-d8 (Figure 2). The aliphatic
protons, however, persist in toluene-d8, broadening only slightly.
This indicates more rotational freedom of the alkyl coil segment
relative to the aromatic rod-like segment.13,36
(28) Claussen, R. C.; Rabatic, B. M.; Stupp, S. I. J. Am. Chem. Soc. 2003, 125,
12680-12681.
(29) Liess, P.; Hensel, V.; Schluter, A. D. Liebigs Ann. 1996, 1037-1040.
(30) Wei, Y.; Yang, Y.; Yeh, J. M. Chem. Mater. 1996, 8, 2659-2666.
(31) Terech, P.; Weiss, R. G. Chem. ReV. 1997, 97, 3133-3159.
(32) Hartgerink, J. D.; Beniash, E.; Stupp, S. I. Science 2001, 294, 1684-
1688.
(34) Schenning, A.; Jonkheijm, P.; Peeters, E.; Meijer, E. W. J. Am. Chem.
Soc. 2001, 123, 409-416.
(35) Whitten, D. G. Acc. Chem. Res. 1993, 26, 502-509.
(33) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11,
371-392.
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Figure 2. NMR of 4T-DRC in THF-d8 (top) and in toluene-d8 (bottom) (the signals in the toluene spectrum at 7 ppm are residual solvent peaks from
toluene-d8).
Table 2. Small-Angle X-ray Diffraction Peaks Taken from Dried
4T-DRC was cast from toluene and dried onto a holey carbon
coated grid. While TEM revealed several different morphologies
Films of DRC Gels ((0.2 nm)a
molecule
spacing (nm)
for the self-assembled structure of 4T-DRC, the dominant one
observed was a twisted, high aspect ratio nanostructure with a
remarkably uniform diameter of 9.3 ( 0.7 nm. The atomic force
microscopy (AFM) images of similarly prepared samples on a
freshly cleaved mica surface show ribbon-like features with
thicknesses of 2-6 nm. Figure 4 shows AFM images of the
ribbon nanostructures of 3PV-DRC and 4P-DRC.
4T-DRC
3PV-DRC
4P-DRC
10.0
10.3
9.1
a Peaks were fit after integration of the 2D diffraction pattern and
background subtraction using a standard Gaussian-Lorentzian curve-fitting
algorithm.
We conclude that DRC molecules dissolved in toluene self-
assemble into ribbon-like nanostructrues. Self-assembly is
evident macroscopically by changes in solvent flow (the
formation of a self-supporting gel) due to an interconnected
network of one-dimensional nanostructures. The nanostructures
have ribbon morphologies with π-π stacked conjugated seg-
ments as evidenced by X-ray, TEM, AFM, optical, and NMR
spectroscopies. These one-dimensional scaffolds formed by
DRC molecules offer a strategy to create ordered arrays of
electronically active molecules.
Conductivity and Alignment of 4T-DRC. As described
above, DRC self-assembly directly affects the physical behavior
of these materials. In addition, we explored the effect of self-
assembly on the electronic properties of solid-state films cast
from gels of 4T-DRC nanostructures. Casting from an as-
sembled state results in a film of supramolecular nanostructures
instead of the amorphous or crystalline films expected when
casting films from oligo(thiophene) solutions.4 Iodine-doped
films of 4T-DRC were prepared from 1 wt % gels (assembled)
and THF solutions (solvated) of 4T-DRC. The films cast from
the self-assembled state were strongly birefringent, and prepara-
tion on a patterned, four-probe electrode (Figure 5) indicated
conductivities of 7.9 × 10-5 S/cm. In great contrast, films cast
from THF solutions in which self-assembly does not occur
exhibit conductivities that are 3 orders of magnitude lower (8.0
× 10-8 S/cm, p-value < 0.01). In our model of 4T-DRC
assembly outlined above, π-π stacking plays a major role. We
believe the 3 orders of magnitude increase in conductivity is
most likely due to improved π-orbital overlap along conductive
pathways in the ribbon-like nanostructures. It is possible that a
similar enhancement could be observed due to conformational
or packing effects in the thiophene segments. In either case,
however, the increase in conductivity is clearly a result of self-
assembly.
Figure 3. (a) TEM micrograph of 4T-DRC ribbons cast on amorphous
carbon. (b) Molecular graphics of two DRC molecules measuring to be 10
nm.
X-ray scattering and microscopy of the series reveals the self-
assembly of DRC molecules into ribbon-like nanostructures.
Small-angle X-ray scattering (SAXS) spectra of dried DRC gels
contain diffraction peaks with characteristic d spacings due to
the solid-state packing of the ribbon structures, as summarized
in Table 2. The major peak matches the ribbon width observed
in transmission electron microscopy (TEM) (Figure 3a) and is
consistent with the fully extended length of two molecules, as
determined from molecular graphics (Figure 3b). While the exact
nature of the dendron interaction is unclear, hydrogen bonding
in the hydroxyl-rich dendron appears to play a significant role.13
For TEM observations, a dilute solution of nanostructures of
Alignment of nanoribbons can be accomplished by solution
casting films in an alternating current (AC) electric field. In an
AC field, the nanostructures become charged and oscillate due
to electrophoresis. Shear forces align the nanoribbons parallel
to the electric field lines, resulting in a uniaxially aligned film
(36) Ahrens, M. J.; Sinks, L. E.; Rybtchinski, B.; Liu, W.; Jones, B. A.; Giaimo,
J. M.; Gusev, A. V.; Goshe, A. J.; Tiede, D. M.; Wasielewski, M. R. J.
Am. Chem. Soc. 2004, 126, 8284-8294.
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A R T I C L E S
Figure 5. Polarized optical micrograph showing birefringent 4T-DRC film
cast from a toluene gel on a four-point probe electrode structure.
Figure 6. Polarized optical microscopy of an AC field aligned 4T-DRC
film. The film switches from light (a) to dark (b) with a 45° rotation between
crossed polarizers, indicating uniaxial orientation of the optic axis of the
film.
Figure 7. Atomic force microscopy image of aligned ribbons of 4T-DRC
on Au/Cr patterned mica substrate. The diagram (left) depicts where the
edge of the gold electrode begins on the substrate. Ribbons are aligned in
the gap.
field (Figure 7). According to the model of assembly, the π-π
stacking direction and the long axis of the ribbon nanostructures
are parallel to each other.
Figure 4. AFM images of 3PV-DRC (a) and 4P-DRC (b) on mica
(scalebar in micrometers).
Conclusions
of ribbon nanostructures, similar to other one-dimensional
systems.37,38 When viewed by polarized optical microscopy, the
birefringent film switches from light to dark upon a 45° rotation
between crossed polarizers (Figure 6a and b, respectively).
Ribbon alignment was also observed directly with AFM, using
a dilute gel on mica aligned between two electrodes by an AC
field. Ribbons in the gap show alignment parallel to the electric
The dendron rodcoil molecular architecture is a unique and
versatile structure to program the self-assembly of high aspect
ratio supramolecular polymers. We have shown that various
conjugated structures can be incorporated into dendron rod coils
without disrupting their one-dimensional self-assembly. The self-
assembly of the conjugated molecules into supramolecular
ribbon nanostructures leads to enhanced electronic conductivity
due to improved π-orbital overlap. Furthermore, these supramo-
lecular polymers can be oriented macroscopically in external
fields, which might be useful in bottom-up device fabrication.
(37) Krupke, R.; Hennrich, F.; von Lohneysen, H.; Kappes, M. M. Science 2003,
301, 344-347.
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2002, 35, 8106-8110.
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Acknowledgment. This work made use of the Keck Biophys-
ics Facility, the NIFTI Center, EPIC, and the Analytical Services
Laboratory at Northwestern University. B.W.M. would like
to thank Prof. E. Zubarev for helpful discussions, Prof. M.
Hersam, and M. Arnold for assistance with conductivity mea-
surements and S. Bull for MS measurements. The work was
supported by the U.S. Department of Energy (DoE) under
Award No. DE-FG02-00ER54810 and the Air Force Office of
Scientific Research (AFOSR) Multidiciplinary University Re-
search Initiative (MURI) under award number F49620-00-1-
0283.
Supporting Information Available: Synthetic information,
additional UV-vis and fluorescence spectra, SAXS 2D integra-
tions, and AFM. This material is available free of charge via
the Internet at http://pubs.acs.org.
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