A supramolecular bundling approach toward the alignment of conjugated polymers

Article abstract of DOI:10.1002/anie.200503128

(Figure Presented) All line up together! A new concept for the alignment and assembly of conjugated polymers through the action of supramolecular bundling ("aligner") molecules is inspired by actin-filament bundling proteins. The approach provides a gener

Full text of DOI:10.1002/anie.200503128

Communications
proteins found in animal cells bind one-dimensional (1D)
actin filaments in high affinity to elicit the formation of actin
bundles.[13] The bundling proteins possess two interactive
modules for cross-linking actin filaments; their distinct
properties determine the type of assembly. If one can
reconstruct such modules interacting with 1D materials in a
supramolecular manner, not only would these systems
provide a new means of aligning the materials but also
would create complex mesoscopic structures and networks
akin to those found in nature. Herein, we report a new
concept for aligning and assembling conjugated polymers
through the action of supramolecular bundling (“aligner”)
molecules. We demonstrate this concept by utilizing the
dative bonds formed between porphyrinatozinc and amine
derivatives because of the high affinity and distinct bonding
geometry of these species.[14] We designed the aligner
molecules 1 and 2, which are porphyrinatozinc oligomers,[14,15]
to elicit positive homotropic allosterism[16–18] during their
binding of the amino-functionalized conjugated polymers CP
and CCP in an effort to organize the polymers into aligned,
rather than random, assemblies (Figure 1). Although the
distances between pairs of porphyrinatozinc units in 1 and 2
aligned in parallel can vary through conformational rear-
rangements about their rotational axes (the butadiyne unit for
1 and the ethylene bridges for 2), the distances between the
binding subunits when in a cofacial orientation are 2.5 and
2.0 nm, respectively. Each pair of cofacially aligned porphyr-
inatozinc tweezers in 1 and 2 binds to a diamine moiety of the
polymer in an allosteric manner to form polymer bundles
(Figure 1B); in this process, the binding of the first polymer to
an aligner molecule facilitates the second binding, which
results in the ready formation of aligned assemblies (Fig-
ure 1C).
To confirm the cooperativity of the binding of CP and
CCP by 1 and 2, we used MCP as a guest molecule for 1 and 2.
We noted the formation of the [1·MCP] or [2·MCP] com-
plexes in CHCl3 from changes in the UV/Vis absorption
spectra that occurred upon the successive addition of MCP
(see the Supporting Information). The values of lmax of the
Soret and Q bands shifted to longer wavelengths with tight
isosbestic points; these changes are consistent with those
observed from studies of other porphyrinatozinc–amine
coordination systems.[14] We estimated the stoichiometries of
the complexes formed between MCP and both 1 or 2 from
molar ratio plots, which clearly indicated the formation of 1:2
[1·(MCP)2] and 1:3 [2·(MCP)3] complexes. Importantly, plots
of absorbance at the Q band (607 nm) versus [MCP] pos-
sessed sigmoidal curvature. From analyses of these binding
isotherm using nonlinear-curve-fitting and Hill-plot[19] meth-
ods, we calculated the association constants (Kn/MCP) and
Hill coefficients (nH) to be K1 = 1.6 ” 105, K2 = 3.0 ” 105, and
nH = 1.9 for [1·(MCP)2] and K1 = 8.8 ” 105, K2 = 9.2 ” 105, K3 =
4.8 ” 106, and nH = 2.8 for [2·(MCP)3]. These results clearly
indicate that highly cooperative binding occurs between MCP
and both 1 and 2. Extrapolating from the cooperativity and
high association constants in the binding of MCP with 1 and 2,
we expected that the diamine moieties in CP and CCP would
become juxtaposed and aligned by the clefts in the porphyr-
inatozinc units in 1 and 2 in a positive homotropic allosteric
Supramolecular Chemistry
DOI: 10.1002/anie.200503128
A Supramolecular Bundling Approach toward the
Alignment of Conjugated Polymers**
Yohei Kubo, Yumiko Kitada, Rie Wakabayashi,
Takanori Kishida, Masatsugu Ayabe, Kenji Kaneko,
Masayuki Takeuchi,* and Seiji Shinkai*
Exploring new methods for controlling the orientation and
electronic state of p-conjugated oligomers and polymers is of
importance for the production of materials with optimized
properties and for their ultimate assembly into molecular
circuitry. In addition to supramolecular assembly schemes,[1–4]
methods for aligning conjugated polymers, which lead to
many new photophysical functions, include the use of
metastable states enforced by liquid-crystalline phases,[5–7]
Langmuir monolayers at the air–water interface,[8] incorpo-
ration into prealigned host matrixes,[9,10] and rubbing.[11,12]
Unlike synthetic macromolecular systems, the bundling
[*] Y. Kubo, Y. Kitada, R. Wakabayashi, T. Kishida, Dr. M. Ayabe,
Dr. M. Takeuchi, Prof. S. Shinkai
Department of Chemistry and Biochemistry
Graduate School of Engineering
Kyushu University, Fukuoka 819-0395 (Japan)
Fax: (+81)92-802-2820
E-mail: taketcm@mbox.nc.kyushu-u.ac.jp
seijitcm@mbox.nc.kyushu-u.ac.jp
Prof. K. Kaneko
HVEM Laboratory
Kyushu University
Fukuoka 812-8581 (Japan)
[**] We thank Dr. N. Fujita and Dr. M. Ikeda for fruitful discussions. M.T.
and Y.K. thank Prof. A. Ikeda for kindly supplying compound 2. Y.K.
thanks the JSPS Research Fellowship for Young Scientists for
financial support. This study was supported partially by a Sumitomo
Chemical Award in Synthetic Organic Chemistry Japan, a Grant-in-
Aid for Scientific Research B (17350071), and the 21st Century COE
Program, “Functional Innovation of Molecular Informatics,” of the
Ministry of Education, Culture, Science, Sports, and Technology
(Japan).
Supporting information for thisarticle isavailable on the WWW
under http://www.angewandte.org or from the author.
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Figure 1. A) Chemical structures of the aligner molecules and conjugated polymers studied herein. CP: number-average molecular mass
(Mn)=48000; CPP: Mn =18000. B) Schematic representation of the modes of bundling of the conjugated polymers with 1 (left) and 2 (right).
Blue platesdenote the porphyrinatozinc complexesin 1 and 2; sky-blue fibers and spheres denote the polymer backbones and methylaminomethyl
moieties, respectively, in the conjugated polymers. C) Schematic illustration of the alignment of the conjugated polymer CP mediated by 1. The
first polymer binding event to the porphyrinatozinc cleft preorganizes the second cleft (denoted by the asterisk) such that it has an even higher
affinity toward the second polymer. The conjugated polymers are expected to be juxtaposed to each other to form aligned assemblies.
manner. That is to say that we expected the first polymer
binding event to enhance the affinity of the second (and, for 2,
the third) polymer binding event to form aligned supramolec-
ular assemblies; such a process is necessary if we are to avoid
the formation of random assemblies. In fact, the addition of
CP to a solution of 1 (3.5 mm) or 2 (0.43 mm) in CHCl3 at 258C
resulted in similar bathochromic shifts in the Soret and
Q bands as those observed for 1 or 2 upon the addition of
MCP (see Figure 2 and the Supporting Information). The
emission intensity of CP ([CPunit] = 24 mm; lex = 400 nm) in
CHCl3 decreased, without a change in its shape when it was
mixed with 1 (see the Supporting Information). This phe-
nomenon is due to efficient energy transfer from CP to 1
within the complex because the emission wavelength of CP
overlaps considerably with the absorption of 1. Deposition of
a homogeneous solution of CP ([CPunit] = 240 mm) in CHCl3
containing 5.0 wt% polystyrene (Mw = 200000) on a quartz
plate provided us with a solution-cast film. The emission
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Figure 2. A) UV/Visspectra of 1 (3.7 mm) upon addition of CP in CHCl3 at 258C. Inset: magnified spectra about the Q bands. B) Fluorescence
spectral change of CP ([CPunit]=240 mm) in the presence and absence of 1 in the film cast from a solution of CHCl3 containing polystyrene
(5 wt%). The fluorescence intensity was normalized with respect to the absorption at 400 nm (lex).
spectrum of this CP film is broadened relative to that
recorded in dilute solution ([CPunit] = 24 mm); this effect is
probably due to random aggregation of the polymer main
chains in the film. Interestingly, the addition of 1 turned the
broad emission—even that obtained in the solid state—into
the sharp one observed for a dilute solution of the [1·CP]
complex (Figure 2B). Because the distance between the two
distinct binding sites in 1 is 2.5 nm, the bundled assemblies of
CP no longer emit the broad emission because of their
random aggregation. Polarized optical microscopy images of
[1·CP], as well as UV/Vis and fluorescence spectroscopic
studies, also support the formation of ordered assemblies in
the solid state (see the Supporting Information). These
findings suggest that the structures of the supramolecular
polymer bundles are maintained in the solid state; we further
confirmed this situation by using atomic force microscopy
(AFM) and transmission electron microscopy (TEM).
Figure 3A displays an AFM image of CP (deposited from
a 1.3 mm solution in CHCl3), in which the CP assembly is well
dispersed in a rectangular shape on highly oriented pyrolytic
graphite (HOPG). We calculated the average height to be
0.3 nm from the height profile. Polymer CP probably lies on
HOPG in a monolayer with a face-on orientation (Fig-
ure 3A). From the height profile of the deposition samples of
1 and 2, their thickness was shown to be 0.6 nm, thus
indicating that porphyrins in 1 and 2 lie on the HOPG in a
face-on orientation as much as possible. By mixing CP
([CPunit] = 1.2 mm) with 1 (0.08 mm), the assemblies assigned
to CP or 1 itself disappeared, and we observed a 20–25-fold
larger (in area ratio) homothetic shape for the CP assemblies
Figure 3. Representative AFM images of CP and [1·CP] assemblies deposited from solution of CHCl3 onto HOPG. Compounds 1 and 2 formed
amorphous assemblies on HOPG. The mixing ratio (a) isdefined as a=[CP]/[1 or 2] (mol/mol). A) CP ([CPunit]=1.3 mm); B) [1·CP] assemblies
(a=15); C) [1·CP] assemblies (a=9.1); D) [1·CP] assemblies (a=3.1); E) [1·CP] assemblies aged for 24 h under the same conditions as those in
(B); F) [2·CP] assemblies prepared under the conditions in which [CPunit]=1.3 mm and [2]=0.17 mm (a=5.9).
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that had a height of 0.35 nm (see Figure 3B and the
Supporting Information). Under the conditions in Figure 3B,
every seventh repeating unit of CP on average was complexed
with one porphyrin cleft of 1, thus showing that 1 was not so
densely packed in [1·CP] assemblies. The fact that the overall
assembly height is larger than observed for CP itself shows
that CP remain aligned in a porous-like solid-state morphol-
ogy rather than collapsing on the surface. Given the degree of
polymerization of CP, 1 must bundle with, and noncovalently
splice, CP to form such aligned supramolecular assemblies.
The polymer bundling and splicing caused by 1 is highly
dependent upon the mixing ratio between CP and 1 and the
aging period in solution. A higher molar ratio of 1 decreased
the size of the assemblies (see Figure 3C and D); that is, the
dimensions of the superstructure are readily controllable by
changing the mixing ratio of CP and 1. Aging the [1·CP]
complex for 24h in CHCl 3 at 258C resulted in further growth
of three-dimensional (3D) assemblies to a height of 1.5 nm
(Figure 3E). From these results, we infer that the bundling
and splicing processes caused by 1 in solution take place
thermodynamically to form initially two-dimensional and
then 3D aligned structures. Compound 2 also aligned CP into
sheetlike assemblies that had an average height of 0.3 nm and
9–16-fold larger areas than that of CP itself (Figure 3F). The
clefts of the porphyrinatozinc units in 1 and 2 are essential for
the bundling and noncovalent splicing of the polymers, as
evidenced by the fact that neither 3 nor 4 was capable of
producing similar assemblies.
Figure 4. Electron micrographs(no staining) of conjugated polymers
aligned by 1 or 2. A) A TEM image of aligned [1·CP] assemblies
consisting of dark and light bands. The periodicity between the dark
bandsis2.0 nm. B) An HRTEM image of [ 1·CP] assemblies with
0.2 nm periodicity, asrevealed from the Fourier-filtered image. C) An
HRTEM image of [2·CP] assemblies displaying multiwalled tubular and
lamellar contrasts. The image in the inset displays a multiwalled tube
that hasbent from the back to the front. D) An HRTEM image of the
[1·CCP] assemblies displaying a kagomꢀ lattice contrast. The perio-
dicity of the kagomꢀ lattice is0.8 nm.
The electron micrograph images of the complexes provide
information regarding how 1 and 2 align the polymer. We
observed highly ordered structures in the TEM and high-
resolution TEM (HRTEM) images (see Figure 4and the
Supporting Information). We prepared solution-cast films of
the polymer—in the presence and absence of 1 or 2—on a
TEM grid without staining. An HRTEM image of the CP
assemblies was observed (see the Supporting Information) in
which the periodicity of the dark layers was 0.4nm, as
determined from the Fourier-filtered image. We suggest that
the dark sections in the electron micrographs to be regions
that contain the ordered p-stacked layers. This hypothesis is
supported by the fact that the periodic distance of 0.4nm is
comparable with the distance between p-conjugated mole-
cules that are associated through p–p stacking interactions.[20]
In the low-magnification micrograph, crystalline sheets (see
the Supporting Information) and a multilamellar morphology
(Figure 4A) with a periodicity of 2.0 nm over a 200-nm-wide
distance was observed for the mixture of CP and 1; this
superstructure is totally different from that formed from CP
itself. We believe that the dark regions observed in the TEM
images of the assembly of CP and 1 to be domains that
contained ordered p-stacked layers and/or the heaviest atom
(Zn) in 1; the results of energy dispersive X-ray spectroscopy
(EDX) support this hypothesis (see the Supporting Informa-
tion). The period of 2.0 nm corresponds to the distance
between CP units when they are bundled in a parallel manner
(Figure 5A). We recorded an HRTEM image of one of the
sections of the assemblies constructed from CP and 1 to
obtain additional evidence for the highly aligned nature of the
polymer units. The micrograph in Figure 4B demonstrates
that the film comprised many dark/light layers and that the
periodicity of the dark contrast was 0.2 nm, which is much
shorter than the p–p-stacking distances of common organic
molecules. We infer that the dark/light contrast arises from
the phase contrast provided by the zinc atoms in 1 aligned in
[1·CP] assemblies when we observe the image in Figure 4A at
an oblique angle. Figure 4C displays the supramolecular
assemblies formed from CP and 2, which we expected to
bundle three CP strands to form 3D superstructures. We
observe in this image unique supramolecular assemblies that
possess both lamellar and multiwalled tubular contrast; the
inside diameters of the tubes varied between 2 and 5 nm. The
dark periodicities of the lamellar and multiwalled tubular
assemblies were both 0.35 nm; this observation indicates that
these assemblies were both constructed from the same
structural unit (Figure 5B). Polymer CCP is also bound to 1
in CHCl3 in a manner similar to that of CP and 1. In addition
to changes in the UV/Vis and fluorescence spectra, our
circular dichroism (CD) spectroscopic measurements also
support the formation of a complex between CCP and 1 (see
the Supporting Information). The split-type CD signal that we
observed in the Soret-band region indicates that the chiral
side chains in CCP influence the orientation of the porphyrin
plane in 1 to produce an induced-CD (ICD) signal, which
should reflect the contrast pattern of the [1·CCP] assemblies.
In fact, Figure 4D represents the HRTEM micrograph of the
aligned assemblies constructed from CCP and 1; the kagomØ
lattice and triangular grid patterns[21,22] with a 0.8-nm-
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Figure 5. Schematic representations of how A) 1 might align CP to form aligned assemblies and how B) [2·CP] assemblies might form multiwalled
tubular structures.
using a Perseptive Voyager RP MALDI-TOF mass spectrometer and/
or a JEOL JMS HX110A high-resolution magnetic sector FAB mass
spectrometer. UV/Vis and fluorescence spectra were recorded using
Shimadzu UV-2500 PC and Perkin-Elmer LS55 spectrophotometers.
Binding isotherm analysis: The cooperative guest-binding process
was analyzed according to the Hill equation: log[y/(1Ày)] = nlog
[guest] + logK, where K, y, and nH are the association constant, the
extent of complexation, and the Hill coefficient, respectively. The
slope and the intercept of the linear (Hill) plots allow K and nH to be
estimated; these values are useful measures of cooperativity. A higher
value of nH is related to a higher degree of cooperativity; the
maximum value is equal to the number of binding sites. In the analysis
of the binding isotherm from the Hill plot, we evaluated the
concentration of the unbound guest ([guest]) by assuming that the
1:2 complex of 1 is formed quantitatively when the absorption change
is saturated.
Sample preparation for AFM and TEM measurements: The
following provides a typical example of the approach. A solution of
CP in CHCl3 ([CPunit] = 290 mm) and 1 (20.0 mm) was prepared; UV/
Vis spectroscopy confirmed that the [1·CP] complex had formed
quantitatively. This solution was then diluted because the higher-
concentration [1·CP] assemblies covered the whole area of the HOPG
or TEM grid. The resultant dilute solution of [1·CP] assemblies in
CHCl3 was cast on HOPG or a copper TEM grid upon a holey carbon-
support film.
periodicity dark contrast regions are clearly apparent. The
differences between the contrast patterns of CP (lamellar)
and CCP (kagomØ lattice) might arise from the orientation of
1 in the assemblies being affected differently by the alkyl
chains of CP and CCP.[23] These micrographs demonstrate that
the conjugated polymers were aligned by both 1 and 2 to form
highly ordered supramolecular assemblies and that the
spacings and arrangements of the polymer binding sites in
the aligner molecules—and the nature of the peripheral alkyl
chains of CP and CCP—should determine the mode of
polymer assembly.
The exploitation of the supramolecular bundling that we
have described herein was inspired by the mode of action of
actin-filament bundling proteins. Our approach provides a
new, general means of preparing complex, ordered assemblies
of conjugated polymers. Introducing interactive sites for the
aligner molecules at special positions along the conjugated
polymers allows control over the dimensions, morphologies,
and interpolymer spacings. The concepts that we have
introduced complement the existing techniques for supra-
molecular and macromolecular assembly; in addition, they
allow the interpolymer spacing and orientation to be con-
trolled so that they can influence the photophysical properties
of the conjugated polymer. Moreover, this approach permits
the construction of alternating arrays of different conjugated
polymers. We believe that such a system could be realized by
utilizing an aligner molecule that displays positive hetero-
tropic allosterism toward such polymers.
TEM and HRTEM images were acquired using a JEOL TEM-
2010 (accelerating voltage: 120 kV) and a TECNAI-20 FEI (accel-
erating voltage: 200 kV), respectively. A sample solution was placed
on a copper TEM grid upon a holey carbon-support film. The TEM
grid was dried under reduced pressure for 6 h prior to TEM
observation.
AFM images were acquired in air using a Topo METRIX SPM
2100 (noncontact mode). The sample was cast on HOPG and dried
for 1 h under reduced pressure prior to AFM observation.
Received: September 2, 2005
Experimental Section
All starting materials and solvents were purchased from Tokyo Kasei
1
Chemicals or Wako Chemicals and used as received. The H NMR
spectra were recorded on a Brucker DRX 600 (600 MHz) spectrom-
eter. Chemical shifts are reported in ppm downfield from tetrame-
thylsilane as the internal standard. Mass-spectral data were obtained
Keywords: alignment · allosterism · conjugated polymers ·
porphyrinoids · supramolecular chemistry
.
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