Conjugated polymers complexed with helical porphyrin oligomers create micron-sized ordered structures

Article abstract of DOI:10.1002/anie.200601493

(Figure Presented) Straight curls: Conjugated polymers are aligned through the use of twining porphyrin oligomers (twimers), which act as helical "hosts" that twine around and are included within a single conjugated polymer. The resulting composites aggre

Full text of DOI:10.1002/anie.200601493

Communications
Porphyrinoids
DOI: 10.1002/anie.200601493
Conjugated Polymers Complexed with Helical
Porphyrin Oligomers Create Micron-Sized
Ordered Structures**
Masayuki Takeuchi,* Chiaki Fujikoshi, Yohei Kubo,
Kenji Kaneko, and Seiji Shinkai*
Recently, oriented polymers and/or polymer nanostructures
have attracted a large amount of attention. Of particular
interest are structures that consist of conjugated polymers
(CPs) because of their potential applications as, for example,
electrochemical switches, electric devices, and sensors.[1,2] In
[*] Dr. M. Takeuchi, C. Fujikoshi, Dr. Y. Kubo, Prof. S. Shinkai
Department of Chemistry and Biochemistry
Graduate School of Engineering, Kyushu University
Fukuoka 812-8581 (Japan)
Fax: (+81)92–802–2823
E-mail: taketcm@mbox.nc.kyushu-u.ac.jp
Prof. K. Kaneko
HVEM Laboratory
Kyushu University
Fukuoka 812-8581 (Japan)
[**] M.T. and C.F. thank Mr. O. Hirata, Mr. S. Tanaka, Mr. M. Shirakawa,
Dr. M. Numata, and Dr. N. Fujita for valuable discussions and
comments. 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 this article is available on the WWW
under http://www.angewandte.org or from the author.
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addition to supramolecular assembly schemes,[3–6] a number of
approaches, including metastable states enforced by liquid
crystalline phases,[7–9] Langmuir monolayers at the air–water
interface,[10] incorporation into prealigned host matrixes,[11,12]
and rubbing,[13,14] have been utilized to align conjugated
polymers, thus leading to some unprecedented and fascinat-
ing photophysical functions. From a supramolecular stand-
point, we recently proposed a supramolecular bundling
approach toward the alignment of CPs. In this approach,
aligner molecules, which elicit positive homotropic alloster-
ism, are used to bundle, noncovalently splice, and align CPs to
form ordered assemblies of CPs.[15]
Herein, we present a new concept toward the alignment of
CPs through the use of twining polymers (twimers) that act as
helical “hosts” that would twine around and include within a
single conjugated polymer. Furthermore, we wanted these
“hosts” to integrate chromogenic groups that could mediate
electron or energy transfer to or from the conjugated
polymer. Taking these factors into consideration, we designed
the oligomeric porphyrins Por-12 and Por-6 (Scheme 1). In
their energy-minimized states, these oligomers tend to form
helical structures in which a coordinative open face of the zinc
porphyrin unit (but not a non-coordinative face covered by a
decamethylene group[16]) is always turned inwards so that the
central metal atom can interact with the included CP. We
expected, therefore, that a CP that possesses appropriate
ligand groups would become included within this helical
strand through coordination with the porphyrinatozinc. We
chose to use CP1 and CP2, which bear coordinative amino
groups, for the CPs.[15] The R groups in the Por oligomers were
introduced to control the spacing between Por/CP composites
and to increase their solubility in organic solvents. From
studies using various spectroscopic and microscopic techni-
ques, we proposed that the Por oligomers twine around the
CP strands and that the resulting composites aggregate into
relatively large two-dimensional (2D) structures in the solid
state that exhibit well-oriented periodic striping.[17]
We used computational methods (Insight II, Discover) to
evaluate whether the porphyrin units in the Por oligomers
could be arranged in helical structures. We confirmed that the
octamer of the Por series possessed helical turns and a 1D
cavity with dimensions of 1.1 ” 1.5 nm, which is adjustable
through the oscillation of meso-aryl groups (note that
Scheme 1a illustrates only the tetramer for the sake of
simplicity). Although it is also possible to construct a double
strand with a 0.74 ” 1.1 nm 1D cavity, this assembly is quite
sterically crowded.
UV/Vis absorption spectra of the complexes formed
between the Por-12 oligomer and CP1 were measured in
CDCl3 at 258C.[18] Figure 1a indicates that shifts to longer
wavelengths occur for both
the Soret (411–420 nm)
and Q bands (e.g., 537–
550 nm) of Por-12 with
several isosbestic points,
thus indicating that the
amino groups in CP1
have coordinated to the
ZnII atoms in Por-12. A
molar ratio plot of A550
versus [diamino units in
CP1]/[porphyrin units in
Por-12] gives
a
break
point at 1.0 (see the inset
of Figure 1a), which sug-
gests that only one of the
two amino groups interacts
with each porphyrin unit;
that is, a single-stranded
Por-12 oligomer could
wrap around a single CP1
chain, probably because
the formation of a doubly
stranded Por-12, in which
all of the amino groups
interact with porphyrin
units (in this case the
break point would occur
at 0.5), is sterically too
crowded (Figure 1c).
When we photoexcited
Scheme 1. Chemical structures of the Por oligomers and conjugated polymers used in this study. CP1:
Mn =48000; CP2: Mn =18000; Por-12: Mn =12000; Por-6: Mn =10000 (Mn =number-average molecular
mass). Energy-minimized structures of a) single- and b) double-stranded conformations of the Por oligomers.
Hydrogen atoms and peripheral alkyl chains have been omitted for clarity.
solutions of Por-12 (0–
0.072 mm unit) and CP1
(0.036 mm unit) in CDCl3
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of Por-12 transformed the broad
emission obtained in the solid state
into the sharp one similar to that
observed for a dilute solution of CP1
(see the Supporting Information),
that is, random aggregates of CP no
longer exist. With this experiment, we
also confirmed the bathochromic
shift of the Q bands of Por-12 in the
solid state, thus indicating that Por-12
formed the complex with CP1 (see
the Supporting Information). The
UV/Vis and emission spectra of the
solution-cast film of the composite
formed between Por-12 and CP1 in
CDCl3 suggest that the structure of
the composite is maintained in the
solid state.
To obtain visual images, we exam-
ined the composite formed between
Por-12 and CP1 in CDCl3 using con-
focal laser scan microscopy (CLSM),
polarized optical microscopy (POM),
and high-resolution transmission
electron microscopy (HRTEM).
From the HRTEM image, we con-
firmed that the Por-12/CP1 compo-
sites grewinto large aggregates (sev-
eral microns in size) that possess
stacked sheetlike 2D structures (see
below). In a separate study, we con-
firmed that the aggregates of Por-12
or CP1 alone are amorphous[15] and
cannot organize into 2D structures as
large as that presented in Figures 2
and 3.
Figure 1. a) UV/Vis spectra of Por-12 (0.050 mm unit) recorded after the addition of CP1. Inset: Plot of the
absorbance change at 550 nm of Por-12 (0.050 mm unit) versus [CP1unit]. b) Fluorescence spectra of CP1
(0.036 mm unit) recorded in the presence and absence of Por-12. c) Schematic illustration of the equilibrium
that exists between the singly and doubly complexed Por-12/CP1 composites. Lower scheme of (c) is a
schematic representation of the equilibrium that exists between the single- and double-stranded Por-12/CP1
composites; the straps and peripheral alkyl chains have been omitted for clarity. The structures were
calculated using Insight II, Discover, software.
We subjected a solution-cast film
of the Por-12/CP1 composite, con-
structed on an indium tin oxide (ITO)
glass, to POM observation. In
Figure 2, we observe a 2D structure
with dimensions greater than 50 mm2.
at lex = 400 nm (an isosbestic point in the UV/Vis spectrum)
at 258C, we observed that the emission intensity (lem
Very interestingly, this structure exhibits a bright pattern,
even between crossed polarizers, thus indicating that this
aggregate displays birefringence; that is, Por-12 and CP1
assemble into a highly ordered crystalline structure on the
micron scale on the ITO glass. When we photoexcited a single
large sheetlike aggregate (20 ” 120 mm) at 354 nm, the CLSM
image displays a blue CP1 emission (400–440 nm) and a red
Por-12 emission (560–640 nm) in the same morphological
domain (see the Supporting Information). It is clear, there-
fore, that the micron-sized superstructure was assembled
through interactions between Por-12 and CP1.
The electron micrographs of the composite provide
information regarding howthe Por-12/CP1 composite self-
organizes. We prepared solution-cast films of the Por-12/CP1
composite on a TEM grid without staining. Figure 3 displays
HRTEM images of the aggregate of the Por-12/CP1 compo-
site in which the periodicity of the dark stripes, over a distance
=
465 nm) of CP1 decreased upon increasing the Por-12
concentration (Figure 1b). At [Por-12] = 0.072 mm, the fluo-
rescence intensity of CP1 decreased to 7% of its value in the
absence of Por-12. This finding clearly indicates that the Por-
12 and CP1 units interact and that efficient energy transfer
from CP1 to Por-12 probably occurs in their complex as a
result of the emission wavelength of CP1 overlapping
considerably with the absorption band of Por-12. We obtained
a solution-cast film after deposition of a homogeneous
solution of CP1 (0.30 mm unit) in CDCl3 containing
5.0 wt% polystyrene (Mw = 200000) on a quartz plate. The
emission spectrum of this CP1 film was broad in comparison
with that recorded in dilute solution ([CP1 unit] = 0.03 mm).
This effect probably is due to random aggregation of the
polymer main chains in the film.[15] Interestingly, the addition
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are probably also ordered
along these stripes. Given
the degree of polymeri-
zation of CP1, the Por-12
oligomers must splice CP1
noncovalently to form such
aligned
supramolecular
assemblies. The addition
of trifluoroacetic acid to a
solution of the Por-12/CP1
composite
in
CDCl3
(namely, to protonate the
amino groups of CP1)
altered the overall mor-
phologies of aggregates of
the Por-12/CP1 composite
on the TEM grid from
sheetlike to amorphous
structures (see the Sup-
Figure 2. Optical microscopy images of the assemblies of the a) Por-12/CP1 ([Por-12unit]=0.020 mm;
[CP1unit]=0.036 mm) and b) Por-12/CP1 ([Por-12unit]=0.20 mm; [CP1unit]=0.40 mm) composites on ITO as
observed between crossed polarizers. c) Optical microscopy image of the assemblies observed after 458
rotation of the sample b). Inset: Images obtained without polarizers. Samples were prepared by drop-casting a
solution of the Por-12/CP1 composite in CDCl3 on the ITO surface and then drying.
porting Information for further details). This phenomenon
is due to decomplexation of the Por-12/CP1 composites into
Por-12 and CP1·nH+ units in solution, which we confirmed
from the appearance of the UV/Vis absorption spectrum of
Por-12 (the value of the lmax value of the Soret band was
412 nm; see Figure 1). Neutralization through treatment of
the acidified solution of Por-12/CP1 in CDCl3 with 1.0m
aqueous NaOH resulted in the Soret band of Por-12 shifting
to 420 nm. Through a process that we ascribe to recomplex-
ation between Por-12 and CP1, TEM analysis clearly
indicates that reorganization occurred to form the sheetlike
morphologies (see the image given in the Supporting
Information). These results indicate that the crystalline
sheet structure is constructed from Por-12/CP1 composites.
Polymer CP2 also formed complexes with Por-12 in a
manner similar to that occurring between Por-12 and CP1.
The UV/Vis and fluorescence spectra support the formation
of a composite between Por-12 and CP2 (see the Supporting
Information). Figure 3c displays a HRTEM micrograph of
the aggregates of the Por-12/CP2 composite; because the
striped pattern possesses a 4.0-nm periodicity, it appears that
the side chains of CP2 do not influence the periodicity
significantly. The following question arises: What does this
distance of 4.0 nm represent? In an attempt to answer this
question, we prepared a composite from Por-6 and CP2 and
its solution-cast film using the same procedure. Interestingly,
we observed an HRTEM image that depicts a similar striped
structure, but with a distance between stripes of only 2.7 nm
(Figure 3d). This result indicates clearly that the distance
reflects the length of the alkyl groups of the Por oligomer. In
fact, if we assume that partially interdigitated packing of the
alkyl groups of Por occurs in the Por/CP composites that were
formed through the twining of Por oligomers around the CP,
then we estimate the ZnII···ZnII distance between adjacent
Por-12/CP composites to be 4.0 nm and that for the Por-6/CP
composites to be 2.7 nm (Figure 4), in which the peripheral
alkyl chains are also thought to participate in the packing
among the composites but to be indecisive.
Figure 3. Electron micrographs (no staining) of aggregates of the Por/
CP composites. a) HRTEM image of the Por-12/CP1 composites that
display dark and light stripes. The periodicity between the dark stripes
is 4.0 nm. b) HRTEM image of a thick plate of Por-12/CP1 composites.
The crossed dark stripes suggest the overlap of a few thin plates that
possess striped structures. c) HRTEM image of the Por-12/CP2
composites. The periodicity between the dark stripes is 4.0 nm.
d) HRTEM image of the Por-6/CP2 composites that display dark and
light stripes. The periodicity of the dark stripes is 2.7 nm.
of a fewhundred nanometers, was 4.0 nm, as determined from
the Fourier-filtered image (Figure 3a). In the image of a thick
plate (Figure 3b), we observe crossed dark-stripe contrast,
thus indicating that a fewthin plates with striped structures
were overlapped. We infer that the dark sections in the
electron micrographs of the Por-12/CP1 composites are
regions that contain ordered p-stacked layers and/or the
heaviest atom (Zn). As mentioned above, the diamino groups
in CP1 coordinate to these ZnII atoms and, therefore, the CP1
From these findings, we propose the following sequential
growth mechanism for the preparation of these micron-sized,
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over the intercomposite spacing.
The concepts that we introduce
herein are complementary to the
techniques that exist currently for
the preparation of supramolecular
and macromolecular assemblies.
Furthermore, our results imply that
“molecular programming”—that is,
utilizing weak intermolecular inter-
actions that tend to occur through
reversible processes in a search for
the thermodynamically most stable
state—can play crucial roles during
such organization processes. This
Figure 4. a) Schematic representation of the influence of the peripheral alkyl chains in the Por oligomers on
the periodicity of the aligned assemblies. The peripheral alkyl chains in CP entwined within the Por oligomers,
which are also considered to participate in the van der Waals packing among the composites in the solid
state, have been omitted for clarity. b) Schematic representation of the formation of the crosslinked network
structure.
twining
association/aggregation
event is a rare example of a molec-
ular assembly technique that results
in a visually recognizable ordered
structure; in this regard, our process
highly ordered assemblies (Figure 4a). In the first step, the
Por oligomer, which is preprogrammed for helix formation in
the presence of the guest, twines around the CP strand and
becomes stabilized through the formation of dative bonds
between the porphyrinatozinc and amino units in solution. In
the second step on the surface, the composites organize
is reminiscent of some of the self-assembly and self-organ-
ization events that occur in biological systems. The applica-
tion for other 1D polymeric materials is nowunder inves-
tigation.
together through attractive van der Waals interactions Experimental Section
All starting materials and solvents were purchased from Tokyo Kasei
between the alkyl groups during the solidifying process. In
the third step, they construct 2D sheetlike assemblies that
display distinctly striped patterns. Other systems that form a
crosslinked network structure constructed from the Por
oligomer and CP (Figure 4b) could not elucidate the effect
of the alkyl chain length in the Por oligomers on the
periodicities and the contrast patterns observed in the
electron micrographs.[19] It is unlikely that the random
crosslinked network structure resulted in the formation of
the crystalline assemblies on the ITO glass and the TEM grid
during the solidification process.
The assembly/aggregation approach we present herein is
also readily applicable for use with nonconjugated, flexible
polymers bearing coordinating moieties. For example, poly(4-
vinylpyridine) (Mw = 60000) also complexed with Por-12 in a
manner similar to its assembly with CP1 through dative
bonds, and the stoichiometry between [pyridine units in
poly(4-vinylpyridine)] and [porphyrin units in Por-12] w as
clearly determined to be 2:1 (see the Supporting Informa-
tion). TEM images and electron diffraction patterns indicate
clearly that the poly(4-vinylpyridine)/ Por-12 composites self-
organized into crystalline supramolecular assemblies with the
periodicity of 5.0 nm that formed stacked thin 2D sheets (see
the Supporting Information).
Chemicals or Wako Chemicals and used as received. 1H NMR spectra
were recorded on a Bruker DRX 600 (600 MHz) spectrometer.
Chemical shifts are reported in ppm downfield from tetramethylsi-
lane, the internal standard. Mass-spectral data were obtained 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.
A typical example of sample preparation for TEM and POM
measurements: A solution of CP1 ([CP1 unit] = 36 mm) and Por-12
([Por-12 unit] = 20 mm) in CDCl3 was prepared. UV/Vis spectroscopy
confirmed that the porphyrinatozinc units in Por-12 had formed
complexes quantitatively. This solution was then cast onto a copper
TEM grid upon a holey carbon support film or onto the ITO glass.
TEM and HRTEM: The images were acquired using JEOLTEM-
2010 (accelerating voltage: 120 kV) and TECNAI-20 FEI (accelerat-
ing voltage: 200 kV) microscopes, respectively. The 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.
Received: April 14, 2006
Published online: July 18, 2006
Keywords: alignment · conjugated polymers · porphyrinoids ·
.
supramolecular chemistry · twining
Herein, we have demonstrated that porphyrin oligomers
(Por series) form homogeneously dispersed composites with
conjugated polymers (CP1 or CP2) and poly-4-vinylpyridine
in CDCl3; the resulting micron-sized aggregates on the
surface possess highly ordered, crystalline structures.[20] It is
quite astonishing that such large, highly ordered assemblies
could be created simply by mixing two polymeric components
together and then drop-casting on a surface. Modification of
the lengths of the alkyl chains in the Por series allows control
[1] a) G. A. Ozin, A. C. Arsenault, Nanochemistry, RSC, Cam-
bridge, 2005; b) D. T. McQuade, A. E. Pullen, T. M. Swager,
Chem. Rev. 2000, 100, 2537 – 2574.
[2] F. J. M. Hoeben, P. Jonkheijm, E. W. Meijer, A. P. H. J. Shen-
ning, Chem. Rev. 2005, 105, 1491 – 1546.
[3] J.-M. Lehn, Supramoleculer Chemistry, VCH, NewYork, 1995.
[4] S. I. Stupp, V. LeBonheur, K. Walker, L. S. Li, K. E. Huggins, M.
Keser, A. Amstutz, Science 1997, 276, 384 – 389.
5498 www.angewandte.org
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[5] S. Park, J.-H. Lim, S.-W. Chung, C. A. Mirkin, Science 2004, 303,
348 – 351.
[6] H. Sirringhaus, P. J. Brown, R. H. Friend, M. M. Nielsen, K.
Bechgaard, B. M. W. Langeveld-Voss, A. J. H. Spiering, R. A. J.
Janssen, E. W. Meijer, P. Herwig, D. M. de Leeuw, Nature 1999,
401, 685 – 688.
[7] H. Goto, K. Akagi, H. Shirakawa, Synth. Met. 1997, 84, 373 – 374.
[8] T. M. Long, T. M. Swager, J. Am. Chem. Soc. 2002, 124, 3826 –
3827.
[9] K. P. Fritz, G. D. Scholes, J. Phys. Chem. B 2004, 108, 10141 –
10147.
[10] J. Kim, T. M. Swager, Nature 2001, 411, 1030 – 1034.
[11] A. Montali, C. Bastiaansen, P. Smith, C. Weder, Nature 1998,
392, 261 – 264.
[12] W. C. Molenkamp, M. Watanabe, H. Miyata, S. H. Tolbert, J.
Am. Chem. Soc. 2004, 126, 4476 – 4477.
[13] M. Hamaguchi, K. Yoshino, Appl. Phys. Lett. 1995, 67, 3381 –
3383.
[14] P. Dyreklev, M. Berggren, O. Inganas, M. R. Anderson, O.
Wennestrom, T. Hjertberg, Adv. Mater. 1995, 7, 43 – 45.
[15] Y. Kubo, Y. Kitada, R. Wakabayashi, T. Kishida, M. Ayabe, K.
Kaneko, M. Takeuchi, S. Shinkai, Angew. Chem. 2006, 118,
1578 – 1583; Angew. Chem. Int. Ed. 2006, 45, 1548 – 1553.
[16] a) A. Osuka, F. Kobayashi, K. Maruyama, Bull. Chem. Soc. Jpn.
1991, 64, 1213 – 1225; b) N. C. Maiti, M. Ravikanth, J. Photo-
chem. Photobiol. A 1996, 101, 7 – 10.
[17] C. Kubel, M. J. Mio, J. S. Moore, D. C. Martin, J. Am. Chem. Soc.
2002, 124, 8605 – 8610.
[18] The spectroscopic data were not as reproducible when we used
CHCl3 as the solvent, probably because the amino groups
induced decomposition of CHCl3 to generate some acidic
species; in contrast, the use of CDCl3, which is somewhat less
reactive than CHCl3, resulted in very reproducible data.
[19] Dynamic light scattering (DLS) analysis of a solution of the
composite in CDCl3 indicated that by mixing the Por oligomer
and CP, somewhat larger assemblies with a high aspect ratio
assignable to the Por/CP composite exist in solution (see the
Supporting Information). At the present stage, we can not fully
rule out the competitive association system (the intersected
model) depicted in Figure 4b; however, we infer that the
formation of the composite as illustrated in Figure 4a is more
likely.
[20] The assemblies of the composites may form a mesophase or
organogel; however, we could not observe gelation phenomena
under the conditions used and liquid crystalline phases.
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