Supramolecular thiophene nanosheets

Article abstract of DOI:10.1002/anie.201210090

Alternating copolymers consisting of phenyl-capped bithiophene (red units) and oligo(ethylene glycol) hierarchically self-assemble into nanosheets through polymer folding in some organic solvents. The lateral size of the nanosheet is controllable by tempe

Full text of DOI:10.1002/anie.201210090

Angewandte
Chemie
DOI: 10.1002/anie.201210090
Self-Assembly
Supramolecular Thiophene Nanosheets**
Yijun Zheng, Haixin Zhou, Dian Liu, George Floudas, Manfred Wagner, Kaloian Koynov,
Markus Mezger, Hans-Jꢀrgen Butt, and Taichi Ikeda*
Recently, two-dimensional (2D) materials, like graphene,[1]
inorganic nanosheets,[2] 2D polymers etc.,[3] attract much
attention owing to their unique structure and potential
applications for nanodevices. These membranes have molec-
ular-scale thickness (less than 5 nm) with huge surface areas,
which offer ample scope for further fabrication or modifica-
tion.[4] Self-assembled supramolecular nanosheets are also
attractive, because their constituents are not crosslinked
covalently, which makes it possible to control the size and
shape of the self-assembled structure by environmental
conditions.[5] Many examples of self-assembled nanosheets
have been reported including the lamellar structure of
hydrogen-bonding gelators,[6] amphiphilic or phase-segre-
gated block copolymers.[7] In those previous studies, the
Figure 1. a) Synthesis of copolymer poly(Ph2TPh-OEG). b) Solvent-de-
pendent UV/Vis spectra of poly(Ph2TPh-OEG). Concentration of the
Ph2TPh unit: 1.0ꢀ10À5 m. c) Solvent-dependent fluorescence spectra
of poly(Ph2TPh-OEG). Concentration of the Ph2TPh unit: 3ꢀ10À6 m.
Excitation wavelength is 365 nm.
molecules and block copolymers were designed to have
a plethora of molecular interactions, which directly lead to the
formation of the sheet structure. In contrast, biological
systems, that is, proteins, adopt a more complex hierarchical
approach. The polypeptides are folded with the help of
secondary structures (a-helices and b-sheets) and subse-
quently organized into higher-order structures, such as fibers
and nanosheets.[8] Achieving a hierarchical self-assembly by
using artificial polymer folding is still challenging.[9]
Artificial polymer folding has been achieved by using
copolymers consisting of a rigid aromatic unit and a flexible
oligo(ethylene glycol) (OEG) unit.[10,11] We applied phenyl-
capped bithiophene (Ph2TPh) as a rigid unit (poly(Ph2TPh-
OEG), Figure 1a), because the Ph2TPh unit has a high
crystallinity[12] and a potential for the application in molecular
electronics.[13] A similar molecular design was reported by
some research groups.[14] O. Henze et al. reported the fiber
formation of OEG-substituted thiophenes,[15] but the copoly-
mer with the same constituents lost the self-assembling
ability.[14b] Although S. Okamoto et al. have reported the
stacked-sheet structure of the folded copolymer on a sub-
strate,[16] self-assembly of the folded copolymer in solution
was unclear. In this study, we found that folded poly(Ph2TPh-
OEG) forms supramolecular nanosheets in some organic
solvents. Interestingly, the monomer unit, Ph2TPh-alkyne
(Figure 1a), did not form well-defined self-assembled struc-
tures under the same conditions. This result indicates that the
folding of the copolymer is a key step to form higher-order
structures, which mimics naturesꢀ strategy. We successfully
modified the nanosheet surface with a fluorescent probe and
observed the suspended nanosheets in solution.
Poly(Ph2TPh-OEG) was synthesized by copper(I)-cata-
lyzed azide–alkyne Huisgen cycloaddition of the alkyne- and
azido-functionalized monomers Ph2TPh-alkyne and Ph2TPh-
azide. (Figure 1a).[17] The feed ratio of Ph2TPh-alkyne/
Ph2TPh-azide was 1.0:0.9 so that most of the copolymers
have acetylene terminal groups. The chemical structure and
molecular weight of the copolymer were characterized by
1H NMR spectroscopy (Figure S4 in the Supporting Informa-
tion) and gel-permeation chromatography (GPC), respec-
tively (Figure S5, Mn = 1.6 ꢁ 104 Da, Mw/Mn = 3.5).
[*] Dr. Y. Zheng, H. Zhou, Dr. D. Liu, Prof. Dr. G. Floudas,
Dr. M. Wagner, Dr. K. Koynov, Dr. M. Mezger, Prof. Dr. H.-J. Butt,
Dr. T. Ikeda
Max Planck Institute for Polymer Research (MPIP)
Ackermannweg 10, 55128 Mainz (Germany)
Prof. Dr. G. Floudas
Department of Physics, University of Ioannina
45110 Ioannina (Greece)
Dr. T. Ikeda
Polymer Materials Unit
National Institute for Materials Science (NIMS)
Namiki 1-1 Tsukuba 305-0044 (Japan)
E-mail: IKEDA.Taichi@nims.go.jp
Solvent-dependent UV/Vis absorption spectra of poly-
(Ph2TPh-OEG) were measured. In dichloromethane, chloro-
form, and 1,1,2,2-tetrachloroethane, poly(Ph2TPh-OEG)
gave a smooth single absorption peak around 380 nm. Since
these spectra are comparable to that of the monomer unit
Ph2TPh-alkyne (Figure 1b and Figure S6a in the Supporting
Information), the absorption at 380 nm is assigned to the
[**] We acknowledge the discussions with Prof. Dr. Werner Steffen, and
we thank Christine Rosenauer for GPC, Thiel Jꢁrgen for DSC and
TGA, and Andreas Best for confocal microscope imaging experi-
ments.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201210090.
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dispersed Ph2TPh unit. In contrast, poly(Ph2TPh-OEG) in
1,2-dichlorobenzene (DCB), N,N’-dimethylformamide
(DMF), and dimethylsulfoxide (DMSO) showed blue-shifted
peaks around 360 nm, which are assigned to the aggregated
Ph2TPh units.[15,18] Fluorescence spectra support the above
assignments. Namely, poly(Ph2TPh-OEG) displays monomer
emission in CH2Cl2, CHCl3, and C2H2Cl4, but excimer
emission in DCB, DMF, and DMSO (Figure 1c).[16] Spectra
of the monomer unit Ph2TPh-alkyne did not depend on the
solvent (Figure S6a). These results indicate that the polymeric
structure is the key for inducing the aggregation of Ph2TPh
units.
To characterize the association behavior of Ph2TPh units
in more detail, we conducted temperature-dependent UV/Vis
spectra measurements at three different concentrations in
DCB (Figure S7 in the Supporting Information). With
increasing temperature, the absorption peak of the aggre-
gated Ph2TPh units decreased, whereas the peak of the
dispersed Ph2TPh units increased. This process was reversible
(Figure S6b). Different sets of isosbestic points were detect-
able below 1008C (374 nm and 431 nm) and above 1008C
(365 nm and 437 nm; Figure S7b,c). This result indicates that
the system contains two different processes below and above
1008C. The relationship between the absorption at 358 nm
and the temperature (Figure S7f) indicates that the first
process below 1008C is concentration-dependent; the second
is not. The former and latter processes are attributed to self-
assembly and folding, respectively, because the folding
process, which is intramolecular, should be concentration-
independent, but the self-assembly, which is intermolecular,
should depend on the concentration.[19]
Figure 2. a) TEM image of the self-assembled poly(Ph2TPh-OEG)
structure. The sample was prepared by using a DCB solution. Concen-
tration of the Ph2TPh unit: 5.0ꢀ10À5 m. b) TEM image of self-standing
nanosheets (blue arrows) on micropores (red arrows). The sample was
prepared by using a DCB solution. Concentration of the Ph2TPh unit:
1.0ꢀ10À4 m. c) Top left: AFM image of supramolecular thiophene
nanosheets on a silicon wafer; top right: height scale bar with the
range of height being 8.5 nm; bottom: section profile of the white line
in the AFM image. The sample was prepared by using a DCB solution.
Concentration of the Ph2TPh unit: 1.0ꢀ10À4 m. d) Electron diffraction
pattern of stacked supramolecular thiophene nanosheets on a carbon
support film.
The temperature dependency of the size of the self-
assembled structure was confirmed by dynamic light scatter-
ing (Figure S8a,b in the Supporting Information) in DCB.
The hydrodynamic radius of the self-assembled structure
decreased with increasing the temperature from 30 to 808C,
thus supporting the assignment of the first process below
1008C to the disassembly of the self-assembled structure. The
solvent dependency of the self-assembly process was also
confirmed by using the DCB and CHCl3 solutions (Fig-
ure S8c,d). In the DCB solution, scattering corresponding to
larger matter with a radius of approximately 280 nm was
detectable, which disappeared in CHCl3.
the TEM support. We succeeded to obtain a TEM image of
the self-standing sheets by using a perforated carbon support
film (Figure 2b). The size of the sheet is comparable to that
determined by dynamic light scattering (Figure S8). These
results suggest that the sheet structure was formed by self-
assembly in solution, not by surface-assisted self-assembly.
Poly(Ph2TPh-OEG) did not form well-defined self-assem-
bled structures in CH2Cl2, CHCl3, and C2H2Cl4 (Fig-
ure S10d,e,f in the Supporting Information).
To measure the thickness of the sheets, we applied atomic
force microscopy. It was confirmed that the self-assembled
sheet has a homogenous height of 3.5 nm and micrometer
lateral size (Figure 2c). The size of these nanosheets depends
on the concentration of the solution (Figure S11 in the
Supporting Information). At a concentration of 1 ꢁ 10À4 m,
large sheets over several mm and some stacked sheets were
observable (Figure S11c,d). Since large nanosheets tend to
form stacks in solution, a further increase in the concentration
resulted in a gel consisting of stacked nanosheets. The
monomer Ph2TPh-alkyne did not self-assemble into well-
defined structures under the same condition.
Temperature-dependent 1H NMR spectroscopic measure-
ments were conducted in the range from 25 to 808C in
a C2D2Cl4/[D6]DMSO solvent mixture (Figure S9 in the
Supporting Information). The mixed solvent lowers the
dissociation temperature of the Ph2TPh units. Although the
thiophene aromatic peaks were not observed at low temper-
ature, sharp peaks appeared at higher temperature. This
result indicates that the mobility of the Ph2TPh unit would be
suppressed at lower temperature owing to the aggregation,
which leads to a shorter T2 relaxation time.
Poly(Ph2TPh-OEG) could self-assemble into sheets in
DCB, DMF, and DMSO. The sheet structure was confirmed
by transmission electron microscopy (TEM; Figure 2a,b and
Figure S10 in the Supporting Information). TEM samples
were prepared by putting a droplet of the sample solution on
a TEM support, followed by removal of the solution by tilting
Wide-angle X-ray scattering (WAXS) measurements
were conducted by using a solid powder sample of the
nanosheets prepared by lyophilization of DCB solution
(Figure 3a). At low angles, a set of two broad reflections
(first order at 2q = 2.348, second order at 4.668) indicates
a lamellar structure with a periodicity of 3.5 nm. The d spacing
of 3.5 nm agrees well with the thickness of the nanosheet
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Figure 3. a) Wide-angle X-ray scattering pattern of a self-assembled
poly(Ph2TPh-OEG) powder sample in the solid state. The correspond-
ing d spacing values in nm are given next to the peaks. b) WAXS iso-
intensity contour plots as a function of temperature. The color scale
corresponds to intensity.
determined by AFM. This result suggests that the self-
assembled nanosheets maintain their structure in the powder
samples. The thickness of the single layer (3.5 nm) is smaller
than the expected length of the repeating unit (ca. 4.0 nm,
estimated from the crystal structures of poly(ethylene
glycol),[20] 1.948 ꢁ (4/7) ꢁ 2 = 2.23 nm, and the Ph2TPh
unit,[13a] 1.78 nm). This deviation may originate from a low
crystallinity of OEG chains or tilting of the OEG chains
against the long axis of the Ph2TPh unit[13a] and even both.
At higher angles, we observed three broad diffraction
peaks corresponding to the d spacings of 0.46, 0.39, and
0.32 nm. An electron diffraction measurement of the stacked
nanosheets (Figure 2d) indicates that these peaks are asso-
ciated with the in-plane structure that is, the molecular
packing of the Ph2TPh units within the nanosheets. Since this
WAXS pattern is comparable to that observed in a vacuum-
evaporated sexithiophene film,[21] these peaks are tentatively
assigned to the (110), (020), and (120) reflexes of a herring-
bone structure (orthogonal, a = 0.57 nm, b = 0.78 nm, Fig-
ure S12 in the Supporting Information). However, it is
impossible to determine the accurate atomic positions of the
Ph2TPh units owing to the low crystallinity and the small
number of significant reflections.
The result of the temperature-dependent WAXS mea-
surement is shown in Figure 3b. Although a slight decrease of
the scattering intensity was detectable above 1808C, the
ordered structure persisted up to high temperatures. The
result of differential scanning calorimetry (DSC) indicated
that poly(Ph2TPh-OEG) starts to melt above 1808C (Fig-
ure S13b in the Supporting Information). The thermal
decomposition temperature (5 wt% loss) was found to be
3758C by thermogravimetric analysis (Figure S13a). How-
ever, a color change of the sample was detectable after the
annealing at 1808C for 24 h (Figure S14 in the Supporting
Information). The 1H NMR spectrum of the annealed sample
indicated a slight deterioration of the Ph2TPh unit.
Figure 4. Illustration of a proposed mechanism for the formation of
supramolecular thiophene nanosheets. T1: Dissociation temperature of
the nanosheets, T2: unfolding temperature of the copolymer. T1 <T2.
strongly depends on the concentration of the solution (Fig-
ure S11). Temperature-dependent UV/Vis spectra at different
concentrations indicated that the self-assembly and folding
are independent processes (Figure S7f). The dissociation
temperature of the nanosheet (T1) is lower than the unfolding
temperature of the copolymer (T2). Although T1 depends on
the concentration of the solution, T2 is always 1108C in DCB.
This indicates that the folded conformation is quite stable in
DCB. Notably, the monomer unit of the copolymer (Ph2TPh-
alkyne) could not form well-defined self-assembled structures
under the same conditions. It is considered that in the case of
Ph2TPh-alkyne, the intermolecular interaction (enthalpy
gain) is not strong enough to overcome the entropy loss. In
the case of the copolymer, Ph2TPh units are covalently
linked. The entropy loss in the folding process of the
copolymer should be smaller than that of the self-assembly
process of the free monomers in the solution. The copolymer
can keep a high local concentration of Ph2TPh units within
a single copolymer chain irrespective of the copolymer concen-
tration in the solution. The 1,4-triazole linker also assists to form
the hairpin conformation of the OEG chains. Owing to these
favorable factors, the folded conformation of the copolymer is
stable in DCB, DMF, and DMSO. Compared to the monomer
unit Ph2TPh-alkyne, the folded copolymer has a much larger
interaction surface with the surrounding copolymers, which leads
to further interpolymer association. Solvent-dependent nano-
sheet formation suggests that the phase segregation of Ph2TPh
units from the solvent is an important driving force for inducing
the folding and the subsequent nanosheet formation. T-type
CH–p interactions between the Ph2TPh units can also stabilize
the nanosheet (Figure S12). Although the structure of the single
folded copolymer is difficult to determine, it is considered that
Ph2TPh units are associated with each other like in Figure 4 so as
to maximize the interaction between Ph2TPh units and minimize
the interface of Ph2TPh units to the solvent. It was reported that
the herringbone arrangement of thiophene/phenylene co-oligo-
mers would induce a blue-shift of the UV/Vis absorption.[22]
The supramolecular thiophene nanosheets reported
herein have many acetylene groups on the surface, which
allow further chemical modification of the nanosheet sur-
Figure 4 shows a model for the formation of supramolec-
ular thiophene nanosheets. The homogeneous thickness of
the nanosheets (3.5 nm) and 2D molecular packing structure
derived from the diffraction patterns (Figure S12 in the
Supporting Information) support the folded conformation of
the copolymer in the nanosheet. There is an equilibrium
between the free copolymer and the self-assembled nano-
sheet. AFM measurements revealed that the equilibrium
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Keywords: click chemistry · nanosheets · polymer folding ·
.
self-assembly · thiophene derivatives
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Received: December 18, 2012
Revised: February 6, 2013
Published online: && &&, &&&&
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Communications
Self-Assembly
Y. Zheng, H. Zhou, D. Liu, G. Floudas,
M. Wagner, K. Koynov, M. Mezger,
H.-J. Butt, T. Ikeda*
&&&& — &&&&
Supramolecular Thiophene Nanosheets
Alternating copolymers consisting of
phenyl-capped bithiophene (red units)
and oligo(ethylene glycol) hierarchically
self-assemble into nanosheets through
polymer folding in some organic solvents.
The lateral size of the nanosheet is
controllable by temperature and concen-
tration of the solution. The nanosheet
surface can be chemically modified by
using copper-catalyzed Huisgen cycload-
dition without disrupting the nanosheet
structure.
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These are not the final page numbers!