A butterfly-shaped amphiphilic molecule: Solution-transferable and free-standing bilayer films for organic transistors

Article abstract of DOI:10.1002/anie.201100712

Butterflies in the stomach: Organic field-effect transistors containing a free-standing film self-assembled from an amphiphilic butterfly-shaped benzodithiophene derivative as the active layer (see picture) were fabricated by a solution transfer process.

Full text of DOI:10.1002/anie.201100712

Communications
DOI: 10.1002/anie.201100712
Molecular Electronics
A Butterfly-Shaped Amphiphilic Molecule: Solution-Transferable and
Free-Standing Bilayer Films for Organic Transistors**
Jie Yin, Yan Zhou, Ting Lei, and Jian Pei*
Ordered crystalline films are one of the most important
elements in high-performance optoelectronic devices.[1] Spe-
cially treated substrates and slow growth speeds are employed
to produce such films by enlarging the grain size and reducing
the grain boundaries. However, highly ordered crystalline
films are difficult to lift off and transfer because of the strong
interactions between the materials and the substrates.[2]
Expensive substrates and low compatibility of the special
treatment constantly limit their applications in optoelectronic
devices. Recently, free-standing films were fabricated on one
substrate and transferred to a medium and placed onto the
desired substrate to produce the final devices.[3] Such sepa-
ration of the film growth and device assembling process can
significantly decrease the cost without sacrificing the perfor-
mance of the devices. However, special substrates and lift-off
processes were needed in these cases. Substrate-free forma-
tion of the self-assembled organic films is thus still challeng-
ing.
Organic nano- and microstructural materials, especially
those self-assembled from p-conjugated molecules, have
exhibited potential applications in electronic and photonic
devices owing to their unique optoelectronic properties.[4,5]
Although various organic semiconductor devices from single
molecules to bulky single crystals were fabricated,[6] ordered
free-standing 2D architectures and their device performances
were rarely reported. A variety of novel supramolecular
architectures can be created from amphiphilic molecules,[7,8]
however, the carrier transport properties of such assemblies
have seldom been investigated. Herein, we report a free-
standing film which was directly self-assembled in solution by
a butterfly-shaped amphiphilic benzodithiophene derivative
through strong p–p interactions between rigid aromatic cores
and van der Waals interactions from both alkyl–alkyl and
triethylene glycol (TEG) chains. Organic field-effect transis-
tors incorporating such 2D free-standing films as the active
layer are fabricated by a solution transfer process. The hole
mobility in the transistors is up to 0.02 cm2 VÀ1 sÀ1. To the best
of our knowledge, this is the first report of an organic
transistor based on a free-standing film which was directly
self-assembled from solution without the assistance of a
substrate.
Scheme 1 illustrates the synthetic approach to amphiphilic
butterfly-shaped benzodithiophene derivative 1. This amphi-
philic molecule has a rigid aromatic backbone with alternat-
ing benzene and thiophene units that are peripherally
substituted with two hydrophobic dodecyloxyl chains and
two hydrophilic TEG chains on the opposite sides. A FeCl3
oxidative cyclization protocol was utilized to construct the
large planar aromatic skeleton by carbon–carbon bond
formation. The oxidative cyclization reaction proceeded
slower because of the polar TEG chains, and was accom-
plished overnight at room temperature to afford 1 in 85%
yield.[9] In our modular approach, the peripheral substituted
group can be easily altered, and thus the intermolecular
interactions and electronic characteristics of the target
molecules can be precisely tuned. Compound 1, which is a
pale-yellow solid, is readily dissolved in common organic
solvents, such as CHCl3, CH2Cl2, THF, and toluene. The
structure and purity of all compounds were verified by 1H and
13C NMR spectroscopy and high-resolution mass spectrome-
try. The thermal decomposition temperature of 4008C under a
nitrogen atmosphere revealed a good thermal stability of 1
(Figure S3 in the Supporting Information).
The self-assembly of 1 was performed by a common
heating–cooling process. Compound 1 (5 mg) was suspended
in CHCl3/MeOH (1:2 v/v, 3 mL) and heated to 508C. A clear
solution was obtained and filtered with a 0.2 mm filter.
Precipitates were formed from the homogenous solution
while it was slowly cooled to room temperature. Scanning
electron microscopy (SEM) and transmission electron mi-
croscopy (TEM) were employed to investigate the morphol-
ogy of the precipitates. As shown in Figure 1a,b, 2D filmlike
assemblies were formed. These assemblies can grow as large
as 50 mm ꢀ 50 mm (Figure S1 and Figure 1a), and have high
surface area, thin thickness, and good flexibility. The sheetlike
assemblies show obvious birefringence under polarized
optical microscopy (Figure 1c), which indicated that these
supramolecular assemblies have highly ordered architectures.
This result was also substantiated by differential scanning
calorimetry (DSC) where two endothermic transitions at
408C and 1508C were observed (Figure 1d).
[*] J. Yin, Y. Zhou, T. Lei, Prof. J. Pei
Beijing National Laboratory for Molecular Sciences
The Key Laboratory of Bioorganic Chemistry & Molecular
Engineering of Ministry of Education
College of Chemistry, Peking University, Beijing 100871 (China)
Fax: (+86)10-6275-8145
To further understand the formation process of the self-
1
assembled 2D films, the concentration-dependent H NMR
E-mail: jianpei@pku.edu.cn
spectra and photophysical properties of 1 were investigated.
In CDCl3 solution, the proton signals of 1 shifted upfield as
the concentration was increased from 10À4 to 10À2 m (Fig-
ure 2a), which is a result of the shielding effect of the ring
current of neighboring aromatic molecules through a cofacial
[**] This research was financially supported by the Major State Basic
Research Development Program (No. 2009CB623601) from MOST
and by the National Natural Science Foundation of China.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201100712.
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stacking.[10] Figure 2b shows
the absorption spectra of 1 in
CHCl3 solution and in the solid
state. The intensities of two
absorption bands at 370 and
388 nm increased as the con-
centration of the solution was
increased, thus indicating the
formation of J-aggregates
because of the strong p–p
stacking interactions among
the conjugated skeletons.[11]
The absorption features in the
drop-cast film of the aggre-
gates displayed
a
broad
absorption at 340 nm, which is
red-shifted about 20 nm rela-
tive to that in the solution.
Such behavior was observed
for the emission maximum
lmax , which also shows a con-
siderable red-shift when chang-
ing from solution to the solid
state (Figure 2c).[12]
X-ray diffraction (XRD) of
bulky films of 1 gave well-
defined diffraction patterns
that are attributed to the
reflections from (001) to (005)
planes and suggest a typical
lamellar structure (Figure 3a).
The lamellar d spacing is
30.9 ꢁ, which is about 7 ꢁ
larger than the molecular
structure models (calculated
Scheme 1. Synthetic route to amphiphilic molecule 1.
1
Figure 2. a) Comparison of H NMR spectra of 1 at different concen-
trations in CDCl3. Changes in the b) normalized absorption and
c) normalized emission spectra of 1 in CHCl3 at different concentra-
tions (open triangle: 1ꢀ10À4 m, open square: 1ꢀ10À5 m, open circle:
1ꢀ10À6 m, filled square: 1ꢀ10À7 m), spin-coated film (filled triangle),
and precipitates (filled circle) formed in CHCl3/MeOH (1:2 v/v).
Figure 1. a) SEM image, b) TEM image (inset: corresponding SAED
patterns), c) polarized optical microscopy image with cross-polarizers
of sheetlike assemblies of compound 1, and d) DSC profile of sheetlike
assemblies of 1 in the first heating/cooling cycle at a rate of
108CminÀ1
.
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Communications
To obtain the detailed information of the repeating unit of
the lamellar structure, the intensities of the five diffraction
peaks obtained from XRD data were employed to reconstruct
the intralamellar electron density profiles (Table S1).[13] As
shown in Figure 4a, the high-electron-density region (shown
in yellow to red) has a width of 7 ꢁ, which is consistent with
the molecular length of the fused-ring conjugated benzodi-
thiophene skeleton region (calculated 7.2 ꢁ). The two high-
electron-density regions are flanked by the low-electron-
density regions, which contain the interdigitated alkyl chains
in the middle (blue band with approximate width of 5 ꢁ) and
the TEG-substituted phenyl group on the outer sides (cyan to
blue; Figure 4a). The electron density profiles indicated that
the parts of the alkyl chains occupy a very small region
(central area, 5 ꢁ; Figure 4a), thus implying that the alkyl
chains are closely packed.
FTIR spectroscopy of the supramolecular assemblies 1
showed strong stretching vibrations of CH2 groups at 2920
(nanti) and 2850 cmÀ1 (nsym; Figure S2), thus contributing to the
crystalline packing of the alkyl chains,[8a,14] which is consistent
with the proposed close-packing model. Based on the above
results, the two phase-transition peaks observed in the DSC
measurements can be explained as follows: the minor
transition (408C) was related to a reorganization of the
alkyl chains from a highly ordered state to a lower ordered
state, as observed in some liquid-crystalline molecules.[15] The
second main transition (1508C) was attributed to the destruc-
tion of the ordered lamellar packing and resulted in an
isotropic state. Therefore, we concluded that the bilayer
sheets were formed through the strong p–p stacking parallel
to the surface of the films, assisted by the van der Waals
interactions of long hydrophobic aliphatic chains (Figure 4b).
Since the outsides of the bilayer sheets were surrounded by
TEG chains, further aggregation was inhibited. Thus the
resulting 2D films are free-standing, and are easily fabricated
on various substrates (e.g., silicon, metal, and glass) by drop-
casting or spin-coating processes.
Figure 3. a) XRD patterns of the films self-assembled from CHCl3/
MeOH (1:2 v/v). b) AFM image and thickness analyses of the self-
assembled films. Scale bar=1 mm.
23 ꢁ). The image of the selected-area electron diffraction
(SAED; Figure 2b, inset) exhibited a crystalline pattern with
two pairs of sharp diffraction spots, (100) and (200) that
correspond to a d spacing of 3.7 ꢁ, which is a typical p–p
stacking distance. The results indicated that the strong p–p
stacking between the molecular skeletons is along the sheet
planes, which is favorable for charge transport. Atomic force
microscopy (AFM) was also employed to study the lamellar
structure of the films. AFM images showed that, with the
exception of filmlike assemblies with thickness in the order of
hundreds of nanometers, individual thin films with a uniform
height of approximately 4.0 nm could also be observed
(Figure 3b), which was close to twice the length of the
molecular structure (calculated 4.6 nm). In view of the
molecular structure properties and dimensions of amphiphilic
1, we proposed that the amphiphile packs into a bilayer
structure, in which the hydrophobic benzodithiophene aro-
matic skeleton with interdigitated long alkyl chains were
packed toward the interior of the film and the TEG chains
were exposed to the outside. The difference of the height of
the XRD and AFM measurements suggested that the alkyl
chains and TEG chains are intercalated or bent in the
multilayer lamellar structures. The proposed structure is
illustrated in detail in Figure 4.
Such free-standing films were employed as the active layer
of organic field-effect transistors (OFETs). The electrodes
were pre-patterned by the standard photolithographic process
and followed by the thermal evaporation of titanium (5 nm)
and gold (30 nm). The space between the two electrodes was
5 mm. The suspensions of the nanosheets were directly spin-
coated onto an n-octadecyltrimethoxysilane(OTS)-treated
SiO2/Si substrate. Figure 5a shows the schematic illustration
of OFETs from an individual film. Figure 5b shows a
microscope image of an Au bottom-contact device con-
structed by a single free-standing film structure of 1. After
assembling with the free-standing films, the devices were
annealed under vacuum at 708C overnight. Typical p-channel
transport characters were observed. The highest mobility was
0.02 cm2 VÀ1 sÀ1 and the on/off ratio was 105. The average
mobility out of 17 devices was about 0.01 cm2 VÀ1 sÀ1 and the
average threshold was about 3.2 V. Figure 5c,d illustrates
transfer and output characteristics of the film-based device.
This is the first report of an OFET that was formed from a
solution processed, self-assembled transferable free-standing
film. It is also the first time that the active films were grown in
solution by direct self-assembly and then transferred to the
Figure 4. a) Reconstructed electron density map of self-assembled 1.
b) Proposed model of the organization of the bilayer structure of 1.
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Figure 5. a) Schematic illustrations of a FET device from an individual
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substrates. Such growth-transfer separation is the key step to
break the limitation of the substrate dependence of the
organic transistors.
In summary, we have developed a novel butterfly-shaped
p-extended condensed benzodithiophene amphiphilic mole-
cule 1 for organic transistors. By taking the advantage of its
amphiphilic properties, this molecule easily forms 2D free-
standing films by direct self-assembly in solution. The detailed
characterization of the free-standing films demonstrate that a
lamellar structure is formed through a bilayer arrangement.
The strong p–p interactions parallel to the surface of the films
not only sustain the film but also make such films electroni-
cally active. This is the first report of the use of highly ordered
free-standing films to fabricate OFET devices. Organic
transistors with the highest mobility of 0.02 cm2 VÀ1 sÀ1 have
been successfully produced. Our results indicate that such an
amphiphilic molecule is a promising organic 2D nanomaterial
that has the predominance of large-area deposition and
solution processability. Moreover, as the bilayer films are very
similar to phospholipids, they hold great promise in biomem-
brane mimetic applications. This work is currently in progress.
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[12] The changes in the bands at 370 nm in Figure 2c arise from the
effect of self-absorption.
Received: January 28, 2011
Published online: May 27, 2011
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Keywords: amphiphiles · bilayers · free-standing films ·
.
molecular electronics · self-assembly
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