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Cite this: Chem. Commun., 2013,
49, 1829
Received 5th November 2012,
Accepted 16th January 2013
DOI: 10.1039/c3cc37990f
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Molecular evidence for the intermolecular S S
interaction in the surface molecular packing motifs of
a fused thiophene derivative†
Xuan-Yun Wang,ab Wei Jiang,a Ting Chen,a Hui-Juan Yan,a Zhao-Hui Wang,a
Li-Jun Wana and Dong Wang*a
www.rsc.org/chemcomm
A microscopic investigation of the molecular packing structures of a
fused thiophene derivative reveals the important role of intermolecular
S S interaction in directing the 2D self-assembly. Thermal annealing of
the assembly results in the irreversible phase transition to a new
structure with different molecular trimeric packing motifs.
In recent years, high performance organic semiconductors based
on fused thiophenes have ignited intensive studies.1–4 Fused
thiophenes and their derivatives are expected to have a more
rigid structure, better conjugation, and a larger band gap. Due to
their efficient p orbital overlap, fused thiophene materials are
reported to have high charge carrier mobility, and have shown
great potential as n-type semiconductor materials in organic
solar cells and other organic electronic devices.5–8 Aside from the
molecular structure, it has been increasingly demonstrated that the
supramolecular packing of organic semiconductors in thin film,
especially the very first few layers, at the organic semiconductor
material/electrode interface has a profound effect on the charge
transport behaviour. For example, the charge transport properties
of several fused thiophene derivatives have been proposed to be
related to molecular packing in the thin film.9–12 As important
weak intermolecular interactions, the S S interactions have been
observed in a number of S-heteroaromatic-ring-based organic
semiconductors and play an important role, in addition to p–p
interactions, in inducing the formation of the self-assembled
structure and providing an alternative charge transport pathway
in the organic semiconductors.13–16
The hole mobility of polythiophene has been shown to be
greatly correlated with crystalline orientation of materials on
substrates.10 Therefore, it is of great interest to investigate the
molecular packing of organic semiconductors on electrodes.
Previously, the self-assemblies of polythiophene, fused thiophene, and oligo-thiophene derivatives on gold and highly
a
Institute of Chemistry, Chinese Academy of Sciences, Beijing National Laboratory
for Molecular Sciences, Beijing 100190, People’s Republic of China.
E-mail: [email protected]; Fax: +86-10-82616935; Tel: +86-10-82616935
b
University of Chinese Academy of Sciences, Beijing, People’s Republic of China
† Electronic supplementary information (ESI) available: Experimental details and
supplementary figures. See DOI: 10.1039/c3cc37990f
This journal is
c
The Royal Society of Chemistry 2013
Scheme 1 Chemical structure of trans-1,2-(dithieno[2,3-b:3 0 ,2 0 -d]thiophene)ethene (TDT).
oriented pyrolytic graphite (HOPG) substrates have been investigated by high resolution scanning tunnelling microscopy (STM)
to understand the effect of intermolecular interactions on the
supramolecular packing motifs and the nanoscale electrical
properties.17–23 In view of the increased interest in fused thiophene
materials, it is of great interest to investigate the assembly
behaviour and structural evolution of fused thiophene derivatives
on electrodes under device operation conditions.
Herein, we investigate the interfacial packing structures of
a fused thiophene derivative trans-1,2-(dithieno[2,3-b:3 0 ,2 0 -d]thiophene)ethene (TDT) (Scheme 1) on HOPG by STM. TDT is
reported to be a high performance organic semiconductor.24 A
high resolution STM image reveals that TDT molecules selfassemble into a honeycomb network on the HOPG surface at
room temperature. The important role of S S interaction,
which is known to facilitate high carrier mobilities in organic
semiconductors, is identified for the first time in the surface
packing motifs of TDT. Thermal annealing of the self-assembly
at 100 1C leads to an irreversible structural transition of TDT
assemblies to a close-packed molecular trimer array, which is
driven by the change in the S S interaction mode in the
building motifs. The present work provides molecular insight
into the supramolecular assembly of fused thiophene derivatives
and helps the understanding of the structure–performance
relationship of thiophene based organic solid materials.
Fig. 1 shows the representative STM images of TDT assemblies
obtained after drop-casting the TDT solution on HOPG at room
temperature. TDT molecules form a highly ordered honeycomb
network, as illustrated in Fig. 1a. The typical domain size can
reach 200 200 nm2. The high resolution STM image of Fig. 1b
provides structural details of the honeycomb network. The sidelength of the network (d, Fig. 1b) is measured to be 1.6 0.1 nm,
Chem. Commun., 2013, 49, 1829--1831
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Fig. 1 (a) A large-scale STM image of TDT showing a honeycomb structure on
the HOPG surface at room temperature. Tunnelling conditions: Vbias = 623 mV,
Iset = 498 pA. (b) A high resolution STM image of the honeycomb structure.
Tunnelling conditions: Vbias = 635 mV, Iset = 541 pA. (c) Structural model for the
honeycomb structure. (d) S S interactions are indicated by blue dashed lines in
the enlarged model.
Fig. 2 (a) A large-scale STM image of the molecular assembly of TDT after
thermal annealing at 100 1C for 10 min. Tunnelling conditions: Vbias = 610 mV,
Iset = 456 pA. (b) A high resolution STM image of (a). Tunnelling conditions: Vbias =
639 mV, Iset = 542 pA. (c) Structural model for the spiral structure. (d) S S
interactions are indicated by blue dashed lines in the enlarged model.
which agrees well with the length of the TDT backbone. The
frameworks of the honeycomb network have a lower contrast
than that of the node positions. On the basis of the above
structural information, we propose that TDT molecules form a
trimer as the basic unit of the self-assembly. Each TDT molecule occupies the framework of the honeycomb network and
appears with a dumbbell shape in the STM image, consistent
with its chemical structure. Every three neighbouring TDT
molecules gather together to form a trimer unit at the node
position of the honeycomb network via the intermolecular S S
interactions between the fused thiophene moieties. The node
position has a high contrast due to the highly delocalized
electrons at the fused thiophene part. The low contrast framework of the network is ascribed to the vinylene linkage of TDT
molecules. The alkyl substitutions of TDT molecules are not
resolved in the STM images and may take a random adsorption
configuration at the vacancy of the honeycomb network or
point towards the solution phase.25–27 The honeycomb network
has a rhombus unit cell with lattice parameters a = 3.0 0.1 nm,
b = 3.0 0.1 nm and a = 60 21.
On the basis of the STM images, a structural model for the
honeycomb structure is proposed in Fig. 1c. For clarity and
because of the uncertainty in the precise position of alkyl chains,
the hexyl chains attached to the TDT core are represented by
methyl groups in the model. The model is in good agreement
with the observed results from STM images. The possible S S
interactions between neighbouring fused thiophene rings are
shown in Fig. 1d (blue dashed lines). It is shown that three S S
interactions occur among the TDT molecules. We note that the
S S interactions have not been observed in the self-assemblies
of oligo-, poly-, and fused thiophene derivatives.17–23
Previous results suggested that the performance of TDT
can be improved by increasing the substrate temperature.24
Therefore, the thermal annealing experiment was carried out to
gain the nanoscale information on molecular assembly after
thermal treatment. After heating the sample at 100 1C for
10 min, a new ordered assembly appears. Fig. 2a shows a
large-scale STM image of the propeller-shaped molecular trimer
array. The typical domain size can reach 200 200 nm2 without
obvious defects. It is worth noting that no chemical change
takes place when the TDT molecules are heated at 100 1C
(Fig. S1, ESI†). The high resolution STM image of Fig. 2b
provides detailed information on the propeller-shaped assembly
structure. In each trimer unit, the core of the molecule takes a
symmetric arrangement with an angle of 1201. According to the
STM observation, a structural model for the propeller-shaped
trimer assembly is proposed in Fig. 2c. The dimensions of the
unit cell outlined in Fig. 2b are a = 3.0 0.1 nm, b = 3.0 0.1 nm
and a = 60 21, which are the same as that of the honeycomb
network. The model is in good agreement with the observed
results from STM images. Based on the high resolution STM
image and structural model, possible S S interactions within
the trimeric motif are shown in Fig. 2d. It is worthwhile to note
that all of three S atoms of a fused thiophene moiety can interact
with the neighbouring S atoms, as shown in Fig. 2d. Compared
with the honeycomb network shown in Fig. 1, three TDT
molecules of a trimer unit rotate and gather together via a
different intermolecular S S interaction mode at the expense
of the 2D network.
Further increasing the annealing temperature up to 150 1C
does not result in any new assembly structures. Then, we further
investigate the structural transition process of TDT assembly at
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Chem. Commun., 2013, 49, 1829--1831
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This work is supported by National Key Project on Basic
Research (grants 2011CB808700 and 2011CB932300), National
Natural Science Foundation of China (grants 91023013,
21121063, 21233010, 21003131), and the Chinese Academy of
Sciences.
Published on 17 January 2013. Downloaded by Institute of Chemistry, CAS on 15/02/2016 03:50:33.
Notes and references
Fig. 3 A large-scale STM topography showing the structural evolution of a
monolayer of TDT after thermal annealing at 60 1C. Tunnelling conditions: Vbias =
587 mV, Iset = 512 pA.
the intermediate temperature of 60 1C. It is noted that TDT has
not been trimerized completely after heating at 60 1C, as shown
in Fig. 3. The packing pattern in most areas is the same as that
in Fig. 1a, corresponding to the honeycomb network of TDT
without thermal annealing. However, at some regions, such as
region A outlined in Fig. 3, the structural transition from
the honeycomb structure to the propeller-shaped structure
can be identified. The result provides direct evidence that the
propeller-shaped trimer array structure is transformed from
the honeycomb phase, rather than growing by itself. Since the
structural transition occurs locally, the unit cell parameters are
kept the same after the annealing process.
In summary, we have demonstrated that intermolecular
S S interaction is important to direct the 2D self-assembly of
a fused thiophene derivative TDT. A molecular trimer based
honeycomb network of TDT is disclosed on HOPG at room
temperature. Thermal annealing of the self-assemblies stimulates
an irreversible phase transition and a new packing motif with a
different S S interaction mode. The present work provides direct
evidence for the supramolecular assembly of fused thiophene
derivatives at the nanoscale, which can correlate well the device
performances of TDT based organic semiconductors. In addition,
the present work demonstrates that the high-resolution STM can
offer molecular insight into the structure–performance relationship of organic solid materials.
This journal is
c
The Royal Society of Chemistry 2013
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