On the transfer of cooperative self-assembled π-conjugated fibrils to a gold substrate

Article abstract of DOI:10.1039/c1cc12799c

The transfer of the cooperative self-assembled fibrils to a gold substrate has been studied by means of scanning probe microscopy techniques revealing the crucial role of the early formation of a monolayer.

Full text of DOI:10.1039/c1cc12799c

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Cite this: Chem. Commun., 2011, 47, 9333–9335
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COMMUNICATION
On the transfer of cooperative self-assembled p-conjugated fibrils to a
gold substratewz
a
a
b
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Martin Wolffs, Mantas Malisauskas, Inge De Cat, Zeljko Tomovic, Jeroen van Herrikhuyzen,
a
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Christianus M. A. Leenders, Evan J. Spadafora, Benjamin Grevin,* Steven De Feyter,*
E. W. Meijer,a Albertus P. H. J. Schenning*a and Philippe Leclere*ad
Received 12th May 2011, Accepted 30th June 2011
DOI: 10.1039/c1cc12799c
The transfer of the cooperative self-assembled fibrils to a gold
substrate has been studied by means of scanning probe micro-
scopy techniques revealing the crucial role of the early formation
of a monolayer.
The development of plastic electronics as an alternative to the
common silicon based electronic circuitry is currently receiving
much attention, where the first applications have become
commercially available.1 This impressive progress has stimulated
the fabrication of electronic devices in the nanometer range
by using self-assembled structures consisting of p-conjugated
oligomers, a research area generally referred to as nanosized or
supramolecular electronics.2 In solution, examples about the self-
assembly of p-conjugated molecules are ubiquitous,3 however,
reports dealing with functional devices are scarce.1,4–6 Especially
transferring self-assembled fibrils from the solution to device-
relevant substrates such as gold or SiO2 while retaining the self-
assembled structure limits the development of nanoelectronics.
This process is shown to be a delicate balance between the solvent,
surface and deposition technique.7–9 In the past, we have shown
the disruptive influence of gold on the deposition of p-conjugated
self-assembled oligo(p-phenylenevinylene) (OPV, Scheme 1) fibers
Scheme 1 Molecular structures of OPV and OPV–s.
reported on the utilization of the gold–sulfur bond to prepare
highly stable nanocolumns of self-assembled hexa-perihexabenzo-
coronenes equipped with a thiol unit on a gold surface in which
the orientation of the molecules could be directed.10 Hence this
approach could be a promising method to obtain stable self-
assembled fibers of p-conjugated oligomers on a gold surface
which are otherwise disrupted.
In the past, we have reported on the self-assembly of chiral
OPV derivatives bearing the self-complementary quadruple
hydrogen bonding ureidotriazine unit. In chlorinated solvents like
chloroform and 1,2,4-trichlorobenzene (TCB) an equilibrium
exists between monomers and their hydrogen-bonded dimers
(OPV, Scheme 1).8 In alkane solvents, such as methylcyclohexane
(MCH), the hydrogen-bonded dimers are self-assembled into
helical fibrils according to the cooperative mechanism.11 The
helical handedness of the assembly is biased by the enantiomeric
purity of the side tails that are attached to the OPV chromo-
phore.3 The presence of fibril-like assemblies has been observed,
which are visualized on the surface with Atomic Force Micro-
scopy (AFM) after dropcasting a dilute alkane solution onto
highly ordered pyrolytic graphite (HOPG).8,11 Remarkably, the
p-conjugated fibrils were destroyed on a gold surface.8
In this report, we aim to transfer the supramolecular fibrils from
the solution to a gold surface in such a manner that the
one-dimensional fibrillar structure is maintained. For this reason,
we have equipped an oligo(p-phenylene vinylene) ureidotriazine
oligomer (OPV, Fig. 1) with a dithiol bridge (OPV–s, Scheme 1).
We show that the structure of the self-assembled fibrils formed by
OPV–s on gold is maintained. Interestingly, the presence of a
monolayer on a gold substrate has been revealed which is shown
to be the crucial factor for retaining the fibrillar nature of the
assemblies on the gold surface.
on the surface.8 Recently, the groups of Mullen and Fichou have
¨
a Institute for Complex Molecular Systems and the Laboratory of
Macromolecular and Organic Chemistry, Eindhoven University of
Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.
E-mail: a.p.h.j.schenning@tue.nl; Fax: +31 (0)40 245 10 36;
Tel: +31 (0)40 247 3101
b Department of Chemistry, Division of Molecular and Nanomaterials,
Katholieke Universiteit Leuven (K.U.Leuven), Celestijnenlaan 200 F,
b-3001 Heverlee, Leuven, Belgium.
E-mail: Steven.DeFeyter@chem.kuleuven.be; Fax: +32 16327990;
Tel: +32 16327921
c CEA-INAC-UMR 5819-SPrAM (CEA-CNRS-UJF), 17 Rue des
Martyrs, F-38054 Grenoble Cedex 9, France.
E-mail: benjamin.grevin@cea.fr; Fax: +33 (0) 04.38.78.50.97;
Tel: +33 (0) 4 76 38 00 46
d Laboratory for Chemistry of Novel Materials, Center for Innovation
and Research in Materials and Polymers (CIRMAP), University of
Mons (UMONS), Place du Parc 20, B 7000 Mons, Belgium.
E-mail: philippe.leclere@umons.ac.be; Fax: +32 (0)65 37 38 61;
Tel: +32 (0)65 37 38 68
While the synthesis of OPV has been described,8 the synthesis
of OPV–s is described in the ESI.z Similar chemical shifts in
1H-NMR for OPV–s are observed when compared to OPV,
w This article is part of the ChemComm ‘Molecule-based surface
chemistry’ web themed issue.
z Electronic supplementary information (ESI) available: Synthesis,
CD Spectra, AFM data. See DOI: 10.1039/c1cc12799c
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 9333–9335 9333

Fig. 1 STM images of OPV (a) and OPV–s (b) on HOPG in TCB,
scan sizes are 42.3 ꢀ 42.3 nm2 and 31.2 ꢀ 31.2 nm2, respectively
(Iset = 39 pA; Vset = ꢁ680 mV and Iset = 33 pA; Vset = ꢁ480 mV,
respectively). (c) CD spectroscopy of a MCH solution of OPV (red)
and OPV–s (black) (c = 1 ꢀ 10ꢁ5 M, T = 298 K). (d) Temperature-
dependent CD measurements of OPV (red) and OPV–s (black)
(c = 1 ꢀ 10ꢁ5 M, DT/Dt = ꢁ60 K hꢁ1, l = 460 nm).
Fig. 2 STM images of OPV (a, b) and OPV–s (c) on gold in TCB,
where (b) is an enlargement of (a) (Iset = 161 pA; Vset = ꢁ412 mV and
Iset = 67 pA; Vset = ꢁ520 mV, respectively). (d) FM-AFM image of a
monolayer of OPV–s on a gold substrate.
probe techniques. In order to elucidate the possibility of mono-
layer formation on the gold surface by OPV and OPV–s we
performed STM measurements using TCB as the solvent. The
presence of a monolayer on a gold substrate has been revealed for
OPV where the molecules are adsorbed as hydrogen bonded
dimers and it has been showed that the molecules are arranged
parallel to the substrate (Fig. 2a and b). These results are
analogous to the results obtained on HOPG (Fig. 1a), except
for some lower degree of ordering. In the case of OPV–s the STM
results do not show any well-organized OPV molecules at all on
the surface and thus prove to be inconclusive for the formation of
a monolayer on the gold terraces (Fig. 2c). Instead, we employ
Frequency Modulation AFM (FM-AFM) in ultra-high vacuum
to confirm the presence of the monolayer.12 In this case the
monolayer has been prepared by dropcasting a chloroform
solution on a gold substrate after which the solvent is evaporated.
Features having a height of B1 nm can be observed. This height
corresponds to the width of the molecules (1.0 nm), and therefore
it can be concluded that the molecules are forming a monolayer
where the molecules are standing upright on the surface (Fig. 2d).
Furthermore, the image shows objects of 50 nm in diameter,
where a zoom in on these structures reveals that they consist of
clusters of single molecules that are either directly on top of the
gold surface or are present on the monolayer of the OPV–s
molecules (see ESIz). Although the FM-AFM measurements
reveal the presence of a monolayer, the molecules are not as
clearly visualized by the STM as in the case of OPV on gold. This
could be the result of a different orientation of the molecules with
respect to the surface resulting from the gold–sulfur attachment.
Since the STM and the FM-AFM results show that the
molecules form a monolayer on gold, the next step is to analyze
if this monolayer is crucial for fibril formation and/or transfer.
Therefore, monolayer formation is achieved by treating the
surface with a solution of OPV or OPV–s prior to dropcasting
the solution containing the self-assembled structures of either
OPV or OPV–s (Fig. 3). Dropcasting a chloroform solution of
OPV–s (c = 1 ꢀ 10ꢁ5 M) on the substrate is followed by
thereby showing the ability to form hydrogen-bonded dimers in
chloroform. Also in the case of monolayer formation from TCB
as studied with Scanning Tunneling Microscopy (STM) at the
liquid/solid (HOPG) interface similar results for OPV and OPV–s
have been revealed (Fig. 1a and b). For both molecules, the
formation of highly organized monolayers on HOPG is observed,
where the images show the presence of hydrogen-bonded dimers
that are arranged with the p-conjugated plane of the molecules
parallel to the surface while following the three-fold symmetry
axis of HOPG. Next the self-assembly of OPV–s in MCH is
investigated with circular dichroism (CD) and UV/Vis spectro-
scopy and compared to OPV. OPV–s assembles into preferential
left-handed helical structures, similar to the previously reported
OPV. However, the lower CD intensity and shape indicate that
the packing of OPV–s in the self-assembled fibril is different with
respect to OPV. The non-sigmoidal shape of the temperature-
dependent CD curves indicates a cooperative self-assembly
mechanism, implying an equilibrium between supramolecular
fibrils and monomers only.11 These OPV–s and OPV fibrils are
successfully transferred to a HOPG surface (ESIz). Hence, the
self-assembly characteristics in solution, as well as monolayer
formation and transfer of the supramolecular fibrils on a HOPG
substrate, are largely unaffected by the sulfur functionality.
As mentioned above, in the past the self-assembled structures of
OPV could not be observed on a gold substrate.8 In sharp
contrast, dropcasting OPV–s from a MCH solution on a gold
substrate, followed by slow evaporation of the solvent in a
saturated MCH atmosphere, leads to the formation of thick
fibril-like assemblies composed of a multitude of single stacks
(see ESIz). This result shows the important role of the sulfur
moiety in retaining the fibrillar nature of the assemblies on the
gold surface.
In order to shed light on the different behaviour of OPV and
OPV–s we have carried out a detailed analysis of the self-assembly
behaviour of the two compounds on a gold surface by scanning
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9334 Chem. Commun., 2011, 47, 9333–9335
This journal is The Royal Society of Chemistry 2011

Fig. 3 TM-AFM images on a gold substrate after dropcasting a
MCH solution of either OPV–s (a) or OPV (b) on a substrate having
a monolayer of OPV–s.
Fig. 4 Proposed model for the transfer of self-assembled p-conju-
gated fibrils on a gold substrate.
evaporation of the solvent to yield the monolayers. Subsequently
the supramolecular assemblies have been transferred to the surface
by dropcasting the MCH solution. Irrespective of the underlying
monolayer, the TM-AFM images show in all cases the presence of
self-assembled fibrils on the surface. However, the morphology
differs for the two systems at hand, where micrometer sized thick
bundles of fibrils are observed for OPV–s, OPV yields much
shorter fibril-like structures that are less high (Fig. 3a versus 3b).
These results show that the presence of a monolayer is crucial for
retaining the fibrils at the surface, while the orientation of the
molecules in the monolayer seems to be less important (see ESIz).
In order to establish the strength of the monolayers formed
by either OPV or OPV–s, the surface containing the mono-
layer was placed back in chloroform for either 10, 20 or
30 seconds and dried. Subsequently a MCH solution having
self-assembled structures of OPV is dropcast to transfer
the self-assembled structures on the surface. Irrespective of
the residence time in chloroform, fibril-like assemblies are
observed when the monolayer consists of OPV–s. However,
when the OPV monolayer is used, already 10 seconds
submerged in a chloroform solution is enough to remove the
OPV monolayer to such an extent that fibril formation is
severely hampered (ESIz). The removal of this monolayer has
been confirmed by FM-AFM. This shows that the stability of
the OPV–s monolayer is significantly enhanced with respect to
the OPV monolayer.
(FNRS-CGRI/CNRS 20182). Ph.L. is a Research Associate of
the FRS-FNRS (Belgium). The Dutch team acknowledges the
Netherlands Foundation for Scientific Research (NWO).
I.D.C. and S.D.F. acknowledge the Fund of Scientific
Research––Flanders (FWO), K.U.Leuven (GOA), the Belgian
Federal Science Policy Office through IAP-6/27. I.D.C. is
grateful to the Agency for Innovation by Science and
Technology in Flanders (IWT).
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In conclusion we have shown that the presence of a mono-
layer can be a crucial factor for retaining the solution
morphology and connectivity of fibrils on a surface. For
cooperative self-assembled systems, the remaining monomers,
inherently present in the solution, significantly influence the
organization of the self-assembled structures on the surface. It
can be anticipated that electronic (nano)devices made from
p-conjugated systems could have a monolayer that is arranged
differently than the bulk of the active layer, thus potentially
influencing the performance of the device.
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The collaboration between Mons and Grenoble has been
carried out in the frame of a joint bilateral research program
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 9333–9335 9335