Synthetic Metals 147 (2004) 43–48

Towards supramolecular electronics
A.P.H.J. Schenninga,∗ , P. Jonkheijma , F.J.M. Hoebena , J. van Herrikhuyzena , S.C.J. Meskersa ,E.W. Meijera , L.M. Herzb , C. Danielc , C. Silvac , R.T. Phillipsc , R.H. Friendc , D. Beljonned ,A. Miurae , S. De Feytere , M. Zdanowskae , H. Uji-ie , F.C. De Schryvere , Z. Chenf ,F. Würthnerf , M. Mas-Torrentg , D. den Boerg , M. Durkutg , P. Hadleyg
a Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlandsb Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UKc Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, UK
d Laboratory of Chemistry of Novel Materials, University of Mons-Hainaut, Place du Parc 20, 7000 Mons, Belgium
e Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200 F, B-3001 Leuven, Belgium
f Institut für Organische Chemie, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany
g Department of NanoScience, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands

Received 28 April 2004; accepted 18 June 2004

Abstract
We have demonstrated that it is possible to program ␲-conjugated molecules to self-assemble into cylindrical aggregates in solution. By
incorporating energy or electron traps in our stacks, energy and electron transfer processes in these one-dimensional assemblies have beenstudied in solution. The transfer of the single OPV cylinders from solution to a solid support as isolated objects was only possible when
specific concentrations and specific solid supports were used. So far, however, we have not been able to measure any current through ourfibers.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Self-organization; Nanotechnology; Oligo(phenylenevinylene)s1. Introduction
In this paper, we will summarize our results on theconstruction of supramolecular assemblies in the 10–100 nm
length scale that act as optoelectronic components. In
the future, we would like to connect these assemblies toelectrodes to construct supramolecular electronic devices(Fig. 1) [1]. Such systems could serve as an alternative fordevices based on carbon nanotubes [2] or inorganic wires[3]. At the moment, there is no methodology to connect
supramolecular structures to electrodes. However, this willbe crucial for the success of supramolecular chemistry∗ Corresponding author. Tel.: +31 40 2473 264; fax: +31 40 2451 036.

E-mail address: a.p.h.j.schenning@tue.nl (A.P.H.J. Schenning).
0379-6779/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.synthmet.2004.06.038

in the field of nanotechnology [4]. The construction andproperties of supramolecular nanosized optoelectronic
devices is of industrial and scientific importance. With
such architectures, scientifically interesting processes likeenergy transfer and solar energy conversion and storage
can be investigated while industrial applications can beforeseen in the nanofabrication of smaller and less expensive(optoelectronic) devices concurrent to silicon technology. Inaddition, the field of plastic electronics [5] could benefit fromthis research since we are combining the advantages of small
molecules (no defects) with that of polymers (easy to process) resulting eventually in improved performance properties [6].
For the fabrication of these supramolecular electronicdevices, ␲-conjugated (chiral) oligo(p-phenylenevinylene)s44

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Fig. 1. Schematic representation illustrating the gap, with respect to length scale, between molecular electronics [7], where single molecules are studied, andplastic electronics [8], which relies on bulk properties of ␲-conjugated polymers. Supramolecular electronics is an unexplored field with the aim to construct
and investigate supramolecular optoelectronic nanoarchitectures that are hooked up to electrodes.

(OPVs) oligomers were synthesized, which self-assemble ina controlled fashion into (helical) wires in solution. In thesestacks, energy and electron traps were incorporated to investigate fundamental issues like light harvesting, exciton
diffusion lengths, and energy and electron transfer processes
within the nanometer dimensions. The structures in solution
were transformed to a solid support and will be sandwiched
between electrodes in future work. The sandwiched architectures can have potential applications in (nanosized) field
effect transistors (FETs), photovoltaic devices, light-emitting
diodes (LEDs) and logic gates.

2. Result and discussion
2.1. Self-assembly in solution
We have found that an OPV derivative bearing a hydrogenbonding diaminotriazine headgroup can form hexameric␲-conjugated rosettes that self-assemble into chiral tubular
objects [9]. The OPV trimer and tetramer (OPVT3 andOPVT4, Fig. 2) are equipped with a tridodecyloxy wedge asend group and substituted with two or four enantiomericallypure (S)-2-methylbutoxy side chains on the OPV backbone.
Scanning tunneling microscopy (STM) shows that hydrogenbond formation exerted by the diaminotriazine moieties leads
to chiral hexameric macrocycles at the solid–liquid interface(Fig. 2). These cycles further organize into aggregates insolution as is demonstrated by UV–vis, fluorescence andcircular dichroism (CD) spectroscopy (data not shown). Disordered OPV molecules self-assemble hierarchically; first
forming hexameric rosettes by hydrogen bond formation,
which subsequently develop into stacks aided by ␲–␲ interactions of the phenylenevinylenes. The chiral side chains
induce supramolecular helicity. It is proposed that the cyclic
hexameric rosettes are not fully planar at the triazine end,
resulting in a propeller arrangement allowing interplanarhydrogen bond interactions from rosette to rosette, hereby
locking the rosettes within the tubules. The tubules of 7 nm
in diameter and 180 nm in length and with perfect spacefilling are still soluble due to the apolar shell that surrounds
the stacks as revealed with small-angle neutron scattering(SANS). Atomic force microscopy (AFM) measurementsof OPVT4 assemblies was used to study the shape of thesupramolecular aggregates in solution when transferredfrom solution to a graphite surface with heights of the stacks
around 6.4 nm (Fig. 2). This thickness is in agreement withthe SANS data and corresponds well to the diameter of thehydrogen-bonded hexamers found by STM pointing to fibers
consisting of stacked rosettes.

Fig. 2. Molecular structures of OPVT3 and OPVT4 (left-hand side), STM image of OPVT4 (18.4 nm × 18.4 nm, Vset = −0.50 V, Iset = 0.45 nA) monolayers(picture in the middle) and AFM image of drop cast OPVT4 solution in heptane (5 × 10−5 M) on HOPG (right-hand side picture, size 604 nm × 604 nm).

A.P.H.J. Schenning et al. / Synthetic Metals 147 (2004) 43–48

45Fig. 3. Hydrogen-bonded triad consisting of one perylenebisimide electron acceptor sandwiched between two OPV electron donors.

As a result of the complementarity of the hydrogen bonding unit in perylenebisimide, which is an electron acceptor, with the diaminotriazine OPV units, supramoleculardonor–acceptor–donor triads, i.e. a p–n–p type junction could
also be created (Fig. 3) [10]. The covalent analogue ofthis triad could be visualized by STM (Fig. 4) [11]. Biasdependent measurements show an electron-acceptor core
sandwiched between two electron donors. The hydrogenbonded triads form helical columnar stacks in apolar solvent as shown by UV–vis, fluorescence and CD titration
experiments [10]. The three techniques prove the existenceof two phases for the triad in solution; aggregates at low
temperatures and molecularly dissolved monomeric species
at high temperatures. Very efficient electron transfer takes
places from the electron donor (OPV, p-type) to the electron
acceptor (perylenebisimide, n-type) in the aggregates. AFM
measurements showed the appearance of chiral, rod-like aggregates with a length of several micrometers and a width of
the smallest resolved fibers of about 7 nm. The large fibers
are presumably composed of multilayers having a width ofmultiples of 7 nm that coil into chiral aggregates (Fig. 5).
The hydrogen bonding strength in our OPV systems
could be increased by increasing the number of hydrogen bond interactions. Three hydrogen-bonded oligo(pphenylenevinylene)s of this type that differ in conjugationlength (an OPV trimer, tetramer and pentamer) have beensynthesized (Fig. 6), only the OPV tetramer is shown [12].
All three compounds contain chiral side chains, long
aliphatic chains and a ureido-s-triazine hydrogen bonding

unit. 1 H NMR and photophysical measurements show that theOPV oligomers grow hierarchically in an apolar solvent; initially dimers are formed by hydrogen bonds that subsequently
develop into stacks by ␲–␲ interactions of the phenylenevinylene backbone with induced helicity via the chiral side chains.
SANS measurements show that rigid cylindrical objects areformed. Stacks of OPV4 have a persistence length of 150 nm
and a diameter of 6 nm. OPV3 shows rigid columnar domainsof 60 nm with a diameter of 5 nm. Temperature and concentration variable measurements show that the stability of thestacks increases with the conjugation length as a result of
more favorable ␲–␲ interactions. When the fraction of aggregated species (φ) versus the temperature (T) is plotted forthe three different photophysical techniques, fairly equivalenttransition curves are obtained (data not shown). The meltingof the stacks takes place in a relatively narrow temperature
range indicating that the thermal denaturation of the stacks isa cooperative process. The stability of the stacks is probably
the consequence of many reinforcing bonds each of whichis relatively weak. The formation of any of these stabilizing
bonds, very much depends on whether adjacent bonds arealso made. The melting of the stacks depends similarly on
concentration as found for the denaturation of DNA in nature. The melting temperature and the persistence length of
the stacks increases with elongation of the ␲-system of theOPV oligomers.

Fig. 4. Bias-dependent STM imaging of a D–A–D triad (left-hand side picture: Vbias = +1.43 V, right-hand side image Vbias = −1.19 V, image size is
10.1 nm × 10.1 nm, Iset = 0.40 nA).

Fig. 5. Tapping mode AFM image (715 nm × 475 nm) of the hydrogenbonded triad (Fig. 3) upon spin-coating from MCH (c = 1 wt.%) on a glass
slide.

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Fig. 6. An OPV tetramer functionalized with a hydrogen-bonding ureidotriazine unit and a schematic representation of a helical stack formed by this molecule.
Bars represent the oligo(p-phenylenevinylene)s and triangles represent the hydrogen-bonding ureidotriazine units.

We have investigated mixed columnar stacks ofhydrogen-bonded oligo(p-phenylenevinylene) oligomers,
where oligomers of longer conjugation length were incorporated into stacks to act as energy acceptors (Fig. 7). Ultrafast energy transfer from the shorter oligomers to the longer
oligomer in these stacks in solution was observed [13]. Suchan efficient energy transfer is normally only seen in adsorbed
monolayers or thin films. This concept could be used in lightemitting color tuning (data not shown).

2.2. Self-assembly in the solid state
The transfer of the single OPV cylinders from solution
to a solid support as isolated objects is only possible when
specific concentrations and specific solid supports are used
as investigated by AFM [12]. Numerous single micrometers
long fibers are found in cast films of OPV on graphite (Fig. 8)
with a uniform height of 5.2 nm and a persistence length of
125 nm. At higher concentrations, an intertwined network isFig. 7. Mixed columnar stacks of OPV dimers in an apolar environment. The formation of mixed stacks is enabled by the identical hydrogen-bonded motif ofboth molecules. Within mixed stacks energy transfer (ENT) occurs from MOPV3 to MOPV4.

A.P.H.J. Schenning et al. / Synthetic Metals 147 (2004) 43–4847Fig. 8. Plot showing the dependence of morphology on both the solution concentration and surface type. Depicted are tapping mode AFM images on glass(repulsive surface), graphite (inert surface) and gold (attractive surface).

formed while at low concentration ill-defined globular objects
are observed. Molecule–surface interactions are also crucial
to transfer successfully the supramolecular stack present insolution to a solid support (inert surfaces: graphite and siliconoxide). When the molecule–molecule interactions present
in the supramolecular stacks in solution are perturbed bystronger molecule–surface interactions (attractive surfaces:gold), the stacks cannot be transferred to the surface. Repulsive surfaces (mica and glass) give rise to clustering of stacks
due to the minimization of the contact area between the stackand the support resulting in lamellar arrays of stacks. Because
the concentration and the temperature of the cast solution also
determine the equilibrium between individual molecules andthe supramolecular stack, it is obvious that many variables
control the successful transfer of stacks from solution to surfaces.
2.3. Electronic devices
Plastic electronic devices have been made from thesupramolecular hydrogen-bonded OPV systems to showthat the recognition sites are electro- and photochemicallyinert. The morphology of the active layer was controlled
by programmed self-assembly of ␲-conjugated oligomers
in solution. The soluble columnar stacks present in solution
could be easily processed by spin-coating resulting in films
in which the organization present in solution is retained withuniform aligned domains with sizes of hundreds of nanome-ters. The optical characteristics are also retained as shown by
photoluminescence and time-resolved photoluminescencemeasurements. Stable light-emitting diodes, field effecttransistor and photovoltaic devices can be constructed [14].
The concept of controlling the supramolecular organizationof the active layer is, in principle, applicable to all kindsof ␲-conjugated oligomers and will probably improve theperformance of opto-electronic devices.
We also investigated isolated OPV fibers as nanosizedwires. A solution containing self-assembled structures was
drop-cast on electrode patterns in order to obtain some stacks
that are oriented across two electrodes. The electrode patterns were produced by electron-beam lithography in which
the electrodes are separated by less than 100 nm from eachother [15]. However, so far no current has been measured[16]. To solve this problem, an electric field will be applied
to align the fibers between the electrodes similar as has been
done with polythiophene fibers [15] carbon nanotubes [17]and inorganic wires [18].

3. Conclusions
This work demonstrates that it is possible to program
␲-conjugated molecules into self-assembled cylindricalaggregates in solution and in the solid state [19]. Ultra-fastenergy transfer has been observed in mixed supramolecular
stacks of oligo(phenylenevinylene)s (OPVs) in solution. We

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also observed collectively and hierarchically self-assembleof oligo(p-phenylenevinylene) with perylenebisimide toconstruct chiral fibers that show photo-induced electrontransfer. It demonstrates that control of nanoscale orderprovides a strategy for tailoring macroscopic electronic
properties of organic semiconductor systems.
The transfer of the supramolecular stacks from solution
to a solid support is a very delicate process. A comprehensive knowledge of all intermolecular interactions gives rise
to controlled transfer of ␲-conjugated assemblies to specificsurfaces. Understanding the interactions in this transfer process will be crucial for the success of applying these semiconducting stacks in the field of nanotechnology. The designof such nanodevices must be such that all components behave
as an inert surface toward self-assembled stacks.

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