Self-assembly and conductivity of hydrogen-bonded oligothiophene nanofiber networks

Article abstract of DOI:10.1039/c1cc10809c

Symmetric oligothiophene derivatives containing hydrogen bond forming segments create self-supporting organogels consisting of self-assembled 1D nanostructures at low concentrations. Hydrogen bond formation and π-π stacking were both found to be crucial f

Full text of DOI:10.1039/c1cc10809c

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COMMUNICATION
Self-assembly and conductivity of hydrogen-bonded oligothiophene
nanofiber networkswz
David A. Stone,ya Alok S. Tayi,yb Joshua E. Goldberger,a Liam C. Palmera and
Samuel I. Stupp*abcd
Received 11th February 2011, Accepted 24th March 2011
DOI: 10.1039/c1cc10809c
Symmetric oligothiophene derivatives containing hydrogen bond
forming segments create self-supporting organogels consisting of
self-assembled 1D nanostructures at low concentrations. Hydrogen
bond formation and p–p stacking were both found to be crucial
for the formation of conductive supramolecular networks of 1D
nanostructures.
orbital overlap is a promising strategy to create large area
devices with conducting soft matter.9
There have been several reports of gelators based on short
oligothiophenes,10 but there are relatively few examples with
four to six repeat units that are typically required for charge
transport.5,11 A significant challenge in materials processing
with the extended thiophene oligomers is their inherently low
solubility.12 In order to solubilize oligothiophene derivates
for solution deposition, chemical substitutions on thiophene
moieties are necessary.3 Unfortunately, substituted oligothio-
phene derivatives often form amorphous films when solution
deposited on substrates as a result of nonspecific aggregation.
We report here the self-assembly of quater- and quinquethio-
phene derivatives into 1D nanostructures in organic solvents.
Quaterthiophene (4TG) or quinquethiophene (5TG) deriva-
tives were designed with an oligothiophene core attached to a
hydrogen-bonding region flanked by alkyl tails for solubility
(see Fig. 1). The objective was to explore the role of hydrogen
bonding and p–p stacking to create ordered conductive assemblies.
The oligothiophenes used were synthesized based on palladium-
catalyzed Stille coupling methodology (see ESIz).
Control over supramolecular structure is essential to achieve
high-performance optoelectrical devices based on electronically
conjugated organic molecules.1 Assemblies of organic molecules
can also be used to template the growth of nanoscale inorganic
semiconductors in order to enhance optoelectrical function.2
Among the most popular classes of small molecules studied for
organic electronic applications are oligothiophene derivatives
due to their low band gaps and high charge mobility.3–5 In
oligo- and polythiophenes, charge conduction is known to
occur by an interchain hopping mechanism through overlapping
p-orbitals between adjacent molecules.6 High conductivity can
be achieved if a continuous pathway of overlapping p–p
orbitals exists; however, this structural feature is largely lost
in amorphous films.7 Langumir–Blodgett, layer-by-layer, and
vacuum sublimation deposition methods have been used to
impart molecular ordering in films, but they are not ideal for
large area depositions.8 For photovoltaics, field-effect transistors,
or light emitting diodes a simple, one-step fabrication from
spin-casting or dip-coating is desired. Therefore, the spontaneous
self-assembly of molecules into ordered systems that promote
Both of the 4TG and 5TG derivatives formed self-supporting
gels in several organic solvents (see ESIz). To induce gel formation
(defined as absence of flow upon inversion of the vial), solutions
a Department of Chemistry, Northwestern University, Evanston, IL,
USA. E-mail: s-stupp@northwestern.edu; Fax: +1 847-491-3010
b Department of Materials Science and Engineering, Northwestern
University, Evanston, IL, USA
c Department of Medicine, Northwestern University, Chicago, IL,
USA
d Institute for BioNanotechnology in Medicine, Northwestern
University, Chicago, IL, USA
w This article is part of a ChemComm ‘Supramolecular Chemistry’
web-based themed issue marking the International Year of Chemistry
2011.
z Electronic supplementary information (ESI) available: Photographs
of gelled samples, AFM and TEM micrographs, variable temperature
absorbance spectra, EDX of 5TG devices, SEM image of films cast
from CHCl3, and I–V curves of P3HT devices. See DOI: 10.1039/
c1cc10809c
Fig. 1 TEM images of films cast from chlorocyclohexane of (a) 4TG
and (b) 5TG showing the presence of bundled nanofiber networks.
Scale bars are 500 nm.
y These authors contributed equally to this work.
c
5702 Chem. Commun., 2011, 47, 5702–5704
This journal is The Royal Society of Chemistry 2011

of either 4TG or 5TG were heated close to the solvent’s
boiling point and allowed to cool to room temperature. The
derivatives formed gels at low concentrations (B0.1 wt%) in
aromatic and halogenated solvents, like toluene and chloro-
cyclohexane (CCH). The formation of a self-supporting gel is
likely due to the presence of an 1D molecular aggregate.
Atomic force microscopy (AFM) and transmission electron
microscopy (TEM) were used to characterize the nanostructures
deposited from dilute solutions in gel-inducing solvents. Micro-
scopy revealed bundled networks consisting of 1D nano-
structures extending to several microns in length (Fig. 1).
Since the nanostructures bundle extensively when cast as a
film, it was difficult to observe isolated, individual fibers. It is
likely that the alkyl tails, known to aggregate in some organic
solvents, on the exterior of the fiber interact to cause the
bundling observed.13 The large fiber bundles were found to be
typically 40–300 nm in diameter for both derivatives.
nanofiber network. FTIR spectra of a dried CCH gel revealed
the NH stretching band (3300 cmꢀ1), amide I (1630 cmꢀ1) and
amide II (1540 cmꢀ1) stretching bands at longer wavelengths
than typical, suggesting the amides are involved in hydrogen
bonding. To further investigate the role of hydrogen bonding
in the self-assembled systems in solution, we added CH3OH to
disrupt intermolecular hydrogen bond (see ESIz). When
CH3OH was titrated into a solution of 5TG in CCH, the
UV-vis spectrum showed a pronounced shift in the maximum
of both the absorption and fluorescence peaks consistent with
the presence of solvated molecules. The fluorescence intensity
recovered upon disassembly. At 10 vol% CH3OH, the absorption
and fluorescence spectra are consistent with fully solvated
molecules. The change from an aggregated state to an un-
assembled state upon addition of CH3OH indicates that
hydrogen bonding drives the formation of ordered aggregates
in these molecules.
To investigate the internal structure of the nanostructures
formed, we performed UV-vis absorbance and fluorescence
spectroscopy using a gel-inducing solvent (CCH) or a solvent
that prevents gel formation (THF) (see Fig. 2). Both 4TG and
5TG showed large shifts in absorbance and fluorescence
maxima in solvents that promote self-assembly. In the case
of 5TG in an assembling solvent, we observed a hypsochromic
shift of 45 nm in the absorption maximum, as well as
significant quenching and a 51 nm bathochromic shift in its
fluorescence maximum. In the self-assembled state, vibrational
fine structure can also be seen on the shoulder of the main
absorption peak indicating increased conformational ordering
as a result of self-assembly.14 The 4TG derivative shows
similar behavior with a 42 nm hypsochromic shift in the
absorption maximum, as well as significant quenching and a
51 nm bathochromic shift in fluorescence maximum. These
results are consistent with face-to-face ordering of the thiophene
cores within the nanofibers (H-aggregation).15
Variable temperature UV-vis absorption spectra showed the
self-assembled structures of 4TG and 5TG formed in CCH
undergo reversible disassembly upon heating above 50 1C (see
ESIz). The melting point of 5TG assemblies was found to be
16 1C higher than that of 4TG. Since the only structural
difference between the 4TG and 5TG is the length of the
thiophene core, the increased thermal stability of 5TG is
attributed to an increase in the p–p stacking of the longer
conjugated thiophene. This finding shows that p–p stacking is
also important in the aggregation of these molecules.
Electrical properties were measured on thin films cast by a
simple drop-cast technique. Solutions of organogelator molecules
were drop cast from chlorobenzene (0.1 wt%) onto a glass
surface by four sequential depositions. To measure conductivity,
devices were fabricated by depositing B200 nm Au electrodes
on the fiber film with a TEM grid shadow mask. This method
formed 5–8 mm channels that did not contain any metal
contamination (see ESIz). Current–voltage (I–V) curves from
a comparable 5TG and 4TG device are shown in Fig. 3. At low
applied voltages, the nonlinear I–V curves indicate contact
resistance due to a charge injection barrier.16 The I–V curves
also showed little hysteresis, suggesting the lack of mobile ion
impurities.17
Because both p–p stacking and hydrogen bonding can occur
among the assembled molecules, it is difficult to ascertain
which forces are responsible for the gel phase formed by the
From the I–V measurements, the average conductivity of
5TG films, measured in a four-point probe configuration, was
found to be 9.9 ꢁ 1.7 ꢂ 10ꢀ6 S cmꢀ1. In contrast, 4TG films
Fig. 3 (a) Schematic of a fabricated device (b) SEM image of a typical
device fabricated from 5TG showing the gold pads and organic
channel. Bundles of self-assembled nanofibers are present throughout
the device as seen in the SEM image. (c) Representative I–V curves of
4TG and 5TG devices. Measured current is due to assembled organic
material as shown by an increase in signal upon photoirradiation at
354 nm (7.32 mW cmꢀ2).
Fig. 2 Absorbance spectra of (a) 4TG and (b) 5TG in CCH and THF.
Fluorescence spectra of (c) 4TG and (d) 5TG CCH and THF. All
spectra were obtained at 3.0 ꢂ 10ꢀ6 M.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 5702–5704 5703

did not reveal any detectable conductivity above the noise
of the apparatus. The hole mobility of 5TG was measured
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films of poly(3-hexylthiophene) prepared by the same deposition
technique (7.5 ꢁ 1.1 ꢂ10ꢀ6 S cmꢀ1). The conductivity was
negligible when films were cast from CHCl3, a solvent that
does not promote extensive self-assembly (see Fig. 3c). This
result indicates the nanofiber networks and the extended
pathways for charge mobility generated by self-assembly are
no longer present in the amorphous films.
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The authors would like to thank the Department of Energy-
BES (DE-FG02-00ER45810) and the National Science Foun-
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of this work. This work used equipment in these following
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5704 Chem. Commun., 2011, 47, 5702–5704
This journal is The Royal Society of Chemistry 2011