Self-assembly and orientation of hydrogen-bonded oligothiophene polymorphs at liquid-membrane-liquid interfaces

Article abstract of DOI:10.1021/ja204811b

One of the challenges in organic systems with semiconducting function is the achievement of molecular orientation over large scales. We report here on the use of self-assembly kinetics to control long-range orientation of a quarterthiophene derivative des

Full text of DOI:10.1021/ja204811b

ARTICLE
pubs.acs.org/JACS
Self-Assembly and Orientation of Hydrogen-Bonded Oligothiophene
Polymorphs at LiquidꢀMembraneꢀLiquid Interfaces
Ian D. Tevis,† Liam C. Palmer,† David J. Herman,‡ Ian P. Murray,‡ David A. Stone,† and
Samuel I. Stupp*,†,‡,§,^
†Department of Chemistry, ‡Department of Materials Science and Engineering, §Feinberg School of Medicine, and ^Institute for
Bionanotechnology in Medicine, Northwestern University, 2220 Campus Drive, Evanston, Illinois 60208, United States
S Supporting Information
b
ABSTRACT: One of the challenges in organic systems with
semiconducting function is the achievement of molecular
orientation over large scales. We report here on the use of
self-assembly kinetics to control long-range orientation of a
quarterthiophene derivative designed to combine intermolecu-
lar πꢀπ stacking and hydrogen bonding among amide groups.
Assembly of these molecules in the solution phase is prevented
by the hydrogen-bond-accepting solventtetrahydrofuran, whereas
formation of H-aggregates is facilitated in toluene. Rapid
evaporation of solvent in a solution of the quarterthiophene in a
2:1:1 mixture of 1,4-dioxane/tetrahydrofuran/toluene leads to self-
assembly of kinetically trapped mats of bundled fibers. In great contrast, slow drying in a toluene atmosphere leads to the homogeneous
nucleation and growth of ordered structures shaped as rhombohedra or hexagonal prisms depending on concentration. Furthermore,
exceedingly slow delivery of toluene from a high molecular weight polymer solution into the system through a porous aluminum oxide
membrane results in the growth of highly oriented hexagonal prisms perpendicular to the interface. The amide groups of the compound likely
adsorb onto the polar aluminum oxide surface and direct the self-assembly pathway toward heterogeneous nucleation and growth to form
hexagonal prisms. We propose that the oriented prismatic polymorph results from the synergy of surface interactions rooted in hydrogen
bonding on the solid membrane and the slow kinetics of self-assembly. These observations demonstrate how self-assembly conditions can be
used to guide the supramolecular energy landscape to generate vastly different structures. These fundamental principles allowed us to grow
oriented prismatic assemblies on transparent indium-doped tin oxide electrodes, which are of interest in organic electronics.
’ INTRODUCTION
leading to precipitation. Introducing solubilizing groups can
frustrate packing, causing disordered film morphologies after
deposition from solution. The organization of such molecules
can be controlled by rationally designing their covalent structure
to assemble in a specific way and also by controlling environ-
mental factors during deposition.
In order to offset the disorder caused by solubilizing groups,
supramolecular interactions, such as hydrogen bonding, can be
used to bring conjugated molecules in close proximity to
promote large spatial π-orbital overlap while maintaining their
solution processability. Numerous π-conjugated molecules have
been found to spontaneously self-assemble in solution into a
variety of one-dimensional (1D) nanostructures by modifying
the chemical structure and varying external factors, like solvent,
temperature, and substrate to form nanofibers, nanotubes, or
nanorods with uniform morphological dimensions.12ꢀ28 Self-
assembling organic molecules have been synthesized by our
group in a variety of architectures, utilizing conjugated
molecules, like oligothiophenes and oligo(p-phenylene vinylene)s,
Highly organized molecules with tailored morphologies across
scales are of great importance to future technologies, such as thin-
film organic electronic devices.1 In order to obtain a semicon-
ducting organic material with properties necessary for efficient
devices there must be spatial overlap of π-orbitals among
adjacent conjugated molecules. This overlap generates large
bandwidths for valence or conduction bands to assist charge
hopping among molecules by shortening intermolecular
distances.2ꢀ9 Reducing grain boundaries or lattice defects can
increase charge mobilities of crystalline organic semiconductors
by lowering the number of carrier traps.10 Research on crystalline
organic semiconductors has also revealed anisotropic carrier
mobilities along crystallographic axes that have different degrees
of π-orbital overlap, thus demonstrating the influence that
molecular packing, orientation, and morphology can have on
electronic properties.2ꢀ9,11,12 Designing solution-processable
organic semiconductors requires innovative approaches to con-
trol ordering and allow efficient charge transport while minimiz-
ing the defects that typically arise from solution casting.2,6
Conjugated molecules tend to have poor solubilities because
strong πꢀπ interactions drive crystalline domain formation,
Received: May 25, 2011
Published: August 31, 2011
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2011 American Chemical Society
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as semiconductor units to form semiconducting 1D nano-
structures.12,20,22,29ꢀ31 Oligothiophene derivatives consist of
thiophene units connected by α-linkages with solubilizing groups
attached at the terminal α-positions imparting the molecule with
a high degree of π-orbital overlap and solubility.11,32 Oligothiophene
has a low band gap33 and can be easily derivatized into many covalent
architectures, and the solution and solid-state aggregation of oli-
gothiophenes has been well characterized for use in opto-electronic
devices.34,35 Oligothiophenes have also been proven to be viable
electronic materials for efficient photovoltaic devices.36,37
Control over morphology has been recently studied in the
n-type semiconductor C60 and its derivatives by amphiphilic
assembly, templated synthesis, and crystallization/precipitation.
Diverse structures ranging from nanowhiskers, nanofibers,
spheres, and flower-like nanoscale objects as well as hexagonal,
rhombohedral, and mixed polygonal sheets have been
obtained.38ꢀ50 A common crystallization/precipitation tech-
nique used to create polymorphs at solvent interfaces has been
termed liquidꢀliquid interfacial precipitation (LLIP).42 In a
typical LLIP experiment, C60 is dissolved into a “good” solvent,
such as toluene, and then a “poor” solvent, such as isopropanol, is
gently poured on top to form a liquidꢀliquid interface. Over time,
the “poor” solvent slowly diffuses into the “good” solvent, decreasing
the solubility of C60 to the point of supersaturation and nucleation of
structures. Following nucleation, myriad structures are obtained
depending on solvent, temperature, and chemical structure of the
C60 derivative. Assembly at solvent interfaces has great potential to
produce unique structures and also to spontaneously organize
molecules into ordered materials.51ꢀ53 Specifically, we recently
found a solid diffusion barrier formed by self-assembly to separate
two liquids that yields a hierarchically ordered membrane.51
Inspired by our interfacial assembly studies, the known poly-
morphism of C60, and examples of supramolecular interactions in
highly organized molecules,54ꢀ57 we explore here self-assembly
pathways of a new small conjugated molecule and the utilization of
a porous solid membrane as a diffusion barrier between two
solutions. Only one solution contained the conjugated molecule,
but each differed in the amount of the “poor” solvent that promotes
self-assembling behavior. Scanning electron microscopy (SEM) and
grazing incidence small-angle X-ray scattering (GISAXS) were used
as tools to identify morphology, internal organization, and orienta-
tion of the resulting structures. The new molecule used in this study
(structure shown in Figure 1) has the ability to aggregate through
both hydrogen bonding and πꢀπ stacking, and it is very similar to a
semiconducting molecule reported recently by us and others.24,58
Molecules that combine both of these interactions have been known
to self-assemble into 1D aggregates and form gels as a result of
network formation at low concentrations.12,59ꢀ61
Figure 1. Molecular structure of a self-assembling oligothiophene
derivative. Quarterthiophene core shown in red and hydrogen-bonding
groups in blue. Solubility is enhanced by substitution of the core by 3,4,5-
tris(20-ethylhexyl)benzoate (shown in green).
of molecules relative to molecules with straight alkyl chains.62 A
molecule similar to 4TG-EH with unbranched alkyl solubilizing
groups was previously found to dissolve in solvents, such as
tetrahydrofuran (THF), that have an affinity for alkyl and
conjugated moieties and can also accept hydrogen bonds to
compete with that molecule’s self-assembly.58 In solvents like
toluene, that same molecule was reported to form solvated
H-aggregates.58 A concentration study of 4TG-EH in both
THF and toluene was performed and monitored by UVꢀvis
spectroscopy in order to probe the aggregation properties of the
molecule. In THF solutions, the absorbance λmax appeared at
approximately 415 nm for all concentrations measured between
10ꢀ5 and 10ꢀ3 M (Figure S2 and Table S1, Supporting In-
formation). In great contrast, the toluene solutions exhibited a
21 nm blue shift in λmax from 416 to 395 nm as the concentration
increased (Figure S2 and Table S2, Supporting Information). A
decrease in fluorescence intensity and 6 nm red shift was
observed when switching from THF to toluene (Figure S2,
Supporting Information). The blue shift in absorption, red shift
in fluorescence, and fluorescence decrease all indicate that
the aromatic oligothiophene core is forming a face-to-face
H-aggregate.63 These markers of solution-phase H-aggregation
of 4TG-EH are less intense than our previously reported
molecule with unbranched alkyl chains; in that system, a
42 nm blue shift in UVꢀvis absorbance, a 51 nm red shift in
fluorescence, and significant fluorescence quenching were
observed.58 We attribute these differences to the packing frustra-
tion provided by the branched alkyl solubilizing groups. We
conclude from these spectroscopic observations that self-assem-
bly of this molecule is inhibited by “good” solvents, such as THF,
and the formation of H-aggregates can be promoted in a “poor”
solvent, like toluene.
LiquidꢀVapor Precipitation. Molecular packing frustration
in 4TG-EH as a result of the six branched alkyl groups was
expected to play a significant role during drying in the self-
assembly of this molecule under kinetic versus thermodynamic
control. The solvent THF has a high vapor pressure and
evaporates rapidly, which may kinetically trap the 4TG-EH
during drying in a morphology with poor π-orbital overlap.
Addition of 1,4-dioxane, which is also a good solvent but has a
lower vapor pressure, should slow evaporation and allow mol-
ecules to access kinetically more stable supramolecular struc-
tures. We blended dioxane and THF in a 2:1 ratio to create a
“good” solvent that could dissolve 4TG-EH. Blending this
“good” solvent mixture with a “poor” solvent, such as toluene,
gave us a solution that dissolves 4TG-EH, evaporates slowly, and
can promote self-assembly by addition of more toluene. The
work described here utilized a blend of 2:1:1 dioxane/THF/
toluene to dissolve 4TG-EH into 2.5 wt % solutions. Solutions
were heated in order to dissolve and disaggregate molecules and
were then allowed to cool to room temperature before use.
We drop cast 10 μL of the 4TG-EH solution onto a clean
’ RESULTS AND DISCUSSION
Oligothiophene Self-Assembly. We synthesized the soluble
self-assembling quarterthiophene molecule, termed here 4TG-
EH, shown in Figure 1, using previously reported synthetic
methodologies.58 A detailed description of the molecular synth-
esis and characterization can be found in the Supporting In-
formation (Figure S1). Assembly of the quarterthiophene core is
assisted by hydrogen bonding among the four amide groups
surrounding the core. The self-assembling quarterthiophene
moiety is converted to a soluble compound by attaching gallic
acid groups at each terminus with six branched 2-ethylhexyl
groups in total. These groups should somewhat frustrate packing
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poly(tetrafluoroethylene) (PTFE) sheet. The drop remained
stationary at the location of drop casting and was allowed to dry
in a covered Petri dish (Figure 2). The resulting dried film was
sputter coated with 10 nm of Au/Pd and imaged by SEM. In the
micrographs, 1D structures can be observed as a mat of fibers
covering the surface of the PTFE (Figure 3a and d). The
observed structures can be tens of micrometers long and range in
diameter from hundreds to thousands of nanometers with
the long axis of the bundles parallel to the substrate. The length
of the 4TG-EH was estimated by molecular modeling to be
5.9 nm in its extended conformation. This length is significantly
smaller than the observed diameters in the SEM, indicating the
occurrence of a high extent of bundling. We expect that the 4TG-
EH does not adhere to the fluorinated substrate because of
PTFE’s low surface energy. This implies that the fibrous assem-
bly of molecules occurs in solution, precipitates, and then grows
as the solution dried. These fibrous structures are common for
self-assembling molecules similar to 4TG-EH, and we assume
that the πꢀπ stacking direction is parallel to the long axis of the
bundled fibers.15
to the sample environment. We drop cast 10 μL of the same 2.5
wt % solution of 4TG-EH onto a piece of clean PTFE and
allowed it to dry in covered Petri dish with a toluene-saturated
atmosphere (see Figure 2b). Over time toluene vapors diffused
into the droplet and lowered the solubility of the 4TG-EH, and
after several hours, the droplet dried. We observed by SEM that
slow drying of the solvent results in the growth of faceted and
regular structures instead of only fibers. The structures had
rhombohedral shapes and ranged in size from 1 μm to a little
over 10 μm across (see Figure 3b). As revealed in Figure 3b, we
also observed bundles of fibers in addition to the rhombohedra.
The rhombohedra typically have one axis longer than the other,
and high-magnification images of their surfaces reveal striations
perpendicular to the short axis of the rhombohedra. We hy-
pothesize that the πꢀπ stacking direction coincides with these
observed striations.15 Rhombohedra were also observed to
precipitate from solutions of 4TG-EH aged overnight in a sealed
vessel. In fact this solution-aging method proved to be a more
reproducible pathway to obtain larger rhombohedra with a
minimum amount of fibers (Figure 3e).
SEM micrographs of the rhombohedra suggest a possible
growth mechanism. We observed fibers growing from the
rhombohedra along the direction of the striations (see
Figure 3e). We believe these fibers are the first structures formed
through a fast process, and toluene is likely to become a part of
the structure around the rapidly formed fibers. The presence of
solvent in initially nucleated self-assembled structures has been
proposed previously by the Meijer group.68 Ostwald’s rule of
order states that the least stable polymorph forms first and then
gets converted to a more stable polymorph.65,69 After the initial
nucleus forms, additional adhering molecules should allow it to
grow, but the initial growth is metastable. The fibers formed are a
metastable state that slowly convert to the more stable rhombo-
hedra, which remain when drying is complete. This argument is
supported by the previously observed fibers that formed from
faster drying droplets. Another possible mechanism for the
formation of rhombohedra starts with initial formation of fibers
that further grow into rhombohedra from dissolved molecules.
Both mechanisms are possible and most likely both occur to
some extent. In micrographs containing both bundled fibers and
rhombohedra (see Figure 3b), the two structures are never
Slow Drying. The interplay between intermolecular forces
used to direct self-assembly and the solvent environment can
have dramatic effects on nanoscale organization.64ꢀ67 We there-
fore repeated the fibrous growing conditions with a slight change
Figure 2. Selective formation of various polymorphs from solvated
4TG-EH (red) on a sheet of PTFE (white). (a) 2.5 wt % 4TG-EH
without toluene vapor (blue). (b) 2.5 wt % 4TG-EH with toluene vapor.
(c) 5 wt % 4TG-EH with toluene vapor.
Figure 3. SEM micrographs of polymorphs formed under different conditions on PTFE. (a and d) Bundled fibers formed from a 2.5 wt % solution
without toluene vapor. (b) Rhombohedra interspersed with bundled fibers formed from a 2.5 wt % solution with toluene vapor. (c and f) Randomly
oriented hexagonal prisms formed from a 5 wt % solution with toluene vapor. (e) Larger rhombohedra precipitated from an aged 2.5 wt % solution with
fibers growing along the striations of the rhombohedra.
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observed to be directly connected, which implies that the
nucleation and growth of rhombohedra occur separately from
bundled fiber formation.
bonding, as expected for a solid cast from low-polarity solvents.
We suspect this hydrogen bonding is intermolecular, since
intramolecular hydrogen bonding would involve formation of a
seven membered ring that would need to be strong enough to
compete with intermolecular hydrogen bonding at the high
concentrations as the solvent evaporates. The lower energy
transitions in the region around 800 cmꢀ1 show vibrations
characteristic of ordered oligothiophenes.70 Amorphous oli-
gothiophenes, such as solvated molecules, should show only a
single peak in this region that can be attributed to one γ-CH-out-
of-plane vibration. This peak would then split as the oligothio-
phenes aggregate.70 There are minor differences in the position
of the peaks, but the general shape is the same for all three
polymorphs, which shows that the oligothiophene core is not
amorphous. This result is significant since the UVꢀvis spectra
show that rhombohedra have a λmax similar to the solvated
molecule, hexagonal prisms with a λmax similar to the aggregated
molecules, and the fibers somewhere in between. Therefore, it
appears that the quarterthiophene units in the three different
morphologies are aggregated but possibly at different angles,
giving slightly different peak positions observed in the FTIR and
different λmax in the UVꢀvis.
Effect of Higher Concentration. Higher concentrations may
also affect morphology by increasing the likelihood of rapid
nucleation and growth. This faster nucleation and growth is likely
to favor the metastable state, and in the case of 4TG-EH, we
assumed it would be the fiber morphology. At high enough
concentrations in the “poor” solvent (toluene), molecules similar
to 4TG-EH have been shown to gel by forming interpenetrating
networks of fibers.24,58 In our experimental solvent blend, gel
formation is not observed even at concentrations as high as 5 wt %
after aging for 30 days. The same precipitation observed in 2.5
wt % solutions was not observed in 5 wt % solutions. This implies
that in aged solutions at the 5 wt % concentration, self-assembly
occurs quickly and traps the 4TG-EH into the metastable fiber
morphology without conversion to a more stable polymorph
over time. We drop cast 10 μL of a heated then cooled solution of
5 wt % 4TG-EH onto a clean flat piece of PTFE in a Petri dish
containing an atmosphere saturated with toluene. This droplet
dried slowly over several hours, resulting in a film. SEM of this
film reveals a third morphology consisting of hexagonal prisms
ranging in size from 3 to 10 μm across and about 1 to 5 μm in
height (Figure 3c and f). Almost all of the 4TG-EH converted to
hexagonal prisms in these samples with fibers appearing at the
periphery of the dried droplet where drying occurred first and
also covering some of the hexagonal structures. As revealed in the
micrograph of Figure 3f, we observe several rectangular objects
that are actually hexagonal prisms on their sides. This side view of
the prisms, as well as the fractured prism in the lower right
portion of Figure 3f, demonstrates clearly the presence of
striations along the long axis of the prism. Based upon our
observations, the rhombohedra appear to be the thermodynamic
product, and the hexagonal prisms and fibers are kinetically
trapped structures. The propensity of 4TG-EH to form a regular
structure like a rhombohedra coupled with its conformational
flexibility may force it to adopt other structures like hexagonal
prisms when conditions favor a kinetically trapped structure.
Structural Characterization. The internal organization of
these dried polymorphs was characterized by UVꢀvis spectros-
copy and Fourier transform infrared spectroscopy (FTIR). The
normalized transmission UVꢀvis data (Figure S3, Supporting
Information) demonstrate that all three dried polymorphs have
different λmax positions. The rhombohedra have a λmax closer to
the solution-phase unassembled molecule at 411 nm. The dried
fibers have a λmax of 402 nm, which is between the solution-phase
assembled and nonassembled states. On the other hand, the
hexagonal pillars have a maximum absorbance at 386 nm, which
is close to the absorbance we attributed to the H-aggregated
solution assembly of 4TG-EH, which was 395 nm. The internal
organization of the quarterthiophene core in the rhombohedra
and fibers is probably not the same aggregated structure that
occurs in the hexagonal prisms and solvated assemblies. In order
to further probe the internal organization of these polymorphs,
powders of all three polymorphs were prepared on PTFE sheets
and transferred to a ZnSe crystal for FTIR analysis. Transmission
FTIR presented in Figure S4, Supporting Information, shows the
similarities of the vibrational profile of all three polymorphs. We
observed a shift of the NꢀH stretch from 3312 to 3314 to
3320 cmꢀ1 from rhombohedra, to fibers, to hexagons, respec-
tively. The position of this NꢀH stretch observed at about
3300 cmꢀ1 is indicative of an amide involved in hydrogen
The three polymorphs are easily accessible by this simple
liquidꢀvapor technique, but orientation of the polymorphs is not
well-defined. The bundled fibers mostly lie in a 2D mat with their
long axis parallel to the substrate; rhombohedra lie flat on the
surface with striations also parallel to the substrate; and the
hexagonal prisms are randomly oriented with respect to the
substrate. Facilitating charge mobility through long-range molecular
orientation with respect to electrodes is of course functionally
important in devices, such as field-effect transistors and solar cells.
For example, in photovoltaic devices the ideal orientation of
conducting pathways is perpendicular to electrode planes.71 We
demonstrate below how a self-assembly pathway was found in the
system investigated here utilizing a liquidꢀmembraneꢀliquid
interface.
LiquidꢀMembraneꢀLiquid Interfacial Precipitation. Re-
cent examples from our laboratory of hierarchically ordered
structures formed at liquidꢀliquid interfaces51 and from the
literature of vertically aligned C60 microtubes by liquidꢀ
membraneꢀliquid interfacial precipitation52 inspired us to de-
velop a different pathway for the self-assembly of the molecule
investigated here. A 5 wt % solution of 4TG-EH in a 1:1 blend of
THF/toluene was gently poured onto a solution of toluene
forming a liquidꢀliquid interface, and the vial was sealed to
prevent evaporation. Over time the toluene diffused into the
4TG-EH, promoting it to assemble. The resulting structure
consisted solely of fibers with no other polymorphs present.
From our liquidꢀliquid interfacial experiments described above,
it is clear that diffusion occurs quickly between these solutions,
favoring metastable fiber formation. We hypothesized that
introducing a porous barrier between the two liquids could
further slow diffusion of the solvent that promotes self-assembly
and allow the selective formation of polymorphs and potentially
the creation of new polymorphs. Anodic aluminum oxide (AAO)
membranes with straight transmembrane cylindrical channels of
200 nm diameters were used as a diffusion barrier between the
4TG-EH solution and the toluene solvent. We placed toluene on
one side of a vertically oriented AAO membrane and then a solution
of 5 wt % 4TG-EH on the other (Figure 4). Over time the two
solutions diffused into each other to create a region of increased
toluene concentration at the liquidꢀmembraneꢀliquid interface,
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and over time the solutions dried. Again, only fibers were observed,
and we concluded the AAO membrane alone was not sufficiently
slowing down toluene diffusion. We considered that diffusion across
the interface of two liquids can also be controlled by increasing their
viscosities and thus chose to control the viscosity of our toluene
solution by dissolving a high concentration (20 wt %) of a high
molecular weight poly(methyl methacrylate) (PMMA) (Mn:
996 000 Da). The high-viscosity solution was used in addition to
the vertically supported AAO membrane to slow diffusion of
toluene into the solution of 4TG-EH, producing a region of
increased toluene concentration at the liquidꢀmembraneꢀliquid
interface (see Figure 4). A detailed description and diagram of the
mounted membrane setup can be found in the Supporting Informa-
tion (Figure S5). We placed the viscous toluene solution on one side
of a mounted AAO membrane and observed it to wet the membrane
making it translucent, but it did not flow to the other side. On the
other side of membrane, we placed 10 μL of a 2.5 wt % 4TG-EH
solution in 2:1:1 dioxane/THF/toluene and allowed it to dry over
hours. The AAO membrane was held vertically, causing the solution
to dry and concentrate downward toward the bottom of the AAO.
The resulting structures are rhombohedral with lengths of the long
axis of about 2 μm (Figure 5a and d). The rhombohedra are the
dominant morphology in this case, appearing as large aggregates
near the bottom of the dried sample. Increasing the concentration to
5 wt % 4TG-EH produces the same hexagonal structure observed
using the original liquidꢀvapor method on PTFE but with one
major difference. We found that the hexagonal prisms are all
anchored to the surface of the AAO and growing perpendicular to
the surface (Figure 5b and e) forming oriented pillars. There are
visible striations in the SEM micrograph of the pillars (see Figure 5e)
that run along a direction perpendicular to the substrate. From these
experiments it is unclear if the directionality of toluene diffusion
through the membrane (see Figure 4) induces the observed oriented
growth of hexagonal pillars on the surface of the membrane. These
hexagonal pillars cover the entire surface of the AAO in contrast to
the rhombohedra, which are found mostly at the bottom of the dried
membrane. Concentration has the same effect on morphology as the
liquidꢀvapor technique, but the AAO substrate and directionality of
solvent flow affords control over long-range orientation of the
hexagonal prisms.
A hexagonal pillar grown on an AAO membrane was thinned
with a focused ion beam and attached to a probe for imaging
by transmission electron microscopy (TEM) (Figure 6). In
Figure 6b the thinned AAO membrane is visible as the darker
pore structure on the bottom, and the 4TG-EH is the solid gray
material on the top. There are visible regular striations in the
4TG-EH pillar running perpendicular to the surface of the AAO
pores and to some extent into the pores. Fast Fourier transform
(FFT) of a section of the TEM micrograph shows that the
striations are indeed regular. Image profiles and the FFT spacing
indicate a spacing of 5.55 nm which indeed corresponds to the
length of the conjugated molecule, which is ∼5.9 nm as
determined by molecular modeling. These TEM micrographs
suggest that the long axis of the molecule lies parallel to the
surface plane of the AAO, and the πꢀπ stacking direction is
perpendicular to the substrate along the direction of the observed
striations.
Figure 4. Schematic of liquidꢀmembraneꢀliquid deposition setup
using an AAO membrane to slow the diffusion of solvent between
two solutions. The compartment to the left of the membrane contains a
viscous toluene solution of PMMA shown in purple. Toluene, shown in
blue, traverses the membrane into the right compartment containing a
4TG-EH solution, shown in red, creating a region of higher toluene
concentration at the interface.
Role of Surface Polarity. Modifying the solvent viscosity and
introducing a porous diffusion barrier have proven to be useful
techniques for controlling the aggregation and orientation of this
Figure 5. SEM micrographs of polymorphs formed on an AAO membrane. (a and d) Rhombohedra precipitated from a 2.5 wt % solution.
(b) Anchored hexagonal prisms growing perpendicular to substrate from a 5 wt % solution. (c and f) Bundled fibers formed from a 5 wt % solution at 0 ꢀC
attached and growing perpendicular to the substrate. (e) Higher magnification micrograph of hexagonal prisms showing striations along the long axis of
the prisms perpendicular to the substrate.
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Figure 7. SEM micrographs of hexagonal prism grown on ITO.
(a) Low-magnification image of several hexagonal pillars. (b) High-
magnification image of hexagonal pillar demonstrating fibrous growth
on the external surfaces of the prism.
Figure 6. Micrographs illuminating internal structure of hexagonal
prisms grown on an AAO membrane with cartoons included for clarity.
(a) SEM micrograph of hexagonal prism coated by a line of platinum and
thinned by a focused ion beam. (b) TEM micrograph of a thinned prism
and the underlying porous membrane. Cross-section of pores is visible
on the bottom of the image, while the thinned 4TG-EH structure can be
seen on the top of the image. (c) FFT of section of TEM micrograph
indicating regularity of striations.
Supporting Information), even at this lower concentration. We
speculate that increasing the surface polarity allows more 4TG-
EH molecules to adsorb to the surface, which coupled with the
higher concentration of toluene at the liquidꢀmembraneꢀliquid
interface increases the likelihood that adsorbed molecules would
favor heterogeneous nucleation of the hexagonal pillars. As
mentioned above, we believe the hexagonal pillars grow from a
nucleus perpendicular to the plane of the membrane by a
heterogeneous nucleation and growth pathway.
Role of Temperature. Lower temperatures should favor the
metastable structure, since molecules have less energy to move
and rearrange into a more stable state. Solvent evaporation is
slower at lower temperatures, which allowed us to use the binary
solvent system of 1:1 THF/toluene instead of the more complex
ternary system used previously. A 5 wt % solution of 4TG-EH
was placed in the vertical setup shown in Figure 4 with UV-ozone
treated AAO membrane and allowed to dry slowly at 0 ꢀC. We
observed twisted bundled fibers growing perpendicular to the
AAO membrane (Figure 5c and f). The UV-ozone treated AAO
membranes allow more 4TG-EH molecules to adsorb to the
surface ultimately leading to nucleation. The initial structure
formed in this case is the metastable fibrous polymorph, but at
these lower temperatures, the metastable phase cannot convert
to the more stable hexagonal polymorph. As the solution dries,
the fibrous growth continues perpendicular to the substrate in a
manner similar to hexagonal prisms, but the entire structure
remains trapped as bundled fibers.
hydrogen-bonded conjugated oligomer and could provide addi-
tional tools to probe the aggregation of these molecules. One
particularly interesting tool is surface polarity modification.72 It is
evident by comparing the hexagonal pillars and rhombohedra
that the AAO surface plays a significant role in their nucleation
and growth. Rhombohedra were not observed directly growing
from the AAO membrane, and this implies that they form
through a homogeneous nucleation pathway. Hexagonal prisms
were observed by SEM to grow either through a homogeneous
nucleation pathway during the previously described experiment
on a PTFE sheet (see Figure 3c and f) or from a heterogeneous
nucleation pathway on an AAO membrane (see Figure 5b and e).
Therefore the surface energy of the substrate was expected to
have a strong impact on the nature of the 4TG-EH structure
nucleated. The amide groups are likely to interact and adsorb
onto the polar aluminum oxide surface, so increasing the polarity
of this surface was expected to increase its affinity for the amide
groups and thus direct the self-assembly pathway toward hetero-
geneous nucleation (we accomplished this surface modification
with a 10 min UV-ozone treatment of the AAO). A 2.5 wt %
solution of 4TG-EH known to form rhombohedra was used to
test the effect that surface modification has on their growth. By
changing this parameter, we observed a transition from exclu-
sively rhombohedra to exclusively hexagonal prisms (Figure S7,
Deposition onto a Nonporous ITO Electrode. From the
experiments described above, it is clear that polymorph directing
factors, like concentration, temperature, and substrate polarity,
can control the structure of this hydrogen-bonded conjugated
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Figure 8. 2D-GISAXS patterns of (a) perpendicularly oriented hexagonal pillars, (b) randomly oriented rhombohedra microcrystals, and (c) mats of
bundled fibers deposited onto ITO substrates.
oligomer. It is not clear, however, to what extent the porous
nature of the substrate surface and the delivery method of a
“poor” solvent affects the polymorphism of this molecule. The
porous membrane made from AAO is an opaque insulator and is
not immediately applicable to optoelectronic devices. Therefore,
we were also interested in self-assembly behavior on a solid
metal-oxide film, such as indium tin oxide (ITO), which is highly
transparent and highly conductive but not porous. Using the
observations made possible from the liquidꢀmembraneꢀliquid
technique that the hexagonal polymorph can be selected over the
others by increasing the 4TG-EH concentration, slowly deliver-
ing toluene, and depositing onto a hydrophilic UV-ozone treated
surface, we attempted to grow hexagonal prisms directly onto an
ITO electrode. A 10 μL drop of 5 wt % 4TG-EH in 1:1 THF/
toluene was drop cast and spread across a UV-ozone treated ITO
electrode and was placed in a Petri dish with a toluene-saturated
atmosphere. After drying, the resulting microstructure is the
hexagonal polymorph growing perpendicular to the ITO sub-
strate (Figure 7). If growth of these structures is allowed to
continue, eventually they coalesce to form sheets (Figure S8,
Supporting Information). These results show that the direction-
ality of solvent flow present in liquidꢀmembraneꢀliquid inter-
facial precipitation (see Figure 4) is not necessary to induce the
observed oriented growth (see Figures 5e and 7). The source of
orientation in this polymorph appears to originate from interac-
tions of the molecules with the metal-oxide surface. Based on
SEM observations, the growth of pillars might proceed through
the metastable fibrous structures as observed in rhombohedra
(Figure 7b). The drying process trapped the metastable states on
the external surfaces of the hexagonal prisms, which stay attached
to the top surface but separated from the sides of the prisms upon
drying. We propose that the external surfaces of the hexagonal
prisms grow by first forming a metastable fibrous state that then
slowly rearranges and converts to the hexagonal structure. We
did not observe these fibrous structures in the case of growth on
an AAO membrane most likely because the solutions cast on ITO
dried at a faster rate.
arrangement of molecules in the xꢀy plane, with a d-spacing of
32.9 Å. This distance corresponds roughly to the length of the
rigid aromatic core of the molecule plus the hydrogen-bonding
amide groups (34 Å), indicating that the long axis of the molecule
runs parallel to the ITO substrate in the xꢀy plane. The (01) and
(02) peaks correspond to a d-spacing of 11.9 Å perpendicular to
the substrate surface in the z-direction. This d-spacing does not
match to any obvious intermolecular distance and is likely due to
a tilted arrangement of molecules within the pillar structure.
Comparing the anisotropic 2D-GISAXS pattern with our TEM
and SEM results, we expect that the z-direction is also the
direction of πꢀπ stacking. The rhombohedra (Figure 8b) show
several spots indicating that the internal structure of the poly-
morph is highly ordered. The (01) and (02) peaks corresponds
to a d-spacing of 24.4 Å. The other bright spots indicate order
along various crystallographic axes, but the polycrystallinity of
this sample resulted in a pattern too complex for complete
analysis. We were unable to grow crystals of sufficient size for
single-crystal analysis. The fibers (Figure 8c) demonstrate less
orientation than the hexagonal pillars with the (01) peak
√
corresponding to 41.0 Å and the (0 3) peak indicating weak
hexagonal packing. In summary, all three polymorphs have
significantly different internal structures. For electronic applica-
tions, the hexagonal pillars (Figure 8a) may have an optimal
structure, with molecular stacking perpendicular to the substrate
surface and H-aggregated πꢀπ stacking between molecules. For
photovoltaic devices, these interactions would provide high-
mobility pathways through πꢀπ stacked structures and short
distances for carriers to travel to electrodes.
’ CONCLUSIONS
We have shown strong polymorphism and control of molec-
ular orientation in the self-assembly of a semiconducting con-
jugated molecule with capacity to form hydrogen bonds.
Selection of a self-assembly pathway that involves nucleation on a
surface and exceedingly slow aggregation of molecules was found
to result in highly oriented prismatic structures that could be of
interest for electronic function. These observations demonstrate
how the supramolecular energy landscape can be guided by
external conditions to modify the structural nature of assemblies
formed by conjugated organic molecules.
Two-dimensional GISAXS (2D-GISAXS) is a powerful tech-
nique for determining internal structure and orientation of self-
assembled structures relative to a substrate.72 2D-GISAXS data
were obtained for the hexagonal pillars grown on ITO, films of
rhombohedra deposited on ITO, and mats of fibers deposited on
ITO (see Figure 8). The hexagonal pillars (Figure 8a) demon-
strated a preferred orientation of molecules with respect to the
’ ASSOCIATED CONTENT
substrate surface, as evidenced by the anisotropic scattering
√
S
intensity in the 2D-GISAXS data. The (10) and ( 20) peaks
along the equatorial direction correspond to a cubic packing
Supporting Information. Complete synthetic details of
b
4TG-EH, UVꢀvis, fluorescence and FTIR spectra, detailed
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Journal of the American Chemical Society
ARTICLE
growth setup of polymorphs on AAO membrane, SEM micro-
graphs of hexagonal structures grown under different conditions,
TEM micrographs of hexagonal structure, SEM of hexagonal
structures grown on ITO, and complete citation for ref 47. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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s-stupp@northwestern.edu
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’ ACKNOWLEDGMENT
This work was supported by the U.S. Department of Energy
Office of Science, Office of Basic Energy Sciences (DE-FG02-
00ER45810) as well as support for L.C.P. from the National
Science Foundation (DMR-0605427). X-ray measurement dif-
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Advanced Photon Source at Argonne National Laboratory. Use
of the Advanced Photon Source at Argonne National Laboratory
was supported by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences, under contract no. DE-
AC02-06CH11357. This work also made use of the Electron
Probe Instrumentation Center (EPIC) for SEM, TEM, and
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