Self-Assembly of a Functional Oligo(Aniline)-Based Amphiphile into Helical Conductive Nanowires

Article abstract of DOI:10.1021/jacs.5b06892

A tetra(aniline)-based cationic amphiphile, TANI-NHC(O)C₅H₁₀N(CH₃)₃⁺Br^- (TANI-PTAB) was synthesized, and its emeraldine base (EB) state was found to self-assemble into nanowires in aqueous

Full text of DOI:10.1021/jacs.5b06892

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Article
Self-assembly of a functional oligo(aniline)-based
amphiphile into helical conductive nanowires
O. Alexander Bell, Guanglu Wu, Johannes S. Haataja, Felicitas Broemmel, Natalie Fey, Annela
M. Seddon, Robert L. Harniman, Robert M. Richardson, Olli Ikkala, Xi Zhang, and Charl F.J. Faul
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b06892 • Publication Date (Web): 23 Oct 2015
Downloaded from http://pubs.acs.org on October 26, 2015
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Self-assembly of a functional oligo(aniline)-based amphiphile into
helical conductive nanowires
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O. Alexander Bell,a Guanglu Wu,b Johannes S. Haataja,c Felicitas Brömmel,a Natalie Fey,a Annela
M. Seddon,d, e Robert L. Harniman,a Robert M. Richardson,d Olli Ikkala,c Xi Zhang,b Charl F.J.
Faul*, a
a School of Chemistry, University of Bristol, Bristol BS8 1TS, UK
b Key Laboratory of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua Univer-
sity Beijing 100084 (China)
c Aalto University, Department of Applied Physics, Molecular Materials, FIN-00076 Espoo, Finland
d H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, United Kingdom
e Bristol Centre for Functional Nanomaterials, Nanoscience and Quantum Information Building, University of Bris-
tol, Tyndall Avenue, Bristol BS8 1FD, United Kingdom
Supporting Information Placeholder
ABSTRACT: A tetra(aniline)-based cationic amphiphile, TANI-NHC(O)C5H10N(CH3)3+Br– (TANI-PTAB) was
synthesized, and its emeraldine base (EB) state was found to self-assemble into nanowires in aqueous solution.
The observed self-assembly is described by an isodesmic model, as shown by temperature-dependent UV/Vis
investigations. Linear dichroism (LD) studies, combined with computational modeling using time-dependent
density functional theory (TD-DFT), suggests that TANI-PTAB molecules are ordered in an anti-parallel ar-
rangement within nanowires, with the long axis of TANI-PTAB arranged perpendicular to the nanowire long
axis. Addition of either S- or R- camphorsulfonic acid (CSA) to TANI-PTAB converted TANI to the emeraldine
salt (ES), which retained the ability to form nanowires. Acid doping of TANI-PTAB had a profound effect on
the nanowire morphology, as the CSA counter-ions’ chirality translated into helical twisting of the nanowires,
as observed by circular dichroism (CD). Finally, the electrical conductivity of CSA-doped helical nanowire thin
films processed from aqueous solution was 2.7 mS cm-1. The conductivity, control over self-assembled 1D struc-
ture and water-solubility demonstrate these materials’ promise as processable and addressable functional mate-
rials for molecular electronics, redox controlled materials and sensing.
tainable processability, is also advantageous for self-
INTRODUCTION
assembly of functional organic materials, where wa-
Self-assembly1 provides facile, bottom-up access to
ter can promote structure formation5–7 and boost the
complex ordered morphologies that can improve
material properties, such as charge carrier mobility
strength of morphology-directing intermolecular π-
π interactions.8 Self-assemblies in aqueous environ-
(µ) and conductivity (σ), and lead to novel emergent
properties. Much work has traditionally been fo-
cused on self-assembly and processing of materials
from organic solvents, however water-soluble mate-
rials present considerable benefits. To this end there
has been increasing interest in water-soluble ‘func-
tional amphiphiles’ - typically an oligomers or small
organic molecules that are covalently connected to a
hydrophilic head groups causing water solubility and
that can undergo self-assembly.2–4 Water solubility,
whilst facilitating routes to biocompatible and sus-
ments of interest for applications include anisotropic
1D fiber- or wire-like structures (to improve charge
transport properties of organic semiconductors),9–12
and 3D gel networks, to modify rheological or me-
chanical properties.13
One class of conjugated oligomeric materials for
which aqueous self-assembly is nearly unknown is
oligo(aniline)s:14 short (4-, 8- or 16-mer) analogues of
the well-known conducting polymer poly(aniline)
(PANI).15,16 Oligo(aniline)s, and especially the syn-
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thetically accessible tetramer, TANI, have been inves-
tigated
for their interesting
and
varied
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Figure 1. (a) Molecular structure and DFT-optimized space-filling model of TANI-NHC(O)C5H10N(CH3)3 Br– (TANI-
PTAB). (b) Concentration dependent UV-Vis spectra of TANI-PTAB in water (arrow indicates change on increasing
concentration), inset: linear dependence of absorbance on concentration (values taken at λmax = 580 nm). (c) Criti-
cal aggregation concentration for aqueous TANI-PTAB solution, measured by pyrene fluorescence, CAC = 1 × 10−4
M.
+
self-assemblies in the contexts of thin films,17,18 crys-
sion inhibition properties.41 More frequently,
poly(ethylene oxide) blocks have been used to pro-
mote water-solubility in adducts with TANI.42,43 We
chose a cationic headgroup to decouple the water-
solubility from effects of pH and doping of the oli-
go(aniline). Previously reported anionic oli-
go(aniline) amphiphiles were self-doped by the acid-
ic sulfonic acid headgroup,41 preventing both study
of the native EB state material or the ability to con-
trol doping to tune structure, as shown here. Further,
where the self-assembly has been investigated, these
attempts have only produced zero-dimensional (0-
D) micelle or vesicle nanostructures. Higher dimen-
sionality is desirable for a host of applications, such
as organic electronics, where a 1-D self-assembling
conducting nanowire, although not often achieved,
is an attractive component.44
talline and block-like
microstructures19–21
structures.22 The acid/base- and redox-switchable
properties of TANI, as well as its switchable transi-
tion to a conductive salt state, echoes that of PANI.23
In addition, the improved processability and control
over molecular structure inherent to oligomers and
small molecules make these materials attractive for
further exploitation. However, very few reports of
self-assembly exist for oligo(aniline)s compared to
other conjugated oligomers such as oli-
go(thiophene), indicative of the challenges posed by
aniline-based materials.
Oligo(thiophene)24 is the most commonly reported
π-conjugated motif on which water-soluble func-
tional amphiphiles are based. Variations in the con-
jugation length from sexi-,25,26 quinque-,27,28 tetra-
,
28,29 to terthiophenes,30,31 architecture from bola-25–27
We report here on the synthesis and self-assembly of
the first TANI-based amphiphile bearing a cationic
head group. TANI was end-functionalized with an
amide-linked five-carbon spacer bearing a trime-
thylammonium head group; together these features
endow water-solubility and cause spontaneous self-
assembly into high aspect-ratio 1D nanowires. The
thermodynamics of self-assembly were studied, with
self-assembly found to be strongly enthalpically fa-
vored. The highly anisotropic structures could be
tuned to form conductive nanowires with induced
helicity from addition of an enantiomerically pure
chiral sulfonic acid dopant, demonstrating the versa-
tility of the TANI motif as a platform for designing
customizable supramolecular structures. Facile pro-
cessing into thin films of nanowires from aqueous
solution was demonstrated, with conductivity meas-
to single-headed amphiphiles28,29 and choice of the
polar head-group from cationic,25 nonionic ethylene
oxide26,29 to peptidic27,28,32 are used to tune the solu-
tion self-assembly, the resultant morphology, and
material properties. Other functional and conjugat-
ed organic motifs that have been applied as func-
tional amphiphiles include derivatives of pyrene,33
perylene,4,34 oligo(fluorene)s35–37,24–26 distyrylben-
zenes27,28,38,39 and p-phenylenevinylenes.40
Solubility of TANI in water is, as is typical for a π-
conjugated oligomer, intrinsically poor. While there
are some reports of attempts to improve water-
solubility by covalently inducing amphiphilicity, no
previous example of a cationic amphiphile exists.
Previously, anionic bola-amphiphiles based on pen-
ta(aniline) oligomers were reported for their corro-
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urements of these films showing the formation of 1D
aggregation was occurring. Solutions exhibited a
shoulder at 490 nm, as well as a 32 nm bathochromic
shift in absorbance at high concentrations (λmax
conducting nanowires. We believe these attractive
features pave the way to further studies on this hith-
erto under-investigated class of material.
=
582 nm for 1 × 10−3 M vs. 550 nm for 1 × 10−5 M solu-
tions, in keeping with previous reports).17 The onset
of these changes was at 1 × 10−4 M, which was con-
firmed as a critical aggregation concentration (CAC)
by pyrene fluorescence measurements (Figure 1c).48–
RESULTS AND DISCUSSION
SYNTHESIS OF OLIGO(ANILINE) AMPHIPHILE TANI-
PTAB
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The bathochromic shifts in absorbance observed
The TANI motif was used here as a functional and
conjugated core for the novel amphiphile TANI-
PTAB (Figure 1a). TANI was synthesized using the
Buchwald-Hartwig amination methodology45–47
combined with an orthogonal protecting-group
strategy to yield an asymmetric tetra(aniline) precur-
sor species (Ph/NH2 TANI). The addition of an am-
ide-linked C5 spacer and a quaternary trime-
thylammonium bromide yielded the water-soluble
functional amphiphile TANI-PTAB in the non-
conducting emeraldine base (EB) state. Full synthet-
ic details are available in the Supporting Infor-
mation.
with increasing concentration were reproduced
when the polarity of the solvent was decreased by
successive addition of aliquots of THF to a sub-CAC
aqueous TANI-PTAB solution, however no shoulder
at 490 nm was seen (Figure S3). This suggests that in
water at concentrations above the CAC, the TANI
chromophore is sequestered within the less polar
interior of aggregates, and aggregation results in vi-
bronic coupling, with emergence of a shoulder peak..
The broad absorbance at 550 nm was assigned to the
HOMO-LUMO transition of aq. TANI-PTAB based
on the main TD-DFT-calculated molecular orbital
(MO) contributions to the energy of the transition to
the first singlet excited state (2.41 eV, 514 nm; CAM-
B3LYP/6-31G* in water PCM (polarisable continuum
model), see the Supporting Information for full
method and further details, Figure S8).
SELF-ASSEMBLY OF EB TANI-PTAB
The aggregate morphology of EB TANI-PTAB was
investigated; transmission electron microscopy
(TEM) and cryo-TEM (Figure 2) showed that highly
anisotropic nanowires were formed with no external
treatment. Nanowires with an individual width of 3
nm (Figure 2c) formed fibrous bundles. The nan-
owire width is comparable to the length calculated
for a single TANI-PTAB molecule (2.9 nm), suggest-
ing truly 1-D aggregates were formed. Further mor-
phological information on the aggregates in solution
was provided by small-angle X-ray scattering (SAXS)
experiments (Figure S5a). Fitting the data to a thin
plate model (detailed further in the Supporting In-
formation) suggested structures with thickness of
approximately 3 nm formed in solution, which is
consistent with measurements from analysis of TEM
images.
Figure 2. (a) TEM (stained with 1% uranyl acetate) and
(b) cryo-TEM (unstained) of EB TANI-PTAB forming
bundles of nanofibers in aqueous solution (4 × 10−3
M
and 1 × 10−3 M, respectively), (c) Histogram of the
measured widths of nanofibers within bundles, (d) en-
larged section of cryo-TEM image showing nanofiber
bundles. Scale bars: 50 nm (a, b), 10 nm (d).
Aqueous solutions of highly water-soluble EB TANI-
PTAB were found to show concentration-dependent
shifts in UV/Vis absorbance (Figure 1b), suggesting
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molecules into the bulk5 and the increased interac-
tion between π-conjugated TANI moieties.
Table 1 - Thermodynamic properties of TANI-PTAB
self-assembly (1 × 10−3 M)
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TANI-PTAB
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Tm / K
ΔH / kJ mol-1
ΔS / J mol-1 K-1
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– 180
– 497
EB TANI-PTAB solutions above the CAC were highly
viscous, likely due to the self-assembled fiber net-
work in solution, and could be shear-aligned. This
was advantageous for linear dichroism (LD) studies,
where aligned samples were used to investigate the
nanowires’ molecular structure in solution. Shear
was applied to a TANI-PTAB solution, causing
alignment of anisotropic aggregates in the direction
of their long axis. Any preference for absorbance of
light polarized either parallel (A∥) or perpendicular
(Aꢀꢁ to the shear direction is indicative of the orien-
tation of electronic transition dipole moments (TDMs)
relative to the fiber axis.
Figure 3. Temperature-dependent UV/Vis for TANI-
PTAB (1 × 10−3 M, aqueous solution). Arrow indicates
direction of change on increasing temperature. Inset:
the mole fraction of aggregated molecules, α(T), as a
function of temperature.
The mechanism of self-assembly for TANI-PTAB was
investigated by temperature-dependent UV/Vis
(Figure 3). Heating a 1 × 10−3 M aqueous solution of
TANI-PTAB at a rate of 1 K min−1 and sampling spec-
tra at 5 K intervals from 288 K up to 338 K revealed a
hypsochromic shift of 32 nm from 582 nm to 550 nm.
The disappearance of a shoulder at 492 nm was also
observed, such that the high-temperature spectra
resembled those of the molecularly dissolved species
in solution at sub-CAC concentrations. The parame-
ter α(T), representing the variation in mole fraction
of the aggregated species with temperature, fitted
well to a sigmoidal curve (Figure 3, inset), indicative
of self-assembly behavior within an isodesmic, or
equal-K, model.51
A negative LD signal was observed for the HOMO-
LUMO45 transition of EB TANI-PTAB, indicating
greater Aꢀ (Figure 4a). To relate LD data to the mo-
lecular structure of TANI-PTAB, TD-DFT modeling
of the excited state determined the orientation of the
TDM for the HOMO-LUMO transition is orientated
along the long axis of the TANI moiety (exact Carte-
sian coordinates for the TDM can be found in the
Supporting Information). For a strong absorbance,
this TDM must be aligned in the same direction as
the polarization of light. The calculated orientation
of the TDM, along with LD results, suggests that
within nanowires TANI-PTAB molecules are ar-
ranged normal to the long axis of nanowires.
ꢃ ꢁ
ꢆ ꢄ ꢇ ꢆꢈꢉꢊ
ꢃ ꢁ
ꢂ ꢄ ꢅ
Eq. (1)
ꢆꢋꢌꢌ ꢇ ꢆꢈꢉꢊ
From Equation 1, A(T) is the measured absorbance at
temperature T, Amon and Aagg are the absorbance val-
ues of the monomer and fully aggregated states, re-
spectively. Values were taken at λmax = 492 nm, corre-
sponding to the shoulder observed for aggregated
species in solution. Thermodynamic parameters
were calculated from a Van’t Hoff plot (Figure S4);
these values and the melting temperature, Tm, of the
aggregates (the temperature, T, at which α(T) = 0.5)
are listed in Table 1. Large enthalpic gains, which off-
set the entropic cost of aggregation, are likely to re-
sult from the release of high-energy interfacial water
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definitive picture of the cationic head-group posi-
tions within nanowires.
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ACID DOPING: CONTROLLING SELF-ASSEMBLY IN
SOLUTION
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Figure 5. UV/Vis spectra showing the transition from
doped ES TANI(CSA)2-PTAB to de-doped EB TANI-
PTAB with dilution. Arrows indicate change on diluting
TANI(CSA)2-PTAB (1 × 10−4 M).
An appealing feature of oligo(aniline)-based materi-
als is their reversible protonic doping, allowing
switching between EB and emeraldine salt (ES)
states via protonation of quinoid N atoms of the EB
state. The acid counter-ion is included in the doped
material,55,56 enabling customization of the mor-
phology of self-assembled structures by selection of
the acid.19,57 Control over self-assembly in an aqueous
environment, with no requirement for solvent mix-
tures, solvent exchange or slow crystallization pro-
cesses, is an advantage that we demonstrate here
with TANI-PTAB.
Figure 4. (a) Calculated orientation of transition di-
pole moment relative to EB TANI-PTAB molecular
structure. (b) LD and UV/Vis spectra of the HOMO-
LUMO spectral region of TANI-PTAB (4 × 10−3 M). (c)
Birefringent optical textures observed under crossed
polars for TANI-PTAB between glass plates (4 × 10−3
M), scale bar: 100 μm. (d) Proposed alignment of TANI
chromophores within nanofibers, with transition di-
pole moment (white arrow) overlaid, relative to fiber
axis (black arrow)
Polarized optical microscopy (POM) investigations
revealed that these solutions displayed birefringent
optical textures typical of chromonic lyotropic ne-
matic phases (Figure 4c).52,53 Chromonic LC phases
consist of co-facial stacks of water-soluble aromatic
molecules that assemble via an isodesmic mecha-
nism. Based on LD, TD-DFT and POM, and by anal-
ogy to other chromonic dyes, amphiphiles based on
perylene diimides54 and oligo(aniline) in the solid
state,19 we suggest the TANI chromophores are
stacked co-facially, normal to the fiber axis. A disor-
dered, loosely cylindrical stack that allows maximal
shielding of the hydrophobic TANI from water by the
cationic headgroups is likely, though further investi-
gation by X-ray diffraction would be required for a
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Figure 7. Exciton-coupled CD spectra of TANI-PTAB
doped with both oppositely-handed CSA enantiomers
and racemic CSA to form TANI(CSA)2-PTAB, with
UV/Vis of the same spectral region .
SAXS confirmed the presence of aggregates of
TANI(CSA)2-PTAB in solution (Figure S5b). Howev-
er, the intensity gradient tended to be flat at low Q
suggesting a significant structure factor resulting
from inter-aggregate interactions. A precise thick-
ness determination is therefore not possible but the
thin plate model fit is consistent with a thickness on
the nanometer scale.
Figure 6. (a) TEM (stained with 1% uranyl acetate) and
(b) cryo-TEM (unstained) of TANI(CSA)2-PTAB (4 ×
10−3 M). (c) Histogram showing comparison of meas-
ured nanowire widths for EB and CSA-doped TANI-
PTAB. Scale bars: 50 nm.
Camphorsulfonic acid is a typical dopant for PANI
that enables high conductivity.58 Addition of enanti-
omerically pure camphorsulfonic acid (S-CSA) to EB
TANI-PTAB in aqueous solution caused a color
change from blue to bright green. This change is as-
sociated with formation of a polaron species contain-
ing cation radicals, typified in PANI systems by ab-
sorbances at 440 nm and, for compact coil confor-
mations at 780 nm.59 As TANI is much shorter than
PANI, and thus unable to form extended coil con-
formations, no true ‘free-carrier tail’ absorbance
band was observed. However, the observed absorb-
ance bands suggest polaron formation (Figure 5) and
the absence of absorbance in the HOMO-LUMO
transition region of EB TANI-PTAB (λmax = 550 nm)
indicated that EB TANI-PTAB was fully doped by
CSA (to yield ES-state TANI(CSA)2-PTAB).
Investigation of the doped TANI(CSA)2-PTAB by CD
spectroscopy (Figure 7) showed exciton-coupled
bisignate spectra indicative of helices in solution. A
positive Cotton effect was observed for TANI(S-
CSA)2-PTAB, indicating right-handed helicity.
TANI(R-CSA)2-PTAB produced a negative Cotton
effect, suggesting left-handed helices, while race-
mically doped TANI(±-CSA)2-PTAB showed no ellip-
ticity by CD. EB TANI-PTAB also showed no helicity
(Figure S7). The exciton-coupled spectra correspond
to the π*
←
polaron transition,60 which strongly sug-
gests the chiral CSA counterion is complexed to the
TANI and inducing local chirality in this chromo-
phore, which is translated into helicity in the nan-
owire as a whole.
TEM and cryo-TEM (Figure 6) confirmed the pres-
ence of nanowires even after protonation. Image
analysis showed that the width of individual wires
within the bundles increased to approximately 6 nm
when CSA was added, suggesting incorporation of
the CSA counter-ions into the fiber structure (Figure
6c).
Thin films of TANI(CSA)2-PTAB could be easily pre-
pared directly from aqueous solution by spin coating
onto glass substrates, without the addition of any
plasticizing additives or further processing. We in-
vestigated the conductivity of these films after drying
in vacuum to remove residual water, with pre-
patterned bottom-contact gold electrodes in a collin-
ear 4-point probe configuration, with electrode sepa-
ration of 250 µm. Conductivity of 2.7 ± 0.3 mS cm−1
was calculated based on 4-point probe resistance
measurements (Figure 8a).61 This is comparable for
previous bulk film measurements of TANI,45,62 alt-
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hough lower than the highest observed values for
Novel tetra(aniline)-based amphiphiles have been
crystalline TANI microstructures.63 However, when
compared to the processing steps for producing pre-
viously reported films or micro-crystals, the facile
processing by which TANI-PTAB nanowire thin films
were obtained may make TANI-PTAB a more attrac-
tive candidate for device applications in future.
prepared and found to self-assemble into nanofibers.
This process was investigated by a variety of com-
plementary methods. Self-assembly fitted an iso-
desmic model, with SAXS and cryo-TEM confirming
the presence of nanowires of 3 nm width in solution.
A model of the structure within the self-assembled
nanowires is suggested based on LD and TD-DFT.
The morphology of the self-assembled nanowires
could be controlled by addition of a chiral acid do-
pant, which induced helicity in the nanowires, with
handedness dependent on the dopant enantiomer
used. These nanowires could be spin-coated from
aqueous solution into thin films of conducting nan-
owires on glass substrates, exhibiting conductivity of
2.7 ± 0.3 mS cm-1. We have demonstrated a new and
promising amphiphilic oligo(aniline) material which,
with facile aqueous processing, tunable conductivity
and supramolecular morphology, has great potential
for use in devices and sensors.
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ASSOCIATED CONTENT
Supporting Information. Instrumentation, experi-
mental details, synthetic methods and characterization,
additional UV/Vis, Van’t Hoff thermodynamic analysis,
SAXS of TANI(CSA)2-PTAB, AFM showing EB TANI-
PTAB and R-CSA-doped TANI-PTAB, and computa-
tional chemistry details.
AUTHOR INFORMATION
CORRESPONDING AUTHOR
Figure 8. (a) Representative I-V curve for TANI(CSA)2-
PTAB spin cast on glass substrates with pre-patterned
gold electrodes in bottom-contact collinear 4-point
probe configuration (separation 250 µm). (b) AFM im-
age of spin-cast TANI(CSA)2-PTAB film (a scratch has
been made in the film to expose the substrate and allow
film thickness calculation).
* charl.faul@bristol.ac.uk
NOTES
The authors declare no competing financial interest
ACKNOWLEDGMENT
This work was supported by the EPSRC, ERC, and the
Academy of Finland. This work made use of the Aalto
University Nanomicroscopy Centre (Aalto-NMC) in-
strumentation. The Ganesha X-ray scattering apparatus
used for this research was purchased under EPSRC
Grant “Atoms to Applications” Grant ref. EP/K035746/1.
F. Brömmel thanks the Deutsche Forschungsgemein-
schaft for funding under Grant no. BR 4346/1-1. The
facilities and staff of the Electron Microscopy Unit,
Mass Spectrometry service and NMR service at the Uni-
versity of Bristol are kindly acknowledged.
AFM measurements show films were networks of
conductive nanowires and had consistent thickness
over large areas (Figure 8b). No conductivity was ob-
served for films of EB TANI-PTAB prepared in an
identical manner, as expected for the typically insu-
lating EB-state TANI materials. The conductivity of
our TANI(CSA)2-PTAB nanowires is over 300 times
greater than reported for a similar self-assembling
fiber system (from organic solvents) based on oli-
go(thiophene)s, further demonstrating the ad-
vantage of our material. 64
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