Block-like electroactive oligo(aniline)s: Anisotropic structures with anisotropic function

Article abstract of DOI:10.1039/c2jm32278a

A tetra(aniline)-alkyl diblock compound was designed, synthesized and fully characterized. By employing suitable conditions, doped, electroactive microstructures could be prepared. The microstructures were characterized in detail and their anisotropic conductivity measured for the first time. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm32278a

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Volume 22 | Number 32 | 28 August 2012 | Pages 16147–16658
ISSN 0959-9428
PAPER
Zhixiang Wei, Charl F. J. Faul et al.
Block-like electroactive oligo(aniline)s: anisotropic structures with
anisotropic function
0959-9428(2012)22:32;1-E

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PAPER
Block-like electroactive oligo(aniline)s: anisotropic structures with anisotropic
function†
b
a
c
b
Zhecheng Shao,‡a Zai Yu,‡bd Jianchen Hu, Saravanan Chandrasekaran, David M. Lindsay, Zhixiang Wei
*
a
*
and Charl F. J. Faul
Received 12th April 2012, Accepted 11th June 2012
DOI: 10.1039/c2jm32278a
A tetra(aniline)-alkyl diblock compound was designed, synthesized and fully characterized. By
employing suitable conditions, doped, electroactive microstructures could be prepared. The
microstructures were characterized in detail and their anisotropic conductivity measured for the first
time.
copolymers, are very well known for (a) forming microphase-
separated nanostructures in the bulk, and (b) to self-assemble
into various micellar structures in selective solvents due to the
immiscibility between the different blocks. A number of block-
like systems have therefore been synthesized by attaching
different types of functional groups to the amine terminus of the
phenyl/amine-capped oligo(aniline)s.20–25 For example, the
attachment of different length alkyl chains has been reported for
phenyl/amine-capped TANI (Ph/NH2 TANI) via simple substi-
tution reactions.26 As the TANI blocks are rigid and the alkyl
chains are flexible, the combinations are electroactive examples
of rod-coil molecules.27 These molecules can self-assemble into
different nanostructures via competing interactions between the
p–p stacking of the rigid TANI conjugation blocks and the
secondary interactions of the flexible alkyl blocks.27,28
Introduction
Oligo(aniline)s are model compounds of the well-known con-
ducting polymer poly(aniline) (PANI).1,2 PANI has retained
a high level of research interest owing to the ease of synthesis,
high electrical conductivity,3 unique redox properties,4 excellent
environmental stability, and potential for use in energy storage
applications.5–9 Oligo(aniline)s not only retain the advantageous
properties of PANI,10 but also exhibit much better processability
than PANI, as these shorter functional analogues are soluble in
most common organic solvents. These attractive properties
prompted us to develop a new, single-step synthesis of a family of
phenyl-capped tetra(aniline) (TANI) derivatives.11 Not only does
this strategy provide a facile route for modification of the elec-
tronic structure of tetra(aniline)s,11 but it also provides inter-
esting materials for the preparation of oligo(aniline) bulk films,12
thin films,13 and discrete self-assembled nanostructures.10 It is
therefore the expectation that these new synthetic developments
will lead to a range of applications for oligo(aniline)s, including
chemical sensors,14,15 gas-separation membranes,16 super-
capacitors,17 and artificial muscles (as already found for PANI).18
Block-like oligo(aniline)s have also attracted considerable
scientific interest recently.19 This interest mainly stems from the
fact that the analogous high molecular weight materials, block
Such oligo(aniline)-alkyl diblock materials are known to
exhibit anisotropic conductive properties, yet in the published
work to date, measurement and quantification of these proper-
ties have not been performed. Here we report our initial inves-
tigations into the anisotropic functionality of microstructure
prepared from TANI-alkyl diblock systems. We describe the
synthesis of amide-linked TANI-alkyl diblock (TANI-C8,
Scheme 1), and the characterization of electroactive anisotropic
microstructures derived from its doped form (using the inorganic
acid HCl), TANI-C8(HCl)0.5. Finally, anisotropic conductivity
was measured for block-like architectures from doped TANI-C8
structures for the first time.
aSchool of Chemistry, University of Bristol, BS8 1TS Bristol, UK. E-mail:
charl.faul@bristol.ac.uk; Fax: +44 (0)117 925 1295; Tel: +44 (0)117 954
6321
bNational Centre for Nanoscience and Technology, Zhongguancun,
Beiyitiao No 11, Beijing 100190, China. E-mail: weizx@nanoctr.cn; Fax:
+86 10 62656765; Tel: +86 10 82545565
cWestCHEM, School of Chemistry, Joseph Black Building, University
Avenue, University of Glasgow, Glasgow G12 8QQ, UK
dAcademy for Advanced Interdisciplinary Studies, Peking University,
Beijing 100871, China
Result and discussion
Design and synthesis
The TANI-C8 molecular architecture was designed to incorpo-
rate both the rigid TANI functional moiety, presenting oppor-
tunities for extensive p–p stacking interactions, and a flexible
alkyl moiety, allowing hydrophobic interactions. Furthermore,
the linking amide group also provides the possibility for
† Electronic supplementary information (ESI) available: Details of
experimental conditions and characterization of TANI-C8, including
synthesis of Ph/NH2 TANI, IR data, TEM, XRD and SAED data. See
DOI: 10.1039/c2jm32278a
‡ These authors contributed equally to the work.
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This journal is ª The Royal Society of Chemistry 2012

quinoid units.11,28,30 When TANI-C8 was doped with HCl, the
UV/Vis-NIR spectra showed peaks at 322, 418 and 846 nm,
which is in good agreement with the previous report in the
literature.31 A free-carrier tail is visible in the NIR region,
a feature characteristic of metallic conductive materials.30,32 The
HCl-doped ES state has a relatively weak localized polaron band
starting from 726 nm, which indicates poor overlapping of the
polarons, but still extends up to almost 2000 nm. The presence of
two bands in the Vis-NIR region may be related to the presence
of non-equivalent radical cations in the TANI chemical
structure.
Wide-angle X-ray diffraction (WAXS) data for pure TANI-C8
and after doping with HCl is shown in Fig. S1, (ESI†), and the
resulting structures are discussed in more detail below.
IR spectroscopy was used to investigate the extent of
hydrogen-bonding behaviour in the solid state for both the
undoped TANI-C8 and doped TANI-C8(HCl)0.5. In order to
ascertain the relevant IR vibration frequencies without any
hydrogen bonding, IR spectra were recorded in dilute CCl4
solutions (i.e., a solvent which does not participate in hydrogen
bonding). These spectra show that, for the TANI-C8, no
hydrogen bonding takes place with the amide C]O
(1652 cmꢀ1).33 A very weak signal was detected at 3405 cmꢀ1 for
the free amine N–H from the TANI backbone. Interestingly, for
the doped TANI-C8(HCl)0.5, very weak hydrogen bonding to the
amide C]O was detected, even in CCl4, as evidenced by the
small shift to higher wave numbers (1657 cmꢀ1) and appearance
of a very weak band at 1602 cmꢀ1 for the C]O stretching
vibration. This very weak splitting of the signal (originally found
at 1652 cmꢀ1) is an indication of very weak partial hydrogen
bonding,34 most probably from the interaction with the HCl
dopant. A number of very weak signals were furthermore
detected (in the range 3219–3377 cmꢀ1), as well as at 3400 cmꢀ1
(the latter for the free amine N–H stretching frequency of the
TANI, see Table 1).
Scheme 1 Synthesis of TANI-C8 (EB).
hydrogen bonding, to further assist the formation of anisotropic
structures. TANI-C8 was synthesized by reaction of the emer-
aldine base (EB) of Ph/NH2 TANI (prepared by a reported
method29) with octanoyl chloride (Scheme 1). To avoid side
reactions with its secondary amines, a slight excess of Ph/NH2
TANI was employed. Upon completion of the reaction, the
ether-soluble EB Ph/NH2 TANI was removed by repeated
washing with diethyl ether. This simple approach provides
a synthetic route to TANI-alkyl diblock compounds comple-
mentary to the reported condensation between Ph/NH2 TANI
and alkyl carboxylic acids.28
Bulk material
Fig. 1 shows the solid-state UV/Vis-NIR spectra of the TANI-C8
in the undoped EB state and in the ES state after full doping with
HCl acid (film cast from 10ꢀ2 M ethanol solution). The EB state
shows the two typical absorption maxima (at 322 and 595 nm),
which are both ascribed to p–p* transitions of the benzenoid and
When solid-state IR data was recorded, surprisingly similar
results were found. No clear indication of hydrogen bonding
interactions with the C]O moiety was seen for the EB TANI-C8,
as this band appeared at essentially the same wavenumber
(1654 cmꢀ1). Investigation of the higher amide and amine N–H
stretching frequencies showed only very weak bands at 3378 and
3284 cmꢀ1, respectively, indicative of hydrogen bonding.28 The
behaviour of TANI-C8(HCl)0.5 in the solid state showed slightly
increased hydrogen-bonding tendencies. This was reflected in the
split of the C]O stretching frequency found at 1656 and
1600 cmꢀ1, and the lowering of the amide and amine N–H
stretching frequencies to 3369 and 3231 cmꢀ1 (very broad),
respectively.
Table 1 Summary of IR vibration frequencies (cmꢀ1) in solution and the
solid state for TANI-C8 and TANI-C8(HCl)0.5
a
TANI-C8 TANI-C8(HCl)0.5 TANI-C8 TANI-C8(HCl)0.5
(CCl4)
(CCl4)
Solid state Solid state
C]O
1652
1657
1602
1654
1656
1600
N–H (amide) —
N–H (amine) 3405
—
3400 (v. weak)
3378
3284
3369
3231 (v.b.)
a
Fig. 1 UV/Vis-NIR of TANI-C8 in the undoped EB, and HCl-doped ES
v.b. ¼ very broad, see ESI† for IR spectra.
J. Mater. Chem., 2012, 22, 16230–16234 | 16231
state.
This journal is ª The Royal Society of Chemistry 2012

Finally, bulk solid-state conductivity of thin pellets of TANI-
C8 in the undoped EB state, as well as TANI-C8(HCl)0.5 was
measured using a standard 4-probe setup (see ESI† for details).
Due to the high resistivity, we were unable to measure the
conductivity of undoped TANI-C8 (i.e., non-conductive and
lower than 10ꢀ9 S cmꢀ1), while the conductivity of the doped
units.11 After doping, two absorption peaks at 423 and 960 nm
were obtained, and are typical for the ES state.30 The absorption
peak in the near infra-red region can be assigned to a localized
polaron band, which again indicates successful doping.30 The
presence of two bands in the Vis-NIR region may, as before, be
related to the presence of non-equivalent radical cations in the
anisotropic microstructures.
TANI-C8(HCl)0.5 was extremely low at 0.38 ꢁ 10ꢀ6 S cmꢀ1
.
To investigate the arrangement of TANI-C8 molecules in the
anisotropic microstructures, further structural characterization
was carried out on the undoped microplates. Fig. 3a shows the
transmission electron microscopy (TEM) image of a representa-
tive microplate. The selected-area electron diffraction (SAED)
pattern obtained from the same microplate is shown in Fig. 3b.
The highly crystalline nature of the microplates was confirmed by
the SAED patterns obtained. The patterns show a d-spacing of
Anisotropic microstructures: preparation and characterisation
With the bulk material characterised and conditions for doping
established, we focussed our investigations on the production
and characterization of anisotropic structures. Anisotropic
microstructures were prepared by dissolving undoped TANI-C8
in a good solvent (THF), and slowly adding water as a poor
solvent. By optimizing the ratio between the good and poor
solvent, microstructures of undoped TANI-C8 were obtained. At
low poor solvent ratio, no precipitate was observed. SEM images
showed that only small irregular aggregates were formed. When
the poor solvent ratio was increased to 80%, regular microplates
were formed and deposited from the solution.
ꢀ
3.95 A from the (020) reflection of the p–p stacking direction
ꢀ
(b-axis) and 5.9 A from the (100) reflection of the edge-on
stacking direction (a-axis).
Fig. 3c shows WAXS profiles of the microplates, with all major
peaks indexed (see ESI, Table S1† for details of the indexation).
It is possible to deduce from these results that the microplates
have an orthorhombic crystalline structure with parameters of
A low-magnification scanning electron microscopy (SEM)
image is shown in Fig. 2a, displaying high concentrations of two-
dimensional microplates. A high-magnification SEM image of
a typical microplate is shown in Fig. 2b. The microplate is
parallelogram-shaped, with typical dimensions of 4 ꢁ 5 mm, and
well-defined angles and sides.35 The UV/Vis-NIR spectrum of the
microplate film was recorded before (in the EB state) and after
doping (in the ES state). Doping was performed by exposing the
drop-cast microplates to HCl vapours over night in a closed
environment. As shown in Fig. 2c, the EB state showed the two
typical absorption peaks (at 318 and 597 nm), which are both
ascribed to p–p* transitions of the benzenoid and quinoid
ꢀ
ꢀ
ꢀ
a ¼ 5.9 A, b ¼ 7.9 A, c ¼ 30.2 A (the calculated length of a fully
stretched TANI-C8 molecules is ca. 3 nm). Fig. 3d shows
a schematic illustration of the arrangement of TANI-C8 mole-
cules in single crystalline microplates. We propose that the main
chain is oriented along the direction of the thickness of the
microplate (c-axis), while the b-axis and a-axis is laying on the
substrate.
We propose that an interplay of non-covalent interactions lead
to the formation of the observed microstructures: the formation
of the microplate structure is ascribed to efficient p–p interac-
tions and the action of hydrogen bonding between the molecules.
The p–p interactions between the TANI backbones lead to the
p-stacking along the b-axis, whereas the arrangement along the
a-axis is the result of intermolecular hydrogen bonding between
amide–amide and amine–imine groups (Fig. 3d, as confirmed by
Fig. 3 (a) TEM image and (b) SAED patterns of undoped TANI-C8
anisotropic microplates, (c) WAXS spectrum of undoped microplates (d)
schematic illustration of TANI-C8 molecules arrangement in single-
crystalline microplates (the ab-plane is on the substrate).
Fig. 2 (a) Low and (b) high magnification SEM images of two dimen-
sional undoped TANI-C8 microplates, (c) UV/Vis-NIR spectra of TANI-
C8 microplates in the EB (undoped) state and the ES (HCl-doped) states.
16232 | J. Mater. Chem., 2012, 22, 16230–16234
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such a confined environment. We are therefore currently
exploring further possibilities for producing fully doped aniso-
tropic structures directly from solution in our laboratories,
while at the same time exploring design principle and molecular
architectures to enhance the anisotropic properties of such
anisotropic structures.
Conclusions
A
tetra(aniline)-alkyl diblock compound, TANI-C8, was
synthesized and self-assembled into single-crystalline two-
dimensional microplates. The microplates showed anisotropic
conductivity owing to the anisotropic arrangement of the TANI-
C8 molecules in the microstructures. This investigation provides
further insight into methods to prepare two-dimensional struc-
tures of oligo(aniline) and showed its potential application in
organic electronics.
Fig. 4 Two dimensional anisotropic conductivity measurements of
single-crystalline microplate structures. (a) AFM height image of
a microplate after evaporating Au electrodes and its section analysis; (b)
current–voltage (I–V) curve and the conductivity of different directions
of the microplate.
Experimental section
Synthesis of N-(4-(4-(4- (phenylamino)phenylamino)
phenylamino)phenyl)octanamide
the IR investigations and a recently published study by Kim and
Park28). In addition, the incorporation of the alkyl chains aid
tuning of the solubility and the degree of aggregation of the
TANI moieties.
To a nitrogen-protected two-neck round-bottom flask equipped
with a dropping funnel was charged tetraaniline (200.3 mg,
0.55 mmol, 1 eq.), triethylamine (253.0 mg, 346.6 mL, 2.5 mmol,
5 eq.) and anhydrous diethyl ether (60 mL). The solid was stirred
while the flask was cooled to 0 ꢂC in an ice bath. A portion
octanoyl chloride (81.3 mg, 85.3 mL, 0.5 mmol, 0.9 eq.) in
anhydrous diethyl ether (20 mL) was slowly added to the reaction
flask through the dropping funnel under nitrogen protection.
The ice bath was then removed and the mixture was allowed to
react overnight. The precipitate was collected by centrifugation
and washed with ether until complete removal of the starting
tetraaniline was achieved, as monitored by TLC analysis. The
solid was dissolved in DMF (25 mL) and treated with phenyl-
hydrazine (54.1 mg, 0.5 mmol) for 2 h. The solution was then
poured into deionised water (250 mL). The precipitate was
collected by centrifugation to afford the product, in the leucoe-
We then proceeded to prepare HCl-doped TANI-C8 micro-
plates by exposure to HCl vapour in an N2 atmosphere, and
investigated their structure and anisotropic conductive proper-
ties (Fig. 4). TEM, WAXS and SAED investigations showed that
the doped anisotropic microplate structures were intact, single
crystalline and possessing an orthorhombic crystalline structure
with similar parameters as for the undoped structure (see ESI,
Fig. S4 and Table S1†).
Electron transport studies on single-crystalline undoped
TANI-C8 microplates were performed in order to determine the
anisotropic functional properties. The microplates were spin-
coated from aqueous solution on Si substrates, which was
capped with a layer of SiO2. Au electrodes (about 20 nm thick)
were deposited on the microplates by thermal evaporation,
using an organic ribbon as a shadow mask. The organic ribbon
was then removed to obtain a conducting channel.36 Fig. 4a
shows the AFM height image of a Au top-contact device con-
structed from a single TANI-C8 microplate (thickness of the
microplate ca. 100 nm). First, it was confirmed that the
microplate in the non-doped EB state was indeed not conduc-
tive (see inset table, Fig. 4). The microplate was then doped with
HCl (similar procedure as above) before conductivity
measurements were performed again. Anisotropic conductivity
was measured across two different directions of the microplate.
Fig. 4b shows the anisotropic conductivity observed after
doping. All of the I–V curves were linear and symmetrical,
which indicated that contact resistance is negligible, i.e. the
contact was ohmic in nature. The results show that the
conductivity along the a-axis is 6.8 ꢁ 10ꢀ5 S$cmꢀ1 and the
conductivity along the p–p stacking direction (b-axis) is
significantly higher, at 2.1 ꢁ 10ꢀ4 S$cmꢀ1 (even if lower when
compared to other reports37,38). We tentatively ascribe this lower
value to a combination of factors, including the possibility that
doping was not uniform throughout the microstructure and the
dilution of the electroactive structures with the alkyl tails within
1
meraldine base state, as a gray powder. (196.9 mg, 80%): H
NMR (400 MHz, DMSO-d6, 25 ꢂC) d ¼ 9.61 (s, 1H), 7.76 (s, 1H),
7.67 (s, 1H), 7.63 (s, 1H), 7.38 (d, 2H, J ¼ 8.8 Hz), 7.14 (t, 2H,
J ¼ 7.9 Hz), 7.00–6.85 (m, 12H), 6.67 (t, 1H, J ¼ 7.3 Hz), 2.23
(t, 2H, J ¼ 7.3 Hz), 1.61–1.52 (m, 2H), 1.34–1.20 (m, 8H), 0.86
(t, 3H, J ¼ 6.5 Hz); 13C NMR (100 MHz, DMSO-d6, 25 ꢂC) d ¼
170.9, 146.0, 129.6, 121.2, 119.8, 118.8, 118.5, 118.0, 116.1, 115.1,
31.8, 29.2, 29.0, 25.9, 22.6, 14.5; IR (neat, cmꢀ1): 3378, 3369,
3324, 2923, 2851, 1656, 1602, 1531, 1514, 1497, 1312, 1302, 1221,
1110, 817, 744, 694; anal. calcd for C32H36N4O: C, 78.01; H, 7.37;
N, 11.37. Found: C, 78.65; H, 7.40; N, 11.35%. HRMS calcd for
C32H36N4O: 492.2889. Found: 492.2897.
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