Electrically conductive hybrid nanofibers constructed with two amphiphilic salt components

Article abstract of DOI:10.1039/c1cc14145g

Electrical conductivity was obtained in hybrid organic nanofibers fabricated with low-molecular-weight amphiphilic azopyridinium and dodecylbenzenesulfonic acid or its salt.

Full text of DOI:10.1039/c1cc14145g

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Cite this: Chem. Commun., 2011, 47, 12768–12770
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COMMUNICATION
Electrically conductive hybrid nanofibers constructed with two amphiphilic
salt componentsw
Weimin Zhou, Takaomi Kobayashi, Huairui Zhu and Haifeng Yu*
Received 11th July 2011, Accepted 28th September 2011
DOI: 10.1039/c1cc14145g
Electrical conductivity was obtained in hybrid organic nanofibers
fabricated with low-molecular-weight amphiphilic azopyridinium
and dodecylbenzenesulfonic acid or its salt.
Supramolecular self-assembly is one of the powerful approaches
being explored for creation of novel nanostructures.1 In the
last few decades, fabrication of self-assembled nanofibers has
been widely studied and developed in the fields of electronics
and biomaterials.2 Fabrication of nanofibers using amphiphilic
salts has become one of the most popular procedures.
Recently, Kimizuka et al. summarized diverse ways of creating
organic nanofibers using the amphiphilic salts as single, double,
triple, quadruple chains, as well as bola-amphiphile, gemini
amphiphile, catanionic amphiphile, and amphiphilic metal
complex types. These nanofibers exhibiting heat-set gel-like
networks in organic media were also studied.3 However,
Scheme 1 Synthesis of LMW compounds for fabricating organic
special functionalities like electrical conductivities of these
self-assembled nanofibers are desired from the viewpoint of
applications.2a–c,3 It is necessary to design novel nanofibers
constructed with different salt components in order to achieve
these unique properties, because it is difficult to realize these
functions for the nanofibers having one single salt in
self-assembled processes.
nanofibers.
does not benefit the flow of electric current.2a–f However,
LMW nanofibers have advantages of being quickly formed
in simple self-assembly processes at room temperature. In this
communication, we report a robust method of fabricating
novel hybrid nanofibers by using different components of
LMW amphiphilic azopyridinium salts and sulfonic-acid
based surfactants, and supplying the obtained nanofibers with
electrically conductive function via improved p–p stacking
interactions. This simple method would help development in
fabrication of conductive LMW nanofibers.
Generally, conductive nanofibers were fabricated with
conductive polyaniline or polyheterocycle polymers by using
various plates.4–7 Gupta et al. created carbon nanofiber
composites with excellent conductivity by dispersing carbon
nanofibers in a polystyrene (PS) matrix.8 Moreover, nano-
wires, nanoribbons, and nanoplates containing polyaniline
were formed by self-assembly in the process of oxidative
chemical polymerization. Their fabrication needed a relatively
long time and their conductivities were acquired by doping
with acids.9,10 Compared with these nanofibers assisted by
conductive polymers, it is difficult to achieve electrically
conductive nanofibers with single low-molecular-weight
(LMW) amphiphilic components because their molecular
arrangement of bilayer structures in the nanofiber formation
As shown in Scheme 1a, LMW azopyridine compounds
were chosen due to their capability of nanofiber formation.2c
Although compound 1 is hydrophobic, it was easily converted
to amphiphilic azopyridinium 2 by addition of excess HCl into
its THF solution. After this azopyridinium-forming reaction, a
new broad peak at 2581 cmꢀ1 was observed in the FT-IR
spectrum due to the formation of pyridinium structures
(Fig. S1, ESIw). Such a structural change is also reflected in
the 1H NMR spectra (Fig. S2, ESIw). Compared with the
results of compounds 1 and 2, the chemical shift of proton b
was obviously shifted from 7.88 to 8.16 ppm when azopyridine
groups were changed into azopyridiniums.
Top Runner Incubation Center for Academia-Industry Fusion, and
Department of Materials Science and Technology, Nagaoka
University of Technology, 1603-1 Kamitomioka, Nagaoka, 940-2188,
Japan. E-mail: yuhaifeng@mst.nagaokaut.ac.jp
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c1cc14145g
Different from the azopyridine compound reported by
Nakagawa et al.,2e,f the azopyridinium 2 lacks hydrogen-
bonding functions. However, its distinctive amphiphilic
c
12768 Chem. Commun., 2011, 47, 12768–12770
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lmax appeared at 371 nm in its film state (Fig. 2b). This red
shift of 15 nm further demonstrates the existence of p–p
stacking interactions of 2 in the solid state of nanofibers.
Accordingly, the organic nanofibers could be formed by the
following processes. Addition of excess HCl into THF solution
of 1 introduced both proton and water, and azopyridinium 2
was formed spontaneously by the reaction in Scheme 1a. Upon
evaporation of solvent, molecular aggregation was induced.
Since THF is more volatile than water, the hydrophobic
hydrocarbon chain should contract and exist in the inner part
of the aggregates. Then the repulsion of positive charges
occurred among the hydrophilic azopyridinium moieties, leading
to the formation of head–tail structures by molecular arrangement,
as shown in Fig. S13a (ESIw). Moreover, p–p stacking inter-
actions existed in the nanofibers obtained with 2.
Fig. 1 Characterization of fabricated nanofibers. (a) and (b) are
optical and SEM images of nanofibers formed with 2. (c) and (d) are
optical images of hybrid nanofibers formed with 4 and 5, or 4 and 5
plus NaCl.
To provide the fabricated nanofibers with special functionalities
like electrical conductivity, a hybrid method was developed by
introducing different components of amphiphilic salts. These
hybrid nanofibers were obtained by the reactions of 1 with
dodecylbenzene-sulfonic acid (DBSA) 3 or its sodium salt
(DBSNa) 6 (Scheme 1c and d). Here, the addition of excess
HCl in the THF solution played a key role in fabricating hybrid
nanofibers. Without this treatment, nanofibers did not form.
To elucidate these processes, the reaction of 3 with 1 (molar
ratio, 1 : 1) in THF solution was first carried out, and then
compound 4 was obtained (Scheme 1b). In the 1H NMR
spectra (Fig. S3, ESIw), all the peaks and their assignments
to 2 and 3 were observed, indicating the formation of a salt
structure shown in Scheme 1c. However, hybrid nanofibers
were not obtained by using compound 4, because its hydro-
philic part influenced the molecular aggregation, leading to
poor capability of nanofibers formation (Fig. S9, ESIw). In
contrast, a mixture of 5 with 4 was prepared by addition of
excess HCl into THF solution of 4. Generally, the DBSA
anion should combine with the azopyridinium cation by
electrostatic interactions, as confirmed by the obvious changes
of the chemical shift of protons in its 1H NMR spectrum
(Fig. S4, ESIw). As the shielding effect of aromatic-ring current
existed between the benzene and pyridine in 5, the chemical
shift of protons (a–e in Fig. S4, ESIw) shifted to high field.
Combining with previous results of Goswami et al.,12 the
structure of 5 is described in Scheme 1c. Meanwhile, the peaks
of 4 were also observed, which indicated that a mixture of 4
and 5 was obtained. The integral ratio of protons (e and e0) of
methylene groups (–CH2O) in 4 and 5 was calculated as 1 : 1.5,
which is the molar ratio of 4 to 5 in the resulted mixture. In
their FT-IR spectra (Fig. S1, ESIw), the characteristic peak of
DBSA 3 at around 907 cmꢀ1 disappeared in the resulted
mixture because of the formation of salts upon combination
of the anion and the cation. More interestingly, the hybrid
nanofibers were successfully fabricated with this mixture
(Fig. 1c), which might be attributed to the existence of
amphiphilic structures of 5 in this mixture.
structure and p–p stacking interactions still enable it to self-
assemble into organic nanofibers. As shown in Fig. 1a, the
nanofiber formed spontaneously upon evaporation of a THF
solution of 2 dropped on the surface of glass substrates.
Furthermore, an obvious birefringence was observed with a
polarizing optical microscope (POM, Fig. S7, ESIw). Both the
optical micrograph and the SEM image (Fig. 1b) clearly
indicate the formation of nanofibers with 2.
As shown in Fig. S5a (ESIw), a high and sharp peak was
observed at 4.53 nm in the X-ray diffraction (XRD) pattern of
the nanofibers formed with 2, which is the width of the
monolayer sheet.2g Other peaks at 0.46 nm, 0.43 nm and
0.41 nm were attributed to the packing of the long hydro-
carbon chain (the dodecyl group). The broad peak at around
2y 4 201 was ascribed to the suppressive effect of p–p stacking
interactions among the azopyridine chromophores.11 Therefore,
the nanofibers obtained with 2 possessed lamellar structures
and p–p stacking interactions.
To investigate the intermolecular interactions of 2, UV-vis
absorption spectra were measured at its different states. As
shown in Fig. 2a, the maximum absorption peak (lmax) was
observed at the band of 356 nm in its THF solution, whereas
Similarly, the hybrid nanofibers were also fabricated with
another mixture, obtained by a reaction of DBSNa 6 with 1
(molar ratio 1 : 1) upon treatment with excess HCl
(Scheme 1d). The optical image in Fig. 1d shows their fibrous
morphologies. Likewise, the hybrid nanofibers were not
obtained without additional treatment of HCl (Fig. S10, ESIw)
Fig. 2 UV-vis absorption spectra of LMW compounds for fabricating
nanofibers. (a) 2 in THF solution, (b) 2 in the film state, (c) a mixture
of 4 and 5 plus NaCl salts in the film state.
c
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because no azopyridinium was formed in this case. This
indicated that the hydrophilic azopyridinium was one of the
most important factors in the fabrication of nanofibers and
their hybrids.
this robust hybrid method. It is expected that more variations
of hybrid nanofibers with multi-components such as various
organic and inorganic compounds can be created via this
simple method, which would advance their future applications
in conductive nanomaterials.2
Besides, two other compounds such as sodium lauryl sulfate
without the aromatic ring and p-toluenesulfonic acid mono-
hydrate (TSAM) with a short alkyl chain were also utilized for
fabrication of hybrid nanofibers. Although the same reactive
conditions were used, neither of them showed nanofiber
formation (Fig. S11, ESIw). For TSAM, solids preferably
precipitated from the solution upon addition of excess HCl.
These indicate that both p–p stacking interactions of aromatic
rings and a long alkyl chain in benzenesulfonic groups are
necessary for fabrication of the hybrid nanofibers.
We thank Dr. M. Masuda in AIST and Dr. Y. Hoshino in
Yokohama National University for helpful discussions.
Notes and references
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Then, electrical conductivity of the fabricated nanofibers
was evaluated via a standard four-probe method. All the
nanofiber films were prepared with a thickness of about
0.4–0.5 mm, but two contrary results were obtained. The
nanofibers with 2 had no electrical conductivity, whereas the
hybrid nanofibers with a mixture of 4 and 5 (or 4, 5 plus NaCl)
showed an electrical conductivity of 1.1 ꢁ 10ꢀ7 S cmꢀ1 (or 1.3 ꢁ
10ꢀ7 S cmꢀ1). Undoubtedly, this additional conductive property
should be achieved from the hybridizing process.
In the UV-vis absorption spectrum of a mixture of 4, 5 plus
NaCl in the solid state (Fig. 2c), lmax was obtained at 417 nm.
In its solution, lmax appeared at 357 nm (Fig. S12, ESIw),
almost the same as that of Fig. 2a, indicating that p–p stacking
interactions existed in the solid state of the hybrid nanofibers.
Moreover, lmax in Fig. 2c showed a remarkable red shift of
46 nm compared with that in Fig. 2b. This demonstrates that
the p–p stacking interactions were strongly enhanced in the
hybrid nanofibers than that without a hybrid, which might be
responsible for the obtained conductivity.
XRD of the obtained hybrid nanofibers is shown in Fig. S5b
(ESIw). Two broad peaks at 2y 4 201 and 151 o 2y o 201
were observed. The former was attributed to p–p stacking
interactions among the azopyridine chromophores, similarly
to that of Fig. S5a (ESIw). The latter is an additional peak,
originated from the hybridization process by introduction of
DBSA or its salt, which should be responsible for the
enhancement of p–p stacking interactions. The sharp peak at
0.28 nm was ascribed to NaCl (Fig. S5b, ESIw). Therefore, a
possible schematic illustration of the enhanced p–p stacking in
the hybrid nanofibers is given in Fig. S13b (ESIw). It is the
strong p–p stacking interactions improved by the hybrid
method that resulted in the additional electrical conductivity.
In summary, hybrid organic nanofibers were successfully
fabricated with LMW amphiphilic azopyridinium and DBSA
or its salt by a simple self-assembled process. The formation of
azopyridinium salts played an important role in the fabrication
of nanofibers and their hybrids. Interestingly, electrical conductivity
was obtained by the enhanced p–p stacking interactions using
c
12770 Chem. Commun., 2011, 47, 12768–12770
This journal is The Royal Society of Chemistry 2011