3D-ordered macroporous poly(ionic liquid) films as multifunctional materials

Article abstract of DOI:10.1039/b921280a

The integration of 3D-ordered macropores with poly(ionic liquid)s can significantly extend the functions of poly(ionic liquid)s, and afford a new type of material, which can serve as not only tunable photonic crystals and anion-directed molecular gating system, but also as a surface with enhanced tunable wettability and unique stable electro-optic switching. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b921280a

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3D-ordered macroporous poly(ionic liquid) films as multifunctional
materialsw
Jing Huang, Chang-an Tao, Qi An, Weixia Zhang, Yiguang Wu, Xuesong Li,
Dezhong Shen and Guangtao Li*
Received (in Cambridge, UK) 13th October 2009, Accepted 24th November 2009
First published as an Advance Article on the web 14th December 2009
DOI: 10.1039/b921280a
The integration of 3D-ordered macropores with poly(ionic liquid)s
can significantly extend the functions of poly(ionic liquid)s, and
afford a new type of material, which can serve as not only
tunable photonic crystals and anion-directed molecular gating
system, but also as a surface with enhanced tunable wettability
and unique stable electro-optic switching.
in this work, we found that the prepared 3D ordered macro-
porous PILs can not only serve as tunable photonic crystals,
electro-optic switches, molecular gating systems and surface
with enhanced tunable wettability (Scheme 1), but also provide
a unique platform for the design of further functional materials.
The colloidal crystal templating method was used to prepare
3D-ordered macroporous poly(ionic liquid)s in three steps:
fabrication of a face centered cubic (fcc) SiO2 opaline
template; infiltration of the template by a solution of ionic
liquid monomer, cross-linker and initiator followed by
polymerization; and selective dissolution of SiO2 template to
produce an inverse opal structure. Details of the fabrication
are given in ESI.w Fig. 1A, B show SEM images of the
used SiO2 opaline template and the resulting 3D-ordered
macroporous poly(ionic liquid)s.
Ionic liquids (ILs) exhibit a series of unique properties
including negligible vapor pressure, nonflammability, high
ionic conductivity, a wide electrochemical window, and good
chemical and thermal stability.1 Moreover, such favorable
properties can be individually customized by rationally
modulating the combination of cations and anions, providing
unprecedented tunability.2 Thereby, these distinct properties
and features make them ideal candidates not only as green
solvents for synthesis, catalysis and separation, but also for the
development of functional materials.1–8
The introduction of nanostructure endows PIL films with
interesting optical properties through interaction with visible
light (Bragg diffraction), affording a new type of photonic
polymers. As a remarkable feature, these films can undergo
rapid and reversible shrinking/swelling by exchanging the
counteranion of the pendant IL moieties, and thus exhibit
tunable optical properties. Fig. 2A shows reflection spectra of
the original PIL film and its response upon anion exchange.
The as-prepared PIL with BrÀ as counteranion displays a
stopband at 625 nm and is red in appearance. With the direct
exchange of more hydrophobic anions (Fig. S1 in ESIw), the
In this respect, polymeric ionic liquids (PILs) constitute a
new class of functional materials with unique properties
combing those of ILs mentioned above and specific properties
of polymers. Though a variety of applications, including solid-
state polyelectrolytes,9 catalysts, biosensors, high CO2 absorbing
resins and microwave absorbing materials,10 is envisaged
based on the combination of macromolecular structure and
ionic liquid moieties, most studies still focus on the synthesis of
various PILs, and less investigated is further functionalization of
PILs through the introduction of ordered nanostructures. In
this communication, we report the fabrication of imidazolium-
based PIL films with ordered macroporous structure, and
demonstrate that the integration of an ordered nanostructure
with PILs can greatly extend functions of PILs to afford a new
type of multifunctional materials.
Three-dimensional ordered macroporous (3DOM) structure
is characterized by high surface-to-volume ratios, periodic
macropore architectures and
a
bicontinuous nature
(interconnected solid skeleton and interconnected pore
system).11,12 Benefiting from these distinctive structural features,
related materials are widely used for the design of photonic
crystals, sensors, power sources, catalysts and various other
applications.12 By association of 3DOM structure with PILs
Key Lab of Organic Optoelectronics and Molecular Engineering,
Department of Chemistry, Tsinghua University, 100084 Beijing,
China. E-mail: lgt@mail.tsinghua.edu.cn; Tel: +86 10 62792905
w Electronic supplementary information (ESI) available: Details of
experimental, FTIR spectra and CV of poly(ionic liquid)s with
different counteranions. See DOI: 10.1039/b921280a
Scheme 1 Schematic illustration of the design of multifunctional
materials based on the combination of the unique properties of both
PILs and 3D-ordered macroporous structure.
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Fig. 1 SEM images of the used SiO2 opaline template (A), the
resulting 3D-ordered macroporous PIL (Br- as counteranion) (B),
and the PIL after the anion exchange with Tf2NÀ (C).
Fig. 3 Electrochemical characterization of the macroporous PIL
films with different counteranions. K3[Fe(CN)6] was used as redox
probe molecule.
stopband shifts gradually to shorter wavelength range. For
example, the PIL with Tf2NÀ has reached a stopband at
420 nm. In concert with the ca. 200 nm large shift of stopband,
the color of the PIL film changes correspondingly from red to
blue (Fig. 2B). In this work, we found that, due to the
interconnected porous structure, the ion response is very fast
(2 min) and under appropriate conditions is completely reversible.
These results suggest the 3D-ordered macroporous PILs can
serve as novel photonics with remarkable dynamic stopband
tunability with potential application in anion detection.10b
For the normal incidence geometry, the Bragg reflection
wavelength (stopband) l of inverse opal is determined by the
lattice constant and the refractive index of polymer network
in the inverse opal structure using the following equation, l =
2d(0.74 + 0.26n2)1/2, where d is the characteristic spacing, and
n is refractive index of the polymer network.13 In our system,
the change of the stopband is mainly resulted from the
diversification of the counteranions, which cause different
degrees of shrinking of the prepared PIL film, leading to the
decrease of the parameter d. It is estimated that the counterion
exchange from BrÀ to Tf2NÀ can induce a decrease of the
volume of spherical voids in the PIL film by 37%. In fact, it is
clearly observed that, due to the strong shrinkage, the macro-
pores as well as the characteristic small window channels
connecting the neighboring macropores in the PIL materials
become closed, when comparing the SEM image of the
original film with that after ion exchange with Tf2NÀ
(Fig. 1B, C). This result implies that the prepared macro-
porous PIL films actually provide also a useful molecular
gating system. Indeed, when a PIL film was mounted on glass
carbon electrode and K3[Fe(CN)6] was used as a redox probe
molecule, we found that the permeability of the film is
controllable by counteranions. The original PIL film (BrÀ as
anion) allows the electron exchange of the probe molecule with
the underlying Pt electrode, showing a couple of well-defined
3À/4À
redox peaks of Fe(CN)6 (Fig. 3). However, the redox
activity of probe molecule was gradually suppressed by PIL
films containinÀg other anions in the following order Tf2NÀ
>
À
À
À
PF6 > ClO4 > BF4 > NO3 (Fig. 3). In the case of
Tf2NÀ as counteranion, for instance, the permeation of probe
molecule in PIL film was completely blocked. Of particular
interest, this anion-directed gating system is reversible; when
the blocked PIL film was exposed to an ethanol solution of
NaBr (0.2 M), the molecular transport was recovered.
In response to particular anions the pore size of the
prepared PIL film is controllable, as demonstrated above.
On the other hand, the degree of hydrophilicity of the pore
channels is also correspondingly adjusted by the associated
anions, thus generating different osmotic pressure. Both the
physical size change and osmotic forces, we believe, are
responsible for the observed molecule gating switch.
Tunability of its physicochemical properties by simply
varying counteranion is one of the most attractive features
of ILs. Such feature can also be utilized for tuning surface
hydrophilicity and hydrophobicity, i.e., wettability. Lee and
co-workers reported the formation of self-assembled mono-
layers (SAMs) with imidazolium-based IL units at the tail
ends, and demonstrated that water contact angles of SAMs
could be easily adjusted from 231 (BrÀ) to 651 (Tf2NÀ).14 In
our system, it is observed that the presence of nanostructures
in the PIL can greatly extend the modulation of surface
wettability (Fig. S2, ESIw). In the case of Tf2NÀ as counter-
anion, the water contact angle of the PIL surface already
reaches 991, indicating that besides composition, the nano-
and microstructure of the surface determines the surface
properties.
Even more interestingly, we found the photonic band
structure of the presented macroporous PILs is very sensitive
to external electric field. Fig. 4A shows reflection spectra of the
PIL film as a function of the amplitude of the applied voltage.
Because BrÀ courteranion is easily oxidized, in this case, the
À
film with BF4 anion was used. As is evident in Fig. 4, the
reflection peak shifts toward shorter wavelengths with an
increase in voltage, saturating with a shift of about 30 nm
for a field of B5 V. Moreover, after the removal of the applied
Fig. 2 (A) Stopband shifts of the 3D-ordered macroporous PIL film
after anion exchanges; (B) the induced color changes of the PIL film
after anion exchanges. All spectra have been normalized.
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for fabricating multifunctional materials has been developed.
We found the integration of 3D-ordered macropores with
poly(ionic liquid)s can significantly extend the functions of
poly(ionic liquid)s, and afford a new type of materials,
which can serve as not only tunable photonic crystals and an
anion-directed molecular gating system, but also as a surface
with enhanced tunable wettability and as a unique stable
electro-optic switch. Because both ionic liquids and photonic
structures offer virtually unlimited tunability, their combina-
tion also provides a unique platform with tremendous
opportunities for design of new functional materials or
chemical systems. Although an imidazolium-based monomer
is used for the present study, the results are expected to be
universal for other types of ionic liquid molecules. Thus, we
believe our findings may open up a new route with great
extendibility to obtain functional materials, which could find
widespread applications.
Fig. 4 (A) Stopband shifts of the macroporous PIL film with an
increase in applied voltage; (B) stopband switching with a pulsed
square-wave voltage (switched every 2 min). The inset in (A) is the
stopband shift with applied voltage while the inset in (B) is the pattern
of the applied pulsed square-wave voltage.
voltage the reflection spectrum is recovered to the initial state.
When a pulsed square-wave voltage was applied, it is observed
that the pulsed electric signals could be easily converted to
pulsed optical signals, providing
a stable electro-optic
switching (Fig. 4B). This response is very fast being of the
order of seconds.
Notes and references
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Due to high ionic conductivities and wide potential
windows, PILs have been explored as solid polyelectrolytes
for electro-chemical applications including in fuel cells
and batteries, sensor and actuators, and high-capacitance
dielectrics for plastic electronics and energy storage. To our
knowledge, however, this is the first example of using such
materials to realize a quick electro-optic switching. The
mechanism underlying this conversion is still not completely
elucidated. The reversible dimensional changes of the macro-
porous PIL films induced by ion transports in response to
external voltages should be probably the main reason. In
comparison with work in literature,15 our PIL system
produces more distinct optical response upon voltage
application, which is much larger than those of other electro-
optic switching systems, such as liquid crystal-infiltrated or
ferroelectric inverse opals, and only a low drive voltage is
required. It should be noted that recently a novel electro-optic
system based on electroactive polymer inverse opals was
developed by Ozin’s group and used for a full color display.16
Besides as multifunctional materials, the above described
PIL films also provide a unique and versatile platform for the
design of further functional materials. Based on the concept of
‘‘task-specific’’ ionic liquids,3 functional groups such as anions
can be introduced into IL units, leading to macroporous PILs
with additional function. As a demonstration, redox-active
polyoxometalate and metal complex anions were incorporated
into the PIL films, respectively, and as a result, 3D-ordered
macroporous PIL films with additional electrochemical activity
or metal nanoparticles were achieved (Fig. S3, ESIw).
In recent years, various stimuli-responsive polymer films
with ordered macroporous structure have been reported,17,18
including solvent, temperature, pH, ionic strength and bio-
molecule responsive films. To our knowledge, however, this is
the first report of producing multifunctional materials based
on nanostructured poly(ionic liquid)s. The distinct properties
of ionic liquids endow our nanostuctured polymer with unique
optical, electro-optic and gating functions.
16 D. P. Puzzo, A. C. Arsenault, I. Manners and G. A. Ozin, Angew.
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(c) M. Harun-Ur-Rashid, T. Seki and Y. Takeoka, Chem. Record,
2009, 9, 87, see also references therein.
In summary, based on the combination of the unique
properties of both ILs and photonic structures, a new concept
18 F. Marlow, M. Parvin Sharifi, R. Brinkman and C. Mendive,
Angew. Chem., Int. Ed., 2009, 48, 6212.
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Chem. Commun., 2010, 46, 967–969 | 969