Clickable inverse opal: A useful platform for fabrication of stimuli-responsive photonic materials

Article abstract of DOI:10.1039/c2cc18081b

Based on azide-containing clickable inverse opal, a strategy for efficiently fabricating functional photonic materials was developed. By using three types of ethynylated compounds as model molecules, it is found that different functional groups can be facilely introduced into the prepared inverse opal via click reaction to access various inverse opaline materials.

Full text of DOI:10.1039/c2cc18081b

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COMMUNICATION
Clickable inverse opal: a useful platform for fabrication
of stimuli-responsive photonic materialsw
Dan Xu, Wei Zhu, Qi An, Weina Li, Xuesong Li, Haowei Yang, Jinxiang Yin and
Guangtao Li*
Received 26th December 2011, Accepted 20th February 2012
DOI: 10.1039/c2cc18081b
Based on azide-containing clickable inverse opal, a strategy for
efficiently fabricating functional photonic materials was developed.
By using three types of ethynylated compounds as model molecules,
it is found that different functional groups can be facilely introduced
into the prepared inverse opal via click reaction to access various
inverse opaline materials.
3DOM (Three-Dimensional Ordered Macroporous) materials,
known as inverse opals, have attracted considerable interest
due to their periodic porous structure, high surface-to-volume
ratios of the skeleton and unique optical properties (Bragg
diffraction).1 The applications of these materials range widely
Scheme 1 Clickable inverse opal as a platform for preparation of
from chemical sensors to chromotropic materials, and anion-
directed molecular gating systems.2–4 In particular, if these
materials are made from stimuli-responsive polymers, they
may show spectral and morphological changes in response to
external environmental changes, usually leading to distinct
optical changes observable by the naked eye through the Bragg
diffraction of light in these ordered macroporous structures.
Thus, such attractive features make them ideal self-reporting
functional materials. However, for the successful formation of
these photonic materials with desirable functions and mechanical
strength, each fabrication is strongly dependent on the screening
and arduous synthesis of functional monomers as well as the
optimization of polymerization conditions. Especially in the case
of sensitive functional moieties (e.g. bioactive units) incompatible
with the conventional thermal- or photo-induced polymerization
processes, the traditional approach described above is inappropriate
or difficult for achieving the corresponding functional inverse opals.
Thereby, developing a new strategy to efficiently access various
inverse opaline materials is highly desirable.
functional inverse opaline materials via a post-functionalization approach.
azide–alkyne 1,3-dipolar cycloaddition reaction is the front-
runner. More importantly, small organic molecules can be easily
decorated with azide or acetylene groups. Inspired by these works,
we report a strategy for efficiently fabricating functional inverse
opaline materials based on a post-functionalization approach
(Scheme 1) in this communication. An acrylate monomer bearing
azide group was synthesized (see ESIw) and employed for the
formation of azide-containing inverse opal. By using model
molecules (see Scheme 1), we demonstrated that different
functional groups could be facilely introduced into the prepared
inverse opal via click reaction, affording the desired functional
materials (Scheme 1). Actually this inverse opal with clickable
reactive groups can serve as a very useful platform for conveniently
accessing various functional photonic materials.
A colloidal crystal templating method was used to prepare
clickable inverse opal with three steps (Scheme 1): fabrication
of a SiO2 opaline template; infiltration of the template with
precursor solution of azide-containing monomer, cross-linker
and initiator followed by photopolymerization; and selective
removal of the SiO2 template to produce an inverse opal
structure. Details of the fabrication are provided in ESI.w
Fig. 1A and B displays the SEM images of the SiO2 opaline
template and the resulting 3D-ordered macroporous polymer
film with a thickness of about 2 mm. It was reported that azide
was not stable upon exposure to ultraviolet (UV) light.7
Fortunately, through controlled polymerization under an
UV light at 365 nm for 40 min in an ice bath, azide groups
survived the polymerization process. Fig. 1D represents the IR
In recent years, ‘‘click chemistry’’,5 with its unique features such
as high efficiency, high aqueous competency, chemical ortho-
gonality, and applicability toward diverse substrates, has attracted
considerable interest for the development of functional materials.6
Among the reactions meeting click standards, the copper-catalyzed
Key Lab of Organic Optoelectronics and Molecular Engineering,
Department of Chemistry, Tsinghua University, 100084 Beijing,
China. E-mail: LGT@mail.tsinghua.edu.cn; Fax: +86-10-62792905;
Tel: +86-10-62792905
w Electronic supplementary information (ESI) available: Experimental
details for synthesis and film formation. See DOI: 10.1039/c2cc18081b
c
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Fig. 2 (A) Shift of Bragg diffraction peaks of the inverse opal
modified by pyridine upon exposure to different pH solutions;
(B) recoverability of the pyridine-modified film.
Electroactive inverse opal holds important applications in
information storage, molecular gating, display and actuator.9–12
Generally, tedious screening of functional monomers as well as
the optimization of polymerization conditions is required. In
this respect, the postfunctionalization shows advantages. As
a demonstration, using the same click-reaction procedure
described above, a type of electroactive inverse opal was easily
achieved by the exposure of N-(2-propargyl)amidoferrocene to
the clickable inverse opal film. Ferrocene immobilization was
correlated with the disappearance of azide absorption peaks
and emergence of the characteristic amide I (1644 cmÀ1) and
Fig. 1 SEM images of silica template (A), the resulting clickable
inverse opal (B) and the cross-section of the inverse opal after click
reaction (C); IR spectra comparison (D) of the precursor (a), the
resulting inverse opal film (b), and the films after click-reaction with
ethynylated pyridine (c) and ethynylated ferrocene (d).
spectra of the precursor (a) and the resulting film (b) after
polymerization, respectively. Clearly, the characteristic band
for azide at around 2100 cmÀ1 was detectable, indicating that
the azide groups were correctly incorporated into the resulting
inverse opaline film.
amide II (1542 cmÀ1 13 and ferrocene (850 cmÀ1) bands14 of
)
As a proof of concept, three types of ethynylated compounds
(pH-responsive, electroactive and bioactive) were synthesized
and used as model molecules to examine the reactivity of the
azide groups integrated in the prepared inverse opaline films.
Click functionalization was first performed by introducing
pH-responsive ethynylpyridine. The click reaction was carried
out at 50 1C by exposure of the prepared polymer film to
water/THF solution of 4-ethynylpyridine containing catalytic
amounts of CuI. After reaction, the polymer film was washed
and soaked in the used solvent repeatedly to eliminate all
physisorbed species during the functionalization step. The
result of the click reaction and the presence of pyridine units
in the polymer film were monitored by IR spectroscopy.
As shown in Fig. 1D–c, the characteristic band at around
2100 cmÀ1 of azide groups in the initial film nearly disappeared,
and new absorption bands located at 1613 cmÀ1 and 1562 cmÀ1
for pyridine8 were detected in the resulting film, indicative of the
covalent attachment of pyridine groups. The SEM detection of
the cross-section of the modified polymer film (Fig. 1C) reveals
that the initial ordered and interconnected porous structure
was still maintained after click reaction. As expected, the
incorporation of pyridine units endowed the inverse opaline
film with pH-responsive property. Fig. 2A shows the shift of
the Bragg diffraction peaks of the pyridine-modified film in
response to pH changes and its recoverability. Clearly, with
the decrease in pH value, the diffraction peak continuously
shifted to a longer wavelength range because of the swelling of
the polymer film induced by protonation of pyridine groups. A
large shift of 60 nm was observed at pH = 2. The film was
recoverable by treatment with base solution, and the pH-response
was fully reversible (Fig. 2B). In a control experiment, 6-azidohexyl
acrylate was chosen as the monomer for the formation of inverse
opal film, which was subsequently subjected to the same acidic
solution described above. No pH-responsive behavior was
observed, as expected.
the attached molecule in IR spectra (Fig. 1D–d). An electro-
chemical method was also used to confirm the presence of
ferrocene moieties in the modified macroporous film. When
the modified film was mounted on a platinum electrode, the
typical electrochemical signature of ferrocene was detected in
comparison with the initial inverse opal film (Fig. 3A). Cyclic
voltammogram (CV) of the attached ferrocene was comparable
to that of the used model molecule [N-(2-propargyl)amido-
ferrocene] (see Fig. S3 in ESIw), and the repeated CV sweeping
showed no decrease of signal after the first CV cycle (Fig. 3A),
indicating that the ferrocene moieties were covalently attached
(but not adsorbed) to the polymer film. Additionally, the
dependence of the peak current on the scan rate was linear
(inset in Fig. 3B), further confirming that the electroactive
Fig. 3 (A) CV sweeps of the clickable inverse opal before and after
the click reaction with ethynylated ferrocene at a scan rate of 50 mV sÀ1
;
(B) CV sweeping of ferrocene-modified film at different scan rates;
the inset in B shows a linear relationship of peak current to scan rates;
(C) shift of diffraction peaks via the oxidation process of ferrocene-
modified film; (D) recoverability of the electroactive film.
c
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high aqueous competency as well as mild reaction conditions,
‘‘click chemistry’’ is found to be especially appropriate and
useful for biochemical systems. Thus, it can be anticipated that
the combination of distinct advantages of click chemistry with
unique photonic properties of inverse opal could provide a
convenient and efficient way for developing biophotonic materials,
which may find great potential applications in various areas.
In summary, we report the first example of clickable inverse
opal. Using three types of ethynylated compounds as model
molecules, it is demonstrated that different functional groups
can be facilely introduced into the prepared inverse opal via
click reaction, affording the corresponding stimuli-responsive
photonic materials. Compared with the traditional methods,
the strategy described here surmounts the arduous process of
synthesis and screening of functional monomers as well as the
optimization for film formation, and provides a useful platform
for efficiently accessing functional inverse opal materials. In
view of the distinct advantages of click chemistry, especially in
biochemical systems, it is expected that different kinds of
functionalized ethynylated aptamers and receptors could be
tethered to the film affording new types of chemo-/biosensors
with inverse opaline structure.
Fig. 4 Shift of Bragg diffraction peaks of the bile acid functionalized
film (blue) upon exposure to polar and nonpolar solvents, upon exposure
to water (green) and upon exposure to dichloromethane (red).
species were immobilized in the film.15 Interestingly, we found
that the photonic band structure of the modified film was very
sensitive to the oxidation process of ferrocene units. Fig. 3C
shows reflection spectra of the ferrocene-modified film as a
function of the applied voltage. With the oxidation of ferrocene
to ferrocenium, a 16 nm red-shift of Bragg diffraction peaks was
observed. When the ferrocenium was reduced to ferrocece, the
reflection spectrum is recovered to the initial state (Fig. 3D).
This response was very fast and took only several seconds. In
this case, electric signals could be easily converted to optical
signals, directly providing an electro-optic switching and
electrically tunable photonic material. Based on these results,
it is expected that different kinds of electroactive molecules
could be tethered to the clickable photonic film, yielding new
material with novel properties.
We gratefully acknowledge the financial support from the
NSFC (20533050, 50673048, and 20972086), MOST Program
(2006CB806200) and transregional project (TRR61).
Notes and references
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To further demonstrate the versatility of the described post-
functionalization approach, the introduction of cholic acid
biomolecules into the clickable inverse opal was also studied.
Although cholic acid has a large molecular size, it was found
that all azide reaction groups in the prepared inverse opals
were accessible, probably due to the very thin pore wall (ca. 50 nm)
as well as the interconnected porous structure. As shown in Fig. S4
(ESIw), the absorption of azide at 2100 cmÀ1 disappeared, and
correspondingly the broad band centered at 3610 cmÀ1 for
hydroxyl groups occurred, indicating that cholic acid was
tethered to the polymer film. Benefiting from the unique facile
amphiphilic property of cholic acid,16 the decorated films
exhibited remarkably adaptive swelling behavior. It is known
that, depending on solvent polarity, cholic acid molecules can
adopt different conformations. Normally, in a nonpolar solvent
the hydrophobic face of cholic acid turns outward, while in a
polar solvent the hydrophilic face points outward. As a result,
different from conventional polymer films, the cholic acid
modified inverse opal could respond to both nonpolar and
polar solvents and shows distinct colour change (Fig. 4).
The preliminary results described above are encouraging.
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platform for efficiently creating various inverse opal-based
functional materials or devices. On the other hand, due to its
high efficiency, tolerance of functional groups and solvents,
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3496 Chem. Commun., 2012, 48, 3494–3496
This journal is The Royal Society of Chemistry 2012