Directional Materials-Nanoporous Organosilica Monoliths with Multiple Gradients Prepared Using Click Chemistry

Article abstract of DOI:10.1002/anie.201502878

The existence of more than one functional entity is fundamental for materials, which are desired of fulfilling complementary or succeeding tasks. Whereas it is feasible to make materials with a homogeneous distribution of two different, functional groups, cases are extremely rare exhibiting a smooth transition from one property to the next along a defined distance. We present a new approach leading to high-surface area solids with functional gradients at the microstructural level. Periodically ordered mesoporous organosilicas (PMOs) and aerogel-like monolithic bodies with a maximum density of azide groups were prepared from a novel sol-gel precursor. The controlled and fast conversion of the azide into numerous functions by click chemistry is the prerequisite for the implementation of manifold gradient profiles. Herein we discuss materials with chemical, optical and structural gradients, which are interesting for all applications requiring directionality, for example, chromatography.

Full text of DOI:10.1002/anie.201502878

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201502878
German Edition: DOI: 10.1002/ange.201502878
Nanoporous Materials
Directional Materials—Nanoporous Organosilica Monoliths with
Multiple Gradients Prepared Using Click Chemistry**
Andreas Schachtschneider, Martin Wessig, Martin Spitzbarth, Adrian Donner, Christian Fischer,
Malte Drescher, and Sebastian Polarz*
Abstract: The existence of more than one functional entity is
fundamental for materials, which are desired of fulfilling
complementary or succeeding tasks. Whereas it is feasible to
make materials with a homogeneous distribution of two
different, functional groups, cases are extremely rare exhibiting
a smooth transition from one property to the next along
a defined distance. We present a new approach leading to high-
surface area solids with functional gradients at the micro-
structural level. Periodically ordered mesoporous organosili-
cas (PMOs) and aerogel-like monolithic bodies with a max-
imum density of azide groups were prepared from a novel sol–
gel precursor. The controlled and fast conversion of the azide
into numerous functions by click chemistry is the prerequisite
for the implementation of manifold gradient profiles. Herein
we discuss materials with chemical, optical and structural
gradients, which are interesting for all applications requiring
directionality, for example, chromatography.
in current literature contains phases, which have to be
described as homogeneous in nature, lacking any gradient.
Homogeneity is regarded rather as a criterion of quality.
One can roughly divide FGMs into two categories: Either
the gradient comprises the bulk-phase of the material, or one
finds a gradient on a surface. The first functional (bulk-phase)
gradient materials found in literature date back to 1978, when
the optical properties of lenses made from materials with
successive refractive indices were studied.[4] Several other
examples followed.[3,5] A very recent example was presented
by Sun et al. addressing the role of a concentration gradient in
Li-ion battery solids.[5g] It took much longer until materials
with surface chemical gradients could be realized, and the
number of known examples is much smaller. In an important
contribution the surfaces of a silicon wafer was modified using
decyltrichlorosilane.[6] A gradient in hydrophobic character
resulted from locally different surface coverage, and this
induced water droplets to run uphill. The system was studied
in detail and the principle was adopted and transferred to
other materials.[7] The latter examples indicate that the
preparation of gradient surfaces represents a difficult but
valuable task. However, it would be highly interesting to
provide gradient materials with much higher specific surface
area than a plane substrate, and it could be promising to
combine this with a much richer variability of functional
groups compared to the discussed “hydrophobic–hydrophilic
theme”. We aim at establishing the concept of functional
gradient materials in the field of nanoporous organosilica
materials. Ultimately, orthogonal gradients of different
groups should be generated (see Scheme 1).
The advantage of nanoporous materials is, that they
provide a large internal surface area, and substantial knowl-
edge exists about their preparation.[8] Methods for chemical
surface modification of SiO2 materials are highly developed.[9]
Amongst the different organosilica materials, the so-called
bridged polysilsesquioxane materials (BPSOs) prepared from
special sol–gel precursors X3Si-R-SiX3 (where R is a bridging
organic group and X is a hydrolysable group) are of
extraordinary importance.[10] Using structure directing
agents the so-called PMOs (periodically ordered mesoporous
organosilica materials) with uniform pore systems have been
established.[11] A key advantage of using BPSOs is that
materials can be prepared without diluting the precursors
entity, and this results in a maximum regarding the degree of
organic modification.[12] The distribution of the functional
organic groups is homogeneous throughout the entire mate-
rial, and yet the number of examples for bifunctional
materials is small.[13] Considering the high stage of develop-
ment in nanoporous materials research, it is also astonishing
B
iological matter has and still is setting important guiding
principles for materials scientists.[1] A key feature of almost
any biological material is a hierarchical architecture,[2] which
is predominantly understood as a structural feature, respec-
tively the organization of interrelated entities on successive
length scales. A less noted, distinct element of hierarchy is the
occurrence of specific gradients over changing dimensions.
Consequently, materials scientists have thought about gra-
dient materials as well, but the field stands quite at the
beginning. In so-called functional gradient materials
(FGMs),[3] a sharp interface is replaced by a gradient at the
microstructural level. This generates a smooth transition from
one property to the next, and it is anticipated that this special
character enables materials to perform more complex tasks.
Despite the large interest in FGMs and their expected
advanced properties, there exist only limited examples. Still
the vast majority of functional materials presented nowadays
[*] A. Schachtschneider, M. Wessig, M. Spitzbarth, A. Donner,
C. Fischer, Dr. M. Drescher, Prof. Dr. S. Polarz
Department of Chemistry, University of Konstanz
Universitaetsstrasse 10
E-mail: sebastian.polarz@uni-konstanz.de
Homepage: http://cms.uni-konstanz.de/polarz/
[**] We thank the German Research Foundation (Deutsche For-
schungsgemeinschaft, DFG) for funding within the SPP 1570 (PO
780/14-1). Further financial support by the DFG (DR 743/7-1). We
thank Prof. M. Tiemann (Paderborn) and Micromeritics for Hg
porosimetry measurements.
Supporting information for this article, including experimental
details, is available on the WWW under http://dx.doi.org/10.1002/
anie.201502878.
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titatively, and the molecular precursor compound was char-
acterized unambiguously (see also the Supporting Informa-
tion S-1). The main signal (m/z = 529.3 gmolÀ1) observed in
electron spray ionization mass spectrometry (ESI-MS) fits
precisely to the protonated form of (1). In the 15N-NMR
spectrum one sees three signals at d = À138, À149 and
À283 ppm, which fit well to phenyl azide as a related
compound,[22] and demonstrates the purity of the compound.
As a BPSO precursor, one should be able to use (1) for the
preparation of PMO materials. True liquid crystal templat-
ing[23] was achieved using an amphiphilic blockcopolymer as
a structure directing agent present. The small angle X-ray
scattering (SAXS) pattern of the obtained material shown in
Figure 1a contains a series of signals with d-spacing in ratios
Scheme 1. Preparation of nanoporous organosilica materials contain-
ing functional surface gradients (for the synthesis of 1 see the
Supporting Information). Using the phenylazide BPSO (white), one
can introduce a multitude of functional groups (blue) by 1,3-dipolar
Huisgen cycloaddition. Ultimately, a second gradient of a different
functional group (yellow) can be generated.
Figure 1. a) SAXS pattern and b) TEM micrograph (scale
bar=100 nm) of the novel PMO material containing phenylazide.
that there is almost nothing known about nanoporous
gradient materials. Mesoporous organosilica materials with
chemical gradients of two different functional groups do not
exist to the best of our knowledge.
For realizing the latter goal, we intend to prepare
nanoporous organosilica materials containing organic
groups imbedded in the pore surfaces, which can be trans-
ferred as easy as possible, rapidly and reproducibly into
a large variety of functional groups. Then, we will explore
methods for generating different chemical gradients (see
Scheme 1). When one is looking for a chemical system of
enormous versatility and high tolerance against numerous
functional groups, one inevitably finds click reactions, and in
pffiffi pffiffiffi pffiffi
of 1:1/ 3:1/ 4:1/ 7, which is typical for the columnar
hexagonal mesophase with p6mm symmetry. Evaluation
using the program package SCATTER[24] yields a value of
15.7 nm for the lattice constant a. The investigation of the
material using TEM (Figure 1b) and physisorption analysis
(S-2) shows that a mesoporous material with an average pore
size of 6.7 nm and a specific surface area of 445 m2 gÀ1 could
be obtained. Because the adsorbed volume at p/p0 = 0.08 is
one third of the total adsorbed volume, it can be expected that
the pore walls contain a substantial amount of micropores.[25]
It is worth noting that BPSO precursors are almost
exclusively used for the synthesis of PMOs, and much less is
known about the generation of alternative porous structures
like aerogels.[26] A particular difficulty is the preparation of
monolithic bodies.[27] The use of precursor (1) for making
aerogel-type monoliths was demanding, also because unlike
to typical sol–gel processes a one-step procedure was not
successful. Basic pH values are not suitable because the bulky
isopropoxy-groups prohibit sufficient hydrolysis rates. Even
after about one week no hydrolysis of the SiOisoPr groups
could be observed at all (see section S-3). Low pH-values
(< 1) are required, but at such conditions polycondensation
and gelation is of the order of weeks. Therefore, a two-step
procedure was developed, and macroscopic gel bodies could
be prepared within 2 days. After supercritical drying, highly
porous aerogel monoliths (Figure 2a) could be obtained (see
also S-4). The porosity of such aerogels can be as high as 92%,
and the specific surface area was 662 m2 gÀ1. Investigation
using scanning electron microscopy (SEM) and TEM shows
the typical, fractal nature of the porous network. Despite the
particular
the
1,3-dipolar
Huisgen
cycloaddition
(Scheme 1).[14] Huesing and Keppeler could modify mesopo-
rous SiO2 with azide groups by post-modification and showed
the general feasibility of click chemistry in pores.[15] An
alternative approach for post-modification of materials by
click chemistry is the use of the thiol-ene reaction, which was
for instance nicely demonstrated by the research group of
Van Der Voort and co-workers.[16] A comprehensive review
about click chemistry in materials science was published very
recently by Bowman and co-workers,[17] and interestingly no
FGMs were mentioned.[18]
1,3-Bis-tri-isopropoxysilyl-phenyl-5-azide (1) can be iden-
tified as a required, new sol–gel precursor (Scheme 1).
À
Because N3 can be regarded as a so-called pseudo-halogen-
ide ion,[19] the exchange of BrÀ with N3À can be achieved easily
by adopting a literature procedure[20] with 1,3-bis-tri-isopro-
poxysilyl-phenyl-5-bromide as a starting compound (see the
Experimental Section in the Supporting Information and
Supporting Information S-1).[21] The substitution works quan-
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cussed. Due to good spectroscopic visibility ethyl propiolate
(see S-7a) was used for the first steps. The success of the click
reaction can be demonstrated by 13C NMR (data shown in S-
8). Additional 13C NMR signals appear at d = 211 (C O), 152
=
(triazol C-R) and 113 ppm (triazol C-H). In FTIR one sees
that nasym(N3) vanishes whereas a band at 1728 cmÀ1 character-
=
istic for C O in propiolate appears (see Figure 3).
Figure 3. Time-dependent FTIR for the click reaction with ethyl propio-
late. The inset shows the fraction of non-reacted azide as a function of
time.
Figure 2. a) Dry monolith of the nanoporous phenylazide organosilica
material, b) EPR mapping of the distribution of TEMPO attached to
the azide groups by click chemistry, and c) photographic image of
a monolith characterized by a gradient in surface-attached fluoresceine
and IR spectra taken from different positions.
Monitoring the rate of conversion shows that there is an
almost linear dependency, which means that the degree of
derivatization can be adjusted precisely with time. Up to 87%
of the azide groups can be converted, which is a fairly high
value. At the same time, the monolithic character and the high
porosity of the material remain intact as indicated by
physisorption measurements (see S-8c for the ethyl-propio-
late derivatization). It should also be emphasized that other,
more functional groups can be introduced whenever a suitable
alkyne is accessible (S-7). A material with superhydrophobic
character obtained by modification with a fluorinated phenyl
entity (see S-7c) is given as an example in the Supporting
Information (see S-9).
Up to this point, the entire monolithic body was exposed
to the solution containing the ingredients for the click
reaction resulting in a homogeneous modification. Achieving
a gradient is imaginably simple. One dips only one side of the
monolith into the solution as shown in S-10. By adjusting the
amount of the alkyne reagent this automatically leads to the
formation of a distribution profile, and also other parameters
like contact time, solvent properties, temperature can influ-
ence the result. The success of our method can be monitored
directly by imaging techniques, for instance electron para-
magnetic spectroscopy (EPR) imaging.[28] For this purpose 4-
ethynyloxy-2,2,6,6-tetramethylpiperidin-1-oxyl (see S-7e), an
alkyne, containing a persistent TEMPO nitroxide radical
derivative, was synthesized and used for the click reaction.[29]
The EPR spectrum of the attached TEMPO derivative shown
in S-11 is characteristic for a probe with an anisotropic and
reduced rotation caused by the successful attachment to the
surface. The spectroscopic information was used for EPR
imaging analysis for separation of the spatial information by
high porosity, the monoliths show a sufficient mechanical
stability to be handled without great care. The force/pressure,
which can be applied to the monolith until it breaks, was
determined and is 3.5 NcmÀ2.
The chemical fingerprint of all prepared, porous materials
(PMO materials and aerogels) was obtained from solid-state
NMR spectroscopy, thermogravimetric analysis (TGA) and
FT-IR spectroscopy and is exemplarily given in section S-5.
The chemical shift of the 13C NMR signals fits very well to
those from the precursor. In FT-IR one can clearly identify
the characteristic vibrations for the azide group (nasym
=
2100 cmÀ1 nsym = 1284 cmÀ1).
;
A
mass loss step (Dm =
À12%) is observed at T= 1888C, which can be assigned to
the loss of one nitrogen molecule from the organosilica
material. Because also using 29Si-NMR one cannot see any
À
cleavage of the Si C bond, one can assign the desired
composition Si2O3C6H3N3 to the obtained materials.
There are two obvious variants for utilizing the azide
entity for click derivatization. One possibility is the reaction
with the sol–gel precursor (1) on the molecular scale, and its
feasibility could be proven by NMR spectroscopy (see S-6).
However, because the latter approach would yield materials
with a homogeneous distribution of functional groups, for
achieving gradient materials we investigated if the click
reaction could be done in the solid state, too (Scheme 1). In
the following, only monolithic aerogel materials are dis-
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performing a deconvolution prior to image reconstruction
using filtered back projection.[30] The intensity of the spatially
resolved signal is proportional to the local density of TEMPO,
the created, and the chemical gradient can be seen (Fig-
ure 2b). There is large freedom regarding the geometric
extension of the gradient, as shown for a different monolith in
S-12. Currently, the gradient can be controlled on the scale of
millimeters up to centimetres.
To show the creation of chemical gradients for an
independent system, we have also prepared alkyne-modified
dyes like fluoresceine (FI; see also S-7g). Figure 2c shows
a photographic image of a porous organosilica monolith
characterized by a FI gradient, additionally proven by
spatially resolved IR spectra. One sees the gradual decrease
of the azide modes accompanied by an increase in bands
corresponding to FI, when one is moving from top to bottom
of the monolith. A second group can be introduced by turning
the monolith and dipping it into a click solution containing
a second functionality (e.g. rhodamine B (RhB); S-7f),
resulting in a material with double-gradient character (for
a photographic image see S-11). As expected, at the top of
such a bigradient material one can only observe the optical
features of FI (lmax,ex(FI) = 499 nm; lmax,em(FI) = 554 nm) and
middle region a new, emergent feature is found (Figure 4b).
Although a wavelength l = 400 nm is not suitable for the
excitation of RhB, exclusively its fluorescence signal can be
detected. This phenomenon can be explained by Foerster
resonance energy transfer (FRET).[31] When the two dyes are
present in a proper ratio and are close enough to each other,
as is obviously the case in middle region of the monolith,
excited FI may transfer its energy to RhB, which then exhibits
fluorescence.
The gradient functionalization of the porous organosilica
materials grants to explore another highly interesting possi-
bility: The creation of materials with structural gradients, for
example, a gradient in pore-size. The network of the as-
prepared phenylazide material can be densified by treatment
with polar solvents. We found that the extent of these
processes correlate to the degree of azide derivatization with
hydrophobic groups, for example, with phenylacetylene.
Treating a material characterized by a gradient of phenyl-
acetylene leads to a new gradient due to the locally varying
magnitude of polycondensation and associated shrinkage (see
Figure 5). The extent of shrinkage directly influences the
at the bottom those of RhB (lmax,ex(RhB) = 560 nm; (lmax,em
-
(RhB) = 624 nm) as shown in Figure 4a,c. However, in the
Figure 5. Pore-size distribution functions (obtained from mercury
intrusion porosimetry) for different segments (s) along a gradient
organosilica monolith also shown as a photographic image.
average pore size of the aerogel. For analytical proof, the
monolith was cut into different parts, and each part was
analysed by mercury intrusion porosimetry (see also S-14).
One sees that the maximum of the resulting pore-size
distribution functions (Figure 5) is shifting gradually from
larger values (for the less shrunk regions) to lower values.
Please also note, that due to the logarithmic scale on the x-
axis the actual shift of the pore-size (Dmax(s1) = 457 nm!
Dmax(s2) = 1872 nm) is pronounced. Furthermore, segments
with a substantial width (see Figure 5a) were needed for
having sufficient material for a measurement. Therefore, the
pore-size distribution function is expected to be narrower for
infinitesimal segments than actually shown in Figure 5. It
should be noted that due to the fragile nature of aerogels and
the high pressure applied during Hg porosimetry, it cannot be
excluded that (partial) changes in the materials occur.[32]
Because our findings are also in agreement with SEM data
(see S-14), one can state that the pore-size gradient is of the
order of 1000 nm per 1 cm extension of the monolith.
Figure 4. Fluorescence image (lexc =400 nm) of a FI (top!bottom)/
RhB (bottom!top) bi-gradient monolith (length of the monolith:
1.2 cm) and PL spectra taken from different positions. Excitation
(hollow symbols) and emission (solid symbols).
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PMOs but also monolithic bodies of aerogel-like materials
could be prepared. Almost any desired functional group can
be attached using click chemistry. Furthermore, it was
possible to create nanoporous solids with either chemical or
structural gradients, which will make the materials promising
for a range of applications, for example, in chromatography or
catalysis.
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Keywords: click chemistry · gradients · hybrid materials ·
nanoporous materials · surface modification
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Received: March 29, 2015
Published online: July 16, 2015
Angew. Chem. Int. Ed. 2015, 54, 10465 –10469
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 10469