Surface area control and photocatalytic activity of conjugated microporous poly(benzothiadiazole) networks

Article abstract of DOI:10.1002/anie.201207163

Conjugated microporous polymers (CMPs) with controlled specific surface area have been prepared through Sonogashira-Hagihara cross-coupling reactions in the presence of silica nanoparticles as templating agents. The CMPs act as heterogeneous photosensitizers for producing singlet oxygen in a continuous flow synthesis (see picture). Copyright

Full text of DOI:10.1002/anie.201207163

Angewandte
Chemie
DOI: 10.1002/anie.201207163
Microporous Polymers
Surface Area Control and Photocatalytic Activity of Conjugated
Microporous Poly(benzothiadiazole) Networks**
Kai Zhang, Daniel Kopetzki, Peter H. Seeberger, Markus Antonietti, and Filipe Vilela*
P-conjugated microporous polymers, such as organic semi-
conductors with additional porosity in the nanorange, are
versatile materials.[1] In addition to typical applications for
high-surface-area materials, such as gas separation[2] and
storage,[2,3] conjugated polymer networks are also potential
heterogeneous catalysts[4] and can be used in optoelectron-
ics[5] and for energy applications.[6] Given the importance of
surface area and porosity, a simple methodology to influence
these parameters is needed. In order to use conjugated
polymer networks as catalysts and catalyst supports their
chemical stability needs to be improved. Therefore we
designed a stable, fully conjugated network by linking
benzothiadiazole as a strong electron-withdrawing moiety
Working under heterogeneous conditions allows for easy
separation of the sensitizer after reaction and reusability.
Traditional systems suffer however from quenching of the
produced singlet oxygen by the solid support, which reduces
the quantum yield.[15] Incorporation of a photosensitizing
structure into a polymer backbone is particularly attractive,
because no support to immobilize the sensitizer is needed
when an insoluble photoactive polymer network is used.
Therefore the ability of the conjugated microporous polymer
(CMP) network to act as a singlet oxygen photosensitizer was
evaluated by employing the oxidation of a-terpinene to
ascaridole. The pore size and structure as well as specific
surface area are beneficial in providing a high accessibility of
exited solid polymer for solubilized oxygen, thus resulting in
high efficiency for singlet oxygen generation in dependence of
the pore architecture.
À
through three Csp Csp bonds to benzene as a weak electron-
donating component. The surface area of these novel
conjugated microporous polymer networks could be adjusted
by a simple synthetic protocol.
A series of polymer networks based on benzothiadiazole
as building block was synthesized through palladium-cata-
lyzed Sonogashira–Hagihara cross-coupling polycondensa-
tion of 4,7-dibromobenzo[c][1,2,5]thiadiazole with 1,3,5-tri-
ethynylbenzen (Scheme 1).
Benzothiadiazole monomers have proven great stability
towards oxidation in photovoltaic applications.[7] The low-
band-gap character, high absorption coefficient, and suitable
energy levels of benzothiadiazole result in a very strong
acceptor to be used in optoelectronic materials, such as low-
band-gap polymers,[7,8] non-fullerene acceptors,[9] or n-type
field effect transistors.[10] The combination of weak electron
donors, such as a phenyl group with benzothiadiazole, may
prevent a fast recombination of excitons and increase the
yield of intersystem crossing to the triplet state of the
polymer, thereby rendering it suitable for photosensitizing.
Conjugated linear polymers with backbones based on the
poly(phenylene ethynylene) structure[11] or the polythio-
phene–porphyrin dyad[12] repeat unit can generate singlet
oxygen in water upon irradiation. Singlet oxygen is used for
a number of applications, such as for treatment of waste water
or in the synthesis of fine chemicals.[13] An industrial process
carried out on a scale of several tons a year is the photo-
chemical oxidation of citronellol to rose oxide.[14] Different
dyes and transition metal complexes can generate singlet
oxygen upon irradiation, and these sensitizers can be used in
homogeneous solution or immobilized on a solid support.
Scheme 1. Synthesis of microporous polymer networks by using
SiO2NPs for surface area control. Reagents and conditions:
a) [PdCl2(PPh3)2], CuI, DMF, Et3N, SiO2NPs (6.25, 12.5, 25.0, 55, and
60 mgmLÀ1 for CMP_6.25, CMP_12.5, CMP_25, CMP_55, and
CMP_60, respectively), 808C, overnight. b) NH4HF2, H2O, overnight.
CMP_0 was synthesized in the absence of templating
agent, whereas different concentrations of silica nanoparticles
(SiO2NP, d ꢀ 12 nm) were dispersed in the reaction mixtures
to produce networks CMP_6.25, CMP_12.5, CMP_25,
CMP_55, and CMP_60 (X in CMP_X indicates the amount
of SiO2NPs in mgmLÀ1 used for templating). The reaction
mixture became too viscous for the coupling reaction to be
completed at concentrations above 60 mgmLÀ1 SiO2NPs. An
aqueous solution of ammonium hydrogen difluoride was used
to dissolve the SiO2NPs after condensation without compro-
mising the polymer networks. The polymers were obtained as
dark yellow powders that were completely insoluble in all
[*] Dr. K. Zhang, Dr. D. Kopetzki, Prof. Dr. P. H. Seeberger,
Prof. Dr. M. Antonietti, Dr. F. Vilela
Max Planck Institute of Colloids and Interfaces
Am Mꢀhlenberg 1, 14424 Potsdam (Germany)
E-mail: filipe.vilela@mpikg.mpg.de
[**] The Max Planck Society is acknowledged for financial support. We
would like to thank Dr. Jꢁrꢂme Roeser and Prof. Xinchen Wang for
the solid-state NMR experiments.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207163.
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solvents investigated. By using the same amount of starting
compounds, all CMPs prepared in the presence of SiO2NPs
were obtained with similar recoveries as for CMP_0, thus
showing that the templating agent does not have a detrimental
effect on the cross-coupling reaction efficiency. The resulting
polymer networks were characterized by elemental analysis,
solid-state 13C CP/MAS NMR, N2 gas sorption, UV/Vis, FTIR
spectroscopy, SEM, and TEM.
In the absence of SiO2NPs, CMP_0 shows carbon signals
at around d = 130 and 122 and 115 ppm, which can be ascribed
to the carbon atoms of the phenyl rings (Figure S1 in the
Supporting Information). The signals at about d = 95 ppm can
be ascribed to the carbon atom next to the phenyl ring of the
the benzothiadiazole monomer, with absorption maxima at
313 and 350 nm, the polymer networks exhibit a large bath-
ochromic shift of around 90 nm. This indicates the effective
enlargement of the p-conjugated system through the poly-
condensation reaction. Owing to the strong light scattering
within the dispersion the absorption edge cannot be easily
determined, and therefore the optical band gap cannot be
easily estimated. Cyclic voltammetry of thin CMP films was
also performed (Figure S12 in the Supporting Information).
Quasi-reversible oxidation and reduction cycles were exhib-
ited, which is similar to linear structures based on benzothia-
diazole and ethynylbenzene oligomers.[16] HOMO–LUMO
band gaps of about 2.10 eV can be calculated.
À ꢁ À
C C group and the signal at approximately d = 87 ppm to
BET surface areas and pore size distributions were
measured for nitrogen adsorption and desorption at 77.3 K
(Table S2 in the Supporting Information), and the adsorption
isotherms were determined (Figure S10 in the Supporting
Information). The BET surface areas range from 270 m2 gÀ1 in
the absence of SiO2NPs (CMP_0) to 660 m2 gÀ1 for a large
excess of 60 mgmLÀ1 SiO2NPs (CMP_60).
The BET surface area increases linearly with the concen-
tration of silica nanoparticles in the reaction mixture (Fig-
ure 2a), thereby proving that this is a simple and efficient
methodology to control surface area whilst retaining the
optoelectronic and chemical characteristics of the polymer.
For all networks, similar pore radii in a range from 6 to 8 ꢀ
are determined (Figure S10 in the Supporting Information).
Likewise, the total pore volumes (Vpore/total) of the polymer
networks show a clear trend. Without using SiO2NPs, polymer
network CMP_0 exhibits a total pore volume of 0.288 cm3 gÀ1.
the carbon atom next to the benzothiadiazole unit, a known
strong electron acceptor that shifts the carbon peak upfield.
CMP_55 was obtained by using a very large amount of
SiO2NPs and shows very similar carbon signals as CMP_0.
This indicates that under the harsh conditions required for
SiO2NP removal using ammonium hydrogen difluoride, the
chemical structure essentially stays unaffected.
The FTIR spectrum of CMP_60 with SiO2NPs shows
a large SiO2 signal at 1051 cmÀ1, which disappeared com-
pletely after removal of the SiO2NPs (Figure 1b). The spectra
of the polymer networks after removal of SiO2NPs exhibit
stretching modes similar to CMP_0. From 2900 to 3400 cmÀ1
=
C-H stretching bands appear. A C C stretching mode at
1600 cmÀ1 is observed. All networks show the typical C ꢁ C
stretching mode at about 2200 cmÀ1
.
The conjugated polymer networks were dispersed in DMF
to obtain UV/Vis spectra (Figure 1a). They show mainly two
absorption maxima at about 320 and 440 nm. Compared to
Figure 1. a) UV/Vis spectra of the polymer networks (dispersed in
DMF). b) FTIR spectra of the polymer networks CMP_0–CMP_60 and
CMP_60 with SiO2NPs.
Figure 2. a) BET surface areas of the polymer networks using different
SiO2NP concentrations. b) Total pore volumes of CMP_0–CMP_60.
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When employing SiO2NPs, similar to the effect on the surface
area, the Vpore/total also increases to 0.951 cm3 gÀ1 for CMP_60
(Figure 2b). Note that the so-called total pore volume does
not contain the silica pores, which are out of the range of these
measurements, that is, it is a total micropore volume.
These data suggest that during the coupling reaction of the
monomers, the cross-linked polymers are covering the surface
of SiO2NPs, leaving the pore space after template removal.
The micropore size in the polymer network obtained did not
change, which can be proven by BET surface area measure-
ments of the CMPs before removal of SiO2NPs, exhibiting
always BET surface areas of about 250 m2 gÀ1. After removal
of SiO2NPs, the micropores simply become more accessible
through the big silica mesopores, showing increasing surfaces
area and total pore volumes.
This methodology of using SiO2NPs is presumably not
only a simple “templating” effect, since the SiO2NPs also
interfere with the mesostructure formation of the original
polymer as observed by electron microscopy. SEM images
show clearly distinct polymer morphologies (Figures S8 and
S9 in the Supporting Information). CMP_0 depicts a more
fibrous morphology, whereas CMP_6.25–CMP_60 are deter-
mined by aggregated smaller spheres to form a porous
structure. The SiO2NPs in the reaction mixture inhibit the
formation of the fiber-shaped structure as for CMP_0,
presumably by suppressing homo-aggregation of precon-
densed oligomers. TEM images of CMPs with SiO2NPs
display domains of different sizes. The polymer networks
are either around or inbetween SiO2NP assemblies.
The photoenergetical states of the CMPs were investi-
gated by using fluorescence spectroscopy. Interestingly, they
did not show any fluorescence. This indicates that the CMPs
could transform into a triplet state under excitation with
a strong intersystem-crossing process within the conjugated
system. Another explanation could be deep traps for one of
the photogenerated charges, thus resulting in effective exciton
splitting and charge stabilization. In case of strong intersys-
tem-crossing, the polymer in the triplet state might excite
triplet oxygen and thus could be employed as photosensitizer.
Photophysical studies are however necessary to fully under-
stand these events. This is work currently under investigation
and will be the focus of a future publication.
Figure 3. Conversion of a-terpinene into ascaridole with CMPs of
different surface areas. a) Flow of oxygen (2 equiv): 5 mLminÀ1, flow of
a-terpinene solution: 1 mLminÀ1; b) flow of oxygen (2 equiv):
10 mLminÀ1, flow of a-terpinene solution: 2 mLminÀ1
.
polymer with highest surface area, the reaction proceeded to
near complete conversion (96%).
Measurements at a doubled flow rate, (2 mLminÀ1 and
half the reaction time), show overall the same trend, with an
offset to lower values as expected from the reduced residence
time. This indicates that the measurements performed under
continuous flow conditions are indeed reproducible.
Although the surface area seems to play an important role
in the photocatalytic event as shown by the increased
performance of the templated polymers, other effects such
as dispersablity and “effective surface area” need to be
accounted for. Indeed, the influences of porosity on micro-
architectural features are manifold and cannot be easily
separated from each other. In any case CMP_0 performed
worst, whereas the templated CMPs show highly improved
efficiencies.
The polymer networks showed different dispersion stabil-
ities. CMP_0 re-aggregated upon storage in chloroform,
whilst CMP_60 was still well-dispersed after the same time
(Figure S11 in the Supporting Information). The lower
density of CMP_60 can be attributed to the higher surface
area and larger pore volume and is the main reason for the
stable dispersions. Enhanced dispersability may be respon-
sible for more efficient singlet oxygen production.
Based on conversion and flow rate, productivities were
calculated. At a flow rate of 1 mLminÀ1, CMP_0 showed the
lowest productivity of 0.03 mmolminÀ1, while polymers with
larger surface area reached productivities up to 0.10 mmol
minÀ1. Productivities of up to 0.12 mmolminÀ1 were obtained
at doubled flow rate suggesting advantages of using high-
surface-area CMPs.
To investigate the feasibility of a CMP network being
applied as a heterogeneous photocatalyst in the production of
singlet oxygen,
a dispersion of cross-linked polymer
(1 mgmLÀ1) containing a-terpinene (0.1m) was mixed with
oxygen (2 equiv) and pumped through a photoreactor con-
sisting of FEP-tubing wrapped around a polycarbonate plate
that was irradiated with blue light at 420 nm (Figure S7 in the
Supporting Information; FEP = fluorinated ethylene propyl-
ene). This efficient continuous flow setup ensures high
throughput at short residence time.[17] The substrate conver-
sion was evaluated by 1H NMR spectroscopy using polymers
of various surface areas (Figure 3). At a flow rate of
1 mLminÀ1, CMP_0 gave the lowest conversion (26%),
whilst CMP_6.25 through to CMP_55 seem to have reached
a plateau in the reaction conversion (70–80%, Figure 3 and
Table S3 in the Supporting Information). With CMP_60, the
Additionally, tetraphenylporphyrin (TPP) was used as
a known sensitizer with high quantum yield at various flow
rates and with two equivalents of oxygen.[14] Complete
conversion was achieved even at 10 mLminÀ1, corresponding
to a productivity of 1.0 mmolminÀ1 and a residence time of
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only seconds, thus demonstrating the high efficiency of the
continuous flow setup. At even higher flow rates the
conversion breaks down, as the photon flow of the light
source is becoming a limiting factor (FP = 2.5 mmolminÀ1).
The selectivity of the reaction was not affected by the choice
of catalyst, and the endoperoxide ascaridole was formed with
a selectivity of 86–88%. Comparison of productivities ach-
ieved show that the maximum singlet oxygen quantum yield
for the CMPs FD = 0.06 is about one order of magnitude
lower than that of TPP. While the polymer network is less
productive, the ease of CMP recovery by simple filtration
presents a clear advantage. FTIR, thermal gravimetrical and
elemental microanalysis of CMP_60 (Table S1 in the Sup-
porting Information) show no modification of the polymer
after the photoreaction, thus indicating that it is stable to both
irradiation and singlet oxygen.
To demonstrate the reusability of these materials, five
repeating experiments of singlet oxygen production were
carried out using the same CMP_55 catalyst (Figure S13 in the
Supporting Information). The polymer can be reused repeat-
edly after a simple filtration, and conversions higher than
50% and selectivities above 80% are achieved. The decrease
in conversion of 3.6% per run on average can be accounted to
photobleaching of the polymer or partial deactivation by
singlet oxygen.
The efficiency and reaction rates are however still about one
order of magnitude lower than the best homogeneous
catalyst.
Nevertheless, this is to our opinion more than compen-
sated by the ease to remove and reuse the catalyst with only
a simple filtration, which is especially relevant for sensitive
substrates. Also, immediate further consecutive all-in-flow
chemistry allows only filtration steps, while cumbersome
purification steps are rather inhibitive.
Received: September 4, 2012
Revised: December 3, 2012
Published online: && &&, &&&&
Keywords: conjugated microporous polymers ·
.
heterogeneous catalysis · polymers · singlet oxygen ·
surface areas
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Communications
Microporous Polymers
Conjugated microporous polymers
(CMPs) with controlled specific surface
area have been prepared through Sono-
gashira–Hagihara cross-coupling reac-
tions in the presence of silica nanopar-
ticles as templating agents. The CMPs act
as heterogeneous photosensitizers for
producing singlet oxygen in a continuous
flow synthesis.
K. Zhang, D. Kopetzki, P. H. Seeberger,
M. Antonietti, F. Vilela*
&&&& — &&&&
Surface Area Control and Photocatalytic
Activity of Conjugated Microporous
Poly(benzothiadiazole) Networks
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