A conjugated porous poly-benzobisthiadiazole network for a visible light-driven photoredox reaction

Article abstract of DOI:10.1039/c4ta03887h

A conjugated porous poly-benzobisthiadiazole network was synthesized via high internal phase emulsion polymerization as a highly active and stable heterogeneous photocatalyst. Under a household energy saving light bulb, dehalogenation of α-bromoacetophenones was performed in an almost quantitative manner at room temperature.

Full text of DOI:10.1039/c4ta03887h

Journal of
Materials Chemistry A
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COMMUNICATION
A conjugated porous poly-benzobisthiadiazole
network for a visible light-driven photoredox
reaction†
Cite this: J. Mater. Chem. A, 2014, 2,
8720
1
Received 28th July 2014
Accepted 16th September 2014
Zi Jun Wang, Saman Ghasimi, Katharina Landfester and Kai A. I. Zhang*
DOI: 10.1039/c4ta03887h
www.rsc.org/MaterialsA
16–21
A conjugated porous poly-benzobisthiadiazole network was synthe- organic synthesis under irradiation of visible light.
High
sized via high internal phase emulsion polymerization as a highly active internal phase polymerization is a well-known technique previ-
and stable heterogeneous photocatalyst. Under a household energy ously employed to prepare highly porous materials. Polymers
saving light bulb, dehalogenation of a-bromoacetophenones was obtained by this technique (polyHIPEs) have found various
2
2–24
25
performed in an almost quantitative manner at room temperature.
applications such as enzyme immobilization,
tissue engineering scaffolds
gas storage,
26
27
and separation media.
Combining the interconnected porosity of polyHIPEs with the
photoactivity of the conjugated polymer backbone, p-conju-
gated porous polyHIPEs have been recently employed as highly
stable heterogeneous visible light-driven photocatalysts for
Photocatalysts, which absorb mainly in the visible range of
light, have established notable prominence in applications such
as water splitting, solar energy storage, and photovoltaics.
1
–3
Among these visible light-driven catalysts developed, rare metal
complexes, especially ruthenium and iridium complexes have
found large applications due to their commercial availability,
28
singlet oxygen generation, highly selective oxidation of
29
organic suldes and effective visible light photoinitiation for
30
free radical polymerization. Taking advantage of the insoluble
4–11
excellent stability and photoredox properties.
However,
nature and monolithic shape of these conjugated porous poly-
mers, they can be readily incorporated into continuous ow
systems, which offer facile automation, precise control over
considering the high cost and toxicity of these rare metals, as
well as their limited availability in nature, their sustainability
still remains a huge challenge. There has been growing interest
in developing metal-free visible light driven photocatalysts in
recent years, where a vast number of organic chromophores and
28
reaction parameters and predictable scale-up.
In this work, we report on the design and preparation of a
conjugated porous polymer via the Sonogashira cross-coupling
reaction using a high internal phase emulsion polymerization
technique. A strong electron acceptor, benzobisthiadiazole
12–14
dyes were employed successfully in photoredox catalysis.
In
particular, uorescein, eosin Y, nile red, and rhodamine B have
15
been studied intensely. A common drawback associated with
most single molecule organic dyes is that they easily suffer from
the solvolytic attack in the reaction medium, the so-called
photo-bleaching. In addition, homogeneous catalysts generally
can be troublesome to remove aer the reaction. Therefore, it is
benecial to develop a new class of organic heterogeneous
visible light driven photocatalysts with high stability and easy
processability.
(
BBT), was incorporated into the porous polymer backbone,
gaining a broad absorption band and the desired energy level.
The visible light-driven photocatalytic activity of the porous
polymer was demonstrated by the reductive dehalogenaton
reaction of a-bromoacetophenones under a 23 W household
energy saving uorescent light bulb.
In Scheme 1, the synthetic pathway of the conjugated
porous polymer B
another porous polymer only containing uorene (B
2
-FL
2
-BBT is displayed. As a comparison,
-FL ) was
Conjugated porous polymers, combining the photoactive
p-electron backbone and porous surface properties, have been
recently introduced as stable heterogeneous photocatalysts for
2
3
synthesized for investigation of the photocatalytic perfor-
mance of the dehalogenation reaction without the electron
withdrawing BBT unit. The experimental synthetic details and
polymer characterization data are described in the ESI.† It
should be noted that, for the Sonogashira coupling reaction,
Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz,
Germany. E-mail: kai.zhang@mpip-mainz.mpg.de
†
Electronic supplementary information (ESI) available: Monomer synthesis,
2 3
K CO seems to be one of the few bases which lead to stable
1
13
13
FT-IR, uorescence spectra, TGA, H, C, solid state C CP/MAS NMR and EPR high internal phase emulsion. By adding amines as the base,
spectra,
N
2
sorption isotherms and pore size distribution. See DOI:
in comparison, only unstable emulsions were formed; rapid
1
0.1039/c4ta03887h
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2 2 2 3
Fig. 1 SEM images of B -FL -BBT (a) and B -FL (b).
1
0 to 20 mm in size, while the interconnected pores show
diameters from 0.5 to 1 mm.
The UV/Vis diffuse reectance spectrum (Fig. 2) of
-FL -BBT showed a broad absorption band ranging from the
B
2
2
visible and into the near infrared (NIR) region. An optical band
gap of ca. 1.5 eV can be derived from the absorption edge. In
comparison, the UV/Vis spectrum of B
2 3
-FL only containing

uorene showed a sharper absorption shoulder, exhibiting an
Scheme 1 Synthetic route and idealized structure of B
2
-FL
2
-BBT and
optical band gap of ca. 2.40 eV. This is probably due to the
strong electron withdrawing properties of the BBT unit, exhib-
iting lower HOMO–LUMO levels, and the resulting band gap of
the polymer. It should be noted that the broad shoulder of the
spectra could partially be due to the light scattering effect,
B
2
-FL
3
. Reaction conditions: (i) Pd(PPh
)
3 4
2
, CuI, K CO , span 80,
3
ꢀ
2
toluene–H O (v/v: 1/9), 80 C, overnight.
phase separation occurred during the polymerization. The giving a broad pore size range inside the porous polymers.
formed monolithic cross-linked polymers were insoluble in Interestingly, by incorporating the electron withdrawing BBT
1
3
all common organic solvents tested. Solid state C/MAS NMR group into the polymer backbone, the electron paramagnetic
spectroscopy showed chemical shis between d ¼ 15 and 60 resonance (EPR) spectrum of B -FL -BBT exhibited signicantly
2
2
ppm that can be assigned to the alkyl groups of the uorene higher intensity than B -FL (ESI, Fig. S4†).
2
3
units. The signals between 110 and 160 ppm can be assigned
to the aromatic carbons in the polymer backbone (Fig. S1 in mers as heterogeneous photocatalysts under visible light was
the ESI†).
investigated by the photoreductive dehalogenaton reaction of
The photocatalytic activity of the conjugated porous poly-
From the elemental analysis, a BBT/uorene ratio of a-bromoacetophenones. A 23 W household energy saving light
approximately 1 : 3 inside the polymer B -FL -BBT was bulb was used as the light source. The simple illumination set-
2
2
observed. This could be caused by the non-quantitative poly- up is shown in Fig. S15 in the ESI.† Various a-bromoacetophe-
merization degree and different relativities of both building none derivatives were investigated (Table 1). B -FL -BBT showed
2
2
blocks during polymerization. Thermal gravimetric analysis notably higher photocatalytic activity than B -FL (entry 1 and 2),
2
3
(
TGA) measurements showed that the conjugated network which proved the importance of incorporation of the electron
ꢀ
remained intact up to 450 C and the weight loss at around withdrawing BBT unit. We further tested different phenone
3
ꢀ
00 C can be attributed to the alkyl groups (ESI, Fig. S2†). The bearing electron-withdrawing (entries 3–6) or electron-donating
Brunauer–Emmett–Teller (BET) surface areas of B -FL -BBT and substituents (entries 7 and 8) on the phenyl ring as substrates
2
2
2
ꢁ1
B -FL are around 23 and 17 m g (ESI, Fig. S4†). Broad pore
2
3
distributions from the meso to macropore range were observed
for both polymers (ESI, Fig. S5†). This is similar to the previ-
ously reported surface area values and pore analyses of this
class of conjugated porous polymers prepared using the HIPE
28,29
technique.
The broad pore size distribution could be caused
by the alkyl side chains of the uorene unit, which block
the micropores and require larger space inside the porous
polymers. As it was shown previously that micropores are too
small to be effectively accessible by diffusion and no enhance-
ment in catalytic efficiency was observed from higher surface
area polymers with additional micropore volumes, the hierar-
chically porous structure of the polymers could be advanta-
29
geous for a higher efficiency of the catalytic reactions.
SEM images (Fig. 1) of two porous polymers illustrate the
interconnected porous structures with cavities ranging from
2 2 2 3
Fig. 2 UV/Vis DRS spectra of B -FL -BBT and B -FL .
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Table 1 Visible light driven reductive dehalogenation of haloketones using the conjugated porous polymers as heterogeneous photocatalysts
under irradiation of a 23 W household fluorescent light bulb
a
b
Entry
1
Substrate
Product
t (h)
Conversion (%)
Yield (%)
4
>99
19
93
15
c
2
4
4
4
3
4
5
95
89
95
93
>99
>99
4
4
5
6
7
8
>99
>99
97
92
5
4
>99
<1
90
d
9
—
e
1
0
4
<1
—
a
1
b
c
Conversion determined by H-NMR using trichloroethylene as the internal standard. Isolated yield by chromatography. Using B
2
-FL
3
as the
d
e
photocatalyst. Blank experiment without B
2
-FL
2
-BBT, but under light. Blank experiment with B
2
-FL
2
-BBT, but in the dark. Reaction
conditions: 10 mg porous polymer, 0.2 mmol substrate, 0.4 mmol DIPEA, 0.22 mmol Hantzsch ester in 1 mL deuterated DMF under a 23 W
household uorescent light bulb at room temperature.
for the reaction. As shown in Table 1, almost quantitative experiments were also carried out as a control (entries 9 and 10).
conversion of the dehalogenation reaction was achieved in every The same reaction with a-bromoacetophenone was carried out
case, demonstrating the general applicability of the conjugated by using B
2
-FL
2
-BBT as a photocatalyst, but without light irra-
-FL -BBT but with light irradiation. The
porous polymer B -FL -BBT as a highly efficient photocatalyst. diation; or without B
2
2
2
2
Five additional repeating experiments of the visible light dehalogenated product, acetophenone, was barely obtained in
induced dehalogenation of a-bromoacetophenone were carried either experiment, indicating that both components are
out using B
Fig. S12 in the ESI†). The dehalogenated product was obtained
in an almost quantitative manner, demonstrating the stability Under visible light irradiation, the excited state of the conju-
and reusability of B -FL -BBT as a photocatalyst. Blank gated porous polymer generates a hole via charge separation,
2
-FL
2
-BBT under the same reaction conditions indispensable.
(
We propose a reaction mechanism as shown in Fig. 3.
2
2
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Conclusions
In summary, we have developed a p-conjugated porous
polymer via Sonogashira-coupling using the high internal phase
emulsion technique as a heterogeneous, visible light-driven
photocatalyst for the reductive dehalogenation of haloketones.
The benzobisthiadiazole-based porous polymer offers a new
class of robust visible light-driven heterogeneous catalyst
systems, with strong and broad absorption from the visible to
the near infra-red range. A household energy saving light bulb
was used as the light source, which provides an environmentally
sustainable and nancially affordable solution for future cata-
lytic processes. The heterogeneous photocatalyst can be easily
recycled and reused without signicant loss in photoactivity.
Further studies of this highly versatile heterogeneous visible
light driven photocatalyst system are expected to nd more
applications in more demanding catalytic processes.
Fig. 3 Proposed mechanism of the visible light-driven dehalogenation
reaction using the conjugated porous polymers as heterogeneous
photocatalysts.
Acknowledgements
The Max Planck Society is acknowledged for nancial support.
which oxidizes the amine DIPEA, leading to a radical anion
inside the porous polymer. The radical anion reacts with
a-bromoacetophenone, generating the phenone radical, as the
Br leaving group cleaves off. The hydrogen radical is then
subsequently donated by the sacricial hydrogen donor
Hantzsch ester to produce the nal product. The dehydro-
genated Hantzsch ester radical reacts with the DIPEA radical
cation, and ammonium bromide and pyridine ester are
obtained as side products.
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