Highly porous conjugated polymers for selective oxidation of organic sulfides under visible light

Article abstract of DOI:10.1039/c4cc02861a

High surface area porous conjugated polymers were synthesized via the high internal phase emulsion polymerization technique and micropore engineering as efficient heterogeneous photocatalysts for highly selective oxidation of organic sulfides to sulfoxides under visible light. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc02861a

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Highly porous conjugated polymers for selective
oxidation of organic sulfides under visible light†
Cite this: Chem. Commun., 2014,
50, 8177
Zi Jun Wang, Saman Ghasimi, Katharina Landfester and Kai A. I. Zhang*
Received 17th April 2014,
Accepted 5th June 2014
DOI: 10.1039/c4cc02861a
www.rsc.org/chemcomm
High surface area porous conjugated polymers were synthesized via polyHIPEs to be used as photocatalysts since the chemical
the high internal phase emulsion polymerization technique and micro- composition of the conjugated polyHIPEs is of vital importance
pore engineering as efficient heterogeneous photocatalysts for highly to their photochemical properties and photocatalytic performance.
selective oxidation of organic sulfides to sulfoxides under visible light.
Organic sulfides are key intermediates in pharmaceutical or
other fine chemical industries.22,23 Selective oxidation of sulfides
Conjugated porous polymers, combining the photoactive p-electron to the corresponding sulfoxides is traditionally achieved using
backbone and porous surface properties, have been recently intro- stoichiometric amounts of organic or inorganic oxidants, which
duced as heterogeneous photocatalysts for the organic synthesis usually generates huge amounts of toxic byproducts or heavy metal
under visible light irradiation.1–5 Polymers obtained by high inter- wastes.24,25 Recent developments in catalysts based on organo-
nal phase emulsion polymerization (polyHIPEs), which is a rela- metallic complexes have been attempted to alleviate this problem
tively new technique for the preparation of highly porous materials, but they still generally require substantial amounts of additional
showed versatile applications such as tissue engineering scaffolds,6 oxidants, such as H2O2 at elevated temperatures,26,27 urea hydro-
enzyme immobilization,7,8 gas storage,9 and separation media.10 peroxide,28 and isobutyraldehyde.29 Thus, conjugated polyHIPEs
Due to the micrometer-sized pore structures, which are highly have the clear advantage of serving as green, effective, chemo-
suitable for mass transfer, polyHIPEs are also used as catalyst selective and recyclable photocatalysts that require only visible light
supports.11 Only a few reports on the synthesis of photocatalysts by and air. Not only can the conjugated polyHIPEs be easily removed
HIPE polymerization have been published.12,13 Recently, the syn- and recycled after use, they can also be readily incorporated into
thesis of the first example of fully conjugated polyHIPEs was continuous flow systems, which offer facile automation, precise
reported; these materials have surface areas of 35–50 m2 gÀ1 and control over reaction parameters and a predictable scale-up.13
could be used as highly efficient and reusable heterogeneous photo-
Herein, we report on the design and preparation of high surface
sensitizers for the singlet oxygen generation under visible light.13 area conjugated porous polymers via high internal phase emulsion
Combining the interconnected porosity of polyHIPEs and the polymerization and micropore surface engineering. By incorpora-
conjugated framework throughout the polymer network, conjugated tion and removal of the cleavable tert-butyl carboxylate (Boc)
polyHIPEs hold great promise in heterogeneous photocatalysis functional group as a spacer into the polymer network, the surface
under visible light. Because conjugated polyHIPEs act as photo- area can be increased up to eight times, gaining extra porosity. The
catalysts having a stable inherent porous skeleton, the use of photocatalytic activity of the polymers for highly selective oxidation
additional support materials such as silica gels14 and zeolites15 of organic sulfides to the desired mono-oxidized sulfoxides under
can be eliminated. Classical polyHIPEs have rather low surface areas visible light irradiation was also described. Furthermore, the
(up to 50 m2 gÀ1).16,17 It was later shown that the surface area of influence of the surface area and energetic level on the photo-
polyHIPEs can be tuned by varying the cross-linker content or by catalytic performance was investigated by introducing a well-
hypercrosslinking the system via chemical functionalization.18–21 studied electron acceptor, the benzothiadiazole (BT) unit into
Nonetheless, these methods are not suitable for conjugated the polymer backbone.
The porous polymers were synthesized via the Suzuki–Miyaura
cross-coupling reaction (Scheme 1). The detailed synthetic pathway
and the characterization of the monomers and polymers are
described in the ESI.† The thermogravimetric analysis (TGA) of
the Boc containing polymers showed a significant weight loss at
250 1C under N2, indicating the thermal cleavage of the Boc group
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,
FT-IR and fluorescence spectra, TGA, 1H, 13C and solid state 13C CP/MAS NMR
spectra, N2 sorption isotherms and pore size distribution, theoretical calculations
and additional SEM and TEM Images. See DOI: 10.1039/c4cc02861a
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Fig. 1 SEM image of the conjugated polyHIPEs before and after the removal
of the Boc group. (a) B-(Boc-NPh2)3, (b) B-(NPh2)3, (c) B-(Boc-CB)3, (d) B-CB3,
(e) B-(Boc-CB)2-BT, and (f) B-CB2-BT. Scale bar: 2 mm.
Scheme 1 Idealized structures and surface area control via micropore
engineering of conjugated porous polymers. Reaction conditions: (i) Pd(PPh3)4,
span 80, toluene/H2O (1/9), 80 1C, overnight. (ii) 250 1C, 12 h under vacuum.
Fig. 2 (a) UV/Vis DR spectra of B-(Boc-NPh2)3, B-(Boc-CB)3, and B-(Boc-
CB)2-BT containing Boc groups. (b) UV/Vis DR spectra of B-(NPh2)3, B-CB3,
and B-(CB)2-BT. (c) An illustrated setup of the glass column photoreactor in a
continuous flow system, the porous polymers are inserted into the gas column.
inside the porous polymers (Fig. S12 in the ESI†). The FTIR
spectra confirmed the disappearance of the typical signals for
{CQO groups at 1750 cmÀ1 (Fig. S7–S9, ESI†). The solid-state
13C/MAS NMR showed a significant decrease of chemical shifts Boc moiety (Fig. 2b), compared to the ones containing Boc
at d = 30 and 80 and 155 ppm, which are assigned to the –CH3, (Fig. 2a). This shift could be caused by the thermal annealing
quart.-C and {CQO groups of the Boc moiety. The signals effect of the conjugated polymer backbone, leading to more ordered
between 100 and 160 ppm, which are assigned to the aromatic rings p–p–stacking, and therefore slightly lower energy levels.
in the polymer backbones, remained unchanged (Fig. S14–S19,
ESI†). This indicates that upon the thermal treatment the aromatic polymer backbone, a bathochromic shift of ca. 55 nm occurred,
structure stays intact. indicating lower HOMO and LUMO levels and narrower band
By introducing the strong electron acceptor BT unit into the
The Brunauer–Emmett–Teller (BET) surface areas of all three gaps. Similarly, the photoluminescence (PL) spectra of B-(Boc-CB)2-
polymers increased significantly after the thermal removal of the Boc BT and B-CB2-BT are largely bathochromically shifted com-
group. In particular, surface areas ranged from 198 m2 gÀ1 for pared to B-(Boc-CB)3 and B-CB3 (Fig. S10 and S11 in the ESI†).
B-(NPh2)3 to 230 m2 gÀ1 for B-CB3 and 53 m2 gÀ1 for B-CB2-BT, An extra fluorescence band at about 520 nm occurred for both
exhibiting an increasing factor of 4, 8 and 2 times from B-(Boc- BT containing polymers. This large Stokes shift reflects a
NPh2)3 with 45 m2 gÀ1, B-(Boc-CB)3 with 27 m2 gÀ1, and B-(Boc-CB)2- stronger p–p*-transition of the electrons under excitation. The
BT with 26 m2 gÀ1, respectively (Fig. S20 and S21, ESI†). Likewise, the NPh-based polymers B-(Boc-NPh2)3 and B-(NPh2)3 exhibited the
total pore volumes of the porous polymers also increased, and the largest optical HOMO–LUMO band gaps of 3.31 and 3.03 eV,
micropore diameter was ca. 1.5 nm, estimated via this micropore followed by the B-(Boc-CB)3 with 3.02 and B-CB3 with 2.83 eV.
modification method. The data are listed in Table S1 of the ESI.†
The lowest bad gaps were exhibited by B-(Boc-CB)2-BT with
In Fig. 1, SEM images of the two series of polyHIPEs are 2.47 and B-CB2-BT with 2.31 (Table 1).
displayed. The polyHIPE series after the Boc cleavage appeared to
To investigate the electronic properties, theoretical HOMO–
have a very similar morphology as that of the series containing Boc, LUMO levels of the repeating units were calculated. The data
showing interconnected pore structures. The cavities of the poly- are listed in Table S3 and the electronic structures are displayed
HIPEs ranged from 10 mm to 20 mm in size whereas the inter- in Fig. S24 and S25 in the ESI.† Similar to the optical band gaps,
connected pores have diameters of about 1 mm. To note is that the the NPh-containing units exhibited the highest HOMO–LUMO
removal of the Boc group upon heat treatment did not affect the gap, followed by the CB-based ones. And the BT-containing
stability of the macrostructure and the porous skeleton remained units showed clearly lower band gaps. In contrast, all repeating
intact. A significant number of micropores (d = B1.5 nm) appeared units exhibited similar HOMO levels ranging from À5.40 to
according to the pore size distribution measurement after the À5.90 eV. However, the LUMO levels of the BT-containing
Boc cleavage.
B-(Boc-CB2)-BT and B-CB2-BT of ca. À3.10 eV were significantly
The UV/Vis diffuse reflectance spectra (DRS) of the polymers lower than the NPh- and CB-based polymers (À1.10 to À1.60 eV).
were slightly bathochromically shifted after the removal of the A better electron transition can be expected.
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Table 1 Photocatalytic oxidation of thioanisole using conjugated poly-
HIPEs as heterogeneous photocatalysts under visible light: (a) thioanisole,
(b) methyl phenyl sulfoxide and (c) methyl phenyl sulfone, and the optical
and electronic properties
Five additional repeat experiments were carried out for inves-
tigating the photooxidation of thioanisole using B-(Boc-CB)2-BT
under the same reaction conditions (Table S2 in the ESI†). The
desired mono-oxidised product was obtained in an almost quanti-
tative manner, demonstrating the stability and reusability of the
polyHIPE. The oxidation of sulfides is likely mediated by photo-
chemically generated singlet oxygen, which should be similar
to a previous study.33 Blank experiments were also carried out
as a control (Table 1). The same reaction was tested by using the
polyHIPE as the photocatalyst, but without light irradiation;
or without the polyHIPE, but under light irradiation. No oxidized
product was obtained in both experiments, indicating that both
components are indispensable.
For the determination of the general applicability of the selective
photooxidation of organic sulfides, a number of various thioanisole
derivatives were examined under the same reaction conditions.
B-(Boc-CB)2-BT was used in all reactions due to its high photo-
catalytic activity. The data are listed in Table 2. We tested different
phenyl sulfides bearing electron-withdrawing (entries 7–9) or
electron-donating substituents (entries 10 and 11) on the phenyl
Selectivity (%)
Band
gap/opt.
(eV)
Conv.
t/(h) (%)
Entry
PolyHIPEs
b
c
1
2
3
4
B-(Boc-NPh2)3
B-(NPh2)3
B-(Boc-CB)3
B-CB3
B-(Boc-CB)2-BT 24
B-CB2-BT
—
48
48
48
48
38
33
91
93
99
98
—
—
499
499
499
499
95
o1
o1
o1
o1
5
3.31
3.03
3.02
2.83
2.47
2.31
—
5
6
24
24
99
o1
Blanka
—
—
Blankb B-(Boc-CB)2-BT 24
—
—
—
a
b
No polymer, but under light irradiation. Using B-(Boc-CB)2-BT as a
photocatalyst, but no light irradiation. General reaction conditions: 10 mg
of polyHIPE, flow rate of sulfides in acetonitrile: 1 mL minÀ1, RT, air.
The photocatalytic activity of the conjugated porous polymers ring as substrates for the oxidation reaction. All reactions exhibited
as heterogeneous photocatalysts under visible light was investi- high selectivity of the mono-oxidated sulfoxides. The conversion
gated by the photooxidation of thioanisole at room temperature in corresponded to the electron-withdrawing ability of the sub-
a continuous flow system as displayed in Fig. 2c. As shown in stituents (–Cl o –Br o –F). In case of the electron-donating
Table 1, an almost quantitative selectivity (499%) of the photo- substituents (entry 10 and 11), the conversion followed the
oxidation of thioanisole to the desired mono-oxidized product opposite order of the electron affinity (–CH3 o –OCH3). Allyl
methyl phenyl sulfoxide was obtained. PolyHIPEs containing phenyl sulfide with a considerable selectivity of 88% (entry 12)
the CB moiety showed higher conversions than those with and ethyl phenyl sulfide (entry 13) with 499% were employed to
NPh2 moieties, and the polymers containing a strong electron demonstrate the chemoselectivity of the photooxidation reaction.
acceptor, BT unit, exhibited the best photocatalytic efficiency.
In conclusion, we reported a novel method to synthesize high
This corresponds with the lower HOMO–LUMO band gaps, surface area conjugated porous polymers with unique electronic
in particular, the low LUMO levels according to the theoretical properties via high internal phase emulsion polymerization and
calculation.
Table 2 Selective oxidation of various sulfides using B-(Boc-CB)2-BT as a
photocatalyst under visible light irradiationa
To note, the series of the polyHIPEs without the Boc moieties,
B-(NPh2)3, B-CB3, and B-CB2-BT, with higher BET surface areas
did not distinguish themselves as more efficient photocatalysts
than their counterparts with Boc. This can be attributed to the
micropores, which after the removal of the Boc moieties were
still too small (d = 1.5 nm) to allow efficient mass diffusion of the
reactants. This finding further proves that in heterogeneous
catalysis, it rather depends on the larger pores (41 mm) that
play a more vital role in providing accessible surface sites for inter-
facial reactions than the micropores (o2 nm).30–32 B-(Boc-CB)2-BT
and B-CB2-BT exhibited almost quantitative conversions within
24 h in acetonitrile (entries 5 and 6 in Table 1) with 95% and 99%
selectivity, respectively. Likely, the reaction time (24 h) might be
too long for B-(Boc-CB)2-BT as an active photocatalyst, and an
overoxidation of 5% occurred.
The photocatalytic experiment suggests that the p-electrons
in the conjugated polymer backbone play a more crucial role in
the photocatalytic feasibility than the surface area. This further
proved a previous study on a series of conjugated porous
polymers as photosensitizers, which showed that the polymers
containing strong electron acceptor units such as BT could
generate singlet oxygen in a more efficient manner.13 And most
importantly, low LUMO levels are mandatory.
Selectivity (%)
Entry
t/(h)
24
Conv. (%)
7
8
98
45
499
499
o1
o1
24
9
24
24
24
33
35
50
499
96
o1
4
10
11
98
2
12
13
a
24
24
39
84
88
12
499
o1
Reaction conditions: 10 mg of B-(Boc-CB)2-BT, flow rate of sulfides in
acetonitrile: 1 mL minÀ1, RT, air.
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