Dispersible porous classical polymer photocatalysts for visible light-mediated production of pharmaceutically relevant compounds in multiple solvents

Article abstract of DOI:10.1039/c9ta11242a

The dispersibility of photocatalytic polymeric materials is currently a limiting factor for wide scale use. Indeed, the dispersibility of photocatalytic material in solvents dictates its use as heterogenous catalysts, due in part to reagent solubility. Here, we describe a new platform for producing porous polymer photocatalytic nanoparticles that are easily dispersible in a broad range of solvents. Photocatalytic porous classical polymer nanoparticles have been designed, based on classical porous nanoparticles copolymerised with photocatalytic moieties. The combination of both classical and photocatalytic monomers results in an affordable highly photocatalytically effective material, that can be easily dispersed in a wide range of solvents. The versatility and useability of this new material has been demonstrated by photocatalysing a number of pharmaceutically relevant compounds in solvents with varying polarity.

Full text of DOI:10.1039/c9ta11242a

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Materials Chemistry A
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Dispersible porous classical polymer
photocatalysts for visible light-mediated
production of pharmaceutically relevant
compounds in multiple solvents†
Cite this: DOI: 10.1039/c9ta11242a
Received 12th October 2019
Accepted 8th December 2019
Calum T. J. Ferguson, Niklas Huber, Thomas Kuckhoff, Kai A. I. Zhang *ab
a
a
a
DOI: 10.1039/c9ta11242a
and Katharina Landfester*a
rsc.li/materials-a
The dispersibility of photocatalytic polymeric materials is currently polymer photocatalysts are restricted to work in either an
a limiting factor for wide scale use. Indeed, the dispersibility of pho- organic or aqueous continuous phase.
tocatalytic material in solvents dictates its use as heterogenous cata-
The study of colloidally stable materials in a range of
lysts, due in part to reagent solubility. Here, we describe a new solvents has been extensively studied in classical polymer
platform for producing porous polymer photocatalytic nanoparticles chemistry. Careful selection of steric stabilisers can enable the
that are easily dispersible in a broad range of solvents. Photocatalytic dispersibility in one or more continuous phases. Notably,
porous classical polymer nanoparticles have been designed, based on polyethylene glycol (PEG) based steric stabilisers have been
classical porous nanoparticles copolymerised with photocatalytic utilised in to stabilise colloidal materials in a range of both
25–28
moieties. The combination of both classical and photocatalytic aqueous and organic continuous phases.
These polymeric
monomers results in an affordable highly photocatalytically effective materials tend to be non-porous, which, if used for catalytic
material, that can be easily dispersed in a wide range of solvents. The purposes, would lead to a low efficiency due to their limited
versatility and useability of this new material has been demonstrated active surface. Recently, however, classical microporous organic
by photocatalysing
a
number of pharmaceutically relevant polymeric materials have been developed which have high
29–31
compounds in solvents with varying polarity.
surface areas.
This enhanced surface area may be utilised in
conjunction with the recently developed organic small molecule
photocatalytic monomers to produce a photocatalytically active
porous classical polymeric material. Moreover, this new
material may provide a platform to produce a new class of
polymer photocatalysts that are colloidally stable in a wide
range of solvents. Easy control of photocatalyst conjugation
length, highest occupied molecular orbital (HOMO) and lowest
occupied molecular orbital (LUMO) levels may also be engi-
neered into these new materials by varying the copolymerised
photocatalytic moiety.
The employment of heterogeneous polymer photocatalysts
under visible light irradiation is of high research interest with
applications in energy, environmental, and biological systems.
These applications require a broad range of dispersing solvents
due to the desired application and solubility. For example there
is need for the use of “green” aqueous based solvents for water
32
1–6
7
8–10
splitting, anti-bacterial treatment, photodynamic therapy,
11,12
13–15
enzyme regeneration
and waste water treatment.
Conversely, organic systems are required for the majority of
Ideally, we would be able to construct a photocatalytic
material that can be easily dispersed in a broad range of
solvents providing versatility. Photocatalytic porous polymer
particles stabilised by PEG chains extending from the surface
may enable the dispersibility in a range of solvents. RAFT-
mediated polymerisation induced self-assembly (PISA) may be
utilised to ensure that the stabilising polymer chains extend
from the surface which ensure the easy dispersibility of the
1
6–19
redox reactions such as: C–C bond formation,
C]C bond
and hydroxyl-
20,21
22,23
cleavage,
oxidative coupling of amines,
24
ations for example. However, the dispersibility of photo-
catalytic polymeric materials in a broad range of solvents is
currently a limiting factor. Indeed, the dispersibility of the
heterogeneous photocatalytic material in solvents dictates what
it may be used for, due in part to reagent solubility. As a result,
29,33
photocatalytic material.
Further, to enhance the potential of
a
the photocatalytic material its surface area should be maxi-
mised. Porous classical polymer particles have previously been
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz,
Germany. E-mail: landfester@mpip-mainz.mpg.de
b
Department of Materials Science, Fudan University, 200433 Shanghai, P. R. China. reported through the combination of divinyl benzene (DVB) and
E-mail: kai_zhang@fudan.edu.cn.
29,34
fumaronitrile (FN).
This produces a highly crosslinked
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ta11242a
polymeric material with a microporous structure. Photocatalytic
1
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units with controllable donor acceptor units as well as conju-
gation length may be incorporated into this porous material.
Here, porous classical polymers containing copolymerised
photocatalytic moieties are produced in the form of photo-
catalytic polymeric material, as illustrated in Fig. 1. These
photocatalytic porous particles are easily dispersible in a broad
range of solvents, ranging from water to toluene. As a result, the
reactions solvent is no longer limited by the photocatalytic
material and the optimum solvent for the reagents can be
selected. The porosity and resulting high surface area of the
designed material leads to a highly efficient photocatalytic
material by maximising the photocatalytic units incorporated.
We have demonstrated the ability of this material to catalyse
a range of highly desired reactions providing functionality that
has importance in pharmaceutical molecules. Specically, we
have used this material to synthesise organosulfones by two
different methods in both aqueous and organic conditions.
Moreover, nitrogen based benzimidazole derivatives have been
synthesised.
Fig. 2 (a) Structure; (b) TEM image; (c) HOMO and LUMO levels; FTIR
spectrum and UV/vis absorption spectrum of the photocatalytic
porous polymer nanoparticles.
Photocatalytic porous classical polymers were synthesised
using RAFT mediated PISA similarly to previously reported,
where a PEG macro chain transfer agent was chain extended
with DVB, FN and the photocatalytic crosslinker 4,7-bis(4-
particles by DVB. The photocatalytic porous material could be
easily dispersed in a spectrum of solvents with polarity ranging
from 0.099 to 1 (water to toluene; Fig. S3†). The diameter of the
photocatalytic material upon dispersion in water was initially
centred around 1700 nm with a large polydispersity. Sonication
of the sample lead to a large reduction in the average diameter
to 375 nm (Fig. S4†).
The porosity of the material was investigated using nitrogen
gas sorption at 77 K (Fig. S5†). The BET (Brunauer–Emmett–
Teller) surface area of the photocatalytic was determined over
29
vinylphenyl)benzo[c][1,2,5]thiadiazole (Fig. 2a, S1 and S2).
This creates a highly crosslinked particle core containing
a small portion of photocatalytic units (0.33 mol%). This core is
stabilised by easily solvated PEG chains that extend from the
surface into the continuous phase. Transmission electron
microscopy (TEM) was utilised to examine the morphology of
the designed material (Fig. 2b). The aggregated particle
morphology observed is not typically observed for particles
produced by PISA; however, similar structures have previously
a relative pressure range of 0.01–0.3 P/P
0
(Fig. S5a† insert). The
surface area of the photocatalytic material was found to be
29,35
been reported for hypercrosslinked systems.
This
2
ꢀ1
around 290 m g , comparable to previous reports of non-
morphology is attributed to the crosslinking of the small
29
photocatalytic material (Fig. S5a†). The pore size of the
material was calculated using NLDFT pore size analysis, the
major peak of the pore size distribution for the photocatalytic
material was centred on 1.7 nm (Fig. S5b†). Along with pores in
the microporous region, meso- and macrospores were also
observed, which are due the highly aggregated morphology of
the material (Fig. S5b†). The HOMO and LUMO of the photo-
catalytic unit were calculated to be 1.6 and ꢀ1.28 EV vs. SCE as
36
previously reported (Fig. 2c).
Solid state NMR spectroscopy was used to determine the
composition of the polymer particles (Fig. S6†). As expected
signals were detected for the quaternary aromatic carbon (145
ppm), the unreacted vinyl group (138 ppm), the aromatic C–H
2
(128 ppm), the nitrile group (114 ppm), the PEG CH group (71
ppm) and the polymer backbone (42 & 30 ppm) (Fig. S6†).
Fourier transform infrared (FTIR) spectroscopy characterisation
was undertaken to determine the chemical composition of the
porous polymeric particles with and without the photocatalytic
moiety. The FTIR spectrum (Fig. 2d) showed typical signals at
ꢀ
1
2
992 and 2950 cm , which are characteristic for the C–H bond
stretch vibrations of the –CH and –CH – groups, respectively.
3
2
Fig. 1 Porous polymer photocatalysts produced by combining clas-
ꢀ
1
sical polymer chemistry and conjugated polymer photocatalysts. The The signal at 2260–2220 cm is attributed to the C^N stretch
designed material is easily dispersible in a spectrum of solvents vibration originating from the FN. Signals from 1700–
enabling a range of reactions to be catalysed.
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ꢀ
1
1
500 cm are due to aromatic C]C bending from the DVB and and in the absence of photocatalyst did not result in measurable
ꢀ
1
photocatalytic monomer. The signal at 1250–1050 cm result conversion (Table 1 entry 4 & 5, Fig S10 & S11†). Interestingly,
from the C–O–C stretch in the PEG chains. The effect of the when the experiment was repeated with the photocatalytic
photocatalytic unit on the FTIR spectrum is limited. As the monomer only in aqueous conditions were the same conver-
photocatalytic unit only constitutes around 0.33 mol% of the sions obtained. Poorer photocatalytic performance was
material its effect is easily masked. The UV/vis spectra of the observed in acetonitrile and a higher yield in toluene (Table 1
photocatalytic polymer particles have a broad absorption peak entry 12–14). Control experiments with scavengers showed that
1
+
in the visible light region up to 700 nm (Fig. 2e).
2
O , cOH and h were integral for the photocatalytic reaction
The versatility of the newly designed porous classical poly- (Fig. S15†). The photocatalytic moiety concentration used in
mer photocatalyst to synthesise pharmaceutically relevant this study was substantially lower than previously reported and
compounds was investigated. Moreover, we have investigated a much lower excess of the alcohol is required to form the
38
the ability of this new material to synthesise compounds in benzimidazole. Interestingly, in previous studies the reaction
three very different solvents water, acetonitrile and toluene. was undertaken in acetonitrile, which was the poorest per-
This range of solvents would insure reagent solubility. forming solvent in this study.
Furthermore, we would expect the lifetime of the photo-
catalytically produced radicals to differ as a result of solvent.
The synthesis of sulfonyl containing molecules is of impor-
tance due to the biological properties they produce.
37
42,43
Initially, we investigated the production of benzimidazole with Furthermore, sulfonyls can be used as important reagents for
functional groups which are of signicant importance due to a number of chemical transformations including Julia olena-
4
4,45
46
their prevalence in pharmaceutically and biologically active tion,
molecules.
Ramberg–B ¨a cklund reaction and Smiles rearrange-
38,39
47
Typically, the synthesis of benzimidazole con- ment. The photocatalytic synthesis of sulfonyl typically
taining compounds requires harsh conditions in strongly acidic proceeds by the oxidation of a sulde containing molecule.
ꢁ
40
conditions with temperatures in excess of 200 C. Alterna- Sulde oxidation has been catalysed by several different pho-
4
8–50
tively, oxidation of benzimidazole intermediates has been tocatalysts,
undertaken, however, this method oen requires a large excess yields. Conversely, Zhang et al. have reported the selective
where the monoxide product is formed in high
51
41
of strong oxidants.
-Substituted benzimidazoles have been synthesised using carbon nitride and an a large excess of aldehyde oxidant.
TiO2 loaded with platinum as a photocatalyst in anaerobic Initially, a high selectivity for the monoxide was observed with
formation of the dioxide product using mesoporous graphitic
2
38
conditions. Here, porous classical polymer photocatalysts this product transforming into the dioxide over time.
have been utilised to produce benzimidazole containing Here, we have utilised the porous classical polymer photo-
compounds under ambient conditions using visible light in air catalyst to induce the sulde oxidation of methyl phenyl sulde
in various solvents. 2-Phenyl-1H-benzo[d]imidazole was syn-
thesised in this study in water, acetonitrile and toluene with
conversions of 96, 50 and 75% conversion respectively (Table 1
Table
2 Photocatalytic conversion using photocatalytic porous
entry 1–3 and Fig S7–S9†). Control reactions in dark conditions polymer particles to form organosulfonyl, either mono or dioxide, in
a range of solvents. All reactions undertaken under blue light irradia-
tion (l ¼ 460 nm)
Table
1 Photocatalytic conversion using photocatalytic porous
polymer particles to form benzimidazole functional group in a range of
solvents. All reactions undertaken under blue light irradiation (l ¼ 460
a
nm)
Selectivity
(b : c)
Entry
Solvent
Cat.
Light
R
Conversion
a
1
H
CH
Tol
2
O
+
+
+
+
+
+
+
—
+
+
+
+
+
+
—
+
Me
Me
Me
Ph
Ph
Ph
>99%
>99%
59%
>99%
>99%
68%
93 : 7
40 : 60
98 : 2
68 : 32
19 : 81
94 : 6
—
Entry
Solvent
Catalyst
Light
Conversion
a
2
3
CN
a
3
1
2
3
4
5
6
7
8
H
CH
Tol
2
O
+
+
+
+
+
+
+
—
+
+
+
+
96%
50%
16%
0%
b
4
H
2
O
3
CN
b
5
CH
Tol
CH
CH
3
CN
b
6
H
2
H
2
H
2
O
O
O
a
7
3
CN
CN
Me
Me
Trace
7%
—
0%
a
9
3
100 : 0
Monomer
Monomer
Monomer
98%
10%
41%
a
CH
Tol
3
CN
Reaction conditions: methyl phenyl sulde (50 mM), isobutylaldehyde
1
ꢀ
ꢀ1
(
100 mM) photocatalytic particles (2.5 mg mL nanogel, 27 mg mL
photocatalytic units), 10 mL solvent, RT, 18 h, conversion determined
a
b
Reaction conditions: 1,2 benzenediamine (50 mM), benzyl alcohol by GC-MS.
Reaction conditions: diphenyl sulde (50 mM),
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
(200 mM), photocatalytic particles (2.5 mg mL nanogel, 27 mg mL
photocatalytic particles (2.5 mg mL
nanogel, 27 mg mL
photocatalytic units), 10 mL solvent, RT, 16 h, conversion determined photocatalytic units), 10 mL solvent, RT, 20 h, conversion determined
by GC-MS.
by GC-MS.
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and diphenyl sulde in water, acetonitrile and toluene. For both sulfonamide and the direct C–C coupling between phenyl rings
reactions high conversion of over 99% was observed when using was observed (Fig. S29–S31†).
water and acetonitrile (Table 2 entry 1, 2, 5 & 6, Fig. S16, S17, S19
&
S20†). Reactions undertaken in toluene had reduced conver-
Conclusions
sion of less than 70% (Table 2 entry 3 & 7, Fig. S18 & 21†).
Differences in the selectivity were observed between water and
acetonitrile, where reactions in water favoured the formation of
the monoxide whereas reactions in acetonitrile favoured the
formation of the dioxide.
To test if this selectivity was due to a faster reaction in
acetonitrile compared to water kinetic testing was undertaken.
It was found that indeed the reaction rate is faster in acetonitrile
and this results in a higher portion of the dioxide (Fig. S22†).
Control reactions without photocatalyst or in the dark did not
proceeded (Fig. S23 and S24†). Reactions in the presence of
In summary, we present here a new class of porous photo-
catalytic polymeric materials comprised mainly of inexpensive
classical monomers. In addition, specically designed divinyl
photocatalytic monomers have been incorporated into the
porous material to produce novel highly efficient photo-
catalysts. This material can easily be dispersed in a range of
solvents with low to high polarity. The designed material has
been utilised to catalyses pharmaceutically relevant reactions in
water, acetonitrile and toluene. We have demonstrated that the
reaction can be conducted in the optimum solvent for the
synthesis, not based on the photocatalyst. This new material
and synthetic route produces highly effective polymeric porous
photocatalytic material in a range of solvents with greater
efficiency.
1
+
ꢀ
ꢀ
scavengers revealed that O
the sulfonyl formation (Fig. S25†). This method does, however,
require the addition of isobutylaldehyde as an oxidant as
previously reported.
2
, h , e and cO
2
are all integral for
51
Recently, a new strategy for the photocatalytic production of
dioxide sulfonyl through the reaction of diazonium salts with
sulfonate salts has been reported. However, our attempts
showed that the reaction proceeded without photocatalyst
Conflicts of interest
The authors declare no conicts of interest.
(Fig. S28†). The photocatalytic in situ formation of the diazo-
nium moiety from aniline and its subsequent reaction has been
52
reported. We have utilised our porous classical polymer pho- Acknowledgements
tocatalysts to undertake similar reactions in a heterogeneous
N. H. acknowledges the Kekul ´e fellowship of Fonds der chem-
metal free nature. The reaction proceeded with the highest
conversion in acetonitrile (84%; Table 3 entry 2 and S30†) with
signicantly lower conversions in water (42%; Table 3 entry 1
and S29†) and toluene (68%; Table 3 entry 3 and S31†). Scav-
ischen Industrie (FCI) and the Gutenberg-Akademie of the
Johannes Gutenberg University of Mainz. K. A. I. Z. acknowl-
edges the Deutsche Forschungsgemeinscha (DFG) for fund-
ing. This work is part of the research conducted by the
MaxSynBio consortium that is jointly funded by the Federal
Ministry of Education and Research of Germany (BMBF) and
the Max Planck Society (MPG).
1
+
ꢀ
ꢀ
enger experiments showed that O
2
, cOH, h , e and cO
2
were
required for the experiment (Fig. S32†). This synthetic route
selectively produces the dioxide sulfonyl over the monoxide
formed by sulde oxidation. However, byproducts of the
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