Modification of conjugated microporous poly-benzothiadiazole for photosensitized singlet oxygen generation in water

Article abstract of DOI:10.1039/c3cc38956a

Water-dispersible alkyne-bearing conjugated microporous poly-benzothiadiazoles were synthesized using thiol-yne chemistry to enhance water compatibility. The water compatible polymer networks were used as heterogeneous photocatalysts to generate singlet oxygen for the conversion of furoic acid to 5-hydroxy-2(5H)-furanone. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cc38956a

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Modification of conjugated microporous
poly-benzothiadiazole for photosensitized
singlet oxygen generation in water†
Cite this: Chem. Commun., 2013,
49, 2353
Received 14th December 2012,
Accepted 5th February 2013
Hiromitsu Urakami, Kai Zhang and Filipe Vilela*
DOI: 10.1039/c3cc38956a
www.rsc.org/chemcomm
Water-dispersible alkyne-bearing conjugated microporous poly-
Previously, we have reported the synthesis of conjugated
benzothiadiazoles were synthesized using thiol–yne chemistry to microporous poly-benzothiadiazole networks that generate
enhance water compatibility. The water compatible polymer networks singlet oxygen under irradiation with blue light at 420 nm in
were used as heterogeneous photocatalysts to generate singlet oxygen a heterogeneous environment.16 The singlet-oxygen mediated
for the conversion of furoic acid to 5-hydroxy-2(5H)-furanone.
conversion of a-terpinene to ascaridole was carried out in
chloroform due to the favorable half-life of singlet oxygen as
Microporous organic polymers (MOPs) are a class of porous well as the dispersibility of CMPs in chloroform. However,
materials that have caught much attention due to their broad these CMPs are highly hydrophobic, and since water soluble
relevance in the field of adsorption, separation, storage, sensing, conjugated polymers have been used to produce singlet oxygen
and heterogeneous catalysis owing to their high surface area and for photodynamic therapy21 and in pursuit of performing
pore dimensions.1,2 Conjugated microporous polymers (CMPs) are singlet oxygen mediated reactions in aqueous heterogeneous
a sub-class of MOPs that combine the high surface area and environments, we have attempted the modification of pre-
porosity of a MOP and electronic and electroluminescent properties synthesized hydrophobic CMPs to allow for enhanced water-
of a conjugated polymer.1,3–14 The structural and electronic proper- dispersibility. Modification of this alkyne bearing network was
ties of CMPs can further be tuned by the selection of monomer carried out by the utilization of thiol–yne chemistry as reported
units,15 as well as applying approaches such as templating.16 The by Kiskan and Weber22 The heterogeneous singlet oxygen
potential control over electronic and morphological properties generation in an aqueous environment was confirmed by the
makes this class of polymers attractive especially for photo-induced high conversion of furoic acid into 5-hydroxy-2(5H)-furanone as
energy applications.17
observed by others.23–25
In order to achieve an economically and environmentally
The polymer networks were synthesized via palladium-catalyzed
friendly chemical process, both homogeneous- and hetero- Sonogashira cross-coupling polycondensation of 4,7-dibromo-
geneous catalysis have been under heavy investigation. benzo[c][1,2,5]thiadiazole with 1,3,5-triethynylbenzene. The
Although both systems have their advantages, heterogeneous polymers were obtained as dark yellow powders, which were
systems are desired over the homogeneous counterpart due to insoluble in all conventional organic solvents tested (CHCl3,
their easy separation and efficient recovery. Considering the toluene, THF, EtOH, MeCN). As shown in Scheme 1, thiol–yne
reaction media for these chemical processes, much effort has chemistry was performed on CMPs by treating the insoluble
been made to utilize ‘‘greener solvents’’ instead of the more solid with 3-mercaptopropionic acid (MPA) and azobis(2-methyl-
hazardous halogenated solvents. Water, in particular, has propionitrile) (AIBN) at 90 1C in DMF. Detailed syntheses and
been considered as a hallmark as a green solvent for organic
reactions.18–20
Department of Colloid Chemistry, Max-Planck-Institute of Colloids and Interfaces,
D-14476 Potsdam, Germany. E-mail: filipe.vilela@mpikg.mpg.de;
Fax: +49 331 567 9502; Tel: +49 331 567 9513
† Electronic supplementary information (ESI) available: General synthetic proce-
dure of post-modification, conditions of the photocatalytic reaction, solid state
NMR spectra of the unmodified CMP and modified WCMP, elemental analysis
data, UV/Vis absorption spectra, BET gas sorption isotherms and pore size
distribution, and 1H and 13C NMR spectra of furoic acid and the resulting
Scheme 1 Thiol–yne modification of WCMPs.
5-hydroxy-2(5H)-furanone. See DOI: 10.1039/c3cc38956a
c
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Chem. Commun., 2013, 49, 2353--2355 2353

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Table 1 Structural properties of the polymer networks
a
b
SBET
/
Vmicropore
/
Vtotalpore
/
Pore
size/nm
Polymers
m2 gÀ1
cm3 gÀ1
cm3 gÀ1
Unmodified CMP
WCMP_0.1
WCMP_0.4
285
280
244
0.184
0.113
0.096
0.263
0.235
0.195
1.4
1.3
1.2
a
Micropore volume was calculated from the N2 adsorption isotherms at
b
P/P0 = 0.15. Total pore volume at P/P0 = 0.90, using the BET method.
more micropores could be filled, which could discourage
the pore volume and micropore size determination. The gas
sorption isotherms and pore size distribution are displayed in
Fig. S3 in the ESI.†
Fig. 1 (a) FTIR of WCMPs before and after the addition of 3-mercaptopropionic
acid. Region I: alkane band; region II: alkyne band; region III: –COOH band.
(b) Aqueous dispersion (1.0 mg mLÀ1) of unmodified (A) and modified WCMPs
(B1: WCMP_0.1 - 0.1 eq. MPA/AIBN, B2: WCMP_0.4 - 0.4 eq. MPA/AIBN).
To investigate the feasibility of the polymer network con-
cerning singlet oxygen generation in water, oxygen was bubbled
into aqueous WCMP dispersion (0.1 mg mLÀ1) containing
0.1 M furoic acid at the rate of 15 mL minÀ1, while irradiated
with blue light at 420 nm. The conversion of the reaction was
evaluated by recording 1H-NMR spectra of aliquots removed
from the reaction mixture at different time points. Unlike
homogenous catalysis systems, our reaction was worked up
simply by filtering off the WCMPs through a syringe filter.
While modified WCMPs exhibited high conversion (B90%)
after 22 h, the use of unmodified CMP only led to moderate
conversion (B50%) of the furoic acid. As with UV/Vis and N2
adsorption–desorption experiments, there seems to be no
difference in the capability of producing singlet oxygen in an
aqueous environment between WCMP_0.1 and WCMP_0.4.
Likely, the addition of 0.1 or 0.4 equivalence of MPA/AIBN
results in no difference in the photocatalytic activity between
the two modified WCMPs (Scheme 2). For both modified and
unmodified CMPs, we were not able to recover the catalyst after
characterization of the water compatible CMPs (WCMPs) and the
subsequent modification are described in the ESI.† The FT-IR
spectra of the unmodified CMP and modified WCMP are
displayed in Fig. 1a. In region I, the band between 2850 and
2950 cmÀ1 can be ascribed to the added –COOH group as well
as to a stronger signal at about 1700 cmÀ1 in region III, which
indicates the addition of carbonyl groups. In region II at
ca. 2200 cmÀ1, the intensity of the alkyne band was observed
to diminish. The modification was also confirmed from the solid
state 13C-NMR (ssNMR), with the emergence of the aliphatic
peaks from MPA (B28 ppm) as well as the small peaks attributed
to carboxylic acid (B170 ppm) (Fig. S1, ESI†). Furthermore,
elemental analysis (Table S1, ESI†) showed a slight increase in
sulfur and hydrogen contents, agreeing with the results observed
from FT-IR and ssNMR. Furthermore, aqueous dispersions of
the polymer networks were only observed for the modified
samples (Fig. 1b). The conjugated polymer networks were
dispersed in DMF, and the UV/Vis spectra were recorded.
The polymer networks show two absorption maxima at about
320 and 440 nm. Modification of the CMPs had no influence on
the absorption maxima (Fig. S2, ESI†).
In the following text, the modified water-dispersible WCMPs
will be labeled as WCMP_X, where X stands for equivalence of
MPA and AIBN used during the thiol–yne reaction with respect
to the theoretical amount of alkyne on the pre-synthesized
CMPs. To maintain the conjugation of the conjugated back-
bone, we avoided the excessive use of MPA/AIBN. WCMP_0.1
and WCMP_0.4 were utilized in the following studies.
Scheme 2 Photocatalytic conversion of furoic acid into 5-hydroxy-2(5H)-
furanone.
The Brunauer–Emmett–Teller (BET) surface area and pore
size distribution of the unmodified CMP were similar to the one
reported.16 After the addition of MPA via thiol–yne chemistry, the
resulting BET surface area of WCMP_0.1 decreased slightly,
whilst the BET surface area of WCMP_0.4 was 40 m2 gÀ1
lower than that of the unmodified CMP. The tendency of the
decreasing micro- and total pore volumes implies the revelation
of micropores on the WCMP surface caused by the modification
(Table 1). Furthermore, the pore diameter was found to diminish
with the addition of MPA. This observation can be caused by
rearrangement of the pore structure and/or the gas diffusion effect,26
which is attributed to the thiol–yne chemistry and the associated
alkyne-to-alkene/alkane transformation.22 In WCMP_0.4, by addition
of a larger amount of MPA (0.4 eq.) than WCMP_0.1 (0.1 eq.),
Fig. 2 Conversion of furoic acid into 5-hydroxy-2(5H)-furanone. Reaction
conducted using 0.1 mg mLÀ1 catalyst loading, 100 mM of furoic acid, O2
bubbling rate was controlled to be 15 mL minÀ1. Squares: unmodified CMP,
triangles: WCMP_0.1, circles: WCMP_0.4.
c
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poly-benzothiadiazole was successfully modified via thiol–yne
chemistry using MPA, transferring a highly hydrophobic polymer
network to a highly hydrophilic material that is completely
dispersible in water. Surface modification resulted in WCMP
with lower surface area and smaller pore volume. The model
reaction chosen for singlet oxygen generation, the conversion of
furoic acid to 5-hydroxy-2(5H)-furanone in water, revealed that
the modified WCMP with higher aqueous compatibility resulted
in higher conversion of the substrate. Although we have not been
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to achieve high conversions (B90%) after 22 h employing
concentrations of only 0.1 mg mLÀ1 of the modified CMP.
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This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 2353--2355 2355