Molecular Structural Design of Conjugated Microporous Poly(Benzooxadiazole) Networks for Enhanced Photocatalytic Activity with Visible Light

Article abstract of DOI:10.1002/adma.201502735

A simple structural design principle and band position alignment of conjugated microporous polymers for enhanced photocatalytic efficiency is presented. The valence and conduction band positions of the polymer networks can be fine-tuned by altering the su

Full text of DOI:10.1002/adma.201502735

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Molecular Structural Design of Conjugated Microporous
Poly(Benzooxadiazole) Networks for Enhanced
Photocatalytic Activity with Visible Light
Zi Jun Wang, Saman Ghasimi, Katharina Landfester, and Kai A. I. Zhang*
Sunlight is a clean and inexhaustible source of renewable
energy. Especially, the visible range of the solar spectrum
accounts for 44% of the total energy as opposed to only 3% from
the ultraviolet (UV) light. Inspired by nature’s ability to convert
solar energy to chemical potentials in photosynthesis, organic
chemists have developed a variety of visible light photocatalysts
to mimic the natural process.[1] As a result, many molecular
inorganic, transition metal-based complexes or organic dye
compounds that absorb significantly in the visible spectrum
were intensely studied to harvest solar energy and catalyze
organic photochemical reactions.[2,3] Nevertheless, there are
some intrinsic drawbacks associated with these homogeneous
systems, such as high cost, toxicity of these rare metals, as well
as limited availability and postreaction purification steps for
catalyst removal. The above-mentioned disadvantages have led
material scientists to pursue the further development of stable,
recyclable, reusable, and transition metal-free photocatalysts for
organic synthesis.
Motivated by the band position tuning methods for tran-
sition metal-based and other inorganic systems, we herein
present a simple structural design principle of conjugated
microporous polymers as pure organic, heterogeneous photo-
catalytic systems to allow the fine alignment of the valence and
conduction band levels for enhanced catalytic activity. As shown
in Scheme 1, we show that by merely altering the substitution
positions on the centered phenyl unit, which functions as the
3D center of the polymer network, the VB and CB positions
can be aligned to optimally bracket the redox potentials of indi-
vidual catalytic reactions, without changing the electron donor
and acceptor moieties in the polymer network backbone. Under
light irradiation, the photogenerated electrons and holes func-
tion as the reductive and oxidative sides of the organic polymer-
based photocatalyst, respectively. The enhanced photocatalytic
activity of the CMPs was demonstrated in the oxidative cou-
pling of amines under irradiation of a 23 W household energy
saving fluorescent light bulb.
In recent years, there have been few responses to the acute
need of heterogeneous nonmetal photocatalytic systems.[4]
Conjugated microporous polymers (CMPs), combining photo-
active π–electron backbone and microporous properties, have
recently emerged as an efficient and stable platform for hetero-
geneous visible light-promoted chemical transformations such
as molecular oxygen activation,[5] selective oxidation of organic
sulfides,[6] C C bond formation,[7] reductive dehalogenation
reaction,[8] oxidative hydroxylation of arylboronic acids,[9] visible
light-initiated free radical and cationic polymerization,[10] and
light-induced hydrogen evolution.[11]
For transition metal-based homogeneous organometallic
complexes, the redox potential can be easily adjusted by modi-
fying the metal center and surrounding ligands, in order to
meet the specific activation energies of the organic reactions.[3]
Similarly, the effective valence (VB) and conduction band
(CB) positions of inorganic heterogeneous metal photocata-
lytic systems can also be modified by the formation of hybrid
or heterostructures with selected metals to achieve enhanced
photoactivity.[12]
The chemical structures of the synthesized CMPs are dis-
played in Scheme 1. Taking a phenyl unit as the 3D center, a
well-known electron acceptor, benzoxadiazole (BO) unit was
connected via a triple bond to the phenyl unit on its 1,3,5-,
1,2,4- and 1,2,4,5-positions, forming the microporous polymer
networks B-BO-1,3,5 B-BO-1,2,4 and B-BO-1,2,4,5, respectively.
The synthetic details and characterization data are described
in the Experimental Section and Supporting Information. The
resulting polymers are insoluble in all common organic sol-
vents tested. Solid state 13C/MAS NMR spectroscopy showed
similar signals for all three CMPs between 80 and 160 ppm,
which can be assigned to the aromatic carbons. The Brunauer-
Emmett-Teller surface areas of B-BO-1,3,5, B-BO-1,2,4 and
B-BO-1,2,4,5 were measured to be 474 m2 g−1, 475 m2 g−1, and
378 m2 g−1, with pore volumes of 0.31 cm3 g−1, 0.37 cm3 g−1,
and 0.20 cm3 g−1, respectively. Similar pore diameters of round
1.5 nm were determined for all three polymers (Table S1
and Figures S1 and S2, Supporting Information). Thermal
gravimetric analysis measurements showed a high thermal
stability of B-BO-1,3,5 and B-BO-1,2,4 up to ≈350 °C, while
B-BO-1,2,4,5 showed a lower thermal stability of up to ≈200 °C
(Figure S3, Supporting Information). The Fourier transform
infrared (FTIR) spectra of the polymers are displayed in Figure
S7–S9 (Supporting Information). the signal at 1620 cm−1 can
be assigned to the C N stretching mode of the BO unit of
the polymer networks. Signals between 1370 and 1440 cm−1
indicate the skeleton vibration of the aromatic rings in the poly-
mers. A C C stretching mode at1600 cm−1 was observed. All
Z. J. Wang, S. Ghasimi, Prof. K. Landfester,
Dr. K. A. I. Zhang
Max Planck Institute for Polymer Research
Ackermannweg 10, 55128 Mainz, Germany
E-mail: kai.zhang@mpip-mainz.mpg.de
DOI: 10.1002/adma.201502735
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Scheme 1. Geometry design principle of valence and conduction band position modification of conjugated microporous poly(benzooxadiazole) net-
works by altering the substitution position on the 3D center.
networks showed the typical C C stretching mode at about
2200 cm−1.
Cyclic voltammetry measurements revealed lowering poten-
tials during the reductive and oxidative cycles within the
polymer series (Figure 2b and Figures S14–S16, Supporting
Information), implying different VB and CB positions of the
polymer series. B-BO-1,3,5 exhibited VB and CB at +1.55 V and
−1.19 V. In comparison, the VB and CB positions of B-BO-1,2,4
and B-BO-1,2,4,5 were determined to be at +1.45 V/−1.13 V and
+1.14 V/−0.89 V, respectively. This is comparable to the redox
potentials of established organometallic photocatalysts such as
[Ru(bpy)3]3+ ( +1.29 V) and [Ru(bpy)3]2+ ( −0.81 V). The electron
paramagnetic resonance (EPR) spectra showed a large increase
in the intensity B-BO-1,3,5 under light irradiation, whereas
B-BO-1,2,4 and B-BO-1,2,4,5 barely exhibited an enhanced
signal (Figure S11–S13, Supporting Information). This prob-
ably indicates that longer living electron–hole pairs were gener-
ated in B-BO-1,3,5 under light irradiation compared to those of
other CMPs.
The scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) images of the three CMPs illus-
trated different morphologies at submicron scale (Figure 1). In
particular, B-BO-1,3,5 appeared as fiber-like structures with a
diameter of less than 100 nm (Figure 1a,d), whereas a porous
flake-like shape was observed for CMP B-BO-1,2,4 (Figure 1b,e)
and B-BO-1,2,4,5 showed a fused irregular particle-like struc-
ture (Figure 1c,f). This formation mechanism could stem from
the steric effect inside the polymer network due to different
substitution positions of their 3D center.
In Figure 2a, the UV/Vis diffuse reflectance (DR) spectra
of the CMPs are displayed. B-BO-1,3,5 show a broad absorp-
tion range until 800 nm, while B-BO-1,2,4 and B-BO-1,2,4,5
absorb significantly up to the near-infrared region, indi-
cating a decreasing optical band gap within the CMP series.
This also corresponds to the color change in the CMP series,
from yellowish, brown to almost black for B-BO-1,3,5, B-BO-
1,2,4 and B-BO-1,2,4,5, respectively (Figure S10, Supporting
Information).
It was previously shown that conjugated macromolecular
systems could mediate the electron transfer from N,N,N′,N′-
tetramethyl-phenylenediamine (TMPD) (Eoxi = 0.12 V vs satu-
rated calomel electrode (SCE)) to molecular oxygen, resulting
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side.[13] As shown in Figure 3a, the intensity variations in UV/
Vis absorption spectra of the resulting blue-color species veri-
fied the superior photooxidative activity of B-BO-1,3,5 com-
pared to B-BO-1,2,4 and B-BO-1,2,4,5, which results from the
highest oxidation potential of 1.55 V and reduction potential of
−1.19 V of B-BO-1,3,5 (Figure 2b). Previously, we reported that
CMPs could generate singlet oxygen species (1O2) under light
irradiation, using triplet electrons in the excited state.[14] Here,
spin trap EPR experiments were conducted using 5,5-dimethyl-
1-pyrroline N-oxide (DMPO) and 2,2,6,6-tetramethylpiperidi-
nyloxyl (TEMPO) as superoxide and singlet oxygen trapping
agents, respectively. As shown in Figure 3c,d, both active oxygen
species O2•− and 1O2 could be determined, with B-BO-1,3,5 gen-
erating the most intense signals among the three polymers.
To further determine the feasibility of CMPs and confirm
the superior catalytic activity of B-BO-1,3,5 in visible light-pro-
moted photocatalytic chemical transformation reactions, the
oxidative coupling reaction of benzylamines was chosen as a
model reaction. The three CMPs were employed under exactly
the same reaction conditions to catalyze the photooxidative
coupling of benzylamine and its derivatives bearing electron-
donating ( OCH3) and electron-withdrawing ( F) moieties
(Table 1, entries 1–3). As expected, the reactions catalyzed with
B-BO-1,3,5 achieved significantly higher conversions (entry
1) than the other two CMPs in all three cases. This further
demonstrates that 3D network structure with 1,3,5-substitu-
tion on the center phenyl unit delivers the highest reduction
and oxidation potentials and therefore the most efficient pho-
tocatalyst. Terao et al. reported structurally defined conjugated
polymers and demonstrated that significantly higher charge
mobility can be attained by simply changing the configuration
of poly(phenylene–ethynylene) from linear (para, via 1,4-pos-
itions) to zigzag (meta, via 1,3-positions) chain through the
phenyl unit.[15] This could explain the superior photocatalytic
performance of B-BO-1,3,5.
Figure1. SEMimagesofa)B-BO-1,3,5, b)B-BO-1,2,4, andc)B-BO-1,2,4,5,
and TEM images of d) B-BO-1,3,5, e) B-BO-1,2,4, and f) B-BO-1,2,4,5.
in blue-colored cationic radical and the activated oxygen species
superoxide (O2•−) (Ered = 0.86 V vs SCE) under light irradiation,
using the photogenerated hole inside the conjugated system as
the oxidative side and the photoexcited electron as reductive
To further investigate the reaction mechanism and the spe-
cific roles of the photogenerated electron–hole pairs during
oxidative coupling reaction of benzylamines, we extended
the study by conducting control experiments and adding diff-
erent hole and electron scavengers into the reaction mixture
(Table 1). From entries 4–6, it could be concluded that all three
components, B-BO-1,3,5 as photocatalyst, light, and oxygen, are
indispensable for achieving high conversion. By adding KI, i.e.,
a hole scavenger, a reduced conversion of 14% was obtained
(entry 7). By adding benzoquinone as superoxide scavenger,
a conversion of only 5% was found (entry 8). Using sodium
azide as a singlet oxygen scavenger led to a conversion of 24%
(entry 9). All these results indicate that both activated oxygen
species (e.g., superoxide and singlet oxygen) took part in the
oxidative coupling reaction of benzylamine, with superoxide
playing a more significant role. Based on our observations from
the screening and control experiments, we propose a modified
reaction mechanism similar to the literature.[16] As displayed in
Figure 3b, under light irradiation, benzylamine loses an elec-
tron to the photogenerated hole of the CMP and then forms
its cationic radical intermediate, which reacts with both photo-
generated active oxygen species to form the intermediate
hydroperoxy (phenyl)methanamine. During the monitoring
experiment (Figure S18, Supporting Information), we did not
Figure 2. a) UV/Vis DR spectra. b) Valence band (VB) and conduction
band (VB) positions of the CMPs and redocx potentials of some sub-
strates such as benzylamine (BA) and molecular oxygen into superoxide
used in the photocatalytic reaction determined via cyclic voltammetry.
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Figure 3. a) UV/Vis spectra and photograph of the cationic radical of TMPD (100 × 10−3 M in 3 mL CH3CN) after photoinduced oxidation by CMPs
(2 mg mL−1) after 1 h under a blue LED in air. Inset: i) B-BO-1,3,5, ii) B-BO-1,2,4, iii) B-BO-1,2,4,5. b) Proposed reaction mechanism of the photo-
catalytic oxidative coupling of benzylamine. ISC: intersystem crossing. c) ESR spectra of DMPO–O2•−, and d) TEMPO–1O2 adducts for deferent CMPs
under light and dark conditions.
Table 1. Oxidative coupling of benzylamines using the CMPs as photocatalyst.
Entrya)
CMP
O2
Light
Additive
Conversionb) [%]
R = H
48
R = F
28
R = OCH3
15
1
B-BO-1,3,5
B-BO-1,2,4
B-BO-1,2,4,5
−
+
+
+
+
+
−
+
+
+
+
+
+
−
+
+
+
−
2
17
8
6
−
3
13
5
4
−
4c)
5d)
6e)
7f)
8g)
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
−
<5
<1
<1
14
B-BO-1,3,5
B-BO-1,3,5
B-BO-1,3,5
B-BO-1,3,5
−
−
Hole scavenger
Superoxide
scavenger
5
9h)
B-BO-1,3,5
B-BO-B
Singlet oxygen
scavenger
24
68
n.d.
n.d.
n.d.
n.d.
+
+
10i)
+
+
−
a)All experiments were conducted with 1 mmol of substrate, 6 mg of CMP in 3 mL CH3CN under the irradiation of blue LED (460 nm, 1.2 W cm−2) for 3 h (1 atm O2, room
temperature). b)Conversion determined by 1H NMR and GCMS, n.d. indicates not detected. c)Without B-BO-1,3,5, under light, 1 atm O2. d)With B-BO-1,3,5, no light, 1 atm
O2. e)With B-BO-1,3,5, under light, no O2. f)KI as hole scavenger. g)Benzoquinone (BQ) as superoxide scavenger. h)NaN3 as singlet oxygen scavenger. i)B-BO-B as homoge-
neous catalyst, concentration: 2 mg mL−1 in CH3CN, blue LED, 3 h.
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Table 2. Screening experiments of the oxidative coupling of benzyl-
amines using B-BO-1,3,5 as photocatalyst.
observe any evidence of benzaldehyde as a possible interme-
diate after the elimination of water and ammonia from hydro-
peroxy (phenyl)methanamine, we thus propose that it could
form directly the imine species after elimination of H2O2,[17]
which then reacts with an additional benzylamine molecule,
forming N-benzyl-1-phenylmethanediamine as intermediate.
Interestingly, after reaching the full conversion of the cata-
lytic reaction, benzaldehyde was determined (Figure S18, Sup-
porting Information). This indicates that benzaldehyde is
probably not an active intermediate, but an overoxidized side
product after the reaction. After elimination of ammonia from
the intermediate N-benzyl-1-phenylmethanediamine, the final
product is obtained. Both photogenerated electrons and holes
actively participate in the photooxidative coupling of amines.
The reductive (−1.19 V) and oxidative (1.55 V) potentials of
B-BO-1,3,5 are sufficiently high to reduce molecular oxygen
into its active species and oxidize benzylamine into its cationic
radical form (Eoxi = +1.08 V) (Figure 2b). In comparison, the
lower reductive (−0.89 V) and oxidative (1.14 V) potentials of
B-BO-1,2,4,5 are rather insufficient for both redox processes.
A small molecule, 4,7-bis(phenylethynyl)benzo[c][1,2,5]-oxa-
diazole (B-BO-B) as a soluble and homogeneous analog was
employed for the same photocatalytic reaction (entry 10).
A higher conversion of 68% under the same reaction con-
ditions was recorded. However, after the reaction, a clear
decrease of the absorption maximum by ≈30%, the so-called
photobleaching effect, was observed (Figure S19, Supporting
Information). In comparison, the heterogeneous counterpart
B-BO-1,3,5 could be used for another five repeating cycles
without significantly losing its catalytic efficiency (Figure S20,
Supporting Information).
Entrya)
11
Substrate
Product
Conversionb)
[%]
>99
12
13
97
82
14
15
16
60
22
8
17
83
a)Reaction conditions: B-BO-1,3,5 (6 mg), substrate (1 mmol),
1 atm O2,
3 mL CH3CN, 23 W energy saving fluorescent light bulb, 24 h, water bath, room
temperature. b)Conversion data were obtained by 1H NMR.
study, for optimizing the polymer networks for high photocata-
lytic efficiency, molecular modeling would be helpful in guiding
research directions and predicting experimental results. More
theoretical studies will be conducted in future projects. Addition-
ally, further studies on different 3D center molecules for more
versatile structural design are on the way.
A number of benzylamine derivatives were also investigated
to demonstrate the general applicability of B-BO-1,3,5 as photo-
catalyst (Table 2). To note, a 23 W household energy saving fluo-
rescent light bulb was used as a low-cost light source. It could
be determined that electron withdrawing groups such as F or Cl
led to higher conversions (entries 12 and 13) than the electron
donating groups such as methyl (entry 14), methoxy (entries 15
and 16). The reason could be the stabilized cationic radical by
the methyl and especially methoxy groups, which decelerated
the formation of the final product. However, this effect was not
observed with the thiophene unit (entry 17).
Experimental Section
Synthesis of B-BO-1,3,5, B-BO-1,2,4 and Synthesis of B-BO-1,2,4,5:
To a solution of 0.02 mmol (23 mg) of tetrakis(triphenylphosphine)
palladium(0), 0.02 mmol (4 mg) of CuI in 20 mL of DMF and Et3N
(vol/vol 50/50), 0.67 mmol (100 mg) of 1,3,5-triethynylbenzene,
In summary, we present a simple structural design prin-
ciple of conjugated microporous polymers as pure organic,
heterogeneous photocatalytic systems to allow the fine justifica-
tion of the photoredox potential for enhanced catalytic activity.
Via merely altering the substitution positions on the centered
phenyl unit, which functions as the 3D center of a series of
poly(benzooxadiazole) networks, the resulted valence and con-
duction band positions of the polymers can be optimally aligned
to bracket the required reductive and oxidative potentials of the
catalytic reactions, without changing the electron donor and
acceptor moieties in the polymer network backbone. It was
shown that the photocatalyst designed with the 1,3,5-substitution
positions of the centered phenyl ring demonstrated the superior
photocatalytic activity in the oxidative coupling of amines. Fur-
thermore, the use of a house hold energy saving light bulb offered
a commercially cost-saving solution for the general application of
the photocatalysts. Overall this report is rather an experimental
1
mmol (278 mg) 4,7-dibromobenzo[c][1,2,5]oxadiazole for B-BO-
1,3,5 were added. To 0.67 mmol (100 mg) of 1,2,4-triethynylbenzene,
1 mmol (278 mg) 4,7-dibromobenzo[c][1,2,5]oxadiazole for B-BO-1,2,4;
0.67 mmol (117 mg) of 1,2,4,5-tetraethynylbenzene, 1.34 mmol (372 mg)
4,7-dibromobenzo[c][1,2,5]oxadiazole for B-BO-1,2,4,5 were added.
The solution was degassed for 20 min with N2 and then was heated to
80 °C for 12 h. The resulting CMPs were then rinsed with Milli-Q water
several times and extracted with 50/50 dichloromethane and methanol
solution in a Soxhlet for 24 h. The insoluble solid was then dried under
vacuum conditions overnight. The final yields were 305 mg (81%), 286
mg (75%), and 379 mg (78%) for polymers B-BO-1,3,5, B-BO-1,2,4,
and B-BO-1,2,4,5 respectively. Inductively coupled plasma spectrometry
analysis showed that all three conjugated polymers have little or trace
amounts of residual Pd (30 ppm) and Cu (20 ppm).
General Procedure for the Photooxidative Coupling of Benzylamine and
Derivatives: 6 mg of photocatalyst and 1 mmol of substrate were added
to 3 mL CH3CN in a glass vial, which was placed in a water bath in front
of the light source: a blue LED lamp (OSA Opto Lights, 1.2 W cm−2)
or a 23 W household energy saving fluorescent light bulb (Osram)
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under 1 atm O2. Samples were taken after a certain period of time and
conversions were determined by 1H NMR using CDCl3 as solvent.
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Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
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The authors acknowledge the Max Planck Society for financial support.
Jan Roth is acknowledged for support with the monomer synthesis.
Z.J.W. is a recipient of a fellowship through funding of the Excellence
Initiative (DFG/GSC 266) of the graduate school of excellence “MAINZ”
(Materials Science in Mainz). K.A.I.Z. thanks Fonds der Chemischen
Industrie, FCI for funding.
Received: June 8, 2015
Revised: July 17, 2015
Published online: September 10, 2015
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