Substituent effect of conjugated microporous polymers on the photocatalytic hydrogen evolution activity

Article abstract of DOI:10.1039/c9ta13212k

The construction of a donor-acceptor (D-A) organic photocatalyst is a facile approach to improve photocatalytic performances because of the efficient separation of light-generated electrons and holes. The rational design of molecular skeletons and the selection of functional groups are of great importance to an organic photocatalyst with a high photocatalytic activity. Herein, we designed two D-A conjugated microporous polymers (CMPs) with different substituent groups of fluorine atoms and methyl groups on the acceptor unit of dibenzothiophene-dioxide (DBTDO). This study reveals that the introduction of fluorine atoms with strong electron-withdrawing ability onto the acceptor unit can further enhance the electron acceptability of the DBTDO unit, leading to the efficient separation of light-generated electrons and holes, while the introduction of methyl with electron-pushing ability decreases the electron acceptability of the acceptor unit, suppressing the separation of light-generated charge carriers. As a result, the polymer PyDF with fluorine atoms shows an excellent photocatalytic activity compared to PyDM with methyl groups. Under UV/Vis light irradiation, PyDF shows an attractive photocatalytic hydrogen generation rate of 18.93 mmol h^-1 g^-1, which is much higher than that of PyDM (3.96 mmol h^-1 g^-1), implying that the functional group acts as a vital role in the catalytic activity of polymer semiconducting photocatalysts.

Full text of DOI:10.1039/c9ta13212k

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Substituent Effect of Conjugated Microporous Polymers on the
DOI: 10.1039/C9TA13212K
Photocatalytic Hydrogen Evolution Activity
Xiaomin Gao,a Chang Shu,a Chong Zhang,a,* Wenyan Ma,a Shi-Bin Ren,b,* Feng
Wang,c Yu Chen,a Jing Hui Zeng,a and Jia-Xing Jianga,*
a
Key Laboratory for Macromolecular Science of Shaanxi Province, Shaanxi Engineering
Laboratory for Advanced Energy Technology, School of Materials Science and Engineering,
Shaanxi Normal University, Xi’an, Shaanxi 710062, P. R. China.
b School of Pharmaceutical and Materials Engineering, Taizhou University, Taizhou 318000, P. R.
China.
c
Key Laboratory for Green Chemical Process of Ministry of Education, School of Chemical
Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan 430073, P. R. China.
* E-mail: chongzhangabc@snnu.edu.cn; renshibin@126.com; jiaxing@snnu.edu.cn
ABSTRACT: The construction of a donor-acceptor (D-A) organic photocatalyst is a
facile appoach to improve the photocatalytic performances because of the efficient
separation of light-generated electrons and holes. The rational design of molecular
skeletons and the selection of functional groups are of great importance to an organic
photocatalyst with a high photocatalytic activity. Herein, we designed two D-A
conjugated microporous polymers (CMPs) with different substituent groups of
fluorine atoms and methyl groups on the acceptor unit of dibenzothiophene-dioxide
(DBTDO). This study reveals that the introduction of fluorine atoms with strong
electron-withdrawing ability onto the acceptor unit can further enhance the electron
acceptability of DBTDO unit, leading to the efficient separation of light-generated
electrons and holes, while the introduction of methyl with electron-pushing ability
decreases the electron acceptability of the acceptor unit, suppressing the separation of
light-generated charge carriers. As a result, the polymer PyDF with fluorine atoms
shows an excellent photocatalytic activity compared to PyDM with methyl groups.
Under UV/Vis light irradiation, PyDF shows an attractive photocatalytic hydrogen

Journal of Materials Chemistry A
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DOI: 10.1039/C9TA13212K
generation rate of 18.93 mmol h−1 g−1, which is much higher than that of PyDM (3.96
mmol h−1 g−1), implying that the functional group acts as a vital role on the catalytic
activity of polymer semiconducting photocatalysts.
Keywords: Conjugated microporous polymers; organic semiconducting
photocatalysts; functional groups; hydrogen evolution
1. Introduction
Considering the ever-increasing energy demands, it is urgent and essential to
develop sustainable energy systems for alleviating the dependence of fossil fuels and
addressing the environmental issues. Hydrogen energy, as a kind of pollution-free
clean energy, has been considered as an ideal energy system in future, and
photocatalytic water splitting has been emerging as a valuable approach to generate
hydrogen by the utilization of solar energy.1,2 Since the discovery of TiO2
photocatalyst,3 there have been significant advances in the development of inorganic
semiconductors photocatalysts.4-7 In recent years, organic polymeric semiconductors
have also attracted much attention as photocatalysts for hydrogen evolution, promoted
partially by the graphitic carbon nitride (g-C3N4)8 and conjugated microporous
polymers (CMPs)9,10 based photocatalysts. Owing to the synthetic diversification and
plentiful building blocks, it is facile to tune the photophysical properties and
electronic structure of organic polymeric photocatalysts by synthetic control. An
important advantage of organic polymeric photocatalysts might be the potential to
introduce various chemical functions into the skeleton that facilitate the improvement
of photocatalytic activity.10-13 To date, a range of organic polymeric
semiconductor-based photocatalysts have been developed for hydrogen evolution by
various design strategies and synthetic methodologies. These mainly include linear

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DOI: 10.1039/C9TA13212K
conductive polymers (CPs),14-16 three-dimensional CMPs,9,17-23 crystalline covalent
triazine frameworks (CTFs),24,25 and periodic ordered covalent organic frameworks
(COFs) photocatalysts.26,27 However, most of these works are concentrated on the
effect of the nature of polymeric backbone on the photocatalytic activity. In
comparison, the influence of substituent groups on photocatalytic activity of
polymeric semiconductor photocatalysts is much less explored,28 although it is an
efficient and universal strategy to tune the electronic and physical properties of
organic semiconducting materials.
We and others demonstrated that designing donor-acceptor (D-A) type organic
photocatalysts is a facile protocol to enhance the photocatalytic activity because of the
efficient separation of light-induced electrons and holes.17,29-33 For example, Yu et al.
synthesized a series of D-A CMP photocatalysts and studied the influence of
electronic properties on the photocatalytic performance.34 He et al. achieved a
hydrogen evolution rate (HER) of 1000 mol h−1 g−1 by constructing a D-A triazine
polymeric photocatalyst.35 Liu et al. prepared a linear polymer consiting of fluorene
donor unit and dibenzothiophene-dioxide (DBTDO) acceptor unit, which showed a
high HER of 5.04 mmol h−1 g−1.36 Tan et al. synthesized a range of D-A1-A2 covalent
triazine frameworks, which exhibited a high HER of 19.3 mmol h−1 g−1.37 We
achieved a high HER of 12986 mol h−1 g−1 by the combination of pyrene donor and
DBTDO acceptor units.38 All of these organic polymeric photocatalysts neither
contain any substituent group nor functional side group. To futher reveal the
correlation between the chemical structure and photocatalytic performance, it is
highly encouraged to enrich the molecular structures of organic polymeric
photocatalysts by the modification of surface chemistry or the introduction of
side-group substitutes.

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DOI: 10.1039/C9TA13212K
With this in mind, we designed here two D-A CMP photocatalysts with different
side-group substitutes of fluorine atoms and methyl groups on the DBTDO acceptor
unit to insight into the effect of substituent groups on photocatalytic activity. It was
found that the introduction of fluorine atoms with strong electron acceptability onto
DBTDO unit can further enhance the electron-withdrawing ability of DBTDO, which
accelerates the separation of light-induced electrons and holes. In contrast,
introducing methyl groups with electron-pushing ability decreases the electron
acceptability of DBTDO unit, suppressing the separation of photo-induced charge
carriers. Therefore, an attractive photocatalytic HER of 18.93 mmol h−1 g−1 under
UV/Vis light irradiation was obtained by the polymer PyDF with fluorine atoms,
which is much higher than that of PyDM (3.96 mmol h−1 g−1). This result
demonstrates that introducing substituent group with strong electron acceptability
onto the acceptor unit is an effective approach to improve the photocatalytic activity
of polymer photocatalysts.
2. Experimental
Chemicals
4,4’-Difluorodiphenyl sulphone, di-p-tolyl sulfone, anhydrous copper (II) chloride,
n-butyllithium
(2.0
M
in
hexanes),
N-bromosuccinimide
and
(NBS),
tetrakis(triphenylphosphine)palladium(0)
(Pd(PPh3)4)
anhydrous
N,
N-dimethylformamide (DMF) were obtained from Sigma-Aldrich or J&K Scientific
Ltd.
The
comonomers
of
1,3,6,8-tetrakis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pyrene,9
2,8-difluoro-3,7-dibromo-dibenzothiophene-S,S-dioxide,
and
in
2,8-dimethyl-3,7-dibromo-dibenzothiophene-S,S-dioxide
accordance the reported methods.39,40
were
prepared

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Journal of Materials Chemistry A
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DOI: 10.1039/C9TA13212K
Synthesis of organic polymer photocatalysts
Synthesis of PyDF: Under N2 protection, K2CO3 solution (2 mL, 2 M) was added
into the solution of 1,3,6,8-tetrakis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)
pyrene (353 mg, 0.5 mmol), 2,8-difluoro-3,7-dibromo-dibenzothiophene-S,S-dioxide
(410 mg, 1.0 mmol) and Pd(PPh3)4 (10 mg) in anhydrous DMF (20 mL). The reaction
mixture was stirred at 150 ℃ for two days. The relulting precipate was obtained by
filtration after the temperaure cooled to ambient temperature. Afterwards, the
resulting polymer was thoroughly washed with ethyl alcohol, deionized water,
tetrahydrofuran and dichloromethane, respectively. After drying in vacuum at 70 ℃
for 24 h, PyDF was obtained as yellow solid powder. Anal. Calcd for (C40H14O4S2F4)n
(%): C 68.80; H 2.02; S 9.18. Found: C 59.23; H 4.56; S 8.14; Pd 0.000327 wt%
obtained from inductively coupled plasma atomic emission spectroscopy (ICP-MS).
Synthesis of PyDM: Under N2 protection, K2CO3 solution (2 mL, 2.0 M) was added
into the solution of 1,3,6,8-tetrakis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)
pyrene (353 mg, 0.5 mmol), 2,8-dimethyl-3,7-dibromo-dibenzothiophene-S,S-dioxide
(402 mg, 1.0 mmol) and Pd(PPh3)4 (10 mg) in anhydrous DMF (20 mL). The reaction
system was stirred at 150 ℃ for 48 h. The relulting precipate was obtained by
filtration after the temperaure cooled to ambient temperature. Afterwards, the
resulting polymer was thoroughly washed with ethyl alcohol, deionized water,
tetrahydrofuran and dichloromethane, respectively. After drying in vacuum at 70 ℃
for 24 h, PyDM was obtained as green solid powder. Anal. Calcd for (C44H26O4S2)n
(%): C 77.40; H 3.84; S 9.37. Found: C 65.67; H 4.48; S 8.87; Pd 0.000276 wt%
obtained from ICP-MS.
Photocatalytic experiments
The photocatalytic hydrogen generation experiments of the polymer

Journal of Materials Chemistry A
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photocatalysts were performed on a set of photocatalytic equipment (BeijingDPOeI:r1f0e.1c03t9/C9TA13212K
Light Co.). Generally, the as-synthesized polymer of 25 mg was suspended in a
mixture of water and triethanolamine (water/TEOA= 80/20) in a 250 mL reactor. The
mixture was ultrasonicated for 30 min to form a well-dispersed polymer suspension.
Then, the reaction flask was connected to the photocatalytic equipment. After
degassing the photocatalytic system to remove air, the reaction mixture was
illuminated vertically by a 300 W Xe lamp with stirring in vacuum. Circulating
cooling water was used to keep the photocatalytic reaction temperature at 20 °C. A
420 nm filter was used to filter out ultraviolet light to obtain the visible light
irradiation. The produced hydrogen was measured online by a gas chromatograph.
O
B
O
B
O
O
O
O
B
O
B
O
O
O
O
O
S
S
Br
Br
Br
Br
F
Me
Me
F
Me
Me
F
O
S
O
O
Me
S
O
F
F
S
O
S
O
O
O
Me
F
F
Me
O
S
O
S
O
O
F
Me
O
S
F
O
S
Me
O
O
PyDF
F
Me
PyDM
Scheme 1. The preparation routes for PyDF and PyDM polymers.

Page 7 of 21
Journal of Materials Chemistry A
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DOI: 10.1039/C9TA13212K
3. Results and discussion
The polymer photocatalysts of PyDF with fluorine atoms and PyDM with methyl
groups were obtained by Pd-catalyzed Suzuki polymerization reaction from
1,3,6,8-tetrakis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pyrene and DBTDO with
fluorine atoms and methyl groups as side-group substitutes, respectively (Scheme 1).
The measurements of Fourier transform infrared spectroscopy (FT-IR) and solid-state
1H-13C cross-polarization magic-angle spinning (CP-MAS) NMR were carried out to
confirm the polymer structures. As shown in Fig. 1a, the peak at around 1603 cm1 as
observed in the FT-IR spectra corresponds to the typical stretching vibration of
aromatic skeletons. In addition, the peak located at 1135 cm-1 should be attributed to
13
the asymmetrically stretching vibration signal of sulphone group (O=S=O). The C
CP-MAS NMR spectra demonstrated the signals of aromatic carbons in the range of
126-144 ppm, while PyDM shows much more distinguished peaks compared with
PyDF (Fig. 1b). The signal of methyl group was obviously observed at 20 ppm for
PyDM, and the weak signal peak at 154 ppm for PyDF could be attributed to the
carbon bonding with fluorine atom. Owing to the highly crosslinked polymer
structures and the rigid skeletons, both PyDF and PyDM are insoluble in conventional
organic solvents, and exhibit highly thermal stability (Fig. 1c). There is no any
obvious diffraction signal in the PXRD profiles of the polymers (Fig. S1), which
implys the amorphous nature of the polymers. Nitrogen sorption curves demonstrated
that both polymers are highly porous with very similar nitrogen adsorption-desorption
isotherms (Fig. 1d). The calculated specific surface areas are 825 and 827 m2 g1 for
PyDF and PyDM (Table S1), respectively, demonstrating that the substitute of
fluorine atoms or methyl groups has a negligible effect on the surface area and pore
size (Fig. S2). However, the polymers show different morphologies as revealed by the

Journal of Materials Chemistry A
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DOI: 10.1039/C9TA13212K
scanning electron microscopy (SEM) images. PyDF shows a mesh-like morphology
with rough surface, while a nanoparticles morphology with smooth surface was
observed for PyDM (Fig. 2).
F
(b)
(a)
PyDF
a
c
F
O
S
d
b
c
c
O
d
d
e
b
PyDF
a
CH3
S
PyDM
a
CH3
c
b
d
d
O
e
c
c
O
d
1603
b
PyDM
a
1135
1298
3500 3000 2500 2000 1500 1000 500
-1
150
100
50
0
Chemical shift (ppm)
Wavenumber (cm
)
500
400
300
200
100
0
PyDF
PyDM
100
80
60
40
20
0
(c)
(d)
PyDF
PyDM
0
100 200 300 400 500 600 700 800
Temperature (oC)
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/Po)
Fig. 1. (a) FT-IR spectra of PyDF and PyDM. (b) The solid-state 13C CP-MAS NMR
spectra of PyDF and PyDM. (c) Thermogravimetric analysis trace for the PyDF and
PyDM under a nitrogen atmosphere with a heating rate of 10 ℃ min1. (d) Nitrogen
adsorption/desorption isotherms.
(a)
(b)
500 nm
500 nm
Fig. 2. SEM images of PyDF (a) and PyDM (b).

Page 9 of 21
Journal of Materials Chemistry A
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DOI: 10.1039/C9TA13212K
(b)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
(a)
0.8
0.6
0.4
2.24 eV
2.46 eV
PyDF
PyDM
0.2
0.0
PyDF
PyDM
300
400
500
600
700
800
1.5
2.0
2.5
E (eV)
3.0
3.5
Wavelength (nm)
10000
1000
100
10
5000
4000
3000
2000
1000
0
(c)
(d)
PyDF
PyDM
PyDF
PyDM
PyDF
PyDM
1
450
500
550
600
650
700
0
10
20
30
40
50
Wavelength (nm)
Time (ns)
1.0
0.8
0.6
0.4
0.2
0.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
LUMO
PyDF
PyDM
(f)
(e)
-1.00
-0.97
H+/1/2H2
HOMO
1.27
1.46
PyDM
PyDF
0
50
100
150
Time (s)
Fig. 3. (a) UV/Vis absorption spectra of the polymers. (b) Taucplots of the
Kubelka−Munk function vs energy. (c) PL spectra of the polymers (inset: photographs
of the polymers under irradiation with UV light). (d) Photoluminescence decay traces
of the polymers. (e) HOMO and LUMO band positions. (f) Transient photocurrent of
the polymers measured in 1.0 M Na2SO4 aqueous solution.
To study the optical characteristics of the polymeric photocatalysts, UV/Vis
absorption spectrum and photoluminescent (PL) emission spectrum were measured in
the solid state powder due to their insoluble nature. Both polymers exhibit a strong

Journal of Materials Chemistry A
Page 10 of 21
light absorption ability in a broad visible light range, while PyDF shows an obviouVsiew Article Online
DOI: 10.1039/C9TA13212K
red-shifted absorption onset by around 50 nm compared with PyDM (Fig. 3a),
indicating that PyDF has a higher conjugation degree along the polymer chain than
PyDM since the less steric effct of fluorine atom than methyl group enhances the
coplanarity of the polymeric chain and improves the electron delocalization along
the polymer skeleton in PyDF. As a result, PyDF exhibits a smaller optical band gap
(Eg) of 2.24 eV than PyDM (2.46 eV) (Fig. 3b). Additionally, PyDF also shows a
narrower band gap in comparison with our previously reported polymer PyDOBT-1
(2.37 eV) without any substituted group,38 while PyDM exhibits a broader band gap
compared with PyDOBT-1, indicating that the introduction of fluorine atom could
extend the light absorption range of the polymer photocatalyst. The PL emmision
spectrum demonstrated that PyDF also exhibits a red-shifted PL emmision peak at
559 nm compared to PyDM (507 nm), while a lower PL emission intensity was
observed for PyDF (Fig. 3c), implying that much more photo-induced electrons could
be involved in photocatalysis reaction rather than recombination with holes through
fluorescent radiation, which could be benefit for the improvement of photocatalytic
activity. As revealed by the time-resolved fluorescence decay spectrum (Fig. 3d),
PyDF exhibits a shorter excited state lifetime (τ1 = 0.6701 s, τ2 = 0.3899 s) than PyDM
(τ1 = 1.1007 ns, τ2 = 0.6743 ns) (Table S2), implying a lower inactivation possibility
of photo-induced excited state electrons in PyDF. The measurement of cyclic
voltammogram (CV) was further performed to obtain the energy band positions of the
polymer photocatalysts (Fig. S3). Both polymers show high enough the lowest
unoccupied molecular orbital (LUMO) level to drive the water reduction half-reaction
with a slight difference in the LUMO level, while PyDF shows a much higher the
highest occupied molecular orbital (HOMO) level than PyDM (Fig. 3e). These results

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DOI: 10.1039/C9TA13212K
reveal that the electronic perpoties and energy band structures of conjugated
microporous polymers could be facilely tuned by the side-group substitutes.
Photocurrent measurement further demonstrated that much stronger photocurrent
response was observed for PyDF than PyDM under UV/Vis light irradiation (Fig. 3f),
implying that much more photo-induced excitons can be generated in PyDF due to its
broader light absorption range compared to PyDM. All of these results indicate that a
higher photocatalytic activity could be expected by PyDF.
20
15
10
5
70
60
50
40
30
20
10
0
(a)
(b)
PyDF
PyDM
PyDF
PyDM
0
0
1
2
3
4
0
1
2
3
4
Time (h)
Time (h)
(c)
5
4
3
2
1
0
1.0
0.8
0.6
0.4
0.2
0.0
80
60
40
20
0
(d)
PyDF
PyDM
0
1
2
3
4
350
400
450
500
550
600
Time (h)
Wavelength (nm)
Fig. 4. (a) Hydrogen generation of the bare polymers (25 mg) under visible light (λ >
420 nm). (b) Hydrogen generation of the polymers with 3 wt% Pt under visible light
(λ > 420 nm). (c) Hydrogen generation of the polymers with 3 wt% Pt under UV/Vis
light (λ > 300 nm). (d) The AQY plots and UV/Vis absorption spectrum of PyDF.
To estimate the photocatalytic activity of the two developed polymers,
photocatalytic hydrogen generation experiment was carried out by employing PyDF
and PyDM polymer powder as photocatalysts in deionized water with a sacrificial

Journal of Materials Chemistry A
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hole-scavenger of triethanolamine (TEOA, 20 v.%). As depicted in Fig. 4a, DbOoI:t1h0.1o0V3fi9ew/CA9rtTicAle13O2n1li2nKe
the bare PyDF and PyDM without loading Pt co-catalyst show a high photocatalytic
activity under visible light with a steady hydrogen production along the photocatalytic
reaction time, while the polymer PyDF with fluorine atom shows a higher HER of
4.09 mmol h−1 g−1 than PyDM (1.28 mmol h−1 g−1) with methyl groups owing to the
higher photo-induced charge transmission ability and separation efficiency of PyDF
than PyDM, as verified by the broader light absorption range, the weaker PL intensity
and the stronger photocurrent response of PyDF discussed above. The energy
dispersive X-ray spectroscopy (EDX) spectra showed the presence of C, O and S
elements for both polymers, and F element in PyDF with fluorine atoms (Fig. S4),
while the signal of Pd from the residue of Pd(PPh3)4 catalyst could not be detected for
both polymers, possibly due to the ultra-low Pd content as revealed by ICP-MS
measurement (0.000327 and 0.000276 wt% for PyDF and PyDM, respestively) (Fig.
S5). Such low Pd content implies the negligible influence of Pd residue on the
hydrogen generation. However, the photocatalytic performance of the polymers could
be considerably improved by loading Pt as a co-catalyst. After optimizing the Pt
content in the hydrogen evolution reaction, 3% Pt was utilized as cocatalyst for the
potocatalytic merasurements (Fig. S6). The photocatalytic HER of PyDF loaded with
3 wt% Pt was increased to 13.47 mmol h−1 g−1 upon visible light (Fig. 4b), and further
to 18.93 mmol h−1 g−1 upon broadband light (Fig. 4c). This could be attributed to the
enhanced photo-generated electron migration from the polymeric photocatalysts to the
Pt nanoparticles surface due to the formation of heterojunctions between polymer and
Pt,30,36 demonstrating the significantly positive role of Pt in the photocatalytic activity.
A maximal apparent quantum yield (AQY) of 4.5% at 400 nm was obtained by PyDF,
which varies with the absorption ability of the polymer to incident light (Fig. 4d). This

Page 13 of 21
Journal of Materials Chemistry A
result matches well with the characteristic of the UV/Vis absorption spectrum oVfiew Article Online
DOI: 10.1039/C9TA13212K
PyDF, implying that the light absorption ability of organic polymer photocatalysts
affects significantly the photocatalytic performance.
We need to point out that the HER of 13.47 mmol h−1 g−1 for PyDF under visible
light exceeds that of most polymeric photocatalysts under the similar photocatalytic
conditions (Table S3), e.g. the linear polybenzothiadiazole polymer of B-BT-1,4
(1160 μmol h−1 g−1),17 the crosslinked CMP of DBTD-CMP1 (4600 μmol h−1 g−1),32
the CTF photocatalyst of CTF-HUST-2 (2647 μmol h−1 g−1),24 the linear
DBTDO-containing conjugated polymer of Flu-SO (6.84 mmol h−1 g−1),36 and the
sulfone-containing covalent organic framework of FS-COF (10.1 mmol h−1 g−1).27 A
direct comparison of the two polymers with PyDOBT-1 that we previously reported38
demonstrated that PyDF has a much higher HER (13.47 mmol h−1 g−1) than
PyDOBT-1 without any substituted group (8.52 mmol h−1 g−1) at visible light
condition, while PyDM shows a lower HER (1.34 mmol h−1 g−1) than PyDOBT-1,
implying the significant effect of substituted group on the photocatalytic hydrogen
evolution activity.

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DOI: 10.1039/C9TA13212K
HOMO
LUMO
PyDF
PyDM
Fig. 5. HOMO and LUMO orbitals distribution of the polymers from DFT
calculation.
To further disclose the influence of substituted group on the photocatalytic
performance, density functional theory (DFT) calculation was performed by using
Gaussian 09 at the B3LYP/6-311G (d, p) level. The simulation results demonstrated
that the HOMO orbitals are highly concentrated on the pyrene segment for both
polymers, while distinct difference in the LUMO orbital distribution was observed.
The LUMO orbital distribution mainly spreads on the DBTDO acceptor unit in PyDF
because of the strong electron-withdrawing ability of fluorine atom. However, the
LUMO orbitals of PyDM mostly distribute on the electron-donor pyrene unit (Fig. 5).
The expanded LUMO orbitals distribution on DBTDO in PyDF implies the efficient
separation of photo-induced electrons and holes and much more photocatalytic active
sites could be produced on the DBTDO unit, promoting the photocatalytic activity. In
addition, PyDM exhibits a more contorted polymer skeleton with a much higher

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dihedral angle of 74.9° between the adjacent pyrene and DBTDO units than
PyDOBT-1 (67.3°) without any substituted group (Fig. S7), 38 which can be attributed
to the large steric hindrance of methyl. Interestingly, this dihedral angle decreases
from 67.3° for PyDOBT-1 to 61.1° for PyDF after introducing two fluorine atoms
onto the DBTDO unit, indicative of the enhanced coplanarity between pyrene and
DBTDO units. This may be ascribed to the interaction between the electron-rich
pyrene unit and the electron-withdrawing fluorine atom enhances the planarity of the
polymer skeleton of PyDF. The relatively contorted polymer chain in PyDM might
block the transmission of photo-induced charge carriers, while the enhanced
coplanarity of the polymeric chain of PyDF will promote the transfer of
light-generated charge carriers, and thus leading to an improved photocatalytic
activity. According to the above results, the proposed charge transfer mechanism for
the two resulting polymer catalysts is displayed in Fig. 6. As for PyDF, the
introduction of fluorine atoms with strong electron-pulling effect further enhances the
electron-acceptor ability of DBTDO unit, promoting the rapid transfer of
photo-induced electrons from the HOMO orbital of pyrene electron-donor unit to the
LUMO orbital of DBTDO electron-acceptor unit, which leads to an efficient
separation of light-generated electrons and holes, and thus resulting in the
improvement of photocatalytic activity. In sharp contrast, the introduction of methyl
groups suppresses the charge transfer from pyrene to DBTDO units and the separation
of photo-induced charge carriers due to the electron-pushing nature of methyl group,
leading to a dramatic decrease of photocatalytic activity for the polymer of PyDM.
These results demonstrated that the introduction of an electron-pulling substituent
group into the electron-acceptor unit is an effective approach for improving the
photocatalytic preformance of organic polymer photocatalysts.

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e-
e-
e-
e-
Me
Me
F
F
S
e-
S
e-
O
O
O
O
Enhanced charge transfer from D to A
Decreased charge transfer from D to A
Fig. 6. The proposed charge transfer routes in the polymers.
To test the reproducibility of the polymer photocatalyst, two additional batches of
PyDF were also synthesized under the same reaction conditions. All the polymers
obtained from different batches show very similar photocatalytic activity for hydrogen
evolution under the identical photocatalysis conditions (Fig. S8), implying the
excellent repeatability of the polymer photocatalyst. The long-term photocatalytic
experiment demonstrated that PyDF has an obvious decrease in photocatalytic activity
in the early stage of 10 h, but it can maintain a high activity after 24 h with a stable
HER of 6.45 mmol h−1 g−1 under visible light (Fig. S9). The decrease of
photocatalytic activity could be partially attributed to the microstructure change of the
polymer as revealed by the measurements of UV/Vis, FT-IR, PL and SEM of the
recovered PyDF after photocatalytic reaction (Fig. S10-13), which could affect the
electronic structure, light absorption and charge carriers’ transmission and separation
abilities of the polymer.30
4. Conclusion
In conclusion, two D-A conjugated microporous polymer photocatalysts with
fluorine atoms and methyl groups on the acceptor unit of dibenzothiophene-dioxide
were developed. The photocatalytic experiment reveals that the photocatalytic activity
is significantly dependant on the side substituted group. It was found that the charge

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transfer and separation abilities could be further enhanced by introducing
electron-pulling substituents on the acceptor unit, thus improving the photocatalytic
performance, while an electron-pushing substituent group will lead to a decreased
photocatalytic activity. An attractive hydrogen generation rate of 18.93 mmol h−1 g−1
was achieved by PyDF with fluorine atoms substituent upon the irradiation of
broadband light. This result verifies the large influence of microstructural unit on the
photocatalytic performance of polymer photocatalysts, and provides guideline for
designing a rational polymer structure for high-efficiency organic photocatalysts.
Electronic supplementary information (ESI) available: Synthesis of the monomers,
pore size distribution, PXRD patterns, CV curves and EDX spectra See DOI:
10.1039/ b000000x/.
Conflicts of interest
There are no conflicts to declare.
Acknowledgments
This work is supported by the National Natural Science Foundation of China
(21574077 & 21304055).
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Graphical Abstract
DOI: 10.1039/C9TA13212K
Two conjugated microporous polymer photocatalysts with different substituent groups
of fluorine atoms and methyl groups on the acceptor unit of dibenzothiophene-dioxide
were developed for hydrogen evolution. PyDF with fluorine atoms shows an attractive
photocatalytic hydrogen generation rate of 18.93 mmol h−1 g−1 due to the enhanced
separation efficiency of light-generated electrons and holes.
R
O
S
O
R
H2O
H2
R
S
O
O
R
R
O
S
O
R
O
S
R
O
R= F, Me
R
HER= 18.93 mmol g-1 h-1
Substituent Effect of Conjugated Microporous Polymers on the
Photocatalytic Hydrogen Evolution Activity
Xiaomin Gao,a Chang Shu,a Chong Zhang,a,* Wenyan Ma,a Shi-Bin Ren,b,* Feng
Wang,c Yu Chen,a Jing Hui Zeng,a and Jia-Xing Jianga,*