Dibenzothiophene Dioxide Based Conjugated Microporous Polymers

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Dibenzothiophene Dioxide-Based Conjugated Microporous Polymers for Visible-Light-Driven Hydrogen Production Zijian Wang, Xiye Yang, Tongjia Yang, Yongbo Zhao, Feng Wang, Yu Chen, Jing Hui Zeng, Chao Yan, Fei Huang, and Jia-Xing Jiang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02607 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 3, 2018

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ACS Catalysis

Dibenzothiophene Dioxide-Based Conjugated Microporous Polymers for Visible-Light-Driven Hydrogen Production Zijian Wang†, Xiye Yang‡, Tongjia Yang†, Yongbo Zhao†, Feng Wang§, Yu Chen†, Jing Hui Zeng†, Chao Yanǁ, Fei Huang‡, Jia-Xing Jiang*,† †

Key Laboratory for Macromolecular Science of Shaanxi Province, Shaanxi Key Laboratory for Advanced

Energy Devices, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an, Shaanxi, 710062, P. R. China ‡

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent

Materials and Devices, South China University of Technology, Guangzhou, 510641, China §

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 ǁ

School of Materials Science and Engineering, Jiangsu University of Science and Technology, Jiangsu,

Zhenjiang 212003, China

ABSTRACT: Conjugated microporous polymers (CMPs) have gained much recent attention as a kind of metal-free organic photocatalysts for photocatalytic hydrogen generation. However, the development of a visible-light-driven CMP photocatalyst with a high photocatalytic activity is still a big challenge. Here, we report a kind of dibenzothiophene dioxide-containing CMPs photocatalysts and demonstrate the influence of the crosslinker length on the photocatalytic performances for hydrogen production. The most active photocatalyst of DBTD-CMP1 with a short crosslinker of benzene exhibits a high hydrogen evolution rate (HER) of 2460 µmol h−1 g−1 under visible light without Pt cocatalyst. Remarkably, the Pt-loaded DBTD-CMP1 shows an attractive HER of 9200 µmol h−1 g−1 under UV-Vis light illumination. This result demonstrates that these dibenzothiophene dioxide-containing CMPs are competitive with the most reported porous organic polymer photocatalysts. KEYWORDS: conjugated microporous polymers, photocatalysis, hydrogen production, 1

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planarity, apparent quantum yield INTRODUCTION Photocatalytic hydrogen evolution from water is great promising for the development of green and sustainable hydrogen energy system. The photocatalytic performances for hydrogen evolution are significantly dependant on the nature of the photocatalysts, such as the molecular structure, electronic property, band gap and surface chemistry.1-4 Therefore, it is crucial to develop efficient photocatalysts for improving the photocatalytic performances for hydrogen evolution. Besides the inorganic semiconductors and metal-based photocatalysts with high carrier mobility and catalytic functions,5-7 organic semiconductor photocatalysts for photocatalytic hydrogen evolution have also gained much recent attention because of their attractive features, such as wide visible light absorption, high carrier mobility, diverse synthetic strategy, and tunable electronic structure.8-11 Graphitic carbon nitrides (g-C3N4)12 have been widely explored for hydrogen evolution as a class of organic photocatalysts. In addition, the photocatalytic performance of g-C3N4 can be enhanced efficiently through the optimization of synthetic strategy.13-16 Recent studies demonstrated that conjugated microporous polymers (CMPs) have been emerging as another class of organic photocatalysts since their chemical and electronic structures could be easily tuned by synthetic control.3,17-20 Various design strategies have been proposed to improve the activity of CMPs photocatalysts for photocatalytic hydrogen production. For example, Cooper et al. demonstrated the systematic control of chemical composition and band gap of CMPs photocatalysts via statistical copolymerization.3 Thomas et al. synthesized the heptazine-containing donor-acceptor (D-A) polymer networks, which exhibited superior photocatalytic performance to g-C3N4 materials due to the efficient separation of light-induced charge carriers.21 Yu et al. also demonstrated that it is an efficient strategy for improving the photocatalytic activity of CMPs to construct a D-A polymer 2


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ACS Catalysis

structure.22 Wang et al. synthesized benzothiadiazole-containing CMPs and investigated the influence of the geometry of the polymer on the photocatalytic performance, and demonstrated that the linear polymer had a higher photocatalytic activity than the crosslinked polymers.23 We previously studied the relationships between the substitution patterns of the crosslinkers and the photocatalytic activity of perylene-based polymers.24,25 Very recently, we demonstrated that the CMPs photocatalysts with a D-π-A molecular structure show enhanced photocatalytic performances due to the improved light-induced charge carriers transport and separation abilities.26 However, most of the reported CMPs photocatalysts show low visible light activity. Therefore, it is highly desirable to explore a visible light active organic photocatalyst with high photocatalytic activity for an efficient photocatalytic process. In this study, a kind of dibenzothiophene dioxide-containing CMPs was developed and applied as efficient organic photocatalysts for visible-light-driven hydrogen generation. A study on the influence of the strut length of the crosslinker on the photocatalytic performances was conducted by changing the crosslinkers from benzene to biphenyl, and to p-terphenyl with four polymerizable functional groups. It was found that the photocatalytic performance of the resulting CMPs decreases with increasing the crosslinker length because the increased twisted polymer skeleton reduces the conjugation degree and planarity of the molecular main chain, which blocks the light-induced charge carriers transporting and separation. As a result, DBTD-CMP1 with the benzene crosslinker exhibits a high photocatalytic activity with an average hydrogen evolution rate (HER) of 2460 µmol h−1 g−1 under visible light (λ>420 nm) without Pt cocatalyst. Remarkably, DBTD-CMP1 loaded with 3 wt% Pt shows an attractive HER of up to 9200 µmol h−1 g−1 under broadband light irradiation

(λ>300

nm).

The

results

demonstrated

that

these

dibenzothiophene

dioxide-containing CMPs organic photocatalysts are great promising for photocatalytic hydrogen production from water.

3


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EXPERIMENTAL SECTION Chemicals 1,2,4,5-Tetrabromobenzene (M1), biphenyl, p-terphenyl, N-bromosuccinimide (NBS) and dibenzothiophene

dioxide

were

purchased

from

TCI.

3,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-5,5-dioxide

(M2),

3,5,3',5'-tetrakis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)biphenyl

(M3),

3,7-dibromo-dibenzothiophene

dioxide

(M4),

3,3'',5,5''-tetrakis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1,1',4',1''-terphenyl

and (M5),

were synthesized according to the literatures.27-30 Synthesis of the conjugated microporous polymers All the dibenzothiophene dioxide-containing CMPs were synthesized by Suzuki-Miyaura polymerization under nitrogen atmosphere. A general synthetic strategy for the polymers is given as below: The K2CO3 aqueous solution (2.0 M) was added into a mixture of the monomers and Pd(PPh3)4 in dimethylformamide (DMF). The reaction solution was degassed by bubbling with nitrogen for three times and stirred for 48 h at 150 oC. Then, deionized water was added into the resulting reaction solution after cooling to room temperature. The collected polymer by filtration was washed thoroughly with methanol, H2O, acetone and dichloromethane, respectively. The product was dried in vacuum at 80 oC overnight. DBTD-CMP1: M1 (197 mg, 0.5 mmol), M2 (468 mg, 1.0 mmol), Pd(PPh3)4 (25 mg, 21.6 µmol), DMF (20 mL), and K2CO3 aqueous solution (2.0 M, 1.5 mL) were used for the polymerization (yield: 94.1%). Anal. Calcd for (C54H26O8S4)n (%): C, 69.68; H, 2.80; S, 13.76; Found C, 64.18; H, 3.27; S, 8.84. The residual Pd catalyst was found to be 0.09 wt% from inductively coupled plasma atomic emission spectroscopy (ICP-MS). DBTD-CMP2: M3 (658 mg, 1.0 mmol), M4 (748 mg, 2.0 mmol), Pd(PPh3)4 (25 mg, 21.6 µmol), DMF (20 mL), and K2CO3 aqueous solution (2.0 M, 3 mL) were employed in this

4


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polymerization (yield: 89.6%). Anal. Calcd for (C60H30O8S4)n (%): C, 71.57; H, 2.98; S, 12.72; Found C, 71.00; H, 3.59; S, 9.51. The residual Pd catalyst was found to be 0.15 wt% from ICP-MS. DBTD-CMP3: M5 (734 mg, 1.0 mmol), M4 (748 mg, 2.0 mmol), Pd(PPh3)4 (25 mg, 21.6 µmol), DMF (20 mL), and K2CO3 aqueous solution (2.0 M, 3 mL) were used in this polymerization (yield: 79.6%). Anal. Calcd for (C66H34O8S4)n (%): C, 73.19; H, 3.14; S, 11.83; Found C, 66.48; H, 3.72; S, 6.87. The residual Pd catalyst was found to be 0.06 wt% from ICP-MS. Photocatalysis experiment The photocatalytic hydrogen generation experiment was carried out on a photocatalytic reaction cell (LabSolar-Ⅲ AG, Beijing Perfect Light Co.). In general, 50 mg polymer was dispersed in a solution of 80 mL water and 20 mL triethanolamine. The photocatalytic reaction suspension was degassed for half an hour and illuminated by a 300 W Xe lamp (Beijing Perfect Light Co.) under stirring. Visible light illumination was obtained using a long-pass cut-off filter (Beijing Perfect Light Co.). The temperature of the reaction solution was kept at 10 oC by the circulating of cool water. The produced hydrogen was detected on an online GC7900 gas chromatograph (Techcomp) with a thermal conductive detector. RESULTS AND DISCUSSION Three dibenzothiophene dioxide-containing CMPs were prepared using Suzuki−Miyaura polymerization (Scheme 1). All the CMPs are chemically stable and insoluble (e.g. in dichloromethane, tetrahydrofuran, and DMF), which is attributed to their highly cross-linked polymer structures. The CMPs also exhibit high thermal stability with a decomposition temperature of approximately 400 oC in N2 atmosphere (Figure S1). Fourier transform infrared (FT-IR) spectrum for the CMPs revealed the characteristic peak of the O=S=O group at around 1155 cm−1, and the peak at 1610 cm−1 from aromatic ring (Figure 1a), confirming 5


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the polymer structures. All the CMPs show a broad peak at 3400 cm−1, which could be attributed to the physisorbed water due to the high microporosity of the CMPs.31 The polymer structure was further confirmed by the solid state

13

C NMR (Figure 1b). All of the CMPs

show the characteristic peak of C bonding to S at ca. 144 ppm. The absent signal of borate groups (at ca. 87 ppm) indicates that the borate groups of the monomers have been consumed and the polymerization was complete. No any sharp diffraction peak could be detected in the powder X-ray diffraction profiles (Figure S2), demonstrating the amorphous structure of the resulting CMPs. Scanning electron microscopy (SEM) images showed that all of the CMPs have a nanoparticle morphology, while DBTD-CMP1 has smaller particle size compared to DBTD-CMP2 and DBTD-CMP3 (Figure S3).

Br

+ Br

O

O

Br

S

O

O

B

O O S

O S O

O

M1

DBTD-CMP1

M2

O B O

O

O

Pd(PPh3)4/K2CO3

S

+

Br

Br

O S O

S O O

O

O S O

DMF/150 oC

B O O

O B O

S O O

B

O

Br

O O B

O S O

O S

M3

M4

DBTD-CMP2

O

O O B

B O

+ O B O

O

O S Br

Br

O S O

S O O

O O S

O S O

B O O

M5

M4

DBTD-CMP3

Scheme 1. The synthesis for the CMPs and the representative structures.

6


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(a)

144

(b)

DBTD-CMP3

DBTD-CMP3

DBTD-CMP2

DBTD-CMP2

DBTD-CMP1

DBTD-CMP1

1155

1610

4000 3500 3000 2500 2000 1500 1000 500

200

150

100

50

0

Chemical shift (ppm)

-1

Wavenumber (cm )

Figure 1. (a) FT-IR spectra; (b) The solid-state 13C CP-MAS NMR spectra of the CMPs.

Nitrogen adsorption measurement revealed that all of the CMPs show a steep nitrogen adsorption at the low relative pressures (Figure 2a), indicative of the microporous nature for the polymers.32,33 The sharp nitrogen adsorption for DBTD-CMP2 and DBTD-CMP3 at high pressures implies the polymers have also some mesopores or macropores derived from the interparticle voids.34,35 The distinct hysteresis loop on the N2 desorption branch of the CMPs can be attributed to the elastic deformation or swelling behavior caused by the gas adsorption.36 All the CMPs show the similar pore size distribution with the micropore diameter at ca. 1.0 nm, while DBTD-CMP2 and DBTD-CMP3 show much more mesopores and/or macropores due to the extended strut length of the crosslinkers (Figure 2b). This is in accordance

with

the

nitrogen

adsorption

characteristic

of

the

CMPs.

The

Brunauer-Emmet-Teller surface areas are 811, 915 and 447 m2 g−1 for DBTD-CMP1, DBTD-CMP2 and DBTD-CMP3, respectively (Table S1). The lower surface area of DBTD-CMP1 than DBTD-CMP2 could be owing to the higher steric hindrance of the 1,2,4,5-tetrabromobenzene monomer, which leads to the lower crosslinked degree of the resulting polymer. While DBTD-CMP3 shows the lowest specific surface area among the three polymers, which could be attributed to its extended monomer strut length, as observed in most previously reported CMPs.17,37,38 7


700

Quantity adsorbed (cm3/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

600

(a)

DBTD-CMP1 DBTD-CMP2 DBTD-CMP3

500 400 300 200 100 0 0.0

0.2

0.4

0.6

0.8

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Differential pore volume (cm3/g)

ACS Catalysis

2.5 DBTD-CMP1 DBTD-CMP2 DBTD-CMP3

(b) 2.0 1.5 1.0 0.5 0.0

1.0

1

10

Pore diameter (nm)

Relative pressure (P/P0)

Figure 2. (a) Nitrogen adsorption (filled symbols) / desorption (open symbols) isotherms for the CMPs obtained at 77.3 K; (b) Pore size distributions of the CMPs.

The resulting CMPs show a blue-shifted tendency in the UV-Vis absorption spectrum from DBTD-CMP1 to DBTD-CMP2 and to DBTD-CMP3 with increasing the monomer strut length from phenyl to biphenyl and to p-terphenyl (Figure 3a), which could be explained by the twisted polymer structure between benzene rings in DBTD-CMP2 and DBTD-CMP3 decreases the conjugation degree of the main chain. All the CMPs have sufficient broad band gaps for water-splitting reaction (Table 1), although DBTD-CMP1 shows a narrower band gap of 2.53 eV compared to DBTD-CMP2 (2.62 eV) and DBTD-CMP3 (2.64 eV). Photoluminescent spectra revealed that the CMPs show different fluorescent emission peaks and intensities (Figure 3b). The phenyl-based polymer DBTD-CMP1 exhibits a red-shifted emission at approximately 479 nm with much lower emission intensity than that of DBTD-CMP2 and DBTD-CMP3, which endows DBTD-CMP1 with higher photocatalytic activity because of the efficient separation of light-induced charge carriers. Cyclic voltammetry (CV) was employed to further investigate the band positions of the CMPs (Figure S4−6). The CMPs show different the lowest unoccupied molecular orbital (LUMO) levels between −0.948 and −0.978 eV (Figure 3c). DBTD-CMP3 exhibits the lower LUMO

8


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energy level of −0.948 eV vs. NHE compared to DBTD-CMP1 (−0.956 eV) and DBTD-CMP2 (−0.978 eV). The time-resolved fluorescent decay spectrum demonstrated that DBTD-CMP1 has a shorter excited state lifetime than DBTD-CMP2 and DBTD-CMP3 (Figure 3d and Table S2), implying the low inactivation probability of light-induced electrons and leading to high photocatalytic activity of DBTD-CMP1.39

(a)


Fluorescence intensity (a.u.)

Absorption (a.u.)

1.0 0.8 0.6 0.4 0.2

3500

300

400

500

600

700

(b)


3000 2500 2000 1500 1000

0.0

500 0 400

800

450

-1.5 LUMO -1.0

-0.956 eV

(c) -0.978 eV

-0.948 eV

0.5

Photon counts

-0.5 0.0

H+ / 1/2H2

HOMO

1.0 1.5

1.574 eV DBTD-CMP1

2.0

500

550

600

650

Wavelength (nm)

Wavelength (nm)

Energy (V Vs. NHE)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

1.642 eV

1.692 eV

DBTD-CMP2

DBTD-CMP3

2.5

10

4

10

3

10

2

10

1

(d) DBTD-CMP1 DBTD-CMP2 DBTD-CMP3

0

10

20

30

40

50

Time (ns)

Figure 3. (a) UV-Vis absorption spectra of the CMPs; (b) Photoluminescent spectra of the CMPs (λexcitation = 365 nm); (c) The band positions of the CMPs; (d) Fluorescent decay spectra of the CMPs.

The photocatalytic experiment was initially carried out by using these bare polymers in a 4:1 mixture of water and triethanolamine (TEOA) as the hole-scavenger. All the CMPs exhibit visible light activity for hydrogen generation, as evidenced by the steady hydrogen production during the visible light (λ > 420 nm) photocatalytic process (Figure 4a). However, 9


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the photocatalytic activity is significantly related to the polymer structure, particularly to the strut length of the crosslinker. The phenyl-based DBTD-CMP1 shows a much higher average HER of 2460 µmol h−1 g−1 than DBTD-CMP2 (186 µmol h−1 g−1) and DBTD-CMP3 (116 µmol h−1 g−1) at the same conditions. The HER value of 2460 µmol h−1 g−1 for DBTD-CMP1 is superior to that of some previously reported CMPs photocatalysts without Pt coctalyst under visible light illumination.2,3,22,40 Some reports revealed that the residual Pd in the CMP photocatalysts obtained from Pd-catalyzed Suzuki-Miyaura polymerization will contribute the H2 evolution.8 However, no any Pd signal could be detected in the resulting CMPs by the energy dispersive X-ray spectroscopy (EDX) measurement (Figure S7−9), which is probably due to the low residual Pd content, as evidenced by the ICP-MS measurement. The Pd contents obtained from ICP-MS are 0.09, 0.15 and 0.06 wt% for DBTD-CMP1, DBTD-CMP2, and DBTD-CMP3, respectively. DBTD-CMP2 shows a slight higher Pd content, while DBTD-CMP1 shows the highest photocatalytic activity among the three CMPs, implying the negligible influence of Pd residue on the photocatalytic activity due to a trace of the Pd residue. This was also observed for some reported CMPs photocatalysts.3,22,23 In addition, the transmission electron microscopy (TEM) image of the bare DBTD-CMP1 showed no any Pd nanoparticles (Figure S10a), again confirming the low Pd residue. The photocatalytic activity for the Pt-deposited CMPs was further explored since Pt is an efficient cocatalyst for hydrogen generation.41-46 The photocatalytic activity of the 3 wt% Pt-modified CMPs was remarkably enhanced (Figure 4b), and an attractive HER of up to 9200 µmol h−1 g−1 was obtained on DBTD-CMP1 under broadband light irradiation. Even under visible light, DBTD-CMP1 also exhibits a high HER of 4600 µmol h−1 g−1 (Table 1, Figure S11). The improved photocatalytic activity of the Pt-modified CMPs can be explained by the loaded Pt nanoparticles, as revealed by the TEM image of DBTD-CMP1 loaded with Pt (Figure S10b), promote the migration of photogenerated electrons from the conductive 10


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ACS Catalysis

band of the CMPs to the surface of Pt nanoparticles due to the heterojunction between the polymer and Pt nanoparticles.47-49 It is noteworthy that the high HER of 4600 µmol h−1 g−1 of DBTD-CMP1 under visible light is superior to the most reported porous organic polymer photocatalysts under the same conditions, such as CTF-1-10min (1072 µmol h−1 g−1),49 TFPT–COF (1970 µmol h−1 g−1),50 N3-COF (1703 µmol h−1 g−1),51 CTF-HUST-2 (2647 µmol h−1 g−1),52 PTO-300 (3030 µmol h−1 g−1),53 and CTF-2 (296 µmol h−1 g−1).54 Table 1. Physical and photocatalytic properties of the CMPs λema

LUMOb

Egc

HERd

HERe

HERf

Polymer

(nm)

(eV)

(eV)

(µmol h−1 g−1)

(µmol h−1 g−1)

(µmol h−1 g−1)

DBTD-CMP1

479

−0.956

2.53

2460

4600

9200

DBTD-CMP2

462

−0.978

2.62

188

440

2820

DBTD-CMP3

463

−0.948

2.64

116

140

1800

a

Fluorescent emission peak for the CMPs powder; b Obtained by cyclic voltammetry; c Obtained from the UV-Vis absorption spectrum; d 50 mg photocatalyst without Pt in a 4:1 mixture of H2O/TEOA (λ > 420 nm); e 50 mg photocatalyst with 3 wt% Pt cocatalyst in a 4:1 mixture of H2O/TEOA (λ > 420 nm); f 50 mg photocatalyst with 3 wt% Pt cocatalyst in a 4:1 mixture of H2O/TEOA (λ > 300 nm).

To evaluate the photostability, the photocatalytic reaction of the most active photocatalyst of DBTD-CMP1 was carried out under visible light for 54 h with evacuation every 6 h. The polymer shows a steady photocatalytic hydrogen production for each cycle (Figure S12). Although the photocatalytic activity has an obvious decrease in the early 36 h, it almost keeps a constant afterwards and the polymer can remain a stable HER of 1830 µmol h−1 g−1 after 54 h photocatalytic reaction. After photocatalytic reaction, the recovered sample did not show any obvious change in the polymer structure (Figure S3, S13−14), indicating that DBTD-CMP1 has a long-term photostability. In addition, we have also synthesized other three batches of DBTD-CMP1 under the same conditions to test the repeatability of the polymer. The photocatalytic experiment for hydrogen generation demonstrated that all of the samples produced from different batches have very similar photocatalytic activity (Figure 4c),

11


ACS Catalysis

indicative of an excellent reproducibility. The apparent quantum yield (AQY) of DBTD-CMP1 was further investigated by using various band pass filter. DBTD-CMP1 shows a maximum AQY of 3.3% at 400 nm, and then the AQY decreases to 1.6% at 450 nm and to 0.2% at 500 nm (Figure 4d), which is in line with the absorption spectra of the CMPs showing the absorption peak at around 400 nm as shown in Figure 3a, implying that light absorption of the CMPs has a big influence on the photocatalytic efficiency.

12000 9000 6000 3000

-1

(a)


15000

Hydrogen evolution (µ mol g )

-1

60000 50000 40000 30000 20000 10000 0

0 0

1

2

3

4

5

0

6

1

2

Time (h)

3

4

5

6

Time (h) 4

70000

(c)

Batch-1 Batch-2 Batch-3 Batch-4

60000 50000

Absorption (a.u.)

-1

(b)


40000 30000 20000

(d)

1.0

2 0.5

10000 0

0.0 0

1

2

3

4

5

300

6

Time (h)

400

500

600

700

Apparent quantum yield (%)


18000


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 800

Wavelength (nm)

Figure 4. (a) Hydrogen evolution of the CMPs without Pt in a 4:1 mixture of H2O/TEOA (λ > 420 nm); (b) Hydrogen evolution of the CMPs with 3 wt% Pt in a 4:1 mixture of H2O/TEOA (λ > 300 nm); (c) Hydrogen evolution of DBTD-CMP1 produced from different batches; (d) The apparent quantum yield of DBTD-CMP1.

We also performed the density functional theory (DFT) calculations to further investigate the influence of the geometry of the polymer skeleton on the photocatalytic performance of the CMPs. The optimized geometries of these polymer structures are shown in Figure 5. All

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ACS Catalysis

of the CMPs show various twisted degrees between the two building blocks. Although the phenyl-based DBTD-CMP1 shows a slight higher dihedral angle of 41.0o between phenyl and dibenzothiophene rings compared to DBTD-CMP2 (38.8o) and DBTD-CMP3 (37.9o), the twisted structure of the crosslinkers of biphenyl and p-terphenyl with the respective dihedral angles of 39.8o and 41o between the phenyl rings increases the twisted degree of the whole polymer main chain in DBTD-CMP2 and DBTD-CMP3, leading to the lower conjugation degree and planarity of the polymer skeletons, which blocks the charges transfer in DBTD-CMP2 and DBTD-CMP3 during the photocatalytic process. Molecular frontier orbital diagrams reveal that the highest occupied molecular orbits (HOMO) are mainly concentrated on the electron donor units (e.g. benzene, biphenyl and p-terphenyl), while the LUMO orbits mainly spread on the electron-withdrawing units (dibenzothiophene dioxide) in the resulting CMPs (Figure S15). Thus, in the photo excitation process, the photogenerated electrons can rapidly transfer from the HOMO orbits on the electron donor units (benzene, biphenyl and p-terphenyl) to the LUMO orbits of the dibenzothiophene dioxide unit along the conjugated polymer chain, and the photocatalytic reaction for hydrogen evolution will occur at the LUMO orbits of the dibenzothiophene dioxide unit. In addition, DBTD-CMP1 shows much more expanded LUMO distribution on the dibenzothiophene dioxide unit than DBTD-CMP2 and DBTD-CMP3, indicating that DBTD-CMP1 can provide much more active reaction sites and has a higher photocatalytic activity, which is in line with the result of the photocatalytic experiment for hydrogen evolution by the resulting CMPs. The great difference in photocatalytic activity of these CMPs was also evidenced by the photo-current measurement. It was found that DBTD-CMP1 shows much stronger transient photocurrent than DBTD-CMP2 and DBTD-CMP3 (Figure S16), indicating that much more light-induced electron and hole pairs could be produced in DBTD-CMP1, which enhances the photocatalytic activity for hydrogen production.

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Figure 5. DFT geometry optimization and dihedral angles for the CMPs.

CONCLUSION In summary, a series of dibenzothiophene dioxide-based CMPs photocatalysts was designed and synthesized. Photocatalytic experiments and DFT calculations revealed that the geometry of the polymer skeleton has a big influence on the photocatalytic activity of the resultant CMPs. DBTD-CMP1 with a shorter crosslinker of phenyl and increased polymer coplanarity shows the better photocatalytic performance than DBTD-CMP2 and DBTD-CMP3 with longer crosslinkers of biphenyl and p-terphenyl, respectively. The Pt loaded DBTD-CMP1 exhibits a high HER of 9200 µmol h−1 g−1 under broadband light illumination and a high AQY of 3.3% at 400 nm. The high photocatalytic activity of DBTD-CMP1 is likely due to the increased conjugation and planarity of the polymer skeleton promoting the photo-induced charge carriers transmission and separation. This work demonstrates that these dibenzothiophene dioxide-containing CMPs are competitive with other organic photocatalysts for hydrogen production. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:xxx.

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Details of the synthesis for the monomers, TGA curves in nitrogen, Powder XRD patterns, Scanning electron microscopy (SEM) images, cyclic voltammetry curves, EDX measurement, molecular orbital diagrams of the polymer structures, transient photocurrent. Tables for pore properties, fitted decay time, DFT geometry optimization, and diheral angles for the polymers. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] ORCID Jia-Xing Jiang: 0000-0002-2833-4753 Notes The authors declare no competing financial interest ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21574077 & 21304055), the National Key Research and Development Program of China (No. 2016YFB0401005) funded by MOST, and the Fundamental Research Funds for the Central Universities (GK201801001). REFERENCES (1) Wang, L.; Wan, Y.; Ding, Y.; Wu, S.; Zhang, Y.; Zhang, X.; Zhang, G.; Xiong, Y.; Wu, X.; Yang, J.; Xu, H. Conjugated Microporous Polymer Nanosheets for Overall Water Splitting Using Visible Light. Adv. Mater. 2017, 29, 1702428. (2) Li, L.; Lo, W.-Y.; Cai, Z.; Zhang, N.; Yu, L. Donor-Acceptor Porous Conjugated Polymers for Photocatalytic Hydrogen Production: the Importance of Acceptor Comonomer. Macromolecules 2016, 49, 6903−6909.

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9200

9000

−1

−1

g )

Graphical Abstract

H2O

H2 S O

HER (µ mol h

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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6000

4600 3000

2460

Bare polymer (λ λ >420 nm) Polymer with 3% Pt (λ λ >420 nm) Polymer with 3% Pt (λ λ >300 nm)

0

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Dibenzothiophene Dioxide Based Conjugated Microporous Polymers

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