Poly(benzothiadiazoles) and Their Derivatives as Heterogeneous

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Poly-benzothiadiazoles and their derivatives as heterogeneous photocatalysts for visible light-driven chemical transformations Run Li, Jeehye Byun, Wei Huang, Cyrine Ayed, Lei Wang, and Kai A. I. Zhang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00407 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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Poly-benzothiadiazoles and their derivatives as heterogeneous photocatalysts for visible light-driven chemical transformations Run Li,§ Jeehye Byun,§ Wei Huang, Cyrine Ayed, Lei Wang and Kai A. I. Zhang* Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany. ABSTRACT: Visible light-driven chemical transformations via photocatalysis have witnessed an explosive growth in the last decade, accompanying with the enormous development of the photocatalyst design. Among the intensely investigated systems, molecular photocatalysts as transition metal complexes and conjugated organic dyes have been established as efficient homogeneous photocatalytic systems for organic photoredox reactions. Nonetheless, heterogeneous photocatalysts possess considerable advantages as being highly stable, easily separable and reusable. Especially the organic, macromolecular heterogeneous photocatalysts have emerged as a non-toxic and potentially more environmentally friendly alternative to the traditional catalytic systems. Among them, poly-benzothiadiazoles and their derivatives have demonstrated the ability to catalyze various organic photoredox reactions under visible light irradiation. In this review, the recent development of metal-free and heterogeneous photocatalytic systems based on poly-benzothiadiazoles and their derivatives is summarized. An overview of organic photoredox reactions mediated via radicals obtained from the photogenerated electron and hole of the photocatalysts is given, and the underlying mechanisms of photochemical transformations are illuminated. The structural design principles of poly-benzothiadiazoles for targeted photoredox reactions are also discussed.

KEYWORDS: poly-benzothiadiazole, visible light, heterogeneous photocatalysis, chemical transformations, photoredox high photocatalytic efficiency for both oxidation and 1. Introduction reduction reactions, to mention but a few, oxidation reactions as oxidation of alkene18, benzene19, alkyne20, alkane21, The direct conversion of solar energy as inexpensive, alcohol22, and amine23, oxidative alkylation24, oxidative non-polluting, abundant, and sustainable source into hydroxylation of arylboronic acids25, oxidative addition26, chemical energy via photocatalysis has been intensively and oxidative cycloaddition27, and reduction reactions as developed during the past decades. Particularly after the reductive dehalogenation28, reductive arylation6, 29, reducpioneering reports on the employment of ruthenium tive cyclization30, and reduction of p-nitrophenol31. Howpolypyridyl complexes as visible light-active photocata1-3 ever, certain disadvantages of the organic molecular pholysts for C-C and C-Br bond activation , chemists and tocatalysts such as photo-bleaching, low redox capacity, material scientists have taken enormous effort for the and limited light responsive nature restrict the broad development of efficient photocatalytic systems to drive range of applications. Further development of stable phoorganic photoredox reactions. Compared to thermally tocatalysts led to the employment of heterogeneous semiactivated reaction mechanisms, photo-induced reactions conductor systems containing metal or metal oxides32-34 undergo mostly the radical bond formation pathways. with TiO2 as the most investigated heterogeneous system. Various reactive radical intermediates have been reported 4-5 Those systems, however, suffer from the finite light abvia photocatalysis, including trifluoromethyl radicals , 6-7 8-9 sorption in the ultraviolet (UV) range. Tremendous studarene radicals , iminium ions , and enone anion radi2, 10 ies on post-modifications showed the enhancement of the cals , which facilitates the construction of chemical optical and electronic properties of TiO2-containing phobonds. 35-37 tocatalysts . Nevertheless, the development of metalMolecular systems based on transition metal complexfree and heterogeneous photocatalytic systems, which can 11 12 13 es, for example, ruthenium , iridium , chromium and efficiently catalyze organic photoredox reactions, is still 14 copper have been established as efficient homogeneous challenging. photocatalysts. Nevertheless, they suffer from some inRecent research activities showed the employment of trinsic drawbacks such as toxicity, high cost, and instabilcarbon nitride as heterogeneous photocatalysts for a ity under aerobic surroundings. Parallel to the transition number of chemical transformations38-41. Pure organic metal complexes, conjugated organic dyes as cypolymer semiconductor-based photocatalytic systems, as anoarenes, xanthenes and acridiniums have been utilized an emerging class of heterogeneous photocatalysts, have for photocatalysis as metal-free alternatives15-17. The employment of conjugated organic dyes has demonstrated a

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very recently demonstrated their ability to be a versatile NS N

N NS

C6H13

C6H13

NS N

m

S

HN NH O

O

HN

OH

N S N

P-3

P-2

P-1

S N N

S

N NS

N S N

SN N

n

N S N

NO N

O

P-4

N Se N

P-5

NO N

N NO

n

S N N O m

NS N

N N Se

ON N

NO N

ON N

NO N

ON N

NO N

n

N O N

N S N

P-6 P-7 P-8 P-9 P-10 P-11

m/2n=100/0 m/2n=75/25 m/2n=50/50 m/2n=30/70 m/2n=10/90 m/2n=0/100

N Se N

O

N

N S N

m

N n SN

S N N

NC O

P-15

NS N

N S N

N S N O

P-14

P-13

P-12

O

N

SN N

CN

N

m

SN N

CN

N NS N

N

N SN

P-17

P-16

P-19 P-20 P-21 P-22

P-18

2m/n=100/0 2m/n=90/10 2m/n=50/50 2m/n=0/100

P-23

C6H13 C6H13 NS N

S N N

N S N

SN N

N NS

N SN

P-24

S

P-34

P-35

C8H17

N NS

SN N n

N S N

N SN

P-28

P-27

P-29 P-30 P-31 P-32 P-33

m/2n=95/5 m/2n=90/10 m/2n=70/30 m/2n=50/50 m/2n=70/30

S N N

NS N C8H17

NS N m

N SN

P-26

P-25

S N N

S N N

SN N

SN N

NS N C6H13 C6H13

NS N

n

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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S

(C6H12) (C6H12) C8H17

C8H17

P-36

Et

N

EtEt N Et

P-37

Figure 1. Chemical structures of the reported photocatalysts based on poly-benzothiadiazoles and their derivatives.

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Table 1. Physicochemical properties of poly(benzothiadiazole) and their derivatives. Compounds

SBET (m2/g)a

Pore Size (nm)

Pore Volume (cm3/g)

Td (oC)a

LUMO (vs. SCE)a

HOMO (vs. SCE)a

Egapel (eV)a

λabsmax (nm)a

λemmax (nm)a

Egapopt (eV)a

P-1

120

-

0.12

554 (air)

-0.67

+1.73

-

395

-

2.40

P-2

44

2.6

0.051

450

-

-

-

410

585

2.36

P-3

280

1.3

0.235

-

-

-

-

493

-

-

P-4

-

-

-

-

-

-

-

435

-

2.46

P-5

-

-

-

-

-

-

-

397

-

2.67

P-6

145

1.5

0.19

~300

-0.62

+1.09

-

425

538

1.71

P-7

226

1.5

0.33

~300

-0.66

+1.42

-

414

536

2.08

P-8

129

1.5

0.18

~300

-0.70

+1.49

-

463

538

2.19

P-9

250

1.5

0.41

~300

-0.74

+1.55

-

456

532

2.29

P-10

199

1.5

0.39

~300

-0.75

+1.62

-

436

542

2.37

P-11

319

1.4

0.60

~300

-0.90

+1.60

-

417

529

2.51

P-12

474

1.5

0.31

380

-1.19

+1.55

2.74

380

-

-

P-13

433

1.5

0.39

380

-1.06

+1.35

2.41

421

-

-

P-14

475

1.5

0.37

400

-1.13

+1.45

2.58

372

-

-

P-15

378

1.5

0.20

250

-0.89

+1.14

2.03

365

-

-

b

+1.43

2.94

404

501

2.47

+1.67

2.42

385

542

2.40

P-16

26

2.9

0.056

420

-1.51

P-17

130

-

0.291

350

-0.75

b

b

P-18

586

1.5

0.29

300

-0.91

+1.75

2.66

430

-

-

P-19

1017

1.5

0.82

491

-1.14

+1.98

-

343

-

3.12

P-20

355

1.5

0.32

383

-0.94

+1.26

-

400

-

2.20

P-21

110

1.5

0.12

236

-0.76

+1.16

-

411

-

1.92

P-22

57

1.5

0.07

237

-0.75

+1.10

-

398

-

1.86

P-23

90

14.3

0.32

500 (O2)

-0.73

+1.55

-

410

542

2.28

P-24

23

3.6

0.06

450

-

-

-

304

-

1.50

P-25

39

0.14

4.0

550

-1.13

+1.04

-

468

-

2.17

P-26

17

0.03

4.5

550

-1.22

+1.03

-

440

-

2.25

P-27

280

0.37

1.5

570

-1.27

+1.17

-

390

-

2.44

P-28

40

0.06

1.5

500

-1.33

+0.99

-

365

-

2.32

P-29

58

0.10

3.8

450

-1.19

+1.01

-

421

-

2.20

P-30

93

0.24

3.8

450

-1.21

+1.00

-

416

-

2.21

P-31

129

0.51

4.5

420

-1.23

+0.99

-

398

-

2.22

P-32

40

0.20

10.7

500

-1.24

+0.99

-

415

-

2.23

P-33

84

0.20

3.8

500

-1.26

+1.03

-

420

-

2.29

P-34

17

-

-

375

-0.93

+0.71

-

414

598

1.64

P-35

-

-

-

-

-1.02

+1.44

-

445

542

2.46

P-36

-

-

-

-

-1.14

+0.84

-

548

669

1.98

P-37

-

-

-

-

-0.96

+1.23

-

420

-

2.19

a

SBET: Brunauer-Emmett-Teller (BET) surface area; Td: decomposition temperature measured by thermogravimetric analysis (TGA) under N2 atmosphere, otherwise noted; LUMO: lowest unoccupied molecular orbital; HOMO: highest occupied molecuel max max lar orbital; Egap : Electrochemical energy band gap; λabs : Maximum absorption wavelength; λem : Maximum emission wave-

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length; Egap : Optical energy band gap; All potentials are referred to saturated calomel electrode (SCE) (E vs. SCE). The LUMO and HOMO value are obtained from density function theory (DFT) calculations.

Benzothiadiazole (BT) and its derivatives, due to their properties of strong electron-withdrawing capacity, excellent stability, strong light absorption efficiency, and tailorable band potentials and excited state lifetimes, have been frequently employed in a variety of optical and electronic applications, for example, biological imaging47-48, organic light emitting diodes (OLEDs)48, field effect transistors, sensors49, and photovoltaic cells50. Lately, the benzothiadiazole unit was firstly utilized into conjugated microporous polymer networks as heterogeneous photocatalysts for visible light-driven singlet oxygen (1O2) generation51. Ever since, substantial research efforts have been undertaken to develop poly-benzothiadiazole-based photocatalysts for organic photoredox reactions. Despite the performance, to date, there has been no review that summarizes recent progresses of heterogeneous photocatalysts based on poly-benzothiadiazoles and their derivatives. In this review, we first describe the general synthetic protocols to build poly-benzothiadiazoles and their derivatives, and identify their basic physicochemical properties. The photo-induced organic redox reactions with underlying reaction mechanism are summarized in details. Our aim is to introduce poly-benzothiadiazoles and their derivatives to enrich the current photocatalyst families.

2. Synthesis of poly-benzothiadiazoles and their physicochemical properties Typical examples of poly-benzothiadiazoles are listed in Figure 1. They are in general constructed by metalassisted cross-coupling reactions between benzothiadiazole monomer as acceptor unit and aromatic comonomer as donor unit. Pd-mediated Sonogashira-Hagihara coupling (P-1, P-3, P-6 to P-15, P-19 to P-22, P-24, P-34) and Suzuki coupling reactions (P-2, P-16, P-25 to P33, P-35 to P-37) are often used, along with the synthetic choices to make conjugated polymers such as Heck-52, Yamamoto-53, Glaser-54, Negishi-55, and Kumada coupling reaction56, and oxidative polymerization42, 57. Metal-free synthetic approaches to build polymeric structure with benzothiadiazole moiety are recently investigated, namely Knoevenagel condensation reaction (P-17) and acidcatalyzed trimerization reaction (P-23). Non-conjugated polymeric systems comprised of benzothiadiazole units are also demonstrated, for instance, by free radical polymerization (P-5) and Friedel-Crafts polymerization (P-18). Via connection to multiple functionalized building blocks, permanent nano-sized pore structures can be obtained, gaining enlarged surface areas and therefore large active catalytic interface of the heterogeneous polymer photocatalysts (Table 1). The resulting polybenzothiadiazoles are thermally stable in harsh conditions up to 300 oC, thus the structural stability are anticipated for repeated use in heterogeneous photocatalytic cycles. The photophysical and electrochemical properties of poly-benzothiadiazoles and their derivatives are also

listed in Table 1. While the benzothiadiazole unit alone shows mainly UV response, by connection to electron donor moieties, the absorption of poly-benzothiadiazoles is tailored into longer wavelength regions, resulting into the efficient visible light response and semiconductive characteristics. The electronic and optical band gaps of the polymers indicate the typical semiconductor-type properties, as shown in Table 1. Furthermore, the photoredox potentials, i.e. energy band positions, could be conveniently modified by structural manipulation during polymerization process. One of the simplest synthetic methods is statistical polymerization, varying the composition of donor and acceptor units on polymers (P-6 to P11 and P-19 to P-22). Typically, poly-benzothiadiazole P-6 to P-11, exhibited a clear tendency of broadening band gap when the amount of electron-withdrawing benzothiadiazole units was increased stepwise, giving the highest redox potential for P-11 only containing benzothiadiazole moiety. Even the change in elements on acceptor unit could change the energy band structure of resulting polymers (P-1, P-12, and P-13). Besides, employment of phenyl ring as a core can further diverse the polymeric structures by altering the substitution sites of benzothiadiazole units (P-25 to P-33). The connection of comonomers in different substitution sites on phenyl ring results in alignment of band position for targeted reactions (P-12, P-14, and P15). Studies showed that the excited state lifetime of the donor-acceptor-type benzothiadiazole materials could be longer than 10 ns, which is comparable to that of the transition metals complex and other conjugated organic dyes58-59. It is worth to note, that the reduction potentials of most benzothiadiazole-based polymers were higher than -0.6 eV, which is enough to activate oxygen into reactive oxygen species60. By incorporation of benzothiadiazole into the polymer matrix, not only the optical and electrochemical properties, but also the morphology and interfacial properties of the polymers can be controlled. The water dispersity of poly-benzothiadiazoles was achieved by employing hydrophilic functional groups at the terminal such as 3-mercaptopropionic acid, nitrile, and diethylamine of P-3, P-17, and P-37, respectively. Truly, by the virtue of being a polymer, polybenzothiadiazoles have a great potential in adjusting their properties with limitless synthetic combinations. 3. Photocatalytic chemical transformations catalyzed by poly-benzothiadiazoles and their derivatives In this part, an overview of the chemical transformation reactions catalyzed by poly-benzothiadiazoles and their derivatives is presented. Organic transformation reactions via redox pathway, referring to the previous definition15, can be divided into two major categories, i.e. oxidation and reduction reactions with respect to the substrates. The listed oxidation reactions include the oxidative degradation of organic pollutants or oxidative coupling for carbon-carbon bond and carbon-heteroatom bond formations. In comparison, the reduction reactions listed

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include the hydrogenation of carbon-heteroatom bonds or reductive bond formations.

1

O2

3.1. Photo-driven oxidation reactions and oxidative couplings For the photocatalytic oxidation reactions, molecular oxygen is frequently involved in the catalytic cycle. Usually, two active oxygen species, superoxide radical (O2•-) and singlet oxygen (1O2), can be formed via the electron transfer and energy transfer pathway from photocatalyst to molecular oxygen, respectively. Especially the photogenerated 1O2 species have been playing a critical role in the synthesis of fine chemicals61, natural compounds and pharmaceuticals62, and photodynamic therapy63. To easily quantify the amount of the photo-generated 1O2, different trapping reagents can be used as illustrated in Figure 2. The first example of poly-benzothiadiazole as efficient heterogeneous photocatalyst (P-1) for visible light-driven 1 O2 generation was reported by Zhang et al 64. In this study, the impact of the surficial properties of the polymer photocatalyst was studied via introduction of SiO2 template during polymerization, demonstrating the larger surface area of the P-1 could lead to higher 1O2 generation rates. In a continuous flow setup, the 1O2 production rate reached up to 1.0 mmol min-1 with the maximum quantum yield of 0.06. With a minute decrease in 1O2 generation efficiency up to 3.6 %, poly-benzothiadiazoles P-1 could be reused for five repeated cycles with high selectivity, proving the advantage of the heterogeneous polymer photocatalyst. The high photocatalytic activity of P-1 with its oxidative capacity led to its employment for a series of organic transformation reactions, which will be discussed in the following sections. Another highly porous benzothiadiazole-containing polymer (P-2) was prepared via high internal phase emulsion polymerization as porous monolith which exhibited extreme structural stability and quantitative conversion of ascaridole in a continuous flow set-up65. P-2 showed no noticeable change in conversion efficiency even after 10th repeating experiment. To gain the applicability of the hydrophobic polymer P-1 and to prevent its aggregation in aqueous solution, a hydrophilic polymer (P-3) was synthesized by a simple postmodification of P-1 with 3-mercaptopropionic acid via thiol-yne chemistry66. P-3 could be well dispersed in water to catalyze the formation of 1O2, which could be determined using furoic acid as a trapper (Figure 2, eqn. 2). The incorporation of the benzothiadiazole unit into nonconjugated polymer matrix such as polyamide and polystyrene offered another synthetic pathway to obtain heterogeneous poly-benzothiadiazole photocatalysts (P-4 and P-5) for the generation of 1O267-68. The structural stability of P-4 and P-5 allowed the repeated use in a continuous flow set-up up to five and thirty cycles, respectively.

O

O

1O 2

O O

(1)

O

HO

O

OH

1

O OH

O2

O

O

(2)

(3)

HO

Figure 2. Typical chemical trapping experiments to detect singlet oxygen by addition reaction. Equation 1 is often used in organic solvent (chloroform) and equation 2 and 3 in aqueous solution.

Figure 3. (a) Schematic representation of mechanism for the inactivation of bacteria by poly-benzothiadiazole. (b) Electron paramagnetic resonance (EPR) spin trapping spectra of 1 TEMP- O2 adducts in CH3CN generated by poly(benzothiadiazole) nanoparticles under visible light irradiation. (c) Photocatalytic inactivation of E. coli K-12 in the presence of different poly-benzothiadiazole nanoparticles. Reproduced with permission from © The Royal Society 69 of Chemistry 2016; Ref. .

One of the important applications of 1O2 locates at photodynamic therapy due to high oxidative capacity of 1O27072 . Owing to the 1O2 generation capability, polybenzothiadiazoles could be attractive for anti-bacterial treatment as non-toxic organic photocatalysts. In order for a favorable incorporation to biological system, bulk poly-benzothiadiazoles were transformed into nanoparticles via the miniemulsion polymerization technique, giving P-6 to P-11 (Figure 3)69. With increasing amount of the benzothiadiazole unit in the polymer backbone, higher amount of 1O2 was determined. The highest 1O2 generation rate reached ca. 0.14 mmol g-1 s-1 with P-11, leading to better performance in photo-inactivation of bacteria as E. coli K-12 and B. subtilis. No apparent change in efficiency was observed after third repeating cycle. Currently, photochemotherapy with benzothiadiazole unit has drawn much attention because of its high efficiency for produc-

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ing 1O2 under the irradiation of visible light and near infrared light73. The photo-degradation of organic pollutants is a green sanitation technology to solve the environmental contamination. Taking advantages from the nano-sized and high dispersity, P-1 nanoparticle was also employed for the photo-oxidative degradation of organic compounds74. Recent studies demonstrated that organic pollutants as rhodamine B (RhB) and N,N,N´,N´-tetramethyl-pphenylenediamine (TMPD) could be photo-degraded efficiently74. Another BT-containing polymer (P-4) showed its ability to photo-degrade bisphenol A, cimetidine, phenol, ciprofloxacin, and sulfathiazole in aqueous solutions67. Hybrid photocatalyst materials of P-1 with metal oxides such as P-1/TiO275 and P-1/Bi2MoO6 were also used for dye degradations45, 76-77.

Figure 4. Photocatalytic oxidative coupling of amines under visible light irradiation.

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The photocatalytic oxidative coupling of amines with oxygen as terminal oxidant is an efficient tool to form imine compounds, which are regarded as useful and active electrophilic intermediates for organic transformations38. Poly-benzothiadiazole network P-1 and its derivatives, i.e. poly-benzooxadiazole (P-12) and polybenzosellenadiazole (P-13), were synthesized via Sonogashira coupling78. While all three polymers could catalyze the oxidative coupling of benzylamines, P-1 exhibited the highest photocatalytic efficiency. The electron paramagnetic resonance (EPR) study revealed that the photo-generation electron/hole separation in P-1 under visible light irradiation was considerably more efficient than in P-12 and P-13. The substitute tolerance of the P-1catalyzed reaction was demonstrated by employing various aryl amines, bearing either electron-donating groups such as methyl and methoxy or electron-withdrawing groups such as fluoride and chloride (Figure 4). With the structural stability, P-1 could be used for five repeating cycles without any loss in photocatalytic efficiency. A synergetic enhancement of the photocatalytic efficiency was found using P-1/TiO2 hybrid materials via improved charge separation and intermolecular electron transfer between P-1 and TiO279. The reaction pathway for the oxidative coupling of amines is illustrated in Figure 5. The benzylamine was firstly oxidized by the photogenerated hole to form cationic radical, which absorbed two reactive oxygen species, O2•- and 1O2, and became imine after H2O2 elimination process. The addition between amine and imine gave the final product through ammonia elimination. As a minor reaction, the generated H2O2 could also oxidize the amine via a hydroxyl radical pathway, which favored the generation of imine intermediate. From the standpoint of structural design, one large advantage of the semiconducting polymer-based photocatalyst is that their energy band position can be fine-tuned via a simple structural variation. Our recent study demonstrated that by employing the same building blocks, the energy band positions of polybenzooxadiazoles (P-12, P-14, and P-15) could be aligned through different substitution positions of the benzooxadiazole unit on the centered phenyl ring (Figure 6)80. As listed in Table 1, all of three structures showed high BET surface area of about 400 m2/g. It could be observed that P-12 with the benzooxadiazole units through the 1,3,5-substitution positions exhibited the largest band gap and therefore the highest redox potentials, leading to its superior photocatalytic efficiency for the oxidative coupling of amines than P-14 and P-15. The high efficiency of P-12 could be attributed to its superior over potentials and enhanced charge mobility from chain configuration with meta position. P-12 showed photostability during the five repeated cycles.

Figure 5. Proposed reaction mechanism for photocatalytic oxidative coupling of amines. s and t indicate the singlet and triplet states of polymer photocatalyst.

Organic sulfides are often used as important intermediates in pharmaceutical and fine chemical production. The selective oxidation of sulfides into sulfoxides without using additives has been achieved with a porous polymer (P-16)81. P-16 could catalyze the sulfide transformation in a

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quantitative manner with excellent selectivity without obtaining the unwished byproduct sulfone caused by over-oxidation. Various sulfide substrates bearing fluorine, bromine, chlorine, methyl, and methoxy functionalities could be selectively oxidized with moderate to high conversions (Figure 7). Furthermore, by incorporating P16 into a flow photo-reactor, sulfoxides could be continuously produced with high selectivity, in which additional separation procedure of the catalyst was unnecessary. Reusability of P-16 was proved by the five successive repeating experiments.

strated that the oxygen on phenol was originated from the molecular oxygen82. The addition of O2•- on phenylboronic acid could form radical intermediate (a), which underwent proton transfer to become radical intermediate (b). The final phenol products could be obtained after successive rearrangement and hydrolysis process. Another study on the photophysical insights of the oxidation process proved that 1O2 only played a limited role for the amine oxidation reaction than for boronic acids, because aliphatic amines could act as efficient 1O2 quencher than that boronic acids25.

Structure Design Principle 3D center

e

Reduction

h

Oxidation

Substitution position Band position alignment of poly(benzothiadiazoles) derivatives -1.19

-1.13 -0.89

-1.0 -0.5

E/V vs. SCE

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

0.0

P-12

P-14

P-15 Figure 7. Photocatalytic selective oxidation of sulfides. The value in parentheses indicates the selectivity of sulfoxides formation.

0.5 1.0 1.14

1.5 1.55

1.45

Figure 6. Geometry design principle of conjugated polybenzothiadiazole photocatalysts through different substitution positions of the central benzene unit, P-12 (through 1,3,5), P-14 (through 1,2,4), and P-15 (through 1,2,4,5), and the corresponding band positions. Reproduced with permission 80 from © 2015 WILEY-VCH; Ref. .

The photocatalytic oxidative hydroxylation of arylboronic acids into phenols involves the generation of O2•25, 42, 82 . A conjugated polymer (P-17) could photocatalytically convert the arylboronic acid into the corresponding phenols with various substitution groups such as cyano, fluorine, bromine, nitro, methoxy, phenyl, naphthyl and hydroxyl (Figure 8)83. A polystyrene-based photocatalyst containing the benzothiadiazole unit was also reported to catalyze the oxidative hydroxylation of arylboronic acids owing to effective generation of reactive oxygen species by benzothiadiazole unit68. The reaction mechanism illustrated in Figure 9 showed that the active oxygen species, particularly O2•-, played the essential role for the reaction initiation process. The 18O-labeling experiments demon-

Figure 8. Photocatalytic hydroxylation of arylboronic acid using P-17 as a photocatalyst.

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B(OH)3 R

O

H2O O2

eLUMO

OH B OH O2 HO

(a)

R

Ar

-

OH B O OH

O

O

O

O

Br

O

O O

Br O

Br

14 h, 50 %

Br

O

O

h+

Figure 9. Proposed reaction mechanism for photocatalytic hydroxylation of arylboronic acid.

Brominated compounds, which are important intermediates in organic synthesis, especially for the carboncarbon and carbon-heteroatom bond construction84-85, could be obtained by a photocatalytic approach in employing a BT-containing polymer photocatalyst (P-18) in our latest study86. P-18 showed a particle-like morphology, which is typical for organic porous polymers (Figure 10). This study also showed that via the incorporation of the BT unit into the polymer backbone, P-18 exhibited a clearly higher photocatalytic efficiency compared to its reference polymer photocatalyst only containing benzene unit in the backbone. Similar to the previous BTcontaining heterogeneous photocatalyst, P-18 also showed high substituents tolerance with various substrates including benzene, thiophene, and benzothiophene etc. (Figure 11). The repeating experiments revealed that P-18 could be easily separated from reaction mixture and reused for several cycles without losing its catalytic activity.

Br 36 h, 55 %

10 h, 81 % S

Br

Br Br

S

S 18 h, 85 %

6 h, 80 %

14 h, 70 %

N

O

Br

8 h, 55 % S

Br

10 h, 75 %

O

O

N

O

Br

12 h, 82 %

15 h, 84 %

HOMO

O O

8 h, 89 %

(b)

N

Ar

CH3CN, visible light, r. t.

B OH OH

OH

HO OH BO O

Br

P-18, HBr, O2

OH

Figure 11. Photocatalytic bromination on electron-rich aromatic compounds using P-18 as a visible light-active heterogeneous photocatalyst.

O e-

O2

s

O O

Br H+

O2

LUMO

O2H

minor side reaction

t 1O

H2O2

2

O

O O

O

H

+ HBr

Br

Br-

O O

HOMO

h+

O

O O

Figure 12. Reaction pathways of photocatalytic bromination of 1,2,4-trimethoxybenzene with P-18 as a photocatalyst. s and t indicate the singlet and triplet states of the polymer catalyst.

Figure 10. (a) Scanning electron microscope (SEM) and (b) transmission electron microscopy (TEM) images of P-18. Adapted with permission from © 2016 American Chemical 86 Society; Ref. .

The study on the reaction mechanism revealed that the bromination of aromatic compounds underwent a radical pathway as illuminated in Figure 12. Under the light irradiation, the photo-generated electron could activate the surrounding oxygen to O2•- and 1O2, while the hole oxidized the starting substrate (1,2,4-trimethoxybenzene, TMB) to its cationic radical which reacted with bromine anion to become a coupled adduct intermediate. Finally, the brominated product was obtained after the following oxidation of the intermediates by oxygen species, accom-

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ACS Catalysis

panying with the generation of H2O2 as side product. The H2O2 could oxidize TMB directly into corresponding bromides as a minor reaction cycle. The photocatalytic oxidation using polybenzothiadiazoles could be applied for the formation of more complex products, i.e. tetrahydroquinolines, an important motif in pharmaceutical compounds87. As a structural analogue to P-1, a series of polymer photocatalysts (P-19 to P-22) was constructed by varying the amount of benzobisthiadiazole unit in the poly-phenyl network. Similar to the BT unit, the introduction of benzobisthiadiazole led to the alignment of the band position of the polymers due to its strong electron-withdrawing property, particularly the HOMO level. With the highest amount of benzobisthiadiazole, the stronger oxidation potential of polymer was achieved by 0.88 eV compared to that without benzobisthiadiazole. As a result of combined effects in the visible light response and the suitable HOMO level, P-20 exhibited the highest photocatalytic activity for the formation of tetrahydroquinolines via the oxidative coupling between N-methylanilines and maleimides, within the polymer series. Various 1,2,3,4tetrahydroquinoline products could be obtained with moderate to high reaction yields as displayed in Figure 13. It is worth to note that P-20 showed a competitive efficiency for the tetrahydroquinolines production in direct comparison to the small molecular catalytic system such as Eosin Y88, N-hydroxyphthalimide89, or the metal catalysts e.g. TiCl290, CuCl291, and Ru(bpy)3Cl292. Thanks to heterogeneous system, P-20 could be recycled for five repeated cycles without compromising its catalytic activity.

The reaction mechanism of the photocatalytic 1,2,3,4tetrahydroquinoline formation is illuminated in Figure 14. N-methylaniline was oxidized by the photo-generated hole to form the cationic radical, which then formed an αaminoalkyl radical after proton transfer. Then, the generated α-aminoalkyl radical reacted with maleimide, leading to the cyclization. The final product was obtained after oxidation of the cyclic radical intermediate by the reactive oxygen species.

Figure 14. Proposed reaction mechanism of oxidative coupling for 1,2,3,4-tetrahydroquinoline production.

Cyclobutanes, which largely contribute to the synthesis of natural products and pharmaceuticals93-94, have been generated through a thermal-driven reaction pathway or photo-driven [2+2] cycloadditions with Lewis acids, amines, and transition metals catalyst95. Among all, cyclobutanes produced by a metal-free photocatalyst have not been much unveiled. In our recent report, we demonstrated that the [2+2] cycloaddition could be achieved using P-1 as a polymer photocatalyst, giving cyclobutanes as the final products96. Using P-1 as a polymer photocatalyst, styrene derivatives including both electron-donating and -withdrawing substituents were tested for cyclobutane production, resulting in a series of cyclobutanes with excellent conversion efficiencies (Figure 15). In addition, two important natural products, endiandrin A and di-Omethylendiandrin A could also be synthesized through the photochemical approach with moderate yields. P-1 exhibited a slight decrease in conversion efficiency after third cycle of the cross cycloaddition reaction, maybe due to the loss of catalyst during recovery process. Figure 13. Oxidative coupling of N-methylanilines with Nsubstituted maleimides using P-20 as a photocatalyst.

The reaction pathways of [2+2] cycloaddition are shown in Figure 16. The photo-generated hole could facilely oxidize the substrate trans-anethole into cationic radical which was added to another part of styrene to form a

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cyclobutane intermediate. Then there are generally three different pathways for the formation of final product; (i) the cyclobutane intermediate could release an electron directly to photocatalyst, (ii) the intermediate could be reduced by photo-generated oxygen species O2•-, or (iii) the intermediate oxidizes another neutral trans-anethole, depending on the chain propagation properties in the cycloaddition reactions97.

Figure 16. Proposed reaction mechanism of [2+2] cycloaddition for cross-coupled cyclobutane production.

Figure 15. Photocatalytic [2+2] cycloaddition of styrene derivatives using P-1 as a polymer photocatalyst. Structures located in the dotted frame indicate natural compound produced in the given reaction conditions.

Polymerization in a photocatalytic way is considered to be a useful tool to produce high quality polymer chains with construable lengths and polymerization degree98-99. In this regard, it is worthwhile to mention that a highly porous poly-benzothiadiazole network, P-16, could catalyze the free radical polymerization of methyl methacrylate (MMA) under the irradiation of household fluorescent light bulb (Figure 17)100. P-16 was used as a photoinitiator in a typical chain initiation process. The study on the polymerization mechanism showed that upon light irradiation, the photo-generated hole could oxidize the triethylamine into a radical cation which combined with another neutral triethylamine via proton transfer, thus forming a reactive amine radical. The generated amine radical could initiate the polymerization of MMA via free radical polymerization pathway, resulting in PMMA. Photopolymerization of MMA could be conducted for at least three repeated cycles with P-16.

Figure 17. Polymerization of methyl methacrylate (MMA) using P-16 as a photo-initiator under the irradiation of household fluorescent light bulb.

3.2. Photo-driven reduction reactions and reductive couplings Reduction reactions also contribute a large part for current photocatalytic applications of the polybenzothiadiazole-based photocatalyst. A useful example of the photocatalytic reduction reaction is photoreduction of hexavalent chromium (Cr(VI)) in wastewater.

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Cr(VI) is a highly toxic inorganic contaminant due to its high mobility and solubility in aqueous solution, thus the reduction of Cr(IV) into less toxic Cr(III) has been widely investigated in the past decades101-102. The photo-reduction of Cr(IV) into Cr(III) using polymer photocatalyst provides a simple approach for wastewater treatment. During the reduction process, Cr(VI) could be reduced either directly by photo-activated electrons or indirectly by O2•generated via electron transfer from photocatalyst to the molecular oxygen. A hybrid photocatalyst containing P1/Bi2MoO6 system76 was reported to directly reduce Cr(IV) into Cr(III). The study showed that, similar to the aforementioned hybrid photocatalytic system containing P1/TiO2, the addition of Bi2MoO6 could also lead to a synergetic enhancement of the reduction efficiency due to the formation of the Z-scheme heterojunction. Recently, other hybrid photocatalytic systems containing P-1 and other semiconductors were designed to show the synergetic catalytic behavior. For example, P-1 was in situ grown on the surface of graphitic carbon nitride (gC3N4)103. According to the band position, the hybrid materials showed catalytic behaviors based on photo-induced electron transfer from P-1 to g-C3N4, thus enhancing the performance in Cr(IV) photo-reduction.

NO2

OH

eLUMO

NH2 + H2O

NO2 OH OH

+H

BH3 or HO

-

BH4 or H2O HOMO

h+

-

BH4

Figure 19. Scheme of reduction process of p-nitrophenol into p-aminophenol. O

O

P-24, DIPEA, Hantzsch ester Br

H DMF, r. t., household light bulb

R O

O

O H

Br

F 4 h, 89 %

O

O H

Figure 18. (a) SEM and (b) TEM images of P-23 with hollow structure templated by SiO2 particles. Adapted with permis104 sion from © 2016 The Royal Society of Chemistry; Ref. .

The reduction of p-nitrophenol (p-NP) into paminophenol (p-AP) is an important method for wastewater treatment, since p-NP belongs to the toxic side-products in agrochemicals and pharmaceutical industry. Previously, the photo-reduction of p-NP was mainly conducted by using the UV-responsive photocatalysts such as TiO2105-106. Recently, we designed a hollow benzothiadiazole-based covalent triazine framework (P23) for the p-NP reduction (Figure 18)104. It was found that the hollow structure of P-23 could improve its photocatalytic efficiency compared to the polymer in its pristine bulk shape, stemming from both enhanced mass transfer and improved light absorption via multi-step reflections. Similar to a previous report105, the reduction of p-NP using P-23 as photocatalyst basically involved successive electron transfer and proton transfer processes (Figure 19). The additional reductant, here NaBH4, functioned as both hydrogen source and sacrificing agent31. During the five repeated cycles for p-NP reduction, P-23 exhibited no change in both catalytic efficiency and hollow structure.

H

H

4 h, 93 %

NC

R

4 h, 95 % O

H

H

O2N

4 h, 93 %

4 h, 97 %

4 h, 92 %

O H O 4 h, 90 %

Figure 20. Visible light-induced reductive dehalogenation of haloketones using P-24 as a polymer photocatalyst under the irradiation of household fluorescent light bulb.

The activation of carbon-halogen bond is an appealing reaction step since it could be employed for the C-H functionalization and C-C bond formation. However, the extremely high dissociation energy of carbon-halogen bonds (C-I, 53 kcal mol-1; C-Br, 67 kcal mol-1; C-Cl, 81 kcal mol-1; C-F 109 kcal mol-1) is beyond the activity of most organocatalysts, thus the hydrodehalogenation reaction is normally carried out using metal catalysts107. Recently, some photocatalysts were reported as a promising candidate for the carbon-halogen bond activation under mild reaction conditions85, 108-109. While most of them involved the transition metals, only a few organic photocatalysts were reported for the carbon-halogen bond activation reaction58, 110-112. A poly-benzothiadiazole derivative (P-24) exhibited catalytic activity for the debromination reaction

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of haloketones113. Various substrates containing electrondonating group such as methyl and methoxy, and electron-withdrawing group such as fluoride, bromide, cyano, and nitro were compatible for the reaction with generally high yields after 4 h of visible light exposure using a household energy saving bulb (Figure 20). The dehalogenated product could be obtained in a quantitative manner even after 5th repeated cycle owing to the stability of P-24. The reaction mechanism for the debromination of haloketones is shown in Figure 21. Under light irradiation, the photo-generated electron was transferred from LUMO of P-24 to α-bromoketone produced a reactive carboradical after the disassociation of C-Br bond. A hydrogenated product could be obtained by the heterogeneous coupling between the ketone radical and hydrogen radical generated from Hantzsch ester. Additional amine was required as a sacrificial electron donor and a neutralizing base for side products. O

brominated substrates such as α-bromomalonate, αbromoacetophenone and p-nitrobenzoyl bromide could be efficiently debromonated to generate the reactive carboradicals for the α-alkylation reaction with high yields and selectivity (Figure 23). The study also demonstrated that the P-1-coated-glass fibers could be used repeatedly in the flow photoreactor without compensating the catalytic efficiency, which might lead to a potential photoreactor design for large-scale production of high value chemicals using metal-free and polymer-based heterogeneous photocatalysts.

O

R

R

Br e

Page 12 of 19

H

-

LUMO EtOOC O

COOEt

-

+ Br

N H

R

EtOOC

COOEt N

-

+ Br

EtOOC

COOEt N

Figure 22. (a) A scheme of fixed-bed photoreactor with P-1 coated glass fibers. (b) Building unit of P-1. (c) Photography of pure glass fiber (d) and P-1-coated glass fiber. (e) SEM images of (e) pure glass fiber and (f-h) P-1-coated glass fiber. Adapted with permission from © 2017 The Royal Society of 117 Chemistry; Ref. . O

N

HOMO

N h+

O

-

+ Br

H

R1 +

Br R2 R3

N H white LED, DMF, r. t.

Figure 21. Proposed reaction pathways for visible light induced reductive dehalogenation.

The photo-generated radical via the reductive dehalogenation reaction could facilely react with nucleophiles such as alkene, alkyne and heteroaromatics to construct the C-C and carbon-heteroatom bonds114-116. On this basis, the α-alkylation of aldehydes has been firstly achieved by MacMillan et al. via an interwoven activation pathway combining a photocatalyst and a chiral organocatalyst1. The obtained products illuminated an excellent enantioselectivity with considerable yields. In a similar manner, we designed a flow photoreactor containing P-1-coated glass fibers for the α-alkylation reaction117. As illustrated in Figure 22, the SEM images revealed a thin layer of P-1 with a thickness of ca. 80 nm on the surface of glass fibers. The effective photocatalyst content was calculated to be about 3.2 wt %. In the flow photoreactor, a series of

R2 R3

O

H O

O O

O

12 h, 96%, ee 83%

H O

O O

R1

H

O

H O

O

HCl

P-1, Lutidine,

O

N

N

O

O O

5 h, >99%, ee 89%

O

O

7 h, 98%, ee 93%

O

H

H O

O

NO2 10 h, 78%, ee 93%

15 h, 83%, ee 95%

Figure 23. Enantioselective α-alkylation reaction in a continuous flow setup using P-1-coated glass fiber as a photocatalyst.

The reaction mechanism for α-alkylation was illustrated in Figure 24. The photo-generated electron could reduce alkyl bromide to produce alkyl radical after C-Br cleavage.

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The obtained alkyl radical was then added to enamine intermediate formed through the condensation of organocatalyst and aldehyde, offering an amine radical, which underwent a further oxidation by photocatalyst to give an unstable imine cation. The final product could be obtained after hydrolysis of iminium cation, accompanying with the regeneration of organocatalyst. Other heteroaryl compounds, such as furan, thiophene, pyrrole, indole and benzofuran, can also act as nucleophiles for the photo-generated radical, forming the arylated products29, 118. A series of molecular benzothiadiazole derivatives were reported as efficient visible light photocatalysts for the arylation reaction between αbromomalonate and heteroarenes with high reaction yields119. However, poly-benzothiadiazole-based heterogeneous photocatalysts have not been reported for this type of arylation reactions so far. Alkyl-R

Br

LUMO O N

+ Br

Alkyl-R

N

t-Bu

R

O O

O N

N N H

N t-Bu

R

H t-Bu

(Organocatalyst)

Alkyl-R

O

R HOMO

The introduction of the diethynyl unit into the linear polymer chain could also improve the H2 evolution efficiency126. In comparison to P-25, P-34 with diethynyl moieties showed 2.4 times higher H2 production rates than the polymer without ethynyl units, likely due to the broader visible light absorption and higher LUMO level. Further improvement could be achieved by combining P34 with an inorganic semiconductor as CdS127, causing the broader light absorption range and the internal Z-scheme electron transfer within the hybrid photocatalyst. Recently, Tian et al. reported surfactant-stabilized conjugated poly-benzothiadiazole dots, which could generate H2 by water splitting under light irradiation (Figure 25)128. Compared to the polymer dots without the benzothiadiazole unit in the main chain, the polymer photocatalysts containing benzothiadiazole, P-35 and P-36, exhibited effectively improved H2 evolution rates129. The corresponding DFT calculation indicated that the nitrogen atom could be the catalytically active site in the benzothiadiazole unit.

e-

-

charge transfer, leading to the higher H2 production rate. P-25 could be recycled for five additional experiments up to 30 h without appearing any loss in catalytic activity. Another study showed that the P-1/TiO2 composite material exhibited the higher H2 production than that of the pure polymer photocatalyst, probably due to improved e/h+ separation by the formation of P-1/TiO2 heterojunction75.

Alkyl-R

H R

h+

O N N

t-Bu

Alkyl-R R

Figure 24. Proposed reaction mechanism of α-alkylation of aldehydes with P-1 visible light photocatalyst.

Among various photocatalytic reduction reactions, the H2 evolution via water splitting using polymer photocatalysts under visible light has been drawn much attention120124 . The polymer photocatalysts have demonstrated their important advantages with tunable energy band positions by simply changing and modifying the building blocks120. In a recent study, we designed various polybenzothiadiazoles (P-25 to P-33) and investigated their structural influence on the photocatalytic H2 production rate125. Interestingly, the linear polymer, P-25, exhibited the highest H2 evolution rate up to 116 µmol h-1 with an apparent quantum yield (AQY) of 4.01% at 420 nm. The P-25 with linear structure showed an improved photocurrent compared to other 3D polymer analogues (P-26 to P33), which indicates enhanced electronic conductivity and

Figure 25. Preparation of water-dispersible polymer dot as a photocatalyst for the light-driven hydrogen evolution via water splitting. Adapted with permission from © 2016 Wiley128 VCH; Ref. .

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Figure 26. Schematic illustration of the switchable hydrophilicity and the photocatalytic applications of the polymer photocatalyst P-37 in water by CO2/N2 swing. P-BT-DEA is referred to P-37 in this review. Adapted with permission from 130 © 2018 Wiley-VCH; Ref. .

Very recently, our group designed a conjugated polybenzothiadiazole photocatalyst (P-37) with tertiary amine terminals, that can reversibly bind CO2 in water, generating a switchable hydrophilicity130. As illustrated in Figure 26, the CO2-assisted hydrophilicity can boost up the photocatalytic efficiency of P-37 in water with minimum dosage. When CO2 was desorbed, the photocatalyst could be isolated from reaction media, the polymer photocatalyst could be simply regenerated for repeated use. Different organic photoredox reactions such as the radicalmediated arylation of heteroarenes, i.e. caffeine, have been examined.

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transformations, for example, the direct aromatic or aliphatic C-H functionalization, or C-O and C-N activation etc. have not yet been reported with the polymer-based photocatalysts. In order to achieve those targets, polybenzothiadiazoles with extremely high redox potentials needs to be designed. Secondly, the precise control of energy level and morphology is a huge challenge for the polymer-based heterogeneous photocatalysts, or in a broader sense, for the class of organic polymer materials in general. Advanced polymerization approaches are required to tailor the structure and morphology of the polymer photocatalyst for target-specific reactions. One possible solution might be the compartmentation of the polymerization environment. Thirdly, the precise charge transfer pathway and reactive site of the polymer photocatalysts still remained unclear on current state of research. More photophysical analysis and molecular simulation should be applied for a better understanding of the reaction mechanism. Lastly, the development of delicate photoreactors should be an important issue in order to realize large-scale chemical production and the final industrial application of heterogeneous photocatalysis. Last but not least, we anticipate that more advanced polymer photocatalysts, especially the poly-benzothiadiazolebased ones, could have the potential to resolve the remaining tasks for efficient photochemical transformations.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected].

Notes §

These authors contributed equally. The authors declare no competing financial interest.

ACKNOWLEDGMENT 4. Summary and outlook The exploitation of poly-benzothiadiazoles and their derivatives as visible light-active, purely organic and heterogeneous photocatalysts has greatly enriched photocatalyst family and synthetic methodologies as a promising alternative to the state-of-art metal photocatalysts. The polymer photocatalysts showed considerable advantages as being easily tunable in their morphology, porosity, energy band positions and therefore the photoredox potentials. The repeating experiments demonstrated the high reusability of the polymer photocatalysts. It is also worth to point out that devices as continuous flow photoreactors could also be constructed using the polybenzothiadiazole photocatalysts. Those factors have made the poly-benzothiadiazoles a promising candidate as highly useful heterogeneous photocatalysts. Although a considerable number of photoredox reactions have been successfully conducted using the polymer-based photocatalysts, there are still remaining challenges, which need to be further addressed and investigated. Firstly, the photocatalytic application of polybenzothiadiazoles and their derivatives is still limited to a narrower range of photoredox reactions compared to transition metal complexes. Highly challenging organic

The authors acknowledge the Max Planck Society for the financial support. R.L. W.H. and L.W. thank the China Scholarship Council (CSC) for the graduate scholarship. J. B. thanks the Alexander von Humboldt Foundation for postdoctoral research fellowship.

REFERENCES (1) Nicewicz, D. A.; MacMillan, D. W. C. Merging Photoredox Catalysis with Organocatalysis: The Direct Asymmetric Alkylation of Aldehydes. Science 2008, 322, 77-80. (2) Ischay, M. A.; Anzovino, M. E.; Du, J.; Yoon, T. P. Efficient Visible Light Photocatalysis of [2+2] Enone Cycloadditions. J. Am. Chem. Soc. 2008, 130, 12886-12887. (3) Narayanam, J. M. R.; Tucker, J. W.; Stephenson, C. R. J. Electron-Transfer Photoredox Catalysis: Development of a TinFree Reductive Dehalogenation Reaction. J. Am. Chem. Soc. 2009, 131, 8756-8757. (4) Studer, A. A “Renaissance” in Radical Trifluoromethylation. Angew. Chem. Int. Ed. 2012, 51, 8950-8958. (5) Alonso, C.; Martínez de Marigorta, E.; Rubiales, G.; Palacios, F. Carbon Trifluoromethylation Reactions of Hydrocarbon Derivatives and Heteroarenes. Chem. Rev. 2015, 115, 1847-1935. (6) Ghosh, I.; Marzo, L.; Das, A.; Shaikh, R.; König, B. Visible Light Mediated Photoredox Catalytic Arylation Reactions. Acc. Chem. Res. 2016, 49, 1566-1577.

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(113) Wang, Z. J.; Ghasimi, S.; Landfester, K.; Zhang, K. A. I. A Conjugated Porous Poly-Benzobisthiadiazole Network for a Visible Light-Driven Photoredox Reaction. J. Mater. Chem. A 2014, 2, 18720-18724. (114) Studer, A.; Curran, D. P. Catalysis of Radical Reactions: A Radical Chemistry Perspective. Angew. Chem. Int. Ed. 2016, 55, 58-102. (115) Yu, J.-T.; Pan, C. Radical C-H Functionalization to Construct Heterocyclic Compounds. Chem. Commun. 2016, 52, 2220-2236. (116) Liu, D.; Liu, C.; Lei, A. Carbon-Centered Radical Addition to C=X Bonds for C−X Bond Formation. Chem. Asian. J. 2015, 10, 2040-2054. (117) Huang, W.; Ma, B. C.; Wang, D.; Wang, Z. J.; Li, R.; Wang, L.; Landfester, K.; Zhang, K. A. I. A Fixed-Bed Photoreactor Using Conjugated Nanoporous Polymer-Coated Glass Fibers for Visible Light-Promoted Continuous Photoredox Reactions. J. Mater. Chem. A 2017, 5, 3792-3797. (118) Furst, L.; Matsuura, B. S.; Narayanam, J. M. R.; Tucker, J. W.; Stephenson, C. R. J. Visible Light-Mediated Intermolecular C−H Functionalization of Electron-Rich Heterocycles with Malonates. Org. Lett. 2010, 12, 3104-3107. (119) Wang, L.; Huang, W.; Li, R.; Gehrig, D.; Blom, P. W. M.; Landfester, K.; Zhang, K. A. I. Structural Design Principle of Small-Molecule Organic Semiconductors for Metal-Free, VisibleLight-Promoted Photocatalysis. Angew. Chem. Int. Ed. 2016, 55, 9783-9787. (120) Sprick, R. S.; Jiang, J.-X.; Bonillo, B.; Ren, S.; Ratvijitvech, T.; Guiglion, P.; Zwijnenburg, M. A.; Adams, D. J.; Cooper, A. I. Tunable Organic Photocatalysts for Visible-LightDriven Hydrogen Evolution. J. Am. Chem. Soc. 2015, 137, 32653270. (121) Zhang, G.; Lan, Z.-A.; Wang, X. Conjugated Polymers: Catalysts for Photocatalytic Hydrogen Evolution. Angew. Chem. Int. Ed. 2016, 55, 15712-15727. (122) 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. (123) Sprick, R. S.; Bonillo, B.; Clowes, R.; Guiglion, P.; Brownbill, N. J.; Slater, B. J.; Blanc, F.; Zwijnenburg, M. A.; Adams, D. J.; Cooper, A. I. Visible-Light-Driven Hydrogen Evolution Using Planarized Conjugated Polymer Photocatalysts. Angew. Chem. Int. Ed. 2016, 55, 1792-1796. (124) Bi, S.; Lan, Z.-A.; Paasch, S.; Zhang, W.; He, Y.; Zhang, C.; Liu, F.; Wu, D.; Zhuang, X.; Brunner, E.; Wang, X.; Zhang, F. Substantial Cyano-Substituted Fully sp2-Carbon-Linked Framework: Metal-Free Approach and Visible-Light-Driven Hydrogen Evolution. Adv. Funct. Mater. 2017, 27, 1703146. (125) Yang, C.; Ma, B. C.; Zhang, L.; Lin, S.; Ghasimi, S.; Landfester, K.; Zhang, K. A. I.; Wang, X. Molecular Engineering of Conjugated Polybenzothiadiazoles for Enhanced Hydrogen Production by Photosynthesis. Angew. Chem. Int. Ed. 2016, 55, 9202-9206.

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(126) Zhang, X.-H.; Wang, X.-P.; Xiao, J.; Wang, S.-Y.; Huang, D.-K.; Ding, X.; Xiang, Y.-G.; Chen, H. Synthesis of 1,4Diethynylbenzene-Based Conjugated Polymer Photocatalysts and Their Enhanced Visible/near-Infrared-Light-Driven Hydrogen Production Activity. J. Catal. 2017, 350, 64-71. (127) Zhang, X.; Xiao, J.; Hou, M.; Xiang, Y.; Chen, H. Robust Visible/near-Infrared Light Driven Hydrogen Generation over ZScheme Conjugated Polymer/CdS Hybrid. Appl. Catal. B 2018, 224, 871-876. (128) Wang, L.; Fernández-Terán, R.; Zhang, L.; Fernandes, D. L. A.; Tian, L.; Chen, H.; Tian, H. Organic Polymer Dots as Photocatalysts for Visible Light-Driven Hydrogen Generation. Angew. Chem. Int. Ed. 2016, 55, 12306-12310. (129) Pati, P. B.; Damas, G.; Tian, L.; Fernandes, D. L. A.; Zhang, L.; Pehlivan, I. B.; Edvinsson, T.; Araujo, C. M.; Tian, H. An Experimental and Theoretical Study of an Efficient Polymer Nano-Photocatalyst for Hydrogen Evolution. Energy Environ. Sci. 2017, 10, 1372-1376. (130) Byun, J.; Huang, W.; Wang, D.; Li, R.; Zhang, K. A. I. CO2-Triggered Switchable Hydrophilicity of Heterogeneous Conjugated Polymer Photocatalyst for Enhanced Catalytic Activity in Water. Angew. Chem. Int. Ed. 2018, 57, 2967-2971.

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