Design Strategies and Redox-Dependent Applications of Insoluble

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Design strategies and redox-dependent applications of insoluble viologen-based covalent organic polymers Tina Skorjanc, Dinesh Shetty, Mark A. Olson, and Ali Trabolsi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20743 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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ACS Applied Materials & Interfaces

Design strategies and redox-dependent applications of insoluble viologen-based covalent organic polymers Tina Škorjanc,1 Dinesh Shetty,1* Mark A. Olson,2 and Ali Trabolsi1* 1Science 2School

Division, New York University Abu Dhabi, Saadiyat Island, PO Box 129188, Abu Dhabi, UAE of Pharmaceutical Science and Technology, Health Science Platform, Tianjin University, Tianjin, China

KEYWORDS: viologens • covalent organic polymers • redox chemistry • gas adsorption • pollutant removal

ABSTRACT: Dicationic quaternary salts of 4,4’-bipyridine, also referred to as the viologen family, are well known for their interesting redox chemistry, whereby they can be reversibly reduced into radical cationic and neutral moieties. Because of this ability to switch between different redox states, viologens have frequently been incorporated into covalent organic polymers (COPs) as molecular switches to construct stimuli-responsive materials. While many viologen-based COPs have been reported, hyper-conjugated insoluble COPs started to emerge fairly recently and have not been comprehensively reviewed. In this review, we investigate the design strategies employed in the synthesis of insoluble viologen-based COPs, which can be broadly classified as those with viologen in the backbone and those with viologen as pendant groups. Chemical reactions used in the synthesis of each category, including Sonogashira-Hagihara cross-coupling, Menshutkin and Zincke reactions, are highlighted. Diverse applications of these COPs are discussed with particular reference to the redox state of viologen in each material. Uses of these materials for gas adsorption, organic and inorganic pollutant removal, catalysis, sensing and film fabrication are explored.

1. INTRODUCTION Covalent organic polymers (COPs) represent a broad class of materials which are, unlike supramolecular1 and coordination polymers,2 formed through covalent bonds between individual organic building blocks.3–5 Owing to their covalent nature, these materials are robust and stable in air as well as in a variety of solvents, and only start to degrade at high temperatures.6 COPs can be both soluble and insoluble, but the latter are particularly interesting on account of:7 (i) insoluble materials are easy to purify by simple washing, whereas soluble materials require more elaborate purification techniques; (ii) insoluble materials are easy to separate from suspension, which greatly facilitates removal without significant loss of material, enhancing cost-efficiency and minimizing waste; (iii) facile separation simplifies regeneration and reuse of materials, which is of particular importance in applications such as catalysis and pollutant removal. All of these combined advantages have stimulated significant interest within the scientific community in the field of insoluble covalent polymeric materials,8 which have been successfully implemented for catalysis,9 removal of pollutants from water10 and organic solvents,11 specialized biomedical applications12 and sensing.13,14 The performance of insoluble COPs in various applications can be enhanced by the incorporation of subunits which are responsive to external stimuli, including light, temperature, pH, force, or the binding of various guests.15 If exposure to a stimulus changes the structure and/or properties of the material, but its removal restores the initial state of the material, the stimulus-responsive subunit is called a molecular switch.16 With incorporation of molecular switches into COPs,

certain properties of materials can be reversibly switched on and off, allowing for the regeneration and recycling of COPs.17,18 A common type of molecular switch incorporated into COPs is a redox switch, which can be altered by a change in the redox potential of its environment. This property has been utilized for the preparation of actuators,19 sensors,20 drug delivery systems,21 electrochemical devices,22 and for environmental applications such as dye, iodine and oxoanion removal.23 Some of the most extensively studied redox molecular switches are derivatives of 4,4’-bipyridine, also known as viologens. As synthesized, they are positively charged, capable of being reversibly reduced to a radical cationic or neutral species by means of chemical or electrical reductions (Scheme 1).16,24

Scheme 1. Redox chemistry of viologen. Each redox state of N,N′-dialkyl-4,4′-bipyridinium is associated with a different color: dicationic is colorless, radical cationic is blue, and neutral is brownish red. R = alkyl chain. The incorporation of viologens into polymeric systems brings numerous advantages: (i) unstable radical cationic species get stabilized by extended polymeric structures, preserving the radical character in open air for several days;25,26 (ii) polymeric viologens can exhibit porosity and therefore be utilized for

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applications that require significant surface areas such as gas adsorption, separation, and reaction catalysis; (iii) polymeric viologens typically outperform monomers and dimers in applications that rely on in situ generation of radical cationic species because of improved charge transfer abilities;27 (iv) the inherent cationic character of viologen-based polymers enhances dispersibility of insoluble materials; (v) controlling the redox state of viologen allows for selective recognition of a variety of different types of guests (hydrophobic, hydrophilic, anionic, cationic etc.).28 Although the literature on viologen-containing polymers is vast and has been the subject of several review articles,24,29–31 these works focus on soluble polymers, likely because insoluble viologen COPs have only recently emerged. In this review, we focus on insoluble viologen-based COPs because it is their insolubility that greatly expands the range of their applications. In the first section, we comprehensively describe the different synthetic protocols used to synthesize insoluble COPs with viologen incorporated into (i) polymer backbones and (ii) into the side chains of the polymers. In the second part of this review, we focus our attention on the applications of insoluble viologen-containing COPs, and how the performance of these materials is affected by the redox state of the viologen subunits. Applications in gas adsorption, pollutant capture, catalysis, sensing, and film fabrication are discussed. We conclude this review with a third section which delves into the challenges that insoluble viologen-based polymeric materials are likely to face in the future and how these could be addressed and overcome.

2. DESIGN OF VIOLOGEN-BASED COPS Several ways of incorporating viologen subunits into insoluble COPs have been reported. Viologens can be part of the polymer backbone (i.e. in the main chain), or included as side chains or pendants. Overall, viologen main-chain COPs tend to be synthesized through solution chemistry whereas COPs with side-chain viologens are commonly synthesized on solid supports such as polymeric films or resins. Many polymers with side-chain viologens are soluble,32,33 so synthesis on a solid support,34,35 or additional crosslinking36 are frequently utilized to produce insoluble materials. This section highlights specific reactions commonly employed in the synthesis of each type of viologen-based COPs.

2.1. COPs with viologen in the backbone Soluble viologen-based monomers have been polymerized into insoluble COPs both through electropolymerization and in chemical reactions. Generally, electropolymerized COPs require a solid support. For instance, electropolymerization of 1,1',1''-(benzene-1,3,5-triyltris(methylene))tris(4-cyanopyridin -1-ium) units through constant potential electrolysis on an indium tin oxide (ITO) substrate generates insoluble anode materials.37 While examples of insoluble main-chain viologen COPs obtained through electrochemical synthesis exist, the majority of insoluble viologen COPs are synthesized in solution. Menshutkin reaction is one of the most common methods used in the synthesis of insoluble viologen COPs. In this reaction, a nucleophilic tertiary amine is converted to a quaternary ammonium salt using an alkyl halide.38 If a symmetric alkyl halide is reacted with 4,4’-bipyridine, a covalent polymer is obtained.39 Such COPs have been synthesized from 4,4’-

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bipyridine and linear terminal alkyl halides, such as 1,8dibromooctane,40 and 1,4-(dibromomethyl)benzene.41,42 These, however, often remain sparingly soluble, so they must be further modified to form insoluble materials. For example, non-crosslinked viologen COPs have been used for selfassembly with palladium nanoparticles, which upon reduction with NaBH4 formed solid-phase self-organized catalysts.41,42 Similarly, a soluble chloromethylated polysulfone film has been converted into an insoluble membrane using 4,4’bipyridine as a crosslinking agent.43 Some of the first porous viologen-based COPs were reported by the Coskun group. Their materials were obtained through Sonogashira–Hagihara cross-coupling, which couples terminal alkynes with aryl halides in the presence of an organic base, and two catalysts, CuI and Pd(PPh3)4. For instance, 1,1’-bis(4halogenophenyl)-4,4’-bipyridinium chloride and tetrakis(4ethynylphenyl) methane44 were reacted to generate a microporous COP with a surface area of 755 m2 g–1.44 The same 1,1’-bis(4-halogenophenyl)-4,4’-bipyridinium chloride has also been coupled with tris(4-ethynylphenyl)amine or 1,3,5-tris(4-ethynylphenyl)benzene,45 to generate porous viologen COPs with surface areas reaching up to 960 m2 g–1 (Figure 1A).45 Synthesis of such highly porous viologen COPs opened the door to applications in gas adsorption, which were previously unreported for viologen-containing COPs. However, this type of chemical coupling is difficult to scale up on account of cost, thus other synthetic approaches have been adopted. In search of cost-effective synthetic methods, our group investigated insoluble COPs from non-linear alkyl halides and 4,4’-bipyridine. We reported a COP with a hexatopic cyclotriphosphazene core linked by 4,4’-bipyridine synthesized through a Menshutkin reaction in acetonitrile both solvothermally46 and under microwave irradiation (Figure 1B).47 While the solvothermal method of synthesis took 72 hours, microwave irradiation shortened the reaction time to just 1 hour because of its ability to induce dielectric heating. The morphology and physical properties of the COPs obtained by both synthetic methods were comparable, but the microwave synthesis approach was much more time-efficient. COP synthesis from viologen-related starting materials have also been investigated. Reticular modifications to the viologen subunit of a COP can help in tailoring materials to specific applications and maximizing their performance. For example, a porous COP formed in a Menshutkin reaction between a viologen-related compound, diazapyrenium dication (DAP2+) and 1,3,5-(tribromomethyl)benzene with a surface area of 41 m2 g–1 has been reported.48 DAP2+ had previously been known to form supramolecular 2:1 adducts with aliphatic amines through a charge-transfer mechanism. This property was exploited at the polymeric level and the COP served as an effective amine sensor. Simultaneously with reports of porous viologen COPs, the material science community also explored the synthesis of crystalline COPs known as covalent organic frameworks (COFs). Synthesis of COFs requires formation of dynamic covalent bonds, which allow for a mechanism of selfcorrection ultimately leading to crystallinity.49 Imine chemistry is one of the most widely known mechanisms of COF formation, and one of the first attempts at synthesizing a viologen COF involved imine condensation between a methyl hydrazine-terminated dendrimer with a cyclotriphosphazene


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core, and 1,1'-bis(4-formylbenzyl)-[4,4'-bipyridine]-1,1'-diium hexafluorophosphate.50 This material, however, turned out to be



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Figure 1. Synthetic methods for COPs with viologen incorporated into the structural backbone. Sonogashira–Hagihara coupling (A, adapted from ref. 45 Published by The Royal Society of Chemistry); Menshutkin reaction (B, adapted with permission from ref. 46. Copyright 2016 The Royal Society of Chemistry); imine condensation (C, adapted with permission from ref. 51. Copyright 2016 The Royal Society of Chemistry); ionothermal trimerization for covalent triazine framework synthesis (D, ref. 52); Zincke reaction (E, ref. 53); diazo coupling (F, adapted with permission from ref. 28. Copyright 2018 Wiley). Overall, there is no single synthetic pathway to generate an non-crystalline. In the same year, the first and so far the only optimal COP with a viologen-based backbone. If a highly imine viologen-containing COF (Figure 1C) was obtained porous material is needed, Sonogashira – Hagihara crossthrough condensation of 1,3,5-tris(4-aminophenyl)benzene coupling or ionothermal trimerization may be suitable. If a and 1,1'-bis(4-formylphenyl)-[4,4'-bipyridine]-1,1'-diium crystalline COP is desired, traditional COF synthetic hexafluorophosphate.51 It formed a 2-dimensional COF with strategies, such as imine condensation, or novel reactions hexagonal pores. whose mechanisms involve dynamic chemistry, such as the A more recent method for generating COPs with high surface Zincke reaction, can be employed. It should be noted that in areas involves the synthesis of covalent triazine frameworks addition to a good understanding of the reaction mechanism, it (CTF). CTFs are a special class of highly porous COPs first is also essential to understand the structures of the starting synthesized in 2008.54 This class of material has attracted materials. Spatial orientation of functional groups, planarity of interest due to their facile and scalable synthesis, high nitrogen the molecule, and its flexibility all influence polymerization 52 content, and structural tunability. A charged CTF from 1,1'and hence physical properties of the synthesized COPs. bis(4-cyanophenyl)-[4,4'-bipyridine]-1,1'-diium dichloride synthesized through ionothermal trimerization with ZnCl2, 2.2. COPs with viologen pendants which serves both as a solvent and a catalyst, has been 52 The incorporation of viologen into pendants of COPs allows published (Figure 1D). The surface area of the obtained for asymmetric functionalization of the bipyridine subunits, a network depended greatly on the reaction temperature and process which typically occurs in two distinct steps. This is varied from 744 m2 g–1 at 400 °C to 1247 m2 g–1 at 500 °C. not the case however for COPs bearing viologen backbones Our group recently found that viologen-based COFs also form because symmetric viologen derivatives are required for through the Zincke reaction, which is the first example of polymerization to proceed in both directions. Polymers with using this reaction to synthesize a crystalline covalent organic side chain viologens can, however, be soluble,32,33 so synthesis 53 material. Zincke reaction is a two-step process to obtain a on a solid support,34,35 or additional crosslinking36 are quaternary pyridinium salt from bipyridine. In the first step, frequently utilized to produce insoluble materials. The use of 4,4’-bipyridine is reacted with 1-chloro-2,4-dinitrobenzene to surface chemistry brings advantages other than insolubility, its generate the Zincke salt in an SN2 reaction. In the second step, related ease of purification, and regeneration. For example, a primary amine attacks the most electrophilic carbon in the reactions on solid supports can be driven to completion more Zincke salt and opens up the pyridine ring, 2,4-dinitroaniline easily than conventional reactions in solution phase through is eliminated, the ring closes and a quaternized pyridinium ion the use of excess reagents in solution.61 Furthermore, solid is formed.55 Ring opening is thought to be the rate-determining supports may add stability to the materials and such reactions step and the reversibility of the reaction likely allows for error may be easier to automate.61 There are two main ways to correction and results in a crystalline material. We reacted the synthesize insoluble COPs with viologen units in the side Zincke salt with 1,3,5-tris(4-aminophenyl)benzene in a chains: (i) polymerization of monomers followed by the EtOH/H2O mixture (1/1, v/v) under microwave irradiation and introduction of viologen, or (ii) introduction of viologen into a 53 this yielded a gel composed of crystalline sheets (Figure 1E). monomer followed by polymerization. We also demonstrated that crystallinity of viologen COFs Articles that report polymerization of monomers followed by synthesized through the Zincke reaction is highly solvent- and the introduction of viologen, introduce viologen on solid amine-dependent. The same reaction in 1,4-dioxane or an supports such as polymeric films34 or beads.36 Typically, EtOH/H2O mixture (4/1, v/v) yielded non-crystalline COPs. polymers contain a monomer with an aliphatic alkyl halide, This could be because a water-ethanol mixture has been found such as chloride. Halides serve as good leaving groups upon to be an optimal solvent for the Zincke reaction for its ability the nucleophilic attack by electron-rich nitrogen centers of to stabilize reaction intermediates most effectively, thereby 4,4’-bipyridine in simple SN2 Menshutkin reactions. Insoluble allowing time for the self-correction mechanism.56 When resins of polystyrene crosslinked by viologens36 have been 1,3,5-tris(4-aminophenyl)benzene was replaced by pyrenesynthesized in this manner (Figure 2A). Firstly, polystyrene based amines, crystallinity was not observed either,57 most functionalized with chloromethyl groups were reacted with likely due to the non-planar structure of the precursor amines. 4,4’-bipyridine to generate a mono-substituted viologen Recently, use of a polyhedral oligomeric silsesquioxane core species. The second nitrogen quaternization was achieved in a in the Zincke reaction performed in DMF under solvothermal subsequent reaction with chloromethylbenzene. Viologenconditions yielded a weakly crystalline material based on grafted low-density polyethylene films34 were similarly PXRD, but no structural model of the material was proposed.58 fabricated by polymerizing chloromethyl ethene, followed by It has recently been shown that the Zincke reaction can also be a Menshutkin reaction with 4,4’-bipyridine to achieve the first used to generate a CTF from 4,4',4''-(1,3,5-triazine-2,4,6quaternization with a subsequent Menshutkin reaction with 59,60 triyl)trianiline and a Zincke salt. Compared to ionothermal chloromethylbenzene completing the second quaternization trimerization,52 this solvothermal method requires significantly (Figure 2B). lower temperatures (120 °C vs. 400 – 500 °C) and slightly Contradistinctively, the introduction of viologen into longer reaction times (72 h vs. 48 h). However, the surface monomers followed by polymerization, 4,4’-bipyridine is area of the CTF obtained through the Zincke reaction was only typically asymmetrically functionalized and one of the 30 m2 g–1.59


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introduced functional groups participates in the polymerization reaction. For example, 1-[3-(2-methacryloxyethyl dimethyl ammono)propyl] 1’-propyl-4,4’-bipyridinium tribromide was synthesized in two steps. First, one of the nitrogen atoms in 4,4’-bipyridine was quaternized in a Menshutkin reaction with 1-bromopropane. Next, the product was subject to another Menshutkin reaction generating the monomer, which was then copolymerized with n-butyl acrylate and 2-(N,N’dimethylaminoethyl)methacrylate (DAEMA) using AIBN as an initiator (Figure 2C).62 Insolubility of this material was achieved by introducing a crosslinking agent between the DAEMA subunits. UV-induced graft copolymerization of Nhexyl-N’-(4-vinylbenzyl)-4,4’-bipyridinium bromide chloride on the surface of a polyethylene terephthalate films similarly generated an insoluble material with asymmetrically functionalized viologen side chains (Figure 2D).35

Figure 2. Design strategies to obtain COPs with pendant viologen. Introduction of viologen into polystyrene (A, adapted with permission from ref. 36. Copyright 1997 Elsevier) or low density polyethylene (B, ref. 34) via Menshutkin reaction; introduction of viologen into monomer via Menshutkin reaction followed by block copolymerization (C, adapted with permission from ref. 62. Copyright 2001 Elsevier) or graft copolymerization (D, adapted with permission from ref. 35. Copyright 2005 Elsevier).

3. APPLICATIONS OF INSOLUBLE VIOLOGENBASED COPS 3.1. Gas adsorption Anthropogenic emissions of carbon dioxide and other greenhouse gases result in global warming, which creates a need to design materials that can effectively sequester and store these gases.63 COPs have been investigated as CO2 adsorbers because of their extensive surface areas, the main requirement for a potent gas adsorption material, which interacts with gas molecules through non-covalent interactions.64 While porous materials such as metal organic frameworks, zeolites, and covalent organic frameworks show good adsorption capacities, COPs are often more stable, and both easier and cheaper to synthesize.65 Certain COPs have been reported to have CO2 uptake capacities as high as 5616 mg g–1 under specific conditions of high pressure (200 bar) and elevated temperatures (65 ℃), but viologen COPs have been generally tested for adsorption at less extreme conditions.66 Although it has been postulated that the cationic charge of viologen and the presence of counterions, which may block pores, impede high porosity of viologen-based COPs,46 several examples of such materials with good surface areas have been reported to date.44,45,52 Dicationic viologens as well as their corresponding counterions in COPs can form electrostatic interactions with CO2 because polar functional groups enhance material’s CO2-philicity.44 Table 1. CO2 uptake capacities of various insoluble viologen COPs. The number in the name of charged triazine framework (CTF) refers to the reaction temperature in K. PCP = porous cationic polymer, POP = porous organic polymer, Red = reduced dicationic viologen units into neutral species. COP

BET surface area (m2 g–1)

Pressure (bar)

Ref

PCPCl

755

101.7

61.4

-

1

44

PCPBF4

586

97.0

58.4

-

1

44

PCPPF6

433

78.1

47.0

-

1

44

VPCIFBr

383

102.5

73.1

-

1

58

VPCIFCl

174

86.7

62.1

-

1

58

POPV1

812

-

40.5

-

1.1

45

RedPOPV1

606

-

29.5

-

1.1

45

POPV2

960

-

55.9

-

1.1

45

RedPOP-

591

-

48.0

-

1.1

45


CO2 adsorption (mg g–1) 273 K 298 K 323 K


V2 cCTF -400

744

126

83

52

1

52

cCTF -450

861

99

62

38

1

52

cCTF -500

1247

133

80

47

1

52

The Coskun group studied the effect of viologen counterions and charges on CO2 capture using a COP obtained by the Sonogashira–Hagihara coupling of tetrakis(4-ethynylphenyl) methane and 1,1’-bis(4-iodophenyl)viologen salts.44 Bulkier counterions, such as BF4– or PF6– lead to decreased surface areas compared to smaller counterions such as Cl– (586 m2 g–1, 433 m2 g–1, and 755 m2 g–1, respectively; Table 1). However, BF4– and PF6– can act as Lewis bases, which compensates for their lower surface areas. As a result, their CO2 uptake capacities at 273 K (97.0 mg g–1 and 78.1 mg g–1, respectively) are comparable to that of the material with a higher surface area (101.7 mg g–1). More recently, it has been found that having a bromide counter ion as opposed to chloride increases CO2 uptake in a COP with a polyhedral oligomeric silsesquioxane core synthesized through the Zincke reaction.58 For all the studied counterions, gas adsorption decreases at higher temperature (Table 1). The effect of the redox state of viologen on CO2 adsorption in a similar system (Figure 1A) was studied by D’Alessandro et al.45 The authors observed that the gas uptake capacity drops after two-electron reduction of dicationic viologen from 0.92 mmol g–1 (≈ 40.5 mg g–1) to 0.67 mmol g–1 (≈ 29.5 mg g–1) in POP-V1 and from 1.27 mmol g–1 (≈ 55.9 mg g–1) to 1.09 mmol g–1 (≈ 48.0 mg g–1) in POP-V2. Based on their theoretical modeling, they postulate that the Cl– counterions form important interactions with CO2 molecules. Removal of the counterion upon reduction to the neutral state thus leads to decreased CO2 uptake. A cyanophenyl-substituted viologen-based porous CTF was also utilized for CO2 adsorption (Figure 1D).52 The surface areas of these frameworks were highly dependent on reaction temperature during their synthesis and reached up to 1247 m2 g–1 at 500 °C (Table 1). A maximum CO2 adsorption level of 133 mg g–1 was achieved with this highly porous COP. By comparing the uptake capacity with those of other triazine framework-based materials of similar surface area, the authors conclude that the cationic moieties of viologen enhance CO2philicity of the COP by up to five times. Taken together, these studies suggest that the charge on viologen and its chloride counterions maximize CO2 adsorption capacity of viologen-based covalent materials. Non-viologen COPs reach CO2 uptake capacities of up to 6.30 mmol g–1 (277 mg g–1) at 273 K and 1 bar,67 which is more than double the capacity of the best-performing viologen COP. Approaches to improving the performance of viologen COPs include further increasing surface areas, functionalizing pores to provide an optimal environment for CO2, and increasing the interaction energy between CO2 and the sorbent.68

3.2. Pollutant removal

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Removal of organic and inorganic pollutants from water is one of the most widely studied topics in environmental science.69 Different processes have been developed to treat polluted waste waters,70–73 but adsorption is particularly promising because of its cost-effectiveness, high uptake capacity, and the ability to be tailored to specific pollutants.74 General requirements of pollutant removal sorbents includes high adsorption ability, fast kinetics, ease of reusability, and good stability.69,75 The main advantage of viologen-based COPs used for such applications is their tunable charge: dicationic materials can interact with a range of anionic toxic dyes and anions, while neutral COPs can serve as Lewis bases and interact with hydrophobic species. Charge-charge interactions between viologen-based sorbents and pollutants can explain most observations of high pollutant uptake capacity for these materials. For example, we observed that dicationic COPs composed of phosphazene cores linked in a network by viologen spacers (Figure 1B)46 show high uptake of toxic oxoanions. These COPs removed 85 % of toxic MnO4– and Cr2O72– anions and 95 % of toxic ReO4– anions from water. Upon one- and in particular two-electron reduction of the bipyridinium groups, the materials become ineffective at removing oxoanions, which strongly suggests that the mechanism for pollutant removal involves anion exchange. This means that the Cl– counter ions get replaced by oxoanions as they dissolve into aqueous solution.

Figure 3. Pollutant adsorption experiments with viologen COPs. (A) Congo red adsorption by viologen- and calixarene-based COPs in their dicationic forms. Adapted with permission from ref. 28. Copyright 2018 Wiley. (B) Redox state of viologen affects the


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preference of a material to adsorb specific organic dyes. Adapted from ref. 47. Published by The Royal Society of Chemistry.

Similarly, materials with cationic viologens perform well in adsorbing anionic dyes. Li et al. studied the uptake of a series of anionic dyes (methyl orange, acid green, acid red 27, indigo carmine, and Direct Fast Brown M) by a crystalline cationic imine-based viologen COP (Figure 1C).51 This material removed >97 % of the various dyes at very low concentrations. The authors rationalize the high uptake capacity using the concept of hard and soft acids and bases.76 Anionic dyes act as soft bases, which pair up with the softly acidic COP, and hard Cl– ions pair up with hard Na+ ions. Our work similarly showed that positively-charged viologen polymers with a macrocyclic calix[4]arene core (Figure 1F) very efficiently adsorb anionic dye Congo red in the pH window of 2 – 10 (Figure 3A).28 COPs with macrocyclic βcyclodextrin cores and cationic viologen linkers also effectively remove anionic Congo red and methyl orange dyes.77 Recently, we carried out a systematic study to investigate the effect of redox state (i.e. charge) of a phosphazene-viologen COP (Figure 1B) on the uptake of various dyes: hydrophilic fluorescein, hydrophilic positively-charged rhodamine B, and hydrophobic Nile red (Figure 3B).47 We found that not only charge, but also the degree of hydrophilicity or hydrophobicity of the sorbent and the dye dictate the uptake capacity. Dicationic COP adsorbs 99 % of fluorescein in only 4 minutes, likely as a result of strong hydrophilic – hydrophilic interactions, which cannot develop between fluorescein and radical cationic or neutral COPs. Similarly, 85 % of Nile red was removed in 1 hour by the neutral COP, likely due to hydrophobic interactions between the two species. Dicationic and radical cationic COPs removed at most 25 % of Nile red. This study demonstrates that the specificity of a viologenbased COP can be tailored to a specific pollutant simply by altering the redox state of the viologen subunit. Table 2. Iodine uptake capacities by various covalent organic polymers with viologen linkers. A range of uptake capacities is given for each core and redox state combination as multiple materials have been reported. COP core

Redox state

Uptake capacity (mg g–1)

Temperature (°C)

Ref.

Phosphazene

Cationic

212 – 258

70

46

Phosphazene

Radical cationic

195 – 211

70

46

Phosphazene

Neutral

277 – 380

70

46

Calix[4]arene

Cationic

147 – 161

70

28

Calix[4]arene

Radical cationic

147 – 197

70

28

Calix[4]arene

Neutral

158 – 176

70

28

TAPB

Cationic

60 – 140

40

53

In addition to being proficient at removing organic pollutants, viologen-based COPs are also potent iodine capturers because of the presence of aromatic rings and quaternized N atoms.78 We studied the effect of viologen redox state on iodine uptake

in multiple COPs with distinct core moieties, including phosphazene, calixarene, and 1,3,5-tris(4aminophenyl)benzene (TAPB). Typically, 10 – 15 mg of a polymer was exposed to iodine vapors at elevated temperatures (40 or 70 °C) in a closed chamber at ambient pressure. Uptake capacities were measured gravimetrically and expressed as % mass increase. The results are summarized in Table 2. Multiple factors contribute to these observations, including the nature of the core, viologen counterions (if any), porosity, redox state, and the presence of known iodine sorption sites. Bulk counterions such as PF6– restrict the surface area available for interaction with iodine and therefore lower the uptake capacity by up to 30 %.28 On the contrary, I– counter ions increase the uptake by about 10 % because of their ability to catalyze formation of polyiodide species on the surface of the COP. In general, fully reduced neutral COPs exhibit high iodine uptake capacity, reaching up to 380 % mass increase. This is likely the result of the Lewis basic character of the neutral polymer, which interacts with Lewis acids I2 and I3–. Furthermore, neutral COPs do not contain counterions, which may allow for additional surface area to interact with iodine.46 Highly stable radical cationic COPs can react with polyiodides such as I3– in a series of reductionoxidation reactions, which ultimately produce a dicationic COP and I–. This also results in high iodine vapor uptake capacity.28 The stability of the radical cationic viologen is crucial for these type of processes to occur.

3.3. Catalysis The insolubility of viologen-based COPs in water and in common organic solvents makes these materials excellent candidates for use as heterogeneous catalysts.79 In the presence of a good heterogeneous catalyst, characterized as being both robust and long-lasting, a chemical reaction proceeds at desirable rates at as low a temperature and pressure as possible. Viologens in these catalysts serve various roles: (i) their redox chemistry enables viologens to serve as electrontransfer catalysts;36 (ii) utilizing their positive charge, viologen COPs form strong complexes with negatively charged metal nanoparticles and aid in stability;41,42 (iii) viologen counterions can act as nucleophiles while also serving as good leaving groups.44,52 In addition, the significant surface areas of certain viologen-based COPs further increase their catalytic efficiency. Insoluble polystyrene-viologen resins have been used for the reduction of nitroarenes into anilines.36 Viologens in this system serve as mediators of electron transfer from sodium dithionate (Na2S2O4) to the substrates. The reduction from nitro to amine occurs in four steps, proceeding through nitroso (NO), hydroxylamine, and NH• radical states, converting radical cationic viologen into the dication at each step (Figure 4A). Efficiency of this conversion depends on the ability of viologen radical cations to dimerize. The more sterically hindered the polymer chain, the less likely the dimerization, and the more efficient the catalyst. Viologen-based polymers have also been used to prevent metal leaching in catalysis. For example, palladium nanoparticles have been coupled to a viologen COP obtained from 4,4’-bipyridine and 1,4-bis(bromomethyl)benzene in a Menshutkin reaction.41,42 Cationic viologen subunits of the COP favorably interact with negatively-charged palladium inorganic materials and prevent leaching, aggregation and



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deactivation. This solid-phase self-organized polymernanoparticle catalyst was used for α-alkylation of ketones with primary alcohols in water and resulted in up to 95 % conversion (Figure 4B).41 The same catalyst was also used for ring-opening alkylation of cyclic 1,3-diketones with primary alcohols with a maximum yield of 81 %.42 In addition to palladium nanoparticles, viologen-based COPs have also formed hybrid materials with gold nanoparticles using NaBH4reduced HAuCl4.80 These hybrids were used as heterogeneous catalysts for the aerobic oxidation of saturated alcohols with high selectivity for cyclohexanol (TOF = 2064 h – 1). Porous polymeric heterogeneous catalysts in which viologen subunits play a direct role in catalysis have also been reported and used in fixing CO2 into cyclic carbonates using various epoxides as starting materials.44,52 In 12 hours, the COPs converted 2-methyloxirane into propylene oxide and 2(chloromethyl)oxirane into epichlorohydrin in 99 and 98 %

Figure 4. Heterogeneous catalysis with insoluble viologen-based COPs. (A) Reduction of nitroarenes by insoluble viologen-based polystyrene resins. Adapted with permission from ref. 36. Copyright 1997 Elsevier. (B) α-alkylation of ketones with primary alcohols in water by Pd-viologen COP system. Adapted from ref. 41. (C) Formation of cyclic carbonates in a reaction between CO2 and epoxides via a viologen COP as a porous catalyst. Adapted from ref. 52. yields, respectively.44 The same two reactions catalyzed by a charged triazine framework from cyanophenyl-substituted viologen dication (Figure 1D) resulted in 99 and 95 % yields, respectively.52 The mechanism of these reactions involves the activation of epoxide through hydrogen bonding with the αprotons of viologen, followed by a nucleophilic attack of Cl– to open up the epoxide ring. The resultant intermediate reacts with CO2 and re-cyclizes while Cl– serves as a leaving group (Figure 4C). Recently, cyclic carbonates have been generated from CO2 and spiro-epoxyoxindole-based small molecules in deep eutectic solvents which have been cited as a greener alternative for CO2 capture. Similar to Coskun’s results, these reactions also proceeded with yields up to 98 %.81

3.4. Sensors A good sensor is a substance that produces a rapid change in one or more of its physical or chemical properties with a high level of sensitivity when exposed to a particular stimulus.

Viologens have a low-energy LUMO, so they can undergo charge transfer as electron acceptors in the presence of electron donors such as ammonia and amines. This charge transfer entails the formation of a radical cation in viologen, which is typically accompanied by a color change. Therefore, viologen-based materials can serve as sensors for the visual detection of ammonia and amines. Because studies on the use of viologen-based COPs are still in their infancy, it is hard to compare them with well-established sensors. It can be noted, however, that the sensitivity can still be greatly improved.82 Our group reported that phosphazene-viologen COPs change color from yellow or red to dark green upon exposure to NH3 for a minute (Figure 5A).46 An EPR signal demonstrates that the change in color is a result of the formation of viologen radical cations. Similarly, Coskun et al. showed that their porous COP composed of diazapyrenium moieties responded to various aliphatic amines, including n-butylamine, diethylamine and trimethylamine,


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with a color change from orange to dark green during 30 minutes of exposure (Figure 5B).48 The reported n-butylamine uptake capacity was 31 wt %, which corresponds to 75 % amine fixing efficiency. Both Coskun and our groups were able to regenerate the COPs by soaking them in aqueous HCl or exposing them to gaseous HCl.

Figure 5. Insoluble viologen COPs as sensors for ammonia and amines. (A) Adsorption of ammonia induces formation of a viologen radical cation, causing a peak in the EPR spectrum. Adapted with permission from ref. 46. Copyright 2016 The Royal Society of Chemistry. (B) Adsorption of n-butylamine by a viologen COP results in formation of a broad charge-transfer absorption band at 595 nm accompanied by a color change from orange to dark green. Adapted with permission from ref. 48. Copyright 2016 The Royal Society of Chemistry.

Viologen-based COPs have also been used as humidity sensors. The humidity-sensitive subunit, 1-[3-(2methacryloxyethyl dimethyl ammono)propyl] 1’-propyl-4,4’bipyridinium tribromide (MDAPBT), was copolymerized with n-butyl acrylate and DAEMA into a block-copolymer with pendant viologen units, which were cross-linked with 1,5dibromopentane on an electrode.62 The humidity sensing activity of this system was measured as resistance: the average resistance at 30, 60 and 90 % relative humidity was 1000, 52 and 3.1 KΩ, respectively, which is a moderate range required for a humidity sensor. Viologens are also sensitive to light and can act as photoswitches, which undergo one-electron reduction accompanied by a color change when exposed to UV light. This property has been utilized for a plausible magic ink application by coating viologen-based COPs on paper or other materials.46 In this manner, different images can be temporarily engraved onto paper by exposing the coated materials to sunlight. Prolonged air exposure can then reoxidize the radical cations resulting in the disappearance of the engraved images

Fabrication of smart windows relies on the phenomenon of electrochromism, which involves a reversible change in color induced by the application of voltage or the passing of current.84 Viologens are well-known electrochromic moieties, so viologen-containing COPs are suitable materials for smart window applications. A low-density polyethylene film with viologen units attached to its surface by graft copolymerization has been reported (Figure 2B).34 Exposure of these films to near-UV radiation for 1 to 3 minutes induces the formation of viologen radicals and causes a color change from faint yellow to blue. A new peak at 615 nm appears in the absorption spectrum upon one-electron reduction, which is characteristic of viologen radicals. When irradiation is stopped, the reverse reaction occurs. The authors show that even after 8 cycles of reduction and re-oxidation, the maximum absorbance at 615 nm does not change, which demonstrates good memory performance of the COP. Similarly, electrochromic properties of a viologen COP-based film were observed in other systems. A series of bis(4-cyano1-pyridinio) derivatives were used for electrode coatings through cathodic electropolymerization, followed by reduction to 4-cyano-1,4-dihydro-4-pyridyl radicals, and coupling to a polyviologen film through elimination of cyanide ions (Figure 6). This film was insoluble because of the high degree of cross-linking.83 In other work, a thin layer of viologen polymer was immobilized on the surface of a PET film through graft copolymerization of N-hexyl-N’-(4-vinylbenzyl)-4,4’bipyridinium bromide chloride and commercial PET (Figure 2D).35 This film also serves as a smart window, turning blue upon irradiation with a Hg lamp and returning to yellow color upon exposure to air. In addition, the film serves as a good antimicrobial agent as demonstrated using an E. coli model. The antimicrobial activity of the film depends on the concentration of viologen units on the surface of the film, with higher concentrations equating to an increase in the antibactericidal property of the material. With a 25 nmol cm–2 surface concentration of N+, less than 1 % of bacteria survive.

3.5. Films Polymeric films exhibit an even distribution of structural elements over an extended area, so they bear better processability and are easier to handle than polymers in powder or suspension forms. Films can also exhibit specific physical properties imposed by the presence of surfaces and interfaces. A good number of viologen-containing polymers have been incorporated into films or membranes and used for a range of applications, including smart windows,34 antibacterial materials,35 and electrochemistry.43,83

Figure 6. The mechanism for the formation of polyviologen film from bis(4-cyano-1-pyridinio) derivatives. Adapted with permission from ref. 83. Copyright 1993 The Chemical Society of Japan.

4. SUMMARY AND OUTLOOK



In conclusion, insoluble COPs can be synthesized with viologens incorporated into the backbone of the polymer or in its side chains. In the former case, solution chemistry and various reactions such as Menshutkin reaction, Sonogashira – Hagihara cross-coupling, ionothermal trimerization, imine condensation, and Zincke reaction have been employed. In the latter case, surface chemistry has been utilized more frequently, particularly in combination with Menshutkin reaction, to generate COPs with asymmetrically quaternized viologen subunits. Applications of these materials tend to depend on the mode of synthesis. Those obtained through solution chemistry serve as gas or small molecule sorbents, catalysts, or sensors, whereas those synthesized using surface chemistry find their use in catalysis and the fabrication of films for various uses. While the insolubility of the COPs discussed herein brings numerous advantages, including facile purification, separability and easy regeneration, these materials also present some challenges.85 Firstly, analytical tools which are available for molecular-level analysis of insoluble materials are limited. Typically, COPs are characterized by means of qualitative Fourier-transform Infrared Spectroscopy (FTIR) and solid state NMR, which is semi-quantitative at most and of far lower precision than its solution counterpart used in the analysis of soluble materials. Therefore, further advancements in existing technology need to be made in order to characterize these materials in greater detail. Secondly, the control over the degree of crosslinking in a COP is non-trivial yet it determines many of the physical properties of the materials, including swellability, surface area, surface charge potential and pore size distribution. As a result, emphasis should be placed on determining the crosslinking density to better understand the performance of COPs in various applications. Thirdly, since COPs are insoluble, they tend to aggregate when their suspensions are made. While this facilitates their removal from a liquid medium and may be highly advantageous in some applications, it poses a general barrier to using COPs for biological applications. Although positive charges in viologen increase water miscibility of COPs, other methods of solubilizing these materials need to be investigated. Additionally, the use of viologen-based materials for biological applications are limited because of the inherent toxicity of viologens.86 Fourthly, even a minimal modification to the structure of one or more of the monomers can greatly alter physical properties of the COPs. If computational methods can be developed so that effects of such structural modifications can be predicted in silico, the chances of obtaining best-performing materials would be greatly enhanced. Computational approaches, however, tend to be more reliable for materials with regular structures such as COFs than for hyper-crosslinked polymers. Lastly, obtaining viologen COPs with high surface areas is still challenging because of the presence of counterions. It is therefore important to develop material-specific protocols to deal with counterions. Overall, the advantages of insoluble COPs currently heavily outweigh their challenges, so future improvements in the performance of these materials are of high interest. In the future, the field of viologen-based polymeric materials will likely shift from bulk 3D polymers toward ordered structures such as 2D and 3D COFs. Such materials are easier to characterize on a molecular level and their structures can be precisely determined by a combination of experimental (X-ray

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diffraction) and theoretical (simulation) approaches. A good understanding of the network structure of the materials would further expand potential applications, as the polymers could then also be used as controllable host-guest systems. Viologen is a well-known guest, which forms strong complexes with various hosts, including cucurbiturils,87 calixarenes88 and pillararenes.89 Adding a macrocycle to an ordered viologenbased polymeric material which serves as an axle would create a stimuli-responsive rotaxane-type system.90,91 Such networks would be useful in studying mechanical properties of viologen materials, which to-date have not been thoroughly investigated. Reticular choice of the axle and macrocycle structures would allow for fine-tuning of mechanical properties. Knowing the strength and elasticity of these materials would be useful in deciding their optimal applications. For example, resilient materials could be incorporated into membranes for pollutant removal in flow experiments.

AUTHOR INFORMATION Corresponding Authors Dinesh Shetty: [email protected] Ali Trabolsi: [email protected]

ACKNOWLEDGMENTS The research described here was sponsored by New York University Abu Dhabi (NYUAD), UAE. T.S., D.S. and A.T. thank NYUAD for its generous support of the research program at NYUAD. We thank all of the co-workers who have contributed to this research as cited. The authors also thank Ms. Khulood Alawadi for 3D cartoons.

ABBREVIATIONS AcOH, acetic acid; AIBN, azobisisobutyronitrile; COF, covalent organic framework; CO2, carbon dioxide; COP, covalent organic polymer; CTF, covalent triazine framework; DAEMA, 2-(N,N’dimethylaminoethyl) methacrylate; DMF, N,Ndimethylformamide; EPR, electron paramagnetic resonance; Et3N, triethyl amine; EtOH, ethanol; DAP, diazapyrenium; FTIR, Fourier-transform Infrared; ITO, indium tin oxide; LUMO, lowest unoccupied molecular orbital; MDAPBT, 1-[3-(2methacryloxyethyl dimethyl ammono)propyl] 1’-propyl-4,4’bipyridinium tribromide; MeCN, acetonitrile; MW, microwave, NMR, nuclear magnetic resonance; PCP, porous cationic polymer; PET, polyethylene terephthalate; POP, porous organic polymer; PXRD, powder X-ray diffraction; ST, solvothermal; TAPB, 1,3,5-tris(4-aminophenyl)benzene; UV, ultraviolet.

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TOC Derivatives of 4,4’-bipyridine or viologen are incorporated into covalent polymers to yield redox-responsive materials, which serve as effective gas sorbents, pollutant removers, catalysts and sensors, and can be incorporated into films.


Design Strategies and Redox-Dependent Applications of Insoluble

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