Table of Contents
Related Titles
Title Page
Copyright
Preface
List of Contributors
Chapter 1: Design and Synthesis of Conjugated Polyelectrolytes
1.1 Introduction
1.2 Poly(Arylene)s
1.3 Poly(Arylene Ethynylene)s
1.4 Poly(Arylene Vinylene)s
1.5 Conclusion
References
Chapter 2: All-Conjugated Rod–Rod Diblock Copolymers Containing Conjugated Polyelectrolyte Blocks
2.1 Introduction
2.2 All-Conjugated, Cationic Polyfluorene-b-Polythiophene Diblock Copolymers
2.3 All-Conjugated Cationic Polyfluorene-b-Polyfluorene Diblock Copolymers
2.4 Conclusion
Acknowledgments
References
Chapter 3: Ionically Functionalized Polyacetylenes
3.1 Introduction
3.2 Polymers from Ionically Functionalized Cyclooctatetraenes
3.3 Polymers from Ionically Functionalized Acetylenes
3.4 Summary
Acknowledgment
References
Chapter 4: Aggregation Properties of Conjugated Polyelectrolytes
4.1 Introduction
4.2 Aggregation: from Disordered Clusters to Structured Vesicles
4.3 Experimental Studies on Aggregation
4.4 Conjugated Polyelectrolyte Aggregation in Solution
4.5 Learning How to Control Aggregation
4.6 Conclusions and Outlook
References
Chapter 5: Sensing via Quenching of Conjugated Polyelectrolyte Fluorescence
5.1 Background and Introduction
5.2 Small Ions/Molecules Sensing
5.3 Protein and Enzyme Activity Sensing
5.4 DNA sensing
5.5 Concluding Remarks
References
Chapter 6: Sensing Applications via Energy Transfer from Conjugated Polyelectrolytes
6.1 Introduction
6.2 DNA and RNA Sensing with Conjugated Polyelectrolytes
6.3 Protein Sensing with Conjugated Polyelectrolytes
6.4 Biological/Chemical Small-Molecules Sensing with Conjugated Polyelectrolytes
6.5 Conclusion
References
Chapter 7: Sensing via Conformational Changes of Conjugated Polythiophenes
7.1 Introduction
7.2 Structural Characteristics of Conjugated Polythiophenes
7.3 Thermochromic Sensors
7.4 Ionochromic Sensors
7.5 Affinitychromic Sensors
7.6 Conclusions
References
Chapter 8: Conjugated Polyelectrolyte-Based Biocide Applications
8.1 Introduction
8.2 Dark Bactericidal Activity of Conjugated Polyelectrolytes
8.3 Light-Activated Biocidal Activity
8.4 Photochemistry, Photophysics, and Modeling
8.5 Conjugated Cationic Oligomers and Polymers as Antimicrobials
8.6 Incorporation into Other Materials and Formats
8.7 Activity against Viruses and Biofilms
8.8 Toxicity toward Mammalian Cells
8.9 Summary and Outlook
References
Chapter 9: Conjugated Polyelectrolyte-Based Imaging and Monitoring of Protein Aggregation
9.1 Introduction
9.2 CPEs for Bioimaging
9.3 Amyloid Fibrils and Protein Aggregation Diseases
9.4 Novel Scaffolds for the Detection of a Diversity of Protein Aggregates
9.5 Conclusion
References
Chapter 10: Charge Injection Mechanism in PLEDs and Charge Transport in Conjugated Polyelectrolytes
10.1 Introduction
10.2 Charge Injection Mechanism in Polymer Light-Emitting Diodes Using Conjugated Polyelectrolytes as Electron-Injecting/Transporting Layers
10.3 Charge Transport in Conjugated Polyelectrolytes
10.4 Conclusion
Chapter 11: Organic Optoelectronic Devices Containing Water/Alcohol-Soluble Conjugated Polymers and Conjugated Polyelectrolytes
11.1 Introduction
11.2 Polymer Light-Emitting Devices Based on Water/Alcohol-Soluble Conjugated Polymers and Conjugated Polyelectrolytes
11.3 Water/Alcohol-Soluble Conjugated Polymers as Efficient Electron Injection/Transport Layer in PLEDs
11.4 Water/Alcohol-Soluble Conjugated Polymers/Polyelectrolytes as Cathode Interlayer for Polymer Solar Cells
11.5 Applications of Water/Alcohol-Soluble Conjugated Polymers and Conjugated Polyelectrolytes in Other Optoelectronic Devices
11.6 Summary
11.7 Conclusion
References
Chapter 12: Optical Processes in Conjugated Polyelectrolytes Dependence on Chain Conformation and Film Morphology
12.1 Introduction
12.2 Hydrophobic and Electrostatic Interactions in CPEs
12.3 Amphiphilic CPEs and CPE-Surfactant Complexes: Toward Ordered Structures and Controlled Photophysics at the Solid State
12.4 Photoluminescence Quenching in CPEs: Fast Exciton Dynamics
12.5 Effect of the Ion and Counterion Choice on the CPE Photoluminescence
12.6 Nature of the Excited States: Charge-Transfer States and Polarons in CPEs
12.7 Conclusions
References
Index
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The Editors
Prof. Bin Liu
National University of Singapore
Department of Chemistry & Biomolecular Engineering
Engineering Drive 4
Singapore 117576
Singapore
Prof. Guillermo C. Bazan
University of California
Department of Chemistry & Materials
MC-CAM 3107 MRL Bldg.
Santa Barbara
CA 93106
USA
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Preface
Conjugated polyelectrolytes constitute a broad class of materials that are being studied because of their unique combination of physical properties and their recent role as key elements in a remarkable range of emerging technologies. The generic molecular structure of these polymers contains a backbone with a π-delocalized electronic structure and pendant substituents with ionic functionalities. As such, conjugated polyelectrolytes describe a class of macromolecules that combines the properties of organic semiconductors with the physicochemical behavior of polyelectrolytes. This combination of physical attributes allows these materials to find applications ranging from biological assays that take advantage of light-harvesting and optical amplification to solid state devices that use the semiconducting and self-assembly properties of the amphiphilic molecular structures. Interesting fundamental questions on science and opportunities for study also arise because of the combination of charge carrier and ion transport within a single phase. Perusal of the literature provides a strong indication that the field of conjugated polyelectrolytes has significantly expanded in the last 10 years and is being studied through a range of multidisciplinary approaches.
This book is the first to specifically focus on the fundamentals and applications of conjugated polyelectrolytes. We have been fortunate that the contributed chapters are written by well-recognized experts in the field. The book is organized into 12 Chapters. The first four chapters introduce the synthesis and optical properties of various conjugated polyelectrolyte structures, and provide a general background for readers, especially for those who are new to the field. Chapter 1 focuses on how to prepare the main types of conjugated polyelectrolyte structures, with strong emphasis on the comparison of synthetic approaches. Conjugated polyelectrolytes can also be incorporated into higher architecture macromolecules, specifically block copolymers, as described in Chapter 2. In Chapter 3, a review is provided on the electrical properties of ionomers based on polyacetylene backbones. Chapter 4 shows that while the structure of single polymer chains is straightforward to describe, the combination of hydrophobic and ionic groups leads to complex aggregation states in solution.
Chapters 5–9 provide a comprehensive account on the application of conjugated polyelectrolytes in optical sensing and imaging, particularly biological systems. This feature of conjugated polyelectrolytes is a result of the ionic component, which allows solubility or dispersion in water and other highly polar solvents. Chapter 5 describes how the facile intra- and interchain electronic communication allows for facile energy transfer mechanisms that are used in optically amplified fluorescence quenching. In Chapter 6, the same physical basis is used to amplify the emission of a reporter dye and thereby enables the sensing of a wide range of targets. The dependence of optical properties of conjugated polyelectrolytes on the main chain conformation, as described in Chapter 7, yields yet another important strategy for detecting various targets in aqueous media. The biocidal applications and mechanism of action by conjugated polyelectrolytes and oligoelectrolytesin in a variety of formats is summarized in Chapter 8. The dependence of optical properties on the state of aggregation, and coordination of electrostatic and hydrophobic interactions can be managed, as detailed in Chapter 9, to form the principal component for monitoring protein aggregation and imaging cells.
The emerging application of conjugated polyelectrolytes in organic optoelectronic devices, and the state of knowledge on how the most relevant properties depend on molecular structure are contained in Chapters 10–12. Of particular reliance is the ability to make multilayer structures with other organic semiconductors through solution processing and thereby improve device characteristics, as described in Chapter 10. Chapter 11 summarizes what is known on the effect of structural variations on electrical properties and describes effective methods of evaluation and testing. Finally, the complex relationship between structural variations, deposition methods, and optical dynamics is discussed in Chapter 12.
Perusal of the contents will leave the reader with an excellent perspective of the significant progress recently made to design and understand conjugated polyelectrolytes, together with the innovative approaches to their applications. As the field has evolved, thanks to close interactions between chemists, physicists, biologists and engineers, we hope that readers with different backgrounds will enjoy reading these new advances. The book also provides an easy entry point for interested researchers that are not yet currently involved with the topic, who may find innovative ideas by having access to the consolidated information in this book.
We cannot end this preface without expressing our gratitude to all those who have made contributions to this book. We thank all authors for their dedicated work and the editorial team at Wiley-VCH for their strong support.
Bin Liu
Guillermo C. Bazan
List of Contributors
Chapter 1
Design and Synthesis of Conjugated Polyelectrolytes
Conjugated polyelectrolytes (CPEs) are π-conjugated polymers with charged ionic side chains, which allow for vital applications ranging from organic light-emitting diodes, solar cells, and field effect transistors to biosensors and bioimaging. This chapter focuses on the design and synthesis of CPEs, which include poly(arylene)s, poly(arylene ethynylene)s, and poly(arylene vinylene)s. In addition, their optical and physical properties, including solubility, absorption, and emission maxima, as well as fluorescence quantum yields, are summarized in tables.
synthesis; conjugated polyelectrolytes; water soluble; palladium-catalyzed reactions; poly(arylene)s; poly(arylene ethynylene)s; poly(arylene vinylene)s
Conjugated polyelectrolytes (CPEs) are a kind of π-conjugated polymers (CPs) containing side chains with ionic functionality [1]. CPEs can be divided into two categories according to the charge of their side chains: cationic conjugated polyelectrolytes (CCPEs) and anionic conjugated polyelectrolytes (ACPEs). Typical cationic groups of CCPEs include quaternary ammonium (
) and pyridinium, while anionic groups of ACPEs include carboxylate (
), phosphonate (
), and sulfonate (
). The solubility of CPEs in polar solvents (e.g., water and methanol) is not only dependent on the ionic side groups but also affected by the hydrophobic aromatic backbones [2].
During the past 20 years, a variety of CPEs have been synthesized, most typically via carbon–carbon bond-forming reactions using organometallic catalysts. The most widely used polymerization methods are shown in Scheme 1.1, which include FeCl3-catalyzed or electrochemical oxidization; the Yamamoto and Suzuki coupling reactions for poly(arylene)s [3]; the Wittig, Gilch, Wessling, and Heck reactions for poly(arylene vinylene)s [4]; and the Sonogashira coupling reactions for poly(arylene ethynylene)s [5]. Through these well-established reactions, CPEs can be obtained directly or via postpolymerization strategy. Another example is the ring-opening metathesis polymerization of cyclooctatetraenes, which is described in more detail in Chapter 3. Among these reactions, palladium-catalyzed coupling methods (the Suzuki, Heck, and Sonogashira methods) are the most popular ones because of their tolerance to various functional groups, mild reaction conditions, and capability to produce different backbone structures.
Scheme 1.1 (a) Illustration of typical structures for CCPE and ACPE. (b) Examples of most widely used polymerization methods; Ar, Ar1, and Ar2 represent aromatic structures.
This chapter summarizes the design and synthesis of various CPEs. The sections are organized according to the backbone structures of CPEs. The chapter starts with poly(arylene)s, which is followed by poly(arylene ethynylene)s and poly(arylene vinylene)s. In addition, reported physical properties of CPEs including solubility, absorption (λabs) and emission (λem) maxima, and fluorescence quantum yields (ΦF) are summarized in Tables 1.1, 1.2, 1.3, respectively. However, it should be noted that these properties can be dependent on purification method, molecular weight, and polymer concentration.
Table 1.1 Physical Properties of Poly(Arylene)-Based CPEsa
Table 1.2 Physical Properties of Poly(Arylene Ethynylene)-Based CPEsa
Table 1.3 Physical Properties of Poly(Arylene Vinylene)-Based CPEsa
The first sulfonated polythiophene was synthesized by Wudl and coworkers [84] in 1987. A neutral polythiophene (P1, Scheme 1.2) was first synthesized from methyl 2-(thiophen-3-yl)ethanesulfonate (2). Subsequent treatment of P1 with NaI in acetone yielded the sulfonated polymer (P2, Scheme 1.2). Later, Leclerc's group [85] reported the synthesis of sulfonated polythiophene (P3, Scheme 1.3) via direct polymerization of a sulfonated monomer. In the first step, 3-methoxy-4-methylthiophene (4) was synthesized from 3-bromo-4-methylthiophene (3) with sodium methoxide and CuBr in N-methyl-2-pyrrolidone (NMP). The methoxy substituent was subsequently reacted with 2-bromoethanol in toluene with sodium hydrogen sulfite to yield 3-(2-bromoethoxy)-4-methylthiophene (5). Treatment of 5 with sodium sulfite in water/acetone mixture yielded sodium 2-(4-methyl-3-thienyl-1-oxy)ethanesulfonate (6), which underwent FeCl3-catalyzed oxidative polymerization to afford polymer P3. By changing 2-bromoethanol to other alcohols bearing different functionalities, such as halogens, carboxylic acids, and amines, various polythiophene-based CPEs have been synthesized [85].
Scheme 1.2 Synthesis of a sulfonated polythiophene (P2).
Scheme 1.3 Synthesis of a sulfonated polythiophene (P3).
Carboxylated polythiophenes were synthesized using the Ni(0)-catalyzed Yamamoto coupling polymerization [86], FeCl3-catalyzed oxidative polymerization [87], or the Stille coupling polymerization [88]. As shown in Scheme 1.4, both the Yamamoto polymerization of methyl 2-(2,5-dichlorothiophen-3-yl)acetate (7) and oxidative polymerization of methyl 2-(thiophen-3-yl)acetate (8) yielded poly(methyl thiophene-3-carboxylate) (P4). Hydrolysis of P4 with NaOH led to poly(sodium thiophene-3-carboxylate) (P5). In addition, a CuO-modified Stille coupling polymerization was performed for 9 to give poly(4,5-dihydro-4,4-dimethyl-2-(2-(thiophen-3-yl)ethyl)oxazole) (P6, Scheme 1.5) [88], which after acid-assisted hydrolysis and base treatment yielded the carboxylated polymer P7. These polymers showed pH-dependent conformational and optical changes.
Scheme 1.4 Synthesis of a carboxylated polythiophene (P5).
Scheme 1.5 Synthesis of a carboxylated polythiophene (P7).
Recently, Wang's group also synthesized a carboxylated polythiophene (P8) [6]. As shown in Scheme 1.6, the key monomer 11 was prepared by reacting the salt of 2-(3-thienyl)ethylamine (10) with methyl acrylate in the presence of boric acid. P8 was obtained through oxidative polymerization of 11 in chloroform, followed by hydrolysis in NaOH aqueous solution.
Scheme 1.6 Synthesis of a carboxylated polythiophene (P8).
In addition, phosphonated polythiophene was synthesized as shown in Scheme 1.7. 13 was synthesized in a way similar to that of 5 and the key monomer, 3-(3′-thienyloxy)propanephosphonic acid diethyl ester (14), was synthesized by treatment of 13 with triethyl phosphite [89]. Electropolymerization of 14 in LiClO4/acetonitrile/CH2Cl2 yielded P9, which after silyl dealkylation and hydrolysis gave poly(3-(3′-thienyloxy)propanephosphonate) (P10).
Scheme 1.7 Synthesis of a phosphonated polythiophene (P10).
An important series of polythiophene derivatives, poly(cyclopentadithiophene)s, were developed by Zotti's group [7, 90]. As shown in Scheme 1.8, the key anionic monomer 16 was prepared from a one-pot reaction between 4H-cyclopenta[2,1-b:3,4-b′]-dithiophene (15) and 1,4-butanesultone in the presence of n-BuLi. Electropolymerization of 16 led to polymer P11. P12 was prepared using the same strategy.
Scheme 1.8 Synthesis of sulfonated poly(cyclopentadithiophene)s (P11) and (P12).
Cationic polythiophenes were synthesized by Leclerc's group [8] on direct oxidation of cationic thiophene monomers. The cationic monomer 17 was synthesized on quaternization of 5 with 1-methyl-1H-imidazole (Scheme 1.9). The cationic monomer 19 was synthesized from the Williamson reaction between 3-bromo-4-methylthiophene and 3-(diethylamino)propanol, followed by quaternization with bromoethane. Oxidative polymerization of 17 and 19 in the presence of Bu4NCl led to P13 and P14, respectively, with chloride counterions [8, 9, 91, 92]. P15–P17 (Scheme 1.9) were synthesized based on a similar approach [10, 11, 93].
Scheme 1.9 Synthesis of cationic polythiophenes and poly(cyclopentadithiophene)s (P13–P20).
As also shown in Scheme 1.9, a series of cationic poly(cyclopentadithiophene)s were also synthesized. 20 was prepared through alkylation of 15 with 1,6-dibromohexane in the presence of n-BuLi. Further quaternization of 20 with trimethylamine, followed by ion exchange afforded the cationic monomer 21 with perchlorate counterion. Electropolymerization of 21 led to cationic polymers P18. P19 and P20 were prepared using the same strategy [7, 94].
Zwitterionic CPEs contain side groups with anionic and cationic functionalities that are covalently bound to each other. Zwitterionic polythiophenes have been synthesized by Inganäs' group [12, 13]. As shown in Scheme 1.10, 1 was brominated with NBS and tosylated to yield a thiophene derivative 22. Displacing the tosyl group of 22 by a Boc-protected amino acid, N-t-Boc-L-Ser, yielded the key monomer 23. Palladium-catalyzed cross-coupling between 23 and thiophene-2,5-bispinacolboronate gave the regioregular terthiophene (24). After removing the Boc groups by trifluoroacetic acid treatment in CH2Cl2, the salt counterpart of 24 was directly polymerized in CHCl3 using anhydrous FeCl3 as the catalyst in the presence of tetrabutylammonium (TBA-OTf) to afford P21. The homopolymer P22 with the same thiophene unit was also synthesized using a similar strategy [95–97].
Scheme 1.10 Synthesis of zwitterionic polythiophenes (P21) and (P22).
The first carboxylated poly(p-phenylene), poly(p-quaterphenylene-2,2′-dicarboxylic acid) (P23, Scheme 1.11), was synthesized by Novak's group using the Suzuki cross-coupling between 2,2′-bis-(4,4′-biphenyl)-1,3,2-dioxaborolane and aryl halide 25 [98]. P23 is insoluble in water and organic solvents but is soluble in dilute aqueous hydroxide solution. A postpolymerization method was also used to synthesize a carboxylated poly(p-phenylene) P25 (Scheme 1.11) via the Williamson reaction between P24 and ethyl p-hydroxybenzoate. After hydrolysis of the ester groups, P26 was obtained and exhibited solubility in polar organic solvents [99].
Scheme 1.11 Synthesis of carboxylated poly(p-phenylene)s (P23) and (P26).
The first sulfonated poly(p-phenylene) P28 was designed and synthesized by Wegner's group through a postpolymerization method (Scheme 1.12) [100]. The key monomer 27 was synthesized via chlorosulfonation of 1,4-dibromobenzene (26) with chlorosulfonic acid, followed by treatment with p-cresol in the presence of pyridine. The Suzuki coupling between 27 and 2,2′-(2,5-dimethyl-1,4-phenylene)-bis(1,3,2-dioxaborinane) afforded P27. Saponification of P27 by BuONa in n-BuOH led to P28, which is soluble in DMSO.
Scheme 1.12 Synthesis of a sulfonated poly(p-phenylene) (P28).
A direct approach to sulfonated poly(p-phenylene)s was reported by Reynold's group [14]. The key sulfonate monomer 30 was prepared by treating 2,5-dibromobenzene-1,4-diol (29) with propane sultone under basic condition (Scheme 1.13). The Suzuki polymerization between 30 and 1,4-phenyldiboronic acid yielded P29 and, similarly, between 30 and 4,4′-biphenyldiboronic acid ester yielded P30 [15]. To endow P29 with biorecognition ability, endcapping reaction was carried out by adding 4-bromobenzaldehyde at the end of the Suzuki polymerization to afford a polymer with aldehyde end groups. The biotin-attached polymer P31 was obtained through hydrazone formation ( − CH—N—NH − ) between the aldehyde groups and biotin hydrazide in aqueous solution [16].
Scheme 1.13 Synthesis of sulfonated poly(p-phenylene)s (P29–P31).
The first cationic poly(p-phenylene) was synthesized by Baullauff and Rehahn [101] through a postpolymerization method. As shown in Scheme 1.14, the cationic poly(p-phenylene)s were synthesized from neutral precursors with phenoxy-substituted alkyl chains. After cleaving the phenoxy groups of P32 with trimethylsilyl iodide, P33 was obtained with alkyl iodide side chains. Subsequent reaction with triethylamine (NEt3) or pyridine gave the cationic poly(p-phenylene)s (P34 and P35) with nearly 100% degree of quaternization. P36–P38 were synthesized via the same postpolymerization strategy. Particularly, P38 with four cationic charges per repeat unit was prepared from consecutive quaternization of P33 with tetramethylethylenediamine and iodoethane [102].
Scheme 1.14 Synthesis of cationic poly(p-phenylene)s (P34–P38).
A more efficient and universal approach toward cationic poly(p-phenylene)s is shown in Scheme 1.15 [17]. The key monomer, 2,5-bis(3-[N,N-diethylamino]-1-oxapropyl)-1,4-dibromobenzene (31), was synthesized via etherification of dibromohydroquinone (29) in the presence of 2-chloroethyldiethylamine hydrochloride [18]. The Suzuki polymerization between 31 and 1,4-bisphenyl-(1,3,2-dioxaborinane) afforded P39. Reaction between the tertiary amine groups and bromoethane yielded the cationic polymer P40 with good water solubility. In addition, the Stille coupling was also used to synthesize cationic poly(p-phenylene)s containing thiophene units [19]. As shown in Scheme 1.15, 2,5-bis(3-[N,N-diethylamino]-1-oxapropyl)-1,4-diiodobenzene (33) was synthesized similarly to that of 31. Copolymerization between 33 and 2,5-bis(trimethylstannyl)thiophene in anhydrous DMF using PdCl2(PPh3)2 as the catalyst gave the neutral polymer, which on treatment with bromoethane yielded P41.
Scheme 1.15 Synthesis of cationic poly(p-phenylene)s (P40) and (P41).
The first cationic poly(fluorene) was based on the key monomer of 2,5-bis[3-(N,N-dimethylamino)-1-oxapropyl)-1,4-dibromobenzene] (34) [20]. As shown in Scheme 1.16, 34 was synthesized via etherification of dibromohydroquinone (29) with 2-chlorotrimethylamine hydrochloride and potassium carbonate in acetone. Subsequently, the Suzuki polymerization between 34 and 2,7-bis-9,9′-dihexylfluorenyl-(1,3,2-dioxaborinane) gave the neutral polymer P42. Treatment of P42 with bromoethane in DMSO/THF mixture led to P43 with a quaternization degree of ∼80%. In an analogous way, P44 was synthesized from 31 [103]. The quaternization degree of P45 could be adjusted from 25 to 80% by varying the amount of bromoethane and the reaction time. P45a has a quaternization degree of 25%, which is almost not soluble in any solvent. However, polymers with quaternization degrees of 60% (P45b) or 80% (P45c) are soluble in DMSO and methanol, and P45c has also shown limited solubility in hot water.
Scheme 1.16 Synthesis of cationic polyfluorenes (P43) and (P45a–c).
To improve water solubility of polyfluorenes, Bazan and coworkers [21] attached charged side chains to fluorene rather than phenylene. The key monomer, 2,7-dibromo-9,9′-bis(6-(N,N-dimethylamino)hexyl)fluorene (37), was synthesized in two steps as shown in Scheme 1.17. Under basic condition, 2,7-dibromofluorene (35) was reacted with 1,6-dibromohexane to afford 2,7-dibromo-9,9-bis(6-bromohexyl)fluorene (36). Treatment of 36 with dimethylamine (Me2NH) in THF led to 37. The Suzuki polymerization between 37 and 1,4-phenyldiboronic acid gave the neutral polymer P46, which on treatment with iodomethane yielded P47.
Scheme 1.17 Synthesis of a cationic polyfluorene (P47).
On the basis of 2,7-dibromo-9,9-bis[3-(N,N-dimethylamino)propyl]fluorene (38), Cao and coworkers [22] reported a series of cationic polymers P48–P53 (Scheme 1.18). The key monomer 38 was synthesized by reacting 2,7-dibromofluorene with 3-dimethylaminopropylchloride hydrochloride in a water/DMSO mixture in the presence of excess NaOH. The Suzuki copolymerization between 38 and different diboronate monomers followed by treatment with bromoethane led to blue-fluorescent alternating polyfluorenes (P48 and P49). To fine-tune the polymer emission, random copolymerization between 38, 2,2′-(9,9-dioctyl-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane), and other dibromide monomers such as 2,1,3-benzothiadiazole, 2,1,3-benzoselenadiazole, 4,7-di-2-thienyl-2,1,3-benzothiadiazole, and the Ir(ppy)3 complex afforded green-to-yellow-emissive P50 [104], orange-to-red-emissive P51 [105], red-emissive P52 [106], and orange-red-emissive P53 [107], respectively. These materials are designed for device applications as discussed in Chapter 11.
Scheme 1.18 Synthesis of cationic polyfluorenes (P48–P53).
To obtain cationic polyfluorene homopolymers, the dioxaborolane monomer, 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9′-bis[3-(N,N-dimethylamino)propyl]fluorene (39), was synthesized by reacting 38 with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane in the presence of n-BuLi (Scheme 1.19) [23]. The Suzuki polymerization between 38 and 39 and subsequent treatment with bromoethane led to the cationic homopolymer P54 with good water solubility. On the basis of 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9′-bis[3′-(N,N-dimethylamino)hexyl]fluorene (40), a cationic polyfluorene copolymer containing oxadiazole units P55 was synthesized to serve as a hole-transporting material in light-emitting devices (Scheme 1.19) [108]. In addition, the boronic acid monomer 41 was synthesized to obtain cationic cross-conjugated P56 [24].
Scheme 1.19 Synthesis of cationic polyfluorenes (P54–P56).
To solve the problem of incomplete quaternization for polymers containing side chains with terminal tertiary amine, a new postpolymerization method was developed based on highly efficient reaction between alkyl bromide and trimethylamine [25]. In the first step, a neutral polymer was synthesized via the Suzuki coupling reaction between 36 and 1,4-phenyldiboronic acid (Scheme 1.20). This was followed by trimethylamine treatment to afford P57 with >95% degree of quaternization. Poly(fluorene-co-phenylene)s (P57a–f) with different amount of meta-phenyl units have been synthesized to adapt to the secondary structure of biomolecules. Cationic poly(fluorene-co-thiophene) (P58) was synthesized similarly from 9,9-bis(6-bromohexyl)-2,7-diiodofluorene (42) and 2,5-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophene followed by trimethylamine treatment [26].
Scheme 1.20 Synthesis of cationic poly(fluorene-co-phenylene)s (P57a–f) and a poly(fluorene-co-thiophene) (P58).
To further facilitate the synthesis of cationic polyfluorene derivatives, 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9′-bis[di(bromoalkyl)]fluorene (43) was synthesized by reacting 36 with excess 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (Scheme 1.21) [27]. The Suzuki coupling between 43 and substituted dibromophenylene, followed by trimethylamine treatment, afforded polymers P59a–d with tunable energy levels of 5.6 ± 0.2 eV for the highest occupied molecular orbital and 2.7 ± 0.1 eV for the lowest unoccupied molecular orbital, respectively. These CPEs have been used for sensor applications as described in Chapter 6.
Scheme 1.21 Synthesis of cationic polyfluorenes (P59a–d).
The availability of 43 allows the facile synthesis of a series of cationic polyfluorenes with different side chains [28, 109]. Different cationic polyfluorenes (P60 and P62a–c) were obtained via the Suzuki polymerization between 43 and dibromomonomers, such as 36 and 44a–c (Scheme 1.22), followed by trimethylamine treatment. Subsequent ion exchange of P60 afforded P61 with different counterions [110].
Scheme 1.22 Synthesis of cationic polyfluorenes (P60), (P61), and (P62a–c).
Apart from blue-emitting polyfluorenes, a new class of polyfluorene copolymers were synthesized via the Suzuki polymerization between dioxaborolane monomer 43 and 4,7-dibromo-2,1,3-benzothiadiazole (45), which was followed by quaternization with trimethylamine to yield cationic poly(fluorene-co-benzothiadiazole) (P63, Scheme 1.23) [29, 30]. Polymers with the same backbone but different counteranions (P64a–e) were also synthesized via ion-exchange reactions [111]. To improve water solubility of P63, 2,2′-(9,9-bis(2-(2-(2-bromoethoxy)ethoxy)ethyl)-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (46) was synthesized via the Miyaura reaction by heating a mixture of 44a and bis(pinacolato)diborane with KOAc in anhydrous dioxane [31]. The Suzuki coupling between 45 and 46, followed by trimethylamine treatment, led to P65.
Scheme 1.23 Synthesis of cationic polyfluorenes (P63), (P64a–e), and (P65).
Random copolymers (P66–P69, Scheme 1.24) have also been synthesized by the Suzuki polymerization of the dioxaborolane monomer (43 or 46) with 45 and other dibromo monomers, followed by quaternization [32–35]. The benzothiadiazole content was adjusted by changing the feed ratio of monomers for polymerization. To increase the benzothiadiazole content and maintain good water solubility, P67a and P67b were synthesized with short side chains on the fluorene monomers and high charge densities for the final polymers. By attaching cationic oligo(ethylene oxide) side chains to the fluorene units, P68 and P69 were synthesized to have 20 mol% benzothiadiazole content with high water solubility. Similar polymer structures with bromide or tetrafluoroborate as the counteranion have also been reported by Friend et al. [112].
Scheme 1.24 Structures of cationic polyfluorenes (P66–P69).
Cationic polyfluorenes with dendritic or bulky water-soluble side chains were also synthesized [36, 113]. As shown in Scheme 1.25, the polymers were synthesized by coupling Boc-protected dendritic fluorene monomers (47 or 49), Boc-protected fluorene (48) with 1,4-bisphenyl-(5,5-dimethyl-1,3,2-dioxaborinane) at a feed ratio of 1 : 1 : 2 in K2CO3/Pd(dppf)Cl2/toluene solution, followed by trifluoroacetic acid treatment to yield P70 and P71, respectively.
Scheme 1.25 Synthesis of cationic polyfluorenes (P70) and (P71).
In addition, a spiro(anthracene-9,9′-fluorene)-based cationic polyfluorene containing an anthracenyl “molecular bumper” has also been reported [37]. As shown in Scheme 1.26, the Suzuki polymerization between 10,10′-bis(6-bromohexyl)-10H-spiro(anthracene-9,9′-(2′, 7′-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)fluorene)) (50) and 1,4-bis(6-bromohexyloxy)-2,5-dibromobenzene (51) afforded the neutral polymer, which on quaternization yielded P72.
Scheme 1.26 Synthesis of a cationic polyfluorene (P72).
Hyperbranched cationic polyfluorene derivatives have been developed by Liu's group [38–40]. As shown in Scheme 1.27, the diyne monomer, 9,9′-bis(6-bromohexyl)-2,7-diethynylfluorene (52), was synthesized from 36 via the Sonagashira coupling reaction in two steps. Homopolycyclotrimerization of 52 under UV irradiation with CpCo(CO)2 as catalyst yielded P73, which after treatment with trimethylamine gave cationic polymer P74. Similarly, homopolycyclotrimerization of 56 afforded P75, which on quaternization yielded P76.
Scheme 1.27 Synthesis of hyperbranched cationic polyfluorenes (P74) and (P76).
The first anionic polyfluorene was synthesized by Scherf's group based on the key monomer 2,7-dibromo-9,9-bis(4-sulfonylbutoxyphenyl)fluorene (59, Scheme 1.28) [41]. 2,7-Dibromofluorene (35) was oxidized with sodium dichromate in acetic acid to yield 57, which reacted with phenol/methanesulfonic acid to give 2,7-dibromo-9,9-bis(4-hydroxyphenyl)fluorene (58). Etherification of 58 with 1,4-butane sultone led to 59. The Suzuki polymerization between 59 and 1,4-phenyldiboronic acid yielded P77. Similarly, a benzothiadiazole-containing random copolymer (P78) was also synthesized in a similar way [42].
Scheme 1.28 Synthesis of anionic polyfluorenes (P77) and (P78).
Another approach to yield sulfonated polyfluorenes was demonstrated by Cao's group [43]. The key monomer 2,7-dibromo-9,9-bis(4-sulfonatobutyl)-fluorene disodium (60) was directly prepared from 2,7-dibromofluorene (35) and 1,4-butane sultone in the presence of excess NaOH aqueous solution (Scheme 1.29). Polymerization between 60 and 1,4-phenyldiboronic acid in the presence of Pd(OAc)2 and NaCO3 in DMF/water yielded P79 with good water solubility.
Scheme 1.29 Synthesis of a sulfonated polyfluorene (P79).
A carboxylated polyfluorene was synthesized by Reynolds' group [44]. The key monomer, 2,7-dibromofluorene-9,9-dipropanoic acid-dibutylester (61), was synthesized from 2,7-dibromofluorene (35) via the Michael addition of the bridge carbon with butyl acrylate (Scheme 1.30) in the presence of triethylbenzyl ammonium chloride (TEBA). 61 was copolymerized with 2,2′-(9,9-diethyl-fluorene-2,7-diyl)-bis-(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) under modified Suzuki reaction conditions using cesium fluoride (CsF) and tetrabutylammonium fluoride (TBAF) as the base to yield a neutral polymer, which was followed by hydrolysis to afford P80.
Scheme 1.30 Synthesis of a carboxylated polyfluorene (P80).
Carboxylated polyfluorenes [45, 46, 114] were also synthesized from 2,7-dibromo-9,9-bis(3′-(tert-butyl propanoate))fluorene (62), which was synthesized via direct alkylation of 2,7-dibromofluorene (35) with tert-butylacrylate in a mixture of toluene and aqueous KOH (Scheme 1.31). Polymerization between 62 and 1,4-bisphenyl-(5,5-dimethyl-1,3,2-dioxaborinane) was conducted under the standard Suzuki coupling condition, which was followed by hydrolysis in CF3COOH/CH2Cl2 to yield P81, which is soluble in water, DMSO, and methanol. The diboronate ester 63 was synthesized under the Miyaura reaction conditions. The availability of this key monomer facilitates the synthesis of the homopolymer P82 and benzothiadiazole-containing polymer P83.
Scheme 1.31 Synthesis of carboxylated polyfluorenes (P81–P83).
Starting from a similar carboxyl-acid-functionalized dibromofluorene, Wang's group synthesized an amino-acid-functionalized polyfluorene (P84, Scheme 1.32) [47]. The key monomer 65 was synthesized by reacting 64 with L-aspartic acid dimethyl ester hydrochloride in the presence of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC · HCl) in dry dichloromethane. The Suzuki polymerization between 65 and 1,4-bisphenyl(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) yielded P84 directly because of in situ hydrolysis of the carboxylic esters during polymerization.
Scheme 1.32 Synthesis of an anionic amino-acid-functionalized polyfluorene (P84).
A phosphonated polyfluorene was also synthesized [48]. The key monomer, 2,7-dibromo-9,9-bis(3-diethoxylphosphorylpropyl)fluorene (67), was synthesized from 2,7-dibromo-9,9-bis(3′-bromopropyl)-fluorene (66) (Scheme 1.33) [115]. Treatment of 67 with trimethylsilyl bromide and subsequently methanol yielded 2,7-dibromo-9,9-bis(3′-phosphonic acid propyl)fluorene (68). The Suzuki copolymerization between 68 and 1,4-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzene with Pd(dppf)Cl2 as catalyst in DMF and Na2CO3 aqueous solution gave P85 as yellow powder.
Scheme 1.33 Synthesis of a phosphonated polyfluorene (P85).
As compared to cationic and anionic polyfluorenes, zwitterionic polyfluorenes have been less developed. However, they have been found useful as electron-transporting/injecting materials in organic light-emitting diodes (OLEDs) [49, 50]. Huck's group synthesized P87 of which the repeat unit contains a zwitterionic fluorene and a neutral fluorene (Scheme 1.34). The Suzuki polymerization between 2,7-dibromo-9,9-bis((N,N-dimethylamino)ethyl)fluorene (69) and 2,7-bis-(1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene led to the neutral polymer P86 with terminal tertiary amine. Quaternization of P86 with 1,4-butane sultone in THF/methanol mixture yielded P87, which was soluble in methanol and DMSO. Meanwhile, Huang's group also synthesized a zwitterionic polyfluorene homopolymer (P89, Scheme 1.35) using a similar postpolymerization strategy. Different from P87 and P89 with zwitterions on the same structural unit, P90 was synthesized to have both cationic and anionic structural units (Scheme 1.36). P90 is soluble in water and partially soluble in DMF and DMSO [24]. The physical properties of poly(arylene)-based CPEs are summarized in Table 1.1.
Scheme 1.34 Synthesis of a zwitterionic polyfluorene (P87).
Scheme 1.35 Synthesis of a zwitterionic polyfluorene (P89).
Scheme 1.36 Synthesis of a zwitterionic polyfluorene (P90).
Poly(phenylene ethynylene)s are typically synthesized via the Pd-catalyzed Sonogashira reaction. Copolymerization between acetylene and 3,5-dioidobenzoic acid (73) in aqueous solution using Pd(0) and CuI as the catalyst directly resulted in carboxylated poly(phenylene ethynylene) (P91) [116]. Another carboxylated poly(phenylene ethynylene) (P92) was synthesized via the Sonogashira reaction between 74 and 75 (Scheme 1.37) [51].
Scheme 1.37 Synthesis of carboxylated poly(phenylene ethynylene)s (P91) and (P92).
Schanze's group synthesized P94 in an analogous way (Scheme 1.38) [52]. The ester-protected diiodo monomer (77) was synthesized in two steps from 32. Then, 77 was reacted with trimethylsilyl acetylene in the presence of (Ph3P)2PdCl2/CuI, which was followed by trimethylsilyl deprotection in a basic solution to afford the diacetylene monomer 78. Copolymerization between 77 and 78 under the Sonagashira reaction conditions yielded the neutral precursor P93, which underwent hydrolysis to give P94. Copolymerization between 77 and other diacetylene monomers followed by base treatment yielded the alternating carboxylated poly(phenylene ethynylene)s P95–P97 [53, 117].
Scheme 1.38 Synthesis of carboxylated poly(phenylene ethynylene)s (P94–P97).
Bunz's group also synthesized a series of carboxylated poly(phenylene ethynylene)s (Scheme 1.39). The homopolymer P94 was synthesized via the Sonogashira reaction between 79 and 80 [118]. Different from Schanze's method, the diiodo monomer 79 was synthesized directly from 32 with ethyl 2-bromoacetate. Using the diacetylene monomer 80, alternating carboxylated poly(phenylene ethynylene)s (P98–P101) were synthesized [54–56, 119].
Scheme 1.39 Synthesis of carboxylated poly(phenylene ethynylene)s (P94) and (P98–P101).
Apart from carboxylated poly(phenylene ethynylene)s, a series of sulfonated poly(phenylene ethynylene)s were also developed by Schanze's group based on the key monomer 81 (Scheme 1.40), which was synthesized analogously as 30 [57]. The Sonogashira coupling between 81 and different diacetylene monomers in aqueous media afforded sulfonated poly(phenylene ethynylene)s (P102–P107) [58, 59]. A diacetylene monomer with four oligo(ethylene glycol) was used by Kim's group to copolymerize with 81 to afford P108. In addition, a meta-linked sulfonated poly(phenylene ethynylene) (P109, Scheme 1.41) was also synthesized from 83, which was prepared on treatment of 3,5-diiodophenol (82) with 1,3-propanesultone in the presence of NaOH. P109 was reported to self-assemble into a helical structure in aqueous solution [60].
Scheme 1.40 Synthesis of sulfonated poly(phenylene ethynylene)s (P102–P108).
Scheme 1.41 Synthesis of a sulfonated poly(phenylene ethynylene) (P109).
Phosphonated poly(phenylene ethynylene)s constitute the third type of anionic poly(phenylene ethynylene)s. P110 was synthesized by Schanze's group through a postpolymerization method (Scheme 1.42) [61]. The key monomer 87 was obtained from 2,2′-(1,4-phenylenebis(oxy))diethanol (8487P110