The “click” type cycloaddition/retroelectrocyclization reaction is an intriguing approach for synthesizing electron donor-acceptor organic polymers. This chapter covers the fundamental reaction mechanism and the basic principles of applying this reaction to the synthesis of organic polymers via postfunctionalization or step-growth polymerization. The electron donor-acceptor moieties can be incorporated into the main-chain and/or side-chain of both conjugated and nonconjugated polymers. These polymers feature attractive properties including intramolecular charge-transfer bands, nonlinear optical properties, redox activities, third-order nonlinear optical properties, and enhanced thermal stability. Because of this, these polymers have found a variety of applications such as colorimetric chemosensors of metal ions, nonlinear optics, and solar cells. This novel “click” chemistry paves a unique path toward the synthesis of next-generation functional materials that cannot be accomplished by the incumbent synthetic methods.
- electronic donor-acceptor polymers
- click reaction
- cycloaddition/retroelectrocyclization reaction
The “click” chemistries such as Diels-Alder cycloaddition, Cu-catalyzed azide/alkyne cycloaddition (CuAAC), and thiol-ene reaction have revolutionized the polymer science over the past two decades and have become an indispensable tool in synthesizing new polymers or incorporating new functionality into macromolecules [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. In recent years, a new “click” chemistry of cycloaddition/retroelectrocyclization reaction has gained momentum in both organic synthesis and polymer synthesis. The “click” type cycloaddition/retroelectrocyclization reaction was first discovered by Bruce et al.  for the synthesis of metal acetylides in the 1980s. The first step of cycloaddition/retroelectrocyclization reaction involves the reaction of electron-rich alkynes activated by strong electron-donating groups (EDGs) with a strong electron-accepting cyanoolefinic molecule via a [2+2] cycloaddition to form the cyclobutene rings, and in the subsequent step, these cyclobutene rings are spontaneously opened to produce the donor-acceptor type chromophores in quantitative yields under mild conditions . The currently available electron-donating groups for almost quantitative yields include aromatic amines [13, 14, 15, 16], ferrocene [17, 18], azulene, and organometallic derivatives [19, 20]. Examples of electron-accepting cyanoolefinic molecules are tetracyanoethylene (TCNE), 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its derivatives [21, 22, 23, 24], as well as dicyanovinyl and tricyanovinyl derivatives [25, 26]. Among these electron-accepting molecules, TCNE is one of the strongest organic electron acceptors, and its high chemical reactivity toward nucleophiles or electron-rich reagents is frequently used to introduce strong acceptor moieties, for example, 1,1,4,4-tetracyanobuta-1,3-diene (TCBD), into organic molecules [27, 28]. The cycloaddition/retroelectrocyclization reaction mechanism between TCNE and electron-rich alkynes activated by an electron-donating group (EDG) is illustrated in Figure 1. By leveraging the same chemistry, TCNQ can also react with electron-donating group substituted alkynes to exclusively yield the thermodynamically stable Z-isomer, as demonstrated in Figure 2 .
Over the past decade, this “click” chemistry of cycloaddition/retroelectrocyclization reaction has been extensively used to synthesize electron donor-acceptor type organic molecules or dendrimers, which exhibit many interesting electrical, electrochemical, or nonlinear optical properties [30, 31, 32, 33]. Very recently, this synthetic protocol has been generalized into macromolecular systems through step-growth polymerization or postfunctionalization due to the high reactivity of this reaction . In particular, most of these polymers were realized by the postfunctionalization reaction of precursor polymers containing activated alkynes with TCNE or TCNQ. These electron donor-acceptor polymers can be classified into two major categories, namely, main chain and side chain, depending on the location of electron donor-acceptor chromophores in the macromolecular chain. For main-chain electron donor-acceptor conjugated polymers, the precursor polyyne polymers often contain electron-donating moieties such as ferrocene , carbazole , thiophene , and metal acetylide . However, only partial adduction of TCNE or TCNQ occurred, which was attributed to the low electron-donating property of electron donors as well as the high steric hindrance in the main chain . To address this challenge, Huang  synthesized a poly(arylene ethynylene) having dialkylanilino groups in the para-position relative to the ethynyl groups, which possess the strongest electron-donating effect on alkynes and thus enable the complete reaction of all alkynes in the main chain with TCNE to afford alternating electron donor-acceptor main-chain polymers. On the other hand, main-chain donor-acceptor nonconjugated polymers often involve a TCNQ-containing molecule and an aniline-activated alkyne molecule by cycloaddition/retroelectrocyclization reaction . Furthermore, side-chain electron donor-acceptor polymers were generally synthesized from precursor polymers bearing dialkylaniline-substituted alkyne side chains [40, 41]. In contrast to main-chain alkynes, side-chain alkynes showed reactivity as high as the corresponding small molecules, probably due to the lowered steric hindrance . Thus, the full TCNE (or TCNQ) addition to the polymer side chains could be achieved.
One of the striking features of cycloaddition/retroelectrocyclization reaction is that it does not involve any metal catalysts, and this reaction typically proceeds rapidly under mild conditions with very good yields . Furthermore, the resulting molecular structure shows not only tunable redox activities in both the cathodic and anodic directions but also strong charge-transfer bands in the visible absorption region . The TCNQ adducts exhibited a more bathochromically shifted absorption (usually a green color) as compared to the counter TCNE adducts (usually a red color) because of the extended π-conjugation [39, 42, 43]. The characteristics of click chemistry meet all the prerequisites required by the polymer synthesis. A very high yield of cycloaddition/retroelectrocyclization reaction is one of the essential parameters to obtain high-molecular-weight polymers by the step-growth polymerization . In addition, a lack of side products for cycloaddition/retroelectrocyclization reaction is desirable. Generally, the synthesis of small molecules can be purified by distillation or chromatography techniques. Unfortunately, it is rather difficult to separate undesired subunits from polymers caused by side reactions. These donor-acceptor polymers display an enhanced thermal stability compared with the precursor polymer, which was attributed to the reinforced intermolecular interactions caused by the cyano groups .
The low bandgap energy of π-conjugated polymers is vital for their applications in many emerging areas such as organic photovoltaic devices, light-emitting diodes, and nonlinear optical devices . The bandgap energy (
Furthermore, another important application of electron donor-acceptor polymers synthesized by cycloaddition/retroelectrocyclization reaction is for the use as colorimetric ion sensors, although their detection limits are usually inferior to those of fluorescent ion sensors. The nonplanar donor-acceptor chromophores in these polymers displayed the selective recognition of certain ions such as Fe3+, Fe2+, Sn2+, and Ag+ ions for the TCNE adducts and Fe3+, Cu2+, Ti4+, Sc3+, and Ag+ ions for the TCNQ adducts [46, 47]. The recognition usually occurs at the aniline nitrogen, resulting in a decrease in the charge-transfer bands accompanied by visual color changes.
This chapter aims to provide an overview of the state-of-the-art development of electron donor-acceptor organic polymers synthesized by the “click” type cycloaddition/retroelectrocyclization reaction, thereby elaborating on the synthetic approaches for both main-chain and side-chain polymers as well as their applications.
2. Synthesis of electron donor-acceptor organic polymers
Over the past decade, a wide variety of functional polymers have been synthesized by cycloaddition/retroelectrocyclization reaction . Among the synthetic protocols, postfunctionalization of precursor polymers bearing electron-rich alkynes with a strong electron acceptor is the most extensively employed technique for preparing electron donor-acceptor polymers because of the high reactivity and high yield of cycloaddition/retroelectrocyclization reaction .
2.1 Main-chain electron donor-acceptor organic polymers
2.1.1 Main-chain electron donor-acceptor conjugated polymers
Low bandgaps of conjugated polymers are highly preferred for many important applications such as solar cells, light-emitting diodes, field-effect transistors, and supercapacitors [49, 50, 51, 52, 53]. The bandgap energy of conjugated polymers can be effectively reduced by introducing electron donor-acceptor chromophores into these polymers, primarily arising from the intramolecular charge-transfer interactions between electron donor and acceptor [44, 45]. TCNE and TCNQ are among the strongest electron acceptors , but they are sparsely employed for synthesizing donor-acceptor type conjugated polymers. The underlying reason lies in the difficulty in synthesizing TCNE or TCNQ derivatives that are suitable for polymerization. In addition, many conventional approaches for synthesizing conjugated polymers, especially those involved with the use of metal ion catalysts (e.g., palladium or nickel), are not appropriate for the use in synthesizing polymers having TCNE or TCNQ derivatives, because these TCNE or TCNQ derivatives would always form strong complexes with these metal ion catalysts leading to the reduction in their catalytic performance . As a result, electron donor-acceptor conjugated polymers involving TCNE and TCNQ must be done by the postfunctionalization approach.
The postfunctionalization approach was first explored by Michinobu for ferrocene-containing poly(aryleneethynylene)s . Slow heating to 120°C for 3 h was required to facilitate the cycloaddition/retroelectrocyclization reaction between ferrocene-containing poly(aryleneethynylene)s and TCNE, and the completion of reaction was evidenced by the color change of the reaction solution from orange to green. However, only partial adduction of TCNE occurred, for example, the TCNE addition amounted to 0.75 and 0.62 for
Furthermore, carbazole was selected as an electron-donating group, and the corresponding poly(arylenebutadiynylene)s were prepared by the acetylenic oxidative polymerization . The substitution pattern of carbazole had a significant influence in the efficiency of the cycloaddition/retroelectrocyclization reaction. For example, 3,6-carbazole-based poly(arylenebutadiynylene) was successfully converted into the donor-acceptor type conjugated polymer, whereas the 2,7-carbazole-based counterpart polymer did not react with TCNE due to the insufficient activation of alkyne moieties. Upon the optimization of reaction conditions, 0.75–0.8 equiv. of TCNE was successfully reacted with main-chain alkynes of 3,6-carbazole-based poly(arylenebutadiynylene), resulting in the donor-acceptor conjugated polymer,
Thiophene is another important electron-donating moiety, and it has been incorporated into poly(thienyleneethynylene) by the Sonogashira polycondensation of 2-bromo-5-ethynyl-3-hexylthiophene [36, 56]. TCNE and TCNQ were employed to react with poly(thienyleneethynylene) in the presence of microwave irradiation to afford
Ohshita et al.  reported the reaction of poly(disilanyleneethynyleneoligothienyleneethynylene)s with TCNE to yield new donor-acceptor type organosilicon polymers (
The cycloaddition/retroelectrocyclization reaction was first demonstrated in metal acetylide compounds , and thus metal-polyyne polymers may also be good candidates for the cycloaddition/retroelectrocyclization postfunctionalization to synthesize metal-containing donor-acceptor conjugated polymers. Yuan and Michinobu  synthesized a main-chain thiophene-based platinum-polyyne conjugated polymer, which was further reacted with TCNE to yield electron donor-acceptor polymer
Huang  reported a facile synthetic route for synthesizing a main-chain donor-acceptor type polymer containing strong electron-donating dialkylamino groups and strong electron-accepting 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) units. The precursor polymer has a dialkylanilino group in the para-position on each main-chain alkyne, affording the strongest electron-donating effect for promoting the highest reactivity between TCNE and main-chain alkynes . This molecular design overcomes difficulty in successfully implementing the postfunctionalization of main-chain ethynyl groups along the macromolecular chain in a complete manner. Specifically, an electron-donating monomer 4,6-diethynyl-N,N,N,N-tetrahexylbenzene-1,3-diamine was successfully synthesized by converting two carbaldehyde groups in the corresponding monomer into acetylene groups using lithium trimethylsilyldiazomethane via a Colvin rearrangement. This electron-donating monomer was then polymerized with a carbonyl-activated diiodide monomer to afford an electron-donating π-conjugated precursor polymer with a reasonably high molecular weight, which was further reacted with TCNE via cycloaddition/retroelectrocyclization reaction under mild conditions to afford the target polymer (
2.1.2 Main-chain electron donor-acceptor nonconjugated polymers
Washio and Michinobu et al.  reported postfunctionalization of TCNQ-containing polyester by cycloaddition/retroelectrocyclization reaction with a small-molecule aniline-activated alkyne to yield a polyester containing electron donor-acceptor chromophores (
By using a similar method to the synthesis of
Washino and Michinobu  described the polyaddition polymerization between electron-rich alkynes and a TCNQ-containing molecule to yield electron donor-acceptor non-conjugated polymers (
Furthermore, Washino and Michinobu  synthesized sequence-regulated linear polymers by multiple click chemistry reactions, which could include cycloaddition/retroelectrocyclization reaction along with other click chemistry reactions such as copper (I)-catalyzed alkyne-azide cycloaddition (CuAAC) and Diels-Alder cycloaddition to create electron donor-acceptor polymers. The orthogonality with CuAAC was explored by investigating the reaction orders. It was only possible to complete the metal-free double click reactions in the order of the Diels-Alder cycloaddition followed by the alkyne-TCNQ addition. The resulting donor-acceptor chromophores in
2.2 Side-chain electron donor-acceptor organic polymers
In contrast to main-chain alkynes, side-chain alkynes showed a reactivity as high as the corresponding small molecules, probably due to the lowered steric hindrance. Thus, the full TCNE addition to the polymer side chains could be achieved. Since click postfunctionalization does not require any tedious purification process, such as column chromatography and reprecipitation, a series of side-chain electron donor-acceptor polymers have been synthesized by this approach [40, 41, 61].
2.2.1 Side-chain electron donor-acceptor conjugated polymers
Michinobu  first synthesized a precursor polyamine bearing the electron-rich alkynes in the side chain, which was subsequently reacted with TCNE and TCNQ to give side-chain electron donor-acceptor conjugated polymers,
The click postfunctionalization of side-chain donor-acceptor conjugated polymers has been leveraged to other conjugated polymer systems, such as polythiophene, poly(p-phenyleneethynylene), and poly(phenylacetylene) derivatives. Yuan et al.  reported the synthesis of a polythiophene electron donor-acceptor polymer
Huang and Chen  developed an effective approach to synthesizing a low bandgap poly(arylene ethynylene) (
2.2.2 Side-chain electron donor-acceptor nonconjugated polymers
Polystyrene-based polymers bearing dialkylanilino-substituted alkynes in the side chain are an important family of precursors for synthesizing electron donor-acceptor nonconjugated polymers by cycloaddition/retroelectrocyclization reaction. The “click” type reaction of TCNE and TCNQ with these polystyrene-based precursors afforded the corresponding new materials (
The orthogonal reactivity is one of the key features of click reactions. In order to illustrate the orthogonal reactivity of the cycloaddition/retroelectrocyclization reaction, double click postfunctionalization of poly(4-azidomethylstyrene) was conducted by CuAAC, followed by a cycloaddition/retroelectrocyclization reaction with TCNE or TCNQ to yield polymers,
Atom transfer radical polymerization (ATRP) of N,N-didodecyl-4-[(4-vinylphenyl)ethynyl]aniline was conducted with bromine-terminated polystyrene to synthesize block copolymers composed of unsubstituted polystyrene and electron-rich alkyne functionalized polystyrene segments. These block copolymers were then reacted with TCNE to introduce electron donor-acceptor chromophores into the side chains of polymers to produce TCNE-adducted polymers (
Fujita et al. reported the synthesis of a block copolymer (
The combination of cycloaddition/retroelectrocyclization “click” chemistry and specific Ag+ ion recognition was later leveraged to create multicolored polyurethanes . Specifically, a colorless polyurethane derivative having electron-rich alkynes substituted by dialkylaniline donors at both sides was synthesized by polyaddition between a diol monomer and tolylene-2,4-diisocyanate. The side-chain alkynes in the precursor polyurethane were then reacted with TCNE and TCNQ to yield orange-colored and green-colored polymers,
3. Summary and perspectives
This chapter gives an overview of electron donor-acceptor organic polymers synthesized by the “click” type cycloaddition/retroelectrocyclization reaction between electron donor-activated alkynes and olefinic acceptors such as TCNE and TCNQ. This “click” chemistry has many unique characteristics including high yields, short reaction times, and without the need for a catalyst or even a solvent. The salient features of the resulting electron donor-acceptor polymers encompass strong intramolecular charger-transfer interactions with tunable electronic absorptions that extend into the near-infrared region, active redox behavior, potent electron acceptor characteristics, high third-order nonlinear optical properties, high thermal stability, good solubility, and sublimation without decomposition, thereby enabling the preparation of thin films by vapor deposition. These electron donor-acceptor moieties have been integrated into the main chain or the side chain of polymers. In particular, the narrower bandgap and unique electrochemical properties of the resulting electron donor-acceptor conjugated polymers showed great potential in various applications such as nonlinear optical devices, organic photovoltaic devices, and light-emitting diodes.
This “click” chemistry was also effective for the synthesis of functional polymers with beautiful colors and successfully furnished the ion sensing abilities by the nonplanar donor-acceptor chromophores. Chemosensors are mainly classified as colorimetric and fluorometric sensors. However, chemosensors with a dual detection ability are very rare. Nonconjugated polymers bearing side-chain electron donor-acceptor chromophores composed of dialkylanilino donor and cyano-based acceptor groups have demonstrated the dual colorimetric detection behavior of several metal ions based on the specific interactions with different nitrogen atoms. Hard to borderline metal ions, such as Fe3+, Fe2+, and Sn2+, are always recognized by the dialkylanilino nitrogen atom, resulting in a decrease in the charge-transfer band intensity of the donor-acceptor chromophores. On the other hand, the recognition site of a soft metal ion of Ag+ is the cyano nitrogen atom due to the readily formed multivalent coordination which produces formed multivalent coordination leading to the bathochromic shift of the charge-transfer band. Chemosensors that can detect specific metal ions based on ligand-metal interactions have attracted increasing attention because of their high selectivity and low cost compared to other precise analytical techniques, such as atomic absorption spectroscopy and mass spectrometry.
The successful stories of electron donor-acceptor polymers enabled by the cycloaddition/retroelectrocyclization “click” chemistry open up a new avenue toward the synthesis of advanced polymers such as low bandgap polymers, intrinsically molecule-based conductive and/or magnetic polymers, organic photovoltaics, molecular batteries, and many other future applications.