Electron Donor-Acceptor Organic Polymers by “Click” Type Cycloaddition/Retroelectrocyclization Reaction

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 [54], 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 [55]. 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 [34]. 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 P1and P2, respectively. This was attributed to the decreased electron-donating power of the ferrocene donor substituted by the TCBD acceptor as well as the high steric hindrance in the main chain. The excess of TCNE did not cause any undesired side reactions and was removed by sublimation or reprecipitation. Both P1and P2featured well-defined intramolecular charge-transfer absorption bands and redox activities in both anodic and cathodic directions.

Furthermore, carbazole was selected as an electron-donating group, and the corresponding poly(arylenebutadiynylene)s were prepared by the acetylenic oxidative polymerization [56]. 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, P3. This conjugated polymer showed strong intramolecular charge-transfer interactions as well as higher thermal stability having a thermal degradation temperature above 350°C.

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 P4and P5, respectively. These two donor-acceptor conjugated polymers displayed an enhanced thermal stability as compared with the precursor polymer, arising from the reinforced intermolecular interactions caused by the cyano groups. UV-Vis-NIR spectroscopy revealed charge-transfer bands in the low-energy region, while electrochemistry confirmed the narrower bandgaps with the elevated HOMO and lower LUMO levels relative to the precursor polymer. After doping, these polymers showed room temperature conductivities of as high as 4.5 × 10⁻⁵ S/cm, which was about ten times greater than that of the precursor polymer.

Ohshita et al. [57] reported the reaction of poly(disilanyleneethynyleneoligothienyleneethynylene)s with TCNE to yield new donor-acceptor type organosilicon polymers (P6) bearing oligothienylene and tetracyanobutadienylene units as the donor and acceptor units, respectively. These polymers exhibited red-shifted absorption bands at λ_max = 513–565 nm in UV-Vis spectrum, whose edges reached approximately 800 nm, indicative of the strong charge-transfer interaction between electron donor and acceptor in the macromolecular backbone. These polymers also displayed solvatochromic behaviors. Specifically, the UV-Vis absorption maxima moved to the longer wavelengths with increasing solvent polarity. P6was photoactive because irradiation of TiO₂ electrodes in the polymer solutions resulted in the attachment of the polymer onto the TiO₂ surface via the formation of Si▬O▬Ti anchor bonds.

The cycloaddition/retroelectrocyclization reaction was first demonstrated in metal acetylide compounds [11], 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 [37] synthesized a main-chain thiophene-based platinum-polyyne conjugated polymer, which was further reacted with TCNE to yield electron donor-acceptor polymer P7. The use of the thiophene and platinum (II) donors in the precursor polymer efficiently improved the reactivity of main-chain alkynes. As a result, P7was obtained under mild heating at 70°C. Moreover, this thiophene-based platinum (II)-polyyne precursor polymer was explored to react with TCNQ to afford P8. P8displayed stronger electron-accepting feature than P7, as revealed by the more bathochromically shifted charge-transfer band and the lower first reduction potential. In order to enhance the electron-withdrawing property of electron donor-acceptor polymers, diketopyrrolopyrrole and benzothiadiazole were incorporated into P9and P10, respectively [58]. The LUMO levels decreased significantly to about −4.2 to −4.5 eV after postfunctionalization with TCNE.

Huang [38] 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 [36]. 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 (P11). P11shows a very low bandgap energy (Egopt=1.40eVand EgCV=1.10eV), because of strong intramolecular charge-transfer interactions between electron donors and acceptors along the macromolecular chain.

2.1.2 Main-chain electron donor-acceptor nonconjugated polymers

Washio and Michinobu et al. [39] 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 (P12). P12possessed the colorimetric chemosensor ability, as evidenced by well-defined color changes of the charge-transfer band) due to the recognition of Ag⁺ and Fe³⁺ ions. In contrast, the precursor TCNQ-containing polyester did not show any spectral changes upon the addition of Ag⁺ and Fe³⁺ ions. Specifically, the Ag⁺ ion was recognized by the cyano nitrogen atoms as a result of the effective multivalent coordination, leading to a bathochromic shift in the charge-transfer band. On the other hand, the Fe³⁺ ion was recognized by the aniline nitrogen atoms, as evidenced by the disappearance of the charge-transfer band.

By using a similar method to the synthesis of P12, polymeric electro-optic materials (P13) were synthesized by simple heating of a mixed thin film of TCNQ-containing polyester and an aniline-activated dialkyne molecule to 140°C under electric poling [59]. A thermal addition reaction between electron-rich alkynes and TCNQ moieties enabled the formation of donor-acceptor chromophores as well as cross-linking networks throughout the polymer film to effectively fix the dipolar orientation induced by electrical poling. It took about 13 h to complete the reaction, and the electro-optic coefficient (d_eff) showed the maximum value of 0.609 pm/V after 2.5 h.

Washino and Michinobu [42] described the polyaddition polymerization between electron-rich alkynes and a TCNQ-containing molecule to yield electron donor-acceptor non-conjugated polymers (P14). The electron-donating monomer possesses two dialkylaniline-substituted alkynes, while the electron-accepting monomer contains two TCNQ moieties. The aforementioned two monomers were then reacted in 1,2-dichloroethane under mild heating conditions to produce the desired linear polymers with high molecular weights. It was reported that high molecular weight polymers were preferentially formed at high monomer concentrations; however, cyclic compounds dominated when the monomer concentration was low. In both cases, the degree of reaction could be readily monitored by observing the shift of the charge-transfer band in UV-Vis spectra. The presence of terminal groups in the polymers were determined by the cyclic voltammetry, demonstrating the linear polymer structures of P14. P14showed an excellent thermal stability with the decomposition temperature exceeding 300°C. It also exhibited strong charge-transfer bands and redox activities due to the produced donor-acceptor moieties.

Furthermore, Washino and Michinobu [60] 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 P15gave rise to optical metal ion sensing, a colorimetric chemosensor behavior toward the Ag⁺ ion and electrochemical properties.

2.2.1 Side-chain electron donor-acceptor conjugated polymers

Michinobu [40] 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, P16and P17, respectively [62]. The successful addition of TCNE or TCNQ to the precursor polyamine was confirmed by the matrix-assisted laser desorption/ionization with time-of-flight (MALDI-TOF) mass spectra and the absence of the acetylene peak signals in the ¹³C NMR spectrum. The π-conjugation length of P16and P17was enhanced in the presence of electron-accepting moieties, thus leading to red shifts in their absorption spectra and significant reduction in their bandgaps. As the TCNE or TCNQ addition increased, both glass transition temperature and thermal decomposition temperature of the polyamines gradually increased. For example, the increment in the glass transition temperature was 27°C, while the increase in the thermal degradation temperature was 118°C for P16. These authors successfully fabricated thin film transistors based on P16and P17and established a clear correlation between the polymer energy levels and hole mobilities [63]. By using a similar strategy, a triphenylamine-based conjugated polymer (P18) with cyano-containing chromophore in the side chain was synthesized and used for probing Hg²⁺ ion. Specifically, fluorescence of P18solution in tetrahydrofuran was specifically quenched by the introduction of I⁻, accompanied by the change of its apparent color from pale brown to light yellow. In the presence of Hg²⁺ ion, the fluorescence of P18/I⁻complex recovered very quickly and even exceeded the initial intensity of pristine P18[64].

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. [61] reported the synthesis of a polythiophene electron donor-acceptor polymer P19. The reaction started from the highly region-regular poly(3-hexylthiophene) prepared by the Grignard metathesis polymerization, bromination at the 4-position of the thiophene ring, and Stille coupling reaction, to TCNE addition to the side-chain alkynes. Chemical and electrochemical n-doping of the TCBD moieties of P19resulted in the formation of poly(radical anion)s and poly(dianion)s. The temperature-dependent magnetic measurements were used to characterize the high spin state of the poly(radical anion)s, because regioregular polythiophene is classified as a non-Kekule and non-disjoint connectivity [61]. Furthermore, a regio-irregular polythiophene derivative with dialkylanilino-substituted alkyne side chains was synthesized by the Suzuki polycondensation, and then it was reacted with TCNE and TCNQ to afford P20and P21, respectively [65]. The study on the poly(phenylacetylene)-based electron donor-acceptor polymers (P22) indicated that the click postfunctionalization had a significant impact on the intermolecular interactions [29, 66]. Upon the completion of cycloaddition/retroelectrocyclization click postfunctionalization of the precursor polymer with TCNE, the film morphology changed from an unstructured homogeneous surface to fibrous nanostructures. As a result, the diffusion coefficients of O₂ and N₂ were dramatically reduced, which was attributed to the decrease in the void spaces. Wang et al. [67] synthesized a series of poly(p-phenylenebutadiynylene) electron donor-acceptor polymers (P23), which were prepared by homocoupling polymerization of asymmetric bifunctional monomers, followed by the “click” chemistry with TCNE to introduce donor-acceptor chromophores in the side chain. The energy level and bandgap could be precisely controlled by the addition of acceptor molecules, as evidenced by the optical and electrochemical characterizations.

Huang and Chen [41] developed an effective approach to synthesizing a low bandgap poly(arylene ethynylene) (P24) having donor-acceptor type chromophores in both side chains. Specifically, a diacetylene monomer and a carbonyl-activated diiodide monomer were synthesized and then polymerized by palladium-catalyzed cross-coupling reaction to afford a π-conjugated precursor polymer, which has approximately 50 repeat units of alternating arylene and ethynylene. The dioctylanilino-activated alkynes in the side chains of the precursor polymer were able to selectively react with TCNE by click-type [2+2] cycloadditions and the subsequent retroelectrocyclization to form the donor-acceptor type chromophores. The deactivated alkynes in the main chain of the precursor polymer remained intact so that the unique features of poly(arylene ethynylene)s such as excellent thermal stability and photostability could be kept in the target polymer. This unique polymer exhibited strong intramolecular charge-transfer interactions as evidenced by UV-Vis spectroscopy and cyclic voltagrammetry. As a result, P24has a low bandgap energy (E_g^opt = 1.59 eV and E_g^CV = 1.35 eV) and enhanced thermal stability.

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 (P25 and P26) bearing electron donor-acceptor moiety in the side chain [46, 47]. Both P25and P26displayed well-defined charge-transfer bands in the visible region at ca. 480 and 730 nm, respectively, which enabled the visual detection of analytes. Specifically, P25selectively detected Fe³⁺, Sn²⁺, and Fe²⁺ ions, while P26preferred Fe³⁺, Sc³⁺, Ti⁴⁺, and Cu²⁺ ions. The decrease in the charge-transfer band in the visible–near-infrared region was a result of the recognition of these metal ions by anilino nitrogen atoms in the side chain of P25and P26, and thus discoloration was a sign for visual recognition. On the other hand, only the Ag⁺ ion could interact with the cyano groups in the side chains of polymers through the multivalent coordination. As a result of this interaction, a bathochromic shift occurred in the charge-transfer bands, leading to the change of the visual colors.

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, P27and P28, respectively [64, 68, 69, 70, 71, 72]. It should be noted that both click reactions proceeded rapidly without any side reactions, and carrying out the reactions in the reverse order also gave the same products. Upon the completion of electric poling, the second-order nonlinear optical responses of the polymer films were observed. Furthermore, all these polystyrene-based electron donor-acceptor nonconjugated polymers exhibited very specific colorimetric ion sensing behaviors [73, 74, 75, 76]. The hard basic aniline nitrogen atoms preferred to interact with hard acidic metal ions like Fe³⁺, leading to the decrease in the charge-transfer absorption band and thereby resulting in decolorization. On the other hand, soft basic cyano nitrogen atoms selectively captured the soft acidic Ag⁺ metal ion, which caused a bathochromic shift of the charge-transfer band wavelength, thus leading to well-defined color changes. Nevertheless, the recognition of Ag⁺ ions by the cyano groups was not possible for the corresponding monomeric electron donor-acceptor molecule, which was attributed to the fact that multiple cyano coordinations were essential for the selective recognition of the Ag⁺ ions.

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 (P29). P29exhibited intense charge-transfer bands, well-defined redox activities, and good thermal stability. In addition, polymer thin films based on P29were prepared by spin coating on a glass slide. The second harmonic generation (SHG) of these thin films was measured before and after electric poling. The results showed that the SHG coefficient (d₃₃) of the poled thin film was as high as 3.0 pm/V [70].

Fujita et al. reported the synthesis of a block copolymer (P30) by combining living polymerization techniques and cycloaddition/retroelectrocyclization reaction [77, 78]. This block copolymer contains a poly(3-hexylthiophene) block and a polystyrene block with an electron donor-acceptor chromophore. P30was used as an interfacial compatibilizer for P3HT/PCBM (herein, P3HT = poly(3-hexylthiophene), and PCBM = phenyl-C61-butyric acid methyl ester) bulk-heterojunction organic photovoltaic cells in order to facilitate the nanophase separation of each semiconducting domain. The TCBD moiety in polystyrene block of P30had higher affinity to the n-type semiconducting domain, while poly(3-hexylthiophene) block in P30had good compatibility with poly(3-hexylthiophene) p-type polymer. Under the optimized blend ratio of the block copolymer, the power conversion efficiency was enhanced, which was attributed to the increased short-circuit current and fill factor. Also, the crystallinity of poly(3-hexylthiophene) polymer was improved upon the addition of P30. The efficient compatibilizing performance of P30at the interface of P3HT/PCBM enlarged and fixed the interfacial area and in turn suppressed the recombination of the generated carriers.

The combination of cycloaddition/retroelectrocyclization “click” chemistry and specific Ag⁺ ion recognition was later leveraged to create multicolored polyurethanes [79]. 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, P31and P32, respectively. P31and P32exhibited the intramolecular charge-transfer bands centered at 476 nm and 698 nm in CH₂Cl₂, respectively. After complexation of Ag⁺ ion with cyano groups in the side chains of P31and P32, the colors of the postfunctionalized polyurethanes were further changed. This was related to the change of the strengths of the intramolecular donor-acceptor interactions. For example, the solution color of P31changed to magenta and that of P32to a yellowish-green upon the Ag⁺ ion addition. Furthermore, the electrochemical oxidation of the precursor polyurethane at >+ 0.25 V (versus Ag/AgCl in MeCN) gave a blue polymer. This color change was attributed to the formation of aromatic amine-based radical cations.