Hai-Yan
Huang
ab,
Dongyang
Fan
*b,
Dong
Wang
b,
Ting
Han
*b and
Ben Zhong
Tang
*c
aCollege of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, Guangdong 518060, China
bCenter for AIE Research, Guangdong Provincial Key Laboratory of New Energy Materials Service Safety, College of Materials Science and Engineering, Shenzhen University, Shenzhen, Guangdong 518060, China. E-mail: dongyang.fan@szu.edu.cn; hanting@szu.edu.cn
cSchool of Science and Engineering, Shenzhen Institute of Aggregate Science and Technology, The Chinese University of Hong Kong, Shenzhen (CUHK-Shenzhen), Guangdong 518172, China. E-mail: tangbenz@cuhk.edu.cn
First published on 23rd January 2025
Conjugated main-chain charged polyelectrolytes (CMCPs) represent an important class of functional polymeric materials exhibiting distinctive structures and properties. By merging the advantages of conjugated polymers with the charge modulation capabilities of electrolytes, CMCPs have found wide applications in different fields and have garnered considerable interest across multiple disciplines such as chemistry, chemical engineering, materials science, and biomedicine. In this review, we emphasize the progress in the synthetic strategies of different types of CMCPs, including pyridinium-containing CMCPs, imidazolium-containing CMCPs, and other related structures. Furthermore, the applications of CMCPs in antimicrobial reagents, fluorescence sensing, fluorescence imaging, cancer cell killing, and fiber preparation are also introduced. To provide insights into the development of novel CMCPs with advanced functionalities, the current challenges and future prospects in this field are addressed at the end of this review.
Specifically reported backbones of MCPs include non-conjugated and conjugated types that contain different ionic motifs such as ammonium, phosphine, sulfonium, selenium, piperidinium, imidazolium, and pyridinium.21–27 Compared to non-conjugated MCPs, conjugated MCPs (CMCPs) exhibit excellent optoelectronic properties, high mechanical strength and thermal stability, and the molecular wire effect, which are well-inherited from the intrinsic advantages of conjugated polymers. The molecular wire effect could facilitate the communication of electrical signals or electrons along the conjugated polymer backbones, thus showing enhanced sensitivity as fluorescence sensors. These properties allow CMCPs to show great potential in the fields of high-performance electronic devices, optoelectronic materials, fluorescent chemo-/biosensors, biological imaging, and biomedicine.8,9,28–31 Current research studies on polyelectrolytes mainly focus on SCPs,10–17 while relevant studies of MCPs, especially CMCPs, remain insufficient, primarily due to the challenges in their synthesis. Based on previous studies, conjugated polyelectrolytes can be achieved through direct polymerization of monomers containing ionic functional groups. Related strategies include Wessling polymerization, oxidative polymerization, the Gilch synthesis method, and Suzuki, Sonogashira, Heck and Stille polycouplings.31–35 Direct polymerization strategies can yield target products efficiently in a one-step manner. However, these methods often require pre-functionalized ionic monomers, resulting in a narrow range of monomers, and the corresponding polymerization often suffers from harsh reaction conditions and poor atomic economy. The other kind of strategy is indirect synthesis, in which neutral conjugated polymer backbones are prepared first, followed by the introduction of charged groups on the backbones by post-modifications.36–38 These strategies feature the advantages of high polymerization flexibility, wider monomer selections, and controllable charge types. However, post-modifications cannot always be accomplished completely and are often accompanied by difficult-to-remove by-products, which may affect the performance and application of polyelectrolytes. Recently, another emerging strategy, named the C–H activation/annulation polymerization (CAAP) strategy, has been applied to the synthesis of CMCPs.29,39 Such polymerizations are highly efficient and environmentally friendly, with broad monomer selectivity and high atom economy, which allow the in situ construction of CMCPs with complex and diverse structures and promote the emergence of more multifunctional polymeric materials.
Despite the emergence of numerous systematic review studies in the field of conjugated polyelectrolytes,32,40–42 a comprehensive review of CMCPs is still lacking. Considering the significance of CMCP materials, this review aims to fill this gap by summarizing the progress in the synthetic methods and application of CMCPs. First, the synthetic strategies for CMCPs containing pyridinium, imidazolium, and other backbones, such as those with phosphonium and cinnolinium salts, are presented. The involved synthetic strategies include ring-transmutation polymerization, nucleophilic substitution, aldol-type polycondensation, CAAP, etc. Furthermore, based on the inherent excellent photophysical properties, high electrical conductivity, good film-forming properties, and biological activities of CMCPs, their applications in antimicrobial reagents, fluorescence sensing, fluorescence imaging, cancer cell killing, and fiber formation are also introduced. Finally, a brief summary and some perspectives on the future development directions of CMCPs are discussed.
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Fig. 1 Synthetic routes of pyridinium-containing CMCPs via ring-transmutation polymerizations. (A) Synthesis routes of pyrylium salts. (B) Polymerizations between pyrylium salts and amines to form pyridinium-containing CMCPs. (C) Chemical structures of different anions (2a and 4a–d) and amine monomers (3a–c) used for the preparation of CMCPs. Adapted with permission from ref. 20 (Copyright 2013 the Royal Society of Chemistry), ref. 43 (Copyright 2006 Elsevier), and ref. 44 (Copyright 2021 MDPI). |
For example, Sun et al. reported the preparation of conjugated polypyridinium salts by ring-transmutation polymerization of a bis(pyrylium salt) (2a) and 3,6-diamino-N-butylcarbazole (3a).20 The polymerization was carried out in DMSO, followed by the addition of toluene, and the resulting water could be removed as a toluene/water azeotrope. The yield was more than 85% with a number-average molecular weight (Mn) of 13800 g mol−1. Notably, the polymer exhibited typical aggregation-induced emission (AIE) properties. Therefore, the fluorescence of the polymer largely increased upon aggregation by electrostatic interactions with DNA, allowing for its application in sensitive DNA detection. Besides, Bhowmik et al. developed the ring-transmutation polymerization of a phenylated bis(pyrylium tosylate) salt (2a) and 2,6-diaminoanthracene (3b), producing an anthracene-containing polypyridinium after reacting for 24 h at 145–150 °C.43 By using lithium triflimide (4a), the counterions of CMCPs could be converted from tosylate to triflimide. In this study, the emission wavelengths of two obtained polymers were lower than 400 nm both in solution and in the solid state, exhibiting UV-light fluorescence. Furthermore, these polymeric products showed lyotropic liquid crystal properties and good film formation abilities, which endowed them with potential in optoelectronic applications. In 2021, this group further developed a series of poly(pyridinium salt)-fluorene CMCPs (4a–d) containing different anions by this method.44 The Mn values of these CMCPs ranged from 96
500 to 107
800 g mol−1. Due to the introduction of bulky counterions, the glass transition temperature (Tg) values decreased for some of the polyelectrolytes, and the thermal stability ranged from 305 to 423 °C. In addition, regardless of the counterion structures, all polyelectrolytes could form aggregates in DMSO, CH3CN, and CH3OH with the addition of water and the emission peaks of these aggregates gradually blue shifted. The above-mentioned approach for preparing pyridinium-containing CMCPs by ring-transmutation polymerization is well established and widely employed. By altering the conjugated backbone and anion structures, the properties of CMCPs can be easily tuned to meet the demands of different practical applications.
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Fig. 2 (A) The common method used for the synthesis of non-conjugated MCPs. (B and C) Synthesis of pyridium-containing CMCPs via (B) nucleophilic substitution reactions and (C) aldol-type polycondensations. Adapted with permission from (B) ref. 48 (Copyright 2009 American Chemical Society), ref. 49 (Copyright 2014 Wiley-VCH GmbH), and (C) ref. 50 (Copyright 1989 Wiley-VCH), respectively. |
Aldol-type polycondensation is another commonly used strategy for the synthesis of pyridinium-containing CMCPs. This method usually employs bisaldehydes and bipyridine derivatives as monomers, which undergo polycondensations under alkaline conditions to produce target products. As shown in Fig. 2C, Merz et al. synthesized a conductive bipyridium polymer via this method.50 The reaction was carried out with methylbipyridine and 1,4-benzenedicarboxaldehyde as monomers in the presence of the organic base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and the dehydrating agent benzoyl chloride to obtain the polyelectrolyte P9, which has a conductivity of 1 × 10−3 S cm−1 and electroactivity in the range from −0.1 to −0.6 V versus Ag/AgCl. More importantly, P9 showed good stability in water and many organic solvents. When exposed to iodine vapors, the conductivity of P9 can be further increased 2–3-fold.
For instance, Qin et al. recently developed a novel spontaneous amino-yne click polymerization reaction with diamine (13) and pyridinium-activated diyne (12) as monomers to prepare a series of pyridinium-containing CMCPs P10 in yields of up to 98% with an Mw of up to 22200 g mol−1 (Fig. 3A).51 The resulting polyelectrolytes all exhibited excellent thermal stability and were made to have good aggregate-state luminescence by introducing AIEgens into the polyelectrolyte backbone. In addition to the click polymerizations, alkyne-based C–H activation/annulation polymerizations (CAAPs) have also been used to synthesize CMCPs. Very recently, Fan et al. reported a cobalt(III)-catalyzed CAAP method using aryl thioamides (14) and internal alkynes (15) as starting materials for the preparation of N, S-doped fused heterocyclic polymers (16) (Fig. 3B).52 The corresponding polyelectrolyte (P11) was then obtained via post-methylation of the polymer. Interestingly, the refractive index values of P11 at 632.8 nm and 1550 nm were up to 1.8464 and 1.7869, respectively, exhibiting low chromatic dispersions. This polymerization strategy was facile, straightforward, and efficient, which enhanced the possibility of constructing CMCPs with various structures and functionalities.
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Fig. 3 (A and B) Alkyne-based two-component polymerization methods. Reproduced with permission from (A) ref. 51 (Copyright 2024 ChemRxiv) and (B) ref. 52 (Copyright 2024 American Chemical Society), respectively. |
Apart from two-component alkyne-based polymerization methods, Tang et al. successfully developed several multicomponent CAAP reactions to synthesize pyridinium-containing CMCPs. For example, a rhodium-catalyzed multicomponent CAAP strategy using readily accessible monomers has been developed (Fig. 4A).2 This method can efficiently synthesize a series of azonia-containing polyelectrolytes in a one-pot manner (up to 99% yield and 28700 Da molecular weight). Owing to the presence of electron-deficient azonia-containing fused rings in the main chains, the emission wavelength of P12 can be tuned by changing the electron donors. Meanwhile, the obtained CMCP films displayed efficient solid-state luminescence with good photosensitivity and high refractive indices. These properties allowed the polymers to be potentially applied in fluorescence photopatterning and advanced optical materials. In 2021, this group further synthesized a series of heteroaromatic hyperbranched polyelectrolytes in a one-pot manner by multicomponent CAAP in up to 97% yield (Fig. 4B).18 Compared to traditional approaches, the method required neither expensive monomers nor strict conditions, and enjoyed the applicability of a wide range of monomers. Similarly, the D–A structures in polymers allowed for tunable luminescence properties.
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Fig. 4 (A and B) Alkyne-based multicomponent polymerization methods. Reproduced with permission from (A) ref. 2 (Copyright 2019 American Chemical Society) and (B) ref. 18 (Copyright 2021 Wiley-VCH GmbH), respectively. |
Overall, alkyne-based polymerization methods have created new development room to synthesize pyridium-containing CMCPs. It can be foreseen that this type of synthetic method and the obtained fused heterocyclic polyelectrolytes will provide more opportunities for the development of advanced functional polymer materials through further exploration of the structural design, synthesis and structure–property relationship.
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Fig. 5 (A) The synthetic method of non-conjugated MCPs with imidazolium-salts. (B and C) The synthetic routes of imidazolium-containing CMCPs via the post-modification reaction. Reproduced with permission from (B) ref. 55 (Copyright 2017 American Chemical Society) and (C) ref. 56 (Copyright 2019 Springer Nature), respectively. |
One of the most popular strategies for constructing imidazolium-containing CMCPs is the post-modification method. For example, Holdcroft et al. synthesized poly(arylene-imidazole) (27) by a microwave-irradiated polycondensation reaction using dialdehyde, bis(dibenzoyl), and ammonium acetate as monomers. The methylation reaction of 27 eventually affords the imidazolium-containing polyelectrolyte P14 with an Mw of 67000 g mol−1 in a yield of 95% (Fig. 5B).55 The resulting polyelectrolyte showed excellent stability with a half-life of more than 5000 h at 100 °C in 10 M KOH aqueous solution. The films of the polyelectrolyte were tough, pliable and transparent. The tensile strength was 43.5 ± 1.4 MPa and elongation at break was 44.3 ± 9.6%. Besides, this group prepared imidazole-containing polymers 30 by Yamamoto homo-polycoupling using synthesized dichloroimidazole compounds as monomers. Then, the imidazole structure of the polymer was alkylated to obtain imidazolium CMCPs (P15) (Fig. 5C).56 The obtained products showed good solubility. These bisimidazolium polymers and membranes exhibited balanced alkaline stability and high ion-exchange capacity, resulting in exceptional chemical stability and hydroxide conductivity with minimal water content.
The CAAP strategy can be employed for the synthesis of imidazolium-containing CMCPs as well. Recently, Wang et al. reported an efficient rhodium-catalyzed cascade CAAP strategy for the synthesis of polymers with multiple ring-fused aza-heteroaromatic structures from aryl imidazoles (31) and internal diynes (32) (Fig. 6A).29 Polybenzoimidazoles (33) containing rigid conjugated spacers of tetraphenylene (TPE) or triphenylamine (TPA) were prepared in up to 96% yield within 2 h. These conjugated polymers showed good thermal and morphological stability, retaining more than 70% of the char residue at 800 °C, with glass transition temperature (Tg) values as high as 315 °C. Imidazolium-containing CMCPs (P17) were further afforded via the simple N-methylation reaction of 33 in nearly quantitative yields with red-shifted fluorescence emission. Due to the highly separated charge distribution in the CMCPs, the energy gap between singlet and triplet states was apparently reduced, which promoted the intersystem crossing process, giving these CMCPs strong ROS-generating ability and excellent performance in cancer cell killing. Following this work, this group further reported an N-heterocyclic carbene-directed cascade CAAP strategy for the direct preparation of fluorescent cationic CMCPs in up to 97.2% yield (Fig. 6B).8 This rhodium-catalyzed polymerization proceeded smoothly under mild conditions with readily available monomers (34 and 35). The generated CMCPs (P18) have good solubility in commonly used organic solvents. By introducing different substituents around the cationic ring-fused core, the donor–acceptor effect in the polymers could be effectively regulated, which further enhances the tuning flexibility of fluorescence properties of these CMCPs.
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Fig. 6 (A and B) Synthesis of imidazolium-containing CMCPs based on the C–H activation/annulation polymerization strategy. Reproduced with permission from ref. 29 (Copyright 2022 American Chemical Society) and ref. 8 (Copyright 2023 American Chemical Society), respectively. |
The abovementioned examples indicated that the CAAP strategy is an attractive method for the development of multi-substituted imidazolium-containing CMCPs with diversified structures in a simple, straightforward, atom-economical, and efficient manner. Further development of novel CAAP reactions will provide more convenience to prepare novel imidazolium-containing CMCPs with fascinating functionalities. Meanwhile, the CAAP strategy also faces several challenges. For instance, the CMCP products obtained through this method are often composed of multiple isomeric repeating units. The poor structural regularity of such CMCPs is favorable for solubility and processibility but may limit their application performance in certain areas like electrical devices.
For example, Anderson et al. reported a strategy for the synthesis of phosphonium-containing CMCPs (Fig. 7).31 First, compound 36 was prepared by a Knoevenagel condensation reaction between bromothiophene aldehyde and 1,4-diacetyl-2,5-diketopiperazine. Subsequently, compound 36 was prepared in the presence of Tf2O to give compound 37. Finally, the substitution reaction of compound 37 with triphenylphosphonium afforded phosphonium-containing monomer 38 (Fig. 7A). The Stille polycoupling of monomer 38 with different tin-functionalized thiophene monomers gave phosphonium-containing CMCPs with different main chains (P19, P20) (Fig. 7B). Optical, electrochemical and theoretical studies revealed that these ionic substituents endowed the polyelectrolytes with low LUMO energy levels and small and highly tunable band gaps. Both polyelectrolytes exhibited NIR absorbance and P19 exhibited excellent photothermal properties under 808 nm laser irradiation, showing great potential in antimicrobial photothermal therapy (PTT). Furthermore, Zhang et al. developed a Rh(III)-catalyzed polymerization process using internal alkynes and azo compounds to obtain cinnolinium-containing CMCPs.57 The synthesized polyelectrolyte had an Mw of 25000 g mol−1 and good thermal stability. Benefiting from the unique luminescence properties and the positive-charged cores, the polyelectrolyte was successfully applied in bacterial imaging and killing.
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Fig. 7 (A and B) The synthetic route of CMCPs with a phosphonium-salt structure. Adapted with permission from ref. 31. Copyright 2019 Wiley-VCH GmbH. |
As a short summary, with the advancement of different synthetic strategies, a number of CMCPs have been developed in the past years. However, the structural diversity of CMCPs still needs to be further enriched.
In recent years, Tang's group reported the synthesis of a series of azonia-containing polyelectrolytes in a one-pot manner.2 The resulting polyelectrolytes exhibited typical AIE characteristics, photostability and rapid bacterial staining capacities. More importantly, they possessed excellent ROS generation ability to kill methicillin-resistant Staphylococcus aureus (MRSA), showing good bactericidal effects both in vitro and in vivo. Susequently, this group further reported a rhodium(III)-catalyzed CAAP strategy for constructing nitrogen-containing fused hyperbranched polyelectrolytes.18 As depicted in Fig. 8A and B, the CMCP P21 containing a tetraphenylethylene (TPE) unit exhibited aggregation-enhanced emission (AEE) with its emission intensity significantly increasing upon the addition of a poor solvent. Using ROS indicators, P21 demonstrated strong 1O2 generation. This capability is ascribed to its conjugated acceptor–donor structure that reduces the energy gap between the singlet and triplet states, while its compact hyperbranched structure stiffens intramolecular motions and thus inhibits the nonradiative decay, both of which together promote the ISC process. The size distribution of P21 was measured by dynamic light scattering and its hydrodynamic diameter ranged from 70 to 150 nm (Fig. 8C) with a zeta potential of 7.79 ± 0.75 mV. The positive charge of P21 enables its accumulation on negatively charged bacterial membranes via electrostatic interactions, facilitating fluorescent labeling of MRSA within 60 min (Fig. 8D). Under dark conditions, P21 moderately inhibited bacterial viability due to its intrinsic dark toxicity. However, upon white-light irradiation, bacterial viability was reduced by two orders of magnitude due to the synergistic effects of phototoxicity and dark toxicity. At a concentration of 30 μg mL−1, P21 achieved a bacterial killing efficiency of 99.2%, indicating its potent inactivation capability (Fig. 8E). Live/dead fluorescence staining further confirmed these effects (Fig. 8F). Under light irradiation, the bacteria displayed red fluorescence, signifying cell death, whereas under dark conditions, surviving bacteria emitted green fluorescence. Scanning electron microscopy (SEM) (Fig. 8G) revealed that, in the absence of light, bacterial membranes exhibited a rough, partially collapsed surface. Under light irradiation, the membranes showed severe splitting and deformation, likely caused by the photodynamic effect of P21. Apart from photodynamic antibacterial effects, some CMCPs have also been utilized in antibacterial photothermal therapy (aPTT). For instance, the previously mentioned P19, upon exposure to an 808 nm laser, raised the temperature of a 30 mM polymer solution to 50.68 °C within 5 minutes, effectively disrupting bacterial membrane proteins and lipids, leading to bacterial death.31
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Fig. 8 (A) Structure of CMCP P21. (B) Plot of the relative emission intensity (I/I0) versus the composition of hexane/chloroform mixtures of P21. Inset: photographs of P21 in hexane/chloroform mixtures with different hexane fractions (fH) taken under UV irradiation from a handheld UV lamp. (C) Size distribution of P21 measured by dynamic light scattering. Inset: a TEM image of P21 aggregates. (D) CLSM images of bacteria incubated with P21 at different times. (E) Statistical analyses of the bacterial viability by the logarithm number in the absence and presence of white-light irradiation. (F) Live/dead staining of bacteria with different treatments. (G) SEM images of bacteria with different treatments. Adapted with permission from ref. 18. Copyright 2021 Wiley-VCH GmbH. |
Lu and co-workers reported preparing a novel conjugated poly(pyridinium salt) derivative (P22) with AIE properties via ring-transmutation polymerization (Fig. 9A). The CMCP aggregated with DNA through the synergistic effects of electrostatic attraction and intercalation, enabling its application in fluorescence turn-on detection. P22 exhibited selective fluorescence enhancement upon interaction with double-stranded DNA (dsDNA), which has also been successfully used to track DNA cleavage by nuclease.28 The cationic charge on P22 facilitated complex formation with highly negatively charged ctDNA via electrostatic attraction. As shown in Fig. 9B, P22 exhibited a maximal absorption at 337 nm, attributed to the π–π* transition of its backbone. Upon increasing the ctDNA content, the absorbance at 337 nm decreased, accompanied by a redshift of approximately 10 nm, indicating aggregate formation between ctDNA and P22. Moreover, with the augmentation of the ctDNA concentration, the fluorescence intensity at 541 nm sharply increased. Compared to the weak emission of P22 in solution, its emission intensity was enhanced 10.2 times upon the addition of ctDNA, reaching saturation at a ctDNA concentration of 23.6 μM. No obvious fluorescence increase at 541 nm was detected with various anions (including Cl−, Br−, HCO3−, HPO42−, HSO4−, I−, P2O74−, PO43−, and SO42−) or ssDNAs containing different base lengths, demonstrating the high selectivity of P22 for ctDNA. This selectivity, driven by synergistic electrostatic attraction and intercalation, enhanced fluorescence emission via the AIE effect. When bovine pancreatic deoxyribonuclease I (DNase I), an endonuclease, was added to the mixture of ctDNA and P22, the fluorescence intensity gradually decreased over time (Fig. 9C). These results suggested that P22 is a promising fluorescent probe for the development of a label-free fluorescence nuclease assay.
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Fig. 9 (A) Structure of CMCP P22. (B) UV-vis absorption spectra of P22 ([P22] = 5 μM) in the presence of different concentrations of ctDNA in CH3CN–H2O (v/v = 1![]() ![]() ![]() ![]() |
As shown in Fig. 10A, Han et al. prepared a CMCP P23 containing a TPA unit and cyano group using the CAAP strategy. P23 was able to achieve highly specific and long-term cytomembrane imaging of plant cells.30 Confocal laser scanning microscopy (CLSM) revealed that P23 exhibited bright fluorescence on the onion cytomembrane, with almost no fluorescence detected in the cytoplasm and intercellular substance. Importantly, P23 displayed long-term imaging of the plant cytomembrane, with no significant change occurring in the cytoplasm and other cellular regions for at least 12 h. The positive charge on P23 greatly promoted its electrostatic interaction with the plant cell membrane, thereby improving its imaging capability. As shown in Fig. 10B and C, Vigna radiata (V. radiata) sprouts were obtained after treatment with P23 at different concentrations for 7 days. The result showed that P23 had minimal impact on the growth of V. radiata even at a concentration of 20 μM, implying its low biotoxicity. Besides, P23 achieved good fluorescence imaging in other plant cells, such as Glycine max (G. max) and V. radiata, indicating its broad applicability. P23 exhibited the advantages of good photostability, wash-free staining, high imaging integrity, durability and low biotoxicity, which provided a promising platform for the development of AIE-active fluorescent probes for plant plasma membranes.
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Fig. 10 (A) Graphical abstract of the fluorescence imaging of CMCP P23. (B) Photographs of representative V. radiata seedlings incubated with different concentrations of P23 after 7 days. (C) Average rhizome lengths of V. radiata seedlings incubated without and with different concentrations of P23 after 7 days. Adapted with permission from ref. 30. Copyright 2024 American Chemical Society. (D) Structure of CMCP P24. (E) ROS generation of polymers (P24 and P1a/P2c, 1 μM) at pH = 5.37 and 7.40 upon white-light irradiation (24 mW cm−2) using dichlorofluorescin (DCFH) as an indicator. (F) Cell viability of 3T3 and 4T1 cells incubated with P24 without or with the irradiation of white light (24 mW cm−2, 10 min). (G) CLSM images of 4T1 and 3T3 cells after incubation with P24 (20 μg mL−1) for 0.5 h. (H) CLSM imaging for intracellular ROS generation (upper row) and the live/dead cell assay (lower row) of 4T1 cells with different treatments. Adapted with permission from ref. 29. Copyright 2022 American Chemical Society. |
In addition to fluorescence imaging, this group developed MCCP P24 for cancer cell killing (Fig. 10D).29 As depicted in Fig. 10E, P24 demonstrated excellent ROS generation capability, with a 730-fold enhancement within 150 seconds. Further experiments confirmed the presence of both type I and type II ROS in P24. The MTT assay revealed that P24 demonstrated a synergistic effect of dark toxicity and high phototoxicity against cancer cells (Fig. 10F). Specifically, in the dark, P24 showed higher dark toxicity towards 4T1 cells compared to normal 3T3 cells. Upon light irradiation (24 mW cm−2, 10 min), the cancer cell viability significantly decreased with the treatment of P24. Differences in cytotoxicity between normal and cancer cells were attributed to the varying cellular uptake levels of P24 (Fig. 10G). As depicted in the upper row of Fig. 10H, using the ROS indicator 2′,7′-dichlorofluorescein diacetate (DCFH-DA), bright green fluorescence was observed in 4T1 cells under light irradiation, whereas no significant fluorescence appeared in the control groups. This result indicated that P24 was able to produce ROS efficiently. The live/dead staining experiment provided visual evidence for the phototherapeutic effect of P24 (Fig. 10H, lower row). While light-only irradiation did not cause severe cell death, red fluorescence indicative of cell death was clearly observed in P24-treated cells, implying its phototoxic effect with reduced side effects. Interestingly, the authors found that green fluorescence was accompanied by faint red fluorescence in the P24 group, which did not entirely align with the MTT assay results. They hypothesized that 4T1 cells treated with P24 for 13 hours under dark conditions were in an early apoptotic state rather than fully dead. This hypothesis was further validated through an Annexin V-APC/PI detection assay.
Recently, Wang et al. reported a series of imidazolium-containing CMCPs (P25–P28) (Fig. 11A). The charge distributions of the repeating units in these CMCPs were analyzed using an electrostatic potential map (EPM), which revealed that the positively charged regions were primarily concentrated in the imidazolium moiety of the cationic ring-fused backbone (Fig. 11B). This provided theoretical support for the formation of IPC fibers. As depicted in Fig. 11C, P28 was brought into contact with sodium alginate (SA) droplets using a pipette tip, and a thin and straight IPC fiber was extracted through the interfacial interaction between polymer solutions, followed by continuous upward traction. The obtained fiber structure was fully characterized by fluorescence microscopy. As shown in Fig. 11D, most of the cationic CMCPs and anionic SA polymers were uniformly distributed in a single fiber. Fluorescence 3D images with corresponding cross-sectional views obtained through CLSM further illustrated the internal structures of fibers, where the fluorescent regions corresponded to the CMCPs, and the dark regions represented the SA components. These IPC fibers exhibited high flexibility and good adhesion, enabling the preparation of stable colorful fiber strands and fluorescent paintings (Fig. 11E). Such properties significantly broaden the application scope of CMCPs. In addition to these common applications, CMCPs also show potential in field-effect transistor devices.48 For instance, the reported CMCP P5 exhibited n-type behaviors with mobilities of 0.24 cm2 (V s)−1 and 3.4 cm2 (V s)−1 at gate voltages of 5–15 V and 15–20 V, respectively. Some CMCPs have also been applied in the detection of various amines. For example, Swager et al. reported a series of CMCPs containing various anions.49 Among them, P7 with perfluorinated tetraphenylborate and P8 with tetrafluoroborate exhibited sensing abilities to simple alkyl amines and aniline, respectively. When they were exposed to the volatile amine vapors, the fluorescence of CMCPs was quenched quickly. The presence of different anions endowed polymers with the ability to differentiate amines. In the future, more CMCPs are expected to be synthesized and applied in diverse fields.
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Fig. 11 (A) Structures of CMCPs (P25–P28). (B) Electrostatic potential surface of the repeating unit of P28. (C) Fluorescence photographs showing the IPC fiber drawing process under 365 nm UV irradiation. (D) Optical and fluorescence images of the freshly formed cationic CMCPs/sodium alginate fibers captured under white light and UV light irradiation (340–380 nm), respectively, using an inverted fluorescence microscope (the front two columns of images), and their fluorescence 3D images together with the corresponding cutaway view obtained by CLSM (the latter two columns of images). All colors are false colors in these images. (E) Diverse forms of the obtained fluorescent cationic polyelectrolyte/sodium alginate fibers. The macroscopic fluorescence images feature real colors and were directly captured using a digital camera under 365 nm UV light irradiation. Adapted with permission from ref. 8. Copyright 2023 American Chemical Society. |
First, the structure types of CMCPs are mainly limited to nitrogen-containing structures, including those based on pyridinium and imidazolium salts. Meanwhile, the charges carried by most of the available CMCPs are positive. More efforts could be devoted to introducing other ionizable heteroatoms into CMCP structures to form charged oxygen and sulfur-containing structural moieties or introducing negatively charged structural units into CMCP structures. Second, traditional methods for the synthesis of CMCPs mainly involve ring-transmutation polymerizations, nucleophilic substitution, aldol-type condensation, etc. These synthetic routes are relatively complex and time-consuming. Developing facile and efficient methods is crucial for the enrichment of structural and functional diversity. Among various methods, the CAAP strategy has shown great potential in the construction of novel CMCPs with the advantages of high atom economy and step economy, a wide monomer scope, and readily functionalized fused heterocyclic units with multiple substituents, which can conveniently modulate the electronic effects of the CMCP backbones and promote the development of multifunctional fluorescent polymer materials. Furthermore, precise control over molecular weights and polydispersities is essential for the study of structure–property relationships as is fine tuning materials properties such as mechanical performance, thermal stability, and photophysical characteristics. However, it still remains challenging to synthesize CMCPs with controllable molecular weights and narrow polydispersities, which could be one of the important research directions of this field that can benefit from further improvements. The properties of CMCPs, such as optical properties, electrical conductivity, and bioactivity, can be altered easily to meet the application demands in different fields. We hope that this review will provide new inspirations and ideas for researchers to develop easier synthetic methods and construct CMCPs with more diversified structures and advanced functionalities.
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