Conjugated polymers developed from alkynes

The numerous merits of conjugated polymers (CPs) have encouraged scientists to develop a variety of synthetic routes to CPs with diverse structures and functionalities. Among the large scope of substrates, alkyne plays an important role in constructing polymers with conjugated backbones. In addition to some well-developed reactions including Glaser-Hay and Sonogashira coupling, azide/thiol-yne click reaction and cyclotrimerization, some novel alkyne-based reactions have also been explored such as oxidative polycoupling, decarbonylative polycoupling and multicomponent tandem polymerizations. This review focuses on the recent progress on the synthetic methodology of CPs in the last five years using monomers with two or more triple bonds and some of their high-technological applications. Selected examples of materials properties of these CPs are given in this review, such as fluorescence response to chemical or physical stimuli, magnetism, white light emission, cell imaging and bioprobing. Finally, a short perspective is raised in regard to the outlook of the preparation methodologies, functionalities as well as potential applications of CPs in the future.


INTRODUCTION
Since the discovery of metallic conductivity in doped polyacetylene (HC=CH) n in 1977 by Heeger, Shirakawa and MacDiamid, conjugated polymers (CPs) become an important category of materials and enjoy a worldwide popularity for many years [1]. CPs have been found to possess distinguished photophysical, electrochemical and magnetic properties. The delocalized electrons migrating along the conjugated backbones result in amplification of fluorescence signals and consequent excellent performance in organic photoluminescence (PL) [2] or electroluminescence devices [3].These discoveries not only lead to the springing of new functional materials, but also inspire chemists to develop versatile synthetic methodology.
Generally, double bonds and triple bonds are major building blocks of pure organic CPs such as polyacetylene and its derivatives [4] and poly(phenylene vinylene)s (PPVs) [5]. These units are linked covalently in a π -conjugated fashion. Some heteroatoms can be involved in the por n-type conjugation as well, such as boron [6], nitrogen [7], silicon [8] and sulfur [9] atoms. For example, PPVs, a classic type of double-bonded CPs, are traditionally prepared through xylyene elimination [10], Wittig-Gilch coupling [11], Knoevenagel condensation [12] and Heck coupling [13] reactions. Most of these reactions are based on alkene monomers and suffer from poor atom economy or limited structural variation in the produced polymers. On the other hand, carboncarbon triple-bonded monomers, namely alkynes, are much superior in reactivity. Alkynes are versatile in terms of constructing CP skeletons, from which plentiful structures can derive [14]. They can undergo self-coupling or cross-coupling to generate diyne-or monoyne-containing CPs, and also form double-bonded repeating units that are consecutive or divided by heteroatoms such as sulfur or silicon. Besides, they can be conveniently coupled into aromatic or triazole ring as well. Some well-known reactions, for instance, Glaser-Hay coupling, Sonogashira coupling and azide-alkyne click reaction, have been widely used to construct functional polymers with desired structures. In addition, a series of novel reactions have been developed to expand the scope of CPs and enrich the methodology of their syntheses, and enormous publications have emerged so far in this field [15]. 494 Natl Sci Rev, 2015, Vol. 2, No. 4 REVIEW For the past decades, polymer chemists, material scientists and engineers have combined efforts to explore the functionality and application of CPs. The domains of semiconductors, luminescent materials and electric devices have all witnessed the development of functional CPs, and many of them have found their industrial value. More high-tech applications regarding stimuli-responsive materials, metallic ceramics, information storage and medical diagnosis [16] are under exploration. The prosperity of CPs always calls for inspiration and dedication on the basis of our fundamental understanding about the realm.
Abundant reviews are available in the literature discussing the basic theories, syntheses and properties of CPs [17]. This short review mainly focuses on the latest research work published during the last five years concerning synthetic approaches based on monomers with two or more triple bonds. For CPs synthesized from monoynes, i.e. polyacetylenes, we are not going to discuss here and interested readers can find related reviews published by us and other groups for details [4]. As shown in Scheme 1, they are grouped into four categories based on the structures of the CPs: (A) polymers with repeating units linked by triple bonds, such as those prepared by Glaser-Hay and Sonogashira polycoupling; (B) double-bonded CPs synthesized through decarbonylative polyaddition, hydrothiolation and hydrosilylation; (C) polyaromatics generated by polycyclization or polyannulation of terminal or internal diyne; and (D) CPs containing Scheme 1. Synthetic routes to conjugated polymers based on alkynes. Liu et al. 495 heterocycles, such as polytriazoles from click polymerization and polythiophenes from tandem polymerization. Among the CPs, those with light emission constitute a significant family with wide applications and intrinsic chromophores in the monomers are usually responsible for their light-emitting properties. Many traditional chromophores suffer from quenching of their solid-state emission due to the formation of excimers/exciplexes. In contrast, molecules with aggregation-induced-emission (AIE) characteristics exhibit strong solid-state emission although they are weak emitters in the solution state. This phenomenon was first discovered by Tang

Triple bonds
Hay-Glaser coupling [20] and Sonogashira coupling [21] reactions are useful tools for direct introduction of triple bonds to the polymer backbones. Hay-Glaser reaction, one of the earliest oxidative homocoupling reaction, is widely used for the construction of polydiynes (PDYs) [22]. The problem of stoichiometric imbalance is avoided since sole component is involved. As a result, PDYs with high polymerization degree can be easily obtained. Conjugated polyelectrolytes (CPEs) are well known for Scheme 2. Glaser-Hay polycoupling reaction.
their distinguished functions as fluorescent transducers or biosensors [23]. The ionic pendants impart good solubility to the CPEs in aqueous media despite their conjugated skeleton. Moreover, the hydrophilic ionic moieties and the hydrophobic backbones endow CPEs with amphiphilic characteristics. As illustrated in Scheme 2, the tetraphenylethene (TPE)-containing diyne monomers 1 and 2 can undergo homocoupling at 50 • C in the presence of CuCl and TMEDA in dichlorobenzene, generating water-soluble CPEs 5 and 6 after subsequent quaternization [24]. Thanks to the typical AIE-active TPE units, the polymers exhibit strong solid-state emission and are responsive to biomolecules such as heparin, bovine serum albumin, human serum albumin, calf thymus DNA and RNA. Additionally, 6 also functions as a fluorescent visualizer for in vivo cell imaging with high contrast and good biocompatibility. The conjugated backbones of CPEs provide plentiful binding sites for biomolecules via hydrophobic interaction, which greatly improves the sensitivity of fluorescent probe to the targeted molecules. Sonogashira coupling is another extensively utilized reaction for the construction of triple-bonded CPs from alkynes [25]. The polymerizations of terminal alkynes and aryl halides are catalyzed by palladium and copper catalysts, generating poly(aryleneethynylene)s (PAEs). Sonogashira polycoupling is highly efficient in terms of technical simplicity and tolerance to numerous functional groups. For example, the silole-containing diyne 7 is reactive to both dibromo-substituted TPE 8 [26] and diiodobenzene derivative 10 [27], generating silole-ethyne-type polymers 9 and 11 with high molecular weights in high yields (Scheme 3A). Slightly different from monodispersed silole molecules [28], 11 is weakly emissive in solution, exhibiting a phenomenon of aggregation-enhanced emission (AEE) characteristic. The reason is that the intramolecular motion of the phenyl rings in  is partially restricted by the entanglement of the polymer chains in the solution state, which enables the polymers to show some sort of light emission. The nanoaggregates of 11 serve as a fluorescent chemosensor for explosives such as picric acid (PA) detection in a superamplification manner with high sensitivity, which will be discussed later.
TPE-containing diyne 12 is a highly versatile monomer for fabricating CPs with AIE features (Scheme 3B). The palladium-catalyzed polycoupling of 12 and p-phthanoylchloride 13 can generate thermally stable poly(aryleneynonylene) (PAY) 14 with a high degradation temperature of 390 • C in air [29]. Owing to its high conjugation, 14 emits a greenish yellow light at 550 nm in the aggregated state. The emission can be quenched by explosives such as PA. Moreover, the ynone groups are wellknown fluorescence quenching sites and are reactive towards nucleophile addition. Once the ynone moieties are transferred into pyrazole rings by reaction with hydrazine (inserted Scheme in Fig. 1), the fluorescence of PAY will be recovered, enabling 14 to serve as a sensitive fluorescent detector for hydrazine detection in a quantitative manner.
Attaching functional side groups to CPs is an effective method to modify their properties. Introduction of chiral pendants to the backbones of CPs is likely to generate materials with circularly polarized luminescence (CPL). For instance, when 12 undergoes cross-coupling with halogenated L-tyrosine REVIEW Liu et al. 497 derivative 15 [30], PAE 16 with both AIE and CPL properties is generated. The intramolecular electronic interaction between the chiral moiety and the rigid polymer backbone results in a specific molecular orientation and consequent high CPL dissymmetry factor. More interestingly, the CPL dissymmetry factor of 16 changes with the solvent used due to the variation in the polymer solvent and polymerpolymer interactions. Apart from linear CPs, a series of hyperbranched polymers can also be constructed by Sonogashira polycoupling of multifunctionalized monomers [31]. The three-dimensional network of the hyperbranched polymers allows the fabrication of microporous systems. Conjugated microporous polymers are a vital category of CPs with unique morphologies and properties [32]. Through polycoupling reaction of 8 and 1,3,5-triethynylbenzene 17, a hyperbranched PAE 18 was obtained (Scheme 3C) [33]. Subsequent solvothermal treatment turns the polymer into a nanoscale conjugated microporous polymer (NCMP) with uniform size and regular morphology. Taking advantage of the high content of cavity in the porous framework, 18 excels in gas adsorption and hosting guest dyes. Polymer 18 also exhibits outstanding light-harvesting nature via polymer-dye energy transfer. By doping with modulated content of fluorescent dyes with complementary emission spectra, the uniform film of 18 can be successfully fabricated into a white emissive material (Fig. 3).

Double bonds
Alkyne-based monomers not only render conjugated systems with triple bonds, but also can form polymers with double bonds (C=C) along the backbones. Double-bonded CPs enjoy better conjugation and higher rigidity than the triple-bonded ones due to the extensive delocalization of π orbitals and steric hindrance of the vinyl substituents. Polyaddition of triple bonds to nucleophiles is one of the most efficient approaches to generate functional vinylene polymers. These synthetic methods are generally atom economical and functional group tolerance.
The introduction of heteroatoms into polymer backbones can bring along many unique properties. For example, the nitrogen and boron atoms with lone pairs can promote the electron mobility along the polymer backbone. These n-type CPs remarkably enrich the community of hole-injection materials for EL devices [34]. CPs containing silicon and sulfur atoms have also attracted great attention for their huge potential as semiconducting materials [35]. Decarbonylative polycoupling, polyhydrothio-lation and polyhydrosilylation are all facile synthetic methods for building conjugated skeletons with heteroatoms.
Linear poly(arylene chlorovinylene) (PACV) 20 was synthesized by the rhodium-catalyzed decarbonylative polyaddition of aromatic diynes 19a-c and 13 (Scheme 4A) [36]. Notably, although the reaction between 12 (19b) and 13 is very similar to the one depicted in Scheme 3B, the catalyst does play an important role in determining the polymer structure. The decarbonylative polyaddition enjoys good regio-and stereoselectivity, and the resulting PACVs 20a-c all possess high molecular weight and thermal stability.
Thiol-yne click polymerization has been actively studied in the past years and is a well-known tool for constructing regio-and stereoselective sulfurrich polymers [37]. Starting from aromatic diynes and dithiols, a series of poly(vinylene sulfide)s (PVSs) can be synthesized under mild conditions in a facile and atom-economic manner. The PVSs 23a-c are constructed from diynes 21a-c and 4,4thiodibenzenethiol 22 in the presence of a catalytic amount of Rh(PPh 3 )Cl at room temperature (Scheme 4B) [38]. The stereostructures of 23a-c can be tuned precisely by either controlling the sequential addition of monomers or post-manipulation by light irradiation. The structures of the monomers are also crucial to the properties of PVSs. For example, 23b inherits the ferrocene moiety from its monomer 21b and acts as an excellent precursor to prepare semiconducting Fe 7 S 8 ceramic. PVS 23c consists of silole units with AIE features and undergoes readily cross-linking reaction in the presence of UV irradiation, generating well-resolved fluorescent photopatterns.
Except transition metals, the thiol-yne click reaction can be triggered also by heat, UV irradiation and organic bases [39]. Recently, a catalyst-free thiol-yne polymerization route was developed [40].
PVSs 23d-f are obtained in high yields (78-97%) under a mild reaction condition in tetrahydrofuran at 30 • C for 2 h (Scheme 4B). The PVSs are bestowed the specific properties of their respective monomers. For example, the TPE-containing PVS 23f shows an AIE characteristic. Thanks to the extended conjugation and large polarity of the backbone, PVS 23e containing nitrogen atoms shows the highest refractive index (RI) value of 1.771 at 632.8 nm. Hence, this catalyst-free thiol-yne 'click' polymerization proves to be quite powerful and applicable for the preparation of a wide range of sulfur-rich functional materials.
Hydrosilylation is another widely utilized method to anchor silicon atoms in CPs. Poly(silylenevinylene)s (PSVs) 26 or 28 are generated by the H 2 PtCl 6 catalyzed polyhydrosilylation of alkyne 24 and dihydrosilafluorene 25 or dihydrosilole 27 (Scheme 4C). These PSVs can be used for surface modification in silica gel thin layer chromatography (TLC) [41]. Taking the advantage of solid-state emission of the silafluorene and silole units (inset in Scheme 4), 26b and 28b coated on TLC plates are effective fluorescent detectors for explosive species such as TNT, DNT and PA. Charge transfer from the electron-rich polymer backbones to the nitroaromatic compounds is responsible for the emission quenching of the polymer-doped TLC plates. Besides, these explosives with different polarities can be easily separated using TLC. Liu et al. 499 Polyhydrosilylation of alkyne monomers 29ad with 1,2-bis(4-dimethylsilanylphenyl)-1,2diphenylethene 30 produce PSVs 31a-d with unique photonic properties (Scheme 4C) [42]. The films of 31a-d exhibit tunable RI values by varying the UV irradiation time. Similar polyhydrosilylation reaction was carried out between silole-containing diyne 7 and aromatic silylhydrides 32a-d [43]. All the PSVs 31 and 33a-d possess high stereoregularity and demonstrate AEE characteristics owing to the embedded TPE or silole moieties inherited from the corresponding monomers. All these reactions prove that polyhydrosilylation is a highly efficient method for constructing silicon-hybrid CPs with diverse functionalities.

Aromatic system
In addition to CPs with repeating units connected by triple or double bonds, alkynes can produce polymers with whole aromatic systems via cyclotrimerization as well. Three C≡C bonds are knitted together to form a benzene ring by cyclotrimerization, generating a large aromatic system with high degree of conjugation.
The polyarylenes synthesized from polycyclotrimerization usually possess a hyperbranched structure, which can be facilely assembled into fine morphologies. For example, TPEcontaining tetrayne 34 can be polymerized into poly(tetraphenylethene) (PTPE) 35 using TaCl 5 as catalyst (Scheme 5A) [44]. This A 4 -type REVIEW monomer renders the resulting polyarylene a highly cross-linked structure but with a good solubility. Abundant terminal groups and binding sites along the polymer skeleton make the polymer to function as fluorescent chemosensor for explosives with superamplification effect and large quenching constant. Hyperbranched PTPE can also be prepared from A 2 -type monomer such as 12 (36c) under similar reaction conditions. Further research found that indium chloride was an efficient catalyst for synthesizing regioregular hyperbranched polymers (Scheme 5A) [45]. The resulting PTPE 37c contains only 1,3,5-trisubstituted benzene linkages, suggesting that it possesses a good regularity.
By means of grafting ionic functional groups, the hyperbranched conjugated polyelectrolytes (HCPEs) are granted with good biocompatibility and found potential applications in gene delivery and cell imaging. For example, when fabricated into nanoparticles, HCPEs 37a can be internalized by cells, leading to transfection with low cytotoxicity (Scheme 5A) [46]. The intrinsic fluorescence of the HCPE core is responsible for their imaging capability. Moreover, HPCEs with desired functionality can be achieved through various post-modification approaches such as hybridization and metal ion complexation. For example, the Gd(III) ion-chelated HPCEs modified from 37b can be applied as an efficient optical/magnetic resonance (MR) dual-model imaging nanoprobe for in vivo cancer diagnosis substrates (Fig. 5) [47].
Besides, other polymerization methods to construct aromatic CPs from alkyne monomers are also reported [48], one of which is the transition metal-catalyzed oxidative coupling reaction [49]. This sort of reaction usually involves the C-H activation of phenyl protons and annulation of two equivalents of triple bonds, forming fusedaromatic systems with multisubstituents. As shown in Scheme 5B, the rhodium-catalyzed oxidative polycoupling of substituted naphthalene 38 and internal diyne 39 leads to the formation of multisubstituted poly(pyrazolylnaphthalene) (PPN) 40 [50]. Although internal triple bonds generally possess a lower reactivity than terminal alkynes, 40 is obtained in a satisfactory yield with a high molecular weight of up to 35 700. The TPE-containing PPN features with AEE characteristics and works as a fluorescent chemosensor to detect PA in solution. The sensing process performs much better in the solid state and the TLC plate doped with PPN 40 can be used as a sensitive portable visualizer for PA detection (Fig. 1).

Heteroatoms
Syntheses of CPs comprising heterocyclic moieties remains an intriguing subject in the realm of material science, mainly due to the unique properties of heterocycles in organic materials [51]. These heterocyclic CPs often find potential applications in the fields of optoelectronics, polymer light-emitting diodes, organic field-effect transistors, organic photovoltaics and organic solar cells [52]. Alkyne-azide click reaction is one of the most popular reactions that can generate triazole rings as aromatic linkages efficiently. Based on the click reaction, a large number of poly(triazole)s (PTAs) with various functions have been developed [53]. Recently, a large number of AIE luminogen-containing PTAs with varied photophysical properties are reported [54]. For example, PTAs 43a and 46a functionalized with TPE moieties show typical AEE phenomenon (Scheme 6A). In contrast, their counterparts 43b and 46b with no R 2 phenyl rings display the aggregation-caused quenching (ACQ) effect. On the other hand, altering the triazole linkage substitution casts little influence on the optical properties of the PTAs. Apart from the traditional copper catalyst, many catalytic systems have been explored for synthesizing PTAs. Particularly, those carried out in the absence of metal make them simpler and thus gain increasing popularity [55]. For example, polymerization of 12 and 4,4 -diazidoperfluorobenzophenone 47 in DMF at 100 • C without any catalyst leads to the generation of PTA 48 in a nearly quantitative yield (97.9%) (Scheme 6A) [56]. PTAs with receptors such as benzochalcogendiazole are selective and sensitive fluorescent detectors for Ni 2+ ion [57]. PTAs 50a-b are prepared by the polycyclotrimerization of 2,7-diazido-9,9-dioctyl-9H-fluorene 41 and 4,7-diethynylbenzoselenadiazole 49a or 4,7-diethynylbenzothiadiazole 49b. The emission of the solutions of 50a-b can be quenched by Ni 2+ ions gradually, and the linear relationship between the emission intensity and the analyte concentration allows quantitative detection of Ni 2+ ions. The nitrogen-rich conjugated polymer backbone provides many binding pockets, which contributes to high sensitivity and low detection limit.
Azide-alkyne click polymerization is also a powerful tool for the construction of hyperbranched conjugated polytriazoles (hb-CPTAs) [58]. Through polymerization of tetrayne 34 and diazide 51, a regular hb-CPTA 52 with high TPE content was facilely obtained [59]. The hyperbranched skeleton of 52 allows facile modification of its properties by grafting the terminal ethynyl groups using thiol-yne click chemistry. Thanks to the rigid structure of the CPTA, the polymer can form unimolecular nanoparticles with a diameter of around 100 nm in solution.
In addition to triazole rings, other heterocycles, such as thiophene, may function as aromatic linkages between the monomeric units of CPs. Derived from the Sonogashira polycoupling reaction to yield polymer 14 shown in Scheme 6, a one-pot three-component polymerization of 12, 13 and ethyl 2-mercaptoacetate 53 was developed [60]. The polymerization can be performed at room temperature, furnishing poly(arylene thiophenylene) (PAT) 54 in high yields of up to 97%. The thiophene units in 54 have endowed its film to exhibit high refractive indices of 1.9461-1.6668 in a wide wavelength range of 400-1000 nm, whose values are larger than those of commercial polymers. Moreover, PAT 54 is an efficient fluorescent chemosensor for detecting Ru 3+ ion with a large quenching constant, thanks to its conjugated backbone and high content of interacting sites.

FUNCTIONS
Conjugated polymers usually possess unique photoelectronic properties owing to their outstanding electron mobility along the polymer backbone. Either by pressure-, temperature-stimulated or template-assisted [61] assembly or by self-assembly via electrostatic, surface energy or other intermolecular interactions [62], the CPs can achieve desired morphologies and structures, which lead to advanced macroscopic shapes and properties. The mechanical strength of the CPs endows them with good processibility and allows facile fabrication of devices. Besides light emission, many other practical functionalities and applications, such as fluorescent chemo-and biosensors, response to physical stimuli and magnetism, can be realized via proper structure design and incorporation of functional monomers into the CPs. Due to the space limitation, we can only give selected examples on their versatile potential and capability in scientific research and realworld applications.

Fluorescent chemosensor
Visual signal is a fast and intuitive indicator for detecting extrinsic stimuli. CPs usually display distinguished luminescence and tend to exhibit superior sensory signal amplification to their fluorescent monomeric counterparts. A large number of chemosensors can be designed and developed in response to analytes by altering the photoelectronic properties of CPs [63], such as absorption, emission and excited-state electron transfer [64]. Many CPs are turn-off sensors driven by the mechanisms of PET, charge transfer or static quenching such as ICT or FRET. In addition to the intrinsic fluorescence from the conjugated backbones, CPs with AIE features are much advantageous as solid-state detectors and free from the problem of analyteinduced ACQ effect. Here, we mainly introduce the sensing performance of AIE luminogen-containing CPs as fluorescent detectors to different categories of analytes.
As mentioned in Scheme 5, the maximum absorption of PPN 40 locates at 327 nm, which is much redder than that of a pristine TPE molecule due to the extensive conjugation along the TPE and naphthalene cores [50]. As shown in Fig. 1A, the solution is barely emissive under UV irradiation, but an enhanced green emission is observed upon water addition owing to the aggregate formation. Therefore, 40 proves to be a potential fluorescent chemosensor in the aggregated or solid state. Indeed, the charge transfer and energy transfer [65] between the electron-rich PPN polymer chain and the electron-deficient PA molecules is responsible for the observed emission quenching. As depicted in Fig. 1D, when the PA concentration increases, the PL intensity decreases accordingly. The Stern-Volmer plot of relative PL intensity versus the explosive concentration shows in an upward bending curve, which is indicative of a superamplification effect of fluorescence quenching (Fig. 1E) [66]. Similar phenomenon is also observed in the solid state (Fig. 1E inset). Apart from the static electronic interaction, the superamplification effect may involve another process. Once the PA molecules manage to enter the three-dimensional network of polymer aggregates, the intermolecular space expands to expose more binding sites and more room is now available for the intramolecular motion of the TPE phenyl rings. In this case, the PL intensity will be further diminished, thus endowing the system with higher sensitivity. This superamplification effect also applies to other circumstances where the analytes are other extrinsic species, such as metal ions. PAT 54 consisting of thiophene rings with AEE characteristic is a good fluorescent sensor for metal ions, especially Ru 3+ [60]. When the concentration of Ru 3+ in the aggregated system of 54 reaches 36.7 μM, the PL intensity decreases to only 4% of the original value. Again, the Stern-Volmer plot of the relative intensity versus the Ru 3+ concentration demonstrates a superamplified quenching effect (Fig. 1F). From the three individual stages of the Stern-Volmer plot, high quenching constants of up to 880 430 M −1 in the concentration range of approximately 20−40 μM are calculated. The sensitivity of 54 towards Ru 3+ is much higher than other metal ions including Co 2+ , Cu 2+ , Fe 2+ , Mg 2+ , Hg 2+ , Ag + , Zn 2+ , Ni 2+ , etc. (Fig. 1G), perhaps because of the relatively higher standard reduction potential of the Ru(III)/Ru(0) couple in addition to the energy transfer from 54 to Ru 3+ .
Another branch of CP detectors show a turnon behavior in response to specific analytes by realtime chemical modification of their structures. PAY 14 is such a functional polymer (Scheme 3B) [29]. The activated ynone units are highly reactive to hydrazine under room temperature and can form pyrazole rings immediately (Scheme in Fig. 1). Originally, the dimethylformamide solution of 14 is barely emissive due to the quenching effect of the ynone groups. Upon addition of hydrazine, the ynone groups form pyrazole rings immediately, which turns on its light emission. With an gradual increase in the hydrazine concentration, the PL intensity experiences a large enhancement followed by a subsequent smooth increment, in agreement with the tendency for the conversion rate ( Fig. 1H and I). The turn-on detection is so efficient that it can be utilized to fabricate TLC strips for real-time solid-state detection for hydrazine (Fig. 1 inset). The faster reaction rate between the ynone groups and hydrazine over other nucleophiles makes 14 a quite selective fluorescent detector for detecting hydrazine.

External stimuli-responsive materials
Apart from chemical analytes, many CPs show fluorescent response to physical stimuli such as temperature or light irradiation. External stimuli such as heat or photon can spur the intramolecular motion or interfere the interaction between the polymer and the environment, and hence influences their optical behavior. As a result, fluorescent polymers serve as straightforward indicators for detecting these Temperature-dependent PL is an acknowledged technique to demonstrate the effect of intramolecular motion on the light emission process. PACV 20c and its hyperbranched counterpart 55 are conjugated polymers with double-bonded backbones prepared from click polymerization (Scheme 4A) [36]. The emission spectra of 20c and 55 in the frozen state are much intensified than those at room temperature ( Fig. 2A and B). Such enhancement is closely related to the limited collision interaction with solvent molecules and the restriction of intramolecular motions.
Light is another environmental stimulus that can cause fluorescence change in CPs. UV irradiation is a powerful energy source for photochemical reactions and an effective tool for RI modulation and formation of photopatterns. Modulation of refractivity is a crucial technology in optical data storage, and hence polymeric materials with tunable refractive indices are of great interest. For example, polymeric sulfur-rich PVS 23c with silole moieties shows RI of 1.7440-1.6848 in the wavelength range of 600-1700 nm [38], whose values are much higher than some commercial polymers [67]. It crosslinks in the presence of UV irradiation, which has lowered its electronic conjugation and backbone's polarizability as well as the RI values. As illustrated in Fig. 2C, after UV irradiation for 15 min, its RI drops by 0.1221. Moreover, the UV-promoted photooxidation allows facile fabrication of well-resolved fluorescent patterns. By UV irradiation of a uniform film of 23c coated on a silica wafer through a designed nanoscale mask, the emission of the exposed parts (black lines) is photobleached, while that of the covered areas remains unaltered. A fluorescent pattern with a high contrast is thus generated (Fig. 2D).

White-light-emitting material
White-polymer-light-emitting devices have aroused huge interest in both scientific and industrial communities [68]. As mentioned previously, the emission of a hyperbranched CP 18 can be facilely tuned by varying the amount of red-emissive dopant [33]. Fabrication into NCMP enables 18 to be easily dispersed in poly(vinyl alcohol) and form a uni-form transparent film. As the emission spectrum of 18 overlaps with the absorption spectrum of the red-emitting dye Phloxine B (PhB), efficient excitation energy transfer from 18 to PhB is realized. Proper adjustment of the dose of PhB can lead to an NCMP/PhB film with white emission. As shown in Fig. 3, when the PhB content in the NCMP/PhB film increases from 0.1 to 0.4 mg/g (from top to bottom in Fig. 3C), the emission color changes from green to white accordingly (Fig. 3B). As shown by the CIE diagram, the emission color locates in the pure white region (Fig. 3D). Such a facile technique of physical dispersion of dopants into CPs is associated with their conjugated structures and microporous skeletons.

Magnetism
Triple bonds are versatile ligands for organometallic compounds [69]. Thus, CPs containing triple bonds are promising candidates for fabricating organometallic materials. The hyperbranched PTPE 35 is converted to organometallic polymer 35-{[Co(CO) 3 ] 2 } x at room temperature, which transforms into magnetic nanoparticles MC35 (Scheme in Fig. 4) [44]. Transmission electron microscopic images show that the Co nanoparticles are wrapped in a carboneous shell ( Fig. 4A and C), while the electron diffraction patterns shown in Fig. 4B and D demonstrate the crystalline nature of MC35. Thanks to the ferromagnetic nature of cobalt, MC35 is magnetic as well. As shown in Fig. 4E, with an increase in the external magnetic field strength the magnetization of MC35 rapidly increases and reaches a saturation value of 82.7 emu g −1 ultimately at high field. Giving that the oxides of Co (CoO and Co 3 O 4 ) are paramagnetic at room temperature, the carboneous shells efficiently prevent the Co nanocrystallites from oxidation. The coercivity is calculated to be 0.08 kOe, suggesting that the ceramics can be utilized as soft magnetic materials for magnetic recording systems.

In vitro and in vivo fluorescent imaging
Many CPs are fluorescent materials with emission located at the long wavelength visible light region. Thus, these CPs are desirable fluorescent probes for intracellular cell imaging with a little interference from cell autofluorescence [70]. Polyelectrolyte 6a is soluble in aqueous media and emits strongly in the aggregated state [24]. It can selectively stain the cytoplasm of HeLa cells with low cytotoxicity ( Fig. 5A-C). Except ionization, grafting biocompatible functional branches is another way to endow CPs with bioactivity. Polyethyleneimine (PEI) is such a cationic polymer widely used for gene delivery [71]. Star-shaped hyperbranched polymer 37a bears abundant terminal alkyne groups, which can facilely react with PEI via click reaction [46]. Benefited from the bioactivity of PEI and AIE property of 37a, the resulting conjugates 37a-PEI1 (M w of PEI = 600) and 37a-PEI2 (M w of PEI = 1800) can internalize by COS-7 cells and light up their cytoplasm (Fig. 5D). Another HCPE 37b with a similar structure and poly(ethylene glycol) terminals can be chelated with gadolinium ion to form the MR imaging agent 37b-Gd [47]. Figure 5E shows that the images of 37b-Gd treated MCF-7 breast cancer cells with their nuclei being stained by propidium iodide. The green fluorescence from the cellular cytoplasm indicates the presence of 37b-Gd internal-ized by the cells. Fluorescent CPs also found useful applications in in vivo imaging. Figure 5F presents the time-dependent biodistribution profile of 37b-Gd in mice carrying H 22 tumor. Clearly, the 37b-Gd nanospheres tend to accumulate in the tumor tissues as time elapses. Therefore, this technique for real-time tumor monitoring enjoys the advantages of high emission efficiency in aqueous solution together with good photostability and low cytotoxicity, which provides an excellent optical/MRI dual-modal imaging technique for in vivo cancer diagnosis.

PERSPECTIVES
Conjugated polymers are a significant class of functional materials for their unique features, such as flexible molecular design, diverse building blocks and corresponding characteristics. Catering to the related scientific research fashion, this review summarizes the most recent development in CPs regarding the synthetic methodology and their functionalities and applications. An emphasis is placed on the alkyne-based chemistry and latest research work from our group. The synthetic routes to linear or hyperbranched CPs categorized by their backbone components are introduced respectively with our wish to provide enthusiasm to scientists for further research in this area. Selected examples are given for illustration of the superb properties and the variety of applications of these polymers.
All the past accomplishment will serve as cornerstone for our future research and new achievements. For conjugated polymers, more efficient, economic and environmental friendly synthetic methodologies are expected to be developed. As for monomers, the scope of triple-bonded monomers will be expanded from mere C≡C bonds to C≡N and B≡N bonds. Our group has set on the foot on the polycyclotrimerization of nitriles [72], though alkyl chain is necessary for the resulting polymer with good solubility and processibility.
Sequence control and structure regulation still remains a problem for many multisegmented copolymers. However, programmable multicomponent reactions based on three or more monomers are highly worth to explore in order to solve the problem. We are longing for new progress in polymer science and polymer engineering to solve the obstacles we are faced so far, such as controllability and regularity in polymer syntheses, and processibility of polymers with high degree of rigidity.
In the respect of polymer construction, novel hybrid structures are anticipated, such as organometallic d-type CPs [73] and polymers REVIEW Liu et al. 507 with covalent-organic framework [74]. The outstanding optoelectronic properties [75] of these hybrid polymers will enable polymer science to take a further step towards real-world applications. The traditional two-dimensional backbone can be evolved to three-dimensional skeleton of CPs [76] with the aid of more sophisticated structure design and advanced synthetic techniques. The application domains of CPs have been extended to an unprecedented level. Yet, there is still large terrain remaining to be explored, such as organic spintronic materials [77]. Hybrid CPs containing magnetic species can be applied in both magnetoresistance and spintronic materials. The large conjugation bestows long spin relaxation time to the polymers and make them promising candidates for spintronic systems [78]. Nanoparticles assembled from hyperbranched CPs hold the advantages of low toxicity, good biocompatibility, high brightness and versatile functions of the attached biomolecules, which make CPs favorable for longterm tracing in cell imaging and other biological applications [79].