AIE polymers: Synthesis and applications

Journal Pre-proof AIE polymers: Synthesis and applications Rong Hu, Anjun Qin, Ben Zhong Tang

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S0079-6700(19)30182-0

DOI:

https://doi.org/10.1016/j.progpolymsci.2019.101176

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JPPS 101176

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Progress in Polymer Science

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1 November 2019

Please cite this article as: Hu R, Qin A, Tang BZ, AIE polymers: Synthesis and applications, Progress in Polymer Science (2019), doi: https://doi.org/10.1016/j.progpolymsci.2019.101176

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AIE Polymers: Synthesis and Applications Rong Hua, Anjun Qin a,*, Ben Zhong Tang a,b,* a. State Key Laboratory of Luminescent Materials and Devices, Key Laboratory of Luminescence from Molecular Aggregates of Guangdong Province, Center for Aggregation-Induced Emission, South China University of Technology, Guangzhou 510640, China. b. Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Centre for Tissue Restoration and Reconstruction, Institute for Advanced Study, and Department of Chemical and Biological Engineering, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China

Corresponding authors: [email protected], [email protected]

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Graphical Abstract

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Organic luminescent materials with aggregation-induced emission (AIE) feature have attracted great attention in diverse fields because of their unique properties. AIE polymers possess advantages of multifunction, well film-forming property and synergistic effect, which could well meet various practical applications. This review briefly introduces the preparation of AIE polymers based on one-, two- and multicomponent polymerizations, and gives a deep insight for the structure-property relationship and the applications in optoelectronic devices, chemo-/biosensors and biomedicines. Furthermore, perspectives on the future direction of AIE polymers are also discussed.

Abstract Organic luminescent materials with an aggregation-induced emission (AIE) feature have attracted great attention in various fields because of their unique properties. Compared to the well studied low-mass AIE luminogens, AIE polymers have been paid less attention even though they present outstanding advantages of high emission efficiency in aggregate and solid states, amplification effect of the signals, good processability and multiple functionalization, etc. This review briefly summarizes the synthetic strategies towards AIE polymers based on one-, two- and multi-component polymerizations. Afterward, the structure-property relationship, such as the effect of

the skeleton and side-chains on the photophysical property is discussed. More importantly, nonconventional luminescent polymers and their mechanisms are specially introduced. Furthermore, the high-tech applications of AIE polymers in optoelectronic devices, chemo-/biosensors and biomedicine are illustrated. Finally, the perspectives on the future development direction of AIE polymers are briefly discussed. It is hoped that this review can serve as a trigger for future innovation in AIE research.

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Keywords: aggregation-induced emission, functional polymer, polymerization methodology, structure-property relationship, high-tech applications

Contents

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1. Introduction ................................................................................................................ 5 2 Synthesis of AIE polymers.......................................................................................... 5 2.1 One-component polymerization........................................................................ 6 2.2 Two-component polymerization ....................................................................... 6 2.2.1 Click polymerization ............................................................................... 7 2.2.2 Other two-component polymerizations................................................... 7 2.3 Multi-component polymerization ..................................................................... 8 2.3.1 Click polymerization mediated multi-component polymerization ......... 9 2.3.2 Multi-component polymerization with green monomer ......................... 9 2.3.3 Other multi-component polymerization ................................................ 10 3. Structure-property relationship ................................................................................ 10 3.1 The effect of backbone on SPR....................................................................... 11 3.2 The effect of the side-chain on SPR................................................................ 12 3.3 Non-conventional AIE polymers .................................................................... 12 4 Applications of AIE polymers ................................................................................... 13 4.1 Optoelectronic applications ............................................................................ 14 4.2 Fluorescent sensor ........................................................................................... 15 4.3 Biomedical applications .................................................................................. 17 5. Conclusions and future perspective ......................................................................... 18 Acknowledgments .................................................................................................... 20 References. ............................................................................................................... 21

Nomenclature

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Aggregation-annihilated circular dichroism Azide–alkyne click polymerization Aggregation-caused quenching Azobisisobutyronitrile Aggregation-induced emission AIE luminogens Atom Transfer radical polymerization Colony forming units Clusteroluminescence Circularly polarized luminescence Conjugated polymers Delayed fluorescence Distryreneathracence Electrochromic device Hexaphenylsilole Intersystem crossing Luminescent liquid crystalline Limit of detection Conjugated microporous polymer films Maleic anhydride Multi-component polymerizations Multi-component reactions Nanoparticles Organic light-emitting diodes Photo-dynamic therapy Photoluminescence Polymeric light-emitting diodes Restriction of intermolecular motion Reactive oxygen species Room temperature phosphorescence Structure-property relationship Tetraphenylethene Tetraphenylpyrazine Volatile organic compound Weight-averaged molecular weight Refractive index Dissymmetry factor Glass transition temperature Torsion angle

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AACD AACP ACQ AIBN AIE AIEgens ATRP CFU CL CPL CPs DF DSA ECD HPS ISC LLC LOD CMP-Fs MAh MCPs MCRs NPs OLEDs PDT PL PLEDs RIM ROS RTP SPR TPE TPP VOC Mw n glum Tg θ

1. Introduction

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Organic luminescent materials have been widely used in optoelectronic devices, chemo-/biosensing and biological applications. But improving their luminescent efficiency has been of ongoing interest to research in the field [1-3]. Conventional materials give bright luminescence in dilute solutions, while the photoluminescence (PL) decreases in concentrated solution or solid state due to an aggregation-caused quenching (ACQ) effect, as documented by Forster in 1954 [4]. Although many strategies, such as chemical modification and physical and engineering methods, have been carried out to prohibit the ACQ effect, only limited achievements have been achieved, as the aggregation is a spontaneous process and natural state. Hence, ACQ as a common phenomenon for traditional organic luminescent materials has greatly limited their broad application. Opposite to the ACQ, a revolutionary concept of aggregation-induced emission (AIE) was demonstrated by our group in 2001, in which some luminogens were faintly emissive in solution but showed strong fluorescence in the aggregate state [6]. Subsequently, by further experiments and theoretical simulations, the mechanism of restriction of intramolecular motion (RIM) was proposed, and has been widely utilized to guide the design of new AIE luminogens (AIEgens) [7]. Notably, the AIEgens have demonstrated their highly promising applications in various research frontiers. AIE polymers possess many unique advantages, such as good film-forming ability and synergistic effects, to meet various practical applications. Besides, the structure, morphology and the composition of polymers can be well regulated to fabricate functional materials to meet the demands of a variety of applications, although existing research is mainly focused on AIEgens with low mass [8-10]. Progress on the preparation of AIE polymers and their applications since the first report of AIE polymer in 2003 by our group has been summarized in several reviews [11-13]. Nevertheless, the development of functional AIE polymers remains deficient, and many fundamental problems should be addressed to promote further progress. This review presents a discussion of the preparation of AIE polymers and the structureproperty relationship, and summarizes promising high-tech applications, along with trends of further development.

2 Synthesis of AIE polymers The general strategy for the preparation of AIE polymers is to incorporate AIEgens into the backbones or as the pendants of polymers through polymerization and modification [14,15]. Generally, typical AIEgens, including tetraphenylethene (TPE), distryreneathracence (DSA), hexaphenylsilole (HPS), 1,8-naphthalimide, 2,4,6triphenylpyridine and tetraphenylpyrazine (TPP) etc. are widely utilized to prepare AIE polymers. Briefly, the synthetic methods towards AIE-active polymers can be summarized into three categories, namely, one-component polymerization, two-

component polymerization and multi-component polymerization. 2.1 One-component polymerization

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One-component polymerization, based on a single monomer, is an important branch of polymerization, enabling the preparation of polymers without tedious strict stoichiometric balance. With the efforts of relevant polymer scientists, various onecomponent polymerizations have been established to prepare AIE polymers, such as radical polymerization, ring-opening polymerization, McMurry polycoupling and metathesis polymerizations [16]. Radical polymerization, for example, has been employed to introduce alkene monomers with AIE features into polymers with the side chain of AIE units. As shown in Fig. 1, Xu and coworkers constructed an AIE polymer P1 through radical polymerization initiated by azobisisobutyronitrile (AIBN) from carbazolyl-substituted triphenylethene-containing monomer with good solubility in commonly used organic solvents [17]. Besides, metathesis polymerization is another method to build AIE polymers bearing AIE units on side chains. Silolylacetylene monomers were polymerized to prepare P2 with high weightaveraged molecular weights (Mw, up to∼70000) in good yield (up to ∼80%) by our group. P2 showed obvious AIE features, high thermal stability and unique photophysical properties [18]. Electropolymerization was employed to prepare AIE porous polymer (P3) film with the monomers of the TPE modified N-substituted carbazoles; the thickness of the films could be precisely controlled with subnanometer [19]. Besides these traditional polymerization strategies, novel methods have been developed to construct functional AIE polymers. A conjugated polydiye, P4, was built via KI-mediated one-component polymerization of bis(iodoalkyne)s with high refractive index (n, 2.1125-1.7747) in a wide range of wavelength (400-900 nm could be regulated by UV exposure because of its photosensitivity [20]. Moreover, we developed a single-component polymerization based on diisocyanoacetates, affording soluble P5 with high Mw (up to 32500) in good yields (up to 94%). The kinetic study showed that the polymerization could be completed in 2 hours, and structural characterization revealed that there were two linking types in the skeleton, leading to the formation of “one structure, two repeating units” [21]. For another example, using monomers containing benzoyl group and organic halide, a Barbier reaction was introduced into polymer chemistry to construct a phenylmethanol group containing P6 by Wan and coworkers. Even though there was no typical AIEgen, an obvious AIE feature was observed with the resultant polymer, that might be a result of inhibition of intermolecular rotations in its aggregate state [22]. 2.2 Two-component polymerization Two-component polymerization is a strategy to linking two difunctional monomers to prepare polymers, perhaps the most traditional and popular approach for AIE polymer synthesis. Many typical and classical polymerization reactions, including Suzuki, Wittig, Heck, MacMurry, Sonogashira, and Hay-Glaser polycouplings, click

polymerization, radical polymerization, metathesis polymerization, polycondensation, ring-opening polymerization, etc, have been developed as two-component polymerizations for AIE polymers construction [15]. Among these polymerization reactions, click polymerization is newly developed and many studies have been reported recently. 2.2.1 Click polymerization

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Click polymerization is an efficient polymerization methodology to construct macromolecules with well-defined structures and unique properties, derived from the click chemistry first employed for polymer preparation in 2004 [23]. Moreover, click polymerization possesses the advantages of click chemistry with regioselectivity, high efficiency, atom economy, functional group tolerance, and so on, making it an ideal candidate for synthesizing AIE polymers. Over the past years, great progress has been made in AIE polymer preparation based on click polymerization, especially in alkynebased click polymerization [24]. Azide-alkyne click polymerization (AACP) is an early method used to build linear and hyperbranched polymers with AIE features. In 2009, our group constructed AIE polymers via Cu(I)-catalyzed ACCP for the first time [25], from which TPEcontaining polytriazoles with good solubility, high Mw and thermal stability were produced. However, the negative effect induced by metal residues in such polymeric products is a crucial issue for their biological and optoelectronic applications. Thus, the development of metal-free click polymerization is a recent research frontier, and environmentally friendly click polymerization is very desirable. An effective strategy to overcome this obstacle is to avoid the use of metalcatalysts. As a result, various metal-free and spontaneous click polymerizations, such as thiol-yne click polymerizations [26], amino-yne click polymerization [27] and hydroxyl-yne click polymerization [28] have been developed. In the quest for new alkyne-based polymerizations, our group introduced electron-withdrawing carbonyl groups to effectively enhance the reactivity of the acetylene triple bonds in monomers, facilitating thermally activated and metal-free click polymerization [29]. These highly efficient click polymerizations were also used to prepare AIE polymers thanks to their excellent functional group tolerance. As shown in Fig. 2, the TPEcontaining diyne could polymerize with dithiols in THF at 30 oC, and AIE-active P7 with a Mw of 85200 was readily produced in a yield of 97% [30]. Similarly, AIEactive P8 and P9 were facilely synthesized by the spontaneous amino-yne click polymerizations of bispropiolates and bis(ethynylsulfone)s with TPE-containing diamines in a regio- and stereo-selective fashion, respectively [31,32]. 2.2.2 Other two-component polymerizations Besides click polymerization, other two-component polymerizations, especially polycouplings, have been widely utilized to prepare AIE polymers with distinct structures and multiple functions. Owing to the sound development of the classical polymerizations, it is quite easy to incorporate different kinds of AIE units into the

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pendant or backbone of polymers by designing the monomer structures. For example, as displayed in Fig. 3, Chujo and coworkers designed and synthesized boron diiminates with various substituents to produce AIE-active P10 with high Mw and scintillation ability based on Suzuki-Miyaura coupling. Its optoelectronic properties could be tuned by choosing an adequate molecular design (Fig. 3A) [33]. Meanwhile, radical polymerization is another normally adopted method for the synthesis of AIE unit-containing polymers from AIE-active monomers. For example, Lu and coworkers used a red-light emitting EtAmPy as the initiator to trigger atom transfer radical polymerization (ATRP) to synthesize the end-functionalized P11 with AIE features (Fig. 3B) [34]. Moreover, many metal-free polymerization reactions have also been used for AIE polymer preparation. For instance, He and coworkers employed a nucleophilic substitution reaction to build a photochromic P12 containing TPE and dithienylethene moieties (Fig. 3C). The open and closed forms of dithienylethene in the polymer could be regulated by irradiation with visible and UV light, resulting in the photoswitching of the fluorescence of P12 in the aggregate state [35]. The AIE polymers mentioned in the preceding were all constructed based on monomers containing an AIE unit. It is more attractive to prepare AIE polymers using non-AIE active monomers. With this idea in mind, our group successfully developed a new strategy to prepare AIE polymers by using non-AIE monomers. As shown in Fig. 3D, internal monoyne and bis(arylboronic acid) were utilized to in situ generate AIE-active P13 by a palladium-catalyzed oxidation polycoupling [36]. Moreover, the in-situ generation of nonconventional fluorescent materials is another type of strategy to construct AIE polymers without using AIE unit-containing monomers. Zhu and coworkers employed a Michael-type polycondensation-addition to build poly(amino amine)s, nonconventional luminophores with AIE feature [37].

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2.3 Multi-component polymerization

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Multi-component polymerizations (MCPs) are a group of unique polymerizations involved in three or more monomers in the synthetic procedures on the basis of multicomponent reactions (MCRs) [38,39]. Complex and functional structures of the polymers can be generated efficiently and economically in a one-pot or sequential way via MCPs. As a fascinating synthetic strategy, MCPs have received much attention from polymer scientists and have an impact on polymeric materials owing to the advantages of high atom economy, mild reaction condition, high efficiency, simple operation procedures, good functional-group tolerance, and diverse structures, etc. Thus, MCPs have been used for the construction of AIE polymers with welldefined and complicated structures extensively, which is not easy to realize through other polymerizations [40].

2.3.1 Click polymerization mediated multi-component polymerization

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Click polymerization mediated multi-component polymerization enjoys remarkable advantages of both click reactions and MCRs, such as high efficiency, simple operation procedures, mild reaction conditions, and good isolation yield and facile monomer availability. This polymerization method is also used to prepare AIE polymers. As shown in Fig. 4, our group developed an efficient multi-component polymerization mediated by the click polymerization in the presence of catalytic system of CuI and triethylamine, to prepare AIE-active heterocyclic polymer P14 with high Mw of 64600 and refractive index n values of 1.9284-1.7734. Moreover, its light refractivity could be regulated by UV irradiation owing to the broken unsaturated bonds in the skeleton [41]. We also reported that by changing the third monomer, an efficient one-pot three-component polymerization of disulfonyl azide, TPE-containing diyne and 2-hydroxybenzonitrile to construct an AIE-active oxindole-bearing P15 in the presence of inexpensive CuI and LiOH at room temperature [42]. In addition to the above polymers with traditional AIE units, nonconventional luminescent polymers with AIE features could also be prepared by click polymerization mediated multi-component polymerization. For example, our group facilely constructed a functional polymer P16 bearing small-ring heterocycles via onepot MCP at room temperature. P16 emitted weakly with the quantum yield of 1.4% in THF solution, and 8-fold enhancement of fluorescent intensity was recorded in the THF/water mixtures with water fraction of 90%. Moreover, the treatment of the polymer with HCl could open the four-membered azetidine rings in the skeleton, further amplifying the emission efficiency to 27.1% in the solid state [43]. 2.3.2 Multi-component polymerization with green monomers

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Green monomers, such as O2, H2O and CO2, possess remarkable merits of being abundant, nontoxic, cheap and environmentally friendly, admirable attributes for reactants for polymer preparation. Therefore, converting natural resources of green monomers into useful polymers shows huge practical implication, and polymer scientists have devoted great efforts to achieve this goal. Among the green monomers, CO2 has been extensively studied and successfully utilized to construct polycarbonates. The introduction of O2 and H2O into polymeric materials is not easy to realize since many polymerization reactions need to be carried out under oxygenand moisture-free conditions. Nevertheless, the utilization of O2 and H2O as the monomer to prepare functional polymers with AIE behavior through MCPs has been reported in recent years. We successfully obtained CO2-based AIE polymer P17 with Mw of 31400 by Ag2WO4-catalyzed MCP of CO2, silole-containing diyne and alkyl dihalide under mild reaction conditions under normal CO2 pressure, in which Ag+ could coordinate with ethynyl groups efficiently, and [WO4]2− could attract and activate CO2 by the formation of the [WO4]2−/CO2 adduct to promote the polymerization. P17 possesses an absolute fluorescence quantum yield of 61% in the solid state, much higher than that of the AIE polymers we obtained previously. Moreover, the polymers can be

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degraded under basic conditions due to the existence of ester groups in the polymer structure [44]. In addition, the introduction of O2 into polymer materials was also realized by the MCP of O2 and TPE-containing diyne in the presence of Pd(OAc)2/ZnCl2 under O2 atmospheric at 70 °C. The resultant P18 in Fig. 4 shows AIE features and possesses a large two-photon absorption cross-section (1570 GM), larger than that of AIEinactive polymers with similar structures [45]. Moreover, our group also used water as one of the three monomers to perform the MCP for the first time. The CsF-mediated MCP of water, diisonitrile and TPE-containing dihalide could be carried out under mild reaction conditions and AIE polyamide of P19 with Mw of 41700 was produced in excellent yield (98.1%). Thanks to its containing bromovinyl groups, P19 could be post-modified efficiently [46]. This work not only provides a facile way to synthesize polyamides but also enriching their properties. 2.3.3 Other multi-component polymerization

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Apart from the aforementioned MCPs, a series of efficient MCPs mediated by Cu(I) [47], Pd(II) [48] and In(III) [49] catalytic systems has been developed to construct AIE polymers with multifunction and diverse applications. To be environmentally friendly, metal-free or spontaneous “green” polymerizations of simple and cheap monomers without solvent or in green solvents under mild reaction conditions for AIE polymer preparation are highly desirable. Recently, several metalfree MCPs have been developed [50]. Our group successfully developed such a catalyst-free MCP (Fig. 4G). Aliphatic amine, elemental sulfur and TPE-containing isocyanide could be polymerized without addition of any catalyst in air and AIEactive polythioureas of P20 with high Mw (56400) could be produced in 10 min at 100 o C or in 2 h at room temperature [51].

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3. Structure-property relationship

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Structure-property relationship (SPR) can be utilized to predict the photo-physical and biological activity from the perspective of molecular structure to help design and prepare desirable materials. Compared with the full study of SPR for low-mass AIEgens, less research has been carried out on AIE polymers. There might be three reasons: (1) AIE polymers possess quite complicated structures and dispersed molecular weights, making it difficult to explore SPR accurately. (2) The inter- and intra-molecular interactions have influence on the relaxation of the excited state to the ground state, making the photo-physical properties of polymers different from those of monomers and AIEgens. (3) The structures of the reported AIE polymers are limited and simple to a certain degree, resulting in the shortage of their systematical and extensive investigation into the SPR. In the following sections, we will briefly discuss the progress of AIE polymers in this aspect.

3.1 The effect of backbone on SPR

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The backbone of an AIE polymer is one of the key factors of photo-physical properties, such as AIE features, absolute quantum yield and fluorescent emission wavelength. Several contributions have been made to explore the relationship between backbone structures and the photo-physical properties of AIE polymers. For example, our group studied the emission behavior of AIE polytriazoles with different skeletons. As shown in Fig. 5A, four polytriazoles (P21-P24) with small changes in structures of the backbone or the linkage manner of the triazoles were constructed. Studies of the photo-physical properties showed that, the polytriazoles of P21 and P23 with phenyl ring substitution groups exhibit the AIE features, whereas, when the phenyl rings were changed to hydrogen, the corresponding polytriazoles P22 and P24 with phenyl rings replaced by hydrogen suffer from the ACQ effect. These results also indicated that linkage manner of the triazoles exerts a smaller impact on the emission behavior [52]. Moreover, our group also found that although the monomers are AIE-active, the polymers might show ACQ effect. As shown in Fig. 5B, two pyrazine-containing polytriazoles of P25 and P26, prepared by the Cu(I)-catalyzed AACP, behave totally different in emission, even though they were both constructed from AIE-active diynes. P25 suffers from the ACQ effect, whereas, P26 is AIE-active. Theoretical calculation and emission behavior of their model compounds demonstrated that P25 might suffer from strong intra- and intermolecular stacking due to the planar units of cyano-substituted pyrazine groups as well as triazole and phenyl rings, which quenches the emission in the aggregate state. However, no such strong interaction was found in P26, making it AIE-active [53]. This study may serve as guidance for further design of AIE polymers. Along similar lines, Dong and coworkers prepared a series of conjugated polyelectrolytes of PTPExFyN with different ratios of TPE and fluorene moieties. The photo-physical property investigation showed that PTPE0F1N without TPE unit was a typical ACQ polymer. By raising the TPE content in the polymer, AIE feature could be detected with dual-emission behavior, in which the emission around 415 nm was from fluorene units and another around 510 nm was from TPE moieties. Further raising the TPE ratio to 0.3, that is, the PTPE0.3F0.7N possessed the most obvious AIE characteristics among all PTPExFyN varients [54]. This study provide a useful way to turn an ACQ polymer to AIE one by enhancing the content of AIE units. Our group further studied the effect of bridge groups on the conjugation of the polymer, and found that the conjugation of the polymers was in the order of P28 > P29 > P27. The difference in the conjugation does not affect their AIE features (Fig. 5C) [55]. It may be noted that Bu and coworkers demonstrated that the degree of the enhancement in emission for conjugated poly(tetraphenylethene)s was in the order of ortho, meta and para positions of TPE [56]. Besides nitrogen, other heteroatoms, such as boron have been incorporated in the main-chains of AIE polymers. Chujo and coworkers synthesized a series of boron

diiminate copolymers with variable connection points, and AIE polymers with diverse colors ranging from green to orange were constructed (Fig. 5D). Because of the D-A structures of these polymers (P30F/T-P33F/T), the accepting and electron-donating ability of the boron diiminate units could be regulated by changing the connection points [57]. Apart from the through-bond conjugated AIE polymers, a series of unique throughspace examples derived from folded TPE were prepared by our group, and the photophysical property investigation revealed that the conjugated backbone was responsible for the absorption, while the PL emission was determined by the folded TPE fragments, representing intramolecular energy transfer [58]. 3.2 The effect of the side-chain on SPR

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Side-chains of the polymers would influence the polarity, intramolecular charge transfer, and even the relaxation pathway of the excited state of the polymers. The effect of type and number of the side-chains as well as the linker length on photophysical properties of the polymers has been studied. For example, in order to explore the correlation between the number of hydrogen bonds and the efficiency of the emission, Ma and coworkers synthesized TPE-containing P34 and P35 with different intensities of hydrogen bond networks (Fig. 5E). By evaluating the quantum yields of the two polymers, they found that P34 possessed much higher value of 62.58% than that of P35 (22.52%), indicating that the hydrogen bonds could restrict intermolecular rotation efficiently to reduce the non-radiative pathway [59]. Bunz and coworkers reported four TPE-based aryleneethynylene polymers P36-P39 with amino or nitrosubstitution on their side chains (Fig. 5F). The maximum emission peaks for P36-P39 are measured at 598, 590, 513 and 552 nm, respectively. P38 with dimethylaminoalkoxyl side-chains exhibits the most blue-shifted emission owing to the weakest electron donating ability of the alkoxyl groups. By evaluating the emission in buffer solutions with different pH values, they found that P36-P38 with amino-substitution are all pH-sensitive, and the directly linked amino groups on TPE would have dramatic effect on the properties. Besides, the strong D-A structure of P39 made it solvatochromic [60]. The effect of the spacer length between the backbone and AIEgens on luminescence efficiency has also been investigated. Chen and coworkers presented a range of TPE-containing polyacrylates with the spacer of methylene number ranging from 0~5. All these polymers exhibited typical AIE features. With the decrease in the spacer length, the fluorescent intensity of the polymer in the aggregate state generally increased, ascribed to the more rigid conformation induced by the strongly coupled jacketing effect compared to the homologues polymer with longer spacers [61].

3.3 Non-conventional AIE polymers Non-conventional polymers with intrinsic fluorescence, such as proteins, starch, cellulose, polyamides and maleic anhydride polymers, have attracted extensive

attention due to their easy preparation, excellent biocompatibility and great potential applications in diverse fields. These non-conventional polymers without any conjugated structures or fluorophores show faint emission in dilute solutions, but intense PL in concentrated solution, or in the aggregated and solid state, hence demonstrating typical AIE behavior. Moreover, with varying the excitation wavelengths, different emission can be observed among the majority of nonconventional polymers. However, the luminescence mechanism is still unclear and under debate.

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By systemic investigation into the photo-physical properties of the pure oxygenic non-conjugated polymer, our group proposed a mechanism of clusteroluminescence (CL) for such emission [62]. However, details on CL, such as the interaction mode, needed to be further explored. Yuan and coworkers demonstrated that nonconjugated P40 possessed strong intrinsic fluorescence, even delayed fluorescence (DF) and room temperature phosphorescence (RTP), ascribed to the cyano cluster formation with through-space electronic interaction (Fig. 6A) [63]. They further studied P41 with dual emission behavior, and proved that the intrinsic fluorescence and RTP originated from the formation of clusters resulting in through-space electronic communication (Fig. 6B) [64]. Our group also prepared P42 and P43, and systematically studied the SPR. As displayed in Fig. 6C, P42 with maleic anhydride (MAh) separated by the bulky t-butyl groups showed non-emission under UV irradiation, which proved that the intrinsic emission was related to the clusters of MAh units. Theoretical calculation further revealed the existence of intra- and interchain n→π* interactions [65]. Similar phenomenon in non-conjugated polyamides with rich hydroxyl and amide groups was also reported by Tang and coworkers [66]. Another phenomenon for non-conventional luminescence was observed by Wang and coworkers based on poly(maleic anhydride-alt-vinyl pyrrolidone) copolymers. They found that the fluorescence intensity increased with the decrease of Mw in Mwdependent fluorescence manner, behavior induced by the enhancement of aggregation with short polymer chains [67].

4 Applications of AIE polymers

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With the efforts of polymer scientists, AIE polymers with considerable structure diversity and multifunction have been generated in recent years since the first report of AIE polymer in 2003. Compared to the organic luminogens with low molecular weights, AIE polymers possess additional advantages, such as strong absorption activity and synergistic effect owing to the delocalized electronic structures. Moreover, the intramolecular motion of the AIE units in the polymers can be further restricted by the steric hindrance from the skeleton and the side-chains, resulting in high fluorescence efficiency and high sensitivity. In addition, the structure, morphology and the composition of polymeric materials can be regulated to prepare functional materials and meet various demands of applications. All these allow for the

rapid development and wide applications of AIE polymers in various fields, ranging from optoelectronic applications to sensing and biomedical areas [68]. 4.1 Optoelectronic applications

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Luminogens for optoelectronic applications are always used in solid or film states. Given the AIE effect, AIE polymers can circumvent the notorious ACQ effect of conventional luminogens and polymers in these states. Taking advantage of this, AIEactive polymers have been used in polymeric light-emitting diodes (PLEDs), circularly polarized luminescence (CPL), luminescent liquid crystalline (LLC), electrochromic devices (ECD), scintillator, etc. Over the past decades, organic light-emitting diodes (OLEDs) have attracted considerable interests because of their great potential in new display devices, and have even been used on smart cellphones and other instruments. Conjugated polymers (CPs) with high efficiency and good solution processability are considered as promising luminescent materials for PLED applications. However, most CPs suffer from the ACQ effect. Thus, the AIE polymers could solve such difficulty. Zhao and coworkers incorporated TPE into the main chain of P44, providing an AIE polymer with high thermal stability (Fig. 7A). Double-layer light-emitting diodes were built using P44 as emitters, with the best performance having a maximum luminance efficiency of 2.11 cd/A and a maximum brightness of 6500 cd/m2 [69]. Li and coworkers developed a TPE containing polyfluorene P45, that emits brightly with the quantum yield of 28.2% in the aggregate state (Fig. 7A). The device with P45 as holetransporting layer showed the best performance with an efficiency of 1.17 cd/A and a maximum luminance of 3609 cd/m2 at 12.9 V [70]. Besides TPE units, the silole moiety was also incorporated into polymer P46 by Chen and coworkers because of its high electron mobility and high electron affinity (Fig. 7A). By utilizing this polymer as the light-emitting layer, the EL device could be turned on at 6.4V with a green light at 520 nm and maximum luminance efficiency, brightness and external quantum efficiency of 7.96 cd/A, 4800 cd/m2, and 3.18%, respectively [71]. These results demonstrated that promising applications of AIE polymers in PLEDs.

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CPL is the selective emission of right- or left-handed circularly polarized light from chiral luminophores, and CPL materials have gained great attention in photonic devices. Compared to chiral low mass molecules, chiral π-conjugated polymers possess higher dissymmetry factor (glum) values, which will facilitate their applications [72-74]. For example, Cheng and coworkers synthesized a chiral binaphthyl-based polymer R-/S-P47 with AIE activity. The R-/S-P47 enantiomers show a high glum value of 0.0145 in the spin-coating film with intense aggregationinduced CPL signal, attributed to the formation of helical nanowires in the aggregate state (Fig. 7B) [75]. By incorporating a chiral amino acid into the polymer skeleton, our group prepared a chiral polymer of P48 with AIE property. As shown in Fig. 7C, by regulating the water fraction in THF and the concentration, the nano-architecture could be tuned from vesicles, “pear-necklace” to helical microfibers, and this transition process could be in-situ visualized by fluorescence microscopy. The glum

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value of this polymer was determined to be 0.0045, revealing it has outstanding CPL performance [76]. EC is a reversible change in color or transmittance induced by reduction or oxidation of materials by electrochemical means. Based on the unique advantages of tunable colors, fast switching capability and high coloration efficiency, polymers especially AIE polymers with synchronous fluorescence switching and EC switching, show great potential in the EC field. For example, a novel alternating polymer P49 with AIE activity and EC properties was prepared based on TPE and thiophene derivatives by Xu and coworkers (Fig. 7D). The color of P49 could be switched from yellow to blue through voltage within ±1.5 V. Furthermore, the green fluorescence could be regulated on/off within ±2.5 V synchronously. In the neutral state, the ECD of P49 showed a bright yellow color and strong fluorescence, but became navy blue with faint fluorescence in the oxidized state [77]. Chen and coworkers also developed an AIE polyamide with the QY of 69.1% in the film state and excellent EC cycling stability. Both the color change from yellow to brown and fluorescence could be switched for hundreds of cycles, and the contrast ratio of the fluorescence on/off could be determined to 417, which is the best value in the reported systems [78]. These results indicated the unique advantages of AIE-active polymers in EC applications. In addition, AIE polymers display advanced properties in other optoelectronic applications. The introduction of AIE effect towards LLCs can overcome the poor efficiency induced by the ACQ effect. For example, Xie and coworkers prepared a series of LLC polymers P50 with different spacer length based on AIE and the “Jacketing” effect (Fig. 7E). The PL measurements revealed that the resultant polymers all exhibited AIE behavior, and high fluorescence could be detected in LC state. Moreover, with the increase of the spacer length, the quantum yield dropped from 52% to 18%, and the glass transition temperature (Tg) decreased as well. Meanwhile, the phase structures of these polymers transformed from smectic A (m = 2, 4, 6) to hexagonal columnar (m = 8, 10, 12) [79]. Scintillation, the key component, always monitors the intensity of high-energy beams or radiation. Chujo and coworkers combined scintillators based on CPs with AIE activity, and the emission colors to PL was realized by X-ray excitation, which suggested that the polymers with AIE properties show a promising platform for the development of tunable plasticscintillator [33]. 4.2 Fluorescent sensor The luminescence properties of fluorescent materials altered in response to the environmental variations or the external stimuli may be utilized in sensing applications. The sensitivity of a sensor depends on not only the photo-physical properties of fluorescent materials, but also the degree and ability of combination between the sensor and analytes. The high fluorescence efficiency, synergistic effect and the large contact area in aggregate state endow the AIE polymers with “superamplification effect”, and make them ideal candidates for constructing sensors with

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high sensitivity. A large variety of AIE polymers have been employed for response to explosives, ions, pH values, pollutants, temperature and multiplex, etc [80-82]. Explosive detection has become an international concern owing to the threat of terrorism in recent years, and the ultra-selective and super-sensitive detection are highly desirable. Many AIE polymers have been used for explosive detection with unique advantages. For examples, the lowest detection limit of 5×10-8 M for trinitrobenzene detection was reported by Scherf and coworkers with quenching constant of 1.26×106 M−1 by using tetraphenylethylene-substituted polycarbazoles [83]. Our group prepared AIE-active hyperbranched poly(aroxycarbonyltriazole)s that could be used to detect picric acid (PA) with the super-amplification quenching effect [84]. Son and coworkers developed TPE-containing conjugated microporous polymer films with macroporous (MA-CMP-Fs) used for the sensing nitrotoluenes. They demonstrated that these MA-CMP-Fs possessed much better sensing ability than that for conventional conjugated microporous polymer films and their building materials because of the additional macroporosity [85]. Apart from these covalently linked polymers, Huang and coworkers constructed a novel supramolecular polymer network with AIE properties (P51) via host-guest recognition for explosive detection, that could be conducted in both solution and thin film (Fig. 8A) [86]. Pollutant detection is of great significance to health and environment issue. The detection of various ions (Pd2+, Al3+, Ca2+, Fe3+, Cd2+, Mg2+, CN-), aromatic pollutants, and volatile organic compounds, etc, based on AIE polymers has been well developed [87-90]. Our group has, for example, constructed an anionic conjugated polytrizole. Thanks to the synergic coordination effect of triazole and sulfonic groups, the detection of Al3+ with high sensitivity and excellent selectivity was realized in a “turn-on” manner, in which the limit of detection (LOD) was deduced to be as low as 31 ppb. It is worth mentioning that its model compound had no response to Al3+, demonstrating the superiority of AIE polymers [91]. Meanwhile, as displayed in Fig. 8B, Ruan and coworkers developed an AIE-active thioketal-containing CP of P52 for Hg2+ detection with the LOD of 2.3 × 10−7 M. After the addition of Hg2+, the fluorescence intensity increased markedly [92]. In addition, toxic VOC pollutants were also efficiently detected by the AIE polymers. For example, Liang and coworkers realized the detection of xylene based on an amphiphilic triblock copolymer bearing TPE moiety. The LOD was determined to be 1 g/L, with a response rate within a few seconds [93]. Yang and coworkers reported that halogen acid gas could be sensitively detected by AIE-active polyurethane [94]. Lu and coworkers fabricated nanoporous fibers with AIE features and switchable fluorescence and used for cyclic oil adsorption [95].

Making the structural and morphological evaluation of materials visible is attracting increasing attention, especially strategies for the in-situ, sensitive and noninvasive visualization [96,97]. Our group developed a facile strategy to monitor the conformational change of the chiral polymers in real-time and in-situ during the aggregation process. As shown in Fig. 8C, four binaphthyl (BN)- and TPE-bearing

chiral polymers (P53-P56) were designed and synthesized. The results revealed that aggregation-annihilated circular dichroism (AACD) could be observed in P53 and P54 with “unlocked” BN moiety, in which the torsion angle (θ) decreased with the relaxation of part conformers from cisoid to transoid upon aggregation. However, the ACCD was suppressed for P55 and P56 with “locked” BN units with slight larger θ, indicating that the twisting of the BN rings might cause the ACCD effect [98]. Our group also made the RAFT polymerization visible by using a TPE-containing initiator (Fig. 8D). With the increase of the conversion ratio, the emission intensity rose gradually, attributed to the mechanism that the fluorescence altered with the restriction of the TPE motion in P57 induced by the varied viscosity. Moreover, the relationship between the PL intensity and molecular weights of the resultant polymers demonstrates the accuracy and sensitivity of this strategy [99].

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4.3 Biomedical applications

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Fluorescent materials play a critical role in biomedical applications, covering analyzing, imaging, diagnosis, therapy and so on. Given with the easy modification and the diversity structures of polymer materials, the incorporation of the AIE units into polymer endows the materials high PL efficiency and excellent sensitivity in the aggregated state. Thus, these polymers have been widely used in diverse biomedical areas, such as in vitro/in vivo imaging, biological analyzing, drug delivery, tumor and infection therapy. The in-situ and on-site biological analysis is of great significance to biotechnology and life science. Conventional fluorescent materials, however, often suffer from the low sensitivity due to the ACQ effect. AIE polymeric materials with high sensitivity and signal amplification activity possess academic and technological superiority in biological sensing and imaging in vitro/in vivo. With these in mind, our group constructed AIE-active polytheterocycles to detect biogenic amines and indicate the pH distribution in vivo. As demonstrated in Fig. 9A, the fluorescence of the polymer dropped when exposed to HCl vapor, behavior induced by protonation on the imine nitrogen (C=N). The recovery of fluorescence could be observed by fuming with NH3 vapor. Thus, the films made by acid-treated polymer could be used for selective response to biogenic amines. Moreover, this polymer exhibited ratiometric pH sensing behavior with the pH values ranging from 1 to 9. In an example, the mapping of the Cladocera Moina macrocopa intestinal pH was performed, in which a pH gradient from 4.2 to 7.8 was observed along the foregut, midgut, and hindgut [100]. Moreover, the sensing and detection of extracellular Ca2+ concentration with high selectivity was achieved by Fukushima and coworkers based on a polymer gel sensor bearing TPE group [101]. Notable, fluorescent probes based on AIE polymers display high photo-stability and low cytotoxicity, and have been utilized in imaging applications in vivo/vitro successfully [102]. Subcellular imaging and long-term tracing have been well realized [67,103]. In addition, AIE polymers are an ideal candidate to function as theranostic agents for fluorescent-guided therapy. Liu and coworkers reported a smart polymer for tracing the process of cancer therapy, in which the uptake by cells and lysosome

escaping to cytoplasm were monitored by observing the fluorescence variation [104]. Cheng and coworkers and Loh and coworkers also prepared polymers with AIE property as drug cargoes for cancer therapy in a self-indicating manner [105]. Our group developed an intracellular polymerization based on our developed spontaneous amino-yne click polymerization and facilely constructed a novel “lab-incell” with P58 (Mw of 7300). As shown in Fig. 9B, by utilizing the AIE effect of TPE and in vivo amino-yne click polymerization, cell imaging was realized in a “turn-on” manner, and efficient cell killing was acquired by in situ destroying the structure of tubulin and actin [106].

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AIE polymers not only exhibit increased fluorescence but also possess enhanced reactive oxygen species (ROS) generation ability in the aggregate state. Moreover, the conjugation structures of polymers will further strengthen the ROS production. Hence, these properties make the AIE polymers particularly appropriate as photosensitizer for image-guided photo-dynamic therapy (PDT) applications. Wu and coworkers reported a useful strategy of polymerization-enhanced photosensitization to improve the ROS generation efficiency (Fig. 9C). They proposed that the enhanced conjugation structure could improve the intersystem crossing (ISC) transition and the light-harvesting ability, resulting in the higher ROS production efficiency [107]. Our group reported two strategies, i.e., the polymerization and the donor-accept even-odd effect to enhance the ROS-generation efficiency by strengthening the ISC process. As shown in Fig. 9D, a conjugated polymer with donor-acceptor structure was used to prepare nanoparticle and employed for cancer therapy. The fluorescence increased gradually at the tumor site after the intravenous injection towards the tumor-bearing mice, and the tumor growth was inhibited partly with the treatment of nanoparticle under white light, revealing that conjugated polymer with AIE property is a powerful platform for imaged-guided PDT [108]. Terrible infections caused by bacteria, especially super-bacteria, are a global health crisis. Due to the unique photophysical property, multifunctional polymer with AIE activity has not only the antibacterial ability but also the bacteria detection activity. Wang and coworkers engineered a peptide-grafted hyperbranched polymer. Thanks to the AIE feature, the fluorescence intensity of the resultant polymer depended on the concentration of E. coli with the LOD of 1 × 104 colony forming units (CFU) mL–1 (Fig. 9E). Moreover, it could penetrate the cell wall of bacteria and cause cell rupture to realize efficient antibacterial effect with no cytotoxicity to mammalian cells [109].

5. Conclusions and future perspectives In this review, we discussed the preparation of AIE polymers by one-, twocomponent and multi-component polymerization. The relationship of the backbones and the side-chains of the AIE polymers with the photo-physical properties are also summarized, respectively. We especially discuss the nonconventional luminogens without chromophore and the mechanism of clusteroluminescence. Thanks to all these scientists’ efforts, a large number of AIE polymers with diverse structures and

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multifunction have been developed, and are playing increasingly important roles in the optoelectronic device, chemo-/biosensor and biomedicine, etc. Encouraged by the remarkable achievements in AIE polymers over the past 17 years, more explorations and breakthroughs with numerous possibilities could be made in this field. Future efforts and challenges might focus on the following aspects. The design and preparation of new AIE-active monomers is desirable for the construction of novel AIE polymers with inspiring properties and exciting functions. Persistent endeavors are demanded for the development of novel and efficient synthetic strategies to construct AIE polymers, especially for metal-free and even spontaneous polymerization reactions. The in-situ generation of AIE polymers from AIE-inactive monomers will be an interesting direction. In addition, AIE polymers with controlled molecular weights and sequence of repeating units as well as welldefined structures are eagerly desired. The SPR of AIE polymers should be investigated more systematically to develop guidelines for the design and preparation of AIE polymers with desirable properties. Furthermore, more polymeric materials without traditional fluorophores but possessing intrinsic photoluminescence need to be explored. However, revealing the mechanism of nonconventional luminescent polymers remains a major challenge, hence, more theoretical simulations and direct experimental evidence should be explored to define the clustering of electron-rich groups. More AIE-active polymers with diverse structures and multifunction should be prepared to meet the demands of various research frontiers. For example, the visualization of the synthesis or aggregations process could enable the testing and validation of classical concepts in polymer science. Even though the utilization of AIE polymers in PLED has been explored, the efficiency of the devices should be further enhanced. Thus, AIE polymers with excellent PL efficiency and strong carrier transporting ability should be designed and prepared to fabricate PLED with outstanding performance. In addition, AIE polymers with various emissions and RTP are highly demanded. Meanwhile, far-red or NIR fluorescent AIE polymers with strong biocompatibility, high PL efficiency and excellent stability are needed for in vivo/vitro imaging so as to develop polymeric materials with strong two-/multi-photon absorption cross sections. The incorporation of other functional elements, such as recognition moieties, stimuli-responsive groups and therapeutic agents into AIE polymers for chemo-/bio-sensing or theranostic will broaden their applications in practical fields and provide strong support for healthcare and environment protection. Last but not least, we can’t address all the opportunities here. Based on the promising vista of AIE polymers, we hope this review can inspire more scientists to focus on the research of AIE polymers, and realize their full potentials. Conflict of interest none

Acknowledgments

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This work was financially supported by the National Natural Science Foundation of China (21788102, 21525417, 21490571 and 51620105009), the Natural Science Foundation of Guangdong Province (2016A030312002, 2018A030313763 and 2019B030301003), the Fundamental Research Funds for the Central Universities (2015ZY013), and the Innovation and Technology Commission of Hong Kong (ITCCNERC14S01).

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Fig. 1 Synthetic routes to P1 (A), P2 (B), P3 (C), P4 (D), P5 (E) and P6 (F) by onecomponent polymerizations.

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Fig. 2 Synthetic routes to P7 (A), P8 (B), and P9 (C) through spontaneous alkyne-based click polymerizations. R = TPE unit.

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Fig. 3 Synthetic routes to P10 (A), P11 (B), P12 (C), and P13 (D) through two-component polymerizations.

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Fig. 4 The synthetic routes to P14 (A), P15 (B), P16(C), P17 (D), P18 (E), P19 (F) and P20 (G) by the multi-component polymerizations.

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Fig. 5 (A) Structures and photographs under a UV lamp for P21, P22, P23, and P24 in THF/water mixtures, with water fractions from 0–90% [52]. Copyright 2014. Reproduced with permission from the Royal Society of Chemistry. (B) The proposed ACQ and AIE mechanism of P25 and P26 [53]. Copyright 2018. Reproduced with permission from the American Chemical Society. (C) The structures of P27-P29. (D) The structures and photographs of P30F/T-P33F/T in the film state under UV irradiation. [57]. Copyright 2018. Reproduced with permission from the Royal Society of Chemistry. (E) Structures and photographs of the solid powder of P34 and P35 [59]. Copyright 2017. Reproduced with permission from the Royal Society of Chemistry. (F) The structures of P36-P39.

Fig. 6 (A) (a) Chemical structure of P40. Possible intra- and intermolecular interactions within cyano clusters. (b) Electron overlap between lone pairs and π electrons, (c) dipoledipole interactions, and (d) n-π interactions [63]. Copyright 2016. Reproduced with

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permission from John Wiley  Sons Inc. (B) The structure and schematic illustration of P41 emission properties at various states [64]. Copyright 2018. Reproduced with permission from the American Chemical Society. (C) (a) Photographs of P42 and P43 in their solid states under UV irradiation. (b) Optimized conformation of P43 at different views. (c) Optimized conformation of P42. (d) The interaction types of carbonyl groups in P43. (e) The proposed model of n→π* interaction [65]. Copyright 2019. Reproduced with permission from Springer Nature.

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Fig. 7 (A) Chemical structures of P44-P46. (B) CPL spectra of R-/S-P47 in nanoparticle and nanowires and their SEM images [75]. Copyright 2018. Reproduced with permission from Elsevier Science Ltd. (C) Schematic representation of the self-assembly and morphology transition processes of P48 in the THF/water mixture at different concentrations and fw [76]. Copyright 2018. Reproduced with permission from the Royal Society of Chemistry. (D) Photographs of the P49 in THF/water mixtures with different water content, and the ECD in its neutral (right) and oxidized (left) states under 365 nm UV light [77]. Copyright 2015. Reproduced with permission from the American Chemical Society. (E) Photograph of P50 under UV light and representative textures at 140 °C [79]. Copyright 2017. Reproduced with permission from the American Chemical Society.

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Fig. 8 (A) Illustration of the formation of an AIE fluorescent supramolecular polymer network and detection of nitro-compound explosives [86]. Copyright 2018. Reproduced with permission from Royal Society of Chemistry. (B) Chemical structure and the Hg2+ sensing process of P52 [92]. Copyright 2018. Reproduced with permission from Elsevier Science Ltd. (C) Circular dichroism spectra of (a) P53, (b) P54, (c) P55 and (d) P56 in THF/water mixtures with different fw [98]. Copyright 2018. Reproduced with permission from Springer Nature. (D) Fluorescent photos of the P57 solutions at different conversion under UV irradiation [99]. Copyright 2018. Reproduced with permission from John Wiley  Sons Inc.

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Fig. 9 (A) (a,b) PL spectra of nanoparticles in polymer buffer solutions with different pH values. Inset: fluorescence photographs of the nanoparticles with different pH values. (c) Plot of relative PL intensity (I/I1, blue line) and (I535/I610, red line) of polymer versus pH values, where I1 = PL intensity at pH = 1, I535 and I610 denote the PL intensity at 535 and 610 nm. (df) Confocal images of Moina macrocopa incubated with polymer for 4 h [100]. Copyright 2018. Reproduced with permission from John Wiley  Sons Inc. (B) Lab-in-cell of spontaneous amino-yne click polymerization [106]. Copyright 2019. Reproduced with permission from Springer Nature. (C) The illustration of the different photosensitization processes of low-mass molecules and conjugated polymer photosensitizers [107]. Copyright 2018. Reproduced with permission from Elsevier Science Ltd. (D) (a) Enhanced photosensitization for application in PDT. (b) Tumor imaging of mice after intravenous injection of PNPs at different times. (c) Tumor growth curves and (d) body weight changes of mice in different treatment groups [108]. Copyright 2018. Reproduced with permission from John Wiley  Sons Inc. (E) Schematic illustration of theranostics integrated combating bacteria and monitoring bacteria in real-time based on NPGHPs. Upon contacting with bacteria, NPGHPs exhibit fluorescence emission because of the AIE effect. Further, NPGHPs also serve as antibacterial agents to kill bacteria [109]. Copyright 2018. Reproduced with permission from the American Chemical Society.