Luminogenic Polymers with AIE Characteristics - American Chemical

Chapter 2

Luminogenic Polymers with AIE Characteristics Downloaded by CORNELL UNIV on October 16, 2016 | http://pubs.acs.org Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1227.ch002

Anjun Qin,*,1 Ming Chen,2 and Ben Zhong Tang*,1,2 1State

Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China 2Department of Chemistry, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China *E-mails: [email protected] (A.Q.); [email protected] (B.Z.T.)

Since conceptually proposed by Tang et al. in 2001, the aggregation-induced emission (AIE) has caused tremendous interests among scientists worldwide. Currently, hundreds of AIE-active luminogens have been designed and prepared, but most of them are small molecular ones. However, the AIE-active polymers possess charming advantages, such as mutable and tunable molecular structure, excellent film-foming ability and facile processability, thus, are potentially applied in diverse high-tech areas, like fluorescent chemo- and bio-sensors, optoelectronic devices and so on. In this chapter, we first summarize the types of the presented AIE-active polymers by incorporating AIE-active units, such as tetraphenylethene (TPE), multi-phenyl substituted siloles and distyrylanthracene (DSA). In addition, a new kind of luminogenic polymers containing unconventional chromophore, such as poly(amido amine) (PAMAM), poly(amino ester) and poly[(maleic anhydride)-alt-(vinyl acetate)], are also discussed. Our aim is to establish a structure-property relationship of AIE-active polymers, and further demonstrate how to control the AIE-activity of the polymers via intelligent and rational molecular design.

© 2016 American Chemical Society Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Downloaded by CORNELL UNIV on October 16, 2016 | http://pubs.acs.org Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1227.ch002

1. Introduction Over the past decades, luminogenic polymers have received considerable attention owing to their potential applications in fluorescent chemo- and bio-sensor, organic light-emitting diodes (OLEDs) and organic lasers, etc. (1–4) Up to now, most developed polymeric luminogens emit intensely in their diluted solution, while the emission is weakened or even quenched upon aggregation. In other words, they suffer from notorious aggregation-caused quenching (ACQ) effect (5). For example, polyfluorene (PFO), which was regarded as one of the most acceptable candidates for blue emitter in OLED, tends to form excimers during device fabrication and operation, therefore leading to red-shift its emission peak and lower its luminescent efficiency (6, 7). Plenty of efforts have been devoted to alleviate the negative ACQ effect caused by aggregation. However, the reported chemical approaches, such as design macromolecule in branched and spiro structure or covalently graft bulky cube onto polymer chain always encounter fussy synthetic steps and painful separation process, whereas, physical doping of luminogens as guest in host inclines to produce phase separation with prolonged operation time (8–11). Actually, such measures only meet with limited success. We all know aggregation is an intrinsic process of molecules in condensed phase. It is envisaged but required urgently whether the aggregation could play a positive instead of negative role in enhancing luminescence. In 2001, Tang et al. found that multi-phenyl substituted silole derivatives are non-emissive in dilute solution but emit intensely upon aggregation. Since the emission of siloles was induce by the aggregation, Tang thus coined this unique photo-physical phenomenon as aggregation-induced emission (AIE) (12, 13). Afterwards, restriction of intramolecular rotation (RIR) has been rationalized as the cause of AIE effect, which has been proved not only by a large amount of experiments but also by theoretical analysis (14–17). Besides silole derivatives, other AIE-active luminogens, such as tetraphenylethene (TPE), tetraphenylpyrazine (TPP), distyrylanthracene (DSA), and tetraphenylpyrrole have been developed subsequently (18–20). Based on these archetypal AIE-active luminogens (AIEgens), lots of work has been conducted to fit various research interests (21–23). In contrast, less work has been focused on AIE-active polymer in spite of its unique advantages of high molecular weight. In device operation, conjugated polymer always possesses a higher glass-transition temperature (Tg), which plays a decisive role in improving device stability. The solid film fabricated by spin-coating, static casting and ink-jet printing instead of vapor deposition could greatly reduce the cost in practical application. And the viscoelastic polymer film is very suitable for utilizing as flexible large-area flat panel display (24, 25). It is also worth noting that polymer is usually partially crystalline or even hard to crystallize due to its piled molecular chain in disorder upon aggregation. To some degree, it favors the formation of porosity in polymer, especially with the branched, hyperbranched or network structures, which provides the void to bind with analyte and induce the signal switch when function as fluorescent sensor (26). For example, hyperbranched AIE-active polymer always shows an unique superamplification quenching 28 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


effect as turn-off sensor to detect explosives, which is much sensitive than small molecule probes. Last but not least, the structure of polymer is multivariate and easy to tailor as it is need. For instance, one or more of the functional groups or moieties with hydrophily or lipophilicity, characteristics of luminescence, and response to photon, temperature or pH, and so on, could be incorporated and coordinated into the polymer, which is of great significance to construct stimuli-responsive materials (27, 28). All these demonstrate the attractive virtues of luminogenic polymer, which is always difficult to achieve in single small molecules. As enlightened by the previous researches, one of the most feasible strategies to design AIE-active polymers is to link, insert and graft typical AIE-active units into or onto the polymer chains. Indeed, by utilization of free radical polymerization, polycondensation, Suzuki polycoupling, ring-opening polymerization, metathesis polymerization, click polymerization, and post-functionalization reaction, etc., a wide variety of AIE-active polymers including polyolefins, polyimides, polyacetylenes, polytrizoles and post-modified polymers with linear, dendritic and hyperbranched structures have been prepared (29). The species diversity, corresponding synthetic methods and diversified applications have been summarized in detail in our previous reviews (29, 30), which thus will be less described here. Instead, in this chapter, we mainly discuss TPE, silole and other typical AIE cores based AIE-active polymers with varying molecular architectures, which include: (a) linear, dendritic, hyperbranched and network structures, (b) partially- and fully-conjugated structures, and (c) AIE-active units linked, grafted and inserted structures. Note that some cluster luminogens without typical luminogenic units, such as poly(amido amine) (PAMAM), poly(amino ester) and poly[(maleic anhydride)-alt-(vinyl acetate)], are introduced here (31–33). However, polymer systems, such as supramolecular polymers and metal-organic frameworks (MOFs) generated by non-covalent interaction are not within the topic. Our aim is to establish a structure-property relationship of AIE-active polymers. We hope, with the aid of the discussion in this chapter, more and more AIE-active polymers with exciting properties could be designed and prepared, which will in turn promote the development of luminogenic polymer in high-tech applications in diverse areas.

2. TPE-Based AIE-Active Polymers Currently, TPE is regarded as the most famous star molecule in AIE research due to its high emission efficiency in solid state, easy preparation and convenient post-functionalization. Recently, more interests have been inclined to the study of AIE-active polymers by incorporating TPE as luminescent units. Thanks to a burgeoning polymerization technique of click polymerization, AIE-active polymers could be prepared with high molecular weights in excellent yields (34, 35). TPE-containing 1,4-regioregular polytriazoles 4 and 5 could be obtained by the organo-soluble Cu(PPh3)3Br-catalyzed click polymerization of diyne 1 and diazides 2 or 3 (Scheme 1). Photo-physical property investigation indicated that both 4 and 5 are AIE-active. In solution state, they are non-luminescent 29 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


because the peripheral phenyl rings of TPE units could freely rotate to annihilate the excitation state energy, whereas, upon aggregation, such rotation is partially restricted accompanying with the emission renovated evidently, with their PL properties resembling to that of TPE. The reason for this emission behavior is that the TPE units incorporated in the polytriazoles are linked with aliphatic alkyl chains, which results in no electron communication between TPE and formed triazole rings (36).

Scheme 1. Synthetic routes to TPE-containing polytriazoles 4 and 5.

It is believed that the residue of copper species in polytriazoles, which probably could coordinate with the form triazole rings and be hard to be completely removed, may cause fluorescence quenching in optoelectronic field and even toxicity in biological application. To overcome this difficulty, one of the approaches of using supported Cu(I) catalyst (CuI@A-21) to catalyze the click polymerization was reported, which not only greatly reduces the metallic residues in the resulting polytriazoles but also makes the catalyst recyclable. More importantly, polytriazole 6 prepared by CuI@A-21 catalyzed click polymerization exhibits increased emissive efficiency in comparison with 5, where both of them possess the same molecular structures (Figure 1) (37). This result demonstrates that the quenching of the fluorescence could be alleviated by reducing the copper residues in the polymers, which further guides to design AIE-active polymer with metal-free or catalyst-free polymerizations. Along this line, the AIE-active polytriazoles 9-11 were prepared by metal-free click polymerizations of activated diynes and TPE-containing diazides (Scheme 2) (38). The electron-withdrawing property of carbonyl group adjacent to alkyne makes it more reactive than normal ones, thus prone to polymerize with diazide in the absence of metal-catalyst systems, which may prompt the application of functional polytriazoles in optoelectronic and biological fields. 30 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 1. (a) PL spectra of 6 in THF/water mixtures with various water fractions. Polymer concentration: 10 μM; λex: 326 nm. (b) Changes of quantum yield (ΦF) of 5 and 6 in THF/water mixtures with various water fractions. Inset: the structure of CuI@A-21.

Scheme 2. Synthetic routes to TPE-containing polytriazoles 9-11. Besides the partially conjugated polytriazoles, the conjugated polytriazoles of 15, 16, 20 and 21 could also be synthesized via Cu(PPh3)3Br-catalyzed click polymerization of aromatic diynes and diazides (Scheme 3). Owing to the presence of TPE unit, 16 and 21 display aggregation-enhanced emission (AEE) characteristics. In solution state, a weak emission of both polytriazoles could be discerned due to the enhanced molecular stiffness, making the phenyls less easy to rotate to annihilate the excitons. It is worth noting that the emission peaks of the aggregates of two polymers formed in THF/water mixture with 90% water fraction located at around 485 nm, which is red-shifted compared with aforementioned aliphatic alkyl chains containing polytriazoles, manifesting that 31 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


the triazole ring could enhance the conjugation length of emitters. Moreover, the similar emission behaviors of 16 and 21 indicate that negligible influence on emission could be exerted by exchanging the substituents on the triazole rings. However, when two phenyl rings of TPE units in 16 and 21 are replaced by hydrogen atoms, both 15 and 20 suffer from ACQ effects. The less congested phenyl rings in stilbene units induce the molecule to adopt a planar conformation, which might favor the formation of π-π interaction in the aggregate states, and thus leads to the quenching of the emission. All these findings suggest that TPE is an ideal unit to construct AIE-active conjugated polymer, and a subtle change in substituents, especially the number of phenyl rotors, may have a great impact on the photo-physical properties of polymers (39).

Scheme 3. Synthetic routes to TPE-containing polytriazoles 15, 16, 20 and 21. In some occasions, polymers with long wavelength emission are much desired because of their huge application potential in biological field. One of the most used strategies to design red emissive AIEgens is to incorporate both strong electron-donating and electron-withdrawing groups into a molecule to induce intramolecular charge-transfer process. As a result, the highest occupied molecular orbital (HOMO) energy level is elevated, while the lowest unoccupied molecular orbital (LUMO) energy level is reduced, which coherently cause a narrow energy gap and resulted red emission. As shown in Scheme 4, alternative copolymer 24 was synthesized via classical Suzuki polycoupling of 22 and 23. In 24, diethylamine and diazosulfide moieties serve as donor and acceptor, respectively, which is consistent with the design principle of red emissive polymers. 24 gave a deep-red emission at 665 nm in thin solid film with ΦF of 6.9%. Compared to its low ΦF (< 1%) in solution, 24 thus features AEE-activity. The contained TPE units in 24 are responsible for overcoming the ACQ factors brought from both diazosulfide and fluorine units (40).

Scheme 4. Synthetic route to red emissive TPE-containing polytriazole 24. 32 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


TPE units could also function as pendant group and be grafted on both flexible and rigid polymer main-chains. For example, 25 and 26 could be prepared from radical copolymerization of TPE-containing vinyl monomer and post-functionalization of TPE unit onto a preformed polymer, respectively (Chart 1). Moreover, the amount of TPE units in polymers could be fine-tuned to fulfill the functionalities as sensors of temperature, pH and bovine serum albumin (BSA). Because only one phenyl ring in TPE unit is dragged onto the main chain, less rotation could be restricted in solution, whereas, the flexible main-chain promotes the full restriction of motion of TPEs in aggregate state, both of which makes resulting polymers behave like archetypal TPE core (41, 42).

Chart 1. Molecular structures of TPE-based polymers 25 and 26.

The TPE unit could also be attached to a rigid polymer main-chain but different emission behaviors were observed (Chart 2). It is interesting to note that the AIE-activity of TPE could remain when it was attached to the rigid polyacetylene main-chain (27) spaced via an alkyl chain, demonstrating that the involved flexible chain plays a crucial role in determining AIE behavior. While, when the spacer is removed, the photo-physical property of 28 is much different. Firstly, the TPE may conjugate with polyacetylene backbone, as evidenced by the emission of polymer in solution was peaked at the longer wavelength of 613 nm, which is more than 60 nm red-shifted than that of 27. Secondly, the AIE effect is discounted seriously, with only a maximum PL increment about 2.8-fold probably because the TPE units are fastened to backbone rigid strand, which makes the whole molecule like a “brush”. Upon aggregation, the polymer chains are hard to pack compactly, where plenty of free volume is produced to induce the rotation of phenyl rings in TPE unit (43). Such negative effect could be alleviated in certain degree when the rigid strand of 28 is replaced by less regular conjugated backbone, such as polydiphenylamine. As expected, the AIE effect of 29 and 30 is improved, and the fluorescence enhancement of 22-fold and 8-fold are recorded, respectively. Additionally, when one more phenyl ring was inserted between polydiphenylamine main-chain and TPE unit, the AIE-activity of 30 is lessened obviously. It is probably due to that the extended conjugated branch-chain may cause more free volume under aggregation (44). 33 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Chart 2. Molecular structure of TPE-based polymers 27-30. The TPE units could also be attached to polymers as terminal groups (45, 46). Are they also AIE-active? Here, we take 31 as an example (Chart 3). Its structure includes a hydrophilic poly(ethylene glycol) (PEG) and a lipophilic end group embracing TPE as luminescent unit. The structure of 31 facilitates it to form spherical micelles in aqueous solution with the hydrophobic TPE units as the core and hydrophilic PEG as the shell. Thus, the TPE terminals behave like self-aggregation, enabling the polymer to be AIE-active.

Chart 3. Molecular structure of TPE end-capped polymer 31. The influence of end-capped TPE units of polymers/oligomers on the emission behavior was also investigated (Chart 4). When the ACQ fluorene unit was covalently bound with two TPE units, the new compound is AIE-active because the peripheral TPE rotors could dissipate the excitation energy transferred from the fluorene units. When the number of fluorene units was increased to 5, the ΦF of 32 in solution was enhanced accordingly by 6-fold, suggesting that the free rotation of TPE units is not enough to quench the emission from five fluorene units (47). This study could guide for further molecule design of AIE-active polymers/oligomers.

Chart 4. Molecular structure of TPE end-capped oligomer 32. 34 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


The TPE units has also been incorporated into hyperbranched polymers. For example, hyperbranched polytriazoles 35 and 36 were prepared by Cu(I)-catalyzed click polymerization of triazides 32 or 33 and diyne 34 (Scheme 5), which display dramatic AIE effect with fluorescence enhancement about 347 and 229-fold, respectively. The ΦF of the aggregate of 35 formed in THF/water mixture with 90% water fraction is up to 38.31%, which is much higher than that of linear analogs of 4-6 and 9-11. The flexible alkyl spacers weaved into the hyperbranched polymer increase their solubility and make them non-emissive in their good solvents. While, in the aggregate state, the alkyl chains may facilitate the TPE units to accumulate more tightly and enable them to emit intensely. Furthermore, more flexible alkyl chains (judged by length), more AIE effect could be achieved (48).

Scheme 5. Synthetic route to TPE-containing hyperbranched polytriazoles 35 and 36. The influence of molecular rigidity of a hyperbranched polymer could be proved by luminescent property of polytriazole 39, which is also prepared by Cu(I)-catalyzed click polymerization (Scheme 6). When inspected the molecular structure of 39, we could find that tetra- and di-substituted TPE units are knitted by triazole rings. The whole molecule is conjugated and therefore its conformation is much rigid than that of 35 and 36, where the phenyl rings in TPE are much harder to rotate in solution. Its branched structures also hamper a tight aggregation of polymer chains. It is also in accordance with the increased ΦF of 4.31% in solution and decreased ΦF of 23.83% in its aggregates compared to 35 (49). More complicated emission behaviors have been recorded from 40-42 (Chart 5). Unlike 39, the enlarged molecular rigidity after polymerization could only provide soluble 40-42 with molecular weights around 4000. The fluorescence enhancement of 40, 41 and 42 was measured to be 2309-, 15- and 4.4-fold, respectively, which is mainly determined by their different ΦF in solution (0.01%, 1.1% and 5.2%). These results manifest that the conjugated joints between branching points have a profound influence on AIE effect of a hyperbranched conjugated polymer. It is speculated that the phenyl rings of TPE units are more restricted by near carbazole and phenyl units due to their strong steric effect, which further reduce their motion in solution. Also, the low molecular weights 35 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


make the hyperbranched polymer possess less generations, which leads to less congestion between branching chains. Therefore, upon aggregation, all the polymer chains are easy to pile up compactly, even though inserting a partial chain of one polymer into the interspace of another one, which agrees with their similar experimental values of ΦF in aggregate state (50).

Scheme 6. Synthetic route to TPE-containing hyperbranched polytriazole 39.

Chart 5. Molecular structures of hyperbranched polymer 40-42. Another topological structure of crosslinked network 43 was prepared via free radical polymerization of TPE containing diacrylates (Chart 6). By fine-tuning the molecular weight, 43 is surprisingly soluble in commonly used organic solvents. 43 is weakly luminescent in solution, which is probably because the RIR process is partially activated when TPE units are knitted into the network and easy to be affected by adjacent chains, which somehow differs from that of TPE incorporated linear and hyperbranched polymers with flexible spacers. Moreover, such flexible network could further restrict the aggregation of luminogens upon aggregation, making the network AEE-active (51). 36 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Chart 6. Molecular structure of TPE-containing crosslinked polymer 43. TPE could also be utilized as a unit to construct conjugated microporous polymers (CMPs) with extended π-conjugation, which have receive much interest for their unique applications in gas storage, light harvesting/emission, catalysis and sensing, etc. (52) To improve the microporosity, morphologies and dispersibility of CMP, 45 has been prepared by mini-emulsion polymerization of dibromo substituted TPE and 1,3,5-triethynyl benzene via Pd-catalyzed Sonogashira polycoupling. Solvothermal treatment of generated hyperbranched prepolymer 44 readily produced CMP 45 (Scheme 7). Although the intraconnection is inevitably present in 45, it could be dispersed in THF, making it feasible to investigate its photo-physical properties both in solution and aggregate states. From 44 to 45, the maximum emission wavelength is red-shifted from 500 to 520 nm, and the ΦF increases from 1.03 to 3.25%, respectively. These results demonstrate that the effective π-conjugation has enlarged but more restriction of rotatable phenyl rings has occurred after solvothermal treatment. Surprisingly, the ΦF of 45 in solid state is determined to be as high as 58.0% (53). Similar high value of 40% was also recorded in CMP film prepared form electrochemical polymerization of four N-carbazole decorated TPE derivatives (54). It seems hard to understand why such effective luminescence could be obtained even when TPE rotors located into hollow three-dimensional rigid scaffolds. Nevertheless, improved floating ability of π-electron in three-dimensional electronic cloud channels may cause a larger transition behavior and result in higher emission efficiency.

Scheme 7. Synthetic route to TPE-containing conjugated microporous polymer 45 from hyperbranched polymer 44. 37 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


3. Silole-Based AIE-Active Polymer Compared to TPE-containing AIE-active polymer, multi-phenyl substituted silole-based polymers are much rare, probably because silole derivatives are troublesome in preparation and purification, and unstable under basic condition. In early studies, multi-phenyl substituted siloles were synthesized by lithiation of diphenylacetylene, followed by the reaction between the intermediate of dilithiotetraphenylbutadiene and chloro-silicane (55). Thus, the substituents at 1,1-positions of silole derivatives were easy to be decorated to generate monomers suitable for polymerization. For example, polysilole 47 can be prepared by reduction of 1,1-dichloro-2,3,4,5-tetraphenylsilole 46 and lithium in THF (Scheme 8). Due to the large steric effect of main chain caused by direct linking of silicon atoms, 47 can only be obtained as an oligomer with a degree of polymerization of around 15. The highest PL intensity of 47 in THF/water mixture with 99% water fraction was recorded to be 18-fold stronger than that in pure THF, indicative of an AEE-activity. Furthermore, compared to the first reported AIEgen of 1-methyl-2,3,4,5-pentaphenylsilole (MPPS), a bathochromic shift of maximum emission about 20 nm was observed in 47, which is ascribed to a slight increase in σ-conjugation along the silicon-silicon main-chain (56, 57). However, another alternative copolymer of 48 (Chart 7) with spirobifluorene units linked to 1,1-position of silole units through carbon-carbon double bond centered its emission at near 500 nm in film, which is much similar to that of MPPS. These results indicate that extended conjugation of silole derivatives through 1,1-position may pose a subtle influence on the emission wavelength, while their AEE-activity could be kept (58).

Scheme 8. Synthetic route to silole-based polymer 47. Actually, it has been proved theoretically and experimentally that the photo-physical property of silole derivatives is mainly dominated by central silole ring and 2,5-substituted groups, but less affected by 1,1-substituted ones, due to the sp3 hybrid of silicon atom (59, 60). Thanks to the synthetic method of intramolecular reduction of diethynylsilane, the substituents at 2,5-position of silole derivatives can be readily tuned or decorated (61). Conjugated polymer 49 could be readily prepared by Sonogashira polycoupling of 2,5-diethynylsilole and 1,4-diiodobenzene derivative (Chart 7). Due to the conjugated effect, the adjacent alkynyl groups may cause partial restraints of rotation of phenyl rings and enhance the molecular luminescence in solution. Indeed, the ΦF values of 49 in solution and aggregate states were measured to be 8.0 and 12.3%, respectively, suggesting 49 is AEE-active (62). 38 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Chart 7. Molecular structures of silole-based polymers 48 and 49. Silole-based units could function as pendants to endow polymers with AIE effect. The silole-containing polymer 50 was readily synthesized by atom transfer radical polymerization (ATRP) of styrene and silole-containing vinyl monomers, in which α-(2-bromo-2-methylpropoyloxy)-PEO functionalizes as an initiator, and CuBr and N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA) serve as catalyst (Chart 8). 50 exhibited a concentration dependent emission behavior. Thanks to its contained PEG unit, no emission could be detected when 50 was at low concentration in aqueous solution, whereas the emission intensity was increased prominently at higher concentration by the formation of micelles. This result suggests that regulating the manner of luminescent polymer aggregation by rational molecular design could also make it AIE-active (63).

Chart 8. Molecular structure of silole-polymer 50. Silole derivative could also be attached to the rigid strand of polyacetylene main-chain through alkoxyl spacers (51, Chart 9). According to the experimental results of polyacetylene bearing TPE units as side groups, it is believed the AIE effect of silole-containing polymers will not be influenced by the rigid main-chain, since the existence of long flexible alkoxyl chains. However, the ΦF of aggregates of 51 formed in chloroform/methanol mixture with 90% methanol fraction was measured to be only 3.0%, though an increase of ΦF about 20-fold could still be obtained when compared to its solution state. It is probably due to the interaction between silole units and polyacetylene main-chains formed upon aggregation, which induced the quenching of emission from silole-based luminogens. 52 is 39 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


an analogue of 51, only with its alkoxy spacer removed. 52 gave a faint red emission at 652 nm, which was attributed to the inefficient radiative decay from polyacetylene backbone. However, with an addition of poor solvent of methanol into the system, only slight emission enhancement was observed, manifesting 52 is AIE-inactive. The direct attachment of silole moieties to the rigid polymer backbone can not pack well under aggregation, where the phenyl rotors can still rotate vigorously to decay excitons (64).

Chart 9. Molecular structures of silole-based polymers 51 and 52. 53 is a quasi-linear AIE-active polyethyleneimine (PEI) end-capped with silole units (Chart 10). The average grafting ratio of silole units is about 4.5, which is much less than conventional AIE-active polymers. Similar to 50, the amphipathy 53 could form nanoparticles in water with hydrophobic silole units as core and hydrophilic PEI chain as crown. The self-assemble structures induced a remarkable emission with ΦF of around 12% due to RIR effect. Further reaction of 53 with 2,3-dimethylmaleic anhydride (DMA) readily produced surface charge-switchable light-up functional nanoparticle, which could be used for targeted biological imaging and selective restraint of cancer cell (65).

Chart 10. Molecular structure of silole end-capped polymer 53. The silole units were also incorporated into hyperbranched polymers. TaCl5-Ph4Sn catalyzed polycyclotrimeriztion of 1,1-diethynyl silole and 1-hexyne readily produced hyperbranched poly(phenylenesilolene) of 54 (Chart 11). Contrary to the expectation, 54 is AIE-inactive although the AIE-active silole units were presented. Its emission intensity in chloroform/methanol mixture 40 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


was weakened gradually with progressive addition of methanol. This is because the 3D rigid scaffold of 54 can not induce a compact intermolecular and intramolecular aggregation, where plenty of free volume was left to provide enough space for free rotation of phenyl rings and for nonradiatively deactivating excitons. Interestingly, cooling-enhanced light emission is THF solution of 54 was observed, suggesting that the phenyl rings inside hyperbranched polymer in solution is rotatable at room temperature, and such motion is prone to freeze at lower temperature, therefore populating excitons decayed via radiative transition channel (66). As discussed above, the conjugated phenyl linkers in 54 have a negligible effect on the electronic property of silole unit, the abnormal luminescent behavior of 54 may be basically determined by the hindering effect of 2,5-position substituted phenyl rings against rotation.

Chart 11. Molecular structure of hyperbranched poly(phenylenesilolene) 54.

By altering ethynyl groups from the 1,1- to 2,5-positions of silole, hyperbranched poly(phenylenesilolene) 56 was prepared under similar polymerization conditions (Scheme 9). Unlike 54, each silole unit of 56 was knitted at its 2,5-positions,which make the whole polymer chain much less congested. Accordingly, the interior phenyl rings can partially rotate, and the polymer chains are easy to be compressed upon aggregation to activate RIR process. As a result, the ΦF of aggregates of 56 formed in THF/water mixture with 90% water fraction is 2.5-fold higher that that in THF, further confirming its AEE-activity, though such effect is weak as displayed in 49 (67). Therefore, both substitution position and polymer chain rigid play important roles in determining the AEE effect of silole-containing hyperbranched polymers.

41 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 9. Synthetic route to hyperbranched poly(phenylenesilolene) 56.

4. Other Typical AIEgen-Based Polymers Except for the most commonly used AIE cores of TPE and silole, other cores, such as distyrylanthracene (DSA), tetraphenylpyrrole, tetraphenylthiophene (TPT), and tetraphenylpyrazine (TPP), have been generated according to RIR mechanism. In principle, all of them could be incorporated into polymers to enables them AIE-active. The development of new types of AIE-active polymers will not only enrich AIE family, but also boost their high-tech applications. Therefore, such research is full of significance, but, still in its infant stage.

a. DSA-Based AIE-Active Polymers DSA is an AIE-active archetypal molecule with its emission centered at near 520 nm in aggregate state (68). The red-shifted maximum emission wavelength of DSA, in comparison to TPE and silole, makes it more favorably applicable in biological imaging. Thus, the construction of DSA-based AIE-active polymers is always associated with biological application. For example, polymer 57 was prepared by AIBN-initiated radical copolymerization of three vinyl monomers with one of them having DSA pendant groups (Chart 12). A very weak emission with ΦF of 0.08% was detected when 57 was dissolved in dimethyl sulphoxide (DMSO). After adding poor solvent of water into the DMSO solution, the emission sustained to enhance. The emission intensity of the aggregates of 57 formed in DMSO/water mixture with 98% water fraction was recorded to be 7.9%, which is 98-fold stronger than that in DMSO. Such remarkable AIE effect of 57 is much like its small molecule derivative, exhibiting a feasible strategy to design AIE-active polymer with DSA as pendants. 57 could be self-assembled into green fluorescent micelles and be used for cell imaging (69).



Chart 12. Molecular structure of DSA-based polymer 57. DSA unit could also serve as a joint to knot two polymer chains together (58, Chart 13). Different from 57, 58 is AEE-active. It is emissive in THF with a ΦF value of 9.3%, which was increased to 17.0% for the aggregates formed in THF/water mixture with 90% water fraction. The AEE effect of 58 is probably ascribed to the end-capped folic acid moieties, which are insoluble in THF, thus induce the polymer chains to form coil conformation, and restrict the rotation of the phenyl rings in DSA. Nevertheless, the introduction of folic acid units onto the polymer may improve its functionalities, especially in targeting cancer cells (70). It is apparent that the properties of attached functional groups as well as the structure of polymer can greatly influence the photo-physical behaviors of DSAcontaining polymers.

Chart 13. Molecular structure of DSA-based polymer 58.

b. 2,4,6-Triphenylpyridine-Based AIE-Active Polymers Besides siloles, other heterocycle-based AIEgens are much desirable owing to their unique electronic properties. A new heterocycle-based AIEgen, named 2,4,6-triphenylpyridine, has been developed recently. Much attention has been inclined to the study of its polymers. 2,4,6-Triphenylpyridine was first used to self-polymerize or copolymerize with fluorene units, and conjugated polymers of 59 and 60 were produced, respectively (Chart 14). The AIE property investigation showed that very weak fluorescent enhancement was observed both in 59 and 60, implying that 2,4,6-triphenylpyridine is not a powerful unit for the design of AIE-active conjugated polymers. Meanwhile, the extended conjugation could red-shift the emissions of polymers into blue-purple light region, which is much preferable to display application, in comparison to 2,4,6-triphenylpyridine archetypal molecule, whose maximum emission is located around 360 nm (71). 43 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Chart 14. Molecular structures of 2,4,6-triphenylpyridine-based polymers 59 and 60. Following question is how to endow 2,4,6-triphenylpyridine-containing polymers with much notable AIE effect? It seems the enhanced rigidity of polymer will deteriorate its AIE effect. Thus, 2,4,6-triphenylpyridine unit was grafted onto the flexible polystyrene as side chains to produce 61 (Chart 15). As 2,4,6-triphenylpyridine units were directly connected by methylene, the rotation of the phenyl rings is free in solution, which has efficiently dissipated the energy of excited state and make the polymer non-emissive, whereas, the flexible main-chain may favor them to pack tightly to induce much enhanced emission in the aggregate state. The ΦF values of 61 in solution and aggregate states were deduced to be 25 and 78%, respectively, proving that such strategy is accessible (72). Moreover, by utilizing diamino-substituted 2,4,6-triphenylpyridine as initiator, 2,4,6-triphenylpyridine unit knotted polymer polytyrosine 62 could be prepared through living ring-opening polymerization of L-tyrosine-N-carboxyanhydride (Chart 15). Similar to 61, the flexible “wing” of 62 could give rise to almost 1-fold of fluorescent enhancement from solution to aggregate states. It is noteworthy that maximum emission wavelength of 62 has red-shifted to around 500 nm compared to 2,4,6-triphenylpyridine, which is mainly derived from a intramolecular charge transfer from peripheral secondary amine to central pyridine ring (73).

Chart 15. Molecular structures of polymers 59 and 60 with 2,4,6-triphenylpyridine unit as side group or knot. 44 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


c. Tetraphenylthiophene (TPT)-Based AIE-Active Polymer Tetraphenylthiophene (TPT) is also a heterocycle-based AIEgen, whose structure resembles that of multi-phenyl substituted siloles, with the silicon atom replaced by sulfur atom. The subtle change of molecular structure, as a result, leads to a huge transition of emission wavelength. The emissions of TPT and silole derivatives are peaked around 400 and 490 nm, respectively (74). The strategy of utilizing TPT to design AIE-active polymers is similar to that of 2,4,6-triphenylpyridine. After grafting TPT unit onto polyethylene as side chains, the resultant polymer 63 exhibits a typical AEE characteristic with more than 8-fold of emission enhancement in aggregate against solution states (Chart 16) (75). Also, by using the method similar in the synthesis of 62, TPT end-capped poly(γ-benzyl-L-glutamate) 64 was obtained. However, only 1-fold fluorescent enhancement of 64 was observed, which is nearly as same as that of 62 (76).

Chart 16. Molecular structures of TPT-based AIE-active polymers 63 and 64.

d. Nitrilevinylphenothiazine-Based AIE-Active Polymers Nitrilevinylphenothiazine-based AIEgen usually emits an orange-yellow light peaked at near 580 nm due to the involvement of strong donor-acceptor groups. Lots of works have been done in designing nitrilevinylphenothiazine-based AIE-active polymers, and exploring their applications in biological imaging. A linear polymer of 65 was prepared by radical copolymerization of nitrilevinylphenothiazine-containing vinyl monomer and glycidyl methacrylate (GM), followed by ring-opening reaction of GM and glucosamine (GLU) (Chart 17). The produced polymer is amphiphilic with hanging nitrilevinylphenothiazine units and glycosyl groups as hydrophobic and hydrophilic moieties, respectively. Meanwhile, the soft alkyl main-chains may help tune the molecular conformation to self-assemble into nanoparticles with core-shell structure, where the lipophilic luminogen aggregates were inside. The ΦF of assembled nanoparticles is as high as 41%, which is rare among the present red-emissive self-assemble materials. As the fluorescent intensity of 65 in diluted solution is very low, its AIE effect is much remarkable, indicative of the practicability to cultivate AIE-active polymers based on nitrilevinylphenothiazine core (77). 45 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Chart 17. Molecular structures of nitrilevinylphenothiazine derivative-based polymers 65-67. Another linear polymer 66 was synthesized by ring-opening polymerization of amino group substituted nitrilevinylphenothiazine and 4,4′-oxydiphthalic anhydride as hydrophobic chain segment, which was then decorated with two PEO chains at terminals as hydrophilic segments. Different from 65, all the luminogenic units in 66 are located in the central chain segments instead of hanging as side groups. Such amphiphilic structure is very prevailing in construction of self-assemble materials. Thanks to RIR process, the formed nanoparticles showed strong red emission at 600 nm (78). In many cases, the nanoparticles self-assembled by aforementioned polymers are subject to disperse at new working conditions below the critical micelle concentration, which hampers their practical biomedical application. To avoid this negative effect, cross-linked amphiphilic polymer like 67, was proposed to make the self-assemblies much more stable. Using nitrilevinylphenothiazine units and PEI as hydrophobic and hydrophilic segments, respectively, 67 is prone to self-assemble into nanoparticles in aqueous solution. The formed nanoparticles give strong emission at 580 nm, with a ΦF of 40%, which is ascribed to the AIE effect of nitrilevinylphenothiazine units in the hydrophobic core. Such emission efficiency is similar with that of 65, demonstrating that the incorporation of nitrilevinylphenothiazine units into the polymer does not influence their aggregation behavior (79). e. Phenyl-Substituted Quinolone-Based AIE-Active Polymer An interesting strategy for designing AIE-active polymer is to postfunctionalize a non-emissive polymer with another non- or weakly luminescent molecules. For instance, both poly(4-acety styrene) 68 and 2-aminobenzophenone 69 are not typical fluorophores. However, after condensation reaction, 70 became AEE-active due to the new generated luminescent phenyl-substituted quinolone 46 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


derivative (Scheme 10). The ΦF of 70 in THF and THF/water (1:9 v/v) mixture were recorded to be 2.1 and 3.7%, respectively. Moreover, by complexing bulky camphorsulfonic acid (CSA) with the quinoline unit of 70, the AEE effect of 71 has almost no change, but the emission efficiencies both in solution and aggregate states increase remarkably, and their ΦF values reached 11.6 and 38.1%, respectively. The hampered intramolecular rotation caused by CSA moiety is the reason for the emission enhancement. Moreover, 71 emits a blue-green light peaked at 500 nm, which has a bathochromic shift about 100 nm compared to that of 70. This phenomenon could be ascribed to the narrowed energy gap manipulated by the lowed LUMO energy level after protonation of quinoline ring (80).

Scheme 10. Synthetic route to phenyl-substituted quinolone-based AIE-active polymer 71.

f. Boron Ketoiminate or Boron Diiminate-Based AIE-Active Polymers Recently, much attention has been focused on organoboron-based AIEgens due to their high emission efficiency in aggregate state and unique electronic property (81). Besides small molecules, AIE-active organoboron-based polymers have also been investigated. For examples, 72-75 are alternative copolymers comprising of boron ketoiminate units and fluorine units. The difference is the R groups connected on the phenyl rings (Chart 18), which make the maximum emission wavelengths of their nanoparticles change from 521 nm to 661 nm. All the polymers here emit faintly with their ΦF < 1% in solutions. However, the values increase notably to be 7-39% in their aggregate states (82). It is worth noting that the enhancement of electron donating ability of substituents of the polymers could not only red-shift their emission, but also lessen their emission efficiency in aggregate states, which is derived from the less electron cloud overlap between ground and excited states of the repeating units (83). Other organoboron-based conjugated polymers 76-78 are different in their backbones (Chart 18). The photoluminescence (PL) measurement showed that the ΦF values of 76-78 in solutions are below 1%, and increase remarkably to 6-14% in their aggregate states, which is similar to that of 74. It is thus concluded that changing the substituents on the phenyl rings connected to the nitrogen atoms, instead of the aromatic repeating units, is a much effective approach to fine-tune the emission behavior of boron ketoiminate unit-based AIE-active polymers (84).



Chart 18. Molecular structures of boron ketoiminate-based conjugated polymers 72-78. By replacing the oxygen with nitrogen atom in phenyl substituted boron ketoiminate moiety, new boron diiminate-based polymers 79-86 were obtained, which featured AIE characteristics (Chart 19) (85). Similar with 72-75, by changing the electronic properties of functional groups in phenyl rings attached to nitrogen atoms, the maximum emission wavelengths of 79-86 varied from 509 nm to 628 nm. The ΦF values of 79-83 in films are in the range of 2-11%, which are much higher than that of their solution states (≤ 1%) (86, 87).

Chart 19. Molecular structures of boron diiminate-based conjugated polymers 79-86.

5. Luminescent Polymer Containing Unconventional Fluorophore It is a general understanding that the emission of polymers is stemmed from their containing fluorophores, otherwise, the polymers are non-emissive. This understanding is, however, not a general dogma. The new luminescent polymers containing no conventional fluorophores are also emissive as reported in recent years, though their mechanism is still under debate. 48 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


An example of such polymer is glacodynamer 87, which was prepared from polycondensation of bishydrazide and oligosaccharide-containing dialdehyde (Chart 20). Both monomers show no fluorescence, however, the yielded polymer displayed remarkable emission at 457 nm in water. The unexpected fluorescent property of 87 is probably due to the tightly packed structure of polymer chains while the inner aromatic chromophores is isolated and inflexibly held in the hydrophobic core. The reason why a deep blue light could be observed by a less conjugated system was not given, but suppression of motion of polymer chains plays a crucial role in enhancing the emission. When the polymer solution heated from 23 to 85 °C, the emission was quenched notably, suggesting that RIR may be one of the factors that induce the polymer to emit. In addition, the dynamic character of these glycodynamers was revealed through covalent exchange reactions of a monomeric component by another, which gave rise to the regulation of the luminescent properties of glycodynamers (88, 89).

Chart 20. Molecular structure of glycodynamer 87.

It is occasionally found that poly(N-isopropyl acrylamide) (PNIPAM, 90) prepared from addition-fragmentation chain transfer (RAFT) polymerization of N-isopropyl acrylamide mediated by the initiator of 89 (Scheme 11), gave a strong fluorescence at near 410 nm in solution. Since no conventional fluorescent species are involved in the polymer, this newfangled emission phenomenon is thus much interesting. After plenty of experimental investigations and theoretical calculation, it is reported that an electron transition between ground state located in phenyl ring and excited state located in benzene ring and adjacent carbonyl group with π-π interaction contributes to the emission. The fluorescence is intensified with the increase of polymerization time, as the longer polymer may form more compact coiled nanostructures, in which the π-π interaction could be 49 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


much stabilized and effectively isolated, and more motions of such emissive π-π interaction- distortion, collision or rotation could be restrained. More importantly, it has been a general strategy to design fluorescent polymers with phenyl unit containing RAFT agent, since the fluorescent 91 and 92 could also be obtained readily by polymerization of methyl acrylate and N,N-dimethylacrylamide, respectively (90).

Scheme 11. Synthetic routes to polymers 90-92. In above two examples, no extended conjugated system is involved and responsible for the visible luminescence, but the phenyl rings undoubtedly participated for the emission. However, some systems even contain no aromatic moieties, but still emit intensely. That is an important and interesting issue that should be clarified. Poly(amido amine) (PAMAM), one of the earliest developed emissive polymers containing no aromatic groups, is extremely worthy being discussed here. It is first reported that Au8 nanoclusters encapsulated on PAMAM dendrimers can induce an obvious emission with peaks at 450 nm. It is believed that the emission is stemmed from the Au8 nanoparticles (91), but this conclusion was ruled out when similar PL behavior was observed without addition of any Au species. Furthermore, simple treatment of OH-terminated PAMAM dendrimers with oxidant of (NH4)2S2O8 (PS) in methanol/water mixture can generate emission, which probably results from the produced blue-emissive chemical species after oxidation reaction of peripheral hydroxyl groups. Similarly, Au3+ is also apt to oxidize OH-terminated dendrimer in the preparation of encapsulated nanoparticles to make it emissive. It was concluded from above experiments that the hydroxyl end groups instead of backbone of PAMAM dendrimer play a vital role in forming the luminescent centers although the underneath mechanism is to be explored (92). Subsequently, besides OH-terminated PAMAM dendrimer, it was reported that NH2 and carboxylate-terminated PAMAM showed similar PL behaviors at pH = 6. For OH-terminated PAMAM dendrimer, increase of the generations from G2 to G4 remarkably enhanced the emission due to the crowed peripheral functional groups, which induces the dendrimers to form a densely packed globular structure. The PL intensity of PAMAM dendrimer (e.g., NH2-G4) could further be intensified when the pH value of solution decreased, and reach maximum at pH ≈ 2.5. According to the reports, several factors could be supposed to explain the behavior: (a) the incorporated tertiary amine groups will be protonated at low pH, while dendritic chains with charge repulse each other, therefore making the structure of PAMAM dendrimer much more rigid; (b) much effective hydrogen bonds could be formed under acid conditions in the dendrimer; 50 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


(c) some emissive species were generated by the reaction of functional groups in polymer chains mediated by acid. It was thus believed that backbone of the dendrimer was crucial for enhancing the emission when hyperbranched PAMAM is subject to acidification (93). To gain deeper insight into the emissive source of PAMAM, triethylamine (TEA) is selected as control for PL studies because it is a typical amino-branching unit inside PAMAM dendrimers. After treating TEA and G4-PAMAM dendrimer with oxidant of ammonium persulfate (APS) in aqueous solution for 3 days, both resulting small molecule and polymer exhibit similar emission with peaks at near 450 nm, manifesting that the tertiary amino-branching points inside the dendrimer are indispensable for emission. Nevertheless, the ΦF of APS-treated TEA and PAMAM are recorded to be 0.25 and 52%, respectively, while each tertiary amino unit of latter was deduced to contribute the luminescent efficiency of 0.84%. This result means 2.4-fold of emission enhancement was occurred when tertiary aminobranching points were incorporated into a dendrimer with more generations. In other words, with the increase of dendrimer generations, tertiary amines located much more compactly into the polymer interior, while the emissive centers are concentrated and restrained in a confined dendrimer “pocket”, thus inducing an enhanced emission due to the lack of rational and translational freedom. It was also proposed a physical interaction (or called as “exciplex”) generated between a tertiary amino unit and a doped oxygen molecule or the formation of a peroxyl radical from two partners may cause intrinsic emission of tertiary amino moieties in blue light region (94). Very recently, the emission behavior of PAMAM has been directly related to the AIE. Linear (l) and hyperbranched (hb) PAMAMs (95 and 96) were prepared by Michael-type polycondensation of acrylamide 93 and amino-substituted piperazine 94 in DMF and water, respectively (Scheme 12). The dilute aqueous solutions of 95 and 96 were nearly non-emissive probably because no oxidation process was involved according to the previous explanation. However, after adding a large amount of poor solvent of acetone into its solution, the emission boosts greatly with peak at around 450 nm, indicative of a typical AIE behavior. The following question is why a significant emission could be observed in such system since no conventional fluorophores was involved and no oxidation or protonation processes was occurred. Theoretical calculation revealed that the lone-pair electron and delocalized π electron dedicated by nitrogen atom and carbonyl group, respectively, could generate a variety of intra- and interchain clusters, where conjugated systems are enlarged though n-π and π-π interactions, which responded to the emission in the aggregate state. Obviously, the emission behavior of PAMAMs here is different from previous ones: (a) no oxidizing agent and acid participated in enhancing the emission; (b) both l- and hb-PAMAM are highly emissive; (c) the emissive wavelength of PAMAMs could be fine-tuned in the range of 463-570 nm by altering the excitation wavelength from 380 to 530 nm, probably due to the various emissive centers determined by different conjugation extent of clusters (95). Another interesting finding is that the intra- and inter-molecular aggregation of carbonyl groups in silicon-containing PAMAMs is an exclusive cause for their luminescence. Si-PAMAMs 97 with generations from G0 to G2 were prepared by 51 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


aza-Micheal and amidation reactions (Scheme 13). Similar to above descriptions, all Si-PAMAM dendrimers gave strong blue emission at around 435 nm in methanol without treatment of any oxidant. The 29Si NMR spectra measurements demonstrated that N→Si coordination bonds existed in Si-PAMAM dendrimers, which facilitate the carbonyl groups to pack together. The emission intensities of polymers enhance promptly as the generations are increased due to more carbonyl groups are involved and crowed conformations. It is worth noting that these polymers possess the unique property of AEE feature. When addition of water into their THF solution, the carbonyl groups tend to aggregate more tightly, and the extended conjugation of cluster induced a red-shifted emission. At the same time, the aggregation well suppressed the rotational or vibrational relaxation of clusters, thereby caused an enhanced emission (31).

Scheme 12. Synthetic routes and cartoon structures of PAMAMs 95 and 96.

The emission mechanism of PAMAMs seems to be complicated. Similar situation is encountering in poly(amino ester)s (PAEs, 100-103) with various terminals, which were synthesized by Michael addition reactions (Scheme 14). The emission of PAEs in aqueous solution may be ascribed to the combination of acidification and protonation of polymer chain. It was explained as the compacted spatial morphologies of polymers become very open at acid atmosphere, and oxygen molecules are easy to approach to the interior tertiary amine branching points. However, such rule does not hold if the PAE, such as 101, is prone to hydrolyze at low pH. The photo-physical properties of 100-103 can not be affected significantly by varying the end groups at neutral pH, and a linear PAE with similar compositions does not show emission at all. These results indicate that a coexistence of tertiary amines/carbonyl moieties in the compacted dendritic architecture could be a key factor for displaying blue emission, and oxidation dose not play an indispensable role in this process. While, reasonable oxidation of hyperbranched PAE may act positively in enhancing emission (32). In another work of sulfur-containing hyperbranched PAE, it was reported that the emission was strengthened with increase of polymerization time owing to the decreased interior mobility of tertiary amine units. Surprisingly, a switching of emission wavelength from 440 to 560 nm was observed after oxidation. Moreover, the emission at 560 nm was enhanced remarkably with prolonging the oxidation time, which was confirmed to stem from the formed ≡N→O groups as proved both by NMR and FT-IR analyses (96). 52 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 13. Synthetic routes to silicon-containing PAMAMs 97. The unique emission behaviors of hyperbranched PAMAM and PAE were also reported in poly(propyl ether imine) (PETIM) in despite of their different structures. When excited at 330 nm, the polymer is emissive with a peak at 390 nm. Furthermore, increasing the generation, oxidation and protonation of PETIM could also remarkably enhance its emission (97). 53 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 14. Synthetic routes and cartoon structures of PAE 100-103.

Interestingly, sole nitrogen-containing polymer of poly(ethyleneimine) (PEI) was also reported to be strongly emissive. The photo-physical properties of 107G0 to 107-G3 (Scheme 15) are also similar to that of PAMAM and PAE. It is undisputed that the amine groups in PEI are the source for generating fluorescence. Increasing the generations of PEI is favorable for emission because the crowed conformation will decrease the mobility of amine moieties. In addition, a notable emission enhancement of PEI after oxidation is directly attributed to a complex formed between amino groups and oxygen as proved by comparative elemental analysis of untreated and treated products (98). Aforementioned emissive polymers containing no conventional chromophores must be designed with much congested dendritic/hyperbranched structure. This strategy is, however, not a general rule. For example, poly(N-vinylpyrrolidone) (PVP, 108) is highly emissive with maximum wavelength at around 380 nm in aqueous solution after polymerization, while its monomer of N-vinyl-2-pyrrolidone (NVP) only shows a very faint emission. Through structural and spectra investigation, it was concluded that the pyrrolidone ring in PVP could be hydrolyzed to generate a product with a secondary amine and a carboxylate spaced by propylidene, in which the stable N→O compound is readily formed between the secondary amine and oxygen molecule (Scheme 16). The resulted secondary amine oxide moiety was regarded as the new luminogens responsible for emission. The PL of PVP and its oxidized hydrolyzate showed stimuli response to metal ions and acid/base, and very possibly some other chemicals (99). 54 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 15. Synthetic routes to PEI 107.

Scheme 16. The oxidized hydrolyzate processes of PVP 108.

As discussed above, in PAMAMs (such as Si-PAMAMs 97), the aggregation of interior carbonyl groups is an intrinsic cause for the emission and AIE process. Such concept has been accepted in the studies of a series of linear polymers containing succinic anhydride (SA) groups. For instance, SA end-capped polyisobutene (PIBSA, 112) (Chart 21) is non-emissive in dilute solution, but exhibits a strong blue-green emission in concentrated heptane solution or viscous fluid, indicating that it is AIE-active. In heptane, the aggregation of carbonyl groups is formed as discerned by UV-vis spectra, where their vibro-rotational motions are restricted and thus increases the radiative recombination of excitons. Moreover, increase the number of SA functional groups in polymer chains may induce more effective aggregation in PIBSA, and thus pose a much significant AIE effect (100). 55 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Chart 21. Molecular structure of SA end-capped PIB 112 and its proposed aggregation mode.

Similarly, it has reported that the colloidal nanoparticles of poly[(maleic anhydride)-alt-(vinyl acetate)] (PMV, 113) emits intensely at 419 nm, whereas, only weak emission was observed in solution (101). Very recently, such phenomenon of PMV was expatiated in detail. To address this issue, poly(maleric anhydride) (PMAh, 114) and poly(vinyl acetate) (PVAc, 115) were prepared for comparison (Chart 22). The experiments showed that PMAh dissolved in THF gave remarkable blue emission centered at 390 nm, whereas, no fluorescence was observed for both PVAc and maleric anhydride (MAh). It is because the bulky anhydride groups of PMAh make the molecular chain much rigid, which in turn favors the formation of heterodox clusters via collection of plenty of carbonyl groups, thereby inducing intense emission. Hence, it is believed the emission behavior of PMAh is closely related to the carbonyl clusters. PMV also possesses intriguing solvatochromism behavior. It appears colorless and gives a blue emission in aromatic and oxygenic solvents of THF, acetone, toluene and dioxane, whereas, it is magenta and displays a redder emission in polar solvents of N-methyl-2-pyrrolidone (NMP), DMSO and dimethylformamide (DMF) etc., indicating that certain interactions between polymer and solvent molecules have taken place. For comparison, polymers 115 and 116 were selected. It was concluded that the vinyl acetate (VAc) moieties in PMV is crucial for its solvatochromic process, and MAh cluster is the origin of emission of PMV (33). 56 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Chart 22. Molecular structures of PMV 113 and other polymers 114-117 for control.

6. Conclusion AIE-active polymers could be designed readily by incorporation of typical AIE-active units, especially of TPE, silole, DSA and boron ketoiminate moieties, into the polymer chains. Generally, introduction of flexible alkyl into polymer main-chains or branching chains may help to keep the AIE effect of resultant products. In conjugated polymers, it is feasible to realize longer emissive wavelength through enlarging effective conjugation or reasonable construction of donor-acceptor structures. Nevertheless, as the conjugation extended, the polymer rigidity increased, thereby making them AEE instead of AIE active. We also summarized the unique AIE-activity of some polymers without typical fluorophores. In most systems, the incorporated nitrogen atom or carbonyl group with lone-pair electrons play a vital role in determining the emission. In the former, tertiary amine groups existed in PAMAM, PAE, PETIM and PEI dendrimers are responsible for the emission, while protonation or oxidation of nitrogen atom may notably strengthen the fluorescence. Increase the generations of dendrimers also acts positively. The emission of these polymers is mainly related to increased rigidity of branching chains or congested environment of tertiary amine, which reduce the rotational and vibrational relaxation of luminescent species. Whereas, in the latter, the formed clusters of carbonyl groups in the aggregate state, mostly in PMV, dominate the emission because of restriction of molecular motion of groups. Though the photo-physical phenomenon of these polymers is quite interesting and some mechanisms were proposed, the underneath mechanism is to be further explored, which will promote the development and application of these systems. It is worth noting that the AIE-active polymers possess quite a few advantages over AIE-active low molecular weight luminogens. For instance, there are numerous possibilities to fine-tune the molecular structure, topology, and morphology as well as functionalities of the polymers. Moreover, the excellent film-forming ability of the AIE-active polymers could facilitate the fabrication large-area films via convenient and simple processes like spin-coating 57 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


or static-casting. In addition, the covalently bonding of the repeating units in polymers often endows the resulting materials with decent mechanical strengths. However, the emission efficiency of AIE-active polymers is generally not as high as AIE-active low molecular weight luminogens mostly because the metal catalyst residues could not be completely removed, which might act as quenchers. What’s more, the property and application of AIE-active polymers should be further explored. Thus, to design AIE-active polymer with very effective emission and to explore their property and applications are still challengeable and will receive keen interest in future research. We hope that with pilot of this chapter, more AIE-active polymers with desired structures and properties could be designed rationally and intelligently, to enrich or embellish the applications in the areas of polymer light-emitting diodes (PLED), chemical and biological probes, biological imaging, porous material, stimuli-responsive material, and so on.

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