Aggregation-induced emission polymer systems with circularly polarized luminescence

2.1 CPL-active polymers with AIEgen in main chains

As shown in Scheme 1, the construction of CPL-active polymers with AIEgen in main chains can be achieved by polymerizing chiral monomers and AIEgen-containing monomers. The chiral moieties could locate in polymer backbones or as pendants in side chains. Numerous functional polymers with good fluorescence and chiral optical properties have been developed based on this strategy.^[52-56] For instance, our group has synthesized a kind of conjugated poly(enaminone)s with high molecular weights through a one-pot three-component sequential polymerization method of alkyne, carbonyl chloride, and amino ester salt. The introduction of optically active chiral amino ester as a side group endowed the polymer products with helical rotating backbones, and the incorporation of tetraphenylethene (TPE) moieties in the polymer main chain allowed the polymer to show obvious AIE behavior, eventually making the resulting polymers exhibit excellent CD and fluorescence signals in aggregate states.^[54]

In the past years, Wang et al. have developed a series of CPL-active AIE polymers in which both the AIEgen and chiral moieties were placed in polymer backbones.^{[57, 58]} In 2018, they synthesized one CPL-active AIE polymer based on the one-step polymerization reaction of chiral 1,1′-binaphthyl-containing diene monomer and TPE-containing difunctional azide monomer.^[59] As shown in Figure 1A, a pair of (S)-/(R)-P1 polymer enantiomers were obtained by this click polymerization. As evidenced by the spectra shown in Figure 1B,C, the produced polymer P1 not only exhibited typical AIE properties with blue aggregate-state fluorescence but also showed strong aggregation-induced circularly polarized luminescence (AICPL). The underlying mechanism for the AICPL response of (S)-/(R)-P1 could be explained by the different self-assembly morphologies in the dispersed and aggregate states. As depicted in Figure 1D, when the water content in THF/water mixtures was increased from 40% to 90%, the self-assembly morphology obviously changed from sphere nanoparticles to left-handed helical nanowires, thus leading to a stronger CPL signal. The g_lum of the nanoaggregates and film of (S)-/(R)-P1 were measured to be up to 6.97 × 10⁻³ and 1.45 × 10⁻², respectively. Furthermore, (S)-/(R)-P1 self-assembled the same left-handed helical nanowires. The formation of the same left-handed helical nanowires could be attributed to the cooperative effects of the π–π interactions influenced by binaphthyl chirality and the presence of multiple weak intermolecular hydrogen bonding as well as the steric effect.

In addition to CPL-active AIE polymers with blue fluorescence, the same research group also developed AIE covalent polymers with deep red AICPL.^[57] As depicted in Figure 1E, a four-component chiral polymer was synthesized via a one-pot and two-step method based on the Pd-catalyzed Sonogashira reaction and Suzuki reaction of monomers incorporating the (R)-/(S)-1,1′-binaphthyl, 4,7-di(thiophen-2-yl)-2,1,3-benzothiadiazole (DTBT), TPE, and fluorene moieties. The molar ratio between the DTBT unit and TPE unit in the obtained polymer (P2) was determined to be 1:19. In this conjugated polymer, the binaphthyl component acts as the chiral source, while the TPE component serves as the AIE-active group and the Forster resonance energy transfer (FRET) relay converter. The DTBT and fluorene components function as the first FRET donor and the second FRET acceptor. It was anticipated that this chiral polymer can undergo two FRET processes with fluorene-TPE as the first FRET pair and TPE-DTBT as the second FRET pair to eventually exhibit deep-red CPL. Indeed, polymer P2 exhibited obvious chirality transfer process from the chiral binaphthyl moiety to the conjugated polymer main chain through the acetylenic bond, and the occurrence of two successive FRET processes was clearly demonstrated due to the appropriate spectra overlapping between the donor and acceptors (Figure 1F). The PL spectrum of the model polymer without DTBT chromophore further verified the occurrence of the FRET process from TPE moiety to DTBT moiety. Benefiting from the twisted polymer backbone and the successive FRET mechanism, P2 showed obvious AIE behavior with bright red fluorescence in the aggregate state. Moreover, the CPL intensity of (S)-P2 and (R)-P2 was also greatly enhanced upon aggregation, showing an AICPL emission signal in the deep red region (Figure 1G). The g_lum of AICPL can reach up to ±2.0 × 10⁻³ in aqueous mixtures with high water contents.

Regarding the CPL-active AIE polymers in which the AIEgen locates in main chains, but the chiral moieties locates as pendants in side chains, the reported design strategies were more diversified. For instance, Liu et al. synthesized a linear polymer (P3) with TPE in polymer backbone and a chiral L-alanine derivative as pendants by click polymerization.^[55] As depicted in Figure 2A, P3 can self-assemble into various morphologies ranging from vesicles, helical nanofibers to microfibers by regulating the water contents and polymer concentration in THF/water mixtures. The formation and transition of the self-assembly processes were clearly characterized by SEM (Figure 2B1‒E1). Taking advantage of the efficient aggregate-state fluorescence of this AIE-active chiral polymer, its self-assembly morphologies can also be observed by a fluorescence microscope (Figure 2B2‒E2), and the microscopic morphology and chirality of P3 can be readily correlated with the visual fluorescence intensity. The rational coexistence of AIEgen and chiral moieties further endowed P3 with obvious aggregation-induced CD and AICPL behaviors. Later on, Liu et al. further designed a series of CPL-active polymers carrying TPE-containing backbones and chiral L-phenylalanine pendants in 2020.^[60] These polymers also showed excellent AIE and AICPL responses and can form helical assemblies with the increase of water contents due to the intermolecular interactions. The obtained CPL-active AIE conjugated polymers were demonstrated to have great potential for applications in miniature optoelectronic devices. Besides, Liu and coworkers have also constructed CPL-active covalent polymers with TPE units in backbones and chiral moieties in pendants from the one-step Sonogashira cross-coupling polymerization of 1,2-bis(4-ethynylphenyl)-1,2-diphenylethene and 3′,5′-diiodo-N-α-tert-butoxycarbonyl-O-octyl-L-tyrosine methyl ester.^[61] The g_lum values of the obtained CPL-active AIE polymer can be remarkably regulated from 0.44 to 0.08 by increasing the water content in THF/water mixtures because the g_lum value of the polymer might be mainly related to the electronic interaction of the isolated molecules and it was beneficial for the higher g_lum values in THF solution.

Apart from the aforementioned CPL-active AIE homopolymers with chiral moieties closely attached to the AIEgen-containing main chains, Hu et al. designed two conjugated block copolymers (P4 and P5) with electron-donating poly(carbazole-ran-acridine) backbones, electron-withdrawing dibenzothiophen-2-yl(phenyl)methanone (DBT-PM) substituent, and chiral alanine moieties at the end of side chains (Figure 3A).^[62] Due to the presence of DBT-PM and the large steric hinderance of side chains, the obtained two polymers with different molar ratios of monomer units exhibited excellent aggregation-induced delayed fluorescence (AIDF) characteristics. Both the decay lifetime and the corresponding ratio of delayed fluorescence of P4 and P5 were increased after adding water as shown in Figure 3B,C. Moreover, obvious CPL signals were observed at around 550 nm in the film state of P4 and P5 (Figure 3D), and the corresponding g_lum values were determined to be −2.01 × 10⁻³ and −1.39 × 10⁻³, respectively. The good chiroptical properties of these two polymers indicated that the chirality successfully transferred from the alanine pendants in side chains to the conjugated polymer main chain, and the chirality of the polymers could be efficiently triggered in the excited state. Based on the AIDF and CPL properties, polymers P4 and P5 were further employed as solution-processed emitting layers for the fabrication of organic light-emitting diodes (OLEDs). Based on the device structure shown in Figure 3E,F, both nondoped and doped OLEDs were fabricated. The constructed OLEDs emitted green electroluminescence with maximum brightness of 1477 cd/m² and maximum current efficiency of 2.52 cd/A. The work provides an effective design strategy to achieve AIDF polymers with CPL activities.

2.2 CPL-active polymers with AIEgen in side chains

CPL-active polymers can also be constructed through the introduction of AIEgen in side chains of polymers, which could avoid the design and synthesis of difunctional AIEgen-containing monomers. The chirality of the obtained AIE polymers could arise from polymer main chains or from the chiral moieties in side chains (Scheme 2).

Optically active poly(isocyanide)s with single-handed helical conformation have attracted extensive attention due to their important applications in molecular recognition, photoconductivity, liquid crystal, asymmetric synthesis, and so forth. In the past years, diverse functional luminescent poly(aryl isocyanide)s (PAIs) have been synthesized and their chiroptical properties have been carefully investigated.^{[63, 64]} For instance, Yan et al.’s research team and Wu et al. have synthesized a series of TPE-containing PAIs under different polymerization conditions.^[65-67] In these optically active AIE polymers, the TPE unit was attached in the side chains and the chirality stemmed from the helical characteristics of the PAI backbones. Diverse types of catalysts were demonstrated to be effective for the homopolymerizations of TPE-containing monoisocyanide monomers, including a series of half-sandwich rare-earth dialkyl complexes^[66] as well as different transition metal catalysts such as AlⁱBu³ and Pd-based complexes.^{[65, 68]} Considering the negative effects of metal residues for the stability and materials performance of PAIs, metal-free initiators have also been adopted for the synthesis of functional fluorescent PAIs. As shown in Figure 4A, Yan et al. have synthesized several TPE-containing PAIs under the catalysis of commercially available, metal-free, and highly efficient [Ph₃C][B(C₆F₅)₄].^[67] The homopolymerization of TPE-containing isocyanide produced P6, while the copolymerization of TPE-containing isocyanide and isocyanide substituted with chiral ester groups (D/L-IMCI) afforded D/L-P7. Unlike typical AIE polymers, the PL intensity of P6 and P7 first increased as the water content rose and then fell down (Figure 4B), which might be related to the formation of aggregate states with looser packing at high water contents. The CD spectra of D/L-P7 showed obvious mirror symmetry with the positive and negative CD signals at 364 nm (Figure 4C), demonstrating that they had good single-handed helical conformations and had great potential in developing CPL signals.

Recently, He et al. synthesized a linear PAI (P8) and a four-armed PAI (P9) that were functionalized with TPE pendants. These two conjugated polymers were prepared by the living polymerizations of the same TPE-functionalized phenyl isocyanide monomer but in the presence of different TPE-based Pd(II) catalysts (Figure 4D).^[68] The resulting PAIs showed typical AIE behaviors (Figure 4E), and their fluorescence properties can be facilely tuned by varying the concentration or molecular weights of the polymers. Moreover, different self-morphologies were observed at different conditions. As shown in Figure 4F (left), the linear polymer P8 indicated uniformly distributed spherical nanostructures at low molecular weight with a degree of polymerization (DP) of 14. When the DP value changed from 14 to 65, stable and well-defined helical nanofibrils were formed (Figure 4F, right), indicating that the molecular weights of polymers played a crucial role on the morphology properties and the subsequent chiral optical properties. By contrast, the four-armed polymer P9 can self-assemble into an apparent bumpy “caterpillar”-like assembly at high concentrations (Figure 4G, right), which was very different from the morphology obtained at a low polymer concentration (Figure 4G, left). These main-chain helical PAIs with efficient aggregate-state fluorescence are promising to be further designed and developed into optical materials with good CPL properties.

The construction of CPL-active polymers with both the AIEgen and chiral moieties in side chains has less limitation for the polymerization types. For instance, recently Lu et al. synthesized chiral fluorescenthelical polyacetylenes based on the copolymerizations of chiral-substituted acetylenic monomers and an achiral acetylenic monomer with TPE pendant.^[69] As shown in Figure 5A−C, the obtained copolymers with chiral side chains and AIEgen-containing side chains showed excellent AIE performance and chiroptical properties. Obvious CD signals were detected at high water fractions from 85% to 95%, but the CD intensities were gradually decreased with the increase of water content. The aggregation-attenuated CD effect might be due to the disruption of the predominantly single-handed helix of the polymer backbone by the strong polarity of water to some extent. No CPL emission was observed in copolymer dispersions. However, the CPL activity of P10 in composite film state was greatly amplified due to the more ordered arrangement of polymer chains in the solid films. As depicted in Figure 5D, by doping a small amount of P10 into the poly(methyl methacrylate) matrix, the obtained fluorescent composite films exhibited strong positive CPL signals, with the maximum g_lum value of ±3.6 × 10⁻². Besides, strong CPL signals can be achieved by simply blending a chiral molecule with the fluorescent polymer (Figure 5E). Different from the common chirality transfer mechanism for CPL-active materials, the underlying CPL generation mechanism of P10 in the composite films was attributed to the “matching rule” between fluorescent moieties and chiral helical polyacetylenes. As illustrated in Figure 5F, no efficient chirality transfer occurred from the helical main chain to the fluorescent side chains, thus the copolymers showed no CPL emission in a solution and dispersed state. When the copolymer was transformed to the film state, ordered arrangement or self-assembly was initiated to endow the composite films with strong Cotton effect in the emission region of P10. The overlapping between the CD and PL spectra at such conditions made P10 show remarkable CPL activities. Through rational structural design to synergistically utilize the chirality transfer and matching rule, the CPL performance could be further amplified. This work provides new perspectives for the design of CPL materials with g_lum values in the solid state.

The commonly used olefin polymerizations are also suitable for the design and synthesis of side chain-type CPL-active AIE polymers. For example, Luo et al. recently reported the highly efficient CPL from a kind of chiral luminescent liquid crystalline polymer with AIE properties.^[70] This polymer was synthesized by the radial polymerization of a methacrylate monomer carrying both TPE moiety and the chiral 4-cholesteryl ester (MT-Chol) group in pendants. As illustrated in Figure 6A, the obtained AIE-active chiral liquid crystalline polymer (PT-Chol) was demonstrated to possess a layered structure. Although the pure polymer did not show CPL behavior in the solid state, the blending mixture of the polymer with a specified amount of 4-cyano-4′-pentyl biphenyl (5CB) exhibited highly efficient CPL. This phenomenon could be explained by the formation of a chiral nematic liquid crystal or smectic C* phase in the mixture of the polymer and 5CB at low doping concentrations. The polarized light microscopic images shown in Figure 6B suggest that blending concentration had a critical effect on the phase structure, and an obvious fingerprint texture was found in the polymer-5CB mixture at low concentrations of PT-Chol. Both the TPE-containing monomer (MT-Chol) and the polymer (PT-Chol) showed enhanced PL intensity with the increase of their concentration in 5CB, and their chiroptical behaviors showed an interesting supramolecular structure-dependent effect. As shown in Figure 6C, with the increase in the content of MT-Chol in 5CB, the absorption dissymmetry factor (g_abs) of the resulting mixture first dramatically increased and then gradually decreased. The maximum g_abs value of −9 × 10⁻³ was collected at 2.5% MT-Chol content due to the formation of the fingerprint texture at this composition. Similar tendency was also observed for the g_abs values of polymer-5CB mixtures (Figure 6D). Nevertheless, the blending mixtures of MT-Chol and 4CB cannot induce CPL behavior, but obvious CPL signals can be detected from the polymer-5CB mixtures, with the PT-Chol content of higher than 20% (Figure 6E). And the highest g_lum value of +0.45 was achieved at 30% PT-Chol content (Figure 6F). These results suggested that the CPL property was derived from the polymer structure, and the formation of PT-Chol aggregates in this liquid crystal hybrid system can simultaneously enhance the luminescence efficiency and CPL performance. The presence of 5CB molecules acted as plasticizers to facilitate the helical ordered packing of the polymer chains. The obtained CPL-active composite material showed excellent stability, which was conducive to applications in optoelectronic devices.

2.3 CPL-active polymers with clusterization-triggered emission

AIE materials generally possess rigid and conjugated aromatic structures, extended π-conjugated systems, or unsaturated heterocyclic systems.^[30] However, a large number of non-conjugated molecules or polymers with abundant electron-rich atoms such as N, O, S, P, and Si were also found to be efficient AIE materials. Zhang et al. and Lai and coworkers proposed that such an unconventional luminescent behavior could be due to the CTE mechanism.^{[32, 71-73]} That is, the clusterization of these electron-rich atoms could result in extended delocalization of the lone pair electrons, and the consequent through-space conjugation and conformation rigidification eventually endow such materials with efficient fluorescence properties. The non-conjugated CTE polymers have the advantages of facile synthesis, good water solubility, low biological toxicity, and so forth, and are attractive alternative materials of the conventional conjugated AIE polymers for diverse applications.

In addition to the covalent AIE polymers with typical AIEgen in the polymer structures, CTE-based polymer systems have also been applied for the generation of CPL through the ordered self-assembly between non-conjugated chiral CTE polymers and the interacting molecules (Figure 7A).^[74-76] For instance, recently Liao et al. reported a purely organic CTE system that can emit strong CPL based on the strategy of solid-phase molecular self-assembly of cationic α-poly-L-lysine (PLL) and oleate ion (OL).^[77] As illustrated in Figure 7B, the ionic assembly between PLL and OL can form cross-linked vesicles in aqueous solution, where PLL displayed typical CTE behavior but no CPL was detected. After applying a mild mechanical pressure to make the composite form a well-defined structure, the vesicle networks could precipitate out upon centrifugation and transform into planar bilayers to show obvious CTE-based CPL. Figure 7C suggested that the mixed system showed dramatically increased turbidity with the increase in OL fraction and became whitish at a molar ratio of 1:1 due to the formation of vesicular clusters at such conditions. However, the formation of such vesicles could not contribute to the well-defined folding of the PLL chains as no CPL was detected. Under mechanical pressures beyond 5 MPa, the cross-linked vesicles can be transformed into a flexible and self-supporting transparent film with strong blue fluorescence, and the CTE QY was up to 5.56%. Moreover, the film exhibited strong CPL signal with a high absolute g_lum value of 0.016. Interestingly, the CTE-based CPL intensity and the g_lum value were responsive to humidity. The increasing humidity can cause significant decrease in g_lum values due to the reduced supramolecular chirality in the solid film (Figure 7D). To further tune the CPL color, a FRET strategy was adopted by doping a yellow-emissive commercial dye (fluorescein sodium, FS) as the energy acceptor into the PLL-OL film. As depicted in Figure 7E,G, the emission of the PLL-OL⊃FS film gradually became stronger and red-shifted with the increasing doping amount of FS. Benefiting from the presence of strong interchain hydrogen bonding, the through-space conjugation of orderly aligned electron-rich O and N atoms leads to CTE-based blue CPL, which is capable of transferring energy to FS acceptor via the FRET process (Figure 7F). By changing the doping amount of acceptors, color-tunable CPL signals were readily achieved. As shown in Figure 7H, compared with PLL-OL, the CPL spectrum of PLL-OL⊃FS film showed brighter and yellower right-handed CPL with the peak at 575 nm, indicating the efficient complexation of FS with the folded PLL chains. This seminal work demonstrated that the chain folding state of CTE-based polymers is critical for the efficient generation of CPL.

Despite of the economic and sustainable advantages of the CTE-based CPL materials, examples on the development of CPL-active polymers based on CTE mechanism are still very limited. One possible reason might be the heterogeneity of clusteroluminogens in a CTE system, which is undesirable for the generation of CPL. The solid-phase molecular self-assembly strategy proposed by Yan and Tang et al. provides an expedient method to transform the CTE materials from random clusters to regular arranged superstructures. With more efforts for the development of CTE-based CPL polymers, the biological applications of CPL materials could be further expanded due to their good biocompatiblity and water solubility.

3.1 CPL-active supramolecular AIE polymers based on host–guest interactions

The reported construction strategies of CPL-active supramolecular AIE polymers based on host–guest interactions could be further divided into several groups. One is based on the host–guest self-assembly interaction between chiral macrocyclic host and achiral luminescent guest. For example, Guo et al. recently reported a CPL-active supramolecular polymer system based on the host–guest complexation between chiral bishelic[6]arenes (P/M-BH6) and different achiral AIEgen guests.^[90] As depicted in Figure 8A, a pair of amide group-bridged P/M-BH6 was synthesized as the chiral host, while blue-emissive AIEgen (BG) and yellow-emissive AIEgen (YG) with two trimethylammonium pendants were selected as the fluorescent guest molecules. With the increasing concentration of host–guest complex, uniform spherical particles and strips were successively observed, demonstrating the formation of linear supramolecular polymer chains via the host–guest interaction. Two-dimensional networks with sheet-like structures were finally formed at the gelation concentration through hydrogen bonding between amide groups of the host linkers. The PL spectra shown in Figure 8B suggested that the fluorescence intensity of AIEgen (BG or YG) was significantly enhanced after the formation of host–guest complexes, which can be explained by the restriction of intramolecular motions of AIEgens in the self-assembled supramolecular gels. Regarding the chiroptical properties, the AIEgen alone showed no CD signals, while the chiral host (P/M-BH6) alone showed mirrored CD signal ranging from 270 to 300 nm. After mixing the achiral AIEgen with the chiral macrocyclic host, new mirrored Cotton effect appeared in the absorption wavelength region of the AIEgen. The CD signal of the new peak gradually increased with the increase of the proportion of guest molecules (Figure 8C), indicating the occurrence of efficient chirality transfer from the chiral host to achiral guests. The enhanced fluorescence and chiral property of AIE supramolecular gels eventually induced obvious CPL signals (Figure 8D). Compared with the host and guest complexes in solution state, supramolecular gels exhibited much better CD and CPL activity. By adjusting the ratio of the blue and green-emitting AIEgen guests (BG and YG) in the complex, CPL-active ternary supramolecular gels with white light emission were readily fabricated with |g_lum| values of around 1.3 × 10⁻³. This work provides a convenient and efficient method for the construction of host–guest supramolecular polymers with CPL activity using chiral macrocyclic hosts and achiral AIEgens.

Another strategy is through the host–guest self-assembly interaction between achiral macrocyclic host and latent chiral AIEgen guest. For instance, in 2021, Li and coworkers^[91] developed a CPL-active supramolecular polymer system taking advantage of the P/M rotational conformation of TPE. As illustrated in Figure 8E, a tetra(6-coumarinylmethyl-pyridinium) TPE derivative was designed and synthesized as a latent chiral AIEgen guest. By using cucurbit[8]uril (CB[8]) as the host macrocyclic molecule to interact with the AIEgen guest via head-to-tail host–guest complexation, an achiral supramolecular organic framework (SOF-1) can be generated. When L-/D-phenylalanine or a small amount of adenosine derivatives (ATP, ADP, and AMP) are encapsulated into the cavities of SOF-1, the achiral CB[8]-based supramolecular network polymer can be induced to show adaptive chirality (Figure 8F,G). The addition of chiral molecules can create an asymmetric environment in this supramolecular system, which could promote the selective homodirectional P or M rotational conformation of TPE units in the supramolecular polymer skeleton, thereby leading to the efficient through-space chirality transfer (TSCT) from the chiral molecules to SOF-1 framework. Mirror-image CD and CPL can be simultaneously achieved in water. The chiral-molecule-induced dynamic adaptive chirality of this supramolecular polymer system makes it an ideal platform for the applications in determining dipeptide sequences and distinguishing some polypeptides/proteins in water.

Recently, Tu's group^[92] ingeniously designed a class of di-pyrene-modified chiral γ-cyclodextrin derivatives that can self-assemble into CPL-active supramolecular polymers via host–guest complexation. Although these monomeric species do not contain any classical AIE group, they exhibit typical AIE properties with efficient aggregate-state fluorescence. The fluorescence quantum yield of the γ-cyclodextrin derivative with butyl amide-linked pyrene substituents was 26.79% in DMF and 59.7% in aqueous suspension with a water content of 99%. The enhanced fluorescence property could be ascribed to the formation of assemblies that avoid collisions with solvents. Moreover, the CD signal of the γ-cyclodextrin derivative was significantly enhanced with the increase of water content. In 99% aqueous solution, the γ-cyclodextrin derivatives can form supramolecular polymers with excellent CPL properties due to the rigid chiral environment created by the hydrophobic complexation-induced aggregation. Significant electronic circular dichroism and CPL with high g_abs (up to 4.3 × 10⁻²) and g_lum values (up to 5.3 × 10⁻²) can be achieved. Furthermore, the dynamic characteristic of the host–guest self-assembly system enabled it to possess remarkable stimuli-responsive chiroptical properties. The aggregation behavior and chiroptical performance of this supramolecular system could be readily manipulated by temperature, pH, and competitive guest complexation. As a brief sum-up, the aforementioned examples of AIE-based host–guest self-assembly strategy can simultaneously achieve excellent fluorescence properties as well as efficient chiral transfer and amplification without very complex synthetic produces. However, investigation on the construction of CPL-active supramolecular AIE polymers by host–guest assembly strategy still remains very slow and limited, possibly due to the lack of appropriate AIE guests to induce efficient CPL generation.

3.2 CPL-active supramolecular AIE polymers based on metal coordination

Chiral coordination polymers with AIE property are another type of attractive candidates for the design of CPL-active polymer materials. Compared with the corresponding monomeric complexes, chiral coordination polymers generally display much higher luminescence efficiencies and larger |g_lum|.^{[93, 94]} As a special group of supramolecular polymers, metal-organic framework (MOF) materials provide a new opportunity to connect chiral units and AIEgens. The dynamic chiral frameworks could enable a different manner of chirality transfer and stimuli-responsive capabilities for CPL materials.

For example, Shang and coworkers reported a chiral reticular self-assembly strategy that combines chiral donors, achiral AIEgens, and metal ions together for the construction of optically pure MOFs with dual mechano-switchable CPL.^[95] As illustrated in Figure 9A, the MOF structure was composed of tetrakis(4-pyridylphenyl)ethylene (a typical AIEgen) as the emissive donor, camphoric acid as the chiral donor, and Cd^II ion as the metal bridge. The formation of the optically pure MOFs with hexagonal structures was clearly verified by the single-crystal X-ray diffraction (Figure 9B). After the chiral reticular self-assembly, the frame-enable emission property and the TSCT from the chiral donor to the achiral AIEgen endowed the resulting MOF material with efficient fluorescence and CPL in solid states. As shown in Figure 9C−E, the MOF crystals formed from D-camphoric acid depicted strong cyan blue fluorescence, with the maximum emission peak at 452 nm and obvious CPL signal with a |g_lum| value of up to 0.012. After grinding and ultrasonication, the fluorescence of D-MOF crystals shifted to 513 and 435 nm, respectively. Meanwhile, the CPL properties of D-MOF changed correspondingly. All of these altered states could fully recover to their original state by suspending the grinded powders into DMF for relaxation or removing the DMF solvent from the ultrasonication system. Such an interesting dual mechano-switchable fluorescence/CPL could be attributed to the influence of external stimuli on the local or global dynamics of the AIEgen rotors in the MOF structure. As indicated by Figure 9F, the dihedral angle between the phenyl ring and the adjacent pyridine moieties in the prefabricated AIE MOF crystals is measured to be about 30°, which contributes to the strong fluorescence (QY = 43%, life time τ = 1.71 ns) and CPL of the crystalline MOF. Grinding made the distance of AIEgen closer and the dihedral angle became smaller (<30°), which further restricted the movement of the AIE units and eventually led to the red-shifted fluorescence, an enhanced QY (65%), and a longer life time. On the contrary, ultrasound promoted the movement of AIEgen compared with the unprocessed state, and the phenyl conformation became more twisted. The nonrigid chiral donor also plays an important role in the reversible mechano-responsive behavior. The coordination framework was demonstrated to be retained under grinding and ultrasonication stimuli to maintain the microscopic dynamics, which explained the reversibility of the mechano-responsive fluorescence/CPL performance. This example suggested that the chiral reticular self-assembly could be an efficient and versatile strategy for constructing CPL-active supramolecular AIE polymers with stimuli responsiveness.

In addition to the MOF-type chiral coordination polymers, AIE-active metal organic clusters formed through metal coordination have also been rationally designed and developed to show excellent CPL properties.^{[76, 96]} For example, Wang et al. successfully constructed novel AIE-active chiral gold clusters using alkynyl alcohol enantiomers.^[97] The fluorescence of the obtained chiral gold clusters was strongest at 60% water content in DMF/water mixtures, and the solid QY can reach up to 73%, which was demonstrated to be conducive to the generation of CPL with high g_lum in the aggregate state. Zhang's group has also prepared diverse chiral metal clusters with both AIE and CPL properties and applied them to biological imaging.^[74] Compared to metal organic clusters, the development of CPL-active supramolecular AIE polymers based on metal coordination still remain very slow. Although coordination polymers have been extensively studied and widely used, examples on coordination polymers with both AIE and CPL properties are rare and there is still a large room to be further explored.

3.3 CPL-active supramolecular AIE polymers based on other interactions

In addition to host–guest interaction and metal coordination interaction, other non-covalent interactions such as hydrogen bonding interaction and electrostatic interaction have also been utilized for the construction of CPL-active supramolecular AIE polymers. One of the representative examples is hydrogen bonding-based supramolecular AIE polymers with finely controlled CPL. Recently, Lee et al. reported a kinetically controlled mechano-responsive CPL supramolecular polymerization system, where a chiral enantiomer (L-1/D-1) was prepared with TPE moiety as the emissive core and the chiral alanine derivative as the branch side chains (Figure 10A).^[98] The presence of TPE moiety not only can endow the resulting supramolecular polymer with AIE properties but also could promote the helical self-assembly process through the C‒H···π stacking interactions of TPE planes in addition to the intermolecular hydrogen bonding of amide groups. As shown in Figure 10B, when a clockwise rotating mechanical force was applied to the THF solution of the monomeric species L-1, its luminescence color remarkably changed from green at 500 nm to blue fluorescence at 455 nm, and the luminescence intensity increased by 500 times. The enhanced luminescence could be explained by the activation of AIE effect during the formation of the supramolecular polymer, while the blue-shifted fluorescence might be due to the more twisted conformation and stacking of TPE moieties in the polymer framework. The supramolecular polymerization was drastically accelerated by increasing the rotation speed (Figure 10C), which was directly related to the mechano-promoted dissociation of the intramolecular hydrogen bonds formed in the monomer L-1. The dissociation of the hydrogen bonding in monomer species consequently facilitates the rapid growth of the supramolecular polymer. Chirality in the target molecule is transferred and amplified during supramolecular polymerization. The time-dependent morphological changes of the self-assemblies under mechanical stimulation suggested that external mechanical stimulus played a key role in the supramolecular polymerization as a helical inducer and accelerator. By finely controlling the duration and strength of the external mechanical stimulus (rotation time and speed), dual enhancement in both fluorescence and chirality can be achieved, which eventually increased the CPL signals and |g_lum| values (Figure 10D,E). As shown in Figure 10E, the CPL intensity rapidly increased with time at a rotation speed of 1000 rpm, and the |g_lum| value can reach up to 2.2 × 10⁻². This mechanical stimulus-accelerated chiral supramolecular polymerization provides a distinctive design strategy for the construction of hydrogen bonding-based AIE supramolecular polymers with CPL activity. Recently, Zheng and coworkers developed a series of CPL-active supramolecular self-assemblies of AIEgens using TPE as the parent nucleus.^[99-101] For instance, they have designed and prepared a TPE-containing macrocyclic system.^[102-104] After being modified based on the alkyl chain engineering, such macrocyclic TPE derivatives could possess excellent self-assembly ability to form a variety of CPL-active AIE supramolecular polymers through different intermolecular interactions. These research results provide a new idea for developing novel AIE supramolecular polymers with CPL activity.

Moreover, temperature-responsive AIE supramolecular polymers with CPL activity have also been developed taking advantage of the dynamic feature of non-covalent interactions. For instance, Wu et al. recently constructed an AIE supramolecular polymer system with temperature-stimulated CPL response by self-assembly technology.^[105] Such AIE supramolecules could form gel through weak interaction forces like hydrogen bonding at room temperature. In this state, the movement of AIE molecule was effectively restricted, which was conducive to enhancing the fluorescence properties of the material. Meanwhile, hydrogen bonding interaction was beneficial to chirality transfer and can thus enhance the chiral properties. Therefore, the obtained supramolecular polymers showed excellent CPL properties in gel state. When the temperature was raised, the intermolecular hydrogen bonds were broken, and the polymer changed from a gel (aggregated state) to a solution (dispersed state), which resulted in a decrease in both chirality and fluorescence intensity, and ultimately decreased the CPL signal.

Besides the above-mentioned examples, in 2019, Cai et al. employed electrostatic interaction for the construction of CPL-active supramolecular helical nanotubes based on TPE-bridged polybissilsesquioxane (TPES).^[106] A supramolecular templating approach was used for the fabrication of such luminescent helical inorganic/organic hybrid nanomaterials. As shown in Figure 10F, a pair of chiral cationic low-molecular weight gelators (L-ValPyBr and D-ValPyBr) were used to form supramolecular assemblies (nanofibers) with single-handed helicity. Then TPE-containing bis(triethoxysilane) (BTSTPE) was dropped into the supramolecular systems. Under basic conditions, the triethoxyl groups of this AIEgen precursor were hydrolyzed to form hybrid silica oligomers, which can be adsorbed by the pyridinium rings of the gelators via electrostatic interactions. With the nanofibers as templates, the oligomers further underwent polymerization on the nanofiber surfaces, and the helical nanostructures of the AIEgen-containing inorganic/organic hybrid assemblies (TPES-M and TPES-P) were finally obtained by removing the templates. The SEM images shown in Figure 10G,H clearly demonstrated that TPES-M and TPES-P possessed left-handed and right-handed screw senses, respectively. The CD analysis further verified that the chirality of the chiral supramolecular templates was transferred to the AIE scaffolds (Figure 10I,J). Combined with the excellent fluorescence efficiencies of the solid TPES-M and TPES-P (QY = 26.2% and 21.2%, respectively), their hybrid nanotubes exhibited CPL activities with good g_lum values of −0.6 × 10⁻³ and +1.6 × 10⁻³, respectively (Figure 10K,L).

Based on the above-mentioned design strategies for the construction of supramolecular AIE polymers with CPL, it is anticipated that diversified AIE polymer systems with more efficient CPL, higher QY, and more intelligent functionalities could be developed in the near future.