English

• 1. Introduction

  As a fundamental and typical class of mechanically interlocked molecules (MIMs) [1-8], a rotaxane [9-16] is composed of two parts: wheel components and dumbbell-like axle components, which are holding together by mechanical bonds (Fig. 1a). Attributed to their unique interlocked structures as well as the controllable nanoscale motion behaviors, rotaxanes have proven to be privileged platforms for the construction of artificial molecular machines [17-21]. For instance, when the axle component of a rotaxane contains multiple stations with different binding affinities with its wheel component, the wheel component can move along the axle under specific external stimuli, thus leading to a molecular shuttle [22-26]. Notably, in 2016, two pioneers in rotaxane-based molecular machines, i.e., Sauvage [27] and Stoddart [28], were awarded the Nobel Prize in Chemistry jointly with B.L. Feringa, opening up a new gold era of research in rotaxanes and other MIMs. During the past few decades, in addition to the construction of novel molecular machines such as molecular muscles, molecular assemblers, molecular pumps, molecular cable cars [29-34], rotaxanes have been also widely applied in diverse fields such as sensing, drug delivery, and catalysis, making them promising platforms for practical applications [35, 36].

  Figure 1

  

  Figure 1.  (a) Cartoon illustrations of [2]rotaxane, (b) [2]rotaxane with AIEgen on the axle component, (c) [2]rotaxane with AIEgen on the wheel component.

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  In particular, aiming at the construction of novel rotaxanes with intriguing photophysical properties, the rapid development of luminescent rotaxanes has been witnessed [37]. By attaching selected fluorophores to the rotaxane skeletons, a series of functional rotaxanes with tunable emissive features have been successfully constructed, which have showed great potential for wide applications such as sensing, bioimaging, and information storage [38-41]. However, most traditional fluorophores would suffer from typical aggregation-induced quenching (ACQ) effect in the aggregate state, thus might hamper the further explorations of the applications of corresponding luminescent rotaxanes [42, 43]. To deal with such key issue, Tang et al. developed a novel class of luminogens (i.e., AIEgens) with attractive aggregation-induced emission (AIE) features, which are typically non-emission in solution but intensively emissive in the aggregate state attributed to the restriction of intramolecular rotation (RIR) [44-46]. During past two decades, the research in AIE has evolved to the birth of aggregology [47]. Many typical AIEgens such as tetraphenylethene (TPE), hexaphenylsilole (HPS), and 9, 10-distyrylanthracene (DSA) have been developed, which have been widely used in bioimaging, sensors, optoelectronic devices and many other fields [48, 49].

  The combination of AIEgens and rotaxanes would give rise to a novel type of luminescent rotaxanes with many attractive features (Figs. 1b and c). First of all, it would further expand the applications of luminescent rotaxanes from solution state to aggregate state, thus offering great opportunities for the developing novel luminescent materials. In addition, the formation of interlocked structures in rotaxanes could influence the AIE behaviors of AIEgens through mechanical bonds, which might efficiently enhance their AIE effect [50]. Notably, for some rotaxanes, it might be also AIE-active even without traditional AIEgens in their structures [51]. More importantly, taking advantage of the unique dynamic feature of rotaxanes would lead to the construction of novel smart luminescent materials with precisely tunable emissions, thus further expanding their application scope. Attributed to these attractive features, recently AIEgen-based rotaxanes have attracted more and more attention and some significant progress has been made. In this minireview, the recent progress of AIEgen-based rotaxanes has been summarized, with an emphasis on the design strategy and potential applications.

  2. Rotaxanes with AIEgens on the axle components

  In 2015, Yin and coworkers demonstrated the first report on the construction of AIEgen-based rotaxanes [52]. In their study, tetraphenylethene (TPE), the most typical AIEgen, was selected as the stoppers for the synthesis of a series of rotaxanes, including [2]rotaxanes 2 and 6 as well as [3]rotaxanes 4 (Fig. 2). Starting from axle components containing both TPE and dialkylammonium moieties, these rotaxanes were prepared through a template-directed clipping approach [53]. By using acetonitrile as a good solvent and water as a poor solvent, the AIE behaviors of these resultant rotaxanes and corresponding axle components was then evaluated. For [2]rotaxanes 2a and 2b, comparing with the axle 1, more obvious AIE effect was observed according to the photoluminescence (PL) spectra since the dramatic enhancement in luminescence for these systems was observed upon increasing the water fraction (f_w) to 85% for 1, 80% for 2a, and 70% for 2b, respectively (Fig. 3a). In addition, according to such result, [2]rotaxane 2b with a long alkyl chain revealed better aggregation then that of 2a. Such phenomenon was possibly attributed to the larger overlap between the host macrocycle in 2b than that of 2a, which has been supported by the theoretical DFT calculation. In the case of [3]rotaxanes 4a and 4b, similar trend in their AIE behaviors as that of the [2]rotaxanes was observed (Fig. 3b). According to these results, the introduction of the wheel components in rotaxanes was proven to promote the formation of aggregate state. Moreover, the existence of long alkoxyl chains would further enhance such process. To confirm such effect, new axle 5 as well as [2]rotaxanes 6a and 6b with longer distance between the TPE and ammonium units were further designed and synthesized. Interestingly, in this new system, due to the long distance between the macrocycle and the TPE units, there was no obvious overlap between them that led to the weakening of the restricted intramolecular rotation (RIR) process of the TPE unit, thus their AIE behaviors were similar (Fig. 3c). According to this study, the formation of rotaxanes, especially ones with functional groups, did influence the AIE behaviors, which offers good opportunities for the constructions of novel rotaxanes with tunable AIE emissions.

  Figure 2

  

  Figure 2.  Synthesis of [2]rotaxanes 2a-2b, 6a-6b and [3]rotaxanes 4a–4b.

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  Figure 3

  

  Figure 3.  (a) fluorescence spectra of 1 (left), 2a (middle), and 2b (right) in CH₃CN-water mixtures with different water fractions. (b) fluorescence spectra of 3 (left), 4a (middle), and 4b (right) in CH₃CN-water mixtures with different water fractions. (c) fluorescence spectra of 5 (left), 6a (middle), and 6b (right) in CH₃CN-water mixtures with different water fractions. Copied with permission [52]. Copyright 2015, Royal Society of Chemistry.

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  In the same year, on the basis of previous work described above, the same group further synthesized another AIE-active [2]rotaxane 7a with TPE as a stopper [54]. Notably, in [2]rotaxane 7a, both the ammonium and amide moieties were introduced as recognition sites, and at the same time N-hetero crown ether ring was introduced as the wheel component. In their design strategy, by using acid and base as external stimulus, the controllable shuttling motion of the wheel component between the ammonium station and the amide station would be realized, which might lead to the modulation of the AIE behaviors of the resultant molecular shuttle (Fig. 4a). To confirm such hypothesis, the AIE behaviors of the rotaxane in both protonated and deprotonated states were investigated (Fig. 4b). It was found that for protonated rotaxane 7a, upon increasing the water fraction (f_w) to 80%, the fluorescence of 7a increased rapidly. However, for the deprotonated rotaxane 7b, no fluorescence was observed until the f_w reached 90%. Such phenomenon might be attributed to that in 7b the wheel component was located at the amide recognition site, which was far from the TPE unit, thus the interaction is weakened, which leads to a reduction in the degree of TPE rotation restriction. It is proved that in rotaxane-based molecular shuttles the interaction between the wheel component and AIEgens could be readily adjusted by external stimuli, which further tuned the aggregation states of AIEgen-based rotaxanes, thereby leading to switchable optical outputs.

  Figure 4

  

  Figure 4.  (a) Chemical and energy-minimized structures of [2]rotaxnes 7a and 7b whose wheel component could be relocated on the axle component though the deprotonation and re-protonation. (b) Fluorescence spectra and photographs of 7a (left) and 7b (right) in CH₃CN-water mixtures with different water fractions (λ_ex = 340 nm). Reproduced with permission [54]. Copyright 2015, Royal Society of Chemistry.

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  Considering the emerging wide biomedical applications of fluorescent probes with AIE characteristics, in 2016, Huang and coworkers demonstrated the successful construction of a novel platform based on AIEgen-based rotaxane as targeting, imaging and therapeutic agents [55]. In their study, TPE and triphenylphosphonium (TPP) groups was introduced as stoppers, which acted as an AIE-active fluorogen and mitochondria-targeting site, respectively (Fig. 5). In addition, pillar[5]arene (P[5]A) macrocycles functionalized with two aldehyde units was selected as the wheel component [56-65]. The targeted [2]rotaxane 8 was synthesized through a one-pot copper(Ⅰ)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. Notably, similar with previous report, the formation of [2]rotaxane significantly enhanced the AIE effect of [2]rotaxane 8 comparing with the corresponding free axle due to the RIR process of TPE unit. Impressively, on the basis of the attractive emission feature of 8, nanoparticles (NPs) self-assembled from it was further prepared a reprecipitation technique, which displayed high specificity to mitochondria and superior photostability. Furthermore, attributed to the existence of aldehyde moieties in the wheel component of [2]rotaxane 8, anticancer drugs containing amine groups such as doxorubicin (DOX) could be further introduced into the rotaxane through the formation of imine bonds, resulting in 9 as a novel drug delivery platform. Notably, due to the Förster resonance energy transfer (FRET) process between TPE and DOX units, the introduction of DOX quenched the fluorescence of 9. Further hydrolysis of the imine bonds in the in the cell led to not only the release of DOX but also the recovery of the fluorescence since the FRET process between these two fluorophores were no longer existed, making such system quite attractive for bioimaging applications.

  Figure 5

  

  Figure 5.  Chemical structure of high-emission [2]rotaxane 8 (top) and schematic illustration of the preparation of the mitochondria-targeting probe-inspired prodrug [2]rotaxane 9 and possible cellular pathways of the dual-fluorescence-quenched 9 nanoparticles (bottom).

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  Through the conventional metal template approach, Lin and coworkers reported the synthesis of [2]rotaxane 10 with TPE and phenanthroimidazole (PIZ) unit as the stoppers (Fig. 6a) [66]. In this study, the DMSO/water solvent system was employed for the investigation of the AIE behavior of 10. It was found that when the water fraction (f_w) was 10%, the PIZ unit contributed to the fluorescence emission at 401 nm. When f_w was above 20% and up to 80%, remarkable AIE effect with bright blue at 470 nm was observed attributed to the aggregation process (Fig. 6b). Moreover, the existence of hetero-atoms "N" and "O" within the unique interlocked structure of [2]rotaxane was further employed for sensing applications. Interestingly, in both the non-aggregated (f_w = 10%) and aggregated states (f_w = 80%) of 10, among many tested alkali and transition metal ions, the addition of Fe³⁺ led to a significant fluorescence quenching (Figs. 6c and d). Further fluorescence quenching experiments also indicated the sensing capability of 10 for Hemin, a real small biomolecule containing Fe³⁺, even with a higher sensitivity (Fig. 6e).

  Figure 6

  

  Figure 6.  (a) Chemical structure of [2]rotaxane 10. (b) Fluorescence spectra of 10 with increasing water fraction (from 25% to 80%). (c) Bar diagram depicting fluorescence quenching of 10 in 10% aq., HEPES (pH 7.4, 10 mmol/L) in DMSO with various metal ions. (d) Fluorescence spectra of 10 in 10% aq., HEPES (pH 7.4, 10 mmol/L) in DMSO with increasing concentration of Fe³⁺ metal ion. (e) Fluorescence titration spectra of 10 in the presence of Hemin in 10% aq., HEPES (pH 7.4, 10 mmol/L) in DMSO. Reproduced with permission [66]. Copyright 2016, Elsevier Ltd.

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  Very recently, by using double TPE units as stoppers and the dibenzo-24-crown-8 (DB24C8) macrocycle as wheel component, the same group demonstrated the construction of a series of AIE-active [2]rotaxanes 11–13 (Fig. 7a) [67]. In addition to the investigations on the formation of various nanostructures upon aggregation, these resultant [2]rotaxanes were also used for ion recognition, but for anions. As shown in Fig. 7, AIE tests indicated that depending on their structures, different [2]rotaxanes started to show AIE behaviors at varied the water fraction (f_w) values (70% for 11, 80% for 12, and 90% for 13) (Fig. 7b). Many key factors such as molecular shuttling of the wheel component and the interactions between the wheel component and TPE units as well as the viscosities of the solvents used for AIE tests was proven to influence their AIE behaviors.

  Figure 7

  

  Figure 7.  (a) Chemical structures of [2]rotaxanes 11–13. (b) Fluorescence spectra of [2]rotaxanes 11 (left), 12 (middle), and 13 (right) (10 mmol/L) in CH₃CN/water solutions with different water fractions. (c) Schematic representations of diverse self-assembled nanostructures of [2]rotaxanes 11–13 in CH₃CN/water solutions at different water fractions. (d) Fluorescence color changes in the presence of various TBA salts in CH₃CN solutions of [2]rotaxane 12. Reproduced with permission [67]. Copyright 2021, Royal Society of Chemistry.

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  Interestingly, upon the aggregation process, for different [2]rotaxanes, varied self-assembled nanostructural morphologies were observed. For instance, when 11 was dissolved in pure acetonitrile, nanospheres with an average diameter of 200 nm were formed. When f_w = 70%, the size of the nanospheres decreased to around 150 nm. When f_w = 90%, nanorods were formed due to the fusion of nanoaggregates. In the case of 12, when dissolved in pure acetonitrile, spheres with a size of 2.0 µm were formed. And when f_w was increased to 80%, nanospheres were formed. For 13, the initial morphology in pure acetonitrile was nanospheres with an average size of 250 nm. When f_w = 90%, a truncated cubic structure with a size of 2.0 µm was found, and the further increase of f_w to 99% resulted in the formation of nanocubic structure (Fig. 7c). Moreover, attributed to the existence of 1, 2, 3-triazolium rings that could interact with anion species through hydrogen bonding or dipole-dipole interactions, the investigations on the anion sensing ability of [2]rotaxane 12 were further performed. The results indicated that 12 revealed a turn-on response towards dihydrogen phosphate (H₂PO₄⁻) with a binding constant of 7.72 × 10³ L/mol calculated by the fluorescence titration profile as well as 1:1 binding stoichiometry determined by Job's plots (Fig. 7d).

  Actually, in 2017, Cao and coworkers have already demonstrated the synthesis of AIE-active [2]rotaxane with double TPE units as stoppers [68]. In their study, cucurbit[10]uril (CB[10]) with large cavity was employed as wheel component. On the basis of the host-guest interaction between CB[10] and viologen moiety, the targeted [2]rotaxane was successfully synthesized in 81% yield through the slipping method by heating the axle component 14 with two TPE units and CB[10] in DMSO at 95 ℃ for 17 days (Fig. 8a). Notably, for the axle component 14, it was nonfluorescent in DMSO. Upon the formation of corresponding [2]rotaxane 15, the RIR process of the TPE stoppers made 15 highly fluorescent in DMSO with the quantum yield up to 0.50. Moreover, when studying the influence of solvent on the AIE behavior of 15, it was found that no matter in which solvent, the fluorescence intensity of [2]rotaxane 15 was always stronger than that of the axle component 14, highlighting the enhanced AIE effect through the formation of [2]rotaxane (Fig. 8b). In particular, upon further addition of chloroform and tetrahydrofuran into the DMSO solution of [2]rotaxane 15, further increase in the fluorescence intensity was observed, indicating an interesting stepwise aggregation-induced emission enhancement.

  Figure 8

  

  Figure 8.  (a) Synthesis of [2]rotaxane 15 from the axle component 14 through the slipping method. (b) Fluorescence intensity at 600 nm of 14 (10 µmol/L, black column) and 15 (10 µmol/L, red column) in different solvents including 1% DMSO (A = acetone; B = CH₂Cl₂; C = CHCl₃; D = 1, 4-dioxane; E = DMF; F = DMSO; G = EA; H = H₂O; I = MeOH; J = THF). λ_ex = 365 nm; Ex/Em slits = 5/5 nm. Copied with permission [68]. Copyright 2017, American Chemical Society.

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  Aiming at the construction of switchable supramolecular amphiphiles based on AIE-active [2]rotaxanes, in 2018, Chung and coworkers demonstrated the synthesis of [2]rotaxanes 16 and 17, both of which were decorated with TPE unit as the capping group and calix[4]arene either with or without t–butyl as the wheel components, respectively [69]. For these [2]rotaxanes, due to the existence of both ammonium and urea moieties as two stations in the axle, the reversible switching between them with the wheel component locating at the ammonium station and corresponding deprotonated [2]rotaxanes 16′ and 17′ with the wheel components staying at the urea station was successfully achieved by using acid/base stimuli (Figs. 9a and b). These four [2]rotaxanes exhibited different AIE effects and unique characteristics of nanostructures under different water contents. For 16 and 16′, upon the gradual increase of water fractions in CH₃CN, they started to aggregate and emit when f_w = 65% and 70%, respectively, and for 17 and 17′ such values were f_w = 70% and 75% respectively (Fig. 9c). For 16, because TPE is closer to the macrocycle, the effect is stronger, and there are multiple interactions such as urea-based intermolecular hydrogen bonding, van der Waals force between tert–butyl groups, which makes it have AIE at relatively low water content effect.

  Figure 9

  

  Figure 9.  (a) Chemical structures of acid/base switchable [2]rotaxanes 16 and 16′. (b) Chemical structures of acid/base switchable [2]rotaxanes 17 and 17′. (c) Fluorescence spectra and emission photographs of [2]rotaxanes 16, 16′, 17, and 17′ in CH₃CN/water cosolvent system with different water fractions. Reproduced with permission [69]. Copyright 2018, American Chemical Society.

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  Further research on its nanostructure found that when the protonated rotaxanes 16 and 17 are in a pure acetonitrile solution, a microsphere structure will be formed; when the f_w is 65% and 70%, respectively, nanospheres will be formed; when the water content is further increased at 90%, nanospheres with reduced size will be formed. For the rotaxanes 16′ and 17′ in the deprotonated state, when f_w is 70% and 75%, hollow nanospheres will be formed; when f_w = 99%, a nanosphere structure will be formed. It proves that the structural transformation can be driven by different f_w values and molecular shuttles. It is worth mentioning that due to the existence of multiple interactions, 16 can form a gel state in methanol solution. When the concentration is low, it will form a spherical structure. When the concentration is high, it will form a dumbbell-shaped 3D cross-linked network structure. The reversible transformation of sol-gel can be realized under the control of acid and alkali.

  In 2020, by further introducing pyrene unit as the other stopper, the same group demonstrated the synthesis of [2]rotaxanes 18 and 19 with both AIE and ACQ fluorophores on their axle components [70]. Similar with previous report, these resultant [2]rotaxanes were also switchable upon the reversible base/acid treatment, resulting in the corresponding [2]rotaxane 18′ and 19′, respectively (Figs. 10a and b). For all [2]rotaxanes, only the emission bands at 395 and 415 nm attributed to the pyrene monomer were observed in pure THF. Upon the addition of water as poor solvent to trigger the aggregation, a new emission band appeared at 485 nm when f_w = 70%, indicating that the AIE effect of TPE unit (Fig. 10c). For 18, when further increasing the f_w to 80%, the AIE effect reached the maximum along with a remarkable blue shift. While for other three [2]rotaxanes, supreme bright fluorescence enhancement was achieved when f_w reached 90%. Due to the formation of nano-aggregates that suppressed the emissive decay pathways, the further increment of water fraction to 99% only led to fainted fluorescence enhancement.

  Figure 10

  

  Figure 10.  (a) Chemical structures of acid/base switchable [2]rotaxanes 18 and 18′. (b) Chemical structures of acid/base switchable [2]rotaxanes 19 and 19′. (c) Fluorescence spectra and emission photographs of [2]rotaxanes 18, 18′, 19 and 19′ in THF/water cosolvent system with different water fractions. Reproduced with permission [70]. Copyright 2018, American Chemical Society.

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  In addition to [2]rotaxanes as described above, the AIEgen could also be introduced into [3]rotaxanes. For instance, in 2021, Wang, He and coworkers demonstrated the preparation of novel AIEgen-based [3]rotaxane 20, from which switchable circularly polarized luminescence (CPL) system was successfully constructed (Figs. 11a) [71]. In their study, another typical AIEgen DSA unit was introduced as the center of axle component in [3]rotaxane 20. In addition, two pillar[5]arene macrocycles (DEP[5]A) was introduced as the wheel components. Due to the existence of urea moiety as stimuli-responsive sites in the axle component, [3]rotaxane 20 revealed unique stimuli-responsiveness towards acetate anions. Upon the addition or removal of acetate anions, controllable motions of DEP[5]A wheels along the axle was realized. AIE tests indicated that [3]rotaxane displayed typical AIE behaviors with the fluorescence quantum yield of (25.8 ± 1.2)% in aggregate state.

  Figure 11

  

  Figure 11.  (a) Design strategy of novel CPL switching system based on the AIE-active chiral [3]rotaxane 20 upon the addition or removal of acetate anions as external stimuli. (b) CPL spectra (left) and g_lum spectra (right) of the cast film of pS-20 and pR-20. (c) CPL spectra (left) and g_lum spectra (right) of the cast film of pS-20 and pR-20 upon the addition of acetate anions. Reproduced with permission [71]. Copyright 2021, Wiley-VCH GmbH.

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  In addition, the switching process triggered by acetate anion led to a slight decrease of fluorescence quantum yield to (24.3 ± 2.6)%. More importantly, owing to the conformation-dependent planar chirality of DEP[5]A wheels, three stereoisomer of [3]rotaxane 20, i.e., a pair of enantiomers and a mesomer, were separated by chiral preparative HPLC. For the resultant chiral [3]rotaxanes, along with the switching process, the relocation of chiral DEP[5]A wheels resulted in expected enhanced chirality information transfer, as revealed by CD spectra. Impressively, upon the addition of acetate anions, CPL tests indicated a 6.4-fold enhancement in high dissymmetry factors (g_lum) from ± 2.14 × 10⁻³ to ± 1.36 × 10⁻² (Figs. 11b and c). Notably, such CPL switching process revealed excellent cycling ability, making it quite attractive for practical applications.

  Aiming at the construction of novel mechanically bonded macromolecules with intriguing photophysical properties, AIEgens have been also introduced into rotaxane dendrimers [72-75]. In an earlier report in 2019, Yang and coworkers demonstrated the construction of novel AIE-active rotaxane-branched dendrimers with DSA unit as the core (Fig. 12a) [76]. Starting from a DSA precursor 22 with two alkyne units, the employment of a neutral organometallic [2]rotaxane 21 as key building block led to the successful synthesis of rotaxane-branched dendrimers 23-Gn (n = 1, 2, 3) through a controllable divergent approach with up to 14 individual [2]rotaxanes as branches. AIE tests indicated that, for all the resultant rotaxane-branched dendrimers, they displayed typical AIE behaviors upon the gradual addition of n-hexane as a poor solvent. Notably, attributed to the varied steric hindrances originated from the surrounding rotaxane branches, these rotaxane-branched dendrimers revealed interesting generation-dependent AIE behaviors. For instance, in the non-aggregate state, the fluorescence quantum yields of all the rotaxane-branched dendrimers were almost the same (for 23-G1, 2.9%; for 23-G2, 2.5%; for 23-G3, 2.6%). While in the aggregate state, more remarkable enhancements in fluorescence quantum yields were observed for higher-generation rotaxane-branched dendrimers (for 23-G1, 11.4%, 3.9-fold; for 23-G2. 16.2%, 6.5-fold; for 23-G3, 17.9%, 6.9-fold) (Fig. 12b). Such phenomenon indicated that the increase of dendrimer generation led to more obvious AIE effect, thus providing a new approach for the construction of novel luminescent functional materials with tunable AIE behaviors.

  Figure 12

  

  Figure 12.  (a) Synthesis of the AIE-active rotaxane dendrimers 23-Gn through the controllable divergent approach from [2]rotaxane 21 and DSA precursor 22 as key building blocks. (b) Fluorescence quantum yields of 23-G1 (left), 23-G2 (middle), and 23-G3 (right) in CH₂Cl₂/n-hexane with various n-hexane fractions. Reproduced with permission [76]. Copyright 2019, Royal Society of Chemistry.

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  Recently, based on their on-going interest in function-oriented rotaxane dendrimers, the same group further reported the construction of novel AIEgen-branched rotaxane dendrimers [77-80], based on which artificial light harvesting systems (LHSs) were further prepared [81]. In this study, TPE unit was introduced into as a stopper [2]rotaxane 24, from which rotaxane dendrimers 25-Gn (n = 1, 2, 3) up to the third generation were synthesized through the same controllable divergent approach (Fig. 13a). In the resultant rotaxane dendrimers, AIEgens were precisely-arranged at each branch, making them the first successful example of AIEgen-branched rotaxane dendrimers. Interestingly, attributed to the existence of urea moiety at the axle of each rotaxane branches, the resultant rotaxane dendrimers revealed a two-step aggregation behavior depending on the fraction of MeOH that was added as not only poor solvent to drive the aggregation but also external stimuli to interact with the urea moieties. Low fraction of MeOH triggered the contraction of rotaxane dendrimers through the MeOH-induced switching translational motions of the pillar[5]arene wheels located at both the branches and branching points. By increasing the MeOH fractions, the further aggregation of the resultant contracted rotaxane dendrimers took place, simultaneously leading to the remarkable enhancement in the emission intensity.

  Figure 13

  

  Figure 13.  (a) Synthesis of the AIEgen-branched rotaxane dendrimers 25-Gn through the controllable divergent approach from [2]rotaxane 24 (left) and fluorescence spectra of in CH₂Cl₂/MeOH with various MeOH fractions (right). Reaction conditions: (Ⅰ): (a) Bu₄NF·3H₂O, THF, r.t., 2 h; (b) 24, CuI, DCM/Et₂NH, r.t., overnight, 70%; (Ⅱ): (a) Bu₄NF·3H₂O, THF, r.t., 2 h; (b) 24, CuI, DCM/Et₂NH, r.t., overnight, 38%. (b) The construction of artificial LHSs based on TPE-branched rotaxane dendrimers 25-Gn and ESY as efficient photocatalysts. Reproduced with permission [81]. Copyright 2021, Wiley-VCH GmbH.

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  Furthermore, by choosing eosin Y (ESY) as the fluorescence energy acceptor, corresponding artificial LHSs were then constructed, which was clearly revealed by the fluorescence spectra. Interestingly, along with the increase of the generation of rotaxane dendrimers, enhancements in both energy-transfer efficiencies (for 25-G1-ESY, 42.5%; for 25-G2-ESY, 68.2%; for 25-G3-ESY, 71.6%) and antenna effect (for 25-G1-ESY, 1.1; for 25-G2-ESY, 2.3; for 25-G3-ESY, 4.1) of the resultant artificial LHSs was observed, revealing an interesting generation-dependent effect. Impressively, these artificial LHSs were proven to act as efficient photocatalysts for both photooxidation reaction and aerobic cross-dehydrogenative coupling (CDC) reaction (Fig. 13b), and their photocatalytic performances were also generation-dependent. According to this study, the integration of multiple AIEgen-based rotaxanes in well-defined arrangements would serve as promising platform for the construction of novel functional supramolecular materials for practical applications.

  In a very recent report, in addition to TPE-based [2]rotaxane 24, another [2]rotaxane 26 decorated with two anthracene units was employed by the same group as functionalized building blocks as well as tetraethynylpyrene unit 27 as both core module and energy acceptor (Fig. 14a), leading to the successful construction of novel rotaxane dendrimers 28-Gn (n = 1, 2, 3) with both AIE and ACQ luminogens as energy donors through the controllable divergent approach (Fig. 14b) [82]. Similar with the AIEgen-branched rotaxane dendrimers 25-Gn described above, the resultant rotaxane dendrimers, including two hetero ones, also revealed solvent-induced switching feature, making them excellent platforms for the construction of artificial LHSs with tunable light harvesting ability. Interestingly, similar generation-dependent effect was observed through the fluorescence spectra, enhanced antenna effects were observed along with the increase of dendrimer generation, thus providing another promising candidate as efficient photocatalysts.

  Figure 14

  

  Figure 14.  Chemical structures of the key building blocks (a) for the synthesis of rotaxane dendrimers 28-Gn as novel LHSs through the controllable divergent approach (b). Reaction conditions: (Ⅰ): (a) Bu₄NF·3H₂O, THF, r.t., 2 h, 87%; (b) 24, CuI, DCM/Et₂NH, r.t., overnight, 70%; Ⅱ): (a) Bu₄NF·3H₂O, THF, r.t., 2 h, 70%; (b) 24, CuI, DCM/Et₂NH, r.t., overnight, 71%. Reproduced with permission [82]. Copyright 2022, Elsevier Ltd.

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  3. Rotaxanes with AIEgens on the wheel components

  In addition to the introduction of AIEgens into the axle components of rotaxanes mainly as stoppers, it can be also attached on the wheel component. For instance, in 2020, Lin and coworkers reported a novel photo-switchable [2]rotaxane 29 with spiropyran (SP) unit, a classical photochromic moiety, as a stopper [83]. In [2]rotaxane 29, crown ether ring modified with two TPE units was selected as the wheel component. In addition, due to the insertion of both the secondary ammonium and N-methyltriazolium units in the axle component, the wheel component could undergo reversible shuttling between these two stations through the deprotonation/protonation process by adding base or acid. Together with the photo-induced transformation between the closed form SP and the open form MC, the reversible transformations between 29, 29′, 29′′, and 29′′′ were successfully realized (Fig. 15).

  Figure 15

  

  Figure 15.  Reversible transformations between [2]rotaxanes 29, 29′, 29′′, and 29′′′.

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  More importantly, attributed to the FRET process between the TPE units in the wheel component and MC stopper as well as the acid-base induced distance changes between them, the emission behaviors of these [2]rotaxanes in different states could be precisely adjusted (Fig. 16). For instance, in the initial state, due to the absence of FRET between the TPE and SP moiety, [2]rotaxane 29 emitted strong blue fluorescence in the aggregate state. Up UV irradiation that induced the formation of MC unit, the FRET between TPE and MC unit resulted in an orange fluorescence emission of [2]rotaxane 29′. Moreover, by deprotonation of the secondary ammonium using DBU to trigger the movement of the wheel component to the N-methyltriazolium station, the reduced distance between TPE and MC units led to enhanced FRET, thus making [2]rotaxane 29′′′ reveal a strong red fluorescence. This report provides a nice example of the precise regulation of emission behaviors of AIEgen-based [2]rotaxanes based on their unique dynamic feature.

  Figure 16

  

  Figure 16.  Tunable emission behaviors of these [2]rotaxanes in different states. Reproduced with permission [83]. Copyright 2020, American Chemical Society.

  DownLoad: Full-Size Img PowerPoint

  4. Conclusion

  During past few years, through the introduction of typical AIEgens into either the axle or wheel components, the successful construction of diverse AIEgen-based rotaxanes have been achieved. The formation of rotaxanes through mechanical bonds did significantly influence the AIE effect, thus leading to a novel type of luminescent materials. More importantly, by using the concept of molecular shuttles, rotaxanes with tunable AIE behaviors have been also constructed, whose emission properties could be well regulated through external stimuli-induced re-locations of AIEgens within rotaxane skeleton. Attributed to these attractive structural and emissive features, AIEgen-based rotaxanes have been applied in diverse fields such as sensing, bioimaging, CPL switching, and light harvesting.

  Although some impressive progress has been made, the research in AIEgen-based rotaxanes is still in an early stage, more efforts are still greatly needed. For instance, the further construction of AIEgen-based rotaxanes with sufficient diversity in both structures and properties is still necessary. According to the in-depth investigations of these diverse rotaxanes, the structure-property relationship of AIEgen-based rotaxanes would be established, which could promote the further function-oriented design of new AIE-active rotaxanes with desired properties and functions. Moreover, in addition to the combination of dynamic feature of rotaxanes with AIEgens toward the construction of smart luminescent materials, the integration of the chiral rotaxanes, particularly the mechanically planar chiral ones [84, 85], with AIEgens would lead to AIEgen-based rotaxanes with switchable chiroptical properties, which would serve as excellent candidates for the construction of novel chiral materials.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  We acknowledge the financial support sponsored by the National Natural Science Foundation of China (No. 22001073), the Fundamental Research Funds for the Central Universities and the Research Fund Program of Guangdong Provincial Key Laboratory of Functional and Intelligent Hybrid Materials and Devices (No. 2020-GDKLFSHMD-07).

  
  
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• 

  Figure 1  (a) Cartoon illustrations of [2]rotaxane, (b) [2]rotaxane with AIEgen on the axle component, (c) [2]rotaxane with AIEgen on the wheel component.

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  Figure 2  Synthesis of [2]rotaxanes 2a-2b, 6a-6b and [3]rotaxanes 4a–4b.

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  Figure 3  (a) fluorescence spectra of 1 (left), 2a (middle), and 2b (right) in CH₃CN-water mixtures with different water fractions. (b) fluorescence spectra of 3 (left), 4a (middle), and 4b (right) in CH₃CN-water mixtures with different water fractions. (c) fluorescence spectra of 5 (left), 6a (middle), and 6b (right) in CH₃CN-water mixtures with different water fractions. Copied with permission [52]. Copyright 2015, Royal Society of Chemistry.

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  Figure 4  (a) Chemical and energy-minimized structures of [2]rotaxnes 7a and 7b whose wheel component could be relocated on the axle component though the deprotonation and re-protonation. (b) Fluorescence spectra and photographs of 7a (left) and 7b (right) in CH₃CN-water mixtures with different water fractions (λ_ex = 340 nm). Reproduced with permission [54]. Copyright 2015, Royal Society of Chemistry.

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  Figure 5  Chemical structure of high-emission [2]rotaxane 8 (top) and schematic illustration of the preparation of the mitochondria-targeting probe-inspired prodrug [2]rotaxane 9 and possible cellular pathways of the dual-fluorescence-quenched 9 nanoparticles (bottom).

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  Figure 6  (a) Chemical structure of [2]rotaxane 10. (b) Fluorescence spectra of 10 with increasing water fraction (from 25% to 80%). (c) Bar diagram depicting fluorescence quenching of 10 in 10% aq., HEPES (pH 7.4, 10 mmol/L) in DMSO with various metal ions. (d) Fluorescence spectra of 10 in 10% aq., HEPES (pH 7.4, 10 mmol/L) in DMSO with increasing concentration of Fe³⁺ metal ion. (e) Fluorescence titration spectra of 10 in the presence of Hemin in 10% aq., HEPES (pH 7.4, 10 mmol/L) in DMSO. Reproduced with permission [66]. Copyright 2016, Elsevier Ltd.

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  Figure 7  (a) Chemical structures of [2]rotaxanes 11–13. (b) Fluorescence spectra of [2]rotaxanes 11 (left), 12 (middle), and 13 (right) (10 mmol/L) in CH₃CN/water solutions with different water fractions. (c) Schematic representations of diverse self-assembled nanostructures of [2]rotaxanes 11–13 in CH₃CN/water solutions at different water fractions. (d) Fluorescence color changes in the presence of various TBA salts in CH₃CN solutions of [2]rotaxane 12. Reproduced with permission [67]. Copyright 2021, Royal Society of Chemistry.

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  Figure 8  (a) Synthesis of [2]rotaxane 15 from the axle component 14 through the slipping method. (b) Fluorescence intensity at 600 nm of 14 (10 µmol/L, black column) and 15 (10 µmol/L, red column) in different solvents including 1% DMSO (A = acetone; B = CH₂Cl₂; C = CHCl₃; D = 1, 4-dioxane; E = DMF; F = DMSO; G = EA; H = H₂O; I = MeOH; J = THF). λ_ex = 365 nm; Ex/Em slits = 5/5 nm. Copied with permission [68]. Copyright 2017, American Chemical Society.

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  Figure 9  (a) Chemical structures of acid/base switchable [2]rotaxanes 16 and 16′. (b) Chemical structures of acid/base switchable [2]rotaxanes 17 and 17′. (c) Fluorescence spectra and emission photographs of [2]rotaxanes 16, 16′, 17, and 17′ in CH₃CN/water cosolvent system with different water fractions. Reproduced with permission [69]. Copyright 2018, American Chemical Society.

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  Figure 10  (a) Chemical structures of acid/base switchable [2]rotaxanes 18 and 18′. (b) Chemical structures of acid/base switchable [2]rotaxanes 19 and 19′. (c) Fluorescence spectra and emission photographs of [2]rotaxanes 18, 18′, 19 and 19′ in THF/water cosolvent system with different water fractions. Reproduced with permission [70]. Copyright 2018, American Chemical Society.

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  Figure 11  (a) Design strategy of novel CPL switching system based on the AIE-active chiral [3]rotaxane 20 upon the addition or removal of acetate anions as external stimuli. (b) CPL spectra (left) and g_lum spectra (right) of the cast film of pS-20 and pR-20. (c) CPL spectra (left) and g_lum spectra (right) of the cast film of pS-20 and pR-20 upon the addition of acetate anions. Reproduced with permission [71]. Copyright 2021, Wiley-VCH GmbH.

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  Figure 12  (a) Synthesis of the AIE-active rotaxane dendrimers 23-Gn through the controllable divergent approach from [2]rotaxane 21 and DSA precursor 22 as key building blocks. (b) Fluorescence quantum yields of 23-G1 (left), 23-G2 (middle), and 23-G3 (right) in CH₂Cl₂/n-hexane with various n-hexane fractions. Reproduced with permission [76]. Copyright 2019, Royal Society of Chemistry.

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  Figure 13  (a) Synthesis of the AIEgen-branched rotaxane dendrimers 25-Gn through the controllable divergent approach from [2]rotaxane 24 (left) and fluorescence spectra of in CH₂Cl₂/MeOH with various MeOH fractions (right). Reaction conditions: (Ⅰ): (a) Bu₄NF·3H₂O, THF, r.t., 2 h; (b) 24, CuI, DCM/Et₂NH, r.t., overnight, 70%; (Ⅱ): (a) Bu₄NF·3H₂O, THF, r.t., 2 h; (b) 24, CuI, DCM/Et₂NH, r.t., overnight, 38%. (b) The construction of artificial LHSs based on TPE-branched rotaxane dendrimers 25-Gn and ESY as efficient photocatalysts. Reproduced with permission [81]. Copyright 2021, Wiley-VCH GmbH.

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  Figure 14  Chemical structures of the key building blocks (a) for the synthesis of rotaxane dendrimers 28-Gn as novel LHSs through the controllable divergent approach (b). Reaction conditions: (Ⅰ): (a) Bu₄NF·3H₂O, THF, r.t., 2 h, 87%; (b) 24, CuI, DCM/Et₂NH, r.t., overnight, 70%; Ⅱ): (a) Bu₄NF·3H₂O, THF, r.t., 2 h, 70%; (b) 24, CuI, DCM/Et₂NH, r.t., overnight, 71%. Reproduced with permission [82]. Copyright 2022, Elsevier Ltd.

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  Figure 15  Reversible transformations between [2]rotaxanes 29, 29′, 29′′, and 29′′′.

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  Figure 16  Tunable emission behaviors of these [2]rotaxanes in different states. Reproduced with permission [83]. Copyright 2020, American Chemical Society.

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