Sequence Engineering in Tuning the Circularly Polarized Luminescence of Aggregation-Induced Emission-Active Hetero[3]rotaxanes
2024; Chinese Chemical Society; Volume: 6; Issue: 10 Linguagem: Inglês
10.31635/ccschem.024.202303738
ISSN2096-5745
AutoresZhiyong Peng, Peipei Jia, Xu-Qing Wang, Xiao‐Li Zhao, Hai‐Bo Yang, Wei Wang,
Tópico(s)Synthesis and Properties of Aromatic Compounds
ResumoOpen AccessCCS ChemistryRESEARCH ARTICLES21 Feb 2024Sequence Engineering in Tuning the Circularly Polarized Luminescence of Aggregation-Induced Emission-Active Hetero[3]rotaxanes Zhiyong Peng, Pei-Pei Jia, Xu-Qing Wang, Xiao-Li Zhao, Hai-Bo Yang and Wei Wang Zhiyong Peng Shanghai Key Laboratory of Green Chemistry and Chemical Processes, State Key Laboratory of Petroleum Molecular & Process Engineering, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062 , Pei-Pei Jia Shanghai Key Laboratory of Green Chemistry and Chemical Processes, State Key Laboratory of Petroleum Molecular & Process Engineering, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062 , Xu-Qing Wang Shanghai Key Laboratory of Green Chemistry and Chemical Processes, State Key Laboratory of Petroleum Molecular & Process Engineering, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062 , Xiao-Li Zhao Shanghai Key Laboratory of Green Chemistry and Chemical Processes, State Key Laboratory of Petroleum Molecular & Process Engineering, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062 , Hai-Bo Yang Shanghai Key Laboratory of Green Chemistry and Chemical Processes, State Key Laboratory of Petroleum Molecular & Process Engineering, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062 Shanghai Center of Brain-Inspired Intelligent Materials and Devices, East China Normal University, Shanghai 200241 and Wei Wang *Corresponding author: E-mail Address: [email protected] Shanghai Key Laboratory of Green Chemistry and Chemical Processes, State Key Laboratory of Petroleum Molecular & Process Engineering, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062 https://doi.org/10.31635/ccschem.024.202303738 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Based on a [2]rotaxane precursor with exchangeable pentafluorophenyl ester stoppers, a new wheel-assembling approach has been successfully developed for the precise sequence control of hetero[3]rotaxanes, leading to the facile and efficient synthesis of both sequence isomers of hetero[3]rotaxanes. More importantly, taking advantage of the chirality retention along with the wheel-assembling process, corresponding sequence isomers of chiral AIEgen-functionalized hetero[3]rotaxanes were further precisely synthesized. Impressively, the resultant hetero[3]rotaxanes revealed remarkable sequence-dependent aggregation-induced emission (AIE) behavior and circularly polarized luminescence performance with large dissymmetry factors up to 0.012, highlighting the great power of the newly coined sequence engineering concept in developing novel AIE-active chiroptical materials. This proof-of-concept study lays the foundation for investigation of the structure-property relationships of heterorotaxanes that can further direct the rational design and precise synthesis of sequence-defined heterorotaxanes with desirable properties for practical applications. Download figure Download PowerPoint Introduction Due to their unique interlocked structure and controllable nanoscale dynamic motion behavior, rotaxanes, a fundamental type of mechanically interlocked molecules (MIMs),1–12 have proven to be excellent candidates for the construction of artificial molecular machines, as recognized by the 2016 Noble Prize in Chemistry for Sauvage13 and Stoddart,14 two pioneers in how to make rotaxanes and other MIMs and how to use them for the construction of molecular machines. Notably, during the past few decades, along with the blooming of rotaxane-based molecular machines,15–30 aiming at the construction of novel functional luminescent materials, we have witnessed rapid development of emissive rotaxanes and their wide applications in such diverse fields as bioimaging, sensing, and information storage.31–38 However, for most emissive rotaxanes with traditional fluorophores which usually suffer from the typical aggregation-induced quenching effect, their further applications in aggregated states are hampered.39,40 To deal with this key issue, attributed to the emerging and rapid development of aggregation-induced emission (AIE), the introduction of AIEgens (i.e. AIE-active luminogens)41–44 into rotaxanes provides an ideal solution, leading to novel AIE-active rotaxanes with attractive emission behavior and expanded application scope in light harvesting,45 photocatalysis,46 and circularly polarized luminescence (CPL).47 In particular, for most of these AIE-active rotaxanes, more attention has been paid to the influence of stimuli-induced rearrangements of AIEgens within the rotaxane skeletons through the movements of the wheel components, that is, the dynamic stereoisomers, in their emission behavior.48–51 However, due to synthetic difficulties, the effects attributed to static stereoisomers such as sequence and orientation of AIE behavior and CPL performance have never been investigated. For instance, for hetero[3]rotaxanes with two different wheel components, when the stoppers are different, there will be two sequence isomers.52,53 The investigation of sequence engineering in AIE-active heterorotaxanes are of great importance since it further enriches understanding of the complexity of rotaxanes in their stereochemical, functional, and dynamic aspects, making them even more attractive for the construction of novel smart luminescent materials for information storage and data processing.54–56 Notably, since the first synthesis of heterorotaxanes in 1995 by Stoddart and coworkers,57 various synthetic strategies such as the orthogonal binding approach,20,54,58,59 the self-sorting approach,36,60–62 the cooperative capture approach,53,63,64 the active metal template approach,65,66 and the molecular pumping approach29,67 have been developed, leading to the successful synthesis of heterorotaxanes with plentiful structures. However, most studies on sequence engineering of rotaxanes have demonstrated the synthesis of only one sequence isomer, and no functional groups have been introduced. Thus, the sequence engineering of their functions has not been achieved.68–71 Therefore, the synthesis of all sequence isomers and the further investigations of the sequence effects of AIE-active heterorotaxanes, particularly the chiral ones, remain to be achieved. To achieve this goal, starting from a [2]rotaxane precursor with exchangeable stoppers, the development of a novel wheel-assembling approach enables the precise synthesis of both sequence isomers of chiral hetero[3]rotaxanes with classical AIEgen as a stopper through the chirality retention process (Figure 1). More importantly, these sequence isomers exhibit remarkable sequence-dependent AIE behavior and CPL performance, attributed to their tunable intra- and intermolecular chirality information transmission, highlighting the great power of sequence engineering for the construction of novel functional rotaxanes with desirable properties for practical use. Figure 1 | Cartoon presentation of the synthesis of both sequence isomers of chiral AIE-active hetero[3]rotaxanes through wheel-assembling approach that reveals remarkable sequence-dependent CPL performance. Download figure Download PowerPoint Experimental Methods Synthesis of the key [2]rotaxane precursor 1 A Schlenk flask was charged with sebacyl chloride (0.47 g, 2.0 mmol), diethoxypillar[5]arene ( DEP[5]A; 4.7 g, 5.0 mmol). The Schlenk flask was then evacuated and backfilled with N2 three times. Next, 10 mL of the freshly distilled CHCl3 was added via syringe. The resultant solution was stirred for 1 h under −15 °C. Then the mixture of pentafluorophenol (0.81 g, 4.4 mmol) and Et3N (0.53 g, 5.0 mmol) in 3 mL freshly distilled CHCl3 was added to the solution under an inert atmosphere, and the reaction mixture was allowed to warm to room temperature and stirred overnight. The solution was concentrated and the residue was purified by column chromatography (SiO2; petroleum ether (PE)/dichloromethane (DCM) = 4:1, v/v) to obtain the white solid 1.8 g with the yield of 72%. In addition, an optimized column-free purification procedure has been also developed. After evaporation of the solvent the crude material was then dissolved in a minimum amount of CH2Cl2 and poured into n-hexane to precipitate DEP[5]A. The resultant filtrate was collected, concentrated and followed by dissolving in a minimum amount of CH2Cl2 again and poured into acetonitrile to precipitate target [2]rotaxane 1 with the yield of 66% on a multigram scale. 1H NMR (CDCl3, 300 MHz): δ = 6.92 (s, 10 H), 4.04–3.79 (m, 20 H), 3.76 (s, 10 H), 2.20–2.15 (m, 4 H), 1.46–1.24 (m, 38 H), 0.69–0.59 (m, 4 H), −0.91 (brs, 4 H), −1.42 (brs, 4 H); 19F NMR (CDCl3, 282 MHz): δ = −153.59, −157.95, −162.64; 13C NMR (125 MHz, CDCl3, 298 K): δ = 170.00, 149.81, 128.59, 114.43, 63.69, 33.13, 31.59, 30.36, 29.32, 27.75, 27.60, 24.94, 15.47, 1.17, 0.14; high-resolution electron spray ionization mass spectroscopy (HR ESI-MS): calcd for [C77H86F10O14]+, 1424.5858; found, 1424.5858. General protocol for wheel-assembling process To a stirred solution of key [2]rotaxane precursor 1 (1.0 equiv) in tetrahydrofuran (THF) were added benzylamine derivative (1.0 equiv). The reaction mixture was stirred for 48 h at room temperature. The solvent was removed under reduced pressure, and the residue was purified by flash column chromatography (SiO2, PE/DCM = 1:1, v/v). The resultant intermediate (1.0 equiv) was dissolved in Tol./DCM with benzylamine derivative (1.0 equiv) and crown ether (6.0 equiv). The reaction mixture was stirred at room temperature for 48 h. The solvent was removed under reduced pressure and the residue was purified by preparative thin layer chromatography (SiO2, DCM/ethyl acetate (EA) = 8:1, v/v). Results and Discussion In our study, inspired by recent developments in postsynthetic modification of rotaxanes,72–77 a wheel-assembling approach was proposed for the efficient, precise, and diverse synthesis of both sequence isomers of hetero[3]rotaxanes. First, to shorten the synthetic routes and improve the synthetic efficiency, a [2]rotaxane with exchangeable stoppers was employed as the key precursor. Along with the stopper-exchange reaction that introduced specific functional groups as new stoppers, additional wheel components were also introduced through a metal-free active template strategy,67,78–80 thus leading to the evolution from the initial [2]rotaxane to corresponding [3]rotaxanes. Starting from the [2]rotaxane with the same two exchangeable stoppers, by controlling the sequence of the stopper-exchange reactions, both the functional groups and wheel components were facilely assembled from either side, thus resulting in the precise synthesis of both sequence isomers of hetero[3]rotaxanes in a controllable divergent way. More importantly, if a chiral [2]rotaxane with a chiral wheel was employed as the starting precursor, the corresponding functional chiral [3]rotaxanes could be obtained since both the stopper exchange and wheel-assembling process would not lead to the dethreading of the original wheel component, thus resulting in the chirality retention of the hetero[3]rotaxanes. With such a design strategy in mind, in this study, we introduced pentafluorophenyl ester moieties81–83 as exchangeable stoppers. On the one hand, the existence of pentafluorophenyl ester stoppers could promote the facile structural analysis of the key intermediates with fluorine NMR spectra. On the other hand, the pentafluorophenyl ester stoppers significantly enhanced the solubility of [2]rotaxane precursor that enabled the chiral resolution on a large scale, thus further making the investigations on the chirality retention in the wheel-assembling process feasible. In addition, considering the unique planar chirality of pillar[5]arene,84–89 its host–guest complex with neutral alkyl chain was employed as the rotaxane moiety for the synthesis of chiral rotaxanes.90,91 The reaction of sebacoyl chloride with pentafluorophenol in the presence of an excess of DEP[5]A and Et3N in CHCl3 afforded the desired [2]rotaxane precursor 1 (Figure 2 and Supporting Information Scheme S1). A small amount of the corresponding axle component byproduct was also generated from the reaction of uncomplexed sebacoyl chloride with pentafluorophenol. The [2]rotaxane 1 was prepared and conveniently separated by silica gel column chromatography in several grams scale with a yield of 72%. More importantly, an optimized column-free purification procedure has also been developed. After confirming the completion of the reaction through thin-layer chromatography monitoring and evaporation of the solvent, the crude material was then dissolved in a minimum amount of CH2Cl2 and poured into n-hexane to precipitate DEP[5]A. The resultant filtrate was collected, concentrated, and followed by dissolving it in a minimum amount of CH2Cl2 again and poured into acetonitrile to precipitate target [2]rotaxane 1 with the yield of 66% on a multigram scale. Figure 2 | The synthetic route and single-crystal X-ray structures of both enantiomers of [2]rotaxanes 1 as the key precursor for the synthesis of sequence isomers of hetero[3]rotaxanes. Download figure Download PowerPoint [2]rotaxane 1 is highly soluble in common solvents and perfectly stable under normal laboratory conditions for several months, which not only makes the resolution of the enantiomers feasible but also lays the foundation for the further synthesis of hetero[3]rotaxanes. The optical resolution of 1 was successfully carried out by preparative chiral stationary phase-high performance liquid chromatography (CSP-HPLC), and the absolute configurations of [2]rotaxane 1 in the first fraction and the second fraction were assigned to be Rp and Sp according to their single crystal X-ray structures, respectively (Figure 2, Supporting Information Figures S1–S5, S41, S42, S54–S61, and Table S2). Notably, both enantiomers were easily resolved on the scale of several hundreds of milligrams, which were sufficient for the synthesis of corresponding chiral hetero[3]rotaxanes, particularly the AIEgen-functionalized ones. With the key [2]rotaxane precursor 1 in hand, the further synthesis of sequence isomers of corresponding hetero[3]rotaxanes were then carried out. In our study, additional crown ether wheel components were further assembled from either side of the initial [2]rotaxane through metal-free active template strategy along with the stopper exchange process ( Supporting Information Scheme S2).67,78,79 Thus from the same [2]rotaxane precursor 1, when one additional crown ether wheel and two different stoppers are introduced, there will be two sequence isomers. In order to firstly confirm the feasibility of newly-developed wheel-assembling approach, 3,5-bis(trifluoromethyl)benzylamine and 3,5-dibromobenzylamine ( Supporting Information Scheme S3 and Figure S53) were selected as two different stoppers. As shown in Figure 3a, treatment of [2]rotaxane 1 with equimolar 3,5-bis(trifluoromethyl)benzylamine at room temperature for 48 h in THF afforded monosubstituted [2]rotaxane 2 with a yield of 67%. By further employing the metal-free active template approach, the reaction of monosubstituted [2]rotaxane 2, 3,5-dibromobenzylamine and dibenzo-24-crown-8 ( DB24C8) in a ratio of 1:2:6 in toluene/DCM (v/v, 1:1) resulted in the successful preparation of one sequence isomer, hetero[3]rotaxane HR-S1, in 43% yield ( Supporting Information Scheme S3). To obtain the other sequence isomer HR-S2, 3,5-dibromobenzylamine was first introduced as stopper to afford the monosubstituted [2]rotaxane 3 through the stopper-exchange reaction in 60% yield. The sequential stopper-exchange reaction with 3,5-bis(trifluoromethyl)benzylamine in the presence of DB24C8 resulted in the successful synthesis of HR-S2 in 80% yield. Notably, for both sequence isomers, the DB24C8 wheel was also introduced in the first stopper-exchange process, resulting in the corresponding hetero[3]rotaxane intermediates 4 and 5 in 21% and 59% yields, respectively. Notably, the chemical structures of these two key intermediates were unambiguously confirmed by single-crystal X-ray analysis (Figure 3c and Supporting Information Figures S43 and S44). The further stopper-exchange reaction would then led to the successful synthesis of targeted sequence isomers in 85% and 84% yields, respectively. Figure 3 | (a) The wheel-assembling approach for the precise synthesis of both sequence isomers of hetero[3]rotaxanes HR-S1 and HR-S2. Reaction conditions: (i) 3,5-bis(trifluoromethyl)benzylamine (1.0 equiv), THF, rt., 48 h, 67%; (ii) 3,5-dibromobenzylamine (2.0 equiv), DB24C8 (6.0 equiv), Tol./DCM, rt., 48 h, 43%; (iii) 3,5-dibromobenzylamine (1.0 equiv), THF, rt., 48 h, 60%; (iv) 3,5-bis(trifluoromethyl)benzylamine (2.0 equiv), DB24C8 (6.0 equiv), Tol./DCM, rt., 48 h, 80%; (v) 3,5-dibromobenzylamine (1.0 equiv), DB24C8 (6.0 equiv), Tol./DCM, rt., 48 h, 21%; (vi) 3,5-bis(trifluoromethyl)benzylamine (4.0 equiv), THF, reflux, 48 h, 85%; (vii) 3,5-bis(trifluoromethyl)benzylamine (1.0 equiv), DB24C8 (6.0 equiv), Tol./DCM, rt., 48 h, 59%. (viii) 3,5-dibromobenzylamine (4.0 equiv), THF, reflux, 48 h, 84%. (b) The 1H NMR spectra (CDCl3, 298 K, 500 MHz) of HR-S1 and HR-S2. (c) Single-crystal X-ray structures of the key intermediates 4, 5, and the resultant hetero[3]rotaxane sequence isomers HR-S1 and HR-S2 (only the Rp isomers are shown) Download figure Download PowerPoint The resultant sequence isomers described above were fully characterized using 1-D multinuclear (1H, 13C, and 19F), 2-D rotating frame overhauser effect spectroscopy (ROESY) techniques, HR ESI-MS, and unambiguous single-crystal X-ray analysis ( Supporting Information Figures S62–S91 and Tables S3–S5). As revealed by 1H NMR spectra (Figure 3b, Supporting Information Figures S82 and S87), for the sequence isomers of both HR-S1 and HR-S2, attributed to the varied shielding effects of wheels in different sequences, remarkable differences in chemical shifts were observed. For instance, for HR-S1, due to the strong shielding effect of DEP[5]A, the highest upfield chemical shifts of ca. −2.18 ppm were observed for H3 in the alkyl chain, while the same case was found for H6 for HR-S2. Such obvious differences were reasonable since these two protons were proven to be located in the middle cavity of DEP[5]A of each sequence by single-crystal X-ray structures (Figure 3c and Supporting Information Figures S45 and S46). According to the above-discussed results, by simply combining different stoppers and wheels as well as programming the synthetic sequences, the synthesis of both sequence isomers of diverse hetero[3]rotaxanes was confirmed to be precisely controllable, thus laying the foundation for further explorations of their unique properties such as the AIE and CPL behavior. After confirming the successful development of the wheel-assembling approach for the synthesis of both sequence isomers of hetero[3]rotaxanes, the further synthesis of the sequence isomers of AIE-active hetero[3]rotaxanes was then carried out. In our study, the typical AIEgen tetraphenylethene (TPE) was selected as the functional stopper.42,92–95 Notably, since we have a sufficient amount of chiral [2]rotaxane precursors Rp- 1 and Sp- 1, they were directly used for the synthesis of sequence isomers of chiral AIEgen-functionalized hetero[3]rotaxanes. However, when synthesizing the targeted sequence isomers with DB24C8, the one with the DB24C8 wheel close to the TPE stopper could not be achieved, possibly because the weaker N–H···O hydrogen bond could not overcome the steric hindrance and so was unable to assemble the DB24C8 wheel, only leading to the synthesis of corresponding [2]rotaxane TPE-2R ( Supporting Information Scheme S4 and Figures S116–S119). In this case, considering the possible steric hindrance effect, a 24-crown-8 ( 24C8) was then employed as the wheel component instead.80 To our delight, this more flexible wheel with less steric hindrance could be successfully assembled, thus leading to the successful synthesis of both the sequence isomers TPE-S1 and TPE-S2 of functional hetero[3]rotaxanes through the key intermediates 2 and 6, respectively (Figure 4a, Supporting Information Scheme S5, and Figures S92–S115). Figure 4 | (a) The wheel-assembling approach for the synthesis of both sequence isomers of AIEgen-functionalized hetero[3]rotaxanes TPE-S1 and TPE-S2, and their single-crystal X-ray structures (only the Sp isomers are shown). Reaction conditions: (i) 3,5-bis(trifluoromethyl)benzylamine (1.0 equiv), THF, rt., 48 h, 67%; (ii) TPE-NH2 (1.0 equiv), THF, rt., 48 h, 50%; (iii) TPE-NH2 (2.0 equiv), 24C8 (6.0 equiv). Tol., 0 °C, 48 h, 10%; (iv) 3,5-bis(trifluoromethyl)benzylamine (2.0 equiv), 24C8 (6.0 equiv). Tol., rt., 48 h, 80%; (b) Chirality retention in the wheel-assembling approach. Chiral HPLC traces (n-Hex:EA = 96:4, v/v) of rac-1, Rp-1 (the first fraction of rac-1), and Sp-1 (the second fraction of rac-1) (left). Chiral HPLC traces (n-Hex:EA = 88:12, v/v) of rac-6, Rp-6 (synthesized from Rp-1) and Sp-6 (synthesized from Sp-1) (middle). Chiral HPLC traces (n-Hex:EA = 75:25, v/v) of rac-TPE-S2, Rp-TPE-S2 (synthesized from Rp-6) and Sp-TPE-S2 (synthesized from Sp-6) (right). Download figure Download PowerPoint Notably, upon the synthesis of these chiral hetero[3]rotaxanes sequence isomers, both the simple stopper-exchange and associated wheel-assembling process would not allow the dethreading of the DEP[5]A wheel since the key reaction intermediates could be regarded as even "bigger" stoppers ( Supporting Information Scheme S2). Thus, the direct employment of chiral [2]rotaxane Rp- and Sp- 1 as starting materials led to the synthesis of corresponding sequence isomers of chiral AIEgen-functionalized hetero[3]rotaxanes through the chirality retention process (Figure 4b, Supporting Information Schemes S6, S7, and Figures S6–S22). As revealed by the chiral HPLC traces, starting from Rp- 1 (99.4:0.6 e.r.) and Sp- 1 (99.5:0.5 e.r.), the successful synthesis of corresponding chiral [2]rotaxane intermediates Rp- 6 and Sp- 6 were realized with e. r. values of 99.5:0.5 and 99.7:0.3, respectively ( Supporting Information Figures S23–S27). Moreover, starting from these chiral intermediates Rp- 6 and Sp- 6, the further stopper-exchange reaction resulted in the successful synthesis of Rp- TPE-S2 and Sp- TPE-S2 with no degradation of e.r. values ( Supporting Information Figures S28–S32), and the chirality of the targeted AIEgen-functionalized hetero[3]rotaxanes were confirmed by single-crystal X-ray structures ( Supporting Information Figures S47–S52 and Tables S6, S7), demonstrating chirality retention during the whole synthetic routes. Having both sequence isomers of AIE-active hetero[3]rotaxanes in hand, their sequence-dependent AIE behavior and CPL performance were then investigated. For these two sequence isomers, the key difference is the arrangements of chiral wheel and AIEgen stopper along the axle components. The different structural features of pillar[5]arene and crown ether (rigid vs flexible and chiral vs achiral) would have great impact in the property engineering of the resultant isomers, thus the sequence-dependent AIE behavior of TPE-S1 and TPE-S2 were first evaluated ( Supporting Information Figures S33, S34, and Table S1). For both isomers, as revealed by the fluorescence spectra, no emission was observed in pure acetone. Upon the increase of the fraction of water as poor solvent (f, vol %) from 10% to 50%, only slight increase of the emission intensity was observed. By further increasing the f to 60% and finally to 95%, the remarkable enhancement in the emission intensity was revealed, thus indicating the typical AIE behavior. However, different from TPE-S1 which maintains cyan emission during the aggregation process (Figure 5a and Supporting Information Figure S33a), TPE-S2 revealed an obvious bathochromic shift from blue (458 nm) to cyan (472 nm) as f was increased from 70% to 80%, indicating a hierarchical aggregation behavior (Figure 5d and Supporting Information Figure S33b). Such interesting phenomenon might be attributed to the fact that the steric hindrance of the rigid pillar[5]arene wheel prevented the intermolecular aggregation of the nearby TPE unit when f < 80%, resulting in a blue emission in the initial aggregate state. The further increase of water made the intermolecular aggregation possible, thus leading to a bathochromic shift in the cyan emission. Figure 5 | Fluorescence spectra (λex = 330 nm) and photographs (upon UV lamp excitation at 365 nm) of (a) TPE-S1 and (d) TPE-S2 in acetone/H2O with different H2O fractions ([c] = 0.01 mM). SEM images of TPE-S1 (f = 70%, b; f = 90%, c) and TPE-S2 (f = 70%, e; f = 90%, f) in different aggregate states. Download figure Download PowerPoint To provide additional supports for such AIE phenomenon described above, scanning electron microscopy (SEM) analysis was further performed to investigate the aggregation process of these two sequence isomers. As revealed by the SEM images, the formation of homogenous nanospheres in the aggregate state were observed for TPE-S1 when the f was 70% (Figure 5b and Supporting Information Figure S35). However, irregular morphology was observed for TPE-S2 when the f was 70% (Figure 5e and Supporting Information Figure S35). Upon further increasing the water content to 90%, both sequence isomers revealed very similar homogenous nanosphere morphologies, which might be attributed to the large steric hindrance of the DEP[5]A wheel and TPE stopper that induced the formation of large curvature ( Supporting Information Figure S36). Such phenomenon were in consistent with the fluorescence spectra (Figure 5c,f), both of which indicated the sequence-dependent hierarchical aggregation behavior that led to varied emission performance. With the targeted AIE-active chiral hetero[3]rotaxane isomers in hand, their chiroptical properties were then evaluated. The circular dichroism (CD) spectra of both Rp-/Sp- TPE-S1 and Rp-/Sp- TPE-S2 indicated that there was no obvious chirality information transfer from the DEP[5]A wheel to the TPE stopper in solution and aggregate state, as revealed by the fact that only CD peaks around 308 nm attributed to the DEP[5]A wheel were observed. Considering that chiral information transfer would be more effective in the solid state, the CD spectra of Rp-/Sp- TPE-S1 and Rp-/Sp- TPE-S2 in solid state were then recorded. As revealed by the CD spectra, for both Rp-/Sp- TPE-S1 and Rp-/Sp- TPE-S2, peaks around 308 nm were observed, which was consistent with that in solution and aggregate state. Moreover, weak bands at 348 nm, which were ascribed to the TPE stopper, were found, suggesting very weak chirality information transfer in the solid state (Figure 6a,b and Supporting Information Figure S38). It is worthwhile noting that the CD peaks at 348 nm of Rp-/Sp- TPE-S2 were slightly higher than those of Rp-/Sp- TPE-S1, suggesting more effective chirality information transfer in Rp-/Sp- TPE-S2. Figure 6 | Circular dichroism and UV–vis spectra of (a) Rp-/Sp-TPE-S1 and (b) Rp-/Sp-TPE-S2 in solution (THF, [c] = 0.1 mM), aggregate (THF:H2O = 10:90), and solid state (KBr pellet), inset: partial CD spectra at the range of 320–420 nm. Download figure Download PowerPoint Encouraged by the sequence-determined chirality information transfer in the ground state, the sequence engineering in the CPL performance of these AIE-active chiral hetero[3]rotaxane isomers was then investigated. CPL tests of these two sequence isomers Rp-/Sp- TPE-S1 and Rp-/Sp- TPE-S2 (Figure 7a,b) indicated that both of them revealed intense CPL signals at 466 nm that originated from the TPE unit, indicating successful chirality information transfer from the chiral DEP[5]A wheel to the TPE unit. As expected, the Rp- TPE-S1/Sp- TPE-S1 resulted in almost mirror-imaged CPL signals with opposite glum. That is, a positive CPL signal for the Rp- TPE-S1 was observed, whereas Sp- TPE-S1 displayed a negative signal, again suggesting a pure CPL response. The glum value of Rp- TPE-S1 and Sp- TPE-S1 were 4.0 × 10−3 and −4.0 × 10−3, respectively. More imp
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