Circularly Polarized Luminescence from a Pyrene-Cyclodextrin Supra

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Interface-Rich Materials and Assemblies

Circularly Polarized Luminescence from a Pyrene-Cyclodextrin Supra-Dendron Yuening Zhang, Dong Yang, Jianlei Han, Jin Zhou, Qingxian Jin, Minghua Liu, and Pengfei Duan Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01035 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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Circularly Polarized Luminescence from a PyreneCyclodextrin Supra-Dendron Yuening Zhang,†,‡ Dong Yang,‡,§ Jianlei Han,‡ Jin Zhou,‡ Qingxian Jin*† Minghua Liu,*‡,§ and Pengfei Duan*‡ †

Henan Provincial Key Laboratory of Surface and Interface Science, Zhengzhou University of Light

Industry, Zhengzhou, Henan 450002, P.R. China ‡

Chinese Academy of Sciences (CAS) Key Laboratory of Nanosystem and Hierarchical Fabrication,

CAS Center for Excellence in Nanoscience, Division of Nanophotonics, National Center for Nanoscience and Technology (NCNST), No. 11 ZhongGuanCun BeiYiTiao, 100190 Beijing, P.R. China. §

Beijing National Laboratory for Molecular Science, CAS Key Laboratory of Colloid, Interface and

Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, No. 2 ZhongGuanCun BeiYiJie, 100190 Beijing P. R. China.

ACS Paragon Plus Environment

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ABSTRACT: Soft nanomaterials with circularly polarized luminescence (CPL) have been currently attracting great interests. Here, we report a pyrene-containing π-peptide dendrons hydrogel, which showed 1D and 2D nanostructures with varied CPL activities. It was found that the individual dendron formed hydrogels in a wide pH range (3-12) and self-assembled into helices with pH-tuned pitches. Through chirality transfer, the pyrene unit could show CPL originated from both the monomer and excimer bands. When cyclodextrin was introduced, different supra-dendrons were obtained with β-cyclodextrin (PGAc@β-CD) and γ-cyclodextrin (PGAc@γ-CD) through host-guest interactions, respectively. Interestingly, the PGAc@β-CD and PGAc@γ-CD supra-dendrons self-assembled into 2D nanosheet and entangled nanofibers, respectively, showing cyclodextrin induced circularly polarized emission from both the monomer and excimer bands of pyrene moiety. Thus, through a simple host-guest interaction, both the nanostructures and the chiroptical activities could be modulated.

1. INTRODUCTION Chiral functional materials showing circularly polarized luminescence (CPL) have been attracting increasing attentions due to the wide applications including sensors,1-3 asymmetric synthesis,4-5 biological probes6-7 and manufacturing new type of optoelectronic devices.8-10 While it is key issue to find the molecules or materials with larger dissymmetry factors (glum), it is also important to regulate the CPL with intensities and signs for the versatile applications. To realize the CPL, a precondition for creating the chiral system is generally necessary. Supramolecular chemistry is used to influence the arrangements of the molecules through noncovalent forces, aimed at designing and implementing functional chemical systems based on molecular components.11-22 Supramolecular gelation, in which small molecules self-assemble into entangled nanostructures to immobilize the solvents, provides various pathways in regulating the supramolecular chirality of the self-assembly systems.23-29 Therefore, it is expected to modulate the CPL as an excellent candidate pathway. Here, we designed a π-peptide dendron, which contains three L-glutamic acid unit and an emissive moiety pyrene, as shown in Figure 1. The former assured the gelation of the dendron, while the latter showed the luminescence. In addition, the pyrene unit could show monomer and excimer emissions under different conditions. We have found that the π-peptide dendron could form hydrogel with a wide-


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pH adaptability and self-assembled into helical nanofibers (Figure 1a). The helical fibers showed supramolecular chirality as well as CPL activity with varied dissymmetry factor depended on the pH. More interestingly, when cyclodextrin was introduced, supra-dendron based on the hostguest interactions between β- and γ-cyclodextrin with pyrene can be obtained (Figure 1b, c). Due to such host-guest interactions, the self-assembled nanostructures could be altered. Previously, we have demonstrated an example of supra-dendron composed of an azobenzenebased dendron and cyclodextrin.30 Similar to the concept of supra-amphiphile, demonstrated by Zhang et al,31-33 supra-dendron also showed excellent assembly behavior in aqueous. In this work, specifically, the PGAc@β-CD showed the CPL exclusively from monomer with the formation of 2D lamellar nanostructures (Figure 1b), while PGAc@γ-CD showed CPL predominantly from excimer with the formation of entangled fibrous structures (Figure 1c). Moreover, the CPL sign of the emissions from monomer and excimer were opposite. So far, various soft chiral materials have been reported to show CPL,34-37 here, we show the first example that both the CPL intensities and the luminescent pathway could be modulated by the helical pitches and the hostguest interactions.


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Figure 1. Illustration of various hydrogels formed by different dendron gelators: (a) π-peptide dendron PGAc could form hydrogel with the uniform twisted fibers; (b) hydrogel formed by PGAc@β-CD supradendron exhibited 2D lameller structures observed from the AFM images; (c) PGAc@γ-CD supradendron formed nanofiber strucutres with excimer-based CPL.

2. MATERIALS AND METHODS 2.1 Chemicals and synthesis: All starting materials were commercially available and used as received. Solvents were of spectral purity and used without further purification. Synthesis of N-Boc-1,5-bis (L-glutamic acid dibenzyl ester)-L-glutamic diamide (BocGBn): L-Glutamic acid dibenzyl ester hydrochloride (10.92 g，0.03 mol，1.2eq) was dissolved in dry CH2Cl2 (125 mL, 250 mL flask), then added triethylamine (TEA; 10 mL) to the solution. The mixture was stirred for 30 min at room temperature. Then tert-butoxycarbonyl (Boc)-


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glutamic acid (3.09 g, 0.012 mol) was added to the above solution, then the mixture was cooled at 0˚C by ice bath and stirred for another 30 min. 1-Ethyl-3-(3-dimethyllamino-propyl) carbodiimide hydrochloride (EDC•HCl) (5.74 g, 0.03 mol) and 1-hydroxybenzotrizole (HOBt, 4.07 g, 0.03 mol) were subsequently added to the mixture, which was stirred at 0˚C for 3 h, then the ice bath was removed and the reaction system was stirred for another 72 h at room temperature. The solution was washed with an aqueous saturated solution of sodium chloride (3×30 mL) and water and the organic phase was dried over magnesium sulfate. The solvent was removed by rotary evaporation and crude product was obtained. After purification by silica column chromatography (DCM/ MeOH 50:1, Rf = 0.26), the target products was obtained as an oil-like compound in the initial, and it would be the white solid after a period of time (9.58 g, 75 %). 1H NMR (400MHz, CDCl3): δ=7.80 (d, J=7.2 Hz, 1H), 7.54 (d, J=7.4 Hz, 1H), 7.31 (s, 20H), 5.06-5.14 (m, 8H), 4.79-4.85 (m, 1H), 4.71-4.77 (m, 1H), 3.99 (s, 1H), 1.93-2.51 (m, 12H), 1.381.42 ppm (m, 9H); MALDI-TOF-MS m/z: calcd for C48H55N3O12: 865.7; found: 888.7 [M+Na]+ , 904.7 [M+K]+. Synthesis of N-(4-(1-pyrene) butyroyl), 5-bis (L-glutamic acid dibenzyl ester)-Lglutamic diamide (PyG3Bn): Step I: The compound Boc-GBn (0.87 g, 1.0 mmol) was dissolved in dried CH2Cl2 (45 mL, 100 mL flask) and trifluoroacetic acid (TFA, 45 mL) was added to the flask and the mixture was stirred at room temperature for 2 h. After the reaction, the solvent was removed by rotary evaporation. The obtained oil-like compound, CF3COO-NH3+GBn could be used without further purification. Step II: CF3COO-NH3+-GBn (1.0 mmol) was dissolved in dry CH2Cl2 (50 mL) and TEA (3 mL) was added to the solution. The mixture was stirred at 0˚C for 30 min. Then 1-pyrenebutyric acid (0.29 g, 1.0 mmol) was added to the above solution and the mixture was stirred at 0˚C for another 30 min. Then EDC (0.23 g, 1.2 mmol) and HOBt (0.16 g, 1.2 mmol) were added to the mixture. The obtained mixture was stirred at 0˚C for another 30 h, and then the ice bath was removed. The reaction system was stirred for another 12 h at room temperature, then the solvent was removed by rotary evaporation at 40˚C and the crude product was obtained. After having been purified by silica column chromatography (DCM/MeOH 20:1, Rf = 0.31), the target product was obtained as a fine yellow solid (0.72 g, 70%). MALDI-TOF-MS m/z: calcd for C63H61N3O11: 1035.4; found: 1058.5 [M+Na]+, 1074.4 [M+K]+; elemental analysis calcd (%) for C63H61N3O11: C 73.03, H 5.93, N 4.06; found: C 72.81, H 5.95, N 4.08.


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Synthesis of N-(4-(1-pyrene) butyroyl), 5-bis(L-glutamic acid)-L-glutamic diamide (PGAc): Compound PyG3Bn (0.52 g, 0.5 mmol) was dissolved in the solution contained tetrahydrofuran (50 mL) and water (10 mL), then catalytically hydrogenated under 0.4 MPa with 10% Pd/C (0.05 g) in a high pressure apparatus for 12 h. After this time, the mixture was filtered and the filtrate was evaporated to remove the solvent to yield a yellow solid (0.31 g, 92%). 1H NMR (400 MHz, DMSO): δ=12.44 (br s, 4H), 7.01-8.39 (m, 12H), 4.14-4.46 (m, 3H), 3.01-3.07 (m, 2H), 2.21-2.29 (m, 4H), 1.74-1.97 (m, 9H), 1.23-1.53 (m, 2H); MALDI-TOF-MS m/z: calcd for C35H37N3O11: 675.7; found: 698.2 [M+Na]+; elemental analysis calcd (%) for C35H37N3O11·2MeOH: C 60.07, H 6.13, N 5.68; found: C 59.15, H 6.40, N 5.70. 2.2 Formation of hydrogels: Fabrication of the PGAc hydrogels at different pH values: PGAc was mixed in a sealed tube with different pH aqueous solutions, which were adjusted by NaOH and HCl solutions. The mixtures were heated up to 80˚C for 2-3 min to form a light yellow solution, and subsequent the sealed hot test tube was cooled down to room temperature unaffectedly. After 5-6 hours, PGAc hydrogel were obtained. We fabricated and tested all the PGAc hydrogels at critical gelation concentration (CGC). Fabrication of the cogels: cyclodextrin and PGAc were added to a capped test tube with water, the mixture were heated up to 80˚C for 2-3 min until the solids were dissolved completely. The solution was subsequently cooled down to room temperature under ambient conditions. After 5-6 hours, the cogel formed (PGAc = 25 mM, molar ratio of PGAc/α-CD = 1/1, PGAc@β-CD = 1/1, PGAc@α-CD = 2/1). The formation of PGAc hydrogels and cogels were determined by the absence of flow of the solvent when the tube was inverted. 2.3 Instruments: UV-Visible spectroscopy study: UV-Vis spectra were measured with a Hitachi U-3900 Spectrophotometer with a 1mm path length quartz cell, and the scan speed and slit width were 1200 nm/min and 1 nm, respectively. All the samples were scanned at room temperature (25°C). Fluorescence spectroscopy study: The fluorescence emission spectra were performed on a Hitachi F-4600 instrument with a 0.1 mm path length quartz cell at room temperature. The excitation and emission slit width were 5 nm and 5 nm, respectively. Atomic force microscopic (AFM) study: AFM images were obtained on a Dimension FastScan (Bruker) using ScanAsyst mode under ambient conditions. Fast Scan B probes were used for the scan. AFM studies were done by placing a small amount of dilute hydrogels on the silicon wafers. The


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materials were allowed to dry in air by slow evaporation first and then under vacuum at room temperature for one day. Transmission electron microscopy (TEM) study: TEM images were obtained on a JEM-100CX II electron microscope operating at an accelerating voltage of 100 kV. Before TEM measurements, the samples were suspended on carbon-coated Cu grids and then allowed to dry under vacuum at room temperature for one day. The rheological properties of the gel were measured at 25±0.05°C with a Thermo Haake RS300 rheometer (cone and plate geometry of 40 mm in diameter). Fourier Transform Infrared Spectroscopy (FTIR) study: FTIR spectra were measured with JASCO FT/IR-660 plus. X-Ray Diffraction (XRD) study: X-ray diffraction (XRD) patterns were achieved on a Rigaku D/Max-2500 X-ray diffractometer (Japan) with CuKα radiation (λ = 1.5406 Å), which was operated at 45 kV, 100 mA. Before FTIR and XRD measurements, the samples were allowed to dry under vacuum at room temperature for one day and then ground into the powder. AFM images, TEM images, FTIR spectra and XRD spectra of hydrogels were recorded in the xerogel state. Circular dichroism (CD) spectroscopy study: Circular dichroism spectroscopy was used for determining the chiral molecular arrangement of assemblies within hydrogels. All CD spectra were recorded by using a quartz cuvette of 0.1 mm path length in a Jasco J-815 spectropolarimeter under a nitrogen atmosphere at room temperature. Circularly polarized luminescence (CPL) study: The CPL spectra were recorded from 350 nm to 650 nm using a JASCO-CPL-810 spectrometer and the excitation/emission wavelength for CPL spectra was 320 nm/475 nm. 0.1 mm cuvettes were used for measuring the CPL spectra. The glum value was evaluated by the formula glum = [(ln10/32980) × ellipticity (in mdeg)]/DC (in V). 1H nuclear magnetic resonance (1H NMR) spectra study: 1H NMR spectra were recorded with a Bruker ARX400 (400 MHz) spectrometer in DMSO by using Me4Si as an internal standard. Mass spectra (MS) study: MS spectra were determined with BEFLEX III for the MALDI-TOF mass spectrometer. Elemental analyses (EA) study: Elemental analyses were performed on a Carlo-Erba-1106 instrument.

3. RESULTS AND DISCUSSION 3.1 Hydrogelation and pH-regulated helical pitch as well as CPL by the individual dendron The π-peptide is shown in Figure 1a, which contains three glutamic acids and a pyrene unit. The multiple hydrogen-bonding sites assist the peptide to form the hydrogels easily while pyrene


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group could show strong π-π stacking in molecular self-assembly.38 PGAc has a high solubility in various organic solvents, such as ethanol, DMF, DMSO, THF. However, the compound could form hydrogels in aqueous solution of various pH values. The hydrogels were fabricated through annealing process in water. The solid PGAc was dissolved in aqueous solution with various pH from 3 to 12 by heating up to 80˚C for 2-3 min to form a transparent solution. After subsequent cooling to room temperature, hydrogels were obtained. The gel formation and photoluminescence behaviour were confirmed respectively by inverse tube tests and irradiation with ultraviolet light. Some of which are shown in Figure 2a. We measured one of the basic parameters for the hydrogels formed at various pH values, the lowest critical gelation concentration (CGC). As shown in Figure 2b, the CGC at pH = 5 is 13.4 mM while the value reach to 33.3 mM at pH = 12. From pH 12 to 3, the transparency and viscosity of the hydrogels decreased gradually. We tried also even lower or higher pH values. They either precipitate at pH < 3 or formed a clear solution at pH > 12. To gain an insight into the nanostructures of hydrogels obtained in various pH values, atomic force microscope (AFM), and transmission electron microscope (TEM) of the xerogels were measured. The AFM images of the xerogels in different pH conditions revealed morphological details of nanofibers (Figure 2c, Figure S1). Right-handed helical nanofiber with different tightness was observed from the hydrogels formed in aqueous solutions at pH 3-12. The helical nanofibers have the smallest pitch at pH = 3 and largest pitch at pH 12. The pitch of these helical nanofibers increased with the pH value. As shown in Figure 2d, the statistical pitch of helical nanofibers become larger with increasing the pH value which suggests that low pH would lead to tight twisting of the helical fibers. In addition, elementary helical nanofibers could selfassembled into larger coiled nanostructure (Figure S1, d). TEM observation confirmed that the 1D nanostructure was in fact a solid structure with the thickness of 30 nm (Figure 2e). These results indicated that PGAc could form helical fibrous nanostructures in all tested pH conditions. Low pH resulted in small helical pitch while loose helix with large pitch formed in high pH condition (Figure 2f). This might be due to the deprotonation in the high pH condition. By adding base to the hydrogel system, the multiple carboxylic acid groups can be gradually deprotonated which will significantly supress the formation of hydrogen bonding. The weak intermolecular interaction will weaken the twisting behaviour of the assembled structures. This


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could be the reason why loose helix with relatively larger pitch could be observed in high pH conditions.

Figure 2. a) Photographs of PGAc hydrogels prepared from aqueous solutions of various pH under an ultraviolet spotlight. b) Plot of the critical gelation concentration (CGC) as a function of pH. c) Height AFM image of PGAc hydrogel obtained from pH 8 (scale bar = 200 nm; the inset photograph is measure of helical pitch in Nanoscope Analysis). d) Statistical chart of helical pitch in AFM images. e)


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TEM image of PGAc hydrogel obtained at pH 7. f) Schematic illustration of the assembly modes of PGAc hydrogels in different pH.

Remarkable observations related to interactions of the chromophores of PGAc in hydrogels were provided by the CD spectra. The compound has chiral centre and luminophore, which could form helical fibers through π-π stacking and hydrogen bonding in water. Figure 3a shows the pHdependent CD spectra of PGAc in various pH conditions. In low pH condition (pH = 3), strong Cotton effect could be observed around 350-375 nm, which is corresponding to the absorption band of pyrene around 350 nm (Figure 3c), suggesting that the local chirality was transferred to the assemblies. Depends on the pH of the environment, the shape of the Cotton effect was not changed. However, the intensities increased with decrease of pH values. Considering the AFM images and the formed helical fibers, the decreased intensities of the CD spectra can be ascribed to the increased helical pitches.39-40 The increased helical pitch means the loosed stacking of the pyrene. We further investigated their fluorescence (FL) and CPL spectra. As shown in Figure 3b and 3d, fluorescence spectra showed the emission at 475 nm upon excitation at 320 nm. Since these nanofibers are helical, they showed also intense CPL signal at the emission band of excimer from pyrene (425-500 nm). Strong packing of pyrene units resulted in the formation of excimer while this also should be the reason why strong CPL could be observed here.41-44 We tested all the samples at critical gelation concentration (CGC). Because PGAc could not dissolve at the extremely high concentration under lower pH condition. Nonetheless, to present the relationship of pH values and excimer formed from strong packing of pyrene units, we also measured the fluorescence spectra of various hydrogels at the same concentration (33.3 mM). As shown in Figure S2, the excimer intensity showed decreasing when increasing the pH. Thus it is not surprising that the CPL signals exhibited decreasing with increasing the pH value, which is consistent with the CD measurements. The luminescence dissymmetry factor (glum), defined as glum = 2 × (IL − IR)/ (IL + IR), where IL and IR refer to left- and right-handed CPL, respectively, be used to scale the magnitude of the CPL activity. The maximum value ranges from +2 for an ideal left CPL to -2 for an ideal right CPL, and zero corresponds to no circular polarization of the luminescence is put forward to provide theoretical basis. Experimentally, the value of glum is specified to glum = [ellipticity/ (32980/ln10)]/ (total fluorescence intensity at the CPL extremum).


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The calculated results showed the maximum glum value around 6.87×10-3 in pH = 3 which is relatively small value compared with reported cases in aqueous. By increasing the pH value, the glum values decreased gradually (Figure S3). Besides, it is no accident that absolute value of gCD of corresponding CD signals appear similar rule. Thus, through simple tuning of the pH values, we provided an approach to control CD and CPL activity of chiral nanomaterials.

Figure 3. a) CD spectra, b) CPL spectra, c) Abs spectra and d) FL spectra of PGAc hydrogels obtained with aqueous solutions of different pH values under identical excitation intensity at 320 nm at room temperature.

3.2 Host-guest interaction Derived supra-dendron and CPL pathway After demonstrating the pH-dependent supramolecular chirality of PGAc hydrogel, we paid our attention to the formation of self-assemblies inducing supramolecular complexes by host-guest interaction with cyclodextrins (CDs), because it was expected to observe intense CPL signal at the emission band of monomer and excimer regions under retention of the monomer emission from pyrene of supramolecular hydrogels. Firstly, 1H NMR spectroscopy was employed to investigate the binding behavior of PGAc with CDs. As shown in Figure S4, in comparison with the spectrum of PGAc, the peaks attributable to protons Ha-d on the pyrene moiety of PGAc displayed pronounced downfield shifts after the addition of β- or γ-CD in a stoichiometric ratio,


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the ∆δ values being 0.2 ppm for Ha-d, whereas the almost negligible downfield shifts with addition of α-cyclodextrin (α-CD) of the pyrene moiety. The results demonstrated that the pyrene moiety of PGAc was embedded in the cavity of β- and γ-CD, but the interaction with α-CD was pretty weak.45 The change of luminescence (Figure 4a) and CPL activity (Figure 4c) upon addition of cyclodextrin to PGAc hydrogels kept consistent with the results of 1H NMR. The emission spectrum of PGAc@β-CD is quite close to monomer emission and CPL spectrum showed significant blue-shift from 475 nm to 440 nm, which were attributed to the complexation between PGAc molecule and β-CD to be around 1:1.46-47 This host-guest combination obviously decreased the strong π-π stacking of the pyrene moieties. In contrast, because two pyrene moieties were embedded within the cavity of γ-CD to form a 2:1 PGAc@γ-CD inclusion complexes,48-49 strong π-π stacking of two pyrenes remarkabley enhanced the formation of excimer. The excimer emission at 480 nm obviously enhanced after adding γ-CD and CPL peak exhibited both monomer and excimer part showing with a splited peak shape. It should be noted that there is no interaction between PGAc and α-CD which could be further confirmed by the fluorescence and CPL spectra. Meanwhile, the interactions of the chromophores of PGAc and cyclodextrin in hydrogels were also confirmed by the absorption (Figure 4b) and CD spectra (Figure 4d). The observed exciton couplet Cotton effect at 325-375 nm showed a splited peak shape in the absence of cyclodextrin. And the exciton couplet Cotton effect for the PGAc@β-CD and PGAc@γ-CD hydrogels were enhanced than PGAc hydrogel, which suggests that chirality transfer from the chiral centre to the pyrene moiety through cyclodextrin with inherent chirality was more significant than that through the short alkyl spacer. Moreover, the host-guest inclusion interaction between the pyrene group and γ-cyclodextrin cavity enhanced the π-π packing of the pyrene groups, and thereby produced a negative Cotton effect exhibited in the PGAc@γ-CD at 381 nm, which is non-covalent dimerization peak. For comparison, we have measured the CD spectrum of PGAc in DMSO (Figure S5). There is no CD signal in diluted solution. This phenomenon is consistent with reported cases that chirality transfer from chiral center to aggregates only occur in the assembled systems.


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Figure 4. a) FL spectra, b) Abs spectra, c) CPL spectra and d) CD spectra of PGAc hydrogel and PGAc/cyclodextrin supramolecular hydrogels under identical excitation intensity at 320 nm at room temperature (PGAc = 25 mM, molar ratio of PGAc/α-CD = 1/1, PGAc@β-CD = 1/1, PGAc@γ-CD = 2/1).

AFM measurement is a straightforward method to reveal the effect of the host-guest inclusion interaction between the pyrene group and the cyclodextrin cavity. All the AFM images of the cogels made from PGAc/α-CD, PGAc@β-CD and PGAc@γ-CD were presented at Figure 5 (Figure S6) and all the AFM images of individual CDs were shown in Figure S7. The AFM images of PGAc/α-CD exhibited clear helical structures (Figure 5a, b), which were similar with the observation of pure PGAc hydrogel. Moreover, many irregular aggregates could be observed all around the scanning view which should be assigned to the self-aggregation of α-CD. This finding also supported the viewpoint that the interaction between PGAc and α-CD was very weak. There is no host-guest interaction between PGAc and α-CD. In contrast, the induction of β-CD to PGAc generated crystalline nature in the formed hydrogel. As shown in Figure 5c and 5d, large size lamellar structures could be observed. This novel structure was rare observed in hydrogel systems. Molecular packing of supra-dendron PGAc@β-CD might adopt a lamellar pathway in the formation of hydrogel. The above results of fluorescence and CPL of PGAc@β-


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CD hydrogel have indicated that pyrene moiety exhibited molecular level emission resulted from the host-guest inclusion interaction. The AFM results further confirm the deduction. The observed lamellar nanostructures should be the hierarchical assembly of supra-dendron of PGAc@β-CD. Interestingly, supra-dendron PGAc@γ-CD in hydrogel generated entangled networks, as shown in Figure 5e and 5f. Thick and straight fibrous structures with an average diameter around 350 nm were observed. This polymer-like structures were consistent with the spectral observation. As demonstrated that two PGAc molecules were included into the cavity of γ-CD which resulted in the formation of enhanced excimer emission due to the intense π-π interaction of pyrene moieties. The specific characteristics of supra-dendron composed of two PGAc molecules and one γ-CD enabled the supramolecular polymerization resulted from the multiple hydrogen bonding sites of PGAc.


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Figure 5. AFM images revealing the morphological changes of PGAc/cyclodextrin hydrogels. For PGAc/α-CD hydrogel (molar ratio of PGAc/α-CD = 1/1): (a) scale bar = 300 nm and (b) scale bar = 200 nm. For PGAc@β-CD supra-dendron (molar ratio of PGAc@β-CD = 1/1): (c) scale bar = 2 µm and (d) scale bar = 1 µm. For PGAc@γ-CD supra-dendron (molar ratio of PGAc@γ-CD = 2/1): (e) scale bar = 2 µm and (f) scale bar = 300 nm.


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We further performed stress-sweep rheological measurement to study the strength of the PGAc hydrogel before and after the addition of the CDs, as shown Figure S8. For PGAc/α-CD composite hydrogel, there is no host-guest interaction between PGAc and α-CD. Thus, adding of α-CD to PGAc hydrogel will weaken the crosslink of PGAc gelators which will weaken the gel strength. The storage modulus of PGAc/α-CD composite hydrogel is lower than the one of PGAc hydrogel. For PGAc@β-CD supra-dendron, host-guest interaction between PGAc and β-CD result in close lamellar packing. Thus, the hydrogel of PGAc@β-CD supra-dendron exhibited high storage modulus than the individual PGAc hydrogel. In the hydrogel composed of PGAc@γ-CD supra-dendron, crosslinked PGAc@γ-CD supra-dendron could assemble into 1D supramolecular polymer, which significantly enhance the hydrogel strength. All the gelformation deduction of the supra-dendrons could be further confirmed by the FTIR and X-ray diffraction measurements. The FTIR spectra of PGAc xerogels under different pH conditions and PGAc/cyclodextrin xerogels was measured to figure out the formation of nanostructures. For PGAc xerogels obtained from various pH, different FTIR spectra were obtained (Figure S9). At pH 7, the stretching N-H vibration was observed at 3294 cm-1, indicating the strong H-bonding formation. The asymmetric and symmetric stretching vibrations of CH2 were observed at 2928 cm-1 and 2862 cm-1 separately. Furthermore, the C=O vibration mainly appeared at 1730 cm-1, which indicated that the four carboxylic acids at the head groups also formed the hydrogen bond. In addition, two vibration bands at 1641 and 1541 cm-1 could be ascribed to amide I (the stretching vibration of carbonyl in amide groups) and amide II (the N-H bending mode in amide groups) bands in the region of 1700-1500 cm-1. These information confirmed that hydrogen bonding were important driving forces for the hydrogel formation. As shown in Figure S10a, for β-CD powder, a broad stretching vibration peak of the hydroxyl group was observed at 3391 cm-1. However, after forming host-guest complex with PGAc, the -OH vibration peak moved to a lower wavenumber at 3307 cm-1 which clearly indicated that strong H-bond formed.50 The vibration peak at 1716 cm-1 should be ascribed to the stretching vibration of C=O in the aggregated state of carboxylic acid group. After forming the inclusion complex, no obvious change was tested which indicated that strong H-bond formed in the inclusion complex. The vibration peak at 1648 cm-1 and 1538 cm-1 should be ascribed to amide-I and -II bands, respectively, suggesting that both C=O and N-H form strong hydrogen bonds, which also is one of the driving force for the self-


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assembly. Similar phenomenon was observed from the spectral measurement of γ-CD and PGAc@γ-CD complex (Figure S10b). However, in the α-CD and PGAc/α-CD systems, no obvious shift could be tested in the vibration peak of -OH (Figure S10c). The structure of PGAc hydrogels could be proved by XRD patterns (Figure 6a and Figure S11). The XRD results revealed weak diffraction patterns for the assemblies and the d-spacing values was estimated to be around 2.98 nm in PGAc xerogel. Based on the CPK models, the molecular length of PGAc was estimated to be 2.94 nm. The d-spacing value is more than one layer of thickness and less than twice of the molecular thickness. This indicated that the dendron molecules formed an interdigitated bilayer structure with anti-parallel arrangement of pyrene moieties. In addition, there is a sharp XRD peak at 0.92 nm which could be assigned to the molecular packing scale between two molecules. For PGAc/α-CD hydrogel, the XRD results also shown weak diffraction peak at 2.98 nm and a sharp diffraction peak at 0.92 nm, which is a similar pattern with pure PGAc xerogel (Figure S12a). In addition, there are some weak characteristic peaks ascribing to α-CD also could be observed. This result suggests that no hostguest interaction occurred between PGAc and α-CD which is consistent with AFM observations. The XRD pattern of supra-dendron PGAc@β-CD hydrogel showed crystalline property with sharp and dense diffraction peaks (Figure S12b). After calculating and comparing the diffraction peaks, it is easy to determine the molecular packing of this supra-dendron. The maximum of dspacing was estimated to be around 1.93 nm which should be assigned to the bilayer length of PGAc@β-CD supra-dendron. In addition, some weak characteristic peaks ascribing to β-CD could also be observed. This result was consistent with the AFM observation of 2D lamellar nanostructures. In the PGAc@γ-CD hydrogel, XRD result exhibited special pattern which could be confirmed as normal bilayer packing mode (Figure S12c). A characteristic peak at 7.47˚ was estimated to 1.81 nm which could be ascribed to the repeating unit of PGAc@γ-CD inclusion complex. Crosslinked PGAc@γ-CD supra-dendron could assemble in a one dimensional order to form supramolecular polymer. This results further confirmed the entangled polymerization observed from AFM images.


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Figure 6. a) X-ray diffraction of PGAc and PGAc/cyclodextrin xerogels. Schematic illustration of PGAc and PGAc@cyclodextrin assembly: b) For PGAc dendron, molecular self-assembling form helical structure through π-π stacking and hydrogen-bond interaction. c) For PGAc@β-CD supra-dendron, hostguest interaction between PGAc and β-CD result in loose packing with the formation of lamellar nanostructures. d) For PGAc@γ-CD supra-dendron, the amphiphile formed a normal bilayer structure first. Then hydrogen bonding drove the bilayer structure to a supramolecular polymer. (molar ratio of PGAc/α-CD = 1/1, PGAc@β-CD = 1/1, PGAc@γ-CD = 2/1)

4. CONCLUSIONS In summary, a newly designed luminescent π-peptide dendron PGAc and the derived supradendron through host-guest inclusion interaction with cyclodextrins were found to form luminescent hydrogels. Individual π-peptide dendron showed wide pH range adaptability with the formation of helical nanofibers. Supra-dendrons in the form of PGAc@β-CD and PGAc@γCD were also found to form luminescent hydrogels. 2D lamellar nanostructures were observed from PGAc@β-CD while entangled network nanostructures were obtained from PGAc@γ-CD hydrogel. Both the supra-dendron hydrogels exhibited novel CPL activities. Since PGAc and βCD could form 1:1 host-guest inclusion complex, molecular CPL emission was observed in PGAc@β-CD hydrogel due to the induced chirality of β-CD. γ-CD was found to encapsulated


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two PGAc molecules to form a supra-dendron PGAc@γ-CD showing entangled network nanostructures in formed hydrogel with intense excimer induced CPL emission. This work provided a new perspective in the design of chiroptical energy system in aqueous.

ASSOCIATED CONTENT Supporting information Synthetic routes of PGAc. Fluorescence spectra of PGAc hydrogels at the concentration of 33.3 mM under various pH values. The gCD values at 360 nm and glum values at 475 nm under various pH values.

1

H NMR spectra of PGAc hydrogel and PGAc/cyclodextrin supramolecular

hydrogels. CD spectra of PGAc hydrogel obtained from pH = 7 and PGAc in DMSO solution. Height

AFM

images

of

PGAc

hydrogels

obtained

from

different

pH

solutions,

PGAc/cyclodextrin supramolecular hydrogels and cyclodextrin water solutions. Stress sweep rheology of PGAc hydrogel and PGAc/cyclodextrin supramolecular hydrogels. FTIR spectra of PGAc hydrogels at different pH values, PGAc/cyclodextrin supramolecular hydrogels and individual cyclodextrin. X-ray diffraction patterns of PGAc hydrogel, PGAc/cyclodextrin supramolecular hydrogels and individual cyclodextrin.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS


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This work was supported by National Natural Science Foundation of China (51673050, 91427302 and U1704149), National Key Basic Research Program of China (2016YFA0203400 and 2017YFA0206600), “Strategic Priority Research Program” of the Chinese Academy of Sciences (XDB12020200). P.D. thanks for the supporting of “New Hundred-Talent Program” research fund from the Chinese Academy of Sciences.

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Luminescent pyrene-peptide dendron (PGAc) and the derived supra-dendrons through host-guest inclusion interaction with cyclodextrins could form luminescent hydrogels. Helical fibers were observed in pure pyrene-peptide dendron hydrogel while two dimensional lamellar nanostructures were obtained in supra-dendron (PGAc@β-CD) and entangled networks in supradendron (PGAc@γ-CD).


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Circularly Polarized Luminescence from a Pyrene-Cyclodextrin Supra

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