Inversion of Circularly Polarized Luminescence of Nanofibrous

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Inversion of Circularly Polarized Luminescence of Nanofibrous Hydrogels through Coassembly with Achiral Coumarin Derivatives Fang Wang,† Wei Ji,† Peng Yang,† and Chuan-Liang Feng*,†,‡

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†

State Key Lab of Metal Matrix Composites, School of Materials Science and Engineering, and School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China ‡ Collaborative Innovation Center of Nano Function Materials & Application, Key Lab For Special Functional Materials, Ministry of Education, Henan University, Kaifeng 475004, China S Supporting Information *

ABSTRACT: Control over the handedness of circularly polarized luminescence (CPL) in supramolecular gels is of special significance in biology and optoelectronics; however, it still remains a great challenge to precisely and efficiently regulate the chirality of CPL. Herein, a chiral phenylalanine-derived hydrogelator and achiral coumarin derivatives can co-assemble into nanofibrous hydrogels with controllable chirality, and the handedness of CPL of these hydrogels can be efficiently inverted by coumarin derivatives through noncovalent interactions, which can be further tuned at will by incorporating metal ions into the co-assembly. The hydrogen bonds, coordination interactions, and steric hindrance are proved to be the crucial factors for the CPL inversion. This study provides feasible strategies to efficiently regulate the handedness of CPL through co-assembly, and these CPL materials may have potential applications in the fields of photoelectric devices, smart chiroptical materials, and biological systems. KEYWORDS: hydrogels, supramolecular chirality, circularly polarized luminescence, coumarin, self-assembly and different achiral fluorophores into the supramolecular assembly system through noncovalent interactions (such as hydrogen bonding, π−π stacking, and electrostatic interactions),34−38 and these supramolecular assemblies with color- or intensity-tunable CPL have been reported.39−41 However, the handedness of CPL of the assemblies is mostly limited to the chirality of molecules, and it is necessary to change the chiral component to its enantiomer to obtain CPL with opposite handedness, while synthetic processes are tedious and such enantiomers are not always well-prepared.42−46 Recently, supramolecular chirality in nonemissive systems has been controlled by changing achiral substituents or adding metal ions,47−51 which provides enlightenment to regulate the chirality of CPL of supramolecular gels through controllable self-assembly based on noncovalent interactions. Inspired by these, herein, a well-developed chiral phenylalanine-derived hydrogelator (LPF) is co-assembled with achiral coumarin derivatives (G), and the formed supra-

C

hiral functional materials with circularly polarized luminescence (CPL) have drawn great attention, not only for understanding of the inherent principles of chirality but also due to their wide potential applications in display devices, biological probes, catalysts for asymmetric photochemical synthesis, and chiroptical materials.1−13 Although various CPL materials have been successfully produced by introducing the chosen chiral moieties into fluorophores through covalent bonds or coordinate covalent bonds, it still remains a challenge to construct CPL materials with controllable handedness from specific chiral building blocks due to the tedious and complicated synthesis of chiral enantiomers.14−23 Even after employing supramolecular gels to regulate the handedness of CPL via a noncovalent bond driven self-assembly process, the handedness of CPL of supramolecular gels is still mostly confined to the chirality of molecules.24−33 To circumvent this issue, rational design of emissive supramolecular gels with tunable self-assembly is becoming important for obtaining CPL materials with controllable handedness. Supramolecular gels provide a convenient platform to construct the CPL materials by introducing chiral sources © 2019 American Chemical Society

Received: April 28, 2019 Accepted: May 31, 2019 Published: May 31, 2019 7281

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Figure 1. Schematic representation of the co-assembly of chiral LPF and achiral fluorescent coumarin derivatives (G) and their response to metal ions. Nanotwists and nanohelices are formed in LPF/G hydrogels, and both supramolecular chirality and circularly polarized luminescence (CPL) of the hydrogels can be inverted by changing the nitrogen spatial positions in the pyridine ring of G. By incorporating metal ions into co-assembly, it can further regulate the handedness of CPL and the chirality and morphologies of the nanofibrous hydrogels.

Figure 2. Combined spectra of UV−vis and CD measured from the hydrogels composed of LPF/G1 (a), LPF/G2 (b), LPF/G3 (c), and LPF/ G4 (d) in different equivalents of Ag+ or Ni2+ ions, respectively. The ground-state supramolecular chirality of the co-assembled hydrogels can be inverted by exchanging nitrogen spatial positions in the pyridine ring of achiral coumarin derivatives. It can be further regulated by adding metal ions because of the coordination bond formation with co-assembly, which is governed by the molecular structure of achiral coumarins.

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Figure 3. CPL and fluorescence spectra excited at 320 nm of LPF/G1 (a), LPF/G2 (b), LPF/G3 (c), and LPF/G4 (d) hydrogels in the absence and presence of different concentrations of Ag+ or Ni2+ ions. The cohydrogels show chiral inversion of CPL by changing nitrogen spatial positions in the pyridine ring of coumarin derivatives, which can be further controlled by adding metal ions.

and G6. However, all six coumarin derivatives could form hydrogels with equimolar LPF. Typically, carboxyl and pyridyl are well-known ligands to coordinate with metal ions,47,53 and hydrogels could be further functionalized through coordination bond formation, e.g., Ag+ or Ni2+ ions with good coordination abilities toward the ligands. Since the LPF hydrogelator has chiral centers, these hydrogels were characterized by circular dichroism (CD) spectra to study the ground supramolecular chirality. LPF hydrogels with and without equimolar Ag+ or Ni2+ exhibited two negative Cotton effects in the 250−320 nm range (Figure S2). However, assemblies of coumarin derivatives in the absence and presence of 1.0 equiv of Ag+ or Ni2+ were CD-silent due to their achiral nature (Figures S3 and S4). When LPF co-assembled with G, significant CD signals were detected in the region of 320−400 nm, which corresponded to the electronic transition of coumarin derivatives (Figures 2 and S5−S7), suggesting that the molecular information on chiral LPF was successfully transcribed to the achiral G chromophores upon co-assembly. A strong positive Cotton effect at 338 nm was observed for coassembled LPF/G1 at a molar ratio of 1:1 (abbreviated as LPF/G1/1:1) and LPF/G2/1:1. While a negative Cotton effect was detected for LPF/G3/1:1, LPF/G4/1:1, LPF/G5/ 1:1, and LPF/G6/1:1 at 334, 361, 325, and 363 nm, respectively. The opposite sign of the CD signals implied that reversed supramolecular chirality could be obtained with varying the location of the nitrogen atoms from ortho to meso or para position in the pyridine ring of achiral coumarin derivatives. In addition, hydrogels were formed at all investigated stoichiometries of LPF/G and the most intense CD signal was observed at the molar ratio of 1:1 except LPF/ G3 (Figures S5−S7). Unexpectedly, the CD signal of LPF/

molecular hydrogels show significant CPL performance which indicates the strongest CPL intensity is obtained in LPF/G1 and LPF/G2 at a molar ratio of 1:1 with glum (luminescence dissymmetry factor) of −1.2 × 10−2 and −1.9 × 10−2. The handedness of CPL of the composite hydrogels can be inverted by changing the nitrogen spatial positions from ortho to meso or para positions in the pyridine ring of coumarin derivatives (Figure 1). Typically, the handedness of CPL can be further regulated at will by metal ions due to their unique directionality and stacking modes,47 and the regulations are influenced by nitrogen spatial positions and 4-methyl substitution on the coumarin derivatives. In addition, the controlled supramolecular chirality and nanostructures (such as nanotwist, nanohelix, and nanotube) certify that the handedness of CPL is largely dependent on the co-assembly way of LPF and achiral coumarin derivatives rather than the inherent molecular chirality. The hydrogen bonds, coordination interactions, and steric hindrance are crucial factors for the CPL inversion. This study provides a better understanding of the relationship between inherent molecular chirality and supramolecular chirality, which develops a methodology to regulate the handedness of CPL by co-assembly based on noncovalent interactions.

RESULTS AND DISCUSSION Chiral supramolecular hydrogelator LPF and six achiral pyridine-substituted coumarin derivatives (G) were synthesized according to previous reports.47,52 The gel formation of the compounds at a concentration of 3 mg·mL−1 was tested through the typical heating−cooling procedure in aqueous solution (Figure S1). LPF, G2, and G4 themselves formed hydrogels, while suspensions were obtained for G1, G3, G5, 7283

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Figure 4. SEM images of the xerogels of LPF/G1, LPF/G2, and LPF/G3 with or without Ag+ or Ni2+ ions at different molar ratios. The SEM images show that the handedness of the co-assembled structures can be switched by changing the substituents of achiral coumarins and further regulated by adding metal ions.

(Figure S11). However, when the nitrogen atoms were located at the para position in the pyridine ring, CD signals remained profiles of plain LPF/G5 and LPF/G6 assemblies in the presence of Ag+ or Ni2+ (Figures S13 and S14), indicating that the coordinated interaction between metal ion and pyridyl ligand may play an important role in the regulation of supramolecular chirality. The regulated CD signals of LPF/G hydrogels with different molar ratios of Ag+ or Ni2+ further verified the effectiveness of chirality modulation (Figures S8− S14). These results suggested that the supramolecular chirality of LPF/G hydrogels was not only determined by the nitrogen spatial positions but also affected by metal ions, which differed from previous approaches.54 The maximum absorption band of G was found at around 320 nm, which corresponded to its large conjugated skeleton (Figure S3). The emission band of G was centered at around 450 nm in the blue light region (Figure S15). Similarly, the same maximum emission wavelength was found for the coassembled hydrogels of LPF/G with and without Ag+ or Ni2+, suggesting no strong π−π stacking interactions between the chromophores.52 Moreover, the blue emission of these hydrogels could be intuitively confirmed by UV light irradiation (Figure S1). The excited supramolecular chirality was measured by CPL spectra. CPL signals with different handedness were obtained in the cogels by excitation at 320

G3/1:0.5 was much stronger than LPF/G3/1:1, suggesting distinct chirality transcription modes for the co-assemblies at different ratios. To investigate the effect of metal ions on the chirality of supramolecular gels, CD spectra were also recorded for the LPF/G/metal hydrogels (Figures 2 and S8−S14). With increasing the amount of Ag+, the Cotton effects of LPF/ G1/1:1 were reduced (Figure 2a). However, LPF/G1/1:1 at 1.0 equiv of Ni2+ (abbreviated as LPF/G1/Ni2+/1:1:1) exhibited an opposite CD signal with a negative Cotton peak at 347 nm, suggesting opposite supramolecular chirality compared to LPF/G1/1:1. Opposite signs to LPF/G2 were obtained upon addition of 1.0 equiv of Ni2+ or Ag+ (Figure 2b), while the CD signal disappeared for LPF/G2/Ag+/1:1:2, indicating that the supramolecular chirality of the hydrogel was tuned by the amount of metal ions. When the location of the nitrogen atom was changed from ortho to meso in the pyridine ring as in the case of G3 and G4, CD signals of both LPF/G3 and LPF/G4 were inverted by adding 2.0 equiv of Ag+ (Figure 2c,d). Moreover, the signal changed from negative to positive at 1.0 equiv of Ni2+ for LPF/G4, while the same sign remained for LPF/G3, suggesting that the steric hindrance from 4methyl substitution on the coumarin derivatives affected the chirality of metal supramolecular aggregates. The CD results of LPF/G3/1:0.5 with Ag+ or Ni2+ were similar to LPF/G3/1:1 7284

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ACS Nano nm, and the λmax of the CPL signals was consistent with the fluorescence emission spectra (Figures 3, S16, and S17). LPF/ G1 and LPF/G2 displayed a negative CPL signal, while LPF/ G3, LPF/G4, LPF/G5, and LPF/G6 showed a positive CPL signal, which implied that the handedness of CPL could be inverted by changing the nitrogen spatial positions from ortho to meso or para in the pyridine ring. The CPL property of LPF/G could be further controlled by metal ions. LPF/G1/ Ni2+/1:1:1, LPF/G2/Ni2+/1:1:1, and LPF/G2/Ag+/1:1:1 exhibited a positive CPL band, which was opposite to that of LPF/G1/1:1, LPF/G1/Ag+/1:1:2, and LPF/G2/1:1, suggesting the inversion of CPL handedness, while LPF/G2/Ag+/ 1:1:2 was CPL-silent. In the presence of Ag+ or Ni2+, CPL signals remained profiles of LPF/G5 and LPF/G6 hydrogels (Figure S17). These results are in good agreement with the CD results. However, although obvious Cotton effects with opposite signs and fluorescence were observed for LPF/G3 and LPF/G4 in the presence of Ag+ or Ni2+, CPL signals were weakened or disappeared without inversion (Figures 3c,d and S16). In contrast, no CPL signals could be detected from G and G/metal assemblies (Figure S18), indicating that the coassembly of LPF and G played an important role for the chirality transfer from the chiral LPF to achiral G, which led to CPL activity of the co-assembled hydrogels. To quantitatively discuss the degree of CPL, the luminescence dissymmetry factor (glum) was used: glum = 2 × (IL − IR)/(IL + IR), where IL and IR denote the intensity of leftand right-handed CPL, respectively. Although these coumarin derivatives are achiral, strong glum values of CPL were observed in the cogels. The strongest CPL intensity was obtained in LPF/G1/1:1 and LPF/G2/1:1 with glum of −1.2 × 10−2 and −1.9 × 10−2, which is much larger than those |glum| values of some isolated organic molecules. The CPL glum of LPF/G1/ Ag+/1:1:2, LPF/G1/Ni2+/1:1:1, LPF/G2/Ag+/1:1:1, and LPF/G2/Ni2+/1:1:1 was about −1.1 × 10−2, +1.0 × 10−2, +2.5 × 10−3, and +1.6 × 10−3, respectively, which are large values among most of the self-assembled CPL systems.40 In addition, the glum values of LPF/G5/1:1 and LPF/G6/1:1 were +1.4 × 10−3 and +3.2 × 10−3. LPF/G5/Ag+/1:1:1, LPF/G5/ Ni2+/1:1:1, LPF/G6/Ag+/1:1:1, and LPF/G6/Ni2+/1:1:1 exhibited similar glum values (Table S1). Furthermore, the glum values for LPF/G3/1:1 and LPF/G4/1:1 were +9.3 × 10−3 and +7.3 × 10−3. Addition of Ag+ or Ni2+ into LPF/G3 and LPF/G4 gels led to weakened glum values which still remained positive (Table S2). Thus, CPL-active hydrogels have been fabricated by using the co-assemble method. In particular, the handedness of CPL is determined by the substituents of coumarin derivatives and the metal ions. To clarify how the substituents and metal ions affect the CPL, the morphology of the gels was studied by scanning electron microscopy (SEM). Flat ribbons and nanofibers were formed for coumarin derivatives in the presence and absence of metal ions except G1/Ni2+/1:1 and G2/Ni2+/1:1, which formed crystals (Figure S19). Additionally, nanofibers for LPF and LPF/Ag+/1:1 and right-handed twists for LPF/Ni2+/ 1:1 were observed. After co-assembly, twists with predominant left-handedness were observed for LPF/G1/1:1 and LPF/G2/ 1:1 (Figures 4 and S20), while twists with right-handedness were observed for LPF/G5/1:1 and LPF/G6/1:1 (Figure S25). These results are in agreement with the observations of the CPL and CD spectra. Interestingly, right-handed twists and higher ordered helices with right-handedness were formed for LPF/G3/1:0.5, while left-handed twists and higher ordered

helices with left-handedness were formed for LPF/G3/1:1 (Figures 4 and S21). This phenomena explained why the sudden decrease of CD signals from LPF/G3/1:0.5 to LPF/ G3/1:1, but no inversion of CD profiles were observed, suggesting that LPF/G3/1:1 formed more than one kind of chirality and a certain percentage of opposite chirality existed in the different levels of assemblies from each form of the molecule.47,55 On further increasing the amount of G3, a mixture of right- and left-handed twists appeared. In addition, twists with predominant right-handedness were found for LPF/G4/1:1 (Figure S24). The structural information on cogels formed with metal ions were also investigated by SEM. Twists with right-handedness were found for LPF/G1/Ni2+/1:1:1, LPF/G2/Ni2+/1:1:1, and LPF/G2/Ag+/1:1:1, which were opposite to LPF/G1/1:1 and LPF/G2/1:1 with left-handed twists (Figure 4). When 2.0 equiv of Ag+ was added, the SEM images showed a mixture of right- and left-handed twisted structures, which led to the weakened or even disappeared CD and CPL properties of LPF/G1/1:1 and LPF/G2/1:1 (Figure S20), whereas for LPF/G5/1:1 and LPF/G6/1:1, the nanotwists retained the same right-handedness upon addition of Ag+ or Ni2+ (Figure S25). These SEM observations are coincident with the CD and CPL spectra. Nanotwists with exclusively right-handedness for LPF/G3/1:0.5 and LPF/G3/1:1 containing 0.5−2.0 equiv of Ni2+ were obtained (Figure S23). However, with an increasing the amount of Ag+, both left- and right-handed nanostructures including twists and helices were simultaneously formed, and even nanotubes with a length of 0.2−1.4 μm and outer diameter of 90−240 nm were obtained (Figure S22). The nanotubes were formed by rolling of the nanobelts which were obtained by the intertwining of a few long slender twisted and helical ribbons. Similarly, both left- and right-handed twists were also obtained for LPF/G4 at 2.0 equiv of Ag+ and 1.0 equiv of Ni2+ (Figure S24). Furthermore, the SEM images showed almost even numbers of left- and right-handed twists, which led to no CD and CPL properties for LPF/G4/Ag+/ 1:1:1. Therefore, for LPF/G3 and LPF/G4 systems, although the CD signals could be inverted by metal ions, their CPL signals weakened or disappeared without inversion, which may be ascribed to the inhomogeneous chiral nanostructures. The diameter of the twisted and helical ribbons obtained in different LPF/G hydrogels with or without Ag+ and Ni2+ ions ranged from 17 to 290 nm (Table S3 and S4). The pitches of chiral twists and helices essentially fell into 0.07−2.70 μm. The diameter and the twist pitch showed a large change depending on the substituents and metal ions used, indicating that these chiral twists were formed in different manners. All these SEM results confirmed that the chirality of the chiral nanostructures was crucial to the CPL, which could be tuned by the substituents of achiral coumarins or metal ions. To gain insight into the noncovalent interactions, 1H NMR in D2O and FTIR spectra were measured. From the 1H NMR spectra (Figures S26−S31), the signals of the heteroaromatic protons of the pyridine ring in LPF/G shifted downfield and became broad, illustrating that carboxylic acid−pyridine hydrogen bonds occurred in the co-assembled hydrogel system. Furthermore, no shifts of the resonance signals were observed for the aromatic protons in the coumarin ring, suggesting no strong π−π stacking interactions between the coumarin derivatives during the self-assembly. FTIR spectra results indicated that the two carboxylic acid groups were involved in hydrogen bonds in LPF/G gels based on the fact 7285

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Figure 5. XRD plots of the xerogels of LPF/G1 (a), LPF/G2 (b), LPF/G3 (c), and LPF/G6 (d) with or without Ag+ or Ni2+ ions at different molar ratios. The basic unit of each of the nanostructures is a lamellar structure, and the spatial arrangements of the component molecules are changed by the substituents and further adjusted by metal ions.

chiroptical properties may be attributed to synergistic effects of these different kinds of hydrogen bonding interactions. When Ag+ or Ni2+ ions were incorporated into LPF/G gels, a new band with different intensity appeared at 1384 cm−1 for all the gels, which could be assigned as the symmetric stretching vibrations of carboxylate, indicating the different coordination efficiencies from Ag+ and Ni2+ (Figures S32−S34). Moreover, two new bands at 1569 and 1540 cm−1 appeared for LPF/G1/ Ni2+/1:1:1, implying that there were two kinds of amide hydrogen bonds existing. These data indicated that coordination with different intensity between the carboxylic acid, pyridine and metal ions occurred, combined with the multifarious hydrogen bonds induced the formation of the different chiral nanostructures. X-ray diffraction (XRD) patterns were investigated to further unveil the molecular arrangement of the assemblies (Figures 5 and S35). The XRD patterns of the co-assembly systems showed big differences from that of the individual component of G, although some small peaks which belonged to G assemblies appeared in the co-assembly systems. LPF/ G1/1:1 showed two sets of Bragg reflections (Figure 5a), and the corresponding d values of these two sets of peaks were d1 = 1.96 nm, d2 = 0.97 nm (1/2) and d′1 = 1.48 nm, d′2 = 0.74 nm (1/2), respectively. These results indicated the presence of two sets of lamellar structures in the gel network with the interlayer spacings of 1.96 and 1.48 nm, respectively.47 XRD plots of the xerogels of LPF/G2/1:1, LPF/G4/1:1, and LPF/G5/1:1 also

that stretching vibration bands of CO from carboxyl groups of LPF at 1744 and 1723 cm−1 disappeared after co-assembly (Figures S32−S34). G1, G2, G3, G5, and G6 showed CO absorption associated with the ester group in the range of 1746−1710 cm−1, which almost disappeared after co-assembly with LPF, and the vibration of LPF/G4/1:1 at 1697 cm−1 was enhanced compared to G4, indicating the formation of hydrogen bonds. Moreover, as for G1 and G2, after coassembly with LPF, the FTIR spectrum displayed a new amide II band at 1542 and 1545 cm−1, respectively, which are typical of amide−amide hydrogen bonds. However, the amide bands in LPF/G system could not be well observed, which may be due to the very strong vibration of CO from ester groups of G. Furthermore, no changes of the bands between 1627 and 1648 cm−1 were observed in the G and LPF/G assemblies, demonstrating that the hydrogen bonds (C−H···OC) remained intact.52 The main driving forces in the process of self-assembly of G were nonconventional intermolecular hydrogen bonds (C−H···OC).52 Based on the above results, it was proposed that, in the co-assembly process, one of the carboxylic acid groups of LPF formed a strong hydrogen bond with pyridine of G, while the other formed hydrogen bond with the CO group of lactone of G, further hydrogen bonds between amide groups and intermolecular hydrogen bonds between the hydrogen atoms of coumarin and the CO group of pyridine assisted the co-assembly. Thus, it was suggested that the formation of chiral nanostructures with different 7286

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Figure 6. Schematic illustration of co-assembly of LPF with achiral fluorescent molecules (G) in the presence and absence of metal ions. The inversion of the supramolecular chirality and the CPL are obtained by changing the nitrogen atom position from ortho (G1, G2) to meso (G3) or para (G5, G6) in the pyridine ring, which alters the spatial arrangement of molecules in cogels. For LPF/G1 (or G2)/1:1 with 1.0 equiv of Ni2+, the coordination of carboxylate anions and pyridine to Ni2+ produces opposite chiral species of the layer membranes, further tuning their twisting and leading to the inversion of the supramolecular chirality and CPL. Helices are formed by the twine of twists in LPF/ G3/1:1, which interwine into nanoribbons and further roll into nanotubes after adding 1.0 equiv of Ag+. The handedness of supramolecular chirality and CPL of LPF/G5 and LPF/G6 cannot be affected by the metal ions.

It was clear from the above results that LPF could form hydrogels with G. The hydrogels exhibited strong CD and CPL signals because of the chirality transfer from the chiral LPF to achiral luminous G. Notably, the CD and CPL signs of the co-assembled hydrogels were successfully inverted by tiny variation of the stereoposition of nitrogen atoms in the pyridine ring. The SEM images further supported the optical properties of the chiral supramolecular assemblies. Underlined mechanisms for such transfer and inversion are illustrated in Figure 6. LPF combines with G through hydrogen bonds of carboxylic acids with pyridyls and the CO groups of lactones and forms ordered lamellar structures. These layer subunits are stacked in a layer-by-layer fashion and further hierarchically self-assemble into higher order nanotwists via amide−amide and C−H···OC hydrogen bonds. However, the transfer of chirality from chiral center to fluorophore is strongly dependent on the spatial arrangement of the component molecules. Although the noncovalent interactions in LPF/G are similar, nitrogen spatial positions in the pyridine ring lead to the different self-assembly mechanisms and packing modes, which further change the distorted direction of the molecules and form opposite handed chiral twists and, thus, cause the chirality inversion of CPL in the cogels. The noncovalent interactions and the molecular packing could be confirmed by the 1H NMR, FTIR, and XRD measurements.

showed the presence of two sets of lamellar structures with the interlayer spacings of 2.00 and 1.48, 2.60 and 2.11, and 2.73 and 2.61 nm, respectively (Figures 5b and S35a,b). However, LPF/G3/1:0.5, LPF/G3/1:1, and LPF/G6/1:1 showed one set of Bragg reflection with interlayer spacing of 2.55, 2.59, and 2.64 nm, respectively (Figure 5c,d). These results suggested that composite layers were formed, and the d spacing value was in the order of para- > meso- > ortho-substituted pyridine in coumarin derivatives (G) after co-assembly. In the presence of metal ions, XRD patterns of LPF/G1/ Ag+/1:1:2 and LPF/G2/Ag+/1:1:2 showed lamellar structures similar to those of LPF/G1/1:1 and LPF/G2/1:1, indicating that the basic layer unit still existed in the LPF/G/metal complexes (Figure 5a,b). However, no obvious diffraction peaks could be found for LPF/G1/Ni2+/1:1:1, LPF/G2/Ag+/ 1:1:1, and LPF/G2/Ni2+/1:1:1. For LPF/G3/1:1 and LPF/ G4/1:1, the XRD patterns did not change significantly after the addition of 1.0 equiv of Ni2+ (Figures 5c and S35a), while there were no obvious Bragg patterns in the presence of Ag+. When Ag+ or Ni2+ were introduced into LPF/G5/1:1 and LPF/G6/1:1, lamellar structures were still formed with different d-spacings (Figures 5d and S35b). These XRD results demonstrated the presence of lamellar arrangements and the different packing modes of aggregation in the coassemblies or complexes. 7287

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total gelators concentration at 0.3 wt %. CPL measurements were performed with a JASCO CPL-200 spectrometer. Cuvettes of 0.1 mm were used for measuring the CD and CPL spectra. SEM was performed on a Sirion 200 microscope. Before SEM measurement, the samples were prepared by depositing dilute solutions of gel on silicon wafers, drying them under vacuum, and coating them with a thin layer of Au to increase the contrast. FTIR studies were performed with a Bruck EQUINOX55 instrument. The XRD patterns were obtained from xerogels and recorded on a D8 Advance instrument from Bruker.

On the other hand, when mixed with achiral metal ions, the metal ion coordination caused the CPL sign of LPF/G to reduce, disappear, or even invert. Similar XRD results were obtained for LPF/G with or without metal ions, revealing that the layer repeat unit was unchanged. For hybrid hydrogels of LPF and G1 or G2 with the nitrogen atom in ortho position, the coordination of metal ions to carboxylic acids and pyridines produced chiral species of the layer membranes with opposite handedness, inverting their twisting and finally achieving the inversion of the supramolecular chirality as well as the inversion of the CD and CPL signals. However, although inverted Cotton effects were observed for LPF/G3 and LPF/G4 with the nitrogen atom in meso position in the presence of metal ions, weak CPL or no CPL signal was detected, which probably could be ascribed to the coexistence of left- and right-handed chiral nanostructures. LPF/G3/1:1 system was special, where the twisted nanoribbons stacked as thinner 1D nanolayers and further twisted into helical assemblies. Upon adding 1.0 equiv of Ag+ into LPF/G3/1:1, these twisted and helical nanoribbons tended to form flat nanobelts, which rolled and further formed nanotubes. The handedness of supramolecular chirality and CPL of LPF/G5 and LPF/G6 was not influenced by the metal ions, and this may be ascribed to the small steric hindrance for the para position of the nitrogen atom in the pyridine ring, which could promote the interaction between LPF and G5 (or G6), in turn increasing the stability of the chiral aggregates.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b03255. Additional experimental details and data including photographs, CD spectra, fluorescence spectra, CPL spectra, SEM images, 1H NMR spectra, FTIR spectra, and XRD (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Chuan-Liang Feng: 0000-0001-6137-5568 Notes

The authors declare no competing financial interest.

CONCLUSIONS In summary, supramolecular hydrogels with strong CPL have been fabricated from chiral phenylalanine and achiral coumarin derivatives. The handedness of CPL of the co-assembled hydrogels is successfully inverted by the alteration of the position of nitrogen atoms in the pyridine ring. The handedness of CPL can be further regulated by introducing metal ions because of extra coordination bond formation, which is mainly governed by nitrogen spatial positions and 4methyl substitution on the coumarin derivatives. The study presents a rare example of opposite CPL properties from the same chiral molecules and enables possibly selective CPL emission of chiral photoluminescence. It develops a methodology to efficiently regulate CPL properties through coassembly by rationally designing chiral molecules and carefully selecting fluorophores, which may have potential applications in photoelectric devices and smart chiroptical materials.

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EXPERIMENTAL SECTION Hydrogel Preparation. Chemical reagents and solvents were purchased from Aladdin and used without further purification. The powder of LPF and G (0.3 wt %) was suspended in a septum-capped 5 mL glass vial. The mixture was gently heated at 80−100 °C for a few minutes until a transparent solution was obtained and then followed by spontaneous cooling to room temperature and incubating for about 2 h. The hydrogel was formed when no gravitational flow occurred upon inversion of the vial. For LPF/G/metal hydrogels, a variety of different equivalents of AgNO3 or Ni(NO3)2 aqueous solutions were prepared first, and then 3 mg of LPF/G and 1 mL of metal ion stock solution were mixed in a septum-capped 5 mL glass vial. The next process was the same as above. Characterization. NMR experiments were performed using a Bruker Advance III 400 Instrument operated at 400 MHz. HRMS were recorded on a Water Q-Tof Mass Instrument. CD spectra were obtained using JASCO J-815 CD spectrometer. CD spectra of hydrogels were recorded in the UV region (200−600 nm) with the 7288

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