Control on Dimensions and Supramolecular Chirality of Self

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Control on Dimensions and Supramolecular Chirality of Self-Assemblies through Light and Metal Ions Guofeng Liu, Jianhui Sheng, Wei Liang Teo, Guangbao Yang, Hongwei Wu, Yongxin Li, and Yanli Zhao J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 07 Nov 2018 Downloaded from http://pubs.acs.org on November 7, 2018

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Journal of the American Chemical Society

Control on Dimensions and Supramolecular Chirality of Self-Assemblies through Light and Metal Ions Guofeng Liu,† Jianhui Sheng,† Wei Liang Teo,† Guangbao Yang,† Hongwei Wu,† Yongxin Li,† and Yanli Zhao*,†,‡ †

Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 637371, Singapore ‡ School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore ABSTRACT: Precise control over helical chirality and dimensions of molecular self-assemblies, a remaining challenge for both chemists and materials scientists, is the key to manipulate the property and performance of supramolecular materials. Herein, we report that a cholesterol-azopyridine conjugate could self-assemble into organogels with photo-controllable dimensional transition from 2D microbelts to 1D nanotubes and finally to 0D nanoparticles. The E/Z-Photoisomerization of the 4-azopyridine unit is the major driving force for the dimensional transformation. Furthermore, the self-assembled structures were observed to exhibit metal ion-mediated helicity inversion through the metal coordination. These observations were collectively confirmed by several techniques including scanning electron microscopy, atomic force microscope, circular dichroism, and X-ray crystallography. The rational design of building blocks for the construction of dimension and chirality controllable self-assembly systems may lead to versatile applications in smart display, advanced optoelectronic device, and supramolecular asymmetric catalysis. INTRODUCTION Helical inversion and dimensional transition in selforganized biological and artificial systems are fundamental and crucial processes that allow for the dynamic control of their structures and functions.1-3 For example, the canonical right-handed B-form double helix DNA and left-handed Zform counterpart present entirely different characteristics to the life.4 In addition, subtle differences in secondary structures, such as α-helix and β-sheet in proteins, are shown to be important signs in predicting the risk of Alzheimer’s disease of individuals.5 Inspired by the fascinating helical structures in nature, various artificial nanostructures with supramolecular or nanoscale chirality have been constructed by the chiral self-assembly of small organic building blocks, displaying potential applications in chemistry,6-10 materials science,11,12 and biomedicine.13,14 As an efficient strategy, chiral self-assembly of building blocks could lead to the formations of various supramolecular polymers through noncovalent interactions including π-π stacking, hydrogen bonding, van der Waals, host-guest, and coordinative interactions.15-21 However, it remains challenging to use noncovalent interactions and external stimuli to control the self-assembly behavior, especially for precise control of helical chirality and dimensions of self-assembled systems.22-25 Dynamic control over the dimensions and helical chirality of supramolecular aggregates by external stimuli is important to manipulate the properties and performance of supramolecular materials and resulted devices. Investigating artificial nanostructures with photo-driven self-assembly process would further the understanding of kinetically controlled behavior and associated spatiotemporal organization. Stimuli responsive supramolecular gels provide a facile platform for studying the above target, since the gel aggregates are collectively driven by reversible noncovalent interactions and their helical chirality could be dynamically regulated by achiral stimuli such as light and metal ions.26-28 Contrast to chemical methods, the utilization of light as an external stimulus offers a unique pathway to tune the handedness of

aggregates with high spatiotemporal precision in a noninvasive manner.29-32 On the other hand, metal ions also play a crucial structural and regulatory role in the organization of supramolecular biological systems.33 For example, metal ions such as Cu2+ and Fe3+ have been demonstrated to promote the aggregation of protein amyloidosis and direct the behavior of metalloproteins involved with Alzheimer’s disease.34,35 Although phototriggered topological changes of azobenzene-based gel systems36-39 and metal-mediated handedness inversion40-42 have been reported, achieving dimensional transition and helicity inversion in single supramolecular gel system triggered by UV light and metal coordination has not been well investigated.

Figure 1. Schematic presentation of photo-regulated dimensional transition of the PAzPCC assembly in DMSO, and metal coordination induced helical superstructures and handedness inversion of the PAzPCC assembly in nbutanol. M and P represent left- and right-handed helical nanofibers, respectively. Herein, we present photo-triggered multidimensional transition and metal-mediated chirality inversion of a selfassembled supramolecular gel based on a cholesterolazopyridine conjugate (PAzPCC). The compound PAzPCC (Figure 1) containing a chiral cholesterol motif, a photoresponsive azo unit, and a pyridyl moiety for metal coordination was rationally designed as an organic building block. It was found that PAzPCC could spontaneously and

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Figure 2. Self-assembly of PAzPCC in DMSO driven by van der Waals' forces. a) Optical photograph and b) SEM image of the PAzPCC gel obtained from DMSO. c) CD spectra of PAzPCC in various organic solvents indicated. d) Temperaturedependent CD spectra of the PAzPCC gel (concentration: 16 mM). e) Single crystal structure and f) lamellar packing structure of PAzPCC. g) Powder XRD patterns of PAzPCC obtained from p-xylene/THF (crystal phase) and DMSO. h) AFM height image of nanosheets on mica. The AFM sample was drop-casted from PAzPCC (16 μM) in DMSO followed by aging overnight. The cross-sectional profile (bottom) was taken along the blue line. selectively self-assemble into two-dimensional (2D) microbelts in dimethyl sulfoxide (DMSO) as a gel and transform into one-dimensional (1D) rolled sheets and finally helical nanotubes upon UV light irradiation. In addition, PAzPCC could also self-assemble into helical nanofibers as gels via metal coordination with various metal ions (Ni2+, Cu2+, Eu3+, Bi3+, etc.), displaying not only amplified supramolecular chirality but also chirality inversion dependent on the solvents and metal ions used. RESULTS AND DISSCUSSION Formation of lamellar organogels driven by van der Waals' force and solvophobic effect. PAzPCC was synthesized and characterized according to the procedure illustrated in the Supported Information (Figures S1 and S2).43 We first studied the gelation of PAzPCC in various organic solvents, including hexane, toluene, p-xylene, dichloromethane (DCM), ethanol (EtOH), n-butanol, tetrahydrofuran (THF), ethyl acetate (EA), acetone, dioxane, methanol, DMSO (with and without 0.5% water), and water-saturated chloroform with a large range of polarity (Figure S3). It was found that PAzPCC can only form self-supportive gels in DMSO by means of heatingto-cooling and inversion tests (Figure 2a). Scanning electron microscopy (SEM) was employed to visualize the

micro-sized morphology of the aggregates obtained from different solvents. When using DMSO as the solvent, lamellar belts with tens of micrometers in width and hundreds of micrometers in length were clearly observed (Figure 2b). The 2D micro-sized nanostructures were further supported by transmission electron microscopy (TEM, Figure S4). Except for the micro-sized belts also observed in p-xylene, the aggregates obtained from other solvents showed amorphous or crystalline morphologies (Figure S3), suggesting that the self-assembly of PAzPCC into gels is highly dependent on solvophobic effect of the solvent.44 This phenomenon was further demonstrated by circular dichroism (CD) studies (Figure 2c). PAzPCC did not show detectable CD signals in most of organic solvents used. In DMSO, however, positive Cotton effects were observed at 250 nm, 381 nm, and 452 nm respectively, corresponding to the absorption region of 250-550 nm. These results reveal that the chiral information from the stereogenic center of the cholesterol motif is successfully transferred to the azo-based chromophore, endowing the selforganization of PAzPCC with a preferred supramolecular chirality. In addition, the UV-Vis absorption of the PAzPCC gel in DMSO displayed remarkable red shift with respect to the ones in other solvents (Figure 2c), indicating


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Figure 3. Photo-induced dimensional transition of the PAzPCC self-assembly. a) Reversible photo-isomerization of E/ZPAzPCC. b) UV absorption changes of E-PAzPCC in dilute solution of ethanol (concentration: 0.04 mM), c) CD and corresponding UV-Vis spectral changes of the PAzPCC gel in DMSO (concentration: 16 mM), and (d-g) SEM images of assemblies upon the irradiation of 365 nm light for d) 0 min, e) 15-30 min, f) 40 min, and g) ≥ 90 min. h) Illustration of photo-triggered dimensional transition of the self-assembly in DMSO. the ordered self-assembly of the azo-based chromophore with J-aggregation in DMSO. The obtained self-assembly from DMSO possessed high thermal stability, as evidenced from negligible changes in CD spectra of the PAzPCC gel when heated from 20 to 80 ℃ (Figures 2d and S5). The high thermal stability suggests that the self-assembly is further stabilized by van der Waal’s interaction between adjacent cholesterol groups. Hence, the supramolecular chirality of the self-assembly is under kinetic control rather than thermodynamic control, since the kinetic control usually gives rise to stable helical structures with definite supramolecular chirality, while thermodynamic control often results in dynamic helical structures that cannot retain their supramolecular chirality in solution.45,46 For this reason, the self-assembly constructed in this work could retain their optical properties in DMSO even at a high temperature of 80 oC. However, in weakly polar solvents such as p-xylene, toluene, and hexane, the self-assembly lost its preferential helicity. Thus, it was proposed that the main driven forces for the self-assembly might be the π-π stacking interaction from aromatic units and/or van der Waals' force originated from neighboring cholesterol groups. To prove this self-assembly mechanism, a temperaturedependent 1H NMR experiment was carried out. If the building blocks would gradually aggregate and form

ordered assemblies upon decreasing the temperature from 105 to 25 °C, remarkable changes of key proton peaks in corresponding 1H NMR spectra should be followed. In the present case, after decreasing the temperature, apart from the intensity variation originated from the aggregation of the PAzPCC building block, the signals of all aromatic protons displayed negligible chemical shifts (Figure S6), suggesting the absence of the π-π stacking from the aromatic unit of PAzPCC during the self-assembly process.47 To further study the gelation mechanism, X-ray crystallography, powder X-ray diffraction (XRD), and atomic force microscope (AFM) were employed. Orange plate crystals of PAzPCC were successfully obtained from the mixed p-xylene/THF solution by slow evaporation (Figure S7). As shown in Figure 2e, only PAzPCC molecules are in the unit cell of the single crystal structure without any solvent molecule. The crystal packing diagram clearly shows the absence of the π-π stacking interaction. The adjacent PAzPCC molecules are stacked in a staggered manner through van der Waals' force as the main driving force (Figure S8). The height of a stacked layer is around 1.6 nm (Figures 2f and S9). These observations are in a good agreement with the results achieved from temperature-dependent CD and 1H NMR spectra. Since the single crystal was obtained from a mixed solvent of p-



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Figure 4. Metal-ligand coordination-driven helical self-assembly of PAzPCC with various metal chlorides in n-butanol. a,d,f) SEM images and b,e,g) CD spectra of PAzPCC+NiCl2, PAzPCC+CuCl2, and PAzPCC+EuCl3 metallogels, respectively. The concentration of PAzPCC was fixed at 16 mM. c) CD absorption of Ni2+ based metallogels at 332 nm with different molar ratios. h) Schematic representation of helical self-assembly and handedness inversion directed by metal ions. xylene/THF, the solvent may affect the packing of PAzPCC. To investigate the interference of solvents, powder XRD patterns of samples obtained from DMSO and the mixed p-xylene/THF solution were measured. Nearly the same diffraction peaks (Figure 2g) suggest the same stacking mode of PAzPCC in DMSO and pxylene/THF. In fact, all the powder XRD patterns of samples obtained from various solvents displayed almost the same diffraction peaks (Figure S10), indicating that the stacking mode of PAzPCC is independent of the solvents used. Specifically, the diffraction peak with 2θ value around 5.60° corresponds to the d-spacing of 1.58 nm, which is agreeable with the layer height of 1.6 nm measured in the single crystal packing structure. The layer height was further supported by the AFM measurement, where the height of nanosheets obtained from dilute DMSO was measured to be 1.54 nm (Figure 2h). To obtain the thin nanosheets, dilute DMSO solution of PAzPCC with a concentration of 16 μM was sonicated thoroughly and drop-casted on the surface of fresh mica. These results suggest that the solvent plays an important role in the determination of the self-assembly behavior. Thanks to no appearance of the solvent molecule in the crystal structure,

we could reasonably conclude that solvophobic effect of the solvent is related to the nucleation of the self-assembly, which collectively drives the lamellar assembly of PAzPCC with van der Waals' force.48,49 Dimensional transition of self-assembly triggered by UV light. By virtue of various photochromic molecules such as azobenzene compounds, light is employed as an effective tool to dynamically regulate the dimension and supramolecular chirality of self-assemblies (Figure 3a).28,50 As shown in Figure 3b, the UV-Vis spectra of E-formed PAzPCC in dilute EtOH exhibited the absorption maxima at 320 nm and 450 nm, corresponding to π–π* and n–π* transitions, respectively. Upon the irradiation of 365 nm light, the intensity of the π–π* band greatly weakened, accompanied by a slight blue shift from 320 nm to 315 nm. After the irradiation for 30 min, the absorption intensity around 450 nm assigned to the n–π* band gradually increased and the absorption peak blue-shifted to 430 nm (inset of Figure 3b), indicating the E to Z photoisomerization of the azo unit in solution upon the UV light irradiation. The photoisomerization of PAzPCC was further confirmed by 1H NMR measurements (Figure S11). In addition, this E to Z transition was also observed in the


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gel state. As shown in Figures 3c and S12, upon the irradiation of UV light, both the intensity of positive Cotton effect and UV absorption at 375 nm gradually decreased, suggesting the dissociation of ordered aggregation and the disappearance of supramolecular chirality after the E to Z photoisomerization. According to the change of UV-Vis absorbance at 334 nm, more than 65% of E-PAzPCC was finally transferred into Z-PAzPCC after irradiation for 90 min (Figure 3c). To better understand the variation of supramolecular chirality upon the Z/E photoisomerization, SEM was employed to carefully monitor the transition process. Impressively, the 2D microbelts or plates (Figure 3d) gradually transformed into 1D nanosized roll-sheets (Figures 3e and S13) after 15-30 min of irradiation. Prolonging the UV irradiation for another 10 min yielded clean-cut nanotubes (Figures 3f and S13), which eventually broke down into amorphous nanoparticles after 90 min (Figure 3g). The process could be illustrated as shown in Figure 3h. Initially, E-formed PAzPCC self-assembles into ordered microbelts or plates via 2D lamellar self-assembly in DMSO. Upon the irradiation of 365 nm light, some Eformed PAzPCC molecules transform into the Zconformation and the co-assembly of both E- and Z-formed PAzPCC forms nanosized roll-sheets and nanotubes.51 Upon prolonging the UV irradiation, nearly all E-formed PAzPCC molecules transfer into the Z-conformation and unordered nanoparticles are obtained. Based on the E→Z photoisomerization of PAzPCC triggered by UV irradiation, the self-assembly system presents multidimensional transitions, i.e., from 2D microbelts to 1D nanotubes and finally to 0D nanoparticles. Since the Z/E photoisomerization of the azo group is reversible under light and heat treatments, this photo-triggered dimensional transition process should also be reversible. Indeed, microbelts were regenerated after heating the irradiated solution of PAzPCC (Figure S11). To better understand the self-assembly pathway, a control molecule (PVPCC) consisting of stilbene and cholesteryl units linked by a carbonate group was synthesized and characterized (Figures S1 and S2). Similar to PAzPCC, the E to Z photoisomerization of PVPCC occurred smoothly under the irradiation of 365 nm light, while having relatively low effectiveness in both solution (Figures S15 and S16) and gel (Figure S17) phases. In addition, the self-assembly of PVPCC was investigated in detail. Different with the multidimensional transitions of PAZPCC in DMSO, lamellar belts of PVPCC were gradually transferred into spherical aggregates with micrometer size under the irradiation of UV light (Figure S18). Metal-coordination-driven 1D helical self-assembly with supramolecular chirality inversion. Inspired by the strong coordination ability of pyridine moiety with metal ions,52-57 we chose various metal chlorides to explore the metal-mediated gelation of PAzPCC and preclude the interference of anions (on account of the insolubility of AgCl in the current conditions, AgNO3 was used). Since cholesterol-based gelators could readily form gels in alcohol solvents, the gelation of PAzPCC was firstly

studied in ethanol, methanol, and n-butanol. PAzPCC showed poor solubility in both ethanol and methanol, while giving a clear solution in n-butanol (Figure 2c). Hence, the gelation behavior in the presence of different metal ions with an appropriate ratio was investigated in n-butanol. Unordered microplate aggregates in n-butanol were confirmed by SEM, AFM, and CD measurements (Figures S19-S21). The inverted test tube method indicated that PAzPCC could form the self-supporting gels after adding metal ions like Ni2+, Cu2+, Mn2+, Mg2+, Fe2+, Bi3+, and Eu3+ (Figure S22). The metal coordination behavior was also supported by the 1H NMR and FT-IR studies (Figures S23 and S24). On account of the presence of paramagnetic species, BiCl3 was employed to study the binding with PAzPCC in DMSO-d6 using 1H NMR experiments. As shown in Figure S23, the peaks of protons a and b gradually shifted to downfield, while the peaks of protons c and d remained unchanged, strongly indicating the coordination interaction between metal ions and pyridyl unit rather than the azo moiety. In addition, the vibrational peaks (e.g., V−C=N− and V−C=C− in the cyan region) of pure PAzPCC were stronger and sharper as compared with those weaker and broader ones from PAzPCC+NiCl2, suggesting a vibration-restricted effect of the helical assembly (Figure S24). By contrast, only flowing solutions were obtained after the additions of K+, Ag+, Co2+, and Zn2+. The dimensional transition and metal-ligand coordinationdriven helical assembly were confirmed by SEM measurements (Figures 4a and S25). As discussed above, microsheets were observed from the dilute solution of PAzPCC in n-butanol. Impressively, exclusively lefthanded (M-type) helical nanofibers were found after adding Ni2+ with the ratio more than 0.2. These results suggested that the microplates of PAzPCC in n-butanol were successfully transformed into helical nanofibers after the coordination with Ni2+. The CD spectra of metallogels with various equivalents of Ni2+ were investigated in detail (Figures 4b and S26). When 0 and 0.1 equivalent of Ni2+ was added, a negligible Cotton effect was observed in the CD spectra. Increasing the Ni2+ concentration from 0.2 to 0.5 equivalent led to enhanced positive and negative Cotton effects at ∼294 nm and ~341 nm respectively, corresponding to the UV absorption peaks of the PAzPCC aggregates at around 320 nm. Compared with the positive CD peaks observed at ~375 nm and ~450 nm in DMSO, the chiroptical activity of PAzPCC was inverted upon the addition of Ni2+ in nbutanol. However, any further increase of the Ni2+ concentration would result in a decrease in the intensity of Cotton effects measured. The CD intensity at 332 nm was summarized as a function of the molar ratio of Ni2+ to PAzPCC (Figure 4c), clearly indicating that the preferred stoichiometry of the coordination mediated assembly between NiCl2 and PAzPCC was about 1:2. When increasing the concentration of the PAzPCC+Ni2+ metallogel under this preferred stoichiometry, an obvious enhancement of the Cotton effects was revealed in the CD spectra (Figure S27). Interestingly, a new negative Cotton peak emerged among 400-500 nm at relatively high concentrations. The CD studies demonstrated that PAzPCC+NiCl2 could form ordered aggregates with



preferred supramolecular chirality, which is in good accordance with the observation of M-helical nanofibers by SEM. Compared with the M-helix and negative CD peak at 471 nm in the PAzPCC+Ni2+ metallogel, P-helical nanoribbons from the metallogel containing Cu2+ were observed in the SEM images (Figure 4d) and confirmed by TEM images (Figure S28), and the corresponding CD spectrum displayed positive, negative, and positive CD signals at 467 nm, 353 nm, and 290 nm, respectively (Figure 4e). Considering the only alteration in these gels was the different metal ions utilized, it was reasonable to infer that the chirality difference was caused by the metal ions. There may be the relationship between the handedness and CD signals of gels, since right-handed nanostructures obtained from the Cu2+ based metallogel exhibited a positive Cotton effect in the range of 400-500 nm, while the Ni2+ based metallogel with left-handed nanofibers showed a negative Cotton effect under the same range. To figure out the relationship between the handedness of chiral nanostructures and the Cotton effect of metallogels in the range of 450-500 nm, PAzPCC complexes with Eu3+, Mn2+, Mg2+, Fe2+, Bi3+, K+, Ag+, Co2+, and Zn2+ were also measured by CD and SEM techniques. As shown in Figure 4f, left-handed twisted nanoribbons were clearly revealed from the Eu3+ gel, corresponding to the negative Cotton effect at 471 nm (Figure 4g). The SEM images of the Mg2+ gel exhibited left-handed worm-like twists (Figure S29) and corresponding CD spectrum displayed a negative Cotton effect above 400 nm (Figure S30). Although the nanofibers from the Fe2+ gel exhibited no twists, its CD spectrum still showed a negative CD peak around 471 nm (Figure S30a). Interestingly, the nanoribbons obtained from the Bi3+ gel showed right-handedness, even though its CD spectrum was silent. Similarly, the CD spectra of K+, Ag+, and Zn2+ gels were silent, but ordered aggregates such as nanorods, nanofibers, and microplates were obtained. By contrast, the CD spectra of Mn2+ and Co2+ gels displayed positive Cotton effects at the range of 400-500 nm, while unordered nanostructures were observed from SEM images. In addition, SEM was also employed to explore if the electrostatic interaction could result in the formation of helical assemblies from the metal-based complexes. As shown in Figure S31, no twists were observed from the complexes of protonated PAzPCC with NiCl2, CuCl2, and FeCl2. Based on the coordination between metal ions and pyridyl unit, the laminar aggregates of PAzPCC in nbutanol were successfully transferred into helical supramolecular polymers with preferred supramolecular chirality (Figure 4h). More interestingly, supramolecular chirality inversion was found in various metallogels. Thus, the relationship between the handedness of chiral aggregates and the CD signals of metallogels was established, i.e., P-type helical aggregates showed positive Cotton effects at 400-500 nm, while M-helical aggregates displayed negative cotton effects at the same range. To better understand the photoirradiation behavior of the metal complexes, complexes based on the coordination of PAzPCC with various metal ions (such as Cu 2+, Fe2+, and Mn2+) in n-butanol were investigated. As shown in Figure

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S32, both the CD and corresponding UV-Vis absorption spectra of these metal complexes did not show obvious changes under UV light irradiation for more than 60 min, presumably because that complete quenching of the excited state of PAzPCC occurs through the resonance energy transfer in the metal complexes.58,59

Figure 5. SEM images of a) Cu2+, c) Mn2+, e) Ni2+ metallogels in p-xylene/EtOH (v/v, 1/1). CD spectra of b) Cu2+ and d) Mn2+ metallogels in p-xylene/EtOH (v/v, 1/1). f,g) CD spectral comparison of Ni2+ and Fe2+ metallogels in various solvents. The concentration of PAzPCC was fixed at 16 mM. Chirality of supramolecular polymers controlled by solvent effect. To probe if metal ions could gelate PAzPCC in other solvents, p-xylene/EtOH (v/v, 1/1) was selected as the model solvent due to high dissolubility of PAzPCC in p-xylene and insolubility in EtOH observed in Figure S3. As shown in Figure S33, PAzPCC was fully dissolved in the mixed solution. After adding different metal ions, complexes based on Ni2+, Cu2+, Mn2+, and Fe2+ formed stable gels, while complexes of Zn2+ and Mg2+ showed transparent solutions. To explore the property of these aggregates, CD and SEM experiments were again conducted. In particular, P-helical nanostructures were obtained from the Cu2+ complex (Figure 5a) and corresponding CD spectrum displayed obviously positive absorption among 400-550 nm (Figure 5b). As compared with the results in n-butanol, the Cu2+ complex remained the same supramolecular chirality. Likewise, the Mn2+ complex also exhibited P-helicity and positive Cotton effect in p-xylene/EtOH (Figure 5c,d). However, in contrast to the M-helicity of the Ni2+ complex shown in n-butanol, nanostructures with P-helix were revealed in pxylene/EtOH (Figure 5e), which was further supported by the positive CD absorption among 400-550 nm (Figure 5f). Thus, supramolecular chirality inversion of the Ni2+ complex triggered by the solvent effect was achieved, i.e., Ni2+ based gel in p-xylene/EtOH showed positive, positive, and negative Cotton effects at ~470 nm, 375 nm, and 320


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nm respectively, while negative, negative, and positive CD absorption peaks were observed in n-butanol at around 470 nm, 350 nm, and 295 nm, respectively (Figure 5f). The inversion of supramolecular chirality was also found from the Fe2+ based gel system. Compared with the negative CD absorption in n-butanol, obviously positive Cotton effect was revealed in p-xylene/EtOH (Figure 5g). It should be noted that the Zn2+, Mg2+, and Co2+ complexes displayed relatively weak CD absorption, corresponding to unordered nanostructures shown in SEM images (Figures S34 and S35). Similarly, the metal complexes in p-xylene were also investigated, and some of them exhibited remarkable Cotton effects and ordered morphologies (Figures S36S39).

not only greatly dependent on the absolute molecular chirality of building blocks, but also highly related to metal ions and the solvent polarity. CONCLUSION In conclusion, we have presented that the rational design of a small molecular building block based on the azocholesterol conjugate could produce layered supramolecular aggregates, exhibiting photo-triggered dimensional transition and controllable supramolecular chirality. Upon the coordination with metal ions in solvents with different polarity, various metallogels based on the metal complexes have been obtained, showing metal ionmediated and solvent-dependent chirality inversion. Thus, the rational design of building blocks for the fabrication of helical nanostructures and supramolecular gel systems could be employed to achieve advanced materials with controllable supramolecular chirality, capable of realizing versatile potential applications in photo-responsive devices, metal-mediated supramolecular catalysis, and biomimetic systems. ASSOCIATED CONTENT Supporting Information Experimental procedures, supplemental figures, and single crystal data. Crystal data for PAzPCC (CIF) AUTHOR INFORMATION Corresponding Author *[email protected] ORCID Guofeng Liu: 0000-0003-1911-8546 Yanli Zhao: 0000-0002-9231-8360

Figure 6. a) Proposed relationship between the CD absorption at 450 nm and corresponding chirality. b) Supramolecular chirality of nanostructures based on PAzPCC and metal ions regulated by metal ion and solvent effects. “+” represents positive Cotton effect, “-” represents negative Cotton effect, and “N” stands for negligible supramolecular chirality. To gain a straightforward view, the relationship between supramolecular chirality and achiral factors such as metal ion and solvent effects based on the CD absorption at 450 nm was summarized in Figure 6. In n-butanol with relatively high polarity, M-type supramolecular chirality corresponding to negative CD absorption at 450 nm was revealed in Ni2+, Fe2+, Mg2+ and Eu3+ based complexes, while P-chirality was found in Cu2+, Mn2+, Co2+, Zn2+, and Bi3+ based complexes. In the solvent of p-xylene or pxylene/EtOH having relatively low polarity, most of the complexes displayed obvious P-chirality. These results are quite different from previous reports that supramolecular chirality usually depends on the enantiopurity of chiral centers.13,28 In our work, the CD band at around 450 nm is not induced by the molecular chirality, but by the aggregate chirality. In addition, the helical chirality of metallogels is

Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This research is supported by the Singapore Academic Research Fund (No. RG11/17 and RG114/17) and the Singapore National Research Foundation Investigatorship (No. NRF-NRFI2018-03). REFERENCES (1) Kim, Y.; Li, H.; He, Y.; Chen, X.; Ma, X.; Lee, M. Collective Helicity Switching of a DNA–Coat Assembly. Nat. Nanotech. 2017, 12, 551. (2) Tao, K.; Makam, P.; Aizen, R.; Gazit, E. Self-Assembling Peptide Semiconductors. Science 2017, 358. (3) Ma, X.; Zhang, S.; Jiao, F.; Newcomb, C. J.; Zhang, Y.; Prakash, A.; Liao, Z.; Baer, M. D.; Mundy, C. J.; Pfaendtner, J.; Noy, A.; Chen, C.-L.; De Yoreo, J. J. Tuning Crystallization Pathways through Sequence Engineering of Biomimetic Polymers. Nat. Mater. 2017, 16, 767. (4) Choi, J.; Majima, T. Conformational Changes of Non-B DNA. Chem. Soc. Rev. 2011, 40, 5893. (5) Selkoe, D. J. Alzheimer's Disease: Genes, Proteins, and Therapy. Physiol. Rev. 2001, 81, 741. (6) Liu, M.; Zhang, L.; Wang, T. Supramolecular Chirality in Self-Assembled Systems. Chem. Rev. 2015, 115, 7304.



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