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Controlling Supramolecular Chirality in Multicomponent SelfAssembled Systems Pengyao Xing† 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
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S Supporting Information *
CONSPECTUS: Chirality exists as a ubiquitous phenomenon in nature, from molecular level L-amino acids, D-sugar, secondary structures of proteins, DNA, RNA, and nanoscale helices to macroscopic conch and even galaxy. The aggregation of molecular building blocks with or without chiral centers might bring about asymmetric spatial stacking, which further results in the appearance of nonsymmetry in extended scales like helical nanofibers. This phenomenon, known as supramolecular chirality, is an important branch of supramolecular and selfassembly chemistry, which relates intimately with biomimetics, asymmetric catalysis, and designing chiroptic advanced materials. One of the important research focuses among supramolecular chirality is about rational manipulation of chirality amplification and handedness, presenting a profound influence on the performance of resulting soft materials such as circularly polarized luminescence and cell adhesion on hydrogels. The control over supramolecular chirality normally relies on two factors, i.e., thermodynamic and kinetic variables dependent on molecular structural parameters and environmental contributions, respectively. Supramolecular chirality in two or more component-based systems places an emphasis on thermodynamic control as it occurs from either integrated coassembly or separated self-sorting, which is more sophisticated than that of single component systems. Thus, the study on supramolecular chirality in multicomponent systems could mimic complicated biosystems, allowing for better understanding about the origin of natural chirality and extended applications as biomimetics. To date, the exploration of supramolecular chirality in multicomponent systems is restricted on both fundamental and application aspects when compared to more matured single component systems. Over the past few years, we have carried out systematic studies on several systems expressing supramolecular chirality from chiral amplification or symmetry breaking. We emphasized more the thermodynamic control by introducing a second component to form noncovalent bonding like hydrogen bonding or coordination interactions. In this Account, we would specifically discuss rational manipulation of the occurrence, transfer, and inversion of supramolecular chirality by taking several of the latest representative examples. In the multicomponent systems, in addition to the building blocks with chiral centers, the second or third components could be structural analogues and achiral small molecules such as bipyridines, melamine, metal ions, inorganic nanomaterials, and even solvents. These second or third components are able to incorporate during the aggregation to form coassembly via noncovalent bonds, influencing spatial arrangements of building blocks within various dimensions from vesicles and nanofibers to organic/inorganic hybrids. Other than chirality, morphology, stimulus responsiveness, and properties could also be well tailored by controlling interactions between different components. flexible control over packing parameters and other characteristics of self-assemblies. Either kinetically or thermodynamically, when the self-assembly prefers an asymmetric molecular packing mode, the as-formed architecture would be chiral at supramolecular scale, i.e., supramolecular chirality or superchirality.3,4 The subject of supramolecular chirality emerges as an important field as it covers or relates to a wide range of research fields from supramolecular chemistry and selfassembly to nanomaterials, asymmetric chemistry, biomimetics, and nonlinear optics. Self-assembled chiral architectures
1. INTRODUCTION Originally defined by Lehn as “chemistry beyond molecules”, supramolecular chemistry exploits noncovalent interactions between molecules to form ordered architectures.1 Molecular self-assembly as an offshoot of supramolecular chemistry emphasizing spontaneous aggregation behavior of supramolecular units. Many biological events such as protein folding and DNA double helix formation are typical self-assemblies supported by hydrogen bonding, in which processes, structural complexity, function, and information storage realize significant evolution. Noncovalent bonds feature a dynamic property whereby self-assembled matters are endowed with environmental adaptivity.2 These noncovalent interactions allow for © XXXX American Chemical Society
Received: June 28, 2018
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DOI: 10.1021/acs.accounts.8b00312 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Accounts of Chemical Research Scheme 1. Schematic Presentation of Supramolecular Chirality Control in Multicomponent Systemsa
a Coassembly is achieved through the participation of structural analogues or small molecules/nanomaterials. Supramolecular chirality with P- and M-handedness could show mirror signals on circular dichroism (CD) or CPL spectra.
Figure 1. Schematic representation of solvent polarity tunable supramolecular chirality. (a) Chemical structure of building block 1. (b) CD spectral comparison of 1-based self-assemblies in THF/hexane and THF/water mixtures. (c,d) TEM images of M- and P-type twist nanofibers observed in hexane and water, respectively. Reproduced from ref 9. Copyright 2016, American Chemical Society.
exist in multiple scales and topologies from chiral cages5 formed with merely several organic ligands to helical nano/ microfibers with infinite aggregation numbers.6 The emergence of supramolecular chirality could be triggered by the chirality transfer, chirality amplification and asymmetry breaking of either chiral or achiral compounds in single or multiple component systems.3 The manipulation of chirality in multicomponent systems could reflect complicated biosystems such as DNA and proteins, providing favorable solutions to biomimetics and better understanding of natural
homochirality, and also presenting considerable meaning to the rational design of advanced chiral materials.7 Compared to single component systems, multicomponent systems are highly complicated, where scenarios like integrated coassembly, separated self-sorting, heterojunction, and their mixtures may exist.8−10 Consequently, it remains challenging to manipulate homochirality in self-assemblies containing two or more components. For example, how to kinetically inverse the chirality of circularly polarized luminescence (CPL) in selfassembled systems, and in what ways plasmonic chiroptical B
DOI: 10.1021/acs.accounts.8b00312 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 2. (a) Chemical structures of compounds 2 and 3. Temperature-variable CD spectra for (b) aqueous assembly of 3 and (c) coassembled vesicles of 2 and 3. (d) Proposed mechanism of dehydration-induced chirality inversion. Reproduced from ref 10. Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
activity could be achieved or enhanced effectively.11,12 These challenges also relate to some key applications of supramolecular chiral systems, such as chiral bioimaging and sensing. In this Account, we present a brief overview regarding the control of chirality in multicomponent supramolecular systems (Scheme 1), including current solutions, limitations, and perspectives, based on some latest representative developments in this field. We shall provide a summary and discussion according to the types of building blocks and noncovalent driving forces between different components. The role of achiral organic molecules in tuning chiral nanostructures, chirality in coassembled structural analogues, and ordered chiral superstructures from organic/inorganic hybrids exhibiting diverse properties and functions shall be highlighted.
transfer, and handedness inversion may occur. The design and selection of achiral components can be flexible with the following basic principles: (1) achiral components have considerable interactions to chiral building blocks, and (2) they form coassembled ordered structures with specific stoichiometric ratios rather than forming self-sorted or separated assemblies. 2.1. Solvent-Precipitated Assembly
Solvent molecules are frequently involved in crystallization, existing as ordered or disordered forms in lattices. Soft selfassemblies are often influenced by solvents. The impact of solvents is mainly expressed as two aspects, including solvent polarity and active precipitation (note that chiral solvent effect is excluded here). In the bottom-up aggregation of building blocks, especially for amphiphiles, the molecular stacking layer that exposes to solvents should be solvophilic. On top of this, the switch of solvents such as from hexane (polarity ∼ 0) to water (polarity ∼ 10) shall lead to tremendous variations in the molecular stacking orientation and supramolecular chirality. We designed and synthesized a highly adaptive building unit 1 bearing naphthalimide and cholesteryl groups (Figure 1a). Compound 1 exhibited diverse self-assembly forms in various solvents, giving rise to self-healing organogel in hexane, decane, and ethyl acetate.9 Gel fibers in apolar solvents showed positive Cotton effects in CD spectrum (Figure 1b) with Mhandedness observed from transmission electron microscopy (TEM) (Figure 1c). When tuning the solvent polarity by increasing water fraction (f w) versus THF, 1 self-assembled into nanotoriod-terminated helices with P-handedness and negative Cotton effects (f w was between 60 and 80%). Further
2. CHIRALITY TUNING IN SUPRAMOLECULAR ASSEMBLY Achiral organic compounds that are able to bind chiral building units to give integrated coassemblies have been utilized to tune supramolecular chirality. Generally, there are a variety of noncovalent interactions between achiral organic compounds and chiral building blocks including hydrogen/halogen bonding,13−18 anion−π interaction,19 π−π stacking interaction,20 coordination,21,22 host−guest interaction,23 electrostatic attraction,24 solvent effect,25,26 and solvophobic interactioninduced physical encapsulation.27 These interactions shall combine multicomponent species into integrated coassemblies with ordered molecular arrangement, on the basis of which some interesting behaviors such as chiral amplification, chiral C
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Accounts of Chemical Research 2.2. Hydrogen-Bonded Complexes
increasing the solvent polarity (f w > 80%) resulted in the formation of vesicles with negative Cotton effects. The interdigitation region within assemblies could be varied from a naphthalimide unit in hexane to a cholesteryl group in water, whereby the directionality of hydrogen bonding between N−H and carbonyl units adopts an opposite manner to invert the supramolecular chirality. This phenomenon was also elucidated by Zhang et al. using a pyridylpyrazole-linked glutamide derivative as the chiral building unit, showing positive and negative Cotton effects in some nonpolar and polar solvents, respectively.25 Interestingly, no supramolecular chirality or CD signals were observed in methanol with moderate polarity. Apart from the polarity, solvent molecules could also behave as an active component that expresses significant influence on self-assembly behavior as well as supramolecular chirality. With respect to this effect, solvent molecules generally exist in an ordered manner within building block packing arrays via directional noncovalent interactions like hydrogen bonding and π−π stacking interactions. Other than organic solvents, water is a very common yet mysterious molecule in tuning the supramolecular chirality, which lies on its extraordinary hydrogen bonding formation capability. Water cluster binding to the pyridine unit was revealed by Lee et al. to tune supramolecular handedness of helical nanotubes constructed from rod-shaped amphiphiles.28 Heating shall induce the disassociation of pyridine−water hydrogen bonding interactions, resulting in the nanotube size contraction and chirality inversion. We recently found that the hydration/dehydration induced chirality inversion could expand to other structures.10 Structural analogues 2 and 3 (Figure 2) bearing chiral cholesteryl groups were synthesized, both of which gave rise to vesicular self-assemblies in aqueous media. Greatly enhanced CD signals were observed in an aqueous mixture of 2 and 3, indicative of their integrated coassembly. Mirror CD spectra of 3-based assembly appeared upon gradual heating (Figure 2b), implying chirality inversion. Further insight into the mechanism for chirality inversion was aided by X-ray crystallography analysis, which showed water participation between dimers via hydrogen bonding. The waterbinding capability of 3 allows specific hydration of hydrophobic vesicle membranes, and the dehydration takes place upon heating to induce the helicity inversion (Figure 2d). Coassembled vesicles of 2 and 3, however, exhibited silent chirality upon heating (Figure 2c) as a result of reorganized packing arrays. The finding drove us to figure out the structural parameters for determining water binding capability of cholesteryl naphthalimide conjugates. We further synthesized a series of cholesteryl naphthalimide derivatives with different alkyl chains (side arms) and spacer groups (Figure 2a).26 The capability of water binding is reflected on three aspects, including (1) ordered water molecules in crystal lattice, (2) moisture-sensitive assembly (a trace amount of water can collapse the gelation into microcrystals in apolar solvents), and (3) thermoresponsive supramolecular chirality inversion. With respect to water binding, the ethylene spacer group is prerequisite, whereas the length of side arms has certain influences. Similarly, the binding and removal of some apolar alkane solvents under the heating−cooling process could reverse supramolecular chirality of coronene bisimide derivatives with alkyl side arms.29 After cooling, solvent molecules embedded in the “molecular pockets” on the periphery of selfassemblies could form clathrate-like complexes and induce stereomutation, which are considered the driving force.
Hydrogen bonding has been considered one of the most important noncovalent interactions in supramolecular selfassembly ascribed to its relatively strong intensity (distance of ∼2 Å) and directionality. It not only provides the basis for helical biomacromolecules such as DNA helices and protein folding but also contributes tremendously to artificial chiral self-assemblies. Hydrogen bonding in organic building blocks is generally observed among amide/imide, ureido, and carboxylic acid groups, which determines chiral spatial orientation in many occasions. For the supramolecular chirality to be manipulated, the introduction of a second component that is able to alter hydrogen bonding is a favorable approach. Some organic molecules comprised of sp2 N atoms such as pyridine and imidazole derivatives (acceptor, A) favorably form hydrogen bonding with carboxylic acids (donor, D). Melamine with 9 D-A-D sites has been utilized to control chirality of carboxylic acid-based self-assemblies by means of duplex bridging-bidentate hydrogen bonding.13−15 We incidentally found that the handedness of N-fluorenyl-9methoxycarbonyl L-glutamic acid (Fmoc-L-Glu) self-assembled nanotubes could be inversed from P to M by melamine.13 We further accomplished the chirality control of other aromatic amino acid-based assemblies using melamine (Figure 3).14 For
Figure 3. (a) Chemical structures of compound 4 and melamine. (b) Powder XRD and (c) CD comparison between 4-based assembly and 4/melamine coassembly, where black and red curves represent 4 and 4/melamine samples, respectively. (d) TEM images of microsheets and twisted fibers from the assembly of 4 and 4/melamine, respectively. Reproduced from ref 14. Copyright 2015, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
instance, compound 4 comprised of pyrene and glutamic acid segments could self-assemble into microscale sheets with welldefined lamellar structure. The addition of two molar equivalents of melamine transformed the microsheets to microscale-twisted fibers (Figure 3d). Powder XRD studies indicated that the lamellar structure was replaced by hexagonal columnar packing, assisted with the mirror CD spectra. This work describes well how an achiral component induces D
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Figure 4. Schematic representation of ditopic phenylalanine building block 5, which of supramolecular chirality is tuned by pyridine coassembly. Reproduced from ref 17. Copyright 2018, American Chemical Society.
Figure 5. (a) Molecular structures of building block 6 and AIEgens and the representation of fabricating CPL emitting nanotubes via coassembly. (b) Digital images of coassembled fluorescent gels. (c) Mirror images of CPL spectra from the coassembly of D/L-6 and AIEgens. Reproduced from ref 27. Copyright 2017, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
macroscopic chirality of a chiral building block. The coassembly behavior including molecular packing, aggregation dimensions, and supramolecular chirality of aromatic amino acids with melamine is influenced by the nature of the amino acids. In this regard, we chose a series of Fmoc-protected Lamino acids as model building blocks to explore the pathway complexity of coassemblies with melamine.15 Two distinguishing pathways of coassemblies, i.e., the enhancement of 1D growth to give hydrogels and 3D growth into precipitates, were observed, featuring columnar and lamellar packing, respectively. The pathway is determined by the type of αsubstituents. Specifically, small dihedral angle between αsubstituents and backbone of aromatic amino acids favors 1D growth and vice versa. Moreover, the supramolecular chirality inversion was only observed from Fmoc−serine, Fmoc− alanine, and Fmoc−glutamic acid coassemblies. This finding may guide us to rationally design multicomponent chiral materials with desired dimensions and sizes.
Similar to melamine, the pyridine-based hydrogen bonding acceptor is an alternative for tuning self-assembly behavior and chirality. Unlike the duplex hydrogen bonding of melamine, the pyridine unit merely binds to carboxylic acid through single hydrogen bonding, which is considered to be weak and vulnerable. Thus, the competition from water−pyridine interactions is nonnegligible in pyridine−carboxylic acid coassembly systems. In this regard, we carried out systematic research on the selective binding of bipyridine derivatives by aromatic amino acids.18 Fmoc amino acids undergo either individual self-assembly or coassembly with some bipyridine derivatives (4,4′-bipyridine and 1,2-di(4-pyridyl)ethylene) depending on steric hindrance of α-substituents and number of hydrogen bonding sites in their solid states. According to small/wide-angle X-ray scattering characterizations, only some of them form coassemblies with bipyridines. Regardless of the selectivity, pyridine and its derivatives are effective components for regulating supramolecular chirality. E
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Figure 6. Schematic representation of ET in chiral assemblies based on aromatic glutamic acid dendrimers (7 and 8) controlled by solvents. Reproduced from ref 38. Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
in the coassembly, diazo bipyridine is organized in a staggered manner with loose stacking mode. Such a delicate difference results in thermodynamic reorganization of hydrogen bonded complexes.
Liu and co-workers revealed that twisted topological structures of self-assembled amphiphilic glutamic acids were responsive to bipyridines.30 We introduced rigid bipyridines during the assembly of ditopic L-phenylalanine gelators with benzene or cyclohexane centers.16 Nanofibers with uniform handedness were obtained from the coassembly. The handedness of nanofibers is unexpectedly dependent on the linkers or spacer in bipyridines. For example, a bipyridine derivative with an amide linker resulted in right-handedness (P), whereas 4,4′dipyridine without the spacer group gave left-handedness (M). The primary reason that the pyridine moiety is able to regulate supramolecular chirality of carboxylic acid building blocks was proposed as the substitution of pristine hydrogen bonding. To authenticate the assumption, we covalently modified ditopic (L,D)-phenylalanine building blocks by esterifying oligo(ethylene glycol) groups to carboxylic acids for eliminating their hydrogen bonding capacity.31 Expectedly, all applied analogues exhibited inversed handedness as compared to the intrinsic unesterified building blocks. Therefore, the key role of intercarboxylic acid hydrogen bonding in determining supramolecular chirality was verified, allowing for rational chiral regulation of self-assembly systems based on multicomponent amino acids or short peptides. In addition, we recently discovered that the stacking modes of bipyridines within coassembled arrays may play another important role for manipulating supramolecular chirality.17 Bipyridine derivatives having a disulfide or diazo spacer were respectively integrated into hydrogels formed by ditopic (L,D)-phenylalanine building block 5 (Figure 4). During the coassembly, disulfide bipyridine is packed with face-to-face H-type π−π stacking, whereas diazo bipyridine is arranged into shoulder-to-shoulder J-type π−π stacking, as confirmed by UV−vis spectroscopy and X-ray crystallography. H- and J-type π−π stacking led to P- and Mchirality for L-5 and M- and P-chirality for D-5, respectively. As compared to relatively compact stacking of disulfide bipyridine
2.3. Other Factors
In addition to the hydrogen bonding, the utilization of other noncovalent interactions allows for flexible control over chirality in host−guest coassembly systems. George et al. discovered the switch of supramolecular chirality in the coordination-driven coassembly of a phosphate receptor, zinc(II) dipicolylethylenediamine, with different enzymatically hydrolyzed adenosine triphosphates.21,22 Unlike coordination and hydrogen bonding with directional feature, electrostatic attraction has been developed as a promising approach to integrate chiral cationic and anionic components into ordered coassemblies.24,25 Incorporating luminescent dyes into chiral assemblies via noncovalent bonding is a useful pathway to realize CPL. As compared to a single-component CPL emitting system, multicomponent systems conveniently afford rich or fullcolor emission. Glutamide amphiphiles bearing a pyridinium headgroup could serve as chiral templates to encapsulate a series of achiral fluorescent dyes to give CPL emission.32 Similarly, Liu et al. observed that C3-symmetric (L/D-) glutamate gelator 6 could form nanotube assembly in mixed solvents (Figure 5).27 They then introduced six aggregationinduced-emission compounds (AIEgens) with blue to orange emission into the nanotube assembly and found that these AIEgens were confined orderly within nanotubes. Chiroptic studies indicate that AIEgens were induced with chirality, demonstrating full-color CPL emission (Figure 5c). They also designed a unique CPL energy transfer system based on the coassembly of the chiral glutamate gelator (donor) and an achiral luminophore acceptor. Weak π−π stacking interactions F
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Figure 7. Molecular structures of building blocks 9−11. (a,b) SEM images of twisted nanofibers from 9/11 and 10/11 coassembly, respectively. (c) Schematic representation of helical nanofiber formation. Hydrogels (d) with and (e) without gold nanowires. (f,g) TEM images of double helical gold nanowires. (h) TEM image of 9/11-based nanofiber. Scale bar is 500 nm for (a,b,f) and 100 nm for (g,h). Reproduced from ref 43. Copyright 2018, American Chemical Society.
the coassembly of two conformational diastereoisomers based on benzophenone-3,3′,4,4′-tetracarboxylic diimide macrocycle in the solid state.35 Fabricating optoelectronic materials from supramolecular chiral coassemblies remains challenging. In a recent effort, Guler and co-workers36 employed pyrene and naphthalenediimide conjugated peptides as n- and p-type semiconductors, respectively, to give β-sheet coassembly. The coassembly exhibited significantly enhanced conductivity relative to that of individual n/p-type nanofibers. N,N′-Bis(1′-phenylethyl)perylene-3,4,9,10-tetracarboxyldiimide (CPDI) is an n-type semiconductor. R-CPDI and S-CPDI were found to form selfdiscrimination (coassembly) nanofibers rather than selforganization (self-sorting) nanofibers.37 Homochiral structures from individual self-assembly showed superior charge transport with better photoresponsivity than heterochiral coassemblies ascribed to large π-planar overlap. We recently synthesized two chiral glutamic acid dendrimers bearing pyrene (7) and naphthalimide (8) moieties as energy transfer (ET) donor and acceptor, respectively (Figure 6).38 It was found that the self-assembly behavior of either self-sorting or coassembly could be determined by the solvent polarity. Polar aqueous media facilitated self-sorting, where 7 and 8 generated Phanded nanofibers and lamellar sheets, respectively. In contrast, 7 and 8 could coassemble into M-type helical nanofibers in apolar decane, showing enhanced ET efficacy relative to that in water. This work provides an excellent example of a rational design of chiral coassemblies for achieving enhanced ET controlled by the solvent polarity. In
facilitate the coassembly, where CPL was transferred on account of considerable spectroscopic overlap.
3. CHIRALITY IN COASSEMBLED STRUCTURAL ANALOGUES Coassembly comprised of structurally similar analogues is another important category of supramolecular multicomponent systems. There are two distinguishing pathways upon the aggregation of analogue mixtures, namely, self-sorting and coassembly.10 The pathway complexity originates from binding equilibrium of components either kinetically or thermodynamically. The realization of coassembly between analogues requires either relatively strong interactions or similar molecular packing parameters between building blocks. The manipulation of supramolecular chirality in coassembled structural analogues is a promising yet challenging research area because it relates to chiral amplification as well as chiroptical and electrical applications of soft materials. Chiroptical properties reflect rich information regarding interactions between chiral analogues, providing solid evidence to the self-sorting or coassembly. On the basis of chiroptical analysis, Meijer et al. corroborated the coassembly of several chiral supramolecular copolymers from C3-symmetric monomers.33 Delicate control of chirality between components is also useful for the fabrication of special soft materials. Recently, Yashima et al. reported an unprecedented “helix-in-helix” superstructure via the coassembly of syndiotactic poly(methyl methacrylate) and fullerene-terminated peptide.34 Stoddart and co-workers observed a double helix superstructure from G
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Figure 8. Chemical structure of building block 12 as well as superstructure fabrication with opposite handedness. Reproduced from ref 46. Copyright 2014, American Chemical Society.
a different manner to our protocol, Nakashima39 and George40 individually studied enantioselective coassembly/self-sorting and ET based on chiral bichromophoric naphthalenediimide derivatives. On account of spatial arrangements, only building blocks with certain stereo conformations (such as enantiomers) were able to form coassemblies with high ET efficacy. Building blocks without matched conformation would result in self-sorting assembly with negligible energy migration. Such enantioselective ET shows great potential in exploring switchable CPL capable of being responsive to chiral guest molecules.
templates are promising materials in optical, catalysis, and therapeutic applications. For instance, organic lipid N,N′bis(octadecyl)-L/D-glutamic diamide (L/D-GAm) could form emissive hybrid gels with CdSe/ZnS QDs41 or perovskite nanocrystals,42 and the chirality transfer from lipid molecules to inorganic nanomaterials enabled the full color tunable CPL with relatively strong intensity (glum up to 10−3). Chiral plasmonic nanostructures with strong chiroptic signals are highly desirable for imaging and sensing applications. On the basis of the delicate predesign, Kawai et al. successfully synthesized double helical gold nanowires using two-component helical hydrogels as soft templates (Figure 7).43 Compounds 9 and 10 with opposite handedness behave as chiral inducers, and compound 11 acts as a stabilizer to control the gold nanowire growth. Semitransparent and thermoreversible hydrogels were obtained in the mixture of 9 or 10 with 11 and LiCl in aqueous media containing a small amount of toluene (Figure 7c,d). The obtained well-defined helical nanofibers with preferred handedness could serve as ideal templates for the growth of helical gold nanowires. After in situ reduction of gold precursors in gel phase, uniform double helical gold nanowires were achieved (Figure 7f,g), and their handedness could be inverted by changing chiral gelators 9 and 10. These gold nanowires also showed broad Cotton signals from 400 to 800 nm.
4. CHIRAL SUPERSTRUCTURES FROM ORGANIC/INORGANIC COASSEMBLY In addition to pure organic building blocks, the coassembly between organic species and inorganic nanostructures represents an important branch of multicomponent systems. Inorganic materials such as polyoxometalates, quantum dots (QDs), silica, and gold/silver nanoparticles have been incorporated into chiral assemblies.3 Ordered organization of organic and inorganic chiral nanostructures generates hybrid nanomaterials with desired functions. The chirality undergoes transfer from chiral organic building blocks to inorganic species, whereby chiral inorganic superstructures could be prepared. These chiral inorganic superstructures with controllable dimensions, size, and helical pitch according to the chiral H
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Accounts of Chemical Research In situ growth of gold nanoparticles on a supramolecular helix is an alternative method to synthesize helical gold superstructures. The positioning of gold nanoparticles on the nanofiber template is controlled by asymmetric arrangement of gold-affinitive groups such as thiol. Rosi et al. reported the preparation of single gold helix using a peptide amphiphile containing sulfide as the helical source.44 Exceptionally, single gold helix demonstrated considerable chiroptical activity at the plasmon frequency, and the absolute g-factor value was measured up to 0.04. This approach is based on helical scaffolds of peptides or other organic building blocks. The structures of these scaffolds feature dynamic and adaptive properties, which are vulnerable to ambient conditions. To address this limitation, Oda and co-workers demonstrated45 a hierarchical building approach to fabricate gold nanoparticlebased helical superstructures. Cationic bis-quaternary ammonium gemini surfactants coassembled with L/D-tartrate counterions to generate helical nanofibers with desired handedness. A sol−gel protocol was applied to the helical nanofiber template, giving rise to silica nanohelix serving as seeds for the deposition of gold nanoparticles. The resultant structurally robust nanoarchitectures exhibited great potential in chiral biosensing. For most of these cases, the handedness of gold nanoparticle superstructures is tuned by changing the molecular chirality (D/L) of building blocks. On the basis of dynamic supramolecular self-assembly, we prepared46 both P- and M-gold nanoparticle helices from one chiral template (Figure 8). The building block 12 bearing chiral cholesteryl, azobenzene, and α-lipoic acid units was synthesized to prepare thermoresponsive organogels in n-butanol, and strong Cotton effect indicated the chiral structure formation. Surprisingly, the addition of 5 mol equiv of HAuCl4 precursor into the solution of 12 (52 °C, in n-butanol) gave mirror CD signals as compared to that at room temperature. Utilizing mass spectrometry and molecular dynamic computation, the binding of Au(III) to the azo group at a relatively high temperature was verified. Au(III)-chlorine electrostatic attraction with a distance of 3.33 Å was considered as a driving force to invert the packing mode and spatial arrangement of complexes. By reducing Au(III) in sol and gel states, P- and M-helical superstructures based on gold nanoparticles were realized.
Despite these developments in manipulating supramolecular chirality, further improvements in this exciting field are still needed. Present characterization techniques for supramolecular chirality mainly depend on electron microscopy and chiroptic spectroscopy, which however are short of quantitative analysis such as chiral enantiomeric excess (ee). It is also difficult to predict the handedness of supramolecular chirality from predesigned molecular chirality of building blocks. For complicated multicomponent systems, the selection between homochirality and heterochirality in enantiomer coassembly remains a challenge. As compared to two-component systems, exponentially increased pathway complexity would exist from more component systems such as three- and four-component systems, showing great difficulty in forming molecularly ordered structures with high fidelity. Current solutions to address this challenge concentrate mainly on metal−ligand coordination and macrocycle-based host−guest chemistry. Therefore, assembled systems containing more than two components with higher complexity require in-depth investigations to mimic biological multicomponent species and design advanced chiral materials. Supramolecular chirality has already been exhibiting tremendous application potentials in several areas. Utilizing synthetic building blocks to construct chirality-controllable coassembled architectures is a promising research direction as it could be applied to alter functions of specific proteins or DNA in biological systems, achieving therapeutic effects (see Supporting Information for details). In preliminary studies, Lee et al. realized reversible B/Z-handedness inversion of DNA in living cells by means of coassembly from synthetic building blocks and DNA.47 In another application, helical nanotubes based on two-component Bi(III)-glutamic amphiphile could be employed to asymmetrically catalyze the Mukaiyama aldol reaction with high enantioselectivity (up to 97% ee).48 Thus, it is reasonable to picture a bright future for supramolecular chirality based on multicomponent systems.
5. CONCLUSIONS AND OUTLOOK Chirality at the supramolecular scale is an important feature of functional molecules and materials, which highly influences their applications. This Account presents a brief overview on the manipulation of supramolecular chirality in multicomponent systems. We describe how achiral organic compounds could actively integrate with chiral structures primarily through hydrogen bonding and coordination interactions as well as their key role in inducing macroscopic chirality and inverting handedness. For example, solvent molecules could affect chiral behavior of building blocks via the polarity and ordered participation in the coassembly. The coassembly between chiral analogues shows a great impact on chiroptic and optoelectronic properties, whereby the CPL and electron mobility of functional systems could be tuned by different chirality. Chiral supramolecular structures are excellent soft matrix and templates for formulating inorganic superstructures. The coassembly of chiral organic structures and inorganic species endows the obtained hybrids with fascinating chiroptic properties, such as CPL and chiral plasmonic activities.
Additional discussion about supramolecular chirality in coassemblies involving biomacromolecules (PDF)
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.accounts.8b00312.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yanli Zhao: 0000-0002-9231-8360 Notes
The authors declare no competing financial interest. Biographies Pengyao Xing graduated with B.Sc. in School of Chemistry and Chemical Engineering, Shandong University. He then received a Ph.D. degree from the same university under the supervision of Prof. Aiyou Hao. He is currently a research fellow in Professor Yanli Zhao’s group at Nanyang Technological University. He is working on functional materials based on supramolecular self-assembly. I
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Accounts of Chemical Research
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Yanli Zhao is currently a professor at Nanyang Technological University. He received his B.Sc. and Ph.D. degrees from Nankai University. He was a postdoctoral scholar with Professor Sir Fraser Stoddart at University of California Los Angeles and subsequently at Northwestern University. His research focuses on self-assembled materials, integrated nanoparticles, and porous materials.
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ACKNOWLEDGMENTS This research is supported by the Singapore Academic Research Fund (nos. RG121/16, RG11/17, and RG114/17) and the Singapore National Research Foundation Investigatorship (no. NRF-NRFI2018-03).
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DOI: 10.1021/acs.accounts.8b00312 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.accounts.8b00312 Acc. Chem. Res. XXXX, XXX, XXX−XXX
