Thermal Chiroptical Switch Based on an Ultrahigh Temperature-Initiated Macrocycle Gel Platform.

Responsive chiral optical materials have gained considerable interests from the fields of sensing, display, and optical devices. Materials that are capable of changing chiral optics under harsh conditions such as strong basic/acidic or ultrahigh temperature provides thoughts for the design of materials working at special environments, which however, are still underdeveloped. Here, a proof-of-concept design of organogel is reported that acts as matrices for thermal chiroptical switch with critical working temperature above 100 °C. The reversible solution-to-gel transition of the specific ß-cyclodextrin/dimethyl formide/LiCl system is initialized at about 130 °C, when the luminophores with aggregation-induced-emission property shall be lighted up with transferred chirality from inherent chiral ß-cyclodextrin. It allows for the controlled emergence of circularly polarized luminescence. This delicate design enables successful fabrication of ultrahigh temperature thermal chiroptical switch.

Multiple-State Control over Supramolecular Chirality through Dynamic Chemistry Mediated Molecular Engineering.

Dynamic chemistry utilizing both covalent and noncovalent bonds provides valid protocols in manipulating properties of self-assemblies and functions. Here we employ dynamic chemistry to realize multiple-route control over supramolecular chirality up to five states. N-protected fluorinated phenylalanine in the carboxylate state self-assembled into achiral nanoparticles ascribed to the amphiphilicity. Protonation promoted one-dimensional growth into helices with shrunk hydrophilicity, which in the presence of disulfide pyridine undergo chirality inversion promoted by the hydrogen bonding-directed coassembly. Further interacting with the water-soluble reductant cleavages the disulfide bond to initiate the rearrangement of coassemblies with a chirality inversion as well. Finally, by tuning the pH environments, aromatic nucleophilic substitution reaction between reduced products and perfluorinated phenylalanine occurs, giving distinct chiral nanoarchitectures with emerged luminescence and circularly polarized luminescence. We thus realized a particular five-state control by combining dynamic chemistry at one chiral compound, which greatly enriches the toolbox in fabricating responsive chiroptical materials.

Pnictogen bonding in imide derivatives for chiral folding and self-assembly.

Pnictogen bonding (PnB) is an attraction interaction that originates from the anisotropic distribution of electron density of pnictogen elements, which however has been rarely found in nitrogen atoms. In this work, for the first time, we unveil the general presence of N-involved PnB in aromatic or aliphatic imide groups and reveal its implications in chiral self-assembly of folding. This long-neglected interaction was consolidated by Cambridge structural database (CSD) searching as well as subsequent computational studies. Though the presence of PnB has limited effects on spectroscopic properties in the solution phase, conformation locking effects are sufficiently expressed in the chiral folding and self-assembly behavior. PnB anchors the chiral conformation to control the emergence and inversion of chiroptical signals, while intramolecular PnB induces the formation of supramolecular tilt chirality. It also enables the chiral folding of imide-containing amino acid or peptide derivatives, which induces the formation of unique secondary structural sequences such as ß-sheets. Finally, the effects of PnB in directing folded helical structures were revealed. Examples of cysteine and cystine derivatives containing multiple N⋯O and N⋯S PnBs constitute an α-helix like secondary structure with characteristic circular dichroism. This work discloses the comprehensive existence of imide-involved PnB, illustrates its important role in folding and self-assembly, and sheds light on the rational fabrication of conformation-locked compounds and polymers with controllable chiroptical activities.

Hypervalent Iodine(III) Mediated Halogen Bonded Supramolecular Chiral System with Cholesteryl Naphthalimides.

Halogen bonding acknowledged as a noteworthy weak interaction, has gained growing recognition in the field of supramolecular chemistry. In this study, we selected structurally rigid diaryliodonium ions (I(III)) with two biaxial σ-holes as halogen-bond donors, to bind with three chiral acceptor molecules bearing cholesteryl and naphthalimides with distinct geometries. The abundant carbonyl oxygen atoms in side-arm substituents function as multiple acceptors for halogen bonding. The self-aggregation of chiral acceptor molecules demonstrates adaptiveness to solvent media, evidenced by the inversion of the Cotton effect and the morphological evolution from spherical to rod-like nanoarchitectures in different solvent systems. The distinct geometries of the acceptor molecules conferred various binding modes with I(III). The introduction of I(III) as a halogen-bond donor regulates the aggregation of the donors, achieving amplification of chiroptical signals and inheriting solvent responsiveness from the self-aggregated assembly. This study successfully utilized rational structural design and multimodal control strategies to achieve regulation of supramolecular chirality.

Diastereoselective Supramolecular Encapsulation and Chirality Transfer Between Cholesteryl Binaphthyl Conjugates and Polyaromatic Hydrocarbon.

Diastereoselective effect plays an important role in the synthesis of chiral complexes and macrocyclic compounds, while its function in selective coassembly and chirality transfer has yet to be unveiled. In this work, two pairs of diastereomers containing R/S- binaphthyl and homochiral cholesteryl domains are synthesized, which provide multiple sites to encapsulate polyaromatic hydrocarbon through π-π and CH-π interactions. X-ray structures and computational studies suggest the binaphthol derivatives feature CH-π folding into butterfly-like open geometry, while binaphthylenediamine derivatives adopt closed geometry supported by van der Waals between cholesteryl domains. Driven by solvophobic forces, the building units self-assemble into vesicles and nanofibers in the aqueous and methanol phases, respectively. Binaphthol derivatives selectively encapsulate pyrene by naphthalene domains in the vesicle phase, while binaphthylenediamine derivatives encapsulate pyrene by cholesteryl domains in the nanofiber phase. Density functional theory-based calculations and circular dichroism spectra evidence the closed geometry of binaphthylenediamine derivatives facilitates a clamp-type host to increase the affinity toward pyrene in spite of the strong solvation competition. This work unveils the diastereoselectivity in the chiral coassembly, deepening the understanding of the precise synthesis of functional chiroptical complexes.

Engineering π-Conjugation of Phenylalanine Derivatives for Controllable Chiral Folding and Self-Assemblies.

π-π stacking interaction is an attractive interaction that involves aromatic groups containing π-conjugated domains. It is a promising strategy for stabilizing folded structures with interesting chiroptical properties and manipulating the supramolecular chiral self-assembly process. In this study, we report the engineering of π-conjugated amino acids that utilize π-π stacking interactions to manipulate chiral folding as well as self-assembly evolution. Stepwise conjugation of phenyl, naphthyl, and pyrenyl to N-terminal phenylalanine derivatives witnessed the folding through intramolecular π-interactions in solution phase, which facilitated the formation of chiral geometry and the emergence of chiral optics. Introduction of aromatic domains efficiently lowers the critical aggregation concentration in the aqueous media. Molecular folding enables a special concentration-dependent self-assembly, whereby the supramolecular chirality accomplished inversion with the evolution of helical nanoarchitectures. This work develops a strategy to engineer π-conjugated amino acids with controllable folding behaviors, which also offers implications for the rational design of functional chiral materials.

Ultrasensitive Solvatochirochromism of Single Benzene Chromophores.

Solvents influence the structure, aggregation and folding behaviors of solvatochromic compounds. Ultrasensitive solvent mediated chiroptical response is conducive to the fabrication of molecular platform for sensing and recognition, which however, remains great challenges in conceptual or applicable design. Here we report a cysteine-based single benzene chromophore system that shows ultrasensitivity to solvents. Compared to the ratiometrically responsive systems, the chiroptical activities could be triggered or inverted depending on the substituents of chiral entities with an ultralow solvent volume fraction (<1 vol %). One drop of dipolar solvents shall significantly induce the emergence or inversion of chiroptical signals in bulky phases. Based on the experimental and computational studies, the ultrasensitivity is contributed to the intimate interplay between solvents and chiral compounds that anchors the specific chiral conformation. It illustrates that structurally simple organic compounds without aggregation or folding behaviors possess pronounced solvatochiroptical properties, which sheds light on the next-generation of chiroptical sensors and switches.

Selective chiral dimerization and folding driven by arene-perfluoroarene force.

Oligomerization and folding of chiral compounds afford diversified chiral molecular architectures with interesting chiroptical properties, but their rational and precise control remain poorly understood. In this work, we employed arene-perfluoroarene (AP) interaction to manipulate the folding and dimerization of alanine derivatives bearing pyrene and a perfluoronaphthalene derivative. Based on X-ray crystallography and nuclear magnetic resonance, the compound with a smaller tether and high skeleton rigidity self-assembled into double helical dimers by duplex hydrogen bonding and AP forces in a less polar solvent. Reversible disassociation occurred upon switching to a dipolar solvent or applying heating-cooling cycles. In comparison, the compound with increased skeleton flexibility folds into chiral molecular clamps in a less polar solvent, and is transformed into planar dimers upon switching to a polar solvent. The dynamic geometrical transformation between dimerization and folding was accompanied by chiroptical switching. Beyond the molecular and supramolecular level, we showed hierarchy control in the self-assembled nanoarchitectures and columnar and lamellar arrangements of their molecular packing. This work utilized AP forces to prepare and manipulate the chiral architectures at different hierarchical levels, enriching methodologies in precise chiral synthetic chemistry.

Chalcogen and Pnictogen Bonding-Modulated Multiple-Constituent Chiral Self-Assemblies.

Chalcogen and pnictogen-based σ-hole interactions have shown limited applications in controlling supramolecular chirality. In this work, we employed chalcogen and pnictogen bonding to control supramolecular chirality in a multiple-constituent system with modulate chiral optics. Phenyl phosphonium-selenium conjugates with electrophilic σ-hole regions were allowed to coassemble with the π-conjugated deprotonated amino acids. Control experimental and computational results evidenced that the chalcogen and pnictogen bonding formed with carboxylates induced morphological transformation from achiral membranes to chiral helical nanotubes with emerging supramolecular chirality. Also, the chiral helical architectures accomplished inverted handedness and chiroptical activities, including circular dichroism and circularly polarized luminescence. Finally, synergistic chalcogen and pnictogen bonding was employed to stabilize the charge-transfer complexation to afford ternary chiral co-assemblies with evolved chiral optics and luminescence. This work, showing the role of chalcogen and pnictogen bonding in manipulating supramolecular chirality and optics, will expand the toolbox in the fabrication and property-tuning of chiral materials containing elements of Group VA and VIA.

Supramolecular axial chirality in [N-I-N]+-type halogen bonded dimers.

Axial chiral molecules are extensively used as skeletons in ligands for asymmetric catalysis and as building blocks of chiroptical materials. Designing axial chirality at the supramolecular level potentially endows a material with dynamic tunability and adaptivity. In this work, for the first time, we have reported a series of halogen-bonded dimeric complexes with axial chirality that were formed by noncovalent bonds. The [N-I-N]+-type halogen bond is highly directional and freely rotatable with good linearity and ultra-high bond energy; this bond was introduced to couple quinoline moieties with chiral substitutes. The resultant dimers were stable in solutions with thermo-resistance. Prominent steric effects from the 2' chiral pendant allowed the chirality to be transferred to aryl skeletons with induced preferred axial chirality and optical activities. Halogen-bonded complexation presented visible emissions to afford luminescent axial chiral materials, whereby circularly polarized fluorescence and phosphorescence were achieved. The [N-I-N]+-type halogen bond performed as a powerful tool to construct functional axial chiral compounds, enriching the toolbox for asymmetric synthesis and optics.

Biogenetic Chiral Deep Eutectic Solvents that Produce Self-Assembled Chiroptical Materials.

Deep eutectic solvents (DESs) show particular properties compared to ionic liquids and other traditional organic solvents. Controlled synthesis of chiral materials in DESs is unprecedented due to the complex interplays between DESs and solutes. In this work, all bio-derived chiral DESs were prepared using choline chloride or cyclodextrin as hydrogen bonding acceptors and natural chiral acids as donors, which performed as chiral matrices for the rational synthesis of chiroptical materials by taking advantage of the efficient chirality transfer between the DESs and solutes. In a very selective manner, building units with molecular pockets could facilitate strong binding affinity towards chiral acid components of DESs disregarding the presence of competitive hydrogen bonding acceptors. Chirality transfer from DESs to nanoassemblies leads to chirality amplification in the presence of minimal amounts of entrapped chiral acids, thanks to the spontaneous symmetry breaking of solutes during aggregation. This work utilizes chiral DESs to control supramolecular chirality, and illustrates the structural basis for the fabrication of DES-based chiral materials.

V-shaped chiral hosts based on π-extended hematoxylin.

Hematoxylin has a V-shaped chiral geometry, but its potential in chiroptical self-assembled materials is underdeveloped. Herein, three novel V-shaped chiral hematoxylin derivatives were synthesized, and they showed extended skeletons as well as photophysical and chiroptical behaviors. Moreover, their host-guest interactions with C60 were investigated. Our findings could aid in the design and synthesis of novel chiral host molecules from natural products.

Chiral Superlattices Self-Assembled from Post-Modified Metal-Organic Polyhedra.

Metal-organic polyhedra (MOPs) are inherently porous, discrete, and solvent-dispersive, and directing them into chiral superlattices through direct self-assembly remains a considerable challenge due to their nanoscale size and structural complexity. In this work, we illustrate a postmodification protocol to covalently conjugate a chiral cholesteryl pendant to MOPs. Postmodification retained the coordination cores and allowed for reaction-induced self-assembly in loosely packed nanosized columns without supramolecular chirality. Solvent-processed bottom-up self-assembly in aqueous media facilitated the well-defined packing into twisted superlattices with a 5 nm lattice parameter. Experimental and computational results validated the role of intercholesteryl forces in spinning the nanosized MOPs, which achieved the chirality transfer to supramolecular scale with chiral optics. This work establishes a novel protocol in rational design of MOP-based chiroptical materials for potential applications of enantioselective adsorption, catalysis, and separation.

Halogen Interaction Effects on Chiral Self-Assemblies on Cyclodipeptide Scaffolds Across Hierarchy.

How halogenation affects protein or peptide folding and self-assembly hierarchically? This study tries to answer this question by using the halogen bonding mediated self-assemblies on cyclodipeptide scaffolds. Single-functionalized cyclodipeptides (Cyclo-GX) based on para-halogenated phenylalanine in the solid state form homochiral helical nanotubes via consecutive X···O bonds (X = Cl, Br, and I) independent of halogen kinds. In contrast, double-functionalized cyclodipeptides (Cyclo-XX) feature versatile self-assembly architectures depending on the para-substituents (X = H, F, Cl, Br, and I), affording nanotubular, lamellar, and triple helical nanotubular architectures. Cyclo-BrBr exclusively adopts intramolecular Type-IV X···X interaction that alters the molecular folding and packing, which also gives rise to opposite chirality at molecular folding (secondary structure), stacking (tertiary structure), and self-assembled nanohelices (quarternary structure) at macroscopic scale. It unveils how halogenation impacts on the self-assembly and chirality at hierarchical levels in specific peptides. Clusteroluminescence is found for the cyclodipeptides, achieving high quantum yield up to 71%, whereby circularly polarized luminescence is realized with tunable handedness by controlling halogen substituents.

Thermal Annealing Triggered Chirality Inversion through Solvent Migration.

Solvent strategy is a powerful tool to manipulate chirality and self-assembly over hierarchical levels, yet the solvent dynamics during thermal annealing in controlling chirality and chiroptical features remain a mystery. Here, we show how solvent migration affects molecular folding and chirality through thermal annealing. Pyrene segments were conjugated to a 2,6-diamide pyridine skeleton, where intramolecular hydrogen bonds anchor the chiral geometry. The orientation of pyrene blades adopted π···π and CH···π stacking, respectively, in organic solvents (dimethyl sulfoxide, DMSO) and aqueous media, leading to the chiroptical inversion. Thermal annealing treatment of the DMSO/H2O mixture homogenized distribution of solvents that further altered the molecular folding from CH···π to π···π modality. Solvent migration from aggregates to bulky phases was evidenced by the nuclear magnetic resonance and molecular dynamic simulations, leading to the rearrangement of molecular packing with luminescent changes. It realized a consecutive chiroptical inversion using solvent strategy and thermal annealing.

Solvent-resolved self-assemblies of cholesteryl-cyanostilbene conjugates with photo- and thermo-responsiveness.

It remains challenging to construct multifunctional chiral stimulus-responsive molecules and to modulate their morphology at the nanoscale. In this paper, we synthesized a novel chiral molecule with both photoactive and potentially bioactive properties and found that the morphological changes of its self-assembly were influenced by solvent polarity and light exposure. This work enabled the synthesized molecule to undergo Z-E isomerization efficiently under light irradiation by introducing highly oriented hydrogen bonds into the cyanostilbene part. The photoisomerization of the cyanostilbene part from Z- to E-type was further exploited, leading to morphological changes from nanohelices to vesicles with chiroptical evolution. The light-modulated supramolecular chirality and nanostructure provide a green and efficient method for the design of responsive chiral materials.

Arene-Perfluoroarene Force Driven Sublimation-Removable Chiral Coassemblies.

Multiple constituent coassembly is an emerging strategy to manipulate supramolecular chirality and chiroptical properties such as circularly polarized luminescence (CPL). However, the second or third constituent could not be removed from pristine self-assembly. Here we developed a constitute-removable chiral coassembly using sublimation that could realize coassembly with tunable supramolecular chirality, luminescence and CPL properties. Octafluoronapthalene (OFN) with small sublimation enthalpy formed coassemblies with perylene-conjugated peptoids via arene-perfluoroarene (AP) interaction that induced the emergence of macroscopic chirality and hypsochromic luminescence from yellow to green. Coassembly with OFN accelerated one-dimensional growth and induced the emergence of macroscopic chirality and CPL. Despite the stability at ambient conditions, vacuum-treatment triggered fast sublimation of OFN, which behaved as a sacrificial template. Physical removal of OFN retained the helical nanoarchitectures as well as the basic features of Cotton effects and CPL activities. X-ray diffraction suggested the back-fill consolidation occurred on the molecular voids by OFN removal that slightly varied the templated molecular arrangements. Sublimation of perfluorinated building units is green and efficient and non-destructive, which is potentially applicable in constructing template-directed chiroptical materials and devices.

Charge-Transfer Complex Doped Photothermal Hydrogels for Discriminating Circularly Polarized Near-Infrared Light.

Hydrogels behave as potential candidates to investigate circularly polarized light (CP)-matter interaction, which however suffer from small sensitivity towards circular polarization. Here we report a general protocol to build hydrogels from π-conjugated amino acids with coassembled charge-transfer (CT) complexes, covering a wide scope of donors and acceptors, which were incorporated into stable hydrogel matrices. CT complexes formed block coassemblies with gelators, induced the emergence of macroscopic chiral helices, where efficient chirality transfer occurs to realize tunable Cotton effects from visible light to NIR-I region depending on the structures of CT pairs. The hybrid hydrogels showed tunable photothermal performances with excellent heating-cooling cycling durability. Circularly polarized NIR light selectively triggered gel-solution phase transition at different timescales. Left- and right-CP illumination generates up to 2.5 folds difference in gel collapse time that allows for direct discrimination by naked eyes.

[N···I···N]+ Type Halogen-Bonding-Driven Supramolecular Helical Polymers with Modulated Chirality.

The [N···I···N]+ type halogen bond has been utilized to synthesize supramolecular architectures, while the applications in constructing helical motifs and modulating supramolecular chirality have been unexplored so far. In this work, the [N···I···N]+ halogen bond was introduced to drive the formation of supramolecular helical polymers via a Ag(I) coordination intermediate, showing tunable supramolecular chirality. Pyridine segments were conjugated to the asymmetric ferrocene skeleton, which show "open" and "closed" geometry depending on the sp2 N positions. Coordination with Ag(I) generated one-dimensional (1D) double helices and 2D helicates featured the [Ag(O)···I···Ag(O)]+ bond, which further stacked into 3D porous frameworks with chiral channels and adjustable pore sizes. Ionic exchange afforded 1D supramolecular helical polymers in solution phases driven by the [N···I···N]+ type halogen bonds, which was evidenced by the experimental results and density functional theory calculation. Fc2 exclusively demonstrated tunable supramolecular chirality in the formation of coordinated and halogen bonded polymers. In addition, solvent change would further inverse the helicity of halogen bonded supramolecular helical polymers depending on the rotation of the ferrocenyl core whose "closed" and "open" states were accompanied by the breakage of intramolecular hydrogen bonds. This work introduces a [N···I···N]+ type ionic halogen bond to prepare supramolecular helical polymers, providing multiple protocols in regulating helicity by ion exchange and solvent environments.

Steroid-Aromatics Clathrates as Chiroptical Materials with Circularly Polarized Luminescence and Phosphorescence.

Solid-state host-guest complexation shows great potential in the fabrication of chiroptical and phosphorescent materials. Developing chiral hosts toward achiral guests with a wide guest scope would expand the chiroptical application, which however remains a major challenge. Here, we report the steroid-aromatic compound complexation in the solid state that could realize effective chirality transfer, circularly polarized luminescence, and room temperature phosphorescence (RTP). Progesterone shows cocrystallization behavior toward a wide scope of guests through CH-π interaction, which also offers a rigid yet chiral microenvironment to entrap aromatic luminophores within the cavities or channels. Depending on the geometry of the guests, the handedness of the Cotton effects and circularly polarized luminescence could be tuned. Host-guest complexation not only gave rise to Cotton effects and circularly polarized luminescence but also stabilized the triplet state of bromo-compounds to achieve RTP and circularly polarized phosphorescence. This work first illustrates the application of steroid complexation in the chiroptical and phosphorescent materials, which shows potential in the display and information aspects.