Self-Assembled Nanohelixes Driven by Host-Guest Interactions and Metal Coordination

Helical nanostructures fabricated via the self-assembly of artificial motifs have been a captivating subject because of their structural aesthetics and multiple functionalities. Herein, we report the facile construction of a self-assembled nanohelix (NH) by leveraging an achiral aggregation-induced emission (AIE) luminogen (G) and pillar[5]arene (H), driven by host-guest interactions and metal coordination. Inspired by the "sergeants and soldiers" effect and "majority rule" principle, the host-guest complexation between G and H is employed to fixate the twisted conformation of G for the generation of "contortion sites", which further induced the emergence of helicity as the 1D assemblies are formed via Ag(I) coordination and hexagonally packed into nano-sized fibers. The strategy has proved feasible in both homogeneous and heterogeneous syntheses. Along with the formation of NH, boosted luminescence and enhanced productivity of reactive oxygen species (ROS) are afforded because of the efficient restriction on G, indicating the concurrent regulation of NH's morphology and photophysical properties by supramolecular assembly. In addition, NH also exhibits the capacity for bacteria imaging and photodynamic antibacterial activities against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli).

Enantioselective Self-Assembly of a Homochiral Tetrahedral Cage Comprising Only Achiral Precursors

How Nature synthesizes enantiomerically pure substances from achiral or racemic resources remains a mystery. In this study, we aimed to emulate this natural phenomenon by constructing chiral tetrahedral cages through self-assembly, achieved by condensing two achiral compounds-a trisamine and a trisaldehyde. The occurrence of intercomponent CH⋅⋅⋅π interactions among the phenyl building blocks within the cage frameworks results in twisted conformations, imparting planar chirality to the tetrahedrons. In instances where the trisaldehyde precursor features electron-withdrawing ester side chains, we observed that the intermolecular CH⋅⋅⋅π forces are strong enough to prevent racemization. To attain enantioselective self-assembly, a chiral amine was introduced during the imine formation process. The addition of three equivalents of chiral amino mediator to one equivalent of the achiral trisaldehyde precursor formed a trisimino intermediate. This chiral compound was subsequently combined with the achiral trisamino precursor, leading to an imine exchange reaction that releasing the chiral amino mediator and formation of the tetrahedral cage with an enantiomeric excess (ee) of up to 75 %, exclusively composed of achiral building blocks. This experimental observation aligns with theoretical calculations based on the free energies of related cage structures. Moreover, since the chiral amine was not consumed during the imine exchange cycle, it enabled the enantioselective self-assembly of the tetrahedral cage for multiple cycles when new batches of the achiral trisaldehyde and trisamino precursors were successively added.

Planar Chirality for Acid/Base Responsive Macrocyclic Pillararenes Induced by Amino Acid Derivatives: Molecular Dynamics Simulations and Machine Learning

Chirality is ubiquitous in nature, ranging from a DNA helix to a biological macromolecule, snail's shell, and even a galaxy. However, the precise control of chirality at the nanoscale is a challenge due to the structure complexity of supramolecular assemblies, the small energy differences between different enantiomers, and the difficulty in obtaining polymorphic crystals. The planar chirality of water-soluble pillar[5]arenes (called WP5-Na with Na ions in the side chain) host triggered by the addition of chiral L-amino acid hydrochloride (L-AA-OEt) guests and acid/base is rationalized by the relative stability of different chiral isomers, being estimated by molecular dynamics (MD) simulations and quantum chemical calculations. As an increase in the pH value, the change from a positive to a negative value of the free energy difference (ΔG) between two conformations, pR-WP5-Na⊃L-AA-OEt and pS-WP5-Na⊃L-AA-OEt, suggests an inversed preference of the pS-WP5-Na conformer induced by the deprotonated L-arginine ethyl ester (L-Arg-OEt) at pH = 14, which is supported by the circular dichroism (CD) experiments. On the basis of 2256 WP5-Na⊃L-Ala-OEt and 3299 WP5-Na⊃L-Arg-OEt conformers sampled from MD, the gradient boosting regression (GBR) model exhibits a satisfactory performance (R² = 0.91) in predicting the chirality of WP5-Na complexations using host-guest binding descriptors, including the geometry matching and binding sites and modes (electrostatics and hydrogen bonding). The machine learning model also performs well on external tests of different hosts (using different side chains and cavity sizes) with the addition of 22 other different guests, with the average chirality prediction accuracy of ML versus experimental CD determinations of 92.8%. The easily accessible host-guest features, binding position coordination and size matching between the cavity and guest, exhibit a close correlation to the chirality of different macrocyclic molecules, water-soluble pillar[6]arenes (WP6) versus WP5, in complexation with different amino acid guests. The exploration of efficient host-guest features in ML displays the great potential of building a large space of various assembled systems and accelerating the on-demand design of chiral supramolecular systems at the nanoscale.

Circular Dichroism Based Chirality Sensing with Supramolecular Host-Guest Chemistry

Optical methods are promising to address the ever-increasing demands for chirality analysis in drug discovery and related fields because they are amenable to high-throughput screening. Circular dichroism-based chiroptical sensing using host-guest chemistry is especially appealing due to the fast equilibrium kinetics, wide substrate scope, and potential for sustainable development. In this Minireview, we give an overview on this emerging field. General aspects of molecular recognition and chirality transfer are analyzed. Chirality sensors are discussed by dividing them into three classes according to their structural features. Applications of these chirality sensors for chirality analysis of the products of asymmetric reactions and for the real-time monitoring of reaction kinetics are demonstrated with selected examples. Moreover, challenges and research directions in this field are also highlighted.

Hysteresis Nanoarchitectonics with Chiral Gel Fibers and Achiral Gold Nanospheres for Reversible Chiral Inversion

Intelligent control over the handedness of circular dichroism (CD) is of special significance in self-organized biological and artificial systems. Herein, we report a chiral organic molecule (R1) containing a disulfide unit self-assembles into M-type helical fibers gels, which undergoes chirality inversion by incorporating gold nanospheres due to the formation of Au-S bonds between R1 and gold nanospheres. Upon heating at 80oC, the aggregation of gold nanospheres results in a disappearance of the Au-S bond, allowing the reversible switching back to M-type helical fibers. The original chirality of M-type fibers could also be retained by adding anisotropic gold nanorods. A series of characterization methods, involving CD, Raman, Infrared spectroscopy, electric microscopy, and small-angle X-ray scattering (SAXS) measurements were used to investigate the mechanism of chiral evolutions. Our results provide a facile way of fabricating hysteresis nanoarchitectonics to achieve dynamic supramolecular chirality using inorganic metallic nanoparticles.

Multilevel Chirality Transfer from Amino Acid Derivatives to Circularly Polarized Luminescence-Active Nanoparticles in Aqueous Medium

Chirality at different levels is widely observed in nature, but the clue to connect it all together, and the way chirality transfers among different levels are still obscure. Herein, a l-/d-lysine-based self-assembly system was constructed, in which two-step chirality transfer among three different levels was observed in aqueous solution. The chirality originated from the point chirality of amino acid derivatives l-/d-PyLys hydrochloride, and was transferred to the planar conformational chirality of water-soluble pillar[5]arene pR-/pS-WP5. Then, with the aid of pR-/pS-WP5, nanoparticles were formed that exhibited L-/R-handed circularly polarized luminescence with a dissymmetry factor of up to ±0.001, arising from pyrene chiral excimers. This multilevel chirality transfer not only provides a perspective to trace potential clues, and to pursue certain ways by which the chirality transfers, but also offers a strategy to create controllable CPL emission in aqueous media.

Saucer[n]arenes: Synthesis, Structure, Complexation, and Guest-Induced Circularly Polarized Luminescence Property

Macrocycles denoted as saucer[n]arenes (n=4,5) were easily synthesized by the one-pot condensation of 2,7-dimethoxynaphthalene (2,7-DMN) and paraformaldehyde in the presence of TFA or catalytic BF₃ ⋅OEt₂ . With 1,1-dimethylpiperidin-1-ium as the template, saucer[4]arene was selectively obtained. Crystal structures show that saucer[n]arenes are all composed of 2,7-DMN moiety bridged by the methylene groups at 1,6-positions: all of the 7-methoxy groups lie on one face, and all of the 2-methoxy groups lie on the other. Saucer[n]arenes exhibit strong fluorescence properties with the quantum yields of 19.6 % and 23.4 %. They form 1:1 complexes with ammonium salts in both solution and solid state (association constant up to 10⁵  M^-1 in CDCl₃ ). Chiral quaternary ammonium salts can induce the chirality of the dynamically racemic inherently chiral saucer[n]arenes in solution, and thus show mirror-imaged circular dichroism signals and circularly polarized luminescence (CPL) properties.

Deracemization of Carbohelicenes by a Chiral Perylene Bisimide Cyclophane Template Catalyst

Deracemization describes the conversion of a racemic mixture of a chiral molecule into an enantioenriched mixture or an enantiopure compound without structural modifications. Herein, we report an inherently chiral perylene bisimide (PBI) cyclophane whose chiral pocket is capable of transforming a racemic mixture of [5]-helicene into an enantioenriched mixture with an enantiomeric excess of 66 %. UV/Vis and fluorescence titration studies reveal this cyclophane host composed of two helically twisted PBI dyes has high binding affinities for the respective homochiral carbohelicene guests, with outstanding binding constants of up to 3.9×10¹⁰  m^-1 for [4]-helicene. 2D NMR studies and single-crystal X-ray analysis demonstrate that the observed strong and enantioselective binding of homochiral carbohelicenes and the successful template-catalyzed deracemization of [5]-helicene can be explained by the enzyme-like perfect shape complementarity of the macrocyclic supramolecular host.

A Self-Assembled Cage for Wide-Scope Chiral Recognition in Water

Herein, we report the self-assembly of an anionic homochiral octahedral cage by condensing six Ga³⁺ cations and four trisacylhydrazone ligands. The robust nature of the hydrazone bond renders the cage stable in water, where it can take advantage of the hydrophobic effect for host-guest recognition. In addition to the internal binding site, namely, the inner cavity, the octahedral cage possesses four "windows", each of which represents an external binding site allowing peripheral complexation. These internal and external binding sites endow the cage with the capability to bind a broad range of guests whose sizes could either be smaller than or exceed the volume of the cage's inner cavity. Upon accommodation of a chiral guest, one of the two cage enantiomers becomes more favored than the other, producing circular-dichroism (CD) signals. The CD signal intensity of the cage is observed to be proportional to the ee value of the chiral guest, allowing a quantitative determination of the latter.

Supramolecular Chirogenesis Induced by Platinum(II) Tweezers with Excellent Environmental Tolerance

Supramolecular chirogenesis has emerged as an effective strategy to access symmetry breaking in artificial systems. However, the chirogenic signals suffer from high susceptibility toward environmental variations. An effective strategy has been developed to address this issue by constructing platinum(II)-based tweezer/guest complexes stabilized by two-fold donor-acceptor and Pt^II -Pt^II metal-metal interactions. Upon guest encapsulation, the two pincers on the achiral Pt^II tweezer undergo a stereospecific twist to minimize steric repulsion, thus locking tweezer/guest complexes into the preferred chiral conformations. The induced chiroptical effects display outstanding solvent and temperature tolerance, ascribed to the balance between electrostatic and desolvation effects for the involved non-covalent interactions. Moreover, hierarchical and multi-component supramolecular assembly of tweezer/guest complexes provide a convenient way to modulate chirogeneic signals for their intensities.

Accelerating Crystallization of Open Organic Materials by Poly(ionic liquid)s

The capability to significantly shorten the synthetic period of a broad spectrum of open organic materials presents an enticing prospect for materials processing and applications. Herein we discovered 1,2,4‐triazolium poly(ionic liquid)s (PILs) could serve as a universal additive to accelerate by at least one order of magnitude the growth rate of representative imine‐linked crystalline open organics, including organic cages, covalent organic frameworks (COFs), and macrocycles. This phenomenon results from the active C5‐protons in poly(1,2,4‐triazolium)s that catalyze the formation of imine bonds, and the simultaneous salting‐out effect (induced precipitation by decreasing solubility) that PILs exert on these crystallizing species.