Macroscopic Helical Assembly of One-Dimensional Coordination Polymers: Helicity Inversion Triggered by Solvent Isomerism

Pathway Complexity of Kinetically Trapped Dipeptide-Based Metastable State: Supramolecular Structural Transformation and Helicity Tuning

Understanding the complexity of nanostructures involved during the supramolecular polymerization process can be achieved by kinetic control rather than thermodynamic stability. Study on supramolecular pathway complexity and associated nanostructures will provide precise control over the materials' properties. This work illustrates the pathway complexity and structural transformation of a naphthalimide-(NMI)-conjugated dipeptide from monomer to thermodynamically stable aggregated state in a binary mixed solvent system (DMSO and water). The self-assembly propensity can be modulated by changing the ratio of water, which offers an effective approach to provide kinetic stability to the on-pathway gel state before reaching its thermodynamically stable crystalline precipitate state. An in-depth spectroscopic and microscopic investigation suggested that the self-assembly process initiated the formation of tiny particles, which further nucleated to form a helical nanofibrilar assembly. At higher water percentages, the supramolecular gel state showed a transient behavior and proceeded toward its thermodynamic stability. However, at lower water percentage, the self-assembly process is kinetically trapped in its gel state. Here, the helicity of nanofibers can be modulated by altering the percentage of water in the mixed solvent. The self-assembled system is completely thermoreversible and can retain its chiral memory even after complete dissolution in the respective solvent composition.

Homochiral layered indium phosphonates: solvent modulation of morphology and chiral discrimination adsorption

Assembling chiral coordination polymers into nano/microflower structures may improve their performance in applications such as chiral recognition and separation. In this study, we chose a chiral metal phosphonate system, i.e., In(NO3)3/R-, S-pempH2 [pempH2 = (1-phenylethylamino)methylphosphonic acid], and carried out systematic work on the self-assembly of this system in different alcohol/H2O mixed solvents under solvothermal conditions. Enantiomeric compounds R-, S-[In2(pempH)2(μ2-OH)2(H2O)2](NO3)2 (R-, S-1) were obtained showing dense layered structures, but their morphologies varied with alcohol solvent. We obtained crystals of R-, S-1C in triethylene glycol (TEG)/H2O mixed solvent and spiral microflowers of R-, S-1MF in isopropanol (IPA)/H2O mixed solvent. Adsorption isotherms using enantiopure R-2-butanol and S-2-butanol as probe molecules revealed that compounds R-1MF and S-1MF had higher adsorption capacity compared to the crystalline sample and displayed enantioselective adsorption behaviour.

Hollow Superhelices Based on Chiral Europium Coordination Polymers

The construction of helical nanotubes based on chiral coordination polymers (CPs) is an intriguing but challenging task, which is important for the development of functional materials that combine macroscopic chirality with tube-related properties. Here, we selected a chiral europium phosphonate system, e. g., Eu(NO3)3/R-,S-pempH2, and carried out a systematic work. By controlling the hydrothermal reaction conditions such as the pH value of the reaction mixture, the molar ratio and concentration of the reactants, we obtained block-like crystals of R/S-1 b, rod-like crystals of R/S-3 r, hollow superhelices of R/S-2 hh, and solid superhelices of R/S-4 sh. In the latter two cases, the chirality has been successfully transferred and amplificated from the molecular level to the macroscopic level. Interestingly, compounds R/S-2 hh and R/S-4 sh have the same chemical composition of Eu(R/S-pempH)3⋅2H2O and show identical PXRD patterns, thus can be considered as the same material except for different morphologies. We further investigated their circularly polarized luminescence (CPL) properties and found that the hollow superhelix of R/S-2 hh had a larger dissymmetry factor than the solid superhelix of R/S-4 sh. This study not only provides the first example of hollow superhelices of chiral CPs, but also offers the possibility of modulating the chiroptical properties of CPs through morphological control.

Mechanically Twisting-Induced Top-Down Chirality Transfer for Tunable Full-Color Circularly Polarized Luminescent Fibers

Circularly polarized luminescence (CPL) materials with rich optical information are highly attractive for optical display, information storage, and encryption. Although previous investigations have shown that external force fields can induce CPL activity in nonchiral systems, the unique role of macroscopic external forces in inducing CPL has not been demonstrated at the level of molecule or molecular aggregate. Here, a canonical example of CPL generation by mechanical twisting in an achiral system consisting of a polymer matrix with embedded fluorescent molecules is presented. By carefully adjusting the twisting parameters in time and space, in conjunction with circular dichroism (CD), CPL, and 2D wide-angle X-ray scattering (2D WAXS) studies, a twisting-induced top-down chiral transfer mechanism derived from the molecular-level asymmetric rearrangement of fluorescent units is elucidated within polymers under external torsional forces. This top-down chiral transfer provides a simple, scalable, and versatile mechanical twisting strategy for the fabrication of CPL materials, allowing for fabricating full-color and handedness-tunable CPL fibers, where the macroscopic twist direction determines the CPL handedness. Moreover, the weavability of CPL fibers greatly extend their applications in anti-counterfeit encryption, as demonstrated by using embroidery techniques to design multilevel encryption patterns.

Helical Polyallenes: From Controlled Synthesis to Distinct Properties

Polyallenes with appropriate pendants can form stable helices and exhibit significant optical activity. These helical polyallenes contain reactive double bonds that allow for further functionalization, making them a class of chiral functional materials with broad application prospects. This review article delves into the intricacies of synthesizing well-defined helical polyallenes through controlled synthetic methodologies, including helix-sense selective living polymerization, regioselective and asymmetric living polymerization, and one-pot block copolymerization of allenes with aryl monomers. The systemically outlined characteristics of the resulting helical polyallenes and related copolymers are summarized include their unique chiroptical properties, stimuli-responsiveness, helix-induced chiral self-assembly, and circularly polarized luminescence (CPL). Additionally, current challenges and future perspectives in the research of controlled synthesis, functionalities, and applications of helical polyallenes are discussed in detail.

Homochiral Covalent Organic Frameworks with Superhelical Nanostructures Enable Efficient Chirality-Induced Spin Selectivity

Despite significant advancements in fabricating covalent organic frameworks (COFs) with diverse morphologies, creating COFs with superhelical nanostructures remains challenging. We report here the controlled synthesis of homochiral superhelical COF nanofibers by manipulating pendent alkyl chain lengths in organic linkers. This approach yields homochiral 3D COFs 13-OR with a 10-fold interpenetrated diamondoid structure (R=H, Me, Et, nPr, nBu) from enantiopure 1,1'-bi-2-naphthol (BINOL)-based tetraaldehydes and tetraamine. COF-13-OEt exhibits macroscopic chirality as right-handed and left-handed superhelical fibers, whereas others adopt spherical or non-helical morphologies. Time-tracking shows a self-assembly process from non-helical strands to single-stranded helical fibers and intertwined superhelices. Ethoxyl substituents, being of optimal size, balance solvophobic effects and intermolecular interactions, driving the formation of superhelical nanostructures, with handedness determined by BINOL chirality. The superhelical nature of these materials is evident in their chiral recognition and spin-filter properties, showing significantly improved enantiodiscrimination in carbohydrate binding (up to six times higher enantioselectivity) and a remarkable chiral-induced spin selectivity (CISS) effect with a 48-51 % spin polarization ratio, a feature absent in non-helical analogs.

Dynamic Dance of Chirality and Morphology: Interplay of Solvent-Sensitive Self-Assembly in Topological Evolution and Chirality Amplification

The building block Pyra-Chol has been designed and synthesized, which exhibits different achiral morphologies in good solvents, forming nanospheres in THF and nanoflowers in 1,4-dioxane. In the presence of water as a poor cosolvent, Pyra-Chol demonstrates an agnostic behavior, generating left-handed superhelices in the water:THF (80:20) solvent system. However, when the good solvent is switched to 1,4-dioxane, a change in chirality is observed in the water:1,4-dioxane (30:70) solvent system, resulting in the formation of fused nanospheres. Interestingly, when the poor cosolvent is changed from water to MCH in THF, the chiral pattern remains unchanged, but the morphology changes completely. Supported by the collective spectroscopic and microscopic analysis, the present study efficaciously demonstrates the remarkable control of hydrophobic building block over the chiral sense and also highlights the fascinating influence of good as well as poor cosolvent in supporting the distinct molecular packing.

Porous Metal Phosphonate Frameworks: Construction and Physical Properties

ConspectusPorous metal phosphonate frameworks (PMPFs) as a subclass of metal-organic frameworks (MOFs) have promising applications in the fields of gas adsorption and separation, ion exchange and storage, catalysis, sensing, etc. Compared to the typical carboxylate-based MOFs, PMPFs exhibit higher thermal and water stability due to the strong coordination ability of the phosphonate ligands. Despite their robust frameworks, PMPFs account for less than 0.51% of the porous MOFs reported so far. This is because metal phosphonates are highly susceptible to the formation of dense layered or pillared-layered structures, and they precipitate easily and are difficult to crystallize. There is a tendency to use phosphonate ligands containing multiple phosphonate groups and large organic spacers to prevent the formation of dense structures and generate open frameworks with permanent porosity. Thus, many PMPFs are composed of chains or clusters of inorganic metal phosphonates interconnected by organic spacers. Using this feature, a wide range of metal ions and organic components can be selected, and their physical properties can be modulated. However, limited by the small number of PMPFs, there are still relatively few studies on the physical properties of PMPFs, some of which merely remain in the description of the phenomena and lack in-depth elaboration of the structure-property relationship. In this Account, we review the strategies for constructing PMPFs and their physical properties, primarily based on our own research. The construction strategies are categorized according to the number (n = 1-4) of phosphonate groups in the ligand. The physical properties include proton conduction, electrical conduction, magnetism, and photoluminescence properties. Proton conductivity of PMPFs can be enhanced by increasing the proton carrier concentration and mobility. The former can be achieved by adding acidic groups such as -POH and/or introducing acidic guests in the hydrophilic channels. The latter can be attained by introducing conjugate acid-base pairs or elevating the temperature. Semiconducting PMPFs, on the other hand, can be obtained by constructing highly conjugated networks of coordination bonds or introducing large conjugated organic linkers π-π stacked in the lattice. In the case of magnetic PMPFs, long-range magnetic ordering occurs at very low temperatures due to very weak magnetic exchange couplings propagated via O-P-O and/or O(P) units. However, lanthanide compounds may be interesting candidates for single-molecule magnets because of the strong single-ion magnetic anisotropy arising from the spin-orbit coupling and large magnetic moments of lanthanide ions. The luminescent properties of PMPFs depend on the metal ions and/or organic ligands. Emissive PMPFs containing lanthanides and/or uranyl ions are promising for sensing and photonic applications. We conclude with an outlook on the opportunities and challenges for the future development of this promising field.

Successively Controlling Nanoscale Wrinkles of Ultrathin 2D Metal-Organic Frameworks Nanosheets

The wrinkles are pervasive in ultrathin two-dimensional (2D) materials, but the regulation of wrinkles is rarely explored systematically. Here, we employed a series of carboxylic acids (from formic acid to octanoic acid) to control the wrinkles of Zr-BTB (BTB=1, 3, 5-(4-carboxylphenyl)-benzene) metal-organic framework (MOF) nanosheet. The wrinkles at the micrometer scale were observed with transmission electron microscopy. Furthermore, high-angle annular dark-field (HAADF) images showed lattice distortion in many nanoscale regions, which was precisely matched to the nano-wrinkles. With the changes of hydrophilicity/hydrophobicity, MOF-MOF and MOF-solvent interactions were possibly synergistically regulated and wrinkles with different sizes were obtained, which was supported by HAADF, molecular dynamics, and density functional theory calculation. Different wrinkle sizes resulted in different pore sizes between the Zr-BTB nanosheet interlayers, providing highly-oriented thin films and the successive optimization of kinetic diffusion pathways, proved by grazing-incidence wide-angle X-ray scattering and nitrogen adsorption. The most suitable wrinkle pore from Zr-BTB-C4 exhibited highly efficient chromatographic separation of the substituted benzene isomers. Our work provides a rational route for the modulation of nanoscale wrinkles and their stacked pores of MOF nanosheets and improves the separation abilities of MOFs.

Chiral Macrocycles for Enantioselective Recognition

The efficient synthesis of chiral macrocycles with highly enantioselective recognition remains a challenge. We have addressed this issue by synthesizing a pair of chiral macrocycles, namely, R/S-BINOL[2], achieving total isolated yields of up to 62% through a two-step reaction sequence. These macrocycles are readily purified by column chromatography over silica gel without the need for chiral separation, thus streamlining the overall synthesis. R/S-BINOL[2] demonstrated enantioselective recognition toward chiral ammonium salts, with enantioselectivity (KS/KR) values reaching up to 13.2, although less favorable separations were seen for other substrates. R/S-BINOL[2] also displays blue circularly polarized luminescence with a |glum| value of up to 2.2 × 10-3. The R/S-BINOL[2] macrocycles of this study are attractive as chiral hosts in that they both display enantioselective guest recognition and benefit from a concise, high-yielding synthesis. As such, they may have a role to play in chiral separations.

Achiral Solvent Inversed Helical Pathway and Cosolvent Controlled Excited-State "Majority Rule" in Enantiomeric Dansulfonamide Assemblies

Achiral solvents are commonly utilized to induce the self-assembly of chiral molecules. This study demonstrates that achiral solvents can trigger helicity inversion in the assemblies of dansyl amphiphiles and control the excited-state "majority rule" in assemblies composed of pure enantiomers, through variation of the cosolvent ratio. Specifically, enantiomers of dansyl amphiphiles self-assemble into helical structures with opposite handedness in methanol (MeOH) and acetonitrile (MeCN), together with inversed circular dichroism and circularly polarized luminescence (CPL) signals. When a mixture of MeOH and MeCN is employed, the achiral cosolvents collectively affect the CPL of the assemblies in a way similar to that of "mixed enantiomers". The dominant cosolvent governs the CPL signal. As the cosolvent composition shifts from pure MeCN to MeOH, the CPL signals undergo a significant inversion and amplification, with two maxima observed at ≈20% MeOH and 20% MeCN. This study deepens the comprehension of how achiral solvents modulate helical nanostructures and their excited-state chiroptical properties.

Macroscopic homochiral helicoids self-assembled via screw dislocations

Chirality is a fundamental property in nature and is widely observed at hierarchical scales from subatomic, molecular, supramolecular to macroscopic and even galaxy. However, the transmission of chirality across different length scales and the expression of homochiral nano/microstructures remain challenging. Herein, we report the formation of macroscopic homochiral helicoids with ten micrometers from enantiomeric pyromellitic diimide-based molecular triangle (PMDI-Δ) and achiral pyrene via a screw dislocation-driven co-self-assembly. Chiral transfer and expression from molecular and supramolecular levels, to the macroscopic helicoids, is continuous and follows the molecular chirality of PMDI-Δ. Furthermore, the screw dislocation and chirality transfer lead to a unidirectional curvature of the helicoids, which exhibit excellent circularly polarized luminescence with large |glum| values up to 0.05. Our results demonstrate the formation of a homochiral macroscopic organic helicoid and function emergence from small molecules via screw dislocations, which deepens our understanding of chiral transfer and expression across different length scales.

Chiral Assembly of Perovskite Nanocrystals: Sensitive Discrimination of Amino Acid Enantiomers

Chirality is a widespread phenomenon in nature and in living organisms and plays an important role in living systems. The sensitive discrimination of chiral molecular enantiomers remains a challenge in the fields of chemistry and biology. Establishing a simple, fast, and efficient strategy to discriminate the spatial configuration of chiral molecular enantiomers is of great significance. Chiral perovskite nanocrystals (PNCs) have attracted much attention because of their excellent optical activity. However, it is a challenge to prepare perovskites with both chiral and fluorescence properties for chiral sensing. In this work, we synthesized two chiral fluorescent perovskite nanocrystal assembly (PNA) enantiomers by using l- or d-phenylalanine (Phe) as chiral ligands. PNA exhibited good fluorescence recognition for l- and d-proline (Pro). Homochiral interaction led to fluorescence enhancement, while heterochiral interaction led to fluorescence quenching, and there is a good linear relationship between the fluorescence changing rate and l- or d-Pro concentration. Mechanism studies show that homochiral interaction-induced fluorescence enhancement is attributed to the disassembly of chiral PNA, while no disassembly of chiral PNA was found in heterochiral interaction-induced fluorescence quenching, which is attributed to the substitution of Phe on the surface of chiral PNA by heterochiral Pro. This work suggests that chiral perovskite can be used for chiral fluorescence sensing; it will inspire the development of chiral nanomaterials and chiral optical sensors.

Solvent modulation of the morphology of homochiral gadolinium coordination polymers and its impact on circularly polarized luminescence

Studying the effect of morphology on the circularly polarized luminescence (CPL) of chiral molecular materials is important for the development of CPL-active materials for applications. Herein, we report that the morphology of Gd(NO3)3/R-,S-AnempH2 [AnempH2 = (1-anthrylethylamino)methylphosphonic acid] assemblies can be controlled by solvent modulation to form spiral bundles Gd(R-,S-AnempH)3·2H2O (R-,S-1), crystals Gd(R-,S-AnempH)3·2H2O (R-,S-2) and spindle-shaped particles Gd(R-,S-AnempH)3·3H2O·0.5DMF (R-,S-3) with similar chain structures. Interestingly, R-,S-1 are CPL active and show the highest value of dissymmetric factor among the three pairs of enantiomers (|glum| = 2.1 × 10-3), which is 2.8 times larger than that of R-,S-2, while R-,S-3 are CPL inactive with |glum| ≈ 0. This work provides a new route to control the morphology of chiral coordination polymers and improve their CPL performance.