Controllable Macroscopic Chirality of Coordination Polymers through pH and Anion-Mediated Weak Interactions

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.

Chirality induction to porphyrin derivatives co-confined at the air-water interface with silica nano-helices: towards enantioselective thin solid film surfaces

A supramolecular approach based on self-assembled structures allows the formation of large structured co-assemblies based on chiral and achiral compounds with original physicochemical features. In this contribution, an achiral and hydrophobic porphyrin was co-assembled at the air-water interface with mesoscopic silica nano-helices dispersed in the water subphase of a Langmuir trough without covalent bond formation. This procedure allowed transferring the porphyrin/nano-helix co-assemblies on a solid support within a thin hybrid layer. The interaction between the two species was characterized using spectroscopic techniques and atomic force microscopy. As evidenced by the circular dichroism measurements performed directly on solid films, tunable chirality was induced to the porphyrin aggregates according to the chirality of the silica nano-helices. When the co-assemblies were transferred on surface plasmon resonance (SPR) slides and exposed to aqueous solutions of histidine enantiomers, selective chiral discrimination was observed which was determined by the matching/mismatching between the chirality of the analyte and the helicity of the nano-helical structure.

Cu12-cluster-based metal-organic framework as a metastable intermediate in the formation of a layered copper phosphonate

The solvothermal reaction of CuSO4·5H2O and a chiral R-pempH2 ligand (molar ratio 6 : 1) first forms the metastable intermediate [Cu24(OH)20(R-pempH)8(SO4)10(H2O)10.5]·35H2O (1), followed by the formation of the stable phase [Cu2(OH)(R-pempH)(SO4)(H2O)]·H2O (2). Compound 1 displays a novel 3D open-framework structure containing Cu12 cluster nodes and sulfate links, which can be converted to the layered compound 2. We also investigated the photothermal effects of both compounds.

Photoisomerization and thermal reconstruction induced supramolecular chirality inversion in nanofiber determined by minority isomer

Here, amphiphile GCH based on glutamide-cyanostilbene is designed and synthesized, it is found that it can assembly in acetonitrile, and shows circular dichroism signals. After Z-E isomerizaition by UV irradiation, the CD signal of the assembly can be inverted. Unexpectedly, after another heating and cooling process, the circular dichroism signals can be totally inverted even though the E-isomers are in minority. Finally, the molecular dynamics (MD) simulations deeply elucidate the supramolecuar chirality inversion mechanism. This work brings some new insights into the control of chirality inversion, which may provide a perspective for the smart chiroptical materials construction.

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.

Stimuli-responsive synthetic helical polymers

Synthetic dynamic helical polymers (supramolecular and covalent) and foldamers share the helix as a structural motif. Although the materials are different, these systems also share many structural properties, such as helix induction or conformational communication mechanisms. The introduction of stimuli responsive building blocks or monomer repeating units in these materials triggers conformational or structural changes, due to the presence/absence of the external stimulus, which are transmitted to the helix resulting in different effects, such as assymetry amplification, helix inversion or even changes in the helical scaffold (elongation, J/H helical aggregates). In this review, we show through selected examples how different stimuli (e.g., temperature, solvents, cations, anions, redox, chiral additives, pH or light) can alter the helical structures of dynamic helical polymers (covalent and supramolecular) and foldamers acting on the conformational composition or molecular structure of their components, which is also transmitted to the macromolecular helical structure.

Enhancing the Circularly Polarized Luminescence of Europium Coordination Polymers by Doping a Chromophore Ligand into Superhelices

Lanthanide-based molecular materials showing efficient circularly polarized luminescence (CPL) activity with a high quantum yield are attractive due to their potential applications in data storage, optical sensors, and 3D displays. Herein we present an innovative method to achieve enhanced CPL activity and a high quantum yield by doping a chromophore ligand into a coordination polymer superhelix. A series of homochiral europium(III) phosphonates with a helical morphology were prepared with the molecular formula S-, R-[Eu(cyampH)3-3n(nempH)3n]·3H2O (S/R-Eu-n, n = 0-5%). The doping of chromophore ligand S- or R-nempH2 into superhelices of S/R-Eu-0% not only turned on the CPL activity with the dissymmetry factor |glum| on the order of 10-3 but also increased the quantum yield by about 14-fold. This work may shed light on the development of efficient CPL-active lanthanide-based coordination polymers for applications.

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

Assembling macroscopic helices with controllable chirality and understanding their formation mechanism are highly desirable but challenging tasks for artificial systems, especially coordination polymers. Here, we utilize solvents as an effective tool to induce the formation of macroscopic helices of chiral coordination polymers (CPs) and manipulate their helical sense. We chose the Ni/R-,S-BrpempH2 system with a one-dimensional tubular structure, where R-,S-BrpempH2 stands for R-,S-(1-(4-bromophenyl)ethylaminomethylphosphonic acid). The morphology of the self-assemblies can be controlled by varying the cosolvent in water, resulting in the formation of twisted ribbons of R-,S-Ni(Brpemp)(H2O)·H2O (R-,S-2T) in pure H2O; needle-like crystals of R-,S-Ni(Brpemp)(H2O)2·1/3CH3CN (R-,S-1C) in 20 vol % CH3CN/H2O; nanofibers of R-,S-Ni(Brpemp)(H2O)·H2O (R-,S-3F) in 20-40 vol % methanol/H2O or ethanol/H2O; and superhelices of R-,S-Ni(Brpemp)(H2O)·H2O (R-,S-4H or 5H) in 40 vol % propanol/H2O. Interestingly, the helicity of the superhelix can be controlled by using a propanol isomer in water. For the Ni/R-BrpempH2 system, a left-handed superhelix of R-4H(M) was obtained in 40 vol % NPA/H2O, while a right-handed superhelix of R-5H(P) was isolated in 40 vol % IPA/H2O. These results were rationalized by theoretical calculations. Adsorption studies revealed the chiral recognition behavior of these compounds. This work may contribute to the development of chiral CPs with a macroscopic helical morphology and interesting functionalities.

Macroscopic handedness inversion of terbium coordination polymers achieved by doping homochiral ligand analogues

Inspired by natural biological systems, chiral or handedness inversion by altering external and internal conditions to influence intermolecular interactions is an attractive topic for regulating chiral self-assembled materials. For coordination polymers, the regulation of their helical handedness remains little reported compared to polymers and supramolecules. In this work, we choose the chiral ligands R-pempH2 (pempH2 = (1-phenylethylamino)methylphosphonic acid) and R-XpempH2 (X = F, Cl, Br) as the second ligand, which can introduce C-H⋯π and C-H⋯X interactions, doped into the reaction system of the Tb(R-cyampH)3·3H2O (cyampH2 = (1-cyclohexylethylamino)methylphosphonic acid) coordination polymer, which itself can form a right-handed superhelix by van der Waals forces, and a series of superhelices R-1H-x, R-2F-x, R-3Cl-x, and R-4Br-x with different doping ratios x were obtained, whose handedness is related to the second ligand and its doping ratio, indicating the decisive role of interchain interactions of different strengths in the helical handedness. This study could provide a new pathway for the design and self-assembly of chiral materials with controllable handedness and help the further understanding of the mechanism of self-assembly of coordination polymers forming macroscopic helical systems.

Effect of Pd(II) uptake on high-temperature phase transitions in a hybrid organic-inorganic perovskite semiconductor

Hybrid organic-inorganic perovskites (HOIPs) have been widely studied for their interesting functions and potential applications. Here, we report a novel sulfur-containing hybrid organic-inorganic perovskite based on a one-dimensional ABX₃-type compound: [C₃H₇N₂S]PbI₃ ([C₃H₇N₂S]⁺ is 2-amino-2-thiazolinium) (1). Compound 1 undergoes two high-temperature phase transitions at 363 K and 401 K, respectively, showing a band gap of 2.33 eV, and has a narrower band gap compared to other one-dimensional materials. Moreover, by introducing thioether groups into the organic component, 1 has the ability to uptake Pd(II) ions. Compared with previously reported low-temperature isostructural phase transition sulfur-containing hybrids, the molecular motion of 1 becomes more intense under the stimulation of high temperature, leading to changes in the space group during the two phase transitions (Pbca → Pmcn → Cmcm), which are no longer the previous isostructural phase transitions. Significant changes in the phase transition behavior and semiconductor properties before and after metal absorption make it possible to monitor the absorption process of metal ions. The study of the effect of Pd(II) uptake on phase transitions may be helpful to reveal the mechanism of phase transitions more deeply. This work will broaden the hybrid organic-inorganic ABX₃-type semiconductor family and pave the way for the development of organic-inorganic hybrid-based multifunctional phase transition materials.

pH and Salt-Assisted Macroscopic Chirality Inversion of Gadolinium Coordination Polymer

The precise adjustment of handedness of helical architectures is important to regulate their functions. Macroscopic chirality inversion has been achieved in organic supramolecular systems by pH, metal ions, solvents, chiral and non-chiral additives, temperature, and light, but rarely in coordination polymers (CPs). In particular, salt-assisted macroscopic chirality inversion has not been reported. In this work, we carried out a systematic investigation on the role of pH and salt in regulating the morphology of CPs based on Gd(NO₃)₃ and R-(1-phenylethylamino)methylphosphonic acid (R-pempH₂). Without extra NO₃^-, the chirality inversion from the left-handed superhelix R-M to the right-handed superhelix R-P can be achieved by pH modulation from 3.2 to 3.8. The addition of NaNO₃ (2.0 eq) at pH 3.8 results in an inversion of chiral sense from R-P to R-M as a pure phase. To our knowledge, this is the first example of salt-assisted macroscopic helical inversion in artificial systems.

Multiple chirality inversion of pyridine Schiff-base cholesterol-based metal-organic supramolecular polymers

Based on a metal coordination driven co-assembly strategy, a metal-organic supramolecular polymer system of pyridine Schiff-base cholesterol and metal ions with multiple supramolecular chirality inversion was successfully achieved by the stoichiometry and exchange of metal ions (such as Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, and Ag⁺), as well as the solvent polarity.